1404 AIAA JOURNAL VOL. 18, NO. 11

Action of Transition Metal Oxides onComposite Solid Propellants

K. Kishore,* V. R. Pai Verneker,f and M. R. SunithaJIndian Institute of Science, Bangalore, India

Introduction

TRANSITION metal oxides (TMO) are known to promotethe thermal decomposition (TD) of ammonium

perchlorate (AP), deflagration of AP, AP-binder sandwichburning, and the combustion of AP-based composite solidpropellants.1 Many attempts have been made to understandhow these catalysts effect decomposition and combustion butno clear-cut picture has emerged, particularly regardingcomposite solid propellants.

In order to understand the nature of catalyst action (in thepresence of TMOs) on the combustion of propellants, twosystems based upon polystyrene (PS) and carboxyl-terminatedpolybutadiene (CTPB) were chosen. The catalysts chosenwere Fe2O3, Ni2O3, MnO2, and Co2O3.

ExperimentalPS/AP propellant was prepared as describe in Ref. 2.

Additives and AP were used at 1% (by weight) level ofpropellant and a particle size of 53-106 /*m was maintained inall the compositions. Perchlorates and perchlorate amines ofiron, nickel, manganese, and cobalt were prepared in thelaboratory as described elsewhere.3'4 A CTPB-basedpropellant (80% AP, 14.4% CTPB, 2% dioctyladipate, 1.8%MAPO, 1.4% epoxy (CIBA GY 252), 0.4% lecithin with andwithout 1% additive) was processed in the usual manner.Pellets of AP and AP with additives were made by pressing 20g of the material at a pressure of 1500 kg/cm ~2 . The pelletsobtained were of 5 cm in diameter and 0.5 cm thick. Thesewere cut carefully to yield strands (4 cm long and 0.5 cmthick). The strands were coated with a homogeneous mixtureof araldite and TiO2 and dried. Burning rate (r)measurements were made at ambient pressure as describedelsewhere.5 r measurements at high pressure (60 atm) werecarried out using a strand burner similar to that devised byCrawford.6 CTPB strands 6 cm long and 0.5 cm in diameterwere used for the /measurements. Polymer rand degradationwere studied as described in Ref. 7. Isothermal ther-mogravimetry experiments were made on a homemadeassembly8 and a Perkin Elmer DSC 1B-TGS unit at tem-peratures of 280-360°C. A homemade differential thermalanalysis (DTA) assembly was used in the present in-vestigation9 at a heating rate of 7°C/min ~ *.

Results and DiscussionDTA of AP shows phase transition at 240°C followed by

two distinct exotherms corresponding to a diffusion-controlled and an electron-transfer process, respectively. Inthe presence of 1 % TMO the DTA traces show that the secondexotherm is sharpened and that the end temperature at which

Received Oct. 15, 1979; revision received April 24, 1980. Copyright© 1980 by K. Kishore. Published by the American Institute ofAeronautics and Astronautics, Inc., with permission.

Index categories: Fuels and Propellants, Properties of; Ther-mochemistry and Chemical Kinetics; Ablation, Pyrolysis, ThermalDecomposition and Degradation (including Referactories).

'Associate Professor, Dept. of Inorganic and Physical Chemistry,High Energy Solids Laboratory.

tProfessor, Dept. of Inorganic and Physical Chemistry, HighEnergy Solids Laboratory.

JPost Doctoral Fellow, Dept. of Inorganic and Physical Chemistry.High Energy Solids Laboratory; present address: Lecturer, ChemistryDepartment, National College, Basanagudi, Bangalore.

the decomposition terminates drops, indicating that theTMOs have a catalytic effect on the decomposition of AP.From the end temperature the order of the effectiveness ofTMO was as follows:

Co2O3 > MNO2 > Ni2O3 > Fe2O3

Detailed studies of AP + MnO2 system reported earlier9 showthat MnO2 facilitates the electron-transfer process since Mnions were found to be present in the lattice of AP. Thishappens during the curing period of the propellant. Similarstudies were made to detect the presence of metal ions in otherTMO systems. Co was found to be present in the lattice, whileFe and Ni could not be detected. It was also observed fromTG studies that the E for the electron-transfer process isreduced in the presence of all TMOs, although the actualextent differs. Based upon these results, the order of activityof TMOs can now be explained. In those systems in which themetal ions enter the lattice, the facilitation of electron-transfer process is much greater (Co2O3 and MnO2), andhence the TD of AP is enhanced to a greater extent comparedtoFe2O3 andNi2O3.

