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Oxidative decomposition of PBAA polymer at high heating rates
OXIDATIVE D E C O M P O S I T I O N OF PBAA P O L Y M E R AT H I G H H E A T I N G RATES JIH-TIEN CIIENG
Pt~illip,~ Petroleum Company, BartlesviUe, Oklahonla* AND
NORMAN W. RYAN AND ALVA D. BAER
Department of Chemical Engineering, University of Ulah, Sail Lake City, Utah The decomposition of polybutadiene-acrylic-acid copolymer, a common rocket-propellant fuel, wins carried out in the presence of distributed fine ammonium pcrchlorate and of hot ambient oxygen, separately and together. The technique employed was one of heating an exposed surface of polymer at rates on the order of 100~ per sec and measuring the temperature 100 u behind the surface. Also, the time of appearance of a flame or of hot particulate matter in the gas phase before the surface was determined. Whichever oxidant was present, a significant exothermic reaction was detected before the appearance of a flame. The rate of this reaction appears to be second order with respect to the amount of polymer surface in contact with oxidant. In experiments with hot ambient oxygen, the temperature at appearance and the rate of the exothermic reaction proved to be dependent on the bulk temperature of the oxygen away from the surface. Precursor reactions involving only small amounts of polymer are postulated. Measured in terms of the rate of heat release by the reaction responsible for the exotherm, ammonium perchlorate proved to be the more effective of the two oxidants. When both oxidants were present, however, hot oxygen appeared to initiate the exotherinic reaction.
Introduction This study of the oxidative decomposition of polymer is motivated by the recognition that this process is a key step in the combustion of composite polymer-ammonium perchlorate propellants. Made beforehand, and amply confirmed in the study, was the assumption that knowledge of the thermal decomposition of polymer in inert atmospheres is not itself sufficient. Indeed, it may not even be relevant because the participation of oxidizing species may occur at temperatures well below those at which unassisted pyrolysis proceeds at a significant rate. The last statement needs qualification because a propellant with a fuel similar to polybutadieneacrylic-acid copolymer may exhibit slow polymer deterioration at temperatures below those at which involvement of incorporated oxidant can be demonstrated. Our interest happens to be in mechanisms that are active at temperatures at * Work performed at the University of Utah.
which oxidant participation can occur, because these temperatures are very quickly attained in normal propellant combustion and in the usual ignition operation. Since, in these processes, heating rates in excess of 1000~ per sec are the rule, it would seem desirable to carry out experiments at comparable heating rates if possible. The established and convenient methods of differential thermal analysis, thermogravimetric analysis, and differential scanning calorimetry, properly supplemented by chemical analysis, are ruled out, because, eml)loying heating rates less than 100~ per rain, they leave open the question of whether the polymer is modified by the lowtemperature processes before the temperatures of concern are attained. On the other hand, heating rates at the level of ignition practice and burning are too high to permit following the process in ally degree of detail. For this work, a compromise was found. Heating rates of the order of 100~ per see were employed, and reaction events at the heated surface were inferred from temperature measurements made 100 t~ beneath the surface. Oxygen, as well as ammonium perchlorate, was
525
526
DECOMPOSITION AND EXPLOSION
studied as an oxidant, partly because decomposition abetted by oxygen is of interest in itself and partly because there has been speculation concerning the possible role of ambient oxygen in the ignition of composite propellants.
Experiment The exI)erimental technique followed was to expose one side of a thin fihn of polymer or polymer containing dispersed solids to blackbody radiation and to record the temperature on the back side of the film as a function of time. Recorded simultaneously was the output of a photocell that viewed the exposed surface during heating. Test samples were cut by microtome to a thickness of 100 ~ from larger bodies of cured material. In an operation carried out under vacuum, film samples were carefully rolled onto copper disks, 345 # thick, previously wetted with liquid polymer. Then the fihn-covered disks, 1.3 cm in diam, were heated in a helium atmosphere to cure the very thin bonding layer. The copper disk served as a thermojunction, copper and constantan wires leading from its uncovered side through a connector plug to external circuitry. The disk assembly was placed in a supporting and protective shield, which exposed only the polymer film to the furnace radiation. The blackbody radiation was provided by a furnace that consisted of a nichrome-wire-wound alumina tube, insulated and placed in a flanged section of 6-in. pipe, which served as the shell of the pressure furnace. Provision was made in one flange for an injection assembly, through which the sample was inserted into the furnace. The insertion operation took from 0.1 to 0.15 sec. Full details of apparatus, experimental procedures, and sample preparation are given by Cheng. ~ The basic polymer system was 85-wt % polybutadiene-acrylic-acid copolymer, supplied by the Thiokol Chemical Corporation, with 15% Epon 828 expoxide curing agent. For all tests reported here, the samples contained three parts of carbon black (Philblack E ) per hundred parts of the rest, to render them opaque and to give their surfaces a high radiation absorptivity. The maximum diameter of the particulate matter in the samples was 10 ~. The thermal diffusivity of the samples was of the order of 0.001 cm 2 sec-1. The thermal transmission time through the polymer, computed as thickness squared divided by diffusivity, was of the order of 0.1 sec; in other words, thermal events observed at the back face of the polymer film indicated events that occurred 0.1 see
earlier at the exposed surface. Computed in the same way, the thermal relaxation time of the copper disk was 0.001 sec, small enough that, in tile time intervals of interest, the disk couhl be assumed to be always uniformly at the temperature of the back face of the polymer film. Figure 1 presents a representative temperaturetime graph traced from a visicorder record. The copper-disk (i.e., copper-polymer interface) temperature increases ahnost linearly (after the first 0.1 sec) during the period in which the polymer is presumed to be passively heated. An increase in the T(t) slope at 1.3 sec and 182~ indicates that an exothermic process began at the exposed surface at 1.2 sec. The corresponding surface temperature, hereafter called the exotherm temperature (or, in instances in which the slope changes the other way, the endotherm temperature), was computed to be 265~ From the linear part of the T (t) curve, the rate of temperature rise for this test was found to be about 120~ per see. Clearly, the problem is to translate the interface temperature, which is recorded, into the surface temperature, which is of interest. The method of translation is given in the next section. If the surface temperature were plotted on Fig. 1, it would show infinite slope at (t, T) = (0, 27) and would initially have the shape of the upper branch of a parabola symmetrical about T = 27. After about 0.2 sec, the surface temperature would also be linear in time, paralleling the interface temperature. The exotherm would be indicated by a break in the slope at (t, T ) = (1.2, 265). Surface temperature so calculated is not reliable after the start of an endotherm or an exotherm; that is, after the period of passive heating. The reaction events are not well enough understood
35~ i 5001
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PBAA IN Oz 0.85 ATMS
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50
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L2 TIME,
1.6
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Fio. 1. Interface temperature and photocell output when polymer is heated in oxygen. Example of data obtained.
