Insoluble Pigments and Preparation Thereof

This invention relates to pigments and, more particularly, this invention relates to water-insoluble pigments which are prepared from water-soluble dyes, and methods for making such pigments.

Pigments and dyes are known in the art as coloring agents and are useful in various applications such as for coloring cosmetics, soaps, food, paints, plastics and polymers. Dyes are generally liquids or soluble solids which are used in solution. Pigments, conversely, are generally solids and are usually insoluble in the medium in which the pigment is being used. Pigments are typically preferred over dyes in applications where color migration or bleeding is undesirable. For example, if a dye is used in a toothpaste formulation, the dye may be absorbed by the tongue, teeth and gums of the user. Similarly, in a two tone bar soap, a dye in one color section of the soap may migrate to a different color portion of the soap. Further, the soap dyes may stain the skin of the user as well as sink fixtures, etc. The use of an insoluble pigment, in these applications prevents these undesirable occurrences.

Although there are a number of known water-insoluble pigments, some are considered to be unsuitable because they have been found to be questionable by government regulators on the ground of potential toxicity or carcinogenicity to the user. In addition, some insoluble pigments which are government-approved may suffer from a lack of color flexibility. These problems vary according to the use to which the pigments are put. In the case of soaps and cosmetics, for example, skin irritation may result from the incorporation of some known pigments.

Another issue involves the coloration of plastics that will be used in contact with food. Pending United States Food and Drug Administration (FDA) regulation will require certification of any colorant used in contact with food, but current plastics colorants are for the most part unlikely to qualify for such certification, and dyes and pigments currently approved for food contact do not generally exhibit the chemical and thermal stability necessary for plastics processing.

Another problem encountered in the pigment and dye area is related to commercial desirability. In producing pigments from dyes, a loss of color brightness and intensity, along with hue changes, is encountered. While use of increased amounts of pigments will help to counteract these effects, the expensive cost of the dye precursor represents significant increases in costs associated with pigment production and, ultimately, of the product in which the colorant will be utilized.

Thus, it would be highly desirable to provide a nontoxic, noncarcinogenic, water-insoluble pigment which is suitable for use in body contact and plastics processing applications and which has a relatively greater color flexibility. It would also be desirable to provide a method of preparing such a nontoxic, noncarcinogenic water-insoluble pigment.

In one aspect, the invention is a water-insoluble pigment comprising a complex of a water-insoluble inorganic anion exchange material with a layered structure and a water-soluble dye, said anion exchange material represented by the general formula:

where M is a metal element or elements each with a positive valence of 1; Q is a metal element or elements each with a positive valence of 2; A.sup.-1, A.sup.-2, A.sup.-3, and A-.sup.-4 are each one or more exchangeable anions each having a negative valence of 1, 2, 3, and 4, respectively; and n, x, y, d, e, f and g are real numbers greater than or equal to zero and satisfy the following:

0<.times.+2y.ltoreq.1

d+2e+3f+4g=x+2y

0.ltoreq.n.ltoreq.10

In another aspect, the invention is a method of preparing a water-insoluble pigment comprising contacting together the aforementioned water-insoluble inorgaric anion exchange material with a layered structure and a water-soluble dye. In yet another aspect the invention is the pigment produced by the aforementioned method.

As used herein the term "pigment" is meant to include any substance, usually in finely divided (i.e., powder) form, that is highly colored and imparts color to another substance or mixture as a result of dispersion therein. The pigment is insoluble in an aqueous medium under normal conditions of use.

Preparation of pigments, according to the present invention, requires, as a first component, an anion exchange material with a layered structure and as a second component, a water-soluble dye.

By "anion exchange material with a layered structure" it is meant an essentially inorganic material characterized as exhibiting anion exchange properties, i.e., anion exchange capacity, under normal conditions of use and a material further characterized as having a layered structure as determined by X-ray diffraction, electron diffraction, electron micrsoopy and micro area X-ray fluorescence analysis. The term "anion exchange material with a layered structure" will be used herein interchangeably with "layered anion exchange material" and "anion exchanger."

