Intumescent acid source

Typical acid source (dehydrating agent) used in numerous intumescent formulations. 

The main components of intumescent compositions are most often a polyacid, a carbon source, and an expansion agent. In some cases, the carbon source is the host polymer itself and the expansion process is generated by the decomposition products of the polymer or the acid source, e.g., melamine pyrophosphate (MPP). The most frequent polyacid used is APP. Polymers such as those mentioned above are generally preferred rather than polyols due to exudation, water solubility, and difficulties with processing for the filled polymer. Several studies mention the use of OMMT as a nanofiller introduced in the polymer playing the role of carbon source. In other studies, the silicate is blended with all the components of the flame-retarded polymer. 

Le Bras and co-workers [50] developed flame retardant intumescent formulations using the association of APP as the acid source and PA-6 as the carbonisation agent in an EVA (8%) copolymer matrix. 

Intumescent flame retardants (IFR) that contains phosphorus are also used in halogen-free flame-retardant systems. Most reported IFRs are mixtures of the three ingredients, an acid source, a polyol, and a nitrogen-containing compound (Halpem et al. 1984). Since processing of ABS resin requires that the additives withstand temperatures in excess of 200 °C, the commonly used intumescent system, ammonium polyphosphate, pentaerythritol, and melamine, which do not have sufficient thermal stability, cannot be incorporated into ABS resin under normal processing conditions they are usually used in polyolefins. 

A patent review [4] showed that early intumescent formulations incorporated in polymers contained a precursor of phosphoric or polyphosphoric acid, a pentaerythritol type char source, and melamine, as typical formulations of intumescent coatings. Further developments tried to reduce the complexity of the additive system, for example by using a binary combination of the add precursor with nitrogen-containing compounds, which also act as a char source. While the add source is generally APP, typical examples of the second component are produds of condensation of formaldehyde with substituted ureas products of readions between aromatic diisocyanates and pentaerythritol or melamine polymers containing the piperazine ring in the main chain, also combined with substituted s-triazine, hydroxyalkyl isocyanurate etc. 

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