Thermal decomposition of oligomeric and polymeric hindered amine light stabilisers

Abstract

In this work pyrolysis-GC/MS studies are presented on two pairs of hindered amine light stabilisers (HALS) oligomers and polymers. Pyrolysis has been performed in the temperature range from 300 to 900°C, also in the presence of PVC in order to observe the possible effect of this hardly avoidable plastic waste component on the production of volatiles from HALS compounds. Intramolecular rearrangement of ester segments of the macromolecular chain leads to succinic acid anhydride and 1-azacyclohexeneethanol in Tinuvin 622 already at 400°C. In the presence of PVC 1-azacyclohexenechloroethane is also formed from this stabiliser. The piperidine moieties bound to the macromolecular core by ester bonds are cleaved through unimolecular β-elimination reaction at around 500°C. Thermal decomposition of HALS composed of only amines starts at higher temperatures and proceeds through the scission of CN bonds. Alkenes are cleaved from secondary amino groups more easily than from tertiary ones. The scission of the pirimidine ring leads to alkylenamines, alkadienes and trienes at above 600°C. In the presence of PVC the formation of alkylenamines is depressed. By CC bond cleavage volatile products originating from alkyl side groups or hindered phenolic moieties also appear in the pyrolysate. Aromatisation of the piperidine rings with methane loss takes place at above 700°C leading to 2,6-dimethylpyridine, 2,6-dimethylanilines and 1,3-dimethylbenzene. In the presence of PVC the formation of pyridine and aniline compounds are depressed. At 900°C volatile alkane- and alkenenitriles evolve together with traces of hydrogen cyanide. The formation of hydrogen cyanide is enhanced in the presence of PVC.