854
B. L. CRAWFORD, JR.,
C. HUGGETT, AND J. J. MCBRADY
THE MECHAKISM OF T H E B C R N I S G OF DOUBLE-BASE PROPELLANTS’, 2 BRYCE L. CRBWFORD, JR., CLAYTON HLGGETT,a*4
ASD
J. J . McBRADYK
School of Chemistry, Institute of Technology, University o j XMinnesota, Minneapolis 14,Minnesota Received J a n u a r y 9, 1960
The application of double-base propellants to jet propulsion devices during and since the recent war focused attention on the burning behavior of these materials a t low and medium pressures, up to 5000 1b.,b2 The burning behavior in this pressure range cannot be described by simple extrapolation from the higher pressures previously studied in guns. As a consequence, the burning mechanism has been the subject of a number of recent investigations with their goal the improvement of propellant characteristics for low-pressure applications. Since these researches have resulted in a considerable increase in our knowledge of the burning process, it seems worth while to present in broad outline our picture of the complex sequence of reactions which constitutes the mechanism of burning. The supporting experiments will be described only briefly. We shall confine our attention to double-base propellants, Le., nitrocellulose plasticized with nitroglycerin, together with various minor constituents. Much of our experimental work was done with a powder having the following simple composition: pcr ccnl
Nitrocellulose (13.25 per cent nitrogen). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitroglycerin.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethyl centralite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
43 3
The centralite, s-diethyldiphenylurea, is present as a stabilizer. Frequently, other constituents were added to this basic composition in order to study their effect on the burning process. Such propellants burn “by parallel layers,” that is, the burning surface tends to maintain its original geometrical form during burning. Thus, for ballistic purposes, the mass burning rate and the burning time may be controlled within 1 This paper is based on work done for the Office of Scientific Research and Development under Contract OEMsr-716 with the University of Minnesota and to some extent on work done for the United States S a v y Department under Contract NOrd-9680. 2 Presented a t the Symposium on Kinetics of Propellants which was held under the auspices of the Division of Physical and Inorganic Chemistry a t the 112th Meeting of the American Chemical Society, S e w York City, September 16, 1947. * This paper is based in part on a thesis submitted by Clayton Huggett to the Graduate Faculty of the University of Minnesota in partial Eulfillment of the requirements for the degree of Doctor of Philosophy, February, 1945. 4 Present address: Rohm and Haas Company, 5ooo Richmond Street, Philadelphia, Pennsylvania. 6 Present address: Central Research Laboratory, Celanese Corporation of America, Summit, New Jersey.
BCKZIXG O F DOUBLE-B.iSE PROPELLASTS
855
limits by the geometry of the propellant grain, independent of the linear burning rate of the particular propellant used. Thus, also, a long cylindrical powder strand, ignited at one end, can be made to burn n-ith a flat burning surface if the sides of the strand are suitably coated to prevent spreading of the flame. We may, neglecting edge effects, consider the burning of such a grain as a one-dimensional process, starting within the solid propellant and proceeding through the burning surface and reacting flame to the final gaseous products. Such end-burning strands, usually in a pressure vessel charged with nitrogen. ivere used in most of our experiments and permitted a number of significant observations. REACTIOSS IK THE SOLID PH.LSE
The thermal decompositions of nitrocellulose, nitroglycerin, and a variety of other nitrate esters hai,e been studied by a number of in1wtigators (1, G , 8). In each case the experimental evidence indicates that the initial step in the decomposition is the unimolecular breaking of the nitrogen-oxygen bond to give nitrogen dioxide and a free radical. The activation ene found to be about 40 kcal. mole. The products of this initial step react further to give the complex mixture cf products usually observed in such decomposition experiments. Reactions of this type undoubtedly take place in the heated powder layers close to the burning surface. The presence of stabilizers, aromatic compounds which are ready acceptors for nitrogen dioxide and which may react directly with nitrate esters, contributes to these solid-phase reactions. If a grain of powder is heated at a temperature too low to produce instantaneous ignition, the temperature vithin the interior of the grain, measured by a theimocouple embedded in the grain, vi11 rise above that of the surroundings. At sufficiently high heating temperatures. this self-heating will cause the grain to ignite. The “ignition temperatures’’ determined in this manner are in the neighborhood of 200°C. While the significance of this figure is debatable, it must certainly set a lower limit, to the temperature of the burningsurface. It is in good agreement Lyith the ignition temperature observed for pure nitroglycerin (9). This self-heating effect is greater in powders containing large amounts of stabilizers. Such powders will burn smoothly in an inert atmosphere at atmcspheric pressure and 3scC., whereas powders containing only nitrocellulcse and nitroglycerin will not burn at pressures beloiv 200 lb.;in.? at 35%. These exothermic reactions, taking place within the solid phase beneath the burning surface, must contribute a significant part of the energy necessary to decompose the surface layers of the propellant and transport it to the fame reaction zone. THE POWDER ELAhfE
