EXPLOSIVES AND THE HAZARDS AND TESTING OF EXPLOSIVES M. A. COOK
nterest in explosives technology has expanded greatly since the advent of the space age owing largely to explosives problems in rode* and new developments in commercial explosives. Although intended to function only by controlled combustion, propellants generally have associated with them various types of explosion hazards (9, 70). For example, even the liquid bipropellants have been involved in numerous launch-pad spillage explosions, although neither the fuel nor the oxidizer may be explosive per se-since the “redox” (oxidation-reduction) mixture is never intended to be formed prior to combustion in the flame itself. Many solid propellants, particularly those of the greatest specific impulse, can detonate powerfully through the action of suitable high explosive boosten. They may therefore be expected, under some potential environmental Conclltlons, to undergo detonation spontaneously via the frequently devastating “deflagration to detonation transition” (D.D.T.), or upon impact via the “shock to detonation transition” (S.D.T.). While some of the solid propellants of lower specific impulse are difficult to detonate, there is still vital concern whether or not they too might detonate on occasion when used in some of the proposed monstrous rockets, such as Saturn and Apollo. This concern is based on the fact that the chance of spontaneous explosion under provocation in general increases with increasing charge size. Again, while great benefits would result by way of simplification of rocket design if liquid monopropellants could be used in place of the bipropellants, unfortunately there are apparently no monopropellants which are not also high explosives susceptible in various degrees to the D.D.T. and S.D.T. Associated with the construction and operation of modem rockets are many devices that depend, in one way or another, on explosives. For example, fast operating explosive switches have become vital to certain devices employed in rockets. Explosive stage separation devices (explosive bolts) of multistage rockets, and safety destruction systems also have become necessary constituents of rockets. Metal forming by ex-
I
-Summary of ACS Symposium, 145th National Meeting, September 1963
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plosives has become practically indispensible in the construction of many rocket parts. Mention should also be made of a current revolution in commercial explosives that began with the discovery of “prills and oil,” an easily formulated, mechanical mixture of coarse, porous, prilled ammonium nitrate and No. 2 fuel oil in a ratio of approximately 94/6. This revolution was significantly extended by the discovery of the novel “slurry explosions” based on aqueous ammonium nitrate solutions (3). These developments have made the consumers of commercial explosives vividly aware of the nature and properties of some explosives where previously this knowledge had been restricted primarily to those associated directly with the manufacture of explosives. The considerable interest shown in the symposium of the Division of Fuel Chemistry on “Explosives and the Hazards and Testing of Explosives” at the 145th meeting of the American Chemical Society, September 9-13, 1963, New York City, was thus not surprising. The most important contributions of this symposium are summarized below. Equation of State
The equation of state in the detonation of condensed explosives has always been a problem of considerable practical and theoretical importance in explosives technology. While it remains an unsolved problem in the minds of many authorities in this field, the paper by M . H. Friedman of 3M Co. provided a valuable correlation in further support of those equations of state employing only a volume-dependent or covolume a(u) description of nonideality. The “virial” equation of state had, in fact, been considered previously (8). I t was shown to agree (at gaseous product densities of about 1.G g./cc.) with the derived empirical equation of state using the thermohydrodynamic theory of detonation and observed velocities for the derivation (9). Refinements described in this paper seemed to extend the applicability of this theoretical equation of state thus adding confidence to the covolume equation :
pu
=
nRT
+ .c(u)p
Hot-Gas Ignition
Adiabatic compression of gases in contact with solid propellants and liquid monopropellants constitutes an important source of ignition from a practical as well as a theoretical viewpoint. A new and reliable adiabatic test method was described by G. A. Mead, Air Reduction Co. This method of measuring sensitivity not only has the advantage of being directly applicable to practical problems, but is amenable to theoretical analysis, and is flexible enough to permit the study of most of the important factors associated with the physical chemistry of ignition. Of particular interest was the influence of chemical composition of the adiabatically compressed gas on ignition of condensed phase explosives, the presence of oxygen in this gas being shown to lower the ignition temperature sharply. 32
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Particle Size Effects
