WO1998047817A1

WO1998047817A1 - A new method for the production of novel powder minerals surface coated with fine titanium dioxide crystals

Info

Publication number: WO1998047817A1
Application number: PCT/US1998/007210
Authority: WO
Inventors: Isaac Yaniv
Original assignee: Baran Advanced Materials (94) Ltd.; Friedman, Mark, M.
Priority date: 1997-04-18
Filing date: 1998-04-13
Publication date: 1998-10-29
Also published as: AU6896698A

Abstract

A process for coating a water insoluble or sparingly water soluble natural and artificial mineral (carrier) powder with titanium dioxide. A slurry is created by milling/mixing titanium dioxide powder, water and a dispersant selected from among low (∫150) molecular weight monocarboxylic acids and their anhydrides. The carrier powder is mixed with the titanium dioxide slurry to produce a coated powder. The scope of the present invention also includes the coated powder, the slurries and the improved titanium dioxide powder of the slurry.

Description

A NEW METHOD FOR THE PRODUCTION OF NOVEL POWDER MINERALS SURFACE COATED WITH FINE TITANIUM DIOXIDE CRYSTALS

FIELD OF THE INVENTION

This invention relates to a new method for the production of extremely stable dispersions of TiO₂ fine powders, of TiO₂ fine powders and of novel powder minerals which are surface coated with separated single crystals of fine TiO₂. These minerals include natural or artificial water insoluble or sparingly soluble materials such as metal oxides, metal hydroxides, and metal salts like carbonates, hydroxycarbonates and sulfates. Examples include MgO, Mg(OH)₂, CaCO₃, MgCO₃, CaMg(CO₃)₂, Ca(OH)₂, BaSO₄, CaSO₄, Al₂O₃, AIO(OH), Al(OH)₃, SiO₂, clays like talc (3MgO4SiO₂ H₂O) and kaolinite (Al₄Si₄O₁₀(OH)₈), and hydrotalcites like Mg₆Al₂CO₃(OH)₁₆4H₂O or hydromagnesite Mg₄(CO₃)₃(OH) ₂3H₂O. This invention improves substantially the state of the art of applying TiO₂ pigments by simplifying and enhancing the operations that are necessary to properly disperse the TiO₂ in e.g. drugs, cosmetics, plastics, coatings, paints and paper and lowers the manufacturing cost of these products, mainly, by cutting the consumption of the expensive constituent - TiO₂. This invention is particularly important in the production of powders for the paper industry, for the cosmetic industry, for the ceramic industry, for the plastic, paint and rubber industries, etc.

BACKGROUND OF THE INVENTION

The literature is replete with articles and reviews concerning the importance of minerals in the plastics compounding and in the production of paper. The following reviews: " CaCOβ Fillers - Market Trends and Developments"; J. Rever e i Vidal; Industrial Minerals; November 1994, " Plastic Compounding - Where Mineral Meets Polymer "; M. O^'Driscoll; Industrial Minerals; December 1994, " Surface Modification of Mineral Fillers "; R. Goodman; Industrial Minerals; February 1995, " Magnesium Hydroxide Flame Retardant (NHFR) for Plastics and Rubber "; O. Kalisky et al; Chimica Oggi/Chemistry Today; June 1995 and references therein, illuminate the importance of the physical properties, and especially the surface characteristics, of fine powders that are used as fillers or flame retardants in a large variety of applications.

Grinding or milling of materials are common technologies for size reduction. However, they require high energy expenditure, especially at the sub-micron range. The high cost of such operations is increased by their low productivity and by the requirement for equipment made of special materials that withstand the high attrition and minimize the contamination of the final fine powders. Generally, two processes are used in the art - dry and wet grinding/milling. In order to increase the production rates of both types of processes and to afford better qualities of grinding/milling, aids, such as dispersants like sodium hexamethaphosphate, fatty acids (long chain carboxylic acids of more than six (6) carbons), fatty acid esters, etc., are usually employed.

The literature concerning the production and uses of TiO₂ is too replete with patents, articles, reviews, books and technical literature (which is issued in abundance by TiO₂ manufacturers) to be covered herein. A short review that summarizes the main issues concerning the production and uses of TiO₂ is given in "Titanium Dioxide"; by A. Skillen, deputy editor; on pages 108-112 in "Raw Materials for Pigments, Fillers and Extenders - an Industrial Minerals Consumer Survey"; Second Edition; Edited by R. Bolger and M. OOriscoll (1995). TiO₂ pigments are used primarily in order to interact with the light that shines on the final products to which they are added. The pigments may scatter the visible light - opacifying the products; or they may absorb the UV light and convert this energy into heat, thereby guarding the products against photochemical damages.

