Next generation green energetic materials (GEMs)

By Mahadev B. Talawar

Next generation green energetic materials (GEMs)

Green chemistry is a topic of interest that is receiving significant attention in recent years due to the importance of environmental issues and has an important goal of making synthesis methods more environmentally benign. Researchers in the field of chemistry have to practice sustainable development, industrial ecology, cleaner synthesis processes, and life-cycle analysis in addition to the new approaches.

Principles of green chemistry

The importance of use of green processes in any chemical industry greatly contributes effectively in the (a) prevention of waste formation, (b) increased consumption of reactants, (c) utilization of less toxic reactants and solvents, (d) enhancement of atom economy in the reaction, (e) use of ambient reaction conditions (ambient pressure and temperature), (f) recycling of raw material and (g) in avoiding the formation of hazardous materials and minimization of hazards and risks. These green principles will help the chemists to carry out their work in a safer and cleaner way. Further, green chemistry has been extended to help the process chemists, chemical engineers/ technologists/ scientists, to make a process more eco-friendly.

The important factors/goals green chemists need to consider in the area of energetic materials/processes are (i) to minimize life-cycle waste generation during manufacture, testing, proofing, and storage (ii) minimize polluting combustion products generated during the utilization of the ammunitions. About 40% of waste production is generated during manufacture.

The strategy adopted to achieve these goals is multi-pronged: (i) assess the environmental issues associated with high-energy materials (HEMs) to identify major areas of concern, (ii) adopt a paradigm shift in the selection of binders for propellants and explosives by replacing the cast-curable polymers by thermoplastic elastomers, which exhibit good recover, recycle and reuse (R3) characteristics, (iii) use HEMs which are inherently non-polluting, like the high nitrogen containing compounds and those devoid of elements like chlorine, (iv) use solvent less methods of manufacture of gun propellants, (v) demonstrate technologies that utilize liquid or supercritical CO2 and enzymes for the synthesis of HEMs and (vi) bio-degradation of HEMs to alleviate the problem of water and soil contamination by HEMs and the decomposition products.

Eco-friendly oxidizers

Oxidizer is a chemical substance with high oxygen content in its chemical structure. A potential oxidizer should possess a high positive oxygen balance (+O.B.) The important function of an oxidizer in a propellant composition is to provide oxygen for the combustion of the fuel elements (C&H) present in the composition into gaseous products. In addition to positive oxygen balance, oxidizers should possess superior heat of formation, density and thermal stability. Oxidizer is an important ingredient in any of the rocket propellant/pyrotechnic compositions. Ammonium perchlorate (AP) is the work-horse oxidizer used in modern rocket propulsion systems. The most widely used solid rocket propellant for space applications consists of ammonium perchlorate (70%), aluminium (16%) and binder (14%). AP-based propellant system produces chlorine rich combustion products, posing environmental hazards such as ozone depletion and acid rain. Further, the main combustion products of aluminized AP based propellants are hydrochloric acid (HCl), aluminum oxide (A12O3), carbon dioxide (CO2) and water. Moreover, the compounds of aluminium are toxic and are harmful to human beings, animals and plants. Research activities have been undertaken for the search of superior and eco-friendly oxidizer for futuristic solid rocket propellants. Ammonium dinitramide (ADN), hydrazinium nitroformate (HNF) and its derivatives are the recent entrants in this class. The task of developing environment-friendly high-energy propellants is assuming top priority. Hydrazinium nitroformate (HNF) and ammonium dinitramide (ADN) propellants are poised to replace modern AP based composite and composite modified double base (CMDB) propellants in vogue, in the coming decades.

Energetic polymers

Path-breaking research on energetic materials has led to the emergence of novel propellants having unique combination of high energy and low vulnerability. Introduction of exothermically decomposing azido groups or oxygen-rich facile nitro/nitrato groups in prepolymers results in realization of formulations with superior performance. Energetic polymers are polymers, which generally contain energetic groups like the nitro, nitrato, azido, etc.; and their combustion products contain significant amount of nitrogen gas. They give out high energy during combustion and thereby increase the performance of the systems that contain them significantly, and because they contain less of carbon compared to conventional hydrocarbon elastomer binders, they are relatively environment friendly.

Supercritical fluid technology: possible green technology for the 21st century

Supercritical fluids (SCF) possess properties that are intermediate between liquids and gases. They act as very good solvent and differ from conventional solvents. The main difference between supercritical fluids and conventional solvents is their compressibility. Conventional solvents in the liquid phase require very large pressures to change the density, whereas for supercritical fluids very significant changes in density can be achieved with small pressure and/or temperature changes near their critical point. The main candidate in this class is supercritical carbon dioxide (SC-CO2) is now well established as a solvent for use in extraction. This is for a number of reasons. It can generally penetrate a solid sample faster than liquid solvents because of its high diffusion rates, and can rapidly transport dissolved solutes from the sample matrix because of its low viscosity. There are also of course less solvent residues present in the products. Further, it is an attractive alternative against to the halogenated solvents, which pollutes the environment. Supercritical fluid recycling would have both economic and environmental advantages over destructive open burning / open detonation processing. Unfortunately, the nitramine ingredients (RDX, HMX) found in many of military explosives and composite low vulnerability ammunition (LOVA) propellants have insufficient solubility in non-reactive supercritical fluids (e.g., CO2). Search for alternate suitable supercritical solvent and suitable conditions for recovery of HEMs ingredients is underway.

Alternate techniques—green context

An expeditious and solvent-free approach for selective organic synthesis is described which involves simple exposure of neat reactants to microwave (MW) irradiation. The coupling of MW irradiation with the use of catalysts or mineral supported reagents, under solvent-free conditions, provides clean chemical processes with special attributes such as enhanced reaction rates, higher yields, greater selectivity and the ease of manipulation.

Lead-free ballistic modifiers: environmentally more compatible materials

Modern/futuristic missiles demand propulsion systems based on advanced high burning rate propellants to realize higher thrust and reduction in action time. Current solid rocket propellants have reached a plateau in terms of ballistics. Efforts are on all over the world to develop super burn rate (>40mm/s at 7MPa) propellants meeting challenging requirements of tomorrow. New ballistic modifiers are being synthesized for this purpose since lead-free compounds are found to be the preferred choice. The transition metal oxides (TMOs) [106], like ferric oxide – Fe2O3 (FO) and copper chromite – CuCr2O4 (CC) are widely used as ballistic modifiers (BMs) in ammonium perchlorate (AP)-based composite propellants. The ferrocenes are relatively new entrants as BMs for this class of propellants and are claimed to be more effective. However, incorporation of these BMs at the cost of active ingredients, like AP beyond 3% level causes undesirable penalty on the energetics of the propellant due to their inherent non-energetic nature and tend to migrate to the propellant surface if incorporated beyond 3% level resulting in change in pre-programmed ballistic profile, and also form sensitive products on air-oxidation. An intensive search is on all over the globe for non-migrating energetic ballistic modifiers (EBMs) to realize desired combustion characteristics without much penalty on energetics even if higher percentages are need to be added to realize ultra-high burning rates. The important compounds considered for development in the last decade are (i) ferrocene-based polymers like FPGO, (ii) carborane-based compounds like n-hexyl carborane, (iii) metal carbohydrazides like Cu, Ni and Co metal nitrates, (iv) energetic metal salts like NTO metal salts, (v) organometallic compounds and (vi) metal salts of nitro or azido group containing carboxylic acids.

Eco-friendly (lead-free) primary explosives

The conventionally reported initiatory compounds are viz., mercury fulminate, lead azide, lead styphnate and tetrazene. Mercury fulminate and lead azide are the foremost primary explosives, which has gained prominence in the military ammunitions as well as civil applications. Despite being an excellent detonating agent, it suffers from certain inherent drawbacks, such as hydrolytic instability, incompatibility with copper or its alloys (commonly used for encapsulation of primary explosive formulations) and high friction sensitivity. Moreover, its high thermal stability (up to 330 ◦C) is undesirable from the point of view of its initiation by thermal stimuli. Search is on for the potential primary explosives with figure of insensitivity (F of I) >20 which are less prone to initiation from eventual mechanical shock during storage, transport or handling of the finished ammunitions and are stable as well as compatible. Lead- and mercury free co-ordination compounds are the choice of tomorrow in view of their additional advantage of being eco-friendly. Another desirable attribute of this class of compounds is the presence of almost stoichiometric fuel and oxidizer moieties. These compounds may enter in all the spectrum of explosives.

Biodegradation of HEMs

Biodegradation of HEMs is becoming an important aspect in the field. This is because; the HEMs are increasingly becoming a common soil and ground water contaminant. The release of HEMs to environment occurs from various commercial and military activities including manufacturing, waste discharge, testing and training, ordnance demilitarization and open burning/detonation. RDX is a major contaminant because RDX is currently widely used, as it is more powerful and less toxic than TNT. This contamination necessitates cost effective technology such as bioremediation. Research has been centered on the biodegradation and metabolic pathways. Previous reports on the biodegradation and biotransformation of RDX and HMX by a variety of microorganisms (aerobic, anaerobic, and facultative anaerobes) and enzymes have shown that initial N-denitration can lead to ring cleavage and decomposition.

Conclusions

Materials listed in the foregoing sections not only improve the performance by leaps and bounds, but also comply with the evolving, soon-to-be mandatory concepts of green energetic materials (GEM), insensitive munitions (IM) and non-destructive demilitarization. As part of a global initiative on these lines, which includes total replacement/ban of environmentally polluting HEMs like ammonium perchlorate in future, every developing and developed nation would have to follow them strictly. The fact that these materials are generally not traded would put enormous constraint not just on the advancements in the field in any country, but the continuation of the existing, albeit outdated technology too. Therefore, the development of these materials, and establishing an indigenous source for the same, pave the solid foundations for harnessing the much-needed strategic initiative. Thus, the development of these materials has direct relevance and has enormous impact on the environment. Currently green energetic materials are 100 times more expensive to produce than conventional ones. However, authors are hopeful the research and development activities will continue in the area of GEMs all over the world irrespective of cost.

 

This blog article is an excerpt from: Talawar, M. B.; Sivabalan, R.; Mukundan, T.; Muthurajan, H.; Sikder, A. K.; Gandhe, B. R.; Subhananda Rao, A.: Environmentally compatible next generation green energetic materials (GEMs). Journal of Hazardous Materials 161 (2-3) pp. 589-607. 

Mahadev B. Talawar
Group Director

Dr. Talawar is currently group director in the Energetic Materials Research Division of DRDO - High Energy Materials Research Laboratory (HEMRL) in Pune, India