wood-flour composites

wood-flour composites

ARTICLE IN PRESS POLYMER TESTING Polymer Testing 26 (2007) 619–628 www.elsevier.com/locate/polytest Material Properties New polymeric-coupling agen...

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POLYMER TESTING Polymer Testing 26 (2007) 619–628 www.elsevier.com/locate/polytest

Material Properties

New polymeric-coupling agent for polypropylene/wood-flour composites Sonia M.B. Nachtigall, Graziela S. Cerveira, Simone M.L. Rosa Instituto de Quı´mica, UFRGS, Av. Bento Goncalves 9500, 91501-970 Porto Alegre, RS, Brazil Received 14 February 2007; accepted 20 March 2007

Abstract The suitability of using polypropylene modified with an organosilane as a coupling agent for polypropylene/wood-flour composites was investigated. The tensile properties, the water-absorption behavior, the thermal degradation properties and the morphology of the composites were analyzed and compared with those of non-coupled composites and of composites coupled with polypropylene modified with maleic anhydride. The coupling agents were prepared in the laboratory and it was verified that the silane showed higher reactivity towards PP chains. The results indicated that the silane-modified polymer increased the interfacial adhesion between the fibers and the polymer matrix and this effect was better than that obtained for the maleated-polypropylene-coupled composites. r 2007 Elsevier Ltd. All rights reserved. Keywords: Wood–plastic composites; Coupling agents; Organosilane; Thermal properties; Mechanical properties

1. Introduction Wood-fiber thermoplastics composites have received considerable attention from industry in recent years. The growing commercial importance of these materials has expanded efforts to understand their structure–properties relations and for exploring new methodologies for their production [1]. Wood fibers are attractive fillers for thermoplastic polymers, mainly because of their low cost, lowdensity and high-specific properties. They are Corresponding author. Tel.: +55 51 3308 7208; fax: +55 51 3308 7205. E-mail addresses: [email protected] (S.M.B. Nachtigall), [email protected] (G.S. Cerveira), [email protected] (S.M.L. Rosa).

0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2007.03.007

biodegradable and non-abrasive during processing, improving the stiffness and the strength of thermoplastics [2]. Because of their wide availability, natural fibers offer a real alternative to the reinforcing fibers presently available [3]. However, the high level of moisture absorption of the wood fibers and the poor adhesion with hydrophobic polymeric matrices (such as PP) can lead to debonding with age and to lowering mechanical properties. Several studies showed that fiber–polymer bonding can be improved by the use of coupling agents [4–7]. In some cases it was verified that the use of coupling agents also served to moderate and somewhat mitigate moisture movement through the composite, thus improving the mechanical properties of the materials [8,9]. However, very limited data are available on the relationship between coupling treatment, surface wettability


S.M.B. Nachtigall et al. / Polymer Testing 26 (2007) 619–628

and interfacial bonding strength of wood and apolar polymers. The low degradation temperature of natural fibers is also a limitation found when considering their use as fillers for thermoplastic polymers. Natural fibers are composed of a variety of chemical substances that present different degradation profiles. The three most important ones are cellulose, hemicellulose and lignin [10]. The degradation of these and other components during processing may produce a detrimental effect on the mechanical properties of the composites, both by changing the structure of the fiber as by producing volatile compounds that create microvoids across the interfaces. On the other hand, the use of coupling agents has also been shown to influence the degradation temperature of the fibers. As an example, Marcovich and co-workers [10] verified that wood-flour treatment with maleic anhydride as well as addition of PP modified with maleic anhydride increased the degradation temperature of PP/wood-fiber composites. Several methods of improving adhesion in natural fiber/polyolefin composites have been described in the literature. Some methods are based on fiber modification (physical or chemical [7]) and others are based on the addition of a coupling agent for interfacial adhesion improvement [7,11–13]. Sanadi et al. [12] reported that small amounts of PP modified with maleic anhydride added to the composites significantly increased the mechanical properties. Coupling agents based on alkoxysilanes are frequently employed for cellulosic fibers. Hydrolysis of the alcoxyl groups with subsequent reaction with hydroxyl groups from cellulose can provide chemical bonding with the fibers [7,14]. However, the bonding of the silane-coupling agents with nonpolar matrices is often weak and/or unexplained. Some authors proposed that the hydrophobic part of the organosilane molecules can interact with carbonyl groups formed by oxidation of the nonpolar matrices at high temperatures [15]. Recently, the literature reported the utilization of functionalized polymers as coupling agents for natural fiber/polyolefin composites [6,13,16]. Pracella et al. [13] observed improved fiber dispersion and higher interfacial adhesion when polyolefins grafted with glycidyl methacrylate were added to PP/hemp composites. Lee et al. [16] verified higher tensile strength and lower creep deflection for PP/ wood-flour composites coupled with maleated

polypropylene. Among others, the effect of acrylic acid grafted PE on the interfacial tension between PE and wood filler was studied by Wang et al. [6]. In the present study, we analyzed the suitability of using PP modified with vinyltriethoxysilane (PPVTES) as coupling agent for PP/wood-flour composites. The results were compared with those obtained for similar composites coupled with PP modified with maleic anhydride (PPMA). A systematic investigation on the effect of fiber loading and coupling agent type and concentration was undertaken to obtain optimum mechanical strength and low water-absorption level. The thermal stability of the composites was analyzed employing TGA/DTG, whereas the fiber–matrix morphology was studied through scanning electron microscope (SEM). 2. Experimental procedures 2.1. Materials Wood flour (0.125–0.210 mm) from Inbrasfama, Brazil, was used as filler. It was previously dried until constant weight under low pressure at 75 1C. Highly isotactic polypropylene (melt flow index 15.4 g/10 min) was donated by BRASKEM SA (Triunfo, Brazil). Maleic anhydride (MA, 98%) from Produtos Quı´ micos Elekeiroz SA (Sa˜o Paulo, Brazil), vinyltriethoxysilane (VTES, Silan GF56) from Wacker Chemie, and dicumyl peroxide (DCP) from Aldrich Chemical Company were used as received. 2.2. Synthesis and characterization of the coupling agents The functionalization reactions were performed in the mixer chamber of a Haake Rheometer 600p, at 170 1C, 50 rpm, for 10 min. The degree of functionalization was determined according to procedures described previously [17,18]. 2.3. Preparation and characterization of the composites PP pellets were introduced into the heated mixer chamber of a Haake Rheometer 600p, at 170 1C, 50 rpm. After 2 min of processing, previously mixed wood flour and coupling agents were added and processed for determined times.

ARTICLE IN PRESS S.M.B. Nachtigall et al. / Polymer Testing 26 (2007) 619–628

2.4. Tensile tests Composite sheets were obtained by hot compression, at 190 1C, 2.5 t, with a Carver press. 10  1 cm specimens were cut from the pressed sheets for tensile measurements. Tensile tests of at least five specimens of each sample were performed with a Universal Testing Machine, Emic DL 10000, equipped with a 50 N load cell, at a crosshead speed of 10 mm/min and at room temperature. Prior to the tests, the specimens were dried at 60 1C for 4 h. 2.5. SEM The photomicrographs were obtained from cryogenic-fractured specimens. The test specimens were attached to an aluminum stub and sputtered with gold to eliminate the electron charging effects. A JEOL microscope JSM 5800 operating at 20 kV was used. 2.6. Water absorption Five specimens (1.0–1.2 g) measuring 1 mm in thickness, 5 mm wide and 7.5 mm long, obtained by hot compression, were used to determine the degree of water absorption. The specimens were first placed in an oven set at 60 1C, under reduced pressure, for 8 h. The oven-dried weight (Wd) was determined and used to calculate the degree of moisture absorption as follows: Water absorption ð%Þ ¼

W  Wd  100, Wd

where W is the weight of the sample after water uptake in deionized water at 30 1C and atmospheric pressure. 3. Results and discussion 3.1. Functionalization reactions Laboratory-made coupling agents were prepared by means of PP functionalization with MA and


VTES initiated by DCP, according to procedures described in Refs. [17,18]. The molar concentration of the vinyl molecules (MA and VTES) was the same in both reactions, with the objective of comparing their effect on the coupling reactions. Table 1 shows the composition of the systems and the degree of functionalization (F) determined for the products. From the results shown in Table 1, it can be concluded that the degree of functionalization obtained with the vinylsilane was higher than that obtained with maleic anhydride. As can be seen, identical molar concentration of MA and VTES was employed for both preparations (1.54 mol%). This means that the number of molecules of MA and VTES were equal with respect to the polymer mass. It was verified that VTES produced a higher concentration of functional groups attached to the polymer chains, showing its good reactivity in these systems (0.88 mol% for VTES and 0.04% for MA). Effectively, the literature relates that the incorporation of maleic anhydride to polyolefins by means of radical reactions shows a limited degree of incorporation [18,19], probably due to parallel reactions. High level of maleic anhydride in the medium produces a low degree of functionalization, due to the low miscibility of this molecule and the polyolefins. As VTES is less polar than MA, it can mix better with polypropylene. Thus, it allows higher degree of functionalization despite its bulkiness. This is a very important result, since we can hope that PP containing higher numbers of attached functional groups can behave even more efficiently as a coupling agent. On the other hand, the functionalization of PP with maleic anhydride produced higher level of chain-breaking reactions in the polymer matrix, as denoted by the final torque values. It is well known that the main parallel reactions in PP radical functionalization are the chain-breaking reactions [18]. As the polymer molecular weight decreases, the observed torque of the system also diminishes. The higher torque measured in the system

