- Email: [email protected]
Reinforced polypropylene composites: Effects of chemical compositions and particle size
Bioresource Technology 101 (2010) 2515–2519
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Reinforced polypropylene composites: Effects of chemical compositions and particle size Alireza Ashori a,*, Amir Nourbakhsh b a b
Department of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Department of Wood and Paper Science, Research Institute of Forests and Rangelands (RIFR), Tehran, Iran
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 26 October 2009 Accepted 5 November 2009 Available online 30 November 2009 Keywords: Aspect ratio Fiber reinforced Mechanical properties Lignocellulosic materials Hot-water treatment
a b s t r a c t In this work, the effects of wood species, particle sizes and hot-water treatment on some physical and mechanical properties of wood–plastic composites were studied. Composites of thermoplastic reinforced with oak (Quercus castaneifolia) and pine (Pinus eldarica) wood were prepared. Polypropylene (PP) and maleic anhydride grafted polypropylene (MAPP) were used as the polymer matrix and coupling agent, respectively. The results showed that pine fiber had significant effect on the mechanical properties considered in this study. This effect is explained by the higher fiber length and aspect ratio of pine compared to the oak fiber. The hot-water treated (extractive-free) samples, in both wood species, improved the tensile, flexural and impact properties, but increased the water absorption for 24 h. This work clearly showed that lignocellulosic materials in both forms of fiber and flour could be effectively used as reinforcing elements in PP matrix. Furthermore, extractives have marked effects on the mechanical and physical properties. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Although there have been quite a number of studies on the wood–plastic composites (WPCs) as new generation of reinforcing materials in recent years (Lai et al., 2003; Jayaraman and Bhattacharyya, 2004; Kim et al., 2006; Salemane and Luyt, 2006; Neagu et al., 2006; Anuar et al., 2008; Ashori, 2008; Nourbakhsh et al., 2009a), so far, not much attention has been paid to the effect of chemical composition of wood on the mechanical and physical properties. The effective use of wood-based particles and fibers as fillers or reinforcements in thermoplastic composites requires a fundamental understanding of the structural and chemical characteristics of wood (Bouafif et al., 2009). Several attempts have been made to correlate properties of wood-based particles and fibers to WPC properties (Stark and Rowlands, 2003; Borysiak et al., 2006). Maldas et al. (1989) investigated the effect of wood species on the mechanical properties of wood/thermoplastic composites. They observed differences in morphology, density and aspect ratios across wood species, which accounted for varying reinforcement properties in thermoplastic composites. Later, Rowell and co-workers (2000) reported a high aspect ratio (length/ width) is very important in fiber reinforced composites, as it indicates potential strength properties. Lu et al. (2005) reported that * Corresponding author. Tel./fax: +98 21 88838337. E-mail address: [email protected] (A. Ashori). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.11.022
the mechanical properties of the resultant WPC increase only at low weight percentages of wood filler. They found that tensile and flexural strengths reach a maximum at 15 and 35 wt.% wood particle contents, respectively, and gradually decrease with a further increase in wood particle content. Wood is a natural and complex polymeric composite, which essentially contains cellulose, hemicellulose, lignin and extractives. A wide range of different substances is included under the extractives heading: flavonoids, lignans, stilbenes, tannins, inorganic salts, fats, waxes, alkaloids, proteins, simple and complex phenolics, simple sugars, pectins, mucilages, gums, terpenes, starch, glycosides, saponins and essential oils. Extractives content in most temperate and tropical wood species are 4–10% and 20% of the dry weight, respectively (Sjöström, 1993). Although extractives contribute merely a few percent to the entire wood composition, they have significant influence on its properties, such as mechanical strength, and the quality of wood can be affected by the amount and type of these extractives. Shebani et al. (2008) have noted that removing extractives improved the thermal stability of different wood species. Therefore using extracted wood for the production of WPCs would improve the thermal stability of WPCs (Shebani et al., 2009). This work is a comparative study to evaluate the effects of wood species (hardwood and softwood), particle sizes (fiber and flour) and hot-water treatment, on some physical and mechanical properties of WPCs.
2516
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519 Table 2 Abbreviation and composition of the studied composites.
2. Methods 2.1. Materials Wood samples from two different species, namely oak (Quercus castaneifolia) and pine (Pinus eldarica) were used as reinforcing fillers. The lignocellulosic materials were physically and chemically characterized (Table 1). The characteristics of materials were determined following the standards outlined in the TAPPI Test Methods (2002). The procedure for the hot-water solubility followed T 207 cm for 3 h. Reinforcing fillers were applied in the form of fiber and particle. The fibers were provided by refiner mechanical pulping process. Particles of the wood flour screened in the range of 60-mesh. The matrix polymer applied for making composites was polypropylene (PP) having a density of 0.91 g/cm3 and a melt flow index of 8 g/10 min at 230 °C and under 2.16 N load. Mechanical properties of used PP were: tensile strength 28.5 MPa, tensile modulus 1250 MPa, flexural strength 38.5 MPa and flexural modulus 1150 MPa. Maleated polypropylene (MAPP) was obtained from Eastman Chemical Products, Inc. (Epolene G-3003), which has an acid number of 9 mg KOH/g, melting point of 158 °C, and average molecular weight of 52,000.
PP/OF-un PP/OF-ex PP/OP-un PP/OP-ex PP/PF-un PP/PF-ex PP/PP-un PP/PP-ex
PP PP PP PP PP PP PP PP
and and and and and and and and
oak – unextracted fibers oak – extracted fibers oak – unextracted particles oak – extracted particles pine – unextracted fibers pine – extracted fibers pine – unextracted particles pine – extracted particles
tam made pendulum impact tester (model SIT-20D). All the tests were done at room temperature (23 °C) at a relative humidity of 50%. The water absorption test was carried out based on ASTM D 570. The sample was oven dried at 50 °C for 24 h to a constant weight (W0). The specimens were immersed in distilled water for 24 h at a temperature 23 ± 1 °C. Subsequently, the excess water on the surface was wiped off by blotting paper and specimens were weighed using an analytical balance with 0.1 mg precision (Wt). The amount of water absorbed (Mt) was determined by using the following equation:
Mt ð%Þ ¼ ðW t W 0 Þ=W 0 100
ð1Þ
2.3. Measurement All physicomechanical tests were performed according to the standard testing methods. Tensile and flexural properties were measured on an Instron computerized testing machine (model 8112) in accordance with ASTM D 638 and ASTM D 790 procedures, respectively. Crosshead speed was 5 mm/min. Notched Izod impact strength was measured according to ASTM D 256 on San-
Table 1 Chemical compositions and physical characteristics of the used materials. Chemical components
Oaka
Pine
Cellulose (%) Lignin (%) Extracts (%)b Ash (%) Fiber morphology Length (mm) Diameter (lm) Aspect ratio (L/D)
42.3 (0.9) 25.9 (0.7) 9.5 (0.3) 1.30 (0.04)
58.1 (1.3) 27.9 (0.5) 3.1 (0.2) 0.31 (0.06)
1.0 (0.2) 38.2 (1.0) 25.6 (0.7)
2.1 (0.3) 42.9 (0.8) 49.0 (0.9)
Numbers in the parenthesis are standard deviations. Hot-water.
3. Results and discussion As can be seen from Table 1, the two investigated wood species are clearly distinguishable by differences in their compositions, and a different mechanical behavior can therefore be expected. 3.1. Tensile properties The tensile properties of composites containing wood, with and without extractives, are presented in Figs. 1 and 2. As shown in Fig. 1, it is evident that moderate increase in tensile strength occurred upon filling the polymer matrix with fiber/flour, as compared with pure PP. It is interesting that tensile strength for samples with two different species of wood had almost same behavior. Similar results have been published by Karmarkar et al. (2007) who studied the properties of wood-fiber reinforced polypropylene composites. Their data show that the tensile strength of PP/wood fiber composites increases with increasing fiber length. The possible reason proposed for this kind of behavior may be the improved interfacial adhesion between the matrix and fibers. In
30
Tensile strength (MPa)
The mass ratio of the lignocellulosic materials to polymer was 40:60 (w:w) for all blends. MAPP as coupling agent was added at 2 wt.% of the batch weight. Composition of the mixes and abbreviation used for the respective mixes prepared are given in Table 2. All the experiments were performed in a Collin corotating extruder having segmented screw and barrels. During the extrusion, temperatures of the four processing zones were chosen as: 165, 170, 175, and 180 °C and the die temperature was 185 °C. The rotational speed of the segmented screw was 60 rpm, and the pressure at the die was 1500 MPa. The product was recovered by guiding the molten extrudate into a cold water stranding bath. The cooled strands were subsequently pelletized into granules, using a pilot scale grinder (Collin model), dried and stored in sealed plastic bags. Next, test specimens were injection molded at 190 °C to produce standard ASTM specimens.
b
Compounds
where Wt and W0 are the weights of the specimen before and after immersion in water, respectively. Five replicates were run for each composition to obtain a reliable average and standard deviations.
2.2. Composites preparation
a
Abbr.
25 20 15 10 5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 1. Comparison of tensile strength for PP/oak and PP/pine composites.
2517
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519
48
Flexural strength (MPa)
Tensile modulus (MPa)
3000 2500 2000 1500 1000 500 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
40 32 24 16 8 0 PP/OF
PP/PP
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 2. Comparison of tensile modulus for PP/oak and PP/pine composites.
3.2. Flexural properties The effect of particle size and hot-water extractives on the flexural strength and flexural modulus for PP/oak and PP/pine composites are given in Figs. 3 and 4, respectively. It is seen that like tensile properties, the extractive-free samples showed higher flexural strength and flexural modulus compared to the unextracted composites. Both flexural strength and modulus showed a steady increase with increasing particle size. Composite type PP/PF-ex reached the maximum values for flexural strength (43.8 MPa) and flexural modulus (2943 MPa). These results are in good agreement with previously reported data (Bouafif et al., 2009). One of the most important parameters controlling the mechanical properties of WPC is the fiber length or more precisely its aspect ratio. A high aspect ratio is very important in fiber reinforced composites, as it indicates potential strength properties. Stark and Rowlands (2003) reported that aspect ratio, rather than particle size, has the greatest effect on strength and stiffness. As can be seen from
3300 2750 2200 1650 1100 550 0
PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 4. Comparison of flexural modulus for PP/oak and PP/pine composites.
