Mechanisms Involved in Impact Sensitivity and Desensitization of RDX
JAMES B. ROMANS, a research chemist with the Naval Re- t search Laboratory, receiued a BA in chemistry f r m Central College of Iowa in 1935. After a relatively brief career tk as a high school science teacher, he joined the National Bureau of Standards.in 1938, where he worked principally on the physical properties of liquid fuels and lubricants and on oil jiltration. Since coming to N R L in 1946, Mr. Romans has been engaged in research on the properties and application of synthetic lubricants, the dielectrie properties of synthetic liquids, the fatigue characteristics of glass fiber-reinforced plastics when flexed while immersed in water, the surface chemical aspects of the shock sensitivity of high explosives, and the adhesional characteristics and water sensitivity of solid rocket propellants. He has coauthored 16 government reports and 11 scientific journal publications in the above fields and has been a member of the ACS for many years.
2 Ind. Eng. Cham. Prod. Res Develop., Vol. 12, No. 1, 1973
WILLIAM A. ZISMAN(BS, M S in physics, M I T , 1927-28; PhD, Harvard, 1932; H o w orary DSc, Clarkson College of Technology, 1965) has been involved in surface physics and chemislq at the Naval Research Laboratory since 1939. He was made Chief Scientist, Laboratory for Chekical Physics in early 1968 and since January 1969 has been the holder of the Chair of Chemical Physics at NRL. He was the r e c i p d of the Hillebrand Award of the Washington Chemical Society, 1964; Distinguished Civilian Service Award of the Department of the N a y , 1954; Carbide and Carbon Award of the ACS, 1955; National Award of the American Society of Lubeation Engineering, f965; A C S Kendall Co. Award in Colloid Chemistry, 1963; Distinguished Civilian Service Award of the Department of Defense, 1964; and the Captain Robert Conrad Dexter Award of the Ofice of Naval Research, Department of the N a y , 1968. He is a member of the ACS and the American Physical Society and was Chairman of the Division of Colloid and Surface Chemistry, ACS, 1958, and President of the Washington Section of the ACS in 1965. From 1961 to 1967, Dr. Zisman was Secretary of the I U P A C Commission on Colloid and Surface Chemistry. He i s also a member of Sigma X i , Washington Academy of Sciences, New York Academy of Sciences, and the American Association for the Advancement of Science.
The impact sensitivity of RDX and RDX-additive compositions was investigated with the aid of an automatic impact-explosion detection system which photographs oscilloscope records of sound pressure as a function of time. Mechanisms involved in the desensitization process were elucidated by modifying the heat-conduction path between the explosive sample and impact apparatus and also b y measuring the impact sensitivity of RDX containing a variety of solid and liquid desensitizers having a wide range of specific heats. Conditions which aid the conduction of heat from the explosive sample decreased impact sensitivity. Effectiveness of the desensitizers was in the same relative order as their specific heats. Desensitizing additives appeared to function primarily by absorbing sufficient heat from any localized hot spots to prevent self-accelerating reactions. However, sensitivity increased with hardness and melting points of solid desensitizers. In addition to absorbing heat, liquids also desensitize by filling some of the voids in the solid explosive, thus reducing the number of gas pockets which are potential sources of the hot spots. An approximate relationship was also found between the specific heats of a variety of pure high explosives and their impact sensitivities, but its extension to primary explosives i s not without exceptions, probably because other mechanisms are important.
