Introduction to Thermally Modified Wood (TMW)
How is TMW made?
Thermal Modification: What does it do?
Thermal modification involves heating wood in a controlled environment with little or no oxygen to prevent combustion. In North America, most thermal modification processes happen in a vacuum. The process consists of three stages. First, kiln-dried lumber is placed into the treatment chamber and gradually heated to high temperatures, while more moisture is removed. Next, during the thermal treatment phase, peak temperatures between 180 and 220°C alter the chemical structure of the wood. Finally, the wood is cooled, and in some processes, reconditioned with moisture and stabilized for use. Research has shown that higher temperatures, those above 200°C, provide greater decay resistance than lower temperatures.
What happens to the wood? Generally speaking, the temperatures and durations of treatment lead to chemical changes in the wood. Wood is composed mainly of three chemical components: cellulose, hemicellulose, and lignin. The thermal modification process leads to the degradation of hemicelluloses, partial modification of lignin, an increase the crystallinity in cellulose and a reduction in the chemical structure that normally binds water.
TMW: Color Changes
One of the most noticeable effects of thermal modification is the color change. As wood is heated, its natural tone shifts to a richer, darker brown that often resembles exotic hardwoods. This darkening occurs uniformly throughout the cross-section, not just on the surface. The intensity of the color depends on the treatment temperature and duration, with higher temperatures producing deeper, browner shades. Like most wood products, TM wood will fade or turn gray over time in outdoor settings unless it is protected with UV-resistant coatings. The TM color changes enable lighter-colored, lower-value, sustainably harvested hardwoods to resemble the color of more exotic tropical hardwoods, creating a new market opportunity for this material. Figure 1 demonstrates the darkening of yellow poplar at different temperatures from a laboratory test. Darker and richer colors can be commercially produced (Figure 1).
TMW: Decay Resistance
The initial goal of TM and one of its most significant advantages is the improvement in biological durability. The degradation of hemicelluloses, which fungi rely on for food, makes the wood less hospitable to decay organisms. As a result, thermally modified wood demonstrates significantly improved durability against brown rot, white rot, and soft rot fungi when compared to untreated wood. Our tests on TM hardwoods indicate that it does not provide increased resistance to termites. Also, while decay resistance is significantly improved, it does not meet the standards for ground contact.
TMW: Dimensional Changes
The chemical changes in wood result in reduced moisture uptake, which in turn reduces swelling, shrinking, and warping. Overall shrinkage and swelling can be reduced by 40–80% depending on the species. For example, in our work on hardwoods, we observed reductions as small as 24% for hickory and as large as 70% for ash. The equilibrium moisture content (EMC) is typically 40–50% lower than that of untreated wood at the same relative humidity.
TMW: Mechanical Properties
While thermal modification enhances specific characteristics of wood, it also reduces its bending strength and impact resistance. The loss of hemicelluloses and the structural changes in lignin reduce the wood’s flexibility, resulting in lower bending strength. Impact resistance is also diminished, making the material more brittle and more prone to cracking under sudden loads. Although the stiffness, or elastic modulus, may remain relatively stable or even improve slightly under moderate treatment levels, higher temperatures typically reduce it as well. In short, thermally modified wood is mechanically weaker than untreated wood of the same species. For this reason, it is not recommended for load-bearing uses; however, it performs well in non-structural and aesthetic applications, such as decking, cladding, and siding.