Prior to the two very interesting contributions by R . H. Dinegar, R. H. Rochester, and M . S. Millican, Los Alamos Scientific Laboratories, it had generally been thought that solid explosives increase uniformly in sensitivity as the particle size decreases, or as the specific surface increases. Composite solid propellants are generally based on finely granulated ingredients, and under some conditions, especially those encountered at some stages of the manufacturing cycle, might thus be expected to become hazardous. Moreover, there is currently considerable interest in ultrafine grained high explosives. The results reported are not only of theoretical interest but also provide some practical reassurance in such cases b>- demonstrating that there is a maximum in the specific surface against sensitivity curve. Apparently there are two opposing factors pertaining to particle size and its influence on sensitivity. One is that the reaction rate in the detonation of granular explosives, being controlled by surface burning, increases with specific surface. This has been generally demonstrated (4,7) and is the only significant factor in the particle size range of conventional granular high explosives. The second factor comes into play apparently only in granular explosives of extremely high specific surface. I n this range of particle sizes new and important explosives technology is developing. iVhile the exact nature of the desensitizing factor in these very fine-grained explosives remains somewhat obscure, the work of Dinegar et al., shows that it definitely is not a reaction rate effect, but rather opposes the normal reaction rate effect. That is, reaction times were shown to decrease in the same range (of increasing surface area) where sensitivity was decreasing. liquid Explosives
From the viewpoint of explosives sensitivity the paper by C. M. Mason, R . W. Van Dolah, and J. Ribovich, U. S. Bureau of Mines. which defined the sensitivity of the trinary liquid explosive “Dithekite” (nitric acid, nitrobenzene, and water), had tJvo points. -It demonstrated relationships between sensitivity, heat of explosion, and, in the region of immiscibility, the influence of distribution of fuel and oxidizer. -It emphasized the hazards involved in a redox mixture comprising nonexplosive ingredients of common usage and great industrial importance, thus demonstrating that caution must be exercised by industrial chemists in handling all liquid redox mixtures because they are likely to be sensitive high explosives. Further development of the Wenograd heat sensitivity method ( 7 7 ) was described by J. M. Rosen and J. R. Holden, U. S. Naval Ordnance Laboratory, together with illustrative induction time against temperature results for nitroglycerine and tetranitromethane-toluene mixtures. This is a technique for measuring the thermal
M . A . Cook is Professor of Metallurgy, University of Utah, Salt Lake City,Utah. He was the Chairman of the symposium summarized here.
AUTHOR
sensitivity of liquid explosives. Like the adiabatic compression method, it is not only a method of practicability, but, if sufficiently refined, will also provide accurate reaction kinetics data that can be interpreted by the fundamental theoretical relationship : log r = A / T f B where A and B are theoretically significant reaction rate constants. The method provides experimental measurements of the induction time r and the absolute temperature T . Usually the spread in observed induction times at a given temperature is too great to make accurate evaluations of A and B from the experimental data. However, improvements described by these authors have apparently reduced this spread to the point where the method is now directly useful. Primary Explosives
The decomposition of silver azide has been shown by Bowden and associates to involve a semiconductor mechanism also directly involved in its explosive sensitivity (2). The same type of semiconductor mechanism is involved in lead azide, as shown in the paper by M. A. Cook, R . T. Keyes, C. H. Pitt, and R. R. Rollins, University of Utah, which also demonstrated a correlation between photoconductivity and sensitivity. The photoconductivity of lead azide is apparently associated primarily with the prominent 4060 A. defect band of lead azide, a band probably associated with VI-centers (interstitial nitrogen atoms). This defect band is destroyed via the formation of NQ by reaction with NT, the former dissociating into 2Nz plus an electron a t active centers (lead specks) on the surface of lead azide crystals ( 5 ) . Thermal decomposition in lead azide and silver azide follows the same general mechanism as the development of the photographic emulsion in which silver specks grow by trapping electrons from the conduction band and by drawing Agf or Pb++ ions, as the case may be, to the specks to neutralize the charge. The primary differences are that in lead and silver azide the electrons reach the conduction band via thermal excitation at and above 150' C., whereas for the silver halide a t these temperatures they are raised to this band only by quantum absorption. Of course, the excitons (electron plus positive hole) differ in each case by the differences in the nature of the positive hole and its modes of formation and decay. The practical advantage of this research is that it affords a much greater control of the sensitivity level and the rate of reaction and induction time of these industrially and militarily important azides. These are factors of considerable importance, for instance, in the design of explosive switches. Solid Propellanl Ignition
