Low density nonwoven aramid sheets

Claim 2

A low density, nonwoven sheet structure consisting essentially of a commingled mixture of from 30 to 90% by weight of aramid fibrids and complementally from 70 to 10% by weight of short aramid fibers, said sheet comprising at least about 30% by weight fusible aramid, the sheet having uniformly expanded portions enclosed by a pattern of fused, densified regions segmented by spaced interruptions of nonfused regions with the expanded portions being comprised of a chamber formed by two opposed, dense, smooth, skin-like surface strata of said fibrids and fibers which two strata enclose a much less dense interior, the sheet having a sufficiently low porosity to provide a Drip Porosity Test time of at least about 10 seconds.

Claim 3

A sheet structure of claim 1 having a basis weight of less than about 7 oz/yd 2 and an abrasion resistance in the Taber abrasion test of at least 1000 cycles to failure.

Claim 4

A sheet structure of claim 1 in which the densified regions have a surface integrity resulting in substantially no loss of material by visual examination to the naked eye in the Tape Pull test.

Claim 5

A sheet structure of claim 1 in which the fused, densified regions enclose discrete expanded portions and are arranged in a geometric pattern of segmented lines.

Claim 6

A sheet structure of claim 4 wherein discrete expanded portions individually occupy a surface area of the sheet within the range of from about 0.1 to about 25 cm 2 each.

Claim 7

A sheet structure of claim 5 having a surface integrity sufficient to provide a loss of material in the Tape Pull test of less than 4 mg/cm 2 .

Claim 8

A sheet structure of claim 1 in which the fibrids and at least some of the short fibers are comprised of poly(m-phenylene isophthalamide).

Claim 9

A sheet structure of claim 7 in which from about 10 to 30% of the sheet by weight consists of short fibers of poly(p-phenylene terephthalamide).

Claim 10

A sheet structure of claim 7 consisting essentially of fibrids and short fibers of MPD-I and short fibers of PPD-T in a ratio by weight of about 60/20/20 respectively, and the PPD-T fibers have been pulped.

Claim 11

A sheet structure of claim 1 in which the material density in the interior of the expanded portions increases with distance from a less dense center towards the surface strata through an increasingly dense sponge-like cellular region and culminating in the more dense skin-like surface strata.

Claim 12

A sheet structure of claim 1 wherein the expanded portions consist essentially of the dense skin-like surface strata and an open interior substantially free of fibrid/fiber matter in a balloon-like configuration.

Claim 13

A sheet structure of claim 11 in which the surface strata are derived from separate sheets of said fibrids and fibers which sheets are fused together in said densified regions.

Claim 14

A sheet structure of claim 12 having a basis weight of less than about 3 oz/yd 2 .

Claim 15

An improved process for preparing an expanded nonwoven sheet comprised of aramid fibrids and short aramid aramid including the steps of forming a wet-laid sheet of said aramid materials, impressing a pattern of nonexpandable, densified regions into the sheet to enclose expandable portions of the sheet, and dielectrically heating the patterned sheet while wet with water to rapidly vaporize the water to create highly expanded portions in the sheet, said sheet comprising at least about 30% by weight fusible aramid wherein the improvement comprises: drying the wet-laid sheet by passing it over a smooth heated surface under tension to remove substantially all water and provide a dry, smooth-surfaced sheet; preparing a layered sheet structure for expansion, comprised of at least one layer of said dried sheet, having a thickness of at least 4 mils, said layer having a thickness greater than 15 mils when there is only a single layer of said dried sheet, by fusing by ultrasonic means the materials together throughout the thickness of the layered sheet structure to form nonexpandable, densified regions, segmented by spaced interruptions of nonfused regions of the sheet structure, in a pattern which encloses expandable portions of the sheet structure, said densified regions occupying less than 50% of the sheet surface; saturating the patterned sheet structure with water; and uniformly expanding the expandable portions of the patterned sheet structure by dielectrically heating the wet sheet to rapidly vaporize water and expand the interior of the expandable portions substantially without disrupting their surfaces.

Claim 16

An improved process for preparing an expanded nonwoven sheet comprised of aramid fibrids and short aramid aramid including the steps of forming a wet-laid sheet of said aramid materials, impressing a pattern of nonexpandable, densified regions into the sheet to enclose expandable portions of the sheet, and dielectrically heating the patterned sheet while wet with water to rapidly vaporize the water to create highly expanded portions in the sheet, said sheet comprising at least about 30% by weight fusible aramid wherein the improvement comprises: drying the wet-laid sheet by passing it over a smooth heated surface under tension to remove substantially all water and provide a dry, smooth-surfaced sheet; preparing a layered sheet structure for expansion, comprised of at least one layer of said dried sheet, having a thickness of at least 4 mils, said layer having a thickness greater than 15 mils when there is only a single layer of said dried sheet, by fusing by ultrasonic means the materials together throughout the thickness of the layered sheet structure to form nonexpandable, densified regions, segmented by spaced interruptions of nonfused regions of the sheet structure, in a pattern which encloses expandable portions of the sheet structure, said densified regions occupying less than 50% of the sheet surface; saturating the patterned sheet structure with water; and uniformly expanding the expandable portions of the patterned sheet structure by dielectrically heating the wet sheet to rapidly vaporize water and expand the interior of the expandable portions substantially without disrupting their surfaces.

Claim 17

A process of claim 14 wherein the fibrids and at least some of the short fibers are comprised of poly(m-phenylene isophthalamide).

Claim 18

A process of claim 15 wherein expandable portions enclosed by said fused, densified regions are discrete and individually occupy a surface area of from about 0.1 cm 2 to about 25 cm 2 .

Claim 19

A process of claim 16 wherein the fused, densified regions are arranged in a geometric pattern of segmented lines.

Claim 20

A process of claim 14 wherein a single dry sheet is used having a thickness greater than about 20 mils.

Claim 21

A process of claim 14 wherein the layered sheet structure consists of two of said wet-laid, dried sheets with each having a thickness of at least about 4 mils.

Claim 22

A process of claim 14 wherein the sheet structure is stress-flexed before dielectric heating.

Claim 23

A process of claim 14 wherein the sheet structure is stress-flexed while dry.

Claim 24

A process of claim 14 wherein the sheet structure is stress-flexed while wet.