The invention relates to paints, lacquers or other coating materials in liquid solution, consisting of binders, pigments, solvents, fillers and additives, the additives at least partly acting against pinhole formation, and the manufacture and use thereof.
The term “silica” is a generic designation for compounds of the general formula SiO2 nH2O, where n is a natural number which indicates the number of chemically bound H2O molecules It should be made clear that this is not water of crystallinity, so that the elimination of H2O may only take place by means of chemical reactions. A subset of this is formed by pyrogenic silicas, which are produced by flame hydrolysis, that is to say pyrogenic methods, in the manner that silicon tetrachloride is decomposed in an oxyhydrogen flame. Because of the high temperatures and the gas phase, coagulation of the monomeric silicas takes place. In the case of pyrogenic silicas, they are highly disperse silicas, which possess an almost entirely non-porous surface and in comparison to precipitated silcas, have significantly fewer OH groups on the surface. Their hydrophilicity is caused by the silanol groups. By corresponding post-treatment processes, a reaction of the OH groups can take place, so that, as a result, pyrogenic silica with hydrophobic, that is to say water-repellent, surfaces is produced. According to the literature (Römpp-Chemielexikon Thieme-Verlag), under the head word “silica”, as uses, there are quoted reinforcing fillers for rubber, carriers for plant protection agents, in silicone oil and mineral oil defoaming agents, in joint sealing compounds, as free-flow aids, for matting of lacquers, as antiblocking agents in plastics and as toothpaste additives.
One of the faults frequently occurring during the application of lacquers and paints is the occurrence of pinholes, wherein, in the context of the present invention, this term is to be widely interpreted and all kinds of surface defects, be they circular or non-round, comparatively shallow in the manner of a dent or depressions, which may extend as far as the substrate of the lacquer film and may occur individually or frequently. One of the most frequent causes of the occurrence of pinholes is the incidence of substances with low surface tension on the not-yet-hardened lacquer film or due to the spontaneous local release of volatile components in a late phase of crosslinking, at a point in time in which the viscosity is already relatively high and the film can thus not completely flow. The formation of pinholes can also occur if one or more lacquer components has a significantly higher surface tension than the rest of the coating. To counteract the formation of pinholes, additives are available which essentially lower the surface tension of the lacquer by accumulating at the surface, with which a control of the viscosity of the lacquer is achieved and wetting agents and solvents with low surface tension are used. It remains to be made clear that other causes that lead to the formation of pinholes may also be present, which may be identified in the unclean pretreatment of the workpiece surface because of residual contamination and foreign materials, but also in the contamination of individual lacquer components. These causes can be effected by an improvement of the cleanliness of lacquer production and/or a correspondingly good pretreatment of the workpiece during painting.
As additives, auxiliary materials or additives are quoted, which are added to the lacquers and paints in small amounts to achieve or improve particular properties. Since the quantity proportion of the additive in the overall formulation is very low, its proportion is usually less than 1 percent of the total formulation. Irrespective of the low quantity proportion, the costs of the additives are often very high, which is caused by the fact that the industrial manufacture of the additives is cost intensive because of the low production quantity.
The surface defects occurring during the production and manufacture of coatings and lacquers, such as pinhole formation, require, after their occurrence, complicated secondary work in order to achieve a smooth surface of the subsequent coats. These measures make continuous production difficult, so that it is necessary to avoid pinhole formation, so that because of the many possible causes, the solution is sought in avoiding pinhole formation by the addition of additives. It is also known, for example from German patent DE 43 03 787, that, after the application of a coating in an electrocoating bath, before the further work steps, the workpiece is removed from the bath and treated with corresponding solutions, for example polyvinyl alcohol solutions (DE 43 03 787).
As anti-pinhole additive, chemical compounds are known, which are attributed to the group comprising polyacrylate resins, polyether resins, polyester resins and silicone resins (DE 68925826 T2).