Copolymer fibers and processes for making same

1. A yarn comprising copolymer derived from the copolymerization of para-phenylenediamine, 5(6)-amino-2-(p-aminophenyl)benzimidazole, and terephthaloyl dichloride, wherein the ratio of moles of 5(6)-amino-2-(p-aminophenyl)benzimidazole to the moles of para-phenylenediamine is 30/70 to 85/15;

said yarn having a sulfur content greater than 0.1%; and

the hydrolytic strength retention of the yarn is greater than 60%;

wherein the hydrolytic strength retention is computed by dividing the strength of a skein of yarn treated with saturated steam at 150° C. for 24 hours by the strength of an untreated skein of yarn and multiplying by 100.

2. The yarn of claim 1 , wherein the ratio of moles of 5(6)-amino-2-(p-aminophenyl)benzimidazole to the moles of para-phenylenediamine is 45/55 to 85/15.

3. The yarn of claim 1 , wherein the hydrolytic strength retention of the yarn is greater than 70%.

4. The yarn of claim 1 , wherein the hydrolytic strength retention of the yarn is greater than 80%.

5. The yarn of claim 1 , wherein at least 20% of the imidazole nitrogens are in a free base state.

6. A process for preparing a yarn derived from the copolymerization of para-phenylenediamine, 5(6)-amino-2-(p-aminophenyl)benzimidazole, and terephthaloyl dichloride wherein the ratio of moles of 5(6)-amino-2-(p-aminophenyl)benzimidazole to the moles of para-phenylenediamine is 30/70 to 85/15; said yarn having a sulfur content greater than 0.1%, said process comprising:

a) forming said yarn by spinning and collecting said yarn from an inorganic acid solvent; and

b) washing said yarn with a basic aqueous solution for at least 5 seconds; wherein the hydrolytic strength retention of the yarn is greater than 60%;

wherein the hydrolytic strength retention is computed by dividing the strength of a skein of yarn treated with saturated steam at 150° C. for 24 hours by the strength of an untreated skein of yarn and multiplying by 100.

7. The process of claim 6 , wherein the ratio of moles of 5(6)-amino-2-(p-aminophenyl)benzimidazole to the moles of para-phenylenediamine is 45/55 to 85/15.

8. The process of claim 6 , wherein the hydrolytic strength retention of the yarn is greater than 70%.

9. The process of claim 6 , additionally comprising:

c) heating said yarn to a temperature of at least 380° C.

10. The process of claim 6 , wherein at least 20% of the imidazole rings are in a free base state.

11. A process for making a yarn from filaments comprising a copolymer derived from the copolymerization of para-phenylenediamine, 5(6)-amino-2-(p-aminophenyl)benzimidazole, and terephthaloyl dichloride having a sulfur content greater than 0.1% comprising the steps of:

a) spinning and collecting an acid-laden yarn; and

b) in a separate step, first washing the acid-laden yarn with a basic solution to form a neutralized yarn, followed by heat treating the yarn;

wherein the hydrolytic strength retention of the yarn is greater than 60%;

wherein the hydrolytic strength retention is computed by dividing the strength of a skein of yarn treated with saturated steam at 150° C. for 24 hours by the strength of an untreated skein of yarn and multiplying by 100.

12. The process of claim 11 , wherein the yarn in step b) is washed with the basic aqueous solution and water for a total time duration of greater than 5 seconds.

13. The process of claim 11 , additionally comprising:

c) heating said yarn to a temperature of at least 380° C.

14. The process of claim 11 , wherein the yarn in step b) is first washed with aqueous media prior to washing with a sodium hydroxide solution.

15. The process of claim 11 , wherein the washing(s) and heat treating is a continuous process.