Synthetic pigment having the color of natural uncalcined umber and its use

Abstract

Pigments having the color of natural uncalcined umber contain more than 85% by weight of iron oxides, expressed as Fe.sub.2 O.sub.3, and less than 0.1% by weight of crystalline silica are obtained by mixing iron oxides of spinel structure and/or hematite structure with or without iron oxide yellow and said pigments are useful in coloring paints, lacquers and building materials.

Description

This invention relates to a synthetic pigment having the color of natural uncalcined umber and to the use of this pigment.

BACKGROUND OF THE INVENTION

Natural umbers differ widely in their composition, depending on origin, and consist of a mixture of various minerals, such as goethite, manganese dioxide, alumosilicates and crystalline quartz. They contain inter alia approximately 40% Fe.sub.2 O.sub.3, 5 to 10% Mn.sub.2 O.sub.3 and 10 to 20% SiO.sub.2.

It is known that naturally occurring pigments show distinct variations in composition and color which are troublesome in many applications. In addition, the presence of crystalline quartz in quantities of more than 1.0% is objectionable on account of the carcinogenic effect of corresponding fine dusts. The MAC value for dusts such as these is 0.15 mg solids per m.sup.3 air. In addition, under the Californian Safe Drinking Water and Toxic Enforcement Act, 1985 (Proposition 65), goods containing more than 0.1% crystalline quartz have to be declared.

For the reasons stated above, many naturally occurring pigments have already been displaced from their applications by synthetic pigments because synthetic pigments are not attended by the disadvantages mentioned.

Natural umbers are used inter alia in paints and lacquers, for example in emulsion paints and multipurpose tinting pastes. The umbers are often used in admixture with other pigments in various quantitative ratios, for example for "breaking" colors. For the applications mentioned, it is very important that a synthetic pigment such as this should correspond to natural umber in color in various mixing ratios (lightening) of pigment and white pigment. There has been no shortage of attempts to adjust a corresponding pigment by mixing commercially available pigments, more particularly by mixing iron oxides. However, corresponding mixtures, such as commercial iron oxide brown pigments for example, undergo an unwanted change of shade with increasing lightening.

Accordingly, the problem addressed by the present invention was to provide a synthetic pigment having the color of natural uncalcined umber which would not have any of the described disadvantages.

BRIEF DESCRIPTION OF THE INVENTION

Pigments which satisfy these requirements have now surprisingly been found. They have the color of natural uncalcined umber and are characterized by an iron content, expressed as Fe.sub.2 O.sub.3 of more than 85% by weight and a content of crystalline silica of less than 0.1% by weight. The pigments according to the invention can be obtained by mixing iron oxides of spinel structure and/or hematite structure and, optionally, an iron oxide yellow.

DETAILED DESCRIPTION

Accordingly, the pigment according to the invention preferably consists of a mixture of synthetic magnetite black (Fe.sub.3 O.sub.4), at least one synthetic maghemite brown (.gamma.-Fe.sub.2 O.sub.3) or hematite red (.beta.-Fe.sub.2 O.sub.3) and, optionally, synthetic goethite yellow (.gamma.-FeOOH) and/or lepidocrocite orange (.gamma.-FeOOH).

The preparation of the starting materials is generally known, cf. for example Ullmann, Enzyklopadie der technischen Chemie, 4th Edition, Weinheim (1979), Vol. 18, pages 600-604. A preferred process for the production of maghemite is described in DE-A 3 820 499.

In one preferred embodiment, the pigment according to the invention contains magnetite black in quantities of 10 to 80% by weight, hematite red in quantities of 0 to 60% by weight, maghemite brown in quantities of 0 to 30% by weight, goethite yellow in quantities of 1 to 40% by weight and lepidocrocite orange in quantities of 1 to 10% by weight as key constituents of the mixture.

In one particularly preferred embodiment, the mixture contains 50 to 75% by weight magnetite black, 1 to 40% by weight goethite yellow, 0 to 10% by weight hematite red and 0 to 30% by weight maghemite brown.

In addition, the low content of soluble salts in the pigment according to the invention is an advantage. According to DIN ISO 787/Part 8, it is less than 1.0% and preferably from 0.5 to 0.9%. The content of soluble salts in natural umbers, at up to 1.5%, is distinctly higher and is therefore a disadvantage for certain applications.

The pigment according to the invention preferably has an iron content, expressed as Fe.sub.2 O.sub.3, of 85 to 99% and a manganese content of less than 1%. The crystalline quartz content was determined by diffractometry after repeated evaporation with hydrochloric acid and after calcination to hematite at 800.degree. C. and is less than 0.1%. Accordingly, the low crystalline quartz content and the low manganese content are particularly advantageous because, in contrast to natural umber, no elaborate measures for avoiding dust emission have to be taken for the pigment according to the invention.

