CA1274658A - Solid, transparent, non-vitreous, ceramic microspheres - Google Patents

Abstract

Abstract of the Disclosure Solid, transparent, non-vitreous, ceramic microspheres comprising polycrystalline titanium dioxide and having an average particle size greater than 200 micrometers, said microspheres being useful as lens elements

Description

4~5~

40226 CA~J 9A

This application is a divisional of application number 486,94~ filed on July 17th, 1985.
The invention of this divisional application relates to solid, transparent, non-vitreous, ceramic microspheres comprising polycrystalline titanium dioxide and having an averaye particle size greater than 200 micrometers, said microspheres beiny useful as lens elements.
The invention of the parent application relates to solid transparent, non-vitreous, dense, ceramic microspheres comprising at least one crystalline phase comprising at least one metal oxide other than oxides of titanium, barium, beryllium, bismuth and boron, although such oxides of titanium, barium, beryllium bismuth and boron may be present in minor amounts, and having an average particle size of more than 125 micrometers, said microspheres being useful as lens elements.
The pavement marking industry has long desired transparent microspheres that would be useful as brighter and more durable retroreflective elements in pavement markings. The transparent microspheres now most widely used for pavement markings are made of certain glasses, which are amorphous vitreous materials generally of the soda-lime-silicate type which, although having acceptable durability, have a refractive index of only about 1.5, which greatly limits their retroreflective brightness.
Higher-index glass microspheres of improved durability have been taught in U.S. Patent No. 4,367,919, but even a higher degree of durability has been desired.
A transparent microsphere is taught in U.S. Patent No.

~ .~

la 60557-2937D
4022~ CAM 9A

3,709,706. These microspheres are ceramic microsphere.s made hy sol-gel processes from silica and zirconium compounds. Generally, a sol-gel process is one which converts a colloidal dispersion, sol, aquasol or hydrosol of a metal oxide (or precursor thereof) to a gel, which is a form of colloid which restrains the mobility of the components. The gelling step is often followed by drying and then firing to obtain a ceramic material.
Although the ceramic microspheres taught in U.S. Patent No. 3,709,70~ have good durability, they have diameters smaller than desirable in certain pavement marking applications.
Microspheres used in pavement markings generally average between about 150 and 1000 iS8

2 60557-2~37D
4022~ CAM 9A

micrometers in diameter, in order to assure that the light-gathering portion of the microsphere protruding from the pavement marking is not obscured by road dirt. The prior art does not teach how to make ceramic microspheres which are both transparent and large enough for these pavement marking applications.
In addition to being rather large and transparent, in order to function in pavement markings, such microspheres need to be resistant to scratching, chipping, cracking, and fracture under the conditions to which they are subjected on the road.
Disclosure of Invention According to the invention of this divisional application, there is provided solid, transparent, non-vitreous, ceramic microspheres comprising polycrystalline titanium dioxide and having an average particle size greater than 200 micrometers, said microspheres being useful as lens elements.
The parent invention provides new large transparent, solid ceramic particles, beads or microspheres which can be made with sufficient clarity, index of refraction, and other properties to make them useful as superior lens elements in retroreflective pavement markingsO The new ceramic microspheres offer a combination of retroreflective brightness and durability that, insofar as is known, has not before been available in a pavement marking lens element. These new particles may be summarized as:
solid, transparent, non-vitreous, ceramic microspheres useful as lens elements in retroreflective pavement markings having at least one metal oxide phase, and an average particle size (minimum particle dimension) of more than 125 micrometers.

~4~8 2a 60557-2~37D
~0226 ~AM 9A

According to one aspect of the parent invention, tnere is provided solid transparent, non-vitreous, 3ense, ceramic microspheres comprising at least one crystalline phase comprising at least one metal oxide other than oxide of titanium, barium, beryllium, bismuth and boron, although such oxides of titanium, barium, berylliuml bismuth and boron may be present in minor amounts, and having an average particle size of more than 125 micrometers, said microspheres being useful as lens elements.
According to another aspect of the parent invention, khere is provided solid, transparent, non-vitreous ceramic mircospheres consisting essentially of silica, having an average particle size of more than 125 micrometers, and being fully dense, said microspheres being useful as lens elements.
According to a still fur-ther aspect of the parent invention there is provided a chemical gelation process for making solid, non-vitreous ceramic particles comprising the steps of:
i) mixing a sol comprising a source of a metal oxide selected from the group consisting of titania, silica, zirconia and mixtures of the foregoing with a gelling agent comprising ammonium acetate;
ii) adding the composition from step i) to a forming fluid in which particles form;
iii) separating the gel particles from the forming agent;

- ~4~
-2b- 557-2937 iv) drying the gel particles from step iii~;
v) firing the particles at temperatures between about 500 and 1200C in order to form Eired, ceramic particles; and vi) allowing the Eired, transparent, ceramic particles from s~c~p v) ~o cooJ..
The term solia means a solid which is not holl~", i.e.
lacking any substantial cavities within the microspheres such as described in U.S. Patent 4,349,456 on ceramic metal oxide micro-capsules.
The term non-vitreous, for purposes of this description, means that the ceramic has not been derived from a melt or mixture of raw materials brought to the liquid state at high temperaturc.
This term is used for the purpose of distinguishing the inventive ceramic

