Characterizing Porous Absorption in Recycled Packaging Paper Grades
Characterizing porous absorption in recycled paper requires measuring dynamic capillary wicking and pore size distribution to control ink set times and glue bonds.
Ink Strike-through
Penetration Defect
Print vehicle migration defines the unwanted capillary movement of liquid ink components through the z-directional thickness of a paper sheet, becoming visible on the unprinted reverse side. Ink strike-through occurs when low-viscosity oils, dyes, or fine pigment fractions travel through the interconnected porous network of the substrate to degrade reverse-side optical quality. The condition reduces the contrast and readability of duplex printing in publication, packaging, and commercial converting applications.
Evaluation involves measuring the visual density or lightness drop on the unprinted back side of a solid print patch relative to unprinted base paper. The definition excludes print show-through, which results from base sheet translucency rather than physical fluid movement through the sheet.
Porous Transport
Capillary pressure inside the cellulosic fiber web drives the liquid phase forward according to the Lucas-Washburn relationship of fluid penetration into porous media. Uncoated papers containing high volumes of mechanical pulp or low internal sizing possess numerous open pores that pull mineral and vegetable oils rapidly away from the surface print film. Pigment particles normally filter out within the upper fiber layers, but soluble dye colorants and sub-micron carbon black fractions follow the vehicle path entirely through the sheet core.
Excessive printing nip impression pressure accelerates this penetration by mechanically forcing the liquid ink into sheet voids during initial contact. Low substrate basis weight and deficient filler distribution worsen strike-through by shortening the physical path length required for fluids to reach the reverse surface. High-speed web offset presses running cold-set newsprint or lightweight directory papers experience elevated strike-through when ink viscosity drops due to press friction.
Substrate Mitigation
Internal sizing agents and barrier surface starches restrict fluid vehicle penetration by elevating the contact angle between paper fibers and ink solvents. Selecting pigments with structured particle sizes prevents colorant migration through open fiber networks without sacrificing ink transfer efficiency.