Breakeven Point in Ultimate Thickness Between Moisture-Reduced Shrinkage and Thickness Recovery of Densified Softwood Species: Part 1: At Room Temperature

Abstract

Thermo-hygro-mechanical (THM) densi fi cation technique has been used to increase the density and mechanical properties of underutilized softwoods. One potential application of the densified softwoods is the substitute of hardwood flooring. This technique is usually employed to mechanically compress the softened wood in the radial direction with the assistance of a high temperature greater than 100°C. Use of high temperature shows that the THM densification process consumes a lot of thermal energy, losing the economic competitivity in comparison with the hardwood floorings. From the energysaving point of view, the authors tried to densify two softwood species at three temperatures of 20°C, 50°C, and 90°C with various compression ratios (CRs), followed by air-drying (AD) the densified wood specimens at room temperature. It was discovered that the decrease of MC would result, during the AD conditioning, in shrinkage of densified wood, which could offset part of the thickness recovery of densified wood. However, this phenomenon was not well addressed. This study was aimed at discussing the effect of shrinkage on the thickness recovery ratio (TRR) of densified softwoods. To ignore the influence of temperature on the shrinkage capability of wood because of the decomposition of hemicellulose, only the data of densified wood specimens compressed and posttreated at room temperature were analyzed in this study. Two softwood species used were eastern white pine (Pinus strobus) and balsam fir (Abies balsamea). The CRs ranged from 0.05 CR and 0.65 CR. The major findings were: 1) the final thickness of densified wood was a result of “recovery” containing elastic deformation and viscoelastic deformation, and “shrinkage” of cell walls; 2) the relationship of TRR of densified pine or fir and CR followed an exponential increase trend at 20°C; 3) after AD conditioning, the breakeven point in thickness between moisture-reduced shrinkage and visco-elastic recovery when the CRs of densified pine and fir at 20°C were 0.12 and 0.13, respectively.