Preparation of a catalyst-free and water-blown rigid polyurethane foam from malic-co-citric acid-based polyols

Bio-based polyols (MCPs) were synthesized from 1,6-hexanediol, malic acid , and citric acid by a series of one-pot esterification reactions. Subsequently, water-blown rigid polyurethane foams (RPUF) were prepared using the MCPs and polyisocyanates by a catalyst-free method. The effect of the molar ratios of malic acid to citric acid on the foaming process, physicomechanical performance, and thermal stability were investigated. Moreover, the thermal degradation behavior of the MCP-based RPUFs was also investigated. The various MCPs exhibited high hydroxyl values (450–515 mg KOH/g) and low viscosities (1550–2150 mPa∙s). In addition, citric acid had a positive influence on the compressive strength and thermal stability of the MCP-based RPUFs. When the ratio of malic acid:citric acid was 95:5, the corresponding RPUF had a density of 42.6 kg/m 3 , a high compressive strength of ∼191 kPa, a low thermal conductivity of 0.0365 W/(m K), and a high initial thermal degradation temperature (> 280°C). Compared to commercial polyether polyols, the MCP-based, water-blown RPUFs demonstrated lower apparent densities, lower thermal conductivities, and higher normalized compressive strengths. These results demonstrated that malic- co -citric acid-based polyols could be an adequate substitute for petrochemical-based polyols for providing water-blown RPUFs with excellent comprehensive properties.