Method for preparing low-free curing agent with solid content of 75% by synthesis of 65/35 toluene diisocynate (TDI)

The invention relates to a method for preparing a low-free curing agent with the solid content of 75% by synthesis of 65/35 toluene diisocynate (TDI). The method is characterized by comprising the following steps of: (1) reacting the TDI with trimethylolpropane (TMP) in a reaction solvent at 70+/-5 DEG C to obtain a prepolymer; (2) adding 2,4,6-tris(dimethylaminomethyl)phenol, namely a trimerization catalyst, to the prepolymer when the weight percentage of a free NCO group is detected to be 15%-16%, and adding phosphoric acid, namely a trimerization inhibitor, to the prepolymer for stopping reaction when the weight percentage of the free NCO group is reduced to 14%-15%; and (3) after 2, 6-butylated hydroxytoluene, namely an antioxidant, is added, diluting a prepolymer by a general solvent for polyisocyanates to discharge when a prepolymer curing agent is changed into transparent liquid, when the complete reaction is finished within 2.5h-4h. The obtained environmental-friendly coating curing agent with low-free TDI and high solid content has the excellent characteristics of high gloss of a coating film as well as high elasticity and toughness, and greatly reduces toxicity and transportation costs.