Chemical Principles Of Polyurethane Catalysts

In fact, polyurethane catalysts are polymers formed from various types of isocyanates and compounds containing active hydrogen. Therefore, during the preparation and curing process of polyurethane adhesives, reactions occur between isocyanates and active hydrogen compounds. Therefore, polyurethane chemistry is based on the reaction of isocyanates.
1) The solubility and permeability of isocyanates can be dissolved in many organic solvents, and the molecular volume of isocyanates is small, making it easy to diffuse and penetrate into the adhesive, thereby improving adhesion
2) Isocyanate chemical reaction
1. The reaction activity between isocyanates and amides is very low. Only at a certain reaction rate at 1000C can acyl urea be generated.
2. Isocyanates can undergo cyclization reactions in the presence of calcium acetate, sodium acetate, sodium formate, triethylamine, and some metal compounds, generating stable isocyanurate trimers. The reaction is irreversible and still exhibits good stability at 150~2000C. Isocyanate trimerization can introduce branched chain and ring structures, improving the heat resistance and chemical medium resistance of polyurethane adhesives.
3. The reaction activity between isocyanate and carbamate is lower than that of urea. Only at high temperatures (120-1400C) or under the action of selective catalysts, can isocyanates and carbamates have sufficient reaction speed to polymerize and generate carbamates.
4. Isocyanate dimerization reaction Aromatic isocyanates react with each other to polymerize dimers; The dimer reaction is a reversible reaction that can depolymerize into isocyanates at high temperatures, and can be used to prepare room temperature stable, high-temperature cured polyurethane adhesives. At room temperature, without a catalyst, it is difficult for MDI and TDI to form dimers. Trialkyl phosphines and tertiary amines (such as pyridine) can be used to catalyze dimerization.
5. Reaction between isocyanate and urea. The biurea polyurethane adhesive prepared by the reaction of isocyanate and substituted urea can be branched or crosslinked at higher temperatures (>1000C), which can improve the bonding strength.
6. Reaction of isocyanate with phenol. The reaction between isocyanates and phenol is slower than the reaction between isocyanates and hydroxyl groups, even at 50-700 ℃. However, tertiary amine or aluminum chloride can catalyze the reaction rate. In the presence of a polyurethane catalyst, this reaction is reversible at high temperatures and can be used to prepare blocked isocyanate adhesives