Diffusion and Permeability of Aldehydes Into Blends

7/28/2019 Diffusion and Permeability of Aldehydes Into Blends

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POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2005; 16: 318322

Published online 15 February 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.583

Diffusion and permeability of aldehydes into blendsof natural rubber and chemically modified low molecular

weight natural rubber

A. K. Akinlabi1*, F. E. Okieimen2 and A. I. Aigbodion1

1Rubber Technology and Quality Control Unit, Rubber Research Institute of Nigeria, P. M. B. 1049, Benin City, Edo State, Nigeria2Department of Chemistry, University of Benin, Benin City, Nigeria

Received 8 July 2004; Revised 27 September 2004; Accepted 19 November 2004

Low molecular weight natural rubber (LMWNR) obtained from natural rubber (NR) by depolymer-

ization of natural rubber latex (NRL) was modified by epoxidation to 35% epoxide level to yield

epoxidized low molecular weight natural rubber (ELMWNR). The ELMWNR was in turn modified

in a solution of thioglycollic acid (TGA) (0.5 mol phr) to yield a product of 20% conversion of its

initial LMWNR epoxide. Blends of NR with LMWNR, ELMWNR and epoxidized low molecular

weight natural rubber thioglycollic acid (ELWMNR-TGA) adduct were made. The mixes were vul-

canized at 1508C for 20 min before determining the system parameters (n and k), the sorption (S),

diffusion (D) and permeability (P) of the vulcanizates in acetaldehyde and formaldehyde solutions

at three different temperatures (25, 40 and 608C) for a period of 90 days. The sorption, diffusion and

permeability of the vulcanizates were found to be temperature dependent. The vulcanizates con-

taining ELMWNR were found not to be easily penetrated by both acetaldehyde and formaldehyde

when compared with base mix A that is vulcanizates with only NR. The reaction system was found

not to be spontaneous but dependent on the activation energies. Copyright # 2005 John Wiley &

Sons, Ltd.

KEYWORDS: rubber; epoxidation; thioglycollic acid modification; diffusion; vulcanization

INTRODUCTION

Low molecular weight natural rubber (LMWNR) constitutes

a new family of polymers chemically derived from natural

rubber (NR) by depolymerization of natural rubber latex

(NRL) with either nitrobenzene or phenyl hydrazine.15

The LMWNR has been found to have similar properties

with NR, e.g. affinity for ingredients during compounding,

chemical behavior and service life performance.1,6

NR being an unsaturated hydrocarbon has been docu-

mented to undergo several reactions due to the level of its

unsaturation (presence of double bonds) in which epoxida-

tion, is one of those reactions.7 Epoxidation, is the reaction ofthe double bond with an active oxygen atom to yield a three-

membered ring structure containing oxygen. Epoxide mod-

ification of macromolecules has been documented as chemi-

cally suitable for the development of various novel

applications7 9 that can combine processing advantageswith

improved solvent resistance and better service life.

Information about sorption, diffusion and permeability of

organic liquids into crosslinked rubber network systems

have been studied by some authors.1013 Siddaramaiah

et al.,14 found that the rate of solvent transport within a

polymer material dependedupon the nature of the functional

group in the solvents and their interaction with the polymer

chain segments. To the best of our knowledge, there is still

scanty information on permeability of organic liquids into

blends of NR and modified epoxidized low molecular weight

natural rubber (ELMWNR). Hence, this paper reports on the

sorption, diffusion and permeability of formaldehyde and

acetaldehyde through blends of NR withdifferentchemically

modified ELMWNRs. It is believed that findings from this

study will serve as a new set of data in the permeability

studies and even provide more information on some limita-tions of NR in the aspect of poor resistance to hydrocarbon

solvents, oils and high permeability to gases.

EXPERIMENTAL

MaterialNRL was obtained from the NIG 805 clone in Rubber

ResearchInstituteof Nigeria and preserved with0.3% ammo-

nia(5% v/v).The latex hasa total solid content (TSC) of 41.0%

and a dry rubber content (DRC) of 38.5%. The reagents used

in the preparation and characterizations of LMWNR,

ELMWNR and epoxidized low molecular weight natural

rubber thioglycollic acid (ELMWNR-TGA) adduct were ofanalytical grades while the rubber compounding chemicals

were of the commercial grades.

