Oligomers in polyethylene naphthalate and polybutylene terephthalate - identification and exploring migration

Polyethylene naphthalate and polybutylene terephthalate were investigated regarding their oligomer content. Based on their accurate mass and fragmentation spectra ten oligomers have been identified in extracts of PBT material and seven oligomers in extracts of PEN material. These oligomers were found to be cyclic and linear for both polymers. They consist of the respective diacid and monoglycol monomers as well as diglycol monomers. The total oligomer content in the polyesters was determined with LC-UV using the cyclic PET trimer as external standard and was found to be 0.81% for PEN and 0.34% for PBT. Migration of PBT oligomers was studied into 20% ethanol (v/v) at 40 °C and 60 °C and the diffusion coefficients thus derived were compared to theoretically determined ones. To estimate the PEN and PBT oligomer migration at 23 °C under long time storage conditions the diffusion coefficients were calculated using the approaches by Welle and Piringer. For cyclic PBT oligomers the Welle model underestimates the migration, for one linear PBT oligomer the Piringer model might be suitable to determine the migration. In the chosen scenario the resulting total oligomer migration was below 30 mg kg−1 for both materials.