studies of energetic compounds, part 29: effect of nto and its salts on the combustion and condensed...

Studies of energetic compounds, part 29: effect of NTOand its salts on the combustion and condensed phasethermolysis of composite solid propellants, HTPB-AP

Gurdip Singha,*, S. Prem Felixa

aChemistry Department, DDU Gorakhpur University, Gorakhpur 273 009, India

Received 22 January 2002; received in revised form 2 July 2002; accepted 22 August 2002

Abstract

The effect of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) and two of its transition metal salts, namelyCu(NTO)2 and Fe(NTO)3, during the combustion of composite solid propellants (CSPs) of hydroxyl-terminatedpolybutadiene (HTPB) and ammonium perchlorate (AP) has been studied. The activities of Cu(NTO)2 andFe(NTO)3 have been compared with those of CuO and Fe2O3 at their equivalent metal concentrations. Theprocessing parameters and mechanical properties of the propellants were also evaluated using Cu(NTO)2 andFe(NTO)3 as additives, and a comparison has been made with that of copper chromite (CC) and active iron oxide(AIO) at pilot plant scale. The safety aspects of using these energetic salts as burning-rate modifiers have beenstudied in terms of the impact-sensitivity of the modified propellants. An attempt has been made to evaluateexperimentally the condensed phase activity of these additives during the slow thermolysis of propellants, as wellas AP. Rapid thermolysis of the propellants and AP has been studied using measurements of ignition delay.© 2003 The Combustion Institute. All rights reserved.

Keywords: Energetic ballistic modifier, NTO, Transition metal salts of NTO, Combustion, Condensed phase, HTPB-AP

1. Introduction

The internal ballistics of a rocket are controlled bythe burning rate (*r ) of the propellants, since thethrust F is related to the mass consumption rate or theweight flow rate ( *w ) by

F � w* Ve/g � (Pe � P0) Ae (1)

where Ve is the exhaust velocity; g a conversionfactor required to make the equation dimensionallyconsistent; Pe the exit pressure; P0 the ambient pres-sure; and Ae the area of the nozzle’s exit. In the case

of solid propellants, *w is related to the linear regres-sion rate (*r ) by

w* � Ab�b r* (2)

where Ab is the burning area and �b is the density ofthe propellant. Thus, any enhancement in the burningrate increases the weight flow rate, which in turnenhances the thrust generated.

Catalysts are required to increase *r of compositesolid propellants (CSPs), as they have low inherentburning rates. Transition metal oxides (TMOs) suchas Fe2O3, copper chromite (CC), MnO2, Ni2O3, etc.enhance *r for CSPs considerably [1]. However, theeffectiveness of TMOs is reported [2] to be concen-tration-dependent, and since they are non-energetic,an increase in their concentration in the propellantmay decrease the total energy. TMOs, owing to their

* Corresponding author. Tel.: 91-551-2202856; fax: 91-551-2340459.

E-mail address: [email protected] (G. Singh).

Combustion and Flame 132 (2003) 422–432

0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved.doi:10.1016/S0010-2180(02)00479-0

large particle size, introduce a certain degree of in-homogeneity and thus alter the effective catalyst con-centration, leading to erratic burning of the propellant[3]. In addition, Fe2O3, one of the catalysts in vogue,is known to increase the viscosity during processing[4,5]. To overcome these disadvantages, research isbeing done to develop new catalysts. Energetic bal-listic modifiers increase the burning rate and at thesame time do not decrease the total energy of thepropellant. When used as ballistic modifiers, transi-tion metal salts and complexes containing energeticgroups are oxidized to the corresponding metal ox-ides, with a substantial release of energy. The TMO,which forms in situ in the system, will be moreactive, and thus, energetic ballistic modifiers haveadvantages over TMOs.

Following our earlier work [6] on burning-ratecatalysts for solid propellants, hexamine metal per-chlorate complexes of some transition metals havebeen evaluated as energetic ballistic modifiers forhydroxyl-terminated polybutadiene (HTPB)-ammo-nium perchlorate (AP) CSPs [7,8]. 5-Nitro-2,4-dihy-dro-3H-1,2,4-triazole-3-one (NTO) is an insensitivehigh explosive [9–11] but, owing to its acidic nature,it forms a wide range of salts, which are also highlyuseful [12]. The metal salts of NTO have been sug-gested as energetic ballistic modifiers for solid pro-pellants [13,14]. The preparation, characterization,and thermal decomposition of some transition metalsalts of NTO have already been reported [15,16]. Theactivity of some of these salts as burning-rate modi-fiers for HTPB-AP CSPs has also been evaluated[17,18]. Our previous studies showed that Cu(NTO)2

is the best additive of the six NTO salts used. Metaloxides have been found to be the final product ofthermolysis for transition metal salts of NTO [15,16].Brill et al. [19] have shown that the metal salts ofNTO form volatile metal compounds during theirthermolysis, which may be further converted to metaloxides in the gas phase. They have hypotheticallysuggested the tailoring of some of the metal salts soas to dampen high-frequency acoustic modes insidethe combustor and thus, avoid unsteady combustion.Moreover, Williams et al. [20] have suggested theuse of NTO as a potential additive in solid propel-lants for suppressing the burning rate and stabilizingcombustion.