Copyright# 2005 John Wiley & Sons, Ltd.

*Correspondence to: A. K. Akinlabi, Rubber Technology andQuality Control Unit, Rubber Research Institute of Nigeria,P. M. B. 1049, Benin City, Edo State, NigeriaE-mail: [email protected]


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Methods

Production of LMWNRLMWNR was obtained by depolymerization of NRL using

different concentrations of nitrobenzene according to the

method described elsewhere.2 The extent of depolymeriza-

tion was determined by viscosity measurements using an

Ubbelhode viscometer and size exclusion chromatography

(SEC).

Epoxidation of the LMWNREpoxidation of the LMWNR was carried out at temperature

of about 58C by performic acid generated in situ by the reac-

tion of formic acid and hydrogen peroxide.7,15 During the

epoxidation reaction,the acid wasslowly added to thestirred

LMWNR solution (10 g of LMWNR in 100ml of chloroform)

for 30 min. The peroxide (135 molper 100 isoprene units) was

introduced drop wise over a period of 30 min. The reaction

was allowed to proceed for various periods of time (ranging

from 1.53 hr). The reaction mixture was poured into a large

excess of water and the coagulated ELMWNR obtained wassteeped in dilute 0.1 M Na2CO3 to neutralize the excess acid.

Themodified LMWNR was later washedwith distilledwater

and dried in an air circulated oven at 558C for 4 hr. The extent

of epoxidation was determined by titration.

Reaction of the ELMWNR with thioglycollic acid16,17

Thioglycollic acid (TGA) (0.12 moll1) of solution was added

to the freshly prepared ELWMNR (15.7 mol%) solution at

298C over 1 hr with occasional stirring. The reaction was

allowed to continue for a further 17hr. At the end of this per-

iod, the modified material (ELWMNR-TGA adduct) was coa-

gulated and dried in air for a period of 24 hr. The unreacted

epoxy groups were estimated by taking the difference

between the percent of epoxide level of the ELMWNR before

and after the reaction with the modifying agent.

Compounding of the mixesFour different mixes A D were prepared with NR, LMWNR

(18500Mw), ELMWNR (35% epoxide content) and

ELMWNR-TGA adduct (20% TGA content) as shown in

Table 1. All the ingredients were mixed with a laboratory

two-roll mill using the recipe given in Table 1. The sulphur

was added after cooling the rubber to prevent scorching.

The mixes were vulcanized at 1508C for 20min using flatmolds of 1.5mm thickness.

Sorption coefficientThe sorption coefficients of the samples were studied using

the gravimetric method1418 at25, 40and 608C. The accurately

weighed (0.1 mg) samples were immersed in airtight glass

bottles containing the respective solvents maintained at the

desired temperature. At an interval of 3 4 hr, the samples

were removed, wiped dry with filter paper and weighed.

The sorption behavior as graphically shown in Fig. 1 was

obtained by plotting the graph of mass uptake (Mt), versus

squarerootof time (t). The maximummass uptake attainable

gave the sorption coefficient (S). The rate constant, k for the

sorption was determined by plotting the graph of log (Mt/

M1

) versuslog t as shown in Fig. 2 usingthis relationship.14,18

logMt=M1 log k n log t 1

where Mt and M1 are the mass uptake values at time t and

at equilibrium respectively, t is the reaction time, n is the

slope of the linear portion of the sorption plot and k is the

intercept.

DiffusionThe diffusion coefficients of the aldehydes were calculated

from the sorption results by using the formula:14,18

D hn=4M12 2

Table 1. Recipe for the preparation of mixes of blends of NRand modified LMWNR

Compound component (phr)

Sample

A B C D

NR 100 50 50 50LMWNR (18 500 Mw) 50 35% ELMWNR 50 20% ELMWNR-TGA adduct 50Zinc oxide (ZnO) 6.0 6.0 6.0 6.0Carbon black (HAF) 40 40 40 40Sulphur 2.5 2.5 2.5 2.5Stearic acid 2.0 2.0 2.0 2.0

Mercaptobenzothiazole (MBT) 1.0 1.0 1.0 1.0Flectol H 2.0 2.0 2.0 2.0

Flectol H-: polymerized 1,2-dihydro-2,2,4-trimethyl quinolene.

