Composites Science and Technology 68 (2008) 3193–3201

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Composites Science and Technology

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Surface functionalization of cellulose fibres and their incorporation in renewablepolymeric matrices

B. Ly, W. Thielemans 1, A. Dufresne, D. Chaussy, M.N. Belgacem *

École Française de Papeterie et des Industries Graphiques, INPG, Laboratoire de Génie des Procédés Papetiers, BP 65, F-38402 Saint Martin d’Hères, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 May 2008Received in revised form 29 July 2008Accepted 30 July 2008Available online 8 August 2008

Keywords:A. Coupling agentsA. Short-fibre compositesB. Fibre/matrix bondB. Mechanical propertiesSurface treatments

0266-3538/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.compscitech.2008.07.018

* Corresponding author. Tel.: +33 4 76 82 69 62; faE-mail address: Naceur.Belgacem@efpg.inpg.fr (M.

1 Driving Innovation in Chemistry and Chemical EChemistry and School of Chemical and EnvironmentalNottingham, University Park, Nottingham NG7 2RD, UK

The surface of model cellulose fibres (Avicell) as well as kraft softwood pulps (ksp) was chemically mod-ified with different coupling agents, namely: two difunctional anhydrides, two diisocyanates, and twoalkoxysilanes. These grafting agents were pyromellitic dianhydride (PMDA), benzophenone-3,30,4,40-tet-racarboxylic dianhydride (BPDA), 1,4-phenylene diisocyanate (PPDI), methylene-bis-diphenyl diisocya-nate (MDI), and c-mercaptopropyltriethoxysilane (MRPS), and c-methacrylopropyltriethoxysilane(MPS). In all cases stiff monomers were used with the aim of ensuring the reaction of only one of the func-tionalities with the cellulose surface, leaving the remaining moiety to react with the polymer matrix dur-ing composite processing to provide a covalent linkage between the matrix and the reinforcing elements,thus enabling perfect stress transfer. Thus, the rigidity of the used molecules and their quite small sizeexclude bridging two fibres or reacting both functionalities at the surface of the same fibres. After mod-ification, the ensuing fibres were submitted to soxhlet extraction, in order to remove all the unboundedand physically adsorbed molecules. The occurrence of the grafting was confirmed by elemental analysis,infrared, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and contact angle mea-surements. The modified fibres were then incorporated into cellulose acetate-butyrate and natural rub-ber matrices and the mechanical properties of the ensuing composites determined. PMDA- and BPDA-treated Avicell were found to reinforce efficiently the mechanical properties of CAB, whereas MPS-treatedcellulose enhanced those made with natural rubber matrix.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The exploitation of cellulose constitutes a rapidly growing issuetoday, not only for the production of high volume commodities liketextiles and paper, but also for novel high-value added materialslike reinforcing elements in natural fibre-based compositematerials. In this context, the application for cellulose differs fromwell-established industrial processes leading to the syntheses ofcellulose esters, ethers and other derivatives, which involve thedestruction of the fibrous morphology and its transformation intothermoplastic materials soluble in some organic solvents [1–3]. Infact, in the context of cellulose fibres reinforced composites, simi-lar chemical processes are applied to cellulose macromolecules,but their impact is limited to the macromolecular layers constitut-ing the fibre surface. In fact, in order to have high-performancereinforcing elements, the fibre integrity should be preserved. In-stead, the surface properties are modified bearing the matrix in

ll rights reserved.

x: +33 4 76 82 69 33.N. Belgacem).ngineering (DICE), School of

Engineering, The University of.

mind, in order to yield the best interface between the two phases(the matrix and the reinforcing fibres). The major driving forcefor the use of cellulose in composite materials [4–8] is the replace-ment of glass-fibres reinforced composites, leading always difficultto recycle materials.

