Effect of chemical modification of wood material on water uptake and wettability of wood flour-polypropylene composites

Background and objectives: In recent years, natural fibers have made significant contributions to the plastic industry, which has led to the emergence of wood plastic composites (WPC) in the construction industry. Products such as decking, fencing, siding, window framing, and roof tiles are being introduced into the market. The uses of wood plastic composites are also increasing in automative, transportation, military, and etc. Despite the widespread application, the natural fibers have some disadvantages, such as lower processing temperature, and incompatibility between the hydrophilic fibers and hydrophobic polymers. This problem can be alleviated by incorporation of coupling agent into the compound and modification of the fibers. Since, the most applications of WPCs exposure to atmosphere or contact with aqueous media has made it necessary to evaluate the water uptake characteristics of these materials. This research was carried out to investigate on the effect of chemical modification of wood material on water uptake and wettability of wood flour - polypropylene composites.

Materials and methods: Firstly, the different chemical treatments on wood flour, namely mercerization, acetylation and benzoylation were done. Then, the chemically treated fibers with polypropylene and coupling agent were compounded by an internal mixer, and samples were fabricated by injection molding. The weight ratio of wood flour was 60% based on the total weight of composites. The amount of PP-g-MA as was fixed at 2 phc for all formulations. The samples were soaked in water for 1000 hours to determine their water absorption and thickness swelling. The wettability of chemically treated samples were also measured by sessile drop method to determine the water repellency, and compared with those of untreated samples.

Results: Results indicated that treated samples had lower water absorption and thickness lower than untreated ones. It is well known that that the chemical modification promoted good wettability and better wood flour and polymer matrix interface adhesion, which consequently results in the moisture diffusion processes, could be much slower because there are fewer gaps in the interfacial region. Furthermore, the distributions speed of the water droplet on the samples decreased by applying chemical treatments due to the increasing of contact angle evolution time of the water droplet on the sample surface. FTIR spectra of treated fibers showed that the intensity of the bonds at 3400 and 1740 cm-1 change due to reaction of O-H groups and formation of ester groups in samples.

Conclusion: Based on the findings, it can be concluded that the chemical treatments have improved the dimensional stability and hydrophobicity of composites.