Initiation and Thermal Decomposition

Ignition occurs when part of a combustible material such as an explosive is heated to or above its ignition temperature. The ignition temperature is the minimum temperature required for the process of initiation to be self-sustaining. 

Tign is the temperature at which the heat generated in the composition is greater than the heat lost to the surroundings, or more accurately, Tign should equal ignition temperature — initial temperature . 

Explosive materials are ignited by the action of an external stimulus which effectively inputs energy into the explosive and raises its temperature. The external stimulus can be friction, percussion, electrical impulse, heat, etc. Once stimulated the rise in temperature of the explosive causes a sequence of pre-ignition reactions to commence. These involve transitions in the crystalline structure. 

The Chemistry of Explosives, 3rd Edition By Jaequeline Akhavan J. Akhavan 2011 

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Ignition, Initiation and Thermal Decomposition Heat generated or lost... 

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

Udupa, M. R., Propellants, Explos., Pyrotech., 1983, 8, 109-111 Unusually sensitive to initiation and of high explosive power [1], it decomposes violently at 350°C [2], The explosive properties have been determined [3], and thermal decomposition at 275-325° was studied in detail [4],... 

J.D. Blackwood, The Initiation, Burning and Thermal Decomposition of Gunpowder , Proc. Roy. Soc. London, 213, 1952. 

For recently prepared terpolymers of PO/CO2 noticeable improvements in thermal stability were thus achieved. The rate of decomposition decreases with the termonomer content. Part of the effect obviously originates from the lower concentration of propylene carbonate entities and another part from the lower flexibility of the main chain. In addition, another explanation for the increase in thermal stability with termonomers (which is, however, not substantiated) could be found in the solubility of water in the product because every recent study indicates that hydrolysis is a major cause of the initiation of thermal decomposition [23]. Also, additives have been identified as slowing down thermal degradation. 

Figure 7. Photolytic and thermal decomposition pathways in crystalline ABP. Initial photolysis at 300-400 nm gives methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemi-cally to give methyl-phenyl radical pairs.

Heating wood to temperatures slightly above 100°C initiates some thermal decomposition. A more active decomposition takes place above 250°C, and for industrial applications temperatures up to 500°C may be used. Above 270°C, thermal decomposition does not require any external heat source because the process becomes exothermic. The thermal decomposition of wood is usually called pyrolysis or carbonization. A number of other terms such as wood distillation, destructive distillation, and dry distillation are used interchangeably for this type of processing. 

Dr. Rudolf Meyer was born on 4. 3. 1908 in Spandau (Berlin) and took his degree in Physical Chemistry. He began his initial studies in the area of energetic compounds in connection with his Doctor s degree in 1931 at Professor Boden-stein s Institute in Berlin with a paper on the enthalpy of formation and thermal decomposition of hydrazoic acid. After taking his Doctor s degree, he entered the Dynamit Nobel Company in 1934 as assistant to Dr. Ph. Naoum. He worked there from 1936-1945 on the development of pourable ammonium nitrate explosives and on hollow charges. 

The same type of decomposition in the presence of NO yields substantial amounts of GeHsOGeHs, lending further support to the idea that reaction 32 is the main process . While the mercury photosensitized reaction is generally accepted to proceed primarily via Ge—H bond cleavage , a different process (equation 33) involving the initial formation of germylene, lGeH2, becomes the most important primary reaction under direct photochemical and thermal decomposition. 

Summary Graft copolymers with poly(organosiloxane) backbone and thermoplastic side chains have been synthesized via the "grafting fi om" method based on azo- and triazene modified poly(organosiloxane)s. Initiation of free radical polymerization is possible from the polymeric azo and triazene initiators after thermal decomposition of the labile frmctions in solution. The graft products have been characterized by NMR, GPC, and DSC. Stable, free standing films can be cast from the graft copolymers. 

Hydroxytelechelic polymer synthesis with redox systems requires hydrogen peroxide as an oxidizing agent and, generally, takes place in aqueous media (to solubilize the salts). This kind of polymerization is possible at lower temperatures compared to polymerizations initiated by thermal decomposition of H202. Therefore, the less frequent transfer reactions improve the polymer functionality and its polydispersity. 

The polymerization initiated by thermal decomposition of hydrogen peroxide has been extended to butenes (1-butene, 2-butene, isobutene) 99). Their molecular weight is below 1000 and their functionality varies between 2 and 4. The weak efficiency (yields < 10 %) of homopolymerization of various monomers initiated by H202 is low and reactivities of the monomers decrease in the series ... 

The initiation rate constant and activation energy have been determined for bulk or solution polymerization of methyl methcrylate initiated by thermal decomposition of hydrogen peroxide154) (Table 3.3). The initiation constant decreases in the following series of solvents ... 

This problem was first treated in detail by Haward (1949). He considered the case of a bulk polymerization that has been compartmentalized by subdividing the reaction system into a large number of separate droplets, each of volume v. Radicals are generated exclusively within the droplets and always in pairs. An example would be the polymerizatiim of styrene in emulsified droplets dispersed in water initiated the thermal decomposition of an oil-soluble initiator which partitions almost exclusively within the monomer droplets. In the model considered by Haward, radicals are unable to exit from the droplets into the external phase. The only radical-loss process is in fact bimolecular mutual termination. It therefore follows that all the droplets must always contain an even number (including zero) of propagating radicals, and that the state of radical occupancy will change in increments of 2. The conclusion reached by Haward is that in this case the effect of compartmentalization is to reduce the overall rate of polymerization per unit volume of disperse phase. The f ysical reason for this is that, as the volume of the droplets is reduced, so are the opportunities for a radical to escape from the others—and hence to avoid mutual... 

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