Process for the preparation of quinacridone solid solutions

Abstract: Solid solutions comprising quinacridone and 6,13-dihydroquinacridone are prepared by oxidation of a finely divided 6,13-dihydroquinacridone salt in an aqueous basic alcoholic medium using hydrogen peroxide as the oxidant. The solid solutions are useful as red organic high performance pigments.

Description: The present invention relates to a new process for the preparation of solid solutions of quinacridone. More particularly it relates to a new process for the preparation of quinacridone solid solutions comprising quinacridone and 6,13-dihydroquinacridone by oxidation of a finely divided 6,13-dihydroquinacridone salt in an aqueous basic alcoholic system using hydrogen peroxide as the oxidant, and the use of the solid solutions as red organic high performance pigments. It is known that under certain conditions, solid solutions of two or more quinacridone pigments having excellent properties can be obtained. Such solid solutions are described for instance in US-3,160,510, US-3,298,847, US-3,647,494 and US-3,681,100. Many processes describe the preparation of quinacridones starting from 6,13-dihydroquinacridones by oxidation with various oxidants like, for example, nitrobenzene derivatives, such as m-nitrobenzene sulfonic acid sodium salt, selenium, sulfur, iodine, oxygen or air. In another method, quinacridone is obtained by cyclization of 2,5-diphenylamino terephthalic acid in polyphosphoric acid. These processes are not environmentally friendly because they generally require large quantities of organic solvents, strong bases or strong acids. Furthermore, they often employ environmentally unfriendly oxidants which afford large quantities of waste materials, such as the reduced organic by-products. The present invention describes a process for the preparation of solid solutions comprising quinacridone, which is also referred to as 5,12-dihydroquino[2,3-b]acridine-7,14-dione, a well-known pigment having the formula (I) and 6,13-dihydroquinacridone of the formula (II) by oxidation of a 6,13-dihydroquinacridone in an aqueous basic alcoholic medium using hydrogen peroxide as the oxidant and the use of the solid solutions as red organic high performance pigments. Quinacridone is known to exist in three major polymorphic modifications: an α form which is disclosed in US-2,844,484, a γ form which is disclosed in US-2,844,581 and US-2,969,366, and a β form, which is described in US-2,844,485 and US-4,857,646. The α and γ polymorphs are red pigments, while the β polymorph has a violet to magenta hue. The γ polymorph itself is known to exist in three forms, a bluish-red γ II form which is disclosed in US-2,844,581, and two yellowish-red forms which are designated as the γ I and γ m forms and disclosed in US-3,074,950 and US-5,223,624, respectively. Each of the three γ forms of quinacridone possesses a distinct X-ray diffraction pattern. The bluish-red γ II form shows an X-ray diffraction pattern with three strong lines at 6.6, 13.9 and 26.3; five medium lines at 13.2, 13.4, 23.6, 25.2 and 28.3; and two weak lines at 17.1 and 20.4° 2 double glancing angles. The yellowish-red γ I form shows an X-ray diffraction pattern with three strong lines at 6.6, 13.9 and 26.5; three lines of medium intensity at 13.2, 13.5 and 23.8, and four weak lines at 17.1, 20.5, 25.2 and 28.6° 2 double glancing angles. The yellowish-red γ III form shows an X-ray diffraction pattern with four strong lines at 6.7, 13.3, 14.0 and 26.6; one medium strength line at 13.6 and seven relatively weak lines at 17.2, 20.6, 21.9, 24.0, 25.3, 28.1 and 28.8° 2 double glancing angles. Several processes are described for the preparation of γ quinacridone pigments, for example, starting from quinacridone crude by milling, followed by recrystallization in certain organic solvents or by using precipitation processes such as, for example, precipitation from basic DMSO or polyphosphoric acid. The present invention is based on the discovery that a high chroma quinacridone / 6,13-dihydroquinacridone solid solution having the crystal structure of a γ I quinacridone is obtained by oxidation of 6,13-dihydroquinacridone in an aqueous basic alcoholic medium in the presence of a quinone catalyst and using hydrogen peroxide as the oxidant if a special form of a 6,13-dihydroquinacridone salt is generated prior to the addition of the oxidant. It was additionally discovered that a highly saturated and opaque quinacridone / 6,13-dihydroquinacridone solid solution having the crystal structure of a γ II quinacridone can be prepared by essentially the same process if a catalytic amount of an aromatic nitro compound is added prior to the addition of the hydrogen peroxide. Thus, the inventive process is valuable because two different quinacridone high performance pigments having different hues can be made by practically the same environmentally friendly process route. The inventive process is also a single-pot process which is easier to control for obtaining a consistent product and which does not requires expensive milling and synthesis equipment for manufacturing. The present invention relates to a process for the preparation of a quinacridone / 6,13-dihydroquinacridone solid solution pigment in the γ I or γ II crystal form, which process comprises (a) preparing a large particle size 6,13-dihydroquinacridone salt, (b) converting the large particle size 6,13-dihydroquinacridone salt to a small particle size 6,13-dihydroquinacridone salt, (c) optionally, adding a catalytically effective amount of an aromatic nitro compound if the γ II crystal form is desired, (d) oxidizing the small particle size 6,13-dihydroquinacridone salt by the addition of hydrogen peroxide in the presence of a catalytically effective amount of a quinone, and (e) isolating the γ I or γ II quinacridone / 6,13-dihydroquinacridone solid solution pigment. The 6,13-dihydroquinacridone starting material of formula (II) is well-known in the quinacridone pigments art, as are procedures for its preparation. The expression "solid solution" is used in this application to mean a pigment composition which has an X-ray diffraction pattern that is different from the sum of the X-ray diffraction patterns of the individual components. Thus, the expression solid solution includes the "guest-host" solid solutions, which have the X-ray diffraction pattern of one of the component pigments, and "solid compounds" or "mixed crystals", which have an X-ray diffraction pattern different from that of any of the individual components and also different from the sum of the X-ray diffraction patterns of the individual components. In general, the quinacridone / 6,13-dihydroquinacridone solid solution in its γ I crystal form contains about up to 25% by weight of 6,13-dihydroquinacridone, and the quinacridone / 6,13-dihydroquinacridone solid solution in its γ II crystal form contains about up to 15% by weight of 6,13-dihydroquinacridone. The resulting solid solutions have an X-ray diffraction pattern with peaks substantially corresponding to those of γ I or γ II quinacridone, respectively. However, the location of the peaks may be slightly shifted. Such solid solutions have been described in the literature as solid compounds. Usually, the quinacridone / 6,13-dihydroquinacridone solid solution in the γ I crystal phase prepared according to this invention contains from about 75 to 95%, preferably from 78 to 94% and most preferably from 80 to 92% by weight, of quinacridone, based on the combined weights of the quinacridone and 6,13-dihydroquinacridone, and 5 to 25%, preferably 6 to 22%, and most preferably 8 to 20% by weight of 6,13-dihydroquinacridone. In contrast to this, the quinacridone / 6,13-dihydroquinacridone solid solution in the γ II crystal phase prepared according to this invention contains usually from about 85 to 98%, preferably from 90 to 98% and most preferably from 93 to 98% by weight, of quinacridone based on the combined weights of the quinacridone and 6,13-dihydroquinacridone, and 2 to 15%, preferably 2 to 10%, and most preferably 2 to 7% by weight of 6,13-dihydroquinacridone. Thus, the present invention relates to a process which provides γ I or γ II quinacridone in the form of a solid solution, wherein the γ I quinacridone / 6,13-dihydroquinacridone solid solution has an X-ray diffraction pattern characterized by strong lines at 6.6±0.2, 13.9±0.2 and 26.5±0.2; medium lines at 13.2±0.2, 13.5±0.2, 23.8±0.2, 25.2±0.2 and 28.6±0.2, and weak lines at 17.1±0.2 and 20.5±0.2 degrees 2 double glancing angles, where the peaks at 13.2 and 13.5 degrees 2 double glancing angle can show up as an overlapped peak, and wherein the γ II quinacridone / 6,13-dihydroquinacridone solid solution has an X-ray diffraction pattern characterized by strong lines at 6.6±0.2, 13.9±0.2 and 26.3±0.2; medium lines at 13.2±0.2, 13.4±0.2, 25.2±0.2, 23.6±0.2 and 28.3±0.2; and weak lines at 17.1±0.2 and 20.4±0.2 degrees 2 double glancing angles, but where the peaks at 13.2 and 13.4 degrees 2 double glancing angle often show up as an overlapped peak at 13.3 degrees 2 double glancing angle. The quinacridone / 6,13-dihydroquinacridone solid solution pigments prepared according to the inventive process are generally polymorphically homogeneous, meaning that the product is substantially free of the other crystalline forms of unsubstituted quinacridone. Advantageously, the salt of the 6,13-dihydroquinacridone of formula (II) is a mono- or dialkali metal salt or a mixture thereof. Dialkali metal salts, particularly, the disodium and/or dipotassium salts are preferred, and the most preferred is the disodium salt. The 6,13-dihydroquinacridone salt is prepared, for example, by stirring the 6,13-dihydroquinacridone in a basic medium, for example a basic mixture of water and an alcohol, at a temperature above 30° C, preferably 40 to 60° C, and most preferably between 50° C and the corresponding reflux temperature, for 5 minutes to 3 hours, preferably 20 minutes to 2 hours. It has been found that the crystalline state of the 6,13-dihydroquinacridone salt has a profound influence on the resulting crystal phase of the quinacridone end product. In the inventive process the 6,13-dihydroquinacridone salt is generated for example, by stirring the 6,13-dihydroquinacridone in a basic medium, for example a basic mixture of water and an alcohol, at a temperature above 30°C, preferably 40-60°C, for 5 minutes to 2½ hours, preferably 20 minutes to 1½ hours, yielding a 6,13-dihydroquinacridone salt having a prismatic shape and a relatively large particle size of up to 25 µm. The resulting 6,13-dihydroquinacridone salt is activated by the addition of an acid, preferably an inorganic acid, for example hydrochloric or phosphoric acid and most preferably sulfuric acid. The amount of the acid and its concentration are both important. Generally, the acid is added in concentrated form. The amount employed should be sufficient to neutralize only a part of the excess of the base used to generate the mono - or preferably dialkali metal salt of the 6,13-dihydroquinacridone. Advantageously, the reaction mixture is then immediately diluted with 30 to 150, preferably 80 to 130 parts water based on 100 parts by weight of 6,13-dihydroquinacridone after the addition of the concentrated acid. The resulting reaction mixture is then stirred, preferably at reflux for 30 minutes to 1½ hours. By this procedure the large particle size 6,13-dihydroquinacridone salt is converted to a small particle size 6,13-dihydroquinacridone salt having a particle size of around 0.05 to 2 µm. Any base capable of forming a salt of the 6,13-dihydroquinacridone is useful in the reaction medium. Preferably, the base is an alkali metal hydroxide such as potassium or sodium hydroxide or a mixture thereof. Most preferably the base is sodium hydroxide. The molar ratio of the base to 6,13-dihydroquinacridone is preferably from 2.2 to 4 moles of base per mole of the 6,13-dihydroquinacridone. Advantageously, the reaction medium contains 2.3 to 3 moles of base per mole of the 6,13-dihydroquinacridone. The concentrated acid, preferably sulfuric acid, is employed in an amount that is sufficient to neutralize at least 20% of the base initially present. Preferably, it is employed in an amount that is at least sufficient to neutralize any excess of base beyond 2 moles base per mole of the 6,13-dihydroquinacridone. Advantageously, it is employed in an amount sufficient to adjust the amount of base to between 1.5 and 2 moles of base per mole of the 6,13-dihydroquinacridone. The generation and the activation of the 6,13-dihydroquinacridone salt is easily observable. The transformation can be readily followed under a light microscope by the formation and change of the crystal shape and size of the 6,13-dihydroquinacridone salt crystals. Generally, the oxidation is carried out in a reaction medium obtained by combining a slurry which consists essentially of the 6,13-dihydroquinacridone, the catalyst(s), the base and a suitable liquid phase, to which an aqueous solution of hydrogen peroxide is added. In general, a suitable liquid phase is any liquid medium which promotes the oxidation reaction, and which does not react to a significant extent with the hydrogen peroxide oxidizing agent. Advantageously, the liquid phase is a mixture of an alcohol and water which contains 150 to 500 parts, preferably 200 to 350 parts of water, and 250 to 750 parts, preferably 300 to 600 parts of the alcohol per 100 parts 6,13-dihydroquinacridone; parts being parts by weight, as the liquid phase. The alcohol is for example a C 1 -C 8 alkyl-alcohol, preferably a C 1 -C 3 alkyl-alcohol, especially methanol. The reaction medium is preferably substantially free of other organic solvents. However, organic solvents are tolerated in the reaction medium as long as they do not impair the 6,13-dihydroquinacridone salt generation or the oxidation reaction. For safety reasons and to avoid potential side reactions as well as for a more controllable process, the oxidation reaction is preferably carried out under an inert gas flow, for example a nitrogen flow. In a preferred process, the oxidation to the γ I or γ II quinacridone / 6,13-dihydroquinacridone solid solution is carried out by adding an aqueous solution of the hydrogen peroxide oxidant to a slurry of the 6,13-dihydroquinacridone in a basic mixture of aqueous alcohol and catalysts over a time interval of from 5 minutes to 8 hours at a temperature of from about 50°C to about 100°C, preferably over 30 minutes to 6 hours at a temperature of 60°C or above, preferably above 70°C and most preferably at reflux temperature, and optionally subsequently maintaining the reaction medium at an elevated temperature with stirring for a period of time to complete the oxidation and promote pigment recrystallization. After the addition of the hydrogen peroxide, the reaction medium is thus more preferably maintained at a temperature of 50°C or above, most preferably at reflux temperature, for from 5 minutes to 5 hours, especially 10 minutes to 2 hours, optionally under pressure. The pigment is then isolated by filtration, washing with an alcohol or hot water and drying. The base and the alcohol can be easily regenerated from the filtrate. Advantageously the same alcohol is used in the reaction medium and for the washing. The aqueous solution of hydrogen peroxide generally contains from 1 to 50 weight%, preferably 5 to 30 weight%, and most preferably 10 to 25 weight%, of hydrogen peroxide. The oxidation of the 6,13-dihydroquinacridone salt to the corresponding quinacridone / 6,13-dihydroquinacridone solid solution by hydrogen peroxide is visually followed by the color change of the reaction mixture from gray to red. Advantageously, a small excess of the hydrogen peroxide is used. The molar ratio is, for example, preferably 1.1 to 5 moles, more preferably 1.2 to 3.5 moles, of hydrogen peroxide per mole of the 6,13-dihydroquinacridone. The presence of an oxidation-promoting amount of a catalyst during the oxidation step leads to a higher yield of quinacridone. Additionally, the presence of the catalyst under the oxidation conditions described above results in a quinacridone product that is substantially free of quinacridonequinone. Typically the quinacridone solid solution product contains less than 2.5% by weight of quinacridonequinone. However, a minor amount of quinacridonequinone is tolerated in the product as long as its presence does not substantially reduce the saturation of the final quinacridone pigment. Any compound capable of catalyzing the oxidation of 6,13-dihydroquinacridone under the present reaction conditions can be utilized as the catalyst. Particularly suitable catalysts for use in the inventive process are, for example, the quinone compounds used for the air oxidation of 6,13-dihydroquinacridone to quinacridone. Such quinone catalysts are well-known in the art. In particular, suitable catalysts include anthraquinone compounds, especially anthraquinone, and anthraquinone sulfonic acid derivatives such as anthraquinone-2,6-disulfonic acid or, preferably, anthraquinone-2-sulfonic acid, or salts thereof, in particular the sodium or potassium salts. The sodium or potassium salt of anthraquinone-2-sulfonic acid is especially preferred. The quinone catalyst is present in the reaction medium in an amount which is effective to catalyze the oxidation reaction, for example from 0.005 to 0.1 times the weight of 6,13-dihydroquinacridone, and most preferably from 0.01 to 0.05 times the weight of 6,13-dihydroquinacridone. Unexpectedly, it was discovered that an aromatic nitro compound such as nitrobenzene or a derivative thereof, for example nitrophenols, nitrobenzoic acids and especially nitrobenzene sulfonic acids, or salts thereof, in particular m-nitrobenzene sulfonic acid, can act as a catalyst and, when used in combination with a quinone catalyst, can enhance the yield of the quinacridone. Surprisingly, the presence of the aromatic nitro compound also directs the crystal phase of the resulting quinacridone / 6,13-dihydroquinacridone solid solution into a γ II form. The aromatic nitro compound should be present in the reaction medium in an amount which is effective to induce the transformation to the γ II form of quinacridone, for example from 0.004 to 0.04 times the weight of 6,13-dihydroquinacridone, and most preferably 0.01 to 0.03 times the weight of 6,13-dihydroquinacridone. After the addition of the aromatic nitro compound and prior to the addition of the quinone catalyst and the hydrogen peroxide, the reaction medium is advantageously stirred under reflux for 3 to 15 minutes, especially 5 to 10 minutes. Without limiting this invention to any particular theory, it is believed that the aromatic nitro compound oxidizes a small amount of the 6,13-dihydroquinacridone, yielding γ II quinacridone crystals which can act as seed crystals during the continuous oxidation by gradual addition of hydrogen peroxide. It is also believed that the quinone catalyst acts to oxidize the 6,13-dihydroquinacridone and is itself reduced to the corresponding leuco compound, which is then regenerated by the hydrogen peroxide. Depending on the composition of the liquid phase, the recrystallization time and temperature, transparent smaller particle size or opaque larger particle size quinacridone pigments are generated. Lower temperatures and shorter times favor a smaller particle size transparent product, while higher temperatures and longer times favor a larger particle size more opaque product. In this regard, an aqueous base, preferably sodium hydroxide, can advantageously be added to the reaction mixture continuously during the addition of the oxidant or immediately after completion of the addition to favor in particular the formation of the inventive γ I or γ II quinacridone solid solution in an especially high degree of cristallinity or in larger particle size opaque forms. The amount of added base is preferably 0.1 to 1.0 mole per mole of the 6,13-dihydroquinacridone salt. Additionally, the addition of a particle growth inhibitor before or after the 6,13-dihydroquinacridone salt generation at step (a) or step (b) to control the particle size of the oxidized quinacridone /6,13-dihydroquinacridone solid solution pigment is tolerated as long as it does not impair the generation of the quinacridone / 6,13-dihydroquinacridone solid solution in its γ I or γ II crystal form, respectively, or the oxidation reaction. Particle growth inhibitors, also known as antiflocculating or rheology-improving agents, are well known. Suitable particle growth inhibitors include, for example, phthalimidomethylquinacridone, imidazolylmethylquinacridone, pyrazolylmethylquinacridone, quinacridone sulfonic acid and its salts, for example the aluminum salt, or 1,4-diketo-3,6-diphenylpyrrolo-[3,4-c]pyrrole sulfonic acid and its salts. To achieve an optimum effect, the particle growth inhibitor is added in an amount of 0.05 to 8 %, preferably 0.1 to 5 % based on 6,13-dihydroquinacridone, preferably before the 6,13-dihydroquinacridone salt formation. Furthermore, the presence of other substituted quinacridones, for example, 2,9- or 4,11-dichloroquinacridone, 2,9-dimethylquinacridone or 2,9-dimethoxyquinacridone can be tolerated as long they do not impair the formation of the quinacridone / 6,13-dihydroquinacridone solid solution in its γ I or γ II crystal form, respectively. Preferably the substituted quinacridones are generated from the corresponding 6,13-dihydroquinacridone derivatives. They are preferably added at the beginning of the process, i.e. prior to the 6,13-dihydroquinacridone salt formation at step (a). Since the 6,13-dihydroquinacridone salt generation and the oxidation reaction are advantageously carried out sequentially in the same vessel, practically no handling losses occur. Thus, the process according to this invention provides a quinacridone product in a high yield. In spite of being carried out in a heterogeneous reaction medium, the inventive process provides quinacridone pigments with a narrow particle size distribution. Due to their high purity and desirably narrow particle size distribution, the quinacridone pigments obtained manifest outstanding pigment properties, such as, for example, a high chroma. Surprisingly, despite the presence of the uncolored 6,13-dihydroquinacridone, the inventive quinacridone / 6,13-dihydroquinacridone solid solutions manifest a high color strength, and particularly a highly saturated, attractive masstone color. Additionally, they show outstanding heat, light and weather stability. Depending on the end use, it may be advantageous to add texture improving agents and/or rheology improving agents, for example before the isolation of the pigment, preferably by blending into the aqueous presscake. Suitable texture improving agents are, in particular, fatty acids of not less than 18 carbon atoms, for example stearic or behenic acid or the amides or metal salts thereof, preferably calcium or magnesium salts, as well as plasticizers, waxes, resin acids such as abietic acid or metal salts thereof, colophonium, alkyl phenols or aliphatic alcohols such as stearyl alcohol or vicinal diols such as dodecane-1,2-diol, and also modified colophonium/maleate resins or fumaric acid/colophonium resins or polymeric dispersants. The use of such texture improving agents with quinacridone pigments is well known. These texture improving agents are preferably added in amounts of 0.1 to 30% by weight, most preferably of 2 to 15% by weight, based on the final product. Suitable rheology improving agents are for example the above mentioned antiflocculating agents, which are added preferably in amounts of 2 to 10% by weight, most preferably of 3 to 8% by weight, based on the final product. The present quinacridone solid solution pigments are suitable as coloring matter for inorganic or organic substrates alone or in the presence of other organic or inorganic pigments or coloring agents. They are highly suitable for coloring high molecular weight materials which can be processed to cast and molded articles or which are used in ink and coating compositions such as solvent or water-based coatings, for example automotive coatings. Suitable high molecular weight organic materials include thermoplastics, thermoset plastics or elastomers, for example, cellulose ethers; cellulose esters such as ethyl cellulose; linear or crosslinked polyurethanes; linear, crosslinked or unsaturated polyesters; polycarbonates; polyolefins such as polyethylene, polypropylene, polybutylene or poly-4-methylpent-1-ene; polystyrene; polysulfones; polyamides; polycycloamides; polyimides; polyethers; polyether ketones such as polyphenylene oxides; and also poly-p-xylene; polyvinyl halides such as polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride or polytetrafluoroethylene; acrylic polymers such as polyacrylates, polymethacrylates or polyacrylonitrile; rubber; silicone polymers; phenol/formaldehyde resins; melamine/formaldehyde resins; urea/formaldehyde resins; epoxy resins; styrene butadiene rubber; acrylonitrile-butadiene rubber or chloroprene rubber; singly or in mixtures. Generally, the pigments are used in an effective pigmenting amount, for example, of 0.01 to 30% by weight, preferably 0.1 to 10% by weight, based on the weight of the high molecular weight organic material to be pigmented. Thus, the present invention also relates to a pigmented plastic composition which comprises a plastic material and an effective pigmenting amount of a pigment or pigment solid solution prepared according to a process of the present invention, and to a process for preparing said pigmented plastic compositions. The present pigments are easily dispersible and can be readily incorporated into organic matrixes to provide homogenous colorations possessing high saturation and excellent light and weather fastness properties. The high molecular weight organic materials are pigmented with the pigments of the present invention by mixing the pigments, if desired in the form of a masterbatch, into substrates using high shear techniques including roll mills or a mixing or grinding apparatus. The pigmented material is then brought into the desired final form by known methods, such as calendering, pressing, extruding, brushing, casting or injection molding. The following examples describe certain embodiments of this invention, but the invention is not limited thereto. In these examples all parts given are by weight unless otherwise indicated. The X-ray diffraction patterns are measured on a RIGAKU GEIGERFLEX diffractometer type D/Maxll v BX. Example 1: A one liter flask, equipped with thermometer, stirrer, condenser and gas inlet tube is charged with 50.0 g 6,13-dihydroquinacridone and 250 ml methanol. Nitrogen is slowly introduced to the stirred mixture via a gas flow outlet positioned well above the reaction mixture. Then 29.8 g 