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Physical and Mechanical Properties of Paulownia tomentosa x elongata Sawn Wood from Spanish, Bulgarian and Serbian Plantations

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The aim of this research is the characterization of physical and mechanical properties of Paulownia sawn wood from three plantation sites in Europe, namely Spain, Bulgaria and Serbia. As a fast-growing wood species, Paulownia has a significant positive forecast for the European markets and a wide range of possible applications that still need to be explored. For this purpose, Paulownia tomentosa(Tunb.) x elongata(S.Y. Hu) wood species was investigated. Sorption behaviour, Brinell hardness, 3-point bending strength, flexural modulus of elasticity, tensile strength, compressive strength and screw withdrawal resistance were examined in detail. The samples from Spain have the higher average bulk density (266 kg/m3), 3-point flexural strength (~40 N/mm2), 3-point flexural modulus of elasticity (~4900 N/mm2), compressive strength (~23 N/mm2), tensile strength (~44 N/mm2) and screw withdrawal resistance (~56 N/mm). The plantation wood from Bulgaria has the highest average of annual ring width (46 mm). Paulownia wood has potential in lightweight applications and can replace successfully expensive tropical species as Balsa.
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Citation: Barbu, M.C.; Buresova, K.;
Tudor, E.M.; Petutschnigg, A.
Physical and Mechanical Properties
of Paulownia tomentosa x elongata
Sawn Wood from Spanish, Bulgarian
and Serbian Plantations. Forests 2022,
13, 1543. https://doi.org/10.3390/
f13101543
Academic Editor: Ian D. Hartley
Received: 2 September 2022
Accepted: 19 September 2022
Published: 21 September 2022
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4.0/).
Article
Physical and Mechanical Properties of Paulownia tomentosa x
elongata Sawn Wood from Spanish, Bulgarian and Serbian
Plantations
Marius Cătălin Barbu 1,2 , Katharina Buresova 1, Eugenia Mariana Tudor 1,2, * and Alexander Petutschnigg 1,3
1Forest Products Technology and Timber Construction Department, Salzburg University of Applied Sciences,
Markt 136a, 5431 Kuchl, Austria
2Faculty of Furniture Design and Wood Engineering, Transilvania University of Brasov, B-dul. Eroilor nr. 29,
500036 Brasov, Romania
3Institute of Wood Technology and Renewable Materials, University of Natural Resources and Life
Sciences (BOKU), Konrad Lorenz-Straße 24, 3340 Tulln an der Donau, Austria
*Correspondence: eugenia.tudor@fh-salzburg.ac.at
Abstract:
The aim of this research is the characterization of physical and mechanical properties of
Paulownia sawn wood from three plantation sites in Europe, namely Spain, Bulgaria and Serbia. As
a fast-growing wood species, Paulownia has a significant positive forecast for the European markets
and a wide range of possible applications that still need to be explored. For this purpose, Paulownia
tomentosa(Tunb.) x elongata(S.Y. Hu) wood species was investigated. Sorption behaviour, Brinell
hardness, 3-point bending strength, flexural modulus of elasticity, tensile strength, compressive
strength and screw withdrawal resistance were examined in detail. The samples from Spain have the
higher average bulk density (266 kg/m
3
), 3-point flexural strength (~40 N/mm
2
), 3-point flexural
modulus of elasticity (~4900 N/mm
2
), compressive strength (~23 N/mm
2
), tensile strength (~44
N/mm
2
) and screw withdrawal resistance (~56 N/mm). The plantation wood from Bulgaria has
the highest average of annual ring width (46 mm). Paulownia wood has potential in lightweight
applications and can replace successfully expensive tropical species as Balsa.
Keywords:
Paulownia tomentosa x elongata; plantation wood; lightweight; physical and mechani-
cal properties
1. Introduction
Of Asian origin, Paulownia is a fast-growing deciduous tree, with at least nine sub-
species [
1
]. In Europe, in the last decade, is growing interest on Paulownia as regards
tree cultivation and agro-forestry plantations for industrial use [
2
]. Paulownia was also
introduced in North America, Australia and Japan [
3
], and is cultivated worldwide in more
than 40 countries [
4
]. Other objectives of Paulownia plantations are to reduce soil hazards
by tree-crop intercropping in farmlands [
5
], to protect systems against erosion, flooding or
wind damage [
6
], to reduce air pollution and to secure the increasing energy demand [
7
].
Paulownia trees have exceptional root systems and can adapt easily to various soil condi-
tions [
8
]. Moreover, Paulownia has high tolerance to drought and salinity, representing
an easy solution for sand fixation, also for water and soil conservation [
9
]. Nevertheless,
the optimum soil and climate conditions and the most suitable plantation sites have not
been clarified yet, and at present there is no best practice guidance for forest farmers who
intent to manage Paulownia plantations [
10
]. This lightweight tree species gained interest
worldwide and is named a miracle tree, empress tree or princess tree [
11
], due to its high
rate carbon absorption and rated as fast-growing energy crop with C4 photosynthesis [
12
],
easy processability, and good fire resistance [13].
