rent, R1, R2 and R3 are a lower alkyl group or hydrogen, and R1 and R2 are the same or different; to an about 60 to about 90 weight percent sulfuric acid or chlorosulfuric acid aqueous solution at about 0??? to about 90??? C. for a period of about 2 seconds to about 10 minutes; and (b) neutralizing the fiber obtained from step (a) to a metal hydroxide or quaternary ammonium ion solution. 11.