Effects of Heat Treatment on Environmental Stability and Dissolution Behavior of Citric Acid-Containing Wood Flour Sheets

Abstract

Wood flour sheets were prepared by wet extrusion molding using hydroxypropyl methylcellulose (HPMC) and citric acid (CA) as a molding aid and crosslinking agent, respectively. The effects of heat treatment (160 to 180 °C, 0.5 to 1.5 h) on moisture absorption, tensile properties, water stability, and dissolution behavior of CA and HPMC were investigated. Moisture absorption decreased from approximately 9% to 8% at 20 °C and 65% relative humidity (RH), and from approximately 15% to 12% at 70 °C and 90% RH. The tensile strength measured at 20 °C and 65% RH remained nearly constant regardless of the heat-treatment severity, whereas the tensile strength at 70 °C and 90% RH was significantly improved by the heat treatment. Water immersion tests showed that the heat-treated sheets exhibited improved structural stability and reduced water absorption compared to the oven-dried control. HPLC analysis revealed that the dissolution ratios of both CA and HPMC decreased markedly with increasing heat treatment severity. The simultaneous reduction in CA and HPMC dissolution suggests that heat treatment promoted CA-mediated crosslinking and insolubilization of HPMC within the sheet structure. These changes contributed to improved water stability and retention of mechanical properties under high-temperature/high-humidity conditions, thereby enhancing the overall environmental stability of the wood flour sheets.