Liquid Extrusion Porosimetry and the Measurement of Pore Size and Volume in Nonwovens

Nonwoven fabric is the name given to fabric-like materials that are made from long fibers (continuous long) and staple fiber (short) bonded together by mechanical treatment, heat, solvent or other chemical application. This method of production distinguishes non-woven materials from woven and knitted fabrics. Fibres may be oriented in one direction or deposited randomly to produce one or more fibre layers. The resulting products are flexible and porous. Felt is probably the most commonly known nonwoven material but the term also includes plastics like nonwoven polypropylene. A variety of manufacturing processes can be used, including spunlaid, staple nonwoven, flashspun and airlaid paper, to produce nonwovens.

Nonwovens have many applications in a wide variety of industries such as healthcare, biotechnology, paper, filtration as well as household uses. When in use they are subjected to compressive stress and the performance of nonwovens in such applications is largely determined by their pore structure. In order to assess this vital property it is important to characterise pore size, pore volume and pore volume distribution of nonwovens under compressive stress.nonwoven

The development of instruments that can measure such pore structure characteristics became a priority in the field and research developed the use liquid extrusion porosimetry to measure pore volume and pore diameter of a nonwoven under compressive stresses. The method uses a wetting agent, a liquid that can spontaneously flow into the pores of a sample. This is possible because the solid/wetting liquid interfacial free energy is less than the solid/gas interfacial free energy. Filling of the pores of the nonwoven sample with the wetting liquid therefore reduces the free energy of the system.

The reversal of the process is not spontaneous because the removal of the wetting liquid from the pore, by a gas, replaces a solid/liquid interface by a solid/gas interface and thereby increases the free energy. For displacement of the wetting liquid from a pore by a gas, the work done by the gas must be equal to the increase in the interfacial free energy. This can be used as the basis for investigating pore structure in the sample because the volume of liquid flowing out of the sample is the volume of the pores. Differential pressure then can yield pore diameter.

From results obtained using 0 psi, 210 psi, 420 psi and 590 psi compressive stresses on a sample it was found that compressive stress decreases pore volume and pore diameter. The small pores of the nonwoven are almost completely eliminated and the influence of stress of the material is great at the beginning and then decreases rapidly with increasing stress.