Pressboard and process for its preparation

Claim 2

High density pressboard comprised of 20-95% by weight aromatic polyamide fibrids and 80-5% by weight high temperature resistant floc, said pressboard having a calculated void volume of 13 to 28% by volume of the pressboard, a thickness of 0.5 to 50 mm, a mercury intrusion volume at low surface/volume, V ml , of less than 0.20 cm 3 /g; a mercury intrusion volume at high surface/volume, V mh , of 0.08 to 0.28 cm 3 /g, an oil absorption by volume in cm 3 /g, V o , of 0.09 to 0.28 and by weight of 8-24 wt.%; and a total available absorption volume in cm 3 /g, V a , equal to the largest of the values for V ml , V mh , and V o ; the ratio of V a to V ml being at least 1.1; said pressboard having a compression set of greater than 0.12 mm but less than 0.35 mm.

Claim 3

Pressboard of claim 1 wherein the high temperature resistant floc is an aromatic polyamide floc and the pressboard has a density of 1.0 to 1.20 g/cm 3 .

Claim 4

Pressboard of claim 2 wherein at least a portion of the floc consists of poly(p-phenylene terephthalamide).

Claim 5

Pressboard of claim 1 wherein at least a portion of the floc is glass fiber floc.

Claim 6

Pressboard of claim 2 wherein the aromatic polyamide fibrids and floc consist essentially of poly(m-phenylene isophthalamide).

Claim 7

Pressboard of claim 5 wherein the pressboard is comprised of 50-70% by weight fibrids and 50-30% by weight floc.

Claim 8

Pressboard of claim 6 wherein the density is 1.02 to 1.17 g/cm 3 .

Claim 9

Pressboard of claim 7 wherein the density is 1.10 to 1.15 g/cm 3 .

Claim 10

Pressboard of claim 6 wherein the compression set is greater than 0.20 mm but less than 0.30 mm.

Claim 11

Process for preparing the high density pressboard of any one of claims 1-9 whereby an aqueous slurry having 0.1 to 2% by weight total solids comprised of 20-95% by weight fibrids of an aromatic polyamide and 80-5% by weight of high temperature resistant floc having a length of 2 to 12 mm, said aromatic polyamide fibrids and said high temperature resistant floc having a melting point higher than 320° C., the slurry is formed into a waterleaf having a water content of 50-95% by weight of the waterleaf; the waterleaf is combined into multiple layers to form a wet lap; the wet lap is pressed at 100° to 200° C. under a pressure of 10 to 60 kg/cm 2 to form a low density pressboard having a calculated void volume of 30 to 60% by volume of the pressboard, the low density pressboard is dried, ultimately at 270° to 320° C. until substantially no further moisture is evolved and then pressed at 8 to 350 kg/cm 2 at 270° to 320° C.

Claim 12

The process of claim 10 wherein the high temperature resistant floc is comprised of an aromatic polyamide.

Claim 13

The process of claim 11 wherein the pressboard is comprised of 50-70% by weight of poly(m-phenylene isophthalamide) fibrids and 30-50% by weight of poly(m-phenylene isophthalamide) floc.

Claim 14

Process of claim 12 wherein the low density pressboard is dried, ultimately at 275°-300° C., and pressed at 275°-285° C. and 15 to 70 kg/cm 2 .

Claim 15

The process of claim 13 wherein the final pressboard is cooled under restraint.