More particularly, the invention relates to a luminescent coating comprising: a luminescent pigment of matrix formula MA1204, wherein M is at least one metal selected from the group comprising calcium, strontium and barium, or on the other hand of matrix MA10, wherein M is a multitude of metal elements, comprising magnesium and at least one element selected from the group consisting of calcium, strontium and barium and a water-borne polyurethane binder dispersion.

A luminescent coating of this kind is known from US-A- 5,665, 793. According to said publication, a suitable amount of a paint containing a colour pigment is added to the aforesaid luminescent coating. This paint mixture is used as a roadway marking coating having enhanced luminescent properties.

The luminescent pigments and the water-borne polyurethane binder dispersions from said publication US-A-5,665,793 are considered to be incorporated in the present application, as are the composition ratios mentioned therein.

The aforesaid luminescent pigments are sensitive to moisture and water. When luminescent pigments come into contact therewith, a heat-emission reaction results, which makes the coating unsuitable in use. This problem is overcome by diluting a suitable polyurethane dispersion as already recognized in the aforesaid patent with another paint which does not contain such water- sensitive pigments.

Another solution which is well-known for screening said luminescent pigments is to cover them with a protective layer.

It is an object of the present invention, however, is to provide a luminescent coating as such, which is not mixed with any other coating such as the paints provided with the new pigment which are known from the aforesaid US patent. Another object of the invention is to provide a coating which can be applied in minimum film thicknesses having sufficient luminescent power. Such film thicknesses can for example range between 15 and 30 ym, preferably they are about 10 ym.

Another object of the present invention is to provide a suitable dilutant, so that a luminescent coating is obtained which is capable of being sprayed.

The problem of the coating referred to in the introduction is that sagging occurs in the form of a two-phase system after a period of rest. The pigments form the heaviest fraction and they will migrate under the lighter water-borne polyurethane dispersion fraction. This makes the coating unsuitable for use.

Surprisingly, this problem is overcome by adding a sagging additive in a low amount of 0.5-10 % by weight to the aforesaid coating. As a result, the luminescent pigments will remain suitably dispersed for a longer period of time, even after a prolonged period of rest of the coating.

Preferably, the anti-sagging additive is a hydrophobic pyrogenous silica. Said anti-sagging has appeared to be highly suitable for forming a permanent dispersion of the aforesaid luminescent pigments in a water-borne polyurethane dispersion.

A preferred embodiment of the luminescent coating according to the invention consists of: -40-80 % by weight of water-borne polyurethane binder dispersion; -19.5-59. 5 W by weight of luminescent pigment; -0.5-10 % by weight of anti-sagging additive.

The hydrophobic pyrogenous silica as the anti-sagging additive is preferably an AEROSIL2, which is marketed by the firm of Degussa.

Besides using the anti-sagging additive, it is advantageous to use an additive in the form of a dispersion additive in an amount of 0.1-5 % by weight.

This dispersion additive will further enhance the dispersion quality by preventing any flocculating activity of the luminescent pigments and keeping the anti-sagging additive dispersed. Preferably, the dispersion additive is DISPERBYK, which is marketed by the firm of Byk Cera.

Preferably, the luminescent coating that is obtained is capable of being sprayed. To this end the invention provides a coating diluted with N-methyl-2-pyrrolidone.

A preferred suitable dilution is N-methyl-2-pyrrolidone and water in a weight ratio of 1 : 12.

A preferred formula of the luminescent coating according to the invention is defined hereafter in Table 1.

TABLE 1

Base Type Supplier Parts by materials weight NeoRez R974 Binder Zeneca Resins 63.31 dispersion RM 4456 Thickener Stahl Holland 0.55 LLA 1698 Flow additive Stahl Holland 0. 1 Byk 028 Defoaming Byk Cera 1.09 agent _ _ Disperbyk Dispersion Byk Cera 1.11 additive Aerosil R Anti-setting Degussa 2.19 972 additive Luminescent Pigment Nemoto & Co. 31.65 pigment bumiNova Ltd. extra fine TOTAL 100 According to another aspect the present invention provides a method for preparing the above-defined luminescent coatings. This method comprises a first step, wherein a base paste for the coating is prepared, after which the luminescent pigment is added to the base paste in a final preparation step. This is done by mixing the luminescent with the base paste through careful stirring so as to ensure that the crystal structure of the luminescent pigments will be retained.

An example of the preparation method is explained in detail hereafter.

First of all, a binder dispersion is weighed in the mixing vessel. Then the defoaming agent, the dispersion additive and the flow additive are added while stirring slowly. Then the anti-setting additive is added, after which it is dispersed to a fineness of < 5 ym by means of a high-speed dispersion apparatus. The dispersion temperature must not rise above 60 °C, however.

Following that, the thickener is added while stirring slowly but firmly. Once the thickener is homogeneously dispersed in the paint, check for clots, the luminescent pigment can be added while stirring carefully.

Another aspect of the present invention is the use of the luminescent coating for coating a light-transmitting or transparent surface, for example a lamp housing.

Said use of a luminescent coating according to the present invention on a light-transmitting or transparent surface is already included in the Dutch patent application NL-A-1009136, which has not been published yet.