Full Paper

Differential Scanning Calorimetric Study of HTPB basedComposite Propellants in Presence of Nano Ferric Oxide

Prajakta R. Patil, V. N. Krishnamurthy, Satyawati S. Joshi*

Department of Chemistry, University of Pune, Pune 411007 (India)DOI: 10.1002/prep.200600059

Abstract

A comparative study of the thermal decomposition of ammo-nium perchlorate (AP)/hydroxy terminated polybutadiene(HTPB) based composite propellants has been carried out inpresence and absence of nano iron oxide at different heating ratesin a dynamic nitrogen atmosphere using differential scanningcalorimetry. The pronounced effect was a lowering of the hightemperature decomposition by 49 8C. A higher heat release up to40% was observed in presence of nano ferric oxide (3.5 nm). Thekinetic parameters were evaluated using the Kissinger method.The increase of the rate constant in the catalyzed propellantconfirmed the enhancement of the catalytic activity of ammoniumperchlorate. The scanning electron micrographs of nano Fe2O3incorporated in HTPB revealed a well-separated characteristicnecklace-like structure of a-Fe2O3 particles at high magnification.

Keywords: a-Fe2O3 Nanoparticles, AP/HTPB, ThermalDecomposition, DSC, Kinetic Parameters

1 Introduction

In composite solid rocket propellants the burning rates ofammonium perchlorate (AP) are routinely adjusted by theaddition of small amounts of ballistic modifiers to thepropellant formulation [1 – 4]. Transition metal oxides(TMO) and their mixtures are most effective and arecommonly used as burn rate modifiers in practical applica-tions of composite solid propellants [5 – 8]. TMO has greatinfluence on thermal decomposition behaviour of ammoni-um perchlorate. The most commonly used TMOs are ironoxide (Fe2O3), copper oxide (CuO), copper chromite(CuO ·Cr2O3) etc. Among these catalysts iron oxide isknown to be structurally simple, highly stable, easy tosynthesize and inert to side reactions. Above all, it imparts ahigh and reproducible burn rate for composite propellants.A comprehensive survey of the literature on burning ratecatalysts suggests that the burn rate depends on catalystconcentration, surface area of added catalyst, particle sizeand state of aggregation [9 – 12]. The burn rate modifierscatalyze the decomposition of oxidizer and its smallerparticle size enhances the catalysis in the gaseous phase ofthe combustion. The optimum concentration of these

catalysts (TMO) produces effective results. High burningrate composite propellants are needed for future pro-grammes; particularly in surface to air mission to reduce theoperational time of the missiles. Commonly used ballisticmodifiers, at their present size do not meet these objectivesas their addition beyond an optimum limit, leads to affectthe processability and the integrity of the propellant.Hence,for achieving a large surface area a new class of catalystsbased on ultrafine particles of iron oxide was developed.With reduced particle size these ultrafine particles havebeen found to be more efficient and active in the thermaldecomposition of AP than the commonly used bulk TMOs[12 – 14].In the present article, we report for the first time the effect

of nano iron oxide particles on the thermal decompositionbehaviour of a composite solid propellant. The synthesizedultrafine iron oxide nanoparticles were incorporated in anAP based composite propellant by using slurry cast tech-nique. The effect of nano a-Fe2O3 on the thermal decom-position behaviour of ammonium perchlorate was studiedby comparing the thermal results with an uncatalyzedAPþHTPB based propellant. Thermal studies were carried outusing differential scanning calorimetry (DSC) to determinethe activation energy and the heat release. The kineticparameters of the solid propellant were computed using theKissinger method.

