Saline Treatment of fibre

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Short natural-fibre reinforced polyethylene and naturalrubber composites: Effect of silane coupling agents and fibres loading

M. Abdelmouleh a, S. Boufi a,*, M.N. Belgacem b, A. Dufresne b

a LMSE, Faculte des sciences de Sfax, BP 802-3018 Sfax, Tunisiab LGP2, Ecole Francaise de Papeterie et des Industries Graphiques (INPG), BP 65, F-38402 St. Martin dHeres, France

Received 5 February 2006; received in revised form 22 June 2006; accepted 2 July 2006Available online 1 September 2006

Abstract

Composites materials based on cellulose fibres (raw or chemically modified) as reinforcing elements and thermoplastic matrices wereprepared and characterized, in terms of mechanical performances, thermal properties and water absorbance behaviour. Four different cel-lulose fibres with different average lengths were used, namely avicel, technical, alfa pulps and pine fibres. Two thermoplastic polymers, i.e.low density polyethylene and natural rubber, were employed as matrices. Cellulose fibres were incorporated into the matrices, as such orafter chemical surface modification involving three silane coupling agents, namely c-methacryloxypropyltrimethoxy (MPS),c-mercaptop-royltrimethoxy (MRPS) and hexadecyltrimethoxy-silanes (HDS). As expected, the mechanical properties of the composites increased withincreasing the average fibre length and the composite materials prepared using both matrices and cellulose fibres treated with MPS andMRPS displayed good mechanical performances. On the other hand with HDS bearing merely aliphatic chain only a modest enhancementon composite properties is observed which was imputed to the incapacity of HDS to bring about covalent bonding with matrix.2006 Elsevier Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites (PMCs); A. Coupling agents; B. Fiber/matrix bond; B. Interface

1. Introduction

Over the past decade there has been a growing interest inthe use of lignocellulosic fibres as reinforcing elements inpolymeric matrix[15]. The specific properties of this nat-ural product, namely low cost, lightweight, renewable char-acter, high specific strength and modulus, availability in avariety of forms throughout the word, reactive surface

and the possibility to generate energy, without residue,after burning at the end of their life-cycle, motivate theirassociation with organic polymers to elaborate compositematerials. However, it is well known that different surfaceproperties between the fibre and the matrix, i.e. the formeris highly polar and hydrophilic while the latter is, generally,non-polar and relatively hydrophobic, impose the surfacemodification of the fibres surface, in order to improve the

fibre/polymer compatibility and their interfacial adhesion[6]. Without such a treatment, natural fibres embedded ina polymeric matrix generate unstable interfaces and thestress applied to the fibre/polymer composite is notefficiently transferred from the matrix to the fibre and thebeneficial reinforcement effect of the fibre remains underex-ploited. Likewise, the poor ability of the polymer to wet thefibre hinders the homogenous dispersion of short fibres

within the polymeric matrix[7].Several strategies of surface modifications aiming atimproving the compatibility between cellulose fibres andpolymer matrices were recently reviewed[8]. The chemicalmodification using coupling agents bearing two reactivegroups, one of which being likely to react with the OHfunction at the fibre surface, whereas the other one is leftto copolymerize with the matrix, constitutes a highly inter-esting way allowing the establishment of covalent bondingbetween fibres and matrix, thus leading to materials withhigh mechanical properties. Many coupling agents have

0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2006.07.003

* Corresponding author. Tel.: +216 74 274 400; fax: +216 74 274 437.E-mail address: [email protected](S. Boufi).

www.elsevier.com/locate/compscitech

Composites Science and Technology 67 (2007) 16271639

COMPOSITES

SCIENCE AND

TECHNOLOGY
mailto:[email protected]:[email protected]


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been investigated, namely anhydrides, maleated polymer[911], isocyanates[1214], triazines[15]and alkoxysilanes[1620], as recently reviewed [8]. Among these differentreagents, maleated polypropylene (MaPP) or polyethylene(MaPE) gave significant enhancement in tensile and flex-ural strength, ranging from 40% up to 80%, was pointed

out when they are blended with cellulose fibres before mix-ing with matrix[9,11].Silane coupling chemicals present three main advanta-

ges: (i) they are commercially available in a large scale;(ii) at one end, they bear alkoxysilane groups capable ofreacting with OH-rich surface, and (iii) at the second end,they have a large number of functional groups which canbe tailored as a function of the matrix to be used. The lastfeature ensures, at least, a good compatibility between thereinforcing element and the polymer matrix or even cova-lent bonds between them. The reaction of silane couplingagents with lignocellulose fibres (mainly: cellulose and lig-nin) was found to be quite different to compare with that

observed between them and glass surface, in the sense thatwith cellulose macromolecules, only prehydrolyzed silanesunderwent the reaction with cellulose surface[21].

