How do tack coat and prime coat emulsions differ in formulation and field performance?

Prime coat is intended to penetrate and bind granular bases, so it is typically formulated for lower viscosity, slower break, and better penetration. Tack coat is designed to create an adhesive bond between asphalt layers, so it breaks faster and leaves a thin uniform residue on the surface rather than soaking in. Tack coat performance is judged by interlayer shear strength and bond uniformity, while prime coat is judged by penetration, dust suppression, and base stabilization. Spray rates and residual binder targets differ, and surface moisture sensitivity also differs.