Report on environmental stress cracking tests
June 20th, 1996
Prosyma Research Ltd report to Weltonhurst Ltd.
S F Bush with J D Tonkin
Introduction
Weltonhurst supply high density polyethylene blow moulded containers to Richard Millington and Co Ltd. The containers are used to hold cleaning fluid under pressure for the purpose of cleaning out beer lines. The HDPE has come from a variety of suppliers including Dow, Atochem and Borealis, with a density around 0.95 gm/cc. The fluid has the specification:
Sodium Hydroxide: 9.2 ± 1 gm/litre
Sodium Hypochlorite: 7.6 ± 2 gm/litre
“Phosphorus butane carboxyl”: 4.8 ± 0.25 gm/litre
The precise chemical formula of the third component is unknown, but it looks like a mild detergent.
An earlier report (Ref 1) investigated the short-term mechanical properties of the polymers used, following the disintegration of a 50 litre container after six years in use. Fragments of the disintegrated container were found to be significantly less ductile than recently made containers, pointing to some long-term degradation of the polymer, exposure to sunlight being a prime possible agent.
The present investigation has been concerned with a second possible agent of long-term degradation, namely environmental stress cracking (ESC). ESC is not a well defined concept, but the essential point is that some environments reduce the mechanical properties over time. Generally these weakening effects are increased with an increase of temperature. Tests have therefore been done at both a working temperature (23 oC) and an elevated temperature (60 oC) which might be seen as the limit that the container could normally attain (by being left out in the sun for example).
References
1: S F Bush & J D Tonkin, Report on failed Bass bottle, PRL, 11.8.95.
2: S Turner, Mechanical testing of plastics, George Godwin, 1983.