Fire Extinguisher

 

A fire retardant is a substance that reduces flame of fuels or delays their combustion. This includes chemical agents, but may also include substances that work by physical action, such as cooling the fuels, such as  fire-fighting foams and fire-retardant gels. Fire retardants may also be coatings applied to an object,such as a spray retardant to prevent Christmas trees from burning.Fire retardants are commonly used in fire fighting.

Physical action

There are several ways in which the combustion process can be retarded by physical action:

• By cooling Some chemical reactions actually cool the material down.
• By forming a protective layer that prevents the underlying material from igniting.
• By dilution: Some retardants release water and carbon dioxide while burning. This may dilute the radicals in the flame enough for it to go out.

Commonly used fire retardant additives include mixtures of huntite and  hydromagnesite, aluminium hydroxide, magnesium hydroxide. When heated, aluminium hydroxide dehydrates to form aluminum oxide (alumina, Al₂O₃), releasing water vapor in the process. This reaction absorbs a great deal of heat, cooling the material into which it is incorporated. Additionally, the residue of alumina forms a protective layer on the material's surface. Mixtures of huntite and hydromagnesite work in a similar manner. They endothermically decompose releasing both water and carbon dioxide  giving fire retardant properties  to the materials in which they are incorporated.....

Fire extinguishers

Class A foam is used as a fire retardant in 2.5 gallon [APW] and [CAFS] extinguishers to contain incipient brush fires and grass fires by creating a fire break. Others chemical retardants are capable of rendering class A material and Class B fuels non-flammable and extinguishing class A, class B, and some class D fires.Fire retardant slurries dropped from aircraft are normally applied ahead of a wildfire to prevent ignition, while fire suppression agents are used to extinguish fires.....

Chemical action

• Reactions in the gas phase: chemical reactions in the flame (i.e. gas phase) can be interrupted by fire retardants. Generally, these retardants are organic halides (haloalkanes) such as Halon and... PhostrEx. However, there are situations where the released gas might be more dangerous when this type of retardant is involved.

• Reaction in the solid phase: some retardants break down   polymers so they melt and flow away from the flame. Although this allows some materials to pass certain flammability tests, it is not known whether fire safety is truly improved by the production of flammable plastic droplets.

• Char Formation: For carbon-based fuels, solid phase flame retardants cause a layer of carbonaceous char to form on the fuel surface. This char layer is much harder to burn and prevents further burning.

Intumescents  : These types of retardant materials add chemicals which cause swelling up behind the protective char layer, providing much better insulation behind the protective barrier. In additions to being added to plastics, these are available as paints for protecting wooden buildings or steel structures. ...