Phosphorus-based char formation

Condensed phase. In condensed-phase modification, the flame retardant alters the decomposition chemistry so that the transformation of the polymer to a char residue is favored. This result could be achieved with additives that catalyze char rather than flammable product formation or by designing polymer structures that favor char formation. Carbonization, which occurs at the cost of flammable product formation, also shields the residual substrate by interfering with the access of heat and oxygen. Phosphorus-based additives are typical examples of flame retardants that could act by a condensed phase mechanism. 

Most phosphorus based FRs tend to promote char formation, which gives several desirable benefits. The char is difficult to ignite, it acts as a barrier between polymer and flame, it restricts oxygen access, and it reduces the rate of heat generation. This works well for polyesters, PU and polyamides, but when it comes to st3Tenic polymers or polyolefins, there is often a need to combine phosphorus systems with other additives. 

The formulation of more recent APP-based systems also seems to be based on phosphorus-nitrogen synergism. Co-additives are polymeric in nature such as poly(triazine-piperazine) materials, essentially substituted melamine rings linked by piperazine groups, and poly(ethylene-urea-formaldehyde) condensates. The sequence of reactions leading to char formation in these systems is poorly understood. It is likely that both water and ammonia are evolved at certain stages, some phosphorus and probably some nitrogen remain incorporated in the char structure, and some phosphorus ends up as polyphosphoric acid. 

Another class of reactive chemical with effective fire-retardant properties is based on phosphorous derivatives (phosphorus, phosphates, etc ). It is mainly used with oxygen or nitrogen-rich polymers like polyesters and polyamides, as it promotes the formation of char during the decomposition of these matrices (Horrocks and Price, 2001 Mouritz, 2007 Laoutid et ah, 2009). 

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