Coating processes – Heat decomposition of applied coating or base material
Patent
1999-10-08
2000-12-19
Beck, Shrive
Coating processes
Heat decomposition of applied coating or base material
427359, 4273977, 427402, B05D 302
Patent
active
061624985
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for providing a metallic surface with a vitreous layer.
Vitreous layers on e.g. steel are generally produced by enamelling processes. For said purpose an adhesive layer is applied thereon, followed by, via a suspension (engobe), a particle-containing composition which upon heating melts to form a glass. Usually said composition is a lead containing glass system (in order to lower the melting point) which is characterized by a relatively high alkali content which serves to match the expansion coefficient of the glass with that of the steel. However, said procedure entails the disadvantage that the resulting enamel layers are usually not completely satisfactory as regards their chemical resistance. In order to obtain closed dense layers, the layer thickness must usually be above 50 .mu.m. This makes the layers inflexible and brittle and thereby sensitive to bending, impact and shock (they chip off). Another method is the electrodeposition of chromium oxide interference layers. However, said layers are not scratch resistant and are very sensitive with respect to staining and fingerprints due to their coloring interference effect.
Numerous studies have been conducted as regards the application of thin layers on steel surfaces by means of sol gel techniques. For example, it has been tried to provide stainless steel surfaces with zirconium dioxide layers in order to improve corrosion resistance. Borosilicate glass layers have also been studied. However, it was found that the refractory systems (high melting oxides such as ZrO.sub.2) do not result in dense layers via said techniques and that the borosilicate glass layers could only be applied in layer thicknesses of significantly below 1 .mu.m so that a sufficient mechanical and chemical protection could not be secured.
According to the present invention it has now been found that by using specific, organically modified systems based on SiO.sub.2 vitreous layers can be formed on metallic surfaces, which layers may be as thick as 10 .mu.m without occurence of cracking upon drying and densification thereof. Said effect is attributed to the improved relaxation behaviour of the organically modified silica gel and silica, respectively, skeletons. Surprisingly it has also been found that such layers can be converted into dense SiO.sub.2 films (for example on stainless steel or steel surfaces) already at relatively low temperatures (generally starting from 400.degree. C.). Such films are usually 3 to 5 .mu.m in thickness and form a hermetically sealing layer which prevents or drastically reduces, respectively, the access of oxygen to the metallic surface and secures an excellent protection against corrosion even at elevated temperatures. Such layers are furthermore abrasion resistant so that they can for example be rubbed with steelwool on the surface thereof without leaving any trace. After 100 cycles of Taber-Abrader-test (CS-10F, 500 g) they also show only hardly visible traces. Furthermore they are flexible, i.e., bending or folding the surface does not result in any cracks or other deterioration of the layers.
Additionally, the layers are usually clear as glass and transparent and may be applied in a manner such that the metallic surface will not be changed in its appearance. For example, it is possible to apply such layers on construction parts made of stainless steel without occurence of a change in appearance.
Finally, further, already known colored vitreous layers, for example thin colloid-colored vitreous layers, may be provided on such vitreous layers. Since such colored vitreous layers are preferably prepared by means of coating compositions containing, for example, metal colloid precursors it may thereby be prevented that the metallic surface (or the metal ions released therefrom, respectively) (adversely) affect the reactions of the metal colloid precursors etc., since there will be no direct contact between the metallic surface and the colored vitreous layer. Moreover, silanes having fluorine containing side chai
REFERENCES:
patent: 4636440 (1987-01-01), Jada
patent: 5182143 (1993-01-01), Holmes-Farley
patent: 5731091 (1998-03-01), Schmidt et al.
patent: 5766680 (1998-06-01), Schmidt et al.
patent: 6017389 (2000-01-01), Schmidt et al.
Jonschker Gerhard
Mennig Martin
Schmidt Helmut
Beck Shrive
Institut fur Neue Materialien gemeinnutzige GmbH
Strain Paul D.
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