The role of stable nitroxyl radical precursors as antioxidants in polyolefins

Various 2,2,6,6-tetramethyl piperidines and their N-alkyl derivatives of stable nitroxyl radical precursors containing acrylic(s) and methacrylic(s) groups were reactively processed in the presence of a peroxide as bound-antioxidant masterbatches for polyolefin stabilisation. It was found that grafting of the antioxidant monomers onto the polymer backbone was inevitably in competition with homopolymerisation of the monomers as well as melt degradation of the polymer and other side reactions. As previously reported, binding efficiency of bisacrylic nitroxyl precursor was maximum due to formation of unextractable homopolymer of the antioxidant. On the other hand, the binding efficiency of monoacrylic derivatives was low and the homopolymers were found extractable, which suggests that the bound monoacrylic derivatives are entirely grafted onto the polyolefin backbone. Application of bis and tri-functional coagents gave improved binding efficiency of the monoacrylic monomers. This may be due to copolymerisation of the antioxidants with the coagents and grafting of the copolymers onto the polymer backbone. Comparison of photostabilising activity of the fully extracted bound antioxidants to those of the corresponding unbound analogous showed lower results for the former. However, thermal stabilising activity of the bound antioxidants was higher than that of the unbound analogous due to better substantivity. Analysis using physical techniques and GPC for molecular weight distribution of masterbatches containing the bound monoacrylic antioxidants showed formation of high molecular weight products. Model reaction of a secondary amine derivative in liquid hydrocarbon and analysis of the product using FTIR and NMR spectroscopy indicated a possibility of side reaction, i.e. involvement of the amine active group (>N-H) of the antioxidant in the binding process to form the high molecular weight product. Implementation of various N-alkylated derivatives did not inhibit the side reaction. The photostabilising activity of the bound-antioxidants can be improved when applied in conjunction with small amounts of a benzophenone uv-stabiliser. The synergistic stabilising activity, however, was diminished when the uv-stabiliser was removed from the system during the service time. Nitroxyl precursors containing methacrylic group(s) gave lower binding efficiency than the corresponding acrylic derivatives. Reversible deploymerisation of the grafted methacrylic antioxidants may be responsible for this. Bis and tris-acrylic coagents improved the binding efficiency, and the presence of methacrylic group improved stabilising activity of the antioxidants. N-methyl derivatives were found to exhibit better stabilising activity than their parent secondary amine derivatives.

Effect of processing on sulphur antioxidants in polyolefins

The effect of processing on the antioxidant activity of sulphur-containing compounds, with particular reference to nickel dialkyldithiophosphates and their corresponding di sulphides, were studied in polyolefins under melt, thermal and photo-oxidative conditions. These compounds were evaluated both at low (normal) and high (concentrates) concentrations. In general, the dithiophosphates were found to be very efficient melt stabilisers at normal concentrtion levels, and compare quite favourably with the best commercially available systems. The nickel dithiophosphates were also found to be very efficient thermal stabilisers for polyolefins, but their activity is highly dependent on the alkyl substituent in the molecule. The corresponding disulphides on the other hand showed very little activity under thermal oxidative conditions, and this was attributed to their inefficiency in scavenging alkyl peroxyl radicals since both compounds possess similar peroxidolytic activity. Furthermore, the nickel dithiophosphates were found to be excellent photo stabilisers for mildly-processed polyolefins while the corresponding disulphides only offer slight protection to the polymer. Oxidative processing of the disulphide, however, results in a dramatic improvement in their photo antioxidant activity. Thionophospho-ric acid, a major oxidation product of dithiophosphates, was also shown to have photo antioxidant activity similar to that of the disulphides. A combination of a U.V. absorber with the nickel complex and/or the disulphide resulted in a synergistic stabiliser system which was further augmented by oxidative processing. Moreover, the dilute analogues of such multicomponent stabiliser concentrates also showed excellent melt, thermal and photo-stabilising activity. The mechanistic studies carried out on the nickel complex and the corresponding disulphide clearly identified the thionophosphoric acid a a major transformation product although various triesters were formed as reaction intermediates. The mechanisms of the antioxidant action of the dithiophosphates, which is believed to involve a cyclical process similar to that shown for simple alkyl sulphides and nitroxyls, are discussed.