DECOMPOSITION OF PBAA that we can account for them in the analysis. Nevertheless, we assume that the initial part of the exotherm-temperature history parallels that for the interface. We then calculate a total flux, the sum of the absorbed radiant flux and the flux contributed by exothermic surface reaction, as the absorbed radiant flux times the T(t) slope for the exotherm divided by the T(t) slope just before the exotherm, both read from the interfacetemperature history. The difference between the total flux and the absorbed radiant flux is called the net exotherm flux, and is taken below as a measure of the rate of the surface exothermic reactions. The assumption that surface and interface temperatures are parallel during the initial part of the exotherm would follow from the straightness of the T (t) line during the exotherm if the process were merely a step increase in total surface flux. There is, however, loss of material from the film; and our interpretation of net exotherm heat flux is an admitted approximation. Also shown on Fig. 1 is the photocell output, to the same time scale but to an arbitrary intensity scale on the ordinate. The photocell trace was recorded, together with the interface temperature, on an oscillograph and was copied onto the visieorder record of the interface temperature. The abrupt rise in the photocell signal at about 1.55 see indicates the appearance of a visible flame at the sample surface.
Analysis of Temperature Records The conversion of interface (copper-disk) temperature to surface temperature was accomplished by solving the one-dimensional energy balance as applied to the polymer film
pc(OT/Ot) = k(O2T/Ox 2) + A exp ( - - E , / R T ) . (1) Here T (t, x) is the local temperature, a function of time t and distance x, measured into the film from the exposed surface; 0 < x < l, where l is the fihn thickness; p, c, k are the density, heat capacity, and thermal conductivity of the fihn material; and the final term is the volumetric rate of heat release by chemical reaction, E~ being the activation energy. The initial and boundary conditions were as follows:
T(O--, x) = To, uniform,
(2)
527
x= 0+:
--k(OT/Ox) = e l + B exp (--Eb/RT) + C e x p (--E~/RT); X ~
(3)
/--;
--k(OT/Ox) = L(pc)d(dT/dt).
(4)
Here, f is the radiative flux from the enclosing furnace, applied at t = 0 and maintained constant thereafter; e is the absorptivity of the film surface; L the thickness; and (pc)a the volumetric heat capacity of the copper disk. The final two terms of Eq. (3) represent energy released by surface reactions--one term for reaction between ingredients of the film material and the other for reaction between ambient oxygen and film material. Equations (3) and (4), as originally written by Cheng,' also included terms for convective-heat input and the several heat losses. Although, partly by auxiliary experiments and partly by analysis--he showed these to be small compared with other terms--they were retained in the numerical calculations. Equation (1) was solved numerically on a digital computer, the explicit Sehmidt method being used. The computer program was proved out by comparison with an analytical solution in which A, B, and C were taken as zero. Constant values (determined at 60~ for each of the film materials heated) of the thermophysical properties were used. Since the thermophysical properties were not known as functions of temperatures and the T(t, l) calculated is not sensitive to changes in them,'the error introduced by assuming them constant is not known. The value of T(t, O) -- T(t, l) (typically 50~ the largest value calculated being about 80~ ) is very nearly inversely proportional to the thermal conductivity at the mean of the two temperatures. That T(t, 0) is reported to three significant figures indicates only the precision of T(t, l) measurements and computer operation. In its application to heating the film, the method involved integrating Eq. (1) with Conditions (2) and (3) and the known flux f, to produce T(t, x), which was then employed in Condition (4) to produce T(t, l) for comparison with the experimental-temperature history. It was found for the early part of the history, when the reaction terms are negligible, that T(t, l) was accurately predicted if the absorptivity e was taken as the very reasonable value 0.82. For the tests in which polymer was heated in nitrogen, A and E, were adjusted to match both the initial temperature and the initial slope of
528
DECOMPOSITION AND EXPLOSION
the endotherm. The Ea value so obtained was used in later analyses of data from the heating of polymer in oxygen and the heating of AP-loaded polymer. In these later calculations, B and Eb or C and Ec were varied to match the initial temperature and the initial slope of the exotherm. Finally, with e and the kinetic parameters thus determined, the solution for the surface temperature T(t, 0) was obtained. Implied in the analysis is the assumption that the nonrefiected incident radiation (el) is absorbed before penetrating to a depth significant compared with the film thickness of 100 #. Although no careful experimental measurement of transmissivity was made, we estimate conservatively, simply from the hiding power of the approximately 2 volume % of very thoroughly dispersed carbon black, that the absorption coefficient must be at least 2000 cm-~. The actual value may be greater than 10,000 cm-~. Taking the absorption coefficient as 10,000 cm-~, or 1 #-~, we find 99% absorption in 5 #. P o l y m e r in N i t r o g e n A thorough study of the decomposition in nitrogen was conducted as a background for the study of oxidative decomposition. 1 Those features of special interest for the present purpose were (1) an endotherm of short duration, clearly indicated on the interface-temperature record, and (2) (at a later time) a photocell signal presumably due to hot particles formed when emissions from the surface migrate into the hot nitrogen. The background experiments also showed that a negligible amount of polymer is gasified before the photocell signal rises and that rapid gasification begins at that time. The surface temperature at the onset of the endotherm observed when the polymer is heated at 4.0 cal/(sec, sq cm) is about 600~ in a vacuum; it is 646~ in 0.85 arm of nitrogen, 696~ in 4.9 atm of
nitrogen. The surface temperature at the time of the photocell signal is similarly pressure dependent. Incorporation of copper chromite in the amount of 9.7% of the mixture results in accelerated pyrolysis, as is indicated by significant reductions in the temperatures of the endotherm and of the photocell signal.
