Cross cationic curing

Photoinduced effects, in hydrogenated amorphous silicon, 22 139 Photoinduced electron transfer, 19 111 Photoinitiated cationic curing, J 0 414-415 Photoinitiated cross-linking, 15 157 Photoinitiated free-radical... 

Figure 8.9 Scheme of the thermally cross-linked products obtained using anhydride and cationic curing methods. 

Condensation cure can also be carried out ia emulsions (200—209). In this case, the cross-linker and polydimethylsiloxanediol are emulsified usiag anionic, cationic, or nonionic surfactants ia water, and a condensation catalyst such as dibutyltin dilaurate is added. The polymer can then undergo cross-linking, forming a continuous film when the water is evaporated. 

Eor instance, exhaust appHcation is possible with cationic finishes which have an affinity for the anionic groups in polymeric materials. After appHcation, the textile is dried. Durable antistatic finishes require cross-linking of the resin. Cross-linking is usually achieved by subjecting the treated, dried material to heat curing. A catalyst is often incorporated to accelerate insolubilization. 

Butyl rubber (HR) is widely used for inner tubes and as a sealant. It is produced using the cationic polymerization with the copolymerization of isobutylene in the presence of a small amount (10%) of isoprene. Thus, the random copolymer chain contains a low concentration of widely spaced isolated double bonds, from the isoprene, that are later cross-linked when the butyl rubber is cured. A representation is shown in structure 5.20 where the number of units derived from isobutylene units greatly outnumbers the number of units derived from the isoprene monomer. The steric requirements of the isobutylene-derived units cause the chains to remain apart giving it a low stress to strain value and a low Tg. 

A considerable number of detailed descriptions on synthesis, production, and applications of epoxy resins exists. Because the aim of this chapter is the application of cationic initiators, and more particularly photoinitiators, to the polymerization of epoxies leading to cross-linked products (curing reaction), only litterature dealing with these aspects will be cited. For the general aspects of epoxy resins the scientific and patent literature may be found in detailed reviews [119,120] and classical books [121-123]. 

The curing by photoinitiated cross-linking and particularly by cationic photoinitiation is of great interest for many technological applications. Cationic photoinitiators have been developed in the past 20 years particularly in the group of Crivello and Lam [125]. Many excellent reviews and books cover the field of cationic photoinitiators. Ln the present paper limited literature is given to facilitate finding other necessary references [126-129]. 

Examples of such photochemical acid generators are shown in Chart 3.2. These onium salts, which are cationic photoinitiators originally developed for curing of epoxy resins (i09), can be used to formulate cross-linking negative resist materials (JOS), are very sensitive to electron beam and X-ray (JOS, 107, 108) radiation, and can be sensitized to longer wavelength radiation (JOS, 110, 111). 

Traces of water from air are sufficient to promote the hydrolysis of =SiOEt groups in a thin polymer film and to give cross-linked material. Alkoxysilane groups hydrolyze rapidly under UV radiation with cationic photoinitiator, which was proved by NMR in the solid state (disappearance of resonances corre onding to OEt groups in cured materials). However, because of the low ratio of water to alkoxysilane, the condensation was relatively slow and might have not... 

TGA analyses were performed for polymer samples having different degrees of cross-linking. The decomposition of the linear oligomer starts at about 200 °C. Once cured and baked, the formed siloxane network is more thermally stable, and the decomposition begins at temperatures higher by 100-150 °C. The results are similar to those reported for analogous Tsi-modified siloxanes cross-linked by means of photo-initiated cationic polymerization of epoxides [8]. 

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