Complete Dissertation for Al-Juhani

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INCORPORATION OF LESS TOXIC ANTIFOULING COMPOUNDS INTO

SILICONE COATINGS TO STUDY THEIR RELEASE BEHAVIORS

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirement for the Degree

Doctor of Philosophy

Abdulhadi Abdullah Al-Juhni

August, 2006



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INCORPORATION OF LESS TOXIC ANTIFOULING COMPOUNDS INTO

SILICONE COATINGS TO STUDY THEIR RELEASE BEHAVIORS

Abdulhadi Abdullah Al-Juhni

Dissertation

Approved: Accepted:

Advisor Department Chair Dr. Bi-min Zhang Newby Dr. Lu-Kwang Ju

Committee Member Dean of the CollegeDr. George G. Chase Dr. George K. Haritos

Committee Member Dean of the Graduate SchoolDr. Lu-Kwang Ju Dr. George R. Newkome

Committee Member DateDr. Gerald W. Young

Committee Member Dr. Teresa J. Cutright





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order of: benzoic acid > capsaicin > sodium benzoate > tannic acid. The solvent-assisted

blending technique was adequate for the cases of sodium benzoate and tannic acid

whereas it was not suitable for benzoic acid and capsaicin. Sodium benzoate/Sylgard®

184 coating was then selected as the model system to obtain the miscibility-release

relationship. The preparation conditions were found to have important effects on the

morphological structure and final distribution of sodium benzoate in silicone, hence the

leaching. The minimum average aggregate size obtained was ~ 3 µm, which had resulted

in the lowest value for the steady leaching rate of ~ 0.1 µg/cm2/day. Empirical

correlations were obtained between the aggregate size as well as the matrix loading of sodium benzoate and the leaching rate. It was found that increasing the aggregate size

had a sharp effect on the increase of the leaching rate, whereas increasing the matrix

loading (up to 5 wt. %) had a mild effect on the leaching rate. The current study did

show that the solvent-assisted blending technique can be an efficient approach for

constructing the miscibility-release correlations.

Both thermodynamic analysis and experimental observations showed that sodium

benzoate has limited solubility in the Sylgard® 184 coating. This, combined with the

mass transfer analysis of the leaching, led us to confirm that the release mechanism of the

monolithic sodium benzoate/silicone coatings generated via the solvent-assisted blending

technique is mainly by the diffusion of the compound through water-filled pores and

constricted channels within the matrix, not through the continuum of the polymer phase.



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ACKNOWLEDGEMENTS

I would like to thank my advisor Dr. Bi-min Zhang Newby, for consistently and

patiently providing me with continues guidance, informative discussion, and educational

help throughout my PhD research. I would like also to thank my committee members;

Dr. Teresa Cutright, Dr. George Chase, Dr. Lu-Kwang Ju, and Dr. Gerald Young, for

their informative discussion and valuable comments and suggestion. I would like also to

thank Dr. Cutright and her research group for assisting me in bacterial attachment study.

I would like also to thank Dr. Sung-Hwan Choi for the AFM scanning. Thanks are also

due to all members of my research group, for being helpful during my entire study.

I want to express my deep thanks to the Ministry of Higher Education, Saudi Arabia,

for granting me a full scholarship to get my PhD degree, without their financial support it

would be difficult to accomplish this study. Appreciations are also due to the Department

of Chemical and Biomolecular Engineering, the Ohio Sea Grant (Project: R/MB-2) and

the Ohio Board of Regents, for partial financial support of my research project.

Finally, I would like to express my deep gratitude to my wife Fatimah, being patient

here with me and supporting me to accomplish my objective, and to my parents, far away

and being patient waiting for me.



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TABLE OF CONTENTS

Page

LIST OF TABLES………………………………………………………………………...x

LIST OF FIGURES……………………………………………………………………...xii

CHAPTER

I. INTRODUCTION………………………………….…………………………...........1

1.1 Introduction…...…………………………………………………………………1

1.2 Importance and scope of the study…………….………………………………...5

1.3 Objectives…………………………………….………………………….............6

1.4 Dissertation outline…………………………...………………………………….7

II. LITERATURE REVIEW………………………………….…………………………8

2.1 Biofouling and biofilms formation…..…………………………………………..8

2.2 Biofouling controls…..…………………………………………………………10

2.2.1 Conventional toxic antifouling coatings……………………………...11

2.2.2 Silicone foul-release coatings………….……………………………..12

2.2.3 Less-toxic antifoulants…………………….………………………….16

2.3 Antifoulant-matrix miscibility………………………………………………….23

2.4 Modeling of the antifoulants release from polymeric matrices…….…………..33

2.4.1 Active compounds with high solubility in the matrix………………...35



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2.4.2 Active compounds with low solubility in the matrix………..………..38

2.4.3 Leaching behaviors of marine antifouling paints……………………..41

III. EXPERIMENTAL APPROACH………..………………………………………….46

3.1 Materials…….………………………………………………………………….46

3.2 Sample preparation……………………………………………………………..48

3.3 Sample processing……………………………………………………………...53

3.4 Characterization techniques…………………………………………………….57

3.4.1 Contact angles technique.…………………………………………….57

3.4.2 The stress - strain technique………….……………………………….593.4.3 The JKR technique…………………………………………………....60

3.4.4 Optical microscopy…………………………………………………...61

3.4.5 Scanning probe microscopy………….……………………………….63

3.4.6 High performance liquid chromatography (HPLC)…………………..64

IV. RESULTS AND DISCUSSION FOR SODIUM BENZOATE– BASED COATINGS……………………………………………………………….66

4.1 Effect of sodium benzoate on surface and bulk properties of silicones….…….66

4.1.1 Effect on wettability…………………………………………………..67

4.1.2 Effect on elastic modulus……………………………………………..71

4.2 Miscibility of NaB in silicones……...………………………………………….73

4.2.1 Effect of composition of the mixed solvent…………………………..74

4.2.2 Effect of solvent/polymer ratio……...………………………………..78

4.2.3 Effect of NaB matrix loading……….………………………………...78

4.3 Thermodynamic analysis for the miscibility study ….………………………...83

4.3.1 Prediction by the Flory-Huggins theory……………………………....83



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4.3.2 Modification of the Flory-Huggins theory to include electrostaticcontribution and concentration-dependent interaction parameters…..………………………………………………………...86

4.3.3 Comparison between the theoretical miscibility trends and the

experimental morphology trends……………………………………...95

4.4 Leaching evaluation……...…………………………………………………....100

4.4.1 Effect of composition of the mixed solvent………………………....100

4.4.2 Effect of solvent/polymer ratio……………………………………...102

4.4.3 Effect of NaB matrix loading……...………………………………...102

4.4.4 Effect of type of the silicone matrix………………………………....105

4.4.5 Empirical correlations for the leaching rate of NaBfrom Sylgard® 184…………………………………………………..108

4.4.6 Effects of continuous stirring and water replacement ………..……..111

4.5 Mass transfer analysis for the leaching study………..………………………..114

4.5.1 Simplified mass transfer model……………………………………...114

4.5.2 Limitation of the simplified mass transfer model…………………...130

4.6 Bacterial attachment evaluations............……………………………………...138

V. RESULTS AND DISCUSSION FOR BENZOIC ACID AND CAPSAICIN-BASED COATINGS…............................................................................................144

5.1 Effects of the compounds on coating’s properties…………...……………….144

5.1.1 Effect of benzoic acid…………………………….............................144

5.1.2 Effect of capsaicin…………………………………………………...147

5.2 Miscibility of the compounds in silicones…….……………………………....150

5.2.1 Miscibility of benzoic acid…………………………..........................150

5.2.2 Miscibility of capsaicin……………………………………………...154

5.3 Leaching evaluation…………………………………………………………..157



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5.3.1 Leaching of benzoic acid……………………………........................157

5.3.2 Leaching of capsaicin……………………………………………….159

5.4 Bacterial attachment evaluations for capsaicin-RTV11 coatings.……………163

5.4.1 Effect of immersion in water on coating’s properties………….…...163

5.4.2 Bacterial attachment evaluations……………………………………169

VI. RESULTS AND DISCUSSION FOR TANNIC ACID-BASED COATINGS…...171

6.1 Effect of tannic acid on coating’s properties……………………………….....171

6.2 Miscibility of tannic acid in silicones……………………………………........173

6.3 Leaching evaluation……..…………………………………………………….176VII. CONCLUSIONS AND RECOMMENDATIONS.……………………………......179

7.1 Conclusions….….…………………………………………...........................179

7.2 Recommendations for future work…………………….…………………….182

REFERENCES…….…………………………………………………………………...187

APPENDICES………………………………………………………………………….196

APPENDIX A MATLAB FILE FOR SOLVING THE GENERAL MASSTRANSFER MODEL (EQUATION 4.19)……………………..197

APPENDIX B SUGGESTION OF A MORE REALISTIC MASSTRANSFER MODEL – APPLICATION OF THEAVERAGE VOLUME THEORY……………………………...201



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LIST OF TABLES

Table Page

3.1 The compositions of the two types of silicones (RTV11 and Sylgard® 184) usedin the current study. All percentage shown here are in mass basis …….…............48

3.2 The actual combinations of (antifoulant/solvent/polymer) mixtures used in thecurrent study to incorporate the antifoulant into the bulk of the polymer matrix by the solvent-blending technique…………………………………………………49

3.3 The detailed concentrations for NaB/ Sylgard®184 samples prepared at differentconditions. All concentration shown here are in mass basis. (Abbreviation: W:water, A: acetone, S: solvent (i.e. water + acetone), P: silicone polymer base)…..51

3.4 The detailed compositions for the samples that were subjected to leachingexperiments in the current study. All concentration shown here are in mass basis. (Abbreviation: W: water, A: acetone, S: solvent, P: silicone polymer base, NaB: sodium benzoate, BA: benzoic acid, TA: tannic acid)………………..55

4.1 Static water contact angles of NaB-entrapped RTV11 films. The (solvent: polymer) ratio and the (water: acetone) ratio were respectively (20: 80) and(50: 50) by mass……………………………………………………………….......69

4.2 Aggregate size distribution of 1 wt.% of sodium benzoate inside the(99 wt.%) bulk of Sylgard® 184 matrix, for samples prepared at differentwater/acetone ( W/A) mass ratios. The solvent/polymer ratio was fixed at20/80 by mass……………………………………………………………………...76

4.3 Aggregate size distribution of 1 wt.% of sodium benzoate inside the(99 wt.%) bulk of Sylgard® 184 matrix, for samples prepared at differentsolvent/polymer (S/P) mass ratios. The water/acetone ratio was fixed at50/50 by mass……………………………………………………………………...80

4.4 Aggregate size distribution of sodium benzoate inside the bulk of Sylgard® 184 matrix, for samples prepared at different NaB matrix loading(wt% NaB in the matrix). The water/acetone ratio was fixed at 50/50 by mass.The solvent/polymer ratio was fixed at 20/80 by mass……………………………82



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4.5 Physical parameters of relevance importance to the miscibility of NaB/PDMS.V and δ are the molar volume and the solubility parameter of the material,respectively. δ12 is the difference in solubility parameters between the materialand PDMS. χ 12 is the interaction parameter between the material and PDMS*…..84

4.6 Data analysis for the results shown in Figure 4.21. The apparent diffusioncoefficient (DA) for NaB/Sylgard® system was obtained by fitting the experimentalleaching data to equation 4.17. The solvent/polymer ratio was 20/80, which wasfixed for all samples. (Abbreviation: W: water, A: acetone).……………………132

5.1 Static water contact angles of BA-entrapped Sylgard® coatings compared tothat of the controlled BA-free SylgardTM coatings. The (solvent: polymer)ratio was (20: 80) by mass………………………………………………..............146

5.2 Static water contact angles of BA-entrapped RTV11 coatings compared tothat of the controlled BA-free RTV11 coatings. The (solvent: polymer) ratio

was (20: 80) by mass……………………………………………………………..1465.3 Elastic modulus of BA-entrapped Sylgard®184 films. The (solvent: polymer)

ratio was (20: 80) by mass………………………………………………………..146

5.4 Elastic modulus of capsaicin-entrapped RTV11 films. The (solvent: polymer)ratio was (20: 80) by mass………………………………………………………..149

5.5 Physical parameters of relevance importance to the miscibility of BA/PDMS.V and δ are the molar volume and the solubility parameter of the material,respectively. δ12 is the difference in solubility parameters between the materialand PDMS.

χ 12is the interaction parameter between the material and PDMS*…153

5.6 Physical parameters of relevance importance to the miscibility of capsaicin/PDMS. V and δ are the molar volume and the solubility parameter of thematerial, respectively. δ12 is the difference in solubility parameters betweenthe material and PDMS. χ 12 is the interaction parameter between the materialand PDMS*………………………………………………………………………156

6.1 Physical parameters of relevance importance to the miscibility of tannicacid/PDMS system. δ is the solubility parameter of the material. δ12 isthe difference in solubility parameters between the material and PDMS.χ 12 is the interaction parameter between the material and PDMS………………..175



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LIST OF FIGURES

FigurePage

1.1 Classical examples for controlled drug-technologies. (a) membrane-reservoir.(b) microencapsulation. (c) monolithic coatings…………………………………….4

3.1 A schematic diagrams for the sample preparation steps used for incorporatingSodium benzoate into the silicone polymer coating……………………………….50

3.2 A simplified sketch for the contact angle concept. (a) the static contact angle:a liquid drop being in equilibrium with a solid substrate in air. γLV, γSV and γSL

are respectively the surface energies at the liquid/vapor, solid/vapor, and solid/liquid interfaces, and θ is the equilibrium (static) contact angle. (b) theadvancing contact angle (θa). (c) the receding contact angle (θa)………………....58

3.3 A simplified sketch for the set-up of the JKR apparatus…………………………..62

4.1 Water contact angles of NaB-entrapped Sylgard®184 films (advancing:,receding:, and static:). The (solvent: polymer) ratio and the (water: acetone)ratio were respectively (20: 80) and (50: 50) by mass, which were fixed at allconcentrations. Error for each data point (average over 12 measurements) is presented by the vertical line………………………………………………………68

4.2 Elastic modulus variations of NaB - entrapped Sylgard® 184 coating. The(solvent: polymer) ratio and the (water: acetone) ratio were respectively(20: 80) and (50: 50) by mass, which were fixed at all concentrations.Error for each data point (average over 6 measurements) is presented by thevertical line………………………………………………………………………...72

4.3 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of 1 wt % NaB/Sylgard® 184 matrix, prepared at differentwater/acetone ratios, keeping the solvent/polymer ratio fixed at 20/80.(a) 20/80 water/acetone; (b) 30/70 water/acetone; (c) 50/50 water/acetone;(d) 80/20 water/acetone; (e) 90/10 water/acetone; (f) 100% water. All thevalues are based on weight………………………………………………………...75



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4.4 Guidelines chart for preparing NaB-incorporated Sylgard® 184 coatings atdifferent set of conditions. The saturation line “ — ” represents the maximumsolubility of NaB in water (0.555 g NaB per 1 g of water). Above this line, NaBis not soluble in the mixed solvent (water + acetone), and hence the bulk entrapment method will not be feasible at this particular set of conditions.

(Abbreviations: S: solvent; P: polymer). All values are in mass basis…………...77

4.5 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of 1 wt % NaB/Sylgard® 184 matrix, prepared at differentsolvent/polymer ratios, keeping the water/acetone ratio fixed at 50/50. (a) 10/90solvent/polymer; (b) 20/80 solvent/polymer; (c) 30/70 solvent/polymer;(d) 40/60 solvent/polymer; (e) 50/50 solvent/polymer. All the values are basedon weight…………………………………………………………………………..79

4.6 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of NaB/ Sylgard® 184 matrix of different NaB concentrations,

keeping the solvent/polymer ratio and the water/acetone ratio fixed at 20/80and 50/50, respectively. (a) 0.5 wt% NaB/ Sylgard® 184; (b) 1 wt% NaB/Sylgard® 184; (c) 2 wt% NaB/ Sylgard® 184; (d) 3 wt% NaB/ Sylgard® 184;(e) 4 wt% NaB/ Sylgard® 184; (f) 5 wt% NaB/ Sylgard® 184; All the valuesare based on weight………………………………………………………………..81

4.7 The miscibility trends for NaB/acetone/water/PDMS mixtures, for all possibleconditions, as predicted by the original Flory-Huggins (FH) model (equation 4.2).(a) Effect of the water/acetone (W/A) ratio; parameters fixed: 1 wt% NaB/polymer.(b) Effect of the NaB matrix loading; parameters fixed: 50/50 water/acetone. Allthe values shown in the legends are based on weight……………………………..87

4.8 The miscibility trends for NaB/acetone/water/PDMS mixtures for all possibleconditions, as predicted by the new model (equation 4.9). (a) Effect of thewater/acetone (W/A) ratio; parameters fixed: 1 wt% NaB/polymer. (b) Effect of the NaB matrix loading; parameters fixed: 50/50 water/acetone. All the valuesshown in the legends are based on weight………………………………………...94

4.9 The free energy of mixing (Gmix/nkT) for NaB/acetone/water/PDMS mixtures prepared at different conditions, as predicted by two models: the Flory-Huggins(F-H) model (equation 4.2), and the new model (equation 4.9). (a) Effect of thewater/acetone ratio, parameters fixed: 20/80 solvent/polymer and 1 wt%

NaB/polymer. (b) Effect of the solvent/polymer ratio, parameters fixed: 50/50water/acetone and 1 wt% NaB/polymer. (c) Effect of NaB matrix loading, parameters fixed: 50/50 water/acetone and 20/80 solvent/polymer. The solventis defined here as water + acetone + NaB. All the values are based on weight.The preparation conditions described here correspond to the actual conditionsfor the morphology experiments performed in the current study. The insert ineach plot represent the corresponding experimental morphology trend ………….96



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4.10 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed in water.The samples were prepared at different water/acetone ratios, keeping the other conditions unchanged (20/80 solvent/polymer, and 1 wt% NaB/matrix). Allthe values are based on weight. Solid lines are for showing the trends.(Abbreviations: W: water; A: acetone). Error for each data point (average

over 2 batches) is presented by the vertical line………………………………….101

4.11 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed in water.The samples were prepared at different solvent/polymer ratios, keeping the other conditions unchanged (50/50 water/acetone, and 1 wt% NaB/matrix). All thevalues are based on weight. (Abbreviations: S: solvent; P: polymer). Error for each data point (average over 2 batches) is presented by the vertical line….........103

4.12 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed in water.The samples were prepared at different wt% NaB/matrix, keeping the other conditions unchanged (50/50 water/acetone, and 20/80 solvent/polymer). All

the values are based on weight. Error for each data point (average over 2 batches) is presented by the vertical line…………………………………………104

4.13 Cumulative leaching of NaB from its incorporated silicone coating:Sylgard® 184 () or RTV11 (▲). The common solvent used was 50/50water/acetone by weight. The initial concentration of NaB in both coatingswas kept constant at 1 wt%., and the solvent/polymer ratios was kept constantat 20/80 by weight for both combinations………………………………………..106

4.14 Empirical correlations for the leaching rate of NaB from Sylgard® 184.(a) Effect of the aggregate size (d, the arithmetic mean size), parameter fixed:1 wt% NaB/matrix. (b) Effect of the NaB matrix loading, parameter fixed:d ~ 3 - 4 µm. The insert in (b) is for enlarging the scale of the y-axis. The symbols(■) and (O) represent the initial and the steady leaching rates, respectively….....110

4.15 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed in water.The samples were prepared at the base case conditions ((1 wt% NaB/Sylgard® 184; 50/50 water/acetone; and 20/80 solvent/polymer)), and the leaching weremeasured at three different conditions: under constant stirring (□), replacing water daily (∆), and at static conditions (O). Error for each data point (average over 3 batches) is presented by the vertical line…………………………………………113

4.16 A simplified sketch (not to scale) for the mass release of antifouling compoundsfrom polymer paint (water-insoluble matrix). In this case, the compound issoluble in the matrix and is initially loaded in excess of its solubility limit inthe matrix. The dissolved zone means that the compound is already absorbed by the polymer phase…………………………………………………………......115



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4.17 A simplified sketch (not to scale) for the leaching of antifouling compoundsfrom polymer paint (water-insoluble matrix). In this case, the compound isinsoluble in the matrix. (Figure re-drawn from Caprari et al . (1990), withslight modification)………………………………………………………………116

4.18 A simplified sketch (not to scale) of a polymer coating incorporated with AFcompound, and immersed in water. The purpose of the sketch is to show themeaning of the axial distance x that was used in equation 4.12………………….117

4.19 Parametric sensitivity analysis for the general mass transfer model (equation 4.19).The dimensionless surface concentration (Ψ|ζ=0 = C/CO |x=0) is plotted againstthe dimensionless time (τ = DA t / L2), for different values of the dimensionless parameter Bm (Bm = k L / DA). The trends were generated by solving equation4.19 numerically………………………………………………………………….122

4.20 Simplified sketch (not to scale) for the possible leaching mechanisms of NaB from

Sylgard

®

184 coating: (a) Perfect packing of the particles; (b) Complete ruptures of the thin membranes; (c) The existence of initial porosity of the matrix, which ismainly composed of constricted narrowed channels. The first column representsthe coatings initially before immersion, the second column after some time t1 > 0,and the third column after some time t2 > t1………………..…………………….126

4.21 Fitting of the cumulative leaching data for NaB/Sylgard® 184 coatings to thesimplified mass transfer model (equation 4.17). (a) Samples were prepared atdifferent NaB matrix loading, keeping the water/acetone ratio and thesolvent/polymer ratio fixed at 50/50 and 20/80, respectively. (b) Samples were prepared at different water/acetone (W/A) ratios, keeping the NaB matrix loadingand the solvent/polymer ratio fixed at 1 wt% NaB/matrix and 20/80 ratio,respectively. Points are experimental data and solid lines are linear fitting of themodel.…………………………………………………………………………….132

4.22 Optical microscope images of the bacterial attachment study for NaB-Sylgard® 184 coating. (a, c) 1 wt% sodium benzoate-blended Sylgard® 184.(b, d) control Sylgard® 184. The coatings were immersed in water containing Lake Erie bacteria for 2 weeks (a, b) and 4 weeks (c, d). Image size is(285 µm x 215 µm)……………………………………………….........................139

4.23 Antibacterial performance of 1 wt% NaB-incorporated Sylgard® 184 coatings,compared to control SylgardTM 184 samples. The % reduction was defined as[(1-A/B) 100), where A and B refer to the area coverage of NaB-containingcoatings and NaB-free coatings, respectively……………………………………140



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4.24 Optical microscopic (reflection bright field) images of bacterial attachment oncontrolled RTV11 coatings after the coatings were immersed in water containing Lake Erie bacteria for 28 days. (a) Half of the coatings surfaces were physicallycleaned by scotch tape, and overall pictures [image size: (2850 x 2150) µm] weretaken showing the cleaned area (right side of picture a) and the un-cleaned area (left

side of picture a). Pictures (b) and (c) are the magnifications [image size: (285 x215) µm] of the two area indicated in picture (a).………………………………..141

5.1 Water contact angles of capsaicin-entrapped RTV11 films (advancing:,receding:, and static:). The (solvent: polymer) ratio was (20: 80) by mass,which was fixed at all concentrations. Error for each data point (average over 12 measurements) is presented by the vertical line………………………………148

5.2 Optical microscope (bright field) images of resulting BA distribution in the bulk of Sylgard® 184 matrix when different solvent were used to mix BA withSylgard® 184: (a) toluene, (b) acetone, (c) acetonitrile, and (d) ether. The

concentration of BA in the matrix was fixed at 1 wt%, and the (solvent: polymer)ratio was fixed at (20: 80) by mass. The image size is 2850 µm x 2400 µmfor (a), and 1140 µm x 960 µm for (b)-(d)………………… ……………………151

5.3 Optical microscope (transmission bright field) image of resulting capsaicindistribution in the bulk of Sylgard® 184 base material. Toluene was used asthe common solvent. The concentration of capsaicin in the matrix was 1 wt%,and the (solvent: polymer) ratio was (20: 80) by mass. The image size is(570 x 480) µm…………………………………………………………………...155

5.4 Cumulative leaching of BA from its incorporated silicone coating: Sylgard® 184

(open symbols) or RTV11 (filled symbols). The common solvent used for BA/silicones were acetone (squares) and Toluene (circle). The initial concentrationof BA in all coatings was kept constant at 1 wt%., and the solvent/polymer ratioswere kept constant at 20/80 by weight for all combinations………………….….158

5.5 Capsaicin cumulative mass per area (Q, in µg/cm2) released from 1 wt %capsaicin-incorporated RTV11 sheet (total mass and surface area of the sheetwere respectively 6.55 g and 114 cm2), plotted against time of immersion in DIwater. Toluene was used as the common solvent to mix capsaicin with RTV11,and the (solvent: polymer) ratio was (20: 80) by mass……………………...…...160

5.6 Effect of the mixing order on the capsaicin cumulative leaching from 1 wt %capsaicin-incorporated RTV11 sheet (total mass and surface area of the sheet wererespectively 1.64 g and 38 cm2). Ethanol was used as the common solvent, and the(solvent: polymer) ratio was kept constant at (20: 80) by mass. The open squarescorrespond to the conditions of mixing capsaicin/ethanol solution with the RTV11 base and drying off the solvent before adding the catalyst. The filled squarescorrespond to the same conditions of the open square data, except that thecapsaicin/ethanol solution was mixed after adding the catalyst………………….162



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5.7 Effect of water immersion time on the wettability of RTV11 films in terms of thestatic contact angles taken for RTV11 immersed in Lake Erie (Δ) and deionizedwater (▲). The advancing (□) and receding () contact angles taken for RTV11immersed in deionized water are also presented. Error for each data point (averageover 12 measurements) is presented by the vertical line………………………....165

5.8 Surface topographic images of Capsaicn-RTV11 coatings. (a) as-preparedcontrolled RTV11 film, surface roughness: 6.7 nm; (b) as-prepared RTV11 filmcontaining 1 wt % capsaicin, surface roughness: 12.3 nm; (c) controlled RTV11film after 14 days of immersion in DI water, surface roughness: 10.5 nm; (d)RTV11 film containing 1 wt% capsaicin after 14 days of immersion in DI water,surface roughness: 88.0 nm. The images (scan size: 80 µm x 80 µm; z-scale: 400nm) were generated using scanning probe microscopy with the non-contact mode ata scan rate of 0.20 Hz……………………………………………………………166

5.9 Effect of water type and immersion time on the elastic modulus of RTV11 films

immersed in different types of water samples (sterilized Lake Erie water:Δ,enriched Lake Erie water: ○, and sterilized Lake Erie water with 20 ppm capsaicin:□). Error for each data point (average over 6 measurements) is presented by thevertical line……………………………………………………………………….168

5.10 Optical microscopic (reflection bright field) images of bacterial attachment for capsaicn-RTV11 coatings. (a) 1 wt% Capsaicin/RTV11, (b) control RTV11. Thecoatings were immersed in water containing Lake Erie bacteria for 14 days. Thesize for both images is (285 x 215) µm…………………………..........................170

6.1 Static water contact angles of TA-entrapped silicone films compared to that of

the controlled TA-free silicones. The concentration of TA in the matrix was fixedat 1 wt% for both combinations. The (solvent: polymer) ratio was fixed at (20: 80) by mass (acetone was the solvent for both combinations). Error for each data point(average over 12 measurements) is presented by the vertical line………….........172

6.2 Optical microscope (transmission bright field) image of resulting TA distributionin the bulk of Sylgard® 184 matrix: (a) 1 wt% TA/Polymer; (b) 4 wt%TA/Polymer. Acetone was used as the common solvent, and the solvent/polymer ratio was 20/80 by mass. The image size is (570 x 480) µm……………………174

6.3 Cumulative leaching of TA from its incorporated silicone coating: Sylgard®

184 () or RTV11 (▲). The common solvent used was acetone for bothcombinations. The initial concentration of TA in both coatings was kept constantat 1 wt%., and the solvent/polymer ratio was kept constant at 20/80 by weightfor both combinations. Error for each data point (average over 2 batches) is presented by the vertical line……………………………………………………..177



1

CHAPTER I

INTRODUCTION

1.1 Introduction

Marine biofouling is the accumulation and growth of marine organisms on

manmade structures immersed in natural water (Evans & Clarkson, 1993; Davis &

Williamson, 1995). Biofouling has resulted in many problems for the marine industry

(Abdul Azis et. al., 2001). The layer of attached organisms on ship’ hulls decreases ship

speed and increases fuel consumption by approximately 30 % (Stupak et. al., 2003). As

an example, the US Navy estimates that biofouling costs over $150 million annually in

excess fuel consumption and cleaning costs for naval vessels. Biofouling is also directly

related to the bio-corrosion and other related problems (Malik et. al., 1996; Melo & Bott,

1997; Wood and Marsh, 1999).

