The degradation of ethylene propylene rubber (EPR) sheets as a cable insulation material for nuclear power plants (NPP) was studied by accelerated thermal ageing, radiation ageing and thermal - radiation combined ageing. The oxidation of EPR proceeded with ageing and the decay of mechanical property was closely related to the content of oxidation products. The antioxidant as a stabilizer in EPR was effective for the thermal oxidation, but not for the radiation oxidation. The distribution of oxidation products across the sheet was changed due to decay of antioxidant by evaporation during thermal ageing. The antioxidant was not effective for radiation oxidation, and it was decomposed by radiation. For the thermal and radiation combined oxidation, the mechanical property and the content of oxidation products were different among the treatment sequences, which was due to the decay of antioxidant. The lifetime of EPR cable is closely related to the remaining content of antioxidant, and the lifetime evaluation would be recommended by the reverse sequential combination (thermal ageing after radiation ageing).

The degradation behavior of SiR for the cable insulation by accelerated thermal and radiation ageing was studied and the degradation mechanism was proposed. The degradation was observed by the change of tensile properties, the distribution of crosslinking, and the change of weight. The chemical reaction under the both ageing in oxidation conditions was crosslinking and the oxidation mechanism was found to be the same between thermal and radiation ageing. The yield of crosslinking was proportional to the ageing time and also the dose. The effect of irradiation temperature on oxidation was accelerated with an increase of temperature above around 120 °C, which might be due to the specific radiation chemical reactions. Therefore, the degradation by simultaneous ageing at higher temperatures above 155 °C was much higher than that for sequential ageing, such as irradiation followed by thermal ageing or thermal ageing followed irradiation. At a high temperature, the degradation by thermal ageing under vacuum (without oxidation) was more progressed than that for the ageing in air (with oxidation). The reason was assumed to be the thermal decomposition of crosslinks between SiR molecules formed by the chemical crosslinking agent. The hardness (Shore hardness) reflected well the degradation of the SiR material for any ageing conditions.

The oxidation products in crosslinked polyethylene for cable insulation formed during thermal and radiation ageing were analyzed by FTIR-ATR. The products were composed of carboxylic acid, carboxylic ester, and carboxylic anhydride for all ageing conditions. The relative yields of carboxylic ester and carboxylic anhydride increased with an increase of temperature for radiation and thermal ageing. The carboxylic acid was the primary oxidation product and the ester and anhydride were secondary products formed by the thermally induced reactions of the carboxylic acids. The carboxylic acid could be produced by chain scission at any temperature followed by the oxidation of the free radicals formed in the polyethylene. The results of the analysis led to formulation of a new oxidation mechanism which was different from the chain reactions via peroxy radicals and peroxides.

The accelerated ageing of cables for nuclear power plant was studied for the life evaluation in the environmental conditions. For the accelerated radiation ageing, the dose rate is the important point, because the oxidation profile in the insulation is much affected by dose rate due to oxygen diffusion limitation during irradiation. The oxidation profile was observed by FTIR for crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) of cable insulation degraded at various irradiation conditions and compared with the mechanical degradation. The oxidation profiles with low dose rate at room temperature and that with high dose rate at elevated temperature (100°C) were very much alike and the mechanical degradations of both irradiations were equal. By increasing the irradiation temperature the oxygen diffusion rate increased exponentially with temperature and the degradation by thermal during irradiation for several hundred hours was very little at around 100°C. Therefore, the dose rate can be increased by 15 times by irradiation at 100°C instead of the irradiation at room temperature for the adequate oxidation throughout the insulation. The experimental result was coincident with the theoretical analysis.

