HIGH TEMPERATURE DRYING AND DIMENSIONAL STABILITY IN FULL SIZE LUMBER
Kare Edvardsen1 and Knut Magnar Sandland2
1. Borregaard Chem Cell, P.O.Box 162, N-1701 Sarpsborg, Norway,
E-mail: kare.edvardsen@borregaard.com
2. Norwegian Institute of Wood Technology,
P.O. Box 113 Blindern, N-0314 Oslo,
Norway, E-mail: knut.sandland@treteknisk.no
Keywords: wood, high temperature drying, dimensional stability, stress and
strain, casehardening
ABSTRACT
Wood is a biological material which contains water. Artificial drying
is well known as a method for reducing the moisture content of wood and
improving various properties.
Among the artificial drying methods, high temperature drying of wood
(HT-drying), which is drying at dry bulb temperatures above 100°C,
has received new interest due to reduced drying time.
Dimensional stability is critical for wood in use, and different methods
are used to obtain improved dimensional stability. Investigations have
shown that heat treatment of wood in the temperature range of 150-250°C
changes such properties of wood as durability and dimensional stability.
A question of interest is therefore whether HT-drying at dry bulb
temperatures of 110-115°C also improves the dimensional stability of wood.
Logs from Norway spruce were split along the pith, and paired planks were
dried. One group was dried using a conventional drying schedule (LT-drying)
while the other was dried using a HT-drying schedule.
Test samples were taken from the planks after drying, and exposed to 17
cyclic climate changes between a humid climate (20°C - 85% RH) and a dry
one (30°C - 30% RH).
The dimensional movement was measured during the climate changes.
There is a significantly lower moisture content in HT-dried wood compared
to LT-dried during the climate changes. However, no apparent difference in
relative movement is observed. It is assumed that the reason for this is
that the modification effect is confounded with other effects, especially
stress and strain relations when drying full size lumber.
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