The modeling of the creep behavior of poly(ethylene terephthalate) bottles

PET bottles containing carbonated beverage show the continuous volume expansion under the internal pressure due to the creep behavior, which causes a reduction of CO₂ concentration in the beverage, because the pressure of CO₂ above the liquid level decrease as the bottle volume increases. In this work, we have developed a mathematical model to describe the volume expansion of PET bottles contained carbonated beverage.; PET bottles containing carbonated beverage are made by the stretch-blow molding process. The process of stretch-blow molding to make a bottle involves biaxially stretching the wall of a perform in the axial and hoop direction. This process is very similar to the process of biaxially stretching the polymer films with a long extension tester(LET). By studying the creep behavior of stretched PET films, we can use the results to predict the volume change of PET bottles under the internal pressure.; The stretched PET films will show the anisotropic properties due to unequal stretch ratios in two directions. In order to describe the change of creep behavior of stretched PET films with direction, we introduce three parameters in the mathematical model: the compliance in the orientation direction, the compliance in the direction perpendicular to the orientation direction and the shear modulus. The change of the creep behavior with directions is related to the stretch ratios of stretched PET films through these three parameters. By assuming the geometry of PET bottle as the cylinder, the mathematical model based on stretched PET films is used to predict the volume change of 2-liter PET bottle. In mathematical model, by replacing the crystallinity and the stretch ratios of stretched PET films with the crystallinity and the stretch ratios of PET bottle, we can use it to predict the volume change of PET bottles containing carbonated beverage with different shapes and sizes.