Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-05-09
2003-04-01
Barr, Michael (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S514000, C427S407100, C427S409000
Reexamination Certificate
active
06541078
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for coating substrates in which one or more coating layers are applied and cured by near infra red (NIR) radiation. The process can be used in the application of automotive and industrial coatings.
DESCRIPTION OF RELATED ART
It is known to use coating compositions curable by UV (ultraviolet light) radiation in automotive and industrial coating. Coating compositions based on binders capable of free-radical polymerization are used in particular. These coating compositions generally contain photoinitiators. Coating compositions curable by UV radiation are described, for example, in DE-A-198 18 735 and U.S. Pat. No. 5,932,282.
A known deficiency of UV-curable coatings is that, particularly in the case of three-dimensional objects to be coated, insufficient curing takes place in shadow regions, that is, in areas which are unexposed or underexposed to UV radiation. Attempts have been made to remedy this problem by the use of so-called dual cure systems, that is, binder systems that cure both by UV irradiation and by means of a further cross-linking mechanism. Depending on the binder system, however, two process steps are sometimes required with these systems for complete curing, namely, UV irradiation and in addition a thermal curing phase. Examples of dual cure systems are described in WO-A-98/00456 and DE-A-197 09 560.
Another general deficiency of UV curable coating compositions is that yellowing of the resulting coating layers occurs after UV irradiation of coating compositions containing photoinitiators. The latter precludes the use of UV curable coating compositions particularly as a clear coat or a pale, e.g., white-pigmented top coat.
UV irradiation and the use of photoinitiators may be avoided by thermally initiated curing of the binders capable of free-radical polymerization. Coating compositions based on binders capable of free-radical polymerization that are cured thermally in the usual way in combination with thermal radical initiators have the disadvantage, however, that insufficient curing of the coating layers, if any, can be obtained if radical scavengers are used, e.g., light stabilizers (light protecting agents) based on HALS products (HALS=hindered amine light stabilizer). The HALS products, acting as radical scavengers, impede free-radical polymerization of the binders. Light stabilizers are a necessary constituent in the production of suitable coating compositions for external applications and for certain interior applications where protection from the influence of light is required.
Moreover, it is known to dry and to cure coating layers with NIR radiation in the context of multi-layer coating. Such processes are described, for example, in DE-A-199 13 446 and DE-A-199 13 442. Binders used here are one-component, physically or oxidatively drying binder systems and two-component binder systems, for example, based on a hydroxyl and a polyisocyanate component or an epoxide and a polyamine component. It is also possible to use binders curable by high-energy radiation, preferably binders capable of free-radical polymerization. In the latter case, photoinitiators are contained in the coating compositions and irradiation with UV rays takes place in addition to NIR irradiation.
This invention provides a process for coating substrates that makes it possible to obtain, using coating compositions based on binder systems capable of free-radical polymerization, low-yellowing, fully curable coatings suitable for external applications. The coatings obtained are capable of curing rapidly with NIR radiation and have sufficient hardness and a good surface quality.
SUMMARY OF THE INVENTION
This invention is directed to a process for coating substrates by applying at least one coating composition to an optionally precoated substrate and then curing the coating layer(s) thus obtained, wherein at least one of the coating layers is produced from a coating composition that contains a binder system having olefinic double bonds capable of free-radical polymerization and having reactive functional groups within the meaning of addition and/or condensation reactions, the resin solids of the coating composition having a C═C-equivalent weight from 300 to 10,000, preferably from 300 to 8,000, and curing of this (these) coating layer(s) is carried out by irradiation with NIR radiation in the wave length range 760-1500 nm.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Surprisingly, it was found that, as a result of the irradiation and curing according to the invention of coating compositions based on binder systems capable of free-radical polymerization and cross-linkable by addition and/or condensation reactions with NIR radiation, coatings are obtained that cure rapidly and completely, even in the presence of radical scavengers, e.g., light stabilizers based on HALS products. Complete curing of coating compositions based on binders capable of free-radical polymerization in combination with, e.g., light stabilizers based on HALS products could not be obtained hitherto with conventional thermal curing methods, e.g., in ovens by means of convection or induction drying.
The NIR radiation used according to the invention is short-wave infra-red radiation in the wave length range from about 760 to about 1500 nm, preferably 760 to 1200 nm. Radiation sources for NIR radiation include, for example, NIR radiation emitters that are able to emit radiation as a flat, linear or point source. NIR radiation emitters of this kind are available commercially (for example, from Adphos). These include, for example, high performance halogen radiation emitters with an intensity (radiation output per unit area) of generally more than 10 kW/m
2
to, for example, 15 MW/m
2
, preferably from 100 kW/m
2
to 800 kW/m
2
. For example, the radiation emitters reach a radiation emitter surface temperature (coil filament temperature) of more than 2000° K, preferably, more than 2900° K, e.g., a temperature from 2000 to 3500° K. Suitable radiation emitters have, for example, an emission spectrum with a maximum between 750 and 1200 nm.
The coating compositions which may be used in the process according to the invention can be cross-linked by chemical means both by free-radical polymerization of olefinic double bonds and by addition and/or condensation reactions of appropriate functional groups.
The olefinic double bonds capable of free-radical polymerization and the functional groups that react together in the manner of addition and/or condensation reactions may be contained, in principle, in the same binder and/or in separate binders.
The functional groups that react together in the manner of addition and/or condensation reactions will be referred to hereinafter as further reactive functional groups. They are reactive functional groups A and reactive functional groups B complementary to the latter. Reactive functional groups A and reactive functional groups B may be present in the same binder and/or in separate binders.
The following variants of the arrangement and combination of the olefinic double bonds and of further reactive functional groups A and B in the binders are possible, in principle:
1. Binders having olefinic double bonds+separate binders containing further reactive functional groups A and B in any combination;
2. Binders having olefinic double bonds and further reactive functional groups A+separate binders having olefinic double bonds and further functional groups B and/or separate binders having olefinic double bonds and further reactive functional groups A and B;
3. Binders having olefinic double bonds and further reactive functional groups A+ separate binders having further reactive functional groups B and optionally further reactive functional groups A;
4. Binders having olefinic double bonds and further reactive functional groups A and B.
In above variant 1, the further reactive functional groups A and B may be present in the same binder and/or in separate binders. The binders containing further reactive functional groups A and/or
Reis Oliver
Rekowski Volker
Wulf Martin
Barr Michael
Benjamin Steven C.
Blanton Rebecca A.
E. I. du Pont de Nemours and Company
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