CA1068878A - Shaped and fired refractory microspheres - Google Patents

Abstract

Abstract of the Disclosure Refractory microspheres of polycrystalline anatase titanium dioxide are disclosed which are useful in making textile and composite articles.

Description

:10681~78 This application is a divisional of our copending application Serial No. 194 473, filed March 8, 1974.
This invention relates to solid, shaped refractory microspheres of polycrystalline titanium dioxide, ard articles made therefrom such as textiles and composites.
In particular, it relates to transparent, solid, shaped and fired, homogeneous, microspheres having diameters in the range of 1 to 200 microns alld comprising predominantly polycrystalline titanium dioxide in its anatase form.
In another aspect, the invention relates to a method for forming refractory microspheres of solid transparent refractory comprising predominant-ly polycrystalline titanium dioxide in its anatase form, which method comprises dispersing as droplets in an organic dehydrating liquid an aqueous acidic titanium oxide sol or aqueous acidic mixture of a titanium compound calcinable in air to titanium dioxide, recovering the resulting amorphous microspheres from said liquid, and heating and firing said recovered microspheres to con-vert them to said refractory microspheres. -In still a further aspect, it relates to an aqueous mixture or sol of titania, which mixture or aol can be shaped, dehydratively gelled, and fired to form solid, shaped, transparent, strong polycrystalline anatase titanium dioxide microspheres.
Briefly, the refract~ry products of this invention are solid, shaped and fired, nonvitreous refractory articles having predetermined shapes in at least two dimensions, i.e. microspheres, comprising predominantly (i.e.
greater than 50 weight percent, e.g. 60 weight percent and greater) poly-crystalline TiO2, in its anatase form. These products are made by a nonmelt process comprising shaping in at least two dimensions and dehydrative-; ly or evaporatively gelling a liquid mixture or sol of titania sr a titanium compound such as tetraisopropyl titanate, calcinable in air to TiO2, to form ~ .. ..

106S87t~3 a t~green~ or nonrefractory amorphous shaped article, such as a fiber or micro-sphere, and heating and firing the shaped green article to remove water, de-compose and volatilize undesired constituents and convert it into said refrac-tory article.
Solid, refractory microspheres of this invention are made by dis-persing droplets of said mixture or sol in an organic dehydrating liquid, such as 2-ethyl-1-hexanol, in which said droplets are immiscible, separating the resulting green microspheres and then heating and firing them to form uni-formly round, smooth, solid, strong, glossy, refractory microspheres of poly-crystalline titanium dioxide, these shaped and fired spheres also having the titanium dioxide content in its anatase form and being transparent to visible light, said spheres being useful, for example, in making reflective signs or traffic marking surfaces or as fillers or reinforcement for plastic composites.
The terms "dehydrative gelling" or "evaporative gelling," as used herein, means that sufficient water and volatile material are removed from the shaped green microspheres so that the form or shape of the article is ` sufficiently rigid to permit handling or processing without significant loss ., or distortion of the desired form or shape. Therefore, all the water in the microsphere need not be removed. Thus, in a sense,this step can be called partial dehydrative gelling. The microspheres in their green form are general- ~-ly transparent to visible light and clear (or perhaps slightly hazy) under an optical microscope. The solidified gel microspheres in their green form are amorphous, i.e. X-ray analysis does not show the presence of any crystal-` line species.
In the accompanying drawing the single figure is a pen-and-in~ -sketch of solid, transparent, polycrystalline, anatase titanium dioxide re-fractory microspheres of this invention which are drawn to the same scale as a photomicrograph (200~) taken with a light or optical microscope, using oblique illumination.

