68
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 68-72
Conclusions The diffusional model for solvent removal from porous rubber particles in aqueous slurries that was derived from batch laboratory experiments gives a good correlation of data obtained in a commercial-scale, continuous-flow vessel. The model accounted for the effects of the major operating variables. Also, the value of the effective crumb radius, the only parameter in the model, was in reasonable agreement between the batch and continuous experiments. However, three simplifying assumptions were required for the analysis of the continuous flow data: (1)the crumb in the vessel is perfectly mixed; ( 2 ) the rubber concentration is the same in the vessel as in the feed; and (3)the crumb size does not affect R , the effective crumb radius for diffusion. Approximation of the crumb residence time distribution by an exponential probably does not lead to serious error. But for the particular vessels used in this investigation, suspension of buoyant, large particles tends to give lower particle concentrations in the vessel than in the feed and, consequently, assumption 2 leads to overestimation of crumb residence time. This in turn would lead to an underestimation of R, but the magnitude of the effect cannot be easily estimated a t present. The assumption of R independent of crumb size causes little error over a twofold variation in mean particle diameter as long as the parameter D8 / R 2 is greater than about 0.007. For larger size variations or smaller values of D8/R2,the diffusion model should be corrected to account for the decrease in R with crumb size. The same is true if the particle size distribution is very broad. Finally, a set of transport, energy, and material balance equations for a continuous-flow stripper is presented that allows the effect of operating conditions to be estimated and solvent removal to be optimized.
Nomenclature C = solvent concentration in rubber, g/cm3 D = diffusion coefficient, cm2/s Q ( 0 ) = diffusion coefficient at 0 solvent concentration D = integral average diffusion coefficient E, = rubber enthalpy E, = steam enthalpy E , = water enthalpy G = rubber flow, wt/time H = hexane flow rate, wt/time P = stripper pressure, kPa absolute Ph = hexane partial pressure, kPa absolute Ph = hexane vapor pressure, kPa absolute P, = water vapor pressure, kPa absolute r = radial position in spherical particle H = effective crumb radius, cm S = steam flow, &/time t = time, s V = solvent concentration in rubber, volume fraction W = water flow, wt/time X = solvent concentration in rubber, wt % 0 = residence time, s p = rubber density, g/cm3 x = Flory-Huggins parameter
Subscripts e = outlet stream from stripper i = inlet stream to stripper eq = equilibrium value Registry No. Hexane, 110-54-3.
Literature Cited Matthews, F. J.; Fair, J. R.; Barlow, J. W.; Paul, D. R.; Cozewith, C. Ind. Eng. Cbem, Prod. Res. Dev. 1986, preceding paper in this issue. Newman, R. D.; Prausnitz, J. M. AIChE J , 1973, 19, 704.
Received for revieu August 16, 1984 Accepted August 29, 1985
Explosives Synthesis at Los Alamos Michael D. Coburn," Betty W. Harris, Klen-Yin Lee, Mary M. Stineclpher, and Helen H. Hayden Explosives Technology, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
During the past two decades, the explosives synthetic effort at Los Alamos has been directed toward energetic heterocyclic compounds and has produced some useful thermally stable explosives. The recent evolution of crystal-density predictive methods is guiding our current efforts toward ultrahigh performance explosives. Much of our effort is now on the development of new methods for preparing unique nitro compounds that are inaccessible by conventional techniques. This paper will review the Los Alamos synthetic program with emphasis on the development of novel thermally stable explosives, and current efforts to synthesize high-density/performance explosives will be discussed.
Introduction The need for a viable explosives synthesis effort is evident when the historically changing requirements of explosives are considered. Since it generally requires about 10 years from the time a useful new explosive is synthesized to qualify it for application, the synthesis effort must be vigorously pursued now or there may be serious deficiencies in our ability t o meet explosives requirements of the next decade. Since the future requirements are not
defined, an inventory of explosives with a broad range of chemical and physical properties is needed. The common explosives 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX), and pentaerythritol tetranitrate (PETN) were considered adequate for all weapons needs until the requirement for an insensitive high explosive (IHE) was implemented. Fortunately, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), which had
0196-4321/86/1225-0068$01.50/00 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986 69 YH2
No, D e n s i t y (glcm3)
TATB
HMX
1.94
1.90
320+
28
C - J P r e s s u r e (GPa)
31.6
39.5
D e t o n a t i o n V e l o c i t y (km's)
7.99
9.15
Impact S e n s i t i v i t y '
(cm)
s e l t i n g P o i n t (OC) Thermal S t a b i l i t y (DTA) ('C)
'Determined w i t h t h e Los Alamos Type 1 2 m a c h i n e ( 2 . 5 kg weight, s a m p l e o n s a n d p a p e r ) . The 5 0 % p o i n t s o f s e v e r a l common e x p l o s i v e s a r e : PETN, 11 cm; RDX, 2 3 cm; TNT, 160 cm.
