Polyurethane catalysts are actually addition polymers of various types of isocyanates and active hydrogen-containing compounds. Therefore, during the preparation and curing process of polyurethane adhesives, the reaction between isocyanate and activated hydrogen compounds must occur, so the basic principle of polyurethane chemistry is the reaction of isocyanate.
1) Dissolution and permeability of isocyanate Isocyanate can be dissolved in many organic solvents, and the isocyanate molecule is small in size and can easily diffuse into the adherend, thereby improving the adhesion
2) Chemical reaction of isocyanate
1. Reaction of isocyanate and amide The reactivity of isocyanate and amide is very low. It only has a certain reaction speed at 1000C and generates acylurea.
2. Trimerization reaction of isocyanate: Isocyanate can undergo cyclization reaction in the presence of catalysts such as calcium acetate, sodium acetate, sodium formate, triethylamine and certain metal compounds to form a stable trimer – Isocyanurate. The reaction is irreversible and still has good stability at 150~2000C. The trimerization reaction of isocyanate can be used to introduce branched and cyclic structures to improve the heat resistance and chemical resistance of polyurethane adhesives.
3. Reaction of isocyanate and urethane The reaction activity of isocyanate and urethane is lower than that of urea. Only at high temperature (120~1400C) or under the action of a selective catalyst, isocyanate and amino Only formate has sufficient reaction speed and generates allophanate through polymerization reaction.
4. Dimerization reaction of isocyanates. Aromatic isocyanates interact with each other to polymerize dimers; the dimer reaction is a reversible reaction and can be depolymerized into the original isocyanate at high temperatures. This reaction can be used to make room temperature Stable, high temperature curing polyurethane adhesive. MDI and TDI are difficult to form dimers at room temperature without a catalyst. Trialkylphosphine and tertiary amines (such as pyridine) can be used to catalyze the dimerization reaction.
5. Reaction of isocyanate and urea. The reaction between isocyanate and substituted urea produces biuret polyurethane adhesive, which can branch or cross-link at higher temperatures (>1000C) and improve bonding strength.
6. Reaction of isocyanate and phenol. The reaction of isocyanate with phenol is slower than the reaction with hydroxyl group, and the reaction rate is very slow even at 50~700C. However, tertiary amines or aluminum chloride can be used to catalyze the reaction rate. This reaction has a polyurethane catalyst and is reversible at higher temperatures, and can be used to prepare blocked isocyanate adhesives.