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- 2 -The refractory microspheres can be made from an aqueous sol of colloidal titania formed by adding to acid (e.g. 37% concentrated hydrochloric acid) a tetraalkyl titanate of the formula Ti(OR)4, where R is lower alkyl, e.g. with 1-8 carbon atoms, preferably 1-4 carbon atoms, such as tetraiso-propyl titanate (the preferred titanate since it yields shaped and fired re-fractory microspheres with superior physical and optical properties). Alkyl titanate compounds used in this invention are known in the art (see United States Patents 3,460,956 and 3,395,203 where they are used in making flakes of titanium dioxide, useful as pigments), me titania sol can also be pre-` 10 pared by slowly adding titanium tetrachloride to water to obtain a clear solution, adding ammonium hydroxide to the solution to preeipitate a titania hydrate, separating and washing the precipitate with water and dispersing the preeipitate in aqueous aeid. Useful titania sols ean also be made by dis-persing eolloidal titania in water admixed with sueh water-miseible, readily volatile, polar organie solvents as methanol, isopropanol, ethylene glyeol, dimethylformamide, and various glycol ethers sold under the trademark ~'Cellosolve". me use of sueh organie solvents, however, is not preferred sinee sueh use inereases the eosts of operating.
The titania sols whi~h are used to form the refractory micro- ~-. 20 spheres of this invention ean also eontain one or more other water-soluble or -dispersible metal eompounds (ealeinable in air to metal oxides) or other ~ metal oxide sols as additives to impart internal color to the final refrac- ~ -`~ tory mierospheres or modify properties thereof, sueh as refraetive index, coefficient of expansion, and the temperature at which anatase TiO2 transforms to rutile TiO2. For example, ferric nitrate ean be added to an aqueous titania sol to impart a red to orange to gold eolor to the final refraetory produet; ehromium diforma~e, trioxide, or chloride to impart a reddish or -amber color; eobalt acetate or ehloride to impart a green color; calcium ; aeetate to impart a yellow color; niekel aeetate to impart a light yellow or , " , . . . .. ..

~06~878 gold color; and copper chloride to impart a light green. The ferric oxide_ containing refractory can be reduced in a hydrogen atmosphere, the resulting reduced iron oxide or iron imparting a black color to the refractory and making it attractive to a magnet. Silicon compounds calcinable to silica, SiO2, or aqueous colloidal silica sols, can also be added to the titania sol, and for purposes of this application they are included in the term "additive metal compounds or oxides". Particularly useful aqueous colloidal silica dispersions or sols which can be used are those available under the trademark "Ludox". The amount of said additive metal compounds or oxides added to the titania sol can vary, depending on their function, but generally the amount to be added will be 0.5 to 10 weight percent, or even up to 50 weight percent, ; based on the total weight of the final refractory microsphere product.
The titania sol as prepared generally will be relatively dilute and a sample of it generally will contain the equivalent of about 10 to 40 weight percent titania solids when calcined in air at 600-800C. The titania sol preferred in the practice of this invention is made by freshly preparing a titania sol from a mixture of tetraalkyl titanate and acid. Acids which are useful are hydrochloric acid ~the preferred acid), nitric acid (diluted with ethanol to prevent a violent reaction of it with the titanate), lactic acid or acetic acids (though organic acids are not preferred since their use means just that much more organic material will have to be removed in sub-J sequent firing). The fresh sol is dried sufficiently to form a clear, firm gel comprising titanium dioxide, and the gel is then dispersed in water. The dispersion can then be concentrated to form a highly viscous aqueous sol of titania. Where the titania sol is prepared from titanium tetrachloride, it can be mixed with an organic viscosifier, such as corn syrup or polyvinyl-pyrrolidone and the mixture can then be concentrated for microsphere forma-tion.
Preferred aqueous colloidal sols which can be used to form 10~;8878 microspheres of this invention are those formed by adding about 5 parts by weight of tetraisopropyl titanate (TIPT) to about 1 part by weight of 37%
concentrated hydrochloric acid, sufficiently evaporating water, HC1, and other volatiles at ambient temperature (20-35C.) or lower (using evacuation, e.g. as with water aspirator) to form a firm gel which contains about 58 to 65 weight percent TiO2, 12 to 20 weight percent HCl, 10 to 30 weight percent H2O, and a small amount (e.g. 0.1 to 2 weight percent) organic material, and redispersing the gel in water to form a clear sol using about 4 parts water to 1 part gel. In some cases, it may be desirable to filter the titania sols to remove large colloids or extraneous particles.
When the green microspheres are fired in air to convert them into refractories, the titanium dioxide content is polycrystalline, i.e. composed of a plurality of crystallites, the size of the crystallites being generally less than 1,000 angstroms and being distinguished from macrocrystals or "whiskers," which are single crystals measured in terms of millimeters or `~ centimeters.
-~ The microspheres in the green or unfired gel form generally com-prise about 60 to 80 weight percent equivalent metal oxide solids (when cal-cined in air, e.g. at 600-800C.) and are dry in the sense that they do not adhere or stick to one another or other substrates and feel dry to the touch.
However, they still contain substantial amounts of water, acid and organics, e.g., 20 to 40 weight percent, and it is necessary to heat and fire in order to remove these remaining fugitive materials and convert the green fibers into refractory microspheres.
In order to remove the balance of water, acid and organics from - the green microspheres and convert them to refractory microspheres, they are heated, e.g. in a furnace, kiln or the like in air or other oxygen-containing atmosphere or in special cases in a neutral or reducing atmosphere at a moder-ately high temperature of up to about 600C. Heating the green articles to about 600C. results in microspheres of polycrystalline anatase titanium ; ~ 5 .. :

