TiO.sub.2 Slurry Stabilization

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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

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.