The sensitiveness of explosives - Mechanical and thermal processes of initiation

Abstract

The sensitiveness of initiators to mechanical action has been examined in relation to heat, for lead azide, mercury fulminate, and in some cases lead styphnate, in order to see how far mechanical action could be equated with local heating. Previous information on mechanical sensitiveness has been extended by measuring friction sensitiveness with grit of varying hardness, and by varying the melting point and hardness of the rubbing surface. The percussion sensitiveness has been compared using as confining metals nickel and tin, as well as the brass normally used. The relation between sensitiveness to heat and sensitiveness to mechanical action has been investigated by determining the percussion sensitiveness of initiators partly sensitized by heating, and also by studying the delay to ignition as a function of the quantity of initiator accessible to the growth of the detonation wave. In these tests, lead azide was found to build up to limiting detonation conditions in considerably smaller quantities than mercury fulminate. It is found that grit does not appreciably sensitize initiators to friction unless it is harder than about 3.5 to 4 on Moh’s scale, and unless the rubbing surfaces have a high hardness and melting point. It is confirmed that mercury fulminate is not appreciably sensitized by grit. Percussion sensitiveness depends to some extent on the confining metal used, being rather lower for the metals of lower melting point. Extension of previous observations confirms that if lead azide or mercury fulminate is heated during half the induction period at any one temperature, and then cooled, these heat sensitized initiators will detonate in approximately half the normal time at any other temperature. However, sensitization of initiators to heat in this way has only a secondary effect on their sensitiveness to percussion. These observations together with previous work indicate the following relationships between mechanical and thermal sensitiveness. (i) In grit sensitveness , the mechanical action involves mainly the formation of 'hot spots’ between the grit and a hard surface. These hot spots acting on the initiator generate the detonation wave more easily with lead azide than with mercury fulminate, so that lead azide is more sensitive to grit than mercury fulminate. Other mechanical effects may be present in a subordinate degree. (ii) Percussion sensitiveness appears to be more complex, and may involve a tribo-chemical ‘trigger’ reaction as well as the formation of hot spots through friction. Tribo-chemical or other mechanical 'trigger reactions' are only indirectly related to the sensitiveness to heat since they involve a more direct transfer between mechanical energy and activation energy, than is involved if the mechanical energy is first converted into heat.