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Effect of pressure and some lead salts on the chemistry of solid propellant combustion
COMBUSTION A N D F L A M E 24, 369-380 (1975)
369
Effect of Pressure and Some Lead Salts on the Chemistry of Solid Propellant Combustion R. A. FIFER and J. A. LANNON Pitman-Dunn Laboratory, Frankford Arsenal, Philadelphia, Pennsylvania 1913 7
Final combustion products have been measured at 1, 20 and 40 atmospheres pressure for laser-ignited thin film samples of nitrocellulose (12.6% N) and 60% NC-40% NG composition, as well as for the single base and double base propellants catalyzed by lead oxide or lead salicylate. Experiments have also been carried out with nitrocellulose samples pretreated with NO2 or water vapor, or burned in an atmosphere containing oxygen. Lead catalysts lead to an increase in CO2 relative to CO at pressures where super rate burning is expected. This is an exothermic process, and explains the higher surface and dark zone temperatures, the higher caloric heat output, and faster burning rates for the leaded propellants. With increasing pressure, CO increases relative to CO2 and NO is reduced to N2. Dissolved • water vapor or pretreatment with NO2 have no measureable effect on the combustion chemistry, but small amounts of O2 catalyze the oxidation of CO to CO2, and the reduction of NO to N2. Mechanisms are proposed to account for these observed features of solid propellant combustion chemistry.
Introduction Most o f the literature o f nitrocellulose-based propellants o f the last 30 years has been concerned with empirical studies o f the effects of propellant composition and burning pressure on such measurable physical properties as burning rate, temperature and caloric heat output. Review articles are available [1, 2] that summarize many o f the results o f these studies. Clearly, however, it would be desirable to have information concerning the sequence o f complex chemical events taking place in the various reaction zones of the burning propellant, since such a mechanism would permit modifications to be made to the propellant composition with predictable results, rather than relying on trial and error techniques as at present. Unfortunately, various experimental difficulties associated with making the required spectroscopic measurements with solid propellants have prevented detailed species concentration profiles from being determined. Consequently, most of what is known has had to be deduced from the results o f experiments involving either final product analysis~ or analysis of products extracted from different zones o f the gas phase reaction. Although a number of studies have been reported employing one or
the other of these general methods [ 3 - 1 0 ] , the results obtained have often been inconclusive or contradictory. In some cases this is due to the fact that .only a small fraction o f the products were analysed. In the case o f experiments involving gas extraction through glass probes, it appears that quenching may not have been fast compared to the rate o f reaction, so that very little change in chemical composition was observed upon moving the probe from one reaction zone to another. Probe experiments are, o f course, limited by the fact that much o f the reaction takes place in the solid phase, which cannot be probed, or very close to the surface in the primary flame ("fizz") zone, which is too thin to be probed, and where the reactions are too fast to be quenched. Finally, some studies have employed analytical techniques that were not quantitative, or not sufficiently sensitive to show the relatively small changes in chemical composition that accompany addition of ballistic modifier or changes of pressure. The results o f those previous studies dealing with the effects of addition of lead modifiers [ 6 10] are particularly confusing. Davis [9] was unable to find any differences between the products of leaded and unleaded double base propellant. Powling, et al. [7] proposed that lead additives Copyright © 1975 by the Combustion Institute Published by American Elsevier Publishing Company, Inc.
370 cause increased reduction of nitric oxide, NO, but their data for the solid propellants showed only a very small effect. Lenchitz and Haywood [10] reported a large increase in CO2 relative to CO in the super burning region but in a more recent paper [8] show data indicating no change in CO2/CO, but rather a large decrease in NO for leaded double base propellant. Although there are slight variations in the compositions of the propellants used by the various above authors, all are essentially nitrocellulose/nitroglycerine double base propellants. The present study makes use of an accurate technique to measure the distribution of species in the final products of combustion. Two phenomena associated with nitrocellulose-based propellants permit useful information to be obtained from final product analysis: (a) the combustion chemistry does not go to thermodynamic equilibrium at pressures of a few hundred psi or less, and (b) lowering the pressure suppresses successively earlier stages of combustion, so that experiments at different pressures permit information to be deduced concerning the sequence of events during the combustion process. We have carried out experiments with both nitrocellulose and double base propellant in order to obtain information about the role of nitroglycerine in the combustion of double base propellants, and in order to determine whether the lead catalysts commonly incorporated in double base propellants principally catalyze the decomposition of the nitrocellulose or the nitroglycerine in the propellant.
Experimental The experimental technique involved infrared analysis of final combustion products for laserignited thin film propellant samples. The propellant samples were burned in a miniature bomb, which also served as the infrared absorption cell for the chemical analysis. This bomb was constructed of stainless steel; it had an 8 cm path length, a 1.83 "cm bore, a total volume (including the valves) of 25 co, and was equipped with two silver chloride windows. Propellant films of 20-25 microns thickness were cast from solutions containing 2% by weight of propellant in acetone. Measured amounts of solution were poured onto a glass plate surrounded with a plexiglass ring. The solvent was then evapo-
R.A. FIFER and J. A. LANNON rated in a slow stream of dry air. The resulting fdms were removed from the glass plate, dried under vacuum at room temperature for several days to remove residual solvent, and stored in dessiccators until used. In the preparation of the leaded films, the lead salicylate (PbSal) was completely'dissolved in the acetone solution, whereas the PbO was only partly dissolved, the remainder being dispersed as fine particles. In the case of the double base films there was some concern that the room temperature vacuum drying might result in a partial loss of nitroglycerin. That is, although the vapor pressure of NG is only 0.25 microns at 20°C [11], there have been reports of significant losses of NG from thin film double base films pumped for short periods of time under vacuum [12]. Experiments were, therefore, performed in which samples of the double base films, dried according to the above procedure, were heated at 90-95°C in a vacuum oven for a day or more, together with control samples of pure NC. The weight loss observed for the double base films typically corresponded to 75% or more of that expected for a 60% NC--40% NG film, indicating that most if not all of the NG remained after the room temperature drying. The nitrocellulose used had a nitrogen content of 12.6% by weight. The nitroglycerine was prepared by standard methods [13], washed, dried and distilled slowly under vacuum, taking only the middle fraction, which was clear and colorless. Normal lead salicylate, Pb(C7 Hs Oa), was prepared by mixing equal volumes of 0.2 M sodium salicylate (pH --- 7) and 0.1 M lead acetate, followed by filtration of the precipitate, washing with water, and drying under vacuum at 50 °C. Commercially available lead oxide (min. 99%) was used without further purification. Accurately weighed pieces of propellant film weighing from 8 to 12 mg were placed in the bomb, which was then evacuated and pressurized to 1, 20 or 40 atm with dry nitrogen. The propellant film was then ignited with a 1-2 sec pulse of light from a 100 W CO2 laser having a beam diameter approximately as large as the bore of the cell (E ,x, 8 cal cm "2 sec"l ). The sample was usually completely burned in 1 sec or less, depending on pressure. The infrared spectrum of the combustion products was then run on a Perkin Elmer Model
CHEMISTRY OF SOLID PROPELLANT COMBUSTION 180 Spectrometer, with a total scan time of onehalf hour. It was found that by using 12 mg or less of propellant in the 25 cc cell, no condensation of water would occur on cooling of the product gases. Experiments demonstrated that the product distribution did not depend on the duration of the laser pulse, or on whether all or just part of the film was hit by the laser. The use of laser heating to initiate the combustion was therefore considered to be merely a convenient means of igniting the samples. Further irradiation of the product gases with the laser after combustion caused no change in product concentrations, indicating that thermal heating of the product gases by the laser was not sufficient to cause reaction. (Of the observed products, only ethylene absorbs near 10.6 microns.) The absorptiQn for selected peaks in the spectra of the product gases was determined using the relation, A = In(loft), where I is the transmittance at the maximum and lo is determined by connecting regions of nonabsorption on either side of the peak of interest. Absorptions were converted to absolute component pressures using calibration spectra obtained at the three pressures, except in the one atmosphere case, which were obtained at 960 Torr total pressure to compensate approximately for the rise in pressure during combustion. The following spectral peaks were used for the quantitative measurement of product concentrations. The absorption bands of CO2 near 2350 cm "~ were usually too intense to be measured; CO2 was therefore measured using the 670 cm "1 peak, as well as at the 650 cm "1 shoulder. The 2120 and 2180 cm"1 band maxima of CO gave identical resuits, indicating that overlap of the latter with the lower frequency component of the 22002240 doublet of N20 did not lead to any significant errors. NO was measured at the 1875 cm"1 (0-0) absorption, and also at the maximum of the band envelope centered at 1905 cm "~ . The 1840 cm "~ band for NO was found to be unreliable due to overlap with water bands in the same region. For most experiments, H20 was determined using the strong peak near 1560 cm "t . CH4 was measured at 3010 cm "~ ; the 1305 cm "1 peak being somewhat less reliable due to overlap with weak N20 bands. HCN, C2H2 and C2H4 were measured at 710, 730 and 950 cm -1 , respec-
371
tively. The absorption doublet centered near 3300 cm "1 is due to C2H2 as well as HCN; the 3300 cm-1 bands were therefore not used for quantitative measurements. Finally, N20 was measured using the very small absorption usually observed near 2240 cm "l . This experimental technique has the advantage that no transfer, or physical or chemical separation of the product gases is required. The principal disadvantage is the inability to measure the homonuclear diatomic molecules (in this case H2 and N2).
Results 1. Qualitative Description of Spectra
Figure 1 shows examples of spectra obtained at 1 atm and 40 atm with nitrocellulose, and the assignment of the observed absorptions in the different spectral regions. (The spectra have been compressed along the abscissa for ease of presentation.) Although the spectra at the two pressures look rather different, this is due as much to the effects of pressure on the absorption spectra of i
,
i
i
I
i
i
i
i
I
i
,
;
i
|
i
i
,
i
I
i
i
i
I
N.C. 112.6%N1 BURNED AT P. 1 ATM
Z < n, I.-
N.C. (12.6 %N) BURNED AT P,40 ATM I , l , q l l l 4000 3O00
i
I
i
i
2000
WAVENUMBER,
i
i
I i 15OO
t
t
i
I
i
i
,
J
500
CM -I
Fig. 1. Infrared spectra of nitrocellulose combustion products at 1 and 40 atm.
372
R . A . F I F E R and J. A. LANNON
the various molecules, as it is to actual changes in the p r o d u c t s with increasing pressure. In every case, the principal products are CO, NO, CO2 and H 2 0 , along w i t h N2 and H2, which are n o t infrared active. A t m u c h lower c o n c e n t r a t i o n s , t h o u g h still very p r o m i n e n t in the spectra, are (HCHO) n , CH4, C2H2, C2H4, HCN and N 2 0 .
These products are qualitatively the same as those reported b y Powling, et al. [7] a n d b y Davis [9], with the following exceptions: We assign bands centered at about 1220, 1360 a n d 1730 cm "I as being due, at least largely, to polymerized and condensed forms o f formaldehyde (i.e., solid paraformaldehyde condensed o n the cell windows).
