Recent advances in new oxidizers for solid rocket propulsion

Djalal Trache *a, Thomas M. Klapötke *b, Lotfi Maiz a, Mohamed Abd-Elghany b and Luigi T. DeLuca cd
aUER Procédés Energétiques, Ecole Militaire Polytechnique, BP 17, Bordj El Bahri, 16111, Algiers, Algeria. E-mail:
bEnergetic Materials Research, Department of Chemistry, Ludwig-Maximilian University, Munich Butenandtstrasse 5-13 (D), D-81377 Munich, Germany. E-mail:; Fax: +213 21863204; Tel: +213 661808275
cDepartment of Aerospace Science and Technology (Retired Professor), Politecnico di Milano, 20156, Milan, Italy
dSchool of Chemical Engineering (Visiting Professor), Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China

Received 27th June 2017 , Accepted 4th August 2017

First published on 22nd August 2017

Ammonium perchlorate (AP), the workhorse of oxidizers in solid rocket and missile propellants, exhibits various environmental issues resulting from the release of perchlorate into ground water, which have been directly linked to thyroid cancer. Furthermore, the generation of hydrochloric acid causes the depletion of the ozone layer and leads to high concentrations of acid rain. Nowadays, considerable efforts have been devoted to developing solid propellants using green oxidizers which demonstrate less hazards and environmentally friendly chlorine free combustion products. Although many candidates for AP replacement have been identified, most of them are far from being practically employed in real applications because of a number of severe difficulties, including cost. In this review, the potential green chemicals for use as oxidizers are highlighted and these reveal interesting physicochemical properties and performance. After a quick definition of green solid propellants and their main ingredients, the current status of AP propellants issues is discussed in light of possible substitution with potential green ingredients. Particular attention will be paid to the recent advances in the green oxidizers, their production and their characteristic properties. The advantages and shortcomings of various green oxidizers for specific and potential propellant uses are also discussed together with the attempts made to overcome these problems. As a consequence, efforts will certainly continue to seek AP alternatives and efficient green oxidizers for solid rocket propulsion in the near future.

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Djalal Trache

Djalal Trache has been working as an Associate Professor at the Ecole Militaire Polytechnique (EMP), Algeria, since 2016. He received his Engineer degree in chemical engineering, his Magister's in applied Chemistry and Doctor of Sciences in chemistry at EMP. He has made several presentations at national and international conferences and published many research papers and one book chapter. He is a reviewer of more than 18 international respected journals. Prof. Trache has particular expertise in energetic materials, bio-based materials and their characterization. He also has interests in nanomaterials and their applications, phase equilibria and kinetics. He has also successfully supervised many engineers, MSc and Doctoral students.

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Thomas M. Klapötke

Since 1997 Klapötke has held the Chair of Inorganic Chemistry at LMU Munich. In 2014 he was appointed an Adjunct Professor at the University of Rhode Island. Klapötke is a Fellow of the RSC, a member of the ACS, a member of the GDCh, and a Life Member of both the IPS and the National Defense Industrial Association. Most of Klapötke's scientific collaborations are between LMU and ARL in Aberdeen, MD and ARDEC in Picatinny, NJ. Klapötke also collaborates with MTC in Cairo, Egypt. He is the executive editor of ZAAC and has published over 750 papers, 30 book chapters and 10 books.

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Lotfi Maiz

Dr Lotfi Maiz obtained a PhD degree in chemistry from the Faculty of Chemistry and New Technology in the Military University of Technology, Warsaw, Poland. He took part in a project developing the new RDX-based composite, and Thermobaric and Enhanced Blast explosives. His research includes the synthesis and the formulation of explosives, the physics of explosions, and the experimental characterization of explosives. He won awards at the NTREM conference in 2015 and at the International Armament Conference on Scientific Aspects of Armament and Safety Technology in 2016.

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Mohamed Abd-Elghany

Mohamed Abd-Elghany obtained his BSc and MSc (Chemical Engineering) in the field of explosives and rocket propellants from the Military Technical College (MTC), Cairo, Egypt in 2009 and 2015, respectively. He is currently studying his PhD at Ludwig-Maximilians Universität München (LMU). His main areas of research include synthesis and characterization of high energy materials, development of insensitive ammunition and high performance plastic explosives, investigation of modern guns and green rocket propellants, ageing and life extension of propellants, and investigation of the thermal behavior and decomposition kinetics of high energy materials.

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Luigi T. DeLuca

Luigi T. DeLuca obtained his PhD in Aerospace and Mechanical Sciences from Princeton University, Princeton, NJ under the supervision of Prof. Martin Summerfield. He founded the Space Propulsion Lab (SPLab) at Politecnico di Milano, Milan, Italy in 1976. Currently, Dr DeLuca is a Retired Professor from the Politecnico di Milano and a Visiting Professor at Nanjing University of Science & Technology, Nanjing, China. He was National Representative for the NATO Propulsion and Power Systems and Propulsion and Energetics Panel. He directed several international research efforts dealing with energetic materials, is member of the Editorial Board of many scientific journals, and organized Workshops (IWCP) attended by the most qualified scientists.

1. Introduction

Solid rocket motors (SRMs) are distinguished from other kinds of propulsion systems (nuclear, electric and radiant) by the fact that their propellants are stored in solid form as a mixture of ingredients, which eject hot gases at high speeds to deliver a payload (e.g., satellite, warhead). Fundamentals and historical overviews on SRMs have been reported elsewhere.1–6 Solid propellants find a wide range of applications in tactical rockets, intercontinental- and submarine-based ballistic missiles, space launcher boosters, airplane ejection seats, and even amateur hobby rockets, to name a few. They are preferred over liquid and hybrid propellants, because of their reliability, simplicity, ready-to-use system availability, lower cost of propulsion system and compactness.7,8 Solid rocket propellants typically fall into one of two broad categories namely, homogenous (double-base) propellants, which contain their oxidizer and fuel in the same molecule, and heterogeneous (composite) propellants, which consist of mechanical mixtures of separate ingredients. These energetic materials liberate their energy through a slow deflagration processes, and it may take up to several seconds to reach complete combustion.9,10 The present review will be limited to numerous types of ingredients used as oxidizers and their effect on the performance and environmental concerns of the composite solid propellant formulations.

Composite propellants, based on ammonium perchlorate (AP, NH4ClO4) and aluminum (Al), have been extensively employed for both military and civilian applications for more than 60 years.7 However, they specifically pose various environmental issues in three main areas: (i) ground-based impacts ranging from groundwater contamination to accidents caused by inopportune processing and handling of propellants, (ii) atmospheric impacts broadly coming from the interaction of the exhaust combustion products with the surrounding atmosphere including the ozone layer, and (iii) biological impacts that encompass toxicity and corrosiveness of propellants.11–13 For example, AP is thought to affect the function of the thyroid gland. AP-based propellants contaminate the atmosphere by releasing hydrochloric acid (HCl) as an exhaust product. The launch of a large space vehicle engenders about 580 tons of HCl in addition to heavy toxic metal oxides, which pollute the stratosphere including land and water sources. The launch of six Titan class vehicles can lead to ozone depletion of the order of 0.024% and stratospheric acidic rain of the order of 0.01%.8 Also, although the compounds of aluminum, for example, aluminium oxide (Al2O3), are not considered harmful, their release as small particles as an Aerosol may present a potential toxic effect to humans, animals and plants.14

Researchers are exploring several approaches to fulfill the increasing requirements of solid rocket propulsion systems that impart enhanced performance, improved mechanical properties, prolonged life span, less vulnerability, and negligible environmental effects during manufacture, processing, handling, transport, storage, usage and disposal. Efforts are being made all over the world to develop modern/futuristic propellants meeting the previously mentioned challenges. In developing new propulsion systems, optimal compromises are often sought that are achievable through synthesis or producing new compounds,15–18 modifying or combining known compounds,19,20 testing of new formulations,21–23 experimental characterization and precise theoretical evaluation.24,25 The development of green energetic materials for propulsion purposes is an emerging area of materials chemistry stimulated by a worldwide need to substitute the propellants currently used, because of the environmental considerations and safety requirements, while at the same time ensuring high performance. This new generation of energetic materials has to meet various standards in order to become largely accepted. In addition to the performance characteristics, other desired criteria are stability, compatibility, reasonable cost, favorable classification hazards as well as the new or adapted technologies to be mastered to bring such green materials into use in ‘real’ applications.

Green chemistry has been widely used in several industrial sectors such as aerospace, energy, automobile, pharmaceutical, cosmetic, electronics, and the propulsion field.11,26–29 A more fundamental definition of green chemistry involves reducing or eliminating the utilization or generation of hazardous substances in the design, manufacture, and application of chemical products.30 It is worth noting, however, that the new class of eco-friendly advanced solid propellants is not totally clean, because its constituent compounds can engender an environmental effect in one way or other.8,31 Nevertheless, a green propellant is viewed as an energetic composition that seeks to minimize or mitigate the environmental and toxicological hazards associated with currently used materials.

