Metal Complex Photo-activators for the Time-Controlled, Photo-Oxidation of Polyolefins

Abstract

The use of antioxidant photo-activator (APA), which is based on a combination of a photo-activator (ideal, e.g., iron dialkyldithiocarbamate (FDMC) and conventional, e.g., iron acetylacetonate (FAcAc)) and U.V. stabilizer, has been examined in both low-density polyethylene (LDPE) and polypropylene (PP). The U.V. stabilizers used were zinc and nickel diethyldithiocarbamates (ZDEC and NDEC), tetraethylthiuram disulphide (TETD), and 2-hydroxy-4-octyloxybenzophenone (HOBP). The two-component APA system (e.g., FDMC + NDEC) gives a more controllable lifetime and at much lower activator concentration than does the ideal photo-activator (e.g., FDMC) alone. During U.V. irradiation, both the induction period and the rate of photo-oxidation can be varied independently. It seems that the stabilizer concentration primarily determines the length of the induction period, while the activator concentration controls the slope of the photo-oxidation curve and hence the embrittlement time. Reactions of ZDEC, NDEC, FDMC, and the corresponding disulphides in the presence and absence of free radical and peroxide initiators, free radical inhibitors, and different bases, in inert and oxidisable substrates, are investigated by oxygen absorption techniques, peroxide decomposition studies, and by product analysis. It is shown that the mechanism of action of these metal complexes involves both free radical scavenging and peroxide decomposition, but that the contribution of each to the overall mechanism is influenced by the nature of the metal centre and its presence. The nature of intermediates involved in reactions of ZDMC and CHP were examined by spectrophotometric methods. Zinc thiocarbomyl sulphenate and sulphinate were found to be amongst the first transformation products formed from the above reactions. There is also evidence of the formation of disulphide from nickel and iron complexes, but not from the zinc dithiocarbamate during reaction with hydroperoxide. Details of the mechanism of action are presented.