Soft Composite Explosives and Process for Making Same

The availability and low cost of ammonium nitrate (AN) have resulted in its widespread use in explosive formulations for commercial blasting. The simplest of these formulations is a mixture of AN and fuel oil (ANFO) in the ratio of approximately 94.5:5.5 by weight. Other formulations include a wide assortment of slurries and emulsions which have been developed to provide advantages over ANFO in handling, water resistance and improved performance.

The success of commercial AN formulations has attracted attention to the possible use of AN as a major ingredient in military explosives. For military applications, however, the developmental thrust has been directed primarily at ways of utilizing more effectively the energy available from explosives containing AN. Typically, AN formulations do not behave ideally in the explosive sense in that the energy release is not sufficiently prompt to yield theoretically possible detonation velocities and pressures. Development of methods to correct this deficiency has been an important part of military oriented research and development efforts over the past several years.

One approach has been to form low-melting eutectics comprised of AN and one or more explosive fuels. Eutectics offer increased intimacy of the ingredients, low melting points near those normally used in loading plants, and melt-cast properties compatible with conventional loading operations. Increased intimacy of the ingredients results in improved performance in some instances.

Typical of the eutectic composite formulations is one comprised of ethylenediamine dinitrate (EDD) and AN in the ratio 49:51 by weight (EA) with a melting point of approximately 103.degree. C. When the AN portion is modified to contain a ratio of 85:15 AN/KNO.sub.3 by weight, the formulation is called EAK. Potassium nitrate (KNO.sub.3) has been added to phase stabilize the AN. Other modifications may include an additional ingredient, such as nitroguanidine, which lowers the melting temperature still further, to approximately 98.degree. C., when present to the extent of 8% by weight of the composition (NEAK). Another formulation typical of an AN based composite explosive is a 2:1 mole ratio of AN:ammonium 3,5-dinitro-1,2,4-triazolate.

A co-pending patent application, entitled Eutectic Microknit Composite Explosives (EMCX), teaches methodology by which essentially anhydrous emulsions of composite formulations such as EAK may be handled as a supercooled fluid while being loaded into containers before setting to a hard consistency.

Two U.S. Pat. Nos. 4,248,644 and 4,391,659, also teach emulsification technology in similar formulations. This prior art differs in part from EMCX technology in that the products retain a grease-like consistency, or are at least extrudeable and do not solidify. The first of the above cited patents limits the fuel concentration to a minimum of 2.5% and the emulsifier to a minimum of 1% by weight for a total of 3.5% by weight. The second patent deals with water-in-fuel emulsions as the final products.

It has not been apparent heretofore that grease-like, oil-continuous emulsified compositions can be made with significantly reduced oil phase and emulsifier concentrations. In fact, some emulsions have been found, as shown in the invention described below, to increase in stability, and to retain grease-like characteristics when the oil phase concentrations have been reduced substantially below those practiced in the prior art.

It is the objective of this invention to obtain soft composite explosive compositions by means of forming oil-continuous, melt-in-fuel emulsions in which the discontinuous phase is comprised of AN and other ingredients which together with AN form a eutectic mixture, and the continuous phase is comprised of a combination of fuel(s) and emulsifier(s) which together constitute less than 2.5% of the explosive formulation by weight. The other ingredients added directly to the discontinuous phase include soluble compounds which can be converted to explosive compounds in situ or soluble self-explosive compounds, along with one or more oxidizer salts. Different combinations of ingredients can be chosen to provide formulations with differing oxygen balances.

Table I lists several example formulations of emulsified explosive compositions containing AN and other ingredients which reduce the melting point of the discontinuous portion of the emulsion.

Examples 1 through 5 show that various types of emulsifiers can be employed for making oil-continuous emulsions using low concentrations of oil phase. Formulations 7 and 8 are examples of formulations which do not contain compound explosives.

Example 6, with an oil phase concentration of 6% by weight, was included to show that a higher oil phase concentration can produce a hard rather than soft product. Examples 1,3 and 4 with oil phase concentrations ranging from 1.2% to 2.4% by weight illustrate that soft grease-like products can be made at these lower oil phase concentrations.

The methodology used to make formulations 1-8 is typical of that used in conventional emulsion technology. Laboratory methods usually involved reverse order addition, i.e., slowly adding the molten eutectic portion to a preheated mixture of oil and emulsifier while stirring continually. Direct order addition was also found to be applicable. The emulsions of this invention can be made by either continuous or batch methods.

A preferred oxygen balance for a given formulation can be achieved by selection of ingredients. The preferred oxygen balance range is from +5 to -30%. If the formulation contains a metallic fuel, the preferred oxygen balance range is from +5 to -50%.