Besides the chemical bonding theory which is the mostevident mechanism by which organosilane coupling agentsare acting, other theories have been proposed, such as theinterpenetrating networks theory which states that thematrix diffuse inside the silane interphase to form an entan-gled network[2225].

Our group has been involved in series of works aiming atunderstanding silanecellulose system [2628]. Thus, theinteraction of silane coupling agents with cellulosic fibres

and the effect of some parameters, such as the pH, the initialamount of silane with respect to cellulose and the adsorp-tion contact time, on their anchoring capability onto thefibre surface have been ascertained. Different spectroscopictechniques have been used to evidence the presence of silaneand to quantify its amount on the substrate and to elucidatethe structure of the anchored siloxane network on the fibressurface[27]. Then, using epoxy and unsaturated polyesterresins we have shown that the fibre treatment with silanecoupling agents bearing functional group able to react withthe matrix enhanced significantly the mechanical strength ofthe final composite[28].

In the present study three silane coupling agents, differ-ent by the functionality of the radical moiety appended tosilicon atoms were used to treat delignified cellulose fibresin order to improve their adhesion to LDPE and NRmatrix. The effect of these treatments on the mechanicalproperties of the ensuing composites and on their wateruptake ability was investigated accordingly.

2. Materials and experimental procedures

2.1. Cellulose fibres

The different types of cellulose fibres used in this work

were:

Commercial microcrystalline fibres (Technocel-50labelled Tech-50) with average length of about 50lm,and specific surface, measured by the BET techniqueusing nitrogen, around 2.5 m2/g.

Technocel-2500 fibres (labelled Tech-2500), with averagelength of about 2.5 mm.

Alfa fibres are bleached soda pulps from the Tunisianannual plant esparto (alfa tenassissima). They had a spe-cific surface (in a dry state) of 3 m2/g and average lengthabout 500lm.

Bleached softwood (pine) fibres with average lengthabout 3.5 mm.

The Avicel fibres used in this work were purchased fromAldrich. The average length of this fibre was about70lm.

2.2. Silane coupling agent

Three commercial silanes chosen for this study (Table 1)

were kindly provided by OSI-WITCO, to whom we areindebted.

2.3. Polymer matrix

The low density polyethylene (LDPE) used in this workhas a melting temperature of 107 C and a degree of crys-tallinity around 2830%, as determined by differential scan-ning calorimetry (DSC) with a heating rate of 10 C/min.Its density at room temperature was 0.9 g/cm3. The naturalrubber (NR) (polyisoprene) was a kind gift of the Technical

Centre, MAPA, Liancourt, France, and was provided as

Table 1The silane coupling agents used in this work

O SiO

O

O O

-Methacryloxypropyltrimethoxysilane

-MPS

Si

O

O

O

Hexadecyltrimthoxysilane

HDS

-mercaptoproyltrimethoxysilane

-MRPS

HS

O

O

O

Si

1628 M. Abdelmouleh et al. / Composites Science and Technology 67 (2007) 16271639


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latex. Its TSC (total solid content), DRC (dry rubber con-tent), and density were 61.58%, 60.1% and 1 g cm3,respectively. The latex particle size was around 1 lm.

2.4. Fibre treatment

The treatment of cellulosic fibres with 3 wt% of the cho-sen silane was carried out in 80/20 v/v ethanol/water med-ium for 2 h. Then, the fibres were filtered and dried at roomtemperature for 2 days, and heated at 120 C under a nitro-gen atmosphere for two hours in order to promote theactual chemical coupling.

2.5. Composite and specimen preparation

Cellulose reinforced LDPE composites were obtained bymixing the matrix with each type of untreated and silane-treated cellulose fibres using a Brabender (FDO 234 Hmodel) with a rotor speed of 50 rpm and mixing chamber

temperature of 170 C during 10 up to 15 min. The ensuingcellulose/LDPE materials were then shaped into 0.5 mmthick sheets by compression moulding at 180 C. Themoulding cycle consisted of 5 min preheating, compressionunder a force of 10 ton for 3 min, and air cooling underload until the mould reached 40 C. CelluloseNR compos-ites were prepared by mixing the cellulose fibres (25 wt%with respect to full composite) into NR latex emulsion.The ensuing dispersion was poured in Teflon mould andsolid films were obtained by evaporating the water at 5060 C for 24 days, until a homogeneous solid film with0.8 mm thickness was formed.

