Study On The Bamboo Drying Properties And Mechanism Of Bamboo Drying Discoloration

Drying plays an important role in the process of manufacturing bamboo products. The quality of bamboo products is determined by the drying effects of bamboo in large extent. Bamboo is a good decorative material with its elegant, soft color and luster. While if bamboo dried under the improper drying condition, it would cause the defects of sintering and spot, making the color of bamboo greyed and darkened. The effects of pretreatment and drying condition on the bamboo drying properties and the differences between discolored bamboo and control group in main chemical composition were studied in this paper. Meanwhile, the optimal bamboo drying process was determined and the mechanism of bamboo drying discoloration was revealed, which might provide the guidance and references for the optimization of pretreatment and drying process. The main conclusions of this study are as follows:(1) The drying rate and the drying shrinkage rate of bamboo could be raised by the pretreatment of hot-water soak, and the hot-water soak can prevent the bamboo from producing spot defects. When the temperature of hot-water soak was set at 100℃ for 4h, the drying rate reached the maximum, which increased by 40.5% than control group. The shrinkage rate of hot-water soaked bamboo is bigger than control group and increased with the soaking temperature rising and time prolonging. What’s more, the shrinkage rate in the radial is bigger than that in the tangential. The temperature and time of hot-water soak have little effect on the chromatism of bamboo that no spot defects occur on the surface. The control group after finish milling all produced spot defects, while the finish milled bamboo treated by hot-water soak with the temperature of 100℃ for 4h didn’t show spot defects, which mean hot-water soak can prevent bamboo from producing spot defects. The optimal condition for hot-water soak pretreatment is 100℃ and 4h for temperature and time, respectively.(2) The drying rate and the drying shrinkage rate of bamboo could be raised by the pretreatment of bleaching, and the bleaching can prevent the bamboo from producing spot defects. When the concentration of H2O2 was set at 4% for 8h, the drying rate reached the maximum, which increased by 25.5% than control group. The shrinkage rate of bleached bamboo is bigger than control group and increased with the concentration of bleach rising and time prolonging. What’s more, the shrinkage rate in the radial is bigger than that in the tangential. The concentration and time of bleaching have little effect on the chromatism of bamboo that no spot defects occur on the surface. The control group after finish milling all produced spot defects, while only the finish milled bamboo treated by bleaching with the concentration of 2% for 2h show spot defects, which mean bleaching can prevent bamboo from producing spot defects. The optimal condition for bleaching pretreatment is 4% and 4h for concentration and time, respectively.(3) The drying rate and drying shrinkage rate of bamboo increase with the drying temperature rising or relative humidity lowering. The drying shrinkage in the radial is bigger than that in the tangential, of which the outer layer is larger than the inner layer. The chromatism of bamboo increases slightly with the drying temperature or relative humidity rising. There are no spot defects on the bamboo surface. When drying temperature is 70℃ or higher than 70℃, the control group and bleached bamboo after finish milling would show some spot defects, which will be heavier with drying temperature increasing. While only the drying temperature of 80℃ would the hot-water soaked bamboo after finish milling produce spot defects. Drying temperature has greater effect on the spot defects than relative humidity. Therefore, the optimal drying process for control group and bleaching bamboo are temperature of 60℃ and relative humidity of 50%, while hot-water soak bamboo is temperature of 70℃ and relative humidity of 50%.(4) The spot defects are caused by the combined action of bamboo chemical composition, high moisture content and the drying medium(temperature, oxygen). Compared to the control group, the amount of cold water extractive, hot-water extractive, 1%NaOH extractive and benzene-alcohol extractive in discolored bamboo are higher, which indicate that the change of extractives relates to discoloration. It can be found from the TG-DTG curve that the largest pyrolysis rate of discolored bamboo occurs earlier than that of control group, and the residual rate of discolored bamboo is lower than that of control group, which mean the discolored bamboo has less lignin than control group. Combined with the FTIR, it can be concluded that the discoloration of bamboo results from the phenolic compounds (-OH) oxidized to quinines (C=O) Moreover, the spot defects don’t affect the crystallinity of bamboo.