Effects Of Preheating Conditions On The Formation And Properties Of Sandwich Compressed Wood Of White Poplar

Wood compression is an efFective way to improve the physical and mechanical properties of low-density wood.But there are some shortages of the traditional wood compression.The overall compression has large volume loss.The thickness of the compressed layer is small for the surface compression,and it would become thinner or disappear after the processing.Searching for a new compression method to improve wood properties has great theoretical significance and practical application value.This paper was mainly to achieve that compressed layer(s)could be formed at any positions from the surface to the center of the wood in the radial direction and the variation law of physical and mechanical propeties of the compressed wood was analyzed.That can lay a good foundation for the later processing and utilization of wood products.In this study,white poplar wood(Populus tomentosa)was selected as experimental material and the formation and density profile of the sandwich compressed wood after soaking,placing,preheating and compressing at different temperatures were investigated.Deformation characteristics of compressed layer,transition layer and uncompressed layer cells of sandwich compressed wood were observed.The physical and mechanical properties of the sandwich compressed wood formed at different preheating temperatures were measured and analyzed.In addition,the sandwich compressed wood(surface compressed wood,center compressed wood)and overall compressed wood were obtained by controlling preheating time.The differences of physical and mechanical properties between the sandwich compressed wood and overall compressed wood were compared and analyzed.Besides that,fixation of the compressed wood was carried out by steam treatment at atmospheric pressure,180? and 0.3 MPa,180?,respectively.The changes of physical and mechanical properties of the compressed wood before and after post heat treatment were analyzed.The main results of this study were summarized below.1)Compressed layer(s)could be formed at any positions from the surface to the center of the wood in the radial direction by controlling moisture and heat distribution(preheating time=12 min)in flat-saw lumber(thickness=25 mm,moisture content=18.3%)by changing preheating temperature only(90,120,150,180 and 210?).It meant that the compressed region could be controlled by changing the preheating temperature.The position of compression layer varied quadratically(y=-0.35x2+3.65x-1.75)or exponentially(y=3.96Ln(x)+1.54)with the increase of preheating temperature.And temperature elevation of 30? resulted in distance increase of 0.45-2.71 mm of the compressed layer and its average density was 0.61-0.71 g/cm3.2)Increasing the preheating temperature could improve dimensional stability of the sandwich compressed wood significantly while it increased the ?C*,-?H*and ?E*of the sandwich compressed wood before and after compression.Additionally,150? was the key temperature point for dimensional stability and color change of the sandwich compressed wood.When the preheating temperature was above 150?,the dimensional stability and color increased significantly.The wood surface hardness increased by 51.4%and the wood hardness decreased by 19.6%when the preheating temperature increased from 90 to 210?.However,the change of the preheating temperature had no significant effect on the modulus of elasticity(MOE)and modulus of rupture(MOR).3)There was no significant difference in dimensional stability and color between the sandwich compressed wood(surface compressed wood,center compressed wood)and overall compressed wood.When the compressed ratio was the same,compressed wood with higher surface hardness,wood hardness,MOE and MOR could be obtained by sandwich compression compared with the overall compression and the surface compressed wood had the highest mechanical properties among them.The increase rates of surface hardness,wood hardness,MOE and MOR of the surface compressed wood to the overall compressed wood were 63.1%,43.5%,19.4%and 21.3%,respectively.4)Compressed wood with good dimensional stability could be obtained by steam treatment at atmospheric pressure,180? for 2 h or 0.3 MPa,180? for 2 h,and compressed wood with better dimensional stability could be obtained by steam treatment at 0.3 MPa,180? for 2 h.Its spring-back ratio was about 1.8%after being conditional at 40? and a 90%relative humidity for 8 d(RH)for 8 d and its spring-back ratio was about 9.1%after being soaked in water for 48 h.Post heat treatment could reduce the surface hardness,wood hardness,MOE and MOR.However,the mechanical properties of the sandwich compressed wood were still significantly improved compared with the control.In summary,compressed wood with good physical and mechanical properties could be obtained by the sandwich compression and post heat treatment.