Short palm tree fibers – Thermoset matrices composites

Composites: Part A 37 (2006) 1413–1422 www.elsevier.com/locate/compositesa

Short palm tree fibers – Thermoset matrices composites Hamid Kaddami a, Alain Dufresne b,*, Bertine Khelifi b, Abdelkader Bendahou a, Moha Taourirte a, Mustapha Raihane a, Nathalie Issartel c, Henry Sautereau c, Jean-Franc¸ois Ge´rard c, Noureddine Sami d a

Universite´ Cadi Ayyad, Laboratoire de chimie bioorganique et macromole´culaire, Faculte´ des Sciences et Techniques Gue´liz, av. A. El Khattabi, BP 618, Marrakech, Morocco b Ecole Franc¸aise de Papeterie et des Industries Graphiques (EFPG-INPG), BP 65, F38402 St. Martin d’He`res Cedex, France c INSA, Laboratoire des Mate´riaux Macromole´culaires/IMP, 21 av. Albert Einstein, Villeurbanne 69621, France d Socie´te´ Excelsa, Appentis Hangar 67, Ae´roport Casablanca Anfa, Casablanca, Morocco Received 28 June 2004; received in revised form 29 June 2005; accepted 29 June 2005

Abstract The use of short palm tree lignocellulosic fibers as a reinforcing phase in polyester and epoxy matrices has been reported. The morphology and the mechanical properties of the resulting composites were characterized using scanning electron microscopy analysis, differential scanning calorimetry, dynamical mechanical analysis and three-point bending tests. It was shown that the interfacial adhesion was better in the case of epoxy-based composites. In order to improve interfacial adhesion the esterification of the lignocellulosic filler in alkaline medium was performed using acetic and maleic anhydrides. Such type of chemical modification, which led to a change in the chemical composition of the filler, only succeeded to improve mechanical properties of the epoxy-based composites.  2005 Elsevier Ltd. All rights reserved. Keywords: A. Thermosetting resin; B. Mechanical properties; B. Interface/interphase; D. Electron microscopy

1. Introduction Natural fibers reinforced composites combine good mechanical properties with a low density. Such composites offer a number of well-known advantages which include low cost, availability of renewable natural resources, biodegradability, etc. The use of natural polysaccharides as a filler in polymers has been gaining acceptance in commodity polymers applications for many years. Enhanced properties have been obtained by using natural cellulosic fibers such as sisal [1,2], cotton [3], bamboo [4], straw [5], jute [6], kenaf [7], banana [8] and wood [9,10]. Only few studies were reported on polymers reinforced with lignocellulosic fibers obtained from palm tree [11–21]. These studies were

*

Corresponding author. Tel.: +33 4 76 82 69 95; fax: +33 4 76 82 69 33. E-mail address: [email protected] (A. Dufresne).

1359-835X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.06.020

mainly performed by research teams from Malaysia and India and concerned oil-palm fibers. Polyolefins, such as polypropylene (PP), are the most widely used matrices for the processing of natural fiber based composites [12,22–25]. Other thermoplastic polymers were also used including poly(vinyl alcohol) [26], polyvinyl chloride [27], polystyrene [28], or natural rubber [14,15, 20,21]. Thermoset polymers can also be used as matrices in the processing of natural fiber composites. Polyester matrices [11,29–35] were more widely used compared to epoxy [36–38]. For both thermoplastic and thermoset based composites, the applications concern mainly automotive industry or domestic objects [39,40]. Unfortunately, the use of natural fibers can be limited in industrial applications due to some well-known drawbacks which may lead to composites with poor final properties. In fact, the inherent polar and hydrophilic nature of lignocellulosics and the non polar characteristics of most of the

