2.1 Introduction

Some basic mechanisms of flame retardancy were recognised as early as 1947 when several primary principles were put forward.1 These included the effect of the additive on the mode of the thermal degradation of the polymer in order to produce fuel-poor pyrolytic paths, external flame retar-dant coatings to exclude oxygen from the surface of the polymer, internal barrier formation to prevent evolution of combustible gases, inert gas evo-lution to dilute fuel formed in pyrolysis and dissipation of heat away from the flame front. Discovery of the flame-inhibiting effect of volatile halogen derivatives subsequently led to the postulation of the radical trap-gas-phase mechanism.2 The gas-phase and the condensed-phase proposals have long been generally considered as the primary, though not the only, effective mechanism of flame retardancy. This situation is now being modified as new mechanisms of new flame-retarding systems, especially those based on physical principles, evolve and as new insights into the performance of flame retardants is being gained. In many cases several mechanistic principles operate simultaneously and consequently it is difficult to identify one dominant mechanism. In such cases modes of action of particular flame-retarding formulation may be defined and described.

This paper attempts to review some of the principles, mechanisms and modes of action which prevail at present in the field of flame retardancy of polymers.

2.2 General considerations

Pyrolysis and combustion of polymers occur in several stages. The poly-meric substrate heated by an external heat source is pyrolysed with the gen-eration of combustible fuel. Usually, only a part of this fuel is fully combusted in the flame by combining with the stoichiometric amount of atmospheric oxygen. The other part remains and can be combusted by

drastic means, e.g. in the presence of a catalyst and by an excess of oxygen. A part of the released heat is fed back to the substrate and causes its con-tinued pyrolysis, perpetuating the combustion cycle. Another part is lost to the environment. The energy needed to heat the polymer to the pyrolysis temperature and to decompose and gasify or volatilise the combustibles and the amount and character of the gaseous products determines the flammability of the substrate. A flame retardant acting via a condensed-phase chemical mechanism alters the pyrolytic path of the substrate and reduces substantially the amount of gaseous combustibles, usually by favouring the formation of carbonaceous char and water.3 In this case the heat released in the combustion decreases with an increase in the amount of the flame-retarding agent.

In the gas-phase mechanism the amount of combustible matter remains constant but the heat released in the combustion usually decreases with an increase in the amount of the flame-retarding agent. The amount of heat returned to the polymer surface is therefore also diminished and the pyrol-ysis is retarded or halted as the temperature of the surface decreases. The flame-retarding moiety has to be volatile and reach the flame in the gaseous form. Alternatively it has to decompose and furnish the active fraction of its molecule to the gaseous phase. The char remaining in the substrate will contain less of the active agent. The pyrolysis of the polymer should, in the limiting case, proceed as if there would have been no flame-retarding agent incorporated in it. In addition presence of the gas-phase active agent should not influence the composition of the volatiles reaching the flame.3

2.3 Gas-phase mechanisms

The gas-phase activity of the active flame retardant consists in its interference in the combustion train of the polymer. Polymers, like other fuels, produce upon pyrolysis species capable of reaction with atmospheric oxygen and produce the H2–O2 scheme which propagates the fuel combustion by the branching reaction:4

H• + O2 = OH• + O• [2.1]

O• + H2 = OH• + H• [2.2]

The main exothermic reaction which provides most of the energy maintaining the flame, is:

OH• + CO = CO2+H•[2.3]

To slow down or stop the combustion, it is imperative to hinder the chainbranching reactions [2.1] and [2.2]. The inhibiting effects of halogen derivatives, usually chlorine and bromine, is considered to operate via the 32 Fire retardant materials gas-phase mechanism. This effect in the first instance occurs either by releasing a halogen atom, if the flame-retardant molecule does not contain hydrogen, or by releasing a hydrogen halide:

MX = M• + X• [2.4]

MX = HX + M• [2.5]

where M• is the residue of the flame-retardant molecule. The halogen atom reacts with the fuel, producing hydrogen halide:

RH + X• = HX + R• [2.6]

The hydrogen halide is believed to be the actual flame inhibitor by affecting the chain branching:

H• + HX = H2 + X• [2.7]

OH• + HX = H2O + X• [2.8]

Reaction [2.7] was found to be about twice as fast as [2.8] and the high value of the ratio H2/OH in the flame front indicates that [2.7] is the main inhibiting reaction.5 It is believed that the competition between reactions [2.7] and [2.1] determines the inhibiting effect. Reaction [2.1] produces two free radicals for each H atom consumed, whereas reaction [2.7] produces one halogen radical which recombines to become the relatively stable halogen molecule.

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