A n investigation of the mechanisms involved in the impact sensitivity of the secondLaryexplosive RDX (hexahydro-1,3,5trinitro-1,3,5-triazine)was initiated by us in 1958. Results of this study subsequently appeared in a series of S R L reports (Bowers and Romans, 1960; Bowers e t al., 1960, 1962) which have not been published in the open literature. This summary of that work and its relationship t o work published sirice then by others are presented because of current scientific and technological interest. Davis (1941), Cook (1.958a), and Bowden and Yoffe (1958a) have discussed the behavior of explosive material when subjected t o shock, vibration, impact, friction, sparks, or flame. Copp et al. (1948) and Bowden and Yoffe (1958a) have shown that the addition of waxes and certain other materials t o explosives renders them less sensitive t o impact. Copp e t al. (1948) proposed that during impact “hot spots” are formed, which if sufficiently intense lead t o progressive thermal decomposition of the esplosive unless the heat center is quenched by the presence of the wax additive. Bowden and Yoffe (1958a) indicated that the heat may originate from the adiabatic compression of small gas bubbles, friction between the esplosive particles, and viscous heating of the explosive. Methods of evaluating the hazards involved in handling esplosives have been a n (essential part of the research and development of explosive compositions (Copp et al., 1948; Bomden and Yoffe, 1958a; Xacek, 1962). Experiments by Linder (1961) led him t o suggest t h a t t’he desensitizers for fi-HXX and P E T N act as a transient thermal insulator, but he concluded that desensitization is a complicated process and that there are other factors of importance involved in the process. h widely used metho’d for determining the sensitivity of a compound is the study of its behavior under well-defined conditions of impact, such as subjecting the explosive t o flying particles or on a more routine basis, to blows resulting from the impact of a falling weight (Departments of the Army and the Air Force, 1955). l l a n y impact sensitivity machines have been devised (llallorj., 1956; Cook, 1958a, Kalker et al., 1967), but regardless of the t’ype used, t o obtain good reproducibility it is essential that the operator be able t o distinguish clearly between esplosion and failure a t the time ot’ impact. &\.is (19411, Braid and Langville (1955), Mallory (1956), Cook (1958a), and Rideal and Robertson (1948) have re-
ported detection methods based on the presence of noise, flame, gas evolvement, or ionization. The most commonly used method of detecting the occurrence of a n explosion in a n impact sensitivity machine on a routine basis is by means of the noise emitted (Nacek, 1962). Because it has been necessary t o rely on the personal judgment of the operator to determine the extent of an esplosion at the time of impact, l\lallory (1956) and Holden (1959) have described acoustically initiated explosion-detection devices which were developed in a n attempt t o improve the resolution of impact sensitivity tests. The need for even greater resolution in the detection of partial decomposition or low-order explosion during impact tests than that available arose during the course of our investigation of the mechanisms involved in the desensitization of RDX. *\s in the destructive testing of many other materials, much valuable information regarding the early and intermediate processes which took place during impact tests was unobt’ainable. Therefore, we developed a method whereby the instantaneous sound pressure vs. time of a n impact test esplosion could be more readily and objectively observed and recorded. It proved particularly useful when the noise level of the esplosion was below that of the impact machine or below the level of detection by ear or by the acoustical devices then available. It also indicated the relative intensities of esplosions and made possible a n estimation of the time interval between impact and explosion. Experimental Apparatus
The impact machine used for our investigation was a n E R L Bruceton KO.12 design developed by the X D R C group at the Explosives Research Laboratory, Bruceton, Pa., and described by Mallory (1956). The fundamental components consisted of a 1.25-in. diam steel “anvil” on which the explosive sample was placed, a 1.25-in. diam st’eel cylinder or “striker” which rested on the sample, and a 2.5-kg steel weight (the hammer) rrhich was dropped from various heights onto the striker. The drop height scale was calibrated from 0 to 320 em. The niachine was located a t the C.S. Naval Ordnance Laboratory, White Oak, Xld., where it was housed in a small air-conditioned room. Peterson (1956) had indicated t h a t because of the complexity and short duration of impact and explosive noise, conventional noise-measuring apparatus such as sound-level Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973
3
Table I. Source and Method of Preparing Solid Additives Source
Additive
Polyethylenes (8417, 8416) Polyethylene (Super Dylan 6600) Acrawax C (mainly octadecenamide) Polyacrylamide (PAM 50) Polyethylene terephthalate (Mylar) Graphite Sulfur (flowers, USP) Sodium acetatea3HzO (conforms to ACS specifications) Lead acetatea3HzO (conforms to ACS specifications Strontium chloride .6Hz0 (“Baker’s analyzed”) Carbowax 1540, 4000, 6000, and 2 0 X (polyethylene glycols) Superla wax (chiefly paraffinic hydrocarbons) Biphenyl (research grade) Fluorowax (experimental sample #DV6020A) Teflon (polytetrafluoroethylene) Carbowax 1000
Treatment before use
Allied Chemical & Dye Corp. Barrett Division Koppers Co., Inc. Glyco Products American Cyanamid Co. E. I. du Pont de Nemours & Co. Acheson Colloid Co. Fisher Scientific Co.; Eimer and Amend Mallinckrodt Chemical Works
a
Merck & Co., Inc.
b
J. T. Baker Chemical Co.
b
Carbide & Carbon Chemicals Co.
C
Standard Oil Co. (Indiana)
C
Monsanto Chemical Co. E. I. d u Pont de Nemours & Co. E. I. du Pont de Nemours & Co. Carbide & Carbon Chemicals Co.
a a
a
a a a
b
C
C
a,d C,d
Material received as a powder and passed through a series of sieves. b Material pulverized with a mortar and pestle before sieving. Material pulverized with Dry Ice with a mortar and pestle before sieving. d Material agglomerated and would not pass through the largest sieve. c
Table II. Source and Method of Preparing liquid Additives Treatment before use
Additive
Source
Polychlorotrifluoroethylene (Kel-F Xo. 3) Polydimethylsiloxane (DC 200 silicone) n-Hexadecane
ill. W. Kellogg Co.
a
Dow Corning Corp.
a
Connecticut Hard Rubber Co.
0
C Water a Used as received from producer. Percolated slowly through column of activated alumina. Triply distilled in quartz.
before impact, which was a function of the drop height, ranged from 0.5 to 5 msec. The microphone was fitted into the end of a cardboard tube 2 in. in diam and 24 in. long which served to reduce background noise, concentrate the sound of the explosions, and direct it to the microphone a t a n angle of 0’. The other end of the tube was placed near the anvil. Since many low-order explosions were known to be directional, a semicircular metal baffle was placed around the anvil of the impact machine t o reflect explosive noise back into the sound tube. Interference by background noise was further reduced by enclosing the microphone, sound tube, and the anvil-striker area with fibrous acoustical tile. This arrangement improved the resolution of the acoustical detection system, but it made it more difficult to detect low-order explosions by ear. Materials
meters, noise meters, and spectrum analyzers gave neither reliable nor sufficient information when applied to sounds of this type. He had also described a n impact-noise analyzer which measured both the peak level and duration of a single impact, but unfortunately no data concerning the frequencies and their individual amplitudes could be obtained or recorded. To obtain more information and in particular t o detect incipient explosions, we used a n hltec-Lansing Type 633A dynamic microphone coupled to a Tektronix Type 512 oscilloscope. The frequency response of the microphone ranged from 30 t o 10,000 Hz. The sweep rate of the oscilloscope was 1 mseclcm scale division, and the vertical gain was held constant. X Polaroid camera was attached t o the face of the oscilloscope to photograph the acoustical display. X photoelectric relay and light source were arranged so that the falling hammer of the impact machine interrupted the light beam and initiated a single sweep of the oscilloscope. The sweep time 4
Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973
Cnless otherwise stated, the RDX used in all these experiments was a commercial grade obtained from the Chemical Engineering Division, Explosives Department, Kava1 Ordnance Laboratory, and identified as Holston Type B (SOL KO.Xl77). It contained a wetting agent as a result of the manufacturing process. The particle-size distribution was not known; however, all of the particles passed through a 44-p sieve. The additives used to desensitize the RDX in these experiments are listed in Tables I and I1 along with the sources and the methods of preparation. They were mixed mechanically with the R D X powder, and unless otherwise noted, the concentration was 10 wt %. The Composition A-3 used was brabash Lot KO. 3-357 made by the Wabash River Ordnance Works; it consisted of R D X and 9yo petroleum wax. The wax, which contained a wetting agent, had been applied in the form of a hot water slurry so as to coat the particles of RDX and act as a bindlng agent (Departments of the Army and the Air Force, 1955).
