International Journal of Adhesion & Adhesives 24 (2004) 43–54

Modification of cellulosic fibres with functionalised silanes:development of surface properties

M. Abdelmouleha, S. Boufia, M.N. Belgacemb,*, A.P. Duartec, A. Ben Salaha, A. Gandinib

a LMSE, Facult!e des sciences de Sfax, BP 802-3018 Sfax, Tunisieb LGP2, Ecole Fran@aise de Papeterie et des Industries Graphiques (INPG), BP 65, Domaine Unviersitaire, F-38402 St. Martin d’H"eres, France

c Department of Paper science and Technology, University Beira Interior, Rua Marques d’Avila e Bolama, 6201-001 Covilha, Portugal

Accepted 7 July 2003

Abstract

The surface modification of cellulosic fibres was carried out using organofunctional silane coupling agents in an ethanol/water

medium. A heat treatment (curing) was applied after reaching the equilibrium adsorption of the prehydrolysed silanes onto the

cellulosic substrate. The modified fibres were then characterised by diffuse reflectance infrared spectroscopy and contact angle

measurements. The presence of Si–O–Cellulose and Si–O–Si bonds on the cellulose surface confirmed that the silane coupling agent

was efficiently held on the fibres surface through both condensation with cellulose hydroxyl groups and self-condensation between

silanol groups. The change of the surface properties after the modification was ascertained by contact angle measurements and

inverse gas chromatographic analysis. It was shown that the silane functional groups appended to the fibre surface could participate

in the chain growth of appropriate monomers to give a covalent continuity between the fibres and the ensuing polymer matrix.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Primers and coupling agents; B. Fibres; B. Surface treatment; C. Infrared spectra; Cellulose

1. Introduction

The interest of using cellulose fibres as reinforcingelements in composite materials based on polymericmatrices is constantly growing, mainly because of themultiple advantages associated with this natural renew-able material [1–9]. In fact, despite a Young moduluswhich is 2–3 times lower than that of glass fibres[5,10,11], cellulose fibres constitute an attractive alter-native to these mineral fibres because of their lowdensity (which makes it possible to obtain lightercomposites), bio-renewable character, ubiquitous avail-ability in a variety of forms, recyclability and modestabrasivity which ensures a greater longevity of theprocessing tools.

However, the preparation of cellulose-based compo-site materials is handicapped by the highly hydrophiliccharacter of these fibres, which is associated with a lowcompatibility with hydrophobic polymeric matrices like

polyolefins, as well as with a loss of mechanicalproperties after moisture uptake. The deficient adhesionleads to a weak load transfer from the matrix to thefibres, which induces a low reinforcing effect [10,11]. Inorder to reduce the hydrophilic character of cellulosefibres and to improve their adhesion properties, it isnecessary to undertake a chemical modification of theirsurface. Several approaches have been studied, amongthem an original procedure [12–14], which calls upon theheterogeneous grafting of bi-functional molecules on thefibre surface, leaving one of the functions available forfurther exploitation.

The present chemical grafting involves functionalisedsilane coupling agents (Scheme 1), commonly used formany purpose [15]. Commercially available tri-alkoxy-silane structures bearing various organic functions areemployed as coupling agents to improve adhesionbetween surfaces in different applications such asmastics or adhesives formulations [16–20]. In glassfibres composites, these reagents constitute the maincoupling agents employed for many purposes, namely (i)the modification the surface properties of glass fibres[15,21], (ii) the reinforcement of the adhesion between

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*Corresponding author. Tel.: +33-4-76-82-69-62; fax: +33-4-76-82-

69-33.

E-mail address: naceur.belgacem@efpg.inpg.fr (M.N. Belgacem).

0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0143-7496(03)00099-X

matrix and fibres [26], and (iii) the reduction of watersensitivity of the composite [22,23]. This treatmentinvolves the condensation of silanol groups with thehydroxy functions on the fibre surface.

Few studies have been devoted to the modification ofcellulose fibres using silane coupling agents [11,24,25]and these investigations were essentially devoted toascertain the impact of these modifications on themechanical properties of the ensuing composites andthe only search for a proof of chemical reaction waspublished by Herrera’s group [25].

Our previous recent study [27] showed that silanecoupling agents display a good affinity toward cellulosicsubstrate. Previously hydrolysed trialkoxysilane werephysically adsorbed onto the fibre surface at roomtemperature providing an effective coverage, which washowever easily removed by washing with ethanol. Thechemical grafting was shown to occur only after a curingprocess at 120�C under an inert atmosphere, thus givinga permanent chemical modification of the surface. Theaim of the present investigation was to acquire moreinformation about the silane structure and conforma-

tion after adsorption and to establish the impact of thesemodifications on the surface properties of the treatedfibres.

