CA1274855A - Metal oxide microspheres and process for making same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Spherical metal oxide particles having diameters in the range of 0.05 - 5 microns are prepared by the direct evaporation of a water-in-oil emulsion composed of an aquasol or an aqueous dispersion of a metal oxide dispersed in a water-immiscible solvent.

Description

~L274855 INTRODUCTION
Methods for the preparation of metal oxide microspheres via sol-gel routes are well known in the art. For example, a metal oxide aquasol may be injected into a dehydrating solvent which forms the aquasol into spherical droplets by surface tension forces and extracts water from the droplets to produce solid microspheres. Alternatively, the aquasol droplets may be dehydrated by contacting them ~ith a heated solvent in which water has a minimal miscibility. Such methods have been described in U.S. 3,312,632 and in U.S. 4,349,456.
By another known method, a salt or aquasol is mixed with an ammonia releasing agent and then introduced as a droplet into a liquid maintained at a temperature sufficient to thermally degrade the ammonia-releasing agent and cause ammonia release which results in gellation of the droplets into firm metal oxide microspheres. Variations of these methods are discussed in U.S.
Pat. No. 2,620,314 and in U.S. Pat. No. 3,312,632.
The methods described above result in metal oxide microspheres which are large, often hundreds of micrometers in diameter. These methods are generally not suitable for preparing submicron particles.
Microspherical metal oxide particles may be prepared by the controlled hydrolysis of the corresponding metal alkoxide compound in dilute solution. The microspherical particles produced by this method are of high purity and are very uniform in size with average diameters ranging from less than 0.05 micrometers to several micrometers. This method, which is discussed by W. Stober, A. Fink and E. Bohn in Journal of Colloid and Interface Science, Vol. 26, 1968, pp. 62-69, and by E.
Barringer and H. K. Bowen in Journal of the American Ceramic Society, Vol. 65, 1982, pp. C199-201, involves the use of

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expensive and reactive organometallic compounds which must be handled ln an air- and water-free environment. These requirements are prohibitive to large-scale implementation o~ the method and restrict its practice to the laboratory.
The use of emulsion techniques in the preparation of metal oxide microspheres by sol-gel routes is known in the art.
Aquasols of metal oxides may be dispersed into lmmiscible organic solvents, often with the aid of surface active agents, and gelled to form spherical particles by the addition of a gellation agent such as ammonia gas or compounds such as amines which are capable of extracting stabilizing ions from the aquasol. Such procedures which are discussed in G.B. 1,412,937 and G.B. 2,116,959A involve several processing steps and produce particles which are generally larger than 1 micrometer.
The method of U.S. Pat. No. 35848,059 comprises the combination of two separate water-in-oil emulsions, each containing a different inorganic salt, to form a spheroidal-shaped particle of an insoluble metathesis product with average particle sizes in the range of û.l to 5.û
micrometers. The distribution of particle sizes is unspecified.
In order to remove the water-soluble salt by-product, it is necessary to filter and extensively wash the insoluble microsphere product.
In U.S. Pat. Nos. 4,011,096 and 4,132,560, methods are described for the preparation of reticulated and pigmented silica microspheres, respectively. The pigmented microspheres (2 to 100 micrometer average diameter) are made by acidiFying a water-in~oil emulsion containing a silaceous aqueous phase whereas the vesiculated microspheres (0.5 to 5û micrometer average diameter) are made by acidifying an oil-in-water-in-oil lZ'7~5~; ~
emulsion prepared by a double-emulsion technique. 30th of these methods involve numerous processing steps such as extensive washing and centrifugation of the silica microspheres and they require the use of undesirably large quantities of surfactants.
These disadvantages render the methods commercially unattractive from a practical and an economic point of view, as stated in U.S.
Pat. No. 4,173,491.
U.S. Pat. No. 3,857,924 describes another emulsion route to spherical silica particles. An alkali silicate solution is treated sequentially with cation and anion exchange materials in order to remove cations and mineral acids, respectively, to give a polysilicious acid solution which is unstable toward gellation. This solution is emulsified in a water-immiscible solvent wherein it gels within 60 minutes to form silica spheres with sizes in the range of l micrometer to 3mm. A problem inherent in this method is that the rapid gellation time requires that the emulsification step be executed soon after the ion exchange steps. This restriction would be disadvantageous in practice. Furthermore, the method does not lend itself to continuous processing.
It can be seen, then, that the methods currently available for the preparation of submicron metal oxide microspheres are unsatisfactory for one or more of the following reasons:
l. The microspheres obtained are too large.
2. The methods entail complicated, multistep procedures which would be impractical and expensive to implement on a large scale.

