chemiluminescence study on thermal decomposition of nitrate esters (petn and nc)

Propellants, Explosives, Pyrotechnics 14, 89-92 (1989) 89

Chemiluminescence Study on Thermal Decomposition of Nitrate Esters (PETN and NC)

J. K' imura

Third Research Center, Technical Research and Development Institute, Japan Defense Agency, Tachikawa, Tokyo 190 (Japan)

Untersuchung der Chemilumineszenz bei der thermischen Zersetzung von Nitratestern (PETN und NC)

Chemilumineszen(CL)-Messungen wurden beim Erhitzen von Nitratestern (PETN und NC) im Temperaturbereich zwischen 40 "C und 90 "C in inerter Atmosphare durchgefiihrt. Ein schwaches Licht wurde in gleichmal3iger Folge von der kondensierten Phase ausge- strahlt. Dieser neue Befund deutet darauf hin, daB die thermische Zersetzung der Nitratester durch einige Oxidationsreaktionen beglei- tet wird. Die Spektraluntersuchung der CL zeigte, dal3 die zuerst angeregten Tripletts carbonylhaltiger Verbindungen und der Singu- lett-Sauerstoff dic lichtaussendenden Teilchen sind. Die Lumineszenz- erscheinung bei niederer Temperatur, die bei PETN und NC beobach- tet wurde, wird durch das Arrheniusgesetz beschrieben und hat cine Aktivierungsenergie von 63 kJ/mol (= 15 kcalimol). Die kinetische Analyse der CL hat ergeben, daB die Bildung von Peroxidradikalen, die durch Oxidation der durch Spaltung der Nitratesterbindung (RO-NO2) entstandenen Primarprodukte gebildet werden konnen, eine fortlaufende Folge von Radikalreaktionen beherrscht.

Etude de la chemoluminescence accompagnant la decomposition thermique des esters nitriques (pentrite et nitrucellulose)

On a mesure les emissions par chemoluminescence produites par I'echauffement entre 40°C ct 90°C d'esters nitriques (pentrite et nitro- ccllulosc) sous atmosph2rc inerte. Une lueur faible est tmise de manikre continue par la phase solide. Cette constatation nouvelle indi- que que la decomposition thermique des esters nitriques est accompa- gnee de quelques reactions d'oxydation. L'analyse spectrale de la lucur &mise montre qu'elle vicnt en premier lieu dc I'excitation de la raie triple des composes contenant Ic groupe carbonyle et de la raie simple de I'oxygkne. Le phknomhe de luminescence a basse temperature, observk pour la pentrite et la nitrocellulose. est decrit par la loi d'Arrhenius ct presente une knergie d'activation de 63 kJ/mol (soit 15 kcalimol). L'analyse de la cinetique de la chemoluminescence montre que la suite continue de reactions entre radicaux est dominee par la formation de radicaux de peroxyde, formes par oxydation des produits primaires resultant de I'ouverture de la liaison (RO-NO2) dans lcs esters nitriques.

Summary

Measurements of chemiluminescence (CL) during heating of nitrate esters (PETN and NC) have been conducted in the temperature range between 40 "C and 90 "C in an inert atmosphere. Faint light was emitted from the condensed-phase in steady-state fashion. This new find- ing implies that the thermal decomposition of nitrate esters is accompanied by some oxidation reactions. Spectral analysis of the CL showed that the light-emitting species will be the first excited triplet of carbonyl-containing products and singlet oxygen. The low-temperature CL phenomena observed for PETN and NC are represented by Arrhenius law, providing the activation energy of 63 kJimol (= 15 kcalimol). Kinetic analysis of the CL has led to the result that the formation of peroxy radicals, which can be produced by oxidation of the primary products of nitrate ester bond (RO-NO,) cleavage, predominates a consecutive series of radical reactions of the CI..

1. Introduction

Nitrate esters have kept their unique positon as a smokeless ingredient in the field of gun and rocket propellants. Accurate aging-out prediction and anti-aging study of smokeless propellants have been our major concern in this laboratory. A better understanding of the decomposition kinetics and the reaction mechanism will be essential for attaining ultimate goal. Although fundamental understanding of the thermal decomposition of nitrate esters has been achieved, their reaction mechanisms are not yet well understood: a number of proposed mechanisms involving beyond the first step or two is little more than speculation as pointed out in a recent review article('). A number of studies on the thermal decomposition of nitrate esters has verified that the decomposition proceeds with homolytic cleavage of nitrate ester groups (RO-N02) and an autocatalytic-type reaction. There was, however, no general agreement between observed kinetic parameters and theoretically predicted ones. Thus, it has long been an open question that reported activation energies ranging from 41 to

47 [kcalimol] differ from an expected value of about 35 kcall mol which is the bond dissociation energy of nitrate ester.