... . . . . .... . ...... . . . ....... . . . .
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1068~il7~

dioxide as determined by X-ray analysis. Above 600C. and in the range of 650-750C. the titanium dioxide undergoes a transformation from the anàtase form to the rutile form, the refractory changing from a transparent, clear, glossy material to an opaque or translucent whitish material. Upon further heating to about 1,000-1,300C., the crystallite grain size of the rutile titanium dioxide increases, the material becoming clear and the crystallites growing at a rapid rate to about 5-10 microns or greater in thickness.
A cl,n Incorporation of additives for color or other effects may causeiu~ increase in the transformation temperature from transparent anatase to opaque or translucent rutile. For example, the presence of silica in a mixture of TiO2-SiO2 in amounts of 0.6, 6.25 and 13 weight percent shifts the anatase-to-rutile transformation to about 800, 900 and 1,100C., respectively (i.e.
the anatase form is retained up to at least 800, 900 and 1,100C., respec-tively). However, opaque or translucent rutile can be transformed to clear rutile upon heating to a higher temperature.
` Firing can be accomplished in a number of ways, for example, by heating in a single step to a desired temperature or by heating in a series of steps at progressively higher temperatures with or without cooling or storage between steps.
; 20 In firing the green microspheres, ignition of combustible material in or evolved from them should be avoided since such ignition may cause a rapid rise in temperature or a catastrophic evolution of volatiles, result-ing in the formation of opaque, fragile microspheres. Ignition may be avoided, for example, by starting out at a low temperature, e.g. room temper-ature and elevating the temperature at a controlled rate. If the green fibers are not to be fired completely in one operation or are not to be fired ' immediately or soon after their formation, it may be desirable or necessary to store the fibers in a relatively dry or protective atmosphere to prevent , them from picking up moisutre or contaminants and deteriorating or sticking together. -~

As indicated by thermogravimetric and differential thermal analyses, the firing step volatilizes the balance of H20 and acid, decom-poses and volatilizes organic material, and removes carbon, the resultant microspheres being homogeneous and refractory. This firing step also causes some shrinking, the amount of linear shrinkage being generally 25 percent or more and the volume shrinkage being generally 50 percent or more. However, the shape of the article during firing remains intact. Rather than firing the green microspheres in air to remove water, acid and organics, they can be heated in an autoclave in an inert atmosphere (e.g. 7 to 140 kg./cm2 helium, argon or nitrogen) for example at 300 to 500C., in order to increase their porosity. Then, they can be refired in air to remove carbon, e.g. at 500 to 600C. or more and convert them into porous refractories.
The titanium dioxide in refractory microspheres resulting from firing the green microspheres in air at about 600C. is polycrystalline in nature, the crystallites being generally less than about 1000 angstroms in . . .
size, and usually less than 500 angstroms. Such polycrystalline microspheres ; are clear, glossy, smooth, uniformly curvilinear in shape, colorless (un-less colorants are deliberately incorporated), and are transparent to visible light. They are flexible and have useful strength and can be handled without breakage.
The solid refractory spherical particles or microspheres of this invention can be prepared from the precursor sols, with or without additive `~ metal compounds or sols thereof incorporated in the titania sols, by using ^ the shaping and dehydrative gelling techniques and equipment of the prior art (e.g. U.S. Patent Nos. 3,329,745 to LaGrange, 3,331,783 to Braun et al,

3,331,785 to Fitch et al, 3,340,567, 3,380,894 to Flack et al, and 3,709,706 .~ .
to Sowman). (This type of dehydrative gelling can be considered in a sense as a solvent-extraction.) For this purpose it is not necessary to concentrate the titania sol and it can have a variable equivalent solids content, for cx~mple, of S to 30 weight percent and a viscosity, for ex~sple, of 10 to ~ ;

