EP0328963A2 - Method of polishing lacquered surfaces - Google Patents

Abstract

A novel process for polishing highly resilient coating surfaces, in which the surface to be polished is kept at a temperature of less than +5 DEG C by means of a cold gas during the polishing process.

Description

The present invention relates to a method for polishing highly elastic paint surfaces, in particular plastic paint surfaces, in which the paint surface to be polished is kept at a temperature of max. + 5 ° C. by passing a cold gas over it.

It is known that highly elastic paints, such as those used in plastic painting, can no longer be touched up properly after drying. That e.g. For metal coatings in the automotive industry, it is usually not possible to grind and polish imperfections in the paint film, which has to be done directly after the oven has dried.

Plastic parts are increasingly being used, especially in the automotive industry, for example to save weight and thus fuel, to avoid minor damage and to allow more freedom in design. These parts generally need to be painted to them to provide sufficient resistance to the effects of weather or chemical substances (fuel, cleaning agents) or to make them scratch-resistant. For body parts made of plastic, painting in body color is also required.

Strict requirements are placed on these plastic paints. The decisive factors are above all the possibility of drying even at temperatures around 80 ° C or below (limited heat resistance of the plastics) and the sufficient flexibility of the paint film. As has been described in detail in the literature (see, for example, M. Schönfelder, "Influence of Polyurethane Varnishes on the Mechanical Properties of Plastics", "defazet" No. 2, 1979, pp. 59-64, Verlag Elvira Moeller GmbH, Filderstadt), If the lacquer layer on a flexible chemical material is too hard, it will damage the bond in the event of impact or impact stress (e.g. in the event of a crash), often resulting in a dangerous splinter fracture. The mechanical values of the unpainted part are reduced by the hard paint film. For this reason, the coating systems customary for painting metals cannot generally be used for painting plastics; Rather, low-temperature drying, highly elastic paint materials are used, usually flexible two-component polyurethane paints in Europe. In the "on-line painting" of motor vehicle bodies made according to the mixed construction, having plastic and metal parts, it is inevitable that not only the plastic parts but also the metal parts are painted with these highly elastic paint materials.

It has long been known, however, that such lacquers cannot be repaired after drying by means of grinding and polishing, as is customary in the case of metal finishes in the automotive industry. The higher the flexibility of the paint film, the more problematic is the ability to repair it.

Sanded, highly elastic clear coat films e.g. cannot be brought back to their original luster even after polishing for several minutes; there are matt stains or scoring marks. With pigmented varnishes, additional haze or color changes often form. The cause of the poor repair ability has not been clearly established; however, it is certain that with increasing flexibility of the paint film, which results from the above-mentioned. But it is essential that the problem arises more and more clearly. It is obvious that the elastic film "evades" the mechanical stress during grinding and polishing. It is also striking that plastic paints heat up more than metal paints during polishing (different chemical structure; in addition, possibly poorer heat dissipation on plastics).

The problems described so far, which have not yet been solved, mean that defects which have been observed after the coating of plastic parts (dust particles; other specks) cannot be repaired; it has to be repainted. The result is high expenditure of time and money and high reject rates.

The lack of polishability of these paint systems is therefore considered one of the biggest obstacles on the way to the increased use of plastics in the automotive sector and the implementation of on-line painting of vehicles, i.e. the simultaneous painting of metal and plastic parts on car bodies with an (elastic) paint system.

The object on which the invention was based was to provide a new process for polishing lacquer surfaces, which also polishes highly elastic lacquer films, in particular those on plastics or, in particular in the case of "on-line lacquering", by the mixed construction method Motor vehicle bodies, also permitted on metals.

This object could be achieved by providing the inventive method described in more detail below.

The invention relates to a method for polishing highly elastic lacquer surfaces, characterized in that the surface to be polished is kept at a temperature of at most + 5 ° C during the polishing process by means of a cold gas.

From DE-AS 2 653 487 it is already known to polish furniture lacquers with air cooling, but it is not highly elastic coatings of the type in question here, so that this publication with the problem underlying the invention cannot be brought into any closer connection. In addition, this publication does not contain any reference to the low temperatures that are mandatory according to the invention. The comparatively high temperatures (uncooled air) which inevitably occur in the process of publication on the paint surface are completely unsuitable for achieving the object according to the invention.

The lacquer surfaces to be polished by the process according to the invention are highly elastic lacquer films which have an elongation at break according to DIN 53 455 of more than 60% (clear lacquers) or more than 30% (pigmented lacquers). Paint films of this type are produced in particular in the case of the known painting of plastics, in particular automobile body parts made of plastic or also of metals in the case of "on-line painting" according to the mixed construction of bodies (the plastics are, for example, such) Polyurethanes, polycarbonates, polypropylene or based on acrylonitrile-butadiene copolymers or their blends with polycarbonates) using two-component polyurethane lacquers with a highly elastic setting, as described, for example, in "Painting car body parts made of flexible plastics with ®Desmodur / ®Desmophen", Information from Bayer AG, Leverkusen, for the paint area, issue 6.80 (order no. KL 44 257) is described.

The polishing is generally carried out using the known polishing aids (polishing pastes) as described, for example, in "Römpps Chemie-Lexikon / Otto-Albrecht-Neumüller", Volume 5, pp. 3269 ff .; Stuttgart 1987; Franckh'sche Verlagbuchhandlung W. Keller & Co. are described.

Suitable polishing devices for the method according to the invention are, for example, compressed air polishing devices or electrically operated polishing devices of the type known per se. The CP 869 P angular polishing device from Chicago Pneumatic, D-6222 Geisenheim / Rhein, for example, is well suited. Grinding tools of the type described in DE-AS 2 145 714 are also very suitable, provided that the grinding wheel described there is replaced by a corresponding polishing wheel, for example covered with a lambskin.

For carrying out the method according to the invention, it is essential that the surface to be polished is kept at a temperature of at most + 5 ° C., preferably at most -10 ° C., by means of a gas passed over the surface to be polished during the polishing process. This can be done, for example, using the tool described in DE-AS 2 145 714 with permanent gassing of the surface to be polished during the polishing process with a cold gas. A variant of the tool described in DE-AS 2 145 714 according to which the cooling gas is introduced in the middle parallel to the rotating axis is also conceivable. Basically the type of conduction is the cooling one Gases not essential to the invention, provided that the gas cooling mentioned ensures that the lacquer surface to be polished during the polishing process is at or below the above-mentioned maximum temperature. The gas used for cooling in turn has a maximum temperature of -5 ° C, preferably from -20 ° C to -60 ° C. Evaporating liquid air or evaporating liquid nitrogen, for example, can be used well as cooling gas.

In order to ensure adequate cooling during the polishing process, it is recommended that the diameter of the polishing disc be max. To hold 30 cm, preferably at about 15 to 25 cm.

The method according to the invention allows for the first time an economical repair of elastic paints, the grinding and polishing process being coordinated.

For example, damaged paint films can first be subjected to a grinding process using customary special abrasive papers, such as those offered by 3M Deutschland GmbH, Neuss, under the name "®Finesse-it" system. The grain size of the sanding paper used here should represent a compromise between the desired high removal rate (short duration of the sanding process) and the avoidance of deep, difficult-to-polish sanding marks, which occur especially with coarse grain. This can be achieved with papers that have a grain size of approximately 800 to approximately 2,000 (preferably 1,200 to 1,500); to avoid bigger ones Damage to these sanding papers should have a small diameter (approx. 3 cm) and be punched out (not cut). Even highly elastic paint systems can be sanded in this way within a few seconds. If a shorter process time is required, the grinding process can be carried out using a rotary grinding device.

Following the grinding process, the polished lacquer surface is then polished according to the invention.

With the method according to the invention, it is possible to repair highly elastic paintwork by means of grinding and polishing, the initial gloss being achieved again and other errors (for example individual remaining grinding marks) being avoided. This means that in the case of highly elastic films, the process according to the invention enables the repair of painted molded parts without the need for a completely new paint job.

Examples

To test the polishability of topcoat systems with graded elasticity, sheets made of an ABS polycarbonate alloy (®Bayblend from Bayer AG, Leverkusen) were first coated with a conventional two-component polyurethane primer and then with a commercially available metallic basecoat (silver gray) coated by Herberts GmbH, Wuppertal. The plates pretreated in this way were then coated with a clear lacquer based on a solvent-containing two-component system (binder base: ®Desmodur N, ®Desmophen 670 and ®Desmophen A 365 from Bayer AG, Leverkusen). The weight ratio of Desmophen A 365 (inelastic hydroxyl-containing polyacrylate resin) and Desmophen 670 (elasticizing polyester) was varied according to Table 1 below. Formulation 1 is very hard, formulation 4 is highly elastic (cf. the elongations at break in the clear lacquer film at room temperature). Formulation 4 corresponds approximately to the highest level of elasticity used in practice (e.g. in the case of clear coats for painting plastic parts in the front and rear areas of vehicles that are subject to impact).

In the clearcoats, the polyisocyanate component was used in an amount corresponding to an NCO / OH equivalent ratio of 1.2: 1. The clearcoats also contained the additives listed below (the percentages relate to percentages by weight, based on binder (solid resin)): Catalyst ¹⁾ 0.2% Leveling aids ²⁾ 0.2% Sunscreen ³⁾ 1.0% Sunscreen ⁴⁾ 1.0% ¹⁾ Triethylenediamine ®Dabco 33 LV from Biesterfeld, Hamburg; ²⁾ ®Baysilon paint additive OL 17 from Bayer AG, Leverkusen; ³⁾ ®Tinuvin 292 from Ciba-Geigy AG, Basel; ⁴⁾ ®Tinuvin 900 from Ciba-Geigy AG, Basel.

The clear lacquers contained as solvent a mixture of ethyl acetate, butyl acetate, methoxypropyl acetate, methyl ethyl ketone and xylene in a weight ratio of 1: 1: 1: 1: 1. The solids content was 49.5% by weight. The clearcoat was applied by compressed air spraying, the dry film thickness of the clearcoat was approx. 30 µm.

After an evaporation time of about 15 minutes, the mixture was dried at 80 ° C. over a period of 45 minutes. The samples painted in this way were taken out of the oven and wet sanded and polished after only a few minutes.

"Finesse-it" sandpaper (⌀ approx. 3 cm; grit approx. 1,500) was used as sandpaper and sanded by hand (duration approx. 10 to 15 s; sanded surface approx. 3 cm x 3 cm).

Subsequently, polishing was carried out using a cold gas polishing device with a rotating polishing surface, care being taken to keep the surface at a temperature of below -10.degree. C. while polishing with evaporating liquid nitrogen (temperature at the outlet opening approx. -40.degree. C.) exhibited. The surface of the polisher consisted of a lambskin, the diameter of the rotating disc was 20 cm, the speed was 2,500 rpm.

"Finesse-it" paste was used as a polishing aid. After a polishing time of 30 seconds, the following results (gloss values according to Gardner; 60 ° C; 20 ° C values in brackets) were achieved. Table 1 Polishability of different elastic top coats (clear coat on metallic base coat) under the influence of cold gas; on primed ®Bayblend plates. Formulation no. 1 2nd 3rd 4th Weight ratio polyacrylate / polyester in the clear coat 1: 0 1: 1 1: 3 0: 1 Elongation at break in% (DIN 53 455) (clear film, room temperature) <5 about 60 approx. 90-100 about 130-150 Initial gloss 95 (89) 95 (86) 92 (84) 95 (86) Shine after sanding 11 (2) 9 (1) 6 (1) 4 (1) Shine after cold gas polishing 95 (87) 95 (86) 90 (83) 95 (83)

The initial shine is restored and there are no traces of grinding or polishing. Even the most elastic setting can be sanded and polished immediately after the oven has dried.

For comparison, the results for conventional polishing are listed in Table 2 for the same paint structures (ie polishing according to the prior art; all boundary conditions are identical, but without continuous cold gas cooling). Table 2 Polishability of different elastic topcoats (analogous to Table 1); without exposure to cold gas. Formulation no. 1 2nd 3rd 4th Initial gloss 93 (85) 95 (85) 91 (84) 93 (85) Shine after sanding 11 (2) 9 (1) 6 (1) 4 (1) Shine after conventional polishing 85 (73) 95 (85) 45 (21) 53 (10)

It can be clearly seen that lacquers which are not very flexible (formulations Nos. 1 and 2) can also be polished relatively well conventionally, so that the use of cold gas can generally no longer bring about any improvement. Even medium-elastic clear coats (elongation at break> 60 to 70% at room temperature) can no longer be polished conventionally. With highly elastic systems the initial shine can in principle no longer be restored by longer polishing (several minutes; which cannot be accepted in practice anyway).

The same polishing tests were carried out with formulations 1 to 4 also on primed aluminum sheets as a substrate instead of on primed plastic plates; the same results were obtained within the scope of the measuring accuracy (both with and without cold gas); i.e. the substrate has little influence on the polishability of the multi-layer coating system.

If the elastic paint systems customary for plastic painting are thus applied to metal (for example in the case of "on-line painting of vehicles in mixed construction), they cannot be polished on the metal parts unless the method according to the invention is used; neither is it applied to one other underground.

The same result is also found when using other (elasticizing) lacquer raw materials than those mentioned above; ie by using the method according to the invention, the polishing time is greatly shortened or (in the case of very high elasticities) the polishability is made possible.

It is also possible to use other sandpaper grits without the results deteriorating (when using the cold gas method). Only with very coarse papers (grit approx. 600) does the sanded area remain visible after the polishing process.

The results thus prove that a new method has been found for the first time to be able to quickly repair the elastic paints used in plastic painting by means of grinding and polishing.

Claims (4)

1. A process for polishing highly elastic lacquer surfaces, characterized in that the surface to be polished is kept at a temperature of max. + 5 ° C during the polishing process by means of a cold gas. 2. The method according to claim 1, characterized in that the lacquer surface to be polished has an elongation at break according to DIN 53 455 of more than about 30% at room temperature. 3. The method according to claim 2, characterized in that the gas used for cooling has a temperature of at most -5 ° C. 4. The method according to claim 1 to 3, characterized in that the paint surface to be polished represents the surface of a plastic paint that has been applied to plastic and / or metal.