3. The methods are inherently limited to specific materials.

-The direct evaporation in vacuo of water-in-oil emulsions containing mixed salt aqueous solution to produce intimately mixed salt particles of unspecified size and shape is described in ~The Use of Emulsions in the Preparation of Ceramic Powders" by P. Reynen, H. Bastius and M. Fiedler in Ceramic Powders, ed. P. Vincenzini, Elsevier Scientific Publishing Company, Ansterdam, 1983. It has, however, never been suggested to apply this method to a water-in-oil emulsion containing an aquasol or dispersion of a metal oxide in order to prepare micron- and submicron-sized metal oxide microspheres.
The method of the present invention provides for the preparation of metal oxide microspheres with submicron diameters, preferably in the approximate range of 0.05 to 5 micrometers, by a simple method which does not entail numerous processing steps.

THE INVEN T ION
In its broadest aspect, the invention comprises a method for the preparation of metal oxide microspheres which comprises evaporating to substantial dryness a water-in-oil emulsion having as its internal phase an aqueous colloidal dispersion of a metal oxide whereby there is produced microspherical particles of the metal oxide having an average particle size diameter within the range of 0.05 - 5 microns.
This invention specifically provides a process for producing micron- and submicron-sized spherical metal oxide gel particles by the steps of:
(i) forming of water-in-oil emulsion composed of fine droplets of a colloidal dispersion of a metal oxide in a water-immiscible organic liquid containing one or more emulsifiers;

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(ii) evaporation of the water-in-oil emulsion of step (i) to remove the oil and water to form a cake or free-flowing powder composed of metal oxide microspheres, and optionally;
(iii) calcining the particles to produce dehydrated metal oxide microspheres which are free of organic contaminants.

The Colloidal Dispersion of the Metal Oxide The aqueous or internal phase of the water-in-oil emulsion of the above step may contain either a true metal oxide aquasol in which the typical particle size is on the order of several millimicrons or a dispersion of the metal oxide composed of larger yet still colloidal particles. The pH of this phase may be either acidic, neutral, or alkaline.
The colloidal dispersions of the metal oxides used to prepare the internal phase of the water-in-oil emulsions contain those metal oxides which are substantially water-insoluble and are capable of forming aquasols. Such metal oxides comprise the oxides of those metals found in Group III-~ through Group IV-A of the Periodic Table. Included in this group of metals are the colloidal oxides of silicon which, for purposes of this invention, are considered to be within the definition of metal oxides. In certain instances mixed or coated metal o~ide dispersions may be used.
Typical of the metal oxides that may be formed into microspherical particles by following the teachings of this invention are the oxides of silicon, aluminum, iron, chromium, cerium, tin, zirconium, titanium, zinc, and the like. These are only considered to be illustrative of the many microspherical ~7~
metal oxides that can be prepared. Also, mixtures of these oxides may be used.
As indicated, these metal oxides or mixtures thereof are employed in the form of aqueous colloidal dispersions. These dispersions contain the metal oxide in a form such that one dimension thereof is within the average particle size range of between 1 up to about 20û millimicrons.
The particle size of the metal oxides in the starting colloidal dispersion will vary depending upon the particular metal oxide used and its method of preparation. Many colloidal dispersions of metal oxides of the type described are available commercially and are of a relatively high degree of purity.
Also, the concentration of the metal oxide in the colloidal dispersion will vary; usually a minimum amount in the starting colloidal dispersion will be about 2% and it may be as high as 50%.
In the case of aqueous colloidal silica sols, typical starting sols that may be used are sold under the tradename, NALCûA ~. These come in a variety of particle sizes and concentrations. Illustrative of such starting aqueous silica sols are those set forth in Table I below.

