Wood Properties And Dynamic Moisture Sorption Of High-temperature Thermlly Treated Larch Gmelinii

Larch gmeiini,as the main species in plantation forests in the Daxinganling area,has the advantages of straight trunk,good mechanical the strength and high density of the wood.However,due to its high resin content,poor dimensional stability and difficulty in drying,the scope of application have been limited,and some modification measures are needed to improve its wood properties.Wood modification techniques are divided into heat treatment,acetylation treatment,furfural treatment,nitrogen hydroxymethyl compound treatment,organic monomer modification,and thermosetting resin modification.Except for high-temperature heat treatment,all types of wood modification techniques use different amounts and degrees of chemical reagents,and high-temperature heat treatment of wood is favored by wood processing enterprises as environmentally friendly and relatively simple process technology.In this study,we used high-temperature heat treatment with atmospheric pressure superheated steam as the modification method,and set three gradient levels(heat treatment temperature of 160?,180? and 200?;heat treatment time of 2h,4h and 6h)with external influencing factors of wood heat treatment and heat treatment time,and a total of nine sets of tests to study the effects of high-temperature heat treatment on the physical and mechanical properties,sorption isotherms characteristics,and moisture absorption of Larch gmelinii larch wood.The effect of high-temperature heat treatment on the physical properties,moisture sorption,color and wettability of Larch gmelinii wood was investigated.Two moisture sorption models,Hailwood-Horrobin and GAB,were used to simulate the dynamic moisture adsorption changes of larch wood before and after heat treatment,and two testing methods,Fourier infrared spectroscopy and X-ray diffraction,were used to analyze the mechanism of wood properties changes of heat-treated wood.The main conclusions of this thesis are as follows.(1)The effects of high-temperature heat treatment temperature level on the mass loss rate and oven-dried density of larch wood were greater than those of heat treatment time.The mass loss rate of sapwood and heartwood of Larch gmelinii showed an obvious increasing trend with the increase of heat treatment temperature and heat treatment time,and the increase of sapwood and heartwood were 1.18%?4.05%and 1.12%?3.84%,respectively;the oven-dried density increased slightly with the increase of heat treatment intensity and then showed a decreasing trend,and the maximum decrease of sapwood and heartwood were 5.86%and 5.32%,respectively.The mass loss rate and oven-dried density of sapwood were slightly higher than those of heartwood.The moisture absorption equilibrium moisture content,shrinkage rate and swelling rate of larch sapwood and heartwood also decreased gradually with the increase of heat treatment intensity,and high temperature heat treatment could reduce the moisture absorption of wood and thus improve its dimensional stability.(2)The flexural strength(MOR)of two groups of Larch gmelinii wood specimens treated at 160? for 2 and 4 h increased slightly by 9.81%and 3.59%,respectively,compared with untreated wood,and then decreased with increasing heat treatment strength;the flexural modulus of elasticity(MOE)and impact toughness of larch wood gradually decreased with increasing heat treatment temperature and time,mainly due to the degradation of wood components and increase of brittleness during the heat treatment process.Compared with untreated Larch gmelinii wood,the compressive strength of heat-treated Larch gmelinii wood increased to different degrees,ranging from 5.08%to 40.63%.The effect of high heat treatment temperature on the mechanical properties of larch wood was greater than that of heat treatment time.(3)The brightness value of larch color decreases with the increase of heat treatment intensity,and the decrease range is from 6.25%to 34.79%;the red-green color index decreases and then increases with the increase of heat treatment intensity,and the change range is from-6.25%to 49.55%;the yellow-blue color index of heat-treated larch shows an overall increasing trend,and the increase range is from 2.34%to 13.84%,and the total color difference is affected by the L*,a*and b*.a*and b*also gradually increased with the increase of heat treatment intensity.The instantaneous average contact angle of untreated lumber was 51.97°,and the maximum average contact angle of larch after heat treatment was 87.19°.(4)The isothermal sorption curves of treated and untreated Larch gmelinii were fitted with both Hailwood-Horrobin and GAB models with high correlation coefficients,and both models could better describe the dynamic water sorption process of treated and untreated wood.The volume of monolayer molecular sorbed water from the GAB model decreased with the increase of heat treatment conditions,and the decrease of this parameter for sapwood and heartwood ranged from 2.4%(160?-2h)to 23.21%(200?-2h).23.21%(200?-6h)and 5.92%(160?-2h)-28.52%(200?-6h).