Лако-красочные материалы — производство

All commercial benzimidazolone azo pigments are produced by classical azo cou­pling, commonly carried out in aqueous media. As mentioned already above, in yel­low and orange pigments the coupling component is 5-acetoacetylaminobenzimida — zolone (1) (apart from one exception, P. O. 64). In red pigments the naphthol deriva­tive 5-(2′-hydroxy-3′-naphthoylamino)-benzimidazolone (2) serves as coupling com­pound. Both coupling components are prepared from 5-aminobenzimidazolone (7) (compare Figure 10.3). This amine is obtained advantageously by the reaction of

4- nitro-1,2-diaminobenzene (5) with phosgene or with urea (in melt) which results in

5- nitrobenzimidazolone (6). Reduction of the nitro compound 6 leads to the required aminobenzimidazolone 7. The coupling component 1 for yellow pigments is pro­duced by reacting diketene or acetoacetic ester with 7. Again, the condensation of 2-hydroxy-3-naphthoic acid (8) with 5-aminobenzimidazolone 7 in the presence of phosphorus trichloride in an organic solvent yields the coupler 2 for red pigments.

For the coupling reaction itself the coupling components 1 or 2 are dissolved in alkali and reprecipitated with acetic or hydrochloric acid, optionally in the pres­ence of a surfactant. This procedure is necessary to facilitate the coupling reaction by conversion of the coupling compound into an “activated form”. This means that it consists of small particles which readily react with the diazonium salt. In

certain cases, auxiliary agents are added during the coupling process. However, regarding the demands of today’s technical applications, the crude pigments are usually in an unsatisfactory physical form. Therefore the initially obtained pig­ments are subject to a post-coupling, thermal aftertreatment called “finishing”. Typically, an aqueous suspension of the crude pigments is heated (under pres­sure) to temperatures of 100 to 150 °C. The purpose of the finishing is to control the physical properties of the pigments, such as crystal shape, crystal phase, crys­tal size and grain size distribution, since these characteristics determine the appli­cation properties of the colorants. In conclusion, finishing provides the basis for pigments with tailor-made application properties including shade, tinctorial strength, transparency/hiding power, light fastness, weather fastness, fastness to solvents and migration resistance as well as dispersibility.

The considerable impact of the finishing on the resulting pigment properties may be best illustrated by comparing two examples from German patent DE1179908. In the patent, it is shown that the pigment prepared by the azo cou­pling of diazotized 3-amino-4-methoxybenzoic acid anilide with 5-(2′-hydroxy-3′- naphthoyl-amino)-7-chloro-benzimidazolone has quite different properties depending on the coupling conditions and aftertreatment. One approach was to carry out the coupling step in the presence of an additive obtained from tributyl — phenol and ethylene oxide. After the coupling was complete, o-dichlorobenzene was added to the resulting pigment suspension prior to heating to 97 °C (207°F). The usual workup yields a bluish red pigment with very soft grain texture. In contrast, the chem­ically identical pigment manufactured without additive and thermal aftertreatment has a hard texture and exhibits inferior fastness properties. The attempt to process the untreated pigment into a printing ink resulted in unsatisfactory tinctorial strength and insufficient flow properties.

Following the finishing process, the pigments are isolated, washed, dried at moder­ate temperatures between 70 to 85 °C, and milled. The same techniques and equip­ment are thus used as for other azo pigments, such as, for example, plate-and — frame filter presses, drying chambers or continuous drying belts and air mills.

10.4