Mechanism And Application Of Compressive Deformantion Fixation Of Poplar By Glycerin Pretreatment

The development and utilization of fast-growing wood became more important with the decreasing wood resources. Wood compression is an effective modification method of wood because it can not only improve the physical and mechanical performance but also can maintain the superior characteristics of wood. However, the utilization of compressed wood has been greatly restricted because of the recovery of compression deformation.In this study, the mechanism of compressive deformation fixation of wood pretreated by glycerin was investigated by stress relaxation, change on structre of cell wall components and chemical compositions at different temperatures. The mechanism was applied to fix compressed deformation of Poplar (Populus cathayana Rehd.). Thus, an environment-friendly and effective method to fix compressive deformation of wood was explored; the process variables were determined by experiments and the physical and mechanical properties, color change, decay resistance and anti-weathering properties of compressed wood were also examined. The main results of this research are as follows:(1)The mechanism of deformation fixation for compressed wood pretreated by glycerin is summarized as follows:Glycerin was adsorbed to hydroxyl groups through hydrogen bonding in amorphous region after pretreatment so the wood was in swollen state and the molecules were easy to slip. The matrix of wood began to flow, the hemicellulose, lignin and a small amount ofβ-cellulose were degraded and the molecular chains were cut so the recovery force of wood was relaxed; the cross-linking was formed between short-chain molecules from wood component degradation or between short-chain molecules and the original molecules, the crystallinity of wood increased, matrix rearranged so that the deformation of compressed wood was fixed.(2) The effect of temperature on the stress relaxation of wood was apparent, and the stress relaxation accelerated by the increase of tempereature and glycerin pretreatment. The stress relaxation during 25-180℃can be divided into two phases acorrding to the apparent activation energy{ΔE). TheΔE1 of PhaseⅠequals 8.24 KJ/mol andΔE2 of PhaseⅡequals 81.38 KJ/mol. PhaseⅠwas the relaxtion caused by the breakage of hydrogen bonding and PhaseⅡwas the relaxation caused by the breakage of hemicellulose and lignin molecular chains.(3) The degradation of wood components was accelerated by glycerin treatment. For glecerin-treated wood (GTW), a part of cellulose began to degrade at 140℃; Hemicellulose bagin to degrade at 120℃and degrade violently at 160℃; Lignin begin to degrade at 120℃and degrade violently at 160℃. FT-IR results showed that the molecular chains of hemicelluloses were broken by decarboxylic reaction and the benzene rings of lignin with small molecular weight were broken in GTW at high temperature.(4) The structural model of cell wall components of wood treated by glycerin was built. Glycerol can only enter the non-crystalline region of cellulose and swell wood between crystalline regions. The crystallinity of wood was improved by heat treatment and was acceletated by glycerin pretreatment at above 140℃, and decreased above 200℃. Glycerol was adsorbed on hydroxyl groups in amorphous region of wood by hydrogen bonding after entering into the amorphous region, which was similar to water molecules. The phenomenon of molecular "cross-linking" of glycerol treated wood at room temperature and 120℃was observed in E'temperature spectra. The relaxation process of glycerol molecule adsorbed to the side chains in amorphous region of glycerol treated wood can be observed in tgδtemperature spectra. Two relaxation processes due to the micro-Brownian motion of the short molecular chains formed by degradation and the new molecular chains formed by crossing-linking and rearrangement can also be observed in tg8 temperature spectra.(5) The recovery ratio of compressed wood pretreated by glycerin (CWPG) is significantly lower than the compressed wood pretreated by hot water (CWPW) and the surface hardness, decay resistance, stability of color after weathering of CWPG were higer than CWPW. The color of CWPG was darker than CWPW. The main process parameters of compressed wood pretreated by glycerin are: pretreat by 50% glycerin solution, compressed at 160℃for 60 min (with optimized MOR) or compressed at 180℃for 30 min (with optimized surface hardness).