2019 Volume 7 Pages 1-9
Wood plastic composite is a relatively new generation of composite material prepared from wood flour, particles, or fibers combined with thermoplastic materials under specific heating and pressure conditions. Wood plastic composite has several advantages, such as ease of maintenance, high durability, and long service life. It can be produced from recycled materials, and several additives can be added to improve its properties. The characteristics of the filler used for manufacturing wood plastic composite influence the physical and mechanical properties of the obtained composite. Filler materials include wood-based filler, other natural fibers, and recycled materials. The composition of the raw material affects the mechanical properties of wood plastic composite. As increasing particle size, melt flow index, flexural and tensile strength, flexural and tensile modulus, heat deflection temperature, and notched impact energy of the composites increase. Chemical treatments such as treatment with NaOH, dilute HCl solution, and chromated copper arsenate can be applied to improve the mechanical properties of wood plastic composite.
Wood plastic composite (WPC) is a common term referring to composite wood-based materials and polymers (Gardner et al., 2015) prepared under specific heat and pressure conditions (Rahman et al., 2013), by various production techniques (Najafi et al., 2011). Table 1 shows the number of reported studies on the WPC theme, in the last three decades, according to the Web of Science database. An increase in research interest on WPC theme is evident. In 1991 only 6 studies were available; a sharp increase since the beginning of 2010 is noticeable. This indicates that the WPC field has good prospects and indeed has many advantages including better thermal and acoustic isolation, better durability, and lower maintenance requirement than other wood-based materials (Garcia et al., 2009).
According to Schwarzkopf and Burnard (2016), the application of WPC can be classified into five categories (see Table 2). These categories include, among others construction, automotive, furniture, highway materials, consumer goods, and other WPC applications. One of the largest commercial applications for WPC is in automotive interior substrates, following by furniture, packaging, housing (English and Falk, 1996), and cladding applications (Friedrich and Luible, 2016). The most abundant profiles made from WPC are boards or lumber used in outdoor decking applications (Gardner et al., 2015). Figure 1 shows an example outdoor flooring application of WPC products.
An example for outdoor flooring application of WPC products
An unused biomass resources can be utilized as a filler of WPC (Isa et al., 2014). When using wood flour as a filler, several factors, such as species and moisture content, must be considered (Stark and Berger, 1997). These parameters can be controlled by the raw material selection in the manufacturing process of WPC. One of the critical parameters influencing the strength properties of WPC is the size of wood flour (Nourbakhsh et al., 2010).
Waste that is difficult to decompose can pose serious problems for the environment. Therefore, significant efforts are needed to reduce the emission of waste and to increase the value of recycled products. The recycled materials and other fibers can be used in the manufacturing of WPC. Sawdust (a waste from wood processing industries), bagasse, rice hull (agro-waste), and recycled plastic can be used as raw materials for WPC. They could improve the value and functionality of WPC and help minimize the waste (Rahman et al., 2013). Agro-wastes such as bagasse, rice straw, and wood flour are very cheap, easily available, and renewable (Naguib et al., 2015). Figure 2 shows the wood flour of Pinus densiflora as a raw material that can be used on manufacturing WPC. The compatibility of the interface between the non-polar plastic and the polar wood flour is improved by using a coupling agent that provides a chemical interaction between the two (Animpong et al., 2017). Mechanical properties of WPC have been reported to be enhanced by adding the coupling agent (Isa et al., 2013; Martins et al., 2017).
(a) Wood flour of Pinus densiflora; (b) and the wood flour SEM image on 30 times of magnification
To improve the resistance of the composite product against fungi and termite attacks, especially for outdoor applications (Tazi et al., 2015), chemical treatments can be applied. For the last 50 years, chromated copper arsenate (CCA) has been the most commonly used commercial wood preservative found in such exterior applications (Tascioglu et al., 2014). Other properties like weatherability and fire retardancy must also be improved to develop safe composites (Garcia et al., 2009).
WPC can be used as an alternative to meet the high demand for limited timber sources caused the deforestation issues. The characteristics of the filler such as filler materials, size and shape, raw material composition, and chemical pretreatment are important factors that must be considered for the manufacturing process of WPC. These factors suggested will affect the quality of the WPC, especially in its physical and mechanical properties. This review highlights the influence of filler characteristics, i.e. sources of fillers, the composition of raw materials, size and shapes of fillers and chemical pretreatment, on the physical and mechanical properties of WPC.
