Environmental stress cracking

Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. Environmental stress cracking may account for around 15-30% of all plastic component failures in service.

ESC and polymer resistance to ESC (ESCR) have been studied for several decades. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air. The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid.

It is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions. It has also been seen that catastrophic failure under stress can occur due to the attack of a reagent that would not attack the polymer in an unstressed state.

Metallurgists typically use the term Stress corrosion cracking or Environmental stress fracture to describe this type of failure in metals.

Although the phenomenon of ESC has been known for a number of decades, research has not yet enabled prediction of this type of failure for all environments and for every type of polymer. Some scenarios are well known, documented or are able to be predicted, but there is no complete reference for all combinations of stress, polymer and environment. The rate of ESC is dependent on many factors including the polymer’s chemical makeup, bonding, crystallinity, surface roughness, molecular weight and residual stress. It also depends on the liquid reagent's chemical nature and concentration, the temperature of the system and the strain rate.

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