The next step was to examine the effect of TMOs on the r ofAP (at 60 atm). It was found that Co2O3 and Ni2O3 con-taining AP do not sustain combustion at this pressure. The rof other systems followed the following order:

r(AP + Fe2O3) > rAP(uncatalyzed) > r(AP + MnO2)

These studies indicate that the TD and f of AP have nosimilarity. This may be because the highest temperature atwhich the AP decomposition was studied is around 400°C,while the temperature encountered during deflagration wasmuch higher. Moreover, the formation of melt has beenobserved during the deflagration of AP and the nature of theaction of the TMOs in the melt could be different than theireffect on the TD of AP.

The f data of the propellant are presented in Table 1 andshow the following order of effectiveness:

r of (PS/AP + 1 % TMO):Ni2O3 * Fe2O3 > MnO2 « Co2O3

Table 1 r values of AP/CTPB and AP/PS propellant systems

Propellant system TMO (1 % wt) Pressure f, cm/s

AP/CTPB (O/F = 4)

AP/CTPB (0/F = 4)

AP/PS(0/F = 3)

60 atmNi203Fe203Mn02Co2O3

- AmbientNi203Fe203MnO2Co203

- AmbientNi203Fe203Mn02Co203Mn(C104)2Co(C104)2Ni(C104)2Fe(C104)2

a

Mn(ClO4)2 amineCo(ClO4)2 amineNi(ClO4)2 amineFe(ClO4)2 amine

0.95 ±0.031.36 ±0.051.09 ±0.021.05 ±0.031.05 ±0.05

0.108 ±0.0040.119±0.0010.120±0.0040.109 ±0.0050.1 10 ±0.005

0.095 ±0.0020.131 ±0.0050.128 ±0.0030.1 14 ±0.0030.109 ±0.0030.083 ±0.0030.102 ±0.0020.097 ±0.003

-0.108 ±0.0010.096 ±0.0020.107 ±0.0020.120 ±0.002

a AP/PS strands with Fe(ClO4)2 did not cure.

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NOVEMBER 1980 TECHNICAL NOTES 1405

and

r of (CTPB/AP + 1 % TMO):

Ni203 *Fe2O3>MnO2«Co2O3

r data were obtained at high pressures for CTPB propellantscontaining TMOs. Table 1 shows that the order of ef-fectiveness of TMOs at 60 atm is the same as that observed atambient pressures, which suggests that the basic mechanismof catalyst action is not affected by the pressure. However,for any particular oxide the effectiveness at higher pressures isnot the same as that observed at lower pressures. The r data ofPS/AP propellant could not be obtained at high pressuressince the burning was very rapid, which may be due to thepolymer melting along the side of the propellant strands. Twointeresting features come into focus from a comparison of ther data of PS/AP and CTPB/AP propellants at ambientpressure: 1) the order of catalytic effectiveness is almost thesame in both cases, which suggests the mechanism of catalystaction to be the same in both the cases; and 2) Ni2O3 appearsto be as good a catalyst as Fe2O3 at both ambient and highpressures.

Studies of the AP/PS propellant containing TMO werecarried out. The propellant was ignited during DTA studies(depending upon the sample mass and heating rate) and theignition temperature was precisely recorded. The followingorder was found for the ignition temperatures of the PS/APpropellant containing TMOs:

Fe>CO>Ni>Mn

It is evident that the effectiveness of the propellant decom-position (leading to ignition) is diffferent than that of APdecomposition and r. From the TD results of AP and thepropellant and the deflagration rate of AP, it is not possibleto shed any light on the mechanism of burning except that theTMOs catalyze AP decomposition via the electron-transfermechanism.

Patil et al.10 have shown that transition metal perchlorateamines (TMPA) are formed as intermediates during thedecomposition of AP. Apart from this, the formation ofmetal per chlorates (TMP) can also take .place. Thus, detailedinvestigations were carried out on r and TD of AP/PSpropellants with 1 % (by weight) of TMP and TMPA. Ad-ditionally, the studies were also done to find the effect ofTMO and TMPA on the degradation and combustion ofbinder PS. PS + TMP could not be studied as they wereimcompatible.

