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ORIGINAL ARTICLE
Evaluation of preservative distribution in thermally modified
European aspen and birch boards using computed tomography
and scanning electron microscopy
Sheikh Ali Ahmed •Margot Sehlstedt-Persson •
Lars Hansson •Tom More
´n
Received: 1 August 2012 / Accepted: 19 September 2012 / Published online: 21 October 2012
ÓThe Japan Wood Research Society 2012
Abstract The aim of this experiment was to impregnate
thermally modified wood using an easy and cost-effective
method. Industrially processed thermally modified Euro-
pean aspen (Populus tremula L.) and birch (Betula pu-
bescens Ehrh.) were collected and secondarily treated at
the laboratory scale with the preservatives tung oil, pine tar
and Elit Tra
¨skydd (Beckers) using a simple and effective
method. Preservative uptake and distribution in sample
boards were evaluated using computed tomography (CT)
and scanning electron microscopy (SEM) techniques. Pre-
servative uptake and treatability in terms of void volume
filled were found the highest in Beckers and the lowest in
tung oil-treated samples. Thermally modified samples had
lower treatability than their counterpart control samples.
More structural changes after thermal modification, espe-
cially in birch, significantly reduced the preservative
uptake and distribution. The differences of preservatives
uptake near the end grain were high and then decreased
near the mid position of the samples length as compared
with similar type of wood sample. Non-destructive evalu-
ation by CT scanning provided a very useful method to
locate the preservative gradients throughout the sample
length. SEM analysis enabled the visualization of the
preservative deposits in wood cells at the microstructural
level.
Keywords Computed tomography Preservative
distribution Scanning electron microscopy Secondary
treatment Thermally modified wood
Introduction
Biodeterioration of wood occurs if the wood serves as a
suitable living environment for biological organisms.
Under certain conditions of exposure, particularly when
wood becomes wet or when there is sufficient moisture and
oxygen, wood may rapidly become susceptible to fungi
and, to some extent, bacteria [1]. To ensure a long, useful
and extended service life, timber needs protection from the
hazards of fungal decay and weathering. Thus, wood must
be treated with preservatives, modified chemically or
modified thermally. During the last decade, wood modifi-
cation with heat has been widely investigated as a method
to reduce its equilibrium moisture content (EMC) and
water absorption and to increase its dimensional stability,
although the durability of thermally modified wood in
outdoor applications remains questionable [2,3]. The
extent of changes in wood properties during heat treatment
depends on the method used for thermal modification, the
wood species, the initial moisture content (MC) of the
wood, the surrounding atmosphere and, most important, the
treatment time and temperature [4]. Of these factors,
temperature has the strongest effect on the thermally
modified wood properties [5,6]. The available –OH groups
present in hemicelluloses have the most significant effect
S. A. Ahmed (&)M. Sehlstedt-Persson L. Hansson
T. More
´n
Department of Engineering Sciences and Mathematics,
Division of Wood Science and Engineering, Wood Physics,
Lulea
˚University of Technology, Campus Skelleftea
˚,
Forskargatan 1, 931 87 Skelleftea
˚, Sweden
e-mail: sheikh.ahmed@ltu.se
M. Sehlstedt-Persson
e-mail: margot.sehlstedt-persson@ltu.se
L. Hansson
e-mail: lars.hansson@ltu.se
T. More
´n
e-mail: tom.moren@ltu.se
123
J Wood Sci (2013) 59:57–66
DOI 10.1007/s10086-012-1299-x
Author's personal copy
on the physical properties of wood, and heat treatment
decreases the number of available –OH groups and inhibits
water adsorption, resulting in higher dimensional stability
[7].