F i g u r e 1. Comparison of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) with HMX. ClfNyCl N
y
N
~
~
f
N
fNY NYN NYN ~
~
H
W
)
r
P
k \ H NHPk N
V
I
I1
Ill
315 300 200 1.74 23.8 1.42
334 330 201 1.88 28.5 7.88
275 250
VI
Welting P o i n t (OC) Thermal S t a b i l i t y (DTA)(*C) I m p a c t S e n s i t i v i t y (cm) Crystal Density (g/cm3) C-J Pressure (CPa) D e t o n e t i o n V e l o c i t y (km's)
___
1.80 25.3 7.55
IV 234 185 50 1.82 27.2 7.73
VI1
v 460 350 53 1.75 24.2 7.45
VI 250 235 65 1.79 25.2 7.53
Vlll VI1 245 245 58 1.78 25.4 7.55
PI11 311 300 92 1.77 24.2 7,45
Figure 3. Effect of structure on thermal stability.
H?So4
CI
NHph
Hpk
TPM
Figure 2. Synthesis of 2,4,6-tris(picrylamino)-1,3,5-triazine (TPM). P h = phenyl. Pk = 2,4,64rinitrophenyl (picryl).
been developed at Los Alamos as a thermally stable explosive under the Plowshare project, was noted for its insensitivity and was readily employed in IHE applications. However, the sacrifice of performance for the sake of insensitivity is apparent when we compare the CJ pressure and the detonation velocity of TATB with that of HMX, the most powerful explosive in current use and the explosive that was replaced by TATB for insensitive applications (Figure 1). Thus, the most challenging immediate problem for explosives chemists is to develop an insensitive explosive with performance approaching that of HMX.
Synthesis of Thermally Stable Explosives The thermally stable explosives program at the Los Alamos National Laboratory was stimulated by the Plowshare project, a program for developing peaceful uses for nuclear devices. In most cases the devices were used to stimulate nonproducing natural gas wells, wherein the devices were subjected to severe geothermal environments for various periods. The first compound prepared under this program was 2,4,6-tris (picrylamino)- 1,3,5-triazine or W,N4,N6-tripicrylmelamine (TPM), shown in Figure 2 (Coburn, 1966). T P M has moderate thermal stability, with performance a little better than that of TNT. Subsequently, the heterocyclic nitrogen atoms of T P M were systematically replaced with the C-nitro function to give nitro-substituted tris(picry1amino) derivatives of pyrimidine (11),pyridine (III),and benzene (IV), as shown in Figure 3 (Coburn and Singleton, 1972; Harris et al., 1973). Note that I1 is more thermally stable than T P M (I); however, the thermal stability decreased with further substitution of C-nitro for heterocyclic nitrogen to give 111 and IV in spite of the increased resonance stabilization of the parent ring systems. The decreased thermal stability was suspected to be the result of increased steric crowding about the rings as one proceeds from I1 to IV.This idea was verified when the bulky 4-picrylamino group was removed from 111 to yield V, 2,6-bis(picrylamino)-3,5-dinitropyridine (PYX), the most thermally stable explosive that we have en-
PYx
Figure 4. (PYX).
