Time to explosion

The Sikarex safety calorimeter system and its application to determine the course of adiabatic self-heating processes, starting temperatures for self-heating reactions, time to explosion, kinetic data, and simulation of real processes, are discussed with examples [1], The Sedex (sensitive detection of exothermic processes) calorimeter uses a special oven to heat a variety of containers with sophisticated control and detection equipment, which permits several samples to be examined simultaneously [2]. The bench-scale heat-flow calorimeter is designed to provide data specifically oriented towards processing safety requirements, and a new computerised design... 

FIG. 5.7 Time to explosion versus temperature for nitrocellulose. As the temperature of the heating bath is raised, the time to explosion decreases exponentially, approaching an instantaneous value. The extrapolated temperature value corresponding to infinite time to explosion is called the spontaneous Ignition temperature, minimum (S.I.T. min). Source of the data reference 6. 

Explosion Temperature The explosion temperature is a qualitative term and depends upon several factors, similar to explosion delay. It is measured mainly for a comparative study. In the present work, it is defined as the temperature necessary to cause explosion in exactly 10 or 5 seconds, that is, 10-/5-second time-to-explosion while determining explosion delay with a few milligrams of an explosive. 

Using this apparatus, it is also possible to determine the 10-/5-second time-to-explosion temperature, as follows. 

Therefore the sensitivity usually ranges between 2 and 20Wkg 1. This heat release rate corresponds to a temperature increase rate of about 4 to 40 °C hour-1 under adiabatic conditions. This also means that an exothermal reaction is detected at a temperature where the time to explosion (TMRJ) is in the order of magnitude of one hour only. 

This reaction scheme is used in two variants, a fast reaction called the addition reaction and a slow synthesis reaction called the substitution reaction. The thermal and kinetic data are summarized in Table 5.1. The decomposition reaction presents a heat release rate of 10 W kg 1 at 150 °C. Together with the activation energy, this heat release rate allows calculating the time to explosion ( I M R id) as a function of temperature. The amounts of reactants to be used in discontinuous operations are summarized in Table 5.2. The solvent used has a boiling point of 140 °C at atmospheric pressure. 

This type of experiment can be repeated at other temperatures, determining the activation energy and the estimation of time to explosion. The concept of time to explosion or TMRad (Time to Maximum Rate under Adiabatic conditions) is extremely useful for that purpose [18]. This TMRad can be estimated by... 

To obtain a more realistic estimation of the behavior of an autocatalytic reaction under adiabatic conditions, it is possible to identify the kinetic parameters of the Benito-Perez model from a set of isothermal DSC measurements. In the example shown in Figure 12.11, the effect of neglecting the induction time assumes a zero-order reaction leading to a factor of over 15 during the time to explosion. Since this factor strongly depends on the initial conversion or concentration of catalyst initially present in the reaction mass, this method must be applied with extreme care. The sample must be truly representative of the substance used at industrial scale. For this reason, the method should be only be applied by specialists. 

Methyl-2,4,5-trinitroimidazole has been synthesized from 4-nitroimidazole using stepwise nitration and further methylation by dimethylsulfate or from commercially available imidazole. l-Methyl-2,4,5-trinitroimidazole is relatively insensitive to impact, and its thermal stability is excellent. The calculated detonation properties point to the fact that its performance is about 30% better than that of TATB. The data of impact sensitivity, friction sensitivity, time-to-explosion tern-... 

This test bridges the gap in the growth from thermal decomposition reaction to explosion and eventually involves fast oxidation reactions. A small sample of explosive is pressed into a blasting cap cup made of gilding metal. The cup is then inserted into a molten Wood s Metal bath. The time it takes from insertion in the bath until some noticeable reaction takes place (usually a mild explosion) is noted. The test is repeated at several different bath temperatures. See Table 6.3. A smooth curve is drawn through the data points (time to explosion versus bath temperature), and the temperatures that cause reaction in 1, 5, and 10 s are interpolated from the graph. 

The validity of Eq. (22.8) is demonstrated by the results of small-scale tests where a slab of explosive of known properties is held between the two heated anvils in a manner that seals in all evolved gases. The anvils are electrically heated and held at a constant temperature. The time it takes to explosion is measured. The test is repeated over a range of temperature, and the results are plotted as reciprocal anvil temperature versus log of time to explosion. The temperature at which the relationship become asymptotic (time approaches infinity) is defined as the critical temperature, Tc, corresponding to the conditions of Eq. (22.8). 

Weber describes an apparatus in vdtich the powder is heated in glass tubes at Ido , 170, 180 and 300 and the time to explosion is measured. 

The importance of impurity on azide sensitivity was also demonstrated by Singh, who observed that [BiNs] (0.24 wt %) increased impact sensitivity (Figure 23) [61], Shorter times to explosion for the more thermally reactive [BiNs ] "-doped azide is expected based on current theory of thermal sensitivity. Increased impact sensitivity along with increased thermal sensitivity and thermal decomposition rate give indirect indication that impact sensitivity is thermal in nature. 

The question of the effects of iron impurities on the explosive properties, of lead azide was addressed by Hutchinson, who subjected pure and Fe" -doped lead azide (0.016 mole % iron) to thermal decomposition and explosive tests [67], Both samples pressed to the same density had identical detonation velocities, 4650 m/sec at a 3.5 g/ml. The iron-doped material was slightly more sensitive to impact (ball-drop test) and to heat (time to explosion tests) than was the pure material. However, the differences in results were close to the scatter in the data. 

Table DC Time to Explosion in Siher Azide as a Function of Frequency [38] ...

Bowden and McLaren also determined the time to explosion as a function of the frequency of the applied electrical field (900 V/cm). The results, summarized in Table IX, showed that the time to explosion was greatly increased if the frequency of the applied field was increased beyond 100 cycles/sec. They suggest that the breakdown is due to field emission from the cathode. If electrons from the cathode enter the crystal with sufficient energy to remove electrons from the ions of the crystal lattice, current will increase rapidly and decomposition will take place, followed by self-heating and explosion. 

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