Radiation-induced cationic curing

Radiation-Induced Cationic Curing of Vinyl-Ether-Functionalized Urethane Oligomers... 

A system that has recently been receiving attention Is radiation induced cationic curing. The interest in cationic curing has been Inspired by the development of onlum salt catalysts. Strong acids are liberated when the coatings are Irradiated In the presence of certain onlum salts. These acids are capable of catalyzing cationic polymerization reactions (7). Onlum salts have been used mostly In UV curable systems. However, It has been shown recently that they may also be used for electron beam Induced curing (8,9). 

In 1982, Crlvello and co-workers published a report on the UV Initiated cationic polymerization of vinyl ether monomers using onlum salt catalysts(14). Vinyl ethers are among the most reactive monomers which polymerize by a cationic mechanism. The radiation Induced cationic curing of vinyl ethers occurs much faster than the cationic curing of epoxy coatings. In fact, cure rates that are at least as fast as the free radical polymerization of acrylates can be achleved(8,14). A recent report Indicates that the cationic polymerization of vinyl ethers can occur even in the presence of certain polar functional groups(15). 

Lapin, S.C., Radiation-induced cationic curing of vinyl ethers, in Radiation Curing, Pappas S.P. (Ed.), (pp. 241-272), Plenum Press (New York), 1992. 

The first coupling of a LINAC with infrared spectroscopy has been performed by Palmese et al. in order to study in situ kinetics of radiation-induced cationic polymerization of epoxy systems. The aim of the study is to understand the curing behavior of polymers under irradiation. A UV light source and an electron beam (10 MeV pulse width of the beam from 2.5 to 10 pm) are coupled to a portable near infrared (NIR) instrument. Briefly, a portable NIR spectrometer (Control Development Incorporated, South Bend, IN, USA) is used,... 

The main reason for the limited industrial application of radiation-induced cationic polymerization is that oligomers that offer high cure rates are not commercially available. So far, due to their excellent combination of chemical, physical and electrical properties, only multifunctional epoxy oligomers have been used, but these have poor cure speeds compared to acrylate based systems. This is a serious limitation. Interesting alternatives to the epoxys are oligomers based on highly reactive vinyl ethers (21. 22) and the recently developed distsrrene ethers (23). which are as reactive as vinyl... 

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

Yamamoto, Y. (1993) Pulse radiolysis of onium salts and radiation-induced cationic polymerization, in Radiation Curing in Polymer Science and Technology, vol. II (eds J.P. Fouassier and J.F Rabek). Elsevier Applied Science, London. 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

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. 

CRIVELLO J.V., Advanced curing technologies using photo- and electron beam induced cationic polymerization. Radiat. Phys. Chem. (2002), 63, 21-27. 

Another mode of dual curing involves the simultaneous occurrence of free radical and cationic radiation-induced cross-linking polymerization of formulations containing appropriate initiators [20, 23, 28]. This method, which is called hybrid curing, leads to coatings with imique properties. A typical hybrid-cure system contains a diacrylate and a diepoxide, the former polymerizing by a free radical and the latter by a cationic mechanism. Exposure of the system to in-... 

FP is not currently in commercial use, although there have been several patents issued related to it. The first is for an In depth curing of resins induced by UV radiation, which used a combination of UV hght to start free-radical FP [63]. In 2001, Gregory patented Ultraviolet curable resin compositions having enhanced shadow cure properties in which the frontal curing of cycloaliphatic epoxides was achieved with a cationic photoinitiator in tandem with a peroxide [64]. Pojman was awarded two patents on functionally gradient materials prepared by FP [65, 66]. The maker of chemical anchors, Hilti, has two patents on FP but has not commercialized either of them [67, 68]. 

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