Fundamentals of thin intumescent coatings for the design of fire-safe structures

Abstract

At present, intumescent coatings represent a mainstream solution for protecting load-bearing structural steel systems during a fire. Upon heating, intumescent coatings swell to form a thick low-density and low-thermal-conductivity porous char that prevents the substrate material from reaching high temperatures that may compromise structural integrity and stability.Within the structural fire safety engineering practice, the performance of intumescent coatings is commonly assessed adopting simplified engineering methods solely based on the standard temperature-time fire curve in fire resistance furnace tests. However, numerous researchers have emphasised the limitations of this methodology due to the influence of several factors, such as the fire heating conditions, on the intumescent process and the overall insulation effectiveness. Consequently, the current design framework does not represent an adequate design practice to ensure the fire safety of structures. In a world moving towards performance-based engineering solutions, there is a need for explicitly understanding how different factors (e.g. heating conditions) may influence the effectiveness of intumescent coatings.This research study aimed at building the fundamentals for developing robust engineering methods for the performance-based design of structural elements protected with intumescent coatings, considering their effectiveness for a wide range of potential conditions. Various studies were carried out in order to understand how different factors affect the performance of intumescent coatings. An experimental methodology was proposed to analyse their performance through a detailed characterisation of the thermo-physical response during thermal exposure. Steel plates coated with a commercial solvent-based thin intumescent coating were exposed to various well-defined and highly-repeatable heating conditions in accordance with the Heat-Transfer Rate Inducing System (H-TRIS) test method. The influence of the applied initial Dry Film Thickness (DFT) and the substrate thermal conditions were also investigated. Moreover, complementary studies using standard experimental methodologies were conducted in order to characterise the intumescent coating by defining its physical, thermal and optical properties and studying the chemical reactions and the thermal decomposition processes at different temperatures and under different heating regimes.Experimental results showed that the tested intumescent coating required incident heat fluxes higher than 20-23 kW/m2 in order to initiate the swelling process. Thresholds for the onset of swelling in terms of steel (100-250°C) and coating (350-500°C) temperatures were defined and it was found that the onset of swelling is directly influenced by the heating conditions and the initial coating thickness. The H-TRIS experiments also evidenced how the swelling process and the resulting swelled coating thickness govern the thermal and physical response of intumescent coatings, therefore their effectiveness. In particular, the heating conditions govern the swelling rate of the intumescent coating, while the initial coating thickness governs the maximum swelled coating thickness.During swelling, the coated steel plates tended to 300-350°C, while their temperature increased above 350°C when the swelling process was completed. As confirmed by the thermo-gravimetric analysis, this temperature range corresponds to the coating swelling reaction, which typically occurs around 350-400°C and it can be considered completed above 400°C. These outcomes suggest that the swelling reaction occurs in the proximity to the substrate-coating interface: the virgin coating swells and protects the substrate by displacing the already-swelled coating towards the direction of the heat source. This was confirmed by the experiments involving different substrate thermal conditions. The physical and thermal properties of the substrate control the capacity of the system to concentrate/dissipate heat at the substrate-coating interface, consequently the temperature evolution of the reacting coating and the swelling process. Accordingly, high swelling rates were recorded for insulating substrate conditions (timber), while low swelling rates for conditions characterised by significant heat losses (water-cooled heat sink).Based on the experimental results, a finite-difference heat transfer model was formulated in order to simulate the thermal and physical response of swelling intumescent coatings. The coating swelling was implemented by adding finite elements at the substrate-coating interface and the intumescent coating was modelled as swelled porous char with constant material properties. Following this approach, the modelling of intumescent coatings mainly became a quasi steady-state physical problem, largely driven by a correct prediction of the coating swelling rate and the evolution of the swelled coating thickness: empirical correlations were derived based on the experimental outcomes. Consequently, the coating material properties have limited influence and the model loses accuracy for transient states. Nevertheless, for the tested experimental conditions, the model is capable of generally describing the heat transfer through swelling intumescent coatings by predicting the evolution of the coating surface and steel temperatures.The experimental and modelling research presented herein shows how the thermal and physical response of intumescent coatings can be predicted by gauging their swelling process and implementing a simplified finite-difference heat transfer model. The swelling process and the resulting swelled coating thickness govern the effectiveness of thin intumescent coatings. Different factors (e.g. heating conditions) affect the swelling process in different ways (e.g. swelling rate), therefore the insulating effectiveness.