There is no oxygen at the nucleus of the flame. Some of the hydrocarbon fragments aromatize to soot particles and, in the luminescent region of the flame, react with water and carbon dioxide to form carbon monoxide. Most of the pyrolysis gases are carried to the exterior of the flame and encounter oxygen diffusing inwards. They react exothermically to produce heat, which melts and decomposes more wax, maintaining the combustion reaction. If there is adequate oxygen, the combustion products from the candle are carbon dioxide and water (Anderson & Christy, 1992).

Natural and synthetic polymers can ignite on exposure to heat. Ignition occurs either spontaneously or results from an external source such as a spark or flame. If the heat evolved by the flame is sufficient to keep the decomposition rate of the polymer above that required to maintain the evolved combustibles within the flammability limits, then a self-sustaining combustion cycle will be established This self-sustaining combustion cycle occurs across both the gas and condensed phases. Fire retardants act to break this cycle by affecting chemical and/or physical processes occurring in one or both of the phases. There are a number of ways in which the self-sustaining combustion cycle can be interrupted. Whatever the method used, the end effect is to reduce the rate of heat transfer to the polymer and thus remove the fuel supply. Troitzsch (1990) described the general physical and chemical mechanisms of flame-retardant action, in both the gas and condensed phases and the behaviour of flame retardants.

Fundamentally, four processes are involved in polymer flammability: preheating, decomposition, ignition and combustion/propagation. Preheating involves heating of the material by means of an external source, which raises the temperature of the material at a rate dependent upon the thermal intensity of the ignition source, the
thermal conductivity of the material, the specific heat of the material, and the latent heat of fusion and vaporization of the material. When sufficiently heated, the material begins to degrade, i.e., it loses its original properties as the weakest bonds begin to break. Gaseous combustion products are formed, the rate being
dependent upon such factors as intensity of external heat, temperature required for decomposition, and rate of decomposition. The concentration of flammable gases increases until it reaches a level that allows sustained oxidation in the presence of the ignition source. The ignition characteristics of the gas and the availability of oxygen are two important variables in any ignition process. After ignition and removal of the ignition source, combustion becomes self-propagating if sufficient heat is generated and is radiated back to
the material to continue the decomposition process. The combustion process is governed by such variables as rate of heat generation, rate of heat transfer to the surface, surface area, and rates of decomposition. 

Flame retardancy, therefore, can be achieved by eliminating (or improved by retarding) any one of these variables. A flame retardant should inhibit or even suppress the combustion process. Depending on their nature, flame retardants can act chemically and/or physically in the solid, liquid or gas phase. They interfere with combustion during a particular stage of this process, i.e. during heating, decomposition, ignition or flame spread (Troitzsch, 1990).

www.natfire.com
fire retardant
flame retardant