Meaning
Polymer durability under mechanical load in aggressive chemical environments is determined by a material’s capacity to withstand slow crack growth. Stress crack resistance measures the time to failure when a notched polymer specimen is subjected to constant tensile stress in a surface-active liquid. In part design, this property determines the lifetime of molded components like plastic pipes and blow-molded bottles.
Failure Mechanism
Environmental stress cracking occurs when a chemical agent accelerates the pulling apart of polymer chains. This process is not a chemical reaction but a physical swelling that reduces the entanglement of the chains, making them prone to brittle failure. High stress crack resistance is achieved by using resins with high molecular weight and optimal comonomer distribution.
Material Specification
Selecting a polymer with high stress crack resistance prevents premature part failure in the field. Test metrics like the bent strip test or the notched constant tensile load test provide a reliable comparison between different grades of polyethylene. Resin datasheets list these values to help designers choose the right material.
Test Metric
Testing involves exposing notched specimens to a detergent solution at elevated temperatures. A high stress crack resistance value ensures that chemical containers do not leak during storage.
