THERMAL-DEGRADATION OF PENTAERYTHRITOL DIPHOSPHATE, MODEL-COMPOUND FOR FIRE RETARDANT INTUMESCENT SYSTEMS .2. INTUMESCENCE STEP

The chemical reactions responsible for the intumescent behaviour of pentaerythritol diphosphate (PEDP) have been studied in detail. On heating, condensation of free acidic hydroxy groups of PEDP takes place with elimination of water. This reaction overlaps with scission of ester bonds by pyrolysis or hydrolysis which frees phosphoric acid groups and gives a foamed carbon-rich residue. The mechanism of formation of this char is discussed on the basis of competing reactions of species resulting from ester pyrolysis or hydrolysis. The foaming effect is due to water evolved from condensation of phosphoric to polyphosphoric acid (12% of PEDP) and gaseous products of partial fragmentation of the charring material, e.g. methane, ethylene, propylene, aldehydes (overall 4%). The result of the intumescent process is a physical mixture of phosphoric-polyphosphoric acid with a foamed char characterised by closed spherical cells of diameter ≤100 γm and wall thickness about 10 γm.