Influence of pre-treatment in citric acid solution on physical and mechanical properties of thermally compressed oil palm board

Abstract

Oil palm trunk is an excellent raw material for thermally compressed wood board. However, improvements to dimensional stability during water absorption and reduced thickness swelling has been tied to losses in other mechanical properties, especially as the compression temperature is increased. Toward solving this trade-off, we analyzed the effects of a 48 h pre-soak in citric acid solutions (0, 5, 15, 25, or 35% w/v in distilled water) on the physical and mechanical properties of oil palm board compressed at 140 °C. The reference benchmark case was compressed at 200 °C without pretreatment. The oil palm board raw materials were obtained from outer, middle and inner parts of trunk. The results showed that the oven-dry density of compressed oil palm board made from different parts of trunk increased with thermal compression (maximum pressure 12.26 MPa for 8 min). The citric acid pretreatment improved water absorption and thickness swelling properties of oil palm board thermally compressed at 140 °C, consistent with the citric acid concentration. The carboxyl groups in citric acid cross-link with the hydroxyl groups in the wood. However, no significant difference was found between the benchmark (200 °C) and pretreatments with 5 or 15% citric acid. The citric acid altered the wood chemistry during hot compressing at 140 °C. Static bending strength, modulus of rupture (MOR), and modulus of elasticity (MOE) slightly decreased with the citric acid pretreatment, matching the effects of high temperature compression at 200 °C.