| In recent years,bamboo culm flattening technologies have been developed rapidly.These technologies can improve the utilization rate of raw materials,reduce the amount of applied adhesive,and retain the natural texture of bamboo to a large extent.However,in the process of processing and utilization,it is found that there is a serious cupping deformation.In this study,steam treatments were used to improve the dimensional stability of flattened bamboo boards.The micro-structure,hygroscopicity and crystallinity were observed and measured,and the mechanism dimensional stability improvement,as well as the effects of saturated steam treatment on physical and mechanical properties were analyzed and discussed.The following main conclusions are drawn:(1)The flattened bamboo boards tangential cupping bent towards the outer layers of the bamboo culm wall when oven-dry,and bent towards the inner layers after water soaking.Although superheated steam treatment reduced oven-dry cupping of flattened bamboo boards,it had no discernible effect on water-soaking cupping.The optimal conditions were 200℃ superheated steam treatment for 90 minutes,and the absolute dry warpage was 0.37%,91.96%lower than that of the control group,and the saturated warpage was-4.38%,20.94%lower than that of the control group.The dimensional stability of flattened bamboo boards improves as the chemical composition of the board changes.The hydrophilic hydroxyl groups in flattened bamboo boards are significantly reduced after superheated steam treatment.(2)The effect of saturated steam treatment was shown to be superior than that of superheated steam treatment.When compared to superheated steam treatment,saturated steam therapy reduced treatment time significantly.It was also discovered that exposing the specimens to high temperatures for an extended period of time caused significant damage.The specimens treated at 185℃ for 10 minutes behaved well under different humidity conditions.The cupping degrees of the treated specimens were 1.48%,0.97%,0.42%,-0.52%,-2.04%,-2.24%at oven-dry condition,30%relative humidity,65%relative humidity,85%relative humidity,water-soaking for 7 days and water-saturated conditions,respectively.At the same time,the saturated steam treatment also reduced the shrinkage rate and swelling rate of the specimens.The radial air-drying shrinkage rate of the 170℃ for 15 minutes treatment group was the lowest,which was 3.27%;the tangential air-drying shrinkage rate,radial air-drying swelling rate and tangential airdrying and swelling rate of the 185℃ for 10 minutes treatment group were the lowest,which were 1.90%,1.44%and 0.80%,respectively.The reason for the decrease of the cupping of specimens is that the hygroscopicity of specimens is reduced,and the cells on the side of inner layers are compressed,which led to the decrease of the structural differences,the large decrease of hydrophilic hydroxyl groups and the increase of crystallinity.(3)After saturated steam treatment,the oven-dry density of specimens did not change much.The color of the specimen changed obviously,and the color of the 185℃for 10 minutes treatment group was the largest,and the total color difference ΔE*value changed 20.97.Saturated steam treatment can reduce the main mechanical properties of specimens.Taking 185℃ for 10 minutes treatment group as an example,its compressive strength,flexural strength,flexural elastic modulus,tensile strength and shear strength are 48.38MPa,76.98MPa,4905.23MPa,84.78MPa and 8.90MPa,respectively.Compared with the control group,they decreased by 0.29%,23.70%,21.39%,26.89%and 49.77%,respectively.(4)After a thorough comparison of all aspects of the specimens’ qualities,it was determined that treated with saturated steam at 185℃ for 10 minutes had the greatest results.They have excellent dimensional stability as well as no significant decrease in major mechanical properties.Flattened bamboo boards treated with saturated steam have higher dimensional stability,which has significant industrial potential. |