Table 1, pine has high fiber length and aspect ratio compared to the oak. 3.3. Izod impact strength Fig. 5 represents the result of notched Izod impact strength measurement with or without extractives. The pine fiber appeared to improve impact strength in comparison with the oak fiber. With respect to the physical properties (Table 1), this was expected because pine fibers should be more resistant to crack propagation in the matrix. The presence of wood flour in the PP matrix provides points of stress concentrations, thus providing sites for crack initiation. Another reason for the decrease in impact strength may be
30
Impact strength (J/m)
addition, various parameters influence the mechanical properties of fiber reinforced composites including the fiber aspect ratio, fiber–matrix adhesion, stress transfer at the interface and mixing temperatures. The PP/oak composites, which had a low aspect ratio, showed inferior strength compared to the PP/pine composites. In other words, the aspect ratio of the pine fibers is much higher than that of the oak, which permits better stress transfer between the matrix and the fibers. Tensile modulus exhibited a similar trend as that of tensile strength and showed a maximum improvement of 2613 MPa for PP/PF-ex sample, which is 210% higher than that of pure PP matrix. The explanation is similar to that of the tensile strength. As expected, tensile modulus increased with the use of fiber, which is believed to be due to better interfacial bonding between the fiber and matrix. In general, results showed that tensile modulus of the composites was moderately enhanced on using extractive-free lignocellulosic materials in both types (fiber and flour). It is due to the chemical compositions of extractives. Hot-water procedure removes a part of extraneous components, such as inorganic compounds, tannins, gums, sugars, starches and fatty derivates (TAPPI, 2002). They can diffuse to the surface, thus blocking cells and reducing contact of the matrix with the hydroxyl groups (– OH) of cellulosic material (Bledzki et al., 2005). This in turn could reduce adhesion at the interface, which causes inferior interfacial bonding strength as compared to the extracted (extractive-free) samples. It is to be noted that hot-water extractives present in oak is about three times more than pine (Table 1).
Flexural modulus (MPa)
Fig. 3. Comparison of flexural strength for PP/oak and PP/pine composites.
25 20 15 10 5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 5. Comparison of notched impact strength for PP/oak and PP/pine composites.
2518
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519
the stiffening of polymer chains due to bonding between wood flour and matrix. Extractives play a major role in determining the crack initiation process by lowering interaction between the lignocellulosic materials and the coupling agent. It is noteworthy that for specific applications, the impact strength can be increased by using impact modifiers or by using natural fibers having higher microfibril angle (cited by Nourbakhsh et al., 2009b). Like tensile and flexural properties, Fig. 5 illustrates increase in the impact strength as the extractive-free samples were used.
ported by Wichman et al. (1993), who studied the influence of moisture content on fiber/matrix adhesion for HDPE/Pine composites by SEM observations, using scanning electron microscopy. The sample PP/PF-ex showed slightly warpage after 24 h of immersion, which may be due to the internal stress developed inside the composites as a result of high moisture content and poor interfacial interaction between PP and the pine fibers.
3.4. Water absorption
The following conclusions can be drawn from the results and discussions presented above:
Fig. 6 shows the values of the water absorption for the composites, which vary depending upon the wood species, particle sizes and extractives. The PP matrix does not absorb any moisture (Bledzki et al., 2005), indicating that moisture is absorbed by the wood component in the composites. Wood contains numerous free –OH groups present in the cellulosic cell wall materials, which are responsible for interaction with water molecules by hydrogen bonding. The absorption of water by different fiber-based composites is largely dependent on the availability of free –OH groups on the surface of the reinforcing fiber. On unextracted samples, some of these –OH groups are blocked, and, as a result the absorption of water gets restricted. Fig. 6 clearly shows that the water absorption of unextracted composites is less than extractive-free samples. As mentioned earlier, extractives may have acted similar to wax which is normally used to control water absorption. It could results in a significant decrease in the degree of moisture absorption of the composite. According to Das et al. (2000), there are three main regions where the adsorbed water in the composite can reside: the cell wall, the lumen (via porous tubular), and the voids between lignocellulosic material and PP in the case of weak interface adhesion. Since voids and the lumens of fibers/flour were filled with extractives, the penetration of water by the so-called capillary action into the deeper parts of composite was prevented. This may suggest that the water absorption has occurred in the surface layer. It is obvious that wood flour causes slightly higher values of moisture uptake, compared to the fiber filled composites. However, there is no significant difference in moisture sorption ability between the two species (oak and pine), and particle sizes (fiber and flour), as can be seen from Fig. 6. This is due to the fact that particles of wood flour are shorter and finer than fibers and consequently have a much larger surface area per unit weight than fibers. On the other side, fibers have large number of porous tubular structures, which accelerate the penetration of water. Moreover, addition of MAPP causes better adhesion between matrix and fibers, since there are fewer gaps in the interfacial region, and also more hydrophilic groups as hydroxyls are blocked by the coupling effect (Espert et al., 2004). Similar results have been re-
Water absorption (%)
9 7.5 6 4.5 3 1.5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 6. Comparison of water absorption for PP/oak and PP/pine composites.