2.6. Characterization of modified fibres

The FTIR spectra were obtained with a PerkinElmerParagon 1000 FT-IR spectrometer working in diffusereflectance mode (DRIFT) with a total of 40 scans andwith a resolution of 4 cm1. 5 mg of fibres was mixed with200 mg of analytical grade KBr and the resulting mixtureground and pressed in order to obtain pellets. All DRIFTspectra were plotted according to the KubelkaMunkfunction.

2.7. Morphological characterization

The average fibre morphology was determined using anoptical microscope equipped with CCD camera and imageanalysis software. These measurements gave the fibrelength and particle diameter distribution.

2.8. Characterization of composites

2.8.1. Differential scanning calorimetry

Differential scanning calorimetry (DSC) was performedwith a PerkinElmer DSC7 equipment, fitted with a coolersystem using liquid nitrogen. Samples, around 10 mg, were

placed in pressure-tight DSC cells and at least two individ-

ual measurements were carried out to ensure perfect reli-ability of measurements. Each sample was heated from 0to +250 C at a heating rate of 10 C/min. The meltingtemperature (Tm) was taken as the peak temperature ofthe melting endotherm.

2.8.2. Tensile testingThe strength of cellulose fibre reinforced LDPE compos-ites was determined using an Instron 4301 universal Testingmachine. Tests were carried out according to ASTM stan-dards D638 using a 100 N load cell and 50 mm/min cross-head speed. The specimens were thin rectangular strips(5 10 0.5 mm of width, length and thickness, respec-tively). Tensile strength and modulus values correspondto the average of four samples.

2.8.3. Dynamic mechanical analysis (DMA)

Dynamic mechanical tests were carried out with a RSA2spectrometer from Rheometrics working in the tensile

mode. The value of 0.05% for the strain magnitude waschosen in order to fall into the linear domain of viscoelas-ticity of the material. The samples were thin rectangularstrips with dimensions of about 30 5 0.5 mm3. Mea-surements were performed in isochronal conditions at1 Hz, and the temperature was varied between 120 upto 120 C at a rate of 3 C/min. This setup measured thecomplex tensile modulus E*, i.e. the storage, E0, and theloss, E00, components, as well as their ratio (E00/E0), i.e.tand.

2.8.4. Scanning electron microscopy (SEM)

The tensile fracture surfaces of the composites, contain-ing 15% (v/v) of alfa fibres, were examined with a scanningelectron microscope (ABT-55). Fracture surfaces of thecomposite samples were coated with gold and then ana-lysed at 7 keV. Before the fracture, the specimens were fro-zen into liquid nitrogen to impede the plastic deformationof the matrix and to get well defined fibrematrix interface.

2.8.5. Water absorption

The sample dimensions for water absorption experi-ments were1 1 cm 0.5 mm. A minimum of two sampleswere tested for each material. Samples were weighted andthen soaked in distilled water at 25 C. The samples wereremoved at specific time intervals, blotted to remove theexcess water on the surface and immediately weighed.The difference between the mass after a given time ofimmersion and the initial mass compared to the initial massled to the determination of the water absorption.

3. Results and discussion

3.1. Fibre modification

The optimal conditions ensuring efficient chemicalanchoring of the organosilane coupling agent onto cellu-

lose fibres have been established previously [26]. In this

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study, the grafting of the coupling agent was performedthrough a physical adsorption of the hydrolyzed silane fol-lowed by a curing process at 120 C under inert atmo-sphere. This treatment guaranteed irreversible chemicalbonding of the silane onto the cellulose surface, as deducedfrom FTIR and ESCA spectroscopy as well as contact

angle and IGC measurements.Fig. 1presents DRIFT spectra corresponding to the cel-lulose samples (Tech-50) treated with MPS, before andafter heat treatment. In both cases, the spectrum corre-sponding to untreated cellulose was deduced, in order toemphasize the band issuing from the silane moiety. Thetwo spectra show different bands at 1712 and 1637 cm1,which are associated with the stretching vibrations of theC@O and C@C groups of the acrylic moiety, respectively.The broad shape of the C@O peak suggests that this groupis in interaction with the surface through hydrogen bondswith the hydroxyl groups. The broad intense bands around1200 and 1135 cm1 were assigned to the stretching of the

SiOCellulose and SiOSi bonds, respectively [1,29].The strong increase in the intensity of these bands afterthe heat treatment suggested that the grafting of silane

onto cellulose as well as the intermolecular condensationbetween adjacent adsorbed SiOH groups were substan-tially enhanced. The peaks near 1100 and 1080 cm1 arerelated to residual unhydrolyzed SiOCH3groups and theirsmall intensity indicates that most of the silane adsorbedunder these conditions was actually hydrolyzed. The large

band around 1015 cm1

, present in the spectrum of theuncured sample, was attributed to SiOH groups. Thisband disappeared after the heat treatment and wasreplaced by a wide band around 1040 cm1, characteristicof SiOSi moieties. These peak assignments are inagreement with those reported in other studies dealing withglass surfaces treated with the same coupling agents[29].

