PAINT COMPOSITION FOR BRITTLE OBJECTS SUCH AS GLASSES AND MIRRORS The present invention relates to a paint composition suitable for brittle objects with a plane and smooth surface, e. g. glasses and mirrors, and objects with rough or variously shaped surface. When a brittle object accidentally falls on the floor or is subjected to substantial thrust, it is liable to be shattered into a plurality of pieces, with the result that, on the one hand, the object becomes unusable and, on the other, a number of pieces might be ejected and spread around with the risk of hurting people standing nearby and damaging objects in the surrounding area. Searches and tests have been conducted to provide a composition which could be applied to brittle object surfaces in order to give them good impact resistance so that after an impact the object, if fractured into pieces, keeps its shape or configuration by preventing the pieces from scattering around and thus becoming loose. EP-1 100 756 discloses a method for adhering a layer of organic paint to the surface of a glass sheet. The organic paint is mainly constituted by a water-soluble sodium silicate and a water-soluble base, e. g. sodium hydroxide and potassium hydroxide. The use of such an organic paint, mostly owing to the presence of the silicate compound, is unsuitable for most kinds of glasses in that between the glass support and the organic paint a link is obtained, which is quite sensitive to temperature changes or liable to quick aging. Another method which has been proposed in the past provides the application of a transparent plastic adhesive layer to one or more surfaces of a brittle object. Such an adhesive layer, although prevent scattering of the various pieces in case of fracture following to an impact of the object, is unsuitable to prevents fracture itself of the object, in that it does not provide any additional impact resistance to the coated object that remains basically brittle or fragile. Moreover, the application of such an adhesive solution does involve the use of suitable machines correctly to apply an adhesive layer to an object to be coated, otherwise air bubbles are formed and trapped between the brittle object surface and the coating adhesive layer. Such a machine is quite expensive to manufacture and involves well trained operators which affects the final cost of the coating operation. Moreover, a method based on adhesive layers cannot be used for brittle objects having curved, undercut or decorated surfaces unless use is made of specific machines. In such a case, however, the production costs of the final objects would be too high with respect to the production costs of the similar objects when obtained by a method other than that providing adhesive layers. When dealing with cut mirrors or glasses the above described methods cannot be used, because any organic paint and adhesive layer covers and hides the cut surface of the objects thus depriving the objects of their aesthetic features. It has already been suggested partly to solve the above mentioned problems by painting a tempered glass with a silicon-based paint. The surface of a painted tempered glass, however, is not often perfectly plane as undulations can result after a painting and tempering operation. Moreover, a tempered glass although becoming fractured upon impact is less liable to scatter fractured pieces or fragments, thus ensuring a minimum safety in case of an accident. A specific drawback connected to the use of tempered glass is the fact that tempered glass cannot be further processed, which is highly inconvenient in many cases. The main object of the present invention is to provide a new polyurethanic paint composition which can be applied to the surface of a brittle object, such as a mirror or a glass, to make it highly resistant to impacts. Another object of the present invention is to provide a paint composition that, when a paint-coated brittle object is subject to an impact, prevents it from scattering fractured pieces while keeping them assembled together to prevent them from damaging people in the vicinity. Another object of the present invention is to provide a paint composition suitable for making brittle objects more resistant to impacts, and applicable to the object in an easy, rapid and cost effective way. Another object of the present invention is that such a paint composition can be also applied to not plane objects, e. g. objects with curved, cut, bent, ground, undercut, rough, smooth surfaces, without affecting their appearance. These and other objects that will better appear below are achieved by a paint composition for at least partly coating brittle glass objects characterized in that it comprises an anionic aqueous solution of at least one aliphatic or aromatic polyurethanic compound. Advantageously, said polyurethanic compound comprises: - at least one glycol compound; - at least one propionic acid; and - at least one diisocyanate compound. More preferably, said polyurethanic compound is a high-solid content, aromatic/polyether-based dispersion obtained without use of solvent. A paint composition according to the present invention comprises an anionic aqueous solution of a polyurethanic compound preferably although not exclusively including: - 4-4' dicyclohexylmethane-diisocyanate (H12MDI); - at least one compound selected from the group comprising polyethylene glycol, polypropylene glycol, polytetrahydrofuran glycol; and - dimethylpropionic acid. Preferably, the 4-4' dicyclohexylmethane-diisocyanate compound is in a percentage ranging from 20 to 50% w/w, the glycol compound is in a percentage ranging from 20 to 50% w/w, and the dimethylpropionic acid is in a percentage ranging from 5 to 20% w/w. Most preferable characteristics of a polyurethanic compound to be used in the paint composition according to the present invention are listed in the following Table A. Table A

A polyurethanic compound that can also be used in a paint composition according to the present invention is a high-solid content, aromatic/polyether-based dispersion produced without use of solvents such as the compounds Syntegra YM 2000 and Syntegra 2100 produced by Dow Chemical Company. Preferably the paint composition comprises solids in a percentage ranging from about 10 to about 80% w/w A glass mirror covered by a layer of 250 μm of a paint composition as specified above according to the present invention has been subjected to an ultimate strength test no. RP006504-02.1 conducted by a testing concern Consorzio Mobile S.p.A. with registered offices in Pesaro (Italy). The mirror was tested by subjecting it to a pendulum-impact test according to the European standard EN 12600, and withstood without fracturing to an impact by a thrust of 50 ± 1 kg of a weight falling from a height of 190 mm, thus the mirror was classified in 2(B)2 classification. This means that the use of a paint composition according to the present invention when applied to a glass object makes it much more resistant to impacts, and thus much more safe. Of course, a paint composition according to the present invention can be opaque, e. g. owing to the presence in it of colouring agents, or preferably transparent so as to better satisfy any aesthetic requirements. Preferably the percentage of one colouring pigment or dye is up to 50% of a dry residue of the paint composition. Moreover, a paint composition according to the present invention can be applied to mirrors or, in general, glass articles by any suitable conventional apparatus, e. g. a spraying or coating machine or a roll coater or a spreading blade. A paint composition according to the present invention is preferably dried, e. g. by making use of an IR (infrared) lamp or a hot-air furnace of any suitable type. The possibility of using conventional apparatus for applying and drying a paint composition according to the present invention makes it possible to obtain competitive painted impact-resistant glass objects. The invention as described above is susceptible to numerous variations and modifications within the scope of the invention, as defined by the claims.