Patent Application
20120022206
The invention relates to a powder composition, particularly for coating a metal substrate.
A composition of the above mentioned kind is known, for example, from WO 00/55268. The powder composition described therein, which is intended for coating a metal substrate such as the wall of a metal can or of another metal container, is notably designed to apply a well adhering, chemically and mechanically resistant coating onto the substrate.
The composition described in WO 00/55268 comprises the following components:
A substantial aspect of the powder composition according to WO 00/55268 appears to be that the blend of polyesters present as the major component comprises two copolyesters with different glass transition temperatures. The glass transition temperature (Tg) of a polymer is the temperature at which a transition from a brittle glassy state to a plastic state occurs. In the powder composition according to WO 00/55268, one of the copolyesters has a relatively high glass transition temperature of at least 45° C. to about 100° C. whereas the other copolyester has a relatively low glass transition temperature of about −10° C. to about 45° C., wherein the said glass transition temperatures differ by at least about 5° C. to 60° C., preferably by about 15° C. to 35° C. and, particularly, about 20° C. to about 30° C. According to WO 00/55268, the coatings that are produced by using such powder compositions are sufficiently flexible so that they can be mechanically deformed without developing cracks. Concomitantly, the said coatings are also sufficiently rigid to ensure an excellent scratch resistance and abrasion resistance.
An object of the invention is to provide further powdery compositions for coating a metal substrate that are particularly suitable as so-called seam protection powders but are also suitable as holohedral powder coatings for protective and esthetic purposes.
This object is achieved according to the present invention with the composition as defined in claim 1.
The powder composition of the present invention comprises:
Surprisingly, it was found that the composition of the present invention is suitable for coating a metal substrate by virtue of the selection of three copolyesters with well defined characteristics, even though, in contrast to WO 00/55268, it does not contain a copolyester having a glass transition temperature above 45° C.
The three copolyesters can be synthesized by esterification of an acid component and a diol component. Advantageously, in all of the three copolyesters the acid component is a blend formed mainly of terephthalic acid and isophthalic acid, whereas the diol component is either butanediol or a blend of butanediol and ethylene glycol. A particularly preferred selection of the three copolyesters is shown in Table 1.
In a preferred embodiment, the first copolyester consists of Griltex® D 2036 E, the second copolyester consists of Griltex® D 1874 E and the third copolyester consists of Griltex® D 1982 E. These commercially available copolyesters can be purchased at Ems-Chemie, Domat-Ems (Switzerland).
The compositions mentioned in the following examples 1 to 3 are processed to a compound. Subsequently, a powder with a particle size of less than about 100 μm, preferably of 15 to 70 μm, is produced by cold grinding and sieving. The powder is sprayed by means of an electrostatic spraying device onto thin metal sheets and then melted onto the same in a oven during 40 seconds at a temperature of 280° C. After cooling to room temperature, a coating with a layer thickness of less than about 100 μm, preferably of 15 to 70 μm, is obtained.
Titanium dioxide can be purchased, for example, in the form of the commercially available product Kronos 2430.
Iron oxide-alpha (FeOOH) can be purchased, for example, in the form of the commercially available product Bayferrox 3910.
Carbon Black, also termed as industrial soot, can be purchased, for example, in the form of the commercially available product Printex V.
Mica can be purchased, for example, in the form of the commercially available product Micro Mica W1.
Aluminum silicate can be purchased, for example, in the form of the commercially available product ASP 400 P.
Barium sulfate can be purchased, for example, in the form of the commercially available product Blanc Fixe N.
Lanco Flow P 10 is a commercially available coating additive based on an acrylate polymer.
It will be understood that, depending on the application, other dye pigments can be chosen instead of the dye pigments mentioned above, in which case another mixing ratio of the dye pigment: filler material may be chosen as needed.
Sections of tin-coated steel sheets were coated with the composition according to example 1 or with one of three conventional compositions. The latter were selected as an epoxide containing polybutylene terephthalate powder (“PBT/Epoxy”, examples 1 and 2) and a polybutylene terephthalate powder without epoxide (“PBT”, example 3).
The coated sheet sections were sterilized at 124° C. during 70 minutes. Subsequently, they were submersed in one of 12 test media at 70° C. during 5 days or 10 days. Finally, the coating was tested by means of a cross incision and by means of a folding test, respectively. The results of these submersion tests are presented in Table 3.
Coated sections of tin-coated steel sheets were produced analogously to those used in the submersion test (see above) and then immersed in one of 29 test media at room temperature during 21 days. The results of the subsequent testing of the coatings are presented in Table 4.
Suitable metal substrates are, in particular, tin-coated steel sheets, ECCS sheets (electrolytic chromium coated steel) and TFS sheets (tin free steel), respectively, but also aluminum parts. As is generally known, tin-coated steel sheets, but also ECCS and TFS sheets, are used for cover plates and for two-part cans; aluminum is also used for cover plates and for two-part cans, but also for mono block aerosol cans and for collapsible aluminum tubes.
The powder composition for coating a metal substrate according to the present invention, particularly has the following advantages in comparison with conventional solvent containing enamel systems:
A powder composition, particularly for coating a metal substrate, comprises about 70% to about 80%, by total weight of the composition, of a blend of polyesters. The blend of polyesters comprises—by total weight of the blend of polyesters—about 57% to about 58% of a first copolyester, about 35% to about 36% of a second copolyester and about 7% of a third copolyester.
| Number | Date | Country | Kind |
|---|---|---|---|
| 288/09 | Feb 2009 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH2009/000391 | 12/11/2009 | WO | 00 | 9/27/2011 |
