Pore Architecture
Structural classification of voids within a coated paperboard surface defines the distribution of capillary paths available for ink and coating absorption. A bimodal pore distribution describes a coating layer or fibre matrix that exhibits two distinct peaks in pore size, typically dividing the structure into narrow micropores and wider macropores.
Absorption Behavior
The absorption rate of fluid depends directly on these different capillary dimensions. During the printing process, the smaller pores exert high capillary pressure that strips the low-viscosity solvent from the ink vehicle, while the larger pores accommodate the pigment particles. Having both pore sizes present improves print gloss and prevents ink rub-off by anchoring the pigment while drying the binder.
High-speed press runs require this dual-action drying to avoid set-off in the delivery pile.
Structural Measurement
Determination of the void profile is performed using mercury intrusion porosimetry or liquid extrusion techniques. A coating with an incorrect distribution of pore sizes will lead to uneven ink absorption, resulting in localized gloss variations or mottle across the print. Standard paperboard specifications often control this distribution to ensure consistent varnish holdout on high-speed folding carton lines.
Coating formulation adjustments allow the mill to fine-tune the ratio of different pore sizes to match specific ink chemistries by blending fine clay and coarse calcium carbonate pigment. This blending creates a tailored capillary network that resists binder migration and optimizes ink holdout during multi-color offset runs.
