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Effect of thermal treatment on physical and mechanical properties of birch and pine wood

Authors:
  • Institute of wood chemistry
78 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
EFFECT OF THERMAL TREATMENT ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
Andis Antons1, Dace Cirule1, Anrijs Verovkins1, Edgars Kuka1,2
1Lavian State Institute of Wood Chemistry, Latvia
2Riga Technical University, Latvia
antonsandis@tvnet.lv
Abstract
No simple method has yet been found for satisfactory wood bio-resistance improvement regarding material
performance in its end-use. An attempt to obtain material with proper strength and bio-durability by combined wood
thermal modication and impregnation with a biocide is being researched. To select the most appropriate treatment
conditions for the combined process, changes in wood physical and mechanical properties depending on the treatment
temperature were investigated in the present study. For the investigation, in Latvia the most widespread wood –
softwood pine (Pinus sylvestris L.) and hardwood birch (Betula spp.) was used. Changes of wood mechanical and
physical properties due to thermal modication were investigated and effect of treatment temperature and relative
humidity on wood characteristics evaluated. It was found that, due to different degree of changes, no identical
treatment conditions suit for birch and pine wood. Birch wood is considerably more sensitive to temperature and
acceptable strength was maintained only for birch wood treated at 150 °C and for pine wood treated at 160 °C.
Nevertheless at higher environmental humidity equilibrium moisture content and consequently radial and tangential
swelling increased for all studied wood types, substantially smaller changes due to elevated humidity were detected
for modied wood.
Key words: thermal treatment, birch, pine, hardness, bending strength, swelling, capillary water uptake.
Introduction
Over recent decades, different treatment methods
have been proposed for restriction of such wood
drawbacks as dimensional instability and low
resistance to biodegradation (Homan & Jorissen,
2004; Gerardin, 2016). Commercially the most
successful has been wood thermal modication and
different treatment processes have been introduced
in production on industrial scale (Hill, 2006).
One advantage of wood thermal modication in
comparison with other wood modication methods is
that no chemicals are used to alter wood properties
as the changes are caused by autocatalytic reactions
of wood chemical components during its exposure
to elevated temperature. This makes wood thermal
modication relatively environmentally friendly
(Sandberd, Haller, & Navi, 2013). Another advantage
is that the equipment is simple and comparatively low
capital expenditure is needed to launch manufacturing.
Therefore, it is foreseen that thermal modication of
wood will continue growing (Militz & Altgen, 2014).
A lot of different wood thermal modication
methods are known which vary with the treatment
atmosphere, temperature, and duration (Hofmann et
al., 2013; Militz & Altgen, 2014; Gerardin, 2016). They
all include exposure of wood to elevated temperature
(150 – 260 °C) in absence of oxygen which result
in complex reactions including certain destruction
of low-molecular substances and hemicelluloses,
reorganisation of lignin and cellulose and evaporation
of volatile compounds (Sandberd, Haller, & Navi,
2013). These wood chemical transformations result in
changed wood colour, improved dimensional stability
and enhanced bio-durability. However, alteration
of wood chemical composition and structure due
to thermal treatment causes reduction in wood
mechanical strength (Boonstra et al., 2007; Arnold,
2010; Welzbacher et al., 2011; Widmann, Fernandez-
Cabo, & Steiger, 2012). Therefore, when selecting the
modication method and processing parameters, it
is important to nd a trade-off between benets and
losses taking into consideration the requirements for
the material end-use.
Despite numerous investigations, no simple
method has been found yet for fully satisfactory result
regarding material potential performance in its end-
use. Thus, different combined treatment processes,
including pre- or post-treatment of thermally modied
wood, are now intensively investigated and the results
are being reported (Ahmed, Hansson, & Moren, 2013;
Ferrari et al., 2013; Wang, Zhu, & Cao, 2013). The
present research is a part of the investigation aimed
at improving wood bio-durability by combined
wood thermal treatment and impregnation with
a commercial biocide. It is found that substantial
enhancement of bio-durability can only be reached
by wood thermal treatment at the temperature range
at which wood mechanical strength signicantly
decreases consequently restricting the application area
of the material (Kamdem, Pizzi, & Jermannaud, 2002;
Metsä-Korteleinen & Viitanen, 2010; Candelier et al.,
2017). It is expected that by applying the proposed
combined treatment process a material with decent
mechanical strength and meeting the requirements of
the use class 3 according to the EN 335-1 standard
will be obtained.
FORESTRY AND WOOD PROCESSING DOI: 10.22616/rrd.24.2018.012
79RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
The objective of the present study was to evaluate
changes in wood physical and mechanical properties
due to thermal modication depending on the treatment
temperature. It will let select the treatment conditions
for the combined process. For the investigation, in
Latvia the most widespread wood - softwood pine
(Pinus sylvestris L.) and hardwood birch (Betula spp.)
was used.
