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higher than that of the neat poplar boards, though the minimum densities are comparable (Fig. 2). The phenomenon results directly from the difference in wood density. The density of wood significantly affects the mat compaction ratio by the reduction of free volume between chips (Medved and Resnik 2006). The final properties of a panel can be even more influenced when different wood species are used in the face and core layers. Xu and Suchsland (1999) observed that MOR, MOE, and IB were higher for panels made of one species when compared with mixed ones. The results obtained in the present work for IB are not consistent with the abovementioned findings.
Fig. 4. Internal structure of the tested panels It is likely that the increase in IB for the mixed poplar-pine boards, when compared with the other two series, comes from the fact that the bulk density of pine chips was higher than that of poplar. Thus, assuming a constant glue rate, the amount of the adhesive loaded on the core layer was higher for the mixed boards than that for the neat poplar ones. On the other hand, the poplar face layers of the mixed boards were compacted to a higher extent because of their lower bulk density, which resulted in a higher pressure transferred onto the pine core layer. A combination of these two factors may have led to the increase in IB observed for the mixed poplar-pine boards. It is worth noting that the differences between mechanical parameters (MOR, MOE, and IB) found for the studied series were statistically significant at 95% confidence interval. The effect of density variation on the analyzed parameters is shown in Fig. 5. The alternations in density affect the strength parameters of the boards (Xu and Suchland 1998). The correlation was true for the investigated poplar and pine boards within the analyzed density range (450 to 550 kg/m 3 ). The strongest correlations were found for the poplar panels, i.e ., r 2 for MOR and MOE was, respectively, 0.4579 and 0.9362. The values achieved for the pine boards were 0.6412 and 0.2581, respectively, for MOR and MOE.
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Boruszewski et al . (2016 ). “Particleboard density ,” B io R esources 11(3), 6909-6919.
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