Effect of Crystal Structural Changes of PET in Can-making Process on Properties of Film Laminated Steel for Containers

Polyethylene terephthalate (PET) film laminated steel sheets with excellent properties are now widely used in beverage cans. In recent years, can weight reduction has been promoted from the viewpoint of resource saving, and deep-drawing and ironing processing has been applied to the laminated steel sheets. Therefore, non-oriented PET films (NO-PET) with excellent formability are required instead of biaxially oriented PET films (BO-PET), but NO-PET films may affect impact resistance and corrosion resistance due to their lack of a crystal structure. This study investigated the effect of the crystallinity of the PET film on various properties required for food cans. PET films with different crystallinities laminated on steel sheets were formed into a can shape by a stretch-drawing process in order to evaluate formability, adhesion, impact resistance and corrosion resistance. The results showed that the BO-PET film laminated steel with high crystallinity was inferior in impact resistance and corrosion resistance compared with the BO-PET/IA (copolymerized with isophthalic acid) and the NO-PET/IA film laminated steel. Film cracks parallel to the can height direction were observed only on the surface of the BO-PET film after heat treatment. Heat treatment increased the crystallinity of the PET and oriented (100) crystal face of the PET parallel to the can height direction, which resulted in the film cracks observed on the surface of the BO-PET film. As a result of this study, it was concluded that the crystal structure change of the PET film in a can making process had significant effects on impact resistance and corrosion resistance, and control of the crystallinity of the PET was the key factor for obtaining excellent properties.