Heat-initiated explosions

Explosives are commonly categorized as primary, secondary, or high explosives. Primary or initiator explosives are the most sensitive to heat, friction, impact, shock, and electrostatic energy. These have been studied in considerable detail because of the almost unique capabiUty, even when present in small quantities, to rapidly transform a low energy stimulus into a high intensity shock wave. 

Silver acetylide is a more powerful detonator than the copper derivative, but both will initiate explosive acetylene-containing gas mixtures [1]. It decomposes violently when heated to 120-140°C [2], Formation of a deposit of this explosive material was observed when silver-containing solutions were aspirated into an acetylene-fuelled atomic absorption spectrometer. Precautions to prevent formation are discussed [3], The effect of ageing for 16 months on the explosive properties of silver and copper acetylides has been studied. Both retain their hazardous properties for many months, and the former is the more effective in initiating acetylene explosions [4],... 

It can be detonated on impact, but is normally considered a stable intermediate (m.p. 288° C), suitable for purification of pyridine [1]. Occasionally explosions have occurred when the salt was disturbed [2], which have been variously attributed to presence of ethyl perchlorate, ammonium perchlorate, or chlorates. A sample was being dried in a glass vacuum desiccator. Over-enthusiastic release of vacuum distributed some of the salt onto the lid. The grinding action as the lid was slid off initiated explosion of this, shattering the lid [6], A safer preparative modification is described [3], It explodes on heating to above 335°C, or at a lower temperature if ammonium perchlorate is present [4], A violent explosion which occurred during the final distillation according to the preferred method [3] was recorded [5],... 

A9 - Heat generated from chemical reaction may initiate explosion... 

Nuclear Explosion Survival Rule 1 Close your eyes and turn away from the initial explosion the intense heat from the fireball can cause temporary or permanent blindness. 

Nuclear Explosion Survival Rule 2 Seek shelter or retreat behind a solid barrier immediately and stay hidden for at least 2 minutes. Note that every fraction of a second out in the open increases exposure to the intense heat, initial radiation, and air blast. 

Andrej Ma ek, "Sensitivity of Explosives , ChemRevs 62, 41-63(1962). "The sensitivity of an explosive can be defined as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition (p 60). This definition 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. If the energy is not supplied directly as heat, but by mechanical means (such as a shock), there is the additional requirement of quantitative assessment of conversion of the stimulus into heat (p60)... 

Since little or no heat is evolved during the induction period, the initial energy input in the "hot spot is not immediately related to the heat of decomposition of the explosive. In such cases, thermal instability is not conditioned by the heat of explosion or the mass of explosive but by other less obvious factors which apparently are related to the nature and amount of the autocatalyst... 

They derive an expression for the ratio of the time lags, between heating and explosion, and the detonation front temperature and at the C-J temperature. They suggest that in the detonation front, chemical reaction is merely initiated, and proceeds to only a slight extent, the major portion occurring at the C-J temperature. 

Originally Bowden s school suggested that heat from the adiabatic compression of such gas bubbles initiated explosion in the surrounding liquid. Johansson coworkers (Ref 4), however, pointed out that heat flow from a compressed gas bubble to the surrounding liquid is much too slow to account for the observed phenomena, particularly at low impact energies. They have shown that to achieve explosion fine droplets... 

Use of high energy radiation to create hot spots. Attempts have been made to initiate explosives by ionizing radiation such a-particles, high speed electrons, y-rays, Pions etc (Refs 8 9). No initiations were observed. Cerny Kaufman (Ref 9) take this absence of initiation to indicate failure of the hot spot model. However a crude preliminary calculation, based on the Friedman model (Ref 15), suggests that the dimensions of the Pion heated regions for Lead Azide (Fig 2 of Ref 9) and for RDX (Fig 3 of Ref 9) are smaller than the critical hot spot dimension at the corresponding temperatures... 

Note Datta Chatterjee used a KHS04 bath which they heated up to 500°. Kast and Haid tested a number of initiating explosives by both methods (Ref 11) and found that, although most explosives give higher results by method b), there are many cases in which both methods give the same results, especially if heating is conducted at a rate of 20° per minute in method a)... 

The main requirements for initiating explosives are 1) sufficient sensitivity to heat, flame or impact etc that they can be readily detonated, but not so sensitive as to make them unsafe to handle and transport 2) sufficient stability at elevated temperatures that they will not decompose while stored in ammunition, thus rendering it unusable. Brisance, power and velocity of detonation are of secondary impor- tance in these explosives... 

KL) Anon, "Military Explosives", TM 9-1300-214/TO 11A-1-34 (1967). Chapter 5. Properties and Tests of High Explosives Sensitivity to Frictional Impact (pp 5 1 to 5-3) Sensitivity to Friction (5 3 to 5-6) Sensitivity to Frictional Impact (5-6) Sensitivity to Heat and Spark, which includes Explosion Temperature Test (5-6 to 5-9) Sensitivity to Initiation (5-9) Stability Tests, which include 75° International Test, 100° Heat Test, Vacuum Stability Test and Potassium Iodide—Starch Test (5 9 to 5-15) Brisance Tests which include Sand Test, Plate Dent Test and Fragmentation Test (5 15 to 5-18 and Fig 5 13 on p 5-19) Initiating Value (5-18 5-20) Sympathetic Detonation (5-20 to 5-21) Power which includes Heat of Explosion Test, Ballistic Pendulum Test and Trauzl Lead Block Test (5-21. to 5-24) Blast Effect (5-24 to 5-27) Cratering Effect (5-28 5 29) and Munroe-Neumann Effect (5-29 to 5 35)... 

The net heat difference between the heats of formation of the reactants and products in a chemical reaction is termed the heat of reaction . For oxidation, this heat of reaction may be termed heat of combustion . The energy liberated when explosives deflagrate is called the heat of deflagration whereas the energy liberated on detonation of explosives is called the heat of detonation in kj mol"1 or the heat of explosion in kj kg"1. In primary explosives, which are used as initiators,... 

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. 

T. Urbanski and Kruszynska [41] made a comparative study of the sensitiveness to impact of lead picrate and other initiating explosives. They found lead picrate to be more sensitive than any other substance. They also examined the decomposition of lead picrate on hot metal plates. On contact with a metal surface heated to 341 °C it explodes after 3 sec, and on one heated to 370°C after 1 sec. 

---