THERMOWOOD is a wood that has been modified by a controlled pyrolysis process during which it is heated to very high temperatures causing real chemical changes to the cell walls of the wood to increase its duration. This type of treatment has been used for over 30 years, especially in northern European countries (Scandinavian countries), and the essences most interested in this process are pine, fir and ash. Nowadays, thermo-treated wood is used in large quantities also in Italy where there are several leading companies for the production of specific plants for the thermo-treatment of wood.
Stages of thermowood treatment
The heat treatment process is divided into three phases:
In the first phase, the wood is dried at a temperature that reaches around 100 ° until it becomes almost anhydrous (0% humidity). The time required for heat treatment depends on the type of wood, the thickness and moisture of the wood before drying. During the drying process, the moisture present in the wood comes out and evaporates.
The second phase is represented by a heat treatment carried out in the plant which is brought to a temperature between approximately 180 and 220 °, during which the wood undergoes a chemical change. The second phase begins immediately after the drying phase and lasts about 4/5 hours. There are two methods to prevent the wood from burning: creating a vacuum or placing the steam in the cell.
In the third phase, the temperature gradually lowers until it reaches room temperature. During this phase, steam is added in order to bring the wood to an ideal humidity for use of about 7/8%; this phase lasts between 5 and 10 hours.
There are two types of heat treatment: "S" which means Stability or stability and "D" which means Durability or durability. For products that only need to maintain a certain stability, the temperature that is used will be a maximum of about 190 ° while, for products that require durability, the temperature must reach about 215 °. Usually class "S" wood is used indoors while class "D" wood can be used both indoors and outdoors.
Characteristics of thermo treated wood
Heat treated wood changes its chemical and physical properties. These transformations begin to manifest themselves around a temperature of 150 ° C and increase with increasing temperature. The heat treatment alters the structure and molecules of the wood, demolishes the hydroxyl groups and modifies the structure of the cell wall of the wood. The result of this treatment is a decrease in the swelling of the wood which loses part of its weight. Instead, it increases its biological durability and changes its natural color which becomes darker. The higher the temperature is, the more intense the final color of the wood will be.
By removing the resin during heat treatment, the wood becomes lighter. Humidity and pH measurement decrease while the thermal-insulating properties increase. Heat treated wood no longer absorbs moisture, remains much more stable and has the advantage of having a long life.
Another advantage of the heat treatment of wood is that with this process the entire structure of the timber is modified, unlike the surface treatment that protects the outside while the wood remains virgin inside.
With heat treatment, soft wood (pine and fir) can be used for applications that require high durability. The same, at the end of the drying process, can reach durability class 1–2, according to the European standard EN 350-2. L
The coloring of heat treated wood
Due to the action of ultraviolet rays, all the wood that is outdoors, over time, undergoes a change in breakfast, which will tend to turn gray. This also happens for wood that has undergone an impregnation or thermal drying treatment. In order to maintain the color it is advisable to use a mordant based on natural products with pigments that do not contain harmful chemicals (ecological), which may also comply with the Minimum Environmental Criteria (CAM).