Physicochemical pretreatments of wheat straw improving fungus corrossion resistance and mechanical properties of wheat straw/polypropylene composites

Abstract: Wood-plastic composite (WPC), whose main raw materials are plant fiber and plastic, is a kind of green low carbon material, prepared by molding, extrusion or injection molding. It has advantages of plant fiber and plastic, widely used in landscape architecture, furniture, logistics and packaging, automotive interiors and other industries. Wheat straw fibers are underutilized agricultural residues with potential for use in reinforced polymer composites that would save existing wood and petroleum resources. The antibacterial property of polymer material is excellent, so WPC is more resistant to corrosion than wood. However, during the pratical application, people found WPC was still affected by fungus corrosion at a certain temperature and humidity, plaque was formed on the surface of the composite and mechanical properties declined. These led the service life of WPC to be shortened greatly. So study on the corrosion resistance of WPC is very necessary. In order to study the effect of different pretreatments of wheat straw on the fungus corrosion resistance of wheat straw / polypropylene (PP) composites, 4 treatments, i.e. sodium hydroxide (NaOH), acetic acid (HAc), hydrotherm and microwave were used to modify the surface of wheat straw, and the accelerated corrosion tests on the treated and untreated composites were carried out. The mechanical properties, color variation and water absorption of the composites after the corrosion were investigated. And the change of functional groups was analyzed by Fourier transform infrared spectroscopy (FTIR), and the fungus growth and the microstructure of the surface of the composites were observed by stereomicroscope. The results showed that the fungus could corrode the cellulose, hemicellulose and lignin in the wheat straw, which led to the cracks and holes generating on the surface of composites. The pretreatments could improve the interface bonding between wheat straw fiber and PP matrix, and the cellulose, hemicellulose and lignin in the wheat straw were effectively prevented from corrosion by fungus. Among 4 pretreatments, the composites under 5% NaOH pretreatment had better fungus corrosion resistance, and the flexural strength, tensile strength and impact toughness were increased by 1.68%, 3.67% and 75.28% compared with untreated composites, respectively. The water absorption and the color variation were decreased by 12.99% and 55.25% compared with untreated composites, respectively. The flexural modulus, tensile strength and impact toughness of the composites under 1% HAc pretreatment were increased by 27.88%, 3.67% and 25.84% compared with untreated composites, respectively; the flexural modulus, tensile strength and impact toughness of the composites under hydrothermal pretreatment were increased by 7.69%, 2.89% and 20.22%, respectively; the flexural modulus, tensile strength and impact toughness of the composites under microwave pretreatment were increased by 11.54%, 0.88% and 14.98% compared with untreated composites, respectively. The water absorption of the composites under 1% HAc pretreatment, hydrothermal pretreatment and microwave pretreatment were decreased by 20.41%, 16.34% and 17.49% compared with untreated composites, respectively. The color variation of the composites under 1% HAc pretreatment, hydrothermal pretreatment and microwave pretreatment were decreased by 73.67%, 49.63% and 62.97% compared with untreated composites, respectively. The composites prepared with pretreated wheat straw had fewer cracks and big holes after corrosion. The results will provide experimental data and theoretical reference for improving mildewproof effect of composites to prolong the service life of WPC materials. The research has great practical significance on improving the fungus corrosion resistance properties of wheat straw/PP WPC by straw fiber surface treatment.