3.2. LDPE matrix

In the first part of our study relatively long fibre Tech-2500 were used to prepare LDPE-based composites bythorough mixing of the fibres with the polymer followed

by a compression moulding step to form a composite filmwith uniform thickness. By this method homogenous com-posites can be obtained up to 50 wt% fibre loading.

1850.0 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1580 1560 1537.4

0.014

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.180

cm-1

K-M1636.36

1713.72

1701.27

C=C

C=O

1250.0 1220 1200 1180 1160 1140 1120 1100 1080 1060 1040 1020 1000 980 960 940 920 900.2

cm-1

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

b

a

Si-O-Si + Si-O-Cellulose

Si-O-Si

Si-O-Si

Si-OH

Si-OCH3

Fig. 1. DRIFT substraction spectra of cellulose MPS-modified fibres (Tech-50) (a) before and (b) after heat treatment (9001250 cm1 region). Inset:

18501550 cm

1 region after heat treatment.



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The evolution of tensile stress vs strain curves at differ-ent fibre contents is shown in Fig. 2. All the curves displaya linear Hookean range at low strain (


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exhibits two relaxation processes located around 20 C(labelled a) and 6065 C (labelled ac). They are associated

with the chain segment mobility of the polymer branchesand to molecular motion within the crystalline phase,respectively. Nitta and Tanaka[30]attributed the acrelax-

ation to a relaxation process occurring in the partially-ordered interface between the lamellae and the amorphousregion and can be defined as the glass transition of the ste-rically hindered interfacial region. The storage modulusE0

(Fig. 5a) decreases by a factor 10 and 100 across the a andacrelaxation zones, respectively. Around 110 C an abrupt

drop inE0

occurred as a result of the melting of the crystal-line phase. The incorporation of Tech-2500 fibres results inan appreciable rise in E0, particularly above the a relaxa-tion temperature range confirming the reinforcing effectof Tech-2500 fibres. Thus, at 20 C, E0 was multiplied bya factor two compared to the unfilled matrix. The arelax-ation did not exhibit any change in its temperature position(Fig. 5b), whereas the acrelaxation was shifted from 60 to67 C after the fibres addition which could be attributed tosegmental immobilization of the polymer chain at the fibresurface.

Depending on the silane structure, the fibre treatmentaffected both E0 and tand curves, as depicted in Figs. 5a

and b, respectively. To analyse this effect, the relative mod-ulus E0r and the relative loss factor tandr were evaluatedusing Eq.(1) for each treatment

E0r

E0c

E0m

and tan dr tan dctan dm

; 1

where E0c and E0mare the storage modulus of the composite

and unfilled matrix, respectively, estimated at the sametemperature, and tandcand tandmthe corresponding mag-nitude of the ac relaxation process.Figs. 6 and 7 show thevalues ofE0rand tandr, respectively, for both untreated andsilane-treated cellulose fibres (Technocel-50, Technocel-

2500 and alfa fibres) reinforced LDPE composites (fibrecontent = 50 wt%). The relative storage tensile moduluswas estimated at T= 20 C.

By comparing the effect of untreated cellulosic fibres, weobserved that the relative reinforcing effect is significantlyhigher for Tech-2500 than for Tech-50. This effect isascribed to the average length of the fibres, which is wellknown to affect the mechanical performances of compos-ites. At the same time, the relative magnitude of the acrelaxation process decreases. It is well known that its valuedepends on both the number of mobile units participatingto the relaxation process and to the magnitude of the mod-ulus drop through the relaxation. The behaviour of

untreated alfa fibres reinforced LDPE is intermediatebetween Tech-50 and Tech-2500.

For treated fibres reinforced LDPE, it can be seen thatonly MPS and MRPS enhanced the storage modulus,except for Tech-50 reinforced composites for which no sig-nificant reinforcing effect was observed. The increase in E0

at 20 C is about 15% and 35% for MPS and MRPS,respectively, in presence of Tech-2500 and about 12%and 20% with alfa fibres. This improvement could beattributed to the increase in the interfacial work of adhe-sion between fibres and matrix through the interface. Asa consequence, the relative damping properties decrease.

One can ask about the possible mechanism of interaction

50

100

150

200

250

300

350

400

LDPE Te ch Te ch -MPS T ech -MRPS T ech -HDS

Mod

ulus(MPa)

0

2

4

6

8

10

12

14

16

18

Strength(MPa)

Modulus

Strength

Fig. 4. Youngs modulus and tensile strength of LDPE-based compositesreinforced with 50 wt% Tech-2500 cellulosic fibres submitted to differentfibre surface treatments.

