radical or a napthyl radical and R3 is a phenyl or napthyl radical, ##STR14## where R1, R2, R3 and R4 are hydrogen, an alkyl radical having 1 to 6 carbon atoms or a substituted or unsubstituted phenyl or napthyl radical, and ##STR15## comprising contacting the filaments with a bath containing 0.01 to 0.5 weight percent of an organophosphorous compound having the formula ##STR16## wherein R is a m