Summary For high performance of hydrocarbon intumescent coatings, interactions between the acid source ammonium polyphosphate (APP) and inorganic fillers can be critical. In the present work, low‐borate concentration (ie, 1 wt% zinc borate) intumescent coatings with different concentration ratios of CaCO3 to APP were studied in an attempt to map the reaction mechanisms. The analysis of the coating performance was conducted using thermal insulation assessment according to the UL1709 fire curve (disregarding substrate load, geometrical diversity, and other full‐scale implications of the complete standard), rheological measurements, thermogravimetric analyses, attenuated total reflection‐Fourier transform infrared spectrometer (ATR‐FTIR), and X‐ray diffraction. Results show that a coating with a CaCO3/APP mass ratio of 0.3 gives the best performance (ie, the longest critical time of steel substrates to reach both 400 and 550°C in the thermal insulation assessment). The dependency on the CaCO3/APP ratio of the dynamic viscosity minimum points to significant interactions between CaCO3 and APP during the softening state (ie, the intumescence process) of the coating. Due to an enhanced anti‐oxidation property, the degradation zone of the intumescent char and the residual weight of the coating after exposure were both improved when increasing the CaCO3/APP ratio. The compositional development of the intumescent char showed that CaCO3 and APP react to form a new polymeric phase, calcium catena‐polyphosphate (CaP2O6). This polyphosphate may contribute to the formation of a compact phase in the intumescent char and benefit the thermal insulation performance of low‐borate concentration hydrocarbon intumescent coatings.

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