The major problems that arise when using TiO₂ pigments emanate from its high price (~$2/Kg) and the basic physical requirements that small and separated single TiO₂ crystals, of the size of one half of the light wavelength that is to be intercepted, be available in the final product. For instant, this requires a crystal size distribution in the range between 400/2 nm (nanometer) to 770/2 nm to scatter the visible light and opacify the products or a crystal size distribution in the range of UVA 2 and or UVB/2 to disperse the UV light that harms the human skin. Different applications require different particle size distributions in order to optimize the cost/performance ratios. However, such small particles have a strong tendency to form agglomerates, leading to reduced efficiencies of the TiO₂ pigments, namely to a reduced "Hiding Power". Therefore, the state of the art technology is aimed at dispersing the fine TiO₂ crystals as thoroughly as possible, at a minimum cost. Yet, massive mixing/milling equipment and a variety of dispersants are still necessary to properly use the TiO₂ - c.f "Finding Dispersant Requirements for TiO₂ Pigment Grades"; R. Bank; Modern Paint and Coatings; pages 36-38 (May, 1996) and "Compatibilization of Titanium Dioxide Powders with Non-polar Media: Adsorption of Anionic Surfactants and Its Influence on Dispersion Stability"; M. Arellano et al; J. of Coatings Technology; Vol. 68, No. 857 (June 1996). At this point it is worthwhile to mention the opinions of W. M. Morgans in "Outlines of Paint Technology"; Charles Griffin & Company LTD; 1969; in Chapter 16; page 267; lines 3-5 regarding the importance of dispersing the pigments in the paint industry and on page 269; line 35 till page 270; line 5 regarding the efficiency of long chain fatty acids as dispersing agents for that purpose compared with the inability of short chain carboxylic acids to disperse pigments (including TiO₂).

Another improvement in the art of dispersing the TiO₂ pigments in the final products is achieved by adding other, less expensive, minerals as spacers or extenders. Usually, the particle size distribution requirement of these extenders (usually larger than that of the TiO₂ ) are not derived from the physical properties of the light that is to be intercepted, but rather from the physical properties and the cost of the final products. Still this improvement does not alleviate the need for extensive mixing/milling. The state of the art dispersants that should have assisted in producing fine dispersions of separated TiO₂ single crystals, produce in most cases slurries of low viscosity that still contain excessive amounts of agglomerated TiO₂ particles, which should be dispersed in the later applications using aggressive mixing/milling.

An additional improvement in the art is to prepare master batches containing TiO₂ pigments for various applications. As each master batch suits quite limited numbers of applications, this technology adds an extra complexity and, naturally, an additional cost to the use of TiO₂ pigments. Even the use of master batches does not solve the problem of agglomeration of TiO₂ crystals during the various processing steps.

It should be mentioned that various tests are available to measure the particle size distribution (e.g. light scattering and Scanning Electron Microscopy (SEM)). However, the use of a simple sedimentation test to determine the particle size and the stability of dispersions is mentioned in the above paper of M. Arellano et al and in "Principles of Mineral Dressing"; A. M. Gaudin; Chapter 8, on page 165; "The Movement of Solids in Fluids"; McGraw-Hill (1939).

The short discussion above leads to the conclusion that still better dispersants are to be sought and to the worthwhile idea of producing universal master batches in which most of the TiO₂ pigments are strongly attached, as separated single crystals, onto the surfaces of common minerals like fillers, flame retardants, extenders, etc.. The TiO₂ single crystals should adhere to the surfaces of these minerals very tightly and not be able to dislocate and agglomerate on further operations with the minerals. The minerals to be coated with TiO₂ are named herein - "carriers". Naturally, these new materials may have more than one function as follows: extenders for the TiO₂ and fillers; extenders for the TiO₂ and flame retardants, etc. Also, the application of such materials will not be so dependent on the intensive mixing/milling and will not be sensitive to small variations of the composition of the formulations that are to be used, as is the case today. However, under most circumstances the powders of the present invention should allow reduction of the consumption and cost of the expensive TiO₂ pigment.

SUMMARY OF THE INVENTION According to the present invention there is provided a process for improving titanium dioxide powder, including the step of: (a) mixing the titanium dioxide powder with water and at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH: (i) having a molecular weight less than about 150, (ii) having at most four carbon atoms connected directly to the COOH group by carbon-carbon bonds, and (iii) passing the Titanium Dioxide Dispersant Selection Test, and anhydrides thereof, thereby creating a slurry; and (b) processing the slurry by a process selected from the group of processes consisting of: (i) extrusion, thereby creating a product that is an extrudate and (ii) filtration, thereby creating a product that is a filter cake.

According to the present invention there is provided a process for coating at least one carrier powder with titanium dioxide, including the steps of: (a) mixing titanium dioxide powder with water and at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH: (i) having a molecular weight less than about 150, (ii) having at most four carbon atoms connected directly to the COOH group by carbon-carbon bonds, and (iii) passing the Titanium Dioxide Dispersant Selection Test, and anhydrides thereof, thereby creating a slurry; (b) selecting the at least one carrier powder from the group consisting of water-insoluble metal oxides, sparingly water-soluble metal oxides, water-insoluble metal hydroxides, sparingly water-soluble metal hydroxides, water-insoluble metal salts and sparingly water-soluble metal salts; and (c) mixing the slurry with the at least one carrier powder, thereby creating a coated powder.

According to the present invention there is provided a coated powder, including at least one carrier powder, having a certain particle size distribution, and a titanium dioxide coating, the coated powder having a particle size distribution identical in modality to the particle size distribution of the at least one carrier powder. According to the present invention there is provided a slurry, including: (a) water; (b) titanium dioxide; and (c) at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH: (i) having a molecular weight less than about 150, (ii) having at most four carbon atoms connected directly to the COOH group by carbon-carbon bonds, and (iii) passing the Titanium Dioxide Dispersant Selection Test, and anhydrides thereof.

