CHAPTER-4 GENERAL DISCUSSION CHAPTER-4 GENERAL DISCUSSION

High-energy dense materials (HEDMs) capable to deliver high VOD compared to today's workhorse explosives RDX and HMX san undesirable vulnerability to unplanned stimuli are the need of tomorrow. CL-20 qualifies for such demands ^ It is also emerging as HEM candidate for clean - burning chlorine - free propellants and offers better alternative to AP compared to ADN and HNF in terms of handling and sensitivity respectively. Consequently, R&D work on CL-20 and formulations based on it is pursued at a high pace all over the globe^' ^ CL-20 can be categorized as caged nitramine class of compounds. The synthesis approach of caged nitramines may involve synthesis of a caged amine followed by nitration or cyclization of nitramine (acyclic) units. A combination of these methods may also be applicable. Owing to the limitations of the second approach for cyclic nitramines with complex structure due to amenable possibilities of side reactions, the first approach is the preferred one"*. However, a viable methodology to introduce nitro groups on all the endocyclic nitrogens of the cage to obtain molecules like CL-20 remains a severe challenge. The most serious one is the vulnerability of aminal nitrogen structures within the cage to ring-opening reactions under nitration reaction conditions as the acidic and oxidizing medium of nitration agent could easily offer the conducive environment for collapse of cage structure. The condensation reactions of an^ines with glyoxal to yield hexaazaisowurtzitane derivatives appear to be limited to benzyl amine. The basic cage structure as prelude to CL-20 is realized by this approach. Nearly

170 stoichiometric quantities of benzyl amine with 40% aqueous glyoxal in aqueous acetonitrile solvent at 25"C in presence of an acid catalyst yield HBIW in high yield. The pH of 9.5 was found optimum for efficijsnt formation of HBIW as reported by other researchers. However, the best results were obtained with formic acid [(formic acid 0.1 molar % of the amine)] during this work (Fig.l)^.

PQ

o 13

40 1- ± 1 0 5 10 ±IS 20 MOLAfl RATIO OP BENZYiAMINE TO FORMIC ACiO

Fig.l: % yield of HBIW vs Molar ratio of Benzyl amine to Formit acid^

The mechanism^ of formation of HBIW is proposed to invMve trimerization of diamine (Scheme-1).

171 QHjCHiNHz + OCHCHO •*• C6H5CH2NHCHOHCHOHNHCH2C6H5

5- S+ CfiHsCHaN = CHCH= NCH2QH5 + 2 H2O II

QHsCHzN = CHCH= NCH2C6H5

5+ 5- C6H5CH2N CHCH=NCH2C6H5 C6H5CH2 N-CHCH=NCH2C6H5

QH5CH2N =CHCH=NCH2C6H5 H IV III II+IV

CH2QH5 CHiCftHs

I I C6H5CH2 CH2C6H5 |-2H*

C5H5CH2 ^ 'CH2C6H5 N N'

QHsCHj—N ;N-CH2C«H5

\^ .N V CH2C6H5 C(iH5CH2 HBIW

Scheme-1: Mechanism of Formation of HBIW

It is inferred that benzyl group exerts its characteristic stabilizing as well as activating influence on ionic intermediates. A substituted

172 benzylamine yields diamines tiiat fail to trimerize due to steric effects like N- alkyl benzylamine of tert-butyl class. Several attempts of synthesizing CL-20 without collapse of cage structure of precursor led to the approach of the reductive debenzylation of HBIW under a wide variety of reaction conditions. Although, palladium on charcoal is a preferred choice over palladium metal alone, it gives yields up to 40-50% level. The best results (-60% yield) are obtained with the catalyst generated by reduction of palladium hydroxide on carbon (Pearlman's catalyst). In the present work, Pearlman's catalyst (20% Pd (OH)2 on C) was used along with HBr introduced as bromobenzene to convert HBIW to TADBIW. The maximum yield of 59 % was obtained with one-fourth the weight of catalyst with respect to HBIW and reaction time of 18 hours.

Hg ,Pd/C

C^OfN

HBIW CL-20 R = H2b) =CH92c)

HBr performs the task of a co-catalyst in the hydrogenation cum acetylation of HBIW to TADBIW. The HBr released under reaction conditions reacts with the acetic anhydride to form acetyl bromide, and thus

173 provides active carbonyl carbon for nucleophilic attack. The concentration of HBr is critical and maximum yields are obtained at its one-eighth molar concentration with respect to the number of moles of HBIW. The solid product, TADBIW remained unaffected after prolonged reaction time, and the reaction was allowed to proceed overnight. The TADBIW was converted to CL-20 by reacting it with NOBF4/ NO2BF4 in sulfolane medium during this work on the lines of reported methods^. It has been found that special nitrating agents like nitrosonium / nitroniun tetrafluoroborate (NOBF4/ NO2BF4) are essential for the conversion of tetraacetyl derivative having lone pair of electrons in conjugatin with carbonyl group to CL-20 as they provide adequate concentration of NOV N02'^ to enable nitration of ring nitrogen. Nitration of amines by nitronium salts was first reported by Olah and Kahn^, and was subsequently investigated by Olsen et al . It is reported to progress as

2 R2NH + N02^BF4_ • R2N-NO2 +R2NH.HBF4

Sulfolane is a relatively good solvent for nitronium salts (7% solubility). Moreover, it does not react with the nitronium salt and provides homogeneous solution. Acetonitrile, another promising solvent for nitronium salts was also assessed. However, the nitrile group strongly interacts with N02^ and causes acetonitrile to slowly oligomerise even at room temperature. Although NOBF4 / NO2BF4 combination is effective for TADBIW ~^ CL-20 conversion, the work up of the product is tedious. Moreover, reagents are cost intensive and are not produced indigenously. Therefore, alternate precursors akin to nitration using normal nitrating agents were investigated.

