Enhancing the flame retardancy for castor oil-based rigid polyurethane foams via silica aerogel

•Silica aerogel was successfully synthesized by sol-gel method under ambient pressure drying.•Two type silica aerogel/castor oil-based rigid polyurethane foam composites were fabricated via blending or coating silica aerogel.•Rigid polyurethane foams with larger cell size via coating silica aerogel on their surface obtained outstanding physical and flame retardant properties. Silica aerogel (SA) was successfully synthesized by sol-gel method under ambient pressure drying. The first and second type silica aerogel/castor oil-based flame retardant rigid polyurethane foam (SA/RPUF) composites were then fabricated via blending SA into the interior of RPUFs and coating SA onto the surface of RPUFs, respectively. Properties of the composites, including density, thermal conductivity, compressive strength, specific modulus, hydrophobicity, chemical component, morphology, thermal stability, limiting oxygen index (LOI), UL-94 tests, cone calorimetry (CC) testing, and char residues stability were characterized. The results showed RPUFs with larger cell size via coating SA onto their surface had cracking properties on the physical and flame retardancy, especially RPUF-4. Compared with neat RPUF, RPUF-4 obtained greatly improved properties with specific modulus an increment of 107.62%, the LOI value an increase of 50.76%, the first and second peak heat release rates with respective reduction of 52.43% and 40.78%, and respective 56.86% and 47.82% reduction in the first and second peak smoke production release rates. Thus, this study shows RPUFs with larger cell size via coating SA onto their surface have great potential for fire safety as building insulation materials.