In-plane and through-plane porosities in nonwovens

Many industrially important nonwovens such as filter materials, felts, paper, battery separators, and hygienic items are produced in the form of sheets. In the past, their porosities have commonly been determined by measuring their characteristics in the direction that points most directly through the sheet. Depending, however, on the process by which the material is manufactured, the porosity measured in the direction piercing the sheet, that is, orthogonal to the plane, may differ from the porosity measured isotropically parallel to the plane of the sheet. We can term these two: Through-plane porosity and In-plane porosity. With certain nonwovens, it appears that there may also be unequal porosities in orthogonal directions within the plane itself. Thus, when seeking to make a proper design of other products using these materials, it can be essential to have the appropriate test data on these different porosities. To accomplish this, test equipment has been devised to measure the Bubble Point in planar nonwovens in both the through-plane and the in-plane directions. The equipment yields reproducible data for both. In a presently used test method for in-plane porosity, the air pressure that is applied is certainly isotropic, however the resulting fluid flow is not. The resulting flow can be greater in some directions than others, depending in part upon the local packing-density of small straight fibers, the loft induced by curled fibers, the primary orientation and length of large fibers, and the uniformity of distribution of all three. The wet-area versus dry-area patterns displayed during in-plane testing provide information on the uniformity of fiber distribution in the manufacturing process.