Study on flame retardancy of melamine/g-C3N4 covalent organic framework nanosheets used in cable sheath material

Covalent organic framework (COF) nanosheets prepared from the direct intercalation reaction between melamine and g-C3N4 are reported for the first time. Scanning electron microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy are applied to characterize the novel COF nanosheets. It is observed apparently from the TEM image that COF nanosheets are obtained. Successful preparation of COF nanosheets is proved further by vanishment of the typical diffraction peak of COFs in the XRD pattern and the appearance of quadrant and semicircle stretching of the s-triazine ring at 1568 and 1469 cm−1 in the FTIR spectra of COF nanosheets. The prepared COF nanosheets are used as a flame retardant for the cable sheath material. The thermal stability, thermal shrinkage, oxygen index, and vertical combustion are tested. The research results show that the COF nanosheet flame retardant increases the thermal stability time of the sheath material from 89 to 125 min, and the thermal shrinkage rate is reduced to 2.17%. The oxygen index and vertical combustion tests show that the COF nanosheet flame retardant makes the oxygen index of the cable sheath material reach 29.5, and the flame retardant grade reaches UL-94 V-0. From the scanning electron microscopy analysis of the carbon residue of the cable sheath material after burning, it is found that the COF nanosheet flame retardant changes the shape of the carbon layer of the cable sheath material into a dense honeycomb structure, which prevents the diffusion of oxygen and combustible gases, thereby inhibiting the transmission of flame and heat and effectively improving the flame retardancy of the cable sheath material.