174 TAIW emerging as potential precursor for alternate nitration route was selected. Being a free amine, TAIW can be smoothly nitrated in strongly acidic nitrating media. The compound was synthesized by acetylation of TADBIW with acetic acid /Pd (0H)2 catalyst combination during this work and was nitrated with HNO3/H2SO4 system at 78 ±2°C to obtain CL-20 in -85% yield^'^. The mechanism of complex multiple concurrent reaction pathways are not understood well because the intermediates have not been isolated. Generally, an amine is expected to be immediately protonated on dissolving it in the acid solution, which is normally expected to prevent subsequent clean nitration. However, nitration of the acetyl substituted positions is likely to release acetyl groups into solution and it is envisaged that during the later stages of nitration, the reaction medium resembles the one in which nitration by acetyl nitrate occurs (a common medium for nitration of secondary amine). In addition, the basicity of the amines gets remarkably reduced as the cage is nitrated allowing nitration in the same way as for non-basic nitrogens such as urethanes. A further possibility is that the basicity and reactivity of the amine nitrogen on the piperazine ring of the substituted hexaazaisowurtzitne cage is unusual because of the conformation into which they are locked by the fused ring system. Despite the clean nitration of most of the amines, there appears to be a degree of decomposition of the cage during nitration, and thereby the yield of CL-20 obtained on nitration of TAIW is not quantitative. The decomposition of the cage and deacetylation of TAIW do not appear to be accompanied with NOx evolution or obvious gassing, and thus the acid stable by-products are envisaged to be derived from hydrolysis of the protonated amine rather than oxidation. HPLC conducted on the lines of the reported method" established >98 % purity of the CL-20 synthesized by both the routes during this work.

175 Although nitric-sulfuric acid combination continues to be the most widely used cost effective practical nitrating reagent, serious environmental problems are posed during spent-acid disposal'^. N2O5 has acquired significance in this scenario as versatile nitrating agent for synthesis. Simple isolation of the products, if N2O5 is used as nitrating agent in solvent medium or reduction in the quantum of required acid even if used in acidic medium are adveintageous. It was attempted as nitrating agent for ADN synthesis as mentioned by Golfier^. In order to assess feasibility of N2O5 nitration of precursors to CL-20, N2O5 was synthesized during this work by clean gas phase reaction of O3 and N2O4. The dichotomy of N2O5 pertaining to non­ selective nitration in acidic medium to nitrate deactivated precursors and selective nitration in an organic solvent (especially chlorinated hydrocarbons) medium was investigated for CL-20 synthesis. The nitration reactions of HBIW, TADBIW and TAIW with N2O5 undertaken during this research work suggest the feasibility of nitration of TAIW by N2O5 dissolved in nitric acid. Particle size of energetic materials plays an important role in the processing of explosives and propellant formulations. Crystallization methods need to be optimized to obtain desired particle size during synthesis/manufacturing stage. The method established during this work offered CL-20 particles of 30±10 |xm size, which is optimum for processing of compositions.

176 (a) (b) CL-20 before crystallization (a) and after crystallization'^ (b) o

(a) (b) CL-20 synthesized in the lab (a) before crystallization and (b) after crystallization

CL-20 exhibits polymorphism like HMX. Its c polymorph is the preferred choice in view of better density as well as relatively low vulnerability and stability characteristics. One of the widely used methods to obtain 8 CL-20 is the non-solvent crystallization'"*. The relative dipole moment of non-solvent has bearing on the yield of 8 HNIW. Crystallization of CL-20 dissolved in ethyl acetate (dipole moment: 1.88) by addition of n-

177 heptane (dipole moment: 0) as non-solvent led to the formation of e CL-20 free from other polymorphs during this work. FTIR spectral feature of the synthesized product obtained during this work matched with that reported'^ for 8 CL-20.

•''*•'*''*'**'*'

I » I • I r F I » I I I I I I I noo nso noo loso (»^)

FTIR Spectrum Reported*^ for CL-20

178 99.5% Pure by HPLC

29 Micron 2Ki .-AH .O.B.H.ra...... OK -- - 3 5 s " s 1 iS s S' i s e Qjmaler trnicron*

FTIR and Crystal Structure of 99.5% Pure e CL-20 of29n Particle'^

-I 1 r—I—It I t t -• 1 1—I—>• I I 1 « -1 1 1111)1 A , -p ) "i ills''

\j >98% Pure by HPLC Epsilon Polymorph

n ... 30 micron X,

.|]]>T)rr 4 t I Ht- H 1 i-4 IJWi' ^

FTIR and Crystal Structure of >98% Pure Synthesized e CL-20 of 30 1-1 Particle

[79 1 I "^ H and C NMR spectra of the synthesized product conform to the structure of CL-20. EIMS pattern of CL-20 obtained during this study, gave peaks corresponding to M-NO2, M-3xN02 and M-6XNO2. DSC of CL-20 brought out that it exothermically decomposes with activation energy of the order of ~ 200 kJ/mol as computed by applying ASTM standard method, based on Ozawa correlation. The Ea values obtained from isothermal (209 kJ/mol) TG experiments of CL-20 were close to it. These Ea values are in agreement with that obtained by Xinhzhong etal'^ (189.8 kJ/mol) from ARC of e CL-20 (heating rate of 5°C/min). It can be inferred from activation energy values that the decomposition is basically dominated by homolysis of N-NO2 bond. It is envisaged that the bond is lengthened in the transition state accompanied with the release of hindered rotation of NO2. prior to decomposition. Patil and Brill'^'^ as well as Bouma etal have inferred that N-N bond cleavage determines the global rate constant for decomposition of CL-20. Patil and Brill^^'^^ proposed the following course of reaction for the decomposition of CL-20.

