Modeling the rheology of commercial reactive fire-retardant coating materials for steel

Fire protection of steel load-bearing building structures by surface-treating them with reactive fire-retardant coating materials is a crucial factor in ensuring fire safety on national scale. Both, the quality and operational characteristics of such materials, which are the subject of this study, require continuous improvement to reduce the cost of fire protection, as it can constitute a large portion of the overall construction expenditure. The aim of this study was to determine optimal rheological parameters for commercial reactive fire-retardant coating materials that ensure that the material can be applied with the maximum wet coating thickness. To achieve this aim, the dynamic viscosity (η) was measured using Brookfield viscometer for a set of commercial reactive fire-retardant coating materials that provide fire resistance of at least R120 for steel load-bearing structures. The dependence of viscosity on shear rate (γ) in the range of (2.09–52.25) s–1 was modeled using the Casson equation. This allowed for the determination of the main rheological parameters of the studied materials – shear stress (τ, Pa), yield stress (τ0, Pa), and viscosity at high shear rates (η∞, Pa · s), which contribute to material’s applicability. With the use of the empirical and calculational data, the approximate viscosity of water-based intumescent coating materials necessary to produce defect-free layer of wet coating on studied surfaces was determined. It was measured by Brookfield viscometer with No. 7 spindle at rotational speeds (30–50) rpm at 20°С, and should preferably be: (30–15) Pa · s, (at 1 mm wet coating thickness); (50–25) Pa · s, (at 1.5 mm wet coating thickness), (80–50) Pa · s, (at 2.0 mm wet coating thickness). These levels of viscosity prevent sedimentation and sagging of the coating during material’s application and can serve as reference markers for optimization of industrially manufactured intumescent fire-retardant products. The obtained results can serve as practical recommendations for manufacturers seeking to improve the rheology of reactive fire-retardant materials in order to increase the wet coating thickness per layer.