Water-pyrotechnic reactions

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

Another matter of concern in pyrotechnic formulations is the possibiHty of exchange reactions occurring between components. Addition of ammonium salts to compositions containing nitrate oxidizers can produce ammonium nitrate, a very hygroscopic material. The composition then becomes quite prone to pick up water and its performance deteriorates. The addition of an ammonium salt to a chlorate-based formulation can lead to the... 

Strontium peroxide, Sr02, is a white powder with a specific gravity of 4.56 that decomposes in water. It is made by the reaction of hydrogen peroxide with strontium oxide and is used primarily in pyrotechnics and medicines. 

A pyrotechnic mixture of aluminium powder with potassium perchlorate, barium nitrate, potassium nitrate and water exploded after 24 h storage under water. Tests revealed the exothermic interaction of finely divided aluminium with nitrate and water to produce ammonia and aluminium hydroxide. Under the conditions prevailing in the stored mixture, the reaction would be expected to accelerate, finally involving the perchlorate as oxidant and causing ignition of the mixture. 

Mixtures of magnesium (or aluminium) powder with water can be caused to explode powerfully by initiation with a boosted detonator [1], The use of organic coatings on magnesium or aluminium powder in pyrotechnic compositions prevents reaction with atmospheric moisture and problems resulting from hydrogen evolution [2],... 

STOICHIOMETRIC MIXTURE A balanced mixture which, on reaction, will yield a stoichiometric compound. For example, two molecules of hydrogen and one molecule of oxygen constitute a stoichiometric mixture because they yield exactly two molecules of water on combustion. Such a balance is important when formulating pyrotechnic compositions. 

Most commercially available methylaluminoxanes are produced by careful reaction of water with trimethylaluminum (TMAL) in toluene. Reaction must be closely controlled to avoid what renowned organometallic chemist John Eisch called "a life threatening pyrotechnic spectacle" (16). Unfortunately, there have been explosions and injuries reported during MAO preparations. Water must be introduced at low temperature and in forms that moderate the potentially violent reaction. For example, water has been introduced as hydrated salts, ice shavings or atomized spray. Even with these precautions, explosive reactions have occurred. The overall reaction is given in eq 6.1. 

Water reactivity is an expression of the likelihood of occurrence of exothermic or other undesirable chemical reactions upon the addition of water to a pyrotechnic composition. In some cases these reactions can lead to ignition. 

The sulfide melts have unique chemical properties. Chemical reactions which occur in water do not always occur in sulfide melts. The author was surprised to find evidence of strong oxidizing ions, such as peroxides, dissolved in sulfide melts. In water, sulfide ion will reduce iodine to iodide and potassium peroxide, and superoxide will oxidize aqueous iodide solutions to iodine, or further to periodate ion. It is not surprising that much of the literature about pyrotechnically produced sulfide melt sparks is filled with reports of chemical species which do not exist at 800°C, such as the thiosulfate ion. These ions are quickly formed when the melts are dissolved in water prior to analysis. The author found radically different chemistry occurring at room temperature for spark droplets only seconds after collection by freezing. Some of the sulfide melts studied were not stable for more than one half second after capture. Special handling techniques were, and are, being developed. As techniques evolve the future may yield better data. Chemistry is an effort to be better informed tomorrow than is possible today. 

A sample of strontium nitrite has been synthesized and thermally characterized by Tuukkanen and coworkers [63] to investigate its role in the ageing of magnesium-strontium nitrate pyrotechnic compositions in the presence of water vapour. Studies by TG-MS and isothermal microcalorimetry showed that the addition of strontium nitrite to a 50% magnesium-50% strontium nitrate composition eliminated the induction reaction normally observed in closed ampoule studies in air at 50 and relative humidities in the range 65-69%. 

When immersed in water, the explosives in water-activated contrivances are initiated by electric current as water (acting as an electrolyte) immerses the electrodes of specially designed batteries by chemical reaction with water and by pressure sensors triggered at certain depths. These contrivances include ammunition, signal flares and other pyrotechnics, sounding devices (which are dropped by ships to determine depth), and actuating cartridges for gas cylinders that automatically inflate life rafts and jackets. 

In a somewhat different manner, the hydrolytic and heat-forming influence of water prevails in some fuel-oxidizer systems that are reactive in the common manner of pyrotechnical combinations, but will also be brought to an intensely-hot flaming reaction by the addition of small amounts of water. Most of these combinations contain sodium peroxide as the oxidizer. Aluminum and magnesium, mixed with sodium peroxide, have been recommended as igniters for the thermite mixtures described later. The magnesium/sodlum peroxide can even be initiated by carbon dioxide. The high reactivity of these combi-... 

The proiecticn of fine zirconium or titanium powders by moistening with water requires the presence of at least 25% of water. If less water is present, the metal might not only be ignitible but the combustion may be extremely violent because of steam formation and reaction of metal and water. Actual use of water in the form of hydrated salts has been claimed for pyrotechnic purposes. ... 

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