Wood is the most abundant biomass resource and has attracted considerable attention as a reinforcement filler combined with polymers in the preparation of WPC (Iwamoto et al., 2014). WPC technology continues to mature, with improvements being made in the manufacturing processes, profiles and parts, durability, and the development of product standards for building construction. New developments for WPC are being made especially in the area of nano-additives like nanocellulose for new types of products and market application areas (Gardner et al., 2015).
The influence of different wood species (8 hardwoods and 2 softwoods) on the appearance and durability performance of WPC was investigated by Kim et al. (2008). The WPC made with different wood species showed the variation on color. However, they became lighter and similarly in appearance after outdoor exposure. The WPC made from eastern red cedar and Osage orange showed a lower water sorption and lower levels of fungal decay than those made from other species (Kim et al., 2008). Tisserat et al. (2013) evaluated the Paulownia wood flour as a reinforcement for thermoplastic composites. The composite made from Paulownia wood flour and maleic anhydride polyethylene (MAPE) pellets showed significantly higher tensile and flexural strengths than neat recycled high-density polyethylene (HDPE). Flexural and impact strengths of WPC with Paulownia wood flour were comparable with or superior to the WPC made from pine filler.
Najafi et al. (2011) studied the effect of lignocellulosic fillers, such as flour of rice hull, wood sawdust, sanding flour from medium density fiberboard (MDF), and sawdust from particleboard, on the mechanical properties of HDPE composite. The composites containing sanding flour from MDF showed higher short-term water absorption and thickness swelling. Composites made from sawdust and rice hull had higher and lowest for water absorption, thickness swelling, and long-term diffusion coefficients, respectively. Composites made from sanding flour of MDF and particleboard sawdust showed higher flexural strength than the other composites studied.
The physical and mechanical properties of WPC made from particleboard and polypropylene were evaluated by Gozdecki et al. (2015). Recycled wood particles, virgin wood particles, and wood flour were used as fillers for the manufacturing of WPC. The properties of WPC made from recycled wood particles did not significantly differ from those of WPC made from virgin wood particles, and were comparable with WPC made from wood flour. These results suggested that the particles derived from milled particleboard proved to be an effective alternative of the wood component of WPC.
Martins et al. (2017) reported the optimization of WPC made from industrial residues of pine sawdust, HDPE, and MAPE as a coupling agent. The results showed that the best properties of composites were obtained for the composition of 55 wt% HDPE, 35 wt% fine wood particle, and 10 wt% MAPE. It can be processed by extrusion and possesses enough tensile strength (22 MPa) for application as a shutter unit in a shading system. The high mechanical strength is due to the high amount of filler used and the good interfacial adhesion obtained by the addition of coupling agent. Physical and mechanical properties of WPC made from Sengon and recycled HDPE were evaluated by Arnandha et al. (2017). WPC had higher compression and shear strength compared to the common Sengon wood itself. They found that the WPC that made from Sengon and recycled HDPE is suitable for both outdoor and indoor applications.
Outdoor usage of WPC requires that its service life and safety must be considered because changes in weather increase the corrosion (Zhang et al., 2016) and the risk of damage due to microbial colonization (Sudár et al., 2013). Catto et al. (2016) investigated the effects of natural weathering on WPC and subsequent material degradation by soil and fungi, and reported that natural weathering by UV radiation, rain water, and high temperatures produced micro- and macro-cracks in the WPC surface, which accelerate the subsequent biodegradation in soil and fungal decay. The raw material (types of wood and the coupling agent) used in the manufacturing of WPC also influences the degradation process. Ogunleye and Aina (2017) found that the density, dimensional properties, and strength of WPC decreased but its elasticity modulus increased with increasing plastic content. Isa et al. (2013) evaluated the effect of three factors—plastic content, coupling agent (two different kind of MAPE: M603 and E265 were used), and micro-fibrillated cellulose on the water resistance of bamboo flour/plastic composite. The water resistance increased with increasing plastic content. As can be seen in Fig. 3, the water resistance of the composite added MAPE was higher compared to the composites without MAPE. Rheological and mechanical properties of the composite were improved by using higher contents of the coupling agent and micro-fibrillated cellulose.
Effect of a coupling agent on water absorption of WPC. M603 added 2wt% of MAPE with 2.5 g/min of melt flow rate and 108 °C of melting temperature. E265 added 2wt% of MAPE with 1.2 g/min of melt flow rate and 131 °C of melting temperature (Isa et al., 2013)
Turku et al. (2017) investigated the possibility of using recycled plastic waste for manufacturing WPC, in terms of flexural, tensile, un-notched impact strength, hardness properties, and water absorption. They found that the strength of the composites was lower than those of the reference WPC manufactured from virgin low-density polyethylene (LDPE), while the hardness of composites was comparable and stiffness was higher than those of the LDPE-based WPC. The plastics recycled from electronic waste and recycled particleboard can be used as a raw material for WPC (Sommerhuber et al., 2016). The water uptake, density, tensile strength, and modulus of elasticity (MOE) of WPC increased by increasing the wood content. WPC made from Norway spruce exhibited reinforced strength and stiffness, while WPC made from particleboard showed reduced strength properties.
Rahman et al. (2013) investigated the technical evaluation of WPC fabricated from sawdust and recycled polyethylene terephthalate (PET) mixed at different ratios. They found that the physical and mechanical properties of WPC depend on the raw material and these mixing ratios. Increasing PET content reduced the moisture content, water absorption, and thickness swelling of the composites. As increasing immersion temperature, water absorption and thickness swelling increased. These results indicated that the fabrication of WPC from mixed sawdust and the recycled PET is technically feasible and additives like a coupling agent can be used for further improvement of the properties. However, heavy metal elements such as cadmium, chromium, copper and lead were found in the recycled resources by elemental analysis. Therefore, more sophisticated waste management systems, policies, and treatment technologies are needed (Sommerhuber et al., 2016).
Currently, many researchers are studying the effect of the composition of raw materials on the manufacturing of WPC. Nourbakhsh et al. (2010) investigated that wood flour could be considered a potential source of low-cost natural fibers for composites. As increasing filler particle size and aspect ratio, the mechanical properties of composites increased due to formation of strong interfacial bonds among the filler, coupling agent, and the matrix. Composites containing 2 wt% maleic anhydride polypropylene (MAPP) showed higher tensile and impact properties than those without treatment. This observation was supported by Stark and Rowlands (2003), who found that the strength of wood flour/polypropylene composites increased upon using higher aspect ratio of wood fibers and adding the coupling agent.
Stark and Berger (1997) studied the effects of wood flour species and particle size on the mechanical properties of polypropylene filled with wood flour. With increasing wood flour content, tensile and flexural moduli, density, heat deflection temperature, and notched impact energy of composites increased. However, tensile and flexural strength, tensile elongation, mold shrinkage, melt flow index, and un-notched impact energy decreased.
Leu et al. (2012) investigated the effects of changing the material composition on the physical and mechanical properties of extruded WPC made from recycled polypropylene and spruce, using pine and fir wood flour. The optimal composition was found to be 47 wt% wood flour (100–120 mesh size), 47 wt% recycled polypropylene, 3 wt% coupling agent (MAPP), and 0–3 wt% ZnSt as the lubricant. Increasing wood content improves the flexural and tensile modulus, but was less favorable for moisture content, thickness swelling, and tensile strength. Adding the proper amount of the coupling agent could improve the mechanical properties and significantly reduce the swelling; however, excessive addition of lubricant significantly increased the swelling and reduced the mechanical properties of WPC.
Tazi et al. (2015) studied the mechanical properties and structure of WPC made from HDPE with different sawdust contents in the presence of a coupling agent. Wood flour addition increased the degree of crystallinity and improved the tensile strength and the ductility of WPC. However, increasing wood flour content reduced water resistance. Although the addition of sawdust improved the mechanical properties, it accelerated the biodegradation of WPC. The addition of wood fibers increases the tensile, flexural, and compression properties of WPC (Garcia et al., 2009).
Poly (methylene (polyphenyl isocyanate)) (PMPPIC) can be used as a coupling agent to improve the compatibility of hydrophilic wood fibers and hydrophobic polymer matrix (Maldas et al., 1989). Adding 2 wt% PMPPIC provided the largest increase in mechanical properties like impact strength compared with those of non-treated composites or composites treated with 8 wt% PMPPIC. Fibers can be incorporated without adversely affecting performance up to a maximum content of 20–30%. They found that flexible softwood spruce pulps provided higher reinforcement to WPC mechanical properties than denser hardwood birch or hardwood aspen pulps.
The effects of material compositions, including different plastic matrices, wood flour, and coupling agents on the mechanical properties of WPC manufactured by injection, have been investigated by Kuo et al. (2009). They found that the tensile strength and modulus of rupture (MOR) of WPC manufactured with LDPE and polypropylene were higher than those of LDPE and polypropylene themselves. However, a significant decrease of MOR was found when the WPC using the polymer matrix of acrylonitrile-butadiene-styrene. WPC manufactured with recycled polypropylene showed higher MOR but lower tensile strength than recycled polypropylene itself. Kuo et al. (2009) reported that superior properties were found for WPC manufactured using polypropylene mixed with 47 wt% wood flour (< 180 µm) and 3–4.5% MAPP.
WPC with high flexural and admirable physical properties can be produced from LDPE, sawn sawdust, and by using automotive engine oil as a coupling agent (Animpong et al., 2017). The optimum properties of WPC, which allowed for outdoor applications, were found for a plastics:sawdust:coupling agent weight ratio of 34:54:10. The optimum amount of fillers used in the manufacturing of WPC was less than 60 wt%.
Two kinds of step, wet milling or dry milling by a disk mill can be applied for filler production of WPC. This procedure makes it easy to produce an uniform size of fibrillated wood flour. The mechanical properties of polypropylene-based WPC were improved by the addition of the produced wood flour (Ito et al., 2017). Tensile and bending properties of the composites containing the wood filler were 10% higher than those of the unfilled composite. The average diameter of the wood flour decreased and the degree of fibrillation of wood fibers increased with the ball milling time (1 to 16 h) at each rotational speed (150, 200, and 250 rpm) (Isa et al., 2014).
The effect of the particle size (Leu et al., 2012), coupling agent, and types of filler on tensile and flexural properties of Paulownia WPC was studied by Tisserat at al. (2014). They found that the particle size distribution of Paulownia wood filler affects the tensile and flexural properties of the composites. Addition of dried distiller's grain with solubles combined with Paulownia wood fillers showed superior tensile and flexural modulus, and impact strength of the WPC. However, the flexural and tensile strengths decreased with increasing size of the wood flour (Leu et al., 2012). Salemane and Luyt (2006) studied the effect of the wood flour and compatibilizer content on the properties of the composites. They found that the wood flour size and content, as well as the presence of MAPP, played a significant role in the tensile properties, thermal stability, and oxygen permeability. The tensile properties of the composites improved owing to the presence of MAPP. The increase of wood flour generated better tensile properties. Isa et al. (2016) evaluated the influence of the wood flour on the physical properties of the wood flour / polypropylene composites. The morphological change of the wood flour in case of the milling process reduced its influence on the physical properties of the composites, such as increased compound fluidity. Addition of the wood flour with nanoscale surface fibrils to the polypropylene composites positively influenced the physical properties of the composites.
Zimmermann et al. (2014) investigated the effects of different particle sizes and content of the wood flour on the mechanical properties of cellular polyethylene - co - vinyl acetate wood flour composites. They found that decreasing the particle size of the wood flour increased the viscosity of the composite. The presence of the wood flour in the cellular composite increased the nucleation of cells (the voids created by the blowing agent in the polymer matrix), providing a larger number of smaller cells with increased filler content. The highest cell density and cell size homogenity were observed in the composites made with wood flour 80–150 mesh. As increasing particle size, melt flow index, flexural and tensile properties, heat deflection temperature, and notched impact energy of the composites increased (Stark and Berger 1997). However, as increasing particle size, the un-notched impact energy decreased and did not affect the specific gravity of the composites.
The effect of oil palm mesocarp flour (OMF) and rubber wood flour of different particle sizes on the physical, mechanical, and thermal properties of recycled polypropylene composites have been evaluated by Ratanawilai et al. (2014). Recycled polypropylene composites based on rubber wood flour have better flexural, tensile, and compressive properties (strength and modulus), and higher hardness and thermal stability than composites prepared using OMF of the same particle size as rubber wood flour. However, rubber wood flour was less homogeneuously distributed in the recycled polypropylene matrix than OMF. Decreasing the particle size of filler for the recycled polypropylene/OMF or rubber wood flour, improved the tensile, flexural, and compressive properties as well as hardness. Thermal stability of the composites was considerably affected by the particle size. This finding is in contrast with that of a previous study that reported that thermal stability was affected by the wood type of raw material used for manufacturing WPC (Ratanawilai et al., 2012).
Bledzki and Faruk (2003) investigated the effect of four types of wood fiber (hardwood fiber, softwood fiber, longwood fiber, and wood chips) on wood fiber reinforced- polypropylene composites with the addition coupling agent of 5% MAPP. They found that wood chips- polypropylene composites had better tensile and flexural properties than the other composites. Hardwood fiber-polypropylene composite had better impact characteristics than others. The damping index decreased to about 60%, while Charpy impact strength increased for long wood fiber- polypropylene composites under this addition. Khonsari et al. (2015) studied the effects of types and mesh–size of wood flour on the physical and mechanical properties of WPC. The mechanical properties of WPC were significantly lower when only sawdust was used. The aspect ratio of ground wood shavings was higher than that of sawdust. Using larger particles for manufacturing WPC increased the water absorption.
