Self-reinforced melt processable composites of sisal

Self-reinforced melt processable composites of sisal

Composites Science and Technology 63 (2003) 177–186 www.elsevier.com/locate/compscitech Self-reinforced melt processable composites of sisal Xun Lua,...

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Composites Science and Technology 63 (2003) 177–186 www.elsevier.com/locate/compscitech

Self-reinforced melt processable composites of sisal Xun Lua, Ming Qiu Zhangb,*, Min Zhi Ronga, Guang Shia, Gui Cheng Yangb a

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Zhongshan University, Guangzhou 510275, PR China b Materials Science Institute, Zhongshan University, Guangzhou 510275, PR China Received 8 March 2002; received in revised form 9 August 2002; accepted 25 August 2002

Abstract Through slight benzylation treatment, skin layers of sisal fibers were converted into thermoplastic material while the core of the fiber cells remained unchanged. On the basis of these modified sisal fibers, self-reinforced composites were prepared using hot pressing, in which the plasticized parts of sisal serve as matrix and the unplasticized cores of the fibers as reinforcement. The paper discussed the influence of various benzylation conditions on the structure, thermal flowability and mechanical properties of modified sisal and the composites. It was found that a balance of melt processability and reinforcing effect of the benzylated sisal fibers should be considered. Unlike the conventional plant fiber composites using petro-polymers as matrices, the current self-reinforced composites based on sisal are characterized by inherent interfacial compatibility and full biodegradability. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibers; Polymer-matrix composites (PMCs); B. Surface treatments; B. Mechanical properties; Environmental

1. Introduction In recent years, plant fibers reinforced polymer composites have attracted more and more research interests owing to their potential of serving as alternative for artificial fiber composites [1–3]. Accordingly, studies on preparation and properties of thermoplastic and thermosetting composites filled with sisal, coir, hemp, flax, etc., were carried out extensively. Compared with glass fibers or carbon fibers used conventionally, plant fibers have many advantages like renewable, environmental friendly, low cost, lightweight, high specific mechanical performance. Nevertheless, it is worth noting that the so-called plant fiber composites are accompanied by the following drawbacks [4]: (1) most of the commercially available polymers are made from petroleum and are non-biodegradable, therefore, composites based on which are still a burden to the environment; and (2) the inherent hydrophilicity of plant fibers used to result in poor interfacial interaction with hydrophobic polymer matri-

* Corresponding author. Tel.: +86-20-84036576; fax: +86-2084036576. E-mail address: [email protected] (M.Q. Zhang).

ces and hence decreased mechanical properties of the composites. To solve the problem, melt processable all-plant fiber composites are developed in our laboratory, in which plant fibers serve as reinforcement and matrix concurrently. The concept originates from the works that provide wood flour with certain thermoplasticity through etherification or esterification [5–7]. So far as we know, plant fibers mainly consist of the following natural macromolecules: cellulose, hemicellulose and lignin. The spirally oriented rigid cellulose microfibrils play the role of reinforcements in soft hemicellulose and lignin matrix (Fig. 1). Due to the high degree of crystallinity of cellulose and the three-dimensional net structure of lignin, plant fibers cannot be processed like thermoplastic polymers in general. Through certain substitution reactions on the side chains of cellulose in association with partial removal of lignin, however, plant fibers can be converted into thermally formable materials. Based on this finding, wood sawdust was cyanoethylated and benzylated respectively to produce thermoplastic material and then reinforced by sisal [4,8]. Such all-plant fiber composites are characterized by full biodegradability, physical heterogeneity instead of chemical heterogeneity in conventional composites and cost effectiveness.

0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00204-X

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The authors of this work manufacture all-plant fiber composites in another way. As the extent of plasticization of a plant fiber is a function of diffusion of the reagent from the fiber surface to the inner layers and is not equal along the radial direction of the fiber and its ultimate cells, a slight modification which provides the fiber with certain thermoforming ability while retaining partial strength properties might yield self-reinforced plant fiber composites. That is, under the joint action of pressure and temperature, the surface layer of the cells (preferentially the primary wall and secondary wall S1, see Fig. 1) of the treated plant fibers can be softened, and then start to flow and mingle with each other. In the meanwhile, the cells core (the secondary walls S2 and S3) maintains its original structure and imparts a reinforcing effect to the composites. In this case, no additional resin is needed to bind the reinforcements together and the resultant composites fall into the category of self reinforced materials. The feasibility of the above idea has been proved in our earlier communication [9]. The present paper is focused on the effect of modification conditions on the structure and properties of sisal based self-reinforced composites, with the objective of obtaining comprehensive understanding of the material.

