The present invention concerns a method according to the preamble
of claim 1 for improving the resistance of cellulosic products against mould and
decay as well as to enhance the dimensional stability of the products.
According to a method of the present kind the cellulosic product
is dried to a moisture content of less than 15% and then subjected to a heat treatment
carried out at an elevated temperature.
It is well-known in the art that the dimensional stability of wood
can be improved by heat treatments. As far as the prior art is concerned, reference
is made to, for instance, the Finnish Patent Specification No. 68,122, which discloses
a method for treating wood products at temperatures of 160 to 240 °C and at pressures
of 3 to 15 bar. As a result of the treatment, the capability of wood to absorb
water and thus to expand is considerably reduced. The effect of heat treatments
on decay resistance of wood has also been studied. Mailun, N.P. and Arenas, C.V
describe in their article "Effect of heat on natural decay resistance of Philippinean
woods" (Philippinen Lumberman, Vol 20, No. 10, 1974, p. 18-19, 22-24) the treatment
of Asian wood species in dry state at temperatures of 90, 110, 130, 150 and 175
°C for 240 hours. As a result of the treatment the colour of the wood changes
to chocolate brown. An extended treatment at 130, 150 and 175 °C increased the
resistance of the wood samples against two brown rot fungi. However, at the same
time it made the wood weaker.
Because all kinds of wood are not suited for the conventional pressure
impregnation methods, using substances which prevent the growth and spreading of
fungi, heat treatment is an interesting alternative for protecting wood against
The prior heat treatment processes, which call for the use of pressure
and extended treatment times, have been too complicated for industrial applicability.
It has also been ascertained that under the influence of high pressures and increased
temperatures wood becomes brittle and it weakens. Furthermore, wood is easily ignited
at high temperatures.
The present invention aims at eliminating the problems related to
the prior art and to provide a completely novel solution for improving the dimensional
stability of and the resistance against decay and mould (i.e. the biodegradation
resistance) of cellulosic products.
The invention is based on the concept of carrying out the heat treatment of the
cellulosic product in two stages: first the product is dried to desired moisture
content, typically to below 15%. Then the temperature is rapidly raised above
150°C (typically to about 180 to 250°C) and the treatment is continued at that
temperature until the weight loss of the treated product amounts to at least 5%.
The products are kept at essentially atmospheric pressure in a moist oven, to which
water steam is fed. By contrast, DE-A-2 263 758 mentioned above, suggests working
at pressures of 5 and 6.5 and the weight loss achieved by the known method is small.
Similarly, it is an objective of WO-A-90/06840 to avoid weight loss of wood caused
by heating at elevated temperatures.
In particular the method according to the invention is principally characterized
by what is stated in the characterizing part of claim 1.
As mentioned above, unseasoned timber or similar cellulosic products
are used as starting materials for the method according to the present invention.
The product can be dried at any suitable conditions (even outdoors at ambient
temperature) to the desired moisture content of less than 15 %. According to a
preferred embodiment of the invention the product is, however, dried at elevated
temperatures. The colour of the wood product will become darker during such drying.
In connection with the drying due care is taken to avoid cracking of the product.
This goal is advantageously attained by constantly determining the temperatures
of the interior and the surface, respectively, of the wood and by maintaining
the temperature difference at a reasonably small value. Preferably said difference
amounts to about 10 to 30 °C. This procedure is followed both when the temperature
is raised and when it is lowered. Surprisingly, it has been found that said solution
will even completely prevent the formation of cracks in (the interior parts of)
the wood material. When larger amounts of wood are to be dried several samples
should be provided with sensors. On an industrial scale, the preferred procedure
comprises determining for each kind of timber a heating programme of its own which
takes into account the influence of the initial moisture content on the process.
In order to protect the wood and improve heat transfer during drying
it is preferred to use steam during drying. According to a preferred embodiment,
the drying process according to the invention comprises the following steps:
- a) first the temperature of the drying oven is raised to at least about 90
°C, preferably to at least 100 °C, and that temperature is maintained until the
wood has at least approximately reached the same temperature,
- b) then the temperature of the oven is gradually raised so that the difference
between the interior temperature of the wood and the temperature of the oven does
not exceed 30 °C until the desired moisture content of the wood is reached, and
- c) finally, the temperature of the oven is lowered gradually while ensuring
that the difference between the temperature of the interior parts of the wood and
the temperature of the oven does not exceed 30 °C until the interior of the wood
has reached the desired temperature.
If needed, stage c can be left out. The heat treatment, which will
be described in more detail below, is then carried out immediately after stage
During the first stage of the present invention (step a), the temperature
of the drying oven is preferable set at a value of about 100 to 150 °C, preferably
100 to 120 °C. In the second stage (step b), the heating is stopped when the humidity
of the wood is below 15 %, e.g. 1 to 15 %. During step b and step c, if any, the
difference between the external temperature and the interior temperature of the
cellulosic product is kept at a value of 10 to 30 °C. Too small a temperature
difference prolongs the drying process, whereas too large a difference increases
the risk of internal cracking. During stage c the temperature of the oven is lowered
until the interior temperature of the wood has decreased below 100 °C.
During stages a, b, and c water steam is fed into the oven to keep
the wet temperature at about 80 to 120 °, preferably at about 100 °C. It is preferred
to use saturated water steam.
When the moisture content of the product has dropped to below 15
%, as a result of the drying, the treatment is continued at an elevated temperature.
During the second stage of the process the temperature is kept higher
than during the first stage of the process. It is preferred to operate the process
at about 180 to 250 °C in an atmosphere of saturated steam. The temperature can
also be raised during the second stage, as will appear from Example 2. The duration
and the temperature of the treatment are interdependent, as explained in connection
with Example 1. Typically, the heat treatment of the second stage takes at least
some 0.5 hours, preferably 1 to 20 hours and in particular about 2 to 10 hours.
The weight loss of the product can be adjusted by varying the heat treatment. This
makes it possible to change the strength and decay resistance properties of the
product as desired. Therefore, the heat treatment is continued until a weight loss
of at least 5% (based on dry matter) has been obtained. Clear improvements of
the dimensional stability of the product are reached at this value already. Mould
and decay resistance will also be improved, and further improvements of said properties
can be obtained by continuing the heating until at least about 6 or even 8%, weight
loss has taken place in the product.
Summarizing, the features obtained by the present invention are:
- Improvement of decay resistance (in comparison to wood which natively has a
good resistance to decay)
- Improvement of mould resistance
- Improvement of the dimensional stability
- Removal of pitch
- Heat conductivity decreased by 25 - 40 %
- Improvement of paint adherence
The heat treatment of the second stage is, according to a preferred
embodiment of the invention, carried out at least essentially under non-pressurized
conditions, i.e. at atmospheric pressure.
The method according to the invention is suited for treatment massive
wood goods, such as logs and pillars. In addition, the method can be applied to
veneer, chips, saw dust, wood fibres and other cellulosic products, such as, for
The wood preservation effect that can be produced is studied in more detail in
Example 2. However, in this connection it should be pointed out that good protection
against decay requires that dried sawn timber of pine is kept for preferably about
2 to 8 hours at a temperature of 200 to 250 °C. The same conditions are used for
birch and larch-tree, whereas good protection against decay can be obtained at
slightly lower temperatures for spruce. Thus, spruce can be treated, for instance,
at about 175 to 210 °C. The method is well-suited for treatment of aspen.
Example 3 explains in detail the decrease of heat conductivity as
a result of a treatment carried out according to the invention.
The invention provides considerable advantages. It will therefore
provide for a shortening of the time required for drying of wood. The colour changes
appearing during drying can be utilized and, at the same time, the resistance of
wood against decay and mould and the dimensional stability can be improved. Detrimental
pitch can be removed from samples of coniferous wood by the treatment. As examples
of products that can be treated with the method according to the present invention,
the following can be mentioned: external cladding, window frames, outdoor furniture,
and boards for sauna platforms.
After a treatment according to the present invention, the dimensional
instability under the influence of moisture is reduced by 50 to 70 %. The resistance
against decay of the products is improved. At its best, the resistance is on the
same level as that obtained by pressure impregnation or even better without any
substantial weakening of the strength properties of the products. The treated wood
forms a good surface for paint.
The preparation process is simple and quick (short treatment times)
and there is no need to use pressure. As far as its weathering resistance, resistance
to decay and mould, and strength properties are concerned, the product can be
modified in a controlled manner by the method. The method is suitable for all kinds
of wood. By means of the heat treatment it is become possible also to improve the
properties of the heartwood, which cannot be done by pressure impregnation. The
durability of those kinds of wood which are difficult to impregnate can be improved.
The improvement of the permeability of wood makes it possible to impregnate the
wood with other colouring agents.
In the following the invention will be examined in greater detail
with the help of the attached drawings and some working examples.
- Figure 1 is a simplified schematic representation of the construction of an
apparatus which can be used for carrying out the present invention,
- Figure 2 indicates the influence of the temperature and treatment time on the
weight loss of the product,
- Figure 3 indicates the reduction of tangential swelling of the wood sample
as a function of the weight loss,
- Figure 4 indicates the reduction of radial swelling of the wood sample as a
function of the weight loss,
- Figure 5 indicates the reduction of moisture taken up by the wood sample as
a function of the weight loss,
- Figure 6 indicates the changes of bending strength caused by the heat treatment,
- Figure 7 shows the moisture contents of bending test samples after conditioning
for 4 weeks,
- Figure 8 shows the weight losses of heat treated and control samples, respectively,
after decay testing,
- Figure 9 shows the drying of unseasoned spruce according to a preferred embodiment
according to the present invention,
- Figure 10 indicates the weight losses of veneer as a function of the duration
of the heat treatment,
- Figure 11 indicates the reduction of thickness swelling of plywood as a result
of a heat treatment, and
- Figure 12 indicates the reduction of the moisture content of plywood under
the influence of a heat treatment.
