Improved water permeability of sugi heartwood by pretreatment with supercritical carbon dioxide

Abstract

Treating wood with aqueous solutions, such as preservatives, fire retardants, and dimensional stabilization agents, generally requires uniform distribution of the agents within the wood. Penetration and diffusion of such solutions into wood, however, are often obstructed in part by extractives and polyphenolic substances, especially in the heartwood. These substances adhering to or accumulating in pit and porous structures of wood may block the pathways for liquid transport. This study investigated the effect of supercritical carbon dioxide (CO2) (critical point; 31°C/ 7.4MPa) pretreatment on the water permeability of sugi heartwood. Supercritical CO2 diffuses and penetrates like gas but has solvating properties that approach those of liquid. It has low surface tension, easily seeps into micro pores and voids, and extracts nonpolar and weakly polar substances effectively. Thus, its uses for washing precision machines and semiconductors as well as dry cleaning have been investigated. Ohira reported that supercritical CO2 extraction was effective in extracting some useful chemical components in wood. Takeshita et al. studied the effects of supercritical CO2 in removing heavy metals from chromated copper arsenate (CCA)-treated wood. Acda et al. and Muin et al. used supercritical CO2 as a carrier solvent for preservative treatment of wood-based composites. In this study, we aimed to improve the water permeability of wood by pretreating the wood with supercritical CO2 that would remove substances that inhibit the permeation. We examined the effects of some treatment conditions on the water permeability of sugi heartwood, and observed bordered pits before and after supercritical CO2 treatment under a scanning electron microscope (SEM).