New Directions in the Area of Modern Energetic Polymers: an Overview

ISSN 0010-5082, Combustion, Explosion, and Shock Waves, 2017, Vol. 53, No. 4, pp. 371–387. c Pleiades Publishing, Ltd., 2017. Original Russian Text c D.M. Badgujar, M.B. Talawar, V.E. Zarko, P.P. Mahulikar. New Directions in the Area of Modern Energetic Polymers: An Overview a b D. M. Badgujar ,M.B.Talawar, UDC 536.46 V. E. Zarkoc, and P. P. Mahulikar a Published in Fizika Goreniya i Vzryva, Vol. 53, No. 4, pp. 3–22, July–August, 2017. Original article submitted November 8, 2016. Abstract: Energetic polymers containing nitro, nitrato, and azido groups release high energy during combustion and thereby increase the performance of the systems. A number of energetic polymers have been found suitable for use as binders in high-performance propellant and explosive formulations. This review describes the synthetic and application aspects of various modern energetic polymers for explosive formulations and propellants. Keywords: energetic polymers, thermoplastic elastomers, energetic binders, plastic bonded explosives (PBX), energetic plasticizers, carborane polymers, energetic polyphosphazenes. DOI: 10.1134/S0010508217040013 INTRODUCTION mers, as, e.g., in the manufacture of nitrocellulose from cellulose [6], picryl nitrocellulose from cellulose [7], Energetic polymers are compounds that generally polyvinyl nitrate from polyvinyl alcohol [8] and nitrated contain energetic groups (explosophores) such as the polybutadiene from polybutadiene [9]. nitro-, nitrato-, azido-, etc., and their combustion products contain a significant amount of nitrogen gas. En- ergetic polymers are of interest for use as binders in 1. ENERGETIC POLYMERS propellants and explosives [1]. The binders can be spe- AND PREPOLYMERS cially synthesized polymers containing explosophores or ordinary polymers in combination with energetic plas- Energetic polymers (Table 1) can be used in the ticizers, namely, nitroesters, nitramines, and nitro- and production of low hazard, high power explosives, thus azido-compounds. The use of these binders is aimed at providing reduced solid loading without loss of power. developing high-energy, smokeless, explosion-proof and The main problem is to obtain practically acceptable low-vulnerability composite energetic systems. polymers with proper energy density. Energetic polymers are usually obtained by poly- A number of energetic polymers containing nitrate merizing energetic monomers, e.g., polyNIMMO from ester functionality have received attention for use as 3-nitratomethyl-3-methyloxetane (NIMMO) [2, 3] and binders in high performance propellant and explosive poly-GLYN from 2-nitratomethyloxirane (GLYN) [4, 5] formulations. Nitrated hydroxyterminated polybuta- or by introducing energetic groups into existing poly- diene (NHTPB) [10] is synthesized by epoxidation of aSchool of Chemical Sciences, North Maharashtra the double bonds of HTPB oligomers with peroxyacetic University, Jalgaon, 425001 India; acid [11], followed by ring-opening nitration of the re- mahulikarpp@rediffmail.com. sulting epoxide functionality with dinitrogen pentoxide b High Energy Materials Research Laboratory, in methylene chloride [12]. Nitrated HTPB with 10% Pune, 411021 India. double bonds is a usable prepolymer with a viscosity bVoevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences significantly low enough to permit easy processing and Novosibirsk, 630090 Russia; zarko@kinetics.nsc.ru. high solids loading. Such prepolymers can be cured 0010-5082/17/5304-0371 c 2017 by Pleiades Publishing, Ltd. 371 372 Badgujar et al. Table 1. Structures of energetic polymers [1] Common name Chemical name Structure NHTPB Nitrated HTPB NIMMO (monomer) 3-Nitratomethyl-3-methyl oxetane PolyNIMMO Poly(3-nitratomethyl-3-methyl oxetane) GLYN (monomer) Glycidyl nitrate PolyGLYN Poly(glycidyl nitrate) GAP Glycidyl azide polymer AMMO (monomer) 3-Azidomethyl-3-methyl oxetane PolyAMMO Poly(3-azidomethyl-3-methyl oxetane) BAMO (monomer) 3, 3-Bis-azidomethyl oxetane PolyBAMO Poly(3,3-bis-azidomethyl oxetane) with aliphatic or aromatic diisocyanates to give ener- this process is a viscous liquid with a very low sensi- getic binders. tivity to impact and well suited for use as an energetic NIMMO [13] is synthesized by selective nitration of binder for rocket propellants and plastic bonded explo- the hydroxy group of 3-hydroxymethyl-3-methyloxetane sives (PBXs). with dinitrogen pentoxide in an inert solvent. The Glycidyl nitrate (GLYN) is synthesized [15] in high cationic polymerization of NIMMO using an initiator yield and purity by selective nitration of glycidol with system of boron trifluoride and diol yields the energetic dinitrogen pentoxide in an inert solvent. The cationic polymer polyNIMMO [14]. PolyNIMMO generated by polymerization of glycidyl nitrate is more difficult than New Directions in the Area of Modern Energetic Polymers: An Overview 373 the polymerization of NIMMO and requires a strong in the area of extruded composite rocket propellants as mineral acid such as tetrafluoroboric acid. The prod- well as pressed/sheet explosives [30]. uct, polyGLYN [16], is a low-molecular-weight hydroxy- Prepolymers are used in energetic formulations terminated prepolymer which reacts with diisocyanates such as cast cured explosives and propellants to give to give energetic polyurethane polymers [17]. Millar mechanical resistance, to increase the stability to im- and co-workers [18] have reported on an improved pro- pact, friction and heat, as well as to provide encapsu- cess for producing poly GLYN which is well suited for lation of crystalline energetic crystals. Energetic pre- use as an oligomer in solid high-energy compositions. polymers also contribute to the amount of released Millar and co-workers [19] have also described the energy. Due to its good performance [31], GAP is use of low-molecular-weight polyGLYN as an energetic a widely used prepolymer that contains azido groups. plasticizer. In this context, polyGLYN has a number Poly (3-azidomethyl-3-methyl oxetane) (PAMMO) and of advantages over traditional nitrate ester plasticizers, poly(3,3-bis-azidomethyl oxetane) (PBAMO) are other including: low volatility, low glass transition tempera- prepolymers with azido groups [32, 33]. Heterocyclic ture (−40◦C), excellent miscibility with the binder and compounds have higher predicted thermal and shock decreased plasticizer mobility. On the basis of perfor- stability and density than carboxylic compounds [34]. mance and the ease with which polyGLYN is prepared The higher nitrogen content also leads to lower oxy- via dinitrogen pentoxide technology, it seems likely that gen requirements in energetic formulations [35]. Under it will prove to be a world leader in the field of ener- conditions of high temperature and pressure, it gives getic polymers. It has been shown [20] that propellants polycyclic systems, which are known to be burning rate with polyGLYN and polyNIMMO binders have a higher modifiers [36]. The compound 2-oxy-4,6-dinitramine- specific impulse than modified composite double-base s-triazine (DNAM) significantly increases the burning systems (≈260 s). rate of PSAN/HTPB formulations, maintaining good Recent interest has turned to nitrated cyclodextrin shock stability properties [37]. The energetic prepoly- polymers (polyCDN) for potential use in insensitive and mers containing the 1,3,5-s-tritriazine ring and azido minimum smoke producing propellants [21]. The syn- groups in their structure were synthesized by the reac- thesis, purification and characterization of the following tion of inert precursors with sodium azide. polymers has been studied in detail [22]: (1) r-cyclodextrin polymer cross-linked with 1.1. Melamine/Epichlorohydrin 1-chloro-2,3-epoxypropane, Based Prepolymers (2) r-cyclodextrin polymer polymer cross-linked with 4,4-methylene-bis (phenyl isocyanate). The reaction of melamine with ethylene oxide or Prominent among the energetic groups is the glycidol (2,3-epoxy-1-propanol) was patented in 1945 azido group [23], which has a heat of decomposi- [38]. The basic catalysts NaOH, KOH, Ca(OH)2 or tion of ≈355 kJ per one N3 group. The feasibility pyridine were used. Subsequently, polyol with at of using low-molecular-weight azido compounds such least two hydroxyl groups was used to produce similar as 1,7-diazido-2,4,6-trinitrazaheptane and 1,3-diazido- compounds. Sodium and potassium methoxides were ◦ 2-nitrazapropane in solid propellants has widened the also used as catalysts at temperatures of 150–200 C. scope of application of azide compounds beyond the Kucharski et al. [40] studied the reaction between class of initiators [24]. It was logical to extend the scope melamine and ethylene oxide or propylene oxide (PO) of azide containing molecules further to the area of poly- in N,N-dimethylformamide (DMF), using tetrabutylam- mers. The first polymer to be developed in this category monium hydroxide (TBAH) as catalyst. Lubczak et was glycidyl azide polymer (GAP), which came into al. [41] used dimethyl sulfoxide (DMSO) as solvent. prominence during the early 1990s. Subsequently, a se- Mixtures of different products were found, revealing un- ries of poly azido oxetanes emerged on the scenario [25]. equal side branches of the triazine ring. It is well known Azido polymers, particularly glycidyl azide poly- that melamine derivatives also react with oxiranes via mer (GAP) [26] and co-polymers of bisazidomethylox- anionic polymerization, with the catalyst TBAH [42, etane (BAMO) [27], have entered the domain of ad- 43], triethylamine,

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