WOOD RESE ARCH
62 (2): 2017
PHYSICAL AND MECHANICAL PROPERTIES OF
PAULoW nIA t oMe ntoS A WOOD PLANTED IN
S K, S F
U S, I W S
U S, I F E T
(R A )
The Paulownia tree (or to its well-known name Chinese empress tree; Paulownia tomentosa) is
classified among the most variable wood species of the world concerning usability. Its cultivation
in Hungary in form of research plantations has just started in the last decade, first of all for
the investigation of energetic properties. Due to this the information related to the physical-
mechanical properties of the wood was still not determined, from which aspect this study is
essential. The investigated wood with an air-dry density of 0.3 g.cm-3 has shown low bending
(42 MPa), compressive (22 MPa), shear (7 MPa), tensile (33 MPa) and impact strength
(1.6 J.cm-2) values, based on which its wooden material properties can be compared to poplars
considering tree species in the region.
KEYWORDS: Paulownia tomentosa, physical properties, mechanical properties, density,
In Asia and in some European countries Paulownia species are preferred in tree cultivations
or agro-forestry plantations of industrial wood purpose, in the latter typically in combination of
industrial wood/agricultural crop plant, rarely in mixed culture of other purpose (e.g. energy).
For example in the Chinese agriculture Paulownia is cultivated as intermediate plant on almost
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1.3 million hectares. The reason for this is that one manages meagre property assets in China, so
food production and industrial wood cultivation can be realized on the same ground (Vityi and
Marosvölg yi 2012).
Literature data and the results of Hungarian investigations performed present that individual
Paulownia species are basically undemanding, have a great yield and can be cultivated relative
simply. It can be well reproduced from grain, root cutting (according to some sources also from
shoot cutting), under favourable conditions it can reach a trunk diameter of 30-40 cm, the height
of 10-12 m and the wood yield of 0.2-0.6 m-3 by the age of 10 years. (Under ideal cultivation
conditions its growth can be even quicker). (Yang 2004).
Its wide utilization spectrum ranges from industrial applications (furniture and building
timber, base material for paper industry, biomass for energy purposes etc.) through apiculture and
medical industry (bark, leaf, f lower cluster) to decoration function (park tree, base material of
exquisite wood-carving). Owing to its machinable timber of decorative texture it is used in Japan
as traditional timber, where a high quality log counts as valuable base material. The less valuable
base material can be utilized on other areas such as for example chipboard (Kalaycioglu 2005),
however it is applied as heat and electric insulation material (AFBI 2008) respectively. From
the properties of a wood density is most essential concerning utilization. From this information
one can conclude on the strength properties (Kiaei 2013). Typically there is a strong correlation
between density and MOR and MOE resp (Zhang 1997). The basic density relates closely to
the quality parameters of the final usage, such as e.g. cellulose yield and strength of construction
materials (Harvald and Olesen 1987). The tree species of higher density generally have more solid
wood than the ones of lower density (Walker and Butterfield 1996).
Paulownia is mainly cultivated on areas with hotter climatic conditions of the world (mainly
on the extended plantations of China and USA), where the usage of P. elongata has spread. For
this reason there is lot of information about this tree species, while one can find much less data
related to P. tomentosa tolerating also more moderate climatic conditions. The purpose of this
research is the discovery of the properties of the wooden material cultivated in Hungary, which
is the essential condition of applicability on some utilization areas.
MATERIALS AND METHODS
The Hungarian cultivations were established first of all for the purpose of plantation
experiences and energetic researches, for this reason older, more mature wood was not available
for investigation. The tree-trunk serving as base material for the specimen required for the
examination is situated at an altitude of 70 m; the average annual temperature ranges between
11-12°C, and the long-term total annual precipitation is approx. 608 mm. The examinations have
covered the following properties in accordance with the relevant standards:
- Annual ring structure
Based on the picture made of the disc cut from the trunk by means of the program Image Pro
Plus the annual ring width was determined along the smallest and biggest radius.
- Physical-mechanical properties
From the examined trunk the following properties were defined based on the relevant
standards: Density (DIN 68364:2003), Shrinkage and swelling (DIN 52184:1979), Compression
strength (DIN 52185:1976), Tensile strength (DIN 52188:1979), Shearing strength (DIN
52187:1979), Static bending streng th (DIN 52186:1978), Modulus of elast icity (DIN 68364:2 003).
Impact bending strength (DIN 52189:1981), Brinell hardness (MSZ EN 1534:2000).
Vol. 62 (2): 2017
The examination were performed under standard climatic conditions (T=20°C; φ=65%)
- until equilibrium moisture content - on stored specimens. The strength examinations were
carried out with universal material examining device type Instron 4208 and Charpy-impact tester
on specimen series of 25 pcs. each.