0

0.05

0.1

0.15

0.2

0.25

-100 -80 -60 -40 -20 0 20 40 60 80 100

Temperature C

Tang

Tech-2500

Tech-2500/MPS

Tech-2500/MRPS

Tech-2500/HDS

LDPE

transition

ctransition

6

6.5

7

7.5

8

8.5

9

9.5a

b

-100 - 80 -60 -40 -20 0 20 40 60 80 100 120

Temperature C

log(E')

LDPE

Tech-2500

Tech-2500/MPS

Tech-2500/MRPS

Tech-2500/HDS

8

8.5

9

0 10 20 30 40 50

Fig. 5. Evolution of the (a) logarithm of the storage tensile modulus E0,and (b) loss angle tangent tan d vs temperature at 1 Hz for LDPE-basedcomposites reinforced with 50 wt% Tech-2500 cellulosic fibres submittedto different fibre surface treatments. The inset is an expanded view of

log E0 vs temperature curves.



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between these agents and LDPE matrix, which gave rise tothe improvement of the interfacial adhesion in this system.Both MPS and MRPS contain functional groups which canreact with radical species to generate a covalent bond;either by an addition reaction with the p-bond (for MPS)or through a transfer reaction with the mercapto moiety.The radical species may be generated through peroxidedecomposition which arises from the thermal oxidationlikely to occur during the processing of the composite.The resulting reaction gives rise to chemical bondingbetween the fibres and the matrix which enhance the inter-facial adhesion. Scheme 1 depicts possible reactionsbetween MPS grafts and LDPE matrix where the radicalspecies result from thermal degradation during the mixingand compressing process. Indeed, even though the process-ing temperature and the kneading time on the Brabenderare relatively low, i.e; 170 C and 15 min, respectively, thepresence of cellulose fibres is cited to promote the thermaldegradation of the composite [31,32]. The significant yel-lowing of the filled sample after mixing and compressionmoulding support this hypothesis. Espert et al.[32]showed

that the addition of cellulose fibres to PP matrix affected

appreciably the thermo-oxidative stability and reducedthe oxidation temperature from 205 C for unfilled 2.5 %ethyl vinyl acetate modified-PP to about 175 C for com-posite containing 40% w/w kraft pulp fibres.

Despite the presence of a long aliphatic chain on HDS,which imparts hydrophobic character to the fibre surface,lowers its surface energy and increases its wettability anddispersion within the non-polar matrix, no significant evo-lution is noted for E0

r

. This effect may be ascribed to the factthat the only van der Waals type interactions could beestablished with the polymer matrix, leading to a modestenhancement of adhesion between the fibres and thematrix. Soxhlet extractions of LDPE or NR matrices usingxylene for LDPE and a mixture THF/xylene for NR werecarried out in order to establish if there are any continuousbonding between the fibres and the matrix. No traces ofdissolution after several days extraction have occurred withLDPE, and some part of NR has been extracted leaving ahigh amount of the matrix as a swelled gel, indicating theoccurrence of cross-linking of polyisoprene chains. Theseexperiments corroborate the proposed mechanism of inter-

action depicted inScheme 1.

0

0.5

1

1.5

2

2.5

3

E'r

LDPE 0 MPS MRPS HDS

Tech-50

Alfa

Tech-2500

Fig. 6. Relative storage tensile modulusE0restimated at 20 C for LDPE-based composites reinforced with 50 wt% Techn-50, Alfa or Tech-2500 cellulosicfibres submitted to different fibre surface treatments. LDPE refers to the unfilled matrix and 0 to untreated fibres-based composite.

0.5

0.7

0.9

1.1

Tan

r

LDPE 0 MPS MRPS HDS

Tech-50

Alfa

Tech-2500

Fig. 7. Relative loss factor tandr for LDPE-based composites reinforced with 50 wt% Tech-50, Alfa or Tech-2500 cellulosic fibres submitted to differentfibre surface treatments. LDPE refers to the unfilled matrix and 0 to untreated fibres-based composite.



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The tandr values undergo an opposite trend than E0r,

since it decreased after treatment with reactive MPS andMRPS silane and remained unchanged in presence ofHDS. Given the fact that tan d (tand= E00/E0) providesan indication of the fractional energy lost in the systemby the deformation, the decrease in tan drcould be ascribedto the movement restriction of the polymer chain in the

vicinity of the fibres surface induced by the chemical bridg-ing through the interface. It is worth noting that, indepen-dently of the silane structure, ac transition did not exhibitany shift in its position after fibre treatment. This resultis in disagreement with other reactive reagents whichinduce a small shift ofa transition in presence of PE matrix[33].

