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Radiation curable silicones have evolved to commercial success in wide use for
release coatings. The chemical literature is rich with descriptions of radiation curable
silicone compositions whose crosslinking chemistry is based on acrylate (1-5) and
mercapto-olefin (6-10) free radical systems, as well as cationic-type systems (11-15).
Silicone release products have common performance requirements regardless of
crosslinking chemistry, chief among which is providing the end-user the means to
predictably vary release (the force needed to remove a pressure-sensitive tape or label
from the silicone coating) from very low (easy, or premium) to very high (tight, or
controlled). Ideally, this is done by providing two silicone polymers, an easy release
silicone and a tight release silicone which can be blended prior to coating in the proper
ratio to provide a desired level of release upon cure. The tight release polymer of this
pair is commonly called a 'controlled release additive' or CRA.
Early industry efforts to furnish radiation-curable silicone CRA products fell short
of customer needs. In part, performance short-falls resulted from the inherent nature of
radiation - curable silicones : low molecular weight (to permit solvent-free application),
highly organofunctionalized (for fast cure response and photocatalyst compatibility)
polydimethylsiloxane molecules. The structure of such polymers can be depicted as:
MDx*DyM or M*D* xDyM, where
M = (CH3)3SiO1/2, D = (CH3)2SiO2/2, D* = (K)(CH3)SiO2/2, and M = ((K)(CH3)2SIO1/2,
where K = a radiation sensitive group including acrylate, mercapto, olefin, epoxide, and
vinylether, and x and y are integers, the sum of which is typically 50 to 150. Release
provided by these cured silicone coatings results from the chemically inert, low surface
energy polydimethylsiloxane surface. Substitution of polar radiation-crosslinkable
functional groups for methyl groups in the polymer necessarily compromises release
characteristics to some extent, so design of radiation-curable silicone polymers is a
blalance between high reactivity-fast cure and low reactivity premium release silicones.
