Progress in radiation cured coatings

Although cure of paint coatings by radiation is no new phenomenon, there is considerable increased interest in development of industrial processes using radiation curing for a variety of reasons. Before studying the developments in detail, it is worth noting that radiated energy can basically be divided into two categories. In the first category we have ionising radiation which for our purposes includes electromagnetic radiation of wave lengths less than visible light and also accelerated electrons. In the second category is radiation, which is essentially thermal in its effect ranging from infra red through to microwave and radio frequency. The use of these longer wave length radiations, whose effect is essentially thermal, will not be discussed in detail since all coatings which can be cured in normal high temperature ovens are basically capable of cure with such methods. It is worth noting however, that improvement in the design of infra red lamps with peak radiation in the near infra red at around 1200 nanometers has led to speed up in infra red curing which has been particularly useful in cure of alkyd‐amino finishes on heat sensitive substrates such as wood. There is also developing interest in high frequency long wave length radiation, particularly in the area usually referred to as microwave. Here we have penetrative radiation which produces heat effects by what is sometimes referred to as molecular friction. The conversion of the microwave energy into heat depends on the loss factor of the material in question and so the process is most efficient with polar materials such as water. The most likely use of microwave heating is with water based coatings on substrates which will not themselves heat up rapidly, such as non‐polar plastic films.