Capillary Absorption Testing Standards in Coated Printing Substrates
Substrate capillary absorption speeds govern ink setting rate, dryer energy consumption, and cross-web print defect thresholds across high-speed converting runs.
Offset Ink Vehicle Filtration
Pigment Concentration
Separating the liquid varnish from the pigments of an ink occurs as the ink makes contact with a porous substrate. Known as offset ink vehicle filtration, this process begins when the capillary pressure of the coating draws the low-viscosity oils and resins into the paper surface. This filtration leaves a concentrated, semi-solid layer of pigment and heavy resin on the top of the sheet, which then dries into a hard print film.
The process stops when the solvent has finished migrating or when the coating pore structure is saturated.
Ink Setting
Rapid separation of the solvent speeds up the setting of the ink, allowing the printed sheet to be handled or turned over quickly without smudging. However, if the filtration is too fast, the surface layer becomes depleted of varnish, which weakens the ink film and causes rubbing off. The balance of this separation rate is critical for high-speed print runs.
Coating Porosity
Pore volume and the surface chemistry of the paper coating control the rate of filtration. Coating formulations containing fine pigments like clay or fine calcium carbonate create narrow pores that accelerate the separation process. This allows paper manufacturers to tune the substrate to match the oil-absorption characteristics of commercial inks.