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Cationic PDF Print E-mail

Cationic Systems

Among different types of monomers which can be polymerized by cationic way, only those functions with epoxy or vinyl ethers have been widely used in cured resins.

  • Vinyl ethers

Among the monomers polymerise by cationic, vinyl ethers and propenyl are considered the most reactive. In the presence of a sulfonium salt, of the photopolymerization of triethyleneglycol divinyléther (Figure 7) runs as fast as a diacrylate, reaching a conversion rate close to 100% within a few tenths of a second. The high reactivity of these monomers from the highly electronegative character of the double bond and stabilization of the carbocation by resonance:

  • Epoxides
In the case of tétrafonctionnels epoxy monomers or oligomers epoxides téléchéliques, curing develops in three dimensions and leads to a strongly bridged polymer. The rate of polymerization is much lower than that of acrylate monomers, mainly due to a lower value of the rate constant of propagation kp. Monomer aliph dicycloépoxyde is one of the most used because of its high reactivity. The addition of difunctional monomers époxysilicones helped increase the speed of polymerization and the final conversion rate, because of the greater mobility of network chains. The crosslinked polymer has excellent mechanical properties, combining flexibility and hardness and a very good resistance to solvents and heat.

Cationic photoamorceur

Salts diaryliodonium or triarylsulfonium give Brönsted acids in presence of a hydrogen donor molecule and the photolysis. These acidic protons are capable of initiating cationic polymerization of monomers which do not polymerize by free radicals such as vinyl ethers or heterocyclic monomers (epoxides, lactones, cyclic ethers, époxysilicones).

These two types of photoamorceur act on different chemical functions. In addition, the photoamorceur cationic are insensitive to oxygen, which inhibits radical reactions on the surface and lead to systems "live" that continue to respond as long as monomers even in the dark.