4. Conclusion
1. The improvement in mechanical properties achieved can be attributed to higher fiber length and aspect ratio of pine compared to the oak. 2. The hydrophilic character of natural fibers is responsible for the water absorption in the WPCs. 3. The difference in absorption of water between extracted and unextracted composites is due to blocking of –OH groups by extractives and MAPP. 4. Hot-water treatment is not recommended for making composites that are exposed to high relative humidity (outdoor applications).
Acknowledgement The authors would like to thank Dr. F. Farhani for pre-reading and useful comments which helped to improve the clarity of the manuscript. References Anuar, H., Wan Busu, W.N., Ahnad, S.H., Rashid, R., 2008. Reinforced thermoplastic natural rubber hybrid composites with Hibiscus cannabinus, L and short glass fiber – part I: processing parameters and tensile properties. Composite Materials 42 (11), 1075–1087. Ashori, A., 2008. Wood–plastic composites as promising green-composites for automotive industries! Bioresource Technology 99 (11), 4661–4667. Bledzki, A.K., Letman, M., Viksne, A., Rence, L., 2005. A comparison of compounding processes and wood type for wood fibre–PP composites. Composites Part A 36 (6), 789–797. Borysiak, S., Paukszta, D., Helwig, M., 2006. Flammability of wood–polypropylene composites. Polymer Degradation and Stability 91 (12), 3339–3343. Bouafif, H., Koubaa, A., Perré, P., Cloutier, A., 2009. Effects of fiber characteristics on the physical and mechanical properties of wood plastic composites. Composites Part A. doi:10.1016/j.compositesa.2009.06.003. Das, S., Sara, A.K., Choudhury, P.K., Basak, R.K., Mitra, B.C., Todd, T., Lang, S., 2000. Effect of steam pretreatment of jute fiber on dimensional stability of jute composite. Applied Polymer Science 76 (11), 1652–1661. Espert, A., Vilaplana, F., Karlsson, S., 2004. Comparison of water absorption in natural cellulosic fibres from wood and one-year crops in polypropylene composites and its influence on their mechanical properties. Composites Part A 35 (11), 1267–1276. Jayaraman, K., Bhattacharyya, D., 2004. Mechanical performance of wood fiber– waste plastic composite materials. Resources, Conservation and Recycling 41 (4), 307–319. Karmarkar, A., Chauhan, S.S., Modak, J.M., Chanda, M., 2007. Mechanical properties of wood fiber reinforced polypropylene composites: effect of a novel compatibilizer with isocyanate functional group. Composites Part A 38 (2), 227–233. Kim, J.P., Yoon, T.H., Mun, S.P., Rhee, J.M., Lee, J.S., 2006. Wood polyethylene composites using ethylene-vinyl alcohol copolymer as adhesion promoter. Bioresource Technology 97 (3), 494–499. Lai, S.M., Yeh, F.C., Wang, Y., Chan, H.C., Shen, H.F., 2003. Comparative study of maleated polyolefins as compatibilizers for polyethylene/wood flour composites. Applied Polymer Science 87 (3), 487–496. Lu, J.Z., Wu, Q., Negulescu, I.I., 2005. Wood-fiber/high-density-polyethylene composites: coupling agent performance. Applied Polymer Science 96 (1), 93– 102. Maldas, D., Kokta, B.V., Daneault, C., 1989. Thermoplastic composites of polystyrene: effect of different wood species on mechanical properties. Applied Polymer Science 38 (3), 413–439.
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519 Neagu, R.C., Gamstedt, E.K., Berthold, F., 2006. Stiffness contribution of various wood fibers to composite materials. Composite Materials 40 (8), 663–699. Nourbakhsh, A., Ashori, A., Kouhpayahzadeh, M., 2009a. Giant milkweed (Calotropis persica) fibers – a potential reinforcement agent for thermoplastics composites. Reinforced Plastics and Composites 28 (17), 2143–2149. Nourbakhsh, A., Karegarfard, A., Ashori, A., Nourbakhsh, A., 2009b. Effects of particle size and coupling agent on mechanical properties of particle-reinforced composites. Thermoplastic Composite Materials. doi:10.1177/ 0892705708340962. Rowell, R.M., Han, J.S., Rowell, J.S., 2000. Characterization and factors effecting fiber properties. In: Natural Polymers and Agrofibers Composites. Sãn Carlos, Brazil, p. 292. Salemane, M.G., Luyt, A.S., 2006. Thermal and mechanical properties of polypropylene–wood powder composites. Applied Polymer Science 100 (5), 4173–4180.