The use of planar stiff molecular configurations bearing tworeactive functions at the opposite end of their structure is basedon the assumption that only one of the functions will react with afibre OH group, whereas the other will be left to copolymerise withthe matrix. Although some publications reporting this original ap-proach are available [9,10], the actual experimental confirmationof its validity was only published by Gandini et al. [11–16], whotreated cellulose with different grafting agents belonging to thisgrafting strategy, namely: pyromellitic dianhydride, 1,4-phenyldiisocyanate, 3-isopropenyl-a,a0-dimethylbenzyl isocyanate,2-isocyanatoethyl methacrylate and methacrylic anhydride.

The purpose of this paper is to graft cellulose fibres with differ-ent coupling agents, in order to use the resulting modified fibres asreinforcing elements in composite materials with the aim of cop-olymerising the matrix and the fibres through these grafting mol-ecules. Pyromellitic dianhydride (PMDA), benzophenone-3,30,4,40-tetracarboxylic dianhydride (BPDA), 1,4-phenylene diisocyanate(PPDI) and 4,40-diphenylmethane diisocyanate (MDI) are used to

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graft fibers, destined to reinforce OH-rich matrices, such as ther-moplastic starch or partially substituted cellulose matrices,whereas c-mercaptopropyltriethoxysilane (MRPS) and c-methac-rylopropyltriethoxy-silane (MPS) will be used to reinforce cellulosefibres to be incorporated in natural rubber or unsaturated styrenecross-linked polyester resins. In this work, cellulose acetobutyrateand natural rubber-based composites were prepared andcharacterized.

2. Experimental

2.1. Materials

The six grafting agents used in this work were purchased fromSigma– Aldrich Inc. and used as received. Their chemical structuresare shown in Fig. 1. Imidazole (99%) and dibutyltin dilaureate,DBTL (95%) were also obtained from Sigma–Aldrich and used ascatalysts for esterification and urethane formation, respectively.They were used as received without any further purification. Anhy-drous tetrahydrofuran and toluene as well as methylene chloridewere also commercial products (Sigma–Aldrich Inc.) with the high-est purity available. n-Dodecane, a-bromonaphtalene, diidome-thane, formamide and water were used as molecule probes forcontact angle measurements. They were high purity commercialproducts, except water which was bi-distilled in our laboratory.The dispersive and polar components of their surface tension arelisted in Table 1.

Microcrystalline cellulose (Avicell) and bleached kraft softwoodpulps (ksp) were commercial and industrial product, respectively.They were used as received. Avicell is used as a modelreinforcement.

Fig. 1. Chemical structure of the coupling agents used for cellulose surfacemodifications.

Table 1Molecule probes for contact angle measurements and their characteristics (in mJ/m2)

Liquid cDL cP

L cTL

Water 21.8 51 72.8Ethylene-glycol 29.0 19 48Formamide 37.6 20.4 58n-Hexadecane 27.5 0 27.8a-Bromonaphtalene 43.5 0.9 44.4Diiodomethane 48.5 2.3 50.8

2.2. Reaction procedures

Since the solvents used are non-swelling, it was calculated fromthe specific surface area of the solid substrate (Avicell particles andpulp fibres), that only 1% of cellulose hydroxyl groups are presentat its surface and therefore potentially accessible for grafting. Thisleads to a total number of 18.5 mmol OH per 100 g of Avicell andksp. A slight excess of the grafting agents was used. The graftingwith dianhydrides was performed in anhydrous tetrahydrofuran(THF, 350 ml per 100 g Avicell particles or pulp fibers). The neces-sary amount of dianhydride was previously dissolved in the anhy-drous THF in a three-necked round-bottom flask and heated at70 �C under nitrogen atmosphere and vigorous stirring. One hun-dred gram of dry Avicell (vacuum dried) and 2 g of imidazole aresubsequently added, and the reaction was allowed to occur for48 h.

