Enamel frits with improved adhesion for steel plate.

0001The present invention relates to enamel frits for the enamelling of steel sheet, in particular unpickled, non-nickel-plated sheet metal, with outstanding enamel adhesion down to the range of low baking temperatures.

0002Basic and direct emails have the primary task of creating good adhesion between the metallic workpiece and the glassy enamel layer. In addition, the enamelling should be fire-resistant over the broadest possible temperature range, have a smooth, flawless surface and should not suffer any reduction in liability even after repeated firing. This is achieved by combining network-forming oxides such as SiO₂, TiO₂, B₂O₃, P₂O₅ with intermediate oxides such as ZrO₂, ZnO, Al₂O₃ as well as network-converting oxides such as Li₂O, Na₂O, K₂O, CaO, BaO and fluorine (the latter in the form of fluorides).

0003Direct emails usually contain a larger proportion of TiO₂ or ZrO₂ in order to make the enamelling resistant to acidic or alkaline attack.

0004To achieve good adhesion between enamel and steel, additions of adhesive oxides such as CoO, NiO, CuO and occasionally also iron oxide, antimony oxide or arsenic oxide are required for all basic and direct emails (enamel and enamelling technology, A. Petzold, H. Pöschmann, Springer Verlag, 1987, pages 58 ff.).

0005This applies in particular to the enamelling of unpickled metal sheets, which are only slightly roughened on the surface, where the interlocking or the reaction with the enamel is difficult. However, this process in particular is of great importance today, since it is becoming more and more important for reasons of ecology and economy.

0006In practice, two different ways are used to develop the enamel adhesion even on degreased metal sheets, some of which are even completely untreated when powder is applied:<ul id="ul0001" list-style="none"><li>1. Development of alkali-rich reactive enamel frits that wet the sheet after a short baking time and bring the adhesive oxides contained in the enamel to optimum effect (W. Podesta, Mitt. VDEfa <u style="single">37</u> (1989), 117).</li><li>2nd Emails with an increased content of adhesive oxides, mainly CoO and NiO (D. Ritchie, Mitt. VDEfa <u style="single">24</u> (1976), 35).</li></ul>

0007In practice, however, both approaches have serious disadvantages: the first method is only suitable for chemically less resistant basic emails, the second, on the other hand, is very cost-intensive (NiO and CoO are the most expensive components of basic and direct emails).

0008For example, in the past there have been repeated attempts to enamel emails without adhesive oxides with acceptable adhesion (see A. Dietzel: Emaillierung, Springer-Verlag, p. 119 ff). However, due to the low tolerances in processing (compliance with very special firing conditions, etc.), these processes are without any practical significance today.

0009Even enamels melted with iron oxide additives can only be enamelled under very drastic conditions so that acceptable adhesion results. In addition, larger additions of iron oxide lead to a significant reduction in the chemical resistance of the resulting enamelling.

0010Antimony and arsenic trioxide also allow good enamel adhesion, but due to ecological considerations and strict legal requirements, their use is no longer to a large extent today.

0011Of the remaining adhesive oxides CoO, NiO or CuO, CoO and NiO are preferably used, both individually and as a combination. The effect of these adhesive oxides is primarily based on an acceleration of the adhesion reaction, preferably by increasing the galvanic corrosion of the steel during enamel baking and the formation of adhesion-promoting FeCo or FeNi mixed crystallites at the phase boundary (cf. E. Millon, dissertation, Lyon, 1988 ). CoO and NiO also promote the wetting of the sheet metal surface with enamel.

0012Like NiO, CuO is only sufficiently effective as the sole adhesive oxide in larger proportions (> 2.5% by weight), the enamel adhesion being poorer than with CoO-containing enamels. This is attributed, among other things, to the higher redox potential of NiO or CuO compared to metallic iron or iron oxide, which in turn leads to a premature reduction of these "noble" adhesive oxides in enamel and thus removes these adhesive oxides from the adhesive reactions taking place at the steel / enamel phase boundary (cf. . A. Dietzel, enamelling, p. 119 ff). So far, CuO has so far only been used as an additional component to CoO and NiO in enamel, sometimes also as a coloring enamel component. Well-adhering basic and direct emails containing CuO as the sole adhesive oxide have so far not been disclosed.

0013Of course, there has been no lack of attempts to exploit the synergistic effects observed in the mixture of CoO and NiO and other oxides having the above-mentioned effect in practice. Basic and direct emails therefore usually contain at least 2, but often 3, 4 or 5, element oxides with adhesive properties, but preferably CoO and NiO.

