Propellants, Explosives, Pyrotechnics 9, 157-160 (1984) 157

Thermoanalytical Studies on CTPB-NH4C104 Composite Solid Rocket Propellants

M. R. R. Prasad and V. N. Krishnamurthy

Propellants Group, Vikram Sarabhai Space Centre, Trivandnun-695022 (India)

Tbennoanalytische Untersuchungen an CTPB-NH,ClO.,-Cornposit- Raketenfesttreibstoffen

Die vorzeitige Wiederzundung bei lenkbaren Raketenfeststoffmo- toren, ein Faktor, der die Zuverlassigkeit des Ausschaltvorgangs beeinflust, wird der ersten exothermen Zersetzungsphase des N&C104 zugeschrieben. Ausfiihrliche Untersuchungen (') zur Unter- driickung dieser Zersetzung des NH4C104 sind bereits durchgeftihrt worden unter Zusatz von Verbindungen wie NHBF,, NH.,PF6, NfiTiF6 usw. Die vorliegende Veroffentlichung behandelt die thermi- sche Zersetzungskinetik von carboxy-terminierten Polybutadien- NfiC104-Treibstoffen mit Zusatz von NH4BF4 zur Unterdriickung der ersten exothermen Zersetzung des NH4C104. Fur die Untersu- chungen wurden eingesetzt die DSC fiir die thermischen Zersetzungs- studien und die Crawford-Bombe (Strands) fiir die Messung der Abbrandgeschwindigkeit. Die DSC-Messungen wurden durchgefiihrt bei Normaldruck und bei Drucken von 17 kg/cmz und 40 kgicm' mit konstanter Aufheizgeschwindigkeit unter Stickstoffatmosphare. Die Abbrandgeschwindigkeit der Treibstoffproben wurde bestimmt im Druckbereich zwischen Normaldruck und 70 kg/cm2 unter Stickstoffat- mosphare. Es wurde gefunden, daR die Abbrandgechwindigkeiten zunehmen bei allen Konzentrationen des Zusatzmittels. Die Abnahme der Abbrandgeschwindigkeit mit zunehmender Konzentra- tion des Additivs bei Drucken von und unterhalb von 35 kg/cm2 wird der Freisetzung von NH, zugeschrieben, das durch Zersetzung von NH4C104 und von NH4BF4 gebildet wird. Bei Drucken oberhalb 35 kg/cm2 wird die Zunahme der Abbrandgeschwindigkeit mit zuneh- mender Konzentration des Additivs auf die Anreicherung des Metall- ions Bor aus N&BF4 zuriickgefiihrt. Diese Beobachtungen decken sich mit den Untersuchungen von Mayer(') und Glaskova(').

Etude par thennoanalyse des propergols composites solides h base de polybutadihne radicaux carboxy en bout de chahe (CTPB) et de perchlorate d'ammonium La rtinflammation prtmaturee d'un moteur-fuste & propergol solide qui met en question la fiabilite de la commande d'extinction, est attri- bute & la premitre phase de dtcomposition exothermique du NH4C104. Des etudes dCtailltes(') ont t t t entreprises pour supprimer cette premitre dtcomposition du NH4C104 par l'addition de substan- ces telles que N&BF4, NH4PF6, W T i F 6 , etc. La prtsente publica- tion traite de la cinktique de la dtcomposition thermique des proper- gols constituts par du polybutaditne & terminaison carboxy en bout de chaine et du perchlorate d'ammonium, avec addition de NH4BF4 pour supprimer la premitre dtcomposition exothermique du N&C104. L'Btude a t t t faite par calorimttrie difftrentielle ii balayage pour ana- lyser les rtactions de dtcomposition thermique et par essais en bombe Crawford pour determiner la vitesse de combustion. Les mesures calo- rimetriques ont BtB effectutes sous pression normale et sous des pres- sions de 17 kglcm' et 40 kglcm' avec une vitesse d'tchauffement con- stante et sous atmosphbre d'azote. La vitesse de combustion des tchantillons de propergol a t t t dtterminte sous atmosphkre d'azote pour une gamme de pression allant de la pression atmosphtrique ti 70 kg/cm2. On a constatt que, quelle que soit la concentration en additif, la vitesse de combustion augmente avec la pression. La diminution de la vitesse de combustion avec une concentration croissante en additif h une pression de 35 kg/cm2 et en-dessous, est attribu6e & la libtration de NH, suite & la dtcomposition de NH4C104 et de NH4BF4. Aux pressions suptrieures & 35 kglcm', l'augmentation de la vitesse de combustion, lorsque la concentration en additif croit, est attribute & la presence croissante de l'ion mttallique bore provenant de N&BF4. Ces observations concordent avec les rtsultats des etudes de Mayer") et Glaskova(').