2. Experimental 2.1. Benzylation of sisal Sisal fibers with diameter ranging from 100 to 200 mm were provided by Dongfanghong State Farm in Guangdong Province, China. Chopped sisal fibers that had been extracted by benzene–ethanol (v/v=2/1) were added into NaOH solution of desired concentration with stirring. After 1–1.5 h, the fibers were transferred into a flask containing benzyl chloride. Bezylation reaction was carried out under vigorous stirring at certain temperature for a period of time to get benzylated pro-

ducts with various reaction extents. Then, the products were purified through washing with distilled water to remove inorganic salts, and with ethanol to remove residues of benzyl chloride and by-products, respectively. Finally the treated sisal was dried under vacuum at 60  C until constant weight was reached. 2.2. Preparation and characterization of self-reinforced sisal composites Thermal flowability of benzylated sisal was assessed by a home-made tester in terms of melt index (MI, defined as the weight of the material flowing out of the nozzle of the tester in gram within 10 min under given pressure and temperature) and flow temperature (defined as the temperature at which the material begins to flow out of the nozzle during hating under a given pressure). To produce self-reinforced sisal composites, the benzylated short sisal fibers were compressed by a hot press at 160  C under 9.8 MPa. The molding conditions were determined according to the aforesaid flowability measurements. Structural variations of sisal fibers were examined by a Bruker Equinox 55 Fourier transform infrared (FTIR) spectrometer and a Rigaku D/Max-3A wide angle Xray diffractometer (WAXD), respectively. Thermal mechanical analysis (TMA) of the modified sisal fibers was conducted on a TA DMA2980 dynamical mechanical analyzer at a heating rate of 5  C under testing pressure of 9.8 N. Morphological observation was carried out using a Hitachi S-520 scanning electron microscope (SEM). Tensile strengths of sisal were measured by a YG001A tester at a rate of 10mm/min. Tensile and three-point bending tests of the composites were conducted on a LWK-5 universal tester at crosshead speeds of 10mm/min and 5mm/min, respectively.

3. Results and discussion 3.1. Relationships between reaction conditions and extent of benzylation of sisal Although both sisal and wood are comprised of cellulose, hemicellulose, lignin, etc., the compositions of the components are quite different (Table 1). Besides, the structural morphologies are also not the same. For example, FTIR crystallinity of sisal is 62.8% while that of fir sawdust is only 46.7% [10]. Therefore, the condiTable 1 Chemical compositions of sisal and wood

Fig. 1. Structural constitution of a plant fiber cell, in which the secondary wall S2 makes up 80% of the total thickness and thus acts as the main load bearing component.

Materials

Cellulose (wt.%)

Hemicellulose (wt.%)

Lignin (wt.%)

Sisal Wood

65.8 50–55

12.0 15–25

9.9 20–30

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tions adapted for benzylation of fir sawdust developed previously [8] cannot be directly applied to the modification of sisal. In fact, benzylation of lignocellulosic materials is a typical Williamson synthesis reaction, which involves nucleophilic substitution of an alkoxide or a phenoxide ion for a halide ion. Since cellulose constitutes the majority of sisal and lignin contains little hydroxyl, benzyl chloride has to react mainly with the hydroxyl of cellulose. For an accurate analysis of the reaction and the resultant, degree of substitution originating from benzylation process should be known. Due to the chemical heterogeneity of sisal fibers, this parameter is hard to be measured. Instead, the extent of benzylation reaction can be evaluated by the percentage weight gain, which has explicit engineering meaning and reflects the incorporation of benzyl groups. Table 2 exhibits the influence of concentration of alkali on benzylation of sisal. In general, pre-treatment with NaOH helps to swell the plant fibers and to partially remove lignin and hemicellulose. Since sisal has a crystallinity higher than that of fir sawdust, concentration of NaOH solution used for the treatment should also be higher so as to fulfill the task of swelling. However, the increase in alkali concentration has a strict limitation. As shown in Table 2, the weight gain of sisal fibers increases with a rise in NaOH concentration when the latter is less than or equal to 35%. When the concentration of alkali exceeds 35%, the weight gain decreases on the contrary. This should be attributed to the significantly intensified rates of secondary reactions of benzylation and dissolving of hemicellulose and lignin. The effect of dosage of NaOH solution at a fixed concentration of 35% is shown in Table 3. There is a peak weight gain when the amount of NaOH solution is 15 ml. Evidently, extremely low dosage of alkali leads to insufficient swelling of sisal while extremely high alkali dosage would accelerate hydrolysis of the benzylated products of sisal. As a result, a sisal-to-NaOH ratio (g/ ml) of about 1:3–5 can give the highest weight gain, i.e. the highest degree of benzylation. It is known that a piece of sisal fiber is not a single filament like carbon or glass fiber but a bundle of cel-