The diisocyanate grafting was performed in a mixture of drydichloromethane and anhydrous toluene (2/3 v/v). First, 500 mlof the solvent mixture was introduced into a three-neckedround-bottom flask under nitrogen atmosphere. The resulting sus-pension was then heated to 70 �C, before the addition of the rightamount of the chosen diisocyanate, in the presence of 0.5 ml DBTLunder vigorous stirring. After diisocyanate dissolution, 100 g ofAvicell or ksp were added and the reaction mixture was left for72 h.

After the grafting reactions, the modified Avicell or ksp werequickly filtered over an extraction thimble and soxhlet-extractedfor 6 h using anhydrous methylene chloride under a nitrogenatmosphere, in order to remove the unreacted molecules and toavoid the hydrolysis of the remaining anhydride or isocyanatefunctions. The extracted fibres were then dried for 48 h at 50 �Cand stored under a dry atmosphere.

The grafting of MRPS and MPS was realized by adding the cho-sen silane, dissolved in a mixture of ethanol and water 80/20 (w/w), to the cellulose suspensions (5% (w/w) with respect to the sol-vent). The resulting suspensions were kept under gentle stirring for2 h, at room temperature. At the end of the adsorption, cellulose fi-bres were isolated by centrifugation at 2500 rpm for 20 min, beforebeing submitted to a heat treatment at 110 �C, under vacuum(2 mmHg) and during two hours, in order to induce the grafting.The detailed description of these experimental conditions has beenreported elsewhere [13].

2.3. Preparation of composite materials

The two matrices used were cellulose aceto-butyrate (CAB) andnatural rubber (NR) (a gift from Antonio Aprigio Da Silva Curveloour Brazilian colleague, working at the University of SaoPaulo–Sao Carlos Institute of Chemistry, to whom we are in-debted). The first one contains about 30% of non-substituted freehydroxyl functions (supplied by Eastmont Co. to whom we wouldlike to express our gratitude) and was used in combination withthe dianhydrides- and diisocyantes-modified fibres, whereas NRcontains residual insaturations which makes it suitable for MPS-and MRPS-treaded cellulose.

Two processing routes were used to prepare composite materi-als, namely: extrusion followed by hot-pressing and film-casting.The extrusion trials were carried out using a twin screw mini-ex-truder allowing working with small quantities of the material(<15 g). The rotation speed of the screw was fixed at 50 rpm, for15 min. The temperature of extrusion varied as a function of thematrix processed. Thus, only CAB-based composites were pro-cessed using extrusion at a temperature of 180 �C. The ensuinghomogeneous mixture was hot pressed between two metallicplates, using a laboratory press (St-Eloi Mécanique), at 180 �C, for5 min. A silicone liner was inserted between the plates and the

B. Ly et al. / Composites Science and Technology 68 (2008) 3193–3201 3195

composite films in order to de-mould easily the prepared materi-als. Films with a thickness of about 300 lm were obtained.

Film casting concerned only natural rubber as a matrix and thereinforced elements were Avicell-modified particles. The couplingagent used for this purpose was c-methacrylopropyl-triethoxysi-lane (c-MPS). Thus, the natural rubber was first dissolved into tol-uene and then the fibres added. The resulting mixture was heatedat 90 �C under magnetic stirring for about 24 h and in the absenceof any traces of oxygen (a stream of nitrogen was maintained dur-ing the full experiment). The prepared mixture was transferredinto a Teflon mould. The filled mould was left into an oven, in orderto evaporate the solvent. The dry composites were then cut in theform of test probes for thermo-mechanical measurements.