Table 1 Functionalization reactions Sample

1 2



DCP (wt%)





7 —

1.54 —

— 3.6

— 1.54

0.1 0.1

Final torque (N m)

0.6 1.6

F wt%


3.98 0.09

0.88 0.04

ARTICLE IN PRESS S.M.B. Nachtigall et al. / Polymer Testing 26 (2007) 619–628


containing silane can be related to lower polymer degradation.

3.2. Composites preparation and characterization 3.2.1. Torque behavior Composites containing 30 wt% of wood flour and up to 10 wt% of the functionalized PPs were prepared. A typical mixing torque profile is shown in Fig. 1. In all cases, an initial loading peak was registered at the beginning of the experiment, reflecting the high viscosity of unmelted PP. The loading peaks decreased due to the PP melting. Addition of the other components after 2 min produced again an increase of the torque, which was followed by a slight decrease as the coupling agent melted and the filler particles were dispersed in the polymer matrix. Once the dispersion was completed, the torque stabilized. 14 PP/WF/PPMA (60/30/10)

Torque, N.m

12 10 8 6 4 2 0 0


4 6 Time, min



Fig. 1. Mixing torque profile of a composite.

The compositions and the final torque determined for each system are given in Table 2. It was observed that the changes in the composition of the systems did not affect the torque values, since they remained nearly constant. This suggests that the functionalized polyolefins showed flow behavior similar to that of the PP homopolymer. 3.3. Mechanical properties Considering practical applications, the mechanical properties of the composites are of major importance. For short-fiber reinforced composites, the adhesion between the fiber and the matrix plays a fundamental role in order to achieve good mechanical properties. In this study, we determined a value of 23 MPa for the tensile strength of virgin PP and 15 MPa for the composite containing 30 wt% WF. These results indicate that WF behaves merely as filler when incorporated into PP, at least at 30 wt% of WF. No reinforcing effect was observed in this case. This occurs because of the chemical incompatibility between the thermoplastic polyolefin and the polar filler, resulting in low interfacial adhesion. Such low adhesion induces the presence of gaps that make breaking under stress easier [20]. However, it was verified that addition of the polymeric coupling agents to this formulation produced composites with better performance, since the tensile strength was increased up to 28 MPa (Fig. 2). This behavior can be attributed to the enhanced chemical compatibility between the components, as it was expected. The presence of hydroxyl groups on the surface of the wood fibers can promote the establishment of strong interactions between the coupling agents and

Table 2 Compositions of the systems and final torque of the composites containing 30 wt% wood fiber Sample

PP (wt%)

PPVTES (wt%)

PPMA (wt%)

Final torque (N m)

C01 C02 C03 C04 C05 C06 C07 C08 C09 C10 C11

70.0 69.0 67.5 65.0 62.5 60.0 69.0 67.5 65.0 62.5 60.0

0 1.0 2.5 5.0 7.5 10.0 0 0 0 0 0

0 0 0 0 0 0 1.0 2.5 5.0 7.5 10.0

3.4 3.1 3.1 3.2 3.3 3.1 3.2 3.2 3.4 3.0 3.4

Conditions: 170 1C, 50 rpm, 10 min.