W. H. Anderson, Aerojet-General Corp., considered the mechanism of ignition of explosives in terms of the minimum required energy of an ignition source. The formulation of Anderson in a convenient form directly
suited for this purpose is noteworthy, although there are many nontractible factors associated with heat transfer in systems of different charge size, particle size distribution, and heterogeneity involved in this problem. Card Gap lest
The paper by P. K. Salzman, Aeroiet-General Corp., attempted to refine the application of the card gap test, a recently standardized test for monopropellants (7). I t described and applied a new reflected wave technique to improve the accuracy of data obtained in the calibrations of the card gap assembly. These calibrations provide the shock pressure of a particular donor as a function of the number of uniform cards between the donor and receptor of the card gap test. One may measure the pressure from a donor system by means of the new aquarium method (6) in which the velocity of propagation of the shock wave is observed in a transparent medium. This velocity is related to the pressure through the equation
P - Pr
= PlVU
where pi is ambient pressure, p l the density, V the shock velocity, and u the particle velocity in the medium in question. Measurements of the initial velocity a t the interface between the donor and the gage are required, but they are difficult to obtain accurately usually because (in small systems) of rapid attenuation of the shock wave in the transparent medium. The wave divides at an interface into a reflected and transmitted wave, the relationships between which may be observed. By studying these two waves and their geometrical relations, the reflected wave technique suggested by Dr. Salzman may be used to improve the accuracy of determining the initial shock velocity. The use of the reflected wave technique is a valuable, though yet incompletely demonstrated, suggestion that deserves careful further consideration. The results presented in this paper should go a long way toward establishing this principle as a valuable means of refinement of the card gap test. Hazards Analyses
Three papers from Picatinny Arsenal presented current concepts and approaches pertaining to hazards and their minimization for the manufacture and storage of explosives and propellants. A paper by S. Wachtell considered the problem from the viewpoint of conventional theory and experimental studies of combustion in a closed system. Measurements of the pressure-time curve for combustion in a closed bomb as a function of burning surface were presented and discussed. The Kistiakowsky concept of critical mass and the development of detonation waves by way of pressure buildup in shocks formed the bases for interpreting the results. A sharp increase in the slope of a pressure-time curve for burning in the closed vessel was considered to provide the main criterion for onset of detonation. Discussions of this paper emphasized that such sharp increases, while still observable at a level enough below infinity VOL.56
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to be measurable, represent conditions very far from detonation. The real onset of detonation cannot be detected by the experimental methods employed in this study. Thus, while the experimental P-T curves are of interest in demonstrating sharp increases in reaction rate at critical stages of the burning of the explosive or propellant in the closed bomb they are in no way characteristic of the D.D.T. The sharp increases observed in the burning rate (or pressure) against time curves were associated largely with fracturing of grains at certain critical pressure levels, thus suddenly exposing much greater specific surface to the burning process. C. E. McKnight applied fundamental probability theory to a consideration of the blast wall or protective wall problem for separation of charges from each other to prevent sympathetic detonation or propagation by influence in storage. Factors pertaining to whether or not the protective wall will fragment, the probability that particles will be ejected from a protective wall into a secondary (or receptor) charge, the probability of reoccurrence of this sequence of events to initiate a third charge by detonation of the second charge, initiation of a fourth charge from a third detonated charge, and so on, and other contributing factors were introduced as factors (to be determined experimentally) into the general probability equation. Predictions of probability of occurrence permit the concept of calculated risk to be intelligently applied. The general scheme developed in this paper should be helpful in many similar problems where explosions cannot definitely be avoided, but where risks of various types may be minimized by proper construction and investigation of the factors involved. The paper by L. W. Saffian and R. M. Rindner attempted to provide the necessary safety design criteria for use in formulating optimum manufacturing and storage facilities. This study was divided into three phases dealing first with prevention of propagation of detonation by influence, second with prevention of propagation by primary high velocity