Abstract
The first part of this series involved mainly commercial HALS. It has been shown that their molecular mass has considerable impact on their performance in polypropylene. The present work pertains to the effect of the number average molecular mass of polymeric HALS. The experimental data have been generated essentially with PP tapes and multifilaments. Both UV stability and thermooxidative stability have been examined. Depending on the particular stabilizer there can be marked decrease of effectiveness with increasing molecular mass. The decrease can already start with number average molecular masses close to 1000. Up to masses of a few thousand the decrease is caused mainly by decreasing compatibility of the stabilizer with the polymer. There will be an additional limiting factor for stabilizer efficiency if the mass reaches values so large that there is no longer one molecule in every amorphous domain of the polymer. This aspect is affecting all the stabilizers if the molecular mass increases beyond some threshold varying with the stabilizer concentration in the polymer. The increase of the number average molecular mass of polymeric HALS has effects on both UV stability and thermooxidative stability. However, there is a significant difference between the corresponding effects. It is found that the contribution of polymeric HALS to thermooxidative stability vanishes completely if the number average molecular mass reaches very high values. This is not so for the contribution to UV stability. In this instance there always remains some limited contribution increasing with HALS concentration. This difference is tentatively attributed to deactivation of catalyst residues or of other species initiating UV degradation but inactive in initiation of thermooxidative degradation.
Published Version
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