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Preignition reactions of AP-HTPB propellants studied by IR spectrometry
P R E I G N I T I O N R E A C T I O N S OF AP-HTPB P R O P E L L A N T S S T U D I E D BY IR SPECTROMETRY ROBERT J. LAW General Electric Corp., Vallecitos Road, Pleasanton, CA 94566 AND
ALVA D. BAER AND NORMAN W. RYAN Department of Chemical Engineering, University of Utah, Salt Lake City, UT 84112
Introduction A great deal of interest, with both diagnostic and design motivation, has been displayed in the preflame chemical reactions of composite solid propellants containing ammonium perchlorate (AP) and organic polymers. In particular, the solid-phase reactions, more accurately the reactions occurring at the phase boundaries, have been of concern. Their character has been inferred largely from thermal experiments, the most relevant being those conducted at high heating rates.l,z Other methods of study include adiabatic heating, ~ measurement of linear regression rates, 4 electronmicroscope examination of burned surfaces, 5 variation of the solid composition,6 and ignition with controlled stimulus. ~ A chemical method of study is infrared spectrometry of specimens before and after pyrolysis,s.9 This paper reports a chemical study in which certain chemical reactions were followed during the course of the pyrolysis. An IR beam was focused on a thin film of reacting material and the transmitted radiation analyzed by a spectrometer operating at the rate of eight scans per second.
Experimental The basic analytical tool employed in this investigation was the Warner and Swasey Model 501 Fast-scanning Spectrometer. Early in the investigation, it was determined that the conventional silicon carbide infrared source did not produce radiation at a high enough intensity to yield an analyzable transmitted beam. The silicon-carbide element was 1271
replaced with a 1/8-inch diameter vitreous carbon rod. This rod was mounted in the water-cooled holder, held between copper electrodes, and electrically heated. The lower electrode clamp, nickel plated, was floated in a water-cooled, mercury-filled well to permit thermal expansion while electrical contact was maintained. Operated at 2750~ in an argon atmosphere, the rod had a lifetime of a few hours. It could be operated at 3200~ for shorter periods of time. A more detailed description of this high temperature infrared radiation source unit is presented in reference
[10]. The test specimens were polymer-ammonium perchlorate films 25.4 ~m thick. The film was sandwiched between two Nichrome V strips, each 25.4 ~xm thick, 1.27 cm wide, and 5 cm long. A slot (0.56 mm wide, 2.0 cm long) was cut in each Nichrome strip to provide a window for passage of radiation. Imbedded in the film 1 or 2 mm from the sl& was a chromel-alumel thermocouple junction 10 Ixm in diameter. The short edges of the composite Nichrome-film-Nichrome strip were clamped between copper electrodes, and the assembly was positioned so that the exposed polymer at the window was at the common focus of the source and analyzer components of the spectrometer. Heating was accomplished electrically, the Nichrome strips serving as resistance heating elements. The thermocouple output was recorded to yield a history of the sample temperature. The signal was also fed into a control circuit which regulated the voltage across the Niehrome strips to maintain the film temperature at the predetermined value after initial temperature rise.
1272
PROPELLANT IGNITION AND COMBUSTION
The spectrometer was operated at the rate of eight scans per second of the spectral range 2.5 to 5.5 txm, a liquid-nitrogen-cooled InSb detector being used. The output signal was displayed as vertical displacement of the beam of an oscilloscope which was operated without sweep. A strip camera with 35-ram film was used to photograph the oscilloscope screen. The spectrometer signal was represented transversely and time (and therefore wavelength) longitudinally on the moving film. Spectra were recorded before and during the heating part of each test, after the film had cooled, and after the film had been cut out of the slot. The last spectra were used to provide a base absorption curve for subsequent data analysis. All tests reported in this study were conducted in air at 0.85 arm. For the test times of interest here, the data of Bouck 1 indicate little air-polymer reaction for this system below 700~ During the transient heating period, the temperature difference A T m between the metallic strips and the center line of the exposed film near the maximum temperature can be estimated from 11 r l "2 ZXTm= - - - - . 2K In this study, the heating rate employed, r, and the half slit, width, l, were respectively, r = 80~ and l = 0.28 ram. For a thermal diffusivity typical of such films, K = 1.2 x 1.0-3 cm ~/sec, the a Tmwould differ from zero principally as a result of radiant heat loss from the films. At about 600~ for a 25-1xm thick film, this maximum temperature difference could be nearly 30~ However, a significant amount of radiation from the IR source was absorbed in the films, and the edge to centerline temperature difference was therefore somewhat less than this value. It was possible to detect the temperature of the orthorhombie to cubic phase change of the ammonium perchlorate from the spectral records. These results indicated that the true mean film temperature was 4 to 8~ less than the measured temperature at 513~ In the following discussion, all temperatures cited are the values measured by the thermocouple, and no corrections have been applied. The test films for this study consisted of 30 weight percent of finely divided ammonium perchlorate (5 txm) and 70 weight percent of a mixture of 92.5 weight percent hydroxyl-terminated polybutadiene (ARCO Chemical Co.
R-25, functionality 2.2-2.3 and 0.85 meq OH/gin) and 7.5 weight percent isophrone diisocyanate (IPDI). The thin films of this material when cured displayed excellent adhesive properties, thus insuring a good thermal contact with the heating strips. The thickness of the films and the quantity of AP added to the polymer were selected to give about equal absorption of the 3.04-txm NH band of the AP and the 3.42-~xm CH band of the binder. Normally, 30 weight percent of AP was used. A typical spectrum from the single-beam fastscanning spectrometer is shown in Figure 1. The two absorption bands of interest are indicated. A detailed discussion of the spectral characteristics of the spectra obtained from various combinations of ingredients before and after 9
yt
x r/
SIGNAL WITHOUT FILM Xl/2 z
d
u3 >
J lxl
t"
z
StGNAL WITH
~_L_J_ t~2_ _ ..n ~ J _ L _ ~ _ ~ p m •
I, I
I
ii
2.5
4.0
5,5
FILM
WAVELENGTH, pm
FIG. 1. A sketch of the output from the singlebeam fast-scanning spectrometer indicating data reduction positions and heights relative to dashed baseline. The 2.5- to 5.5-1xm range was scanned in 0.1 second. The intensity symbols on this plot are interpreted as: (primed symbols indicate intensities without film present) a--2.74 microns, window used in rapid-scanning baseline reduction method, x--3.04 microns, residual transmission after NH band absorption by NH4C10 4 in a loaded polymer; a window for HTPB-polymer, y--3.42 microns, residual transmission after CH band absorption by polymer, r--4.03 microns, window used in rapid-scanning baseline reduction method, d--4.38 microns, window used in rapid-scanning baseline reduction method, z--4.45 microns, NCO absorption by free isocyanate, Io--3.04 microns, projected transmission without NH band absorption, Iio--3.42 microns, projected transmission without CH band absorption.