Materials and Methods
For the thermal modication, kiln-dried boards
of birch (Betula spp.) and pine (Pinus sylvestris L.)
wood measuring 700 × 100 × 25 mm were used. The
modication was carried out in a multifunctional
wood modication device of WTT (Denmark)
production. The boards were thermally modied
in a water vaper medium under elevated pressure
(0.6 – 0.8 MPa depending on the temperature) for
1 h at the peak temperature. Each of the wood species
was treated at three peak temperatures: birch wood at
150, 160 and 170 °C and pine wood at 160, 170 and
180 °C. The boards were weighed before and after
modication and mass losses calculated. The modied
boards were conditioned (RH 65 ± 5%; 20 ± 2 °C)
for at least two weeks before preparing specimens for
testing mechanical and physical properties.
Determination of wood equilibrium moisture
content (EMC) and wood radial and tangential
swelling was performed with specimens measuring
20 x 20 x 10 mm (r x t x l) and having the annual ring
orientation strictly parallel to the edge. Ten replicates
were used per each modication temperature and the
untreated wood. Before starting the test, specimens
were oven-dried (102 ± 2 °C) and their mass (with
accuracy of 0.0002 g) and dimensions (with accuracy
of 0.02 mm) were determined. Further the specimens
were conditioned until reaching constant weight at
20 ± 2 °C temperature and xed relative humidity
(RH) conditions in increasing sequence:
45%, 65%, 85%. The specimen equilibrium mass
and dimensions were recorded for each tested RH and
calculations according to DIN 520184 were performed
to establish wood swelling characteristics. The
moisture exclusion efciency (MEE) was calculated
as given below (Eq. 1).
DOI: 10.22616/rrd.24.2018.012
combined process. For the investigation, in Latvia the most widespread wood - softwood pine (Pinus sylvestris L.)
and hardwood birch (Betula spp.) was used.
Materials and Methods
For the thermal modification, kiln-dried boards of birch (Betula spp.) and pine (Pinus sylvestris L.) wood
measuring 700 × 100 × 25 mm were used. The modification was carried out in a multifunctional wood modification
device of WTT (Denmark) production. The boards were thermally modified in a water vaper medium under
elevated pressure (0.6 0.8 MPa depending on the temperature) for 1 h at the peak temperature. Each of the wood
species was treated at three peak temperatures: birch wood at 150, 160 and 170 °C and pine wood at 160, 170 and
180 °C. The boards were weighed before and after modification and mass losses calculated. The modified boards
were conditioned (RH 65 ± 5%; 20 ± 2 °C) for at least two weeks before preparing specimens for testing
mechanical and physical properties.
Determination of wood equilibrium moisture content (EMC) and wood radial and tangential swelling was
performed with specimens measuring 20 x 20 x 10 mm (r x t x l) and having the annual ring orientation strictly
parallel to the edge. Ten replicates were used per each modification temperature and the untreated wood. Before
starting the test, specimens were oven-dried (102 ± 2 °C) and their mass (with accuracy of 0.0002 g) and
dimensions (with accuracy of 0.02 mm) were determined. Further the specimens were conditioned until reaching
constant weight at 20 ± 2 °C temperature and fixed relative humidity (RH) conditions in increasing sequence:
45%, 65%, 85%. The specimen equilibrium mass and dimensions were recorded for each tested RH and
calculations according to DIN 520184 were performed to establish wood swelling characteristics. The moisture
exclusion efficiency (MEE) was calculated as given below (Eq. 1).
MEE =

×100, %
(1)
Capillary water uptake (CWU) through radial and tangential surfaces was tested using cubic specimens (20 x 20
x 20 mm) with annual ring and grain orientations strictly parallel to the edges. All sides, except two opposite faces
(radial or tangential), one of which was intended for CWU evaluation, were sealed with waterproof coating. After
conditioning (RH 65 ± 5%; 20 ± 2 °C) until constant weight, the specimens were installed into a frame that
restricted water evaporation from the container and fixed the specimens in a position in which the contact surface
was 2 ± 0.2 mm under the water. The container was filled with distilled water the level of which was monitored
and adjusted every day. After 10 days, the specimens were removed from the water, excess water wiped off with
a paper towel and the specimen weight recorded with an accuracy of 0.0002 g. The experiment was carried out in
a room with controlled RH (65 ± 2%) at 20 ± 2 °C. Ten replicates were used per each wood type.
Wood bending strength was determined according to DIN 52186 in a three-point bending test using a material
strength testing device ZWICK Z100. Before the test was carried out, the specimens measuring 360 x 20 x 20 mm
with the fibres parallel to the sample longitudinal axis were conditioned at two different RH (65% and 85%) till
constant mass. At least 30 specimens obtained from different boards were tested for each modification and RH.
The loading speed was adjusted for each of wood type to reach the destruction maximum within 90 ± 10 sec.