0014It is also known that by mixing hard (filler base) or soft (mesh base) enamel frits, sometimes with the addition of melt-effective substances or inert substances, the enamel adhesion can be influenced positively. The primary purpose of such mixtures, however, is to be able to influence surface tension, thermal expansion, chemical resistance, viscosity, burning stability and color in a targeted manner - according to the requirement profile.

0015The task now was to provide enamel frits that adhere very well to pickled, but also to pickled, non-nickel-plated steel sheets, are fire-resistant over a wide temperature range and, in the case of direct emails, are also resistant to acid and alkaline attack,

0016This task could be solved by providing the enamel frits according to the invention.

0017Surprisingly, it was found that standard basic and direct email fries with the following composition <tables id="tabl0001" num="0001"><table frame="all"><title /><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">SiO₂</entry><entry namest="col2" nameend="col2" align="right">30th - 60% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">ZrO₂</entry><entry namest="col2" nameend="col2" align="right">0 - 15% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">TiO₂</entry><entry namest="col2" nameend="col2" align="right">0 - 10% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">P₂O₅</entry><entry namest="col2" nameend="col2" align="right">0 - 4% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">B₂O₃</entry><entry namest="col2" nameend="col2" align="right">7 - 27% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Al₂O₃</entry><entry namest="col2" nameend="col2" align="right">0 - 11% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">F</entry><entry namest="col2" nameend="col2" align="right">0 - 8% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Na₂O</entry><entry namest="col2" nameend="col2" align="right">6 - 22% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">K₂O</entry><entry namest="col2" nameend="col2" align="right">0 - 10% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Li₂O</entry><entry namest="col2" nameend="col2" align="right">0 - 6% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">BaO</entry><entry namest="col2" nameend="col2" align="right">0 - 20% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">CaO</entry><entry namest="col2" nameend="col2" align="right">0 - 18% by weight</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">ZnO</entry><entry namest="col2" nameend="col2" align="right">0 - 15% by weight</entry></row></tbody></tgroup></table></tables> and optionally additions of up to 10% by weight of MgO, Fe₂O₃, MnO, Cr₂O₃ and / or MoO₃ lead to excellent enamel adhesion on enamelled, preferably unpickled steel, if these frits additionally 0.8 to 5% by weight of CoO and contain 1.2 to 6% by weight of CuO, the CuO: CoO molar ratio being between 1 and 5, preferably 1.5 and 3 and the sum of CuO and CoO being at least 2.3% by weight and maximum 10% by weight, preferably 3 to 5% by weight.

0018The enamel frits can additionally contain up to 0.5% by weight of NiO. This addition leads to a significant improvement in adhesion in individual cases.

0019It seems to be particularly important that the CuO: CoO molar ratio is at least 1, since a higher CoO content could reduce the CuO melted in the enamel prematurely. This also applies to enamels that contain nickel oxide (NiO and CoO are less noble than CuO).

0020Surprisingly, it was found that the enamels according to the invention are particularly suitable for application on unpickled, non-nickel-plated steel substrates and are superior to standard and direct e-mails in terms of enamel adhesion, but also in terms of raw material costs. This applies in particular to enamelling on low-carbon steels (&lt;0.006% C).

0021The enamels according to the invention are suitable for the slip application by flooding, dipping or spraying, but can also be applied to the sheet to be enamelled by electrophoretic application by dip or powder electrostatic application. In the latter case, in extreme cases, degreasing can even be omitted in the sheet metal pretreatment, which makes this process particularly economical.

0022The enamels according to the invention can be applied and baked either in the one-layer process or - together with a cover enamel - in the two-layer 2-fire or in the two-layer 1-fire process (cf. DE-A 3 117 706).

0023The enamel frits according to the invention are melted from commercially available enamel raw materials such as borax (Na₂B₄O₇ · 5H₂O), quartz powder, sodium polyphosphate, feldspar, zircon sand, fluorspar, alkali and alkaline earth carbonates as well as the corresponding heavy metal oxides at 1,100 - 1,250 ° C and chilled over water-cooled steel rollers. The resulting flakes are applied both as enamel powder in the electrostatic field and as an aqueous suspension in slurry form by dipping or spraying on test sheets of 0.5 to 3 mm thickness and can be in a box furnace or in a temperature gradient furnace in a temperature range between 780 and 880 ° C can be branded.

0024The layer thickness of the single-layer enamelling was between 0.15 and 0.2 mm in the tests and between 0.25 and 0.5 mm for the two-layer enamelling. The enamel adhesion was tested using the ball-drop device in accordance with DEZ Leaflet F 6.2, deforming the steel sheet and visually assessing the adhesion pattern.

0025The subject matter of the present invention will be explained in more detail with reference to the following examples.