It is a purpose of the present invention to provide an inexpensive and simple method to produce a novel series of mineral powders coated with TiO₂ single crystals. It is a further purpose of the present invention to provide a method to produce free flowing fine powders and stable dispersions of mainly single crystals of TiO₂, using common and inexpensive equipment and raw materials. It is a further purpose of the invention to provide a method to produce TiO₂- coated powders, in free flowing and stable dispersions or as dry free flowing powders, using common and inexpensive equipment and raw materials.

It is a further purpose of the invention to provide simple methods to determine which dispersants are suitable to produce these fine powders.

Other purposes and advantages of the invention will appear as the description proceeds.

Surprisingly, it has been found that mixing/milling of TiO₂ pigments in water in the presence of short chain organic monocarboxylic acids, like formic acid, acetic acid, propionic acid, methoxyacetic acid, butanoic and acrylic acid and their anhydrides, gives rise to extremely stable, pumpable (viscosity less than about 1000 cp) and acidic (pH less than 7) slurries, in which most of the TiO₂ particles are separated single crystals. However, the invention is not limited to low viscosity slurries of TiO₂. These TiO₂ slurries can either be used as such or the TiO₂ particles can be surface coated with hydrophobic materials (optionally), filtered off and/or extruded (optionally), dried (optionally) and disintegrated (optionally). Alternatively, the slurries of mainly separated TiO₂ crystals can be mixed quite easily with carriers (as fine powders or slurries in water) to form minerals surface coated with TiO₂ single crystals. These materials can then be used right away as slurries in water or may be further surface treated/coated with paraffin, silicone rubber, long chain fatty acids, long chain fatty acid esters, polymers, waxes and/or greases (optionally), filtered off and/or extruded (optionally), dried (optionally) and disintegrated (optionally) to form a novel series of powders. The free flowing coated powders, whose flowability is mainly derived from the physical properties of the carriers and whose optical properties are mainly derived from the separated TiO₂ crystals, can be used to prepare paints and coatings, as well as many other products, containing substantially reduced amounts of TiO₂ compared to those amounts required to reach the same optical characteristics when mixed according to the state of the art methods.

This discovery is particularly surprising in the light of the conventional wisdom cited above regarding the inability of short chain carboxylic acids to disperse pigments. The carriers that can be used in this invention include natural or artificial water insoluble or sparingly soluble minerals such as metal oxides, metal hydroxides, and metal salts such as sulfates, carbonates and hydroxycarbonates. Examples include MgO, Mg(OH)₂, CaCO₃, MgCO₃, CaMg(CO₃)₂, Ca(OH)₂, BaSO₄, CaSO₄, Al₂O₃, AIO(OH), Al(OH)₃, SiO₂, clays like talc (3MgO 4SiO₂ H₂O) and kaolinite (Al₄Si₄O₁₀(OH)₈), and hydrotalcites like Mg₆Al₂CO₃(OH)₁₆4H₂O or hydromagnesite Mg₄(CO₃)₃(OH)₂3H₂O.

A process for the production of TiO₂ fine powders and carriers surface coated with TiO₂ separated single crystals, comprises the following steps: A. Milling or mixing TiO₂ particles in water with at least one dispersant selected from the group of organic monocarboxylic acids which have the formula:

R-COOH with at most four carbon atoms connected directly to the COOH group by carbon- carbon bonds, whose molecular weight is at most 150, and which passes the "Titanium Dioxide Dispersion Selection Test" defined herein, and anhydrides thereof; R preferably being H, alkyl (linear or branched; saturated or unsaturated; cyclic or acyclic); wherein one or more of its carbon or hydrogen atoms may be replaced by oxygen, nitrogen, phosphorus or sulfur atoms;

B. (optionally) mixing the resulting slurry of step A. with least one additive selected from the group including paraffins, silicone rubber, long chain fatty acids, long chain fatty acid esters, polymers, waxes and/or greases;

C. (optionally) filtering off and or extruding the resulting fine particles of steps A. or B;

D. (optionally) drying the resulting filtered cake or pellets of step C; E. (optionally) disintegrating the resulting dry material of step D. to form a powder;

F. (optionally) mixing the resulting slurry of step A. with at least one carrier, in the powder form or as a slurry in water, selected from the group consisting of water insoluble or sparingly soluble metal oxides, metal hydroxides, and metal salts to form a TiO₂-coated powder; G. (optionally) mixing the resulting TiO₂-coated powder of step F. with at least one additive selected from the group including paraffins, silicone rubber, long chain fatty acids, long chain fatty acid esters, polymers, waxes and/or greases;

H. (optionally) filtering off and or extruding the resulting TiO₂-coated powder of steps F. or G. ;

I. (optionally) drying the resulting filtered cake or pellets of step H.; and

J. (optionally) disintegrating the resulting dry TiO -coated powder of step

I.

Because of the exceptionally good adherence of the TiO₂ particles of the present invention to the carrier particles, and in the absence of excessive amounts of

TiO₂, unique TiO₂-coated powders can be produced that have particle size distributions of the same modality as the carrier powders. For example, if the carrier powder has a unimodal particle size distribution, then the resulting TiO₂-coated powder also has a unimodal particle size distribution; if the carrier powder has a bimodal particle size distribution, then the resulting TiO₂-coated powder also has a bimodal particle size distribution. The size of the resulting composite particles of such TiO₂-coated powders is the combined size of the (usually) larger carrier particles and the smaller TiO₂ particles adhering to their surfaces. In many cases the particle size distribution of such a TiO₂-coated powder is close to the original particle size distribution of the carriers, as the result of size reduction due to attrition of these particles during milling/mixing offset by size increase due to the TiO₂ surface coating.

