Cationic photocurable systems

The mfichanisms for free-radical type photocurable and cationic photocurable systems can be discussed individually. 

In general, cationic photocure systems (4) are often deemed superior to free-radical photocuring processes since they are ... 

Dual Free Radical/Cationic Photocurable Systems... 

One of the more interesting possibilities of developing truly unique photocurable films lies in the ability of certain onium salts to generate both a reactive radical species and a strong acid upon photolysis. Indeed, several papers describe dual radical/cationic photocurable systems (16-18). The possibilities afforded by dual cure systems provide a unique mechanism for generating cured films with expanded properties for a number of applications. 

The choice to use one photocurable system over another is dictated by the particular requirements of the application and consideration should be given to both free radical and cationic photocurables. 

With the basic mechanism of free-radical and cationic photocuring presented in Schemes II and III, we turn to consideration of each of the individual components in UV curable systems. It is this aspect over which the formulator has control in producing a resin system which will cure in an appropriate environment (temperature, air, etc.) at reasonably fast rates to generate the type of film desired. 

Cho and Hong (2005) used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated cationic photopolymerization of 1,4-cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4-diethylthioxanthone (DETX) in the presence of a diaryliodonium-salt photoinitiator. Two kinetic parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system using an auto-catalytic kinetics model as shown in the following equation ... 

Photopolymer systems are photocurable resins incorporating reactive liquid monomers, photoinitiators, chemical modihers and hhers. Typically stereolithography utilizes UV radiation, so UV-curable systems are used. Free-radical-photopolymerizable acrylate systems were originally used however, newer cationic epoxy-resin and vinyl ether systems (based on iodinium- or sulfonium-salt cationic initiators) are now being utilized. 

Photocuring kinetic of UV-initiated cationic photo-pol5rmerization of divinyl ether shows a very low activation energy comparing to the two epoxies studied above. The Epiclon HP-7200/ Rapi-Cure DVE-3 system has been optimized by Differential Photo Calorimetry (DPC) in presence of cationic photoinitiator Cyracure UVI-6976. 

Crivello and Lam have demonstrated the use of diaryliodonium salts as photoinitiators for polymerization of electron-rich olefins, cyclic ethers, cyclic sulfides, lactones and spiro orthoesters, but a vast majority of their published work concerns polymerization of substituted oxiranes, illustrating the potential of such systems in photocuring of epoxy resins. Such polymerizations can be quite fast in the most favorable example, a 93% yield of polymer of Mn 10,700 was obtained from 3-vinylcyclohexene oxide after only 90 seconds of irradiation at room temperature with 4,4 -di-tert-butyldiphenyliodonium hexafluoroantimonate as initiator (3). The substituted salts are often preferred to the simple unsubstituted diphenyliodonium compounds for reasons of solubility (2). The use of diaryliodonium salts in combination with various dyes allows one to initiate cationic polymerization with visible light (5). 

CHO 05] Cho J.-D., Hong J.-W., Photocuring kinectics of UV-initiated cationic polymerization of a cycloaliphatic diepoxide system photosensitized by thioxanthone , European Polymer Journal, vol. 41, no. 2, pp. 367-374, 2005. 

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