The influence of veneer modification on the properties of plywood glued with thermoplastic LDPE film

Abstract

Plywood glued with thermoplastic films is a promising material in furniture production. After all, the replacement of liquid thermosetting adhesives with dry thermoplastic films in the technological process of plywood production not only provides a change in the emission class, but also improves production conditions. That is why the production of plywood glued with thermoplastic films is becoming increasingly important today. However, the wood composites glued with thermoplastic films have low water resistance compared to the thermoreactive adhesives. In addition, low adhesion to wood is characteristic for thermoplastic polymers. In this paper, a modification of veneer in the production of plywood glued with thermoplastic film of low density polyethylene (LDPE) is proposed. The main objective of this study was to investigate an effect of veneer modification on the properties of plywood glued with LDPE film. Citric acid as a modifying substance in concentrations of 5, 15, and 25% and thermoplastic LDPE film with a thickness of 100 µm were used in the experiments. The dosage of acid solution was 50, 100 and 150 g/m2. To compare the results, the plywood from unmodified veneer glued with a LDPE film and urea-formaldehyde adhesive (UF) was made. The results of this study showed that modification of veneer has a positive effect on the properties of plywood. It was found that water absorption and thickness swelling of plywood improve, while shear and bending strength increase. It was also found that the concentration of the citric acid solution significantly affects the properties of plywood, while the dosage of the citric acid solution does not. It was found that plywood made from veneer modified with a 15% solution of citric acid and glued with a LDPE film has the lowest water adsorption (38,5 - 39,24%). Plywood samples made from modified veneer glued with LDPE film met the requirements of the DSTU EN 314-2 in terms of shear strength. Modification of the veneer with an acid solution increased the shear strength of plywood to 2,11 MPa. It was noted that the modulus of elasticity of plywood made from modified and unmodified veneers bonded with LDPE film is somewhat lower than that of plywood bonded with UF adhesive, which is important for the manufacture of various bent complex structures. It was found that the surface treatment of birch veneer by citric acid in the production of plywood bonded with LDPE thermoplastic film improves the compatibility between differently polar materials, such as hydrophilic wood and hydrophobic thermoplastic polymer. This leads to improved bonding quality of veneer. The use of high concentrations and dosage of citric acid for surface treatment of birch veneer changes its color, which also indicates the possibility of chemical reactions between wood and citric acid compounds. Modification of veneer with citric acid in the production of plywood glued with a LDPE film has a great potential due to its environmental friendliness, availability, cost and ability to improve the properties of plywood, which is important for maintaining high operational characteristics of products.

Keywords

• plywood
• rotary-cut veneer
• veneer modification
• citric acid
• thermoplastic film
• water absorption
• thickness swelling
• shear strength
• bending strength
• modulus of elasticity