Dynamic viscoelasticity of wood after various drying processes

In this study, specimens of heartwood from Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) plantation trees were dried by high-temperature drying (HTD), low-temperature drying (LTD), and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried and untreated wood specimens with various moisture contents were investigated in the temperature range between-120 and 40 degrees C at 1Hz using a dynamic mechanical analysis (DMA). The results indicated that the relative storage modulus and relative loss modulus were both the highest for HTD wood and the lowest for FVD wood, and that two mechanical relaxation processes developed. The alpha relaxation process in the higher temperature range was presumably a result of surpassing the glass transition of hemicelluloses with low molecular weight, whereas the beta relaxation process occurring in the lower temperature range was most probably due to the motions of both methyl groups in the amorphous region of wood cell wall and adsorbed water molecules in wood. As moisture content increased, the decrease of relative storage modulus with increasing temperature became more dramatic, and the loss peak temperatures of the relaxation processes shifted to lower temperature range. The difference of dynamic mechanical behavior among untreated and dried specimens reduced with the increase of moisture content.