Inhibition of Aluminium Pigments for Waterborne Coatings
Doctoral thesis, 2009
Micrometer-sized aluminium flakes for use as pigments in waterborne coatings need protection, since the reaction between aluminium and water causes formation of aluminium hydroxide and hydrogen gas. The aluminium hydroxide destroys the metallic appearance of the pigment flakes and the evolution of hydrogen gas leads to a dangerous pressure build-up in the paint containers. The protection of the aluminium pigment particles is referred to as inhibition and three different inhibition techniques have been developed and evaluated in this thesis.
In the first method, aluminium pigment flakes were dispersed in aqueous surfactant solutions and studied over time. Phosphate esters like sodium dodecyl phosphate (SDP) were shown to be efficient inhibiting agents for aluminium pigments, and storage stabilities of several weeks were obtained. Aluminium pigments treated with the structurally similar sodium dodecyl sulphate (SDS), as well as pigments treated with non-ionic surfactants reacted within one day. The difference in inhibition capacity between the two anionic surfactants was explained by a greater amount of adsorbed SDP compared to SDS at a model surface of aluminium oxide, but also by greater adsorption strength. This was seen by quartz crystal microbalance with dissipation monitoring and atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) and reflectometry analyses, respectively.
In the second method, the inhibition obtained with phosphate ester surfactants was further refined by formation of a polymer layer inside the adsorbed surfactant layer, a technique called admicellar polymerisation. Poly (methyl methacrylate) (PMMA) was evaluated as polymer for this application, and it was shown using thermogravimetric analysis and XPS that a substantial amount of polymer was formed around the aluminium pigment. Furthermore, the PMMA-encapsulated aluminium pigment flakes were stable for several weeks in aqueous solution, even when SDS was used as surfactant.
The third inhibition technique studied protected the aluminium pigment flakes by means of an inorganic silica layer, which was deposited on the aluminium pigment surface by condensation of hydrolysed tetraethylorthosilicate. The silica-encapsulated pigment flakes could be stored in water for several weeks. Hence, all three inhibition techniques have been proven useful for protection of aluminium pigments in waterborne coatings.