Improvement of the properties of birch plywood with thermo-hydro treatment method. Summary of the doctoral thesis

Plywood has a good ratio between weight and strength, therefore it is widely used in building and transport industry and for furniture production. In high humidity conditions applicability of plywood is limited due to low biological durability, dimensional instability and decrease in strength. It limits plywood application without special finish or structure modification. Thermal modification (TM) is potential method to improve plywood properties without using biocides. TM is relatively wide used for improvement of solid wood properties, but there is small information about TM of plywood. Therefore local birch (Betula spp.) wood was selected as research object and thermo-hydro treatment (THT) of veneers and plywood in water vapour at elevated pressure was performed. Two main process variable parameters – maximum treatment temperature and time were used to develop suitable THT regime. According to our practical experience, mild treatment parameters were chosen for THT of veneers compared to information available in literature. Identical treatment was used for industrial birch plywood. The aim of the research was to obtain birch plywood with improved service properties – dimensional stability, biological durability and without serious loss in mechanical strength. Therefore our task was to establish which method for obtaining THT plywood is optimal – THT of industrial plywood or THT of veneers and further gluing with suitable adhesive to obtain as much positive properties for final product as possible. In most cases THT at 160°C ensures better properties for plywood than after 150°C treatment. Both obtained plywood products have 3rd use class (for use in outdoor conditions without contact with soil); 4th – 1st durability class against rot fungi, reduced equilibrium moisture content (10-53%), improved dimensional stability (35-65%). Hardness and bending strength of THT plywood diminishes by 32-46%, but glue bond strength by 71-75%. Bending strength of plywood obtained from THT veneers could be improved by 28-34%, but hardness diminishes by 6-10% and glue bond strength by 7-57%. The results from the doctoral thesis could be used for improvement of THT technology, to create production unit which will produce plywood with high added value.