An apparatus shown, for instance, in Figure 1 is used in the present
invention. The apparatus comprises an oven 2 surrounded by an oven jacket 1. The
samples 3 are placed in the oven, which is provided with inlet 4 and outlet 5
channels for air 5, for conducting moist air through the oven. The outlet channel
5 is combined with a steam feed pipe 6 for feeding more water steam into the outlet
air coming from the oven. In order to form a closed cycle the inlet and outlet
channels are joined each to its end of a set of ducts 7 provided with a fan 9 and
with heating means 8. The air flowing through said ducts are heated by electric
resistances 8 to the set temperature and conducted via the fan 9 to the inlet
channel 4 of the oven. The recycling direction of the air in the apparatus is indicated
with an arrow.
By using the present apparatus it is possible to make sure that the
samples placed in the oven are heated to the desired temperature by moist air.
By changing the amount of steam, which is being fed, the moisture content of the
air can be altered. Usually, the air of the oven is saturated with water steam.
Heat treatment of wood
Moist wood is dried in the above-described apparatus at 120 to 140
°C either with steam or without it. As a result of the treatment, there is some
darkening of the colour of the wood sample, but no cracking. When the moisture
content of the wood is below 15 % the temperature is raised to at least 175 °C,
preferably to 180 to 250 °C. The treatment is continued for 2 to 10 hours. Saturated
steam is conducted to the apparatus. By varying the temperature and the time, the
desired result can be obtained. The colour of the wood darkens further.
Figure 2 shows the influence of temperature and time on the reduction
of wood weight.
By adjusting the weight losses the properties of the wood can be
changed as desired. Figures 3, 4 and 5 depict the reduction of tangential swelling
of the wood, the reduction of the radial swelling of the wood, and the reduction
of the amount water absorbed by the wood (wood moisture content) in comparison
to the control samples. The graphs of Figures 4 and 5 correspond to graph model
of Figure 1.
The heat treatment weakens the bending strength of wood after a certain
weight loss. On the other hand, the experiments show that the bending strength
properties of some of our samples were even better than the corresponding properties
of the control samples (Figure 6). This is due to the fact that, depending on
ambient humidity, some of the heat treated samples clearly adsorbed less water
than the control sample (Figure 7).
The decay test was carried out according to European Standard EN
113 modified as follows: the number of parallel test specimens was four, the sizes
of the test specimens were 5x20x35 mm, and they were not rinsed before the test.
The samples were subjected to the test rot fungus, cellar fungus (Coniophora
puteana), for 2, 4, 8, and 12 weeks.
The test specimens were sawn from planks of pine, birch, larch-tree
and spruce, treated according to example 1. Table 1 contains a summary of the conditions
prevailing during the heat treatment.
Treatment conditions of the test specimens of the decay test
Heat treatment (time/temp.)
1.5 h/200 °C + 1 h /220 °C
2 h/220 °C
4 h/210 °C
1 h/160 °C
4 h/210 °C
1 h/160 °C + 2 h/220 °C
1 h/160 °C + 4 h/220 °C
4 h/210 °C
4 h/210 °C
1 h/180 °C
After the heat treatment the dry matter of the wood specimens were
determined. The test specimens were sterilized by radiation (Co-60), the sterilized
test specimens were inserted in kolle dishes on a fungus culture growing on malt
agar medium. At least one heat treated test specimen and one untreated control
sample were inserted into each dish.
At the end of the decay test the specimens were dried at 103 °C and
the weight losses of the specimens were calculated according to EN 113. For pine
a weight loss of less than 10 % was achieved by the heat treatment; the weight
losses for untreated wood were over 30 %. The smallest weight losses for heat
treated birch, larch-tree and spruce were close to zero.
The results of the decay tests are indicated in Figure 8. It is apparent
from the figure that a mild heat treatment (160 °C) does not yet significantly
improve the decay resistance of the timber.
Drying of unseasoned spruce
When a specimen of wet spruce (50 x 100 x 1500 mm), initial moisture
content about 40 %, was heated according to the preferred drying embodiment of
the invention for 24 hours by operating the drying system in such a way that the
difference between the internal and external temperatures was 10 to 20 degrees,
no cracks were found in the test specimen (Figure 9). The final moisture of the
dry test specimen was below 5 %.
