LAUSR

Abstract Steel slag (SS) waste was microencapsulated by a 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived silane via sol–gel reaction to improve its compatibility with polymer as well as to enhance flame retardant efficiency. The modified steel slag (mSS) was combined with expanded graphite (EG) as flame retardant system for rigid polyurethane foams (RPUFs). The RPUF-3 sample with 10 wt% mSS and 10 wt% EG showed an optimal comprehensive performance including a compressive strength of 206.9 kPa, a thermal conductivity of 0.0304 W/(m·K), a limiting oxygen index (LOI) value of 24.0% and UL-94 V-0 classification. The peak heat release rate (pHRR) and total heat release (THR) of the RPUF-3 dropped by 55% and 47%, respectively. By comparison, the RPUF-6 sample with 10 wt% SS and 10 wt% EG exhibited poorer flame-retardant behaviors. The superior flame retardancy of RPUF-3 over RPUF-6 could be ascribed to phosphorus-silicon complex in mSS promoted charring that provided better barrier effect during combustion.