Significant advances have been made in the synthesis, production, characterization and development of green energetic compounds for their employment in solid propellant formulations. Fig. 1 shows that the investigations on green energetic materials for solid rocket propulsion are increasing with a greater number of scientific papers being published in this field. Although many green propellant formulations have been tested, most of them are far from being practically usable in the near future because of a number of difficulties, including cost considerations, and consequently further effort is needed to produce mature green propulsion systems. Currently work is continuing worldwide to overcome the existing problems and to find reasonable solutions. Several reviews,14,30–36 books6,11,13,15 and patents37,38 have been published in the last two decades covering numerous aspects related to green energetic materials for solid rocket propulsion, including, synthesis procedures, theoretical evaluation, characterization, propellant formulation, processing and testing. However, the focus of the present paper differs from the published literature and where suitable, specific points covered in the published literature are summarized and/or referenced to the relevant paper/patent/book. This review paper firstly provides an overview of the green chemistry principles followed by the main ingredients used in composite solid propellants showing the potential green substances that could be substituted for the current ones. Furthermore, a critical and analytical examination is offered of the advantages and disadvantages of the various oxidizers developed so far.

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Fig. 1 Illustration of the annual number of scientific publications since 2006, using the search terms “solid propellant and green propellant”. Data analysis completed using Scopus search system in June 2017.

2. Green chemistry and propulsion

The concept of green chemistry is not new, and dates back in the 1960s, but it has only received urgent attention over the last 25 years, and this continues today, with the focus on minimizing the environmental impact of manufacturing processes through the control of products, energy and wastes. The 12 principles of green chemistry, as described by Anastas and Eghbali,29 provide useful context to highlight self-evident challenges related to the chemical production and utilization, where several sectors of industry are associated encompassing various scientific disciplines.28,29,39 These principles demonstrate a multidimensional matrix to guide the manufacturing process and the design of individual components, and to make a process eco-friendlier. They will help the chemical engineers/technologists/scientists carry out their work in a safer and cleaner manner.14 The essential factors that should be considered in the area of energetic materials for propulsion systems are: (i) the substitution of the current energetic materials that generate polluting combustion substances with others, which exhibit green characteristics and comparable performance, (ii) the development of safer and cleaner methods for synthesis of substances, (e.g., utilization of supercritical carbon dioxide (CO2), enzymatic procedures, microwaves, ionic liquids, ultrasound) or formulation manufacturing (e.g., solventless methods, clean solvents), (iii) efficient monitoring of the life-cycle during manufacturing, storage, testing and disposal, and (iv) reduction of costs to determine whether the processes can be commercialized.7,11,14,31,32,34,36,40,41 The development of green propellants is under a significant cost pressure, because it is problematical for emergent products to compete with the relatively low cost of well-developed conventional formulations. Currently, green energetic materials are 100 times more expensive to produce than conventional ones.14 Therefore, green energetic materials for propulsion purposes require further government and other external support to succeed and address all of the previously mentioned challenges. Perhaps this review paper can convey part of this process by providing an overview of some of the challenges in this domain and their potential solutions.

It is worth noting that recent research efforts have been made towards environmentally friendly and non-toxic composite solid propellants. Nonetheless, adopting green energetic materials for solid propulsion does not mean that these propellants are totally clean and without any impacts on the environment, because the combustion of the so-called green propellants generate exhausts which may typically encompass alumina, nitrogen oxides, water vapor, CO2, inorganic chlorine, sulfates, and soot.8,42 Thus, these green formulations do impact on the environment but at a relatively lower level. Recently, many activities have been conducted through several projects worldwide to develop green propellants. Various candidates such as green composite solid rocket propellant ingredients have appeared and continue to be developed in different laboratories and research centers all over the world.

3. Composite solid rocket propellant formulations

Propellants are considered the most influential factor in the design of rockets, missiles and launch vehicles. These energetic materials generate mainly hot gaseous products ejected at high velocity from the gas dynamic nozzle to produce forward thrust to the vehicles. Solid rocket propellants consist, in one form or another, of a blend of fuels and oxidizers with some structural rigidity.2,5,6 They are prepared as a slurry, and are commonly cast and cured into the motor as a solid mass known as the grain. The engineered geometry of the grain is a crucial parameter that determines the thrust level and profile of the motor.

Modern rockets and missiles broadly employ composite propellants which are essentially made up of an oxygen-rich solid oxidizer (65%–90%) that provides oxygen (O2) for oxidation purposes, an organic polymer that serves as both binder and gas fuming combustible (8%–15%), and a metal fuel (10%–20%) that generates additional thermal energy to increase the propellant performance.35,43,44 In addition to these primary ingredients, other minor substances such as plasticizers, crosslinking agents, curing catalysts, antioxidants, bonding agents, process aids, and burning rate catalysts are added to the propellant formulation. The following sections will especially focus on composite solid propellants oxidizers. They will give detail on substitutes for traditional toxic compounds such as AP. Recent advances on green ingredients used as new propellant oxidizers are also presented and discussed.

4. Green oxidizers

The main oxidizer that has been consistently utilized in all rocketry until now is AP.45,46 Because of its oxygen balance of 34%, high density, high thermal stability, low sensitivity to shock, good compatibility and long shelf-life, this oxidizer is used for applications in amateur rocketry, airbag inflators, aircraft injection seats and pyrotechnic devices such as warning flares.6,10,47 The greatest benefit of using AP is the huge experience and widely available information on AP-based propellants collected over many decades, which gives a sound confidence in this ingredient.48–51 Unfortunately, this low cost salt has some toxic issues, especially when considering its good solubility. Perchlorate anions (ClO4) have been detected in drinking water supplies throughout the south-western United States, where it is mistakenly taken up in place of iodide leading to dysfunction and affecting both growth and development of humans and animals. Furthermore, amphibians’ normal pigmentation and growth is altered by the exposure to AP.6 This oxidizer might be also toxic to various marine life forms. Further problems are generated by AP-based solid propellants during their combustion. For example, the burning of the space shuttle boosters produces a huge amount of exhaust products containing mainly HCl and other compounds which are highly toxic and corrosive in nature. It is estimated that each flight of the Ariane 5 space launcher liberates about 270 tons of concentrated HCl as well as alumina, thereby polluting the atmosphere and causing ozone depletion in the stratosphere. Acid rains can be caused by this enormous quantity of HCl emission.10,35 Additionally, for military applications, the smoke trail caused by AP is a very serious tactical disadvantage, because it adversely affects guidance and control systems.

Currently, AP has no suitable alternatives, this is why intensive efforts have been devoted and continue to be made to produce eco-friendly propellants with a reduced component of such pollutants or altogether free from them. The most important benefit of developing chlorine-free propellants is that they eradicate smoke formed by the condensation of atmospheric water vapor and the exhaust plume, and avoid the creation of a visible signature plume (Fig. 2). Several promising candidates as oxidizers to substitute for AP have been developed such as phase-stabilized ammonium nitrate (PSAN), ammonium dinitramide (ADN), hydrazinium nitroformate (HNF), hexanitrohexaazaisowurtzitane (HNIW or CL-20), some molecules containing the trinitromethyl functionality or fluorodinitromethyl derivatives, polynitro-substituted pyrazoles and triazoles, polynitroazoles, tetrazole derivatives, carbamate derivatives and tetranitroacetimidic acid.6,41,52–54 These chlorine-free alternatives, which at present are being tested, can overcome most of the previously mentioned shortcomings, but at the same time bring up new challenges as it will be described later in the paper. Some properties of conventional and advanced oxidizers, which have been investigated in the present review, are given in Table 1.

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Fig. 2 Smokeless and smoky exhaust products of solid propellant during combustion. Reprinted from ref. 8 with permission. Copyright © 2017, Springer International Publishing, Switzerland.
Table 1 Properties of some green and conventional oxidizers from various sources
Oxidizera Chemical formula Oxygen balance (%) Molar mass (g mole−1) Density (g cm−3) ΔHf (kJ mol−1) Environment impact Reference
a The acronyms are identified in the respective sections in the text.
AP NH4ClO4 +34.00 117.50 1.95 −295.8 ClHNO DeLuca et al. (2013)91
HNF N2H4HC(NO2)3 +25.00 183.00 1.86 −71.0 CHNO
AN NH4NO3 +20.00 80.04 1.73 −367.5 HNO Chaturvedi and Dave (2013)57
ADN NH4N(NO2)2 +25.80 124.10 1.81 −134.6 HNO DeLuca (2016)1
CL-20 (NNO2)6(CH)6 −10.90 438.20 2.04 +372.0 CHNO
FOX-7 C2H4N4O4 −21.6 148.10 1.88 −134.0 CHNO Axthammer et al. (2017);118 Krause (2006)158
TNAA C2HN5O9 +30.00 239.10 1.87 −322.6 CHNO Vo et al. (2014)53
NTNAA C2HN5O9 +30.00 239.10 2.03 −415.3 CHNO Zhang and Gong (2016)131
TNENCA C3H3N5O10 +32.70 269.08 1.73 +343.9 CHNO Axthammer et al. (2014)148
TNEF C7H7N9O21 +30.40 553.18 1.81 −519 CHNO Abd-Elghany et al. (2017)151
BTNEO C6H4N6O16 +30.80 416.12 1.84 −688.0 CHNO Abd-Elghany et al. (2017)151
TKX-50 C2H8N10O4 −27.10 236.15 1.92 +446.6 CHNO Fischer et al. (2012)159
HADNMNT C2H8N8O7 +6.35 256.15 1.87 +299.4 CHNO Fan et al. (2017)153
DNDNT C2N18O8 +15.84 404.00 1.95 +1210.0 CHNO Keshavarz et al. (2017)152
TTBTE C2H2N20O8 +11.06 434.00 1.92 +1274.0 CHNO
ANNPA C5H4N11O10 +14.85 378.23 1.82 +491.7 CHNO Yin et al. (2014)52
DNNPDA C5H6N9O10 +11.40 354.57 1.81 +124.1 CHNO
DNPDN C3H4N6O4 −8.51 188.04 1.82 +173.0 CHNO
NNTAA C4H5N8O8 +10.95 292.00 1.79 +160.6 CHNO

