Condensed phase fire-retarding

Zinc borates are predominately a condensed phase fire retardant. In a halogenated system such as flexible PVC, it is known to markedly increase the amount of char formed during polymer combustion whereas the addition of antimony trioxide, a vapor-phase flame retardant, has little effect on char formation. Analyses of the char show that about 80%-95% of the antimony is volatilized, whereas the majority of the boron and zinc from Firebrake ZB remains in the char (80% and 60%, respectively).48-56 The fact that the majority of the boron remains in the condensed phase is in agreement with the fact that boric oxide is a good afterglow suppressant. The mode of action can be summarized in the following equation (not balanced). 

The fire-retardant mechanism associated with nanoclays has recently been studied and is likely to involve the formation of a ceramic skin which catalyzes char formation by thermal dehydrogenation of the host polymer to produce a conjugated polyene structure. " The nanocomposite structure present in the resulting char appears to enhance the performance of the char through reinforcement of the char layer. These effects would explain the apparent fire-retardant synergy observed when nanoclays are incorporated into polymer formulations containing condensed phase fire-retardant systems, including coated fillers. 

A condensed phase fire retardant mechanism is proposed for APP in nylon-6 [141]. In fact, an intumescent layer is formed on the surface of burning nylon-6/APP formulations which tends to increasing content of APP. 

The Flamtard grades function as both vapor phase and condensed phase flame retardants and smoke suppressants. This dual phase action acts synergistically in halogenated plastic and rubber formulations. For non-halogenated polymers, the addition of Flamtard with a halogen source will improve fire performance and reduce smoke emission. 

Solid (Condensed) Phase Sohd phase fire retardant(s) alter the physical burning characteristics by either forming an insulative fire barrier or by changing the surface morphology to interfere with the release or generation of combustible gases. This mechanism is commonly seen with phosphorus-based flame retardants. 

The studies of fire-retardant action of halogenated compounds in the condensed phase were performed mostly on mixtures of chloroparaffin with vinyl polymers such as polyethylene,... 

Concluding, the destabilization of the above polymers by chloroparaffin should be beneficial in terms of fire retardance because it induces the formation of fuel at the temperature at which HC1 is evolved. Thus, the occurrence of a polymer additive interaction in the condensed phase... 

Indeed, radical trapping in the gas phase performed by HX is bound to increase production of CO which would otherwise be oxidized by OH radicals. Furthermore, restriction of oxidation increases the amount of nonoxidized products which may condense into droplets or particles when they leave the flame, increasing the optical density of the smoke. Finally HX and metal halides are highly corrosive. The ensuing threat to people, structures, and goods involved in the fire may discourage the use of these fire retardants in spite of their high effectiveness and versatility which... 

Mechanistic studies described above show that halogenated fire-retardant systems can act by a condensed phase mechanism that in some cases could be induced by a halogen-free compound. 

Costa, L. Camino, G. Luda, M. P. Mechanism of condensed phase action in fire retardant bismuth compound-chloroparaffin-polypropylene mixtures Part I—The role of bismuth trichloride and oxychloride, Polymer Degradation and Stability, 1986, 14(2), 159-164. 

Phosphorus-containing fire retardants can be active in the condensed phase or in the vapor phase, or in both phases.88 The relative predominance of the different mechanisms actually operating in a... 

Their fire-retardant mechanism is predominantly due to condensed phase action involving a combination of endothermic decomposition, water release, and oxide residue formation. 

In an attempt to look for alternatives to the use of halogenated fire retardants, which function in the gas phase, an approach has been pursued which controls the polymer flammability by modifying the condensed phase chemistry. Silica gel combined with potassium carbonate have been reported to be an effective fire retardant for a wide variety of common polymers, such as polypropylene, nylon, poly(methylmethacrylate), poly(vinyl alcohol), cellulose, and to a lesser extent, polystyrene and styrene-acrylonitrile.49 The cone calorimeter data shown in Table 8.5 indicate that the PHHR is reduced by up to 68% without significantly increasing the smoke or carbon monoxide levels during the combustion. 

Fire-retarded materials functioning in the condensed phase, such as intumescent systems, form, on heating, foamed cellular charred layers on the surface, which protects the underlying material from the action of the heat flux or the flame. It is recognized that the formation of the effective char occurs via a... 

Pawlowski and Schartel92 have added 1 or 5 wt % of boehmite to blends of PC/ABS with PTFE and RDP or bisphenol A bis(diphenylphosphate). The release of water from AlOOH influences the decomposition of the material by enhancing the hydrolysis of PC and RDP. Consequently, the condensed action of RDP or BDP is perturbed. The reaction of the arylphosphate with boehmite replaces both the formation of anhydrous alumina and alumina phosphate on the one hand, and the cross-linking of arylphosphate with PC on the other hand, since less phosphate is available to perform condensed-phase action. The reaction with arylphosphate therefore decreases the char formation, but the formation of aluminum phosphate could enhance barrier properties. On the whole, even high levels of fire retardancy can be achieved (V-0 ratings) the combination of boehmite with arylphosphates acting in the condensed phase seems very complex, particularly when the host polymer can undergo hydrolysis reactions due to water release. 

Price D, Anthony G, Carty P. Introduction Polymer combustion, condensed phase pyrolysis and smoke formation. In Fire Retardant Materials. Horrocks AR, Price D, Eds. Woodhead Publishing Cambridge, U.K., 2001 chap. 1, pp. 1-30. 

Ravey, M. Weil, E.D. Keidar, I. Pearce, E.M. Flexible polymethane foam. II. Fire retardation by tris(l,3-dichloro-2-propyl) phosphate. Part B. Examination of the condensed phase (the pyrolysis zone). J. Appl. Polym. Sci. 1998, 68, 231-254. 

---