Abstract
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.
Highlights
With the continuous development of flame retardant technology of polymer materials, the requirements for the comprehensive performance indicators of flame retardants are higher, to achieve the specified flame retardant level, and to have good physical and mechanical strength, non-corrosiveness, smokeless nature, light stability, aging resistance, heat stability, etc
From the scanning electron microscopy analysis of the carbon residue of the cable sheath material after burning, it is found that the Covalent organic framework (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
The specific process is as follows: first, 5 g melamine was put into a ceramic crucible, it was covered with a crucible cover, heated to 500 ○C at 50 ○C/min in a muffle furnace, and baked for 1 h; the temperature rapidly raised to 520 ○C within 5 min, and the roasting continued for 1 h, after which light yellow g-C3N4 powders were obtained by cooling and grinding at room temperature
Summary
With the continuous development of flame retardant technology of polymer materials, the requirements for the comprehensive performance indicators of flame retardants are higher, to achieve the specified flame retardant level, and to have good physical and mechanical strength, non-corrosiveness, smokeless nature, light stability, aging resistance, heat stability, etc. Halogen-free intumescent flame retardants (IFRs) have excellent thermal stability, high char formation, good flame retardant and smoke-suppressing effects, and low toxicity, which are in line with environmental protection requirements, but its large amount of addition in flame retardant materials, poor dispersion, and a serious impact on the mechanical properties of materials need to be modified (Ref. 1). Inspired by the fact that graphene is prepared from graphite, a large number of attempts have been made to achieve two-dimensional COF nanosheets. In this study, Schiff-based COF nanosheets prepared from the condensation reaction between melamine and g-C3N4 are obtained. This work provides a low-cost method to synthesize COF nanosheets
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