Thus, it was found interesting to assess the activ-ity of NTO as a burning-rate modifier experimentally.In this study, NTO, Cu(NTO)2, and Fe(NTO)3 havebeen evaluated as burning-rate modifiers forHTPB-AP CSPs. A comparison between transitionmetal salts of NTO and the corresponding TMOs asburning-rate modifiers for CSPs also seemed to berelevant. In-depth study of the condensed phase be-

havior of these salts during the thermolysis of AP andHTPB-AP propellants has also been made.

2. Experimental

2.1. Materials

NTO, Cu(NTO)2, and Fe(NTO)3 were prepared inour laboratory. CuO (Sarabhai Chemicals), Fe2O3

(BDH), CC (BL Industries), HTPB (NOCIL IndiaLtd.), IPDI (Anabond Ltd.), AP (Central Electro-Chemical Research Laboratory, Karaikudi), and di-octyl adipate (DOA) (S. D. Fine Chem. Ltd.) wereused as received. The active iron oxide (AIO) usedwas an in-house product of Vikram Sarabhai SpaceResearch Centre (VSSC), Thiruananthapuram.

2.2. Preparation of propellants

A bimodal particle distribution of AP (coarse,100–200 mesh : fine, 200–400 mesh � 3:1) wasused for preparing the propellants in our laboratory.Both aluminized and non-aluminized propellantswere prepared. The solid loading was 80 wt % in allcases. Aluminum powder 18 wt % was used formaking aluminized propellants. DOA (25 wt % ofbinder) was the plasticizer and IPDI was the curingagent in all cases. Finely powdered additives (200–400 mesh), viz., NTO, Cu(NTO)2, and Fe(NTO)3,had concentrations of 2 wt %. The solid componentswere dry-mixed [21] first and were then added slowlyto the binder at 50°C with constant stirring using avertical stirrer. The stirring was continued for 1 hafter the completion of mixing, and the slurry wasvacuum-cast over stainless steel plates and cured at60°C for 8 days.

Propellants were also made at pilot plant scale atVSSC. The details of mixing, curing, as well as thecomposition are given in Table 1. Mixing was donefor 3 h under a residual pressure of 4 mm Hg in amicro vertical-kneader with twin blade configuration.IPDI was used as the curing agent. Bimodal AP ofaverage particle size 215 �m was used for 86% solidloading and unimodal AP, 40 �m, was used for 81%solid loading. Curing was done at 60°C for 8 days.

2.3. Burning-rate measurements

Propellant strands (10 � 0.7 � 0.7 cm3) were cutfrom the castings, and the sides were inhibited byapplying paint. The steady burning rates of thestrands were measured using the fused-wire tech-nique at Birla Institute of Technology (BIT), Ranchi.The value of *r was measured under ambient pressure,as well as 7 kg/cm2 and 14 kg/cm2 of N2. For each

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Table 1Propellant data using NTO, Cu(NTO)2, Fe(NTO)3, CC, and AIO as ballistic modifiers in HTPB-AP composite solid propellants

Experiment No.

1 2 3 4 5 6 7 8 9 10

1. CompositionSolid loading % 86 86 86 86 86 86 86 81 81 81AP % 68.0 67.5 67.5 67.5 67.9 67.5 67.5 79.0 77.0 77.0Al % 18 18 18 18 18 18 18 2 2 2Binder % 14 14 14 14 14 14 14 19 19 19Additive Nil NTO Fe(NTO)3 AIO Cu(NTO)2 Cu(NTO)2 CuCr2O4 Nil Cu(NTO)2 CuCr2O4

Concentration - 0.5% 0.5% 0.5% 0.1% 0.5% 0.5% - 2.0% 2.0%2. Mix details

Mix size (kg) 1.0 1.5 1.5 3.0 1.0 1.0 3.0 1.5 1.5 3.03. End of mixing

viscosity, poise5440 25280 9600 5760 6400 6080 4800 4800 5120 6000

4. Burning rate (mm/s)40 kg/cm2 4.95 � 0.05 5.2 � 0.07 6.8 � 0.06 7.8 � 0.05 6.31 � 0.07 6.91 � 0.06 8.0 � 0.06 8.2 � 0.05 11.3 � 0.2 13.8 � 0.370 kg/cm2 6.03 � 0.06 - 8.2 � 0.09 10.2 � 0.06 7.9 � 0.08 8.7 � 0.08 10.5 � 0.07 10.1 � 0.07 14.0 � 0.4 17.1 � 0.5

