Epoxy-based coating for fire protection of steel structures

• Main Author: Siow, Jonathan Jun Yang
• Other Authors: Aravind Dasari
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/166517
• Institution: Nanyang Technological University

The use of (structural) steel is widely prevalent in the construction industry. Even though steel is not flammable, it loses up to ~50% of its yield strength and ~70% of its Young’s modulus when heated beyond 600°C. Numerous solutions to insulate steel during a fire scenario have been developed; however, the use of intumescent coatings is growing at an increasing trend because it can be applied on complex shapes and has a nice aesthetic finish. Intumescent coatings swell during a fire scenario to produce a foamy char that has low thermal conductivity, which delays the heat transfer to the steel substrate underneath. However, the foamed char is susceptible to thermo-oxidative erosion at high temperatures due to its carbonaceous nature that causes the loss of the protective char over time. While this may be addressed through the addition of inorganic additives/fillers to the intumescent coating composition, the resulting inorganic residue is friable that makes it susceptible to cracking and delamination. In this thesis, a fire protective coating has been developed that forms an inorganic binding phase that maintains the cohesion and adhesion of the residue to the substrate throughout the duration of the fire scenario, and lead to superior fire performance. It was found that the flammability of the developed coating plays a significant role at influencing the rate of heat transfer to the steel substrate and consequently its fire performance. Since little information existed in the literature on effective strategies that would be useful to mitigate it during furnace testing, an initial study on different epoxy-based coating model systems possessing different flame-retardant strategies (aluminium hydroxide (ATH), ammonium polyphosphate (APP), 9,10-dihydro-9-oxa-10-phosphaphenanthrene oxide (DOPO), and tetrabromobisphenol-A (TBBA)), were evaluated. Fast heating rates during the start of the furnace tests result in rapid production of volatiles that lead to autoignition and sustained flaming and a sudden rise in the heating rate of the steel substrate. Condensed-phase mechanisms that include char formation with APP addition were shown to be more effective than gas-phase mechanisms (DOPO, TBBA, ATH) at mitigating the effects of flaming combustion. A high loading (~67wt.%) of inorganic additives that largely comprised of APP and ATH was produced as a coating formulation employing the concept of in-situ ceramisation to form a rigid thermal barrier of high thermo-oxidative stability that would not possess the fragile nature of conventional intumescent char. Its fire performance and flammability were assessed and showed that such a char residue may protect the underlying steel substrate at high application thicknesses (>25mm) against the effect of flaming combustion during a constant heat flux exposure of 50kW/m2 due to its high thermal conductivity, which also affected the flammability and subsequent fire performance. The rigidity of the char residue formed was observed to be dependent on the type and amount of metal oxide present in the coating composition, which lead to a study to determine their role on the fire performance. Four different types of metal oxides (zinc, iron (iii), titanium, and zirconium) and three different loading amounts (4, 8, and 14 wt.%) of titanium dioxide were investigated. As expected, the extent of char expansion was reduced with increasing metal oxide loading and reactivity of the metal oxide to APP due to increased amount of metal phosphate formation, which was due to catalytic effect of the metal oxide on the decomposition of APP and intumescent additives. However, the improvements in fire performance observed with inorganic residues formed was not observed in heat transfer experiments and the improvement is hypothesised to be attributed to increasing the heat dissipation capability of the formed char. The role of the inorganic residue during furnace tests was better understood as a result, and this may enhance future heat transfer models using intumescent coatings. In conclusion, the work in this thesis helped provide insight into the effect of inorganic additives/fillers in an intumescent coating, and highlight the factors affected by their addition that are critical for fire performance. These concepts (i.e. reducing flammability, limiting expansion of the char residue, and careful selection of metal oxide type and loading) contributed to the successful development of an epoxy-based intumescent coating for the fire protection of steel structures producing a char residue that could provide up to ~95 minutes of fire performance during large-scale fire tests with an application thickness ~7.5mm that demonstrates a successful proof of concept.