Four steps are involved in biofouling formation (Al-Ahmed et. al., 2000;

Steinberg et. al., 1997). Once a surface is immersed in water, it is immediately covered

with a thin “conditioning film” consisting mainly of different proteins and dissolved

organic matters. It is followed by the adhesion of single floating bacteria. Once



2

firmly attached, bacteria start to generate extracellular polymeric substances (EPS),

which connect the cells in between each others as well as connecting to the surface. The

cells plus EPS is what we called the “biofilm”. Finally, the biomass continues to grow,

with some large macro fouling organisms attach to, while other cells detach from the

film. The first step, conditioning films adsorption, is very fast and occurs within hours of

immersion, whereas the later step, production of EPS, is very slow and needs weeks or

perhaps months to complete.

Antifouling paints have long been the most effective method to prevent biofouling, where biocides or heavy metal compounds such as TBTO (Tributyltin oxide)

are released from the coatings and inhibit organism’s attachment. TBT compounds are

the most effective compounds for biofouling prevention. Unfortunately, they are also the

most toxic compounds against non-target marine organisms (Axiak et. al., 2000;

Haslbeck et. al., 1996). As a result, the International Maritime Organization (IMO)

banned the application of TBT compounds on 2003, and the entire removal of TBT

coatings by year 2008 will be required worldwide (Champ, 2000).

Recognizing the harmful effects and the already-started bans for TBT compounds,

considerable efforts are being committed worldwide to find new non-toxic or less-toxic

antifouling alternatives. Possibilities which have been considered include dissolution of

adhesive substance by various enzymes, biogenic agents, smart polymer coatings,

polymer coatings with defined surface-microstructures (Jelvestam et. al , 2003), foul-

release coatings (Brady, 1999, 2000), natural product antifoulants (NPAs) (Hellio et. al .,



3

2001, Ponasik et. al. 1998), and less-toxic and commercially available preservatives,

pesticides, drugs, and insecticides.

Upon the use of the less toxic antifouling compounds, the selection of a less toxic

antifouling compound is the first step toward biofouling control. The next important step

is to control its release from the coating. Control-release technologies that were

originally developed for drug-release are expected to be useful for controlling the release

of antifouling compounds. Three drug-release technologies are widely used: membrane-

reservoir, microencapsulation, and monolithic coatings (Figure 1.1). In a membrane-reservoir device, the active compound is highly concentrated into a middle layer of the

polymer coating, which is surrounded by a second layer of another polymer coating. In

microencapsulation, the active compound is encapsulated into microcapsules, which are

distributed in a polymer coating made of a material differed from that of the capsules. In

a monolithic coating, the active compound is distributed within the layer of the polymer

coating. Depending on the solubility and loading of the compound in the monolithic

coating, the compound can be molecularly dissolved in the polymer phase or it can be

uniformly dispersed as a separate droplet phase (as shown in Figure 1.1c). The

monolithic coating technology is the most widely used due to its simplicity. Several

experimental approaches exist to prepare monolithic coatings; the solvent casting

technique and the compressing molding technique (Fan and Singh, 1989) are two

common ones. In the current study, monolithic coatings prepared using a solvent-assisted

blending technique will be focused.





5

1.2 Importance and scope of the study

Many non-toxic or less-toxic antifouling (AF) compounds have been identified in

the literature. Some of these compounds are directly blended with different polymer

coatings and tested for biofouling prevention. Although direct blending is a simple

method, in most cases, the release rate of AF compound will be very fast and hence the

service life of the coating will be very short. Different methods have been suggested in

literature to extend the service life of the coating, either by chemically attaching the

compound to the coating surface or by employing sophisticated controlled releasetechniques such as the micro-encapsulation method or the membrane reservoir method.

However, these methods are highly complicated and, in some cases, are challenging for

certain compounds. In the current study, we are proposing a simple experimental

approach (dissolving the compound in a solvent or in a blend of solvents and then

homogenizing the solution with the polymer) to generate monolithic coatings that are

expected to result in the desired slow-controlled release of the AF compound. This

experimental approach is termed as the “solvent-assisted blending technique”. For the

current study, a systematic combination of the compound/solvents/polymer will be

carried out, and a systematic evaluation of the miscibility and leaching behavior of the

compound from the coatings will be conducted. The main focus of the current study is to

obtain the relationship between the miscibility of the compounds in the coating matrix

and their leaching in water, for the purpose of controlling the release rate and maintaining

the antifouling performance of the coatings for their long term service.



6

1.3 Objectives

The primary objective is to obtain the relationship between the miscibility of less-

toxic antifouling compounds in a monolithic, hydrophobic polymer coating and their

release in water. A secondary simultaneous objective is to evaluate the possibility of

applying the solvent-assisted blending technique as a simple preparation method for the

purpose of controlling the release of four antifouling compounds. Specifically, the study

aims to:

(1) Evaluate the feasibility of the solvent-assisted blending technique for

incorporating four less-toxic antifouling (AF) compounds (sodium benzoate,

benzoic acid, capsaicin, and tannic acid) into two silicone matrix coatings

(Sylgard® 184, and RTV11) by examining experimentally the morphological

structure of the compounds in the matrix and their leaching into Deionized (DI)

water.

(2) One of the combinations (sodium benzoate in Sylgard®

184) is selected as the

model system to perform more detailed investigations on the miscibility-leaching

relationship, as listed below:

(a) Compare, experimentally, the role of varying the preparation conditions

on the miscibility and the leaching of the compound, and obtain

correlations between the miscibility and the release rate.

(b) Perform thermodynamic analysis on the miscibility of various coating

combinations to explain the effect of the preparation conditions on the

distribution and morphology of the compound inside the coating matrix.



7

(c) Perform a mass transfer modeling on the leaching to explain the

experimental data and propose a suitable leaching mechanism of the

monolithic sodium benzoate/silicone coating generated using the solvent-

assisted blending method.

1.4 Dissertation outline

The outline of the dissertation is as follow. In chapter 2, the dissertation starts with

providing backgrounds and literature review about various issues of relevance to thecurrent study, including biofouling and its control, foul-release coatings and less toxic

antifoulants, antifoulant-coating miscibility, and antifoulant leaching modeling. The

experimental approach is described in chapter 3. The results and discussion for sodium

benzoate-based coatings are presented in chapter 4. The results and discussion for

benzoic acid and capsaicin-based coatings are presented in chapter 5, and the results and

discussion for tannic acid-based coatings are presented in chapter 6. Finally, in chapter 7,

the dissertation concludes with summarizing the key results and providing

recommendations for future works.



8

CHAPTER II

LITERATURE REVIEW

This chapter provides the necessary background and literature review about the

subject of the current study. In section 2.1, background about biofouling and biofilm

formation is provided. Current and proposed solutions to combat biofouling are reviewed

in section 2.2, including conventional toxic antifouling coatings, nontoxic foul-release

coatings, and less/non toxic antifouling compounds. The miscibility of additives and

polymeric matrices is reviewed in section 2.3. The mechanisms and mathematical

models describing the release of the active compounds (i.e. antifoulants) from the matrix

into water are reviewed in section 2.4.

2.1 Biofouling and biofilms formation

Four steps are involved in biofouling (Al-Ahmed et. al., 2000; Steinberg et. al.,

1997). Once a surface is immersed in water, it is immediately covered with a thin

“conditioning film” consisting mainly of different proteins and dissolved organic matters.

Then, it is followed by the adhesion of single floating bacteria. Once firmly attached,

bacteria start to generate extracellular polymeric substances (EPS), which connect the

cells in between each others as well as connecting to the surface. The



9

cells and EPS form the “biofilm”. The biomass continues to grow, and in the meantime

some large macrofoulers attach while some adhered cells detach from the film (due to

shear force of the bulk phase, or due to partial cohesive failure of the biofilms). The first

step, conditioning films adsorption occurs within hours of immersion, whereas the later

step, production of EPS needs weeks or maybe months to finish. The conditioning step

cannot be avoided for any kind of surface immersed in the natural water. Therefore, an

effective strategy to minimize biofilm formation and the subsequent macro-foulers

attachment is to prevent or reduce bacterial adhesion.

The attachment of bacteria to solid surfaces is a complicated process with many

variables contributing to the final result. These variables can be subdivided into

properties of the substrates (e.g.: surface hydrophobicity and roughness), properties of the

bacterium, and properties of the liquid media (e.g.: flow velocity and temperature). A

concise review on the effects of these variables on bacterial adhesion was written by An

and Friedman (1998).

Marshall and his co-worker (Marshall et al. 1971) are the first to describe that

bacterial adhesion to solid surfaces as a two phase process: an initial, instantaneous, and

reversible physical phase (phase 1), and a time-dependent and irreversible molecular and

cellular phase (phase 2). This classification is largely accepted by the majority of

researchers. In the first phase of adhesion, floating bacteria move to a material surface by

means of physical forces, such as Brownian motion, van der Waals attraction forces,

gravitational forces, electrostatic forces, and hydrophobic interactions. These physical



10

forces are further classified into long-range and short-range interactions (Dankert et al .

1986). The long-range interactions are nonspecific interactions, which are effective when

the distance between the bacterium and the substrate is greater than 150 nm. Short-range

interactions become effective when the distance between the bacterium and the substrate

is less than 5 nm. These short-range interactions include chemical bonds (such as

hydrogen bonding), ionic and dipole interactions, and hydrophobic interactions (An and

Friedman, 1998; Gottenbos et al ., 2002). The short-interactions forces from phase 1

provide the suitable ground for the second phase of adhesion to occur, where here

molecular reactions between specific bacterial surface structure and substratum surface become predominant. The active sites of the bacterium surface that are responsible for

firm adhesions are those polymeric surface structures that contain adhesins as parts of

their structures, which include capsules, fimbriae (or pili), and fibrillae. At the

irreversible stage of adhesion, bacteria start to produce slime extracellular polymeric

substances (EPS), which are exopolymers composed of mainly polysaccharides. EPS can

be regarded as external adhesives that connect the bacteria between each others as well as

connecting the bacteria to the substrate.

2.2 Biofouling controls

Biofouling has caused serious problems to the maritime industry including

enormous economic loss due to considerable increase in ship’s fuel consumption and

maintenance costs, and damages and harmful effects resulted from biocorrosion of the

immersed infrastructures. Consequently, the prevention or minimization of biofouling is





12

Yebra et al ., 2004). For the former, the compound is incorporated into a polymer matrix

that is water-insoluble and does not erode considerably over time in water. For the later,

the matrix erodes slowly over time in water. Both classes of matrices are used for

incorporating TBT compounds, and it is found that the TBT-self polishing coatings are

having better long-term antifouling performance than that of the TBT-insoluble coatings,

mainly due to the fact that TBT-self polishing coatings are having a slower and controlled

mass release. However, due to the extreme toxicity of TBT compounds, the ban for

using TBT compounds includes not only the TBT-insoluble coatings but also the TBT-

self polishing coatings.

2.2.2 Silicone foul-release coatings

Silicones belong to the water-insoluble matrices class, and have unique properties

that make them distinguished as foul-release (FR) coatings (Brady & Singer, 2000;

Estarlich et. al ., 2000; Watermann et. al., 1997; Wynne et. al ., 2000). They consist

mostly of Polydimethylsiloxane (PDMS), which has methyl (–CH3) side chains results in

its low surface energy ( 20 - 24 mJ/m2) and a flexible, inorganic -Si-O backbone linkage

leads to its extremely low elastic modulus (~ 1 MPa), both are essential to the extreme

low adhesion of silicone coatings. Thus, biofilms can be easily removed from the surface

by simple mechanical cleaning or during vessel movement. Marine coatings industries

have recently shown much interest in silicone coatings as a non-toxic alternative. In

general, two classes of PDMS are available commercially with distinct curing chemistry.

The first one is the hydrosilation cured PDMS, such as Sylgard® 184. The second class is



13

the condensation cured PDMS, such as RTV11. This subsection reviews some of the

recent works done for investigating silicone coatings as a nontoxic alternative for marine

biofouling control.

Wynne et al . (2000) evaluated two types of PDMS coatings (RTV11, and an in-

house unfilled hydrosilation cured PDMS). They evaluated the surface hydrophobicity

and roughness of the coatings, and the mass loss of the coatings upon immersion in

water. They found that the hydrosilation cured PDMS was fairly stable in water; with the

mass loss was less than 0.2 % after 50 days of immersion. RTV11, on the other hand,was not stable in water, with a mass loss of about 0.8 % after 50 days of immersion.

However, the bulk moduli of the two coatings in water were not evaluated. The two

coatings were also evaluated for barnacle adhesion. The barnacle adhesion strength to

the hydrosilation cured PDMS and to the RTV11 coatings were found to be about 0.5 and

0.78 kg/cm2, respectively. In another publication of the same research group (Bullock et

al ., 1999), the surface properties of RTV11 (with different catalyst concentration) were

investigated in more details, and a mechanism for the mass loss of RTV11 in water was

proposed.

Arce et al. (2003) performed a comparative study for the microelastic properties

of RTV11 and Intersleesk TM elastomers (Intersleek TM is the trade name of a PDMS-based

coating, which is already of being used as a foul release coating in some vessels). The

measurements were done by using AFM and other techniques, which gave valuable

information about the local structural and mechanical properties of the as-prepared



14

coatings. However, the behavior of the coatings in water and their antifouling

performance were not evaluated.

The nature and failure mechanism of bioadhesive bonding between barnacles

and two types of PDMS coatings (RTV11 and RTV1556) were investigated by Berglin

and Gatenholm (1999). Analysis of the fracture surfaces indicated that the failure

mechanism was a cohesive failure within the PDMS coatings. As a control, they

compared their results with PMMA coatings, and they found that the failure mechanism

of PMMA coatings was more complex than the failure mechanism of PDMS coatings.Also, the surface energies of the coatings were calculated from contact angles data, and

the values were 23.3 mJ/m2 and 22.4 mJ/m2 for RTV11 and RTV1556 coatings,

respectively.

Edwards et al . (1994) evaluated the hydrophobicity and antifouling performance

of some room-temperature-vulcanizing PDMS and polydimethyldiphenylsiloxane

(PDMDPS) coatings. The authors selected these two classes of silicones to relate their

hydrophobicity to their antifouling performances. PDMDPS showed higher

hydrophobicity (contact angle ~ 123) and better antifouling performance than PDMS

(contact angle ~ 112). Incorporation of silicone oils into the coatings was also

investigated in the range of 0 — 20 %. It was concluded that silicone oil had only a

significant positive effect for enhancing the antifouling performance if its concentration

was sufficiently high (between 10 to 20 %), and the enhancement here was speculated to

be by the formation of a surface film.



15

Estarllich et al . (2000) studied the change in surface properties of some PDMS

coatings (RTV11, RTV160, and RTV655) and flourosilicones as a function of immersion

time in different types of water. Bacteria and microalgae attachment tests were also

performed, where it was found that the early attachment was least on RTV11 and greatest

on flourosilicones.

Barnacle release mechanism for two silicone coatings (a single layer coating consists

of Sylgard® 184, and a duplex coatings consist of RTV11 as a top coat and Silgan ® J-501

as the bond coat) were studied by Singer et al . (2000). The results suggested that thecoatings with lower modulus and thicker thickness had a better foul release performance.

This was confirmed by the theoretical fracture-mechanical analysis of Brady and co-

authors (Brady and Singer, 2000; Brady, 2001).

The two classes of foul release coatings (fluorinated and silicone coatings) have

shown to be partially effective and both have advantages and limitations. Brady and

Aronson (2003) synthesized a new foul release coating that combined the best features of

the two classes of materials. After optimizing the preparation conditions, the new

material (an elastomeric fluorinated polyurethane coating) was found to be effective as

foul release coating with the desired surface and mechanical properties. It should be

noticed that this new product is a polyurethane-based coating, not a silicone-based

coating.

Stein et al. (2002) evaluated systematically model silicone coatings (both

condensation-cured and hydrosilylation-cured types) with controlled molecular



16

architectures to determine the effect of compositional variables such as filler loading and

cross-linking density on Pseudobarnacle attachment strengths. The results suggested that

there was a trade-off between mechanical integrity of the coatings and foul release

performance.

2.2.3 Less-toxic antifoulants

Scientists have searched for non-toxic or less toxic antifoulants, to replace the

harmful toxic antifoulants such as TBT compounds. Less-toxic antifoulants include Natural Products Antifoulants (NPAs), food-preservatives, antibacterial drugs, pesticides,

insecticides, and other related compounds.

NPAs are natural compounds extracted from plants and marine species. The

discovery of the potential of NPAs came from the observation that the biomass of foulers

is usually higher on non-toxic artificial substrates than on the surface of living objects.

Natural defenses against biofouling involve secondary organic exometabolites: phenolic

or polyphenolic and halogenorgnic compounds, terpenes, heterocyclic compounds, and

other compounds (Railkin, 2004). These compounds are easily bio-degradable, thus

making the antifouling process ecologically safe. More than 100 potential NPAs have

been identified nowadays (Clare, 1996; Fustian, 2004). However, most of them have not

yet been systematically incorporated into suitable coatings and evaluated. In addition,

there is a substantial lack of information on the mechanisms of actions on how these

NPAs work against marine fouling species.



17

The structure of any NPA is usually complex with many functional groups, and

usually difficult to be commercialized. Efforts are being made to examine simplified

structures of a lot of NPAs for antifouling activity and then synthesizing them. The

resulted synthesized compound, no longer now called NPAs, could have potentials as

antifoulants, antibacterial drugs, and food-preservatives. Based on this strategy, different

analogues compounds have been identified, synthesized, and tested for antifouling

activity. The synthesized compounds with proved antifouling activity and much less

toxicity (compared to heavy metal compounds) include benzoic acid, nicotinic acid,

picolinic acid, 2-furyl-n-pentyl ketone, 3-acetyl-2,5-dimethyl furan, and their derivatives(Stupak et. al ., 2003; Clare, 1996; Sundberg et. al ., 1997; Railkin, 2004). Again, most

evaluations are concerned with the biological side, and a detailed study on the

compatibility, miscibility, leaching and material properties of the antifoulant/polymer

systems is still lacking.

Understanding the mechanism of action for the antifouling compounds is

necessary. For toxic compounds such as TBT and heavy metals, the mechanism of action

is acute toxicity, which results simply in killing the attached microorganisms. For less or

nontoxic compounds, the identification and experimental validation of the mode of action

is rather complicated (Railkin, 2004). In general, two antifouling mechanisms are

suggested for the effectiveness of the non-toxic compounds: repellency and chemical

anti-adhesive (Sundberg et. al ., 1997, Railkin, 2004). As emphasized by Railkin (2004),

the following exact definition should be applied to assign the non-toxic repellency

mechanism for a certain antifouling compound: “Repellents are cues inducing a negative



18

motor response, taxis, or kinesis in organisms at a certain stage of development, which

causes them to move away from the source of these cues” (Railkin, 2004). This precise

definition differentiates the true repellent property (a non-toxic mode of action) from

other toxic properties of the antifouling material. For this definition to be applied, special

experimental behavioral tests should be performed. For example, the repellent effect can

be evaluated using the special test developed by Railkin (1995), where here chemotactic

chambers made of Plexiglas and measuring 36 x 40 x 80 mm can be used, with each

chamber consists of 3 sections, separated by 0.92-micro nucleopore filters. The

microorganism is placed in the middle section; the antifoulant solution is filled in oneside and a reference solution (e.g. sea water) is filled in the other side. However, in most

studies, the true repellent function of various natural and synthetic antifoulants is

hypothetically assumed rather than experimentally verified (Railkin, 2004). Among a

few studies that followed the standard behavioral tests, benzoic acid and tannic acid were

proved to have a true repellent mechanism as a non-toxic mode of action. (Mitchell and

Kirchman, 1984; Railkin et. al ., 1993; Railkin, 1995, Railkin and Dobretsov, 1994). In

addition to the repellency mechanism, the non-toxic antifouling compounds can also

exhibit the chemical anti-adhesive mechanism, where molecules of the antifouling

compound exist freely in water and act as catalytic inhibitors for the biochemical

reactions involved in cell adhesion. The word “chemical” is used here to distinguish this

mechanism from the physical anti-adhesive mechanism, which usually refers to the easy-

release silicone coatings. The chemical anti-adhesive mechanism, a terminology used by

Railkin (2004), appears to be the same as described by Sundberg and his co-workers

(1997) but in different terminology: “the blocking of the attachment sites mechanism”,



19

where Sundberg et al . (1997) hypothesized that the molecules of the compound exist

freely in water could bind to the attachment sites on the microorganism cell wall and thus

prevent cell adhesion. Again, special experimental behavioral tests should be performed

in order to confirm the chemical anti-adhesive mechanism, such as the standard tests

developed by Ina et. al . (1989) or Hellio et. al . (2000). Among a few studies that applied

these controlled tests, benzoic acid was also proved to have chemical anti-adhesive

properties (Railkin et. al ., 1993; Railkin, 1995, Railkin and Dobretsov, 1994).

Among the environmentally benign NPAs, capsaicin, a stable monocliniccrystalline alkaloid extracted from chili peppers, is being considered as a potential.

Capsaicin, or 8-methyl-N-vanillyl-6-nonenamide, and its analogues have been used as

active ingredients in medicinal products, and are found to be effective in inhibiting

bacterial growth (Jones et. al, 1997; Tsuchiya, 2001; Cichewicz & Thorpe, 1996; Molina-

Torres et. al., 1999). The studies done indicate that both capsaicin and its structural

analogs, all containing the phenolic ring and the amide group but with different

hydrocarbon tails, have similar inhibitory capability towards bacteria, suggesting that the

phenolic ring and the amide group are responsible for the antibacterial activity of

capsaicin. It was found that the presence of capsaicin in an aqueous solution reduced the

bacterial attachment on a coating (Xu et al ., 2005). In addition, capsaicin is significantly

less toxic (EC50, the concentration to kill 50% of bacterial population, is ~ 5 to 20 ppm

towards various bacteria [Xu et al ., 2005]) as compared to organotin compounds (EC50 <

0.01 ppb). Furthermore, capsaicin is biodegradable in the marine environment, and

approved by the EPA as an active ingredient for insect, bug and animal repellents.



20

Therefore, it is an attractive candidate to be utilized for generating environmentally

benign coating alternatives. For the past 10 years, there exist a lot of patents that

document the potential of using capsaicin-based marine antifouling coatings (e.g. Yao et.

al ., 2004; Hatch, 2003; Veech, 1997; Watts, 1995). Based on our knowledge, only two

journal papers (Shi et. al ., 2004; Yu et. al . 2004) exist that report the incorporation of

capsaicin into polymer coatings for inhibiting biofilm formation, where a good

antifouling performance was observed after the exposure to marine environment.

However, based on our knowledge, the incorporation of capsaicin into a foul-release

coating has not been published or patented.

Benzoic acid and its sodium salt (sodium benzoate) are most common safe food

preservatives and antimicrobial agents. Benzoic acid and sodium benzoate are classified

in the United State as Generally Recognized as Safe (GRAS) and their use in food is

permitted up to the maximum level of 0.1 % (Sagoo et. al. 2002). From Microtox

studies, the toxicity, in term of the concentration that kills 50% of bacterial population or

EC50, of benzoic acid and sodium benzoate are determined to be ~ 7 ppm and ~ 560 ppm

(Haque et al ., 2005), respectively, which are three to five orders of magnitude lower than

that of organotin compounds. Recognizing also the commercial availability and

cheapness of benzoic acid and sodium benzoate, they are attractive candidates to be

incorporated into coatings as environmentally benign alternatives.

Previous works (Lueck, 1980) for evaluating the antibacterial behavior of benzoic

acid have concluded that, at different points of the citric acid cycle, benzoic acid



21

deactivates the enzymes that control acetic acid metabolism and oxidative

phosphorilation in yeast and bacteria. In a recent review on antibacterial mechanisms

(Chapman, 2003), benzoic acid is believed to act by interfering with the ability of the cell

membrane to maintain a suitable pH level, which consequently leads to acidification of

the cell interior and widespread disruption of the metabolism process (Eklund, 1985). As

mentioned before (Railkin, 2004), the mode of action of benzoic acid has been identified

experimentally by standard behavioral tests, where benzoic acid is proved to exhibit both

non-toxic modes of actions (repellency and chemical anti-adhesive). This finding

highlights the possibility of benzoic acid to be effective against broad spectra of microand macro fouling species. In addition, field studies had shown that benzoic acid when

added to vinyl-rosin coating was effective in inhibiting different species of both micro

and macro foulers (Railkin, 1995), making it a very attractive non-toxic or less toxic

antifouling candidate. However, due to the extremely fast leaching of the compound, the

effectiveness of the coating with benzoic acid incorporated was only for a short period of

time (1 month). Therefore, for further utilization of benzoic acid, understanding the

causes of the fast leaching is essential and seeking techniques to incorporate benzoic acid

into a coating to control its leaching is also important.

The salt form of benzoic acid, such as sodium benzoate (NaB) is even more

environmentally benign as compared to benzoic acid. Static biological assays have also

proved that NaB showed a narcotic (non-toxic) effect on the investigated microorganisms

(Vetere et. al ., 1999). In addition to sodium benzoate, other different benzoic acid salts

(calcium benzoate and aluminum benzoate) have also been tested by biological assays,



22

and antifouling effectiveness similar to NaB were observed (Vetere et. al., 1999). The

mode of action for sodium benzoate is not precisely identified as for the case of benzoic

acid, but the above study could indicate that the benzoate anion freely floating in water is

important for the antifouling of NaB, and that the antifouling mechanism of NaB might

be similar in some aspect as that of benzoic acid (Vetere et. al., 1999, Stupak et. al .,

2003). Nevertheless, field studies had shown that sodium benzoate when incorporated

into a marine paint was successful for inhibiting marine microorganism attachments

(Stupak et. al ., 2003). However, based on our knowledge, the incorporation of sodium

benzoate or benzoic acid into a foul-release coating has not been published or patented.

Tannic acid or tannins are naturally accruing polyphenolic compounds of high

molecular weight in the range of 500 to 3000 g/mol. They are important in industry,

food, and environmental science (Ho, 1992). The anticorrosive properties of tannins

were known at least 50 years ago, and several tannin-based products exist in the market

for corrosion treatment application. The antifouling properties of tannic acid and its

derivatives – when exist in solution or entrapped into a coating - were also reported in

several studies (e.g. Perez et al ., 2006; Stupak et. al. 2003; Lau and Qian, 2000).

However, up to our knowledge, the incorporation of tannic acid or its derivatives in

silicone coatings has not been reported. In addition, tannic acid is significantly less toxic

than heavy metals-based compounds [the EC50 of tannic acid is about 118 ppm (Xu et al .,

2005)]. As mentioned before, Railkin (2004) emphasized that tannic acid was one of a

few compounds that were verified by standard experimental behavioral tests to have true

repellent mode of action.





24

∆Gmix / RT = n1 ln Φ1 + n2 ln Φ2 + n3 ln Φ3 + n4 ln Φ4

+ n1 Φ2 χ 12 + n1 Φ3 χ 13 + n1 Φ4 χ 14 + n2 Φ3 χ 23

+ n2 Φ4 χ 24 + n3 Φ4 χ 34 (2.1)

where R and T are respectively the ideal gas constant and the absolute temperature, ni and

Φi are respectively the number of moles and the volume fraction of component i, and χ ij is

the binary interaction parameter for the component pair i-j. However, in their

calculations, they did not measure or calculate the χ ij parameters. Instead, they assumed

some constant values for the χ ij parameters (either 1.0, 1.5, 0.5, or -1.0).

Venkataraman and Hajra (1999) developed a complicated model to describe the

solid-liquid equilibrium for their quaternary system. They derived the excess free energy

function for the liquid mixture in terms of 37 parameters pertaining to six of the

constituent binaries, four ternaries and the quaternary interactions of the system. Their

model is basically an extension of the four-parameter model available for binary system.

The four-parameter model for binary system is:

∆GE / RT = y1 y2 [ a1 y1 + a2 y2 + y1 y2 ( a3 y1 + a4 y2)] (2.2)

where ∆GE is the excess free energy and a1 ─ a4 are the binary constants. Consequently,

the partial excess property for component 1 and 2 are:

RT lnγ1 = ∆GE - y2 (∂∆GE / ∂ y2) (2.3)



25

RT lnγ2 = ∆GE + (1-y2 ) (∂∆GE / ∂ y2) (2.4)

where γ1 and γ2 are the activity coefficients of component 1 and 2, respectively.

