Electron beam curing radical reactions

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. 

The polymeric systems are usually composed of a polymer which Imparts the majority of physical properties and actinic additives. In simple systems such as curing films or electron beam resists, the polymer is also the radiation sensitive species. In most cases, the formulations behave simllarily in their response to high energy irradiation. Practically any polymer can be made radiation sensitive by direct exposure to ionizing energies or by formulation with additives such as free radical precursors. Thermally sensitive polymers are also likely to undergo a similar reaction when exposed. 

In 1983, radiation curable 100 % solvent-free silicone acrylates were introduced into the market [4], This system provide the opportunity to be cured by either ultraviolet light (UV) or electron beam (EB). Similar in concept to peroxide initiation, silicone acrylate systems employ photoinitiators to generate free radicals and initiate cure, which is based on the polymerization of the acrylic C=C double bond via a radical chain reaction. 

Both electron beams and ultraviolet light initiate free-radical polymerizations with very low activation energy. This allows high polymerization rates at room temperature because the rates are not temperature dependent. Once initiated, free-radical polymerizations follow typical paths. It is, however, peculiar to radiation curing of coating materials that the gel states form at very early stages of the reactions. This is due to extensive use of polyfunctional monomers and prepolymers. In fact, it was demonstrated that the gel points occur at around 5% conversion of the prepolymers in typical commercial formulation, yet conversions to about 63% of the prepolymers to polymers are required to obtain dry films. ... 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

In radiation curing processes, addition reactions are activated by various forms of radiation infrared, microwave, radio frequency, gamma rays, ultraviolet (UV), and electron beam [2,3]. Basically, the energies of the first three are such that they simply thermally activate the system, that is, heat it, and are used in conjunction with the usual free-radical initiators. 

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