Environmental Stress Cracking (ESC) is one of the most common causes of unexpected fragilities of currently known thermoplastic (especially amorphous) polymers. Environmental stress cracking can account for approximately 15-30 of all plastic component failures in service.

ESC and polymer resistance (ESCR) to ESC. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the brazing process, and the madness starts at much lower stresses than the stresses that cause airborne cracking. The effect of tensile stress or an abrasive fluid alone will not be sufficient to cause failure, but in ESC, the initiation and growth of a crack is due to the combined effect of stress and an abrasive environmental fluid.

The fact that this stress cracking does not break the polymer bonds is slightly different from polymer degradation. Instead, it breaks the secondary connections between the polymers. They break when mechanical stresses cause small cracks in the polymer and spread rapidly under harsh environmental conditions. It has also been found that catastrophic failure under stress may occur due to the attack of a reagent capable of attacking the polymer in an unstretched state.

Metallurgists typically use the term Stress corrosion crack or Environmental stress break to describe such a failure in metals.

Although the ESC phenomenon has been known for decades, the research has not predicted such a failure for all environments and for all types of polymers. Some scenarios are well known, documented or predictable, but there is no complete reference to all combinations of stress, polymer and media. The ESC ratio depends on many factors such as chemical structure, bonding, crystallinity, surface roughness, molecular weight and residual stress of the polymer. It also depends on the chemical structure and concentration of the liquid reagent, the temperature of the system and the strain rate.

There are various views on how certain reagents behave in stressed polymers. Since ESC is generally seen in amorphous polymers rather than semi-crystalline polymers, theories related to ESC mechanism generally revolve around fluid interactions with amorphous regions of polymers. Such a theory is that the liquid diffuses into the polymer and causes swelling which increases the chain mobility of the polymer. The result is a reduction in yield stress and glass transition temperature (Tg), as well as plasticization of the material leading to lower stress and perforation in the strains. A second view is that the liquid can reduce the energy required to create new surfaces in the polymer by wetting the surface of the polymer, and thus may help create voids which are considered to be very important in the early stages of caries formation.

Once a polymer has cracked, this creates an easy propagation path so that environmental cracking can continue and the cracking process can be accelerated.
Chemical compatibility between the environment and the polymer governs the amount that the environment causes the polymer to swell and plasticize.
The effects of ESC are reduced when the crack growth rate is high. This is primarily because the liquid cannot keep up with the growth of the crack.

A number of different methods are used to evaluate the resistance of the polymer to environmental stress cracking. A common method in the polymer industry is the use of Bergen jig, which exposes the sample to variable stress during a single test. The results of this test show the critical stress for cracking using only one sample. Another commonly used test is the "Bell Telephone" test, where the bent strips are exposed to the liquids of interest under controlled conditions.

Environmental Tension Crack Resistance You can work with our laboratory EUROLAB for ESCR tests.