Quantifying Specular Gloss Drift in Silica Matte Finishes
Quantifying specular gloss drift in silica finishes requires 85-degree grazing geometry to detect micro-roughness variations before binder leveling elevates sheen.
Pore Volume
Void Fraction
The total volume of unoccupied space distributed between solid mineral pigment particles and cellulose fibres within a paper or coating matrix constitutes pore volume. Quantified typically as cubic centimeters per gram of dry substrate, this metric captures the internal capillary capacity that governs the uptake, transport, and retention of fluids across paperboard. Standard characterization methods utilize mercury intrusion porosimetry or nitrogen gas sorption to establish the volume and size distribution of internal voids.
The value reflects the structural packing density achieved during the forming, consolidation, and calendering stages of papermaking. Pore volume governs physical phenomena distinct from sheet density, as two substrates with identical total void space can exhibit vastly divergent pore size distributions and permeability profiles. It excludes closed, inaccessible voids that are impermeable to probing gases and penetrating liquids.
Microstructural Geometry
Pigment morphology and binder concentration dictate the spatial architecture of internal voids in coated cartonboard. In coating formulations, the blending of fine platy kaolin clay with rhombohedral precipitated calcium carbonate creates an irregular packing structure, yielding a higher pore volume than uniform pigment arrays that pack tightly. The addition of synthetic latex or starch binders reduces available void spaces by filling the interstitial channels between pigment minerals.
Heavy supercalendering compresses these void networks, collapsing large macro-pores into tight capillaries and reducing overall volume. In base paper webs, chemical refining splits wood fibres into flexible fibrils that pack tightly, depressing void space, whereas unrefined mechanical pulps maintain rigid structures that generate voluminous void networks. High void fractions accelerate the capillary transport of low-viscosity liquid carriers into the porous network.
Ink Setting Control
Offset lithography and flexographic printing processes rely on controlled pore volume to freeze ink films quickly without bleeding. Deficient void capacity in the surface coating prevents the rapid absorption of ink oils, causing wet ink to set slowly and transfer onto adjacent sheets in the delivery pile, a defect known as set-off. Conversely, an excessively high pore volume draws ink binders deep into the substrate, leaving dry pigment particles unanchored on the surface, which causes chalking and poor scuff resistance.
In barrier coating applications for food packaging, minimizing substrate void space reduces the consumption of expensive polymer emulsions required to achieve a pinhole-free functional film. Porosimetry data enables paper mills to balance ink setting rates against coating strength, ensuring stable press runnability across varied print methods. Substrate buyers verify these void characteristics to guarantee consistent varnish holdout and glue absorption across production runs.