What is the Test Methods for Evaluation of ESCR of Plastics ?

3. Test Methods for Evaluation of ESCR of Plastics

A common laboratory request for ESC-prone polymers is to check ESCR performance for quality control, competitive product evaluations, and research and development work. There is a variety of test methods available for assessing the ESCR of thermoplastics and they can be divided into two groups: tests at constant strain and tests at constant load (stress). It should be remembered, that any test that involves the application of a constant strain is less severe than the apparently equivalent test involving the application of a constant load because the strain is not maintained constant during the test.[2] The stress in the sample induced by constant strain will decay with time due to stress relaxation, which makes the ESC conditions less severe.

3.1. Tests at Constant Strain

Constant strain methods are most commonly used because they are cheap to perform and the investment in equipment is small. The main limitation of using constant strain tests with plastics is that the stress will decay with time due to stress relaxation. It is important for the accuracy of the ESC tests to select the most appropriate strain applied on the sample, because high strain will result in cracking too quick to observe, and lower strain will cause long-term experiments. Wang et. al.[45] carried out investigations to determine the appropriate values of strain to be exerted in the ESC test of different kinds of plastics. They found that for the brittle plastics the strain should be selected in the elastic region of the stress-strain curve, while for toughened plastics the plastic region is the best selection.

3.1.1 Three-Point Bending Test

This normally involves the application of a mid-point deflection δ which generates a maximum surface strain. There are two major variants of the test which are shown in Figure 3.1.[2, 46] Samples are placed in the test device and the desired strain is attained by adjusting the screw. Deformed samples (strips) are immersed in the stress cracking agent. After a predetermined test period the samples are removed, rinsed with distilled water and allowed to dry at room temperature for 24 hours. Following this, the samples are inspected for crazing and their tensile properties are investigated.

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