Coating apparatus – With means to apply electrical and/or radiant energy to work...
Reexamination Certificate
2001-10-15
2004-05-18
Crispino, Richard (Department: 1734)
Coating apparatus
With means to apply electrical and/or radiant energy to work...
C118S300000, C118S302000, C427S492000, C427S512000, C427S596000
Reexamination Certificate
active
06736898
ABSTRACT:
The present invention relates to a method and a means of producing cured, especially radiation-cured, coating films on a substrate surface.
Coating films are nowadays produced, inter alia, with the aid of the method of radiation curing. In radiation curing, a readily processible mixture of reactive starting materials and additives is converted by exposure into a three-dimensional, mechanically stable polymer network. In this procedure, the reactive coating formulation is first applied to the corresponding substrate and in a second step is crosslinked by means of optical exposure, preferably with a UV exposure unit, or by means of electron beam curing. Examples of this are the optically initiated (using photoinitiators) polymerizations of low-viscosity coating formulations of reactive monomers, oligomers and prepolymers, an example being free-radical acrylate polymerization or cationic vinyl ether or epoxy polymerization, or the optical crosslinking of linear polymers having reactive side chains. Use is also made of polymers based on (meth)acrylates, (meth)acrylamides, maleimide-vinyl ethers, hydrogen abstraction systems, unsaturated polyesters, and acid-curable resins. Typical applications are coatings of paper, skis, furniture, floorings, metals, plastics, and adhesives.
In the case of a radiation-curable coating system, such as, for example, the UV coating or the electron beam curing of three-dimensional surfaces of complex configuration, such as, for example, that of motor vehicles, exposure must take place uniformly in order to avoid uncured areas remaining at critical points such as, for example, on edges or on internal surfaces. Residual uncured areas can result, among other things, in instances of sticking, in the emission of low molecular mass compounds, in some cases associated with an odor nuisance and/or a health hazard, and in deficient gloss and inadequate protection by the coating. This often necessitates expensive reworking, or even the disposal of valuable substrates, which involves high costs. In order to be able to ensure uniform exposure of substrates of large surface area, it has hitherto been necessary to use large-area radiation sources, especially UV lamps, in combination with 3D robotics. This requires high levels of investment in customized exposure units with correspondingly high operating costs and slow cycle times and, possibly, expensive thermal aftertreatment, such as in the case, for example, of dual-cure formulations. A further problem of conventional coating methods occurs when using pigmented coating formulations or coating formulations which have been provided with light stabilizer additives. Such formulations are used primarily for exterior applications. In both of these cases there may be interactions with the light irradiated for exposure: for example, there may be absorption or scattering of UV light. This has the consequence, in turn, that, owing to the “shadow effect” of the light required for activation, the activation of the crosslinking reaction by the photoinitiator system is possibly inadequate. It is therefore very difficult to obtain homogeneous through-curing, especially in relatively deep coating films.
It is an object of the present invention to provide a method and means with the aid of which a uniform coating film can be produced simply and fairly rapidly without the occurrence of the problems set out above.
We have found that this object is achieved by the method of the invention as claimed in claim
1
and the corresponding means of the invention as claimed in claim
10
. Advantageous developments are specified in the subclaims.
The method of the invention constitutes a method of producing at least one coating film, preferably a cured coating film, on at least one area of a substrate surface, said method comprising at least the following steps in the following order:
a) initiating at least one crosslinking reaction in at least one reactive coating formulation;
b) applying, preferably homogeneously, said at least one reactive coating formulation before the onset of said at least one crosslinking reaction on said at least one area of said substrate surface.
Initiating at least one crosslinking reaction here means that, although at this point in time the crosslinking reaction is not yet proceeding, a state is created in said at least one reactive coating formulation on the basis of which, after a certain period of time, the crosslinking reaction will proceed.
The method of the invention is notable, accordingly, in particular for the fact that said at least one crosslinking reaction, in contrast to coating methods known from the prior art, is now initiated even prior to the application of the coating formulation to the corresponding substrate surface. This permits homogeneous initiation of the crosslinking reaction and so avoids non-uniform crosslinking of, for example, three-dimensional substrates of complex shape, with which it is often necessary in the case of conventional coating methods to expend considerable effort in order to treat in fact every area of the substrate surface equally, in order thereby to obtain a uniform coating film.
In one preferred embodiment of the method of the invention, in step a) the crosslinking reaction is initiated optically in said at least one reactive coating formulation. This preferably takes place by means of UV exposure or electron beam irradiation of said at least one reactive coating formulation. In a reactive coating formulation which can be used in this embodiment it must be possible to activate a crosslinking reaction optically, so that from a coating formulation of low viscosity it is possible for a highly viscous, mechanically stable coating film to form.
In the method of the invention, said at least one reactive coating formulation preferably comprises at least one photoinitiator. Said at least one photoinitiator is able to interact with appropriately irradiated light in a manner such that it is made able to initiate the crosslinking reaction in said at least one coating formulation. Examples of this are the optically initiated (using photoinitiators) polymerizations of low-viscosity coating formulations of reactive monomers, oligomers and prepolymers, or the optical crosslinking of linear polymers having reactive side chains. In this case it is possible, inter alia, to mention free-radical acrylate polymerization and cationic vinyl ether or epoxy polymerization. The coating formulation, which at this point is still of low viscosity, is irradiated with light, preferably with UV light, prior to its application to the substrate surface in question. In this case it is relatively simple to achieve homogeneous UV exposure. For example, homogeneous flooding with UV light can be performed, for example, at the spray nozzle for the reactive coating formulation, or in the corresponding feed line, by carrying out exposure from different sides or configuring the feed line as a UV photoconductor. Here too it is advantageous that with these small dimensions and geometries it is possible to operate not only with conventional UV lamps but also with UV lasers. The latter are used with preference owing to their ease of beam guidance and the possibility of tailoring the laser wavelength to the absorption of the photoinitiator system contained in the reactive coating formulation, as described, for example, in J. -P. Fouassier,
Photoinitiation, Photopolymerization and Photocuring
, Hanser Publishers, Munich, 1995.
In a further preferred embodiment of the process of the invention, in step a) the crosslinking reaction in said at least one reactive coating formulation is initiated thermally. This means that in this case the crosslinking reaction within said at least one reactive coating formulation is initiated through the establishment of a certain temperature. Here again, as in the case of optical initiation, it is relatively easy to bring the coating formulation not yet applied to the corresponding substrate to a uniform temperature required to initiate the crosslinking reaction, something which is
Beck Erich
Meisenburg Uwe
Schrof Wolfgang
Schwalm Reinhold
BASF - Aktiengesellschaft
Crispino Richard
Keil & Weinkauf
Lazor Michelle Acevedo
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