ASTM D1693 Environmental Stress Cracking Test of Ethylene Plastics
The standard test method ASTM D1693 evaluates Environmental Stress Cracking Resistance (ESCR) of Ethylene plastics, by chemical immersion of pre-stressed specimens at elevated temperatures. The resulting ESCR value indicates Slow Crack Growth susceptibility of Polyethylene resins.
The ASTM D1693 test for ESCR (also termed Slow Crack Growth Resistance), is useful in selecting polyethylenes for applications involving exposure to chemicals and mechanical stresses at elevated temperatures. Stress factors include residual stresses from the component manufacturing process, imperfections, and stress concentration at small cracks. Typical chemicals of concern for Environmental Stress Cracking (ESC) of Ethylene plastics include acids, alcohols, chlorine, chlorine dioxide, dissolved oxygen, ozone, chloramines, detergents, and fatty oils.
The ESC damage is initiated at existing cracks or microstructure defects in polymer structures, causing brittle-mechanical type of failure. High ESCR value is desired for the types of polyethylene used in buried and submarine pipelines, lined pipes, chemical storage vessels and conduits for underground electrical and communication cables. ESC testing by ASTM D1693 method is applicable to various types of polyethylene such as low-density polyethylene (LDPE), high density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). However, ASTM D1693 has time limitations and polyethylene resins with extremely high ESCR values (greater than 1000 hours) should be evaluated by alternative tests.
The ASTM D1693 test requires at least 10 rectangular specimens of specified dimensions. The specimens are cut from a compression moulded plaque made from the polyethylene resin to be tested. The specimens are initially stress relieved in boiling water, after which a specified notch is cut in each specimen to create a surface defect. The specimens are then slowly bent into U-shapes and placed in grooved clamps, locking each specimen in the U-bend configuration. Each bent strip specimen is then inserted into a test tube containing a known solution of stress-cracking chemical such as IGEPAL CO630 (a non-ionic detergent). The ASTM D1693 prescribes three different environmental conditions, with varying chemical concentrations and immersion levels. The sealed tubes are placed into an elevated temperature bath at either 50 0C or 100 0C, depending on the density of the polyethylene sample and testing time. The specimens are visually examined periodically, at least up to 360 hours, till ESC failure is observed.
Each moulded specimen is 38 mm long and 13 mm wide with a notch of 19 mm length. The required thickness of specimens and the notch depth depend upon the properties of the Polyethylene sample being tested.
The time in hours when 50% of the samples exhibit cracks is recorded as ESCR value.
ASTM D1693 specifies the characteristics of ethylene plastics by conducting environmental stress cracking tests on them. This helps decide their applications and gives a hint of their durability.
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