Abstract
For environmental reasons, the paints industry shifts from solvent-borne towards water-borne formulations. This change is challenging the business of aluminum pigments, as the hydrogen released by the reaction of aluminum with water degrades the optical properties, besides being a safety concern. In this work, industrial-grade aluminum pigments are encapsulated, by a well-known method, in a silica matrix by sol-gel process using isopropanol - a more suitable solvent for the industry. The effectiveness of the encapsulation process is proven by advanced physical methods (Scanning Electron Microscopy, Energy Dispersive X-Ray Analysis, Selected Area Electron Diffraction, Fourier Transformed InfraRed Spectroscopy, Thermo-Gravimetric Analysis) and by industry-relevant tests (stability in water, hiding power, flop and granulometry). Moreover, advanced surface-applied physical methods (High Resolution Transmission Electron Microscopy combined with Selected Area Electron Diffraction and Scanning Transmission Electron Microscopy, and FT-IR microscopy) clearly show the homogeneity of the resulting pigments, a quality which is highly desirable for practical applications. The results demonstrate that stability comparable to that of pigments passivized by chromium-based inhibitors is easily achieved, for a variety of operating conditions. However, accomplishing a homogeneous silica layer of the right thickness is the determining factor for good optical properties.
Highlights
Aluminum Pigments (AP) are used in solvent-borne metallic paints due to their optical properties [1], with wide applications in automotive and motorcycle industry, general industrial coatings, printing inks [2, 3], and decorative and plastic industry [4]
Advanced surface-applied physical methods (High Resolution Transmission Electron Microscopy combined with Selected Area Electron Diffraction and Scanning Transmission Electron Microscopy, and FT-IR microscopy) clearly show the homogeneity of the resulting pigments, a quality which is highly desirable for practical applications
The encapsulation and chemical inhibition greatly enhanced the stability of the pigment
Summary
Aluminum Pigments (AP) are used in solvent-borne metallic paints due to their optical properties [1], with wide applications in automotive and motorcycle industry, general industrial coatings, printing inks [2, 3], and decorative and plastic industry [4]. The surface of the Aluminum Pigment particles is covered by a thin layer of oxide that, at pH neutral conditions, protects the pigment against corrosion. A sustainable alternative is encapsulating the aluminum flakes in a protective layer [9]. The encapsulation can be achieved with an inorganic layer (usually SiO2 obtained by sol-gel process). The sol-gel process is an economic and easy way to form inorganic nanostructures on the aluminum flake surface [15,16,17]. Sol-gel films offer hydrophobic properties to the substrate, while being an environmentally-friendly alternative to chromate-based surface treatments [16]. The sol-gel process includes hydrolysis and condensation reactions of different kind of precursors, generating an inorganic network around the aluminum flake [18, 19].
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