Influences of polymerization degree of ammonium polyphosphate on fire protection of waterborne intumescent fire resistive coating

Six kinds of ammonium polyphosphate (APP) samples with different polymerization degrees (DP) were used as fire retardant additives for preparation of waterborne intumescent fire resistive coatings. The DP of APP was measured by 31P NMR. The FTIR spectra proved that all APP samples had same chemical structure in addition to the different DP. The uncertain influences made by the particle size distribution of the APP samples was excluded by the results of laser particle size analysis. The influences of the DP of APP on the properties and char formation of the coatings were investigated in detail by TGA, FTIR, XRD, EDS, SEM and fire protection test. The TGA and EDS results proved that APP with high DP could enhance anti-oxidation of the char layer at high temperature. The SEM images demonstrated that the cell size of the char layer decreased and the cell size distribution of the char layer became narrow with increase of the DP of APP. When the DP of APP was less than 100, the fire protection of the coating was significantly improved with the increase of DP of APP gradually when the DP of APP was less than 100. But the fire protection of the coating improved slowly with increase of the DP of APP when the DP of APP was more than 100. The results showed that the coating using APP with DP of 184 had best fire protection in our field of research.