2519
Shebani, A.N., van Reenen, A.J., Meincken, M., 2008. The effect of wood extractives on the thermal stability of different wood species. Thermochimica Acta 471 (1– 2), 43–50. Shebani, A.N., van Reenen, A.J., Meincken, M., 2009. The effect of wood extractives on the thermal stability of different wood–LLDPE composites. Thermochimica Acta 481 (1–2), 52–56. Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Academic, San Diego, USA. Stark, N.M., Rowlands, R.E., 2003. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites. Wood and Fiber Science 35 (2), 167–174. TAPPI, 2002. Tappi Test Methods. Tappi Press, Atlanta, USA. Wichman, I.S., Oladipo, A.B., Hermann, I., 1993. In: Proceedings of Ninth Annual ASM/ESD Advanced Composites Conference, ESD, The Engineering Society, pp. 265–276.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Reinforced polypropylene composites: Effects of chemical compositions and particle size Alireza Ashori a,*, Amir Nourbakhsh b a b
Department of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Department of Wood and Paper Science, Research Institute of Forests and Rangelands (RIFR), Tehran, Iran
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 26 October 2009 Accepted 5 November 2009 Available online 30 November 2009 Keywords: Aspect ratio Fiber reinforced Mechanical properties Lignocellulosic materials Hot-water treatment
a b s t r a c t In this work, the effects of wood species, particle sizes and hot-water treatment on some physical and mechanical properties of wood–plastic composites were studied. Composites of thermoplastic reinforced with oak (Quercus castaneifolia) and pine (Pinus eldarica) wood were prepared. Polypropylene (PP) and maleic anhydride grafted polypropylene (MAPP) were used as the polymer matrix and coupling agent, respectively. The results showed that pine fiber had significant effect on the mechanical properties considered in this study. This effect is explained by the higher fiber length and aspect ratio of pine compared to the oak fiber. The hot-water treated (extractive-free) samples, in both wood species, improved the tensile, flexural and impact properties, but increased the water absorption for 24 h. This work clearly showed that lignocellulosic materials in both forms of fiber and flour could be effectively used as reinforcing elements in PP matrix. Furthermore, extractives have marked effects on the mechanical and physical properties. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Although there have been quite a number of studies on the wood–plastic composites (WPCs) as new generation of reinforcing materials in recent years (Lai et al., 2003; Jayaraman and Bhattacharyya, 2004; Kim et al., 2006; Salemane and Luyt, 2006; Neagu et al., 2006; Anuar et al., 2008; Ashori, 2008; Nourbakhsh et al., 2009a), so far, not much attention has been paid to the effect of chemical composition of wood on the mechanical and physical properties. The effective use of wood-based particles and fibers as fillers or reinforcements in thermoplastic composites requires a fundamental understanding of the structural and chemical characteristics of wood (Bouafif et al., 2009). Several attempts have been made to correlate properties of wood-based particles and fibers to WPC properties (Stark and Rowlands, 2003; Borysiak et al., 2006). Maldas et al. (1989) investigated the effect of wood species on the mechanical properties of wood/thermoplastic composites. They observed differences in morphology, density and aspect ratios across wood species, which accounted for varying reinforcement properties in thermoplastic composites. Later, Rowell and co-workers (2000) reported a high aspect ratio (length/ width) is very important in fiber reinforced composites, as it indicates potential strength properties. Lu et al. (2005) reported that * Corresponding author. Tel./fax: +98 21 88838337. E-mail address: [email protected] (A. Ashori). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.11.022
the mechanical properties of the resultant WPC increase only at low weight percentages of wood filler. They found that tensile and flexural strengths reach a maximum at 15 and 35 wt.% wood particle contents, respectively, and gradually decrease with a further increase in wood particle content. Wood is a natural and complex polymeric composite, which essentially contains cellulose, hemicellulose, lignin and extractives. A wide range of different substances is included under the extractives heading: flavonoids, lignans, stilbenes, tannins, inorganic salts, fats, waxes, alkaloids, proteins, simple and complex phenolics, simple sugars, pectins, mucilages, gums, terpenes, starch, glycosides, saponins and essential oils. Extractives content in most temperate and tropical wood species are 4–10% and 20% of the dry weight, respectively (Sjöström, 1993). Although extractives contribute merely a few percent to the entire wood composition, they have significant influence on its properties, such as mechanical strength, and the quality of wood can be affected by the amount and type of these extractives. Shebani et al. (2008) have noted that removing extractives improved the thermal stability of different wood species. Therefore using extracted wood for the production of WPCs would improve the thermal stability of WPCs (Shebani et al., 2009). This work is a comparative study to evaluate the effects of wood species (hardwood and softwood), particle sizes (fiber and flour) and hot-water treatment, on some physical and mechanical properties of WPCs.
2516
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519 Table 2 Abbreviation and composition of the studied composites.
2. Methods 2.1. Materials Wood samples from two different species, namely oak (Quercus castaneifolia) and pine (Pinus eldarica) were used as reinforcing fillers. The lignocellulosic materials were physically and chemically characterized (Table 1). The characteristics of materials were determined following the standards outlined in the TAPPI Test Methods (2002). The procedure for the hot-water solubility followed T 207 cm for 3 h. Reinforcing fillers were applied in the form of fiber and particle. The fibers were provided by refiner mechanical pulping process. Particles of the wood flour screened in the range of 60-mesh. The matrix polymer applied for making composites was polypropylene (PP) having a density of 0.91 g/cm3 and a melt flow index of 8 g/10 min at 230 °C and under 2.16 N load. Mechanical properties of used PP were: tensile strength 28.5 MPa, tensile modulus 1250 MPa, flexural strength 38.5 MPa and flexural modulus 1150 MPa. Maleated polypropylene (MAPP) was obtained from Eastman Chemical Products, Inc. (Epolene G-3003), which has an acid number of 9 mg KOH/g, melting point of 158 °C, and average molecular weight of 52,000.
PP/OF-un PP/OF-ex PP/OP-un PP/OP-ex PP/PF-un PP/PF-ex PP/PP-un PP/PP-ex
PP PP PP PP PP PP PP PP
and and and and and and and and
oak – unextracted fibers oak – extracted fibers oak – unextracted particles oak – extracted particles pine – unextracted fibers pine – extracted fibers pine – unextracted particles pine – extracted particles
tam made pendulum impact tester (model SIT-20D). All the tests were done at room temperature (23 °C) at a relative humidity of 50%. The water absorption test was carried out based on ASTM D 570. The sample was oven dried at 50 °C for 24 h to a constant weight (W0). The specimens were immersed in distilled water for 24 h at a temperature 23 ± 1 °C. Subsequently, the excess water on the surface was wiped off by blotting paper and specimens were weighed using an analytical balance with 0.1 mg precision (Wt). The amount of water absorbed (Mt) was determined by using the following equation:
Mt ð%Þ ¼ ðW t W 0 Þ=W 0 100
ð1Þ
2.3. Measurement All physicomechanical tests were performed according to the standard testing methods. Tensile and flexural properties were measured on an Instron computerized testing machine (model 8112) in accordance with ASTM D 638 and ASTM D 790 procedures, respectively. Crosshead speed was 5 mm/min. Notched Izod impact strength was measured according to ASTM D 256 on San-
Table 1 Chemical compositions and physical characteristics of the used materials. Chemical components
Oaka
Pine
Cellulose (%) Lignin (%) Extracts (%)b Ash (%) Fiber morphology Length (mm) Diameter (lm) Aspect ratio (L/D)
42.3 (0.9) 25.9 (0.7) 9.5 (0.3) 1.30 (0.04)
58.1 (1.3) 27.9 (0.5) 3.1 (0.2) 0.31 (0.06)
1.0 (0.2) 38.2 (1.0) 25.6 (0.7)
2.1 (0.3) 42.9 (0.8) 49.0 (0.9)
Numbers in the parenthesis are standard deviations. Hot-water.
3. Results and discussion As can be seen from Table 1, the two investigated wood species are clearly distinguishable by differences in their compositions, and a different mechanical behavior can therefore be expected. 3.1. Tensile properties The tensile properties of composites containing wood, with and without extractives, are presented in Figs. 1 and 2. As shown in Fig. 1, it is evident that moderate increase in tensile strength occurred upon filling the polymer matrix with fiber/flour, as compared with pure PP. It is interesting that tensile strength for samples with two different species of wood had almost same behavior. Similar results have been published by Karmarkar et al. (2007) who studied the properties of wood-fiber reinforced polypropylene composites. Their data show that the tensile strength of PP/wood fiber composites increases with increasing fiber length. The possible reason proposed for this kind of behavior may be the improved interfacial adhesion between the matrix and fibers. In
30
Tensile strength (MPa)
The mass ratio of the lignocellulosic materials to polymer was 40:60 (w:w) for all blends. MAPP as coupling agent was added at 2 wt.% of the batch weight. Composition of the mixes and abbreviation used for the respective mixes prepared are given in Table 2. All the experiments were performed in a Collin corotating extruder having segmented screw and barrels. During the extrusion, temperatures of the four processing zones were chosen as: 165, 170, 175, and 180 °C and the die temperature was 185 °C. The rotational speed of the segmented screw was 60 rpm, and the pressure at the die was 1500 MPa. The product was recovered by guiding the molten extrudate into a cold water stranding bath. The cooled strands were subsequently pelletized into granules, using a pilot scale grinder (Collin model), dried and stored in sealed plastic bags. Next, test specimens were injection molded at 190 °C to produce standard ASTM specimens.
b
Compounds
where Wt and W0 are the weights of the specimen before and after immersion in water, respectively. Five replicates were run for each composition to obtain a reliable average and standard deviations.
2.2. Composites preparation
a
Abbr.
25 20 15 10 5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 1. Comparison of tensile strength for PP/oak and PP/pine composites.
2517
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519
48
Flexural strength (MPa)
Tensile modulus (MPa)
3000 2500 2000 1500 1000 500 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
40 32 24 16 8 0 PP/OF
PP/PP
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 2. Comparison of tensile modulus for PP/oak and PP/pine composites.
3.2. Flexural properties The effect of particle size and hot-water extractives on the flexural strength and flexural modulus for PP/oak and PP/pine composites are given in Figs. 3 and 4, respectively. It is seen that like tensile properties, the extractive-free samples showed higher flexural strength and flexural modulus compared to the unextracted composites. Both flexural strength and modulus showed a steady increase with increasing particle size. Composite type PP/PF-ex reached the maximum values for flexural strength (43.8 MPa) and flexural modulus (2943 MPa). These results are in good agreement with previously reported data (Bouafif et al., 2009). One of the most important parameters controlling the mechanical properties of WPC is the fiber length or more precisely its aspect ratio. A high aspect ratio is very important in fiber reinforced composites, as it indicates potential strength properties. Stark and Rowlands (2003) reported that aspect ratio, rather than particle size, has the greatest effect on strength and stiffness. As can be seen from
3300 2750 2200 1650 1100 550 0
PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 4. Comparison of flexural modulus for PP/oak and PP/pine composites.