Synthesis of 2,6-bis(picrylamino)-3,5-dinitropyridine
Table I. Physical a n d Explosive Properties of PYX molecular formula crystal density, g/cm3 melting point, "C thermal stability (DTA), "C vacuum stability, cm3/g impact sensitivity, cm spark sensitivity, J friction sensitivity failure diameter, mm
C17H7N11016 1.75 460 350 0.5/h a t 300 "C 0.9148 h a t 250 "C 0.7/91 days a t 200 "C type 12: 63 type 12B: 84 1.175, 0.076-mm foil negative a t all angles less than 2.54
countered (Baytos, 1975). In each case, removal of a picrylamino or nitro group led to an increase in thermal stability, as illustrated in Figure 3. Hercules, Inc., obtained a license to manufacture T P M under our patent (Coburn, 1968), and they carried its development through the pilot-plant stage. At the completion of their study in 1976, Hercules estimated that they could produce T P M on a large scale for about the cost of RDX. Los Alamos conducted a pilot-plant study of the preparation of PYX according to Figure 4. The study demonstrated that PYX could be produced in high yield from relatively inexpensive starting materials. Some of the physical and explosive properties of PYX are given in Table I. Chemtronics, Inc., is currently producing PYX under license of the Los Alamos patent (Coburn, 1972) in 3000-4000 lb/year quantities for use in thermally stable perforators for oil and gas wells. Recent information indicates that PYX is gradually replacing hexanitrostilbene (HNS) for most commercial thermally stable explosives applications. Some other potentially useful thermally stable explosives synthesized during this period are shown in Figure 5 . P A T 0 (3-picrylamino-1,2,4-triazole) is very inexpensive and is as insensitive as TATB (Coburn, 1969), but its
70
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986 H\
N-N
Figure 7. Sulfilimine oxidation method No, PAT0 310 300 320C 1.82 24.7 7.44
M e l t i n g p o i n t (OC) Thermal S t a b i l i t y (DTA) )C'( I m p a c t S e n s i t i v i t y (cm) C r y s t a l D e n s i t y (gicm3) C - J P r e s s u r e (GPa) D e t o n a t i o n V e l o c i t y (kmis)
ATNI 248 253 50 1.84 33.0 8.56
1
NH2
H",
$-CH3 CH3
XI
Xlll
I
N-
PADP 215 210 26 1.86 25.4 7.62
M e l t i n g P o i n t ('C) Thermal S t a b i l i t y (DTA)('C) I m p a c t S e n s i t i v i t y (cm) C r y s t a l D e n s i t y (gicm3) C - J P r e s s u r e (GPa) D e t o n a t i o n V e l o c i t y (kmls)
BTX
N-
NH2
263 260 33 1.74 23.4 7.17
I
/
s \i
CH3
XI1
CH3
XIV
Figure 8. Preparation of sulfilimines.
Figure 5. Thermally stable explosives for special applications.
,g CH3 CH3
IX
X 1.c7 34.2 9.14
Figure 6. Calculated properties of 2,4,6-trinitro-1,3,5-triazine (IX) and 3,6-dinitro-l,2,4$-tetrazine (X).
performance is much lower than that of TATB. The ammonium salt of 2,4,5-trinitroimidazole (ATNI) (Coburn, 1977) was considered as an ingredient in castable ammonium nitrate compositions. Although ATNI would not form a low-melting eutectic with ammonium nitrate (Cady et al., 19771, this work led us to evaluate the analogous ammonium salts of 3,5-dinitro-1,2,4-triazole and 5-nitrotetrazole, which form useful eutectics with ammonium nitrate that give near-ideal performance (Fowler et al., 1978; Stinecipher and Coburn, 1981; Lee and Coburn, 1983). The other two compounds in Figure 5, 2,6-bis(picrylazo)-3,5-dinitropyridine (PADP) (Coburn, 1974) and 5,7-dinitro-l-picrylbenzotriazole(BTX) (Coburn, 1973), are both thermally stable detonator explosives (Carlson et al., 1976). Synthesis of High-Density Explosives Our current goal is the synthesis of high-density explosives. This effort could lead to the exceptional compound, the explosive with both insensitivity and high performance. Increasing the oxygen balance and heat of formation will generally increase the sensitivity of an explosive as well as the performance; however, increasing the density should do more to improve the performance without increasing the sensitivity because the detonation pressure is proportional to the cube of the density. The recently developed crystal-density predictive methods (Stine, 1981) have been very valuable in directing our efforts toward high-density explosives. These calculations predict unusually high densities for two general
XIV
CH3
xv
,
NH2
w 3
XVI
xvn
Figure 9. Oxidation of 3,6-bis(S,S-dimethylsulfilimino)-l,2,4,5-tetrazine (XIV).
