Thermal data, explosives

In a nutshell, it may be concluded that DTA, DSC and TGA have been used mainly to determine the thermal properties of explosives like melting points, thermal stability, kinetics of thermal decomposition and temperatures of initiation and ignition etc. Further, the properties which can be calculated quantitatively from the experimentally obtained values are reaction rates, activation energies and heats of explosion. DTA data of some explosives are given [46] in Table 3.6. 

Another problem may arise in the case of loss of control starting from 40 °C, the MTSR is 216°C. This temperature is much higher than the two limits of T024 = 113°C and Tm = 122 °C. This means that the secondary reaction is immediately triggered. Thus, a lack of control of the reactor temperature results in a thermal explosion. Moreover, the boiling point of 140 °C is reached during runaway, which would result in a pressure increase. Eventually the reactor will burst and there will be a flammable vapor release that may lead to a secondary room explosion. The data are summarized in the scenario presented in Figure 6.6. 

The predetonation distance (the distance the decomposition flame travels before it becomes a detonation) depends primarily on the pressure and pipe diameter when acetylene in a long pipe is ignited by a thermal, nonshock source. Figure 2 shows reported experimental data for quiescent, room temperature acetylene in closed, horizontal pipes substantially longer than the predetonation distance (44,46,52,56,58,64,66,67). The predetonation distance may be much less if the gas is in turbulent flow or if the ignition source is a high explosive charge. 

The evaluation of ehemieal reaetion hazards involves establishing exothermie aetivity and/or gas evolution that eould give rise to inei-dents. Flowever, sueh evaluation eannot be earried out in isolation or by some simple sequenee of testing. The teehniques employed and the results obtained need to simulate large-seale plant behavior. Adiabatie ealorimeters ean be used to measure the temperature time eurve of selfheating and the induetion time of thermal explosions. The pertinent experimental parameters, whieh allow the data to be determined under speeified eonditions, ean be used to simulate plant situations. 

It is very endothermic (AH°f (g) +150.2 kJ/mol, 3.66 kJ/g). A sample exploded when heated in a sealed ampoule [1], and during redistillation at 59°C/1 bar, a drop of liquid fell back into the dry boiler flask and exploded violently [2], The explosive decomposition has been studied in detail [3], and existing data on thermal explosion parameters have been re-examined and discrepancies eliminated [4], See Ethyl isocyanide... 

It is somewhat endothermic (AH°f (g) +87.5 kJ/mol, 1.0 kJ/g), the liquid may explode on pouring or sparking at 2°C, and the gas readily explodes on rapid heating or sparking [1,2], on adiabatic compression in a U-tube, or often towards the end of slow thermal decomposition. Kinetic data are summarised [3], The spontaneously explosive decomposition of the gas was studied at 42-86°C, and induction periods up to several hours were noted [4], Preparative precautions have been detailed [5],... 

With pressure-time data from spontaneous deflagrations (thermal explosions), the maximum expected pressure, and the time for pressure-rise can be estimated. Furthermore, the so-called "specific energy" (F) or "explosive power" of substances [24, 31] by Equation (2-22) from experiments in which the sample mass is varied. 

If DSC data have been obtained for a pure material or a reaction mixture, several thermal stability indicators (ASTM E 1231-96) may be estimated from the data. These are adiabatic temperature rise, explosion potential, instantaneous power density, time to maximum rate, and NFPA instability index (Leggett 2002). 

Reaction. va/efy-thermal test data, hazardous bond groups, and exothermic reactions. Powder handling/milling-dust explosion issues. 

Four nitrosamines, seven nitramines, three nitroesters and the explosives Semtex 10 and Composition B have been investigated by TGA. Linear dependence was confirmed between the position of the TGA onsets, as defined in the sense of Perkin-Elmer s TGA-7 standard program, and the samples weights. The slope of this dependence is closely related to the thermal reactivity and molecular structure. The intercept values of the dependence correlate with the autoignition temperatures and with the critical temperatures of the studied compounds, without any clear influence from molecular structure. Results show that Semtex 10 exhibits approximately the same thermostability as its active component pentaerythrityl tetranitrate (PETN, 274). Results also show that TGA data for Composition B do not correlate with analogous data for pure nitramines564. 

