Consumption and loss of commercial stabilisers in polyethylene exposed to different natural environments

Abstract: A procedure for the extraction and analysis of commercialstabilisers in low density and medium density polyethylene(LDPE and MDPE) films was developed and used to study theconsumption of stabilisers in polymers exposed to differentnatural environments. An ultrasonic technique was applied toextract HALS-3 (Chimassorb 944) from LDPE film and antioxidants(Irganox 1010 and Irgafos 168) from MDPE film. This wasfollowed by a quantitative analysis of Chimassorb 944 by UVspectroscopy at a detection wavelength of 250 nm for theabsorption of the 1,3,5-triazine group and a quantitativeanalysis of the antioxidants by High Performance LiquidChromatography (HPLC) using acetonitrile as mobile phase and adetection wavelength of 280 nm.The migration of the stabilisers from the polymeric matrixwas studied when LDPE and MDPE films were exposed to asimulated landfill leachate (aqueous media, pH5) as well asrainfall (aqueous media, pH7) both under accelerated conditionsand in real time (exposure temperatures of 25°C,50°C, 80°C and 105°C) with a solid-to-liquidratio of 1:5 (a ratio of 1:10 was also used in LDPE for thepurpose of comparison). The amount of stabilisers, whichmigrated from the polyethylene matrix, increased continuouslywith time. An increase in temperature as well as a lowersolid-to-liquid ratio accelerated the migration process,whereas a rise in pH retarded it. The kinetics of the migrationwere well described by first order equations, and anArrhenius-type relationship was applicable for thetemperature-dependence of the migration rate constants. Thetime required for the complete release of the stabilisers underdifferent landfill conditions was predicted.The consumption and conversion products of the stabilisersas well as the degradation behaviour of the polymeric matriceswere investigated after long-term exposure for a period of 4years to different natural environments such as open air andsunlight, composting mixture and chemical hydrolysis at pH5 and7, in all cases at room temperature. Loss of the stabilisersand structural changes occurred in the polymeric matricesduring this exposure time and were strongly influenced by theexposure conditions.The structural changes in the polyethylene films both afteraccelerated exposure to simulated landfill leachate andrainfall and after long-term exposure to natural environmentswere investigated by different methods such as ScanningElectron Microscopy (SEM), Fourier Transform InfraredSpectroscopy (FTIR), Differential Scanning Calorimetry (DSC)and Size Exclusion Chromatography with Viscosity detection(SEC-Viscosity). Degradation of the polymers caused changes incrystallinity, an increase in melting temperature and theformation of crosslinks.A variety of degradation and transformation products fromboth the polymeric matrices and the stabilisers were identifiedby Gas Chromatography-Mass Spectrometry (GC-MS). The amounts ofdegradation products were remarkably greater from the LDPE thanfrom the MDPE film, indicating a much higher extent ofdegradation in the LDPE matrix than in the MDPE matrix. Themain degradation products found were linear and branchedalkanes, alkenes, alcohols, carboxylic acids, aldehydes,ketones and esters. Different products formed from Chimassorb944 were found in all the tested environments as a result bothof degradation of the stabiliser to smaller fragments and ofreactions between these fragments and other additives presentin LDPE. The transformation products of the antioxidants formedas result of processing and/or exposure to thetested mediawere also identified. The transformation of the phenoxylradical of the Irganox 1010 led to ester, acid, dealkylatedcinnamate and quinone products, whereas conversion oxidationproducts and the phenolic hydrolysis by-product 2,4-di-tert-butylphenol were formed from Irgafos 168.Keywords: LDPE, MDPE, ultrasonic extraction, polymeradditives, stabilisers, antioxidants, HALS, Irganox 1010,Irgafos 168, migration, consumption, GC-MS, additivedegradation products.