Standard Laboratory Rub Testing for Matte Overprint Varnishes
Laboratory rub testing evaluates matte overprint varnish durability by measuring specular gloss shifts and receptor color transfer under standardized shear loads.
Matting Silica
Optical Modification
Amorphous synthetic particulates act as surface topography modifiers when incorporated into aqueous paper coating formulations. Matting silica lowers specular gloss by creating microscopic surface roughness that scatters incident light in diffuse directions. The inorganic additive consists of porous particles with high oil absorption values and controlled pore volumes.
It governs print readability and tactile finish on coated board without altering underlying sheet brightness or shade. Its application stops when coating formulations require high-gloss mirror finishes or when particulate abrasion damages printing plates.
Particle Dispersion
Particle size distribution dictates the efficiency of gloss reduction and the resulting surface feel. Coarser particles above five micrometers produce strong scattering but increase ink demand and blade wear on continuous coating lines. Matting silica must remain evenly suspended within the binder matrix, requiring precise dispersant chemistry to prevent agglomeration during storage.
When agglomeration occurs, high shear during blade coating creates streaks along the web direction and causes uneven ink absorption in print areas. Fine pore structures within the silica particles absorb liquid carriers rapidly, accelerating coating consolidation on porous fiber bases.
Rheological Boundary
High pore volume restricts the maximum solids content of the coating color due to excessive water absorption by the silica network. Matting silica increases coating viscosity at low shear rates, which limits runnability if chemical formulation limits are exceeded.