Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 304-307
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Molecular Decomposition of Polymeric Binders and Energetic Materials Maurice H. Miles' Department of physics, Washington State University, Pullman, Washington 99 164
Kenneth L. DeVrles Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah 84 1 12
The proven use of ESR to monitor molecular bond rupture in polymers has been applied to explosives, polymeric binders, and their mixtures. The production of free radicals has been proposed as one of the preconditions for subsequent initiation or growth of a self-sustaining chemical reaction. Generation of detectable concentration of free radicals in cyclotetramethylene tetraniiramine (HMX), cyckh'imethylene trinitramine (RDX), triaminotrinitrobenzene (TATB), pentaerythritol tetranitrate (FETN), and polymeric binders loaded with HMX is presented. Bond rupture was attempted with mechanical impact and y irradiation. The stability of free radicals and radical reactions in these systems is investigated. Charged particle detectors have been employed to detect electrons and positive ions during and following fracture of PETN and HMX. The advantages of fracto-emission in investigating bond rupture in these systems is presented.
Introduction Interest has been expressed in the possible role of free radicals in energetic material decomposition (Walker, 1982; Owens and Sharman, 1980; Miles et al., 1983). Special interest is devoted to free radical reactions that may provide the necessary energy release for initiation of fast decomposition in energetic materials. The significant energy released in energetic materials is not generated from the initiating stimulus but is produced from the decomposition and rearrangement of atoms. Semenov (1959) studies the chemical kinetics of explosions. Semenov distinguished between thermal explosions and chain explosions. The detailed atomic kinetics of gaseous reactions is much better understood than corresponding kinetics of energetic materials in the condensed state. In condensed energetic materials energy from the initiating stimulus must be concentrated into small localized regions in order to produce significant local temperature increases. When this local increase in temperature is sufficient, a chemical reaction may commence and the reaction becomes selfsustaining. It has been proposed that at this stage free radicals are involved in the complicated chemistry of these reactions (Walker, 1982; Miles et al., 1983). What is perhaps not appreciated is the possible role of free radicals controlling the very start of the processes that eventually lead to initiation and explosion. If the first important step in initiation is endothermic bond breaking, then diagnostic techniques such as ESR and the newly applied fractoemission studies may have important value in studying energetic materials (Miles and Dickinson, 1982). Sufficient endothermic bond breaking may produce runaway free-radical chain reactions producing initiation of an explosion. Here one views the hot spot more as a consequence than the cause of initiation. The obvious evidence for the free-radical initiation hypothesis would be a rapid increase in free radical concentration upon either warming an activated sample or simply letting it stand for some period of time at a given temperature. The former has been observed in RDX by Stals et al. (1971) and the latter has been observed in both RDX and HMX
maintained near their melting points (Pugh). Explosions follow for some but not all of the observations of free radical increases. In fracto-emission, corresponding evidence would show a freshly fractured surface containing large surface concentration of free radicals producing an increasing fracto-emission of electrons, ions, neutrals, etc., after the fracture event in contrast to the expected immediate decay of such emissions. Experimental Procedure Low-temperature y irradiation of cyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX), triaminotrinitrobenzene (TATB), pentaerythritol tetranitrate (PETN), and polymeric binders containing HMX was conducted using a 5050-Ci cesium source. Samples were sealed in evacuated quartz tubes. The sample tubes were immersed in liquid nitrogen inside a specially constructed irradiation dewar. Samples were irradiated up to doses of 10 Mrad. After irradiation, the sample end of the quartz tube was immersed in a small liquid nitrogen dewar so that the protruding portion of the quartz tube could be heated with a torch. This heating completely removed the y-ray-induced color centers and their associated ESR detectable unpaired electrons from the heated end of the quartz. The quartz tube containing the sample was then immersed in a large liquid nitrogen bath and inverted, allowing the sample to slide down to the color center free end of the quartz tube. The quartz tube containing the sample could then be quickly transferred to the precooled temperature accessory in the microwave cavity of the Varian E-3 ESR spectrometer. Temperature control was achieved using a Varian E-4540 variable-temperature controller. Many organic free radicals are stable at liquid nitrogen temperatures so that by working at low temperatures it is often possible to produce and accumulate large concentrations of radicals. The kinetics and reactivity of these radicals can then be studied upon increasing the temperature. Some information concerning the more reactive radicals that might be produced but are not stable at liquid
0196-4321/84/1223-0304$01.50/00 1984 American Chemical Society
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Figure 2. ESR spectra of polycrystalline PETN measured only 12 min after removal from y-ray source.
Figure 1. ESR spectra of polycaprolactone-10% HMX annealing sequence: (1) 123 K; (2) 143 K,5 min; (3) 153 K, 5 min; (4) 153 K, 10 min; (5)173 K, 5 min.
Figure 3. ESR spectra of Figure 2 with NO2 radical spectrum removed by computer extraction showing the reactive radical spectrum.
nitrogen temperature were obtained by reducing the time between cessation of the irradiation and completion of the ESR measurement. This time was reduced to 12 min. Various techniques for mechanical generation of free radicals were attempted. These methods ranged from mechanical grinding using a dental drill to grinding in an alumina mortar with a pestle. Several mechanical impact methods were performed. A major fraction of the impact work was performed with a Picatinny Arsenal type fall hammer. Sample temperatures for the mechanical radical generation attempts were usually either at room temperature or near liquid nitrogen temperatures. In the fracto-emission technique, small crystals were fractured in a vacuum chamber at a pressure of lo-’ torr. Charged particles were detected with Galileo Electro-Optics Model 4039 channel electron multipliers (CEM). The pulse output (50 ns pulse width) of the CEMs were amplified and fed to 100-mHz discriminators whose outputs were counted by two multichannel scalers (MCS) providing counts vs. time. The CEM could be biased to detect either electrons or positive ions.
this material. Ultraviolet photons, y irradiation, and mechanical work all produce the same very stable singlet ESR signal in TATB. In PETN, y irradiation produced a more reactive radical than the NO2radical. The spectrum for this radical was observed after relatively short but intense y irradiation from the 5050-C source at liquid nitrogen temperature. This radical is converted to the NOz radical quite rapidly even at liquid nitrogen temperatures. Hence the spectra of samples irradiated at liquid nitrogen for extended periods of time w i l l be dominated by the NO2radical. Figure 2 shows the ESR signal about 12 min after cessation of the y irradiation. Computer extraction of the NO2radical from the spectra of Figure 2 results in the spectrum shown in Figure 3. To date we have not been able to identify the radical responsible for this spectrum. The search for mechanically generated radicals was made in a variety of ways. In TATB mechanical stresses were applied by frictional grinding of TATB powders in a standard alumina mortar with a pestle. The TATB was also impacted using a drop hammer. An 8-lb weight was dropped from heights of 18 and 28 in. In this system mechanically generated radicals are identical with radicals produced by radiation (y or ultraviolet rays). Mechanical grinding or impact of polymer binders and polymer binders loaded with HMX produced the peroxy radical. This radical is easily produced, and impact yields detectable concentrations of radicals for peak impact loads as low as 0.4 kbar. Despite extensive systematic effort, mechanical generation of detectable concentrations of free radicals in HMX and RDX powders have not yet been successful. During the work on radicals produced by y rays, occasional “accidents”were encountered wherin radiated HMX and RDX crystalline powders spontaneously exploded. These samples were irradiated up to 10 Mrad at liquid nitrogen temperature accumulating concentrations of radicals in the range of 10l8/g. The “spontaneous explosions” occurred during removal of the quartz tubes from the nitrogen-cooled irradiation dewar usually accompanied by minor mechanical or thermal disturbances of the sealed evacuated samples. Definite explosions were observed even while the sample tubes were still immersed in liquid nitrogen. When the irradiation exposure was reduced from 10 to 6.6 Mrad, the apparent spontaneous explosions ceased entirely. The total number of HMX samples radiated at 10 Mrad was over 30, whereas over 300
Experimental Results The major results produced to date might be summarized as follows: farily large concentrations of NO2radical spectra have been achieved for RDX, PETN, HMX, and polymeric binders containing 50% HMX. The NOz radical in HMX is reactive at low temperatures in the presence of polycaprolactone and polyethylene glycol type binders. With sufficient HMX present (50% or greater) this reaction continues until the binder radicals appear exhausted. This stabilizes further decay of the NO2radical and it will then decay only with significant increase of temperature. For polymeric binders with 10% HMX the NOz radical decays away leaving the polymer radical identical with that produced for irradiation of the binder material only. Figure 1 shows the decay of the HMX radical in the presence of polycaprolactone binder. Low-temperature y irradiation of TATB produces a very stable singlet radical similar to that produced by ultraviolet light irradiation as reported by Britt et al. (1981). Dissolved free radicals in TATB have been reported by Britt et al. (1981) as a simple H-atom adduct of the TATB molecule. The free radical in the solid has not yet been identified. I t was also possible to produce radicals in TATB by mechanical deformation with a fall hammer. To date we have not been able to produce the NO2radical in
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Figure 4. Liquid nitrogen 0.95 kg impact sensitivity of HMX: (0) as received HMX; (A)vacuum liquid nitrogen irradiated, impacted with frozen-in y radicals; (A) vaccum liquid nitrogen irradiation impacted with y radicals removed by annealing.