lular aggregate consisting of more than 100 irregular hexagonal hollow ultimate cells [11] (Fig. 2). Substitution reaction of sisal fibers with such structures has to proceed layer by layer. An increase in reagent dosage would certainly result in a rise in the diffusion rate of the reagent and facilitate the reaction. However, excess benzyl chloride might speed up the side reactions. It suggests that a balance should be considered. As manifested by Table 4, the weight gain of sisal fibers is nearly the same when the sisal-to-benzyl chloride ratio (g/ml) is equal to or higher than 1:5. Only by getting rid of water and other by-products, the extent of benzylation can be further increased. Similar phenomenon can be observed in Fig. 3, where reaction time is adjusted. A prolonged reaction time is factually detrimental to the decomposition of sisal fibers. Studies on benzylation of wood powders indicated that reaction temperature is an important influencing factor [10]. Considering the fact that cellulose in sisal fibers has higher crystallinity than that in wood, its reactivity should be worse accordingly. A higher reaction temperature is thus required. This is evidenced by the experimental results in Fig. 4. When reaction temperature is raised from 100 to 115  C, weight gain of sisal fibers is increased remarkably. In the case of 125  C, however, the extent of benzylation is close to that obtained under 115  C. An increase in reaction time at 125  C would even result in a gradual reduction in weight gain due to the intensified side reactions. On the basis of earlier investigation, it is concluded that the following conditions should be used when dealing with benzylation of sisal fibers: concentration of alkali solution: 35%, sisal-to-NaOH ratio (g/ml): 1:3, sisal-to-benzyl chloride ratio (g/ml): 1:5, reaction temperature: 115  C. Under these prerequisites, different weight gains (i.e. different extent of benzylation reaction) of sisal fibers can be yielded only by changing reaction time. The materials discussed in the following sections were prepared according to this route. 3.2. Characterization of benzylated sisal fibers Fig. 5 compares FTIR spectrum of untreated sisal with those of benzylated sisal. After benzylation, the

Table 2 Effect of alkali concentration on benzylation of sisala NaOH concentration (%)

Dosage of benzyl chloride (ml)

Reaction temperature ( C)

Reaction time (h)

Weight gain (wt.%)

15 20 25 30 35 40

25 25 25 25 25 25

115 115 115 115 115 115

4 4 4 4 4 4

0.2 8.0 15.4 26.3 32.4 28.0

a

Weight of sisal=5 g, dosage of NaOH solution=15 ml.

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Fig. 2. Schematic drawing of (a) cross-section of a piece of sisal leaf and (b) inner construction of bunched sisal fibers. Table 3 Effect of dosage of alkali on benzylation of sisala Dosage of NaOH (ml)

Dosage of benzyl chloride (ml)

Reaction temperature ( C)

Reaction time (h)

Weight gain (wt.%)

5 10 15 20 25 30

25 25 25 25 25 25

115 115 115 115 115 115

4 4 4 4 4 4

16.2 20.0 32.4 36.4 32.6 23.7

a

Weight of sisal=5 g, concentration of NaOH solution=35%.

