Effect of Low Temperature Cyclic Treatments on Modulus of Elasticity of Birch Wood

This study aimed to investigate the effects of growth ring width, height from the tree base, and loading direction on the transverse compressive strength of Japanese larch wood, which is commonly used in wood structures in China. Plantation wood is often used to replace natural forest woods for reconstruction purposes, despite significant differences in properties (e.g., growth rings, density, strength) between them. The ends of transversely compressed wood members in such structures are prone to damage by breaking or crushing. A transverse compressive test was conducted following Chinese national standards, which revealed the following key findings. (1) There was a significant difference in the transverse compressive strength of wood with different growth ring widths (p < 0.05). The radial and slant compressive strength of wood increases with growth ring width, while the tangential compressive strength decreases as growth ring width increases. (2) The transverse compressive strength of wood decreases as the height from the tree base increases. The radial, tangential, and slant compressive strength at a lower height were 18.39%, 22.58%, and 18% higher than those at a greater height in the stem, respectively, with significant differences at the 0.05 level. (3) The load–displacement curve of Japanese larch wood under radial and slant compression follows a “three-segment” form. In contrast, the load–displacement curve of tangential compression is a continuous curve that drops sharply upon reaching its highest point. (4) There is a significant difference in the transverse compressive strength of Japanese larch wood in different loading directions when growth ring width and height from the tree base are constant (p < 0.05), which fall into order as tangential > radial > slant.

Heat treatment is an environmentally friendly method that improves the moisture-resistant properties and increases the service life of timber. In this work, Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) wood was heat-treated in a chamber with steam at temperatures of 160, 180, 200 and 220 °C for 2 h, and the absorption of water was studied by gravimetric analysis and time domain nuclear magnetic resonance (TD-NMR). The results show that both the amount of bound water and free water decreased with the increasing treatment temperature. The water absorption of wood in the radial direction was faster than that in the tangential direction due to the existence of rays, and this difference remained after the heat treatment. The heat treatment at 220 °C had a significant effect on water absorption in the tangential direction of wood, and the moisture content (MC) was approximately 20% lower than that of samples absorbing water in the radial direction. T₂ (spin–spin relaxation time) distributions showed two main components which were associated with bound water and free water, and for samples absorbing water in the radial or tangential direction, there was only a difference in the amount of free water. The amount of free water significantly decreased for the samples that were heat-treated at 220 °C and absorbed water in the tangential direction, indicating that the high-temperature heat-treated samples tended to close the pits in wood cells.

In order to evaluate the physical and mechanical properties of plantation Japanese larch wood, various physical and mechanical indicators were measured with reference to Chinese national standards. The characteristics of the growth ring on the end face of wood samples were captured, with the mean latewood percentage being 21.4% and the mean ring width being 3.24 mm. Relationships between growth ring characteristics, latewood percentages, and the physical and mechanical properties of the plantation Japanese larch wood were investigated. The results revealed that it is most appropriate to use growth ring width to evaluate MOE and MOR, and to use latewood percentage to evaluate air-dry density, radial air-dry shrinkage, IBS and UTS. Regression analysis confirmed that air-dry density (R² = 0.99), radial shrinkage (R² = 0.97) and UTS (R² = 0.96) had significant positive correlations with latewood percentage, while MOE (R² = 0.88) and MOR (R² = 0.90) had significant negative correlations with RW. IBS was not significantly related to either characteristic. RW can be used to define juvenile wood and mature wood, with the dividing ring width being 4.85 mm. There is a large difference in MOE and MOR between the two wood types.

High-temperature heat treatment (HTHT) is an effective method for improving the wood dimensional stability and biological durability at present. However, the quantitative control of the wood color during the HTHT has been a more difficult problem for a long time. To solve the problem of quantitative control of wood color under the thermo-vacuum treatment (TVT) process, the mathematical model of heat and moisture transfer in alder birch wood (Betula alnoides) during the TVT process was built, and its application in the quantitative control of the wood color was also studied. (1) The experimental values and the model values of heat and moisture transfer in wood were found to be in good agreement. (2) The changes of wood temperature and moisture content (MC) were influenced obviously by the heat treatment temperature, the initial MC, and the thickness of specimen as well. The higher the heat treatment temperature was, the more rapid was the rise in the wood temperature and the drop in MC. The higher the initial MC of the wood was and the thicker the wood got, the slower the rise in wood temperature and the drop in MC. (3) Based on the mathematical model of heat and moisture transfer during the TVT process and the wood color index difference regression equations in the function of the temperature and time, the quantitative control of alder birch wood color during the TVT process was achieved. Therefore, it is feasible that heat and mass transfer principle applied during the TVT process guided the wood color quantitative control.