In the manufacture of paints, it is important to mix the pigment with the coating composition In this case, the pigment is dispersed into a vehicle to form a dispersion paste, and then the paste is mixed with the coating composition to obtain a pigment dispersing composition. However, a greatest problem in such a pigment dispersing composition lies in that the pigment particles are apt to be agglomerated. This agglomeration may occur in the dispersion step, dissolution step, storing step or painting step. As a result, the gloss, sharpness and color strength of the finally obtained paint film are injured.
In order to improve the above drawbacks, therefore, there are proposed various pigment dispersing agents and grinding aids and also methods for improving dispersion of the pigment are attempted. For instance, there are known (1) a method of dispersing the pigment by using non-ionic, anionic or cationic surfactant or a wetting agent such as aliphatic polyvalent carboxylic acid or the like as a grinding aid; (2) a method of dispersing the pigment by using an amphipathic substance obtained by bonding a pigment-affinitive substance disclosed in British Pat. Nos. 1108261, 1159252 and 1346298 to a medium-affinitive substance; (3) a method of reducing surface tension with a surfactant such as alkyl silicone to prevent the occurrence of pigment floating; (4) a method of mixing the pigment with a certain substituted derivative of the pigment to conduct the dispersion of the pigment as disclosed in Japanese Patent laid open No. 51-18736; (5) a method of using as a dispersing agent a compound obtained by reacting an organic compound having two or more isocyanate groups, an organic compound having one or more functional group (amino group or hydroxyl group) capable of reacting with the isocyanate groups, and a polymer having substantially one functional group capable of reacting with the isocyanate group as disclosed in U.S. Pat. No. 3,684,771, British Pat. No. 1393401 and the like; (6) a method of dispersing the pigment by using a dispersing agent obtained by bonding a basic substance disclosed in European Patent laid open No. 208041, U.S. Pat. No. 4,647,647 and the like to a polyester compound; and the like.
However, the grinding aids or dispersing agents used in the methods (1).about.(3) do not sufficiently improve the aforementioned drawbacks, and the harmful effects derived from the use of these agents, i.e. secondary evil influences of degrading the paint film performances or painted state should be encountered. The pigment derivative used in the method (4) is naturally colored, so that it can not generally be used. Further, the basic thought of the dispersing agent used in the methods (5) and (6) is a technical idea of making a block structure to a polymer portion capable of solvating with a solvent as described by A. Topham in Progress in Organic Coatings, vol. 5, (1977) pp 237.about.243. In this technique, it is important that the polymer portion capable of solvating with the solvent is excellent in the compatibility with a resin used as a film forming component of the paint film and has adsorption point to the pigment as much as possible. When the polymer portion capable of solvating with the solvent is poor in the compatibility with the resin added as a film forming component of the paint film, the solvated polymer portion agglomerates to cause the degradation of pigment dispersing performances. Further, when the number of adsorption points to the pigment is small, the desorption of the dispersing agent from the pigment is apt to be caused to degrade the pigment dispersing performances. From these reasons, it is most important how to make the polymer portion having excellent compatibility with a wide range of resins, and further how to give many adsorption points to the pigment. And also, the polymer portion solvating with the solvent is important to have one functional group reacting with the isocyanate group. Because, when the polymer portion has two or more functional group, the adsorption state of the pigment onto the dispersing agent tends to produce a loop form, and consequently the efficiency of steric stability of the solvated polymer portion becomes small as compared with the tail-like adsorption state in case of one functional group and the pigment dispersing performances are degraded. Therefore, it is unfavorable to use (A) a polyester compound obtained from dialcohol starting from monoalcohol and dicarboxylic acid in the polymer portion as disclosed in U.S. Pat. No. 3,684,771 and British Pat. No. 1393401 as a polymer component of the dispersing agent because such a polyester compound includes not only a polyester compound having substantially one hydroxyl group at its terminal but also polyester compounds having hydroxyl groups at both terminals or no hydroxyl group. For this end, almost of the polymer component in the dispersing agent are (B) polyester compounds obtained by starting from monoalcohol and ring-opening polymerizing with lactones, and (C) polyester compounds obtained by polycondensation of an acid having hydroxyl group such as .alpha.,.omega.-hydroxyl propionic acid, 12-hydroxy stearic acid or the like. An example of these reactions may be mentioned as follows: ##STR3##
As the starting material in the reactions (B) and (C), there are restricted propiolactone, .epsilon.-caprolactone, valerolactone and the like in case of (B) and .alpha.,.omega.-hydroxy propionic acid, .alpha.,.omega.-hydroxy decanoic acid, 12-hydroxy stearic acid and the like, so that there is a problem that the range of compatibility of the dispersing agent is considerably limited.