Effects of High Radiation Environments on Polymer Composite Epoxies

Abstract

High polymer epoxy adhesives are more and more used in an increasing number of domains such as the naval and aerospace industries. In addition to excellent adhesion and high strength properties, they exhibit good weather resistance, low shrinkage upon cure, low toxicity, excellent resistance to corrosion and ease of use. Before using these high polymer epoxies in environments subjected to high gamma and/or neutron radiation, it is essential to acquire a good knowledge of their resistance to high fluences of ionizing radiation. Applications under such environments include composite materials in space vehicles and in the nuclear industry for nuclear reactor components or spent fuel storage containers. In this work, the adhesive performance of a high polymer epoxy (a fast-setting 5-minute epoxy glue) was examined, resulting from various exposure durations to a high radiation (fast and thermal neutron, and gamma) flux produced by the SLOWPOKE-2 research reactor at Royal Military College of Canada. The tests used the ASTM D897 procedure on samples of epoxies subjected to various doses of radiation. These tests consisted in tensile strength measurements done on a fully computerized Instron Model 4206 tensiometer. The samples were made with two aluminum cylinders with flanges held together by a fine layer of epoxy glue prepared and applied in a very carefully controlled manner. The first tests involved a batch of a dozen unirradiated samples which exhibited an average adhesive strength of 4 +/- 2 kN. Batches of 12 similar samples were then irradiated in the SLOWPOKE-2 reactor pool using an "elevator" designed to bring and maintain samples against the reactor vessel at a position coincident with the mid-plane of the reactor core. Irradiation durations ranged from 1 hour to 4 hours, giving neutron doses calculated as ranging from 3 +/- 2 mGy to 130 +/- 50 mGy. The gamma ray doses were estimated as from 11 +/- 5 mGy to 2.S +/- 1 Gy. The tensile strength tests performed on the most irradiated samples gave tensile strengths of more than 9 +/- 1 kN. The graphs of the tensile strength versus irradiation times exhibited a sharp rise after three hours of irradiation, with the reactor at halfpower, producing a thermal neutron flux of 5 x 1011 n/cm2-sa t an inner irradiation site (or 10kWth power). Additional experimentation was performed in order to understand this unexpected tensile strength increase, in terms of damage mechanisms. Examination of both unirradiated and irradiated samples was carried out using methods such as neutron activation analysis and Fourier Transform Infrared spectroscopy, among others. These analyses reveal that the epoxies are affected by radiation even at relatively low fluences, and that the main effects are the modification of chemical bonds within the molecules into some complex cross-linking. The next phases of this research to be carried out include longer irradiations and selective exposures to either neutrons alone (thermal or fast) or to gammas alone, aiming at determining the effects of each type of radiation.