Polymer in O x y g e n When the l)olymer is heated in oxygen, the first manifestation of surface reaction is the appearancc of a sharp exotherm on the calorimeter temperature record. The exothermic surface reaction thus indicated always precedes the photocell signal, which, when oxygen is present, indicates a visible flame. Also, the exotherm occurs at temperatures far below the endotherm temperatures noted for heating in nitrogen. The complex process manifested in the exotherm cannot be explained in simple kinetic terms. As Table I shows, for experiments at an oxygen pressure of 0.85 arm, the exotherm temperature does not increase as the applied flux (determined by furnace temperature) is increased. Instead, it drops significantly. We postulate that precursor reactions, involving too little mass and energy to influence the temperature record perceptibly, release fragments that can diffuse far enough away from the surface to react with hot oxygen. Then, as thus altered or activated, the fragments return to the surface to promote an exothermic oxygen attack on the polymer. At the lower flux levels (and, accordingly, at lower bulk-oxygen temperatures), the fragments must diffuse further to encounter sufficiently reactive oxygen; and a smaller fraction of them returns to the surface. Higher surface temperatures are needed to compensate for the reduced concentration of promoter. Table I also shows the effect of changing pressure for tests conducted at an absorbed radiant
TABLE I Polymer heated in oxygen
O~ pressure, arm 0.85 0.85 0.85
1.5 2.9 4.9
Absorbed radiant flux, cal/(sec, sq cm)
Net exotherm flux, cal/(sec, sq cm)
Initial exotherm temperature, ~
1.5 :t: 0.05 2.5 4.0 4.0 4.0 4.0
1.3 • 0.2 1.5 1.9 2.3 3.8 5.1
598 4- 5 548 532 531 530 523
DECOMPOSITION OF PBAA flux of 4.0 cal/(see, sq cm). Since all samples had very similar surface-temperature histories up to the time of the exotherm, the pressure effect should be evident. A nearly sixfold increase in oxygen pressure more than doubles the net exotherm flux, which is taken as the measure of reaction rate. Our data do not allow distinguishing the several presumed effects of pressure: to alter the vaporization, the diffusion and reaction history of precursor fragments and to provide a mass action effect at the surface. Some additional light is shed on mechanism by experiments in which the polymer was diluted with fine glass beads. The oxygen pressure was 0.85 atm, and the applied flux was 3.9 cal/(see, sq cm). Figure 2 displays the results as
~p40
PBAA § GLASS BEADS in 02 (0.85 ATMS, IlO0"C) 3.C - -
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20
30
40
50
60
WEIGHTPERCENTAP,(~ FIG. 3. Surface temperatures at times of characteristic reaction events: polymer with AP heated in nitrogen; furna~:e temperature, 1100~
about 100~ higher temperature, apparently also promotes oxidative decomposition. The exotherm temperature is not significantly changed; only the net exotherm flux is.
x~
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When the oxidant is incorporated in the polymer as small crystals of ammonium perchlorate (AP) instead of being present as ambient oxygen, the phenomena observed are different. For an absorbed radiant flux of 4.0 cal/(set, sq em), a nitrogen pressure of 0.85 atm, and less than 20 wt % of AP, an endotherm is the first indication of reaction but at lower temperatures than when AP is absent, as is shown on Fig. 3. I t is followed, however, b y an exotherm, indicating reaction with AP or its decomposition products.
FIG. 2. Dependence of net exotherm flux on volume frartion of polymer containing small glass beads.
700~(F~
net exotherm heat flux, taken as proportional to rate, vs volume fraction and, therefore, fraction of exposed surface of polymer. The samples had nearly identical temperature histories up to the time of the exotherm, except for a slight increase in exotherm temperature as the fraction of beads was increased. Whatever the rate-controlling process is, it is apparently approximately second order with respect to accessible polymer. The inclusion of 5% of copper chromite resulted in an increase in the net exotherm flux by about 200-/0 over that for the same polymer-bead mixture uncatalyzed, even though the polymer was correspondingly further diluted. The catalyst, which promotes the endothermic decomposition when heating occurs in nitrogen but does so at
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FIG. 4. Surface temperatures at times of characteristic reaction events: polymer with AP and catalyst, in 20/1 weight ratio, heated in nitrogen.
530
DECOMPOSITION AND EXPLOSION
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According to Fig. 5, copper chromite acts to increase the net exotherm flux at all AP concentrations.
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Polymer with Ammonium Perchlorate Heated in Oxygen ~
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FIG. 5. Net exotherm heat flux as a function of AP content, all data for 0.85-atm pressure, ll00~ furnace temperature. AP density is proportional to both the fraction of surface that is AP and the APpolymer contact surface. Finally, the photocell signal is observed, indicating hot particles at tow AP loadings and a visible flame at the higher loadings. The pronounced exotherm precedes the flame, as when oxygen is the oxidant. At AP loadings above 20 wt %, the endotherm is no longer observed. The effect of including conventional burningrate catalysts, 1 part per 20 of AP, is to reduce the significant temperatures, as is revealed by a comparison of Figs. 3 and 4, the effect being greater at the higher AP loadings. The time and the temperature lag of the photocell signal behind the exotherm diminish as AP loading is increased. Extrapolation to propellant loading levels of S0 or more (wt (~o) suggests that the exotherm and the flame will occur ahnost simultaneously and, therefore, that the appearance of a flame is an acceptable indicator of the chemical events of ignition. The net exotherm flux, the measure of the rate of the exothermic reaction, is plotted on Fig. 5 against the density of AP in the composite material. The graph indicates a reaction order of 1.8 with respect to polymer-AP contact surface9 The order is likely greater because the effect reduced exotherm surface temperature at the higher loadings is not accounted for.