Wood preservation with either oil-borne or water-borne
preservatives can eliminate biological attack by fungi,
insects and termites and can increase mechanical strength
[8]. However, no single preservative is available that can
provide all the desired kinds of protection. Though the
natural durability of wood offers a certain degree of
resistance to wood degrading agents, there is evidently no
ideal wood species that can be used universally in any
application without eventual break-down [9]. As a result, a
wide range of techniques and methods have been devel-
oped for wood preservation [10–13]. Preservation is an
important part of wood technology, encompassing every
process of chemical or physical treatment that is under-
taken to extend the service life of wood by optimizing its
resistance to biological attack, fire, shrinkage and swelling
due to changes in MC. One of the greatest advantages of
wood treatment is that it reduces the ability of wood to
absorb moisture and thus improves its dimensional stabil-
ity. Water repelling characteristics have been shown to
increase after the impregnation of Scots pine (Pinus syl-
vestris L.) sapwood with linseed oil [14], coconut oil and
various tall oils [15]. Even though the decreased hygro-
scopicity in heat-treated wood is reported to increase decay
resistance [16–18], thermally modified wood is not found
to be suitable for ground contact outdoor applications [3,
19]. The decay resistance depends on several factors, such
as wood species, treatment method and exposure condi-
tions. For example, a noticeably lower mass loss due to
fungal attack is observed for oil–heat-treated wood com-
pared with air–heat-treated wood [20]. Nevertheless, heat-
treated wood is found to be susceptible to termites as well
as marine borers [21,22]. No single method has been
shown to protect wood from all types of biodegradation.
Thus, heat treatment in combination with oil can provide
more effective protection than single treatment methods
alone [13,20].
The studies cited above emphasize the susceptibility of
thermally modified wood to different biodegrading agents
and the performance improvements that can be obtained by
combining heat treatment with a secondary treatment, such
as oil treatment. For example, water uptake by borate-
treated wood can be slowed down by treating wood with
secondary water repellent substances [23]. To the authors’
knowledge, however, the published literature on the sec-
ondary treatment of thermally modified wood with a pre-
servative to improve the durability of wood in outdoor
conditions is still lacking. Additionally, it is important to
know the distribution pattern of a chemical preservative in
wood, because deep and uniform penetration of the
preservative in the wood is crucial for achieving good
retention and performance [9,24]. We conducted this study
to provide insight into the uptake and distribution of dif-
ferent oil-borne wood preservatives in thermally modified
European aspen and birch boards. Durability testing in
outdoor conditions involves many parameters, the com-
plexities of which are outside the scope of this article.
However, the preservative distribution in thermally modi-
fied boards was assessed using high-value techniques such
as computed tomography (CT) and scanning electron
microscopy (SEM).
Materials and methods
Industrial materials and thermal modification
Commercial thermally modified European aspen (Populus
tremula L.) (ca. 27 9165 94000 mm) and birch (Betula
pubescens Ehrh.) boards (ca. 27 992 94000 mm) were
purchased from Thermoplus (Arvidsjaur, Sweden). There,
green boards were kiln-dried to 18 % MC prior to thermal
modification at 170 °C for 2.5 h. Saturated steam at
approximately 8 bar was used during drying and thermal
treatment as a protective vapor to prevent the wood from
burning. More about this thermal modification process,
which was introduced by the Danish company Wood
Treatment Technology (WTT), can be found in Dagbro
et al. [5]. Treated boards were brought to the laboratory for
subsequent preservative impregnation. Additional green
samples from aspen and birch were also collected before
thermal modification to serve as control samples after
attaining the EMC at room conditions.
Secondary treatment
Preservatives used
Three different oil-borne preservatives were used. The first
was a water-miscible commercial product, Elit Tra
¨skydd
(Beckers, Stockholm, Sweden). This preservative contains
additives such as propiconazole (0.6 %) and 3-iodo-2-
propynyl butylcarbamate (IPBC, 0.3 %), with modified
linseed oil as a binder and water as a solvent. The second
preservative was a commercially produced pine tar mixed
in boiled linseed oil with turpentine (Claessons Tra
¨tja
¨ra
AB, Go
¨teborg, Sweden) for thinning the oil-based preser-
vatives at a ratio of 1:4:2, respectively. The third pre-
servative was commercial 100 % tung oil (Pelard AB,
Stockholm, Sweden). This oil, pressed from nuts of tung
tree (Aleurites fordii Hemsl.), is reported to confer fungal
resistance to treated wood [25]. All preservative solutions
were stirred properly before use. Elit Tra
¨skydd and pine tar
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are referred to as Beckers and tar, respectively, in the
following sections.
Test samples and preservative treatment
Three commercially treated boards from each species were
planed, and three end-matched samples from each ther-
mally modified board and control board were prepared. The
sample dimensions were 25 990 9300 mm. The samples
were free of knots, cracks and other visible defects, and
they were numbered consecutively. Three matched ther-
mally modified and control samples from each species
(aspen and birch) for each of the three treatments (tung oil,
tar and Beckers) were used for preservative uptake, for a
total of 36 samples tested. Samples were impregnated with
preservatives following a simple and easy method descri-
bed by Ahmed and More
´n[26]. The ultimate goal is to
implement oil impregnation in commercial plants during
the cooling phase of the thermal modification process. The
tests described here are part of a series of experiments to
evaluate the efficacy of secondary treatment at a laboratory
scale. Here, additional heating at 170 °C for 1 h in a dry
oven was performed to reach the sample target temperature
(170 °C), as is performed in commercial plants during the
heat treatment phase. Then, the still-hot samples were
quickly submerged in preservatives for simultaneous
impregnation and cooling for 2 h. We defined this process
as the secondary treatment of thermally modified wood. No
external vacuum or pressure was applied before or during
the preservative impregnation.
Measurement of preservative solution uptake
and treatability of wood
After 2 h of soaking, additional preservative liquid was
wiped off gently from the surfaces with a paper towel, and
the sample mass was recorded using a balance accurate to
the nearest 0.01 g. The amount of preservative absorbed by
the sample was calculated as follows: uptake (kg m
-3
)=
1000G/V, where Gis the grams of preservative solution
absorbed by the sample (the mass difference between
preservative-treated and oven-dried samples) and Vis the
volume of the sample in cm
3
.
Preservative retention is often used as an indication of
the treatment level. Maximum retention is always influ-
enced by the permeability and wood density. Thus, reten-
tion is not the best descriptor for treatability, since it is
influenced by density. Treatability in this study is so
expressed as the ratio of preservative uptake to the poten-
tial volume that could be occupied if the samples were
completely filled. Void volume filled (VVF) will, therefore,
be a direct measure of treatability, as long as none of the
samples are fully treated.
VVF%¼
MtMu
V
P
"#
100:ð1Þ
In this equation, M
t
is the mass of the treated sample (g),
M
u
is the mass of untreated sample (g), Vis the untreated
sample volume (cm
3
), and Pis the porosity as void volume
of wood, P=[{1 -(oven-dry density/1.50)} 9100]
considering the average density of the solid wood
substance 1.50 g cm
-3
[9]. This calculation represents
the amount of space available in each sample and provides
an estimate of the maximum volume of preservative that
could potentially be absorbed by the wood.
CT scanning and image processing
A Siemens CT scanner (SOMATOM Emotion Duo,
Munich, Germany) located at Lulea
˚University of Tech-
nology, Campus Skelleftea
˚, Sweden, was used for the non-
destructive density measurements. The density accuracy in
a CT scanner is ±2kgm
-3
for dry wood and ±6kgm
-3
for wet wood, with moisture content ranging from 6 to
100 % in a 2 9291.5 mm sample, and larger measuring
volumes give more accurate density measurements [27].
The 512 9512 pixel CT images were obtained using scan
settings of 110 kV, 260 mAs, 1.5 s scan time and 3.0 s
delay time. The intensity value of each pixel corresponds to
the measured density in that measuring volume. The vol-
ume of each voxel was 0.98 90.98 mm through the cross-
section and 5 mm in the grain direction. Thirty scans at
5 mm intervals were performed, covering half of the
sample length and producing a total of 2160 scan images
before and after impregnation. Scanning on the other half
was not performed, but the other half was assumed to have
the same distribution of density values. To ensure mea-
surement at the same position of each sample, reference
marks were used. Preservative uptake was calculated by
taking the density values obtained after preservative
treatment and subtracting the values obtained for the oven-
dried wood before treatment. The subtraction was per-
formed using the image analysis software, ImageJ 1.46i
[28]. Averaged data from the triplicate samples were used
to plot the density graphs. To calculate preservative dis-
tribution profiles, the UnwarpJ plugin in ImageJ was used
to correct slight swelling effects in the treated sample
images so that they would more closely resemble the
untreated sample images in terms of their physical
dimensions.
Scanning electron microscopy
SEM specimens were prepared after sawing wood cubes
measuring ca. 5 (radial, R)95 (tangential, T)95 (lon-
gitudinal, L) mm from thermally modified and control
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samples. Thermally modified wood became brittle [29] and
thus required embedding in order to preserve microscopic
structures. Artifacts due to the embedding and microtoming
are expected to be minimized as PEG 1500 is not reported
to penetrate cell wall [30]. Samples were immersed over-
night in melted polyethylene glycol (PEG 1500, Sigma-
Aldrich Chemie GmbH, Schnelldorf, Germany) at 60 °C.
The transverse surfaces of embedded specimens were fin-
ished with a sliding microtome (Reichert, Vienna, Austria).
Specimens were washed with several changes of warm
distilled water to remove the PEG embedding matrix, and
they were subsequently air dried. Samples were then
observed under SEM following the technique mentioned
elsewhere [31]. Without embedding, radial and tangential
surfaces of the thermally modified samples impregnated
with preservatives were split along a tangential or a radial
plane with a razor blade and were examined to observe the
preservatives deposition. Three sample blocks from each
control samples were examined and captured SEM images
were analyzed by ImageJ 1.46i for the following quantified
details: aperture diameter of vessel–ray pits in R–Lsec-
tions, number of uni-, bi- or tri-seriate rays per mm
2
in
T–Lsection (considering all complete plus incomplete rays
crossing one vertical and one horizontal side of the square
and ignoring all incomplete rays crossing the other vertical
and horizontal sides) and rays contacted with vessel in
T–Lsection measured by total counted rays divided by the
ray in contact with vessel in 1 mm
2
area.
Data analysis
Uptake and treatability data were analyzed using the sta-
tistical software package IBM SPSS Statistics, Version 20
(IBM Corporation, New York, USA). An analysis of var-
iance (ANOVA) was carried out, and a 5 % level of sig-
nificance was used to detect differences. A one-way
ANOVA was applied to determine whether tung oil, pine
tar and Beckers uptake were significantly different between
wood samples. A similar test was performed to assess the
effect of preservative on the treatability (VVF%) of wood.
When significant differences were found, Duncan’s multi-
ple-range test was performed, and significant differences
were marked by different letters.
Results and discussion
Wood microstructure
Microscopic analysis of aspen and birch wood before and
after thermal modification is presented in Fig. 1. Micro-
scopic observation indicates that vessels, fibers, parenchyma
Fig. 1 Scanning electron micrographs of aspen and birch on
transverse sections. Aspen before (a) and after (band c) thermal
modification. Birch before (d) and after (eand f) thermal modifica-
tion. White and black arrowheads indicate intrawall and transwall
fracture on thermally modified samples, respectively. Vvessel,
Ffiber, Rp ray parenchyma. Scale bars (a,b,dand e) 100 lm,
(cand f)10lm
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and rays are still obvious, though wood anatomy was
slightly affected and some shrinkage occurred after thermal
modification. A similar observation was also reported by
Mburu et al. [18]. Moreover, intrawall and transwall frac-
tures were observed after thermal modification (Fig. 1c, f).
