ESR Study of Irradiated Ethylene-Propylene Rubber S. BACCARO*, U. BUONTEMPO”, B. CACCIA+, S. ONORI+, M. PANTALONI+ *ENEA - C.R.E. Casaccia - S. Maria Di Galeria, Roma, Italy “Universiti de L’Aquila, Italy +Istituto Superiore di Sanita (ISS) and INFN - Roma, Italy The ESR technique was used to investigate the resonant pammagnetic properties of an ethylene-propylene based material and of its basic components irradiated with a 6oCo source up to 0.1 MGy. A qualitative model for the radical formation in a polymeric material with antioxidant as an additive is proposed. KEYWORDS: Radiation damage; polymer; ethylene-propylene; free radicals; ESR. INTRODUCTION One of the most powerful techniques suitable to study the post-irradiation modifications in polymers is the electron spin resonance (ESR) spectroscopy because most of the chemical reactions involve the production of free radicals. Unfortunately the interpretation of ESR spectra and in particular the assignment of the observed signals to well defined chemical species, is rather difficult because of the presence of simultaneous reactions involving formation and change of many different radicals (Sanby et al., 1977). Regardless of the nature of the radicals produced during or just after the irradiation, the usual behavior at long times after the irradiation is a decrease of the ESR signal due to the recombination processes of the radicals. An opposite behavior has been recently found (Baccaro et al., .1990) in ethylene-propylene rubber (EPR) irradiated in air. Indeed in such a material a monotonous increase of the ESR signal with time is recorded after the end of irradiation and such an increase is still appreciable after some thousands of hours. In order to clarify the processes which determine such unusual phenomena, and in particular to identify the nature of the stable radicals, we have performed a new quantitative ESR study of the radiation effects on the rubber components (ethylene-propylene copolymer and antioxidant) as well as on the formulated rubber both in presence and absence of oxygen. A quantitative comparison between the observed effects on the components and on the rubber gave the chance to assign the observed ESR signal to a well defined chemical species and to suggest a process to take into account for this characteristic behavior. EXPERIMENTAL The materials tested in this work were Afumex@, an insulation for electrical cables (Pirelli Oavi S.p.A.-Italia Div.), and its basic components, i.e., an ethylene-propylene copolymer and an antioxidant with the NH functional group. Irradiations were carried out at the ISS 6oCo Gammacell facility. samples was about 0.3 Gy s-l. The dose rate at the position of the The samples were placed in 2.5mm thick polyvinyl chloride build- up holders for electronic equilibrium. The nominal dose values were monitored with the ISS 331