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS curing is outlined in reference [10] for the H T P B - I P D I - A P system.
Data Processing The large quantity of information generated with the fast-scanning spectrometer d u r i n g each test required a data reduction technique that w o u l d correct for the spectral variations, yet retain the important spectral features. In using a single-beam instrument, one takes the spectrum of interest (polymer film in place) a n d the spectrum of the source (polymer film cut away) at different times, then attempts to compare them. F i g u r e 1 shows the source spectrum and that w i t h the film in place. A projected base line is established b y scaling d o w n the source spectrum to match the film spectrum at the w i n d o w s at 2.74, 4.03, and 4.38 txm, then at all other wavelengths taking into account the w a v e l e n g t h d e p e n d e n c e of the scaling factor. , With reference to F i g u r e 1, we w o u l d take the absorbance (hereby defined) of N H as ( I 0 - x)/Io, a n d of C H as ( H o - y ) / H o. The fractional absorbance, s h o w n on Figures 3 and 4, is the value obtained for a particular spectrum d i v i d e d b y that taken for a scan for the cold film before heating. If the extinction coefficient in the BeerLambert law were a constant at a given wave-
1273
length, then the concentrations of N H and C H could be calculated as a function of time. Unfortunately, the extinction coefficient is a function of temperature, at least for NH, and some d e c o m p o s i t i o n occurred before the desired test temperature was attained. Concentrations could not be computed.
Results If, for the AP-polymer system of interest here, the coefficient of absorption, e, in the Beer-Lambert law were i n d e p e n d e n t of temperature, it w o u l d be p o s s i b l e to relate infrared absorption measurements to species concentrations as the film was heated. However, it was f o u n d that the coefficient of absorption b y N H was not a constant, and further that the transmissivity of the films even at the w i n d o w regions, was a complex function of temperature. At a constant temperature, the absorption coefficient was a p p a r e n t l y constant. It was thus possible to make a valid interpretation of tests for times after a film was heated to a preselected temperature. T h e reactions of the AP and p o l y m e r were f o l l o w e d b y noting the changes in the absorbance at the 3.04-txm (NH) and 3.42-1xm (CH) wavelengths. A discussion of these tests and a d i s p l a y of the results in reaction kinetic terms are presented in the last part of this section.
20 :~D~......~
4.03~m ( r ) ~
!5
~
o--....~ o .~...~
' 0 - - ' - - - 0 ~ 0_..~_0 2.74prn(a) ~-'-'--0 ~_,_._0..... ~ - 0 ~ 0 ~ 0 ~
n~ <
~--5 uO
0
3 . 0 4 ~ m (NM) .~--0--o--o--o--o--o--o
z LU
3 , 4 2 jum ( C H ) - - ~ o
o 600 soo
W
~- .400 Ld P 300
~) oddeoa:m:o~-o
o--o--o--o--o--o
_/ o
590*K-.J
I
I
5
10
I
I
15 20 TIME, SEC.
I
25
30
FIG. 2. The magnitude of the signals (intensities) at the various wavelengths measured as indicated in Figure 1 are shown here as a function of time and temperature for a test on a 25-~m thick film containing 30 percent AP in the HTPB fuel-binder.
1274
PROPELLANT IGNITION AND COMBUSTION
Figure 2 presents results from a test on a 70/30 polymer/AP film 25 p~m thick. This film was heated at a rate of 80~ to 490~ and held at this temperature for several seconds. Very similar results were noted with films formed with other polymers. The transmitted signal intensities are plotted as a function of time. The intensities are the strengths of the transmitted signals, e.g., (as on Figure 1) 1o - x f o r N H . The effect of increasing temperature is to increase the infrared transmittance at all wavelengths. A similar effect for visible radiation was noted by Mihlfeith. lz At about 513~ the temperature of the orthorhombic to cubic phase change for AP, a decrease in the transmittance or signal intensity was noted. This decrease is likely the result of the changes in crystal density and refractive index. As the temperature was increased above 513~ the signal intensities again increased at all wavelengths. Once the film reached a constant temperature, the signal intensity at the window regions (4.03 and 2.74 ixm) decreased slowly with time as the film darkened at the high temperature. The signal intensities at the NH and CH bands would increase, decrease, or remain relatively constant, as in Figure 2, depending on the relative rates of film darkening and oxidizer-polymer reaction. The possibility that these temperature effects were due solely to the HTPB was checked by making tests on films of polymer without AP. In this case, the transmission of radiation was dependent on temperature, though in a different way. When a polymer was heated a second time, it was found that the transmissivity of the film was essentially constant with temperature. It is postulated that a thermal annealing process eliminated optically inhomogeneous regions in the polymer. The pyrolysis of the polymer has been studied by this technique, and the results of these tests are presented in references [10] and [13]. Unfortunately, the effects illustrated in Figure 2 for the AP-loaded films persisted in second and later cycles of heating. In experiments intended for constanttemperature observations at temperatures above 600~ ignition of the AP-containing film sometimes occurred. The probability of ignition increased with increasing preselected temperature, being virtually certain at 620~ The available temperature range was limited accordingly. L. S. Bouck, l working with 30% AP in similar polymers heated at similar rates, observed ignition between 600 and 620~ Figure 3 shows typical results from the test of a 25-p,m thick HTPB-30% AP film heated
at 80~ to (and held at) 590~ Here, and in all subsequent discussions, time is measured from the start of film heating. The CH band absorbance was constant and the NH band absorbance decreased as the temperature rose to 513~ After the phase-change temperature was exceeded, both absorbances decreased. In this case, when the film was maintained at a constant temperature of 590~ both the NH and the CH band absorbances decreased monotonically with time. The film absorbances after cooling are shown at the righthand side of the longtime-scale plot. The agreement between the last high-temperature value of the CH absorbance and the room temperature value illustrates the apparent constancy with temperature of the Beer-Lambert absorption coefficient for the CH band. A
~c 1.0 o o_~39o(L~q
3.42pm (CH)
<~0.9 - U ~ m
0.8
(NH)
~_ 0.7 0.6 ~600 0.:'50O ][ 400
3~
300
590~ I
1
2
3
I
I
[
L
4 5 6 7 TIME, SEC. (a)
8
1.0
c5 0.9, ;)-% rY
0 d ) 0.8 rn <[ 0.7 ,( 0.6
o
I
3.42.pm (C H ) \
~176
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o.-
~176176176
0.3
I
0
I
__
9 lO
I
I
,'
I
I
10 20 30 40 50 60 ~' C O O L E D -a TIME, SEC. (b)
FIG. 3. Typical short- and long-time histories of the CH and NH fractional absorbances as a function of time and temperature. Cooling of the film started at 65 seconds. The darkened symbols on the right of the plot correspond to the room temperature fractional absorbances of these bands after cooling the reacted films.