3 605~7-2937D
4022~ CA~1 9A
microspheres over glass beads which are maae by a melt proce g9 .
The term transparent, for purpo0e3 o~ this dl0cussion means that the ceramlc microsphere~ when viewed under an optlcal mlcroscope ~e.g., at lOOX) have the property of tran~m1tting rays of vlsil~le light 80 ~hat bodle~ beneath the microspheres, such as bodies of the ~arne nature as the microspheres can be clearly seen through the micro~pheres, ~hen both ar0 immersed in oil Oe approxi-mately the 3ame refractive index as the micro~pherea.~lthough the oil should have a refractive lndex approxi-matlng that of the microspheres, it ~hould not be so close that the microsphere3 seem to dlsappear (as they would in the case of a perfect index match). The outline, periphery or edges of bodles beneath the micro~phere~ are clearly di~cernible.
The inventlve micro3pheres can be made fully dense. The term fully dense mean~ close to theoretical density and having sub~tantially no open poro3ity detectable by qtandard analytical techniques such as the s.E.T. nitrogen technique (ba~ed upon adsorption of N2 molecules from a gas with which a ~pecimen is contacted).
SUCh measurements yield data on the surface nrea per unlt weight of a sample (e.g. m2/g) which can be compared to the surface area per unit weight for a ma~ of perfect mlcrospheres of the same ~ize to detect open porosity.
Illgher specific surface (m2/g) lndicates higher surface irregularities and/or poroslty. Such measure~ents may be made on a ~uantasorb*apparatu~ made by Quantachrome Corporation of Syos0et, New York. Density mea~urements may be made u0ing an air or water pycnometer.
The microspheres de~cribed herein may be truly ~pherical but may also be oblate or prolate.
The preferred ceram~c microspheres are also generally characterized by: an average hardness greater than ~and, ~hich is an abraslve material oten found on roads; toughne3s, crush resistance, sphericity and *Trade-mark retroreflectivity a~ great or greater than those of conventional glass beads havlng a similar size and a refractive index of about 1.5: and an index of refractlon of between about 1.4 and 2.6. The inventive microsphere~
al~o have fewer internal imperfections and inclu~ions than conventlonal glass beads of a similar size.
The present invention also proviaes sol-gel proce~es for making the inventive ceramic microspheres.
One lmproved sol-gel process i8 a chemical gelation proce~s (ns distingui~hed ~rom a dehydrative gelation proce~s) in whlch gelation of a ~ol i9 induced by chemlcal 3isruption of the ~tability of the sol. For colloidal metal oxide3 in aqueous 9019, such as zirconia and silica, this often entails adjusting the pH to an un~table regime by the addition of a gelling agent. Prior to gelatlon, which may take from a few 3econd~ to ~everal minutes, the composition can be added to a particle forming fluid under agitation to form spheres. The 3ize of the spheres can be controlled by the degree oE aqitation. The re~ulting solid, gelled microspheres are then recovered, dried and fired in an air atmo~phere to convert them lnto the aforesaid ceramic microspheres.
Surprislngly, these techniques provide uncracked ceramic particles in large size ~greater than 125 mlcrometers) with the previously mentioned combination of retroreflect~ve brightnes~ and durability not available from known glass microspheres. The sol-gel process al90 has the advantage of lower proce~ing temperature than glass ~orming proces~e~ and thus le~s energy consumption per unit welght produced.
The ceramic micro~phere~ of this invention are useul not only in pavement marking mater~als but also in other fields such as: peening materials (becau~e of their toughne~s), high temperature ball bearings filler~ and reinforclng agents in ~uch materials as gla~3, refractory materlals, ceramic~, metal matrix materials and polymers.
reflective sheeting and media for attrition mills such as 7 4 ~ ~

-5- 605a7-2937D
40226 CA~7 ~A
sand mlll~. The inventive ceramic microsphere3 can be crushed or otherwise pulverlzed and the partlculate product u~ed ~9 an abra3ive. ~ter helng thllfl reduced in size, the partlcles are no longer spherical but would have irregular ~hapes .

8rief ~
The Figure 1~ a graph of Reflectivity Retention v.s. Number of Pa33es of a sand blast wear te3t for variou3 pavement marking sheet materlal~. Curve~ A-D represent data on the performance of pavement marking 3heet materials slmilar in all resp~cts except ~or ~he followlng difeerence~ ln the 300-500 micrometer diameter retroreflectlve len~ element~ used: Curve A is repre3entative of data for zirconla-silica microspheres;
curve B - 1.5 index of refraction glass beads; curve C - 1.75 refractive index gla33 beaas, and curve D - 1.9 refractive lndex glass bead3.

Detalled Description The ~ollowing list exemplifies ~he metal3 wh$ch form oxides u3eful in maklng the ceramic microspheres of this invention: aluminum, silicon, thorium, tin, titanium, yttrium, zirconium and mixture~ of these oxides with each other and with other additlves. The formulas for the3e oxide3 are exemplified by: A12O31 SiO2, ThO2, SnO2, Tio2~
Y2O3 and ZrO2. Of the3e, the oxides of zirconium, silicon and titanium are preferred.
The following list exQmplifies other metals whose oxides can serve a3 useful materials in admixture with the above mentloned oxlde3: antlmony, barium, beryllium, bl~muth, boron, calclum, lead, magne~ium, strontium, tantalum, zinc, and mixture~ of the foregoing. The3e oxide~ are for the most part colorles3 or only wea~ly colored and are exemplifled by ~aO, BeO, B~2O3~ ~23~ CaO, PbO, Sb2Os, SrO, Ta2Os, MgO, and ZnO.

