Residual Stress in Injection Stretch Blow Molded PET Bottles

Abstract

Injection stretch blow molded poly(ethylene terephthalate) bottles show evidence of residual stresses. Some of these may develop as a result of cooling rate gradients through the material thickness. Others result from stretch crystallization as well as other forms of reduced entropy caused by rapidly stretching and quenching the amorphous PET matrix. A wide range of interdependent variables can change the magnitude and sign of residual stresses in bottles including the shape of the mold, resin properties, blowing pressure, preform temperature, thickness of the bottle wall, and cooling rate. Residual stresses can significantly influence bottle dimensional stability, top load, permeability, and consumer tactile feel. Here, the residual stresses were observed with photo-elasticity, which measures the changes in optical properties of the polymer in the presence of residual stresses. For observation of the time-dependence of stresses in the polymer film, a ring-cutting method was implemented. Depending on process and mainly on cooling rate, the rings have a tendency to open up or collapse under relaxation. X-ray diffraction crystallinity measurements with different incident angles were performed on both sides of the film to obtain information from different depths. XRD helped to explain the cause of collapsing instead of an expected opening in some ring tests. The change in distance between the two edges of the cut was measured with an image analyzing program. For the initial cut and 30 days after cutting, the residual stress was calculated. A chemical etching technique was used for validating the presence of tensile residual stress on one side of the polymer film. Applying chemicals to the surface of a polymer creates fracture on the more amorphous surface side of the film, depending on the magnitude of the tensile stress.