Composition containing first treatment agent for synthetic fiber, method for preparing diluted solution of treatment agent for synthetic fiber, method for treating synthetic fiber, method for producing synthetic fiber, and method for producing short fiber

A first agent-containing composition for treating synthetic fiber that is combined in use with a second agent for treating synthetic fiber containing a nonionic surfactant (C) described below and is separate from the second agent for treating synthetic fiber when not in use, the first agent-containing composition for treating synthetic fiber comprising a first agent for treating synthetic fiber described below and a solvent (S) described below, wherein

• assuming that the sum of the amounts of the first agent for treating synthetic fiber and the solvent (S) contained in the first agent-containing composition for treating synthetic fiber is 100 parts by mass, the amount of the first agent for treating synthetic fiber contained in the first agent-containing composition for treating synthetic fiber is not less than 35 parts by mass and not more than 55 parts by mass,
• the first agent for treating synthetic fiber contains an organic phosphoric acid ester salt (A) described below, an organic phosphoric acid ester salt (B) described below, and optionally a nonionic surfactant (C) described below,
• the first agent for treating synthetic fiber has a mass ratio of the sum of the contents of the organic phosphoric acid ester salt (A) and the organic phosphoric acid ester salt (B) and the content of the nonionic surfactant (C) such that (organic phosphoric acid ester salt (A)+organic phosphoric acid ester salt (B))/nonionic surfactant (C) is 95/5 to 100/0,
• assuming that the sum of the amounts of the organic phosphoric acid ester salt (A) and the organic phosphoric acid ester salt (B) contained in the first agent for treating synthetic fiber is 100 parts by mass, the amount of the organic phosphoric acid ester salt (A) contained in the first agent for treating synthetic fiber is not less than 80 parts by mass but less than 100 parts by mass and the amount of the organic phosphoric acid ester salt (B) contained in the first agent for treating synthetic fiber is more than 0 parts by mass but not more than 20 parts by mass,
• the first agent for treating synthetic fiber has an acid value of more than 0 mg KOH/g but not more than 18 mg KOH/g, as measured by a potentiometric titration method,
• the solvent (S) has a boiling point at atmospheric pressure of not more than 105° C.,
• the organic phosphoric acid ester salt (A) is an alkali metal salt of an organic phosphoric acid ester having a hydrocarbon group with not less than 16 and not more than 18 carbon atoms,
• the organic phosphoric acid ester salt (B) is an alkali metal salt of an organic phosphoric acid ester having a hydrocarbon group with not less than 4 and not more than 8 carbon atoms, and
• the nonionic surfactant (C) has a (poly) oxyalkylene structure in the molecule.