Stress-cracking of PET bottle as an unpredictable factor limiting the commercial life of carbonated soft drinks

The shelf life of carbonated beverages mainly depends on the carbon dioxide retention performances of the PET bottle, however the commercial life of such products can be compromised by ruptures and crackings of the material, which can occur in certain conditions. The so-called "stress cracking" of PET bottles occurs in correspondence with the injection point, which is more susceptible due to its amorphous nature, unlike the sides of the bottles, whose strength is a result of a certain degree of orientation. Stress cracking can be a consequence of various factors, which usually act in combination and are difficult to isolate: excessive internal pressure (excess carbonation), material distribution, altered intrinsic viscosity, exposure to UV rays and/or high temperatures, but most important is the contact with caustic substances (for example, the lubricant of the conveyors belts), etc. The present contribution focuses on a case study occurred at SIBEG s.r.l., concerning the non conformity of 1.5 L PET bottles of a popular carbonated soft drink, which caused the loss of an important amount of bottles, the disposal of defective products, and the careful inspection and selection of in-house stocks. Inspections revealed no failure in the blowing process, leading to hypothesize over-carbonation and transport conditions as the triggering events. Moreover, the analysis of intrinsic viscosity (IV) of bottles and preforms revealed the non-conformity with specification. Despite the product conformity concerning thermal stability, burst test and stress cracking test performed in alkaline conditions, PET bottles quality and performances may become altered if the IV of the material does not fulfill specifications. Therefore, the IV determination plays a key role, complementary with other routine determinations, in the quality control of PET bottles addressed to carbonated soft drinks packaging. The use of such tool is fundamental for the prevention of severe problems which might occur along the distribution chain, such as claims, withdrawal and disposal of defective products.