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-106887~

30 cps. Rather, the sol can be dispersed in the form of small droplets in an organic dehydrating liquid having a low water solubility (e.g., 1 to 30 volume percent), such as C4 to ClO alkanols, e.g. butanol, hexanol, ethyl-butanol and ethylhexanol. In order to ensure formation of the solid micro-spheres, the alcohol, such as butanol, may have to be nearly saturated or mixed with a minor amount of water, e.g. n-butanol mixed with 18 to 20 weight percent water or used in anhydrous form, e.g. 2-ethyl-1-hexanol.
These partially water-immiscible alcohols are preferred dehydrating liquids to be used in making the microspheres of this invention, and they have sufficiently low solubility for water that water is extracted from the dis-persed droplets at a rate low enough to allow the droplets to dehydratively `~ gel into solid microspheres of uniform surface and internal structure. The amount of dehydrating liquid used should be sufficient to prevent the drop-lets or spherical particles formed therein from sticking together. In the case of 2-ethyl-l-hexanol, the amount of water to be extracted in the dehydrating liquid is maintained at less than 2 volume percent. Alternative-ly, an oil, such as mineral oil, can be used as the dehydrating medium, such oil being heated, e.g. to 60-90C., to dehydrate the droplets dispersed in the heated oil.
Where the sol used to make the microspheres contains significant amounts of an alcohol which would be miscible with the dehydrating liquid, it will be necessary to remove sufficient alcohol from the sol so that the sol ~ -.~ will be immiscible in the dehydrating liquid when dispersed therein. The .~ above discussed procedure of drying freshly prepared sol and then redisper- --sing the resulting gel in water will be particularly useful in preparing a sol for microsphere formation.
The addition of the sol to the dehydrating liquid can be made by injecting or jetting a stream of the sol into the body of the dehydrating liquid either above or below the surface thereof, for example, with a hypo- -dermic needle. The dehydrating liquid is preferably agitated by stirring or :

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swirling during the addition of the sol thereto. After addition of all of the sol to the dehydrating liquid, the mixture can be stirred further, for example, for 20 to 30 minutes, until the resultant spherical particles of the dispersion are sufficiently dehydrated and firm. The spherical particles can be separated from the dehydrating liquid, for example, by filtering or by centrifuging, and allowed to dry further in air at ambient room temper-atures or higher, for example 60 to 80C., to a solids content of about 60 to 80 weight percent. The particles can then be fired to convert them into ; hard refractory particles, e.g. fired in air at about 600C. The particles in the green form or their fired form will generally be water clear, trans-parent and spherical under an optical microscope, and they can also be in-ternally colored in manner described above by adding various water-soluble metal salts to the initial precursor liquid. Generally, the green and the ~; fired spherical particles will have diameters in the range of 1 to 200 microns, usually 20 to 100 microns, depending upon the degree of agitation used to form them, more vigorous agitation giving smaller spheres. The spheres will be solid and can be screen-classified to obtain fraction with desired diameters.
Another technique for making green spherical particles is to spray-dry the precursor sol in a dilute or concentrated, nonviscaus form. Atomiz-ing of the precursor liquid can be carried out, for example, with pressure - nozzles, the droplets or spheres as made descending in a countercurrent of dry air at ambient room temperature or in a flowing stream of warm air. -In describing microspheres of this invention as "transparent", this ` term means that they have the property of transmitting rays of visible light. ~-Transparency of microspheres is indicated by the ability of the microspheres ~
to function as the optical component in reflective sheeting made, for example, - ~-in accordance with U.S. Patents 2,407,680 or 2,326,634. "Opaque" articles, . on the other hand, are those which are impervious to light, e.g., the bodies or substrate beneatn an opaque microsphere are obscured and cannot be seen . : . ~ . . , : ,.

~068878 therethrough. The "translucent" articles are those which fall between trans-parent and opaque and though translucent articles have the property of trans-mitting light to some degree, and therefore are somewhat or partly trans-parent, bodies beneath cannot be seen in a clearly distinguishable or sharp manner. Sometimes, because of vagaries in firing, an article or product may be a mixture of these various types of products, though generally one will be present in a predominant amount, indicative of the true nature of the mixture, the other products present in minor amounts having their particular appearance due to nonuniform firing conditions or due to localized overheat-ing because of hot spots in the furnace or undesirable combustion.
Microspheres of this invention are those refractory articles con-taining anatase titanium dioxide which are transparent, though for some particular applications, for example, where the article is used as a rein-- forcement for composites, transparency may not be important. The transparent quality of a refractory microsphere product of this invention is coincident with other desirable properties, such as strength and flexibility, and thus transparency can be considered in a sense as a gross measure of the quality of the refractory product. In some applications of the refractory micro- -spheres of this invention, e.g. where microspheres are used in reflective -sign surfaces, transparency will be of special importance.
A particularly useful application for the refractory microspheres of this invention is that of reinforcement for structural plastic, elasto-meric, metallic or ceramic composites especially those composites used in ~-high temperature environments found in the aero-space industry and in ablative environments. The matrix materials which can be so reinforced include any of those heretofore used in making such composites, such as those disclosed in the above-cited '~odern Composite Materials" text and "Handbook of Reinforced Plastics," by Oleesky and Mohr, Reinhold Pub. Co., N.Y. (1964).
The plastics may be either of the thermosetting or thermoplastic types.
Representative plastics which can be used include epoxy resins, polyester -. .

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resins, acetal resins, acrylics, especially methyl methacrylate polymers, amino resins, especially urea-formaldehyde and melamine-formaldehyde, alkyds, cellulosics, especially ethyl cellulose, cellulose acetate, and cellulose proprionate, fluorocarbons, furanes, polyurethanes, phenolics, polyamides, polycarbonates, vinyl aromatics such as styrene, polyolefins, especially polyethylene and the like. The refractory microspheres of this invention can be made with a wide useful range of indices of refraction, e.g. about 1.8 to 2.6 or higher. In the form of particulate materials, the refractory products can be used as fillers and/or coloring agents or pigments for paints and enamels, such as water-based paints or alkyd-resin paints.
Metal matrix composites have had generally only limited application heretofore, one major reason being the lack of reinforcement materials which will withstand the elevated temperaturesencountered in processing, e.g.
casting and sintering temperatures. The refractory products of this inven-tion, because of their th0rmal stability, strength, flexibility and other properties are useful as reinforcements for metal composites, such as shaped or cast articles made of aluminum, copper, magnesium, lead and nickel. Here, `-too, the prior art methods of incorporating reinforcement, in metal matrix composites can be used, reference being made to "Fiber-Streng~hened Metallic Composites," ASTEM Spc. Tech. Pub. No. 427, published by the American Society for Testing and Materials, Philadelphia, Pa. (1967).
The refractory products of this invention can also be used as reinforcement for ceramic composites, such as silica, glass, aluminum silicate and other inorganic materials such reinforced ceramics being in the form of blocks, paper and other shaped articles used in high temperature environments.
The refractory products of this invention can also be used as re-inforcing agents for elastomeric materials, such as rubber, e.g. natural -rubber, styrene-butadiene rubber, acrylonitrile-butadiene rubber and neoprene, for example where such rubbers are used in making passenger-car or truck tires.

, ~068878 This invention is further illustrated in the following examples.
In these examples, all parts are by weight unless otherwise noted, the viscosities recited are Brookfield viscositites measured at ambient room temperature. In the examples, the firing of green articles and the firing of amorphous refractory articles to high0r temperatures were all carried out by firing in air in an electric resistance furnace unless otherwise noted.
Example 1 Five parts tetraisopropyl titanate was added slowly to 1 part 37%
conc. ~ICl cooled in a water bath. The resulting sol was drled in air at room temperature to a gel containing approximately 63 wt.~ TiO2 determined by calcination of a sample.
A TiO2 sol was made by dispersing 15 g. of the dry gel in 60 g.
water. Microspheres were formed by injecting the sol with a hypodermic syringe into 3500 ml. of 2-ethyl-1-hexanol agitated by a laboratory stirrer at 1200 rpm. The dispersion of microspheres was stirred for 20 min., filtered through No. 54 '~hatman" paper, dried at 90C., and the recovered microspheres were heated in air from room temperature to 300C. over a 3-hr.
period to give transparent, brown-colored, solid microspheres with diameters predominantly in the range of 50 to 100 microns. Portions of the fired microspheres were soaked in various aqueous solutions or sols of metal - compounds, filtered, dried by heating to 90C. for about 1 hr., and fired in air, and the refractive indices of the fired microspheres were measured.
` Control portions of the microspheres were fired without soaking to determine changes in refractive index due to the soaking treatment and firing temper-ature. The results are tabulated in Table I. Some of the microspheres were placed in the furnace at the desired firing temperature and others were placed in the furnace at room temperature (or, in one run, at 300C.) and the temperature raised to the desired temperature. The aqueous solutions or `
. sols of metal compounds, used for soaking the microspheres, contained the ; A 30 equivalent of 5 wt.% of metal oxide. Where the soaking media were ~ 3 ;

