Combined use of adiabatic calorimetry and heat conduction calorimetry for quantifying propellant cook-off hazards

Recent work performed at DERA (now QinetiQ) has shown how accelerating rate calorimetry (ARC) can be used to obtain time to maximum rate curves using larger samples of energetic materials. The use of larger samples reduces the influence of thermal inertia, permitting experimental data to be gathered at temperatures closer to those likely to be encountered during manufacture, transportation or storage of an explosive device. However, in many cases, extrapolation of the time to maximum rate curve will still be necessary. Because of its low detection limit compared to the ARC, heat conduction calorimetry can be used to obtain data points at, or below, the region where an explosive system might exceed its temperature of no return and undergo a thermal explosion.Paired ARC and heat conduction calorimetry experiments have been conducted on some energetic material samples to explore this possibility further. Examples of where both agreement and disagreement are found between the two techniques are reported and the significance of these discussed. Ways in which combining ARC and heat conduction calorimetry experiments can enhance, complement and validate the results obtained from each technique are examined.     

Sample geometry as critical factor for stability research

Stability research on gun propellants has been widely performed by microcalorimetry since the 1980s. TNO Prins Maurits Laboratory has already a broad experience since the early 1970s. In the past many studies were performed, to investigate the influence of oxygen, humidity etc. Less attention was paid to two other important aspects, namely the sample geometry and the filling degree during the microcalorimetric test.A statement in the old Dutch military literature presents the following “It is a well-known fact that the free surface influences the decomposition rate of the gun powder, i.e. unground propellant decomposes slowly in comparison to ground propellant”. This is the same for all types of propellant (Amsterdam, February 1924), which implies research on this topic, related to the stability prediction measured by microcalorimetry is important.Since the decomposition of nitrocellulose is influenced by the amount of oxygen and the surface area, the best way to investigate the stability of gun propellants is to measure it ‘ammunition like’. This means that a combination of the propellant grain size and the filling degree of the ammunition should be used for investigation. For small caliber ammunition the filling degree is close to 95%, and for large caliber bag ammunition (e.g. 155 mm) it is around 60%. As a result on these important aspects, TNO has developed five different sizes of sample vessels, to investigate the propellant grains in the most ‘ammunition like’ condition.In this paper, an overview is given of the influences of relative humidity, different grain sizes and available oxygen. The available oxygen is adjusted by changing the oxygen content of the air or by changing the filling degree.     

Detonation wave initiation of ram accelerator propellants

The current ram accelerator operations have shown that data on the ability of the propellants to detonate are required. Previous studies examined the efficacy of initiation techniques based on piston impact. The purpose of the present work is to analyze the effects of detonation wave transmission from a detonating mixture into a low sensitivity mixture. One-dimensional modeling based on the analysis of pressure vs particle velocity for the mixtures is used to interpret experimental data. Furthermore, calculations based on chemical kinetics (CHEMKIN code) are provided. Experimental data together with the modeling of the detonation transmission provide some new insight into the limiting conditions necessary to establish a Chapman-Jouguet (CJ) wave in a detonable mixture. Received 11 January 2000 / Accepted 15 September 2000     

Ignition of single nickel-coated aluminum particles

A thin coating of nickel on the surface of aluminum particles can prevent their agglomeration and at the same time facilitate their ignition, thus increasing the efficiency of aluminized propellants. In this work, ignition of single nickel-coated aluminum particles is investigated using an electrodynamic levitation setup (heating by laser) and a tube reactor (heating by high-temperature gas). The levitation experiments are used for measurements of the ignition delay time at different Ni contents in the particles. The high-temperature gas experiments are used to measure the critical ignition temperature. It is reported that the Ni coating dramatically decreases both the ignition delay time of laser-heated Al particles and the critical ignition temperature of gas-heated Al particles. A heat balance analysis of the levitated particles shows that the lower ignition temperature of Ni-coated Al particles is the most probable reason for the observed reduction in the ignition delay time. Exothermic intermetallic reactions between liquid Al and solid Ni are considered as the main reason for the lowered ignition temperature of Ni-coated Al particles.     

A Simple Method for Initial Condensed-Phase Combustion Reactions Predictions

Abstract

Combustion modeling of energetic materials has undergone a tremendous amount of development in recent years. Gas-phase combustion properties predictions are possible and have been shown to agree with experimental results. For solid energetic materials, the gasification reaction products must be determined if gas-phase models are to be applied. The majority of actual models still involve a guessing step for gasification products due to the lack of knowledge in the condensed phase. Quantum dynamical calculations are presently the only way to predict initial condensed-phase reactions but require tremendous computing power, thus limiting their application. In this work, an empirical method that uses spectroscopic measurements of energetic molecules as the source for the complete modal molecular picture is presented. The method enables a qualitative prediction of the first combustion reaction in the condensed phase. The application of the methods with nitrocellulose and nitroguanidine shows that it predicts initial reactions matching the current ideas used in modeling.     

