Sustainable and low-toxicity pyrazolyl N-methyliminodiacetic acid boronate as a nitrogen–boron flame retardant for functional silk

Abstract

Owing to growing environmental and human health concerns, halogen-free flame retardants have attracted significant attention. Boron compounds have garnered considerable interest due to their low toxicity and outstanding flame-retardant properties in recent decades. Herein, we used a low-toxicity pyrazolyl boronate BN3 as a flame retardant to reduce the flammability of silk. With the incorporation of BN3, the limiting oxygen index value of silk was increased from 24.1% to 28.3%. Compared to neat silk, the total smoke production was significantly decreased by 82% and the smoke production rate was also remarkably reduced by 56% with the incorporation of BN3. Additionally, the tensile strength of the BN3-silk increased from 93.9 cN to around 102 cN, while the elongation at break and softness remained nearly unchanged compared with those of neat silk. The moisture permeability of BN3-silk was slightly higher than that of neat silk, but its air permeability decreased by 12.4%. Different measurements revealed that BN3 acts as an intumescent flame retardant, functioning in both the condensed and gaseous phases during combustion. This work presents a novel approach to producing silk with both flame-retardant and smoke-suppression properties without the use of halogens and phosphorous, demonstrating the great potential of N-heteroaryl boronates in environmentally friendly flame-retardant materials.