Photo-stabilising action of metal chelate stabilisers in polypropylene: Part V—Light stability as a function of concentration and further studies in metal stearate-stabiliser systems

The effect of concentration on the light stabilising efficiency of three metal chelates in polypropylene film has been examined. Both the calcium and nickel complexes of bis(ethyl-3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate (Irganox 1425 and Irgastab 2002, respectively) exhibit the same behaviour and have a limiting concentration above which no significant further photo-protection occurs. However, in the case of the nickel complex of 2,2′-thiobis(4-tert-octyl-phenolato) n-butylamine (Cyasorb UV 1084), there is a linear relationship between light stability and concentration up to 0·1% w/w of additive. Deviations from this linearity appear at higher concentration levels. The results confirm earlier conclusions that the photo-oxidation products of the first two chelates are photo-active, whereas those of the latter are themselves photo-stabilisers.Further studies on the effects of metal stearates on the stabilising action of the two nickel complexes have been carried out. In the case of Cyasorb UV 1084, synergism was observed with strontium stearate whereas antagonism was present with all the other stearates, their order of stability being Zn > Na > Mg > Co. On the other hand, all the stearates were antagonistic with Irgastab 2002, their order of stability being Na > Co ≈ Zn > Sr > Mg. The effects are accounted for on the basis of metal exchange, supporting our earlier findings.