Wood surface hardness was determined according to the Brinell test procedure and meeting the requirements of
the EN 1534 standard. The hardness for radial and tangential directions of specimens conditioned till constant
mass at two RH (65% and 85%) were evaluated. A force of 1 kN was applied with reaching it within 15 sec and
maintaining it for 25 sec by using a universal test device ZWICK Z100 and a metal ball of diameter 8 mm as an
intender. Ten specimens were examined for each wood type and eight measurements were performed on radial
and tangential surfaces of each specimen.
Results and Discussion
Density of wood before and after THT as well as mass loss due to thermal treatment, are presented in Table1.
Table 1
Oven-dry wood density and mass losses due to thermal treatment (THT)
Wood
Birch
Pine
Treatment
unmodified
THT 150
THT 160
THT 170
unmodified
THT 170
THT 180
ρ, g cm-3
0.618
0.593
0.578
0.577
0.481
0.444
0.467
STDEV
0.046
0.045
0.07
0.072
0.053
0.055
0.049
Mass loss, %
-
0.1
6.6
15.7
-
6.8
10.6
The wood mass loss increases with increasing THT temperature for both species at the temperature range used in
the study. However, birch wood is considerably more sensitive to temperature compared to pine as equal mass
(1)
Capillary water uptake (CWU) through radial and
tangential surfaces was tested using cubic specimens
(20 x 20 x 20 mm) with annual ring and grain
orientations strictly parallel to the edges. All sides,
except two opposite faces (radial or tangential), one
of which was intended for CWU evaluation, were
sealed with waterproof coating. After conditioning
(RH 65 ± 5%; 20 ± 2 °C) until constant weight, the
specimens were installed into a frame that restricted
water evaporation from the container and xed the
specimens in a position in which the contact surface
was 2 ± 0.2 mm under the water. The container was
lled with distilled water the level of which was
monitored and adjusted every day. After 10 days,
the specimens were removed from the water, excess
water wiped off with a paper towel and the specimen
weight recorded with an accuracy of 0.0002 g. The
experiment was carried out in a room with controlled
RH (65 ± 2%) at 20 ± 2 °C. Ten replicates were used
per each wood type.
Wood bending strength was determined according
to DIN 52186 in a three-point bending test using a
material strength testing device ZWICK Z100. Before
the test was carried out, the specimens measuring
360 x 20 x 20 mm with the bres parallel to the sample
longitudinal axis were conditioned at two different
RH (65% and 85%) till constant mass. At least
30 specimens obtained from different boards were
tested for each modication and RH. The loading
speed was adjusted for each of wood type to reach the
destruction maximum within 90 ± 10 sec.
Wood surface hardness was determined according
to the Brinell test procedure and meeting the
requirements of the EN 1534 standard. The hardness
for radial and tangential directions of specimens
conditioned till constant mass at two RH (65% and
85%) were evaluated. A force of 1 kN was applied
with reaching it within 15 sec and maintaining it for
25 sec by using a universal test device ZWICK Z100
and a metal ball of diameter 8 mm as an intender. Ten
specimens were examined for each wood type and
eight measurements were performed on radial and
tangential surfaces of each specimen.
Results and Discussion
Density of wood before and after THT as well as
mass loss due to thermal treatment, are presented in
Table1.
The wood mass loss increases with increasing
THT temperature for both species at the temperature
range used in the study. However, birch wood is
considerably more sensitive to temperature compared
to pine as equal mass losses are detected for birch
at 160 °C and pine at 170 °C. Other authors have
also reported similar ndings (Rowell et al., 2009;
Chaouch et al., 2010). It is explained by higher
content of hemicelluloses, lower content of lignin and
high content of syringyl groups in hardwood. Unlike
the mass loss, the density dependence on temperature
is signicantly less pronounced and, compared to the
initial material, the decrease for both woods varies in
the range of 7 – 8%.
Examination of wood mechanical properties
showed that both bending strength and hardness
Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
80 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
decrease with increasing treatment temperature
(Table 2 and 3). For samples conditioned at RH of
65% (20 °C), it was found that untreated pine wood
is relatively soft with its Brinell hardness being
approximately 16 – 18 N mm-2. Treating of pine at the
THT temperature range 160 – 180 °C, results in wood
hardness decrease by 18 – 23% in the radial direction
and by 19 – 23% in the tangential direction. However,
increase of the THT temperature by 20 °C, causes
hardness decrease by only 4 – 5%, whereas hardness
reduction by 16 – 33% in the radial direction and by
27 – 48% in the tangential direction was detected for
birch after THT treatment. Moreover, birch wood is
affected to a greater extent by THT with hardness
decrease almost twice due to rising the treatment
temperature by 20 °C (from 150 °C to 170 °C). The
birch wood surface hardness loss is signicant, and
it is an important indicator, especially for the articles
subjected to horizontal loads (e.g., terraces).