TABLE 1 Computed Equilibrium Mole Fractionsa
C(s) CO CO2 H20 H2 N2 NO NO 2 N20 CH4
C2H4 C2H2 HCN H OH O 02 CH20 NH 3
C(s) CO CO2 H20 H2 N2 NO NO 2 N20 CH4
C2H4 C2H2
HCN H OH O 02 CH20 NH 3
1000 °K
2000 °K 1 atm
Pure Nitrocellulose (12.6% N) 3000 °K 1000 °K 2000 °K 3000 °K 40 atm
1000 °K
2000 °K 1000 atm
3000 °K
0 0.308 0.237 0.119 0.223 0.112 10"Is 10"26 10 -20 0.0011 10-9 10"11 10"7 10-9 10"12 I0 "20 10-21 10-7 10"s
0 0.402 0.143 0.211 0.131 0.111 10-6 10-12 10-10 10-10 10-16 10-13 10 -'/ 0.0006 0.0001 10"7 10-7 10-a 10-7
0 0.414 0.100 0.180 0.104 0.103 0.0031 10-7 10-7 10 -12 10-19 10-14 10 -8 0.0505 0.0293 0.0089 0.0063 10 "8 10 .7
0.104 0.112 0.333 0.195 0.095 0.120 10-is 10-25 10-19 0.0402 10-7 10-10 10-6 10-1° 10-13 10 -21 10"21 10-6 0.0003
0 0.421 0.120 0.225 0.109 0.110 0.0006 10-7 10-7 10 "9 10-14 10-11 10"6 0.0081 0.0056 0.0002 0.0002 10`6 10 -6
0.160 0.023 0.368 0.245 0.022 0.124 10 -15 10-25 10-18 0.057 10-7 10-11 10-'/ 10"11 10"13 10"22 10"21 10-6 0.0008
0 0.402 0.143 0.211 0.131 0.111 10-7 10-13 10-1o 0.0002 10-7 10-7 10-s 10-s 10-6 10-1o
0 0.423 0.122 0.230 0.111 0.111 0.0001 10-8 10-7 10-6 i0-1o
10-s 0.0003
10-s 0.0017 0.0011 10-s 10-s 10-s 0.0001
0 0.177 0.329 0.195 0.152 0.146 10"15 10-26 10"19 0.0001 10-11 10 "12 10-8 10-9
0 0.263 0.244 0.280 0.067 0.146 10 -s 10-11 10-1o 10 -ll 10-I 8 10-18 10-8 0.0004 0.0002 10-6 10-6 10-8 10-7
0 0.314 0.149 0.213 0.062 0.130 0.0068 10-6 10-7 10 -13 10-20 10-1s 10 -s 0.039 0.045 0.017 0.024 10-8 10-8
60% NC (12.6% 0 0.122 0.385 0.208 0.096 0.156 10-15 10-26 10-19 0.033 10-7 10-1o 10-6 10 -1° 10 -12 10"21 10-21 10-6 0.0003
N)-40% NG 0 0 0.263 0.283 0.244 0.218 0.281 0.284 0.067 0.051 0.146 0.144 10 -6 0.0018 10-11 10-6 10-1o 10-7 10-8 10 "1° 10-13 10-16 i0-I1 i0"12 10 7 10"7 0.0001 0.0056 10-s 0.010 10 -8 0.0007 10 -8 0.0016 10-7 10-7 10-s 10-6
0.051 0.027 0.430 0.254 0.023 0.161 10-1s 10-2s 10-18 0.053 10-7 10"11 10-7 10"11 10-13 10-22 10-21 10-6 0.0008
0 0.263 0.244 0.280 0.067 0.146 10-7 10-12 10-1o lO-S 10 "9 10-9 10-5 lO-S lO-S 10-9 10-9 10-s 0.0001
0 0.279 0.226 0.295 0.050 0.146 0.0004 10-7 10-7 lO-S 10-12 10-9 lO-S 0.0011 0.0022 10-s 0.0001 10-s lO-S
I 0 -12
10-20 10"20 10-8 10"s
aAll other species 10-5 or lower at these temperatures
0 0.403 0.143 0.211 0.132 0.111 10 -7 10-12 i0-1o 10 -7 10-11 10-10 10-6 0.0001 10"5 10-8 10"8 10-6 10-5
10-1o
10-8
CHEMISTRY OF SOLID PROPELLANT COMBUSTION
373
Davis, on the other hand, assigned a band at 1730 cm "I to acetaldehyde, and Powling, et al. do not identify the aldehyde component for their products. We occasionally observed an absorption band system centered at 1100 cm "1 , which appears to be due to small amounts of formic acid. We did not observe NO2 in any of the experiments reported here, except those in which the film was burned in the presence of air. Davis, on the other hand, reported some NO2 for his experiments at 1 arm and below (possibly due to air leaks: 2NO + 02 2NO2), and Powling, et al. observed some NO2 for nitrocellulose at 1 atm. Nitrogen dioxide is widely believed to be the primary product of nitrate ester decomposition, but is evidently rapidly destroyed by secondary reactions in the solid phase and/or fizz zones.
product relative to that for CO, the largest product. This relative basis was chosen because small pieces of propellant were sometimes blown away from the fdm during burning, and did not burn. However, we have shown for each experiment the millimoles of CO observed per gram of propellant, which can be used to convert all of the results to an absolute basis if desired. The concentration of N2 in the products was calculated by assuming that all of the N not accounted for in NO, HCN and N20 is present as N2 ; this obviously represents an upper limit for those cases in which some of the propellant did not burn. Also shown is the millimoles per gram, and the corresponding percentage, for each of the four atomic species not accounted for in the measured products.
2. Equilibrium Calculations
This appears to be the first determination of NC products as a function of pressure. It can be seen that the CO2/CO ratio decreases from 0.46 (0.39) at 1 atm, to 0.28 (0.28) at 20 atm, and 0.24
Theoretical equilibrium compositions have been computed for comparison with the experimental results. These were computed using a free energy minimization procedure [14], and JANNAF thermodynamic data [15] for 51 atomic and molecular species. Figure 2 is a graphical representation of the temperature dependence of the equilibrium products for a double base propellant burned at 40 atm, and Table 1 shows data innumerical form at 1, 40 and 1000 atm for nitrocellulose and double base propellant. It can be seen from Table 1 that the compositions of the major equilibrium products are essentially independent of pressure above 1000°K. With increasing temperature, CO and H20 increase, and CO2 and H2 decrease. The double base equilibrium products contain much more CO2, H20 and N2, and less CO and H2, than for pure NC. This reflects the increased O : C and N: C content upon NG addition. Finally, we note that all of the other species observed experimentally (NO, CH4, C2 H2, C2 Ha, HCN, N20 and HCHO) are completely negligible at equilibrium, and are therefore present in greater than equilibrium amounts during the combustion process.
A. Effect of Pressure
.
.
.
.
__.zo [
3. Quantitative Product Concentrations
Results for 34 product analyses are shown in Table 2. The data are expressed in terms of moles (or concentration or pressure) of each measured
TEMPERATURE. *K
Fig. 2. Computed equilibrium composition for double base propellant products at 40 atm.
374
R . A . FIFER and J. A. LANNON
TABLE 2 Measured Propellant Combustion Productsa 1 Atmosphere Experiments Exp Comp
13 NC
7CO 7CO2 7H20 7NO 7CH4 ')'C2 H 2 ")'C2 H 4 7HCN TN20 7N2
1.0 0.464 0.149 0.827 0.053 0.035 0.036 0.056 0.019 0.088
19 11 35 12 48 46 24 N C + 1 % N C + 2 % N C + 3 % N C + 2 % NC+Air N C + N O 2 N C + N G PbO PbO PbSal PbO+H20
37 40 29 36 NC+NG NC+NG+NC+NG+ NC+NG+ 1% PbO 3%PbSal 3%PbSal
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.454 0.162 0.722 0.082 0.043 0.045 0.064 0.007 0.040
0.494 0.145 0.837 0.057 0.037 0.033 0.063 0.011 0.049
0.460 0.200 0.685 0.105 0.039 0.042 0.069 0.007 0.022
0.507 0.771 0.056 0.029 0.032 0.041 0.014 0.084
2.760 0.407 0 0 0 0 0.082 0 0.80
0.490 0.155 0.712 0.077 0.050 0.039 0.113 0.007 0
0.416 0.216 0.873 0.045 0.050 0.023 0.061 0.006 0.049
0.361 0.231 0.912 0.027 0.038 0.019 0.039 0.010 0.034
0.381 0.243 0.873 0.036 0.037 0.021 0.039 0.006 0.047
0.412 0.200 0.861 0.042 0.052 0.026 0.048 0.004 0.013
0.410 0.237 0.828 0.046 0.047 0.025 0.049 0.005 0.015
Millimoles CO per g 8.21
10.2
8.82
11.1
8.93
5.36
11.1
10.2
10.3
10.5
11.3
11.7
~c ¢o CN CH
7.96/36 12.4/34 1.44/16 21.3/77,
3.84/17 6.57/30 2.16/9.8 6.64/30 7.68/21 10.1/28 5.22/14 0,81/9.0 0.87/9.6 0.50/5.5 1.50/17 17.6/64 20.7/75 15.1/55 -
1.47/7 -0.73/-2 8.56/95 22.9/83
1.52/7 4.83/13 -0.28/-3 16.8/61
1.43/7.7 2.68/14 7.72/20 8.13/22 1.00/9.3 0.70/6.5 16.5/65 17.6/69
Exp Comp
20 NC
20 Atmosphere Experiments 21 10 34 6 9 49 NC+1% NC+1% NC+3% NC+H20 NC+I% NC+Air PbO PbO PbSal PbO + H20
25 NC+NG
39 NC+NG+ 1% PbO
30 NC + NG + 3% PbSal
7 CO 1.0 ")'CO2 0.279 'yH20 0.151 ')'NO 0.452 "yC H 4 0.056 9' C2H2 0.009 '),C2 H 4 0.016 "/HCN 0.061 "/N20 0.014 7N2 0.082
1.0 0.348 0.242 0.567 0.046 0.012 0.017 0.059 0.008 0.151
1.0 0.423 0.234 0.700 0.039 0.011 0.019 0.069 0.010 0.132
1.0 0.389 0.259 0.628 0.024 0.018 0.012 0.040 0.003 0.097
Millimoles CO per g 12.7
1.0 0.353 0.221 0.469 0.078 0.016 0.026 0.079 0.007 0.080
1.0 0.294
2.01/11 -0.28/-1.5 -0.70/-3.8 7.41/20 4.95/13 3.98/11 0.99/9.3 0.29/2.7 0.35/3.2 16.7/66 16.1/63 14.9/59
0.531 0.064 0.008 0.019 0.020 0.024 0.141
1.0 0.379 0.697 0.035 0.009 0.015 0.011 0.017 0.186
1.0 0.402 0.126 0 0.033 0.015 0.018 0.093 0.022 -
1.0 0.284 0.218 0.712 0.019 0.017 0.012 0.049 0.010 0.047
1.0 0.300 0.264 0.664 0.022 0.014 0.011 0.055 0.006 0.141
9.53
8.54
12.5
10.2
8.08
12.3
12.2
10.5
12.3
~c ~o CN
3.60/16 8.63/24 2.10/23 19.2/69
7.64/35 12.4/34 2.88/32 19.9/72
8.43/38 12.5/34 2.25/25 20.9/75
2.11/9.6 6.36/17 2.00/22 15.6/56
7.38/33 2.88/32
10.1/46 3.01/33 -
2.48/11 12.4/34 7.33/81 20.6/74
1.28/6.9 7.06/19 1.15/11 17.5/69
3.45/19 10.9/29 2.98/28 17.5/69
-0.11/-0.6 4.83/13 2.39/22 16.3/64
Exp Comp
14 NC
16 NC
15 NC+1% PbO
32 33 42 26 27 41 43 28 31 N C + 3 % N C + 3 % N C + N G N G + N G NC+NG NC+NG+NC+NG NC+NG+ NC+NC+ PbSal PbSal 1% PbO +l%PbO 3%PbSal 3%PbSal
1.0 0.241 0.201 0.368 0.088 0.009 0.018 0,062 0.010 0.146
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.249 0.214 0.338 0.072 0.007 0.018 0.053 0.012 0.119
0.300 0.195 0.330 0.075 0.009 0.023 0.060 0.007 0.151
0.333 0.201 0.351 0.093 0.014 0.023 0.082 0.0 0.087
0.326 0.217 0.356 0.094 0.013 0.025 0.079 0.0 0.104
0.282 0.251 0.721 0.019 0.014 0.009 0.046 0.009 0.018
0.284 0.255 0.692 0.018 0.015 0.011 0.051 0.012 0.027
0.293 0.228 0.615 0.016 0.012 0.010 0.021 0.010 0.084
0.328 0.276 0.713 0.026 0.012 0.011 0.027 0.008 0.045
0.335 0.275 0.661 0.21 0.013 0.011 0.045 0.006 0.041
0.344 0.302 0.662 0.020 0.013 0.010 0.038 0.007 0.122
0.353 0.304 0.705 0.018 0.014 0.012 0.045 0,006 0.t70
40 Atmosphere Experiments
3'CO ")'CO 2 ")'H20 ,')'NO 7CH4 7C2H 2 "YC2H4 -yHCN 7N20 7N2
CHEMISTRY OF SOLID PROPELLANT COMBUSTION
375
TABLE 2 (continued) Millimoles CO per g 12.1 ~bC ~bO eN ~H
4.54/21 11.4/31 3.53/39 16.7/60
13.8 2.40/11 7.91/22 3.27/36 15.9/57
12.8 2.91/13 9.16/25 3.85/43 16.7/60
14.9
14.0
13.0
13.0
-1.47/-6.7-0.05/-0.20.37/2.0 3.42/9.4 5.18/14 4.53/12 2.57/29 2.91/32 0.46/4.3 13.2/48 13.4/49 16:4/65
0.25/1.3 4.80/13 0.70/6.6 16.2/64
13.0
12.7
13.4
0.70/3.8 6.07/16 2.17/20 17.6/69
0.46/2.5 4.12/11 1.13/11 15.9/63
-0.86/-4.7 2.36/13 2.74/7.3 8.05/21 1.10/10 2.72/25 15.4/61 16.6/65
11.1
9.70 4.25/23 11.3/30, 3.29/31 17.6/69
a,), is [X]/[CO] ; ~bis the millimolesper gram, and percent, of each atomic species lost.