fragments, and third with the use of barricades and dividing or protective walls to protect against propagation by influence. Criterion I was based on sympathetic detonation limits, using as a theoretical foundation the familiar law of similitudes (d, = KW:l3 where d, is the sympathetic detonation limit, K a constant, and Wethe effective donor charge weight). The effective weight W e was related to the real weight W through four multiplicative factors, a composition factor F,, a confinement factor F,, a reflection coefficient FT, and a shape factor Fa;W , is given as the product F,F,F,F, W . The laws governing initiation by fragment impact were next considered in order to define limits of propagation via high velocity fragment impact and thus criteria for design against impact by high velocity particles. The behavior of protective walls was finally considered in interrupting blast waves, in stopping high velocity fragments, and as a potential source itself of high velocity fragments by rupture or spalling. The ultimate objective of this study is a safety design manual to which anyone may turn to design a particular 34
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
type of explosives facility. I t was indicated that data sufficient for design purposes are all available at present. Data were considered to be available also for propagation via primary fragments, but a testing program is in progress to complete the design criteria to cope with propagation via high velocity impact. While designs of protective walls were stated to be complete, again a testing program is under way to evaluate the designs. This program apparently represents a formidable one which, if successful, should provide a safety design manual of great utility and importance. While many figures of sound and well supported data and design criteria were presented in this paper, one may readily recognize important gaps in the data required to fulfill the ultimate objective, in addition to serious gaps in the theoretical formulations needed to correlate these data, illustrating the magnitude of the task remaining. V. T. Dinsdale, Thiokol Chemical Corp., outlined in detail the safety concepts and extensive testing facilities and procedures employed by Thiokol in discovering and coping with hazards in the manufacture of composite solid propellants. Sensitivity to friction, impact, heat, and electrostatics and the critical diameter for the propagation of detonation were given major consideration. Recognizing specific environmental conditions and the inadequacy of conventional friction, impact, heat, and electrostatic sensitivity testing methods, extensive well engineered new methods developed and standardized at Thiokol were outlined. While some of the new impact and friction methods described for evaluating explosive hazards are evidently best suited for the type of materials dealt with in the manufacture of composite solid propellants they may also constitute important methods of studying the hazards in other types of explosives, particularly those which (like these propellants) are of threshold detonation level in which explosive hazards are hard to recognize. Commercial Blasting
While dealing less directly with hazards, the paper on “Shock Coupling, Loading Density and the Efficiency of Explosives in Commercial Blasting’’ by R. B. Clay, M. A. Cook, and V. 0. Cook, Intermountain Research and Engineering Co., seemed appropriate to a symposium emphasizing hazards because it described applications of the modern blasting agents wherein hazards in commercial blasting (although still existing) have been reduced almost to the ultimate minimum. The novel slurry explosives (3) are inherently among the least sensitive and safest yet developed in commercial blasting. The new on-site slurry formulation and borehole placement method (pump truck) described in this paper is evidently capable of operation under conditions where explosive hazards are practically nonexistent. For instance, when using in a 9-inch diameter borehole, a nitro-carbo-nitrate or NCN-type of slurry (one sensitized with a nonexplosive fuel and containing no explosive ingredients per se) that has also been designed with a critical diameter of around 5 inches, this formulation and placement method deals
with an explosive only when it actually is placed in the borehole. The paper by Clay and coworkers outlined first a theory of rock fragmentation by blasting based on basic principles of modern rock mechanics ; second, physicochemical conditions for optimizing efficiency in hard rock blasting; and finally, the best, most modern methods of achieving these conditions by use of the modern on-site pumping of slurry blasting agents. l o w Pressure Explosives
