Patent Publication Number: US-6902780-B2

Title: Coating composition comprising colloidal silica and glossy ink jet recording sheets prepared therefrom

Description:
This application claims the benefit of provisional application Ser. No. 60/365,616, filed on Mar. 19, 2002. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to coated ink jet recording sheets and coating compositions used to prepare the same. In particular, the invention relates to coating compositions suitable for preparing glossy ink jet recording sheets which possess good printability characteristics. 
   Ink jet printing processes are well known. Such systems project ink droplets onto a recording sheet, e.g., paper, at varying densities and speed. When using multi-color ink jet systems, the process projects in very close proximity a number of different colored inks having varying properties and absorption rates. Indeed, these multi-color systems are designed to provide images which simulate photographic imaging, and such images require high resolution and color gamut. Accordingly, ink jet recording sheets must be able to absorb ink at high densities, in a capacity such that the colors deposited are bright and clear, at rates to effect quick drying, absorb ink so that it does not run or blot, and in a manner that results in smooth images. 
   To meet these goals, highly porous pigments, e.g., porous silicas, have been incorporated into paper coatings. Silica-based coating systems have been successful in meeting the printability goals. However, it has been difficult to obtain such properties and produce a non-matted, or glossy, finish typically seen in traditional photographic systems. The aforementioned porous pigments typically have porosities above 1 cc/g and have average particle sizes greater than 1 micron. Such particle sizes and porosities increase the surface roughness of the finished coating, thereby deflecting incident light so that it is scattered, thereby matting the coating. 
   To enhance the glossiness of such coatings, second gloss layers are provided on top of ink receptive layers prepared from the aforementioned porous pigments. These top layers are prepared from binder systems that are inherently glossy, or from layers comprising binder and much smaller sized inorganic oxide particles, e.g., conventional colloidal silica. The colloidal silica in the latter approach tends to enhance the ink receptive nature of the top coating, but are not large enough to cause surface deformations. There is, however, a tendency for the colloidal particles to agglomerate at high concentrations, thereby causing imperfections and surface roughness in the top layer, and thereby reducing gloss. Accordingly, lower concentrations (i.e., lower ratios of colloidal solids to binder solids) have been used when employing this approach. 
   It would therefore be quite desirable to increase the amounts of solid inorganic oxides in these top layers to further improve printability. Indeed, it would be desirable to use coating layers having at least 1:1 colloidal solids to binder solids ratios, and even more preferable to employ coatings having colloidal to binder ratios as high as 4:1, yet at the same time attain acceptable gloss. 
   Furthermore, coating systems for ink jet paper are frequently designed to have an overall cationic charge. Many of the inks employed in ink jet processes possess a negative charge; and it, therefore, is desirable for the coating components to have an opposite charge to affix the ink. Colloidal aluminum possesses a positive charge and has been widely used as a coating pigment for that purpose. Cationic dye fixing components and cationic binders are also employed. Indeed, the presence of these latter cationically charged materials usually require that the pigment components in the coating be cationic or at least nonionic. Otherwise the materials in the coating tend to aggregate, thereby creating surface imperfections and reducing gloss. It, therefore, would be desirable, and it is a goal of this invention, to provide a coating layer comprising a relatively high content of silica solids which are cationic. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  illustrates the particle size distribution of a polydispersed colloidal silica employed in a preferred embodiment of invention. 
       FIG. 2  illustrates a colloidal silica&#39;s silica solids to alkali metal ratio versus gloss achieved from coatings containing the same. 
   