Bending strength is more affected by thermal
modication than hardness. Already at lower treatment
temperatures, the bending strength for birch and pine
decreases by 33% and 34%, respectively (Tables 3
and 4). The decrease in bending strength is close to
linear within a modication temperature range of
160 – 180 °C for pine. For birch, the largest decrease
in bending strength is caused by the increase in
temperature from 150 °C to 160 °C. At the maximum
treatment temperature of 170 °C, the bending strength
for birch wood decreases by 56%. Bending strength
for pine wood at this temperature decreases by 51%
and by 60%, if the treatment temperature is increased
up to 180 °C.
It is well known that the RH of the environment
substantially affects wood properties (Simpson
& TenWolde, 1999). For both unmodied and
thermally modied samples conditioned at higher
RH, the mechanical strength is signicantly reduced
comparing with samples conditioned at lower RH
(Tables 2 and 3). At RH 85% for untreated pine,
bending strength decreases by 25%, but for birch –
by 32%, compared to similar parameters at RH
65%. However, for thermally modied wood, the
decrease in bending strength at elevated humidity is
Table 1
Oven-dry wood density and mass losses due to thermal treatment (THT)
Wood Birch Pine
Treatment unmodied THT 150 THT 160 THT 170 unmodied THT 160 THT 170 THT 180
ρ, g cm-3 0.618 0.593 0.578 0.577 0.481 0.471 0.444 0.467
STDEV 0.046 0.045 0.07 0.072 0.053 0.043 0.055 0.049
Mass loss, % - 0.1 6.6 15.7 - 1.2 6.8 10.6
Table 2
Brinell hardness, HBS (N mm-2), for unmodied and modied (THT) birch and pine
Birch RH 65% RH 85% Pine RH 65%
Rad direct. Tg direct. Rad direct. Tg direct. Rad direct. Tg direct.
unmodied 27.5 (3.1) 24.0 (2.7) 19.7 (2.0) 18.4 (1.7) unmodied 16.8 (2.8) 18.3 (3.2)
THT150 22.3 (3.4) 17.7 (3.0) 17.9 (2.7) 15.4 (1.5) THT160 14.4 (3.2) 13.8 (2.5)
THT160 18.7 (3.2) 15.7 (3.2) 13.4 (1.6) 13.6 (1.5) THT170 13.4 (2.0) 13.8 (2.9)
THT170 18.3 (2.3) 14.2 (1.9) 13.2 (1.4) 11.5 (1.3) THT180 13.1 (2.2) 13.4 (1.9)
(standard deviation in parentheses).
Table 3
Bending strength (MPa) for unmodied and modied (THT) birch and pine
Birch RH 65% RH 85% Pine RH 65% RH 85%
unmodied 116.8 (16.4) 79.2 (10.4) unmodied 93.4 (13.9) 69.6 (8.9)
THT 150 78.7 (18.9) 60.2 (14.1) THT 160 61.5 (15.9) 47.8 (11.7)
THT 160 57.9 (15.5) 41.2 (8.6) THT 170 46.2 (18.3) 40.3 (18.1)
THT 170 59.2 (17.2) 40.7 (13.2) THT 180 37.8 (19.6) 33.2 (12.7)
(standard deviation in parentheses).
Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
81RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
signicantly smaller. Similar trend has been reported
by Arnold (2010). However, different results were
obtained for thermally modied pine and birch wood
regarding the effect of treatment temperature on
strength reduction due to elevated RH. The reduction
in bending strength of pine caused by rising RH
decreased with increase in THT temperature. For the
THT treated birch wood the decrease does not depend
on the modication temperature. Similar effect of
elevated RH was observed also with respect to the
surface hardness of both tree species. The surface
hardness of unmodied pine decreases by 29% in
the radial direction and by 34% in the tangential
direction. The surface hardness of THT pine wood
also decreased but to a level that measuring failed and
therefore no data on pine at RH 85% are presented
in the table. The unmodied birch surface hardness
decreases by 34% in the radial direction and by 23%
in the tangential direction, compared to the samples
conditioned at RH 65%. For the birch modied at
150 °C, the conditioning at RH 85% reduces the
hardness in the radial and tangential direction by 24%.
However, elevation of RH almost does not change
the hardness for THT treated birch wood at higher
(160 – 170 °C) temperatures. The differences between
both species regarding the inuence of humidity on
hardness in different directions may be explained by
the signicantly different anatomical structure.
The changes in the wood component composition
as a result of the thermal action have signicant
inuence on the wood/moisture/water interaction. As
a result of the degradation and/or mutual interactions
in wood, with decreasing hydrophilic components, the
equilibrium moisture, linear swelling and capillary
water uptake decrease. The changes in wood EMC
at different RH and depending on the modication
temperature are presented in Fig. 1 and 2.
For both unmodied and THT wood, the EMC
increases with rising RH, but for the modied one,
EMC is signicantly lower and decreases with
increasing treatment temperature.
MEE is an important characteristic of modied
wood that shows how much the equilibrium moisture
decreases by modifying wood (Van Acker et al.,
2015). This characteristic implies on improvement.
In accordance with the normative documents, MEE at
RH 85% must be > 40%. Figures 3. and 4. give MEE
values for THT birch and pine.
