Structure–property relationships of hydrothermally treated western hemlock

Abstract

Western hemlock (Tsuga heterophylla), mostly found in west coast of North America, is a highly underutilized and lowvalue timber species, partially because of the presence of wet pockets, which makes wood processing challenging, especially when drying and sawing. The present work studies the modification of western hemlock using a hydrothermal process, which induced significant changes in the chemical structure and physical characteristics of the wood. The treatment caused the degradation of hemicellulose and reduced the quantity of adsorbed water and free hydroxyl groups. As a result, the wood became less hygroscopic and exhibited improved dimensional and thermal stability with a darker and more attractive color. The hydrothermally-induced variations in the wood structure also influenced its mechanical properties. Flexural bending tests revealed that the specific modulus of rupture and specific maximum load decreased after treatment, while the specific modulus of elasticity increased. In addition, the hydrothermal process was successful in eliminating case hardening, which is typically caused by residual compression stress during conventional kiln drying of wood. These results reveal that hydrothermal treatment is a promising method to expand utilization and develop value-added wood products using low quality wood, such as western hemlock.

Durability of Thermally Modified Western Hemlock Lumber Against Wood Decay Fungi

“…Despite the limited improvements in durability seen here, thermal modification of western hemlock has been shown to make the wood less hygroscopic, a characteristic that is would suggest better durability in above ground contact applications (Song et al, 2019). Modest durability increases seen here are lower than what is expected based on some previous testing of thermally modified wood (Kamden et al, 2002;Hakkou et al, 2006).…”

“…Additionally, mass losses of about 10% are necessary to fully inhibit brown rot decay on thermally modified wood (Candelier et al, 2015). Treatment temperature and mass loss was kept low for wood tested in this study to maintain structural integrity of the modified hemlock, which does deteriorate at higher mass loss values (Song et al, 2019). Further alteration of the modification cycle in future work would help develop a process for western hemlock that balances greater durability increases with the maintenance of structural properties of wood necessary for load-bearing applications such as decking.…”

“…One piece of the matching lumber specimens was designated as the kiln-dried KW sample, while the second matching specimen was designated as the thermally-modified TW sample. The TW specimens were loaded into a hydrothermal reactor and treated according to the conditions listed in Table 1 (Song et al, 2019). The treatment conditions, including parameters like temperature, time and pressure, were based on previous research conducted in the Natural Resources Research Institute at the University of Minnesota in Duluth (UMDNRRI).…”

“…Treatment conditions for the thermal modification process used for western hemlock in this study derived fromSong et al (2019). *Cooling water is injected into the kiln during the cooling and conditioning phase.…”