Method of making cylindrical fibrous filter structures

A method of manufacturing a cylindrical fibrous structure comprising a fibrous mass of nonwoven, synthetic, polymeric, thermoplastic microfibers having a voids volume in the range of from about 64 to about 93 percent, said microfibers having diameters in the range of from about 1.5 to about 20 micrometers and being substantially free of fiber-to-fiber bonding and secured to each other by mechanical entanglement or intertwining, comprising the steps of:

• (a) extruding synthetic, polymeric, thermoplastic resin from a fiberizing die and attenuating said extruded polymeric, thermoplastic resin to form said synthetic, polymeric, thermoplastic microfibers by the application of one or more gas streams directed toward a rotating, reciprocating mandrel and a rotating forming roll in operative relationship with said mandrel;
• (b) cooling said synthetic, polymeric, thermoplastic microfibers prior to their collection on said mandrel to a temperature below that at which said microfibers bond or fuse to each other, thereby substantially eliminating fiber-to-fiber bonding;
• and
• (c) collecting said cooled microfibers on said mandrel as a nonwoven, synthetic fibrous mass while applying a force to the exterior surface of the collected microfibers by said forming roll;
• wherein at least the major portion of said synthetic, polymeric, thermoplastic microfibers contacts said forming roll prior to collection on said rotating, reciprocating mandrel and the process variables are controlled to form said cylindrical fibrous structure with said fibrous mass having a substantially constant voids volume and varying fiber diameter over at least a substantial portion thereof as measured in the radial direction to achieve a varying pore size over said portion.