2. Materials and methods

2.1. Starting materials

The cellulose fibres used in this work were in the formof commercial microcrystalline particles (Technocel-150DM). Their length was about 50 mm, and theirspecific surface, measured by the BET technique usingnitrogen, was found to be 2.5 m2/g. In order to have flatsurfaces suitable for contact angles and wetting mea-surements of the unmodified and modified surfaces,commercial additives-free, ash less filter papers (What-man No. 5, WH5), supplied by Whatman Co., were alsoused. The four commercial silanes, kindly provided byOSI-WITCO, are shown in Scheme 1. All other reagentsand solvents were high purity commercial products.

2.2. Adsorption isotherms and fibre treatments

The adsorption isotherms were constructed by addingdifferent amounts of the given silane to a 5 w/w%cellulose suspension in a 80/20 v/v ethanol/water mix-ture and by stirring for 2 h. The silane were previouslyprehydrolysed operating: (i) at room temperature andthe pH of silane solution (ii) at the correspondingconcentrations used to built the adsorption isotherms,(iii) for 2 h, and (iii) in the same medium as that used toconstruct adsorption isotherms. The cellulose fibreswere then centrifuged at 2500 rpm for 20 min. Theamounts of silane adsorbed were assessed by FTIR andcolorimetry for MPS and APS, respectively, as reportedpreviously [27]. Subsequently, the fibres were dried atroom temperature for 2 days. The heat treatment towhich fibres were submitted consisted of curing at 120�Cunder a nitrogen atmosphere for 2 h. All heat-treated,i.e., cured samples were submitted to a 24 h soxlhetextraction with THF and dried.

2.3. Copolymerisation or grafting experiments

Cellulose fibres which were modified using a given bi-functional silane coupling agent were suspended in asuitable polymerisable medium in order to synthesise co-continuous matrix–fibre composites. Styrene (ST) ormethylmethacrylate (MMA) were used with fibrestreated with MPS, whereas an epoxy formulation basedon bis-phenol-A-diglycidylether was used to graft fibresmodified by APS and HPS. The epoxy formulationcontained a isophorone diamine as a hardner. Thecopolymerisations (second grafting) were carried outfor 12 h in a refluxing toluene/monomer solution

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O SiO

O

O O

γ -Methacryloxypropyltrimethoxysilane

MPS

H2N SiO

O O

γ-Aminoproyltriethoxysilane

APS

SiO

O O

Hexadecyltrimethoxysilane

HDS

HS SiO

O O

γ-Mercaptopropytrimethoxylsilane

MRPS

Scheme 1. The silane coupling agents used in this work.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5444

(80/20 w/w) in which 10 w/w% of the oven-dried(110�C) treated and cured fibres were suspended.Reactions involving free radical initiation (with ST orMMA) were carried out in the presence of benzoylperoxide (4 w/w% with respect to the monomer). Then,the cellulose fibres were soxhlet extracted for 24 h withtetrahydrofuran in order to wash off the residualhomopolymer and unreacted monomer.

2.4. Infrared spectroscopy

The FTIR analysis was performed using a Perkin-Elmer Paragon 1000 FTIR spectrometer. DRIFTspectra of the cellulose fibres before and after treatmentwere obtained using the diffuse reflectance accessory.The fibres were mixed with analytical grade KBr at aweight ratio of 5/200. Each spectrum was recorded witha resolution of 4 cm�1, with a total of 40 scans.Background scans were obtained using the KBr powder.All DRIFT spectra were plotted according to theKubelka-Munk function.

2.5. Contact angle

Contact angle measurements were carried out bydepositing calibrated liquid drops on the WH5 sheets.The apparatus used for these measurements was ahome-made static/dynamic instrument (1� precision)equipped with a CCD camera working at up to 200images per second. Various liquids with differentdispersive and polar properties were used and thecalculation of the dispersive and polar contributions tothe surface energy of the cellulose samples wasperformed using Owens–Wendt equation [28]. The studyof the dynamic contact angle as a function of the post-reaction time in the dry state of the silane-treatedcellulose was conducted by leaving the WH5 sheets atambient temperature for different periods.

