Effect of crystal structural changes of PET in can-making process on properties of film laminated steel for containers

PET film laminated steel sheets with excellent properties have now widely used in beverage cans. In recent years, can-weight reduction has been advanced from the viewpoint of resource saving, so that deep-drawing and ironing are performed to the laminated steel sheets, which requires non-oriented PET films with excellent formability instead of BO (Biaxially Oriented) PET films. However, non-oriented PET films have the possibility of any effects on impact and corrosion resistance due to the lack of crystal structure. In this study, we investigated the effect of the crystallinity of PET film on various properties required for beverage and food cans. Steel sheets on which PET films with different crystallinities were laminated were formed into the can shape by stretch-drawing processing in order to evaluate formability, adhesion property, impact and corrosion resistance. Here we found that BO-PET film laminated sheet with high crystallinity was inferior in impact and corrosion resistance compared with BOPET/ IA and NO-PET/IA film laminated ones. Film cracks parallel to the can-height direction were observed only on the surface of BOPET film after the heat treatment, which caused deterioration of the properties. The heat treatment increased crystallinity degree of PET and aligned (100) crystal face of PET parallel to the can-height direction, which resulted in the occurrence of the film cracks. The study concluded that crystal-structural changes of PET film in can-making process had a significant effect on corrosion and impact resistance, and that the control of crystallinity of PET was the key factor to obtain the excellent properties.