Phosphate-Functionalized Waterborne Acrylic Binders for Inhibitor-Free Direct-to-Metal Paints

Purpose：The study develops waterborne acrylic binders for direct-to-metal coatings without corrosion inhibitors and compares their adhesion, water sensitivity and corrosion protection with a commercial DTM paint.

System and methods：Two phosphate-functionalized acrylic binders were formulated, one containing crystalline polystearyl acrylate nanodomains and the other an amorphous methyl methacrylate-butyl acrylate copolymer. Water sensitivity, mechanical stability, adhesion, high humidity, accelerated weathering and salt spray were evaluated.

Key findings and mechanism：Two phosphate-functional waterborne acrylic binders were compared with commercial DTM coatings. Both inhibitor-free phosphate systems outperformed the inhibitor-free commercial control in humidity and accelerated weathering and approached the inhibitor-containing benchmark. The amorphous MMA-BA binder performed particularly well in salt spray, even exceeding the inhibitor-containing control. Phosphate groups provide chemical adsorption and anchoring at the metal, helping the coating resist adhesion loss after moisture ingress. The amorphous binder may form a more continuous film with fewer water pathways, while crystalline nanodomains can tune barrier and mechanical behavior.

Application value and limits：Phosphate functionalization can support interfacial anchoring and barrier performance and may reduce inhibitor dependence, but morphology, film integrity and the complete coating formulation remain decisive. The results support inhibitor-free direct-to-metal coatings, but performance still depends on glass transition, coalescents, pigments, fillers and cure. Different metals, film thicknesses, outdoor exposure and low-temperature film formation must be validated rather than relying on salt spray alone.