2.4. Characterisation

FTIR spectra were obtained by preparing pellets of dried KBrpowder containing 1% w/w of the investigated samples. Sixteenscans were taken between 4000 and 400 cm�1, with 4 cm�1 inter-vals, on a Perkin-Elmer PARAGON 1000 FTIR spectrometerequipped with a spectrum software. X-ray photoelectron spectros-copy (XPS) experiments were performed using a XR3E2 apparatus(Vacuum Generators, UK) equipped with monochromated MgKa X-ray source (1253.6 eV) and operating at 15 kV under a current of20 mA. Samples were placed in an ultrahigh vacuum chamber(10�8 mbar) with electron collection by a hemispherical analyzerat an angle of 90 �. Signal decomposition was realised using Spec-trum NT, and the CAH signal was used as a reference peak at285.0 kV.

Static and dynamic measurements of contact angles were car-ried out with various liquids. The apparatus used was a DataPhys-ics instrument, equipped with a CDD camera, which allowed boththe determination of contact angle at equilibrium, with a precisionof ±1�, and the kinetics of its evolution, by taking up to 1000images per second, starting within a few tens of milliseconds afterthe deposition of the drop. The contact angle at equilibrium of pureliquids having different polarity, namely water, formamide, ethyl-ene glycol and diiodomethane were used to calculate the disper-sive and polar contributions to the surface energy according tothe approach proposed by Owens and Wendt [17]. Elemental anal-ysis were carried out at the Laboratoire Centrale d’Analyses de

Fig. 2. FTIR spectra of cellulose before

Vernaison. This technique is based on atomic absorption of theinvestigated elements.

2.5. Characterisation of composite materials

The prepared composite materials were characterised using dy-namic mechanical analysis (DMA). DMA measurements were car-ried out with an apparatus DMA RSA3 from TA Instrumentsworking in the tensile mode. The specimen was a thin rectangularstrip with dimensions around 40 mm � 5 mm � 1 mm. Tests wereperformed under isochronal conditions at 1 Hz and the tempera-ture was varied between �120 and 200 �C by steps of 2 �C.

Scanning electron microscopy (SEM) was also used to charac-terise the quality of the fibre/matrix interface. Thus, the compos-ites materials were fractured after their immersion into nitrogenliquid and the resulting surfaces characterised. The SEM usedwas an ESEM Quanta 200 (FEI) equipped with a motorized Pelletierplating (�5 �C to 55 �C) and an infrared camera. The image resolu-tion could reach 3584 � 3094 pixels.

3. Results and discussion

3.1. Surface modification

The modified and solvent extracted fibres were characterized byFTIR spectroscopy which showed that anhydride-grafted cellulosedisplayed the appearance of ester and anhydride signals (1650,1682, 1698, and 1726 cm�1), and that corresponding to isocyanatefunction (2275 cm�1) in the case of PDI and MDI modification,respectively. In both anhydride and isocyanate grafting, the pres-ence of a peak at 831 cm�1 associated with the presence of di-substituted aromatic ring confirmed the occurrence of chemicalbonds between cellulose and the four coupling agents studied.The urethane linkage signal, which should appear due to the reac-tion of the isocyanate with the cellulose hydroxyl groups, ismasked by a large signal at 1634 cm�1. Fig. 2, taken as an example,shows the FTIR of Avicell before and after grafting with BPDA, fromwhich one can observe the appearance of peaks at 1726 and1662 cm�1, associated with anhydride, and carboxylic acid groups(the hydrolysed function after the esterification reaction with cel-lulose), respectively,. These data are in agreement with pioneering

and after modification with BPDA.

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works, reported by Gandini’s group, using the same or similar cou-pling agents [11–16]. For silane-grafted cellulose samples, thebands at 1712 and 1637 cm�1 are associated with the stretchingvibrations of the C@O and C@C groups of the acrylic moiety,respectively. The signal at 1637 cm�1 may result from water,therefore the FTIR spectrum of the investigated samples were re-corded before and after drying at 105 �C for different time (includ-ing overnight) of the KRr pellets, which were kept in a dessicator tobe cooled. The resulting spectra were similar indicating that thesignal at 1637 cm�1, belongs to the grafted cellulose.

The broad intense bands around 1200 and 1135 cm�1were as-signed to the stretching of the ASiAOACellulose and ASiAOASiAbonds, respectively [18–21].

Table 2Elemental analysis of Avicell and bsk, before and after modification with differentcoupling agents

C (%) H (%) O (%) N (%) Si (%)

Avicel 42.4 6.5 49.3 – –PMDA_Av 43.2 6.4 49.4 – –BPDA_Av 45.2 5.9 49.5 – –PPDI_Av 44.5 6 48.4 1.1 –MPS_Av 42.9 6.4 50.9 – 0.1Bsk 41.7 6.2 51 – –BPDA_bsk 43.1 6.1 48.5 – –MDI_bsk 44.3 6.2 47.2 1.1 –c-MPS_bsk 42.0 6.1 49.1 1.3

Fig. 3. Full XPS spectra of cellulose surface, before and

The elemental analyses of some samples studied here are givenin Table 2. Most importantly, it confirms the presence of nitrogenfor PPDI-treated cellulose. As discussed above, for the other graft-ing agents, no significant difference could be noticed, because thereis no big difference in oxygen and carbon atom content betweenthe coupling agents and the cellulosic substrate.

XPS analyses were also carried out in order to ascertain moreprecisely the chemical composition of the surface, since the depthof penetration associated with this technique is a few nanometers.The XPS spectra of cellulose before and after treatment are given inFigs. 3 and 4. Theoretically, pure cellulose exhibits two peaks in thefull XPS spectra: carbon and oxygen, with an O/C ratio of 0.83. TheXPS spectra of unmodified Avicell and ksp displayed the expectedtwo signals (oxygen and carbon atoms at around 532.6 and 285 eV,respectively) [22] (Fig. 3a and b). For modified cellulose samples,new peaks appeared, namely: the nitrogen signal at around400 eV, for PDI and MDI, respectively (Fig. 3g and h), those corre-sponding to silicon atoms in MPS and MRPS at 102 and 150 eV,respectively (Fig. 3i and j) and that associated with the presenceof sulphur atom at 160 eV, for MRPS (Fig. 3j).

As expected, anhydride-grafted substrates did not show theappearance of new peaks (Fig. 3c–f). The value of O/C ratio for purecellulose is lower than that deduced from theoretical formula (seeTable 3). In fact, this ratio was found to be 0.56, 0.8 and 0.64, forAvicell before and after extraction and ksp, respectively. This dif-ference can be attributed to non-oxidized alkane-type carbon

after modification with different coupling agents.

Fig. 4. Deconvoluted C1s XPS spectra of cellulose surface before and after modification with different coupling agents.

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atoms and originates from impurities like lignin, extractive sub-stances and fatty acids, as reported previously [22–26]. The graft-ing with anhydride decreased slightly this parameter, i.e., from0.8 to 0.74 and 0.66, for PMDA and PBDA, respectively for Avicell

and from 0.64 to 0.51 and 0.52 for PMDA and PBDA, respectively,for ksp. This modest change can be attributed to the chemicalstructure of both molecules, in which the number of carbon atomsis slightly higher than that of oxygen. Diisocyanates have induced a

Table 4Surface energy (in mJ/m2)of cellulose samples as well as the contact angle of a drop ofwater (Hw in �) on their surfaces

Hw cDS cP

S cTS

Av 47 29.8 22.9 52.4PPDI_Av 86 30.7 5.1 35.8ksp 65 34.9 10.0 44.9MDI_ksp 90 26.9 5.8 32.7

Fig. 5. The evolution, within time, of the contact angle of a drop of water depositedat the surface of cellulose before and after modifications with diisocyanates.

Table 3XPS analysis of cellulose samples, before and after modification with different coupling agents

C1 C2 C3 C4 O/C N/C Si/C

CASi CAC/CAH CAN CAO C@O OAC@O283.3 284.9 286.0 286.6 287.8 289.1

Av – 26.0 – 56.9 14.8 2.3 0.56 – –Av (purified) – 15.0 – 72.7 12.4 0.0 0.80 – –PMDA_Av – 11.4 – 69 17.1 2.4 0.74 – –BPDA_Av – 23.5 – 9.6 50.5 16.4 0.66 – –PPDI_Av – 46.4 21.9 9.3 12.0 10.4 0.28 0.22 –ksp – 30.1 – 46.7 18.7 4.4 0.64 – –ksp (purified) – 20.3 – 61.5 16.4 1.7 0.69 – –PMDA_ksp – 50.4 – 27.2 11.4 12.5 0.51 – –BPDA_ksp – 58.4 – 22.6 10.4 9.6 0.52 – –MDI_ksp 0.9 75.1 19.6 2.8 0.4 3.4 0.16 0.09 0.19MPS_ksp 0.8 42.2 – 35.4 13.3 9.2 0.70 – 0.10MRPS_ksp 26.5 – 53.6 17 2.8 0.68 – ?

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dramatic decrease of the O/C value (three times lower), becausethese molecules are richer in carbon atoms. Moreover, they aremuch more reactive compared with anhydrides, which inducedprobably a better coverage of the surface. The quite high N/C valuesconfirm these results, as summarised in Table 3. The two silanesdid not alter the O/C value, because they bear an O/C ratio closeto that of pristine cellulosic substrate, but the value of Si/C ratioclearly shows grafting of ksp fibres.

The deconvoluted C1s XPS spectra of pure cellulose should exhi-bit only two peaks (CAO of alcohols and ethers groups and OACAOfor acetal moieties). Actually, the XPS analysis of cellulose revealsalways three C1 peaks at 285.0, 286.7 and 288.3 eV, attributed toC1 (CAH), C2 (CAO) and C3 (OACAO and/or C@O), respectively[22–24]. The additional C1 peak corresponds to non-oxidized ali-phatic carbon atoms originating from impurities, as discussedabove [22–26]. Thus, the decomposed signals of unmodified Avicelland ksp (Fig. 4a and b), respectively confirmed the features previ-ously established for similar materials [22–26]. The purification ofcellulose by soxhlet successive extractions with methylene chlo-ride and acetone removed a substantial part of the aliphatic andaromatic impurities described above. In fact, when cellulose wassubmitted to such purification procedure, the intensity associatedwith C1 peak decreased from 26% to 15% and from 30% to 20%,for Avicell and ksp, respectively, as summarised in Table 3.

Modification of cellulose with PBDA (Fig. 4c and d), for Avicelland ksp, respectively showed an increase in carbon linked withtwo and three bonds to an oxygen atom (C@O/OACAO andOAC@O) at 288.3 eV. The use of BPDA in which the CH moietiesare quite high yielded a significant increase of the C1 peak at285.0 eV, i.e., from 15% to 23% for BPDA-Avicell and from 30% to50.4% and 58.4% for PMDA- and BPDA-ksp, respectively.

The deconvoluted carbon signal for PPDI- and MDI- modifiedcellulose also showed that the magnitute of both C1 and C4 peakshas increased (see Table 3 and Fig. 4e and f), respectively. Moreover,a CAN signal appeared at 286.0 with intensity around 20% for bothisocyanates. Finally, the spectra of the MPS-modified and MRPS-treated cellulose are shown in Fig. 3g and h, respectively. Thedeconvolution of the peak corresponding to carbon atoms in theXPS spectra of the MPS-treated samples revealed that C1 and C4 sig-nals increased significantly (from 30% to 42% and from 4% to 9%, forC1 and C4, respectively). This large variation was attributed to themethylene groups from MPS. The grafting with MRPS thus appearsto be less efficient. It is also worth noting that it seems that ksp wassystematically grafted more effectively compared to cellulose,which can be associated with the more amorphous morphologyof the kraft pulps and their higher contents in hemicelluloses. Bothaspects favour the accessibility of surface hydroxyl groups.

