[1]'United States Patent Officie 395229079 p a t e n t e d J u l y 2 8 , 1 9 7 0 2 3,522,079 a monovalent anion, particularly such a salt of an inorganic acid, for example aluminium trichloride or nitrate, which are readily watersoluble, hydrolyzable salts of aluminum, although aluminum salts containing polyvaThomas James Wiseman, Darlington, England, assignor 5 lent anions such as aluminium sulphate may be used, if desired. The initial aqueo us solutio n conve niently contai ns betwe en 0.5 and 200, and particu larly betwe en I and 150, grams/ litre of the salt (expre ssed as the oxide) for solu10 tions containi ng monoval ent anions and prefera bly a concent ration in the range of 2 to 50 grams/lit re for solutions containi ng polyvale nt anions (particul arly for aluminiu m sulphate solutions ). Such aqueous solutions normally have a pH value below about 3.7. 15 The pH value is then raised by the addition of a water soluble alkali, for example an alkali metal hydroxi de or carbonat e, until a white precipita te forms. Such a precipita te generall y begins to form in an aqueous aluminiu m salt solution at a pH value in excess of about 3.4 in a 20 solution of an aluminiu m salt having a monoval ent anion and at a pH in excess of about 3.2 where the aluminiu m salt contains a polyvale nt anion, and,it is preferre d, for example when coating with alumina, to adjust the pH value at this stage to one in the range 3.8 to 5.0, and particula rly 25 to, one in the range 4.2 to 4.8 for solutions of salts having a monoval ent anion, and to one in the range 3.4 to 4.5, and particula rly 3.4 to 4.2 for solutions of salts having a polyval6 nt anion. Altern atively , the prefer red amoun t of alkali to be 30 added may be express ed as that sufficien t to give a ratio of the metal (e.g. aluminiu m) to hydroxy l groups in the solution in the range of 1:1.5 to 1:2.8 and particula rly a ratio in the range 1:2 to 1:2.5 for solutions of salts having a monoval ent anion and in the range 1: 0.2 to 1: 1.5, par35 ticularly 1:0.3 to 1:0.9, for solutions of a salt having a polyvale nt anio n: When the desire d amoun t of alkali has been added to form a precip itate, the mixtur e is heated to rediss olve the precip itate. It is believ ed that the heatin g may also 40 "age" the solution thereby making it more suitable for the process of the present inventio n. It is prefer red not to boil the solutio n at this stage and a conve nient maxim um tempe rature is about 90' C. Gener ally, heatin g to a tempe rature below 25' C. is not 4,5 favour ed. It is prefer red to heat the solutio n to a tempe rature in the range 401 C. to 90' C. and to heat to a higher tempe rature in this range the more conce ntrate d the solutio n. Heatin g is conve niently carrie d out for a period in the range 5 to 200 minute s and prefer ably for about 30 to 60 50 mi nut es. The solution, after heating to redissol ve the precipita te, is normally diluted, if necessa ry, to a suitable concent ratio-n for use in the coating process. The degree of dilution will depend to some extent upon the amount of hy55 dr ou s me tal oxi de to be ap pli ed to the par ticl es. Ge ne rall y, dil uti on, wh er e ne ces sar y, to giv e a sol uti on co nta ini ng bet we 'en ab out 0.5 an d 6 gra ms/ litr e of the salt (ex pr ess ed as the oxi de) is pr efe rre d. The insolubl e metal oxide particles , parfticul arly pig60 mentary titanium dioxide particles , are then suspend ed in the solution. It has been found that aqueo us solutio ns prepar ed as descri bed above axe gener ally excell ent disper sin,g agents for metal oxide partirles and provid e - accept able disper - 65 sion Of the particles without the addition of another dispersi ng agent such as silicates, polypho sphates, metapho sphates and Organic dispersi ng agents such as isopropa nolamin es (which may have an adverse effect upon the subsequ ent use of the metal oxide particles ) although 70 the latter may, of course, be added if desired. After dispersi on of the metal oxide particles in the PRECIPITATION COATING PROCESS FOR C'OAT. ING METAL OXIDE PARTICLES WITH A HYDROUS METAL OXIDE to British Titan Products Company Limited, BilUngham, Durham, England, -a corporation of the United Kingdom No Drawing. Ffled Aug. 17, 1967, Ser. No. 661,194 Claims priority, application Great Britain, Oct. 25, 1966, 47,832/66; Feb. 14, 1967, 7,086/67 Int. Cl. C09c 1136, 3100, B44d 1102 U.S. Cl. 117-100 24 Claims ABSTRACT OF THE DISCLOSURE A process for coating metal oxide particles with a hydrous metal oxide comprising forming an aqueous solution of a hydrolyzable metal salt, raising the pH value of the solution until a precipitate forms, heating the solution to redissolve the precipitate, thereafter mixing the solution thus prepared with metal oxide particles and precipitating a hydrous metal oxide upon the particles from the solution. The present invention relates to a process for coating metal oxide particles, particularly pigmentary titanium dioxide particles, with hydrous metal oxide(s). Metal oxide particles such as pigmentary titanium dioxide particles, are commonly coated with one or more hydrous metal oxides, for example of aluminium, titanium, zirconium and/or cerium. Such coatings are generally applied to modify the dispersion properties of the metal oxide particles and/or to improve their photochemical reactivity, in the medium in which they are dispersed for use. It is desirable that such hydrous metal oxide coatings should be firmly adherent to the surface of the metal oxide particle and should preferably be as dense as possible (as opposed to a loosely adherent "fluffy" coating). It is also desirable, in appropriate cases, to be able to apply a single coating of, for example, alumina of this type to the metal oxide particles without the necessity for first coating the particle with another oxide such as silica (which may be undesirable) and this has hitherto proved very difficult to accomplish. The quality of the hydrous metal oxide coating applied may be readily determined from an electron micrograph of the coated particles and/or by the performance of the coated pigment particles in certain tests, for example those described later in this specification. It is, therefore, an object of the present invention to provide an improved process for coating metal oxide particles with hydrous metal oxide(s). Accordingly, the present invention is a process for coating metal oxide particles comprising forming an aqueous solution of a hydrolyzable metal salt; raising the pH value of the solution until a precipitate forms; heating the solution io redissolve the precipitate; mixing the resulting metal-containing solution with insoluble metal oxide particles; precipitating hydrous metal oxide from the solution upon the particles and thereafter recovering the coated metal oxide particles. The process of the present invention is hereinafter described in the coating of pigmentary titanium dioxide particles with alumina but the process is of general applicability in the coating of metal oxide particles with hydrous metal oxide(s). Where the particles are to be coated, for example, with alumina, the aluminium salt is preferably a salt containing
[2]3;522,079 3 solution the pH value of the dispersion is slowly raised by the addition of a water-soliible alkali, for example a water-soluble alkali metal hydroxide or carbonate. This adjustment is conveniently carried out over a period of from 0.25 to 10, hours and preferably between 3 and 5 hours to a pH value of at least 5.7 and preferably to a value in the range 5.7 to 6.0 in order to ensure precipitation of alumina upon the oxide particles in the desired form for soltitions of salts having a monovalent anion. A.-itition of the suspension may be provided during the period but should preferably not be too vigorous. In the case of solutions of salts having a polyvalent anion the initial adjustrnent is preferably to a value in the ran,-e 5.1 to 6.3 and particularly to one in the range 5.7 to 6. 1. After this initial adjustment has been accomplish,-d more alkali is preferably added, for example over a period of I to 3 hours, to give a pH value of at least 6.5, after which the pH value is conveniently adjusted, rapidly if desired, to a substantially neutral pH value (i.e. one in the range 7 to 8.5) to ensiire that the coated particles do not exhibit excess acidity or alkalinity. Dtiring these adjustments of pH value the temperatiire of the suspension may be maintained at any suitable value but it is preferred to maintain it in the range 10' C. to 50' C. and particularly in the range 15' C. to 45' C. The coated metal oxide particles may then be recovered, for example by ffitration, decantation or by the use of a centrifuge, washed and dried. If the coated particles are to be used for pigmentary purposes they will normally be subjected to milling, for example to wet sand milling or to fluid energy milling before packing. It is preferred that the process of the present invention be applied to uncoated metal oxide particles or to particles which have already been coated by the process of the present invention (and which, in the case of titanium dioxide particles may be anatase or rlitile and may have been produced by the so-called "sulphate?' or "chloride" processes). @If desired, it may be possible to precipitate the hydrous metal oxide from the metalcontaining solution upon the oxide particles by means of the passage of an electric current rather than by the addition of an alkali. Such a method is particularly attractive, for example, where the solution formed by raising the pH value of the metal salt soltition until a precipitate forins, followed by heating the solution to redissolve the precipitate, is also used as a dispersing agent for the milling of the particles. ()nce the milling has been accomplished the oxide particles in the suspension may be recovered by means of elei, ,trophoretic separation. During the passage of the current the hydrous metal oxide may be deposited upon the metal oxide particles, thereby achieving the coatin-. and recovery of the particles in a single step. It may, of coiarse, be desirable to dilute the suspension before subjectiiig it to the electrophoreticrecovery/coating step. Alternatively, if desired, the hydrous metal oxide can be precipitated upon the metal oxide particles by the addition of alkali in the manner described in U.S. application 565,077, i.e. by the simultaneous addition of metal-containing solution (prepared as described in the preceding paragraph) and alkali. It has been found that by this method of carrying out the present invention the quality of the hydrous metal oxide coating on the metal oxide particles may be even further improved. The following examples show embodiments of the present invention. EXAMPLE 1 An aqiieous aluminium trichloride solution (M/1) was prepared and 300 ml. of this was mixed with 340 ml. of an aqueous 2 N solution of sodiumbydroxide. The mixttire had a pH value of 3.8. A white precipitate formed 4 to redissolve the precipitate. The final solution was diluted to a molarity of 0.26 aluminium (expressed as AlpO3). A quantity (400 n-d.) of this solution was diluted with 4.5 litres distilled water to form the coating solution and in this were suspended (with stirring but without another dispersing agent) 600 g. of uncoated pigmentary rutile titanium dioxide particles giving a susppnsion having a pH value of 4. I. Aqueous sodium hydroxide (500 ml. NIIO@) was added 10 to give a pH of 5.6. Thereafter, over a period of four hours 230 ml. of NIIO aqueous sodium hydroxide were added to bring the pH value to 6.3. During this period the alumina was deposited upon the titanium dioxide particles. 1 5 Th-@ pH of thesuspension was finally raised to 7.5 by the addition of NIIO aqueous sodium hydroxide (over a period of 1 hour) and the titanium dioxide particles were recovered by filtration, washed and dried. The particles contained 1.82% A1203 and electron 20 micro-rapbs showed the particles to have a closely adherent'dense layer of aluniina uniformly deposited upon them wbich was quite unlike the rather diffuse, "fluffy" and non-uniform layer of alumina deposited on similar particles by conventional alumina coating techniques in 25 the absence of silica. EXAMPLE 2 An aqueous solution of aluminium, nitrate of similar molarity to the aluminium chloride solution in Example 30 1 was formed mixed with sodiuni hydroxide and heated to redissolve the precipitate as described in Example 1. The coating solution was prepared by mixing 435 ml. of the heated solution with 4 litres of distilled water and 500 g. tincoated rutile titanium dioxide were suspended 35 therein. The pH of the solution was initially raised to 5.7 by the addition of 327 ml. NIIO aqueous sodium hydroxide and from 5.7 to 6.3 over a period of 4 hours by the addition of 493 ml. of N/10 sodium hydroxide solution. 40 The pH value was finally adjusted to 8 with N/10 sodium hydroxide solution (over a period of I hour) and the alumina coated TiO2 pigtnent filtered off, washed and dried. The dried pigment contained 2.1% A1203 and the ap45 pearance of the particles in electron micrographs was indistinguishable from the product of Example 1. The pigments produced as described in Examples 1 and 2 were tested for (a) Reactivity in paint (aft6r one day's storage) '50 (b) Drying tirne in paint media (after twelve days' storage), and by the Jacobsen Test and the results were compared with those obtained from: 5.5 (1) an iincoated rutile pigment, and (2) a rutile pigment coated with silica, titania and allimina by conventional means (i.e. by suspending the pigment in aqtieous sodium silicate solution followed by 60 the addition of aqueoiis aluminium and titanium sulphate solutions and neutralisation) as shown in the following table: D l@ 3-acobs 65 Pigment Reactivity Yggp Test, percent E, xample 1 -------------------- 34 6y, 83 Example 2 ------------- ------ 34 6 82 Uncoated -------------------- 22 7y, 66 coated with SiO2, TiO2 and A1203 ----------------------- 137 17 73 70 EYAMPLE3 An aqueous aluminium trichloride solution (M/1) was prepared and 1,000 ml. was mixed with 1.130 ml. of aqueous 2 N soditmi hydroxide to give a mixture having a and the mixttire was heated to 9@0' C. for about 1 hour 7,5 pH of 3.8. A white precipitate for-med which redissolved
[3]31522,079 5 after heating to 90' C. for 1 hour. The solution was diluted to a molarity of 0.26 (expressed as A1203) and 1,20G nil. were added to 680 g. TiO2 particles. The resulting slurry was -milled without any other dispersing agent by agitation with 2 litres of Ottawa sand in a sand mill consisting of a container of 6" internil diameter in which rotated (at 3,360 r.p.in.) an impellier shaft bearing 3 circular impellers 3" diameter. During mflling the pH value rose from 3.6 to 4.Z. The sand was separated from the slurry of grouind TiO2 particles and the latter was diluted to a volume of 5 litres. The pH value of the slurry was raised to 5.7, then from 5.7 to 6.2 over a period of 6 hours with NI IO sodium hydroxide and finally from 6.2 to 8 over a period of 30 minutes. The TiO2 was then filtered off, washed and dried. The pigment, when tested by the Jacobsen test, gave a value of 72.4%. A similar pigment without the alumina coating gave a value of 64%. EXAMPLE4 An aqueous aluminium sulphate solution (0.2 M) was prepared and 100 ml. diluted to I litre with distilled water. Aqueous normal sodium hydroxide (29.4 ml.) was added and a white precipitate formed which redissolved after heating for 2 hours at 90' C. The alwninium content (expressed as A12,03) was 4.48 g./litre; the pH value 3.4 and the Al: OH ratio 1:0.75. A portion of this solution (224 ml.) was added to a slurry of 50 g. TiO? in 500 ml. distilled water and aqueous sodium hydroxide (NI 1 0) was added to the resulting mixture over a period of 6 hours to give a pH of 7.75. This pH value was maintained for 3G minutes before the TiO2 was filtered off and washed to remove sulphate ions. The pigment particles contained 2.18 % A1203 (on TiO2) and when tested by the Jacobsen test gave a lialue of 71.6%. A similar uncoated pigment gave a value of 64%. EXAMPLE5 The process of Example 4 was repeated except that a solution containing only 2.2 g./litre of aluminium (expressed as A1203) having a pH value of 3.2 and an Al: OH ratio of 1:0.375 was prepared. A volume (395 ml.) of this solution was added to a slurry of 50 g. rutile TiO2 in 500 ml. distilled water and aqueous N/IG sodiiun hydroxide added over a peiriod of 6 hours to give a flnal pH value of 7.7. The TiO2, when filtered off, washed and dried, had an A1203 content of 2.80% (on TiO2) and gave a result by the Jacobsen test of 70.7% (compared with a value of 64% for a similar uncoated TiO2 pigment). EY.AMPLE 6 An aqueous suspension of pigmentary rutile TiO2 particles was prepared by stirring. The suspension contained 125 g./litre TiO2. The pH value of the suspension was adjusted to a value of 5.7 by the addition of normal nitric acid. An aqueous solution of aluminium nitrate (M/1) was prepared and 300 ml. were mixed with 340 ml. of aqueous sodium hydroxide (2, N). A white precipitate formed which redissolved on heatina A quantity (400 nil.) of the solution was diluted with 4 litres of distilled water to form the coating solution. The coating solution and NIIO aqueous sodium hydroxide were then simultaneously run into the stirred suspension of TiO2 particles maintained at 50' C. over a period of 3 hours and in such relative quantities as to maintain a pH value of 5.7 throughout the addition. The suspension was stirred for 15 minutes after the additions were complete and aqueous sodium hydroxide was then added to give a pH value of 7.5 after which the alumma,-coated pigment was filtered off, washed, dried and Ruid energy ini.Ued before testing for drying time (after 14 days), reactivity (after I day's storage) and by the Jacobsen test. The results are given in the table below. 6 EY@AMPLE 7 The process described in Example 6 was repeated but the pH value of the aqueous TiO2 suspension was initially adjusted to 5.9 and this value was maintained during the simultaneous addition of the aluminium-containing coating solution and aqueous sodium hydroxide. After recovery, washing, drying and rnilling the alumina-coated pigment was tested as described in Example 6. The results are given in the table below. 10 EYAMPLE 8 The processes described in Examples 6 and 7 were repeated but the aluminiumcontaining coating solution was prepared from an aqueous aluniinium chloride solution 15 and the pH value of the TiO2 suspension before, and during, the simultaneous addition of the coating solution and aqueous sodium hydroxide was maintained at 5.1, The coated pigment was also treatment and tested as described in Example 6 and the results are given in the 20 table below. The results obtained from similar tests on a similar TiO2 pigment coated with 0.5 SiO2 and 2% alumina and 1.5% TiO2 (the latter coatings from a mixed aluminium and titanium sulphate solution by conventional rapid neutrali25 sation) are also given as a control for comparison. Drying J-acobsen Pigment Reactivity Time Test, percent Example 6 -------------------- 36 63/4 80.7 30 Example 7 -------------------- 37 6Y4 84.4 Example 8 -------------------- 36 6 72.8 Control ---------------- 78 22 73.1 'Me Reactivity of the pigments is tested by milling the pigment for 16 hours with a copal vamish/white spirit 35 medium of known viscosity at a pigment/medium ratio of 1:1. After milling the effluxion time of the product from a No. 4 Ford cup is measured and the product is stored for 24 hours at Zl' C. and the effluxion time is again measured. T'he difference in efflux time (in seconds) 40 between the latter two measurements is the figure given in the tables-the higher the figure the greater the reactivity. The Drying time is the time required for the drying of 0.0 03" thick wet paint films containing the pigment as 45 measured on a Beck-koller drying time, recorder at 21' C. and a relative h-amidity of 65%. The Jacobsen test (which is a test for resistance to discolouration under UV light) is carried out by mixing a predetermined amount of the pigment with glycerol and 50 scaling an opaque film of the mixture between glass slides. The reflectance of the film is measured through the green filter of a Harrison Colourimeter and the film is exposed to a source of ultraviolet light for a predetermined period. The reflectance is again measured as de55 scribed above. The result is expressed as the percent of green reilectance retained after exposure to ultraviolet light-a lower figure indicates a greater discolouration. Wbat is