In a final series of experiments, AP-loaded polymer was heated in oxygen at a flux of 4.0 cal/(sec, sq cm) and a pressure of 0.85 aim. Figure 6 shows the effect of AP loading on the exotherm and the photocell surface temperatures. According to Fig. 6, small amounts of AP tend to inhibit the oxygen-polymer reaction, probably by reducing the available polymer surface just as the glass beads reduced it. Once the exothermic reaction starts, however, the AP begins to participate, as is shown by the reduction in the temperature of the photocell signal, which indicates a flame. Figure 5 displays the net exotherm fluxes for these experiments along with those for the experiments in nitrogen. I t shows that AP is the more effective oxidant at the higher AP concentrations studied. If extrapolation to propellant loading levels is permitted, it shows that the AP-polymer reaction becomes dominant and that the hot oxygen contributes very little. Keller, Baer, and Ryan 2 reached the same conclusion in their study of the ignition of A I ' - P B A A propellants having smooth surfaces.
Conclusions The most obvious conclusion, trivial because it was expected, is that polymer decomposition in a reactive environment is very different from 660[
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WEIGHT PERCENT Ate, (%) FIG. 6. Surface temperatures at, times of characteristic reaction events: polymer with AP heated
in oxygen.
DECOMPOSITION OF PBAA decomposition in an inert environment. More significant, exothermic, presumably heterogeneous reactions occur before a flame appears in the gas phase. Part of the energy required for the ignition process, at rates up to 20 cal/(sec, sq cm), inferred from our data, is provided before feedback from hot gas-phase reactions is available. It is reasonable to postulate that these preflame reactions are significant in steady propellant burning, though so compressed in time as to be probably undetectable. The role of oxygen is intricate, and the combustion mechanism very complicated. To rationalize the results, we found it necessary to invoke participation by oxygen close to the surface but at a temperature greater than that at the surface. We would expect quite different results if polymer were heated by radiation but in the presence of oxygen colder than the surface. If an A P - P B A A propellant were being ignited in cold oxygen, we would expect that AP would be the dominant oxidizer, with ambient oxygen playing a negligible role. Indeed, we infer that even hot oxygen may play only a minor role. The situation may well be different with other polymeric fuels, especially if they del)olymerize readily or are unusually vulnerable to oxygen attack at 500~ or below.
531
Our most firm conclusion with respect to mechanism is that the rate-controlling preflame process contributing to the exotherm is probably second order with respect to the polymer surface accessible to the oxidant. This result emerges from an examination of data both from the reaction between oxygen and polymer containing inert solids and from the reaction between polymer and incorporated AP in a nitrogen environment. ACKNOWLEDGMENT
This work was performed under the sponsorship of the Air Force Office of Scientific Research, Propulsion Division; B. T. Wolfson, monitor; Grants AF-AFOSR 40-66, -67. REFERENCES 1. CHENG, J. T.: Thermal Effects of Composite Propellant Reactions, Ph.D. thesis, University of Utah, 1967; also available, under the same title, as AFOSIr 68-0243, August, 1967. 2. KELLER, J. A., BAER, i . D., AND RYAN, N. W. AIAA J..~, 1358 (I966).
COMMENTS E. W. Price, U.S. Naval Weapons Center, China Lake, California. I understood you to state that your results resolve in the affirmative a long-standing question as to whether optical observation of the first f a m e was a reliable indication of attainment of ignition. It is my impression that the long-standing question related to whether combustion would self-sustain upon removal of the internal heating, and whether appearance of the flame assured a self-sustaining situation. This issue has been studied extensively, and it is well established that appearance of the flame may occur considerably before self-sustaining conditions are achieved. Your results apparently do not reveal anything regarding this question.
N. W. Ryan. This comment has made a valid point, although it is not central to the thesis of the paper. In our experiments, a strong exothermic reaction appeared a very short time before a visible flame. When this is the case, the appearance of a visible flame is a sufficient criterion of ignition. However, there are some ignition conditions in which it is apparently not the case. Perhaps we contribute to resolving the issue by directing attention to the exothermic
surface reaction and demonstrating that it can be experimentally detected. II
R. F. McAlevy III, Stevens Institute of Technology, Hoboken, N.J. (1) Considering the probable error introduced in surface temperature calculations, due to incomplete knowledge of optical and thermophysical prol)erties, is it reasonable to make any kinetics arguments based on temperatures known with such a low confidence level ? (2) How do the subject data compare with the DTA, TGA, and other low-heating rate data (where strong effects of gaseous oxygen have been reported) or the high-heating rate d a t a - - b y Hansel and myself in the AIAA Journal in 1966 and 1967--(which show no effects)? (3) Considering that the heating rates employed were an order-of-inagnitude or so below those typical of solid-propellant motor ignition, is it reasonable to make statements concerning the motor situation based on the subject data? N. IV. Ryan. The confidence level of the sin'face temperature is not so low that the modest kinetics arguments advanced should be ques-
532
DECOMPOSITION AND EXPLOSION
tioned for that reason. Whatever might be said about the absolute values of the temperature, comparative values from test to test or of the temperatures for two events in the test are certainly significant. Perhaps a more fundamental question should be asked: W h a t do we mean by "temperature"? I t is possible that the molecular relaxation rate of the polymer is not large compared with the heating rates we employed. If so, refinement of thermophysical data, determined in the usual ways, would be no help in determining the temperature relevant to the chemical processes.
We have compared the behavior of PBAA polymer in the experiments reported here to its behavior in a differential scanning calorimeter, and found them to be different. This observation confirmed our opinion that DSC, TGA, and D T A experiments contribute little to understanding propellant ignition and burning. We do not know if our heating rates are fast enough that the chemical events of realistic ignition practice do indeed occur in our experiments. We think it is safer to assume that they do than to put confidence in T G A or DTA.
Pt~illip,~ Petroleum Company, BartlesviUe, Oklahonla* AND
NORMAN W. RYAN AND ALVA D. BAER
Department of Chemical Engineering, University of Ulah, Sail Lake City, Utah The decomposition of polybutadiene-acrylic-acid copolymer, a common rocket-propellant fuel, wins carried out in the presence of distributed fine ammonium pcrchlorate and of hot ambient oxygen, separately and together. The technique employed was one of heating an exposed surface of polymer at rates on the order of 100~ per sec and measuring the temperature 100 u behind the surface. Also, the time of appearance of a flame or of hot particulate matter in the gas phase before the surface was determined. Whichever oxidant was present, a significant exothermic reaction was detected before the appearance of a flame. The rate of this reaction appears to be second order with respect to the amount of polymer surface in contact with oxidant. In experiments with hot ambient oxygen, the temperature at appearance and the rate of the exothermic reaction proved to be dependent on the bulk temperature of the oxygen away from the surface. Precursor reactions involving only small amounts of polymer are postulated. Measured in terms of the rate of heat release by the reaction responsible for the exotherm, ammonium perchlorate proved to be the more effective of the two oxidants. When both oxidants were present, however, hot oxygen appeared to initiate the exotherinic reaction.
