VTT Technical Research Centre of Finland has developed new pretreatment and curing methods that can be used with the sol-gel coating process. With the new pretreatment method, the adhesion of sol-gel coatings can be improved significantly. The method is based on the use of modified CO2 dry ice with particles. It is environmentally friendly and does not cause as much waste as traditional cleaning methods. Another new method is connected with curing sol-gel coatings. It makes use of the thermal energy stored in pieces, instead of separate curing process. VTT has submitted a patent application for both methods.
The advantage of pretreatment with modified dry ice is that after treatment, the surface does not become oxidized as easily as when other methods such as chemical pretreatment are used. Another advantage for any substrate is that the treatment does not give rise to the large amounts of waste produced using traditional cleaning wet methods, such as chemical baths and generated wastewater.
In the new coating curing method, the thermal energy stored in the piece to be coated is used in the thermal curing of the sol-gel coating. This method has already been applied successfully to hot-dip galvanized and thermally sprayed products. This was demonstrated several times by applying the coating to hot-dip galvanized trailers, bicycle frames, timber sills, and pieces with thermally sprayed coating.
The project family also developed the mouldability properties and manufacturability of the coatings for use at an industrial scale. The project demonstrated the suitability of the sol-gel coating and the preceding pretreatment phases for a manufacturing process involving the roll-to-roll coating of sheet metal strips.
For the pretreatment of the sol-gel coating in production, it was found that electropolishing (the PET method, electropolishing with electric discharge) can also be used in continuous processes in where environmentally friendly bath are used as electrolytes. The use of carbon dioxide dry ice blowing in the roll-to-roll process was also found to give the coating sufficient adhesion and lend itself to the process in other respects, too. In terms of the environmental friendliness of the sol-gel coatings, all the coatings used in the projects were found to be below the current limit values of the VOC Directive (Volatile Organic Compounds).
With the sol-gel coatings, it was even possible to essentially improve the functional properties of different material surfaces, such as dirt repellence, easy to clean, corrosion and oxidization resistance, anti-scratch properties and wear resistance. However, thin film coatings of the type investigated here do not always provide sufficient corrosion resistance in demanding outdoor conditions. The best results in field corrosion tests were obtained for sol-gel coatings almost as thick as a coat of paint/varnish (even as much as 50 ìm). The best combination of thin coating and base material in outdoor conditions was sol-gel coating applied to hot-dip galvanized steel.