Green Energetic Materials

Tore Brinck

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9781119941293
• eBook ISBN: 9781118676462
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica industriale e tecnica

This comprehensive book presents a detailed account of research and recent developments in the field of green energetic materials, including pyrotechnics, explosives and propellants. This area is attracting increasing interest in the community as it undergoes a transition from using traditional processes, to more environmentally-friendly procedures. The book covers the entire line of research from the initial theoretical modelling and design of new materials, to the development of sustainable manufacturing processes. It also addresses materials that have already reached the production line, as well as considering future developments in this evolving field.

1

Introduction to Green Energetic Materials

Tore Brinck

Applied Physical Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Sweden

1.1 Introduction

The first energetic materials were developed in ancient China. Around 200 BC, Chinese alchemists were already starting to experiment with heating mixtures of saltpeter and sulfur. In the seventh century, saltpeter and sulfur were combined with charcoal to create an explosive material resembling what we today refer to as gunpowder. It was originally used for fireworks but soon became increasingly important for a range of military applications. The use of gunpowder in mining and civil engineering did not begin until the seventeenth century. Gunpowder remained the base for all energetic materials in practical use until the isolation of mercury(II) fulminate in 1799 by Edward Charles Howard. However, the first revolution in the development of energetic materials since the discovery of gunpowder started with the inventions of nitrocellulose (NC) in 1846 and nitroglycerine (NG) in 1847. NC was used as a propellant, whereas NG was mainly an explosive. Both these compounds had greatly enhanced performance compared to gunpowder. In 1866, Alfred Nobel introduced the original dynamite, a mixture of 75% NG with 25% kiselguhr, with a minor addition of sodium carbonate. It had much reduced sensitivity compared to pure NG and was, in contrast to NG, relatively safe to handle and transport. Nobel later developed gelatinous dynamite by combining NG with NC in a jelly. This material performed considerably better than the original dynamite and additionally improved safety. These examples illustrate the two main objectives, to improve performance and safety, that have traditionally driven research on energetic materials. It is important to remember that, in the wider definition of energetic materials – which include propellants, explosives, and the large area of pyrotechnics – the definition of performance depends largely on the purpose of the intended device. For example, in the area of pyrotechnics, performance can relate to light intensity, gas generation or smoke production.

Since the end of the twentieth century it has been increasingly realized that the use, or production, of many energetic materials leads to the release of substances that are harmful to human health or to the environment. In some instances the use of certain compounds has been restricted, or even banned, as a consequence of legislative actions. The result is that new objectives have been enforced on the development of energetic materials. Today, almost all research in the area is focused on designing new materials that can be considered “green.” This book intends to summarize the most recent developments in the area of green energetic materials, and to introduce the reader to some tools that are used in the research. However, before we embark on this journey, it may be valuable to try to define “green”, and how this extra requirement relates to the objectives of maximizing performance and safety of handling.

1.2 Green Chemistry and Energetic Materials

The concept of “green chemistry” was first introduced in 1990s by the US Environmental Agency (EPA), and is briefly defined on their web site as [1]:

To promote innovative chemical technologies that reduce or eliminate the use or generation of hazardous substances in the design, manufacture, and use of chemical products.

Since its first appearance, green chemistry has gradually evolved due to organized efforts in both Europe and the USA, and has been widely adopted within the chemical industry as a method to promote sustainability in the design and manufacturing of new chemicals. The basic ideas behind green chemistry were concretized in 1998 by Anastas and Warner by their definition of the Twelve Principles of Green Chemistry [2]:

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The Twelve Principles of Green Chemistry

(Reproduced with permission from [2] © 1998 Oxford University Press)

1. Prevention

It is better to prevent waste than to treat or clean up waste after it has been created.

2. Atom Economy

Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

3. Less Hazardous Chemical Syntheses

Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Designing Safer Chemicals

Chemical products should be designed to affect their desired function while minimizing their toxicity.

5. Safer Solvents and Auxiliaries

The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.

6. Design for Energy Efficiency

Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

7. Use of Renewable Feedstocks

A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

8. Reduce Derivatives

Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

9. Catalysis

Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

10. Design for Degradation

Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.

11. Real-time analysis for Pollution Prevention

Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

12. Inherently Safer Chemistry for Accident Prevention

Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.

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The principles of green chemistry are largely geared towards guiding the design of the manufacturing process, since for many chemicals the manufacturing has the largest impact on human health and the environment. Energetic materials are to some extent different, in that their use prohibits recycling and proper waste disposal; they disintegrate, and the decomposition or combustion products are directly released into the environment. It is, therefore, particularly important to consider the health and environmental effects of the final product and its usage. With this aspect in mind, the principles of green chemistry can be applied to the design of energetic materials and their manufacturing. It may seem very difficult to adhere to some of the principles, such as 2, 5, 8, and 9. After all, most energetic materials are complex structures that are very high in energy. Energy and complexity are often afforded by employing reactive reagents, specialized solvents, extreme reaction conditions, and through the use of protecting groups or other derivatization. However, most drugs are of equal or larger complexity, and therefore it is encouraging that green chemistry has been very successfully implemented in the pharmaceutical industry [3,4]. It is also obvious from reading Chapter 9 that great progress is currently being made in adopting green chemistry principles to the design of manufacturing processes for energetic materials. In some cases the achievements are clearly at the forefront of sustainable manufacturing, such as the use of biocatalysts or the implementation of continuous processes. The implementation of electrochemical methods is also likely to become of increased importance. Electrochemical processes often constitute energy efficient approaches for synthesis and for remediation of chemical waste. The use of water as the prevalent solvent in many such processes is an added advantage.

The principles of green chemistry provide no direct indication of how to determine the sustainability of a chemical or manufacturing process. One attempt to remedy this deficiency is the E-factor, which has been introduced as a method for quantifying the greenness of processes and products [5–7]. It is defined as the quotient of mass of waste over the mass of product, that is, m_waste/m_product. The waste is often considered to include all compounds formed during the process, including gases and water. It is generally a better measure for comparing different processes for making the same product than for comparing products, since it does not explicitly consider the constitution of the waste or its toxicity. Even if the E-factor is a relatively blunt tool, it provides a rapid and often very revealing method for assessing different process alternatives from an environmental perspective. A lifecycle assessment (LSA) is a much preferred approach as it attempts to give an assessment of the overall environmental impact of a product. It considers the entire process from the extraction and acquisition of the raw material, via the manufacturing and use of the product, to the end-of-life management [5,8]. There are also methods that attempt to combine LSA with lifecycle cost analysis to get a total assessment of the costs for a product. Such a technique was recently used to analyse the life-cost of a toxic monopropellant (hydrazine) propellant versus a green one [9]. The analysis demonstrated that replacing the toxic propellant would give large cost reductions, even though the actual manufacturing cost for the green replacement is higher. It is a common observation that it pays off, from a direct cost perspective, to replace old products with greener alternatives. This also holds for the manufacturing of chemical products; converting to processes that adhere to the green principles of chemistry is often cost effective.

Reduction of costs can actually be considered one of main forces that drive the implementation of green products and manufacturing technologies; the others are societal pressure due to public awareness and government legislation. In Europe, chemicals and their use are regulated through the European Community regulation REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances) [10]. REACH was introduced in 2007 and will be gradually phased in over 11 years. “The aim of REACH is to improve the protection of human health and the environment through the better and earlier identification of the intrinsic properties of chemical substances” [10]. REACH significantly increases the responsibilities of manufacturers and importers of chemicals. They are required to gather information on the properties of their chemical substances and provide safety information that ensures their safe handling. The Regulation further calls for the progressive substitution of the most dangerous chemicals when suitable alternatives have been identified. This also has implications for the energetic materials industry. It is becoming increasingly important to identify early on energetic materials that are in danger of being phased out, and to begin the development of green replacements.

So far we have not touched on the subject of whether the considerations according to the principles of green chemistry should be prioritized over performance and safety of handling when designing green energetic materials. Safety of handling is partly considered in principle 12. However, the potential consequence of an accident in the case of an energetic material is often of such magnitude that safety of handling must take precedence over other priorities. The prioritization of performance is a slightly more complicated matter, and is somewhat dependent on the application. It is obvious that the performance of a product will affect its atom-economy, for example, if a new material has half the performance of an old material, we need to use twice the amount of the new material to accomplish the same task. However, for some applications the consequence of lowered performance can be detrimental. In the case of rockets for space exploration, the mass of the propellant can easily be up to 90% of the total weight, whereas the payload typically constitutes only a few percent; even a minor reduction in performance (specific impulse) will significantly reduce the size of the payload. Thus, high performance is essential for efficient energy utilization and is often needed for mission completion. There are many other applications of energetic materials where high performance is also of great importance.

With this discussion in mind we can att...

Table of contents

• Cover
• Title Page
• Copyright
• List of Contributors
• Preface
• Chapter 1: Introduction to Green Energetic Materials
• Chapter 2: Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance
• Chapter 3: Some Perspectives on Sensitivity to Initiation of Detonation
• Chapter 4: Advances Toward the Development of “Green” Pyrotechnics
• Chapter 5: Green Primary Explosives
• Chapter 6: Energetic Tetrazole N-oxides
• Chapter 7: Green Propellants Based on Dinitramide Salts: Mastering Stability and Chemical Compatibility Issues
• Chapter 8: Binder Materials for Green Propellants
• Chapter 9: The Development of Environmentally Sustainable Manufacturing Technologies for Energetic Materials
• Chapter 10: Electrochemical Methods for Synthesis of Energetic Materials and Remediation of Waste Water
• Index