TiO.sub.2 slurry stabilization

Abstract

A stable aqueous pigment slurry and a process for preparing same. The slurry contains about 50 to 90 wt. %, based on the weight of the slurry, of a pigment such as titanium dioxide and about 0.2 to 3.0 wt. %, based on the weight of the slurry, of a smectite clay such as bentonite, hectorite or montmorillonite. Preferably, the pigment slurry also contains about 0.1 to 2.0 wt. %, based on the weight of the slurry, of a dispersant. The process involves admixing a pigment slurry and a smectite clay slurry and preferably also the dispersant by milling the three components together under high shear conditions.

Description

FIELD OF THE INVENTION

The invention relates to stable aqueous pigment slurries and processes for preparing same. The stabilized pigment, e.g. TiO.sub.2, slurries of the invention exhibit dramatically reduced syneresis and settling out.

BACKGROUND OF THE INVENTION

When any pigment such as TiO.sub.2 is dispersed in water, there is a tendency for the pigment to immediately start to flocculate. One prior art approach to overcome such problem is to use surfactants. However, the present trend is to avoid, if at all possible, the use of surfactants. Guar gums have been used with some pigment slurries to help prevent settling. Although guar gums are helpful in reducing the settling-out problem, such approach is not efficient. Intermittent stirring in the storage tank is still required and the presence of the guar gums renders the slurry open to enzyme attack.

SUMMARY OF THE INVENTION

It has now been found that it is possible to prepare stable aqueous pigment slurries, especially TiO.sub.2 slurries, which have a marked resistance to settling out and syneresis. The stable slurries of the invention do not require significant amounts of dispersants and need not be continuously or intermittently stirred to overcome settling-out and syneresis while in the storage tank.

The pigment slurries of the invention comprise an admixture of the pigment (preferably titanium dioxide) and a smectite clay. One or more dispersants are also preferably present to wet-out the pigment; however, syneresis and settling-out control are not as dependent on the dispersant when a smectite clay is present.

DETAILS OF THE INVENTION

The invention encompasses a stable aqueous pigment slurry comprising an admixture of about 50 to 90 wt. %, preferably 60 to 85 wt. %, based on the weight of the slurry of a pigment and about 0.2 to 3 wt. %, preferably 0.5 to 2.5 wt. %, based on the weight of the slurry, of a smectite clay. Preferably, the smectite clay is a naturally-occurring or synthetic bentonite, hectorite or montmorillonite clay.

Preferably, the pigment comprises titanium dioxide which is present in an amount of 60 to 85 wt. %, based on the weight of the slurry.

The average particle size of the smectite clay in the final slurry will typically be in the range of about 0.025 to 1.0.mu., and the average particle size of the titanium dioxide in the final slurry will typically be in the range of about of about 0.25 to 0.5.mu.. Such average particle size ranges may be achieved by admixing the titanium dioxide with the smectite clay and milling the admixture under high shear conditions, as more particularly described below.

Preferably, the pigment slurry will also contain one or more dispersants, present in an amount of about 0.1 to 2.0 wt. %, based on the weight of the slurry. Useful dispersants include alkali polyphosphates, aliphatic carboxylic acids and alkali salts thereof, polyacrylic acids and alkali salts thereof, polyhydroxy alcohols, amino alcohols and mixtures thereof.

The stable pigment slurry of the invention is preferably prepared by admixing the pigment, e.g. titanium dioxide, with an aqueous slurry of the smectite clay containing 5 to 20 wt. % and preferably 8 to 12 wt. % of smectite clay based on the weight of the aqueous smectite clay slurry. Such admixing is readily accomplished by milling (or grinding) the components under conditions such that the average particle size of the smectite in the final slurry will be in the range of about 0.025 to 1.0.mu., and the average particle size of the titanium dioxide in the final slurry will be in the range of about of about 0.2 to 0.5.mu.. Suitable admixing techniques for achieving such average particle sizes are well known in the prior art, e.g. mechanical grinding, steam milling, micronization, high shear milling, ball milling, pug milling, disc milling, dispersion, colloidal milling, Manton-Gaulin milling, horizontal media milling, etc.

The invention is illustrated by the following Example which to be considered as illustrative and not delimiting of the invention otherwise described.

EXAMPLE 1

A titanium dioxide slurry having 76.2 wt. % solids was admixed with the smectite clay slurries (10 wt. % solids) in the weight %, based on the weight of the final slurry, indicated in Formulation I below. "Gelwhite L" is a smectite clay of the montmorillonite type available from Southern Clay Products, Inc.; it is a fine powder having a GEB brightness of 83, a moisture content of 10%, a pH of 9.0 and a viscosity of 525 cps (10% solids, Brookfield, 20 RPM). "Bentolite L-10" is a naturally occurring off-white calcium bentonite having a moisture content of 8%, a pH of 7.5 (10% slurry) and a loose bulk density of 30.0 lb/ft.sup.3. "HSD" signifies high shear dispersing; "M-G" signifies milling by hand using the Manton-Gaulin mill.