5. Mechanical propertiesTensile strength(kg/cm2)

7.5 8.1 7.4 6.8 6.8 6.2 7.3 5.6 4.4 6.7

Elongation (%) 38 34 27 47 30 30 48 60 49 65Modulus(kg/cm2)

42 51 61 40 55 55 42 21 16 20

Hardness (Shore A) 75 77 75 65 70 70 70 60 50 60

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sample, *r was measured for five strands at eachpressure, and the average values are presented inTable 2. With the propellant compositions in Table 1,*r was measured using the acoustic emission tech-nique. The cured propellant strands (8-cm length;0.6-cm2 cross-sectional area) were ignited electri-cally by a hot wire and burned under water in astainless steel bomb. The requisite N2 pressure wasmaintained in the bomb. An acoustic transducer wasmounted on the side of the combustion chamber, and*r was measured from the acoustic output. For everytest pressure, four or five strands were burned, andthe average value of the burning rates was taken;these are reported in Table 1.

2.4. TG-DTG studies of propellants

Non-isothermal TG-DTG analyses of non-alumi-nized HTPB-AP propellant (control) and propellantscontaining NTO, Cu(NTO)2, and Fe(NTO)3 as addi-tives were done with a Dupont 2100 TG instrument ata heating rate of 10°C/min (sample mass �2 mg, at-mosphere � flowing N2 gas at a rate of 60 mL/min).The TG-DTG thermograms are shown in Fig. 1, and thephenomenological data are summarized in Table 3.

2.5. DSC studies of propellants

DSC analyses on the propellant samples sealed inaluminum pans were done (using a Dupont 2100DSC) at a heating rate of 10°C/min (sample mass �2mg, atmosphere � flowing N2 gas at a rate of 60mL/min). The corresponding thermograms are shownin Fig. 2, and the phenomenological data are summa-rized in Table 3.

2.6. Non-isothermal TG

Non-isothermal TG was done on aluminized aswell as non-aluminized propellants, using an in-digenously fabricated TG apparatus [21], in anatmosphere of static air (sample weight �25 mg,heating rate � 5°C/min). TG thermograms for non-aluminized propellants are given in Fig. 3, and TGthermograms for aluminized propellants are shownin Fig. 4. Non-isothermal TG studies on AP andAP containing 2 wt % of additives have also beendone using the indigenously fabricated TG appa-ratus, and the corresponding thermograms areshown in Fig. 5.

Table 2Steady burning rate r, impact-sensitivity (h50%), and pressure exponent (n) of propellants

Pressure(kg/cm2)

Additive h50% (cm) *r (mm/s) *raa/*r0

b n

Non-aluminized1.033 Nil 102 1.63 � 0.02 1.00 -1.033 NTO 109 1.92 � 0.02 1.18 -1.033 Cu(NTO)2 63 2.51 � 0.03 1.54 -1.033 Fe(NTO)3 55 2.04 � 0.03 1.25 -1.033 CuO 85 2.02 � 0.03 1.24 -1.033 Fe2O3 67 1.94 � 0.02 1.19 -

Aluminized1.033 Nil 110 1.26 � 0.02 1.007 2.79 � 0.04 1.0014 3.58 � 0.05 1.00 0.40

1.033 NTO 110 1.43 � 0.02 1.147 3.55 � 0.02 1.2714 4.44 � 0.05 1.24 0.44

1.033 Cu(NTO)2 86 1.67 � 0.02 1.337 5.09 � 0.04 1.8214 6.90 � 0.06 1.93 0.55

1.033 Fe(NTO)3 105 1.56 � 0.03 1.247 4.81 � 0.03 1.7214 5.71 � 0.04 1.60 0.52

a *ra � burning rate of modified propellant.b *r0 � burning rate of un-modified propellant.


Fig. 1. TG-DTG thermograms of HTPB-AP propellantscontaining 2% additives: (a) control, (b) NTO, (c)Cu(NTO)2, and (d) Fe(NTO)3.

Table 3TG-DTG and DSC phenomenological data on non-aluminized HTPB-AP propellants

Additive TG-DTG DSC peaktemperatures (°C)

�H (J/g)

Ti (°C) Ts (°C) Tf (°C) % Weight loss Endo Exo Endo Exo

Nil 271.99 284.61 287.98 16.36 242.53 364.42 73.07 2908332.00 357.39 372.74 66.95 405.10438.16 447.98 463.28 5.77

NTO 267.95 282.49 289.12 18.53 242.57 370.55 76.72 2897327.55 342.67 342.07 63.34 406.83444.33 449.41 461.49 5.75

Cu(NTO)2a - 322.04 - 96.40 245.38 345.84 67.86 2918

366.54Fe(NTO)3 270.52 284.23 289.95 28.60 242.78 352.97 66.43 2137

334.80 351.09 364.23 52.50 390.68440.10 448.23 462.25 5.50

a Ti and Tf in this case are too close to Ts and hence, clear distinction is not possible.

