Adiabatic temperature mode. Prediction of the reaction progress (-), development of the temperatures and adiabatic induction times for selected starting temperatures and T-adiabatic (Hr/Cp with Hr :heat of reaction and Cp :heat capacity).

Reaction progress , adiabatic induction times.

Comments:

Quantitative Predictions of Thermal Hazards

All energetic materials, e.g. explosives or propellants, evolve heat during decomposition. The rate of the decomposition depends on the temperature. From a box full of oily rags or a barn full of hay to a rocket motor filled with solid propellant, energetic materials can self-heat with unfortunate results. Processing, design, quality control, and operational applications of systems using energetic materials all require an understanding of thermal hazards and an ability to predict safety limits.

The calculation of adiabatic reaction progresses and/or explosion conditions from results of DSC/DTA measurements is often desirable because of the small amounts of material available. The precise predictions of such reaction progresses can be required in safety analysis of many technological processes. Calculations of an adiabatic temperature-time curve of the reaction progress can be also used to determine the decrease of the thermal stability of materials during storage at temperatures near the temperature of the reaction beginning. Due to insufficient thermal convection and limited thermal conductivity, a progressive temperature increase in the sample can easily take place, resulting in a thermal explosion. The determination of:

• the critical starting temperatures,
   
• development of the temperatures
   
• and adiabatic induction times

are important parameters both for the production as well as for the storage of potentially explosive materials.

Thermal Stability

Several methods have been presented for predictions of the reaction progress of exothermic reactions under adiabatic conditions [1-4]. However, because solid state reactions usually have a multi-step nature, the accurate determination of the kinetic characteristics strongly influences the ability to correctly describe the progress of the reaction. For adiabatic self-heating reactions, incorrect kinetic description of the process is usually the main source of serious errors in its interpretation.

The assumption that the decomposition of an energetic material will obey a simple rate law is not often true. Solid state reactions are often too complex to be described in terms of a single pair of Arrhenius parameters and the traditional set of reaction models [5]. As a general rule, solid state reactions demonstrate profoundly multi-step characteristics. They can involve several processes with different activation energies and mechanisms. Changes in mechanism associated with complex reactions can cause changes in heat of reaction (Hr) and in the kinetic description of the proceeding reaction. When mechanisms change during the course of a reaction, it is not valid to "linearize" the rate data for the entire process to obtain a single rate constant. In addition, methods of the prediction of the summary reaction progress which consider only one kinetic parameter, namely the "activation energy" and ignore the others such as the pre-exponential factor and the model function, are an over simplification of reality. A reliable numerical technique applied in solid state kinetics should be able to consider several activation energy values, preexponential factors and corresponding models for the description of the solid state reaction extent. The correct choice of all the kinetic parameters strongly influences the ability to properly describe the progress of the reaction. The validity of approaches, which consider exclusively the activation energy values for the determination of the kinetic characteristics of solid state reactions, can be hardly accepted [6]. It is hazardous to develop safety predictive models that are based on simplified kinetics determined by DSC, DTA or any other methods. It is extremely dangerous to use such simplified models for large-scale predictions.

The determination of the appropriate rate equations is a prerequisite for the correct analysis of the kinetics of the decomposition of energetic materials. The reaction products formed during the early (induction), intermediate (acceleratory), and late (decay) periods of a self-heated reaction can be considerably different. Changes in mechanism during decomposition can result in changes in the kinds of gas produced as well as their amounts.

Adiabatic induction time

The assumption that it is safe to handle an energetic material at any temperature below the first appearance of an exothermic signal on the DSC or DTA curve can be often false. Under perfect adiabatic conditions, there is some delay at any temperature before the materials reach their maximum rate of decomposition. When the temperature of any energetic material is increased, it will either decompose quietly (ultimately rupturing its confinement as a result of the production of gaseous products), self-heat to explosion or detonation, or ignite and burn.

One very important criterion for thermal safety is the "critical temperature" (Tc), defined as the lowest constant temperature at which a specific material of a specific size and shape will self-heat catastrophically. It must be remembered, however, that energetic materials still decompose at temperatures below the critical temperature. Determination of critical temperature is a very important test to be run on energetic materials. If they are sealed in a container, gas pressure will build up until the container ruptures and the result may look like an "explosion". Extremely violent responses can be expected at temperatures above Tc.

The highest safe temperature for handling any energetic material depends essentially on its size, shape, and previous thermal history. The critical temperature is very sensitive to changes in size in the small-size range. It can change by several degrees for smaller sizes. If we carry out measurements in 50 cm geometry and make the assumption that an 8 m charge would survive the same temperature, an accident would almost certainly be the result. It is therefore very important to predict the induction time under adiabatic conditions. The adiabatic induction time is defined as the time which is needed for the self-heating from the start temperature to the time of maximum rate. The adiabatic induction time is an important parameter to determine when the thermal safety of any material or process is in question. The time to maximum reaction rate (or explosion in some cases) can also be used for quality control purposes. A reduced adiabatic induction time for a new sample of a specific energetic material indicates a less stable material. Similarly, when considering compatibility problems, reductions in time to explosion and/or critical temperature when another material is mixed with the explosive one indicate incompatibilities. Slight changes in purity or composition, introduction of defects into crystal due to pressing, past history of a sample, and/or fabrication or formulation methods can cause major changes of the time to explosion. These changes can be very large and unexpected.

One important aspect of AKTS-Thermokinetics software is that it can provide a small-scale test for quantitative thermal-hazard predictions. The adiabatic induction time of an energetic material can be determined rapidly for any starting temperature.

Important: A normal DSC crucible is partly sealed; therefore, the testing proceeds often in a self-generated atmosphere. Important secondary reactions can often be observed. Tests carried out at reduced pressure or with purging gas can lead to different results. However, with energetic materials it is advantageous to carry out both types of experiments: in a self-generating atmosphere and with the purge gas. The differences between the results of both tests will often help to identify the safety problems involving confinement.

Caution: For explosives, explosion or partial detonation of a few mg sample can be extremely destructive. Tests must be run with adequate shielding.

[1] P.H. Thomas, Trans. Faraday Soc., 54 (1942), 60.
[2] N. Semenov, Einige Probleme der chemischen Kinetik und Reaktionsfähigkeit, Akademieverlag, Berlin, 1961.
[3] D.A. Frank-Kamenetskii, Diffusion and Heat Transfer in Chemical Kinetics, Plenum Press, New York, London, 1969.
[4] T. Grewer, Thermochim. Acta, 225 (1993) 165.
[5] B. Roduit, Thermochim. Acta, 355 (2000) 171.
[6] M. Maciejewski, Thermochim. Acta, 355 (2000) 145.