DEGRADATION OF LDPE LLDPE AND HDPE IN （退化的LDPE和LLDPE HDPE）.pdf

DEGRADATION OF LDPE LLDPE AND HDPE IN FILM EXTRUSION Thorbjörn Andersson* and Bengt Wesslén** * Department of Material Development, Tetra Pak Research Development AB, SE 221 86 Lund, Sweden ** Polymer Science and Technology, Lund University, SE 221 00 Lund, Sweden ABSTRACT The degradation of different polyethylenes, LDPE, LLDPE, and HDPE, with and without antioxidants and at different oxygen concentrations in the polymer granulates have been studied in extrusion coating processing. The degradation was followed by On Line Rheometry, Size Exclusion Chromatography, Surface Oxidation Index measurements, and GC-MS Chromatography. The degradations starts in the extruder where primary radicals are formed which are subject the auto oxidation when oxygen is present. In the extruder, cross-linking and chain scission reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is over all dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice and the quenching point is mainly related to the exposure time to air-oxygen. Melt temperatures above 280°C give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching. A number of degradation products were identified, for example, aldehydes and organic acids, which were present in homologous series. INTRODUCTION Polyethylene is the most frequently used polymer in packaging. The polyethylene is converted into packaging materials by thermal processes such as film blowing, sheet extrusion, or extrusion coating on substrates such as paperboard. An important prerequisite for fo