There were no observed artifacts on control sample due to
PEG 1500 embedding and microtoming. And thus fractures
on thermally modified wood were supposedly the effect of
heat treatment. These fractures appear to be the result of
stress caused by differential shrinkage due to heat treat-
ment. Compared to birch, aspen wood showed fewer
structural changes due to thermal modification (Fig. 1) and
developed smaller cracks in the cell walls and small
changes in the shape of vessels, i.e., less vessel wall
buckling. In contrast, birch samples showed substantial
shrinkage of vessels leading to vessel wall buckling
(Fig. 1e). While birch showed structural changes such as
the disintegration of ray parenchyma resulting from cell
wall thinning and vessel wall buckling could reduce the
preservative uptake and alter the distribution of pre-
servative (Table 1). Further observation indicated that both
types of thermally modified samples were more brittle than
their untreated counterparts.
Treatability to preservative liquids
The mechanism of oil impregnation is as follows: as wood
is preheated prior to secondary treatment, whatever air
that is contained within the cell cavities and voids
becomes hot and expands. Immersing the hot wood in a
room temperature preservative solution causes the rapid
contraction of the air within the cell cavities and voids,
resulting in the solution being drawn into the wood void
structures. It is believed that the performance of treated
wood is closely connected with the oil properties, the
amount of absorption and the extent of oil leaching during
use. Although we did not investigate leaching in this
study, more details on that phenomenon can be found in
Ahmed and More
´n[26].
Table 2shows the average preservative uptake in dif-
ferent wood samples. For both species, thermally modi-
fied samples had lower preservative uptake than their
corresponding control samples. In particular, thermally
modified birch had the lowest preservative uptake, and
that difference was found to be statistically significant. In
Table 2, it is also evident that Beckers had the highest
penetration in both types of wood samples. However,
previous results from Ahmed and More
´n[26]andAhmed
et al. [32]. indicate that Beckers has lower penetration in
Scots pine (Pinus sylvestris L.) and Norway spruce (Picea
abies [L.] Karst.). This discrepancy could be attributable
to the tert-butylhydroquinone that is added to the com-
mercial formulation. The previous studies also suggested
that a separation of the solute (e.g., tert-butylhydroqui-
none) from the solvent could cause reduced penetration
[32]. In this experiment, however, we used only com-
mercial Beckers solution, which contains both non-polar
and polar substances (see ‘‘Preservatives used’’ in
‘‘ Materials and methods’’). It is well known that non-polar
liquids penetrate by bulk flow, mainly through the cell
lumens and pits, whereas polar liquids penetrate by both
bulk flow and diffusion through the wood cell wall [33,
34]. Thus, uptake differences may exist among tung oil,
tar and Beckers.
Birch and aspen vary in wood density. The average
oven-dry densities for control aspen and birch samples
were 452 and 577 kg m
-3
, respectively, as measured by
the CT scanner. This density parameter is an important
factor influencing the theoretical maximum amount of
preservative liquid that can be absorbed in a given block
volume. The gross pore space or porosity (void volume) of
each sample was calculated as an estimate of the maximum
volume of preservative that could be absorbed by the wood.
Table 3shows the variation in treatability expressed as
VVF in different samples. Because Beckers showed greater
penetration than tung oil and tar, the highest treatability
was obtained for Beckers-treated samples, especially in
aspen. Birch had lower preservative uptake, and we
observed no significant differences in treatability between
tung oil, tar and Beckers.
Assessment of preservative distribution
CT scanner
Non-destructive quantitative analysis of preservative
uptake using CT scanning was performed for samples
comprising the part of the board from the non-sealed end
grain to the middle (Fig. 2). Near the end grain (0–5 mm),
the preservative uptake was the highest, and the uptake
gradually decreased through the length of the sample, in
agreement with Banks [35] and Johansson et al. [2]. The
effect of the non-sealed end grain was quite prominent in
Beckers-treated samples, which showed greater pre-
servative uptake up to 125 mm deep into the sample
boards. This high uptake could be caused by the higher
permeability of Beckers in wood. For tung oil and tar, the
end grain effect was limited to 35 mm, and the preservative
uptake gradually declined and became steady near the
middle of the samples (Fig. 2). Lateral penetration was
mainly responsible for distributing the preservative along
the length of the sample. Interestingly, regardless of pre-
servative type, the preservative uptake in a particular kind
of wood sample was similar in the middle position, indi-
cating the effectiveness of the secondary treatment process.