Paulownia trees can be harvested in 6–7 years for low-quality lumber and in 15 years
for worthwhile timber. The height of an adult Paulownia tree is from 10 to 20 m, its growth
Forests 2022,13, 1543. https://doi.org/10.3390/f13101543 https://www.mdpi.com/journal/forests
Forests 2022,13, 1543 2 of 12
rate is up to 3 m in one year (in ideal conditions), allowing exploitation in short rotation
periods. The diameter of a 10 year Paulownia is 30–40 cm with a timber volume of 0.3–
0.5 m
3
per log [
3
]. Paulownia wood is semi-ring porous to ring porous, soft, frequently
knot free, with an average density under 300 kg/m
3
[
14
]. The wood is light coloured, soft,
lightweight and easy to handle [
15
]. It dries quickly and is easy to shape; these properties
recommend it for industrial applications as furniture, building timber, packaging, plywood,
insulation [
16
], for sculptures and handicrafts [
17
] and as reinforcing filler for thermoplastic
composites [
18
]. The primary use of Paulownia includes solid wood products, veneer,
and pulp as a source for fine papers [
19
]. Products made from Paulownia wood do not
warp, crack, deform or decay easily [
18
], and are rot resistant [
20
]. It has a low thermal
conductivity and a high ignition point [
14
,
19
]. Less valuable stems can be chipped to
produce biofuels, biomass, electricity and contribute to air purification [
21
] or can be used
as raw materials for particleboard, OSB and MDF production [
22
]. Other uses of Paulownia
stems were introduced by the Chinese for medical purposes as a component of remedies
for infections, e.g., poliovirus [23], due to its antioxidant properties [24].
The variability of Paulownia species depends on plantation site, climatic conditions,
irrigations and forestry management [
25
]. Agro-forestry Paulownia plantations ensure
sustainability for small rural communities. The trees represent a source of lumber, firewood,
compost and coal and can easily adapt to new places. The Paulownia leaves are rich in
nitrogen and can be introduced as feed for livestock [26].
Although Paulownia has a significant value for agroforestry, wood processing in-
dustry and ensuring ecological maintenance, this wood species is still under-rated and
understudied [
9
]. Paulownia tomentosa is anywise considered invasive species, so its future
spread should be attentive managed [21,27].
The objective of this study is to determine the physical and mechanical properties
of Paulownia tomentosa x elongata sawn wood, grown in Spain, Bulgaria and Serbia and to
compare it with similar Paulownia plantation wood from Europe and with other lightweight
wood species as Balsa and poplar.
2. Materials and Methods
The Paulownia wood (Paulownia tomentosa x elongata) was provided from Glendor
Holding GmbH (Kilb, Austria) and originates from plantations from Spain, Bulgaria and
Serbia. Mostly juvenile wood samples from 5–7 years old trees were used for the tests. The
plantation wood was delivered as rough-sawn lumber of approximately 150 cm length, 20–
30 cm width and thickness of 20–25 mm. Prior to testing, the raw material was conditioned
to constant weight at 20
C and 65% relative air humidity for at least 14 days, until constant
weight was achieved.
Several tests were carried out to determine the physical and mechanical properties of
Paulownia wood from Spain, Bulgaria and Serbia (Table 1).
Table 1.
Tests, norms, sample dimension and number of testing specimens for Paulownia wood from
Spain, Bulgaria and Serbia.
Test Norm n
Samples nr.
Sample Dimension
[mm]
Swelling and shrinkage
Bulk density (kg/m3)
DIN 52184:1979-05
ISO 3131:1996
12
12 20 ×20 ×10
Brinell-hardness (N/mm2)EN 1534:2011-01 10 -
3-point modulus of elasticity (MOE) (N/mm2)
3-point modulus of rupture (MOR) (N/mm2)
DIN 52186:1978-06
DIN 52186:1978-06 12 20 ×20 ×360
Compressive strength (N/mm2)DIN 52185:1976-09 12 20 ×20 ×50
Tensile shear strength (N/mm2)DIN 52188:1979-05 15 20 ×6 at predetermined
breaking point
Screw withdrawal resistance (N/mm) EN 320:2011-07 9 50 ×50
Forests 2022,13, 1543 3 of 12
Physical and mechanical properties of the Paulownia wood samples were evaluated
according to European and German norms, specific for massive wood testing.
The differential swelling and shrinking (in all cutting directions of wood) after 24 h water
immersion was determined according to DIN 52184:1979-05 [
28
]. To measure the density
according to ISO 3131:1996 [
29
], the weight and dimensions (20 mm
×
20 mm
×
10 mm) of
samples were measured (i) after conditioning and (ii) after 24 h kiln drying.
The annual ring width was calculated as mean value from two opposing radii manually.
The authors would like to emphasize that the reported values are indicative.
The test for the Brinell-hardness, with respect to EN 1534:2011-01 [
30
], was performed
with the hardness tester Emco Test Automatic (Kuchl, Austria). The metal ball, with a
diameter of 10 mm, was pressed with the force ranging from 199–207 N into the three main
directions (radial, tangential and axial).
