Air Permeability
Sheet porosity defines void volume fraction through the ratio of unfilled pore space to total volume within a cellulosic matrix. Papermakers control void volume fraction by adjusting refining intensity and wet pressing loads during sheet formation. High void volume fraction accelerates liquid absorption rates in corrugated medium and printing paper, whereas denser compaction restricts fluid migration.
Pores distribute internal air channels through the paper web. Papermaking refiners modify fibre flexibility to alter pore architecture before pressing. Void volume fraction drops when heavy nip pressures squeeze free water out of the wet web.
Press section loading sets the final porosity baseline before the sheet enters dryer cylinders.
Porosity Metric
Converting plants measure void volume fraction to predict how corrugated paperboard absorbs starch adhesive during flute lamination. High void volume fractions absorb excessive moisture and cause board warping, while low void volume fractions produce weak bond lines. Adhesive penetration depth depends directly on internal void volume fraction networks.
Corrugating speeds require precise adhesive viscosity matching to account for varying sheet porosity. Fluting medium specifications mandate strict porosity ranges for consistent converting performance.
Void Structure
Printability demands balanced void volume fractions to prevent ink strike through on lightweight coated papers. Surface pores accept ink solvents while retaining pigment particles on top of the coating layer. Excess void volume fraction lets liquid vehicle drain deep into the sheet interior, which leaves printed dots starved of binder.
Sheet calenders compress the surface layer to reduce superficial voids without crushing internal bulk. Coating formulations fill remaining surface voids with mineral pigments to create uniform ink receptive planes.