The author has answered the question in the previous paper(2); we obtained the activation energy for the homolysis to be 35-37 [kcalimol] by exploiting the Taliani test (a man- ometric method) and the Abel test (a KI-starch paper test). The aim of this paper is to elucidate the reaction mechanism of so-called autocatalytic reaction. The autocatalytic-type reaction is believed to proceed by a complicated series of consecutive radical reactions which are not well-studied due to the lack of adequate diagnostic technique. Any new phenomena which may aid in the study of this process are therefore of consider- able interest and warrant investigation.

The homolysis of nitrate esters produces alkoxy radicals (RO 0 ) and nitrogen dioxide, which is a strong oxidizer. Conse- quently, the alkoxy radicals can readily be oxidized to alkyl peroxy radicals (ROO.). It is known that termination reactions of two peroxy radicals are strongly exothermic process frequently accompanied by emission of light, which is referred as chemiluminescence or oxyl~minescence(~-~). Thus, it is natural to think about that the thermal decomposition of nitrate esters possibly emits faint light.

Recent development in photo-counting apparatus has facili- tated to measure very faint light(4). In the present study, a chemiluminescence analyzer was employed as a tool to verify the hypothetical mechanism stated above. We now wish to record the occurence of chemiluminescence from condensed- phase nitrate esters during heating up to about 100 "C.

2. Experimental

2.1 Apparatus and procedure

The luminous intensities and spectra of chemiluminsescence were obtained on a type OX-7-TC chemiluminescence ana-

0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1989 0721-31 15/89/0306-0089$02.50/0

90 J. Kimura Propellants, Explosives, Pyrotechnics 14, 89-92 (1989)

3. Results and Discussion

Thermoelectronic Photanultiplier (R878; 2!5&650nm)

Figure 1. Schematic representation of a chemiluminescence analyzer. A sample placing in the sample cell inside a light-tight box can be heated to a desired temperature in predetermined atmosphere, and resulting emission of light is detected with photomultiplier being cooled.

lyzer (Tohoku Electronic Industrial Co. Ltd.). Figure 1 shows a schematic representation of the apparatus; this is similar to that used in the most recent studies of polymer chemiluminescence. A stainless steel cell, having an inner diameter of 50 mm and a height of 12 mm, is placed on a thermostatically controlled heating block inside a light-tight box. A ring with a quartz optical cover provides a chamber in which a sample could be heated at a desired temperature under an atmosphere predetermined. After placing a sample of about 0.2 g in the chamber and being reached at a test temperature, dark noise alone was measured first followed by the measurement of the sum of dark noise and signal due to chemiluminescence. The very weak light emitted by a sample passes up through glass filters, if any were used, the shutter openinig, and finally to a photomultiplier tube being kept cooled to reduce the dark noise. Photo-counting numbers during a predetermined time period are recorded by a digital counting system; luminous intensities are plotted as a function of time on a chart recorder. Spectral distribution of chemiluminescence can be measured by the change of a series of glass filters having a band width of 10 nm in the spectral range of 420 nm-600 nm.

2.2 Material

The nitrate esters to be tested were a low-molecular weight nitrate ester (PETN) and a high-molecular weight nitrate ester (NC). Powdered PETN (mp = 141 "C) was a product of Asahi Chemical Co. Ltd. A commercially available NC (N = 12.6%), provided also by Asahi Chemical, was purified by repeating the following treatment three times; boiling with 50- fold excess amount of deionized water followed by filtration. The treated NC samples were finely dried in vacuum at 40 "C to a constant weight. The NC sample indicated a molecular weight of the order lo6 by gel permeation chromatography (GPC). The experimental activation ener y, determined by the temperature-varied Abel (TVA) test('f was 35 kcalimol, which agrees well with the theoretical value. Whereas, the untreated NC gave an activation energy of 15 kcaVmol probably due to the presence of water soluble impurities.

Anticipated chemiluminescence has been observed for the two nitrate esters (PETN and NC) during heating in a flowing nitrogen and air, as illustrated in Figs. 2 and 3, where luminous intensity versus time are plotted. The shape of the luminosity-time plot is known to give a clue to the nature of radical reactions involved in chemilumine~cence(~). The observed time-independent luminosity indicates that steady- state concentration of light-emitting species is reached at the experimental conditions used. This may enable one to conduct simple kinetic treatment of the chemiluminescence data, as shown later in this paper.

