Stress cracking environmental

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 sequence of events for ESC can be observed in transparent glassy plastics, loaded in three-point bending, with the surface in tension in contact with the active liquid. They are that 

Environment Volume fraction liquid T,CQ Craze stress (MPa) 

Experimental data on the growth of methanol-filled crazes in PMMA confirms Eq. (10.7). The length increases with /i until the equilibrium length according to Dugdale s model (Eq. 9.15) is reached. 

Semi-crystalline polymers such as polyethylene are less affected by organic liquids, but nevertheless, the amorphous phase is susceptible to attack. Both alcohols and surface active agents can eventually lead to crack formation. Severe conditions are used for laboratory quality control tests of the 

Another issue is the stress that a part experiences in application. Tensile loads cause less tendency for stress cracking than compressive loads. Fluoropoly-mer-lined equipment and parts are examples of objects which may contain residual stresses due to their design and/or fabrication. 

Because this type of failure is associated with relatively mobile (i.e., low molecular weight) Uquids which are normally present in small concentrations the technique of GC-MS is very well suitable to its investigation. A particularly effective approach is dynamic headspace GC where any stress cracking fluid can be removed from the plastic matrix without the use of another solvent and long heating times enable it to be concentrated up to aid detection. An area of plastic that has not stress cracked can be used as a control. 

There are cases where stress cracking can occur due to the migration of a less volatile liquid (e.g., a plasticiser). An example of such a case is the stress cracking of ABS due to the migration of plasticiser from PVC, and this can be approached by the detection of phthalate in the ABS matrix by solvent extraction and then analysis of the extract using HPLC. 

Where the liquid is relatively non-volatile and complex in composition (such as silicone oil which causes cracking of polyethylene) again a solvent extraction technique has to be performed with either infrared microscopy analysis of a cast micro-film or a liquid chromatography method (i.e., LC-MS or HPLC) as the analysis technique. 

A review of the environmental stress cracking of plastics has been produced by Wright (315). 

More often than not this contamination is in the form of solid, discreet entities which makes their removal from the product relatively easy. Once removed the microscopic techniques IR and Edax are excellent at obtaining assigmnent data in a cost effective way. Where enough of the contaminant is obtainable (e.g., 1 mg), and if a mixture of chemicals is suspected, a bulk composition by TGA can be obtained. If identification problems are still occurring, more sophisticated techniques such as GC-MS, SIMS and LIMA can be employed. Where the contamination is in a liquid form, a similar analytical regime can be used. A number of plastic products can suffer from solid inclusions, but it is probably most common in extruded products such as car door seals. 

This test was prepared and is limited to type 1 (low-density) polyethylenes. Specimens are annealed in water or steam at 212°F (100°C) for 1 h and then equilibrated at room temperature for 5-24 h. After conditioning the specimens are nicked according to directions given. The specimens are bent into a U shape in a brass channel and inserted into a test tube that is then filled with fresh reagent (Igepal). The tube is stoppered with an aluminum-covered cork and placed in a constant temperature bath at 122°F (50°C). 

This is the phenomenon which is mentioned in several of the case studies as a failure hazard. It is of concern in the selection of materials for the Flymo impellor and mower hood and is a consideration in the selection of materials for gears. It was also important in changing from polycarbonate to acetal in the control knob of the TV selector switch. 

Environmental stress cracking, or ESC, is the phenomenon in which a product can fail when exposed to specific combinations of stress and an aggressive environment. 

The susceptibility of polycarbonate to ESC led to its rejection for Flymo impellors and hoods, where the aggressive environments would again have been petrol. It was replaced by acetal in the selector switch where the aggressive environment was switch-cleaning fluid. 

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MSTM D1693, ed. Test Methodfor Environmental Stress-Cracking of Ethylene Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1988. 

ISO 4599, Plastics Determination of Resistance to Environmental Stress-Cracking, Pent Strip Method, ISO, Geneva, Swit2edand, 1986. 

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

For reasons that are not fiiUy understood, PPSF exhibits generally improved compatibiUty characteristics over either PSF or PES in a number of systems. An example of this is blends of PPSF with polyaryletherketones (39,40). These blends form extremely finely dispersed systems with synergistic strength, impact, and environmental stress cracking resistance properties. Blends of PPSF with either PSF or PES are synergistic in the sense that they exhibit the super-toughness characteristic of PPSF at PSF or PES contents of up to 35 wt % (33,34). The miscibility of PPSF with a special class of polyimides has been discovered and documented (41). The miscibility profile of PPSF with high temperature (T > 230° C) polysulfones has been reported (42). 

Pacemaker Interfaces and Leads. Problems of existing pacemaker interfaces and pacemaker lead materials made from siUcones and standard polyurethanes are environmental stress cracking, rigidity, insulation properties, and size. 

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA. Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point. 

The susceptibility of low molecular weight grades to environmental stress cracking. 

Polypropylene appears to be free from environmental stress cracking problems. The only exception seems to be with concentrated sulphuric and chromic acids and with aqua regia. 

It is less resistant to aliphatic hydrocarbons than polyethylene and polypropylene and in fact pipes may be solvent welded. At the same time the resistance to environmental stress cracking is excellent. 

The homopolymer finds a variety of uses, as an adhesive component, as a base for chewing gum, in caulking compounds, as a tackifier for greases, in tank linings, as a motor oil additive to provide suitable viscosity characteristics and to improve the environmental stress-cracking resistance of polyethylene. It has been incorporated in quantities of up to 30% in high-density polyethylene to improve the impact strength of heavy duty sacks. 

From this table it will be noted that in terms of the mechanical and thermal properties quoted the copolymers are marginally inferior to the homopolymers. They do, however, show a marked improvement in resistance to environmental stress cracking. It has also been shown that the resistance to thermal stress cracking and to creep are better than with the homopolymer.This has led to widespread use in detergent bottles, pipes, monofilaments and cables. 

The styrene-based terpolymers were originally used to the extent of some 2-9% in order to reduce the notch sensitivity of the polycarbonate and to improve the environmental stress cracking resistance. More recently emphasis has been on alloys with 10-50% of SAN or ABS. Alloys of polycarbonates with ASA have also become available (Luran SC-BASF)... 

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