classes of compounds: hydrogen-free nitroheterocycles and polynitro-cage compounds. We have chosen to concentrate on the nitroheterocycles because this is our area of expertise and the US.Army is funding a significant effort in nitro-cage compounds. Two compounds that have been the focus of explosives chemists over several decades are 2,4,6-trinitro-1,3,5-triazine (IX) and 3,6-dinitro-1,2,4,5-tetrazine (X) (Figure 6). Over the past twenty years we have attempted the synthesis of these compounds by conventional techniques without success. We had abandoned hope of obtaining either of these compounds until a new method of converting aminoheterocycles to nitroheterocycles was reported by Taylor et al. (1982). This method involves converting the amine to a sulfilimine, followed by oxidation to the nitro compound (Figure 7). We were optimistic that this method would lead us to nitrotriazines and nitrotetrazines because it was successful in converting 2aminopyrimidine to 2-nitropyrimidine, another conversion that was not possible by other methods. We have prepared the intermediate sulfilimines, 2,4,6tris(dimethylsulfilimino)-1,3,5-triazine(XIII) and 3,6bis(dimethylsulfilimino)-1,2,4,5-tetrazine (XIV), in good yield by treating melamine (XV) and 3,6-diamino1,2,4,5-tetrazine (XII), respectively, with dimethyl sulfide ditriflate (Hartman et al., 1983) (Figure 8). Compound XIV was isolated and characterized as the free sulfilimine, but XI11 could be isolated only as its triflate salt (Coburn et al., 1986). Thus far, we have been able to obtain 3(dimethylsulfilimino)-6-nitroso-1,2,4,5-tetrazine (XV) from the oxidation of XI1 with m-chloroperoxybenzoic acid. Compound XV is a remarkably stable solid that can be
N \o/N Crystal Density
Detonation V e l o d t v
Figure 10. Calculated properties of dinitrofurazan (DNF): crystal density, 1.98 g/cm3; CJ pressure, 41.3 GPa; detonation velocity, 9.20 cm/s.
xvm
XIX
(g/cm3)
xx
(kmls)
1.99 42.2 9.50
41.5 9.16
Figure 13. Calculated properties of pernitrobiheterocycles.
DNAT
DNF
Figure 11. Synthesis of dinitrofurazan (DNF).
1.08
1.98 41.7 9.34
c-J PreSsYre ( m a )
DNF
Figure 14. Calculated properties of 5,5’-dinitr0-3,3’-azo-l,2&triazole (DNAT): crystal density, 1.94 g/cm3; CJ pressure, 35.1 GPa; detonation velocity, 8.65 km/s. Hy+ kNJNH, XXI
XXI
DNBF xeelting Point (OC) Thermal S t a b i l i t y (DTA)(’C) Impact S e n s i t i v i t y (cm) c r y s t e l Density ( p i e d )
254 12 1.92 (calculated)
85
C - J Pressure (GPe) Detonation Velocity (km’s)
1.85 ( f l o a t a t i o n ) 35.6 ( c a l c u l a t e d ) 8.80 (calculated)
Figure 12. Properties of 4,4’-dinitro-3,3’-bifurazan(DNBF).
stored indefinitely in a desiccator at room temperature. Hydrolysis of XV gives poly(azo-1,2,4,54etrazine) (XVII), which is the condensation polymer of 3-amino-6-nitroso1,2,4,5-tetrazine (XVI) (Figure 9). Further oxidation of XV with a variety of oxidants gives dimethyl sulfone as the only isolatable product (Coburn, 1984). We assume that X is also formed during the oxidations but is too unstable to isolate. Another target compound that is predicted to have high density and performance is 3,4-dinitrofurazan (DNF) (Figure 10). Our proposed synthetic route to DNF is shown in Figure 11. Oxidation of 3,4-diaminofurazan (XVIII) with peroxytrifluoroacetic acid gave 3-amino-4nitrofurazan (XIX), which is resistant to further oxidation (Coburn, 1968). However, it was easily converted to the sulfilimine (XX) (Coburn et al., 1986), which can be oxidized with a number of reagents to give dimethylsulfone, but DNF could not be detected in the reaction mixtures with 13C NMR (Coburn, 1984) (Figure 11). We have subsequently examined the natural abundance 13C NMR spectrum of 4,4’-dinitro-3,3’-bifurazan (DNBF) to discover that the nitrocarbons are split by the 14N of the nitro groups such that they can hardly be detected. By contrast, the 14N NMR spectrum of DNBF contains a very sharp singlet (width a t half-height of 12 Hz) corresponding to the nitro nitrogen. In addition, the nitrocarbon of 13Cenriched DNBF appeared as a l,l,l-triplet (J = 19 Hz) in the 13C NMR spectrum, and the 14NNMR signal was split into a doublet with the same coupling constant (Coburn, 1985). These results suggested that 14NNMR spectroscopy may be the tool of choice for detecting DNF in solution. Therefore, the reaction mixture obtained by treating XX with anhydrous peroxytrifluoroacetic acid in dichloromethane was analyzed by 14NNMR spectroscopy to show a sharp singlet (width at half-height of 17 Hz) with about the same chemical shift as that of DNBF. In addition, dimethyl sulfone was identified as the only other product
DNAT
N-DNAT
F i g u r e 15. DNAT).
Synthesis of l,l’-dinitro-3,3’-azo-1,2,4-triazole (N-
Ofl\
/NO2
