Oxidative Degradation of Polyethylene - A Cause of Off-flavour
Abstract: Polyethylene is the most frequently used polymer material in food packaging today. The polymer is often processed by film extrusion and extrusion coating between 250°C and 325°C. Although polyethylene has high thermal stability some degradation takes place and the degradation products formed will migrate to the packed food, which will give food off-flavour tastes when consumed. With an increase in extrusion temperature the risk of detecting off-flavour in the packed product would increase. Additives in the polyethylene might reduce degradation but the effects are unclear and probably depend on the additive’s stability to the high processing temperatures used in extrusion coating. The aim of this research was to understand the behaviour of polyethylene and polyethylene degradation products, from the inlet of the extruder to the final packaging film and the packed product. The degradation is initiated in the extruder during the melting and metering of the polymer. The polymer degradation starts with chain scission reactions forming radicals, and via auto-oxidative process the oxygen dissolved in the polymer forms peroxides and other products. Some of the radicals are combined resulting in higher molar mass and an increase in the viscosity of the melt. At an extrusion temperature of below 300°C cross-link reactions predominate while at higher process temperatures chain scission reactions predominate. When the melt exits the die orifice the radicals react with the oxygen present in the atmosphere nearest to the melt surface leading to an auto-oxidative degradation. The low molar mass degradation products will evaporate from the melt forming “extruder smoke”. Higher molar mass degradation products will remain at the film surface as alcohols, aldehydes, ketones, carboxylic acids and esters depending on the temperature of the melt, the exposure time to the atmosphere and the oxygen concentration in air-gap gas. This makes the passage through the air-gap the most critical processing step from degradation point of view. The oxidative degradation products are shown to give the highest off-flavour, the aldehydes and ketones being the most prominent ones. The presence of carboxylic acids seems to be necessary to carry the hydrophobic carbonyl compounds into the product and to generate off-flavour when reaching the mouth. There are clear synergistic effects between aldehydes and ketones and between aldehydes and carboxylic acids. The most off-flavour giving products present in the extruded film are C7-C9 ketones and C5-C8 aldehydes together with the carboxylic acids. Different types of additives can be used as scavengers to adsorb degradation products or radicals as well as antioxidants to stabilise the radicals, which slows down the degradation reaction rates. It is important to select the additive from extrusion process temperature because different additives have different thermal stability. It is possible to control the off-flavour of the polyethylene films when controlling the extrusion temperatures and exposure time and oxygen content in the air-gap or by using selected additives.
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