Table 1, pine has high fiber length and aspect ratio compared to the oak. 3.3. Izod impact strength Fig. 5 represents the result of notched Izod impact strength measurement with or without extractives. The pine fiber appeared to improve impact strength in comparison with the oak fiber. With respect to the physical properties (Table 1), this was expected because pine fibers should be more resistant to crack propagation in the matrix. The presence of wood flour in the PP matrix provides points of stress concentrations, thus providing sites for crack initiation. Another reason for the decrease in impact strength may be
30
Impact strength (J/m)
addition, various parameters influence the mechanical properties of fiber reinforced composites including the fiber aspect ratio, fiber–matrix adhesion, stress transfer at the interface and mixing temperatures. The PP/oak composites, which had a low aspect ratio, showed inferior strength compared to the PP/pine composites. In other words, the aspect ratio of the pine fibers is much higher than that of the oak, which permits better stress transfer between the matrix and the fibers. Tensile modulus exhibited a similar trend as that of tensile strength and showed a maximum improvement of 2613 MPa for PP/PF-ex sample, which is 210% higher than that of pure PP matrix. The explanation is similar to that of the tensile strength. As expected, tensile modulus increased with the use of fiber, which is believed to be due to better interfacial bonding between the fiber and matrix. In general, results showed that tensile modulus of the composites was moderately enhanced on using extractive-free lignocellulosic materials in both types (fiber and flour). It is due to the chemical compositions of extractives. Hot-water procedure removes a part of extraneous components, such as inorganic compounds, tannins, gums, sugars, starches and fatty derivates (TAPPI, 2002). They can diffuse to the surface, thus blocking cells and reducing contact of the matrix with the hydroxyl groups (– OH) of cellulosic material (Bledzki et al., 2005). This in turn could reduce adhesion at the interface, which causes inferior interfacial bonding strength as compared to the extracted (extractive-free) samples. It is to be noted that hot-water extractives present in oak is about three times more than pine (Table 1).
Flexural modulus (MPa)
Fig. 3. Comparison of flexural strength for PP/oak and PP/pine composites.
25 20 15 10 5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 5. Comparison of notched impact strength for PP/oak and PP/pine composites.
2518
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519
the stiffening of polymer chains due to bonding between wood flour and matrix. Extractives play a major role in determining the crack initiation process by lowering interaction between the lignocellulosic materials and the coupling agent. It is noteworthy that for specific applications, the impact strength can be increased by using impact modifiers or by using natural fibers having higher microfibril angle (cited by Nourbakhsh et al., 2009b). Like tensile and flexural properties, Fig. 5 illustrates increase in the impact strength as the extractive-free samples were used.
ported by Wichman et al. (1993), who studied the influence of moisture content on fiber/matrix adhesion for HDPE/Pine composites by SEM observations, using scanning electron microscopy. The sample PP/PF-ex showed slightly warpage after 24 h of immersion, which may be due to the internal stress developed inside the composites as a result of high moisture content and poor interfacial interaction between PP and the pine fibers.
3.4. Water absorption
The following conclusions can be drawn from the results and discussions presented above:
Fig. 6 shows the values of the water absorption for the composites, which vary depending upon the wood species, particle sizes and extractives. The PP matrix does not absorb any moisture (Bledzki et al., 2005), indicating that moisture is absorbed by the wood component in the composites. Wood contains numerous free –OH groups present in the cellulosic cell wall materials, which are responsible for interaction with water molecules by hydrogen bonding. The absorption of water by different fiber-based composites is largely dependent on the availability of free –OH groups on the surface of the reinforcing fiber. On unextracted samples, some of these –OH groups are blocked, and, as a result the absorption of water gets restricted. Fig. 6 clearly shows that the water absorption of unextracted composites is less than extractive-free samples. As mentioned earlier, extractives may have acted similar to wax which is normally used to control water absorption. It could results in a significant decrease in the degree of moisture absorption of the composite. According to Das et al. (2000), there are three main regions where the adsorbed water in the composite can reside: the cell wall, the lumen (via porous tubular), and the voids between lignocellulosic material and PP in the case of weak interface adhesion. Since voids and the lumens of fibers/flour were filled with extractives, the penetration of water by the so-called capillary action into the deeper parts of composite was prevented. This may suggest that the water absorption has occurred in the surface layer. It is obvious that wood flour causes slightly higher values of moisture uptake, compared to the fiber filled composites. However, there is no significant difference in moisture sorption ability between the two species (oak and pine), and particle sizes (fiber and flour), as can be seen from Fig. 6. This is due to the fact that particles of wood flour are shorter and finer than fibers and consequently have a much larger surface area per unit weight than fibers. On the other side, fibers have large number of porous tubular structures, which accelerate the penetration of water. Moreover, addition of MAPP causes better adhesion between matrix and fibers, since there are fewer gaps in the interfacial region, and also more hydrophilic groups as hydroxyls are blocked by the coupling effect (Espert et al., 2004). Similar results have been re-
Water absorption (%)
9 7.5 6 4.5 3 1.5 0 PP/OF
PP/OP
Extracted Unextracted PP/PF
PP/PP
Fig. 6. Comparison of water absorption for PP/oak and PP/pine composites.
4. Conclusion
1. The improvement in mechanical properties achieved can be attributed to higher fiber length and aspect ratio of pine compared to the oak. 2. The hydrophilic character of natural fibers is responsible for the water absorption in the WPCs. 3. The difference in absorption of water between extracted and unextracted composites is due to blocking of –OH groups by extractives and MAPP. 4. Hot-water treatment is not recommended for making composites that are exposed to high relative humidity (outdoor applications).