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS significant change in the N H b a n d absorbance after cooling was noted however. If the room-temperature values of the fractional absorbances b y a previously heated film are assumed to be equal to the fraction of the unreacted AP and H T P B (treated here as C 2 H a ) , it is possible to evaluate the ratio of AP to H T P B reacting in the films. F r o m Figure 3, values of 0.14 a n d 0.43 are obtained for the reacted fractions of C H and NH. T h e n about 2.5 moles of C H reacted for each mole of NH, a typical value for the tests performed. If we equate three moles of C H to one mole of C 2 H 3 and four moles of N H to one mole of AP, then 3.3 moles of C 2 H a are c o n s u m e d for each mole of AP d e c o m p o s e d . Such a large figure makes it i m p o s s i b l e to account for C H loss as conversion to C O and H z or H 2 0 . We infer that some volatile h y d r o c a r b o n fragments are released b y oxidative pyrolysis. Experiments with p o l y m e r not containing AP support this view, there b e i n g negligible C H d i s a p p e a r a n c e w h e n the films were heated to and held at 625~ F o r the part of the test period at constant temperature, the data of F i g u r e 3 should be a m e n a b l e to kinetic analysis. If the N H decay is a first-order process, the initial concentration and the d i s p l a c e m e n t of the time scale are not relevant, and the logarithm of fractional absorbance should be linear in time. As F i g u r e 4 shows for the tests analyzed, the a s s u m p t i o n 1.0 o.g ~ 0.8 0.7 0.6
O--O~o ~-~- 0 ~
0 560~
~-~n
o5a--~o~_
"~'~
~3 0.5 z ca 0.4
~o. ~._.\
~
~ 0
571~
@60~K~..~ 611*K ~ (
113 <( ~zO.3
Q
W c~ LL
0.2
0.1 O
_
L
10
20
30 40 TIME, SEC.
50
60
FIG, 4. A semi-logarithmic plot of the fraction absorbances of the 3.04-1xm NH band as a function of time for several tests of 25-1xm thick HTPB-30 films at various temperatures.
1 xlO-
1275
1 \
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/
\
~
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lx10-2
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1.7
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FIG. 5. An Arrhenius plot is shown here of the first-order rate constants. The solid lines are the least-squares representations. The dashed lines labeled 1, 2, and 3 are derived from data presented in references [15], [3], and [7], respectively. of a first-order process appears to be valid. T h e slopes of the lines on Figure 4 are rate constants w h i c h are d i s p l a y e d on an Arrhenius plot, Figure 5. F r o m the slope of the N H line on F i g u r e 5, we obtain a value of 19 k c a l / g mole for the activation energy. W h e n the data for C H decay are treated in the same way, we find that the least-squares line on Figure 5 leads again to 19 k c a l / g mole for the activation energy. C o n s i d e r i n g the scatter of C H points on Figure 5, one is inclined to consider o b t a i n i n g this same value as sheer accident. Nevertheless, it is the value to be expected because very p r o b a b l y the N H a n d the C H reactions are c o u p l e d a n d sequential. By our present interpretation the AP decomposes first. The C 2 H 3 then reacts at the same rate (adjusted for stoichiometry) as fixed b y the s u p p l y of oxidizing species from the AP. The ratio of the a p p a r e n t first-order rate constant for C H d i s a p p e a r a n c e to the firstorder rate constant for N H disappearance is given b y
1276
PROPELLANT IGNITION AND COMBUSTION kc
1
2
x
kN
3
3 1-x'
where x is the fractional conversion of CH at which kc is determined, The numerical constants contain the presumably constant ratio of rates of disappearance, 2.5, and the ratio of initial concentrations, 7.5. The likely range of x is 0.1 to 0.2, giving 0.17 to 0.26 as the predicted range for k c / k N. This variation, arising randomly in data analysis, could explain the exaggerated scatter of CH results on Figure 5. The value of k c / k N given by the least-squares lines on Figure 5 is 0.25. Figure 5 also displays results adapted from three other studies. Waesche 15 presented data from DSC tests on several propellants; Inami, Ros ser, and Wise 3 made adiabatic heating tests on propellant slabs; and Baer and Ryan v studied hot-wire ignition of propellants. The adaptation of their results required assumptions which discourage the direct comparison of rate constants, but still permit comparison of the slopes and the activation energies obtained from them. In the other three studies, an activation energy of about 30 kcal/g mole was reported, and this study one of 19 kcal/g mole. The precision of the results of this study is such that the difference is probably not attributable to random error. The first temptation is to assume that the other three studies dealt with a reaction-limited process, and this one dealt with a diffusionlimited process. We have focused on the later stages of reaction, when a vapor gap had developed between the two phases and oxidizing species had to diffuse from AP to polymer. Such a mechanism implies an activation energy equal to half the heat of dissociation of AP. However, since the heat of dissociation is about 58 kcal/g mole, 16 we predict an activation energy of about 30 kcal / g mole even for the diffusion-limited process. A possible explanation does come from a more detailed consideration of the diffusion process. We consider product diffusing from a spherical AF particle of decreasing diameter through a gaseous film to react with the polymer at the surface of a spherical bubble of increasing diameter. Such an approach requires postulating a temperature or narrow range of temperature at which the reaction first starts, and the data from these tests suggest that significant reaction is initiated at the phase change. If one further assumes that the diffusion process is essentially equimolar counter diffusion, that the instantaneous diffusion rate is describable by the steady state solution for
the existing AP to polymer surface separation (a film thickness), and that an average diffusion coefficient is applicable, a straight-forward development leads to the result that
f
t
~dF
F F 2 / 3 -~- 2 8 F 1 / 3
= C l t.
Here F is the fraction AP unreacted, 8 is the ratio of the gas film thickness to original AP particle diameter, t is time, and C l is a constant whose value depends on system parameters. The relationship between ~ and F is easily obtained and involves the fraction of the volume of the gas film generated by AP decomposition, taken as about 0.4 on the basis of the observed N H / C H ratio of reaction. This value was used in the numerical evaluation of the integral. The resulting F ( t ) is appropriately described by a first-order rate equation only after F has fallen below about 0.6. A superficial first-order rate constant obtained for greater values of/7, as in this study, will be too large. It is possible, therefore, that rate constants for NH disappearance at temperatures below about 590~ may be too large. As seen on Figure 5, lowering the values of the lowtemperature constants will result in a steepening of the line and an approach to a 30-kcal/g mole value for the activation energy. It is likely that we have studied a diffusionlimited reaction between AP and the HTPB polymer. We cannot, however, on the basis of activation energy, distinguish between the process we studied and the processes studied by the other workers cited. We have, however, reached our conclusions on the basis of chemical rather than thermal measurements.
Conclusions Infrared absorption spectrometry, employing a high-temperature IR source developed for this study and a fast-scanning spectrometer, has been used to follow the disappearance of NH and CH bonds during the pyrolysis of HTPB-IPDI polymer containing 30 weight percent ammonium perchlorate. The conclusions reached are that the reactions are coupled, polymer decomposition being induced by products of AP decomposition, and about 2.5 CH bonds disappear for each NH bond that disappears. The activation energy obtained, 19 kcal/g mole, differs markedly from the 30 kcal/g mole expected. The difference is tentatively attributed to the possibility that
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS the diffusion mechanism thought to be active is not properly describable as a first-order rate process at the low levels of conversion observed at the lower temperatures studied.
Acknowledgment This work was sponsored by NASA, Grant NGL45-003-019. REFERENCES 1, BoucK, L. S., BAER, A. D., AND RYA~, N. W., "Pyrolysis and Oxidation of Polymers at High Heating Rates," Fourteenth Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute, 1139 (1973). Also L. S. Bouck, "Propellant Reactions," Ph.D. Dissertation, University of Utah (1971). 2. CHENC, J. T., RYAN, N. W., AND BAER, A. D., "Oxidative Decomposition of PBAA Polymer at High Heating Rates," Twelfth Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute 525 (1969). 3. INAMI,S. H., ROSSER,W. A., JR., AND WISE, J., "Heat-Release Kinetics of Ammonium Perchlorate in the Presence of Catalysts and Fuel," Combustion and Flame, 12, 41 (1968). 4. SIMON,D. R., "Rapid Linear Pyrolysis of Composite Solid Ingredients," Report NASA CR66856 under Contract No. NAS1-8450, Vitro Laboratories, Silver Springs, MD., September 1970. 5. Boccs, T. L., "Deflagration Rate, Surface Structure and Subsurface Profile of Self-Deflagrating Single Crystals of Ammonium Perchlorate," AIAA Journal, 8, 867 (1970). 6. SCrtM1DT, W. G., "The Effect of Solid Phase Reactions on the Ballistic Properties of Propellants," Report No. NASA CR 112083, (N7221809) under Contract NAS 1-9463, Aerojet Solid Propulsion Co. Sacramento, California, August 1972.
1277
7. BAER, A. D. AND RYAN, N. W., "Evaluation of Thermal-Ignition Models from Hot-wire Ignition Tests," Combustion and Flame, 15, 9 (1970). 8. DAUERMAN,L., "Solid State Reactions in Solid Propellants," Report under Contract AFOSR1491-68, Department of Chemical Engineering, New York University, University Heights, New York, N.Y., March 1970. 9. HARTMAN,K. O. ANDMusso, R. C., "The Thermal Decomposition of Nitroglycerine and Its Relationship to the Stability of CMDB Propellants," Paper 72-30, Fall Meeting of the West Coast Section, The Combustion Institute, Monterey, California, October 1972. 10. Law, R. J., "Reactions in Solid Propellants from Infrared Spectra," Ph.D. Thesis, Department of Chemical Engineering, University of Utah (1976). 11. CARS~W, H. S. AND JAEGER, J. C., Conduction of Heat in Solids, 2nd Ed., London: Oxford University Press, p. 104 (1959). 12. MIHLFEITH,C. M., BAEa, A. D., AND RYAN, N. W., "The Response of a Burning Solid Propellant Surface to Thermal Radiation," AFOSR Scientific Report No. TR-71-2664, AD-736049 (1971). 13. Law, R. J., BAER, A. D., AND RYnn, N. W., "A Spectrometric Study of Pyrolysis Reactions of Polyurethane," West Coast Section, The Combustion Institute, Paper 73-11 (1973). 14. JAco~s, P. W. M. aND RUSSELL-JoNES,A., "'The Decomposition and Ignition of Mixtures of Ammonium Perchlorate and Copper Chromate," Eleventh Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute, 457 (1967). 15. WAESCHE,R. H. W., "Research Investigation of the Decomposition of Composite Solid Propellants," United Aircraft Research Laboratories Report G910476-24 on Contract NAS7-481 (1969). 16. INAMI,S. H., W. A. ROSSEa,ANDH. WIsE,~'Dissociation Pressure of Ammonium Perchlorate," J. Phys. Chem., 67, 1077-1079 (1963).