(0.28) at 40 atm, while at the same time the NO/ CO ratio decreases from about 0.8 (0.9) at 1 atm to 0.5 (0.7) and 0.4 (0.65) at 20 and 40 atm, respectively. The values in parentheses are those for the double base propellant. The H20/CO ratio is approximately 0.15 (0.22) at 1 and 20 atm, and 0.21 (0.25) at 40 atm. Of the minor products, C2H2, C2H4 and'N20 (C2H4 and CH4) are significantly reduced in going from 1 to 20 or 40 atm. Although it was not possible to measure (HCHO)n quantitatively, it was observed (see Fig. 1) that there was a decrease in formaldehyde condensed on the windows with increasing pressure. At equilibrium (see Table 1) much higher CO2/ CO ratios are predicted for the double base propellant, due to the higher ratio of oxygen to carbon compared to pure NC: NC C:.232 O:.382 N:.095 H:.291 60% NCC:.201 O:.408 N:.116 H:.275 40% NG However, what is actually found from the analysis of the nonequilibrium products is that the CO2/ CO ratios are about the same as those for pure NC; the higher nitrogen and oxygen content for NG leads instead to an increased NO yield for the double base propellant. Compared to nitrocellulose, the double base propellant yields less CH4 and more C2 H2, particularly at elevated pressures, and significantly more H20. A high percentage (i.e., 25-35) of the carbon initially in the NC is not accounted for in the measured products at 1 atm. At high pressure, where a secondary flame zone is expected, there is a decrease in the amount of carbon "lost". This is reflected in an increased CO yield in going from 1 atm to the higher pressures. The large increase in CO is probably accompanied by a small decrease in CO2. Thus, at high pressures less formaldehyde
and perhaps less soot remain, the increased oxidation of the carbon-containing species, and the increased NO reduction, resulting principally in increased CO production. The carbon "loss" for the double base propellant is apparently smaller, being roughly 10% at 1 atm, 6% at 20 arm and 2% at 4 0 atm. For both propellants, the ratio of H to O "missing" is approximately 2, which is consistent with HCHO formation if the concentration of H2 in the products is small, as indicated by the 1 atm data of Powling, et al. [7]. However, the millimoles per gram of missing carbon is always less than for oxygen, but this may be due to a systematic error in the determination of the six measured carboncontaining species, as well as to unburned propellant and small amounts of HCO2 H, both of which would give carbon to oxygen ratios less than unity. In any case, it appears that soot (i.e., solid carbon) formation is small compared to formaldehyde formation. It is interesting to consider the degree of reduction of NO to N2 as a function of pressure. We find the NO/N2 ratio to be at least 9 at 1 atm, at least 5 at 20 atm, and about 2.5 at 40 atm. This corresponds to at most 18% reduction of NO to N2 at 1 atm and no more than 40% at 40 atm. For the double base propellant only about 10% or less of NO is reduced to N2 at 1 arm and no more than 21% at 40 atm. It therefore appears that NO reduction is far less complete than for NC. These results suggest that the temperatures for burning NC are greater than for the double base propellant at these pressures. Apparently, the solid phase and/or surface reactions are faster for the double base propellant since the burning rate for NC is lower than for the double base propellant even though the surface temperature is higher [5]. Of course, at equilibrium the flame
376 temperature must be lower for NC than for the double base propellant.
B. Effect of Lead Modifiers At one atmosphere, the effects observed upon addition of lead compounds are very small. However, at 20 and 40 atm, for both nitrocellulose and double base propellant, addition of either 1% PbO or 3% Pb Salicylate leads to a significant increase in the CO2/CO ratio, and in most cases to an increase in H2 O/CO. For example, at 40 atm CO2/CO is increased from 0.25 to 0.33 on addition of 3% PbSal to NC, while for the double base propellant it is increased from 0.29 to 0.35 (with H~O/CO increasing from 0.24 to 0.30). The effect is not large enough to determine from the data how CO or COs individually are altered. Similarly, there are fluctuations in the NO yields from one experiment to another which make it impossible to say with certainty whether lead additives lead to increased reduction of NO. It is clear, however, that any increase in NO reduction is quite small for the propellants and pressures examined; certainly less than 10%. It is interesting to note that a close examination of the tabulated data of Powling, et al. [7] shows a small increase in CO2 relative to CO for NC and double base propellants at 1 arm, upon addition of lead catalysts, although no mention was made of it in their paper. Davis [9] likewise was apparently looking for much larger effects and failed to notice this effect in comparing his tabulated data for the products of leaded vs unleaded double base propellant. Lenchitz and Haywood [ i 0 ] , however, have shown product analysis for a propellant of unspecified ("classified") double base composition, which shows a very large increase in the COs/CO ratio in the 300 psi pressure region upon addition of lead stearate, although Lenchitz, et al. report a decrease in NO and no change in COs/CO in a more recent study with a double base propellant
[81. In many cases, the ratio of H to O "lost" appears to be larger for the leaded compounds. This may indicate increased H2 formation for the catalyzed propellants. It is perhaps not surprising that the observed chemical effects of lead catalysts should be
R.A. FIFER and J. A. LANNON smaller at 1 atm than at the higher pressures, since it has been shown [1, 7] that the burning rates of leaded and unleaded double base propellants approach one another as the pressure is lowered towards atmospheric. There is no evidence here of any difference between the effect of PbO and that of the less stable, organic lead salts like PbSal. Powling [7], as a matter of fact, has shown that PbO produces super rate burning curves very much like those for the organic lead compounds. It is significant that lead additives have the same effect qualitatively and quantitatively on nitrocellulose as on the double base propellant, since this suggests that the lead catalyses principally the nitrocellulose decomposition rather than the nitroglycerine decomposition in double base propellants.
C. Effect of Water, NO2 Pretreatment and Oxygen. Dissolved Water. For these experiments, water was admitted to the bomb containing the NC film up to its room temperature vapor pressure, 1 or 20 atm of N2 was added, and the sample then allowed to stand for a couple of hours in order to allow the H20 to dissolve in the propellant. The results in Table 2 show that water addition does not cause significant changes in product concentrations, except perhaps for a decrease in HCN and an increase in N20 at the higher pressures. NO2 Pretreatment. We have found that over a 15-20 hour period, up to 60 Torr (3.7 rag) of NO2 can be consumed by reaction with a 10 mg NC film in the 25 cc cell. The reacted NO2 is converted almost quantitatively to NO, but smaller amounts (2-5 Torr each) of CO2, N20 and H 2 0 are also formed. During this reaction, the film developes a new absorption band at 1740 cm "~ , as well as a broad, unresolved absorption in the 3100-3400 cm "1 region. Qualitatively, these spectral changes are consistent with a number of previous reports [16, 17] indicating that NO2 can oxidize the primary hydroxylic groups in cellulose to carboxylic groups. In these experiments with nitrocellulose (12.6% N), however, we found that the number of NO2 molecules consumed is several times greater than the maximum possible number of unnitrated primary hydroxylic groups, thus indicating a much more complex reaction. The formation of CO2 in-
CHEMISTRY OF SOLID PROPELLANT COMBUSTION dicates that some of the oxidation is degradive in nature. Even for the most heavily treated samples, the concentrations of the major combustion products were not significantly different from those for untreated NC, although the treated samples gave somewhat less N 2 O, and somewhat more CH4 (see exp. #46). This was true even if excess NO2 (1020 Torr) was not removed from the bomb prior to ignition. Thus, under our experimental conditions, we are unable to confirm the results of Dauerman and Tajima [5], who report significant changes in products for thick strands of NC burned at 10 Torr following treatment with 9 Torr of NO2 for only 5 min. Oxygen Catalysis. Dramatic changes in products are observed when NC is burned in air instead of inert gas. At one atmosphere (see #48), CO~/CO is increased from 0.46 to 2.76, NO is completely reduced to N2, and no hydrogen-containing species remain. At 20 atm (1 atm air + 19 atm N2), however, CO2/CO is only slightly increased, and the hydrogen-containing species are not greatly changed (see #49); NO is completely absent, and large amounts of NO2 are observed. The interpretation of these results is as follows. At one atmosphere, oxygen from the air can diffuse into the reacting gases and greatly catalyze the combustion process, leading to complete reduction of NO and greater amounts of CO2 relative to CO. At 20 atm, on the other hand, diffusion is too slow to permit oxygen to enter the flame to any appreciable extent during combustion and very little catalysis occurs. After the combustion, however, the oxygen reacts with the NO formed and converts it to NO2. From experiments with smaller amounts of air, we have found that the principal changes induced by oxygen catalysis at 1 atm can be represented approximately by the overall reaction CO + NO 02 CO2 + ½N2
AH = -89 kcal/mole (I)
(The H20 yield is only slightly increased.) Since at least 7 millimoles of NO are formed per gram of propellant in the absence of 02, the heat output is increased by a minimum of 600 cal/g by oxygen catalysis, or by more than 100%. Clearly, it is the lack of oxygen that prevents the com-
377
bustion of nitrocellulose from going to completion except at very high pressures. Discussion 1. Effect of Catalysts
It is easy to show that the higher CO2/CO ratios do explain the higher energy output for the catalyzed propellant in the super-burning region. This can be seen by writing any reasonable reaction involving conversion of CO to CO2, for example: Pb CO + CO ~ CO2 + C (solid) AH =-41 kcal/mole
(2)
Pb CO + HCHO ~ CO2 + C(solid) + H2 AH = -40 kcal/mole
(3)
Pb C O + N O ~ CO2+½N2 AH = -89 kcal/mole
(4)
That is, every such reaction is very exothermic and will lead to higher temperatures. At first glance, the fact that catalysis by lead compounds is greater at 20 or 40 atm than at 1 atm would suggest that the catalytic activity is located in the secondary flame zone, as suggested by Camp, et al. [18]. However, this requires radiative feedback to the burning surface, and it has been deafly pointed out [1], that this is inconsistent with a large number of experimental fmdings. It appears, therefore, that catalysis takes place mainly in or on the burning surface and/or in the f e z zone. Powling, et al. [7] proposed that lead compounds caused increased NO reduction by carbon in the fizz or dark zones, leading to increased heat transfer to the surface. Lenchitz, et al. [8] have presented some data indicating increased NO reduction for their leaded propellants. However, neither our data, or that of Davis [9], nor indeed that of Powling provides any evidence that this is occurring to any appreciable extent. (However, we cannot rule out the possibility that some NO reduction accompanies the increase in CO2/CO and presumably carbon formation for the leaded propellant.) It appears rather that NO reduction
378 takes place mainly in the secondary flame zone. Lenchitz, et al. hypothesize that lead compounds containing chelating ligands catalyze the initial decomposition of the nitrate ester propellant molecules via an electronic interaction with the nitrate group oxygen atoms. Although interesting, there appears to be no evidence for this hypothesis, and it fails to explain how a nonchelate type lead compound, like PbO, can cause super-rate burning characteristics, as shown by Powling, et
al. [7]. Kubota, et al. [I] have shown that leaded propeUants exhibit a steeper temperature gradient in the fizz zone, and propose that this is due to an increased NO2/aldehyde ratio resulting from increased surface carbon formation. They also proposed that the catalyst does not change the solid phase heat output. They correctly point out that a steeper temperature gradient will lead to greater heat conduction back to the surface. However, it appears to us that carbon formation must be too small to cause an appreciable change in the NO2/ aldehyde ratio. However, carbon formation or increased CO2 production relative to CO is an exothermic process (see Reactions (2) and (3) above), and will lead to higher temperatures for the gases entering the fizz zone, and therefore, to a faster reaction rate in this zone. We would therefore prefer to say that increased solid phase (or surface) heat output, coupled with increased conductive heat transfer to the surface, accounts for the catalytic effect of lead compounds. Salooja [19] has published some interesting results that provide a possible explanation for the catalytic effect of lead compounds in nitrocellulose. based propellants. He showed that whereas lead oxide inhibits the combustion of hydrocarbons, it strongly promotes the combustion of oxygen derivatives of hydrocarbons (e.g., methanol, formaldehyde, methyl acetate). The catalyzed reactions were characterized by enhanced formation of carbon dioxide and in some cases hydrogen, and reduction of lead oxide to elemental lead (which was not an effective catalyst). To explain his observations, Salooja proposed that lead monoxide interacts with the reactant molecule to produce a H-bonded surface complex, which subsequently dissociates. For example, for formaldehyde
R.A. FIFER and J. A. LANNON HN,N / O - - - P C
-~
CO2 + I-I2 + Pb
(5)
(The Pb can be reoxidized to PbO by oxygencontaining radicals, and thereby be available for further reaction.) In the case of hydroxyl compounds, which were even more strongly catalyzed, Salooja proposed that PbO first abstracts the hydroxyl hydrogen atom, producing a radical that loses another hydrogen atom to form an aldehyde, which then further reacts with PbO to produce CO2, as for formaldehyde. If similar reactions catalyze the primary or secondary solid phase decomposition reactions of nitrocellulose, then the higher CO2/CO ratios for the leaded propellants are explained. That all lead compounds except the lead halides appear to act as catalysts is then clear, since almost all lead compounds except the halides can probably form PbO at the temperatures encountered at or just beneath the surface of the burning propellant. This scheme would predict less catalysis for more highly nitrated propellants, assuming that the unnitrated hydroxyl groups of the propellant are directly involved in some rate-determining step of the solid phase decompositions. It would also explain the observation [20] that PbO does not catalyze the gas phase decomposition of ethyl nitrate, which has no hydroxyl or carbonyl groups. Similarly, nitroglycerine decomposition should not be catalyzed, which is consistent with out observation that the chemical effects of lead catalysts are the same for both single and doube base propellants. 2. Effect of Pressure
Our results show that the CO2/CO ratio decreases with increasing pressure. Since this is the opposite of what might be expected, it must be considered in more detail. It has been suggested to us that this might be due to a shift in the "water-gas" equilibrium during cooling of the product gases. This reaction is CO + H20 = CO2 + H2.
(6)
However, an examination of the equilibrium constant for this reaction as a function of temperature
CHEMISTRY OF SOLID PROPELLANT COMBUSTION shows that cooling shifts this equilibrium to the right. Since the reaction is approximately second order with respect to the partial pressures of the reacting species and has an activation energy of about 30 kcal/mole [21 ] any quenching shift for high temperature flame zone products at high pressure would be larger than for low temperature dark zone products at 1 atm. The net effect would be an increase in CO2/CO with increasing pressure, which is the opposite of what is observed. Since less carbon is lost at high pressures, we interpret the apparent increase in CO on going from 1 atm dark zone products to flame zone products at 20 or 40 atm as being due to oxidation of formaldehyde and hydrocarbon species. Since the increase in CO is almost as large as the decrease in NO (at least for NC), it is interesting to hypothesize the following reaction: HCHO + NO ~ CO + H 2 0 + ~ N 2 AH = -78 kcal/mole
(7)
This reaction would also explain the increase in H 2 O, which is observed to accompany the increase in CO with increasing pressure, and is also consistent with the results of Powling, et al. [22, 23], which show that NO is extensively reduced by decomposing formaldehyde in the flames of methyl nitrite and simple dinitrate esters. The much smaller decrease in CO2 with increasing pressure may be due to the reaction C(solid) + CO2 ~ 2CO = 41 kcal/mole
AH
(8)
That is, carbon particles formed via solid phase exothermic reactions are blown off the burning surface by the evolving gases, pass through the secondary flame zone, and are there oxidized by reaction with CO2. Conclusion In conclusion, we have shown that the catalytic effect of lead additives is the same in both nitrocellulose and double base propellants. Lead catalysts lead to an increased CO2/CO ratio in the combustion products, which accounts for the higher energy output and burning rate observed
379
at moderate pressures. Similarly, evidence has been presented which indicates that in the later stages of combustion CO is formed while C02 is di, rninished, and that the reduction of nitric oxide by formaldehyde results in the initial fast temperature rise at the flame front. Dissolved water vapor and pretreatment with NO2 vapor do not significantly affect the combustion chemistry, but small amounts of oxygen can cause the gas phase reactions to go to completion even at 1 atm pressure. We wouM like to thank Dr. Y. Carignan o f Picantinny Arsenal f o r supplying us with the bulk nitrocellulose samples.
References 1. Kubota, N., Ohlemiller, T. J., Caveny. L. H., and Summerfield, M., Princeton U., No. AD 763 786, March 1973.
2. Adams, G. K., Proc. Fourth Syrup. on Naval StructuralMechanics, Purdue University, 1965, p. 117147. 3. Crawford, B. L., Huggett, C. L., and MeBrady, J. J., J. Phys. Colloid Chem. 54, 854 (1950). 4. Heller, C. A., and Gordon, A. S., J. Phys. Chem. 59, 773 (1955). 5. Dauerman, L., and Tajima, Y. A.,A.I.A.A.J. 6, 1468 (1968). 6. Dauerman, L., and Tajima, Y. A.,A.LA.A.J. 6, 678 (1968). 7. Hewkin, D. J., Hicks, J. A., Powling, J., and Watts, H., Comb. Sci. Tech. 2, 307 (1971). 8. Suh, N. P., Adams, G. F., and Lenchitz, C., Combust. Flame 22, 289 (1974). 9. Davis,B. E., Ph.D. Thesis, University of Sheffield, 1970. 10. Lenchitz, C., and Haywood, B., Combust. Flame 10, 140 (1966). 11. Marshall, A.,and Peace, G.,J. Chem. Soc. 109, 298
(1916). 12. Aleksandtov,V. V., and Khlevnoi, S. S., Fiz. Goreniya i Vzryva 4, 438 (1970). 13. Naoum, P., Nitroglycerine and Nitroglycerine Explosives, The Williams and Wilkins Co., Baltimore, 1928, p. 27f. 14. Gordon, S., and McBride, B. J., Computer Program for Calculation of Complex Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman-Jouguet Detonations, NASA SP-273, 1971. 15. Stull, D. R., and Prophet, H., JANAF Thermochemical Tables, National Bureau of Standards, NSRDSNBS 37, 1971.
380 16. Urb~nski, T., Chemistry and Technology of Explosives (W. Ornaf and S. Laverton, trans.), Pergamon Press, 1965, Vol. II, p. 347-355. 17. Yasnitskii, B. G., Dol'berg, E. B., and Oridotoga, V.A.,Zh. Prikl. Khim. 44, 1615 (1971). 18. Camp, A. T., Haussmann, H. K., MeEwan, W. S., Henry, R. A., Olds, R. H., and Besset, E. D., U. S. Naval Ordnance Test Station, NAVORD Report 5824, 1958. 19. Salooja, K. C., Combust. Flame 11,247 (1967). 20. Ellis, W. R., Smythe, B. M., and Theharne, E. D., Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1955, p. 641.
R . A . F I F E R and J. A. LANNON 21. Howard, J. B., Williams, G. C., and Fine, D. H., Fourteenth Symposium/International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 975. 22. Powling, J., and Smith, W. A. W., Combust. Flame 2,157 (1958). 23. Arden, E. A., and Powling, J,, Combust. Flame 2, 55 (1958).
Received 3 September 1974; revised 6 January 1975
369
Effect of Pressure and Some Lead Salts on the Chemistry of Solid Propellant Combustion R. A. FIFER and J. A. LANNON Pitman-Dunn Laboratory, Frankford Arsenal, Philadelphia, Pennsylvania 1913 7
Final combustion products have been measured at 1, 20 and 40 atmospheres pressure for laser-ignited thin film samples of nitrocellulose (12.6% N) and 60% NC-40% NG composition, as well as for the single base and double base propellants catalyzed by lead oxide or lead salicylate. Experiments have also been carried out with nitrocellulose samples pretreated with NO2 or water vapor, or burned in an atmosphere containing oxygen. Lead catalysts lead to an increase in CO2 relative to CO at pressures where super rate burning is expected. This is an exothermic process, and explains the higher surface and dark zone temperatures, the higher caloric heat output, and faster burning rates for the leaded propellants. With increasing pressure, CO increases relative to CO2 and NO is reduced to N2. Dissolved • water vapor or pretreatment with NO2 have no measureable effect on the combustion chemistry, but small amounts of O2 catalyze the oxidation of CO to CO2, and the reduction of NO to N2. Mechanisms are proposed to account for these observed features of solid propellant combustion chemistry.
Introduction Most o f the literature o f nitrocellulose-based propellants o f the last 30 years has been concerned with empirical studies o f the effects of propellant composition and burning pressure on such measurable physical properties as burning rate, temperature and caloric heat output. Review articles are available [1, 2] that summarize many o f the results o f these studies. Clearly, however, it would be desirable to have information concerning the sequence o f complex chemical events taking place in the various reaction zones of the burning propellant, since such a mechanism would permit modifications to be made to the propellant composition with predictable results, rather than relying on trial and error techniques as at present. Unfortunately, various experimental difficulties associated with making the required spectroscopic measurements with solid propellants have prevented detailed species concentration profiles from being determined. Consequently, most of what is known has had to be deduced from the results o f experiments involving either final product analysis~ or analysis of products extracted from different zones o f the gas phase reaction. Although a number of studies have been reported employing one or
the other of these general methods [ 3 - 1 0 ] , the results obtained have often been inconclusive or contradictory. In some cases this is due to the fact that .only a small fraction o f the products were analysed. In the case o f experiments involving gas extraction through glass probes, it appears that quenching may not have been fast compared to the rate o f reaction, so that very little change in chemical composition was observed upon moving the probe from one reaction zone to another. Probe experiments are, o f course, limited by the fact that much o f the reaction takes place in the solid phase, which cannot be probed, or very close to the surface in the primary flame ("fizz") zone, which is too thin to be probed, and where the reactions are too fast to be quenched. Finally, some studies have employed analytical techniques that were not quantitative, or not sufficiently sensitive to show the relatively small changes in chemical composition that accompany addition of ballistic modifier or changes of pressure. The results o f those previous studies dealing with the effects of addition of lead modifiers [ 6 10] are particularly confusing. Davis [9] was unable to find any differences between the products of leaded and unleaded double base propellant. Powling, et al. [7] proposed that lead additives Copyright © 1975 by the Combustion Institute Published by American Elsevier Publishing Company, Inc.
370 cause increased reduction of nitric oxide, NO, but their data for the solid propellants showed only a very small effect. Lenchitz and Haywood [10] reported a large increase in CO2 relative to CO in the super burning region but in a more recent paper [8] show data indicating no change in CO2/CO, but rather a large decrease in NO for leaded double base propellant. Although there are slight variations in the compositions of the propellants used by the various above authors, all are essentially nitrocellulose/nitroglycerine double base propellants. The present study makes use of an accurate technique to measure the distribution of species in the final products of combustion. Two phenomena associated with nitrocellulose-based propellants permit useful information to be obtained from final product analysis: (a) the combustion chemistry does not go to thermodynamic equilibrium at pressures of a few hundred psi or less, and (b) lowering the pressure suppresses successively earlier stages of combustion, so that experiments at different pressures permit information to be deduced concerning the sequence of events during the combustion process. We have carried out experiments with both nitrocellulose and double base propellant in order to obtain information about the role of nitroglycerine in the combustion of double base propellants, and in order to determine whether the lead catalysts commonly incorporated in double base propellants principally catalyze the decomposition of the nitrocellulose or the nitroglycerine in the propellant.
Experimental The experimental technique involved infrared analysis of final combustion products for laserignited thin film propellant samples. The propellant samples were burned in a miniature bomb, which also served as the infrared absorption cell for the chemical analysis. This bomb was constructed of stainless steel; it had an 8 cm path length, a 1.83 "cm bore, a total volume (including the valves) of 25 co, and was equipped with two silver chloride windows. Propellant films of 20-25 microns thickness were cast from solutions containing 2% by weight of propellant in acetone. Measured amounts of solution were poured onto a glass plate surrounded with a plexiglass ring. The solvent was then evapo-
R.A. FIFER and J. A. LANNON rated in a slow stream of dry air. The resulting fdms were removed from the glass plate, dried under vacuum at room temperature for several days to remove residual solvent, and stored in dessiccators until used. In the preparation of the leaded films, the lead salicylate (PbSal) was completely'dissolved in the acetone solution, whereas the PbO was only partly dissolved, the remainder being dispersed as fine particles. In the case of the double base films there was some concern that the room temperature vacuum drying might result in a partial loss of nitroglycerin. That is, although the vapor pressure of NG is only 0.25 microns at 20°C [11], there have been reports of significant losses of NG from thin film double base films pumped for short periods of time under vacuum [12]. Experiments were, therefore, performed in which samples of the double base films, dried according to the above procedure, were heated at 90-95°C in a vacuum oven for a day or more, together with control samples of pure NC. The weight loss observed for the double base films typically corresponded to 75% or more of that expected for a 60% NC--40% NG film, indicating that most if not all of the NG remained after the room temperature drying. The nitrocellulose used had a nitrogen content of 12.6% by weight. The nitroglycerine was prepared by standard methods [13], washed, dried and distilled slowly under vacuum, taking only the middle fraction, which was clear and colorless. Normal lead salicylate, Pb(C7 Hs Oa), was prepared by mixing equal volumes of 0.2 M sodium salicylate (pH --- 7) and 0.1 M lead acetate, followed by filtration of the precipitate, washing with water, and drying under vacuum at 50 °C. Commercially available lead oxide (min. 99%) was used without further purification. Accurately weighed pieces of propellant film weighing from 8 to 12 mg were placed in the bomb, which was then evacuated and pressurized to 1, 20 or 40 atm with dry nitrogen. The propellant film was then ignited with a 1-2 sec pulse of light from a 100 W CO2 laser having a beam diameter approximately as large as the bore of the cell (E ,x, 8 cal cm "2 sec"l ). The sample was usually completely burned in 1 sec or less, depending on pressure. The infrared spectrum of the combustion products was then run on a Perkin Elmer Model
CHEMISTRY OF SOLID PROPELLANT COMBUSTION 180 Spectrometer, with a total scan time of onehalf hour. It was found that by using 12 mg or less of propellant in the 25 cc cell, no condensation of water would occur on cooling of the product gases. Experiments demonstrated that the product distribution did not depend on the duration of the laser pulse, or on whether all or just part of the film was hit by the laser. The use of laser heating to initiate the combustion was therefore considered to be merely a convenient means of igniting the samples. Further irradiation of the product gases with the laser after combustion caused no change in product concentrations, indicating that thermal heating of the product gases by the laser was not sufficient to cause reaction. (Of the observed products, only ethylene absorbs near 10.6 microns.) The absorptiQn for selected peaks in the spectra of the product gases was determined using the relation, A = In(loft), where I is the transmittance at the maximum and lo is determined by connecting regions of nonabsorption on either side of the peak of interest. Absorptions were converted to absolute component pressures using calibration spectra obtained at the three pressures, except in the one atmosphere case, which were obtained at 960 Torr total pressure to compensate approximately for the rise in pressure during combustion. The following spectral peaks were used for the quantitative measurement of product concentrations. The absorption bands of CO2 near 2350 cm "~ were usually too intense to be measured; CO2 was therefore measured using the 670 cm "1 peak, as well as at the 650 cm "1 shoulder. The 2120 and 2180 cm"1 band maxima of CO gave identical resuits, indicating that overlap of the latter with the lower frequency component of the 22002240 doublet of N20 did not lead to any significant errors. NO was measured at the 1875 cm"1 (0-0) absorption, and also at the maximum of the band envelope centered at 1905 cm "~ . The 1840 cm "~ band for NO was found to be unreliable due to overlap with water bands in the same region. For most experiments, H20 was determined using the strong peak near 1560 cm "t . CH4 was measured at 3010 cm "~ ; the 1305 cm "1 peak being somewhat less reliable due to overlap with weak N20 bands. HCN, C2H2 and C2H4 were measured at 710, 730 and 950 cm -1 , respec-
371
tively. The absorption doublet centered near 3300 cm "1 is due to C2H2 as well as HCN; the 3300 cm-1 bands were therefore not used for quantitative measurements. Finally, N20 was measured using the very small absorption usually observed near 2240 cm "l . This experimental technique has the advantage that no transfer, or physical or chemical separation of the product gases is required. The principal disadvantage is the inability to measure the homonuclear diatomic molecules (in this case H2 and N2).
Results 1. Qualitative Description of Spectra
Figure 1 shows examples of spectra obtained at 1 atm and 40 atm with nitrocellulose, and the assignment of the observed absorptions in the different spectral regions. (The spectra have been compressed along the abscissa for ease of presentation.) Although the spectra at the two pressures look rather different, this is due as much to the effects of pressure on the absorption spectra of i
,
i
i
I
i
i
i
i
I
i
,
;
i
|
i
i
,
i
I
i
i
i
I
N.C. 112.6%N1 BURNED AT P. 1 ATM
Z < n, I.-
N.C. (12.6 %N) BURNED AT P,40 ATM I , l , q l l l 4000 3O00
i
I
i
i
2000
WAVENUMBER,
i
i
I i 15OO
t
t
i
I
i
i
,
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500
CM -I
Fig. 1. Infrared spectra of nitrocellulose combustion products at 1 and 40 atm.
372
R . A . F I F E R and J. A. LANNON
the various molecules, as it is to actual changes in the p r o d u c t s with increasing pressure. In every case, the principal products are CO, NO, CO2 and H 2 0 , along w i t h N2 and H2, which are n o t infrared active. A t m u c h lower c o n c e n t r a t i o n s , t h o u g h still very p r o m i n e n t in the spectra, are (HCHO) n , CH4, C2H2, C2H4, HCN and N 2 0 .
These products are qualitatively the same as those reported b y Powling, et al. [7] a n d b y Davis [9], with the following exceptions: We assign bands centered at about 1220, 1360 a n d 1730 cm "I as being due, at least largely, to polymerized and condensed forms o f formaldehyde (i.e., solid paraformaldehyde condensed o n the cell windows).
