Ammonium perchlorate, decomposition mechanisms

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

A considerable amount of research has been conducted on the decomposition and deflagration of ammonium perchlorate with and without additives. The normal thermal decomposition of pure ammonium perchlorate involves, simultaneously, an endothermic dissociative sublimation of the mosaic crystals to gaseous perchloric acid and ammonia and an exothermic solid-phase decomposition of the intermosaic material. Although not much is presently known about the nature of the solid-phase reactions, investigations at subatmospheric and atmospheric pressures have provided some information on possible mechanisms. When ammonium perchlorate is heated, there are three competing reactions which can be defined (1) the low-temperature reaction, (2) the high-temperature reaction, and (3) sublimation (B9). 

Catalysts which enhance the burning rate of composite propellants are generally believed to accelerate the decomposition of ammonium perchlorate, but the catalytic mechanism is still not very clear. The important observed aspects of this catalysis can be summarized as follows ... 

Materials for a batch of ammonium perchlorate castable propellant were charged into a mechanical mixer. A metal spatula was left in accidentally, and the contents ignited when the mixer was started, owing to local friction caused by the spatula. A tool-listing safety procedure was instituted [1]. The literature on the kinetics of thermal decomposition has been reviewed critically [2],... 

In a review of the course and mechanism of the catalytic decomposition of ammonium perchlorate, the considerable effects of metal oxides in reducing the explosion temperature of the salt are described [1], Solymosi s previous work had shown reductions from 440° to about 270° by dichromium trioxide, to 260° by 10 mol% of cadmium oxide and to 200°C by 0.2% of zinc oxide. The effect of various concentrations of copper chromite , copper oxide, iron oxide and potassium permanganate on the catalysed combustion of the propellant salt was studied [2], Similar studies on the effects of compounds of 11 metals and potassium dichromate in particular, have been reported [3], Presence of calcium carbonate or calcium oxide has a stabilising effect on the salt, either alone or in admixture with polystyrene [4],... 

The activation energy of decomposition of both irradiated and unirradiated ammonium perchlorate in the orthorhombic form below 240°C is ca. 18.0 kcal/mole according to Freeman. This seems to indicate that irradiation of ammonium perchlorate does not change the decomposition mechanism but provides many more nucleation sites. 

Ya Karpenko, O.P. Korobeinitchev, V.N. Panfilov, E.F. Khairetdinov, On the Mechanism of the Thermal Decomposition of Ammonium Perchlorate, Combust. Flame, 15(1970) 71-78. 

A. V. Rayevskii, in Mechanisms of Thermal Decomposition of Ammonium Perchlorate, Inst. Chem. Phys. Acad. Sci. USSR, Chernogolovka Branch, 1981, p. 30. 

Mechanism of Deflagration of Hydrazine Perchlorate. One approach to the mechanism of hydrazine perchlorate deflagration is to consider whether it fits the classification of a vaporization-type process like ammonium perchlorate where the material vaporizes without decomposition, and exothermic gas phase reactions occur with resultant heat transfer to the condensed phase. The alternative to a process of this type is one wherein heat production occurs in the molten zone as a result of condensed phase reactions. 

Nitronium perchlorate, as an ion complex may exist as a transitory intermediate, which may either revert to ions, sublime, or decompose. A similar mechanism has been proposed for the decomposition of ammonium perchlorate (3). 

Reviews of the application of electrical measurements in solid state decompositions have been given by Kabanov [52]. Electrical conductivity measurements, both a.c. and d.c. studies, have been used to characterize the species that participate in the thermal decomposition of ammonium perchlorate [53,54] (this reaction is discussed in Chapter 15). Other studies have been concerned with the mechanisms of oxide decompositions [55,56]. Torkar et al. [56] conclude from electrical conductivity evidence that the decompositions of alkali oxides are more complicated than exciton formation processes. 

Ammonium perchlorate decomposes over the wide temperature range of 200 C to 440 C by two different mechanisms.[40, 43] Between 200 C and 300 C the decomposition takes place by an autocatalytic process which ceases after about 30% decomposition. The decomposition proceeds via a second mechanism in the high-temperature regime (300-430°C), where the reaction is not autocatalytic and the decomposition goes to completion. Bircumshaw and Newman [47, 48] were the first to report that simultaneous with decomposition, sublimation of AP takes place throughout both the low- and the high-temperature decomposition regions. 

Mechanism of low-temperature decomposition t>f AP Explosive properties of NH4CIO4 Manufacture of ammonium perchlorate Specification Perchlorate of metals Other perchlorate Hydrazine perchlorate I iydrazinc dipcrchlorate... 

Baumgartner et al. (9) compared the thermal and mechanical degradation of filled and unfilled elastomers. They were particularly interested in the long-term aging and fatigue behavior of solid propellants filled with ammonium perchlorate or potassium chloride. They reported that the mechanisms for thermally and mechanically induced decomposition of the propellant binder appear equivalent. At low temperatures, mechanical processes control the decomposition rates of the polymers, whereas thermal processes control the decomposition at high temperature. They further note that, "Equivalence of thermal and mechanical degradation... 

The decomposition of a number of perchlorates containing substituted ammonium groups (various methylamines, guanadine etc.) [935—939] resemble the mechanism proposed for AP decomposition in that proton transfer is identified as the initial step. 

Studies of the decomposition of this substance have been carried out in order to further understanding of the mechanism of the deflagration of ammonium perchlorate440 and of the reactions occurring in perchloric acid fuel flames441. 

The findings concerning mechanism and kinetics of thermal decomposition of nitric and perchloric acids and their salts (ammonium, hydrazinium, hy-droxylammonium and metal salts) are presented in the monograph [11]. 

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