a novel method for determination of the concentration of active antioxidant in stored htpb-based...

Summary

The aim of this work has been to ®nd a simple, non-destructivemethod for determination of the concentration of active antioxidant inHTPByAP composite propellants. Differential Scanning Calorimetry(DSC) was used for determination of Oxidation Induction Time (iso-thermal OIT) and Oxidation Induction Temperature (dynamic OIT) atdifferent antioxidant concentrations. The ammonium perchloratecontent in the propellants were 80% by weight. The antioxidant wasBKF with concentration up to 2.5% by weight of the polymer. Ironoxide was added to some of the samples, and to one of these samplesaluminium was also added.

The results show that the dynamic OIT depends on the activeantioxidant concentration, and useful calibration curves were obtainedfor the propellants with and without iron oxide. Concentrations ofactive BKF up to 1.5% by weight of polymer could be determinedusing this method. Aluminium content in the propellant had no effecton the results. The results also indicate that the isothermal OIT cannotbe used for determination of the active antioxidant concentration.

1. Introduction

The physical and mechanical properties of polymers

deteriorate with time. This aging of the polymers may cause

problems when old materials are used. In order to prevent

aging, antioxidants are added to polymeric materials.

Composite propellants, which are used in rocket engines,

are polymer-based. Even though normally the polymer

content of the propellant is not more than 10±20% by

weight, the physical and mechanical properties of the pro-

pellant depend largely on the polymer. The most used and

studied polymer in the ®eld of composite propellants is

hydroxy-terminated polybutadiene (HTPB), Figure 1. The

unsaturation of polybutadiene makes it sensitive to oxida-

tion reactions as the hydrogens in a-position to the double

bonds are highly reactive. The hydrogens bonded to tertiary

carbons are also highly reactive, Figure 2. Ammonium

perchlorate (AP) is dominating among oxidizers. A typical

HTPB-propellant also contains an antioxidant to prevent

aging of the polymer(1). BKF [2,20-methylene-bis-(4-

methyl-6-tert-butylphenol)], Figure 3, is an effective and

therefore widely used antioxidant for HTPB(2). BKF is a

hindered phenol which gives a stable free radical (less

reactive than peroxy radicals) when it donates the hydrogen

of one of its hydroxyl groups to a radical. This radical can

react with another peroxy radical and give a dienone, Figure

3. Iron oxide is a common catalytic additive for increasing

the burning rate. Composite propellants often also contain

aluminium, which increases the speci®c impulse(2).

Oxidation is sustained by atmospheric oxygen and is

diffusion controlled in thickwalled products. This explains

A Novel Method for Determination of the Concentration of ActiveAntioxidant in Stored HTPB-Based Composite Propellants

Mattias Lokander and Bengt Stenberg

Department of Polymer Technology, The Royal Institute of Technology, SE-100 44 Stockholm (Sweden)

Roland SandeÂn*

FOA, Defence Research Establishment, SE-147 25 Tumba (Sweden)

Eine neue Methode zur Bestimmung der Konzentration vonaktiven Antioxidantien in gelagerten HTPB-haltigen Komposit-treibstoffen

Ziel dieser Arbeit war, eine einfache nichtzerstoÈrende Methode zu®nden fuÈr die Bestimmung der Konzentration an aktiven Anti-oxidantien in HTPByAP-Treibstoffen. Die DSC-Methode wurdeangewendet zur Bestimmung der Oxidations-Induktionszeit (isothermeOIT) und der Oxidations-Induktionstemperatur (dynamische OIT) beiunterschiedlichen Antioxidans-Konzentrationen. Der Ammonium-perchlorat-Gehalt in den Treibstoffen lag bei 80 Gew.%. Antioxidanswar BKF mit Konzentrationen bis zu 2,5 Gew% des Polymers. Eise-noxid wurde zugefuÈgt zu einigen Proben und zu einer dieser Probenauch noch Aluminium. Die Ergebnisse zeigen, daû die dynamischeOIT von der aktiven Antioxidans-Konzentration abhaÈngt und daûbrauchbare Kalibrierungskurven erhalten wurden fuÈr Treibstoffe mitund ohne Eisenoxid. Konzentrationen des aktiven BKF bis zu 1,5Gew.% des Polymers konnten bestimmt werden mit dieser Methode.Der Aluminiumgehalt im Treibstoff hatte keinen Ein¯uû auf dieErgebnisse. Diese zeigen auch, daû die isotherme OIT nicht verwendetwerden kann zur Bestimmung der Konzentration des aktiven Anti-oxidans.

Une nouvelle meÂthode pour deÂterminer la concentration d'anti-oxydants actifs dans des propergols composites aÁ base de HTPBstockeÂs

L'objectif de cette eÂtude eÂtait de trouver une meÂthode simple nondestructive permettant de deÂterminer la concentration d'antioxydantsactifs dans des propergols HTPByAP. La meÂthode DSC a eÂteÂ utiliseÂeen vue de deÂterminer le temps d'induction d'oxydation (OIT iso-therme) et la tempeÂrature d'induction d'oxydation (OIT dynamique)pour diffeÂrentes concentrations d'antioxydant. La teneur en per-chlorate d'ammonium dans les propergols eÂtait de l'ordre de 80% enpoids. L'antioxydant eÂtait le BKF avec des concentrations allant jus-qu'aÁ 2,5% en poids du polymeÁre. De l'oxyde de fer a eÂteÂ ajouteÂ aÁcertains eÂchantillons ainsi que de l'aluminium dans l'un de ceseÂchantillons. Les reÂsultats montrent que l'OIT dynamique deÂpend dela concentration d'antioxydant actif et que des courbes de calibrageutiles ont eÂteÂ obtenues pour des propergols avec et sans oxyde de fer.Des concentrations de BKF actif allant jusqu'aÁ 1,5% en poids dupolymeÁre ont pu eÃtre deÂtermineÂes par cette meÂthode. La teneur enaluminium du propergol n'a pas eu d'in¯uence sur les reÂsultats. Cesderniers montrent eÂgalement que l'OIT isotherme ne peut eÃtre utiliseÂpour la deÂtermination de la concentration d'antioxydant actif.

*Correspondence author

# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998 0721-3115/98/0306±0155 $17.50�:50=0

272 Propellants, Explosives, Pyrotechnics 23, 272±278 (1998)

why an oxidation pro®le is observed in an aged propellant.

The propellant is more oxidized on the surface than in the

core. The higher the reaction rate, the closer the reaction

zone is to the surface. Higher temperature or the presence of

a catalytic additive therefore lead to a steeper oxidation

pro®le. When propellants are stored, the BKF is slowly

consumed and the protection against aging decreases. As

the structural differences between active and reacted BKF

are very small on the molecular level, it is dif®cult to

measure the amount of active BKF in the propellants.

The purpose of this investigation is to ®nd a simple non-

destructive method for determination of the amount of active

BKF in stored HTPB-propellant. With such a method, it may

be possible to see if the concentration of active BKF has

decreased before the properties of the propellant have dete-

riorated. Introductory studies with ordinary microcalorimetry

were not successful. After 14 days at 65�C no exotherm had

appeared even for samples with antioxidant concentrations as

low as 0.05% by weight of the polymer. Isothermal and

dynamic OIT, which can be determined by differential scan-

ning calorimetry, DSC, are known to be useful for determi-

nation of the active antioxidant concentration in pure uncured

HTPB(3). Therefore, these methods were tested on cured

HTPByAP composite propellants. It seems possible that a

metal ion like Fe3� also to some extent can catalyse the

reactions between oxygen and HTPB under the conditions

existing during the DSC measurements. For this reason, the

experiments were carried out on HTPByAP propellants both

with and without added iron oxide.

2. Methods and Materials

2.1 Isothermal and Dynamic OIT

Isothermal and dynamic OIT can be determined by DSC

measurements, and can be used to determine the con-

centration of active antioxidant in pure uncured HTPB(3). In

the isothermal case, a sample is kept at a high constant

temperature in oxygen atmosphere in the DSC apparatus.

The time until the exotherm appears is measured and

de®ned as the Oxidation Induction Time, isothermal OIT.

The isothermal OIT is related to the concentration of

effective antioxidant, and the more antioxidant, the longer

the isothermal OIT. A more ef®cient antioxidant also

gives(4) longer OIT:s. In the dynamic case, a sample is

heated at a constant rate of oxygen atmosphere. The tem-

perature at which the exotherm appears is de®ned as the

Oxidation Induction Temperature, dynamic OIT. Similarly

as in the isothermal case, the dynamic OIT is higher for a

sample with higher content of antioxidant, or a more ef®-

cient antioxidant(3). A problem in both cases is how to

Figure 1. Molecular composition of the used HTPB.

Figure 2. Reactive hydrogens in HTPB.

Figure 4. Ways of ®nding the isothermal or dynamic OIT.

Propellants, Explosives, Pyrotechnics 23, 272±278 (1998) Determination of the Concentration of Active Antioxidant 273

de®ne where the exotherm begins. One way is to use the

tangent at a certain threshold level. Another is to use the

tangent at the in¯exion point, Figure 4.

2.2 Sample Preparation

The investigated propellants were mixed by hand. A ®rst

mixture was made from approximately 20% HTPB with iso-

phorone diisocyante (IPDI) as curing agent, 20% ammonium

perchlorate with particle size 20 mm, and 60% ammonium

perchlorate with particle size 100 mm. This mixture was kept

in vacuum for a few minutes in order to get rid of possible

humidity before it was divided into smaller parts. Then the

antioxidant, BKF, was added with the concentrations 0, 0.05,

0.1, 0.25, 0.5, 1.0, 1.5, 2.0 and 2.5% by weight of the polymer.

All propellants were thoroughly stirred and placed in vacuum

again before curing.

Anothermixture, identical to the®rstone,wasalsoprepared.

To this mixture 1% of iron oxide, `̀ karbonyl-eisenoxyd rot''

from BASF (BET-surface 90 m2yg), was added before it was

divided into smaller parts. The BKF concentrations were the

same as in the propellants without iron oxide, but two samples

with 2.5% BKF were prepared. Finally, approximately 10%

aluminium powder, with particle size about 10 mm, was added

to one of the mixtures with 2.5% BKF. All of these samples

were also stirred thoroughly and kept in vacuum for a few

minutes, before and after adding the antioxidant.

All samples were cured at 70�C for one week. A small piece

of each sample was kept in the oven to age after curing. After

ten weeks, the aging temperature was raised to 90�C and after

another week the temperature was raised to 100�C.

2.3 Differential Scanning Calorimetry

The DSC instrument was a Mettler DSC 30. The samples

were kept in aluminium crucibles with lids pierced several

times. The temperatures used in the case of isothermal OIT

were 170, 180, 190 and 210�C. Tests were carried out on all

samples without iron oxide and on one with iron oxide. The

heating rates used in the case of dynamic OIT were 20, 10

and 2�Cymin. There is a crystal transformation of ammo-

nium perchlorate(5) at about 240�C, which results in an

endotherm on the thermogram. Therefore, it is no use in

going further in the temperature range. For the measure-

ments at heating rate 2�Cymin, the start temperature was

150�C for the samples without iron oxide, and 140�C for the

samples containing iron oxide. When the heating rate was

10 or 20�Cymin, the start temperature was 120�C. In order

to minimize possible in¯uence of the sample weight on the

results, the tangent for ®nding the isothermal and dynamic

OIT were drawn from the in¯exion point, Figure 4. In order

to obtain approximately the same amount of polymer in

each sample, the sample weights were 4±6 mg except for

the one with aluminium, which had sample weights of 7±

8 mg. In order to determine the in¯uence of the sample

weight on the results, measurements with sample weights 1

and 10 mg were also made with the heating rate 10�Cymin.

The oxygen ¯ow over the crucibles was approximately

5 cm3ymin.

The aged samples were tested the ®rst time after four

weeks, and then every second week. Dynamic OIT was

determined, and the heating rate was 10�Cymin. When the

temperature had been raised to 90�C, the samples were

tested after one week. Then the temperature was raised

again.

3. Results

3.1 Isothermal OIT

It was not possible to get useful results from the iso-

thermal OIT measurements. The lowest concentrations

required temperatures lower than 180�C, otherwise the peak

Figure 5. A typical exotherm peak for propellants without iron oxide, from the isothermal method at 190�C.

274 M. Lokander, B. Stenberg, and R. SandeÂn Propellants, Explosives, Pyrotechnics 23, 272±278 (1998)

appeared before the measuring temperature was reached.

The higher concentrations, on the other hand, required

temperatures above 190�C to give a useful exotherm peak.

At 190�C this peak was ¯at and therefore not distinct,

Figure 5. In order to provide a useful peak, the temperature

had to be 210�C. However, this peak was not very distinct

either.

3.2 Dynamic OIT

The exotherm peaks from the dynamic OIT measure-

ments, Figure 6, looked far better than those from the iso-

thermal measurements. However, when the heating rate was

10 or 20�Cymin, and the concentrations of BKF were also

high, the whole peak did not appear before the endotherm of

the ammonium perchlorate transformation appeared. This

caused some dif®culties in the determination of the in¯ex-

ion point. It is possible that the in¯exion point in some cases

did not even appear before the endotherm, and therefore the

dynamic OIT may be too low. When the heating rate was

2�Cymin, the whole peak appeared before 240�C, and a

calibration curve could be drawn, Figure 7. The calibration

curve for the measurements at heating rate 10�Cymin,

Figure 8, is similar to the one from the 2�Cymin measure-

ments. When the heating rate was 20�Cymin, the in¯exion

Figure 6. A typical exotherm peak for propellants without iron oxide, from the dynamic method, heating rate 10�Cymin

Figure 7. Calibration curve HTPByAP, heating rate 2�Cymin. Figure 8. Calibration curve HTPByAP, heating rate 10�Cymin.


point did not appear before the endotherm for high BKF

concentrations.

Useful calibration curves for the propellants which

included iron oxide were obtained at the heating rates 2 and

10�Cymin. These curves were similar to the ones for the

propellants without iron oxide but gave other dynamic

OIT:s for the same BKF concentrations. The dynamic OIT:s

did not differ between different BKF concentrations, Figure

9, as much as between the propellants without iron oxide,

Figure 8. The shape of the exotherm peaks for propellants

with iron oxide, Figure 10, was totally different from those

for the propellants without iron oxide, Figure 6. Aluminium

did not in¯uence the dynamic OIT or the shape of the peaks.

The deviation of the results was in all cases very small, in

most cases less than � 1�C.

The sample weight is not critical for the results. For all

propellants, approximately the same dynamic OIT was

obtained for the sample weight 1 mg as for the sample

weight 10 mg, see Table 1. The diffusion effect, for these

small samples, is negligible.

The dynamic OIT:s for the aged samples were approxi-

mately the same after ten weeks as before aging. Thus, the

amounts of active antioxidant had not changed. Some of the

measurements gave higher dynamic OIT:s after aging than

before aging, Figure 11. This is probably because the BKF

molecules move within the propellant. The highest BKF

concentrations were found in the surface of the propellant.

As the aged samples were too small, it was not possible to

measure the tensile strength of the propellants after aging or

to determine if the properties of the propellants had deteri-

orated. However, there were no signs indicating deteriora-

tion before the temperature was raised to 100�C. After two

weeks at 100�C, the propellants with low BKF concentra-

tions had partly been visibly destroyed, and the dynamic

OIT had decreased, Figure 11. This can also be seen on the

exotherm peaks, Figure 12, which have become much

smaller. The heights of the exotherm peaks of the pro-

Figure 9. Calibration curve HTPByAPyFe2O3, heating rate10�Cymin.

Figure 10. A typical exotherm peak for propellants containing iron oxide, from the dynamic method, heating rate 10�Cymin. At temperatureshigher than 200�C exotherms from other reactions can be seen.

Table 1. Weight Influence on OIT on HTPByAP Propellants

Sample 0.25% BKF 1.5% BKF 0.25% BKF 2.0% BKF(with Fe2O3) (with Fe2O3)

dynamic OIT 185.3 197.0 184.3 218.9(�C), 1 mgdynamic OIT 186.8 197.8 186.4 218.6(�C), 10 mg


pellants containing iron oxide have decreased much more

than the heights of those without iron oxide. All the peaks

of the propellants without iron oxide, with BKF con-

centrations less than 1.5% before aging, have decreased in

height.

4. Discussion

The method using isothermal OIT was considered

unsuitable for this application because of the difference in

temperatures required for different BKF concentrations, the

inaccuracy of the results at high BKF concentrations, and

the long measuring time.

The calibration curves obtained using the dynamic OIT at

the heating rates 2 and 10�Cymin, Figures 7±8, are useful

for determination of the amount of active antioxidant in

HTPB composite propellants with and without iron oxide or

aluminium. Since the differences in the precision of the two

curves are quite small, the heating rate 10�Cymin might be

preferable because it is so much faster than 2�Cymin. If a

Mettler DSC 30 apparatus with liquid nitrogen as cooling

agent is used, the total time for analysis of one specimen is

less than 20 minutes when the heating rate is 10�Cymin, and

about one hour when the heating rate is 2�Cymin.

A calibration curve could be established for the heating

rate 20�Cymin, but it is not as accurate as the ones for the

heating rates 10 and 2�Cymin. With high heating rates, most

of the time needed for analysis is the time for cooling the

apparatus after each measurement. This means that the time

for analysis, at heating rate 20�Cymin, is not much shorter

than at heating rate 10�Cymin. Since the heating rate

10�Cymin gives more accurate results, this heating rate may

be preferable to use.

BKF is a useful antioxidant in the sense that it is quite

stable and effective at temperatures up to 70�C. At higher

temperatures, however, both the stability and effectivity

decrease. During storage the BKF molecules move within

the propellant so that the concentration of BKF is not uni-

form throughout the propellant. Iron oxide seems to catalyse

the aging reactions at high temperatures.

5. Conclusions

The results are as follows:

� It is possible to use dynamic OIT for determination of the

concentration of active antioxidant in HTPByAP pro-

pellants which have no other additives than iron oxide,

aluminium, and the antioxidant BKF. With this method,

concentrations of active BKF up to about 1.5% can be

determined.

� Isothermal OIT method is not useful in this application.

� Iron oxide content in the propellant in¯uences the results.

This means that different calibration curves must be used

depending on whether the propellant contains iron oxide

or not.

� Aluminium content in the propellant does not in¯uence

the results.

� The sample weight can vary between 1 and 10

mg without in¯uencing the results.

Figure 12. Thermogram for propellant with iron oxide and 0.5% BKF after aging, from the dynamic method, heating rate 10�Cymin. Theoxidation peak at about 200�C is partly hidden by the heat from other reactions (for comparison see Figure 10).

Figure 11. Dynamic OIT after storage at high temperatures.


� BKF is a useful antioxidant at temperatures up to 70�C.

Up to this temperature, the concentration remains stable

and aging of the propellant is prevented. At higher tem-

peratures, the BKF concentration decreases faster and the

polymer is decomposed quicker.

6. References

(1) A. Bailey and S. G. Murray, `̀ Explosives, Propellants &Pyrotechnics,'', Brassey's, UK, 1989, pp 109±114.

(2) J. S. Chhabra, J. Athar, J. P. Agrawal, and H. Singh, ComparativeStudy of Various Antioxidants for HTPB Prepolymer', Plastics,Rubber and Composites Processing and Applications, 20, 305±310(1993).

(3) H. Berg, B. Stenberg and R. SandeÂn, `̀ Study of Stabilization ofHydroxy-Terminated Polybutadiene by Differential ScanningCalorimetry and Viscosity Measurements'', Plastics and RubberProcessing and Applications 12, 235±239 (1989).

(4) H. E. Bair, `̀ Thermal Analysis of Additives in Polymers'' in:`̀ Thermal Characterization of Polymeric Materials'', E. A. Turi(ed.) Academic Press, New York, 1981, pp. 845±909.

(5) L. L. Bircumshaw and B. H. Newman, `̀ The Thermal Decom-position of Ammonium Perchlorate'', Proc. Roy. Soc. 227, 115±132, 228±241 (1955).

AcknowledgementThe authors gratefully acknowledge the Defence Material Admin-

istration, FMV, Sweden, for the ®nancial support.

(Received August 25, 1997; Ms 44y97)


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