Initiation by electron beams

Initiation by Electron Beams. According to Bowden Yoffe ( Fast Reactions in Solids Acad Press, 1958, p 114) the initiation of lead silver azides by an electron beam is at least in part due to bulk heating of the... 

The primary effect of any ionizing radiation is based on its ability to excite and ionize molecules, and this leads to the formation of free radicals, which fhen initiate reactions such as polymerization and cross-linking or degradation. Accelerated electron beams have energy sufficient to affect the electrons in the atom shell, but not its nucleus, and can therefore only initiate chemical reactions. Typically, the reactions initiated by electron beam are extremely fast and are completed in fractions of a second. 

Polymerization and polymer grafting to substrate surface is initiated by electron beam, plasma, gamma ray, and ultraviolet irradiations in the presence of monomer (and photoreactive compounds). 

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. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Silicone acrylates (Fig. 5) are again lower molecular weight base polymers that contain multiple functional groups. As in epoxy systems, the ratio of PDMS to functional material governs properties of release, anchorage, transfer, cure speed, etc. Radiation induced radical cure can be initiated with either exposure of photo initiators and sensitizers to UV light [22,46,71 ] or by electron beam irradiation of the sample. 

Initiation by these methods can be carried out by either gamma rays or by electron beam. Usually grafting has been carried out either by the mutual or the preirradiation method using Co as a source of gamma rays. 

Therefore one pair of ions produces one OH and one HO2 radicals. The total amount of radicals, which are produced in flue gas by electron beam irradiation, is possible to calculate using reported G-values. The main radicals produced initially through direct and ionic decomposition processes are OH, N, HO2, O, and H. 

Polyethylene may be crosslinked by electron beams or by free-radical initiators. These polymers have elastic memory, i.e., stretched crosslinked Aims or tubing shrinks to the original dimensions when heated. 

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

Electron beam processing of solvenf-free liquid systems for coatings, inks, and paints involves essentially polymerizahon and cross-linking using electrons with energies between 120 and 300 keV. Initiation by electrons leads primarily to free radical reactions. Cationic polymerization is only found in rare cases. ... 

An alternative is a cationically initiated epoxy resin system cured by electron beam. The experiments with this process showed the following advantages ... 

In view of the above, the initiation of polymerization and cross-linking reactions in polymeric systems is different from events induced by photons. Most of the events initiated by UV irradiation occur near the surface, because the absorption is governed by the Beer-Lambert Law, whereas the reactive species produced by electron beam are dispersed randomly throughout the entire thickness of the material (see Figure 5.1). 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

With the development of the TSP interface for LC-MS (Ch. 4.7), Vestal et al. [4, 16-18] also introduced a new ioiuzation technique. While the analyte ionization in their first experiments was initiated by electrons from a filament, they subsequently demonstrated that collision of the vapour-droplet beam from the TSP nebulizer with a nickel-plated copper plate leads to soft ioiuzation of analytes. Next, the collision was found not to be a vital step in the process [18]. The presence of a volatile buffer or acid in the mobile phase appeared more important in TSP, i.e., in charging the droplets generated by TSP, and in generation of preformed ions in solution. The ionization phenomena were explained in terms of the ion evaporation (lEV) model [4]. 

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