Table 4 Effect of dosage of benzyl chloride on benzylation of sisala Dosage of benzyl chloride (ml)

Dosage of NaOH (ml)

Reaction temperature ( C)

Reaction time (h)

Weight gain (wt.%)

10 20 25 30 40 25+5b

15 15 15 15 15 15

115 115 115 115 115 115

4 4 4 4 4 4+0.5

16.4 25.8 32.4 33.7 35.2 42.5

a

Weight of sisal=5 g, concentration of NaOH solution=35%; Having reacted for 4 h, the by-products were removed from the system. Then, fresh benzyl chloride (5 ml) was added for a further reaction of 0.5 h. b

stretching mode of O–H at 3443 cm1 diminishes and the peak at 1110 cm1 due to associated hydrogen disappears. The appearance of the absorptions at 1800–1950, 1600, 736 and 695 cm1 is indicative of the mono-substituted benzene rings. With a rise in the weight gain of the benzylated products, the peaks representing benzyl group

become stronger and the characteristic absorptions of C–H in ester are perceivable at 3029 and 3060 cm1. The earlier variations reveal that some hydroxyl groups of cellulose have been substituted by benzyl groups. In addition, the absence of the band at 1730 cm1 corresponding to carbonyl stretching and the significant

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Fig. 3. Effect of reaction time on extent of benzylation of sisal. Reaction conditions: weight of sisal=5 g, dosage of benzyl chloride=25 ml, concentration of NaOH=35%, dosage of NaOH solution=15 ml, reaction temperature=115  C.

reduction in the peak intensity of the resonance mode of C¼O at 1636 and 1250 cm1 demonstrate that lignin and hemicellulose are partly removed. Damage of lignin networks helps to improve the extent of substitution of cellulose. Changes of supramolecular structures of sisal are examined by X-ray diffractometer (Fig. 6). With a rise in the extent of benzylation, the WAXD peak at 2=22.6 originating from the reflection of (002) plane of cellulose I lattice shifts towards low angle regime (at around 19.5 for the weight gain of 43.0%) and becomes more and more weaker. The (101) peak at 16.0 disappears when the fiber is benzylated. It means that the introduction of benzyl groups into the crystalline regions of cellulose in sisal results in a decrystallization effect [12]. Such a decrystallization process improves thermoplasticity of cellulose by breaking hydrogen bonds between cellulose molecules. The large benzyl groups brings in more free volumes and increase the mobility of cellulose segments as a result. Since concentrated alkali, high reaction temperature and long reaction time have to be employed during benzylation of sisal, the strength performance of the fiber would be inevitably influenced due to the variations in both the chemical and physical structures of the components (Table 5). Although lower degree of benzylation results in less damage of the fiber, the acquired thermoplasticity might be insufficient to produce composite sheets through thermoforming. Therefore, the modification conditions should be optimized. In principle, a complete benzylation, that converts sisal into a fully thermoplastic material and leaves no fibrous rein-

Fig. 4. Effect of reaction temperature on extent of benzylation of sisal. Reaction temperature: (1) 100  C, (2) 115  C, (3) 125  C. Other reaction conditions: weight of sisal=5 g, dosage of benzyl chloride=25 ml, concentration of NaOH=35%, dosage of NaOH solution=15 ml.

Fig. 6. WAXD patterns of (1) sisal fiber as-received and (2, 3) benzylated sisal fiber with different weight gains (curve 2: 10.5wt.%, curve 3: 25.8 wt.%, curve 4: 43.0 wt.%).

Table 5 Tensile strength of sisal fibers as a function of extent of benzylation reaction Fig. 5. FTIR spectra of (1) sisal fiber as-received and (2, 3) benzylated sisal fiber with different weight gains (curve 2: 10.5 wt.%, curve 3: 25.8 wt.%).

Weight gain (wt.%) Tensile strength (MPa)

0 481.2

1.5 334.6

3.8 288.2

10.8 236.5

25.8 153.6

43.0 90.5

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forcements with load bearing ability after molding, is disadvantageous to the production of self-reinforced sisal composites as described in the Introduction part. Morphologies of sisal fibers before and after benzylation modification are shown in Fig. 7. It is clear that the cementing materials (hemicellulose, lignin, pectin, etc.) of the bundled sisal cells are removed during the alkali pre-treatment [Fig. 7(b)]. The fibers that took part in benzylation reaction for 1h begin to split into single ultimate cells (Fig. 7(c)]. With a rise in reaction time, the surfaces of the cells become rather rough [Fig. 7(d)], manifesting degree of the reaction is gradually lowered from the skin to the core of sisal. The SEM micrographs in Fig. 7 also reveal the effects of alkali pre-treatment: swelling the fibers, partially removing hemicellulose and lignin, disrupting the lignin networks and improving the

