Enhanced flame retardancy of polyurethane foam with alginate-based flame-retardant coating

Polyurethane (PU) foam is widely used in industrial and civil fields, but it is highly flammable. An eco-friendly flame-retardant coating has been fabricated from sodium alginate (SA) and mica powder, it has been applied to PU foam using a facile direct dip coating method, followed by crosslinking with Ca²⁺ and modification with polydimethylsiloxane (PDMS), respectively. The original porous network structure is maintained in the coated PU (SMPU) foam with a porosity of 90.51 %, and exhibits good thermal stability, hydrophobicity and excellent flexibility. Moreover, the as-prepared SMPU foam exhibits reduced flammability, e.g., the limiting oxygen index (LOI) for combustion of SMPU foam is 30.5 %, whereas that for unmodified PU foam is 16 %. The SMPU foam also exhibits a self-extinguishing effect without melt dripping and passes UL-94 V-0 rating. A significant reduction in the release of heat as well as total smoke and CO₂ formation during combustion was noted using cone calorimetry. These improvements in combustion behavior may be ascribed to a physical barrier formed by non-flammable mica powder, SA and PDMS which displays char forming ability. A facile strategy for improvement of the fire safety of PU foam using a green and efficient flame-retardant coating has been demonstrated.