Possibilities and limits of synergism in with light stabilizers in polyolefins 1. in polyolefins

Synergistic combinations of HALS have gained increasing importance for stabilization of polyolefins. Some of the effects observed have been presented to illustrate the possibilities of such combinations. The study has been extended beyond the solutions used commercially. This was aimed at gaining better understanding of the physico-chemical phenomena involved in synergism between stabilizers of the same basic structure. It has beers found that synergism or antagonism between two HALS can be detected by examining one single combination of the two HALS. It is the 1:1 combination of the two components that gives the most reliable results. In addition, one single test criterion seems sufficient for valid conclusions, Furthermore, it looks as if the results obtained in one PP type were valid with all the PP types so far available and polyolefins in general. There is no synergism usually if two low molecular mass HALS are combined. Many of these combinations show just additive effects but there can be strong antagonism with specific combinations. Combinations of low and high molecular mass HALS,give much more often synergism than combinations of two low molecular mass HALS. This has been attributed to the complementary effects resulting from rapid diffusion of the low molecular mass HALS to the surface layers where the need for stabilizers is highest with the resistance to migration and extraction of high molecular mass HALS. Nonetheless, synergism is not general with such combinations, they may give just additive effects or even antagonism. Of course, this explanation of synergism is not valid for combinations of two high molecular mass or polymeric HALS. If synergism is observed with such combinations, it must result from very specific interaction or specific protection mechanisms of the components. Although there are many examples for such synergistic combinations in the patent literature, the effect is not at all general. As a consequence of a considerable amount of work, synergism can be understood to some extent for combinations of the polymeric HALS-2 with HALS-3 and high molecular mass HALS of similar structure. However, most experimental observations of synergism and antagonism cannot yet be explained. (C) 2002 Elsevier Science Ltd. All rights reserved.