Method for removing sulfur from fiber using an aqueous acid
The present invention concerns methods for removing sulfur from a fiber made from a polymer comprising imidazole groups, said method comprising: contacting never-dried sulfate anion-containing polymeric-fiber with an aqueous acid having a pKa of less than 5 to displace at least a portion of the sulfate anions; and b) rinsing the fiber to remove the displaced sulfate ions.
1. A method for removing sulfur from a fiber made from a polymer comprising imidazole groups, said method comprising:
a) contacting never-dried sulfate anion-containing polymeric-fiber with an aqueous acid wash comprising an acid having a pKa of less than 5to displace at least a portion of the sulfate anions; and
b) rinsing the fiber to remove displaced sulfate anions.
2. The method of claim 1 , wherein said polymer comprises derivatives of 5(6)-amino-2-(p-aminophenyl)benzimidazole, aromatic diamine, and aromatic diacid-chloride.
3. The method of claim 2 , wherein said aromatic diacid-chloride is terephthaloyl dichloride.
4. The method of claim 2 , wherein said aromatic diamine is para-phenylenediamine.
5. The method of claim 2 , wherein the molar ratio of 5(6)-amino-2-(p-aminophenyl)benzimidazole to aromatic diamine is in the range of from 30/70to 85/15.
6. The method of claim 5 , wherein the molar ratio of 5(6)-amino-2-(p-aminophenyl)benzimidazole to aromatic diamine is in the range of from 45/55to 85/15.
7. The method of claim 1 , wherein said aqueous acid wash comprises an acid having a pKa of less than 3.5.
8. The method of claim 7 , wherein said aqueous acid wash comprises an acid having a pKa of less than 2.2.
9. The method of claim 1 , wherein said aqueous acid wash comprising an acid having a pKa of less than 5 comprises nitric acid, nitrous acid, acetic acid, halogenated acetic acids, propanoic acid, formic acid, phosphoric acid, citric acid, ascorbic acid, oxalic acid, lactic acid, carbonic acid, butyric acid, hydrocyanic acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, and hydroiodic acid, or mixtures of two or more of these.
10. The method of claim 9 , wherein said aqueous acid wash comprising an acid having a pKa of less than 5comprises nitric acid, hydrochloric acid, or phosphoric acid.
11. The method of claim 1 , wherein in step b), at least a portion of residual aqueous acid is removed.
12. The method of claim 1 , wherein after step b), the fiber has less than 3.0 weight percent sulfur based on the weight of the fiber.
13. The method of claim 12 , wherein after step b), the fiber has less than 2.5 weight percent sulfur based on the weight of the fiber.
14. The method of claim 13 , wherein after step b), the fiber has less than 1.0 weight percent sulfur based on the weight of the fiber.