4.1. Phase-stabilized ammonium nitrate (PSAN)

Ammonium nitrate (AN, NH4NO3) is one of the most important ammonium chemicals in the agricultural and chemical industries.10,55,56 It has been widely used as a fertilizer component and as an industrial explosive ingredient, as well as an oxidizer for solid propellants, gas generator systems and emergency starters because of its low cost, availability, chemical stability, low sensitivity to friction and impact, plus, it releases almost 100% gaseous products during decomposition, and has a positive oxygen balance.54,55 Although it plays the role of a source of ammonia and nitrate ion vital to plants in the form of nitrogen fertilizer, in industrial explosives and propellants the nitrate ion is considered as a source of oxygen.57 AN is broadly produced by the neutralization reaction of synthetic ammonia and nitric acid (HNO3), followed by evaporation to the melt that is subsequently treated by a prilling process or a granulator to generate the commercial product in the form of prills (pellets or granules).58 These high density prills are frequently used in the fertilizer industry. However, their low liquid absorption means they cannot be used in energetic material compositions.59 Subsequently, Kim et al. have developed a process, for manufacturing spherical AN particles with a uniform distribution, which is the melt spray.60 The AN particles are considered to be better for energetic material compositions because of their morphology, surface roughness, uniformity and particle size.

Much has been published about the physicochemical properties of AN, its thermal decomposition, its coating, the effect of additives, its applications, its disasters and the different challenges associated with its practical uses. For a more complete view of this green energetic oxidizer, there are many excellent review papers57,58,61–63 and books10,55,56 that have been published in recent years. In this section, this will not be repeated but the most important aspects applicable to AN as oxidizer for solid rocket propellants will be mentioned. However, this review will concentrate on providing recent progress in using AN as a replacement for AP in solid propellant formulations.

In spite of its hygroscopic nature, low performance, low burning rate, and the near room temperature polymorphic transitions involving a volume change, AN is actually considered as one of the most attractive oxidizers.20,54,64–66 Recently, there has been renewed and growing interest in developing smokeless, chlorine-free, and environmentally benign propellants based on AN, because much progress has been achieved in surmounting the previously mentioned shortcomings. The hygroscopicity of AN has been recognized as the main cause for caking and has been considered as the most serious obstacle for its utilization in solid propellants. Many researchers have suggested various surface modification methodologies based on successful coating to decrease this hygroscopicity. These methods can be physical, chemical or encapsulating coatings (Table 2), and have been described well in the recent reviews by Elzaki and Zhang,61 and Jos and Mathew.54 Another major drawback of AN is the presence of temperature dependent phases at atmospheric pressure that are characterized by continuously more motion freedom of the NH4+ and NO3 ions. The phase transformation which occurs around room temperature can be accompanied by a substantial volume contraction and expansion which gives rise to undesirable crack formation in the propellant grains. AN shows at least five polymorphic transitions below its melting point at atmospheric pressure (Fig. 3).67 Among the phase transformations of AN, IV–III phase transition happens at ambient temperature and is followed by a volume change of about 3.8%. Therefore, many research activities have been devoted to preventing such phenomenon. It is commonly achieved by adding organic or mineral modifiers into the AN crystal lattice. For example, diamine complexes of transition metals, alkali metal salts, potassium salts and magnesium nitrate, potassium ferrocyanide, copper nitrate, potassium nitrate (KNO3), poly(vinyl pyrrolidine), poly(ethylene oxide), poly(acrylamide), trioxy purine and crown ethers have been extensively used to phase-stabilize AN.10,54,58,64 Phase stabilization of AN with metal oxides is demonstrated to be beneficial with respect to burning rate, ignition, and hygroscopicity.57 Furthermore, in spite of the stability of AN at ambient temperature, a small amount of ammonia can be evolved, leaving the salt slightly acidic.57 It is worth noting that the thermal decomposition of AN strongly depends on temperature, pressure, sample purity, state of confinement, monitoring techniques, and amount of additives, and experimental conditions such as sample size, sample mass and heating rate.54,68 It is noticeable that no simple mechanism can be used to elucidate all of the aspects of its decomposition features. It is broadly accepted that the thermal decomposition of AN is initiated by an endothermic proton transfer reaction, followed by an exothermic reaction at around 200–230 °C.57 Other reactions may be undergone under different conditions. During decomposition, several products may appear such as water (H2O), nitrogen (N2), nitrous oxide (N2O) and HNO3. Other minor by-products could be detected such as nitric oxide (NO) and nitrogen dioxide (NO2).54 Recently, Cagnina et al. studied the gas phase decomposition mechanism of AN using CBS-QB3 ab initio calculations.69Scheme 1 displays the mechanism of the formation of decomposition products H2O, N2, O2, OH, HNO and NO3. The authors proposed, as a first step, the dissociation of AN into ammonia and HNO3. It was suggested that the hemolytic breaking of the NO bond in HNO3 engendered hydroxyl radicals and NO. The reaction between the hydroxyl radical and ammonia generated an amidogen radical. The successive reaction of the amidogen radical with NO led to the final decomposition products of AN. Other detailed mechanisms, kinetics and the thermal decomposition of AN, and the effect of different additives have been widely investigated, and some comprehensive reviews have been written.57,58,63

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Fig. 3 (a) Phase transitions of ammonium nitrate. Reprinted from ref. 155 with permission. Copyright © 2017, Springer Science; (b) low-temperature differential scanning calorimetry identification-test thermograms of ammonium nitrate, showing the different phase transitions. Reprinted from ref. 67 with permission. Copyright © 2013, Elsevier Limited.

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Scheme 1 Reaction paths for the decomposition of ammonium nitrate. Reprinted from ref. 69 with permission © 2013, The Royal Society of Chemistry.
Table 2 Advantages and drawbacks of different coating methods of AN61
Method Advantages Disadvantages
Physical coating (1) Simple. (1) Uses a large amount of coating agent.
(2) Convenient and easy to manufacture. (2) Thickness of coating layer not easy to control.
(3) Improves the stability of the particles. (3) Large difference of interfacial tension between the surface coating layer and the polarity of AN.
(4) Safety.
(5) Enhances the compatibility of particles with other materials.
Chemical coating (1) Small dosage of coating agent. (1) The surfactant and coupling agent have low molecular weight.
(2) Strong binding force. (2) The surfactant has a small solubility in water.
(3) The hydrophobic group makes the thin layer on the surface which prevents hygroscopicity. (3) Low hygroscopicity properties.
Encapsulation coating (1) Improves the physical properties of coated particles on the surface. (1) The brittle polymer is susceptible to cracking during the drying process.
(2) Protects the particles from external moisture. (2) There are sticky polymer adhesives which are not dispersed.
(3) Polymer hygroscopicity was zero. (3) Low polarity of polymer makes it difficult to stick on the surface of AN particles.
(4) Coating layer is thin.

In order to surmount the low reactivity and low energetics of AN in a propellant formulation, different approaches have been adopted. The first concerns the incorporation of additives to enhance the thermal decomposition of AN. Some inorganic salts such as chromium nitrate, iron nitrate, aluminum nitrate and iron salts have been revealed to promote the thermal decomposition of AN because of the high charge to radius ratio of the metal ions.70 Some monometallic catalysts such as platinum (Pt), copper (Cu), zinc (Zn) and iron (Fe) supported on silica doped alumina shifted the endothermic decomposition of AN into exothermic decomposition.71 Other transition metal oxides such as MOx (M = manganese (Mn), cobalt (Co), nickel (Ni), Cu) acted on the endothermic decomposition as well.54 Some nanocatalysts such as nano titanium dioxide (TiO2) and nano copper oxide (CuO) have been tested by Vargeese's group.64,66,72 The incorporation of TiO2 to AN led to the decrease of the activation energy and a plausible mechanism for the catalyzed AN is depicted in Fig. 4a. The reaction starts with the dissociation of AN (step 1) followed by the adsorption of ammonia (NH3) on TiO2 (step 2). The dissociation of HNO3 produced from AN generates OH and NO2 (step 3), and that subsequently dissociates to NO and O2 (step 4), which interacts with TiO2 as well (step 5). Like TiO2, the addition of nano CuO into AN also reduced the activation energy of the thermal decomposition. As shown in Fig. 4b, the authors demonstrated that the CuO nanorods provide Lewis acid and/or active metal sites, enabling the elimination of AN decomposition inhibition species such as NH3 and thereby improve the rate of decomposition. More recently, some nanocomposites such as CuO or copper iron oxide (CuFe2O4) anchored on graphene oxide (GO) sheets have been tested as catalysts.73 It was shown that the decomposition temperature and the activation energy were notably decreased when CuO/GO was added to AN, whereas no synergetic effect was found when CuFe2O4/GO was added. However, the effect of carbonaceous materials on the thermal decomposition of AN has been widely investigated. Lurie and Lianshen deduced that incorporation of carbon black into AN, augmented the AN decomposition rate intensely.74 It is caused by the reduction of HNO3, produced from the dissociation of AN, to HNO2 by the carbon black and the following reactions generate N2. Recently, Atamanov et al. have revealed that the addition of 5 mass% of dextran (acting as a catalyst) reduces the activation energy of AN to 64 kJ mol−1.75

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Fig. 4 (a) Mechanism of catalytic decomposition of ammonium nitrate. Reprinted from ref. 72 with permission. Copyright © 2011, Elsevier Limited; (b) the possible mechanism of the adsorbed ammonia surface reactions. Reprinted from ref. 64 with permission. Copyright © 2012, Elsevier Limited.

