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Environmental stress cracking

Environmental stress cracking

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Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

 failure of thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 (especially amorphous) polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s known at present. Environmental stress cracking may account for around 15-30% of all plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 component failures in service.

ESC and polymer resistance to ESC (ESCR) have been studied for several decades. Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing
Crazing
Crazing is a network of fine cracks on the surface of a material, for example in a glaze layer.Crazing is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which...

 process, initiating crazes at stresses that are much lower than the stress causing crazing in air. The action of either a tensile stress or a corrosive
Corrosive
A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...

 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.

It is somewhat different from polymer degradation
Polymer degradation
Polymer degradation is a change in the properties—tensile strength, colour, shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts...

 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
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

 that would not attack the polymer in an unstressed state.

Metallurgists typically use the term Stress corrosion cracking
Stress corrosion cracking
Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...

 or Environmental stress fracture
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, composites, plastics and ceramics.Metals and alloys exhibit...

 to describe this type of failure in metals.

Predicting 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
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...

, surface roughness, molecular weight and residual stress
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...

. It also depends on the liquid reagent's chemical nature and concentration, the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 of the system and the strain rate.

Residual stress and residual strain are especially important because they can drive crack growth. They frequently arise from poor injection moulding
Injection moulding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...

, especially where cold tools are used in the process. Polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

 is 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 strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 of the polymer, the lower the molecular weight, the lower the strength. So lower grades of a polymer will be more sensitive to organic fluids than higher mass grades.

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 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 (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
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

 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.

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
    Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

     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.

Measuring 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.

Examples


An obvious example of the need to resist ESC in everyday life is the automotive industry
Automotive industry
The automotive industry designs, develops, manufactures, markets, and sells motor vehicles, and is one of the world's most important economic sectors by revenue....

, 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
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...

 such as Igepal was developed to give a warning of ESC.

SAN 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 adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 was used, and excess adhesive which had spilled onto areas where it was not required was removed using a ketone solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

. 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.

To determine the cause of the fracture, the SAN piano key was heated above its glass transition 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 from the spring action and the presence of the ketone solvent.

See also

  • Forensic engineering
    Forensic engineering
    Forensic engineering is the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability. The field also deals with...

  • Forensic polymer engineering
    Forensic polymer engineering
    The study of failure in polymeric products is called forensic polymer engineering. The topic includes the fracture of plastic products, or any other reason why such a product fails in service, or fails to meet its specification...

  • Fracture Mechanics
    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 characterize the material's resistance to fracture.In...

  • Structural failure
    Structural failure
    Structural failure refers to loss of the load-carrying capacity of a component or member within a structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations...


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