Silicone compositions have become widely accepted as release coatings, which are useful to provide a surface or material which is relatively nonadherent to other materials which would normally adhere closely thereto. Silicone release compositions may be used as coatings which release pressure sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM., and other such substrates are also useful to provide non-stick surfaces for food handling and industrial packaging.
Previously developed silicone release products have been heat-curable, however, the high energy costs and safety and environmental considerations associated with high temperature oven curing has provided the incentive for developing alternative technologies, such as ultraviolet radiation-curable silicones.
Ultraviolet (UV) radiation is one of the most widely used types of radiation because of its low cost, ease of maintenance, and low potential hazard to industrial users. Typical curing times are much shorter, and heat-sensitive materials can be safely coated and cured under UV radiation where thermal energy might damage the substrate.
Three basic UV-curable silicone systems have been developed: Epoxysilicone systems, acrylic-functional silicone systems, and mercaptofunctional silicone systems.
Epoxysilicone systems, such as those described in U.S. Pat. No. 4,279,717 (Eckberg et al.) and commonly assigned copending U.S. application Ser. No. 375,676, filed May 6, 1982, feature epoxy-functional diorganopolysiloxane base polymers catalyzed by onium salt photoinitiators. The compositions exhibit extremely high cure rates but depend on expensive materials which are not widely available.
Acrylic-functional silicone systems such as described in commonly assigned copending U.S. application Ser. Nos. 239,297, filed Mar. 2, 1981, and 375,676, filed May 6, 1982, and also modified systems described in U.S. Pat. No. 4,048,036 (Prucnal) and U.S. Pat. No. 4,017,652 (Gruber), provide serviceable coatings that are UV-curable in the presence of free radical-type photoinitiators but require complex, multistep preparation.
Many mercapto-functional systems are known: For example, U.S. Pat. Nos. 4,064,027 (Gant), 4,107,390 (Gordon et al.), 4,197,173 (Curry et al.), Japan Kokai Tokkyo Koho No. 79 48,854 (to Takamizawa; Chem. Abstracts 91: 58888r, 1979), U.S. Pat. Nos. 3,661,744 (Kehr et al.), 4,070,526 (Colquhoun et al.), 4,052,529 (Bokerman et al.), 3,873,499 (Michael et al.), and 3,816,282 (Viventi) disclose mercaptofunctional polysiloxane or polythiol compositions which are UV-curable when combined with an ethylenically unsaturated organic compound, which curable compositions also contain, variously, mercaptoalkyl polysiloxane cure accelerators, acetophenone-type photosensitizers, silacyclopentenyl curing agents, cure rate accelerators and gellation inhibitors. These provide a wide range of serviceable release coating compositions, however, the acceptance of this technology has been hindered by several persistent disadvantages including dependence on scarce or expensive starting materials, unserviceably slow curing rates, complex processing, and offensive odors (associated with the mercaptan group) which persist in the cured products. Some of these specific disadvantages have been addressed, for example, in the Gant patent, photosensitizers such as acetophenone are added to assist radiation curing, in U.S. Pat. No. 4,171,252 (Fantazier), photopolymerization of unsaturated compounds is catalyzed by peroxy-napthalenic compounds, and in the aforementioned Kehr et al. and Colquhoun et al. patents aromatic ketones are employed to accelerate the cure; however, there is still a need for improvement and reduction of costs in mercaptofunctional compositions and related UV-cure technologies.
New mercaptoalkoxyalkyl-functional silicones have now been discovered which can be synthesized in a two-step, one-vessel process from readily available, inexpensive materials and which do not emit disagreeable odors. In addition, it has been discovered that certain perbenzoate esters are suitable photoinititators for polymerization reactions between mercaptofunctional compounds and vinyl-functional compounds; and the cure characteristics of the novel reactive polysiloxane/photoinitiator blends of the present invention may be enhanced or modified by certain aromatic photosensitizers or by judicious selection of the molar ratios of reactive incredients in such blends.
All of the patents and patent applications mentioned above are hereby incorporated by reference.
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