N - NO2 • N- + NO2

N -1-NO2 • N-O-i-NO

HC + NO2 • HC-O. + NO

HC + NO2 • HONO • Vi NO +V2 NO2 +V2 H2O

Although much of the NO2 liberated is released to the gas phase, an increase in NO/NO2 ratio as the reaction proceeds indicates that the NO2

180 increasingly reacts with radical sites in the back-bone at higher temperature due to its retention in cage. It is the involvement of such energy releasing reactions involving -NO2 substituent which may be the cause of overall exothermicity as brought out by the fact that precursor of CL-20 devoid of such units do not exhibit exotherm in DSC. Analysis of weight loss pattern in TG experiments conducted during this work revealed that a residue (-20 %) is left after decomposition of CL- 20 in the temperature region of 235-240 °C. This is in line with the findings of Brill and Patil '^'l It is again attributed to the hindrance to the escape of decomposition products because of cage structure of CL-20 resulting in radical recombination in its backbone. Such radical combination in the backbone of CL-20 could stabilize the C-N bond after N-NO2 homolysis by multiple bond formation leading to polymerization, and thereby residue formation, unlike in case of RDX and HMX having two-dimensional cyclic structure. Patil and Brill'^'^ have reported that residue has stoichiometry corresponding to C6H5N5O2 up to 205 °C. They observed that the heating of residue up to 285 °C results in evolution of NO2 as one of the gaseous products suggesting that some of the NO2 functional groups are retained by it. The evolution of additional amount of CO2 observed by these researchers was explained as a consequence of oxidation of C in the residue by NO2. The detection of gaseous species containing C=N, N=0 and C=0 moieties during TG-FTIR experiments conducted for the work can be explained on the lines of the findings of these researchers. Patil and Brill'^''^ inferred that evolution of HNCO and HCN may be an outcome of decomposition of amide (- C=0=NH) and polyazine units (-CH=N-) of residue respectively. They suggested polyazine may be similar to melons which are known to remain

181 stable beyond 600 °C. IR of residue observed during this work also is indicative of presence of amide moiety in the residue. Patil and Brill'^'^ proposed a general reaction pathway for decomposition of CL-20 (Fig. 2). They proposed homolysis at N (2) leading to weakening of C (3) and N (4) bond (2-1) facilitating the insertion of NO2 in cyclic structure resulting in creation of C-O-N bond (2-II). The intermediate product could react in several ways. It may rearrange to (2-III), which may in turn cleave leading to the formation of N2O and carbonyl residue. Parallely, C (1) - C (7) bond could cleave resulting in adjacent radical sites which may recombine to form C=N (2-V). In addition to HCN, CO formed due to the decomposition of amide residue in the condensed phase, HO-NO may result due to radical scavenging by NO2.

o^ ^b m il n

/\ /\

IV V

Fig.2: Decomposition Pathways of CL-20

182 An attempt was made to shift the decomposition temperature of CL-20 to higher limits by incorporating thermally stable explosives like triamino trinitro benzene (TATB) and tetranitrodibenzotetraazapentalene

(TACOT)^''^^. However, thermally stable compounds did not influence the decomposition pattern of CL-20, probably because of a wide difference in their decomposition temperatures and mechanism.

CL-20 being an HEDM, aluminized CMDB compositions based on it can offer relatively higher Isp than that of corresponding RDX and HMX based compositions due to the relatively higher heat of formation and superior oxygen balance of CL-20. Considering that CL-20 is a nitramine, the combustion structure of the CL-20 propellant will be homogeneous like that of double base matrix despite heterogeneous system, in corollary with

RDX/HMX based CMDB system^^. The basic features of flame structure of

CL-20 CMDB propellant may be depicted as in Fig.3 on the lines of that of

RDX/HMX systems.

183 Burning Gas Surface Phase

Flame Zone

Subsurface reaction zone

Dark zone reaction

RNN02 N02,R-CH0, NO, N2O, N2,H2 0,CO,C02 MONO, NO CO, CO2

Subsurface Fizz zone Flame zone Reaction Reaction Reaction

Fig.3: Basic Features of Flame Structure of CL-20 Propellant

A relatively higher oxygen balance of CL-20 molecule compared to RDX and HMX is expected to have relatively less adverse effect on NO2 to aldehyde ratio with respect to double base system. Further, CL-20 appears to decompose without transition to melt phase as reported by Korsounskii^'* and observed during hot stage spectroscopy as well as SEM studies undertaken in this research programme. Thus, the dilution effect on propellant surface is expected to be less in case of CL-20 than that in case of RDX/HMX. Further, overall decomposition followed by exothermic secondary reactions near the

184 condensed phase are expected to lead to effective heat feed back to the surface compared to that in case of RDX/HMX. These reasonings offer an explanation to the superior burning rate and LPCL combination of CL-20 based compositions than that of corresponding RDX based compositions studied earlier. DSC of CL-20 incorporated CMDB propellant gave exotherms corresponding to the decomposition of DB matrix (Tmax 203°C) and CL-20 (Tmax 243 °C) suggesting their independent decomposition. TG also showed two-stage decomposition. It may be an outcome of their widely different decomposition temperatures and almost similar combustion products, which are overall oxygen lean. The FTIR of decomposition gases evolved during TG of CL-20 incorporated aluminized CMDB formulation showed features observed on decomposition of both CL-20 and DB matrix alone further indicating lack of significant mutual interaction. There is a possibility of -CN containing species getting adsorbed on the amphoteric Al in the condensed phase or disfavour of their formation in the presence of Al as indicated by the absence of absorption band corresponding to -CN containing species. During this work, copper chromite was found superior to Fe203 as burning rate enhancer (9-40% in the pressure range of 2-9 MPa) for CL-20 based CMDB compositions studied. Metal oxides are reported to function by accelerating gas phase reaction as well as by modifying the decomposition mechanism of the condensed phase. It is also suggested that the catalysts provide active sites facilitating exothermic reaction of gases evolved from decomposition of oxidizer / HEMs and binder or heterogeneous reactions on the surface. They may also provide catalytic sites in the gas phase for such reactions. It has been established by a large number of researchers that Fe203 does not act in sub-surface region or condensed phase. Pittman^ supported

185 this reasoning on the basis of the observation that Fe203 co- precipitated with AP did not cause greater burning rate enhancement compared to physically added Fe203. The encapsulation of AP also did not diminish its catalytic effect. According to Beckstead et af^, the primary flame region is the logical site for the catalytic reaction of FezOs. Flanagan and Pearson^^' ^^ found supporting experimental evidences in this direction. Wang et aP^ reported that iron catalysts serve as in-situ source of a Fe203 particles which eject in the flame and provide the catalytic site for primary combustion reaction by heterogeneous mechanism. Experimental work undertaken by various researchers suggests that CC catalyzes both condensed and gas phase reaction with a major contribution to the latter. Boggs et al^^ have inferred that CC releases heat on the propellant surface due to the establishment of exothermic redox cycle between CuO-Cu20-Cu. Findings of Pearson^^ support this reasoning. He observed a pronounced heat release on exposing copper chromite to fuel vapour, and subsequently to oxidizing species. Such exothermic reactions can also occur in the gas phase, and may catalyze fuel and oxidizer reactions at these sites. 2CuCr204 • Cu2Cr204 + CrjOs + V2 O2 Pearson^^ proposed three modes of catalysis by CC. He suggested activation of fuel molecules by absorption on the surface of the catalyst followed by the subsequent reaction with oxygen and reaction of fuel molecules with catalyst-oxygen in the surface layer as modes of catalysis. The reaction of fuel molecules with "catalyst-oxygen" throughout the bulk of catalyst is proposed as the third mode. Similar processes are also likely to play a role in the ballistic modifier incorporated CL-20 based propellants. BLS + CU2O + C black combination was also effective as burning rate modifier for CL-20 propellants studied during this work. Duterque et al

186 opined that the catalytic effect of lead salts and carbon black on combustion behaviour of ntiramine based propellants can be explained on the basis of the mechanism operative in double base systems. Kubota and Hirata^^ observed a decrease in reaction time in the preparation zone of lead catalyzed nitramine propellants to the extent of 25 % and inferred that the reaction rate in this zone is increased by about four times in presence of catalysts. The catalytic activity in the gas phase was indicated by higher temperature gradient for catalyzed propellants during experimental work of these researchers. This reasoning may be extended to CL-20 incorporated CMDB propellant.

The role of lead salts on combustion pattern of double base propellants is well studied and can be explained on the basis of variety of theories by various propellant technologists^^'^^. However, carbon and carbonaceous matter formation theory proposed mainly by Kubota and Eisenrich^^ explains most of the experimental findings. According to this theory, C/NO ratio plays a prominent role in combustion pattern of double base propellant. It is suggested that the super burning rate region appears if C/NO is greater than 1 whereas plateau region is observed if C/NO amounts to 1. The C/NO ratio < 1 corresponds to post plateau region. The mechanism of catalytic effect of lead based ballistic modifiers has been reviewed by Youfang^^. He suggested that the Pb salts need to be heated to 700 °C to decompose to lead monoxide for initiating catalytic process during combustion of the propellant. This may be the reason of increase in LPCL observed for BLS modified CL-20 propellant during this work despite enhancing effect of BLS system on burning rates. Lead monoxide can split the decomposition products resulting in the formation of a large number of evenly distributed carbon nuclei leading to rapid growth of the deposit of charred layer. It is proposed that the heterogeneous reactions

187 involving the reduction of part of NO occur near the burning surface due to the large surface area provided by porous structure of the deposited char. An increase in burning rate on incorporation of Pb salts can be directiy correlated with such processes. The reactions proposed by him are given below. NO + 2Cs • Cs(N)p + Cs(0) [where Cs is surface carbon atom of the char deposited, Cs (N)p is the nitrogen atom physisorbted on the carbon and Cs (O is oxygen atom bonded to the carbon ] NO + Cs + Cs (H) •Cs (N)p + Cs (OH) [where Cs(H) and Cs(OH) are hydrogen atom and hydroxyl group bonded to the carbon respectively] NO + Cs + Cs(OH) • Cs (N)p + C3 (COOH) [Cs (COOH) is carboxyl group bonded to the carbon] 2Cs (N)p • 2Cs + N2 As the pressure increases, the flame front approaches the burning surface and the probability of reactions involving carbon nuclei as well as PbO-C reactions increases resulting in super burning rates. The plateau region may be outcome of subsequent stage in which the formation and consumption of carbon nuclei occur at similar rate. Beyond a particular stage, the char layer disappears due to the involvement of the following set of reactions leading to the post plateau region. Cs(0) • CO Cs (COOH) • Cs (H) fierecely + CO2 Addition of an optimum quantity of finely divided carbon black generally results in enhancement of the effect of Pb catalysts. It is expected to contribute towards formation of homogenous char layer. Thus, carbon

