plasticizers. Plasticizers are mixed into polymers to increase flexibility and workability [35,36]. Aryl phosphate plasticizers are used in PVC based products, used as lubricants for industrial air compressors and gas turbines. Other uses of aryl pho psphates are as pigments , dispersants and peroxide carriers, and additives in adhesives and wood preservatives.
2.2.2.4 Halogenated phosphates
These are flame retardants that contain phosphorus and bromine or chlorine. They normally combine the flame retardancy properties of phosphorus and those of the halogens (bromine & chlori ne).
Among this group, is tris (1 – chloro – 2 – propyl) phosphate (TCPP) used in polyurethane foam. The other is the Tris (2 – chloroethyl) phosphate used in the manufacture of polyester resins, polyacrylates, polyurethanes and cellulose derivatives [16]. In the bromine – phosphorus group, the commonest is tris (2,3 – dibromopropyl) phosphate which has been withdrawn from use in many countries due to carcinogenic properties in animals [19].
2.2.2.5 Nitrogen – based flame retardants
Nitrogen-based flame retardants are used primarily in nitrogen containing polymers such as polyurethanes and polyamides. They are also utilized in PVC and polyolefins and in the formulat ion of intumescent paint system s [37].
Melamine, melamine cyanurate, other melamine salts are currently the most used group of nitrogen – containing flame retardants. Melamine, melamine cyanurate are used f or polypropylene, polyethylene, polyamindes epoxy and polyurethane [38].
2.3 Mechanism of action of flame retardants Fundamentally, four p rocesses are involved in polymer flammability: preheating, decomposition, ignition or combustion and propagation [16]. Preheating involves heating of the material by means of an external source. This raises the temperature of the material at a rate depende nt 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 vapourization of the material. When sufficiently heated, the material begins to degrade as the weakest bond begins 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 [11].
The ignition characteristics of the ga s and the availability of oxygen are the two important variables in any ignition process.
The combustion process is governed by such variables as rate of heat generation, rate of heat transfer to the surface, surface area, and the rates of decomposition [29].
Flame retardancy, therefore, can be achieved by eliminating or retarding any one of these variables. Depending on their nature, flame retardants can act chemically and/or physically in the solid, liquid or gas phase. They interfere with combustion dur ing a particular stage of this process,
i.e. during heating, decomposition, ignition or flame spreading.
2.3.1 Physical action of flame retardants
Combustion process can be retarded by physical action which includes cooling, formation and dilution [ 17].
Cooling is the endothermic processes triggered by additives ; (cooling the substrates to a temperature below that required to sustain the combustion process ). Formation of a protective layer or coa ting is the situation by which the condensed combustible layers are shielded from the gaseous phase with a solid or gaseous protective layer. Flame retardants that exhibits such actions include, antimony trioxide and boric acid-borax systems for flame – retarding cellulosic fabrics [3]. The condensed phase is thu s cooled, smaller quantities of pyrolysis gases are evolved, the oxygen necessary for the combustion process is excluded and heat transfer is impeded. Dilution is by incorporation of inert substances e.g. fillers and other additives that evolve inert gases on decomposition dilute the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not exceeded.
2.3.2 Chemical reactions
The chemical reactions interfering with the combustion process take place in the solid and ga s phases [17].
2.3.2.1 Reaction in the gas phase:
The free radical 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 sup ply of flammable gases are reduced and eventually completely suppressed. This mechanism involves the formation of free radicals inhibitors, or radical trap agents in the flame front and causes the material to recede from the flame with the resultant of lowering of melting point of the material. The flame retardants that can function in this mechanism are halocarbons mainly organochlorides such as polychl orinated biphenyls (PCBS), chloren dic acid e.g. dibutyl chlorendate and dimethyl chlorendate and chromate paraffins.
Organobromides such as polybrom inated diphenyl ether (PBDEs) which are made up of pe ntabromodiphenyl ether (Penta BD E) octabromodiphenyl ether (octaBDE ), decabromodiphenyl ether (deca BDE) and hexabromocyclo - dodecane (HBCD) . Organophosphat es in form of halogenated phosphorus compounds such as tri – o – cresyl phosphate, tris (2,3 dibromopropyl) phosphate tris, bis (2,3 dibromopropyl) phosphate.
The reactions involved in the gas phase mechanism are dilution of gas phase. In this mechanism inert gases (e.g. carbondioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases lowering the partial pressure of oxygen, and slowing the reaction rate.
The other is the gas phase radical quenching. In this case chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride (H Cl) and hydrogen bromide (HB r). These react with the highly reactive
H. and OH. radicals in the flame, resulting in an inactive molecule and a Cl . or Br . radical [38].
The halogen radical has much lower energy than H . or OH. And therefore has much lower potential to propagate the radical oxidation reactions of combustion.
H. + HBr H2 + Br.
OH. + HBr H2O + Br .
2.3.2.2 Condensed phase mechan isms.
The phosphorus compounds are effective flame retardants through the formation of char . This is regarded to be the key mechanism for phosphorus salts of volatile metals and most organophosphorus compounds. These include triphenyl phosphate and triphe nyl phosphine. For example, in flame retardant action of phosphorus compounds on cellulose and thermoplastics, the retardant act s by forming phosphoric acid, which changes the course of the decomposition of cellulose to form carbon char and water [17].
In polyethylene terephthalate and poly methyl methacrylate the mechanism of action of phosph orus – based flame retardants are also shown to be a similar decrease in the amount of combustible volatiles and increase in the amount of residues (aromatic residues and char). The char formed also acts as physical barrier to heat and gases. In rigid and flexible polyurethane foams the actions of phosphorus flame