40226 ~'ALI 9A
All of these metal o~ide~ can be furnishe-3 in the form of aqueous 9019 or solutlon~ of metal oxide precursor~
that are stable in a normal air environment, e.g , 23C and 50~ relative humidity. More information on such metal oxides appear3 ln U.S. Patent 4,349,456 Column 3, line 32 -Column 4, line 5.

The inventlve ceramlc articles which are made with a silica ~ol have an a~orphous sillca phase. Mo~t otller metal oxide~ eorm a polycrystalline or microcrystalline phase. ~here are many u~e~ul combination~
of a polycryatalline metal o~ide phase with an amorphoua phane such n9 ~ilica.
A study of the effect of sllica on zirconla-~llica microspheres has indicated that clarlty i9 lncreased with decreasinq silica colloid particle size.
The size of tha colloidal silica partlcle~ in the starting material can vary, for example from 0.001 to 0.1 micrometer~ in largest dimen~ion. A silica colloid partlcle size of les3 than about 200 ang~troms (0.020 micrometers) is believe~ to yield ceramic mlcrospheres having better transparency.
Wherea3 prlor-art glass microspheres used as retroreflective elements have a generally uniform, continuou~, glassy structure, sub~tantially free of crystallinity (often speci~ied to have less than 5 percent cry~tallinity), microspheres of the invention preferably have a sub~ivided or grainy ~tructural nature, comprlsing a multlplicity of grains such a~ amorphous remnants of colloidal particles from a ~ol used in preparlng the mlcrospheres Oe the inventlon, or cryst~llites. The amorphous gralns may be ~olned to one another (e.g., through covalent bondlng), as ln an amorphou~ 3ilica mlcrosphere of the inven~ion or, they may be joined to themselves and/or crystallite~, a~ in microspheres in which there is an amorphous matrix having crystallite~ di3persed throughout. Also, the crystalllte grains may be joined to ~lf~74~'~5f~
`~o one another, as in micro~pheres comprising crystallites, or they may be joined to themselves and/or amorphous grain~, as in microsphere~ in which crystallites are di~persed in an amorphous matrix.
The grainy nature of microspheres of the invention i~ desired because it ma~es the micro~pheres more to~gh and resistant to fracture. In contrast to the straight l1ne fracture~ that can occur in a continuous glassy structure, fracture in a micro~phere of the invention typically proceea~ in a tortuous path along the boundaries between the grain~, which requires greater energy. An important advantage of micro~phere~ of the lnv~ntion iB their superior toughness.
The term grain ~Jill be used hereinafter a~ a term generic to crystallite3 in crystalline materials and to domain3 or colloidal particles in amorphou~ materials. For beqt reflective brightness, it i8 preferred that the size of grains in the microspheres be no larger than lO00 angstroms (cry~tallites preferably 50-400 angstrom3) to m~nimize the effect of grain boundaries on light transmittance and also to minimize the effect of larger areas on light scattering especially with large difference~
in refractive lndex between different phases (e.g. ZrO2 and SiO2). In order to minimize light scattering, crystallites of a light t~ansmi~sive materlal preferably have a size less than one quarter of the transmitted light. lOO0 Ang~troms ~9 well below one quarter of the average wavelength of visible light which is about S500 angstroms.
By u3ing microsphere~ with the grain size specified above, one obtain~ the toughness and transparency which has long been de~ired in a large bead.
Void~ in microspheres of the invention are de~irably avoided, i.e., by firing the gelled precursors to a den~e 3tate, to improve transparency, to avoid the weakening effect that s~ructural gap3 can cause/ and to avoid ab~orption of moisture or other liquids that can degrade the microspheres, e.g., through freeze-thaw cycles.

1 ~7~5~
_ ._ In the present invention, large particles haYe been fired to a dense state while avoi~ing undesired cry~tal gro~th that woul~ take away needed tran~parency.
From X-ray analy~e~ of zirconia-silica micro~pheres of the invention, it appear~ that the zirconia initially cry.~tallizes in a predominantly pseudo-cubic form which then converts to tetragonal zirconia between about 910 and 1000C. The micro~phere~ become slightly more cry~talline between 1000 and 1100C., and it i8 within this temperature range, when the zirconia is mainly in the tetragonal form, that optimum hardne~s ia achieved. A~ the microspheres are fired to higher temperatures (above 1100C) the cooled 3ample~ ar~ found to con~ain increa~ing amounts of the ~onoclinic modiflcation, and these micro~phere~ dlsplay a 1089 of clarity and hardne~. Al~o, cry~tallite ~ize increases with higher temperature and longer firing times.
In some of the zirconia-~$1ica microspheres, there ha3 been found a relatively thin (e.g. about 10 micrometers thlck on a 200 micrometer diameter microsphere) continuou~ uniform zirconia rich layer at the surface of the micro~phere. Zirconia content at the surface has been found to be up to about 40 mole percent higher than it~
proportion in the center region, and silica content ha~
b~en found 3ubstantially lower in the exterlor region than the average ~ilica content ~or the entire bead.
Silica-containing compo~ition3 of thi~ invention can be for~ed from a two phase sy~tem comprising an aqueou~
colloidal di~persion of silica (i.e., a ~ol or aqua~ol) and an oxygen containing metal compound wh~ch can be calcined to the metal oxide. The colloidal silica i~ typically in a concentration of about 1 to 50 weight percent in the ~ilica 901. A number of colloidal silica sols are available commer~ially having different colloid ~izes, ~ee Surface &
Colloid Science, Vol. 6, ed. Matijevic, E., Wiley Interscience, 1973. Preferred ~ilicas are tho3e which are supplied a~ a dispers~on of amorphou~ ~ilica in an aqueou~

