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

► The test method of average DP of APP by NMR was used in our work. ► The uncertain influences made by particle size of APP was excluded by the results of LS analysis. ► APP with higher DP could enhance anti-oxidation of the coating and strength of the char layer. ► When DP was less than 100, fire protection performance was significantly improved with DP increased. ► When the DP of APP was more than 100, the improvement was not remarkable.