Citation:He, Z.; Qi, Y.; Zhang, G.;
Zhao, Y.; Dai, Y.; Liu, B.; Lian, C.;
Dong, X.; Li, Y. Mechanical Properties
and Dimensional Stability of Poplar
Wood Modied by Pre-Compression
and Post-Vacuum-Thermo
Treatments.Polymers2022,14, 1571.
https://doi.org/10.3390/
polym14081571
Academic Editors: Shang-Tse Ho and
Han-Chien Lin
Received: 2 March 2022
Accepted: 6 April 2022
Published: 12 April 2022
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4.0/).polymers
Article
Mechanical Properties and Dimensional Stability of Poplar
Wood Modied by Pre-Compression and
Post-Vacuum-Thermo Treatments
Zaixin He
1,†
, Yanran Qi
1,†
, Gang Zhang
1,†
, Yueying Zhao
2
, Yong Dai
3
, Baoxuan Liu
4
, Chenglong Lian
1,2,
*,
Xiaoying Dong
1,
* and Yongfeng Li
1,
*
1
Key Laboratory of State Forestry Administration for Silviculture of the Lower Yellow River, College of
Forestry, Shandong Agricultural University, Tai'an 271018, China; [email protected] (Z.H.);
[email protected] (Y.Q.); [email protected] (G.Z.)
2
Postdoctoral Innovation Practice Base, Shandong Xiaguang Group Co., Ltd., Jining 277600, China;
[email protected]
3
Jiangsu Longyuan Decoration Materials Co., Ltd., Suqian 223900, China; [email protected]
4
Shandong Laucork Development Co., Ltd., Jining 272100, China; [email protected]
*Correspondence: [email protected] (C.L.); [email protected] (X.D.); [email protected] (Y.L.);
Tel.: +86-538-8240610 (Y.L.)
† These authors contributed equally to this work.
Abstract:
Fast-growing poplar wood has the bottleneck problems of inferior mechanical strength
and poor dimensional stability. In this study, the wood was modied by combined treatments of
pre-compression and post-vacuum-thermo modication to improve its mechanical strength and
dimensional stability, simultaneously; in addition, the variation law of mechanical properties of the
wood with compression ratio as well as the improvement effect of dimensional stability of the treated
wood were mainly studied. The results show that the optimal temperature and time of the vacuum-
thermo modication were 190

C and 10 h, respectively. Under these conditions, the structure of
pre-compressed and post-vacuum-thermally modied wood (CT wood) is gradually densied with
the increase in the compression ratio, which results in the continuous enhancement of mechanical
properties. Meanwhile, the anti-swelling efciency (ASE) of the CT wood after water absorption is
correspondingly better than that of the compressed wood before thermal modication, indicating that
the dimensional stability of compressed wood was improved by the thermal modication. When the
compression ratio was 70%, the modulus of rupture (MOR) and impact toughness of CT wood was
176 MPa and 63 KJ/m
2
, which was 125% and 59% higher than that of untreated wood, respectively.
The ASE was also 26% higher than that of the wood with sole compression. Therefore, this method
improves the mechanical strength and dimensional stability of wood simultaneously, and it provides
a scientic basis for optimization of the reinforcing modication process of fast-growing wood.
Keywords:
compression; vacuum-thermo treatment; mechanical properties; dimensional stability;
poplar wood
1. Introduction
Forest resources play a fundamental role in regulating ecology, improving climate,
and providing abundant and renewable woody resources as a substitute to non-renewable
resources, such as steel and plastic, to reduce the environmental pollution caused by their
processing and utilization [1]. Therefore, fully utilizing wood resources aids in the green
and sustainable development of the economy and society. However, trees grow slowly,
resulting in a long period before initially providing wood materials, especially for high-
quality wood resources, which makes it difcult to meet the urgent demand for wood
resources supplied for rapid economic and social development. Although fast-growing
trees have alleviated the tight supply of wood resources, they have the drawbacks of lower
Polymers2022,14, 1571.

Polymers2022,14, 1571 2 of 13
strength and unstable dimension of moisture absorption and water absorption, particularly
for wood materials such as poplar wood (PopulusL.), which restricts the wider applications
of such resources as substitutes for non-renewable resources [2–4].
Various modication methods have been explored and applied to fast-growing wood
for simultaneous improvement in mechanical strength and dimensional stability such
as resin impregnation [5–7], densication combined with pretreatment of partial lignin
removal [8], chemical modication with low molecular agents [9], and a combination of
densication and thermal modication [10–15]. However, until now, the combination of
densication and thermal modication has only been in line with the concept of sustainable
development. Compression treatment is used to densify wood, mainly through pressure
combined with/without hydrothermal treatment to improve its mechanical strength [15,16].
Thermal modication mainly degrades and converts partially hydrophilic hemicelluloses
into hydrophobic substances to improve their dimensional stability through high tempera-
ture with/without inert gas protection [17–19]. The extent of these changes in wood during
the heat treatment depends on the thermal modication method, the wood species and its
typical properties, the initial moisture content of the wood, the surrounding atmosphere,
treatment time, and temperature [20]. Combining the two methods could simultaneously
improve the mechanical strength and dimensional stability of wood [10,11]. The whole
process does not involve chemical substances that pollute the environment or require high
energy consumption during the manufacturing process. The wood modication mainly
relies on the hydrothermal effect with lower energy consumption, which is environmentally
friendly, green, low carbon and, thus, has been widely explored. For example, the me-
chanical strength and dimensional stability of wood could be improved simultaneously by
the following treatments: combined segmented heating and hot-pressing treatments [12],
combined hot steam and compression treatments [16], combined pre-compression and
post-hot oil modication [21], combined pre-compression and post-nitrogen-protected
thermal modication [22].
However, due to the variety of the heat treatment methods, they often have different
degrees of negative effects on the mechanical strength of the treated wood. Okon's [23]
research found that under the condition of treatment at 210

