Definition

Porosity is the measure of how easily air can pass through a sheet of paper. The Gurly test measures the time needed to pass a given volume of air through the sample.

Influence

The porosity of linerboard affects the rate at which air vapor, and (to some extent) liquids will pass through or will be absorbed into the substrate. Thus, porosity can have an effect on corrugator operations by affecting the rate at which moisture, in the form of water vapor, can be removed from the liners or how receptive the linerboards are to the adhesive. It can also affect converting operations with respect to ink and the glue absorption rates. In product use, porosity can affect vacuum-type box setup equipment and case sealing operation.

Potential Problems

"Closed" linerboards (high Gurley numbers) create more difficulty in removing excess moisture on corrugator preheaters because the low porosity inhibits the movement of water vapor through the sheet. Low porosity may also inhibit liquid penetration into the surface, leading to retardation of liquid ink penetration into low-porosity liners may lead to machine slowdowns to avoid smearing, and high hold-out of adhesive can increase compression time requirements on the gluing operations for both the converter and the product-user.

Highly porous linerboards (low Gurley numbers) can create (a) corrugator bonding problems through dewatering of the adhesive before gelation, (b) glueability problems through excessive variation from the color expected of a liquid ink or an increase in ink consumption to achieve that color. Very porous linerboards also have contributed to problems on product-user setup equipment employing vacuum cups to lift or square boxes. Excessive airflow through the facing inhibits the creation of sufficient suction.

Source
McGrattan, W. "Key characteristics of linerboard, corrugating medium, and roll stock mechanical condition and their influence on the manufacture of corrugated products, part 1." Tappi Journal (1990): 99-108.

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