CA1038135A - Shaped and fired fibers of tio2 - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Solid, shaped and fired refractory articles, such as fibers and microspheres, of titanium dioxide are made by shaping, gelling and firing titanin sols.
Said fibers can be made by extruding in air a viscous aqueous titania sol and heating and firing the resulting green fiber to remove water, decompose and volatilize undesired constituents and form a refractory fiber of polycrystalline titanium dioxide which is useful, for example, to form refractory fabrics or as reinforcement for composites. Said microspheres can be made by dis-persing droplets of an aqueous titania sol in an organic dehydrating liquid, separating the resulting green microspheres from the dehydrating liquid and heating and firing the green microspheres to form solid refractory microspheres of polycrystalline titanium dioxide, which are useful, for example, in reflective signs or traffic marking surfaces. Said fired fibers and microspheres are preferably transparent to visible light with the titanium dioxide content thereof in its anatose form.

Description

- F.N.go9,934 1038~35 SHAPED AND FIRED ARTICLES CF TiO2 This invention relates to solld, shaped refractory articles of polycrystalline titanium dioxide, suc-h as fibers and microspheres and articles made there~rom such as textiles and composites. In another aspect, it relates to continuous, transparent, strong, flexible refractory fibers of polycrystalline anatase titanium dioxide. In another aspect, it relates to solid, uniformly shaped, ~ ~ -transparent refractory microspheres of polycrystalline anatase titanium dioxide. In another aspect, it relates to processes for the preparation of said shaped refractory articles. In still a further aspect, it relates to an ~ -~
aqueous mixture or sol of titania, which mixture or sol can be shaped, dehydratively gelled, and fired t~ form -~ -solid, shaped, transparent, strong polycrystalline anatase titanium dioxide articles, such as fibers and microspheres.
Briefly, the refractory products of this inven-tion are solid, shaped and fired, nonvitreous refrac-tory articles having predetermined shapes in at least two dimensions, such as fibers and microspheres, comprising predominantly (i.e. greater than 50 weight percent, e.g.
60 welght percent and greater) polycrystalline TiO2, preferably in its anatase form. These products are made by a non~elt process comprlsing shaping in at least two dlmensions and dehydratively or evaporatlvely gell-ing a llquid mixt~re or sol of titania or a titaniumcompound such as tètralsopropyl tltanate, calcinable in air to T102, to form a "green" or nonrefractory amorphous shaped article, such as a ~lber or mlcrosphere, and heat-ing and firing the shaped green article to remove water, decompose and volatilize undesire-d constituents and ~ .
' .._~, -- ~ - - . . - : , lQ~13~i convert it into said refractory article.
Shaped and fired refractory fibers of this in-vention can be made by extruding in air a viscous, fiber~
izable concentrate of said mixture or sol and then heating and firing the resulting green fibers to form continuous uniformly round or oval, strong, flexlble, smooth, glossy, refractory fibers of polycrystalllne titanium dioxide, said fibers preferably havlng the titanium dioxide in its anatase form and being transparent to vislble light, said fibers being useful in making refractory textile fabric or as fillers or reinforcement for plastic composites. -Solid, refractory microspheres of this invention are made by dispersing 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 --uniformly round, smooth, solid, strong, glossy, refractory microspheres of polycrystalline titanium dioxide, these shaped and fired spheres also preferably having the tltanium dioxide content in lts anatase form and being transparent to vlslble light, said spheres being useful, for example, ln making reflectlve slgns or traffic marking surfaces or as flllers or relnforcement for plastlc composites.
The terms "dehydrative gelling" or "evaporatlve gelllng", as used herein, means that sufficient water and volatlle materlal are removed from the shaped green artl-cles (e.g., the green flbers or microspheres) 60 that the form or æhape of the artlcle ls surriclently rlgld to permit handling or processing without slgnlficant loss or distortlon .

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lQ3~35 of the deslred form or shape. Therefore, all the water in the shaped article need not be removed. Thus, in a sense, this step can be called partial dehydrative gelling. The -' shaped articles in their green form are generally trans-parent to vislble light and clear tor perhaps slightly hazy) under an optical microscope. In the case of fibers, ~ -unless coloring additives are included in the vlscous concentrate, the green flbers appear to look llke color- `
less glass fiber. The solidified gel artlcles of this ~ ~ -10 invention in their green form are amorphous, i.e., X-ray ~-analysis does not show the presence of any crystalline -species.
In the accompanying drawlng:
FIG. 1 is a pen-and-ink sketch of solld, trans-parent, polycrystalline, anatase titanium dioxide refractory microspheres Or this invention which are drawn to the same scale as a photomicrograph (200X) taken with a llght or optlcal microscope, uslng obllque lllumlnatlon;
FIG. 2 ls a pen-and-lnk sketch Or solld, trans-~ 20 parent, polycrystalllne, anatase tltanlum dioxlde refrac-tory rlbers Or this invention fired to 550C. which are drawn to the same scale as a photomicrograph (150X) taken wlth a llght or optical microscope using incldental and transmitted light; and FIG. 3 is a pen-and-ink sketch of solid, opaque, polycrystalline, rutile titanium dloxide refractory fibers of this invention fired to 800C. under the same conditions as those of FIG. 2 and drawn in a llke ~.anner.

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10;~135 Turning attention first to the refractory fibers of this invention, they 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 tetraisopropyl titanate (the preferred titanate since it yields shaped and fired refractory fibers with superior physical and optical properties). Alkyl titanate com-pounds used in this invention are known in the art (seeU. S. Patents 3,460,956 and 3,395,203 where they are used in making flakes of titanium dioxide, useful as pigments). The titania sol can also be prepared by-slowly - adding titanium tetrachloride to water to obtain a c~ear solution, adding ammonium hydroxide to the solution to precipitate a titania hydrate, separating and washing the precipitate with water and dispersing the precipi-j tate in aqueous acid. Useful titania sols can also be made by dispersing colloidal titania in water admixed with such water-miscible, readily volatile, polar organi~
solvents as methanol, isopropanol, ethylene glycol, dimethylformamide, and various glycol ethers sold under the trademark "Cellosolve". The use of such organic solvents, however, is not preferred since such use in-crease~ the costs of operating.
The titania sols which are used to form therefractory fiber~ of this invention can also contain one or more other water-soluble or -dispersible metal compounds (calcinable in air to metal oxides) or other metal oxlde s019 as additives to impart internal color .., ~038135 to the final refractory fibers or modify properties~thereof, -~ -such as refractive index, coefficient of expansion, and the temperature at which anatase TiO2 transforms to rutile TiO2. For example, ferric nitrate can be added to an ~
aqueous titania sol to impart a red to orange to gold ~ ~-color to the final refractory product; chromium difor-mate, trioxide, or chlorlde to impart a reddish or~amber color; cobalt acetate or chloride to i~part a green color;
calcium acetate to impart a yellow color, nickel acetate to impart a light yellow or gold color; and copper chloride to impart a light green. me ferric oxide-containing re-fractory can be reduced in a hydrogen atmosphere, the resulting reduced iron oxide or iron imparting a b~ack color to the refractory and msking it attractive to a magnet. Sillcon compounds calcinable to silica, SiO2, or aqueous colloidal silica sols, can also be added to the titania 801, and for purposes of this application they are included in the term "additive metal compounds or oxidesn. Psrticularly useful aqueous colloidal sillca dispersions or sols which can be used are those available under the trademark "Ludox". The amount of ~ald additlve metal compounds or oxides added to the tltanla 801 can vary, depending on their functionj 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 t~e final refractory fiber product.
The titania sol as prepared generally will be relatlvely dilute and a sample of lt generally will con- ~`
tain the equivalent of about lO to 40 weight percent titania solids when calcined in air at 600-800C.

