Chemical Principle Of Polyurethane Catalyst

In fact, polyurethane catalysts are polymers formed from various types of isocyanates and compounds containing active hydrogen. Therefore, during the preparation and curing of polyurethane adhesives, reactions occur between isocyanate and active hydrogen compounds. Therefore, the chemistry of polyurethane is based on the reaction of isocyanates.
1) Solubility and permeability of isocyanate Isocyanate can be dissolved in many organic solvents, and the molecular volume of isocyanate is small, so it is easy to diffuse and penetrate into the adhesive, thus improving the adhesion
2) Chemical reaction of isocyanate
1. Isocyanate and amide reaction Isocyanate and amide reaction activity is very low. Only at 1000C, with a certain rate of reaction, will acyl urea be formed.
2. In the presence of calcium acetate, sodium acetate, sodium formate, triethylamine and some metal compounds, isocyanate can undergo cyclic reaction to form stable trimers of isocyanuric acid. The reaction is irreversible and stable at 150~2000C. Isocyanate tripolymerization can introduce branch chain and ring structure to improve the heat resistance and chemical medium resistance of polyurethane adhesive.
3. Reaction of isocyanates with carbamate The reaction activity of isocyanates with carbamate is lower than that of urea. Only at high temperature (120~1400C) or under the action of selective catalyst, isocyanate and carbamate have sufficient reaction speed, can be polymerized into carbamate.
4. Isocyanate dimerization aromatic isocyanate reaction polymerization dimer; The dimer reaction is reversible and can depolymerize into protoisocyanate at high temperature. It can be used to prepare polyurethane adhesive which is stable at room temperature and curable at high temperature. At room temperature, it is difficult for MDI and TDI to form dimers without a catalyst. Trialkyl phosphine and tertiary amines such as pyridine can be used to catalyze dimerization.
5. Reaction of isocyanate with urea. The biurea polyurethane adhesive prepared by reaction of isocyanate with substituted urea can be branched or crosslinked at higher temperature (>1000C), which can improve the bond strength.
6. Reaction of isocyanate with phenol. The reaction between isocyanate and phenol is slower than that between isocyanate and hydroxyl group, even at 50~700℃. However, tertiary amine or aluminum chloride can catalyze the reaction rate. In the presence of polyurethane catalyst, the reaction is reversible at high temperature and can be used to prepare closed isocyanate adhesives