   SUMMARY OF INVENTION 
   The present invention provides ink jet recording sheets comprising a support and at least one coating layer thereon, said at least one coating layer (a) having a specular surface gloss of at least 30 at 60°, (b) comprising cationic colloidal silica having a silica solids to alkali metal ratio of at least the sum of AW(−0.013SSA+9), and (c) binder, wherein the colloidal silica solids and binder solids are present at a ratio of at least 1:1 by weight, AW is the atomic weight of the alkali metal and SSA is the specific surface area of the colloidal silica. 
   Preferably, the ratio of colloidal silica solids to binder solids is in the range of about 6:4 to about 4:1. 
   Preferably, the colloidal silica has a silica solids to alkali metal ratio of at least 150. 
   Preferably, the colloidal silica has an average particle size in the range of about 1 to about 300 nanometers. 
   Even more preferably, the silica solids to alkali metal ratio is at least the sum of −0.30SSA+207, and the alkali metal is sodium. 
   A goal of this invention is also a coating composition comprising (a) cationic colloidal silica having a solids to alkali metal ratio of at least the sum of AW(−0.013SSA+9), and (b) binder, wherein silica solids of (a) and binder solids of (b) are present at a ratio of at least 1:1 by weight, AW is the atomic weight of the alkali metal and SSA is the specific surface area of the colloidal silica. 
   Preferably, the ratio of silica solids of (a) to binder solids of (b) of the coating is in the range of about 6:4 to about 4:1. 
   Preferably, the colloidal silica has a silica solids to alkali metal ratio of at least 150. 
   Preferably the colloidal silica has an average particle size of about 1 to about 300 nanometers. 
   More preferably, the silica solids to alkali metal ratio is at least the sum of −0.30SSA+207, and the alkali metal is sodium. 
   Even more preferably, the colloidal silica has a median particle size in the range of 15 to 100 nm and has a particle size distribution such that at least 80% of the particles span a size range of at least 30 nanometers and up to about 70 nanometers. 
   It has been discovered that cationic colloidal silica having relatively low amounts of alkali metals, e.g., sodium, provides for colloidal silica which does not aggregate at relatively high solids contents, and thereby reduces deformation and matting of the coating surface. 
   DETAILED DESCRIPTION OF THE INVENTION 
   By the term “colloidal silica” or “colloidal silica sol” it is meant particles originating from dispersions or sols in which the particles do not settle from dispersion over relatively long periods of time. Such particles are typically below one micron in size. Colloidal silica having an average particle size in the range of about 1 to about 300 nanometers and processes for making the same are well known in the art. See U.S. Pat. Nos. 2,244,325; 2,574,902; 2,577,484; 2,577,485; 2,631,134; 2,750,345; 2,892,797; 3,012,972; and 3,440,174, the contents of which are incorporated herein by reference. Colloidal silicas having average particle sizes in the range of 5 to 100 nanometers are more preferred for this invention. Colloidal silicas can have a surface area (as measured by BET) in the range of 9 to about 2700 m 2 /g. 
   A colloidal silica particularly suitable for this invention is what is known as polydispersed colloidal silica. “Polydispersed” is defined herein as meaning a dispersion of particles having a particle size distribution in which the median particle size is in the range of 15-100 nm and which has a relatively large distribution span. Preferred distributions are such that 80% of the particles span a size range of at least 30 nanometers and can span up to 70 nanometers. The 80% range is measured by subtracting the d 10  particle size from the d 90  particle size generated using TEM-based particle size measurement methodologies described later below. This range is also referred to as the “80% span.” One embodiment of polydispersed particles has particle size distributions which are skewed to sizes smaller than the median particle size. As a result, the distribution has a peak in that area of the distribution and a “tail” of particle sizes which are larger than the median. See FIG.  1 . The lower and upper particle size of the span encompassing 80% of the particles can be −11% to −70% and 110% to 160% of the median, respectively. A particularly suitable polydispersed silica has a median particle size in the range of 20 to 30 nanometers and 80% of the particles are between 10 and 50 nanometers in size, i.e., 80% of the distribution has a span of 40 nanometers. 
