Study On Characteristics And Dimensional Stabilization Mechanism Of Press Dried Quercus Spp.Wood

White oak,as one of the world's most important specie of commercial wood,have the advantages of high mechanical strength,high corrosion resistance and good machinability and is a kind of excellent raw material for furniture and wood flooring manufacturing.However,because of the inherent shrinkage and swelling characteristics and the resulting dimensional instability of wood,white oak still can not meet the requirements of solid wood flooring manufacturing for ground with heating system.In this study,press drying wasselected as a dimensional stabilization method for wood,and effects of varying initial MCs,press temperatures,pressures and cutting widths on the drying characteristics,color,hygroscopicity,and dimensional stability of press dried white oak wood were studied.Meanwhile,in order to analyze the modification mechanism on hygroscopicity of white oak by press drying,the changes in the water adsorption property and the chemical structure and composition of wood before and after press drying were determined.The main conclusions are as follows:(1)Press drying wood showed a markedly improved drying rate compared with kilndrying wood but easily developed severe drying defects.20-mm-thick wood cuttings with initial moisture contents(MCs)varying from 14%to 75%and were press dried at 140?,0.1 MPa to a final MC below 2%in 120-210 min..The average drying rate were increased with increased initial MC.Press dried cuttings generally contained severe honeycomb and collapse with a initial MC above the fiber saturation point(FSP)while developed no honeycomb and only slight collapse when the initial MC of wood was lower than 15%.With a increased initial MC,the wood thickness shrinkage coefficient increased,while the width shrinkage coefficient decreased.Honeycomb generated along the wood ray and obviously compressed pores in early wood could be observed on the microscopic transverse section of wood with high initial MC.According to the relationship between the temperature and water content of wood during press drying,the method of monitoring the drying ending by measuring the central layer temperature of wood was put forward.(2)Wood air-dried to equilibrium moisture content(EMC)indoor was press-dried withhigh drying rates which increased with a increased press temperature.Cuttings after press drying appeared obvious color change that was bigger with a higher temperature but irrelevant of pressure and cutting width.The equilibrium moisture content(EMC)and swelling of white oak lumber could be significantly reduced by press drying and the reduction was more with a higher temperature in general,while there was no regular effect of pressure and cutting width on the EMC and swelling of wood.In this study,compared with the un-dried cuttings,the EMC,radial and tangential volume swelling of the wood after press drying could be decreased by about 9.97-33.67%,8.54-33.96%,11.26-30.02%,respectively.The optimized conditions of press drying of white oak boards are as follows:a temperature of 140-150?,a pressure of 0.1 MPa and a original board width.Based on the optimized parameters,the factory pilot research of press drying white oak was conducted and the floor detection results showed that the heating performance of the floors has met the requirements of the wood compound floor in the forestry industry standard.(3)Wood specimens before and after press drying were studied to determine their moisture sorption isotherm using a dynamic vapor sorption apparatus.The results showed that all moisture adsorption isotherms were characteristic sigmoid IUPAC type ? curves.The equilibrium moisture content(EMC)of wood at each target relative humidity could be significantly reduced by press drying and the EMC was decreased with a higher temperature.The adsorption isotherms were analyzed using the Hailwood-Horrobin model with excellent fits to the experimental data.The monolayer water(Mh)and polylayer water(Md)of the treated samples were both reduced at various relative humidity values and the reduction in Md generally became more dominant as the relative humidity increased.Compared with the untreated wood,the percentage decrease in projected FSP for wood press dried at 140,150 and 160? was 8.89%,11.76%and 13.62%respectively.(4)Adsorption kinetics were analyzed using the parallel exponential kinetics(PEK)model with excellent fits to the experimental data.The PEK model divides the adsorption kinetics curve into a fast and slow sorption processes.Press drying generally had a impact on characteristic times associated with the slow process but little influence on characteristic times associated with the slow process.The parameters obtained by PEK were interpreted based on the Kelvin-Voigt(K-V)model to determine elasticity and viscosity values of the wood cell wall.The moduli and elasticity showed a decreasing tendency in relation to relative humidity but were not regularly affected by press drying.The moduli was higher in the fast process than in the slow one while the elasticity was higher in the slow process.(5)The decreased hygroscopicity of press dried wood,which entailed improved dimensional stability compared with untreated wood was presumably caused by the reduced accessible hydroxyl groups and the increase in matrix stiffness of the cell walls.Characteristics of Fourier Transform Infrared Spectroscopy(FTIR)and X-ray Diffraction(XRD)of wood showed that press drying resulted in degradation of hemicellulose,esterification of the hydroxy of lignin and an increased cellulose crystallinity,which reduced the contents of free hydroxyl groups and other hydrophilic groups in wood.However,the reason of a increasd cell wall stiffness still needs further research.