Figure 1. A typical graph of mass increase per 100 g of the

rubber material (Mt, %) versus the square root of time for the

sorption of acetaldehyde by rubber mixes A, B, C and D at

608C.

Figure 2. A typical plot of log (Mt/M1) versus log t for the

sorption of acetaldehyde by rubber mixes A, B, C and D at

608C.

Copyright# 2005 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2005; 16: 318322

Blends of natural rubber 319


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where D is the diffusion coefficient, p is 3.142 and h is the

thickness of the samples.

PermeabilityPermeability coefficient was calculated by using the relation-

ship:14,18

P DS 3

where D and S are diffusion and sorption coefficients

respectively.

Activation energiesThe activation energies (Ea) of the system was calculated by

adopting the Arhennius equation:

log D log Do Ea log RT 4

where Ea is obtained from the slope of the plot of log D

against T, R is the gas constant and T is the absolute tem-

perature in Kelvin.

RESULTS AND DISCUSSION

SorptionThe behavior of the vulcanizates in acetaldehyde at 608C is

graphically represented in Fig.1. An initial linear progression

was observed at the onset of the experiment but before the

systemattains equilibrium there wasa deviationfrom thelin-

earity, giving the graph a sigmodial shape. The linear part of

the graph was due to the fast rate at which the solvent pene-

trates the vulcanizates, because initial swelling concentration

at the onset of the experiment was low but as the experiment

progresses, the penetration power of the solvent reduces as aresult of the relief that was experienced by thesolvent uptake

power, which continues until an equilibrium is reached. This

equilibrium state is the optimum points on the graph, which

corresponds to thesorptionvalue, that is, themaximum mass

uptake for the system to attain equilibrium. The sorption

results in Tables 2 and 3 show that the vulcanizates swell

more in formaldehyde solution than in acetaldehyde, since

the later is structurally lighter than acetaldehyde and thus

penetrates the vulcanizates more. Previous studies19 had

shown that lighter solvents gather compressive lateral force

and exert the force on the surface of the material to be pene-

trated, thereby causing surface wrinkles of the material due

to instability, which subsequently allows the penetration of

the material. The extent of this penetration varies with differ-

ent solvents. However, as the liquid penetrates further, thelateral resistance of the material weakens and the surface of

the rubber will become stretched, then the concentration of

the solvent in the surface layer will continue to increase until

an equilibrium is reached. The time taken to attain this equi-

librium was found to be influenced by the composition of the

blends, that is, higher molecular mass materials will attain

equilibrium faster. For example, mixes C and D have higher

molecular mass than mixes A or B because of the presence of

the epoxidesin their molecular structure. The sorptionresults

of these mixes as shown inTables 2 and 3 confirmedthatthey

attained equilibrium faster than mixes A or B.

Theresults of therate constant (k)inTables2and3showan

increasing trend as temperature increases. It was observedthat k valuesat258C arealways theleastwhencompared with

k valuesat 608C, which arealways thehighest. The k values of

the four mixes (AD) in the two solvents show mix A (NR)

havingthe least values, while mixD has thehighest. Thehigh

k value observed for mix D, could have been possibly due to

the additional molecular mass with an increase in crosslink

density due to the presence of TGA on the rubber, which

would have required extra force from thesolvent to penetrate

the vulcanizate and thereby resulting in high k value for mix

D. The k values for mix C were slightly lower than the values

for mix D but higher than the k values for mix B. It was

observed that the sorption increases with a rise in tempera-ture, which follows the conventional theory that an increase

in temperature leads to an increase in free volume due to an

increase in movement of the chain segments of theelastomer.

Diffusion coefficientThe diffusion coefficient (D) results in Tables 2 and 3 show

an increase with temperature rise. The diffusion results of

Table 2. Transport of formaldehyde through vulcanized NR/chemically modified LMWNR blends: values of sorption coefficient

(S), sorption rate (k), diffusion coefficient (D), diffusion mechanism (n) and permeation coefficient (P) at various temperatures

Vulcanizates

Temperature

(8C)

S102

(g/g) k (min1

)

D 105

(mm2

min1

) n

P 101

(mm2

min1

)

NR25 90 50 1.19 0.70 1.07140 94 50 0.86 0.62 0.80860 94 52 0.83 0.61 0.780

NR/LMWNR25 64 60 3.38 0.84 2.16340 69 65 1.25 0.55 0.86360 72 65 1.27 0.58 0.915

NR/ELMWNR25 51 63 3.00 0.63 1.53040 53 66 2.78 0.63 1.47360 53 65 2.78 0.63 1.473

NR/ELMWNR-TGA

25 62 66 3.61 0.84 2.23840 64 69 0.89 0.43 0.57060 64 65 0.89 0.43 0.570

320 A. K. Akinlabi, F. E. Okieimen and A. I. Aigbodion



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all the mixes, at 258C gave the least value when compared

with the values observed for similar mixes at 40 and 608C.

