Process for preparing aramid copolymer yarn having low residual sulfur

1. A process for producing a yarn comprising the steps of:

(a) producing a plurality of dope filaments by spinning a polymer solution through a multi-hole spinneret, the polymer solution being formed from sulfuric acid and a polymer in the form of a neutralized polymer crumb, said polymer comprising residues of 5(6)-amino-2-(p-aminophenyl)benzimidazole, terephthaloyl dichloride, and para-phenylenediamine;

(b) coagulating the plurality of dope filaments into an as-spun yarn;

(c) contacting the yarn with an aqueous base having a pKa less than or equal to 11;

(d) rinsing the yarn; and

(e) heating the yarn to a temperature of at least 350° C.

2. The process of claim 1 , wherein in step c) the yarn is contacted with aqueous base comprising ammonium hydroxide, sodium bicarbonate, sodium carbonate, or mixtures thereof.

3. The process of claim 1 , wherein said polymer comprises copolymer derived from the copolymerization of para-phenylenediamine, 5(6)-amino-2-(p-aminophenyl)benzimidazole; and terephthaloyl dichloride wherein the molar ratio of 5(6)-amino-2-(p-aminophenyl)benzimidazole to aromatic diamine is 30/70 to 85/15.

4. The process of claim 3 wherein the molar ratio of 5(6)-amino-2-(p-aminophenyl)benzimidazole to aromatic diamine is 45/55 to 85/15.

5. The process of claim 1 , wherein in step d) the yarn is rinsed with additional quantities of aqueous base, said base being the same or different than the base used in step c).

6. The process of claim 1 , wherein in step d) the yarn is rinsed with an aqueous solution.

7. The process of claim 1 , wherein in step d) the yarn is rinsed with water.

8. The method of claim 1 , where after step b), the yarn has a sulfur content of 0.01 to 3 percent, based on the weight of the yarn.

9. The method of claim 8 , where after step b), the yarn has a sulfur content of 0.1 to 2.5 percent, based on the weight of the yarn.

10. The method of claim 8 , where after step b), the yarn has a sulfur content of 0.05 to 1.75 percent, based on the weight of the yarn.

11. The method of claim 10 , where after step b), the yarn has a sulfur content of 0.05 to 1.0 percent, based on the weight of the yarn.

12. The method of claim 8 , where after step b), the yarn has a sulfur content of 0.01 to 0.08 percent, based on the weight of the yarn.

13. The method of claim 12 , where after step b), the yarn has a sulfur content of 0.01 to 0.05 percent, based on the weight of the yarn.

14. The process of claim 1 , wherein the yarn, after heating, has a tenacity of 32 cN/dtex (35.6 gpd) or higher.

15. The process of claim 14 , wherein the yarn, after heating, has a tenacity of 34 cN/dtex (37.8 gpd) or higher.

16. The process of claim 15 , wherein the yarn, after heating, has a tenacity of 36 cN/dtex (40 gpd) or higher.