Studies On The Mechanism And Properties Of Superheated Steam Heat-treated Oak Wood

High Temperature Heat Treatment (HTHT) on wood is one of the most well-establishedtechnologies to reduce hygroscopicity, increase dimensional stability and bio-durability ofwood. While the mechanical strength and oven density of wood are generally reduced, andcolor difference (△E*) increases during this process. It’s an urgent issue for relatedresearching academies and wood processing enterprises to seek a good balance among thosecontradictory effects induced by HTHT through selection and optimization of related processin accordance with the end-users’ demand on properties of Heat Treatment Wood (HTW).Therefore it’s of critically practical and theoretical importance for upgrading HTHT technologyand promoting the development of related wood processing enterprises to master the HTW’sproperties changing patterns and relationship amongst various properties, and to explore themechanism on dimensional stability and mechanical strength of HTW through application ofdifferent analysis tools and approaches.The paper studies the effect of such parameters as temperature, time, oxygenconcentration and steam pressure on the wood properties and in-laboratory termite resistanceproperty of Oak (Quercus mongolica) samples treated in an environment of superheated steam.A non-linear regressive model is established to optimize the processing technology for HTWapplied in flooring for ground with heating system. Apart from this, various testing approaches,including Scanning Electron Microscopy, Gas Chromatography, X-ray Diffraction andNano-indentation are adopted to explore the mechanism of change in dimensional stability andmechanical strength of HTW.The key conclusions are drawn as follows:1) The relationships between the parameters (including temperature, oxygen concentrationand steam pressure inner kiln) and the properties of THW (including the Modulus ofElasticity (MOE), Modulus of Rupture (MOR), Anti-Swelling Efficiency in Radialdirection (ASE-R), Anti-Humidity Efficiency in width (AHE), Equilibrium MoistureContent (EMC) and color difference (△E*)) are established. An cubic polynomial relationship exists between temperature and MOE, a linear one between temperature andMOR, ASE-R, AHE, EMC and△E*, a quadratic polynomial for oxygen concentration andthese six properties mentioned above, and a linear for steam pressure inner kiln and heattreatment time with these six properties.2) Six regression equations (temperature, oxygen concentration, steam pressure and time asfunctions of MOE, MOR, ASE-R, AHE, EMC and△E*) are developed for the estimationand a nonlinear programming model is derived with operation research theory to obtain themost desirable HTW properties under heat flooring constraints. The results indicate that amaximum MOR of80.02MPa for the HTW can be obtained when heat treated at atemperature of195℃, oxygen concentration of8%, steam pressure of0.1MPa for1h.Under these production conditions, MOE, ASE-R, AHE, EMC and△E*are17.79GPa,18.19%,0.28%,7%and16.06respectively, which meet the requirements of the standards.3) In the practice, when the heat treatment temperature was less than180℃, oxygenconcentration was not over10%. While the temperature was higher than200℃, theoxygen concentration was less than8%.4) Compared with HTW treated at atmospheric pressure, the heat treatment temperature ofless than180℃has a significant impact on improving the dimensional stability of HTW atsteam pressure of kiln, where the heat treatment temperature of larger than200℃has aslight impact on dimension stability of HTW at steam pressure of kiln. However, with anincrease in steam pressure of kiln, the MOE, MOR and surface hardness decrease.5) When the heat treatment temperature was less than180℃for2h, no significant differencein△E*between the first layer and other four layers was found. When the temperature washigher than200℃, there was a significant difference in△E*between the first layer andother four layers. However, there was no significant difference in△E*among the otherfour layers. The△E*value become gradually small from first to fifth layer, but the airdensity was gradually large. The linear relationship between△E*and MOR and thepolynomial relationships of△E*with EMC and ASE-R were obtained, but there was norelationship between△E*and hardness in tangential, hardness in radial and MOE. Therefore, the EMC, ASE-R and MOR can be estimated by△E*.6) In terms of termite (Coptotermes formosanus Shiraki) resistance of HTW in laboratory, thevisual rating of the heat treated Oak was No.4grade, the HTW did not resist termite.7) The reasons why the dimension stability of HTW was improved are as follows: Firstly,when treated at temperature of180℃, the hemicellulose was decomposed into varioussugars. The content of sugar reached maximum level at temperature160~220℃, mainlyE-carbon sugar. As the temperature increased to220℃, the cellulose would bedecomposed into hexose. The hygroscopicity was reduced due to decomposing. Secondly,the crystallinity of HTW increased as a result of decomposition of non-crystalline regionof cellulose. HTW in steam pressure could produce formic acid, acetic acid and sugars,which created an acid environment within kiln, the HTW would be further decomposed, sothe crystallinity was higher than the HTW in atmospheric pressure. The hygroscopicitywas reduced due to an increase in crystallinity.8) The reasons why the mechanical strength of HTW was changed are as follows: Firstly, theHTW could be intensified because the vessel shape of late wood converted from round orrectangular to oval after HTHT, and the crystallinity of HTW was increased, so the MOEof HTW treated at200℃for2h was higher than un-treated wood. Secondly, with anincrease in the heat treatment temperature, time and steam pressure, predominant collapseor even cracks of the early wood vessels were observed in HTW, and so the MOR, MOEand Compressive strength parallel to grain were reduced. Thirdly, as the heat treatmenttemperature increased, the longitudinal elastic modulus of HTW cell increased, and thenreduced, which followed the same changing patterns with cross-section hardness of smallclear wood. The longitudinal and cross-section hardness of HTW cell were larger thanun-treated wood.