Chernetskyi O. Patterns of influence of the wood species and the thickness of the thermoplastic film on the properties of plywood

In the dissertation work, the scientific and technical task was solved, which consisted of improving the adhesion between wood and thermoplastic polymer by modifying the veneer with citric acid and establishing the regularities of the influence of the wood species and the thickness of the thermoplastic film on the properties of plywood. Replacing conventional thermosetting adhesives, which are usually toxic due to the presence of formaldehyde, with thermoplastic polymers will allow reducing the amount of formaldehyde emissions into the environment during the plywood production and the harmful impact on human life, as well as reduce the volume of plastic waste accumulation. One of the promising directions for solving the plywood toxicity is the use of thermoplastic polymers, especially recycled ones, instead of toxic thermosetting adhesives. The great advantage of these polymers is that formaldehyde emission from such plywood is practically zero. This not only improves the environmental performance of plywood and the conditions of its production but also reduces the negative impact of such polymers on the environment due to the long process of biodegradation. The use of a thermoplastic film as an adhesive for bonding veneers, in addition to the fact that the film does not contain formaldehyde, greatly simplifies the technology compared to the use of liquid adhesives. In addition, recycled polymers are production waste and are cheaper than virgin polymers. Thus, the utilization of recycled thermoplastic synthetic waste allows the economical use of primary polymer resources and at the same time protects the environment. The effect of the type of thermoplastic polyethylene film, its thickness, the wood species, and the method of veneer surface modification on the properties of plywood is clarified in the dissertation work. It has been demonstrated that the recycled film of low-density polyethylene (rLDPE) is suitable for obtaining environmentally friendly plywood panels of various wood species with characteristics ideal for successful indoor use. In addition, the properties did not show statistical differences between the plywood samples bonded with rLDPE and virgin thermoplastic low-density polyethylene film (LDPE). The main advantage of these polymers as an adhesive for plywood is proven, which is that the amount of formaldehyde emitted by plywood is close to zero (0.01– 0.10 mg/m2 ·h). These results confirm the effectiveness and environmental benefits of using recycled plastic waste. It has been proven that rLDPE and LDPE of different thicknesses can be successfully used for bonding plywood using various wood species - beech, birch, hornbeam, and poplar. Birch, beech, and hornbeam veneers provide plywood with higher density, bending strength, modulus of elasticity, and thickness swelling of plywood, but lower water absorption than poplar veneer. In contrast, plywood made using poplar veneers showed bond strength identical to samples made using birch and hornbeam veneers. Microscopic studies have confirmed that the main mechanism of bonding wood veneer with thermoplastic polymer is a "mechanical connection" formed by the flow of molten polymer into the cavities of wood and the formation of adhesive locks. To improve the interfacial adhesion between the wood veneer and the thermoplastic film, preliminary surface treatment of the veneer with a solution of citric acid (CA) is proposed. This treatment improves the compatibility of hydrophilic wood and hydrophobic thermoplastic polymer. The indicators of water absorption and thickness swelling of plywood made with veneers modified with a solution of CA decreased, and the shear strength and bending strength increased. In particular, treatment of the veneer with CA allows to increase the bonding strength by up to 31% compared to plywood from untreated veneer. It was established that the properties of plywood bonded with the proposed types of films are not inferior to the corresponding indicators of plywood bonded with typical urea-formaldehyde adhesive. It was found that the shear strength of plywood bonded with films of LDPE meets the requirements of the EN 314-2 standard. Environmentally friendly plywood bonded with film can be successfully used indoors. The technology of plywood is proposed, in which instead of liquid synthetic adhesive between adjacent sheets of veneer, the use of thermoplastic polymers in the form of a film is provided, as well as the treatment of the surface of the veneer with CA. The method of manufacturing such plywood is protected by a patent of Ukraine for a utility model. Based on the conducted studies, the laws of the influence of the wood species, the type and thickness of the thermoplastic film on the properties of plywood were established, and a method of improving the adhesion of the thermoplastic polymer to wood was proposed.