   Most colloidal silica sols contain an alkali. The alkali is usually an alkali metal hydroxide from Group IA of the Periodic Table (hydroxides of lithium, sodium, potassium, etc.) Most commercially available colloidal silica sols contain sodium hydroxide, which originates, at least partially, from the sodium silicate used to make the colloidal silica, although sodium hydroxide may also be added to stabilize the sol against gelation. 
   Generally speaking, colloidal silica possesses a net negative charge and therefore is anionic as a result of the loss of protons from silanol groups present on the silica&#39;s surface. For the purposes of this invention, colloidal silica is considered cationic if an anionic colloidal silica has been physically coated or chemically treated so that the colloidal silica possesses a net positive charge. A cationic silica thus would include those colloidal silicas in which the surface of the silica contains a sufficient number of cationic functional groups, e.g., a metal ion such as aluminum, or an ammonium cation, such that the net charge is positive. 
   Several types of cationic colloidal silica are known. Such cationic colloidal silicas are described in U.S. Pat. No. 3,007,878, the contents of which are incorporated by reference. Briefly, a dense colloidal silica sol is stabilized and then coated by contacting the sol with the basic salt of a trivalent or tetravalent metal. The trivalent metal can be aluminum, chromium, gallium, indium, or thallium, and the tetravalent metal can be titanium, germanium, zirconium, stannic tin, cerium, hafnium, and thorium. Aluminum is preferred. 
   The anions in the polyvalent metal salt, other than hydroxyl ions, are so selected as to make the salt soluble in water. It will be understood that when reference is made herein to the fact that the salt has a monovalent anion other than hydroxyl, the intention is not to exclude hydroxyl from the salt but to indicate that another anion is present in addition to the hydroxyl which the salt contains. Thus all basic salts are included, provided they are water-soluble and can produce the required ionic relationships as hereinafter described. 
   Preferably colloidal sols of positively charged silica are prepared by depositing alumina on the surface of colloidal silica particles. This may be achieved by treating an aquasol of negatively charged silica with basic aluminum salts such as basic aluminum acetate or basic aluminum. Processes for preparing these positively charged silica sols are disclosed by Moore, U.S. Pat. No. 3,620,978; Moore, U.S. Pat. No. 3,956,171; Moore, U.S. Pat. No. 3,719,607; Moore, U.S. Pat. No. 3,745,126; and Bergna, U.S. Pat. No. 4,217,240, all of which are incorporated herein by reference. The aluminum treatment results in aluminum:silica ratios at the surface of the colloidal particles ranging from about 1:19 to about 4:1. Preferred for use herein are aluminum:surface silica ratios of from about 1:2 to about 2:1. The sol is stabilized by a slightly acidic pH which may be achieved by adding small amounts of an acid, e.g., acetic acid, or by passing the sol through a bed of a strongly acidic ion exchange resin. 
   As indicated above, the cationic colloidal silica sols of this invention have significantly lower levels of alkali metal ions than commercially available colloidal silica sols. Lower alkali levels can be shown by calculating the silica solids to sodium weight ratio of the colloidal silica sol, as shown in the Equation 1.  FIG. 2  shows that acceptable gloss can be obtained from colloidal silica sols using the equation below:
 