1o68878 solution, BaC12 solution, and a ZrO2 sol and the microspheres soaked then fired from room temperature to 640,680 and 700 C., respectively, the fired microspheres were found to contain o.6, 0.15 and 0.3 wt.% of Pb, Ba and Zr, respectively, as determined by spectrographic analysis.
TABLE I
Fired Microspheres Stereoscopic Metal Firing Duration Microscope Refractive Compd. Temp.C. of Firin~ APPearance _ _ Index NONE 525 15 min. very clear, transp. 2.36 NONE 580 20 min. clear, transp. 2.385 ` NONE 580 60 min. clear, transp. 2.385 ` NONE 640 15 min. mostly clear, 2.42 transp.
NONE RT*-640 3 hr. very clear, transp. 2.59 -~
NONE RT-720 4-1/2 hr. mostly clear, 2.625-transp.** 2.65 Pb(N3)2 525 15 min. very clear, transp. 2.37 ~ ;
'r Pb(N3)2 580 20 min. clear, transp. 2.41 -3 2 5 60 min. clear, transp. 2.45 ` -:-(N3)2 640 15 min. mostly clear, . transp. 2.42 Pb(N03)2 RT-640 3 hr. very clear, transp. 2.525 Pb(N03)2 RT-720 4-1/2 hr. slightly diffuse 2.64-2.67 , BaC12 300-575 1-1/2 hr. clear, transp. 2.37 -BaC12 RT-650 2-1/2 hr. mostly clear, 2.515 transp.**
' B C1 RT-680 4 hr. very clear, transp. 2.65 J 2 RT-720 8 hr. diffuse 2.65 Zr2 100-550 2 hr. very clear, transp. 2.39 sol 3 Zr2 620 20 min. very clear, transp. 2.40 sol :~068878 TABLE I (cont) Fired Microspheres Stereoscopic Metal Firing Duration Microscope Refractive Compd. Temp.C. of Firing Appesrsnce Index Zr2 RT-630 4 hr. very clear, sol transp. 2.525 ~r2 RT-700 3-1/2 hr. clear, transp. 2.65 sol Zr2 RT-780 2 hr. cloudy ---sol ---- .. _ * "RT" means firing star~ed at room temperature.
** These fired microspheres were mostly clear and transparent but the fired product contained some microspheres larger than 50 microns which were slightly diffuse.

These data indicate that the refractive index is somewhat dependent upon the additive used for soaking the microspheres prior to firing but appears to be more dependent on the firing temperature.
Example 2 ~ A sol was made by adding 5 parts tetraisopropyl titanate to 1 ; part 37% concentrated Hcl. The sol was coated on a polyester film and allowed to dry in air at room temperature. Ten g. of the dry gel, in the 20 form of clear, transparent, colorless flakes, were dispersed in 23.3 g. of an aqueous colloidal silica sol ("Ludox" SM) containing 15 wt.% SiO2. The resultant mixed sol wss calculated to contain the equivalent of 65 wt.%
TiO2 snd 35 wt.% SiO2. Four g. of this mixed sol were diluted with 4 g.
water snd the diluted sol was poured into 500 ml. 2-ethyl-1-hexanol which ~`
was being rapidly stirred. ~he resulting dispersion of microspheres was filtered and the recovered microspheres were fired in air at room temperature.

The dried, transparent microspheres were fired in air from room temperature to 530C. and held at the latter temperature for 15 min. The resultant fired :
solid microspheres were fairly clear, transparent, polycrystalline with the titanium dioxide in the anatase form and had a refractive index of 1.815.

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After firing in air further from room temperature to 630C. and holding at 630C for 30 min., the microspheres were still fairly clear and transparent with no change in refractive index and the titanium dioxide was still in -the anatase form, the microspheres having a diameter ranging from less than 30 microns to 100 microns. A similarly prepared batch of dried microspheres ` was fired in air from room temperature to 730C. and held at that temperature for 30 min.; these fired microspheres were also transparent, fairly clear and had an index of refraction of 1.815, the titanium dioxide also being present in the anatase form.
Example 3 About 2.5 g. of a dry gel containing the equivalent of 61 wt.%
TiO2 and made according to the procedure of Example 1, were dispersed into 9,24 g. of an aqueous colloidal silica sol ("Ludox" Sh~ containing 15 wt.%
SiO2. The mixed sol was poured into 750 ml. 2-ethyl-1-hexanol which was being rapidly stirred and the stirring continued for 20 min. The resulting dispersion of solid microspheres was filtered and the recovered microspheres were dried. The dried microspheres were transparent, clear and colorless.
They were fired in air from room temperature to 600C., held at that temperature for 15 min. and further fired to 1000C. and held at that temp-erature for 30 min. The resultant fired solid microspheres were transparent,slightly cloudy and had an index of refraction of 1.75 to 1.76 and they had a calculated composition of 52.5 wt.% TiO2 and 47.5 wt.% SiO2 with the titanium dioxide being in the anatase form.
Example 4 Green, porous, transparent titanium dioxide microspheres were formed from a TiO2 sol ~using the procedure of Example 1) that contained about 30 wt.% TiO2. The resulting green microspheres were fired in air at three different temperatures and the surface areas measured using the BET
method. The data are given in Table II.

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~068878 TABLE II
Firing Firing Surface Area Temp.C. Time (hr.) (m2/g) 200 1 107.0 310 2 133.0 370 2 68.6 Example 5 Sixty-five g. of a TiO2 g01, made according to the procedure of Example 1 were dispersed in 260 g. water to form a TiO2 sol, which was filtered through a 0.25 micron "millipore" filter. Sixty g. of the filtered sol was poured slowly into the vortex formed in 3700 ml. of 2-ethyl-1-hexanol being stirred at 1240 rpm. in a 4-liter beaker. Stirring was continued for 20-25 min. The dispersion of microspheres was filtered and the recovered microspheres were dried overnight at 80C. The dried green microspheres were fired in air from room temperature to 530C. over a l-hr. period and held at 530C. for 1 hr. The resulting solid, clear, shiny microspheres had an average diameter of about 45 microns and had an average index of re-fraction of 2.32. The TiO2 content of these fired microspheres was poly-crystalline anatase. The drawing illustrates what these fired microspheres look like under a light microscope, where they are designated by reference number 10 (reference number 11 denoting light spots caused by reflection ; from the illumination source on the surface of the microspheres and the shading shown by stippling).
Example 6 A solution was made by dripping 504 g. TiC14 into 500 g. water -~;
being constantly stirred in a beaker and maintained at about 20C. in a cold water bath, the reaction mixture being cooled to prevent elevation of the ~
temperature due to the exothermic reaction. A clear yellow liquid resulted. ~ ~-To 481 g. of the solution, 350 ml. of water and 250 ml. of ammonium hydroxide (28-30% NH3) was added with agitation, forming a thick white precipitate.
., ~

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- 10f~l~878 Water was added to disperse the precipitate which was filtered off and washed repeatedly with water. The precipitate was then dispersed in water.
A portion, 250 g. (containing about 10 wt.% TiO2) of the dispersion was digested with 45 ml. of concentrated HCl over a period of 4 hrs. The result-ing sol was filtered through No. 54 and then No. 50 "Whatman" filters. The filtered sol was slightly hazy and had a pH of about 0.5 with a TiO2 content of about 8.6 wt.%. Three ml. of the sol were injected with a hypodermic syringe into 250 ml. 2-ethyl-1-hexanol and the dispersion of solid micro-spheres was stirred as in Example 1. The green, water-clear microspheres were recovered by filtration and dried in air at about 80C. After firing from room temperature to 550C. in about 1 to 1-1/2 hrs., the microspheres were shiny, transparent and clear to slightly hazy. The fired microspheres ` averaged 45-60 microns in diameter and had a refractive index of 2.24.
Example 7 ; Transparent and colorless polyester sheeting (0.10 mm. thick) is coated with a 30% solution of vinylidene fluoride perfluoropropene fluorin-ated elastomer ("Viton" A) having an index of refraction of about 1.38 to ~` 1.39 when cured to a thickness of about 0.015 mm. in methylisobutylketone using a wire-wound bar (28 winds per cm.) to accomplish this coating. The coating is allowed to dry to a slightly tacky state and solid titanium dioxide microspheres, made as described in Example 5 (with diameters ranging from 15-30,u and an index of refraction of 2.58), are cascade coated onto ; and partially embedded in the tacky coating or web as a monolayer, and the ` assembly is then oven dried in air at 95C. for 15 min. The exposed surfaces of the partially embedded microspheres are then vapor coated with aluminum to yield a composite sheeting of the type described in U.S. Patent No.
2,407,680 the sheeting being retro-reflective to visible light when covered with water.
Example 8 A glossy polyacrylate treated paper carrier web is knife-coated :

