Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2001-08-13
2003-04-15
Beck, Shrive P. (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S377000
Reexamination Certificate
active
06548119
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for producing two-layer automotive top coats consisting of a base lacquer layer and a clear lacquer finishing layer.
BACKGROUND OF THE INVENTION
Two-layer automotive top coats are produced by wet-on-wet application of a transparent finishing lacquer layer to a colour- and/or effect-giving base lacquer layer that has been pre-dried by exposure to air. The colour- and/or effect-giving base lacquer layer is produced using either non-aqueous or aqueous base lacquers in the lacqering units of the individual motor vehicle manufacturers. Non-aqueous base lacquers contain organic solvents in an order of magnitude of, for example, from 55 to 80 wt. %, while aqueous base lacquers contain only, for example, from 10 to 25 wt. % organic solvents. For reasons of environmental protection, therefore, there is a trend towards replacing base lacquers based on organic solvents by aqueous base lacquers. For example, older lacquering units in which non-aqueous base lacquers are processed are being replaced by new lacquering units constructed for the processing of aqueous base lacquers.
The air-exposure conditions for base lacquer layers in lacquering units for the application of base lacquers based on organic solvents are different from those in lacquering units designed specifically for the application of water-borne lacquers. In lacquering units designed for the application of base lacquers based on organic solvents, the air-exposure conditions that prevail are characterised by short air-exposure times of, for example, from 30 to 180 seconds at air temperatures of, for example, from 20 to 30° C. Aqueous base lacquers require longer air-exposure times at higher air temperatures in order to obtain a pre-dried base lacquer film on completion of the air-exposure operation. In lacquering units for the application of aqueous base lacquers, therefore, the air-exposure times are, for example, from 5 to 10 minutes and the air temperatures prevailing during the exposure to air are higher at, for example, from 40 to 80° C. For example, for the purposes of adequate pre-drying suitable for the subsequent wet-on-wet application of a clear lacquer coating composition, coating layers applied from aqueous base lacquers are exposed to air first for from 1 to 2 minutes at from 20 to 40° C. and then for from 3 to 5 minutes using warm air of from 40 to 80° C. The short air-exposure times in lacquering units designed for the application of base lacquers based on organic solvents are the result of the only short overall length of the air-exposure region and the belt speed given by the production target of the vehicle body coating process. In the case of base lacquers based on organic solvents, such short air-exposure times are sufficient to obtain a pre-dried base lacquer film on completion of the air-exposure operation, but that is not the case with aqueous base lacquers. Accordingly, it is not possible to process aqueous base lacquers in lacquering units that allow only short air-exposure times.
SUMMARY OF THE INVENTION
The object of the invention is to provide a process for producing two-layer motor vehicle top coats by the wet-on-wet application of an aqueous base lacquer coating composition and a clear lacquer coating composition while allowing only short air-exposure times of, for example, from 30 to 180 seconds for the base lacquer layer. In particular, the process is to allow aqueous base lacquers to be processed in automated motor vehicle series lacquering units that are designed for the processing of non-aqueous base lacquers and allow only short air-exposure times for the base lacquer layers.
The object is achieved by observing the following conditions in the formulation of aqueous base lacquer coating compositions and observing the following process conditions during the exposure to air of the coating layers applied from such aqueous base lacquer coating compositions.
Accordingly, the invention provides a process for producing a two-layer colour- and/or effect-giving top coat on motor vehicle bodies in an automatic motor vehicle series lacquering unit, wherein a transparent finishing lacquer layer of a clear lacquer coating composition is applied to a base lacquer layer that has been applied from an aqueous colour- and/or effect-giving base lacquer coating composition and exposed to air, and the two coating layers are together baked, which process is characterised in that there is used an aqueous base lacquer coating composition that contains organic solvents according to a high solid value of from 40 to 70%, and the base lacquer layer, after it has been applied and before the transparent finishing lacquer layer is applied, is exposed to air for from 30 to 180 seconds using circulating air at from 25 to 45° C. with an air throughput of from 0.10 to 0.70 m/s, based on the area provided with the aqueous base lacquer layer.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, aqueous base lacquer coating compositions are used. For example, in addition to water they contain one or more conventional binders, organic solvents and pigments as well as, optionally, fillers, crosslinking agents, and/or additives conventionally employed in lacquers. The aqueous base lacquer coating compositions used in the process according to the invention have, for example, solids contents by weight of from 15 to 50 wt. %; for aqueous effect base lacquers that value is, for example, preferably from 15 to 30 wt. %, and for aqueous single coloured base lacquers it is preferably higher, for example from 20 to 45 wt. %. The solids content by weight of the aqueous base lacquer coating compositions is formed from the sum of the solids contents of the binders, pigments, fillers, crosslinking agents, and non-volatile additives conventionally employed in lacquers. The weight ratio of pigment to binder in the aqueous base lacquer coating composition is, for example, from 0.05:1 to 3:1; for aqueous effect base lacquers it is, for example, preferably from 0.1:1 to 0.6:1, and for aqueous single coloured base lacquers it is preferably higher, for example from 0.1:1 to 2.5:1, in each case based on the weight of the solids. When calculating the ratio of pigment to binder, the sum of the amounts by weight of colour-giving pigments, effect pigments and fillers is related to the sum of the amounts by weight of binder solids, paste resin solids and crosslinking agent solids in the finished aqueous base lacquer.
In addition to the air-exposure conditions for the base lacquer layer which are to be observed according to the invention and which are explained hereinbelow, it is essential to the invention that the aqueous base lacquer coating compositions used in the process according to the invention have a high solid value of from 40 to 70%. In the case of aqueous effect base lacquers, that value is, for example, from 40 to 60%, and in the case of aqueous single coloured base lacquers it is, for example, from 40 to 70%. The high solid value of the aqueous base lacquer coating compositions is calculated according to the formula
solids
content
in
wt
.
%
×
100
%
solids
content
in
wt
.
%
+
content
of
organic
solvents
in
wt
.
%
The solvent content and the solids content are therefore to be so matched that a high solid value of from 40 to 70% is obtained according to the above formula.
The aqueous base lacquers used in the process according to the invention contain conventional ionically or non-ionically stabilised binder systems. Such systems are preferably anionically and/or non-ionically stabilised. Anionic stabilisation is preferably achieved by means of at least partially neutralised carboxyl groups in the binder, while non-ionic stabilisation is preferably achieved by means of lateral or terminal polyethylene oxide units in the binder. The aq
Minko Peter
Schmidt Holger
Siever Ludwig
Beck Shrive P.
E. I. du Pont de Nemours and Company
Fletcher, III William Phillip
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