Summary

Premature reignition of controllable solid propellant motors, a fac- tor affecting the extinguishment reliability, has been attributed to the first exotherm of NH4C104. Detailed studies were made(') on suppres- sing the first exbtherm of N&C104 with the use of materials like NH4BF4, N&PF6, (N&)*TZ6 etc. The present paper deals with the thermal decomposition kinetics of carboxy-terminated polybutadiene (CTPB)-N&CIO, propellant with NH4BF4 as an additive to suppress the first exotherm of NH&104.

The techniques employed in this study include: differential scan- ning calorimetry (DSC) for the thermal decomposition studies, and strand-burner for the measurement of propellant burning rates. DSC experiments were conducted on the propellants at pressures of ambient, 17 kg/cm2 and 40.82 kg/cm'; at a constant heating rate under an inert atmosphere of nitrogen. The burning rate of the propellant samples were determined in the pressure range of ambient to 70 kg/ cm2 in an atmosphere of nitrogen.

The burning rates were found to increase with pressure at each of the concentrations of the additive. The decrease in burning rate with the increase in concentration of the additive, at pressures of 35 kg/cm2 and below is attributed to the excess of ammonia formed during the decomposition of both NH4C104 and m B F 4 . At pressures above 35 kg/cm2, the increase in burning rate with the increase in concentration

0 Verlag Chemie GmbH, D-6940 Weinheim, 1984

of the additive is attributed to the increased concentration of the metal ion (boron) in NH.,BF4. These observations are in agreement with those of Mayer(') et al., and Glaskova").

1. Introduction

Premature reignition of the solid propellants can cause mis- sion failure in controllable solid propellant motors. It has been postulated(3, 4, that this inadvertent reignition may be pre- vented by limiting the heat radiated to the propellant surface after extinguishment, or reducing the heat generated in the residual propellant through shifting the exothermic decompo- sition to its elevated temperatures. The investigations reported herein are based on the later approach. The computations using thermal models(4, 5, of extinguishable motors had shown that the first exotherm characteristic of NH4C104 was a major factor causing this premature reignition. It is reported(') that additives like NH4BF4, N&PFs, (N&)2TiF6 etc., can suppress the first exotherm of NH4C104 decomposition.

The combustion of NH4C104 based composite solid pmpel- lants involves a complex Series of interaetions between the

0721-3 1 15/84/05 1O-O157$02.50/0

158 M. R. R. Prasad and V. N. Krishnamurthy Propellants, Explosives, Pyrotechnics 9, 157-160 (1984)

various components of the propellant system. There is no gen- eral agreement as to whether the individually definable proc- esses which regulate the rate of combustion occur in the con- densed-phase or in the gas-phase. Some researcheda) in combustion chemistry are quite convinced that the condensed- phase reactions contribute very little. However, the later studies(%'5) have supported the existence and the importance of these condensed-phase reactions.

Applicability of dynamic enthalpimetric analysis to basic combustion studies of composite solid propellants has been demonstrated b Summons('3) and Waesche et a1.(16. ").

(DSC) techniques to obtain values for the rate of heat release at the surface temperature of NH4CI04. Although, certain degree of extrapolation was necessary to obtain the values for rate of heat release at the surface temperature, excellent agreement with experimentally determined pyrolysis rates at ambient pressure has been achieved.

In earlier studies(") it was shown that those formulational variables which effect thermal decomposition also effect burn- ing rates of solid propellants. The present paper aims at a correlation between the kinetic data obtained through the application of DSC technique and the observed burning rates of composite solid propellants based on CTPB polymer with NH4BF4 as an additive.

Waesche et aI.(' ?y have used differential scanning calorimetry

2. Materials

The propellant used in this study is based on carboxy-termi- nated polybutadiene (CTPB). It is an aluminised composition with a solid loading of 86%. The propellant is cured with conventional diepoxide and a trifunctional aziridine (MAPO) system. The concentration of the additive, NH4BF4, was var- ied between 0 and 1.5% by weight of the propellant.

3. Experimental

The differential scanning calorimetry (DSC) experiments were conducted in an DuPont-990 thermal analyser system at a heating rate of 20 "Clmin under ambient conditions and also in nitrogen pressures of 17 kg/cm2 and 40.82 kg/cm2. The kinetic parameters were evaluated using Borchardt and Daniels(") equation. The burning rates of the propellant samples were determined in an conventional strand-burner in the pressure range between ambient and 70 kg/cm2.

4. Results and Discussion

Figure 1 gives the strand burning rates for propellants with- out and with different concentrations of W B F 4 . The burning rates of control sample was found to increase with pressure at all pressure levels studied. In the case of the propellant with 0.5% NWF, , the burning rate was found to be less than the control sample at all pressures upto 70 kg/cmz. The propellant with 1.0% of the additive W B F 4 showed a decreasing tend- ency in burning rate with increase in pressure upto a pressure of 50 kg/cm2. At 50 kg/cm2, the burning rate has leveled off with that of the control sample. Beyond this pressure, the burning rate was found to increase progressively with pressure. For the propellant with 1.5% NH4BF4, the burning rate was found to decrease with increase in pressure level upto

25 k g / d

0 0.5 10 15 NH4BF4Concentratton [Wt.o/,l -

Figure 1. Effect of NH$F4 concentration on the burning rate of CTPB-NJ&C104 propellant.

35 kg/cm2. At 35 kg/cm2 it levels off with that of the control sample. Further increase in pressure resulted in the increase in burning rate.

Figure 2 shows the DSC thermograms of propellants with 0, 0.5% and 1.5% N€&BF4 at ambient pressure. It can be seen from these thermograms that the low temperature decomposi- tion (LTD) peaks occur almost at the same temperature. Eventhough, the high temperature decomposition (HTD)

0 Temperature [ O C 1 -

Figure 2. DSC thermograms of CTPB-NJ&ClO, propellants without and with NHBF, at ambient pressure.

Propellants, Explosives, Pyrotechnics 9, 157-160 (1984) Thermoanalytical Studies on Solid Rocket Propellants 159

peaks in the case of the propellant with 0.5% NKBF, and that of the control propellant occur at the same temperature, the peak height in the case of the propellant with 0.5% NH4BF4 is more.

The shoulder peak seen in the case of the control propellant and in the propellant with 0.5% N W F 4 has resolved into a clear peak in the case of the propellant with 1.5% NH4BF4. Also, the high temperature decomposition peak has shifted to the higher temperature side by about 25 "C-35 "C, and both the LTD and HTD peaks are broader in this case.

The negligible effect on the LTD at lower concentrations of NH4BF4 can be attributed to the following reasons:

The NH4BF4 on thermal decomposition forms gaseous fluorides and ammonia according(') to the equation:

NKBF4 = NH3 (g) + HF (g) + BF3 (g) (1)

The rate controlling step in the decornpositioda) of NH&104 is the transfer of a proton in solid NH4C104.

NH&104 = NH3 (a) + HC104 (a) (2)

The ammonia produced by the decomposition of NH4BF4 inhibits the dissociation of HClO, into H+ and ClO;. Simi- larly, the proton-donor ability of the HF-Lewis acid pair could inhibit the suggested(21) rate controlling step for NH&104 decomposition in which an electron is released from the per- chlorate ion.

ClO; = ClO, + e- (3)

The marked ability of the HF-Lewis acid pair to transfer protons to very weak bases could inhibit reaction of the above type by promoting the competing reaction:

H+ + C10, = HC104 (4)

in which a proton is transfered to the very weak base ClO;. The proton released by the HF-Lewis acid pair can neutralize electrons in N&C104 to inhibit the decomposition(').

The increased areas of the peaks observed in the case of the propellant with 1.5% N€€J3F4 can be explained on the lines that the BF3 gas released during the decomposition of N&BF4 can react exothermically with HzO produced during the exothermic decomposition(') of NH4C104. Also, the shift in the HTD peak to the higher temperature region indicates

Table 1. Kinetic Data on CTPB-NH4C104 Propellant with N&BF4

NHBF, concentration Pressure Activation Enerev. E [wt.%]

", [ kcal/mol] LTD HTD

0.0 0.5 1.5

0.0 0.5 1.5

0.0 0.5 1.5

ambient ambient ambient 17.0 17.0 17.0 40.82 40.82 40.82

28.6 19.4 30.0 38.0 47.3 58.8

38.2 30.5 48.0 50.0 25.0 - 44.5 26.0 91.5 33.0 Sudden energy release takes place at about 310 "C

Table 2. Burning Rates of CTPB-NH4C104 Propellant with Different Concentrations of N&BF4 at Ambient Pressure

N&BF4 concentration [wt.%]

0.0 0.5 1.5

~

1.11 1.10 1.08

higher activation energy (Table l), which means a lower burn- ing rate. However, the observed burning rates at ambient pressure indicate that there is no change in the burning rate with increasing concentration of NH8F4 (Table 2).This sug- gests that only the intermediate processes are influenced and as such there is not much contribution from these intermediate processes in controlling the burning rate at ambient pressure.

Figure 3 shows the DSC thermograms of propellants with 0, 0.5% and 1.5% NH,,BF4 at 17.0 kg/cm2 of nitrogen pressure. From these thermograms it can be seen that both the LTD and HTD peaks occur almost at the same temperature, in the case of the propellant with 0.5% NHJ3F4 and control propellant. This suggests that the propellant decomposition mechanism has not been altered in the presence of N W F 4 upto this pressure level. In the case of the propellant with 1.5% NH4BF4 at 17.0 kg/cm2 pressure level, the LTD peak has shifted to the higher temperature region and, that of the HTD peak to the lower temperature region.

Inspite of the fact that the LTD and HTD peaks of the propellants without and with 0.5% NhBF4 show no shift in the peak temperatures the burning rates were found to decrease in the presence of 0.5% NHJ3F4 at all pressure levels studied. The higher activation energy (E) for these propellants as seen from Table 1, are in agreement with this observation.

200 300 4 0 0 500 Temperature [ "C I -

Figure 3. DSC thermograms of CTPB-NH4CI04 propellants without and with N&BF4 at 17 kg/cm2 of nitrogen pressure.

160 M. R. R. Prasad and V. N. Krishnamurthy Propellants, Explosives, Pyrotechnics 9, 157-160 (1984)

200 300 400 500 Temperature [ “C 1 -

Figure 4. DSC thermograms of CTPB-NH4C104 propellants without and with NH4BF4 at 40.82 kg/cm2 of nitrogen pressure.

However, in the case of the propellant with 1.5% Nl&BF4, increase in burning rate was noticed after an initial decrease upto a pressure of 35.0 kg/cm2. Also, the activation energy for the LTD has decreased. Probably,this is the point at which the concentration of boron comes into play. This line of thinking is in conformity with the observations of Glaskova(’) that the presence of boron in a molecule of a salt like NH4BF4, strongly reduces the inhibiting effect of the additive.