As depicted in Figs. 2 and 3, the effect of oxygen on the chemiluminescence was significant but not great; at 90 "C, for example, a change from an atmosphere of pure nitrogen to air produced appoximately twofold increase in the luminosity for both PETN and NC. Enhancement of luminous intensity in the presence of oxygen indicates the predominance of reactions with resultant formation of light-emitting species. The

I 1501 NC ( 1 2.6 %N 1

0 0 0 0

0 O OO O 0 0 0 0 oo ooo 3 1 2 0 O O 0 0 0 0 c 2

= " I , , I , 0 0 2 4 6 a 10

Time [ m i n ] - Figure 2. Luminous intensity-time plot for PETN during heating at 90" in a flowing nitrogen (A) or air (0).

90 1- A i r I

0 1 I I I I 0 2 4 6 a 10

Time [ m i n ] - Figure 3. Luminous intensity-time plot for NC during heating at 90 "C in a flowing nitrogen (A) or air (0).

Propellants, Explosives, Pyrotechnics 14, 89-92 (1989) Chemilumi

WAVE LENGTH [ n m ] +

Figure 4. Spectral distribution of chemiluminescence from NC during heating at 150 "C.

oxygen effect, moreover, will provide sound evidence for the view that the thermal decomposition of nitrate esters in the condensed phase is accompanied by some oxidative reactions.

We also succeeded in obtaining the emission spectra of a nitrate ester NC at relatively high temperatures above 150 "C. Measurements of the emission spectra of PETN have failed due to insufficient luminosity in the temperature range below its melting point.

Figure 4 shows a typical emission spectra of NC during heating at 150 "C, where relative luminous intensities are plotted against wave length between 420 nm and 610 nm. Strong emission peaks can be seen at 460, 510, 560, 580, and 600 [nm]. The emission peak at 510 nm is known to be due to carbonyl- containing oxidation products at the first excited triplet state. The emission peaks appearing in the the range from 560 nm to 600 nm, are due to electronic transitions of singlet oxygen ('Oi), where asterisk denotes an excited state. These results of the assignments of the emission peaks strongly suggest that the chemiluminescence observed in the thermal decomposition of the nitrate esters can be generated in the course of recombination of peroxy radicals as illustrated in the following scheme;

2 ROO* -+ '0; + 3(> C=O)* + 3 0 2 + '(> C=O) + hv (1)

There are many evidences for that the oxyluminescence of hydrocarbons and polymers is interpreted using(4) the reaction scheme (1).

A kinetic scheme for the observed chemiluminescence will be proposed in the following section based on the assumptions and the experimental facts below: (1) Nitrate esters initially decompose by rupture of one of the RO-NO2 bonds to form oxidizers (NO2) and alkoxy radicals (RO 0 ) .

( 2 ) The generated NO2 oxidizes ROO to form peroxy radicals (ROO a) and nitrogen oxide (NO). (3) Termination reaction occurs by the recombination of the peroxy radicals as demonstrated by the spectral analysis. (4) Decomposition of hydroperoxides is a possible step for chemiluminescence since hydroperoxides (ROOH) can be formed through reactions of peroxy radicals and nitrate esters. If the contribution of degenerately branching through

inescence Study on Thermal Decomposition of Nitrate Esters 91

hydroperoxides becomes dominant, the chemiluminescence curve would show build-up of luminosity. The observed luminosity-time plots, however, showed steady-state profiles. Thus the contribution of hydroperoxide decomposition to the chemiluminescence is neglected probably due to a low accumulation of hydroperoxides at the very early stage of the decomposition of the nitrate esters.

We propose a simplified kinetic scheme for the chemiluminescence accompanied by the thermal decomposition of nitrate esters as follows:

ki RO-NO2 - RO* + NO2 (Initiation) (2)

RO. + NO^ 2 ROO.+NO (Propagation) (3)

2 ROO* kt - (R' C=O)* + 0; + ROH

(Termination) (4)

where ki, kp, and kt are the rate constants for initiation, propagation, and termination, respectively; R and R' denote alkyl groups.

The simplified kinetic scheme ( 2 ) to (4) enables one to conduct a kinetic analysis as illustrated in the following. The luminous intensity, I, will be proportional to the rate of termination by the bimolecular recombination reaction of peroxy radicals:

I = @ Kt [ROO *I2 (5)

where 4, the chemiluminescence quantum yield, is known to be typically in the oxyluminescence of hydrocarbon^(^). According to the steady-state assumption (d [ROO *]/dt = 0):

Kp [RO *] [NO*] - 2 Kt [ROO *I2 = 0 (6)

Consequently, substituting the concentration of ROO. in Eq. (6) for that in Eq. (5 ) , so that, from Eq. (5):

I = 4' Kp [ROO] [NO21 (7)

By assuming an Arrhenius rate law, and differentiating the substituted form of Eq. (7), one can obtain:

where R is universal gas constant, and T is temperature (K), and Ep is activation energy for the propagation reaction.

