This invention relates to the radiation curing of polymerizable coatings, such as is commercially practiced on many types of substrates. Radiation curing speeds up the application of coatings and generally requires the use of photoinitiators.
There are two principal types of such photoinitiators, cationic and free radical, which are selected depending upon the species of polymerizable materials used. For example, cationic photoinitiators are required by vinyl ethers and epoxy compounds, while free radical photoinitiators are employed with acrylates. When free radical photoinitiators are used, it is generally preferred to carry out the polymerization in an inert atmosphere such as nitrogen. Otherwise, the polymerization reaction is slowed by the presence of oxygen. Materials which are radiation cured with cationic photoinitiators need not avoid air and thus have an advantage over materials which require an inert atmosphere to be most efficiently cured.
In the past, where both types of polymerizable compounds have been used together, both types of photoinitiators have been included. For example, in U.S. Pat. No. 4,156,035 mixtures of epoxy resins and acrylates are shown to be cured by radiation in the presence of both cationic and free radical photoinitiators. Dougherty and Vara in Rad Tech 90, North America, Conference Proceedings, confirmed this and reported on their experiments with mixtures of acrylates and vinyl ethers in which they employed both types of photoinitiators. They found that the use of cationic photoinitiators alone gave inferior results compared to mixtures of both types. More importantly, they found that free radical initiators failed to provide sufficient curing when both acrylates and vinyl ethers were present.
In U.S. Pat. No. 4,999,216 the patentees reported on a formulation which employed only free radical initiators with mixtures of vinyl ethers and maleate or fumarate esters which are known in the art to copolymerize. The coating formulation was intended for use on concrete floors and, as the patentees pointed out, cationic photoinitiators could not be used on a basic substrate since they involve the release of acid compounds. The assignee of the '216 patent stated in PCT/US90/01243 that vinyl ethers do not homopolymerize in the presence of free radical initiators and that they are therefore not replacements for (meth)acrylates.
Many of the radiation cured coatings in commercial use today include acrylate compounds and these are often blended with undesirable reactive diluents, such as N-vinyl pyrrolidone and acrylate monomers. Consequently, it is important to limit the use of such undesirable compounds and the substitution of vinyl ethers has been considered. Since vinyl ethers are radiation cured in the presence of cationic photoinitiators and acrylates are cured using free radical initiators, one might expect that mixtures of both types would be preferred. The present inventors have found that mixtures of multifunctional acrylates and vinyl ethers can be satisfactorily cured by radiation using only free radical photoinitiators, as will be seen in the discussion below.