Cationic cure

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Fig. 20. UV initiated cationic cure of epoxidized block copolymer in the presence of alcohol (Kraton Polymer s EKP-207 and L1203 inono-ol).

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... 

In comparison with cationic curing systems, hybrid curing systems increase the rate of cure, produce cured films with improved solvent resistance, and offer a greater formulation latitude. When compared to free radical cure, better... 

The chemistry involved depends on the oligomer used silicone acrylates cure by radical mechanism epoxy-silicones require cationic curing systems. Silicone... 

Oxman et al. (3) determined that cure speed and enthalpy were improved in photopolymerizable compositions when using anthracene derivatives containing electron donors as photoinitators for cationic curing. Dimethoxy-, diethoxy-, and diphenoxyanthracene were especially preferred. 

Because of their low viscosity, cycloaliphatic epoxies are often used to dilute other epoxy resins. These resins, however, have not achieved general importance in adhesive formulations because of relatively low tensile strength and because they do not cure well at room temperature. One major application for cycloaliphatic epoxies, however, is for adhesives and coatings that can be cationically cured by exposure to uv light. 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

Cationic Cure Epoxies. As with free radical uv chemistry, the same principles of formulation component selection apply to cationic cured uv epoxy adhesives. Although different monomers and oligomers are normally required for this type of chemistry, the main difference lies with the photoinitiator system. 

Cationic cured epoxies may also be crosslinked by electron beam radiation. A major application for this technology is the repair of composite aerospace structures. Direct benefits of EB processing include rapid cure, allowing completion of a permanent repair in the same or less time than a traditionally temporary repair, and ease of material handling. Other... 

Cationic cured systems have traditionally been aimed at niche applications or where ink based on free radical acrylate curing is not suitable. The raw materials used in cationic systems are generally different than those used in free radical cured systems. ... 

Other uses for the arylborates are UV cationic cure [108,109], biological activity as fungicides (structure 3) [110], and marine antifoulants for nylon fishing nets (structure 4) [111]. In the UV cationic cure, the counteranion forms, after photolysis, a superacid intermediate that catalyzes an epoxide or vinyl ether polymerization. 

A purely cationic curing can be induced by Lewis acids such as AICI3, BF3, ZnCl2, TiCU, or FeBr3. The cationic polymerization of the epoxy resins has received little attention and is mentioned only in connection with photo- or radiation chemistry initiation. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

Timpe classified the onium salts as bivalent initiators , that is, capable of initiating both radical and cationic cures simultaneously, hence capable of so-called dual cures of mixtures of radical and cationically polymerizable monomers [18]. This theme has been developed in the more recent review by Rabek [19]. More recently, use of photogenerated Bronsted or Lewis acids to effect decomposition of acid-labile polymers has found utility in photoimaging and been reviewed [20],... 

Mechanistically these two cases are quite different. For cationically cured epoxies, the initiator moiety (cation) cannot directly terminate the cure propagation but species created in association with the initiation step (anions) may carry the seeds of the termination process. For free radical initiation and propagation cures, the initiator can also fimction as a chain terminator. Thus the kinetics and mechanisms in the two case are quite different. We will discuss the two cases separately. 

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