The second approach consists of the use of dual oxidizers to increase the performance of AN-based propellants. Chaturvedi and Dave57 have shown that AN-based composite propellants are attractive because of the clean burning and smokeless exhaust. However, this propellant presents some drawbacks such as poor ignition and low burning rate. AP-based composite propellants, however, have outstanding ignition and burning characteristics, although the combustion gases contain HCl. It was anticipated that an AN/AP dual oxidizer-based propellant would have an acceptable performance for practical applications because each oxidizer would compensate for the flaws of each other.57 In another work, propellant formulations using double-oxidizers such as (PSAN + AP)/hydroxyl terminated polybutadiene (HTPB)/Al (40 + 28)/14/18, with PSAN in turn including 5% phase stabilizer, were also tested by DeLuca's group.76 The measured burning rates were in the range of 5 to 8 mm s−1 at 7 MPa, with a pressure sensitivity n = 0.58–0.64. Recently, Kohga and Handa65,77 studied the thermal decomposition behaviors and burning rate characteristics of composite propellants prepared using combined AP/AN particles. They tested two methods to combine both oxidizers (physical and freeze-drying) before propellant formulation. They deduced that the burning characteristics of the propellants produced with the combined AP/AN samples varied from those of the propellants manufactured by physically mixing AP and AN particles. The burning characteristics of some of the propellants produced by physically mixing AP and AN particles exhibited unsteady combustion, whereas the propellants manufactured with the combined AP/AN samples burned steadily. The use of the combined AP/AN particles reduced the heterogeneity of the combustion wave of an AP/AN propellant. In a separate work, Kumar et al.20 prepared a mixture of AN and potassium dinitramide (KDN) using a co-crystallization method. The authors revealed that KDN presents an excellent phase stabilizing effect on AN and has a positive effect on the burning characteristics. The thermal analyses of different co-crystals of AN/KDN have shown that the ratio 50/50 is the best one, because the KDN plays a double role as both a phase stabilizer and an energy enhancer. The morphology of the differently prepared propellants is shown in Fig. 5. It is clear that the morphology differs from one propellant to another, because the composition is different. As depicted in Fig. 6, the combustion characteristics were affected as well. It is revealed that the propellant AN/KDN (50/50)/HTPB/catalyst (copper–cobalt based metal oxides, Cu–Co*) shows the highest burning rate with an acceptable pressure index (n) value of 0.746, compared to other formulations. The authors concluded that the propellant containing AN/KDN (50/50) is the most promising green formulation.

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Fig. 5 Scanning electron microscopy images of: (a) pure AN propellant, (b) AN + Cu–Co* propellant, (c) AN/KDN (75/25) + CuO propellant, (d) AN/KDN (75/25) + Cu–Co* propellant, (e) AN/KDN (50/50) + CuO propellant, and (f) AN/KDN (50/50) + Cu–Co* propellant. Reprinted from ref. 20 with permission. Copyright © 2016, Elsevier Limited.

image file: c7gc01928a-f6.tif
Fig. 6 Burning rate vs. pressure for different propellant samples. Reprinted from ref. 20 with permission. Copyright © 2016, Elsevier Limited.

To avoid redundancy, other propellant formulations based on AN as oxidizer have been recently discussed and reviewed,54 where several fillers [cyclohexamethylene trinitramine, cyclotetramethylene tetranitramine, HNIW], binders [polytetrahydrofuran, poly(3,3-bis(azydomethyl) oxetane)], plasticizers [nitroglycerine, 1,2,4-butanetriol trinitrate, and trimethylolethane trinitrate (TMETN)] and catalysts (Fe2O3, Cr2O3, MnO2, SiO2, PbC, CuC, potassium dichromate, ammonium dichromate, transition metal [Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II)] salts of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one, aminoguanidinium 5,5′-azobis-1H-tetrazolate, and triamino guanidine nitrate) have been tested. It was consequently concluded that in spite of the fact that much progress has been achieved in the AN-propellant formulations studies, further endeavor should be undertaken in future research to overcome the remaining shortcomings and offer efficient propulsion systems using AN.

4.2. Ammonium dinitramide (ADN)

Another promising, relatively new, candidate considered as a replacement for hazardous AP in solid rocket propellant is the ammonium dinitramide [ADN, NH4N(NO2)2].6,10 This dinitramide was first synthesized in the Soviet Union in the early 1970s, where it was tested in various missile programs, and it has been developed in the western world since the 1990s.56,78,79 Its synthesis has been extensively studied and an overview of the different methods has been given by Venkatachalam et al.80 These authors deduced that the most practical procedure for scaling up is that invented by Langlet et al. (Scheme 2), which is based on the direct nitration of ammonia sulfate derivatives using an ordinary sulfo-nitric acid mixture followed by reaction with NH3.160 The current commercially available ADN from Eurenco Bofors in Sweden is produced using this method developed and patented by FOI.78,81 More recently, Kim et al. have used potassium sulfamate as an ADN precursor and have proved that the prepared ADN has a high purity of 99.2% with a high reaction yield of 57.2%.82 However, the commonly resulting crystals of crude ADN have low purity, high cost, irregular morphology and a high aspect ratio with some agglomerates, which can complicate the processing.6,81,83,84 Such issues make ADN unsuitable for propellant formulation and the feasibility of the compositions are greatly compromised because of the large increase in viscosity as well as the high loading rates that are envisaged. In this sense, two methods stand out in re-shaping the ADN crystals, which are spray crystallization and prilling in suspension (Fig. 7).83,85 The disadvantage of the first method is the use of molten ADN which is known to be unstable above its melting point, whereas the second method presents a technical complexity in order for it to be scaled-up. To overcome the previously mentioned drawbacks and improve the properties of the final ADN crystals, some researchers have recently patented two other crystallization methods.84,86 These methods are not based on converting the crude crystals obtained, but either on crystallization in the presence of an added chemical element (crystal modifier) or on crystallization in solution with controlling nucleation and crystal growth in a high viscosity solvent. It is shown that the first method based on the modified crystallization is a simple, easily comparable operation whose implementation does not necessitate exceptional equipment and does not exhibit any particular pyrotechnic hazard. Furthermore, it can be performed in inexpensive and non-toxic solvents. This crystallization is revealed to be much more valuable than the prilling process. On the other hand, it is demonstrated by the second approach that the crystals obtained present a low shape factor of 1 to 1.5 and are perfectly suitable for use in energetic material formulations.
image file: c7gc01928a-s2.tif
Scheme 2 Synthesis of ammonium dinitramide. Reprinted from ref. 80 with permission © 2004, John Wiley and Sons.

image file: c7gc01928a-f7.tif
Fig. 7 Principal steps of the ADN-prilling process. Reprinted from ref. 85 with permission © 2009, John Wiley and Sons.

ADN has attracted the attention of researchers, as a solid rocket propellant or liquid monopropellant oxidizer, for many reasons.13 It combines a positive oxygen balance, high enthalpy of formation, high burning rate, does not evolve chlorine or mimic iodide, clean burning properties, low signature combustion and does not show any phase transition like AN or density modification under temperature stress.87 Nevertheless, ADN exhibits a moderate thermal stability and high hygroscopicity. It is chemically reactive with some curing systems and this can be problematic, and it does not exhibit simple ballistic control. Consequently, extensive research has been devoted to understanding its behavior and possibly correct such problems. Benazet and Jacob84 demonstrated that less hygroscopic ADN crystals can be obtained by optimizing and improving the crystallization process. In another work, Ting et al.88 have successfully applied an alumina coating on the surface of ADN using an atomic layer deposition technology in order to build a water molecule diffusion barrier layer on the surface and improve its stability in humid air. However, the thermal behavior and combustion of ADN have not been adequately elucidated because of the numerous and complex phenomena that can occur. Subsequently, several pieces of research have been conducted and currently continue to be done in various laboratories all over the world.

Recently, Ermolin and Fomin79 have published a comprehensive review on the mechanisms of thermal decomposition of ADN. It was demonstrated that ADN can decompose in either the liquid or solid phase. It was reported that ADN decomposition occurring in the liquid phase proceeds through two paths, where the initial steps are monomolecular decomposition of the anion over the N–NO2 bond and equilibrium dissociation of the salt into the acid and base. The second path, however, occurs at 100 °C. In contrast, the path of salt decomposition in the solid phase proceeds through its dissociation into the acid and base. It was shown that the monomolecular decomposition of the anion into NO3 and N2O occurs at a higher rate than in the melt process. In addition to being thermally labile, ADN is also light sensitive.56 Furthermore, it was found that the synthesis method and conditions may certainly affect the ADN structure and its physicochemical properties. Matsunaga et al.89 have also studied the thermal decomposition behavior of ADN under pressurized conditions. The pressure differential scanning calorimetry (PDSC) curves of ADN at each pressure value are displayed in Fig. 8a. An endothermic peak was observed at around 92 °C and two exothermic events appeared between 135 °C and 220 °C at each pressure. The melting temperature reported in the literature ranges from 83 °C to 95 °C.56 This large variation in the published values is most likely to be from impurities, which can significantly affect the thermal behavior of ADN even at very low concentrations. The first exothermic peak became more significant with the pressure increase. Raman analysis revealed the formation of AN during the decomposition. The authors found that the AN inhibited the decomposition of ADN at a low decomposition temperature, and contributed to the reaction at high temperature. In a related work, a similar research group has recently investigated the thermal decomposition of ADN using simultaneous thermogravimetry-differential thermal analysis-mass spectrometry-infrared TG-DTAMS-IR spectroscopy.90Fig. 8b shows the evolution of gas and the thermal behavior, obtained using the simultaneous analyses. They have shown that the main evolved gases were H2O, N2, N2O and NH3, and they indicated that the activation energies decreased with increasing progress of the reaction, and the decomposition of ADN exhibited an autocatalytic behavior. The proposed mechanism of the decomposition reaction of molten ADN is shown in Scheme 3. This intricate decomposition of ADN confirmed the reported results of the complicated combustion behavior of its energetic compositions as discussed and summarized by DeLuca et al.76,91 However, a severe chemical instability can be caused by the aggressive oxidizing ability of ADN. The low symmetry structure of the dinitramide anion is one of the main reasons for the reactivity and the instability of ADN. However, AP is nearly not reactive because of the high symmetry and low energy of the tetrahedral structure of the perchlorate anion.6 Thus, different reactions and compatibility behavior of ADN with other substances that could be added to propellant formulations should be well understood. Also, some stabilizers can be added to ADN to improve its chemical stability.56,78

image file: c7gc01928a-f8.tif
Fig. 8 (a) PDSC of ADN. Reprinted from ref. 89 with permission. Copyright © 2014, Springer Science; (b) TG-DTA-MS curves for ADN at a heating rate of 4 k min−1. Reprinted from ref. 90 with permission. Copyright © 2017, Springer Science; (c) DSC measurement of ADN at a heating rate of 0.5 k min−1. Reprinted from ref. 156 with permission. Copyright © 1997, Elsevier Limited.

image file: c7gc01928a-s3.tif
Scheme 3 Reaction mechanism of the decomposition behavior of molten ADN. Reprinted from ref. 90 with permission. Copyright © 2017, Springer Science.

Broadly, ADN does not attack C–H or C–C single bonds, and displays good compatibility with compounds having double-bonded carbon.78 However, ADN exhibits severe compatibility issues with isocyanates, and it easily reacts and decomposes in their presence.92 Thus, the polymers that undergo polyurethane bonding (Scheme 4) can negatively affect the compatibility and the chemical stability of the ADN-based energetic composition.93 Therefore, two main approaches have been adopted to avoid such a problem.

image file: c7gc01928a-s4.tif
Scheme 4 Overview of curing mechanisms, curing systems, and curing agents for glycidyl azide polymer. Reprinted from ref. 96 with permission © 2015, John Wiley and Sons.

The first one consists of the use of polymeric coating materials to increase the compatibility of ADN with the common curing agents used in most binder systems. Several microencapsulating and coating processes have been developed since the first work of Green and Schleicher in 1953.56 The technological procedures are widely used in nearly all industrial and commercial fields. Teipel et al.78,94 have employed a conservation process according to the core–shell principle. They have utilized ethylcellulose and cellulose acetobutyrate (CAB) as coacervate capsules (Fig. 9). They revealed that the choice of materials is important for a successful microencapsulation process and showed that non-polar organic solvents were suitable for water soluble cores such as ADN and AN. A few years later, Heintz et al.85 developed a coating method based on fluidized bed technology. They demonstrated that the use of polyacrylate, glycidyl azide polymer (GAP) and HTPB were leading to increased compatibility of ADN. The second approach, which consists of the use of free-isocyanate curing systems, was the most explored pathway. The motivation of this approach was not only to improve the compatibility of ADN in a propellant formulation, but to overcome other flaws as well. Isocyanates are both moisture sensitive and hazardous. They react with moisture to liberate CO2 and form voids in the cured propellant, leading to poor mechanical properties during storage. In systems including energetic nitrate ester plasticizers, isocyanates generate toxic nitroso derivatives. Some of potential replacements for isocyanate-based curing agents that have been reported are: bispropargylhydroquinone (BPHQ), bisphenol A bis(propargyl ether) (BABE), bis-propargylsuccinate (BPS), 1,4-bis(1-hydroxypropargyl)benzene (BHPB), and other bis(propargyl) aromatic esters and ethers.92,93,95,96

image file: c7gc01928a-f9.tif
Fig. 9 Spherical ADN particles in CAB-containing coacervate capsules. Reprinted from ref. 56 with permission © 2006, John Wiley and Sons; ADN-Prills coated with 5% polyacrylate/silane. Reprinted from ref. 85 with permission © 2009, John Wiley and Sons.

The development of ADN-based green propellant was driven by civilian requirements for environmental respect in space propulsion systems and by military needs for high performance and minimum smoke propellant in tactical missile applications. Several formulations have been tested to satisfy a number of critical tasks such as compatibility, performance, curing, mechanical properties and stability. Flon et al.97 have evaluated the substitution of AP by ADN in solid rocket propellants, containing HTPB as binder and Al as fuel, for large space launch boosters. The results from the performance computations revealed that, by replacing AP with ADN, the theoretical specific impulse increases by 3% and the combustion temperature decreases by 4%. The authors deduced the presence of a little reactivity between ADN and HTPB, thus the use of such a formulation needs improvement. To further improve the ADN-based propellant performance, some researchers suggested the utilization of GAP which is an energetic binder, to compensate for the lower oxygen balance of ADN (+25.8%), with respect to AP (+34.04%). Thermodynamic calculations of the theoretical gravimetric specific impulse under frozen equilibrium assumption have been reported for the systems ADN/GAP/Al and AP/HTPB/Al. It was noticed that the gravimetric specific impulse of the system ADN/GAP/Al features higher values with a maximum of 296 s at 59% ADN, 20% GAP, and 21% Al when compared to the system AP/HTPB/Al (maximum 284 s at 68% AP, 12% HTPB and 20% Al). Even more notable is the fact that the compositions with higher specific impulses are in the region of larger binder contents of 20%–30% instead of 10%–20% for AP/HTPB/Al and this will permit the manufacture of such formulations with better mechanical properties. The combustion behavior of a propellant formulation containing ADN/GAP filled with 16% of Al was investigated by Weiser et al.98 They demonstrated that the combustion of such propellant obeys Vieille's law with a pressure exponent of 0.58 and a multiplication factor of 8.82 mm s−1. That is quite high for practical applications and thus further efforts are needed to decrease the absolute burning rate. Cerri et al.99 have studied the aging behavior of several ADN/GAP-based propellant formulations. They revealed that the ADN/GAP-based formulations show evidence of a high porosity of the propellants and strong dewetting phenomena, as shown in Fig. 10. Also, the dynamic mechanical analysis measurements revealed a high glass transition of 40 °C to 50 °C, which is higher than the ones of the current HTPB/AP/Al formulations. Thus, it was concluded that they cannot fulfill the NATO specification for the very wide in-service temperature range of −54 °C to +71 °C, which means that intensive efforts should be focused to address such problems. Wingborg100 has recently substituted 1-diamino-2,2-dinitroethene (FOX-7, C2H4N4O4) and guanylurea dinitramide (FOX-12) for ADN to decrease the sensitivity of the ADN/GAP propellant. It was concluded that the amount of FOX should be kept below 30% in order to obtain a reasonable pressure exponent. In a separate work, propellant formulations containing a combination of dual oxidizers AN (coated with KNO3)/ADN with GAP and HTPB binders have recently been tested.101 It was shown that varying the ratio of the ADN/AN oxidizer mixture, the burning rate of the aluminized propellant can be tuned. For GAP-based propellants, the increase of the content of AN led to a decrease of the burning rate and the impact sensitivity, whereas an increase of the pressure exponent to unacceptable values was found for the HTPB-based propellant. It was noted that further research is required to solve some compatibility issues as well.

image file: c7gc01928a-f10.tif
Fig. 10 Scanning electron microscopy analysis of the ADN-based propellant after the tensile test. Reprinted from ref. 99 with permission © 2014, John Wiley and Sons.

