1. Field of the Invention
The invention relates to aqueous, hydroxy-functional binder dispersions with a low solvent content and based on hydroxy-functional copolymers, to a process for preparing such binders, to binder combinations based thereon and to the use of the binders in coating materials.
2. Description of the Prior Art
The use in coating systems (Paint & Resin 12/83, p. 34 ff., DE-A 3 209 421, EP-A 95 263, EP-A 105 293, EP-A 133 949, EP-A 288 763, DE-A 202 212 and literature cited therein) of copolymer-based, water-dilutable binders is known. Generally, however, these systems include emulsifiers for stabilizing and/or relatively large fractions of organic cosolvents.
The emulsifiers normally affect the properties of the coating materials or of the coatings, such as water resistance, film appearance (gloss), pigmentability, for example, adversely.
The use of relatively large amounts of organic solvents is undesirable on environmental grounds. Generally, however, it cannot be avoided in order to ensure sufficient stirrability and heat removal from the reaction mixture in the course of polymer preparation and also to ensure a certain minimum filling level of the reactor. Organic solvents additionally lead in aqueous coating materials to advantageous effects such as enhanced storage stability, pigment wetting, film appearance and levelling.
Removing subsequently from copolymers or copolymer dispersions the solvents present therein for process-related reasons entails much energy consumption and great complexity of apparatus, and hence also high costs, so that there is a need for aqueous polymer dispersions in whose preparation the use of organic solvents can be largely avoided without impairment to the performance properties.
Copolymer dispersions which are to be cured by means of a chemical reaction, as for example with an amino resin, a blocked polyisocyanate or a polyisocyanate, must contain a certain amount of reactive groups, hydroxyl groups for example. These groups are generally incorporated into the copolymer through the use of hydroxy-functional (meth)acrylic esters during the copolymerization. In comparison to the non-functional (meth)acrylic esters or else to styrene, however, these raw materials are very expensive. Additionally it is often necessary to use relatively large amounts of these raw materials as compared with copolymers in organic solution, in order to compensate the hydrophilicity of the coating films by means of a relatively large crosslinking density.
One way to prepare hydroxy-functional secondary copolymer dispersions that largely avoids the use of solvents in the polymerization is shown by the teaching of EP-A 0 758 007. According to that publication the solvents normally used are replaced in whole or in part by hydroxy-functional polyethers. The hydroxy-functional polyethers remain as reactive diluents in the secondary dispersion and in the course of subsequent crosslinking they too react with isocyanates or blocked isocyanates to form urethane. Consequently they do not contribute to the VOC. A disadvantage experienced with these products, however, is their poor stability.