Introduction This study of the oxidative decomposition of polymer is motivated by the recognition that this process is a key step in the combustion of composite polymer-ammonium perchlorate propellants. Made beforehand, and amply confirmed in the study, was the assumption that knowledge of the thermal decomposition of polymer in inert atmospheres is not itself sufficient. Indeed, it may not even be relevant because the participation of oxidizing species may occur at temperatures well below those at which unassisted pyrolysis proceeds at a significant rate. The last statement needs qualification because a propellant with a fuel similar to polybutadieneacrylic-acid copolymer may exhibit slow polymer deterioration at temperatures below those at which involvement of incorporated oxidant can be demonstrated. Our interest happens to be in mechanisms that are active at temperatures at * Work performed at the University of Utah.
which oxidant participation can occur, because these temperatures are very quickly attained in normal propellant combustion and in the usual ignition operation. Since, in these processes, heating rates in excess of 1000~ per sec are the rule, it would seem desirable to carry out experiments at comparable heating rates if possible. The established and convenient methods of differential thermal analysis, thermogravimetric analysis, and differential scanning calorimetry, properly supplemented by chemical analysis, are ruled out, because, eml)loying heating rates less than 100~ per rain, they leave open the question of whether the polymer is modified by the lowtemperature processes before the temperatures of concern are attained. On the other hand, heating rates at the level of ignition practice and burning are too high to permit following the process in ally degree of detail. For this work, a compromise was found. Heating rates of the order of 100~ per see were employed, and reaction events at the heated surface were inferred from temperature measurements made 100 t~ beneath the surface. Oxygen, as well as ammonium perchlorate, was
525
526
DECOMPOSITION AND EXPLOSION
studied as an oxidant, partly because decomposition abetted by oxygen is of interest in itself and partly because there has been speculation concerning the possible role of ambient oxygen in the ignition of composite propellants.
Experiment The exI)erimental technique followed was to expose one side of a thin fihn of polymer or polymer containing dispersed solids to blackbody radiation and to record the temperature on the back side of the film as a function of time. Recorded simultaneously was the output of a photocell that viewed the exposed surface during heating. Test samples were cut by microtome to a thickness of 100 ~ from larger bodies of cured material. In an operation carried out under vacuum, film samples were carefully rolled onto copper disks, 345 # thick, previously wetted with liquid polymer. Then the fihn-covered disks, 1.3 cm in diam, were heated in a helium atmosphere to cure the very thin bonding layer. The copper disk served as a thermojunction, copper and constantan wires leading from its uncovered side through a connector plug to external circuitry. The disk assembly was placed in a supporting and protective shield, which exposed only the polymer film to the furnace radiation. The blackbody radiation was provided by a furnace that consisted of a nichrome-wire-wound alumina tube, insulated and placed in a flanged section of 6-in. pipe, which served as the shell of the pressure furnace. Provision was made in one flange for an injection assembly, through which the sample was inserted into the furnace. The insertion operation took from 0.1 to 0.15 sec. Full details of apparatus, experimental procedures, and sample preparation are given by Cheng. ~ The basic polymer system was 85-wt % polybutadiene-acrylic-acid copolymer, supplied by the Thiokol Chemical Corporation, with 15% Epon 828 expoxide curing agent. For all tests reported here, the samples contained three parts of carbon black (Philblack E ) per hundred parts of the rest, to render them opaque and to give their surfaces a high radiation absorptivity. The maximum diameter of the particulate matter in the samples was 10 ~. The thermal diffusivity of the samples was of the order of 0.001 cm 2 sec-1. The thermal transmission time through the polymer, computed as thickness squared divided by diffusivity, was of the order of 0.1 sec; in other words, thermal events observed at the back face of the polymer film indicated events that occurred 0.1 see
earlier at the exposed surface. Computed in the same way, the thermal relaxation time of the copper disk was 0.001 sec, small enough that, in tile time intervals of interest, the disk couhl be assumed to be always uniformly at the temperature of the back face of the polymer film. Figure 1 presents a representative temperaturetime graph traced from a visicorder record. The copper-disk (i.e., copper-polymer interface) temperature increases ahnost linearly (after the first 0.1 sec) during the period in which the polymer is presumed to be passively heated. An increase in the T(t) slope at 1.3 sec and 182~ indicates that an exothermic process began at the exposed surface at 1.2 sec. The corresponding surface temperature, hereafter called the exotherm temperature (or, in instances in which the slope changes the other way, the endotherm temperature), was computed to be 265~ From the linear part of the T (t) curve, the rate of temperature rise for this test was found to be about 120~ per see. Clearly, the problem is to translate the interface temperature, which is recorded, into the surface temperature, which is of interest. The method of translation is given in the next section. If the surface temperature were plotted on Fig. 1, it would show infinite slope at (t, T) = (0, 27) and would initially have the shape of the upper branch of a parabola symmetrical about T = 27. After about 0.2 sec, the surface temperature would also be linear in time, paralleling the interface temperature. The exotherm would be indicated by a break in the slope at (t, T ) = (1.2, 265). Surface temperature so calculated is not reliable after the start of an endotherm or an exotherm; that is, after the period of passive heating. The reaction events are not well enough understood
35~ i 5001
~
i
I
PBAA IN Oz 0.85 ATMS
~ 200--INTERFACE 182~ SURFACE 265"C L~"'150 __~V_~I~ ,oo z
50
PHO
OCELL
0 0
0.4
0.8
L2 TIME,
1.6
2.0
2.4
($ec)
Fio. 1. Interface temperature and photocell output when polymer is heated in oxygen. Example of data obtained.
