Porosity
Also: porosity, air permeability, Gurley, Bendtsen porosity
How easily air passes through a paper, measured as time for a fixed air volume (Gurley) or as flow rate at fixed pressure (Bendtsen).
Porosity tells you how open the paper's pore structure is. The Gurley method times how long 100 ml of air takes to pass through a 6.4 cm² sample at fixed pressure: high times mean low porosity (denser paper). The Bendtsen method measures air flow rate at fixed pressure and gives the inverse reading.
Porosity matters in printing (ink penetration), packaging (barrier performance), filter paper (filtration efficiency), and specialty grades like cigarette paper (controlled porosity is essential for smoking behavior).
Related
- Smoothness. The surface regularity of a paper, measured by air leak under a pressurized head (Parker Print Surf, Bendtsen).