2 Experimental

The synthesis of a-Fe2O3 nanoparticles was carried outusing the modified electrochemical route originally report-ed by Reetz et al. at a higher current density of 40 mA cm�2

[15 – 16]. Propellant formulations were made using 2% ofsynthesized 3.5 nmsizednano ironoxide and comparedwiththe same propellant without a catalyst. Propellant compo-sitions containing 85% AP and 15% binder were preparedusing slurry cast technique. Hydroxy terminated polybuta-dine prepolymer (HTPB) was obtained from HEMRL,DRDO, Pune. HTPB along with plasticizer dioctyl adipate(DOA), pyrogallol and lecithin was de-aerated undervacuum (1.3 kPa) for half an hour in amixerwith continuous* Corresponding author: ssjoshi@chem.unipune.ernet.in

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hot water (� 60 8C) circulation in the outer jacket of themixer. After one hour, the ingredients were added ininstallments with intermittent mixing for 15 min successive-ly at 65 8C. The content was allowed to cool down to roomtemperature and toluene di-isocyanate (TDI) was added in1 :1 stoichiometry (NCO:OH – 1 :1). Final mixing wasfurther continued for 15 min without vacuum, subsequentlyunder vacuum at 0.6 – 1.3 kPa for 15 min. The propellantslurry was then cured at 55� 5 8C for one week in a waterjacketed oven. Similarly, another composition was curedwithout nano iron oxide catalyst. Hereafter we will desig-nate APþHTPB formulation as composite A (withoutcatalyst) and APþHTPBþNano Fe2O3 formulation ascomposite B.After casting both the composite A and B, the thermal

decomposition behavior of ammonium perchlorate i.e. heatrelease and exothermic nature of the propellant wasmeasured by differential scanning calorimetry (DSC) on aPerkin Elmer TAC 7/DX. 1 – 2 mg of sample were takenover a temperature range from 50 to 500 8C at differentheating rates viz. 5, 10, 15, 20 and 25 8C/min. An inertnitrogen atmosphere was maintained throughout the DSCrun. In order to know the morphology, the homogeneity ofthe mixture and the agglomeration of iron oxide nano-

particles in HTPB, scanning electron micrographs weretaken on a Quanta 200 FEI.

3 Results and Discussion

Figures 1 and 2 show the DSC results for composites Aand B at different heating rates viz. 5, 10, 15, 20, 25 8C/min.Each DSC curve shows first an endothermic and then anexothermic stage. For both the composites A and B,ammonium perchlorate shows an endothermic peak ataround 247 8C, which represents the transition from ortho-rhombic form to cubic form. Earlier reports for AP showtwo exothermic peaks, one due to incomplete combustion ataround 322 8C and a high temperature peak at about 477 8C.While we observed only one high temperature exothermicpeak for both the composite A and B, which indicatescomplete decomposition. For composite A (APþHTPB)exothermic peaks show different maximum temperatures(Tp) as 393.5, 404.43, 413.72, 421.67, 433.25 8Ccorrespondingto 5, 10, 15, 20, 25 8C/min heating rates respectively. Alsofrom Table 1 we can see that the heat release (DH) isincreasing with increasing heating rate in the range 1.59 to2.10 kJ/g.

Figure 1. DSC of composite A at different scan rates a) 5 8C/min, b) 10 8C/min, c) 15 8C/min, d) 20 8C/min, e) 25 8C/min.

Figure 2. DSC of composite B at different heating rates a) 5 8C/min, b) 10 8C/min, c) 15 8C/min, d) 20 8C/min, e) 25 8C/min.

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Inami et al. reported that catalysts play a dual role inaccelerating the decomposition of AP and promote theoxidation of the fuel by heterogeneous reactions [17]. It isknown in metal oxide catalyzed oxidation, that the metalions in the lattice undergo redox cycle involving interactionsof hydrocarbons with their high valence state followed byreoxidation by molecular oxygen. Hence significant alter-ation in the decomposition characteristics of AP wasobserved by addition of ultrafine iron oxide nanoparticlesas burn rate modifier in composite B. The endothermicphase transition was unchanged by addition of nano Fe2O3while large differences were observed in high temperatureexothermic peak (Figure 2). The position of this exothermstrongly depends on the size of nanomaterials [11]. Theaddition of nano iron oxide lowered the high decompositiontemperature of AP by 47.12, 48.7, 53.96, 53.94, 61.5 8C at 5,10, 15, 20, and 25 8C/min heating rates respectively ascompared to uncatalyzed AP (composite A). The loweringin temperature is due to the catalytic effect of nano ironoxide. As the surface area of nano Fe2O3 is larger due to thehigh surface to volume ratio, there is less sputtering ofparticles during decomposition. This causes less mechanicalloss and more efficient heat transfer within the composite.Asa result, an interesting featureofa40%higherheat releasewas observed in presence of nano Fe2O3 (composite B) thanin composite A. Another reason for the higher heat releasemay be attributed to higher concentration of Fe3þ moiety atthe interphase between AP and organic hydrocarbon due tovery strong ion-ion interactions. The concentration of thiscatalyst at the interphase and the catalytic decomposition ofvapors of fuel and oxidizer further enhance the exothermicreaction and consequently increase the burning rate [13]. InTable 1 and 2 it has been observed that for a slowheating rate(at 58C/min) the heat release is less while higher heat releasewas obtained at 258C/min. In composites A and B theasymmetry in the peaks of the DSC curves confirms thecomplex process in the decomposition of AP.