ALVA D. BAER AND NORMAN W. RYAN Department of Chemical Engineering, University of Utah, Salt Lake City, UT 84112
Introduction A great deal of interest, with both diagnostic and design motivation, has been displayed in the preflame chemical reactions of composite solid propellants containing ammonium perchlorate (AP) and organic polymers. In particular, the solid-phase reactions, more accurately the reactions occurring at the phase boundaries, have been of concern. Their character has been inferred largely from thermal experiments, the most relevant being those conducted at high heating rates.l,z Other methods of study include adiabatic heating, ~ measurement of linear regression rates, 4 electronmicroscope examination of burned surfaces, 5 variation of the solid composition,6 and ignition with controlled stimulus. ~ A chemical method of study is infrared spectrometry of specimens before and after pyrolysis,s.9 This paper reports a chemical study in which certain chemical reactions were followed during the course of the pyrolysis. An IR beam was focused on a thin film of reacting material and the transmitted radiation analyzed by a spectrometer operating at the rate of eight scans per second.
Experimental The basic analytical tool employed in this investigation was the Warner and Swasey Model 501 Fast-scanning Spectrometer. Early in the investigation, it was determined that the conventional silicon carbide infrared source did not produce radiation at a high enough intensity to yield an analyzable transmitted beam. The silicon-carbide element was 1271
replaced with a 1/8-inch diameter vitreous carbon rod. This rod was mounted in the water-cooled holder, held between copper electrodes, and electrically heated. The lower electrode clamp, nickel plated, was floated in a water-cooled, mercury-filled well to permit thermal expansion while electrical contact was maintained. Operated at 2750~ in an argon atmosphere, the rod had a lifetime of a few hours. It could be operated at 3200~ for shorter periods of time. A more detailed description of this high temperature infrared radiation source unit is presented in reference
[10]. The test specimens were polymer-ammonium perchlorate films 25.4 ~m thick. The film was sandwiched between two Nichrome V strips, each 25.4 ~xm thick, 1.27 cm wide, and 5 cm long. A slot (0.56 mm wide, 2.0 cm long) was cut in each Nichrome strip to provide a window for passage of radiation. Imbedded in the film 1 or 2 mm from the sl& was a chromel-alumel thermocouple junction 10 Ixm in diameter. The short edges of the composite Nichrome-film-Nichrome strip were clamped between copper electrodes, and the assembly was positioned so that the exposed polymer at the window was at the common focus of the source and analyzer components of the spectrometer. Heating was accomplished electrically, the Nichrome strips serving as resistance heating elements. The thermocouple output was recorded to yield a history of the sample temperature. The signal was also fed into a control circuit which regulated the voltage across the Niehrome strips to maintain the film temperature at the predetermined value after initial temperature rise.
1272
PROPELLANT IGNITION AND COMBUSTION
The spectrometer was operated at the rate of eight scans per second of the spectral range 2.5 to 5.5 txm, a liquid-nitrogen-cooled InSb detector being used. The output signal was displayed as vertical displacement of the beam of an oscilloscope which was operated without sweep. A strip camera with 35-ram film was used to photograph the oscilloscope screen. The spectrometer signal was represented transversely and time (and therefore wavelength) longitudinally on the moving film. Spectra were recorded before and during the heating part of each test, after the film had cooled, and after the film had been cut out of the slot. The last spectra were used to provide a base absorption curve for subsequent data analysis. All tests reported in this study were conducted in air at 0.85 arm. For the test times of interest here, the data of Bouck 1 indicate little air-polymer reaction for this system below 700~ During the transient heating period, the temperature difference A T m between the metallic strips and the center line of the exposed film near the maximum temperature can be estimated from 11 r l "2 ZXTm= - - - - . 2K In this study, the heating rate employed, r, and the half slit, width, l, were respectively, r = 80~ and l = 0.28 ram. For a thermal diffusivity typical of such films, K = 1.2 x 1.0-3 cm ~/sec, the a Tmwould differ from zero principally as a result of radiant heat loss from the films. At about 600~ for a 25-1xm thick film, this maximum temperature difference could be nearly 30~ However, a significant amount of radiation from the IR source was absorbed in the films, and the edge to centerline temperature difference was therefore somewhat less than this value. It was possible to detect the temperature of the orthorhombie to cubic phase change of the ammonium perchlorate from the spectral records. These results indicated that the true mean film temperature was 4 to 8~ less than the measured temperature at 513~ In the following discussion, all temperatures cited are the values measured by the thermocouple, and no corrections have been applied. The test films for this study consisted of 30 weight percent of finely divided ammonium perchlorate (5 txm) and 70 weight percent of a mixture of 92.5 weight percent hydroxyl-terminated polybutadiene (ARCO Chemical Co.
R-25, functionality 2.2-2.3 and 0.85 meq OH/gin) and 7.5 weight percent isophrone diisocyanate (IPDI). The thin films of this material when cured displayed excellent adhesive properties, thus insuring a good thermal contact with the heating strips. The thickness of the films and the quantity of AP added to the polymer were selected to give about equal absorption of the 3.04-txm NH band of the AP and the 3.42-~xm CH band of the binder. Normally, 30 weight percent of AP was used. A typical spectrum from the single-beam fastscanning spectrometer is shown in Figure 1. The two absorption bands of interest are indicated. A detailed discussion of the spectral characteristics of the spectra obtained from various combinations of ingredients before and after 9
yt
x r/
SIGNAL WITHOUT FILM Xl/2 z
d
u3 >
J lxl
t"
z
StGNAL WITH
~_L_J_ t~2_ _ ..n ~ J _ L _ ~ _ ~ p m •
I, I
I
ii
2.5
4.0
5,5
FILM
WAVELENGTH, pm
FIG. 1. A sketch of the output from the singlebeam fast-scanning spectrometer indicating data reduction positions and heights relative to dashed baseline. The 2.5- to 5.5-1xm range was scanned in 0.1 second. The intensity symbols on this plot are interpreted as: (primed symbols indicate intensities without film present) a--2.74 microns, window used in rapid-scanning baseline reduction method, x--3.04 microns, residual transmission after NH band absorption by NH4C10 4 in a loaded polymer; a window for HTPB-polymer, y--3.42 microns, residual transmission after CH band absorption by polymer, r--4.03 microns, window used in rapid-scanning baseline reduction method, d--4.38 microns, window used in rapid-scanning baseline reduction method, z--4.45 microns, NCO absorption by free isocyanate, Io--3.04 microns, projected transmission without NH band absorption, Iio--3.42 microns, projected transmission without CH band absorption.
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS curing is outlined in reference [10] for the H T P B - I P D I - A P system.
Data Processing The large quantity of information generated with the fast-scanning spectrometer d u r i n g each test required a data reduction technique that w o u l d correct for the spectral variations, yet retain the important spectral features. In using a single-beam instrument, one takes the spectrum of interest (polymer film in place) a n d the spectrum of the source (polymer film cut away) at different times, then attempts to compare them. F i g u r e 1 shows the source spectrum and that w i t h the film in place. A projected base line is established b y scaling d o w n the source spectrum to match the film spectrum at the w i n d o w s at 2.74, 4.03, and 4.38 txm, then at all other wavelengths taking into account the w a v e l e n g t h d e p e n d e n c e of the scaling factor. , With reference to F i g u r e 1, we w o u l d take the absorbance (hereby defined) of N H as ( I 0 - x)/Io, a n d of C H as ( H o - y ) / H o. The fractional absorbance, s h o w n on Figures 3 and 4, is the value obtained for a particular spectrum d i v i d e d b y that taken for a scan for the cold film before heating. If the extinction coefficient in the BeerLambert law were a constant at a given wave-
1273
length, then the concentrations of N H and C H could be calculated as a function of time. Unfortunately, the extinction coefficient is a function of temperature, at least for NH, and some d e c o m p o s i t i o n occurred before the desired test temperature was attained. Concentrations could not be computed.
Results If, for the AP-polymer system of interest here, the coefficient of absorption, e, in the Beer-Lambert law were i n d e p e n d e n t of temperature, it w o u l d be p o s s i b l e to relate infrared absorption measurements to species concentrations as the film was heated. However, it was f o u n d that the coefficient of absorption b y N H was not a constant, and further that the transmissivity of the films even at the w i n d o w regions, was a complex function of temperature. At a constant temperature, the absorption coefficient was a p p a r e n t l y constant. It was thus possible to make a valid interpretation of tests for times after a film was heated to a preselected temperature. T h e reactions of the AP and p o l y m e r were f o l l o w e d b y noting the changes in the absorbance at the 3.04-txm (NH) and 3.42-1xm (CH) wavelengths. A discussion of these tests and a d i s p l a y of the results in reaction kinetic terms are presented in the last part of this section.
20 :~D~......~
4.03~m ( r ) ~
!5
~
o--....~ o .~...~
' 0 - - ' - - - 0 ~ 0_..~_0 2.74prn(a) ~-'-'--0 ~_,_._0..... ~ - 0 ~ 0 ~ 0 ~
n~ <
~--5 uO
0
3 . 0 4 ~ m (NM) .~--0--o--o--o--o--o--o
z LU
3 , 4 2 jum ( C H ) - - ~ o
o 600 soo
W
~- .400 Ld P 300
~) oddeoa:m:o~-o
o--o--o--o--o--o
_/ o
590*K-.J
I
I
5
10
I
I
15 20 TIME, SEC.
I
25
30
FIG. 2. The magnitude of the signals (intensities) at the various wavelengths measured as indicated in Figure 1 are shown here as a function of time and temperature for a test on a 25-~m thick film containing 30 percent AP in the HTPB fuel-binder.
1274
PROPELLANT IGNITION AND COMBUSTION
Figure 2 presents results from a test on a 70/30 polymer/AP film 25 p~m thick. This film was heated at a rate of 80~ to 490~ and held at this temperature for several seconds. Very similar results were noted with films formed with other polymers. The transmitted signal intensities are plotted as a function of time. The intensities are the strengths of the transmitted signals, e.g., (as on Figure 1) 1o - x f o r N H . The effect of increasing temperature is to increase the infrared transmittance at all wavelengths. A similar effect for visible radiation was noted by Mihlfeith. lz At about 513~ the temperature of the orthorhombic to cubic phase change for AP, a decrease in the transmittance or signal intensity was noted. This decrease is likely the result of the changes in crystal density and refractive index. As the temperature was increased above 513~ the signal intensities again increased at all wavelengths. Once the film reached a constant temperature, the signal intensity at the window regions (4.03 and 2.74 ixm) decreased slowly with time as the film darkened at the high temperature. The signal intensities at the NH and CH bands would increase, decrease, or remain relatively constant, as in Figure 2, depending on the relative rates of film darkening and oxidizer-polymer reaction. The possibility that these temperature effects were due solely to the HTPB was checked by making tests on films of polymer without AP. In this case, the transmission of radiation was dependent on temperature, though in a different way. When a polymer was heated a second time, it was found that the transmissivity of the film was essentially constant with temperature. It is postulated that a thermal annealing process eliminated optically inhomogeneous regions in the polymer. The pyrolysis of the polymer has been studied by this technique, and the results of these tests are presented in references [10] and [13]. Unfortunately, the effects illustrated in Figure 2 for the AP-loaded films persisted in second and later cycles of heating. In experiments intended for constanttemperature observations at temperatures above 600~ ignition of the AP-containing film sometimes occurred. The probability of ignition increased with increasing preselected temperature, being virtually certain at 620~ The available temperature range was limited accordingly. L. S. Bouck, l working with 30% AP in similar polymers heated at similar rates, observed ignition between 600 and 620~ Figure 3 shows typical results from the test of a 25-p,m thick HTPB-30% AP film heated
at 80~ to (and held at) 590~ Here, and in all subsequent discussions, time is measured from the start of film heating. The CH band absorbance was constant and the NH band absorbance decreased as the temperature rose to 513~ After the phase-change temperature was exceeded, both absorbances decreased. In this case, when the film was maintained at a constant temperature of 590~ both the NH and the CH band absorbances decreased monotonically with time. The film absorbances after cooling are shown at the righthand side of the longtime-scale plot. The agreement between the last high-temperature value of the CH absorbance and the room temperature value illustrates the apparent constancy with temperature of the Beer-Lambert absorption coefficient for the CH band. A
~c 1.0 o o_~39o(L~q
3.42pm (CH)
<~0.9 - U ~ m
0.8
(NH)
~_ 0.7 0.6 ~600 0.:'50O ][ 400
3~
300
590~ I
1
2
3
I
I
[
L
4 5 6 7 TIME, SEC. (a)
8
1.0
c5 0.9, ;)-% rY
0 d ) 0.8 rn <[ 0.7 ,( 0.6
o
I
3.42.pm (C H ) \
~176
I-C)
0.5 i:z LL 0,4
I
o.-
~176176176
0.3
I
0
I
__
9 lO
I
I
,'
I
I
10 20 30 40 50 60 ~' C O O L E D -a TIME, SEC. (b)
FIG. 3. Typical short- and long-time histories of the CH and NH fractional absorbances as a function of time and temperature. Cooling of the film started at 65 seconds. The darkened symbols on the right of the plot correspond to the room temperature fractional absorbances of these bands after cooling the reacted films.
