Quinacridone pigments have been known for a considerable time and have acquired great industrial importance for pigmenting coating materials and plastics. In practice, therefore, stringent requirements are placed on their fastness properties and coloristic properties. Consequently, the preparation and fine-division process is accorded great importance. For the preparation of quinacridone pigments there are two preferred synthesis routes. On the industrial scale, they are prepared by oxidizing dihydroquinacridones in an alkaline medium in the presence of solvents and then dry- or wet-grinding the resultant coarsely crystalline crude pigments or by subjecting 2,5-dianilinoterephthalic acids to ring closure in polyphosphoric acid or polyphosphoric ester and subsequently finishing the resultant finely divided crude pigment with organic solvents.
Depending on the synthesis route, the crude pigments are obtained either in finely divided or coarsely crystalline form. The crude pigments obtained in finely divided form do not require further fine division, whereas the coarsely crystalline crude pigments must undergo fine division prior to an optional finish treatment. Examples of such fine-division and finish procedures are acid pasting and dry and wet grinding. Combinations of these methods are also described.
The preparation of quinacridone pigments, mixed-crystal quinacridone pigments and quinacridone pigment preparations is described in the following patent documents:
DE-C-1 261 106 describes a process for improving the pigment properties of linear substituted quinacridones by subjecting substituted dianilinoterephthalic acids to ring closure in polyphosphoric acid and then subjecting the wet, finely divided crude pigments obtained after hydrolysis to a solvent finish at elevated temperature and pressure. This process employs large quantities of solvent, which require to be worked up and which make the process expensive.
JP-A-9-110 867 describes the preparation of substituted and unsubstituted quinacridone pigments by slow addition of 50-95% strength phosphoric acid to the polyphosphoric acid ring closure melt at from 140 to 170.degree. C. until a final phosphoric acid concentration of from 90 to 95% is obtained. The coarsely crystalline crude pigments obtained must subsequently be converted to the pigment form by fine division and finishing. These additional steps make the process expensive.
EP-A-0 799 862 describes a process for preparing substituted and unsubstituted quinacridone pigments by high-temperature hydrolysis of the quinacridone ring closure mixtures at more than 110.degree. C. in water or dilute aqueous phosphoric acid, until a final phosphoric acid concentration of less than 50% is obtained. In accordance with this procedure, the unsubstituted quinacridones are obtained in the .beta.-phase.
A number of polymorphic phases of linear, unsubstituted quinacridone are known. Described are the .alpha.-phase (U.S. Pat. No. 2,844,484), the .beta.-phase (U.S. Pat. No. 2,844,485), the .gamma.-phase (U.S. Pat. No. 2,844,581), the .delta.-phase (U.S. Pat. No. 3,272,821), the .epsilon.-phase (JP-Patent 69-22 420), and the .zeta.-phase (DE-A 2 435 219).
Three phases of y-quinacridone are described. The .gamma.I-phase is described in U.S. Pat. No. 3,074,950 and EP-A-0 267 877. In the X-ray spectrum, it shows three strong lines for twice the glancing angle 2.theta., at 6.6.degree., 13.9.degree. and 26.5.degree., three moderate lines at 13.2.degree., 13.5.degree. and 23.8.degree. and four weak lines at 17.1.degree., 20.5.degree., 25.2.degree. and 28.6.degree.. The .gamma.II-phase is described in U.S. Pat. No. 2,844,581 and in EP-A-0 267 877 and in DE-A-1 184 881. In the X-ray spectrum, it shows three strong lines for twice the glancing angle, 2.theta., at 6.6.degree., 13.9.degree. and 26.3.degree.; five moderate lines at 13.2.degree., 13.4.degree., 23.6.degree., 25.2.degree. and 28.3.degree., and two weak lines at 17.1.degree. and 20.4.degree.. The .gamma.III-phase is described in JP-A 53-39 324. In the X-ray spectrum it shows three strong lines for twice the glancing angle, 2.theta., at 6.2.degree., 13.6.degree. and 26.5.degree.; three moderate lines at 12.5.degree., 25.8.degree. and 27.7.degree., and three weak lines at 16.5.degree., 20.5.degree. and 24.0.degree.. The moderate lines at 25.8.degree. and 27.7.degree. can be attributed to small amounts of .alpha.-phase.
DE-C 1 184 881 describes a process for preparing linear unsubstituted .gamma.II-phase quinacridone pigments by ring closure of 2,5-dianilinoterephthalic acid in polyphosphoric acid and subsequent hydrolysis in ice-water. The resultant crude quinacridones are subsequently treated with aqueous alkali and following isolation in the form of aqueous neutral pastes are heated at from 120 to 200.degree. C. under pressure. The process is complex, since the finish is carried out in two stages.
JP-A 53-39324 describes the preparation of novel pigments of the .gamma.III-phase of unsubstituted quinacridone by slow addition of water or dilute phosphoric acid to the polyphosphoric acid ring closure melt at from 70 to 140.degree. C. until a final phosphoric acid concentration of from 62 to 90% is obtained. The coarsely crystalline crude pigments present following hydrolysis are subsequently converted to the pigmentary form by fine-division and finish processes.