The optimal relative amounts of TiO₂ and carriers, and the optimal particle size distribution of the TiO₂ powder and the carrier powders, should be determined by experts skilled in the art in order to obtain the best cost performance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A proper choice of the dispersant(s) is the key for a successful preparation of a

"true" dispersion of separated TiO₂ single crystals. This slurry can be used as obtained for subsequent processes or purposes. The slurry of separated TiO₂ single crystals can be further mixed with paraffin, silicone rubber, long chain fatty acids, long chain fatty acid esters, polymers, waxes and/or greases to turn the surface of the TiO₂ particles hydrophobic and be used as obtained for subsequent processes and purposes, or it can be further used for coating other mineral particles with these tiny TiO₂ single crystals. A set of criteria, the "Titanium Dioxide Dispersion Selection Test", is given in Example 1 (in the experimental section), which defines the suitable dispersants for producing the TiO₂-coated powders of the present invention.

Generally, the suitable dispersants should lead to a stable and pumpable dispersions of separated TiO₂ single crystals in order to be able to coat the surface of the carrier(s) with the TiO₂, in the formulation, and to obtain the majority of the TiO₂ particles as single crystals on these surfaces. In order to achieve this goal and to obtain an adherent coating, the pH of the TiO₂ slurry should be in the range below 7. This allows an efficient precipitation of the separated TiO₂ single crystals onto the surface of the carrier(s).

There are many methods to operate and control the preparation process of the TiO₂ slurries and the formation of TiO₂-coated carrier(s). Some of these methods are proposed in the experimental section. However, any one skilled in this art may operate it differently, according to the general guidelines that are depicted herein.

The dispersants in the present invention are only limited by the guidelines that were described and the set of criteria that is given in the Titanium Dioxide Dispersant

Selection Test (c.f. Example 1; experimental section). However, organic carboxylic acids of low molecular weight (less than 150, preferably less than 130, most preferably less than 100) and their respective anhydrides lead to outstanding results.

In a preferred embodiment of the invention, each carboxylic acid is selected from the group comprising formic acid, acetic acid, propionic acid, methoxyacetic acid, butanoic and acrylic acid and their anhydrides. Carboxylic acids of higher molecular weight and of longer carbon-to-carbon chains (containing >4 carbons) attached to the COOH are much less effective in producing stable dispersions of separated TiO₂ single crystals. However, such molecules may be used in some applications in which hydrophobic surfaces are desired.

Carboxylic acid halides of the general formula R(COX), X= Halogen, undergo very fast hydrolysis under the regular conditions of the applications to give the respective carboxylic acids (R(COOH)). However, the HX that forms along with the carboxylic acid contaminates the product with the residual halide and destabilizes the TiO₂ dispersions.

The addition of paraffins, silicone rubber, long chain fatty acids, long chain fatty acids esters, polymers, waxes, greases and other additives to impart hydrophobic surfaces to the fine powders is also well known in the art, and may well be used in the present invention.

Easily pumpable slurries containing up to 75% - 85% wt TiO₂ can be formed using mills/mixers (e.g. ultra twrax high shear mills, ball mills, dissolvers, dispersers, etc.). The concentration of the dispersant(s) is in the range between 0.3% to 5% wt/wt of dry TiO₂. Too low concentrations of the dispersant(s) (<0J% wt/wt of dry TiO₂) may not allow to obtain stable slurries loaded with enough TiO₂; and too high concentrations (>20% wt/wt of dry TiO₂) of the dispersant(s) may lead to slurries of excessive viscosity and/or harm the final products. The milling/mixing of the TiO₂ mixture should continue until most, preferably >80% wt, of the TiO₂ particles in the aqueous media have been turned into separated single crystals of the right size distribution.

The carriers' optimal particle size distribution is determined by many factors. This determination may be controlled by a person skilled in this art. The range of D₅₀ may exceed 350 nm and in most cases that value may exceed 1000 nm. In some cases the technology of the present invention can be applied just to improve the whiteness of certain minerals, though it is mainly intended to produce the powders of the present invention.

Mixing of the suitable TiO₂ slurry with the proper carrier(s) can be done using mills as well as mixers (e.g. ultra turrax high shear mills, ball mills, dissolvers, dispersers, etc.). The TiO₂ slurry may be mixed with the carrier(s) in the powder form or in a slurry. Generally, simultaneous addition of both components into stirred containers lead to the best results, as the pH of the newly formed slurry is constant during the process. However, as the weight of the TiO₂ to weight of the carrier(s) ratio may vary considerably, better results may be obtained by adding one component into the other stirred component. All the above and other characteristics and advantages of the invention will better be understood from the following illustrative and non limiting description of preferred embodiments, with reference to the examples given below.