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conventional industrial polymers (PP, unsaturated polyester, etc.) result in a lack of compatibility which leads to difficulties in achieving a high level of dispersion of the reinforcing phase within the host polymer matrix and causes a poor interfacial adhesion. These two factors determine to a large extent the final performances of the composite. Several methods have been tested to enhance adhesion between the lignocellulosic filler and the polymer matrix. They generally involve fiber and/or matrix modification by physical or chemical methods. In chemical methods, for apolar thermoplastic matrices such as PP, several coupling agents have been used to modify the nature of the interactions at the fiber surface. Among these coupling agents, maleic anhydride graft PP homopolymers or copolymers have been widely used [23,24]. On the other hand, silicon alkoxides such as alkyl triethoxysilanes [8,29,36,41] or pure organic coupling agents [32,42] have been widely used to modify lignocellulosic fibers for processing with thermosets/lignocellulosic fibers composites. These modifications allow to enhance properties of the composite materials such as the mechanical properties and the water uptake. It is well known in date producing countries that the date palm trees need to be maintained each year after the fruit harvesting. The upkeep of the date palm trees results in the production of a huge renewable amount of palms. These renewable resources are used as biofuel for domestic purposes. The valorization of this abundant lignocellulosic residual source as reinforcement in polymer composites worth to receive attention during the last three years. The aim of the present study is to process thermoset matrix/ palm tree lignocellulosic fibers composites and characterize their morphological, thermal, and mechanical behavior. For such a purpose, two thermosetting matrices have been used: (i) an unsaturated polyester and (ii) an epoxy/amine. Moreover, the influence of the chemical modification of the palm tree lignocellulosic fibers on the composites performances was also studied. 2. Experimental 2.1. Materials 2.1.1. Polymer matrices The polyester used for the study was an unsaturated polyester resin G154TB (containing 31 wt% of styrene monomer and having a gelation time at 25 C for 30 min). It was obtained from Cray Valley/Total. Methyl ethyl ketone peroxide, MEKP, and cobalt octanoate, used as initiators, were obtained from Aldrich. 0.2 wt% of cobalt octanoate and 2 wt% of MEKP were added to the UP resin before introducing the palm tree fibers. The poly epoxy matrix was obtained by the polymerization reaction of epoxy prepolymer (50 vol%) and amine as a curing agent was HV 953U. The epoxy resin and the curing agent were supplied by CIBA-GEIGY. For both matrices, the curing was done at room temperature for 24 hours. Post curing was per-

formed according to the following cure schedule: 60 C for 2 hours, 90 C for 1.5 hour, 120 C for 1 hour, and 150 C for 30 min. 2.1.2. Filler preparation The lignocellulosic filler was obtained by cutting the palm tree leafs into pieces of about 1.5 cm long and 1 mm width. The filler was then extracted for 24 h in the soxhlet reflux of a solvent mixture composed of acetone/ ethanol (75/25). The bleached fibers were then dried at 80 C for 2 h. The resulting fibers were denoted as unmodified fibers. The modification of fibers was done with acetic anhydride (Aldrich) and maleic anhydride (Aldrich) for 30 min according to the protocol detailed in Scheme 1. The resulting fibers were denoted as modified fibers. The modified fibers were then dried under vacuum at 60 for 24 h. The chemical modification of the fiber was checked by FT-IR in a transmission mode. 2.1.3. Chemical composition of the filler The chemical compositions of the dried palm fibers were determined according to the French Standards (NF T 12011). It allows to determine the weight fraction of cellulose, hemicelluloses, lignin, extractible and ashes. 2.1.4. Composites processing Both the acetate-modified and maleic anhydride-modified fibers were used as a reinforcing phase for composite preparation. The former was used to reinforce the epoxy matrix and the latter was used in association with the unsaturated polyester matrix. Unmodified fibers were also used to reinforce both matrices and the properties of resulting composites were used as references to display the effect of the chemical modification. Therefore, four composites were prepared. The filler content was fixed at 17 wt% for all the composite materials. The composites were prepared using the classical ‘‘contact method’’ which consists on the deposition of the fibers in a rectangular mold and its physical impregnation with the liquid resin. At the end of the stratification the mold was closed and a pressure of 2 bars was applied for 24 h. MODIFICATION WITH ACETIC ANHYDRIDE

MODIFICATION WITH MALEIC ANHYDRIDE

unmodified fibers

unmodified fibers

NaOH (18%)

NaOH (18%)

activated fibers

activated fibers

activated fibers in NaOH 5%

activated fibers in DMF/N(C2H5)3

acetic anhydride acetate modified fibers

maleic anhydride maleic anhydride modified fibers

Scheme 1. Description of the chemical modification of the lignoceullulosic fibres.

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Post curing was made following the temperature schedule given previously. 2.2. Experimental methods 2.2.1. Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was used to investigate the morphology of the different types of materials and the filler/matrix interface by using a ABT-55 microscope. The specimens were frozen under liquid nitrogen, fractured, mounted, coated with gold/palladium and observed using an applied tension of 10 kV. 2.2.2. Differential scanning calorimetry (DSC) A Mettler TA 3000 calorimeter was used to measure the glass transition temperature, Tg. It was taken as the onset temperature of the specific heat increment. The heating rate was fixed at 10 C min1 and scans were recorded under argon atmosphere between 100 and 200 C. 2.2.3. Dynamic mechanical analysis (DMA) Dynamic mechanical analysis (DMA) experiments were performed with a Rheometric viscoelasticimeter, RDA II, equipped for rectangular samples and working in the shear mode testing to measure shear storage and loss moduli, i.e., G 0 and G00 , respectively. This apparatus is especially dedicated to the study of films and composite materials. The average typical dimensions of the composite samples were 20 · 4 · 1 mm3. Tests were performed in isochronal conditions at 1 Hz and each sample was heated from 120 to 200 C at a heating rate of 2 K min1. The maximum shear strain was equal to 0.2%. 2.2.4. Non linear mechanical properties Three-point bending tests were performed according to the international standard ISO 178, to determine the stress at break (MPa), the flexural modulus (GPa) and the total absorbed energy (J). The testing machine was a 2/M type supplied by MTS (load cell: 10 kN). Specimens were parallelepipedic bars with dimensions close to 60 · 10 · 5 mm3 and the distance between supports was fixed at 50 mm. Tests were carried out at the glass–rubber transition (Tg) of the corresponding neat matrix and at Tg 40 K. The results were averaged from data collected on five tested samples. 3. Results and discussion 3.1. Chemical analysis of the fibers Results of the chemical composition of the different fibers are presented in Table 1. It can be clearly seen that the solvent extraction step (purification treatment) results in the elimination of CH2Cl2 extractibles. The surface chemical modification induces a significant decrease of the hemicelluloses content and an increase of the cellulose content. The former observation results from the high solubility of hemicelluloses in alkaline solution. The amount

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Table 1 Chemical composition of the palm tree fibers before (raw dried palm tree fiber) and after purification treatment (unmodified fibers) and chemical modification (modified fibers) Constituent

Raw dried palm tree fiber (wt%)

Unmodified fiber (wt%)

Modified fiber (wt%)

Cellulose Hemicelluloses Lignine Ashes CH2Cl2 extractibles

33.9 ± 1.9 26.1 ± 2.0 27.7 ± 1.2 6.9 ± 0.1 3.5 ± 1.0

33.5 ± 0.5 28.5 ± 1.5 26.5 ± 0.5 6.5 ± 0.5 0

43.0 ± 0.5 20.3 ± 1.2 26.0 ± 1.0 5.2 ± 0.2 0

of lignin and ashes also decreases but not significantly. This modification induces a shortening and a thinning effect of the filler as can be clearly seen after the fibers chemical modification (not shown). It should have an influence on the mechanical properties of the resulting composites. This shortening and thinning effect was also reported by Angle`s et al. [23] and Kallavus and Gravitis [43]. 3.2. Morphological investigation of the interfaces Figs. 1 and 2 show SEMs of freshly fractured surface for composite materials based on unsaturated polyester and epoxy matrices, respectively. Both unmodified and modified fibers reinforced materials were investigated. For each composite material, two different magnifications were used to evidence the effect of the fiber treatment on the interfacial adhesion. For unfilled materials, i.e., thermoset matrices, the fracture surface is rather smooth as expected for brittle polymers, especially for the polyester matrix. By comparing these micrographs with those of the composite materials, the fibers can be clearly identified. The SEM micrographs in Fig. 1 clearly indicate that the interfacial adhesion between the filler and the matrix is poor for unsaturated polyester based materials. This can be readily seen from the absence of any physical contact between both components. The fibers are pulled out from the UP matrix and their surface remains practically clean. On the other hand fracturing the samples did not lead to break the palm tree fiber. Holes result from debonding occurring along the fiber due to the lack of interactions at the interface. It results in a poor stress transfer between the matrix and the filler. It is also well known that unsaturated polyester networks display a large shrinkage after curing [48–50]. This shrinkage reduces the specific volume of the matrix, and due to the weak interfacial interactions it results in free spaces between the matrix and the filler. On the other hand, the SEM analysis shows that the chemical modification did not succeed to improve the adhesion between the polymer and the palm tree filler. Other coupling agents, i.e., organo functional alkoxysilane like vinyl silane or acrylate silanes, should be used for this purpose. The silanols resulting from the hydrolysis of the silanes are expected to react with the hydroxyl groups from the cellulosic fiber surface as reported previously [51].