It was reported that any binder normally used in solid rocket propellants could be used allowing for the fact that the curing agent needs to be chemically compatible with ADN. Thus, the utilization of non-isocyanate curing agents is of great interest. Thus, an alternative methodology suggested to exploit the 1,3-dipolar cycloaddition reaction (Huisgen reaction) between the azide group of a new generation of energetic binders and triple bond of alkynes forming 1,2,3-triazoles.92 This is considered as a versatile tool in polymer chemistry for forming crosslinked networks without any side reaction, and thus is a prime example of Click chemistry.102 In addition to GAP, several other binders can be employed in ADN-based propellants such as poly(3-nitratomethyl-3-methyloxetane) [poly(NiMMO)], poly(glycidyl nitrate) [poly(GLyN)], poly(3,3 bis(azidomethyl)oxetane) (poly(BAMO)) and poly(azidomethyl methyl oxetane) [poly(AMMO)].78 Consequently, this kind of ADN/binder-propellant may yield enhanced performance in terms of specific impulse as well as producing clean environmentally acceptable combustion products.14

4.3. Hydrazinium nitroformate (HNF)

A potential eco-friendly oxidizer, hydrazinium nitroformate (HNF, N2H5C(NO2)3) has entered the field of advanced propulsion systems some decades ago.103,104 Although HNF has a relatively lower oxygen balance with respect to AP, it has a noticeably superior heat of formation leading to a higher specific impulse.105 Additionally, it undergoes an intense exothermic combustion reaction near the burning surface of the HNF-based propellant, giving rise to an effective heat feedback which augments the burning rate.14 This energetic material has further benefits over AP, such as clean combustion, a low signature, non-hygroscopic nature, high density, and easy method of synthesis.35 Furthermore, the melting point of HNF lies in the range of 115–124 °C depending on its purity and is suitable for processing propellant formations, because the curing process is commonly performed at high temperature. In Europe, HNF was actively produced in the Netherlands in a pilot plant that had a maximum capacity of 300 kg per year.10 Today, it is mainly India and China who continue its production.1 HNF is reported to have been discovered in 1952. The synthesis of HNF involves a two-step process with an acid–base reaction of nitroform (NF) and hydrazine, as shown in Scheme 5.10 NF is the key starting material for the preparation of HNF.104 According to Joo and Min,106 Hantzsch was considered as the pioneer in the production of NF by the nitration reaction of acetic anhydride to tetranitromethane, followed by its conversion to NF using sulfuric acid (H2SO4) and potassium hydroxide. Several other procedures for the synthesis of NF have been developed by the use of a number of substrates, comprising acetylene, acetone, isopropanol, pyrimidine-4,6-diol, acetic anhydride, and cucurbit[6]turil (CB[6]).106–108 But several production technology methods had to be abandoned because of the environmentally unfriendly substances, high costs and many explosions occurring during the production process.108 The only efficient procedure for large-scale manufacturing is the reaction of isopropanol and HNO3. Recently, the reaction conditions were optimized and a NF yield of 53.6% was obtained from isopropanol by Ding et al.109 More recently, Yan et al.108 have synthesized NF using acetylacetone as substrate and fuming HNO3 with acetic acid as the nitrating system. This new synthesis route for NF is expected to be an alternative method for the industrial production of the first step process of HNF manufacturing. This method is considered inexpensive, has mild reaction conditions with a satisfactory yield. Recent different procedures of NF synthesis are shown in Scheme 6.
image file: c7gc01928a-s5.tif
Scheme 5 Synthesis procedure of hydrazinium nitroformate. Reprinted from ref. 109 with permission © 2014, The American Chemical Society.

image file: c7gc01928a-s6.tif
Scheme 6 (a) Synthetic routes for the synthesis of nitroform using different substrates. Reprinted from ref. 157 with permission © 2014, The American Chemical Society; (b) reaction mechanism for the synthesis of nitroform from acetylacetone. Reprinted from ref. 108 with permission © 2016, The American Chemical Society.

However, despite the progress in the last few decades, there are various unresolved issues concerning the thermal stability, and friction and impact sensitivity of HNF. Furthermore, the use of the hydrazinium cation may be critical because of the eventual liberation of highly cancerogenic hydrazine as a consequence of thermal stress or alkaline reaction conditions, but the latter is free of chlorine, thus the combustion reaction is considered to be clean. The purity of the HNF produced is also crucial because the presence of solvents or impurities leads to serious safety problems during handling, transport and storage.6,10,35 Recently, several production procedures have been optimized to overcome this drawback.108–110 The incorporation of stabilizers will probably be required to improve the thermal stability of HNF as well.10 However, it has long been considered that the main problem of HNF was its sensitivity. The large length to diameter ratio (L/D) of HNF is expected to be the reason for its high sensitivity. Several methodologies have been tested to decrease the L/D of HNF crystals, but no important progress was achieved using simple crystallization procedures. Recent research, by several researchers, has developed the industrial manufacturing of HNF crystals, using advanced crystallization or coating methods.109–111 Athar et al.,111 for example, have successfully desensitized HNF by changing its crystal size, shape and coating it with nanocomposites. The authors have employed a number of methods such as mechanical stirring, ultrasound and using crystal shape modifiers. The optimized conditions generate a preferential axial crystal growth, where the long needles with sharp edges and corners, which have a very high L/D and high impact as well as friction sensitivity, have been transformed to near cubic shape crystals with a lower L/D and improved sensitivity, as shown in Fig. 11. They reported that the best coating agent was the hydroxyl-terminated poly(butadiene)-based clay nanocomposites. In 2014, Ding et al. synthesized HNF using NF derived from isopropanol.109 The morphology of HNF crystals was modified by a number of crystallization procedures to decrease the sensitivity of HNF toward environmental stimulus. The morphology of HNF crystals obtained using different test methods is displayed in Fig. 12. The solvent/non-solvent (S/NS) crystallization using methanol/dichloromethane displayed crystals with small L/D ratio with gentle edges and corners. However, the obtained value of L/D was still comparable with that reported previously. The utilization of sono-crystallization produced promising L/D values, whereas sharp edges and corners still persisted. However, the best L/D value was, provided using the sequential cooling crystallization method leading to uniform crystals with soft edges and corners. The synthesized HNF using the latter procedure exhibited lower sensitivity toward friction and impact.

image file: c7gc01928a-f11.tif
Fig. 11 (a) Virgin HNF crystals with sharp edges and corners and very high L/D ∼ 6.0, friction/impact sensitivity of 2.0 kg/25 cm; (b) Modified HNF crystals with rounded edges and lower L/D ∼ 2.0. Friction/impact sensitivity of 4.0 kg/28 cm. Reprinted from ref. 111 with permission © 2010, John Wiley and Sons.

image file: c7gc01928a-f12.tif
Fig. 12 HNF crystals from: (a) antisolvent crystallization using S/NS, acetonitrile/dichloromethane; (b) sono-crystallization; (c) sequential cooling crystallization. Reprinted from ref. 109 with permission © 2014, The American Chemical Society.

Another flaw of HNF, that has been recently resolved, is its compatibility with the most extreme binder which is commercially available and used, which is HTPB. The existence of the carbon double bond in the HTPB backbone was reported to be the main cause of the HNF/HTPB incompatibility, because the carbon–carbon double bonds are easily oxidized by HNF leading to a decrease of the mechanical properties of the binder.104 This incompatibility can also be caused by the presence of isocyanates, used as crosslinking agents, where a hydrogen transfer from HNF to the nitrogen of the isocyanate group –N[double bond, length as m-dash]C[double bond, length as m-dash]O can occur.10 A recent patent by Deppert et al.38 showed a new approach to desensitize HNF and to improve its compatibility with HTPB and its curing isocyanate agents. HNF particles have been dispersed in a polymeric binder and there is a bonding agent that plays the role a of Lewis acid to form an encapsulating film on at least a portion of the HNF particle surfaces. A bonding agent is a component of propellant formulation that improves processing, mechanical properties, safety, ballistic and stability characteristics, eliminates voids and micro porosity, and enables higher solids loading.9,10,43 Several bonding agents can be used, such as boron-based compounds, halides, some metals, enone compounds, and any monomer or polymer containing an atom or group that acts as Lewis acid. The bonding agent is added to HNF in an amount of 0.1 to 1.0 mass%. The molecular mass of the binder that holds the two components (HNF and bonding agent) can be in the range between about 600 and 3000 g mol−1. Additional additives can be incorporated into the coated HNF to improve its properties such as other fillers [e.g., cyclotrimethylene trinitramine (RDX)], fuels (e.g., Al), stabilizers (e.g., diphenylamine), and processing aids (e.g., catalysts).10,112–114 Briefly, the hydrazinium cation (N2H5+), of the HNF salt, that has a nitrogen atom with a lone pair of electrons can play the role of a Lewis base. Accordingly, in the presence of a Lewis acid, this hydrazinium cation of the HNF salt will donate a pair of electrons to form a Lewis adduct. The procedure developed leads to the chemical reaction of the bonding agent with the surface of the HNF and during the curing step, the bonded oxidizer and other compounds, if present, will react with a polymeric binder to produce the required propellant formulation without compatibility problems.38 In another piece of research, Sonawane et al.95 have tested a new isocyanate-free curing agent (BPHQ) for GAP and investigated the compatibility of HNF with this curing agent. It was found that the isocyanate-free curing system was more suitable in a chlorine-free composite solid propellant formulation, and the BPHQ showed good compatibility with HNF.

