Bernard J. Lyons

Radiation crosslinking of fluoropolymers—a review

Fluorinated polymers containing hydrogen and copolymers of tetrafluoroethylene and hexafluoropropylene are crosslinked with varying degrees of effectiveness by ionizing radiation. The open literature on the radiolysis of these polymers is summarized in this update of a review originally published about 8 years ago [Lyons, B.J. (1984) The crosslinking of fluoropolymers with ionizing radiation: a review.Second Int. Conf. on Radiation Processing for Plastics and Rubber, Canterbury, Plas. Rub. Inst., London, pp. 4-1 to 4-8]. Surprisingly little can be concluded with certainty about the radiolytic processes occurring in these polymers with the possible exception of poly(vinylidene fluoride) (PVF2), which has been studied most extensively. An analysis of published gel data for this polymer, irradiated at various temperatures, shows that the sol fraction of some PVF2 resins varies almost linearly with reciprocal radiation dose (as does the sol of many vinyl terminated polyethylenes). It is postulated that the similar sol behavior of PVF2 indicates that terminal unsaturation is initially present in these resins and that this endlinks during the initial stages of irradiation, thereby producing a molecular weight distribution which tends to the weight random distribution. A corollary of this postulate is that the published values forG (chain scissions) in this and, by extension, other similar fluoropolymers may be too high. Use of certain unsaturated additives in many instances considerably increases gel formation rates in these polymers during irradiation.

The effect of radiation on the physical and chemical properties of polyethylene containing an antioxidant and/or other additives

Abstract This paper describes the effect of an antioxidant, a monomer and fillers (silica or carbon black), alone or in certain combinations with each other, on the mechanical and chemical response of a low density polyethylene to exposure to ionizing radiation. If chain scission is a significant factor in the radiolysis of a polymer, then the relation between sol fraction and Youngs modulus or ultimate elongation (both at 160°C) should change depending on whether an additive inhibits or enhances crosslinking. Such additives do not significantly or consistently alter the relation between these properties in a low density polyethylene, indicating that chain scission is not a significant feature of the radiolysis of this polymer.

Radiolytic crosslinking and chain scission in aliphatic and alkyl-aromatic polyamides—II

Abstract Regression analysis of the radiation parameters of nine aliphatic polyamides exposed to ionizing radiation leads to the conclusion that the decline in the ratio of chain scission to crosslinking in higher aliphatic polyamides is best related to the linear increase in the methylene content of, or the number of methylene groups in, the polyamide repeat unit. G (crosslink) [ G (X)] and G (chain scission) [ G (CS)] values, however, do not correlate well with either of these parameters. Rather it is found that the major determinant of yields [about 80–85% of the variation for G (X), 70% for G (CS)] is the number of hydrogen atoms or methylene groups in the amine residue. A minor determinant [15% of the variation for G (X) and 30% for G (CS)] is the number of hydrogen atoms or methylene groups in the acid residue of the repeat unit. Significantly, although there was little sign of a decrease in G (CS) in the higher aliphatic homologues (up to 11 methylene residues per amide group), G (CS) is found to be positively related to the number of methylene groups in the amine residue but negatively related to the number of methylene groups in the acid residue. Thus although the amine residue dominates the radiolytic response, the acid residue plays a critical if lesser role in determining chain scission yields in polyamides. Thus a polyamide with the repeat unit —(CH 2 ) 3 n CONH(CH 2 ) n NHCO— would be predicted to have a very low or zero G (CS) and, if n is 10 or more, a G (X) comparable to that of polyethylene. Although, logically, the yields of crosslinks and chain scissions in polyamides would be expected to tend to that of polyethylene as the number of methylene groups in the repeat unit increases, use of two models assuming an exponential trend to the G (X) value characteristic of polyethylene in the analysis did not provide better fits to the data than the simple linear model referred to above. Indeed, the assumption of a significant exponential trend factor led to a marked drop in the goodness of fit.

The physical and chemical effects of ionizing radiation on polyethylene containing a phenolic scavenger and/or other additives☆

Abstract This paper describes the effect of an antioxidant, a monomer and fillers (silica or carbon black), alone or in certain combinations with each other, on the mechanical and chemical response of a low density polyethylene (and, more briefly, that of other polyethylenes), to exposure to ionizing radiation. It is shown that, if chain scission is a significant factor in the radiolysis of a polymer, then the inter-relations between sol fraction, elastic modulus and ultimate elongation (both at 160°C) should change depending on whether an additive inhibits or enhances crosslinking. It was found that such additives do not significantly or consistently alter the inter-relations between these properties in polyethylene, indicating that chain scission is not an important feature of the radiolysis of this polymer.

The role of intramolecular crosslinking in the radiolysis of bulk crystallized high density polyethylene

Abstract Intramolecular crosslinks have been suggested to occur in bulk crystallized, irradiated, high density polyethylene (HDPE) and to account for the low rates of gel formation, especially those of previously annealed samples when compared with that manifested by the same resin when previously quenched from the melt. Such crosslinks do not contribute to the development of gel and contribute to only a limited extent to the elastic properties above the crystalline melting point when compared with intermolecular crosslinks, but, if the mesh size of the intra- and inter-molecular networks are comparable, are fully reflected in the rupture elongation. The rupture elongations of a wide range of HDPE resins, for a given sol fraction or elastic modulus, are found to be at least as high as and often higher than those of low (LDPE) or linear low (LLDPE) polyethylene resins, indicating that intramolecular crosslinking of this type does not occur to a significantly greater extent in these higher crystallinity resins. Other factors more likely to account for the reduced rates of inter alia gel formation in some HDPE resins are discussed.