Flame retardant surface treatments for rigid polyurethane foams used in the building sector: Current state-of-the-art and perspectives

Rigid polyurethane foams are widely employed in the building sector, mainly as thermal insulating materials. On the one hand, they can easily be sprayed or applied as rigid insulation panels, providing exceptional thermal resistance, lowering heat transfer, and contributing to building energy efficiency. However, polyurethane foams are easily flammable when exposed to a direct flame or an irradiative heat flow. This drawback significantly limits their exploitation, especially when flame retardancy is an undeniable prerequisite. Notwithstanding the possibility of providing flame retardant features to polyurethane foams through either the incorporation of effective flame retardant additives into the foam formulation or the chemical modification of the monomers (i.e., the diols or polyols) with flame retardant species (usually containing P and/or N elements) before the synthesis of the polyurethane, one of the current strategies relies on the use of flame retardant surface treatments directly applied to the foamed materials. Indeed, the flammability of polyurethane foams is a surface-localized phenomenon that can be quite effortlessly and effectively controlled by tailoring their surface through the deposition of efficient flame retarded treatments (e.g., layer-by-layer architectures, coatings (also UV-curable), and “onepot” deposited complexes/colloids). This work is aimed at reviewing the latest research outcomes about the use of effective surface treatments for rigid polyurethane foams, providing the reader with the current limits and some perspectives for the forthcoming years.