Unlike HTPB, several pieces of research have shown that HNF is compatible with the recent developed binders such as GAP, poly(NiMMO), poly(GLyN), polynitromethyloxetane (PLN) and poly(BAMO). Potential benefits can be obtained when using this oxidizer, because it not only gives high performance, but also produces an environmentally benign exhaust as the gases released during combustion are free from chlorine.6,10,14,115 The friction and impact sensitivity of HNF-based propellants are acceptable with respect to other formulations being used presently.10 An overview of the HNF-based propellant formulations is summarized by Dendage et al. in their review article.104 Propellant formulations based on HNF demonstrated a relatively high burning rate (30 mm s−1 at 7 MPa). For non-catalyzed HNF-based propellants, the pressure exponent n ranged between 0.81–1.12.35,115 The n value can be decreased to more acceptable values, i.e., 0.4 < n < 0.6, by using suitable ballistic modifiers or by reducing the mean size of the HNF.10,116 It is predicted that by utilizing HNF-based propellants, an increase in specific impulse of more than 7% and a payload capacity gain of 10% can be reached.6,35

4.4. FOX-7 and its derivatives

Some of the new energetic materials, which have been prepared during the last two decades, have led to new possibilities not only for military applications but also for civilian ones, because of environmental considerations and safety requirements while at the same time securing high performance.15,17,117 FOX-7, a relatively new high energetic material, presents high thermal stability, high performance, low sensitivity, high heat of formation, favorable oxygen balance, high density, clean decomposition products and good compatibility with oxidizers, polymers, plasticizers and isocyanates.118–120 The synthesis procedures of FOX-7 (Scheme 7), together with its structural, spectroscopic, thermal and explosive properties, have been thoroughly reviewed.119–122 This compound can be used as an insensitive high energy density material because its performance is comparable to the common secondary explosives such as cyclotetramethylene tetranitramine (HMX), RDX and pentaerythritol tetranitrate (PETN). The typical structural features of FOX-7 are established by alternating amino and nitro groups in the solid state.
image file: c7gc01928a-s7.tif
Scheme 7 Typical methods used to synthesize FOX-7. Reprinted from ref. 122 with permission © 2016, The Royal Society of Chemistry.

Since 1998, when FOX-7 was first synthesized by Latypov et al.,123 it has caused a great deal of interest and has been considered as one of the potential candidates to be used in solid propellants. Florczak124 performed thermodynamic calculations and studied the thermochemical and ballistic properties of aluminized composite propellants containing AP/Al/binder with and without FOX-7. The author concluded that the incorporation of FOX-7 instead of AP led to a decreased heat of combustion, burning temperature, specific impulse and burning rate of the propellant. In a separate work, Chen et al.125 investigated the properties of some propellant formulations based on HTPB/FOX-7 using DSC and a sensitivity test apparatus. They showed that the apparent activation energy of FOX-7 propellant was about 245.2 kJ mol−1 and the friction sensitivity was less than 68% and the impact sensitivity was over 25.0 J. Compared with RDX propellant formulations, mechanical sensitivities and electrostatic discharges of HTPB/FOX-7 significantly decreased. Recently, Lempert et al.126 have theoretically compared the effect of FOX-7 and HMX on the properties of AP/Al/binder (inert or active) propellant formulations. They reported that the FOX-7-based composites containing an inert binder (C73.17H120) had energy characteristics which were too low, as would be expected in view of the low oxygen content of FOX-7. They demonstrated that the values of specific impulse of propellant formulations with FOX-7 were lower than those of composites with HMX by 10 s and 4 s at levels of 60% FOX-7 and 30% HMX in the composite, respectively. They concluded, however, that a solid composite propellant formulation, with a specific impulse of 251 s, density of 1.91 g cm−3, and burning temperature of 3600 K, can be created using 60% FOX-7 and 19% of an active binder (C18.96H34.64N19.16O29.32). In another work, Lips et al.127 successfully prepared FOX-7/AP/GAP propellants with 68% to 70% solid loading including 20% to 42% of AP and TMETN/1,2,4-butandiole trinitrate (BTTN) nitrate ester plasticizers. The prepared formulations exhibited the highest thermodynamic performance with a high burn rate up to 20 mm s−1, interesting mechanical properties, convenient chemical stability, low thermal sensitivity, and less toxic combustion products. More recently, Jensen et al.128 showed that FOX-7 is an attractive, but less than ideal, substitute for nitramine in smokeless GAP-RDX composite rocket propellants that exhibit low shock sensitivity and good mechanical properties.

However, because of the abundance of FOX-7 chemical reactivity through acid–base reactions, coordination reactions, reduction reactions, oxidation reactions, acetylate reactions, nucleophilic substitution reactions and electrophilic halogenation reactions, more than 130 derivative compounds of this energetic material have been published (Scheme 8). Recent reviews have comprehensively collected all of these reactions.119,121,122,129,130 In spite of the numerous reactions involving FOX-7 described in the past ten years, new reactions continue to be found.118,129,131 Derivatives of FOX-7 were not known to act as oxidizers before 2013, and the discovery of this behavior presents a new episode in the chemistry of FOX-7. While these chemicals are not proposed as potential alternatives for current propellant oxidizers, this is a valuable discovery in the chemical life and behavior of FOX-7.132 Therefore, finding new oxidizers with desirable properties is needed so that they can be substituted for the currently reported ones (AP, PSAN, ADN, HNF), which are limited in numbers and present many drawbacks in practical applications.

image file: c7gc01928a-s8.tif
Scheme 8 Selection of FOX-7 derivatives. Reprinted from ref. 121 with permission © 2015, John Wiley and Sons.

Incorporating further nitro groups is a practical strategy to produce energetic FOX-7 derivatives. Nitro groups are essential chemical groups of high energy density materials and their presence in molecules contributes significantly to the overall energetic performance and enhances the density and oxygen balance of energetic materials. The Shreeve group proposed a new oxidizer, tetranitroacetimidic acid (TNAA, Scheme 9, 42).53,122 It was demonstrated that TNAA is a very attractive and promising replacement for AP. Its melting point of 91 °C is comparable to that of ADN (93 °C) and its decomposition temperature of 137 °C is higher than that of HNF (131 °C). Despite its lower thermal stability and friction sensitivity with respect to AP, it presents comparable properties and acceptable stability to that of ADN and HNF, and it can be added to propellant formulations. Compared to AP, TNAA has a considerably enhanced oxygen and nitrogen content and higher positive oxygen balance because of the presence of the trinitromethyl group. A similar research group pointed out that the calculated value of the specific impulse of the formulation of HTPB/TNAA/Al (12/68/20) was 261 s. Another very promising candidate compound to be considered as a replacement for AP is tetranitroacetamide (NTNAA).131 It was demonstrated by a computational study that the synthesis of NTNAA from TNAA is thermodynamically possible. Zhang and Gong revealed that NTNAA presents properties similar to TNAA, and consequently it could be a potential replacement for AP as oxidizer in composite propellants.131

image file: c7gc01928a-s9.tif
Scheme 9 Polynitro derivatives of FOX-7. Reprinted from ref. 122 with permission © 2016, The Royal Society of Chemistry.
Other high energy dense oxidizers (HEDOs). The challenge in the development of new HEDOs or improvement of those studied during the last few years is to seek a good compromise between performance and physicochemical properties, in this case between high specific impulse and high oxygen balance on the one hand and acceptable thermal stability and low sensitivity on the other hand, in addition to low cost, simplicity of synthesis, and low hazards.

HNIW or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-tetracyclo[5,5,0,03.11,05.9]dodecane, commonly known as CL-20, is regarded as one of the HEDOs for the next generation of propellants.35,133–140 It was found to be eco-friendlier as well as to reduce the propellant exhaust plume signature without generating combustion stability problems. Under ambient conditions, CL-20 has four polymorphs, α, β, γ, and ε, as shown in Fig. 13.134 These different polymorphs lead to various physicochemical features such as thermal stability, sensitivity, density, and performance, which govern its application. Thermodynamically, the ε-phase is the most thermodynamically stable and is considered as the favored phase for propulsion applications because of its high density (2.04 g cm−3).

image file: c7gc01928a-f13.tif
Fig. 13 (a) Ball-and-stick model of ε-CL-20; (b) the four polymorphs of CL-20. Adapted from ref. 134 with permission © 2016, The Royal Society of Chemistry.

The synthesis of CL-20 is considered to be one of the most complicated chemical procedures. Several papers have been published demonstrating various procedures involving multiple steps and methodologies for the synthesis.133,141 However, HNIW has high impact and friction sensitivity as well as high production costs. Consequently, controlling the crystal density, decreasing the sensitivity and the production cost have received much attention in order to produce a promising candidate CL-20 for several energetic applications. In addition to a review by Nair et al.,133 another interesting recent review paper by Viswanath et al.141 summarizes different synthesis strategies to produce CL-20 with reasonable properties and its characterization. The commonly known methods are based on the same starting material, hexabenzylhexaazaisowurtzitane (HBIW).142 Nevertheless, conversion of HBIW directly to CL-20 is a major challenge. A rather low yield of HNIW and high costs of the implemented nitronium tetrafluoroborate (NO2BF4) and nitrosonium tetrafluoroborate (NOBF4) catalysts necessitate improvements of the HNIW synthesis method.1 More recently, Simakova and Parmon142 developed a new approach based on the two-step HBIW debenzylation with separately repeated use of a palladium-based catalyst in each catalytic stage. This approach is considered as a promising way to increase the catalyst productivity and to reduce the CL-20 production costs. In a separate work, Zhang et al.143 developed a coating agent of paraffin wax/molybdenum disulfite as a desensitizer of ε-CL-20. They demonstrated that this coating process generated a greatly lower friction and impact sensitivity of ε-CL-20. All these improvements would certainly increase the number of its applications as an AP substitute for solid rocket propulsion.

