Effect of lanthanum 4-hydroxy cinnamate on the polymerisation, condensation and thermal stability of hybrid sol–gel formulations

Hybrid organic–inorganic sol–gel based coatings are of increasing interest nowadays as conversion coating replacements due to their promising performance. They provide good barrier properties; however, they lack active corrosion protection. Lanthanum 4-hydroxy cinnamate [La(4-OHCin)3] was incorporated into a known sol–gel formulation with the aim of improving the overall corrosion resistance of the derived coating. Since the final properties of these sol–gel coatings are dependent on the hydrolysis and condensation reactions of the alkoxide precursors, as well as the polymerisation processes that occur in the organic component, the effect of adding an inhibiting agent, such as La(4-OHCin)3, on the sol–gel chemistry and the polymerisation process needs to be investigated. This study focuses on the role of La(4-OHCin)3 on the sol–gel formation when it is incorporated into two different sol–gel formulations (Si sol and Si–Ti sol). Its influence on hydrolysis and condensation reactions has been studied by 29Si NMR and the opening of the epoxide ring from one of the precursors has been monitored by 1H NMR and Fourier-transformed infrared. Differential scanning calorimetry and Thermogravimetric analysis have also been performed to investigate the effect of this compound on the thermal properties of the final materials synthesised. It has been observed that the addition of La(4-OHCin)3 plays an important catalytic role both in the organic and inorganic polymerisation of the different metal alkoxide precursors. Furthermore, this effect is enhanced when only silicon alkoxides are present in the formulation, as the titanium precursor also plays a role on this catalysis. [Figure not available: see fulltext.].