Aspects of the impact of stabilizer mass on performance in polymers: 3. Performance of HALS in polyethylene

Abstract

This work examines the effect of HALS mass on performance in polyethylene. The first part compares the effect of low molecular mass HALS to the effect of medium and high molecular mass HALS. The second part of the work pertains to the effect of increasing number average molecular mass of a polyacrylate HALS on its performance in polyethylene. The results show that the relative performance of low and high molecular mass HALS is depending on sample form and exposure conditions. Under ideal conditions there can be slight superiority of low molecular mass HALS over medium and high molecular mass HALS in polyethylene. Hence, the difference between polypropylene (see Parts 1 and 2 of this series) (Gugumus F. Polym Degrad Stab 66;1999;133 and Gugumus F. Polym Degrad Stab 67;2000:299) and polyethylene is of quantitative rather than of qualitative nature. Protection of the surface layers is less important with polyethylene than with polypropylene but is nevertheless significant. An increase of the number average molecular mass of the polyacrylate HALS leads to significant decrease of performance in polyethylene as found previously for polypropylene. However, there are pronounced differences between the various polyethylene types, e.g. between PE-HD and PE-LD. Thus, for PE-HD as for PP, the performance is decreasing only to some minimum level function of the HALS concentration. It is not affected further by any increase in molecular mass. However, with PE-LD the performance of HALS is decreasing to zero for sufficiently high number average molecular mass. The difference between PE-LD and PE-HD or PP is attributed to the important contribution of deactivation of catalyst residues (Ti) to UV stability of the corresponding polymers. This deactivation seems to be independent of HALS number average molecular mass. Of course, there are no catalyst residues to be deactivated usually in PE-LD. The work allows better understanding of the effect of molecular mass in various polyolefins. It shows also the importance of a particular stabilization mechanism of HALS or rather, of specific HALS, i.e. deactivation of polymerization catalyst residues.