TABLE 1 Computed Equilibrium Mole Fractionsa
C(s) CO CO2 H20 H2 N2 NO NO 2 N20 CH4
C2H4 C2H2 HCN H OH O 02 CH20 NH 3
C(s) CO CO2 H20 H2 N2 NO NO 2 N20 CH4
C2H4 C2H2
HCN H OH O 02 CH20 NH 3
1000 °K
2000 °K 1 atm
Pure Nitrocellulose (12.6% N) 3000 °K 1000 °K 2000 °K 3000 °K 40 atm
1000 °K
2000 °K 1000 atm
3000 °K
0 0.308 0.237 0.119 0.223 0.112 10"Is 10"26 10 -20 0.0011 10-9 10"11 10"7 10-9 10"12 I0 "20 10-21 10-7 10"s
0 0.402 0.143 0.211 0.131 0.111 10-6 10-12 10-10 10-10 10-16 10-13 10 -'/ 0.0006 0.0001 10"7 10-7 10-a 10-7
0 0.414 0.100 0.180 0.104 0.103 0.0031 10-7 10-7 10 -12 10-19 10-14 10 -8 0.0505 0.0293 0.0089 0.0063 10 "8 10 .7
0.104 0.112 0.333 0.195 0.095 0.120 10-is 10-25 10-19 0.0402 10-7 10-10 10-6 10-1° 10-13 10 -21 10"21 10-6 0.0003
0 0.421 0.120 0.225 0.109 0.110 0.0006 10-7 10-7 10 "9 10-14 10-11 10"6 0.0081 0.0056 0.0002 0.0002 10`6 10 -6
0.160 0.023 0.368 0.245 0.022 0.124 10 -15 10-25 10-18 0.057 10-7 10-11 10-'/ 10"11 10"13 10"22 10"21 10-6 0.0008
0 0.402 0.143 0.211 0.131 0.111 10-7 10-13 10-1o 0.0002 10-7 10-7 10-s 10-s 10-6 10-1o
0 0.423 0.122 0.230 0.111 0.111 0.0001 10-8 10-7 10-6 i0-1o
10-s 0.0003
10-s 0.0017 0.0011 10-s 10-s 10-s 0.0001
0 0.177 0.329 0.195 0.152 0.146 10"15 10-26 10"19 0.0001 10-11 10 "12 10-8 10-9
0 0.263 0.244 0.280 0.067 0.146 10 -s 10-11 10-1o 10 -ll 10-I 8 10-18 10-8 0.0004 0.0002 10-6 10-6 10-8 10-7
0 0.314 0.149 0.213 0.062 0.130 0.0068 10-6 10-7 10 -13 10-20 10-1s 10 -s 0.039 0.045 0.017 0.024 10-8 10-8
60% NC (12.6% 0 0.122 0.385 0.208 0.096 0.156 10-15 10-26 10-19 0.033 10-7 10-1o 10-6 10 -1° 10 -12 10"21 10-21 10-6 0.0003
N)-40% NG 0 0 0.263 0.283 0.244 0.218 0.281 0.284 0.067 0.051 0.146 0.144 10 -6 0.0018 10-11 10-6 10-1o 10-7 10-8 10 "1° 10-13 10-16 i0-I1 i0"12 10 7 10"7 0.0001 0.0056 10-s 0.010 10 -8 0.0007 10 -8 0.0016 10-7 10-7 10-s 10-6
0.051 0.027 0.430 0.254 0.023 0.161 10-1s 10-2s 10-18 0.053 10-7 10"11 10-7 10"11 10-13 10-22 10-21 10-6 0.0008
0 0.263 0.244 0.280 0.067 0.146 10-7 10-12 10-1o lO-S 10 "9 10-9 10-5 lO-S lO-S 10-9 10-9 10-s 0.0001
0 0.279 0.226 0.295 0.050 0.146 0.0004 10-7 10-7 lO-S 10-12 10-9 lO-S 0.0011 0.0022 10-s 0.0001 10-s lO-S
I 0 -12
10-20 10"20 10-8 10"s
aAll other species 10-5 or lower at these temperatures
0 0.403 0.143 0.211 0.132 0.111 10 -7 10-12 i0-1o 10 -7 10-11 10-10 10-6 0.0001 10"5 10-8 10"8 10-6 10-5
10-1o
10-8
CHEMISTRY OF SOLID PROPELLANT COMBUSTION
373
Davis, on the other hand, assigned a band at 1730 cm "I to acetaldehyde, and Powling, et al. do not identify the aldehyde component for their products. We occasionally observed an absorption band system centered at 1100 cm "1 , which appears to be due to small amounts of formic acid. We did not observe NO2 in any of the experiments reported here, except those in which the film was burned in the presence of air. Davis, on the other hand, reported some NO2 for his experiments at 1 arm and below (possibly due to air leaks: 2NO + 02 2NO2), and Powling, et al. observed some NO2 for nitrocellulose at 1 atm. Nitrogen dioxide is widely believed to be the primary product of nitrate ester decomposition, but is evidently rapidly destroyed by secondary reactions in the solid phase and/or fizz zones.
product relative to that for CO, the largest product. This relative basis was chosen because small pieces of propellant were sometimes blown away from the fdm during burning, and did not burn. However, we have shown for each experiment the millimoles of CO observed per gram of propellant, which can be used to convert all of the results to an absolute basis if desired. The concentration of N2 in the products was calculated by assuming that all of the N not accounted for in NO, HCN and N20 is present as N2 ; this obviously represents an upper limit for those cases in which some of the propellant did not burn. Also shown is the millimoles per gram, and the corresponding percentage, for each of the four atomic species not accounted for in the measured products.
2. Equilibrium Calculations
This appears to be the first determination of NC products as a function of pressure. It can be seen that the CO2/CO ratio decreases from 0.46 (0.39) at 1 atm, to 0.28 (0.28) at 20 atm, and 0.24
Theoretical equilibrium compositions have been computed for comparison with the experimental results. These were computed using a free energy minimization procedure [14], and JANNAF thermodynamic data [15] for 51 atomic and molecular species. Figure 2 is a graphical representation of the temperature dependence of the equilibrium products for a double base propellant burned at 40 atm, and Table 1 shows data innumerical form at 1, 40 and 1000 atm for nitrocellulose and double base propellant. It can be seen from Table 1 that the compositions of the major equilibrium products are essentially independent of pressure above 1000°K. With increasing temperature, CO and H20 increase, and CO2 and H2 decrease. The double base equilibrium products contain much more CO2, H20 and N2, and less CO and H2, than for pure NC. This reflects the increased O : C and N: C content upon NG addition. Finally, we note that all of the other species observed experimentally (NO, CH4, C2 H2, C2 Ha, HCN, N20 and HCHO) are completely negligible at equilibrium, and are therefore present in greater than equilibrium amounts during the combustion process.
A. Effect of Pressure
.
.
.
.
__.zo [
3. Quantitative Product Concentrations
Results for 34 product analyses are shown in Table 2. The data are expressed in terms of moles (or concentration or pressure) of each measured
TEMPERATURE. *K
Fig. 2. Computed equilibrium composition for double base propellant products at 40 atm.
374
R . A . FIFER and J. A. LANNON
TABLE 2 Measured Propellant Combustion Productsa 1 Atmosphere Experiments Exp Comp
13 NC
7CO 7CO2 7H20 7NO 7CH4 ')'C2 H 2 ")'C2 H 4 7HCN TN20 7N2
1.0 0.464 0.149 0.827 0.053 0.035 0.036 0.056 0.019 0.088
19 11 35 12 48 46 24 N C + 1 % N C + 2 % N C + 3 % N C + 2 % NC+Air N C + N O 2 N C + N G PbO PbO PbSal PbO+H20
37 40 29 36 NC+NG NC+NG+NC+NG+ NC+NG+ 1% PbO 3%PbSal 3%PbSal
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.454 0.162 0.722 0.082 0.043 0.045 0.064 0.007 0.040
0.494 0.145 0.837 0.057 0.037 0.033 0.063 0.011 0.049
0.460 0.200 0.685 0.105 0.039 0.042 0.069 0.007 0.022
0.507 0.771 0.056 0.029 0.032 0.041 0.014 0.084
2.760 0.407 0 0 0 0 0.082 0 0.80
0.490 0.155 0.712 0.077 0.050 0.039 0.113 0.007 0
0.416 0.216 0.873 0.045 0.050 0.023 0.061 0.006 0.049
0.361 0.231 0.912 0.027 0.038 0.019 0.039 0.010 0.034
0.381 0.243 0.873 0.036 0.037 0.021 0.039 0.006 0.047
0.412 0.200 0.861 0.042 0.052 0.026 0.048 0.004 0.013
0.410 0.237 0.828 0.046 0.047 0.025 0.049 0.005 0.015
Millimoles CO per g 8.21
10.2
8.82
11.1
8.93
5.36
11.1
10.2
10.3
10.5
11.3
11.7
~c ¢o CN CH
7.96/36 12.4/34 1.44/16 21.3/77,
3.84/17 6.57/30 2.16/9.8 6.64/30 7.68/21 10.1/28 5.22/14 0,81/9.0 0.87/9.6 0.50/5.5 1.50/17 17.6/64 20.7/75 15.1/55 -
1.47/7 -0.73/-2 8.56/95 22.9/83
1.52/7 4.83/13 -0.28/-3 16.8/61
1.43/7.7 2.68/14 7.72/20 8.13/22 1.00/9.3 0.70/6.5 16.5/65 17.6/69
Exp Comp
20 NC
20 Atmosphere Experiments 21 10 34 6 9 49 NC+1% NC+1% NC+3% NC+H20 NC+I% NC+Air PbO PbO PbSal PbO + H20
25 NC+NG
39 NC+NG+ 1% PbO
30 NC + NG + 3% PbSal
7 CO 1.0 ")'CO2 0.279 'yH20 0.151 ')'NO 0.452 "yC H 4 0.056 9' C2H2 0.009 '),C2 H 4 0.016 "/HCN 0.061 "/N20 0.014 7N2 0.082
1.0 0.348 0.242 0.567 0.046 0.012 0.017 0.059 0.008 0.151
1.0 0.423 0.234 0.700 0.039 0.011 0.019 0.069 0.010 0.132
1.0 0.389 0.259 0.628 0.024 0.018 0.012 0.040 0.003 0.097
Millimoles CO per g 12.7
1.0 0.353 0.221 0.469 0.078 0.016 0.026 0.079 0.007 0.080
1.0 0.294
2.01/11 -0.28/-1.5 -0.70/-3.8 7.41/20 4.95/13 3.98/11 0.99/9.3 0.29/2.7 0.35/3.2 16.7/66 16.1/63 14.9/59
0.531 0.064 0.008 0.019 0.020 0.024 0.141
1.0 0.379 0.697 0.035 0.009 0.015 0.011 0.017 0.186
1.0 0.402 0.126 0 0.033 0.015 0.018 0.093 0.022 -
1.0 0.284 0.218 0.712 0.019 0.017 0.012 0.049 0.010 0.047
1.0 0.300 0.264 0.664 0.022 0.014 0.011 0.055 0.006 0.141
9.53
8.54
12.5
10.2
8.08
12.3
12.2
10.5
12.3
~c ~o CN
3.60/16 8.63/24 2.10/23 19.2/69
7.64/35 12.4/34 2.88/32 19.9/72
8.43/38 12.5/34 2.25/25 20.9/75
2.11/9.6 6.36/17 2.00/22 15.6/56
7.38/33 2.88/32
10.1/46 3.01/33 -
2.48/11 12.4/34 7.33/81 20.6/74
1.28/6.9 7.06/19 1.15/11 17.5/69
3.45/19 10.9/29 2.98/28 17.5/69
-0.11/-0.6 4.83/13 2.39/22 16.3/64
Exp Comp
14 NC
16 NC
15 NC+1% PbO
32 33 42 26 27 41 43 28 31 N C + 3 % N C + 3 % N C + N G N G + N G NC+NG NC+NG+NC+NG NC+NG+ NC+NC+ PbSal PbSal 1% PbO +l%PbO 3%PbSal 3%PbSal
1.0 0.241 0.201 0.368 0.088 0.009 0.018 0,062 0.010 0.146
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.249 0.214 0.338 0.072 0.007 0.018 0.053 0.012 0.119
0.300 0.195 0.330 0.075 0.009 0.023 0.060 0.007 0.151
0.333 0.201 0.351 0.093 0.014 0.023 0.082 0.0 0.087
0.326 0.217 0.356 0.094 0.013 0.025 0.079 0.0 0.104
0.282 0.251 0.721 0.019 0.014 0.009 0.046 0.009 0.018
0.284 0.255 0.692 0.018 0.015 0.011 0.051 0.012 0.027
0.293 0.228 0.615 0.016 0.012 0.010 0.021 0.010 0.084
0.328 0.276 0.713 0.026 0.012 0.011 0.027 0.008 0.045
0.335 0.275 0.661 0.21 0.013 0.011 0.045 0.006 0.041
0.344 0.302 0.662 0.020 0.013 0.010 0.038 0.007 0.122
0.353 0.304 0.705 0.018 0.014 0.012 0.045 0,006 0.t70
40 Atmosphere Experiments
3'CO ")'CO 2 ")'H20 ,')'NO 7CH4 7C2H 2 "YC2H4 -yHCN 7N20 7N2
CHEMISTRY OF SOLID PROPELLANT COMBUSTION
375
TABLE 2 (continued) Millimoles CO per g 12.1 ~bC ~bO eN ~H
4.54/21 11.4/31 3.53/39 16.7/60
13.8 2.40/11 7.91/22 3.27/36 15.9/57
12.8 2.91/13 9.16/25 3.85/43 16.7/60
14.9
14.0
13.0
13.0
-1.47/-6.7-0.05/-0.20.37/2.0 3.42/9.4 5.18/14 4.53/12 2.57/29 2.91/32 0.46/4.3 13.2/48 13.4/49 16:4/65
0.25/1.3 4.80/13 0.70/6.6 16.2/64
13.0
12.7
13.4
0.70/3.8 6.07/16 2.17/20 17.6/69
0.46/2.5 4.12/11 1.13/11 15.9/63
-0.86/-4.7 2.36/13 2.74/7.3 8.05/21 1.10/10 2.72/25 15.4/61 16.6/65
11.1
9.70 4.25/23 11.3/30, 3.29/31 17.6/69
a,), is [X]/[CO] ; ~bis the millimolesper gram, and percent, of each atomic species lost.
