First of all, let's understand the reaction mechanism of isocyanate. The reaction between isocyanate and active hydrogen compound is a stepwise addition polymerization of hydrogen transfer, which is caused by the nucleophilic center of active hydrogen compound attacking the positive carbon ion in the isocyanate group. The reaction is carried out on the more reactive NCO double bond. The hydrogen atoms in the active hydrogen compound are transferred to the N atom of the NCO group, and the remaining groups are combined with the C atom of the carbonyl group to form the carbamate compound.

There are generally nine different types of reactions involved, some specific to specific applications and synthetic reactions, but the most common are the following three.

Secondly, the core of calculation is the reaction of -----NCO and ----OH.

One mole of NCO reacts perfectly with one mole of OH to form one mole of polyurethane.

The reaction process can be divided into two parts, volatilization process and reaction process. First volatilize solvent, resin and curing agent contact probability will be greatly increased; After the cross-linking reaction between NCO and OH in the coating, the molecular weight increases, the viscosity increases, and the TG point increases. The urea produced by the surface and its trace amount of water is rigid and has the function of self-catalysis, and the TG point increases more obviously.

If it is in a reasonable range of 1-1.2, 1.05 is generally selected and can improve the performance and stability of the product. This is because the air contains a small amount of water or the diluent contains water, which will consume part of the NCO, so a little excess curing agent will be used to make up for the part consumed by the water in the air or the diluent. However, if the curing agent is seriously excessive (some people desperately add curing agent for high hardness), the drying speed of the film will be greatly reduced, it is likely to foam, luster decline and many other ills. In the case of severe excess, excess NCO can only react with water in the air, while the reaction between water and curing agent is a "slow reaction". As the solvent in the surface layer evaporates faster than the solvent in the inner layer, the drying speed of the surface will be faster than that of the bottom layer, and the NCO in the bottom and surface layer will react with trace water to produce carbon dioxide gas. Cause the coating foaming tendency to increase (whether it is a large bubble or a small bubble), special anti-corrosion requirements of the occasion, it is easy to cause rust, and the formation of arbazate group in some occasions prone to stress cracking (curved surface) and will reduce the luster of the film (because the arbazate polarity is large, the degree of phase separation is higher, the degree of crystallization will increase, so the luster decline and hardness increase).