Simultaneously Enhancing Mechanical Strength, Toughness, and Fire Retardancy of Biobased Polyurethane by Regulating Soft/Hard Segments and Crystallization Behavior

In this work, biobased flame-retardant polyurethanes were designed. First, a vanillin-based diol (VDP), as the hard segment, was prepared by condensation and addition reactions of vanillin with DDM (4,4′-diaminodiphenylmethane) and DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide). Then, polyurethane materials FRBPU-xP were prepared by a polycondensation reaction of different contents of VDP, crystallized polycaprolactone diol, and HDI by modulating the amount (x) of P. Interestingly, when the amount of VDP was increased from 0 to 14.5 wt % (P = 1.0%) and the amount of PCL diol was decreased from 82.4 to 64.8 wt %, the Tg of the prepared FRBPU-xP was significantly increased from −34.7 to 2.8 °C, while the tensile strength increased from 10.7 to 15.6 MPa and the elongation at break increased from 822% to 930%, showing simultaneous mechanical strengthening and toughening. These behaviors can be ascribed to (1) the increase of the hard VDP diols and the enhancement of intermolecular interactions and (2) the decrease of the crystallinity from 21.2% to 14.5% due to the decreasing content of PCL diols and the inhibiting crystallization behavior induced by VDP, which helped to improve the ductility of the polyurethane material. Furthermore, the LOI value of FRBPU-0.5P reached 29.6%, and this material achieved a UL-94 V-0 rating. In addition, the polyurethanes showed good reprocessability, and FRBPU-1.0P had a retention of 72.6% and 93.3% for its tensile strength and elongation at break after thermal remolding, respectively. This work provides an idea for the preparation of high-performance biobased flame-retardant polyurethane.