Fig. 2. DSC thermograms of HTPB-AP propellants con-taining 2% additives: (a) control, (b) NTO, (c) Cu(NTO)2,and (d) Fe(NTO)3.

426 G. Singh, S.P. Felix / Combustion and Flame 132 (2003) 422–432

2.7. Impact-sensitivity

The propellant samples were scratched with em-ery paper to produce a fine powder. The impact-sensitivity of this fine powder was assessed at BITusing a falling-weight (2 kg) method [22]. The heightfrom which the drop caused 50% explosions (h50%)has been calculated and is reported in Table 2.

2.8. Ignition-delay measurements

The ignition delay (tid) of the propellant samples(both aluminized and non-aluminized) was deter-mined using the tube furnace (TF) technique [23], asreported earlier [18,23]. Thus, the values of tid re-ported are the average of three experiments andagreed within experimental error. The values of tidfitted the equation [24–26]:

tid � A exp (E*/RT) (3)

where E* is the activation energy for ignition and T isthe absolute temperature. The value of E* was ob-tained from the slope of the Arrhenius plot. Thus,plots of ln tid against (1/T) for non-aluminized andaluminized propellants are given in Figs. 6 and 7,

respectively. Values of tid and E* for propellants aregiven in Table 4.

Ignition-delay measurements were also performedon pure AP and AP plus additives (2 wt %), and thecorresponding data are summarized in Table 5. Theplots of ln tid vs. 1/T for AP and AP plus additives aregiven in Fig. 8.

2.9. Mechanical properties

The mechanical properties of the propellants pre-pared in the pilot scale plant were determined andreported in Table 1. The tensile strength, elongation(%), modulus, and hardness (Shore A) were mea-sured for 5-mm-thick dumb-bells of ASTM specifi-cation D 412 (Die – C), cut from the cured propel-lants. Measurements were made using the InstronUniversal Testing Machine at VSSC, at a cross-headspeed of 50 mm/min at 25°C.

3. Results and discussion

The data reported in Tables 1 and 2 clearly showthat NTO, Cu(NTO)2, and Fe(NTO)3 enhance the

Fig. 3. Non-isothermal TG thermograms (under static airatmosphere) of non-aluminized HTPB-AP propellants.

Fig. 4. Non-isothermal TG thermograms (under static airatmosphere) of aluminized HTPB-AP propellants.

Fig. 5. Non-isothermal TG thermograms (under static airatmosphere) of AP and AP plus additives (2% by weight).

Fig. 6. Plot of ln tid vs. 1/T for non-aluminized HTPB-APpropellants.


burning rate of HTPB-AP CSPs considerably. Of thethree, Cu(NTO)2 is the best additive, followed byFe(NTO)3 and NTO. The activity of these additives isevident in both aluminized and non-aluminized pro-pellants. The fact that NTO itself enhances the burn-ing rate is notable and contrary to what is theoreti-cally suggested in the literature [20]. Williams et al.[20], after their pyrolysis studies on NTO, showedthat NTO forms polymeric cyclic azines when it isdecomposed below �280 °C. They have suggestedfrom their studies that, since the polymeric azine isthermally stable up to �700 °C, it can accumulatetransiently at the burning surface and thus, may retardmass transfer from the condensed phase to the gasphase and reduce heat transfer from the gas phase tothe condensed phase. This was the basis for them tohypothesize that NTO can act as a burning-rate sup-pressant and can be used as an additive for enhancingcombustion stability. However, it is reported [27,28]that •NO2 is liberated during the thermolysis of NTO.This may add to the radical chemistry of HTPB-APdecomposition. The liberated NO2 may form

NO2�ClO4

- as an intermediate [29], the decomposi-tion of which increases *r considerably [30]. In addi-tion, the thermolysis of NTO is highly exothermic[10,28], which may also contribute toward the sur-face temperature of the propellants during combus-tion and enhance the burning rate.

Of the variables, pressure (P) is the most impor-tant parameter on which *r of CSPs depends [31].Although there are many empirical relationships pro-posed between *r and P [32], Vielle’s law (*r � a Pn)is the most commonly used one for CSPs [33] in thepressure domain up to around 100 atm, where a is aconstant and n is the exponent. The measured steadyburning rates for aluminized propellants given inTable 2 obey Vielle’s law. The plot of ln *r vs. ln P isgiven in Fig. 9. The values of n determined from theslope of the plots are given in Table 2. The highestvalue of n is for Cu(NTO)2.