Table Ill. Effect of Various Insulators (Experimental Procedures) on Impact Sensitivity of RDX Conditions of testing RDX powder
No. of trials
50% ht, ~ r n
0
Bare anvil and striker 5/0 flit paper on a n d 5/0 flint paper on anvil (grit removed) 5/0 flint paper reversed on anvil (Le., grit toward anvil) Bond paper on anvil Between two squares of bond paper Two squares of bond paper above and two below Between two pieces of 0.005-in. copper Between two pieces of 0.010-in. copper
35 50 40
74 20 32
0.27 0.09
43 25 30 40 60 40
16 28 21 30
0.15 0.13 0.08 0.08 0.17 0.19
The sensitivity test procedure employed with the impact machine was the “up-and-down” method described by Dixon and Massey (1969). An initial trial was made a t an arbitrary drop height estimated to he near the 50% height, i.e., t h e height a t which on+half of all trials would result in an explosion. Each subsequent trial was made a t the next lower height if explosion occurred and a t the next higher height if there was no explosion. The test heights were spaced a t 0.1 log intervals. The impact sensitivity, expressed as the 50% height, and the standard deviation u were then calculated. Unless otherwise indicated, t h e impact machine test samples each weighed 35 i: 2 mg. They were placed directly on the anvil, on sandpaper or on or between 1-in. square sheets of 0.004-in. thick white hond paper containing 25y0 rag. The sandpaper was made by Abrasive Products, Inc., and was designated as “5/0 A flint finishing paper.” The average number of trials made in the impact machine in evaluating the R D X powder (both the sensitized and solid desensitized compositions) was 38, with no evaluations based on less than 25 trials. The impact sensitivities of most of the liquid desensitized R D X compositions were determined hy making 60-75 trials with no evaluations based on less than 50 trials. The standard deviation u was less than 0.2 for most of the materials studied. However, the deviation was approximately onethird greater for those samples evaluated on the hare anvil. Studies of Mechanisms Involved in Impact Sensitivity Influence of Explosion-Detection Methods. The impact sensitivity value of 20 em obtained for R D X powder evaluated on 5/0 flint paper, after the practice of Svadeha and Duck (1955), agrees well with the value of 19 em ohtained by them. However, their value of 87 om for Composition A-3 evaluated on 5/0 flint paper was much higher than t h a t of 32 em obtained by us under t h e same conditions hut by use of our acoustical recording system for detecting the explosions. This difference is believed to he inherent in the methods used to determine whether or not a n explosion had occurred. Examination of Figure l a shows the sound pressure vs. time oscillogram produced by our apparatus when the impact machine hammer fell from a height of 64 em, and no explosion occurred. Figure l h is an oscillogram produced under the same conditions when a mild explosion took place as indicated by the large downward deflection. This explosion, typical of many we observed, was inaudible to the operator. Any noise recorded on the oscilloscope in excess of background noise was considered evidence of an explosion, even though not all of the explosive charge was consumed. Impact sensitivities obtained by this method of detection were considerably lower
84
204
0.08
Figure 1. Sound pressure vs. time of impact sensitivity test machine operated from height of 64 cm when: (a) no explosion occurred; (b) explosion occurred. Time scale along horizontal axis is 1 msec per scale division. Instantaneous sound pressure is displayed on vertical axis
than those obtained by conventional methods. When most of the sample was consumed, the explosions were audible, and close agreement between the two testing methods was obtained. Effect of Sample Form. Impact sensitivity measurements were made on R D X powder, pellets, and single crystals placed on 5/0 flint paper. The pellets were about ‘ / z om in diam, and each weighed from 30 to 40 mg. The calculated density of several pellets was 1.7 g/cm3. This compares well with 1.8 g/cma for the single RDX crystals grown by us from a solution in N,Ndimethylformamide by allowing a nearly saturated solution a t 80°C to cool slowly to room temperature. Only the most perfectly developed crystals in the weight range 25-40 mg were used. The calculated 50y0 heights were 20, 18, and 17 om for the powder, pellets, and crystals, respectively. Although the sensitivity appeared to increase slightly as the density of the sample increased, the differencesinvolved were probably not significant. Effect of Thermally Insulating Sample. Table I11 gives the sensitivities of R D X evaluated in the impact machine when the experimental procedure was varied by placing t h e powder either on or between sheets of either good or poor heat-insulating materials. When R D X was tested on 5/0 flint paper from which all the grit had been removed, the 50% height, although higher than when tested on grit paper, was much lower than when tested on the bare anvil. This demonstrated that, although the grit may have caused a r e duction in the 50% height (increased sensitivity), the paper (0.004 in. thick) when used hy itself also caused a large r e duction. Since the paper was a good heat insulator, it was thought to cause an increase in sensitivity by reducing transfer to the anvil of the heat generated by the adiabatic compression of occluded gas in the explosive-the hot spots discussed fully by Bowden and Yoffe (1958a). When the 5/0 paper was reversed, i.e., grit side toward the anvil, the 50% height was Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973 5
SUPERLA WAX 0 110
LCRAWAX C.
-
GRAPHITE\
201
__
TEFLON.
