Secondary reactions in the analysis of microplastics by analytical pyrolysis

Mixtures of plastic particles of different types were analysed by Py-GC-MS in order to investigate the effect of polymer interactions on the composition of pyrolysates. Pyrolysis products associated to secondary reactions were observed when particles of poly(ethylene terephthalate) (PET) were pyrolysed in the presence of poly(vinylchloride) (PVC) and the polyamides PA6 (polycaproamide) and PA66 (poly(hexamethylene adipamide)). Co-pyrolysis of PET in the presence of PVC particles produced mono and dichloroethyl esters of terephthalic acid due to the addition of HCl to the double bond of the vinyl group. Benzyl chloride, benzoyl chloride, 4-chlorobenzoic acid were also tentatively identified. The interaction between PET and PA6 and above all PA66 brought about the formation of aromatic nitriles including methyl and ethyl benzonitrile, 1,4-benzenedicarbonitrile, 4-cyanoethyl benzoate, 4-cyano benzoic acid. In addition, co-pyrolysis of PET and PA66 particles produced N-alkyl amides of benzoic acid. Pyrolysis of benzoic acid generated benzene and diphenyl, while N-hexylbenzamide was observed from the pyrolysis of benzoic acid with hexyl-1-amine or hexan-1,6-diamine. PE (polyethylene), PP (polypropylene), PS (polystyrene) and PVC did not exhibit significant interactions. Calibration curves built up with single plastic types in the 10–120 µg range were used to quantify each polymer in the mixture using typical pyrolytic markers. The highest deviations of recovery and the lowest RSD were observed for PET and PA6, while polyolefins produced satisfactory recovery and RSD. The detection of pyrolysis products from secondary reactions can be used as markers for the co-presence of different polymers supporting their identification.