Abstract:
The mechanism of urethane formation using organotin carboxylate and titanium acetylacetonate chelate catalysts is investigated using computational and experimental methods. In this work, computational chemistry and statistical experimental design were selected as the computational modelling methods. Chemical reaction monitoring, Fourier Transform Infar Red (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy analysis were used along with model compound studies for verification of predictions from computational modelling and vice versa. Results show that for organotin carboxylate catalysis of urethane reactions, the mechanism for aliphatic isocyanates is different to that for aromatic isocyanates. For aliphatic isocyanates, the carboxylate ligand contributes to the reaction rate in non-polar media; however this is not found in aromatic systems. The addition of carboxylic acid at low concentrations also increases the reaction rate for aliphatic systems. The concentration of the catalyst also affects the catalysis rate of urethane formation for aliphatic isocyanates. This is as a result of some of the free carboxylic acid formed in the catalytic cycle taking part as a co-catalyst in another catalytic cycle. However, the free carboxylic acid concentration is not sufficient for this function at lower catalyst concentration. There is also evidence that catalysis for aliphatic isocyanate increases from non-polar to polar media due to a change in the catalysis mechanism for urethane formation. For aromatic systems, the organotin alkoxide that forms as an intermediate takes part in the catalysis of urethane formation. Therefore for aromatic systems in polar and non-polar media the same reaction mechanism takes place. Information through modelling suggests that the addition of stoichiometric excess of carboxylic acid to organotin catalyst content during and after polymerization improves the storage stability of aliphatic pre-polymers when synthesised using organotin carboxylate catalysts. Experimental and computational results prove that organotin alkoxides lead to poor stability of both aromatic and aliphatic urethane pre-polymers due to polymerization of the isocyanate. The information gained through this study was implemented in a coating system for the marine sector and marketed globally under a world leading coating brand. Titanium chelates as catalyst show better control over the urethane reaction in the presence of excess chelating agent compared to the organotin carboxylate catalysts in aliphatic 2k (two component) polyurethane coating systems. The titanium chelate catalyst has the advantage in high solid coating systems due to its high catalytic activity after evaporation of the excess chelating agent after application of the coating. The retarding effects due to excess chelating agent increases the pot life of the coating mix and reduces the tack free time significantly after application due to evaporation of the excess chelating agent. Computational and experimental results show that catalysis takes place due to the alcoholysis of the coordinately saturated titanium complex to give a five-coordinated intermediate. The nitrogen atom on the isocyanate molecule then coordinates with the titanium centre to undergo an insertion reaction to form the titanium carbamate. This reaction is stabilised by alcohol molecules. The titanium carbamate is further alcoholised to form the starting active titanium complex and the urethane. Catalysis also shows that the reaction rate depends on the type of isocyanate and alcohol in the system. Since the titanium chelate catalyst leads to the formation of nano TiO2 and other isocyanate adducts of titanium, the final coating is sensitive to UV radiation. The addition of UV absorber and HALS (Hindered Amine Light Stabilizer) overcame the sensitivity of the titanium chelate catalyzed coating when exposed to UV radiation and performed better than the control catalysed with the organotin carboxylate catalyst. The titanium chelate catalyst system was commercialised under a further world leading coating brand as an additive to promote the cure for a 2k high solid urethane system at low temperatures. Modelling methods provided insights into existing catalysts and hence the knowledge to develop catalysts that have specific functions in urethane reactions.