Exploratory thermal modification of western hemlock

Abstract

ABSTRACT This research focuses on exploring commercial heat treatment on western hemlock product quality for potentially new applications and markets for this abundant Pacific Northwest coast softwood species. In this regard, flatsawn and quartersawn kiln-dried hemlock boards were subjected to the ThermoWood® process in a commercial unit at three temperatures, i.e. 170°C, 212°C, 230°C and for 2 h. Samples were cut from treated and untreated boards for evaluation of basic density, hygroscopicity, water absorption, anti-swelling efficiency, color change, Janka hardness, and dynamic modulus of elasticity. Results revealed that basic density, hygroscopicity, and water absorption decreased at higher treatment temperatures, while dimensional stability considerably increased. Treatment temperatures had a minor effect on hardness and stiffness, but they made the samples darker in color. No significant property differences between the two cuts were observed. Within the scope and limitations of this study, the treatment that enables improved dimensional stability and provides a darker color without significantly affecting hemlock strength was the 212°C/2 h combination. Further research is required to fully determine the performance of thermally modified hemlock under expanded treatment combinations for interior and exterior applications.