All of us are familiar with applications of wood used outdoors. Logs in a log cabin, wood siding, a wood deck, a mailbox post. There are hundreds of examples. In almost all these examples, we use preservatives to extend the useable life of the wood. We apply stain to the exterior of a log cabin and wood siding. We purchase pressure treated wood to build a deck or install a mailbox post. These chemical treatments can extend the life of the wood by years even decades. What if I told you there is a way to extend the useable life of wood used in exterior applications without the use of chemicals. Is it too good to be true? No, it is called thermally modified wood.
Thermally modified wood has been around for decades, but it has found expanded use in the United States in recent years. Its market is growing extremely quickly in Europe because the European Union has more restrictive policies on chemically treated wood. After treatment, the chemical structure of wood is modified so it is no longer palatable to decay fungi or insects. Thermally modified wood is popular because you get decay resistance without the use of added chemicals.
The process used to thermally modified wood is a controlled pyrolysis process. There are 5 or 6 methods for thermally modifying wood, but the basics are the same. Wood is heated to more than 350 degrees Fahrenheit in an extremely low oxygen environment. If oxygen were present, combustion would occur at that temperature. The process used here in the Lake States uses 3 phases of treatment. Phase 1 consists of a gradual increase in temperature of the wood to reduce the moisture content. Kiln dried lumber at around 8% moisture content must be used in this process. Phase 2 uses a rapid increase in temperature to reach the 350 degree mark. The cellular composition of the wood is altered in this high-heat, oxygen-deprived environment, which modifies the lignin, cellulose, and hemicellulose in the wood cell walls. The 3rd and final phase introduces steam to cool the wood down and increase the final moisture content of the lumber to around 4%.
The main advantage of thermally modified softwood lumber and hardwood lumber is they can be used for applications that require high durability. We are able to take non-durable species and transform them into durable products. The main mechanism is the change in the wood chemistry. Decay fungi use an enzyme called cellulase to break the chemical bonds in the wood structure. Since the wood chemistry is modified, the cellulase enzyme is no longer effective in breaking up the cellulose for digestion by the fungi. A second advantage is that shrinking and swelling is reduced from 50 to 90%. This makes the wood more stable for certain building applications. Another advantage is the color change. Thermally modified wood is darkened ranging from light brown to a coffee color. The color often resembles more expensive imported tropical woods. As a principle the heat treatment process can be done on all wood species.
The main disadvantage is that the strength is decreased as a result of the high temperatures. In general the bending strength is reduced up to 30% with more reduction at higher temperatures. Thermally modified wood also becomes more brittle, which can make it more difficult to machine. The final disadvantage is that thermally modified wood is more expensive than unmodified wood. The modification process is energy intensive and is one more step added to the manufacturing process.
Thermally modified wood is one more amazing product derived from Wisconsin’s forests. Demand for these sustainable products helps manage our forests and provide economic opportunity in the Northwoods for generations to come.
