Electron-beam radiation photoinitiators

Films of polyphthalaldehyde, sensitized by cationic photoinitiators, have been imaged at 2-5-mJ/cm in the deep ultraviolet (DUV) (see Section 3.10), at 1 pC/cm (20 kV) electron beam radiation and at an unspecified dose of Al-A x-ray radiation. The ultimate utility of this "self-developing" resist system will depend upon its efficacy as an etch barrier. It seems clear that such materials would not serve as adequate etch masks for... 

In the past, electron beam radiation was applied to produce PSA exclusively however, recent improvements in UV curing technology (precise UV dose control, suitable photoinitiators) permit UV to be used to produce pressure-sensitive adhesives. PSA formulations can vary in consistency from low-viscosity liquids up to solids melting at 80°C (176°F). Therefore, applications may vary from screen printing to roll coating to melt extrusion. Coat weights for most PSA materials vary from 1 to 10 g/m. ... 

The majority of commercial methacrylic ester polymers are produced by free-radical initiators. Peroxides and azo compounds ftinction as t5ipical initiators for this type of polymerization. Other possible routes for producing methacrylic polymers with radicals include photoinitiation and radiation-induced polymerization. Both Y ray and electron-beam radiation have been employed in the production of methacrylic ester polymers (36-38). At constant temperature, there is a first-order dependence of the polymerization rate on monomer concentration and a one-half-order dependence on initiator concentration. Rate data for the polymerization of various methacrylic monomers using the azo compoimd 2,2 -azobisisobut5ironitrile [78-67-1] (AIBN) are shown in Table 8. 

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 type of crosslinking achieved with electron beam and uv radiation is very similar, but the way curing initiates is different. Electron beams have the higher energy, and the electron itself has sufficient energy to initiate polymerization. In uv curable materials, the polymerization reaction is not directly initiated by uv light, and a photoinitiator is required to interact with the uv radiation and produce the initiating species. 

Examples of such photochemical acid generators are shown in Chart 3.2. These onium salts, which are cationic photoinitiators originally developed for curing of epoxy resins (i09), can be used to formulate cross-linking negative resist materials (JOS), are very sensitive to electron beam and X-ray (JOS, 107, 108) radiation, and can be sensitized to longer wavelength radiation (JOS, 110, 111). 

Electron Beam-Cured Inks. Electron beam-cured inks are similar in principle to ultraviolet light-cured inks except that no photoinitiator is needed. Vinyl polymerizations may be initiated Iqr any form of ionizing radiation, e. g., neutrons, a-particles, y-rays, and x-rays, as well as by high-energy electrons ( 5-rays). The mechanism of initiation is more complex than that of photochemical initiation in that radiation of vinyl monomers gives cations and anions as well as free radicals however, most radiation-initiated polymerizations are radical-initiated because the cations and anions formed are not stable at the temperature of polymerization and therefore dissociate to form radicals. 

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. 

Alessi et al. (2005) examined the y-irradiation and electron-beam processing of an epoxy-resin system in the presence of a photoinitiator. They showed that increasing the irradiation dose frequency and photoinitiator concentration greatly increases the temperature reached by the samples. The increase in temperature of the system during irradiation is related to the balance between the heat-evolution rate, due to both polymerization reaction and radiation absorption, and the rate of heat release towards the environment. High dose rates and high photoinitiator concentmtions increase the reaction rate and the difference between the heat produced and the heat released to the environment (Alessi et al. (2005). 

Free-radical, radiation-curable systems generally consist of monomers, oligomers, photoactivators, other resins, and fillers or tackifiers. An ultraviolet source or electron-beam generator is used to cure the systems. As an illustration Stueben (37) describes a typical UV-cured PSA system containing acrylates and polyvinyl ether. McGinniss (38) discusses formulation design related to UV-curable systems. The cationic photoinitiators are shown in Fig. 1. 

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

Due to their main chain double bond, polydiene rubbers are prone to oxidative degradation and can be cross-linked by reactive resins and sulfur compounds [227, pp. 233-261). Cross-linking can be achieved further by an electron beam (EB) and in presence of photoinitiators by UV radiation [228]. 

The polymerization of the monomer in acrylic engineering adhesives can be initiated by electron beam or ultraviolet (UV) radiation, provided that the adherends or the fillers in the adhesives are not barriers to radiation. Acrylic monomers are generally more reactive to radiation than methacrylates. Electron beams generate free radicals directly in the adhesive, whereas UV curing requires a photosensitizer or photoinitiator to provide free radicals. 

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