Modified Gallic Acids as Both Reactive Flame Retardants and Cross-Linkers for the Fabrication of Flame-Retardant Polyurethane Elastomers

Graphical Two modified gallic acids (GAP and GAN) are prepared through a simple esterification reaction and used as reactive flame retardants and cross-linkers for the fabrication of flame-retardant polyurethane elastomers. Polyurethane elastomers (PUEs) serve a vital role in our daily life and various industrial applications, but their flammability has been proven to be a significant drawback. In this work, we have successfully fabricated two modified gallic acids (GAP and GAN) by simple esterification, both of which are effective flame retardants and cross-linkers for PUEs. According to thermogravimetric analysis (TGA) and limiting oxygen index (LOI) tests, the introduction of GAP and GAN conspicuously improves the char yield (4.9~5.8 wt.%) and LOI value of PUEs (33~35 %). The cone calorimetry tests suggest that the peak heat release rate (PHRR) and total heat release (THR) of PUE decrease significantly, while the fire performance index (FPI) value increase after GAP or GAN modification. Both GAP and GAN enhance the density and strength of carbon residues, and inhibit the generation of combustible volatile gas during the combustion process, thereby improving the fire safety of PUEs. In addition, both GAP and GAN contain multiple benzene rings and hydrogen bonding, which can improve the cross-linking density of PUEs to enhance their mechanical properties (the elongation at break: 844~951 %; tensile strength: 6.37~8.13 MPa).