Treatment agent for polyester-based synthetic fibers, composition containing treatment agent for polyester-based synthetic fibers, first treatment agent for polyester-based synthetic fibers, composition containing first treatment agent for polyester-based synthetic fibers, second treatment agent for polyester-based synthetic fibers, composition containing second treatment agent for polyester-based synthetic fibers, diluted solution of treatment agent for polyester-based synthetic fibers, method

A first component of polyester synthetic fiber treatment agent, comprising a (poly)oxyalkylene derivative (A), wherein

• the first component of polyester synthetic fiber treatment agent is used in combination with a second component of polyester synthetic fiber treatment agent or a composition containing second component of polyester synthetic fiber treatment agent,
• the second component of polyester synthetic fiber treatment agent contains an organic phosphoric acid ester compound (C),
• the composition containing second component of polyester synthetic fiber treatment agent contains the second component of polyester synthetic fiber treatment agent, which contains an organic phosphoric acid ester compound (C), and a solvent(S),
• either one or both of the first and second components of polyester synthetic fiber treatment agent contain an organic acid compound (B),
• a 5% by mass water-diluted liquid of a mixture (containing no solvent) of the first component of polyester synthetic fiber treatment agent and the second component of polyester synthetic fiber treatment agent has a pH at 25° C. of 5.5 or more and 8.5 or less,
• the mixture (containing no solvent) of the first agent component of synthetic fiber treatment agent and the second component of polyester synthetic fiber treatment agent contains 5% by mass or more of the (poly)oxyalkylene derivative (A), 1% by mass or more of the organic acid compound (B), and 5% by mass or more of the organic phosphoric acid ester compound (C),
• the organic acid compound (B) is at least one selected from the group consisting of an organic acid, an organic acid salt, and an organic acid anhydride,
• the organic phosphoric acid ester compound (C) is at least one selected from the group consisting of an organic phosphoric acid ester and a salt thereof having a hydrocarbon group with 16 or more and 20 or less carbon atoms in a molecule, and
• the solvent(S) has a boiling point of 105° C. or lower at atmospheric pressure.

A second component of polyester synthetic fiber treatment agent, comprising an organic phosphoric acid ester compound (C), wherein

• the second component of polyester synthetic fiber treatment agent is used in combination with a first component of polyester synthetic fiber treatment agent or a composition containing first component of polyester synthetic fiber treatment agent,
• the first component of polyester synthetic fiber treatment agent contains a (poly)oxyalkylene derivative (A),
• the composition containing first component of polyester synthetic fiber treatment agent contains the first component of polyester synthetic fiber treatment agent, which contains a (poly)oxyalkylene derivative (A), and a solvent(S),
• either one or both of the first and second components of polyester synthetic fiber treatment agent contain an organic acid compound (B),
• a 5% by mass water-diluted liquid of a mixture (containing no solvent) of the first component of polyester synthetic fiber treatment agent and the second component of polyester synthetic fiber treatment agent has a pH at 25° C. of 5.5 or more and 8.5 or less, the mixture (containing no solvent) of the first component of polyester synthetic fiber treatment agent and the second component of polyester synthetic fiber treatment agent contains 5% by mass or more of the (poly)oxyalkylene derivative (A), 1% by mass or more of the organic acid compound (B), and 5% by mass or more of the organic phosphoric acid ester compound (C),
• the organic acid compound (B) is at least one selected from the group consisting of an organic acid, an organic acid salt, and an organic acid anhydride,
• the organic phosphoric acid ester compound (C) is at least one selected from the group consisting of an organic phosphoric acid ester and a salt thereof having a hydrocarbon group with 16 or more and 20 or less carbon atoms in a molecule, and
• the solvent(S) has a boiling point of 105° C. or lower at atmospheric pressure.