Gander et. al. (1995) proposed a different approach for predicting the miscibility

between the polymer and the incorporated compound. Experimentally, they incorporated

a macromolecular additive into PLLA polymer using different solvents, where they first

dissolved the additive into water and then they mixed the aqueous solution with the

polymer/solvent mixture. Theoretically, they hypothesized that the maximum entrapment

quality (i.e. maximum additive/polymer miscibility) would be obtained if:

| ∆E1 + ∆E2 | = minimum (2.5)

where ∆E1 is the interaction energy between the solvent (S) and the aqueous

additive/water solution (B), and ∆E2 is the interaction energy between the solvent (S)

and the polymer (P), and given by:

∆E1 (J/mol) = ─ 2 VS (δd,S δd,B + δ p,S δ p,B) ─ (ES EB + CS CB ) (2.6)

∆E2 (J/mol) = ─ 2 VS (δd,S δd,P + δ p,S δ p,P) ─ (ES EP + CS CP ) (2.7)

where Ei and Ci are the Drago’s constants (Drago et. al ., 1993) for component i. In their

analysis, however, they assumed that the properties of the aqueous additive/water

solution are equivalent to the properties of pure water, which might not be correct for



26

concentrated aqueous solutions. In addition, the applicability of the model is limited to

systems of known Ei and Ci constants, which are unavailable for most polymeric

materials.

Nesterov and Lipatov (1999) used slightly a different form of Flory-Huggins

equation. In their analysis for the ternary system, the change in the free energy of mixing

three components was expressed per unit volume, and given by:

∆Gmix / V = (Φ1 / V1) ln Φ1 + (Φ2 / V2) ln Φ2 + (Φ3 / V3) ln Φ3 + Φ1 Φ2

χ 12 + Φ1 Φ3 χ 13 + Φ2 Φ3 χ 23 (2.8)

where V is the mixture volume, and Vi is the molar volume of component i. Based on the

above equation, they defined the overall interaction parameter χ 123 by the following

approximate relation:

χ 123 ~ Φ1 Φ2 χ 12 + Φ1 Φ3 χ 13 + Φ2 Φ3 χ 23 (2.9)

Consequently, they implied that a positive value for χ 123 is an indication for an

immiscible system, whereas a negative value is an indication for a miscible system.

Equations 2.1 and 2.8 are basically the same “the Flory-Huggins equation”, but in

different forms and unit basis. Another more convenient form of the Flory-Huggins







29

Most of the above models are based on the Flory-Huggins theory. The Flory-

Huggins theory was originally derived to describe the miscibility of polymer-solvent and

polymer-polymer mixtures (Flory, 1953). Although still widely used due to its

simplicity, it is now well-known that the Flory-Huggins theory has many limitations,

which make its prediction weak for many situations. One weakness of the Flory-

Huggins theory came from the original cubic lattice model that was used to derive the

mixing functions (entropy and enthalpy of mixing) of a solvent (or a polymer) with

another polymer. According to the Flory-Huggins theory, for a binary solvent-polymer

system, the lattice is subdivided into n number of sites, each site is filled by either asolvent molecule or a segment of polymer chain (the polymer chain is divided into N

number of segments, each segment is assigned to be equivalent in length to the size of the

solvent molecule). Similarly, for a binary polymer-polymer system, each site is filled by

either a segment of the chain of the first polymer or a segment of the chain of the second

polymer. In other words, no sites are allowed to be empty because the polymers are

assumed to be simple incompressible fluids and hence there is no volume change upon

mixing, which is not a true assumption for a polymeric material due to its huge size. The

weakness of the Flory- Huggins theory can be further understood by elaborating more on

the binary interaction parameter, χ 12. According to the Flory-Huggins theory, χ 12 is

always greater than zero, a smaller χ 12 indicates a higher chance that the binary solvent-

polymer system would be miscible, and a value of χ 12 < 0.5 is the Flory-Huggins criterion

for a solvent/polymer system to be completely miscible. However, it is now believed that

χ 12 is qualitatively described in the following more general form:











34

for controlled drug release delivery systems, and most of these models are empirical or

semi-empirical. A review of this modeling effort is the subject of section 2.4.3.

The mass release mechanism of an antifoulant from a polymer matrix immersed

in water depends largely on the solubility of the antifoulant in the matrix. If it has a high

solubility, the release primarily follows a diffusion-dissolution mechanism, which takes

place in the continuum of the polymer phase. If the solubility is very low, the mechanism

is controlled by the channeling/pores formation, where the pores and channels are formed

progressively due to water diffusion within the aggregate phase and/or at the polymer/aggregate interface and dissolution of the particles and generation of more

empty space (voids), and therefore the media of diffusion here is water that fills the

pores, not the continuum of the polymer phase. If the solubility is intermediate, both

mechanisms contribute to the overall release and the medium of diffusion are both in the

polymer phase and the water-filled pores phase.

Sections 2.4.1 and 2.4.2 review the mathematical modeling efforts published for

the case of very high and very low antifoulant/polymer miscibility, respectively. For

both sections 2.4.1 and 2.4.2, the review of the modeling efforts is strictly referred to the

situations where the following conditions are always satisfied: the polymer matrix is

monolithic; the polymer matrix is neither soluble nor swell-able in water; and the active

compound is initially loaded in the matrix at a concentration in excess of its solubility

limit in the matrix. Also, unless specified, the geometry of the matrix is always in

rectangular coordinate.









38

2.4.2 Active compounds with low solubility in the matrix

If the solubility of the active compound in the matrix is very low, then the media

of diffusion is the liquid that fills the pores within the matrix, and hence diffusion through

the continuum of the polymer phase could be neglected. Therefore, the polymer matrix

could be visualized as a porous membrane. Known to be the first mathematical model

published in the literature to describe the drug mass release from porous polymeric

matrix, Higuchi (1963) applied the pseudo-steady state approach to come with:

M = A [ (D b ε / Ө) C bs (2Ca0 ─ ε C bs) t ]1/2 (2.27)

where A is the total surface area of exposure; D b is the diffusion coefficient of the

compound in the pore-filled fluid (i.e. water in our systems); ε and Ө are respectively the

matrix porosity and tortuosity factor; Ca0 is the initial concentration of the compound in

the matrix; and C bs is the solubility of the compound in water. However, equation 2.27 is

derived based on pseudo-steady state assumption, and on the assumption that the

compound aggregates are quite small comparing to diffusion distance and are evenly

distributed in the matrix. Also, the pseudo-steady state assumption only holds if (Ca0 >>

ε C bs). Another concern for equation 3.1 is the porosity ε, which is actually not constant

but is a function of time as more particles of the active compound are dissolved. For

simplicity, to account for the time functionality of ε, one may assume that [ε ~ Ca0 / ρa],

where ρa is the density of the active compound, provided that the initial porosity could be

neglected, and if the initial volume fraction of the compound in the matrix is high.



39

Desai et al (1966) slightly modified equation (2.27) in order to account for the

drug binding phenomena, and came with:

M = A { (D b ε / Ө) C bs [2Ca0 ─ [ε + K eq(1 ─ ε)] C bs] t ]1/2 (2.28)

where K eq is the equilibrium partition coefficient of the active compound in the matrix,

which is defined as the ratio of the concentration of the compound in the polymer phase

to its concentration in the fluid inside the pores. Hence, if K eq is zero, equation (2.28) is

reduced back to equation (2.27).

Miller and Peppas (1982) applied the moving boundary approach to describe the

release from a polymer with continuous porosity formation. In their analysis, the

compound dissolves in the fluid medium (e.g. water) at the interface which moves into

the pore as the dissolution progresses. After defining the position of the interface by [2α

(D b t / Ө)1/2 ], they derived analytical expression for the mass release rate (dM/dt) as:

(dM/dt) = [ε 2/3 A C bs / erf (α)] (D b / Ө π t) 1/2 (2.29)

where α is obtained from:

π 1/2 α exp (α2) erf (α) = C bs / (ρa ─ C bs) (2.30)

The model of Miller and Peppas (1982) is much more accurate than the Higuchi model.

However, it is still limited to the case when (Ca0 > ε C bs), and in their model they

neglected the kinetic of the dissolution step assuming that it is very rapid.





41

M/M0 = 3K t1/2 ─ 3(K t1/2) 2 + (K t1/2) 3 (2.33)

In equation (2.33), K is the release rate constant whose expression is also dependent on

the geometry of the matrix. For a spherical matrix, K is defined by (Gerstl et al ., 1998):

K = (1/ Ca0 r) [Dm (2 Ca0 ─ C bs) C bs]½ (2.34)

where r is the radius of the matrix (which is a microsphers here), Ca0 and C bs are as

defined before, and Dm is the diffusion coefficient of the compound in the matrix(including the effect of the porosity and tortuosity).

2.4.3 Leaching behaviors of marine antifouling paints

With respect to their behaviors in seawater, marine antifouling paints can be

classified into three categories: insoluble matrix, soluble matrix, and self-polishing

matrix. This section considers only the insoluble matrix category, summarizing the

mathematical modeling efforts specifically conducted for quantifying the leaching

behaviors of insoluble antifouling paints in water. The models summarized here are from

the perspective of antifouling paints researchers, not from the perspective of controlled

drug delivery researchers. Although most of the models presented here are empirical or

semi-empirical, they are useful since they are supported by real field leaching

experiments in seawater. In the following discussion, the pigment (a nomenclature



42

frequently used by antifouling paints researchers) is always referred to the antifouling

compound.

Insoluble matrix coatings are those that do not erode over time in water, and

sometimes called continuous contact or contact leaching coatings, because of the leaching

mechanism involved. Marson (1969) was the first who mathematically modeled the

antifoulant leaching from insoluble matrix paints. In order to come up with his model, he

postulated the following leaching mechanism, with an assumption that the antifoulant

particles (Cu2O in his study) were spherical of equal size and uniformly distributed inmulti layers in the matrix (a rubber resin in his study). First, when the coating is initially

immersed in water, the pigment particles at the surface layer of the film dissolve forming

a saturated solution of the pigment at the pigment/leachate interface. Then, the saturated

solution diffuses outward through the diffusion layer in contact with the coating surface.

When a particle dissolves in seawater to reveal the thin binder membrane separating it

from the un-dissolved particles, water diffuses into the thin membrane and dissolves

some of the un-dissolved particles. Consequently, the resulting osmotic pressure ruptures

the membrane and the pores become interconnected. After many simplifications, Marson

was able to come up with a simple analytical equation that shows the dependency of the

leaching rate on the parameters of the coating and of the leaching media:

F = B υ / [1 + Cd/P) (2.35)

where F is the leaching rate, B and C are constants, υ is the pigment volume fraction, d is

the thickness of the matrix, and P is the fraction of interconnected holes that depends on υ



43

and the particle size distribution. P could also be defined as the packing factor (a

parameter defining the number of voids between interconnecting pigment particles).

However, the Marson model is empirical and not predictive since it does not

include the dependency of the leaching rate on time. Later on, Marson (1974) modified

his original model to account for the dependency of the number of interconnecting holes

on the initial Cu2O loading. However, his modified model is still empirical and not time-

dependent. It should be noticed that, since Cu2O is insoluble in the rubber matrix, the

leaching mechanism is similar to the one described for drug release from porous matrix(section 2.5.2).

De La Court and De Vries (1973) modified the Marson model to account for the

shape and distribution of the antifoulant particles in the matrix. However, they applied a

pseudo steady state assumption in their analysis.

Monaghan et al . (1978) derived an empirical relation to quantify the leaching of

organotin compounds form insoluble matrix coatings. However, it appears that their

model is empirical and maybe applicable only for specific organotin compounds.

One disadvantage of the Marson model is that it is not predictive because the

independent variable is the film thickness. Caprari et al (1990) re-derived and modified

the Marson model to let the immersion time to be the independent variable, hence their

model turned to be predictive (Caprari et al , 1990):



44

F = B υ / [1 + C e n/ε) (2.36)

where e is the thickness of the leached layer, and n is the number of leached layers. Here,

n is a function of the immersion time (t), and given by the solution of the quadratic

equation (Caprari et al , 1990):

0 = (Cd pe2 / 2KBθ) n2 + [d pe / KB + Cd pe

2 / 2KBθ] n + [d pe / KB ─ t] (2.37)

where d p is the pigment density, K is a constant (K = 10

-6

M p / M b, where M p and M b arerespectively the molecular weight of the pigment and the bulk fluid), and ε is the

porosity, given by the following empirical equation (Caprari et al , 1990):

ε = exp [ ─ (1 ─ υ)2 / A υ] (2.38)

where A is a constant. The Caprari model is much better than the original Marson model.

However, it is still a semi-empirical relation.

Vasishtha et al . (1995) performed a leaching experiment to study the leaching of Sea

Nine 211 from VYHH matrix (a commercial polymer from Union Carbide, which is a

copolymer of vinyl chloride and vinyl acetate). They varied the initial loading of Sea

Nine 211 in VYHH from 20-35 wt%, and prepared the coatings on cylindrical rods. In

their mathematical analysis, they selected the model of Crank (1975) for describing the

release form cylindrical matrix coating:



45

M / M0 = 4 (π)1/2 [D t / a2]1/2 (2.39)

where a is the radius of the cylinder. By fitting the leaching data to the above equation,

they calculated the diffusion coefficient, D, to be 1.7 x 10 -14 and 3.8 x 10-13 cm2/s at 20

wt% and 35 wt% loading, respectively. However, the model that they selected was

strictly derived by Crank for a special case when the compound is initially loaded in the

matrix at a concentration well below the compound/matrix solubility, thus the compound

is always molecularly dissolved in the polymer phase. However, in their work, Vasishtha

et al . (1995) did not justify if the solubility of Sea Nine 211 in VYHH is greater than 35wt%, in order to use the Crank equation. In fact, it is very rare to have an antifouling

compound with such a high solubility in the matrix.





47

carbonate). Sylgard®184 also came in two parts: a polymer base containing vinyl-

terminated-PDMS and a curing agent having SiH-terminated-PDMS and the Pt-catalyst.

Table 3.1 summarizes the compositions of the two types of silicones.

Sodium benzoate (99% pure, in powder form) was purchased from Sigma Aldrich

Chemical Inc. and used as received. It has a molecular weight of 144.1 g/mol and a

melting point of above 300 o C. It is highly soluble in water (0.555 g / 1 g water) but has

a limited solubility in most organic solvents.

Benzoic acid (99% pure) was purchased from Sigma Aldrich Chemical Inc. and

used as received. It has a molecular weight of 122.1 g/mol and a melting point of 122 o

C. It is highly soluble in most organic solvents and has a limited solubility in water. The

solubility of benzoic acid in water is about 3.4 mg/ml (Farrell and Sirkar, 1999)

Capsaicin is a naturally extracted mixture containing approximately 65%

capsaicin and 35% dihydrocapsaicin. It was purchased from Sigma Aldrich Chemical

Inc. and used as received. It has a molecular weight of 305.4 g/mol and a melting point

of 63.5 o C. It is highly soluble in most organic solvents and has a very low solubility in

water (about 60 ppm).

Tannic acid was purchased from Sigma Aldrich Chemical Inc. and used as

received. According to the data sheet for the commercial product obtained from Sigma



48

Table 3.1 The compositions of the two types of silicones (RTV11 and Sylgard® 184)used in the current study. All percentage shown here are in mass basis.

Component RTV11 Sylgard® 184

Polymer base OH-PDMS (66.4 %) * Vinyl-PDMS (90.9 %)

Curing agent ES40 (1.6 %) * SiH-PDMS (9.1 %)

Inorganic Filler CaCO3 (32 %) * -

Catalyst DBT (0.5 %) ** Pt (included in the curing agent)

* comes in one part (part 1).

** per 99.5 % of part 1

Aldrich, the molecular weight is specified as 1700 g/mol. It has a melting point of 210 0

C. It is highly soluble in water (1 g/ 0.35 ml water) and also has good solubility in some

organic solvents such as acetone and ethanol.

The organic solvents used in the current study (toluene, acetone, acetonitrile,

ethanol and ether) were purchased from VWR and used as received. Microscopic glass

slides (pre-cleaned, size: 25 x 75 x 1 mm) were obtained from VWR and used as

received.

3.2 Sample preparation

The antifoulants (capsaicin, benzoic acid, sodium benzoate, and tannic acid) were

incorporated into two polymeric silicone coatings (Sylgard® 184 and RTV11) by the



49

Table 3.2 The actual combinations of (antifoulant/solvent/polymer) mixtures used inthe current study to incorporate the antifoulant into the bulk of the polymer matrix by thesolvent-blending technique.

Antifoulant Silicone matrix Solvents

Sodium benzoateSylgard®184,

RTV11water/acetone blends

(different ratios)

Benzoic acidSylgard®184,

RTV11

Acetone,toluene,

acetonitrile,

ether

Capsaicin RTV11Toluene,Ethanol

Tannic acidSylgard®184,

RTV11Acetone

solvent-blending technique. Briefly, in order to obtain homogeneous antifoulant/silicone

blending by the solvent-blending method, the antifoulant was first dissolved in a solvent

or blend of solvents, and then the solution was homogenized with the polymer base, then

the curing agent was added to the mixture after drying off the solvent used. Table 3.2

summarizes the actual combinations of materials (antifoulant/solvent/matrix mixtures)

used in the current study.



50

For incorporating sodium benzoate (NaB) into silicones, the following procedure

was followed, which is shown schematically in Figure 3.1, and the detailed

concentrations of the samples prepared are summarized in Table 3.3. NaB is much more

inorganic in nature than the other antifoulants, and consequently has low solubility in

organic solvents. On the other side, it has a high solubility in water (0.555 g of NaB per

1 g of water). Therefore, it was first dissolved in de-ionized water at a certain

concentration (the exact concentration is shown in Table 3.3). Next, a water-miscible

organic solvent, acetone, was added to the aqueous phase at a certain water/acetone mass

ratio (the exact ratio is shown in Table 3.3) while maintaining the complete solubility of NaB in the mixed-solvent. Then, the NaB/(water + acetone) solution was homogenized

Figure 3.1 A schematic diagrams for the sample preparation steps used for incorporating sodium benzoate into the silicone polymer coating.

NaB Acetone

Water

+ NaB

Water

mixing

polymer

base

drying

solvent

mixing

Curing

agent



51

Table 3.3 The detailed concentrations for NaB/ Sylgard®184 samples prepared atdifferent conditions. All concentration shown here are in mass basis. (Abbreviation: W:

water, A: acetone, S: solvent (i.e. water + acetone), P: silicone polymer base).

Sample

#wt% NaB/W W/A ratio Wt% NaB/S S/P ratio wt% NaB/P

1 7.77 50/50 4.0 20/80 1

2a 16.67 50/50 9.1 10/90 1

2b 4.61 50/50 2.4 30/70 1

3a 17.39 20/80 4.0 20/80 1

3b 12.31 30/70 4.0 20/80 1

3c 9.52 40/60 4.0 20/80 1

3d 5.00 80/20 4.0 20/80 1

3e 4.47 90/10 4.0 20/80 1

3f 4.04 100 % W 4.0 20/80 1

4a 3.94 50/50 2.0 20/80 0.5

4b 15.09 50/50 8.2 20/80 2

4c 22.02 50/50 12.4 20/80 3

4d 28.57 50/50 16.7 20/80 4

4e 34.78 50/50 21.1 20/80 5





53

dissolved in the organic solvents at a concentration of 4 wt.%. Next, 1.5 g of the 4 wt. %

antifoulant/solvent solution was homogenized with 6 g of the silicone polymer base with

rigorously mixing. In other words, a solvent/polymer mass ratio of 20/80 was used,

which was fixed for the case of capsaicin, benzoic acid, and tannic acid throughout the

study. This procedure produced a matrix with 1 wt% of antifoulant entrapped in the

silicone matrix after drying off the solvent. Then, all mixtures were left inside the fume-

hood at ambient conditions for 2 days, then under vacuum at room temperature for 20

min, to remove the solvent. After that, the samples (after drying off the solvent) were

cured and processed exactly the same way as described above for NaB/silicone coatings.

3.3 Sample processing

After making sure that the samples prepared were apparently cured, the samples

were preceded for further experimentations and processing, as follow.

First, the surface and bulk properties of the samples were evaluated by a variety of

techniques. The wettability of the samples was evaluated in terms of measuring the water

contact angles. The bulk property of the samples was evaluated by measuring the elastic

modulus, which was done either by the JKR technique and/or the stress-strain technique.

The surface morphology of the samples was examined by Optical Microscopy

techniques. The details of the surface morphology and surface roughness were evaluated

by SPM technique. SPM was employed here for two set of samples only: RTV11



54

samples, and 1 wt% capsaicin/RTV11 samples, and the SPM scans were done here for

both as-prepared samples and for samples after being immersed in DI water for 14 days.

Second, the miscibility of the antifouling compounds with the silicone matrix was

described through the analysis of the aggregate size of the antifouling compounds in the

matrix. One advantage of Sylgard®184, compared to RTV1, is that it is highly transparent

so any material entrapped could be observed easily by the Optical Microscopy.

Therefore, Optical microscopic images were taken for the bulk of Sylgard®184 contained

the incorporated compounds, after drying off the solvent. The aggregate size wasanalyzed by Scion Image Software

Third, the leaching of the incorporated compounds from the silicone coatings into

water was evaluated, as follow (the detailed compositions for the samples that were

subjected to leaching experiments in the current study are summarized in table 3.4). For

benzoic acid, tannic acid, and sodium benzoate - incorporated silicones, large sheets

(total mass: ~ 6.0 g; size: ~0.09 cm x 64 cm2) of silicones with the incorporated

compounds were immersed in large glass beakers each containing 300 ml de-ionized (DI)

water. To determine the amount leached out, the conductivities of the solutions at

different time intervals were measured and their corresponding concentrations were

entrapolated via standard calibration curves. For capsaicin/RTV11 system, large sheet

(total mass: ~ 6.0 g; size: ~0.09 cm x 64 cm2) of RTV11 with the incorporated compound

was immersed in large glass beakers containing 500 ml DI water. Then, the



55

Table 3.4 The detailed compositions for the samples that were subjected to leachingexperiments in the current study. All concentration shown here are in mass basis.(Abbreviation: W: water, A: acetone, S: solvent, P: silicone polymer base, NaB: sodium benzoate, BA: benzoic acid, TA: tannic acid).

Sample

#Compound Polymer Solvent

S/P

ratio

wt%

compound/P

1 NaB Sylgard®184 50/50 W/A 20/80 1

2a NaB Sylgard®184 50/50 W/A 10/90 1

2b NaB Sylgard®184 50/50 W/A 30/70 1

3a NaB Sylgard®184 20/80 W/A 20/80 1

3b NaB Sylgard®184 30/70 W/A 20/80 1

3c NaB Sylgard®184 40/60 W/A 20/80 1

3f NaB Sylgard®184 80/20 W/A 20/80 1

3g NaB Sylgard®184 90/10 W/A 20/80 1

3h NaB Sylgard®184 100 % W 20/80 1

4a NaB Sylgard®184 50/50 W/A 20/80 0.5

4b NaB Sylgard®184 50/50 W/A 20/80 2

4c NaB Sylgard®184 50/50 W/A 20/80 3

4d NaB Sylgard®184 50/50 W/A 20/80 4

4e NaB Sylgard®184 50/50 W/A 20/80 5

5 NaB RTV11 50/50 W/A 20/80 1



56

Table 3.4 The detailed compositions for the samples that were subjected to leachingexperiments in the current study (Continued)

Sample

#Compound Polymer Solvent

S/P

ratio

wt%

compound/P

6a BA Sylgard®184 Toluene 20/80 1

6b BA Sylgard®184 Acetone 20/80 1

6c BA RTV11 Acetone 20/80 1

7a Capsaicin RTV11 Toluene 20/80 1

7b Capsaicin RTV11 Ethanol 20/80 1

8a TA Sylgard®184 Acetone 20/80 1

8b TA RTV11 Acetone 20/80 1

concentrations of capsaicin in the water bath at different time intervals were evaluated by

HPLCE technique where the unknown concentrations were interpolated via standard

calibration curve.

Fourth, bacterial attachment in fresh water containing enriched bacteria isolated

from Lake Erie was conducted to assess the coating’s antibacterial performance of

capsaicin-treated coatings and sodium benzoate-treated coatings. The bacterial water

solutions were prepared and provided by the research group of Dr. T. Cutright, Civil

Engineering Department, and the detailed procedure for culturing the bacteria was

described elsewhere (Xu et al., 2005; Xu, 2004). The coatings prepared on glass slides

were placed in amber bottles half-filled with the water having the isolated bacteria. Care

was taken to ensure all the coatings were arranged facing the bottom of the bottle to avoid



57

the settlement of foreign species and organic matter. Coatings were observed at periodic

intervals up to one month. At each time interval, a coating was removed, dipped gently

in fresh de-ionized water several times to remove loosely attached objects, and after

drying to remove the DI water, they were subjected to optical microscope observation

immediately.

3.4 Characterization Techniques

3.4.1 Contact Angle technique

Contact angle is a quick and easy method to evaluate the wettability of a solid

surface (Chan, 1994). It is also an indirect quantative method for measuring the surface

energy of a solid surface. By comparing the contact angles of two surfaces, higher

contact angle indicates lower wettability and higher hydrophobicity of the surface, which

implies lower surface energy. The physics behind contact angle phenomena is best

described through considering the situation of a liquid drop being in equilibrium with a

solid substrate in air, which is frequently described mathematically through the famous

Young’s equation:

γLV cos θ = γSV ─ γSL (3.1)

where γLV, γSV and γSL are respectively the surface energies at the liquid/vapor,

solid/vapor, and solid/liquid interfaces, and θ is the equilibrium contact angle (as shown



58

in Figure 3.2a). Therefore, the Young’s equation relates the contact angle to the surface

energy of the substrate. Experimentally, the contact angles can be measured by applying

the sessile drop method where both dynamic and static contact angles can be measured.

For dynamic angles, the advancing and receding angles are achieved by the addition and

removal, respectively, of water from the drops formed on the coating surface, whereas for

static angles the water drop is placed on the coating surface and let to equilibrate without

external force (as shown in Figure 3.2b).

Figure 3.2 A simplified sketch for the contact angle concept. (a) the static contactangle: a liquid drop being in equilibrium with a solid substrate in air. γLV, γSV and γSL

are respectively the surface energies at the liquid/vapor, solid/vapor, and solid/liquidinterfaces, and θ is the equilibrium (static) contact angle. (b) the advancing contact angle(θa). (c) the receding contact angle (θa).

advancing

θa Substrate

(b)receding

θr

Substrate

(c)

air

γSL

γLV

SubstrateγSV θ

(a)



59

In the current study, the contact angle technique was applied to evaluate the

surface wettability of the coatings. Deionized water was the probe liquid, and both

dynamic and static contact angles were measured via the sessile drop method. Images of

the drops were captured using the Dazzle DVC (Digital Video Creator) and its software,

and data were processed using the Scion Image Software.

It should be noticed that both advancing and receding contact angles slightly

depend on the rate of fluid injection (for advancing) and removal (for receding). For

advancing angle, it slightly increases as the rate of injection increases. For recedingangle, it slightly decreases as the rate of removal increases. Therefore, for our

experiments, the rate of injection was gradually increased at three rates and the

corresponding advancing contact angles were measured and averaged all together.

Similarly, the rate of removal was gradually decreased at three rates and the

corresponding receding contact angles were measured and averaged all together.

3.4.2 The stress - strain technique

The stress – strain technique is a common and widely used technique for

measuring the elastic modulus of most types of materials, including both soft and hard

materials. According to this method, the elastic modulus (E) is defined as ratio of the

stress applied on a sample over the strain resulted in the shape of the sample.

Experimentally, the stress is usually varied at a certain range, and at each time the

elongation in the sample length is measured. Strain is defined as [(L ─ L0)/L0)], where L



60

and L0 are the initial length and the elongated length of the sample, respectively. Stress is

defined as [mg/(w x t)], where m is the mass of the load used, g is the acceleration of

gravity, w and t are respectively the width and thickness of the sample. Hence, E is

obtained as the slope of the linear plot of stress v.s strain data.

For the current study, the stress – strain technique was applied to measure the

elastic modulus of antifoulant-blended RTV11 coatings, due to their opaque nature. A

small rectangular sheet (length ~ 25 mm, width ~ 6 mm, and thickness ~ 1 mm) of

coating was vertically hung in air, its elongation under a particular weight was measuredfrom the magnified images captured using the goniometer video system and the Dazzle

DVC and its software. Stress was varied gradually up to ~ 0.13 MPa.

3.4.3 The JKR technique

The JKR technique is a specialized technique for measuring the elastic modulus

of soft materials. It is also a direct quantative method for measuring their surface

energies. Its name came on behalf of the three scientists (Johnson-Kendall-Roberts)

whose originally established the theoretical framework for this method in 1971 (Johnson

et al., 1971), and sometimes called contact mechanics technique. The detailed theory

behind the JKR technique can be found elsewhere (Chaudhury and Whitesides, 1991;

Johnson et al., 1971). Briefly, a soft elastic lens is brought into contact with an elastic

surface, and the deformation of the contact zone (normally a circular area) under a certain

load can be related to the elastic modulus of the system, thus the modulus of the coating.



61

In the current study, the JKR method was used to measure the elastic modulus of

Sylgard® 184 coatings and antifoulant-incorporated Sylgard® 184 coatings. The modulus

of antifoulant-incorporated Sylgard®184 coatings could also be measured via the

stress-strain technique. However, the JKR technique was selected here because of the

extensive usage of the JKR technique for evaluating the properties of Sylgard®184, a

highly transparent coating, and the values measured in this study could be compared to

the reported values. The elastic modulus of control Sylgard®184 coating was also

measured by the stress-strain method to compare between the accuracy of the two

methods. For the JKR method, the procedures developed by Chaudhury and Whitesides(1991) were followed in the current study. Briefly, as shown schematically in figure 3.3,

the radii of contact areas for 8 to 10 different compression loads were measured,

and the contact radius vs. load was plotted to extrapolate the modulus of the system

from the slope of the plot. With the known modulus of the lens and assuming the

materials were perfect elastic, the modulus of the coating was deduced from the modulus

of the system.

3.4.4 Optical Microscopy

Optical microscopy is a quick nondestructive tool for providing useful

information about the overall morphology – in two dimensions – of the sample surface.

The principle behind it is that it employs a visible light. Because optical microscope

requires a visible light source, it has a limitation that it can not scan very small area of

less than tens of microns, therefore localized morphology information in nanometer scale



62

Figure 3.3 A simplified sketch for the set-up of the JKR apparatus.

can not be obtained by this technique. Also, only images can be produced by this

technique and no quantative data can be generated directly. The basic components of

optical microscope are the light source, the condenser (to condense the light), the

objective, and the eyepiece. Common types of Optical microscopy involve reflected light

microscopy (for non-transparent samples) and transmitted light microscopy (for

transparent samples). Transmitted light microscopy can be further classified into:

brightfield microscopy, phase contrast microscopy, polarized light microscopy, and

differential interface contrast microscopy.

Optical

Microscope

Analytical balance

Movingstage

Glass slide

Hemisphere lens

Sample



63

In the current study, variations in the morphology of the AF compounds-

incorporated silicone films were observed using an optical microscope (Model IX-70,

Olympus) having video and still image capturing capabilities.

3.4.5 Scanning Probe Microscopy

Scanning probe microscopy (SPM) involves several types, such as scanning

tunneling microscopy (STM), atomic force microscopy (AFM), lateral force microscopy

(LFM), and magnetic force microscopy (MFM). The basic components of SPM are thelaser diode, the piezoelectric scanner, the cantilever and tip probe, and the position

sensitive photo detector. The tip is usually made of silicon or silicon nitrile. AFM was

the technique used in the current study. In AFM, a force probe is applied, which detects

the van der Waals interaction force between the probe tip and the surface, to scan over

the surface of a sample. AFM has the advantages over the optical microscopy technique

is that it can provide quantitative information about the localized surface topography of

the samples both in two dimensions and three dimensions, and it can also provide

quantative information about the surface roughness (Magonov and Reneker, 1997). The

resolution of AFM is very high, close to the atomic level, where a surface area of as small

as 1µm x 1µm can be scanned with a high image quality. However, opposite to optical

microscopy technique, AFM has a limitation that it is not suitable for scanning large

surface area (e.g. > 100µm x 100µm) to provide overall morphology of the sample

surface. AFM is commonly operated in either a contact mode (where repulsive force is



64

used) or a non-contact mode (where attractive force is used). The non-contact mode is

generally preferred because it usually gives better resolution than the contact mode.

In the current study, the details of the coating surface were examined with the

non-contact mode AFM (Metrology 2000, Molecular Imaging), where a Si3 N4 cantilever

with a spring constant of 21 – 78 N/m was used. All the AFM images presented in this

study have a scan size of 80 µm x 80 µm and obtained with a scan rate of 0.20 Hz.

3.4.6 High Performance Liquid Chromatography (HPLC)

HPLC is a popular method of analysis. It has many applications such as

separation, identification, purification, and quantification of various compounds. The

basic components of the HPLC set-up are the solvent reservoir (where the mobile phase

comes from), the pump, the injection port (where the samples are injected into the mobile

phase), the column (where the stationary phase is placed), the detector, and the waste

reservoir. The basic principle behind HPLC is that certain compounds have different

migration rates given a particular column and mobile phase. Thus, the chromatographer

can separate compounds from each other, and the degree of separation is mostly

determined by the selection of the mobile phase and the stationary phase. The mobile

phase is the solvent being continually applied to the column, and acts as a carrier for thesample solution. As a sample solution flows through the column with the mobile phase,

the components of that solution migrate and separate according to the non-covalent

interactions of the compound and the mobile phase with the stationary phase. For



65

example, those samples which have stronger interactions with the stationary phase than

with the mobile phase will elute from the column slower and therefore will have a longer

retention time, whereas the reverse is also true (Schoeff and Williams, 1993).

In the current study, HPLC technique was employed to determine the unknown

bulk concentrations of capsaicin in water, as part of the leaching experiment. The

solutions of unknown capsaicin concentrations were subjected to HPLC analysis (Model

LC-10AT from Shimadzu with the symmetry C-18L column from Walters) using a

mixture of acetonitrile + DI water (50:50 vol/vol) with a pH value of 2.1 as the mobile phase. 10 µL of the solution was injected into the column and flown at a constant rate of

1 ml/min. In order to obtain the calibration curve, a set of standard solutions with

concentrations in the range of 13 – 5000 ppm of the purchased capsaicin were prepared

using the same mobile phase as the solvent.



66

CHAPTER IV

RESULTS AND DISSCUSSION FOR SODIUM BENZOATE-BASED COATINGS

In this chapter, the results obtained for sodium benzoates (NaB) - incorporated

silicone coatings are presented and discussed. The effect of incorporating the compound

on the surface and bulk properties of the coatings is presented in section 4.1. The effectof varying the preparation conditions on the bulk morphology/miscibility of NaB-based

coatings is discussed in section 4.2. Theoretical thermodynamic analysis for the

miscibility study is presented in section 4.3. The effect of varying the preparation

conditions on leaching of the compound in water is presented in section 4.4. Theoretical

mass transfer analysis for the leaching study is presented in section 4.5. The antibacterial

performance for the NaB-incorporated coating is presented in section 4.6.

4.1 Effect of sodium benzoate on surface and bulk properties of silicones

Sodium benzoate (NaB) was incorporated into two types of silicones (Sylgard®

184 and RTV11). For Sylgard® 184 coating, the concentration of NaB in the matrix was

varied from 0 to 5 wt%. For RTV11 coating, only one concentration was prepared (1

wt% NaB in the matrix). For all of the samples prepared, it was observed that the NaB-

blended coatings were cured similarly as the NaB-free coatings alone. Further examining



67

the effect of incorporated compound on the surface and bulk properties of the silicone

coatings qualitatively confirmed this observation, as to be discussed in sections 4.1.1 and

4.1.2.

4.1.1 Effect on wettability

The wettability of the coatings was investigated in terms of measuring the water

contact angles. Sylgard® 184 coatings of various amounts of NaB (0 - 2 wt %) were

subjected to contact angle measurements, and the results are shown in Figure 4.1. For control samples of NaB - free Sylgard® 184 coatings, the advancing, static, and receding

contact angles were measured to be 110°, 106° and 80°, respectively. As seen in Figure

4.1, the incorporation of NaB in Sylgard® 184 (up to 2 wt. %) had little effect on the

wettability of Sylgard® 184 coating. This could suggest that most of NaB molecules

were entrapped inside the bulk of Sylgard® 184 rather than aggregated to the surface.

Otherwise, the contact angle was expected to decrease considerably due to the fact that

NaB has much higher surface energy than silicones (the surface energy for different types

of silicones is in the range of 20 - 24 mJ/m2), and the contact angle hysteresis (difference

between the advancing and receding angles) should increase due to the in-homogeneity of

aggregates if presented on the surface.

In addition, the contact angles of NaB-free RTV11 coating and 1 wt%

NaB/RTV11 coating were also measured, and the results are shown in Table 4.1. For

control samples of NaB - free RTV11 coatings, the static contact angle was 101°,



68

70

80

90

100

110

120

0 0.5 1 1.5 2 2.5

wt% NaB in Sylgard® 184

C o n t a c t a n g l e s

Figure 4.1 Water contact angles of NaB-entrapped Sylgard®184 films (advancing: ,receding: , and static: ). The (solvent: polymer) ratio and the (water: acetone) ratiowere respectively (20: 80) and (50: 50) by mass, which were fixed at all concentrations.Error for each data point (average over 12 measurements) is presented by the verticalline.



69

Table 4.1 Static water contact angles of NaB-entrapped RTV11 films. The (solvent: polymer) ratio and the (water: acetone) ratio were respectively (20: 80) and (50: 50) bymass.

Coating Static Contact angles

Control RTV11 101.2 ± 0.5

1 wt% NaB/RTV11 100.7 ± 0.7

and maintained around this value for the 1 wt% NaB-blended RTV11 coating. The

indifferent contact angle values of 1 wt% NaB-blended RTV11 as those of pure RTV11

could give us the indication that most of NaB molecules were entrapped inside the bulk

of RTV11 rather than aggregated to the surface for the same reasoning mentioned above

for NaB- Sylgard® 184. To summarize, the indifferent contact angle values of NaB

blended coatings as those of pure silicone is the first indication that the NaB-blended

silicone coatings could be cured.

It could be of interest also to notice here the slight difference in the wettability of

pure (i.e. NaB-free) Sylgard® 184 and pure RTV11 coatings. By comparing the static

contact angle data for control samples of NaB-free coatings, it could be suggested thatSylgard® 184 silicone has slightly a higher hydrophobicity than RTV11 silicone. The

same conclusion could be drawn from the surface energy data for the two matrices, as

follow. From literature, the surface energy of RTV11 is 23.3 mJ/m2 (Berglin and



70

Gatenholm, 1999), which is slightly higher than the surface energy of Sylgard® 184

silicone (~ 20 mJ/m2).

Another interesting observation is the effect of the curing temperature on the

contact angle hysteresis for the two matrices. During the initial stage of trying different

schemes to prepare the samples, it was observed that the curing temperature had a strong

effect on the contact angles hysteresis for Sylgard® 184 coating but had no effect on the

contact angles hysteresis for RTV11 coatings. Both control samples of RTV11 and

Sylgard

®

184 were cured at two temperatures: 25° C and at 100° C. For RTV11, thetemperature had no effect on the advancing (103 °) and receding (95°) contact angles.

For Sylgard® 184, however, the advancing contact angle was about 110° at both

temperatures while the receding contact angle increased significantly from about 80° at

the lower temperature to about 95° at the higher temperature. In other words, for

Sylgard® 184, the contact angle hysteresis decreased from 30° at the lower curing

temperature to 15° at the higher curing temperature. Although it is not intended in the

current study to explore this effect in details, it could be anticipated that there were more

un-cross linked chains aggregated on the surface of Sylgard® 184 at the lower

temperature compared to the higher temperature, which would result in increasing the

contact angle hysteresis. For the purpose of enhancing the antifouling performance of the

silicone coating by means of minimizing its contact angle hysteresis, it might be better

technically to process Sylgard® 184 at higher temperatures, although this might not be an

easy task practically. However, for the current study henceforth, all the results associated

with Sylgard® 184 were corresponding to samples cured at room temperature.



71

4.1.2 Effect on elastic modulus

The elastic modulus of a coating is a good measure of its bulk properties. Thus,

Sylgard® 184 coatings of various amounts of NaB (0 - 2 wt %) were subjected to elastic

modulus measurements, and the results are shown in Figure 4.2. For control NaB-free

Sylgard® 184 coating, the elastic modulus was 0.95 MPa, which was in agreement with

the literature value reported (Eddington et. al ., 2003). As shown in Figure 4.2, after

incorporating NaB into Sylgard® 184, it was observed that the elastic modulus increased

slightly from those of control value. This slight variation of the modulus could beattributed to the final distribution and aggregate size of the compound inside the bulk of

the matrix. As to be seen in the next section, sodium benzoate has a uniform distribution

with small aggregate size about 3 µm. Consequently, Sodium benzoate could behave

here as a fine reinforced filler that resulted in increasing the elastic modulus.

Nevertheless, the elastic modulus measurements for the above systems indicated that the

low content of NaB was insignificant in affecting the bulk properties of the silicone

coatings. Also, it confirmed that NaB had little effect on the curing behaviors of

Sylgard® 184 coatings, as the bulk modulus was expected to drop significantly for the

uncured, liquid like coating. To summarize, the incorporation of NaB into Sylgard® 184

coatings did not considerably affect the surface and bulk properties of the Sylgard® 184

coatings, suggesting that the foul-release property of Sylgard® 184 likely be retained.



72

0.0

0.6

1.2

1.8

2.4

3.0

0 0.5 1 1.5 2 2.5

wt% NaB in Sylgard® 184

E l a s t i c M o d u l u s ( M P a )

Figure 4.2 Elastic modulus variations of NaB - entrapped Sylgard® 184 coating. The(solvent: polymer) ratio and the (water: acetone) ratio were respectively (20: 80) and (50:50) by mass, which were fixed at all concentrations. Error for each data point (averageover 6 measurements) is presented by the vertical line.



73

The elastic modulus of NaB-free RTV11 coating was also measured, and found to

be 1.56 MPa. This value is in good agreement with the literature value reported for

RTV11 (Kohl & Bolstes, 2001). The higher modulus of RTV11 as compared to that of

Sylgard® 184 is due to the fact that RTV11 has a high content of enforced fillers (32 wt%

CaCO3) whereas Sylgard® 184 do not have. For 1 wt% NaB/RTV11 coating, the elastic

modulus was not measured, but at this low concentration the modulus do not expected to

differ two much from the corresponding value of NaB-free RTV11 coating. It was

physically observed that the 1 wt% NaB-RTV11 coating was cured. This is supported by

the fact that RTV11 is much more resistant to poisoning than Sylgard

®

184 (the resultsobtained for the other compound, capsaicin; support the last statement, as to be discussed

in Chapter 5).

4.2 Miscibility of NaB in silicones

After confirming that the coating was cured, it is necessary to analyze the

miscibility of the compound with the matrix, because it plays an important role for

compound distribution within the silicone coatings, and consequently on its leaching into

water. The miscibility was related to the aggregate size and distribution of the entrapped

antifouling compound. One advantage of Sylgard® 184, compared to RTV1, is that it is

highly transparent so any material entrapped could be observed easily by Optical

Microscopy. Therefore, to roughly examine the antifoulant/silicones miscibility; optical

microscopic images were taken for the bulk of Sylgard® 184 contained the incorporated

compound after drying off the solvents. For NaB/ Sylgard® 184 system, the preparation



74

conditions were systematically varied to investigate their effects on the morphological

structures. The parameters varied were: solvent composition (acetone/water ratio),

solvent/polymer ratio, and concentration of NaB in the matrix (i.e. after drying off the

solvent). To facilitate the comparisons, we selected the base case conditions to be: 50/50

water/acetone ratio, 20/80 solvent/polymer ratio, and 1 wt% NaB in the matrix, with all

ratios in mass basis. Thus, when varying any one parameter, the other parameters were

fixed at the base case conditions.

4.2.1 Effect of composition of the mixed solvent

The water/acetone ratio was increased from 20/80 water/acetone to 100 % water,

keeping the other conditions unchanged (20/80 solvent/polymer, and 1 wt%

NaB/Polymer). The results are shown in Figures 4.3 and Table 4.2. For the samples

prepared with 20-50 %, the particles had a narrow size distribution and there were no

aggregate observed with a size greater than 15 µm. However, for the samples prepared

with the solvent of 90/10 water/acetone and 100% water, the particles had a wide size

distribution and more than 22% of the aggregates had sizes greater than 15 µm, indicating

non-uniform NaB dispersion in the matrix. It is clear that adding acetone will help to

uniformly disperse NaB in the matrix with smaller aggregates. However, it is not useful

to decrease the water/acetone ratio less than a certain liming ratio, because below this

ratio NaB will not be soluble in the mixed solvent and hence the aggregate size will not

be fine and uniform. From literature, the maximum solubility of NaB in water is 0.555

g/g water, and NaB is practically insoluble in acetone (Bustamante et al., 2000).



75

Figure 4.3 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of 1 wt % NaB/Sylgard® 184 matrix, prepared at different water/acetoneratios, keeping the solvent/polymer ratio fixed at 20/80. (a) 20/80 water/acetone; (b)30/70 water/acetone; (c) 50/50 water/acetone; (d) 80/20 water/acetone; (e) 90/10water/acetone; (f) 100% water. All the values are based on weight.

(a) (b)

(c) (d)

(e) (f)





77

Based on this fact, the limiting acetone/water ratios for different sets of conditions are

calculated and presented in Figure 4.4, which serves as a practical guide for preparing the

samples.

0.0

0.5

1.0

1.5

2.0

0% 20% 40% 60% 80% 100%

water/(water + acetone) mass %

m

a s s N a B / m a s s

w a t e r

1 wt% NaB/P; 20/80 S/P

saturation line

5 wt% NaB/P; 20/80 S/P

7 wt% NaB/P; 20/80 S/P

Figure 4.4 Guidelines chart for preparing NaB-incorporated Sylgard® 184 coatings at

different set of conditions. The saturation line “ — ” represents the maximum solubilityof NaB in water (0.555 g NaB per 1 g of water). Above this line, NaB is not soluble inthe mixed solvent (water + acetone), and hence the bulk entrapment method will not befeasible at this particular set of conditions. (Abbreviations: S: solvent; P: polymer). Allvalues are in mass basis.



78

4.2.2 Effect of solvent/polymer ratio

The solvent/polymer ratio was increased from 10/90 to 50/50 solvent/polymer,

keeping the other conditions unchanged (50/50 water/acetone, and 1 wt% NaB/Polymer).

As shown in Figures 4.5 and Table 4.3, the solvent/polymer ratio of 20/80 was the

optimum ratio that resulted in the minimum aggregate size with the most uniform and

narrower distribution. Decreasing the solvent/polymer ratio to 10/90 resulted in having 8

% of the aggregates with sizes greater than 15 µm. On the other side, increasing the

solvent/polymer ratio to 30/70 – 50/50 resulted in having 14 - 19 % of the aggregateswith sizes greater than 15 µm.

4.2.3 Effect of NaB matrix loading

The wt% NaB/polymer was varied from 0.5 wt% to 5 wt%, keeping the other

conditions unchanged, and results are shown in Figure 4.6 and Table 4.4. While the size

of the aggregates (~ 3 - 4 µm) and the observed narrow size distribution were similar for

all concentrations, the number of aggregates increased and hence the distance between

aggregates decreased as the concentration increased. For all the samples of different NaB

matrix loadings, there were no aggregates observed with sizes greater than 15 µm. The

aggregate size remained at its minimum value because the solvent/polymer ratio and

water/acetone ratio were fixed at their optimum values (20/80 solvent/polymer and 50/50

water/acetone), and at these ratios, NaB was still soluble in the mixed solvent even for

the samples of highest NaB matrix loading (5 wt% NaB/matrix) prepared.



79

Figure 4.5 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of 1 wt % NaB/Sylgard® 184 matrix, prepared at different solvent/polymer ratios, keeping the water/acetone ratio fixed at 50/50. (a) 10/90 solvent/polymer; (b)20/80 solvent/polymer; (c) 30/70 solvent/polymer; (d) 40/60 solvent/polymer; (e) 50/50solvent/polymer. All the values are based on weight.

(a) (b)

(c) (d)

(e)



80

Table 4.3 Aggregate size distribution of 1 wt.% of sodium benzoate inside the (99wt.%) bulk of Sylgard® 184 matrix, for samples prepared at different solvent/polymer (S/P) mass ratios. The water/acetone ratio was fixed at 50/50 by mass.

Sample0-5 µm

size range*

5-10 µmsize range

*

10-15 µmsize range

*

> 15 µmsize range

*

Arithmeticmeansize(µm)

**

Quadraticmeansize(µm)***

10/90S/P 3.1 (42.5) 6.9 (37.0) 12.3 (12.7) 18.6 (8.1) 6.9 8.3

20/80S/P

2.1 (65.8) 7.1 (30.1) 11.5 (4.1) (0) 4.0 4.8

30/70S/P

2.8 (58.9) 7.3 (19.8) 12.2 (7) 33.3 (14.3) 8.7 13.6

40/60S/P

2.8 (42.6) 7.2 (29.5) 12.3 (8.2) 31.0 (19.7) 10.4 14.8

50/50S/P

3.2 (38.4) 7.3 (36.6) 12.1 (11.6) 39.2 (13.4) 10.5 15.7

* The number outside the parenthesis is the average aggregate size evaluated in that particular size range. The number inside the parenthesis is the percentage of number of aggregate at that particular size range to the total number of aggregates.** The arithmetic mean size is defined by summing the sizes of all aggregates anddividing by the total number of aggregates.*** The quadratic mean size is defied as: (∑ di

2 ni/ntot)1/2 , where di and ni are,

respectively, the average size and the number of aggregates at that particular size range,and ntot is the total number of aggregates.



81

Figure 4.6 Optical microscope (bright field) images (570 µm x 480 µm) of the bulk morphology of NaB/ Sylgard® 184 matrix of different NaB concentrations, keeping the

solvent/polymer ratio and the water/acetone ratio fixed at 20/80 and 50/50, respectively.(a) 0.5 wt% NaB/ Sylgard® 184; (b) 1 wt% NaB/ Sylgard® 184; (c) 2 wt% NaB/ Sylgard®

184; (d) 3 wt% NaB/ Sylgard® 184; (e) 4 wt% NaB/ Sylgard® 184; (f) 5 wt% NaB/Sylgard® 184; All the values are based on weight.

(a) (b)

(c)

(e) (f)

(d)



82

Table 4.4 Aggregate size distribution of sodium benzoate inside the bulk of Sylgard®

184 matrix, for samples prepared at different NaB matrix loading (wt% NaB in thematrix). The water/acetone ratio was fixed at 50/50 by mass. The solvent/polymer ratiowas fixed at 20/80 by mass.

Sample0-5 µm

size range

*

5-10 µmsize range

*

10-15 µmsize range

*

> 15 µmsize range

*

Arithmeticmeansize

(µm)**

Quadraticmeansize

(µm)***0.5 wt

%2.4 (67.1) 7.1 (28.8) 12.5 (3.2) (0) 4.1 4.8

1 wt % 2.1 (65.8) 7.1 (30.1) 11.5 (4.1) (0) 4.0 4.8

2 wt% 2.2 (70.4) 6.5 (27.7) 11.8 (1.9) (0) 3.6 4.2

3 wt % 2.2 (70.8) 6.8 (27.0) 12.1 (2.2) (0) 3.6 4.4

4 wt % 1.7 (71.9) 6.7 (24.7) 11.8 (3.4) (0) 3.3 4.2

5 wt % 2.0 (62.4) 6.7 (32.6) 11.7 (5.1) (0) 4.0 4.9

* The number outside the parenthesis is the average aggregate size evaluated in that particular size range. The number inside the parenthesis is the percentage of number of aggregate at that particular size range to the total number of aggregates.** The arithmetic mean size is defined by summing the sizes of all aggregates anddividing by the total number of aggregates.*** The quadratic mean size is defied as: (∑ di

2 ni/ntot)1/2 , where di and ni are,

respectively, the average size and the number of aggregates at that particular size range,

and ntot is the total number of aggregates.



83

4.3 Thermodynamic analysis for the miscibility study

4.3.1 Prediction by the Flory-Huggins theory

The miscibility of a substance (1) with a polymer (2) can be roughly predicted by

the substance-polymer interaction parameter, χ 12. According to the “Flory-Huggins”

theory, χ 12 is given by (Flory, 1953):

χ 12 = (V1/RT) (δ1 – δ2)

2

(4.1)

where V1 is the molar volume of the smaller specie, the substance in our case, R and T

are respectively the ideal gas constant and the temperature, and δ1 and δ2 are respectively

the solubility parameters of the substance and the polymer. A smaller χ 12 indicates a

higher chance that the system would be miscible, and a value of χ 12 < 0.5 is the Flory-

Huggins criterion for a solvent/polymer system to be completely miscible. Table 4.5

summarizes the χ 12 values of various substances and silicone (i.e. PDMS). First, the χ 12

values for NaB/PDMS system is 17.6, indicating that NaB is likely not miscible with

PDMS, as observed experimentally. For the two solvents used in NaB/PDMS system, it

is clear that water is highly immiscible with PDMS (χ 12 = 8.0) while acetone has some

miscibility with PDMS (χ 12 ~ 0.9), hence the size of sodium benzoate aggregates

increased as the water content (in the mixed solvent of water/acetone) increased, as

observed experimentally.



84

Table 4.5 Physical parameters of relevance importance to the miscibility of NaB/PDMS. V and δ are the molar volume and the solubility parameter of the material,respectively. δ12 is the difference in solubility parameters between the material andPDMS. χ 12 is the interaction parameter between the material and PDMS*.

MaterialV

(cm3/mol)

δ (MPa)

½

∆δ12 (MPa)

½ χ 12 Boilingpoint

(oC)

Water 18.2 47.9 a 33.0 8.000 100

Acetone 74.0 20.3 a 5.4 0.871 56

NaB 100.1 35.8 b 20.9 17.648 -

a, b Values obtained from (Rodriguez, 1989) and (Bustamante et. al., 2000), respectively* For PDMS, the literature value of its solubility parameter (14.9 MPa½, from: Rodriguez, 1989) was used

The interaction parameter calculated by equation 4.1 provides a rough prediction for

the miscibility of the mixture. However, the effect of composition of the mixture is not

explicitly accounted for by this method. In a more general form, the miscibility of the

mixture can be predicted by calculating the free energy of mixing (∆Gmix) for the system

at all compositions. As thermodynamics states, the system becomes more heterogeneous

as ∆Gmix increases, and becomes spontaneously homogeneous when ∆Gmix is less than

zero. NaB/water/acetone/PDMS is a quaternary system. According to the “Flory-

Huggins” theory, the free energy of mixing for a quaternary system is given by ((Flory,

1953) :



85

∆Gmix / nkT = Φ1 ln Φ1 + (Φ2 / R 21) ln Φ2 + (Φ3 / R 31) ln Φ3 + (Φ4 / R 41) ln Φ4

+ Φ1 Φ2 χ 12 + Φ1 Φ3 χ 13 + Φ1 Φ4 χ 14 + (Φ2 / R 21) Φ3 χ 23

+ (Φ2 / R 21) Φ4 χ 24 + (Φ3 / R 31) Φ4 χ 34 (4.2)

where “∆Gmix / nkT” in equation (4.2) is the dimensionless free energy of mixing per unit

segment or per site of the lattice model, k and T are respectively the Boltzman constant

and temperature, n is the total number of sites in the lattice model, Φi is the volume

fraction of component i, and R i1 is the size ratio of molecule i to that of molecule 1, given

by:

R i1 = Vi / V1 (4.3)

where Vi and V1 are the molar volume of component i and component 1, respectively.

Component 1 should be chosen to be the one with the smallest molecular size. For our

system, the following subscripts were used: 1, water; 2, acetone; 3, NaB; and 4, PDMS.

In equation (4.2), the first four terms in the right-hand side represent the configurational

entropy of mixing whereas the last five terms in the right-hand side represent the

enthalpic terms. In equation (4.2), χ ij is the interaction parameter between component i

and j in the system, given by:

χ ij = (Vi/RT) (δi – δ j)2 (4.4)

where δi and δ j are respectively the solubility parameters of component i and j.





87

0.0

0.3

0.6

0.9

1.2

1.5

1.8

0.0 0.2 0.4 0.6

(Φ1 +Φ2)

( ∆ G m i x / n k

T ) F H

50/50 W/A

20/80 W/A

30/70 W/A

40/60 W/A

80/20 W/A

90/10 W/A

100 % W

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6

(Φ1 +Φ2)

( ∆ G m i x /

n k T ) F H

1 wt% NaB/Polymer

5 wt% NaB/Polymer

0.5 wt% NaB/Polymer

3 wt% NaB/Polymer

Figure 4.7 The miscibility trends for NaB/acetone/water/PDMS mixtures, for all possible conditions, as predicted by the original Flory-Huggins (FH) model (equation4.2). (a) Effect of the water/acetone (W/A) ratio; parameters fixed: 1 wt% NaB/polymer.(b) Effect of the NaB matrix loading; parameters fixed: 50/50 water/acetone. All thevalues shown in the legends are based on weight.

(b)

(a)



88

where χ ij, d accounts for the dispersion interactions, χ ij, fv accounts for the interactions

resulted from the free volume effect, and χ ij, sp accounts for the specific interactions such

as acid-base interactions or H-bonding. However, the “Flory-Huggins” theory only

considers the dispersion interactions and totally neglects the other two types of

interactions, which might result in errors in estimating the values of χ ij and consequently

some errors could result in the value of ∆Gmix. For example, neglecting the free volume

effect is based upon the assumption that there is no volume change upon mixing, which is

rarely satisfied for mixing large macromolecules with solvents or additives of much

smaller size. Specific interactions could exist in our system and affect the value of thefree energy of mixing. One specific interaction that could be identified here is the

electrostatic interactions, because our system is in fact containing electrolyte solution

(NaB, which dissociates into salt ions in water). Other specific interactions are the polar

interactions for water/acetone and NaB/water. Another source of error is the assumption

that the interaction parameters are not functions of concentrations. All these factors

together could affect the miscibility prediction. In the current subsection, we are

modifying the Flory-Huggins thermodynamic model (equation 4.2) to include

electrostatic contribution term to the total free energy and concentration-dependent

interaction parameters.

The NaB aqueous solution is an electrolyte solution. Thus, when mixed with

polymers, specific interaction like electrostatic interaction will exist that could change the

phase behavior of the mixture. In this section, we present a simple lattice thermodynamic

model that accounts for this electrostatic interaction. The model was originally



89

developed by Hino et al . (1998) for a ternary (water, salt, and polymer) system. We will

extend the equation to a quaternary (water, salt, polymer, and organic solvent) system,

and then apply the resulting model for our particular NaB/water/acetone/PDMS

quaternary system.

Considered a ternary system consisting of water (1), salt (2), and polymer (3),

Hino et al . (1998) proposed the free energy of the mixture as:

Gmix / nkT = G

FH

/ nkT + G

DH

/ nkT (4.6)

where the (GFH / nkT) term is the Flory-Huggins expression for the free energy of

mixing, given by an expression similar to equation (4.2), and the (GDH / nkT) term is the

electrostatic contribution to the free energy of mixing, given by the Pitzer extension

(Pitzer, 1973) of the Debye-Huckel function. Specifically, (G DH / nkT) is given by

(Hino et al ., 1998):

G DH / nkT = Φw (Mw/1000) [— A (4 I / b) ln (1 + b I1/2)] (4.7)

where Φw and Mw are, respectively, the volume fraction and molecular weight of water,

A and b are constants (A = 0.392, b = 1.2), and I is the ionic strength of the mixture,

expressed as:

I = 0.5 [Φsalt (1000/Mw) / (1 — Φw)] │zM zX│ (4.8)



90

where Φsalt is the volume fraction of the salt, and zM and zX are, respectively, the valences

of the cation and anion. In the above model (equation 4.6-4.8), several assumptions are

employed. The polymer is assumed to be neutral. The ternary system is assumed to be

incompressible. It is also assumed that the salt is completely dissociated into ions.

In the following, we extend the above model (equation 4.6-4.8) to our quaternary

system consisting of water (1), acetone (2), NaB (3), and PDMS (4). The complete

equation is:

∆Gmix / nkT = Φ1 (W/1000) [— A (4 I / b) ln (1 + b I1/2)]

+ Φ1 ln Φ1 + (Φ2 / R 21) ln Φ2 + (Φ3 / R 31) ln Φ3 + (Φ4 / R 41) ln Φ4

+ Φ1 Φ2 χ 12

* + Φ1 Φ3 χ 13 + Φ1 Φ4 χ 14 + (Φ2 / R 21) Φ3 χ 23

+ (Φ2 / R 21) Φ4 χ 24* + (Φ3 / R 31) Φ4 χ 34 (4.9)

Another new feature of the new model (equation 4.9) is the introduction of the

concentration-dependent interaction parameters for water-acetone pair, χ 12*, and acetone-

PDMS pair, χ 24*. These two parameters are obtained from the literature for experimental

data collected specifically for these two particular systems. Therefore, they are expected

to give better values than the values predicted by equation (4.1). The reason why

specifically the above pairs (water/acetone, and acetone/PDMS) are consideredconcentration-dependent is because acetone is highly miscible in water and acetone has

some miscibility in PDMS. On the other hand, the interaction parameter for water-

PDMS system (χ 14) is considered constant and calculated by equation (4.1). This is



91

because water is highly immiscible with PDMS, and for this situation this assumption is

reasonable (Yilmaz and McHugh, 1998). Similarly, the interaction parameters for NaB-

water system (χ 13), NaB-acetone system (χ 23), and NaB-PDMS system (χ 34) are

considered constants and calculated by equation (4.1). Experimentally, we incorporated

NaB in PDMS up to only 5 wt% NaB/PDMS, and therefore Ф3 was always very small in

equation (4.9). Consequently, the terms Φ1 Φ3 χ 13, (Φ2 / R 21) Φ3 χ 23, and (Φ3 / R 31) Φ4 χ 34

are negligible in equation (4.9), and hence the assumption of constant (χ 13 , χ 23 , and χ 34)

will not strongly affect the results of equation (4.9).

For acetone-water system, the experimental correlation for the interaction

parameter is (Yilmaz and McHugh, 1986):

χ 12* = 0.661 + 0.417 / (1 — 0.755 Ф2) (4.10)

and for acetone-PDMS system the experimental correlation is (Singh et. al, 1998):

χ 24* = 16.0 — 34.9 (1 — Ф2) + 20.7 (1— Ф2)

2 (4.11)

It should be noticed that the correlation given here for acetone-PDMS was obtained for a

set of data in the range of 0 — 20 volume % acetone. Fortunately, this is the range of our

experimental conditions for preparing the samples. Therefore, this correlation can be

safely used in our model in this range.



92

To criticize the validity of equation (4.1) for acetone-PDMS system and acetone-

water system, the following can be mentioned. The experimental value for acetone-

PDMS interaction parameter is in the range of 1.3 to 1.9 ((Singh et. al, 1998), indicating

that acetone is not a good solvent for PDMS. To verify this experimentally, we did a

simple experiment of dissolving a small amount of PDMS in acetone (1 wt%

PDMS/acetone mixture), and it had been observed that the mixture was not miscible even

after two days of continuous stirring. However, the prediction of the interaction

parameter by equation (4.1) gives a value of 0.87, which is lower than the measured

value, mainly because of the reason described by equation (4.5). Similarly, theexperimental value for the water-acetone interaction parameter is in the range of 1 to 2

(Yilmaz and McHugh, 1986), whereas it is 5.5 when predicted by equation (4.1). Again,

this predicted value was higher than the measured value, since water and acetone are

highly miscible with each other, and the reason for the disagreement is also as described

by equation (4.5). Therefore, it is always better, whenever possible, to use experimental

values for the interaction parameters.

In summary, it should be noticed that the new model (equation 4.9) as well as the

conventional Flory-Huggins model (equation 4.2) contain no adjustable parameters. Two

of the binary interactions, χ 12* and χ 24

*, are concentration-dependents and obtained form

empirical correlations, and the others are concentration-independent and calculated byequation (4.1). In summary, in equation (4.9), the first term of the right hand side is the

electrostatic contribution term (which is always negative). The remaining terms are

Flory-Huggins terms but with two concentration-dependent interaction parameters (χ 12*



93

and χ 12*). The second to the fifth terms of the right-hand side are the entropic terms

(always negative), and the sixth to the eleventh terms are the enthalpic terms (always

positive). Therefore, compared to equation (4.2), the free energy of mixing could be

negative if the favorable electrostatic term is high enough to overcome the unfavorable

enthalpic terms.

Equation (4.9) is evaluated for our particular system and compared to the results

obtained by the conventional Flory-Huggins model (equation 4.2), and the results are

shown in Figure 4.8. Compared to the trends obtained by equation (4.2), the change inthe trends is observed, but does not change the earlier conclusions that the mixtures are

immiscible because the free energy of mixing is positive at all the conditions. This is

expected because the unfavorable enthalpic interaction between water and PDMS is too

large to be compensated by the favorable electrostatic interaction. The observed

difference between the trends obtained by the new model and the Flory-Huggins model is

due mainly to the effect of introducing the concentration-dependent interaction

parameters, χ 12* and χ 24

* , and not because of the introduction of the electrostatic term.

Nevertheless, the new model (equation 4.9) is useful because it isolates most of the

possible interactions existing in the system and thus gives us practical guidelines to tailor

the system. For example, we could slightly modify the polymer from being strongly

hydrophobic to be partially hydrophobic. In this case, the unfavorable enthalpic terms

will be reduced and the system will be more miscible.



94

0.0

0.3

0.6

0.9

1.2

1.5

1.8

0.0 0.2 0.4 0.6

(Φ1 +Φ2)

∆ G m i x

/ n k T

50/50 W/A

20/80 W/A

30/70 W/A

40/60 W/A

80/20 W/A

90/10 W/A

100 % W

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6

(Φ1 + Φ2)

∆ G

m i x

/ n k T

1 wt% NaB/Polymer

5 wt% NaB/Polymer

0.5 wt% NaB/Polymer

3 wt% NaB/Polymer

Figure 4.8 The miscibility trends for NaB/acetone/water/PDMS mixtures for all possible conditions, as predicted by the new model (equation 4.9). (a) Effect of thewater/acetone (W/A) ratio; parameters fixed: 1 wt% NaB/polymer. (b) Effect of the NaBmatrix loading; parameters fixed: 50/50 water/acetone. All the values shown in thelegends are based on weight

(a)

(b)



95

4.3.3 Comparison between the theoretical miscibility trends and the experimentalmorphology trends

In this subsection, we are making comparison between the experimental

morphology trends (aggregate size of NaB as function of preparation conditions) and the

theoretical miscibility trends (Gmix as function of preparation conditions), in an attempts

to explain the role of thermodynamics on the aggregate size and also to compare between

accuracy of the Flory-Huggins model and the new model. To do this, Gmix calculated

by both models is plotted as function of one preparation parameter, keeping the other

parameters fixed. At the same time, the insert in the same plot shows the experimentallymeasured aggregate size as a function of this preparation parameter. The results are

shown in Figure 4.9. As can be seen from Figure 4.9, both models do capture

qualitatively most of the important effects of the preparation conditions on the aggregate

size of NaB in PDMS. In addition, both models correctly predict that NaB matrix

loading has no strong effect on the aggregate size, as long as NaB is soluble in the mixed

solvent. To compare between the accuracy of the two models, the following can be

mentioned. Experimentally, the aggregate size increased strongly as the solvent/polymer

ratio increased from 20/80 to 30/70, but after the ratio of 30/70 the increase was little

(8.7, 10.4, and 10.5 µm for samples prepared at 30/70, 40/60, and 50/50 solvent/polymer

ratio, respectively). The trend for Gmix calculated by the new model in this

solvent/polymer ratios range is more similar to the experimental morphology trend than

the trend of Gmix calculated by the Flory- Huggins model (see Figure 4.9b). Therefore,

the new model is shown to be more accurate than the Flory-Huggins model.



96

0.1

0.3

0.5

0.7

0.9

0% 20% 40% 60% 80% 100%

water/(water+acetone) wt%

∆ G m i x / n k

T

F-H model

new model

0.1

0.3

0.5

0.7

0.9

0% 10% 20% 30% 40% 50%

solvent/(solvent+polymer) wt%

∆ G m i x / n k T

F-H model

new model

Figure 4.9 The free energy of mixing (Gmix/nkT) for NaB/acetone/water/PDMSmixtures prepared at different conditions, as predicted by two models: the Flory-Huggins

(F-H) model (equation 4.2), and the new model (equation 4.9). (a) Effect of thewater/acetone ratio, parameters fixed: 20/80 solvent/polymer and 1 wt% NaB/polymer.(b) Effect of the solvent/polymer ratio, parameters fixed: 50/50 water/acetone and 1 wt% NaB/polymer. All the values are based on weight. The preparation conditions describedhere correspond to the actual conditions for the morphology experiments performed inthe current study. The insert in each plot represents the corresponding experimentalmorphology trend.

(b)

(a)

0

5

10

15

20

0 25 50 75 100

water/(water+acetone) wt %

d ( u m )

0

5

10

15

0 20 40 60

solvent/(solvent+polymer) wt %

d ( u m )



97

Figure 4.9 The free energy of mixing (Gmix/nkT) for NaB/acetone/water/PDMSmixtures prepared at different conditions, as predicted by two models: the Flory-Huggins(F-H) model (equation 4.2), and the new model (equation 4.9). (c) Effect of NaB matrixloading, parameters fixed: 50/50 water/acetone and 20/80 solvent/polymer. The solvent isdefined here as water + acetone + NaB. All the values are based on weight. The preparation conditions described here correspond to the actual conditions for themorphology experiments performed in the current study. The insert in each plotrepresents the corresponding experimental morphology trend. (continued)

However, both the Flory-Huggins model and the new model could not explain

two experimental observations. First, the aggregate size at the smallest experimental

solvent/polymer ratio used (10/90) was 6.9 µm, which was larger than the corresponding

aggregate size (4.0 µm) at 20/80 solvent/polymer. However, Gmix at the (10/90) ratio is

lower than Gmix at the (20/80) solvent/polymer ratio. Second, experimentally, the

0.1

0.3

0.5

0.7

0.9

0% 1% 2% 3% 4% 5%

wt% NaB/Polymer

∆ G m

i x

/ n k T

F-H model

new model

0

3

6

9

0 2 4 6

wt % NaB/Polymer

d ( u m )

(c)



98

water/acetone ratio in the range of 20/80 to 50/50 water/acetone did not affect

considerably the aggregate size (the aggregate size was in the range 3-4 µm), but it

affected the aggregate size sharply after the ratio of 80/20 water/acetone (the aggregate

size was 15.6 for samples prepared with 90/10 water/acetone ratio). However,

theoretically, Gmix increases almost linearly as the water/acetone ratio increases. The

reasons will be discussed in the next paragraph. Before that, it should be noticed that the

comparison made in Figure 4.9 between the experimental morphology curves and the

theoretical miscibility curves is qualitative because the y-axis is not the same. Our

objective here is to prove that as Gmix for any set of preparation conditions increases,the aggregate size increases, which were true for most of the sets of the preparation

conditions.

The difference between the predictions and the above two observations could be

explained as follow. The values of ∆Gmix shown in Figure 4.9 and predicted by equations

(4.2) and (4.9) are the description for a situation of thermodynamic mixing of an isolated

system. That is, the NaB/water/acetone/PDMS system is placed inside a closed vial that

is perfectly insulated, the mixture is well-mixed, and then the mixing is stopped and

enough time is allowed for the system to equilibrate. ∆Gmix shown in Figure 4.9 is,

hence, of what is described above. Under the situation described above, the total

composition of the entire system is presumably the same, in other words, water and

acetone do not evaporate in this isolated system. However, this was not the case for our

NaB/PDMS morphology experiments. That is, in the process of incorporating NaB into

PDMS by the solvent blending technique and during the process of heating and drying

the system, the composition of the system was continually changing due to evaporation of



99

water and acetone because the system here was open to the environment. This resulted in

the nucleation and growth of new solid phase (solid NaB particles) as the solvent became

more concentrated, until the solvent was completely dried off. This dynamic drying step

would strongly affect the final morphology of the NaB/PDMS composites, but equation

4.2 and equation 4.9 do not consider such a dynamic effect. In order to account for this

dynamic effect, a mass transfer model has to be combined with the thermodynamic

model. The mass transfer model would need to account for the phase change

(vaporization) of acetone and water, and to account for the phase change (solidification)

of NaB, whereas PDMS can be considered as an inert material (i.e. no phase change for PDMS) at all time. As more water vaporizing, the NaB volume fraction will be closer to

its solubility limit in water, and hence start to precipitate out. Once precipitation occurs,

the unfavorable enthalpic terms (in equations 4.2 and 4.9) will increase and the favorable

entropic terms (in equations 4.2 and 4.9) will decrease, both will result in the

enhancement the phase separation of the system. It has been shown that when mixing

solid particles with solvent and polymer, the entropy-driven phase separation is possible

even for athermal (i.e. zero χ ij) system (Schaink and Smith, 1996). Since water and

acetone are not good solvents for PDMS, the system can not form a homogeneous phase.

Instead, a two phase or even a multi-phase system is expected, and the development for a

multi-phase transport phenomena model for this system and combining it with the

thermodynamic model is too complicated and beyond the scope of the current study.

Nevertheless, the rough prediction based on the thermodynamic models presented

here did qualitatively describe most of the features of our system, and still useful as a

preliminary guide for selecting the components of the quaternary system.



100

4.4 Leaching evaluation

The NaB molecules incorporated into the coatings should leach out in order for

them to antifoul. Therefore, the NaB-entrapped silicone coatings were subjected to

leaching studies in static cells. The effects of different preparation conditions on the

leaching behaviors are presented in the following subsections. The parameters varied

were: solvent composition (acetone/water ratio), solvent/polymer ratio, wt% NaB in the

matrix, and type of the matrix. We selected the base case conditions to be: 50/50

water/acetone ratio, 20/80 solvent/polymer ratio, 1 wt% sodium benzoate in the matrix,and Sylgard® 184 as the base-case matrix. Thus, when varying any one parameter, the

other parameters were fixed at the base case conditions.

4.4.1 Effect of composition of the mixed solvent

NaB-incorporated Sylgard® 184 samples prepared at different water/acetone

ratios, whose bulk morphology are shown in Figure 4.3, were subjected to leaching

evaluations, and the results are shown in Figure 4.10. For all these samples, the

solvent/polymer ratio was fixed at 20/80, and the wt. % NaB in the coating was fixed at 1

wt. %. As shown in Figure 4.10, the general trend, regardless water/acetone ratio used,

was observed. Sodium benzoate leached out in two stages, a first fast stage occurred in

the initial few days and followed with a second steady stage having a much slower rate.

This leaching behavior is not unique to the sodium benzoate/Sylgard® 184 coating. In



101

0

100

200

300

400

500

0 5 10 15 20

time (day)

Q

( µ g / c m 2 )

20/80 W/A

50/50 W/A

80/20 W/A

90/10 W/A100% W

Figure 4.10 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed inwater. The samples were prepared at different water/acetone ratios, keeping the other conditions unchanged (20/80 solvent/polymer, and 1 wt% NaB/matrix). All the valuesare based on weight. Solid lines are for showing the trends. (Abbreviations: W: water;A: acetone). Error for each data point (average over 2 batches) is presented by thevertical line.

0

20

40

0 5 10 15 20

time (day)

Q

( µ g / c m

2 )



102

fact, it is a characteristic of hydrophobic monolithic coatings ((Fan and Singh, 1989),

which sodium benzoate/Sylgard® 184 coating belongs to. Back to Figure 4.10, the

cumulative leaching amount increased as the water/acetone ratio increased, and this

increase was very sharp at the water/acetone ratio higher than 80/20. This is directly

related to the size of the aggregates, as to be shown later.

4.4.2 Effect of solvent/polymer ratio

NaB-incorporated Sylgard

®

184 samples prepared at different solvent/polymer ratios, whose bulk morphology are shown in Figure 4.4, were subjected to leaching

evaluations, and the results are shown in Figure 4.11. For all these samples, the

water/acetone ratio was fixed at 50/50, and the wt. % NaB in the coating was fixed at 1

wt. %. As shown in Figure 4.11, at any time interval, the cumulative amount leached out

was highest for samples prepared at 30/70 solvent/polymer ratio, lowest for samples

prepared at 20/80 ratio, and intermediate for samples prepared at 10/90 solvent/polymer

ratio. This is also directly related to the size of the aggregates, as to be shown later. This

also confirmed that the solvent/polymer ratio of 20/80 was the optimum solvent/polymer

ratio that resulted in minimum aggregate size and consequently in minimum leaching.

4.4.3 Effect of NaB matrix loading

To explore the effect of NaB loading in the coating, Sylgard® 184 sheets of

different concentrations (form 0.5 to 5 wt% NaB in Sylgard® 184) were prepared and



103

Figure 4.11 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed inwater. The samples were prepared at different solvent/polymer ratios, keeping the other conditions unchanged (50/50 water/acetone, and 1 wt% NaB/matrix). All the values are based on weight. (Abbreviations: S: solvent; P: polymer). Error for each data point(average over 2 batches) is presented by the vertical line.

0

20

40

60

80

100

0 5 10 15 20

time (day)

Q ( µ g / c m

2 )

10/90 S/P

20/80 S/P

30/70 S/P



104

Figure 4.12 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed inwater. The samples were prepared at different wt% NaB/matrix, keeping the other conditions unchanged (50/50 water/acetone, and 20/80 solvent/polymer). All the valuesare based on weight. Error for each data point (average over 2 batches) is presented bythe vertical line.

0

20

40

60

80

0 6 12 18 24 30

t (day)

Q ( µ g

/ c m 2 )

1 wt%

2 wt%

3 wt%

4 wt%

5 wt%

0.5 wt%



105

subjected to leaching study, and the results are shown in Figure 4.12. For all the above

samples, the water/acetone ratio and the polymer/solvent ratio were fixed at 50/50 and

20/80, respectively. As shown in Figure 4.12, the cumulative amount leached out

increased as wt% of NaB in the matrix increased. However, the increase was quite

gradual rather than to be very sharp. This is also related to the aggregate size, as it is

shown in the previous section that the water/acetone ratio and the solvent/polymer ratio

had stronger effect on the aggregate size than the NaB/polymer ratio, as long as NaB was

soluble in the mixed solvent.

4.4.4 Effect of type of the silicone matrix

To explore this effect, NaB was incorporated into Sylgard® 184 and RTV11

coatings at the same preparation conditions (50/50 water/acetone, 20/80 solvent/polymer,

and 1 wt% NaB in the matrix), and subjected to leaching evaluations, and the results are

shown in Figure 4.13. For sodium benzoate / Sylgard® 184 coatings, slow leaching was

observed compared to RTV11, with the leached percentages to its initial mass of 6.7, 7.3

and 9.8 %, respectively, after 1 week, 1 month, and 4 months being in water. Changing

the coating carrier to RTV11 did considerably affect the leaching. For 1 wt% sodium

benzoate/RTV11 after 1 week, 1 month and 4 months of immersion, it was found that

about 16%, 31% and 88% of the initial mass, respectively, had leached out. As compared

to 1 wt% sodium benzoate / Sylgard® 184 coatings prepared under the same conditions,

the leaching rate (slope from the curve) for sodium benzoate from RTV11 was about 23

times and 3 times higher during the slow and fast leaching periods, respectively.



106

0

50

100

150

200

250

0 6 12 18 24 30

time (day)

Q ( µ g / c m

2 )

Figure 4.13 Cumulative leaching of NaB from its incorporated silicone coating:Sylgard® 184 ( ) or RTV11 (▲). The common solvent used was 50/50 water/acetone byweight. The initial concentration of NaB in both coatings was kept constant at 1 wt%.,

and the solvent/polymer ratios was kept constant at 20/80 by weight for bothcombinations.



107

Therefore, the effect of changing the matrix type on NaB leaching was clearly

observed, where the leaching of NaB from RTV11 was higher than that from Sylgard®

184. This could be attributed to the following reasons. First, partial degradation and

erosion of the RTV11 matrix in water, which were confirmed experimentally by Bullock

et. al. (1999) and Brady (2000), could be contributing and facilitating the leaching.

Wynne et al . (2000) evaluated two types of PDMS coatings (RTV11, and an in-house

unfilled hydrosilation cured PDMS), and they found that the hydrosilation cured PDMS

was very stable in water, whereas RTV11 was not stable in water with a mass loss of

about 0.8 % after 30 days of immersion. This mass loss from RTV11 was attributed tothe leaching of the RTV11 filler (RTV11 has a high content of inorganic fillers including

CaCO3 (32 wt.%)), and was also attributed to the continuous loss of small amounts of

RTV11 constituents, such as Me2SiO, other than CaCO3 (Bullock et. al., 1999). Second,

as mentioned before, RTV11 has a high content of inorganic fillers including CaCO 3 (32

wt. %), which has a pore volume of 0.1 ─ 0.8 cm3/g (Wypych, 1999). At this high filler

content, filler agglomeration, which is a result of incomplete dispersion or flocculation, is

highly possible, which could lead to “voids” in-between the filler particles and at the

filler/polymer interface (Wel and Adaned, 1999). Thus, these voids allow water

molecules to seep into the silicone matrix more easily through these empty spaces and

carry the dissolved antifoulants molecules with them as they leave the coating. Third, the

reported solubility of water in PDMS is 7000 ppm and 700 ppm for filled and unfilled

silicone, respectively (Banerjee et al ., 1997), which implies that the water uptake of

RTV11 is higher than that of Sylgard® 18 matrix. The finding of the current study on the

effect of the matrix type on NaB leaching was supported by the leaching results obtained



108

for incorporating another compound (tannic acid) into Sylgard® 184 and RTV11 coatings

at the same preparation conditions, where the same matrix type effect was observed: the

leaching of tannic acid from RTV11 was higher than that from Sylgard® 184 (the results

for tannic acid are to be discussed in chapter 6). The matrix type effect observed in the

current study was also in agreement with the data reported by Barrios (2004) for the

leaching of zosteric acid form Sylgard® 184 and RTV11 coatings, where he reported that

the leaching of zosteric acid from RTV11 was about 10 times higher than the leaching

from Sylgard® 184 (Barrios (2004)).

4.4.5 Empirical correlations for the leaching rate of NaB from Sylgard® 184

Apart from the specific properties of the coating carrier, the preparation

conditions may also affect the morphological structures and the distribution of the

antifouling compound in the matrix, which can play important roles in leaching when

carriers are immiscible with the antifoulant compounds. To explore this effect in more

details, the preparations conditions were systematically varied for a particular

combination (NaB and Sylgard® 184). For each set of conditions, the bulk morphology

was analyzed before immersing the coating in water (results are shown in section 4.2).

Then, the cumulative leaching in water was measured (results are shown in sections 4.4.1

– 4.4.3). Consequently, the leaching rate was obtained as the slope of the cumulative

leaching curve for each set of preparation conditions. The slope for the cumulative

leaching data in the 0-2 day immersion period is defined as the initial leaching rate. The

slope for the cumulative leaching data in the 14-30 day immersion period is defined as



109

the steady leaching rate. Then, attempt was made to correlate the leaching rate to the

preparation conditions, and the results are shown in Figure 4.14. Two factors of the

preparation conditions were considered here: the size of the NaB aggregate and the NaB

matrix loading. For the first factor, the NaB matrix loading was fixed at 1 wt%

NaB/matrix, whereas the aggregates size was varied because of varying the water/acetone

ratio and the solvent/polymer ratio. For the second factor, the NaB matrix loading was

varied from 0.5 wt% to 5 wt%, whereas the aggregate size was basically fixed at its

minimum narrower distribution by fixing the water/acetone ratio and the solvent/polymer

ratio at their optimum ratios (50/50 water/acetone and 20/80 solvent/polymer).

As shown in Figure 4.14a, the leaching rate sharply increased as the NaB

aggregate size increased and linear correlations were obtained. For the initial leaching

rate, the correlation obtained was: F (µg/cm2-day) = 3.54 t (day) – 6.30, with R 2 value of

0.97. For the steady leaching rate, the correlation obtained was: F (µg/cm2-day) = 0.49 t

(day) – 2.16, with R 2 value of 0.91. On the other side, as shown in Figure 4.14b, the

leaching rate gradually increased as the NaB matrix loading increased, and linear

correlations were also obtained. For the initial leaching rate, the correlation obtained

was: F (µg/cm2-day) = 1.75 t (day) + 5.59, with R 2 value of 0.91. For the steady leaching

rate, the correlation obtained was: F (µg/cm2-day) = 0.02 t (day) + 0.12, with R 2 value of

0.79. In summary, by comparing Figure 4.14a to Figure 4.14b at the same scale for the y-

axis, it could be concluded that the increase of the NaB matrix loading (up to 5 wt %) had

a smooth “gradual” effect on the increase of the leaching rate; whereas the increase of the

NaB aggregate size had a sharp effect on the increase of the leaching rate.



