Document npZVmRq2XyBoDRzBRLbBybzyG

BROOKES & RTIN CHARTERED PATENT AGENTS TRADE MARK AGENTS EUROPEAN PATENT ATTORNEYS RICHARO C. MARTIN, m.a .c a n y m. DAVJO C-WOODCRAFT, *... (Cmc m.1 ERIC E. BARNARD, C.e*-.M.l.C.E.,C.Oii*. A.r. JOHN F. SYMONDS, LC-a two-, s o l ic it o r JOHN H. SLAKE. o .c .,Ic h L) HUGH R. WRIGHT, m.a .Ox b n . PETER C. SANDERS. B.Sc. OONALO J. ATTFIELO. B.Se.C-CNew M.R 5 C-lBIRMINOHAM) as s is t ed by : TIMOTHY M. A. RlCKAR0.fl.8e.PMO. GRAHAM F. MATTHEWS, B.A , C.Eh p . F.I.C.C. (CONSULTANT) H. NICHOLAS MATTHEWS. M.iwar. T. n .a . (c o n s u l t a n t ) GEOFFREY E.BROOME [CONSULTANT) Orncs Ma n a o c r - A. J. JELLY ACCOUNTS-Mfifl C.OLSlNIA TELEPHONES (071)242 9S3J _ in t e r n a t io n a l +44 71 242 9631 FACSIMILE (071) 631 0566 INTERNATIONAL +44 71 631 0586 t e l e k 267541 BROOKMAR G l o x 3)6 HIGH HOLBORN HOUSE 52/5-4, HIGH HOLBORN LONDON, WCIV 6SE JR/CHI 370 YOUR REF -s 1q 6 February 1991 E.I. Du Pont de Nemours & Company Legal Department, Patent Records Center, 1007 Market Street, Wilmington, Delaware 19898, U.S.A. For the attention of Mr. David J. Gould Ft3lSIS9, <a^oesB Dear Sirs, . 0 245 984 We have pleasure in enclosing the Certificate of Grant of European Patent No. 0245984, granted on Application No. 87303682.6, together with a copy of the published patent specification. Publication of the grant took place on 16 January 1991 and the patent will be open to opposition for a period of nine months from that date. The patent will remain in force for a term of 20 years from the application filing date, i.e. from 27 April 1987 subject to the payment of annual renewal fees in each of the designated states in which it has been registered, i.e. Belgium, Germany, Spain, France, Great Britain and the Netherlands. The patent may be maintained in all or any of these designated states. The Associates in each of these countries have been instructed to attend to the payment of the first renewal fee after grant and also to forward the certificate of renewal and their debit notes direct to Du Pont. We have entered the European Patent (UK) on our maintenance system. ijurs faitjifully. \: Miss Jill Routh BROOKES & MARTIN i.- -H i ,. ? jfe jt;* I'*'* rj? " TUNBRIDGE WELLS: I BOYNE PARK, TUNBRIOGE WELLS, KENT, TN4 BEL. TEL.[0602) 610600 TELEX 057076 FACSIMILE [06021 510666 BIRMINGHAM: PRUDENTIAL BUILDINGS, S ST. PHILIPS PLACE, BIRMINGHAM, B3 2AF. TEL. [02') 230 0262 TELEX 33S38I FACSIMILE (021} 2001473 > MUNICH: IS ZWEIBRUCKENSTRASSe. 6000 MUNICH 22. foM>a TEL. (099) 22 20 IO locrr oP&A7D q .C. 7,-IK-Qi <;,? ; N 30919 Europaisches- IP Patentamt European Patent Office if Office europeen des brevets m lit URKUNDE CERTIFICATE CERTIFICAT Es wird hiennit bescheinigt, dafi fur die in der beigefiigfen Patentschrift beschriebene Erfmdung ein europaisches Patent fiir die in der Patentschrift bezeichneten Vertragsstaaten erteilt worden ist. It is hereby certified that a European patent has been granted in respect of the invention described in the annexed patent specification for the Contracting States designated in the specifica tion. II est certifie qu'un brevet europden a dte ddlivre pour l'invention decrite dans le fascicule de brevet ci-joint, pour les Etats contractants designds dans Ie fascicule de brevet. Europaisches Patent Nr.: European patent No.: Brevet europeen n: 0245984 Patentinhaber: Proprietor of the patent: Titulairedubrevet: E.I. DU PONT DE NEMOURS AND COMPANY 1007 Market Street Wilmington Delaware 19898/US Munchen, den Munich, Fait a Munich, Ie 16.01.91 F.PA/F.PO/OEB Form 2031 03.83 Jf A. Goeller Generaldiiektion 2 - FormalpriifungssteUe Directorate-General 2 - Formalities Section Direction gdnerale 2 - Section des formalites il it N 30919.01 DUP050449329 Europaisches Patentamt 0 245 984European Patent Office Publication number: B1 '" >! JOffice europeen dee brevets EUROPEAN PATENT SPECIFICATION () Dateofpublicationof patentspecification: 16.01.91 (B> Application number: 87303682.6 <g) Date of filing: 27.04.87 () Int. CL6: C 09 Cl/36, C 09 C 3/06 I!- @ Titanium dioxide pigment coated with boria-modffred silica. (g) Priority: 30.04.86 US 857324 () Dateofpublicationof application: 19.11.87 Bulletin 87/47 (6) Publication of the grant ofthe patent: 16.01.91 Bulletin 91/03 (g) Designated Contracting States: BEDEESFRGBNL () References cited: EP-A-0 073 340 DE-A-2 740 561 CHEMICAL ABSTRACTS, vol. 88, no. 2,9th January 1978, page 85, abstract no. 8602k, Columbus, Ohio, US; & FN-A-53133 (KEM1RA OY) 31-10-1977 Proprietor: E.l. DU PONT DE NEMOURS AND COMPANY 1007 Market Street Wilmington Delaware 19898 (US) (72) Inventor: Jacobson, Howard Wayne 2009 Longcome Drive Wilmington Delaware 19810 (US) Representative: Woodcraft, David Charles et_al BROOKES & MARTIN High Holborn House 52/54 High Holborn London, WCIVESE(GB) EP 0245984 B1 Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European patent convention). Courier Press, Leamington Spa, England. 'ill fel N 30919.02 DUP050449330  Description EP 0 245 984 B1 Background of the Invention The present invention relates generally to coated TiOz particles for use as pigments, and particularly to 5 Ti02 particles having densified silica coatings, TiOj has a high refractive index for its density, which renders it a superior pigment for use in coatings, e.g,, paints. However, Ti02 is photoactive; exposure to ultraviolet radiation results in generation of free radicals on the surface of Ti02 particles. Thus, where a Ti02 pigment is employed in a paint exposed to sunlight, free radicals are generated which can migrate to the film-forming component of the paint, 10 resulting in coating degradation or failure. Therefore, minimizing free radical migration is desirable in order to provide lightfast and stable pigments for coating use. A widely-used approach to the problem of providing lightfastness involves coating Ti02 particles with a layer of densified silica. The following patent references disclose conventional silica coating densification techniques. >s Werner, U.S. Patent 3,437,502, discloses Ti02 pigment having high opacity and dispersibility, which is obtained by applying a dense silica coating to Ti02 followed by topcoating with alumina. The silica coating is applied by increasing the pH of a slurry of Ti02 particles to above 7. A predetermined quantity of sodium silicate is then added to the slurry, resulting in a rise in pH of the slurry to above 8, generally above 11. The pH is then gradually reduced over a period of several hours by addition of dilute acid, and the resulting 20 silica-coated particles are cured at pH 6.0 to 7.5, at 60 to 100C, for 30 minutes to several hours. The resulting product is then coated with alumina. West, U.S. Patent 4,125,412, discloses preparation of durable Ti02 pigments by slurrying silica-coated Ti02 particles at pH 9 to 10.5 while maintaining slurry temperature at 80 to 100C. European Patent Application 73,340 describes a similar method for coating Ti02 particles with 25 amorphous dense silica prior to topcoating with alumina. Silica is applied by adding a soluble silicate to a Ti02 slurry at a temperature of at least 85C and at a pH of from 9.8 to 10.1. The slurry is then neutralized in at least three neutralization steps by addition of acid. Common to the teachings of the foregoing references is a requirement for relatively high temperatures and high alkalinity during the silica densification step. An extremely lightfast white pigment for use in paint 30 systems can be made by depositing 4-6 weight percent amorphous dense silica on rutile Ti02 pigment. To coat the base Tt02 pigment with this quantity of dense silica requires several hours at 85-90C, during which mineral acids are used to precipitate silica from Na2Si03. This high temperature coating operation requires use of steel tanks, since less costly fiberglass tanks are not rated for use in the 85-90C temperature range. Methods for providing dense coatings of more than 4 weight percent silica at lower 35 temperatures, e.g., 65-80C, to allow use of fiberglass tanks, would be of significant interest to the Ti02 pigment industry. ' It has now been found that codeposition of B203 with Si02 provides dense silica coatings at process temperatures which permit use of fiberglass slurry tanks. The boria-containing pigments resulting from this process are highly lightfast, and exhibit excellent gloss and dispersibility. 40 Summary of the Invention The present invention provides a pigment consisting essentially of Ti02 particles bearing coatings comprising Si02 and B203. In a process aspect, the present invention provides a process for preparing Ti02 particles bearing coatings of boria-modified silica, comprising: 45 (a) forming an aqueous siurry of rutile Ti02 at a temperature from about 65C to about 90C; (b) adjusting pH of the slurry to the range 7-10.5; (c) adding a predetermined quantity of a solution comprising Na2Si03 and B^Og, under conditions which maintain silicate and boria ions in solution; (d) gradually lowering pH of the slurry to about 7.5-8.5 by addition of acid, thereby depositing a coating so of silica and boria; and (e) curing the resulting coated pigment at a temperature from about 65C to about 90C for a period of at least 15 minutes. Detailed Description of the Invention 55 In accordance with the present invention, B203 is codeposited with Si02 on Ti02 particles, providing lightfast, durable pigments. The Ti02 particles making up the pigments bear dense silica coatings, yet are produced at temperatures significantly lower than those required by other silica densification processes capable of providing dense silica coatings at greater than 4 weight percent. in practice, silica/boria layers are codeposited from a master solution of Na2Si03 and Na2B204- A 60 standard acid solubility test can be to measure effectiveness of coating densification. In addition to enhanced densification of Si02 in the presence of B203, an improvement in pigment brightness is provided. The Ti02 used to prepare the pigment of the Invention can be of the conventional rutile variety, prepared by either the chloride or sulfate process. The amount of silica or silica topcoated with alumina which the particles bear as coatings can be varied 55 depending primarily upon the pigment's intended use. Generally, the silica coatings will constitute from 2- 2 * jl f I j| . f i*( is1r" . 4 DUP050449331 EP 0 245 984 B1 10% of the total pigment weight, and an alumina topcoat, if present, will constitute 0.5-5% of the total weight of the particles. Preferably, the silica coating constitutes 4-8% of the total weight of the pigment particles. Preferably, the alumina topcoat, if present, constitutes 1-4% of the total weight. The amount of alumina the particles bear as coatings, expressed as percent by weight, is calculated by s first determining the alumina content of the coated pigment by ion plasma spectroscopy. The alumina content of the uncoated rutile Ti02 is similarly determined, and the alumina content attributable to the coating is determined by computing the difference between coated and uncoated alumina contents. The pigment of the present invention can be prepared from an aqueous slurry containing 200-450 grams per liter Ti02. This slurry is brought to 65-90C and is held at that temperature throughout the to preparation procedure. The slurry is adjusted to pH 7-10.5, and a sufficient quantity of solution containing Na2Si02 and Na2B204, or separate solutions of Na2SI03 and Na2B204, is added to provide a coating of the desired composition and weight. Upon addition of the Na2Si03 and Na2B204 solutionis), pH of the slurry * increases to about 11. Generally, slurry pH must be maintained above about 10 to maintain silicate and boria ions in solution. is Generally, the weight and composition of the coatings to be deposited will be determined by the composition of the master Na2Si03/Na2B204 solution, the quantity added to the slurry containing Ti02, and quantity of Ti02 in the siurry. For Ti02 slurries containing 400 g/L Ti02, about 40-50 mL of a master aqueous solution, containing 400 g Si02 equivalent and 40 g B203 equivalent per liter, should be added per liter Ti02 slurry to provide coatings approximating 5% of pigment weight. The compositions of the coating solutions 20 can be varied as required to provide variations in coating weight and composition. The coatings applied to Ti02 particles in accordance with the present invention can contain from 60-95 percent by weight Si02 and 0.5-30% by weight B203, based upon coating weight. Preferably, coatings will contain 80-90 percent by weight Si02 and 2-20 percent by weight B203. Upon addition of the coating solutions, the pH oftheTi02 slurry will rise to the range 10.5-11.5. Over a 25 period of 2-4 hours, 15% aqueous HC1 or other mineral acid is added to gradually decrease slurry pH to about 8.0. As acid is added, Si02/B203 will be deposited and densified on the surfaces of the Ti02 particles. After all acid is added, the pigment should be cured at 65-90 gC for 30 minutes. If desired, alumina can be precipitated on silica/boria coated Ti02 particles by adding enough sodium aluminate to the slurry, at a temperature from about 50-90C, to provide a concentration of about 2-8% by 30 weight of the Ti02. Acid, normally sulfuric or hydrochloric, is added at the same time to maintain the pH of the slurry within the range 6-9 as the alumina is precipitated. After addition of aluminate is complete, the slurry is allowed to cure, with continuous stirring, for 15-30 minutes. The resulting pigment is then separated from the liquid by filtration or centrifugation, washed with water, and dried. 35 Coating compositions can be prepared with the pigments of the invention in conventional ways, e.g., by blending the pigment with a film-forming component and a liquid carrier. The following examples illustrate particular aspects of the present invention. In the examples, all parts and percentages are by weight and all degrees are Celsius unless otherwise indicated. 40 Example 1 in a large plastic vessel equipped with stirring apparatus and a pH probe, 9000 parts water are mixed with 3000 parts rutile Ti02. The resulting slurry is heated to 75, and the pH is adjusted to about 7.5 by addition of NaOH. 400 parts *' of an aqueous solution containing 400 parts Si02 equivalent (as Na2Si03) and 40 parts B203 equivalent (as 45 Na2B204) per 1000 parts water are added. The pH of the resulting mixture is then determined, and sufficient 15% aqueous HCI is added, over a period of about 3 hours, to reduce the slurry pH to about 8.0. After all HCI has been added, the siurry is cured for 30 minutes at pH 8.0 and 75. The resulting coating, which constitutes about 5.5% by weight of the total pigment, contains SiOz and B203 in a ratio by weight of about 90:10. so A coating of 2.5% hydrous alumina is than deposited on the pigment by simultaneous gradual addition of 200 ml NaAI02 solution (containing NaAI02 equivalent to 400 g Al203 per liter solution) and HCI, such that the pH of the slurry is continuously maintained from about 7.0 to about 7.5. The resulting coated pigment is cured by holding for about 30 minutes at ambient temperature and pH 7.5. The pigment is then filtered from the slurry, washed free of salts with water, and air-dried. The pigment S5 is then micronized, using 3 lbs. superheated steam per lb. pigment. The resulting pigment is suitable for use in paint systems where iightfastness is required. Example 2 In this series of experiments, several batches of pigment were prepared to evaluate the effect of B203 60 codeposition and various processing temperatures upon acid solubility. Generally, acid solubility is inversely related to chalk-fade resistance for Ti02 pigments. The pigments described below were prepared by procedures substantially similar to that described in Example 1, above, except that processing temperatures were varied as indicated in Table 1, below. Acid solubilities were determined as follows. 65 10 mL 66% sulfuric acid is added to a test tube containing a magnetic stirring bar, and the tube is 'i:* : ,.:.J 3 DUP050449332 * EP 0 245 984 B1 placed in an aluminum heating block and heated to 175. A 0.2000 g pigment sample is added to the tube, and digested for 1 hour with stirring. At the conclusion of the digestion period, the tube containing the pigment sample is cooled by pouring the acid mixture into a beaker of ice, and the residue in the tube and beaker is washed with distilled water. The pigment residue in the tube and beaker is collected in a 100 mL s volumetric flask, the volume made up to 100 mL by addition of distilled water, and the contents mixed thoroughly. The contents of the volumetric flask are then filtered, and 10 mL of the resulting filtrate are added to a 25 mL volumetric flask, to which 2 mL 20% hydrogen peroxide, and sufficient 10% sulfuric acid to make 25 mL, are added. The resulting solution is allowed to stand one hour. Absorbance of the solution is then read at 400 nm using a 10 mm cell path. Soluble Ti02 is determined by reference to a previously to prepared spectrophotometric curve obtained by measurement of samples containing known quantities of dissolved Ti02. TABLE 1 Acid Solubility: (% dissolved Ti02) Processing Temp. (C) Coating Composition: 100% Si02 90% SiO/10% B203 90 20 80 0.11 0.10 0.27 0.12 70 25 65 3.34 6.0 0.17 0.22 Example 3 30 A mill base is made by mixing Pigment of Example 1 387 parts alkyd resin (Syntex 3145; Celanese Coatings Co.) 149.6 parts 35 n-butanol 9.4 parts xylol 79 parts 40 This mixture is sand ground, and then the sand is filtered from the mill base. A paint is made by mixing 100 parts of the mill base with the following: xylol 45 alkyd resin (Syntex 3145; Celanese Coatings Co.) 1.4 parts 71.8 parts melamine resin solution {Cymel 248-8, 55% solids, American Cyanamid Co.) 50 Claims 38.5 parts 1. A pigment comprising rutile Ti02 particles bearing coatings comprising Si02 and Bi03. 2. A pigment according to claim 1, wherein the Ti02 particles have coatings comprising from 60-98 ss percent by weight Si02 and 0.5-30 percent by weight B203 based on coating weight. 3. A pigment according to claim 2, wherein the Ti02 particles have coatings comprising from 80-90 percent by weight Si02 and 2-20 percent by weight B203, based on coating weight. 4. A pigment according to claim 3, wherein the Ti02 particles have Si0j/B203 coatings constituting 2 10% of the total pigment weight. so 5. A pigment according to claim 4, wherein the Ti02 particles have Si0j/B203 coatings constituting 4 8% of the total pigment weight. 6. A pigment according to any one of the preceding claims, wherein the Ti02 particles have outer coatings of alumina constituting 0.5-5% of the total weight of the particles. 7. A pigment according to claim 6, wherein the Ti02 particles have outer coatings of alumina 65 constituting 1-4% of the total weight of the particles. 4 - i: i. DUP050449333 EP 0 245 984 B1 8. A process for preparing Ti02 particles bearing coatings of boria-modrfied silica, comprising: (a) forming an aqueous slurry of rutile TiOz at a temperature from about 65 to about 90C; (b) adjusting pH of the slurry to the range 7-10.5; (c) adding a predetermined quantity of a solution comprising Na2Si03 and B503, under conditions s which maintain silicate and boria ions in solution; (d) gradually lowering pH of the slurry to abut 7.5-8.5 by addition of acid, thereby depositing a coating of silica and boria; and (e) curing the resulting coated pigment at a temperature of from about 65C to about 90C for a period of at least 15 minutes. 10 9. A process according to claim 8, wherein sufficient Na2Si03 and Bs03 are added in step (e) to provide a coating comprising from 60-98 percent by weight Si02 and 0.5-30 percent by weight B203 based on coating weight. . . 10. A process according to claim 9, wherein sufficient Na2Si03 and B303 are added in step (c) to provide a coating comprising from 80-90 percent by weight Si02 and 2-20 percent by weight B203, based on coating is weight. 11. A process according to claim 10, wherein sufficient Na-^iOj, and B203 are added in step (c) to provide a SiOg/BsA, coating constituting 0.5-10% of the total pigment weight. 12. A process according to claim 11, wherein sufficient Na^iO;, and B203 are added in step (c) to provide a SiOg/BaO;, coating constituting 4-8% of the total pigment weight. 20 13. A process according to any one of claims 8 to 12 comprising the following additional step:(f) adding sufficient NaAI02 to the slurry, at a temperature from about 50-90C, to provide a concentration of about 2-8% by weight of the Ti02, while adding mineral acid to maintain the pH of the slurry from 6-9. 14. A coating composition comprising a pigment having a densified SiOa/B203 coating according to any 25 one of claims 1 to 7, a film-forming material, and a liquid carrier. Patentanspr&che 1. Pigment, welches Rutii-Ti02-Teiichen enthait, die Si02 und B203 enthaltende Beschichtungen 30 aufweisen. 2. Pigment nach Anspruch 1, worin die TiOj-Teilchen Beschichtungen aufweisen, die 60-98 Gew.-% Si02 und 0,5 bis 30 Gew.-% B203, bezogen auf das Beschichtungsgewicht, enthalten. 3. Pigment nach Anspruch 2, worin die Ti02-Teilchen Beschichtungen aufweisen, die 80-90 Gew.-% Si02 und 2-20 Gew.-% BjO,, bezogen auf das Beschichtungsgewicht, enthalten. 35 4. Pigment nach Anspruch 3, worin die TiOa-Teilchen Si03/B203-Beschichtungen aufweisen, die 2-10% vom gesamten Pigmentgewicht ausmachen. 5. Pigment nach Anspruch 4, worin die TiO:-Teilchen SiOs/B2Q3-Beschichtungen aufweisen, die 4-8% vom gesamten Pigmentgewicht ausmachen. 6. Pigment nach einem der vorstehenden Anspruche, worin die Ti02-Teilchen auBere Beschichtungen 4o aus Aluminiumoxid aufweisen, die 0,5-5% vom Gesamtgewicht der Teilchen ausmachen. 7. Pigment nach Anspruch 6, worin die Ti02-Teilchen aulJere Beschichtungen aus Aluminiumoxid aufweisen, die 1-4% vom Gesamtgewicht der Teilchen ausmachen. 8. Verfahren zur Herstellung von Ti02-Teiichen mit Beschichtungen aus Boroxid-modifiziertem Siliziumdioxid durch 45 (a) Bilden einer wassrigen Aufschlammung von Rutil-TiOa bei einer Temperatur von etwa 65 bis etwa 90C; <b) Einstellen des pH-Werts der Aufschlammung im Bereich von 7-10,5: (c) Zugeben einer vorbestimmten Menge einer Na2Si03 und B203 enthaltenden Losung unter Bedingungen, die Sifikat- und Borationen in der Losung halten; so (d) alimahliches Senken des pH-Werts der AufschlSmmung auf etwa 7,5-8,5 durch Zugabe von Saure, wodurch eine Beschichtung aus Siliziumdioxid und Boroxid niedergeschlagen wird; und (e) Harten des resultierenden beschichteten pigments bei einer Temperatur von etwa 65C bis etwa 90C uber einen Zeitraum von wenigstens 15 Minuten. 9. Verfahren nach Anspruch 8, worin in Schritt (c) ausrelchend Na2Si03 und B203 zugesetzt warden, um 55 eine Beschichtung mit 60-98 Gew.-% Si02 und 0,5-30 Gew.-% B203, bezogen auf das Beschichtungsgewicht, zu ergeben. 10. Verfahren nach Anspruch 9, worin in Schritt (c> ausreichend Na2Si03 und B20} zugesetzt werden, um eine Beschichtung mit 80-90 Gew.-% Si02 und 2-20 Gew.-% B203, bezogen auf das Beschichtungsgewicht, zu ergeben. so 11. Verfahren nach Anspruch 10, worin in Schritt (cl ausreichend Na2Si03 und B203 zugesetzt werden, um eine Si02/B203-Beschichtung zu ergeben, die 0,5-10% vom gesamten pigmentgewicht ausmacht. 12. Verfahren nach Anspruch 11, worin in Schritt (c) ausreichend Na2Si03 und B203 zugesetzt werden, um eine SiCyB203-Beschichtung zu ergeben, die 4-8% vom gesamten Pigmentgewicht ausmacht. 13. Verfahren nach einem der Anspruche 8 bis 12, welches den folgenden zusatzlichen Schritt umfaBt 65 (f) Zugabe von ausreichend NaAI02 zur Aufschlammung bei einer Temperatur von etwa 50 bis 90C, 5 DUP050449334 EP 0 245 984 B1 um eine Konzentration von etwa 2-8% vom TiOj-Gewicht bereitzustellen, wobei Mineralsaure hinzugefugt wird, um den pH-Wert der Aufschlammung bei 6-9 zu halten. : 14. Uberzugszusammensetzung, welche ein Pigment mil einer verdichteten Si02/B203-Beschichtung gemaG einem der Anspriiche 1 bis 7, ein filmbildendes Material und einen flusslgen Trager umfaGt 5 Revendications 1. Un pigment comprenant des particules de Ti02 du type rutile, portant des revetements comprenant Si02 et B203. 10 2. Un pigment selon la revendication 1, dans lequel les particules de Ti02 portent des revetements comprenant 60 a 38 pour cent en poids de Si02 et 0,5 30 pour cent en poids de B203r par rapport au poids de revetement. 3. Un pigment selon la revendication 2, dans lequel les particules de Ti02 portent des revetements comprenant 80 & 90 pour cent en poids de Si02 et 2 a 20 pour cent en poids de B203r par rapport au poids de 15 revetement. 4. Un pigment selon la revendication 3, dans lequel les particules de Ti02 portent des revetements de Si02/B203 constituant 2 a 10% du poids total du pigment. 5. Un pigment selon la revendication 4, dans lequel les particules de Ti02 portent des revetements de Si02/B203 constituant 4 a 8% du poids total du pigment. 20 6. Un pigment selon I'une quelconque des revendications prcedentes, dans lequel les particules de Ti02 portent des revStements sxterieurs d'alumine constituant 0,5 5% du poids total des particules. 7. Un pigment selon ia revendication 6, dans lequel les particules de TI02 portent des revetements exterieurs d'alumine constituant 1 4% du poids total des particules. 8. Un proced pour preparer des particules de Ti02 portant des revStements de silice modifies par de 25 I'anhydride borique, consistant a: (a) former une suspension aqueuse de Ti02 du type rutile une temperature d'environ 65 a environ 90C. (b) ajuster le pH de la suspension dans I'intervalle de 7 10,5; (cj ajouter une quantite predetermines d'une solution comprenant Na2Si03 et B203, dans des 30 conditions qui maintiennent en solution les ions silicate et d'anhydride borique; (d) abaisser progressivemant le pH de la suspension par addition d'acide jusqu' atteindre environ 7,5 8,5, en dSposant ainsi un rev&tement de silice et d'anhydride borique; et (e) faire murir le pigment revStu resultant une temperature d'environ 65C environ 90C pendant une periode d'au moins 15 minutes. 35 S. Un procdde selon la revendication 8, dans lequel on ajoute suffisamment de Na2SI03 et de B203 dans I'etape (c) pour former un revetement comprenant 60 98 pour cent en poids de Si02 et 0,5 30 pour cent en poids de B203, par rapport au poids de revetement. 10. Un precede selon la revendication 9, dans lequel on ajoute suffisamment de Na2Si03 et de 8203 dans l'6tape (c) pour former un revetement comprenant K) 90 pour cent en poids de Si02 et 2 20 pour 40 cent en poids de B203, par rapport au poids de revetement. 11. Un procddd seion ia revendication 10, dans lequel on ajoute suffisamment de Na2Si03 et de B2Oa dans I'etape (c) pour former un revetement de Si<VB203 constituant 0,5 10% du poids total du pigment. 12. Un precede seion la revendication 11, dans lequel on ajoute suffisamment de Na2Si03 et de B203 dans i'etape (el pour former un revetement de Si02/B203 constituant 4 8% du poids total du pigment 45 13. Un precede seion I'une quelconque des revendications 8 12, comprenant I'etape supplementaire suivante; (f) ajouter suffisamment de NaAI02 la suspension, une temperature d'environ 50 90C, pour obtenir une concentration d'environ 2 8% par rapport au poids du T!02, tout en ajoutant un acide mineral de fagon maintenir le pH de la suspension entre 6 et 9. 50 14. Une composition de rev&tement comprenant un pigment portant un revtement densifie de SiCV B203 selon I'une quelconque des revendications 1 7, une matire fllmogene et un vghicule liquide. 55 60 * ; i / i f 65 6 if .* 5* i DU P050449335