Examples 1-6

0026Enamel frits of the following composition were melted from commercially available enamel raw materials in chamotte crucibles with a volume of 700 ml at about 1,200 ° C (values ​​in% by weight):<tables id="tabl0002" num="0002"><img file="EP0425927A1_D0001.tif" /></tables>

0027Depending on the enamel, the melting time was between 25 and 45 minutes. After melting, the enamels were cooled using steel rollers, the glass ribbon was broken into flakes, ground together with conventional additives and applied to degreased enamelling sheets using one of the usual methods listed below. Firing was carried out either in a gradient oven at temperatures from 780 to 880 ° C or in box ovens at 800 to 850 ° C.

Slip application:

00281,000 g flakes of frit 1 or 3 were ground in a ball mill filled with 3,500 g heavy grinding balls to give a slip after the following mill offset: <tables id="tabl0003" num="0003"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Frit</entry><entry namest="col2" nameend="col2" align="right">100 % By weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Quartz flour</entry><entry namest="col2" nameend="col2" align="right">5 % By weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Shade of blue</entry><entry namest="col2" nameend="col2" align="right">1 % By weight</entry></row><row><entry namest="col1" nameend="col1" align="left">White tone</entry><entry namest="col2" nameend="col2" align="right">3.5% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Calcium silicate (like)</entry><entry namest="col2" nameend="col2" align="right">0.1% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Sodium nitrite</entry><entry namest="col2" nameend="col2" align="right">0.1% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Boric acid</entry><entry namest="col2" nameend="col2" align="right">0.2% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">water</entry><entry namest="col2" nameend="col2" align="right">approx. 50% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Fineness of grinding</entry><entry namest="col2" nameend="col2" align="right">3rd on 3,600 mesh screen</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">density</entry><entry namest="col2" nameend="col2" align="right">1.67 g / cm³</entry></row></tbody></tgroup></table></tables>

0029So much of this slip was applied to sheet metal strips of 6 x 45 cm (for gradient oven) or 10 x 10 cm (for box oven) by spraying or dipping that a layer thickness of 0.15 mm resulted after baking. After drying, the sheets were either baked in the gradient oven for 10 minutes or in the box oven for 4 minutes. The adhesion test with the drop device showed good adhesion over the temperature range of 800 to 880 ° C (gradient oven) or 820 to 850 ° C (box oven).

Electrophoretic Diving Order (ETE):

00301,000 g flakes of frit 2 or 3 were ground in a ball mill filled with 3,500 g heavy grinding balls to give a slip after the following mill offset: <tables id="tabl0004" num="0004"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Frit</entry><entry namest="col2" nameend="col2" align="right">100 % By weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Shade of blue</entry><entry namest="col2" nameend="col2" align="right">2.5% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Bentonite</entry><entry namest="col2" nameend="col2" align="right">0.8% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Relatin</entry><entry namest="col2" nameend="col2" align="right">0.1% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">NaCl</entry><entry namest="col2" nameend="col2" align="right">0.01% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">water</entry><entry namest="col2" nameend="col2" align="right">approx. 50% by weight</entry></row><row><entry namest="col1" nameend="col1" align="left">Fineness of grinding</entry><entry namest="col2" nameend="col2" align="right">3rd on 16,900 mesh screen</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">density</entry><entry namest="col2" nameend="col2" align="right">1.64 g / cm³</entry></row></tbody></tgroup></table></tables>

0031The slip is adjusted to a viscosity of approx. 200 mPa · sec or a conductivity of 3000-4000 µS by adding water or sodium aluminate solution and electrophoretically degreased 7 at a current / voltage of 5-10 amperes / 50 volts x 11 cm sheets of a thickness of 1 mm deposited, so that after firing a layer thickness of 0.18 mm results. After drying, the test panels were baked in the box oven at 800 or 820 ° C for 3 minutes. The liability test with the drop device showed good liability.

Powder application:

0032In a porcelain ball mill, 100 g of enamel flakes of frit 4 or 5 and 0.4 g of methyl hydrogen siloxane were ground until the residue of the powder on the 16,900 mesh sieve (mesh size 40 μm) was less than 20%. The powder, which had a surface resistance of more than 10 12 Ω, was sprayed with an electrostatic gun with a 70 kV voltage onto 6 x 45 cm steel sheets of 1 mm thickness in a uniform layer. After 14 g of puler had been applied (corresponding to 5.2 g / dm 2), the steel sheets were baked in the gradient oven for 10 minutes. Both degreased and non-degreased sheets were used. The adhesion test with the drop device showed good adhesion for both frits in the temperature range from 800 to 880 ° C and particularly good adhesion to sheets made of decarburized steel.

0033Frit 6, which does not meet the parameters according to the invention, shows a significantly poorer enamel adhesion than the frits 1 - 5 according to the invention.