PHOTODEGRADATION OF THYLENE-CO-PROPENE-CO-(5-ETHYLIDENE-2-NORBORNENE)] RUBBERS .1. REAPPRAISAL OF THE PHOTOOXIDATION MECHANISM UNDER ACCELERATED CONDITIONS

In this work the reexamination of the photo-oxidation mechanism of thr ee (ethylene-co-propene-co-5-ethylidene-2-norbornene) (EPDM) terpolyme rs containing different amounts of 5-ethylidene-2-norbornene (ENB)a) ( 2-8 wt.-%) is discussed. The photo-oxidation depends on the initial co ntent of ENB units in the chains. The formation of a band at 923 cm-1 is attributed to the CH out-of-plane deformation of a new double bond in terminal position in the ethylidene moieties. The presence of doubl e bond migration suggests the. formation of stable tertiary hydroperox ides which show a new band at 870 cm-1. The kinetic behaviour of a ban d with similar spectral pattern in the UV at 235 nm has been correlate d with double bond migration. The identification of photoproducts both by 2,4-dinitrophenylhydrazin and SO2 confirms the proposed spectrosco pic assignments. The gel measurements of the studied terpolymers, and some comparisons with a poly(ethylene-co-propene) (EPR) copolymer havi ng a similar ethylene to propene ratio, suggest that the presence of E NB can strongly influence both the photo-oxidation and the crosslinkin g yields of these rubbers.