The detergent stress‐cracking of polythene

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Abstract
Whenever the fracture of a polymer is preceded by crazing or other types of cavity formation, energy must be supplied both for local yielding and for the generation of a new surface. However previous calculations for rigid glassy polymers have shown that the surface energy contribution required for the formation of a craze, compared with that associated with plastic and elastic strains is small. In order to study this problem further, we have now made a finite element analysis of the cavitation of an elastic‐plastic solid introducing a surface energy term. This shows that the ratio of the plastic energy requirement to surface energy depends, not unexpectedly, on (I) the ratio of yield stress/ surface tension and (II) cavity size. Thus for polymers with a low yield stress such as polythene, the surface tension becomes significant if the hole diameter is below 100 nm. In these circumstances crazing and cracking is predictably accelerated by dilute aqueous detergents which do not need to penetrate the polymer, provided of course that the detergent has access to the cavities.Hole growth also depends on the extent of orientation hardening during plastic strain and this contributes to the greater stress crack resistance of high molecular weight polythene. The occurrence of a larger amount of plastic deformation on the fracture surfaces of high molecular weight polymer has been demonstrated by Harman for high density polythene and is confirmed here in scanning electron micrographs of low density material.Other electron micrographs show that very small cacities are indeed formed when high density polythene is broken in the coventional detergent stress cracking test, thus accounting for the accelerating effect of the detergent.

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