Process for coating substrates

Details Process for coating substrates Process for coating substrates

2001-02-12

2002-08-13

Pianalto, Bernard (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S140000, C427S261000, C427S287000, C427S388100, C427S388200, C427S407100, C427S407300, C427S510000, C427S557000, C427S559000

Reexamination Certificate

active

06432490

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for coating substrates, in which one or more layers of coating that are applied are cured by means of NIR (near infra red) radiation. The process can be used in the application of automotive and industrial coatings.
DESCRIPTION OF RELATED ART
It is known that coating compositions cured by UV (ultraviolet light) radiation can be used as automotive and industrial coatings. These coating compositions contain film forming binders that are capable of radical polymerization and generally contain photoinitiators. Coating compositions that can be cured by means of UV radiation are described in DE-A-198 18 735 and in U.S. Pat. No. 5,932,282.
A known deficiency of coating compositions that can be cured by UV radiation is that poor curing is obtained in shadow areas, i.e. the areas which are not reached by the UV radiation. Also, yellowing of the coating occurs after UV irradiation when the coating compositions contain photoinitiators. The latter is a particularly disadvantage when these coating compositions are used as clear coats or lightly pigmented coats and especially, when they are used as white top coats.
The use of UV irradiation and photoinitiators in coating compositions having binders that are capable of radical polymerization can be avoided by a thermally initiating curing of the binders. Coating compositions containing binders that are capable of radical polymerization that are thermally cured in a conventional manner in combination with thermal radical initiators, present a problem when radical scavengers are used. The radical scavengers, e.g. light protecting agents based on HALS-products (HALS=hindered amine light stabilizer), significantly lower the level of curing of the layers of coating. The HALS-products impede the radical polymerization of the binders due to their function as radical scavengers. Light protecting agents, however, are a necessary component of coating compositions that are to be used for exterior and also for interior use, where protection against the influences of light is necessary.
It is further known to dry or to cure multilayer coatings with NIR radiation. Such processes are disclosed in DE-A-199 13 446 and in DE-A-199 13 442. One-component binder systems that dry physically or in an oxidative manner and two-component binder systems that are, for example, based on a hydroxyl and a polyisocyanate component or an epoxide and a polyamine component are used as the binders in these cases. Binders that can be cured by means of high-energy radiation, preferably binders that are capable of radical polymerization, can also be used. In the latter case, the coating compositions contain photoinitiators and UV radiation in addition to the NIR irradiation is used to cure the coating compositions.
This invention provides a process for coating substrates and forms coatings that do not exhibit yellowing or only slight yellowing, are completely cured and are suitable for exterior use, by using coating compositions based on binder systems that are capable of radical polymerization. The coatings obtained are capable of curing rapidly with NIR radiation and form coatings that are sufficiently hard and have a good quality surface.
SUMMARY OF THE INVENTION
The process of this invention applies at least one coating composition to an optionally precoated substrate and then cures the thus obtained coating layer(s), whereby at least one of the coating layers is prepared from a coating composition that can be chemically cross-linked solely by radical polymerization of olefinic double bonds, the resin solid of the composition has a C═C equivalent weight from 100 to 10,000, preferably from 100 to 5,000, and the curing of these coating layer(s) is carried out with NIR radiation in the wave length range from 760-1500 nm.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Surprisingly, it has been found that by irradiating according to the invention and by curing the coating compositions based on binders that are capable of radical polymerization with NIR radiation, coatings are formed that exhibit slight or no yellowing when cured rapidly and completely even in the presence of radical scavengers, e.g. light protecting agents based on HALS-products (hindered amine light stabilizer products). It has not been possible hitherto to obtain complete curing of coating compositions based on binders that are capable of radical polymerization in combination with light protecting agents based on HALS-products, by using conventional thermal curing methods, for example, in ovens by means of convection or induction drying.
The NIR radiation used according to the invention is a short wave infrared radiation in the wavelength range from about 760 to about 1500 nm, preferably 760 to 1200 nm. Radiation sources for NIR radiation are, for example, conventional NIR radiators that emit a beam with a planar, linear or spot focus. Such NIR radiators are commercially available (for example, from the Adphos Company). These high-performance halogen radiators have an intensity (radiation capacity per surface) from in general over 10 kW/m
2
to 15 MW/m
2
, preferably between 100 kW/m
2
and 800 kW m
2
. The radiators reach a temperature at the surface of the radiator (filament temperature) of more than 2000 K, preferably more than 2900 K and typically are in a temperature range between 2000 and 3500 K. Suitable radiators have an emission spectrum with a maximum between 760 and 1200 nm.
The coating composition used in the process of this invention contains a binder that can be chemically cross-linked solely by radical polymerization of olefinic double bonds of the binder and optionally, contains a reactive thinner that is capable of radical polymerization and/or contains a physically drying binder.
The resin solid of the coating composition consists of binders that can be chemically cross-linked by radical polymerization of the olefinic double bonds, of optional reactive thinners and optional physically drying binders that can be incorporated therein. The resin solid has a C═C equivalent weight from 100 to 10,000 and preferably from 100 to 5,000.
The coating compositions suitable for use in the process of this invention contain one or more binders having olefinic double bonds capable of radical polymerization and are known to persons skilled in the art. These binders are prepolymers, such as polymers and oligomers, containing one or more, preferably, on average 2 to 20, and more preferably, 3-10 olefinic double bonds capable of radical polymerization per molecule.
The double bonds that can be polymerized, can be present in form of (meth)acryloyl, vinyl, allyl, maleinate and/or fumarate groups.
(Meth)acryloyl and (meth)acryl, respectively, shall mean here and in the following acryloyl and/or methacryloyl and acryl and/or methacryl, respectively.
Examples of useful prepolymers or oligomers are (meth)acryloyl-functional poly(meth)acrylates, polyurethane (meth)acrylates, polyester (meth)acrylates, polyether (meth)acrylates, silicone (meth)acrylates, epoxy(meth)acrylates, amino(meth)acrylates, melamine (meth)acrylates, unsaturated polyesters and unsaturated polyethers. The number average molecular mass Mn of these compounds can be, for example, from 500 to 10,000 g/mol, preferably, from 500 to 5000 g/mol. The binders can be used individually or in a mixture.
The prepolymers can be used in combination with reactive thinners, that are low molecular compounds capable of radical polymerization having a molecular mass below 500 g/mol. The reactive thinners can be mono-unsaturated, di-unsaturated or poly-unsaturated. Examples of mono-unsaturated reactive thinners are: (meth)acrylic acid and esters thereof, maleic acid and half esters thereof, vinyl acetate, vinyl ethers, substituted vinylureas, styrene, vinyl toluene. Examples of di-unsaturated reactive thinners are: di(meth)acrylates, such as, alkylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, vinyl (meth)acrylate, allyl (meth)acrylate, divinylbenzene,

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