Thermally modified wood, is wood that has been modified by a controlled pyrolysis process of wood being heated (> 180 °C) in absence of oxygen inducing some chemical changes to the chemical structures of cell wall components (lignin, cellulose and hemicellulose) in the wood in order to increase its durability. Low oxygen content prevents the wood from burning at these high temperatures. Several different technologies are introduced using different media including nitrogen gas, steam and hot oil.
The first technological approach of thermal modification processes of wood is the paper of Burmester.
There are five different thermal modification processes: Thermowood (or Premium wood) in Finland, Retification process (Retiwood, New Option Wood) and Les Bois Perdure in France, Plato process in Netherlands, Oil-heat treatment in Germany (OHT-Process).
Description of the process
Three of the processes are performed in one step, using oil (oil-heat treatment), nitrogen (Reti wood) and steam (Le-Bois Perdure). The Thermo wood process consists of drying, heat treatment and finally cooling/conditioning, and takes up to 72 hours. The Plato process consists of hydrothermolysis, dry curing and conditioning, and can take up to 7 days. The required time depends on wood species, width and initial moisture content.
Westwood is the most advanced modern thermo-modification process, patented in 2004. The process has initially developed for treatment hardwoods, which are more complicated compared to softwoods due to thermo-chemical reactions appeared in hardwoods during thermo-treatment. The fully automated Westwood control system allows to manage thermo-treatment of any hardwoods and softwood species.
A two step process which first subjects wood saturated with water to hydrothermal treatment at 160 °C–190 °C at a high pressure. After the moisture content in the wood is reduced to 10% then a curing step is performed at 170 °C–190 °C under 1 atm of pressure.
Retification refers to the French word rétification, which is the portmanteau of réticulation (creation of chemical bonds between polymeric chains) and torréfaction (roasting). Wood in this process must have a moisture content at 12% or lower which can be attained through simple drying processes. The wood is then placed in a high nitrogen atmosphere with no more than 2% oxygen content.
Les Bois Perdure
Freshly cut timber must first be dried and then will be heated in a steam environment using the leftover moisture in the wood.
Oil Heat Treatment
The wood is placed in an oil bath between 180 °C and 220 °C. This process can also be used with chemical additives to alter the wood in other ways.
This process is similar to the Les Bois Perdure treatment in that it uses a steam environment at atmospheric pressure to treat the wood. However, this process can also be used on “green” wood and was the most widely used commercial process as of 2004.