There is no abstract available for this thesis.

Thesis (M.S.)--University of North Texas, Dec., 2009. / Title from title page display. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2004. / Title from first page of PDF file. Document formatted into pages; contains xxi, 238 p.; also includes graphics Includes bibliographical references (p. 231-238). Available online via OhioLINK's ETD Center

Thesis (Ph. D.)--Ohio State University, 2004. / Title from first page of PDF file. Document formatted into pages; contains xviii, 226 p. : ill. (some col.). Advisor: Jose M. Castro, Dept. of Industrial and Systems Engineering. Includes bibliographical references (p. 225-226).

Thesis (M.F.A.)--Rochester Institute of Technology, 1981. / Typescript. Includes bibliographical references (leaf 45).

No description available.

Compared with metals, polymer materials have Iow strength and stiffness. However, molecular orientation can enhance many mechanical properties of polymer materials in the direction of orientation. Studies on solid state polymer extrusion (Le. extrusion carried out at temperatures below the melting point of the polymer) through convergent dies show that it is possible to produce extrudates exhibiting a high degree of monoaxial orientation in the extrusion direction. Although the strength of these extrudates has been greatly enhanced in the orientation direction, the strength decreases in the transverse direction to the axial orientation. Biaxially oriented polymer materials, on the other hand, show increased mechanical properties in more than one direction. But so far, extrusion processes that confer orientation in more than one direction have not received much attention. The present work is concerned with the development of biaxial orientation in thick thermoplastics extrudates by extrusion through dies exhibiting simultaneously converging and diverging walls perpendicular to each other and with a cross-section area at the die entry being the same as at the exit. Four die designs are examined, known respectively as the dual-taper die, the expansion fish-tail die, the constant fish-tail die and the cross die, using polytetrafluoroethylene and ultra-high molecular weight polyethylene. Measurements of birefringence and tensile strength on sections of the extrudates have shown that a preferential orientation along the transverse direction is normally achieved with the fish-tail dies and the dual-taper die, while the extrudates obtained with the cross die were found to exhibit a cross-ply orientation pattern with a bias in the extrusion direction for outerlayers and a preferential orientation in the transverse direction for the middle layers. The mechanics of the processes has been analysed by a plasticity approach for solid state extrusion, and by using variable wall boundaries for the melt extrusion analysis. The extrusion pressure predicted by the analysis compares very well with the values measured experimentally.

Aspects of the degradation of cellulose-based plastics have been studied, with work concentrating on cellulose nitrate and cellulose acetate. Because prior history of the majority existing cellulose-ester based artifacts was unknown, a range of samples were fully characterised and used as controls for subsequent ageing studies. The initial part of the work concentrated on an evaluation of relative rates of degradation when the esters were subjected to various temperatures and relative humidities in the presence and absence of light. Viscometric methods were employed to determine changes in molar mass of samples and infra-red to evaluate functional group changes. The results of these studies demonstrated that not only did the rate of degradation depend primarily on humidity, but that the interplay between de-esterification and chain-scission by hydrolysis versus by oxidation was both humidity and temperature dependent, leading to subtle changes in mechanism. The role of additives, namely the plasticisers was determined by Thermogravimetric Analysis and Differential Scanning Calorimetry. Plasticisers were also isolated from samples by extraction and solution methods. The extracts were analysed by infra-red and nuclear magnetic resonance spectroscopy. The results obtained from these techniques along with visual observation of samples indicated that in humid conditions camphor was exuded from cellulose nitrates as a creamy coloured, 'mushy' deposit Whilst for cellulose acetates diethyl phthalate was itself subject to degradation. Stabilisation of samples was attempted and several additive combinations were found to retard degradation. Cleaning the surface of the samples with natural oils was also beneficial in that it not only improved the appearance of samples but enhanced their subsequent stability. Implication of the work for conservation of cellulosebased artifacts is discussed.

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.

No description available.