The tests for mechanical properties as bending strength, compressive strength, tensile
strength and screw withdrawal resistance were carried out using a universal testing ma-
chine Zwick Roell Z 250 (Ulm, Germany). For the bending test, the specimen was placed
on two bearings and continuously loaded with vertical force until breakage, according to
DIN 52186:1978-06 [
31
]. During the test, a sensor records the bending stress in N/mm
2
and
thus determines the bending strength—modulus of rupture (MOR) and flexural modulus
of elasticity (MOE).
According to DIN 52185:1976-09 [
32
], for the compression test the specimen is loaded
in vertical direction with an increasing force until breakage.
The tensile strength was measured in the fibre direction with samples prepared as
DIN 52188 [33] requires (Figure 1).
Forests 2022, 13, x FOR PEER REVIEW 3 of 12
3-point modulus of rupture (MOR) (N/mm2) DIN 52186:1978-06
Compressive strength (N/mm2) DIN 52185:1976-09 12 20 × 20 × 50
Tensile shear strength (N/mm2) DIN 52188:1979-05 15
20 × 6 at predetermined
breaking point
Screw withdrawal resistance (N/mm) EN 320:2011-07 9 50 × 50
Physical and mechanical properties of the Paulownia wood samples were evaluated
according to European and German norms, specific for massive wood testing.
The differential swelling and shrinking (in all cutting directions of wood) after 24 h
water immersion was determined according to DIN 52184:1979-05 [28]. To measure the
density according to ISO 3131:1996 [29], the weight and dimensions (20 × 20 × 10 mm) of
samples were measured (i) after conditioning and (ii) after 24 h kiln drying.
The annual ring width was calculated as mean value from two opposing radii
manually. The authors would like to emphasize that the reported values are indicative.
The test for the Brinell-hardness, with respect to EN 1534:2011-01 [30], was performed
with the hardness tester Emco Test Automatic (Kuchl, Austria). The metal ball, with a
diameter of 10 mm, was pressed with the force ranging from 199–207 N into the three
main directions (radial, tangential and axial).
The tests for mechanical properties as bending strength, compressive strength, tensile
strength and screw withdrawal resistance were carried out using a universal testing
machine Zwick Roell Z 250 (Ulm, Germany). For the bending test, the specimen was
placed on two bearings and continuously loaded with vertical force until breakage,
according to DIN 52186:1978-06 [31]. During the test, a sensor records the bending stress
in N/mm2 and thus determines the bending strength—modulus of rupture (MOR) and
flexural modulus of elasticity (MOE).
According to DIN 52185:1976-09 [32], for the compression test the specimen is loaded
in vertical direction with an increasing force until breakage.
The tensile strength was measured in the fibre direction with samples prepared as
DIN 52188 [33] requires (Figure 1).
Figure 1. Measuring of the tensile strength of a Paulownia sample from Bulgaria with universal
testing machine Zwick Roell Z 250 (in the background the device Emco Test Automatic for testing
of Brinell hardness).
The screw withdrawal resistance was tested on samples cut according to EN 320:2011
[34] (Figure 2). The force was applied perpendicular to the screw. The standardized screw
with a diameter of 4.2 mm and a nominal length of 38 mm was countersunk into the wood
Figure 1.
Measuring of the tensile strength of a Paulownia sample from Bulgaria with universal
testing machine Zwick Roell Z 250 (in the background the device Emco Test Automatic for testing of
Brinell hardness).
The screw withdrawal resistance was tested on samples cut according to EN 320:2011 [
34
]
(Figure 2). The force was applied perpendicular to the screw. The standardized screw with
a diameter of 4.2 mm and a nominal length of 38 mm was countersunk into the wood
at a right angle to the grain. In this case predrilling was carried out in the pillar drilling
machine with a 2.8 mm drill bit. The specimens were fixed to the universal testing machine
Zwick Roell Z 250 (Ulm, Germany) in a special fixture with the screw clamped at the top.
The screw was pulled out with a specific and constant force.
Forests 2022,13, 1543 4 of 12
Forests 2022, 13, x FOR PEER REVIEW 4 of 12
at a right angle to the grain. In this case predrilling was carried out in the pillar drilling
machine with a 2.8 mm drill bit. The specimens were fixed to the universal testing machine
Zwick Roell Z 250 (Ulm, Germany) in a special fixture with the screw clamped at the top.
The screw was pulled out with a specific and constant force.
Figure 2. Measuring of the screw withdrawal resistance of a Paulownia sample from Spain with
universal testing machine Zwick Roell Z 250.
3. Results and Discussion
The results of this study include density (Table 2), sorption behaviour (Table 3),
width of annual rings (Table 4), Brinell hardness (Table 5), 3-point-bending strength
(Table 6), flexural 3-point-modulus of elasticity (Table 7), compressive strength (Table 8),
tensile strength (Table 9) and screw withdrawal resistance (Table 10). For each physical
and mechanical property, Paulownia wood sourced from Spain, Bulgaria and Serbia was
compared with the values of Paulownia wood from other European plantations and with
Balsa, poplar and spruce from the literature sources.
3.1. Density (ISO 3131:1996)
Physical properties of wood, especially density and water-related properties, are
important factors affecting wood quality [35].
Table 2. Basic statistics for bulk density of Spanish, Bulgarian and Serbian Paulownia plantation
wood and other Paulownia species from the literature; n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) =
12 (standard deviation in parentheses) (kg/m3).
Wood Type
Mean Value
Bulk Density
(kg/m3)
Min/Max
(kg/m3) Source
Paulownia tomentosa x elongata (Spain) 266 (22) 238/297 Present study
Paulownia tomentosa x elongata (Bulgaria) 250 (26) 198/307 Present study
Paulownia tomentosa x elongata (Serbia) 259 (31)
201/313
152/237
262/360
178/270
179/270
Present study
Paulownia tomentosa (Hungary) 246 Koman and Feher [1]
Paulownia tomentosa (Hungary) 300 (26.59) Koman and Vityi (2017)
Paulownia tomentosa (Türkiye) 272 Akyildiz and Kol [3]
Paulownia tomentosa (Portugal) 460 Estevez et al. [11]
Paulownia COTE-2 (Spain) 216 Lachowiz et al. [36]
Paulownia Sp. Siebold and Zucc. (Bulgaria) 220 Bardarov and Popovska [37]
Figure 2.
Measuring of the screw withdrawal resistance of a Paulownia sample from Spain with
universal testing machine Zwick Roell Z 250.
3. Results and Discussion
The results of this study include density (Table 2), sorption behaviour (Table 3), width
of annual rings (Table 4), Brinell hardness (Table 5), 3-point-bending strength (Table 6), flex-
ural 3-point-modulus of elasticity (Table 7), compressive strength (Table 8), tensile strength
(Table 9) and screw withdrawal resistance (Table 10). For each physical and mechanical
property, Paulownia wood sourced from Spain, Bulgaria and Serbia was compared with
the values of Paulownia wood from other European plantations and with Balsa, poplar and
spruce from the literature sources.
3.1. Density (ISO 3131:1996)
Physical properties of wood, especially density and water-related properties, are
important factors affecting wood quality [35].
Table 2.
Basic statistics for bulk density of Spanish, Bulgarian and Serbian Paulownia plantation
wood and other Paulownia species from the literature; n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) = 12
(standard deviation in parentheses) (kg/m3).
Wood Type Mean Value Bulk Density
(kg/m3)
Min/Max
(kg/m3)Source
Paulownia tomentosa x elongata (Spain) 266 (22) 238/297 Present study
Paulownia tomentosa x elongata (Bulgaria)
250 (26) 198/307 Present study
Paulownia tomentosa x elongata (Serbia) 259 (31)
201/313
152/237
262/360
178/270
179/270
Present study
Paulownia tomentosa (Hungary) 246 Koman and Feher [1]
Paulownia tomentosa (Hungary) 300 (26.59) Koman and Vityi (2017)
Paulownia tomentosa (Türkiye) 272 Akyildiz and Kol [3]
Paulownia tomentosa (Portugal) 460 Estevez et al. [11]
Paulownia COTE-2 (Spain) 216 Lachowiz et al. [36]
Paulownia Sp. Siebold and Zucc.
(Bulgaria) 220 Bardarov and Popovska [37]
Balsa 160 Wiekiping and Doyle [38]
Poplar 440 Grosser [39]
Spruce 430 Grosser [39]
Table 2shows the values of density for Spanish, Bulgarian and Serbian Paulownia
plantation wood measured according to ISO 3131:1996 [
29
]. Results from the technical
literature about Paulownia and other lightweight wood species are listed for comparison.
Forests 2022,13, 1543 5 of 12
The average density of wood from all three sites for Paulownia tomentosa x elongata is
258 kg/m
3
. The Paulownia wood from Spain had the highest average density of 266 kg/m
3
,
followed by the Serbian wood with 259 kg/m
3
, and the lowest average value had the
Bulgarian wood with 250 kg/m3(Table 2).
In their study, Akyildiz and Kol [
3
] determined an average density of 272 kg/m
3
for
the basic species Paulownia tomentosa from Türkiye. This value is lower for Paulownia
wood from Hungary at 246 kg/m
3
[
1
] or from Spain at 215 kg/m
3
[
36
]. Estevez et al. [
11
]
reported a value of 460 kg/m
3
for Portuguese Paulownia wood, which is even higher than
the average density of spruce with 430 kg/m3according to [39].
Lachowicz et al. [
36
] measured the lowest value (Paulownia wood sourced from Spain)
with a mean density of 216 kg/m
3
. Considering the values reported from [
40
,
41
], Balsa
wood has a lower density of 160 kg/m
3
. The lightweight hardwood species poplar, has a
density of 440 kg/m3[39,42].