Now that it is established that NTO itself in-creases *r, the additional enhancement by the metalsalts must be due to the metal oxides formed. Sincethese metal oxides are formed in situ during thecombustion of the propellants, they must be of fineparticle size and have defective sites in their crystal-lites. Thus, they may be more active than commercialCuO and Fe2O3. In order to settle this point, propel-lants were made with CuO and Fe2O3 as catalysts.The concentration of these oxides were chosen asequivalent to the effective metal concentration inCu(NTO)2 and Fe(NTO)3 in their respective propel-lant samples. Thus, propellants were made with 0.5wt % CuO and 0.36 wt % Fe2O3. The measured *r forthese propellants are reported in Table 2. The *r ofpropellant modified with CuO is lower than that ofCu(NTO)2, and that of Fe2O3 is lower than that ofFe(NTO)3, as expected. The difference in *r for pro-pellants modified with Cu(NTO)2 and CuO is con-

Fig. 7. Plot of ln tid vs. 1/T for aluminized HTPB-APpropellants.

Table 4Ignition delay (tid) and activation energy for ignition (E*) for HTPB-AP propellants

Additive tid (s) at temperatures (°C) E* (kJ/mol)

275 300 325 350 400 450 500

AluminizedNil - 109.9 � 0.21 - 68.0 � 0.14 47.9 � 0.09 36.6 � 0.08 28.7 � 0.08 24.6NTO - 96.5 � 0.20 - 61.2 � 0.16 43.4 � 0.08 34.1 � 0.09 26.0 � 0.07 23.8Cu(NTO)2 - 84.7 � 0.23 - 60.8 � 0.13 40.2 � 0.12 32.6 � 0.08 23.8 � 0.07 23.3Fe(NTO)2 - 81.5 � 0.19 - 56.8 � 0.19 39.8 � 0.09 29.7 � 0.10 23.2 � 0.06 23.4

Non-aluminizedNil 156.3 � 0.24 133.0 � 0.20 96.0 � 0.20 81.5 � 0.15 59.0 � 0.15 - - 24.7NTO 154.5 � 0.26 124.4 � 0.19 94.8 � 0.16 81.2 � 0.09 57.6 � 0.11 - - 24.4Cu(NTO)2 141.5 � 0.21 104.4 � 0.20 85.7 � 0.16 57.6 � 0.14 53.0 � 0.12 - - 23.4Fe(NTO)2 109.1 � 0.20 95.9 � 0.19 75.1 � 0.14 63.5 � 0.12 45.8 � 0.10 - - 21.9CuO 151.8 � 0.25 119.0 � 0.21 89.2 � 0.16 80.0 � 0.09 57.4 � 0.10 - - 23.8Fe2O3 120.3 � 19 96.5 � 0.18 78. � 0.15 66.7 � 0.11 48.4 � 0.08 - - 22.3


siderably large, whereas that between Fe(NTO)3 andFe2O3 is much less. The lesser activity of Fe(NTO)3

may be due to the presence of two molecules of waterof hydration [16] in it. It is reported that the presenceof water molecules reduces *r [34]. In the case ofCu(NTO)2, it may be seen from a comparison of *r forpropellants modified with CuO, Cu(NTO)2, andNTO, that the metal oxide formed in this case isindeed more active than commercial CuO.

Since the additives are energetic compounds, as-sessment of safety aspects is necessary. The impact,friction, as well as spark-sensitivity of these com-pounds was reported on earlier [16,35]. It was foundthat these salts are as insensitive as the parent com-pound, NTO. The values of h50%, reported in Table 2,show that modification enhances the sensitivity of thepropellant. Interestingly, a reduction in h50% has alsobeen observed for propellants modified with puremetal oxides. It is known that hard and high-melting-point grits sensitize energetic materials by creating“hot spots” through a frictional mechanism [36,37].There is no considerable enhancement in sensitivityfor propellants modified with energetic salts beyondthat imparted by TMOs, via the frictional mecha-nism. Thus, impact-sensitivity reveals that the use ofNTO and its salts does not affect sensitivity to anydetrimental level and so may be used safely.

The steady burning rates of propellants made inpilot scale plant were determined at high pressures,comparable to what exist in rocket motors.

Cu(NTO)2 enhances *r even at very low concentra-tions such as 0.1 wt %, and from experiments 5, 6,and 9 (in Table 1), it may be seen that its activity isconcentration-dependent. The overall results summa-rized in Table 1 show that CC is more active thanCu(NTO)2 and AIO is more active than Fe(NTO)3, atthe same concentrations (by weight). However, itmust be borne in mind that the effective metal con-centrations of NTO salts are much less than CC andAIO. Hence, it may be inferred that, in order toachieve the level of enhancement in *r attained by CCor AIO, Cu(NTO)2 and Fe(NTO)3 must be used athigher concentrations.

NTO is not a good burning-rate modifier, not onlybecause of its comparatively lesser activity, but alsodue to its detrimental effect on processing parame-ters. A considerable build-up in viscosity, as deter-mined by a Brookfield viscometer, was observed atthe end of mixing in the case of NTO (see Table 1).This may be due to an attack of the acidic proton ofNTO [9] on unsaturated butadiene chains. Viscositybuild-up is also relatively high for Fe(NTO)3. How-ever, the processing parameters of Cu(NTO)2 arecomparable to those of AIO and CC. The mechanicalproperties of the propellants modified by NTO and itssalts are comparable with those of CC and AIO.

There is no consensus regarding the regime for acatalyst’s action during the combustion of CSPs.Some studies have proposed that the action of the

Fig. 8. Plot of ln tid vs. 1/T for AP and AP plus additives(2% by weight).

Fig. 9. Plot of ln r vs. ln P for non-aluminized HTPB-APpropellants.

Table 5Ignition delay (tid) and activation energy for thermal ignition (E*) for AP and AP plus additives

Additive tid (s) at temperatures (°C) E* (kJ/mol)

400 425 450 475 500 550

Nil - - 128.0 � 0.21 88.2 � 0.16 75.2 � 0.12 50.2 � 010 44.7NTO - - 104.9 � 0.20 75.5 � 0.16 59.7 � 0.11 43.3 � 0.11 43.1Cu(NTO)2 53.0 � 0.12 49.3 � 0.12 42.3 � 0.10 37.6 � 0.09 34.7 � 0.09 27.4 � 0.08 20.6Fe(NTO)3 87.1 � 0.20 68.9 � 0.15 60.6 � 0.14 55.2 � 0.12 45.5 � 0.12 34.3 � 0.09 27.6


catalysts takes place primarily in the gas phase [38].However, most studies have suggested that the bal-listic modifier is active mainly in the condensedphase [39,40]. Therefore, we have tried to evaluatethe activity of the additives in the condensed phaseduring thermolysis of the propellants. The TG-DTGthermograms shown in Fig. 1 and also the data sum-marized in Table 3 show that thermolysis of thecontrol propellant sample and propellants modifiedwith NTO and Fe(NTO)3 takes place in three steps.The first step corresponds to the low-temperaturedecomposition (LTD) of AP [32,39]. It may be com-bined with the first stage in the decomposition ofHTPB [41]. The second step corresponds to the high-temperature decomposition (HTD) of AP [32,39].The third and final stage, occurring at 440–460°C, isdue to the decomposition of residual HTPB, left overafter the first stage of thermolysis. In addition to thesethree steps, initially there is a small weight loss start-ing at �150°C and continues up to �225°C; this stepis due to the evaporation of the plasticizer. The smallpeak in the DTG at �257°C for the propellant mod-ified with NTO may be due to the decomposition ofNTO. Activity of the additives in the condensedphase is evident in all three cases. Both NTO andFe(NTO)3 increase the extent of conversion duringLTD, Fe(NTO)3 being more effective than NTO. Theon-set (Ti), inflection (Ts), and end-set (Tf) tempera-tures for LTD are not altered by the additives. How-ever, Ts and Tf for HTD have been lowered consid-erably by these additives. For the propellant modifiedwith Cu(NTO)2, thermolysis occurs in a single rapidstep. Cu(NTO)2 affects both LTD and HTD, and thedemarcation between these processes has been com-pletely lost. It can also be inferred from the TG-DTGdata for propellants modified with Cu(NTO)2 that theheat released by the condensed phase due to thethermolysis ultimately led to “auto-ignition” of thepropellant; that is evident from the sharp weight lossin the TG. The fact that both stages of thermal deg-radation of HTPB were complete after the step alsosupports the occurrence of ignition.

The multi-step thermolysis of the propellants isalso observed in the DSC thermograms shown in Fig.2 and the data summarized in Table 3. Any peakcorresponding to evaporation of the plasticizer hasnot been observed in the DSC thermograms. Thismay be due to the use of sealed aluminum pans, inwhich the pressure rise might have offset evapora-tion. The first endothermic peak at �242°C is due tothe polymorphic transition of AP from orthorhombicto cubic [32]. Two exothermic peaks correspondingto LTD and HTD have also been observed in all fourcases. In the case of Cu(NTO)2 and Fe(NTO)3, man-ifestation of their activity is evident from the lower-ing of the peak temperatures for both LTD and HTD.

However, such an effect is absent in the case of NTO.The peak temperatures for LTD and HTD are veryclose to each other in the case of Cu(NTO)2. Theexothermic heat release (�H) for LTD and HTDtogether has been calculated from the area of the peakand is reported in Table 3. �H for propellants mod-ified with Cu(NTO)2 is slightly higher than that forunmodified propellants, whereas lower values wereobtained for propellants modified with NTO andFe(NTO)3. The heat release is considerably less inthe case of Fe(NTO)3, probably due to the presenceof water of hydration.

The activity of NTO salts is also evident from thenon-isothermal TG thermograms in Figs. 3 and 4. Inthe case of Cu(NTO)2, the demarcation between LTDand HTD is almost indistinguishable, and ignition-like reaction occurs. It can also be seen from Fig. 3that the activity of NTO, its metal salts, and TMOsduring the thermolysis is in the order:

Cu�NTO2 Fe(NTO)3 CuO Fe2O3 NTO

The order is that for combustion. Thus, there exists arelation between the activity during the condensed-phase thermolysis of propellants and the enhance-ment in burning rate. NTO and its salts also affect thethermolysis of AP (see Fig. 5). The order of activityis again the same as that for combustion, indicatingthat these additives mainly affect the condensed-phase thermolysis of AP.

Measurement of ignition delays is an effectivemethod of studying the thermal stability of energeticcompounds and mixtures [15,16,22,23,26]. It is alsoan effective tool for investigating the catalytic ther-molysis of energetic materials [18,42]. During mea-surements of the ignition delay, response of the ma-terial to a sudden high temperature is measured; thisis similar to pyrolytic studies. It is evident from thevalues of tid in Table 4 that the additives reduce thethermal stability of the propellants. Fe(NTO)3 seemsto be the most efficient additive in reducing tid, fol-lowed by Fe2O3. However, the values of E* do notseem to be much affected. A comparison of Tables 4and 5 shows that the thermal stability of pure AP isconsiderably higher than that of HTPB-AP mixture(propellant). The activity of NTO, as well as its salts,is also clearly evident from the lowering of tid duringthe thermal ignition of AP (see Table 5). The order ofE* for AP and AP plus additives is:

AP � AP � NTO

� AP � Fe�NTO3 AP � Cu(NTO)2

Cu(NTO)2 reduces the value of E* for AP to less thanhalf. Thus, the activity of additives is mainly on thethermolysis of AP. The rapid thermolysis of a pro-pellant is mainly controlled by thermolysis of the


binder (HTPB). Hence, there is no reduction in E*for propellants. It may be inferred that the addi-tives do not affect the thermolysis of HTPB, exceptFe(NTO)3 and Fe2O3, for which there is a smalleffect. The tid for aluminized propellants is lowerthan that for non-aluminized propellants at thesame temperature (see Table 4). This may be dueto more efficient heat transfer by aluminum parti-cles in the propellant.

4. Conclusions

NTO, Cu(NTO)2, and Fe(NTO)3 enhance thesteady burning rates of HTPB-AP CSPs consider-ably. The activity of Cu(NTO)2 and Fe(NTO)3 isgreater than that of CuO and Fe2O3 at equivalentconcentration of metal. This result shows that metaloxides formed in situ during combustion are moreactive than the commercial ones. The activity of NTOsalts is concentration-dependent. To achieve a higherburning rate than that attainable by CC or AIO, theNTO salts must be used at higher concentrations.NTO cannot be used as an additive in HTPB-basedpropellants, because it adversely affects the process-ing parameters. The processing parameters, mechan-ical properties, and safety aspects of Cu(NTO)2 arecomparable to those of CC and AIO. Activity ofNTO, as well as its salts, is also evident during thecondensed-phase thermolysis of the modified propel-lants. From the TG analyses, the condensed-phaseactivity of the additives seems to be related to theiractivity during the propellant’s combustion. Therapid thermolysis of a propellant seems to be con-trolled by the thermolysis of the binder (HTPB).NTO and its salts also affect the rapid thermolysis ofAP.

Acknowledgment

The Head of the Department of Chemistry isthanked for laboratory facilities. We are grateful toISRO, Bangalore, DRDO, New Delhi, and CSIR,New Delhi, for financial assistance. Dr. K. N. Ninanand Dr. T .L. Varghese, VSSC, Thiruananthapuram,are thanked for propellant processing and data there-from. Help rendered by Prof. A. K. Chaterjee, Prof.Mohan Verma, and Mr. Manoranjan Pandey of BIT,Ranchi, in conducting some of the experiments, isalso gratefully acknowledged. Prof. G. N. Mathur,Director, and Dr. D. K. Setua, Jt. Director, DM-SRDE, Kanpur, are also thanked for thermal analysisdata.

References

[1] K. Kishore, M.R. Sunitha, AIAA J. 17 (1979) 1118.[2] C.H. Burnside, AIAA Paper No. 75-234 (1975).[3] A.T. Nielson, US Patent 3,878,233 (1975) p. 4.[4] M. Proebster, R. Strecker, I. Harrer, Eighteenth Int.

Jahrestag. Fraunhofer-Inst. Treib-explosivst., 1987, p.65/1.

[5] R. Sanden, Report No. FOA-C-20436-D1, 1982.[6] G. Singh, http://www.quicksitebuilder.cnet.com/sing-

hgurdip /drgurdipsingh/.[7] G. Singh, D.K. Pandey, Thirteenth Symposium (Na-

tional) on Thermal Analysis, Indian Thermal AnalysisSociety, Bhabha Atomic Research Centre, Mumbai,2002, p. 86.

