Epoxy resin curing free radical systems

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. 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acryHc (52). The acryhc-vinyl monomers are grafted onto preformed epoxy resins in the presence of a free-radical initiator grafting occurs mainly at the methylene group of the aHphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, soHd epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino—formaldehyde resin. 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

In 1987, UVEXS [145] claimed simultaneous cationic and free radical polymerization of a mixture of a cycloaliphatic epoxy resin, a hydroxy functional polyether terminated polysiloxane, an acrylate functional resin, a triarylsulfonium salt, and a free radical photoinitiator. A simultaneously cured cationically and free radically polymerized system consisting of an epoxy resin, a methacrylate monomer, an onium salt, a carbonyl type free radical photoinitiator, and tetrahydrofurfuryl alcohol accelerator was patented by Cook Paint and Varnish [146] in 1987. 

The compositions of materials photocrosslinkable by cationic mechanism consist of mixtures of various vinyl ethers, or epoxides, or both. Difunctional cycloaliphatic epoxides have been used extensively in some UV curable systems, often as diluents for the various epoxy resins described in Chapter 6. Use of various divinyl ethers is also extensive. Because some cationic photoinitiators also generate free radicals, some compositions may contain mixtures of both types of materials, those that cure by cationic and those that cure by free-radical mechanisms. 

Adhesive systems based on free-radical curing epoxymethacrylates are cited (16). And Dudgeon (17) has covered cationic, heat-curable rubber-modified epoxy resin systems which very likely have latent-cure, structural adhesive capability. 

VE resin exhibits desirable mechanical properties like epoxy and simultaneously offers processability like a polyester resin. Like UPE resins, VE resins are cured using a free-radical intiator in combination with an accelerator [78-80], The commonest room temperature curing VE resin system consists of MEKP (1-2 wt%) and cobalt... 

A curing agent for polyurethanes and epoxies, as well as a chain extender for polyurethane and polyurea http //www.albemarle.com/n)S/Curatives/SC7003L ETHACURE 100.pdf An amine cure accelerator that is used to promote free-radical formation in addition polymerizations such as unsaturated polyester resin, viny ester, and acrylate systems http //www.albemarle.com/ TDS/Curatives/SC0006F FlRSTCURE MHPT.pdf... 

Allylacetone, methallylacetone, 2-allylcyclohexanone, l-phenyl-4-penten-l-one and other allyl substituted ketones, in the presence of free-radical initiators, copolymerize with MA to give equimolar copolymers in good yields.Copolymerization with maleimides also occur but non-equimolar copolymers are obtained. The anhydride copolymers exhibit good solubility in many solvents, are compatible with many resin systems, and are useful for curing epoxy resins for castings and adhesives. 

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