Agents for and methods of processing synthetic fibers

A method of processing synthetic fibers which are subjected to a heat treatment, said method comprising the step of applying an agent to said synthetic fibers at a ratio of 0.1-3 weight % with respect to said synthetic fibers, said agent comprising a polyether compound which has average molecular weight 700-20000 and is obtained by addition polymerization of alkylene oxide with 2-4 carbon atoms to monohydric-hexahydric alcohol, a straight-chain polyether modified polyorganosiloxane shown by Formula (1) given below, and an ionic surfactant, containing said straight-chain polyether modified polyorganosiloxane in an amount of 0.1-12 weight % and said ionic surfactant in an amount of 0.1-12 weight %, said Formula (1) being:

• ##STR5## where R.sup.1 and R.sup.4 are each either alkoxy group with 1-10 carbon atoms or alkenoxy group with 2-10 carbon atoms;
• R.sup.2 and R.sup.3 are each alkylene group with 3 or 4 carbon atoms, R.sup.5 is methyl group, A.sup.1 and A.sup.2 are each alkylene group with 2 or 3 carbon atoms, p and q are each an integer 20-150, m is an integer 5-20, and n is an integer 1-9;
• said ionic surfactant being quaternary onium salt given by:
• ##STR6## where X is N or P, R.sup.6, R.sup.7, R.sup.8 and R.sup.9 are each alkyl group with 1-25 carbon atoms, alkenyl group with 2-25 carbon atoms or hydroxyalkyl group with 1-6 carbon atoms, B.sup.y- is anion group of valence y obtained by removing hydrogen ions from a compound selected from the group consisting of acidic phosphoric acid esters with 1-30 carbon atoms, acidic sulfuric acid esters with 1-30 carbon atoms, acidic sulfonic acid esters with 1-30 carbon atoms and carboxylic acids with 1-30 carbon atoms, and y is an integer 1-3.