The yields of gases evolved from three types of polyethylene and ethylene-propylene copolymer during radiation were precisely measured after gamma ray irradiation under vacuum over a wide range of temperatures (−196 to 200°C). For all polymers the major gas evolved was H2 and the minor products were C1, C2, C3 hydrocarbons and the oxidation compounds CO2 and CO. The total gas yield increased with an increase in the irradiation temperature, but the ratio of the yields among the gas components was not greatly changed. The H2 would originate from H-bond scission and the concomitant formation of crosslinks and double bonds in the polymer chains. The minor products of C1, C2, C3 hydrocarbons were products of chain scission at the chain ends, including branched chains, and the oxidation compounds of CO and CO2 were the products formed by reactions of oxygen remaining trapped in the polymer matrix. The yield of H2 increased with increasing irradiation temperature, which is closely related to the molecular motions of the polymer chains during irradiation.

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.

Radiation and thermal degradation of ethylene–propylene rubber (EPR) and crosslinked polyethylene (XLPE) as cable insulation materials were investigated by evaluating tensile properties, gel-fraction, and swelling ratio, as well as by the infrared (FTIR) analysis. The activation energy of thermal oxidative degradation changed over the range 100–120°C for both EPR and XLPE. This may be attributed to the fact that the content of an antioxidant used as the stabilizer for polymers decreases by evaporation during thermal ageing at high temperatures. The analysis of antioxidant content and oxidative products in XLPE as a model sample showed that a small amount of antioxidant significantly reduced the extent of thermal oxidation, but was not effective for radiation induced oxidation. The changes in mechanical properties were well reflected by the degree of oxidation. A new model of polymer degradation mechanisms was proposed where the degradation does not take place by chain reaction via peroxy radical and hydro-peroxide. The role of the antioxidant in the polymer is the reduction of free radical formation in the initiation step in thermal oxidation, and it could not stop radical reactions for either radiation or thermal oxidation.

Improvement of the comfort and esthetics of artificial plastic teeth is desirable for the recently increasing numbers of elderly in society. Plastic teeth made of polycarbonate (PC) were modified by electron beam (EB) irradiation under specific conditions, and the change in the chemical properties of the PC was investigated. The water absorption, glucose attachment, level of bis-phenol-A (BPA) extraction, maltose adhesion, and mucin adhesion on the PC teeth were measured before and after EB irradiation. EB irradiation to a dose of 3.5kGy at 150°C in a nitrogen gas atmosphere reduced the water absorption by 20%, glucose absorption by 40%, maltose adhesion by 20%, and the amount of various amino acids, formed as the hydrolysis products of mucin, adhering on the PC teeth were reduced by 60–99%. The BPA content was lower than the detection limit for analysis of both the original and the EB irradiated PC teeth.

The irradiation temperature effect on polytetrafluoroethylene (PTFE) was investigated by tensile test and thermal analysis. The behavior of mechanical properties and changes of crystallinity on irradiation indicated the network formation in PTFE molecules by radiation induced crosslinking under oxygen-free atmosphere in the molten state around 613K. The radiation resistance of PTFE was much improved by crosslinking, that is, the irradiation dose at a half value of ultimate elongation was about 1MGy for 500kGy-crosslinked PTFE, while the dose for non-crosslinked PTFE was only 3.5kGy, so it is expected that the crosslinked PTFE can be applied for the nuclear facilities with relatively high radiation environment. Also, the crosslinked PTFE had light transparency due to the decrease of crystallites, and held the electrical insulation and heat resistance. The abrasion factor in frictional test was greatly improved by crosslinking.

An SiC fiber with excellent thermal stability has been developed by means of a reduction in its oxygen content. This low oxygen content SiC fiber was synthesized using a radiation curing process. Polycarbosilane (PCS) fiber was cured by irradiation with an electron beam in a helium atmosphere. The cured polycarbosilane fiber was pyrolyzed and an SiC fiber with a 0.4 wt% oxygen content was obtained. The low oxygen content SiC fibers were then heat-treated at 1773–2273 K and compared with an SiC fiber having 10 wt% oxygen cured by oxidation. The low oxygen content SiC fiber kept its fibrous form and was flexible even after the 2273 K treatment, while oxidation-cured fiber changed to a powder-like material with extreme crystal growth above 2073 K. The surface appearance and crystal structure of the fibers were analyzed using SEM and XRD, respectively.