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Generally where fibers are to be made from the sol, it will be desirable to concentrate or otherwise viscosify the titania sol that it will readily gel (i.e. hold its shape as a fiber sufficiently for further handling) when ex-truded and drawn in air. The titania sol preferred inthe practice of this invention is made by freshly pre-paring a titania sol from a mixture of tetraalk~l 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 subsequent 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 fiberizable 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 poly-- vinylpyrrolidone and the mixture can then be concentrated for fiber formatlon.
Preferred aqueous colloidal sols which can be used to form fibers of thls invention are those formed by addlng about 5 parts by weight of tetraisopropyl titanate (TIPT) to about 1 part by weight of 37~ concen-trated hydrochloric acid, sufficiently evaporating water, HCl. 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 .

in3~l3s percent TiO2, 12 to 20 weight percent HCl, lO to 30 weight percent H20, and a small amount te.g., 0.1 to 2 welght 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 501s to remove large colloids or extraneous partlcles.
The concentration of the titania sol for fiber ~ "
formation can be carried out by techniques known in the art, 10 various details as to the preferred technlques being disclosed in U.S. Patent 3,709,706. For example, the sol can be concentrated with a "Rotavapor" flask under water-aspirator vacuum, the vacuum ad~usted to prevent ~ -or minimize frothing or loss of the sol. Sufficient 15 concentration will be obtained when the equivalent~ -solids content of a calcined sample is 35 to 55 weight -~
percent titania solids and the viscosity (Brookfield at ambient room temperature) of the concentrate is in the range of 15,000 to 1,000,000 cps., preferably 45,000 to - 20 500,000 cps. m e size Or the co~Didal titania particles in the concentrated sol will generally be below lO milli- -microns in diameter. The viscous concentrates are rela-tlvely stable but low temperature storage or refrigeration may be preferred if the concentrate is not to be used shortly after preparation, e.g., wlthin 24 hours. Prior to extrusion, the cohcentrate can be centrifuged to remove air bubbles. The particular equivalent solids content or viscosity used for fiber formation will be dependent ;
on the particular apparatus and conditions used to extrude ; 3 the viscou3 concentrate. For example, when the vlscous :, :

", ' . , . ' , ., ' ., , concentrate is extruded under pressure, e.g., 3.5 to 70 kg/cm2, using a conventiQnal spinnerette with a plurality of orifices (e.g,, 15 to 1,000 or more orlfices wlth diameters of 0.025-0.25 mm), such as used in the rayon industry, the vlscoslty of the concentrate should be such that flbers are formed ln a continuous manner wlthout breaklng of the extruded flber as lt ls formed.
The extruded green fibers formed by this inven-tion can be allowed to fall in air by the force of gravity or drawn mechanically in air by means of rolls or a drum or winding device rotating at a speed faster than the rate of extrusion, or the concentrate can be extruded through orlfices from a stationary or rotating head and blown by parallel, oblique or tangential streams of air - 15 such as in the making of cotton candy, the blown fibers being collected on a screen or the like ln the form Or a mat. Any of these forces exerted on the extruded ` fibers, e.g., gravity, drawing or air streams, cause ; attenuation or stretching of the fibers, reduclng thelr cross-sectional area by about 50 to 90 percent or more and increaslng thelr length by about 100 to 1,000 percent and serve to hasten or ald the drying of the fibers.
The dehydrative gelling of the green fibers is carried out ln ambient air or heated air can be used ir desirable or necessary to obtain faster drying. The relative humidity Or such air should not be too high, since large amounts of molsture will prevent drylng and cause the gelled green flbers to stick together. Generally, relative humidity in the range of 20 to 60 percent can be used, at temperatures of 15 ~o 30C. If the humidity .

., ~......
;.. ~ -i5 high and must be tolerated, compensations can be made by uslng a concentrate with a greater equivalent solids content or a higher viscosity, extrudlng at a lower rate, using a lower drawlng rate, using a smaller extrusion orifice, exposing the green fibers to heated air as they are formed and/or increasing the distances between ~ -the extrusion orifice and the polnt where the lndl~ldual extruded flbers come lnto contact. On the other hand, lf the relatlve humldlty ls too low, e.g., 10 to 15 per-cent, or lower, the green flbers may dry too fast and they wlll tend to break or fracture durlng splnnlng or handling before they can be flred. Low hlmldlty condi-tlons may be compensated for by extruding at a faster rate, using larger extrusion oriflces, decreaslng the distance between the oriflces and the point where the fibers come into contact with one another or the drawlng rolls, and/or uslng concentrates wlth lower equivalent solids content or lower viscosities. Air currents should be minimized or controlled because they may cause the individual extruded fibers to come into contact before they are surficiently dry or cause flber breakage. A
thln coatlng of grease, lubrlcant or slzlng, such as "Halo-carbon" 25-5S (halogenated polychlorotrifluoroethylene thlckened wlth slllca gel) or "ANTIFOAM A SPRAY" (slll-cone defoamer) can be applled to the face of the spln-nerette to minimlze the stlcklng Or the concentrate or extruded fibers to the splnnerette face. In any event, the extruded flbers should be made or handled under condltlons whlch will prevent or mlnimize thelr contact wlth one another before they are sufrlciently dry to .~ _ g _ .....
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~ (~3~135 to prevent sticking.
The green fibers can be brought into contact to form a strand of multi-fibers and the strand can be sized to hold the fibers together without fiber-to-fiber bonding. Where a size is used, the strand (or extruded fibers) can be mechanically drawn over a size applicator, like that used in the textile industry, and a conventional heat fugitive size or lubricant, such as oil, applied.
Controlled rates of heating can be used to volatilize the size so as to avoid combustion of the size when the green fibers are fired, such combustion tending to cause overheating of the fibers (i.e. the temperature and rate of temperature rise caused by combustion may be higher than desired). The size may also require longer firlng to completely remove lt from the fired fiber.
When the shaped green articles are fired in air to convert them into refractories, the titanium dioxide content is polycrystalline, i.e. composed Or a plurality of crystallites, the size of the crystallltes being generally less than 1,000 angstroms and belng dlstinguished from macrocrystals or "whiskers," which are single crystals measured in terms of millimeters or centimeters.
Further detail on the extruding of fibers from the viscous concentrate will be omitted here in the lnterest of brevlty slnce appllcable shaping procedures are known in the art, reference belng made to U. S.
Patent 3,709,706, Belglum Patent 778,966 and Chapter 8 Or "Modern Composite Materlals" text, supra, which illu-strates and describes apparatus which can be used in ' ~ :
, . ' ` -~03~135 this invention to form fibers from viscous concentrates.
The fibers in the green or unfired gel form generally comprise about 60 to 80 weight percent equiva-lent metal oxide solids (when calcined in air, eOg 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 sub-- stantial amounts of water, acid and organics, e.g. 20 to 40 weight percent,and it is necessary to heat and fire the green fibers in order to remove these remaining fugi-tive materials and convert the green fibers into refractory fibers.
In order to remove the balance of water, acid and organics from the green fibers and convert them to refractory fibers, they are heated, e.g. in a furnace, kiln or the like in alr or other oxygen-containing at-mosphere or in special cases in a neutral or reducing atmosphere at a moderately high temperature of up to -. about 600C. Heating the green fibers to about 600C.
results in a fiber of polycrystalline anatase titanium dioxide as determined by X-ray analysis. Above 600C.
and ln the range of 650-750C. the titanium dioxide undergoes a transformation from the anatase form to `~ the rutile form, the refractory changing from a trans parent, clear, glossy material to an opaque or translucent whltish material.l 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 grcwing at a rapid rate to about 5-10 microns or greater in thickness. Incorporation of --3.1--- . . . . . . .
.. .- - -: ~............... .. .
- - : , . ., : .

~.0381;~5 additives for color or other effects may cause an Increase in the transformation temperature from transparent anatase to opaque or translucent rutile. For example, the pre-sence of silica in a mixture of T102-SiO2 in amounts of o.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., respectively). However, - opaque or translucent rutile can be transformed to clear - 10 rutile upon heating to a high0r temperature.
Firing can be accomplished in a number of ways, ior example, by heating in a single step to a de~ired temperature or by heating in a series of steps at pro-gressively hlgher temperatures with or without cooling or storage between steps. The green fibers can be fired in the form Or individual ribers or collected ln a regular s or random order and heated or heated in the form of strands (a plurality of untwisted, parallel-allgned ribers),or fired ln the form of hanks (a bunch of flbers or strands), or they can be chopped in the form of staple and fired in that manner. Also, the green strands or rlbers can be twlsted to form yarn and fired as such or can be woven to form a cloth and heated in the latter form. In order to ensure the production of continuous refractory fibers with lengths a~ great as 3 to 6 meters or longer, the green fibers are preferably heated in the form of a multi-fiber strand which is accumulated - or collected in a loose, relaxed, unrestrained or slack configuration such as offset or superimposed loops, as disclosed in said Belgium Patent 779,966.

_ ~_ In firing the green fibers, ignition of com-bustible material in or evolved from the fibers should be avolded since such ignition may cause a rapid rise in temperature or a catastrophic evolution of volatiles, resulting in the formation of opaque, fragile fibers.
Ignition may be avoided, for example, by starting out at a low temperature, e.g. room temperature 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 forma-tion, it may be desirable or necessary to store the fibers in a relatively dry or protective atmosphere to -prevent them from picking up moisture or contaminants ! ' and deteriorating or sticking together.
As indicated by thermogravimetric and differ-ential thermal analyses, the firing step volatilizes the - balance of H20 and acid, decomposes and volatilizes or-ganic material, and removes carbon, the resultant fiber being homogeneous and refractory. This firing step also causes some shrinking Or the fiber, the amount of linear shrinkage being generally 25 percent or more and the . -.. . ..
volume shrlnkage being generally 50 percent or more. ~ :
However, the fibrous shape of the article durlng firing remains intact and fibers when so fired are still of essen-tially continuous length. Rather than firing the green fibers in air to ~emove 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 ,, .
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carbon, e.g., at 500 to 600C., or more and convert them lnto porous refractories.
The titanlum dioxide ln refractory fibers re-sulting from firing the green fibers 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 fibers are clear, glossy, smooth, uniformly curvilinear in shape, colorless (unless colorants are deliberately incorporated), and, when the titanium dioxide is present in predominantly its anatase form, are transparent to visible light. They are flexible and have useful strength and can be handled without breakage, the refractory fibers generally havlng a tensile strength of 7000 kg/cm2 or higher and a modulus of elasticity of about 1 x 106 to 3 x 10 kg/cm or higher.
The polycrystalline refractory fibers are con-tinuous and generally have a rounded or ovoid cross section. The term "continuous fiber" as used in this application means a fiber ( or multi-fiber article such as a strand) which has a length which is lnfinite for practical purpose as compared to its diameter. The con-tinuous fibers of this invention, in green or refractory form, can be as long as 3 to 6 meters or longer, fibers of shorter length than this arising only from occasional flaws due to minute inhomogenities, such as foreign par-ticles or bubbles, stemming from their presence in the viscous concentrate precursor, or due to restraint during drying as by drying on a cylinder or from inadvertent mechanical fracture. By bringing a plurality of the Pibers , : . , : .

l~J3~ S
together in the form of a continuous strand, tow, yarn or other multi-fiber article, the occasional breakage or - fracture of a continuous flber does not affect the prac-tlcal utlllty of the multi-fiber article contalnlng a fiber whose length is relatively short. In any event, the fibers of thls lnventlon, even lf broken or fractured for reasons glven above, can be made in lengths which are slgnlficantly longer than the length of a staple flber.
Flrlng at low temperatures, e.g., 300C., results in a porous refractory fiber. Because of the intercon-nected poroslty Or the refractory flbers, solutlons of soluble metal compounds can be absorbed thereln and dried and fired ln alr to convert the compounds to metal oxlde deposits which enhance or change the color, index of re-fractlon, modulus of elastlclty and magnetlc or electrlcal propertles of the flbers; by using this technique, the flred flbers can serve as a support for cata-lytlc metals or metal oxldes.
The solld refractory spherical particles or mlcrospheres of this invention can be prepared from the same precursor sols used to make fibers, with or without addltlve metal compounds or sols thereof incorporated in the titania sols, by using the shaping and dehydrative 25 gelling techniques and equipment Or 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 a~ 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 .30 a sense as a solvent-extraction.) For this purpose _ 15 -- . . - . ~ - . ~ . . .

~3~135 lt is not necessary to concentrate the titania sol and lt can have a varlable equlvalent solids content, for example, of 5 to 30 welght percent and a v~scosity, for example, of 10 to 30 cps. Rather, the sol can be dispersed ln the form of small droplets ln an organic dehydrating llquld having a low water solublllty (e.g., 1 to 30 volume;per-cent), such as C4 to C10 alkanols, e.g., butanol, hexanol, ethylbutanol and ethylhexanol. In order to ensure forma-tlon of the solld microspheres, 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 lnvention, and they have sufficiently low solubility for water that water is ex-tracted from the dispersed droplets at a rate low enough to allow the droplets to dehydratively gel lnto solld mlcrospheres of uniform surface and internal structure.
The amount of dehydrating liquid used should be sufficient to prevent the droplets or spherical particles formed therein from stlcklng together. In the case of 2-ethyl-l-hexanol, the amount of water to be extracted ln the dehydrating liquld is maintained at less than 2 volume percent. Alternatively, an oil, such as mineral oil, can be used as the dehydrating medium, such oil belng heated, e.g., to 60-90C., to dehydrate the droplets dispersed in the heated oil.
Where the sol used to make the mlcrospheres contalns slgnlflcant amounts of an alcohol which would .~, .

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--~03~135 be miscible with the dehydrating liquid, it will be necessary to remove sufficlent alcohol from the sol so that the sol wlll be immiscible in the dehydrating llquld when dispersed therein. The above discussed procedure of drying rreshly prepared sol and then redispersing the resulting gel in water will be particularly useful in preparing a sol ror microsphere formation.
The addition of the sol to the dehydrating liquid can be made by in~ecting or ~etting a stream of the sol into the body of the dehydrating liquid either above or below the surface thereof, for example, with a hypodermic needle. The dehydrating liquid is preferably agitated ;~
by stirring or swirling during the addition of the sol thereto. hfter addition Or all of the sol to the dehy-drating llquld, the mixture can be stirred further, for example, for 20 to 30 minutes, until the resultant spheri-cal particles Or the dlspersion are sufficiently dehy-drated and firm. The spherical particles can be separated from the dehydratlng liquld, for example, by filtering or by centrlfuging, and allowed to dry further in alr, (like the green flbers described above) at ambient room temperatures or higher, for example, 60 to 80C,, to a æolids content Or about 60 to 80 weight percent.
The particles can then be fired to convert them into hard rerractory particles in the same manner described above for refractory fibers, e.g., fired in air at about 600C. The particles in the green form or their fired form will generally be water clear, transparent and spherical under an optical microscope, and they can also be internally colored in the same way as described , ~' . ' lV3~1~5 for the colored fibers by addlng various water-soluble metal salts to the initlal precursor llquld. Generally, the green and the fired spherlcal particles wlll 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 agitatlon glvlng smaller spheres. The spheres will be solld and can be screen-classlfled to obtaln fractlon with deslred dlameters.
The crystallographlc ldentity of the mlcrospheres wlll be the same as that described above for fibers flred under the same condltlons.
Another technlque for maklng green spherlcal partlcles ls to spray-dry the precursor sol ln a dllute or concentrated, nonviscous form. Atomlzlng of the precursor llquld can be carrled out, for example, with pressure nozzles, the droplets or spheres as made de-scending ln a countercurrent of dry alr at amblent room temperature or ln a flowing stream of warm air.
In describing refractory products of this ln-- 20 ventlon as "transparent", this term means that they have the property of transmittlng rays of vislble llght.
In the case of a transparent flber, bodles beneath and contlguous to the flber can be clearly seen through lt, the outllne, periphery or edges Or contlguous bodies beneath the flber being sharply discernlble. In the case of microspheres!, transparency thereof is indlcated by the ablllty of the mlcrospheres to functlon as the optlcal component in reflectlve sheetlng made, for example, ln accordance wlth U.S. Patents 2,407,680 or 30 2,326,634.

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"Opaque" articles, on the other hand, are those which are - impervious to light, e.g., the bodles or substrate be-neath an opaque fiber are obscured and cannot be seen therethrough. The "translucent" artlcles are those which fall between transparent and opaque and though translucent artlcles have the property of transmittlng light to some degree, and therefore are somewhat or partly transparent, bodies beneath cannot be seen in a clearly distinguishable or sharp manner. Sometimes, because of vagaries in firlng, an article or product may be a mlxture of these various types of products, though generally one will be present - in a predominant amount, indlcative of the true nature Or the mixture, the other products present in mlnor amounts havlng their particular appearance due to nonuniform firing conditions or due to localized overheating because of hot spots in the furnace or undesirable combustion.
Articles of this invention are preferably those refractory articles containing anatase titanium dioxide which are transparent, though for some particular appli-cations, for example, where the article is used as arelnfGrcement for composltes, transparency may not be lmportant. The transparent quallty of a refractory pro-duct of thls lnventlon ls colncldent wlth other deslrable propertles, such as strength and flexlblllty, and thus transparency can be consldered ln a sense as a gross measure of the quallty Or the refractory product. In some appllcatlons of the refractory products of thls lnventlon, e.g., where a flber or bundle of flbers are used ln flber optlcs or where mlcrospheres are used ln reflectlve slgn surfaces, transparency wlll be Or speclal importance.

.~ -- 19 --10~135 The refractory fibers of this invention are particularly useful in fabricating woven, felted, knitted, and other types of textiles such as braids. Such textiles generally will have the same properties, such as high strength, flexibility, refractoriness, and chemical resistance, as the fibers from which they are made. The refractory fibers colored with additlve metal or metal oxldes wlll find a partlcularly useful application in decorative fabrics, such as used in wall coverings.
Fibers or yarns of thls invention of different colors and/or compositions can be used together in making fabrics with decorative designs. Fibers or ~arns Or this lnven-tion can be plied or interwoven with fibers of other materials, such as metal fibers, silica fibers, carbon, graphite, polytetrafluoroethylene or fiberglass, if desired. Woven cloths made from the refractory fibers can be firmly bonded as wall covering to various sub-strates. For example, such cloths can be bonded with molten glass, or refractory cements such as zircon, alumlnum oxide, phosphates, and sllicates to aluminum or other metal substrates and used as the lnterlor wall coverings of airplanes. The woven cloths (or mats) can also be used as layups in plastlc, metal or ceramlc laminates. The flbers can be also bonded wlth such cements, -as well as colloidal sllica, to form flexible ceramic papers or mats useful as thermal insulation or preforms for relnforced resin composites.
The refractory flbers Or this lnventlon can be used in the form of fabrics, mats and batting as light-weight acoustlcal or thermal insulatlon for hlgh temperature '. ' , 103~135 equipment, such as resistance and induction furnaces, and for purpose of heat shielding or reflecting, such as heating mantles and thermal curtains.
In their porous form, the refractory fibers are useful in filtering or absorption applications, for example, a filter to remove solids from hot gases, as a chromato-graphic column packing to selectively separate or resolve liquids or gases, or as catalyst supports.
Another particularly useful application for the refractory products of this invention is that of reinforcement for structural plastic, elastomeric, metal lic or ceramic composites especially those composites used in high temperature environments found in the aero-space industry and in ablative environments. As composite reinforcement, the refractory products of this invention are preferably used in the form of fibers (either in continuous or staple form), though other particulate forms, such as microspheres, aggregates and powders can be used for such purposes. 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 "Modern 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 resins, acetal resins, acrylics~
especially methyl methacrylate polymers, amino resins, especially urea-formaldehyde and melamine-formaldehyde, alkyds, cellulosics, especially ethyl cellulose~ cellulose

-2~-1()3~ S
acetate, and cellulose proprianate, r luorocarbons, furanes, polyurethanes, phenolics, polyamides, polycarbonates, vinyl aromatlcs such as styrene, polyoleflns, especially polyethylene and the like. The refractory products of this invention can be made with a wide useful range of indices Or 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 ma~or reason being the lack of reinforcement materials wh~ch wlll withstand the elevated temperatures encountered in processing, e.g., casting and sintering temperatures. The refractory pro-ducts of this invention, because of their thermal stability, strength, flexibility and other properties are useful as reinforcements, particularly, in their fiber form, 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 "Flber-Strengthened Metallic Composites", ASTEM
Spc. Tech. Pub. No. 427, published by the American Society for Testlng and Materlals, Phlladelphia, Pa. (1967).
The refractory products of thls invention can also be used as reinforcement for ceramlc composltes, such as sllica, glass, alumlnum sllicate and other inor-ganic materials such relnforced ceramics being in the form of blocks, paper and other shaped articles used in ~ . . : , :

1~3t~135 high temperature environments.
The refractory products of this invention can also be used as reinforcing agents (especial~y as fibers or in particulate form) 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.
- The ob~ects and advantages of this invention are further illustrated in the following examples. In these examples~ all parts are by weight unless otherwise noted, the viscosities recited are Brookfield viscosities measured at ambient room temperature. In the examples, the firing of green articles and the firing of amorphous refractory articles to higher temperatures were all carried out by flring in alr in an electric resistance furnace unless otherwise noted.

Esmple 1 A sol was made by slowly adding tetraisopropyl tltanate to 37% concentrated hydrochloric acid in the weight ratio of 5 parts tetraisopropyl titanate to 1 part acid. The aqueous resulting mixture was con^entrated ir~ a "Rotavapor" flssk evacuated by water aspiration to a vacuum of 380-710 Torr. The resulting concentrate was a clear sol havlng a viscosity of about 50,000 cps and an equivalent TiO2 solids content of 36.2 wt.~. me concen-trate was allowed to stand at room temperature for 16 hrs.
to eliminate bubbles. The sol was extruded into ambient air (22C.) through a gold-platinum spinnerette having ~3--- - . .. ~. ~

10~813S
six holes (o.o76 mm. diameter). The green fibers as spun were essentially continuous, smooth, round, flexible, shiny, clear and transparent and were drawn in alr and continuously wound on a variable speed takeup drum covered with polyester film. m e green fibers so spun were essen-tially dry such that they did not stick together. The fibers were removed from the drum as a bundle and fired in air in that form in an electric furnace at 570C. for 15 min. The refractory fibers so formed were continuous, smooth, round, flexible, transparent and had an average -~
diameter of about 12 microns and an average tensile - strength of about 6,540 kg/cm2. The titanium dioxide of the fibers was polycrystalline and in the anatase form.
Example 2 Five parts tetraisopropyl titanate was added slowly to 1 part 37% conc. HCl cooled in a water bath.
The resulting sol was dried in air at room temperature to a gel containing approximately 63 wt.% TiO2 determined by calcinatlon of a sample. A clear sol was made by adding 251 parts of the gel to 1,000 parts water and stirring. The 801 was filtered through No. 54 "Whatman"
filter paper yielding a light yellow sol having a pH of less than 1Ø The filtered sol was concentrated using a water-aspirated "Rotavapor" flask (under a vacuum of 710 Torr) to formla clear, yellowish concentrate having a viscosity of 140,000 cps.
A portion of the concentrated sol (452 grams) was extruded under 39 kg/cm2 through a 30-hole gold-platinum spinnerett (with o.o76 mm. diameter holes).

.. . . . ..

-:: , - . . ..
, : : - ,, . : :

~03813S
The fibers were spun into ambient air (about 22C.) and drawn for abou~ m. by and wound on a 61 cm. diameter drum at a linear rate of about 60 m./min. The green fibers were cut and removed from the drum as a bundle 5 about 1.8 m. in length. ~le green fibers were continuous, round, smooth, glossy, clear and transparent.
The green fibers were draped over a sllica - rod placed in an air atmosphere electric kiln and fired from room temperature to 550C. in 3 hrs. and held at 550C. for an additional 30 min.
- The resulting fibers were continuous, flexible, strong, smooth, shiny, clear, transparent, round (average diameter 15 microns) and essentially colorless when ob-served at 48 magnificiations with a stereoscopic micro-15 scope. The titanium dioxide in the fibers was polycrystal-- line (crystallite size predominantly in the range of 130-560 angstroms) and in the anatase form. The density of the fibers was 3.81 g/cc, which is essentially the theoretical density of anatase titanium dioxide. Mod-llus 20 of elasticity values of several fibers ranged from about 1.05 x 106 kg/cm2 to 2.8 x 106 kg/cm2.
FI~. 2 shows the nature of the refractory fibers when they are immersed in microscope oil having a refrac-tive index of 1.515. Fibers 12 are clear and transparent, 25 the boundaries or outline of the underlying fiber being visible through the overlaying fibers at their inter-sections 13. The outline of the underlying fibers is sharp and clear although the outline of the underlying fibers may be diffracted. When 550C.-fired fibers 30 are fired to the high tempera'ures of 800C., and the -2g-- anatase titanium dloxide transformed to rutile titanium dioxide, they turn opaque as shown in FIG. 3 where the 800C.-fired flbers 16 do not transmlt llght and under-lylng fibers are not vlslble through the top flbers.
A relnforced resin composite was made comprlsing 48 vol.~ of the above-described 550C-fired fibers and 52 vol.% epoxy novolak resin. The fibers used in making the composlte were sized by coating them, as they were oriented in straight parallel layers with a mixture of 4.75 g. hexamethoxymethylmelamine ("Cymel" 301), 1.5 g.
"PEI" 12 (polyethylene imine), 4.3 g. tetraisopropyl titanate, 225 g. dlacetone alcohol, and 9 drops of con-centrated HN03. The coated fibers were dried in alr for two hours and heated overnight at 120C. The layers of sized flbers were cut into strips (2.5 cm. x 10 cm), with the fibers aligned parallel to the length of the strips.
The strips of sized fibers were plied with layers of epoxy novolak resin placed between and on the top and bottom of the outermost strips of sized fibers. The resulting laminate was heated to 177C. ln a steel die and pressed to 17.6 kg/cm , these conditions being maintained ~or 1 hour. The laminate was then maintained overnight at 138C. The resulting composite (1 mm. thick) was found to have a cross-bending modulus of elasticity of 0.67 x 10 kg/cm2.
Example 3 A titania sol was made by adding 480 g. tetra-isopropyl titanate cautiously into 100 g conc. (37%) HCl with stirring. This sol was refrigerated at about .
5C overnight, removed and allowed to return to room . . ', , : " ' ' ' .' ~ . . . ~ , .

11~3~35 temperature. To this sol, 4.53 g. ethyl sillcate, having an equlvalent SiO2 content of 28.8 wt.% was added wlth stirring to dlsperse. The sol mixture w~s concentrated in a "Rotavapor" flask under water aspirator vacuum to a vlscosity of 114,000 cps. and centrifuged to remove air bubbles. Fibers were formed by extruding the con-centrate at a pressure of about 14-21 kg/cm2 through a 30-hole gold-platinum spinnerette having 0.076 mm.
dlameter holes. They were drawn by and collected contin-uously on a 15 cm. diameter drum located about 1 m.
below the spinnerette. The coil of green flbers was removed and flred ln air from room temperature to 500C.
over a perlod of about 45 minO The fired polycrystalline anatase Tio2 flbers were transparent, clear, shiny, round and contlnuous. The fiber composition was calculated to be 99 wt.% TlO2 and 1 wt.% SiO2.

Example 4 Forty g. of a gel containing the equlvalent of 62 wt.% TiO2, made according to the procedure described in Example 2, were dlspersed ln 200 g. water. To this sol, 0.75 g. of chromlum acetate, containing the equiva-lent of 34 wt.% Cr2O3 was added, the equlvalent solids in the mixture being present ln the ratlo of 99 parts TlO2 to 1 part Cr203. The mlxture was stirred for 1 hr., flltered through a 0.5 mlcron "Mllllpore" fllter and concentrated in a 250 ml. "Rotavapor" rlask, rotated in a water bath at 25-40C. under water aspirator vacuum.
A clear, dark green concentrate resulted which was per-mltted to slt overnlght. Vlscosity lncreased markedly ~ 0;~8135 and 10 g. H20 was added and blended with the concentrate by rotation of the "Rotavapor" flask without evacuation until homogenized. The diluted mixture was concentrated again by evacuation to a fiberizablé mass extruded through a 6-hole gold-platinum spinnerette with o.o76 mm. diameter holes. The fibers were drawn by and collected on a 15 cm.
diameter drum located about 1 m. below the spinnerétte.
The dry green flbers had a greenish tint and were clear.
m e dry coil of flbers was cut and removed from the drum. One bunch o~ the flbers was inserted and suspended in an alr atmosphere furnace at lOO-C. and heated up to 600C. in 45 min. The fired flbers of polycrystalline anatase TiO2 and Cr203 were-shiny, coppery-bronze to the unaided eye, amber under the mlcroscope, clear and trans-parent. Fiber diameters were mostly about 15 to 20 microns.Tensll strength of the 600C.-fired fibers averaged about 4600 kg/cm2.
Part of the fibers which had been flred to 600-C.were further fired in air to 800C. These further fired fibers were opa~ue and greenish-gray ln color, the -TiO2 being ln the rutile form.
Example 5 A titania sol was made by dispersing 150 g. of gel containlng 62.4 wt.~ TiO2 equivalent, made acc~ording to t~e procedure described in Example 2, into 1200 ~.
water. A chromic ~cid solution, made by dissolving 11.2 g. CrO3 in 30 g. water,was stirred together with the titanla sol and the mixture flltered through a #54 "Whatman"filter paper. The resulting ~lltered sol was concentrated in a "Rotavapor" flask with water aspirator vacuum to a clear, -2~-~:

1(~38~3S
orange-red color and an estimated viscosity of about 60,000 cps. This concentrate was extruded through a 30-hole gold-platinum spinnerette with o.76 mm. diameter holes at a pressure of about 32 kg/cm2 and the extruded filaments were drawn by and accumulated on a 61 cm. di-ameter reel at a linear rate of 76 m./min. The reel was loc&ted about 1.8 m. below the spinnerette.
The coil of fibers was cut into a single bundle length of about 1.8 m. and this bundle divided into seven different tows or bundles which were then draped over a horizontal silica rod and dried at 50C.
overnight. Fiber bundles were fired from 50C. to various higher temperatures and samples removed at 300, 400, 500, 600, 700, 800 and 1000C. and their appearance noted. The following table summarlzes the results.
TARTF I
Time Lapse Appearance From 8tart Visual With Stereo-Firin~ at Appearance Scopic Micro-Temp.~C. 50C., min. to Eye Scope at 140X
0 yellow color ---200 40 reddish brown ---300 75 reddish purple transp.*, clear-dark red 400 140 purple transp., clear-red purple 500 180 darker purple transp., clear-dark red 600 255 ! purple transp., clear-dark red 700 315 grey-green transp., clear-red-green ~.0;~13S
TAELE I (cont.) Time Lapse Appearance From Start Visual With Stereo- ~
Firin~ at Appearance Scopic Micro- -Temp.~C. 50C., min. to Eye Scope at 140X
800 350 duller than mostly opaque at 700C. and grainy 1000 390 dull green opaque and grainy * "transp." means transparent The composition of the fired titanium dioxide-chromia fibers was calculated to be 91.7 wt. % TiO2 and 8.3 wt.% Cr203.
E~ample 6 A titania sol was made by dispersing in 200 g.
water 40 e. of gel prepared according to the procedure of Example 2 with an equivalent TiO2 content of 61.5 wt.
To this sol, 0.75 g. chromlum acetate having 34 wt.%
equivalent Cr203 content and 1.0 g. of gamma-aminopropyl- `
triethoxysilane (Union Carbide A-llO0) with an equivalent of 26.8 wt.~ SiO2 were added and stirred until dispersed.
me resultant dispersion was filtered through ~ 0.5 micron filter and a clear, green mixture resulted. This liquid was concentrated in a "Rotavapor" flask under water as-plrator vacuum to a viscosity of about lO0,000 cps, the resultant concentrate having an equivalent total oxide solids content oflabout 54.4 wt.~. Fibers were readily -formed by inserting and withdrawing a glass rod. me concentrate was extruded at 17.6 kg/cm2 pressure through a gold-platinum spinnerette having o.o76 mm. diameter holes and the extruded fibers were continuously drawn by and - ` . , ~0;~E~135 collected on a 15.24 cm. diameter drum located 1 m. below the spinnerette at a rate of 21 linear m./min. Several coil bundles were collected and removed from the drum.
Separate samples of the dry fibers were fired by suspending in an air-atmosphere furnance and firlng to 350, 700, 750, 800, 850 and 900C. About 1 to 1-1/4 hrs. were required to reach 700C. The fired composition of the fibers was calculated to be equivalent to 98 wt.%
TiO2, 1 wt.% S102 and 1 wt.~ Cr203. Results are shown in the following table, the densities of the 700, 750 and 850C.-fired fibers being 3.18, 3.40 and 3.52 g/cm3, respectively.
TABLE II

Modulus Or Elas-15 Firing X-ray Dif- ticlty Temp. Visual ~raction Tensile kg/c~2 C. Appearance Analysis*** kg/cm2* x 10 350 clear, gold --- 1550 o.48 color, transp., shiny, contin-uous 700 clear, copper- anatase, 6000 0.99 gold color, ~9OOA
transp., shiny contin-uous 750 same as 700 anata~e, 4360 0.93 fibers <lOOOA

800 same as 700 anatase 5100 1.38 flbers C lOOOA

850 same as 7~0 anatase 7380 1.42 flbers but < lOOOA
darker and clear amber color 900 shiny, brown, predomi- 8300 ---translucent, nantly stiff anatase**

- - ' ~38~35 TABLE F (cont) Modulus - of Elas-Firing X-ray Dif- tic~ty;- - -Temp. Vi~ual fraction Tensile kg/c~Z
C. Appearance Analysis*** kg/cm2* X 10 1080 shiny, brown, rutile Rtiff, opaque ~lOOOA

* Fiber test specimens were 15-17 ~1 in diameter and 2.54 cm in length.
*~ The relative intensity of anatase was 100 and rutile was 5.
*** Crystallite size is es$imated by line broadening.
Example 7 ~
A titania sol was made by dispersing 30 g. of ~ - -a gel made according to Example 2 with an equivalent of 61.5 wt.% TiO2, into 60 g. of anhydrous methyl alcohol.
Thls 801 was mixed with an alumina sol made by dlspersing 7.5 g. alpha-alu~lna monohydrate ("Dispaln-M) into 92.5 g. water and about 0.75 gO con. Hcl (37~). Forty g. of -white syrup (nKAR0") were added to the mixed sols to asslst in fiber forming. The dispersion was concentrated ~-ln a "Rotavapor" flask to a fiberizable mass (determined by withdrawing glass rod) and extruded at a pressure of 14 kg/cm2 through a gold-platinum spinnerette with six ~ -holes (o.o76 mm. diameter). ~e extruded fibers were drawn downwar~ by and collected on a 15 cm. diameter drum. A heat lamp was used to dry fibers before collection on the drum. m e dry coil was removed and fired in air to 550C. me fired fibers were predominantly clear, trans-parent, and ~hiny and the only polycrystalline species found by X-ray analysis was anatase TiO2 with a crystallite s size estimated to be less than lOOOA. The composition of these fired fibers was calculated to be about 76.o wt.% TiO2 and 24.0 wt.% A1203.

