The ignition of a thin layer of explosive by impact; the effect of polymer particles

Abstract
In an earlier paper Heavens and Field described the use of high speed photographic and pressure-measuring techniques to study the behaviour of thin layers of pure explosive under drop-weight impact. When ignition occurred it invariably did so at local sites after bulk plastic flow of the sample. The present paper investigates the effect of added particles to the explosive layer. The case of the hard, high-melting-point particle is reasonably well understood. As shown by Bowden and Gurton the 'hotspot' temperature is then usually controlled by the melting point of the particle. However, as this paper confirms, some relatively soft, low-melting-point solids such as certain polymers can also sensitize explosive layers. Both chemical and mechanical explanations are examined. Chemical effects were assessed by using thermogravimetric analysis. High speed photography, with the sample between transparent anvils, was used to photograph impacts on polymers, explosive samples and layers of explosive with polymer added. The sensitizing effect is shown to be a mechanical one with the production of free radicals by the polymer only of secondary importance. Polymers that sensitize are those that fail catastrophically either by fracture or localized adiabatic shear, and that have a low specific heat, latent heat and thermal conductivity. Hot spots in these polymers, produced during rapid deformation, can greatly exceed the polymer's softening point. This was confirmed by separate experiments with a friction apparatus with hot-spot temperatures recorded by using infrared techniques. The experiments with the polymeric samples also have implications for studies of the fracture, friction, erosion and wear of polymers.

This publication has 0 references indexed in Scilit: