Process for preparing gamma quinacridone from polyphosphoric acid and alcohol

A method is provided for the direct production of gamma quinacridone pigment comprising ring closure of 2,5-dianilinoterephthalic acid in polyphosphoric acid and diluting the mixture by addition thereof into a water-miscible alcohol containing 1-3 carbon atoms, e.g. methanol, while maintaining the temperature of the alcohol-acid at between 0.degree. and 25.degree. C.

This invention relates to quinacridone pigments. More particularly this 
invention relates to a process for producing the gamma crystalline phase 
of quinacridone (hereinafter termed gamma quinacridone). 
Linear quinacridone is represented by the structure 
##STR1## 
and, as is disclosed in U.S. Pat. No. 2,844,581, exists in a number of 
crystalline phases, including the alpha phase, the beta phase and the 
gamma phase. 
Recently, gamma quinacridone has found a ready market in the automotive and 
architectural industries in view of the fact that this pigment products a 
deep red metallic finish in the quinacridone color region. Heretofore, 
gamma quinacridone pigment has geen commercially produced by milling 
quinacridone prepared by the oxidation of dihydroquinacridone. Note U.S. 
Pat. No. 2,844,581. Other patents in this same general area are U.S. Pat. 
Nos. 3,160,510, 3,257,405, 3,265,699, 3,342,823, 3,362,957 and 3,547,925. 
The milling of quinacridone to produce the gamma quinacridone pigment is, 
of course, an additional step in the production of the pigment and 
requires additional energy input, time, equipment, etc. 
It is an object of this invention to produce gamma quinacridone. It is a 
further object of this invention to produce gamma quinacridone directly 
from the ring closure of 2,5-dianilinoterephthalic acid. These and other 
objects will become apparent from the description which follows. 
SUMMARY OF THE INVENTION 
In accordance with this invention there is provided a process for producing 
gamma quinacridone pigment which comprises heating, at above about 
70.degree. C., 2,5-dianilinoterephthalic acid in the presence of 
polyphosphoric acid and thereafter cooling the mixture to below 25.degree. 
C. and diluting the mixture by addition thereof to a water-miscible 
alcohol containing 1-3 carbon atoms, e.g. methanol, at a rate such that 
the temperature does not rise above 25.degree. C. 
When operating in accordance with the present invention, i.e., the ring 
closure of 2,5-dianilinoterephthalic acid followed by dilution of the 
reaction mixture into an alcohol under controlled temperature conditions, 
it has been found that the product produced is essentially pure gamma 
quinacridone pigment, i.e., exhibits an X-ray diffraction pattern 
consistent with that generally recognized as identifying the gamma 
crystalline form of quinacridone. Thus, the process of the present 
invention provides a process for the direct production of gamma 
quinacridone pigment and eliminates the need for the extra milling step 
which has theretofore been used for the production of this crystalline 
phase of the quinacridone pigment.

