Radiation Physics and Chemistry 68 (2003) 925–931 Influence of gamma radiation on hindered phenols in LDPE, paraffin oil and paraffin wax Shamshad Ahmed*, Tariq Yasin, Abdul Ghaffar Polymer Processing and Radiation Technology Lab, ACL, PINSTECH, Nilore, Islamabad, Pakistan Received 21 August 2002; accepted 30 December 2002 Abstract Extraction and high performance liquid chromatography procedures have been developed for the determination of two hindered phenols—Irganox-1010 and Santonox s , incorporated in low density polyethylene (LDPE), paraffin oil and paraffin wax at varied concentrations. These blends were gamma irradiated from 25 to 400 kGy and the radiation and thermal stability of the antioxidant under the experimental conditions has been determined. Upon increase in radiation dose from 25 to 400 kGy, a gradual diminution in the extractable level of each antioxidant was observed. The fate of both the antioxidants in various matrices has been compared. It has been shown that both the antioxidants can influence the polyethylene network formation and the radical yield in different ways resulting in retardation in the rate of crosslinking, as determined by gel-content analysis. r 2003 Elsevier Science Ltd. All rights reserved. 1. Introduction Antioxidants are of great importance in the manu- facture of thermoplastic materials, but primarily in their stabilization as required by the intended use of the final product. In polymer blends exposed to ionizing radia- tion, for modifying their mechanical properties, the antioxidants have to satisfy some requirements such as stability in radiation field and minimum inference with the crosslinking process. Several authors have discussed why certain antioxidants are more efficient stabilizers than the others and to correlate the structure of an antioxidant with its activity in blends (Gugumus, 1985; Shimada et al., 1974; Layer, 1985). Some problems related to irradiated polyethylene-antioxidant systems such as thermo-oxidative degradation of irradiated blends and the influence of stabilizers on the yield of crosslinking have been previously investigated (Nova- kovic et al., 1985; Gal et al., 1983; Jaworska et al., 1991). Most studies are related to the effects of concentration and the influence of only a narrow range of radiation dose. We have been engaged in the development of a radiation crosslinkable thin wire insulation based on LDPE using different additives including Irganox 1010 as primary antioxidant (Shamshad et al., 1995). In this work a comprehensive study has been carried out to investigate comparative fate of a phenolic antioxidant (Irganox-1010) and a thio-phenolic antioxidant, Santo- nox s admixed with LDPE at various concentrations and exposed to wide range of radiation doses ranging from 25 to 400 kGy. Molecular formulae of antioxidants used are shown in Table 1. The antioxidants used in this work are high molecular weight hindered phenols, which have one or more t-butyl substituents at ortho positions. The work has also been extended to include the monitoring of the fate of the antioxidants in paraffin oil and wax for comparing the behavior of antioxidant in closely related polymeric matrices. These investiga- tions permit us to calculate the actual amount of carryover available for stabilization during post-irradia- tion storage. This, in turn, may allow us to determine the optimum amount of an antioxidant required to stabilize LDPE or other matrices being irradiated. ARTICLE IN PRESS *Corresponding author. Fax: +92-51-9290275. E-mail address: shamshad zaki@hotmail.com (S. Ahmed). 0969-806X/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00010-0