Toluene Diisocyanate and Its Derivatives: Modified TDI and TDI Trimers

The urethane modification of TD mainly involves the reaction of the 4-position NCO of TDI, forming TDI prepolymers containing urethane structures and mixtures of TDI. The application of urethane-modified TDI in flexible foams was initially focused on the two-step method for preparing flexible foams. It can also be used in cold-cured molded foams and integral skin foams: by reacting a portion of polyol with all or part of the TDI prepolymer, the viscosity of the isocyanate component is increased to improve the compatibility of the foam materials, and it can also reduce the volatility of TDI, improving the working environment.

The purpose of modification is to increase the functionality of TDI and enhance the crosslinking of foam plastics while maintaining low viscosity. In the early stages of manufacturing HR foam plastics, modified TDI was often used to improve the elasticity and load-bearing properties of the foam plastics.

Excess TDI reacts with ethylene glycol or polyether diol to first form low molecular weight end-NCO prepolymers, which then react at higher temperatures in the presence of catalysts (such as zinc acetate and acetone) to produce carbamate-modified TDI. Its average functionality is generally between 2.2 and 2.3. Adding a small amount of alkylating agents (such as dimethyl sulfate, methyl benzenesulfonate, and phenyl sulfonate) can reduce side reactions such as the formation of isocyanurates and uretidione. The use of carbamate-modified TDI can increase the hardness of foam plastics, making it particularly suitable for low-density, high-load automotive cushions and cold-cured molded foams.

In the presence of catalysts such as 2,4,6-tris(dimethylaminomethyl)phenol, TDI undergoes trimerization at lower temperatures to form isocyanurate-modified TDI (TDI trimer). When the temperature exceeds 60℃, the catalyst decomposes and loses its activity. A small amount of acid can be added after the reaction to deactivate the catalyst. Its average functionality is generally around 2.3. Isocyanurate trimer-modified TDI not only increases the strength of foam plastics but also improves their flame retardancy.