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Radiation-curable, water-borne polyurethane dispersions combine the best properties from various welldeveloped coating techniques. For example, the radiation curing of water-borne UV coatings takes seconds and is of similar efficiency to that of solvent-borne or 100% UV coatings. It is generally not necessary to use expensive solvents or irritating UV monomers to reduce the viscosity of water-borne UV coating materials. The viscosity of water-borne UV binders is independent of the molecular weight of the polymer and can be adjusted to be virtually as low as that of water. Correspondingly, UV coatings enable maximum application freedom from rolling via curtain coating to spraying. Water-borne UV dispersions are solvent-free and therefore meet the strict requirements of VOC legislation. Water-borne, radiation-curable coating materials are preferably based on high-molecular-weight polyurethanes, which lead to physically drying coatings even before radiation curing. Advantages of such high-molecular-weight dispersions are lower susceptibility to dust, better resistance properties and lower shrinkage during radiation curing. In addition, highly pigmented systems in which, under certain circumstances, perfect curing is not achieved do at least give a blocking-resistant film with good properties. Owing to the advantages mentioned, water-borne UV coatings are replacing existing coating systems in the area of interior wood and furniture coating. They have been growing strongly and increasing their market share continuously for many years. At the same time, it is foreseeable that new applications will also be tapped by water-borne UV coatings. An important field here is water-borne UV coatings for exterior applications, for example cladding elements, door and window frames. UV coatings were long regarded as unsuitable for exterior applications. It was assumed that UV absorbers used as light stabilizers, for example benzotriazoles, hydroxyphenyltriazines or hydroxybenzophenones, would compete too strongly with the photoinitiators, for example benzophenones and α-hydroxyketones, for the UV light employed in curing, and consequently initiation of free-radical polymerization would not occur. In addition, the free radical scavengers, generally sterically hindered amines of the HALS type (HALS = hindered amine light stabilizers), likewise employed as light stabilizers could hinder free-radical polymerization. In spite of these reservations, formulations which give weathering-stable coatings were developed for 100% and solvent-containing UV coatings. The essential factor here is the correct combination of photoinitiator and UV absorber (1). Application of this valuable knowledge is of crucial importance for the development of weathering-stable water-borne UV coatings. The aim here is to take into account the particular properties of water with respect to solubility, but also hydrolysis of the light stabilizers.
