Optimizing expansion and char strength in steel structure intumescent fire-resistant coatings via resin copolymerization

Intumescent fire-resistant coatings (IFRC) are widely used for steel structures. The fire protection performance of the coating is closely related to its expansion and the strength of the char residue. However, these properties are often in contradiction. Recent studies suggest that resin properties play a crucial role in expansion and charring processes. Here, copolymer resins are synthesized using styrene (St), 2-ethylhexyl acrylate (2Eha), and isobutyl methacrylate (iBma), via suspension polymerization, to regulate the expansion behavior and char strength of the IFRC. Using experimental characterization combined with density functional theory (DFT), multilevel structures of the terpolymer are analyzed, consisting of plasticizing fillers formed by polymerization-induced phase separation (PIPS) and a crosslinked layer of hydrogen bonding. The IFRCs made of the copolymer resins are then evaluated for expansion and char strength, and the positive effects of the multilevel structures on performance are discussed. The results show that 2Eha significantly enhances the expansion performance, whereas iBma and St improve the char strength. The optimized IFRC, prepared from the terpolymer resin (10 wt% iBma, 20 wt% 2Eha, and 70 wt% St), extends the time to reach the steel failure temperature of 535 °C from 1276 s (uncoated) to 3236 s, a 253 % increase in the safe period. Additionally, the IFRC exhibits good flame retardancy and low smoke emission, with a peak heat release rate (PHRR) of 30.15 kW/m², fire performance index (FPI) of 4.48 m²s/kW, fire growth index (FGI) of 0.195 kW/m²/s, and smoke production rate (SPR) of 0.01649 m²/s.