(0.28) at 40 atm, while at the same time the NO/ CO ratio decreases from about 0.8 (0.9) at 1 atm to 0.5 (0.7) and 0.4 (0.65) at 20 and 40 atm, respectively. The values in parentheses are those for the double base propellant. The H20/CO ratio is approximately 0.15 (0.22) at 1 and 20 atm, and 0.21 (0.25) at 40 atm. Of the minor products, C2H2, C2H4 and'N20 (C2H4 and CH4) are significantly reduced in going from 1 to 20 or 40 atm. Although it was not possible to measure (HCHO)n quantitatively, it was observed (see Fig. 1) that there was a decrease in formaldehyde condensed on the windows with increasing pressure. At equilibrium (see Table 1) much higher CO2/ CO ratios are predicted for the double base propellant, due to the higher ratio of oxygen to carbon compared to pure NC: NC C:.232 O:.382 N:.095 H:.291 60% NCC:.201 O:.408 N:.116 H:.275 40% NG However, what is actually found from the analysis of the nonequilibrium products is that the CO2/ CO ratios are about the same as those for pure NC; the higher nitrogen and oxygen content for NG leads instead to an increased NO yield for the double base propellant. Compared to nitrocellulose, the double base propellant yields less CH4 and more C2 H2, particularly at elevated pressures, and significantly more H20. A high percentage (i.e., 25-35) of the carbon initially in the NC is not accounted for in the measured products at 1 atm. At high pressure, where a secondary flame zone is expected, there is a decrease in the amount of carbon "lost". This is reflected in an increased CO yield in going from 1 atm to the higher pressures. The large increase in CO is probably accompanied by a small decrease in CO2. Thus, at high pressures less formaldehyde
and perhaps less soot remain, the increased oxidation of the carbon-containing species, and the increased NO reduction, resulting principally in increased CO production. The carbon "loss" for the double base propellant is apparently smaller, being roughly 10% at 1 atm, 6% at 20 arm and 2% at 4 0 atm. For both propellants, the ratio of H to O "missing" is approximately 2, which is consistent with HCHO formation if the concentration of H2 in the products is small, as indicated by the 1 atm data of Powling, et al. [7]. However, the millimoles per gram of missing carbon is always less than for oxygen, but this may be due to a systematic error in the determination of the six measured carboncontaining species, as well as to unburned propellant and small amounts of HCO2 H, both of which would give carbon to oxygen ratios less than unity. In any case, it appears that soot (i.e., solid carbon) formation is small compared to formaldehyde formation. It is interesting to consider the degree of reduction of NO to N2 as a function of pressure. We find the NO/N2 ratio to be at least 9 at 1 atm, at least 5 at 20 atm, and about 2.5 at 40 atm. This corresponds to at most 18% reduction of NO to N2 at 1 atm and no more than 40% at 40 atm. For the double base propellant only about 10% or less of NO is reduced to N2 at 1 arm and no more than 21% at 40 atm. It therefore appears that NO reduction is far less complete than for NC. These results suggest that the temperatures for burning NC are greater than for the double base propellant at these pressures. Apparently, the solid phase and/or surface reactions are faster for the double base propellant since the burning rate for NC is lower than for the double base propellant even though the surface temperature is higher [5]. Of course, at equilibrium the flame
376 temperature must be lower for NC than for the double base propellant.
B. Effect of Lead Modifiers At one atmosphere, the effects observed upon addition of lead compounds are very small. However, at 20 and 40 atm, for both nitrocellulose and double base propellant, addition of either 1% PbO or 3% Pb Salicylate leads to a significant increase in the CO2/CO ratio, and in most cases to an increase in H2 O/CO. For example, at 40 atm CO2/CO is increased from 0.25 to 0.33 on addition of 3% PbSal to NC, while for the double base propellant it is increased from 0.29 to 0.35 (with H~O/CO increasing from 0.24 to 0.30). The effect is not large enough to determine from the data how CO or COs individually are altered. Similarly, there are fluctuations in the NO yields from one experiment to another which make it impossible to say with certainty whether lead additives lead to increased reduction of NO. It is clear, however, that any increase in NO reduction is quite small for the propellants and pressures examined; certainly less than 10%. It is interesting to note that a close examination of the tabulated data of Powling, et al. [7] shows a small increase in CO2 relative to CO for NC and double base propellants at 1 arm, upon addition of lead catalysts, although no mention was made of it in their paper. Davis [9] likewise was apparently looking for much larger effects and failed to notice this effect in comparing his tabulated data for the products of leaded vs unleaded double base propellant. Lenchitz and Haywood [ i 0 ] , however, have shown product analysis for a propellant of unspecified ("classified") double base composition, which shows a very large increase in the COs/CO ratio in the 300 psi pressure region upon addition of lead stearate, although Lenchitz, et al. report a decrease in NO and no change in COs/CO in a more recent study with a double base propellant
[81. In many cases, the ratio of H to O "lost" appears to be larger for the leaded compounds. This may indicate increased H2 formation for the catalyzed propellants. It is perhaps not surprising that the observed chemical effects of lead catalysts should be
R.A. FIFER and J. A. LANNON smaller at 1 atm than at the higher pressures, since it has been shown [1, 7] that the burning rates of leaded and unleaded double base propellants approach one another as the pressure is lowered towards atmospheric. There is no evidence here of any difference between the effect of PbO and that of the less stable, organic lead salts like PbSal. Powling [7], as a matter of fact, has shown that PbO produces super rate burning curves very much like those for the organic lead compounds. It is significant that lead additives have the same effect qualitatively and quantitatively on nitrocellulose as on the double base propellant, since this suggests that the lead catalyses principally the nitrocellulose decomposition rather than the nitroglycerine decomposition in double base propellants.