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l ~ o~o o _, _, o o ~j N N

O O O ~
Is~ ~ ~
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l u~ o o o O a~
O O O
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N

Ir~ u-\ ~) N O ll~
~ r~
O a) I ~ N
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~ cC ~--I N O
I,LJ ~ ~ N C~
~ O ~O I . .
m _~ ~ -I o O O N ~O O ~ ~t . ~ 0 O O I N N O
O
O~

N O
O
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t~ _I h ~ ~
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.,, ~ ta U) ~
U~ ~ ~
a~ >.
O ~ ~ > ~
n~ c a~
o ~ c t~ ~ C
c~ ~ ~ a~ Q.
c: ~ C ~ cn ~
o ~ ~n C.~ ur) ~ h C~ O Ul J ~ ~ a) ~ o N a c~ a) ~I ~ >CL .~a Z Q QQ 5 c~ Z r-~

In the case o~ silica sols of the type described above, there average particle size ranges between about 5 millimicrons up to about 150 millimicrons. A preferre~ particle size range is within the range of about 10-60 millimicrons.
Another useful starting colloidal dispersion of the metal oxides of the type described above are the alumina sols described in detail in U.S. 3,141,786. These materials are described as an aqueous sol composed of alumina fiburles having a surface area of 200-400 square meter per gram and an average length of 25-1500 millimicrons.
In addition to the specific silica and alumina sols of the type described above, there are listed below properties and characteristics of other commercially available colloidal dispersions of metal oxides that may be used as a starting material in the practice of the invention:
Alumina Sol Typical Physical Properties Surface Area (BET) 320 m /gm.
Particle Size Powder - by sieving greater than 45 microns 15%
less than 45 microns 85%
Dispersion - by X-ray diffraction .0048 micron Loose Bulk Density 45 lbs./ft.3 Packed Bulk Density 50 lbs./ft.3 g_ ~ ~27qL~5~

Cerium Oxide Sol Typical Properties Formula CeO2 Wt. % 18 Particle Size, millimicrons 10-20 pH 2.7 Specific Gravity 1.19 Viscosity, cps 10 r~
~ c~
~ ~ l r~
~ o N ~:t O ' ' ' >- ~ N ~ ~ --~

a~ ~~o ~-1 a C t.) O ~
O N O ~
O ~ O N
h O ~
I`J ~ N

O Oz ~
O N O 0 I C`~
h O
.,_1 h O
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O
c ~ z h f^l ~ O ~ ~
I~J 1~1 N --I O --I ~ I`
h Q
h Ir~ ~1 O x 0 ' a7 C O~ r`_, O
N Q

~ O
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h O I -t NIt~ N
tL)~ O O O
C~ ~ --I O

~1 O ~:
._~~ ~ O C~l ~-1 O I I ~ O N
U)~D O O ~ O
O ON --I O ~ ~

a~ h Q
N ~
.~ ,c c ~n o o .) OO O ~7 E O
V ~)C ~ , ~1 ~h o~ ~a ~ta o o I ' '-I
h. Z ~ E C_ O E Q ~ ~

Aquasols of alumina and titania sold under the tradename, NALCO; aquasols of zirconia and iron oxide sold under the tradenames, Nyaco ~ and P ~; and dispersible alumina sold as Disperal~ are particularly useful in this invention.
While the colloidal dispersions of the metal oxides have been described as being finely divided particles in water7 it is understood that instead of water, there may be substituted therefor highly polar liquids such as the water-soluble liquids, ethanol, isopropanol, water-soluble or immiscible glycol such as ethylene or propylene glycol, and the like. Such colloidal dispersions are considered equivalent to the aqueous colloidal dispersions of the metal oxides.

The Water Immiscible Organic Liquids The preferred water-immiscible organic liquids which constitute the oil phase of the water-in-oil emulsion of step (i) above are sufficiently volatile to evaporate at temperatures below that which will cause de-emulsification of the emulsion, yet it is not so volatile as to evaporate at a rate much greater than water. Commonly available commercial hydrocar~on mixtures such as those sold under the trade names, Napthol Spirits and Low ûdor Paraffin Solvent, for example, are useful in this invention. The amount of organic liquid used in relation to water may be varied.
While it is preferred to use hydrocarbon liquids of the type described above, it is understood that other organic liquids may be also used. Such organic liquids must be hydrophobic and be sufficiently free of polar groups such that they would either not be capable of forming the water-in oil emulsions or would cause extractive dehydration of the colloidal dispersion Qf the ~2~
hydrous metal oxide to occur. Thus, certain chlorinated liquids such as perchloro ethylene, vegetable oils, and the like may be used.
Preferred emulsions are prepared with water-to-oil ratios of l:l to 1:2. The ratios are illustrative of emulsions that can be prepared, although it should be understood that the invention is not limited thereby.

The Water-in-Oil Emulsifier ._ Suitable emulsifying systems useful in producing the water in-oil emulsions of step (i) above are obtained by choosing a so-called low ~LB material or by combining a low HLa material ¦with a higher HLB material in ratios such that the resultant HLB
¦of the combined materials is su~ficiently low to form a ¦water-in-oil emulsion with fine aqueous droplet size. Although ¦these emulsifying systems are useful in producing good ¦water-in-oil emulsions, other emulsifiers may be used as long as ¦they produce the desired oil-in-water emulsions. The HLB
¦classifications of various emulsifiers are summarized in The HLB
System, ICI Americas, Inc., Wilmington, DE, 1976, and in McCutcheon's Detergents and Emulsions, 1983 North American Edition, McCutcheon Div., MC Publishing Co., Glen Rock, NJ.
Also, see Surfactants and Interfacial Phenomena, by Milton J.
Rosen, John Wiley & Sons, NY, 1978, Chapter 8.
As a general rule, the HLB of the emulsifier or emulsifier system will be below 8 and is preferably within the range of 4-6.
Typical water-in-oil emulsifiers that have been used successfully in the invention are the well known commercial emulsifiers:

~ lZ7~85~i 1 Span 8 O Sorbitan Monooleate Tween 6 ~ Sorbitan Monostearate Rx 4 moles E~
Ethoduomeen T/13~ N,Tallow 1,3-propylene diamine Rx 3 moles E0 the combination of Span 8 ~ with Tween 6 ~, Tween 61~ or Ethoduomeer~ T/13 form useful emulsifying systems for the present invention. It is generally desirable to minimize the amount of emulsifier used. The preferred total emulsifier level is usually less than 10% of the oil phase by weight. It may range, however, between 0.5 - 20% of the oil phase.
The emulsions of step (i) may be prepared by any of a number of techniques known to those skilled in the art. For example, the emulsions may be prepared by using high-speed agitation to disperse the aqueous phase into the oil phase containing the emulsifier or emulsifiers.

Dehydration of the Emulsion The emulsion is then subjected to temperatures and pressures which are sufficient to achieve evaporation of both the water and the oil phases at approximately similar rates. The temperature to which the emulsion is subjected must not be so high as to disrupt or break the aqueous droplets within the oil phase. When the emulsion is properly formulated~ the rate of evaporation is not critical and whereas it is convenient to rapidly evaporate the emulsion, it is possible to prolong the evaporation or even interrupt the evaporation at some intermediate stage and continue it at a later time.
The evaporation of the emulsion may be conducted in any device which will accomplish vacuum or atmospheric drying of solids or distillation of liquids. In the pr~ferred methods for ~Z7~ ;i5 preparation of the microspheres of this inventiont evaporation of the emulsion may be accomplished in thin-fllm dryers and in thin-film evaporators, each of which may be o~ the mechanically agitated or non-agitated type, or in stills, dryers or evaporators of the wiped-film type. For example, the emulsion may be evaporated in a rotary evaporator. Other methods of evaporating the emulsion may be employed, however, it should be understood that the invention is not limited thereby.
The removal of the water and oil phases by evaporation affords a solid composed of metal oxide microspheres which are coated with the emulsifiers. If the oil phase contains a high-boiling component, the microspheres which form upon evaporation of the free water will be fully or partly suspended in the high-boiling component and may be recovered by settling, centrifugation or filtration. Since high boiling organic liquids are often used, vacuum evaporation is preferred since it allows low temperatures to be used.
The particular temperature required to dehydrate the emulsion cannot be described with certainty since as indicated above, it would be dependent upon the nature of the emulsion being evaporated, the liquids present in the emulsion, and the like. It may be said, however, that the temperature should be below that temperature which would cause breaking of the emulsion during the dehydration step.
~ y stopping the dehydration step prior to producing substantially dry metal oxide particles, it is possible to produce a dispersion which has as its continuous phase the organic liquid and having dispersed therethroughout the fine metal oxide microspheres.

Expressed in another fashion, this aspect of the invention comprises a method for the preparation of metal oxide microspheres suspended in a water immiscible organic liquid which comprises evaporating the water and a small amount of the organic liquid from a water-in-oil emulsion having as its external phase a water immiscible organic liquid and as its internal phase an aqueous dispersion of a metal oxide whereby there is produced microspherical particles of the metal oxide having an average particle size diameter within the range of 0.05 - 5 microns.

The Microspheres Produced by the Process of the Invention The microspheres which are produced by the method of this invention are observed by electron microscopy to be discrete and entirely round in shape. The average particle diameter and size distribution is determined by the size of the aqueous droplets in the emulsion which, in turn, is controlled largely by the method of dispersion employed to prepare the emulsion and by the emulsion formulation. By the method of the present invention, it is possible to prepare spherical partlcles with an average diameter in the range of 0.05 to 5 micrometers and, preferably, 0.1 - 3 micrometers. The ratio of the diameter of the largest sphere to that of the smallest sphere is equal to less than approximately 5. Preferably it is less than 20.
Since the method of the present invention does not involve treatment of the emulsified metal oxide aquasol or dispersion with a precipitant or gellation agent which could contribute contaminating ions to the metal oxide microspheres, the purity of the microspheres is dependent largely upon the composition of the metal oxide aquasol or dispersion. It is, -therefore, preferable to employ a metal oxide aquasol or dispersion containing stabilizing ions such as nitrate or acetate which may be decomposed upon calcination. Additionally, it is highly desirable to employ emulsifiers which are substantially free o~ metal const~tuents or contaminantsS
The microspheres of the present invention may be directly calcined to remove residual emulsifiers, oil phase and water. The temperature and duration of calcination will be dependent on the particular precursor materials usçd and on the particular final properties desired. The microspheres may, in some cases, be washed with certain organic solvents possessing sufficient polarity to remove residual emulsifiers, oil phase and water. for example, methyl ethyl ketone or diethyl ether may be employed to wash residue from the microspheres. The microspheres may then be dried and calcined if desired.
The temperature at which the calcination is performed will vary depending upon the particular microspherical metal oxide particle being treated. Generally the minimum temperature will be about 300F. but may range as high as 2300F.
It is understood that the temperature will depend upon the end use to which the calcined metal oxide particles will be employed. for example, hydrous alumina microspheres must be calcined above 2000F to convert them to microspheres o~
alpha-aluminum oxide or ceramic alumina. Partially hydrated alumina phases like gamma alumina are obtained when the microspheres are calcined below 2000F. Hydrous iron oxide microspheres may be converted to the hematite phase by calcination at approximately 1000F.

-In summary, calcination temperature ranges for common oxide particles are:
A1203 600 -- 2300F.
SiO2 600 -- 15û0F.
Fe203 600 -- lû00F.
FeOOH 600 -- 1000F.
In the case of silica, the temperature rangP can vary widely, e.g. rather low temperatures when simple uses such as fillers or binders are contemplated up to approximately 1500F.
when it is sought to produce a fused silica microsphere.
One of the interesting phenomena of the invention is that when the emulsions are evaporated to dryness as described above, the particle size of the produced microspheres is less than or equal to about 80% of the size of the particles in the internal phase. The degree of shrinkage upon evaporation increases with increasing dilution of the metal oxide dispersion prior to emulsification. The term, "evaporating," or "evaporated to substantial dryness," is meant to include those products which have been treated to produce microspheres which are substantially free of both the internal and external phase of the starting water-in-oil emulsion. It also includes cases where only the water has been removed from the starting emulsion and the colloidal dispersions of the metal oxides are suspended in the external phase of the emulsion.

Uses The microspherical particles of the invention can be employed in a variety of uses. The calcined alumina particles can be used in the preparation of catalysts. The silica particles may be used as refractory binders, o'pacifying agents, and the like. The iron particles can be used in the preparation of magnetic tapes and the like.

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I Another feature of the invention is that prior to ¦calcination, the particles contain, in most instances, a thin ¦coating of the low HLB emulsifier which allows them to be readily ¦blended into organic systems such as plastics and the like, ¦thereby making many of the particles excellent inorganic fillers ¦and components of finished plastic materials and products.

Evaluation of the Invention To illustrate the invention, the following are given by way of example:
¦ Example 1 Two hundred 9. of a dispersion containing 22%
¦Dispera ~ Alumina were added to 210 9. of Napthol Spirits, 5 9.
¦of Span 8 ~ and 5 9. of Tween 61~. The mixture was ¦mechanically agitated in a blender to produce a water-in-oil ¦emulsion. The emulsion was evaporated on a rotary evaporator at la vacuum of 50mm Hg and at a bath temperature of 8û-90C. A
¦free-flowing powder composed of microspherical alumina gel ¦particles was obtained in approximately 10 minutes. The powder ¦was then stirred in an excess of methyl ethyl ketone to remove ¦residual emulsifiers, solvent and water. After allowing the alumina to settle, the methyl ethyl ketone was siphoned and the alumina is first air-dried and then calcined in air at 500C
for 60 minutes. The powder is composed of spherical particles with a mean diameter of û.5 micrometers and a standard deviation ¦of 0.2 micrometers. The surface area of the sample is 233 M2/g ¦and the total pore volume is 0.4 CC/9.

- 1 ~7~ i5 ¦ Example 2 I
l One hundred 9. of Nalco silica sol containing 30% SiO2 ¦were added to an oil phase composed of 100 g. Napthol SpiritsJ
¦1.8 9. Span 8dE~ and 3.2 9. Ethoduomeer~ T/13. The mixture ¦was agitated in a blender to form a water-in-oil ernulsion. The ¦emulsion was then evaporated on a rotary evaporator at a vacuum ¦of 50mm Hg and at a bath temperature of 95C, resulting in a solid composed of spherical particles with a mean diameter of 0.15 micrometers and a standard deviation of 0.04 micrometers.
Calcination at 800F to remove organic residue affords a microspherical powder with a surface area of 177 M2/g and a pore volume of 0.3 cc/g.