reactivity of the residual lignin by introducing hydroxyl groups so that a substitution of benzyl groups for the hydroxyl groups of lignin is facilitated and the ultimate cells are further expanded. 3.3. Thermoplasticity of benzylated sisal The above discussion only deals with sisal nodification and the resultant structural variation. To check whether the benzylated sisal fibers have acquired thermoforming ability, TMA measurements should be made first. From Fig. 8 it is seen that with increasing temperature a rapid deformation appears on the TMA curves of the fibers with different weight gains. It corresponds to the softening of the fibers. The onset temperatures, determined from the intersecting point of the

Fig. 7. SEM micrographs of (a) sisal fiber as-received, (b) alkali treated sisal, (c) sisal experienced benzylation reaction for 1 h, and (d) sisal experienced benzylation reaction for 4 h.

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Fig. 8. TMA traces of benzylated sisal fiber with different weight gains (curve 1: 25.8 wt.%, curve 2: 43.0 wt.%).

two tangential lines, are 118 and 87  C, respectively. That is, the softening temperature of benzylated sisal decreases with increasing substitution as expected. As the glass transition temperature of cellulose lies in the neighborhood of 250  C [13] and the present plasticization of sisal does not change the chemical structure of the backbone of cellulose, softening of the modified sisal fibers should not be attributed to the amorphous a-transition. Inter-molecular slip might be responsible for it. A more careful study down to molecular level of this phenomenon is required to reveal the mechanism. Further investigation of the flowability of benzylated sisal melts in Figs. 9 and 10 reveals that the processing window is a function of molding temperature and pressure like conventional thermoplastics. Therefore, thermally manufacturing of the modified sisal is feasible. Fig. 11 illustrates the thermal flowability of treated sisal as a function of weight gain of the fibers. Similar to the results of TMA (Fig. 7), the more hydroxyl groups of cellulose are substituted by benzyl groups, the higher the melt index of the fibers. Fig. 12 shows the morphologies of a bundle of benzylated sisal fibers that had experienced melt processing.

Fig. 9. Effect of temperature on thermal flowability of benzylated sisal fiber with a weight gain of 23.2 wt.% under 9.8 MPa.

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In the case of lower degree of benzylation, pieces of plasticized materials appear on the surfaces of the cells [Fig. 12(a, b)]. With increasing reaction time, cell surface is wrapped by a plasticized layer (Fig. 12(c, d)] and fibrous structures gradually disappear due to the expanding of the plasticized layer [Fig. 12(e)]. Eventually, a fully thermoplastic material is obtained as a result of high degree of benzylation [Fig. 12(f)]. It is interesting to see the split of the plastisized portion out of the unplasticized core of a sisal cell [Fig. 12(d)]. It explicitly confirms the concept of the structure of slightly plastisized plant fibers proposed in the introductory part. Considering the dual role acted by benzylated sisal in self-reinforced composites as well as strength retention of sisal in response to benzylation treatment and thermal proceesability of the treated fibers, a weight gain higher than 20% is suitable for the subsequent composites preparation.

Fig. 10. Effect of pressure on thermal flowability of benzylated sisal fiber with a weight gain of 23.2 wt.% at temperatures of (1) 160  C and (2) 170  C.

Fig. 11. Effect of weight gain on melt index and flow temperature of benzylated sisal fiber (pressure=9.8 MPa).

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Fig. 12. SEM micrographs of a bundle of benzylated sisal experienced hot pressing at 160  C under 9.8 MPa. Reaction time of benzylation: (a) 30 min, (b) 1 h, (c, d) 4 h, (e) 8 h, (f) 10 h.

