A theme running through several presentations made at COST E49’s recent one day conference in Istanbul was the use of heat treatment to improve the dimensional stability and water-resistance of different wood based
panels. The idea is not a new one as it has been used by the hardboard industry since the 1950’s.
Heat treatment is also applied to solid wood. Examples of commercial products include: Retiwood; PlatoWOOD; and Termawood. Although there are differences in the techniques applied, the basic principal is the same in that the wood is heated to high temperature in an oxygen-depleted atmosphere. The temperature used will depend on the degree of modification of the wood, but it should exceed 200°C and temperatures of 230 to 240°C are common. The risk of fire around these temperatures is high and so a reduced oxygen level is an essential safety requirement. However, a low-oxygen environment also alters the chemistry of the modification reactions occurring in the wood polymers.
A heat treatment process will chemically modify wood without the addition of chemicals. In other words, some of the polymers and molecules present in the wood will be changed through a wide variety of chemical mechanisms. The upshot is that heat-treated wood (HTW) does not have the same chemistry as non-treated wood.
The potential advantages of using heat-treated wood for a panel manufacturer include: enhanced dimensional stability; greater bio-resistance; and lower density.
All three of these benefits are related to the fact that HTW adsorbs much less water than non-treated wood. For example, the fibre-saturation point of HTW is about 15%, which is around half that of untreated wood and so the swelling is also about half. Fungi and bacteria need water in order to attack wood. If the moisture content of wood can be kept below 18%, then it is not normally attacked. It can be concluded that when HTW is not in ground contact it should be very resistant to fungi and insects.
The lower density of HTW is partly due to its lower moisture content and partly to the loss of various organic compounds which evaporate or are degraded during the heat treatment process. This could be an advantage to panel manufacturers in that there is a market advantage for
products which have a lower density at a given
mechanical performance.
Of course, all processes have some disadvantages. Heat treatment causes significant darkening of the wood, a reduction in the mechanical properties and especially toughness, an increase in costs and reduced resin efficiency. However, there is the potential to apply a mild heat treatment to particles and fibres since they must be dried anyway. Perhaps, existing manufacturing lines could be modified to obtain some of the benefits without markedly increasing costs.
I wait to hear from anyone who is attempting to do this.