The Effects of Multiple Extrusion Passes During the Recyling of High Density Polyethylene

Abstract

<p>Plastics recycling has received significant attention within the past decade. Positive or negative, the attention has provided an impetus for the plastics industry to advance recycling technology. High density polyethylene blow molding resins have been identified as a primary material for solid waste minimization and recycling. This thesis is concerned with the effects of multiple extrusion passes on the processing and product application properties of high density polyethylene blow moulding resins. The objective is to provide a comprehensive analysis on the system of properties as they are affected by multiple extrusion passes.</p> <p>A justification for this present work is provided by a description of the current state of plastics recycling in terms of the history, legislation and technology. The role of political and global market pressures, relative to legislative initiatives such as the Green Dot program in Germany, the 3 R's legislation in Ontario, the content legislation in California and the decision to ban products such as polystyrene foam clamshell packaging, and the impact of these events on the research initiatives in plastics recycling is discussed.</p> <p>An experimental study into the effect of multiple extrusion cycles on the properties of a virgin homopolymer, virgin copolymer, natural post consumer and mixed colour post consumer blow moulding resin was conducted. Rheological properties such as shear and elongational viscosity and elastic modulus were studied in the context of changes experienced during recycling. The G' - Gil (elastic storage and loss modulus) crossover point was used to measure relative changes in the polydispersity index and molecular weight distribution. It is also shown how parameters such as the Bagley correction factor, extrudate swell and sag are sensitive to the effect of multiple extrusion passes. Strength properties such as tensile strength and environmental stress crack resistance were measured. A rationale for the significant decrease in the environmental stress crack resistance of the virgin copolymer resin is presented. The results are analyzed in terms of known degradation mechanisms such as chain scission and cross-linking, and their relationship to the Phillips and Ziegler-Natta catalyst systems. Principal component analysis, a multivariate statistical technique, was applied to this plastics recycling study to provide data based support in the identification of those responses, within a system of correlated responses, which are most affected by multiple extrusion passes.</p>