In our case, this requirement is ensured for birch,
modifying at 160 °C, whereas for pine – at only 180 °C.
DOI: 10.22616/rrd.24.2018.012
RH 85% are presented in the table. The unmodified birch surface hardness decreases by 34% in the radial direction
and by 23% in the tangential direction, compared to the samples conditioned at RH 65%. For the birch modified
at 150 °C, the conditioning at RH 85% reduces the hardness in the radial and tangential direction by 24%. However,
elevation of RH almost does not change the hardness for THT treated birch wood at higher (160 170 °C)
temperatures. The differences between both species regarding the influence of humidity on hardness in different
directions may be explained by the significantly different anatomical structure.
The changes in the wood component composition as a result of the thermal action have significant influence on
the wood/moisture/water interaction. As a result of the degradation and/or mutual interactions in wood, with
decreasing hydrophilic components, the equilibrium moisture, linear swelling and capillary water uptake decrease.
The changes in wood EMC at different RH and depending on the modification temperature are presented in Figs.
1 and 2.
Figure 1. Birch equilibrium moisture (EMC) % with standard deviation, depending on the air RH and THT
temperature.
Figure 2. Pine equilibrium moisture (EMC) % with standard deviation, depending on the air RH and THT
temperature.
For both unmodified and THT wood, the EMC increases with rising RH, but for the modified one, EMC is
significantly lower and decreases with increasing treatment temperature.
MEE is an important characteristic of modified wood that shows how much the equilibrium moisture decreases by
modifying wood (Van Acker et al., 2015). This characteristic implies on improvement. In accordance with the
normative documents, MEE at RH 85% must be > 40%. Figures 3. and 4. give MEE values for THT birch and
pine.
2
4
6
8
10
12
14
16
18
RH 45 % RH 65 % RH 85 %
EMC, %
unmodified THT 150 THT 160 THT 170
2
4
6
8
10
12
14
16
18
RH 45 % RH 65 % RH 85 %
EMC, %
unmodified THT 160 THT 170 THT 180
Figure 1. Birch equilibrium moisture (EMC) % with standard deviation,
depending on the air RH and THT temperature.
DOI: 10.22616/rrd.24.2018.012
RH 85% are presented in the table. The unmodified birch surface hardness decreases by 34% in the radial direction
and by 23% in the tangential direction, compared to the samples conditioned at RH 65%. For the birch modified
at 150 °C, the conditioning at RH 85% reduces the hardness in the radial and tangential direction by 24%. However,
elevation of RH almost does not change the hardness for THT treated birch wood at higher (160 170 °C)
temperatures. The differences between both species regarding the influence of humidity on hardness in different
directions may be explained by the significantly different anatomical structure.
The changes in the wood component composition as a result of the thermal action have significant influence on
the wood/moisture/water interaction. As a result of the degradation and/or mutual interactions in wood, with
decreasing hydrophilic components, the equilibrium moisture, linear swelling and capillary water uptake decrease.
The changes in wood EMC at different RH and depending on the modification temperature are presented in Figs.
1 and 2.
Figure 1. Birch equilibrium moisture (EMC) % with standard deviation, depending on the air RH and THT
temperature.
Figure 2. Pine equilibrium moisture (EMC) % with standard deviation, depending on the air RH and THT
temperature.
For both unmodified and THT wood, the EMC increases with rising RH, but for the modified one, EMC is
significantly lower and decreases with increasing treatment temperature.
MEE is an important characteristic of modified wood that shows how much the equilibrium moisture decreases by
modifying wood (Van Acker et al., 2015). This characteristic implies on improvement. In accordance with the
normative documents, MEE at RH 85% must be > 40%. Figures 3. and 4. give MEE values for THT birch and
pine.
2
4
6
8
10
12
14
16
18
RH 45 % RH 65 % RH 85 %
EMC, %
unmodified THT 150 THT 160 THT 170
2
4
6
8
10
12
14
16
18
RH 45 % RH 65 % RH 85 %
EMC, %
unmodified THT 160 THT 170 THT 180
Figure 2. Pine equilibrium moisture (EMC) % with standard deviation,
depending on the air RH and THT temperature.
Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
82 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
The effect of thermal modication on the linear
swelling of wood is shown in Table 4. As it can be
seen, both the pine and birch wood swell less after
thermal treatment and the swelling at similar RH
decreases with increase in THT temperature. However,
the results, obtained by soaking the wood in water up
to maximum linear swelling was reached, show that
there is no effect of the modication temperature
on the wood dimensional stability above wood bre
saturation point. It agrees with the ndings that
changes in some other wood properties are dependent
on the modication temperature only up to a certain
DOI: 10.22616/rrd.24.2018.012
Figure 3. Moisture exclusion efficiency (MEE) with standard deviation for modified birch depending on the air
RH.
Figure 4. Moisture exclusion efficiency (MEE) with standard deviation for modified pine depending on the air
RH.
