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Environmentally friendly coating method from Finland
09.02.2010
Additional value for metal products with sol-gel coatings
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 μm). The best combination
of thin coating and base material in outdoor conditions was sol-gel coating
applied to hot-dip galvanized steel.