H. Kudoh

Radiation induced crosslinking of polytetrafluoroethylene

Abstract The irradiation effect of polytetrafluoroethylene (PTFE) was examined by tensile test and thermal analysis in a wide range of irradiation temperature. The tensile strength and elongation at break changed very much by irradiation temperature, especially around the PTFE melting temperature (327°C). By electron beam irradiation under vacuum in the molten state at around 340°C, the dose required for the half decay of the elongation at break was about 1 MGy, and the modulus and yield strength increased with dose, whereas the dose for the half decay at room temperature irradiation was less than 50 kGy, and the modulus and yield strength decreased. The crystallinity decreased by the irradiation in the molten state, but increased with dose below the melting temperature. It is concluded that network is formed with the crosslinking of PTFE by irradiation under vacuum in the molten state just above the melting point.

Temperature effects on radiation induced phenomena in polytetrafluoroetylene (PTFE) : Change of G-value

Abstract Temperature dependencies on the radiation induced phenomena and G-value of polytetrafluoroethylene (PTFE) have been studied in a temperature range 77–653 K. It is well known that main chain scission occurs very effectively below the melting temperature of PTFE (600 K). We have found in our experiments that G-value of chain scission increases significantly with increasing irradiation temperature, until 600 K. In addition to that, we have realized that crosslinking occurs by irradiation in the molten state at 613 K (Tabata, 1992; Oshima et al. , 1995; Tabata et al. , 1996). In the molten state, G-value of crosslinking was found to be 0.35 (number of crosslinking/100 eV absorption), as a lower limit, and the apparent G-value of chain scission is drastically reduced. Above 633 K, radiation induced crosslinking mainly occurs, however parallel thermal depolymerization or decomposition takes place to some extent.

High-energy-ion-irradiation effects on polymer materials: 3. The sensitivity of cellulose triacetate and poly(methyl methacrylate)

Abstract The changes in sensitivity of a cellulose triacetate (CTA) film dosimeter is reported as a function of linear energy transfer (LET). The change in molecular weight of poly(methyl methacrylate) (PMMA) is also reported. For both materials, little or no LET effect was observed up to a threshold LET, but the sensitivity or radiation yield decreased with increasing LET above this threshold level. The threshold LET level was similar for both polymers, occurring at around a few hundreds of MeV cm2g−1, with this level probably corresponding to the overlapping of spurs along the ions path.

Pulsed e− beam irradiation of polymers—A comparison of dose rate effects and let effects

Linear Energy Transfer (LET) effects in radiation chemistry are ascribed to the high density of active species in the track structure, resulting in overlapping of spurs. We studied the possibility of spur overlapping in electron beam irradiation at extremely high dose rate, both theoretically and experimentally. Considering differences in overlap mode leads to the concept of a threshold dose rate, above which spur overlapping may occur, and an overlapping dose, which is necessary to cause overlapping of spurs even at higher dose rate than the threshold, depending on lifetime and effective volume of reactive intermediates. Using Sandias pulsed power e - beam system, we irradiated cellulose triacetate (CTA), polymethylmethacrylate (PMMA), polycarbonate (PC) and high density polyethylene (HDPE) at a dose rate as high as 4 x 10 10 Gy/s at room temperature in the absence of oxygen. Comparison of the e - beam results with data obtained from gamma irradiation at 0.5 Gy/s showed no dose rate effects based on discoloration sensitivity for CTA, or on scission probabilities of PMMA and PC. For HDPE, the results indicated a slightly lower rate of gel formation under e - beam irradiation, implying that the crosslinking efficiency may be somewhat reduced at the high dose rate. Published by Elsevier Science Ltd

Degradation of polypropylene under gamma irradiation: protection effect of additives

Abstract The protection mechanism of antioxidants and an antirad agent dissolved in isotactic polypropylene (PP) irradiated under practical conditions has been investigated by the analysis of gas products and the measurement of mechanical properties. In vacuum radiolysis, protection operating through energy and charge-transfer processes, as observed in the model alkane liquids previously reported, results in the reduction of H2 evolution, in proportion to the number of benzene rings in the additive, and retardation of the film deterioration. Under oxygen atmosphere, G-values of more than 50 for oxygen uptake were measured in pure PP, which is attributable to a chain reaction. Although the uptake was significantly reduced by the addition of additives, the uptake recovered quickly after the consumption of additives at higher doses. In addition, it was found that the reduction of H2 evolution in the presence of additives decreases to nil under severe oxidation. The mechanism of the degradation of PP and the protection by additives is discussed by comparison with the results previously obtained in the model experiment by using liquid alkanes.