SiO 2 /Alkali Metal≧AW(−0.013*SSA+9)  Equation 1.
 
The SiO 2 /alkali metal is the weight ratio of silica solids and alkali metal in the colloidal silica sol. AW is the atomic weight of the alkali metal, e.g., 6.9 for lithium, 23 for sodium, and 39 for potassium, and SSA is the specific surface area of the colloidal silica particles in units of square meters per gram (m 2 /g). When the alkali metal is sodium, the SiO 2 /Alkali Metal ratio is at least the sum of −0.30SSA+207.
 
   The low alkali cationic colloidal silicas can be prepared by deionizing them to an extent such that the colloidal silica has a silica solids to alkali metal ratio referred to in Equation 1. By “deionized,” it is meant that any metal ions, e.g., alkali metal ions such as sodium, have been removed from the colloidal silica solution. Methods to remove alkali metal ions are well known and include ion exchange with a suitable ion exchange resin (U.S. Pat. Nos. 2,577,484 and 2,577,485), dialysis (U.S. Pat. No. 2,773,028) and electrodialysis (U.S. Pat. No. 3,969,266). 
   As indicated below, the colloidal silicas can be incorporated in conventional coating binders. The binder not only acts to bind the colloidal silica and to form a film, it also provides adhesiveness to the interface between the gloss-providing layer and the substrate or any intermediate ink-receiving layer between the glossy layer and substrate. 
   Cationic and nonionic binders are particularly suitable in the present invention. Suitable binders include, but are not limited to, styrene-butadiene or styrene-acrylate copolymers having functional cationic groups and/or cationic polyvinyl acetates, cationic polyvinyl alcohols or their copolymers. 
   Furthermore, the binder can be selected from the group of decomposed and native guars, starches, methyl celluloses, hydroxymethyl celluloses, carboxymethyl celluloses, alginates, proteins and polyvinyl alcohols which are present in cationic form. Proteins are also suitable because they are amphoteric. 
   Specific examples of cationic water-soluble binders include, for example, diethylaminoethylated starch, trimethylethylammonium chloride-modified starch, and diethylaminoethyl ammonium-methyl chloride salt-modified starch; and cation-modified acrylic ester copolymers. 
   Suitable non-ionic, water-soluble binders include, but are not limited to, polyvinyl alcohol, hydroxyethyl cellulose, methyl cellulose, dextrin, pluran, starch, gum arabic, dextran, polyethylene glycol, polyvinyl pyrrolidone, polyacrylamide, and polypropylene glycol. 
   Water-insoluble or poorly water-soluble cationic or nonionic binders in the form of an aqueous emulsion, include but are not limited to, acrylic and methacrylic copolymer resins, for example, methyl methacrylate-butyl acrylate copolymer resins, methyl methacrylate-ethyl acrylate copolymer resins, methyl methacrylate-2-ethylhexyl acrylate copolymer resins, methylmethacrylate-methyl acrylate copolymer resins, styrene-butyl acrylate copolymer resins, styrene-2-ethylhexyl acrylate copolymer resins, styrene-ethyl acrylate copolymer reins, styrene-methylacrylate copolymer resins, methyl methacrylate-styrene-butyl acrylate copolymer resins, methyl methacrylate-styrene-2-ethylhexyl acrylate copolymer resins, methyl methacrylate-styrene-ethyl acrylate copolymer resins, methyl methacrylate-styrene-ethyl acrylate copolymer resins, methyl methacrylate-styrene-methyl acrylate copolymer resins, styrene-butyl acrylate-acrylonitrile copolymer resins, and styrene-ethyl acrylate-acrylonitrile copolymer resins. 
   Other suitable binders include casein, gelatin, a maleic anhydride resin, a conjugated diene-type copolymer latex such as a vinyl-type polymer latex such as an ethylene-vinyl acetate copolymer; a synthetic resin-type binder such as a polyurethane resin, an unsaturated polyester resin, a vinyl chloride-vinyl acetate copolymer, polyvinyl butyral or an alkyd resin. 
   The binder can be combined with the colloidal silica using conventional blenders and mixers. The components can be combined and mixed at ambient conditions. 
   It is desirable for the colloidal silica solids and binder solids to be present in the coating at relatively high ratios. It has been found that in certain embodiments higher silica to binder ratios provide good printability, as well as provide advantageous mechanical properties to the finished ink receptive coating sheet. It is particularly desirable for the colloidal silica and binder solids to be present at a ratio of at least 1:1, and more preferably 6:4 to 4:1 by weight. The ratio can be as high as 9.9:1. The colloidal silica to binder solids ratio is also referred to herein as pigment to binder ratio. 
   It may also be desirable to include additional components in the coating composition of this invention. The coating of this invention can contain one or more of the following: dispersant, thickener, fluidity-improving agent, defoaming agent, foam-suppressing agent, release agent, blowing agent, penetrating agent, coloring dye, coloring pigment, fluorescent brightener, ultraviolet absorber, anti-oxidant, preservative, ash-preventing agent, waterproofing agent, and wet-strength agent. 
   Cationic dye mordant is a preferred additive. Examples of suitable mordants include, but are not limited to, a polymeric quartenary ammonium compound, or a basic polymer, such as poly(dimethylaminoethyl)-methacrylate, polyalkylenepolyamines, and products of the condensation thereof with dicyanodiamide, amine-epichlorohydrin polycondensates; lecithin and phospholipid compounds. Specific examples of such mordants include the following: vinylbenzyl trimethyl ammonium chloride/ethylene glycol dimethacrylate; poly(diallyl dimethyl ammonium chloride); poly(2-N,N,N-trimethylammonium)ethyl methacrylate methosulfate; poly(3-N,N,N-trimethyl-ammonium)propyl methacrylate chloride; a copolymer of vinylpyrrolidinone and vinyl(N-methylimidazolium chloride; and hydroxyethylcellulose derivatized with 3-N,N,N-trimethylammonium)propyl chloride. In a preferred embodiment, the cationic mordant is a quaternary ammonium compound. 
   The mordant which may be used in the invention can be employed in any amount effective for the intended purpose. In general, good results are obtained when the mordant is present in an amount of from about 0.1-10% by weight of the total coating formulation. These mordants are especially preferred when the binder is nonionic. 
   A portion of the relatively alkali metal free cationic colloidal silica also can be replaced by one or more other colloidal materials, e.g., those materials containing alkali metals in larger amounts, provided the total amount of alkali present in the combination of colloidal silica and the other material is such that the silica solids to alkali metal ratio is that given by Equation 1, and the amount of such colloidal material does not detract from the overall cationic nature or gloss desired for the finished coating. These other colloidal materials can be silica, as well as inorganic oxides other than silica, e.g., titania, zirconia, and the like. Such additional inorganic oxide colloidal particles can be added as a filler and/or as additional pigment. 
   The coatings of this invention have a gloss of at least thirty (30) at 60° according to a BYK Gardner measuring instrument. Preferable coatings according to this invention have a gloss of at least 80 at a 6:4 colloidal silica to binder ratio and at least 50, and preferably at least 70 at a 4:1 colloidal silica to binder ratio. Even more preferred, the coating has a gloss of at least 90 at a 4:1 colloidal silica to binder ratio. 
   Suitable supports for preparing the ink recording sheet of this invention can be those typically used in the art. Suitable supports include those having a weight in the range of about 40 to about 300 g/m 2 . The support may be base paper produced from a variety of processes and machines such as a Fourdrinier paper machine, a cylinder paper machine or a twin wire paper machine. The supports are prepared by mixing its main components, i.e., a conventional pigment and a wood pulp including, for example, a chemical pulp, a mechanical pulp, and a waste paper pulp, with at least one of various additives including a binder, a sizing agent, a fixing agent, a yield-improving agent, a cationic agent and a paper strength-increasing agent. Other supports include transparent substrates, fabrics and the like. 
   Further, the support may also be size-pressed paper sheets prepared using starch or polyvinyl alcohol. The support can also be one which has an anchor coat layer thereon, e.g., paper already having a preliminary coating layer provided on a base paper. The base paper may also have an ink-receiving layer applied prior to applying the coating of this invention. 
   Coatings comprising colloidal silica, binder and optional additives can be applied online as the support is being prepared, or offline after the support has been finished. The coating can be applied using conventional coating techniques, such as air knife coating, roll coating, blade coating, bar coating, curtain coating, die coating, and processes using metered size presses. The resulting coatings can be dried by ambient room temperature, hot air drying methods, heated surface contact drying or radiation drying. Typically, the coating composition of the invention, and any optional intermediate layers, is applied in a range of 1 to 50 g/m 2 , but more typically in the range of 2 to 20 g/m 2 . 
   The examples below show that a glossy ink jet recording sheet having good printability can be prepared essentially from a support and one layer of the invention. However, it may be desirable in certain instances to place another layer, which is ink receptive, between the gloss providing layer of the invention and the support to enhance the printability of the final sheet. For example, sheets coated with a certain deionized colloidal silica would preferably contain a separate ink receptive coating between the gloss layer and the substrate in order to improve the printability of the finished ink jet recording sheet. 
   Suitable ink receptive layers are those identified as such in U.S. Pat. No. 5,576,088, the contents of which are incorporated herein by reference. Briefly, suitable ink receptive layers comprise a binder such as the water soluble binders listed above, and an ink receptive pigment. Such pigments include a white inorganic pigment such as light calcium carbonate, heavy calcium carbonate, magnesium carbonate, kaolin, talc, calcium sulfate, barium sulfate, titanium dioxide, zinc oxide, zinc sulfide, zinc carbonate, satin white, aluminum silicate, diatomaceous earth, calcium silicate, magnesium silicate, synthetic amorphous silica, colloidal silica, alumina, colloidal alumina, pseudo boehmite, aluminum hydroxide, lithopone, zeolite, hydrolyzed halloysite or magnesium hydroxide, or an organic pigment such as a styrene-type plastic pigment, an acrylic plastic pigment, polyethylene, microcapsules, a urea resin or a melamine resin. Suitable pigments for the ink receptive layer have average particle sizes in the range of 0.5 to 3.0 microns (measured by light scattering) and pore volumes ranging from 0.5 to 3.0 cc/g and preferably pore volumes of 1.0 to 2.0 cc/g, as measured by nitrogen porosimetry. In order to obtain an ink jet recording sheet having a high ink absorptivity, it is preferred that the pigment in the ink-receiving layer contains at least 30 vol. % of particles having a particle size of at least 1.0 μm. 
   The preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular embodiments disclosed, since they are to be regarded as illustrative rather than restrictive. Variations and changes, therefore, may be made by those skilled in the art without departing from the spirit of this invention. 
   Further, any range of numbers recited in the specification or claims, such as that representing a particular set of properties, conditions, physical states or percentages, is intended to literally incorporate expressly herein any number falling within such range, including any subset ranges of numbers within any range so recited. 
   ILLUSTRATIVE EXAMPLES 
   The parameters listed below and/or indicated earlier were measured as follows:
         Average Particle Size—unless indicated otherwise is a number average particle size determined by the equation SSA=3100/d n  wherein d n  is the average particle size and SSA is the specific surface area defined below   Median Particle Size—is a number weighted median measured by electron microscopy (TEM).   Gloss—measured using a BYK Gardner micro-TRI-gloss instrument which has been calibrated on a transparent film. The gloss values were measured using a 60° geometry.   Alkali metal (e.g., Na) Content—percentage by weight based on alkali metal ion content measured using the inductively coupled plasma-atomic emission (ICP-AES) spectroscopy technique. The sample is first dissolved at ambient conditions, e.g., 25° C. and 75% relative humidity, in hydrofluoric acid and nitric acid (at a 30/70 weight ratio) before applying this technique. The sample was allowed to dissolve for sixteen hours before measurements were taken.   Silica Solids Content—measured in an Ohaus furnace at 205° C., with the end point for the solids measurement being when the sample weight change is less than 0.01 g for sixty (60) seconds.   Specific Surface Area—titrimetric method correlated to surface area by nitrogen adsorption as given by G. W. Sears, Jr., Analytical Chemistry, Vol. 28, p. 1981, (1956).       

   Example 1 
   Comparison 
   Martoxin® GL3 (SSA=332 m 2 /g) alumina was peptized according to the manufacturer&#39;s procedures. Martoxin® GL3 powder was added to deionized (DI) water at a 15% solids level and stirred for 5 minutes. Then the pH was adjusted to 4.5 with acetic acid and the slurry stirred for 10 more minutes. At the end the pH was again adjusted to 4.5 with acetic acid. 21.015 g (15 wt. %) of the above prepared colloidal alumina slurry was placed in a beaker. To that, 4.85 g of Airvol® 523 (15.5 wt. % solution) polyvinyl alcohol were added. Then, 0.19 g of Agefloc® B50 dye mordant (50 wt. %), diluted with 0.768 g of deionized water was added to the mixture. The resulting formulation was coated as a 100 micron wet film on Melinex™-534 polyester, opaque white film, from E. I. DuPont de Nemours &amp; Co. using a TMI coater (K control coater) with a number 8 rod. The obtained coating had a gloss of 93% at 60 degrees. 
   Example 2 
   Comparison 
   10.01 g of Ludox® CL-P (40% solids; 140 SSA; 22 nm average particle size; % (by weight) Na=0.250; SiO 2 /Na=160) colloidal silica from W. R. Grace &amp; Co.-Conn. were placed in a beaker and diluted with 10.31 g of deionized water. To that, 5.81 g of Airvol® 523 (15.5 wt. % solution) polyvinyl alcohol were added followed by 0.22 g of Agefloc® B50 (50 wt. %). The resulting formulation was coated on polyester film as described in Example 1. The obtained coating had a gloss of 4% at 60 degrees. This relatively low gloss is consistent with Equation 1 which indicates that SiO 2 /NA must be 165 or greater to obtain acceptable gloss. 
   Example 3 
   Comparison 
   12.06 g of Ludox® CL (30% solids; 230 SSA; 12 nm average particle size; % Na=0.260; SiO 2 /Na=115) colloidal silica from W. R. Grace &amp; Co.-Conn. were placed in a beaker and diluted with 6.31 g of deionized water. To that, 5.26 g of Airvol® 523 (15.5 wt. % solution) polyvinyl alcohol were added followed by 0.20 g of Agefloc® B50 (50 wt. %). The resulting formulation was coated on polyester film under conditions described in Example 1. The obtained coating cracked. This result could be expected in view of Equation 1 indicating that the SiO 2 /Na should be at least 138 to obtain an acceptable glossy coating. 
   Example 4 
   84 g of deionized water was added to 329 g of Ludox® HS-40 (W. R. Grace) colloidal silica containing 40.0% SiO 2  with average particle size=22 nm and having specific surface area=220 m 2 /g. The mixture was heated to 40-50° C. and Amberlite® IR-120 Plus cation exchange resin in the hydrogen form was added with stirring in small amounts until the pH dropped to 2.5. Stirring and temperature were maintained for 1 hour, during which small amounts of resin were added to maintain pH in the range of 2.5-3.0. The mixture was filtered through coarse filter paper to separate the deionized colloidal silica sol from the resin. 1% ammonium hydroxide solution was added to the deionized colloidal silica sol dropwise, with stirring, until the sol reached the range of pH 7.2-7.5. 
   The resulting colloidal silica sol was added dropwise into a beaker containing 87.2 g of 45% aluminum chlorohydrol (20.7% Al 2 O 3  and Al:Cl atomic ratio of 2:1) with rapid stirring. After the addition was complete, the mixture was allowed to equilibrate for about 12 hours, then filtered through fine filter paper. The resulting sol contained 30% solids, exhibited a pH of 3.5, had a sodium content of 0.06% by weight and a SiO 2 /Na ratio of 500. 
   14.51 g of the above product (30 wt. %) were placed in a beaker and diluted with 7.52 g of deionized water. To that, 6.27 g of Airvol® 523 (15.5 wt. % solution) polyvinyl alcohol were added followed by 0.22 g of Agefloc® B50 (50 wt. %). The resulting formulation was coated on polyester film under conditions described in Example 1. The obtained coating had a gloss of 93% at 60 degrees. The gloss is in agreement with Equation 1 which indicated that the SiO 2 /Na ratio should at least be 141 to obtain acceptable gloss. 
   Example 5 
   62 g of deionized water was added to 367 g of Ludox® TM-50 (W. R. Grace) colloidal silica grade containing 50.6% SiO 2  with average particle size=22 nm and having specific surface area=140 m 2 /g. The mixture was heated to 40-50° C. and Amberlite® IR-120 Plus cation exchange resin in the hydrogen form was added with stirring in small amounts until the pH dropped to 2.5. Stirring and temperature were maintained for 1 hour, during which small amounts of resin were added to maintain pH in the range of 2.5-3.0. The mixture was filtered through coarse filter paper to separate the deionized colloidal silica sol from the resin. 1% ammonium hydroxide solution was added to the deionized colloidal silica sol dropwise, with stirring, until the sol reached the range of pH 7.2-7.5. 