SEM observations of fractured surface for LDPE-basedcomposite reinforced with 50 wt% Tech-2500 fibres werealso carried out to investigate the fibrematrix interfaceand compare their aspect after silane treatment. The sur-face of fracture for untreated fibres-based composite is

shown inFig. 8a for two different magnifications. It is clearthat the interfacial adhesion between the fibres and thematrix is poor. Indeed, the fibres are pulled out from thematrix giving rise to holes and their surface seems to beclean, intact and free from any adhering polymer. Whenusing MPS treatment (Fig. 8b), the fibres are broken offnear the surface and do not leave any voids on the frac-tured surface. Likewise no discontinuity is reportedbetween the two phases and the fibres seem to be totallylayered by the matrix. This observation gives direct evi-dence about the adhesion improvement at the interface inthe presence of reactive silane. Compared to untreatedfibres, the treatment with HDS (Fig. 8c) did not improvethe adhesion in the interfacial region as indicated by theconsiderable pulling-out of the fibres from the surfaceand by the absence of any physical contact between bothcomponents. These observations are in agreement withthe previous results and support further the hypothesis for-mulated from DMA experiments.

3.3. Natural rubber matrix

Natural rubber (NR) is one of the most important elas-tomers, widely used in different industrial sectors. NR isoften reinforced by the incorporation of filler such as car-

bon black silica and short fibres. The use of natural fibres

to reinforce rubber was the subject of many reports [3438]. As reported in Section 2, NR-based composites wereprepared by direct dispersion of cellulosic fibres in theNR latex. The presence of water makes easier the disper-sion of the fibres within the matrix without prerequisiteheating. However, with such a procedure the fibres loadingcannot exceed 27 wt%.

The plot of the logarithm of the storage tensile moduluslog E0 and tand at 1 Hz vs temperature for unfilled NRmatrix, and both unmodified and modified Tech-2500 rein-forced composites are shown inFig. 9a and b, respectively.At low temperatures, the NR matrix is in the glassy stateand E0 remains roughly constant around 2 GPa. Then atca. 60 C a sharp drop in E0, by more than 2 decadesoccurs, associated with the glassrubber transition of theelastomer. This relaxation phenomenon involves coopera-tive motions of long chain sequences which induce dissipa-tion energy revealed by a maximum in tand (Fig. 9b).Beyond this relaxation process, a rubbery plateau is

attained for which the modulus slightly decreases whenfibres are incorporated in the matrix and then remainsroughly constant above 0 C over a wide temperaturerange. This relaxation was attributed to the melting ofthe low amount of crystalline regions of NR generated bythe fibres incorporation which act as a nucleating agent.The crystalline regions behaving as filler particles due totheir finite size, which would increase the modulus substan-tially. In a previous study [39], we have shown that themodulus drop around 0 C was attributed to the meltingof the low amount of crystalline regions of NR duringthe temperature scan.

The incorporation of cellulose fibres does not increasesignificantly the composite modulus below Tg. This effectmay be ascribed to the fact that in its glassy state the differ-ence between the modulus of the matrix (2 GPa) and theone of the fibres (1015 GPa) is not high enough to gener-ate a remarkable reinforcement effect at 25 wt% loading.However, above Tg, a higher increase in the modulus isobserved, i.e. it shifts from 0.9 to 3.6 MPa at 20 C. Thesurface treatment of the fibres with silane coupling agentsgreatly affects the rubbery plateau modulus (Fig. 9a). From3.6 MPa for the untreated fibres-based composite, it jumpsto 10, 12.6 and 6.3 MPa for the MPS, MRPS and HDStreated fibres, respectively. Likewise, the magnitude of

the tan d peak decreases significantly upon the addition

Cellulosicfiber

MPS

modified fiber

PE

Matrix

Interface

Scheme 1. Schematic illustration of the interfacial zone in LDPE-based composites containing MPS-modified cellulose fibres. Note that the relative size ofthe fibres and of the chemical species are not on scale.



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of the filler and its treatment emphasizes this effect. How-ever, the temperature position of the relaxation did notundergo any appreciable evolution. As in the case of theLDPE matrix, the appreciable improvement of the modu-lus in presence of reactive MPS and MRPS reactive silaneis related to their ability to react with the matrix during thecuring process and their capacity to establish covalentbonds between the fibres and the matrix through the inter-face, thus enhancing the adhesion between the two phases.Evidence of a better enhancement of the fibreNR adhe-

sion is also confirmed by SEM observation of frozen frac-

tured surface of specimen (Fig. 10) which reveals that thefibres are totally fractured under at the surface level whenthey are treated with MPS or MRPS. On the other handsome pulling out occurs when the fibres are treated withnon-reactive HDS.

