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

Smoke suppressants

Smoke production is determined by numerous parameters. No comprehensive theory yet exists to describe the formation and constitution of smoke. Smoke suppressants rarely act by influencing just one of the parameters determining smoke generation. Ferrocene, for example, is effective in suppressing smoke by oxidizing soot in gas phase as well as by pronounced charring of the substrate in the condensed phase. Intumescent systems also contribute to smoke suppression through creation of a protective char. It is extremely difficult to divide these multifunctional effects into primary and subsidiary actions since they are so closely interwoven [17].

1.7.1 Condensed phase

Smoke suppressants can act physically or chemically in the condensed phase [24]. Additives can act physically in a similar fashion to flame retardants, that is, by coating or dilution thus limiting the formation of pyrolysis products and hence of smoke. Chalk (CaCO3) frequently used as a filler acts in some cases not only physically by effecting cross-linking so that the smoke density is reduced in various ways. Smoke can be suppressed by the formation of a charred layer on the surface of the substrate, for example, by the use of organic phosphates in unsatwurated polyester resins. In halogen containing polymers such as PVC, iron compounds cause charring by the formation of strong Lewis acids.

Certain compounds such as ferrocene cause condensed phase oxidation reactions that are visible as a glow. There is pronounced evolution of carbon (ii) oxide and carbon (iv) oxide, so that less aromatic precursors are given off in the gas phase.

Compounds such as molybdenum oxide can reduce the formation of benzene during the thermal degradation of PVC, probably via chemisorption's reactions in the condensed phase [24].

Relatively stable benzene-MoO3 complexes that suppress smoke development are formed.

1.7.2 Gas phase

Smoke suppressants can also act chemically and physically in the gas phase. The physical effect takes place mainly by shielding the substrate with heavy gases against thermal attack. They also dilute the smoke gases and reduce smoke density. In principle, two ways of suppressing smoke chemically in the gas phase exist; the elimination of either the soot precursors or the soot itself. Removal of soot precursors occurs by oxidation of the aromatic species with the help of transition metal complexes [25]. Soot can also be destroyed oxidatively by high energy OH radicals formed by the catalytic action of metal oxides or hydroxides.

Smoke suppression can also be achieved by eliminating the ionized nuclei necessary for forming soot with the aid of metal oxides. Finally, soot particles can be made to flocculate by certain transition metal oxides.

Performance criteria and choice of flame retardants

At present, the selection of a suitable flame retardant depends on a variety of factors that severely limit the number which are acceptable materials [26].

Many countries require extensive information on human and environmental health effects for new substances before they are allowed to be put on the market.

The following information regarding human and environmental health is essential in understanding a chemical potential hazards.

Data from adequate acute and repeated dose toxicity studies is needed to understand potential health effects.

Data on biodegradability and bioaccumulation potential is needed as a first step in understanding a chemical's environmental behaviour and effects.

Since flame retardants are often processed into polymers at elevated temperatures, consideration of the stability of the material at the temperature inherent to the polymers processing is needed as well as on whether or not the material volatilizes that temperature.

Uses of flame retardants [11] [27] a. Plastics

The plastic industry is the largest consumer of flame retardants estimated at about 95% for the USA in 1991 [28].

About 10% of all plastics contain retardants. The main applications are in building materials and furnishings (structural elements, roofing films, pipes, foamed plastics for insulation, furniture and wall, floor coverings) transportation (equipment and fillings for air craft, ships, automobiles and railroad cars) and in electrical industry (cable housing and components for television sets, office machines, household appliances and lamination of printed circuits).

b. Textile/furnishing industry

In contrast to the plastics industry, the textile industry is much smaller market for flame retardants. However, rather than employing just one flame retardant, the use of a combination of chemicals is usually necessary for textiles.

Phosphorus-containing materials are the most important class of compounds to impart durable flame resistance to cellulose. Flame retardant finishes containing phosphorus compounds usually also contain nitrogen or bromine or sometimes both. Another system is based on halogens in conjunction with nitrogen or antimony.

Formation of toxic products on heating or combustion of flame retarded products [26]

Natural or synthetic materials that burn produces potentially toxic products. There has been considerable debate on whether addition of organic flame retardants results in the generation of a smoke that is more toxic and may result in adverse health effects on those exposed. There has been concern in particular about the emission of polybrominated dibenzofurans (PBDF) and polybromintated dibenzodioxins (PBDD) during manufacture, use and combustion of brominated flame retardants.

1.10.1 Toxic products in general [29]

Combustion of any organic chemical may generate carbon monoxide (CO) which is a highly toxic non-irritating gas and a variety of other potentially toxic chemicals. Some of the major toxic products that can be produced by pyrolysis of flame retardants are CO, CO2, HCl, HBr, phosphoric acid etc.

In general the acute toxicity of fire atmosphere is determined mainly by the amount of CO, the source of which is the amount of generally available flammable material [25]. Most fire victims die in post flash-over fires where the emission of CO is maximized and the emission of HCN and other gases is less. The acute toxic potency of smoke from most materials is lower than that of CO. Flame retardant significantly decreases the burning rate of the product, reducing heat yields and quantities of toxic gas. In most cases, smoke was not significantly different in room fire tests between flame-retarded and non flame -retarded products.