Environmental stress cracking, crazing

All polyethylenes are soft, flexible and resistant to acids and alkalis up to 60°C. They retain this flexibility down to —40°C. Hence they have good resistance to impact even at low temperatures. However, unless correctly formulated they can suffer from environmental stress cracking (ESC), poor adhesion and UV degradation. ESC is the phenomenon which occurs when a thermoplastic is put under stress, e.g. bent, in a particular environment and prematurely cracks or crazes. Alcohol and detergent are examples of agents that can cause ESC in polyethylenes. 

The other effect of having a stretched area is a reduction in resistance to stress cracking. Crazing is a possibility in such areas such as in polystyrenes, and environmental stress cracking caused by solvent substances will occur in the stretched areas. This is a particularly important consideration in vacuum formed products used for packaging food that frequently has some solvent action on the plastics. 

When environmental stress cracking tests are carried out to detect the time to onset of crazing, results are generally obtained as a function of applied strain, and possibly also of temperature. There is then an apparent threshold strain below which crazing does not occur for a very long time. A discussion of how this data can be treated has been given by Wright [18]. 

A TP s molecular orientation can be accidental or deliberate. Accident can occur during the processing where unwanted excessive frozen-in stresses develop, however with the usual proper process control, there is no accidental orientation. The frozen-in stresses with certain TPs can be extremely damaging with products being subjected to environmental stress cracking or crazing in the presence of heat, chemicals, etc. 

Li J, Arnold JC, Isaac DH (1994) Environmental stress cracking behavior of urethane methacrylate based resins - I. Environmental crazing and cracking under bending conditions. J Mater Sci 29(12) 3095—3101... 

The profound effect of water on tree growth that is so widely reported may be expected with the electrokinetic model on the basis of three principal effects. Water filling the crazes as they develop will help prevent their collapse. Water, as a good solvent for ionic species, will be an excellent medium to facilitate entry of surface-active agents, which, by a process similar to that of environmental stress cracking, will advance the void and craze formation caused by the electric field. Water, with its high relative permittivity, will distort and locally enhance electric fields in the neighbourhood of the voids and crazes where it accumulates. Whether one or other of these effects dominate in a particular situation depends on the exact nature of the specimen and its environment. 

Environmental crazing and environmental stress cracking are often the limiting factors in determining how long the properties (and especially the strength) of the polymer will remain sufficient for a plastic part to be useful. These phenomena are dependent upon the solution and diffusion of environmental agents in the polymer [15,16], and thus upon 5. 

Environmental Stress Cracking - Cracking or crazing that occurs in a thermoplastic material subjected to stress or strain in the presence of particular chemicals or weather conditions or as a result of aging. Also called ESC. 

In addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate due to exposure to water (hydrolysis) or different types of chemical agents. Condensation polymers like nylons, polyesters, and polycarbonates are susceptible to hydrolysis. Structural alteration of some polymers may occur as a result of exposure to different chemical environments. Most thermoplastics in contact with organic liquids and vapors, which ordinarily may not be considered solvents for the polymers, can undergo environmental stress cracking and crazing. This may result in a loss of lifetime performance or mechanical stability and ultimately contribute to premature mechanical failure of the polymer under stress. 

Environmental stress cracking is similar, but not identical to, stress corrosion cracking of metals. Corrosion involves chemical reactions that produce corrosion products, whereas, in ESC, a liquid is absorbed by the polymer, promoting crazing and crack formation. Corrosion reactions are rare in polymers. ESC can typically cause a factor-of-ten reduction in strength. The two conditions for it to occur are that... 

The crazing effect can be indirectly determined by testing chip impact resistance of specimens which crack either because of weathering or environmental stress cracking. ... 

Many molded plastics, such as acrylic, will show crazing when cleaned with solvent if they have residual stresses in the molded part. Annealing is a way of removing the internal stresses to minimize crazing. See also anneal, environmental stress cracking.)... 

Furthermore, the crazes in PP show other similar characteristics to those of amorphous polymers. They grow apparently normal to the direction of major tensile stress which somewhat deviates from the tensile direction because of spherulitic structure. There are similar environmental effects on craze initiation (see also Environmental stress cracking of polypropylene in this book). Crazing is also an important source of toughness in toughened PP alloy systems such as propylene-ethylene block copolymers. 

Environmental stress cracking (ESC) is the premature cracking of a polymer due to the combined action of a stress and a fluid. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. The embrittlement by oxidative or other chemical degradation is not included under ESC, but is classed as corrosion stress cracking (CSC). 

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