2.6. Inverse gas chromatographic (IGC)

IGC experiments were carried out using a DELSI 121DFL chromatograph with a flame ionization detector.The stationary phases of cellulose fibres were packed inpreviously degreased, washed and dried Pyrex columns,typically 30 cm long and 0.4 cm in internal diameter.Between 0.5 and 1 g of dried modified fibres were packeddirectly into the column and conditioned overnight at30�C or 90�C (depending on the treatment history of thesample) in a stream of nitrogen. The dispersivecomponent of their surface energy was determined usinga series of n-alkanes probes. Their acid/base propertieswere evaluated using tetrahydrofuran and chloroform asdonor and acceptor probes, respectively. The relevantcharacteristics of these probes, namely the dispersivecomponent of their surface tension, their molecular

surface, and their Gutman’s donor and acceptornumbers were taken from the literature [29]. The IGCmethod of calculating the dispersive component gD

S ofthe surface energy of a solid and the acid/base propertiesof its surface, expressed as the donor (AN) and acceptor(DN) numbers ratio, has been described in detailelsewhere [29]. Thus, the interactions of the n-alkanemolecules with the substrates under investigation areonly caused by dispersive forces which enable todetermine the dispersive component of the surfaceenergy of the solid under investigation [30]. Concerningthe acid–base properties of the cellulose surfaces beforeand after modification, as mentioned before, we usedchloroform as an electron acceptor probe and tetra-hydrofuran (THF) as an electron donor one. The freeenergy of adsorption of THF or chloroform onto thesesurfaces, which are assumed to be the acceptor (AN) orthe donor number (DN), respectively, can be obtainedaccording to Schlutz’ approach [30]. The ratio betweenAN and DN gave an indication of the acid–baseproperties of our surfaces. According to the Lara’sarbitrary scale [31], this parameter is close to unity foramphoteric substrates, higher than 1 for acidic ones andlower than 1 for basic surfaces.

3. Results

In our previous work [27], the study of adsorptionisotherms revealed that the same prehydrolysed silanesused here were adsorbed onto the surface of cellulosicfibres. This adsorption followed a mono- and a multi-layer processes depending on the ratio between thequantity of the silane and that of the substrate. Thisadsorption was essentially driven by the formation ofhydrogen bonds between the hydroxy groups of silanesand those of cellulose. However, the functional groupborne at the end of the short aliphatic moiety of thesilane structure also contributed to the adsorptionprocess through specific interactions. The ESCA spec-troscopy analysis confirmed that the thermal treatmentof modified fibres at 120�C induced the chemicalbonding of silanes. However, little information wasobtained on the structure and the configuration of thecoupling agent grafted at the surface. The complexity ofsilane chemistry and the coexistence of self-condensa-tion reactions made it difficult to obtain such evidence.

Therefore, we decided to call upon the FTIR spectro-scopic analysis using the diffuse reflectance mode(DRIFT), which is well adapted to analyse the chemicalgroups present on a surface. Due to the small quantitiespresent on the fibre surface, the analysis was based onthe spectral differences between treated and untreatedsamples. All the spectra were taken at the same substrateconcentration (i.e., the quantity of KBr and that of thesubstrate under scrutiny were constant which enables to

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avoid the use of any internal standard) and the cellulosefibres were treated by a silane solution capable ofpromoting the adsorption of only a monolayer, basedon the adsorption isotherms established previously [27].

The subtraction spectra corresponding to the cellulosesamples treated with MPS, before and after heattreatment (curing) and extraction, are shown in Fig. 1.The bands at 1712 and 1637 cm�1 are associated,respectively, with the stretching vibrations of the C=Oand C=C groups of the acrylic moiety. From the broadshape of the C=O peak, it was deduced that it hadestablished hydrogen bonds with the hydroxy groups atthe cellulose surface. The broad intense bands around1200 and 1135 cm�1 were assigned to the stretching ofthe –Si–O–Cellulose and –Si–O–Si– bonds, respectively[1,33–35]. The strong increase in the intensity of thesebands after the curing suggested that both the graftingof silane onto cellulose and the intermolecular con-densation between adjacent adsorbed –Si–OH groupshad been substantially enhanced.

The peaks near 1100 and 1080 cm�1 are related toresidual unhydrolysed Si–OCH3 groups and their smallintensity, compared to those of the spectrum of MPS,suggested that most of the silane adsorbed under our

conditions had been hydrolysed. The large band around1015 cm�1, present on the spectrum of the uncuredsample, was attributed to –Si–OH groups. This banddisappeared after the curing and was replaced by a wideband around 1040 cm�1, characteristic of –Si–O–Si–groups. These peak assignments are in agreement withthose reported in other studies dealing with glasssurfaces treated with the same coupling agents [32,34].

