UV Speciality Coatings

Gloss is specular reflection, and specular reflection needs a surface smoother than the wavelength of light. A 60° gloss of 90 GU or more requires a coating film that levels completely before it cures, which is why gloss UV coatings are low in viscosity, have long open time relative to the moment of cure, and are applied at a film weight that fills the micro-roughness of the ink and board. Lower film weight, absorbent stock, over-dilution or a hot pressroom that thins the coating all lower gloss for the same reason.

Matt scatters light with a matting agent — precipitated or fumed silica, polymer beads or wax — dispersed in the liquid. As the film shrinks on cure the particles protrude from the surface, creating micro-roughness that diffuses reflection. Three consequences follow: film weight has a ceiling, because a heavier film buries the particles and raises gloss; rub resistance is inherently lower than gloss, because the particles that create the effect are the first thing a rub tester touches; and matting agents settle, so the coating must be shaken before use and kept agitated on long runs.

Drip-off contrast uses the mutual repulsion of a matt primer and a gloss coating: where the primer is printed, the gloss coating de-wets and pulls away, giving high-contrast gloss and matt from a single coating unit.