Effect of DOP Mobilizer on the Radiolysis of Semi-Crystalline Ethylene-Propylene Copolymers

Copolymers of ethylene and propylene (PP) have a wide range of application in medical disposables, such as syringes, catheters and dialysis units. During radiation sterilization, formation of radiolysis products may cause yellowness, and loss of mechanical properties due to damages in chemical structures. We have recently reported a study of radical formation at low radiation doses and 77K and rheology of the high-energy radiolysis in a commercial PE/PP copolymer, Elpro. The co-polymer is comprised principally of PP and has only small ethylene content, ∼1 %. In the present report, in order to assess further the role of the stabilizer, we have studied the effect of mobilizer dioctylpthalate (DOP) by FTIR, XPS, and retention in mechanical properties by Instron's mechanical testing method for semicrystalline PE/PP copolymer. The FTIR spectra of Elpro and Elpro+Ca exhibited characteristic absorption bands of -C-H and -C-C of PP/PE. The spectra of the samples containing DOP were similar to those for Elpro but with additional charctaristic ester peaks, including carbonyl peak at 1735-1740 cm-1. When the co-polymer was irradiated with 25 kGy or 50 kGy at room temperature, the carbonyl peak was lost for samples having 0.5 phr of DOP probably due to chemical removal with out-gassing of volatile products. However, at higher concentrations of DOP, shifting of ester peak at 1735 cm-1 to 1719 cm-1 reflects chemical change at surface, most probably due to conversion of ester to aldehyde. C. Albano has reported the simultaneous presence of different degradation products, the band attributed to C=O group appears broader with an increase in radiation dose in various polyethylenes. XPS studies showed the presence of O 1s on the surface in presence of mobilizer, but decrease in intensity of O 1s relative to un-irradiated samples. At lower dose (25 kGy), 40-70 % of Eb retention was observed with no significant loss in T.S, but at 50 kGy mechanical properties could not be retained