Mechanism of stress cracking of poly(ethylene terephthalate) beverage bottles: A method for the prevention of stress cracking based on water hardness

A recently described bottle test method was used to evaluate the dependence of stress crack failure of poly(ethylene terephthalate) (PET) carbonated soft drink bottles on water hardness. Although the industry belief is that water hardness is irrelevant to stress cracking, it was found that hardness ions exert a tremendous positive impact by deactivating water alkalinity through precipitation as harmless carbonate minerals. This mitigating effect of hardness means that no complete understanding of stress cracking as a function of alkalinity is possible without also considering water hardness. A useful concept is that of excess alkalinity, which is defined as alkalinity that is not precipitated during solution evaporation. Limiting excess alkalinity by using water with sufficient hardness is an effective means of stress crack prevention in PET soft drink bottles. Evaluation of compositions which are typical of those used for lubricating PET bottles on production conveyors showed that the role of these compositions in stress cracking was that of spectators, that is, they neither cause failure if water does not otherwise cause it, nor stop failure if water otherwise causes it.