The sensitiveness of explosives - The sensitiveness of initiators to friction. Temperature coefficient

Abstract

In part III (2), a machine was described for subjecting explosives to grazing friction between a stationary and a moving surface, rubbing one over the other. By arranging Service initiators in a scale of increasing sensitiveness, it was shown that a different order was obtained: ( a ) when the rubbing surfaces were coated with standard emery paper, so that the friction was due to particles of hard grit; ( b )when the rubbing surfaces were of steel; ( c ) when grit was introduced between steel surfaces. Initiators such as mercury fulminate show much the same sensitiveness in all three cases, but others, such as Service azide, are notably more sensitive under the action of grit. The experiments described in this section were designed to give additional information on the nature of grit sensitiveness in particular, by investigating: (1) the effect of raising the temperature of the rubbing surfaces; (2) the deadening produced by wetting the explosive with various liquids. The relative probability of initiation by grazing friction was determined for a range of velocities of the rubbing surface, as described in the part quoted above. Principal changes in determining grit sensitiveness were: (i) Graded emery powder was mixed with the initiator which was then rubbed between steel surfaces. One reason for eliminating the use of paper was the uncertainty arising from the effect of heating or wetting on the standard emery paper. (ii) An initiator balance (described below) was used to weigh out standard quantities of compositions for each test. These were sprinkled over a constant area of the steel on the tilting table through a small multiple sieve. (iii) The temperature of the tilting table, and of the thin layer of powder on it was maintained by electric heaters, at various temperatures up to 300° G. (iv) To investigate the deadening effect of liquids, drops or a fine spray were applied to the layer of composition. Liquids used included water and glycerine solutions with and without the addition of capillary active substances, and ethyl and butyl alcohols. Principal results obtained were: (i) Effect of temperature on grit friction sensitiveness . On heating, mercury fulminate, lead di-nitroresorcinate (L. D. N. R.), lead styphnate, Service azide and A. S. A. show no definite increase in sensitiveness to friction between steel surfaces, with or without the admixture of grit. These results were in most cases confirmed up to 145° C; much above this temperature the heat sensitiveness of the initiators can lead to spontaneous ignition before the rubbing takes place. A small increase in sensitiveness at 145° C was observed with the initiator 664 rubbed between steel surfaces. This was confirmed with a 664-grit mixture. (ii) Effect of wetting on grit sensitiveness . Thorough wetting with water + glycerine solution or with ethyl or butyl alcohol largely deadened the sensitizing action of grit. Notable effects were observed even when the quantity of liquid was reduced to a very small amount by using a fine spray. The breaking of crystals in rubbing was substantially the same, e.g. in the case of Service azide, where the only effect of wetting was observed to be a small increase in the number of unbroken crystals. From these results, it is concluded that: (1) If the effect of rubbing initiators were merely to raise the overall temperature, till the decomposition induced led to self-heating and detonation, heating of the tilting table should provide some of the energy, and should result in a greater probability of detonation at the same velocity of rubbing. Since no such effect is observed as a general rule, the conclusion is that the friction sensitiveness cannot be represented as due to an overall heating. Initiation by friction may be due to the formation of a few local hot spots, or to shearing of the crystals, but if these effects are operative, the conclusion is that they do not show a marked temperature coefficient within the range of the present experiments. (2) Wetting of the initiator crystals does not appear to lessen the gross breakage sufficiendy to account for the observed desensitization. The observed deadening must be due either to the prevention of local hot spots, or to the damping down on propagation from marginal initiation.