Khen a polvder strand is ignited in air by an open f!ame, the strand burns xvith a large luminous flame and the burning surface assumes a conical form as the reaction moves down the sides of the strand (figure 1). If the strand is ignited in an inert atmosphere, or even in air by gentle ignition, such as a glowing wire, it burns quietly with a nonluminous flame (figure 2 ) . The burning surface is flat
MMnu lLlb?IL 200
300
600
700
400
800
500
900
Ib
/I$
BURNIh-G O F DOUBLE-B.&SE PROPELLASTS
857
As the pressure of inert gas surrounding the burning strand is increased to 200 or 300 lb./in.*, a luminous region appears in the flame at some distance from the burning surface. Further increases in pressure cause the luminous flame to approach the surface more closely, until at 1000 lb./in.* it is difficult to detect the dark zone between the powder surface and the luminous flame (figure 3). The length of the dark zone \vas determined at various pressures from motion picture films of the burning strands. The pressure dependence can be represented quite accurately by the expression: s = c/P3
(1)
in which s is the length of the dark zone, c is a constant whose value depends somewhat on the povder composition, and P is the pressure of inert gas. At 1000 lb./in.* the luminous flame is about 0.25 mm. from the burning surface. The
Prow.
-
Ib/n
FIG.4 FIG5 FIG.4. Variation of the heat of explosion with pressure FIG.5. Variation of the gaseous products of explosion with pressure
interpretation of this pressure dependence is complicated by the uncertain effect of diffusion of the inert gas into the reaction zone under the conditions of our experiments. However, the occurrence of the dark and the luminous reaction zones indicates that the gas-phase reactions take place in a t least two welldefined stages. The rate of the second, at least, shows a strong dependence on pressure. THE HEAT O F EXPLOSION
Further evidence for the stepTvise nature of the flame reaction is found in the variation of the heat of explosion with pressure. Fennimore and Kuhn (3) have shown that nitrocellulose does not develop its full heat of explosion a t low loading densities. Double-base poLvders behave similarly. Our experiments were carried out by pressurizing the calorimeter with nitrogen and keeping the density of loading constant and low enough so that burning took place at nearly constant pressure. The result of a typical series of measurements is shown in figure 4.
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B. L. CRAWFORD, JR., C. HUGGETT, AND J. J. MCBRADY
The heat of reaction remains nearly constant a t about one-half of its maximum value from below atmospheric pressure to the point a t Thich the luminous flame appears, 300-400 1 b . / h z At this point it increases sharply and approaches the maximum value observed at high pressures. About one-half of the total energy of the propellant must come from reactions taking place in the luminous flame zone. THE PRODUCTS OF REACTION
As is to be expected, the products of reaction shorn a corresponding change with pressure. Figure 5 shows the results of a series of analyses of the gaseous reaction products a t various pressures. As in the calorimetric experiments, the sample in the form of an end-burning strand was burned in nitrogen at a low loading density. At loiv pressures an oily residue remains in the combustion chamber. Similar residues have been analyzed by others and found to consist largely of glyoxal and other low-molecular-weight oxygenated organic compounds (IO). At higher pressures this residue disappears along with the nitric oxide. The amounts of carbon monoxide and carbon dioxide increase with the appearance of the luminous flame, and more hydrogen is formed by the Tvater-gas reaction. I n one experiment a t 1000 1b.,h2 the concentrations of hydrogen, water, carbon monoxide, and carbon dioxide were found to correspond to the water-gas equilibrium value a t 1750°K. This presumably represents the temperature a t which equilibrium is frozen on cooling, since the calculated flame temperature is mqch higher, in the neighborhood of 3000°K. At high pressures and high loading densities, the products are known to be in approximate thermodynamic equilibrium. THE BURNING RATE
Measurements of burning rate TYere made on long end-burning strands at constant pressure. Burning times were observed visually a t low pressures and recorded electrically a t higher pressures. Details of the experimental method have been reported elsewhere (2). The burning rate is determined by three fundamental variables: pressure, temperature, and powder composition. Secondary effects, such as those due to radiation and to erosive burning, depend on the geometry of the propellant grain and the combustion chamber and were not important in our experiments. It is customary, for the purpose of ballistic calculation, to express the dependence of the burning rate on the pressure a t constant temperature by one of the two equations: r=a+bP (2)
r
=
CP"
(3)
where a, b, c, and n are constants characteristic of the powder composition. The two forms reproduce the available high-pressure data about equally well. Our own more precise measurements at pressures up to 6500 1 b . / h 2 show that the burning rates of conventional double-base compositions can be represented very accurately down almost to atmospheric pressure by the expression: T
= a
+ bPn
(4)
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BURXIXG OF DOUBLE-BASE PROPELLANTS
where TZ is approximately unity. Figure 6 is a logarithmic plot of the burning rate over the entire pressure range covered in these experiments. Figure 7 s h o w how well equation 4 fits the experimental data. It is significant that the burning rate
L ,o 100 1000
0.01
Pressure -lb./ine
FIG.6. The burning rate of a double-base powder as a function of pressure: log
T
FIG.7 FIG.7. The burning rate of a double-base powder as a function of pressure: log
-
(T
us.