Few models have been advanced to explain the mechanism of the thermal explosion, and of these, only one seems to fit most of the available experimental data. 

It is possible that this theory can be adapted to explain molten metal-water thermal explosions although many needed data are still unavailable. One might presume that, at the molten metal-wet surface interface, there is some chemical reaction. Possibly that of the metal plus water or metal plus surface to lead to localized formation of salt solutions. These may then superheat until homogeneous nucleation occurs. The local temperature and pressure would then be predicted to be far in excess of the critical point of pure water (220 bar, 647 K) and a sharp, local explosion could then result. Fragmentation or subsequent other superheat explosions would then lead to the full-scale event. 

Of the explosives listed in Table 4, only those such as NG with vapour pressures greater than 10 Pa at 25°C are good candidates for the direct detection of vapour by current instrumental techniques. However, vapour pressure rises markedly with temperature. In addition, consideration of the thermal stability data in Table 4 offers the possibility of heating samples containing traces of involatile explosives such as RDX or PETN to increase their vapour pressure and render them detectable. This is the basis of the common technique of combining a heated inlet system with a vapour-type detector, for example, the method of desorption from a swab on a heated stage often used with IMS or TEA systems. This approach has greatly broadened the scope of what were previously viewed as vapour-type detectors and is now standard practice such instruments are now known as particle detectors. 

The sensitivity of explosives has been defined by Koenen et al (Ref 10) as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition. This definition is, accdg to Ma ek (Ref 13, p 60) meaningful and can serve as a basis of quantitative fundamental treatments provided the imparted energy is thermal and provided its initial distribution in time and space is known. The accuracy of treatments of thermal explosion described in Section IIA of Mafcek s paper is then limited mainly by the accuracy of chemical kinetic data... 

Under adiabatic decomposition. Cook (Ref 8, p 178) states that F.P. Bowden et al E.K. Rideal A.J.B. Robertson J.L. Copp, A. Yoffe and their coworkers (See Refs listed on pp 42-3 of Cook s book) have made outstanding contributions to the knowledge of the sensitiveness of explosives. Their preliminary investigation showed that all types of sensitivity measurements may be understood in terms of thermal decomposition and laws of adiabatic decomposition. Moreover, they developed A B data for equation ... 

To illustrate the use of the heat data in the preceeding section for estimating thermal explosion parameters, let us determine the critical thickness of a semi-infinite slab of Tetryl kept at 445°K. From Ref (2) we take E = 35000 cal/mole and Z = 1013sec . Substituting these values in eqn (3) and then into eqns (1) and (2) gives ... 

These examples of explosives show beyond doubt that the m.p. is raised by the introduction of amino group/s. Further, data on thermal decomposition show that the thermal stability is associated with high melting point and low vapor pressure... 

The data indicate that BTATNB is slightly more thermally stable (m.p. 320 °C as compared with 310°C for PATO) coupled with better insensitivity toward impact and friction. Similarly, 5-picrylamino-l,2,3,4-tetrazole [72] (PAT) [Structure (2.28)] and 5,5 -styphnylamino-l,2,3,4-tetrazole [73] (SAT) [Structure (2.29)] have been synthesized by condensing picryl chloride and styphnyl chloride respectively with 5-amino-l,2,3,4-tetrazole in methanol. A comparison of thermal and explosive properties of newly synthesized PAT (deflagration temperature 203 °C and calc. VOD 8126ms"1) and SAT (deflagration temperature 140 °C and calc. VOD 8602 ms"1) reveals that PAT is more thermally stable than SAT but more sensitive to impact and friction. 

The m.p. data show that II is more thermally stable than I. However, thermal stability decreases with further substitution of C-nitro for heterocyclic nitrogen to give III and IV in spite of increased resonance stabilization of the parent ring. The decrease in thermal stability appears to be the result of increased steric crowding about the ring as we proceed from II to IV. This is supported by the fact that when the bulky 4-picrylamino group is removed from III, it yields PYX, one of the most thermally stable explosives [78]. 

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