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For many years it has been clear that an obvious home for radiation curing is the coil coating industry. The process involves the application of paint to a rapidly moving flat substrate. Advantages such as reduced energy costs, shorter curing zones and a greater degree of control over the curing process are often cited. So why have no coil coaters switched to this technology?The answer is performance! Coil coatings are high performance systems which provide a combination of flexibility, abrasion resistance, aesthetic appeal and durability. Many coatings are applied to metal destined for architectural uses. They thus have to maintain gloss and colour for as long as possible in the face of often adverse weather conditions and of course they must remain adhered to the metal surface.
Adhesion is not a simple phenomenon, particularly when the metal substrate is formed after painting has taken place. Radiation cured systems need the right thermo-mechanical properties, discussed previously, in order to withstand the forming process. In addition the adhesion test which involves pulling off adhesive tape with a sudden jerk requires a certain amount of the energy to be dissipated by viscous loss within the coating. Hence the high crosslink densities attained with radiation curable systems are not conducive for passing the various tests which combine adhesion and flexibility.
Acid adhesion promoters have been suggested in the past for acrylate based but the increased susceptibility to corrosion and hydrolysis of ester linkages limits their use. Hence the burden has fallen on cationic chemistry as proponents of these systems have consistently claimed that the system has better adhesion due to a reduced tendency to shrink on cure. This paper will discuss how sophisticated surface analytical techniques have been used to interrogate the interfaces between these types of coating and the metal substrate and thereby identify the effects that the components have on the performance. In addition one particular pretreated surface will be charactensed and the consequences this has for good adhesion discussed.