Results on the r of AP/PS propellants with TMP andTMPA are presented in Table 1. It may be observed thatperchlorates do not have much effect on r with the exceptionof Mn (C1O4)2 which shows a desensitizing effect. Thereforethe results on the r of AP/PS propellants with TMOs couldnot be explained on the basis of the formation of perchloratesas intermediates. From Table 1 it may be seen that Fe, Ni, andMn perchlorate amines (PA) sensitize the r of propellants,while Co PA appears to have no effect. TMP As have thefollowing order of effectiveness:

FePA>MnPA>NiPA>CoPA

This order is almost the same as that observed for TMOcatalyzed propellants. In order to explain the above trend inthe r\ the role of TMPA was examined on the binder com-bustion and degradation. Binder r was found to be slightlydesensitized and degradation was desensitized to a greaterextent in the presence of both TMO and TMPA. The resultsdid not have any bearing with propellant r. Thus, the effect ofthese additives on the combustion of the propellant could notbe explained on the basis of the combustion of the binderalone.

An attempt was then made to understand the effect ofTMPAs on the r of propellants based upon their effect on theTD of AP. Patil etal.10 have shown that these compoundssensitize the TD of AP and the effectiveness of TMPAs on theTD of AP has the following trend.

CoPA>MnPA>NiPA>FePA

and the thermal stability of these amines is in the followingorder:

FePA>NiPA>MnPA>CoPA

The r data as well as the ignition temperature data (obtainedfrom DTA) of the propellant containing TMPA have the sametrend as that indicated by the stability of the amines, fromwhich it may be inferred that during the combustion ofpropellants it is the stability of the amines which determinetheir effect on r. This could be because at the temperaturesencountered during combustion, which are quite high, onlythose compounds which are thermally more stable can acteffectively. This appears to be quite sound since the formationand decomposition of TMPA is exothermic and therefore thechances of formation of this compound (which is more stableat higher temperatures) are greater and will release more heatin the propellant matrix. Therefore, the effect will be greater.Another observation made during these studies was the effectof Mn PA on the r of AP/PS propellants. From Table 1 it isevident that the r of the Mn PA catalyzed propellant is of thesame order as the r of the Ni PA catalyzed propellant. Basedupon this, MnO2 would be expected to be just as effective acatalyst as Ni2O3; however this is not true. This could beperhaps due to the combined effect of Mn perchlorate and MnPA on the r.

In conclusion it may be stated that one of the ways by whichTMOs act is through the formation of reactive TMPA in-termediates as indicated by the correlation between thermalstability of TMPAs and r of the propellants containing TMOsand TMP As.

ReferencesAshore, K. and Sunitha, M. R., "Effect of Transition Metal

Oxides on Decomposition and Deflagration of Composite SolidPropellant Systems," AIAA Journal, Vol. 17, Oct. 1979, p. 1118.

2Kishore, K., Pai Verneker, V. R., and Prasad, G., "Effect ofStorage Temperatures on the Mechanical Properties of the CompositeSolid Propellants," Combustion Science Technology, Vol. 19, 1979,pp. 107-118.

3Schumacher, J. C., Perchlorates, Their Properties, Manufactureand Uses, Reinhold Publishing Corp., New York, 1960.

4Patil, K. C., Pai Vernecker, V. R., and Jain, S. R., "MetalPerchlorate Amines: Thermal and Infrared Studies," ThermochimicaActa, Vol. 15, 1976, p. 257.

5Kishore, K., Pai Verneker, V. R., and Sunitha, M. R., "Effect ofCatalyst Concentration on Burning Rate of Composite SolidPropellants," AIAA Journal, Vol. 15, Nov. 1977, pp. 1649-1651.

6Crawford, B. L. Jr., Hugget, C., Daniels, F., and Wilfong, R. E.,"Direct Measurement of Burning Rates of Propellant Powders,"Industrial and Engineering Chemistry, Analytical Edition, Vol. 19,No. 9, 1947, p. 630.

7Kishore, K., "Polymer Flame Inhibition in Presence of TransitionMetal Oxides," Colloid and Polymer Science, Vol. 257, No. 9, 1979,pp. 984-986.8Kishore, K., Pai Verneker, V. R., Sunitha, M. R., and Prasad, G.,"Contribution of Condensed Phase Reaction in the Combustion ofSolid Rocket Fuels," Fuel, Vol. 56, July 1977, pp. 347-348.

9Kishore, K., Pai Verneker, V. R., and Sunitha, M. R., "Effect ofManganese Di-Oxide on the Thermal Decomposition of AmmoniumPerchlorate," Journal of Applied Chemistry and Biotechnology, Vol.27, 1977, pp. 415-422.

10Patil, K. C., Pai Verneker, V. R., and Jain S. R., "Role of MetalPerchlorate Amines on Ammonium Perchlorate Decomposition,"Combustion and Flame, Vol. 25, 1975, p. 387.

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