Investigation Of Effects Of Alum And Potassium Sesquicarbonate On The Fire Characteristics Of Flexible Polyurethane Foam

This self sustaining combustion cycle occurs across both the gas and condensed phases. Fire retardants act to break this cycle by affecting chemical and physical processes occurring in one or both of the phases.

Fundamentally, four processes are involved in polymer flammability

Preheating

Decomposition

Ignition

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, that is, 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 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 [17]. Combustion process is governed by such variables as rate of heat generation, rate of heat transfer to the surface, surface area, rates of decomposition [19]. Flame retardancy can be achieved by eliminating (or improved by retarding) any one of these variables.

Depending on their nature, flame retardants can act chemically or physically in the solid, liquid or gas phase.

1.4.1 Physical action

There are several ways in which the combustion process can be retarded by physical action [4];

By cooling:- Endothermic processes triggered by additives cool the substrate to a temperature below that required to sustain the combustion process.

By formation of a protective layer:- The condensed combustible layer can be shielded from the gaseous phase with a solid or gaseous protective layer. The condensed phase is thus cooled, smaller quantities of pyrolysis gases are evolved, the oxygen necessary for the combustion process is excluded and heat transfer impeded.

By dilution:- The incorporation of inert substances (e.g. fillers) and additives that evolve inert gases on decomposition dilutes the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not exceeded.

1.4.2 Chemical action

a. Reaction in the gas phase:- The free mechanism of the combustion process which takes place in the gas phase is interrupted by the flame retardant. The exothermic processes are thus stopped, the system cools down, and the supply of flammable gases is reduced and eventually completely suppressed.

b. Reaction in the solid phase:- Here, two types of reaction can take place; firstly, breakdown of the polymer can be accelerated by the flame retardant causing pronounced flow of the polymer and hence its withdrawal from the sphere of influence of the flame which breaks away. Secondly, the flame retardant can cause a layer of carbon to form on the polymer surface. This can occur through the dehydrating action of the flame retardant generating double bonds in the polymer. These form the carbonaceous layer by cyclizing and cross linking.

Improvement of the flame retardancy

Flame retardancy is improved by flame retardants that cause the formation of a surface film of low thermal conductivity and high reflectivity which reduces the rate of heating. It is also improved by flame retardants that might serve as a heat sink by being preferentially decomposed at low temperature.

Finally, it is improved by flame retardant coatings that upon exposure to heat, form into a foamed surface layer with low thermal conductivity properties. A flame retardant can promote transformation of a plastic into char and thus limit production of combustible carbon-containing gases. Simultaneously, the char will decrease thermal conductivity of the surface [18-20].

Structural modification of the plastic or use of an additive flame retardant might induce decomposition or melting upon exposure to a heat source so that the materia shrinks or drips away from the heat source [21]. It is also possible to significantly retard the decomposition process through selection of chemically stable structural components. One mechanism of improving the flame retardancy of thermoplastic materials is to lower their melting point. This results in the formation of free radical inhibitors in the flame front and causes the material to recede from the flame without burning.

Free radical inhibition involves the reduction of gaseous fuels generated by burning materials. Heating of combustible materials results in the generation of hydrogen, oxygen, hydroxide and provides radicals that are subsequently oxidized with flame [22]. Certain flame retardants act to trap these radicals and thereby prevent their oxidation. Bromine is usually more effective than chlorine, for example;

HBr + HO◦ → Br◦ +H2O HBr + O◦ → HO◦ +Br HBr + H◦ → H2 + Br ◦

HBr + ROH2 → ROH3 + Br◦

RBr → R◦ + Br◦

Co-additives for use with flame retardant [23]

Brominated flame retardants are in some cases used on their own but their effectiveness is increased by a variety of co­additives, so that in practice they are more often used in conjunction with other compounds or with other elements incorporated into them. Thus, for example, the addition of small quantities of organic peroxides to polystyrene greatly reduces the amount of hexabromocyclodecane needed to give a flame retardant foam [15]. These compounds appear to act by promoting depolymerization of the hot polymer giving a more fluid melt. More heat is therefore required to keep the polymer alight, because there is a greater tendency for the more molten material to drip away from the neighbourhood of the flame.

The flame-retardant properties of bromine compounds, like those of chlorine compounds will be considerably enhanced when they are used in conjunction with other hetero-elements notably phosphorus, antimony and certain other metals. The simultaneous presence of phosphorus in bromine-containing polymer systems usually serves to improve their degree of flame retardance, sometimes the two elements are present in

the same molecule, e.g. tris (2, 3-dibromopropyl) phosphate. In other systems, however it is more convenient to use mixtures of a bromine compound and a phosphorus compound so that the ratios of the elements are readily adjusted. Brominated flame retardants on their own act predominantly in the gas phase while phosphorus compounds act mainly in the condensed phase especially with oxygen containing polymers.

Bromine-phosphorus compounds affect primarily the condensed phase processes. However, studies of the flammability of rigid polyurethane foams show that the inhibiting effect of tris (2 , 3 - dibromopropyl) - phosphate on combustion depends on the nature of the gaseous oxidant, suggesting that the flame retardant acts here at least in part by interfering with reactions in the gaseous phase.

Antimony is a much more effective co-additive than phosphorus, generally in the form of its oxide, Sb2O3. On its own this compound has no flame retardant activity and is therefore always used in conjunction with a halogen compound [16]. The use of antimony trioxide reduces the high levels normally needed for effective flame retardance of bromine compounds on their own. The principal mode of action is in the gas phase [7].