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Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
(especially amorphous)
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s known at present. According to ASTM D883, stress cracking is defined as "an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength". This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces. Environmental stress cracking may account for around 15-30% of all
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adapta ...
component failures in service.H. F. Mark (2004). Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc. This behavior is especially prevalent in glassy, amorphous thermoplastics. Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperature higher than their glass transition temperature (Tg) due to the increased free volume. When Tg is approached, more fluid can permeate into the polymer chains.


Exposure of polymers to solvents

ESC and polymer resistance to ESC (ESCR) have been studied for several decades.Xiangyang Li (2005). Environmental Stress Cracking Resistance of a New Copolymer of Bisphenol-A. Polymer Degradation and Stability. Volume 90, Issue 1, 44–52. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the
crazing Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing frequently precedes fracture in some glassy thermoplastic polymers. As it only takes place under tensile stre ...
process, initiating crazes at stresses that are much lower than the stress causing crazing in air.J. C. Arnold (1998). The Effect of Diffusion on Environmental Stress Crack Initiation in PMMA. Journal of Materials Science 33, 5193 – 5204. The action of either a tensile stress or a corrosive liquid 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 corrosive environmental liquid. These corrosive environmental liquids are called 'secondary chemical agents', are often organic, and are defined as solvents not anticipated to come into contact with the plastic during its lifetime of use. Failure is rarely associated with primary chemical agents, as these materials are anticipated to come into contact with the polymer during its lifetime, and thus compatibility is ensured prior to use. In air, failure due to creep is known as creep rupture, as the air acts as a plasticizer, and this acts in parallel to environmental stress cracking. It is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions. It has also been seen that catastrophic failure under stress can occur due to the attack of a
reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
that would not attack the polymer in an unstressed state. Environmental stress cracking is accelerated due to higher temperatures, cyclic loading, increased stress concentrations, and fatigue. Metallurgists typically use the term
Stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. S ...
or
Environmental stress fracture In materials science, environmental stress fracture or environment assisted fracture is the generic name given to premature failure under the influence of tensile stresses and harmful environments of materials such as metals and alloys, composi ...
to describe this type of failure in metals.


Factors influencing ESC

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, polymer and environment. The rate of ESC is dependent on many factors including the polymer's chemical makeup, bonding,
crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic stru ...
, surface roughness,
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
and residual stress. It also depends on the liquid reagent's chemical nature and concentration, the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
of the system and the strain rate.


Mechanisms of ESC

There are a number of opinions on how certain reagents act on polymers under stress. Because ESC is often seen in
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language, Gr ...
polymers rather than in semicrystalline polymers, theories regarding the mechanism of ESC often revolve around liquid interactions with the amorphous regions 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 The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or ru ...
(Tg), as well as a plasticisation of the material which leads to crazing at lower stresses and strains. A second view is that the liquid can reduce the energy required to create new surfaces in the polymer by
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with ...
the polymer's surface and hence aid the formation of voids, which is thought to be very important in the early stages of craze formation. ESC may occur continuously, or a piece-wise start and stop mechanism There is an array of experimentally derived evidence to support the above theories: * Once a craze is formed in a polymer this creates an easy
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
path so that the environmental attack can continue and the crazing process can accelerate. * Chemical compatibility between the environment and the polymer govern 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. *Once separated from the other chains, the polymers align, thus allowing embrittlement. ESC generally occurs at the surface of a plastic and doesn't require the secondary chemical agent to penetrate the material significantly, which leaves the bulk properties unmodified. Another theory for the mechanism of craze propagation in amorphous polymers is proposed by Kramer. According to his theory, the formation of internal surfaces in polymers is facilitated by polymeric surface tension that is determined by both secondary interactions and the contribution of load-bearing chains that must undergo fracture or slippage to form a surface. This theory provides and explanation for the decrease in the stress needed to propagate the craze in the presence of surface-active reagents such as detergents and high temperature.


ESC mechanism in polyethylene

Semi-crystalline polymers such as polyethylene show brittle fracture under stress if exposed to stress cracking agents. In such polymers, the crystallites are connected by the tie molecules through the amorphous phase. The tie molecules play an important role in the mechanical properties of the polymer through the transferring of load. Stress cracking agents, such as detergents, act to lower the cohesive forces which maintain the tie molecules in the crystallites, thus facilitating their “pull-out” and disentanglement from the lamellae. As a result, cracking is initiated at stress values lower than the critical stress level of the material. In general, the mechanism of environmental stress cracking in polyethylene involves the disentanglement of the tie molecules from the crystals. The number of tie molecules and the strength of the crystals that anchor them are considered the controlling factors in determining the polymer resistance to ESC.


Characterizing ESC

A number of different methods are used to evaluate a polymer's resistance to environmental stress cracking. A common method in the polymer industry 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 test where bent strips are exposed to fluids of interest under controlled conditions. Further, new tests have been developed where the time for crack initiation under transverse loading and an aggressive solvent (10% Igepal CO-630 solution) is evaluated. These methods rely on an indentor to stress the material biaxially, while preventing a radial stress concentration. The stressed polymer sits in the aggressive agent and the stressed plastic around the indentor is watched to evaluate the time to crack formation, which is the way that ESC resistance is quantified. A testing apparatus for this method is known as the Telecom and is commercially available; initial experiments have shown that this testing gives equivalent results to ASTM D1693, but at a much shorter time scale. Current research deals with the application of
fracture mechanics Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics ...
to the study of ESC phenomena. In summary, though, there is not a singular descriptor that is applicable to ESC—rather, the specific fracture is dependent on the material, conditions, and secondary chemical agents present . Scanning electron microscopy and fractographic methods have historically been used to analyze the failure mechanism, particularly in high density polyethylene (HDPE). Freeze fracture has proved particularly useful for examining the kinetics of ESC, as they provide a snapshot in time of the crack propagation process.


Strain hardening as a measure of environmental stress cracking resistance (ESCR)

Many different methods exist for measuring ESCR. However, the long testing time and high costs associated with these methods slow down the R&D activities for designing materials with higher resistance to stress cracking. To overcome these challenges, a new simpler and faster method was developed by SABIC to assess ESCR for high density polyethylene (HDPE) materials. In this method, the resistance of slow crack growth or environmental stress cracking is predicted from simple tensile measurement at a temperature of 80℃. When polyethylene is deformed under a uniaxiial tension, before yield, the stiff crystalline phase of the polymer undergoes small deformation while the amorphous domains deforms significantly. After the yield point but before the material undergoes strain hardening, the crystalline lamellae slips where both the crystalline phase and the amorphous domains contribute to load bearing and straining. At some point, the amorphous domains will stretch fully at which the strain hardening begin. In the strain hardening region, the elongated amorphous domains become the loading bearing phase whereas the crystalline lamellae undergoes fracture and unfold to adjust for the change in strain. The load-bearing chains in the amorphous domains in polyethylene are made of tie-molecules and entangles chains. Because of the key role of tie-molecules and entanglements in resisting environmental stress cracking in polyethylene, it follows that ESCR and strain hardening behaviors can very well be correlated. In the strain hardening method, the slope of strain hardening region (above the natural draw ratio) in the true stress-strain curves is calculated and used as a measure of ESCR. This slope is called the strain hardening modulus (Gp). The strain hardening modulus is calculated over the entire strain hardening region in the true stress strain curve. The strain hardening region of the stress-strain curve is considered to be the homogeneously deforming part well above the natural draw ratio, which is determined by presence of the neck propagation, and below the maximum elongation. The strain hardening modulus when measured at 80℃ is sensitive to the same molecular factors that govern slow crack resistance in HDPE as measured by an accelerated ESCR test where a surface active agent is used. The strain hardening modulus and ESCR values for polyethylene have been found to be strongly correlated with each others.


Examples

An obvious example of the need to resist ESC in everyday life is the
automotive industry The automotive industry comprises a wide range of companies and organizations involved in the design, development, manufacturing, marketing, and selling of motor vehicles. It is one of the world's largest industries by revenue (from 16 % ...
, 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 weight of the polymer. A test of exposure to a strong
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are m ...
such as Igepal was developed to give a warning of ESC.


Styrene acrylonitrile susceptibility to ketone solvent

A more specific example comes in the form of a
piano key The piano is a stringed keyboard instrument in which the strings are struck by wooden hammers that are coated with a softer material (modern hammers are covered with dense wool felt; some early pianos used leather). It is played using a keyboa ...
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
adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...
was used, and excess adhesive which had spilled onto areas where it was not required was removed using a
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
. Some vapour from this solvent condensed 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. (2007). Unexpected and Unusual Failures of Polymeric Materials. ''Engineering Failure Analysis'', Volume 14, 1153–1165. To determine the cause of the fracture, the SAN piano key was heated above its
glass transition The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or ru ...
temperature for 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 In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...
from the spring action and the presence of the ketone solvent.


See also

* Corrosion engineering *
Creep (deformation) In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high lev ...
*
Crocodile cracking Crocodile cracking, also called alligator cracking and perhaps misleadingly fatigue cracking, is a common type of distress in asphalt pavement. The following is more closely related to fatigue cracking which is characterized by interconnecting ...
* Embrittlement *
Environmental stress fracture In materials science, environmental stress fracture or environment assisted fracture is the generic name given to premature failure under the influence of tensile stresses and harmful environments of materials such as metals and alloys, composi ...
* Forensic engineering * Forensic polymer engineering *
Fracture Mechanics Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics ...
* Season cracking *
Stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. S ...
*
Structural failure Structural integrity and failure is an aspect of engineering that deals with the ability of a structure to support a designed structural load (weight, force, etc.) without breaking and includes the study of past structural failures in order t ...


References


Further reading

* Ezrin, Meyer (1996). ''Plastics Failure Guide: Cause and Prevention'', Hanser-SPE. * Wright, David C. (2001). ''Environmental Stress Cracking of Plastics'' RAPRA. * Lewis, Peter Rhys, Reynolds, K. and Gagg, C. (2004). ''Forensic Materials Engineering: Case studies'', CRC Press. {{DEFAULTSORT:Environmental Stress Cracking Polymer physics Thermoplastics Polymers