*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**


**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<sup>3</sup> ), 3-point flexural strength (~40 N/mm<sup>2</sup> ), 3-point flexural modulus of elasticity (~4900 N/mm<sup>2</sup> ), compressive strength (~23 N/mm<sup>2</sup> ), tensile strength (~44 N/mm<sup>2</sup> ) 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 mechanical properties

#### **1. Introduction**

Of Asian origin, Paulownia is a fast-growing deciduous tree, with at least nine subspecies [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 conditions [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

**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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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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<sup>3</sup> per log [3]. Paulownia wood is semi-ring porous to ring porous, soft, frequently knot free, with an average density under 300 kg/m<sup>3</sup> [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 industry 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.


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 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/mm<sup>2</sup> 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 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/m<sup>3</sup> ).


Table 2 shows 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.

The average density of wood from all three sites for Paulownia *tomentosa x elongata* is 258 kg/m<sup>3</sup> . The Paulownia wood from Spain had the highest average density of 266 kg/m<sup>3</sup> , followed by the Serbian wood with 259 kg/m<sup>3</sup> , and the lowest average value had the Bulgarian wood with 250 kg/m<sup>3</sup> (Table 2).

In their study, Akyildiz and Kol [3] determined an average density of 272 kg/m<sup>3</sup> for the basic species *Paulownia tomentosa* from Türkiye. This value is lower for Paulownia wood from Hungary at 246 kg/m<sup>3</sup> [1] or from Spain at 215 kg/m<sup>3</sup> [36]. Estevez et al. [11] reported a value of 460 kg/m<sup>3</sup> for Portuguese Paulownia wood, which is even higher than the average density of spruce with 430 kg/m<sup>3</sup> according to [39].

Lachowicz et al. [36] measured the lowest value (Paulownia wood sourced from Spain) with a mean density of 216 kg/m<sup>3</sup> . Considering the values reported from [40,41], Balsa wood has a lower density of 160 kg/m<sup>3</sup> . The lightweight hardwood species poplar, has a density of 440 kg/m<sup>3</sup> [39,42].

At 12% moisture content, Paulownia wood density varies from 220 to 350 kg/m<sup>3</sup> , with an average of 270 kg/m<sup>3</sup> [5]. This variability in density is determined by growth conditions. Higher Paulownia densities, about 400 kg/m<sup>3</sup> , 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 compared to other Paulownia species from the literature; n (Spain) = 12; n (Bulgaria) = 12; n (Serbia) = 12 (standard deviation in parenthesis) (%).


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.

#### *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).


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, Bulgarian and Serbian Paulownia wood with other wood species. n (Spain) = 10; n (Bulgaria) = 10; n (Serbia) = 10 (standard deviation in brackets) (N/mm<sup>2</sup> ).


In the case of Paulownia wood source from Bulgaria, Brinell hardness in axial direction was 18.7 N/mm<sup>2</sup> , 5.6 N/mm<sup>2</sup> in radial direction and 5.3 N/mm<sup>2</sup> in tangential direction. For Paulownia wood from Spain was measured the Brinell hardness in axial direction of 21.22 N/mm<sup>2</sup> , 6.1 N/mm<sup>2</sup> in radial direction and 5.81 N/mm<sup>2</sup> in tangential direction. For Paulownia wood from Serbia were measured the highest values of Brinell hardness: 21.22 N/mm<sup>2</sup> in axial direction, 6.1 N/mm<sup>2</sup> in radial direction and 5.8 N/mm<sup>2</sup> in tangential 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<sup>2</sup> [45], Paulownia

has significantly increased hardness. Other lightweight hardwood species is poplar, with a hardness of 25–33 N/mm<sup>2</sup> [43], in concordance with the results of [16] of 26.74 N/mm<sup>2</sup> .

#### *3.5. Modulus of Rupture and Modulus of Elasticity (DIN 52186:1978)*

Table 6 shows 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 6 show 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/mm<sup>2</sup> ).


Paulownia wood from Spain achieved the highest flexural modulus of elasticity of 4866.49 N/mm<sup>2</sup> and the highest flexural strength of 39.77 N/mm<sup>2</sup> (Tables 6 and 7). The Bulgarian Paulownia wood had the lowest MOR of 35.53 N/mm<sup>2</sup> . Paulownia wood from Serbia is in the middle range with 37.54 N/mm<sup>2</sup> (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<sup>2</sup> [5]. Lachowitcz et al. [36] measured a similar bending strength ranging from 23.89 N/mm<sup>2</sup> to 53.17 N/mm<sup>2</sup> with a mean value of 38.63 N/mm<sup>2</sup> and Esteves et al. [11] found a higher mean value of 53.5 N/mm<sup>2</sup> 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<sup>2</sup> [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<sup>2</sup> [44] that is at least two fold higher as MOR for Paulownia, which is also the case of oak (95 N/mm<sup>2</sup> ) [44].

The modulus of elasticity (MOE) for Paulownia ranges from 2651 to 4917 N/mm<sup>2</sup> [5] (Table 7).


**Table 7.** Flexural modulus of elasticity (MOE)—comparison of Spanish, Bulgarian and Serbian Paulownia wood with other wood species (N/mm<sup>2</sup> ).

Lachowicz et al. [36] measured the lowest modulus of elasticity of Paulownia wood with 1899 N/mm<sup>2</sup> . 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<sup>2</sup> and an MOR of 80 N/mm<sup>2</sup> [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<sup>2</sup> and a MOR of 99 N/mm<sup>2</sup> [39]. This value is almost three times higher than that of Paulownia wood. MOE of Balsa wood is lower, averaging 2900 N/mm<sup>2</sup> [45], but still higher than the values resulted from the study of [36], namely 1900 N/mm<sup>2</sup> .

#### *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/mm<sup>2</sup> ).


Paulownia wood from Spain has a compressive strength of 22.53 N/mm<sup>2</sup> . The Bulgarian Paulownia wood has a compressive strength of 18.77 N/mm<sup>2</sup> and the Serbian Paulownia wood has a compressive strength of 21.41 N/mm<sup>2</sup> . Other values of compressive strength for Paulownia from other plantations are ranging from 25.55 N/mm<sup>2</sup> [3] and

35.56 N/mm<sup>2</sup> [46] for Paulownia from Türkyie and significant lower, of 14.24 N/mm<sup>2</sup> , as results from the research of [36].

In comparison, spruce exhibits a compressive strength of 45 N/mm<sup>2</sup> , 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<sup>2</sup> [39]. The Balsa wood has a the lowest mean value for compressive strength of 10 N/mm<sup>2</sup> [38],

#### *3.7. Tensile Strength (DIN 52188:1979-05)*

Table 9 shows 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/mm<sup>2</sup> ).


The Spanish Paulownia wood has a tensile strength of 44.12 N/mm<sup>2</sup> . The tensile strength of Paulownia wood from Bulgaria was 36.17 N/mm<sup>2</sup> and the tensile strength for the wood from Serbia reached a value of 40.14 N/mm<sup>2</sup> . For Paulownia sourced from Hungary Koman and Vityi [16] reported a tensile strength of 33.25 N/mm<sup>2</sup> , which is consistent the values presented in this study. The tensile strength of Balsa wood is considerable lower with 14 N/mm<sup>2</sup> [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).


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 lightweight 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 properties 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.

#### **References**

