Thermal Decomposition Kinetics of PBAN-Binder andComposite Solid Rocket Propellants

THORSTEN SELL, SERGEY VYAZOVKIN, and CHARLES A. WIGHT*Center for Thermal Analysis, Department of Chemistry, University of Utah, Salt Lake City, Utah 84112

Thermogravimetric analysis at heating rates between 0.5 and 10°C min21 has been used to study thedecomposition kinetics of the PBAN binder and three propellants based on ammonium perchlorate (AP) withPBAN, HTPB, or BAMO-AMMO binders. Thermal runaway was observed for the PBAN and HTPBpropellants at heating rates faster than 4.5 and 3°C min21, respectively. The multistep decomposition kineticswere analyzed by an advanced isoconversional method that showed that the effective activation varies with theextent of decomposition of the studied systems. For PBAN the activation energy increases from 100 to 200 kJmol21 throughout the polymer decomposition. In propellants the major mass loss step primarily relates to APdecomposition accompanied by some degradation of the polymer binder. For the PBAN and HTPB propellantsthis step shows respective increases in the activation energy from 60 to 180 kJ mol21 and from 100 to 230 kJmol21. The major decomposition step of the BAMO-AMMO propellant has a fairly constant activation energyof 120 kJ mol21. © 1999 by The Combustion Institute

LIST OF ABBREVIATIONS

Al aluminum powder (particlesize)

AP ammonium perchlorate(particle size)

BAMO-AMMO Bis-azidomethyloxetane/azidomethyl-methylazido-copolymer

Desmodur N-100 crosslinking agent, hexane1-6 diisocyanatehomopolymer(C8H12N2O2)x

DOA plasticizer, dioctyladipateor bis (2 ethylhexyl)adipate

GAP glycidyl azide polymerHTPB hydroxy terminated

polybutadienePBAN polybutadiene-acrylic acid-

acrylo nitrile terpolymer

INTRODUCTION

The thermal decomposition of solid rocket pro-pellants is a complex process that may involvevarious chemical (solid- and gas-phase reactionsas well as reactions of gaseous products with thesolid) and physical (phase transitions, diffusion,adsorption, desorption) phenomena. In a rocket

motor, extreme conditions are predominantleading to temperatures of 2000–3500 K, pres-sures of 5–10 MPa, and estimated heating ratesas high as 106 K s21. To simulate these condi-tions, pyrolysis studies of solid rocket propel-lants and polymeric binders should be con-ducted at heating rates greater than 100°C s21

[1–3]. Unfortunately the extracted kinetic infor-mation is limited because of the high uncer-tainty in measuring the temperature and reac-tion rate. Slow heating rate experiments allowone to get a better insight into the reactionkinetics and mechanisms. However, the rele-vance of these data to the fast heating rate andhigh temperature conditions of combustion isnot obvious [4]. The slow heating rate data mayhappen to be irrelevant if the reaction mecha-nism changes with temperature. However, thedifference in the temperature regions is not thesufficient condition of the change in the reac-tion mechanism. For instance, Vyazovkin andWight [5] studied thermal decomposition ofammonium dinitramide at heating rates of 1022

to 1021 °C s21 by using regular thermal analysistechniques and at a heating rate of 107 °C s21 byusing laser pyrolysis technique. Both techniqueshave shown formation of the same gaseousproducts that suggests that the decompositionmechanism does not change in spite of thedramatic change in the temperature. In such asituation, the reaction kinetics and mechanismsderived from the slow heating rate experiments

*Corresponding author. Email: wight@chemistry.chem.utah.edu

COMBUSTION AND FLAME 119:174–181 (1999)0010-2180/99/$–see front matter © 1999 by The Combustion InstitutePII S0010-2180(99)00036-X Published by Elsevier Science Inc.

can be used to predict decompositions underconditions of combustion [6]. However, reliablepredictions cannot be made without extractingreliable kinetic information from slow heatingrate data. This study focuses on obtaining reli-able kinetic information on the thermal decom-position PBAN and three different solid propel-lants (Table 1).

Experimental Section

Samples of PBAN, PBAN-AP, HTPB-DOA-AP, and BAMO-AMMO propellants werekindly supplied by the Thiokol Corp. The chem-ical composition of the propellants is shown inTable 1. The thermogravimetric analysis (TGA)experiments were carried out using a Rheomet-rics 1000M1 thermobalance. To reduce ther-mal gradients and self-heating the experimentswere performed on small samples (PBAN ;2.9mg, PBAN-AP ;0.8 mg, HTPB-DOA-AP ;0.9mg, BAMO-AMMO ;1.0 mg). Samples of thepropellants were placed in open aluminum pansand heated in a flowing atmosphere of nitrogen(100 ml min21). The instrument was pro-grammed for heating at constant rates fromroom temperature to 590°C.

Kinetic Analysis of TGA Data

The kinetics of heterogeneous decompositionsof solids are customarily described by the basickinetic equation

da

dt5 k~T! f~a! (1)

where a represents the extent of reaction (0 #a # 1), t is time, k(T) is the rate constant, and

f(a) is the reaction model, which describes thedependence of the reaction rate on the extent ofreaction. The value of a is experimentally de-rived from the global mass loss in TGA exper-iments. In most cases the temperature depen-dence of k(T) can be satisfactorily described bythe Arrhenius equation, whose substitution intoEq. 1 yields

da

dt5 A exp S2E

RTD f~a! (2)

where E is the activation energy and A is thepreexponential factor.

To evaluate E and A in Eq. 2, one has toseparate the temperature and conversion de-pendencies of the reaction rate. Here, a com-monplace approach is force fitting of experi-mental data to assumed reaction models. Whenapplied to decomposition data obtained undernonisothermal conditions, the model-fittingmethod gives highly uncertain values of theArrhenius parameters [7]. Another major flawof this approach is that usually the model-fittingmethods produce a single pair of Arrheniusparameters for the whole process. However, thethermal decompositions of solid materials areknown to involve multiple steps that are likely tohave different activation energies. Then, thecontributions of these steps into the overalldecomposition rate measured by TGA shouldvary with both T and a. This means that theeffective activation energy determined fromTGA experiments will also be a function ofthese two variables. The model-fitting methodsare simply incapable of accounting for thisdegree of complexity.

The above-mentioned problems can be avoidedby using model-free isoconversional methods

TABLE 1

List of Ingredients Used in the Composite Propellants Tested

Ingredient Weight % Ingredient Weight %

BAMO-AMMO: PBAN-AP:BAMO-AMMO polymer 7.58 PBAN 29.76GAP plasticizer 11.99 AP (2 mm) 70.24AP (200 mm) 49.60AP (20 mm) 12.40 HTPB-DOA-AP:Al (5 mm) 18.00 HTPB (R45M) 23.94Desmodur N-100 0.42 DOA 5.81Triphenyl tin chloride 0.01 AP (2 mm) 70.25

175DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS

[8–10]. These methods allow the activation en-ergy to be evaluated without making any as-sumptions about the reaction model. Addition-ally, the isoconversional methods evaluate theeffective activation energy as a function of theextent of conversion, which allows one to ex-plore multistep kinetics [11]. Vyazovkin devel-oped an advanced isoconversional method [12],which is based on the assumption that thereaction model is independent of the heatingprogram, T(t). According to this method, for aset of n experiments carried out at differentheating programs, the activation energy is de-termined at any particular value of a by findingthe value of Ea that minimizes the function

F~Ea! 5 Oi51

n OjÞi

n J@Ea, Ti ~ta!#

J@Ea, Tj ~ta!#(3)

Henceforth, the subscript a denotes the valuesrelated to a given extent of conversion. In Eq. 3,the integral

J@Ea, Ti ~ta!# ; E0

ta

exp F 2Ea

RTi~t!Gdt (4)

is evaluated numerically for a set of experimen-tally recorded heating program, Ti(t). The min-imization procedure is repeated for each valueof a to find the dependence of the activationenergy on the extent of conversion. An advan-tage of the advanced isoconversional method isthat it can be applied to study the kinetics underarbitrary temperature programs (e.g., under alinear heating program distorted by self-heat-ing) [12]. To our knowledge, this is the firstwork to apply such a method to thermal decom-position of composite rocket propellants.

RESULTS

PBAN Polymer Binder

Thermal decomposition experiments on theneat PBAN binder were performed at constantheating rates of 0.9, 1.8, 2.7, 4.6, 7.3, and 9.1 °Cmin21. The representative TGA curves areshown in Fig. 1. The PBAN polymer showsthree major steps of mass loss. The steps arestrongly overlapped. The first step covers a

temperature region from ;110 to ;220 °C andinvolves a mass loss of ;8%. The second stepdemonstrates a mass loss of 15% in the temper-ature region of ;220–380 °C. The thermaldecomposition noticeably accelerates in thethird step, in which the sample loses ;75% ofmass in a temperature region of ;380–500 °C.After decomposition we found a small amountof char that accounted for 1% of the initialmass. There is also a minor step of the mass loss(;2%) with the maximum rate at ; 100°C. Thisstep is hardly noticeable at faster heating rates.

Figure 2 compares the Ea-dependenceagainst the experimentally observed mass losscurve. The three decomposition steps are asso-ciated with variations in the Ea-dependence.

Fig. 1. TGA curves for decomposition of PBAN.

Fig. 2. TGA curve for decomposition of PBAN at heatingrate of 0.9°C min21 (upper trace) compared to the depen-dence of the activation energy on the extent of decomposi-tion conversion (lower trace). Open symbols representliterature data [13].

176 T. SELL ET AL.

The first step (a , 0.08) is characterized by asmall activation energy of 50–70 kJ mol21. Thesecond step (a 5 0.08–0.25) is characterized byan increase in Ea from ;100 to 260 6 40 kJmol21. In the third step (a 5 0.25–0.99) theactivation energy rapidly decreases from 260 640 kJ mol21 to a practically constant value of200 6 30 kJ mol21.

PBAN-AP Composite Propellant

The composite PBAN-AP propellant was stud-ied at constant heating rates of 0.5, 0.9, 2.3, 3.2,4.1, 4.5, 7.3, 8.9, and 9.0 °C min21. The TGAexperiments showed (Fig. 3) thermal runaway atthe heating rate of 4.5°C min21 and faster. Atslower heating rates the TGA curves show twomajor steps of mass loss. As in the case of neatPBAN, we can also see a minor mass loss(;2%) with maximum rate at ; 100°C. Itprecedes the first major step that shows a fastmass loss of around 70% in the temperatureregion of 100–350°C. The second step occursabove 350°C and demonstrates a slow mass lossof ;15%. The slower heating rate (,4.5°Cmin21) decompositions resulted in formation ofa char residue that accounted for ;15% of theinitial mass.

The Ea-dependence was evaluated for theslow heating rate decompositions, which do notundergo thermal runaway. The results are pre-sented in Fig. 4. The major decomposition step(a , 0.8) shows a rise in the activation energyfrom ;20 to 180 kJ mol21. The slow second step(a . 0.8) starts with an abrupt drop in the

activation energy. The average activation energyfor this step is around 40 kJ mol21.

HTPB-DOA-AP Composite Propellant

The thermal decomposition of HTPB-DOA-APcomposite propellant was examined at constantheating rates of 0.5, 0.9, 1.5, 1.8, 2.3, 2.5, 2.7, 3.2,3.6, 4.5, 6.8, and 9.0 °C min21. Thermal runawaywas observed at the heating rates faster than 3°C min21 (Fig. 5). At slower heating rates, theTGA curves show 3 major steps of the mass loss.During the first step the propellant loses 8% ofits mass in the temperature range of 100–230°C.The second step demonstrates a fast mass lossof ;70% at temperatures between 230 and 380

Fig. 3. TGA curves for decomposition of PBAN-AP com-posite propellant. Fig. 4. TGA curve for decomposition of PBAN-AP at

heating rate of 0.9°C min21 (upper trace) compared to thedependence of the activation energy on extent of decompo-sition (lower trace).

Fig. 5. TGA curves for decomposition of HTPB-DOA-APcomposite propellant.

177DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS

°C. The third step (T . 380°C) shows a veryslow decomposition accompanied by 5% of lossin mass. When the temperature reached thefinal value of 590°C, the samples had less than10% of the initial mass remaining.

The Ea-dependence was determined for de-compositions conducted at heating rates slowerthan 3°C min21. The results are shown in Fig. 6.The first step of decomposition (a , 0.08)exhibits an activation energy of about 100 kJmol21. The activation energy suddenly drops atthe beginning of the second step (0.08 , a ,0.9), which shows an increase in Ea from ;100to 230 kJ mol21. The third step exhibits anabrupt change of the activation energy to verylow values that could not be reliably evaluated.

BAMO-AMMO Composite Propellant

The BAMO-AMMO composite propellant wasdecomposed at constant heating rates of 0.9,2.3, 4.5, 7.2, and 9.0 °C min21. The results arepresented in Fig. 7. The TGA curves show threemajor steps of the mass loss. As in previouscases, the first step is preceded by a minor massloss with the maximum rate around 100°C.During the first step the propellant loses ;15%of its mass in the temperature range 150–250°C. In the second step (T 5 250–320 °C), thepropellant loses more than 50% of its mass. Thethird step (T . 320°C) is a slow decomposition

accompanied by a small mass loss of 4%. By590°C about 4% of the propellant mass re-mained.

The thermal decomposition of this propellantis characterized by the Ea-dependence shown inFig. 8. The first stage of decomposition showsan increase of the activation energy from 100 to160 kJ mol21 at a 5 0.2. At the beginning of thesecond step the activation energy suddenlydrops to 80 kJ mol21. The activation energy forthe second step (a 5 0.2–0.9) is practicallyconstant (120 6 20 kJ mol21) at a . 0.3. Thetransition to the third step of decomposition ischaracterized by some increase in activationenergy that is followed by the rapid drop at a 5

Fig. 6. TGA curve for decomposition of HTPB-DOA-AP-propellant at heating rate of 0.9°C min21 (upper trace)compared to the dependence of the activation energy onextent of decomposition (lower trace).

Fig. 7. TGA curves for decomposition of BAMO-AMMOcomposite propellant.

Fig. 8. TGA curve for decomposition of BAMO-AMMOcomposite propellant at heating rate of 7.2°C min21 (uppertrace) compared to the dependence of activation energy onthe extent of decomposition (lower trace).

178 T. SELL ET AL.

0.95. The third step has an activation energy of;20 kJ mol21.

DISCUSSION

The neat PBAN as well as PBAN-AP andBAMO-AMMO propellants showed a minorinitial mass loss (;2%). This minor step isabsent in the thermal decomposition of HTPB-DOA-AP propellant. The values of the activa-tion energy related to that step are too small(20–50 kJ mol21) for a chemical reaction (bondbreaking). Based on this fact and on the prox-imity of the maximum rate of mass loss to100°C, we assume that this initial step is associ-ated with vaporization moisture from the abovesystems.

As was noticed, the evaluated Ea-dependen-cies sometimes show abrupt drops in the activa-tion energy. We feel that generally they repre-sent a computational artifact. Note that thesedrops occur at the point of transition from onemass loss step to another. The reaction rate atthis point markedly decreases and does not varymuch with temperature. The low temperaturesensitivity of the reaction rate manifests itself ina small value of the activation energy.

PBAN Polymer Binder

The first mass loss step (a , 0.08) shows theactivation energy (;50–60 kJ mol21) that isuncharacteristically small for the thermal de-compositions. This step may result from vapor-ization of a residual organic solvent or someother species of low molecular mass.

The second decomposition step (a 5 0.08–0.25) demonstrates an increasing Ea-depen-dence. It suggests that this step is likely toinvolve two decomposition pathways [11]. Withincreasing the temperature, the pathway havinga smaller activation energy (;100 kJ mol21) istaken over by the pathway that has a greateractivation energy (;200 kJ mol21). Given thefact that the third decomposition step (a 50.25–0.99) is strongly overlapped with the sec-ond one and that the activation energy for thethird step is 200 6 30 kJ mol21, it is reasonableto assume that the third and second steps mayinvolve a common reaction pathway. Therefore

the overall kinetics of PBAN decompositionappears to be determined by two overlappingprocesses with the respective activation energiesof ;100 and 200 kJ mol21.

Rao and Radhakrishnan [13] studied thethermal decomposition of cured PBAN by usingTGA at heating rates between 2 and 50 °Cmin21. The use of the Ozawa method [9] re-sulted in a Ea-dependence (Fig. 2) that has asimilar shape as that obtained by us. The some-what higher values of the activation energy maybe explained by the effect of curing that is likelyto result in formation of a more stable polymernetwork. However, it may also be due to thelimitations of Ozawa’s method, which uses anoversimplified approximation for the tempera-ture integral (4) that holds only for ideally linearheating program and cannot account for appre-ciable self-heating.

Cohen et al. [3] used Xe lamp radiation tosurface pyrolyze PBAN. By plotting the loga-rithm of the mass loss flux against the reciprocalsurface temperature, they found the activationenergy of 70 kJ mol21. The validity of this valueis obviously limited by an arbitrary assumptionof zero-order kinetics. The value is too small torepresent a chemically controlled decomposi-tion. Besides, Cohen et al. [3] did not report theextent of pyrolysis reached in their experiments.This is a crucial factor because according to thepresent results the activation energy varies withthe extent of PBAN degradation. Overall, wecannot meaningfully compare our results withthose obtained by Cohen et al. [3]. This alsoapplies to the data of Rao and Radhakrishnan[14] who measured the total amount of C2hydrocarbons released from PBAN, which waspyrolyzed at different temperatures. Under as-sumption of first-order kinetics, they obtainedthe activation energy of 52 kJ mol21.

PBAN-AP Composite Propellant

The thermal decomposition of the PBAN-APpropellant does not show the mass loss between110 to 220 °C that we observed for the neatPBAN binder. The major mass loss step starts at;200°C and accounts for about 80% of the massloss. It should be kept in mind that thePBAN-AP propellant is mostly (70%) com-posed of AP. When heated at 5°C min21, neat

179DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS

AP completely decomposes in the temperatureregion of ;240–350°C [7]. At this heating rate,neat PBAN decomposes at the temperaturesbetween 200 and 480 °C. However, by 350°Cneat PBAN loses only about 10% of its mass(Fig. 1). We may therefore expect that the firststep of the propellant decomposition representsthe decomposition of all AP and about one-third of PBAN.

The Ea-dependence determined for the firstand major step of the propellant decomposition(Fig. 4) is suggestive of a complex reactionmechanism. Note that this dependence showsan increase in the activation energy that issimilar to that observed for the PBAN decom-position at a , 0.25 (Fig. 2). However, themaximum value of the activation energy for thepropellant decomposition is lower (180 kJmol21) than that for the decomposition of neatPBAN (260 kJ mol21). On the other hand, themaximum activation energy for the propellantdecomposition is markedly higher than that fordecomposition of neat AP. According to Vya-zovkin and Wight [7] the thermal decomposi-tion of AP is characterized by a complex Ea-dependence for which the maximum value ofthe activation energy does not exceed 120 kJmol21. Therefore, the maximum activation en-ergy of the PBAN propellant decomposition islikely to characterize the reaction betweenPBAN and decomposition products of AP. Tak-ing into account that the PBAN propellantbegins to decompose at the temperature that is;40°C lower than the decomposition tempera-ture of neat AP, we should ascribe the initialsmaller activation energy (;80 kJ mol21 at200°C) to the thermal decomposition of PBAN.Therefore, the overall kinetics of the first andmajor decomposition step can be reduced totwo parallel pathways, which are decompositionof individual PBAN and reaction of PBAN withdecomposition products of AP. The slow rateand low activation energy for the second decom-position step (a . 0.8) suggest that furtherdecomposition may be limited by diffusion [11].

HTPB-DOA-AP Composite Propellant

The first decomposition step (a , 0.05) has themaximum rate at 150°C. The correspondingactivation energy is about 100 kJ mol21. Note

that the mass loss is practically equal to thecontent of the DOA plasticizer in the propel-lant. It does not seem to be unreasonable toassume that the first step relates to the vapor-ization or decomposition of the plasticizer. Wecannot, however, rule out that the HTPB mayalso contribute to this first step. Unfortunately,pure HTPB was not available for this study toclarify this situation.

