Synergistic enhancement of flame retardancy and thermal energy storage in rigid polyurethane foams by optimizing isocyanate index with expandable graphite, aluminum hydroxide, and phase change microcapsules

Abstract

With increasing awareness of fire safety, research on the flame retardancy of rigid polyurethane foams (RPUFs) has attracted extensive attention. However, previous studies on flame retardant formulations are not fully applicable to energy-storing polyurethane foams that require both high flame retardancy and thermal energy storage capabilities for thermal management. In this study, a flame-retardant RPUF containing 12.5 wt% expandable graphite (EG) and 2.5 wt% aluminium hydroxide (ATH) was first prepared. The Limiting Oxygen Index (LOI) of the foam increased from 20.3% for the non-flame-retardant sample to 27.4% after the addition of flame retardants, while the compressive strength and thermal conductivity were 271.10  kPa and 0.02942 W/(m⋅K), respectively. Based on this formulations, flame-retardant RPUFs with different performance characteristics were subsequently fabricated by varying the polyisocyanate index. Performance analysis results revealed that, at a constant flame retardant content, the foam with a polyisocyanate index of 1.44 exhibited the best overall performance compared with those having lower indices (e.g., 1.23); at this optimal index, the LOI reached 28.0%, with a compressive strength of 322.72 kPa, an apparent density of 80.0 kg/m 3 , and a thermal conductivity of 0.03179 W/(m⋅K). Subsequently, phase change microcapsules (PCMs) with polyurethane shells encapsulating n-octadecane were incorporated into the flame-retardant RPUF. Performance evaluation of the resulting RPUF showed that at a PCM loading of 6 wt%, the thermal energy storage performance was significantly enhanced while good flame retardancy was maintained. Under this condition, the LOI and phase change enthalpy were 26.3% and 10.79 J/g, and the compressive strength and thermal conductivity were 219.22 kPa and 0.03671 W/(m⋅K), respectively. These findings were further supported by thermogravimetric, dimensional stability and cone calorimetric analyses.