Cross-linking by ionizing radiation

Polyethylene Is well known to be cross-linked by ionizing radiation through the recombination of polyethylene radicals produced by the irradiation (1). The efficiency of the reaction is not so high and the G-value has been reported to be 1-5 (2, 3). 

Normally, TPEs are not cross-linked because the thermoplastic nature is their desired property in most cases. However, in some cases cross-linking is used to improve mechanical properties, influence flow to reduce swelling in oils and solvents, eliminate dissolution of fhe polymer in oils and solvenfs, increase heat resistance, and influence ofher performance characteristics. Examples where cross-linking by ionizing radiation is necessary for fhe given processes are ... 

Polystyrene is a clear, amorphous polymer with a high stiffness and good dielectric properties. It is easily cross-linked by ionizing radiation. Often small amounts of divinyl benzene may enhance the degree of cross-linking.98... 

Fluorocarbon elastomers based on vinylidene fluoride can be cross-linked by ionic mechanism. However, if the polymer has been prepared in the presence of a cure site monomer (CSM) it can be cross-linked (cured) by a free radical mechanism. Moreover, many flnoroelastomers can be cross-linked by ionizing radiation (see Section 5.1.3.3). 

Fig. 7.1 Plot of melting point depression (AT) as a function of fraction of units cross-linked, p, for polymer networks formed from random chains. A molten polyethylene cross-linked by ionizing radiation o natural rubber cross-linked with sulfur natural mbber cross-linked with di-t-butyl peroxide. (Data from Refs. (13) and (14))...

Polybutene can be cross-linked by irradiation at ambient temperature with y-rays or high energy electrons in the absence of air. The performance of articles manufactured from polybutene is only slightly affected by ionizing radiation at doses below 30 kGy (3 Mrad) (26). PMP is also relatively stable to P-and y-radiation employed in the sterilization of medical suppHes (27). 

The use of ionizing radiation to induce cross-linking is another important technique for producing hydrogels from linear water-soluble polymers. When such polymers are bombarded by ionizing radiation, either in the bulk state or in solu-... 

In earlier work preformed polymer was cross-linked, e.g. by ionizing radiation (39, 103, 104) with suitable assumptions as to randomness of the reaction, the molecular statistics of product can be calculated as a function of the degree of cross-linking. An alternative method is to co-polymerize with small amounts of tetra-functional monomer, e.g. divinyl benzene (36, 105, 107). Both these methods produce highly poly-disperse products, having tetra-functional branch-points. 

Simic M, Dizdaroglu M (1985) Formation of radiation-induced cross-links between thymine and tyrosine Possible model for cross-linking of DNA and proteins by ionizing radiation. Biochemistry 24 234-236... 

Continuous upper service temperature of commercial ETFE is 150°C (302°F).64 Physical strength can be maintained at even higher temperatures when the polymer is cross-linked by peroxide or ionizing radiation.65 Highly cross-linked resins can be subjected to temperatures up to 240°C (464°F) for short periods of time.64... 

Fluorocarbon elastomers, particularly copolymers of HFP and VDF, are cross-linked by low-level y radiation (up to 20 Mrad) in the same fashion as PVDF. Radiation degradation of a VDF-HFP copolymer was studied by Zhong and Sun.35 Their finding was that there is a linear relationship between the dose and the weight loss of the polymer. Fluorocarbon elastomers based on copolymers of IFF and propylene were found to be more resistant to y radiation than Viton elastomers.36 Cross-linking of VDF-HFP, VDF-TFE, and tetrafluoroethylene-propylene (TFE/P) copolymers as well as of terpolymers VDF-HFP-TFE by ionizing radiation (gamma... 

Perfluoroelastomers (ASTM designation FFKM) are essentially copolymers of two perfluorinated monomers, TFE and PMVE with a CSM, which is essential for cross-linking. Perfluoroelastomers can be cured by ionizing radiation without any additives. The advantage of radiation cured FFKM is the absence of any additives, so that the product is very pure. The disadvantage is the relatively low upper-use temperature of the cured material, typically 150°C, which limits the material to special sealing applications only [32]. 

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