a)
v s . log p .
FIG.8. The burning rate of various powders as a function of pressure
increases smoothly through the pressure region where the heat of reaction, the products of reaction, and the character of the flame are undergoing a rather abrupt change (compare figures 3 , 4 , and 5 with figures 6 and 7 ) . The final stages of the reaction which are responsible for these changes have little effect on the burning rate in this low-pressure region.
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B . L. CRAWFORD, JR., C. HUGOETT, AND J. J. MCBRADY
The burning rate a t low pressures becomes nearly independent of pressure, in accord with equation 4. However, the burning rate falls off suddenly a t about atmospheric pressure, and the strands fail to burn at much lower pressures, even a t elevated temperatures. We cannot suggest a reason for this abrupt change a t this time, since we were unable to observe any discontinuity in the other properties of the reaction in this pressure region. We are tempted to associate the pressure-independent term in the burning rate equation with the energy liberated by the reactions taking place within the solid powder surface. The pressure-dependent term can be associated in similar fashion with the pressure-dependent transfer of energy from the hot flame zone to the burning surface. Such a picture undoubtedly represents a Considerable oversimplification of the process. It has been pointed out that at high pressures an approximate linear relationship holds between the heat of explosion and the linear burning rate at constant temperature and pressure (4).A correlation of burning rate and flame temperature, which roughly parallels the heat of explosion, should be more sound, since a t high pressures the burning rate is largely controlled by energy transfer to the burning surface from the flame. However, at lower pressures, as we have shown, powders may develop only a fraction of their heat of explosion, or the flame temperature may reach its ultimate value only at a considerable distance from the burning surface. Under these conditions, reactions taking place beneath the burning surface or in the gas layers close to the surface may contribute a major portion of the energy necessary to support the reaction. Consequently, it is not surprising that we have found powders with similar total heats of explosion but widely different compositions to have burning rates differing by more than a factor of 2 at 1000 lb./in.* The heat of explosion and flame temperature may be useful guides in predicting the effect of small variations in composition on the burning rate, but their usefulness is questionable when considering major changes in composition or low-pressure burning. THE BURNING PROCESS
The experimental results described in the preceding section provide us with a qualitative picture of the burning process. This model has been the subject of theoretical investigations by others (5, 7 ) . The heated surface layers of the propellant grain decompose into volatile fragments and pass into the gaseous flame region. The energy necessary to bring about this initial decomposition is supplied in part by exothermic reactions taking place within the surface layers and in part by energy transferred to the surface from the hotter flame zone. The burning surface probably reaches a temperature in the neighborhood of 300°C. before decomposition is complete. The rate of volatilization of the surface layers, the burning rate, is controlled by the rate a t which energy is supplied to the reacting powder layers from all sources. These primary fragments react further in the gas phase very close to the powder surface. The products are nitric oxide and simple organic molecules, such as glyoxal, together with some of the stable end products: nitrogen, water, carbon monoxide, and carbon dioxide. Approximately half of the total heat of
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861
reaction is liberated up to this point and the temperature of the reaction zone rises to perhaps 1500°K. I n the final stages of the reaction, nitric oxide reacts with the remaining oxidizable material and the products approach thermodynamic equilibrium. The flame temperature reaches its maximum value, perhaps 3000°K., and the reaction zone is brightly luminous. This high-temperature region is too far from the burning surface at low pressures to supply much of the energy from the primary decomposition; radiation is of minor importance compared to conduction in the powder flame. However, at high pressures the high-temperature luminous flame zone approaches close enough to the burning surface to become a controlling factor in determining the burning rate. I t should be pointed out that while most of our experiments were carried out with pressure as the variable, time or distance in the reaction zone is probably the more fundamental quantity. Under the conditions of our experiments, using slender end-burning strands in an atmosphere of nitrogen, reactions undoubtedly failed to go to completion, because