Benign design of intumescent fire protection coatings for steel structures containing biomass humic acid as carbon source

The application of biomass materials in engineering has received a great deal of attention. In this work, a new eco-friendly biomass-based intumescent coating, composed of natural and sustainable biomass humic acid (HA), ammonium polyphosphate (APP), melamine (MEL), and zinc borate (ZB), was developed to endow steel structures with excellent fire resistance and fire retardancy. When exposed to a butane torch, the intumescent coating (1.5 mm thickness) reduced the maximum temperature observed on the backside of a 3 mm thick steel plate by over 350 °C. Moreover, the cone calorimeter results showed that thanks to the formation of an extremely intumescent and dense char layer, the peak heat release rate (pHRR) and total heat release (THR) of intumescent coatings (S5) were reduced by 86 % and 61 %, respectively, as compared to that of the control epoxy coating, meanwhile, significant reductions in fire growth rate (FIGRA, −89 %) and maximum average rate of heat emission (MARHE, −86 %) were also observed, suggesting that 2 wt% ZB loading is required to achieve optimal level of fire resistance and fire retardancy. Furthermore, the mechanism of action was elucidated by SEM-EDX, FTIR, Raman, and XRD analyses. Results revealed that the combination of APP, MEL, HA, and ZB promoted the formation of a continuous and cohesive graphitized intumescent char layer on the surface of the steel plate, reinforced by borophosphate and zinc-containing species, which effectively prevents the transfer of heat and fuel, thereby enhancing the fire resistance and fire retardancy of the intumescent coating. This eco-friendly intumescent coating provides a strategy for developing new green fire protection systems using widely available, low-cost biomass materials without using hazardous ingredients.