Figure 3 shows scanning electron micrographs of nanoFe2O3 incorporated in HTPB at 600�and 5000�magni-fications. At lower magnification (Figure 3a), well-separat-ed nano iron oxide particles were observed with lessagglomeration. While at high magnification (Figure 3b)necklace-like structures of Fe2O3 has been retained whichare a characteristic feature of a-Fe2O3 particles.

3.1 Kinetics of Thermal Decomposition

The thermal decomposition of solid composite propel-lants is a multistep process and the reaction mechanismchanges with temperature and hence activation energy (Ea)varies. The Arrhenius activation parameters viz. energy ofactivation (Ea), frequency factor (A) and rate constant (k)were calculated fromDSC results using theASTM standardmethod based on the Kissinger correlation [18];

Ea¼R d ln[b/T2p]/[d(1/Tp)]

where b is the heating rate in 8C/min, Tp the peak temper-ature (K) and R the ideal gas constant. The slope of thekinetic plot of ln[b/T2

p] against 1/Tp gives the activationenergy Ea in kJ mol

�1. Once Ea is known, the values of thefrequency factor are calculated by the equation

A¼b E eEa/RT/(R T2p)

The specific rate constant (k) for high temperaturedecomposition was calculated using Arrhenius equation

k¼A e�Ea/RT

The values of Ea, A and k for high temperature decom-position are shown in Table 1 and 2.

Table 1. DSC of composite A (APþHTPB).

Heating Rate Peak Temperature Tp DH Ea A k(8C min�1) (8C) (kJ g�1) (kJ mol�1) (min�1) (s�1)

5 393.50 1.59 3.62� 101010 404.43 1.66 4.62� 101015 413.72 1.67 143.8 2.28� 1011 4.75� 10�320 421.67 2.01 2.25� 101125 433.25 2.10 3.75� 1010

Table 2. DSC of composite B (APþHTPBþNanoFO).

Heating Rate Peak Temperature Tp DH Ea A k(8C min�1) (8C) (kJ g�1) (kJ mol�1) (min�1) (s�1)

5 346.38 2.54 5.77� 101010 355.73 2.79 6.63� 101615 359.76 2.82 181.5 7.87� 1014 6.11� 10�320 367.73 3.0 6.66� 101125 371.75 3.1 6.65� 1014

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Many researchers have reported a higher activationenergy (Ea) for catalysed AP than that of pure AP [9, 19 –21]. Similarly, Dubey et al. [22] observed that the catalystlowers Ea in the temperature range 220 – 260 8C andincreases Ea in the temperature range 220 – 380 8C. Thusthere are contradictory reports about the effect of Fe2O3 onEa. From Table 1 and 2 we can see that the value of theactivation energy for high temperature decomposition ishigher for the catalyzed composite B (181.5 kJ mol�1) thanfor the uncatalyzed composite A (143.8 kJ mol�1). Thoughthis observation is in agreement with Dubey et al. it iscontrary to the generally observed trend of lowering of Ea,for a reaction whose rate is increased by a catalyst. Thisdiscrepancy may be attributed to the facts like stronginteraction and binding between AP and HTPB andsecondly alongwithEa the frequency factor (A) also showedvariations. The increase in activation energy and thecorresponding increase in the A value is due to the kineticcompensation effect as reported earlier by Ninan et al. [23].In a reaction catalyzedbynano ironoxide catalyst there is anincrease in reactant concentration at the catalyst surface, asconfirmed earlier by observing high heat release. Thus thereaction gets accelerated through a relatively high frequen-cy factor in catalyzed composite B (6.63� 1016 min�1 at10 8C/min) compared to uncatalyzed composite A (4.62�1010 min�1 at 10 8C/min) (Table 1 and 2). As a result a directcorrelation of Ea with the reaction rate becomes difficultsince both Ea and A are altered in the reaction. Thus thefrequency factor plays an important role in increasing thereaction rate of the thermal decomposition of AP. Table 1and 2 shows average values for the specific rate constant k.The specific rate constant increases by addition of nano ironoxide as expected. In iron oxide nanoparticles there aremany active sites on the surface; due to their smaller particlesize and high surface area. During exothermic decomposi-tion of AP, nano Fe2O3 absorbs the gaseous reactivemolecules present on its surface and catalyzes the reaction.