______________________________________FORMULATION I Weight %No. Smectite Smectite Mixing Process______________________________________A None None 2600 rpm, 40 mm blade, HSD, 5 minB Gelwhite L 1 3000 rpm, 40 mm blade, HSD, 5 minC Gelwhite L 1 3000 rpm, 40 mm blade, HSD, 5 min then hand M-GD Gelwhite L 2 4000 rpm, 40 mm blade, HSD, 5 minE Gelwhite L 1 3000 rpm, 40 mm blade, HSD, 5 minF Gelwhite L 1 3000 rpm, 40 mm blade, HSD, 5 min then hand M-GG Gelwhite L 1 3000 rpm, 40 mm blade, HSD, 5 minX Bentolite 1 3000 rpm, 40 mm blade, HSD, 5 min L-10______________________________________ The Formulation I slurries were examined for syneresis at 55.degree. C. after 10 and 27 days and contrast ratios and y reflectance values of paint formulations prepared from the slurries. The results are shown in Table I. TABLE I______________________________________Syneresis, % Contrast y ReflectanceNo. 10 days 26 days Ratio, % Value______________________________________A 8.93 12.5 97.98 94.14B 0.00 0.00 98.38 94.40C 0.00 0.00 98.49 94.70D 0.00 0.00 100.00 96.79E 0.00 0.00 98.65 95.07F 0.00 0.00 98.74 95.01G 1.85 5.61 98.62 94.71X <1.00 <1.00 98.76 95.07______________________________________

EXAMPLE II

A titanium dioxide slurry was prepared from the ingredients set forth in Formulation II below. "Polywet ND-2" is an anionic dispersant obtained from Uniroyal Corporation. "Bubblebreaker 748" is a silicone defoamer.

______________________________________FORMULATION IIIngredient Weight, grams______________________________________Water 210.18NaOH 0.23Potassium tripolyphosphate 3.00Polywet ND-2 2.13Bubblebreaker 748 0.06Thickener As indicated in Table IIBiocide 0.39Titanium dioxide 354.00______________________________________

Paint formulations were prepared from the Formulation II slurries by adding 100 grams of the slurry to 59.0 grams of paint base and mixed. The paint base consisted of 40.0 grams of "UCAR 376" Latex resin, a vinyl-acrylic resin obtained from Union Carbide Corporation; 1.4 grams of "Texanol" (a high molecular weight alcohol); 0.3 gram of "Bubblebreaker 748"; 0.3 gram of "Triton CF-10" (a nonionic surfactant having an HLB value of about 13); and 17.0 grams of water. Paint formulations employing Formulations III, IV and V described below were prepared in the same manner. Table II set forth below contains the results obtained from the use of Formulation II.

TABLE II__________________________________________________________________________ Brookfield % Syneresis/ Thickener/ Viscosity, 60 rpm % Syneresis, Settling, Hegman Contrast ReflectanceSample Amount #3 spindle 10 days, 50.degree. C. 34 days, 50.degree. C. Grind Ratio Value__________________________________________________________________________A BTLT/0.0054% 362 1.5 <2.0/some 7.5+ 97.85 93.13B BTLT/1.0% 10,000+ 0 0/none 7+ 99.35 95.33C BTLT/0.5% 2,650 0 0/none 7.5+ 99.54 95.05D BTLT/0.25% 960 0 0/none 7.5+ 99.66 95.06E None 200 3.5 5/severe 7.5+ 98.10 93.57AA BTLT/0.0054% 204 1.5 5/some 7.5+ 98.49 94.26BB BL10/1.0% 470 1 <1/none 7.5+ 99.45 95.25CC BL10/0.5% 300 2.0 <2/some 7.5+ 99.10 94.74DD BL10/0.25% 224 3.3 3/severe 7.5 98.98 94.59EE BL10/0.5% 320 1.7 <2/some 7.5+ 99.56 94.99FF BL10/0.0054% 252 3.2 3.2/severe 7.5+ 98.41 94.16GG BL10/1.5% 594 0.5 0/none 7.5+ 99.37 96.54HH BL10/1.75% 736 0 0/none 7.5+ 99.13 96.13__________________________________________________________________________ All percentages of thickener are based on the weight of titanium dioxide added to the slurry, not the entire slurry weight. "BTLT" is a 50:50 blend of hectorite and hydroxyethylcellulose. "BL10" is a naturally occurring offwhite calcium bentonite having a moisture content of 8%, a pH of 7.5 (10% slurry) and a loose bulk density of 30.0 lb/ft.sup.3.

