Determination of polyethylene in biodegradable polymer blends andin compostable carrier bags by Py-GC/MS and TGA

In this paper, the amount of polyethylene (PE) contained in complex biodegradable polymer blends andin carrier bags claimed to be compostable was determined by thermogravimetry analysis (TGA) andpyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). To validate the method, polymer blendsbased on polylactide (PLA), poly(butylene adipate-co-butylene terephthalate) 53/47 (PBAT), native starch(NS), and calcium carbonate, with known amount of PE ranging from 0 to 10%, were prepared by meltmixing. Py-GC/MS parameters were optimized. In the first step at 400oC, pyrolysis products originatedfrom thermal cracking of PLA, NS and PBAT were detected. In the second step carried out at 500oC, tripletpeaks characteristic for PE, consisting of ,-dienes, -alkenes and n-alkanes, were revealed togetherwith some signals related to PBAT. TGA under nitrogen atmosphere was used to determine the amountof PE in the blends. An overestimation of about 1.2%, constant for all the blends, was revealed. The over-estimation was attributed to the partial overlapping of one of the steps of degradation of the commercialPBAT and that of PE. Additionally, the analytical approach was applied in the characterization of carrierbags claimed to be compostable, recovered directly from the market. The correct determination of thecomposition of these biodegradable blends is important and crucial to comply with the compostabilitystandard EN 13432-2000. According to this standard, each significant organic constituent of the packag-ing material must be biodegradable. "Significant" means any constituent present in more than 1% of dryweight of that material. This implies that maximum allowed PE concentration that can be detected in acompostable blend is 1%. In two commercial carrier bags, TGA revealed a component with a degradationtemperature of about 455oC corresponding to that of PE. The relative weight loss percentages of thesetwo carrier bags have been estimated 7.6 and 14%. Optimization of the Py-GC/MS conditions allowedidentifying its chemical structure. In fact, the Py-GC/MS at 500oC provided a pyrogram that was a fin-gerprint for PE with triplets corresponding to linear chains of ,-dienes, -alkenes and n-alkanes withchain lengths in the range of 9-36 C atoms. Overall, the results show that the combination of TGA andPy-GC/MS can be applied with practical implications.