To utilize agricultural and forestry wastes and to improve environmental protection, polypropylene (PP) wood–plastic composites were prepared with rice straw powder by using two methods of formation. The effects of formation conditions and rice straw powder content on the mechanical properties, water absorption, and moisture absorption performance of the composites were studied. The tensile sections of the composites were observed by using a stereo-microscope. The results showed that the mechanical properties, water absorption, and moisture absorption performance of the composites formed by mixing compression molding were better than those obtained by layer compression molding. Mixing produced more uniform blending of PP with the fiber compared with using layers of PP with powder. With a rice straw powder content of 50 percent, composites formed by mixing compression molding had good mechanical properties, water absorption, and moisture absorption performance.ABSTRACT
Wood–plastic composites (WPCs) are made with agricultural and forestry waste, including waste wood flour, wood shavings, saw dust, rice husks, crushed rice straw, peanut shells, coconut shells, sugarcane, flax, jute, and hemp. The types of thermoplastics that are blended with these wastes include polypropylene (PP), polyethylene (PE), and polyvinyl chloride. Thus, some measure of thermoplastics can replace a portion of the wood products. Given the current interest in advocating for a low carbon economy, greater attention has been paid to the study of WPC (Zhang et al. 2002). We summarize the literature on thermoplastic composites being manufactured with agricultural wastes.
WPCs have shown to display better mechanical properties compared with pure plastic (Guo et al. 2005, Peng et al. 2007). Wood powder improves the heat resistance and mechanical properties of PP (Li 2005, Taividi et al. 2006) and the impact strength of linear low-density PE. The amount of wood powder and the type of xylem filler have a great influence on the mechanical properties of WPCs. The mechanical properties of PP composites were good when bamboo powder content was 30 to 40 percent (He et al. 2010). The mechanical properties of wheat stalk–based composite were superior to those of composites made with corn stalks or wood flour (Panthapulakkal and Sain 2006) when high-density PE is used as the matrix. The water resistance, water absorption, thickness swelling, and flexural properties of rice straw–recycled tire composite was better than wood particleboard (Yang et al. 2004). When the proportions of wheat husks, rye husks, and softwood were 45, 43, and 42 percent, respectively, the uniformity between matrix and reinforcement of PP-based WPCs was improved. Composites using wheat husks had superior impact strength compared with softwood-based composites (Kuang et al. 2010). Research indicated that coupling agents have a positive effect on the interfacial bonding of WPCs (Bengtsson et al. 2005, Bengtsson and Oksman 2006). Wang et al. (2005) prepared composites using recycled PP plastic and poplar fiber and showed that the composites demonstrate the best performance when the ratio of wood to plastics is 50:50.
Plastic film is widely used in packaging daily necessities, causing a serious “white pollution” problem. Therefore, utilizing waste plastic has important practical significance in environmental protection and recycling (Wei 2005, Fu et al. 2006, Sun et al. 2006, Wang 2010).
The potential advantages of crop straw have drawn attention, and the utilization of rice straw has important significance in environment protection. Thermoplastics are usually used as a matrix for manufacturing WPCs with wood flour and rice husk powder, but there are a limited numbers of studies that use waste plastic film as a matrix. This study aimed to protect the environment and utilize natural resources, using rice straw powder as filler material and PP as a matrix. PP-based WPCs in this study were prepared by layer spread molding and by mixing molding. The influence of formation conditions and rice straw powder content on the mechanical properties, water absorption, and moisture absorption performance of the composites were analyzed. The morphological differences based on rice straw content and the interface bonding of PP-based WPCs were characterized by microscopic analysis of the composites.
Materials and Methods
Raw materials
Rice straw powder was chopped with a straw stalk grinder with a particle size of 60 mesh. Recycled PP film was obtained from Suzhou Jielilai Electric Appliance Limited Company. The silane coupling agent KH570 was obtained from Nanjing Sanpai Fine Chemical Industry Limited Company.
Preparation of rice straw powder–PP composites
After being washed with water, recycled PP film was air dried and cut into 100 by 100-mm pieces and then put in the baking oven at 80°C for 12 hours. The rice straw powder was surface treated using the silane coupling agent KH570 and then dried. The mass fraction of coupling agent content was 2 percent (wt/wt). The PP WPCs were prepared using two methods of formation to produce pieces that were 100 by 100 by 7 mm. The composites pieces were then cut into test specimens.
Layer compression molding composites were formed by putting the treated rice straw powder and PP films alternately into the mold. PP WPCs were then prepared by hot press molding at 180°C and 12.5 MPa for 12 minutes on a plate vulcanization machine.
Mixing compression molding composites were made by putting the treated rice straw powder and PP films alternately into a mixing roll at 165°C to 175°C for 6 minutes. PP WPCs were prepared by hot press molding at 180°C and 12.5 MPa for 12 minutes on a plate vulcanization machine.
Property testing and characterization of rice straw powder–PP composites
Mechanical properties of the composite specimens were tested following GB/T 17657‐1999: tensile properties, with a tensile speed of 50 mm/min; flexural strength and elastic modulus, with a loading speed of 2 mm/min; and impact properties. Both experiments were carried out at room temperature, and the result was the average of three specimens. The microstructure of the surface of fractured samples was observed using a SMZ1000 stereo-microscope.
Moisture content of the specimens was tested following GB/T 17657‐1999. Specimens were dried to a constant weight at a temperature of 103°C ± 2°C. Moisture content was calculated by:
where H is moisture content of specimens (%), mv is the weight before drying (g), and mo is the weight after drying (g). The result was the average of five specimens.
To determine water absorption property, PP-based WPCs were immersed in room temperature water according to GB/T 17657‐1999 (Shiying et al. 1999). Specimens were removed from the water after 24 hours and dried with filter paper. The water absorption rate was calculated using the following formula:
where W is the 24-hour water absorption rate of specimens (%), M0 is the weight before soaking (g), and M1 is the weight after 24 hours of soaking (g). The result was an average of five specimens.
To calculate the hygroscopic property, the PP-based WPC specimens were placed into a controlled temperature and humidity chamber and then weighed at specific time points (6, 18, 42, 66, 114, and 282 h). Once the weight change remained constant between time points, the moisture absorption had reached a balance. The moisture absorption followed the formula
where Q is the moisture absorption after a period of time (%), m1 is the weight when specimens were dry (g), and m2 is the weight after moisture absorption (g). The result was an average of five specimens.
Results and Discussion
Mechanical properties
Figure 1 shows mechanical properties of PP composites with different percentages of rice straw content manufactured by two different methods. Figure 1 shows that the tensile strength, bending strength, and especially the impact strength of the composites made by layer molding significantly declined as rice straw content increased. The mechanical properties of the composites made by mixing molding initially increased but then decreased with increasing rice straw content. The maximum tensile strength, bending strength, and modulus of elasticity were 21.8 MPa, 24.0 MPa, and 0.85 GPa, respectively, when the rice straw content was 50 percent; the tensile strength and bending strength were 14 and 11 percent higher than composites made by layer molding. However, the mechanical properties of composites made by mixing molding decreased with higher rice straw content. The minimum tensile strength of composites made by mixing molding was 13.5 MPa and by layer molding was 12.7 MPa when the rice straw content was 80 percent. The greatest impact strength for a composite made by mixing molding was 5.0 kJ/m2, which was associated with a rice straw content of 65 percent. These results suggest that there is an optimal range of the rice straw content when using mixing molding.
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00113
Composites formed by mixing molding generally had superior mechanical properties compared with those formed by layer molding. When the content of rice straw powder was 50 to 65 percent, mechanical properties (except modulus of elasticity) of the composites formed by mixing compression molding were better than those formed by layer compression molding. This means that a rice straw powder content between 50 and 65 percent was suitable. When the proportion of rice straw powder was greater than PP, the plastic matrix could not blend adequately with the powder. Clumps of rice straw powder occurred within the product, which reduced the mechanical properties of the composites. When the PP content was proportionately greater than rice straw powder content, the rice straw fiber had no enhancement effect, so the mechanical properties of the composites were poor.
Moisture absorption and water absorption performance
Figure 2 shows 24-hour water absorption, thickness swelling, and moisture absorption versus time. When the rice straw powder content was the same, the properties of the PP composites formed by mixing molding were superior to those of the product formed by layers.
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00113
In Figures 2a and 2b, we show that 24-hour water absorption and thickness swelling of rice straw powder–PP composites formed by layer molding gradually increased as the rice straw powder content increased. The 24-hour water absorption and the thickness swelling of mixing molded composites increased slowly when the rice straw powder content was less than 50 percent and increased quickly when the content of rice straw powder was more than 65 percent. When the rice straw powder content was 35 percent, the 24-hour water absorption of the mixing molded composite material was 0.3 percent, only 1/20 of the water absorption of the product formed by layer molding. The thickness swelling of the mixing molded composites was 0.9 percent, which was 1/5 that of the layer molded product. Moisture absorption of rice straw powder–PP composites made by mixing molding was less than that of composites made by layer molding. Water absorption of composites increased rapidly when rice straw powder content increased. When the rice straw powder content was 35 percent, and time was between 0 and 282 hours, moisture absorption of composites with mixing compression molding was low, only 0.4 percent. When the rice straw powder content was between 50 and 65 percent, the moisture absorption of rice straw powder–PP composite was 1.2 and 2.6 percent, respectively. When the rice straw powder content was 80 percent, the moisture absorption of rice straw powder–PP composites was high, reaching 5.8 percent, which was close to that of the composite with layer compression molding. In conclusion, when the rice straw powder content was between 50 and 65 percent, composites formed by mixing compression molding had good water absorption and moisture absorption performance.
Microstructure of PP composites
Figure 3 shows the microstructure of the product formed by layer molding, and Figure 4 shows the product formed by mixing molding. Figures 3a and 4a reveal that when the straw content was 35 percent, the PP matrix dominated. In a comparison of Figures 3c and 3d with Figures 4c and 4d, the mixing process is seen to provide better contact between the PP and rice straw and a more uniform distribution of the same, compared with the coarser, less uniform product obtained layer molding. The coarse-textured product obtained by using layers contains voids where the powder was absent, producing weaker mechanical properties compared with product with similar PP and rice straw contents but formed by mixing. Figures 3d and 4d show that rice straw powder dominates the structure of the composite, and the mechanical properties of the composites made by using different forming molding methods were similar when the rice straw content was 80 percent.
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00113
Citation: Forest Products Journal 63, 1-2; 10.13073/FPJ-D-12-00113
Conclusions
The mechanical properties, water absorption, and moisture absorption performance of the composites formed using mixing compression molding were superior to the properties exhibited by the composites formed in layers. These results were due to the uniform distribution and good interfacial bonding of the two phases with mixing compression molding and the nonuniform distribution with layer compression molding as revealed by microstructure analysis.
When rice straw powder content was 50 percent, rice straw powder–PP composites made by mixing compression molding displayed good mechanical properties, water absorption, and moisture absorption performance, and the tensile and bending strengths were 14 and 11 percent higher, respectively, than those of composites made by layer molding.
Mechanical properties of polypropylene composites containing different percentages of rice straw by two different formation methods. (a) Tensile strength, (b) impact strength, (c) bending strength, and (d) modulus of elasticity.
Moisture absorption of polypropylene composites containing different percentages of rice straw by two different formation methods. (a) 24-hour absorption, (b) thickness swelling, and (c) moisture absorption.
Microstructure surface of the rice straw powder–polypropylene composites formed by the layer molding process. Rice straw content: 35 (a), 50 (b), 65 (c), and 80 (d) percent.
Microstructure surface of the straw powder–polypropylene composites formed by the mixing molding process. Rice straw content: 35 (a), 50 (b), 65 (c), and 80 (d) percent.
Contributor Notes
The authors are, respectively, Professor, PhD Candidate, Master's Candidate, and Master's Candidate, College of Engineering, Nanjing Agric. Univ./Key Lab. of Intelligence Agric. Equipment, Jiangsu Province, Nanjing, China (chunxiahe@tom.com [corresponding author], jessierlhou@hotmail.com, xuejiao3336129@163.com, huan0804@163.com). This paper was received for publication in November 2012. Article no. 12‐00113.