The pigment mixtures may be obtained in the usual way by mixing the various components and grinding the mixture in a dismembrator or in a vibrating disk mill. To determine the color tones, the pigments are colorimetrically evaluated both in pure form and after lightening in various ratios in Alkydal.RTM. F48 or L64 (alkyd resins, products of Bayer AG) in accordance with DIN 6174 (equivalent to ISO DIN 7724, 1-3 Drafts). Various quantities of pigments and TiO.sub.2 R-KB-2.RTM. (a product of Bayer AG) were used to prepare the lightened forms. The color values are expressed in CIELAB units either as absolute values or in relation to a commercially available uncalcined umber.

The color of the pigments according to the invention is characterized in that the color angle h both for the pure form and for the form lightened with TiO.sub.2 in a ratio of 1:10 may assume values of 60 to 90, the saturation C* being from 1 to 20 and preferably from 2 to 12. Accordingly, these pigments are clearly distinguished from known iron oxide pigments. To evaluate lightening behavior, the residual color differences between the pigments according to the invention and commercial umbers lightened in various ratios were determined. The pigment according to the invention shows very little difference in color from natural umbers both in pure form and in the lightened forms.

Commercially available iron oxide black pigments in the form of magnetite, red pigments in the form of hematite, yellow pigments in the form of goethite and/or lepidocrocite and a brown pigment in the form of maghemite (.gamma.-Fe.sub.2 O.sub.3) were used for the mixtures. Pigments showing neutral lightening behavior corresponding to natural umber were mainly obtained in the case of mixtures containing an iron oxide black as principal component and an iron oxide brown in the form of maghemite, an iron oxide red in the form of hematite and an iron oxide yellow in the form of goethite as secondary components.

The pigments according to the invention are strongly colored and show coloring strengths of 100 to 160% compared with natural umbers. The specific surface of the pigments according to the invention, as measured by the BET method, is from 10 to 30 and preferably from 15 to 20 m.sub.2 /g (DIN 66 131, nitrogen one-point method).

The pigments according to the invention are readily dispersible in binders of the type used for the production of multipurpose tinting pastes. The lower binder demand compared with natural umbers and the high solids content of the pastes obtained are particular advantages.

The present invention also relates to the use of the pigment according to the invention for the production of paints and lacquers, such as complete paints and tinting paints, to its use in multipurpose tinting pastes and to its use for pigmenting building materials.

The following Examples are intended to illustrate the invention without limiting it in any way.

COMPARISON EXAMPLES

The properties of commercial, natural, uncalcined umbers are investigated in Comparison Examples C1 to 4. The results of these investigations are set out in Tables 1.1, 1.2 and 1.3.

TABLE 1.1______________________________________Contents and physical data of natural uncalcined umbers Comparison Examples 1 2 3 4______________________________________% Fe.sub.2 O.sub.3 40.0 47.6 47.4 --% Mn.sub.2 O.sub.3 6.9 9.5 6.6 --% SiO.sub.2 21.4 13.3 20.8 --% Al.sub.2 O.sub.3 3.3 -- 3.5 3.6% Quartz, crystalline 1.3 0.4 -- 0.9Spec. surface, m.sup.2 /g 64 -- 82 103Oil number, g/100 g 53 -- -- 40______________________________________ -- = not determined TABLE 1.2______________________________________Color values of the natural umbers of Comparison Example1 lightened in various ratiosLighteningpigment: TiO.sub.2 L* a* b* C* h______________________________________Pure form 26.5 0.6 1.7 1.8 70.61:1 50.5 1.0 5.4 5.5 80.11:2 50.5 0.9 5.4 5.4 80.1 1:3.33 58.1 0.6 4.0 4.0 81.51:5 63.0 0.5 3.7 3.7 82.31:7 66.9 0.4 3.3 3.3 83.1 1:15 74.2 0.4 2.9 3.0 82.3 1:30 80.2 0.3 2.3 2.3 81.9______________________________________ TABLE 1.3______________________________________Color values of the natural umbers of Comparison Examples1-4 in pure form and in lightened form (1:10)ComparisonExample L* a* b* C* h______________________________________C1 Pure form 26.5 0.6 1.7 1.8 70.6C2 Pure form 28.0 1.7 3.5 3.5 64.6C3 Pure form 26.6 0.3 2.3 2.3 82.4C4 Pure form 26.5 1.0 2.6 2.8 69.0C1 Lightened 70.7 0.4 3.0 3.9 82.4C2 Lightened 72.0 2.0 11.1 11.3 80.0C3 Lightened 71.7 0.7 4.2 4.3 80.5C4 Lightened 70.8 1.7 4.8 5.1 70.5______________________________________

EXAMPLES

For the Examples, various commercial pigments were mixed with one another. Maghemite brown, as described in DE-A 3 820 499, was also used for some mixtures.

To prepare the mixtures, the components were combined, homogenized and ground in a dismembrator or in a vibrating disk mill. The compositions of the mixtures are shown in Table 2.1.

Bayferrox 318 is a synthetic magnetite black (Fe.sub.3 O.sub.4). Bayferrox 930 and 415 are synthetic goethite yellows (.alpha.-FeOOH). Bayferrox 110 is a synthetic hematite red (.alpha.-Fe.sub.2 O.sub.3). Bayferrox 943 is a synthetic lepidocrocite orange (.gamma.-FeOOH).

TABLE 2.1______________________________________Composition of the mixtures of Examples 1-10Mixture components in % by weight Bayferrox .RTM.*Example 318 930 415 110 943 Maghemite brown______________________________________1 70 4 -- -- 262 70 -- 4 -- -- 263 59 -- 10 -- -- 404 40 -- 20 10 -- 305 50 -- 30 10 -- 106 40 -- -- 20 -- 407 10 -- 10 20 -- 608 55 -- 40 5 -- --9 58 -- 37.5 4.5 -- --10 60 -- 30 5 5 --______________________________________ *a product of Bayer AG TABLE 2.2______________________________________Color values of Examples 1-10, pure formExample L* a* b* C* h______________________________________1 27.5 0.6 2.8 2.9 77.92 27.2 0.9 3.5 3.6 76.63 30.0 2.4 6.6 7.0 70.04 30.0 1.9 6.2 6.5 73.05 28.8 0.6 4.1 4.1 81.76 28.7 2.1 4.9 5.3 66.87 32.2 5.0 9.8 11.0 63.08 32.9 1.8 10.1 10.3 79.99 29.5 1.1 5.6 5.7 78.910 29.2 1.1 5.5 5.6 78.7______________________________________ TABLE 2.3______________________________________Color values of Examples 1-10 in lightened form (1:10)Example L* a* b* C* h______________________________________1 67.3 0.8 3.3 3.4 76.42 68.6 1.3 3.2 3.5 67.93 70.4 2.9 7.1 7.7 67.84 70.1 2.4 6.9 7.3 70.85 70.3 0.8 4.2 4.3 79.26 67.3 2.9 5.6 6.3 62.67 69.1 5.4 11.3 12.5 64.68 74.0 1.3 7.7 7.8 80.49 71.3 1.0 3.7 3.8 74.910 71.8 1.1 3.5 3.7 78.7______________________________________ TABLE 2.4______________________________________Color values of Example 1 for various lightening ratiosLighten-ing ratio L* a* b* C* h______________________________________Pure form 27.5 0.6 2.8 2.9 77.91:2 47.2 1.0 2.4 2.6 68.31:5 58.7 0.9 3.3 3.4 74.71:7 62.6 0.8 3.2 3.3 76.0 1:15 70.2 0.7 3.0 3.1 77.0 1:30 76.8 0.6 2.8 2.9 77.7______________________________________ TABLE 2.5______________________________________Color values of Example 9 for various lightening ratiosLighten-ing ratio L* a* b* C* h______________________________________Pure form 29.5 1.1 5.6 5.7 78.91:10 71.3 1.0 3.7 3.8 74.91:20 77.7 1.0 3.2 3.4 72.61:30 80.9 1.0 2.9 3.1 71.0______________________________________ TABLE 2.6______________________________________Color differences of certain pigment mixtures of Examples1-10 lightened in a ratio of 1:10 in relation to thenatural umber of Comparison Example 1 ColoringExample strength (%) .DELTA.a* .DELTA.b* .DELTA.C*______________________________________1 151 0.3 0.1 0.12 154 0.5 -0.9 -0.69 107 0.1 0.2 0.2______________________________________ TABLE 2.7______________________________________Color differences of the pigments mixtures of Examples 1and 2 in relation to the natural umber of ComparisonExample 1 for various lightening ratios (experimentallightness match)Lighten-ing ratio .DELTA.L* .DELTA.a* .DELTA.b* .DELTA.C*______________________________________Example 11:3 0.0 0.4 -1.1 -0.91:5 0.0 0.5 -0.6 -0.41:10 0.0 0.4 0.9 0.11:15 0.0 0.4 -0.2 -0.11:20 0.0 0.4 0.2 0.31:30 0.0 0.3 0.3 0.4Example 2Pure form 0.8 0.1 1.1 1.11:5 0.0 0.3 0.1 0.21:10 0.0 0.3 0.2 0.31:20 0.0 0.2 0.5 0.351:30 0.0 0.2 0.4 0.4______________________________________ TABLE 2.8______________________________________Contents of the mixtures of Examples 1-10 % Cryst. % Water-Example % Fe.sub.2 O.sub.3 % Mn quartz soluble salts______________________________________1 94 <0.1 0.82 94 0.5 <0.1 0.93 93 <0.1 --4 92 <0.1 --5 91 <0.1 --6 95 <0.1 --7 94 <0.1 --8 89 <0.1 0.59 90 <0.1 0.410 90 <0.1 --______________________________________ -- = not determined TABLE 2.9______________________________________Other characteristic data of the mixtures of Example 1- 10 S.sub.BET Oil numberExample m.sup.2 /g g/100 g______________________________________1 14 162 14 153 15 234 15 255 14 256 14 257 19 288 14 269 14 2610 -- 26______________________________________

