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The purpose of this paper is to investigate the effect of char on the flame retardancy of fabrics by a cone calorimeter, which is an important factor to compare the flame retardancy of different fabrics.

Cone calorimeter measurements were carried out in a Fire Testing Technology (UK) apparatus at the heat fluxes of 50 and 75 kW/m². Fabrics with one and three layers were employed, with the name of cotton1, cotton3, FR cotton1, FR cotton3, PMIA1 and PMIA3. The dimension of the fabric was 100×100 mm². A cross-steel grid was used to prevent the fabrics from curling during burning. The distance between the bottom of the cone heater and the top of the sample was 25 mm.

This work was generously supported by National Key R&D Program of China (Project No. 2017YFB0309000), Natural Science Foundation of Shandong Province of China (Project No. ZR2019BEM026), Natural Science Foundation of China (Project No. 51803101) and China postdoctoral science foundation funded project (Project No. 2018M632619).

The present research provides insight into the effect of the char formation on the flame retardancy of the fabrics, and a method to comprehensively investigate the char influence in the flame retardancy of the fabrics by a cone calorimeter is proposed.

The theoretical analysis indicates that the main factors of cover‐shelter properties of fabrics are cover factor of fabrics, fabric thickness, optical reflectance and absorbance of yarn. With a combination of distances and luminances, 512 experiments have been made to find out the relationship between the least distinguishable distance and reflectance coefficient.