It has been found in the present study that, in laboratory fungi tests after leaching with water (EN 84), exterior birch (Betula spp.) plywood, as birch solid wood, is subjected to degradation by rot fungi. This limits the use of such plywood outdoors or in building structures in high humidity conditions. The possibility to improve the birch plywood fungal resistance was investigated in two ways: 1) thermo-hydro treatment (THT) of birch veneers, followed by its gluing with adhesive (experimental plywood), 2) THT of the industrial plywood (reference plywood). Taking into account the sensitivity of birch solid wood against the thermal treatment in water vapour medium at elevated pressure established in the previous tests, and the small thickness of veneers, four mild modification regimes were chosen (°C /min): 150/10, 150/50, 160/10, 160/50. For THT veneers, improvement of the physical properties (equilibrium moisture content, capillary water absorption, antiswelling efficiency) was achieved, but the samples’ fungal resistance conformed only to the durability class 3 according to EN 350-1. The plywood glued from THT veneers with a phenol-formaldehyde (PF) film did not demonstrate essentially better durability. The industrial untreated plywood in the test according to EN 12083 showed the durability class 1 according to EN 350-1, but after leaching, mass loss was similar to the control birch wood indices. Better durability was achieved, thermally modifying the industrial plywood in the regime 160/50 (class 3, moderately durable). The chemical changes of veneers (wood components, methoxyl groups) as a result of the THT action were analysed by chemical analysis and instrumental methods (elemental composition, Py-GC/MS analysis). To understand the effect of water leaching on the plywood fungal resistance, water leachates of veneers, plywood and the polymerised adhesive were analysed. It has been suggested that the durability of untreated plywood is influenced by the water soluble phenolic and/or furan type compounds, but the effect of fungi on THT birch veneers can be influenced by the low-molecular carbohydrate and lignin degradation products, formed in the thermal treatment.

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