Abstract
This study presents a simple but effective process route for the production of transparent coatings on glass substrates from inorganic pigment goethite. For this purpose, coating suspensions were prepared by wet milling with a stirred media mill. A water/ethanol mixture was used as the liquid medium to take advantage of the resulting low surface tension for the coating process. In this manner, stable suspensions with particles of down to 50 nm in size were obtained, which already showed a significant increase in transparency. With regard to grinding characteristics, particularly low stress energies proved to be energetically reasonable. The coating step was performed by wet film deposition, achieving coating thicknesses in a range of 0.5–2.5 µm via dip coating. Highly transparent coatings were obtained by applying small particles of 50 nm, which exhibited a significantly lower scattering loss of light (≈3%) in comparison to particles of around 300 nm (70–80%). Additionally, the film color could be adjusted through a variation of the drying temperature due to a conversion of goethite to hematite by dehydration. Since transparency was not affected, this provides an easy-to-implement process adaptation for controlling coating colors.
Highlights
Transparent coatings play a major role in various fields of application, both in research and industry
This study presents a simple but effective process route for the production of transparent coatings on glass substrates from inorganic pigment goethite
The film color could be adjusted through a variation of the drying temperature due to a conversion of goethite to hematite by dehydration
Summary
Transparent coatings play a major role in various fields of application, both in research and industry. If the particle component cannot be directly synthesized in the desired, small particle size range or if only coarse particle structures are available from natural resources, a top-down formulation represents the only method to prepare coating suspensions. Bearing this in mind, grinding in stirred media mills provides a well-established and suitable method to obtain nanoparticulate suspensions [10–13]. The investigation was targeted to be able to vary function-oriented properties (e.g., film color) by rather simple strategies Within this context, the presented studies were conducted on the exemplary inorganic material goethite, which is a widely used yellow pigment. Taking formulation properties (e.g., solid content in the suspensions, particle size) and process parameters (e.g., withdrawal speed, drying temperature) into account, an in-depth evaluation was carried out, focusing on the homogeneity, transparency and color of the coatings
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