Abstract
In this study, kiri wood (Paulownia spp.) is assessed as potential core layer material for blockboards. Blockboards with core layers made of kiri and spruce (Picea abies) were produced using Anthocephalus cadamba as crossband veneer and beech (Fagus sylvatica) as face veneer. With spruce as core layer material, higher values for bending strength and modulus of elasticity, tested in grain direction, and screw withdrawal resistance were obtained. In return, blockboards with kiri as core layer material were 31% lighter and showed a more homogeneous core layer material with regard to density.
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1 Introduction
The aim of producing blockboards is to create a building material that is fully made of wood but exhibits enhanced material properties, like higher stability against deformation and reduced weight. With respect to mass production, the blockboard has already been replaced many years ago by other wood-based panels due to high labour costs, but it can still be found in handcrafted furniture or furniture for special purposes. The reduced weight makes it particularly interesting for purposes, where low weight is considered a crucial factor such as caravanning, trade fair construction, automotive industries and interior constructions for shipbuilding (Marutzky et al. 2008). Paulownia is a very fast-growing species, which is cultivated on plantations. In China, it has already been cultivated for over 2000 years (Barton et al. 2007). For several years, a growing interest in this species can be noted also in Europe, as many plantations have been cultivated throughout Europe with the purpose of timber production. Thinning on a regular basis is crucial to obtain high-quality timber (Rao 1986), which makes low-density wood assortment available in high quantities. The assortment is still difficult to market, because of uncertainties due to a lack of knowledge regarding the material properties. Nelis et al. (2018) stated that kiri wood is a very interesting raw material for the wood-based panel industry and thus, it could also be of interest for the blockboard production due to its low density. The low hardness of this species should not be a problem, when using a face veneer of high density and considerably higher hardness such as beech wood. Kiri could replace other light species from exotic origins, which could increase the consumers acceptance and also play its part in the conservation of the tropical rainforests (Marutzky et al. 2008).
2 Materials and methods
2.1 Material
For the production of the blockboards, single sticks with dimensions of 500 (L) × 20 (W) × 15 (H) mm3 of Paulownia spp. (from Shaanxi province, China) and Picea abies were used as core layers. Each core layer consisted of 17 sticks. Veneers of 2 mm thickness from Anthocephalus cadamba served as crossband veneers and Fagus sylvatica veneers of 0.5 mm thickness as face veneers.
2.2 Panel and specimen manufacturing
Four variants of blockboards were produced. While crossband and face veneers were kept the same for all variants, two variants of core layer (PG and NG) were used for each species. For variant PG, the single sticks of the core layers were glued among each other and in a second step, they were glued onto the crossband veneer. The utilized glue was Jowacoll® 103.30 by Jowat SE. For variant NG, the single sticks were directly glued onto the crossband veneer without preceding pre-gluing. The further production parameters were kept the same for all variants. First, the core layers were glued onto the crossband veneers and then pressed in a hot press at 60 °C, 70 N mm−2 and a pressing time of 15 min. After the pressing process, the boards were cooled to ambient temperature. Second, a uniform face veneer with dimensions of 600 × 500 mm2 was put together from several face veneer strips with a width of 150 mm. The face veneers were then glued onto the crossband veneers at a 90° grain angle and pressed at 60 °C, 70 N mm−2 and a pressing time of 15 min. After the pressing process, the blockboards were cooled down to ambient temperature and cut to dimensions of 560 × 500 mm2. Each blockboard was cut into seven specimens with dimensions of 500 × 50 mm2 and five specimens with dimensions of 50 × 50 mm2.
2.3 Mechanical properties
All specimens were conditioned to constant mass at 20 °C and 65% RH. Density was determined according to EN 323 (1993); bending strength (modulus of rupture, MOR) and modulus of elasticity in bending (MOE) were tested with the core layer in longitudinal grain direction and perpendicular to grain according to EN 310 (1993). Screw withdrawal resistance (SWR) was assessed according to EN 320 (2011). The group “rad/tan” shows SWR perpendicular to grain, “long” shows SWR with the screw orientation in grain direction (longitudinal) and “face” shows SWR perpendicular to grain and additionally effected by crossband and face veneer.
3 Results and discussion
All blockboards could be produced without any technical problems. The mean densities and the corresponding standard deviations of the blockboard variants differed significantly (Table 1). Blockboards containing spruce exhibited a 31% higher mean density with 54% higher standard deviation than those based on kiri. Blockboards with kiri as core layer showed a more homogeneous density. Statistical analysis revealed that variants PG and NG with the same core layer did not show significant differences with respect to density, bending strength and screw withdrawal resistance. Therefore, all results were distinguished only between the two species.
Boards containing spruce (S) showed significantly higher modulus of rupture (MOR) and modulus of elasticity (MOE) than kiri (K), when tested in grain direction (Fig. 1). Respective boards of spruce (S.PTG) and kiri (K.PTG) tested perpendicular to grain revealed statistically insignificant results for MOR and MOE. While MOR results of spruce tested in grain direction were 37% higher than kiri, for MOR tested perpendicular to grain a 9% higher mean value of spruce was noted. This shows that for blockboards, bending strength tested perpendicular to grain is much less susceptible to changes in density than that tested in grain direction. Considering the generally low bending strength of wood when tested perpendicular to grain, the crossband veneer contributes significantly to the strength values. First, the crossband veneer is tested in grain direction and second, it has a high influence because of the generally low bending strength tested perpendicular to grain. The respective specimens with the same core layer material did not show significant differences between variants with and without pre-gluing. This indicates the high influence of the crossband veneer on the bending strength perpendicular to grain. The much higher standard deviation for the density of the spruce is also reflected by MOR and MOE. According to DIN EN 636 (2015), blockboards with spruce as core layer would be ascribed to class F35/E60 (min. MOR of 52 N mm–2 / min. MOE of 6300 N mm−2) and with kiri as core layer to class F20/E40 (min. MOR of 30 N mm−2 / min. MOE of 3600 N mm−2).
Boards containing spruce showed about 40% higher screw withdrawal resistance (SWR) than those containing kiri for all screw locations (Fig. 2). These results confirm the strong correlation between density and SWR. The orientation “long” showed the lowest and “face” the highest mean density for both species. For kiri and spruce, statistical analysis revealed significant differences between “face” and “long”.
4 Conclusion
The choice of the core layer wood species is crucial for bending strength and MOE, when tested in grain direction, and for screw withdrawal resistance. While spruce as core layer material caused higher bending strength, MOE and SWR, blockboards with kiri as core layer material were much lighter and more homogeneous with regard to density. Bending strength tested perpendicular to grain did not show significant differences between spruce and kiri. This can be attributed to the generally much lower bending strength of wood perpendicular to the grain and the corresponding high influence of the crossband veneer, which is tested in grain direction and, therefore, equalizes the results. Blockboards with kiri as core layer material are very interesting products for lightweight applications with low requirements with respect to mechanical strengths.
References
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Acknowledgements
The authors wish to acknowledge funding by the AiF Projekt GmbH (project no. KF2454610WZ4).
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Nelis, P.A., Henke, O. & Mai, C. Comparison of blockboards with core layers made of kiri (Paulownia spp.) and of spruce (Picea abies) regarding mechanical properties. Eur. J. Wood Prod. 77, 323–326 (2019). https://doi.org/10.1007/s00107-019-01381-3
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DOI: https://doi.org/10.1007/s00107-019-01381-3