The subtraction spectra relative to the g-APS modifiedcellulose surfaces before and after curing and extractionare shown in Fig. 2. The bands at 1575 and 1484 cm�1

are typical of the deformation modes of the NH2 groups[34] hydrogen bonded to the OH functions of bothsilanol moieties and cellulosic substrate. The largeintense bands around 1190 and 1140 cm�1, are relatedto the –Si–O–Si– linkage and –Si–O–Cellulose bonds, asdiscussed above for MPS treated cellulose. Again, theband at 1013 cm�1 (Si–OH) disappeared after heattreatment and was replaced by the Si–O–Si band at1040 cm�1.

Finally, the subtraction spectra corresponding to theHDS modified cellulose before and after heat treatmentand extraction are displayed in Fig. 3. Before thetreatment, the spectrum revealed some differences

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(b)

(a)

Si-O-Si + Si-O-Cellulose

Si-O-Si

Si-O-Si

Si-OH

Si-OCH3

1250 1220 1200 1180 1160 1140 1120 1100 1080 1060 1040 1020 1000 980 960 940 920 900

K-M

1181.43

1134.76

1038.741001.08

968.69

1201.79

1204.05

1181.83

1130.62

1099.36

1083.93

1051.92

1036.85

1013.88

968.32

C=C

C=O (c)

cm-1

Fig. 1. DRIFT substraction spectra of cellulose MPS-modified fibres (a) before and (b) after heat treatment (900–1250 cm�1 region) and (c) that of

1850–1550 cm�1 region after heat treatment.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5446

compared to those of MPS or APS modified cellulose,since the two intense bands at 1120 and 1090 cm�1,characteristic of the Si–O–CH3 groups, suggested thatHDS had only been partially hydrolysed, probablybecause of its marked hydrophobic character. Thedifferent features in Figs. 3a and b followed the samepattern as discussed above in terms of the chemicalgrafting produced by the heat treatment. An additionalpiece of evidence was revealed by the two intense bandsat 2924 and 2854 cm�1 in Fig. 3c, characteristic of thesymmetric and asymmetric stretching vibrations of theCH2 groups, respectively. Thus, the fact that afterextraction these aliphatic moieties would have beenretained on the cellulose surface strongly corroboratesthe interpretation based on the occurrence of a chemicalreaction between Si–OH and C–OH, strongly acceler-ated by the temperature increase.

The change of the contact angle values of a drop ofwater deposited on the WH5 sheets after their treatmentwith MPS, APS and HDS are shown in Figs. 4–6,respectively. These figures represent the dynamic acqui-sition of the values of contact angles within the time.The treated sheets were left to dry at room temperaturefor different times before being submitted to the curingat 120�C and the extraction to remove any physisorbedsilane. As mentioned before, the cellulose fibres weretreated by a silane solution capable of promoting theadsorption of only a monolayer, as established pre-viously [27]. In all these figures, regardless the coupling

agent used the contact angle values on a freshly treatedsheet (few hours of drying at room temperature) was aslow as that measured on an untreated substrate. Thislow value denoted the hydrophilic character of thesurface, despite the presence of the physisorbed silane,probably because of the presence of the free –Si–OHcoming from the hydrolysed silanes.

Except for the MPS-treated sheets, the contact anglevalues increased very slightly with drying time at roomtemperature (Fig. 4). After 2–3 days, the contact anglevalues reached a plateau situated near that of thecorresponding thermally treated material. As pointedout previously, the change of the contact angles could beexplained by the progressive disappearance of Si–OHgroups resulting from their condensation with thecellulose hydroxy groups and from their self-condensa-tion giving a siloxane polymer. The heat treatment(curing) of the paper accelerated these reactions allow-ing efficient reaction of grafting agents on the surface.

In the case of MPS, the reason for the modest contactangle change was attributed to the relatively low MPScoverage of the surface, i.e. 0.23 mmol g�1 against0.91 mmol g�1 for APS [27], which left an importantfraction of cellulose hydroxy groups exposed and/or ahigh proportion of residual Si–OH groups, since the rateof their self-condensation was much reduced by theirhigh intermolecular distance.