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS significant change in the N H b a n d absorbance after cooling was noted however. If the room-temperature values of the fractional absorbances b y a previously heated film are assumed to be equal to the fraction of the unreacted AP and H T P B (treated here as C 2 H a ) , it is possible to evaluate the ratio of AP to H T P B reacting in the films. F r o m Figure 3, values of 0.14 a n d 0.43 are obtained for the reacted fractions of C H and NH. T h e n about 2.5 moles of C H reacted for each mole of NH, a typical value for the tests performed. If we equate three moles of C H to one mole of C 2 H 3 and four moles of N H to one mole of AP, then 3.3 moles of C 2 H a are c o n s u m e d for each mole of AP d e c o m p o s e d . Such a large figure makes it i m p o s s i b l e to account for C H loss as conversion to C O and H z or H 2 0 . We infer that some volatile h y d r o c a r b o n fragments are released b y oxidative pyrolysis. Experiments with p o l y m e r not containing AP support this view, there b e i n g negligible C H d i s a p p e a r a n c e w h e n the films were heated to and held at 625~ F o r the part of the test period at constant temperature, the data of F i g u r e 3 should be a m e n a b l e to kinetic analysis. If the N H decay is a first-order process, the initial concentration and the d i s p l a c e m e n t of the time scale are not relevant, and the logarithm of fractional absorbance should be linear in time. As F i g u r e 4 shows for the tests analyzed, the a s s u m p t i o n 1.0 o.g ~ 0.8 0.7 0.6
O--O~o ~-~- 0 ~
0 560~
~-~n
o5a--~o~_
"~'~
~3 0.5 z ca 0.4
~o. ~._.\
~
~ 0
571~
@60~K~..~ 611*K ~ (
113 <( ~zO.3
Q
W c~ LL
0.2
0.1 O
_
L
10
20
30 40 TIME, SEC.
50
60
FIG, 4. A semi-logarithmic plot of the fraction absorbances of the 3.04-1xm NH band as a function of time for several tests of 25-1xm thick HTPB-30 films at various temperatures.
1 xlO-
1275
1 \
[
/
\
5x 10- 2 l
/
\
~
2xlO-2~\
lx10-2
T d ~ 5x1~ z m
,,
, o\
\
\
\
u \
1x10-3
\\
SYM B A N D o D
lX10_ 4
1.4
\\3
DEI
D \\~ \
2•
\
\~, [3\
~ 5•
\
\
X \~ ~176
~_ 2 x 1 0 -3
u_ oU w
\
\
\ \
\ \
3.42#m I
(C~)"
I
I
I
1.5
1.6
1.7
1.8
1.9
l I T , ~ -1
2.0 x10-3
FIG. 5. An Arrhenius plot is shown here of the first-order rate constants. The solid lines are the least-squares representations. The dashed lines labeled 1, 2, and 3 are derived from data presented in references [15], [3], and [7], respectively. of a first-order process appears to be valid. T h e slopes of the lines on Figure 4 are rate constants w h i c h are d i s p l a y e d on an Arrhenius plot, Figure 5. F r o m the slope of the N H line on F i g u r e 5, we obtain a value of 19 k c a l / g mole for the activation energy. W h e n the data for C H decay are treated in the same way, we find that the least-squares line on Figure 5 leads again to 19 k c a l / g mole for the activation energy. C o n s i d e r i n g the scatter of C H points on Figure 5, one is inclined to consider o b t a i n i n g this same value as sheer accident. Nevertheless, it is the value to be expected because very p r o b a b l y the N H a n d the C H reactions are c o u p l e d a n d sequential. By our present interpretation the AP decomposes first. The C 2 H 3 then reacts at the same rate (adjusted for stoichiometry) as fixed b y the s u p p l y of oxidizing species from the AP. The ratio of the a p p a r e n t first-order rate constant for C H d i s a p p e a r a n c e to the firstorder rate constant for N H disappearance is given b y
1276
PROPELLANT IGNITION AND COMBUSTION kc
1
2
x
kN
3
3 1-x'
where x is the fractional conversion of CH at which kc is determined, The numerical constants contain the presumably constant ratio of rates of disappearance, 2.5, and the ratio of initial concentrations, 7.5. The likely range of x is 0.1 to 0.2, giving 0.17 to 0.26 as the predicted range for k c / k N. This variation, arising randomly in data analysis, could explain the exaggerated scatter of CH results on Figure 5. The value of k c / k N given by the least-squares lines on Figure 5 is 0.25. Figure 5 also displays results adapted from three other studies. Waesche 15 presented data from DSC tests on several propellants; Inami, Ros ser, and Wise 3 made adiabatic heating tests on propellant slabs; and Baer and Ryan v studied hot-wire ignition of propellants. The adaptation of their results required assumptions which discourage the direct comparison of rate constants, but still permit comparison of the slopes and the activation energies obtained from them. In the other three studies, an activation energy of about 30 kcal/g mole was reported, and this study one of 19 kcal/g mole. The precision of the results of this study is such that the difference is probably not attributable to random error. The first temptation is to assume that the other three studies dealt with a reaction-limited process, and this one dealt with a diffusionlimited process. We have focused on the later stages of reaction, when a vapor gap had developed between the two phases and oxidizing species had to diffuse from AP to polymer. Such a mechanism implies an activation energy equal to half the heat of dissociation of AP. However, since the heat of dissociation is about 58 kcal/g mole, 16 we predict an activation energy of about 30 kcal / g mole even for the diffusion-limited process. A possible explanation does come from a more detailed consideration of the diffusion process. We consider product diffusing from a spherical AF particle of decreasing diameter through a gaseous film to react with the polymer at the surface of a spherical bubble of increasing diameter. Such an approach requires postulating a temperature or narrow range of temperature at which the reaction first starts, and the data from these tests suggest that significant reaction is initiated at the phase change. If one further assumes that the diffusion process is essentially equimolar counter diffusion, that the instantaneous diffusion rate is describable by the steady state solution for
the existing AP to polymer surface separation (a film thickness), and that an average diffusion coefficient is applicable, a straight-forward development leads to the result that
f
t
~dF
F F 2 / 3 -~- 2 8 F 1 / 3
= C l t.