188 black acts as co-catalyst and also shifts the plateau burning to higher- pressure regions. Although, copper compounds alone are not much effective burning rate enhancers, they produce a synergetic effect in combination with lead salts. Almost no change in the AH recorded in the DSC of the ballistically modified CL-20 compositions studied during this work despite remarkable burning rate enhancement brings out that the exothermic processes in the condensed phase/ near gas phase region compensate the heat losses associated with heating of the catalyst to the temperature essential for them to exert the catalytic effect, and the main site of the catalytic action of ballistic modifiers is mainly the gas-phase. Incorporation of GAP at the cost of DEP led to a remarkable increase in burning rate in the pressure range of 2-9 MPa. The burning rates obtained for GAP plasticized CL-20 based aluminized CMDB formulation (9.5 - 37 mm/s) evaluated during this programme are close to those reported by Golfier^ for the CL-20 - GAP propellant. The availability of heat evolved on exothermic cleavage of pendant azide group of GAP appears to play an important role in increasing the surface temperature of the propellants. It is reported that the exothermic cleavage of an azide group occurs as per the following reaction leading to release of 355 kJ/ azido group favoring the formation of N2 as one of the products.

— CH2—Ch+—O ^ —CH2—CH—O— +N2+H2

CH2—N3 C ^ N

-30 Initial Decomposition Process of GAP

189 It has been established by Hori and Kimura^^ that such exothermic processes occur in the sub-surface region. An increase in AH of the GAP plasticized CL-20 formulation observed in DSC conducted during this work with respect to the corresponding DEP plasticized formulation is an indication of manifestation of such processes. The site of cleavage of azide bond with the liberation of N2 has been established by comparing the IR of the degraded product with that of the polymer. The combustion wave structure of GAP propellant is reported to comprise of non reactive heat conducting zone, condensed phase reacting zone and gas phase reacting zone. Hori and Kimura^^ have predicted that the exothermic cleavage of azido group occurs at the surface melt phase of GAP which controls the burning rate, particularly in lower pressure regimes. The final combustion process may occur in the gas phase reaction zone. Hori and Kimura have established the structure of GAP combustion as depicted in Fig.4.

Virgin Preheat • ^urface GAP Zone

Rubber Phase •f Gas Phase (+ Residue) Melt Phase Fig.4: Schematic illustration of structure of GAP combustion3 8

190 Incorporation of BDNPF/A also led to an increase in burning rate albeit to a lesser extent than in case of GAP. BDNPF/A containing oxygen rich energetic NO2 group can promote oxidative reactions in the condensed- / near - surface gas-phase reactions resulting in much higher temperature gradient on propellant surface than in case of inert plasticizer DEP'^^. It is reported that the combustion wave structure and the combustion mechanism of NC-TMETN system are more or less similar to that of standard NC/NG propellants. Kubota et al'*' reported effective reaction order of about 2.5 for TMETN based formulations which is close to that for double base propellants. However, TMETN/TEGDN plasticized NC based composition studied by them exhibited sluggish decomposition leading to relatively lower burning rates than those of DEP desensitized NG plasticized propellant. The thermocouple measurements undertaken by Kubota et al'*' are also indicative of lower surface temperature of TMETN based propellants than that of NC/NG system. It may be the cause of relatively lower burning rates of the TMETN/TEGDN plasticized CL-20 incorporated composition evaluated during this work. TMETN (OB: -34 %) / TEGDN (OB: - 66%) are oxygen deficient unlike NG (OB: +3.5%). Thereby, combustion of TMETN/TEGDN system is expected to result in higher proportion of CO/CO2 compared to that in case of NG as reported by Roos and Brill'^l Percentage of NO2 evolved during decomposition /combustion is also envisaged to be relatively lower for the former. The analysis of decomposition products of combustion of TMETN /TEGDN and NG undertaken by Roos and Brill"*^ supports these reasoning (Table-1).

191 Table -1: Gaseous Decomposition Products of Plasticizers 42

Gaseous TMETN TEGDN NG Species evolved (400 °C, 5 atm of Ar) (450 °C, (400° C, 5 atm of Ar) 5 atm of Ar) NO2 0.016 0.009 0.025 NO 2.19 1.023 2.82 N2O 0.026 0.035 0.002 CO 2.72 2.814 1.956 CO2 0.58 0.699 1.022 CH2O 0.63 0.160 0.033

HCN 0.245 0.250 - H2O 0.27 0.274 0.377

The physical model of combustion of CL-20-CMDB propellant may be depicted as in the Fig.5 on the basis of the findings of this research work.

192 /—" ^"1 PN < 0 •p1^« o ;N -41^ CQ s 1n ri I 9\ o 1 U O s e u o o ^ en •-c •\ S o 0u S CO u 0\ O o o PH iz; fl *\ o o s o iz; 09 &e g o i o0^ -a en 0 ! e^ «e a09 I "•i J I

O ^ In general, polymer coating desensitizes explosives. However, chemical structure of polymer has bearing on effectiveness of polymer as desensitizer.In case of polymer coated CL-20, TPEs (EVA, Hytrel, estane) desensitized CL-20 to a remarkable extent. O O OH H H O [11 II 111/^1/7^ Ml HO-(CH,),-0- -C-(CH,),-C-0-(CH,),-0 "C-NY QJ \ -C-/Q VN-C-0

H Poly (Urethane-ester-MDI) (Estane)

r \ r \ •^"\OV-°- ^^"^^^-^ • • • • ^ • • \0/ ^ • •" o{(CH,), -

CH2= CH2 + CH^ CH /^CH2-CH2-CH,-CH- ^ i . b O 'C - CH, 0='C^ CH V

Ethylene Vinyl Acetate (EVA)