DECOMPOSITION OF PBAA that we can account for them in the analysis. Nevertheless, we assume that the initial part of the exotherm-temperature history parallels that for the interface. We then calculate a total flux, the sum of the absorbed radiant flux and the flux contributed by exothermic surface reaction, as the absorbed radiant flux times the T(t) slope for the exotherm divided by the T(t) slope just before the exotherm, both read from the interfacetemperature history. The difference between the total flux and the absorbed radiant flux is called the net exotherm flux, and is taken below as a measure of the rate of the surface exothermic reactions. The assumption that surface and interface temperatures are parallel during the initial part of the exotherm would follow from the straightness of the T (t) line during the exotherm if the process were merely a step increase in total surface flux. There is, however, loss of material from the film; and our interpretation of net exotherm heat flux is an admitted approximation. Also shown on Fig. 1 is the photocell output, to the same time scale but to an arbitrary intensity scale on the ordinate. The photocell trace was recorded, together with the interface temperature, on an oscillograph and was copied onto the visieorder record of the interface temperature. The abrupt rise in the photocell signal at about 1.55 see indicates the appearance of a visible flame at the sample surface.
Analysis of Temperature Records The conversion of interface (copper-disk) temperature to surface temperature was accomplished by solving the one-dimensional energy balance as applied to the polymer film
pc(OT/Ot) = k(O2T/Ox 2) + A exp ( - - E , / R T ) . (1) Here T (t, x) is the local temperature, a function of time t and distance x, measured into the film from the exposed surface; 0 < x < l, where l is the fihn thickness; p, c, k are the density, heat capacity, and thermal conductivity of the fihn material; and the final term is the volumetric rate of heat release by chemical reaction, E~ being the activation energy. The initial and boundary conditions were as follows:
T(O--, x) = To, uniform,
(2)
527
x= 0+:
--k(OT/Ox) = e l + B exp (--Eb/RT) + C e x p (--E~/RT); X ~
(3)
/--;
--k(OT/Ox) = L(pc)d(dT/dt).
(4)
Here, f is the radiative flux from the enclosing furnace, applied at t = 0 and maintained constant thereafter; e is the absorptivity of the film surface; L the thickness; and (pc)a the volumetric heat capacity of the copper disk. The final two terms of Eq. (3) represent energy released by surface reactions--one term for reaction between ingredients of the film material and the other for reaction between ambient oxygen and film material. Equations (3) and (4), as originally written by Cheng,' also included terms for convective-heat input and the several heat losses. Although, partly by auxiliary experiments and partly by analysis--he showed these to be small compared with other terms--they were retained in the numerical calculations. Equation (1) was solved numerically on a digital computer, the explicit Sehmidt method being used. The computer program was proved out by comparison with an analytical solution in which A, B, and C were taken as zero. Constant values (determined at 60~ for each of the film materials heated) of the thermophysical properties were used. Since the thermophysical properties were not known as functions of temperatures and the T(t, l) calculated is not sensitive to changes in them,'the error introduced by assuming them constant is not known. The value of T(t, O) -- T(t, l) (typically 50~ the largest value calculated being about 80~ ) is very nearly inversely proportional to the thermal conductivity at the mean of the two temperatures. That T(t, 0) is reported to three significant figures indicates only the precision of T(t, l) measurements and computer operation. In its application to heating the film, the method involved integrating Eq. (1) with Conditions (2) and (3) and the known flux f, to produce T(t, x), which was then employed in Condition (4) to produce T(t, l) for comparison with the experimental-temperature history. It was found for the early part of the history, when the reaction terms are negligible, that T(t, l) was accurately predicted if the absorptivity e was taken as the very reasonable value 0.82. For the tests in which polymer was heated in nitrogen, A and E, were adjusted to match both the initial temperature and the initial slope of
528
DECOMPOSITION AND EXPLOSION
the endotherm. The Ea value so obtained was used in later analyses of data from the heating of polymer in oxygen and the heating of AP-loaded polymer. In these later calculations, B and Eb or C and Ec were varied to match the initial temperature and the initial slope of the exotherm. Finally, with e and the kinetic parameters thus determined, the solution for the surface temperature T(t, 0) was obtained. Implied in the analysis is the assumption that the nonrefiected incident radiation (el) is absorbed before penetrating to a depth significant compared with the film thickness of 100 #. Although no careful experimental measurement of transmissivity was made, we estimate conservatively, simply from the hiding power of the approximately 2 volume % of very thoroughly dispersed carbon black, that the absorption coefficient must be at least 2000 cm-~. The actual value may be greater than 10,000 cm-~. Taking the absorption coefficient as 10,000 cm-~, or 1 #-~, we find 99% absorption in 5 #. P o l y m e r in N i t r o g e n A thorough study of the decomposition in nitrogen was conducted as a background for the study of oxidative decomposition. 1 Those features of special interest for the present purpose were (1) an endotherm of short duration, clearly indicated on the interface-temperature record, and (2) (at a later time) a photocell signal presumably due to hot particles formed when emissions from the surface migrate into the hot nitrogen. The background experiments also showed that a negligible amount of polymer is gasified before the photocell signal rises and that rapid gasification begins at that time. The surface temperature at the onset of the endotherm observed when the polymer is heated at 4.0 cal/(sec, sq cm) is about 600~ in a vacuum; it is 646~ in 0.85 arm of nitrogen, 696~ in 4.9 atm of
nitrogen. The surface temperature at the time of the photocell signal is similarly pressure dependent. Incorporation of copper chromite in the amount of 9.7% of the mixture results in accelerated pyrolysis, as is indicated by significant reductions in the temperatures of the endotherm and of the photocell signal.
