Abstract
The objective of this study was to explore some of the physical and mechanical properties of 9-layer birch (Betula spp.) plywood with the addition of phenol-formaldehyde glue, in cases in which the cutting edges of the samples are coated with the damp-proof mastic Fibergum, and in case in which they remain unprocessed (uncoated), following a total of ten cycles of soaking/oven-drying. The properties to be determined were the bending strength (BS), modulus of elasticity in bending (MOE), thickness swelling (TS) and restore dimensions (RD), which were tested according to the European standards (EN). A linear-fractional equation and linear relationship were used for the approximation of any change in the physical and mechanical properties of the samples depending upon the number of soaking/oven-drying cycles. It was shown that the values of the properties investigated were most affected by the first soaking and drying cycle. Thereafter, BS and MOE levels decreased smoothly at a low rate, but the values of TS became stabilised. The BS and MOE values for the wet samples with coated cutting edges were higher than when they were uncoated, as the moisture levels in the former case were lower. After the first soaking of the samples with coated cutting edges, the retention values were as follows: BS at 52.8 % and 66.7 % for the major and minor axes, respectively, with the same applying to MOE at 61.9 % and 64.2 %, while TS was at 105.2 %. To clarify the phenomenon that causes a decrease of the properties, the face plies and edge structures of the initial dry samples and of the samples after the first, second and ninth soaking/oven-drying cycles were studied using the X-Ray technique.Ključne riječi
Highlights
After the first soaking of the samples with coated cutting edges, the retention values were as follows: bending strength (BS) at 52.8 % and 66.7 % for the major and minor axes, respectively, with the same applying to modulus of elasticity in bending (MOE) at 61.9 % and 64.2 %, while thickness swelling (TS) was at 105.2 %
The mean values obtained for one cycle of BS and MOE in the directions of the II-major axis and Tminor axis are presented in Figure 1, for oven-dry (7.7 ± 1.8) % and wet (25.3 ± 1.7) %, where the cutting edges are coated with mastic, and in Figure 2, for wet (40.0 ± 4.0) %, where the cutting edges have remained unprocessed
The mechanical and physical properties of 9-layered birch plywood after soaking/oven-drying cycles were investigated according to European standards
Summary
Birch is considered a form of hardwood and has good mechanical properties resulting in an increasing demand when it comes to its use in products such as lumber, sawn timber and plywood. Plywood is made by bonding together several hardwood plies (mostly for indoor products) or softwood plies (mostly for outdoor products) (Dieste et al, 2008; Cosereanu et al, 2010; Lipinskis et al, 2011; Zalemanis et al, 2018). There are a good many important factors that affect the physical properties of plywood (such as moisture, thickness swelling and density) and its mechanical properties (strength characteristics), such as: trees species, wood density, slope of grains, shakes, checks, number of plies, type of adhesives, thicknesses of plies and the technological parameters involved in the manufacturing process
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