Polyurethane foams feature a variable polymer structure that can meet a wide range of application needs. This structural difference is not only caused by the difference between the isocyanate used as a raw material and the polyol, but also related to the different reactions carried out by these raw materials. These reactions are strongly influenced by the type and amount of catalyst used. The reaction of the isocyanate with the polyol is referred to as a gelation reaction and finally produces a carbamate. 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. Flexible block foam Flexible block polyether polyol based foams are typical tertiary amine-catalyzed reaction products used with organotin catalysts. The colloidal catalyst can be a dilution of a pure compound such as triethylenediamine and bis(dimethylaminoethyl)ether, or a blend of properties optimized. Typical tin catalysts for foaming of soft blocks are pure or diluted stannous octoate. Dilution products (including amines and tin) are designed to solve the problems of raw material handling, metering accuracy, and pumping viscosity limitations. The use of high-quality blends for specialized foaming equipment can improve processing, broaden the range of formulations, and make small differences in the physical properties of the foam.