C for 4 h, the mechanical
properties of silicone oil thermally modied Masson pine wood were two times lower than
those of untreated samples. Pelit [24] studied the effect of densied and heat post-treated
on the mechanical properties of wood. The results showed that the MOR values of Uludag
r, linden, and black poplar wood were reduced by 48%, 56%, and 39%, respectively, when
they were compressed at a 100

C and 50% compression ratio and then heat treated at
212

C. Bal [25] studied the thermal modication of black pine wood at 180, 200, and
220

C for 150 min in vacuum, nitrogen, and air atmospheres. The results showed that
with the increase in the modication temperature, the loss of mechanical properties was
greater. At 220

C, the MOR of thermally modied wood in air, vacuum, and nitrogen
atmospheres decreased by 23.8%, 12.9% and 19.8%, respectively. Therefore, it is of great
practical signicance to promote wide application of the combined densication and
thermal modication treatment by selecting an appropriate thermal modication method
to reduce the decline in the mechanical strength.
High-temperature thermal modication partially degrades hemicellulose into volatile
substances with small molecules, such as organic acids, aldehydes, and furans, which re-
duces the mechanical properties. Among them, acidic small molecules could accelerate the
degradation of polysaccharide macromolecules and, thus, adversely reduce the mechanical
strength of wood [26–28]. Therefore, during thermal modication, timely removal of such
small molecular substances by proper methods, such as vacuum treatment, is helpful for
protecting the mechanical properties of the compressed wood. Therefore, combing the two
ways by pre-compression and post-vacuum-thermo modication could theoretically obtain
high-quality poplar wood (CT wood) with improved mechanical properties and desired
dimensional stability [10]. However, such treatment has rarely been reported. Therefore,
this study employed this method to modify wood with a focus on optimizing the process

Polymers2022,14, 1571 3 of 13
of vacuum thermal modication and the variation laws of wood mechanical properties and
dimensional stability with the compression rate under the optimized thermo modication
in order to provide a scientic basis for the wider application of such treatment (Figure).Polymers 2022, 13, x FOR PEER REVIEW 3 of 13


theoretically obtain high-quality poplar wood (CT wood) with improved mechanical
properties and desired dimensional stability [10]. However, such treatment has rarely
been reported. Therefore, this study employed this method to modify wood with a focus
on optimizing the process of vacuum thermal modification and the variation laws of wood
mechanical properties and dimensional stability with the compression rate under the op-
timized thermo modification in order to provide a scientific basis for the wider application
of such treatment (Figure 1).

Figure 1. Schematic diagram of wood modification with combined pre-compression and post-vac-
uum-thermo treatments: (a) vacuum-thermo modification; (b) combined pre-compression and post-
vacuum-thermo treatment.
2. Materials and Methods
The poplar wood board employed in this study was bought from the wood market
in Tai’an city, Shandong Province. The contents of hemicellulose and lignin in poplar
wood are about 21% and 24%, respectively [29,30]. Temperature and time were explored
as the main indexes for the optimization of vacuum thermal modification conditions. Four
temperatures (i.e., 170, 180, 190, and 200 °C) and six thermal modification times (5, 10, 15,
24, 36, and 48 h) were explored. The experiment was designed by the all-factor method,
and three parallel groups of data were tested to obtain the average value for evaluating
each physical property. Firstly, the samples were dried with a drying oven (DZF-6050,
Shanghai Huitai Analytical Instrument Co., Ltd., Shanghai, China) at 103 °C for 48 h; then,
vacuum treatment was conducted in the oven with a vacuum pump until the absolute
pressure reached 0.01 MPa; after, the temperature was raised to the explored values and
kept for the whole thermal modification treatment. Finally, the samples were naturally
cooled to room temperature under the vacuum condition.
The treatment of pre-compression and post-vacuum-thermo modification was con-
ducted as follows. Poplar blocks of 100 × 100 × 20 (L × T × R mm
3
) were employed and
placed in a water bath (HH-2, Jintan Baita Xinbao Instrument Factory, Changzhou, China)
at 90 °C for 2 h to soften the wood components. Then, the softened samples were further
compressed by a hot-pressing machine (YLJ-HP88V, Hefei Kejing Material Technology
Co., Ltd., Hefei, China) for different compression ratios of 30%, 50%, and 70%. The total
pressing temperature was first set to 60 °C and maintained for 1 h, and then it was raised
to 100 °C for 4 h. Next, the wood blocks were moved to a drying oven and dried at 103 °C
for 48 h. Finally, the wood samples were vacuumed at 0.01 MPa and hot treated under the
optimized thermal modification conditions (i.e., temperature and time). Before further
evaluation of the properties, all samples were naturally cooled to room temperature under
the vacuum condition and placed under a ventilation condition for more than 1 week.
Density was calculated from the sample’s volume and weight. The dimensional sta-
bility was evaluated mainly based on the anti-swelling efficiency (ASE) of wood after
Figure 1.
Schematic diagram of wood modication with combined pre-compression and post-
vacuum-thermo treatments: (a) vacuum-thermo modication; (b) combined pre-compression and
post-vacuum-thermo treatment.
2. Materials and Methods
The poplar wood board employed in this study was bought from the wood market
in Tai'an city, Shandong Province. The contents of hemicellulose and lignin in poplar
wood are about 21% and 24%, respectively [29,30]. Temperature and time were explored as
the main indexes for the optimization of vacuum thermal modication conditions. Four
temperatures (i.e., 170, 180, 190, and 200

