Environmental stress crazing

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

The evaporation rate of a solvent is determined to obtain relative value to some standard, selected solvent. The solvent selection depends on reasons for solvent use and the type of solvent and it is usually agreed upon between interested parties. In Europe, diethyl ether is the most frequently used reference solvent and in the US butyl acetate. The evaporation rate of other solvents is determined under identical conditions and the resultant values are used to rank solvents. The most obvious requirement is that the determination is done without excessive drafts and air currents. The evaporation rate is the ratio of the time required to evaporate a test solvent to the time required to evaporate the reference solvent under identical conditions. The results can be expressed either as the percentage evaporated within certain time frame, the time to evaporate a specified amount, or a relative rate. Relative rate is the most common. 

For halogenated solvents used in cleaning applications, the relative evaporation rate is compared either to xylene or perchloroethylene. The determination is done on a test panel using 10 ml of solvent. The relative evaporation rate is calculated. 

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White SA, Weissman SR, Kambour RP (1982) Resistance of a polyetherimide to environmental stress crazing and cracking. J Appl Polym Sci 27(7) 2675-2682... 

Although liquid environments often induce environmental-stress crazing or crazing in glassy plastics, not all liquids are deleterious to fatigue performance, and some are beneficial. Typical liquids, eg, heptanes, esters, and alcohols,... 

It mainly remains to be discussed in this section the nature of the effect which the simultaneous action of mechanical and environmental parameters may have on mechanical properties. One of the most interesting phenomena in this field, the environmental stress crazing (ESC), will be treated in the final chapter. 

Environmental stress crazing in polystyrene the roles of chain length and architecture in craze initiation... 

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. 

Applied stress There are TPs that will craze or crack under certain environmental condition. Products that are highly stressed mechanically must be checked very carefully. Polypropylene, ionomer, chlorinated polyether, phenoxy, EVA, and linear polyethylene are examples that offer greater freedom from stress crazing than some other TPs. Solvents may crack products held under stress. TSs is generally preferable for products under continuous loads. 

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. 

The influence of solvents has been touched on in Sect. 2.3.2. In fact, the influence of environment on crazing and fracture properties of polymers is of major importance in the practical uses of these materials. There are many ways for the environment to induce fracture by means of stress cracking, stress crazing, chain scission, chain crosslinking, etc. Therefore, environmental fracture has been widely studied, specially from the experimental point of view. Reference is a review of environmental cracking of polymers. Most work on environmental crazing has been done in liquid environments - (solvents and non solvents of the material), or high pressure gas environment, near condensation pressure (liquid nitro-... 

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. 

Optical KLenents. Problems which are common to many solar-related optical elements Include dirt retention, cleaning, surface abrasion, and photodegradation. A common feature of some of these problems Is that the deleterious effects occur at an Interface. Ultraviolet radiation, atmospheric components, mechanical stress, etc., can have a profound effect on performance by changing surface characteristics. The lifetimes of UV stabilizers can be limited by exudation permeability can cause harmful reactions at Interfaces and mechanical properties can be Influenced by surface crazing. In other applications mechanical behavior of the bulk polymer Is critical and virtually all applications require that the polymer system withstand multiple environmental stresses simultaneously. 

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... 

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