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ThermoWood® Process

ThermoWood® - thermal modification process has been developed and patented by VTT Technical Research Centre of Finland.

Process can be divided into three phases:

1. Temperature increase

Wood temperature is raised rapidly using heat and steam to a level of around 100 °C. Thereafter the temperature is increased steadily to 130 °C. Raw material can be green of pre-dried wood. Steam is used to prevent cracking of the wood. It also affects the chemical changes taking place in the wood. Moisture content reduces to nearly zero.

2. Actual thermal modification

During actual thermal modification phase the temperature is increased to a level of 185-215 °C. Temperature depends on the desired treatment class. When the target level has been reached, the temperature remains constant 2-3 hours. Steam is used to prevent the wood from burning in high temperatures and it also affects to the chemical changes in the wood.


ThermoWood® process



Diagram of the production process


3. Cooling and re-conditioning

The temperature is reduced by using water spraying as cooling media. Conditioning and re-moisturing takes place to bring the wood moisture content to a level of 4-6 percent.

When the temperature is raised or lowered, a special adjustment system is used in order to prevent surface and inside splitting and checking. Customised adjustment values are used for different wood species and dimensions.

The raw material can be green or kiln-dried wood. If the process is begun with green wood, the wood can be dried in a very fast high-temperature drying process. The method is suitable for softwood and hardwood species. However, the process must be optimised separately for each wood species.

Standard ThermoWood® treatment classification

Softwood and hardwood species have a separate classification since their properties clearly differ. There are two classes of heat treatment. Having more than two classes is not reasonable since wood properties change slowly at first as the temperature increases. Once the treatment temperature exceeds 200 °C, the properties change rapidly. Using more than two classes would generate a risk of mixing properties of different classes. 215 °C is sufficient as a maximum temperature value yet is not so high that the effects of heat treatment on the wood's structural properties would be significant.
In the standard class of ThermoWood treatment, swelling and shrinkage of wood due to moisture, colour change, and biological durability are emphasised as key properties.

Since the ThermoWood material to be supplied to industrial customers is heat?treated in accordance with the agreements between the purchaser and producer, the processing level can be carefully optimised according to the end use application. In this case, the material will be ThermoWood that is not categorised according to the standard treatment classification scheme.

Standard ThermoWood treatment classes

ThermoWood has two standard treatment classes, Thermo-S and Thermo-D.

Thermo-S

The letter 'S' in 'Thermo-S' stands for 'stability'. Along with appearance, stability is a key property in the end use applications of the products in this treatment class. The average tangential swelling and shrinkage due to moisture for Thermo-S class treated wood is 6-8%. Thermo-S class ThermoWood is classified as relatively durable according to the standard EN 113; i.e., its natural resistance to decay meets class 3 requirements.

Recommended end use applications for Thermo-S class heat-treated timber:

Thermo-S Softwood Thermo-S Hardwood
- building components
- furnishing in dry conditions
- fixtures in dry conditions
- furniture
- garden furniture
- sauna benches
- door and window components 		- furnishing
- fixtures
- furniture
- flooring
- sauna structures
- garden furniture

Thermo-D

The letter 'D' in 'Thermo-D' stands for 'durability'. Along with appearance, biological durability is a key property in the end use applications of products in this treatment class. The average tangential swelling and shrinkage due to moisture for Thermo-D class treated wood is 5-6%. Thermo-D class ThermoWood is classified as durable according to the EN 113 standard; i.e., its natural resistance to decay meets class 2 requirements.

Recommended end use applications for Thermo-D class heat-treated timber:

Thermo-D Softwood Thermo-D Hardwood
- cladding
- outer doors
- shutters
- environmental constructions
- sauna and bathroom furnishing
- flooring
- garden furniture 		End use applications as in Thermo-S.
If a darker colour is desired, Thermo-D should be used.

Summary of the effects of the ThermoWood process on wood properties, by treatment class

Softwoods (pine and spruce)

Thermo-S Thermo- D
Treatment temperature 190 °C 212 °C
Weather resistance + ++
Dimensional stability + ++
Bending strength no change -
Colour darkness + + +

Hardwoods (irocco, tali, ash, oak, birch and aspen)

Thermo-S Thermo- D
Treatment temperature 185 °C 200 °C
Weather resistance no change +
Dimensional stability + +
Bending strength no change -
Colour darkness + + +



ThermoWood® Handbook - pdf

ThermoWood® Brochure - pdf

ThermoWood® Planing Handbook - pdf

ThermoWood® Manual on Surface Treatment


     
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