Pyrotechnic composition

A pyrotechnic composition is a substance or mixture of substances designed to produce an effect by heat, light, sound, gas/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.

Types

Basic types of pyrotechnic compositions are:

flash powder – burns very fast, produces explosions and/or bright flashes of light
gunpowder – burns slower than flash powder, produces large amount of gases
solid propellants – produce large amount of hot gases, used as sources of kinetic energy for rockets and projectiles
pyrotechnic initiators – produce large amount of heat, flames, and/or hot sparks, used to ignite other compositions
gas generators – produce large amount of gas, either high volume at short time or controlled flow rate
ejection charges – burn fast, produce large amount of gas at short time, used to eject payloads from containers
burst charges – burn fast, produce large amount of gas at short time, used to fragment a container and eject its content
smoke compositions – burn slowly, produce smoke, plain or colored
delay compositions – burn at constant slow speed, used to introduce delays into the firing train
pyrotechnic heat sources – produce large amount of heat and little to no gases, slow-burning, often thermite-like compositions
sparklers – producing white or colored sparks
flares – burn slowly, produce high amount of light, used for illumination or signaling
coloured fireworks compositions – produce light, white or colored

Some pyrotechnic compositions are used in industry and aerospace for generation of large volumes of gas in gas generators, 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; e.g. 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, also a combustion instability suppressant. Less energy per mass than carbon but less gas evolution, retaining heat in the reaction mixture. High-temperature flame with solid particles, which interfere with flame colorants. Reacts with nitrates, except ammonium nitrate, yielding nitrogen oxides, ammonia, and heat ; the reaction can be inhibited by a weak acid, e.g. boric acid. Corroded by alkaline substances. Flake particles easier to ignite and better for pyrotechnics than spherical ones. In presence of moisture reacts with potassium chlorate and perchlorate, yielding hydrogen. Particle size selected according to the required burn rate.
* Magnesium – more sensitive and violent than aluminium, increases probability of spontaneous ignition in storage. Used in fireworks to increase flame temperature. Less interference with flame color than aluminium.
* Magnalium – aluminium-magnesium alloy, more stable and less expensive than magnesium; less reactive than magnesium, easier to ignite than aluminium
* 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, e.g. the NASA Standard Initiator, also a combustion instability suppressant
* 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; coarse powder produces branching blue-white sparks
* Ferrotitanium – iron-titanium alloy, produces bright yellow-white sparks, used in pyrotechnic stars, rockets, comets, and fountains
* Ferrosilicon – iron-silicon alloy, used in some mixtures, sometimes replacement of calcium silicide
* Manganese – used to control burn rates, e.g. in delay compositions
* Zinc – used in some smoke compositions, together with sulfur used in some early amateur rocket fuels, also in pyrotechnic stars; heavy, zinc-based compositions may require additional lift to fly high enough; moisture-sensitive; can spontaneously ignite; rarely used as primary fuel except in smoke compositions, can be encountered as a secondary enhancement fuel
* Copper – used as a blue colorant with other fuels
* Brass – a zinc-copper alloy used in some fireworks formulas, as a blue colorant for its copper content
* 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 :
* Titanium hydride – together with potassium perchlorate it is used in some igniters
* Zirconium hydride – together with potassium perchlorate it is used in some igniters
* Aluminum hydride – unstable for storage and reacts dangerously in contact with water
* Decaborane – experimented with for some rocket fuels
Metal carbides
* Zirconium carbide – used in some rocket fuels, also a combustion instability suppressant
Metalloids
* Silicon – high flame temperature, burns producing molten glass, used in some ignition compositions and delay charges, commonly with lead tetroxide
* Boron – used in some ignition mixtures
* Antimony – used in some fireworks for glitter effects, toxic, burns bright white; usually used as 200–300 mesh; with potassium nitrate and sulfur produces white fires
Non-metallic inorganic
* Sulfur – ignition promoter, increases burn rate; increases sensitivity to temperature, impact and friction, dangerous in combination with chlorates; commonly used with nitrates; used as an additive; may contain residual acids, combination with carbonates or other alkaline stabilizers is advised in acid-sensitive compositions
* Red phosphorus – extremely dangerous, especially in combination with chlorates ; used in caps; also used in matches and some military infrared flares; toxic
* White phosphorus – used in incendiary weapons and to make some military smoke screens, ignites spontaneously in air; even more toxic
* Calcium silicide – used in some special compositions
* Antimony trisulfide – ignition promoter; fine powder increases sensitivity, sharpens the boom of salutes; toxic and sensitive to static electricity; emits bright white light, crystals also used as a fuel in glitter compositions and in white comets and pyrotechnic stars. Sensitive to friction and impact; the degree of sensitisation depends on the oxidizer.
* Arsenic sulfide – toxic, sensitive to impact and friction. Used for report compositions due to its sensitivity with chlorate even in small amounts. Used in yellow smoke compositions due to its low boiling point.
* 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
** Carbon 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. Some forms contain ammonia; should not be combined with chlorates.
* 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
* Potassium picrate – used in some whistle compositions, safer than gallic acid but still dangerous, with heavy metals forms explosive salts
* 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; cheap low-reactivity accessory fuel
* Dextrose – used in some amateur solid 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 – accessory fuel, a possible replacement for charcoal and/or sulfur in some compositions; lengthens flames, can reduce friction sensitivity; phlegmatizing agent
* Hexachloroethane – used in many military smoke compositions
Organic polymers and resins, also sometimes serving as binders
* Teflon, Viton and other fluoropolymers – sometimes also working as oxidizer – used in military pyrolant compositions, e.g. Magnesium/Teflon/Viton; extremely reactive in contact with some fine metal powders
* Hydroxyl-terminated polybutadiene, used together with aluminium and ammonium nitrate in composite rocket fuels as a fuel and binder
* Carboxyl-terminated polybutadiene, used 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
* Polysulfide, used in composite rocket fuels as a fuel and binder
* Polyurethane, used in composite rocket fuels as a fuel and binder
* Polyisobutylene
* Nitrocellulose
* Polyethylene
* Polyvinyl chloride, also serving as chlorine donor and a binder
* Polyvinylidene chloride, also serving as chlorine donor
* Shellac, good especially for colored flame compositions
* Accroides resin, higher burning rate than shellac, burns well even with potassium perchlorate. Suitable for chrysanthemum stars.

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, have higher reaction surface and therefore are ideal when faster reaction is desired. Using nanoparticles can drastically affect the reaction rates; metastable intermolecular composites exploit this.
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.