C) and six thermal modication times (5, 10, 15,
24, 36, and 48 h) were explored. The experiment was designed by the all-factor method, and
three parallel groups of data were tested to obtain the average value for evaluating each
physical property. Firstly, the samples were dried with a drying oven (DZF-6050, Shanghai
Huitai Analytical Instrument Co., Ltd., Shanghai, China) at 103

C for 48 h; then, vacuum
treatment was conducted in the oven with a vacuum pump until the absolute pressure
reached 0.01 MPa; after, the temperature was raised to the explored values and kept for the
whole thermal modication treatment. Finally, the samples were naturally cooled to room
temperature under the vacuum condition.
The treatment of pre-compression and post-vacuum-thermo modication was con-
ducted as follows. Poplar blocks of 10010020 (LTR mm
3
) were employed and
placed in a water bath (HH-2, Jintan Baita Xinbao Instrument Factory, Changzhou, China)
at 90

C for 2 h to soften the wood components. Then, the softened samples were further
compressed by a hot-pressing machine (YLJ-HP88V, Hefei Kejing Material Technology
Co., Ltd., Hefei, China) for different compression ratios of 30%, 50%, and 70%. The total
pressing temperature was rst set to 60

C and maintained for 1 h, and then it was raised
to 100

C for 4 h. Next, the wood blocks were moved to a drying oven and dried at 103

C
for 48 h. Finally, the wood samples were vacuumed at 0.01 MPa and hot treated under
the optimized thermal modication conditions (i.e., temperature and time). Before further
evaluation of the properties, all samples were naturally cooled to room temperature under
the vacuum condition and placed under a ventilation condition for more than 1 week.
Density was calculated from the sample's volume and weight. The dimensional stabil-
ity was evaluated mainly based on the anti-swelling efciency (ASE) of wood after moisture
or water absorption [26]. The treated wood samples were put into a sealed tank under
different moisture levels or into water solution, and then taken out after moisture/water
treatment for 2, 5, 10, 24, 48, 72, 96, and 120 h, respectively. Each sample volume change
was nally calculated in terms of the sample volume before and after moisture/water treat-
ment [26]. Hardness, modulus of rupture (MOR), and impact toughness were measured us-

Polymers2022,14, 1571 4 of 13
ing a universal mechanical testing machine (CMT4104, Xinsansi Material TestingCo., Ltd.,
Shenzhen, China), respectively, according to theGB/T 1929-2009andGB/T 1936.1-2009.
The thermogravimetric (TG) behavior was characterized by a synchronous thermal ana-
lyzer (TGA Q500, TA company, Boston, MA, USA). The microstructure, component, and
surface elements of the samples were characterized by SEM (JEM-6610LV, JEOL, Akishima,
Japan), XRD (D/MAX 2200, Rigaku, Tokyo, Japan), FTIR (Nicolet Magna 560, ThermoFisher
Scientic, Waltham, MA, USA), and XPS (STA 449F3, ThermoFisher Scientic, American).
3. Results and Discussion
3.1. Optimization of Vacuum Thermal Modication Process
3.1.1. Density
Figurea shows that the wood density, overall, decreased with the thermal modi-
cation time and temperature. The density change was mainly due to the mass change
caused by component degradation, while the volume was basically unchanged in this
temperature and time range. This is the pyrolysis of hemicellulose in the amorphous region
and the rapid pyrolysis of lignin, which formed charcoal and volatile intermediates and
decreased the wood density. At temperature of 170

C, the change in wood density was
not obvious during the entire treatment period, which indicates that such temperature and
time had no signicant effect on wood density. Within the range of 180

C and 200

C, the
wood density decreased obviously with the thermal modication time; especially at 200

C,
the density declined obviously during the entire experimental period, indicating that the
wood components were degraded continuously at this temperature, which would have an
obviously negative impact on the mechanical properties of wood. Within the temperature
range of 180

C and 190

C, the wood density remained stable during the treatment stage in
the rst 10 h, and began to decrease signicantly after 15 h of treatment, indicating that the
wood components were not signicantly degraded within treatment time of 10 h, which
was conducive to the maintenance of the wood's mechanical strength. These results match
research by Hajihassani [12], Bal [25], andCabalov¡[27], showing the same variation law.Polymers 2022, 13, x FOR PEER REVIEW 5 of 13


affected the mechanical strength and, subsequently, both were positively correlated
[28,31]. The decrease in MOR may mainly be due to the degradation of hemicellulose,
which weakens the fixation of cellulose chain [26,32,33]. Figure 2e−f show that both the
ASE after moisture absorption and water absorption increased with the vacuum temper-
ature; in other words, the dimensional stability was positively correlated with the thermal
modification temperature. During the early treatment time of 10−15 h, it increased signif-
icantly with the treatment time and then plateaued, indicating that the effect of thermal
modification time on ASE was mainly restricted within the first 15 h [28]. This was prob-
ably due to the degradation of hydrophilic chemical components, such as hemicellulose,
after thermal modification; the reduction in water absorption capacity; the increase in
ASE. At approximately 15 h, with the completion of the degradation of a large amount of
hemicellulose, the change of ASE also became flat.