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were radiated with exposure of 6.6 Mrad. Impact tests of sensitivity of irradiated HMX were performed. The results suggest a marked increase in sensitivity of HMX to impact when irradiated in a vacuum at liquid nitrogen temperature. Figure 4 shows the enhanced sensitivity for cold impact of 20 mg of HMX near liquid nitrogen temperature. Each data point gives the fraction that exploded in twenty attempts. Recently the emission of electrons, positive ions, neutral particles, and photons has been observed from the fracture of many different types of solids (Miles and Dickinson, 1982; Dickinson et al., 1981; Hauser et al., 1983). In hard brittle solids, cracks can propagate with velocities as large as 5000-8000 m/s, and it appears clear that “mechanochemical”rupture of bonds occurs. What is not so evident is that intense particle emission and high electric fields can accompany the fracture events. Further, the emission observed frequently continues in a vacuum for a surprisingly long time after fracture, which is considered evidence for very high reactivity of the fractured surfaces. One solid type which only recently has been investigated for fracto-emission activity is the molecular solid or crystal (Miles and Dickinson, 1982). Small crystals (20 to 50 mg) of PETN and HMX were fractured in a vacuum chamber at a pressure of lo-’ torr. Charged particles were detected with two channel electron multipliers (CEM), Galileo Electro-Optics Model 4039, positioned on opposite sides of the sample at a distance of 1cm. The front of the CEM was biased at +600 V for efficient detection of electrons and at -2500 V for detection of positive ions. Two detectors were used so that both electrons and positive ions could be observed during fracture. Fracture of the small crystals was accomplished by placing them between two parallel metal surfaces in the vacuum system and closing the parallel surfaces by means of a bellows arrangement, crushing the crystals in compression. Thus, the detectors were sampling only a thin layer of compressed materials. On some occasions, after compression, the parallel metal surfaces were opened, increasing the effective area of surface sampled for emission. Both PETN and HMX crystals were fractured in this way. The emission from single crystal HMX was more intense and longer-lasting than that for PETN. The EE decay of HMX is shown in Figure 5a at 0.8 s/channel. The first peak was generated by compression and the second peak occurred when the gap was opened a few millimeters. The emission is seen to last for several minutes. A third sample yielded the emission curve shown in Figure 5b, where the tail of the decay is seen to be quite long lasting. The gap
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Figure 5. Electron emission vs. time for two HMX single crystals on a time scale of 0.8 s/channel.
between the metal surfaces was closed and opened at the arrows marked C and 0, respectively. Conclusions The chemical kinetic understanding of initiation and fast reactions in condensed energetic materials remains for future development. We are not sure of the first step nor can we identify important intermediates at the start or during the reaction. This is in contrast to the detailed molecular understanding of gas-phase reactions (Semenov, 1959; Kaufman, 1982) and the partially successful still promising molecular theory of polymer deformation and fracture pioneered by Zhurkov et al. (1966 and 1972). The commonality of the above phenomena is that original bonds must be broken and new bonds formed. The atomic rearrangements in energetic material transformations are so intensive as to require complete atomization if the reaction proceeded in a single step. It therefore seems necessary for the fast decompositions to proceed via a sequence of elementary steps such that many intermediate species are formed and destroyed as the energetic materials react. It is here that free radicals and radical atoms may play decisive roles. As an example, in the hydrogen-oxygen reaction, it has been calculated that each initial hydrogen radical generates 1015hydrogen radicals or atoms in 0.3 s at 700 K and 11kPa (Pilling, 1975). In other words, this radical chain reaction produces an explosion. Brown et al. studies the growth curves for atoms and radicals in irradiated pure nitrogen and methane at low temperatures (Brown et al., 1962). They obtained evidence for autoaccelerated recombination of atoms through self-heating. As a function of radiation dose the mole percent of nitrogen in solid pure Nz,of hydrogen, and of CH3radicals in frozen CH, increased toward saturation and then dropped abruptly toward zero concentration. They proposed that radical reactions in these systems constituted a branched chain reaction having the potential to become explosively unstable even at very low temperatures. This is a likely initiation mechanism to account for our spontaneous explosions in HMX at liquid nitrogen temperature. Using time-of-flight mass spectrometry, Hauser et al. (1983) have presented evidence that the fracture-induced decomposition products depend upon the energy transmitted to the crack. Low energy fracture produced decomposition similar to thermal decomposition whereas
Ind. Eng. Chem. Prod. Res. Dev., Vol.