Examp e 8 A TiO2 sol was made by dispersing 15 g. dry gel (made as described in Example 2) in 60 g~ water.
Microspheres were formed by injecting the sol with a hypodermic syringe into 3500 ml. of 2-ethyi-1-hexanol agitated by a laboratorystirrer at 1200 rpm. The dispersion of microEpheres was stirred for 20 min., filtered through No. 54 "Whatman" 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 micro-spheres were measured. Control portlons of the micro-spheres were fired without soaking to determine changesin refractive index due to the soaking treatment and firlng temperature. The results are tabulated in Table III. Some of the microspheres were placed in the fur-nace at the desired firlng temperature and others were placed ln the furnace at room temperature (or, in one run, at 300C) and the temperature ralsed to the deslred temperature. The aqueous solutions or sols of metal compounds, used for soaklng the mlcrospheres, contalned the equlvalent of 5 wt.% of metal oxide. Where the soaking media were PbN03 so ution, BaC12 solution, and a Zr2 sol and the microspheres soaked then fired from room temperature to 640, 680 and 700C., respectively, the fired microspheres were fo~nd to contain o.6, 0 15 5 and 0.3 wt.~ of Pb, Ba and Zr, respectively, as deter-mined by spectrographic analysis.
TABLE III
Fired Microspheres Stereoscopic Metal Firing Duration Microscope Refractive Compd. Temp.C. of Firing 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, transp. 2.42 NONE RT*-640 3 hr. very clear, transp. 2.59 NONE RT-720 4-1/2 hr. mostly clear, 2.625-transp.** 2~65 PbN03 525 15 minO very clear, transp. 2.37 PbN03 580 20 min. clear, transp. 2.41 PbN03 580 60 min. clear, transp. 2.45 PbN03 640 15 min. mostly clear, transpO 2.42 PbN03 RT-640 3 hr. very clear, transp. 2.525 PbN03 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, transp.** 2.515 BaC12 RT-680 4 hr. very clear, transp. 2.65 BaC12 RT-720 8 hr. diffuse 2.65 TABLE III (cont) Fired Microspheres Stereoscopic Metal Firing Duration Microscope Refractive Compd. Temp.C. of Firing Appearance Index Zr2 100-550 2 hr. very clear, sol transp. 2.39 Zr2 620 20 min very clear, sol transp. 2.40 ZrO2 RT-630 4 hr. very clear, sol transp. 2.525 Zr2 RT-700 3-1/2 hr. clear, transp. 2.65 sol Zr2 RT-780 2 hr. cloudy ---15 * ''RTI' means firing started at room temperature.
** These fired mlcrospheres were mostly clear and transparent but the ~ired product contained so~e microspheres larger than 50 microns whlch were slightly diffuse.
These data indicate that the refractive index is 20 somewhat dependent upon the addit've used for soaking the microspheres prior to firing but appears to be more dependent on the firing temperature.
Example 9 A sol was made by adding 5 parts tetraisopropyl titanate to 1 part 37% concéntrated Hcl. The sol was 25 coated on a polyester film and allowed to dry in air at room temperature. Ten g. of the dry gel, in the 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 30 was calculated to contain the equivalent of 65 wt.%
TiO2 and 35 wt.% SiO2. Four g. of this mixed sol were diluted with 4 g. water and the diluted sol was poured ..

into 500 ml. 2-ethyl-1-hexanol which was belng rapldly stlrred. The resultlng dispersion of microspheres was filtered and the recovered microspheres were dired in alr at room temperature. m e dried, transparent micro-spheres were flred in air from room temperature to 530C.and held at the latter temperature for 15 min. m e re-sultant fired solid microspheres were fairly c~ear, transparent, polycrystalline with the titanium dloxide in the anatase form and had a refractive index of 1.815.
After firing in air further from room temperature to 630C. and holding at 630C for 30 minO~ the microspheres ~ere 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 alr from room temperature to 730C. and held at that temperature for 30 min.; these rired micro~pheres were also transparent, fairly clear and had an index of re-fraction of 1.815, the titanium dioxide also belng present in the anatase form.
Example 10 About 2.5 g. Or a dry gel containing the equiYa-lent Or 61 wt.% TiO2 and made accordlng to the procedure o~ Example 2, were disper~ed into 9.24 g. Or an aqueous colloidal silica s~ol ("~udox" SM) 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 dlspersion of solid microspheres was filtered and the recovered microspheres ~. ', ' ' ;"" ' ~
- . . .

lQ3~3S
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 5 temperature for 30 min. The resultant fired solid micro-spheres were transparent, slightly cloudy and had an index of refraction of 1.75 to 1.76 and they had a cal-culated composition of 52.5 wt.~ TiO2 and 47.5 wt.%
SiO2 with the titanium dioxide being in the anatase form.
Example_ll Forty g. of TiO2 gel (containing the equivalent of 24 g. TiO2), prepared by the procedure described in Example 2, were dispersed in about 80 g. water. To this sol, 5.65 cc. of aqueous ferric nitrate solution (equiva-lent to 1.2 g. Fe203)was added. The resulting dispersion was concentrated for several hours in a "Rotavapor" flask under water aspirator vacuum to a fiberizable mass determlned by inserting and withdrawing a glass rod. The concentrated sol was extruded through a six-hole die (with o.o76 mm.
diameter holes) at a pressure of 4.9-5.6 kg/cm2. The extruded fibers were continuously drawn on a 15 cm. di-ameter wheel at the linear rate of lô to 37 m./min.
The green fibers were removed from the wheel.
One bundle of the fibers was fired in air from RT to 500C. and another bundle of fibers fired in air from RT to 550C. The!fibers were calculated to contain 95.2 wt,% TiO2 and 4.8 wt.% Fe203. The fibers of both fired bundles were continuous, round, shiny, transparen~, clear, and gold in color (the fibers fired to 550c. being a darker gold color). Some of the fibers were further IU3bl;~5 flred ln Eir to 750C. and the resulting flbers were red-brown in color and opaque. The titanium diox~de in the 550C-fired fibers was in the anatase form and that in the 750C-flred fibers was in the rutile form.
A sol was formed in a similar manner containing the equivalent Or 40 wt.~ TiO2. To 10 ml. of this con-centrate, containing theequlvalent of 5.6 g. TiO2, 1 g.
of Co(N03)2 6H20 dissolved in a small amount of water was added. The resultant viscous concentrate was ex-truded at 4.9-5.6 kg/cm2 through a die with six 0.076 mm. holes and the fibers drawn at about 27-30 m./min.
One bundle of the resulting fibers was fired at 350C.
for 1/2 hr. and then refired at 500C. for 1/2 hr. The resulting fibers were continuous, round, shiny, transparent and green in color with the T102 ln the anatase form.
Another bundle of the flbers flred from room temperature to 500C. produced slmllar flbers which were ollve green ln color and about 15 mlcrons in dlameter. Another bundle fired to 750C. produced fibers which were opaque and ocean-green ln color, the T102 content belng ln the rutlle form.
Example 12 A bundle of the 750C-rired T102-Fe~03 flbers of Example 11 was flred ln a hydrogen atmosphere to 700C.
and held at 700C for about 1/2 hr. and then cooled ln the furnace to about 200C. ln the hydrogen atmosphere, and further cooled to room temperature ln nltrogen. The resultant rlbers were opaque, shiny black, stifr, magnetic and sllghtly electrlcally conductlve, the rlbers belng composed predominantly Or rutile TiO2 with a small amount ',:, , ~ . -, ~ ~03813S
Or alpha iron being present~
Example 13 Green, porous, transparent titanium dioxide micro-spheres were formed (using the procedure of Example 8) from a TiO2 sol (made from a tltania gel as in Example 2) that contained about 30 wto% TiO2. me resl~lting green microspheres were ~ired in air at three different temperatures and the surface areas measured using the B T method. m e data are given in Table IV.
TA~LE IV
Firin~ Firing Surface Area Temp. C. Time (hr.) (m?/g) - 310 2 133.0 370 2 68.6 - Example 14 Si~ty-five g. Or a TiO2 gel, made according to the procedure of Example 2 were dispersed in 260 g. water to form h TiO2 sol, which was filtered through a 0.25 A micron nm~ p~e ~I filter. Sixty g. of the ~ilteréd 801 waB poured slowly into the vortex formed in 3700 ml.
Or 2-ethyl-1-hexanol being stirred at 1240 rpm. in a 4-liter beaker. Stirring was contlnued ~or 20-25 min.
The dispersion of microspheres was filtered and the recovered microspheres were dried overnight at 80C.
The dried green ~ crospheres were fired in air from raom 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 refraction of 2.32. m e :