EXAMPLE 3

A titanium dioxide slurry was prepared from the ingredients set forth in Formulation III below. "Tamol 1124" is a sodium carboxylate dispersant and "AMP 95" is 2-amino-2-methyl-1-propanol.

______________________________________FORMULATION IIIIngredient Weight, grams______________________________________Water 140.60Thickener As indicated in Table IIITamol 1124 0.53AMP 95 1.28Titanium dioxide 450.00______________________________________

The ingredients in Formulation III were mixed on a high-speed disperser for 10 minutes and thereafter were dispersed in a media mill for 30 minutes (1,000 rpm) using glass beads. Table III set forth below contains the results obtained from the use of Formulation III.

TABLE III__________________________________________________________________________ Brookfield % Syneresis, Thickener/ Viscosity, 60 rpm 10 days/30 Settling, Hegman Contrast ReflectanceSample Amount #3 spindle days, 50.degree. C. 30 days, 50.degree. C. Grind Ratio Value__________________________________________________________________________183A BT10/1% 650 0/0 some 7.5+ 98.66 94.21183B BL10/0.5% 316 1/1 some 7.5+ 98.68 94.51183C BL10/0.75% 346 0.5/0.5 some 7.5+ 98.87 94.49183D BL400/1% 356 0/0 some 7.5+ 97.87 93.80183E BLWH/0.5% 500 0.5/0.5 slight 7.5+ 98.50 94.27183F BLWH/0.75% 740 0/0 slight 7.5+ 98.25 92.76183G BL400/0.75% 278 1/0.5 some 7.5+ 98.41 94.20184A 3X/0.5% 430 0/0 some 7.5+ 98.79 94.77184B SCPX944/0.5% 524 0/0 some 7.5+ 99.77 94.67184C 3X/0.75% 646 1.7 some 7.5+ 99.78 94.60182A-1 None 166 8.6/8.7 hard 7.5+ 98.79 94.58001D None 174 11.7/20.3 hard 7.5+ 94.31 91.62__________________________________________________________________________ All percentages of thickener are based on the weight of titanium dioxide added to the slurry, not the entire slurry weight. "BL10" is a naturally occurring offwhite calcium bentonite having a moisture content of 8%, a pH of 7.5 (10% slurry) and a loose bulk density of 30.0 lb/ft.sup.3. "BLWH" is a sodium ionexchanged version of BL10. "BL400" is a micronized version of BLWH. "3X" is a Wyoming bentonite slurry milled 3 times in a MantonGaulin mill. "SCPX" is 3X treated with a quaternary ammonium compound.

EXAMPLE 4

A titanium dioxide slurry was prepared from the ingredients set forth in Formulation IV below.

______________________________________FORMULATION IVIngredient Weight, grams______________________________________Water 140.6Thickener As indicated in Table IVTamol 1124 0.9Potassium tripolyphosphate 0.6AMP 95 1.5Bubblebreaker 748 0.6Titanium dioxide 350.0______________________________________

The ingredients in Formulation IV were mixed on a high-speed disperser for 10 minutes and thereafter were dispersed in a media mill for 30 minutes (1,000 rpm) using glass beads. Table IV set forth below contains the results obtained from the use of Formulation IV.

TABLE IV__________________________________________________________________________ Brookfield % Syneresis, Thickener/ Viscosity, 60 rpm 10 days/31 Settling, Hegman Contrast ReflectanceSample Amount #3 spindle days, 50.degree. C. 31 days, 50.degree. C. Grind Ratio Value__________________________________________________________________________185A BL10/0.64% 858 0/0 none 7+ 96.11 90.25185B BL10/0.32% 880 0/0 none 7+ 96.54 90.50185C BL10/0.96% 860 0/0 none 7+ 97.47 91.52185D BLWH/0.64% 1,028 0/0 none 7+ 97.22 91.17185E BLWH/0.32% 948 0/0 none 7+ 97.00 91.24185F BL400/0.64% 832 0/0 none 7+ 97.00 91.09185G BL400/0.96% 888 0/0 none 7+ 96.93 91.27185H 3X/0.64% 1,240 0/0 none 7+ 97.78 92.03185I 3X/0.32% 1,014 0/0 none 7+ 96.55 90.86185J None 1,318 0/0 none 7+ 97.03 91.51185K None 1,040 0/0 none 7+ 96.34 90.67185L None 602 0/0 none 7+ 96.04 90.27185M SCPX944/0.25% 1,712 0/0 none 7 97.41 91.32185N BL10/0.64% 932 0/0 none -- -- --185O BLWH/0.32% 948 0/0 none -- -- --001C BL10/1.0% 610 8.3/12.7 slight 7 96.37 93.59001E None 270 18.5/25.5 hard 7 90.12 87.06__________________________________________________________________________ All percentages of thickener are based on the weight of titanium dioxide added to the slurry, not the entire slurry weight.