Composites made from bark particles showed lower water absorption than those made by wood particles. It might be due to the different chemical compositions of the particles for both of them. Increasing the fiber content that using on manufactured WPC, the strength and stiffness of the WPC were improved. However, it reduced elongation and the energy to break. As increasing fiber content, the water uptake of the WPC increased (Bouafif et al., 2009).
The water sorption and diffusion properties of WPC were investigated by Kaboorani (2017) as a function of the formulation design. Increasing wood content, increased the susceptibility of WPC to water. Improving the adhesive forces between the wood and polymer by adding a coupling agent, decreased the water absorption, diffusion coefficients, and thickness swelling. The optimum formula of the coupling agent was dependent on the size and content of wood. Composites with large particles in their formulations absorbed water rapidly, resulting in high moisture content at the saturation point, high diffusion coefficients, and high thickness swelling (Kaboorani 2017).
The micro-fibrillated cellulose was surface treated with silicate hydrate to develop an effective filler material for the production of WPC. It was under water heated condition at temperature 175 ° C with varying time range from 0, 3, 6, 18, and 24 hours. This modification resulted in high-heat resistance and prevented irreversible aggregation. The composite materials demonstrated high mechanical strength and elastic modulus when the modified micro-fibrillated cellulose filler was added to polypropylene -based WPC (Ito et al., 2013).
Koohestani et al. (2017) investigated the influence of silanized micro-sized silicate-based minerals on the mechanical, thermal, and rheological properties of WPC. They found that the wood filler is primarily attributable for the increase in viscosity, MOE, tensile, and flexural strength. Any changes in the other properties of the WPC are more dependent on the wood filler content than on the mineral filler content. Mineral fillers modified with vinylsilane could improve tensile and flexural strength more effectively but were less effective on rheological behavior. Both the surface-modified minerals could improve the thermal stability of the WPC; amine-modified minerals had no negative effect on the rheological performance. However, it decreased the rigidity of the WPC considerably.
Tascioglu et al. (2014) investigated the dimensional stability, mechanical and biological performance, and thermal degradation of the WPC made from HDPE and recycled wood treated with CCA. These CCA-treated were taken out after 20 years of service durations. The influence of the coupling agent (MAPE) was also investigated. The recycled CCA-treated wood flour and coupling agent could improve the dimensional stability and mechanical performance of the WPC, excluding the Izod impact strength. The biological test showed that resistance to termites and fungus was improved with this treatment. This treatment can be utilized as an alternative recycling method for chemically treated wood. Further, chemically treated bagasse fibers can be used as a raw material for manufacturing WPC (Naguib et al., 2015). Composites prepared with chemically treated as follows: first with 5% NaOH for 60 minutes at room temperature and second diluted HCl solutions for 30 minutes at 60 ° C. The bagasse fibers exhibited stronger interaction at the fiber-matrix interface than composites prepared with untreated bagasse fibers. Additionally, increasing the content of chemically treated bagasse fibers, increased water adsorption by the composites and enhanced their mechanical properties.
The characteristics of the filler used for manufacturing the WPC influenced its physical and mechanical properties. The characteristics of wood-based fillers are strongly influenced by the wood species that used and different wood-based fillers give rise to WPC with different properties. Types of wood and the coupling agent that used on manufacturing of WPC also influenced the degradation process. Furthermore, recycled materials including recycled wood particles, sawdust, and recycled composites can be used as fillers. The composition of the raw materials affects the mechanical properties of the WPC. Adding sawdust improved the mechanical properties and accelerated the biodegradation rate of the WPC. Furthermore, adding a coupling agent at an appropriate amount can improve the mechanical properties and considerably reduce the swelling rate of composites. For the manufacture of WPC, the optimum amount of fillers is less than 60 wt%. As increasing the particle size of wood flour, the melt flow index, flexural and tensile (strength and modulus), heat deflection temperature, and notched impact energy of composites increases. However, the viscosity of composite increased as decreasing the particle size. Chemical treatment with NaOH, diluted HCl solutions, and chromated copper arsenate can improve the mechanical properties such as the dimensional stability and water resistance of WPC. Mineral fillers modified with vinyl-silane could improve the tensile, flexural strength and thermal stability of WPC.