What Is a Stress Crack Test? A Guide to Testing PET Bottle Bases
A stress crack test measures how well the base of a PET bottle resists cracking when it is under pressure and in contact with an aggressive liquid. It is used for blow-molded PET bottles for carbonated soft drinks, whose base stays under the pressure of the CO2 for the whole life of the product. The test accelerates a failure that could otherwise appear long after filling, so that bottle makers and bottlers can estimate the long-term behavior of their bottles.
How does a stress crack tester work?
The bottle is filled with water and pressurized, and its base is immersed in a dilute solution of sodium hydroxide (NaOH). AT2E recommends a 0.200 % NaOH solution. The tester then records, for each bottle, the time and the pressure at which it fails, until the predefined test time is over.
The AT2E SCT-PLC and SCT-D offer two test sequences:
PS mode: the bottle is pressurized first and held for a set time, for example 5 minutes. The base is then immersed in the solution while the pressure is maintained.
SP mode: the base is immersed in the solution first, then the bottle is pressurized according to the cycle settings.
A failure shows as a leak. On the AT2E SCT-D, the position of a leaking bottle is flagged on the screen, its pressurization stops, and the leak pressure and time are displayed.
Main applications
Carbonated soft drink bottles: checking the stress crack resistance of PET bottle bases.
Bottle and preform design: comparing base designs or preform designs before production.
Blow molding control: checking bottles after a change of settings, molds or material.
Contact with alkaline liquids: evaluating how the bottles behave in contact with alkaline products, such as some conveyor lubricants and cleaning agents.
Key concepts: stress, chemical contact and time to failure
Stress: the base of a carbonated soft drink bottle carries the pressure of the CO2, and its center is the least stretched part of the bottle during blow molding. This combination makes it the zone most likely to crack.
Environmental stress cracking: a plastic under stress cracks much faster in contact with certain chemicals than in air. For PET, alkaline liquids are typical, which is why the test uses sodium hydroxide.
Time to failure: the result of the test is the time each bottle holds before it leaks, and the number of bottles that fail before the end of the test. Results scatter from one bottle to the next, so several bottles are tested at once: AT2E stress crack testers have multiple positions, up to 26 on the SCT-D.
Pressure accuracy: the SCT-D controls the test pressure from 0 to 6 bar (0 to 87 psi) with an accuracy of ±0.5 % F.S. and a resolution of 0.01 bar, while the mechanical SCT-ECO has an accuracy of ±2.5 % F.S.
Pressure plus chemistry: why bottles crack
Hang a weight from a sheet of paper and it holds. Put a drop of water where the paper is stretched, and it tears there. The weight alone and the water alone did no damage; together, they found the weakest point.
Stress cracking works the same way. Neither the pressure in the bottle nor the alkaline liquid is enough to break a well-made base on its own, but together they open small cracks at the most stressed points, and the cracks grow until the bottle leaks. Temperature also speeds up the process, so the test is run under controlled conditions, which makes results comparable from one test to the next.
Calibration of a stress crack tester
A stress crack tester has three things to keep under control: pressure, solution and temperature.
Compare the pressure reading of each position with a certified reference indicator at several pressures.
Prepare the solution at the specified concentration, check it, and check the solution volume in each vessel.
Check the test temperature, record all deviations, and adjust or recalibrate if they exceed the acceptable limit.
Fresh solution and clean vessels are part of the method: a solution that has changed concentration gives results that cannot be compared with earlier tests.
AT2E stress crack testers
AT2E stress crack testers range from a mechanical model to a fully automatic system:
SCT-PLC: PLC-controlled stress crack tester with automatic water and solution filling, PS and SP modes, 0 to 6 bar, test time 0 to 200 minutes, single or multi-position.
SCT-D: multi-position stress crack tester, 1 to 26 positions, 0 to 6 bar, accuracy ±0.5 % F.S., up to 100 hours per step.
SCT-ECO: fully mechanical stress crack tester that needs no electricity (compressed air supply 7 to 8 bar), 1 to 12 positions, 0 to 6 bar, accuracy ±2.5 % F.S.
Conclusion
Stress cracking is a slow failure that starts in a place that is hard to see: the center of the base. A stress crack test reproduces it in the laboratory under controlled conditions. With a controlled pressure, a solution at the right concentration and enough bottles per test, it gives an estimate of how a base design and a blow molding process will behave over the life of the product.
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