Figure 2. The (a) density change rate; (b) hardness; (c) modulus of rupture; (d) impact toughness of
thermally modified wood at different temperatures; the ASE of wood after (e) moisture absorption
at 98% humidity and (f) after water absorption with only thermal treatment.
3.1.3. Optimum Thermal Modification Process
Totally, the temperature of 200 °C had the greatest improvement in dimensional sta-
bility, however, it had a largely negative effect on the mechanical strength of wood. While
in the temperature range of 170 °C and 190 °C, the best dimensional stability was attained
at 190 °C. Under this temperature and during the thermal modification time of the early
10 h, the mechanical strength of wood did not decrease significantly but only fluctuated
slightly. After a period of time, the overall strength of the wood began to decline signifi-
cantly. Similarly, the wood’s dimensions become stable after the 10 h. Therefore, combin-
ing the mechanical strength and dimensional stability, 190 °C and 10 h were employed as
the optimal conditions for vacuum thermal modification. This temperature was the same
as the optimal thermal modification temperature reported in the literature [11,20,25].
3.1.4. SEM
Figure 3a,b show that the modified cell wall did not change significantly and still
maintained a stable cell arrangement structure, which was almost the same as that of the
untreated wood. Therefore, the thermal modification treatment at 190 °C for 10 h could
not destroy the integrity of the cell wall structure, which further proved that the physical
and mechanical properties of wood did not decrease significantly after the thermal mod-
ification [34].
Figure 2.
The (a) density change rate; (b) hardness; (c) modulus of rupture; (d) impact toughness of
thermally modied wood at different temperatures; the ASE of wood after (e) moisture absorption at
98% humidity and (f) after water absorption with only thermal treatment.

Polymers2022,14, 1571 5 of 13
3.1.2. Mechanical Properties
Figureb–d show that the hardness, modulus of rupture, and impact toughness of the
wood decreased with the thermal modication temperature and time. At the treatment tem-
perature of 200

C, the three mechanical strengths continued to decrease with the thermal
modication time, indicating that this temperature could adversely affect the mechanical
properties. Under thermal modication, pyrolysis of hemicellulose in the amorphous
region and rapid pyrolysis of lignin occurred, resulting in reduced mechanical properties.
At the same time, with the prolongation of the thermal modication time, the degradation
of hemicellulose and lignin increased; thus, there was a continuous downward trend. When
the treatment temperature was in the range of 170

C and 190

C, the mechanical strength
uctuated in varying degrees during the rst treatment time of 10 h, but there was no
obvious decrease overall. This may be due to the fact that the initial wood had a certain
amount of moisture or other substances that absorbed the heat during thermal modication
and delayed the degradation time. As the moisture or other substances evaporated or
dissipated, the wood gradually reached the thermal modication and then the mechanical
properties began to decline in varying degrees after 10 h. This was basically consistent with
the variation law of wood density, indicating that change in density affected the mechanical
strength and, subsequently, both were positively correlated [28,31]. The decrease in MOR
may mainly be due to the degradation of hemicellulose, which weakens the xation of
cellulose chain [26,32,33]. Figuree,f show that both the ASE after moisture absorption and
water absorption increased with the vacuum temperature; in other words, the dimensional
stability was positively correlated with the thermal modication temperature. During
the early treatment time of 10–15 h, it increased signicantly with the treatment time and
then plateaued, indicating that the effect of thermal modication time on ASE was mainly
restricted within the rst 15 h [28]. This was probably due to the degradation of hydrophilic
chemical components, such as hemicellulose, after thermal modication; the reduction in
water absorption capacity; the increase in ASE. At approximately 15 h, with the completion
of the degradation of a large amount of hemicellulose, the change of ASE also became at.
3.1.3. Optimum Thermal Modication Process
Totally, the temperature of 200

C had the greatest improvement in dimensional
stability, however, it had a largely negative effect on the mechanical strength of wood. While
in the temperature range of 170

C and 190

C, the best dimensional stability was attained at
190

C. Under this temperature and during the thermal modication time of the early 10 h,
the mechanical strength of wood did not decrease signicantly but only uctuated slightly.
After a period of time, the overall strength of the wood began to decline signicantly.
Similarly, the wood's dimensions become stable after the 10 h. Therefore, combining the
mechanical strength and dimensional stability, 190

C and 10 h were employed as the
optimal conditions for vacuum thermal modication. This temperature was the same as
the optimal thermal modication temperature reported in the literature [11,20,25].
3.1.4. SEM
Figurea,b show that the modied cell wall did not change signicantly and still
maintained a stable cell arrangement structure, which was almost the same as that of
the untreated wood. Therefore, the thermal modication treatment at 190

C for 10 h
could not destroy the integrity of the cell wall structure, which further proved that the
physical and mechanical properties of wood did not decrease signicantly after the thermal
modication [34].

Polymers2022,14, 1571 6 of 13Polymers 2022, 13, x FOR PEER REVIEW 6 of 13



Figure 3. SEM morphologies of the (a) untreated wood and the (b) thermally modified wood; (c) TG
curves of the untreated wood and the thermally modified wood; (d) FTIR spectra; XPS spectra of (e)
untreated woods (f) the carbon element peak of untreated wood, (g) thermally modified wood, and
(h) carbon element peak of the thermally modified wood; (i) X-ray diffraction pattern of untreated
wood and the thermally modified wood with the temperature of thermal treatment of 190 °C.
3.1.5. Thermogravimetry
Figure 3c shows that the thermal degradation mainly occurred in the temperature
range of 200 °C and 415 °C, hemicellulose and cellulose began to degrade at 200 °C and
250 °C, respectively, and the maximum loss rate appeared at 268 °C and 355 °C. The ther-
mal degradation temperature of lignin started from 180 °C [35–37]. This was the pyrolysis
of hemicellulose in the amorphous region and the rapid pyrolysis of cellulose and lignin,
which formed charcoal and volatile intermediates (CO2, CO, CH4, CH3OH, CH3OOH, hy-
drocarbons, hydroxyl compounds, carbonyl compounds, etc.) [38,39]. Comparing the TG
curves of untreated wood and thermally modified wood, it can be observed that the onset
temperature of the thermally modified wood was similar to the peak temperature of
weight loss, which was slightly higher than that of untreated wood, indicating partial
degradation of hemicellulose or structural rearrangements caused by volatilization of
some substances in wood during the thermal modification process.
3.1.6. FTIR Spectra
Figure 3d depicts that the absorption peak of the thermally modified wood near 3363
cm
−1
corresponded to the stretching vibration of the −OH group, and the absorption peak
intensity of the thermally modified wood decreased slightly compared to the untreated
wood, indicating that the number of hydroxyl groups in the wood decreased to a certain
extent after thermal modification. The hemicellulose has abundant and active hydroxyl
groups that can condensate at higher temperatures to form −O− bonds, removing water
molecules and thereby eliminating part of the free hydroxyl groups [40]. Additionally,
hydroxyl and carbonyl groups of various components in wood cell walls could strongly
interact with each other by hydrogen bonds or van der Waals force at high temperatures,
which further reduced the number of free hydroxyls groups [41].
Figure 3.
SEM morphologies of the (a) untreated wood and the (b) thermally modied wood; (c) TG
curves of the untreated wood and the thermally modied wood; (d) FTIR spectra; XPS spectra of
(e) untreated woods (f) the carbon element peak of untreated wood, (g) thermally modied wood,
and (h) carbon element peak of the thermally modied wood; (i) X-ray diffraction pattern of untreated
wood and the thermally modied wood with the temperature of thermal treatment of 190

C.
3.1.5. Thermogravimetry
Figurec shows that the thermal degradation mainly occurred in the temperature
range of 200

C and 415

C, hemicellulose and cellulose began to degrade at 200

C and
250

C, respectively, and the maximum loss rate appeared at 268

C and 355

C. The thermal
degradation temperature of lignin started from 180

C [35–37]. This was the pyrolysis of
hemicellulose in the amorphous region and the rapid pyrolysis of cellulose and lignin,
which formed charcoal and volatile intermediates (CO2, CO, CH4, CH3OH, CH3OOH,
hydrocarbons, hydroxyl compounds, carbonyl compounds, etc.) [38,39]. Comparing the
TG curves of untreated wood and thermally modied wood, it can be observed that the
onset temperature of the thermally modied wood was similar to the peak temperature
of weight loss, which was slightly higher than that of untreated wood, indicating partial
degradation of hemicellulose or structural rearrangements caused by volatilization of some
substances in wood during the thermal modication process.
3.1.6. FTIR Spectra
Figured depicts that the absorption peak of the thermally modied wood near
3363 cm
1
corresponded to the stretching vibration of theOH group, and the absorption
peak intensity of the thermally modied wood decreased slightly compared to the untreated
wood, indicating that the number of hydroxyl groups in the wood decreased to a certain
extent after thermal modication. The hemicellulose has abundant and active hydroxyl
groups that can condensate at higher temperatures to formObonds, removing water
molecules and thereby eliminating part of the free hydroxyl groups [40]. Additionally,
hydroxyl and carbonyl groups of various components in wood cell walls could strongly
interact with each other by hydrogen bonds or van der Waals force at high temperatures,
which further reduced the number of free hydroxyls groups [41].

