Condensed phase mechanism, halogen

The question as to whether a flame retardant operates mainly by a condensed-phase mechanism or mainly by a vapor-phase mechanism is especially comphcated in the case of the haloalkyl phosphoms esters. A number of these compounds can volatilize undecomposed or undergo some thermal degradation to release volatile halogenated hydrocarbons (37). The intact compounds or these halogenated hydrocarbons are plausible flame inhibitors. At the same time, thek phosphoms content may remain at least in part as relatively nonvolatile phosphoms acids which are plausible condensed-phase flame retardants (38). There is no evidence for the occasionally postulated formation of phosphoms haUdes. Some evidence has been presented that the endothermic vaporization and heat capacity of the intact chloroalkyl phosphates may be a main part of thek action (39,40). 

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. 

As in polyester resins, reactive halogens containing fire-retardant chemicals are most often used in epoxy materials. Tetrabromobisphenol A is perhaps the most widely used component for flame-retarding epoxy resins. Nara and Matsuyama (24) and Nara et al. (25) described the thermal degradation and flame retardance of tetrabrominated bisphenol A diglycidyl ether compared to the nonbrorainated structure. Their results indicate that bromine acts by vapor-phase as well as condensed-phase mechanisms of flame inhibition. 

Thus, the condensed phase mechanism of fire retardant intumescent systems aims at reducing the rate of pyrolysis of the polymer below the threshold for self-sustained combustion. This limits the production of volatile moieties and hence reduces undesirable secondary effects of volatiles combustion such as visual obscuration, corrosion and toxicity, which are typical of the widely used halogen containing fire retardants. 

Either mechanism can be used to describe how antimony—halogen systems operate in both the condensed and vapor phases. In the condensed phase a chat that is formed during the reaction of the polymer, antimony trioxide, and the halogen reduces the rate of decomposition of the polymer therefore, less fuel is available for the flame (16). 

Mechanism of Action of Halogenated Compounds in the Condensed Phase.82... 

These systems are of great importance in spite of the current cautious attitude, which will be discussed below, toward the use of halogen-based systems. Recent mechanistic results show that the active species formed by halogen compounds in the condensed phase may play a fundamental role in the hre-retardant mechanism, which could be reproduced using halogen-free moieties. 

MECHANISM OF ACTION OF HALOGENATED COMPOUNDS IN THE CONDENSED PHASE... 

MECHANISM OF CONDENSED PHASE ACTION OF SYNERGISTIC SYSTEMS BASED ON HALOGEN-METAL COMPOUNDS... 

Costa, L. Luda, M. P Trossarelli, L. Mechanism of condensed phase action in flame retardants. Synergistic systems based on halogen-metal compounds, Polymer Degradation and Stability, 2000, 68(1), 67-74. 

Phosphorus promotes char formation to protect substrate, and halogen acts in vapor phase. Good thermal stability. Process with modified PPO up to 550-600°E Flame-retardant mechanism condensed phase. Flame retardant induces reactions in host resin that lead to charring and insulation against further burning. 

Bromine is believed to perform most of its flame retardant function in the gaseous phase by means of two mechanisms redirection or termination of the chemical reactions involved in combustion, and the evolution of heavy bromine-containing gases which tend to protect the condensed phase by inhibiting access of oxygen and transfer of heat [68]. The chemical reactions may involve halogenation followed by dehydrohalogenation to yield a polymeric residue rich in double bonds which is then converted to carbon. 

Antimony compounds are generally used as fire retardants in synergistic combinations with halogen compounds. The mechanism of action is not well understood, but may be associated with effects due to the release of the hydrogen halide, acting possibly both in the condensed phase to modify the decomposition of the polymer and in the gas phase as a flame quencher. 

The mechanism by which tin flame retardants function has not been well defined, but evidence indicates tin functions in both the condensed and vapor phases. In formulations in which there is at least a 4-to-l mole ratio of halogen to tin, reactions similar to those of antimony and halogen are assumed to occur. Volatile stannic tetrahaUde may form and enter the flame to function much in the same manner as does antimony trihaUde. 

Boron Mechanism. Boron functions as a flame retardant in both the condensed and vapor phases. Under flaming conditions boron and halogens form the corresponding trihalide. Because boron (rihalides are effective Lewis acids, they promote cross-linking, minimizing decomposition of the polymer into volatile flammable gases. These trihalides arc also volatile thus they vaporize into the (lame and release halogen which Ihen functions as a Maine inhibitor. 

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