Preparation of flame-retardant rigid polyurethane foam with bio-based phosphorus-containing polyols and expandable graphite

In the last decades, rigid polyurethane foam has focused on increasing renewable components and enhancing flame retardancy due to sustainable development and fire safety requirements. Herein, we report a bio-based phosphorus-containing polyester polyol (PMCP) that is synthesized with dimethyl methylphosphonate, malic acid, citric acid, and 1,6-hexanediol in one pot. The hydroxyl value of PMCP is 485 ~ 512 mg KOH/g. As a result, it can be used as the sole polyol in the production of RPUF. Moreover, expandable graphite (EG) was incorporated into RPUF-PMCP20 to enhance flame retardance. The limited oxygen index of RPUF increases from 18.3% to 24.0% with the rise in DMMP moiety in PMCP. Moreover, the peak heat release rate (PHRR) significantly decreases from 264.2 to 129 kW/m 2 . Meanwhile, the time to ignition increases from 5 to 18 s. The flame retardant mechanism indicated that PMCP plays a dual role in the gaseous and condensed phases. The addition of EG exhibits an excellent blocking effect. The LOI increases from 22.5% to 27.3% with increasing EG content. In addition, both the PHRR and smoke release rate of RPUF are suppressed by incorporating EG.