Organometallic and Other Metal Compounds

Metal compounds, and especially organotin compounds, are much more efficient catalysts than the amines, especially for the hydroxyl/isocyanate reaction. This allows the polymer-forming polyol/isocyanate reaction to proceed at a sufficient rate to increase viscosity rapidly to a state where gas is effectively trapped, as well as to develop enough gel strength to present any cell structure from collapsing after gas evolution has ceased. 

Derivatives of both two- and four-valent tin have found use as catalysts in urethane foams. In addition to the more commonly used dibutyltin (IV) dilaurate, dibutyltin (IV) diacetate, dialkyltin (IV) oxide or salts of two-valent tin with a variety of carboxylic acids such as stannous octoate, hexoate and naphthenate, there are also available chlorine-containing compounds (dibutyltin dichloride, SnCl2, SnClJ, sulphur-containing compounds (e.g. tributyltin laurylmercaptate), and complexes such as stannous quinolinolate or dibutyltin bis(acetylacetonate). Other catalysts 

Carboxylates of divalent tin are used almost exclusively for the commercial production of flexible foams. Catalysts containing tetravalent tin, e.g. the dialkyltin dicarboxylates, catalyze oxidation at elevated temperatures. Since temperatures exceeding 140° are often reached during the production of polyurethanes, this can lead to severe degradation unless these catalysts are used in conjunction with small amounts of antioxidants such as substituted phenolics. 

Tin compounds do not catalyze hydrolysis of polyurethanes. Many, especially the divalent tin types, are, however, susceptible to hydrolytic attack and are therefore unstable in the presence of water. 

An excellent account of the effect of tin catalysts on the physical properties of cellular urethanes is given by Mack (1964), who found that stannous tin catalysts are extremely sensitive to exposure to air, and to contamination by ferrous iron these result in chemical oxidation of stannous to stannic tin carboxylate, which in itself was found to be a very poor gelation catalyst. 

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