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with a non-oxidizing alkyd resin solution (an oil-free, baking alkyd solution containing adipic acid, maximum acid number of 15, said solution containing about 68 wt.% resin solids and 32 wt.% aromatic solvent) to provide a wet coating of about 0.20 mm. thick. The resin is cured at 65C. for 10 min.
and then 120C. for 10 min. A second coating (about 0.05 mm. thick) of the same alkyd resin solution is knife-coated on top of the first coating and allowed to become tacky by evaporation of the solvent at room temperature.
` Solid titanium dioxide microspheres made in accordance with Example 5 (having diameters ranging from 20-45 ~u and a reflective index of 2.30) are cascade coated on and partially embedded in the tacky resin as a monolayer and the assembly is then cured in place for 30 min. at 95C. A xylol-butanol solu-tion containing 20 wt.% polyvinyl butryal (having an index of refraction of about 1.50-1.53 when dry) was knife-coated directly over the microsphere coating to provide a final layer of resin (0.025-0.038 mm. in thickness).
After this final layer was dried at 95C. for 30 min., the resin surface is vacuum vapor coated with aluminum. The paper carrier web is stripped from - the assembly to yield a composite sheeting of the type described in U.S.
Patent 2,407,680 which is retro-reflective to visible light when covered with water.
Example 9 A solution of a blend of 5 parts by weight of nitrile rubber (a high acrylonitrile-butadiene copolymer), 6.6 parts by weight of phenolic resin ("Durez" 14296), 1 part by weight of dioctyl phthalate plasticizer and 2.9 parts by weight of aluminùm powder in 16 parts by weight of methyl isobutyl ketone is knife-coated to a thickness of about 0.05 mm. onto a glossy polyacrylate treated paper carrier web and the coating allowed to dry to a tacky state. Solid titanium dioxide microspheres made in accordance with Example 5 (the majority of the microspheres having a diameter of about ; 20-75,u and an index of refraction of 2.54) are cascade coated onto and partially embedded into the tacky resin and the assembly is cured in a 120C.

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oven for 20 min. The resulting sheeting is retro-reflective when covered with water and is of the type described in U.S. Patent No. 2,326,634.

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Claims (6)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. Transparent, solid, shaped and fired, homo-geneous, microspheres having diameters in the range of 1 to 200 microns and comprising predominantly polycrystalline titanium dioxide in its anatase form.

2. The microspheres of claim 1 further comprising one or more other metal oxide.

3. The microspheres of claim 1 further comprising one or more of the oxides of silicon, chromium, aluminum, iron and cobalt.

4. A method for forming refractory microspheres of solid transparent refractory comprising predominantly polycrystalline titanium dioxide in its anatase form, which method comprises dispersing as droplets in an organic de-hydrating liquid an aqueous acidic titanium oxide sol or aqueous acidic mixture of a titanium compound calcinable in air to titanium dioxide, recovering the resulting amorphous microspheres from said liquid, and heating and firing said recovered microspheres to convert them to said refractory microspheres.

5. A method according to claim 4, wherein the material which is dispersed as said droplets to form said amorphous microspheres comprises an aqueous titanium dioxide sol prepared by dispersing in water a gel formed by drying an acidified aqueous mixture of tetraalkyl titanate.

6. The method according to claim 5, wherein said tetraalkyl titanate is tetraisopropyl titanate.