- Environmental stress cracking
Environmental Stress Cracking (ESC) is one of the most common causes of unexpected
brittlefailure of thermoplastic(especially amorphous) polymers known at present. Environmental stress cracking may account for around 15-30% of all plasticcomponent failures in service.H. F. Mark. Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc. 2004]
polymerresistance to ESC (ESCR) have been studied for several decades.Xiangyang Li. Environmental Stress Cracking Resistance of a New Copolymer of Bisphenol-A. Polymer Degradation and Stability. Volume 90, Issue 1, October 2005, Pages 44-52 ] Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazingprocess, initiating crazes at stresses that are much lower than the stress causing crazingin air.J. C. Arnold. The Effect of Diffusion on Environmental Stress Crack Initiation in PMMA. Journal of Materials Science 33 (1998) p 5193 – 5204] The action of either a tensile stress or a corrosiveliquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosiveenvironmental liquid.
It is somewhat different from
polymer degradationin that stress cracking does not break polymerbonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymerand they propagate rapidly under the harsh environmental conditions. [Michigan University – College of Engineering, [http://www.engin.umich.edu/labs/EAST/me589/gallery/bioplastics_f01/599Website/plastics_properties.htm Properties of Plastics] . Accessed 22 April 2008.] It has also been seen that catastrophic failure under stress can occur due to the attack of a reagentthat would not attack the polymerin an unstressed state.
Metallurgists typically use the term
Stress corrosion crackingor Environmental stress fractureto describe this type of failure in metals.
Although the phenomenon of ESC has been known for a number of decades, research has not yet enabled prediction of this type of failure for all environments and for every type of
polymer. Some scenarios are well known, documented or are able to be predicted, but there is no complete reference for all combinations of stress, polymerand environment. The rate of ESC is dependent on many factors including the polymer’s chemical makeup, bonding, crystallinity, surface roughness, molecular weightand residual stress. It also depends on the liquid reagent's chemical nature and concentration, the temperatureof the system and the strain rate. Residual stressand residual strainare especially important because they can drive crack growth. They frequently arise from poor injection moulding, especially where cold tools are used in the process. Polycarbonateis a particular problem, and tool temperatures in excess of 80 C must be used to lower residual stresses and strains. Molecular weight influences the tensile strengthof the polymer, the lower the molecular weight, the lower the strength. So lower grades of a polymer will be more sensitive to organic fluids tha higher mass grades.
Mechanisms of ESC
There are a number of opinions on how certain
reagentsact on polymers under stress. Because ESC is often seen in amorphous polymers rather than in semicrystalline polymers, theories regarding the mechanism of ESC often revolve around liquid interactions with the amorphousregions of polymers. One such theory is that the liquid can diffuse into the polymer, causing swelling which increases the polymer’s chain mobility. The result is a decrease in the yield stress and glass transition temperature(Tg), as well as a plasticisation of the material which leads to crazingat lower stresses and strains. A second view is that the liquid can reduce the energy required to create new surfaces in the polymerby wettingthe polymer’s surface and hence aid the formation of voids, which is thought to be very important in the early stages of craze formation.
There is an array of experimentally derived evidence to support the above theories:
* Once a
crazeis formed in a polymerthis creates an easy diffusionpath so that the environmental attack can continue and the crazingprocess can accelerate.
Chemicalcompatibility between the environment and the polymergovern the amount in which the environment can swell and plasticise the polymer.
* The effects of ESC are reduced when crack growth rate is high. This is primarily due to the inability of the liquid to keep up with the growth of the crack.
A number of different methods are used to evaluate a
polymer’s resistance to environmental stress cracking. A common method in the polymerindustry is use of the Bergen jig, which subjects the sample to variable strain during a single test. The results of this test indicate the critical strain to cracking, using only one sample. Another widely used test is the Bell Telephone testwhere bent strips are exposed to fluids of interest under controlled conditions.
An obvious example of the need to resist ESC in everyday life is the
automotive industry, in which a number of different polymers are subjected to a number of fluids. Some of the chemicals involved in these interactions include petrol, brake fluid and windscreen cleaning solution. Plasticisers leaching from PVC can also cause ESC over an extended period of time, for example.One of the first examples of the problem concerned ESC of LDPE. The material was initially used in insulating electric cables, and cracking occurred due to the interaction of the insulation with oils. The solution to the problem lay in increasing the molecular weightof the polymer. A test of exposure to a strong detergentsuch as Igepalwas developed to give a warning of ESC.
AN piano key
A more specific example comes in the form of a piano key made from injection moulded styrene acrylonitrile (SAN). The key has a hook end which connects it to a metal spring, which causes the key to spring back into position after being struck. During assembly of the piano an
adhesivewas used, and excess adhesivewhich had spilled onto areas where it was not required was removed using a ketone solvent. Some vapour from this solventcondensed on the internal surface of the piano keys. Some time after this cleaning fracture occurred at the junction where the hook end meets the spring.Ezrin, M & Lavigne, G. Unexpected and Unusual Failures of Polymeric Materials. Engineering Failure Analysis, Volume 14, Pages 1153-1165, January 2007]
To determine the cause of the fracture, the SAN piano key was heated above its
glass transition temperaturefor a short time. If there is residual stress within the polymer, the piece will shrink when held at such a temperature. Results showed that there was significant shrinkage, particularly at the hook end-spring junction. This indicates stress concentration, possibly the combination of residual stress from forming and the action of the spring. It was concluded that although there was residual stress, the fracture was due to a combination of the tensile stress from the spring action and the presence of the ketone solvent.
Environmental stress fracture
Forensic polymer engineering
Stress corrosion cracking
* Ezrin, Meyer, "Plastics Failure Guide: Cause and Prevention", Hanser-SPE (1996).
* Wright, David C., "Environmental Stress Cracking of Plastics" RAPRA (2001).
* Lewis, Peter Rhys, Reynolds, K and Gagg, C, "Forensic Materials Engineering: Case studies", CRC Press (2004)
* [http://materials.open.ac.uk/mem/index.htm Museum of failed products]
* [http://www.elsevier.com/wps/find/journaldescription.cws_home/30190/description#description The journal Engineering Failure Analysis]
* [http://www.forensic-courses.com/wordpress/?p=42; Forensic science and engineering]
* [http://www.open2.net/forensicengineering/modern_methods.html Analytical tools]
* [http://openlearn.open.ac.uk/file.php/2980/formats/print.htm New course]
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