Here F is the fraction AP unreacted, 8 is the ratio of the gas film thickness to original AP particle diameter, t is time, and C l is a constant whose value depends on system parameters. The relationship between ~ and F is easily obtained and involves the fraction of the volume of the gas film generated by AP decomposition, taken as about 0.4 on the basis of the observed N H / C H ratio of reaction. This value was used in the numerical evaluation of the integral. The resulting F ( t ) is appropriately described by a first-order rate equation only after F has fallen below about 0.6. A superficial first-order rate constant obtained for greater values of/7, as in this study, will be too large. It is possible, therefore, that rate constants for NH disappearance at temperatures below about 590~ may be too large. As seen on Figure 5, lowering the values of the lowtemperature constants will result in a steepening of the line and an approach to a 30-kcal/g mole value for the activation energy. It is likely that we have studied a diffusionlimited reaction between AP and the HTPB polymer. We cannot, however, on the basis of activation energy, distinguish between the process we studied and the processes studied by the other workers cited. We have, however, reached our conclusions on the basis of chemical rather than thermal measurements.
Conclusions Infrared absorption spectrometry, employing a high-temperature IR source developed for this study and a fast-scanning spectrometer, has been used to follow the disappearance of NH and CH bonds during the pyrolysis of HTPB-IPDI polymer containing 30 weight percent ammonium perchlorate. The conclusions reached are that the reactions are coupled, polymer decomposition being induced by products of AP decomposition, and about 2.5 CH bonds disappear for each NH bond that disappears. The activation energy obtained, 19 kcal/g mole, differs markedly from the 30 kcal/g mole expected. The difference is tentatively attributed to the possibility that
PREIGNITION REACTIONS OF AP-HTPB PROPELLANTS the diffusion mechanism thought to be active is not properly describable as a first-order rate process at the low levels of conversion observed at the lower temperatures studied.
Acknowledgment This work was sponsored by NASA, Grant NGL45-003-019. REFERENCES 1, BoucK, L. S., BAER, A. D., AND RYA~, N. W., "Pyrolysis and Oxidation of Polymers at High Heating Rates," Fourteenth Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute, 1139 (1973). Also L. S. Bouck, "Propellant Reactions," Ph.D. Dissertation, University of Utah (1971). 2. CHENC, J. T., RYAN, N. W., AND BAER, A. D., "Oxidative Decomposition of PBAA Polymer at High Heating Rates," Twelfth Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute 525 (1969). 3. INAMI,S. H., ROSSER,W. A., JR., AND WISE, J., "Heat-Release Kinetics of Ammonium Perchlorate in the Presence of Catalysts and Fuel," Combustion and Flame, 12, 41 (1968). 4. SIMON,D. R., "Rapid Linear Pyrolysis of Composite Solid Ingredients," Report NASA CR66856 under Contract No. NAS1-8450, Vitro Laboratories, Silver Springs, MD., September 1970. 5. Boccs, T. L., "Deflagration Rate, Surface Structure and Subsurface Profile of Self-Deflagrating Single Crystals of Ammonium Perchlorate," AIAA Journal, 8, 867 (1970). 6. SCrtM1DT, W. G., "The Effect of Solid Phase Reactions on the Ballistic Properties of Propellants," Report No. NASA CR 112083, (N7221809) under Contract NAS 1-9463, Aerojet Solid Propulsion Co. Sacramento, California, August 1972.
1277
7. BAER, A. D. AND RYAN, N. W., "Evaluation of Thermal-Ignition Models from Hot-wire Ignition Tests," Combustion and Flame, 15, 9 (1970). 8. DAUERMAN,L., "Solid State Reactions in Solid Propellants," Report under Contract AFOSR1491-68, Department of Chemical Engineering, New York University, University Heights, New York, N.Y., March 1970. 9. HARTMAN,K. O. ANDMusso, R. C., "The Thermal Decomposition of Nitroglycerine and Its Relationship to the Stability of CMDB Propellants," Paper 72-30, Fall Meeting of the West Coast Section, The Combustion Institute, Monterey, California, October 1972. 10. Law, R. J., "Reactions in Solid Propellants from Infrared Spectra," Ph.D. Thesis, Department of Chemical Engineering, University of Utah (1976). 11. CARS~W, H. S. AND JAEGER, J. C., Conduction of Heat in Solids, 2nd Ed., London: Oxford University Press, p. 104 (1959). 12. MIHLFEITH,C. M., BAEa, A. D., AND RYAN, N. W., "The Response of a Burning Solid Propellant Surface to Thermal Radiation," AFOSR Scientific Report No. TR-71-2664, AD-736049 (1971). 13. Law, R. J., BAER, A. D., AND RYnn, N. W., "A Spectrometric Study of Pyrolysis Reactions of Polyurethane," West Coast Section, The Combustion Institute, Paper 73-11 (1973). 14. JAco~s, P. W. M. aND RUSSELL-JoNES,A., "'The Decomposition and Ignition of Mixtures of Ammonium Perchlorate and Copper Chromate," Eleventh Symposium (International) on Combustion, Pittsburgh, Pennsylvania: The Combustion Institute, 457 (1967). 15. WAESCHE,R. H. W., "Research Investigation of the Decomposition of Composite Solid Propellants," United Aircraft Research Laboratories Report G910476-24 on Contract NAS7-481 (1969). 16. INAMI,S. H., W. A. ROSSEa,ANDH. WIsE,~'Dissociation Pressure of Ammonium Perchlorate," J. Phys. Chem., 67, 1077-1079 (1963).