Environmental Stress Cracking (ESC) Test

Environmental stress cracking (ESC) of polymers is a fracture failure mode that occurs as a result of exposure to mechanical stress and in the presence of a chemical that initiates stress relief.

When designing parts from polymers, environmental stress cracking (ESC) must be taken into account. Permeability variables have a strong influence on stress cracking, which must be considered in part design and material selection. Different polymers vary in their propensity for environmental stress cracking, depending primarily on their degree of crystallinity.

Resin processing affects crystallinity. Reducing the processing temperature and time and rapid cooling or quenching at the end of the production process reduces the crystal content, thereby increasing the amorphous content. If cooling is too rapid, residual stresses develop in the parts that reduce their resistance to environmental stress cracking. Increasing the molecular weight of the polymer reduces its crystallinity and increases its stress crack resistance. Longer chains have higher tensile strength, i.e. load carrying ability.

The environmental stress cracking effect of chemicals on polymers can be measured by exposing the polymer to the chemical under desired conditions. The tensile properties of the exposed sample are then measured. Any loss of elongation and tensile strength indicates environmental stress cracking.

Briefly, the ability of a polymer to resist slow crack growth or environmental stress cracking is known as environmental stress cracking (ESC). Different polymers exhibit varying degrees of environmental stress cracking. Some have very good resistance to environmental stress cracking, while others have marginal flexibility.

Environmental stress cracking is an example of material degradation by the simultaneous action of stress and chemical action. Chemical materials do not cause any chemical degradation of the polymer but accelerate the process of macroscopic brittle crack formation. Similarly, the applied voltage is normally below the level that would cause mechanical damage to the materials. However, when their synergistic effects come together, they cause financial failure.

Environmental stress cracking occurs in amorphous polymers such as polystyrene (PS) and polyvinyl chloride (PVC), but also in some semicrystalline thermoplastics such as polyethylene (PE), polypropylene (PP), and polybutylene (PB). The amorphous structure of the polymer facilitates the solvent diffusion necessary for the damage process to occur.

The stresses that cause environmental cracking are now caused by heat treatment such as cold working, welding and grinding, or can be applied externally during service and must be tensile (rather than compressive) to be effective.

Cracks form and propagate at approximately right angles to the direction of tensile stresses, at stress levels much lower than those required to fracture the material in the absence of a corrosive environment. As cracking penetrates further into the material, it reduces the supporting cross-section of the material to the point of structural failure due to overload.

The environmental stress cracking (ESC) testing process involves applying a known stress to the polymer under test conditions and exposing the polymer to the relevant environmental stress cracking environment. In advanced laboratories, a testing program is designed to evaluate the performance of the material when exposed to both primary and secondary stress cracking agents.

Some of the stress-relieving agents tested are: paints, adhesives, cleaning agents, lubricants, plasticizers, inks, aerosol sprays, leak detection fluid, fruit essences and vegetable oils.

The main international standards based on environmental stress cracking (ESC) tests are:

• “ASTM D543-20 Standard practices for evaluating the resistance of plastics to chemical reagents”. These applications include evaluating all plastic materials, including cast, hot molded, cold molded, laminated resin products and sheet materials, for resistance to chemical reagents. Three procedures are presented in this standard. Two of these are under immersion testing, one under mechanical stress and reagent exposure under standardized conditions of applied voltage. These applications include guidelines for reporting changes in weight, size, appearance, color, durability and other mechanical properties.
• “ISO 22088-3 Plastics - Determination of resistance to environmental stress cracking (ESC) - Part 3: Spun strip method”. This standard describes a test method for determining the environmental stress cracking (ESC) resistance of thermoplastics when subjected to a constant bending stress in the presence of chemicals. Environmental stress cracking is indicated by the change of an appropriately selected indicative property of samples stressed for a period of time in the environment. The test method is suitable for determining the resistance of sheets and flat test specimens, in particular the susceptibility of localized surface areas of the specimens to environmental stress cracking.

Environmental stress cracking (ESC) test services are among the numerous testing, measurement, analysis and evaluation studies provided to businesses by our organization.