Isocyanate catalysts

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

Figure 1. Effect of relative permitivity D of the solvent system on cyclotrimeriza-tion of isocyanates Catalyst cyclic sulfonium zwitterion VI 25°C solvent system, acetonitrile (D = 37.5) and ethyl acetate (D = 6.02).

With the use of suitable catalysts, it is possible to produce a polyurethane product in a single step. This involves the mixing in the correct ratios of the polyol(s), isocyanate, catalyst, and any pigment and plasticizer. 

A comparison of the activity of various tin compoimds and of other isocyanate catalysts is shown in Table XI. The wide variation in the activity... 

A-Ethylmorpholine, a relatively weak isocyanate catalyst, was found to be ineffective for these reactions. Other f-amines appear to have little effect. 

Entelis assumes the formation of an activated alcohol-isocyanate binary complex during the catalysis of the methanol-phenyl isocyanate reaction by dibutyltin dilaurate (DBTDL) (3, 5) Activated alcohol-isocyanate-catalyst ternary complexes have also been proposed by others. However, significant differences can be noted in the structures of either the postulated one (2, 4, 6, 7) or two (8) coordination positions of the isocyanate to the metal. To explain the synergistic effects observed when tertiary amine and organometallic compounds are combined, several authors suggest the formation of an activated quaternary complex I, II or III (2, 6, 9, 10, 11, 27). 

The bulk of the flexible polymethane foam is now being manufactured by the one-shot process using polyether-type polyols because they generally produce foams of better cushioning characteristics. The main components of a one-shot formulation are polyol, isocyanate, catalyst, surfactant, and blowing agent. 

Reegen SL, Frisch KC (1970) Isocyanate-catalyst and hydroxyl-catalyst complex formation. Journal of Applied Polymer Science Part A Polymer Chemistry 8 2883-2891. 

The chemical components (Table II) in integral skin, rigid urethane foams are similar to those used in conventional, high density, rigid urethane wood foams, and include polymeric isocyanates catalysts blowing agents surfactants and polyols. 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

The most important types of two-component adhesives are moisture-sensitive polyurethane, comprising polyol resin and isocyanate catalyst, and heat-sensitive epoxy, comprising bisphenol A or F resins and polyamine catalyst. Both single- and two-component adhesive systems are available in the market. Single-component reactive adhesives, which include cyanoacrylate and silicone, are also widely known, with the latter often being used as a sealant. 

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