Patent ID: 8034146

Claim:
A filter element comprising: a pleated composite substrate having at least a gas removal sheet and a nonwoven fabric sheet; the nonwoven fabric sheet having a first thickness; the nonwoven fabric sheet comprising thermoplastic resin fibers, wherein the nonwoven fabric sheet is lapped or laminated on the gas removal sheet; the filter element further comprising at least one shape retaining member for retaining a pleated shape of the composite substrate, wherein the thermoplastic resin fibers contained in said nonwoven fabric sheet have been bound to one another by heat treatment at a first temperature which is equal to or higher than the melting point of the thermoplastic resin fibers, and wherein said bound thermoplastic resin fibers have been heat treated at a second temperature which is lower than the melting point of the thermoplastic resin fibers under compression such that strain is generated and retained in the thermoplastic resin fibers and the state of the bound and compressed thermoplastic resin fibers is stabilized so as to form the first thickness of the nonwoven fabric sheet, and wherein when heated to substantially 60 degrees C., the generated strain is released thus causing the first thickness of the nonwoven fabric sheet to increase to a second thickness; and the second thickness is at least five percent (5%) greater than the first thickness.