At 12% moisture content, Paulownia wood density varies from 220 to 350 kg/m
3
, with
an average of 270 kg/m
3
[
5
]. This variability in density is determined by growth conditions.
Higher Paulownia densities, about 400 kg/m
3
, were reported for Paulownia tomentosa [
3
,
11
],
and for Siebold and Zucc. (Bulgaria) [37].
3.2. Sorption Behavior (DIN 52184:1979)
The measurement of the sorption behaviour for Spanish, Bulgarian and Serbian
Paulownia wood was carried out according to DIN 52184:1979 [
28
] and are shown in
Table 3. From raw 4 to raw 7 (Table 3) are listed comparative results from the literature.
Table 3.
Differential swelling and shrinkage of Spanish, Bulgarian and Serbian Paulownia wood com-
pared to other Paulownia species from the literature; n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) = 12
(standard deviation in parenthesis) (%).
Wood Type Mean Value Axial (%) Mean Value
Radial (%)
Mean Value
Tangential (%) Source
Paulownia (Spain) 0.375 (0.048) 0.504 (0.077) 1.58 (0.234) Present study
Paulownia (Bulgaria) 0.157 (0.057) 0.52 (0.17) 0.978 (0.181) Present study
Paulownia (Serbia) 0.199 (0.050) 0.456 (0.087) 1.266 (0.277) Present study
Paulownia (Hungary) 0.69 3.2 5 Koman and Feher [1]
Paulownia (Türkiye) 0.07 0.17 Akyildiz and Kol [3]
Paulownia (Spain) 0.172 (0.118) 1.99 (0.44) 5.19 (0.62) Lachowicz et al. [36]
Paulownia (Croatia) 0.35 (0.332) 2.47 (0.631) 5.3 (0.969) Sedlar et al. [35]
Spanish Paulownia wood had shrinkage in the axial direction of 0.375%, in the radial
direction of 0.50%. In the tangential direction, with 1.58%, Paulownia wood has the highest
average values for shrinkage. Bulgarian Paulownia wood has an axial shrinkage of 0.157%,
in radial direction 0.52%, and in tangential direction 0.978%. Serbian paulownia wood has
the shrinkage in axial direction of 0.199%, a radial shrinkage of 0.456%, and a tangential
shrinkage of 1.266%.
It can be observed that the Bulgarian Paulownia wood swells and shrinks the least
in all cutting directions. Compared to the Paulownia, Balsa wood has a lower sorption
behaviour. In tangential direction, it shrinks and swells between 3.4%–7%. Radial shrinkage
is 1.4%–2.1% and volume shrinkage is 5.1%–9.3% [38].
It is important to emphasize the lower ratios of swelling. This behaviour of Paulownia
wood can be attributed to narrower core rays. The rays are narrow, occupying a single row
up to 0.5 mm, but also multi-seriate rays can occur [
5
]. Firstly, the core rays control the
wood in a radial direction and ensure values of swelling up to 4% [
35
], such as for most
species (at this density). Secondly, the small width of core rays did not influence higher
rates of swelling in tangential direction.
Forests 2022,13, 1543 6 of 12
3.3. Width of Annual Rings
The widths of annual rings for Spanish, Bulgarian and Serbian Paulownia wood are
shown in Table 4.
Table 4.
Tree ring width—comparison of Spanish, Bulgarian and Serbian Paulownia wood.
n (Spain) = 59; n (Bulgaria) = 7; n (Serbia) = 18 (standard deviation in parentheses) (cm).
Wood Type Mean Value Annual Ring Width (cm) Min./Max.
(cm)
Paulownia (Spain) 2.8 (1.08) 1.2/7.5
Paulownia (Bulgaria) 4.6 (0.62) 3.7/5.7
Paulownia (Serbia) 1.7 (6.77) 0.6/3.1
The average annual ring width of entire batch of Serbian Paulownia wood was 1.7 cm.
Paulownia trees from Spain had larger annual ring width of 2.83 cm, but the largest annual
ring width was measured for Bulgarian Paulownia, namely 4.6 cm.
Serbian Paulownia wood had the smallest annual ring width, which is due to soil and
climatic conditions. The tree ring width decreases as the height of the tree increases. The
diameter of the tree tapers with increasing height. Thus, the diameter decreases, and the
annual ring width consequently decreases. As already noted by [
16
], there are very large
fluctuations in tree ring width within the first five years (up to 30%) (from 1 to 3.5 cm).
From the beginning of the fifth year, the annual ring width becomes constant and is hardly
subject to fluctuations anymore.
3.4. Brinell Hardness (DIN 1534:2022)
The testing of Brinell hardness for Spanish, Bulgarian and Serbian Paulownia wood
was carried out according to DIN 1534:2022 (Table 5). In this Table 5, after the third row, are
listed comparative results from the literature.
Table 5.
Brinell hardness—in axial, radial and tangential directions—comparison of Spanish, Bul-
garian and Serbian Paulownia wood with other wood species. n (Spain) = 10; n (Bulgaria) = 10; n
(Serbia) = 10 (standard deviation in brackets) (N/mm2).