Polymer in O x y g e n When the l)olymer is heated in oxygen, the first manifestation of surface reaction is the appearancc of a sharp exotherm on the calorimeter temperature record. The exothermic surface reaction thus indicated always precedes the photocell signal, which, when oxygen is present, indicates a visible flame. Also, the exotherm occurs at temperatures far below the endotherm temperatures noted for heating in nitrogen. The complex process manifested in the exotherm cannot be explained in simple kinetic terms. As Table I shows, for experiments at an oxygen pressure of 0.85 arm, the exotherm temperature does not increase as the applied flux (determined by furnace temperature) is increased. Instead, it drops significantly. We postulate that precursor reactions, involving too little mass and energy to influence the temperature record perceptibly, release fragments that can diffuse far enough away from the surface to react with hot oxygen. Then, as thus altered or activated, the fragments return to the surface to promote an exothermic oxygen attack on the polymer. At the lower flux levels (and, accordingly, at lower bulk-oxygen temperatures), the fragments must diffuse further to encounter sufficiently reactive oxygen; and a smaller fraction of them returns to the surface. Higher surface temperatures are needed to compensate for the reduced concentration of promoter. Table I also shows the effect of changing pressure for tests conducted at an absorbed radiant
TABLE I Polymer heated in oxygen
O~ pressure, arm 0.85 0.85 0.85
1.5 2.9 4.9
Absorbed radiant flux, cal/(sec, sq cm)
Net exotherm flux, cal/(sec, sq cm)
Initial exotherm temperature, ~
1.5 :t: 0.05 2.5 4.0 4.0 4.0 4.0
1.3 • 0.2 1.5 1.9 2.3 3.8 5.1
598 4- 5 548 532 531 530 523
DECOMPOSITION OF PBAA flux of 4.0 cal/(see, sq cm). Since all samples had very similar surface-temperature histories up to the time of the exotherm, the pressure effect should be evident. A nearly sixfold increase in oxygen pressure more than doubles the net exotherm flux, which is taken as the measure of reaction rate. Our data do not allow distinguishing the several presumed effects of pressure: to alter the vaporization, the diffusion and reaction history of precursor fragments and to provide a mass action effect at the surface. Some additional light is shed on mechanism by experiments in which the polymer was diluted with fine glass beads. The oxygen pressure was 0.85 atm, and the applied flux was 3.9 cal/(see, sq cm). Figure 2 displays the results as
~p40
PBAA § GLASS BEADS in 02 (0.85 ATMS, IlO0"C) 3.C - -
529
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7 ~"
I-
i
C~ k(~ I
~""~"--.~
I
I
'
I
PBAA+AP m Nz
! ~..,. /
(0,8.5 ATMS, HO0~ I / PHOT'OCEL L i
ENDOTHERM
v)
i
0
I0
I
!
20
30
40
50
60
WEIGHTPERCENTAP,(~ FIG. 3. Surface temperatures at times of characteristic reaction events: polymer with AP heated in nitrogen; furna~:e temperature, 1100~
about 100~ higher temperature, apparently also promotes oxidative decomposition. The exotherm temperature is not significantly changed; only the net exotherm flux is.
x~
P o l y m e r with A m m o n i u m P e r c h l o r a t e H e a t e d in N i t r o g e n
--I "
2.C
xl.
CATALYST NONE
W
CuOzO~
O. 0.5 VOLUME
0.8
I:0
FRACTION PBAA
When the oxidant is incorporated in the polymer as small crystals of ammonium perchlorate (AP) instead of being present as ambient oxygen, the phenomena observed are different. For an absorbed radiant flux of 4.0 cal/(set, sq em), a nitrogen pressure of 0.85 atm, and less than 20 wt % of AP, an endotherm is the first indication of reaction but at lower temperatures than when AP is absent, as is shown on Fig. 3. I t is followed, however, b y an exotherm, indicating reaction with AP or its decomposition products.
FIG. 2. Dependence of net exotherm flux on volume frartion of polymer containing small glass beads.
700~(F~
net exotherm heat flux, taken as proportional to rate, vs volume fraction and, therefore, fraction of exposed surface of polymer. The samples had nearly identical temperature histories up to the time of the exotherm, except for a slight increase in exotherm temperature as the fraction of beads was increased. Whatever the rate-controlling process is, it is apparently approximately second order with respect to accessible polymer. The inclusion of 5% of copper chromite resulted in an increase in the net exotherm flux by about 200-/0 over that for the same polymer-bead mixture uncatalyzed, even though the polymer was correspondingly further diluted. The catalyst, which promotes the endothermic decomposition when heating occurs in nitrogen but does so at
u.I
! I
ICATALYST
[
rl IRON OXIDE
I ~
i
o cow
: \'s..
I
mITE
i550~- PBAA+AP+CATALYSTin N, c~
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! (0.85 ATMS, ilOOaC) ,o
WEIGHT PERCENT AP, (%)
FIG. 4. Surface temperatures at times of characteristic reaction events: polymer with AP and catalyst, in 20/1 weight ratio, heated in nitrogen.
530
DECOMPOSITION AND EXPLOSION
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io
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o~
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According to Fig. 5, copper chromite acts to increase the net exotherm flux at all AP concentrations.
|
Polymer with Ammonium Perchlorate Heated in Oxygen ~
=
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AP DENSITY, pwAp (gm/cm a)
FIG. 5. Net exotherm heat flux as a function of AP content, all data for 0.85-atm pressure, ll00~ furnace temperature. AP density is proportional to both the fraction of surface that is AP and the APpolymer contact surface. Finally, the photocell signal is observed, indicating hot particles at tow AP loadings and a visible flame at the higher loadings. The pronounced exotherm precedes the flame, as when oxygen is the oxidant. At AP loadings above 20 wt %, the endotherm is no longer observed. The effect of including conventional burningrate catalysts, 1 part per 20 of AP, is to reduce the significant temperatures, as is revealed by a comparison of Figs. 3 and 4, the effect being greater at the higher AP loadings. The time and the temperature lag of the photocell signal behind the exotherm diminish as AP loading is increased. Extrapolation to propellant loading levels of S0 or more (wt (~o) suggests that the exotherm and the flame will occur ahnost simultaneously and, therefore, that the appearance of a flame is an acceptable indicator of the chemical events of ignition. The net exotherm flux, the measure of the rate of the exothermic reaction, is plotted on Fig. 5 against the density of AP in the composite material. The graph indicates a reaction order of 1.8 with respect to polymer-AP contact surface9 The order is likely greater because the effect reduced exotherm surface temperature at the higher loadings is not accounted for.