Polymers2022,14, 1571 7 of 13
The absorption peak near 1731 cm
1
corresponded to the stretching vibration of the
C=O group, which was mainly originated from the acetyl and carboxyl groups of hemicel-
luloses. The absorption peak of wood slightly decreased after the thermal modication,
which was mainly due to the pyrolysis of hemicellulose and the change in chemical com-
position in the wood during the process [42]. The acetyl groups of hemicellulose were
cleaved at high temperatures and underwent a deacetylation reaction with free hydroxyl
groups to form acetic acids. With the increase in temperature, the pyrolysis of hemicellulose
was further promoted, resulting in a decrease in the number of hydroxyl groups; some
polysaccharides in hemicellulose were cleaved into furfural or certain carbohydrates with
short-chain structures, which further polymerized into some water-insoluble polymers
under the high temperatures [34,40]. Nicolas believed that the thermal modication process
was accompanied by the cleavage of lignin-aryl-ether bonds, followed by condensation
reactions and cross-linking reactions between lignin biopolymers that formed a novel
cross-linked network structure [43].
3.1.7. X-ray Photoelectron Spectroscopy
The elemental composition of the wood before and after thermal modication were
determined by X-ray photoelectron spectroscopy (XPS), and the results are shown in
Figuree–h. There were no novel peaks presented in the thermally modied wood,
indicating that new elements were absent in the process. The content of carbon element
increased after thermal modication, which was due to the decrease in the oxygen element
content caused by the decrease in the number of hydroxyl groups [40]. The increase in C1
content mainly resulted from the reaggregation of lignin components, which is consistent
with the results in Figured.
3.1.8. X-ray Diffraction Pattern
Figurei shows that the diffraction peak of the I002crystal plane of the thermally
modied wood was still located near 22

, indicating that the cellulose crystalline was stable
without crystal form change during the treatment. However, the crystallinity decreased
from 49% to 44%. This was mainly due to the acetyl groups on hemicellulose fall off and
acetic acids formed, which resulted in the partial acidolysis of the cellulose molecular
chain at high temperatures, and further destroyed the cellulose aggregation to reduce the
polymerization degree of cellulose and, accordingly, led to the crystallinity reduction [43].
A previous study reported that the crystallinity of larch wood decreased at 230

C under
nitrogen atmosphere as protective gas [44]. This result explains, to a certain extent, the
decline in the physical and mechanical properties of the thermally modied wood.
3.2. Combining Pre-Compression and Post-Vacuum-Thermo Treatments
3.2.1. SEM
Based on the above optimized thermal modication, the poplar wood was modied
by the combined treatments of pre-compression and post-vacuum-thermo modication,
and the variation law of mechanical properties and dimensional stability of the wood
with the compression ratio were focused on and explored. The internal microstructures of
CT wood were observed by SEM to analyze the changes in the physical and mechanical
properties. Figurea–d show that the diameter of the cell lumen gradually decreased
with the increase in the compression ratio, that is, the porosity gradually decreased, and
the wood cell wall accordingly became denser. A previous study [45] described that
the cell structure, in a cross-section, was distorted but was not signicantly damaged
after compression, which intuitively explains the density increase of the wood caused
by compression and, therefore, improved the hardness, bending strength, and impact
toughness. Additionally, the densied structure also demonstrated that the improvement
in the mechanical properties caused by compression treatment was greater than the negative
effect of the thermal modication on the mechanical properties [45,46].

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Figure 4. SEM of (a) untreated wood; CT wood with a (b) 30%, (c) 50%, and (d) 70% compression
ratio; (e) FTIR spectrum of untreated wood and CT wood with different compression ratios; (f) X-
ray diffraction patterns of untreated wood and CT wood with different compression ratios.
3.2.2. FTIR Spectra and X-ray Diffraction Pattern
Figure 4e shows that the absorption peak intensity of CT wood near 3363 cm
−1
(−OH
group) was reduced, indicating a reduction in the −OH group that was mainly caused by
the degradation of hemicellulose during the thermal modification process. The absorption
peak decreased at 1731 cm
−1
(C=O group), further confirming the degradation of hemicel-
lulose. Figure 4f describes that the diffraction peak of the I002 crystal plane was still located
near 22° after the combined treatments, indicating that the cellulose crystal form did not
change without dissolution or chemical treatment [47]. The crystallinity of the untreated
wood and the CT wood with compression ratios of 30%, 50%, and 70% was 44%, 46%,
49%, and 49%, respectively. With the increase in the compression ratio, the crystallinity
also increased. During the compression process, the distance between the cellulose fibrils
in the amorphous region was reduced so that the fibrils were arranged more closely; the
transverse compression caused the fibrils to only move in the cross-section, which aided
the fibrils orientally arranged along the direction of the crystalline region [48–50]. In ad-
dition, the thermal modification itself removed partial hemicellulose in the amorphous
region, which promoted the improvement in the crystallinity. All these effects will com-
prehensively contribute to the improvement in the mechanical properties of wood.
3.2.3. Density
Figure 5a shows that the density of the wood increased almost linearly with the com-
pression ratio. When the ratio reached 70%, the poplar wood’s density increased by ap-
proximately two times. Although the density of the compressed wood decreased after the
post-vacuum-thermo modification due to the degradation of hemicellulose, there was no
significant change. When the compression ratio was 70%, the thermal modification treat-
ment only reduced the wood’s density by 4%. This indicates that such a combined treat-
ment could effectively reduce the density change in wood and, accordingly, promote the
dimensional stability and mechanical properties of the modified wood [51–53].
Figure 4.
SEM of (a) untreated wood; CT wood with a (b) 30%, (c) 50%, and (d) 70% compression
ratio; (e) FTIR spectrum of untreated wood and CT wood with different compression ratios; (f) X-ray
diffraction patterns of untreated wood and CT wood with different compression ratios.
3.2.2. FTIR Spectra and X-ray Diffraction Pattern
Figuree shows that the absorption peak intensity of CT wood near 3363 cm
1
(OH group) was reduced, indicating a reduction in theOH group that was mainly
caused by the degradation of hemicellulose during the thermal modication process. The
absorption peak decreased at 1731 cm
1
(C=O group), further conrming the degradation
of hemicellulose. Figuref describes that the diffraction peak of the I 002crystal plane was
still located near 22

