Tannic Acid As Cross-Linker and Flame Retardant for Preparation of Flame-Retardant Polyurethane Elastomers

Bio-based tannic acid (TA) has been first applied as a reactive flame retardant for the preparation of polyurethane elastomers (PUEs). The flame retardant properties of PUEs are evaluated by limiting oxygen index (LOI), cone calorimeter test (CCT), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and Raman tests, which confirm the flame retardant of TA for PUE owing to its polyphenol and cross-linked aromatic structure. When 5 wt% of TA is involved, the LOI value of the sample (PUE4) increases to 40%. According to CCT, TGA, SEM and Raman tests, the introduction of TA can reduce the heat release of PUE and greatly promote the generation of carbon residue during combustion. The flame retardant mechanism of PUEs is also investigated by TGA-FTIR test, indicating that TA inhibits the generation of combustible gas during combustion. Compared with PUE1, the elongation at break of TA-modified PUEs decreases obviously due to the presence of benzene ring structure (474.5–632.7%), but their tensile strength increases significantly owing to the hydroxyl groups and cross-linked structure of TA (14.8–31.1 MPa).