Coating articles using radiation-curable powder coatings

Details Coating articles using radiation-curable powder coatings Coating articles using radiation-curable powder coatings

1999-01-08

2000-08-22

Cameron, Erma

Coating processes

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

Polymerization of coating utilizing direct application of...

427519, 427520, 427195, B05D 306

Patent

active

061069051

DESCRIPTION:

BRIEF SUMMARY
This is a National Stage of PCT/EP 96/05864, filed Dec. 30, 1996.
The present invention relates to a method of coating articles with powder coatings which contain polymers and which are applied to the surface of the substrate to be coated, are melted and are crosslinked by UV radiation.
The term radiation-curable powder coatings generally refers to finely particulate, organic polymer powders which do not stick together during storage (ie. are blocking-resistant), are in general electrostatically chargeable, and which can be applied by spraying to substrates in an electric field or applied to the substrate to be coated by other methods, eg. by fluidized-bed sintering. On the substrates the powders are melted by heating to convert them into even coatings which are then crosslinked by a photochemically initiated reaction.
The powder coatings in use include both partially crosslinked coating materials and those which can be melted while exhibiting purely thermoplastic behavior. For applications requiring high degrees of weather resistance, scratch resistance, corrosion prevention, elasticity and gloss retention, for example in automotive finishing, only crosslinked systems are suitable; purely thermoplastic sintered powders are unable to satisfy these requirements. Crosslinking of the powder coatings currently employed is almost exclusively by thermally initiated reactions of the polymeric film-forming binders with themselves or with an added crosslinking agent. Examples of such thermally activated crosslinking reactions are the reaction of copolymerized epoxide groups with solid dicarboxylic acid anhydrides such as phthalic anhydride (cf. eg. U.S. Pat. No. 3,919,347) or of copolymerized hydroxyl groups with blocked isocyanates.
A major problem with powder coatings is their poor flow in comparison to liquid systems. This poorer comes about since it is not possible with thermally activated systems to make a clean division between the melting process and the crosslinking reaction; a crosslinking reaction resulting in an increase in viscosity occurs even during melting, so that the low viscosity which is desirable for optimum flow is not achieved or, if achieved, is not maintained for a sufficient period. This problem cannot be solved by the use of polymers having a low melting point, since powder coatings produced from such polymers stick together during storage, ie. they do not have an adequate blocking resistance.
There have therefore been attempts to separate the melting process from the crosslinking reaction, by using a reaction which is not thermally activated for the crosslinking process. For this purpose, binders having mainly acrylic double bonds have been proposed in combination with photoinitiators. The intention is first to melt these mixtures at a sufficiently high temperature and then to carry out crosslinking with UV light.
However, this method is also unable to separate the thermal crosslinking from the melting process to a sufficient extent, since the acrylic double bonds too are able to undergo thermal polymerization. Further problems result from the possibility of thermally initiated polymerization of the double bonds during preparation of the powders, for which it is generally necessary to employ melt processes, ie. relatively high temperatures.
EP-A-346 734, EP-A-367 054, EP-A-377 199, EP-A-395 990, EP-A-417 564, EP-A-448 741, EP-A-458 164 and EP-A-486 897 disclose specific aromatic ketones which can be used for the preparation of UV-crosslinking adhesives, especially contact adhesives. These documents do not disclose the transfer of this principle to the separation of the melting and crosslinking process with regard to powder coatings.
DE-A-24 36 186 describes the preparation of UV-crosslinkable powder coatings from polymers containing from 0.5 to 3.5 polymerizable C--C double bonds per 1000 molecular weight units. These polymers are converted to powder, mixed if desired with from 0.1% to 0.5% of photoinitiators, applied to the substrate to be coated, melted, and then crosslinked by the acti

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