The research report on catalysts for polyurethane foaming mentions that the reaction of isocyanates with polyols is called gelation and eventually produces carbamates. The reaction of isocyanate with water, known as the foaming reaction, ultimately produces dolphins. When the carbamate is reacted further with the isocyanate in the evening, cross-linking may occur, and the methyl formate and shrinking eyes may be generated separately. Isocyanates can self-condense in several different ways to become trimers, dimers, and carbodiimides. The choice of catalyst affects the reactivity of the entire foaming system and the selectivity of some of the individual reactions described above. The reactivity of the foaming system is expressed by the activation time of the system, the curing process, the demoulding or the curing time. The change in the selectivity of the reaction as a function of the choice of catalyst influences the balance of the reactions that occur, the type and sequence of polymer chains formed, and the fluidity of the foaming system, thereby affecting the final foam processing and physical properties.
The most commonly used catalysts for polyurethane foaming are tertiary amines, quaternary amines, amine salts, and metal nucleic acid salts (usually SnII, SnIV, or K+). Tertiary amines are used to promote gelation. Foaming and cross-linking reactions. Limb salts and thermosensitive amines, such as diazobicyclo undecane, are used to provide retardation. Metal salt strongly affects the gelling reaction. The stannous compound (SnII) has a low cost but is easily hydrolyzed and unstable. Its typical use is for applications where it is possible to meter separate streams, such as soft blocks. Tin compounds (SnIV) are not easily hydrolyzed and can be incorporated into systems such as soft molding and rigid foaming. For example, a specific class of compounds such as quaternary amines, potassium nucleic acids, tris(dimethylaminomethyl)phenol, and 2,4,6-tris[3-(dimethylamino)-n-propyl]hexahydrotriethylammonium etc. Trimerization is highly selective.