Additives interactions in the thermo-oxidative degradation of metallocene polyethylene
During the 1990's the development of metallocene catalysts has made a dramatic
impact on the polyethylene industry. A large amount of work can be found in the literature
regarding the degradation and stabilisation of conventional polyethylenes. However, very
few studies have been devoted to these novel metallocene polymers. The first part of this
thesis deals with the thermo-oxidative degradation of various metallocene polyethylenes
(mPE's) in the solid state (oven ageing at 90°C in air) and the melt state (via mastication in
air in a Brabender® mixer). For each mPE, essentially one characteristic (melt index,
molar mass distribution, density and ash content) was changed at a time. In the second
part, the effect of different grades (animal versus vegetable) and types (calcium versus zinc
stearates) of metallic stearates in combination with phenolic and phosphite antioxidants
was investigated in the melt stabilisation of a standard metallocene LLDPE (mLLDPE).
Finally, interactions between additives present in a typical stabiliser package used for film
grade LLDPE were assessed in the stabilisation of the mLLDPE with help of a two-level
factorial design of experiments. The response included melt flow protection and
discolouration during processing, as well as long-term stability.
Outstanding thermo-oxidative stability was exhibited for all the mPE's in the solid
state. This remarkable oxidative stability was believed to be due to the presence of low
concentrations of innocuous metal catalyst residues, as well as low initial vinyl
unsaturation content and degree of branching. Furthermore, polymer density/crystallinity
appeared to reduce the spreading rate of oxidation. In the case of thermo-oxidation in the
melt state, the polymer melt viscosity appeared to govern the processing stability of the
metallocene polymers. In the second part, it was found that the quality of the metal
stearates, i.e., peroxide content, metal oxide and unsaturation, may affect the performance
of the phenolic and phosphite antioxidants in the melt stabilisation of the mLLDPE.
Finally, the statistical design of experiments proved to be a very useful tool for screening
the effects of each additive and the possible interactions between additives present in the
investigated stabiliser system.