Effects of Simulated Recycling on Polyolefin Materials

Doctoral thesis, 2003

Production and handling of waste is a matter of concern in most countries, and one way to reduce the amount of waste is recycling. Since synthetic polymers are made from non- renewable resources and are used in large amounts, often for short periods of time, they should be recycled in a sustainable society. When polymer materials are recycled through re-melting and shaping of new products (mechanical recycling), they degrade both during the reprocessing due to the shear and heat, and also during use due to UV-radiation, heat e t c. In order to increase the use of recycled materials it is of great importance to understand the degradation behaviour in order to determine the quality of a recyclate and prevent further degradation. In this study some of the most common polyolefin materials (some recycled) have been investigated by using a simulated recycling procedure. The materials were processed repeatedly in combination with intermediate accelerated thermo-oxidative ageing and the polymer degradation was measured by several different analysing techniques. Also upgrading of some materials by addition of antioxidants was investigated with promising results. Some important knowledge has been achieved by subjecting polyolefin materials to simulated recycling. Ageing steps at high temperatures caused a major drop in elongation-at-break for a recycled polypropylene material. This may be explained by a combination of morphological changes, such as re-crystallisation and probably also surface degradation leading to micro-cracks acting as stress raisers. After a subsequent extrusion step the material recovered almost to the initial elongation-at-break value. Beside formation of new crystallites, probable explanations are dilution of degraded parts of the material and formation of a new surface layer during the mixing in the extruder. Hence, recycled materials may have better quality than first expected if only the surface layer is degraded. Moreover, the simulated recycling degraded the material faster compared repeated extrusion only, or accelerated ageing performed separately. We have shown that hydroperoxides are formed during accelerated thermo-oxidative ageing steps and decompose rapidly forming radicals at high temperatures during processing steps. When the hydroperoxides decompose they form two highly reactive radicals, which accelerate the oxidation. Simulated recycling should therefore be a valuable tool to evaluate the recycling potential for materials, without over-estimating the lifetime of materials.