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without coupling agent with PP-MA

Tensile strength, MPa


with PP-VTES

25 20 15 10 5 0







Coupling agent, wt% Fig. 2. Tensile strength of PP composites containing 30 wt% WF as a function of the coupling agent concentration.

the fibers, while the non-polar part of the coupling agents interacts with the polymer matrix. Results shown in Fig. 2 indicate that PPVTES was more efficient than PPMA as coupling agent. The highest value of tensile strength for composites containing 30 wt% WF was obtained with 7.5 wt% PPVTES. The tensile strength value determined for this sample was more than 80% higher than that determined for the non-coupled composite. On the other hand, the behavior of the systems coupled with PPMA showed a tendency to get worse at high PPMA concentration. For PPVTES this behavior was not so evident. It was verified that 10 wt% PPMA produced a material with tensile strength even lower than that of the non-coupled composite. This can be explained by the intrinsic tensile properties of PPMA, since the PP radical functionalization reactions usually produce high level of polymer chain breaking, as observed through the torque values at the end of these reactions. An elevated concentration of coupling agent with low molecular weight can confer lower mechanical properties, if the chemical compatibility among the phases is not adequately improved. Fig. 3 shows the relationship between the tensile strength and the filler loading for the composites prepared with and without PPVTES. It was found that the tensile strength decreased with increasing filler loading, as usually related in the literature [21]. The concentration of the coupling agent was 1 wt% with respect to the PP content. We observed that up to 30 wt% WF the tensile properties of the coupled

composites were better than those of the noncoupled ones. However, coupled composites containing 40 and 50 wt% WF showed lower strengths. These results can be explained by the low concentration of PPVTES with respect to the higher level of WF, thus resulting in an inefficient coupling effect. Since the PP concentration diminishes with increasing concentration of the filler, so does the coupling agent concentration. At 40 and 50 wt% WF, the PPVTES level was not sufficient to impart adequate adhesion improvements. 3.4. Water absorption WF contains numerous hydroxyl groups available for interaction with water molecules by hydrogen bonding. So, wood-fiber reinforced polymers can take up high amounts of water, which generally causes a reduction in mechanical properties [22]. As polypropylene is an apolar polymer matrix, its penetration into pits, cracks, and other voids in wood fiber tends to be limited. Under moist conditions, these spaces can be filled with water. The water-absorption behavior determined for the composite containing 30 wt% wood flour without coupling agent and for similar composites containing 5 and 10 wt% of PPMA and PPVTES is shown in Fig. 4. It was observed that all composites containing coupling agents showed lower degree of water absorption as compared to the non-coupled one. This indicates that PPMA and PPVTES improved the compatibility between the

ARTICLE IN PRESS S.M.B. Nachtigall et al. / Polymer Testing 26 (2007) 619–628


without coupling agent


with 1wt% PP-VTES


(related to PP concentration)

22 Tensile strength, MPa

20 18 16 14 12 10 8 6 4 2 0 10


30 Wood flour, wt%



Fig. 3. Tensile strength of the composites as a function of the filler level.

8 1 hour

Water absorption, wt%

2 hours 18 hours


24 hours



0 Without coupling agent

With 5% PPMA

With 10% PPMA

With 5% PPVTES

With 10% PPVTES

Fig. 4. Water absorption of PP/WF composites (30 wt% WF).

fibers and the matrix. Establishment of bonds between hydroxyl groups on the filler surface and functional groups from the coupling agents prevents bonding of the cellulosic filler with water, thus limiting water absorption. When comparing the systems containing 5 and 10 wt% PPMA it could be verified that the lowest concentration exhibited better effect on the wateruptaking behavior. This agrees with the general knowledge that only small amounts of PPMA are

necessary for improving adhesion in this kind of composites [12]. We can believe that an excess of PPMA could increase water affinity, since it is more polar than the matrix PP. On the other hand, excellent results of water uptake were obtained for the composites compatibilized with PPVTES. It was verified that the water absorption of the composite containing 5 wt% PPVTES was near 35% of the amount absorbed by the non-coupled composite. This is a very important

ARTICLE IN PRESS S.M.B. Nachtigall et al. / Polymer Testing 26 (2007) 619–628


result, since water absorption usually affects mechanical properties, dimensional stability and other properties, lowering the applicability of the materials. By changing the PPVTES concentration from 5 to 10 wt%, a slight increase in water absorption was verified. The low level of water absorption of the composites coupled with PPVTES indicates that these materials are very suitable to use in damp places where wood cannot be employed, such as the interior of bathrooms, decking, etc. 3.5. Morphology The effect of addition of the macromolecular coupling agents on the morphology of the composites was studied by SEM. Representative SEM micrographs are reported in Figs. 5–7. Non-coupled composites displayed a rough morphology with the presence of many voids and cavities resulting from fiber pullout (Fig. 5). This indicates poor interfacial adhesion, thus revealing the low affinity between the polymer matrix and the wood-flour filler. Water can be easily absorbed by the voids and cavities, thus explaining the high level of moisture absorption found for the non-coupled composite. The presence of the coupling agents changed the morphology of the materials. Addition of PPMA to the composites produced a more homogeneous surface with less voids and cavities. This indicated that PPMA had a positive effect on the interfacial adhesion between filler and matrix (Fig. 6). However, some voids and cavities from fiber pullout could still be found, and these are responsible for part of the water absorption observed in these composites. The use of PPVTES as coupling agent greatly improved the interfacial bonding between the wood-

Fig. 6. SEM micrograph of the PP/WF/PPMA composite (60/30/10).

Fig. 7. SEM micrograph of the PP/WF/PPVTES composite (60/30/10).

flour filler and the PP matrix, as it can be observed through the surface photomicrograph in Fig. 7. This composite showed a more homogeneous and a smoother surface with very small voids and few cavities. The better swelling properties and tensile strength of the composites coupled with PPVTES can be explained by the decrease in these irregularities, as well as by the increase in the interfacial adhesion between the phases as compared to the non-coupled composites and to the composites coupled with PPMA. 3.6. Thermal properties

Fig. 5. SEM micrograph of the PP/WF composite (70/30).

The thermal stability of lignocellulosic-filled polymer matrix composites is a very important

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degradation rate was shifted to a lower temperature (around 423 1C) showing that the presence of the wood flour lowered the thermal stability of the polymer. Another important feature observed was the higher degradation temperature of the polymer matrix in the composites containing the coupling agents. The temperature of degradation of the polymer matrix increased about 60 1C in comparison to the non-coupled composite, indicating that PPMA and PPVTES improved the thermal stability of the polymer. This indicates that the compatibility and the interfacial bonding increased by mixing both components in the presence of the coupling agents [23].

parameter for the processing and usage of these materials. The manufacture of such composites requires the mixing of fibers and matrix at high temperatures, so the degradation of the biomaterial can produce undesirable effects on the properties. The thermogravimetric analysis of virgin PP showed a single-mass loss step with maximum degradation rate centered at 475 1C. The TGA and DTG curves a non-coupled composite and of composites coupled with PPMA and PPVTES under nitrogen are represented in Figs. 8 and 9. For the non-coupled PP/WF composite prepared at 30% fiber loading, it was verified that the maximum

100 no coupling agent with PPMA with PPVTES

Weight, %





0 0






Temperature,°C Fig. 8. TGA curves of the composites (30 wt% WF).




Deriv. weight, %/°C



no coupling agent with PPMA with PPVTES

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0




Temperature,°C Fig. 9. DTG curves of the composites (30 wt% WF).


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All the fiber-filled systems showed multi-stepped degradation profiles due to the various species present: polymer matrix, cellulose, hemicellulose, lignin and other minor substances from the filler and the coupling agent. Some degradation observed below 220 1C in the composites was attributed to dehydration or degradation of lignin and hemicellulose [24]. The degradation peak of PP in the DTG curve of the non-coupled composite showed a shoulder at about 383 1C that could be attributed to the cellulose degradation. Single peaks at similar temperatures were also observed in the coupled composites, showing that the degradation behavior of the wood flour did not significantly change in this range of temperature in the presence of PPMA and PPVTES. Above 450 1C the noncoupled composite and the composite coupled with PPMA showed some peaks of degradation corresponding to approximately 6.5 wt% mass. These peaks are probably related to further breakage of decomposition products formed during the thermal analysis.

4. Conclusions In this study, we assessed the possibility of using pre-prepared PP functionalized with vinyltriethoxysilane as a coupling agent for PP/WF composites. The resulting properties were compared to those obtained with a traditionally employed coupling agent, PP modified with maleic anhydride. Both coupling agents were prepared in the laboratory using the same concentration of silane and maleic anhydride, in the presence of peroxide. The determination of the degree of functionalization indicated that the silane showed higher reactivity with the polymer. We observed that both coupling agents improved the properties of the composites. However, the PP modified with silane produced composites with better tensile strength, lower level of water absorption and more homogeneous morphology than the composites coupled with PP modified with maleic anhydride. The thermal degradation profiles of both kinds of coupled composites were similar and increased the thermal stability of PP in comparison to the non-coupled composite. These results indicate that PP modified with vinyltriethoxysilane can be an interesting coupling agent for PP/wood-flour composites due to the good properties determined for these materials.


Acknowledgments The authors thank PROPESQ/UFRGS for financial support and BRASKEM SA for PP supplying.

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