The Influence Of Environmental Factors On Color Stability Of Wood

Heat-treated wood,also known as thermally modified wood,is wood with improved properties such as dimensional stability and decay resistance obtained by heat treatment at180°C to 240°C in an oxygen-deficient environment.After heat treatment,the color of the wood becomes darker and the brightness decreases,which can cover some of the surface defects of the wood and present an appearance similar to that of precious tropical wood.Since color is the most important characteristic of wood and largely determines its commercial value,the study of color of heat-treated wood is of great application.In this paper,spruce and alder were heat-treated at 180°C,200°C,and 215°C,respectively,and then placed in three scenarios:outdoor,transition(bay window),and indoor space,to record the changes of brightness and color indexes during a cycle of 1 year and 4 months.The corresponding component and structural changes of the test wood were then analyzed with the aid of Fourier infrared spectroscopy and scanning electron microscopy to provide objective data and suggested references for the expanded application of heat-treated wood.The test results show that:(1)The color change of the specimens placed outdoors was the most significant,and the surface eventually appeared dark gray regardless of the initial state of the specimens,indicating that outdoor weathering conditions had a significant reshaping effect on the wood color of spruce and alder.The color change of heat-treated wood was less than that of control wood,but the final color difference between heat-treated wood and control wood of spruce and alder was less than 8.67,but with the extension of time,the brightness and chromaticity indexes were eventually similar to those of control wood,and the color difference between each other was less than or close to the limit of naked eye discrimination.The combined effect of light and precipitation,fungus,wind and atmospheric pollutants was an important reason for eliminating the structural differences between the two types of test materials and homogenizing the color of each type of test material.(2)The color of spruce and alder specimens did not change significantly in the room.The longitudinal color difference between spruce and alder before and after the test was 5.86,while that of heat-treated wood was only 2.77-3.57.The longitudinal color of alder before and after the test was only 0.97,and that of heat-treated wood was 3.83-6.07,indicating that the color of wood can be retained for a long time in an environment without UV irradiation and stable temperature and humidity.(3)Unlike the outdoor and indoor environments,spruce and alder showed obvious differences: the color change of the heat-treated wood of spruce was less than that of the control wood,and the color difference between the heat-treated wood at 180℃ and 200℃ was only4.33 and 6.12;while the color difference of the control wood of alder was only 4.37.The color change of the heat-treated wood was greater than that of the control wood,and the color difference was more significant as the heat-treatment temperature increased.The color variation of heat-treated wood was greater than that of control wood,and the color difference was more significant with increasing heat treatment temperature.This indicates that for spruce,heat-treated wood has performance advantages and application potential in transitional environments such as drift windows.(4)In both spruce and alder,there were large differences in the absorbance of the IR spectra after one year and four months in the three scenarios: most of the peaks were flattened or even disappeared in the outdoor environment,indicating that lignin and hemicellulose were degraded in the outdoor environment,and the degradation products were washed away by rainwater,resulting in the remaining non-chromatic cellulose and hemicellulose,which turned the wood gray in color,and there was no significant difference between treated and untreated wood after one year and four months.There was no significant difference between treated and untreated wood after one year and four months,which was also consistent with its wood color;the test wood in the drift window environment showed a completely opposite trend of content change at 1643 cm-1 from that outdoors,which was due to the reaction of phenoxy radicals generated from lignin photodegradation with oxygen to produce conjugated carboxyl groups,which manifested as yellowing of wood.The b* of spruce and alder showed the same(steadily increasing)trend after one year and four months because their conjugated C=O content also increased significantly.(5)The degree of spruce cell wall rupture was more easily observed outdoors,and the roughness of the light surface increased significantly due to various complex and harsh environmental influences.From the cross-section(vertical side of the light surface),it was already difficult to observe the intact cell morphology in the outermost layer of the light surface.Probably due to the strong UV light irradiation,cell wall fissures could be clearly seen along the light surface toward the inner layer,and the gap between the cell wall and the cell wall increased.Both spruce and alder were most severely damaged outdoors,where only tiny cracks in the cells could be observed in the floating windows,while cell wall changes could hardly be observed indoors.