With APS, the contact angle values increased rapidlyto reach a relatively high limit of about 75�, despite the

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(b)

(a)

NH2

Si-O-Si + Si-O-Cellulose

Si-O

-Si

Si-OH

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600

cm-1

K-M

1575.24

1484.82

1306.56

1280.73 1191.601142.51

1098.60

1039.18

1000.43

961.67792.46

742.08

702.04

771.79

911.30

1013.34

1037.89

1562.32

1291.06

Fig. 2. DRIFT substraction spectra of APS-modified cellulose (a) before and (b) after heat treatment.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–54 47

presence of a high polar amine end group. We attributethis result to the APS configuration at the surface whichis assumed to bend and orient its polar NH2 headtoward the surface, driven by the formation of stronghydrogen bonds with the cellulose hydroxy groups. Suchconfiguration would leave the methylene sequence

exposed at the surface, thus providing the hydrophobiccharacter measured through the high water contactangles. This interpretation has been previously proposedin the case of the glass surface treatment with APS [16].

The second point which caught our attention was therelatively rapid reach of the maximum contact angle

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1250 1220 1200 1180 1160 1140 1120 1100 1080 1060 1040 1020 1000 980 960 940 920 900

cm-1

K-M

1037.45

1004.03

1100.59

1145.15

1187.24

1130.67

1216.03

1198.54

1121.58

1097.79

1039.02

1012.43

973.95

909.58

Si-OC2H5

Si-O-Si +

Si-O-Cellulose Si-O-Si Si-O-Si

Si-OH

- CH2 -

3100 3000 2900 2800 27000.0394

0.045

0.050

0.055

0.060

0.065

0.070

0.075

0.080

0.085

0.090

0.095

0.100

0.105

0.110

0.115

0.120

0.1253

cm-1

-M

2924.75

2854.14

(a)

(b)

(c)

Fig. 3. DRIFT substraction spectra of HDS-modified cellulose (a) before and (b) after heat treatment and (c) that of 2700–3100 cm�1 region.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5448

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0

5

10

15

20

25

30

35

40

45

50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Con

tact

ang

le

Cellulose

15 h

3 days

Heat treatment

Time (S)

Fig. 4. Water contact angle changes on MPS-modified cellulosic substrate as a function of drying time at room temperature or after heat treatment.

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Con

tact

ang

le

Cellu lo se2h30 h2 daysHeat treatment

Time (S)

Fig. 5. Water contact angle changes on APS-modified cellulosic substrate as a function of drying time at room temperature or after heat treatment.

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Con

tact

ang

le°

Cellulose3 h

8 h24 h

Heat treatment

Time (S)

Fig. 6. Water contact angle changes on HDS-modified cellulosic substrate as a function of drying time at room temperature or after heat treatment.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–54 49

with APS, compared with MPS or MRPS, viz. two daysinstead of more than four. This effect could be explainedby the relatively high adsorption plateau (0.91 mmol g�1

[27]) which allows an efficient –Si–OH self-condensa-tion, but also by the catalytic role of the amine group,capable of accelerating both –SiOH+–SiOH and –Si–OH+C–OH condensations. This catalytic effect is well-established in silane chemistry [20,36] and was con-firmed here by the fact that the cellulose treatment by anMPS solution in the presence of triethylamine (TEA)(20% molar with respect to MPS) accelerated thecontact angle increase, as shown in Fig. 7. Thus, theaddition of TEA seems to be an interesting way toimprove the modification of cellulosic fibres by MPS. Inaddition to its catalytic effect, the addition of TEAenhanced the MPS adsorption (Fig. 8) to about0.35 mmol g�1 against 0.23 mmol g�1 in its absence.

In the presence of HDS (Fig. 6), the rapid increase ofthe contact angle and its high limiting value (whichreached 110� after heat treatment) clearly indicated theimportant hydrophobic contribution of the long alipha-tic chain to the surface property through its masking ofthe OH groups.

Fig. 9 shows the contact angle values for each silaneafter (i) 2 h immersion and (ii) heat treatment at 120�Cand extraction. As discussed above, the surface proper-ties were affected by the silane structure only aftercuring or prolonged standing at ambient temperature.The trend in the maximum contact angle value in theorder of HDS>MPS-TEA>MRPS>APS was in tunewith the increasing polar character of the fourth moietyattached to the trialkoxy silane coupling agent.

To quantify the change of the surface energy after thevarious silane treatments, we measured the contactangles using four liquid probes, namely water, forma-

mide, ethylene glycol and diiodomethane. By applyingthe Owens–Wendt approach [28], both the dispersiveand the polar contribution to the surface energy of thesematerials were calculated using the equation:

gLð1 þ cos yÞ ¼ 2ffiffiffiffiffiffiffiffiffiffiffigD

S gDL

qþ 2

ffiffiffiffiffiffiffiffiffigP

SgPL

q;

where gDL and gP

L were known for each liquid. Table 1gives the surface energies related to the unmodified andmodified fibres. Untreated cellulose paper exhibited thewell-known high polar and dispersive energy values. Justfollowing the treatment with MPS, the fibres showed asignificant increase in gP

S compared with that of nativecellulose. This effect is the consequence of the presenceof a high concentration of free –Si–OH groups. Heat

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0

20

40

60

80

100

120

0 0.1 0.2 0.3 0.4 0 .5 0.6 0.7 0.8Time (S)

Con

tact

ang

le°

Cellulose5 h24 h4 days7 daysHeat treatment

Fig. 7. Water contact angle evolution on MPS-modified cellulosic substrate in presence of TEA and as a function of drying time or after heat

treatment.

