Patent Application
20080050535
Further embodiments and modifications of the method according to the invention can be derived from the specification and the subordinate claims.
Surprisingly, it has become apparent that completely or partially thermosetting powder paints of any shade on any objects, particularly poorly thermally conductive ones with a thermal conductivity of between 0.05 and 5 W/mK and/or heat-sensitive objects and, especially preferred, objects with profiled surfaces, can be completely and efficiently cured—that is, without noteworthy heating of the object itself and hence without any damage for the coating and/or the substrate—using medium-wave to long-wave infrared radiation whose maximum radiation flux density lies above wavelengths of 2.0 μm, if the powder paints contain substances with the characteristic of absorbing medium- and/or long-wave IR radiation. The paint surfaces thus achieved are free of imperfections and show no indication of outgassing due to the emission of moisture or thermal disintegration of the undersurface, which manifest as surface blemishes of the powder coating—so-called “pinholes.” In addition, it was possible to observe that UV- and electron beam-setting powder paints which are provided with appropriate additives which are absorbent in the medium- to long-wave IR range melt and form films reliably using the abovementioned type of radiation at a radiation intensity which, without the respective additive, merely brings about an agglomeration of the powder particles. This powder coating thus melted in the liquid phase is then subjected to a UV or electron beam radiation in order to cure it.
The use of antimony tin oxide (Minatec® 230 A-IR of the Merck Co.) in its commensurate powder paint formulations, for example, has proven to be very successful for an efficient curing with the aid of medium-wave to long-wave IR radiation while still protecting the MDF undersurface. Moreover, tin antimonate, vanadium oxide, tin oxide, indium tin oxide (Nano® ITO of the company Nanogate Technologies GmbH, Saarbrücken, Germany) and C nanotubes (of the company Nanoledge, Montpelier, France) as well as C nanofibers (of the company Electrovac GesmbH, Klosterneuburg, Austria) have exhibited outstanding efficiency.
Other noteworthy classes of material with high effectiveness in terms of the present invention are the oxides of the rare-earth metals. They exhibit a clearly pronounced effect in pure form, in the form of mixtures of the individual pure substances and as oxides of the respective rare-earth metal mixtures as well.
Furthermore, organic substances with a high component of hydroxyl groups of at least 0.5 hydroxyl groups per C atom have also exhibited medium- and/or long-wave IR radiation absorption effectiveness in terms of the present invention. Examples of such organic substances are carbohydrates (cellulose fibers or powder, starch, lactose) or polyalcohols such as pentaerytlrite, di-pentaerythrite, for example.
The required quantity of additive, with respect to the powder paint, depends on the substance which absorbs in the medium to long-wave IR range, the energy required by the powder paint system in order to melt and to cure, the heat-sensitivity of the substrate to be coated and the emission spectrum of the radiation source being used.
When using an IR radiator which is operated with electrical energy (Heraeus Co.), additions of Nano® ITO, C nanofibers and C nanotubes in the range of 0.01% with respect to the total powder paint formulation exhibited outstanding effectiveness with regard to the curing of such formulations, while the non-modified compositions of such powder paints do not cure at all under these conditions. The radiator (of the companies Infragas Nova Impianti, Italy; Vulcan Catalytic, USA) which are operated with gas as an energy carrier preferably emit in the wavelength range of 2-12 μm. Compared with electric radiators, that distinguish themselves by their very low operating costs. They are suited to the gentle curing of powder coatings on profiled surfaces of objects made of heat-sensitive materials, particularly if the respective formulations contain, for example, 2.5% ytterbium oxide and 2.5% neodymium oxide. The addition of 1-5% Minatec® 230 A-IR fulfills the same purpose. It is of course possible to prepare powder paints through the combined use of respective additives in such a manner that they are suitable for curing according-to the invention with different radiation sources for medium- and/or long-wave IR radiation.
The aforementioned additives with the characteristic of absorbing medium- and/or long-wave IR radiation can be used individually and in combination with each other in the thermal treatment of powder paints which can be applied to substrates.
Due to the substantially lower energy density of the medium- and/or long-wave IR radiation used—in comparison to the methods based on NIR and/or shortwave IR—the curing times for the powder paints on temperature-sensitive substrates which are prepared with the respective absorbers lie in the order of minutes (1-5 minutes, for example) and not—as is the case there—in the order of seconds. Since medium-wave and long-wave radiation, especially that which is generated with gas-catalytic radiators, has a predominantly diffuse character, it copes much better with the demands of curing profiled surfaces of temperature-sensitive substrates than NIR and IR radiation, which can be essentially characterized as being focussed. The doping of the powder paints with absorbers for medium- and/or long-wave IR brings with it the characteristic that the coatings preferably absorb the radiation, quickly melt and cure as desired.
