Лако-красочные материалы — производство

8.4.1.3 Physically Setting Systems

The ‘workhorse’ of the wood processing industry, apart from the manufacture of engineered wood products made from strands and fibers, is white glue, an adhesive based on polyvinyl acetate dispersions, either in the form of homopolymers or copolymers in combination with acrylates or functional monomers (e. g. malein acid derivatives). The adhesive film is physically produced by the evaporation of water. This process requires a minimum temperature at which the film is formed; this is known as the MFFT (minimum film-forming temperature), and typically ranges between 2 and 15 °C. If the drying process of the bond-line takes place at a temperature below the MFFT, a chalky or crumbly layer without any efficient strength will be formed rather than a compact adhesive film. Generally, the setting of adhesives is accelerated by heat (i. e. by using heated presses). However, this method does not work efficiently in the case of a greater thickness of the wood layer, because wood is a good heat insulator and hinders heat transfer from the pressing jaws to the adhesive bond-line. Owing to the high dipole moment of water, radiofrequency fields may be used instead. These are absorbed by the bond-line to a greater extent than by the surrounding wood, and are able to penetrate even thicker components.