The temperature region of the second andmajor mass loss step (0.1 , a , 0.9) practicallycoincides with the temperature region of APdecomposition [7]. During this step the propel-lant loses ;80% of its mass. Based on thesefacts we may conclude that this step involves thethermal decomposition of all AP as well as of;10% of HTPB. The activation energy for thisstep increases from ;100 to ;280 kJ mol21.Because the average activation energy for de-composition of the propellant is significantlygreater than that for decomposition of pure AP[7], we may expect that the overall kinetics ofthis step is determined by decomposition ofHTPB or/and by its reaction with the productsof AP decomposition. The low rate and smallactivation energy (we were unable to reliablydetermine its value) of the third step suggestthat the decomposition of the residual polymeris likely to be diffusion-controlled.

Ninan and Krishnan [15] used TGA to studythe kinetics of thermal decomposition of HTPBbinder at heating rates of 1–100 °C min21. Toevaluate Arrhenius parameters, they fitted datato the reaction-order model. The reported val-ues [15] of the activation energy correspond to areaction of zero order and randomly vary from80 to 150 kJ mol21 depending on the heatingrate as well as on the computational methodused. These values are noticeably smaller thanthose found by us. However, the reliability ofthe reported values [15] is rather questionablebecause of the flawed nature of the model-fitting methods [7].

Cohen et al. [3] studied the kinetics of thesurface pyrolysis of HTPB. Assuming zero-or-der kinetics, they found the activation energy of71 kJ mol21. Rao and Radhakrishnan [14] mea-sured the total amount of C2 hydrocarbons thatwas released from pyrolyzed HTPB at differenttemperatures. By assuming first-order kineticsfor this process, they obtained the activation

180 T. SELL ET AL.

energy of 53 kJ mol21. We have already ques-tioned the validity of such values in our discus-sion of PBAN data.

BAMO-AMMO Composite Propellant

Interpretation of the overall kinetic data for thispropellant is extremely difficult. The propellantcontains seven components, including two poly-mers (BAMO-AMMO and GAP), two sizes ofAP, aluminum, and curing agents. The overallkinetics for this propellant may be determinedby decomposition of individual polymers andAP, as well as by the reaction of the polymerswith decomposition products of AP. Addition-ally, the overall decomposition kinetics may becomplicated by curing of the polymers. Al-though the nature of the mass loss step isunclear, the second step occurs in the sametemperature region as the temperature decom-position of neat AP [7]. This step represents themajor mass loss of ;70%. Since this amountexceeds the overall amount of AP in the pro-pellant, we may conclude that the thermal de-composition of AP is accompanied by decom-position of the polymers. The activation energyfor the second step remains fairly constant at120 6 20 kJ mol21 (Fig. 8). The constancy of theeffective activation energy suggests that theoverall kinetics of this step is limited by a singlereaction. For the thermal decomposition of neatAP the activation energy varies within the limitsof 80–120 kJ mol21 [7]. Therefore, the Ea-dependence (Fig. 8) observed for the propellantdecomposition is likely to represent the reactionof AP decomposition products with the polymerbinder. The third decomposition step (a . 0.95)has a low activation energy and reaction ratethat are characteristic of diffusion control.

CONCLUSIONS

As a result of this study we found that thethermal decompositions of the PBAN binderand PBAN, HTPB, and BAMO-AMMO pro-pellants follow multistep kinetics. The use of theadvanced isoconversional method allows one to

observe variation of the effective activationenergy with the extent of decomposition. Themajor mass loss step for decomposition of pro-pellants is the thermal decomposition of APaccompanied by partial degradation of the poly-mer binder. Comparison of the activation ener-gies for this process with the activation energiesfor decomposition of individual AP or/andbinder suggests that the overall kinetics of themajor mass loss is determined by the reactionbetween binder and decomposition products ofAP.

This research is supported by the Ballistic Mis-sile Defense Organization and the Office of NavalResearch under MURI Contract No. N00014-95-1-1339. T.S. has been supported by a scholarshipfrom Deutsche Akademischer Austauschdienst(DAAD).

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Received 5 August 1998; revised 25 February 1999; accepted10 March 1999

181DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS