IE43470B1 - Tio2 pigments coated with hydrous oxides - Google Patents

Abstract

A coated titanium dioxide pigment and the process for its preparation consisting essentially of a core of pigmentary titanium dioxide (TiO2), a dense inner coating of dense amorphous silica (SiO2) in an amount from 1% to 10% by weight, based on the uncoated pigment, and, optionally, up to 5% of alumina (Al2O2) and a porous outer coating of porous silica (SiO2) and alumina (Al2O3), said porous silica being present in an amount from 0.5% to 15% by weight and said alumina being present in an amount from 0.5% to 11% by weight, based on the uncoated pigment. As a component of coating compositions, this pigment provides a high degree of hiding power and durability for indoor and outdoor applications.

Description

This invention relates to TiO2 pigments coated with hydrous oxides.

The treatment of Digmentary titanium dioxide (TiClg) in the rutile or anatase form with hydrous oxides, especially silica and alumina, is known in the art to provide resistance to chalking and discoloration in paints, good opacity in paper and good hiding power in flat paints. Generally, the silica is applied to the pigment by precipitation from sodium silicate with acid in an aqueous slurry of the pigment. The resulting pigments, having a dense, i.e., nonporous silica coating, have good durability but do not provide as high a degree of hiding power as is desirable in many paint compositions. Applying a loose, i.e., porous, alumina to dense silica-coated TiOg improves the dispersibility of dense silica-coated TiC>2 without sacrificing durability, but does not significantly enhance the hiding power of the dense silica-coated TiOg. Applying a porous coating consisting essentially of alumina and silica to pigmentarv titanium dioxide Drovides a coated pigment having generally better hiding power than the densely coated pigment, but lower durability than is desired for many applications.

This invention provides for a pigmentary TiOg coated in such a way as to combine the durability of dense silica coatings with the hiding power of loose hydrous coatings.

For purposes of this application pigmentary means that the material is pulverulent and of good whiteness with an average particle diameter of from about 0.15 to about 0.3 micron.

This invention provides for a coated Ti02 pigment consisting essentially of a core of pigmentary TiO2, as hereinbefore defined -2an Inner coating of from 1% to 10% by weight of dense amorphous silica, calculated as SiO2, and based on the uncoated nigmentary TiOp, and from 0% to 5% by weight of alumina, calculated as Al20^, and based on the uncoated pigmentary TiO2, and an outer coating of porous silica and alumina. The porous silica in the porous outer coating is present in an amount from 0.5% to 15% by weight, calculated as SiOp, and based on the uncoated pigmentary TiOj, and the alumina in the porous outer coating is present ln an amount from 0.5% to 11% by weight, calculated as AI2O3, and based on the uncoated pigmentary T102. The preferred amount of dense amorphous silica in the dense inner coating is from 3.5% to 6% hy weight. When alumina is present in the dense inner coating the preferred amount is from 1% to 3% by weight. In the porous outer coating it is preferred that from 5% to 10% by weight of porous silica and from 5% to 10% by weight of alumina be present.

In accordance with the present invention there is nrovided a process for preparing coated T1O2 pigment by the steps of (1) adjusting the pH of an aqueous slurry of pigmentary TIOj to from about 9 to 10, (ii) adding to said slurry an aqueous solution of soluble silicate in an amount calculated to provide from 1% to 10% by weight of silica, calculated as SiO2, and based on said pigmentary TIOj, (iii) slowly adding dilute aqueous acid to said slurry until the pH of said slurry is from 6 to 7.7, (iv) adding to said slurry, maintained at a pH from 5 to y, an aqueous solution of a soluble silicate in an amount calculated to provide from 0.5% to 15% by weight of silica, calculated as S102, and based on said pigmentary TiO2, (v) adding to -343470 said slurry, maintained at a pH from 4 to 9, an aqueous solution of soluble aluminate in an amount calculated to provide from 0.5% to 11% by weight of alumina, calculated as AljO-j and based on said pigmentary TiO2, (vi) adjusting the pH of said slurry to from 7 to 9, and (vii) separating the resulting coated TiO2 pigment from said slurry. To incorporate alumina in the inner coating, the above described process comprises the additional step (iiia) of adding to said slurry, after step (iii) and prior to step (iv), an aqueous solution of soluble aluminate in an amount calculated to provide from 0.5% to 5% by weight of alumina, calculated as A12O3, and based on said pigmentary TiO2, while the pH of said slurry is maintained from 6 to 9. The dense silica is precipitated generally in the manner described in British Patent Specification No. 1,179,171. In this specification the . term 'dense' as applied to the inner coating of silica has the same meaning as in U.S. Patent Specification No. 2,885,366.

The order of process steps (iv) and (v)is not particularly critical in that a suitably coated pigment results when the order of these steps is reversed or performed simultaneously. Also the porous silica and alumina of the outer coating may be precipitated separately or simultaneously.

Although curing the slurry at various stages in the process is not essential to the attainment of good pigment properties, in applications where curing is deemed desirable, the slurry can be cured, for example at temperatures of at least 40°C., and preferably at least 70°C., for at least 15 minutes, and preferably at least 30 minutes, after any or all of process steps (i), (iii), (iv) and (v). Curing following step (iii) for at least 30 minutes at about 90°C while maintaining pH of the Slurry from 6 to 7.7 (v) is desirable.

The base titanium dioxide pigment may be prepared by the high temperature vapor phase oxidation of TiCl^the vapor phase lydrolysis of TiC^, or the hydrolysis of colloidally seeded sulfuric acid solutions of titaniferous raw materials such as ilmenite. The hydrolysate of the sulfuric acid process must be washed and calcined to develop the crystalline characteristics and particle size needed for the good light scattering characteristics of pigments. The temperature of the slurry being treated may vary from room temperature to above 90°C., but it is preferred that the temperature be from 80°C to 90°C. Preferably, the slurry is maintained in this temperature range during at least steps (i), (ii) and (iii). The slurry should be agitated throughout the precipitation of both the silica and the alumina to ensure general uniformity of the pH conditions within the specified ranges.

Any soluble silicate may be used in the process including sodium or potassium silicate. Commercially available water soluble sodium silicates with SiC^/NajO weight ratios from 1.6 to 3.75 and varying from 32% to 54% by weight of solids, with or without further dilution are the most practical. To apply dense amorphous silica to the pigmentary Ti02 the slurry must be basic during the addition of the effective portion of the soluble silicate, then slowly acidified over a period of preferably at least 45 minutes until a pH from 6 to 7.7 is reached. To apply a porous silica to the coated pigment the slurry should preferably be acid during the addition of the effective portion of the soluble silicate. The acid used may be any acid, such as HC1,H2SO4, HNO^, or H-jPO^ having a dissociation constant sufficiently high to precipitate silica and used in an amount sufficient to maintain an acid condition in the slurry. Preferably, dilute aqueous acid having a normality of less than about 4 is used. Compounds such as TiOSO^ or TiCl^ which hydrolyze to form acid may also be used. Alternative to adding all the acid first, the soluble silicate and the acid may be added simultaneously so long 1470 as the acidity of the slurry is preferably maintained at a pH of below 7.5.

The alumina of the inner coating or outer coating must be precipitated in an environment having a pH from 4 to 9. To avoid subjecting the treated pigment to unduly acid or alkaline conditions, a pH frcm 6 to 8 is preferred. For purposes of this invention It is preferable to add about 75% of the total desired alumina as sodium aluminate, while maintaining the slurry pH between 6 and 8.

The remainder of the sodium aluminate can be added, allowing the slurry pH to rise to between 8 and 9.

This process may be applied to pigments which have been previously treated with oxides such as titania, but their presence is not essential to the benefits herein obtained. After treatement according to this process the pigment is recovered .by known procedures including neutralization of the slurry if necessary, filtration, washing, drying and frequently a dry grinding step.

Drying is not necessary, however, as a thick slurry of the product can be used directly in preparing emulsion paints where water is the liquid phase. The process provides a method for obtaining good hiding power and good durability in titanium dioxide pigments for both indoor and outdoor applications.

In the examples which follow, the test results are obtained by the procedures described below.

Acid solubility of the coated T.1O2 pigments of this invention is measured by digesting a predetermined quantity of coated pigment in sulfuric acid and spectrophotometrically comparing it to a standard TiOj sample -6<3470 similarly treated. The standard spectrophotometric curve is prepared as follows: a standard solution is prepared by dissolving 15 g. of ammonium sulfate and 1.0118 g. of TiO2, containing 98.83% by weight TiO2, in 20 ml. of 66° B£. sulfuric acid and diluting to 800 ml. with water.

Eighty milliliters of the sulfuric acid solution is then added to this solution. After allowing the resulting solution to cool to room temperature, it is diluted to one liter with water. The solution contains 0.001 g. of TiOj per ml. and εΐιομίΰ be allowed to stand for one week before using.

Samples of 2, 4, 6 and 8 ml. of the aboveprepared standard solution are each mixed with 10 ml. of 30% hydrogen peroxide and diluted to 100 ml. with 10% sulfuric acid. After standing for one hour, the absorbance of these solutions is read using a Beckman (Trade Mark) Model DK, DU or B Spectrophotometer against a reference solution, prepared by diluting 10 ml. of 30% hydrogen peroxide to 100 ml. with 10% sulfuric acid at AOO mp using 10 mm. cells. A plot of titanium peroxide concentration in mg./l. versus optical density is made for these samples.

To determine acid solubility of the coated TiO, pigments, 0.2000 g. of coated TiO2 pigment is added with stirring to 10 ml. 66° B6. sulfuric acid at 175°C. The pigment is digested at 175°C. for one hour. After digestion, the samnle is quenched by pouring into crushed ice, made from distilled water. The sample is diluted to 100 ml. with water and filtered. Ten milliliters of the filtrate is mixed with 2 ml. of 30% hydrogen peroxide and diluted to 25 ml. with 10% sulfuric acid. After one hour the -770 absorbence of the sample is read against a reference solution, prepared by diluting 2 ml. of 30% hydrogen peroxide to 25 ml. with 10% sulfuric acid.

The concentration of soluble Ti02 is determined from the above-prepared standard spectrophotometric curve from the measured optical density and the percent soluble Pi02, i.e., the acid solubility, is calculated by dividing the concentration of soluble TiOp by 8.

The tinting strength test compares the green ’eflectanee of paints which contain the same ratio of TiOj Jigment to tinting gel. Although any color tint could be ised for this test, green or blue tints are preferred >ecause the eye is more sensitive to changes in strength Mth these colors. The tinting gel is prepared by mixing ;he following ingredients: Parts by weight iistilled water 77.6 ι commercially available organic 20.0 pigment sold by the Du Pont Comnany under the Trade Mark Monastral Green B lydroxyethyl cellulose 2.4 Ten grams of the above-described tinting gel is lxed with 100 g. of paint. The paint is a conventional ndoor water-based emulsion paint containing 2.5 lbs./gal. 0.36 kilograms/liter) T102 pigment, about 1.4 lbs./gal. 0.19 kilograms/liter) of extender pigment and about 1.5 lbs./gal. (0.20 kilograms/liter) of vinyl acetate!thylene emulsion. The pigment volume concentration of his paint is about 50%. l'he Ti02 pigment used in the tandard paint is a commercial TiO2 pigment produced by he vapor phase oxidation of TiCl4 and treated with Al203 -8<3470 and SiO2 according to the procedure of Angerman U. S. Patent Specification No. 3,591,398. This TiO2 pigment, designated as Control ID in Table I, consists essentially of 87.6% by weight of Ti02, 6.2% by weight of Al203 and 6.2% by weight of SI02, based on the weight of the treated pigment.

A film of prepared paint is drawn on white lacquered paper using a draw-down blade with 0.008 inch (0.020 cm.) clearance. The film is dried for at least two hours. The reflectance of the dried film is measured using a green filter on Neotec Du-Color Model 220 Reflectometer. From the value of this reflectance, R^ the value of the ratio of the absorption coefficient, K, to the scattering coefficient, S, is found using the Kubelka-Murik Table (D. B. Judd arid G. Wyszecki, Color in Business, Science, and Industry, John Wiley, Dr. Sans, 1963, Appendix D). Values beyond the range shown in the Table can be calculated by K/S = 2R„ Expressing the tinting strength on a relative basis, a value of 100 is assigned as the tinting strength of the standard. The relative value for the sample is calculated as follows: Standard Relative Tinting Strength - (k/sj Sample x 100 The indoor hiding power is determined using a conventional indoor water-based emulsion paint containing 3 lbs./gal. (0.36 kilogram/liter) of Ti02 pigment, about 1.6 lbs./gal. (0,19 kilogram/liter) of silicate extenders and about 1.7 lbs./gal. (0.20 kilogram/liter) of vinyl 170 acrylic resinous binder in an aqueous emulsion, lhe TiO2 pigment volume concentration of this paint is about 49%, the remaining 51% by volume of solids consisting essentially of silicate extenders and benders. The T102 pigment used in the standard indoor paint is the commercial TiO2 pigment used for the tinting strength test described above.

The outdoor hiding power is determined using a conventional outdoor paint and a commercial TiO2 pigment produced by the vapor phase oxidation of TiCl4 and treated with dense amorphous silica and alumina according to the procedure of U.S. Patent Specification No. 3,437,502. This outdoor standard TiO2 pigment, designated as Control OD in Table I, consists essentially of 92.6% by weight of TiO2, 5.7% by weight of dense amorphous silica, calculated as SiO2, and 1.9% by weight of alumina, calculated as Al203.

The conventional outdoor paint used is a waterbased emulsion containing about 2.5 lbs./gal. (0.30 kilogram/liter) of Ti02 pigment, about 2.0 lbs./gal, (0.21) kilogram/liter) of magnesium silicate (talc) extender, and about 4.6 lbs./gal. (0.55 kilograms/liter) of an acrylic emulsion (50% by weight of solids). The T102 pigment volume concentration of this paint is about 18.5% and the total pigment volume concentration is about 40%.

A film of prepared paint is drawn on a Morest Chart Form 09, a glossy paper chart having black and white areas, using a draw-down blade with an 0.0025 inch (0.0064 cm.) clearance, at 50% R.H. and 72°F. (22eC.). When the film is dried, the reflectances over the white and black backgrounds are measured using a Neotec Du-Color Model 220 Reflectometer using a green filter. One reading is taken -1043470 on each of two white and black sections of each of four charts.

The scattering power (SX) is determined, using the above-measured reflectances, from the appropriate Kubelka5 Munk Opacity Chart (D. B. Judd, Optical Specification of Light Scattering Materials, Journal of Research of the National Bureau of Standards, Vol, 19 j 1937, p. 287).

The relative hiding power (H.P.) of the samples is calculated as: TO SX of sample U.P. = ρ,χ 0£ standard x 100 The titanium dioxide used ir. all the following examples is produeid by the vapor phase oxidation of titanium tetrachloride. The percentage compositions of alumina and silica, referred to hereinunder and elsewhere in the specification, are by weight, based on the weight of the TiO2 pigment, and are calculated as SiOs and Als03, respectively, unless otherwise specifieu.

The determination of chalk./fade index is in 20 accordance with the procedure described by W. H. Daiger and W. H. Madson in Vol. 39, July 1967, of the Journal of Paint Technology, pp. 399-410, Specifically, the tests used for obtaining data in this application are made on paints of the following formulation: Grams TiOs pigment 48.0 Blue organic pigment 6.0 (a commercial organic pigment sold by the Du Pont Company under the Trade Mark Rampo Blue BP-366-D) Alkyd resin (a commercial resin sold by 120.0 Celanese Coatings Company under the Trade Mark Syntex 3533, 50% N.V.) -11470 Organic solvent, boiling range l6o°C.- 36.0 193°C. (commercial solvent sold by Humble Oil and Refining Co. under the Trade Mark Varsol /1) Xylene 36.0 24^ Pb naphthenate drier 0.75 6% Co naphthenate drier 0.40 6% Mn naphthenate drier 0.20 These ingredients are ground in a rotating glass jar containing 400 g. of 0.5 inch (1.27 cm.) glass balls for about 42 hours and the resulting paint sprayed, without reduction, onto aluminum panels for exposure tests. Control panels prepared using the standard outdoor pigment described above are placed in each set for exposure.

Degradation on exposure causes the blue to become white due to chalking. The degree of degradation is measured by recording the red reflectance at various intervals. Red reflectance increases as the chalking progresses. To determine the index numbers reported, the panels are exposed outdoors until the instrument gives a fixed reading, arbitrarily set to indicate a well-defined degree of chalking. The chalk/fade index is the ratio of time units required for the test sample to reach this reflectance divided hy the time units required by the control times 100. The instrument used is a spectrophotometer adapted to read light intensity reflected from the panel surface through a red filter having o a peak transmission at about 5760 A.

Examples 1-18 and Controls Three thousand grams of pigmentary Ti0? Is slurried in sufficient water to achieve 300 g. TiO^ Der liter. The slurry is heated to 90°C. and the pH is adjusted to 9.5 with -12a 50% by weight aqueous solution of sodium hydroxide. At this temperature a solution of sodium silicate (weight ratio of S10p/Na?0 is 3.25/1) at a concentration of 400 g. of SlOp per liter is gradually added with agitation in an amount sufficient to Drovide the percent of dense SiOp shown in Table I. Maintaining the temperature at about 90°C., a 10% by weight aqueous solution of HpSOj) is added to the slurry over a period of about one hour until the pH of the slurry drops to from 7.3 to 7.6. Then additional sodium silicate solution (described above) is added to the slurry in an amount sufficient to provide the percent of porous SiO2 shown in Table I. During the addition of the sodium silicate solution the pH of the slurry is maintained between 5 and 7 by the simultaneous addition of 96% H2SO4. After the addition the slurry is allowed to stand for 30 minutes. Then 96% sulfuric acid and a sodium aluminate solution containing 383 g./l. of Al203 are simultaneously added at rates which maintain the pH of the slurry between 6 and 8. The sodium aluminate solution is added in an amount sufficient to provide the percent of Al203 shown in Table I. The pH of the slurry is then adjusted to from 7·5 to 8.0. The pigment is recovered by filtering, washing, drying at about 120°C., and micronizing. The properties of the pigments so prepared and the Controls are shown in Table I.

Examples I9-S6 Four thousand grams of pigmentary Ti02 is slurried in sufficient water to achieve 300 g. TiO2 per liter. The slurry is heated to 90°C. and the pH is adjusted to 9·5 with a 50% by weight aqueous solution of sodium hydroxide. At this temperature a solution of sodium silicate, described in -134S470 Example 1, at a concentration of 200 g. of SiOg per liter is gradually added with agitation in an amount sufficient to provide the percent of dense silica shown in Table II. Maintaining the temperature at about 90°C., a 5% by weight aqueous solution of H2S04 is added to the slurry over a period of about 2 hours until the pH of the slurry drops to from 7.3 to 7-6. The slurry is maintained at about 90°C. for 30 minutes. Then 50% sulfuric acid and a sodium aluminate solution containing 370 g./l. of Al203 are silumtaneously added at rates which maintain the pH of the slurry between 8 and 9· The sodium aluminate solution is added in an amount to provide the percent of AI2O3 shown in the third column of Table II. Then additional sodium silicate solution, described above, is added to the slurry in an amount sufficient to provide the percent of porous Si02 shown in Table II. During the addition of the sodium silicate solution the pH of the slurry is maintained between 8 and 9 by the simultaneous addition of 50% H2SO4. To the amount of additional Al203 shown in the fifth column of Table II, 50% sulfuric acid and the sodium aluminate solution, described above, are simultaneously added at rates which maintain the pH of the slurry between 8 and 9· Then the slurry is cured for 30 minutes at about 9θ°0· The pH of the slurry is then adjusted to about 7. The pigment is recovered in the manner described in Example 1. All the pigments exhibit good hiding power with respect to the indoor and outdoor standards, The acid solubility and chalk/fade index of the pigments so prepared are shown in Table II. «ϋ β fc •H p co r-t co kOb-cnb-iPCT'CrrHCM c^b-mco A’ O CHOChCcoHChr-IHOOOa O\VO C— t-t- O Η Η H rH i—I r—I H r-1 i—It—I r-t r-1 fc V & (fc hO C •H rt Ή tc be c •H 4.0 (0 o D E o hO fc •H P ftj O ϋ fc o o rt P β o Ό r-i •rd »rd ο O < β rd C w CC O CM rH PJ e rH co « a o fc P-.

CM O •H co φ w C Q b-oco mippn^r o cvco^r CHCC)OOOHHCCI r-l Η H r—I H rIp-I rH ι—I rH Η ΟΆ C\J O OCu O rd LT. (Ό b- itf· ocrc- σ. rH rH LPS fP b-VC 0 0.000 rH Η Η H zrco ΓΛΟλΟ b-co ovo o rH rd ruir. N-xrLPvc\jc\j^-mcn b'lPr^iPPAA’Pin rdkr>njcJcocf\jf\jmojHC\JOjr4rv!:tC\Jrc\ ι ι rH H (\J Pinoca c coLmf.ipiO coC'^Qoo CMCMOLPOLPOLPCMCMCMIPICIC tP1^ fjOO O ip in c ο o c c o ip in lpvo vo vc voO in & cj vo rH rH r-H ip ip u\ m σ o mining cm c> a > rH rH rH rH rH ιηιηίΛο >d d d IPIPLPLPLP lAOlA Vp rH VO VO rH rd ΓΡ ΓΡ fP rH fP IP b-KYO ΚΥθ O §a rH rd rd 0 0 rH CM fp-=r IP VC b-CO Ολ O rH CM LPVO b-CO fc fc HHHHH Η^πΗρ p -15470 CQ 5 c Q τ, S ti H n CD cu Ό a) fc tf co Λ4 0 H ft tf XS O a \O -=t MD -tfK\ F\ K\ ΝΛ K\ in \O -=t m tn tn o Ch CO co CO CO σ\ rH rH rd rH rH rH rH rH ΤΤ .'T'TfTtf.T. w o •H w o o o o in in o o -sf rH rH rH cu cu cu rH o o » o in o in 4 o o cu in cu tn cu tn rn © o o in in o o O o tn rH rH H H w o •rH w Φ CG C CD Q -=t -tf· -tfΦ rH I i σ\ o h oj rH CU W CM K\ m kO w w w w

Claims (16)

1. Coated TiO 2 pigment which comprises a core of pigmentary Ti0 2 ,as hereinbefore defined, an inner coating of from 1% to 10% by weight of dense amorphous silica, as hereinbefore defined, calculated as SiO 2 and based on said pigmentary TiC> 2 , an ^ optionally up to 5% by weight of alumina, calculated as A1 2 O 3 and based on said pigmentary TiO 2 , and an outer coating of porous silica and alumina, said porous silica being present in an amount from 0.5% to 15% by weight, calculated as SiO 2 and based on said pigmentary TiO 2 , said alumina being present in an amount from 0.5% to 11% by weight, calculated as A1 2 O 2 and based on said pigmentary TiO 2 .

2. Coated TiO 2 pigment according to Claim 1 wherein said dense amorphous silica of said inner coating is present in an amount from 3.5% to 6% by weight.

3. Coated TiO 2 pigment according to Claim 1 or Claim 2 wherein alumina is present in said inner coating in an amount from 1% to 3% by weight.

4. Coated Ti0 3 pigment according to any one of the preceding Claims wherein said porous silica in said outer coating is present in an amount from 5% to 10% by weight and wherein said alumina in said outer coating is present in an amount from 5% to 10% by weight.

5. Coated TiO 2 pigment according to any one of the preceding Claims wherein said dense amorphous silica of said inner coating is present in an amount of 3.5% by weight, said alumina in said outer coating is present in an amount of 5.25% by weight, and said porous silica in said outer coating is present in an amount of 5% by weight,

6. A process for preparing coated TiO 2 pigment comprising the steps of 18470 (i) adjusting the pH of an aqueous slurry of pigmentary TIO2 to from 9 to 10, (ii) adding to said slurry an aqueous solution of soluble silicate in an amount calculated to provide from 155 to 10% by weight of silica, calculated as SiO 2 and based on said pigmentary TI0 2 , (iii) slowly adding dilute aqueous acid to said slurry until the pH of said slurry is from 6 to 7.7, (iv) adding to said slurry, maintained at a pH from 5 to 9, an aqueous solution of a soluble silicate in an amount calculated to provide from 0.5% to 15% by weight of silica, calculated as SiO 2 and based on said pigmentary Ti0 2 , (v) adding to said slurry, maintained at a pH from 4 to 9, an aqueous solution of soluble aluminate in an amount calculated to provide from 0.5% to 11% by weight of alumina, calculated as AI2O3 and based on said pigmentary TiO 2 , (vi) adjusting the pH of said slurry to from 7 to 9, and (vii) separating the resulting coated Ti02 pigment from said slurry.

7. A process for preparing coated TiO 2 pigment according to Claim 6 wherein after step (iii) and prior to step (iv) , an.additional step (iiia) is carried out comprising adding to said slurry, maintained at a pH from 6 to 9, an aqueous solution of soluble aluminate in an amount calculated to provide from 0.5% to 5% by weight of alumina, calculated as Al 2 0g and based on said pigmentary TiO 2 .

8. A process for preparing coated TiO 2 pigment -1843470 according to Claim 6 or Claim 7, wherein said dilute aqueous acid of step (iii) has a normality of less than about 4.

9. A process for preparing Ti0 2 pigment according to any one of Claims 6 to 8 wherein said aqueous slurry of pigmentary TiO 2 is heated to a temperature of from 80C. to 90°C, prior to step (i).

10. A process for preparing coated T10 2 pigment according to any one of Claims 6 to 9 wherein-said aaueous slurry of pigmentary Ti0 2 is maintained at a temperature- of ou°C. to 90°C. during steps (1), (ii), and (ill).

11. A process for preparing coated Ti0 2 pigment according to Claim 10, wherein said slurry is cured for at least 30 minutes prior to sten (v).

12. A process for preparing coated Ti02 pigment according to Claim 11,wherein following step (ill) said slurry is cured for at least 30 minutes at about 90°C. while maintaining the pH of said slurry from 6 to 7.7.

13. a process for preparing coated TiO^ pigment according to Claim 10, wherein in step (iii) the pH of said slurry is from 7.3 to 7.6, in step (iv) the pH of said slurry is maintained from 5 to 7, in step (v) the pH of said slurry is maintained from 6 to 8, and in step (vi) the pH of said slurry is adjusted to from 7-5 to 8.0.

14. A process for preparing coated Ti0 2 pigment according to Claim 10, wherein in step (iii) the pH of said slurry is from 7.3 to 7.6, in sten (iv) the pH of said slurry is maintained from 8 to 9, in step (v) the pH of said slurry is maintained from 8 to 9, and In step (vi) the pH of said slurry is adjusted to about 7. -193470

15. A process for the preparation of coated TiO 2 pigment having a dense silica inner coating and a porous outer coating of silica and alumina substantially as described with reference to the Examples 1 to 26.

16. Coated TiO 2 pigment, whenever prepared by a process according to any one of claims 6 to 15.