Pyrotechnic composition

A pyrotechnic composition is a substance or mixture of substances designed to produce an effect by heat, light, sound, gas or smoke or a combination of these, as a result of non detonative self-sustaining exothermic chemical reactions. Pyrotechnic substances do not rely on oxygen from external sources to sustain the reaction and include flash powder, gunpowder, pyrotechnic initiators, gas generators, smoke compositions, coloured fireworks compositions, and solid propellants.

Some pyrotechnic compositions are used in industry and aerospace for generation of large volumes of gas in gas generators (e.g. in airbags), in pyrotechnic fasteners, and in other similar applications. They are also used in military pyrotechnics, when production of large amount of noise, light, or infrared radiation is required; eg. missile decoy flares, flash powders, and stun grenades. A new class of reactive material compositions is now under investigation by military.

Many pyrotechnic compositions - especially involving aluminium and perchlorates - are often highly sensitive to friction, impact, and static electricity. Even as little as 0.1-10 millijoules spark can set off certain mixtures.

Materials used

Pyrotechnic compositions are usually homogenized mixtures of small particles of fuels and oxidizers. The particles can be grains or flakes. Generally, the higher the surface area of the particles, the higher the reaction rate and burning speed. For some purposes, binders are used to turn the powder into a solid material.

Fuels

Typical fuels are based on metal or metalloid powders. A flash powder composition may specify multiple different fuels. Some fuels can also serve as binders. Common fuels include:

* Metals
** Aluminium - most common fuel in many classes of mixtures
** Magnesium - more sensitive and violent than aluminium, increases probability of spontaneous ignition in storage
** Magnalium - aluminium-magnesium alloy, more stable and less expensive than magnesium
** Iron - makes gold sparks, frequently used
** Steel - an alloy of iron and carbon, makes branching yellow-orange sparks
** Zirconium - produces hot particles, good for ignition mixtures, eg. the "NASA Standard Initiator"
** Titanium - produces hot particles, increases sensitivity to impact and friction; sometimes the Ti4Al6V alloy is used which gives a bit brighter white sparks; together with potassium perchlorate it is used in some pyrotechnic igniters
** Ferrotitanium - iron-titanium alloy, produces bright yellow-white sparks, used in pyrotechnic stars, rockets, comets, and fountains
** Manganese - used to control or delay burn rates
** Zinc - used in some smoke compositions, together with sulfur used in some early amateur rocket fuels, also in pyrotechnic stars
** Copper - also a blue colorant
** Brass - a zinc-copper alloy used in some fireworks formulas
** Tungsten - used to control and slow down burn rates of compositions, also in delay compositions
** Zirconium-nickel alloy - used in some military delay compositions
* Metal hydrides (generally safer than the corresponding metals, release hydrogen during burning which may serve as a secondary fuel):
** Titanium(II) hydride - together with potassium perchlorate it is used in some igniters
** Zirconium hydride - together with potassium perchlorate it is used in some igniters
** Decaborane - used in some rocket fuels
* Metalloids
** Silicon - high flame temperature, burns producing molten glass, used in some ignition compositions
** Boron - used in some ignition mixtures
** Antimony - used in some fireworks, toxic, burns bright white
* Non-metallic inorganic
** Sulfur - ignition promoter; increases sensitivity to temperature, impact and friction, dangerous in combination with chlorates; used as an additive
** Red phosphorus - extremely dangerous, especially in combination with chlorates (Armstrong's mixture), also used in matches and some military infrared flares
** White phosphorus - used in incendiary weapons and to make some military smoke screens, ignites spontaneously in air
** Calcium silicide
** Antimony trisulfide - ignition promoter; fine powder increases sensitivity, sharpens the boom of salutes; toxic and sensitive to static electricity [http://www.chemistry.org/portal/a/c/s/1/acsdisplay.html?DOC=HomeMolecule%5Carchive%5Cmotw_antimonytrisulfide_arch.html] ; emits bright white light, crystals also used as a fuel in glitter compositions and in white comets and pyrotechnic stars
** Phosphorus trisulfide - used to make matches
** Calcium phosphide - liberates phosphine when wet, used in some naval signal flares
** Potassium thiocyanate
* Carbon-based
** Carbon
** Charcoal - makes dim gold sparks
** Graphite - also used as opacifier in rocket fuels to prevent heat transfer by radiation into lower layers of fuels and avoid the related explosions
** Lamp black - produces long lasting fine gold sparks in fireworks, also used as opacifier in rocket fuels
** Asphaltum - carbon-based fuel, also used as a binder
** Wood flour
* Organic chemicals
** Sodium benzoate - often used in whistle mixes together with potassium perchlorate
** Sodium salicylate - used in some whistle mixes
** Gallic acid - used in some whistle mixes; sensitive to impact and friction, there are safer alternatives
** Terephthalic acid - a fuel in some smoke compositions
** Hexamine - a low-reactivity, accessory fuel
** Anthracene - a fuel in some smoke compositions, produces black smoke
** Naphthalene - a fuel in some smoke compositions
** Lactose - used together with potassium chlorate in many smoke compositions
** Dextrose - used in some amateur rocket fuels
** Sucrose - used in some smoke compositions
** Sorbitol - used together with potassium nitrate as an amateur solid rocket fuel
** Dextrin - also a binder
** Stearin, stearic acid - a possible replacement for charcoal in some compositions
* Organic polymers and resins, also sometimes serving as binders
** Teflon and other fluoropolymers - sometimes also working as oxidizer - used in military pyrolant compositions, eg. Magnesium/Teflon/Viton; extremely reactive in contact with some fine metal powders
** Hydroxyl-terminated polybutadiene (HTPB), used together with aluminium and ammonium nitrate in composite rocket fuels as a fuel and binder
** PBAN, used together with aluminium and ammonium nitrate in composite rocket fuels as a fuel and binder
** Polyisobutylene
** Nitrocellulose
** Polyethylene
** Polyvinyl chloride, also serving as chlorine donor
** Polyvinylidene chloride, also serving as chlorine donor
** Shellac, good especially for colored flame compositions

When metallic fuels are used, the metal particle size is important. A larger surface area to volume ratio leads to a faster reaction; this means that smaller particle sizes produce a faster-burning composition. The shape also matters. Spherical particles, like those produced by atomizing molten metal, are undesirable. Thin and flat particles, like those produced by milling metal foil, are ideal.

A suitable metal fuel may be dangerous on its own, even before it is mixed with an oxidizer. Careful handling is required to avoid the production of pyrophoric metal powders.

Oxidizers

Perchlorates, chlorates and nitrates are the most commonly used oxidizers for flash powders. Other possibilities include permanganates, chromates, and some oxides. Generally, the less the oxidizer, the slower the burning and the more light produced. For use at very high temperatures, sulfates can be used as oxidizers.

Oxidizers in use include:
* Perchlorates (also serving as chlorine donors):
** Potassium perchlorate - common, relatively stable
** Ammonium perchlorate - the most common oxidizer for modern solid rocket fuels; more sensitive to mechanical stimuli than potassium perchlorate
* Chlorates (also serving as chlorine donors):
** Potassium chlorate - much less stable than perchlorate, hazardous
** Barium chlorate - also serves as a green colorant in fireworks
** Sodium chlorate - much less stable than perchlorate, hazardous, also serves as yellow colorant, hygroscopic
* Nitrates:
** Potassium nitrate
** Sodium nitrate - also a yellow colorant, hygroscopic
** Calcium nitrate - also a red-orange colorant
** Ammonium nitrate - used in some less common composite rocket propellants
** Barium nitrate - most common oxidizer/colorant for green colors; also used in flash powders and some military infrared flares
** Strontium nitrate - most common oxidizer/colorant for red colors in flares, fires, and stars
** Caesium nitrate - used in some military infrared flare compositions
* Permanganates:
** Potassium permanganate - used in early mixtures, regarded to be sensitive and unstable
** Ammonium permanganate - a moderate explosive
* Chromates:
** Barium chromate - used in delay compositions, e.g. in fireworks rockets
** Lead chromate - used in delay compositions
** Potassium dichromate - used infrequently as an oxidizer; can be used as a surface treatment for passivation of magnesium particles, also as a catalyst and in some matches
* Oxides and peroxides:
** Barium peroxide - unstable, spontaneously decomposes, compositions with its content should not be stored
** Strontium peroxide
** Lead tetroxide
** Lead dioxide - used in friction-sensitive compositions, eg. matches
** Bismuth trioxide - used as a safe alternative to lead tetroxide in some compositions
** Iron(III) oxide - a high temperature oxidizer, a catalyst
** Iron(II,III) oxide - an oxidizer in Thermite and Thermate
** Manganese(IV) oxide - an oxidizer in manganese thermite, a catalyst
** Chromium(III) oxide - an oxidizer in chromium thermite
* Sulfates (require high temperatures):
** Barium sulfate - a high-temperature oxidizer for e.g. strobe compositions, a green colorant
** Calcium sulfate - a high-temperature oxidizer for e.g. strobe compositions, a red-orange colorant
** Potassium sulfate - a high-temperature oxidizer, a purple colorant
** Sodium sulfate - a high-temperature oxidizer, a yellow colorant
** Strontium sulfate - a high-temperature oxidizer, a red colorant
* Organic chemicals
** Guanidine nitrate - used in some high power rocket fuels, propellants, and blue firework compositions
** Hexanitroethane - used in some special military compositions
* Others
** Sulfur - oxidizer for zinc in zinc-sulfur fuels
** Teflon - oxidizer for some metal fuels

