Electron-beam-irradiation processing

Electron accelemtors are utilized in industrial processes such as crosslinking of polymers, curing rubbers, curing paints and adhesives, sterilization of medical devices and pasteurization of foodstuffs. 

The interaction of high-energy electrons and materials depends on the kinetic energy of the electron and the composition of the irradiated materials, so effective operation is conditioned by this interaction and knowledge of the effective penetration depth of the electrons into the material. This penetration depth is the depth at which the absorbed dose has fallen to an arbitrary level (ideally similar to that of the unirradiated surface). An electron processing facility consists of 

Polymer Application examples Processing Stability up to 25 kGy Stability 25 kGy Comments 

ABS Closure-piercing devices, roller clamps Injection moulding Good To 1000 kGy Rigid, most often translucent/ opaque 

Acrylics Luer connectors, injection sites Injection moulding Good To 1000 kGy Clear with slight yellowness after irradiation 

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As indicated earlier, irradiation with 20 Mrad can induce some heating of the samples. An approximate calculation of the temperature rise due to electron beam irradiation process should be useful in this regard(8). In brief, an energy balance based on a unit volume element of the medium provides the following equation ... 

Electron beam treatment of natural fibers of interest is carried out before biocomposite fabrication. A relatively large amount of raw natural fibers, bundles, and/or woven fabrics contained and spread in a polyethylene bag can be irradiated separately or simultaneously. Different levels of electron beam dosages, for example, from 1 to 100 kGy, or even higher, may be applied. Use of too high intensity of electron beam may cause some damages and microstructural defects of the natural fibers, resulting in deterioration of their mechanical properties. Eventually, the treatment effect on the property improvement of biocomposites may depend on the treatment level, as reported earlier with different natural fiber/polymer biocomposite systems by Cho et al. [105, 107-110, 112]. The electron beam irradiation processes can normally be performed at ambient temperature in air. 

Figure 4.5 Illustration of electron beam irradiation processing for treating natural fibers. (After S.G. )i [107].)...

It is clear from this discussion that the dose requirement and unit cost will be lower if the material has a higher molar mass M and the reaction has a high G value. Thus, the best candidates will be a polymeric material and a chain reaction. Quite often, a free-radical irradiation is used. The radiation source of choice is usually a 60Co - y facility, although electron beam irradiation is also used. Since most radiation-chemical reactions used in industry can also be brought about by other conventional means such as thermal, or photochemical processes, the processing cost must be below 10irradiation cost one has to include the cost of operation, maintenance, and the like. (Danno, 1960). 

Continuous processes such as electron beam irradiation o less penetrating... 

To avoid this problem, Wills and Boyd have applied a semi-empirical method [18]. They used relative cross sections for 100 eV electrons and calculated the yields for the various processes based on the ff -values. This method is rather simple and contains many assumptions nevertheless, the results are a good indication for radiation chemists to provide a specific amount of initial species. The following equations [19] show the amounts of initial species from the main components of flue gas with electron beam irradiation based on the data listed in Ref. 18. 

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. 

The flue gas from municipal waste incinerator boilers contains SO2, and HCl. To remove these harmful components simultaneously by dry process, electron beam treatment method was investigated. The pilot-scale test was conducted in Matsudo, Japan, in 1992 with a flue gas of 1000 m /hr [34]. Recently, dioxins, namely, poly-chlorinated-di-benzo-paradioxins (PCDDs) and poly-chrorinated-di-benzo-furan (PCDFs), from incinerators have become a very serious problem because of their high toxicity. Pilot-scale tests to decompose dioxins by electron beam irradiation were conducted in Karlsruhe, Germany [35], and in Takahama, Japan [36], using almost the same capacity of flue gas, 1000 m /hr. Very promising results were obtained with decomposing more than 90% of dioxins. 

Plastics are by far the largest group of polymeric materials being processed by electron beam irradiation. Cross-linking of polyolefins, PVC, polyesters, polyurethanes, fluoropolymers, and fiber-reinforced composites is a common practice. 

Two-sided irradiation of a wire by electron beam. (Radiation Processing of Polymers (Singh, A., and Silverman, J., Eds.), Carl Hanser Publishers, p. 82 (1992). With permission from Carl Hanser Publishers.)... 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

Cooper, W.J., Nickelsen, M.G., Meacham, D.E., Kurucz, C.N., and Waite, T.D., High energy electron beam irradiation an advanced oxidation process for the treatment of aqueous based organic hazardous wastes, Water Pollut. Res., 27(1), 69-95, 1992a. 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

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