Table 1

Surface energies (mJ m�2) of cellulose fibres before and after different

treatment

Cellulose gPS gD

S gTS

20 30 50

MPS

MPS 0 37 32 69

MPS-1.5 h (120�C) 31 30 61

MPS-3 h (120�C) 20 34 64

MPS-1.5 h (150�C) 8.5 33 41.5

MPS-TEA 1.5 33 34.5

MRPS

MRPS 0 28 34 62

MRPS-1.5 h (120�C) 12 31 43

MRPS-3 h (120�C) 4 32 36

APS

APS 0 19 29 48

APS-2 h (120�C) 9.2 33.3 43.5

HDS

HDS-2 h (120�C) 0.5 37 37.5

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5450

treatment at 120�C during 3 h led to a small decrease ingP

S : A significant decrease could only be attained after 3 htreatment at 150�C, which was too high for cellulose, orby addition of TEA as a catalyst.

In the presence of MRPS, the initial value of gPS was

again higher than that of native cellulose. Heattreatment at 120�C produced a continuous decreaseuntil a limiting value of 4 mJ m�2. APS treatment led toan initial gP

S value close to that of untreated cellulose,even though more polar groups (NH2, –Si–OH) werepresent. It is probable that the rapid rate of –Si–OHcondensation, induced by the catalytic effect of theamine group, resulted in a lower concentration of free –Si–OH groups at the surface. After curing, gP

S attained aminimum value of 9 mJ m�2. Cellulose treated with

HDS displayed virtually no polar surface energy afterheat treatment, which indicated again that the aliphaticchain of HDS totally masked the surface hydroxygroups and imposed its own the surface properties.

The order of gPS evolution after heat treatment

followed a trend opposite to than of the contact anglevalues with water (HDSoMPS-TEAoMRPSoMPS),as expected in terms of the decrease in the hydrophobiccharacter of this series of silanes. As for the values of gD

L ;they remained roughly unchanged with all these silanes,as expected from the nature of the dispersive forcesinvolved.

The modified cellulose fibres were also analyzed byIGC after the curing treatment. The results of this studyare presented in Table 2. The analysis of these data

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 50 100 150 200 250

Initial concentration C0 mmol/l

Ads

orbe

d am

ount

mm

ol/g

cellu

lose

100% adsorbed lineMPS-TEAMPS

Fig. 8. Adsorption isotherms of MPS on cellulose surface in the presence or not of TEA as grafting reaction catalyst.

0

20

40

60

80

100

120

cellulose MPS MPS-TEA APS HDS MRPS

Con

tact

ang

le°

2 h

Heat treatment

Fig. 9. Contact angles values of silane-modified cellulose before and after heat treatment.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–54 51

confirmed again that gDS remained practically unchanged

after the treatment with all silanes, except in the case ofHDS, where gD

S attained a value of 22 mJ m�2, which isindeed that of the surface energy of a linear 16 carbonaliphatic chain. The relatively high value of the AN/DNratio for untreated cellulose reflected the acid characterresulting from the strong density of surface hydroxygroups, as previously reported [6]. After modific-ation with MPS, MRPS and HDS, the decrease inthe AN/DN ratio, which remained however higherthan unity, reflected the progressive reduction insurface accessibility of hydroxy groups to the probes.The APS treatment reversed the interaction balance andled to a surface which displayed a modestly basiccharacter, generated by the presence of the aminogroups.

In order to explore the aptitude of the fibres modifiedwith reactive groups to undergo chemical reaction withmonomers, a series of experiments were carried out inwhich styrene (ST), methylmethacrylate (MMA) or bis-phenol-A-diglycidylether/diamine (epoxy) were poly-merised in the presence of the modified fibres. In Table3, we report the monomer selected for each specificsilane functionality. Evidence for the participation of thesilane functional groups as comonomer in these reac-tions was obtained by FTIR. In all instances, thespectral differences clearly indicated the presence ofpolyST, polyMMA or epoxy chains on the extractedfibres, as shown in Fig. 10 (presence of C=O band ofpolyMMA at 1718 cm�1) and Fig. 11 (presence ofaromatic peaks of polyST at 1017, 725 and 689 cm�1)for the case of MPS-modified fibres, after copolymeriza-tion (grafting) with MMA and ST, respectively. It isimportant to underline that the same grafting procedureapplied to untreated fibres did not reveal any trace ofpolyST, or polyMMA after the removal of thesehomopolymers by extraction.

This grafting of polymer chains on the silane modifiedfibres greatly enhanced their hydrophobic character, asindicated by the high value of the contact angle withwater (Fig. 12) and the low values of gP

S (Table 3). Itfollows that the use of the appropriate silane canprovide a continuous path of covalent bonds between

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Si-O-Si +

vSi-O-Cellulose +

vC-O-C PolyMMA

vC=O (MPS + PolyMMA)

�CH

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 4000.30

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.60

cm-1

K-M

1718.37

1633.96

1442.82

1328.141296.28

1184.78

1140.18

1086.03

1057.36

998.42

976.13

915.60

888.52

823.22

730.83

700.57

662.34

622.52

592.26

555.62

498.28

468.02

541.29

Fig. 10. DRIFT substraction spectra of MPS-modified cellulose followed by polymerisation with MMA monomer.

Table 2

Dispersive component of the surface energy and acid/base properties

of cellulose surfaces before and after different modification, as

determined by IGC

Samples gDS AN/DN

Cellulose 30.9 3.1

Cell+MPS 29.6 1.4

Cell+APS 31.2 0.8

Cell+MRPS 29.1 1.6

Cell+HDS 22.6 1.1

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5452

the fibre surface and the polymer matrix, thus enhancingthe fibre/polymer compatibility.

This study confirmed that the adsorption of silanes bythe surface of cellulosic fibres, followed by heattreatment at 120�C for 1–2 h induces chemical bonding.Even though the –Si–OR is prone to hydrolysis, thepermanent bonding of the silane to the cellulose surfaceis ensured by the fact that the self-condensation of –Si–OH groups generates a siloxane network which shieldsthe surface from hydrolysis of the C–O–Si moieties, asschematically and hypothetically illustrated in Fig. 13.

4. Conclusion

Silane coupling agents adsorbed from diluted solutionon cellulose fibre surfaces, followed by heat treatment,were shown to condense both among themselves andwith the OH groups of the substrate to give Si–O–Si andSi–O–C couplings, respectively. These reactions ensuredefficient and irreversible chemical bonding of the silaneonto the cellulose surface. Contact angles and IGCmeasurements revealed that the hydrophilic character ofthe cellulose fibres can be strongly decreased after thesilane treatment. A particularly hydrophobic surfacecould thus be obtained after treatment with HDS orMPS-TEA. Moreover, polymerisation tests, carried outin the presence of different monomers, showed that thepresence of adequate functional groups on the silanemolecule enable them to participate in the polymerchain growth and give rise to chemical grafting of thefibres. This double modification represents an effectiveapproach to the optimisation of quality of the fibre/

ARTICLE IN PRESS

Si-O-Si +

Si-O-Cellulose

vC=O (MPS) �CH (aromatic)

δCH � CH2

� C=C (aromatic)

� sCH2 (aliphatic)

vC=C( PolyST)

1871.3 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 441.9

0.0165

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

0.075

0.080

0.085

0.0917

cm-1

K-M

1716.171702.26

1312.70

1162.45

1017.75

1040.01

970.45

909.23

689.41

642.11

726.541080.43

1111.20

1200.45

1241.99

1281.991371.24

1455.86

1338.92

Fig. 11. DRIFT substraction spectra of MPS-modified cellulose followed by polymerisation with ST monomer.

0

20

40

60

80

100

120

0 0.1 0.2 0.3 0.4 0.5

Con

tact

ang

le

MPS-MMAMPS-StyreneMRPS-EpoxyAPS-Epoxy

Time (s)

°

Fig. 12. Water contact angle evolution of silane-modified cellulosic

substrate and polymerized with different monomers.

Table 3

Surface energies (mJ m�2) of cellulose fibres after copolymerization

with different monomers

Silanes MPS MRPS APS

Monomers MMA ST MMA Epoxy Epoxy

gPS 3.4 0.1 2.3 0.8 0.8

gDS 38.9 36.6 40.3 43.5 37.9

gTS 42.4 36.7 42.6 44.4 38.6

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–54 53

matrix interface for composite materials containingnatural fibres.

