Oxidation studies OIT test

This approach was adopted for a study of polypropylene used as the filler in the interstices between the cores of some types of three-phase power distribution cables. The cores consisted of a conductor with polymeric insulation and extruded screens which were in turn screened with copper tape. Polypropylene twine is used to pack the interstices as the cable is laid up and then the outer parts of the cable are applied later. Thus the polypropylene is in direct contact with the copper tape. 

Copper is well known as a catalyst of the autocatalytic oxidation of polyolefins, catalysing hydroperoxide breakdown and chain scission polypropylene is particularly susceptible to this degradation route and it was therefore vital that grades were selected which contained a suitable metal deactivator as well as adequate antioxidant. 

OIT was measured at temperatures in the range 180-210°C for a range of samples, either in aluminium pans alone or with 3% of metallic copper also added to the pan. Sample size was kept as close as possible to 10 mg for the sake of consistency between tests. Samples were heated to the test temperature xmder nitrogen and the purge changed to oxygen at the start of the isothermal part of the measurement. 

In addition some samples were reformed by pressing into sheets small samples were then cut for ageing in air drculated ovens at various temperatures, between 120 and 150°C, both in and out of contact with copper. These oven ageing trials were conducted in order to validate the extrapolation to higher temperatures so that future material selection based solely on DSC measurement of OIT could be vahdated. The time to failure in the oven ageing trials was assessed by a simple three-point bending test, since it was found that the polypropylene lost aU mechanical integrity rather rapidly once the oxidation process was under way. The time to failure was reported as OIT at the lower temperatures and these results incorporated into the Arrhenius plots. 

This second trial demonstrated the necessity of making adequate measurements at oven ageing temperature ranges to test the validity of using data obtained at higher temperatures in the DSC for extrapolation purposes. It is clearly vital to ensure that the reaction mechanism remains the same over the whole range of interest. 

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