Postirradiation effects

Silicone Good Phenyl-methyl silicones are more stable than are methyl silicones. Platinum cure is superior to peroxide cure full cure during manufacturing can eliminate most postirradiation effects. 

Three types of reactive species are formed under irradiation and may become trapped in polymers ionic species, radicals, and peroxides. Little is known about the role of ions in the chemical transformations in irradiated polymers. Long-lasting ions arise, as demonstrated by radiation-induced conductivity, and may become involved in postirradiation effects. The presence of trapped radicals is well-established in irradiated polymers, but certain problems remain unsolved concerning their fate and particularly the migration of free valencies. Stable peroxides are produced whenever polymers are irradiated in the presence of oxygen. Both radicals and peroxides can initiate postirradiation grafting, and the various active centers can lead to different kinetic features. 

T wo aspects of the radiation chemistry of polyethylene terephthalate (PET) are reviewed here the dependence of product yields on radiation dose and on dose rate. The review is limited to work with thin films from which air and water were pumped prior to irradiation. Moreover, it is judged that in the experiments described postirradiation effects were negligible. 

Postirradiation Effects. Several experiments were conducted in which parallel runs were made. In one case the tubes were frozen in... 

Figure 2. Postirradiation effects in grafting of styrene to wool 3.06M styrene in dioxane containing 10% methanol...

Radiolysis of HN03 generates different radicals (305), but at high acidity, HN03 can also act as a hydroxyl radical scavenger (306). Moreover, postirradiation effects have been observed (variation of the concentration of radicals, for example) (305). 

Small doses of sunlight have a postirradiation effect in laboratory and field-scale photoreactors. The solar photocatalytic disinfection is correlated to the accumulated energy (Q, kJL-1) experimental time always plays an important role some groups are at the moment working in this aspect. 

Figure 4. Effect of temperature during postirradiation heating of 91-9 PVC-styrene...

The effect of postirradiation heating on the course of the polymerization itself can be seen in Figures 2 and 3. Within the scatter of experi-... 

Since Schultz (7) found that ultimate conversion depended considerably on temperature (in a highly polymerized methyl methacrylate system), similar effects would be expected in the postirradiation-heating of the PVC-styrene system. Such effects have indeed been found. Figure 4 shows the effect of heating temperature on the conversion level at two different radiation doses. No increased conversion is found for a temperature higher than 75 °C. This seems to indicate that a more or less definite melting point of the partially polymerized mixture exists. When this temperature is reached during the postirradiation treatment, the reaction runs to a point of termination and is unaffected by further temperature increases. 

The effect of radiation intensity was investigated for three compositions of PVC-styrene. The data for the 77-23 composition are shown in Figure 5. A radiation intensity effect is present even when postirradiation heating is used. When the 84-16 and 91-9 PVC-styrene compositions were similarly studied, no significant dose rate effect was found. 

The previous study was confined to only one dose rate and a limited range of methanol contents. Furthermore, the effects of oxygen and postirradiation were examined only briefly. These aspects of the grafting process were investigated before embarking on the more detailed ESR study. 

A more important phenomenon is that after low-temperature postirradiation incubation vegetative cells show a marked recovery. In some cases the effect is equivalent to reducing the dose by one third (S17, H22). A recent British symposium on radiation after effects is available (S24). 

The effect of various doses of radiation upon the internal ethylene concentration of climacteric McIntosh apples for the 10-day postirradiation period is indicated in Figure 2. Increases in ethylene concentration at doses of 5 and 10 krads were obtained immediately. At 50 and 100 krads there was an initial reduction in concentration and a subsequent rise, followed by a rather sharp decline. Experiments conducted by irradiating the fruit under anaerobic conditions indicate that whereas some of the destruction may be oxidative in nature, a considerable amount may take place under reducing conditions. 

A number of studies concerned with the effect of irradiation upon intermediary metabolism in carrots have been conducted. One such study utilizing the methods of Ap Rees and Beevers (1,2) involved measuring the relative radiation susceptibility of the pentose phosphate (PP) and the Embden-Meyerhof (EM) systems in the postirradiation period. 

Effect of Postirradiation Warming Rate. The effect of postirradiation warming rate was studied at — 196°C. and 6.0 megarads for oxygen- and vacuum-packed steaks under conditions designed to maximize any possible effects. The postirradiation warming was carried out under the following conditions ... 

Table VI. Effect of Postirradiation Warming Rate on Irradiation Flavor Scores of Steaks Irradiated at — 196°C. at 6.0 Megarads...

Effect of Packaging Environment. Because of the opposing effects of postirradiation warming rate on steaks packed in an oxygen atmosphere and on vacuum-packed steaks discussed above, it was not possible to establish a clear preference on the basis of initial irradiation flavor intensity scores. 

Rincdn, A.G. and Pulgarin, C. (2007b) Solar photolytic and photocatalytic disinfection of water at laboratory and field scale. Effect of the chemical composition of water and study of the postirradiation events. Solar Energy Eng. 129, 100-110. 

Radiation (Co-60) is used to sterilize many rubber items, especially those used in devices. The effect of gamma radiation on rubber closures is a function of the elastomer, dose, and postirradiation time. 

One weakness of postirradiation methods, i.e., analysis of the ablation properties and surface analysis, as tools for studying the ablation mechanisms of polymers is the possibility that reactions after irradiation cause the observed effect. These results are therefore only indirectly related to the abla-... 

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