Environmental stress cracking polyethylene

Environmental stress-cracking, polyethylene, detergent, creep, molecular mass... 

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. 

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. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

Polypropylene It has similar chemical properties to polyethylene but is less susceptible to environmental stress cracking. It may also be used at somewhat higher temperatures. 

Polyethylene and polystyrene are examples of plastics subject to environmental stress cracking. Crack resistance tests have shown that surfactants, alcohols, organic acids, vegetable and mineral oils, and ethers provide an active environment for stress cracking of polyethylene. Table 6 lists typical sterile devices and plastic materials used to fabricate them, while Tables 7-9 list the potential effects of sterilization processes on polymeric materials. The effect of gamma irradiation on elastomeric closures has been studied by the Parenteral Drug Association [15]. 

ISO 8796 2004 Polyethylene PE 32 and PE 40 pipes for irrigation laterals - Susceptibility to environmental stress cracking induced by insert-type fittings - Test method and requirements ISO 9625 1993 Mechanical joint fittings for use with polyethylene pressure pipes for irrigation purposes... 

ISO 16770 2004 Plastics - Determination of environmental stress cracking (ESC) of polyethylene - Full-notch creep test (FNCT)... 

When heated. PP dissolves in nonpolar solvents with solubility parameters in the range of 6.5 to 9.5 H, but it is resistant to polar solvents, such as ethanol. PP is more resistant to environmental stress cracking than polyethylene. 

The largest volume of polymeric materials used for wire and cable insulation is thermoplastics, namely polyethylene (PE) and polyvinylchloride (PVC),4 and, to a lesser degree, elastomeric compounds. The main reason for the prevalence of the PE and PVC in wire and cable insulation is their easy processing and relatively low cost. However, their main disadvantage is that their physical properties, such as plastic flow at elevated temperatures, environmental stress cracking, poor solvent resistance and low softening temperatures,3 cannot always meet demands imposed on them by modern applications. Cross-linking of these materials improves their... 

Stress Crack Resistance. Failure caused by environmental stress cracking may be attributed to stored stresses acquired in the molding or exlrnding operation. These dormant stresses may release themselves by cracking under the combined influence of an adverse environment and polyaxial stretching dunng use. Polyethylene of narrow molecular weight distribution tends to crack less under environmental stress. 

Schellenberg J, Fienhold G (1998) Environmental stress cracking resistance of blends of high-density polyethylene with other polyethylenes. Polym Eng Sci 38(9) 1413-1419... 

Qian R, Lu X, Brown N (1993) The effect of concentration of an environmental stress cracking agent on slow crack growth in polyethylenes. Polymer 34(22) 4727-4731... 

Ward AL, Lu X, Huang Y, Brown N (1991) The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent. Polymer 32(12) 2172-2178... 

Soares JBP, Abbott RF, Kim JD (2000) Environmental stress cracking resistance of polyethylene the use of CRYSTAF and SEC to establish structure-property relationships. J Polym Sci Part B Polym Phys 38(10) 1267-1275... 

Fillers are not used to any extent in products made by the rotational molding process. Rotational molding is dominated by polyethylene (close to 90% volume) to which even small addition of pigments or fillers (less than 2 wt%) causes a decrease in tensile and impact properties of the products manufactured in this process. Polyethylene is vulnerable to environmental stress cracking which is made worse if fillers are present. 

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