Sheets and fibrids comprising a mixture of poly(m-phenylene isophthalamide) and copolymer made from 5(6)-amino-2-(p-aminophenyl)benzimidazole

1. A sheet comprising fibers and fibrids, the fibrids bonding individual fibers together to provide wet and/or dry strength to the sheet, wherein the fibers and/or the fibrids are formed from a polymer mixture of at least a first polymer and a second polymer;

the first polymer having a structure derived from the reaction of one or more amine monomers and a plurality of acid monomers, wherein

i) the one or more amine monomers includes at least 60 mole percent 5(6)-amino-2-(p-aminophenyl)benzimidazole, based on the total amount of amine monomers; and

ii) the plurality of acid monomers include those having a structure of

Cl—CO—Ar 1 —CO—Cl and Cl—CO—Ar 2 —CO—Cl

wherein Ar 1 is an aromatic group having para-oriented linkages and Ar 2 is an aromatic group having meta-oriented linkages, and wherein the plurality of acid monomers has at least 50 mole percent of the monomer containing aromatic group Ar 2 ; and

the second polymer having a structure derived from the reaction of metaphenylene diamine and isophthaloyl chloride wherein no more than 10 percent of other diamines are substituted for metaphenylene diamine.

2. The sheet of claim 1 , wherein the one or more amine monomers is 100 mole percent 5(6)-amino-2-(p-aminophenyl) benzimidazole.

3. The sheet of claim 1 , wherein the one or more amine monomers include metaphenylene diamine.

4. The sheet of claim 1 comprising fibrids derived from wholly aromatic synthetic polymer.

5. The sheet of claim 4 wherein the wholly aromatic synthetic polymer is poly (metaphenylene isophthalamide).

6. The sheet of any one of claims 1 to 5 , wherein the mixture of first polymer and second polymer contains 25 to 80 weight percent of the first polymer, based on the total amount of first polymer and second polymer.

7. The sheet of claim 6 , wherein the mixture of first polymer and second polymer contains 25 to 50 weight percent of the first polymer, based on the total amount of first polymer and second polymer.

8. A fibrid having a length and width of 100 to 1000 micrometers and a thickness only on the order of 0.1 to 1 micrometer formed from a polymer mixture of at least a first polymer and a second polymer;

the first polymer having a structure derived from the reaction of one or more amine monomers and a plurality of acid monomers, wherein

i) the one or more amine monomers includes at least 60 mole percent 5(6)-amino-2-(p-aminophenyl)benzimidazole, based on the total amount of amine monomers; and

ii) the plurality of acid monomers include those having a structure of

Cl—CO—Ar 1 —CO—Cl and Cl—CO—Ar 2 —CO—Cl

wherein Ar 1 is an aromatic group having para-oriented linkages and Ar 2 is an aromatic group having meta-oriented linkages, and wherein the plurality of acid monomers has at least 50 mole percent of the monomer containing aromatic group Ar 2 ; and

the second polymer having a structure derived from the reaction of metaphenylene diamine and isophthaloyl chloride wherein no more than 10 percent of other diamines are substituted for metaphenylene diamine.

9. The fibrid of claim 8 , wherein the one or more amine monomers is 100 mole percent 5(6)-amino-2-(p-aminophenyl) benzimidazole.

10. The fibrid of claim 9 , wherein the one or more amine monomers include metaphenylene diamine.

11. The fibrid of any one of claims 8 to 10 , wherein the mixture of first polymer and second polymer contains 25 to 80 weight percent of the first polymer, based on the total amount of first polymer and second polymer.

12. The fibrid of claim 11 , wherein the mixture of first polymer and second polymer contains 25 to 50 weight percent of the first polymer, based on the total amount of first polymer and second polymer.