Лако-красочные материалы — производство

The manufacture of Twist-off style closures requires the application of an internal sealing compound for good vacuum retention after sealing onto glass jars. To ensure good compound adhesion, the internal lacquer must be the final application, thus requiring the external system to show minimal discoloration when subjected to this re-bake. For this reason acrylic and polyester coatings have been traditionally used in this area of metal decorating.

The most common system comprised vinyl size, acrylic coating and epoxy ester varnish. This system was successfully modified to eliminate the use of size, but more recently, polyester coatings and varnishes have become the most commonly used systems, because of their greater flexibility.

Disperse items 1 & 2 by a Cowles mixer or bead mill. Uralac SN808 S2G3-50 (Ex DSM) is a saturated polyester resin specially designed for metal decorating and for cure by a benzoguanamine in the ratio 90/10 — 80/20 (solid/solid). In this example a ratio of 87/13 is used. To optimise flexibility, crosslinking with benzoguanamine, and also blocked isocyanate Desmodur BL 3175, are used to increase adhesion. Adhesion is further enhanced by the use of an epoxy resin (Epikote 836 (Ex Shell)).

The high flexibility requirements of aluminium closures have commonly been met through the use of a three coat vinyl system of size, white and varnish. In striving for improved applied costs, size coats have been eliminated through the use of retreated aluminium and the replacement of vinyl coatings by polyesters.

A requirement for external closure coatings is low discoloration when it is subjected to phenolic fumes. Closures traditionally have a phenolic containing internal lacquer. This is normally applied after the external decoration. During the prolonged stoving of the internal lacquer, phenolic fumes are generated and may cause discoloration. The application of the internal lacquer before external decoration minimises this problem. However, the practice of applying internal lacquers as the final coat is considered to perform a ‘functional barrier’ against any offsetting of the external coating onto the underside of the sheet immediately above it in the stack. Different resin systems have different yellowing characteristics in the presence of phenolic fumes. It is generally considered that acrylic resin based systems suffer less from yellowing when exposed to phenolic fumes than polyester based systems.

One of the reasons that water based coatings find limited usage in sheetfed systems is the difficulty of applying them by rollercoater methods, where they tend to give inferior film properties in comparison with solvent based ones. As the VOC is reduced, so the problems increase. Nonetheless, water based systems will gradually make inroads into the sheetfed metal decorating market. The high capital cost of incinerators necessary for VOC compliance in Europe and the USA will force may smaller operations either to use water based VOC compliant systems or close their operation.