Process for coating workpieces, in particular vehicle bodies

Abstract

When coating workpieces, in particular vehicle bodies, the following coating layers are applied in succession and in a manner known per se: a primer layer in an electrophoretic dip coating process, a functional layer, an aqueous base coating layer and a clear coating layer. In order to configure this process in an environmentally compatible manner, in particular in order to avoid solvent emissions and coating sludge, the functional coating layer is also applied by an electrophoretic dip coating process and the clear coating layer is applied as a powder coating by electrostatic spraying, which coating is subsequently stoved. In this manner, it is possible to bring about an overall reduction in solvent emissions to approximately one sixth of the previous value of known processes and in coating sludge formation to approximately one third of the previous value.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a process for coating workpieces, in particular vehicle bodies, comprising the following process steps:

[0003] a) pretreatment, in particular cleaning, of the workpiece surface;

[0004] b) application of a primer layer in an electrophoretic dip coating process,

[0005] c) application of a functional coating layer;

[0006] d) application of an aqueous base coating layer;

[0007] e) application of a clear coating layer;

[0008] wherein rinsing and drying steps are inserted as required between process steps b), c), d) and e).

[0009] 2. Background Art

[0010] In known, currently used coating processes of the above-stated type, the functional coating layer, which is frequently also known as a “surfacer”, is produced by spray application of a water-based coating, to which solvents are optionally added. Although electrostatic processes are generally used for this purpose, a comparatively large proportion of the sprayed coating misses the surface of the workpiece and gives rise to sludge which must be disposed of. Moreover, comparatively large quantities of solvent vapours are liberated during this type of application of the functional coating. In this known conventional process, the clear coating is applied by spraying as a two component coating, which is likewise associated with comparatively high solvent emissions and the production of coating sludge. A typical value for total solvent emissions is approximately 35 g/m2, relative to the total surface area.

[0011] The object of the present invention is accordingly to develop a process of the above-stated type in such a manner that, overall, it exhibits greater environmental compatibility and is lower in cost.

SUMMARY OF THE INVENTION

[0012] This object is achieved according to the invention in that

[0013] f) in process step c) the functional coating layer is also applied in an electrophoretic dip coating process; and

[0014] g) in process step e) the clear coating layer is applied by spray application of a clear powder coating.

[0015] According to the invention, wherever possible, the processes used to apply the coating layers are systematically those which result in low solvent emissions and in the production of small amounts of coating sludge. This applies, for example, to the application provided in process step f) of the functional coating layer in an electrophoretic dip coating process which operates virtually completely without producing sludge, since any coating particles entrained from the dip coating tank are recovered by returning the rinsing water to the dipping bath. In addition, the solvent content of the coating used in this case is lower than that which had to be applied by spraying according to the prior art.

[0016] The same applies to process step e), in accordance with which, instead of the two component clear coating of the prior art, a clear powder coating is applied by spraying. Virtually no solvent emissions occur in this case and the “overspray” of the clear powder coating may furthermore be collected, worked up and reused. In this manner, total solvent emissions may be reduced to approximately one sixth of those which occur in prior art coating processes. Replacing the electrostatic spraying process for the functional layer with a dipping process and replacing the solvent-based clear coating layer with a clear powder coating reduces the total quantity of coating sludge produced to one third of that hitherto produced.

[0017] Overall, a coating process is obtained in this manner which is not only highly environmentally compatible but also low in cost.

[0018] It is particularly advisable to use a coating material in process step b) which possesses elevated electrical conductivity for performance of process step f). In this manner, it is ensured that the primer layer applied in process step b) does not, by the insulating action thereof, hamper application of the functional coating layer in process step d) in an electrophoretic dip coating process.

[0019] It is furthermore convenient if differently coloured functional coating layers may alternatively be applied in process step f). In this manner, it is possible to select a different colour of the functional coating layer for each article to be coated; in particular, it is thus possible to achieve a colour shade in the functional coating layer which exhibits a certain similarity to the colour shade of the aqueous base coating, which ultimately determines the appearance of the coated workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] An exemplary embodiment of the invention is shown in greater detail below in the drawing; FIG. 1 is a schematic flow chart of a coating process according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0021] While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and described herein in detail a specific embodiment with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated.

[0022] The coating process illustrated in the flow chart of FIG. 1 is in particular used for coating vehicle bodies. In step 1, the surfaces of the vehicle body are prepared for the actual coating operation, i.e. are in particular degreased, rinsed, phosphated and optionally passivated. In step 2, the surface of the vehicle body pretreated in said manner is cataphoretically dip coated with a water-based synthetic resin coating to a layer thickness of approximately 20 &mgr; on external areas and to a layer thickness of approximately 10 &mgr; in cavities. This “primer” bonds tightly to the workpiece surface and effectively excludes oxygen. For reasons which will become clear below, a coating material is selected which, in comparison with those hitherto conventional, exhibits elevated electrical conductivity.

[0023] Rinsing operations, which are not shown separately in the flow chart, are then performed repeatedly, in which loose coating material is removed by rinsing water which is obtained by ultrafiltration from the dipping bath.

[0024] The vehicle body is then dried in step 3 in a one- or two-phase drying operation. In both cases, temperatures of between 80 and 100° C. are initially used, water substantially being removed from the primer. The primer is then optionally stoved in a second phase which proceeds at a temperature within the range between 175 and 200° C.

[0025] If required, a sanding operation could, in principle, be performed after this drying step. However, in the coating process shown in the drawing, such a sanding operation is omitted. Instead, a functional coating layer is applied in process step 4, again in a cataphoretic dip coating process. Since, as mentioned above, the primer layer consists of a material with comparatively good electrical conductivity, cataphoretic deposition of the functional coating layer is not hampered by any insulating action of the primer layer.

[0026] As is indicated by the plurality of boxes in process step 4, this functional coating layer may be applied in a different colour in different dipping baths 4a, 4b, 4c which are alternatively passed through. A functional coating layer is obtained which has a colour similar to the colour of the aqueous base coating layer which is subsequently to be applied.

[0027] The total layer thickness in process step 4 is approximately 35 &mgr;, a water-based synthetic resin coating again being used. The functional coating layer levels out the surface roughness of the workpiece and a sandable substrate is provided. Thanks to its resilience, the functional coating layer also provides protection from stone impact and ensures resistance to UV radiation.

[0028] Rinsing operations, which are not shown in the flow chart, again follow the cataphoretic dip coating operation in process step 4.

[0029] In process step 5, the functional coating layer applied in process step 4 is dried in a similar manner as in process step 3. This drying operation is now followed by a sanding operation in process step 6.

[0030] Thereafter, two operations proceed in process step 7 which have nothing to do with the actual coating process and may, in principle, also be inserted at another point in the coating process: firstly, the seams are sealed (“NAD”) by application of beads of resilient plastics material or rubber, coarse sealing (“GAD”) preferably being performed with robots and fine sealing (“FAD”) also being performed manually. An underbody sealant layer of resilient plastics material (“UBS”) up to 500 &mgr; in thickness is also applied by robot in the necessary areas; this layer provides acoustic insulation and protection from stone impact.

[0031] A further drying operation follows in process step 8, which is performed as “gelation drying”, i.e. the resilient plastics material is only polymerised until it is touch- and dust-dry.

[0032] Finally, in process step 9, an aqueous base coating, which comprises the “value-adding”, colour-imparting coating layer, is applied by spraying. Layer thickness generally varies as a function of colour shade: white colour shades have greater layer thicknesses, while thinner layers are sufficient for colour shades with good opacity, in particular red. Moderate layer thicknesses are used for metallic coatings.

[0033] In process step 10, the aqueous base coating is preferably dried in a condensation dryer, i.e. in a stream of very dry air which is circulated and dehumidified outside the drying chamber. This drying operation proceeds at comparatively low temperatures of for example 30°. Substantially only the water is removed during said operation.

[0034] In the following process step 11, a synthetic resin based clear powder coating is applied by electrostatic spraying. Said coating itself exhibits elevated resistance to chemicals, imparts gloss to the coated surface and, having a layer thickness of approx. 60 &mgr;, provides a polishable layer. This coating layer is stoved in process step 12.

[0035] At this point, the coating operation is essentially complete. Final finishing, in particular inspection and touch-up operations, are performed in process step 13.

[0036] The foregoing description merely explains and illustrates the invention and the invention is not limited thereto except insofar as the appended claims are so limited, as those skilled in the art who have the disclosure before them will be able to make modifications without departing from the scope of the invention.

Claims

1. A process for coating workpieces, in particular vehicle bodies, comprising the following process steps:

a) pre-treatment of the workpiece surface;

b) application of a primer layer upon the workpiece surface in an electrophoretic dip coating process;

c) application of a functional coating layer upon the primer layer in an electrophoretic dip coating process;

d) application of an aqueous base coating layer upon the functional coating layer; and

e) application of a clear coating layer by spray application of a clear powder coating upon the aqueous base coating;

wherein further steps of at least one of rinsing and drying may be optionally applied between process steps b), c), d) and e.

2. The process according to claim 1, wherein in step b), a coating material is used which possesses elevated electrical conductivity for performance of process step c).

3. The process according to claim 1, wherein differently coloured functional coating layers may alternatively be applied in process step c).

4. A process according to claim 1, wherein the step of pre-treatment comprises a step of cleaning the workpiece.