Resistance to Stress Cracking

Plastics are subject to stress-crack formation. The stresses that initiate these cracks are either internal stresses resulting from processing conditions or external stresses resulting from external mechanical load or the superposition of both types of load [243], 

In order to differentiate more precisely, we must distinguish between stress-crack formation in a vacuum and stress-crack formation in air, and environmental and media assisted stress-cracking in the presence of liquid or gaseous media. For true , media assisted stress-cracking ( environmental assisted stress cracking (ESC)), simultaneous loading and the presence of a (crack)initiating medium are required [193], 

In contrast to metals, plastics are mostly subject to a purely physical process in which time-dependent diffusion and swelling processes play a considerable role. At first, fine crazes are observed at the surface from which fracture will later develop. Crazes are crack-like damage zones within which no complete material separation takes place as is seen in cracks, but where load transfer is still possible via fibrillated or homogeneously stretched material between the craze walls. Such crazes represent weak points under impact load and can develop into true cracks under long-lasting load that will lead to fracture. Such craze formation is visible to the eye in transparent plastics and sometimes also in unfilled, non-transparent plastics [13]. 

The following factors facilitate stress-crack formation  

In media assisted stress-crack formation, diffusion and forces interacting between polymer and medium molecules play a decisive role. The forces of interaction are characterized by solubility parameters. If the solubility parameters of the medium and the polymer are the same, the medium may act as a solvent. 

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The excellent electrical insulation properties of polyethylene have led to extensive use in cable and other wire-covering applications. Spectacular early uses included undersea cables and airborne radar and the materials continue to be used in substantial quantities. One particular trend is the increasing use of cross-linked polyethylene for this area of use. Such materials have improved heat resistance and in addition have given generally better resistance to stress cracking. Cellular polyethylene is used as the insulator for television downlead aerials. 

Interest in EVA as a cable-insulating material has arisen because of the good resistance to stress cracking and because the polymer may be more easily cross-linked (see Table 11.12). 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

Low water absorption and good chemical resistance, including resistance to stress cracking. 

Polyethylene can be chlorinated in solution in carbon tetrachloride or in suspension in the piescnce ot a catalyst. Below 55-60% chlorine, it is more stable and more compatible with many polymers, especially polyvinyl chloride, to which it gives increased impact strength. The low pressure process copolymerizes polyethylene with propylene and butylene to increase its resistance to stress cracking. Copolymerization with vinyl acetate at high pressure increases flexibility, resistance to stress cracking, and seal ability of value to the food industry. 

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. 

Another successful development was based on morphological studies of traditional impact-resistant polystyrenes. Products with unusually big particles and a certain combination of composition and properties are particularly resistant to stress cracking (84). They have achieved considerable success on the... 

Linear PE same properties as the equivalent branched PE with an improvement in the mechanical properties, thermal and creep behaviour, and resistance to stress cracking. 

Polybutenes absorb little water and are not very sensitive to it. They are generally fairly resistant to stress cracking. 

Compared with other transparent plastics, they can be more resistant to stress cracking for some chemicals and sometimes less resistant for others. 

EVAs absorb water the more so as VA content rises, but for a 30% VA the water uptake is acceptable, for example 0.15% for a given grade. EVAs are generally more resistant to stress cracking than comparable LDPE. 

Neat ABS absorbs little water and is not very sensitive to it. Resistance to stress cracking is better than that of polystyrene. 

Acrylate rubbers which are employed in ASA contain no double bonds. For that reason, ASA is substantially more resistant to weathering than ABS. Owing to the polar acrylate component, ASA is also more resistant to stress cracking than ABS. ABS in turn has significant advantages in low-temperature impact resistance on account of the very low glass transition temperature of the polybutadiene rubber. 

As is the case for other polymers, the properties are affected by the average molecular weight and molecular weight distribution. As relative molecular mass (or molecular weight) increases, tensile strength, impact strength and resistance to stress cracking increase. 

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