110

0

15

30

45

60

0 5 10 15 20

aggregate size (µm)

L e a c h i n g r a t e ( µ g / c m 2 / d a y )

Figure 4.14 Empirical correlations for the leaching rate of NaB from Sylgard® 184.(a) Effect of the aggregate size (d, the arithmetic mean size), parameter fixed: 1 wt% NaB/matrix. (b) Effect of the NaB matrix loading, parameter fixed: d ~ 3 - 4 µm. Theinsert in (b) is for enlarging the scale of the y-axis. The symbols (■) and (O) representthe initial and the steady leaching rates, respectively.

0

15

30

45

60

0.0 1.0 2.0 3.0 4.0 5.0

wt% NaB in Sylgard®

184

L e a c h i n g r

a t e ( µ g / c m 2 / d a y )

0.0

0.5

1.0

0 1 2 3 4 5

(b)

(a)



111

4.4.6 Effects of continuous stirring and water replacement

All the leaching experiments done so far were performed in static cells. Each

coating sample was immersed in an-unstirred water bath of a constant volume and the

concentration of the water bulk was monitored as a function of time. Two questions arise

here. First, is there a build-up of concentration in the water volume, which may result in

lowering the leaching considerably? Second, since the water bath was not continuously

stirred, is it possible that there is a strong establishment of boundary layer close to the

coating surface, which may also lower the leaching considerably?

To answer the above two questions, other sets of leaching experiments were also

performed at three different conditions. All the samples considered here were the ones

that were prepared at the base case conditions (1 wt% NaB/Sylgard® 184; 50/50

water/acetone; and 20/80 solvent/polymer). Each set of samples consisted of three

coatings. In the first set, the samples were immersed in constant volume baths but with

continuous constant stirring. In the second set, the samples were immersed in un-stirred

water baths but with replacing the water daily. In the third set, which was the control

static cell set done here for comparison; the samples were immersed in un-stirred water

baths and without replacing the water. The cumulative leaching was measured as a

function of time up to 19 days. The results are shown in Figure 4.15. It was observed

that the continuous stirring and replacing the water bath did not considerably affect the

cumulative leaching of NaB from Sylgard® 184. For example, after 19 days of

immersion, the cumulative leaching was 29.3 µg/cm2, 31.6 µg/cm2, and 34.1 µg/cm2 for



112

samples corresponded to static conditions, water replacement conditions, and continuous

stirring conditions, respectively. Based on the mass balance analysis, about 5.1 %, 5.4 %,

and 5.9 % of the NaB original mass had leached out after 19 days of immersion for

samples corresponded to static conditions, water replacement conditions, and continuous

stirring conditions, respectively.

A final note is made here about the experimental method used to evaluate the

leaching of NaB. The amount of NaB leached out into the solution was determined via

conductivity measurements. The conductivity meter used in the leaching experiments isof high precision and accuracy. It gives readings in two digits, and its accuracy is

specified by the manufacturer as ± 0.4%, and it has the ability to detect as small as 0.1

ppm of dissolved matters. For example, for one of the NaB/Sylgard® 184 samples

prepared at the base-case conditions, the concentration of NaB in solution was 0.83 ppm,

6.59 ppm, and 10.01 ppm after 1 hr, 1 day, and 1 week of immersion, respectively. The

change in concentrations here was higher than the range of the error of the equipment.

Therefore, we believe that the increase in conductivity of the solution is because of the

real change of NaB concentration in solution, which is resulted from leaching of NaB

from the coating.



113

0

15

30

45

60

0 5 10 15 20

time (day)

Q ( µ g / c m 2

)

static condition

stirring condition

water replacement condition

Figure 4.15 Cumulative leaching (Q) of NaB from Sylgard® 184 matrix immersed inwater. The samples were prepared at the base case conditions ((1 wt% NaB/Sylgard® 184; 50/50 water/acetone; and 20/80 solvent/polymer)), and the leaching were measuredat three different conditions: under constant stirring (□), replacing water daily (∆) , and atstatic conditions (O). Error for each data point (average over 3 batches) is presented by

the vertical line.



114

4.5 Mass transfer analysis for the leaching study

4.5.1 Simplified mass transfer model

The previous subsection (section 4.4.5) provides two empirical correlations for

the effect of NaB aggregate size and NaB matrix loading on the leaching rates. Here, we

are trying to provide more fundamental mass transfer analysis for the leaching process.

The antifoulant release mechanism can be classified according to the solubility of the

compound in the polymer phase. As shown in Figure 4.16, if it has a high solubility andinitially loaded in excess of its solubility limit, the release primarily follows a diffusion-

dissolution mechanism, which takes place in the continuum of the polymer phase

(Cardarelli, 1980; Fan and Sigh, 1989). On the other hand, as shown in Figure 4.17, if

the solubility is very low, the media of dissolution and diffusion is water that fills the

porosity of the matrix, not the continuum of the polymer phase (Cardarelli, 1980; Fan and

Sigh, 1989). Since NaB is not soluble in silicones (the χ 12 value for sodium

benzoate/PDMS is 17.6), most likely the release is by the porosity formation mechanism.

In this case, the release process can be described mathematically based on the basics of

diffusion in porous media as follow. The concentration, C, of the compound in the water-

filled pores at time t and axial distance x can be described by (see Figure 4.18):

(∂C / ∂t) = DA (∂2 C / ∂x2) (4.12)

where DA is the apparent diffusion coefficient of the compound in the water-filled pores,

given by:



115

Figure 4.16 A simplified sketch (not to scale) for the mass release of antifoulingcompounds from polymer paint (water-insoluble matrix). In this case, the compound issoluble in the matrix and is initially loaded in excess of its solubility limit in the matrix.The dissolved zone means that the compound is already absorbed by the polymer phase.

Water bulk

Boundary layer

Dissolved zone

Un-dissolved zone

Impermeable substrate



116

Figure 4.17 A simplified sketch (not to scale) for the leaching of antifoulingcompounds from polymer paint (water-insoluble matrix). In this case, the compound isinsoluble in the matrix. (Figure re-drawn from Caprari et al . (1990), with slightmodification).

Water bulk

Boundary layer

Leaching holes

Exhausted matrix

Unleached matrix

Impermeable substrate



117

Figure 4.18 A simplified sketch (not to scale) of a polymer coating incorporated withAF compound, and immersed in water. The purpose of the sketch is to show themeaning of the axial distance x that was used in equation 4.12

Impermeableobject

BoundaryLayer

Water bulk

x=Lx=δ x

Polymer coating of thickness L

x=0



118

DA = D ε / θ (4.13)

where D is the molecular diffusion coefficient of the compound in water, and ε and θ are,

respectively, the porosity and tortuosity of the matrix. The porosity here is the initial

porosity of the matrix (generated by the preparation conditions) plus the empty spaces

generated progressively with time when the compound is released out. The tortuosity is

the deviation of the diffusion path from ideality, where for ideal case the diffusion path is

straight and hence θ is equal to unity, whereas for nonideal case θ is greater than one (Fan

and Sigh, 1989; Welty et al., 1984). In other words, the tortuosity is a measure thatdescribes how much the diffusion path is zigzagging. The initial and boundary

conditions associated with equation (4.12) are:

C = CO at t = 0, all x (4.14a)

DA (∂C / ∂x) = k (C — C b), at x = 0 (4.14b)

(∂C / ∂x) = 0, at x = L (4.14c)

where CO is the initial concentration of the compound in the matrix, L is the thickness of

the matrix, C b is the bulk concentration of the compound in the water bath, x=0 is the

coating surface -water bath interface, and k is the external mass transfer coeffieicent

(which is associated with the convective bulk flow). For simplicity, C b can be assumed

zero at all time, and then equation (4.14b) is replaced by:

DA (∂C / ∂x) = k C, at x = 0 (4.15)



119

An approximate analytical solution was obtained for the above PDE by Gurny et

al. (1982) for a special case of applying the boundary condition of:

C = 0 at x=0 (4.16)

and the solution is:

Q = 2 CO (DA t/ π)1/2 (4.17)

Due to its simplicity, equation (4.17) is desired to analyze our experimental leaching data.

However, its applicability for our system has to be tested. Equation (4.17) is originally

derived for a special case called “perfect sink conditions”, which is based on two

assumptions. First, the volume of the water bulk is very big “i.e. infinite volume” and

consequently the bulk concentration is almost zero at all time. Second, the water bulk is

perfectly and continually stirred so that the bulk concentration is not a function of

distance and also there is no boundary layer exists near the coating surface, consequently

the surface concentration is equal to the bulk concentration. Based on these two

assumptions, the boundary condition (C=0 at x=0) is applicable, which will lead to the

simple solution as shown in equation (4.17). To justify the above assumptions for our

experimental setup used to study the leaching of NaB, the following could be mentioned.

First, our system was not an infinite volume, but it was experimentally observed that the

bulk concentration of NaB in the water bath was very low. For example, after 1 month of

immersion of 0.5 wt% NaB/matrix and 5 wt% NaB/matrix sheets, the bulk concentrations



120

were 8 ppm and 23 ppm, respectively. These concentrations are close enough to zero to

justify the first assumption. Second, our system was not continually and perfectly stirred,

but at each time of measurement the water bath was gently stirred to get an average bulk

concentration. This gentle stirring could be enough to satisfy the second assumption.

The difference between perfect stirring and gentle stirring is elaborated more on the next

paragraph.

Equations (4.15) is a general form whereas equation (4.16) is a special form, both

of them are boundary conditions for the same system of equations. Equation (4.15) isapplicable for any degree of stirring whereas equation (4.16) is originally assumed for

perfect stirring. It is of interest to examine mathematically the difference between the

two situations. To do so, the PDE with the general boundary condition (equation 4.15)

has to be solved, and then the situations can be identified where the general boundary

condition (equation 4.15) can be reduced to the special boundary condition (equation

4.16). This procedure is called “model sensitivity analysis or model parametric study”,

and we apply it here for our system as follow. Equations (4.13) to (4.15) are rewritten in

dimensionless forms by defining the following dimensionless variables:

ζ= x / L; Ψ = C / CO; τ = t DA / L2 (4.18)

Hence, the PDE with its boundary conditions in normalized form are:

(∂ Ψ / ∂ τ) = (∂2 Ψ / ∂ ζ 2) (4.19a)

Ψ = 1 at τ = 0, all ζ (4.19b)



121

(∂ Ψ / ∂ ζ) = Bm Ψ, at ζ = 0 (4.19c)

(∂ Ψ / ∂ ζ) = 0, at ζ = 1 (4.19d)

where the dimensionless parameter Bm is the Biot number for mass transfer, and defined

as: (Bm = k L / DA ). Equations (4.19a) to (4.19d) are difficult to solve analytically.

Therefore, we solved it numerically by Orthogonal Collocation technique. Details about

the theory and application of the Orthogonal Collocation technique can be found

elsewhere (Ruthven, 1984; Rice and Do, 1994), and for solving the problem a computer

program in MATLAB was written (the program is shown in the appendix). Thesimulations are performed here for all range of Bm, from a very small value (almost zero)

to a very large value (almost infinity). These two limits have physical meanings. If Bm

goes to infinity (i.e. k L >> DA), it means that the mass transfer process is diffusion-

limited and there is large bulk convection due to high bulk velocity, and hence there will

be no boundary layer at the surface at all because of having perfect mixing or because

that the coated object (e.g. a moving ship) is moving with very high speed, and

consequently the surface concentration is equal to zero [i.e. the problem is reduced to the

special case of equation (4.16)]. If Bm goes to zero ((i.e. k L << DA), it means that the

mass transfer process is bulk convection-limited and the coated object and the water bath

are perfectly stagnant and there is a strong boundary layer established, and in this case the

surface concentration is high. This physical meaning is understood more clearly with

referring to the simulations trends shown in Figure 4.19. In Figure 4.19, the normalized

surface concentration is plotted as a function of the normalized time, for different values

of the normalized parameter Bm. As can be seen in the figure, the surface concentration



122

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 4.19 Parametric sensitivity analysis for the general mass transfer model (equation4.19). The dimensionless surface concentration (Ψ|ζ=0 = C/CO |x=0) is plotted against thedimensionless time (τ = DA t / L2), for different values of the dimensionless parameter Bm (Bm = k L / DA). The trends were generated by solving equation 4.19 numerically.

Bm = 1

Bm = 2

Bm = 10

Bm = 20

Bm = 100

τ

Ψ|ζ=0



123

becomes close to zero when the value of Bm is greater than 100, which implies that the

applicability of equation (4.16) could be valid if Bm is greater than 100.

Bm is a function of k, the external mass transfer coefficient. In standard tests

available in the literature (Court and Vries, 1973), k is determined experimentally by

connecting the coated object to a motor thus the object is rotating with different velocity

(this is to simulate a ship moving with different speeds), and hence k can be calculated by

correlating with the angular velocity. We do not have this kind of setup and therefore we

do not know the exact value of k and Bm for our particular system, but for our experimental setup where we have gentle stirring at each time of leaching measurement,

we assume that this gentle mixing is enough for Bm to be greater than 100 and therefore

the validity of equation (4.17) is assumed. The estimation for the value of Bm for our

particular experimental setup is elaborated more on the next paragraph.

The value of Bm for our particular NaB system is estimated as follow. The external

mass transfer coefficient, k, can be estimated by the correlation (Welty et al., 1984;

Skelland, 1985):

Sh = k l / D = 0.664 (Re)1/2 (Sc)1/3 (4.20)

where Sh, Re, and Sc are the Sherwood number, Reynolds number, and Schmidt number,

respectively, and the latter two are defined as:





125

will be shown that the apparent diffusion coefficient, DA, is in the order of 10-11 cm2/s

(see Table 4.6). Thus, from the relation Bm = k L / DA, with L ~ 1 mm and DA ~ 10-11

cm2/s, a value of about 600 for Bm is obtained. As demonstrated in Figure 4.19, this

value for Bm is high enough for the surface concentration to be close to zero and hence

the validity of equation 4.17 could be justified. This estimated high value for Bm is not

because the bulk velocity is high, but because the parameter (ε/θ) is very small, as to be

discussed more on the next paragraph.

In order to understand the physical meaning of the porosity that we are talkingabout and its relation to the aggregate size and the matrix loading, we need to elaborate

more on the leaching mechanism. During the immersion of NaB-incorporated Sylgard®

184 coatings, there is more than one possible mechanism for the compound to release.

First, in the most ideal case, the aggregates are spherical and uniformly distributed

throughout the matrix, and perfectly packed such that the particles are smoothly touching

each others (Figure 4.20a). In this case, a sharp boundary (Caprari et al . (1990) between

the dissolved zone and the un-dissolved zone of the matrix will be established during

immersion, and the location of this boundary is moving inward into the matrix as

immersion time proceeds. The second possibility is that all the particles are not touching

each other initially (Figure 4.20b). In this case, a sharp boundary between the dissolved

zone and the un-dissolved zone will not be observed. Instead, the following mechanism

is hypothesized. Initially, the particles are separated from each others because they are

encapsulated in thin membranes of binder (the polymer phase). The particles at the

surface layer of the film are first dissolved, forming a saturated solution that diffuses



126

Figure 4.20 Simplified sketch (not to scale) for the possible leaching mechanisms of NaB from Sylgard® 184 coating: (a) Perfect packing of the particles; (b) Completeruptures of the thin membranes; (c) The existence of initial porosity of the matrix, whichis mainly composed of constricted narrowed channels. The first column represents thecoatings initially before immersion, the second column after some time t1 > 0, and thethird column after some time t2 > t1.

(a)

(b)

(c)



127

outward through the diffusion layer in contact with the coating surface. When such a

particle dissolves in water and being removed, a cavity separated by thin membranes

from other cavities and from the un-dissolved particles is resulted. Water diffuses into

the cavity and through the thin membrane to dissolve some of the un-dissolved particles.

Consequently, the resulting osmotic pressure ruptures the membrane and the cavities

become interconnected. This mechanism (the second mechanism) was first postulated by

Marson (1969). The third mechanism is that the matrix has an initial porosity generated

by the preparation condition (Figure 4.20c). Therefore, it could be possible for water to

diffuse through this initial spacing even though the particles are not initially connected,and then the porosity is increasing with time as more particles are dissolved. For the

third mechanism, it is possible that the initial porosity is very small, because it might be

composed mainly of constricted channels of very narrow spacing which spread out

throughout the matrix and connect the particles between each others. Water diffuses

through these channels from the dissolved particles region to the un-dissolved particles

region to dissolve the un-dissolved NaB particles, and then carries the dissolved NaB

molecules back out toward the surface layer of the coating. These constricted channels

exist initially could also be progressively increased during time of immersion. It is

possible that the width of these channels is very narrow (<< 1 µm), which is much

smaller than the diameter of particles, and it is also possible that the length of these

channels is very tortuous (i.e. θ >> 1); therefore the diffusion process slows downconsiderably during the slow leaching stage. In addition, Sylgard® 184 does absorb water

to approximately 0.1 wt% water/polymer (Banerjee et. al., 1997). The water absorbed by

the polymer phase may create molecular level pores which may be responsible for





129

wall interface (Adamson, 1990). If the wall is purely hydrophobic (i.e. purely PDMS), φ

is equal to or greater than 900, which implies that the pressure drop is zero or negative

and hence water flow will not be possible. If there is some NaB exist on the wall, which

is our speculation, φ will be less than 900 and hence the pressure drop will be positive and

consequently water flow will be possible through the capillary.

The above proposed mechanism is based on the assumption that NaB is totally

insoluble in the polymer phase. To check for the validity of this assumption, the

solubility of NaB in PDMS was estimated by the following equation, which wassuggested by Zaikov et al . (1988) for the estimation of electrolytes solubilities in

hydrophobic polymers:

— ln ФS = 1 + (VS / RT) (δS — δP)2 (4.24)

where ФS is the volume fraction of the solute (i.e. NaB in our case), VS is the molar

volume of the solute, and δS and δP are the solubility parameters of the solute and

polymer, respectively. By evaluation in equation (4.24), the solubility of NaB in PDMS

is 8 x 10-7 vol. %, which is close enough to zero to justify the above assumption.

In summary, in all the above possible mechanisms, the key point is that NaB is

insoluble in the polymer phase; therefore NaB diffusion is taking place through pores

(empty space) within the matrix, not through the continuum of the polymer phase. For

this case, as the porosity of the matrix increases it will be easier for water to diffuse into

the matrix and carry with it the dissolved compound, and hence the cumulative release of

the incorporated compound will increase.



130

4.5.2 Limitation of the simplified mass transfer model

The simplified model applied in section 4.5.1 has the advantage that it has explicit

analytical solution for the cumulative leaching as function of time:

Q = 2 CO (DA t/ π)1/2 (4.17)

However, it has several limitations. Before listing the limitations, the simplified model is

first applied to fit our experimental data, followed by the extraction of the model parameters, then more elaborations is made on the physical meaning of these extracted

parameters, as follow.

The experimental cumulative leaching data of the NaB/Sylgard® 184 samples

prepared at different set of conditions were fitted to equation (4.17) by plotting the Q data

against t1/2 at the early leaching time period (0 – 4 days), and the results are shown in

Figure 4.21 and Table 4.6. For the fitting analysis, only the sets of preparation conditions

that resulted in the aggregate size smaller than 15 µm were considered here, because

these were the optimized preparation conditions that resulted in samples of fine and

uniform dispersion. As shown in Figure 4.21 and Table 4.6, in most cases, a good linear

fit was observed, which could suggest that diffusion is the rate limiting step for

NaB/Sylgard® 184 system. From the slope of the linear fit, the apparent diffusion

coefficient (DA) for NaB/Sylgard® system for each set of preparation conditions was

calculated, and the results are summarized in Table 4.6. For all the samples, the apparent



131

0

20

40

60

0.0 0.5 1.0 1.5 2.0

t1/2

(day1/2

)

Q ( µ g / c m 2 )

1 wt%

2 wt%

3 wt%

4 wt%

5 wt%

0.5 wt%

0

10

20

30

40

0.0 0.5 1.0 1.5 2.0

time1/2

(day1/2

)

Q

( µ g / c m 2 )

20/80 W/A

30/70 W/A

40/60 W/A

50/50 W/A

Figure 4.21 Fitting of the cumulative leaching data for NaB/Sylgard®

184 coatings tothe simplified mass transfer model (equation 4.17). (a) Samples were prepared atdifferent NaB matrix loading, keeping the water/acetone ratio and the solvent/polymer ratio fixed at 50/50 and 20/80, respectively. (b) Samples were prepared at differentwater/acetone (W/A) ratios, keeping the NaB matrix loading and the solvent/polymer ratio fixed at 1 wt% NaB/matrix and 20/80 ratio, respectively. Points are experimentaldata and solid lines are linear fitting of the model.

(a)

(b)



132

Table 4.6 Data analysis for the results shown in Figure 4.21. The apparent diffusioncoefficient (DA) for NaB/Sylgard® system was obtained by fitting the experimentalleaching data to equation 4.17. The solvent/polymer ratio was 20/80, which was fixed for all samples. (Abbreviation: W: water, A: acetone).

Sample#

Solvent NaBmatrixloading

Slope of thelinear fitting

(µg/cm2/day1/2)<R 2> value

DA (cm2/sec)

x 1011

1 20/80 W/A

1 wt % 11.71 0.98 1.1

2 30/70 W/A 1 wt % 11.46 0.99 1.1

3 40/60 W/A 1 wt % 12.76 0.98 1.3

4 50/50 W/A 1 wt % 12.57 0.96 1.3

5 50/50 W/A 0.5 wt % 8.02 0.99 2.1

6 50/50 W/A 2 wt % 15.03 0.90 0.5

7 50/50 W/A 3 wt % 18.31 0.85 0.3

8 50/50 W/A 4 wt % 20.16 0.93 0.2

9 50/50 W/A 5 wt % 24.03 0.89 0.2



133

diffusion coefficient was estimated to be in the order of 10 -11 cm2/s. The diffusion

coefficient value is in the same order of magnitude for all these samples because these

samples were shown previously in section 4.2 to have similar fine and uniform size

distribution, with no observed aggregate with size greater than 15 µm. However, this

value for the diffusion coefficient is extremely small, close to the typical values known in

solid-solid diffusion. However, it should be noticed that we define DA as the apparent

diffusion coefficient of NaB in the water-filled pores, not as the molecular diffusion of

NaB in the continuum of the polymer phase, because we believe that NaB has limited

solubility in the polymer phase. Therefore, the value of DA estimated here could bereasonable only if the porosity of the matrix is extremely small, where in this case the

physical meaning is that there is a large geometrical obstruction for NaB to diffuse

through the matrix.

Next, by applying the relation associated with the simplified model (DA = D ε / θ),

with considering DA to be ~ 10-11 cm2/s and D to be ~ 10-5 cm2/s (D is the molecular

diffusion of NaB in water, which is estimated here by an independent theory (equation

4.22) as described in section 4.5.1), this implies that the value of (ε/θ) is in the range of

10-6. This suggests that the system has very small porosity (ε) and very high tortuosity

(θ). However, we should here retain the physical meaning of the tortuosity. The

tortuosity is the ratio of the actual diffusion length to the ideal diffusion length.

Therefore, in most nonideal cases, the tortuosity most likely cannot be more than 10, and

in many examples in the drug-release literature it was assumed a value of 3 to 5.

Although many examples in the drug-release literature have reported a tortuosity value



134

above 1000, these values appear unrealistic and have no physical meanings. Thus, in the

most nonideal case, let’s assume a value of 10 for the tortuosity of our system. This

would predict an extremely low value of the porosity in the order of 10-5. This predicted

value for the porosity is an indication for the limitations of the simplified model, because

if the porosity is almost zero, NaB will not leach out due to the fact that NaB has limited

solubility in the polymer phase and has to find some porous space to get through.

One clear limitation for the simplified model is the incomplete description of the

porosity, and considering it as a time independent. From its physical meaning, the porosity is the empty volume inside the coating over the total volume. The empty

volume is any volume available inside the coating other than the continuum of the

polymer phase itself and the un-dissolved solid NaB particles phase. Therefore, it can be

qualitatively described as:

ε = ε0 + εn + εw

where ε0 is the initial porosity of the coating, εn is the empty spaces generated

progressively with time when the compound is released out, and εw is the porosity due to

water absorption by the polymer (as to be described in the next paragraphs). ε0 is

constant whereas εn and εw are time-dependent. However, the simplified model lumped

all these types together as a single constant value for the porosity, which is a clear

limitation of the model.



135

The initial porosity, ε0, is the porosity generated by the preparation procedure,

which is already exist before immersing the coating in water. For our model system,

NaB/Sylgard® 184, this could originate from various sources, such as the existence of air

bubbles. Another source is because that packing of NaB is not expected to be perfect or

ideal. As a result, in-perfect dispersion is possible, which might lead to tiny “voids” in-

between the NaB particles and at the NaB/polymer interface. The magnitude for this

initial porosity is not expected to be zero; otherwise it would be very hard for NaB to

leach out. Nevertheless, the value of ε0 should still be very small, as an estimation, ε0 for

our model system could be in the order of 0.0001 - 0.01.

The porosity generated by the release of NaB, ε N , is a function of immersion time.

Approximately, it could be expressed as:

ε N = (mt / ρn ) / (m0 / ρ p)

where mt is the mass of NaB released at time t, ρn is the density of NaB, m p is the initial

total mass of the sample (6.0 g for our experiments), and ρ p is the density of Sylgard®

184. For our leaching data for the model system prepared at the base-case conditions, ε N

was varied from 0 initially to 0.0004 after 1 month of immersion.

The porosity due to water absorption by the polymer, εw , may also be important and

contribute to the total porosity. Sylgard® 184 does absorb water to approximately 0.1

wt% water/polymer (Banerjee et. al., 1997). This value could suggest that εw is in the



136

range of 0.001. The water absorbed by the polymer phase may create molecular level

pores (Miller and Peppas, 1982). Also, Miller et al. (1983) have shown experimental

evidences from the porosomitery analysis that very small pores, in the order of 100 Å, are

present in their system (a water-soluble drug incorporated into a hydrophobic, monolithic

coating), which is likely possible for our system. εw is time-dependent because it depends

on the kinetics of water absorption and diffusion in Sylgard® 184. The diffusivity of

water in silicone should play a role here. The diffusivity data of water in silicone

membrane can be obtained from literature. The literature value for the diffusivity of

water in silicone membrane was reported to be 8.6 x 10

-7

cm

2

/s at 25

o

C (Banerjee et. al.,1997). However, the simplified model totally neglected the existence of εw and the role

of water absorption and diffusion in silicone membranes. This is a clear limitation of the

simplified model.

Summing up the components of the total porosity (ε = ε0 + εn + εw ), it is expected

that, based on physical meaning, the magnitude of ε to be in the order of 0.01. However,

the value of ε predicted by the simplified model is in the order of 10-5. This discrepancy

is a clear indication for the limitations of the simplified model.

To summarize, the limitations of the simplified model are listed briefly. The first

major limitation is that it treats the coating system as one homogeneous phase whereas in

fact the coating system is a heterogeneous (multi-phase) system. To account for the

heterogeneity of the system, the simplified model applies the parameter (ε and θ), where ε

and θ are the porosity and tortuosity of the system, respectively. Although the



137

application of these parameters partially accounts for the heterogeneity of the system, two

problems arise here. First, the porosity is, in real situations, a function of time as more

active compound is released from the system, but the simplified model treats the porosity

as constant. Second, the tortuosity becomes at the end of the analysis almost as an

empirical parameter because it cannot be measured independently, and in many examples

in the drug-release literature it is either assumed a certain value or determined by fitting

the release data to the model. Although many examples in the drug-release literature

have reported a tortuosity value above 1000, these values appear unrealistic and have no

physical meanings. The second limitation is the over-simplification of treating the problem as a fixed-boundary problem. However, the problem is a moving boundary

problem because as more pores and channels are generated with time the boundary

condition inside the coating is moving inward and its location is not known in prior. The

third limitation is that it does not explicitly include the resistance to mass transfer in the

boundary layer surrounding the coating. The fourth limitation of the model is that it does

not explicitly account for the role of the diffusivity of water through the coating. In other

words, the diffusion coefficient of water in silicone membrane is absent in the simplified

model. The fifth limitation of the model is that it does not account for the dissolution

step resistance to mass transfer. These limitations can be avoided by re-deriving the mass

balance equations for each phase in the coating (the polymer phase, the solid NaB

particles phase, and the water-filled pores phase) separately, as shown in Appendix B.



