Post-irradiation oxidative effects

Post-irradiation oxidative effects lead to a considerable deterioration of the mechanical properties of polypropylene and of its blends with a low content of polyethylene. The elongation at the breaking point of the polymer falls sharply even at doses of lower than 100 kGy which are used for example during the sterilization of medical instruments. In blends consisting of more than 30% w. of low density polyethylene, the influence of post-irradiation oxidative effects is markedly diminished [151]. 

Studies of the influence of irradiation on the kinetics of oxidation have been confined to post-irradiation work. In general, prior irradiation increases reactivity, although there are considerable inconsistencies in the enhancements obtained The effects can be derived from an increased surface area associated with the swelling voids produced in the metal by the irradiation, and can also probably arise to a lesser extent from chemical effects of the fission products. 

Due to the free-radical character of both the y-radiation induced transformation and oxidation in the post-irradiation phase, free-radical scavenging stabilizers - phenols and amines - effectively protect PO [11,226,227]. This was confirmed by excellent antioxidant/antiradiant effects of DP A, PNA and PD in particular in the post-y-irradiation phase of PO and elastomers [11]. Oligomeric DHQ 21 or its combination with 9b efficiently stabilized EPDM or X-LPE used as insulation materials for electric cables exposed to irradiation doses up to 2MGy at a dose rate 300 Gy h 1 [242]. 

First, vitamin E scavenges free radicals propagating oxidation. It is hence not surprising to observe that it significantly reduces the post irradiation effects observed in UHMWPE. Kinetic curves in post irradiation phase always display the same characteristic shape with a maximal increase rate at the beginning of post irradiation (Figure 3.13). 

On the other hand, inhibiting effect of nitroxylic radical at irradiation is not observed owing to its strong action as photosensitizer. Photosensitization is absent after irradiation in dark reaction and nitroxylic radical displays its inhibiting action. That is why oxidation after irradiation (post-effect) is not observed (Figure 2.7, curve 2). 

When a polymer is irradiated below the melting temperature of crystallites, the polyfunctional monomer remains in the amorphous regions. Gels of polypropylene crosslinked in the presence of butadiene are crystalline which is a manifestation of the formation of crosslinks at the lamellar surfaces [79]. The presence of hexadecane and hexadecene-1 in isotactic polyiroiylene increases the rate of termination of radicals formed in the irradiated polymer and the oxidative post-effect is reduced as well as the deterioration of mechanical properties is less than in a polymer without additives [80]. 

The type of pre- and post-treatment can also influence the measurement results of irradiation in inert atmosphere, because residual oxygen with long living radicals causes oxidative degradation effects following irradiation. However, this effect can be prevented by sufficiently long evacuation prior to irradiation or by annealing in inert atmosphere subsequent to irradiation [711]. 

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