1~7~ J~
_g_ mediurn (such an the Nalcoag~ olloidal ailicas made by ~lalco Chemical Company) ana tho~e which are low in soda concentration and can be acidified by admixture with a ~uitable acid (e.g. Ludox~LS colloidal 3ilica made by e. I.
DuPont de Nemours ~ Co.).
The zlrconium compounds useful in making zirconia-slllca 901 gel ceramic~ can be organic or lnorganic acid water-soluble ~alts, 3uch as the zirconium 9alt9 of aliphatic or acyclic mono or di-carboxylic acids (e.g. formic, acetic, oxalic, citric, tartaric, and lactlc acids). Zirconyl acetate compound~ are partlcularly - useful. For the chemical gelation proce3s, acid deficient inorganic saltY of zirconium are u~eful (e.g. nitrate, chloride, chlorate or sulfate salt~). Colloidal zirconia 9019 are commercially available, for example nitrate stabllized (0.~3 moles nitrate per mole of zirconia marketed by Nyacol, Inc. of Ashland, Ma~sachusetts).
U~eful lnorganic zirconium compounds are zirconium 3ulfate and zirconium oxychloride. See U.S. patent 3,709,106 20 Column 4 line 61 - Column 5 line 5 for further details on zirconia ~ources.
The other metal oxide~ mentioned earlier (e.g.
A1203 or MgO) can be supplied as water soluble 3alt3 such as nitrates, sulfates, halides)oxyhalide~, phosphates, borate~, carbonate~, or sal~s of organic acids (mono- or dl-carboxylic acids, oxoacld~, hydroxy aclds, amino acids or mlxture~ thereof).
In the case of zirconia-silica ceramics, the two major raw materials are usually pre~ent in amounts sufficient to provide equivalent ZrO2~SiO2 mole ratio in an aqueous dispersion in the range of about 10:1 to 1:10, preferable 5:1 to 1:5. As the proportion of the material having the higher index of refraction (ZrO2) i~ increased, the refractive index of the resulting microspheres increases, thu~ allowlng an adju~tment of refractive index to ~uit different purpose~. The refractive index ~' /~` I

difference between the two pha~es may impart a slight tran~lucency.
The di~persion can be prepared by admixing ~
~llica aquasol with an aqueous metal oxide solution under agitation. ~or some starting materials, rever3e order of adaition (i.e. ar3ding the metal oxi~e solution to the silica aqua~ol under agitation) can lead to non-uniform interspersal of the amorphous ~nd cry~talline grains in the 1nal microsphere. The mixture i9 agitated in order to obtain a uniform dispersion without forming a floc or precip~tate and may be filtered to remove extraneous material. The a~ueou~ mixture of colloidal 3ilica ~u~pen~ion and zlrconium compound will generally be relatively dilute (e.g. 15 to 30 weight percent solids).
Gelling may be by a number of techniques, some of whlch are described hereinafter. After the microspheres have been gelled and formed, they are collected (e.g. by filtration) and fired or exposed to high temperature~ in an oxidizing (e.g. ~ir) atmosphere. Firing may be done at temperatures ranging between 500 and 1200C. It is preferred that, in the case o~ zirconia ceramics, most of the zirconia component be in the tetragonal form and thus higher temperatures (950-1100C) are preferred. In the case ot tltania, temperatures no higher than about 650C
are preferred. Above that temperature some of the titania may convert to the rutile crystal form which results in translucency. The lower temperatures yield the anatase form. The addition of other metal oxldes to the titania may alter the preferrea Eiring temperature which can be determined experimentally. In general, higher firing temperature~ also help to achieve microspheres which are fully ~ense. In the fir~n~ proce~s, the unfired ceramic micro~phere~ should be loosely packed in order to obtain a uniform, free flowing fired product.
One variation of the 901 gel proce~s which has been euccessfully u~ed to make large transparent ceramic microspheres of this invention is chemlcal gelation, one ~7~

method of which compri~e~ adding to a metal oxide 901 a gelling agent which alters the p~l of the sol and induce3 gelation by a variety of chemical mechanisms. One variant of this method u8e9 a colloidal zirconia pre~erably stabilize~ with an inorganic anion or an acid deficient z~rconium salt (preferably inorganic) a~ ~he zirconia source and a silica sol as a silica 30urce. These may be mlxed ~nd conc~ntrate(l to obtaln a mixcd ny~tem of the denlred viscoslty. The 801 mixture can be lnduced to gel 0 by r~lslng it~ pll, Eor example by a~ltllnq ammonium acetate.
The chemlcal gelation proce~s enable3 the formation of a maas of di3crete, ~ol~r3, transparent, non-vitreous, fully dense ceramic microspheres, having an average diameter of more than 200 micrometers. The term formed mas~ means a mas~ of micro~phere3 yielded by one batch of the manufacturing process or a random ~ample taken from a continuous manufacturing proce9s which proce~s (in either case, batch or continuou~) doe~ not include a size classification step to eliminate a ma~or portion of the formed microspheres. When made with a zirconia source, these microspheres are filled with zirconia crystallites, no larger than 1000 angstroms in size, as a polycrys~lline phase comingled throughout the spheres with the amorphous ~llica pha~e.
To be ~uccessfully applied to the preparation of ceramic beads, a chemical gelation technique shoula use a gelling agent which: (1) can be thoroughly mixed with the 301 system without produclng localized gelation, (2) qives su~ficient working time to allow for the formation of 3~ sphere~ prior to the onset of gelation: and ~3) leaves no appreciable residue upon combustion which might opacify or deg~a~e the microspheres. The quantity of gellin~ agent to be used is found empirically, one method being to add gelling agent to the sol in small incremental amounts until gelatlon occurs within the desired time.
There is no univer3al particle forming fluid, but mo~t are substantially water immi~cible. Some usable 1~7~ J~3 particle forming fluids are: hydrocarbon~ ~uch a~ hexane and toluene, dichloroethane, carbon tetrachloride, 2-ethylhexanol, n butanol, and trichloroethane. Oils are also useful in this process. The appropriate forming fluid for a particular sol-gel system i8 found by experlmentation. ~enerally, it i.5 pre~erred that the volume of sol be 1 to 10 percent of the volume of forming fluid used.
Ammonium acetate solutions have been ~uccessfully used ~n a chemical gelation process for controllably gelling titania 9013 yielding titan~a ceramic microspheres greater than 200 m~crometers in diameter. The adaitlon of small controlled amount~ of glacial acetic acid to titania 301 B prior to the addltion of ammonium acetate produces a lS material which can be readily and controllably gelled with a high degree of homogeneity. The quantity of acetic acid is roughly on a 1:1 mole ratio to the chloride ion present in the titania 901 (Yee next paragraph for titania sol formation). This, in turn, has resulted in the ability to 20 generate large reflective spherical particles.
The manufacture of titania 8019 and gels i8 described in U.S. Patent 4,166,147 at column 2, line 54-colu~n 4, line 15 and other passages within that patent.
By way of example, a titania 901 may be made by adding 5 25 part~ tetraisopropyl titanate ~lowly to one part of 373 concentrated hydrochloric acid cooled in a water bath.
Water and other volatiles are removed at ambient temperature (20-35~C) uslng evacuation (water aspirator) or evaporation to form a gel whieh contains about 58 to 65 30 weight percent TiO2, 12 to 20 weight percent HCl, 10 to 30 weight percent H20 and a small amount of organic material.
Thls gel 1B redispersed in water to form a clear sol using about ~our part~ water to one part gel.
The hardness of the particles of thi~ invention 35 is typically above 400 Knoop, preferably greater than 500 knoop. Knoop hardness (50 and 100 g. load~) measurements have been made of the inventive ceramic microsphere~ and ~7a,~
.~ _ certain controls. Repre~sntative hardnes~ measurement~ are given in Table 1 below:

Table 1 KnooP
- S Sample Firing Hardness Number Main Constituents Temperature Range Average 1 Zro2-sio2 1100C703-863 797 2 Zr02-siO2 1000C*834-1005~94 3 slo~ 1000C6~5-7g2 726 Control Sam~les 1.5 ND gla~s beads** 770 1.75 ND gla~s beads*~ 602 1.9 ND glass beads*~ 566 road sand 141-955 573 .~and blast 3and 1,117 ~ 50 g. indenter load *~ 150-210 micrometer particle size ND i~ refractive index.

Crush resistance of the inventive microspheres has also been mea~ured on an apparatus the major feature of which is two parallel plates made of very hard, non-deforming ma~erial (e.g., sapphire or tungsten carbide). A single microsphere of known diameter is placed on the lower plate and the upper plate lowered until the microsphere fail~. Crush resistance is the force exerted on the microsphere at failure divided by the cro~-sectional area of the mi~rospheres (~r23. Ten microspheres of a given composition are t2sted and the average re3ult is reportea as the crush resistance for the compo~ition.
Cru~h resiYtance of the inventive microsphereq has been measured at about 175,000 to 230,000 psi ~1200 - 1600 mega-- Pascals). Glasses typically have a crush resistance of about 50,000 to 75,000 psi ~350-525 megaPascals).

The inventions of this and the parent applic~tion~, will be ~urther clarified by a consideration of the follo-~/ing examples which are intended to be purely exemplary.
Example I
30 g. of titanla gel was mixed in 60 g. of water.
Three grams of carbon black were added to remove organlc impuritle3 and tlle mixture waB filtered tllrough a 1.5 micrometer Millipore~ fllter. To 20 g oE the a~ove nol ~an added 3 9. Oe glacial acet~c acla, followed by ~.6 g. of a solutlon ~on~iYting of one part by weight ammon~um acetate dissolved in two parts by weight of water. The resultlng mlxture wa~ then added immefliately to 300 q. of 2-ethylhexanol under agitation (3-blade propellor mixer at about 300 rpm~ in a 500 ml. beaker. The rsAultln~ mixture was st~rred for 30 minute~ and filtered to separate the gel particles from the alcohol. Tr~nsparent, hard ~pheres were reco~ered and observed to be up to about 1 mm. in diameter.
The~e partlcles were driea at room temperature and at 90DC.
After heating to 500C, the particles were ob3erved to be very clear to somewhat transluscent, depending on ~ize and were up to 500 micrometers in diameter.
A portion of the microsphere3 made in this example waa heated to 550C for fifteen minute~ and cooled.
Whe~ placed in a shallow aluminum pan and covered with water, the.~e particles were observed to ~hine brightly when viewed wltll a flashl~ght beam from a side angle. ~ secon~
sample of these microspheres were mounted on a white vinyl tape surface, and when thl~ surface was coated with water, the particles were observed to shine brightly when viewed w~th a fla~hlight beam from a wide range of viewing angles.
Another sample heated to 625C for 30 mlnutes was found to have a hardness of 536 to ~40 Knoop (650 average). SurEace area measurement~ showed the s~mple to be close to fully dense.

~x7a~ ;8 Example II
~ nitrate stabilized zirconium oxide 901 containing about 20% ZrO2 by weight and about 0.83 M No3 per mole ZrO2 (obtained from Nyacol Product~ Compan-y~ was ~on exchan~,ed with an anion exchange re~in (Amberlyst~A-21 resin made by Rohm and llaas Company) at a ratio of about 15 g. sf resin to 100 9. of the 901. One gram of the ion exchanged zirconia ~ol wa~ added to 20 g. of a filtered TiO2 gel-wa~er mixture prepared as in Example I. The molar ratio of the two oxides was approximately 97 percent TiO2 and 3 percsnt ZrO2. Three grams of glacial acetic acid wa~
added to the above 901 mixture with agitation. Three gram~
of a 1:1 (weight ratio~ solution of ammonium acetate in water was then added with ag1tation. The above mixture was poured lnto a one liter beaker containing 900 ml. of 2-ethylhexanol saturated with water and under agitation.
After stirring for five minutes, the mixture was filtered to separate the gelled partlcles from the alcohol. The recoverQd hard gelled spheres were clear, of excellent qu~llty and had diameters ranging up to and over 1000 micrometers.
A sample of the gelled microspheres was placed in a Pyrex d~sh and placed in a box furnace, and the temperature was ra~ed from room temperature to 400C over two hours. The 400C temperature was maintained for 30 minutes after which the dish was removed and cooled. The f~red beads were very clear. ~ sample of the spheres wa~
then placed in a crucible and heated to 620C in a box furnace. After 20 minutes at 620C, the crucible wa~
removed and cooled. Examination of the particles under the microscope at 80X magnification ~ho~ed that they were tran~parent with a large proportion be~ween 200 and 350 micrometer~ in d~ameter.

1 ~'74~ 8 Example_III
Approximately 0.6 g. of a ~ilica 801 ( having about 31 weight percent solids, about 150 ~ng~trom prlmary particle ~ize, pH of about ~.2, Na2O content of about 0.10 percent and obtained as Ludox LS from E. I. DuPont de Nemours ~ Co.) was added to 20 9. of a titania 301 prepared as ln Exa~ple I. This mixture wa~ about 97 mole percent ~i2 and 3 mole percent sio2. After addition of glacial acetic acid and ammonium acetate, a~ in Example II, gel particle~ were ~ormed. The hard gellsd sphere~ which were recovered were very clear and up to and over lO00 micrometer~ in diameter. A sample wa~ fired as in Example II and appeared less transparent than the æirconia titania partlcles but ~till had retroreflectivity in a flashlight beam. The overall particle size wa~ larger than those of Example II.

Example IV
A stable, ion-exchanged zirconia ~ol was prepared by mixing a nitrate stabilized zirconia sol (as used in Example II) with an ion exchange resin (as described in Example II) in a ratio of about lO0 9 of 901 to 15 9 resin.
To about 21 9 of the resultlng stable zirconia 801 were added about seven grams of silica 901 (Ludox LS), and then about 2.5 9 o a S0~ aqueou~ ammonium acetate solution were added to the sol with agita~ion. The resulting mixture (having a ZrO2:SiO2 mole ratio of about 1:1) was immediately added to 500 ml of 2-ethylhexanol under agltation in a 600 ml beaker. After stirring for about 5 minutes, the mixture was filtered to separate the gel particles from the alcohol. very tran~parent, rigid gelled ~phere~ up to and exceeding 1 mm in diameter were re~overed. These particles were dried and ~ubsequently fired to 1000C. Intact, transparent to ~lightly translucent spheres up to and over 500 micrometers in diameter were obtained. A micro-hardness test performed on the microspheres which had been fired at 1000C mea~ured ~.~7~
, ~ O~ n~ ~0 ln~ knoo~. Their ~urrace nren wnn me~n~lre-l an-~ ~oun-l to he ahout ~.~15 m~-/g, 3ndicatinq that the~ were e~q~entially Eully den.~e. A sample of the microspheres from thl~ example wa~ mountc-l on an a(llle~ive-co~te~l wh~te vinyl strip. When observed in a Elashlight beam, the particle~
reElected brilliantly from a wi~e range of viewing angle~.
Other mixtures of zirconia 9018 and silica 9019 were made a~ ~n thi~s example to yield microsphere~ with indicQs of reEraction up to 1.~1.

~xample V
625 ml (510 g) of water saturate~ 2-ethyl hexanol wa~ added to an ~00 ml. beaker and wa~ stirred with a three-blade propellor mixer at about 100~ rpm. 2.5 g. of a nonionic wettin~ agent (Terg;tol~ TMN) was added. In a ~econd heaker, 4 qram~ of a solution compri~ing one part by weight ammonium acetate in two parts by wei~ht water were added to 50 9. of an agitated ~ilica 801 obtained as Nalco 41D01 from ~alco Chemical Company. The contents of the second beaker ~as poured into the agitated 2-ethyl hexanol, and agitation was continued for about five minutes, during which gel particles formed. The qelled particles were removed by ilt~ation. A~ter dryin~ at 9~C a sample of the particles wa~ placed in an electric furnace at 1000C
and that temperature maintained for 3~ minutes~ Upon cooling, the particles were examine~ unde~ a micro~scope.
They were very clear and range~ in si~e from about 60 to 1000 micrometer~ in diameter. A few of the particles had crystallized and become opaque. The transparent microspheres produce~ were measured at an avera~e hardne~
of 726 knoop and w~re ob~erve~ to re~lect light brlghtly in a ~lashlight beam. .Surface area mea.surement~ of these particles showed them to be ~ub~tantially fully dense.

It i~ within the ~cope of thi~ invention to impart color to the transparent ceramie microspheres. The aqueous ai~ersions which are u~ed to ~orm the ceramics of 1~7~

this invention can contain variouA other water-~oluble metal compounds which will impart internal color to the finished ceramic without ~acrificing clarity. The adaing of colorants to the inventive ceramic~ may be done in accordance with the teaching of U.S. Patent No. 3,795,524 found in Col~ 4, line 72-Col. 5, line 27. Colorant~ ~uch a~ ferric nitrate (for red or orange) rnay be added to the di~perAion in an amount of about 1 to 5 weight percent of the total metal oxide present. Color can al30 be impartea hy the interaction of two colorle3~ compoun~3 under certain proce~aing conditions (e.g., To2 and ~rO2 may interact to produce a yellow color).

Industrial Applicabllity The transparent, ceramic micro~phere~ of thi3 invention are quite useful in pavement marking sheet materials (i.e. sheeting to be applied to road ~urfaces).
The mlcrospheres of thi3 invention can alqo be incorporated into coating composition~ which generally compri~e a film-forming material in which a multiplicity of the microspheres are dlspersed (e.g., see Palmqui~t U.S. Patent No. 2,963,37B). The micro~pheres may also be u~ed in drop-on applications for ~uch purpose~ as highway lane ~triping in which the beads are simply dropped onto wet paint or hot thermoplastic and adhered thereto.
There are several type~ of retroreflective sheeting in which the inventlve microsphere~ may he used, ~uch a~ exposed lens (a~ taught for exampl~ in U.S. Patents 2,326,634 and 7,354,018), embedded lens (see for example V.S. Patent 2,407,6~0j and encapsulated lsn~ (3ee U.S.
Patent 4,025,159) sheeting. These ~heeting type~ and method3 for manufacturing them are known to the art. The draw'ng~ of the aforement~oned patents (4,025,1$9:
2,407,6~30: and 2,326,634) lllustrate the variou~ ~heeting types and are incorporated by reference herein.
One type of retroreflective ~heet material useful for traffic sign~ comprise~ a polymeric binder film in which a monolayer of the inventive micro~phere3 are embedded to about half their diameter or more. The mlcrospheres are in optical connection with a reflecting means, s~ch as an alumlnum coating on their embedded surfaces. ~Such retroreflective sheet material can be made hy: t) ~nrt~Ally emhnddin~ a monolnyer o~ the lnvent3ve microspheres into a treated carrier web (e.g., polyethylene-coated paper1: ii) coating the mlcrospheres with aluminum by vacuum vapor depo~ition; iii~ applying a binder coating (e.g., 68 weight percent ~olid~ alkyd resin solution in aromatic solvent); iv) curing the binder ~e.g., 30 minute~ at 95C) v) applying a clear polymeric base layer (e.g, 20 weight percent solution of polyvinyl butyral in xylene-butanol solvent~ over the binder: vi) drying the base layer (95C for 30 minutes); and vii) stripping away the carrier webO
One typical pavement marking ~heet i3 de~cribed in U.S. Patent No. 4,24~,932. This sheet material is a prefabricated ~trip adapted to be laid on and secured to pavement for ~uch purpose~ as lane dividing lines and comprises:
1. A base sheet, such a~ a soft aluminum foil which is conformable to a roadway surface;
2. A top layer (also called the support film or binder film) adhered to one surface of the base sheet and being very flexible and reaistant to rupture; and 3. A monolayer of particles ~uch as tran~parent microsphere lens elements partially embedded in the top layer in a scattered or randomly separated manner.
The pavement marklng sheet construction may also ~nclude an adhesive (e.g. t pre~sure sensltive, heat or ~olvent activated~ or contact adhesive) on the bottom of the base ~heet.
The ba3e sheet may be made of an elastomer ~uch a~ acrylonitrile-butadlene polymer, polyurethane, or neoprene rubber.