138

4.6 Bacterial attachment evaluations

To evaluate the coatings antibacterial behaviors, bacterial attachment studies

using fresh water containing indigenous enriched microbial consortium isolated from

Lake Erie water were performed. Pure silicone coatings and 1 wt% NaB containing

silicone coatings were submerged in the above water at periodic intervals up to one

month. Some representative biofilm morphologies are shown in Figure 4.22 for pure

Sylgard® 184 coatings and 1 wt% NaB-blended Sylgard® 184 coatings, for samples

prepared at the base-case conditions. As shown in Figure 4.22, the bacteria can be easilyidentified and differentiated, and it can be concluded that a clear reduction of bacterial

attachment was achieved for 1 wt% NaB/Sylgard® 184 coatings compared to Sylgard®

184 alone. The bacterial attachment images for NaB/Sylgard® 184 systems were further

quantified by counting the pixels to approximate the area coverage, and hence the %

reduction (% reduction = (1-A/B) 100, where A and B refer to the area coverage for NaB-

containing coatings and NaB-free coatings, respectively). As shown in Figure 4.23, for a

particular period of immersion, an average of 45 – 55% reduction in bacterial coverage

was achieved for 1 wt% NaB/Sylgard® 184 coatings as compared to Sylgard®184 alone.

For NaB-containing RTV11, the morphology was hard to be observed and the

bacteria were difficult to be identified from the pictures directly. To differentiate the

bacteria from other objects, the RTV11 coating surfaces were physically cleaned by

scotch tape, and pictures were taken before and after cleaning. For control RTV11

coatings, the surface became very clean after applying scotch tapes, indicating that the



139

Figures 4.22 Optical microscope images of the bacterial attachment study for NaB-Sylgard® 184 coating. (a, c) 1 wt% sodium benzoate-blended Sylgard® 184. (b, d) controlSylgard® 184. The coatings were immersed in water containing Lake Erie bacteria for 2weeks (a, b) and 4 weeks (c, d). Image size is (285 µm x 215 µm).

a b

c d



140

Figure 4.23: Antibacterial performance of 1 wt% NaB-incorporated Sylgard® 184coatings, compared to control Sylgard® 184 samples. The % reduction was defined as[(1-A/B) 100), where A and B refer to the area coverage of NaB-containing coatings and NaB-free coatings, respectively.

0

25

50

75

100

1 2 3 4

time (week)

% r e d u c

t i o n



141

Figure 4.24 Optical microscopic (reflection bright field) images of bacterial attachmenton controlled RTV11 coatings after the coatings were immersed in water containing Lake

Erie bacteria for 28 days. (a) Half of the coatings surfaces were physically cleaned byscotch tape, and overall pictures [image size: (2850 x 2150) µm] were taken showing thecleaned area (right side of picture a) and the un-cleaned area (left side of picture a).Pictures (b) and (c) are the magnifications [image size: (285 x 215) µm] of the two area

indicated in picture (a).

a

b c



142

removed objects were likely bacterial biofilm (Figure 4.24). For NaB-RTV11 coatings,

however, the morphology did not change much after applying the scotch tape, suggesting

that the irregularities seen were parts of the coatings surface and were not bacterial

biofilm. The irregularities seen in/on NaB-RTV11 coatings were mostly holes likely

generated by NaB leaching, and the reason that they were much larger than the holes seen

in/on NaB-Sylgard® 184 coatings was the much faster leaching of NaB from RTV11 than

from Sylgard® 184.

By comparing control samples of NaB-free RTV11 and NaB free-Sylgard

®

184, itwas observed that RTV11 had a higher tendency for biofilm formation than that of

Sylgard® 184, attributing to the fact that RTV11 has a slightly higher surface energy, bulk

modulus, and surface roughness than those of Sylgard® 184. Another reason for this

observation is that RTV11 experienced an increase in surface roughness upon immersion

in water due to slow surface erosion. Our scanning probe microscopy verified the

increase in the surface roughness value (R q, with a scan size of 80 µm x 80 µm) from 6.9

nm for as prepared RTV11 films to 11.8 nm for 14 day water-aged RTV11 films. For

Sylgard® 184, on the other hand, a previous study (Barrios et al ., 2005) verified that

Sylgard® 184 was fairly stable in water with no observed increase in surface roughness.

To summarize, a clear reduction in bacterial attachment on the NaB-treatedSylgard® 184 coatings was observed, which suggested that NaB could be effective in

inhibiting bacterial attachment when entrapped into Sylgard® 184 coating. Also, the

attachment study demonstrated that the antibacterial performance of NaB/Sylgard® 184



143

coating was better than that of NaB/RTV11 coating. In additions, by referring to the

leaching data presented previously, it was estimated here that the bacterial water solution

had a bulk concentration of NaB of about 1 ppm after 1 month of immersion. This

concentration is much below the EC50 of NaB, which is about 560 ppm towards various

types of bacteria (Haque et al ., 2005, Xu et al, 2005). Therefore, the reduction in

bacterial attachment observed here for NaB treated surfaces was most likely not because

the bacteria were simply died off, but because of a nontoxic mode of action of the

compound. This emphasizes the benefit of NaB to be a nontoxic antifoulant.



144

CHAPTER V

RESULTS AND DISSCUSSION FOR BENZOIC ACID AND

CAPSAICIN– BASED COATINGS

In this chapter, the results obtained for benzoic acid (BA) and capsaicin -

incorporated silicone coatings are presented and discussed. The effect of incorporating

the compounds on the surface and bulk properties of the coatings is presented in section

5.1. The miscibility/bulk morphology of the compounds with the polymer matrix is

discussed in section 5.2. The leaching of the compounds in water is presented in section

5.3.

5.1 Effect of the compounds on coating’s properties

5.1.1 Effect of benzoic acid

Benzoic acid (BA) was incorporated into two types of silicones (Sylgard® 184 and

RTV11), and the concentration prepared was fixed at 1 wt% BA/matrix. For both

combinations, it was observed that the BA-blended coatings were cured similarly as the

BA-free coatings alone. Further examining the effect of the incorporated compound on





146

Table 5.1 Static water contact angles of BA-entrapped Sylgard® 184 coatings

compared to that of the controlled BA-free Sylgard

®

184 coatings. The (solvent: polymer) ratio was (20: 80) by mass.

Coatings Static contact angle

Control Sylgard® 184 106.1 ± 0.6

1 wt% BA/ Sylgard® 184 104.9 ± 1.8

Table 5.2 Static water contact angles of BA-entrapped RTV11 coatings compared tothat of the controlled BA-free RTV11 coatings. The (solvent: polymer) ratio was (20:80) by mass.

Coatings Static contact angle


1 wt% BA/ RTV11 98.7 ± 1.0

TABLE 5.3 Elastic modulus of BA-entrapped Sylgard® 184 films. The (solvent: polymer) ratio was (20: 80) by mass.

Coatings Elastic modulus (MPa)

Control Sylgard® 184 0.95 ± 0.20

1 wt% BA/ Sylgard® 184 0.52 ± 0.02



147

5.1.2 Effect of Capsaicin

Capsaicin was incorporated into RTV11 coatings, up to a concentration of 1 wt%

in the matrix, using toluene as the common solvent. It was observed physically that the

capsaicin-blended RTV11 coatings were cured similarly as the capsaicin-free RTV11

coatings. Further examining the surface wettability, surface roughness, and elastic

modulus of the coatings confirmed this observation, as discussed below. The wettability

of the coatings was evaluated in terms of measuring the water contact angles. The

contact angles for RTV11 films containing various concentrations of capsaicin (0.1 – 1wt %) were measured, and the results are shown in Figure 5.1. RTV11 surfaces without

capsaicin had advancing, static, and receding contact angles of 103°, 100°, and 95°,

respectively. As shown in Figure 5.1, the advancing and static contact angles were

almost unaffected by the addition of capsaicin. The receding contact angles decreased

slightly as capsaicin concentration increased, a value of 87° for 1 wt% capsaicin was

observed. The indifference in the advancing and static contact angles between both

controlled RTV11 and capsaicin-incorporated RTV11 samples suggested that most

capsaicin molecules were entrapped inside the bulk of the polymer matrix rather than

aggregated to the surface. Otherwise, the advancing and the static contact angle are

expected to drop significantly due to the fact that capsaicin has a higher surface energy (~

45 mJ/m2) than PDMS (~ 20-24 mJ/m2). The surface energy of capsaicin reported here

was our predicted value, predicted by the group-contribution method following the

procedures of van Krevelen (1990).





149

The elastic modulus of control RTV11 coatings and 1 wt % capsaicin/RTV11

coatings were measured, and the results are shown in Table 5.4. The elastic modulus for

RTV11 film was measured to be 1.56 MPa, which is consistent with the literature value

reported (Kohl & Bolstes, 2001). As shown in Table 5.4, the indifference in the elastic

modulus for the capsaicin free-RTV11 and capsaicin-blended RTV11 could indicate that

the low content of capsaicin (up to 1 wt %) was insignificant in affecting the curing

behaviors and bulk properties of RTV11, as the elastic modulus is expected to drop

significantly for the uncured coating. Based on this finding and comparing it with the

curing behaviors of the other compounds/matrices combinations that were tried in thecurrent study, and combined also with previous findings for other combinations tried in

our research group [zosteric acid in Sylgard® 184 and RTV11 matrices (Barrios, 2005),

and capsaicin in Sylgard® 184 matrix (Jaggari, 2003)], it is apparent to us now that

RTV11 matrix is much more resistant to poisoning than Sylgard® 184 matrix, because all

the above combinations were cured except capsaicin/ Sylgard® 184. This information is

useful to the coating formulation industry.

Table 5.4 Elastic modulus of capsaicin-entrapped RTV11 films. The (solvent: polymer) ratio was (20: 80) by mass.

Coating Elastic modulus (MPa)


1 wt% Caps/RTV11 1.57 ± 0.11



150

5.2 Miscibility of the compounds in silicones

5.2.1 Miscibility of benzoic acid

For benzoic acid-incorporated Sylgard® 184 coatings, the preparation conditions

were systematically varied to examine their effects on the distribution and morphological

structures of benzoic acid inside the coating matrix. The coatings were prepared using

four different organic solvents (toluene, acetone, acetonitrile, and di-ethyl ether), whereas

keeping the other preparation conditions unchanged (1 wt% benzoic acid in the coating,and 20: 80 solvent: polymer mass ratio). As shown in Figure 5.2, large crystals were

observed inside the polymer matrix for all types of solvents used. Toluene was found to

result in the largest crystals (~ 600 ─ 1000 µm), and di-ethyl ether produced the smallest

ones (50 ─ 100 µm), whereas acetone and acetonitrile resulted in crystals somewhere in

between (~ 200 ─ 500 µm). The estimated size here refers to the average length (the

largest dimension) of the crystal. The number densities of the benzoic acid crystals were

(1 – 2) /mm2 and (7 – 8) /mm2 using toluene and acetone, respectively. For the case of

using toluene as the solvent, it is possible that occasionally, some large crystals could

span the entire thickness of the coating. From Figure 5.2, it is also clear that ether

resulted in a much uniform distribution of smallest crystals (the number density was (90 –

95)/mm2) compared to the other solvents.



151

Figure 5.2 Optical microscope (bright field) images of resulting BA distribution in the bulk of Sylgard® 184 matrix when different solvent were used to mix BA with Sylgard® 184. (a) toluene, (b) acetone, (c) acetonitrile, and (d) ether. The concentration of BA inthe matrix was fixed at 1 wt%, and the (solvent: polymer) ratio was fixed at (20: 80) bymass. The image size is 2850 µm x 2400 µm for (a), and 1140 µm x 960 µm for (b)-(d).

(a) (b)

(c) (d)





153

Table 5.5 Physical parameters of relevance importance to the miscibility of BA/PDMS. V and δ are the molar volume and the solubility parameter of the material,respectively. δ12 is the difference in solubility parameters between the material andPDMS. χ 12 is the interaction parameter between the material and PDMS*.

MaterialV

(cm3/mol)

δ

(MPa)½

∆δ12

(MPa)½ χ 12

Boiling

point

(oC)

Toluene 106.8 18.2 a 3.3 0.469 110.6

Acetone 74.0 20.3 a 5.4 0.871 56

Acetonitrile 52.6 24.6 a 9.7 1.998 81.5

Ether 105 15.1 a 0.2 0.002 34.6

BA 92.5 22.9 b 8.0 2.389 -

a, b Values obtained from (Rodriguez, 1989) and (Bustamante et al ., 2000), respectively* For PDMS, the literature value of its solubility parameter (14.9 MPa½, from: Rodriguez, 1989) was used

and the results are presented in Table 5.5. First, the χ 12 values for benzoic acid/PDMS

mixture is 2.389, indicating that BA is likely not miscible with PDMS, as observed

experimentally. For the four solvents used in benzoic acid/PDMS systems, it is clear that

ether is most miscible with PDMS (χ 12 = 0.002), thus the domains of (ether + benzoic

acid) would likely be the smallest as ether being completely evaporated and resulting in

the smallest benzoic acid crystals as compared to other solvents. Experimentally, the

miscibility of PDMS in the four solvents used was tested in the current study by

dissolving PDMS in the four solvents used. It was observed that both ether and toluene

were very good solvents for PDMS; up to a concentration of 20 wt% PDMS/ether and 20

wt% PDMS/toluene were prepared, and excellent dissolution was observed. This



154

observation is agreed with the χ 12 values predicted for ether and toluene, where both

values are less than the critical value (χ 12 < 0.5) defined by the Flory-Huggins theory for

a polymer solution system to be completely miscible. On the other hand, it was observed

that acetone and acetonitrile were not good solvents for PDMS compared to toluene and

ether. This observation is also agreed with the χ 12 values predicted, which are 0.87 and

2.00 for acetone and acetonitrile, respectively.

5.2.2 Miscibility of capsaicin

Although direct blending of capsaicin with Sylgard® 184 resulted in uncured

coating (Jaggari, 2003), the blend is still useful to roughly examine the miscibility of

capsaicin with RTV11, because Sylgard® 184 is transparent whereas RTV11 is not.

Thus, capsaicin was blended with Sylgard® 184 base by the simple blending technique

using toluene as the common solvent following the same mass ratios and conditions as

we did with RTV11, but without adding the curing agent. The mixtures were kept inside

the air hood for 4 days to dry off the solvent at room temperature, and then optical

microscope imaging was taken. As shown in Figure 5.3, the resulted mixture did show

the formation of large capsaicin crystals (~ 30 — 50 µm), confirming that capsaicin is not

miscible with silicones.

The miscibility of capsaicin with silicones can be predicted by calculating the

interaction parameter, χ 12, which can be calculated by applying equation 5.1. However,

equation 5.1 can not be used directly without knowing the solubility parameter of



155

Figure 5.3 Optical microscope (transmission bright field) image of resulting capsaicindistribution in the bulk of Sylgard® 184 base material. Toluene was used as the commonsolvent. The concentration of capsaicin in the matrix was 1 wt%, and the (solvent: polymer) ratio was (20: 80) by mass. The image size is (570 x 480) µm.

capsaicin, δ2, which is not available in the literature. Alternatively, we predicted the

solubility parameter of capsaicin by the group contribution method, according to the

relation:

δ = (∑ Fi) / (M/ρ) (5.2)

where Fi is the molar attraction constant of group i in capsaicin structure, and M and ρ are

respectively the molecular weight (305.4 g/mol) and density (1.15 g/cm3) of capsaicin.

Two sets of Fis were used (Hoy’s and van Krevelen’s, both found in van Krevelen



156

(1972)), and the solubility parameter of capsaicin was 22.44 MPa1/2 and 25.77 MPa1/2 by

applying Hoy’s and van Krevelen’s data, respectively. Using the average of the two

estimated solubility parameters of capsaicin into equation (6.1), the χ 12 value for

capsaicin/PDMS system was estimated to be 9.092, indicating that capsaicin is

immiscible with PDMS. In Table 5.6, the properties of the solvent used (toluene) are also

summarized with the properties of capsaicin for comparison. Therefore, even when

capsaicin was mixed with silicone using a common miscible solvent (toluene, χ 12 =

0.469), it phased separated from silicone upon removal of the solvent, and resulted in the

formation of large capsaicin aggregates/crystals, as observed experimentally.

Table 5.6 Physical parameters of relevance importance to the miscibility of capsaicin/PDMS. V and

δare the molar volume and the solubility parameter of the

material, respectively. δ12 is the difference in solubility parameters between the materialand PDMS. χ 12 is the interaction parameter between the material and PDMS*.

MaterialV

(cm3/mol)

δ

(MPa)½

∆δ12

(MPa)½ χ 12

Toluene 106.8 18.2 a 3.3 0.469

Capsaicin 265.6 24.1 b 9.2 9.092

a, Values obtained from (Rodriguez, 1989). b` Value predicted in the current study by the group-contribution method (see text).* For PDMS, the literature value of its solubility parameter (14.9 MPa½, from: Rodriguez, 1989) was used.



157


5.3.1 Leaching of benzoic acid

The BA-entrapped silicone coatings were subjected to leaching studies in static

cells. The effects of different preparation conditions on the leaching behaviors are

presented in the following subsections. The parameters varied were: solvent type

(acetone v.s. toluene), and matrix type (RTV11 v.s. Sylgard® 184). For all the above

combinations, the solvent/polymer ratio was fixed at 20/80 by mass, and the wt. % BA inthe matrix was fixed at 1 wt %.

The cumulative leaching of benzoic acid from silicones is shown in Figure 5.4.

Benzoic acid had shown the highest leaching amongst all the antifoulants used in the

current study, regardless of the matrix (Sylgard® 184 or RTV11) or the solvent (acetone

or toluene) used. For benzoic acid/Sylgard® 184 coatings, prepared using acetone as the

solvent, after 1 week and 1 month of immersion, about 73 and 85 % of the initial benzoic

acid content, respectively, had leached out from the coating. When toluene was used as

the solvent or RTV11 as the carrier (acetone as the solvent), benzoic acid completely

depleted from the coating in about one month. The leaching rate (slope from the curve)

for BA from Sylgard® 184 coating were 83 µg/cm2/day and 2.4 µg/cm2/day during the

slow and fast leaching periods, respectively. Similarly, the leaching rate (slope from the

curve) for BA from RTV11 coating were 86 µg/cm2/day and 5.9 µg/cm2/day during the

slow and fast leaching periods, respectively.



158

0

100

200

300

400

500

0 5 10 15 20 25 30

time (day)

C u m u l a t i v e l e a c h i n g , Q

( µ g / c m

2 )

Figure 5.4 Cumulative leaching of BA from its incorporated silicone coating: Sylgard® 184 (open symbols) or RTV11 (filled symbols). The common solvent used for BA/silicones were acetone (squares) and Toluene (circle). The initial concentration of BA

in all coatings was kept constant at 1 wt%., and the solvent/polymer ratios were keptconstant at 20/80 by weight for all combinations.



159

The crystal formation behavior of benzoic acid, as illustrated using various

solvents, could likely be the primary reason for its considerably high leaching. It is

possible in some occasions that some large crystals could span the entire thickness of the

coating, and therefore there will be little mass transfer resistance for BA to leach out.

The crystal formation and consequently high leaching of BA could be the main reason for

the relatively short period of antifouling effectiveness reported in the literature (Railkin,

1995), even though in that study a vinyl-rosin coating was used. Therefore, bacterial

attachment studies were not performed and not recommended in the current study for

BA/silicone coatings, because of three reasons. First, the bulk concentration of the bacterial solution at the initial days of immersion will be most likely much above the

EC50 of BA, which is about 7 ppm towards various types of bacteria (Haque et al ., 2005),

and consequently the coating will effectively inhibit bacterial attachment but by a toxic

mechanism. Second, after one month of immersion, most of the compound will leach out

from the coating and hence the coating will not be effective for reducing the bacterial

attachment for a longer time. Third, a considerable increase in surface roughness will be

expected for BA/silicone coatings during immersion in water because of the high

leaching rate of the compound, a factor that will accelerate bacterial attachment after

longer time.

5.3.2 Leaching of capsaicin

The capsaicin entrapped RTV11 coating (using toluene as the common solvent)

was subjected to leaching studies in static cells, and the results are shown in Figure 5.5.



160

0

50

100

150

200

250

0 5 10 15 20 25 30

immersion time (day)

Q ( µ g / c m 2

)

Figure 5.5 Capsaicin cumulative mass per area (Q, in µg/cm2) released from 1 wt %capsaicin-incorporated RTV11 sheet (total mass and surface area of the sheet wererespectively 6.55 g and 114 cm2), plotted against time of immersion in DI water. Toluenewas used as the common solvent to mix capsaicin with RTV11, and the (solvent: polymer) ratio was (20: 80) by mass.

As shown in Figure 5.5, capsaicin leached out rapidly from RTV11 within the first 7

days, and then slowed down as time proceeded. The cumulative mass leached out after

the first and fourth weeks were about 161 and 198 µg/cm2, respectively. Approximately

35 % of capsaicin original mass had leached after one month of immersion. The leaching

rate (slope from the curve) for capsaicin from RTV11 coating were 75 µg/cm2/day and

1.6 µg/cm2/day during the slow and fast leaching periods, respectively. By extrapolation,

it would take approximately 8 months for capsaicin to leach out completely.



161

Capsaicin was also blended with RTV11 using ethanol as the solvent, and the

corresponding leaching data are shown in Figure 5.6. For this particular system, two

cases were considered: capsaicin was homogenized with RTV11 base polymer before (or

after) mixing with DBT catalyst. The objective here was to investigate the effect of

mixing order during the preparation conditions on the leaching behavior. To accurately

compare the mass flux, the surface area and thickness of the coatings were consistent for

both cases. As shown in Figure 5.6, at any particular time, the cumulative leaching was

higher if capsaicin was mixed afar adding the catalyst to the polymer base. This

experiment did show the importance of mixing the antifoulant/solvent mixture to the polymer base before adding the curing agent in order to get a more homogeneous coating

and consequently a more controllable leaching, although for the case of capsaicin its

leaching was just slowed down slightly. To summarize, the above experiments did also

show that the capsaicin leaching from RTV11 was relatively high, regardless of using

toluene or ethanol as the solvent, or regardless of the mixing order.

The relatively high leaching of capsaicin from RTV11 could be the results of the

following reasons. First, capsaicin is immiscible in silicones as discussed above (χ 12 =

9.09). Second, partial degradation and erosion of the RTV11 matrix in water, which has

been confirmed experimentally (Bullock et. al., 1999), could contribute and facilitate the

leaching. Third, RTV11 has a high content of inorganic fillers (32 wt% CaCO3), which

could lead to presence of “voids” in-between the filler particles and at the filler/polymer

interface and allow water molecules to seep into the silicone matrix through these empty

spaces and carry the dissolved capsaicin molecules with them as they leave the coatings.





163

5.4 Bacterial attachment evaluations for capsaicin-RTV11 coatings

5.4.1 Effect of immersion in water on coating’s properties

Before evaluating the antibacterial performance of the capsaicin - incorporated

RTV11 coating, it is worthwhile to examine the effects of the water type and the

immersion time on the coating properties. This is important in order to confirm that the

difference in bacterial attachment – if exist – is due solely to capsaicin leaching and not

due to a change in the coating properties. This factor was investigated by immersingcontrol samples of RTV11 coatings in different types of water and evaluating the surface

and bulk properties of the coatings, as discussed below.

Control capsaicin-free RTV11 coatings were immersed in two types of water

(sterilized DI water and enriched LE water) for up to 14 days to study the effect of water

type and immersion time on the wettability of RTV11. For coatings in sterilized DI

water, both static and dynamic water contact angles were taken; while for coatings in

enriched LE water, only static contact angles were taken. The results are presented in

Figure 5.7. The static contact angles almost remained constant at a value around 100° for

the 14 day immersion period, irrespective to the type of water used. The advancing

contact angles also remained almost constant at a value around 103°. In general, the

static contact angles resemble the advancing contact angles with slightly lower values

(Adamson, 1990). The receding contact angles, however, showed a gradual decrease,

down to a value of 80° at the end of 14 day period. Consequently, the contact angles



164

hysteresis (difference between the advancing and receding angles) increased from 8°

initially to 23° after 14 days of immersion. The increase in contact angles hysteresis is

possibly due to slow surface erosion, which would result in a slight increase in surface

roughness. Surface erosion could be the result of a continuous small mass loss of fillers

such as CaCO3 from RTV11, and the micro-pit formation on RTV11 surfaces upon

immersion in water (Bullock et. al., 1999). Our scanning probe microscopy also verified

the increase in the surface roughness value (R q, with a scan size of 80 µm x 80 µm) from

6.9 nm for as prepared RTV11 films (Figure 5.8(a)) to 11.8 nm for 14 day water-aged

RTV11 films (Figure 5.8(b)).

In addition, the wettability and surface roughness for water-aged 1 wt%

capsicin/RTV11 coatings were also evaluated. For 1 wt% capsaicin-blended RTV11

coatings immersed in DI water for 14 days, the advancing contact angles increased to a

value of 109°, whereas the receding contact angle decreased to a value of 79°. As a

result, the contact angles hysteresis increased from 16° initially to 30° after 14 days of

immersion, indicating an increase in surface roughness. The increase in surface

roughness for 1 wt % capsaicin-incorporated RTV11 coatings was confirmed by scanning

probe microscopy, where it was observed that the surface roughness increased

considerably from ~ 12 nm for as prepared capsaicin-treated RTV11 films (Figure 5.8(c))

to ~ 88 nm after 14 days of immersion (Figure 5.8(d)). The increase in surface roughnesshere is mainly due to the high leaching of capsaicin, where it is possible that capsaicin

could leave behind irregular surface geometries (mostly holes) when it leached out,

which would cause the surface roughness to increase significantly. By refereeing to the



165

70

80

90

100

110

0 3 6 9 12 15


c o n t a

c t a n g l e s

Figure 5.7 Effect of water immersion time on the wettability of RTV11 films in termsof the static contact angles taken for RTV11 immersed in Lake Erie (Δ) and deionized

water (▲). The advancing (□) and receding () contact angles taken for RTV11immersed in deionized water are also presented. Error for each data point (average over 12 measurements) is presented by the vertical line.





167

leaching data, about 30 % of capsaicin original mass had leached after 14 days of

immersion. This amount is high enough for the surface roughness to increase

considerably.

A 42 day immersion period was used to study the effects of water immersion time

as well as bacteria and dissolved capsaicin on the bulk modulus of RTV11. As-prepared

RTV11 coatings were immersed in three different waters (sterilized DI water, enriched

LE water with bacteria, and sterilized LE-20 ppm water). As shown in figure 5.9, the

bulk modulus slightly decreased from 1.56 MPa to 1.37 MPa after the 42 days of immersion in all three waters, with no observed effect from the water type. This

indicates that bacterial attachment or dissolved capsaicin has minimum contribution to

the variation of the elastic modulus of RTV11. Two factors may contribute to the slight

decrease in bulk modulus. First is a slow leaching of CaCO3 filler from the bulk of the

RTV11 coating. Second reason could be a continuous loss of small amounts of RTV11

constituents other than CaCO3 (Bullock et. al., 1999). Previously, Wynne et. al. (2000)

performed a quantative mass loss experiment for RTV11 coatings immersed in DI water,

where they proved that the mass loss of RTV11 was about 0.8 wt% after 30 days of

immersion. This is in accordance with the general conclusion made by Brady (2000) that

hydrosilylation-cured PDMS – such as Sylgard® 184 - are stable in water whereas

polycondensation – cured PDMS – such as RTV11 - are not stable in water.

In summary, it can be summarized from this subsection that the type of water will

not have effect on the properties of the coatings and consequently will not have effect on



168

0.0

0.6

1.2

1.8

2.4

3.0

0 10 20 30 40 50


E

l a s t i c M o d u l u s ( M P a

Figure 5.9 Effect of water type and immersion time on the elastic modulus of RTV11films immersed in different types of water samples (sterilized Lake Erie water: Δ,

enriched Lake Erie water:

○,and sterilized Lake Erie water with 20 ppm capsaicin:

□).

