Types and Activities of Organometallic Salt Catalysts

Various organometallic compounds, including alkyl compounds and carboxylates of metals such as lead, tin, titanium, antimony, mercury, zinc, and bismuth, exhibit catalytic activity in the reaction between isocyanates and hydroxyl groups. Among these, organotin compounds, particularly stannous octoate and dibutyltin dilaurate, are crucial in the production of polyurethane foams. Organotin compounds are generally used for flexible polyurethane foams due to their significantly higher catalytic activity for the gelation reaction compared to the foaming reaction.

Alkali and Alkaline Earth Metal Compounds

Strongly basic compounds of alkali and alkaline earth metals, such as sodium methoxide, potassium isooctanoate, and potassium oleate, also serve as catalysts in polyurethane synthesis. For instance, potassium acetate and potassium oleate are catalysts for polyisocyanurate foam production.

Classification of Organotin Compounds

Organotin compounds are diverse and can be classified based on the number of carbon atoms attached to the tin atom and the heteroatoms linked to it. Examples include dibutyltin dilaurate, stannous octoate, stannous oleate, di-2-ethylhexanoate dibutyltin, tributyltin chloride, and tributyltin trichloride.

Catalytic Activity of Tin Catalysts

The activity of tin catalysts correlates with their molecular structure, following the order:

R₂SnX₂, R₂SnO, R₂SnS > RSnX₃, RSnOOH, R₃SnX > R₄Sn

The activity varies with different functional groups, where R: CH₃ > C₄H₉ > C₆H₅ and X: OH > OC₄H₉, SC₄H₉, OCOCH₃ > Cl > F.

Mechanism of Organotin Catalysis

Organotin compounds act as Lewis acids, interacting with the basic centers of reactants. The vacant electron orbitals of tin atoms coordinate with the electron pairs of hydroxyl oxygen atoms in polyols or isocyanate oxygen atoms through π-bonding. This coordination forms polyol or isocyanate complexes, enhancing the nucleophilicity of hydroxyl oxygen or the electrophilicity of the isocyanate's carbon atom, thus accelerating the reaction. Polyol hydroxyl groups, having a higher electron density than water due to their electron-donating groups, exhibit stronger coordination with tin atoms. Consequently, organotin catalysts are more effective in catalyzing the reaction between isocyanates and polyols than between isocyanates and water.

Practical Implications in Production

The stronger catalytic effect of organotin compounds on the NCO-OH reaction compared to the NCO-H₂O reaction is a crucial feature, offering utility in industrial applications. Since both reactions involve active hydrogen in isocyanates, most catalysts exhibit comparable activities for these reactions. However, industrial processes often demand different reaction rates, necessitating flexibility in control. This adaptability is challenging with tertiary amine catalysts but is feasible with organotin compounds, enabling convenient mixed catalytic systems.

Stability and Degradation in Polyurethane Foams

Unlike amine catalysts, tin catalysts remain in the foam post-formation and undergo chemical changes over time, such as the oxidation of divalent tin to tetravalent tin or hydrolysis. Dibutyltin dilaurate, while effective, impacts the thermal aging properties of polyether-based polyurethane foams. Studies indicate that prolonged exposure of such foams containing dibutyltin dilaurate to air at approximately 140°C results in thermal degradation, reducing foam performance. This degradation does not occur in the absence of air. Rigid polyurethane foams, due to their closed-cell structure, limit air and moisture circulation, making thermal degradation less significant compared to flexible foams. To mitigate these effects, stabilizers like 2,6-di-tert-butyl-p-cresol (antioxidants) are added when using dibutyltin dilaurate in foam systems.

In contrast, stannous octoate (a divalent tin compound) has minimal impact on foam properties. Some studies suggest that the oxidation of divalent tin compounds to tetravalent tin within the foam may provide anti-aging benefits, potentially extending the product's lifespan.