-
-.. \
/ 0
~sUL;u;MyLAR
-
/
)./;,CARBOWAX .'CARBOWAX
\
6000
20M *POLYETHYLENEIDYLAN)
PAM 5 0
10
b
(LEAD A C E T A T E W ~ O ~ Oil
0!2
0!3 d 4 0!5 0!6 S P E C I F I C HEAT OF A D D I T I V E
017
d8
Figure 2. Impact sensitivity of RDX as function of specific heat of additive
then even lower than that obtained with the paper used in the conventional manner. This increased sensitivity may have been caused by the additional heat insulation resulting from the presence of a layer of air in the spaces between grit particles. It is also possible t h a t heat was generated in the grit when the impact forced it against the anvil, thus reducing the thermal gradient across the insulating paper. It is improbable that any grit was forced through the paper by the impact, since an inspection of the flint paper after each nonexplosion failed to disclose any evidence of such behavior a t all impact heights. Results similar to those found with the flint paper from which the grit had been removed were obtained by substituting a 1-in. square of white bond paper of the same thickness (0.004 in.). The 5Oy0 height was even less when the sample was placed between two such squares of bond paper. Since the 5Oy0height was much less than that with the bare striker and anvil in all the above experiments, any cushioning effect of the paper must have been relatively smaller than the effect of thermal insulation. When two squares of bond paper were placed below the powder and two above, the resultant 5oy0 height was somewhat greater than for one square above and one below. Presumably, any increased sensitivity arising from additional heat insulation was less than the effect of the increased cushioning caused by the extra sheets of paper. Placing the samples between thin sheets of a good heat conductor (copper) resulted in a much greater 50% height than that obtained with the bare steel anvil and striker. Although both thermal conduction and cushioning effects could have caused this increase, it is believed that thermal conduction was the more important factor. Any effect of cushioning was lessened by placing each copper sheet in the impact machine and dropping the weight on it from a height of 320 cm to flatten and compress the sheet before it was used. A11 of the data summarized in Table 111 demonstrate that if conditions are changed so that the rate of heat transfer from the R D X to the striker and anvil is decreased, the 50Oj, height will be decreased, i.e., the explosive will be more impact sensitive. Effect of Abrasives. Since the influence of the sandpaper abrasive on impact sensitivity was uncertain, its effect was further examined by mixing with R D X powder various concentrations of the grit from the 5/0 flint paper. The grit was removed by soaking the paper in water, washing, decanting several times to remove the water-soluble glue, 6 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973
and finally drying the residue in a n oven a t 110°C. The abrasive, which ranged in particle size from 36% smaller than 44 p to 9% larger than 350 p , was mixed with RDX powder in concentrations of 0.05, 0.2, 1.0, and 10 w t %. Impact sensitivities were determined on samples placed on the bare anvil or on or between single sheets of bond paper. When the bare anvil was used, the height decreased from a high of 74 cm for the unadulterated powder to only 7 cm for the mixture containing 10% abrasive. Over 80% of this decrease had occurred by the time the abrasive content reached 0.2y0. Copp et al. (1948) also found that the addition of grit sensitized R D X markedly when tested between steel surfaces. When bond paper was used, no such dramatic decrease in 50% height occurred. The values obtained with one (28 em) or two (21 cm) sheets of bond paper for unadulterated R D X (Table 111) may be compared with 14 and 17 cm, respectively, for R D X containing 1% abrasive. The results obtained with R D X containing a n abrasive are attributed to the greater probability of surface hot spots being developed when the grit can rub against both the bare striker and anvil than when it can rub only against the striker (one square of paper). When the grit can rub only against another grit particle (two squares of paper), the probability of generating a surface hot spot is reduced further. Effect of Adsorbed Monolayer of Boundary Lubricant. Levine (1956) had shown t h a t cetyltrimethylammonium bromide (CTAB) could readily be adsorbed as a closepacked monolayer on R D X crystals by a brief immersion in a n aqueous solution of appropriate concentration. Levine and Zisman (1957) had proved that such a monolayer of CTAB adsorbed on glass is an effective boundary lubricant. However, in studying the frictional and hardness properties of large, clean, single crystals of RDX, Bowers and Zisman (1958) demonstrated that it was not possible to detonate the crystals by friction under the most extreme conditions of pressure and speed obtainable with the apparatus employed. Thus, to determine the effectiveness of CTAB in desensitizing R D X through the reduction of friction, it was necessary to coat each particle of the powdered explosive and determine the impact sensitivity. Purified crystals of R D X (supplied by the Xaval Ordnance Laboratory) were pulverized with a remotely controlled mortar grinder and passed through a Xo. 40 sieve (particle size
