Tunnicliffe's ThermoWood®
Thermally Modified NZ Radiata Pine

Tunnicliffe's has been developing markets for “thermally modify” New Zealand grown Radiata pine using the ThermoWood® process since 2008. In effect the process creates a new timber species with its own specific properties. Tunnicliffe's purchased a high-tech, small-scale treatment chamber, previously located at Scion (former Forest Research Institute) in Rotorua, New Zealand where the technology has been trialled since 2003.

Tunnicliffe's ThermoWood® is a durable timber product, free of any chemical. The timber used is thermally modified NZ Radiata pine, which is comparable with naturally durable timbers such as Cypress Macrocarpa, Western Red Cedar and Redwood. Apart from increased high durability properties the other main feature is added stability due to the reduction of equilibrium moisture content (EMC). ThermoWood® modified Radiata pine will display approximate half the amount of dimensional change when compared with non-modified Radiata pine under the same conditions.

The technology can be seen as revolutionary for our New Zealand Radiata pine resource as the timber will be able to enter areas of application which were previously the domain of imported timbers.

Tunnicliffe's ThermoWood® is a truly sustainable and environmentally friendly, long lasting product of the future for New Zealand and beyond.

Tunnicliffe's first ThermoWood® product is in its beehive box range, which has proven very successful ThermoWood® Beehive Boxes

ThermoWood® is now increasingly common in several exterior timber joinery applications as an alternative for both Radiata pine and Western Red Cedar.

It has been known for centuries that burning the surface of wood in open fire will make it more durable in exterior use. The Vikings (700 -1050) used this method in outdoor structures such as fences.

The heat treatment of wood was scientifically studied in Germany and the USA during the 1930's and 40's. Research carried on in Germany in the 1950's through to the 1970's. More recently research work was carried out in Finland, France and the Netherlands in the 1990's when the technology gained momentum and was successfully developed for commercial use. The technology Tunnicliffe's is using originates from Finland, the ThermoWood® process has been developed by VTT Technical Research Centre of Finland in Helsinki Tunnicliffe's established an association with the Finnish company Jartek Invest Oy who are treatment chamber manufacturers and a member of the Finnish Thermowood Association

Today there are several brands established in the European market where thermally modified timber is used to replace tropical hardwood timbers, naturally durable timbers such as Western Red Cedar and Redwood and also softwood timbers treated with chemical preservatives.

The Process
New Zealand grown Radiata pine is very suitable for the heat treatment process. The timber is modified in a special chamber where it is exposed to temperatures up to 230 degrees Celsius in a computer controlled process. Steam is added to prevent the timber from combusting but also has an effect on the chemical changes taking place in the timber.

The process permanently changes the chemical and physical properties of the timber. In other words the wood structure is re-formed or “modified”. These changes are mainly caused by thermal degrading of hemicelluloses (a sugar compounds in the timber). It increases the biological durability, stability and also the thermal insulation properties are improved. On the other hand there is some loss of strength.

High Durability
In principle the durability is increased due to the fact that the edibles (sugar compounds), on which fungi live on, have been taken out by burning-off. But there are a number of other changes occurring during the modification process that also contribute to the increased durability such as a lower level of moisture.

Click here to view
ThermoWood®230 Durability Test Update March 2009
ThermoWood®230 Durability Test Update June 2011
ThermoWood®230 Durability Test Update June 2015

Dimensional Stability
The equilibrium moisture content (EMC) is reduced, which increases the dimensional stability of the timber as there is less swelling and shrinkage caused by moisture uptake / release.

The timber darkens, right through, towards an attractive chocolate brown colour. The higher the maximum temperature treated to, the darker the timber looks.

Modification Level
The level of modification depends on the end-use application. The three factors to be considered are durability, stability and strength. The modification process is not suitable for structural timber such as framing, beams and trusses. A compromise is needed between maximum durability and strength. With a higher durability level, there is some loss of strength. For example, Tunnicliffe's ThermoWood® Beehive boxes are treated with the maximum durability in mind because the loss of strength is less significant. The varying level of modification are achieved using different schedules with different maximum temperatures ranging from 185 to 230 degrees Celsius. To prevent the timber from combusting steam is added.

Working with Tunnicliffe's ThermoWood®
Working with ThermoWood® is easy. It machines and finishes well. In general the timber can be nailed and screwed as with the untreated version, keeping in mind that it is a little more brittle. For nailing it is recommended to use a small pneumatic nail gun with adjustable drive depth. Self-tapping screws can be used without pre-drilling. Normal PVA, PU, MUF glues and RF resins can be used as well.

ThermoWood® can be used unpainted for exterior use. However it will weather, turning grey as any other timber. It is recommended to seal the timber with at paint system to keep it in long lasting good condition.


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