Environmental stress cracking is the term used to describe the microscopic cracks that occur to polyethylene as a result of storing chemicals. Over time, the smaller cracks combine to form large cracks, which can result in leaks or even tank failure. The polyethylene’s resistance to these cracks is what is known as environmental stress crack resistance (ESCR).
How Stress Cracks Occur
Traditionally, when polyethylene is processed, it is done via common plastics processes such as injection molding, extrusion, thermoforming, or blow molding. These processes result in molded-in stress due to pressures involved, part design, cooling rates, and molecular orientation. Molded in stress can result in a possible weak/failure point in the part. Although rotational molding creates a virtually stress-free part, chemical exposure as well as exposure to temperature, pressure, and the elements can lead to environmental stress cracking. Choosing the correct material for tank construction as well as following Poly Processing’s installation and operation recommendations can minimize this risk.
Chemicals like soaps, detergents, and any sort of wetting agents or surfactants are particularly hard on polyethylene and can cause cracking over time. Additionally, because polyethylene tanks expand and contract with the cycling of the tank, they can experience operational stress that occurs during discharging of chemical or when the tank is filled. Luckily, there is a way to test the strength of polyethylene and its resiliency to cracking over time.
Environmental Stress Crack Resistance (ESCR) Test
The most common Environmental Stress Crack Resistance Test is the ASTM method D-1693 for polyethylene plastics. It shows how susceptible a particular polyethylene is to stress cracking when storing surfactants. The test requires ten 1/8th inch thick samples of compression-molded plastic. Compression-molded polyethylene samples are specified by the test method because compression molded samples are stress-free; molded-in stress would compromise the test and the results would represent the process rather than the material. Using samples of the tank wall itself would not work because the samples are thicker than what the test calls for and would lead to skewed results.
The pieces of polyethylene are notched with a razor blade to create an imperfection in the sample, and they are bent and inserted into a channel to keep them stressed. They are then placed in an aggressive surface active agent, or surfactant, at an elevated temperature, and monitored.
Measuring ESCR Test Results
Results are measured hourly. If a sample cracks, it usually cracks perpendicular to the razor blade notch. Samples that crack fail the resistance test. Each of the ten samples represents 10% of the sample group. The test is typically stopped when the sample group reaches a 50% failure rate. The ASTM D-1693 has proven that XLPE has excellent stress crack resistance. It can hold up for thousands of hours with 0 failures in the test opposed to linear polyethylene’s hundreds of hours (when tested at the harshest conditions for the test method).
The test ensures that the polyethylene will perform as it should over time for a safe and durable chemical storage tank.