Environmental Stress Cracking - Mechanisms of ESC

Mechanisms of ESC

There are a number of opinions on how certain reagents act on polymers under stress. Because ESC is often seen in amorphous polymers rather than in semicrystalline polymers, theories regarding the mechanism of ESC often revolve around liquid interactions with the amorphous regions of polymers. One such theory is that the liquid can diffuse into the polymer, causing swelling which increases the polymer’s chain mobility. The result is a decrease in the yield stress and glass transition temperature (T_g), as well as a plasticisation of the material which leads to crazing at lower stresses and strains. A second view is that the liquid can reduce the energy required to create new surfaces in the polymer by wetting the polymer’s surface and hence aid the formation of voids, which is thought to be very important in the early stages of craze formation.

There is an array of experimentally derived evidence to support the above theories:

• Once a craze is formed in a polymer this creates an easy diffusion path so that the environmental attack can continue and the crazing process can accelerate.
• Chemical compatibility between the environment and the polymer govern the amount in which the environment can swell and plasticise the polymer.
• The effects of ESC are reduced when crack growth rate is high. This is primarily due to the inability of the liquid to keep up with the growth of the crack.