Degradation of polyethylene during extrusion. II. Degradation of low‐density polyethylene, linear low‐density polyethylene, and high‐density polyethylene in film extrusion

Abstract

The degradation of different polyethylenes—low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (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 online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography–mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto‐oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall 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 too. A number of degradation products were identified—for example, aldehydes and organic acids—which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7≫C2 for LDPE, C5>C6>C4>C7≫C2 for LLDPE, and C5>C6>C7>C4≫C2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1525–1537, 2004

Degradation of low density polyethylene during extrusion. III. Volatile compounds in extruded films creating off‐flavor

“…In our previous work 4,8,15 it was discussed how the degradation of polyethylene is initiated at temperatures of 260 -325°C. We identified the type of volatile degradation products being formed and where in the process most of the oxidative degradation occurs.…”

“…The tube was then placed in the sample rack of the thermo desorption autosampler. 8 Water. An internal standard was prepared by weighing 100.0 Ϯ 1.0 mg of 1,4-dibromobenzene (CAS#106 -37-6, 99%, Acros Organics, Geel, Belgium) into a vial, dissolved and diluted with dichloromethane (diChloromethane, Enviroscan 99.9%, Lab-scan, Stillorgan, Dublin, Ireland) to 10.0 mL.…”

“…8,15 All volatile compounds found were in the sub ppb to low ppb concentrations in all evaluated films. This was in accordance with our conclusions from previous works.…”

“…They also reported a synergistic behavior of aldehydes, ketones, and acids, giving offflavor sensations even though the different components were below their threshold concentrations. Based on our previous work regarding degradation products found in extruder smoke 4 and in extruded PE films, 8 the present study was aimed at finding a correlation between the experienced off-flavor in packaged foods and the presence of specific degradation products in PE packaging films. In the present work, the possibilities to trap degradation products by chemical reactions with scavengers, thus preventing these components from migrating to the polymer film surface, and further into food in contact with the film, were investigated.…”

“…The best correlation between off-flavor and oxidized components were found for C 7 -C 9 ketones, and aldehydes in the range of C 5 to C 8 . © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 847-858, 2005 …”