The resistance to chemicals of polymers is usually determined by immersing specimens in the media in question at various temperatures. After a given time has elapsed, the specimens are removed and changes in their mass, dimensions and mechanical properties are measured. However, this is only of limited practical use in determining the suitability of a polymer for a given application, because the internal and external stresses experienced in service can have a dramatic effect on the nature of the chemical attack.
When embrittlement of polymers occurs under these conditions it is termed environmental stress cracking (ESC) and the environment is frequently a liquid or vapour that would not attack the polymer if the stress were absent. ESC, a time dependent phenomenon, often first makes itself apparent on the polymer surface in the form of fine hairline cracks that reflect light. These fine cracks are in fact bridged by extended polymer chains which are still able to transfer some load. However, if subjected to impact loads, they will act as stress concentrators, resulting in failure. Over a long period of time they can develop into cracks in their own right.
Poor chemical resistance is often considered the Achilles heel of amorphous thermoplastics. Crystalline resins tend to have much better chemical resistance, and in particular solvent resistance, because of their insoluble nature. However, crystalline polymers are particularly susceptible to ESC.