Price: 10.00
Radiation curable release coatings contain no solvent and have the unique ability to be
cured at, or slightly above, room temperature. Polymerization is initiated by ultraviolet
light (UV) or electron beam (EB) exposure. Many successful release coating formulations
have been prepared using silicone based materials. Free radical systems based on
"thiol-ene" reactions have been described1. These systems, however, cure slowly and
have an offensive mercaptan odor. The free radical polymerization of acrylate functional
silicones offer a viable alternative and numerous patents on this technology have
appeared2. Free radical polymerizations, however, are inhibited by the presence of
oxygen and the high oxygen permeability of silicones amplifies this effect. Thus,
commercially acceptable cure speeds are best obtained with expensive nitrogen
inerting 3,4. Cationically curable epoxy5 and vinyloxy organo polysiloxanes have recently
been described 6. These coatings cure rapidly with UV exposure and, unlike free radical
polymerizations, the cationic polymerization is not inhibited by the presence of oxygen
thus eliminating the need for nitrogen inerting. In practice, however, these UV-curable
release coatings have several limitations. First, while UV-curable silicone release
coatings can readily provide a low energy surface giving low or "easy" release coatings;
controlling the level of release is difficult. Control release additives have been only
partially successful in developing moderate or "tight" release coatings. Secondly, many
potential end users will not use silicone based products due to potential contamination
of conventional coating lines. Silicones can cause adhesion and wetting problems when
present as a contaminant in other coating resins. Finally, silicones are frequently less
economically attractive than other organic resins.
A single example of a UV-curable nonsilicone release coating has been reported7. UV-curable
compositions based on mixtures of long-chain alpha olefin oxides are reported
to be useful in paper release applications. The cationic polymerization of alpha olefin
oxides, however, is much slower than UV-curable silicones. Nevertheless, the concept
of providing a nonsilicone release coating based on monomers containing long alkyl
groups deserves further consideration. The photoinitiated cationic polymerization of long-
chain alkylvinylethers represents a particularly attractive avenue of research. First,
vinylethers are one of the most reactive monomers toward cationic polymerization, and,
as mentioned earlier, cationic polymerizations are not inhibited by te presence of
oxygen. Second, long alkylvinylethers may be readily prepared from inexpensive
starting materials, Finally, the polymers derived from long-chain alkylvinylethers are
known to have a low surface energy8 and their copolymers have release properties 9. The
limited availability of soluble cationic photoinitiators has heretofore hindered the
development of vinylether based nonsilicone release coatings. However, photoinitiators
with improved hydrocarbon solubility have recently been reported.