Several groups worldwide have intensively investigated other potential chemicals to substitute the current widely used oxidizer (AP). Such compounds include azide, azo, nitro, amino functionalities on triazine, azole, furazano, carbamite backbones to name a few.15,17,117,144 Some of the most promising classes of materials were poly-nitro moiety compounds. One of the most well-known groups in this field is that of Klapötke, who investigated several classes of materials such as orthocarbonates, 2,5-disubstituted tetrazoles, bi-1,2,4-oxadiazoles, carbamites and nitrocarbamites (Scheme 10).17,145–147 These different compounds demonstrated several advantages but also flaws in their suitability for use for propulsion purposes and as replacements for AP. Based on experiments and computing results, it was shown that propellants based on some of the synthesized chemicals broadly exceed the performance of the AP-based ones, but fail to meet other requirements such as low sensitivity or thermal stability. More recently, the Klapötke group revealed that 2,2,2-trinitroethyl nitrocarbamite (TNENCA) prepared using a simple synthesis procedure can be a potential replacement of AP.148 This nitrocarbamite presents a melting temperature of 109 °C which is higher than that of ADN (93 °C) and a decomposition temperature of 153 °C which is higher than that of HNF (131 °C). Although it has a lower thermal stability and friction sensitivity when compared to AP, it exhibits comparable properties and acceptable stability to those of ADN and HNF. Furthermore, TNENCA has a very positive high oxygen balance and presents a specific impulse comparable to a composition using AP. Advantageously, the burning of TNENCA with aluminum produces no toxic substances such as hydrogen chloride. However, it is worth noting that the main issue of this new oxidizer is its synthesis procedure that involves a toxic synthesis step using phosgene.148 The same group has developed a new synthesis procedure to overcome this flaw.145 Thus, a less hazardous synthesis route for 2,2,2-trinitroethyl carbamate, used to produce TNENCA, is performed using chlorosulfonyl isocyanate in a one-step synthesis with a yield of 96% compared to the previously toxic procedure (71%).

image file: c7gc01928a-s10.tif
Scheme 10 Selected compounds synthesized by Klapötke group (a) 5,5′-bis-(trinitromethyl)-3,3′-bi-(1,2,4-oxadiazole), (b) 1-(trinitroethylamino)tetrazole, (c) 2,2,2-trinitroethyl nitrocarbamite, (d) trinitroethane.

Klapötke's group also revealed two different HEDOs that can replace AP in the homogenous and heterogeneous solid rocket propellants which are bis(2,2,2-trinitroethyl) oxalate (BTNEO) and 2,2,2-trinitroethyl formate (TNEF).149,150 These new high energy dense oxidizers “oxalate and formate” have shown very good properties as green oxidizers for solid rocket propellants with melting temperatures of 115 °C and 127 °C, respectively, and decomposition temperatures of 186 °C and 192 °C, respectively. This means that they can be used in the production of solid propellants using the casting method because of the difference between their melting and decomposition temperatures, which is near 70 °C. Although they are perform less well than AP in the thermal and impact sensitivity, BTNEO showed a higher value than AP in the friction sensitivity test.149 In the same way, oxalate and formate oxidizers have a high density, which are 1.84 g cm−3 and 1.81 g cm−3, respectively, with a high positive oxygen balance. Also the specific impulse of these oxidizers which are 231 s and 228 s, respectively, are higher than that of AP. The synthesis of these oxidizers is easy (Schemes 11 and 12) and also their decomposition is eco-friendly. All these four different HEDOs showed clear, homogenous, smokeless burning with high burning rates when added to nitrocellulose (NC) as an oxidizer (Fig. 14) instead of the extremely dangerous and sensitive nitroglycerine.151 Klapötke's group synthesized these different HEDOs and fully characterized them. New formulations of green, solid rocket propellants based on those oxidizers with different fuel binders are currently under study.

image file: c7gc01928a-s11.tif
Scheme 11 Synthesis procedure of bis(2,2,2-trinitroethyl) oxalate.

image file: c7gc01928a-s12.tif
Scheme 12 Synthesis method of 2,2,2-trinitroethyl formate.

image file: c7gc01928a-f14.tif
Fig. 14 Homogenous smokeless burning of different green energetic compositions: (a) BTNEO/NC; (b) TNENCA/NC; (c) TNEF/NC.

Recently Shreeve's group52 has investigated the synthesis of polynitro-substituted pyrazoles and triazoles as potential propellant oxidizers. The group revealed that 5-azido-3,4-dinitro-N-(2,2,2-trinitroethyl)-1H-pyrazol-1-amine (ANNPA), 3,5-dinitro-N-(2,2,2-trinitroethyl)-1H-pyrazol-1,4-diamine (DNNPDA), 3,4-dinitro-1H-pyrazole-1,5-diamine (DNPDN) and 3-nitro-N-(2,2,2-trinitroethyl)-1H-1,2,4-triazol-1-amine (NNTAA) can be considered as promising candidates as replacement oxidizers for AP. These compounds with an oxygen balance over 10% have good thermal stability, high density and favorable performance. More recently, Keshavarz et al.152 have introduced novel tetrazole derivatives as energetic performance compounds, as an oxidizer in solid propellants. These tetrazols {5,5′ [(1Z,5Z)-3,4-dinitrohexaaza-1,5-diene-1,6-diyl]bis(1-nitro-1H,tetrazole), (DNDNT), 3,3′,7,7′-tetranitro-3,3a,3′,3′a-tetrahydro7H,7′H-6,6′-bitetrazolo[1,5-e] pentazine, (TTBTE)} are considered as good candidates, because they have good thermal stability, good performance and less sensitivity.

Fan et al.153 studied two new green tetrazole salts, hydroxyl ammonium 2-dinitro methyl-5-nitro tetrazolate (HADNMNT) and dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (HATO or TKX-50). Theoretically, it was revealed that HADNMNT is a promising oxidizer to replace AP in composite solid propellants. Safety tests demonstrated that HATO shows excellent thermal stability and low mechanical sensitivities. The compatibility tests of TKX-50 with HTPB, AP, RDX, and Al powder in vacuum stability tests were acceptable. Results from comparative investigations of TKX-50 and RDX as ingredients for composite solid propellants showed that TKX-50 formulations offer the advantages of high burning rate and low mechanical sensitivities.

Sinditskii et al.154 focused his attention on high nitrogen energetic materials. Among polynitrogen energetic materials, 1,2,4,5-tetrazine derivatives are of particular interest for the propulsion community because of their high density, thermostability, and remarkable insensitivity to electrostatic discharge, friction, and impact. High enthalpy of formation and good thermal stability of the tetrazine cycle generate tetrazine-based energetic materials, which can be utilized as an insensitive, thermostable, environmentally friendly ingredient in various energetic material applications such as propellants and gas generating compositions. Investigations of the combustion behavior revealed that most tetrazines are low-volatile components with high surface temperatures, which can play a dominant role of the condensed phase in the combustion of several tetrazine derivatives.

5. Conclusion

With regard to the reference ingredient AP, which is the most used oxidizer in rocket science and is the compound to be replaced, it is notable that this chemical is an ionic material with properties which appear to be perfect, except for its toxicity. The currently developed substitutes need optimization to fully satisfy all of the requirements of ideal oxidizers. Thus, finding green, high-performing replacements for AP is a top priority internationally and needs further endeavor to reach its aim.

For the near future, propellant formulations including green oxidizers are expected to advance the state-of-the-art solid rocket propellants and reduce the environmental concerns caused by the use of AP. In the last few decades, several energetic oxidizers have appeared but most of them are far from being practically employed in real applications because of their various drawbacks. In this review, the most potential green oxidizers that show interesting properties and potential use in solid rocket propellants have been focused on. In addition, the advantages and disadvantages of various green oxidizers have also been discussed. The main challenges of green ingredients as propellant oxidizers and many of the attempts made to overcome these problems have been also highlighted.

To overcome the flaws of the current potential green oxidizers such as PSAN, HNF and ADN, several procedures and methods of synthesis, crystallization and coating have been recently developed to improve the physicochemical properties, decrease sensitivity, enhance compatibility, improve stability and performance, and reduce the production costs. Furthermore, the use of dual oxidizers, such as PSAN/AP or ADN/AP with less environmental impacts has been shown to be a good solution for improving the propellant formulations’ performance. FOX-7 and its two derivatives TNAA and NTNAA were revealed to be very attractive and environmentally friendly replacements for AP with interesting properties. CL-20 also proved to be a potential candidate as a replacement for AP. The development of other HEDOs such as TNENCA, BTNEO, TNEF and TKX-50 has demonstrated that these candidates can meet the specific performance goal and allay the environmental concerns while presenting a helpful classification of hazards.

It is believed that the studies presented in this review will increase the interest of researchers on green oxidizers for solid rocket propulsion as well as provide a basic understanding of these green ingredients.


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