~ ~7~

The top layer in which the transparent microspheres are embed~ed may typically be a polymer such as vinyl polymers, polyurethanes, epoxie3, and polye3ters.
The mlcrosphere lenses may alternatively be completely embedded in a layer of the pavement marking ~heet. Another patent describing such pavement marking sheet material i~
U.S. Patent No. 4,117,192.
Pavement mark;ng Aheets may be made by processes known ln the art (see e.g. U.S.Patent 4,248,932), one example comprising the steps of: i) coating onto a base she~t of soft aluminum ~50 micrometers thick) a mixture of re~ins (e.g., epoxy and acrylonitrile butadiene elastomer mixture), pigment (Tio2~ and solvent (e.g., methylethylketone) to form the support film; ii) dropping onto the wet surface of the support film lngr2dients a multiplicity of the ~ol gel microsphere~ (160 microns and larger in diameter) of thi~ invent~on: and curlng the ~upport film at 150C for about 10 minutes. A layer of adhesive is then usually coated on the bottom of the base sheet.
The microsphere~ may be treated with an agent which improves adhesion between them and the top layer, or such an agent may be included in the top layer where it contacts the microspheres. Silane coupllng agents are useful for this purpose.
Pigments or other coloring agents may be lncluded in the top layer in an amount sufficlent to color the sheet materlal for use as a traffic control marking. Titanium dioxide will typically be used for obtaining a white color;
whereas, lead chromate will typically be used to provide a yellow color.
In some useful embodiments of the invention, a specular reflective means is provided by a layer of metal (e.g. aluminum) vapor-deposited on the microspheres.
Another useful specular reflective means is a dielectric reflector which comprises one or more layers of a transparent material behind the microspheres, each layer having a refractive index of about 0.3 higher or lswer than that of the ad~a~ent layer or beads and each layer having an optical thickness corresponding to an odd numbered multiple of about 1/4 wavelength of light in the visible range. ~ore detail on such dielectric reflectors is found in U.S. Patent 3,700,305.
Pavement marking sheet material~ of this invention have been tested in a 3and blast test. This test utilizes an apparatus comprised of a channel about 156 mm wide and 508 mm long in which is mounted a flat me'cal te.g.
alum~num) plate about 152 mm wide. The pavement marking sheet material sample being te~ted i9 adhered to the metal plate which i3 moved down the channel by engagement with an electric motor having a speed control. A commercial compre~sed air ~and blast gun having a compressed air supply at 10 p3i9~ (69 kPa) ana u~ing common ~and bla~t sand (e.g., 70% 250/425 m~crometer particle size) is directed towaed a portion of the channel which must be pas~ed by the sample. The ~and blast gun is placed with lts tip 762 mm from the point where sand will impact the sample, and it is oriented at about 75 to the channel such that the sampl~ is moving toward the point where sand is impacting at a rate of about 0.04 m/sec (1-1~2 in/sec.) The sample passes the point of the impacting sand repeatedly, and reflectivity of the sample i3 measured after a number of passes to te~t the durab;lity of the microsphere len~es. The figure represents sana bla~t data for pavement marking sheet materials similar in all re~pects except that there were different lens elementa used in the four types of sheeting te~ted. The plotted curves present retained reflectivity data as indicated prevlously under Brief Des~ription of the Drawings. The inventive sample retained substantially more of i~5 original reflectivity than any of the glass bead samples.
The actual data from which retained reflectivity wa~ calculated for the Figure is shown in Table 2 below.
Reflective brightness or retroreflectivity is recorded in lX7~ 8 unit~ of millicandela/foot candle/square foot (mcd/fc/ft as measured by a photometer. All mea~urements were made with incident light at an angle of 86~5 from normal to the sample surface and with a divergence, angle between the l~ht 30urce and the photocell, of 1Ø

Table 2 Retroreflectivity (mc/fc/ft23 Microsphere zr02/si2 ~ Glass Type _ Ceramlc 1.5 Nn 1.75 Nn1.9 ~n No. of Sand slast Pa~se3 0 118~ 308 1303 1776 S 1073 2~2 833 ~35 15 15 994 19~ 57g 132 9S~ 18~ 341 10 The inventive zirconla/silica 801 gel ceramic tested had a mole ratio of ZrO2:SiO2 of about 1:1 and an index of refraction o about 1.76-1.77. The above table ~hows that, in addition to retaining more of its retroreflectivity, the pavement marking sheet made with the inventive microspheres had a higher absolute retroreflectivity after only five passes of the sand blaster than any of the control samples, and continued to have ~uperior retroreflectivity after 20 pas~es.

O~her embodiments of this invent~on will be apparent to those ~killed ln the art from a consideration of this ~peclfication or practice of the invention d~elo~ed herein. Variou~ omis~ion~, modifications, and alteration~ of this invention may be made without departing from the true scope and spirit of this ~nvention which is indicated by the following claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Solid, transparent, non-vitreous, ceramic microspheres comprising polycrystalline titanium dioxide and having an average particle size greater than 200 micrometers, said microspheres being useful as lens elements.