   The resulting colloidal silica sol was added dropwise into a beaker containing 70.8 g of 45% aluminum chlorhydrol (20.7% Al 2 O 3  and Al:Cl atomic ratio of 2:1) with rapid stirring. After the addition was complete, the mixture was allowed to equilibrate for about 12 hours, then filtered through fine filter paper. The resulting sol contained 39% solids and exhibited a pH of 3.5. The sodium content of the sol was 0.099% by weight and had a SiO 2 /Na ratio of 394. 
   10.77 g of the above product (39 wt. %) were placed in a beaker and diluted with 10.56 g of deionized water. To that, 6.23 g of Airvol® 523 (15.5 wt. % solution) were added followed by 0.24 g of Agefloc® B50 (50 wt. %) The resulting formulation was coated on polyester film. The obtained coating had a gloss of 86% at 60 degrees. The gloss is in agreement with Equation 1 which indicated that the SiO 2 /Na ratio should at least be 165 to obtain acceptable gloss. 
   Example 6 
   35 g of deionized water was added to 422 g of a polydispersed colloidal silica (50 wt. % solids, median particle size of 22 nanometers and 80% particle span of about 40 nanometers) having a specific surface area of 70 m 2 /g and silica solids to sodium ratio of 179 The mixture was heated to 40-50° C. and Amberlite® IR-120 Plus cation exchange resin in the hydrogen form was added with stirring in small amounts until the pH dropped to 2.5. Stirring and temperature were maintained for 1 hour, during which small amounts of resin were added to maintain pH in the range of 2.5-3.0. The mixture was filtered through coarse filter paper to separate the deionized colloidal silica sol from the resin. 1% ammonium hydroxide solution was added to the deionized colloidal silica sol dropwise, with stirring, until the sol reached the range of pH 7.2-7.5. 
   The resulting colloidal silica sol was added dropwise into a beaker containing 43.6 g of 45% aluminum chlorhydrol (20.7% Al 2 O 3  and Al:Cl atomic ratio of 2:1) with rapid stirring. After the addition was complete, the mixture was allowed to equilibrate for about 12 hours, then filtered through fine filter paper. The resulting sol contained 42% solids and exhibited a pH of 3.5. The sodium content of the sol was 0.110 wt. % and the SiO 2 /Na ratio was 382. 
   10.22 g of the above product(41.9 wt. %) were placed in a beaker and diluted with 11.53 g of deionized water. To that, 6.22 g of Airvol® 523 (15.5 wt. % solution) were added followed by 0.20 g of Agefloc® B50 (50 wt. %). The resulting formulation was coated on polyester film. The obtained coating had a gloss of 81% at 60 degrees. The gloss is in agreement with Equation 1 which indicated that the SiO 2 /Na ratio should at least be 186 to obtain acceptable gloss. 
   
     
       
         
             
             
          
             
                 
                 
             
             
                 
               Gloss at 60° 
             
          
         
         
             
             
             
             
             
          
             
                 
               20% (by 
               40% (by 
               60% (by 
               80% (by 
             
             
               Colloidal 
               weight) 
               weight) 
               weight) 
               weight) 
             
             
               Material 
               Colloidal 
               Colloidal 
               Colloidal 
               Colloidal 
             
             
                 
             
             
               Example 1—Colloidal 
               92 
               86 
               95 
               93 
             
             
               Alumina (Comparison) 
             
             
               Example 2—Ludox ® 
               96 
               95 
               71 
                4 
             
             
               CL-P Silica 
             
             
               (Comparison) 
             
             
               Example 3—Ludox ® 
               ˜ 
               94 
               87 
               cracks 
             
             
               CL Silica 
             
             
               (Comparison) 
             
             
               Example 4 
               98 
               ˜ 
               ˜ 
               93 
             
             
               Example 5 
               98 
               ˜ 
               ˜ 
               86 
             
             
               Example 6 
               98 
               ˜ 
               ˜ 
               81 
             
             
                 
             
             
               ˜—a measurement was not taken for this product