The preferred anion exchange material with a layered structure used in the present invention may be described as a layered aluminate having the general formula:

where M is a metal element or elements each with positive valence of 1; Q is a metal element or elements each with a positive valence of 2; A.sup.-1, A.sup.-2, A.sup.-3, and A.sup.-4 are each one or more exchangeable anions each having a negative valence of 1, 2, 3, and 4, respectively; and n, x, y, d, e, f, and g are real numbers greater than or equal to zero and satisfy the following:

0<x+2y.ltoreq.1

d+2e+3f+4g=x+2y

0.ltoreq.n.ltoreq.10

In the above equation, the metal element or elements M may be, preferably, selected from the group consisting essentially of lithium, potassium, sodium, cesium, and rubidium and mixtures thereof with lithium being the most preferred. The metal element or elements Q may be, preferably, selected from the group consisting essentially of magnesium, calcium, strontium, barium, iron, cobalt, manganese, nickel, copper, zinc and mixtures thereof, with magnesium and calcium being the most preferred, and even more preferably, magnesium.

The exchangeable anions of the composition of the present invention may be selected from any inorganic or organic exchangeable anions commonly known in the art of anion exchangers. The exchangeable anions may be selected from monovalent, bivalent, trivalent, tetravalent anions, or mixtures of two or more of these exchangeable anions. In the above formula, the anion A.sup.-1, for example, may be an inorganic anion selected from halides such as chlorides (Cl.sup.-1); bromides (Br.sup.-1); iodides (I.sup.-1); and fluorides (F.sup.-1); carbonates such as HCO.sub.3.sup.-1 ; nitrates (NO.sub.3.sup.-1); sulfates such as HSO.sub.4.sup.-1 ; phosphates such as H.sub.2 PO.sub.4.sup.-1 ; hydroxides (OH.sup.-1); and mixtures thereof. For example, the anion A.sup.-1 may be a combination of two or more exchangeable anions described above such as a mixture of Cl.sup.-1 and HCO.sub.3.sup.-1 anions. In the above formula, the anion A.sup.-2, for example, may be an inorganic anion selected from carbonates such as CO.sub.3.sup.-2 ; sulfates such as SO.sub.4.sup.-2 ; phosphates such as HPO.sub.4.sup.-2 ; and mixtures thereof. For example, the anion A.sup.-2 may be a combination of two or more exchangeable anions described above such as a mixture of SO.sub.4.sup.-2 and CO.sub.3.sup.-2. In the above formula, the anion A.sup.-3, for example, may be a phosphate such as PO.sub.4.sup.-3. An example of the anion A.sup.-4 used in the present invention may include organic anions such as ethylenediaminetetraacetic acid (EDTA) and diphosphates such as ##STR1## Other organic exchangeable anions used in the composition of the present invention may include, for example, acetates, stearates, formates, benzoates and mixtures thereof. In addition to the above anions used in the present invention, the composition of the present invention may include a combination of two or more exchangeable anions selected from the group A.sup.-1, A.sup.-2, A.sup.-3 and A.sup.-4 as described above. For example, the composition may include a mixture of exchangeable anions such as Cl.sup.-1 and CO.sub.3.sup.-2 anions or Cl.sup.-1 and SO.sub.4.sup.-2 anions. The total negative charge of the exchangeable anion or mixture of exchangeable anions selected for the composition should be sufficient to balance the excess positive charge of the above composition.

Typical examples of the anion exchange material used in the present invention include, for example, (LiCl).Al(OH).sub.3, (LiOH).sub.0.5.Al(OH).sub.3, and [Ca(OH).sub.2 ].sub.0.25.Al(OH).sub.3. The above anion exchange materials have a layered structure and may be produced by methods known in the art such as described, for example, in U.S. Pat. Nos. 4,348,295, 4,348,297, 4,392,979 and 4,333,846. U.S. Pat. No. 4,348,295 describes a method of preparing lithium aluminates with a two-layer or three-layer structure. A method of preparing magnesium aluminate is disclosed in U.S. Pat. No. 4,392,979. Other aluminates and their preparation are described in U.S. Pat. No. 4,333,846. The layered anion exchangers of the present invention are advantageously used over non-layered anion exchanger because the layered materials provide greater surface area for contacting the dye and greater structure flexibility for substitution of metal elements into the structure which results in a higher anion exchange capacity.

One process of preparing the layered anion exchange materials used in the present invention involves a basic coprecipitation process. The basic coprecipitation method generally involves contacting basic salts of metals desired to be coprecipitated in an aqueous environment. As an example of the basic coprecipitation method, and not to be limited thereby, a sodium aluminate is mixed with lithium hydroxide in water. A precipitate is formed which is filtered and washed. The precipitate is then mixed with a magnesium chloride in water at a pH of about 9 to about 11, preferably, at a pH of about 9.5 to about 10.5, to form a layered anion exchange material having the following general formulation:

All terms in this formula are defined as previously noted.

Another method useful in preparing the anion exchangers includes an acidic coprecipitation method. The acidic coprecipitation method, generally, involves contacting soluble acid salts of the metals desired to be coprecipitated in an aqueous basic environment and thereafter titrating with a base to a pH suitable for precipitation. As an example of the acidic coprecipitation method, and not to be limited thereby, an aqueous solution of an aluminum chloride and lithium chloride is titrated with a base such as sodium hydroxide to form a precipitate. After the precipitate is filtered and washed, it is mixed with magnesium chloride at a pH of about 9 to about 11, preferably at a pH of about 9.5 to about 10.5, to form a layered anion exchange material having the following general formula:

All terms in this formula are defined as previously noted.

The second component of the present invention is a water-soluble dye. The term "dye," as used herein, is meant to include material that will bind, by an anion exchange mechanism, to the material to which it is applied and which will impart the appearance of a solid color to a formulation with which it is mixed. "Water-soluble dye" means that the dye is at least slightly soluble (i.e., at least about 0.01 weight percent soluble) in either pure neutral water, an aqueous salt solution, or an aqueous alkaline or acidic solution in which the dye remains chemically stable.

Any dye and mixtures of two or more dyes which will complex with the layered anion exchange material herein may be used. Preferably, dyes which are designated by the United States Food and Drug Administration (FDA) as Food, Drug and Cosmetic (FD&C) colors, Drug and Cosmetic (D&C) colors, and mixtures thereof may be used. For example, FD&C Blue No. 1, Blue No. 2, Green No. 3, Red No. 3, Yellow No. 5, Yellow No. 6, and mixtures thereof are preferred dyes. Examples of preferred D&C dyes include Green No. 8, Yellow No. 7, Yellow No. 10, and mixtures thereof. These dyes, and their structures and properties are well known to those skilled in the art. Further information may be obtained for instance, in "Kirk-Othmer Encyclopedia of Chemical Technology," Third Edition, Volume 6, pages 561-596.

In carrying out the method according to the present invention, the aforementioned layered anion exchange material is contacted with the dye under conditions in which a water-insoluble pigment is obtained. "Water-insoluble pigment" means that the pigment is not appreciably soluble (i.e., less than about 0.01 weight percent soluble) in either pure neutral water, an aqueous salt solution, or an aqueous alkaline or acidic solution in which the pigment remains chemically stable. Desirably, the layered anion exchange material and the water-soluble dye may be contacted together in a liquid medium in which the dye has been dissolved. The dye may be dissolved in water and the layered anion exchange material added to the water. In another embodiment, the dye may be dissolved in water and then passed through a bed of anion exchanger in an ion-exchange column. Other methods which provide sufficient intimate contact between the dye and the anion exchanger may be used. Once the dye and anion exchanger are contacted together, an insoluble complex forms almost immediately.

The amount of layered anion exchange material and water-soluble dye used may vary. Typically, a ratio by weight of layered anion exchange material to water-soluble dye used is in the range of from about 1.5:1 to about 20:1.

Another method of preparing the pigment of the present invention is to precipitate the layered anion exchange material in the presence of the dye and other desired additives. The method may be carried out in a batch or continuous operation. Preferably, an aqueous solution of the dye and the other desired components is formed. Then an aqueous solution of the water-soluble salts of M and Q, such as previously described, is formed. Thereafter, a base such as sodium hydroxide or ammonia, the salt solution and the solution of the dye and other components are substantially simultaneously contacted together to form a precipitate. The precipitate is the pigment material according to the present invention. The metal salts of M and Q may be dissolved in water separately or together and thereafter contacted with the above solutions as described above. Sufficient base should be used to maintain the desired pH constant and to precipitate the layered anion exchange material.

The size of the pigment particles obtained can vary widely. Typically, for some applications, such as coloring agents for soaps, the particle size is preferably from about 200 angstroms to about 20 microns in diameter. The size of the precipitated particle obtained according to the present invention is preferably in the range of from about 200 angstroms to about 20 microns. The size of the particles can be controlled through techniques such as reagent selection, controlled temperature, pH concentration, and stirring speed during precipitation steps. It is also to be understood that larger size particles than 20 microns can be ground to the desired size.

The various pigments produced by the method of the present invention may be used separately or in combination with each other to form different color pigment materials. For example, a yellow pigment may be blended with blue pigment to form a green pigment.

The pigments may be added to toiletries such as soaps (e.g., soap bars), toothpastes and cosmetics in amounts such that the desired color is obtained using techniques known in the art. The pigments of this invention can also be added to other items such as plastics, polymers or fabrics which are desirable to be colored.

When the pigments are to be used in plastics, one method of incorporation would be to use the pigment desired in either a dry powder form or as a paste or slurry in a suitable solvent. The particular solvent will depend on the plastic used. The pigment is then added or mixed with the pellets or granules of either a thermoplastic or thermoset resin at the time of processing. Because of the significant thermal stability of pigments formed from insolubilized dyes by the formulation of the present invention, a number of processing methods may be employed. These include molding by methods such as: injection molding; compression molding; vacuum forming; blow molding; structural foam, including conventional low pressure, high pressure and expanding mold using either chemical or physical blowing agents; extrusion, including profile, pipe, wire and cable, sheet and coextrusion; coinjection molding; and thermoforming. The incorporation of the insoluble pigments is accomplished by whatever method would be used to incorporate a dye colorant into the selected high polymer. These pigments, in being incorporated into the plastic material, are chemically and thermally stable. The insolubility of the dye is maintained throughout processing and in the final polymer material, so that the dye cannot be adsorbed onto any food with which it comes into contact. This feature will most likely satisfy any FDA regulations and avoid any problems of suspected or actual carcinogenicity associated with dye ingestion. Typical applications will contain a level of pigment within the range of from about 0.001 to about 0.1 percent by weight; however, this amount will vary according to the tint strength of the pigment, the resin selected, and a number of other variables.

The pigment produced by the present invention may be used either alone or in combination with other additives which are not detrimental to the pigment properties. Other materials or components which may be mixed with the pigment may include, for example, fillers such as clays; and extenders such as TiO.sub.2, Al.sub.2 O.sub.3, and Al(OH).sub.3 which will not substantially adversely affect the pigment properties. Other additives may be used, for example, to pelletize, agglomerate or coat the pigment, provided the pigment properties are not substantially adversely affected. The various additives used with the pigment will depend on the application in which the pigment is used.

In addition to using other materials as additives along with the pigments of the present invention in a designated application, certain materials may be complexed with the pigment components to increase the efficacy of the colorant. One problem encountered with pigments made from insolubilized dyes is a loss of color brightness and intensity and also color hue changes occurring during the insolubilization process. An effective way of counteracting this problem is through the complexing of a water-soluble polymer with the basic pigment complex. Modified suspending agents such as cellulosic materials work particularly well. These include carboyxmethyl methylcellulose, carboxymethylcellulose, and other water-soluble polymers having an anionic functionality. The polymer serves to stabilize the color hue and maintain the original brightness and intensity of the dye precursor. An added advantage is that the polymer is insolubilized along with the other pigment components and thus does not affect the inert nature of the pigment particularly desired for many applications, such as in food, cosmetics, and food-contact plastics. The celluloses are of themselves generally physiologically inert anyway.

It is preferable when adding a water-soluble polymer to use up to about 3 percent by weight of the polymer and up to about 40 percent by weight of the dye, the remainder being the inorganic anion exchange material. The small amount of dye needed to produce a pigment capable of imparting a given brightness level and hue stability represents a substantial cost savings in many applications, and the addition of the polymer does not affect other properties of the pigment, such as its chemical and thermal stability and physiological inertness.

The general preferred process for adding the water-soluble polymer comprises steps including dissolving the components of the selected inorganic anion exchange material in water; dissolving the water-soluble polymer in water; combining the two solutions with a source of alkalinity to precipitate the anion exchange material in the presence of the water-soluble polymer; and adding a selected water-soluble dye to the suspended precipitate. The pigment product may then be washed with suitable pH distilled water to remove excess reactants and by-products. Alternatively, the pigment can be completely synthesized first, then the water-soluble polymer added to it in aqueous solution.

The following examples are intended to illustrate the invention and are not intended to limit the scope thereof. In the examples, all parts and percentages are by weight unless otherwise specified.

A 100 gram (g) sample of NaAlO.sub.2.3H.sub.2 O was dissolved in 1,000 milliliters (ml) of water. The aqueous solution was then heated to 90.degree. C. and stirred. To the aqueous solution was added a solution of 50 g LiCl in 200 ml of water while stirring. After a few seconds, a precipitate formed. The precipitate was filtered and washed with water. The washed precipitate was then suspended in 1,000 ml of water. A solution of 250 g of MgCl.sub.2.6H.sub.2 O was then added to the suspension while stirring. The resultant solid precipitate was filtered and washed with water and then dried in an oven in air at 125.degree. C. for three hours. An x-ray diffraction analysis of the solid showed the solid to be a crystalline lithium magnesium aluminate having a layered structure. The solid was suspended in 1,000 ml of water and then 10 g of FD&C Blue No. 1 dye was added to the suspension while stirring. The resultant pigment was washed with water until all excess dye was removed. The pigment was then dried in an oven in air at 125.degree. C. for three hours.

A 500 g sample of AlCl.sub.3. 6H.sub.2 O was dissolved in 2,000 ml of water. A 175 g sample of LiCl was added to the aqueous solution and then the resultant solution was titrated with 50 percent NaOH to form an aqueous solution having a pH of 7.0 and a solid precipitate. The solid precipitate was then filtered and washed with water. The washed solid precipitate was then suspended in 2,000 ml H.sub.2 O. To the aqueous suspension, a 500 g sample of MgCl.sub.2.6H.sub.2 O was added with stirring. The resultant precipitate was then filtered and washed with water and then dried at 125.degree. C. for three hours.

An X-ray diffraction analysis of the solid showed the solid to be a layered lithium magnesium aluminate. The solid was suspended in 1,000 ml of water and then 10 g of FD&C Blue No. 1 dye was added to the suspension while stirring. The resultant pigment was washed with water to remove excess dye. The pigment was then dried in a.n oven in air at 125.degree. C. for three hours.

A sufficient amount of pigment (formed by the method of Example 1) to produce a 1 weight percent concentrate was dispersed in a water and 2 weight percent soap (20 percent sodium cocoate and 80 percent sodium tallowate) slurry. A colored soap was obtained which, after rapid stirring for three days at 60.degree. C., did not lose its color.

A 100 g sample of NaAlO.sub.2.3H.sub.2 O was dissolved in 1,000 ml of water. The aqueous solution was then heated to 90.degree. C. and stirred. To the aqueous solution was added a solution of 50 g LiCl in 200 ml of water. After a few seconds a precipitate formed. The precipitate was filtered and washed with water. An X-ray diffraction pattern showed the material to be a crystalline lithium aluminate material. The washed precipitate was then suspended in 1,000 ml of water and 10 g of FD&C Blue No. 1 dye was added with stirring. The resulting pigment material was filtered, washed with water, and dried at 110.degree. C. for three hours.

A 500 g sample of AlCl.sub.3.6H.sub.2 O was dissolved in 2,000 ml water. A 175 g sample of LiCl was added to the aqueous solution and then the resultant solution was titrated with 50 percent NaOH to form a precipitate at pH 7.0. The precipitate was filtered, washed, and dried at 110.degree. C. for three hours. X-ray diffraction of the solid precipitate showed it to be a crystalline lithium aluminate layered material. A 100 g portion of the solid precipitate was then suspended in 2,000 ml of H.sub.2 O. To the aqueous suspension 10 g of D&C Green No. 8 dye was added with stirring. The resultant pigment was filtered, washed with water, and dried in an oven in air at 125.degree. C. for three hours.

About 100 g MgCl.sub.2.6H.sub.2 O and about 29.7 g AlCl.sub.3.6H.sub.2 O and about 5 g MgCO.sub.3 were dissolved in 250 ml of H.sub.2 O and then coprecipitated in 300 ml of 1 percent carboxymethyl methylcellulose at pH 11 by the addition of 50 percent sodium hydroxide. The resulting precipitate was diluted to 800 ml. A 100 ml portion was mixed with 0.125 g D&C Green No. 8 dye. The pigment product was centrifuged, washed with distilled water at pH 11 and recentrifuged. The final product was a wet cake material with uniform color, smooth texture and fluorescent-like brightness which was not soluble in water from pH 7 to 12. In particular, the pigment retained a hue and brightness very similar to that of the water soluble dye.