Radiation resistance polyethylene

Table V presents average results on impurity levels of 16 polyethylene vials. Vials were 5 cm long, with 6-mm i.d., and 9-mm o.d. The average weight of the sealed vial was 2.00 g. High density polyethylene was used because of its better radiation resistance. The high impurity levels in these vials for S, Na, Cl, K, Al, and Ca severely limited the sensitivity to these elements in gasoline and other volatile materials which had to be counted in the vials.

In general, plastics are superior to elastomers in radiation resistance but are inferior to metals and ceramics. The materials that will respond satisfactorily in the range of 1010 and 1011 erg per gram are glass and asbestos-filled phenolics, certain epoxies, polyurethane, polystyrene, mineral-filled polyesters, silicone, and furane. The next group of plastics in order of radiation resistance includes polyethylene, melamine, urea formaldehyde, unfilled phenolic, and silicone resins. Those materials that have poor radiation resistance include methyl methacrylate, unfilled polyesters, cellulosics, polyamides, and fluorocarbons. 

HPLC—MS with atmospheric pressure chemical ionization (APCI) was recently used to evaluate the potential for the analysis of radiolysis products in polyethylene tere-phthalate (PET) irradiated with to gamma rays in order to evaluate the effect of such radiation on this polymer. Results indicate that highly radiation-resistant polymers, such as PET, form measurable amounts of low-molar-mass degradation compounds, with their yield depending on the radiation conditions (presence or absence of air). 

The functional properties of polymers can be ameliorated by the irradiation of their nanocomposites [86, 87]. The addition of MWCNT to low density polyethylene increases the radiation resistance in comparison to the pure LDPE, which was dependent on the MWCNT content [88]. WCNT nanocomposites were gamma irradiated at 90 kGy to improve the interaction between MWCNTs and the polymer matrix [89]. The irradiation produced a 38 % decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. 

The various materials (often polymers) have to be chosen such that they are sufficiently radiation resistant to withstand the sterilization dose. Under the dose conditions, needed for sterilization some polymers such as polyethylene, polystyrene, poly(ethylene terephthalate), polyurethanes, and epoxides maybe stable, whereas others such as polypropylene, poly(methyl methacrylate), polyvinyl chloride (PVC) may not. When the radiation stability of the plastic... 

S. Giorgio and R. Kern (J. Polym. Sci., Polym. Phys. Edn 22 (1984) 1931) claim that polyethylene crystals under 0.1 /Ltm across are as radiation resistant as the aromatic materials and that a lattice image can be formed directly. 

Figure 5.128 Radiation resistance ofPOiJm thick polyethylene films irradiation in vacuum at 5000 kGy/h with 1 MeV electrons from a van de Graaf generator and irradiation in air at WkGy/h with y-radiation from a °Co source [711]...

PVC, another widely used polymer for wire and cable insulation, crosslinks under irradiation in an inert atmosphere. When irradiated in air, scission predominates.To make cross-linking dominant, multifunctional monomers, such as trifunctional acrylates and methacrylates, must be added. Fluoropolymers, such as copol5miers of ethylene and tetrafluoroethylene (ETFE), or polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), are widely used in wire and cable insulations. They are relatively easy to process and have excellent chemical and thermal resistance, but tend to creep, crack, and possess low mechanical stress at temperatures near their melting points. Radiation has been found to improve their mechanical properties and crack resistance. Ethylene propylene rubber (EPR) has also been used for wire and cable insulation. When blended with thermoplastic polyefins, such as low density polyethylene (LDPE), its processibility improves significantly. The typical addition of LDPE is 10%. Ethylene propylene copolymers and terpolymers with high PE content can be cross-linked by irradiation. ... 

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