In principle, all binding systems commonly used for the preparation of thermosetting powder paints can be used for the preparation of the thermosetting powder paint formulations according to the inventive method, including polyesters, polyurethanes, polyester epoxy hybrids, polyester polyacrylate hybrids, pure epoxides based on bisphenol A or epoxied phenol novolacs, epoxy polyacrylate hybrids, pure polyacrylates, etc., for example. Suitable as curing agents of the aforementioned polymers are substances such as triglycidyl isocyanurate, diglycyl terephthalate either in pure form or in combination with triglycidyl trimellitate, isocyanate curing agents which are based on diisocyanate adducts, di- or trimers which are optionally inhibited from a premature reaction through the use of detachable blocking agents such as ε-caprolactam, glycoluril (Powderlink® 1174 of the company Cytec Industries Inc.), β-hydroxy alkylamides, imidazoles and their epoxide adducts, imidazolines and their epoxide adducts, polyamines and their epoxide adducts, dicyanamide, novolacs, dodecanedioic acid polyanhydride, etc., for example.
Powder paints for curing according to the inventive method can be laced with all known pigments, including titanium dioxide, carbon black, iron oxides, chromium (III) oxide, ultramarine blue, phthalocyanine blue and green, for example. Likewise, the use of effect pigments, for example those based on aluminum, brass and copper platelets, is possible, as well as mineral effect agents such as mica or iron mica, for example. Barite, calcite, dolomite, quartz, wollastonite, aluminum hydroxide, kaolin, and talc, for example, can also be used as fillers.
In addition, the thermosetting powder paints according to the inventive method can contain, for example, additives for degassing (benzoin, amide waxes, for example), leveling agents (polyacrylates, for example), curing accelerators (tertiary amines, imidazoles, imidazolines, quaternary ammonium and/or phosphonium salts, organic tin compounds, tin soaps, lithium soaps, sulfonic acid and its salts, for example), light-protection agents [UV absorbers, HALS compounds (=hindered amine light stabilizer)], tribo additives (tertiary amines, HALS compounds), waxes, polymer particles such as Teflon, polyamide, polyethylene, or polypropylene as structuring agents and/or to increase the scratch-resistance.
The preparation of powder paints which contain the IR-absorbent substances is done, according to the most common method, through extrusion of the intimate dry mixture of the powder paint raw materials being used, grinding of the extrudate and subsequent sifting. Solvent processes can also be used in place of the melting process in the extruder for the purpose of materially homogenizing the powder paint components. The coating powder can be obtained through the use of a solvent through spray-drying. If, in the place of a solvent, supercritical gas (supercritical carbon dioxide, for example) is used, the process step of the classic spray-drying can be skipped; it is sufficient in this case to expand the obtained mixture via a nozzle to normal conditions.
The application of the powder paints which are suitable for the method according to the invention onto the objects to be coated is done through spraying of the powder paint particles under a simultaneous electrostatic or tribostatic charge. Special variations of this form of application are, for example, application using the EMB® process (electromagnetic brush method) or using the so-called powder cloud method.
After successful application of the powdery coating material on non-developable three-dimensional objects made of heat-sensitive materials, it can then be melted and cured in a trouble-free manner according to the invention with the aid of medium-wave to long-wave IR radiation in all areas of these objects and without damaging them.
The following examples are intended to illustrate the usefulness of the invention in more detail without limiting it to the explanations provided here.
Treatment of thermosetting powder-paints
1Cellulose of CFF GmbH, D-90708 Gehren
The initial components of the C 1 formulations above (comparison example 1) as well as 1-10 (examples according to the invention) are mixed in a Prism Pilot 3 laboratory mixer for one minute at 1500 rpm and then extruded in a laboratory extruder of the type Theyson TSK PCE 20/24D (zone temperatures 40/60/80/80° C.) at 400 rpm. The extrudates obtained are then ground to a grain size of <100 μm.
