Properties And Characterization Of Polyethylene Glycol/Heat Treated Poplar

Heat treatment can decrease the equilibrium moisture content of wood and then improve the dimensional stability by reducing the amount of hydroxyl groups in wood during thermal degradation at high temperatures. PEG impregnation is also a popular method for dimensional stabilization by "bulking" effect. This study investigated the water aborsption, hygroscopicity, swelling rate, modulus of rupture (MOR), modulus of elasticity (MOE) and color of PEG/heat treated wood. The combination effect of PEG and heat treatment on viscoelasticity, composition, and structure was also investigated by using stess relaxation, dynamic mechanical analysis (DMA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM). The main results of this study are as follows:(1) High temperature heat treatment also decreased the water aborsption, hygroscopic ity, and swelling rate. The PEG treatment could accelearte the color change and had a positive effect on dimensional stability. The MOR and MOE of heat treated wood dropped at high temperatures. After heat treatment at same temperature, the MOR and MOE of PEG treated wood were lower than those of the untreated wood.(2) Temperature plays an important role in stress relaxation of wood. At higher temperatures, less time was required to reach the stable state. The stress could completely release at140℃for both untreated and PEG treated wood. PEG accelerated the stress relaxation process but reverse effect was observed above180℃. According to the apparent activation energy, the stress relaxation could be divided into two parts with140℃as the critical point. PEG impregnation could reduce the apparent activation energy of wood. Temperature is an important factor influencing the DMA of wood. With increasing temperature, the storage modulus decreased. The relaxation peak of tgδ for PEG treated wood would move to lower temperature region after180℃and200℃heat treatment, which indicated the degradation of wood components during heat treatment at180℃and200℃.(3) The spectrum of FT-IR showed that little degradation of lignin degraded after heat treated at200℃. However, the lignin in PEG/heat treated wood began degradation at200℃. The results of XRD indicated that PEG could only enter the non-crystalline region of cellulose and swell wood between crystalline regions. The characteristic diffraction peak of PEG could be found in XRD spectrum of PEG/heat treated wood. The crystallization of heat treated wood without PEG impregnation increased at first, descreased at160℃, and then increased again, while the crystallization of PEG treated wood descreased at140℃followed by an increase. SEM Image showed that PEG could deposit at cell wall of vessels and fibers, or block the pits. The PEG molecules would redistribute during the process of heat treatment at200℃to achieve a more uniform distribution.(4) The bending MOR and MOE increased after heat treatments below140℃. But when wood was treated at160or180℃, the physical and mechanical properties would be changed greatly with improved dimensional stability and reduced hygroscopicity. The stress could be completely released quickly but the bending strength would be reduced. At same temperature of heat treatment, PEG impregnated wood showed more severe reaction and also more degradation of chemical components, which was confirmed in by the XRD and property tests.