Reduction of heat conductivity
Table 2 presents the heat conductivities of heat treated samples
of spruce, pine and aspen. The table also indicates the conditions of the heat
Heat conductivity of the test specimens
Wood/treatment, temperature and time
Density at time of measuring, kg/m3
Heat conductivity λ10, W/(mK)
Aspen, 4 h 210 °C
Aspen, 10 h 210 °C
Spruce, fresh, 26 h, heat treatment: 3 h / 220 °C
Spruce, 8 h 230 °C
Pine, fresh, heat treatment: 3 h 220 °C
Pine, 30 h 230 °C
Birch veneer, thickness 1.5 mm, was heat treated in an oven of the
kind shown in Figure 1. The temperature of the treatment was 200 °C and the time
2 to 7 hours.
The test specimens were selected by dividing the veneer into two
parts and by choosing one half of the veneer as a control. The other half was heat
treated. A 3-ply plywood was prepared from the veneer. The gluing was made by FF
glue, which was applied to the surfaces of the veneer by a brush. The veneers
were pressed together at 130 °C for 6 minutes. The compression load was 1.7 MPa.
The control plywood and the plywood prepared from the heat treated veneers were
kept in the same pressing.
In order to determine the thickness swelling, the test specimens
were dried in an oven at 102 °C. Then they were immersed into 20 °C water for 2,
6, 26, and 168 hours. The test specimens were prepared for the strength testing
by conditioning them at a relative humidity of 65 %, whereinafter they were evaluated
for wood failure, tensile strength and bending strength. The tests included two
parallel test specimens.
The weight loss of the wood (calculated on basis of the dry matter)
caused by the heat treatment is indicated in Figure 10. As a result of the treatment
the weight of the wood decreased by 3.4 to 8.4 %.
The thickness swelling of the plywood is indicated in Figure 3.
Thickness swelling of plywood and moisture content of plywood after immersion
2 h heat
2 h control
3 h heat
3 h control
4 h heat
4 h control
5 h heat
5 h control
6 h heat
6 h control
7 h heat
7 h control
The thickness swelling of the control samples varied to a large extent.
For this reason, the swelling reduction results presented in Figure 11 have been
calculated in relation to the control samples of each test series. Figure 12 shows
the reduction of the amounts of water absorbed by the wood samples compared to
the untreated samples.
As far as thickness swelling is concerned the best results were obtained
by the treatment having the longest duration, i.e., by a 7 hour heat treatment.
After a 2 hour immersion the thickness swelling was then 80 % smaller than that
of the control samples. An almost equally good a result was reached by a 4 hour
treatment. 2 and 3 hour heat treatments reduced thickness swelling after a 2 hour
immersion to 50 or 70 %. After a 24 hour immersion the thickness swelling of plywood
which had been heat treated for 7 and 4 hours was 50 % smaller than that of the
The heat treatment reduces the amount of water absorbed by the wood
sample (= moisture content of wood). Subject to immersion into water for 24 hours,
the moisture content of plywood which had been heat treated for 7 hours was about
38 % smaller than that of the control plywood.
Table 4 indicates the strength properties of the plywood articles.
Strength properties of the plywood
Shear strength of glue line
Tensile strength N/mm2
Moisture content (%) during strength testing
Bending strength /mm2
Wood failure %
Resist, to shear N/mm2
2 h heat
2 h control
3 h heat
3 h control
4 h heat
4 h control
5 h heat
5 h control
6 h heat
6 h control
7 h heat
7 h control
Requirements for a 3-ply plywood:
- Shear strength of glue line, dry, strength = 2.1 N/mm2. If the strength
is less than that, the wood failure percentage should be more than or equal to
- Tensile strength 54 N/mm2
- Bending strength 72 N/mm2
The tensile strength of plywood prepared from heat treated veneer
was almost always less than the required 2.1, but because the wood failure % exceeded
50, it should be noted that the requirements regarding shearing strength were
The bending strength of the plywood prepared from heat treated veneer
was inferior to that of the control plywood, but even so it met the requirements.
The required bending strength was not reached with heat treated veneer which had
been heat treated for 5 or 6 hours.
Test specimens (50 x 25 x 500 mm) were heat treated for 4 hours at
220 °C. The samples were placed on test field in contact with the earth. After
a time of one year the test specimens were checked and evaluated.
The results were evaluated using the following scale: 1 = some beginning
decay (25 %), 2 = 50 %, 3 = 75 %, 4 = the test specimen breaks under a weight.
Average values of the results:
- Pine, control = 0.3. Heat treated pine = 0.
- Spruce control = 1. Heat treated spruce = 0.2.
- Birch, control = 3.6. Heat treated birch = 2.5.