Hence, an increase in the rate constant was observed whichis a direct indication of the enhancement in the catalyticactivity of composite B.

4 Conclusions

The differential scanning calorimetric studies of AP/HTPB based composite propellants in presence of nanoferric oxide exhibit the best catalytic effect on the hightemperature decomposition ofAP. The lowering ofTp in thehigh temperature decomposition, a higher heat release andan increase in rate constant in the catalyzed propellantconfirmed the enhancement in catalytic activity of ammo-nium perchlorate. The scanning electron micrographs ofnano Fe2O3 incorporated in HTPB revealed a well-sepa-rated characteristic necklace-like structure of a-Fe2O3particles at high magnification.

5 References

[1] a) P. W. M. Jacob, H. M. Whitehead, Decomposition andCombustion of Ammonium Perchlorate, Chem. Rev. (Wash-ington, DC, U.S.) 1969, 69, 551. b) V. V. Boldyrev, ThermalDecomposition of Ammonium Perchlorate, Thermochim.Acta 2006, 443, 1.

[2] S. W. Mayer, E. K. Weinberg, L. Schieler, Procedures forSuppressing Premature Exothermic Decomposition in Am-monium Perchlorate, AIAAJ 1970, 8(7), 1328.

[3] Y. Wang, J. Zhu, X. Yang, L. Lu, X. Wang, Preparation ofNiO Nanoparticles and their Catalytic Activity in theThermal Decomposition of Ammonium Perchlorate, Ther-mochim. Acta 2005, 437, 106.

[4] A. K. Galwey, M. A. Mohamed, D. S. Cromie, Role of SilverCompounds in Promoting the Thermal Decomposition ofAmmonium Perchlorate, React. Solids 1986, 1, 235.

[5] G. S. Pearson, Composite Propellent Catalysts: Copper Chro-mate and Chromite, Combust. Flame 1970, 14, 73.

Figure 3. SEM of Nano Fe2O3 incorporated in HTPB at a) 600�magnification, b) 5000�magnification.

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G 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim www.pep.wiley-vch.de Prop., Explos., Pyrotech. 31, No. 6, 442 – 446

[6] S. M. Shen, S. I. Chen, B. H. Wu, The Thermal Decomposi-tion of Ammonium Perchlorate (AP) containing a BurningRate Modifier, Thermochim. Acta 1993, 223, 135.

[7] R. P. Rastogi, G. Singh, B. L. Dubey, C. S. Shukla, Solid StateChemistry of Copper Chromite Used as a Catalyst for theBurning of Ammonium Perchlorate/Polystyrene Propellants,J. Catal. 1980, 65, 25.

[8] A. A. Said, Thermal Decomposition of Ammonium Metava-nadate Doped with Iron, Cobalt or Nickel Hydroxides,Thermal. Anal. 1991, 37, 849.

[9] R. Rajeev, C. Gopalkrishnan, K. Krishnan, K. G. Kannan,K. N. Ninan, Studies on the Effect of Concentration of FerricOxide Catalyst in the Thermal Decomposition of AmmoniumPerchlorate, 3rd International High Energy Materials Confer-ence and Exhibition (HEMCE), December 2000, Thiruva-nanthapuram, India, 2000, p. 121.

[10] E. Santacesaria, A. Morini, S. Carra, Ammonium PerchlorateDecomposition in Presence of Metallic Oxides, Combust.Flame 1978, 31, 17.

[11] L. Liu, F. Li, L. Tan, L. Ming, Y. Yi, Effects of Nanometer Ni,Cu, Al, and NiCu Powders on the Thermal Decomposition ofAmmonium Perchlorate, Propellants, Explos., Pyrotech. 2004,29(1), 34.

[12] P. R. Patil, S. K. Hait, V. N. Krishnamurthy, S. S. Joshi,Characterization of Nano Iron Oxide and its Effect onThermal Decomposition of Ammonium Perchlorate, 5thInternational High Energy Materials Conference and Exhibi-tion (HEMCE), November 23 – 25, 2005, Hydrabad, India,2005, p. 111.

[13] A. K. Nema, S. Jain, S. K. Sharma, S. K. Nema, S. K. Verma,Mechanistic Aspect of Thermal Decomposition and BurnRate of Binder and Oxidizer of AP/HTPB CompositePropellants Comprising HYASIS-CAT, Int. J. Plast. Technol.,2004, 8, 344.

[14] J. Gao, F. Guan, Y. Zhao, W. Yang, Y. Ma, X. Lu, J. Hou, J.Kang, Preparation of Ultrafine Nickel Powder and itsCatalytic Dehydrogenation Activity, Mater. Chem. Phys.2001, 71, 215.

[15] M. T. Reetz, W. Helbig, Size-Selective Synthesis of Nano-structured Transition Metal Clusters J. Am. Chem. Soc. 1994,116, 7401.

[16] S. S. Joshi, P. R. Patil, M. S. Nimase, P. P. Bakare, Role ofLigands in the Formation, Phase Stabilization, Structural and

Magnetic Properties of a-Fe2O3 Nanoparticles, J. Nano. Res.2006, 8, 635.

[17] S. H. Inami, Y. Rajaphase, R. Shaw, H. Wise, Solid PropellantKinetics. I: The Ammonium Perchlorate-Copper Chromite-Fuel System, Combust. Flame 1971, 17, 189.

[18] J. A. F. F. Rocco, J. E. S. Lima, A. G. Frutuoso, K. Iha, M.Ionashiro, J. R. Matos, M. E. V. Suarez-Iha, Thermal Degra-dation of a Composite Solid Propellant Examined by DSC, J.Therm. Anal. Calorim. 2004, 75, 551.

[19] F. Solymosi, T. Bansagi, Catalysis of Reactions in Solid Phase,Thermal Disintegration of AP in Presence of Fe2O3, Kinet.Catal. 1963, 4(1), 73.

[20] K. Kishore, V. R. Pai Verneker, M. R. Sunitha, Effect ofManganese Dioxide on the Thermal Decomposition ofAmmonium Perchlorate, J. Appl. Chem. Biotechnol. 1977,27, 415.

[21] N. B. Singh, A. K. Ojha, The Catalytic Behavior of NiFe2�xCrxO4(0.0� x� 2.0) during the Thermal Decomposition ofAmmonium Perchlorate, Polystyrene and their CompositePropellants, 3rd International High Energy Materials Confer-ence and Exhibition (HEMCE), December 2000, Thiruva-nanthapuram, India, 2000, p. 154.

[22] B. L. Dubey, N. Nath, A. Tripathi, N. Tiwari, CatalysedCombustion of Ammonium Perchlorate, Polystyrene andtheir Composite Propellants, Ind. J. Eng. Mater. Sci. 1994, 1,341.

[23] K. N. Ninan, A Thermogravimetric Study on the CatalyticDecomposition of Ammonium Perchlorate from ActivationEnergy Normalised Through Kinetic Compensation, Indian J.Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem.1998, 37, 295.

Acknowledgements

Prajakta R. Patil thanks the funding agency ISRO for thefinancial support and HEMRL, Pune for availing DSC facility.

(Received July 17, 2006; MS 2006/029)

446 P. R. Patil, V. N. Krishnamurthy, S. S. Joshi

Prop., Explos., Pyrotech. 31, No. 6, 442 – 446 www.pep.wiley-vch.de G 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim