CHAPTER-1 INTRODUCTION CHAPTER-1
INTRODUCTION
It has always been the aim of explosives technologists to achieve higher performance leading to enhanced lethality of system. Increase in oxygen balance (OB) and heat of formation (AHf) are the most sought after features to augment the performance of explosives. Most of the approaches to increase performance involve introduction of a large number of nitro / nitramino groups into a stable carbocylic or heterocyclic parent molecule. However, an increase in performance level is often accompanied by increase in sensitivity of the explosives. An increase in density offers an option to improve the performance of an explosive without increasing the sensitivity. Although gunpowder developed by Roger Bacon finds first mention as an explosive, it was the discovery of TNT in 1863, which led to tremendous development in the area of high explosives.The attractive feature of TNT is large gap between its melting point (80.8°C) and decomposition temperature (250°C) rendering it castable in wide range of munitions. TNT was used as a major explosive during World War I. Insatiable demand for high-energy potential brought nitramine class of explosives into focus ' . Cyclotrimethylene trinitramine (RDX) also referred as cyclonite / T4 / hexogen offered much higher velocity of detonation (VOD) compared to TNT due to its superior heat of formation and oxygen balance combination as well as higher density (Table-1). The cyclotetramethylene tetranitramine (HMX) also known as octogen provided still superior alternative. However, both RDX and HMX have melting points close to their decomposition temperature rendering them unsuitable for melt casting unlike TNT. As D (mixture) = I (^i Di [where D (mixture) is the VOD of mixture, D\ is the VOD of i•i h component and (j)i is the volume fraction of mixture of i" component of mixture], a combination of RDX/HMX with TNT in the form of castable slun-y (at 80-85"C) emerged as practical solution to meet the demand of high lethality for modern projectiles and warheads. RDX/HMX occupied prominent position as components of powerful new high explosives developed during World War II. Cyclotols and octols are the most sought after explosive formulations even today. Composition B comprising ~ 60% RDX and ~ 40 % TNT finds wide applications due to ease in casting (Table- 2). Innovative approaches leading to improved casting technologies render it possible to increase the percentage of RDX/HMX in TNT based compositions resulting in improved performance . Table -1: Characteristics of Widely used High Explosives', ^
Compound *MP Structure Density AHf OB YOD Detonation
(°C) ig/cm') (kj/mol) (%) (m/s) Pressure (GPa) TNT 80.8 1.65 -63 -74 6950 18.9
NO, -22 8700 34.3 RDX 204 1 " 1.82 61.78
NO, 1 • 75.30 -22 9100 39.5 HMX 286 CH, N^GH, 1.91 1 r O^N N N — NO^ CH.. N CH,.
'MP- Meltinsi Point Table -2: Performance Potential of Selected Explosive Compositions'
Composition Density VOD *Impact *Friction (g/cm^) (m/s) Sensitivity Sensitivity (hso. cm) for 2 kg (Insensitive up to fall weight load of kg) RDX/TNT 1.72 7900 106 24.0 (60/40) RDX/TNT 1.77 8250 95 14.8 (75/25) HMX/TNT 1.73 8150 96 19.2 (60/40) HMX/TNT 1.81 8640 61 16.8 (75/25) * Determined at HEMRL Applications demanding air blast effect for long duration led to the introduction of aluminium (Al) metal powder as a component of explosive formulation. A large amount of energy is liberated by aluminized explosives beyond the C-J plane as a result of reaction of Al with primary detonation products of high explosives. The basic chemical processes involving Al in this phenomenon can be summarized as
2A1 + 3H.0 AI2O3 +3H2 +866kJ/mol
2A1 + 3C0. AI2O34-3CO +741kJ/mol
2A1 + 3C0 Al.O, + 3C +1251 kJ/mol The well-known aluminized TNT based explosive is tritonal which is reported to produce impulse of the order of 115 kbar. However, aluminized RDX/TNT compositions like dentex and toipex acquired a great significance in modem projectiles and warheads aimed at producing blast effect^ (Table-3).
Table -3: Performance Potential of Widely reported Aluminized Explosive Compositions'*
Composition Density VOD *Peak *Impulse (g/cm') (m/s) pressure (kg ms/ (kg/ cm') cm') H-6 (RDX/TNT/AlAVax 1.76 7490 5.03 1.51 45.1/29.2/21/4.7) HTA (HMX/TNT/Al 1.90 7866 5.29 1.97 /CaCl. 49/29/22/0.5 added) Dentex (RDX/TNT/Al/Wax 1.81 7780 5.27 1.45 48.5/33.5/18/1 added) Torpex -4B 1.76 6700 4.25 1.47 (RDX/TNT/Al/Wax 40.5/37.5/18/4)
*(Experimentally determined at HEMRL for charge weight 1 kg at a distance of 1.5 m) 1.1 Low Vulnerable Explosives (LOVEXs) Although TNT-based explosives have enormous advantages such as cost effectiveness and ease of large-scale manufacturing as well as possibility of demilitarization of filled munitions by remelting of explosive charges, their disadvantages cannot be overlooked. The most glaring drawback of TNT based formulations is their high vulnerability to mechanical and shock stimuli. Thus, TNT based explosives are susceptible to sympathetic detonation in magazines and storage areas as well as can undergo premature initiation upon impact with hard targets. Shrinkage after casting resulting in large internal voids as well as lack of structural integrity and possibility of exudation of TNT during extended storage due to temperature cycling, particularly in tropical countries as well as due to the shock encountered in supersonic systems mainly due to its low melting point are other areas of concern. Moreover, like most of the other aromatics, TNT is toxic. The issue of safety assumed great significance with the signature of STANAG 4439, which provided thrust to the implementation of Insensitive Munitions (IM) policy all over the world. For certain applications, low vulnerability even at the cost of power (VOD) is of utmost importance. Introduction of IM with added feature of superior structural integrity has been identified as major goal since last decade^. These developments have led to the emergence of plastic bonded explosives (PBXs) on global arena ' . Polymer matrix introducing flexibility (rubbery material) can absorb shocks, rendering the PBXs less prone to accidental detonation. Further, PBX can be cast into even a geometrically complicated shape as a liquid at near room temperature, unlike TNT based compositions rendering the processing less hazardous. The systems requiring large quantum of explosive formulation are preferably based on cast PBXs. PBXN-106, PBXN-107 and PBXN-110 based on such flexible binders are widely used particularly in Naval Ordnance^ Hydroxyl terminated polybutadiene (HTPB) is the preferred choice as binder for cast compositions due to its high filler loading capability
Q and clean curing reaction . Rowanex range of PBX compositions containing nitramine loaded in HTPB matrix were the first entrant to this class^. Aluminized castable compositions are being pursued for underwater applications. In order to enhance bubble energy and to achieve complete oxidation of Al, AP is also incorporated in these formulations (PBXN-IU / PBXW-115). Royal Ordnance (RO) Defence has planned spending of £ 2 million to establish the facility of filling of Rowanex-1100 in 105 mm projectiles'^. Filling of 155 mm shells/projectile is also envisaged. Table - 4 and 5 include properties of widely studied castable PBXs"^''. The polymers have also made in-roads into the field of pressed explosives conventionally comprising of 90-97% RDX/HMX coated with 10-3% wax. PBX based pressed explosives offer superior mechanical properties and retain their structural integrity even at higher temperatures comp£ired to wax based compositions. PBXN series of pressed compositions based on Laminac-Styrene binder found application in early stages. However, they became obsolete due to brittle nature resulting in low load bearing capabilities as well as high sensitivity. Polyethylene glycol (PEG) binder also evinced interest. Plastisol grade nitrocellulose emerged as moderately energetic alternative. However, its sensitivity to thermal inputs from impact, friction and electrostatic stimulations resulted in fewer applications. Thermoplastic Elastomers (TPEs) particularly, ethylene vinyl acetate (EVA) and Estane have acquired great significance as binder for pressed PBXs'l LX-14 comprising of 95.5% HMX and 4.5% Estane binder is under production for application in anti-tank weapons. Viton is also emerging as a promising binder for futuristic explosives demanding higher thermal resistance (Table- 6). O in c ^ ^ ;4 CLN Q O o o O •n ^ O in V} ;> 1 00 m '^H ot-~- •~-. in ''•^ r- 00 00 in t\ t\ pxa PLH T5 M >M ^—^ -s3 Sid o C/3 s r^ oo in c ^ (^ -- "--i' "3 ^ -^ -^ < •p-o^ > C^M O + r^ r^ LI ^^ (-<-; r^> in .§ »o in in c ,,• 00 CU IS r's t—I in "^ 1—Q1 U u C< a < CQ ^ 2 in in ^ iJ Qi H ^,>-> ^ u Qz 00 1 in 00 o -5 CM 00 OX) ^ O in u '53 r^j X s -s .o s o OO CN (^1 OO U o o C a; Vi o o o o oc 00 o oc !/3 c o o in 51 OO oc OO OO OO '^ S in IT) I 3 H in in in o in o o Ci in o OO CQ oc Cu OO OO CQ CQ CQ CQ H 0- a, H a. H < X H >< >< >< u Q Q Q oo CO 00 in CM I ^ £ S ^ ss &£ V3 •1^ O oo 1/3 s U (U in o t\ O) 0^ a is ? a © &X) c H .^^ a; hH rq -4-O1 QH .C ISJO ON o O o o o ON o OO oo oo o I oo 2 in •n t^ in U r- R: c o o •4—« »n a. in > > ON >< o >< >< ^ X Q Q § S =< O K a >^ o X S Q rr! U© Q< 3-nitro l,2,4-triazol-5-one (NTO) has evinced great interest as component of extremely insensitive compositions'^. A wide variety of NTO based aluminized cast PBXs with HTPB binder have been developed by SNPE, France. l,r-diamino -2,2'-dinitro ethylene (FOX-7) is entering in this area in a big way due to superior performance combined with low vulnerability''^. Guamylureadinitramide (GUDN) or FOX-12 is the recent entrant to this class'°' '^. l,3,5-triamino-2, 4,6- trinitrobenzene (TATB) is the most sought after thermally stable insensitive explosive due to its typical structural features resulting in decomposition temperature exceeding 300°C (Table-7). It is being explored under Safe High Energy Explosive (SHEE) program. HMX/TATB compositions with Kraton, Estane, Kel-F 800'^ and Viton as binder have been processed and characterized by LLNL (Lawrence Livermore National Laboratory). Tetranitro dibenzo tetraazapentalene (TACOT) is another thermally stable explosive of interest'^. NH2 0 + I II .NO2 N N NH2 ^NH'^ NH2 NO2 FOx-12 10 Table -7: Sensitivity Performance of Selected insensitive High Explosives 1-2.13-17 u 9\ 9i a is •So d s s o f u .gf s S 2 .2 O a O a w — U !/5 S > •2 'S o 0^ •r 4i Q C/5 H NTO 1 >252 1.91 8120 30.7 87 >36 ^C N \ // / N -C\ H NO, TATB NH , 350 1.86 8000 29.1 175 36 H , N "'^^Y^''^ N H , TACOT 410 1.85 7200 24.5 68 36 J(2)5.«> FOX-7 HN ,N3: 254 1.88 8800 36 126 36 HN ^Ch RDX 1 " 204 1.81 8700 34.1 22 19 1.2 Emerging Trends A new era of high-energy materials (HEMs) has dawned on the sector of explosives as in case of propellants. Revolutions in synthesis and manufacturing technologies have opened the possibilities of realizing the molecules, which were considered a non-starter till yesterday. However, developing new energetic materials is a complex process in which many candidate molecules are considered, a few synthesized, even fewer formulated and only a handful are adopted by the military or industry. Although a large number of candidate molecules are envisaged, their actual synthesis in the laboratory can require long painstaking efforts. "It requires laborious trial and error to get the synthesis reactions to go," says organic chemist Phil Pagoria'^. Many a times, a synthesis scheme arrived at after relentless work cannot be considered for full-scale production as it may require too many steps or rarely available reagents that are too costly." Ever increasing knowledge of structure - property relationships, coupled with computer codes to predict the explosive properties from molecular structure, has led to the development of new energetic materials with increased performance and reduced sensitivity to stimuli as well as enhanced chemical and thermal stability depending upon application. 1.2.1 Binders/Plasticizers The most sought after approach in the area of HEMs is to introduce energetic polymers containing exothermally decomposing azido groups (-N3) or oxygen rich nitro / nitrato (C-NO2 / O-NO2) / nitramino (N- NO2) groups. Prominent among the energetic groups is the azido group, which contributes to heat release close to 355 kJ/Ns unit. The first polymer to be developed in this category was glycidyl azide polymer (GAP)'^ under US Army program, which came into prominence during early 1990s. GAP has the advantage of positive heat of formation (+117 kJ/mol) over HTPB (-62kJ/mol). Another azido polymer, which has come into prominence, is poly bis- azido methyl oxetane (Poly-BAMO). However, Poly-BAMO being a solid cannot be used as a binder in propellants and need to be copolymerized. Copolymers of BAMO paved way for their application as potential replacement of HTPB 12 due to almost similar glass transition temperatures. BAMO-AMMO (azidomethyl methyl oxetane) copolymers £ire being explored by Thiokol, US as energetic thermoplastic elastomers (ETPEs). Research on oxygen rich nitrato group containing polymers led to the emergence of polyglycidyl nitrate (PGN) and nitrato methyl-methyl oxetane (NIMMO) as promising candidates. However, their stability problems need to be tackled. UK scientists claim to have overcome this draback. Co-polymers of BAMO with NIMMO are gaining importance with wide range of applications in the area of extruded composite rocket propellants as well as pressed / sheet explosives. r ~l CHoNo HO—CH—CH2—O H r \ \ 1 H—O—HgC—C—CH2--OH CH2N3 I L CH2N3 J^ GAP Poly BAMO r CH2N3 •\ H—0-fiC—C—Cl-b- -OH H- -H2C—C—CH2 •(CH2)4- -OH CH2 N3 m ^ V. O^Nb J J n BAMO-*THF Copolymer Poly AMMO *THF-Tetrahydrofuran CH3 HO- -CH—CH2- H- O H3C- -CHc -OH _ CH2ONO2 -In CH2ONO2 Poly NIMMO PGN 13 Attempts are also being made to develop energetic plasticisers with the aim of improving the energetics of the formulations. Again the molecules having nitro/nitrato substituents, and thereby superior oxygen balance as well as energy content compared to commonly used plasticizers (organic phthalates) are preferred choice. Among nitro plasticisers, bis- (2,3 dinitro propyl) formal/acetal (BDNPF/A) combination has emerged as the choice material^°. BDNPF/A is being manufactured for over 30 years. Compounds having nitroxy ethyl nitramine (NENA) group like R-N(N02)CH2CH20N02, where R is an alkyl substituent, have also evinced interest as energetic plasticizers. Butyl nitrato ethyl nitramine (Bu-NENA) having better thermo chemical characteristics than other members of this class is a potential candidate. Low molecular weight azido polymers and azido esters also find application in this regard. Trimethylol ethane trinitrate (TMETN) in combination with triethylene glycol dinitrate (TEGDN) is also emerging as promising plasticizer. H OC HgC (N02)2C H3 H OCH2C(N02)2CH3 H ^ ^OC H2C (N0 2)2C H3 H3C ^^OCH2C(N02)2CH3 BDNPF BDNPA CH2ONO2 I NO2OCH2CH2O CH2CH2OCH2CH2ONO2 CH3—C HONO2 CH2ONO2 TMETN TEGDN 14 1.2.2 High Energy Dense Materials (HEDMs) Programmes on powerful explosives focus on the molecules with compact strained ring and three-dimensional cage structure. Conversion of such speciality materials to nitro / nitrato/ nitramino derivatives results in realization of power packed storehouses of energy. Polycyclic compounds of this class containing interconnected close packed atoms are of great interest as HEDMs. However, straightforward nitration of caged hydrocarbon precursors is not viable due to formation of by-products and often cleavage of carefully constructed cage structure. Such complexities led to strong opinion in past that HEMs with cage structure will remain a laboratory curiosity. However, innovative nitration approaches and selection of highly specialized nitrating agents as well as reaction conditions led to realization of HEDMs with improved properties during recent times. The most powerful HEDM having the tongue-twisting name 2,4,6,8,10,12-hexanitro-2, 4, 6, 8, 10, 12 - hexaazatetracyclo [5.5.0. 0^'^ .O^'^'l-dodecane or Hexanitrohexaazaisowurtzitane (HNIW), populeirly known as CL-20, has evinced great interest as today's most powerful, non-nuclear, explosive. Its acronym originates from the fact that it was first synthesized, at Naval Air Warfare Center (NAWC) in China Lake. The breakthrough was achieved by Dr. Nielsen during late 1980s under Office of Naval Research (ONR) basic research support . Strain introduced due to cage like structure and presence of six N-NO2 groups render it a rich source of potential energy as evident from its heat of formation of 419 kJ/mol in comparison to 63 and 76 kJ/mol for RDX and HMX respectively. Its close packed structure results in higher 15 density. CL-20 based formulations can outstrip the performance of modem RDX / HMX based systems as brought out by its higher VOD. They offer a means to increase lethality of the specified weapon systems upward by 15- 25%. This increase would translate into either increased mission capability, given the same number of weapon systems, or significant logistic cost avoidance due to a fewer weapons being required to accomplish the same number of missions. Production technology breakthrough by Thiokol Corporation and SNPE^^ provided impetus to R&D work on CL-20, resulting in its emergence as a viable alternative to RDX/HMX^^. CL-20 is fast emerging as the most sought after HEDM for a wide range of warheads and projectiles owing to its manageable stability and resistance to external stimuli despite high performance capacity. CL-20 based PBX formulations are being optimized to fit in the Navy's 1989 decree for development of "insensitive munitions," capable of withstanding unplanned exposure to external forces^"^. These qualities also translate into safety requirement of warheads that are less likely to react violently if dropped, involved in carrier deck fires, subjected to bullet impacts or exposed to any of the other disasters that might befall a weapon in the course of its service life. CL-20 is also emerging as a tough competitor to hydrazinium nitroformate (HNF) and ammonium dinitramide (ADN) as component of clean buming/pollution-free propellant compositions to propel missiles and space vehicles. In most of the laboratories, HNF and ADN are widely investigated as alternative to ammonium perchlorate (AP) as both of them 16 are chlorine free compounds with superior heat of formation than today's workhorse oxidizer AP^^'^l ADN based propellants are reported to be in operation in Russia, whereas work on HNF is pursued largely in TNO, The Netherlands. The thrust to R & D efforts aimed at CL-20 based propellants is provided by the fact that ADN is highly hygroscopic and HNF is highly vulnerable to friction posing processing and handling hazards. CL-20, free from such limitations can offer propellants of equivalent energetics due to much superior heat of formation, despite being oxygen lean compared to ADN and HNF. N02 >N NH^-^ ^^N-N -H I C^N ^ I C-NO^ L H L NO2 J ADN HNF Thiokol is commercially marketing CL-20 as well as end-product formulations. Candidate applications span standard missile and advanced gun systems, as well as land attack missiles and precision-guided mortar munitions. CL-20 appears to be the only option in the next 10-15 years capable to meet goals of weapon system size reduction without loss of performance. Today, CL-20-based products are moving towards "dual use" technology category with the potential applications in commercial sector in addition to defence sector^"^. Wardle, supervisor of the Propellant Research Section at Thiokol reports that the oil well industry has already begun to use CL-20 based explosive charges and considers them ideal for 'down hole' 17 applications. "Special demolition devices" and "high-rate detonating cord" are seen as other commercial applications. In the early 80s, Sollot etal of the US Army Armament Research and Development Command (ARDEC) projected octaniro cubane (ONC) as an ultra high potential energetic material. Its density was predicted as 2.1 -2.2 g/cm on the basis of statistical and computational approaches. Recently published reports estimate heat of formation of 594 kJ/mol for ONC, which may be attributed to the strain energy of the cubane. VOD of ONC with stoichiometric oxygen balance is estimated 9900 m/s (Table-8). Although, synthesis of cubane and bishomocubane was reported way back in 1966, ONC was realized in sub-gramme level at Chicago Laboratory during 2003 as a stable white solid compound which was found soluble in hexane and polar organic solvents. The synthesized compound is reported to sublime unchanged at atmospheric pressure at 200°C and its density was found 1.979 g/cm^ which is still lower than that calculated. The synthesized sample of ONC has survived 14 months in sealed glass tubes. Currently, preparation of ONC is expensive and research is focused on economizing it. One of the projected inexpensive methods of synthesis of ONC is the tetramerization of dinitro acetylene. The state of the art theoretical calculation on this methodology by Politzer etal^° indicated that the free energy changes for dinitro-acetylene to octanitrocubane is thermodynamically downhill by 4171g/mol. 18 Table -8: Potential Futuristic High Explosives vis- a- vis Todays Benchmark Explosive -•^^•^'^•^^ S -o .2 ^ g /-v 1/3 s s ^ ^1 # 1 o Q 0- 'S ^^ K ^ ^^ a PN B u ST CQ Q .2 0 0 0 o J/5 S a S ^1 > Q S Q 0 H 9i CL-20 O^N- NAN-NO, 0,N lO -N-NO, >220 2.04 419 -11 9400 41.9 OjN- N N-NO2 ONC N"2 NO, 02N\^ A 1 N<>2 >200 2.2 594 0 9900 137 O2 •K. V^ --\Y>T 0-)\ \ N()2 DNAF 128 2.0 668 0 10000 0 0 RDX N02 204 1.82 62 -22 8240 34.1 HMX 1 1 286 1.91 75 -22 9100 39.5 0,N N1 1N—NO , CH2 N^CHj NO,, 1.2.3 High nitrogen content (HNC) Materials 1,2,5- oxadiazoles (furazans)^' and their 2-oxides^^ are highly promising building blocks for the creation of high performance materials due to high density of their molecular crystals and positive enthalpy of formation because of simultaneous active oxygen atoms present inside the ring. The introduction of explosophoric groups to furazan and furoxan rings allows wide range of density and energetics. One of the interesting furoxan based high-energy materials obtained at Zelinsky Institute of Organic Chemistry (ZIOC), Russia by Makhova et aJ^^ is 4,4-dinitro-3, 3'-dizenofuroxan (DNAF). They reported experimentally determined velocity of detonation for DNAF of the order of 10 km/s at single crystal density of 2.002g/cml The ZIOC team synthesized DNAF in three steps from a key synthon 3- azidocarbonyl-4-aminofuroxan (AzCAF). It involved the oxidative condensation of AzCAF with 3,3-bis (azidocarbonyl)-4,4'-diazenofuroxan (BAzCDF) followed by its Curtius rearrangement to 4,4'-diamino-3, 3'- diazenofuroxan (DADF). DADF on oxidation with hydrogen peroxide yielded DNAF. However, its worthiness in comparison to CL-20 and ONC need to be proved particularly in view of its low decomposition temperature (127-128°C), and high impact sensitivity. HNCs san oxygen are becoming focal point of research in the area of advanced HEMs aimed at futuristic defence and space sector needs ' . The high energy content of HNCs despite lack of oxygen stems from the presence of adjacent nitrogen atoms poised to form nitrogen (N=N). Such transformations are accompanied with an enormous energy release due to a wide difference in the average bond energies of N-N (160 kJ/mol) and N=N (418 kJ/mol) compared to that of N = N (954 kJ/mol). As a natural 20 consequence of their chemical structure, HNCs also generate large volume of gas (N2) per gramme of HEM bracketing them as choice material, for clean burning gas generators. Storage of maximum amount of energy in a polynitrogen molecule would mean having the largest number of single N-N bonds in their molecular structure. In order to realize isolable and manageable homoleptic polynitrogens, compounds need to possess a sufficiently large energy barrier to decomposition. Researchers^^, ^^ at US Air Force office of Scientific Research in their HEDMs programme have explored unusual polynitrogen molecules with the aim of finding systems surpassing delivered performance of ONC based explosives and cryogenic propellant systems. Quantum mechanical calculations suggest that N4, N5, Ng and N|o are the viable molecules^^. Although, N4 and Ns^ are conceptualized to contain, only cumulated linear N=N bonds like azides, the neighbouring positive charge unlike in the case of N3', renders both N4 and Ns^ energetically unfavorable. However, such limitations can be remedied for Ns^ considering the resonance structures resulting in bent structure of C2v symmetry with bond order of 1.5 for the central N-N bonds. As a matter of fact, a tremendous thrust to research in the area of polynitrogen compounds has been provided by technological breakthrough achieved at Air Force Research Laboratory (AFRL) on realizing Nj^. The new discovery, announced on 19"" January 1999 at the Winter Flourine Conference of the Division of Flourine Chemistry of the American Chemical Society, was made by Christie etal . They obtained novel Ng^ cation by the reaction of N2F and AsFe with HN3 in anhydrous solution. Nj^AsFg is a highly energetic white solid with strong oxidizing nature. It detonates violently with detonation pressure four times 21 to that of HMX and has Isp potential double to that of mono-propellants of today. In a significant development, Christie et al^^ have also succeeded in preparing Ns"^ SbF6" salt in high purity and yield. This salt is surprisingly stable up to TO^'C and exhibits little shock sensitivity unlike Ns"^ AsFe' which is reported marginally stable at room temperature. Attempts to recrystallize the compound from SO2/SO2 CIF solution yielded another new N5F salt. The crystallographic studies confirmed V shaped configuration of these compounds. Presence of the delocalized electronic structure of N5' seems to offer stronger N-N bonds, and it could have been the building block for polynitrogen molecules from an electronic structure viewpoint. However, its extreme sensitivity combined with the fact that it cannot be stabilized in a structure without a large, delocalized aromatic ring has made its use difficult. Ns is considered another promising polynitrogen^^ envisaged to combine Ns^ and N3". However, azapentalene is not envisaged thermodynamically stable enough, whereas its cubane analogue referred as octaazacubane appears to be relatively stable with theoretical energy barrier of 84 kJ/mol for its decomposition. The performance predictions project Ng as ultra high energy material anticipated to be more than 1.5 times powerful than CL-20. It is speculated that N5" having transient existence can be trapped by interacting with Ns"^ to realize Nio. Manna from Livermore National Laboratory, USA, claims that bridging of N-N bond in Nio may be remarkably strong considering energy barrier of 391 kJ/mol for its cleavage to 2 N5 moieties^^. It will also permit a facile rotation of one ring with respect to the other ring to acquire stable configuration. 22 Nio is being projected as a key polynitrogen, which can serve as a building block for clustering into the nitrogen analog of buckminster fuUerene comprising of 32 - 60 carbon atoms. According to Manna, it might be possible to join six Njo molecules into a soccer-ball-shaped 60 atom molecule, on the lines of carbon based buckminster fullerene by subjecting Nio to ultra high pressure induced process. Huge amounts of energy are bound to be released on cleavage of tight bonds of a typical dense Neo molecule generating high thrust (energy release). Engelke and Stine'^^ estimated its density in the range of 2.25 -2.67 g/cm^ and predicted AHf of the order of + 546 kcal/mol. 0 0 0 0 0 0 0 0 0 0 0 0 N=N=N N=N=^M=N N=N= N N4 Ns" N •N. ^N—^N N-^ \ / ^N L-l N N-N I Ni x N Ng N 10 N60 Despite, extensive theoretical investigation indicating that polynitrogen molecules are vibrationally stable, lack of single successful synthesis of a new species on a macro scale is surprising. It may be a testament to the great experimental difficulties resulting from their high endothermicities. Polynitrogens if realized in large quantum can drastically change the technology of high-energy rocket propellants and explosives. 23 1.3 CL-20: Resume of work done The paucity of new energetic materials of practical application is offshoot of the fact that they must meet such diverse needs as high energy density (VOD x p) and insensitivity to mechanical/ shock stimuli as well as resistance to chemical decomposition in addition to the possibility of inexpensive synthesis from readily available reagents. The ability to be formulated with other materials for fabrication into practical devices is another major requirement. Thus, despite the emergence of a large number of advanced HEMs, AP / RDX/HMX based systems hold sway as component of rocket propellants/ high explosive formulations. Novel technologies need to be mastered to bring ultra high power materials into realm. Considering the current scenario, CL-20 appears to be the most promising molecule for application in advanced explosives and propellants of tomorrow. Consequently, enormous amount of data has been generated on synthesis and characterization of CL-20 as well as processing and evaluation of CL-20 incorporated compositions. 1.3.1 Synthesis Approaches The first step in CL-20 synthesis involves creation of basic cage structure through condensation of glyoxal with benzyl amine leading to the formation of 2,4,6,8,10,12-hexa benzyl-2, 4,6,8,10,12-hexaazatetracyclo [5.5.0.0^'^.0^''']- dodecane commonly referred as hexabenzyl hexaazaisowurtzitane (HBIW)"^'. Ouet al"^^ found acetonitrile to be a reaction medium superior to ethanol for HBIW synthesis from the point of view of yield, quality and reaction rate. Conversion of HBIW to CL-20 poses a major challenge. Direct nitration of HBIW to CL-20 by nitrolysis is unsuccessful because of competing nitration of phenyl rings'*^ and thereby debenzylation 24 by catalytic hydrogenation prior to nitration is a necessity. Hexaaza isowurtzitane itself being unstable, hydrogenation in the absence of reactive acetylating agent leads to collapse of the cage structure. Several attempts to develop the synthesis process of CL-20 in this direction led to the emergence of alternate routes of reductive debenzylation. Bellamy'^'^ has investigated reductive debenzylation of HBIW under a wide variety of hydrogenation conditions in the presence of palladium catalyst. HBr is recommended as a co-catalyst. It is most convenient to introduce the HBr in the reaction medium as bromobenzene / acetyl bromide / benzyl bromide or other bromine containing compounds, which are dehydrohalogenated during the hydrogenation to form HBr. The extent of hydrogenation depends on concentration and type of catalyst. Pd on C is preferred over Pd metal alone. The best results (yield of -60%) is obtained with the catalyst generated by reduction of Pd (OH)2 on C (Pearlman's catalyst). Hydrogenation in presence of excess of acetic anhydride results in formation of 4,10-dibenzyl-2, 6,8,12-tetraacetyl - 2, 4, 6, 8, 10, and 12 - hexaazaisowurtzitane (TADBIW) derivative as reported by Nielson ' . Further reaction in acetic anhydride containing 10-20 % acetic acid leads to the formation of noticeable amounts of 4, 10 - diethyl - 2, 6, 8, 12 - tetraacetyl - 2, 4, 6, 8, 10, 12 - hexaazaisowurtzitane, but there was no evidence of formation of hexaacetyl hexaazaisowurtzitane. TADBIW can be obtained in 75 % yield by reductive (H2 and Pd/C) acetylation of HBIW in ethyl benzene employing N-acetoxysuccinimide and acetic anhydride combination^^ Further hydrogenation of TADBIW with palladium acetate and acetic acid results in 2,6,8,12-tetraacetyl 25 2,4,6,8,10,12 - hexaazaisowurtzitane (TADAIW or TAIW) in 73 % yield. Reductive debenzylation of HBIW (H2, Pd/C and acetic anhydride) in the presence of N, N-dimethyl acetamide"*^ lead to the formation of TADAIW as major product along with TADBIW and tetraacetyl monobenzyl hexaazaisowurtzitane. Hydrogenation'^^ of HBIW in formic acid as solvent (with Pd catalyst) yields 4,10-diformyl-2, 6,8,12-tetraacetyl -2,4,6,8,10,12 - hexaazaisowurtzitane (TADFIW). Attempts are being made to optimize the requirement of Pd catalyst '*^"^° as an economy measure. The conversion of TADBIW to tetraacetyl dinitroso hexaazaisowurtzitane (TADNIW) by N2O4 or nitrosonium salt (NOBF4) followed by nitration of TADNIW with NO2BF4 to CL-20 in a 90 % yield is well reported^'- ^l Both TADAIW and TADFIW can be directly nitrated with a mixture of nitric acid and sulphuric acids ^^'^'^ to obtain CL-20 offering a means of its cost effective synthesis. One-pot synthesis reported by Wang et al^^ is a modified approach of preparation of CL-20 from TADBIW (82% yield and purity up to 98%) through TADNIW. Kawabe et al^^ claimed that the mixed acid nitration of TADAIW at 60° C for 24 hours results in 98% yield of CL-20. Sanderson et al^^ established manufacture of CL-20 by mixed acid nitration of TADAIW at 85°C (99% conversion within ten minutes). Nitration of TADFIW with 98% HNO3 at higher temperature (125°C) is reported to produce CL-20 in 90-97 % yield^^ Recendy, Chung et al^^ made an attempt to synthesize pentaacetyl hexaazaisowurtzitane or pentaacetyl formyl hexaazaisowurtzitane as precursor to CL-20. 26 1.3.2 Polymorphism The focus of the researchers investigating polymorphs of CL-20 is on their relative stability and structural features^^"^^. The P modification (orthorhombic space groups pb2ia) is found to be less stable with respect to polymorphic interconversion whereas the e modification (monoclinic space group p2i/c) is reported to be thermodynamically most stable at normal ambient conditions. The a and y modifications have the same molecular conformation as P modification but are packed differently into the unit cell (orthorhombic pbca and monoclinic p2i/c packings respectively). The crystal structure of e modification of CL-20 is reported to correspond to monoclinic packing P2,/c with the parameters a=8.848 (2) A°, b= 12.567 (3) A°, c= 13.387 (3) A°, (3 = 106.90 (3)°, V= 1424.2(6) A°, Z=4, Dc: 2.044 cm"'. The transition from e to y modification is observed in the temperature range of 56.5±1.5°C. Non-solvents with low dipole moment (petroleum ether/ isooctane/cyclohexane: 0) offer e CL-20 whereas those with high dipole moment (triethylene glycol: 5.0) yield the a polymorph. Nonsolvents having intermediate dipole moment (trichloromethane: 1.55,ethyl alcohol: 1.68) result in a mixture of a and e polymorphs^. A high-pressure ^ polymorph was observed by Russel^^ during reversible phase transition of y polymorph at a pressure of 0.7±0.05 GPa. 27 o OVN;;C- O <^" '~ o O O /N-N N-N^^ 0 ,>N—N N—N^'v o O 'P ^ ° 0 Npo 0 o o a polymorph Y polymorph 8 polymorph 1.3.3 Particle size and crystallization CL-20 is often obtained in wide range of particle size in the form of polycrystalline particles with sharp comers and microscopically visible defects. It is possible to control the particle shape and size of CL-20 by optimizing crystallization process. Thome et al^^ investigated various solvents (diisopropyl ether / methyl isobutyl ketone / nitrobenzene / water) for CL-20 crystallization and observed blocking of energetically favourable sites by solvent molecules resulting in odd lookmg morphology in some cases. 28 Generally, a single solvent offers particle size between 50-100 |J,m whereas particles of 10 to 150 |xm size may be obtained by varying the rate of addition of less efficient solvent to the solution of CL-20 in an efficient solvent, provided both the solvents are miscible. Evaporation of solvent from CL-20 solution results in coarse or fine particle size depending upon the type of seed and rate of evaporation. Golfier et al recommended acetone, esters and aromatic solvents to realize e polymorphs of particle size in the range of 10 to 150 )xm. They claimed that the smallest particle size (10|xm) of CL-20 might be obtained by grinding e modification of CL-20. Wardle"^^ discussed the results of alternate crystallization processes examined at Thiokol. Both fine (35 - 50 |im) and coarse (> 200 |xm) fractions of e CL-20 were obtained without poly crystalline features and microscopic defects. Variations in the grinding parameters offered a material with an average particle size ranging from 2 to 15 |i.m. Johnston and Wardle found that temperature of crystallization influences the purity of the product, probably due to e to y transition. 1.3.4 Spectroscopy Extensive work on spectral analysis (FTIR, H, C and N NMR) of CL-20 including 2D experiments (using COSY, HETCOR and NOESY) of CL-20 and its precursors is reported in literature ' . Foltz established FTIR as a powerful tool for identification of CL-20 polymorphs. Detailed measurements were performed in the frequency range of 1200-700 cm' for different polymorphs. The e polymorph displays a distinct splitting of absorption band in the range of 780 to 740 cm' unlike P and y polymorphs. It also shows a doublet in the region of 840 to 820 cm"' instead of a normal singlet obtained for the latter. 29 Table-9: Characteristic IR Peaks of CL-20 Polymorphs in the 1200-700 cm ' range^"* e a P Y 705.1 938.3 718.0 951.7 1121.9 718.9 719.5 1043.8 723.9 944.4 746.5 953.0 1168.1 746.4 741.0 1080.4 738.2 980.9 751.1 989.2 1169.6 835.4 755.8 1106.1 738.3 998.7 757.7 990.7 882.9 764.1 1153.0 744.5 1022.1 764.9 1052.4 907.7 831.7 1180.4 751.4 1051.6 825.3 1072.8 944.9 834.4 758.2 1087.2 835.4 1077.0 959.2 858.3 820.2 1125.1 860.2 1078.3 991.8 879.2 831.6 1139.2 881.7 1080.3 1052.3 892.0 855.4 1182.5 904.3 1082.0 1094.7 909.4 883.8 1191.7 945.2 1094.9 1154.4 938.1 900.3 947.7 1118.3 1171.8 958.8 913.2 949.1 1119.8 1178.6 970.6 'H NMR of CL-20 gives two signals at 8.4 ppm (corresponding to 4Hb) and 8.2 ppm (corresponding to 2H;,). The "N NMR spectrum shows well- resolved resonance for the two non-equivalent NO2 groups at - 42.1 ppm (N,) and - 45.1 ppm (Nj) in the ratio of 2:Usee Scheme -1]. A very broad resonance barely visible in region around - 190 ppm is assignable to the nitrogen atoms belonging to ring nitrogens (of nitramine groups). 'C NMR of CL-20 shows singlets at 75.0 and 72.1 ppm respectively. 30 t^rt Scheme -1 The mass spectrum of CL-20 in the chemical ionization (CI) mode^'exhibited molecular ion peaks at m/e of 467 (M+ 29, 24 %) and 439 (M+1, 30%). The other signals were observed at m/e of 347 (M+l-2xN02, 26), 301 (M+l-3xN02, 28), 255 (M+1- 4xN02, 14) and 209 (M+l-5xN02, 18). 1.3.5 Chromatography Jacob et al recommended high pressure liquid chromatography (HPLC, C-18 column) as a preferred technique to establish purity (0.5 % precision) of CL-20. A mixture of acetonitrile, water and phosphoric acid as eluent was found effective by them in this regard. Tian^^'^^ also analyzed CL- 20 by HPLC using a Supelco LC-18 column, Liu et al^° determined the CL- 20 content in explosive mixtures using external standard by applying HPLC on a Nova-Pak C18 column with methanol-water mobile phase at a flow rate of 0.8 cm^/min. Other components of the mixture (RDX, HMX, TNT, 2,4- DNT, benzotrifuroxan, tetryl, and 1,3,5-trinitrobenzene) did not interfere in the determination of CL-20 content. 31 1.3.6 Thermal Studies Thermal analysis results with respect to specific CL-20 polymorphs are well reported. Simpson^^ found that e-CL-20 exhibits two endotherms in the vicinity of ~65°C in DSC attributable to e -> a and a ^ 7 transitions. Nedelko et al inferred that the earlier stages of decomposition (<1%), involve (a-^y ^nd e-^y) phase transitions and the decomposition process is related to that of Y-CL-20. The rate of thermal decomposition appeared to increase in the order of a > y > 8. Under linear heating rates of 2-4°C/min, ignition of a- CL-20 was observed at 216°C whereas ignition temperatures obtained for the y and 8 polymorph were 226 and 230°C, respectively. Wardle et al"^^ reported onset decomposition temperature of 220°C for p polymorph and ~ 240°C for 8 CL-20. The maximum decomposition temperature recorded by these researchers for a, {3 and 8 conformers were 250, 240 and 253°C respectively. Bouma et al reported the results of thermogravimetric analysis (TG), differential thermal analysis (DTA) and differential scarming calorimetry (DSC) of CL-20 at heating rates of 2 and 10°C/min in nitrogen and air environment. They observed an endotherm with the beginning of the temperature decrease between 107 and 113°C with a minimum in the region of 116-129°C followed by an exotherm with the beginning of the temperature increase in the region of 202-217°C with maxima in the range of 220-239°C. They inferred that slow decomposition under isothermal conditions is dominated by N-NO2 homolysis, and subsequently the back bone (cage) of the CL-20 molecule collapses. Korsounskii et al^^ studied thermal decomposition of CL-20 under non- isothermal conditions (4°C/min) in air, argon and with continuous evacuation (under dynamic vacuum) by TG. CL-20 was observed to ignite in air at 243°C. In dynamic vacuum, rapid loss of weight began at 215 °C, but ignition was not observed, probably because the exothermic oxidative 32 reaction was impossible owing to escape of NOi. Ignition occuiTcd at a lower temperature (224'^C) in argon, which is heavier than air, and thereby reduces extent of escape of NO2 favouring pronounced oxidative interaction. Nedelko et al ' established that thermal decomposition of CL-20 is purely a solid-state process proceeding with self-acceleration and can be described by a kinetic law of first order with auto catalysis. The kinetic parameters of non-catalytic and catalytic decomposition of CL-20 polymorphs as reported by these researchers using isothermal TG are summarized in Table-10. Higher activation energy values of e CL-20 for non-catalytic stage are indicative of its superior stability. Bohn also observed auto-catalytically accelerated thermal decomposition of CL-20. He inferred that the decomposition is not accompanied by liquefaction. Table -10:Kinetic Parameters of Non-Catalytic and Catalytic Thermal Decomposition of CL-20 **' Kinetic parameters E ( kj/mol ) A (s^ ) Temperature Polymorph Non- Non- (°C) Catalytic Catalytic catalytic catalytic stage stage stage stage a 166-194 149.5±10.5 232.7+8 j(y,..i,.3 1 Q23.0+0» wy7.7+[.6 ip.(9.6±(). The kinetic equation was adopted in the form drj/dt = ki(l-r}) +k2t](I'r}),ki A,exp{-Ea/RT}, i^ 1,2 33 Patil and Brill^^ also investigated weight loss patterns of CL-20 under isothermal conditions in the temperature range of 190-204°C. These researchers plotted -log (1-a) against time (min) and obtained activation energies of 152.5 kJ/mol for n=l and 160.4 kJ/mol for n=2. The corresponding log A (s"^) values were 13.6 and 14.1 respectively. They inferred that CL-20 does not undergo autocatalysis in contrast to other nitramines as evidenced by the fact that the data do not fit into the Prout - Tompkins model. Isothermal studies conducted by these researchers bring out that decomposition proceeds at slower pace below 190 °C whereas it is quite rapid above 204 °C. Lobbecke et al^^ found relatively higher Ea for the non-catalytic (172± 25 kJ/mol) and for the catalytic (184± 8 kJ/mol) process in the temperature range of 160-180°C. Xingzhong et al^^ also reported relatively higher activation energy of 189.8 kJ/mol on the basis of accelerating rate calorimetery (ARC) experiments. Korsounskii etal found that a residue amounting to 16 % of the initial CL-20 sample was left after completion of exothermic decomposition of CL-20. The residue remained stable up to 300°C, and subsequently decomposed to the extent of 60 % at temperatures exceeding 610°C. FTIR investigations established polymeric nature of the residue and suggested its SO formation after elimination of five nitro groups. Patil and Brill also reported the formation of a dark-colored residue, at temperature above 285°C during rapid thermal decomposition of P-CL-20 in T-jump Fourier-transform IR spectroscopy experiment. Krauetle^° found that the residue has an elemental composition averaging to about C3H3N2O2. IR spectrum of the residue formed at 215°C exhibited bands corresponding to C=0 (1821, 1703cm"'), 34 C=N (1594, broad) and NH (3277 cm', broad).The presence of broad band (1594 cm"') and absorbances at 1292 and 1348 cm' suggest the presence of amide moiety. Korosounskii etal reported absorption bands corresponding to amino groups forming hydrogen bonds (3100-3210 cm"'), carbonyi groups (1753cm"') and nitro groups (1625,1390cm"') in IR spectrum of the condensed products of CL-20 decomposition in air at 202°C. The product of decomposition in vacuum also exhibited similar spectrum albeit with lower intensity of carbonyi bands. Patil and Brill found that at higher temperature, the residue released residual NO2 groups resulting in oxidation of C. HNCO and HCN were evolved probably on decomposition of amide and polyazine in residue respectively. They suggested the presence of cyclic azines like melon in the residue, which are reported to be stable up to 700°C. The microscopic observations suggest initial cracking of crystals during decomposition process followed by disorientation of formed fragments. High pre exponential factor for CL-20 may be accounted for the lengthening of the bond in the transition state accompanied by the release of hindered rotation of NO2 in N-NO2 bond. A newly developed^' gem anvil cell technique (combination of high-pressure and thermal-shock conditions with low- temperature matrix isolation) revealed the formation of HjO, CO and NO2. According to Korosounskii etal^^ manometric measurements bring out that complete decomposition of 1 g of CL-20 results in generation of 570 cm^ of gaseous products (llmol of gas from 1 mol of CL-20). Nitrogen contributes to 47% (vol) of products corresponding to 5.2 mol of N2 / CL-20 mol. Gravimetric experiments have shown that average molecular weight of gaseous products of the reaction is -33.5. 35 Oxley et al reported that the decomposition of CL-20 in acetone (1% solution) is governed by the kinetic equation of the first order with activation energy of 177 kJ/mol in the temperature region of 146-226 °C. They observed that, 9.61 mol of gaseous products are evolved from I mol of CL-20 at 240 °C comprising of: N2 (4.9), N2O (0.82), CO2 (3.30) and CO (1.20). In acetone solution, 0.97. 0.07, 0.20 and 0.19 mol respectively of the same gases are formed. Simpson et al obtained heat of combustion of e modification of CL- 20 as 3.596 x 10^ J/mol in comparison to that of 3.649 x 10^ J/mol for P-CL- 20. The calorimetric heat of detonation obtained for e modification of CL-20 (-12200 J/cm^) was found to be ~ 8% greater than that for HMX (-11700 J/cm ). The time of explosion of CL-20 and RDX samples stored at a given temperature was fairly close to each other. However, CL-20 exhibited much more violent reactions and its critical temperature (163°C) was found relatively lower than that of RDX and HMX (184 and 190°C). 1.3.7 Sensitivity of CL-20 Simpson et al^^ reported that CL-20 exhibits impact (hso p-CL-20: 14 cm and e-CL-20: 12-21 cm) as well as friction sensitivity (insensitive up to 6.2-7.2 kg) close to those for PETN (hso 13-16 cm and insensitive up to 8 kg ) whereas it is more sensitive to these stimuli than HMX (32 cm and 11.6 kg respectively). Mezger et af ^ also reported higher sensitivity of CL-20 to mechanical stimuli than that of HMX. However, Dudek et af "^ found that impact sensitivity of CL-20 (hso =28 cm) is comparable to that of HMX (30 cm) and TNAZ (26 cm). Bouma et al^^ established that CL-20 is friction insensitive up to 84 N whereas Mueller^^ reported its insensitivity up to 5kg. 36 The morphology of CL-20 plays an important role in this regard^^ Thus, rounded crystals of e-CL-20 are reported to have lower sensitivity to impact (h5()=36cm) as compared to polycrystalline materials (hso^ 19.4 cm). Daoud ' reported the relative sensitivity of polymorphs of CL-20 of Thiokol origin (Table-11). However, impact sensitivity results obtained by him are on much higher side compared to those reported by other researchers. Table-11: Sensitivity Characteristics of CL-20 polymorphs (Thiokol) 96 Polymorph Impact Sensitivity, Friction Electrostatic (hsccm) Sensitivity discharge (ESD) (Insensitive up to (J) kg) a 55.2 088 28.12 P (30 ^im) 48.0 039 >29.03 c (35 jim) 86.4 036 25.40 a (wet) >110.4 >29.03 >8 >8 P (wet) >110.4 28.57 >8 c (wet) >110.4 >29.03 37 As expected, water wet CL-20 is much safer with respect to sensitivity. As regards sensitivity to static charge, CL-20 is found to be safe (ESD 0.36- 0.88J) than HMX (ESD 0.2-0.3J)^^ Ultrasound aided crystallization^^ of CL- 20 (frequency 10-100 kHz and amplitudes of 0.4-l)im) is reported to decrease the sensitivity and improve the stability of CL-20. Manning et al suggested graphite coating (<2%) as an effective means of reduction of sensitivity. 1.3.8 Formulations A large number of CL-20 based plastic bonded explosives (PBXs) as well as clean burning propellants with variety of binder systems are reported in literature (Table-12). PBXs HTPB, polyglycol adipate (PGA) and GAP find application as binders of cast PBXs whereas thermoplastic elastomers (TPEs) like Estane / Ethylene Vinyl Acetate (EVA) / Hycar are widely employed for pressed PBX. Golfier^^ reported 12-15% higher VOD of cast CL-20/HTPB high explosives compared to corresponding HMX/HTPB compositions depending on filler content. Dudek et al^'* formulated a cast composition containing CL- 20 in combination with insensitive explosive TEX (4,10-dinitro-2,6,8,12- tetraoxa 4,10-diazatetracyclo-[5.5.0.0^'^.0^''']- dodecane. Mezger et al^^obtained pressed CL-20 - Estane composition (LX-19) corresponding to today's most widely used HMX based LX-14 composition. LX-19 delivered higher VOD compared to LX-14 by about 650 m/s. He also formulated another series of CL-20 - Estane compositions designated as PATHX-1, PATHX-2 and PATHX-3 (currently known as PAX-11, 12 and 13) as 38 alternative to LX-14. The delivered VOD of the compositions ranges from 8890 to 9500 m/s (Table-12). Table-12: Density and VOD of Selected CL-20 Based pBXs ^2. 93-94,99 Composition Density (g/cm ) VOD(m/s) 66.8-72.1 % CL-20; HTPB 1.618-1.710 8325-8470 66.8-72.1 %HMX; HTPB 1.575-1.648 8030-8107 LX-19: 95% CL-20; Estane 1.959 9440 LX-14: 95 % HMX; Estane 1.835 8790 PATHX-1: 88-95% CL-20; Estane 1.868-1.944 8890- 9370 PATHX-2: 92-95%CL-20; Estane 1.869-1.923 8850- 9220 PATHX-3:85-94%CL-20 ; Estane 1.871-1.958 8910-9500 RX-39-AA&AB: 1.942+0.001 9208+10 95.5 - 95.8% CL-20; Estane PBXC-19 :95% CL-20; EVA 1.896±0.002 9083+9 96% HMX; 1% Hy Temp; 3% DOA 1.817 8748 96% CL-20; 1% Hy Temp; 3% DOA 1.901 9018 PBXCLT-1: 1.906 8384 49-70% CL-20; 48-27% energetic material (HNJ); 3 % polymeric binder (PVB) PBXCL-1: (97 % CL-20; 3 % PVB) 1.921 9102 39 Simpson^^ reported estane and EVA based compositions with 95.5-95.8 % CL-20 designated as RX-39-AA/AB and PBXC-19 respectively offering VOD exceeding 9000 m/s. "rk-iO^^^^ Dudek etal^"^ established higher VOD of pressed ((|) 21mm) CL-20 based composition (9018 m/s) based on commercially available Hy Temp binder compared to that of corresponding HMX composition (8748 m/s). Daoud ^^ evaluated CL-20 - Hycar formulation (CL-20 95%, Hycar 3%, TA 5%) in comparison to PBXW-11 (90% HMX, 1% Hycar and 3% DOP) and reported 300 m/s gain in VOD. They found that EVA and PIE binder based CL-20 composition offer marginally lower VOD (by 30-40 m/s) than Hycar-based formulation (9060-8687m/s). Tian etaf^ formulated 49-70% CL-20 incorporated compositions based on PVB binder with energetic plasticizer (PBXCLT-1 and PBXCLT-2) apart from CL-20/PVB 97/3 ( PBXCL-1). Cylindrical pressed charges of the composition ((|) 10 nmixlO mm and (j) 20 mmx20 mm) delivered VOD of the order of 8384-9102 m/s. Pressable / extrudable CL-20 based compositions with polymeric binders like cellulose acetate butyrate (CAB)/ nylon/ Hytrel 8184 and plasticizers like isodecyl pelargonate (IDF)/ BDNPA/F/ GAP are also reported'^. Wendy etal'"' studied aluminized CL-20 based compositions (Aluminised PAX-11 and PAX-29) with BDNPF/A plasticized GAP binder. They reported TMD of 2.002-2.023 as compared to that of 1.859 for aluminized HMX based composition PAX-3. VOD of the compositions was 8770-8870 m/s compared to 8060 m/s for PAX-3 and CJ pressure was 38.3 - 39.5 GPa compared to 28.1 for PAX-3 (Table-13). 40 Table-13: Aluminized CL-20 based PBX compositions 101 Composition Density CJ pressure VOD 99% TMD (GPa) (m/s) (g/cm') PAX-11 (Aluminised) CL-20: 79,A1: 15. CAB: 2.023 39.5 8870 2.4,BDNPF/A: 3.6 PAX-29 CL-20: 77,A1: 15, CAB: 2.002 38.3 8770 3.2,BDNPF/A: 4.8 A-3 (Aluminised) 1.824 21.0 7420 RDX: 64AI: 30, wax: 6 PAX-3 HMX: 64M: 2(1 CAB: 1.859 28.1 8060 6.5MDNPF/A: 9.5 Sato et al studied CL-20 as base charge for detonators and as filling of detonation cords. Simpson et al'"^ projected probable application of CL-20 based PBXs in explosively formed projectile (EFP). Propellants Eisele amd Menke'"' reported the probability of application of CL-20 based compositions in rocket motors for high velocity ballistic / guided tactical missiles (HVMs or HFKs). Golfier" reported that CL-20 incorporated propellants offer much higher burning rates (-35-110%) than 41 those of HMX propellants. The burning rates of CL-20/PGA systems obtained were 11.5-23 mm/s in the pressure range of 7-15 MPa in comparison to 6-11 mm/s of HMX/PGA composition. The incorporation of GAP as replacement of PGA resulted in enhancement of burning rates (17- 18 %). He compared CL-20- glycidyl azide polymer (GAP) propellant with corresponding RDX based propellant and reported higher value of Isp for former (251s and 242s respectively). An improvement in burning rates (-17%) of CL-20-GAP propellants was achieved by him on incorporating ballistic modifiers (Table-14). Wagstaff'^^ also studied GAP/ BDNPF/A based CL-20 composition for application in propellant system. Table-14: Burning Rates of CL-20 Incorporated Rocket Propellants 22 Composition Burning Rate (mm/s) n 60 % CL-20; 40 % PGA 11.5-23 (7-15 MPa) 0.92 60 % HMX; 40 % PGA 6-11 (7-15MPa) 0.89 60 % CL-20; 40 % GAP 13.4-27.2 (7-15 MPa) 0.94 60 % HMX; 40 % GAP 7.2-13.6 (7-15 MPa) 0.91 60 % CL-20; 40 % GAP 20.0-32.4 (7-20 MPa) 0.48 (5-25 MPa) (Ballistically modified) 60% RDX; 40 % GAP 14.6-21.4 (7-20 MPa) 0.37(4-20 MPa) (Ballistically modified) 42 Chan et al^^^studied the effect of replacement of ADN by CL-20 in polycaprolactone (PCP) - nitrate ester (NE) propellant. The propellant containing a combination of ADN and CL-20 exhibited burning rates similar to those of ADN propellant during their study. This may be an outcome of greater influence of ADN on burning rate of the propellant than CL-20, probably because ADN bums much faster than CL-20, and thereby dictating the bum rate of the propellant. The propellants containing ORP-2A [poly diethyleneglycol-4, 8-dinitraza undeconate] and poly glycidyl nitrate (poly- Glyn) as binder bumt at a faster rate than the corresponding PCP propellants, which may be attributed to the higher chemical energy content of the Poly- Glyn £ind ORP-2A binder than that of PCP which is an inert polymer. \f\Q As regards gun propellants, Harris et al recommended hytrel (copolymer of polybutylene terephthalate-polyether glycol) and BAMO- azido methyl methyl oxetane (AMMO) copolymer based CL-20 containing propellants as superior alternative to M30 composition. Wardle et al reported higher impetus of oxetane based CL-20 gun propellant (1297 J/g) compared to that of RDX propellant (1182 J/g). Mueller^^ found that CL-20 propellant formulations based on NC plasticized with BDNPF/A or EPX are safe enough to be tested in the 40-caliber gun simulator (Table-15). 43 Table-15: Combustion Characteristics of CL-20 Incorporated Gun Propellant 95,109,110 Composition Impetus Flame Burn rate (J/g) Temperature (cm/s) (K) 76%CL-20; BAMO / AMMO 1297 3412 11.48-29.97 (75.8-179 MPa) 76 % RDX; BAMO /AMMO 1182 2827 3.86-10.97 (75.8-179 MPa) CL-20;NC; BDNPF/A 1253 3698 RDX;NC; BDNPF/A 1220 3390 CL-20;5-30% BAMO- 1278- 20.32-27.94 AMMO/BAMO- NMMO/ 1349 BEMO-NMMO Lewis et al'" studied a CL-20 based pyrotechnic gas-generating material that burns down to produce one or more reaction products that suppress fire. 1.3.9 Processing Prevailing casting, extrusion and pressing techniques find application in processing CL-20 based formulations. Mueller'^^ made an attempt to use a continuous twin-screw extrusion technology for propellant processing. Simpson et al"^ reported injection moldable formulations containing -80% CL-20, 10 % bis (2,2,2-nouro dinitro) formal (FEFO) and 10% FM-1 (a 44 mixed nitroformal). Simpson et al"^ used sol-gel process for producing formulations based on CL-20. Application of modem process technology can lead to realization of 98- 99% of TMD. It is a highly desirable feature because even small increase in density significantly increases the explosive "punch" due to enhancement of VOD, and consequently penetration performance^^.Certain binders/plasticizers function synergistically in combination with certain CL- 20 polymorphs, either separately or together and yield high % of TMD. Mezger et al^^ established that there is no phase transition of CL-20 during processing in the temperature range of 30-90°C at 138-345 MPa. However, Foltz et af found that polymorphs other than e undergo phase change during processing causing decrease in density of CL-20-estane formulation. The p phase transition was observed at 60°C. A long duration heating of CL-20-estane based formulations at 100-105°C led to a-y as well as e-y phase transition. This has been attributed to high solubility of CL-20 in estane owing to high carbonyl content of latter. 1.3.10 Compatibility Wardle et al^^ reported that CL-20 is compatible with HTPB and polyethylene glycol (PEG) as well as azido polymers and nitrate esters, whereas it is incompatible with polybutadiene acrylic acid acrylonitrile terpolymer - (PBAN). Vacuum stability test (VST) of PBAN based compositions revealed evolution of gases in larger volumes than for CL-20 alone and CL-20/estane formulations [PATHX-3 (96) and LX-19]. However, the overall amount of gas evolved (0.1-0.3 cm^) at 100°C for 48 hours remained at an acceptable level for CL-20-PBAN compositions containing graphite, zinc stearate, metals (copper, molybdenum, aluminium, iron and 45 tantalum) and resins (RTV 3104, Ultem plastic) as additives except for compositions containing loctite (1 cm^) suggesting incompatibility^^ of the latter with CL-20. A lower onset temperature of the exotherm for loctite- containing CL-20 based formulations (199-220°C) in comparison to that of other formulations is another indication of its incompatibility. Heintz and 119 Teipel found that micro encapsulation results in improvement of stability and processibility. Tappan and Brill''^ observed that a mechanical mixture of CL-20 and NC decomposed as discrete units during DSC experiments, as was evidenced by two exotherms with maxima at 200 and 233°C. However, in CL-20 /NC cryogels a single exotherm was observed at 205°C suggesting the influence of NC decomposition on decomposition of CL-20. T- jump/FTIR experiments also bring out the dominance of NC decomposition except for cryogels. Mueller^^ investigated chemical stabilities of CL-20/NC based gun propellant compositions with BDNPF/A and EPX (a nitramine plasticizer). He observed that the best results are obtained for BDNPF/A binder in terms of gas evolution (2.1 cm^, at 100°C in 48 hours). ADN/CL-20 composites based on PCP/ORP/polyGlyn binder exhibited exotherm corresponding to decomposition of both ADN and CL-20 with maxima at 161-177°C and 199-222°C respectively suggesting that these molecules decompose as discrete units. The volume of gas evolved during VTS (1-0.14 cm^) established the compatibility of ADN and CL-20 ^^\ 1.3.11 Sensitivity of formulations Among pressed compositions, Mezger et af ^found impact and friction sensitivity of CL-20 based LX-19 and PATHX-3 (94) almost comparable to that of HMX incorporated LX-14. The compositions with a lower CL-20 content (PATHX-3 (90) were found superior in this regard. Card Gap test 46 results reported by Wardle et af ^ suggest comparable safety characteristics of CL-20 (93%) / binder and HMX (93%) / estane based formulations (card gap 5.4-5.44 cm and 5.184-5.256 cm respectively) in terms of shock susceptibility. However, Dudek et al^"^ found that the cast CL- 20/TEX/HTPB (32/48/20) formulation undergoes detonation by a shock wave of 5.6 GPa in comparison to that of 10.2 GPa for HMX/TEX/HTPB (32/48/20) formulation. Tarver et al'^'^also found higher shock sensitivity of LX-19 in the wedge and embedded manganin pressure gauge tests. Mezger et al^^ applied a variable confinement cook off test, to determine the response of CL-20 based formulations. In this test the wall thickness was considered as a means of confinement and the sample (2.54 cm in diameter and 6.35 cm long) was placed in to a steel casing of thickness ranging from 0.0381 to 0.19 cm. LX-19 underwent partial detonation with 0.076 cm confinement in contrast to the detonation of LX-14 at 0.19 cm confinement further indicating higher sensitivity of the former. On the other hand, PATHX-3 (90) containing less CL-20 content underwent only deflagration at 0.19cm confinement implying its low vulnerability. The PATHX-3 (94) was found comparable to LX-14 in this regard. CL-20 propellant developed by Mueller^^ exhibited much better friction (16kg) and impact insensitivity (4Nm) than CL-20 alone. Eisele and Menke^°^ reported that despite safety classification of CL-20 propellants (CL-20/ AP/GAP/TMETN/ butanetrioltrinitrate- BTTN) being 1.1, their sensitivity to detonation is lower than that of pressed explosives as well as high energy double-base /composite double-base propellants. 47 1.3.12 Modeling Theoretical modeling is a valuable aspect of synthetic chemistry and enormous work has been undertaken in this direction for CL-20. PoroUo et al adopted combinatorial enumeration as an approach to model thermal decomposition of CL-20 on the basis of recombination reaction networks (RRN) considering every particle (mol/ ion/ radical) as potential reactant. Zhang et al ''^'^'^ predicted the molecular geometries, electronic structures, IR spectra and thermodynamic properties of CL-20 conformations in the temperature range of 298-1000 K by using ab initio and density functional theory (DFT) methods at HF/6-31G* and B3LYP/6-31G* level, respectively. The results obtained are in good agreement with the experimental values. He also carried out theoretical study on pyrolysis initiation reactions of CL- 20 polymorphs in gas phase using quantum-chemical UHF-SCF-PM3 MO method to obtain transition states as well as activation and potential energies. Thome et al'^^ predicted high reactivity for e-CL-20 compared to that of P HMX based on semi-empirical quantum chemical calculations. The semi- empirical AMI method to optimize the geometry of CL-20 and to calculate and analyze the vibration frequency is suggested by Wen et al . The bond- shaping characteristics of the nitramine (N-NOj) groups were analyzed by the NBO and BO methods. Sorescu et al*^^ used isothermal-isobaric molecular simulations and molecular packing calculations to accurately reproduce changes in the crystallographic parameters of CL-20 crystals as functions of pressure for the 48 entire range of investigated pressures. Pivina etal^^'"'^^ reported a retrospective analysis of theoretical approaches to establish the energy content as well as heat of formation of CL-20 and discussed the advantages as well as disadvantages of different calculation methods. They established an approach for the ab initio prediction of crystal structures (by packing optimization) as well as crystal structures of six crystalline polymorphs of CL-20. During their work, chemical structures were generated as graphs, and were subsequently converted to 3-dimensional representations to evaluate CL-20 for practical purposes. Li et al '^"^ applied the PM3 (MO) method to optimize the structure of e-CL-20. Yong et al calculated the combustion performances of CL-20 incorporated CMPB (carboxymethyl-terminated polybutadiene) propellants on the basis of a combustion model for plateau propellants and found that CL-20 was the most effective HEDM in increasing the burning rate as well as reducing the pressure index. The geometric structure as well as heat of formation of CL-20 and its variants were calculated by Wang et al'^^ using AMI program. The most stable structure was identified and the interaction energies of nitro groups were obtained by these researchers. Bohn'^^ discussed the models which consider auto catalytic decomposition and evaporation of minor components in addition to those of the main component. Morphology prediction and 1 Oft simulation of high energy explosives developed by Han et al simulate the dynamic crystal morphology of CL-20. 49 1.4 Objective of the Present Work A review of the science and technology of HEMs brings out that CL-20 will occupy a prominent place among HEMs in coming years. Thereby, it is of interest to continue work on synthesis schemes for generating database. The work in this direction need to focus on establishing well defined reaction parameters to obtain high purity product. The polymorphs of CL-20 are well characterized and it is essential to tailor the method to obtain its relatively more thermally stable and low vulnerable e form. Control on particle size is of significance from point of view of processing of formulation. The present work was undertaken to select the viable synthesis routes and establish the reaction conditions to obtain e CL-20 of high purity. The synthesized compound was characterized by spectroscopic and allied techniques. The results obtained were compared with the reported literature to confirm the structural features of synthesized product. Although CL-20 has been subjected to detailed thermal studies, the temperatures and activation energy values of decomposition process reported by various researchers are at variance. It may be due to difference in synthesis and manufacturing approaches as well as variations in thermal techniques adopted by researchers. A detailed thermal study was undertaken during this work on the compound synthesized and characterized in the laboratory to generate data as well as analyze evidences to understand mechanism of energy release of this HEDM in light of reported literature. Sensitivity aspects, which are of great relevance from handing and processing point of view were also investigated in view of variation in the 50 findings. In order to achieve a combination of improved thermal stability and performance level, variants of CL-20 were also prepared by combining it with TATB and TACOT. The influence of TATB and TACOT on thermal decomposition pattern of CL-20 was investigated during this programme. The resume of work done on CL-20 brings out that research and development efforts are on to realize clean burning high Isp propellant formulations based on it. However a limited information is available on this class of formulations. During the present work, an attempt was made to obtain CL-20 based composite modified double base (CMDB) propellant formulations with Isp exceeding 260s. Effect of selected ballistic modifiers on bummg rate pattern of CL-20 based propellants was also studied as this aspect has not been investigated in detail. Copper chromite (CC) was selected as one of the ballistic modifiers as it was found effective during earlier work on metallized RDX based compositions, which like CL-20 belongs to the nitramine class. Effect of BLS + CU2O -1- C black combination, widely used for double base propellants and Fe203, a commonly used ballistic modifier for composite class of propellants were also assessed. BDNPF/A and low molecular weight GAP were evaluated as co- plasticizers with NG as substitute of inert phthalate plasticizer to optimize energetics. NG-free TMETN-TEGDN plasticized propellants were also investigated with the objective of achieving low vulnerability. Global trends suggest that CL-20 will acquire prominence as high explosive filling for advanced warheads and projectiles in view of its current status as the most powerful explosive (TME). Pressable CL-20 incorporated 51 explosive charges are widely reported particularly as potential replacement of LX-14. Different polymers were evaluated during this work as coating material for CL-20 in order to assess their potential as component of pressed charges. As limited information is available on application of CL-20 based compositions in explosive devices, CL-20 composition filled miniature shaped charges envisaged to have superior penetration power were evaluated during this work to obtain practical data. It is well reported that a HEM may exhibit typical thermal behavoiur in presence of other components in a formulation. 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