Properties of wood powder sheets extruded with hydroxypropyl methylcellulose and citric acid after heating

Plastic products generally have excellent thermal plasticity and water resistance. However, their adverse environmental effects have become a severe problem. To overcome these problems, a 100% plant-derived plastic substitute material was developed by mixing wood powder, hydroxypropyl methylcellulose (HPMC), and citric acid solution, followed by vacuum extrusion and heating to insolubilize the HPMC. In this study, the effect of heating on physical properties was investigated. The extruded wood powder sheets were heated under a wide range from 0.5 to 5 h at 150 to 210 °C, which covers the conditions required for esterification between HPMC and citric acid. Water absorption, tensile strength, puncture resistance, and wettability were then tested. The sheet became tolerant of water and developed slightly higher tensile strength upon adequate heating, although it was more easily punctured when rewetted. Excessive heating at 210 °C was found to damage the sheet. The overall activation energy, calculated from the weight loss during heating, was as low as 46 kJ/mol, indicating that the dehydration and crosslinking of HPMC could occur easily. The curing process improved the water resistance and did not considerably worsen other physical properties; therefore, the possibility of using wood powder/HPMC/citric acid composite sheets has potential.