Finally, contact angle measurements were carried out, assummarized in Table 4, which gives the contact angle at the equi-librium formed by a drop of water on both cellulose surfaces. As,expected the initial substrates were quite polar and hydrophilic(polar component of the surface energy of about 23 and10 mJ/m2, for Avicell and ksp, respectively, and a contact angle ofwater droplet of less than 50� and 65�, for Avicell and ksp, respec-tively). The contact angle of a drop of water increased up to 85� and89�, for PPDI- and MDI-treated samples, respectively (see Table 4).Moreover, the evolution of the contact angle of a drop of waterdeposited on the surface of paper before and after treatment showsthat the penetration of the liquid was reduced significantly afterdiisocyanate coupling, as shown in Fig. 5, which gives the evolu-tion, within the first 2 s, of the contact angle of a drop of water

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deposited onto the cellulose surface and shows clearly that the sur-face became much more hydrophobic.

3.2. Composite materials

The estimated at 50 �C storage modulus of CAB was increasedfrom 0.72 to 0.91 MPa, when 30% w/w of Avicell was added, assummarized in Table 5. The chemical modification of these micro-crystalline elements yielded a further improvement of themechanical properties. Thus, the storage modulus of the compos-ites reinforced by BPDA- and BMDA-treated Avicell reached around1.04 and 1.24 MPa, respectively. Treatment with BMDA gave betterresults, probably, because of its higher length and consequentlybetter ability to establish a covalent link between with the residualOH functions of the matrix. It is worth to emphasis that enhancingthe mechanical properties of CAB-based composites results mostprobably from a better dispersion of the modified Avicell into thematrix, which consequently influences the reinforcement effect.Thus, the ”effective” surface area of the fibres has probably in-

Table 5Storage modulus of CAB-based composites (30% w/w of reinforcing Avicell), at 50 �C

CAB Avicell

Pristine BPDA-grafted PMDA-grafted

Storage modulus (MPa) 716 909 1040 1240

Fig. 6. SEM Micrographs of fractured surfaces of CAB-based composites: (a

creased inducing an increased contact surface between the fibresand the matrix.

The chemical modification of Avicell seems to improve the qual-ity of the fibres/matrix interface. In fact, as shown in Fig. 6, beforemodification, the wettability between Avicell and CAB (Fig. 6a),seems to be poor, because of the presence of a gap between thetwo components. This gap is much less pronounced in both caseswere the fibres were modified by the two anhydrides (Fig. 6band c). Such an improved interface corroborated well the enhance-ment of the previously observed mechanical properties observed.

As mentioned in experimental section, this family of composites(natural rubber-based) were prepared by solvent casting process.The mechanical properties of Avicell-reinforced natural-rubbercomposites were also tested and showed that the incorporationof 30% of microcrystalline cellulose particles enhanced themechanical properties of the initial matrix from about 0.5 to lessthat 3 MPa, as summarised in Table 6 (storage modulus taken at50 �C). The treatment of Avicell with MPS further improved the

Table 6Storage modulus of Avicell-reinforced natural rubber composites (at 50 �C)

Treatment Amount of added Avicell (% w/w) Storage modulus (MPa)

None 0 0.48MPS 20 1None 30 2 .9MPS 30 3.8MPS 40 6.2MPS 60 17.2

) untreated Avicel; (b) PMDA-treated Avicel; (c) BPDA-treated Avicel.

Fig. 7. Logarithm of the storage modulus of NR-based composites reinforced withMPS-treated Avicell, as a function of temperature.

Fig. 9. Tg (d) of NR-based composites prepaed in this work.