The low oxygen SiC fibers were prepared by curing of polycarbosilane fibers with electron beam irradiation and pyrolysis. These SiC fibers were continuous, in multi-filament form, and consisted of Si-13C-0.02O by atomic ratio. They had a high tensile strength and an elastic modulus of 2.8GPa and 270GPa, respectively. The low oxygen SiC fiber retained high strength and modulus after 10 hours exposure at 1873K in argon. It exhibits outstanding thermal stability as compared to other polymer-derived ceramic fibers. This fiber should be one of the best candidates for the reinforcement of ceramic matrix composites. At 1773K, tensile strength and modulus of the SiC fiber were lower than that at room temperature, because of plastic deformation. The SiC fiber with larger crystallite size tends to retain higher modulus at elevated temperature.

A super radiation-resistant motor was developed for a project of ITER, in which the insulation materials consist of high radiation-durable organic materials such as poly(benz-imidazole) (PBI), poly(arylate) (VECRUS ® , VECTRA ® ), and poly(phenyl-ether/urea) grease. The motor was tested by γ-rays irradiation for 2 years under power supply, and confirmed to have high radiation resistance over 100 MGy, which is 50 times higher than an ordinary radiation durable motor for a nuclear power station. The key technology was the selection of radiation resistant polymers and further improvement of radiation resistance by mixing aluminum (Al) micro-flakes in organic polymers. The effects of Al flake to prevent radiation degradation of polymer were assumed to be channeling of secondary electron induced by γ-ray irradiation.

Blended organosilicon precursors containing 10 or 20% of poly(vinylsilane) (PVS) in polycarbosilane (PCS) were prepared and shaped into fiber form by melt-spinning. The influence of PVS addition on the spinning, radiation-curing, and pyrolysis processes was investigated. The addition of PVS increased the spinability of the precursor melt and increased the oxidation sensitivity of the precursor. By adjusting the precursor compositions and the radiation conditions, highly heat-resistant silicon carbide fibers were obtained.

The distribution of the local transformations induced in a low density polyethylene irradiated with various ion-beams was observed by using micro-FT-IR system. The predominant transformations induced by irradiation are crosslinking, trans-vinylene, end-vinyl and vinylidine. The depth profile of the trans-vinylene resembles the Bragg curve, but that each of the end-vinyl and the vinylidine shows rather complicated profile. The irradiation effect of H+ ions is similar to that of γ-rays, but on irradiation with the ions heavier than H+, the species induced are found to depend on the beam fluence and the stopping power of the ion.

Fiber reinforced plastics (FRPs), reinforced with carbon fiber, SiC fiber and glass fiber, were prepared by using a new thermosetting silicon-containing polymer, poly[(phenylsilylene) ethynylene-1,3-phenyleneethynylene](MSP), as a matrix resin. In MSP composite processing, no solvent is needed, no by-products are generated, and the curing temperature is low (150–210°C). The FRPs (MSP composite) showed high heat-, burn- and radiation-resistant properties. Bending strengths (110–140 MPa) and modulus (30 GPa) at 200°C and 400°C were almost equal with those at room temperature, and were not affected by 100 MGy of irradiation. Dynamic viscoelasticity and creep properties of MSP composite were also determined and compared with those of a polyimide composite.

Gamma-ray or electron beam irradiation at high temperature and at a small dose improved the Rockwell hardness and resistance to wear for polycarbonate and polysulfone. The effective temperature during irradiation was the glass transition temperature (Tg) of respective polymer, and the dose at maximum hardness was only 3–5 kGy. The effect for hardness was same between 60Co-gamma ray and electron beam and the main chain scission was predominant for both the polymers. The improvement of hardness and wear resistance was supposed to be dense molecular packing in matrix by rearrangement of molecules with synergistic effect of radiation and temperature.