In our case, this requirement is ensured for birch, modifying at 160 °C, whereas for pine at only 180 °C.
The effect of thermal modification on the linear swelling of wood is shown in Table 4. As it can be seen, both the
pine and birch wood swell less after thermal treatment and the swelling at similar RH decreases with increase in
THT temperature. However, the results, obtained by soaking the wood in water up to maximum linear swelling
was reached, show that there is no effect of the modification temperature on the wood dimensional stability above
wood fibre saturation point. It agrees with the findings that changes in some other wood properties are dependent
on the modification temperature only up to a certain temperature range (Welzbacher, Brischke, & Rapp, 2007).
Table 4
Birch and pine swelling in the radial (Rad) and tangential (Tg) direction depending on the humidity (RH)
Modification
temperature
Rad direction
Tg direction
RH 45%
RH 65%
RH 85%
(max)
RH 45%
RH 65%
RH 85%
(max)
Birch
untreated
1.5 (0.1)
2.4 (0.3)
4.0 (0.5)
5.0 (0.7)
1.7 (0.2)
2.8 (0.3)
4.7 (0.5)
8.2 (0.7)
THT 150
0.9 (0.2)
1.3 (0.2)
2.0 (0.4)
2.5 (0.3)
1.2 (0.2)
1.8 (0.3)
3.0 (0.5)
5.2 (0.6)
THT 160
0.8 (0.1)
1.1 (0.2)
2.0 (0.4)
3.1 (0.9)
1.0 (0.1)
1.5 (0.2)
2.9 (0.4)
6.9 (1.3)
THT 170
0.7 (0.2)
1.1 (0.2)
1.8 (0.4)
3.6 (0.8)
0.9 (0.1)
1.4 (0.2)
2.4 (0.3)
5.0 (0.9)
Pine
untreated
1.4 (0.3)
2.0 (0.3)
3.2 (0.5)
5.2 (0.9)
2.3 (0.2)
3.4 (0.3)
5.4 (0.5)
9.9 (0.9)
THT 160
0.7 (0.1)
1.1 (0.1)
2.0 (0.2)
3.4 (0.4)
1.4 (0.2)
2.2 (0.2)
3.9 (0.3)
6.9 (0.5)
THT 170
0.7 (0.1)
1.1 (0.2)
1.9 (0.5)
3.0 (0.8)
1.1 (0.2)
1.8 (0.3)
3.4 (0.7)
5.5 (1.4)
THT 180
0.6 (0.1)
1.0 (0.3)
1.8 (0.4)
3.4 (0.8)
0.9 (0.1)
1.8 (0.3)
3.2 (0.6)
6.1 (0.9)
(standard deviation in parentheses).
0
10
20
30
40
50
60
RH 45 % RH 65 % RH 85 %
MEE, %
THT 150 THT 160 THT 170
0
10
20
30
40
50
60
RH 45 % RH 65 % RH 85 %
MEE, %
THT 160 THT 170 THT 180
Figure 3. Moisture exclusion efciency (MEE) with standard deviation
for modied birch depending on the air RH.
DOI: 10.22616/rrd.24.2018.012
Figure 3. Moisture exclusion efficiency (MEE) with standard deviation for modified birch depending on the air
RH.
Figure 4. Moisture exclusion efficiency (MEE) with standard deviation for modified pine depending on the air
RH.
In our case, this requirement is ensured for birch, modifying at 160 °C, whereas for pine at only 180 °C.
The effect of thermal modification on the linear swelling of wood is shown in Table 4. As it can be seen, both the
pine and birch wood swell less after thermal treatment and the swelling at similar RH decreases with increase in
THT temperature. However, the results, obtained by soaking the wood in water up to maximum linear swelling
was reached, show that there is no effect of the modification temperature on the wood dimensional stability above
wood fibre saturation point. It agrees with the findings that changes in some other wood properties are dependent
on the modification temperature only up to a certain temperature range (Welzbacher, Brischke, & Rapp, 2007).