high energy fracture involved the breakage of the stronger C-C bond yielding the unstable CH20N02free-radical fragment. Our fracto-emission studies on HMX and PETN and the recent investigation by Hauser et al. are a revival and extension of the work initiated by Fox and Soria-Ruiz (1970 and 1970) and summarized by Alster et al. (1977). Fox studied fracture-induced decompositions by observing the evolution of nitrogen accompanying crack propagation in single crystals of lead azide. The evolved nitrogen gas was detected by mass spectrometry. Fracture induced surface reactions to the extent that almost twenty atomic surface layers decomposed on the fractured surface. Our work presented here contains three unusual observations: the “spontaneous” explosions of irradiated HMX in evacuated quartz tubes while immersed in liquid nitrogen; second, the unusual radical decay and interactions encountered in polymeric binders loaded with HMX; no explanation of this observation is attempted here; and finally, the continued fracto-emission of electrons, ions, and presumably neutrals from fractured surfaces of HMX and PETN for long times after the fracture event. Pace and Moniz (1982) have studied in detail the NOz radical in single-crystal RDX. Their results indicate three different orientations of the NO2 radicals in the crystal lattice. Cleavage of N-N02 groups in HMX and RDX and 0-NO2 groups in PETN result in NO2 radicals having several possible orientations in the crystal lattice. Choi and Bouten (1970) have determined that the hydrogen atoms in HMX are located close to nearby oxygen atoms at distances indicating weak hydrogen bonding. The results of Pace and Moniz (1982) where the oxygen atoms of the NO2 radical remaining fixed by hydrogen bonding appear very reasonable. The decay of the NO2 radical evidence in Figure 1would seem to indicate a remarkable ability of radicals to diffuse and react a t interfaces in HMX-polymer binder systems for temperatures as low as 150 K. Modern fracto-emission techniques offer excellent potential for studying decomposition of energetic materials and explosives. Initiation and “hot spot” may be studied on small samples in controlled and sensitive ways by
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monitoring emissions of charged particles, neutrals, and photons as the sample is subjected to various external excitations. Certain critical questions can best be answered by combining ESR, emission studies, optical, and Fourier transform infrared probes.
Acknowledgment University of Utah portions of this study were supported by the NSF Polymer Program Grant DMR 79-25390. Washington State University portions of this work were supported by the Office of Naval Research. Registry No. HMX, 2691-41-0; RDX, 121-82-4; TATB, 3058386; PETN, 78-11-5; polyethylene glycol (SRU), 25322-68-3; poly(caprolactone) (homopolymer), 24980-41-4; poly(capro1actone)
(SRU), 25248-42-4.
Literature Cited Aisfer, J.; Downs, D. S.; Gora, T.; Iqbai, 2.; Fox, P. G.; Mark, P. I n Energetic Materials”, Fair, H. D.; Walker, R. F., Ed.; Plenum: New York 1977; Vol. 1, Chapter 9. Britt, A. D.; Moniz, W. B.; Chinagas, G. C.; Moore, D. W., Heiler, C. A,, KO, C. L. Propellents Explos. 1081, 6 , 94. Brown D. W.; Florln. R. E.; Wall, L. A. J . Phys. Chem. 1062, 66, 2602. Choi, C. S.; Bouten, H. P. Acta. Ctyst. 1070, 826. 1235. Dlcklnson. J. T.; Donaidson, E. E.; Park, M. K. J . Mat. Sci. 1081, 16, 2897. Fox, P. G. J. SoM State Chem. 1070. 2 , 491. Fox, P. G.; Soria-Ruiz, J. R o c . R. SOC. London, Ser. A 1070, 377, 79. Hauser, H. M.; Field, J. E.: Mohan. V. K. Chem. Phys. Lett. 1083, 99, 66. Kaufman, F. “Nineteenth Symposium (International) on Combustion”; The Combustion Institute: Pittsburgh, 1982; p 1. Miles, M. H.; DeVries, K. L.; Britt, A. D.; Moniz, W. B. Propellants, Explos., fyrotech. 1082, 7, 100. Mlles, M. H.; DeVries, K. L.; Britt, A. D.; Moniz, W. B. Propellants, Explos., pvrotech. 1083, 8 , 49. Miles, M. H.: Dickinson, J. T. Appl. fhys. Lett. 1082, 4 7 , 924. Ownes, F. J.; Sharma, J. J. Appl. fhys. 1080. 4 7 , 1494. Pace, M. D.; Monk, W. B. J. Mag. Reson. 1082, 4 7 , 510. Pllling, M. J. “Reaction Kinetlcs”; Oxford University Press: London, 1975; p 107. Pugh, H. L., Jr., private communication. Semenov, H. N. “Some Problems in Chemical Kinetics and Reactivity”; Princeton University Press: Princeton, 1959; Vol. 1 and 2. Walker, F. L. Propellants, Explos., fyrotech. 1082, 7 , 2. Zhurkov, S. N.; Tomasheuskii, E. E. I n “Proceedings of the Conference on the physical Basis of Yield and Fracture”; Stlckland, A. C., Ed.; Oxford: London, 1966; p 200. Zhurkov, S. N.; Zakrevskyi, V. A,; Korsukov, V. E.; Kuksenko, V. S. Po/ym. Sci. A-2 1072, 70, 1509.
Received for review S e p t e m b e r 19, 1983 Accepted J a n u a r y 16, 1984
