Impact of MDF Board Side Plane Instability on Lacquer Film Crack Appearance

This paper presents the research of a specific problem met in woodworking practice by the company K that manufactures door frames made of MDF boards (medium density fi breboards). Visible side surfaces of MDF door frames were lacquered with white polyurethane lacquer. Thirty days after installation, longitudinal surface cracks appeared in the lacquer fi lm on side planes of the door frame. Within the research of the said appearance of cracks, testing was carried out of technical properties and free formaldehyde concentration of the original door frame made of MDF boards by the manufacturer K, MDF boards of the same manufacturer from door frame plant, MDF boards made by the manufacturer H and lacquered experimental MDF boards of both manufacturers. The lacquered surface structure was tested by applying 97 % sulphuric acid (H2SO4) on 5 mm thick lattices that were cut out from side planes of experimental and original door frame samples. The action of acid destructed the lacquer final layer, and pigmented base with other additives was scraped off the surface. It was noticed that the
lacquer film on the original door frame made of MDF panels by the manufacturer K had no two-component polyurethane clear base, or that it was applied in a very small quantity. Lacquer film cracking on side planes is the consequence of in-depth cracks in the structure of MDF panels that occur thirty days after lacquering. Experimental boards designated KE, produced by MDF manufacturer K, have an optimum layer of two-component polyurethane clear base, two-component polyurethane white base and two-component polyurethane semi gloss enamel paint. On side planes of these boards no distinctive cracks were observed. In lacquering MDF boards, it is necessary to apply two-component polyurethane clear base on side planes, and to fully protect the door frames with polyurethane films, and not only the visible planes. The use of load bearing MDF boards is necessary in dry conditions - MDF.LA type, and in increased relative humidity, fibreboards are required for general use in humid conditions - MDF.H type or load bearing boards for use in humid conditions - MDF.HLS type. Thin MDF board, glued to a thick MDF board, must be resistant to water or be an HDF fibreboard.