Photoinitiators hybrid curing

Initiation of UV Hybrid Curing Kinetics of Photoinitiated Reactions... 

Recently, Endo and coworkers have developed novel sulfonium type initiators that can initiate polymerization either upon irradiation or upon thermal treatment. In addition, these photoinitiators are shown to be functional for both cationic and radical polymerizations. This dual activity is particularly important in hybrid curing systems for coatings and adhesions [28]. Scheme 11.6 contains the general steps for the synthesis of these photoinitiators from sulfides. 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

Hybrid systems are understood to be photocurable compositions that cure by both ionic and free radical mechanisms. As can be seen in Chapter 2, section 2.3.4. photoinitiators were actually designed for use with systems that contain both type of monomers, or prepolymers, and both to cure by both mechanisms. One such example are combinations of phosphonium and sulfonium salts described in that chapter ... 

Another hybrid system has been proposed for resist applications the simultaneous chain polymerization of N-methylolacrylamide and the acid-catalyzed crosslinking of the resulting polymer with poly(vinyl alcohol) In dual cure plastisols, finally, a solid thermoplastic resin is dispersed in a multifunctional aciyrlate mixture which contains a thermal and a photoinitiator. These plastisols have been proposed for the encapsulation of integrated circuits... 

The in situ intercalative polymerization associated with the UV curing is a technique which was successfully employed in the synthesis of hybrid films, when fast polymerization of liquid monomers yielded in solid materials with designed properties. It was proved to have high eflftciency for epoxy oligomers, vinyl ethers, oxetanes in the presence of onium salts as photoinitiators [232]. The literature is not abundant in reports on the in situ UV-initiated polymerization of epoxides in the presence of layered silicates [233-237], as compared to data on thermally cured or melt compounded nanocomposites. In some studies, the clays were used either unmodified [235, 237] or organically modified [233, 236, 238] or treated with various reagents able to change their surface properties [239-243]. 

The introduction of so-called hybrid resins represents the beginning of the next phase of materials development. Hybrid resins comprise a minor fraction of acrylic-based component that guarantees rapid aoss-linking to reach form stability within short processing times. The majority of the resin is constituted by epoxy-ftmctional compounds exhibiting comparatively low polymerization shrinkage during slow cationic cure. Development of hybrid resin formulations requires careful choice and adjustments of the included compounds, especially of cationic and radical photoinitiators. 

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