Pyrotechnic reaction

Pyrotechnics is based on the estabflshed principles of thermochemistry and the more general science of thermodynamics. There has been Httle work done on the kinetics of pyrotechnic reactions, largely due to the numerous chemical and nonchemical factors that affect the bum rate of a pyrotechnic mixture. Information on the fundamentals of pyrotechnics have been pubflshed in Russian (1) and English (2—6). Thermochemical data that ate useful in determining the energy outputs anticipated from pyrotechnic mixtures are contained in general chemical handbooks and more specialized pubHcations (7-9). 

A recent review relating the pyrotechnic reaction mechanism, particle size, stoichiometry, temp and compaction density to burning rate is Ref 66, and a study of the effect of multidimensional heat transfer on the rate of flame propagation is Ref 120, which showed that the material of the delay body has no effect on the performance of most delay compns, a finding which agrees with test data... 

As with most pyrotechnic reactions, the energetic properties which are demonstrated involve a comprehensive mixture of inorganic, physical and solid-state chemistry and these have been well documented by authors such as Conkling, McLain and Ian von Maltitz. 

For pyrotechnic reactions, many of which proceed at constant pressure and temperature, the two quantities that define the free energy of a system are the entropy, as discussed above, and the enthalpy, H, which represents the heat content of a system. 

At room temperature, entropy effects are so small that they have little effect on the direction of a chemical reaction unless the difference in AG or 2s.H between reactants and products is correspondingly small. But at the high temperature encountered in pyrotechnic reactions such as the combustion of gunpowder, the relative importance of the change in entropy increases until it becomes a dominant factor. Hence, the importance of the temperature term in the free energy equation. 

Finally it should be appreciated that pyrotechnic reactions usually follow a sequence of SOLIDS GASES. This is in accordance with a positive value for AS being associated with an increase in the disorder of a system. Quantitatively S(soiid)<5 (iiquid) S (gas)-... 

Thermal analytical techniques such as thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) have all been successfully employed in studying the pyrotechnic reactions of energetic materials such as black powder, as well as of binary mixtures of the constituents. 

As well as acting as a support for the pyrotechnic composition, the steel wire serves as a heat conductor, promoting the smooth propagation of the pyrotechnic reaction along the sparkler. 

M.E. Brown and R.A. Rugunanan, A Thermo-analytical Study of the Pyrotechnic Reactions of Black Powder and its Constituents , Ther-mochim. Acta, 1988, 134, 413. 

Inert diluents such as clay and diatomaceous earth can also be used to retard burning rates. These materials absorb heat and separate the reactive components, thereby slowing the pyrotechnic reaction. 

If the propagation of the pyrotechnic reaction is a solid-solid or solid-liquid phenomenon, without the significant involvement of gas-phase components, then an increase in loading pressure should lead to an increase in burn rate (in grams per second). An example of this possibility is given in Table 4.6. 

Alternatdy, the pyrotechnic reaction can occur in a separate container, and the heat that is produced volatilizes a smoke-forming component contained in an adjacent compartment. The vaporization and dispersion of heavy oils to create white smoke uses this technique. 

Finally, a product of a pyrotechnic reaction may vaporize from the reaction zone and subsequently condense as fine particles in air, creating a smoke. Potassium chloride (boiling point 1407°C) produces smoke in many potassium chlorate and potassium perchlorate compositions, although smoke is usually not a goal sought from these mixtures. 

The processes used to generate a white smoke by means of a pyrotechnic reaction include ... 

Volatilization of oil A pyrotechnic reaction produces the heat needed to vaporize high molecular weight hydrocarbons. The subsequent condensation of this oil in air creates a white smoke cloud. The toxicity of this smoke is probably the least of all the materials discussed here. 

The famous Chinese firecracker uses a mixture of potassium chlorate, sulfur, and aluminum. The chlorate combined with sulfur makes this mixture doubly dangerous for the manufacturer. The ignition temperature of the potassium chlorate/sulfur system is less than 200°C The presence of aluminum - an excellent fuel - guarantees that the pyrotechnic reaction will rapidly propagate once it begins. Safety data from China is unavailable, but one has to wonder how many accidents occur annually from the preparation of this firecracker composition. The preparation of potassium chlorate/sulfur compositions was banned in Great Britain in 1894 because of the numerous accidents associated with this mixture ... 

These concepts permit the chemist to examine chemical reactions and determine the mass relationships that are involved. For example, consider the simple pyrotechnic reaction... 

A pyrotechnic reaction generates a substantial quantity of heat, and the actual flame temperature reached by these mixtures is an area of study that has been attacked from both the experimental and theoretical directions. 

McLain has proposed that the maximum in performance centered at approximately 15% boron by weight indicates that the principal pyrotechnic reaction for the BaCrO /B system is... 

Most explosive and pyrotechnic reactions produce significant quantities of smoke, and this visible phenomenon may or may not be desirable. Smoke can obscure colored flames, and therefore attempts are made to keep the production of smoke to a minimum in such mixtures. However, a variety of smoke-producing compositions are purposefully manufactured for use in daytime signalling and troop and equipment obscuration, as well as for amusement and entertainment purposes. 

Two basic processes are used to create smoke clouds the condensation of vaporized material and the dispersion of solid or liquid particles. Materials can either be released slowly via a pyrotechnic reaction or they can instantaneously be scattered using an explosive material. Technically, a dispersion of fine solid particles in air is termed a smoke, while liquid particles in air create a fog. A smoke is created by particles in the 10 -10 " meter range, while larger suspended particles create a dust (1). ... 

The molecules creating the colored smoke must be of low toxicity (including low carcinogenicity). Further, they must readily sublime without decomposition at the temperature of the pyrotechnic reaction to yield a dense smoke of good color quality [3]. 

Water-Pyrotechnic Reactions. See in Vol 8, M150-R to M252-L under Moisture , and the following Addnl Refs ... 

G.J. Rees, Pyrotechnic Reaction of Silicon and Red Lead , Comb Flame 31 (2), 105... 

Study of Pyrotechnic Reactions by Temperature Profile Analysis and Differential Thermal Analysis , Combustn Flame 24 (1), 137—38 (1975) CA 83, 30450 (1975) 38) A.G. 

A mixture with boron incandesces on heating, and with silicon the reaction is vigorous. If aluminium is present the mixture explodes on heating (but the same is true if silicon is absent) [1]. The pyrotechnic reaction of boron and lead oxide mixtures has been studied by DSC [2]. 

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