DETAILED DESCRIPTION OF THE INVENTION 
The gamma quinacridone pigments produced by the process of the present 
invention have excellent outdoor durability and are useful in coloring 
automotive finishes, inks and house paints. They are particularly valuable 
in preparing finishes and transparent automotive metallic finishes. 
The process of the present invention provides a simple economic route to 
gamma quinacridone which eliminates the need for certain operations 
heretofore thought necessary for the production of this crystalline phase 
of quinacridone. Furthermore, it is quite unexpected that the process of 
the present invention would produce gamma quinacridone inasmuch as present 
quinacridone technology would ordinarily predict that drowning of 
polyphosphoric acid solution of quinacridone into an alcohol would produce 
the violet, beta phase type of quinacridone. It has been found that this 
is not the case and that by operating within the confines of the present 
invention the quinacridone produced exhibits a pure gamma type of X-ray 
pattern. 
In carrying out the process for the present invention, the 
2,5-dianilinoterephthalic acid is dissolved in polyphosphoric acid. The 
use of polyphosphoric acid as a ring closing agent is well known in the 
quinacridone art as exemplified by U.S. Pat. Nos. 3,257,405 and 3,342,823. 
As indicated in that prior art, the polyphosphoric acid should have an 
acid content (H.sub.3 PO.sub.4) of at least 100 percent. Preferably, the 
polyphosphoric acid strength is between 114-120 percent. The mixture is 
heated while being stirred at a temperature of above 70.degree. C., 
preferably between 80.degree. and 120.degree. C. After the ring closure 
has been accomplished, which usually takes from 4-16 hours, the mixture is 
added to alcohol which is maintained at a temperature of between 0.degree. 
and 25.degree. C., the rate of addition being such that the temperature 
does not rise above 25.degree. C. Thus, for faster addition, the amount of 
external cooling will be greater. The amount of alcohol necessary is, of 
course, dependent on the volume of the mixture. Enough alcohol must be 
used which is sufficient to dissolve all the phosphoric acid. Preferably 
excess alcohol is present to facilitate agitation of the mixture. After 
stirring the mixture at below 25.degree. C. for a time sufficient to 
dissolve the acid mixture, the total mixture is then heated to reflux, 
drowned into water, filtered, washed alkali free and dried to yield the 
essentially pure, red, solid, gamma phase quinacridone pigment. 
The addition of the polyphosphoric acid solution to the alcohol, rather 
than vice versa, is critical to the process of this invention, as in 
maintaining the temperature below 25.degree. C. While it is possible to 
obtain gamma quinacridone by the time-regulated addition of alcohol to the 
polyphosphoric acid solution, the results of such a process are not easily 
reproducible and very often large amounts of beta quinacridone are 
produced with the gamma quinacridone. In following the order of addition 
of the process of this invention, only gamma quinacridone is produced. 
Whether the acid solution is added over a period of three minutes or 
thirty minutes, the same results are obtained as long as the temperature 
is maintained within the critical range. 
The alcohols useful in the process of the present invention are 
water-miscible alcohols containing 1 to 3 carbon atoms. The alcohols can 
contain one, two or three hydroxy groups, and representative examples are 
methanol, ethanol, propanol, isopropanol, ethylene glycol, 1,2-propylene 
glycol, 1,3-propane diol and glycerol. Alcohols that contain more than 
three carbon atoms, or are water-immiscible, tend to initiate the 
formation of betaquinacridone such that mixtures of beta and gamma 
quinacridone are obtained, as opposed to essentially pure gamma 
quinacridone. Methanol is a preferred embodiment due to its ready 
availability, ease of handling and low cost. 
As has been indicated above, the temperature of the alcohol and the 
alcohol-acid mixture, during the drowning step, must be between 0.degree. 
and 25.degree. C., preferably 0.degree. to 15.degree. C. If the 
temperature is not controlled, it will rise, due to the higher temperature 
of the polyphosphoric acid mixture, above 25.degree. C. and beta phase 
quinacridone will result. The 0.degree. C. lower limit is selected for 
convenience since it is most economical to keep the alcohol temperature 
low by means of an ice bath. 
DESCRIPTION OF PREFERRED EMBODIMENT 
The following examples are given by way of illustration only; all parts are 
by weight unless otherwise indicated. 
EXAMPLE I 
To 130 parts of 118 percent polyphosphoric acid (118 percent H.sub.3 
PO.sub.4) are added 18 parts of 2,5-dianilinoterephthalic acid and the 
mixture is heated at 102.degree. to 106.degree. C. for 16 hours. The 
resulting thick solution is cooled to 90.degree. C. and poured into 250 
parts of methanol over a period of 5 minutes at a rate such that the 
temperature of the methanol is not allowed to exceed 15.degree. C. The 
methanol is maintained at between 0.degree. and 15.degree. C. by means of 
an ice bath. After addition is complete, the mixture is stirred for 15 
minutes at 15.degree. C. or below and then heated to reflux for 11/2 
hours. After allowing the mixture to cool to about 90.degree. C it is 
drowned into 400 parts of water at 20.degree. C., heated at 60.degree. C. 
and boiled for 1/2 hour. The solid is filtered, washed acid free, 
reslurried with 1000 parts of water and boiled with 50 parts of 50 percent 
NaOH for 1 hour. The solid is again filtered, washed acid free and dried 
at 180.degree. F. to yield 15 parts of pure pigmentary gamma quinacridone. 
Example II 
The procedure of Example I was repeated with the exception that the ice 
bath was removed. Addition of the acid solution to the methanol caused the 
temperature of the methanol to rise to reflux. The product recovered was 
the violet, beta phase quinacridone containing only minor amounts of gamma 
quinacridone.