In a final series of experiments, AP-loaded polymer was heated in oxygen at a flux of 4.0 cal/(sec, sq cm) and a pressure of 0.85 aim. Figure 6 shows the effect of AP loading on the exotherm and the photocell surface temperatures. According to Fig. 6, small amounts of AP tend to inhibit the oxygen-polymer reaction, probably by reducing the available polymer surface just as the glass beads reduced it. Once the exothermic reaction starts, however, the AP begins to participate, as is shown by the reduction in the temperature of the photocell signal, which indicates a flame. Figure 5 displays the net exotherm fluxes for these experiments along with those for the experiments in nitrogen. I t shows that AP is the more effective oxidant at the higher AP concentrations studied. If extrapolation to propellant loading levels is permitted, it shows that the AP-polymer reaction becomes dominant and that the hot oxygen contributes very little. Keller, Baer, and Ryan 2 reached the same conclusion in their study of the ignition of A I ' - P B A A propellants having smooth surfaces.
Conclusions The most obvious conclusion, trivial because it was expected, is that polymer decomposition in a reactive environment is very different from 660[
]
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--
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~
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~n
]-- -C~,T~YST
T -
'
T ....
T-
-
!
'~EXOTHERM ~ ' - - ~ g e - " ,
I PBAA § AP in Oz 46~-- (0.85 ATMS, IIO0~
-I
Io
I
COPPER CHROMITE I
~ r oToCE,L
~
~"
o
]
ONO.E
" !
i i
L
!
2o
30
40--
5o
60
WEIGHT PERCENT Ate, (%) FIG. 6. Surface temperatures at, times of characteristic reaction events: polymer with AP heated
in oxygen.
DECOMPOSITION OF PBAA decomposition in an inert environment. More significant, exothermic, presumably heterogeneous reactions occur before a flame appears in the gas phase. Part of the energy required for the ignition process, at rates up to 20 cal/(sec, sq cm), inferred from our data, is provided before feedback from hot gas-phase reactions is available. It is reasonable to postulate that these preflame reactions are significant in steady propellant burning, though so compressed in time as to be probably undetectable. The role of oxygen is intricate, and the combustion mechanism very complicated. To rationalize the results, we found it necessary to invoke participation by oxygen close to the surface but at a temperature greater than that at the surface. We would expect quite different results if polymer were heated by radiation but in the presence of oxygen colder than the surface. If an A P - P B A A propellant were being ignited in cold oxygen, we would expect that AP would be the dominant oxidizer, with ambient oxygen playing a negligible role. Indeed, we infer that even hot oxygen may play only a minor role. The situation may well be different with other polymeric fuels, especially if they del)olymerize readily or are unusually vulnerable to oxygen attack at 500~ or below.
531
Our most firm conclusion with respect to mechanism is that the rate-controlling preflame process contributing to the exotherm is probably second order with respect to the polymer surface accessible to the oxidant. This result emerges from an examination of data both from the reaction between oxygen and polymer containing inert solids and from the reaction between polymer and incorporated AP in a nitrogen environment. ACKNOWLEDGMENT
This work was performed under the sponsorship of the Air Force Office of Scientific Research, Propulsion Division; B. T. Wolfson, monitor; Grants AF-AFOSR 40-66, -67. REFERENCES 1. CHENG, J. T.: Thermal Effects of Composite Propellant Reactions, Ph.D. thesis, University of Utah, 1967; also available, under the same title, as AFOSIr 68-0243, August, 1967. 2. KELLER, J. A., BAER, i . D., AND RYAN, N. W. AIAA J..~, 1358 (I966).
COMMENTS E. W. Price, U.S. Naval Weapons Center, China Lake, California. I understood you to state that your results resolve in the affirmative a long-standing question as to whether optical observation of the first f a m e was a reliable indication of attainment of ignition. It is my impression that the long-standing question related to whether combustion would self-sustain upon removal of the internal heating, and whether appearance of the flame assured a self-sustaining situation. This issue has been studied extensively, and it is well established that appearance of the flame may occur considerably before self-sustaining conditions are achieved. Your results apparently do not reveal anything regarding this question.
N. W. Ryan. This comment has made a valid point, although it is not central to the thesis of the paper. In our experiments, a strong exothermic reaction appeared a very short time before a visible flame. When this is the case, the appearance of a visible flame is a sufficient criterion of ignition. However, there are some ignition conditions in which it is apparently not the case. Perhaps we contribute to resolving the issue by directing attention to the exothermic
surface reaction and demonstrating that it can be experimentally detected. II
R. F. McAlevy III, Stevens Institute of Technology, Hoboken, N.J. (1) Considering the probable error introduced in surface temperature calculations, due to incomplete knowledge of optical and thermophysical prol)erties, is it reasonable to make any kinetics arguments based on temperatures known with such a low confidence level ? (2) How do the subject data compare with the DTA, TGA, and other low-heating rate data (where strong effects of gaseous oxygen have been reported) or the high-heating rate d a t a - - b y Hansel and myself in the AIAA Journal in 1966 and 1967--(which show no effects)? (3) Considering that the heating rates employed were an order-of-inagnitude or so below those typical of solid-propellant motor ignition, is it reasonable to make statements concerning the motor situation based on the subject data? N. IV. Ryan. The confidence level of the sin'face temperature is not so low that the modest kinetics arguments advanced should be ques-
532
DECOMPOSITION AND EXPLOSION
tioned for that reason. Whatever might be said about the absolute values of the temperature, comparative values from test to test or of the temperatures for two events in the test are certainly significant. Perhaps a more fundamental question should be asked: W h a t do we mean by "temperature"? I t is possible that the molecular relaxation rate of the polymer is not large compared with the heating rates we employed. If so, refinement of thermophysical data, determined in the usual ways, would be no help in determining the temperature relevant to the chemical processes.
We have compared the behavior of PBAA polymer in the experiments reported here to its behavior in a differential scanning calorimeter, and found them to be different. This observation confirmed our opinion that DSC, TGA, and D T A experiments contribute little to understanding propellant ignition and burning. We do not know if our heating rates are fast enough that the chemical events of realistic ignition practice do indeed occur in our experiments. We think it is safer to assume that they do than to put confidence in T G A or DTA.