Table 4
Birch and pine swelling in the radial (Rad) and tangential (Tg) direction depending on the humidity (RH)
Modification
temperature
Rad direction
Tg direction
RH 45%
RH 65%
RH 85%
(max)
RH 45%
RH 65%
RH 85%
(max)
Birch
untreated
1.5 (0.1)
2.4 (0.3)
4.0 (0.5)
5.0 (0.7)
1.7 (0.2)
2.8 (0.3)
4.7 (0.5)
8.2 (0.7)
THT 150
0.9 (0.2)
1.3 (0.2)
2.0 (0.4)
2.5 (0.3)
1.2 (0.2)
1.8 (0.3)
3.0 (0.5)
5.2 (0.6)
THT 160
0.8 (0.1)
1.1 (0.2)
2.0 (0.4)
3.1 (0.9)
1.0 (0.1)
1.5 (0.2)
2.9 (0.4)
6.9 (1.3)
THT 170
0.7 (0.2)
1.1 (0.2)
1.8 (0.4)
3.6 (0.8)
0.9 (0.1)
1.4 (0.2)
2.4 (0.3)
5.0 (0.9)
Pine
untreated
1.4 (0.3)
2.0 (0.3)
3.2 (0.5)
5.2 (0.9)
2.3 (0.2)
3.4 (0.3)
5.4 (0.5)
9.9 (0.9)
THT 160
0.7 (0.1)
1.1 (0.1)
2.0 (0.2)
3.4 (0.4)
1.4 (0.2)
2.2 (0.2)
3.9 (0.3)
6.9 (0.5)
THT 170
0.7 (0.1)
1.1 (0.2)
1.9 (0.5)
3.0 (0.8)
1.1 (0.2)
1.8 (0.3)
3.4 (0.7)
5.5 (1.4)
THT 180
0.6 (0.1)
1.0 (0.3)
1.8 (0.4)
3.4 (0.8)
0.9 (0.1)
1.8 (0.3)
3.2 (0.6)
6.1 (0.9)
(standard deviation in parentheses).
0
10
20
30
40
50
60
RH 45 % RH 65 % RH 85 %
MEE, %
THT 150 THT 160 THT 170
0
10
20
30
40
50
60
RH 45 % RH 65 % RH 85 %
MEE, %
THT 160 THT 170 THT 180
Figure 4. Moisture exclusion efciency (MEE) with standard deviation
for modied pine depending on the air RH.
Table 4
Birch and pine swelling in the radial (Rad) and tangential (Tg) direction
depending on the humidity (RH)
Modication
temperature
Rad direction Tg direction
RH 45% RH 65% RH 85% (max) RH 45% RH 65% RH 85% (max)
Birch
untreated 1.5 (0.1) 2.4 (0.3) 4.0 (0.5) 5.0 (0.7) 1.7 (0.2) 2.8 (0.3) 4.7 (0.5) 8.2 (0.7)
THT 150 0.9 (0.2) 1.3 (0.2) 2.0 (0.4) 2.5 (0.3) 1.2 (0.2) 1.8 (0.3) 3.0 (0.5) 5.2 (0.6)
THT 160 0.8 (0.1) 1.1 (0.2) 2.0 (0.4) 3.1 (0.9) 1.0 (0.1) 1.5 (0.2) 2.9 (0.4) 6.9 (1.3)
THT 170 0.7 (0.2) 1.1 (0.2) 1.8 (0.4) 3.6 (0.8) 0.9 (0.1) 1.4 (0.2) 2.4 (0.3) 5.0 (0.9)
Pine
untreated 1.4 (0.3) 2.0 (0.3) 3.2 (0.5) 5.2 (0.9) 2.3 (0.2) 3.4 (0.3) 5.4 (0.5) 9.9 (0.9)
THT 160 0.7 (0.1) 1.1 (0.1) 2.0 (0.2) 3.4 (0.4) 1.4 (0.2) 2.2 (0.2) 3.9 (0.3) 6.9 (0.5)
THT 170 0.7 (0.1) 1.1 (0.2) 1.9 (0.5) 3.0 (0.8) 1.1 (0.2) 1.8 (0.3) 3.4 (0.7) 5.5 (1.4)
THT 180 0.6 (0.1) 1.0 (0.3) 1.8 (0.4) 3.4 (0.8) 0.9 (0.1) 1.8 (0.3) 3.2 (0.6) 6.1 (0.9)
(standard deviation in parentheses).
Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
83RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
temperature range (Welzbacher, Brischke, & Rapp,
2007).
The amount of water absorbed by birch and pine
samples during 10 days of the CWU experiment is
shown in Figs. 5 and 6. The unmodied pine absorbs
more water than birch. However, the amount absorbed
through the tangential surface is considerably larger
for both unmodied birch (1.6-fold) and unmodied
pine (1.9-fold). For birch, as a result of modication,
the absorbed water quantities decrease through both
radial and tangential surfaces. Moreover, for the
modied birch, the differences between the amounts
absorbed through the two surfaces are insignicant.
However, the HTM treatment of pine does not reduce
the CWU through the radial surface, and increases the
CWU through the tangential surface. Similar effect of
CWU increase has been observed also by Johansson,
Sehlstedt-Persson & Moren (2006). The explanation
of the differences between the THT birch and THT
pine CWU could be the differences in structural
changes during thermal treatment, but it needs
further research. However, these results suggest that
impregnation regimes should be adjusted for each of
THT treated species.
Conclusions
1. The results showed that, due to the signicant
strength losses during wood thermal treatment,
only modication at 150 °C for birch wood and
at 160 °C for pine wood is admissible to obtain
intended material by the combined wood treatment.
2. The higher treatment temperatures resulted in
greater improvement of wood hydrofobicidy;
however, substantial decrease in wood swelling
and equilibrium moisture content is obtained also
at lower temperatures at which hardly any losses
of wood mass were detected.