EXAMPLE 1

The pigment mixture of Example 1 both in pure form and in lightened form shows minor differences in color from the pigment of Comparison Example 1 (see Tables 1.2 and 2.4) and, with a coloring strength of 151%, is dinstinctly stronger in color. The pigment mixture contains less than 0.1% crystalline quartz and 0.8% water-soluble salts. The oil number and the specific surface, at 16 g/100 g and 14 m.sup.2 /g, respectively, are distinctly lower than in the natural uncalcined umbers.

EXAMPLE 2

The pigment mixture corresponds in its properties to Example 1, but contains a smaller amount of soluble salts.

EXAMPLES 3-7

The pigment mixtures of Examples 3-9 both in pure form and lightened in a ratio of 1:10 show a relatively high yellow component and thus correspond to the natural uncalcined umbers with an increased yellow tinge as represented, for example, by Comparison Example 2.

EXAMPLES 8-10

The pigments of these Examples correspond coloristically to the natural umber of Comparison Example 1. The coloring strength is approximately 107%.

Claims

1. A synthetic pigment having the color of natural uncalcined umber which has an iron content, expressed as Fe.sub.2 O.sub.3, of more than 85% by weight and a content of silica of less than 0.1% by weight.

2. A synthetic pigment as claimed in claim 1 which consist essentially of a mixture of synthetic magnetite black (Fe.sub.3 O.sub.4) and at least one synthetic maghemite brown (.gamma.-Fe.sub.2 O.sub.3) or hematite red (.alpha.-Fe.sub.2 O.sub.3), with or without at least one synthetic goethite yellow (.alpha.-FeOOH) or lepidocrocite orange (.gamma.-FeOOH).

3. A synthetic pigment as claimed in claim 2 which contains magnetite black in quantities of 10 to 80% by weight, hematite red in quantities of 0 to 30% by weight, maghemite brown in quantities of 0 to 60% by weight, goethite yellow in quantities of 0 to 40% by weight and lepidocrocite orange in quantities of 0 to 10% by weight as constituents of the mixture.

4. A synthetic pigment as claimed in claim 2 which contains 50 to 75% by weight magnetic black, 1 to 40% by weight goethite yellow, 0 to 10% by weight hematite red and 0 to 30% by weight maghemite brown.

5. A synthetic pigment as claimed in claim 2 which contains less than 1.0% by weight soluble salts.

6. A synthetic pigment as claimed in claim 2, which contains from 0.5 to 0.9% by weight soluble salts.

7. A synthetic pigment as claimed in claim 1 wherein the color, expressed in CIELAB units by the color angle h and the saturation C*, has values of 60 to 90 for h both in the pure form and in the form lightened with TiO.sub.2 in a ratio of 1:10 and the saturation C* value is from 1 to 20.

8. A synthetic pigment as claimed in claim 7 wherein the saturation C* value is from 2 to 12.

9. A synthetic pigment as claimed in claim 1 which has a specific surface (S.sub.BET) of 10 to 30 m.sup.2 /g.

10. A synthetic pigment as claimed in claim 9 which as a specific surface (S.sub.BET) of 15 to 20m.sup.2 /g.

11. In an improved pigmented paint or lacquer, the improvement comprises said paint of lacquer containing a pigmenting amount of synthetic pigment as claimed in claim 1.

12. In an improved building material containing a pigment, the improvement comprises said pigment being a pigment as claimed in claim 1.