. -. ~

S
The TiO2 content of these rired microspheres was poly-crystalllne anatase. FIG. 1 illustrates what these flred microspheres look llke under a light microscope, where they are deslgnated by reference number 10 (reference number 11 denoting light spots caused by reflection ~rom the lllumination source on the surface Or the microspheres and the shadlng shown by stippling).
Example 15 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 o~
the temperature due to the exothermic reaction. A clear yellow liquid resulted. To 481 g. of the solution, 350 ml.
Or water and 350 ml. of ammonium hydroxide (28-30% NH3) was added with agitation, forming a thick white precipi-tate. Water was added to disperse the precipitate whlch was rlltered Orr and washed repeatedly with water. The preclpltate was then dispersed ln water. A portlon, 250 g. (containing about 10 wt.% TiO2) Or the dispersion 20 was digested with 45 ml. Or concentrated HCl over a period ~:
of 4 hrs. The resultlng sol was flltered through No. 54 and then No. 50 "Whatman" rllters. The filtered sol was slightly hazy and had a pH Or about 0.5 with a T102 content Or about 8.6 wt.%. Three ml. Or the sol were 25 ln~ected wlth a hypodermlc syrlnge lnto 250 ml. 2-ethyl- -l-hexanol and the dlsperslon Or solld microspheres was stirred as in Example 8. The green, water-clear micro-spheres were recovered by filtration and dried in air at about 80C. After flrlng from room temperature to ' .
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~038135 550Co in about 1 to 1-1/2 hrs., the microspheres were shiny, transparent and clear to slightly hazy. The rlred ~icro-spheres averaged 45-60 mlcrons in diameter and had a refractive index Or 2024.
- Example 16 Transparent and colorless polyester sheeting (OolO mm. thick) is coa~ted with a 30~ solutlon of ~inyli-dene fluoride-perfluoropropene fluorinated elastomer (~Vlton" A) having an index oi rerract~on Or about 1~38 to L 39 when cured to a thlckness Or about 0.015 mm.
ln methylisobutylketone using a wire-wound bar (28 winds pcr cm.) to accomplish this coating. The coating is sllowed to dry to a slightly tacky state and solid tltanium dioxide microspheres, made as described in ~xample 14 (with diameters ranging from 15-30Jl and an index Or refraction Or 2.58)~ are caficade coated onto - and partially embedded in the tacky coating or web as a nolayer, and the assembly is then oven dried in air at 95C~ rOr 15 min. m e exposed surraces Or the par-tially embedded microspheres are then vapor coated with alu~inum to yield a composite sheeting Or the type descrlbed in U. S. Patent No. 2,407,680 the sheeting being retro-rerlective to visible light when covered with water.
Example 17 A glossy polyacrylate treated paper carrier web i8 knire-coated with a non-oxidizing alkyd resin solutlon (an oll-free, baking alkyd soluvion containing adipic acid, maximum acid number Or 15~ said solutlon containing -about 68 wt.~ resin solids and 32 wt.~ aromatic ~olvent) - ~' ' '" :

~` ~038135 to provlde a wet coating o~ about 0.20 mm. thick. The resln i~ cured at 65C. for 10 min. and then 120C.
ror 10 minO A second coating (about 0.05 mm. thick) Or the same alkyd resin solution is knlfe-coated on top Or the flrst coating and allowed to become tacky by evaporatlon of the solvent at room temperature. Solid - tltanium dioxide microspheres made in accordance with Example 14 (havlng diameters ranging from 20-45~ and a reflectiYe index of 2.30) are cascade coated on and partially embedded in the tacky resin a~ a monolayer and the assembly ls then cured in place for 30 min. at 95-C. A xylol-butanol solutlon containing 20 wt.%, polyvinyl butryal (having an index of refractlon of about 1.50-L 53 when dry) was knife-coated directly over the microsphere coating to provlde a final layer of resin - (0.025-0.038 mm. ln thlckness)0 After this flnal layer wa~ drled at 95C~ for 30 min., the resin surface i5 vacuum vapor coated wlth aluminum. The paper carrier web is strlped from the assembly to yleld a composlte sheeting of the type described in U. S. Patent 2,407,680 which ls retro-reflectl~e to vlslble llght when covered with water.
Example 18 A solutlon of a blend of 5 parts by weight of nitrlle rubber (a hlgh acrylonitrlle-butadiene copolymer), 6.6 parts by welght of phenolic resin ("Durez" 14296), 1 part by weight of dioctyl phthalate plasticizer and 2.9 parts by Neight of aluminum powder in 16 parts by weight of methyl isobutyl ketone is knlfe-coated to a ~hickness of about 0.05 mm. onto a glossy pclyacry;ate 103b~i35 treated paper carrler web and the coating allowed to dry to a tacky state. Solld titanium dioxide microspheres made in accordance with Exa~ple 14 (the ma~ority of the microspheres havlng a diameter of about 20-75~ and an lndex of refractlon of 2. 54) are cascade coated onto and partlally embedded into the tacky resin and the assembly is cured in a 120Co over for 20 mln. The resulting sheetlng is retro-reflective when covered with water and is of the type described in U. S. Patent No.
2~326~634~
The following terms, used herein above, are trademarks: "Cellosolve", "Ludox", "Rotavapor", "Halocarbon", "ANTIFOAM A SPRAYn, "Cymeln, "PEI", "Millipore", "Whatman", "Dispal", "KARO", "Vitonn, and "Durez".

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Claims (19)

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

1. A transparent, solid, shaped and fired, homo-geneous, continuous refractory fiber comprising predominantly polycrystalline titanium dioxide in its anatase form.

2. A solid, shaped and fired, homogeneous, trans-parent, continuous refractory fiber comprising predominantly polycrystalline titanium dioxide in its anatase form, and one or more other metal oxides.

3. The fiber of Claim 2, wherein said other metal oxides are chromium oxide, aluminum oxide, iron oxide, cobalt oxide and silica.

4. The fiber of Claim 2, wherein said other metal oxide is aluminum oxide.

5. The fiber of Claim 2, wherein said other metal oxide is chromium oxide.

6. The fiber of Claim 2, wherein said other metal oxide is iron oxide.

7. The fiber of Claim 2, wherein said other metal oxide is cobalt oxide.

8. The fiber of Claim 2, wherein said other metal oxide is silica.

9. The fiber of Claim 2, wherein said other metal oxide is chromium oxide and silica.

10. The fiber of Claim 2, wherein said other metal oxide is present in an amount sufficient to impart a color to said fiber.

11. A method for forming continuous fibers of solid, transparent refractory comprising predominantly polycrystalline titanium dioxide in its anatase form, which method comprises extruding in air an aqueous acidic titanium oxide sol or aqueous acidic mixture of a titanium compound calcinable in air to titanium dioxide, and heating and firing the resulting amorphous fibers to remove water, acid, organic material, and carbon therefrom and form said continuous fibers.

12. A method according to Claim 11 wherein the material which is extruded to form said amorphous fibers is a viscous, fiberizable concentrate comprising an aqueous titanium dioxide sol prepared by dispersing in water a gel formed by drying an acidified aqueous mixture of tetraalkyl titanate.

13. A method according to Claim 12 wherein said firing is carried out at a temperature up to about 600°C.

14. The method of Claim 12 wherein said tetraalkyl titanate is tetraisopropyl titanate.

15. The method of Claim 12, wherein said aqueous mixture of tetraalkyl titanate further comprises a further metal oxide or metal compound which is calcinable in air to its corresponding metal oxide and the resulting shaped and fired refractory article further comprises said metal oxide.

16. The method of Claim 15 wherein said metal oxide is silica.

17. The method of Claim 15 wherein said metal oxide is alumina.

18. The method of Claim 15 wherein said metal oxide is chromia.

19. The method of Claim 15 wherein said metal oxide is chromia and silica.