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• Coatings with expandable graphite typically rely on the physical flame-retardant mechanism. • Higher loadings can improve the fire-resistance time in a bench-scale test. • Significant differences in the fire-resistance time were observed when tested in 1D and 3D heat transfer tests. • Factors like char cohesion and cracking along the edges are not well-captured in bulk-scale furnace tests. This work explores the importance of varying the ratio of conventional flame-retardant additives and the scale of testing on the thermal and flammability/fire performance of acrylic-based coatings. Ammonium polyphosphate (APP), pentaerythritol (PER), and expandable graphite (EG) are used as intumescent additives by varying their ratios as 1:1:3 or 1:3:1 or 3:1:1. APP, PER and EG are used as acid source, carbonising agent and blowing agent, respectively. Despite the different roles of APP, PER, and EG, in all the compositions, the physical mechanism of exfoliation of graphite played an important role in offering the fire protection. With higher loadings of EG, the fire-resistance time was higher. However, there were clear differences in the protection extent when tested in a furnace under one-dimensional heat transfer conditions (bench-scale) as opposed to three-dimensional large-scale testing. Parameters that are not intrinsic to the coating system like char cohesion, cracking, delamination from the substrate, rapid and non-directional expansion, and even higher heat fluxes experienced by the edges of the I steel section affect the fire performance. [ABSTRACT FROM AUTHOR]

Material-scale tests involving milligrams of samples are used to optimize fire-retardant coating formulations, but actual applications of these coatings require them to be assessed with structural-scale fire tests. This significant difference in the scale of testing (milligrams to kilograms of sample) raises many questions on the relations between the inherent flammability and thermal characteristics of the coating materials and their "performance" at the structural scale. Moreover, the expected "performance" requirements and the definition of "performance" varies at different scales. In this regard, the pathway is not established when designing and formulating fire-retardant coatings for structural steel sections or members. This manuscript explores the fundamental relationships across different scales of testing with the help of a fire-protective system based on acrylic resin with a typical combination of intumescent additives, viz. ammonium polyphosphate, pentaerythritol, and expandable graphite. One of the main outcomes of this work dictates that higher heat release rate values and larger amounts of material participating in the pyrolysis process per unit time will result in a rapid rise in steel substrate temperature. This information is very useful in the design and development of generic fire-retardant coatings. [ABSTRACT FROM AUTHOR]