Zinc Oxide and Expandable Graphite-Based Flame Retardant Coatings of Unsaturated Polyester Resin for Composite Applications

Halogen-free multifunctional composite coatings modified with zinc oxide (ZnO) and expandable graphite (EG) on glass fiber–reinforced unsaturated polyester resin (UPR) composites, developed for applications in electronics and other areas where local heat loads and mechanical stress can affect the composite surface, were investigated. The primary objective of the coatings was to provide mechanical protection and enhanced flame retardancy without the use of halogen-based flame retardants. The combined use of ZnO and EG reduced the maximum average rate of heat emission by 64% compared to the reference (465 kW/m2). Furthermore, the effective heat of combustion was reduced from 3156 to 41 MJ/kg with the combined use of the flame retardants. In glow wire flammability index tests, the ignition temperature increased by 310°C compared to the uncoated composite. When ZnO and EG were used together, a stable, intumescent char layer formed that effectively protected the substrate under local heat stress. The additives did not influence the adhesion of the coatings, while the combined ZnO–EG system demonstrated higher toughness and better crack dispersion under bending loads, maintaining coating integrity even after local deformation. These results indicate that halogen-free ZnO–EG coatings can provide both flame retardancy and mechanical robustness, making them promising for structural and electrical composite components exposed to heat and mechanical stress.