Determination of Safety Data for Azides Using Model Free Kinetics (MFK)

Structure and Properties of Azides

Azides are salts and organic compounds of hydrazoic acid. Their general structure is shown in Figure 1. R can be a metal ion (e.g. Na+) or even a hydrocarbon chain.

Sodium azide (NaN3) is a typical example of an azide. The substance is used as a propellant in many car airbag systems.

The decomposition reaction is triggered by heat or an electrical impulse. The reaction almost instantaneously produces a large volume of nitrogen. A well-known organic azide is azidothymidine, an antiretroviral drug used in the treatment of AIDS. In this study, two organic azides namely C₁₄H₁₉N₃O₆ (C14) and C₂₀H₃₀N₆O₁₄ (C20) were analyzed to determine their thermal behavior and to make predictions about their reactivity.

Assessment of the hazard potential of chemicals Information about the enthalpy and kinetics of a reaction is important for planning the heating or cooling capacity in a chemical process and for assessing the hazard potential of the reaction. If the heat generated in an exothermic reaction in a technical process is not removed sufficiently quickly, the temperature of the reactor increases and the reaction rate accelerates. Increasing reactor temperature leads to an ever-increasing reaction rate. This can result in an explosion if the reaction enthalpy is sufficiently large.

The worst case is adiabatic conditions in which no heat is removed. In practice, this situation is reached when the rate of heat generation due to the reaction is greater than the removal of heat through the cooling system. This means that exothermic reactions with a large reaction enthalpy are potentially dangerous. A useful value for estimating the hazard potential is the maximum (adiabatic) temperature increase (Δ_Tadiabat) where Δh is the specific reaction enthalpy and cp the mean specific heat capacity of the reaction contents. Equation (1) states that the temperature change of the sample is proportional to the reaction enthalpy produced and inversely proportional to the heat capacity of the sample.

The enthalpy of a reaction is easily measured in a DSC experiment by integrating the area under the reaction peak. Typically, the heat capacity of organic substances can be assumed to be about 1.5 J/g. If more accuracy is required, cp is determined by DSC. A classification of the hazard potential of exothermic reactions is shown in Table 1. Besides the reaction enthalpy, the temperature at which decomposition occurs is also often used to characterize a decomposition reaction. In fact, however, the decomposition rate increases continuously with increasing temperature. 

It therefore makes sense to analyze the kinetics of the reaction in order to determine the relationship between reaction temperature, reaction time and reaction conversion. In general, kinetic studies then provide important information about the stability of a substance.

Instrumental Details

Samples of 0.5 to 1 mg of the powdered substance were weighed into 40-μL aluminum crucibles and closed with a lid with a hole. The measurements were performed using a DSC 1 with an FRS5 sensor in an air atmosphere. The samples were measured from 30 to 400 °C at heating rates of 2, 5 and 10 K/min. The reaction kinetics were evaluated using Advanced Model Free Kinetics (AMFK) software program [1, 2].

Measurement Results and Interpretation

The DSC curves in Figure 2 show that C20 melts from about 120 °C onward. Decomposition begins immediately after. The total reaction enthalpy from the beginning of the reaction to about 370 °C is 1220 J/g (Table 2). The adiabatic temperature increase is 820 K. C14 melts from about 70 °C onward. The decomposition reaction begins above 140 °C and produces a total reaction enthalpy of about 900 J/g with an adiabatic temperature increase of 600 K. The hazard potential of both substances is thus very high (see Table 1). 

The reaction shifts to higher temperatures at higher heating rates. To estimate the influence of combustion on the measured reaction enthalpy, C14 was also measured at 5 K/min in nitrogen (dashed curve in Figure 2). The reactions in air and nitrogen first proceed identically. The measurement in air however shows a broad oxidation peak above 270 °C that contributes about half of the total reaction enthalpy. 

The peak temperatures and reaction enthalpies determined from the measurement curves in Figure 2 are summarized in Table 2. The oxidation reaction was also taken into account in the determination of the reaction enthalpy and the adiabatic temperature increase.

Kinetic Evaluation

It is not necessary to take into account the oxidation that occurs at high temperatures to obtain information about the stability. Only the first part of the reaction (the shaded area in Figure 2) was therefore used for the kinetic analysis. Conversion curves were calculated from these areas and evaluated by AMFK. This yields the apparent activation energy as a function of conversion for the first reaction step (Figure 3). 

^{Determination of safety data for azides using model free kinetics (MFK) | Thermal Analysis Application No. UC 334 | Application published in METTLER TOLEDO Thermal Analysis UserCom 33}