Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2001-04-24
2003-07-08
Beck, Shrive P. (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S409000, C427S419200
Reexamination Certificate
active
06589604
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the production of multi-layer coatings from a thin primer surfacer substitute layer, a base coat layer and a clear top coat layer.
BACKGROUND OF THE INVENTION
Modem automotive coatings comprise mostly an electrodeposition coat primer, a primer surfacer layer and a color- and/or special effect-imparting base coat/clear coat top coating.
Processes are known, for example, from WO 96/13537 and U.S. Pat. No. 5,976,343 in which the primer surfacer layer normally to be applied in a relatively high layer thickness is replaced by so-called primer surfacer substitute layers which may be applied in dry layer thicknesses of, for example, only 10 &mgr;m to 25 &mgr;m.
WO 00/71596 discloses clear coats which contain a combination of anti-sag urea compounds and silica. The urea compound content is given therein as 0.1 to 5 wt-%, preferably 0.2 to 2.5 wt-%, most preferably 0.6 to 1.8 wt-%, and the silica content is given as 0.1 to 10 wt-%, preferably 0.2 to 2.5 wt-%, most preferably 0.6 to 2.0 wt-%, in each case based on the total solids content.
There is a desire to find an improved process for the production of multi-layer coatings from a thin primer surfacer substitute layer, base coat layer and clear coat layer. It should be possible, with the process, to apply a perfectly satisfactory clear coat layer in terms of its optical surface quality from a clear coat with good sagging resistance and at the same time a low clear coat wetting limit.
Surprisingly, this can be achieved when, in such a process for the production of the clear coat layer, a clear coat is used having a very low content of at least one anti-sag urea compound and along with a very low content of highly dispersed silica.
SUMMARY OF THE INVENTION
The invention relates to a process for the production of multi-layer coatings in which a substrate is provided with a 10 &mgr;m to 25 &mgr;m thick primer surfacer substitute layer, a base coat layer determining the color shade of the multi-layer coating is applied without baking or after baking the primer surfacer substitute layer, and a clear coat layer is applied thereto and cured, wherein a solvent-containing clear coat containing from 0.1 to 0.3 wt-% of at least one anti-sag urea compound and 0.1 to 0.4 wt-% of highly dispersed silica, in each case based on the clear coat solids, is used to prepare the clear coat layer.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The substrates coated with multi-layer coatings in the process according to the invention are preferably metal substrates, particularly automotive bodies or parts thereof that usually have a baked electrodeposition coat primer layer.
The primer surfacer substitute layer is applied to these substrates by spraying to form a dry layer having a thickness from 10 &mgr;m to 25 &mgr;m, preferably 15 &mgr;m to 23 &mgr;m. It may be overcoated in the unbaked state with the base coat layer but it is preferably baked initially at temperatures from, for example, 120° C. to 160° C.
In order to prepare the primer surfacer substitute layer, conventional waterborne or solvent-based coating agents may be used, for example, conventional primer surfacers known to the skilled person, or, in particular, coating agents conventionally used for this purpose and likewise known to the skilled person. Examples include the coating agents disclosed in WO 96/13537. In particular, the primer surfacer substitute layer may also be applied, for example, in the form of a first base coat layer, from a coating agent that may be produced from the actual base coat determining the color shade of the multi-layer coating, by adding suitable components, for example, a filler paste or a binder, as known, for example, from U.S. Pat. No. 5,968,655 or 5,976,343.
A base coat layer determining the color shade of the multi-layer coating is applied by spraying to the substrate provided with the baked or unbaked primer surfacer substitute layer. This base coat layer is a conventional color-and/or special effect-imparting waterborne or solvent-based base coat known to the skilled person and applied in a dry layer thickness dependent on the color shade, for example, from 8 &mgr;m to 30 &mgr;m.
The base coat layer may be baked before the subsequent application of the clear coat, but the clear coat is applied to the base coat layer preferably by the known wet-in-wet method, for example, after a brief flash-off phase for the base coat, e.g. at 20° C. to 80° C. The clear coat is applied by spraying in a dry layer thickness from, generally, 30 &mgr;m to 50 &mgr;m and optionally flashed off briefly. The substrate is then brought to the curing process, particularly a baking process in which the clear coat layer is baked together with the base coat layer at elevated temperatures, for example, from 80° C. to 160° C.
The clear coats used in the process according to the invention are liquid clear coats based on organic solvents. They contain, as constituents forming the resin solids, one or more conventional binders, optionally in addition one or more reactive thinners (compounds that are chemically incorporated in the clear coat film during curing) and, if the binders are not self-cross-linking, one or more cross-linking agents.
The clear coat cross-linking system that constitutes the resin solids may be a cross-linking system for clear coats that can be cured by free-radical polymerization and/or preferably by addition and/or condensation reactions, of the kind that may be used in the production of base coat/clear coat two-layer coatings. Thus, the clear coats may be cured by actinic radiation and/or by heating.
The clear coats are preferably externally cross-linking systems with a stoichiometric ratio adjusted to the desired degree of cross-linking of, generally, 50 to 90 wt-% binders, 0 to 20 wt-% reactive thinners and 10 to 50 wt-% cross-linking agents, the sum being 100 wt-%.
Neither the binders nor the reactive thinners are subject to any restriction, in principle. Examples of suitable film-forming binders include polyester, polyurethane and/or (meth)acrylic copolymer resins. There is no restriction on the choice of cross-linking agents, it depends on the functionality of the binders, i.e. the cross-linking agents are selected such that they have a reactive functionality that complements the functionality of the binders.
Clear coats containing cross-linking systems capable of free-radical polymerization are clear coats that cure by thermal and/or photochemical means.
Apart from thermal radical initiators and/or photoinitiators, they contain binders having olefinically unsaturated groups capable of free-radical polymerization and optionally, further components capable of free-radical copolymerization. Examples include polymers or oligomers with olefinic double bonds capable of free-radical polymerization, particularly (meth)acryloyl groups, such as, (meth)acrylic-functional (meth)acrylic copolymers, epoxy resin (meth)acrylates, polyester (meth)acrylates, polyurethane (meth)acrylates, unsaturated polyesters or unsaturated polyurethanes, for example, with number-average molecular masses in the range from 500 to 10,000.
Examples of reactive thinners include (meth)acrylic acid and esters thereof, maleic acid and half esters thereof, vinyl esters, vinyl ethers, ethylene and propylene glycol di(meth)acrylate, butane diol di(meth)acrylate, vinyl (meth)acrylate, allyl (meth)acrylate, glycerol tri-, di- and mono(meth)acrylate, trimethylol propane tri-, di- and mono(meth)acrylate, styrene, vinyltoluene, divinylbenzene, pentaerythritol tri- and tetra(meth)acrylate, di- and tripropylene glycol di(meth)acrylate, and hexane diol di(meth)acrylate.
Examples of addition reactions suitable for cross-linking clear coat cross-linking systems that can be cured by addition reactions include the addition of an epoxy group to a carboxyl group, a hydroxyl and/or an amino group to an isocyanate group, an amino group and/or CH-acidic group to an alpha,beta-unsaturated carbonyl group, particularly (meth)acryloyl group, and the addition of an amino g
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