Heat Treatment
The treatment method we use was worked out
in Finland and it's based on heating wood at high temperatures, 180 - 250° C
(356 - 482 F), by using a water vapour as shielding gas.While heating wood at
temperatures over 200° C wood undergoes a large number of chemical changes, like
degradation of wood hemicelluloses.
Process is computer controlled and we can
recheck the process after treatment.
When raising or decreasing the temperature a
special adjustment system is used in order to prevent inside cracking. The
temperature rise in the kiln is regulated by the wood’s inside temperature. The
difference between kiln and wood temperature is dependent on the dimensions of
the wood specimens.
Before heat treatment we kiln dry lumber
up to 10-14%
During the heat treatment wood degrades and the
degradation products are mainly acetic and formic acid, a small amount of
phenolic compounds and other aromatic compounds and wood extractives. The gases
which evolve during treatment are mainly carbon monoxide and carbon dioxide and
methanol. This means that the equipment has some special requirements, it must
be built of acid resistant stainless steel and it needs a washing system where
the degradation products are absorbed.
Wood properties
The properties of
heat treated wood are dependent on the treatment process: treatment time and
temperature. Temperature has greater influence on many properties than time.
Treatment in lower temperatures for longer times does not bring corresponding
properties.
The colour of wood changes easily and colour
changes do not tell anything about how much the other properties have changed
compared to untreated wood.
Colour change into brown or dark brown.
The colour of
wood changes already in a mild heat treatment.
The colour is not very stable
for UV-light, unless the surface is treated with UV-resistant coating.
Reduction of equilibrium moisture content of wood by 40-50 %
Heat treatment slowers
water uptake and wood cell wall absorbs less water,because of the
decrease of the amount of wood’s hydroxyl groups.
Reduction of shrinking and swelling 50 - 90 %
As a consequence of the
reduced number of hydroxyl groups the swelling and shrinking are lower.
Improvement of biological
durability
The biological
resistance in a laboratory test showed very good durability depending on the
treatment temperature and time.
The resistance in ground contact is not
acceptable.
The improved biological durability is based on the
chemical degradation in wood components and formation of new compounds.
The
essential changes in wood chemistry are not exactly known.
Resin flows out of
wood
Wood looses some of it’s weight (5 - 15 %)
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