Corresponding sodium salts can be substituted for potassium ones.

Additives

Coolants. For some purposes it is necessary to lower the burning temperature of the mixture, and/or slow down the reaction rate. For such purpose, inert materials (eg. clay, diatomaceous earth, alumina, silica, magnesia, or others) or endothermically decomposing materials (eg. carbonates) are added.

Colorants, sometimes in combination with sources of chlorine. Usually salts of suitable metals, often barium, strontium, calcium, sodium, copper, etc. The salt may simultaneously serve as an oxidizer.

Chlorine donors. Presence of chlorides increases the volatility of the color-imparting metals and enhances the color intensity. Polyvinyl chloride, polyvinylidene chloride, chlorinated paraffins, chlorinated rubber (e.g. Parlon), hexachloroethane, hexachlorobenzene, and some other organochlorides and inorganic chlorides (e.g. ammonium chloride, mercurous chloride) are used as chlorine donors. Perchlorates and chlorates play this role together with their main use as oxidizers. Chlorine donors are often used also in smoke compositions, e.g. hexachloroethane together with zinc oxide to produce smoke based on zinc chloride.

Catalysts. Propellant formulas often require a catalyst to burn faster and more stable. Certain oxidizers often serve as catalysts. E.g. ammonium dichromate is used as a catalyst in ammonium nitrate based propellant formulas. Other catalysts are e.g. iron(III) oxide, manganese dioxide, potassium dichromate, copper chromite.

Stabilizers. Some mixtures, eg. containing chlorates, tend to degrade and create acidic byproducts. Carbonates (eg. sodium, calcium, or barium carbonate) or other mildly alkaline materials can be added to scavenge such acids. Boric acid can be used to inhibit the sensitivity of aluminium to moisture, and to stabilize mixtures of metals with nitrates (which can otherwise form amides which react exothermically with metals and can cause spontaneous initiation). Many organic nitrated amines are used as stabilizers as well, e.g. 2-nitrodiphenylamine. Petroleum jelly, castor oil, linseed oil, etc. can be used as stabilizers, also to add hydrophobicity to particles and protect metals (especially iron and magnesium) from corrosion.

Anticaking agents. Eg. fumed silica. For powder compositions, e.g. flash powder or gunpowder. Graphite is used in some cases to coat the grains, lubricate them, and dissipate static electricity.

Binders. Often gums and resins, e.g. gum arabic, red gum, guar gum, copal, carboxymethyl cellulose, nitrocellulose, rice starch, cornstarch, shellac, dextrin. Binders can also serve as fuels. Camphor can be used as a plasticizer. Binders are used in manufacture of compact compositions, e.g. pyrotechnic stars. Polymers like HTPB and PBAN are often used for rocket fuels. Other polymers used are e.g. polyethylene or polyvinyl chloride can be encountered as well.

External links

* [http://www.pyroguide.com/index.php?title=Pyrotechnic_composition A pyroguide article on Pyrotechnic composition]


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