Acknowledgements
The authors gratefully acknowledge the nancial
support by the European Regional Development Fund
project No. 1.1.1.1/16/A/133.
DOI: 10.22616/rrd.24.2018.012
The amount of water absorbed by birch and pine samples during 10 days of the CWU experiment is shown in Figs.
5 and 6. The unmodified pine absorbs more water than birch. However, the amount absorbed through the tangential
surface is considerably larger for both unmodified birch (1.6-fold) and unmodified pine (1.9-fold). For birch, as a
result of modification, the absorbed water quantities decrease through both radial and tangential surfaces.
Moreover, for the modified birch, the differences between the amounts absorbed through the two surfaces are
insignificant. However, the HTM treatment of pine does not reduce the CWU through the radial surface, and
increases the CWU through the tangential surface. Similar effect of CWU increase has been observed also by
Johansson, Sehlstedt-Persson & Moren (2006). The explanation of the differences between the THT birch and
THT pine CWU could be the differences in structural changes during thermal treatment, but it needs further
research. However, these results suggest that impregnation regimes should be adjusted for each of THT treated
species.
Figure 5. Capillary water uptake (CWU) with standard deviation of birch wood in the radial (Rad) and tangential
(Tg) direction.
Figure 6. Capillary water uptake (CWU) with standard deviation of pine wood in the radial (Rad) and tangential
(Tg) direction.
Conclusions
1. The results showed that, due to the significant strength losses during wood thermal treatment, only
modification at 150 °C for birch wood and at 160 °C for pine wood is admissible to obtain intended material
by the combined wood treatment.
2. The higher treatment temperatures resulted in greater improvement of wood hydrofobicidy; however,
substantial decrease in wood swelling and equilibrium moisture content is obtained also at lower temperatures
at which hardly any losses of wood mass were detected.
Acknowledgements
The authors gratefully acknowledge the financial support by the European Regional Development Fund project
No. 1.1.1.1/16/A/133.
References
0
500
1000
1500
2000
2500
3000
3500
4000
unmodified THT 150 THT 160 THT 170
CWU, g m-2
Rad direction Tg direction
0
1000
2000
3000
4000
5000
6000
7000
8000
unmodified THT 150 THT 160 THT 170
CWU, g m-2
Rad direction Tg direction
Figure 5. Capillary water uptake (CWU) with standard deviation of birch wood
in the radial (Rad) and tangential (Tg) direction.
DOI: 10.22616/rrd.24.2018.012
The amount of water absorbed by birch and pine samples during 10 days of the CWU experiment is shown in Figs.
5 and 6. The unmodified pine absorbs more water than birch. However, the amount absorbed through the tangential
surface is considerably larger for both unmodified birch (1.6-fold) and unmodified pine (1.9-fold). For birch, as a
result of modification, the absorbed water quantities decrease through both radial and tangential surfaces.
Moreover, for the modified birch, the differences between the amounts absorbed through the two surfaces are
insignificant. However, the HTM treatment of pine does not reduce the CWU through the radial surface, and
increases the CWU through the tangential surface. Similar effect of CWU increase has been observed also by
Johansson, Sehlstedt-Persson & Moren (2006). The explanation of the differences between the THT birch and
THT pine CWU could be the differences in structural changes during thermal treatment, but it needs further
research. However, these results suggest that impregnation regimes should be adjusted for each of THT treated
species.
Figure 5. Capillary water uptake (CWU) with standard deviation of birch wood in the radial (Rad) and tangential
(Tg) direction.
Figure 6. Capillary water uptake (CWU) with standard deviation of pine wood in the radial (Rad) and tangential
(Tg) direction.
Conclusions
1. The results showed that, due to the significant strength losses during wood thermal treatment, only
modification at 150 °C for birch wood and at 160 °C for pine wood is admissible to obtain intended material
by the combined wood treatment.
2. The higher treatment temperatures resulted in greater improvement of wood hydrofobicidy; however,
substantial decrease in wood swelling and equilibrium moisture content is obtained also at lower temperatures
at which hardly any losses of wood mass were detected.
Acknowledgements
The authors gratefully acknowledge the financial support by the European Regional Development Fund project
No. 1.1.1.1/16/A/133.
References
0
500
1000
1500
2000
2500
3000
3500
4000
unmodified THT 150 THT 160 THT 170
CWU, g m-2
Rad direction Tg direction
0
1000
2000
3000
4000
5000
6000
7000
8000
unmodified THT 150 THT 160 THT 170
CWU, g m-2
Rad direction Tg direction
Figure 6. Capillary water uptake (CWU) with standard deviation of pine wood
in the radial (Rad) and tangential (Tg) direction.
Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
84 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
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EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
85RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
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Andis Antons, Dace Cirule, Anrijs Verovkins, Edgars Kuka
EFFECT OF THERMAL TREATMENT
ON PHYSICAL AND MECHANICAL
PROPERTIES OF BIRCH AND PINE WOOD
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