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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:289422 |
| Date | January 2003 |
| Creators | HoaÌ€ng, Eric MiÌnh |
| Publisher | Manchester Metropolitan University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0051 seconds