Process for coating

Details Process for coating Process for coating

2001-07-31

2003-06-24

Cameron, Erma (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Heating or drying

C427S388100, C427S407100

Reexamination Certificate

active

06582770

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for coating with coating agents that cure by free-radical polymerization of olefinic double bonds.
BACKGROUND OF THE INVENTION
The use of coating agents curable by ultraviolet light (UV) irradiation and based on binders capable of free-radical polymerization is known in automotive and industrial coating; cf. U.S. Pat. No. 5,425,970 and U.S. Pat. No. 5,486,384. Coatings prepared in this way are characterized by outstanding chemical and mar resistance, properties that are particularly desirable for exterior top coat, clear coat or sealing layers.
Coating agents used for the preparation of coating layers for exterior or high-grade interior applications contain light stabilizers based on radical scavengers from the group of sterically hindered piperidines (so-called HALS compounds, HALS=hindered amine light stabilizer), generally in combination with UV absorbers, in order to guarantee a long service life of coating layers produced therefrom. This also applies to coating agents capable of free-radical polymerization under UV irradiation.
Whereas the curing of coating agents capable of free-radical polymerization and containing HALS light stabilizers takes place without problems under UV irradiation, curing by thermally induced free-radical polymerization takes place only incompletely, if at all. Thermal curing is possible if HALS light stabilizers are dispensed with in the coating agent, but leads to a coating with only a limited service life under the action of UV rays, as is the case, in particular, in exterior applications.
If there were not the problem of the limited service life, it would be inherently desirable to use coating agents curing thermally by free-radical polymerization of olefinic double bonds in coating, particularly industrial coating, for example, automotive coating, because of the outstanding properties of the coating layers produced therewith, and the possibility of being able to use these in conventional coating plants equipped for the application of thermally curing coating agents. In short, it would be possible to provide substrates, particularly industrially produced substrates, such as, automotive bodies, with coatings that have the outstanding range of technological properties of weather-resistant coatings chemically cross-linked under UV irradiation by free-radical polymerization of olefinic double bonds without having to use UV curing technology.
3,3,5,5-polysubstituted morpholin-2-one derivatives are known from U.S. Pat. No. 6,140,326 as light stabilizers, inter alia, in coating agents. The coating agents may be coating agents capable of free-radical polymerization under UV irradiation.
Surprisingly, coating layers with a long service life even when exposed to UV rays may be prepared if they are applied from coating agents capable of free-radical polymerization and containing certain sterically hindered morpholin-2-one derivatives as light stabilizers, and are thermally cured.
SUMMARY OF THE INVENTION
The invention provides a process for the preparation of a coating layer comprising the steps:
(a) applying a coating layer to a substrate from a coating agent of which the resin solids comprise a binder system curable by free-radical polymerization of olefinic double bonds and contains from 0.1 to 4 wt-%, based on resin solids, of a morpholin-2-one derivative sterically hindered by 3,3,5,5-polysubstitution as a light stabilizer; and
(b) thermal curing of the applied coating layer.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The coating agent applied in step a) of the process according to the invention is also referred to hereinafter as “coating agent” for brevity. The resin solids of the coating agent are composed of i) a binder system chemically cross-linkable by free-radical polymerization of olefinic double bonds (hereinafter also referred to as “binder system capable of free-radical polymerization” for brevity) and optionally ii) components not capable of free-radical polymerization. The resin solids have an olefinic double bond content corresponding to a C═C-equivalent weight of from 300 to 10,000, preferably from 350 to 3,000, more particularly from 400 to 1,500.
The binder system capable of free-radical polymerization is composed of one or more binders capable of free-radical polymerization and optionally one or more reactive thinners capable of free-radical polymerization. The optionally contained components not capable of free-radical polymerization are one or more binders not capable of free-radical polymerization, one or more reactive thinners not capable of free-radical polymerization and/or one or more hardeners not capable of free-radical polymerization.
The binder system capable of free-radical polymerization is a binder system of the kind that is also conventionally used in coating agents chemically cross-linkable under UV irradiation by free-radical polymerization of olefinic double bonds. Such binder systems are known to a person skilled in the art. They contain one or more binders with olefinic double bonds capable of free-radical polymerization. Suitable binders with olefinic double bonds capable of free-radical polymerization include, for example, all the binders known to the skilled person that can be cross-linked by free-radical polymerization of olefinic double bonds. These binders are prepolymers, such as, polymers and oligomers which contain, per molecule, one or more, preferably on average 2 to 20, particularly preferably 3 to 10 olefinic double bonds capable of free-radical polymerization.
The polymerizable double bonds may be present, for example, in the form of (meth)acryloyl, vinyl, allyl, maleinate and/or fumarate groups. The double bonds capable of free-radical polymerization are particularly preferably present in the form of (meth)acryloyl, vinyl, and/or maleinate groups.
Examples of prepolymers or oligomers include (meth)acryloyl-functional (meth)acrylic copolymers, polyurethane (meth)acrylates, polyester (meth)acrylates, unsaturated polyesters, polyether (meth)acrylates, silicone (meth)acrylates and epoxy resin (meth)acrylates having number-average molecular masses from, for example, 500 to 10,000, preferably 500 to 5,000.
The binder system chemically cross-linkable by free-radical polymerization of olefinic double bonds may contain one or more reactive thinners with olefinic double bonds capable of free-radical polymerization. The reactive thinners are low molecular weight compounds with a molecular mass of below 500. The reactive thinners may be mono- di- or polyunsaturated. Examples of monounsaturated reactive thinners include (meth)acrylic acid and the esters thereof, maleic acid and the half esters thereof, vinyl acetate, vinyl ethers, styrene, vinyl toluene. Examples of diunsaturated reactive thinners include di(meth)acrylates, such as, alkylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, butane 1,3-diol di(meth)acrylate, vinyl (meth)acrylate, allyl (meth)acrylate, divinyl benzene, dipropylene glycol di(meth)acrylate, hexane diol di(meth)acrylate. Examples of polyunsaturated reactive thinners include glycerol tri(meth)acrylate, trimethylol propane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate.
Both the binders capable of free-radical polymerization and the reactive thinners capable of free-radical polymerization may contain, in addition to the olefinic double bonds, one or more further, identical or different functional groups. Examples of functional groups include hydroxyl, isocyanate (optionally blocked), N-methylol, N-methylolether, ester, carbamate, epoxy, amino (optionally blocked), acetoacetyl, alkoxysilyl and carboxyl groups.
This will be explained on the basis of the example of binders and reactive thinners capable of free-radical polymerization, in each case having hydroxyl groups as functional groups present in addition to the olefinic double bonds. For example, polyurethane resins with (meth)acryloyl and hydroxyl groups are binders capable of free-radical polymerization having hydroxyl groups as further funct

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