Porosity Measurement
Determination of the structural density of a sheet involves measuring the resistance to the passage of air through the fibre network. The gurley air permeance value is expressed as the time in seconds required for a specific volume of air to pass through a circular area of paper. It provides a direct measure of the openness of the substrate.
Flow Calculation
Testing is performed by placing the sample in a device where the weight of a cylinder creates a constant pressure. The speed at which the cylinder falls indicates the air flow rate. A high gurley air permeance reading signifies a dense sheet with few voids while a low reading indicates a highly porous material.
Application Limit
Control of this property is vital for papers that must function in vacuum fed printing presses or automatic filling lines. If the paper is too porous, the vacuum grippers will fail to pick up individual sheets and cause a jam in the machine. Conversely, papers used for sterile medical packaging must have a controlled gurley air permeance to allow for gas sterilization while maintaining a bacterial barrier.
Engineers use these values to specify the correct grade for high speed industrial processes where air handling is a primary factor.