Acknowledgement The authors would like to thank Dr. F. Farhani for pre-reading and useful comments which helped to improve the clarity of the manuscript. References Anuar, H., Wan Busu, W.N., Ahnad, S.H., Rashid, R., 2008. Reinforced thermoplastic natural rubber hybrid composites with Hibiscus cannabinus, L and short glass fiber – part I: processing parameters and tensile properties. Composite Materials 42 (11), 1075–1087. Ashori, A., 2008. Wood–plastic composites as promising green-composites for automotive industries! Bioresource Technology 99 (11), 4661–4667. Bledzki, A.K., Letman, M., Viksne, A., Rence, L., 2005. A comparison of compounding processes and wood type for wood fibre–PP composites. Composites Part A 36 (6), 789–797. Borysiak, S., Paukszta, D., Helwig, M., 2006. Flammability of wood–polypropylene composites. Polymer Degradation and Stability 91 (12), 3339–3343. Bouafif, H., Koubaa, A., Perré, P., Cloutier, A., 2009. Effects of fiber characteristics on the physical and mechanical properties of wood plastic composites. Composites Part A. doi:10.1016/j.compositesa.2009.06.003. Das, S., Sara, A.K., Choudhury, P.K., Basak, R.K., Mitra, B.C., Todd, T., Lang, S., 2000. Effect of steam pretreatment of jute fiber on dimensional stability of jute composite. Applied Polymer Science 76 (11), 1652–1661. Espert, A., Vilaplana, F., Karlsson, S., 2004. Comparison of water absorption in natural cellulosic fibres from wood and one-year crops in polypropylene composites and its influence on their mechanical properties. Composites Part A 35 (11), 1267–1276. Jayaraman, K., Bhattacharyya, D., 2004. Mechanical performance of wood fiber– waste plastic composite materials. Resources, Conservation and Recycling 41 (4), 307–319. Karmarkar, A., Chauhan, S.S., Modak, J.M., Chanda, M., 2007. Mechanical properties of wood fiber reinforced polypropylene composites: effect of a novel compatibilizer with isocyanate functional group. Composites Part A 38 (2), 227–233. Kim, J.P., Yoon, T.H., Mun, S.P., Rhee, J.M., Lee, J.S., 2006. Wood polyethylene composites using ethylene-vinyl alcohol copolymer as adhesion promoter. Bioresource Technology 97 (3), 494–499. Lai, S.M., Yeh, F.C., Wang, Y., Chan, H.C., Shen, H.F., 2003. Comparative study of maleated polyolefins as compatibilizers for polyethylene/wood flour composites. Applied Polymer Science 87 (3), 487–496. Lu, J.Z., Wu, Q., Negulescu, I.I., 2005. Wood-fiber/high-density-polyethylene composites: coupling agent performance. Applied Polymer Science 96 (1), 93– 102. Maldas, D., Kokta, B.V., Daneault, C., 1989. Thermoplastic composites of polystyrene: effect of different wood species on mechanical properties. Applied Polymer Science 38 (3), 413–439.
A. Ashori, A. Nourbakhsh / Bioresource Technology 101 (2010) 2515–2519 Neagu, R.C., Gamstedt, E.K., Berthold, F., 2006. Stiffness contribution of various wood fibers to composite materials. Composite Materials 40 (8), 663–699. Nourbakhsh, A., Ashori, A., Kouhpayahzadeh, M., 2009a. Giant milkweed (Calotropis persica) fibers – a potential reinforcement agent for thermoplastics composites. Reinforced Plastics and Composites 28 (17), 2143–2149. Nourbakhsh, A., Karegarfard, A., Ashori, A., Nourbakhsh, A., 2009b. Effects of particle size and coupling agent on mechanical properties of particle-reinforced composites. Thermoplastic Composite Materials. doi:10.1177/ 0892705708340962. Rowell, R.M., Han, J.S., Rowell, J.S., 2000. Characterization and factors effecting fiber properties. In: Natural Polymers and Agrofibers Composites. Sãn Carlos, Brazil, p. 292. Salemane, M.G., Luyt, A.S., 2006. Thermal and mechanical properties of polypropylene–wood powder composites. Applied Polymer Science 100 (5), 4173–4180.
2519
Shebani, A.N., van Reenen, A.J., Meincken, M., 2008. The effect of wood extractives on the thermal stability of different wood species. Thermochimica Acta 471 (1– 2), 43–50. Shebani, A.N., van Reenen, A.J., Meincken, M., 2009. The effect of wood extractives on the thermal stability of different wood–LLDPE composites. Thermochimica Acta 481 (1–2), 52–56. Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Academic, San Diego, USA. Stark, N.M., Rowlands, R.E., 2003. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites. Wood and Fiber Science 35 (2), 167–174. TAPPI, 2002. Tappi Test Methods. Tappi Press, Atlanta, USA. Wichman, I.S., Oladipo, A.B., Hermann, I., 1993. In: Proceedings of Ninth Annual ASM/ESD Advanced Composites Conference, ESD, The Engineering Society, pp. 265–276.