References

[1] Valadez GA, Cervantes U, Olayo R, Herrera-Franco PJ.

Composites Part 1999;30:309.

[2] Matos RM, Cavaill!e JY, Dufresne A, G!erard JF, Graillat C.

Mat!er Compos 2000;7:63.

[3] Mataana LM, Woodhams RT, Balatinecz JJ, Park CB. Polym

Compos 1989;19:446.

[4] Kuruvilla J, Varghese S, Kalaprasad G, Thomas S, Prasannaku-

mari L, Koshy P, Pavithran C. Eur Polym J 1990;32:1243.

[5] Kokta BV, Maldas D, Daneault C, B!eland P. Polym Plast Tech

Eng 1990;29:87.

[6] Belgacem MN, Czeremuskin D, Saphieha S. Cellulose 1995;

2:145.

[7] Felix JM, Gatenholm P, Schreiber HP. J Appl Polym Sci

1994;51:286.

[8] Favier V, Chanzy H, Cavaill!e JY. Macromolecules 1995;28:

6365.

[9] Singh B, Gupta M, Verma A. Polym Compos 1996;17:910.

[10] Zadorecki P, Flodin P. J Appl Polym Sci 1985;30:3971.

[11] Felix JM, Gatenholm P, Schreiber HP. Polym Compos

1993;14(6):449.

[12] Matias M, De La Orden MU, Gonzalez Sanchez C, Martinez UJ.

J Appl Polym Sci 2000;75:25.

[13] Botaro VR, Gandini A. Cellulose 1989;5:1.

[14] Gandini A, Botaro V, Zeno E, Bach S. Polym Int 2001;50:7.

[15] Daniels MW, Francis LF. J Colloid Interface Sci 1989;205:191.

[16] Chiang CH, Ishida H, Koing JL. J Colloid Interface Sci

1980;74:396.

[17] Underhill PR, Goring G, Duquesnay DL. Int J Adhes Adhesives

2000;20:195.

[18] Mattson B, Aksmes E, Huse J, Tveten C, Redford K, Stori A.

Compos Interfaces 1996;4:77.

[19] Demjen Z, Pukanszky B, F #oldes E, Nagy J. J Colloid Interface Sci

1997;190:427.

[20] Pluddeman EP. Silane coupling agents, 2nd ed. New York:

Plenum; 1991.

[21] Rosen MR, Goddard ED. In: Proceeding of the 34th Annual

Technical Conference. Sect. 19-E. SPI Reinforced Plastics/

Composites Institute, 1979.

[22] Bascom WD. J Adhes 1970;2:161.

[23] Mukherjea RN, Pal SK, Sanyal SK. J Appl Polym Sci

1983;28:3023.

[24] Mieck KP, Nechwatal A, Knobelsdorf C. Makromol Chem

1995;224:73.

[25] Cazaurang MM, Franco HP, Gonzalez-Chi PI, Aguilar-Vega

MP. Appl Polym Sci 1991;43:749.

[26] Koenig JL. In: Leyden DE, editor. Silanes, Surfaces, and

Interfaces: Proceeding of the Silanes, Surfaces, and Interfaces

Symposium. New York: Gorden and Breach; 1986.

[27] Abdelmouleh M, Boufi S, Ben Salah A, Belgacem MN, Gandini

A. Langmuir 2002;18:3203.

[28] Owens DK, Wendt RC. J Appl Polym Sci 1969;13:1741.

[29] Belgacem MN, Gandini A. IGC as a tool to characterize

dispersive and acid–base properties of the surface of fibers and

powders. In: Pefferkorn E, editor. Interface phenomena in

chromatography. New York: Marcel Dekker; 1998. p. 41–124.

[30] Schultz J, Lavielle L, Martin C. J Adhes 1987;23:45.

[31] Lara J, Schreiber HP. J Coat Technol 1991;63:81.

[32] Salmon L, Thominette F, Pays MF, Verdu J. Polym Compos

1999;2:715.

[33] Miller JD, Hoh K, Ishida H. Polym Compos 1984;5:18.

[34] Chiang CH, Ishida H, Koenig JL. J Colloid Interface Sci

1980;24:396.

[35] Britcher L, Kehoe D, Matisons J, Swincer G. Macromolecules

1995;28:3110.

[36] White LD, Tripp CP. J Colloid Interface Sci 2000;232:400.

ARTICLE IN PRESS

Fig. 13. Schematic illustration of the silane structure on the cellulosic substrate.

M. Abdelmouleh et al. / International Journal of Adhesion & Adhesives 24 (2004) 43–5454