Chemical modification of wood : dimensional stabilization of viscoelastic thermal compressed wood

Abstract

The tendency of wood to shrink and swell with changing moisture content remains as one of the most significant challenges to using wood in its many applications. Viscoelastic Thermal Compression (VTC) has been shown to significantly increase the density, strength and stiffness of wood. However, dimensional stability is still a concern. Active and passive chemical modifications have been developed which impart dimensional stability by chemically altering the wood substrate or physically blocking the vital pathways of water through the wood microstructure. The efforts of this research have been to develop an approach which combines the VTC process with a chemical modification process resulting in a novel wood-based product that exhibits improved structural properties, as well as a high degree of dimensional stability. Low-grade, plantation-grown hybrid poplar (Populus spp.) was impregnated with low molecular weight phenol-formaldehyde resin, acetic anhydride or tung oil and then densified in the VTC process. Water soak and boil tests were performed to investigate the influence of each treatment on thickness swell, antiswelling efficiency (ASE), irreversible swelling, and thickness recovery. Modulus of elasticity (MOE) was also examined for each treatment. Fluorescence microscopy was applied to determine the physical location and distribution of the impregnating reagents to better understand their role in imparting dimensional stability. PF and acetylation treatments were shown to increase the stability of VTC treated samples. ASE values for both PF and acetylation treatments were high, with a maximum value of 86% for the PF treatment and 56% for the acetylation treatment. Two different low molecular weight PF resins were tested and it was found that the higher MW resin was retained within the cellular structure to a greater degree and imparted greater dimensional stability. MOE was positively correlated to density but negatively correlated to weight percent gain for PF treated samples. All chemically modified samples had lower increases in MOE than unmodified control specimens compressed to the same final thickness. A sub-sample of higher density PF treated specimens had significantly higher MOE values and similar stability values compared to lower density samples. Tung oil treated samples showed no ability to swell the wood cell wall and remained in the cell lumens. Although dimensional stability may have increased on a very short-term basis due to physical obstruction of moisture, long-term stability was not improved with tung oil treatments.