An amphoteric resin composed of an acidic resinous component such as oil-free polyester resin, alkyd resin, acrylic resin and the like, and a basic resinous component such as urea resin, melamine resin, polyamide resin, polyurethane resin and their prepolymers and/or a basic compound as hydroxylamines, amino acids, primary or secondary amine compounds and the like, possess, in its molecule, both the characteristics of the and basic groups and therefore, such a resin can exhibit excellent compatibility with a number of resins customarily used for the preparation of coating compositions and good affinity toward various types of pigments. For these reasons, it is being watched with keen interest as general purpose resin.
The combination of amphoteric resin and etherified amino-formaldehyde resin and/or isocyanate compound is characterized by having excellent compatibility with other coating resins, improved coating workability, finishing appearance (gloss, glamorous) and weather resistance, and hence is superior in many points to the aminoalkyd or aminoacrylic resin type paints heretofore used as top coats for automobile bodies and the like.
The present inventors have previously found that a coating composition comprising an amphoteric resin obtained by the reaction of polyester resin, 10 to 80 mole % of the acid component being composed of saturated alicyclic polycarboxylic acid and 10 to 100 mole % of the carboxyl groups capable of developing resinous acid value being derived from the polycarboxylic acid which will give a titration midpoint potential in non-aqueous potentiometric titration when developing a resinous acid value, of more than -350 mV, and a basic compound, and amino-formaldehyde resin etherified with C.sub.1 to C.sub.4 monohydric alcohol can exhibit excellent weather resistance, mechanical properties of the coating, interlaminar adhesion, curability and pigment dispersibility and is especially useful as a top coat for automobile bodies and the like. This is the subject of Japanese Patent Application No. 154210/82. The disclosed invention, while making the most of the merits possessed by polyester resin as weather resistance, mechanical properties of the coating, interlaminar adhesion and the like, seeks to overcome the drawbacks of the prior art such as poor compatibility with amino resin, by the adoption of amphoterization, thereby providing an excellent coating composition for top coat use. Since the resinous composition contains the abovementioned amphoteric resin, fairly good results are obtained even in respect of pigment dispersibility, too. Such effect can never be attained with the mere mixture of acidic resin and a basic compound.
However, the subsequent studies revealed that even with the aforesaid resinous composition, when use was made of the so-called hard dispersible pigments having a quinacridone structure as Cinquacia red and the like, no satisfactory results could be obtained, often resulting in gloss down, blushing and the like, and that such pigment dispersibility problem was significantly related to the kind and amount of basic resin and/or basic compound contained in the amphoteric resin. The present invention has been made during the course of such studies.