Wood Species Mean Value Brinell Hardness (N/mm2)Source
Axial Radial Tangential
Paulownia (Spain) 20.6 (5.56) 5.6 (1.53) 4.8 (1.19) Present study
Paulownia (Bulgaria) 18.7 (3.1) 5.6 (1.35) 5.3 (1.35) Present study
Paulownia (Serbia) 21.22 (7,64) 6.1 (3.23)
5.81 (2.13)
9.13 (2.16)
9.016 (0.23)
Present study
Paulownia (Hungary) 26.74 (3.22) 9.51 (2.17) Koman and Vityi [16]
Paulownia (Bulgaria) 20 Bardanovand
Popovska [37]
Paulownia (Türkiye) 19.7 (0.37) 8.23 (0.09) Akyildiz and Kol [3]
Balsa 7 Finger [43]
Black poplar 25–33 10–15
Richter and Ehmke [
44
]
Spruce 32 12
Richter and Ehmke [
44
]
In the case of Paulownia wood source from Bulgaria, Brinell hardness in axial direction
was 18.7 N/mm
2
, 5.6 N/mm
2
in radial direction and 5.3 N/mm
2
in tangential direction.
For Paulownia wood from Spain was measured the Brinell hardness in axial direction of
21.22 N/mm
2
, 6.1 N/mm
2
in radial direction and 5.81 N/mm
2
in tangential direction.
For Paulownia wood from Serbia were measured the highest values of Brinell hardness:
21.22 N/mm
2
in axial direction, 6.1 N/mm
2
in radial direction and 5.8 N/mm
2
in tan-
gential direction. The latter values in axial direction are consistent with the results of [
37
]
and [
16
]. Compared to Balsa wood, with a Brinell hardness of 7 N/mm
2
[
45
], Paulownia
Forests 2022,13, 1543 7 of 12
has significantly increased hardness. Other lightweight hardwood species is poplar, with a
hardness of 25–33 N/mm2[43], in concordance with the results of [16] of 26.74 N/mm2.
3.5. Modulus of Rupture and Modulus of Elasticity (DIN 52186:1978)
Table 6shows the measured values of the three-point bending tests (modulus of
rupture, MOR) for Paulownia wood sourced from Spain, Bulgaria and Serbia, measured
according to DIN 52186:1978. The fourth to eight rows in Table 6show the comparative
results from the literature.
Table 6.
3-point bending strength (MOR)—comparison of Spanish, Bulgarian and Serbian Paulownia
wood with other wood species. n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) = 12 (standard deviation
in parentheses) (N/mm2).
Wood Species Mean Values MOR
[N/mm2]
Min./Max.
[N/mm2]Source
Paulownia (Spain) 39.77 (6.98) 28.96/50.5 Present study
Paulownia (Bulgaria) 35.53 (5.53) 24.57/43.99 Present study
Paulownia (Serbia) 37.54 (8.54) 24.84/59.48 Present study
Paulownia (Bulgaria) 35 Baranov and Popovska [37]
Paulownia (Türkiye) 43.56 (7.00) 33.36/60.37 Akyildiz and Kol [3]
Paulownia (Hungary) 32.3 (4.68) 28.65/48.65 Koman and Vityi [1,16]
Paulownia (Portugal) 53.5 (6) - Esteves et al. [11]
Paulownia (Spain) 38.63 23.89/53.17 Lachowicz et al. [36]
Balsa 16.63 (1.72)
-
Kotlarewski et al. [45]
Spruce 80 Richter and Ehmcke [44]
Oak 95 Richter and Ehmcke [44]
Black poplar 55–65 Richter and Ehmcke [44]
Paulownia wood from Spain achieved the highest flexural modulus of elasticity of
4866.49 N/mm
2
and the highest flexural strength of 39.77 N/mm
2
(Tables 6and 7). The
Bulgarian Paulownia wood had the lowest MOR of 35.53 N/mm
2
. Paulownia wood from
Serbia is in the middle range with 37.54 N/mm2(Table 6).
Jakubowski [
5
] analysed in a review article the mechanical properties of Paulownia
wood and reported a range for static bending strength from 23.98 to 43.56 N/mm
2
[
5
].
Lachowitcz et al. [36] measured a similar bending strength ranging from 23.89 N/mm2to
53.17 N/mm
2
with a mean value of 38.63 N/mm
2
and Esteves et al. [
11
] found a higher
mean value of 53.5 N/mm
2
for Paulownia from Portugal. All these values are at least
two-fold higher compared to MOR for Balsa wood, which is about 17 N/mm
2
[
44
]. The
higher value for MOR achieved by the Palownia from Türkiye [
3
] was at least 20% lower
than that the one for black poplar [
44
]. Spruce has an MOR of 80 N/mm
2
[
44
] that is at
least two fold higher as MOR for Paulownia, which is also the case of oak (95 N/mm
2
) [
44
].
The modulus of elasticity (MOE) for Paulownia ranges from 2651 to 4917 N/mm
2
[
5
]
(Table 7).
Forests 2022,13, 1543 8 of 12
Table 7.
Flexural modulus of elasticity (MOE)—comparison of Spanish, Bulgarian and Serbian
Paulownia wood with other wood species (N/mm2).
Wood Species Mean Values
MOE [N/mm2]
Min./Max.
[N/mm2]Source
Paulownia (Spain) 4866.49 (797.84) 3580/5941 Present study
Paulownia (Bulgaria) 3714.14 (588.51) 2685/4899 Present study
Paulownia (Serbia) 4532.49 (900.92) 2733/6492 Present study
Paulownia (Spain) 1898.75 1167/2690 Lachowicz et al. [36]
Balsa 2900 - Sell [46]
Black Poplar 8800 - Grosser [39]
Spruce 11,000 - Richter and Ehmke [44]
Larch 13,800 Grosser [39]
Oak 13,000 - Grosser [39]
Lachowicz et al. [
36
] measured the lowest modulus of elasticity of Paulownia wood
with 1899 N/mm
2
. This mean value is lower than the minimum value of the wood which
was tests in this study.
When non-destructive methods were employed, a higher modulus of elasticity has
been reported for trees with larger diameters [5].
In comparison, spruce has an MOE of 11,000 N/mm
2
and an MOR of 80 N/mm
2
[
44
].
Thus, spruce achieves a value twice as high. MOE for larch has even higher values, with
a bending MOE of 13,800 N/mm
2
and a MOR of 99 N/mm
2
[
39
]. This value is almost
three times higher than that of Paulownia wood. MOE of Balsa wood is lower, averaging
2900 N/mm
2
[
45
], but still higher than the values resulted from the study of [
36
], namely
1900 N/mm2.
3.6. Compressive Strength (DIN 52185:1976)
The values of the compressive strength tests for Spanish, Bulgarian and Serbian
Paulownia wood measured according to DIN 52188:1979 are shown in Table 8, together
with other Paulownia species from Hungary, Spain and Türkyie and Balsa, spruce and
black poplar.
Table 8.
Compressive strength—comparison of Spanish, Bulgarian and Serbian Paulownia wood
with other wood species. n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) = 12 (standard deviation in
parentheses) (N/mm2).
Wood Species Compressive Strength
[N/mm2]
Min./Max.
[N/mm2]Source
Paulownia (Spain) 22.53 (3.17) 18.7/28.12 Present study
Paulownia (Bulgaria) 18.77 (1.5) 16.25/21.71 Present study
Paulownia (Serbia) 21.41 (4.55) 14.39/32.01 Present study
Paulownia (Hungary) 19.9 (1.78) 19.63/25.24 Koman and Vityi [1,16]
Paulownia (Spain) 14.24 (1.52)
10.45/18.29
20.35/29.42
Lachowicz et al. [36]
Paulownia (Türkyie) 35.56 (6.95) Kaymakci et al. [47]
Paulownia (Türkyie) 25.55 (2.25) Akyildiz and Kol [3]
Balsa 10 Wiekiping and Doyle [38]
Spruce 45 Richter and Ehmke [44]
Black poplar 30 Grosser [39]
Paulownia wood from Spain has a compressive strength of 22.53 N/mm
2
. The Bul-
garian Paulownia wood has a compressive strength of 18.77 N/mm
2
and the Serbian
Paulownia wood has a compressive strength of 21.41 N/mm
2
. Other values of compressive
strength for Paulownia from other plantations are ranging from 25.55 N/mm
2
[
3
] and
Forests 2022,13, 1543 9 of 12
35.56 N/mm
2
[
46
] for Paulownia from Türkyie and significant lower, of 14.24 N/mm
2
, as
results from the research of [36].
In comparison, spruce exhibits a compressive strength of 45 N/mm
2
, which is twice
as high as the determined compressive strength of Paulownia wood [
44
]. The value is
comparatively similar for black poplar, which has a minimum value of 30 N/mm
2
[
39
].
The Balsa wood has a the lowest mean value for compressive strength of 10 N/mm2[38],
3.7. Tensile Strength (DIN 52188:1979-05)
Table 9shows the measured values of the tensile strength tests for Spanish, Bulgarian
and Serbian Paulownia wood according to DIN 52188:1979. The fourth line in Table 9
shows the comparative results from the literature.
Table 9.
Tensile strength—comparison of Spanish, Bulgarian and Serbian Paulownia wood with other
wood species. n (Spain) = 15; n (Bulgaria) = 15; n (Serbia) = 15 (standard deviation in parentheses)
(N/mm2).
Wood Species Tensile Strength
[N/mm2]
Min./Max.
[N/mm2]Source
Paulownia (Spain) 44.12 (10.66) 28.1/64.59 Present study
Paulownia (Bulgaria) 36.17 (6.69) 27.60/51.08 Present study
Paulownia (Serbia) 40.14 (9.11) 25.62/62.27 Present study
Paulownia (Hungary) 33.25 (8.9)
21.86/52.96
Koman and Vityi [16]
Balsa 14 Forest Products Laboratory
[48]
Spruce 95 Richter and Ehmcke [44]
Oak 110 Richter and Ehmcke [44]
Black poplar 77 Richter and Ehmcke [44]
The Spanish Paulownia wood has a tensile strength of 44.12 N/mm
2
. The tensile
strength of Paulownia wood from Bulgaria was 36.17 N/mm
2
and the tensile strength
for the wood from Serbia reached a value of 40.14 N/mm
2
. For Paulownia sourced
from Hungary Koman and Vityi [
16
] reported a tensile strength of 33.25 N/mm
2
, which
is consistent the values presented in this study. The tensile strength of Balsa wood is
considerable lower with 14 N/mm
2
[
47
]. The tensile strength of lightweight species as
black poplar is about 40% higher than that of Paulownia and for spruce is two-fold higher.
In the case of oak, its tensile strength exceeds with at least 60% the values for Paulownia [
44
].
3.8. Screw Withdrawal Resistance (EN 320:2011)
Table 10 shows the results of screw withdrawal resistance (SWR) measurements
according to EN 320:2011-07.
Table 10.
Screw withdrawal resistance—comparison of Spanish, Bulgarian and Serbian Paulownia
wood with other wood species. n (Spain) = 9; n (Bulgaria) = 10; n (Serbia) = 9 (standard deviation in
parentheses) (N/mm).
Wood Species Screw Withdrawal
Resistance (N/mm) Min./Max. (N/mm) Source
Paulownia (Spain) 55.56 (6.6) 41.48/63.47 Present study
Paulownia (Bulgaria) 51.95 (13.66) 31.4/87.74 Present study
Paulownia (Serbia) 56.55
34.24/91.72
Present study
Paulownia (Türkiye) 50.5 (7.87) Akyildiz [49]
Black pine 152 Aytekin [50]
Fir 108 Aytekin [50]
Oak 170 Aytekin [50]
Forests 2022,13, 1543 10 of 12
Paulownia wood from Spain measured a screw withdrawal resistance of 55.56 N/mm.
The screw pull-out resistance of Bulgarian paulownia wood was 51.95 N/mm. Serbian
Paulownia wood reached a value of 56.55 N/mm. The results for SWR for the Paulownia
from Bulgaria are consistent with the findings of [
48
], where plantation wood was extracted
from Türkiye, therefore it can be supposed that Paulownia from Black Sea region exhibits
similar properties. Compared with SWR of hardwood species, the overall values for
Paulownia are at least two or three fold lower [49].
4. Conclusions
The physical and mechanical properties of Paulownia wood have shown that the
location of these plantations (Iberian Peninsula and Balkans), the type of soil and the
environmental conditions strongly influence the wood properties. The density is directly
corelated with the mechanical properties. The low density of all these tested samples
ensures that the wood is filled with a lot of air and thus has heat-insulating and light-
weight properties.
As expected, Paulownia wood achieved significantly lower values in physical and
mechanical properties compared to conventional species such as spruce, oak or poplar.
Paulownia wood can be classified very low and low for MOR, MOE and compression
strength. Paulownia is not recommended for structural uses, which require high mechanical
strength and stiffness.
For further investigations, it is important to pay close attention from which log section
was extracted the sample. There are large variations in strength within a log and therefore
different mechanical properties, depending on the spot of the log where the test specimen
was cut. There are significant differences in the width of annual rings which greatly affects
the woods properties.
In view of all the results, the conclusion is that Paulownia has enormous potential
for special lightweight application in construction, model making and thermal insulating.
Paulownia offers many possibilities in non-load-bearing structures and can successfully
replace other tropical wood species, which are more expensive and rarer. Paulownia wood
bears resemblance to Balsa wood concerning its lightweight. It is known that Balsa is one
of the best core materials embedded in lightweight sandwich structures, with distinctive
stiffness-to-weight and strength-to-weight ratios. The comparisons of mechanical proper-
ties of these two species demonstrates that it might be suitable to focus on the possibility of
using Paulownia wood as a substitute for Balsa wood as core material for composites.
Author Contributions:
Conceptualization, M.C.B. and E.M.T.; methodology, E.M.T.; validation, A.P.
and E.M.T.; formal analysis, K.B.; investigation, K.B.; resources, K.B. and E.M.T.; data curation,
A.P.; writing—original draft preparation, K.B. and E.M.T.; writing—review and editing, E.M.T.;
visualization, M.C.B. and A.P.; supervision, A.P. and E.M.T.; project administration, M.C.B. All
authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
The authors would like to thank to Arien Crul, for providing all Paulownia
lumber from three sources (Spain, Bulgaria, Serbia), FH-Thomas Schnabel (FH Salzburg) for the
support with design of experiment, Thomas Wimmer, for the measurements conducted in the facilities
of FH Salzburg and Helmut Radauer. for the support with sample preparing.
Conflicts of Interest: The authors declare no conflict of interest.
Forests 2022,13, 1543 11 of 12
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