Study On Temperature And Vapor Pressure Variations During Hot-pressing Of Bamboo Scrimber

Bamboo scrimber is the flagship product of the Chinese bamboo industry,considerably increasing the added value of bamboo resources and expanding the application areas for bamboo materials.However,the long hot-pressing time and high energy consumption of reconstituted bamboo,as well as the lack of clarity of changes in the internal temperature and vapor pressure of the slab during hot-pressing,have led to a lack of scientific basis for innovation in energy-saving hot-pressing processes.In this paper,the initial moisture content(5%～20%),target density(0.9 g/cm3～1.2 g/cm3),target thickness(15 mm～60 mm),hot-pressing temperature(140℃～170℃),holding time(15 min～30 min)and hot-pressing pressure(4 MPa～7 MPa)on the hot-pressing process of bamboo bundle slabs were investigated.The effects of surface layer and core layer temperature and core layer vapor pressure on the hot-pressing process of bamboo scrimber were investigated,and a model of heat-moisture coupling during hot-pressing of bamboo scrimber was constructed.The theoretical basis for the innovation of the bamboo scrimber hot-pressing process can be provided by the research.The main conclusions are as follows:(1)The temperature of the surface and core layers of the impregnated bamboo bundle slab and the variation in vapor pressure of the core layer during the hot-pressing process were measured using thermocouples and homemade pressure transducers.The results show that there was a temperature difference in the horizontal direction between the surface and core layers of the impregnated bamboo bundle slab,and that the temperature pattern was as follows:layer centre>layer edge>layer end;the vapor pressure variation curve of the core layer can be divided into normal pressure section,rapid pressure increase section,slow pressure increase section and rapid pressure drop section.(2)The higher initial moisture content(IMC)can promote the heat transfer of bamboo bundle slabs,but the effect of moisture content over 10%was limited.20%initial moisture content of the slab represents 3.3 times the maximum central vapor pressure of 5%during hot pressing,and there was residual vapor pressure(33.4 kPa,relative pressure)at the end of hot pressing.The increase in target density and thickness slowed the rate of slab heating and increased hot-pressing time.Target density had no significant effect on the rate of steam pressure rise in the core and on maximum pressure;increasing target thickness reduced the rate of steam pressure rise,but had no significant effect on the maximum pressure.(3)The higher the hot-pressing temperature,the faster the heating rate of the impregnated bamboo bundle slab,the significantly shorter the hot-pressing time and the higher the maximum vapor pressure in the slab’s core layer,with the maximum vapor pressure at 170℃ increasing by a factor of 1 compared to 140℃.Extending the holding time had a lesser effect on the increase in maximum temperature in the surface and core layers of the slab,but significantly increased the maximum vapor pressure in the core layer,with an increase of 33.6%for a thickness of 30 mm compared with a thickness of 15 mm.The increase in hot-pressing pressure was beneficial for slab heating,but had less effect on maximum surface and core layer temperatures;hot-pressing pressure had a greater effect on maximum vapor pressure in the slab’s core layer,increasing 1.8 times for 7 MPa compared with 4 MPa,and the former had a residual vapor pressure(19.1 kPa)at the end of hot-pressing.(4)A coupled wet-thermal model of the bamboo scrimber hot-pressing process was developed by COMSOL software,and the model was solved by defining initial moisture content,target thickness and hot-pressing temperature as variation parameters.The results show that the simulated core layer temperature and vapour pressure curves match the measured curves to a large extent(correlation coefficient>0.97,highly significant),that the maximum core layer temperature was basically consistent,and that the simulated maximum vapour pressure was 8-10%higher than the measured value;the simulated surface layer temperature curves do not overlap well with the measured curves.