Boards longer than the experimental samples considered
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here could also be effectively impregnated. Of the three
types of wood preservatives, Beckers certainly showed the
highest treatability, and tung oil showed the lowest
(Table 3). The likely reason for the greater treatability with
tar rather than tung oil is that turpentine is used as a thinner
for oil-based media. Turpentine actually reduces the
Table 1 Comparison of ray features (n=30) between aspen and birch
Properties Aspen Birch
Rays
Control
Thermally modified
Aperture diameter of vessel–ray pit (lm)
8.5 2.8
Number of uniseriate rays per mm
2
31.2 10.9
Number of bi- or tri-seriate rays per mm
2
– 26.2
Vessel–ray contact (%) 56.8 46.6
Table 2 Average preservative uptake (kg m
-3
)byaspenandbirchboards
Wood species Tung oil Pine tar Beckers
Control aspen 267.07 a 283.59 a 371.18 a
TM-aspen 245.00 a 250.87 a 349.82 a
Control birch 241.67 a 247.10 a 269.17 b
TM-birch 67.58 b 95.34 b 118.68 c
TM thermally modified
Mean values followed by different letter within a column indicate that
there is a significant difference (PB0.05) as determined by the
Duncan’s multiple range test
Table 3 Void volume filled (VVF%) by different preservatives in
aspen and birch boards
Wood species Tung oil Pine tar Beckers
Control aspen 37.90 b 40.46 b 52.45 a
TM-aspen 35.24 b 35.58 b 50.14 a
Control birch 38.99 a 39.76 a 42.88 a
TM-birch 10.72 a 14.88 a 18.66 a
TM thermally modified
Mean values followed by different letter within a row indicate that
there is a significant difference (PB0.05) as determined by the
Duncan’s multiple range test
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viscosity of tar, and it has been reported that an even dis-
tribution was observed when the preservative viscosity was
kept low [34].
Structural changes due to thermal modification, such as
cracks in the cell wall and delamination of the middle
lamella, should increase the treatability of wood. However,
as mentioned above, the treatability of thermally modified
wood was low for aspen and birch compared to their cor-
responding control samples. These results suggest that it
may be the chemical changes in wood cell material that
affect preservative treatability, rather than the structural
changes caused by thermal modification alone. Chemical
changes during thermal modification such as degradation
of hemicellulose, ramification of lignin, crystallization of
cellulose, formation of furfural or 5-(hydroxymethyl) fur-
fural take place depending on the treatment condition [7,
36]. Chemical decomposition of birch is reported to be
more affected than aspen [7]. This could be the reason why
the thermally modified wood has lower especially the birch
sample had the lowest preservative uptake.
Visualization of preservative penetration at different
points along the sample length is shown in Fig. 3. Greater
amounts of preservative were impregnated both in control
and thermally modified aspen, and preservatives were
found to be evenly distributed. In contrast, birch had lower
amounts of impregnated preservative, especially in the
thermally modified samples. Anatomical differences
between the two species are thought to be responsible for
these distribution differences. Properties such as a greater
number of vessels or larger vessel–ray pit aperture in aspen
could make it more permeable than birch. The movement
of liquid through wood occurs primarily along the grain. It
is also evident that rays play an important role in liquid
penetration and distribution [37]. A subsequent SEM
examinations revealed that aspen has exclusively uniseriate
rays and that birch has additional bi- and tri-seriate rays.
Aspen has a higher percentage of vessels with ray contact
than birch (Table 1). Moreover, the larger vessel–ray pits
in aspen could affect the preservative distribution. In
addition, the cells of the uniseriate rays of aspen have more
direct contact with the vessels and other longitudinal ele-
ments than the bulk of the cells in the bi- or tri-seriate rays,
so uniseriate rays are expected to be more permeable.
These results explain the better treatability and distribution
of preservatives in aspen (Fig. 3). In contrast, approxi-
mately 71 % of rays in birch were bi- or tri-seriate, and
those rays seemed to be discontinued or collapsed
(Table 1) after thermal treatment. This discontinuation
probably was responsible for the poor preservative uptake
and distribution in birch.
Scanning electron microscopy
SEM analyses visualized the penetration path of preser-
vatives in wood cells through different pit membranes and
the pattern of preservative deposition in cell lumen. SEM
observations show that the intervessel pit membranes in
hardwood are permeable despite the lack of margo-like
structures, which are present in softwood (Fig. 4b, e). The
observed preservative deposition in different pit membrane
of thermally modified samples indicates their permeability.
Apparently, hardwood pit membranes acts like filters in
which the fluid flows circuitously in the membrane. Thus,
uptake may be higher due to better access to the remaining
axial pathway and better ‘bridging’ of flow pathways that
may occur between the ray cells and axial elements,
especially vessels and fibers. SEM observation showed that
0
100
200
300
400
500
600
Tung oil uptake (kg m-3)
0
100
200
300
400
500
600
Pine tar uptake (kg m-3)
0
100
200
300
400
500
600
5 203550658095110125140
Beckers uptake (kg m-3)
− Position to center (mm)
Control aspen
TM-aspen
Control birch
TM-birch
Fig. 2 Preservative uptake at different positions along aspen and
birch boards, estimated by CT scans. Each position is the average of
three estimates. TM thermally modified
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pit membranes in vessel–ray pits, intervessel pits and fiber
pits were permeable (Fig. 4a–c, e). It is impossible for
preservative to move great distances along the vessel only
[34]. Thus, transverse movement is important to achieve an
even distribution of preservative throughout the sample
board. In this case, ray parenchyma facilitated radial pen-
etration (Fig. 4d). Moreover, vessels connected by inter-
vessel pits, especially on the tangential walls of aspen and
birch, also allow the transverse flow of preservative liquids.
Regardless, the highest uptake and most prominent depo-
sition were not observed under SEM for the Beckers, and
hence, those micrographs are excluded. As water is used as
a solvent in Beckers formulation, it could be absorbed by
the cell wall materials leaving no visible traces.
Despite the nature of the deposition observed in wood
cells, it can be concluded that preservative uptake and
Fig. 3 Preservative distribution
at different positions along
aspen and birch boards,
estimated by CT scans and
image analysis. TM, thermally
modified
Fig. 4 Scanning electron
micrographs of thermally
modified aspen and birch after
tar (a–c) and tung oil (dand
e) impregnation, respectively.
aVessel–ray pits in aspen.
bIntervessel pits in aspen.
cFiber pits in aspen. dRay
parenchyma in birch.
eIntervessel pits in birch with
coalescent apertures. Arrow
head indicating preservative
deposition in the pit membrane.
Vvessel, Ffiber, Rp ray
parenchyma, Llongitudinal,
Rradial, Ttangential. Scale bars
10 lm
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distribution depend on the wood structure and the nature of
the preservative used. Wood species with little or no
commercial value can be thermally modified and subjected
to secondary treatment with preservative, and in this way, a
new use can be found for those species. The durability of
thermally modified wood after secondary treatment in
outdoor conditions remains an important area for future
investigations.
Conclusions
After thermal modification, a greater number of anatomical
changes in birch led to reduced preservative uptake and
treatability (as measured by VVF%) compared to coun-
terpart control samples, whereas this deviation was found
to be quite low in aspen. A non-destructive analysis of the
preservative distribution by CT scanning revealed great
differences in preservative uptake at the end grain, but
quite small differences near the middle of the wood sam-
ples. SEM analysis showed that preservatives especially tar
and tung oil deposited in the cell lumen by passing through
different pit membranes. Beckers was found to be absorbed
by cell wall materials. The preservative treatability of all
types of wood followed the order Beckers [tar [tung oil,
and the preservative distribution was found to be more
even in the aspen samples.
Acknowledgments Financial support from the European Union and
the European Regional Development Fund, the County Administra-
tion of Va
¨sterbotten, the municipality of Skelleftea
˚and Tra
¨Centrum
Norr is highly appreciated and acknowledged.
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