C. Effect of Water, NO2 Pretreatment and Oxygen. Dissolved Water. For these experiments, water was admitted to the bomb containing the NC film up to its room temperature vapor pressure, 1 or 20 atm of N2 was added, and the sample then allowed to stand for a couple of hours in order to allow the H20 to dissolve in the propellant. The results in Table 2 show that water addition does not cause significant changes in product concentrations, except perhaps for a decrease in HCN and an increase in N20 at the higher pressures. NO2 Pretreatment. We have found that over a 15-20 hour period, up to 60 Torr (3.7 rag) of NO2 can be consumed by reaction with a 10 mg NC film in the 25 cc cell. The reacted NO2 is converted almost quantitatively to NO, but smaller amounts (2-5 Torr each) of CO2, N20 and H 2 0 are also formed. During this reaction, the film developes a new absorption band at 1740 cm "~ , as well as a broad, unresolved absorption in the 3100-3400 cm "1 region. Qualitatively, these spectral changes are consistent with a number of previous reports [16, 17] indicating that NO2 can oxidize the primary hydroxylic groups in cellulose to carboxylic groups. In these experiments with nitrocellulose (12.6% N), however, we found that the number of NO2 molecules consumed is several times greater than the maximum possible number of unnitrated primary hydroxylic groups, thus indicating a much more complex reaction. The formation of CO2 in-
CHEMISTRY OF SOLID PROPELLANT COMBUSTION dicates that some of the oxidation is degradive in nature. Even for the most heavily treated samples, the concentrations of the major combustion products were not significantly different from those for untreated NC, although the treated samples gave somewhat less N 2 O, and somewhat more CH4 (see exp. #46). This was true even if excess NO2 (1020 Torr) was not removed from the bomb prior to ignition. Thus, under our experimental conditions, we are unable to confirm the results of Dauerman and Tajima [5], who report significant changes in products for thick strands of NC burned at 10 Torr following treatment with 9 Torr of NO2 for only 5 min. Oxygen Catalysis. Dramatic changes in products are observed when NC is burned in air instead of inert gas. At one atmosphere (see #48), CO~/CO is increased from 0.46 to 2.76, NO is completely reduced to N2, and no hydrogen-containing species remain. At 20 atm (1 atm air + 19 atm N2), however, CO2/CO is only slightly increased, and the hydrogen-containing species are not greatly changed (see #49); NO is completely absent, and large amounts of NO2 are observed. The interpretation of these results is as follows. At one atmosphere, oxygen from the air can diffuse into the reacting gases and greatly catalyze the combustion process, leading to complete reduction of NO and greater amounts of CO2 relative to CO. At 20 atm, on the other hand, diffusion is too slow to permit oxygen to enter the flame to any appreciable extent during combustion and very little catalysis occurs. After the combustion, however, the oxygen reacts with the NO formed and converts it to NO2. From experiments with smaller amounts of air, we have found that the principal changes induced by oxygen catalysis at 1 atm can be represented approximately by the overall reaction CO + NO 02 CO2 + ½N2
AH = -89 kcal/mole (I)
(The H20 yield is only slightly increased.) Since at least 7 millimoles of NO are formed per gram of propellant in the absence of 02, the heat output is increased by a minimum of 600 cal/g by oxygen catalysis, or by more than 100%. Clearly, it is the lack of oxygen that prevents the com-
377
bustion of nitrocellulose from going to completion except at very high pressures. Discussion 1. Effect of Catalysts
It is easy to show that the higher CO2/CO ratios do explain the higher energy output for the catalyzed propellant in the super-burning region. This can be seen by writing any reasonable reaction involving conversion of CO to CO2, for example: Pb CO + CO ~ CO2 + C (solid) AH =-41 kcal/mole
(2)
Pb CO + HCHO ~ CO2 + C(solid) + H2 AH = -40 kcal/mole
(3)
Pb C O + N O ~ CO2+½N2 AH = -89 kcal/mole
(4)
That is, every such reaction is very exothermic and will lead to higher temperatures. At first glance, the fact that catalysis by lead compounds is greater at 20 or 40 atm than at 1 atm would suggest that the catalytic activity is located in the secondary flame zone, as suggested by Camp, et al. [18]. However, this requires radiative feedback to the burning surface, and it has been deafly pointed out [1], that this is inconsistent with a large number of experimental fmdings. It appears, therefore, that catalysis takes place mainly in or on the burning surface and/or in the f e z zone. Powling, et al. [7] proposed that lead compounds caused increased NO reduction by carbon in the fizz or dark zones, leading to increased heat transfer to the surface. Lenchitz, et al. [8] have presented some data indicating increased NO reduction for their leaded propellants. However, neither our data, or that of Davis [9], nor indeed that of Powling provides any evidence that this is occurring to any appreciable extent. (However, we cannot rule out the possibility that some NO reduction accompanies the increase in CO2/CO and presumably carbon formation for the leaded propellant.) It appears rather that NO reduction
378 takes place mainly in the secondary flame zone. Lenchitz, et al. hypothesize that lead compounds containing chelating ligands catalyze the initial decomposition of the nitrate ester propellant molecules via an electronic interaction with the nitrate group oxygen atoms. Although interesting, there appears to be no evidence for this hypothesis, and it fails to explain how a nonchelate type lead compound, like PbO, can cause super-rate burning characteristics, as shown by Powling, et
al. [7]. Kubota, et al. [I] have shown that leaded propeUants exhibit a steeper temperature gradient in the fizz zone, and propose that this is due to an increased NO2/aldehyde ratio resulting from increased surface carbon formation. They also proposed that the catalyst does not change the solid phase heat output. They correctly point out that a steeper temperature gradient will lead to greater heat conduction back to the surface. However, it appears to us that carbon formation must be too small to cause an appreciable change in the NO2/ aldehyde ratio. However, carbon formation or increased CO2 production relative to CO is an exothermic process (see Reactions (2) and (3) above), and will lead to higher temperatures for the gases entering the fizz zone, and therefore, to a faster reaction rate in this zone. We would therefore prefer to say that increased solid phase (or surface) heat output, coupled with increased conductive heat transfer to the surface, accounts for the catalytic effect of lead compounds. Salooja [19] has published some interesting results that provide a possible explanation for the catalytic effect of lead compounds in nitrocellulose. based propellants. He showed that whereas lead oxide inhibits the combustion of hydrocarbons, it strongly promotes the combustion of oxygen derivatives of hydrocarbons (e.g., methanol, formaldehyde, methyl acetate). The catalyzed reactions were characterized by enhanced formation of carbon dioxide and in some cases hydrogen, and reduction of lead oxide to elemental lead (which was not an effective catalyst). To explain his observations, Salooja proposed that lead monoxide interacts with the reactant molecule to produce a H-bonded surface complex, which subsequently dissociates. For example, for formaldehyde
R.A. FIFER and J. A. LANNON HN,N / O - - - P C
-~
CO2 + I-I2 + Pb
(5)
(The Pb can be reoxidized to PbO by oxygencontaining radicals, and thereby be available for further reaction.) In the case of hydroxyl compounds, which were even more strongly catalyzed, Salooja proposed that PbO first abstracts the hydroxyl hydrogen atom, producing a radical that loses another hydrogen atom to form an aldehyde, which then further reacts with PbO to produce CO2, as for formaldehyde. If similar reactions catalyze the primary or secondary solid phase decomposition reactions of nitrocellulose, then the higher CO2/CO ratios for the leaded propellants are explained. That all lead compounds except the lead halides appear to act as catalysts is then clear, since almost all lead compounds except the halides can probably form PbO at the temperatures encountered at or just beneath the surface of the burning propellant. This scheme would predict less catalysis for more highly nitrated propellants, assuming that the unnitrated hydroxyl groups of the propellant are directly involved in some rate-determining step of the solid phase decompositions. It would also explain the observation [20] that PbO does not catalyze the gas phase decomposition of ethyl nitrate, which has no hydroxyl or carbonyl groups. Similarly, nitroglycerine decomposition should not be catalyzed, which is consistent with out observation that the chemical effects of lead catalysts are the same for both single and doube base propellants. 2. Effect of Pressure
Our results show that the CO2/CO ratio decreases with increasing pressure. Since this is the opposite of what might be expected, it must be considered in more detail. It has been suggested to us that this might be due to a shift in the "water-gas" equilibrium during cooling of the product gases. This reaction is CO + H20 = CO2 + H2.
(6)
However, an examination of the equilibrium constant for this reaction as a function of temperature
CHEMISTRY OF SOLID PROPELLANT COMBUSTION shows that cooling shifts this equilibrium to the right. Since the reaction is approximately second order with respect to the partial pressures of the reacting species and has an activation energy of about 30 kcal/mole [21 ] any quenching shift for high temperature flame zone products at high pressure would be larger than for low temperature dark zone products at 1 atm. The net effect would be an increase in CO2/CO with increasing pressure, which is the opposite of what is observed. Since less carbon is lost at high pressures, we interpret the apparent increase in CO on going from 1 atm dark zone products to flame zone products at 20 or 40 atm as being due to oxidation of formaldehyde and hydrocarbon species. Since the increase in CO is almost as large as the decrease in NO (at least for NC), it is interesting to hypothesize the following reaction: HCHO + NO ~ CO + H 2 0 + ~ N 2 AH = -78 kcal/mole
(7)
This reaction would also explain the increase in H 2 O, which is observed to accompany the increase in CO with increasing pressure, and is also consistent with the results of Powling, et al. [22, 23], which show that NO is extensively reduced by decomposing formaldehyde in the flames of methyl nitrite and simple dinitrate esters. The much smaller decrease in CO2 with increasing pressure may be due to the reaction C(solid) + CO2 ~ 2CO = 41 kcal/mole
AH
(8)
That is, carbon particles formed via solid phase exothermic reactions are blown off the burning surface by the evolving gases, pass through the secondary flame zone, and are there oxidized by reaction with CO2. Conclusion In conclusion, we have shown that the catalytic effect of lead additives is the same in both nitrocellulose and double base propellants. Lead catalysts lead to an increased CO2/CO ratio in the combustion products, which accounts for the higher energy output and burning rate observed
379
at moderate pressures. Similarly, evidence has been presented which indicates that in the later stages of combustion CO is formed while C02 is di, rninished, and that the reduction of nitric oxide by formaldehyde results in the initial fast temperature rise at the flame front. Dissolved water vapor and pretreatment with NO2 vapor do not significantly affect the combustion chemistry, but small amounts of oxygen can cause the gas phase reactions to go to completion even at 1 atm pressure. We wouM like to thank Dr. Y. Carignan o f Picantinny Arsenal f o r supplying us with the bulk nitrocellulose samples.
References 1. Kubota, N., Ohlemiller, T. J., Caveny. L. H., and Summerfield, M., Princeton U., No. AD 763 786, March 1973.
2. Adams, G. K., Proc. Fourth Syrup. on Naval StructuralMechanics, Purdue University, 1965, p. 117147. 3. Crawford, B. L., Huggett, C. L., and MeBrady, J. J., J. Phys. Colloid Chem. 54, 854 (1950). 4. Heller, C. A., and Gordon, A. S., J. Phys. Chem. 59, 773 (1955). 5. Dauerman, L., and Tajima, Y. A.,A.I.A.A.J. 6, 1468 (1968). 6. Dauerman, L., and Tajima, Y. A.,A.LA.A.J. 6, 678 (1968). 7. Hewkin, D. J., Hicks, J. A., Powling, J., and Watts, H., Comb. Sci. Tech. 2, 307 (1971). 8. Suh, N. P., Adams, G. F., and Lenchitz, C., Combust. Flame 22, 289 (1974). 9. Davis,B. E., Ph.D. Thesis, University of Sheffield, 1970. 10. Lenchitz, C., and Haywood, B., Combust. Flame 10, 140 (1966). 11. Marshall, A.,and Peace, G.,J. Chem. Soc. 109, 298
(1916). 12. Aleksandtov,V. V., and Khlevnoi, S. S., Fiz. Goreniya i Vzryva 4, 438 (1970). 13. Naoum, P., Nitroglycerine and Nitroglycerine Explosives, The Williams and Wilkins Co., Baltimore, 1928, p. 27f. 14. Gordon, S., and McBride, B. J., Computer Program for Calculation of Complex Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman-Jouguet Detonations, NASA SP-273, 1971. 15. Stull, D. R., and Prophet, H., JANAF Thermochemical Tables, National Bureau of Standards, NSRDSNBS 37, 1971.
380 16. Urb~nski, T., Chemistry and Technology of Explosives (W. Ornaf and S. Laverton, trans.), Pergamon Press, 1965, Vol. II, p. 347-355. 17. Yasnitskii, B. G., Dol'berg, E. B., and Oridotoga, V.A.,Zh. Prikl. Khim. 44, 1615 (1971). 18. Camp, A. T., Haussmann, H. K., MeEwan, W. S., Henry, R. A., Olds, R. H., and Besset, E. D., U. S. Naval Ordnance Test Station, NAVORD Report 5824, 1958. 19. Salooja, K. C., Combust. Flame 11,247 (1967). 20. Ellis, W. R., Smythe, B. M., and Theharne, E. D., Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1955, p. 641.
R . A . F I F E R and J. A. LANNON 21. Howard, J. B., Williams, G. C., and Fine, D. H., Fourteenth Symposium/International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 975. 22. Powling, J., and Smith, W. A. W., Combust. Flame 2,157 (1958). 23. Arden, E. A., and Powling, J,, Combust. Flame 2, 55 (1958).
Received 3 September 1974; revised 6 January 1975