Error for each data point (average over 6 measurements) is presented by the vertical line.

the antibacterial performance of the coatings. However, immersion in water will cause

the surface roughness for capsaicin-blended coatings to increase considerably. This

considerable increase in surface roughness could enhance bacterial attachment, unless the

amount of capsaicin leached out is high enough to inhibit bacterial attachment, as to be

discussed in the next section.



169

5.4.2 Bacterial attachment evaluations

In order to evaluate the coatings antibacterial behaviors, bacterial attachment

studies using fresh waters containing indigenous enriched microbial consortium isolated

from Lake Erie water were performed. Some representative bacterial attachment images

are presented in Figure 5.10. As shown in the figure, much less bacteria were attached to

capsaicin-blended RTV11 coating as compared to RTV11 coating alone. By defining the

% reduction in bacterial coverage to be (1-A/B) 100, where A and B refer to the area

coverage for capsaicin-blended RTV11 coating and control RTV11 coating, respectively,the % reduction was estimated to be (58 ± 11) %. However, based on the leaching data

shown previously, the concentration of capsaicin in solution for the immersion period

shown in Figure 5.10 was approximately 4-6 ppm. This concentration was very close to

the EC50 of capsaicin [~ 5 to 20 ppm towards various bacteria (Xu et al. 2005)].

Therefore, we are not sure if the reduction of bacterial attachment on the capsaicin-

treated surface shown here is because of the bacteria simply died off or because of the

non-toxic mode of action of capsaicin.

In summary, the clear reduction in bacterial population on the capsaicin-treated

RTV11 coating suggests that capsaicin can effectively inhibit the attachment of bacteria

we tested. However, the antibacterial effectiveness of the capsaicin-treated RTV11

coating is likely short lived due to the relatively high leaching of capsaicin coupled with

the dramatic increases in surface roughness of the coatings when immersed in water.









173

the modulus do not expect to differ two much from the corresponding value of that of

control silicone coatings.

6.2 Miscibility of tannic acid in silicones

To roughly examine TA/silicones miscibility; optical microscopic images were

taken for the bulk of Sylgard® 184 contained the incorporated compound after drying off

the solvent. The solvent used here was acetone, and the solvent/polymer ratio was 20/80

by mass. As shown in Figure 6.2, small aggregates (~ 1 — 3 µm) were distributeduniformly throughout Sylgard® 184 matrix. The resulted aggregate size here was

considerably small, much smaller than the crystal size of benzoic acid and capsaicin, and

comparable to the minimum aggregate size obtained for sodium benzoate in Sylgard®

184. This could be attributed to the following reason. Acetone is a good solvent for

tannic acid. Also, acetone is quickly dried off, and has some miscibility with silicones.

However, as shown in figure 6.2, phase separation was observed clearly for the tannic

acid/ Sylgard® 184 system. This implies that, despite the excellent and fine distribution

of the compound inside the matrix, tannic acid is not soluble in the polymer phase. Also,

Figure 6.2 demonstrates the effect of the compound matrix loading on the aggregate size.

While increasing TA matrix loading from 1 wt% TA/polymer to 4 wt% TA/polymer had

resulted in increasing the number of aggregates, the aggregate size did not change

considerably. This result had some similarity with the effect of NaB matrix loading on

the aggregate size (Figure 4.6 in Chapter 4), and therefore they supported each others.



174

(a) (b)

Figure 6.2 Optical microscope (transmission bright field) image of resulting TA

distribution in the bulk of Sylgard® 184 matrix: (a) 1 wt% TA/Polymer; (b) 4 wt%TA/Polymer. Acetone was used as the common solvent, and the solvent/polymer ratiowas 20/80 by mass. The image size is (570 x 480) µm.

The miscibility of tannic acid with silicones can be predicted by calculating the

interaction parameter, χ 12, which can be calculated according to the same previous

equation discussed before in chapter 4:

χ 12 = (V1/RT) (δ1 – δ2)2 (6.1)

However, equation 6.1 can not be used directly without knowing the solubility parameter

of tannic acid, δ2, which is not available in the literature. Alternatively, the solubility

parameter of tannic acid was predicted here by the group contribution method, according

to the relation:



175

δ = [(∑ Ecoh, i) / (∑ Vi)]1/2 (6.2)

where Ecoh, i and Vi are respectively the molar cohesion energy and molar volume of

group i in the structure of tannic acid. The values used here for Ecoh, i and Vi were

obtained from Fedors’ table (found in van Krevelen (1990)). Consequently, the predicted

value for the solubility parameter of tannic acid was 36.60 MPa1/2. By evaluating the

predicted solubility parameter of tannic acid into equation 7.1, the χ 12 value for tannic

acid/PDMS system was estimated to be 14.06, indicating that tannic acid was not soluble

in the polymer phase, as observed experimentally. In Table 6.1, the properties of thesolvent used (acetone) are also summarized with the properties of tannic acid and PDMS

for comparison.

Table 6.1 Physical parameters of relevance importance to the miscibility of tannicacid/PDMS system. δ is the solubility parameter of the material. δ12 is the difference insolubility parameters between the material and PDMS. χ 12 is the interaction parameter between the material and PDMS.

Material∆

(MPa)½

∆δ12

(MPa)½ χ 12

Tannic acid 36.6 a 21.7 14.06

Acetone 20.3 b 5.4 0.871

PDMS 14.9 b - -

a` Value predicted in the current study by the group-contribution method (see text). b, Values obtained from (Rodriguez, 1989).



176


The tannic acid-entrapped silicone coatings were subjected to leaching studies in

static cells. In this experiment, only one preparation condition parameter was varied,

keeping the other parameters unchanged. The parameter varied was the matrix type

(RTV11 v.s. Sylgard® 184). For all the above combinations, the solvent/polymer ratio

was fixed at 20/80 by mass, and the wt. % TA in the matrix was fixed at 1 wt%, and the

solvent type was fixed (acetone). The cumulative leaching of tannic acid from both

matrices is shown in Figure 6.3. For tannic acid/Sylgard

®

184 coatings, a very slowleaching was observed. About 0.14% and 0.17 % of the initial tannic acid content had

leached out from Sylgard® 184 coating after 1 week and 1 month of immersion,

respectively. Changing the coating carrier to RTV11 did considerably affect the

leaching. For 1 wt% tannic acid/RTV11 after 1 week and 1 month of immersion, it was

found that about 1.3% and 1.6% of the initial mass, respectively, had leached out. For

the two combinations, a general behavior was observed: tannic acid leached out fast in

the initial few days, and continued to leach out but with a much slower rate. The

leaching rates for tannic acid form Sylgard® 184 were 0.15 µg/cm2/day and 0.01

µg/cm2/day at the initial leaching stage and the final leaching stage, respectively. For

tannic acid/RTV11 system, on the other hand, the initial and final leaching rates were 2.0

µg/cm2/day and 0.10 µg/cm2/day, respectively.

Compared to the other antifouling compounds investigated in the current study,

tannic acid had shown to have the slowest leaching behavior from both matrices. At least



177

0

5

10

15

20

0 6 12 18 24 30

time (day)

Q ( µ g / c m

2 )

Figure 6.3 Cumulative leaching of TA from its incorporated silicone coating:Sylgard® 184 () or RTV11 (▲). The common solvent used was acetone for both

combinations. The initial concentration of TA in both coatings was kept constant at 1wt%., and the solvent/polymer ratio was kept constant at 20/80 by weight for bothcombinations. Error for each data point (average over 2 batches) is presented by thevertical line.

two reasons could be attributed here to explain this observation. First, tannic acid had

shown the formation of fine distribution of very small aggregates (~ 1 — 3 µm) inside the

silicone matrix, and we have enough evidence now that the leaching will decrease if the

aggregate size decreases. Second, tannic acid has the largest molecular weight amongst



178

all the compounds investigated in the current study (the molecular weight of tannic acid

is 1700 g/mol, which is about 12 times higher than the molecular weight of sodium

benzoate), and it is well-known that in general the heavier molecule diffuses slower than

the lighter molecule.

The above results clearly showed the effect of the matrix type on the leaching of

tannic acid, where the leaching of tannic acid from RTV11 was higher than the leaching

from Sylgard® 184. This experimental result supports the previous finding for sodium

benzoate/silicones regarding the effect of the matrix type (section 4.4.4), where it wasobserved that the leaching of sodium benzoate from RTV11 coating was higher than the

leaching from Sylgard® 184. Therefore, the reasons for the observed matrix type on the

leaching of tannic acid are similar to what we discussed before in section 4.4.4 and need

not to be repeated.

In summary, it can be evident from the leaching data presented here that leaching of

TA from silicone coatings into water is very slow. This could be an advantage for TA-

incorporated coatings. For example, in the case if TA/silicone coatings are to be

immersed in bacterial water solution for evaluating the antibacterial performance, the

bulk water concentration of TA will be certainly much below the EC50 of TA [~ 118 ppm

(Xu et al., 2005)], and hence bacteria will not die, and hence if a reduction in bacterial

attachment observed most likely it will be by a non-toxic mode of action. However,

bacterial attachment was not performed here for TA/silicone coatings; because this is

beyond the scope of the current study.



179

CHAPTER VII

CONCLUSIONS AND RECOMMENDATIONS

7.1 Conclusions

As stated in the objectives section of this dissertation, the major objective is to

arrive at the connection between the miscibility/distribution of less-toxic antifouling

compounds in a monolithic, hydrophobic polymer coating and their leaching in water. A

minor simultaneous objective is to assess the applicability of applying the solvent-

assisted blending technique as a straightforward incorporation method for the purpose of

controlling the release of four less-toxic antifoulants.

The secondary objective was achieved first. Four significantly less toxic

compounds (sodium benzoate, benzoic acid, capsaicin, and tannic acid), as compared to

tin-based antifoulants, were incorporated into two types of silicone coatings (Sylgard®

184 and RTV11). The incorporation was achieved by the solvent-assisted blending

technique. The feasibility of this technique was assessed by examining experimentally

the morphological structure of the compounds in the matrix and their leaching into

Deionized (DI) water. The solvent-assisted blending technique was found to be useful

for the cases of sodium benzoate and tannic acid, evidenced by the even dispersion of the

small and uniform aggregates of the compounds inside the coating, which had shown the



180

advantage of controlling the release. On the other hand, the solvent-assisted blending

technique was not suitable for the cases of capsaicin and benzoic acid, owing to the

observation that these two compounds had shown the tendency of forming large crystals

inside the coatings, regardless the different solvents used to control the crystal size. The

formation of large crystals was the main reason for the fast leaching of benzoic acid and

capsaicin observed from the coating carriers, thus truncated their usage as antifoulants to

be incorporated into a coating by the solvent-assisted blending technique.

Based on the main findings obtained for the secondary objective, the toxicity of the compounds, and the costs of the compounds, sodium benzoate-incorporated Sylgard®

184 coating was selected in the current study as the model system. Such a model system

was used to determine, based on detailed experimental observations and theoretical

analysis, the miscibility-leaching relationship. Experimentally, sodium benzoate was

found to exhibit slow and controllable leaching by tuning the preparation conditions (the

solvent composition, solvent/polymer ratio, and compound/polymer ratio). A fine and

uniform aggregate size distribution (~ 3 µm) was obtained at a 20/80 solvent/polymer

mass ratio and at a solvent composition of 50/50 water/acetone mass ratio, which had

resulted in the lowest value for the steady leaching rate of about 0.1 µg/cm2/day.

Empirical correlations between the effects of the aggregate size and the matrix loading of

sodium benzoate and its leaching rate were obtained. It was concluded that increasing

the aggregate size had a sharp effect on increasing the leaching rate, whereas increasing

the matrix loading (up to 5 wt. %) had a mild effect on the leaching rate. Moreover, as a

supplementary corollary to the study, 1 wt% sodium benzoate/Sylgard® 184 coatings with



181

fine and uniform aggregate distribution exhibited enhanced antibacterial behaviors as

compared to Sylgard® 184 coatings alone. This suggested that sodium benzoate could be

an environmental friendly alternative to the currently used toxic biocides in antifouling

applications, and highlighted the benefit of applying the solvent assisted blending

technique as the incorporation method.

Theoretical thermodynamic analysis was performed for predicting the miscibility

of sodium benzoate with Sylgard® 184 matrix using acetone/water blends as the solvent.

The quaternary Flory-Huggins model was extended to include the electrostaticcontribution and the concentration-dependent interaction parameters. Comparison was

made between the theoretical miscibility trends and the experimental morphology trends.

The extended Flory-Huggins model was found to be more accurate than the original

Flory-Huggins model, and both did predict that the system was not miscible, as observed

experimentally. Both the original Flory-Huggins model and the extended Flory-Huggins

model also captured qualitatively most of the important effects of the preparation

conditions on the aggregate size of NaB in Sylgard® 184 matrix, and the limitations of

both models to accurately predict all the effects of all the preparation conditions were

likely attributed to the total ignorance of the dynamic drying. Nevertheless, the rough

prediction based on the thermodynamic models did qualitatively describe most of the

features of our system, and still useful as as a preliminary guide for selecting the

components of the coating system.



182

Theoretical mass transfer analysis was performed for the leaching of sodium

benzoate from Sylgard® 184 matrix, in an attempt to elucidate on the leaching

mechanism. Based on the analysis, and combing it with the results from the miscibility

and thermodynamic study, the following mechanism was proposed: “Sodium benzoate is

insoluble in the polymer phase; therefore, the diffusion of the compound is taking place

through pores (empty space) filled with water within the matrix, not through the

continuum of the polymer phase”. For our experimental conditions, where the highest

matrix loading was only 5 wt%, the small particles (~3 µm in size) uniformly distributed

in the matrix may not necessarily be connected to each other. Instead, constrictedchannels of very narrow spacing could spread out throughout the matrix and connect the

particles between each others to allow for water to diffuse through these channels and

dissolve the particles. In this case, the porosity would be very small and the diffusion

path would be very tortuous. This would slow down the leaching process extremely after

a longer period, unless the capillary rise is capable of enhancing the flow of water

through these constricted channels, which is our speculation.

7.2 Recommendations for future work

The current study concluded that sodium benzoate is the most attractive

antifoulant among the four antifoulants investigated in the current study. The followings

(recommendations 1-8) are recommended as a continuation of the current study to





184

divide the agglomerates into small aggregates. In this case the leaching will be

assured to be continuous until most of the compound is leached out, and the

leaching mechanism will be similar to mechanism (a) and mechanism (b)

described in Figure 4.20. The second recommendation is, if we want to keep the

matrix loading below 5 wt%, is that the initial porosity has to be increased.

Techniques exist in the drug delivery literature (e.g. Miller and Peppas, 1983) to

generate initial porosity of the matrix, which proved to be effective for controlling

the release. The initial porosity that we seek for our system may not have to be

high; a value of 0.1 could be satisfactory for the initial porosity.(4) The above recommendations are made for the combination sodium

benzoate/silicone coating, where the leaching process is governed by the property

that sodium benzoate is insoluble in the polymer phase. It is recommended also

to incorporate sodium benzoate into a polymer matrix in which the compound has

some solubility on it. In this case, the release mechanism will be totally different

from what we proposed for the system of the current study, because compound

diffusion through the continuum of the polymer phase will take place and govern

the release. One advantage for applying recommendation (4) is that it is possible

to incorporate low matrix loading (< 5 wt. %) and at the same time to assure that

the compound will leach out continually for a long period of time.

(5) The thermodynamic model applied in the current study is useful for roughly

predicting the miscibility of the system. However, it does not account for the

dynamic effect (solvent vaporization and the solidification of sodium benzoate),



185

which appears to be crucial. Further theoretical work is needed to combine the

dynamic effect with the thermodynamic model.

(6) Mass transfer analysis for the leaching study is also recommended if

recommendations 3 and 4 are to be applied. The existing drug release models

described in chapter 2 are expected to be adequate for this purpose, and the

selection of a model depends on the concentration of sodium benzoate in the

matrix and on the compound/polymer solubility (as described in details in section

2.4.1 and section 2.4.2).

(7) It is also recommended to incorporate sodium benzoate into real marine coatings,which belong to the class “self-polishing polymers”. In this case, the leaching

mechanism will be totally different from silicone coatings, because here both

matrix erosion and compound diffusion contribute to the release of the compound.

Mass transfer analysis for the leaching study is also recommended in parallel.

The drug release models described in section 2.4.1 and section 2.4.2 are not

adequate for this purpose. Instead, the model developed by Kill et al . (2002) for

the analysis of self-polishing antifouling coatings will be useful.

(8) The current study assessed the antifouling performance of sodium

benzoate/silicone coatings by conducting bacterial attachment studies. It is

recommended to asses the antifouling performance of the coatings by using the

common fouling organisms, such as algae, tubeworms and diatoms.

For benzoic acid and capsaicin, the current study proved that the leaching rates of

the compounds were high, due to large crystal formation of the compounds inside the



186

matrix. In order to utilize any of them as an effective antifoulants, alternative

incorporation methods other than the method described in the current study are required

in order to control the release of the compounds. Sundberg et. al . (1997) described two

methods to achieve constant slow release rate for a long period of service life. They

applied these two methods for controlling the release of Sea-nine 211 (a commercial

antifoulant). The first method, the “reservoir membrane” method, was a two-layer

coating: the base layer composed of a highly concentrated Sea-nine 211 homogenouesd

with a plasticizer, and a top layer composed of RTV silicone. The second method was by

microencapsulating the active compound before dispersing it in the polymer coating. Byapplying these methods, they were able to slow down and control the release rate of Sea-

nine 211 considerably. Recently, Xing et. al . (2004) reported an efficient experimental

procedure for producing capsaicin microcapsules. It will be useful to investigate the

possibility of applying these two methods in order to control the release of benzoic acid

and capsaicin.

Finally, a previous work in our research group (Barrios et al ., 2005) proved that

zosteric acid - another nontoxic antifouling compound – incorporated into silicone

coatings was effective in inhibiting bacterial attachment. Zosteric acid has shown some

similarity with sodium benzoate regarding the miscibility and leaching behavior from

silicone coatings, and continues to be attractive nontoxic antifouling compound.

Therefore, the recommendations listed for sodium benzoate (recommendations 1-8) are

also recommended for zosteric acid.



187

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196

APPENDICES



197

APPENDIX A

MATLAB FILE FOR SOLVING THE GENERAL MASS TRANSFER MODEL(EQUATION 4.19)

% program written by Abdulhadi AL-Juhni (2006) % file name: pores.m % this file is to calculate the concentration profiles of the compound

% inside % the coating, considering the boundary condition of non-zero surface % concentration %************************************************************

The m.file: pores.m

% M=20; no. of nodes % ISOTHERMAL % with external film resistance % no dissulotion % this case is when the AF compound has zero solubility in the

% matrix,and % Co << Cs(sol. of A in water) % therfore the mass transfer mechanism is by channelling/pore formation

function yd=pores(t,y); %data******************************* bm = 5; % bm = kl/D; dimensionless no % %************************************************************

% call the m file: matrixab20.m (to get the coefficients of A and B

matrix) matrixab20;

%matrix size [m,n]=size(y); yd=zeros(m,n);

%assign dummy variables; [h] denote the conc. of the AF compound at

the 20 internal nodes;



198

h2=y(1); h3=y(2); h4=y(3); h5=y(4); h6=y(5); h7=y(6); h8=y(7); h9=y(8); h10=y(9); h11=y(10); h12=y(11); h13=y(12); h14=y(13); h15=y(14); h16=y(15); h17=y(16); h18=y(17); h19=y(18); h20=y(19); h21=y(20);

% boundary conditions for mass balance eq of component A (the key

point: derive external nodes as function of internal nodes):

hin = [ h2; h3; h4; h5; h6; h7; h8; h9; h10; h11; h12; h13; h14; h15;

h16; h17; h18; h19; h20; h21]; % internal nodes

a1= AX(1,1) - bm; a2= AX(22,1); b1= AX(1,22); b2= AX(22,22);

phi1= - AX(1,2:21)*hin; phi2= - AX(22,2:21)*hin;

xx = [a1, b1 a2, b2];

zz = [phi1 phi2];

hh = xx\zz; h1 = hh(1);

h22 = hh(2);

h = [ h1; h2; h3; h4; h5; h6; h7; h8; h9; h10; h11; h12; h13; h14; h15;

h16; h17; h18; h19; h20; h21; h22]; % mass fraction at all nodes

%ODE

%conc. of component A in the diffusion length



199

yd(1)= (BX(2,:)*h);

yd(2)= (BX(3,:)*h);

yd(3)= (BX(4,:)*h);

yd(4)= (BX(5,:)*h);

yd(5)= (BX(6,:)*h);

yd(6)= (BX(7,:)*h);

yd(7)= (BX(8,:)*h);

yd(8)= (BX(9,:)*h);

yd(9)= (BX(10,:)*h);

yd(10)=(BX(11,:)*h);

yd(11)= (BX(12,:)*h);

yd(12)= (BX(13,:)*h);

yd(13)= (BX(14,:)*h);

yd(14)= (BX(15,:)*h);

yd(15)= (BX(16,:)*h);

yd(16)= (BX(17,:)*h);

yd(17)= (BX(18,:)*h);

yd(18)= (BX(19,:)*h);

yd(19)= (BX(20,:)*h);

yd(20)= (BX(21,:)*h);

%**********************************************************************

%**********************************************************************

**************************************

The main file to plot surface conc=f(time)

[T,Y] = ode23s('pores',[0 1],[1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1]);



200

matrixab20; m=size(Y);

bm = 5;

hin = Y;

a1= AX(1,1) - bm; a2= AX(22,1); b1= AX(1,22); b2= AX(22,22);

phi1= - AX(1,2:21)*hin'; phi2= - AX(22,2:21)*hin';

xx = [a1, b1 a2, b2];

zz = [phi1

phi2];

hh = xx\zz; cc = hh';

c5 = cc(:,1); t5 = T;

plot (t5,c5)



201

APPENDIX B

SUGGESTION OF A MORE REALISTIC MASS TRANSFER MODEL – APPLICATION OF THE AVERAGE VOLUME THEORY

To overcome the limitations of the simplified model, we propose here a more

realistic model based on the average-volume quantum theory. The major thrust of the

new model is that it treats the coating system as multiphase system by applying the

average volume quantum theory, where the necessary equations are written for each

phase rigorously, thus eliminating the main concern of the old simplified model which

treats the coating system as a “lumped” one phase. Therefore, the new model removes

the uncertainty arises from applying the “lumped” tortuosity factor in the old simplified

model.

The new model also treats the problem as a moving boundary problem. The new

model also explicitly accounts for the resistance to mass transfer from the dissolution step

and from the boundary layer. Also, the new model clearly includes the role of water

diffusion in silicone membranes. In addition, the new model also considers the porosity

to be time-dependent.

The formulation of the new model shown below follows the procedure outlined in

Chase (2002) and Willis et al. (1991), where details about the average-volume quantum



202

theory and its application can be found, and the nomenclature used here are kept the same

as in the mentioned references. In our new model, the coating system is considered to be

heterogeneous and comprising of three distinct phases: the α-phase, the β-phase, and the

γ-phase, where they represent the polymer phase, the particle (i.e. solid NaB) phase, and

the porous phase (i.e. the water-filled pores and channels), respectively. Both the α-phase

and the γ-phase are considered to be multi-components systems whereas the β-phase is a

single component system. The following subscripts are used for the components: 1:

water, 2: NaB, and 3: polymer. Hence, the mass continuity equations for the three phases

are:

∂ (εα ρα) / ∂t + Emα = 0 (A.1a)

∂ (εβ ρβ) / ∂t + Gmβ = 0 (A.1b)

∂ (εγ ργ) / ∂t + Emγ + Gm

γ = 0 (A.1c)

where εi is the volume fraction of phase i, ρi is the total concentration (in mass per

volume of phase i), and Emi is the total mass exchange between phase i and its adjacent

phase. Gmi is the total mass produced (or consumed) in phase i due to phase change. The

physical meaning for Gmi is that Gm

β is the mass lost in the NaB solid particle phase due

to dissolution of the particles, which is simultaneously added to the porous phase as an

aqueous water solution mass, Gmγ. Similarly, the mass absorbed into the polymer phase

comes from the porous phase. Hence, the following relations are applied:

Emα = — Em

γ (A.2a)

Gmγ = — Gm

β (A.2b)



203

The chemical species balances for the α-phase are:

∂ (εα ρ1α) / ∂t + ∂ (εα J1x

α) / ∂x + E1α = 0 (A.3a)

∂ (εα ρ2α) / ∂t + ∂ (εα J2x

α) / ∂x + E2α = 0 (A.3b)

∂ (εα ρ3α) / ∂t + ∂ (εα J3x

α) / ∂x + E3α = 0 (A.3c)

where ρk α is the concentration of component k (in mass per volume of phase α), Jkx

α is the

mass flux – in axial direction - of component k in phase α, and Ek α is the mass exchange

– of component k – between phase α and its adjacent phase. The following relation is

applied:Ei

α = Emα ρi

α (A.4)

For the β-phase, the chemical species balance is not needed because the β-phase is

a single component system (only contains solid NaB). For the γ-phase, the chemical

species balances are:

∂ (εγ ρ1γ) / ∂t + ∂ (εγ J1x

γ) / ∂x + E1γ = 0 (A.5a)

∂ (εγ ρ2γ) / ∂t + ∂ (εγ J2x

γ) / ∂x + E2γ + G2

γ = 0 (A.5b)

where ρk γ is the concentration of component k (in mass per volume of phase γ), Jkx

γ is the

mass flux – in axial direction - of component k in phase γ, Ek γ is the mass exchange – of

component k – between phase γ and its adjacent phase, and G2γ is the mass produced of

component 2 in phase γ due to phase change. The following relations are applied:



204

E1γ = Em

γ ρ1γ (A.6a)

E2γ + G2

γ = (Emγ + Gm

γ) ρ2γ (A.6b)

The constitutive relations are:

J1x α = — D1

α ∂ (ρ1α) / ∂x (A.7a)

J2x α = — D2

α ∂ (ρ2α) / ∂x (A.7b)

J3x α = — D3

α ∂ (ρ3α) / ∂x (A.7c)

J1x γ = — D1

γ ∂ (ρ1γ) / ∂x (A.7d)

J2x

γ

= — D2

γ

∂ (ρ2

γ

) / ∂x (A.7e)Gm

β = — k S (ρβ — ρ2γ) (A.7f)

Emα = k 1 (ρ1

γ — ρ1α) + k 2 (ρ2

γ — ρ2α) (A.7g)

where here the constititve relations for the mass flux (equations A.7a through A.7e) are

obtained by applying Fick’s second law, the constitutive relation for Gmβ (equation A.7f)

is expressed in terms of resistance to mass transfer due to dissolution of NaB particles,

and the constitutive relation for Emα (equation A.7g) is expressed in terms of resistance to

mass transfer due to absorption of the aqueous NaB water solution into the polymer

phase. In the above equations, Dk i is the diffusion coefficient of component k in phase i,

and k S is the dissolution coefficient of NaB in water. k 1 and k 2 are the mass exchange

coefficients of component 1 and 2, respectively, which are related to the mass exchange

between the α-phase and the γ-phase. Hence, as shown in equation (A.7a), the inclusion

of the diffusivity data of water through silicone membranes is explicitly considered in the

new model.



205

The jump mass balance at the moving boundary (i.e. at x=h) is obtained by equating

the physical definition of the mass flux to the definition that is based on Fick’s first law.

Hence, this is expressed as:

— εγ D2γ ∂ (ρ2

γ) / ∂x = (εβ ρβ — εγ ρ2γ) (dh / dt) (A.8)

The following condition is always applied:

εα + εβ + εγ = 1 (4.9)

Equations (A.1 – A.9) describe the mass transport phenomena inside the matrix.

It is possible also to add to this set of equations an equation that describes the resistance

to mass transfer in the boundary layer region. The equation that describes the boundary

layer region, δ, is:

(∂C2δ / ∂t) = D2

water (∂2 C2δ / ∂x2) (A.10)

The above new model explicitly incorporates the diffusivity of water in silicone

membrane (D1α, which appears in equation A.7a). The diffusivity data of water in

silicone membrane can be obtained from literature. The literature value for the

diffusivity of water in silicone membrane was reported to be 8.6 x 10

-7

cm

2

/s at 25

o

C(Banerjee et. al., 1997). This temperature is the same temperature of our leaching

experiments, and therefore this reported diffusivity data can be safely used in the model.



Equations (A.1 – A.10) have to be solved simultaneously. Most likely analytical

solution is challenging for this set of equations. Therefore, numerical solution is

recommended here. Due to time limitation of the current study, it was not possible to

develop the numerical solution code for the above set of equations, and it is

recommended to be accomplished in future work.

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Complete Dissertation for Al-Juhani