It is proposed that hot spots are created at the point of initiation of explosion reaction under impact. Wen et af ^ reported that from the point of initiation of explosion reaction under impact, hot spots propagate from one crystal to another resulting in the advancement of explosion phenomena. In

194 case of polymers capable of coating of explosive particles, the reaction front meets the surface coating prior to high-energy material, leading to decrease in probability of appearance of hot spots, and thereby decrease in impact sensitivity. The effective coating can contribute towards efficient quenching of the hot spots because of the formation of the protective/molten layer around CL-20 particles. Lower vulnerability of EVA, estane and hytrel based formulations may also be attributed to inherent characteristic of thermoplastic elastomers (TPEs). Increase in the elasticity of TPEs due to presence of soft blocks during hot spot formation by impact/friction stimuli may be responsible for the reduction of crystalline friction of solid filler. Viton was not effective in this regard. Minimum desensitization effect obtained with viton may be an outcome of presence of highly polar F in its structure. PU was also as effective as TPEs. The effective use of PU coating in reducing the probability of generation of hot spots under impact may be due to its capability to absorb and disperse the impact energy"^"^ as reported by Akhavan and Burke.

O H H O H H II II II II -C—N—C—C—N—C—O—C—C—O- I I I I II H H HPol3rurethan H e H H

In DSC, the polymer coated CL-20 exhibited single exotherm with T^ax close to that of CL-20. The TG of PU / Hytrel / Estane / EVA coated CL-20 also revealed single stage decomposition. These results suggest that molten/softened polymer matrix forms a unified energetic mix with CL-20,

195 which decomposes as a homogeneous mass. These results also bring out that the decomposition of CL-20 plays a major role during the decomposition of coated CL-20. Relatively higher Tmax observed for exotherm of viton coated CL-20 in DSC and continuation of weight loss up to higher temperature in TG may be attributed to inherent temperature resistance of former. The decrease in AH for polymer coated CL-20 can be accounted for dilution of CL-20 by binder (10%). FTIR of the decomposition gases evolved during TG of EVA, Hytrel, estane and PU revealed prominent absorption attributable to -C=0 group (1720-1750 cm"') containing species. In case of EVA, reduction in the intensity of the band, with the progress of the decomposition suggests that cleavage of the acetyl group predominates in the initial stages. This is in line with the two-stage decomposition reported'*^ for EVA as per following scheme.

.^^ -• R—C—R" + CH3Q II "" O

or H

I -H I +H R—C—R" + CH3C O »• O *' II ^H O c=o I CH3 CH3 Decomposition of EVA

196 Dutta et al"^^ as well as Sultan and Sorvick"*^ also observed a decrease in the intensity of band at 1737 cm"' at a high rate and explained it by same reasoning. Appearance of new band at 962 cm"' observed by these researchers is attributed to the formation of trans-vinylene bonds.

RCH2 + R'CH-CHgR" ^ RCH3 + R'-CH= CH-R"

R—CH2 + R'-CH-CH2—R" *- R—CH3 + R'—CH-CH—R• • " H' shift

R'—CH=CH—R"

Formation of Transvinylenes (Disproportionation of Secondary Radicals)"*^

In case of hytrel, CO2 may evolve due to cleavage of ester linkage whereas formation of CO2 in estane and PU may be due to the cleavage of urethane linkage - the primary degradation process. Estane and PU gave an additional absorption band at 3265 cm' which is attributable to - NH containing species envisaged to be arising from the cleavage of urethane linkage. Products of decomposition of viton exhibited peak corresponding to -F as expected. FTIR of coated material showed the products similar to those obtained for CL-20 and polymer individually. However, appearance of new bands in the region of 2300-2400 cm"' suggests possibility of abstraction of proton of pyrolyzed binder by -NO2/NO resulting from decomposition of CL-20 leading to the formation of acetylinic species. This aspect needs to be investigated in detail.

197 Polymer bonded CL-20 are choice material for shaped charges to defeat armours by means of high velocity jet formed as a result of detonation of explosives and flow of metal liner as fine particulate'*^'^^. It focuses all of its energy on a single line, rendering it accurate and controllable. The jet causes the target material to yield and flow plastically. The power of the detonating explosive as well as density and homogeneity of the charge play an important role in determining the penetration capability of die shaped charge.

In case of miniature shaped charges prepared during this study, superior penetration level observed for CL-20 based shaped charge compared to HMX based shaped charge can be attributed to high potential of CL-20 as HEDM. ~ 6-8% increase of penetration capability of CL-20 based shaped charge matched with the theoretical predictions made during this study. The findings are in agreement with those of Daniels etal^^. They performed modeling of HMX and CL-20 loaded 125 nmi warheads using Picatinny Arsenal Shaped Charge Code - one dimensional analysis (PASCCl). They found a significant increase in jet tip velocity and accumulated jet mass on replacement of HMX by CL-20. The experimental results obtained by these researchers also indicated increased jet mass profile on incorporating CL-20.Copper lined shaped charge loaded with powerful explosive like CL-20 were also found to produce increased jet length over the base line warheads.

198 References 1. R. L. Simpson, P. A. Urtiev, D. L. Omellas, G. L. Moody, K. J. Scribner and D. F. Hoffman, "CL-20 performance exceeds that of HMX and its sensitivity is moderate", Propellants, Explosives, Pyrotechnics, 22, No.5, 249-255, 1997. 2. M. Golfier, H. Graindorge, Y. Longevialle and H. Mace, "New energetic molecules and their applications in the energetic materials", 29^** International Annual conference of ICT, Karlsruhe, 3/1-3/17, June30-3 July 1998. 3. U. R. Nair, R. Sivabalan, G. M. Gore, M. Geetha, S. N. Asthana and Haridwar Singh, "Hexa nitro hexaaza isowurtzitane (CL-20) and CL-20 Based Formulations: A Review", Combustion, Explosion and Shock Waves, Vol.41, No.2, 121-132, 2005. 4. A.T.Nielsen, "Polycyclic amine chemistry". Chemistry of Energetic Materials, Chapter 5, 95-124, edited by G. A. Olah, D.R.Squire, Academic Press Inc, 1991. 5. A.T. Nielsen, R. A. Nissan, D. J. Vanderah, C. L.Coon, R. D. Gilardi, C. F. George, J. F. Anderson "Polyazapolycyclics by condensation of Aldehydes with amines. 3. Formation of 2,4,6,8,10,12-hexabenzyl-2, 4,6,8,10,12-hexaaaza tetracyclo [5.5.0.0^ ^0^'"]-dodecanes from Glyoxal and Benzyl amine", J. Org. Chem., 57, 1459-1466,1990. 6. A. T. Nielsen, A. P. Chafin, S. L. Christian, D. W. Moore, M. P. Nadler, R. A. Nissan and D. T. Vanderah, "Synthesis of polyazapolycyclic caged polynitramines". Tetrahedron, 54, No. 39, 11793-11812,1998. 7. G. A. Olah, S. J. Kahn, " Friedel-Crafts and Related Reactions", Olah G.A., Ed.; Wiley Interscience; New York, Vol.3, Chapter 43,1964.

199 8. R.E.Olsen, D.W. Fisch and E. E. Hamel, "Advanced Propellant Chemistry", Gould. R.E., ed.; Advances in Chemistry Series 54, Chapter 6, American Chemical Society: Washington, 1966. 9. R. S. Hamilton, A. J. Sanderson, R. B. Wardle and K. F. Warner, "Studies of the synthesis and crystallization of CL-20", 31" International Annual Conference of ICT, Karlsruhe, 21/1 - 21/8, June 27-30, 2000. 10. S. Kawabe, H. Miya, T. Kodama and N. Miyake, "Process for the preparation of hexanitrohexaazaisowurtzitanes", PCT Int. Appl WO 9805666 Al, 70 pp, 12 Feb 1998. 11. G. Jacob, G.Lacroix, V. Destombes, "Identification and analysis of impurities of HNIW", 31" International Annual Conference of ICT, Karlsruhe, 106-1,2000. 12. M.B. Talawar, R. Sivabalan, B. G. Polke, U. R. Nair, G. M. Gore and S. N. Asthana, "Establishment of process technology for the manufacture of dinitrogen pentoxide and its utility for the synthesis of most powerful explosive of today-CL-20", Journal of Hazardous materials, B124, 153-164,2005. 13. Pyrotechnics and energetic materials, examples of crystallized energetic materials, TNO-PML, internet communication (www.pml. tno.nl/en/pe/products/crystallisation_of_energetic_materials.html) 14. M. F. Foltz, "Thermal stability of e Hexanitrohexaazaisowurtzitane in an estane formulation", Propellants, Explosives and Pyrotechnics, 19, 63-69, 1994. 15. E.von Holtz, D.Omellas, M.F.Holtz, J.E. Clarkson, "The solubility of 8 CL-20 in selected materials", Propellants, Explosives and Pyrotechnics, 19,206-212, 1994.

200 16. S.Hamilton/'Studies of the crystallization of CL-20", www.dtic.mil/ndia/2003insensitive/hamilton.pdf 17. W. Xinhzhong, Y. Ou, B. Chen and C. Feng, "Determination of thermal stabilities of CL-20 and HMX using Accelerating Rate Calorimeter (ARC)", 3'^ Beijing International Symposium on Pyrotechnics and Explosives, 520-525, Nov.6-9, 1995. 18. D. G Patil and T. B Brill, "Thermal decomposition of energetic materials 59. Characterization of the residue of hexanitrohexaazaisowurtzitane". Combustion and Flame, 92, No. 4, 456-458, 1993. 19. D. G. Patil and T.B. Brill, "Thermal decomposition of Energetic materials, 53. Kinetics and mechanisms of Thermolysis of Hexanitrohexaazaisowurtzitane", Combustion and Flame, 87, 145 - 151,1991. 20. R. H. B. Bouma, W. Duvalois, A. E. D. M. Vander Heijden, A. C. Van der Steen, "Characterization of Commercial grade CL-20: Morphology, crystal shape, sensitivity and shock initiation testing by Flyer Impact", 3r' International Annual conference of ICT, 105/1-105/9, June27-30, 2000. 21. T. Urbanski, "Heat Resistant Explosives", Chapter 7, Chemistry and Technology of Explosives, Vol 4, 202-217, Pergammon Press, USA, 1964. 22. R. A. Carboni, J. C. Kauer, J. E. Castle and H. E. Simmons, " Aromatic Azapentalenes I Dibenzo-1, 3 a, 4,6a-tetraazapentalene and dibenzo-1, 3a, 6,6a-tetraazapentalene: New heteroaromatic systems", J.Amer.Soc, 89,2618, 1967. 23. N. Kubota, "Survey of Rocket Propellants and Their Combustion Characteristics," Edited by K. K Kuo, M. Summerfield, Fundamentals of Solid Propellants Combustion, Vol. 90, AIAA, New Jersey, 1-47, 1984. 24. B. Korsounskii, V. V. Nedelko, N. V. Chukanov, T. Larikova and F. Volk, "Kinetics of thermal decomposition of hexanitro hexaaza isowurtzitane", 30"' International Annual Conference of ICT, Karlsruhe, 64/1-64/20, June 29-02 July, 1999. 25. C.U. Pittman "Location of Action of Burning Rate Catalyst in Composite Propellant Combustion", AIAA Journal Vol.7, No.2, 328- 334, Feb 1969.

201 26. M.W. Beckstead, R.L. Derr and C.F. Price, "A Model of Composite Solid Propellant Combustion Based on Multiple Flames", AIAA Journal, Vol.8, 2200-2207, Dec 1970. 27. D. A. Flanagan, Technical Report AFRPL-TR-67-18, 1967. 28. G.S. Pearson 'The Role of Catalyst in Ignition and Combustion of Solid Propellants", Combustion, Science and Technology, 3, 155-163,1971. 29. S. Y. Wang, S. S. Wang, F. Liu, and H. S. Chiu, "An Investigation of Catalysis in the Combustion of Iron Catalyzed Composite Propellants," Proceedings of the 19^^ International Annual Conference oflCT, (A89- 1950 106-28), Karlsruhe, FRG, 8-1 to 8-14,1988. 30. T.L. Boggs "The Thermal Behaviour of Cyclotrimethylene trinitramine (RDX) and Cyclotetramethylene tetranitramine (HMX)" Chapter 3 in Fundamentals in Solid Propellant Combustion, Edited by KK Kuo and Martin Summerfield, Progress in Aeronautics and Astronautics, Vol.90, AIAA, 121-175,1984. 31. J. Duterque and G. Lengelle, "Combustion mechanism of nitramine based propellants with additives". Journal of Propulsion, Vol.6 No.6, 1990. 32. N. Kubota and N. Hirata, "Super rate burning of catalyzed HMX- Propellants", 21" Symp. (Int) on Combustion, The Combustion Institute, 1913-1951,1986. 33. R.F. Preckel, "Plateau ballistics in Nitrocellulose Propellants", ARS Journal, Vol.31, 1286-1287, 1961. 34. D.J. Hewkin, J.A. Hicks, J. Fowling and H. Watts, "Combustion of Nitric esters based propellants - Ballistic modification by Lead Compounds", Combustion Science and Technology, Vol.2, No.5-6, 307- 327, 1971.

202 35. N. Kubota and Eisenrich, "Determination of Plateau Burning Effect of Catalysed Double based propellant", 7"^ International symposium on Combustion, The Combustion Institute, USA, 1435-1441. 36. Haridwar Singh and KRK Rao "Mechanism of combustion of catalysed double based propellants", Combustion and Flame, Vol 71, 205-212, 1998. 37. C. Youfang "Combustion Mechanism of Double Base Propellants with Lead Burning Rate Catalysts", Propellants, Explosives, Pyrotechnics, 12,209-214,1987. 38. N. Kubota, T.S. Sonobe "Combustion mechanism of Azide Polymer", Propellants, Explosives, Pyrotechnics, 13, 172-177, 1988. 39. K. Hori, M. Kimura "Combustion mechanism of Glycidyl Azide Polymer", Propellants, Explosives, Pyrotechnics, 160-165, 1996. 40. G. M. Gore, K. R. Tipare, C. N. Divekar, R. G. Bhatewara and S. N. Asthana, "Studies on effect of BDNPF/A on burning rates of RDX/AP/Al filled CMDB propellants". Journal of Energetic Materials, Vol. 20, 255-278, 2002. 41. N. Kubota, T.J.Ohlemiller, L.H.Caveny, M.Summerfield, "Site and action of Platonizers in Double Base Propellants", AIAA Journal, Vol.12, 1709-1714, 1974. 42. B. D. Roos and T. B.Brill, "Thermal decomposition of energetic materials 82. Correlations of gaseous products with the composition of aliphatic nitrate esters". Combustion and Flame, 128: 181-190, 2002. 43. Li Wen, Song Quancia, Wang Wembin and Chen Yongke "The influence of DDT-preventing agents upon the impact sensitivity of the polyurethane bonded RDX" Propellants, Explosives, Pyrotechnics, 21, 247-250, 1996.

203 44. J. Akhavan and T.C.Burke, "Polymer Binder for High Performance Explosives", Propellants, Explosives, Pyrotechnics, 17 (6), 271-274, 1992. 45. S.K. Dutta, A.K. Bhoumik, P.J. Mukunda and T.K. Chaki, "Thermal Degradation Studies of Electron Beam Cured Ethylene Vinyl Acetate Copolymer", Polymer Degradation and Stability, 50, 1995, 75-82. 46. B.A. Sultan and E. Sorvik, "Thermal Degradation of EVA and EBA - A Comparison, Volatile Decomposition Products" Journal of Applied Polymer Science, 43, 1737 - 1745, 1991. 47. J. Carleone, "Mechanics of shaped charges". Tactical missile warheads. Vol. 155, Progress in Astronautics and Aeronautics, AIAA, Tactical Missile Warheads, Edited by J. Carleone, 1993. 48. A. S. Kumar, S. S. Samudre, N. G. Waghmare, J.S. Gharia, "A facile pressing technique for the preparation of small calibre dual role explosive charges", Propellants, Explosives, Pyrotechnics, 21, 106- 110,1996. 49. W. P. Walters, J. A. Zukas ''Fundamentals of shaped charges" Wiley, New York, NY, 1989. 50. A.S. Daniels, E. L. Baker, K. W. Ng, T.H. Vuong and S.E. DeFisher, "High performance Trumpet lined Shaped Charge Technology", 27^' International symposium on Ballistics, Adelaide Convention Center, DSTO, Australian government, 19-23 April, 2004.

204 CHAPTER-5 SUMMARY