RESULTS AND DISCUSSION
Due to the eccentric structure of the examined disc along the biggest radius the growth is
wider by approx. 30% than the value of the smallest radius. The increase cycle becomes constant
beginning from the 5th year (Fig. 1) under the growth ring width value of 1 cm. In the previous
years significantly bigger values can be found. The average growth ring width is 14.7 mm along
the biggest radius, while 8.3 mm in the direction of the smallest radius .
Fig. 1: Growth per year.
Paulownia tomentosa has an expressly light, soft wood, which is proven by the measured
density values. The air-dry density (Tab. 1.) received corresponds with the average of the values
in the literatures (Kanehira 1933, Senelwaa and Sims 1999, Flynn and Holder 2001, Kalaycioglu
2005, Akyildiz 2010). This low density does not even exceed that of the poplars for example.
Tab. 1: Values of the measured densities (kg.m-3).
Density type Average Min. Ma x. St . dev.
Air-dry (u=12%) 300.18 262.33 360.18 26.59
Oven-dry (u=0%) 275.46 240.00 328.15 23.52
Basic 264.2 224.39 309.09 23.04
In case of the Paulownia tree we have received favourable results (Tab. 2.) from aspect
of dimensional change caused by moisture. Firstly the radial and tangential values are very
favourable, which almost completely comply with the ones communicated by Meier (2015).
However related to the dimensional change of volume higher literature values by more than
20-30% can be found as well (Akyildiz 2010).
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Tab. 2: Values of the measured densities.
Longitudinal Radial Tangential Volu met ric
Shrinkage 0.69 2.20 3.89 6.94
Swelling 0.68 2.17 3.73 6.28
From the low density value of the wooden material it is inferred that the different strength
values are not high as well. It is proven by the results received (Tab. 3.), which often have not
reached that of the poplars with similarly soft wooden material.
Tab. 3: Mechanical properties (parallel to the grain (MPa)).
Test type Average Min. Ma x. St .dev.
Compressive 22 .14 19.63 25.24 1.78
Tensile 33.23 21.86 52.96 8.90
Shearing 7.0 3 6.08 8.00 0.52
MOR (3 points) 41.51 28,65 48.65 4.68
MOE (3 points) 3492 2595 4142 472
Cross 26.74 20.78 31.89 3.22
Tangential 9.13 5.59 13.61 2.16
Radial 9.51 6.53 13.39 2.17
Impact bending strength
(J.cm-2)1.59 1.27 1.90 0 .17
The strength values of the investigated paulownia correspond in some cases, while in other
cases these deviate more or less from the ones indicated by others. The values of bending strength
as maybe the most important strength property are practically identical with the values found in
some literatures (Akyildiz 2010, Meier 2015, Shim 1948). However regarding flexibility we have
received significantly lower values than communicated by other sources (Akyildiz 2010, Meier
2015, Shim 1948).
Regarding compressive strength the average 22 MPa value can be considered as very low,
which is however connected to favourable deviation values. The average shear strength value
received can be higher by even 20% compared to some earlier researches (Shim 1948, Barton
The determination of the tensile strength values is generally performed rarely in case of
wooden material examination, contrary to e.g. bending or compressive strength. In this case the
very high deviation value was connected to relatively low (33 MPa) strength.
The determination of the impact-bending strength – which is already a dynamic strength
property - is performed also relative rarely, for this reason values in this regard were not available.
The values received are extremely low, even in case of wood species with similar low density.
The Brinell hardness values measured in the main anatomical directions of the wood have
produced similar values as the similarly soft poplars. The axial hardness is almost the triple of the
side hardness values, which are practically the same.
Vol. 62 (2): 2017
The wooden material properties of Paulownia tomentosa cultivated in Hungary do not show
any significant deviation related to the ones cultivated in other parts of the world. Due to its
low density (air-dry: 300 kg.m-3) and strength values it is not suitable for structural purposes;
however it can be a serious competition for broad-leaf tree species, f irst of all due to its growth
characteristics. Based on our investigations its properties can be compared first of all to hybrid
poplars. The density of hybrid poplars wood is between 300-400 kg.m-3. The cultivation
experiences and the current research related to the physical-mechanical properties of wooden
material can establish the more significant extension of the species in the region.
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current knowlwedge, Occasional publication No. 7. Agri-Food and Bioscience Institute,
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231, 9pp. Rotorua, New Zealand, Forest Research Institute.
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5. DIN 52188, 1979: Testing of wood - Determination of ultimate tensile stress parallel to
6. DIN 68364, 2003: Properties of wood species - Density, modulus of elasticity and strength
7. DIN 52186, 1978: Testing of wood - Bending test
8. DIN 52187, 1979: Testing of wood - Determination of ultimate shearing stress parallel to
9. DIN 52189, 1981: Testing of wood - Determination of impact bending strength
10. DIN 52185, 1976: Testing of wood - Compression test parallel to grain
11. DIN 52184, 1979: Testing of wood - Determination of swelling and shrinkage
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19. Meier, E., 2015: Wood. Identifying and using hundreds of woods worldwide, ISBN 978-0-
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B Z. S.
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