1. Field of the Invention
The present invention is directed to a process for improving the coatings properties of aqueous emulsion paints based on homo or copolymers of olefinically unsaturated monomers by adding organic polyisocyanates containing aliphatically and/or cycloaliphatically bound isocyanate groups.
2. Description of the Prior Art
Emulsion paints owe their widespread use mainly to the fact that they can be diluted with water and the ease with which they can be applied. They are used inter alia as wall paints for both internal and external use on a wide variety of mineral surfaces such as concrete, plaster, rendering, cement, sand limestone, gypsum, etc. Numerous aqueous polymer dispersions are available to the manufacturer as binders for such emulsion paints. These binders usually have a solids content of from 40 to 60% by weight. The polymer binder dispersed in water is generally based on homo or copolymers of olefinically unsaturated monomers such as polyvinyl acetate, polyvinyl propionate, copolymers of styrene/butadiene or styrene/acrylic acid esters, or homopolymers or copolymers of various acrylic acid esters.
These binders are aqueous dispersons of thermoplastic polymers which are made up into emulsion paints ready for use by the addition of conventional auxiliary agents and additives such as pigments, fillers, thickeners, emulsifiers, wetting agents and other additives.
The paints are applied by roller, brush, spray gun, airless spray gun or immersion, but preferably by roller. The coating is dry 10 to 30 minutes after application and hardens within a further 6 to 12 hours to a decorative film with a matte to silky surface finish.
One disadvantage of such coatings is that their use in places such as hospitals and the like is very limited due to their insufficient resistance to chemicals, organic solvents and numerous domestic chemicals with solvent properties such as disinfectants, cleaning agents, etc. The possibility of cleaning the surface with hot steam is also considerably limited by the thermoplastic behavior of these coatings.
The ability to withstand mechanical action such as scrubbing of the surfaces is in many cases insufficient for practical purposes. Lastly, emulsion paints when used for exterior decorating have the disadvantage of soiling very rapidly and in many cases succumbing to rapid chalking with the resulting disintegration of the film.
Various methods have been applied to improve the wear resistance of emulsion paints. It is customary to seal the surfaces with resistant clear lacquer films, e.g. based on polyurethanes. Transparent films based on polymer binders have also been used to increase the surface smoothness and thereby reduce the tendency of the otherwise rough surface to accumulate dirt. These procedures, however, involve additional expenditure of time and material.
It was thereby an object of the present invention to provide emulsion paint systems which may be used for preparing coatings with improved properties, in particular improved resistance to scrubbing, solvents and chemicals, increased weather resistance and reduced swelling without sacrificing the characteristic advantages of emulsion paints, in particular the possibility of diluting them with water, their ease of application and their lack of environmentally harmful characteristics.
It was surprisingly found that this problem could be solved by the use according to the invention of certain organic polyisocyanates which will be described in more detail below.
Although it was already known from DE-OS No. 2,708,442 that the ease of application of aqueous polymer emulsions containing polyurethanes could be improved by the addition of organic diisocyanates if certain criteria are observed in the addition of the diisocyanates, the process disclosed in the said prior publication is strictly limited to dispersions containing polyurethanes and the polyisocyanates to be added must strictly be difunctional diisocyanates. The prior publication therefore gives no suggestion for adding higher functional polyisocyanates to emulsion paints ready for use which are based primarily on homo or copolymers of olefinically unsaturated monomers free from urethane groups in order to improve the lacquer technical properties of the emulsion paints.