EXAMPLE 5

Titanium slurries were prepared to illustrate the effect of different dispersants. The ingredients are shown in Formulation 5:

______________________________________FORMULATION 5Ingredient Weight, grams______________________________________Water 140.6AMP 95 1.3Dispersant As indicated in Table VTitanium dioxide 450.0BL10 As indicated in Table V______________________________________ The ingredients in Formulation IV were mixed on a high-speed disperser for 10 minutes and thereafter were dispersed in a media mill for 30 minutes (1,000 rpm) using glass beads. Table V set forth below contains the results obtained from the use of Formulation V. TABLE V______________________________________ Surfactant/ Thickener/ Brookfield % Syneresis Settling Amount, Amount, Visc., 60 10 days/20 20 days,Sample grams grams rpm, #3 days, 50.degree. C. 50.degree. C.______________________________________190A TSG-1/0.71 none 226 3.4/5.1 slight190E TSG-1/1.42 none 972 -- --190H TSG-1/0.71 BL10/2.25 988 <0.5/<0.5 none190B S104/0.52 none 112 7/8.6 some190F S104/1.04 none 564 -- --190G S104/0.52 BL10/2.25 958 0.5/0 none190C T165/1.14 none 192 5.6/4.6 slight190D T165/2.28 none 374 -- --190I T165/1.14 BL10/2.25 908 0/0 none______________________________________ "TSG1" is "Tamol SG1", an anionic surfactant. "S104" is "Surfonyl", a hydrophobic surfactant. "T165" is "Tamol 165", a nonionic surfactant.

Claims

1. A stable aqueous pigment slurry comprising an admixture of:

(a) about 50 to 90 wt. %, based on the weight of the slurry, of a pigment; and

(b) about 0.2 to 3.0 wt. %, based on the weight of the slurry, of a smectite clay.

2. The slurry of claim 1 further comprising about 0.1 to 2.0 wt. %, based on the weight of the slurry, of a dispersant.

3. The slurry of claim 2 wherein the dispersant is selected from the group consisting of alkali polyphosphates, aliphatic carboxylic acids and alkali salts thereof, polyacrylic acid and alkali salts thereof, polyhydroxy alcohols, amino alcohols and mixtures thereof.

4. The slurry of claim 1 wherein the pigment is present in an amount of 60 to 85 wt. %, based on the weight of the slurry.

5. The slurry of claim 1 wherein the pigment comprises titanium dioxide.

6. The slurry of claim 4 wherein the average particle size of the titanium dioxide is in the range of about 0.2 to 0.5.mu..

7. The slurry of claim 1 wherein the smectite clay is present in an amount of 0.5 to 2.5 wt. %, based on the weight of the slurry.

8. The slurry of claim 1 wherein the average particle size of the smectite clay is in the range of about 0.025 to 1.0.mu..

9. The slurry of claim 1 wherein the smectite clay is selected from the group consisting of bentonite, hectorite and montmorillonite clays.

10. A process for preparing a stable aqueous pigment slurry which comprises admixing about 50 to 90 wt. %, based on the weight of the pigment slurry, of a pigment with about 0.2 to 3.0 wt. %, based on the weight of the pigment slurry, of a smectite clay, said smectite clay being present in the form of an aqueous smectite clay slurry containing about 5 to 20 wt. % smectite clay, based on the weight of the aqueous smectite clay slurry.

11. The process of claim 10 wherein about 0.1 to 2.0 wt. %, based on the weight of the pigment slurry, of a dispersant is also present during the admixing.

12. The process of claim 11 wherein the dispersant is selected from the group consisting of alkali polyphosphates, aliphatic carboxylic acids and alkali salts thereof, polyacrylic acid and alkali salts thereof, polyhydroxy alcohols, amino alcohols and mixtures thereof.

13. The process of claim 10 wherein the pigment is present in an amount of 60 to 85 wt. %, based on the weight of the pigment slurry.

14. The process of claim 10 wherein the pigment comprises titanium dioxide.

15. The process of claim 10 wherein the smectite clay is present in the aqueous smectite clay slurry in an amount of 8 to 12 wt. %, based on the weight of the aqueous smectite clay slurry.

16. The process of claim 10 wherein the smectite clay is present in an amount of 0.5 to 2.5 wt. %, based on the weight of the pigment slurry.

17. The process of claim 10 wherein the admixing of the pigment, dispersant and aqueous smectite clay slurry is accomplished by milling under high shear conditions.

18. The process of claim 17 wherein the pigment comprises titanium dioxide and the milling is carried out such that the average particle size of the titanium dioxide is in the range of about 0.2 to 0.5.mu..

19. The process of claim 10 wherein the milling is carried out such that the average particle size of the smectite clay is in the range of about 0.025 to 1.0.mu..

20. The process of claim 10 wherein the smectite clay is selected from the group consisting of bentonite, hectorite and montmorillonite clays.