Wednesday, October 19, 2011: 2:10 PM
Conrad D (Hilton Minneapolis)
Starting in 1978 with the introduction of one piece poly(ethylene terephthalate) (PET) soft drink bottles with molded-in feet, there has been the problem of environmental stress cracking in bottles used for packaging carbonated beverages. Environmental stress cracking is delayed failure in which a large crack forms in the amorphous base portion of the bottle through which all of the liquid contents are lost. Using a full package test method first described in 2008, the mechanism of stress cracking was determined to be alkaline hydrolytic cleavage of ester bonds in the PET backbone and not a Volynskii crazing mechanism as was previously thought. Hydrolysis leading to failure does not happen in the case that naturally occurring water alkalinity is precipitated as carbonate minerals during evaporation. For this reason, eliminating contact of filled PET bottles with softened water has proven to be a highly effective means of stress crack prevention. Physical aging greatly increases amorphous PET susceptibility to hydrolytic stress crack failure and minimizing exposure of empty bottles to environmental conditions which promote physical aging provides another important strategy for the elimination of stress crack failure.