GB2042574A - Process for preparing a stable inorganic pigment composition - Google Patents

Abstract

In an aqueous slurry of an inorganic pigment other than ultramarine (eg a chromate, iron oxide, lead oxide, titanium oxide or metal powder pigment), an alkali silicate is reacted with at least one of an organic acid, organic acid derivative, a phosphoroxyacid, a borate ester, a phosphate ester, an alkali metal salt, an ammonium salt, boric acid and ethylene carbonate to form an amorphous silica sol in the slurry. The slurry is maintained at a pH of 7 to 11 and at a temperature of not less than 60 DEG C so as to deposit the sol on the surfaces of the pigment particles.

Description

SPECIFICATION Process for preparing a stable inorganic pigment composition This invention relates to a process for preparing a stable inorganic pigment composition, the particles of which are coated with fine amorphous silica and has good pigment characteristics, i.e. high chemical resistance, hydrogen sulfide resistance, light resistance, weather-ability, heat resistance and storage stability.

Pigments are widely used as colorants for paints, printing ink and many varied articles such as plastics, rubber, construction materials, cosmetics and paper. They are also used as an agent for providing protective coatings because of their advantageous physical and chemical properties such as hiding power, covering power, reflecting properties, insulating effect and rust preventing power. However, the pigments have undesired properties of their own, for example, chalking of titanium oxide, heat- or light-induced discolouration of chrome yellow and yellow oxide and decomposition of inorganic pigments (e.g. cadmium pigments) with acid.

Hence, the type and amounts of pigments to be used are essentially limited by the manufacturing process and use of the articles to which the pigments are applied. On the other hand, the type of pigments determines what articles they should be applied to.

In addition, with the recent improvement in the technology of forming and processing olefinic plastics as well as the use of forming temperatures higher than 250"C, the demand for highly heat resistant pigments has increased. Under these circumstances, various industrial sectors are expressing a strong need for the improvement in pigment characteristics such as light resistance, weatherability, chemical resistance, hydrogen sulfide resistance, heat resistance, storage stability and dispersibility. It is true that many processes for improving the characteristics of pigments have been proposed and implemented in commercial plants, but the fact is they have their own advantages and disadvantages.

For example, Japanese Patent Publication No.

16531/74 teaches a method of covering pigment particles with a metal oxide by hydrothermal treatment with heat and pressure, but the method does not fully achieve the intended improvement of various resisting properties for the probable reason that the particles of the metal oxide grow during the hydrothermal treatment. The method described in Japanese Patent Publication No. 9555/71 covers the particles of a lead chrome pigment with the fine silica produced from a dilute aqueous solution of sodium silica and dilute sulfuric acid. The resulting silica cover makes the pigment very resistant in various aspects, but because of the strong acidity of sulfuric acid, the method has the potential hazard of damaging the pigment and therefore, cannot be used in general applications.The inventors of this invention conducted many runs of experiment and confirmed that the fine silica sol produced by decomposing an aqueous solution of alkali silicate in the presence of a specific decomposer was very active and had extremely high capability of covering the particles of a pigment. Therefore, a technique that can cover the particles of whatever type of pigment with such silica sol will be a great contribution to the pigment industry where the demand for improved pigment characteristics is strong. However, it turned out that the attempt at covering the particles of various pigments with such reactive silica sol of minimum particle size without adversely affecting the particles involved a considerable difficulty. Some combinations of pigment and alkali silicate decomposer may damage the pigment.The slightest variation in the temperature and pH used in decomposition of the aqueous sodium silicate will result in a considerable change in the covering power of the silica, and particularly, the choice of the decomposer which is to be reacted with the aqueous silicate is the most predominant of the factors that affect the covering power of the silica.

As a result of various studies on a method of covering the particles of a pigment with a protective coat which is free from the defects described above, the inventors of this invention have found that certain compounds react with an aqueous solution of alkali silicate and can form an extremely fine and reactive silica which will not damage the particles of an inorganic pigment other than ultramarine. The inventors have also found that the pigment, the particles of which are coated with such silica has remarkably high resisting properties.

Therefore, this invention provides a process for preparing a stable inorganic pigment composition which comprises reacting an alkali silicate with one or more compounds capable of reacting said alkali silicate to form an amorphous silica sol in an aqueous slurry of an inorganic pigment other than ultramarine with maintaining the pH and the temp erature of the slurry at7 to 11 and not lowerthan 60"C, respectively, to deposit said amorphous silica sol on the particle surfaces of said inorganic pigment, said compound being selected from the group consisting of organic acids and derivatives thereof, phosphor oxyacids, borate esters, phosphate esters, alkali metal salts, ammonium salts, boric acid and ethylene carbonate.

The process of this invention is applicable to any type of inorganic pigments but ultramarine, and illustrative typical pigments include the following: chromate pigments, such as chrome yellow, chrome vermilion and barium chromate; iron oxide pigments such as yellow oxide, red oxide and iron black; titanium oxide pigments such as titanium oxide and titanium yellow; lead oxide pigments such as red lead oxide, white lead and litharge; calcium salt pigments such as calcium carbonate; barium salt pigments such as barium carbonate and barium sulfate; magnesium salt pigments such as magnesium carbonate; cobalt pigments such as cobalt violet, cobalt blue and cobalt green, manganese pigments such as manganese violet and manganese blue; cadmium pigments such as cadmium yellow and cadmium red; vermilion, antimony trioxide, lead sulfate, zinc oxide, aluminium oxide, lithopone, viridian and chromium oxide green; and metal powder pigments such as silver, copper, zinc, tin and copper-zinc alloy. Calcined pigment and silicate inorganic powder pigments such as talc, kaolin, aluminumd silicate, magnesium silicate, and calcium silicate may also be employed.

Prior to covering with fine silica, the particles of these pigments may be covered with hydrous metal oxides such as zirconium, aluminum, titanium, cerium, antimony and magnesium as described later. Metal powder pigments may be subjected to the conventional chromate treatment or heat treatment using boric acid. For details of these treatments, see for instance, Japanese Patent Publications Nos: 6568166,2594167, 15468167 and 408/78.

These pigments must, either directly or after the preliminary treatment, be formulated as a uniform slurry before the particles are subjected to the step of covering with a fine amorphous silica. Inorganic pigments can be adequately dispersed in an aqueous medium to form a slurry of a desired concentration, but most pigments are preferably dispersed under alkaline condition. Forthis purpose an alkalifying agent can be used. A surfactant having a wetting effect may be used in dispensing a metal powder pigment since the surfaces of the metal powder pigments are usually coated with various oily coatings in the course of production or application of the pigments. Suitable alkalifying agents include caustic alkali, alkali carbonate, alkali silicate, alkali phosphate and ammonia as well as basic organic matter such as ethanolamine.Less than 2 molll of the alkali is sufficient to achieve uniform dispersion of pigment particles. These alkalifying agents are mentioned here for illustrative purposes only, and they may even be omitted from the treatment of certain types of pigment. Mechanical shearing and ultrasonic waves may advantageously be used to redisperse secondary particles of pigment. The slurry of pigment is generally subjected to hydrothermal treatment, i.e. heating at 60 to 250"C under atmospheric or superatmospheric pressure for a period of at least 30 minutes, preferably from 1 to 5 hours under alkaline condition (e.g. less than 2 molll of alkali).Low slurry concentration requires a drain system and reactor of large scale, whereas high concentration either prevents uniform dispersion of pigment particles or uniform deposition of silica sol in the subsequent step. Therefore, the slurry concentration should be determined at a practical level which is in most cases between about 50 and about 300 gull. The thus prepared slurry of pigment particles is held at a pH between 7 and 11, preferably between 8 and 11, and by adding, if so required, an alkalifying agent set forth above at a temperature of at least 60"C, preferably between 70 and 100 C so as to be subjected to the step of silica sol deposition.If the pigment is subjected to the preliminary treatment described above prior to the step of silica sol deposition, the following method will be used: an aqueous slurry of the proper concentration is mixed with one or more fine hydrous oxides of zirconium, aluminum, titanium, cerium, antimony and magnesium so that the pigment particles are covered with the deposit of such hydrous oxides. To achieve this purpose, an aqueous solution of at least one salt selected from the group consisting of zirconium salt, aluminum salt, titanium salt, cerium salt, antimony salt and magnesium salt is added to the slurry.

Examples of the zirconium salt are zirconium sulfate, zirconium chloride, zirconium nitrate, and basic salts of zirconium; examples of the aluminum salt are aluminum sulfate, aluminum chloride, aluminum nitrate, aluminum phosphate and basic salts of aluminum such as sodium aluminate; examples of the titanium salt are titanium chloride and titanium sulfate; examples of the cerium salt are cerium chloride, cerium sulfate and cerium nitrate; an example of the antimony salt is antimony chloride; and examples of the magnesium salt are magnesium sulfate, magnesium chloride and magnesium nitrate. The concentration of the aqueous solution of these salts if preferably as low as possible, and in most cases, it is preferably less than 10 wt%, more preferably less than 5 wot%, in terms of the corresponding oxide.The aqueous solution of these salts if gradually added to the slurry while it is being stirred at normal temperature (20 ~ 30"C) or while heating. The slurry may have any level of pH so long as it enables the aqueous salt to be hydrolyzed in the slurry. Therefore, a suitable amount of acidifying or alkalifying agent is to be contained in the slurry, and ordinarily, hydrolysis is performed under acidic condition until a fine hydrous oxide is deposited on the pigment particles. The maximum amount of the aqueous solution of metal salt added is 5 wt%, in terms of the corresponding oxide, of the total weight of the pigment and solution.After deposition of the hydrous metal oxide, the pigment particles are redispersed in water or warm water either directly or after separation from the mother liquor, and the pH and temperature of the slurry are adjusted in the manner described above, and the thus prepared slurry is ready for the subsequent step of covering with silica sol. Alternatively, the cake obtained by filtering said mother liquor is dehydrated, redispersed in water or warm water, and the pH and temperature are adjusted likewise to prepare an aqueous slurry suitable for the subsequent step.

Therefore, whether the pigment is subjected to the preliminary treatment or not, the pigment slurry to be subjected to the step of covering with silica desirably has a temperature of at least 60"C, preferably between 70 and 100 C, and a pH of at least 7, preferably between 8 and 11.

The thus prepared pigment slurry is mixed with specific components to form a fine silica sol which covers the pigment particles uniformly. The formation of the silica sol is achieved by adding an aqueous solution of alkali silicate and one or more certain compounds (hereafter referred to as "silica solforming compound") described hereunder while the pH and temperature of the slurry is controlled within a specified range.

In accordance with the present invention, any type of the alkali silicate may be used independently or as a mixture thereof in the form of an aqueous solution.

Generally, it is such that the molar ratio of SiO2 to M2O (M is Na or K) is between 2 and 4, and sodium silicate having a molar ratio of from 3 to 3.5 is advantageously used. For effective production of the silica sol, the concentration of the aqueous solution of alkali silicate should be as low as possible, but it should not be so low as to decrease the operating efficiency and drainage efficiency; in most cases, the practical concentration is between 1 and 10 wt% as SiO2.

The "silica sol-forming compound" used in this invention is a water-soluble compound which is capable of reacting with said aqueous solution of an alkali silicate to form fine particles of silica sol without attacking the pigment crystal or the surfaces of the pigment particles, that is to say, a water-soluble compound containing a hydrogen ion active enough to convert an alkali silicate to silicic acid and condense its silanol group to form a siloxane bond.

Suitable silica sol-forming compounds are those being water-soluble at lower than 100"C, and stated specifically, those having a solubility Qf at least 0.01 g/100 g H2O at200C and a pH of at least 1.0 in 3 wt% aqueous solution thereof.

The silica sol-forming compounds include organic acids and derivatives thereof, phosphor oxyacids, borate esters, phosphate esters, alkali metal salts, ammonium salts, boric acid (orthoboric acid) and ethylene carbonate.

The organic acids which can be used in the invention include carboxylic acids which may have a heterocyclic ring, phenols, sulfonic acids, amino acids and the like, and also include those substituted with a halogen atom, a hydroxy group, a cyano group, an amino group, a nitro group, a lower alkyl group, an allyl group, an alkoxy group and phenyl group. Illustrative organic acids will hereunder be given, but it will be understood from the foregoing that they are by no means intended to limitthe invention.

Carboxylic acids: aliphatic saturated monovalent carboxylic acids, preferably having 1 to 10 carbon atoms, such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid and caproic acid, and their substitution products such as mono-, di- and trifluoroacetic acids, mono-, di- and tribromoacetic acids, sulfoacetic acid, cyanoacetic acid, di- and trimethylacetic acids, acetoacetic acid, mono- and diphenylacetic acids, mono- and difluoropropionic acids, mono- and dichloropropionic acids, monoand dibromopropionic acids, phenylpropionic acids and methylbutyric acid; aliphatic unsaturated carboxylic acids, preferably having 3 to 18 carbon atoms, such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, vinylacetic acid, and cinnamic acid; aliphatic saturated divalent carboxylic acids such as oxalic acid, mesoxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pymelic acid, suberic acid, azelaic acid, sebacic acid, and their substitution products such as oxamin acid, and chlorosuccinic acid; oxycarboxylic acids, preferably having 2 to 14 carbon atoms and having 1 to 4 hydroxy groups such as glyceric acid, glycolic acid, lactic acid, ss-oxypropionic acid, (p-, y-) oxybutyric acid, oxymalonic acid, malic acid, citric acid, tartaric acid, mandelic acid, and aldehyde-acid and keto-acid such as glyoxalic acid, pyruvic acid, oxalacetic acid and levulinic acid; aromatic carboxylic acids, which may be mono or bicyclic such as benzoic acid, (o-, m-, p-) chlorobenzoic acid, dichlorobenzoic acid, (o-, m-, p-)nitrobenzoic acid, dinitrobenzoic acid, (o-, m-, p-)toluylbenzoic acid, ditoluylbenzoic acid, (o-, m-, p-)oxybenzoic acid, (o-, m-, p-)aminobenzoic acid, aminosalicylic acid, sulfosalicylic acid, protocatechuic acid, aspirin, anisic acid, benzilic acid, vanillic acid, sulfobenzoic acid, sulfinic acid, gallic acid, and hippuric acid; and aromatic polyvalent carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid, trimellitic acid, trimesic acid, prehnitic acid, mellophanic acid, pyromellitic acid, benzenepentacarboxylic acid and mellitic acid.

Phenols: such as phenol, (o-, m-, p-)cresol, (o-, m-, p-)chlorophenol, dichlorophenol, trichlorophenol, (o-, m-, p-)nitrophenol, dinitrophenol, (o-, m-, p-)bromophenol, dibromophenol, picric acid, (o-, m-, p-)aminophenol, catechol, resorcin, hydroquinone, pyrogallol, oxyhydroquinone and phloroglucinol.

Sulfonic acids: such as methylsulfonic acid, ethylsulfonic acid, sulfamic acid, methyl sulfate, dimethyl sulfoxide, benzenesulfonic acid, (o-, m-, p-)phenolsulfonic acid, naphthalenesulfonic acid and sulfanilic acid.

Amino acids (preferably containing 2 to 15 carbon atoms: such as glycine, dimethylglycine, alanine, cerin, valine, leucine, isoleucine, phenylalanine, threonine, proline, cysteine, methionine, glutamic acid, aspartic acid, ornithine, lysine and arginine.

Organic acids having a heterocyclic ring: such as furancarboxylic acid, furandicarboxylic acid, thiophenecarboxylic acid, thiophenedicarboxylic acid, pyrrolecarboxylic acid, pyrroledicarboxylic acid, pyridinemonocarboxylic acid, pyridinedicarboxylic acid, pyridinetricarboxylic acid, pyridinetetracarboxylic acid, pyridinepentacarboxylic acid, pyridinemonoacetic acid and pyridinediacetic acid.

The derivatives of the organic acids which can be used in the invention include salts, esters, amides, aldehydes, anhydrides and peroxides. Illustrative derivatives of the organic acids are given below, but not limited thereto.

Salts: carboxylic acid salts of Na, K, Mg, Ba, Zn, Al, Ni, Fe, Cu, Ti and ammonium such as zinc formate, potassium hydrogenformate, ammonium formate, zinc acetate, aluminum acetate, iron acetate, nickel acetate, barium acetate, ammonium acetate, zinc valerate, magnesium valerate, ammonium valerate, zinc malerate, sodium hydrogenmaleate, barium maleate, copper maleate, diammonium maleate, zinc oxalate, sodium hydrogenoxalate, potassium hydrogenoxalate, aluminum oxalate, potassium antimony oxalate, titanium oxalate, copper oxalate, nickel oxalate, zinc sebacate, barium sebacate, magnesium sebacate, ammonium sebacate, sodium hydrogencitrate, potassium hydrogencitrate, aluminum citrate, iron citrate, nickel citrate, ammonium citrate, copper glycolate, ammonium mandelate, zinc benzoate, aluminum benzoate, ammonium benzoate, ammonium carbamate, and potassium hydrogenphthalate.

Esters (preferably of aliphatic alcohols having 1 to 4 carbon atoms): isoamyl formate, ethyl acetate, amyl acetate, glycerol acetate, ethylene glycol acetate, methyl acrylate, ethyl acrylate, ethyl maleate, ethyl oxalate, ethyl hydrogentartrate, dimethyl malonate, dimethyl succinate, dimethyl glutarate, dimethyl adipate, dibutyl sebacate, butyllactone, caprolactone, methyl furan - 2 - carboxylate, methyl picolinate, and methyl nicotinate.

Amides: formamide, dimethylformamide, acetamide, dimethylacetamide, propionam ide, butylamide, valeramide, acrylamide, cinnamamide, maleinamic acid, maleuric acid, oxamide, oxamic acid, sebacic amide, citramide, glycollic acid amide, mandelic acid amide, benzamide, phthalamic acid, terephthalic acid amide, caprolactam, a-pyrrolidone, furan - 2 - carboxylic acid amide, pyrrole - 2 - carboxylic acid amide, picolinic acid amide and nicotinamide.

Peroxides: performic acid, peracetic acid, monoperphthalic acid, perbenzoic acid, permaleic acid and persuccinic acid.

Aldehydes: glutardialdehyde, malonic acid aldehyde and glyoxal.

Illustrative examples of the phosphor oxyacids include orthophosphoric acid, pyrophosphoric acid, phosphinic acid, phosphorous acid, hypophosphoric acid and amidophosphoric acid.

Illustrative examples of the borate esters and the phosphate esters include methyl borate, ethyl borate, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, dimethyl phosphate, diethyl phosphate, dipropyl phosphate, monomethyl phosphate, monoethyl phosphate and monopropyl phosphate.

The alkali metal salts include sodium or potassium salts of phosphor oxyacids, sulfur oxyacids and carbonic acid. Illustrative examples of the alkali metal salts are sodium hydrogensulfite, potassium hydrogensulfite, sodium hydrogensulphate, potassium hydrogensulfate, sodium hydrogencarbonate, potassium hydrogencarbonate, sodium dihydrogenphosphate, potassium dihydrogenphosphate and sodium dihydrogenpyrophosphate.

The ammonium salts include ammonium salts of halogen acids, sulfur oxyacids, phosphor oxyacids, carbonic acid, boron oxyacids and nitrogen oxyacids. Illustrative examples of the ammonium salts are ammonium chloride, ammonium nitrate, ammonium chlorate, ammonium bromate, ammonium sulfate, ammonium hydrogensulfate, aluminium ammonium sulfate, ammonium zinc sulfate, ammonium sulfite, ammonium hydrogensulfite, ammonium amidosulfate, ammonium thiosulfate, ammonium persulfate, ammonium hydrogencarbonate, ammonium carbonate, ammonium dihydrogenphosphate, ammonium pyrophosphate, ammonium metaphosphate, ammonium hydrogenphosphite, ammonium borate and ammonium perborate.

Of these silica sol-forming compounds listed above, aliphatic mono- or divalent carboxylic acids having 1 to 4 carbon atoms and derivatives thereof, phosphate esters, ethylene carbonate, phosphoric acid, boric acid, sodium hydrogensulfate, sodium hydrogensulfite, sodium hydrogencarbonate, sodium dihydrogenphosphate, ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium hydrogensulfate, ammonium sulfite, ammonium hydrogensulfite, ammonium carbonate, ammonium hydrogencarbonate, and ammonium dihydrogenphosphate are particularly preferred in view of solforming capability, influence on pigments, treatments for exhaust water and commercial standpoint.

The silica sol-forming compounds may be used independently or as a mixture in the form of an aqueous solution. Like the alkali silicate, these compounds are preferably used in low concentration, but this limitation is not so strict as with the aqueous solution of an alkali silicate if their type and manner of adding them to the slurry of pigment are properly selected. It is absolutely necessarythatthe reaction for the formation of silica sol in the slurry prepared in the manner described above proceed throughout at the pH of 7 toll, preferably 8 toll, and at a temperature not lowerthan 60"C, preferably 70 to 100 C. The silica sol may be formed in the slurry by adding a separately prepared silica sol to the slurry.

In most cases, however, the aqueous solution of alkali silicate is generally reacted with an aqueous solution of the specified silica sol-forming compounds in the slurry under stirring and atmospheric pressure.

The pH control may advantageously be accomplished by adding the two aqueous solutions simultaneously and over an adequately extended period of time such that the pH of the slurry does not vary more than +0.5 from a predetermined pH within the aforementioned range. However, this is only one example of the method of controlling the pH and temperature of the slurry within the specified range, and if the aqueous solution of the silica sol-forming compound to be added is either neutral or alkaline, addition of the silica sol-forming compound may precede gradual addition of the aqueous alkali silicate. In any event, it is preferred to control the pH of the reaction system by adjusting the rates of addition of the two aqueous solutions. Though not so preferred, alkalifying agents as described above may also be added for the purpose.It is thus essential that the reaction of the formation of the silica sol be carried out slowly under the specified conditions, but 6 hours are sufficient to achieve the purpose and, generally, the reaction may be completed within 1 to 4 hours.

In consequence of the reaction described above, many particles of silica sol are formed within the aqueous slurry of the pigment without causing any adverse effect on the pigment particles. The pea articles obtained under atmospheric pressure and under the conditions controlled within the stated narrow range contain a lot of silanol and/or low polymer siloxane groups and therefore are very active. The particle size is generally less than 200 my, and in most cases, it is less than 100 m. Because of this fineness, the particles of silica sol specifically deposit on the pigment particles and form a substantially continuous coat on them. As a matter of fact, one can observe with an electron microscope the way they cover the pigment particles. According to the many experiments conducted by the inventors of this invention, if the reaction is performed under conditions outside the indicated range, rapid decomposition of the alk ali silicate tends to form a porous siloxane bond con taining silca gel which results in a heterogeneous coat on the pigment particles. The gel is of high molecular weight and inactive, and it not forms a heterogeneous coat with respect to the pigment particles but also it fails to deposit on the pigment and assumes to be a free form.Therefore, neither the method wherein all of the aqueous solution of alkali silicate is added to the slurry at one time and then decomposed through reaction with the silica solforming compound, nor the method wherein the alkali silicate is decomposed rapidly, nor the method wherein it is decomposed under superatmospheric pressure can provide an effective silica coat that increases the resisting properties of the pigment.

The amount of silica with which the pigment particles are to be covered according to this invention varies with the use of the pigment, but its content is at least 1 wt% of the total weight of the pigment composition in terms of SiO2, and in most cases, less than 40 wt% of silica will serve the purpose. The particularly preferred range is from 3 to 25 wt%.

Silica contained in an amount less than 1 wt% is not capable of covering all particles of the pigment and therefore it cannot enhance the resisting properties of the pigment to the intended degree. If the silica content exceeds 40 wt%, the concentration of the pigment becomes so low as to reduce its coloring power.

After formation of the deposit of silica sol, the slurry may, if necessary, be held for a suitable period under the conditions used for the reaction, and is thus neutralized, followed by filtration of the mother liquor. The pigment composition in which the particles are covered with a uniform coat of the silica sol is thoroughly washed by conventional means until salts are no longer observed, and is then dried. The thus obtained composition has remarkably high light resistance, weatherability, chemical resistance, hydrogen sulfide resistance, heat resistance and storage stability.As described in the foregoing, this invention is applicable to a wide variety of inorganic pigments other than ultramarine, and the particles of the pigment compositions prepared by this invention are covered with a continuous coat which is far strongerthan expected and demonstrates the great improvement achieved by this invention.

This invention will hereunder be described in greater detail by reference to the following examples which are given here for illustrative purposes only and are by no means intended to limitthe scope of the invention. Unless otherwise indicated all parts and percents are by weight.

Example 1 A hundred parts of dried and ground particles of chrome yellow (color index No.77600, No. 77601 or No. 77603) were dispersed in 840 parts of water and stirred at room temperature to form a uniform slurry.

An aqueous solution of sodium silicate (JIS No.3) was added to the slurry to bring the pH within the range of 9.0 to 10.0. The slurry was then passed through a homogenizer to make a uniform dispersion of the pigment particles which was held at a temperature between 85 and 90"C and controlled at a pH between 9.0 and 10.0 by adding, as required, an aqueous solution of sodium hydroxide under stir ring. To the slurry being stirred, 500 parts of an aqueous solution of sodium silicate JIS No.3 having a concentration of 4.83 wt'/o as SiO2 (hereafter refer red to as Solution A) and 471 parts of a 2.74 wt% aqueous solution of formic acid were added simultaneously at rates of 3.4 parts/min and 2.6 parts/min respectively.Throughout the addition of the two solutions, the slurry was controlled at a pH between 9.0 and 10.0. Afine silica sol resulted to form a uniform deposit on the pigment particles. The slurry was allowed to stand at a pH between 6.5 and 7.0, washed with water by decantation until no more sodium salt was detected, and filtered and dried at 110 C for 6 hours to provide about 120 parts of a chrome yellow composition the particles of which were covered with fine, dense amorphous silica.

Example 2 A hundred parts of dried and ground particles of chrome vermillion (color index No. 77605) were dispersed in 840 parts of water and stirred at room temperature to form a uniform slurry. An aqueous solution of sodium silicate (JIS No. 3) was added to the slurry to bring the pH within the range of from 9.0 to 10.0. The procedure in Example 1 was repeated to prepare a slurry having a temperature between 90 and 95"C and a pH between 9.0 and 10.0. To the slurry being stirred, 500 parts of Solution A and 675 parts of a 3.6 wt% aqueous solution of orthobovic acid were added simultaneously at rates of 3.4 parts/min and 3.7 parts/min, respectively.

Throughout the addition of the two solutions, the slurry was controlled at a pH between 9.0 and 10.0. A fine silica sol resulted to form a uniform deposit on the pigment particles. The procedure of Example 1 was repeated to provide about 120 parts of a chrome vermilion composition the particles of which were covered with fine, dense amorphous silica.

Example 3 A hundred parts of a synthetic yellow iron oxide pigment (color index No. 77492) were dispersed in 480 parts of water containing 0.48 parts of ferric chloride, and stirred at room temperature to form a uniform slurry. The procedure of Example 1 was repeated to prepare a slurry having a temperature between 90 and 95"C and a pH between 9.0 and 10.0.

To the slurry being stirred, 128 parts of Solution A and 123 parts of a 5.0 wt'/o aqueous solution of crotonic acid were added simultaneously at rates of 0.9 parts/min and 0.7 parts/min respectively.

Throughout the addition of the two solutions, the slurry was controlled at a pH between 9.0 and 10.0.

The procedure of Example 1 was repeated to provide about 106 parts of a synthetic yellow oxide pigment composition the particles of which were covered with fine, dense, amorphous silica.

Example 4 The procedure of Example 3 was repeated to treat a hundred parts of red oxide (color index No.77491) except that 281 parts of a 3.5 w1% aqueous solution of orthophosphoric acid were added at a rate of 1.5 parts/min instead of the crotonic acid solution.

About 106 parts of a red oxide composition the parti cles of which were covered with fine, amorphous silica was provided.

Example 5 The procedure of Example 3 was repeated to treat a hundred parts of iron black (color index No. 77499) except that 287 parts of a 2.55 wt% aqueous solution of valeric acid were added at a rate of 1.6 parts/min instead of the crotonic acid solution. About 106 parts of an iron black composition the particles of which were covered with fine, amorphous silica was provided.

Example 6 The procedure of Example 1 was repeated to disperse 100 parts of cadmium yellow (color index No.

77199) in 840 parts of water and prepare a slurry of cadmium yellow having a temperature between 85 and 90"C and a pH between 9.0 and 10.0. To the slurry being stirred, 456 parts of Solution A and 452 parts of a 5.0 wtO/o aqueous solution of butyric acid were added simultaneously at rates of 3.0 parts/min and 2.5 partslmin respectively. A fine silica sol resulted to form a uniform deposit on the pigment particles. The slurry was allowed to stand at a pH between 6.5 and 7.0, and the procedure of Example 1 was repeated to provide about 119 parts of a cadmium yellow composition the particles of which were covered with a fine, dense amorphous silica.

Example 7 A hundred parts of well ground antimony trioxide (color index No.77052) were dispersed in 1,000 parts of water and stirred at room temperature to form a uniform slurry. The procedure of Example 1 was repeated to prepare a uniform dispersion of anti monytrioxide having a temperature between 90 and 95"C and a pH between 9.0 and 10.0. To the slurry being stirred, 128 parts of Solution A and 123 parts of a 4.07 wt% aqueous solution of citric acid were added simultaneously at rates of 0.9 parts/min and 0.7 partslmin, respectively. Throughout the addition of the two solutions, the slurry was controlled to have a pH between 9.0 and 10.0.The procedure of Example 1 was repeated to provide about 106 parts of an antimony trioxide composition the particles of which were covered with fine, dense amorphous silica.

Examples 8 to 21 A hundred parts of the pigment listed below were dispersed in 1,000 parts of water, and the procedure of Example 1 was repeated except using, instead of the formic acid solution, Solution B, under the conditions specified in Table 1 below, whereby the respective pigment having particles coated with fine, dense amorphous silica were provided.

Example 8: white lead (color index No. 77597) Example 9: titanium dioxide (color index No.

77891) Example 10: titanium yellow Example 11: lithopone (color index No.77115) Example 12: zinc oxide (color index No.77947) Example 13: red lead (color index No. 77578) Example 14: litharge (color index No. 77577) Example 15: cobalt violet (color index No.77360) Example 16: cobalt blue (color index No.77346) Example 17: cobalt green (color index No.77335) Example 18: manganese violet (color index No.

77742) Example 19: manganese blue (color index No.

77112) Example 20: viridian (color index No.77289) Example 21: chromium oxide green (color index No.77288).

Table 1 Aqueous solution of sodium silicate having Pigment slurry cont of 4.83 wtO/. as Example No: reaction system SiO2 (Solution A) Solution B amount rate of amount rate of temperature added addition cont added addition pH {9C} (parts) (partslmin) component Iwtol (parts) (partslmin) 8 9.5-10.0 90-95 312 2.1 ammonium sulfate 4.2 382 2.1 9 " " " " pimelic acid 2.73 533 2.9 10 " " " " ammonium pensulfate 4.3 642 3.6 11 9.0- 9.5 85-90 ,, " phenol 3.3 500 2.8 12 " 90-95 ,, ,, ammonium nitrate 2.6 745 4.1 13 9.5-10.0 " 210 1.4 trimethyl phosphate 2.3 237 1.3 14 9.0- 9.5 " ,, ,, ammonium borate 3.6 663 3.7 15 9.5-10.0 " " " tartaricacid 4.33 202 1.1 16 9.0- 9.5 " " " pyridinetetracarboxylic acid 3.2 243 1.4 17 9.5-10.0 " ,, ammonium hydrogencarbonate 4.8 269 1.5 18 ,, ,, ,, ,, thiophenecarboxylic acid 1.0 1496 8.3 19 ,, ,, ,, ,, furancarboxylic acid 3.4 385 2.1 20 " " ,, methylsulfonicacid 5.0 224 1.2 21 9.0- 9.5 " ,, ,, pyrogallol 1.0 485 2.7 Example 22 The procedure of Example 1 was repeated to treat a hundred parts of chrome yellow except that a mixture of 549 parts of a 3.5 wt% aqueous solution of orthophosphoric acid and 337 parts of a 3.6 wt% aqueous solution of orthobromic acid was added simultaneously with Solution A at a rate of 4.9 parts/min instead of the formic acid solution. About 120 parts of a composition having fine, amorphouse silica-covered chrome yellow particles was provided.

Example 23 The procedure of Example 2 was repeated to treat a hundred parts of chrome vermilion except that a mixture of 294 parts of a 4.1 wt% aqueous solution of butyrlactone and 260 parts of a 3.1 wtO/o aqueous solution of glyoxal was added simultaneously with Solution A at a rate of 3.0 parts/min instead of the orthoboric acid solution. About 120 parts of a composition having fine, amorphous silica-covered chrome vermilion particles was provided.

Example 24 The procedure of Example 3 was repeated to treat a hundred parts of a synthetic yellow iron oxide pigment except that a mixture of 111 parts of a 5.4 wt% aqueous solution of sodium hydrogensulfate and 157 parts of a 2.1 wt% aqueous solution of ammonium sulfate was added simultaneously with Solution A at a rate of 1.5 partlmin instead of the crotonic acid solution. About 106 parts of a composition having fine amorphous silica-covered synthetic yellow iron oxide particles was provided.

Example 25 The procedure of Example 7 was repeated to treat a hundred parts of antimony trioxide except that a mixture of 72 parts of a 2.3 wt% aq aqueous solution of trimethyl phosphate and 99 parts of a 3.13 wtO/o aqueous solution of ethylene glycol monoacetate was added simultaneously with Solution A at a rate of 1.0 partlmin instead of the citric acid solution.

About 106 parts of a composition having fine amorphous silica-covered antimony trioxide particles was provided.

Example 26 The procedure of Example 13 was repeated to treat a hundred parts of red lead except that a mixture of 128 parts of a 3.5 wt% aqueous solution of ammonium acetate and 216 parts of a 2.0 wt% aqueous solution of glutamic acid was added simultaneously with Solution A at a rate of 1.9 parts/min instead of the trimethyl phosphate solution. About 109 parts of a composition having fine, amorphous silica-covered red lead particles was provided.

Example 27 A hundred parts of dried and ground particles of chrome yellow were dispersed in 840 parts of water and stirred at room temperature to form a uniform slurry. An aqueous solution of sodium silicate (JIS No. 3) was added to the slurry to bring its pH to within the range of from 9.0 to 10.0. The slurry was then passed through a homogenizerto make a uniform dispersion of pigment particles. An ultrasonic oscillating generator (manufactured by Branson Co., Ltd., U.S.A., resonance freq.: 50 KHz, generator: lead titanate zirconate) was used to generate ultrasonic waves which minimized the formation of secondary particles of chrome yellow, while the temperature of the slurry was held between 90 and 95"C and the pH was held between 9.0 and 10.0 by adding, if required, an aqueous solution of sodium hydroxide.To the slurry being stirred, 500 parts of Solution A and 689 parts of a 6.5 wt% aqueous solution of a 1:1 (by volume) mixture of monopropyl phosphate and dipropyl phosphate were added simultaneously at rates of 3.4 parts/min and 3.8 parts/min, respectively. Throughout the addition of the two solutions, the slurry was controlled to have a pH between 9.0 and 10.0. The generation of ultrasonic waves was continued to the end of the reaction. The procedure of Example 1 was repeated to provide about 120 parts of a chrome yellow composition the particles of which were covered with fine, dense amorphous silica.

Example 28 A hundred parts of a synthetic yellow iron oxide pigment were dispersed in 480 parts of water con taining 0.48 parts of ferric chloride and stirred at room temperature to form a uniform dispersion. To the slurry prepared as in Example 26, 128 parts of Solution A and 107 parts of a 2.0 wt% aqueous solution of acetamide were added simultaneously at rates of 0.9 parts/min and 0.6 parts/min, respectively.

The generation of ultrasonic waves was continued throughout the addition of the two solutions, as in Example 27. The procedure of Example 3 was repeated to provide about 106 parts of a synthetic yellow oxide pigment the particles of which were covered with a fine dense amorphous silica.

Example 29 The procedure of Example 1 was repeated to make aslurryofa hundred parts of chrome yellow having a temperature and pH held between 85 and 909C, and between 9.0 and 10.0, respectively. To the slurry, 607 parts of a 4.0 wt% aqueous solution of ethylene carbonate was added over a period of 20 minutes, stirred for 10 more minutes, and mixed with 500 parts of Solution A which was added art a rate of 3.4 parts/min. Throughoutthe addition of the two solutions, the slurry was held at a pH within the range of from 9.0 to 10.0. A fine silica sol resulted to form a uniform deposit on the pigment particles. The procedure of Example 1 was repeated to provide about 120 parts of a chrome yellow composition having its particles covered with fine, dense amorphous silica.

Examples 30 to 35 The procedure of Example 29 was repeated using Solution B2 instead of the ethylene carbonate solution under the conditions indicated in Table 2 below to provide six pigment compositions.

Table2 Aqueous solution of sodium silicate having Pigmentslurry cont of4.83 wto/oas Example No. reaction system SiO2 (Solution A) Solution B2 amount rate of amount temperature added addition cont added pigment pH ('C) (parts) zpartsiminJ (wt%) (parts) 30 chrome 9.0-10.0 90-95 500 3.4 glycerol 7.0 287 vermilion triacetate 31 synthetic 9.5-10.0 85-95 128 0.9 ethyl acetate 5.0 126 yellow iron oxide 32 red oxide ,, ,, 0.9 L-proline 5.88 139 33 iron black ,, ,, ,, 0.9 L-lysine 1.59 657 34 antimony 9.0-10.0 90-95 " 0.9 hydroquinone 3.5 113 trioxide 35 red lead 9.5-10.0 " 210 1.4 aminophenol 2.0 628 Example 36 A thousand parts of a 0.4 wt% aqeuous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 5-hour hydrothermal treatment at 180"C and 10 kg/cm2. After thorough washing with water by decantation, a dispersing mill was used to redisperse the pigment particles to obtain the initial slurry concentration, and an aqueous solution of sodium silicate (JIS No. 3) was added to the resulting slurry to bring its pH to between 9.5 and 10.0.The slurry was held at a temperature between 85 and 90"C and held at a pH between 9.5 and 10.0 by adding an aqueous solution of sodium hydroxide, if necessary. To the slurry being stirred, 128 parts of Solution A and 527 parts of a 1.2 wt% aqueous solution of tributyl phosphate were added simultaneously at rates of 0.9 parts/min and 2.9 parts/min respectively. Throughout the addition of the two solutions, the pH of the slurry was controlled between 9.5 and 10.0. The slurry was allowed to stand at a pH between 6.5 and 7.0, washed with water by decantation until no more sodium salt was detected, and filtered and dried to provide about 106 parts of a synthetic yellow iron oxide pigment having particles covered with fine, dense amorphous silica.

Example 37 A thousand parts of a 0.4 wt/o aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 2-hour hydrothermal treatment at 180"C and 10 kg/cm2. To the resulting slurry, 167 parts of an aqueous solution of zirconyl sulfate hav ing a concentration of 0.3 wt% as ZrO2 were added gradually at room temperature.After the addition, hydrolysis occurred in the slurry to form the fine deposit of zirconium hydroxide on the pigment par tiles. The slurry was then neutralized to a pH of 6.0, washed with water, the particles redispersed to obtain the initial slurry concentration, and the slurry was mixed with an aqueous solution of sodium sili cate[JIS No. 3] to adjust the pH to a level between 9.5 and 10.0. The slurry temperature was held bet ween 85" and 90"C, and the pH was controlled to rest between 9.5 and 10.0 by adding an aqueous solution of sodium hydroxide, if necessary.To the slurry being stirred, 128 parts of Solution A and 249 parts of a 6.1 wt% aqueous solution of aluminum citrate were added simultaneously at rates of 0.9 parts/min and 1.4 parts/min respectively. Throughout the addition of the two solutions, the pH was held within the range of from 9.5 to 10.0. The procedure of Example 36 was repeated to provide about 106.5 parts of a synthetic yellow iron oxide composition having particles covered with zirconium-silica.

Example 38 A thousand parts of a 2 wt% aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 2-hour hydrothermal treatment at 150"C and 5 kg/cm2, the slurry was washed with water, and 100 parts of an aqueous solution of aluminum sulfate having a concentration of 0.5 wtD/o as Al203 were added to the slurry at room tempera ture. After the addition, the slurry was neutralized to a pH of 6.0, washed with water, and a dispersing mill was used to redisperse the pigment particles to obtain the initial slurry concentration. The procedure of Example 36 was repeated to prepare a slurry having a temperature between 85 and 90"C and a pH between 9.5 and 10.0.To the slurry being stirred 128 parts of Solution A and 173 parts of a 3.6 wt% aqueous solution of orthoboric acid were added simultaneously at rates of 0.9 parts/min and 1.0 parts/min respectively. Throughout the addition of the two solutions, the pH of the slurry was held between 9.5 and 10.0. The procedure of Example 36, was repeated to provide about 106.5 parts of a synthetic yellow iron oxide composition having particles covered with aluminum-silica.

Example 39 A thousand parts of a 2 wt'/o aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 5-hour hydrothermal treatment at 150"C and 5 kg/cm2. The slurry was washed with water, and 100 parts of an aqueous solution of titanium sulfate having a concentration of 1.0 wt% as TiO2 were added to the slurry at room temperature.

After the addition, the slurry was neutralized to a pH between 6.0 and 6.5. The procedure of Example 36 was repeated to prepare a uniform slurry having a temperature between 85 and 90"C and a pH between 9.5 and 10.0. To the slurry being stirred, 128 parts of Solution A and 133 parts of a 3.1 wl% aqueous solution of glyoxal were added simultaneously at rates of 0.9 parts/min and 0.7 parts/min respectively.

Throughout the addition of the two solutions, the pH of the slurry was held between 9.5 and 10.0. The procedure of Example 36 was repeated to provide about 107 parts of a synthetic yellow iron oxide composition having particles covered with titanium-silica.

Example 40 A thousand parts of a 4 wt% aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 2-hour hydrothermal treatment at 150"C and 5 kg/cm2. The slurry was washed with water, and 100 parts of an aqueous solution of cerium chloride having a concentration of 0.5 wt% as CeO2 were added to the slurry at room temperature.

After the addition, the slurry was neutralized to a pH between 6.0 and 6.5. The procedure of Example 36 was repeated except that 128 parts of Solution A and 216 parts of a 2.4 wt% aqueous solution of ethylene glycol diacetate were added simultaneously to the slurry at rates of 0.9 parts/min and 1.2 parts/min, respectively, under stirring. About 106.5 parts of a synthetic yellow iron oxide composition having particles covered with cerium-silica were provided.

Example 41 A thousand parts of a 8 wt% aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 3-hour hydrothermal treatment at 95"C and under atmospheric pressure. The slurry was washed with water, and 100 parts of an aqueous solution of antimony chloride having a concentra tion of 0.3 WtO/o as Sub203 were added to the slurry at room temperature. After the addition, the slurry was neutralized to a pH between 6.0 and 6.5.The proce- dure of Example 36 was repeated except that 128 parts of Solution A and 150 parts of a 4.1 wt% aque ous solution of butyrolactone were added simul taneouslyto the slurry at rates of 0.9 parts/min and 0.8 parts/min, respectively, while the slurry was stir red and controlled to have a pH between 9.5 and 10.

About 106.3 parts of a synthetic yellow iron oxide composition having particles covered with antimony-silica were provided.

Example 42 A thousand parts of a 8 wt% aqueous solution of sodium hydroxide were added to 100 parts of a synthetic yellow iron oxide pigment, and the mixture was subjected to a 5-hour hydrothermal treatment at 95"C and under atmospheric pressure. The slurry was washed with water, and 100 parts of an aqueous solution of magnesium chloride having a concentration of 1.0 wt% as MgO were added to the slurry at room temperature. After the addition, the slurry was neutralized to a pH between 6.0 and 6.5. The procedure of Example 36 was repeated except that 128 parts of Solution A and 191 parts of a 5.0 wt% aqueous solution of ammonium carbonate were added simultaneously to the slurry at rates of 0.9 parts/min and 1.1 parts/min, respectively, while the slurry was stirred and controlled for its pH.About 107 parts of a synthetic yellow iron oxide composition having particles covered with magnesium-silica was provided.

Example 43 A slurry of chrome yellow (containing 200 parts of the pigment in 1680 parts of water) was added to 330 parts of water containing one part of zirconyl sulfate (ZrO.SO4 nH2O) as ZrO2, and an aqueous solution of sodium carbonate was added to the mixture to adjust its pH to 6.4. After washing with water by decantation, the slurry was filtered, and the cake was dried with a dryer (95-100"C) for a period of 12 hours to provide 201 parts of chrome yellow covered with zirconium. 100 parts of ground particles of the dried pigment were dispersed in 840 parts of water, and stirred at room temperature to form a uniform mix ture. An aqueous solution of sodium silicate (JIS No.

3) was added to the slurry to bring its pH to between 9.0 and 10.0. The slurry was then passed through a homogenizer to make a uniform dispersion of the pigment particles which was held at a temperature between 85 and 90"C and held at a pH between 9.0 and 10.0 by adding, as required, an aqueous solution of sodium hydroxide. To the slurry being stirred 500 parts of Solution A and 476 parts of a 4.5 wt% aque ous solution of sodium hydrogenoxalate were added simultaneously at rates of 3.4 parts/min and 2.6 parts/min, respectively. Throughout the addition of the two solutions, the slurry was held at a pH bet ween 9.0 and 10.0. A fine silca sol precipitated to form a uniform deposit on the pigment particles. The slurry was left to stand at a pH between 6.5 and 7.0 washed with water by decantation until no sodium salt was detected, and filtered and dried to provide about 120 parts of a chrome yellow composition hav ing particles covered with zirconium-silica.

Example 44 A slurry of ch rome vermilion (containing 200 parts of the pigment in 1680 parts of water) was added to 200 parts of an aqueous solution of cerium chloride having a concentration of 0.5 wt% as CeO2. The procedure of Example 43 was repeated except that 500 parts of Solution A and 583 parts of a 3.7 wt'/o aqueous solution of peracetic acid were added simultaneously to the slurry at rates of 3.4 parts/min and 3.2 parts/min, respectively at a controlled pH under stirring. About 120 parts of a chrome vermilion composition having particles covered with cerium-silica was provided.

Example 45 A slurry of synthetic yellow iron oxide pigment (containing 200 parts of the pigment in 960 parts of water was added to 200 parts of an aqueous solution of magnesium chloride having a concentration of 1.0 WtO/c as MgO. After the addition, an aqueous solution of sodium carbonate was added to the slurry to adjust its pH to between 6.0 and 6.5. The slurry was washed with water, filtered and dried to provide 202 parts of a magnesium-covered synthetic yellow iron oxide. 100 parts of ground particles of the dried pigment were dispersed in 480 parts of water containing 0.48 parts of ferric chloride.The Procedure of Example 43 was repeated except that 128 parts of Solution A and 150 parts of a 4.2 wt% aqueous solution of ethyl acetate were added simultaneously to the slurry at rates of 0.9 parts/min (Solution A) and 0.9 parts/min, respectively under stirring. About 106 parts of a synthetic yellow iron oxide composition the particles of which were covered with magnesium-silica was provided.

Example 46 A slurry of red oxide (containing 200 parts of the pigment in 960 parts of water) was added to 200 parts of an aqueous solution of aluminum sulfate having a concentration of 0.5 wtO/o as Al2O3. The procedure of Example45 was repeated exceptthat 128 parts of Solution A and 196 parts of 3.5 wtO/o aqueous solution of glutardialdehyde were added simultaneously to the slurry at rates of 0.9 parts/min and 1.1 parts/min respectively, under stirring. About 106 parts of a red oxide composition having particles covered with aluminum-silica was provided.

Example 47 A slurry of red lead (containing 200 parts of the pigment in 2,000 parts of water) was added to 200 parts of an aqueous solution of antimony chloride having a concentration of 0.3 wt% as Sb2O3. The pro cedure of Example 43 was repeated except that 210 parts of Solution A and 251 parts of a 2.96 wt/o aqueous solution of oxalic acid were added simul taneouslyto the slurry at rates of 1.4 parts/min and 1.4 parts/min, respectively, under stirring. About 109 parts of a red lead composition having particles covered with antimony-silica was provided.

Example 48 Two hundred parts of well ground particles of antimonytrioxide were dispersed in 2,000 parts of water and stirred at room temperature to form a uniform slurry. The slurry was then passed through a homogenizer to made a uniform dispersion of the pigment particles. Two hundred parts of an aqueous solution of titanium sulfate having a concentration of 1.0 wt% as TiO2 were added to the slurry, and then an aqueous solution of sodium hydroxide was added to the slurry to bring its pH to about 6.0. The procedure of Example 43 was repeated except that 128 parts of Solution A and 115 parts of a 3.0 wtO/o aqueous solution of triethyl borate were added simultaneously to the slurry at rates of 0.9 parts/min and 0.6 parts/min, respectively, while the pH of the slurry was held between 9.0 and 10.0.About 106 parts of an anti monytrioxide composition having particles covered with titanium-silica was provided.

Example 49 Three types of bronze powder (89% Cu and 11% Zn, or 74% Cu and 26% Zn, color index No.77440) were prepared. One type was subjected to the conventional preliminary heat treatment using boric acid as in similar manner as in Comparative Example 1 the second was subject to the conventional chromate treatment as in the similar manner as in Comparative Example 2 and the third was subjected to neither surface treatment. A hundred parts of each type were dispersed in 700 parts of water containing 0.7 parts of a surfactant ("Nonionic OD 100", manufactured by Emulsol Co., Ltd.,) The ultrasonic oscillating generator used in Example 27 was used to generate ultrasonic waves that thoroughly dispersed the pigment particles in water, while the slurry was held at a temperature between 90 and 95"C and a pH between 9.5 and 10.2.To the slurry being stirred, 103 parts of Solution A and 92 parts of a 3.75 wtO/o aqueous solution of acetic acid were added simultaneously at rates of 0.69 parts/min and 0.5 parts/min, respectively, while the pH was controlled to rest between 9.5 and 10.2. A fine silica sol resulted to form a uniform deposit on the pigment particles. The pH of the slurry was adjusted to a level between 7.0 and 7.5 by the conventional method, and then the slurry was washed with water, filtered and dried, About 103 parts of a bronze powder composition having particles covered with fine amorphous silica was provided from each type of bronze powder.

Examples 50 to 73 A hundred parts each of the metal powder pigments listed in Table 3 below and having average particles sizes of 18 p, 20 to, 25 L and 35 Er were treated by the procedure of Example 49 using solution B, under the conditions specified in Table 3, and the respective metal powder pigment compositions having particles covered with fine amorphous silica was provided.

Table3 Metal powder pigment slurry reaction system Solution B Amount Rate of Metal powder Temp. Cont. Added Addition Experiment No: Pigments pH ("C) Component (wt'/o) (parts) (partsimin) 50 Bronze powder 9.5-10.2 90-95 Sodium hydrogencarbonate 4.2 159 0.9 51 " ,, ,, Citric acid 4.7 99 0.55 52 " " 85-90 Ammonium chloride 3.1 139 0.8 53 " 9.0- 9.5 90-95 Melliticacid 2.4 139 0.8 54 " " 85-90 Sodium hydrogensulfate 2.7 356 2.0 55 ', " " ammonium perchlorate 7.0 134 0.7 56 " " 90-95 O-cresol 1.3 476 2.6 57 " 9.5-10.2 " Methylsulfonicacid 5.0 110 0.6 58 ,, " " ,, Butyrolactone 4.1 121 0.7 59 " " " ammonium hydrogenphosphite 3.3 239 1.3 60 " 9.0-9.5 85-90 Zinc acetate 5.0 210 1.2 61 " ,, r. pyro-phosphoricacid 1.8 200 1.1 62 " " " Acetamide 2.0 86 0.5 63 Pure silver " 90-95 Furancarboxylic acid 3.4 189 1.0 64 64 " 9.5-10.2 Phenol 3.3 165 0.9 64 " 9.5-10.2 " Phenol 3.3 165 0.9 65 Zinc durst 8.0- 9.5 85-90 ammoniumamidosulfate 2.9 159 0.9 66 " rr rr 2-pyridinecarboxylic acid 4.4 161 0.9 67 Tin powder 9.0- 9.5 " ammoniumcarbonate 5.0 154 0.9 68" " " " Trimethyl phosphate 2.3 116 0.6 69 Copper powder " " Oxalic acid 2.96 123 0.7 70 " " " Sodium hydrogensulfile 2.9 286 1.6 71 " 9.5-10.2 " Pyrogallol 1.0 238 1.3 72 " 9.0- 9.5 90-95 sodium dihydrogen phosphate mono- 7.4 150 0.8 hydrate 73 2 9.5-10.2 " Glyoxal 3.1 107 0.6 Note: For the concentration of Solution A, the amount added, and the rate of addition see Example 49.

Example 74 The procedure of Example 1 was repeated to pre pare a slurry of 100 parts of chrome yellow, and Sol- ution A and 3.5 wt% aqueous solution of acetic acid 5 (Solution B4) were added simultaneously to the slurry in the amounts and at the rates of addition specified in Table 4 below. Chrome yellow compositions having particles covered with different proportions of fine amorphous silica were provided as shown in Table 4.

Table 4 Solution A Solution B Rate of Rate of Amount addition Amount addition SiO2 Run No. (parts) (partsimin) (parts) (parts/min) (wit%) 1 82.8 2.1 78.8 1.7 3.9 2 290.4 3.0 276.4 2.5 12.3 3 668.2 3.7 636.1 3.0 24.4 4 1,269.0 4.3 1,208.0 3.8 38.0 Example 75 The procedure of Example 3 was repeated to pre pare a slurry of 100 parts of a synthetic yellow oxide pigment, and Solution A and a 3.1 wtO/o aqueous sol15 ution of ammonium chloride (Solution B5)were added simultaneously to the slurry in the amounts and at the rates of addition specified in Table 5 below. Synthetic yellow oxide pigment compositions having particles covered with different proportions of fine amorphous silica were provided as shown in Table 5.

Table 5 Solution A Solution B6 Rate of Rate of Amount addition Amount addition SiO2 Run No. (parts) (parts/min) (parts) ZpartsiminJ (wt%) 1 82.8 2.1 111.9 2.2 3.9 2 207.0 2.3 279.8 2.5 9.2 3 518.0 2.9 700.1 3.4 20.2 4 828.0 3.4 1119.1 3.8 28.8 Example 76 The procedure of Example 49 was repeated to prepare a slurry of 100 parts of a bronze powder 25 (89% Cu and 11% Zn, average particle size 25 u), and Solution A and a 3.5 wtO/o aqueous solution of ammonium acetate Solution B8 were added simultaneously to the slurry in the amount and at the rates of addition specified in Table 6 below. Bronze powder compositions having particles covered with different proportions of fine amorphous silica were provided as shown in Table 6.

Table6 Solution A Solution B8 Rate of Rate of Amount addition Amount addition SlO2 Run No. (parts) (parts/min) (parts) (parts/min) (wit%) 1 31.5 0.8 38.6 0.8 1.5 2 178.0 1.9 266.7 2.4 8.0 3 366.5 2.4 549.1 3.0 15.0 4 652.2 3.6 977.1 4.6 24.0 Comparative Example 1 200 ml of a methyl alcohol solution or ethyl alcohol solution containing 20 g of boric acid, or 200 ml of a methyl alcohol aqueous solution or ethyl alcohol aqueous solution (alcohol: water = 1:1 by volume) containing 20 g of boric acid and unhydrous borax (1:1 by weight) and bronze powder (Cu: 89% and Zn: 11%; or Cu: 74% and Zn: 26%) having average particle sizes of 18p, 20Cl, 25CL and 35cm, respectively, were mixed and allowed to stand for about 1 hour. After filtration of the resulting mixture, the particles obtained was dried and subjected to heattreatment for about 5 minutes at a temperature of 250-300"C in the case using the former alcohol solution and at a temperature of 450 to 500"C in the case using the latter alcohol aqueous solution, respectively, followed by cooling to obtain heat-resistant bronze powder.

Comparative Example 2 35 g of dioxane was added to a mixture of 15 g of water, 1 g of chromic acid anhydride and 0.1 ml of 85 wt% phosphoric acid. Bronze powder (Cu: 89% and Zn: 11%; or Cu: 74% and Zn: 26%) having average particle sizes of 18cm, 20CL, 25CL and 35,a, respectively, were immersed in the resulting mixture at 60"C for 1 hour with stirring. After filtration and drying, the par ticles were subjected to a chromate-treatment to obtain an anti-corrosive bronze powder.

Tests and thefr results The following tests were carried out using the pigments prepared in Examples 1-5, 13, 14 and 49 and Comparative Examples 1 and 2 1. Acid resistance test (A): A painted plate was immersed in 5 wtO/o H2SO4 at500Cfor8 hours, and the resulting change was checked.

2. Acid resistance test (B): A pigment powder was immersed in 1 watt' H2SO4 at 50"C for 8 hours, filtered, washed with water and dried, and the resulting weight loss and change in the color of the recovered pig ment were checked.

3. Alkali resistance test: A pigment powder was immersed in 1 wtO/o aqueous solution of potassium hydroxide at room temperature for 12 hours, filtered, washed with water and dried, and the result ing weight loss and change in the color of the recovered pigment were checked.

4. Hydrogen sulfide resistance test: Atest piece was immersed in saturated hyd rogen sulfide water at room temperature for 1 hour, and the resulting change in hue was checked.

5. Heat resistance test: A mixture of 100 g of polyethylene powder from medium-low pressure polymerization process and 0.5 g of a sample pigment was held at different temperature between 200 and 260"C for 5 minutes during which it was injection-molded to form panels, and the resulting change was checked.

6. Light resistance test: A sample was exposed to a fadeometer for 500 hours, and the resulting change was checked.

7. Weatherabilitytest: A sample was exposed to aweatherometerfor 1,000 hours, and any chalking and change in color were checked.

8. Dispersibilitytest: A pigment formulated as a paint was applied by an applicatorto a glass plate in a thickness of 3 mils, baked at 1500Cfor30 minutes, and the surface of the film was observed.

9. Thickening test: A pigment formulated as a paint was placed in an air-tight container and left to stand at 50"C for a given period of time. The resulting increase in the viscosity was checked by a vis cosimeter.

(I) The paints and painted plates tested in Test Nos. 1, 6, 7, 8 and 9 were prepared by the following method: 80 g of a melamine alkyd resin were mixed with 20 g of the sample pigment, 20 g of xylol and 200 g of beads, and the mixture was stirred in a paint shaker for 30 minutes to form a uniform dispersion. The dispersion was filtered to separate the beads. The filtrate was applied to a specified steel sheet by an applicator, left to stand for 30 minutes, and baked at 150"C for 30 minutes to thereby obtain a test piece.

(Il) Preparation of the test pieces used in test No.

4 A 50 g sample of a mixture of 100 g of PVC (soft), 50 g of DOP, 0.5 g of barium stearate, and 0.5 g of calcium stearate was mixed with 0.5 g of a sample pigment and milled with a heated roll at 1600C for 3 minutes until a sheet was provided. The sheet was cut into test pieces of a suitable size.

Test results The results obtained are shown in Table 7.

Table7 Test No.

Example No. 1 2 3 4 5 6 7 8 9 1 555555555 2 555555555 3 555555555 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 13 5 5 5 5 5 5 5 5 5 14 5 5 5 5 5 5 5 5 5 49 5 5 5 5 5 5 5 5 5 Comparative Example No.

1 1-2 1-2 1-2 1 2-3 2 1-2 3 1-2 2 2 2 2 1-2 2-3 2-3 2-3 3 2-3 Note: Evaluation 1: Bad 2: Fair 3: Good 4: Very good 5: Excellent The pigments prepared in the examples of this invention were very stable and resistant to the hostile testing conditions. On the other hand, the pigments prepared in Comparative Examples 1 and 2 deteriorated to a great extent. The effect of this invention was particularly conspicuous with chromate pigments, iron oxide pigments, lead oxide pigments, and metal powder pigments.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims (12)

1. A process for preparing a stable inorganic pigment composition which comprises reacting an alkali silicate with one or more compounds capable of reacting said alkali silicate to form an amorphous silica sol in an aqueous slurry of an inorganic pig mentotherthan ultramarine with maintaining the pH and the temperature of the slurry at 7 to 11 and not lower than 60"C, respectively, to deposit said amorphous silica sol on the particle surfaces of said inorganic pigment, said compound being selected from the group consisting of organic acids and derivatives thereof, phosphor oxyacids, borate esters, phosphate esters, alkali metal salts, ammonium salts, boric acid and ethylene carbonate.

2. The process of claim 1, wherein said aqueous slurry of the inorganic pigment is prepared by hydrothermal treatment with alkali.

3. The process of claim 1, wherein said aqueous slurry of the inorganic pigment is a slurry of pigment covered with a fine amorphous hydrous oxide of metal selected from the group consisting of zirconium, aluminum, titanium, cerium, antimony and magnesium.

4. The process of any preceding claim, wherein said silica sol is formed under atmospheric pressure and under conditions so controlled that the temperature is between 70"C and 100"C and the pH is between 8 and 11.

5. The process of claim 1, wherein said com pound has a solubility of at least 0.01 g/100 g H2O at 20"C and a pH of at least 1.0 in its 3 WtO/o aqueous solution.

6. The process of claim 1, wherein said organic acid is a carboxylic acid, a phenol, a sulfonic acid or an amino acid.

7. The process of claim 6, wherein said derivative is a derivative selected from the group consisting of a salt, an ester, an amide, an aldehyde, an unhydride and a peroxide.

8. The process of claim 1, wherein said phosphor oxyacid is phosphoric acid.

9. The process of claim 1, wherein said alkali metal salt is sodium hydrogensulfate, sodium hyd rogensulfite, sodium hydrogencarbonate, or sodium dihydrogenphosphate.

10. The process of claim 1, wherein said ammonium salt is an ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium hydrogensulfate, ammonium sulfite, ammonium hydrogensulfite, ammonium carbonate, ammonium hydrogencarbonate, or ammonium dihydrogenphosphate.

11. The process of any preceding claims, wherein the inorganic pigment is selected from the group consisting of a chromate pigment, an iron oxide pigment, a titanium oxide pigment, a lead oxide pigment, a calcium salt pigment, a barium salt pigment, a magnesium salt pigment, a cobalt pigment, a manganese pigment, a cadmium pigment, a metal powder pigment, vermilion, antimony trioxide, zinc oxide, viridian and chromium oxide green.

12. A pigment composition when produced by the method as claimed in any preceding claim.