Scheme 2gives a schematic representation of the differ-ent possible reactions occurring in presence of MPS andMRPS silanes which are promoted by radical grafting reac-tion between the C@C unsaturations of NR and terminalacrylic moieties of MPS or SH groups of MRPS . The

radical species could be generated from peroxy (or similar)

Fig. 8. SEM micrographs of freshly fractured surfaces of LDPE-based composites reinforced with 50 wt% Tech-2500 cellulosic fibres: (a) untreated,(b) MPS-, and (c) HDS-treated fibres.



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formed during the evaporation period. These results con-firm again that reactive silane coupling agents are suitablefor the modification of cellulose fibres, as reinforcing ele-ments for non-polar thermoplastic polymeric matrices.

3.4. Effect of the fibre length

One of the most important parameters controlling the

mechanical properties of short fibres composite besidesthe interfacial shear strength (determined by the interfacialadhesion) is the fibre length or more precisely its aspectratio, viz. the ratio between its length and diameter. Thisparameter has not been much explored in the case of natu-ral fibres and deserves some attention. With this aim inview, cellulose fibres with different lengths were used, assuch or after silane-treatment, to prepare LDPE and NR-based composites. For the former, the amount of fibreswas fixed to 50 wt%, whereas for the latter, only 25 wt%of reinforcing elements was incorporated.

Tech-50, avicel, alfa, Tech-2500 and pine fibres having

an average length around 0.05, 0.07, 0.5, 2.5 and 3.5 mm,

respectively, have been used to prepare LDPE and NR-based composites. The relative modulus E0r at 20 C andloss angle tangent magnitude tandr were determined fromEq.(1) for untreated fibres-based composites. Results areshown inFigs. 11 and 12, respectively. It is observed thatfor LDPE-based composites reinforced Tech-50, avicel

and alfa fibres whose average length is lower than0.5 mm, the modulus is raised by a factor 150% withrespect to the unfilled matrix, whereas in the presence oflonger fibres (2.5 and 3.5 mm), this factor is multiplied by200% and 300%, respectively. NR-based composites pre-pared using avicel and Tech-2500 were also investigated(Figs. 11 and 12). Again, higher stiffness of the compositewas reported for longer fibres. For a given fibre type, therelative reinforcing effect is higher for NR-based compos-ites than for LDPE-based materials despite the higher fillercontent for the latter. This is ascribed to the crystallinity ofLDPE.

This trend is quite expected for short fibre reinforced

composites, as predicted by Cox model which correlatedthe modulus to the fibre volume fraction and length, as fol-lows[40]:

Ec Ef/f 1tanhbl=2

bl=2

Em/m; 2

b 2 GmEf r2fLnR=rf

1=2; 3

lcr rmaxf

s ; 4

where Ef, and Emand correspond to the fibre and the ma-

trix modulus, respectively; /f, /m to their respective vol-ume fraction. l, r, rmaxf and s are the fibre length, thefibre radius, the tensile strength of the fibre and the shearstrength of the fibrematrix interface, respectively. GmandR are the fibres shear modulus and the interval amongthe fibres. According to Cox model (Eq.(2)), the modulusof the composite remains roughly constant up to a criticallength lc, above which it grows rapidly. Then it reaches asecond plateau whose value is close to that of a compositewith continuous fibres. Belowlcthe reinforcing effect is lowand the fibres are under-exploited. Even though, Cox ap-proach makes the assumption of parallel fibres, the depen-dence of the modulus as a function of the length follows thesame trend with fibres having different orientations. Itshould also be noted that the fibres radius remainedroughly constant (about 57 lm), as confirmed by SEMobservations.

Referring to Cox approach, one could ask howEccouldevolve after the silane treatment, since it depends only onthe aspect ratio of the fibres and should not reflect the cou-pling efficiency. This discrepancy could be rationalized ifwe assume that treatment with reactive silanes improvedthe fibre matrix adhesion and therefore led, according tothe expression oflc(Eq.(4)), to a decrease in the fibres crit-ical length lc. Hence, for the same fibres length Ec will be

higher for the composite with lower lc.

6

6.5

7

7.5

8

8.5

9

9.5a

b

-80 -60 -40 -20 0 20 40

Temperature C

l

Tang

og(E')

Tech-2.5/NR

Tech-2.5/HDS-NR

Tech-2.5/MPS-NR

Tech-2.5/MRPS-NR

NR

0

0.2

0.4

0.6

0.8

1

1.2

-100 -80 -60 -40 -20 0 20

Temperature (C)

Tech-2.5/-NR

Tech-2.5/-HDS-NR

Tech-2.5/MPS-NR

Tech-2.5/MRPS-NR

Fig. 9. Evolution of the (a) logarithm of the storage tensile modulus E0,

and (b) loss angle tangent tand vs temperature at 1 Hz for NR-basedcomposites reinforced with 25 wt% Tech-2500 cellulosic fibres submittedto different fibre surface treatments.

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Fig. 10. SEM of fractured surfaces, frozen under liquid nitrogen, of natural latex composites based on long viscose fibres: before treatment (a) and aftertreatment with MPS (b), and HDS (c).

Cellulosicfiber

MPSmodified fiber

NRMatrix

Interface

peroxyde radical

Scheme 2. Schematic illustration of the interfacial zone in NR-based composites containing MPS-modified cellulose fibres. Note that the relative size of

the fibres and of the chemical species are not on scale.



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The effect of the silane treatment on the mechanical per-formances of the LDPE-based composites in presence offibres with different length was investigated and the relative

storage modulus E0rat 20 C for both unmodified and mod-ified fibres was determined. A similar trend as the onereported inFigs. 6 and 7 was observed provided that thefibres average length was higher than 0.5 mm. For shorterfibres, the improvement induced by the silane treatmentwas modest. Thus, the treatment of the cellulosic filler withMRPS results in an increase of the modulus by a factor of250% for Tech-2500 and only 25% for Tech-50 in LDPE-matrix at 50 wt%. The same behaviour was reported forTech-2500 and avicel reinforced NR composites. As thefibres length is lower than lc, the reinforcing action is mod-est and any improvement in the interfacial adhesion would

not provoke noticeable effect on E0

.

3.5. Water absorption properties

The most serious handicap related to the use of lignocel-lulosic fibres in composite materials is their extreme sensi-tivity to water, which reduces dramatically theirmechanical performances in a damp atmosphere. The evo-lution of the water absorption as a function of the immer-sion time is shown inFig. 13for LDPE-based compositesreinforced with 50 wt% alfa fibres submitted to differentfibre surface treatments. For all composites, the waterabsorption was found to increase with immersion timereaching a plateau after about 6 days. For unfilled LDPE

the water absorption is very low due to the apolar natureof this polymer. For composites, the equilibrium wateruptake was found to depend on the treatment of the fibres.Thus, the LDPE composites containing untreated fibresabsorbed 3.5% of water, whereas the composites basedon HDS-, MPS- and MRPS-treated fibres absorbed 2.2%,2.6% and 3% of water, respectively. The treatment of thefibres with HDS did not bring about significant reductionin water absorption compared to MPS or MRPS, despitethe marked hydrophobic character of the ensuing modifiedfibre. It seems therefore that the covalent bonding betweenthe fibres and the matrix is also necessary to reduce thewater absorption by the composite. If one considers thatwater molecules will be accumulated within the fibres struc-ture, than chemical linkage between the fibres and thematrix will inevitably reduce the expansion possibility ofthe fibres which in turn reduces the water uptake by thecomposite material.

4. Conclusion

This work clearly shows that cellulose fibres can be effec-tively used as reinforcing elements in thermoplastic LDPEand NR matrices. It constitutes a good alternative to glassinorganic fibres commonly used as a reinforcing material.

This approach became successful thanks to coupling agents

0

0.5

1

1.5

2

2.5

3

3.5

4

E'r

Tech50 Avicel Alfa Tech2500 Pine Avicel Tech2500

LDPE-matrix NR-matrix

Fig. 11. Relative storage tensile modulus E0r estimated at 20 C for LDPE-and NR-based composites reinforced with 50 wt% and 25 wt%, respec-tively, untreated cellulose fibres presenting different average lengths.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Tan

r

Te ch 50 Avi ce l Al fa Te ch 25 00 Pi ne Avi ce l Te ch 25 00

NR-matrixLDPE-matrix

Fig. 12. Relative loss factor tandr for LDPE- and NR-based compositesreinforced with 50 wt% and 25 wt%, respectively, untreated cellulose fibrespresenting different average lengths.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12 14 16 18 20

Time (days)

Wateruptake(%)

Tech2500 + MPS

Tech2500 + MRPS

Tech2500 + HDS

Tech2500

PE

Fig. 13. Water uptake versus immersion time at 25 C for LDPE-basedcomposites reinforced with 50 wt% alfa fibres submitted to different fibresurface treatments. The lines serve to guide the eye.



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from silane family. It can be believed that from the combi-nation of adequate fibres (especially in terms of aspectratio) and optimal grafting agents and reaction conditions,one can succeed in the preparation of good performancecomposite materials containing renewable naturalresources. Works are under progress to verify whether

reactive silane coupling agents could effectively enhancethe interfacial adhesion in the presence of polypropylenematrix.

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