they were quenched by the diffusion of cool inert gas into the reaction zone before sufficient time had elapsed for reaction. A grain of powder burning under the pressure of its decomposition products instead of under pressure of nitrogen might give different results, although data on the burning rate shoTv no significant difference. At the same time, our experimental technique allowed us to isolate the intermediate reaction steps which otherwise would be difficult to observe. There can be little doubt that the sequence of reactions described here actually takes place, perhaps with a slightly modified scale of distances, when a propellant grain functions at high pressures. The function of an increase in pressure is to compress the reaction zone into a smaller distance about the burning surface. This takes place through an acceleration of chemical processes and also through the compression of the gaseous products. The detailed structure of the reaction zone and the form of the temperature gradient through it will depend on the composition of the propellant. Through a consideration of this dependence of the structure of the reaction zone on composition, we are able to predict, in a qualitative way, the effect of specific compositional changes on the burning rate and the pressure dependence of the burning rate. Compositional changes 1% hich favor the exothermic solid-phase reactions should increase the burning rate at low pressures and thus decrease the effect of pressure on the burning rate in this region. Stabilizers, such as centralite and diphenylamine, are effective, p-phenylenediamine has been found to be even more reactive. In figure 8 the burning rate of the standard composition (curve I) is compared to that of a sample to which 5 per cent of p-phenylenediamine has been added (curve 11).Although this addition decreases the heat of explosion, the burning rate at low pressures is increased, and the pressure dependence of the burning rate is significantly lower. At high pressures the burning rate of the p-phenylenediamine powder is lower than that of the standard because of the lower heat of explosion, but the pressure dependence, which is controlled primarily by the flame reaction, is normal Cooling the burning surface by the addition of some substance which decom-
862
B. L. CRAWFORD, JR., C. HUGGETT, AKD J. J. MCBRADY
poses with the absorption of energy should have the opposite effect, reducing the burning rate a t low pressures and increasing the pressure dependence. Curve 111 of figure 8 shows the effect on the burning rate of adding 5 per cent of paraformaldehyde to the standard composition. The paraformaldehyde presumably dissociates to formaldehyde and then to hydrogen and carbon monoxide close to the burning surface, absorbing some 400 cal./g. in the process. This powder fails to burn a t low pressures, but since the final flame temperature is less affected than the surface temperature, the burning rate increases rapidly as the hot flame zone is forced closer to the burning surface by increased pressure. The addition of a strong oxidizing agent, such as potassium perchlorate, causes the final step in the reaction to take place closer to the burning surface. As a result, the burning rate increases rapidly in the low-pressure region and attains a relatively high value. Thereafter it increases only slowly, since increased pressure can have little further effect on the structure of the reaction zone. Curve IT: of figure 8 s h o w the burning rate of a sample containing 40 per cent of a mixture containing 85.2 per cent perchlorate and 14.8 per cent carbon black. I t appears that at high pressures the burning rates of the various powders shown in figure 8 draw together, when due allowance is made for small differences in the heats of explosion. The flame temperature is undoubtedly the most important factor in determining the burning rate a t high pressures. SUMMARY
The results of experiments dealing with various aspects of the burning of double-base powders a t pressures between 10 and 6500 lb./in.? are described. Based on these experimental results, a qualitative picture of the burning process is presented. This mechanism is shown to account for the observed effects of various compositional changes on the burning rate and the pressure dependence of the burning rate. REFERESCES (1) . ~ P P I N ,A , , TODES, O . , A M D ~ < H . A R IYu. T O NB,. : J. Phys. Chem. (U.S.S.R.) 8, 866 (1936). (2) CRAWFORD, B. L., JR., HUGGETT, C., DANIELS, F., A N D WILFOSG,R. E . : Anal. Chem. 19, 630 (1947). (3) FENNIMORE, C., ASD HUHN,L.: Unpublished work. (4) GIBSON,R. E . : Unpublished work. ( 5 ) PARR,R. G., ASD CRARFORD, B. L., J R . : J. Phys. b- Colloid Chem. 64, 929 (1950). (6) PHILLIPS, H.: S a t u r e 160, 753 (1947). ( 7 ) RICE,0.K., A N D GIZIELL, R.: J. Phys. & Colloid Chem. 64, 585 (1950). (8) ROGINSKY, S . : Physik. Z. Sowjetunion 1, 640 (1932). (9) SNELLISG, W. O., AXD STORM,C. G . : U. S. Bur. Mines Techn. Paper N o . 12 (1912). (10) WOLFROM, XI. L., A S D COLLABORATORS: Unpublished work.