A plot of the luminosity against the reciprocal temperature measured will provide a straight line having a slope of - Ep/R. In Figs. 5 and 6 are depicted the Arrhenius plots of the isothermal chemiluminescence data of PETN and NC measured in the temperature range between 40 "C and 90 "C. Cal- culated values of the activation energy (kcal/mol) are as follows: 15(N2) and 10(air) for PETN; 14(N2) and 10(air) for NC. It should be pointed out that the same activation energy was obtained for the two nitrate esters in the same atmosphere. This result may imply that the chemiluminescence of nitrate esters develops chiefly through the intramolecular mechanism, producing blocks of peroxy radicals.

It should be simply pointed out that oxygen decreased the activation energy but increased luminosity. Detailed discussion on the oxygen effects is beyond the scope of this paper. It

92 J . Kimura Propellants. Explosives, Pyrotechnics 14, 89-92 (1989)

Figure 5. Temperature dependence of chemiluminescence from Figure 6. Temperature dependence of chemiluminescence from NC PETN in nitrogen or air; activation energies calculated are 10 kcaVmol in nitrogen or air; activation energies calculated are 10 kcalimol (0) (0) and 15 kcalhol (A). and 14 kcaUmol (A).

has been reported that the activation energies for polymer oxyluminescence are mostly in the range‘“’) of about 7 kcali mol to 15 kcalimol. The formation of a peroxy radical (CHZOO .) has been suggested(’) to be a rate-determing step of the oxidative reaction (E = 15.1 kcalhol) between formal- dehyde and nitrogen dioxide. The obtained activation energies for the chemiluminescence of the nitrate esters fall in the same range. We shall refer to the oxidation in the thermal decomposition of nitrate esters as “self-oxidation” hereinafter.

4. Conclusion

Nitrate esters emit faint light from the condensed-phase during heating in an inert or air atmosphere. Emission spectral analysis verified that the observed chemiluminescence is mainly due to a bimolecular recombination of peroxy radicals, which can be formed by oxidation of alkoxy radicals (RO-) with nitrogen dioxide (NO2); both are produced by homolytic cleavage of nitrate ester bonds (RO-NO2). Kinetic analysis of the temperature-dependent chemiluminescence has led to the conclusion that the rate-determining step of the light-emitting process will be the propagation reaction (oxidation of RO

We have demonstrated that the chemiluminescence method used in this study is a promissing diagnostic technique to inves- tigate the thermal decompsotion of nitrate esters and possibly other explosives.

with NOz).

5. References

(1) R. A. Fifer, “Chemistry of Nitrate Ester and Nitramine Propel- lants”, in “Fundamentals of Solid Propellant Combustion”, Ed. K . K. Kuo and M. Summerfield, AIAA Inc., New York 1984, Chap. 4, pp. 177-237.

(2) J. Kimura, “Kinetic Mechanism on Thermal Decomposition of a Nitrate Ester Propellant”, Propellants, Explos., Pyrotech. 13,

(3) S. S. Stivala, .I. Kimura, and L. Reich, “The Kinetics and Degra- dation Reactions”, in “Degradation and Stabilization of Poly- mers”, V d . 1, Ed. H. H. G. Jellinek, Elsevier, New York, 1983, Chap. 1, pp. 1-63.

(4) G. A. George, “Use of Chemiluminescence to Study the Kinetics of Solid Polymer” in Developments in Polymer Degradation-3”, Ed. N. Grassie, Applied Science, London, 1981, Chap. 6, pp.

(5) M. P. Schard and C. A. Russel. “Oxyluminescence of Polymers. I. General Behavior of Polymers”, J . Appl. Polymer Sci. 8,

(6) M. P. Schard and C. A. Russell, “Oxyluminescence of Polymers. 11. Effect of Temperature and Antioxidants”, J . Appl. Polymer

(7) F. Yoshii, T. Sakai, K. Makuuti, and N. Tamura, “Durability of Radiation-Sterilized Polymers. I. Estimation of Oxidative Degra- dation in Polymers by Chemiluminescence”, J . Appl. Polymer

(8) F. H. Pollard and M. H. Wyatt, “Reactions between Formal- dehyde and Nitrogen Dioxide”, Trans. Faraday SOC. 45, 760-767 (1949).

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(Received July 5 , 1988; Ms 26/88)

chemiluminescence study on thermal decomposition of nitrate esters (petn and nc)

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