Energetic polymers as binders in composite propellants and explosives

The principles behind the use of polymeric binders in composite propellants and explosives are described with emphasis on the properties which they should possess in order to satisfy the requirements for inclusion in a composition. The desirability of using energetic polymers as binders in terms of both performance and safety, and the problems associated with their preparation and properties, are discussed. The contributions of chemical synthesis within DRA to overcome these problems will be shown. Preparation of energetic polymers both by polymer modification and by polymerization of an energetic monomer is described. We have developed three energetic polymers: poly-3-nitratomethyl-3-methyloxetane (polyNIMMO), polyglycidyl nitrate (polyG-LYN) and nitrated hydroxy-terminated polybutadiene (NHTPB). Two of these (polyNIMMO and polyGLYN) have shown excellent properties in propellant and explosive formulations and proved that low-vulnerability, high-performance compositions are possible. The properties of the products from our work are compared with those of other groups and a glimpse of the future in this area is given to show the potential for new energetic polymers.     

Application of a nanostructured supramolecular solvent for the microextraction of diphenylamine and its mono‐nitrated derivatives from unburned single‐base propellants

A supramolecular solvent made up of nano‐sized inverted hexagonal aggregates of 1‐octanol is proposed for the microextraction of diphenylamine and its mono‐nitrated derivatives in unburned single‐base propellants. The procedure included the extraction of sub‐gram quantities (30 mg) of homogenized propellant with 1.5 mL of the supramolecular solvent. Several conditions affecting extraction efficiency, for example the concentrations of the major components of the supramolecular solvent (tetrahydrofuran and alkanol), alkanol type, solvent pH, and extraction time, were investigated and optimized. The main forces for the microextraction of analytes in the nanostructured supramolecular solvent include both dispersion and hydrogen bond interactions. This mixed‐mode mechanism resulted in high extraction efficiencies reaching low method detection limits (0.005–0.012 mg/g) without the need for extract evaporation. Under the optimum conditions, recoveries in samples ranged between about 82.6 and 98.7%. Compared to the reference method, the proposed method is simple and rapid, delivering accurate and precise results, and can be applied for routine determination of diphenylamine and its derivatives in propellants. The precision of the method, expressed as relative standard deviation, was about 4.3–10.9%.     

Optimization of global kinetics parameters for heterogeneous propellant combustion using a genetic algorithm

We examine the combustion of heterogeneous propellants for which, necessarily, the chemical kinetics is modelled using simple global schemes. Choosing the parameters for such schemes is a significant challenge, one that, in the past, has usually been met using hand-fitting of experimental data (target data) for global burning properties such as steady burning rates, burn-rate temperature sensitivity, and the like. This is an unsatisfactory strategy in many ways. It is not optimal; and if the target set is large and includes such things as stability criteria, or bounds, difficult to implement. Here we discuss the use of a general optimization strategy which can handle large data sets of a general nature. The key numerical tool is a genetic algorithm that uses MPI on a parallel platform. We use this strategy to determine parameters for HMX/HTPB propellants and AP/HTPB propellants. Only one-dimensional target data are used, corresponding to the burning of pure HMX (AP) or a homogenized blend of fine HMX (AP) and HTPB. The goal is to generate kinetics models that can be used in the numerical simulation of three-dimensional heterogeneous propellant combustion. The results of such simulations will be reported in a sequel.     

Numerical simulation of the 3-dimensional combustion of aluminized heterogeneous propellants

We report the first 3-dimensional simulations of aluminized propellant combustion, accounting for heat conduction in the solid, combustion in the gas-phase, and coupling of these via the irregularly moving propellant surface, one that can not be defined by a single-valued height function. The simulations are used to examine the dynamics of aluminum particles in the near-neighborhood of the surface after detachment, and to provide an estimate of the time to ignition of the particles, and their speed and height above the surface at ignition. In addition, we examine the temperature history of the particles during their rise to the surface, determine whether they melt or not, and in this way test Cohen’s well-known melting criterion. And, we discuss a simple model which provides insights into how aluminum particles floating on a binder melt layer would migrate because of surface tension effects, and calculate an average migration distance that is consistent with previous agglomeration studies.     

Analysis of propellant stabilizer components via packed and capillary supercritical fluid chromatography/Fourier transform infrared spectrometry

The separation of model mixtures of nitrated diphenylamine and nitrated aniline via supercritical fluid chromatography employing both capillary and packed columns is described. With highly deactivated stationary phases each mixture could be separated with 100% CO₂ pressure programmed. Peak identification and peak purity were afforded by on-line Fourier transform infrared spectrometric detection. Nitrosoaniline derivatives were observed via FTIR to have been converted to nitro derivatives on storage.