Generally mix C was more resistant in the two solvents by

having the lowest D values. This was followed by mix D,

then mix B and finally mix A. The observed resistances of

mixes C and D might have been enhanced by the epoxides.

Epoxidation has been documented as having an increasing

influence on the glass transition temperature (Tg) of rub-

bers20,21 due to the ease of movement of the rubber seg-

ments, which determines the rate of progress of the

diffusing molecules. In addition, concentration depen-

dence and diffusion coefficient of liquids in different rub-bers have been documented to be affected by the Tgvaluesof rubber, that is,when Tg is low, there is lowconcen-

tration dependence and when Tg ishigh,thereisstrongcon-

centration dependence.19 It was also observed that the

diffusion values in Table 2 were higher than the values in

Table 3, which still support the previous statement made

that formaldehyde penetrates more than the acetaldehyde.

The diffusion mechanism of the system, which was related

to the calculated slope of the sorption plot (n) is as shown

in Tables 2 and 3. The n values obtained do not show any

variation with an increase in temperature. The n values

forformaldehyde variedfrom 0.43 to 0.84 while in acetalde-

hyde, n varies from 0.32 to 0.79. The independence of n

on temperature was earlier reported by Siddaramaiah

et al.14

Permeability coefficientIt was noticed that increasing the temperature of the system

from25to608C leads to an increase in thepermeability values

of the system as shown in Tables 2 and 3. This effect follows

the convention that at higher temperatures an increase in the

free volume occurs thereby increasing the rate of permeabil-

ity of the solvents. However, the extent of permeability

would have resulted from the nature of the penetrant mole-

cule and the structural characteristics of the blends, whichaccounted for the various values observed for the mixes

under different solvents.

Energies of activationThe data on diffusion coefficient (D) was treated with the

Arhennius type of expression22 to obtain the Ea values of

thesystem. Theobtainedfrom theslopes of thecurves forfor-

maldehydeare given in Table 4 while the results for acetalde-

hydes are given in Table 5. Comparing the results in Tables 4

and 5, it can be observed that Ea values depend on the nature

of thesolvents. This is so because theresults obtained foracet-

aldehydes were generally higher than the results for formal-

dehydes. This observation is in line with previous

reports,19,22 where the dependence of Ea values on solvent

molecular masses were documented. Hence the observedlower Ea values of vulcanizates in formaldehyde when com-

pared with values of vulcanizates in acetaldehyde shown in

Table 5.

In determining the enthalpies (DH) and entropies (DS) of

the system, the equilibrium adsorption constant (Ks) was

treated with Vant Hoff expression:22

log Ks S=2:303R H=2:303RT 5

where Ksmaximum sorption quotient/mass of polymer.

The intercept and slope of the linear plots of log Ks against

1/T gave values for the entropy DS and enthalpy, DH. The

DS and DH values obtained are given in Tables 4 and 5.

The DH and DS values were observed to be solvent

dependent, which suggests that enthalpies decrease with

increase in molecular mass of solvent. The positive values of

enthalpies show that the reaction is endothermic. The

Table 3. Transport of acetaldehyde through vulcanized NR/chemically modified LMWNR blends: values of sorption coefficient

(S), sorption rate (k), diffusion coefficient (D), diffusion mechanism (n) and permeation coefficient (P) at various temperatures

VulcanizatesTemperature

(8C)S 102

(g/g) k (min1)D105

(mm2 min1) nP101

(mm2 min1)

NR25 89 50 1.02 0.64 0.90840 92 50 0.95 0.64 0.874

60 92 50 0.88 0.61 0.801NR/LMWNR

25 62 62 1.08 0.46 0.67040 68 62 0.65 0.39 0.44260 70 63 0.74 0.43 0.518

NR/ELMWNR25 48 54 6.30 0.86 3.02440 51 59 0.51 0.26 0.26060 52 59 3.98 0.74 2.070

NR/ELMWNR-TGA25 59 59 3.52 0.79 2.07740 62 68 0.52 0.32 0.32260 62 69 0.53 0.32 0.329

Table 4. Activation energies (Ea), enthalpies (DH),

entropies (DS) and free energies (DG) of absorption of the

rubber mixes in formaldehyde

Rubber mixesEa

(J mol1)DH

(J mol1)DS

(J mol1)DG

(J mol1)

A 100.0 66.7 0.025 74.2

B 66.7 166.7 0.125 204.2C 366.7 33.3 0.280 117.3D 200.0 66.7 0.180 120.7

Blends of natural rubber 321



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negative value of entropy is in agreementwith thetheory that

sorbed solvent molecules remainin theliquid state20 through

out the reaction.

The free energychange (DG) ofthe system was obtained by

adopting Gibbss thermodynamics expression:22

G H TS 6

where DG is the Gibbss free energy. The values of DG

obtained are as shown in Tables 4 and 5. The DG values

were observed to be positive in all cases.

CONCLUSION

This study has shown epoxidation as a chemical method that

can be used to improve the solvent resistance of materials.

Some inherent limitations of NR like: poor resistance to sol-

vents and high permeability to gases were improved by

blending NR with modified ELMWNR. Above all, transport

characteristics of the systems were influenced by; the nature

and molar mass of the solvent; nature of the material; and the

temperature of the system. An increase in the temperature

leads to faster molecular mobility and higher permeability.

AcknowledgmentsThe authors are grateful to the authorities of the French

Embassy in Nigeria for sponsoring part of this work.

Appreciation also goes to the Centre de cooperation interna-

tionale en recherche agronomique pour le developpement

(CIRAD)-CP, Montpellier, France and the Rubber Research

Institute of Nigeria, Benin City, Nigeria, for access to their

laboratory facilities.

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1070.3. Pautrat R. Caoutchoucs & Plastiques 1980; 38: 91.4. Brosse JC. Production of Liquid Natural Rubber. Faculte des

Science, University du Maine: Le Mans, 1989.5. Azeddine EH. Degradation du caoutchouc par oxydation

controle. In Material Science. AlUniversite du Maine: LeMans, 1991.

6. Meker T. Rubber Technol. Int. 1999; 45: 57.7. Okieimen FE, Akinlabi AK, Aigbodion AK, Oladoja NA,

Bakare IO. Nigerian J. Polym. Sci. Technol. 2003; 3(1): 223.8. Darey JE, Loadman MJR. Polym. J. 1984; 16: 134.9. Aigbodion AI, BakareIO, Okieimen FE,AkinlabiAK.Indian

J. Chem. Technol. 2001; 8: 1.10. AlfreyT, Gurnce EF, Lloyd WJ.J. Polym. Sci. 1996; C-12: 249.11. Aithal US, Aminabhari TM. J. Chem. Ed. 1990; 67: 82.12. Aithal US, Aminabhavi TM, Cassidy PE. Polym. Prepr. J. Am.

Chem. Soc. 1989; 307(1): 17.13. Koros WJ, Hellums MW. Encyclopedia of Polymer Science and

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2000; 77: 1413.16. Orszulik ST. Tetrahedron Lett. 1986; 27(32): 3781.17. Okwu UN, Okieimen FE. Eur. Polym. J. 2001; 37: 2253.18. Ouddane M, Rancourt Y. J. Appl. Polym. Sci. 2001; 79: 1178.19. Muniandy K, Southern E, Thomas AG. Diffusion of liquids

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Table 5. Activation energies (Ea), enthalpies (DH),

entropies (DS) and free energies (DG) of absorption of the

rubber mixes in acetaldehyde

Rubber mixesEa

(J mol1)DH

(J mol1)DS

(J mol1)DG

(J mol1)

A 166.7 66.7 0.025 74.2B 133.3 200.0 0.130 239.0

C 400.0 133.0 0.265 212.5D 233.3 66.7 0.205 128.2

322 A. K. Akinlabi, F. E. Okieimen and A. I. Aigbodion


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Diffusion and Permeability of Aldehydes Into Blends