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storage modulus of the prepared composites. Thus, compositesreinforced by 30% of Avicell and Avicell-MPS-treated displayed astorage modulus of 2.9 and 3.8 MPa, respectively. Increasing theamount of reinforcing elements induces a systematic increase ofthe storage modulus of the corresponding composites, as shownin Fig. 7. In fact, E0 was found to be 1 MPa, 3.8 MPa, 6.2 MPa and17.2 MPa, for 20%, 30%, 40% and 60%-reinforced composites. Thestorage modulus was plotted as a function of the amount of addedAvicell-MPS and showed that it follows an exponential behaviourwith a good correlation coefficient (R2 = 0.97), as shown in Fig. 8.Such an increase could be assigned to the percolation phenomenawhich takes place after a certain amount of added reinforcing ele-ments. Of course, here an assumption was made concerning theweight fraction related to reinforcing elements. Thus, it was as-sumed that, in these composite materials, the weight and volumefractions are very close, given the similar densities of celluloseand the matrices used.

Fig. 9 shows the evolution of the tangent of the loss angle as afunction of temperature for NR-based composites. It displays amaximum between �60 and �50 �C corresponding to energy dissi-pation. This relaxation process, labelled a, involves cooperativemotions of long chain sequences and is therefore associated tothe anelastic manifestation of the glass-rubber transition of theNR matrix in agreement with Tg values reported from DSC mea-

Fig. 8. Dependence between the storage modulus of NR-based composites and theamound of the added reinforcing elements.

surements. It is characterized by its magnitude (Ia), half-heightwidth (La) and temperature position (Ta). When adding lignocellu-losic fibres, La remains roughly constant, but both a decrease of Iaand an increase of Ta are observed.

The magnitude of the a relaxation process, which is related tothe magnitude of the storage modulus drop, depends upon boththe number of mobile entities and their contribution to the compli-ance. It decreases significantly as the fibre loading increases. Thismeans a decrease of the mobile units at Tg. This result may be ex-plained by the decrease of matrix material amount, responsible fordamping properties. However, the relative damping is not relatedsimply to the filler volume fraction and is also influenced by inter-facial conditions in the filled compounds.

The temperature position of the a peak is directly linked to Tg ofthe matrix and to the drop of the composite storage modulus(mechanical coupling effect). The latter tends to decrease the Tavalue. Then, the increase observed for Ta is inevitably ascribed toan increase of Tg. This evolution is most probably attributed tofavourable interactions between both components in the compos-ite material.

In this case, the chemical modification of Avicell seems, also, toimprove the quality of the fibres/matrix interface, as shown inFig. 10. Thus, before modification, the matrix seems to be far fromAvicell elements (Fig. 10a), as witnesses the presence of a gap be-tween the two components. When Avicell was treated with MPS,the resulting modified Avicell (Fig. 10b) is much closer to the ma-trix, which is in agreement with the enhanced mechanical proper-ties discussed above.

4. Conclusions

Cellulose particles were successfully modified using six differ-ent difunctional molecules. The stiff backbone of these graftedagents ensured that one reactive group was preserved and directedoutwards from the fiber surface. The grafting and the preservationof the second function were proven by several techniques. Thesefunctionalized fibers were then exploited in the preparation of cel-lulose-reinforced composites based on OHA or C@C-rich matrices,such as cellulose derivatives with a degree of substitution lowerthan 3 or natural rubber. Thus, fully sustainable composites withgood performances were prepared. They could be considered aspotential candidates to substitute composite materials made upwith classical matrices derived from fossil resources.

Fig. 10. SEM Micrographs of fractured surfaces of NR-based composites: (a) untreated Avicel, (b) MPS-treated Avicel (the amount of reinforcing elements is 30%).

B. Ly et al. / Composites Science and Technology 68 (2008) 3193–3201 3201

Acknowledgements

This research forms an integral part of the SUSTAINPACKproject, funded by the European Community under the 6th Frame-work Program. Wim Thielemans also greatly acknowledges theEuropean Community for his Marie Curie Intra-European Fellow-ship (MEIF-CT-2005-025125).

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