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Figure 2.

Schematic of mass spectrometric system for sampling 1 Atm flames. A representative gas sample passes through a cone system (c) into an evacuated chamber (region II); the beam then passes through an adjustable orifice into region III, which contains a wheel (w) for mechanical modulation of the beam, an ion source (g) for partial conversion of the molecular species into positive ions, and a mass filter and ion detection system (p).

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We find that addition of antimony tribromide to such a flame gives compoThe hydrogen bromide and sition profiles of the type indicated by figure 5. methyl bromide intermediates are only stable in the pre-flame region. The temperatures in this region and the rate constants for reactions of these bromides with hydrogen atoms are known, and it is generally agreed that at least However, we also believe that some inhibition occurs in this pre-flame region. there is a strong inhibition effect at the reaction zone where the radicals all In this antimony case, we believe that it have their maximum concentration.

is the antimony monoxide species which is affecting the radicals by interacting with hydrogen atoms and catalyzing a recombination of hydrogen atoms to molecular hydrogen. This lowers the flame speed and causes inhibition.

The primary Let me sum up this discussion by indicating a reasonable approach to this problem of chemically controlling flames as summarized by table 3. problem that we have been dealing with up to now is to identify the flame species. Once these are established, then we have to worry about the kinetics of the reactions of these species in flames. Unfortunately, very little of this data is available and we will need to obtain some of it using the mass spectrometric Once the flame kinetics are known, one can apply well When the model has been flame sampling system: developed flame theory and test a molecular model. established, one can then look at basic data to see what other kinds of species For instance, we think that species such as would be good flame inhibitors. antimony oxide, phosphorus monoxide and tin monoxide are good flame inhibitors Once we know what the various optimum flame retarding at the present time. such as antimony trihalide, the molecular precursor to species should be We then have to be concerned with the solid state chemistry antimony monoxide. such as the interaction of antimony oxide and the organic halogens to generate these molecular precursors. Finally, there is the problem of how to incorporate Clearly, there are many chemical problems these additives into real polymers.

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to be solved before a definitive understanding of chemical fire retardance can result.

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NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 411, Fire Safety Research, Proceedings of a Symposium Held at NBS, Gaithersburg, Md., August 22, 1973, (Issued November 1974)

MECHANISM OF FLAME RETARDANT ACTION IN TEXTILES

Robert H. Barker

Clemson University, Clemson, South Carolina

Flame retardants may exert their effect on textile materials
by either modifying the pyrolysis reactions of the polymer sub-
strates so that smaller quantities of flammable gases are produced
or by interfering with the oxidation reactions in the flame. The
modes of action for several common types of retardants on cellulose,
nylon, and polyester have been determined by use of thermal analysis,
pyrolysis-gas chromatography, oxygen index, and calorimetric techni-

ques.

Key words:

Calorimetry; cellulose; flames; flammable gases; nylon; oxidation reactions; oxygen index; phosphorus; polyester; polymer substrates; pyrolysis-gas chromatography; textiles; thermal analysis.

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For combustible textile materials the most serious hazard is usually that associated with their propensity toward flaming combustion. It is therefore the flaming reaction which is most commonly the focus of inhibitory action.. This flaming process is basically cyclic in nature. In the initial stages of the process heat is supplied to the non-volatile polymer substrate to initiate an endothermic degradation reaction which for most polymer systems is thought to be predominantly pyrolytic in nature. There seems to be little evidence of significant oxidative processes occurring in the condensed phase except in the case of afterglow. The products of this polymer pyrolysis migrate to the surface of the fabric and are released into the atmosphere immediately above the fabric. As they diffuse away from the surface of the fabric, they begin to mix with the oxygen of the air so that combustion can take place. The combustion is, of course, an exothermic process, and the heat thus liberated can be returned in part to the fabric surface. This heat causes further pyrolysis of the polymer, assuring a continuous supply of fuel for further flame propagation.

Because of the cyclic nature of this process, it is usually amenable to attack at any of several points. Effective flame retardants may, therefore, act either in the condensed phase, or in the vapor phase above the decomposing polymer. Retardants which act in the gas phase exert their effect by functioning as either inert diluents or as free radical inhibitors which slow the oxidation processes and decrease the heat returned to the fabric surface. Those retardants which act in the condensed phase may operate by several mechanisms. They may inhibit the polymer pyrolysis so that it does not break down to produce the small volatile molecules necessary for flame propagation. More commonly, however, they act to alter rather than inhibit this pyrolysis reaction. The alteration is such that the mode of pyrolysis is changed and lesser quantities of flammable gas are produced. Finally, they may also exert their effect in a physical rather than a chemical manner. In this case they act as a shield to prevent the transfer of heat from the flame back to the fabric surface. This reduces the rate of polymer pyrolysis and fuel production is decreased.

In order to evaluate a particular flame retardant system, it becomes necessary to determine the mechanism of action of the various flame retardants on the fabric. This usually requires a knowledge of whether the flame retardant is gas phase or condensed phase active. Once this is known an investigation may be begun to determine the actual chemical mechanism involved in the retardation process.

2.

DETERMINATION OF THE SITE OF FLAME RETARDANT ACTIVITY

A wide variety of phosphorus-containing flame retardants are known to be effective on cellulosic substrates. Of these, phosphoric acid is one of the simplest and most effective. For many years, it has been postulated that this material acts completely in the condensed phase to alter the fuel producing reaction. That this is actually the case has been shown in a recent study. [1] Thermal analysis of cotton fabrics treated with various amounts of phosphoric acid are shown in figure 1. The DTA curves show that the endothermic pyrolysis

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reaction of the cellulose occurs at progressively lower temperatures as increasing amounts of phosphoric acid are present. In fact, the endothermic decomposition reaction becomes two endotherms in the presence of phosphoric acid. These endotherms correspond to catalyzed decomposition and catalyzed phosphorylation of the cellulose. This is possible in the case of cellulose

because of its ability to undergo decomposition by at least two competing pathways, as shown in figure 2. The decomposition to carbon dioxide water proceeds only very slowly in the absence of catalysts. In the presence of a catalyst such as phosphoric acid, however, this reaction becomes the predominant one at the expense of the fuel producing reaction.

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