after the combined treatments, indicating that the cellulose crystal
form did not change without dissolution or chemical treatment [47]. The crystallinity of the
untreated wood and the CT wood with compression ratios of 30%, 50%, and 70% was 44%,
46%, 49%, and 49%, respectively. With the increase in the compression ratio, the crystallinity
also increased. During the compression process, the distance between the cellulose brils
in the amorphous region was reduced so that the brils were arranged more closely; the
transverse compression caused the brils to only move in the cross-section, which aided the
brils orientally arranged along the direction of the crystalline region [48–50]. In addition,
the thermal modication itself removed partial hemicellulose in the amorphous region,
which promoted the improvement in the crystallinity. All these effects will comprehensively
contribute to the improvement in the mechanical properties of wood.
3.2.3. Density
Figurea shows that the density of the wood increased almost linearly with the
compression ratio. When the ratio reached 70%, the poplar wood's density increased by
approximately two times. Although the density of the compressed wood decreased after
the post-vacuum-thermo modication due to the degradation of hemicellulose, there was
no signicant change. When the compression ratio was 70%, the thermal modication
treatment only reduced the wood's density by 4%. This indicates that such a combined
treatment could effectively reduce the density change in wood and, accordingly, promote
the dimensional stability and mechanical properties of the modied wood [51–53].

Polymers2022,14, 1571 9 of 13Polymers 2022, 13, x FOR PEER REVIEW 9 of 13



Figure 5. Results of (a) density; (b) hardness; (c) MOR; (d) impact toughness; (e) TG curves; (f) ASE
after the water absorption of the CT wood.
3.2.4. Mechanical Properties
Figure 5b shows that the hardness of the compressed wood without post-thermal
modification increased significantly with the compression ratio, and the maximum was
approximately six times that of the uncompressed wood (i.e., untreated wood) [51,53,54].
An increase in the compression ratio resulted in an increase in density, which led to the
significant increase in hardness. The hardness of the CT wood was slightly lower than that
of the compressed wood, but the decrease in the compressed wood, reduced by thermal
modification, was not obvious when compared to the hardness increase in the wood
caused by compression. When the compression ratio reached 70%, the maximum hard-
ness was 69 N/mm
2
, which was four times that of the original wood. Similar to the above
hardness change, all the MOR and impact toughness of the compressed wood increased
with the compression ratio, no matter before and after the post-vacuum-thermo treat-
ments (Figure 5c,d; when the compression ratio was 70%, the MOR and impact toughness
attained 206 MPa and 73 KJ/m
2
, which was 164% and 86% higher than that of the untreated
wood, respectively [55,56]. In the same way, the MOR and impact toughness of the CT
wood was slightly lower than that of the compressed wood without the post-thermal
modification, respectively. However, compared with the increase in the MOR and impact
toughness of the compressed wood, the decrease of both values of the CT wood reduced
by thermal modification was not significant. When the compression ratio reached 70%,
the MOR and impact toughness still achieved 176 MPa and 63 KJ/m
2
, which was 125% and
59% higher than that of the untreated wood, respectively. With the progress in compres-
sion, the arrangement of cellulose fibers became closer and the interaction force became
stronger; the thermal modification removed partial hemicellulose in the amorphous re-
gion, and the compression process induced a more oriented fiber arrangement, resulting
in a relative increase in crystallinity; therefore, the mechanical properties showed a slope-
increasing improvement [10–12,14]. Consequently, combining pre-compression and post-
vacuum-thermo treatments could significantly improve the hardness, bending strength,
and impact toughness of wood [24,57].
3.2.5. Thermogravimetry
In addition, the maximum thermal degradation temperature of CT wood also
changed, to a certain extent, with the increase in the compression ratio (Figure 5e). The
maximum degradation temperature was 9 °C higher than that of untreated wood at a
compression ratio of 50%. When the compression ratio was 30% and 70%, it was slightly
lower than that of untreated wood, respectively, but the difference was not significant.
Figure 5.Results of (a) density; (b) hardness; (c) MOR; (d) impact toughness; (e) TG curves; (f) ASE
after the water absorption of the CT wood.
3.2.4. Mechanical Properties
Figureb shows that the hardness of the compressed wood without post-thermal
modication increased signicantly with the compression ratio, and the maximum was
approximately six times that of the uncompressed wood (i.e., untreated wood) [51,53,54].
An increase in the compression ratio resulted in an increase in density, which led to the
signicant increase in hardness. The hardness of the CT wood was slightly lower than that
of the compressed wood, but the decrease in the compressed wood, reduced by thermal
modication, was not obvious when compared to the hardness increase in the wood
caused by compression. When the compression ratio reached 70%, the maximum hardness
was 69 N/mm
2
, which was four times that of the original wood. Similar to the above
hardness change, all the MOR and impact toughness of the compressed wood increased
with the compression ratio, no matter before and after the post-vacuum-thermo treatments
(Figurec,d); when the compression ratio was 70%, the MOR and impact toughness attained
206 MPa and 73 KJ/m
2
, which was 164% and 86% higher than that of the untreated wood,
respectively [55,56]. In the same way, the MOR and impact toughness of the CT wood was
slightly lower than that of the compressed wood without the post-thermal modication,
respectively. However, compared with the increase in the MOR and impact toughness of
the compressed wood, the decrease of both values of the CT wood reduced by thermal
modication was not signicant. When the compression ratio reached 70%, the MOR
and impact toughness still achieved 176 MPa and 63 KJ/m
2
, which was 125% and 59%
higher than that of the untreated wood, respectively. With the progress in compression, the
arrangement of cellulose bers became closer and the interaction force became stronger;
the thermal modication removed partial hemicellulose in the amorphous region, and the
compression process induced a more oriented ber arrangement, resulting in a relative
increase in crystallinity; therefore, the mechanical properties showed a slope-increasing
improvement [10–12,14]. Consequently, combining pre-compression and post-vacuum-
thermo treatments could signicantly improve the hardness, bending strength, and impact
toughness of wood [24,57].

Polymers2022,14, 1571 10 of 13
3.2.5. Thermogravimetry
In addition, the maximum thermal degradation temperature of CT wood also changed,
to a certain extent, with the increase in the compression ratio (Figuree). The maximum
degradation temperature was 9

C higher than that of untreated wood at a compression
ratio of 50%. When the compression ratio was 30% and 70%, it was slightly lower than that
of untreated wood, respectively, but the difference was not signicant. From what is shown,
we can see that the combined treatments of pre-compression and post-vacuum-thermo
modication had little signicant effect on the thermal stability of wood.
3.2.6. ASE
Figuref showed that all the ASE of the compressed wood with different compression
ratios were improved after the thermal modication. Especially, the ASE increase of CT
wood with compression ratio of 50% was 28% higher than that of uncompressed and the
only thermally modied wood. Therefore, thermal modication improved the dimensional
stability signicantly, especially for the compressed wood [34].
In short, combining pre-compression and post-vacuum-thermo treatments effectively
and simultaneously improved the mechanical properties and dimensional stability of wood,
which was the expected outcome of the study. Generally, with increase in the compression
ratio, the mechanical properties of the wood improved, while the ASE declined, and the
dimensional stability accordingly decreased. In this study, with the thermal modication
combined the pre-compression, bending strength, and impact toughness of the CT wood
increased signicantly with the compression ratio; meanwhile, it maintained a satised
ASE, indicating a simultaneously improved dimensional stability of the wood [14,45,58].
4. Conclusions
Focusing on the defects of poor mechanical properties and dimensional stability of
poplar wood caused by its soft substrate, this study explored the method of combining
pre-compression and post-vacuum-thermo modication and analyzed the variation law of
mechanical properties of CT wood with different compression ratios and the improvement
in the dimensional stability. The conclusions are as follows:
1.
With the increase in thermal modication temperature and time, hemicellulose and
other substances gradually decomposed and changed, resulting in decreases in the
mechanical properties of wood and improvement in the dimensional stability of wood.
Considering both the mechanical properties and dimensional stability, 190

C and
10 h were determined as the optimum conditions for thermal modication;
2.
With the increase in the compression ratio, the cell porous structure of the poplar wood
became denser, which improved the density, hardness, MOR, and impact toughness
but adversely affected the dimensional stability. After further treatment with thermal
modication, all of the poplar wood with different compression ratios presented im-
proved ASEs and a reduced negative impact on the mechanical properties. The MOR
and impact toughness of CT wood at a compression ratio of 70% was 176 MPa and
63 KJ/m
2
, which was 125% and 59% higher than that of untreated wood, respectively.
The ASE of the CT wood was 26% higher than that of the only compressed poplar
wood. Consequently, such a method could improve the mechanical properties and
dimensional stability of poplar wood simultaneously and signicantly, and it can
be applied to oors, load-bearing walls, etc. Further research on larger-scale poplar
and other wood species will provide a scientic basis for large-scale application of
this method.
Author Contributions:
Conceptualization, X.D., C.L. and Y.L.; methodology, G.Z., X.D. and Y.L.;
investigation, G.Z., Z.H. and Y.Q.; resources, Y.Z., Y.D. and B.L.; writing—original draft preparation,
Z.H., Y.Q., G.Z., C.L. and Y.L.; visualization, Z.H. and G.Z.; writing—review and editing, Z.H., Y.Q.,
C.L. and Y.L. All authors have read and agreed to the published version of the manuscript.

Polymers2022,14, 1571 11 of 13
Funding:
This research was funded by the Project of Shandong Provincial Natural Science Foundation
(Funder: Natural Science Foundation of Shandong Province; Grant No. ZR2021MC095), the Project
of Shandong Provincial Agricultural Science and Technology Foundation (Forestry Science and
Technology) (Funder: Department of Natural Resources of Shandong Province; Grant No. 2019LY008),
Science and Technology Support Program for Youth Innovation Team in Colleges and Universities of
Shandong Province, China (Funder: Shandong Education Department; Grant No. 2020KJF012), Key
Laboratory of Bio-based Material Science & Technology (Northeast Forestry University), Ministry
of Education (Funder: Northeast Forestry University; Grant No. SWZ-MS201912), and Jiangsu
Provincial Policy Guidance Program (Special Project of Science and Technology for Northern Jiangsu)
(Funder: Jiangsu Provincial Department of Science and Technology; Grant No. SZ-SQ2020031).
Institutional Review Board Statement:Not applicable.
Informed Consent Statement:Not applicable.
Conicts of Interest:The authors declare no conict of interest.
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