Experimental Section

Raw Materials

In the examples given hereinafter, the following raw materials were used:

-Formic acid of Aldrich -Acetic acid of Aldrich

-Propionic acid of Aldrich

-Acrylic acid of Aldrich

-Butanoic acid of Aldrich

-Pentanoic acid of Aldrich -Hexanoic acid of Aldrich

-Octanoic acid of Aldrich

-2-Ethylhexanoic acid of Aldrich

-Fumaric acid of Aldrich

-Adipic acid of Aldrich -Oxalic acid of Aldrich

-Citric acid of Aldrich

-Oleic acid of Aldrich

-Sodium Oleate of Aldrich

-Palmitic acid of Aldrich -Sodium palmitate of Aldrich

-Isostearic acid Emersol - 875 of Emery

-Sodium stearate of Aldrich

-Tall oil of Aldrich

-Acetyl chloride of Aldrich -Acetic acid anhydride of Aldrich

-Phthalic acid anhydride of Aldrich

-Maleic acid anhydride of Aldrich

-Methoxyacetic acid of Aldrich

-Polyacrylic acid of Fluka (#81140) -Tamol 731 of Rohm & Haas

-Tamol 681 of Rohm & Haas -p-Toluenesulfonic acid of Aldrich -Dodecylbenzenesulfonic acid of Aldrich -Sodium dodecylbenzenesulfonate of Aldrich -Sodium dioctylsulfosuccinate OT - 100 of Cyanamid -Sodium octansulfonate of Aldrich

-HC1 of Frutarom, Israel -H₃PO₄ of Frutarom, Israel

-TiO₂ (Kronos 2160)of Kronos (However, similar TiO₂ pigments, like Tioxide R-TC90 and Tioxide TR92, of which their D₅₀= 220 nm ± 20 nm may serve equally well)

-CaCO3 powder (d5Q=3.5 microns) of Polychrom, Israel- "Girulite-8"

-Al(OH)₃ - FR -10 of Alcan -Al(OH)3 - A.T.H-X of Riedel-de Haen

-Propylene glycol of Frutarom, Israel -Butyl diglycol acetate of Union Carbide

-Nopco NDW of Henkel

-Ammonia (25%) of Frutarom, Israel

-Kathon LXE of Rohm & Haas

-Synthetic sodium aluminum silicate (p820) of Degussa -Copolymer vinyl acetate acrylate emulsion (55% N.V.) of Cerafon, Israel

-Talc (D₅₀ = 12.3 micron) of Lusenac Val Chisone

-Kaoline clay (D₅₀ = 3.1 micron) of Engelhard

-Disperse One (45% N.V.) of Tambur, Israel

-Synperonic NPlO oflCI -Cellosize QP 15000 (hydroxy ethyl cellulose) of Union Carbide

Instruments

-Cilas Model 930 was used to determine the D₅₀ Example 1

The Titanium Dioxide Dispersant Selection Test

Part I

200g water and 300g TiO₂ powder (Kronos 2160) were added to a beaker (d = 8.2 cm; h = 14.5 cm). A laboratory ultra turrax ( Ika; Model T-25) was dipped into the mixture and operated at 9000 rpm and the dispersant (1-2 % wt/wt of dry TiO₂) was added. After 5 mins. the mixing was stopped. The viscosity (Brookfield) and the pH of the slurry obtained were measured. Mixtures that showed viscosity below 1000 cp and pH values below 7 passed this first test. The results are given in Table 1 :

Table 1

- near y ₁₂ enzene u onc c o ecy enzene su onc ac ¹ - 32% HC1 solution

3

- Usually, slurries that failed part I were not subjected to part II. However, some slurries that failed part I were tested in order to clearly demonstrate the differences between common dispersants, which are regularly used in the prior art, to the dispersants of the present invention.

- Active ingredient.

Part II

The first experiment was repeated using 487.5g water and 12.5g TiO₂ powder (Kronos 2160) and a dispersant that passed the first test. The slurry that formed was transferred into a 250 ml graduate cylinder and allowed to settle. The volume of the phase containing the TiO₂ particles was measured at various intervals, depending on their precipitation rate. The measurement of some samples could be stopped rather soon, as it was obvious that the results were not satisfactory. Most samples were allowed to stand for 48 hrs.. Thereafter, the volume of the phase containing the TiO₂ was measured.

Samples that exhibited lower volumes than 200 ml after 48 hrs. were deemed to have failed the test. Under such conditions the SEM measurements, at any time between stopping the mixing and 48 hrs. later, showed that >80% (wt) of the TiO₂ particles were separated single crystals for slurries that showed values >200 ml after 48 hrs.. The results are given in Table 2:

Table 2

e voume o e p ase a conans e ₂ par ces.

This test was performed though it failed the previous one, as being too basic

BenzeneSulfonic Acid (dodecylbenzene sulfonic acid)

Example 2 Production of AKOH)? surface coated with Fine TiO₂

1. A 70%) wt of Al(OH)₃ sluπy in water was prepared in a beaker (d = 8.2 cm; h = 14.5 cm) by mixing 350g dry Al(OH)₃ (FR - 10 of Alcan) and 150g water, using a laboratory mixer (IKA, Model RW20) for 10 mins.. The particle size distribution of the Al(OH)₃ in the resulting slurry (D₅₀=20 micron) was measured using a Cilas model 930.

2. A 70% wt of TiO₂ slurry in water was prepared in a beaker (d = 8.2 cm; h = 14.5 cm) by mixing 350g dry TiO₂ (2160 of Kronos), 150g water and 5g dispersant (Tamol 681 of Rohm & Haas), using a laboratory ultra turrax (IKA, Model T-25) for 5 mins.. The particle size distribution of the TiO₂ in the resulting slurry (D₅₀=l .0 micron*) was measured using a Cilas model 930.

3. 300g of the Al(OH)₃ slurry in 1. and 300g of the TiO₂ slurry in 2. (above) were added simultaneously into a 1000 ml. beaker while mixing with a laboratory mixer (IKA, Model RW20). The particle size distribution of the resulting slurry was measured using a Cilas model 930. The histogram showed the two distinct peaks of the carrier and the TiO₂ particles, indicating poor coating of the TiO₂ onto the Al(OH)₃ particles.

* - Cilas model 930 is unable to measure precisely the D₅₀ of particles in the range between 0.1 to 1 micron. In such cases the results are higher than the actual ones. However, Cilas model 930 is an excellent device to indicate whether the Ti0₂ particles and the carriers coalesce.

Example 3

Production of Al(OH) surface coated with Fine TiO

1. A 70% wt of Al(OH)₃ slurry in water was prepared in a beaker (d = 8.2 cm; h = 14.5 cm) by mixing 350g dry Al(OH)₃ (A.T.H - X of Riedel-de Haen) and 150g water, using a laboratory mixer (IKA, Model RW20) for 10 mins.. The particle size distribution of the Al(OH)₃ in the resulting slurry (D₅₀=10 micron) was measured using a Cilas model 930.

2. A 70%) wt of TiO₂ slurry in water was prepared in a beaker (d = 8.2 cm; h = 14.5 cm) by mixing 350g dry TiO₂ (2160 of Kronos), 150g water and 3.5g dispersant (propionic acid of Aldrich), using a laboratory ultra turrax (Ika, Model T- 25) for 5 mins.. The particle size distribution of the TiO₂ in the resulting slurry (D₅₀=0.9 micron*) was measured using a Cilas model 930.

3. 300g of the Al(OH)₃ slurry in 1. and 300g of the TiO₂ slurry in 2. (above) were added simultaneously into a 1000 ml beaker while mixing with a laboratory mixer (EKA, Model RW20). The particle size distribution of the resulting slurry was measured using a Cilas model 930. The histogram showed that practically all of the TiO₂ particles (at D₅₀=0.9 micron) disappeared, indicating excellent coating of the TiO₂ onto the Al(OH)₃ particles**.

* - Cilas model 930 is unable to measure precisely the D₅₀ of particles in the range between 0.1 to 1 micron. In such cases the results are higher than the actual ones. However, Cilas model 930 is an excellent device to indicate whether the Ti0₂ particles and the carriers coalesce. ** - In this Example, the particle distributions of both ie Al(OH)₃ carrier and the coated powder were unimodal. The analogous prior art coated powder of Example 2 was bimodal. Example 4

Production of Paint (Exterior White Paint - Hercules Inc. Stage I - Preparation of TiOi-coated CaCO₃

Water, dispersant (propionic acid) and TiO₂ were added to a plastic container (d=20 cm; h=30 cm) equipped with a disk (d=8 cm) attached to a disperser (Homo Dispers Model HD-550 (0.75 HP) of Hsiangtai Machinery Industry Co. Ltd; Taiwan). The slurry was mixed at 3500 rpm for 10 mins.. Then a dry CaCO₃ powder was added to the stirred TiO slurry and this slurry was stirred at 3500 rpm for further 10 mins.. The resulting TiO₂-coated CaCO3 is ready for further uses.

Three TiO₂-coated CaCO₃ compositions were prepared as follows in Table 3:

Table 3

Stage II - Preparation of Exterior White Paint - Hercules Inc.

The procedure for the preparation of this paint was obtained from Hercules Inc.; Cellulose & Protein Products Dept.; Wilmington, DE 19899 (USA). The procedure followed quite closely the Celanese Resins Formulation No. EP-48-222 for the production of this Exterior White Paint (Vinyl Acetate & Acrylate). The raw materials used are listed in the same numerical order as in the original procedure. Note that ingredient no. 8 (thickener) of the original formulation was not used. Though some ingredients were replaced by others, their function is still stated as follows:

(1) -Tap water

(2) -Cellosize QP 15000 (hydroxy ethyl cellulose) (thickener)

(3) -Propylene glycol (solvent) (4) -Disperse One (45% N.V.) (dispersant)

(5) -Ammonia (25%)(base) (6) -Kathon LXE (preservative)

(7) -TiO₂ (Kronos 2160) (pigment)

(7') -TiO₂-coated CaCO₃ of Stage I (pigment)

(9) -Synthetic sodium aluminum silicate (p820) (spacer) (10) -CaCO₃(spacer)

(10) -Talc(D₅₀=12.3 micron)(spacer)

(10) -Kaolin clay (D₅₀ = 3.1 micron)(spacer)

(11) -Nopco NDW (defoamer)

(12) -Butyl diglycol acetate solvent (coalescent agent) (13) -Copolymer vinyl acetate acrylate (55% N.V.) (emulsion)

(14) -Synperonic NP10 (surfactant; wetting agent)

A. Tap water (1), defoamer (11) and thickener (2) were added to the plastic container, using the same facilities as in Stage I. The mixture was stirred at 500 rpm for 10 mins.. B. The dispersant (4) and the wetting agent (14) were added to the mixture of A. and stirring is continued at 500 rpm for additional 5 mins..

C. This basic (reference) formulation was prepared by further addition of the pigment (7) and mixing at 1500 rpm for 5 mins.. Then the spacers (9) and (10) were added and the mixing continued for additional 10 mins. at the same speed. Alternatively this step (C) was replaced by step C as follows:

C. The improved formulation was prepared by further addition of the pigment (7') powder formed in stage I, and mixing at 1500 rpm for 5 mins.. Then the spacers (9) and (10) were added and the mixing continued for an additional 10 mins. at the same speed. D. Then the emulsion (13) was added and the mixing continued at 1000 rpm for 3 mins..

E. Then the solvent (3) was added and the mixing continued at 600 rpm for 1 min..

F. Then the coalescent agent (12) was added and the mixing continued at 600 rpm for 1 min.. G. Then the preservative (6) was added and the mixing continued at 600 rpm for 1 min..

H. Then the ammonia (instead of potassium carbonate (5)) was added and the mixing continued at 600 rpm for 1 min.. I. Then tap water (1) was added to bring the mixture up to 100 parts total of paint and the mixing continued at 600 rpm for 1 min.. The paint was ready for testing.

The contrast ratio (%) and the whiteness (%) of 120 micron layers of paint were determined with an ACS instrument (Applied Color Systems) and the results are given in following Table 4:

Table 4

Note: Substantial amounts of TiO₂ can be saved by using the TiO₂-coated CaCO₃ of the present invention without sacrificing the quality of the resulting paint. On the contrary, the other key properties, like scrub resistance, mentioned in the copy of the proposed Hercules Inc. formulation (attached) are at least as good or better than the paint obtained by following the basic formulation. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A process for improving titanium dioxide powder, comprising the step of:

(a) mixing the titanium dioxide powder with water and at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH:

(i) having a molecular weight less than about 150, and

(ii) having at most four carbon atoms connected directly to said

COOH group by carbon-carbon bonds, and anhydrides thereof, thereby creating a slurry having a pH less than about 7; and

(b) processing said slurry by a process selected from the group of processes consisting of:

(i) extrusion, thereby creating a product that is an extrudate and (ii) filtration, thereby creating a product that is a filter cake.

2. The process of claim 1, wherein said mixing is effected by milling.

3. The process of claim 1, further comprising the step of:

(c) drying said product.

4. The process of claim 3, further comprising the step of:

(d) disintegrating said dried product.

5. The process of claim 1, wherein said R is selected from the group consisting of:

0) H,

(ii) linear saturated acyclic alkyl,

(iii) linear saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (iv) saturated cyclic alkyl, (v) saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (vi) linear unsaturated acyclic alkyl,

(vii) linear unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (viii) unsaturated cyclic alkyl, (ix) unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (x) branched saturated acyclic alkyl, (xi) branched saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xii) substituted saturated cyclic alkyl, (xiii) substituted saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xiv) branched unsaturated acyclic alkyl, (xv) branched unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xvi) substituted unsaturated cyclic alkyl, and (xvii) substituted unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur.

6. The process of claim 1, wherein said at least one dispersant has a molecular weight less than about 130 and has at most three carbon atoms connected directly to said COOH group by carbon-carbon bonds.

7. The process of claim 6, wherein said at least one dispersant is selected from the group consisting of formic acid, acetic acid, propionic acid, methoxyacetic acid, butanoic acid and acrylic acid.

8. The process of claim 6, wherein said at least one dispersant has at most two carbon atoms connected directly to said COOH group by carbon-carbon bonds.

9. Titanium dioxide powder produced by the process of claim 1.

10. The process of claim 1, wherein each of said monocarboxylic acids is such that:

(i) a mixture of about 200 parts by weight of water, about 300 parts by weight of a diagnostic titanium dioxide powder having a D₅₀ between about 200 nm and about 240 nm, and between about 3 and about 6 parts by weight of said each acid has a viscosity below about 1000 cp and a pH below about 7; and

(ii) in a volume of a mixture, of about 1900 parts by weight of water, about 100 parts by weight of said diagnostic titanium dioxide powder, and between about 1 and about 2 parts by weight of said each acid, allowed to stand for about 48 hours, said diagnostic titanium dioxide powder remains suspended in at least 80 percent of said volume.

11. A process for coating at least one carrier powder with titanium dioxide, comprising the steps of:

(a) mixing titanium dioxide powder with water and at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH:

(i) having a molecular weight less than about 150, and

(ii) having at most four carbon atoms connected directly to said

COOH group by carbon-carbon bonds, and anhydrides thereof, thereby creating a slurry having a pH less than about 7;

(b) selecting the at least one carrier powder from the group consisting of water-insoluble metal oxides, slightly water-soluble metal oxides, water-insoluble metal hydroxides, slightly water-soluble metal hydroxides, water-insoluble metal salts and slightly water-soluble metal salts; and

(c) mixing said slurry with the at least one carrier powder, thereby creating a TiO₂-coated powder.

12. The process of claim 11 , wherein said mixing of said titanium dioxide powder with said water and said dispersant is effected by milling.

13. The process of claim 11 , wherein said mixing of said slurry with the at least one carrier is effected by milling.

14. The process of claim 11, further comprising the step of:

(d) processing said slurry by a process selected from the group of processes consisting of:

(i) extrusion, thereby creating a product that is an extrudate, and (ii) filtration, thereby creating a product that is a filter cake.

15. The process of claim 14, further comprising the step of:

(e) drying said product.

16. The process of claim 15, further comprising the step of:

(f) disintegrating said dried product.

