Effects of the Cooling Rate on the Crystallinity and Seal Strength of Sideweld Sealed Polypropylene Bags

ABSTRACT We have investigated the mechanical strength of sideweld sealing for biaxially oriented polypropylene bags. The tensile strength of the seal of the biaxially oriented polypropylene films showed a smaller deviation with increasing crystallinity in the sealed zone. Because the distribution of the crystallinity indicated sufficient homogeneity within the sealed zone at the microscopic level, the smaller deviation of the sealing strength with increasing crystallinity is ascribed to the formation of macroscopic defects under stretching. The defects around the sealed zone were visualized by photoelasticity observation under tensile stretching. We observed fluctuation of the strain near the sealed zone, accompanied by wavy patterns owing to the troughs induced by the shrinkage stress perpendicular to the stretching direction. Cross‐sectional observation of the sealed zone in the sideweld sealed specimen after the tensile test suggested that the fracture was initiated by ductile fracture. The subsequent fracture process became more brittle for specimens with lower strength, presumably because of the higher crystallinity. We also confirmed that the crystallinity can be controlled by the cooling rate in a rapid‐cooling experiment mimicking the air cooling in the bag forming process, where the crystallinity was significantly reduced by about 15%.