[8] G. Singh, D.K. Pandey, J. Energ. Mater. 20 (2002)223.

[9] K.Y. Lee, L.B. Chapman, M.D. Coburn, J. Energ.Mater. 5 (1987) 27.

[10] G. Singh, I.P.S. Kapoor, S.K. Tiwari, S.P. Felix, J.Hazard. Mater. B 81 (2001) 67.

[11] G. Singh, I.P.S. Kapoor, S.M. Mannan, S.K. Tiwari, J.Hazard. Mater. A 68 (1999) 155.

[12] G. Singh, S.P. Felix, J. Hazard. Mater. A 90 (2002) 1.[13] L. Shangwen, W. Jiangming, F. Xiayun, Z. Jihua,

Energ. Mater. 1 (1993) 22.[14] G. Dalin, L. Shangwen, Y. Cuimei, Z. Shangjian, D.,

Yongzhan, Tuijin. Jishu. 20 (3) (1999) 91.[15] G. Singh, I.P.S. Kapoor, S.K. Tiwari, S.P. Felix, In-

dian J. Eng. Mater. Sci. 7 (2000) 167.[16] G. Singh, I.P.S. Kapoor, S.P. Felix, J.P. Aggrawal,

Propellants Explos. Pyrotech. 27 (2002) 16.[17] G. Singh, I.P.S. Kapoor, S.K. Tiwari, J. Kaur, O.P.

Singh, S.P. Felix, D.K. Pandey, Fifteenth Convention(National) of Aerospace Engineers, Institute of Engi-neers India, Birla Institute of Technology, Ranchi,2001, p. IV 3.1.

[18] G. Singh, I.P.S. Kapoor, S.K. Tiwari, S.P. Felix, T.L.Varghese, K.N. Ninan, J. Energ. Mater. (communicat-ed, 2001).

[19] T.B. Brill, T.L. Zhang, B.C. Tappan, Combust. Flame121 (2000) 662.

[20] G.K. Williams, S.F. Palpoli, T.B. Brill, Combust.Flame 98 (1994) 197.

[21] G. Singh, R.R. Singh, Res. Ind. 23 (1978) 92.[22] G. Singh, I.P.S. Kapoor, S.M. Mannan, J.P. Aggrawal,

Combust. Flame 97 (1994) 355.[23] G. Singh, I.P.S. Kapoor, Combust. Flame 92 (1993)

283.[24] N. Semenov, Chemical kinetics and chemical reac-

tions. Clarendone Press, Oxford, England, 1935, chap-ter 18.

[25] E.S. Freeman, S. Gordon, J. Phys. Chem. 60 (1956)867.

[26] J. Zinn, R.N. Rogers, J. Phys. Chem. 66 (1962) 2646.[27] H. Ostmark, H. Bergman, G. Acqvist, Thermochim.

Acta 213 (1993) 165.[28] K.V. Prabhakaran, S.R. Naidu, E.M. Kurien, Thermo-

chim. Acta 241 (1994) 199.[29] A.K. Galwey, P.J. Herley, M.A. Mohamed, Thermo-

chim. Acta 132 91) (1988) 205.


[30] G.E. Heckman III, H.C. Beachell, AIAA J. 6 (3)(1968)561.

[31] R.E. Waich Jr., R.F. Strauss, Fundamentals of rocketpropulsion-Reinhold space technology series. Rein-hold Publishing Corporation, New York, 1960, p. 30.

[32] K. Kishore, K. Sridhara, Solid propellant chemistry: con-densed phase behaviour of ammonium perchlorate basedsolid propellants. DESIDOC, New Delhi, 1999, p. 10.

[33] N.R.R. Kumar, in: Fundamentals of solid propellantcombustion, AIAA Inc., New York, 1984, p. 409.

[34] G.B. Manelis, V.A. Strunin, Combust. Flame 17(1971) 69.

[35] G. Singh, Progress report of the ISRO RESPONDproject No. 10/3/313, 2001.

[36] F.P. Bowden, A.D. Yoffe, Initiation and growth ofexplosion in liquids and solids, Cambridge UniversityPress, Cambridge, U.K., 1952, p. 19.

[37] J.P. Agrawal, S.M. Walley, J.E. Field, Combust.Flame 112 (1998) 62.

[38] H. Wise, S.H. Inami, L. McCally, Combust. Flame 11(6) (1967) 483.

[39] S. Ramamurhty, P.G. Shrotri, J. Energ. Mater. 14(1996) 97.

[40] S.R. Chakravarthy, E.W. Price, R.K. Sigman, J. Pro-pulsion Power 13 (4) (1997) 471.

[41] D. Tingfa, Thermochim. Acta 138 (1989) 189.[42] G. Singh, I.P.S. Kapoor, J. Energ. Mater. 11 (1989)

293.


studies of energetic compounds, part 29: effect of nto and its salts on the combustion and condensed...

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