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Fig. 1. Scanning electron micrograph of freshly fractured surface for an unsaturated polyester film filled with (a) 0 wt%, (b,c) 17 wt% of unmodified and (d,e) 17 wt% of acetate modified palm tree fibers.

On the contrary, for epoxy-based composites, the micrographs in Fig. 2 evidence a good adhesion between the matrix and the filler. One can observe the absence of holes around the fillers on the fractured surface, i.e., no debonding occurs, and the break of fibers during fracture. On the other hand, the region surrounding the cellulosic filler seems to be continuous with the matrix phase. No evidence of the changes in the interfacial adhesion was observed by comparing SEM micrographs of the fractured composites based on modified or unmodified palm tree fillers. This difference in interfacial adhesion between the unsaturated polyester-based materials and the epoxy-based ones is the result of the difference in the nature of physico-chemical interactions which can be created at the interface. This difference can be explained from the values of the solubility parameters of the two polymers. In fact the polarity of the

epoxy resin [48], which is expressed by it high solubility parameter, makes easier its adsorption on the lignocellulosic fibers. As a consequence, the wettability of the fiber surface with the epoxy resin which is a necessary condition for good interfacial adhesion, is better for the epoxy-amine reactive mixture. On the contrary the low solubility parameter of the unsaturated polyester resin [49] makes more difficult its adsorption at the lignocellulosic fiber surface whatever the fiber treatment is. It results in a poor interfacial adhesion. 3.3. Thermal behavior of palm tree fiber-based composite materials As mentioned in Section 2 the thermal behavior of palm tree fibers based composites was investigated using DSC.

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Fig. 2. Scanning electron micrograph of freshly fractured surface for an epoxy film filled with (a) 0 wt%, (b,c) 17 wt% of unmodified fibers and (d,e) 17 wt% of maleic anhydride modified palm tree fibers.

The glass transition temperatures, Tg, of these materials are collected in Table 2. Tg of the unfilled epoxy matrix is close to room temperature and the one of the unsaturated polyester matrix is slightly higher. Table 2 clearly shows that Tg of the unsaturated polyester matrix is almost not affected by the presence of unmodified fibers, whereas the surface chemical modification of the filler with acetate results in a significant decrease of Tg down to 35 C. It could be ascribed to a hindering of the cross-linking process of the matrix in the vicinity of the modified filler/matrix interface. It can be noticed that the free radical polymerization process of such unsaturated polyester matrix is very sensitive to the reactive mixture composition. In fact, the components, i.e., polyester resin, styrene monomer, initiator, etc, can have specific interac-

Table 2 Glass transition temperature, Tg, determined from DSC measurements, main relaxation temperature, Ta, storage shear modulus in the rubbery state, G0c , of the composite materials and relative modulus, G0c =G0m (G0m refers to the storage shear modulus of the neat matrix in the rubbery state) determined from DMA experiments Tg (C)

Ta (C)

Unsaturated polyester-based materials Neat matrix 46 95 Unmodified filler 49 93 Modified filler 35 93 Epoxy-based materials Neat matrix 24 Unmodified filler 32 Modified filler 38

51 57 61

G0c (MPa)

G0c =G0m

12.5 ðG0m Þ 5.5 5.4

1.00 0.44 0.43

3.2 ðG0m Þ 5.2 8.2

1.00 1.63 2.56

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tion with the cellulosic fiber surface leading to changes in local concentrations and formation of microgels. For the epoxy matrix based materials, a significant increase of Tg from 24 to 32 C is reported upon palm tree fibers addition. Modifying the filler with maleic anhydride results in a further increase of Tg. It could be ascribed to strong interactions between both components and possible etherification reactions between carboxylic groups of the esterified cellulosic fibers and epoxy groups of the epoxyamine reactive mixture. As a consequence, the segmental motion of the epoxy chains is reduced. This result is particularly interesting because to our knowledge such an effect was never observed even with polar matrices.

1.00E+10

Shear Modulus (G' / Pa)

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a

1.00E+09

1.00E+08

1.00E+07

1.00E+06

1.00E+05

1.00E+04 -50

-30

-10

10

30

50

70

90

110

130

150

Temperature (˚C) 1

3.4. Mechanical behavior

0.9

3.4.1. Dynamical mechanical analysis The dependence of log G 0 , i.e., logarithm of the storage shear modulus, and tan d, the loss factor, vs. temperature at 1 Hz are displayed in Figs. 3 and 4, for unsaturated polyester based materials and epoxy based materials, respec-

0.8

Tan (delta)

The mechanical behavior of all specimens was investigated in both linear (DMA measurements) and non linear conditions (three-point bending experiments).

b

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -110

-60

-10

40

90

140

Temperature (˚C)

Shear Modulus (G'\Pa)

1.00E+10

Fig. 4. (a) Storage shear modulus G 0 , and (b) loss factor tan d vs. temperature at 1 Hz for epoxy based composites filled with (·) 0, () 17 wt% of non-modified and () 17 wt% of maleic anhydride modified palm tree fibers.

a

1.00E+09

1.00E+08

1.00E+07

1.00E+06 -110

-60

-10

40

90

140

Temperature (˚C) 1 0.9

b

Tan (delta)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -110

-60

-10

40

90

140

Temperature (˚C) Fig. 3. (a) Storage shear modulus G 0 , and (b) loss factor tan d vs. temperature at 1 Hz for unsaturated polyester based composites filled with (·) 0, () 17 wt% of non-modified fibers and () 17 wt% of acetate modified palm tree fibers.

tively. All materials exhibit a relaxation process which is associated to the glass–rubber transition of the matrix. It is displayed through a sharp modulus decrease and the concomitant maximum of the loss factor. This relaxation process, denoted a, involves the release of cooperative motions of the chains between crosslinks. The relaxation temperature values, Ta, corresponding to the maximum of the loss factor are collected in Table 2. It is about 93–95 C for the unsaturated polyester-based materials and 51–61 C for epoxy based materials. An increase of Ta is observed for epoxy-based composites in the presence of the lignocellulosic filler, especially as its surface is chemically modified. This increase confirms the previous conclusions given from the increase of Tg observed from DSC measurements. The magnitude of the main relaxation process of the epoxy matrix is also strongly reduced in composite materials. This is due to strong interfacial phenomena. On the contrary the magnitude of the loss factor is almost unaffected by the incorporation of the lignocellulosic filler within the unsaturated polyester matrix. In the temperature range between 100 and 10 C a secondary relaxation, denoted b relaxation, can be observed for the epoxy-based materials. This relaxation process was reported for numerous epoxy polymers and is associated with the motions of hydroxyether groups. In

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surface and the hydroxyl groups of the poly epoxy matrix. This restriction of the organic chains motions is confirmed by the decrease in the main relaxation peak magnitude and its broadening towards higher temperatures in the composite materials compared to the neat epoxy matrix [47]. However, it should be noted that the increased G0c =G0m values for the epoxy-based composites reported in Table 2 could be also due, as least partially, to an increase of the cross-link density. This assumption can be supported by the increase in Tg values. Generally, the expected reinforcing effect depends on the interfacial adhesion between both components, i.e., matrix and filler, which allows an efficient stress transfer from the matrix to the filler. This adhesion restricts the motions of the polymer chains in the vicinity of the filler surface and leads to an increase of their glass–rubber transition temperature. Based on this argument we can expect a good adhesion between the lignocellulosic filler (modified or unmodified) and the epoxy matrix and a rather bad adhesion between the lignocellulosic filler (modified or unmodified) and the unsaturated polyester matrix. The lack of adhesion in the last case causes the creation of debonded zones at the polyester matrix/filler interfaces which lower the mechanical properties. Moreover, we should

120

a

Neat Polyepoxyde Matrix Composite with modified fillers

100

Composite withnonemodified fillers

Force (N)

80

60

40

20

0 0

4

8

12

16

20

24

Displacement (mm) 80

b

Force (N)

the case of unsaturated polyester based materials, a shoulder on the low temperature side of the a relaxation can be observed. It is also ascribed to a b secondary relaxation of the unsaturated polyester. This relaxation accounts for the motions of molecular groups in the vicinity of the residual maleic groups [42,43] and the corresponding relaxation temperature depends on the amount of styrene monomer in the unsaturated polyester formulation. On the other hand, at very low temperature (around 100 C) an illdefined relaxation process is observed and attributed to a c relaxation which accounts for the restricted motions of the phenyl groups of the styrene cross-links [44–46]. From the dependence of log G 0 vs. temperature, it is difficult to observe any significant effect of the filler at low temperature, i.e., in the glassy state. A simple mixing rule allows accounting for this fact. As it is well known the exact determination of the sample glassy modulus depends on the precise knowledge of the sample dimensions. On the other hand, the water absorption could affect the exact determination of the glassy modulus. Therefore, the reinforcing effect of the filler was estimated in the rubbery region of the polymer matrix. The values of the rubbery shear moduli are reported in Table 2, as well as the relative rubbery modulus values corresponding to the ratio of the rubbery modulus of composites, G0c , divided by the one of the neat matrix, G0m . Because the modulus is not perfectly constant against the temperature, G 0 values reported in Table 2 correspond to an average value. The rubbery shear modulus is lower for the unfilled epoxy matrix than the one of the unfilled polyester matrix. For the unsaturated polyester matrix based composites, the introduction of the lignocellulosic filler (modified or unmodified) results in a decrease of the rubbery modulus. It could be ascribed to the poor adhesion between both components reported from SEM observations. As a consequence, the composite material can be assimilated to a foam or a polymer matrix filled with holes. As mentioned previously, the introduction of the palm tree fibers in the reactive polyester system could also affect the free radical polymerization process. The loss factor is unaffected by the lignocellulosic filler introduction. On the contrary a significant reinforcing effect is observed for epoxy-based composites in the rubbery state. It can be quantified through the relative rubbery modulus, G0c =G0m , which increases up to 1.63 and 2.56 for the unmodified and maleic anhydride modified filler, respectively. It can be ascribed to favorable interactions and possible reactions between the matrix and the filler surface zone which lead to an improved adhesion for chemically-modified fibers. This result is in good agreement with SEM observations and the resulting conclusions. The main a relaxation process is shifted towards higher temperatures as already discussed from DSC data. It could be explained by more restricted motions of the epoxy chains in the composite, due to the good adhesion between the polymer and the filler. As mentioned, it could be attributed to the additional hydrogen bounds between the acetyl groups on the filler

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Neat Unsaturated PolyesterMatrix

70

Composite with modified fillers

60

Composite with nonemodified fillers

50 40 30 20 10 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Déplacement (mm) Fig. 5. Load versus displacement curves obtained from three-point bending tests performed at Tg for (a) epoxy-based composites and (b) unsaturated polyester-based composites.

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Table 3 Mechanical properties of the neat matrices and composite materials filled with the lignocellulosic filler obtained from three-point bending experiments: flexural modulus (EF, data into square brackets corresponds to the storage tensile modulus values deduced from DMA experiments and taking a Poisson’s ratio of 0.35), stress at break (rB), deflection at break (F) and energy at break (Eb) (values are given ± the standard deviation) Temperature of the experiment (Texp)

Tg of neat matrix 40 K

Tg of the neat matrix EF (GPa)

rB (MPa)

F (mm)

Eb (J)

EF (GPa)

rB (MPa)

F (mm)

Eb (J)

1.30 ± 0.16 [1.55] 1.92 ± 0.27 [1.00] 2.49 ± 0.31 [2.00]

No break 1.27 ± 0.28 1.76 ± 0.13

– 10.51 11.44

– 0.44 ± 0.18 0.72 ± 0.23

1.92 ± 0.06 [2.60] 2.04 ± 0.10 [1.45] 2.23 ± 0.16 [2.60]

No break 1.16 ± 0.25 1.34 ± 0.27

– 6.64 7.82

0.22 ± 0.12 0.34 ± 0.13

Unsaturated polyester-based materials Neat matrix 1.70 ± 0.17 [1.55] Unmodified filler 1.92 ± 0.25 [2.20] Modified filler 1.71 ± 0.15 [2.20]

2.12 ± 0.19 0.86 ± 0.16 0.72 ± 0.06

15.48 5.42 5.70

0.78 ± 0.14 0.124 ± 0.058 0.118 ± 0.013

3.50 ± 0.64 [2.25] 3.70 ± 0.46 [3.00] 3.69 ± 0.29 [3.00]

2.11 ± 0.61 0.83 ± 0.09 0.74 ± 0.10

3.66 2.28 2.19

0.13 ± 0.06 0.043 ± 0.014 0.050 ± 0.012

Epoxy-based materials Neat matrix Unmodified filler Modified filler

consider the change in the chemical composition of the fibers after the chemical modifications. In fact the chemical modification of the lignocellulosic fibers lead to the dissolution of the hemicelluloses, thus direct interactions between the polymer matrix and the crystalline cellulose are possible. As a consequence, the possibility of stress transfer from the matrix to the fiber is better and the mechanical properties are improved. 3.4.2. High strain behavior (three-point bending test) Typical load versus displacement curves obtained from the three-point bending experiments are given in Fig. 5(a) and (b) for epoxy and unsaturated polyester based materials, respectively. These curves were obtained at the glass transition temperature of the unfilled matrix, i.e., 46 and 24 C for the unsaturated polyester and epoxy-based materials, respectively. The mechanical properties derived from these experiments as well as those performed at Tg 40 K are presented in Table 3. Storage shear modulus values measured from DMA experiments were determined at these temperatures (Tg and Tg 40 K). Storage tensile modulus values were deduced assuming a Poisson’s ratio of 0.35. Values are reported in Table 3 and show good correlation with the elastic modulus. In the case of the epoxy-based materials, a significant effect of the filler on the modulus is reported as the test is performed at a temperature, Texp, equal to the Tg of the matrix, Tg(matrix), or to Tg(matrix) 40 K. As expected, the neat matrices and the composite materials are brittle at low temperatures. The composite material reinforced with the modified filler displays higher mechanical properties compared to the composite filled with the non-modified filler. In fact, the elastic modulus determined in bending mode, the deflection and the energy at break are higher for the composites based on treated palm tree fibers. For unsaturated polyester based materials, a significant deterioration in the mechanical properties is obtained by the introduction of the lignocellulosic filler. Even if the bending modulus of the neat matrix and the composite materials was of the same order of magnitude, the stress at break, the deflection and the energy at break are seriously lowered upon the introduction of the filler. On the

other hand, no significant difference in the mechanical properties, between the composite materials reinforced with unmodified and chemically-modified lignocellulosic filler, was reported. These results are in good agreement with the DMA measurements. In fact, the reinforcing effect obtained for the epoxy-based composites is confirmed by the higher mechanical performances at different temperatures. This results from the good interfacial adhesion and as a consequence from the better dispersion of the filler within the epoxy-amine reactive mixture, i.e., the improved wettability. 4. Conclusions Thermosets reinforced with lignocellulosic fibers extracted from palm tree were processed. Two matrices, namely unsaturated polyester and epoxy, were used. To improve the interfacial adhesion the filler esterification was done. The surface of the filler was treated with acetate when using a polyester matrix and maleic anhydride when using an epoxy matrix. All the results lead to the conclusion that good interactions exist between the filler and the epoxy matrix. These interactions can be improved by the chemical treatment of the filler. Improved thermomechanical properties, bending modulus, stress at break and maximum absorbed energy were reported. These strong interactions and/or reactions occurring at the epoxy/lignocellulosic fiber interface result in a significant increase of the glass–rubber transition temperature of the neat matrix. On the contrary, the adhesion level is very weak when using an unsaturated polyester as matrix. The chemical treatment of the filler with acetate does not improve the interfacial adhesion. It results in very low mechanical properties. Other coupling agent should be tested such as vinylsilane and/or acrylatesilane. Acknowledgments The authors thank the Moroccan National Center of Technical and Scientific Research (CNRST) for its finan-

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