Polypropylene (PP) fibers are widely used in filter manufacturing. PP can be formed into fleeces and mats of various fiber diameters that are bonded permanently cross link by heat, being joined by melting, to minimize or eliminate media migration. The melt-spinning method of permanently fixing the fibers to one another, replaced the adhesives and mechanical manipulations of earlier and less effective methods of mat formation.
The melt-blown technique now supersedes even this fabrication of polypropylene fleeces by varying the mean fiber size during the fashioning of the prefilter. This enables the progressive changing of the mat’s pore size while it is being constructed. The result is a highly graded asymmetric composition having a constant packing density. One advantage is that, in effect, a series of prefilters is combined into the making of the single prefilter composite. The asymmetric morphology provides less resistance to flows, requires lower differential pressure in their operations, provides for the accommodation of higher particle loadings, and offers greater particle removal due to the efficiency of thinner fibers.
Fibers of diameters as small as 0.2um in their mean size provide the retention of particles from a fluid stream down to 1 um in size. The graded pore size format is derived by decreasing the fiber diameters rather than by increasing the mat’s density by tighter packing. This enhances the prefilter porosity and provides the additional advantages of lower operational differential pressure, and larger load accommodations.