Major recent advances. Particle formation and design of solid particles and powdery composites with unique properties is at the moment a major development of supercritical fluids (synonyms: dense gasses, dense fluids, high pressure) applications. This review will focus on recent advances and on fundamentals of these processes and their applications.

Particles formation and particle design using supercritical fluids

A global increase in environmental pollution demands the development of new "cleaner" chemical processes. Among urgent improvements, the replacement of traditional hydrocarbon-derived toxic organic solvents with neoteric solvents less harmful for the environment is one of the most vital issues. As a result of the favorable combination of their unique properties, ionic liquids (ILs), dense gases, and supercritical fluids (SCFs) have gained considerable attention as suitable green chemistry media for the preparation and modification of important chemical compounds and materials. In particular, they have a significant potential in a specific and very important area of research associated with the manufacture and processing of high-energy materials (HEMs). These large-scale manufacturing processes, in which hazardous chemicals and extreme conditions are used, produce a huge amount of hard-to-dispose-of waste. Furthermore, they are risky to staff, and any improvements that would reduce the fire and explosion risks of the corresponding processes are highly desirable. In this Review, useful applications of almost nonflammable ILs, dense gases, and SCFs (first of all, CO2 ) for nitration and other reactions used for manufacturing HEMs are considered. Recent advances in the field of energetic (oxygen-balanced and hypergolic) ILs are summarized. Significant attention is paid to the SCF-based micronization techniques, which improve the energetic performance of HEMs through an efficient control of the morphology and particle size distribution of the HEM fine particles, and to useful applications of SCFs in HEM processing that makes them less hazardous.

Prospective Symbiosis of Green Chemistry and Energetic Materials

Extraction of brominated flame retardants from polymeric waste material using different solvents and supercritical carbon dioxide

Al2O3/graphene reinforced bio-inspired interlocking polyurethane composites with superior mechanical and thermal properties for solid propulsion fuel

This paper describes the mechanical and thermal characterisation of cross-linked glycidyl azide polymer (GAP) and GAP–hydroxyl terminated polybutadiene (HTPB) networks. Cross-linked GAP and GAP–HTPB networks were prepared by reacting GAP diol and GAP–HTPB diol mixture with different diisocyanates. The physical and mechanical characteristics were found to be influenced by the type of isocyanate curing agents, [NCO]/[OH] equivalent ratios and concentration of GAP. For all the three types of curing agents, GAP–HTPB blends of 50 : 50 to 30 : 70 ratios show higher mechanical strength over the virgin networks of GAP or HTPB. Thermal decomposition of cross-linked GAP–HTPB networks was evaluated by thermogravimetric analysis (TGA). The kinetic parameters for the decomposition of GAP–HTPB blends were found to be dependant on the concentration of GAP and HTPB in the blend. The cross-linked GAP–HTPB blends were subjected to dynamic mechanical analysis (DMA). The glass transition characteristics of the blends were evaluated by DMA and it was found that blends prepared with GAP content up to 30% showed single transition in the loss tangent trace indicating no phase separation in the cured network.

Thermomechanical and Morphological Characteristics of Cross‐Linked GAP and GAP–HTPB Networks with Different Diisocyanates

Currently formulated propellants comprise RDX and polymeric binders, such as hydroxy-terminated polybutadiene (HTPB) and cellulose-acetate butyrate (CAB) as well as the energetic substances glycidyl azide polymer (GAP) and nitrocellulose (NC). Propellants based on GAP are often brittle if they are formulated with a high content of cyclotrimethylene trinitramine (RDX) and due to the usually insufficient mechanical properties of GAP. On the other hand formulations based on RDX and NC may exceed the tolerable burning temperature with increasing RDX concentration. Therefore, in this study propellants with a high force and with relatively low burning temperature has been formulated by using a compound of NC and GAP as energetic binder. According to thermodynamic calculations GAP/NC composite propellants can be formulated with up to 15 percent more specific energy than seminitramines at the same burning temperature. By choosing appropriate polymerization conditions chemical stable compositions can be produced. ARC experiments give evidence that at temperatures from 120°C to 160°C the binder decomposes similar to NC. At higher temperatures the behaviour switches from NC type to GAP type decomposition. In comparison to GAP bound propellants the compressive strength of propellants bound by the GAP/NC compound can be significantly increased by up to 420 percent at room temperature. Although the examined seminitramine propellants bound with NC show a compressive strength which is about 10 percent higher at room temperature, the GAP/NC compositions are quite superior at elevated temperature.

Compounding of Glycidyl Azide Polymer with Nitrocellulose and its Influence on the Properties of Propellants

As solvent supercritical carbon dioxide offers unique possibilities, like non-toxicity and therefore reduction of environmental pollution or access of low-temperature processing resulting in additional process safety. Therefore, knowledge to the influence of modifiers due to better solubility of polar substances is important for evaluating novel manufacturing techniques like the RESS- or GAS-process. Dynamic supercritical fluid extraction of PETN, nitroguanidine (NIGU) and RDX, respectively, with pure as well as with modified carbon dioxide indicated that all explosives except nitroguanidine were extracted though for RDX and 3-nitro-1,2,4-triazole the use of modifiers proved to be necessary. The results show the high capacity of modified supercritical carbon dioxide as a solvent in RESS- and GAS-processes.

Suitability of modified Supercritical carbon dioxide as solvent for polar substances

Production of microcapsules comprises spraying drops or fine particles of a solution of the coating agent in a supercritical fluid onto <100 mu m particles of the substrate in a fluidised bed.

Method for microencapsulation of particules

The development of new propellant binder systems requires the thermodynamic calculation of physico-chemical data as well as the adaption of the mechanical properties in order to achieve a reliable innerballistic profile of the resulting propellant. Therefore, this study focuses on the mechanical properties of a binder system comprising GAP, NC as well as the energetic plasticizer DNDA and the curing agent Desmodur N100. By applying statistical methods, the mechanical properties of the binder can be predicted as a function of its composition. A macroscopic analysis of the interrelations between the E-modulus, the ultimate tensile strength as well as the elongation at break and the specific deformation energy lead to the creation of failure master curves, which can be used as a rule of thumb criterion for the formulation of propellant binders.

Optimization of Propellant Binders – Part Two: Macroscopic Investgation of the Mechanical Properties of Polymers

The development of new propellant binder systems requires the thermodynamic calculation of physico-chemical data as well as the adaption of the mechanical properties in order to achieve a reliable innerballistic profile of the resulting propellant. However, in most cases the mechanical data may not be easily predicted due to the complex interactions between the components of the binder like the resin, the curing agent or possibly plasticizers and curing catalysts. Therefore, this study focusses on the capability of the multivariate analysis on the prediction of the E-modulus of a system comprising nitrocellulose as well as GAP, Desmodur N100 and an energetic plasticiser. Using this method, an equation has been derived which, within the regression intervall, may be used for the prediction of the E-modulus as a function of the components mentioned above.

Michael Dipl Chem Niehaus

Papers

Compounding of Glycidyl Azide Polymer with Nitrocellulose and its Influence on the Properties of Propellants

Suitability of modified Supercritical carbon dioxide as solvent for polar substances

Method for microencapsulation of particules

Optimization of Propellant Binders – Part Two: Macroscopic Investgation of the Mechanical Properties of Polymers

Optimization of Propellant Binders – Part I: Statistical Methodology