Polyethylene terephthalate （PET） sheets, which are now widely used in beverage cans, have excellent properties of formability, corrosion resistance, and adhesion to steel sheets. To expand their application, we have sought to develop a new PET film laminated steel sheet for use in food cans.
However, PET films are known to contain cyclic oligomers as natural impurities that are by-products generated during polymerization. Food cans are subject to steam sterilization, called retort treatment, during which cyclic oligomers are precipitated on the PET film surface. Increased amounts of oligomers on the film cause difficulties such as degraded appearance and difficulty in the adherence of product labels.
This study investigated the effects of the oligomer contents in PET films and crystallinity of PET films on the amounts of precipitated oligomers after steam sterilization. They were analyzed using high-performance liquid chromatography. The analyses showed that the amount of precipitated oligomers increased with the oligomer contents in PET films, and decreased with the co-monomer ratio of PET. The PET film （oligomer content: 0.90 wt%） with more than 6 mol% cyclohexanedimethanol （CHDM）, suppressed oligomer precipitation on the film surface.
Results show that the precipitated oligomer amount is related closely to the amount of amorphous material in PET film. With the amount of amorphous material controlled to be 72% or more, it was suppressed to less than 0.5 μg/cm2, which caused no difficulty.