Figure 4 shows the DSC thermograms of the propellants without and with 0.5% and 1.5% of NH4BF4 at 40.82 kg/cm2.

The similarities of the thermograms of control propellant and that with 0.5% NH4BF4 show that the decomposition mechanism has not been affected. A third exotherm at about 460 “C in the case of the control propellant occurs, which has been shifted to 440°C in the presence of 0.5% of NI&BF4. In the case of the propellant with 1.5% NH4BF4, there is a sud- den release of energy at about 310°C and the whole material decomposes all of a sudden.

5. Conclusions

The change in pattern of the burning rate behavior with the concentration change in N&BF4 around 35 kg/cm2 suggests

that there is a change in the mechanism of heat release in the propellant. It is probable that sub-surface reactions of NH4C104 in the propellant do not control the burning rate at pressures above 35 kg/cm2. Caveney and Pittman’d’) observa- tions about the influence of NH.,C104 solid-phase decomposi- tion rate on the burning rate support our above statement. Probably, it may be true, as suggested by that the exothermic reactions occuring in the pseudo condensed-phase coupled with gas-phase reactions are responsible for the heat transfer to the deflagrating NH4C104 crystals between the pressures of 20 and 50 atmospheres.

6. References

(1) S. W. Mayer, E. K. Weinberg, and L. Schieler, AZAA J. 8, 1328

(2) A. P. Glaskova, AZAA J . 13, 438 (1975). (3) L. W. Carlson and J. D. Seader, AZAA 5,1272 (1967). (4) G. M. Lehmann and G. R. Schneiter, TOR-0158(S3801-04)-7,

Aerospace Corporation, San Bernardino, California (1968). (5) G. M. Lehmann, TR-0200 (S4309)-1, Aerospace Corporation,

San Bernardino, California (1969). (6) M. Summerfield et al., AZAA 4th Propulsion Joint Specialists

Conference, AIAA Paper No. 68-658 (1968). (7) L. H. Caveny and C. U. Pittman Jr., AZAA J. 6, 1461 (1968). (8) E. W. Price, Naval Weapons Center, Technical Note 508-45,

(1967). (9) H. Wise, S. H. Inami, and W. A. Roser, Stanford Research

Institute, Contract NONR-3415(00), Final Report (1967). (10) J. Wenograd, 3rd ICRPG Combustion Conference, CPIA Publi-

cation No. 138 (1967) p. 89. (11) Lockheed Propulsion Company, Report No. AFRPL-TR-68-191

(1968). (12) R. F. Landell, B. G. Moser, and R. E. Weich, Combustion

Institute, Western States Section, Fall Meeting, Santa Barba, California, October (1965).

(13) G. D. Sammons, “Analytical Calorimetry”, Plenum Press, New York, P-305 (1968).

(14) C. E. Hermance, AZAA J. 4, 1629 (1966). (15) M. W. Beckstead, R. L. Derr, and C. F. Price, AZAA J. 8, 2200

(1970). (16) M. W. Beckstead, R. L. Derr, and C. F. Price, 13th Symposium

(International) on Combustion, the Combustion Institute, Pitts- burgh, Pennsylvania, (1971), p. 1047.

(17) R. H. W. Waesche and J. Wenograd, AZAA 7th Aerospace Sci- ences Meeting, New York, Jan. 20-22 (1969).

(18) R. H. W. Waesche, J. Wenograd, and R.L. Feinauer, ZCRPGI AZAA Solid Propulsion Conference, June (1967).

(19) H. J. Borchardt and F. Daniels, J. Amer. Chem. SOC. 79, 41 (1957).

(20) P. W. M. Jacobs and H. M. Whitehead, Chemical Reviews 69, 551 (1969).

(21) A. G. Keenan and R. F. Siegmund, Quarterly Reviews 23, 430 (1969).

(22) T. L. Boogs, AZAA J . 8, 867 (1970).

(1970).

(Received September 7, 1983, Ms 20/83)