Environmental stress cracking (esc) is not the result of the continual battle that the plastics industry has in convincing environmentalists and politicians that plastics can be the solution to environmental problems and not the cause.
Esc is the consequence of a combination of mechanical stress and exposure to an ‘aggressive’ chemical. The mechanical stress can arise from service loading or from internal strain created during the moulding process. The ‘aggressive’ chemical can be crudely classified as a poor solvent or non-solvent for the thermoplastic material. The esc first manifests itself as minute cracks as the strain is relieved by the chemical. The cracks will eventually grow and lead to catastrophic failure. As with many polymer properties, time, temperature and even uv exposure can all be contributory factors.
I recall one example where blow-moulded PVC sweet jars literally disintegrated in the warehouse, leading to the collapse of stacks of jars of sweets. Because the same jars had been performing well for years, the customer blamed the moulder for a ‘bad’ batch. It transpired that the customer had added to the sweet recipe a new vegetable oil, which just happened to be aggressive to the PVC, a fact that was able to be demonstrated by laboratory tests to convince the customer.
My next blog will look at how we can avoid environmental stress cracking.