Thermally modifying wood involves kiln-drying lumber in a high-temperature, low oxygen environment. This process changes the chemical structure of wood, resulting in improved dimensional stability and durability while darkening the color. The Finnish ThermoWood treatment is the most commercially successful thermal modification treatment within the different methods and technologies available. The ThermoWood method does not require chemicals or a high-pressure vessel, instead using steam and airtight stainless steel kilns to modify wood.
Thermally Modified Wood
The Finnish ThermoWood Association has patented the following three-phase thermal modification process:
Figure 1. Thermal Modification Process, Courtesy of Jartek
Phase 1–High Temperature Drying
Thermally modified Hickory Hardwood flooring.
* Heat and steam raise temperature to 100oC (212oF) * Moisture content of wood reduced to approximately 0% * Phase 2–High Intensity Thermal modification * Temperature increased between 180oC (356oF) to 210oC (410oF) for 2-3 hours (temperature and time depend on end-use application) * Phase 3–Cooling and Moisture Conditioning *Temperature decreased with water spray to increase moisture content to a usable 4–7%
The thermal modification process caramelizes hemicellulose chains causing a reduction in the amount of water absorbing hydroxyl groups. The wood displays a 40-50% lower equilibrium moisture content, reducing tangential and radial swelling, while a lower permeability decreases finishing and adhesive requirements. Modification also improves appearance, preventing sap stain through the evaporation of extractives and concealing any stain already present in the wood by darkening the color.
In addition to dimensional stability, improved biological durability also increases the appeal of thermally modified wood. Testing shows improvements in durability with temperatures as low as 180oC (356oF), with resistance characteristics comparable to cedar. In addition, the color of softwoods modified at 180-200oC (356-410oF) closely emulates the appearance of cedar. This color change, the increased dimensional stability and an improvement in durability make the market occupied by cedar and tropical species an ideal target.
The high capital cost of a thermal modification system requires producers to find markets with substantial profit margins. For Hardwoods, the price gap between Oak, Birch, Hickory, Ash and Maple with Black Walnut and tropical imports, including Wenge, Teak and Mahogany creates this market.
Thermally Modified Wood in the Market Today
Over the last decade, thermal modification has been developed in five main product markets in Europe; Hardwood flooring, siding/cladding, decking, saunas/wall paneling, and specialties. Thermal modification allows high-grade lower-cost species to compete with naturally durable and higher-cost species, including tropical Hardwoods and cedar, resulting in competitive advantage in these markets. North American manufacturers have the opportunity to benefit from the experiences of their European counterparts and create a commercially viable market for thermal modification in North America.
While thermal modification opportunities exist for wood manufacturers, certain barriers will need to be overcome. The high capital cost of a thermal modification system requires producers to find markets with substantial profit margins. For Hardwoods the price gap between Oak, Birch, Hickory, Ash and Maple with Black Walnut and tropical imports, including Wenge, Teak and Mahogany creates this market. This price gap also exists in the softwood market when comparing western red cedar against spruce, pine, fir and hemlock. The other consideration involves the wood itself. Manufacturers need to deal with quality issues, and the end product requires some form of UV protection to prevent fading. Although thermal modification significantly reduces rot problems, it does not eliminate them. However, these challenges are not new, as the organic and anisotropic nature of wood has always presented these hurdles to the wood industry.
Jartek’s 10,000 board foot capacity Thermal Modification Chamber with a small research chamber.
The Future of Thermally Modified Wood in North America
Those likely to participate in the development of thermal modification in North America will be midsize value-added manufacturers and innovative companies that have the opportunity to capitalize on their established markets and new product potential of thermally modified wood.
* Flooring manufacturers will be able to reduce raw material costs and begin to use wider planks, with color changes offering competition to imported tropical species
* Siding, cladding and decking producers will have a less expensive substitute to western red cedar with better durability and stability than regular SPF, while also eliminating resin bleed
* Value-added manufacturers find new opportunities to provide specialty goods to an industry constantly needing an exciting new color or product
* Cedar manufacturers will diversify, offering distributors lower cost alternatives, while decreasing their reliance on inconsistent cedar material and availability
As companies begin to embrace these new opportunities, an entire forest industry trying to rediscover its competitive advantage will be rejuvenated as thermal modification provides it with a product that will compete successfully with expensive imports. The path has been laid out, and if North American wood manufacturers are able to follow it they will discover an environmentally responsible and economically viable product with untold future possibilities.
Jartek is a manufacturer of thermal modification chambers, with ThermoWood chambers operating in 13 countries including chambers in Quebec, Minnesota and British Columbia. Jartek is dedicated to researching thermal modification of local sustainable North American species and is willing to process samples for interested companies through its partner SeasonWood. If interested contact Dallin Brooks at 604-762-7075 or by email at info@seasonwood.com. For more information on thermal modification visit www.thermowood.fi, www.Jartek.com and/or www.seasonwood.com.
