Rapid Investigation of Thermally Hazardous Substances
Differential Scanning Calorimetry (DSC) is used to measure the enthalpy of reaction and reaction rates of chemical reactions. This information is important for designing processes, for identifying and assessing potential hazards in chemical syntheses and decomposition reactions, and for thermodynamic and kinetic calculations. Whether a product is thermally unstable or represents a potential explosion hazard (thermal run away) must be determined at a very early stage. DSC and TGA screening measurements using small amounts of substance provide the necessary data for assessing the potential risks.
Introduction
Assessing the potential hazards presented by chemicals and chemical reactions is a never-ending analytical task in the laboratory and in production. An important question concerns the potential danger that could arise through an uncontrolled temperature increase in the handling of materials and their storage.
The risk of a thermal runaway and an explosion with a large amount of damage is significant if
- the reaction enthalpy is large and exothermic,
- the rate of increase of temperature is high or even self-accelerating,
- gaseous products are produced, whether through decomposition or vaporization,
- the reactor system cannot withstand high pressure and/or high temperatures,
- subsequent complications such as fires or environmental pollution are caused.
The potential danger not only has to be estimated with regard to the points mentioned above but also with regard to the probability that such an effect can occur and how large the possible damage would be. In such risk analyses, the determination of the reaction enthalpy of a chemical synthesis or a decomposition reaction is often the starting point for further investigations. The focus is naturally on high risks.
Past experience has however shown that all chemicals and processes must be investigated, not just the ones that are thought to be potentially dangerous. Processes that seem at first sight harmless are synthesis reactions, standard operations such as drying or grinding, and storage.
A rapid screening method is very desirable in view of the fact that a large number of tests have to be performed. DSC has proven to be an ideal technique for providing the necessary information at an early stage of the chemical product development. The method requires very little sample material [1]. The DSC results help one to assess the risks with regard to thermal stability as shown schematically in Figure 1.
DSC measurements provide data on the specific reaction enthalpy (∆hr ) and specific heat capacity (cp). These two quantities can be used to estimate the highest temperature, ∆Tadiab, that could be reached in of a thermal runaway:
If the temperature increase is more than 50 K, the situation is potentially dangerous. If the increase is however 200 K, the situation becomes extremely dangerous [2].
The boundaries of course depend on the particular circumstances. In fact, the rate of production of heat is just as important as the reaction enthalpy itself. Dangerous, almost adiabatic behavior occurs if the heat exchange with the surroundings is inadequate. This is particularly critical with fast reactions [3].
The determination of kinetic behavior is therefore essential. The conversion of a reaction can be described as a function of temperature and time with the aid of just a few DSC measurements followed by evaluation using model free kinetics (MFK). The results allow predictions to be made about the long-term behavior (e.g. storage conditions) or short-term behavior (explosions) of substances.
The time to maximum reaction rate (TMR) can also be determined with relatively simple calculations [4]. calorimetric investigations (DSC) of open systems can however lead to wrong conclusions.
This is for example the case when exothermic reactions and endothermic vaporization processes occur simultaneously and balance each other out. In this respect, thermogravimetric analysis (TGA) is very helpful because it can be used to determine the amount of gas liberated in the process. If the TGA instrument is connected to a gas analyzer (MS, FTIR), the nature of the evolved gases can be identified.
DSC samples are usually measured in pressure-tight crucibles to avoid problems arising from interfering vaporization effects. The following sections describe some typical examples of safety investigations (screening) using these techniques.
Experimental Details
The measurements were performed using METTLER TOLEDO STARe System DSC 1 and TGA/DSC 1 instruments. The chemicals and analytical conditions are noted in the examples.
Results
Routine Investigation of a Substance
Figure 2 displays typical DSC measurements of two thermally hazardous substances. The melting peak (here at about 50 °C, Figure 2, left) helps to identify the substance in question and determine its purity.
Summary
A chemical substance can be quickly analyzed by DSC and TGA to check whether there is a risk of rapid decomposition or explosion. DSC provides the basic thermoanalytical information such as specific heat capacity, reaction enthalpy and reaction rate under different conditions. TGA shows how much gas is produced in a decomposition reaction. The gases can be identified by connecting the TGA to a gas analyzer such as a mass spectrometer or a Fourier transform infrared spectrometer.
Kinetic evaluation by model free kinetics requires at least three dynamic DSC or TGA measurements. Predictions can be made for situations in which it is not possible to perform direct measurements, for example for very fast or very slow reactions. The predictions must be verified with additional measurements.
There is a potential risk for a reaction runaway when exothermic reaction enthalpies are high, when the reaction is self-accelerating, and when large amounts of gases are produced. If a survey measurement yields such results, further investigations will be necessary to make sure that the synthesis reaction and other steps in the production process as well as storage of the products can be safely implemented.
Rapid Investigation of Thermally Hazardous Substances | Thermal Analysis Application No. UC 332 | Application published in METTLER TOLEDO Thermal Analysis UserCom 33