Porosity - What is porosity?
A measure of the void (empty) spaces within a material (such as paper). When fibres are bonded together to produce paper, they create tiny passages that may penetrate completely through the sheet or only extend part of the way down. Porosity can be controlled to an extent by binding the fibres together more tightly during manufacturing. Porosity of paper is measured either by the time it takes for a particular quantity of air to pass through a sample (using a Gurley densometre) or how quickly air passes through a sample (using a Sheffield porosimetre).
Porosity can determine how much adhesive or ink will soak into a sheet of paper. A highly porous paper will have good adhesive or ink receptivity, resulting in a high quality adhesive bond or print respectively. Porous papers are ideal for high speed printing processes, as when printing newspapers, where the ink needs to be absorbed rapidly to reduce the risk of smudging. However, it can also result in strike-through. Low porosity papers can lead to print smudging, poor adhesion, and usually have greater problems with dimensional stability due to issues with moisture content.