Synergism of antioxidants in high density polyethylene

The effect of several commercial antioxidants (viz. phosphites and hindered phenols) used in synergistic formulations in two grades of high density polyethylene (HDPE), viz. a Ziegler type and a Philips type sample, has been examined using infra-red (ir) spectroscopy. The results show that, in general, and, as expected, synergism is obtained between hindered phenols and phosphites in HDPE films during oven ageing and uv irradiation conditions. However, the degree of synergism is not the same for both grades of HDPE, which differ mainly in their unsaturation and metal impurity content. In some cases, antagonism is observed in one of the HDPE samples whilst synergism is exhibited by the other. There is also a difference regarding the optimum concentration ratio of the antioxidants in each polymer. Formulations which include a commercial metal deactivator (Naugard XL 1) also show synergistic behaviour. The reasons for the observed behaviour are discussed on the basis of the mode of action of the antioxidants and their rate of consumption in the polymer medium.