Sorption of flavour compounds by polyethylene terephthalate and copolyester films for food packaging

The sorption of flavour compounds by PET and 15 copolyesters (PET copolymerized with isophthalic acid, adipic acid, sebacic acid, tetramethylene glycol, diethylene glycol, triethylene glycol and neopentyl glycol in the range of 5 to 18 mol%) was studied. Biaxially oriented films of each resin were immersed in aqueous solutions containing 37 types of flavour compounds for 4 weeks at 37°C. The amount of flavour compounds sorbed by the films was highly dependent on the copolymer composition. Among the tested films, the copolyester film containing 11 mol% sebacic acid exhibited the highest sorption of the 37 compounds, which was approximately 40 times that of PET. The copolyester film containing 11 mol% isophthalic acid exhibited the lowest sorption, approximately half that of PET. The relationship between sorption and the characteristic values of the polyester films was studied. Sorption was negatively correlated with the solubility parameter and density of the amorphous area and strongly negatively correlated with the glass transition temperature. It was concluded that the dynamic free volume involved in diffusion dominates the sorption of flavour compounds by copolyesters. The log P value and molecular size of the compounds had a strong effect on the sorption of individual flavour compounds by the films in aqueous solutions.