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PEER-REVIEWED ARTICLE
The highest heat transfer rate observed for pine mats was associated with a moisture content higher than that of the poplar material (5.0% face layer and 4.5% core layer vs . 3.5% face layer and 2.9% core layer), lower compression of the core layer (minimum density 402 kg/m 3 , Fig. 2), and subsequent easier steam penetration, which is the main heat carrier from face layer to core layer upon overheating (Sokolovs`kyi and Petriv 2007). In addition, the heat transfer coefficient for pine wood was higher than that for poplar (pine, 0.14 W/mK; poplar, 0.10 W/mK (Niemz 1993)), which explains the higher heat transfer rates. Moreover, the lower heat transfer rates observed for poplar might come from higher compression of the mats (minimum density 431 kg/m 3 , Fig. 2), which is a hindrance to steam penetration. Because of the lower heat transfer rate, the plasticizing of the chips took more time. Slower heating resulted in more uniform compression of the boards (Fig. 2). In effect, a higher pressure was necessary to hold the target thickness of the mat for the poplar boards. For the poplar-pine boards, middling heating times and moderate steam penetration were observed. This may have been a result of the mediocre compression of the core layer (minimum density 386 kg/m 3 ) and its fairly porous structure.
Fig. 2. Density profiles of the tested panels Considering the mechanical properties of the manufactured panels (Fig. 3), it can be summarized by stating that the requirements for a P1 panel type (EN 312 (2010) - tab.1) were met to some extent by the respective panel series. Thus, the modulus of rupture (MOR) requirement was met by the poplar panels, pine panels, and mixed poplar- pine panels at 92%, 60%, and 71%, respectively, while the internal bond strength (IB) requirement was met respectively at 60%, 73%, and 100%.
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Boruszewski et al . (2016 ). “Particleboard density ,” B io R esources 11(3), 6909-6919.
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