50% sodium hydroxide are added, followed by heating to 50 to 55° C. The gray suspension is stirred at 50 to 55°C for one hour, yielding a 6,13-dihydroquinacridone salt with a particle size of up to 25 µm. Then 5.0 g concentrated (96%) sulfuric acid are added to the stirred suspension, followed by 40 ml water. The mixture is heated to reflux and stirred for one hour at reflux, resulting in a 6,13-dihydroquinacridone salt having a particle size of around 0.02 to 2 µm. Then 0.5 g of anthraquinone-2-sulfonic acid sodium salt are added into the reaction mixture. A 150 ml beaker is charged with 44.0 g 35 % hydrogen peroxide and 40 ml cold water, resulting in 84 g 18.33 % aqueous hydrogen peroxide solution. The hydrogen peroxide solution is transferred from the beaker to the stirred reaction mixture in the flask, which is under reflux and nitrogen flow, using a Cole-Parmer Masterflex® peristaltic pump, with tubing type 6404-14, at a pumping rate of 0.3 ml/min. The total transfer time for the hydrogen peroxide is around 3 h 50 min. The 6,13-dihydroquinacridone is oxidized continuously. The oxidation is practically complete at the end of the hydrogen peroxide addition. The reaction mixture is thick in the beginning but easily stirrable. Generally, the suspension gets thinner as a function of time. No foaming is observed, even where the hydrogen peroxide hits the reaction mixture. The pigment suspension is stirred at reflux under nitrogen flow for another 10 min. Then it is diluted with 70 ml cold water and vacuum filtered hot. The press cake is washed with 5 l of hot water to a pH of 8.0 to 8.5. The filtration and washing is relatively fast. The product is dried over night in an oven at 90°C yielding 48.8 g red pigment. An analytical evaluation of the red quinacridone pigment by spectrophotometric methods shows a quinacridone content of 79% and a 6,13-dihydroquinacridone content of 21%. The X-ray diffraction pattern, which displays the pattern of a γ I quinacridone, with no peaks of the 6,13-dihydroquinacridone present, is characterized by the following diffraction lines: scattering angle [°2] relative intensity [%] 6.84 92 13.35 57 13.70 60 14.16 82 17.36 16 20.63 19 23.84 23 25.45 26 26.49 100 28.68 24 By rubout according to ASTM method D-387-60 in a lithographic varnish, the pigment shows an opaque yellowish-red masstone and a strong red color in TiO 2 extension. When incorporated into plastics or paints, the pigment imparts a yellowish-red color with excellent fastness properties. Example 2: A one liter flask, equipped with thermometer, stirrer, condenser and gas inlet tube is charged with 50.0 g 6,13-dihydroquinacridone and 250 ml methanol. To the stirred mixture nitrogen is slowly introduced with the gas flow outlet well above the reaction mixture; then 29.8 g 50% sodium hydroxide are added, followed by heating to 50 to 55°C. The gray suspension is stirred at 50 to 55°C for one hour, yielding a 6,13-dihydroquinacridone salt with a particle size of up to 25 µm. Then 5.0 g concentrated sulfuric acid are added followed by 40 ml water. The mixture is heated to reflux and stirred for one hour at reflux, resulting in a 6,13-dihydroquinacridone salt having a particle size of around 0.02 to 2 µm. Then 1 g m-nitrobenzenesulfonic acid, sodium salt is added, the reaction mixture is stirred for 5 min at reflux, followed by the addition of 0.5 g of anthraquinone-2-sulfonic acid, sodium salt. A 150 ml beaker is charged with 44.0 g 35% hydrogen peroxide and 40 ml cold water, resulting in 84 g 18.33% aqueous hydrogen peroxide solution. The hydrogen peroxide solution is transferred from the beaker to the stirred reaction mixture in the flask, which is under reflux and nitrogen flow, using a Cole-Parmer Masterflex® peristaltic pump, with tubing type 6404-14, at a pumping rate of 0.3 ml/min. The total transfer time of the hydrogen peroxide is around 3 h 50 min. The 6,13-dihydroquinacridone is oxidized continuously. The oxidation is practically complete at the end of the hydrogen peroxide addition. The reaction mixture is thick in the beginning but easily stirrable. Generally, the suspension gets thinner as a function of time. No foaming is observed, even where the hydrogen peroxide hits the reaction mixture. The pigment suspension is stirred at reflux under nitrogen flow for another 10 min. It is then diluted with 70 ml cold water and vacuum filtered hot. The press cake is washed with 51 of hot water to a pH of 8.0 to 8.5. The filtration and washing is relatively fast. The product is dried over night in an oven at 90°C yielding 48.8 g red pigment. An analytical evaluation of the red quinacridone pigment by spectrophotometric methods shows a quinacridone content of 94.6%, a 6,13-dihydroquinacridone content of 5.3% and a quinacridonequinone content of 0.1%. The X-ray diffraction pattern displays the characteristic pattern of a γ II quinacridone, with no peaks of the 6,13-dihydroquinacridone present. It is characterized by the following diffraction lines: scattering angle [°2] relative intensity [%] 6.54 100 13.24 (overlapped) 66 13.86 86 17.15 13 20.32 16 23.56 22 25.18 26 26.23 92 28.31 23 By rubout according to ASTM method D-387-60 in a lithographic varnish, the pigment shows an opaque bluish-red masstone and a strong red color in TiO 2 extension. When incorporated into plastics or paints, the pigment imparts a bluish-red color with excellent