The powder paints thus obtained are applied using Gema Easy Tronic coating equipment onto furniture store fronts made of MDF with marked profiling (final layer thickness approx. 80 μm) and then subjected to thermal treatment for curing through medium-wave to long-wave IR irradiation with a maximum radiation flux density of >2 μm wavelength. For this purpose, two IR baking units are available:
A) Electrically operated:
4 radiators of the Heraeus Co. (2 medium wave carbon radiators, 2 conventional medium wave radiators, both with a maximum radiation temperature of <1000° C., hence maximum radiation flux density of >2 μm wavelength) are arranged perpendicular to the transporting direction such that they are distributed over the length. For the experiments, the radiators are operated at 60% of their maximum output, which shifts the emitted radiation further into the long-wave range. The belt speed is selected in such a manner that the specimens pass over the curing segment in approximately 3.5 minutes. For the first 30 seconds, the surface temperature lies at approximately 100° C., and then at an average of 135° C.
B) Gas-catalytically operated:
A curing segment of about 10 m is provided with gas-catalytic IR radiators of the Vulcan Co. According to information from the manufacturer, the maximum radiation flux density is approximately 4.5 μm at a radiator temperature of 400° C., and approximately 3 μm at a radiator temperature of 530° C. For the experiments, the radiator output is set in such a manner that an uncoated MDF plate does not exhibit any changes after a pass through the system lasting 4.5 minutes.
Experimental results for the curing of powder paint formulations (determination of the resilience to methyl ethyl ketone):
The characteristic of chemical resilience is used in order to evaluate the curing density of the powder paint which is achieved through baking.
Execution: Methyl ethyl ketone is dripped at room temperature onto the surface to be tested and the time is measured in minutes after which the paint can be wiped away at least partially with a cellulose cloth under moderate pressure or at least partially from the undersurface. If the powder paint resists the solvent for 10 minutes, the test is terminated and [the paint] is considered to have passed the test.
In non-inventive comparison example C (without addition of absorbers), no curing is achieved through the thermal treatment in the electrical system (the powder paint can be washed off), and the curing of this powder paint is clearly insufficient in the gas-catalytic system at any rate.
The following examples 1-10 according to the invention, both on the flat and the profiled parts of the test objects, show that, with the selected doping of IR absorbers which absorb in the medium and/or long-wave IR range, a good to complete curing of the respective powder paints can be achieved in at least one of the baking units used.
Thermal treatment of UV-setting powder paints
Numbers indicate raw material quantities in grams
As has already been described above, the initial components of the C 2 formulations above (comparison example 2) as well as 11 (example according to the invention) are mixed in a Prism Pilot 3 laboratory mixer for one minute at 1500 rpm and then extruded in a laboratory extruder of type Theyson TSK PCE 20/24D (zone temperatures 40/60/80/80° C.) at 400 rpm. The extrudates obtained are then ground to a grain size of <100 μm.
The powder paints thus obtained are applied using Gema Easy Tronic coating equipment onto furniture store fronts made of MDF with marked profiling and then subjected to thermal treatment in the previously described system A. For these experiments, the radiators are operated at 50% of their maximum output, and the belt speed is selected in such a manner that the specimens pass over the treatment segment in approximately 2 minutes.
A visual inspection of the test pieces provided with the coating powders in formulation no. 11 (according to the invention) results in a homogeneously flowing coat of paint which is homogeneously melted throughout all areas and cures using UV without problems; in formulation no. C 2 (non-inventive), one sees an only partially melted layer in the profile areas in which the individual powder grains are still visible. UV setting cannot be carried out on powder paint which is in such a state.
Determination of the surface temperatures of powder-coated MDF parts during transport through System A:
Two MDF test plates of the same kind are each provided with a temperature sensor, and one is provided with the comparison formulation C 1 while the other is provided with the formulation from example 7. Subsequently, the two test plates are run through the constantly operating baking system A at the same speed, and the resulting surface temperatures are recorded with the aid of a DataPaq 11 measurement and recording device. In so doing, it became apparent that the. coating produced from formulation 7 exhibited a temperature throughout the process which was approximately 20±5° C. higher than that of coating produced from formulation C 1.
This measurement proves, in an impressive manner, how a sufficiently moderate medium-wave to long-wave IR radiation intensity, which is unable to damage parts and cannot cure powder paint, leads to the complete curing of the powder paint when a suitable absorber is used in it.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 1898/2003 | Nov 2003 | AT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AT04/00418 | 11/26/2004 | WO | 00 | 2/8/2007 |
