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• The addition of MgAlCO 3 -LDHs to the IFR coating effectively reduced the TSP and SPR from the cone calorimetry. • Thermal decomposition of MgAlCO3-LDHs could promote and catalyze esterification between APP and PER. • The C-N-P-MgAl coating could improve the thermal insulation performance of the char layer. MgAlCO 3 -layered double hydroxides (MgAlCO 3 -LDHs) were introduced in a waterborne intumescent fire retardant (IFR) with nano-coating containing ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (MEL). The effects of MgAlCO 3 -LDHs were evaluated based on thermogravimetry (TG), differential thermal analysis (DTA), Fourier transform infrared spectroscopic analyses (FT-IR), X-ray power spectroscopy (XPS), scanning electron microscopy (SEM), fire performance test, and cone calorimetry. The 1.8 wt% of MgAlCO 3 -LDHs added to the IFR coating promoted the expansion of the char layer to improve its thermal insulation performance during the fire performance test. The results characterized by TG, DTA, XPS, and FT-IR revealed that MgAlCO 3 -LDHs enhanced the residual weight, the thermal stability and the oxidation resistance of the IFR coating. There was statistical evidence that the heat-shielding performance was reinforced by adding an appropriate amount of MgAlCO 3 -LDHs to the IFR coating to form uniform and compact foams in the char layer by using the digital image to process SEM images. The cone calorimetry revealed that a 46% reduction in total smoke production (or total smoke release) and a 25% decrease in the smoke production ratio were achieved in the coating with added MgAlCO 3 -LDHs compared with that without. It can be concluded that the addition of MgAlCO 3 -LDHs to IFR coating actively interacts with APP, PER, and MEL, thus improving the fire retardant properties of the coating. [ABSTRACT FROM AUTHOR]

• An analysis method for SEM images of intumescent fire retardant char layers is proposed. • Foams in an intumescent fire retardant char layer are characterized quantificationally. • Appropriate addition of CaAlCO 3 -LDHs makes the char layer compact and uniform. • Unsatisfactory result image after color transformation is returned to object segmentation. A new characterization method is proposed to investigate the microscopic morphology of intumescent fire retardant (IFR) char layer observed under a scanning electron microscope (SEM). The method consists of object segmentation extracting the object, color transformation improving the recognition of subtle details, and statistical analysis obtaining the features (area, average, and standard deviation) of the object in the SEM images. There is a descending trend in the standard deviation of the foams with the increase of CaAlCO 3 -LDHs (layered double hydroxides) added to the coatings. Specifically, the number of the foams in the IFR coating with 5 wt% of CaAlCO 3 -LDHs addition is increased from 102 (that of the char layer without CaAlCO 3 -LDHs) to 123. Meanwhile, the standard deviation of the foams in the char layer is decreased from 6383.6 to 4304.5, demonstrating that an appropriate addition of CaAlCO 3 -LDHs can improve the foam quality of the IFR coating by making it more compact and uniform, and enhance the thermal insulation of the char layer covered on the substrate surface. [ABSTRACT FROM AUTHOR]