| abstract |
An improved copolyester fiber capable of being dyed with cationic dyes and having an excellent brightness, heat resistance and a satisfactory mechanical strength comprises a copolyester resin comprising a polymerization product of at least one aromatic dicarboxylic acid compound, at least one alkylene diol compound and at least one quaternary phosphonium organic sulfonate of the formula (I): ##STR1## wherein A=an aromatic or aliphatic radical, X 1 =H or an ester-forming organic radical, X 2 =an ester-forming organic radical, R 1 , R 2 , R 3 and R 4 =an alkyl or aryl radical, n=1 or more, and having an [η] f of 0.5 or more, the copolyester fiber having a silk factor of 25 or more and exhibiting the number of scission of backbone chains of the copolyester molecules of 140×10 23 or less per 10 6 g of fiber when wet heat-treated in a solution of 0.4 g/l of acetic acid in distilled water at 130° for 60 minutes. |