Compositions comprising inorganic UV absorbers

#The present invention relates to compositions comprising inorganic UV absorbers and to coatings produced from them for the long-term protection of materials, especially plastics, against photochemical degradation.

#Many materials such as plastics or natural substances (wood) must, for use outdoors, be protected against photochemical degradation by means of an appropriate coating. Appropriate primarily for this purpose is coating of the surface in a wet-chemical operation, although in the case of substantially two-dimensional substrates such as sheets of plastic it is also possible to achieve corresponding protection by coextrusion (on both sides) of a plastic which contains UV absorber. The wet-chemical application of a coating, particularly of an inorganic coating, however, has the advantage that it generally also enhances the chemical resistance and scratch resistance of the plastic. Coextrusion, although not providing these advantages, is nevertheless much easier to implement from a technical standpoint.

#Both in the (wet-chemically) applied coatings and in the coextruded coats, it is common to use organic UV absorbers, which usually provide satisfactory protection of the underlying material (see, for example, Paint & Coatings Industry 2001, July, 64-76). In the case of very prolonged exposure to UV radiation (sunlight), however, organic UV absorbers are slowly degraded and so lose their protective effect. Through effects of weathering (humidity, high temperatures), moreover, there may be a loss of UV absorber through migration and leaching.

#The polyfunctional organosilane used in the experiments below was oligomeric cyclo-{OSi[(CH2)2Si(OC2H5)2(CH3)]}4 (“D4 diethoxide oligomer”). It was prepared as described in U.S. Pat. No. 6,136,939, Example 2. Nano-CeO2 was used in the form of a commercially available (Aldrich) 20% dispersion, stabilized with 2.5% by weight of acetic acid. The 0.1 N aqueous solution of p-toluenesulphonic acid used was prepared in-house by dissolving corresponding amounts of p-toluenesulphonic acid in a corresponding amount of water; all other starting materials were available commercially and were used without further purification. The mixtures were prepared using glass bottles of appropriate size, stirring being carried out using a magnetic stirring rod; the bottles were kept closed as far as possible.

#The substrates used were extruded polycarbonate sheets of Makrolon® 3103 (Bayer AG, Leverkusen). Prior to coating, the sheets were cut to a format of 7.5×15 cm, cleaned by rinsing with isopropanol and provided with an adhesion promoter. The adhesion promoter, an alkoxysilane-modified polyurethane, was prepared as follows:

#a) Preparation of the Polyol Component: