Radiation crosslinking dose rate

Much research into radiation effects on polymers is done with samples sealed under vacuum. However, polymer materials may, in practical applications, be subjected to irradiation in air. The effect of irradiation is usually substantially different in air, with increased scission at the expense of crosslinking, and the formation of peroxides and other oxygen-containing structures. Diffusion rates control the access of oxygen to radicals produced by the radiation, and at high dose rates, as in electron beams, and with thick samples, the behaviour may be similar to irradiation in vacuum. Surface changes may be quite different from bulk due to the relative availability of oxygen. 

The increase in the modulus for Bis A PSF and Hq/Bp PSF with irradiation indicated that crosslinking predominated for both polymers and that the crosslink structures were probably basically similar. Hq/Bp(50) PSF was considerably more radiation resistant than Bis-A PSF, as shown by the rate of decrease in the elongation at failure. For both polymers, there was an initial rapid decrease in the elongation at failure followed by a slower decrease. This effect was also demonstrated by the variation in the fracture toughness (KI(.) with irradiation for Bis-A PSF. This work with cobalt-60 gamma radiation complements earlier studies of these materials using high dose rate electron beam irradiation (6). 

In principle, detailed information about the radiation chemistry of a polymer may be obtained using sol-gel analysis as a function of dose to provide estimates of both crosslinks (x) and fractures (F) (5). Such estimates have been made in only one case for PET (18) but are supplemented by estimates made from other data obtained at high dose rates in Table II. The data used are scanty, generally only three points, and more detailed work is desirable. In addition, further work is required to... 

Smetania and coworkers [53] proved that iV-phenylmaleimide (N-PMI) acted as a sensitizer in radiation crosslinking of EPR (63 mole% C2H4) the rate of gel formation being directly proportional to the quantity of additive up to 20 Mrad dose. A comparison of radical ion concentration up to 3 Mrad dose is shown in Fig. 2. The sensitizing effect of the A-PMl is not of a free-radical or ionic nature but is due to the fact that under y-radiations, A-PMI becomes a hydrogen acceptor and is reduced to A-phenylsucdnimide... 

The degradative radiation-recycling of PTFE led to a successful pilot-scale plant producing 12 tons/year recycled powder at Sumitomo, Japan [9], For similar polymerdegrading industrial developments several other candidates are very promising. Among other synthetic polymer products, discarded automobile tires represent a major environmental concern, in an amount close to 10 Mtons/a. A promising method is mentioned in the literature [9] in which the vulcanized rubber product is crushed at low temperature, irradiated at a dose rate of 100 kGy, and milled repeatedly, if necessary. The reclaimed de-crosslinked material can be added to an extent 10 - 15% to various new rubber blends. 

PA 11 and PA 12 are easily sterilized with ionizing radiation. For instance, exposing 30 to 100 mm thick polyamide 12 films to irradiation at a dose rate of 50 kGy did not result in changes in the film. Only above a dose rate of 100 kGy signs of slight crosslinking were observed. Thicker test specimens of heat stabilized PA 12 were treated with 400 kGy electron radiation without any measurable changes. 

Fig. 18 Change of electrical conductivity of UPR during radiation induced crosslinking at the dose rate B = 0.345 kGy/h at 290 K in non-logarithmic (solid symbols) and logarithmic scale (open symbols). The logarithm of conductivity is calculated from the data presented in non-logarithmic form. The changes of free styrene content after extraction are also shown (large solid squares). Reprinted from (1999) Radiat Phys Chem 54 95 [159] with permission...

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