THE ANTIOXIDANT ROLE OF ALPHA-TOCOPHEROL IN POLYMERS .1. THE NATURE OF TRANSFORMATION PRODUCTS OF ALPHA-TOCOPHEROL FORMED DURING MELT PROCESSING OF LDPE

The antioxidant role of alpha-tocopherol (vitamin E) in low-density po lyethylene (LDPE), and the nature of its transformation products forme d during extrusion of the polymer are investigated. The melt stabilizi ng effectiveness of alpha-tocopherol was found to be very high, higher than that of the commercial hindered phenol antioxidants, Irganox 107 6 and 1010, after single and multiple extrusion. The high antioxidant activity of alpha-tocopherol as a melt stabilizer is due, at least in part, to its transformation products. The importance of the processing history and the parent antioxidant concentration on the transformatio n products is discussed. Transformation products of alpha-tocopherol w ere analyzed after each of the four extrusion passes of the polymer. T hese were fractionated, analyzed, and characterized by HPLC and spectr oscopy, respectively. The main products formed are diastereoisomers of dimers and trimers, as well as aldehydes; the relative concentration of each was shown to depend on processing severity (number of extrusio n passes) and the initial concentration of alpha-tocopherol. The dihyd roxydimer was found to be formed at a high concentration relative to t he other products and proportional with the initial concentration of t ocopherol. Based on both the identity and distribution of transformati on products, a mechanism is proposed for the melt stabilization effect of alpha-tocopherol in LDPE. (C) 1994 John Wiley & Sons, Inc.