17. The process of claim 11, wherein said R is selected from the group consisting of:

0) H,

(ii) linear saturated acyclic alkyl,

(iii) linear saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (iv) saturated cyclic alkyl, (v) saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (vi) linear unsaturated acyclic alkyl, (vii) linear unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (viii) unsaturated cyclic alkyl, (ix) unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (x) branched saturated acyclic alkyl, (xi) branched saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xii) substituted saturated cyclic alkyl, (xiii) substituted saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xiv) branched unsaturated acyclic alkyl, (xv) branched unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xvi) substituted unsaturated cyclic alkyl, and (xvii) substituted unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur.

18. The process of claim 11 , wherein said at least one dispersant has a molecular weight less than about 130 and has at most three carbon atoms connected directly to said COOH group by carbon-carbon bonds.

19. The process of claim 18, wherein said at least one dispersant is selected from the group consisting of formic acid, acetic acid, propionic acid, methoxyacetic acid, butanoic acid and acrylic acid.

20. The process of claim 18, wherein said at least one dispersant has at most two carbon atoms connected directly to said COOH group by carbon-carbon bonds.

21. The process of claim 11 , wherein the at least one carrier powder is an oxide selected from the group consisting of MgO, Al₂O₃ and SiO₂.

22. The process of claim 11 , wherein the at least one carrier powder is a hydroxide selected from the group consisting of Mg(OH)₂, Ca(OH)₂, AIO(OH) and Al(OH)₃.

23. The process of claim 11, wherein the at least one carrier powder is a salt selected from the group consisting of sulfates, carbonates and hydroxycarbonates.

24. The process of claim 23, wherein the at least one carrier powder is selected from the group consisting of CaCO₃, MgCO₃, CaMg(CO₃)₂, BaSO₄, CaSO₄, and hydrotalcites.

25. The process of claim 24, wherein said hydrotalcites are Mg₆Al₂CO₃(OH)₁₆4H₂O and Mg₄(CO₃)₃(OH)₂3H₂O.

26. The process of claim 11, wherein the at least one carrier powder is a clay.

27. The process of claim 26, wherein said clay is selected from the group consisting of talc and kaolinite.

28. A TiO -coated powder produced by the process of claim 11.

29. The TiO₂-coated powder of claim 28, wherein a majority of said titanium dioxide is in the form of single crystals.

30. The TiO₂-coated powder of claim 29, wherein at least about 80%_> of said titanium dioxide is in the form of single crystals.

31. The TiO₂-coated powder of claim 28, wherein the at least one carrier powder and said coated powder have particle size distributions with identical modalities.

32. The process of claim 11, wherein each of said monocarboxylic acids is such that:

(ii) in a volume of a mixture, of about 1900 parts by weight of water, about 100 parts by weight of said diagnostic titanium dioxide, and between about 1 and about 2 parts by weight of said each acid, allowed to stand for about 48 hours, said diagnostic titanium dioxide powder remains suspended in at least 80 percent of said volume.

33. A slurry, comprising:

(a) water;

(b) titanium dioxide; and

(c) at least one dispersant selected from the group consisting of organic monocarboxylic acids of the formula R-COOH:

(i) having a molecular weight less than about 150, and

(ii) having at most four carbon atoms connected directly to said

COOH group by carbon-carbon bonds, and anhydrides thereof; the slurry having a pH less than about 7.

34. The slurry of claim 33, having a weight proportion of said at least one dispersant to said titanium dioxide between about 0.1% and about 20%.

35. The slurry of claim 34, having a weight proportion of said at least one dispersant to said titanium dioxide between about 0.3% and about 5%.

36. The slurry of claim 33, wherein said R is selected from the group consisting of:

0) H,

(ii) linear saturated acyclic alkyl,

(iii) linear saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (iv) saturated cyclic alkyl, (v) saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (vi) linear unsaturated acyclic alkyl, (vii) linear unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (viii) unsaturated cyclic alkyl,

(ix) unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (x) branched saturated acyclic alkyl, (xi) branched saturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xii) substituted saturated cyclic alkyl, (xiii) substituted saturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xiv) branched unsaturated acyclic alkyl, (xv) branched unsaturated acyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur, (xvi) substituted unsaturated cyclic alkyl, and (xvii) substituted unsaturated cyclic alkyl wherein at least one atom is replaced by at least one atom selected from the group consisting of oxygen, nitrogen, phosphorus and sulfur.

37. The slurry of claim 33, wherein said at least one dispersant has a molecular weight less than about 130 and has at most three carbon atoms connected directly to said COOH group by carbon-carbon bonds.

38. The slurry of claim 37, wherein said at least one dispersant is selected from the group consisting of formic acid, acetic acid, propionic acid, methoxyacetic acid, butanoic acid and acrylic acid.

39. The slurry of claim 37, wherein said at least one dispersant has at most two carbon atoms connected directly to said COOH group by carbon-carbon bonds.

40. The process of claim 33, wherein each of said monocarboxylic acids is such that:

(i) a mixture of about 200 parts by weight of water, about 300 parts by weight of a diagnostic titanium dioxide powder having a D50 between about 200 nm and about 240 nm, and between about 3 and about 6 parts by weight of said each acid has a viscosity below about 1000 cp and a pH below about 7; and (ii) in a volume of a mixture, of about 1900 parts by weight of water, about 100 parts by weight of said diagnostic titanium dioxide powder, and between about 1 and about 2 parts by weight of said each acid, allowed to stand for about 48 hours, said diagnostic titanium dioxide powder remains suspended in at least 80 percent of said volume.