3.4. Mechanical properties of self-reinforced sisal composites According to the study on the thermoplasticity of benzylated sisal fibers, self-reinforced sisal composites were hot compressed at 160  C under 9.8 MPa. Tensile properties of the composites are measured and illustrated in Figs. 13 and 14. As indicated by the stress–strain curves (Fig. 13), elongation-to-break of the composites is low. With a rise in the extent of benzylation, the value is increased marginally. Basically the composites can be regarded as brittle type materials because there is no obvious yielding prior to final rupture. Fig. 14 shows the dependence of tensile strengths and Young’s moduli of the composites on weight gain of the benzylated sisal fibers. As can be imagined, maximums appear on the two curves, respectively. This is the result of the contradictory effects of benzylation induced thermoforming ability and preservation of supramolecular structure of cellulose. In comparison with the tensile properties of the molding sheets from thoroughly thermoplasticized sisal (strength=30.3 MPa, modulus=2.6 GPa) reported in

ref. [4], the current self-reinforced sisal composites exhibit evident strengthening effect provided by the unmodified microfibrils. Flexural performance of the composites is shown in Figs. 15 and 16. According to the criteria that distinguish failure modes based on the force-deflection curves

Fig. 13. Typical tensile stress-strain curves of self-reinforced sisal composites. Weight gains of the benzylated sisal: (1) 25.8 wt.%, (2) 43.0 wt.%, (3) 59.8 wt.%.

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Fig. 14. Dependence of tensile properties of self-reinforced sisal composites on weight gain of benzylated sisal.

Fig. 15. Typical stress–deflection curves of self-reinforced sisal composites in three-point bending tests. Weight gains of the benzylated sisal: (1) 25.8 wt.%, (2) 43.0 wt.%, (3) 59.8 wt.%.

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structures of cellulosic microfibrils. The composites made from these sisal fibers possess approximately homogeneous microstructure instead of inhomogeneous one of the composites of slightly benzylated sisal. Therefore, typical flexural failure is detected. Unlike the results of tensile testing, the peak flexural strength and modulus appear at a weight gain of 43.0% (Fig. 16). It reflects the structural characteristics of the composites in response to different stress states. As suggested by Rong et al. [15], two types of interface of sisal composites should be taken into consideration: one between fiber bundles and the matrix and the other between the ultimate cells. In the case of tensile tests, enhanced strength and modulus of the composites result from higher adhesion strength at the first type of interface that prevents transverse failure and weaker intercellular bonding of the second type of interface that facilitates pull-out of ultimate cells. Under the circumstances of three-point bending, shear failure resistance controls the ultimate performance of the composites. With respect to the present self-reinforced sisal composites, the aforesaid first type of interface no longer exists. The bonding at the intercellular region is improved due to the thermoplasticization effect of the cell walls that fill up the interstitial volumes. Consequently, enhancement of composites’ tensile strength and modulus is limited. When the composites are subjected to flexural testing configuration, the improved adhesion between plasticized skin walls and unplasticized core of sisal cells might provide higher shear strength as a result of the chemically homogeneous interphase. Thus the flexural strength of the composites is relatively high.

4. Conclusions

Fig. 16. Dependence of flexural properties of self-reinforced sisal composites on weight gain of benzylated sisal.

recorded during three-point bending [14], it is known that the contribution made by shear deformation can be neglected when the weight gain of sisal is as high as 50.5% (Fig. 15). This might be due to the high degree of benzylation that destroys most of the orderly arranged

1. By using the technique of slightly etherification (benzylation in the present work), the skin of sisal cells is transformed into thermoplastic material while the original core structure is maintained. Under certain temperature and pressure, sisal fibers modified following the above route can be processed like conventional thermoplastics, producing self-reinforced composites. In such all-plant fiber composites, the plasticized parts of sisal act as matrix and the unplastized parts play the role of reinforcement. As a result, interfacial compatibility is guaranteed owing to the chemical homogeneity of both matrix and reinforcement. 2. The degree of modification of sisal is a function of concentration of NaOH solution, dosage of NaOH solution, dosage of benzyl chloride, reaction time, reaction temperature, etc. Therefore,

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structure, melt processability and mechanical properties of the treated fibers and the self-reinforced composites can be tailored accordingly. 3. As the self-reinforced sisal composites still consists of cellulose, hemicellulose and lignin, the biodegradability associated with plant fibers would be retained. They can thus be decomposed by fungi or enzyme at the end of their lifetime instead of incineration for conventional composite materials. Another paper on this topic will be published in the near future.

Acknowledgements The financial support by the National Natural Science Foundation of China (Grant: 50173032), the Key University Teachers Training Program of the Ministry of Education of China, the Team Project of the Natural Science Foundation of Guangdong Province, and the Talents Training Program Foundation of the Higher Education Department of Guangdong Province are gratefully acknowledged.

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