¦ Example 3 ¦ Fifty 9. of Nyacol~) iron oxide sol containing 7.5 wt.%
¦Fe203 were added to an oil phase composed of 100 9. Napthol ¦Spirits, 3.75 9. Span 8~, 3.75 9. Tween 61~ and 2.50 9. of Rapisol B246~ polymeric surfactant from ICI Americas Inc. The mixture was agitated with a laboratory disperser until a water-in-oil emulsion containing droplets of the desired size was formed. The emulsion was evaporated on a rotary evaporator at a vacuum of 50mm Hg and at a bath temperature of 85C. The wet ¦residue which was produced was treated with excess methyl ethyl ¦ketone and allowed to settle. After removal of the supernatant liquids9 the wet solids were allowed to dry. The resulting rust-orange powder was shown by electron microscopy to contain spherical particles with diameters in the range of û.5 to 4 micrometers. The powder was calcined at 8ûûF to afford a powder with a surface area of 21 M2/g and a pore volume of l û.l cc/g.

--Example 4 One hundred 9. of a Nyaco ~ ~irconia sol containing 20~ ZrO2 were added to an oil phase composed of lOO 9. of Napthol Spirits 7 0 .ao g~ Tween 6 ~, 1.70 9. Span 8 ~. The mixture was agitated in a blender to afford a ~ater-in-oil emulsion. The emulsion was then evaporated at a vacuum of 50mm Hg and at a bath temperature of 85 to 90C for 10 minutes, resulting in a free-flowing solid. Examination of the solid with an electron microscope reveals it to be composed of spheres with a mean diameter of 0.52 micrometers and a standard deviation of 0.25 micrometersL The sample was calcined for 1.5 hours at 800F and ground to afford a powder composed of discrete microspheres with a surface area of 7 MZ/g and a pore volume o~
0.02 cc/g.

Example 5 Twenty-five g. of cerium dioxide sol from Rhone-Poulenc containing 18% CeO2 were added to 50 9. of Span 8 ~ and 0.40 9. of Tween 61~. The mixture was agitated in a blender to form a water-in-oil emulsion. The emulsion was evaporated on a rotary evaporator at 50mm Hg vacuum and at a bath temperature of 80-85C. After the free water was evaporated and no further oil phase evaporated, the CeO2 microspheres were recovered from the unevaporated oil phase by centrifugation. The diameters of the microspheres were observed by scanning electron microscopy to be in the range 0.1 to 0.5 microns.

Example 6 Fifty g. Nalco titania sol containing 12% TiO2 were added to an oil phase composed of lûO 9. Low Odor Paraffin Solvent, 5 9. Span 8 ~ and 5 9. Tween 6 ~. The mixture was agitated in a blender to form a water-in-oil emulsion which was subsequently evaporated on a rotary evaporator at 50mm Hg. A
solid composed of spherical titania particles with diameters of approximately 2 micrometers was obtained.

Example 7 An emulsion of colloidal alumina was prepared according to the method described in Example 1. This emulsion was then evaporated in an agitated thin-film evaporator at 61mm Hg pressure and elevated temperature to afford a slurry of alumina microspheres in the organic liquid. The average diameter of these microspheres as measured on a Leeds and Northrup MICROTRA ~ Particle Size Analyzer is 1.25 micrometers and the relative standard deviation o~ the size distribution is 45%.

Example 8 Thirty 9. of alumina sol containing lû% A12O3 were added to an oil phase composed of lOO 9. Napthol Spirits, 2.5 9.
Span 8 ~ and 2.5 9. Tween 61~ The mixture was agitated with a disperser to form a water-in-oil emulsion. The emulsion ~as evaporated on a rotary evaporator at 30mm ~9 and at elevated temperature to form a wet cake which was washea with methyl ethyl ketone and acetone and dried. The resulting white powder was found by scanning electron microscopy to be composed of discrete spheres with diameters ranging from about O.l micrometers to 2 micrometers.

Example 9 Twenty-five 9. of a 22% colloidal alumina dispersion were combined with 75 9. of a 7.5~ colloidal iron oxide dispersion and added to an oil phase composed of 100 g. Napthol Spirits, 2.5 9. Span 8 ~ and 2.5 9. Tween 6 ~. The mixture was agitated in a blender to form a water-in oil emulsion. This was evaporated in a rotary evaporator at 50mm Hg and at elevated temperature to afford a slurry of mixed-metal oxide microspheres in the organic liquid. The microspheres were observed by scanning electron microscopy to have diameters ranging from about 0.1 micrometer to about 1.5 micrometers.

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G L O S S A R Y

Tradename Nyacol Philadelphia Quartz PQ Philadelphia Quartz Disperal Remet Chemical Corp.

Span 80 ICI America Sorbitan monooleate, N.F.
(water-in-oil emulsifier) Tween 60 ICI America POE(20 sorbitan monostearate (polysorbate 60) -- emulsifier Tween 61 ICI America POE(4) sorbitan monostearate -- emulsifier Ethoduomeen Tl3 Armak Ethylene oxide condensation product of Duomeen T.

Duomeen T Armak N-tallow trimethylene diamine -- corrosion inhibitor Nalcoag Nalco Chemical Co. See Table I

Rapisol B246 ICI America Polymeric surfactant

Claims (14)

Claims:

1. A method for the preparation of metal oxide microspheres which comprises evaporating to substantial dryness a water-in-oil emulsion having as its internal phase an aqueous colloidal dispersion of a metal oxide whereby there is produced microspherical particles of the metal oxide having an average particle size diameter within the range of 0.05 - 5 microns.

2. The method of Claim 1 where the hydrous metal oxide is from the group consisting of alumina, silica, zirconia, titania, iron oxide, and ceria and the external phase of the emulsion is a hydrocarbon liquid.

3. The method of Claim 1 where the evaporation is conducted under a vacuum.

4. The method of Claim 2 where the metal oxide is silica.

5. The method of Claim 1 where the dried microspherical particles of the metal oxide are in the form of a free flowing powder.

6. A method for the preparation of metal oxide microspheres suspended in a water-immiscible organic liquid which comprises evaporating the water and a small amount of the organic liquid from a water-in-oil emulsion having as its external phase a water-immiscible organic liquid and as its internal phase an aqueous dispersion of a metal oxide whereby there is produced microspherical particles of the metal oxide haying an average particle size diameter within the range of 0.05 - 5 microns suspended in the water-immiscible organic liquid.

7. A method for the preparation of metal oxide microspheres which comprises evaporating to substantial dryness a water-in-oil emulsion having as its internal phase an aqueous colloidal dispersion of a metal oxide whereby there is produced microspherical particles of metal oxide and then calcining such particles which, after calcination, have an average particle size diameter within the range of 0.05 - 5 microns.

8. A method for the preparation of microspherical mixed-metal oxide particles which comprises evaporating to substantial dryness a water-in-oil emulsion having as its internal phase a mixture of two or more aqueous colloidal dispersions of metal oxides whereby there is produced microspherical particles composed of a mixture of the two metal oxides and having an average particle size diameter within the range 0.05 - 5 microns.

9. The method of Claim 8 where the metal oxides are the oxides of iron and aluminum.

10. The method of Claim 5 wherein the microspherical particles have been calcined at a temperature sufficient to remove any organic impurities contained in said particles.

11. Microspherical particles of a metal oxide having an average particle size within the range of 0.05 - 5 microns, said particles having been produced in accordance with the method of Claim 1.

12. The microspherical particles of claim 11 in the form of a free flowing powder wherein the particles have been calcined at a temperature sufficient to remove any organic impurities contained in said particles.

13. Microspherical particles of Claim 11 prepared by the method of Claim 1 where the evaporation of the emulsion is accomplished by thin-film or wiped-film methods.

14. Microspherical particles of Claim 11 where metal oxide is a mixture of metal oxides.