fastness properties. Example 3: This Example illustrates the incorporation of the quinacridone / 6,13-dihydroquinacridone solid solution in its γ II crystal form, obtained according to Example 2, into an automotive paint system. • millbase formulation The following ingredients are added into 1/2 pint jar containing, as dispersion medium, 980 g 4 mm rods: 66.0 g of acrylourethane resin 14.5 g of AB-dispersant, 58.1 g of SOLVESSO 100, and 26.4 g quinacridone / 6,13-dihydroquinacridone solid solution according to Example 2. The pigment is dispersed by rolling the above mixture for 64 h. The pigment dispersion is separated from the dispersion media providing a homogenous "millbase dispersion" containing 16.0% quinacridone pigment solid solution having a total solids content of 48.0%, and a pigment to binder ratio of 0.5. Example 3A: 47.3 g of the above millbase, 36.4 g of a solution containing a melamine resin catalyst, a non-aqueous dispersion resin, a UV absorber, and 16.3 g of a resin solution containing a polyester urethane resin are mixed and diluted with a solvent mixture containing 76 parts xylene, 21 parts butanol and 3 parts methanol to a spray viscosity of 20-22 s as measured by a #2 Fisher Cup. The red resin/pigment dispersion is sprayed onto a panel twice at 1.5 min intervals as basecoat. After 2 min, clearcoat resin is sprayed twice at 1.5 min intervals onto the basecoat. The sprayed panel is then flashed with air in a flash cabinet for 10 min and then "baked" in an oven at 121°C (250°F) for 30 min, yielding a high chroma bluish-red colored panel with excellent weatherability. Example 3B: (tint color) • white base A TiO 2 dispersion is prepared by mixing the following ingredients in a can: 604.1 g of a TiO 2 pigment, 129.8 g of acrylourethane resin, and 161.1 g of SOLVESSO 100. 473 ml (1 US pint) of 1.26 cm ceramic balls are then added. The dispersion is then milled for 24 h. The white pigment dispersion is separated from the balls yielding a "TiO 2 dispersion" containing 67.5% pigment with a total solids content of 77.4%. • tint shade A 10/90 tint shade is prepared by mixing the following ingredients: 15.7 g mill base, 33.4 g white base, 20.0 g of resin solution containing a melamine resin catalyst, a non-aqueous dispersion resin, a UV absorber and 30.9 g of a resin solution containing a polyester urethane resin. The paint contains 25.1% pigment with a pigment/binder ratio of 0.7 and a total non-volatile content of 60.9%. The red pigment/TiO 2 /resin dispersion is sprayed onto a panel followed by a clearcoat as described in Example 3A, yielding a high gloss red tinted panel with excellent weatherability. Example 4a (incorporation of the quinacridone / 6,13-dihydroquinacridone solid solution pigment into a PVC sheet): 63.0 g of polyvinylchloride, 3.0 g epoxidized soy bean oil (PARAPLEX G-62 from The C.P. Hall Company), 2.0 g of barium/cadmium heat stabilizer, 32.0 g dioctyl phthalate and 1.0 g of the γ I crystal phase quinacridone / 6,13-dihydroquinacridone solid solution according to Example 1 are mixed together in a glass beaker using a stirring rod. The mixture is formed into a soft PVC sheet with a thickness of about 0.4 mm by rolling for 8 min on a two roll laboratory mill at a temperature of 160°C, a roller speed of 25 rpm and friction of 1:1.2 by constant folding, removal and feeding. The resulting soft PVC sheet is colored in an attractive yellowish-red shade with excellent fastness to heat, light and migration. Example 4b: A PVC sheet having a bluish-red colored shade is achieved following the procedure of Example 4a, however using the γ II crystal phase quinacridone /6,13-dihydroquinacridone solid solution pigment of Example 2. Example 5a (incorporation of a quinacridone / 6,13-dihydroquinacridone solid solution pigment into HDPE): 5 g of the γ I crystal form quinacridone / 6,13-dihydroquinacridone solid solution pigment according to Example 1, 2.5 g hindered amine light stabilizer, 1.0 g benzotriazole UV absorber, 1.0 g hindered phenol antioxidant and 1.0 g phosphite process stabilizer, all from CIBA Specialty Chemicals Inc., are mixed together with 1000 g of high density polyethylene from U.S.I. QUANTUM Chemicals at a speed of 175-200 rpm for 30 s after flux. The fluxed pigmented resin is chopped up while warm and malleable, and then fed through a granulator. The resulting granules are molded on an injection molder with a 5 min dwell time and a 30 s cycle time at temperatures of 204, 260 and 316°C. Homogeneously colored chips are obtained which show a bright yellowish-red color with similar color shades at each of the temperatures and with an excellent light stability. Example 5b: HDPE chips having a bluish-red colored shade and equally good light and heat stability are obtained following the procedure of Example 5a, however using the γ II crystal phase quinacridone / 6,13-dihydroquinacridone solid solution pigment of Example 2. Example 6a (incorporation of the quinacridone / 6,13-dihydroquinacridone solid solution pigment in its γ II crystal form into polypropylene fibers): 1000 g of polypropylene granules (DAPLEN PT-55® , from Chemie Linz) and 10 g of the quinacridone / 6,13-dihydroquinacridone solid solution pigment according to Example 2 are thoroughly mixed in a mixing drum. The granules so obtained are melt spun at 260-285°C to afford red filaments of good light fastness and textile fiber properties.

Claims: A process for the preparation of a quinacridone /6,13-dihydroquinacridone solid solution in the γ I or γ II crystal form of quinacridone comprising quinacridone of the formula (I) and 6,13-dihydroquinacridone of the formula (II) which comprises (a) preparing a 6,13-dihydroquinacridone salt, (b) converting the 6,13-dihydroquinacridone salt having a particle size of up to 25 µm to a 6,13-dihydroquinacridone salt having a particle size of 0.05 to 2 µm by the addition of an acid, preferably concentrated sulfuric acid, (c) optionally, adding a catalytically effective amount of an aromatic nitro compound if the γ II crystal form is desired, (d) oxidizing the 6,13-dihydroquinacridone salt having a particle size of 0.05 to 2 µm by the addition of hydrogen peroxide in the presence of a catalytically effective amount of a quinone catalyst, and (e) isolating the γ I or γ II quinacridone / 6,13-dihydroquinacridone solid solution pigment. A process of claim 1, wherein said quinacridone / 6,13-dihydroquinacridone solid solution has the crystal phase of a γ I quinacridone and consists of 80 to 92% of quinacridone and 8 to 20% of 6,13-dihydroquinacridone based on the combined weights of the quinacridone and 6,13-dihydroquinacridone. A process of claim 1, wherein said quinacridone / 6,13-dihydroquinacridone solid solution has the crystal phase of a γ II quinacridone and consists of 93 to 98% of quinacridone and 2 to 7% of 6,13-dihydroquinacridone based on the combined weights of the quinacridone and 6,13-dihydroquinacridone. A process of claim 1, wherein the 6,13-dihydroquinacridone salt is a mono- or dialkali metal salt, preferably the disodium salt, and wherein from 2.2 to 4 moles of an alkali metal hydroxide, preferably from 2.3 to 3 moles of sodium hydroxide, are utilized per mole of the 6,13-dihydroquinacridone to prepare the large particle size 6,13-dihydroquinacridone salt. A process of claim 1, wherein the oxidation step is carried out by combining a slurry comprising the 6,13-dihydroquinacridone salt, the quinone catalyst, the aromatic nitro compound if present, the base and a liquid phase, with an aqueous solution of hydrogen peroxide at a temperature above 60°C. A process of claim 5, wherein the liquid phase consists of 150 to 500 parts by weight of water and 250 to 750 parts by weight of a C 1 -C 8 alkyl-alcohol per 100 parts by weight of 6,13-dihydroquinacridone, preferably consists essentially of from 200 to 350 parts by weight of water and from 300 to 600 parts by weight of the alcohol per 100 parts by weight of 6,13-dihydroquinacridone. A process of claim 6, wherein the alcohol is a C 1 -C 3 alkyl-alcohol, preferably methanol. A process of claim 1, wherein the quinone catalyst is present in an amount of from 0.005 to 0.1 times the weight of the 6,13-dihydroquinacridone and is preferably selected from the group consisting of anthraquinone, anthraquinone monosulfonic add and anthraquinone disulfonic acid, or a salt thereof, most preferably is anthraquinone-2-sulfonic acid, sodium or potassium salt. A process of claim 5, wherein the oxidation step is carried out with a 5 to 30% by weight aqueous solution of hydrogen peroxide, and wherein 1.1 to 5 moles of the hydrogen peroxide are combined per mole of 6,13-dihydroquinacridone. A process of claim 5, wherein the aqueous solution of hydrogen peroxide is added to the slurry over a time interval of from 5 minutes to 8 hours at a temperature of from 60°C to reflux and the reaction medium is subsequently stirred at a temperature of from 50°C to reflux for from 10 minutes to 2 hours. A process of claim 3, wherein an amount of 0.01 to 0.03 times the weight of 6,13-dihydroquinacridone of an aromatic nitro compound is added prior to the addition of the quinone catalyst and the hydrogen peroxide oxidant. A process of claim 11, wherein the aromatic nitro compound is selected from the group consisting of nitrophenols, nitrobenzoic acids and nitrobenzene mono sulfonic acids, and salts thereof. A process of claim 1, wherein the oxidation step is carried out in the presence of from 0.05 to 8 % by weight based on the 6,13-dihydroquinacridone of a particle growth inhibitor, preferably of a particle growth inhibitor selected from phthalimidomethyl-, imidazolylmethyl-or pyrazolylmethyl-quinacridone, quinacridone monosulfonic acid or a salt thereof, and a 1,4-diketo-3,6-diarylpyrrolo[3,4-c]pyrrole sulfonic acid, or a salt thereof. A composition comprising a high molecular weight material and an effective pigmenting amount of a quinacridone / 6,13-dihydroquinacridone solid solution pigment of claim 1, with the proviso that said solid solution has the crystal phase of γ I quinacridone. A composition of claim 14, wherein said quinacridone / 6,13-dihydroquinacridone solid solution in the γ I crystal phase consists of 80 to 92% of quinacridone and 8 to 20% of 6,13-dihydroquinacridone based on the combined weights of the quinacridone and 6,13-dihydroquinacridone.

Inventors: BAEBLER FRIDOLIN, BAEBLER, FRIDOLIN, Bäbler, Fridolin
Applicants: CIBA SC HOLDING AG, CIBA SPECIALTY CHEMICALS HOLDING INC.
Application year: 1998
Main classification: C09B67
IPC codes: C09B67, C07D471, C09B48