A valuable, practical, and theoretically interesting study describing new plastic explosives of controlled detonation pressures as low as 10 kilobars still sensitive enough to detonate consistently was contributed by M. T. Abegg, Sandia Corp., and H. J. Fisher, H. C. Lawton, and W. T. Weatherill, Areojet-General Corp. T o achieve adequate sensitivity, either a finely ground primary explosive (lead azide or thallium azide) or a superfine, sensitive secondary explosive (PETN or RDX) was plasticized with a suitable plastic binder, polyurethane (PU), nitropolyurethane (NPU), or dinitropropyl acrylate (DNPA) . Plastic explosives having detonation pressures in the range from about 160 to 10 kilobars were thereby obtained. The practical advantage of these plastic explosives is that they are militarily safe and suitable for generating any prescribed “brisance” or detonation pressure in this range. The latter property is important in specialized military applications where low detonation pressure and close tolerances are necessary. Detonation pressures, p,, were measured in this study by means of a plate dent method. This method was calibrated by using measured densities p1 and detonation velocities D together with the approximate equation p2 = 0.0025p1D2 for pressure in kilobars, density in g./cc., and velocity in m./sec. While this approximation is fairly good at ordinary detonation pressures (100 to 250 kb.), there is serious doubt that it is good at very low pressures. The above method of controlling detonation pressure in a relatively incompressible explosive may be termed the dilution method. An alternate method is the chemical method discussed by M. T. Abegg and W. J. Meikle, Sandia Corp., and J. W. Fronabarger, C. W. Hoppesch, and C. T. Rittenhouse, Universal Match Corp. Detonation pressure in an explosive at its maximum density-i.e., with effectively no free space-is determined by its energy density (heat of explosion Q times density pl or plQ). One should thus be able to select an explosive with any particular energy density to fill a specific detonation pressure requirement. A third method, perhaps less easily controlled, is to formulate any particular explosive at or near its crystal density with a particle size distribution such as to prescribe detonation a t the required pressure. This is the reaction rate control method. The dilution and chemical methods may (but need not) involve ideal detonations, but the reaction rate method must involve nonideal detonation (4). This paper describes studies of coordination compounds in which the energy density may be varied by using oxidizing and reducing subgroup
ions in prescribed proportions to control the oxygen balance. They considered ions of ammonia, ethylenediamine (en), propylenediamine (pn), and trimethylenediamine (tmen) as the reducing ions of the complex and ions of iodates, periodates, perchlorates, nitrates, and nitrites as the oxidizing components of the complex. Additionally, ions of no particular fuel or oxidizing value (for example, chloride ion) were also considered, evidently in order to be able to introduce explosive dilution character into the complex if need be. The general formula of the coordination compounds was, therefore, MR,0,D,.nH20 where M is the metal ion (cobalt, copper, zinc, platinum, mercury, and cadmium being mentioned) ; R the reducing ion; 0 the oxidizing ion; D the dilution ion; the subscripts x, y, and z are appropriate integers; and n is zero, l/z, 1, or ”2 as the case may be. Oxygen balance was based on per cent of the required oxygen for perfect balance (to carbon monoxide), an unconventional but useful definition. The results showed that the desired detonation pressures below 100 kb. could be achieved at near crystal density only by maintaining the oxygen balance in the range 20 to 30%. However, at this oxygen balance propagation of detonation near crystal density is difficult to achieve. Measurements of detonation pressure were made as in the previous paper, by means of plate dent method calibrated by the approximation p G 0.0025p1O2. Anomalies in the results may perhaps be traced to failure of this approximation. For example, the linear p ( p 1 ) curves indicated for T N T and C O ( N H ~ ) ~ I O ~ * ‘ / z HzO disagree with the calibration equation itself in that detonation velocity (usually) follows the equation D = a b p l so that the detonation pressure should b p ~ in ) ~ follow the approximation fi S 0.025pl(a which the px2 and pI3 terms are important.
+
+
LITERATURE CITED (1) American Rocket Society Committee on Mono ropellant Methods Recom-
mended Test No. !r “Card-GaS, Test for Shock gensitivity of Liquid Monopropellants,” American Rocket ociety, New York, July 1955. (2) Bowden, F. P., Proc. Roy. Sac. London, No. 1245, July 29. 1958. (3) Cook, M. A., Sciancc 132, 1105.(1960). (4) Cook, M. A., “The Science of High Explosives,” Reinhold, New York, 1958. (5). Cook, M. A., Head, N . L Keyes, R. T., Thornley, G. M., Pitt, C. H. “Sensitlvlty of Lead Azide ” Intekational Conference on Sensitivity and Hdrards of Explosives, London, bctober 1963. (6) Cook, M. A., Udy, L. L., A.R.S. Journal, January 1961. (7) Eyring, H., Powell, R. E., Duffey, G. H., Parlin, R. B., Cham. REP.45,69 (1949). ( 8 ) Paterson, S., Rusaarck London 1, 221 (1948). ( 9 ) Vance, R. W., “Cryogenic Technology,” Chapter 13, pp. 375-423, Wiley, New York, 1963. (10) Vance,-R. W., Duke, W. M., “Applied Cryogenic Engineering,” Chapter 11, pp. 293-318, Wiley, New York, 1962. (11) Wenograd, J., T7aru. Faraday SOC.57, 1612 (1961).
Complete Copies The articles rummorlzed here are available as Preprints of the Division of Fuel Chemistry, 145th National Meeting, ACS, September 1963, Volume 7, No. 3. Copies may be obtained from: Dr. R. A. Glenn, ACS Divn. of Fuel Chemistry, Bituminous Coal Research, Inc., 3 5 0 Hochberg Road, Monroeville, Pa. 1 5 1 4 6 Price
$3.00 (Please include with order)
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