High energy ion irradiation effects on polymer materials: 2. Proton irradiation effects on PMMA and GFRP

The changes in mechanical properties of poly(methyl methacrylate) (PMMA) and glass fibre reinforced plastic (GFRP) induced by high energy (30, 45 MeV) protons were studied and compared with those induced by Co-60 gamma rays. The flexural strength at break of PMMA and GFRP showed the same degradation behaviour as a function of dose, indicating no difference between proton and Co-60 gamma ray irradiation.

Low temperature gamma-ray irradiation effects of polymer materials on mechanical property

Abstract A low temperature γ-ray irradiation equipment was constructed for the evaluation of the radiation resistance of polymer materials used in low temperature environment. The change of mechanical property by irradiation at 77 K was studied for glass fiber reinforced plastic (GFRP), poly(methyl methacrylate) (PMMA) and poly(tetra fluoroethylene) (PTFE), and compared with that by irradiation at room temperature. The decrease in flexural strength or in elongation at break showed a big difference between 77 K and room temperature irradiation. The ratio of dose at half strength or half elongation by 77 K and room temperature irradiation was 25 for GFRP, 17 for PMMA and 5 for PTFE. For GFRP the annealing effect at room temperature after irradiation at 77 K was observed scarcely by the flexural strength measurement at 77 K, which means that the degradation on mechanical property depends mainly on the temperature during irradiation.

The distribution profile of the chemical structural changes in ion-irradiated polyolefins

The distribution profiles of the chemical structural changes induced in low density polyethylene(LDPE) irradiated by various ion-beams were obtained by micro-FT-IR measurement. Predominant species induced by ion-beam irradiation were trans-vinylene, hydroxyl group and carbonyl group. It was found that the depth profiles of these species resemble the Bragg curve, but they are rather different from the depth profile of the stopping power calculated by TRIM code. The terminal of the chemical reaction was observed to be deeper than the range calculated by TRIM code for all ion particles. This suggests that the energy profile in the region which the energy of the ion particle becomes lower is very complicated.

Low temperature gamma-ray irradiation effects on polymer materials—3. Gas evolution and change of molecular weight

Abstract Gas analysis was carried out after gamma-irradiation at room temperatures (RT) and 77K for poly(methyl methacrylate) (PMMA) and glass fiber reinforced plastic (GFRP). The yield of hydrogen from irradiated PMMA and GFRP was nearly the same at RT and 77K. The yield of CH 4 , CO and CO 2 , however, was much less at 77K than at RT; these results were in good accordance with results of mechanical properties. Changes in molecular weight and network structure were also studied. The radiation degradation mechanism and its temperature dependence is discussed.

High energy ion irradiation effects on mechanical properties of polymeric materials

The authors prepared ion irradiation system to study irradiation effect on polymer materials, and irradiated carbon/glass fiber reinforced plastic (CFRP, GFRP), polymethylmethacrylate (PMMA), polyethylene (PE), polytetrafluoroethylene (PTFE), and cellulose tri-acetate (CTA), with high energy ions using cyclotron under vacuum at room temperature. Flexural strength in bending test or elongation at break in tensile test decreased with absorbed dose, but the behavior to the dose was the same as that in gamma or 2 MeV electron irradiation. We have found that there is little LET effect for polymers used in terms of change in mechanical properties within the LET range in this work. However, the change in molecular weight for PMMA and optical density for CTA in UV region showed a clear LET dependence above a certain LET, which probably means overlapping between spurs.