WO2012039729A1

WO2012039729A1 - Paper laminates comprising tungsten treated titanium dioxide having improved photostability

Info

Publication number: WO2012039729A1
Application number: PCT/US2010/055902
Authority: WO
Inventors: John Davis Bolt; Eugene M. Mccarron; Charles David Musick
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2010-09-21
Filing date: 2010-11-09
Publication date: 2012-03-29
Also published as: US20120216976A1; AU2010361147A1; CN102686800A

Abstract

This disclosure relates to a resin-impregnated, opaque, cellulose pulp-based sheet comprising an inorganic particle, wherein the inorganic particle comprises at least about 0.002 % of tungsten, based on the total weight of the inorganic particle, and has a photostability ratio (PSR) of at least about 2, as measured by the Ag+ photoreduction rate, and color as depicted by an L* of at least about 97.0, and b* of less than about 4. The disclosure also relates to paper laminates prepared from these resin-impregnated, opaque, cellulose pulp-based sheets.

Description

TITLE

PAPER LAMINATES COMPRISING TUNGSTEN TREATED TITANIUM DIOXIDE HAVING IMPROVED PHOTOSTABILITY BACKGROUND OF THE DISCLOSURE

Field of the Disclosure:

The present disclosure relates to resin-impregnated, opaque, cellulose pulp-based sheet and paper laminates, and more particularly to resin-impregnated, opaque, cellulose pulp-based sheet and paper laminates prepared therefrom comprising tungsten.

Background of the Disclosure:

Paper laminates are in general well-known in the art, being suitable for a variety of uses including table and desk tops, countertops, wall panels, floor surfacing, tableware and the like. Paper laminates have such a wide variety of uses because they can be made to be extremely durable, and can be also made to resemble (both in appearance and texture) a wide variety of construction materials, including wood, stone, marble and tile, and can be decorated to carry images and colors.

Typically, the paper laminates are made from papers by

impregnating the papers with resins of various kinds, assembling several layers of one or more types of laminate papers, and consolidating the assembly into a unitary core structure while converting the resin to a cured state. The type of resin and laminate paper used, and composition of the final assembly, are generally dictated by the end use of the laminate.

Decorative paper laminates can be made by utilizing a printed decorative paper layer as upper paper layer and various support paper layers in the unitary core structure. The decorative paper is typically highly opaque so that the appearance of the support layers below the decorative paper does not adversely impact the appearance of the decorative paper laminate. A decorative paper is also known as a decor paper.

To achieve required abrasion, scuff, and mar resistance, typically, a separate overlay is used as the top layer for paper laminates. An overlay usually comprises the same resin as the one that is used for the resin impregnated decorative paper. A paper laminate has been made by applying to the outer layer of a composite structure a mixture of an additive amount of a fluorourethane additive, available from E. I. du Pont de Nemours and Company and a melamine resin slurry. Paper laminates may be produced by both low- and high-pressure lamination processes.

Various methods can be employed to provide paper laminates by low-pressure lamination. For example, a single opening, quick cycle press can be used where one or more resin-saturated paper sheets are laminated to a sheet of plywood typically with a 1A face, particle board, or fiberboard.

In a high-pressure lamination process, a melamine overlay and a melamine resin-impregnated decor paper are usually laminated onto a phenolic sheet, which provides additional mechanical support. For example, a "continuous laminator" can be used where one or more layers of the resin-saturated paper are pressed into a unitary structure as the layers move through continuous laminating equipment between plates, rollers or belts. One or two laminated sheets (continuous web or cut to size) may be pressed onto a particle or fiberboard, etc. and a "glue line" used to bond the laminated sheet to the board. Single or multiple opening presses may also be employed which contain several laminates.

The decor paper in such paper laminates generally comprises a resin-impregnated, cellulose pulp-based sheet, with the pulp being based predominantly on hardwoods such as eucalyptus, sometimes in

combination with minor amounts of softwood pulps. Pigments (such as titanium dioxide) and fillers are added in amounts generally up to and including about 45 wt. % (based on the total dry weight prior to resin impregnation) to obtain the required opacity. Other additives such as wet- strength, retention, sizing (internal and surface) and fixing agents may also be added as required to achieve the desired end properties of the paper. The resin can be a thermosetting resin selected from the group consisting of a polymer of diallyl phthalate, epoxide, urea formaldehyde, urea-acrylic acid ester copolyester, melamine formaldehyde, melamine phenol formaldehyde, dicyandiamide-formaldehyde, urethane, curable acrylic, unsaturated polyester and phenol formaldehyde and mixtures thereof.

Titanium dioxide pigments are prepared using either the chloride process or the sulfate process. In the preparation of titanium dioxide pigments by the vapor phase chloride process, titanium tetrachloride, TiCI₄, is reacted with an oxygen containing gas at temperatures ranging from about 900 °C to about 1600 °C, the resulting hot gaseous suspension of ΤΊΟ₂ particles and free chlorine is discharged from the reactor and must be quickly cooled below about 600 °C, for example, by passing it through a conduit, i.e., flue, where growth of the titanium dioxide pigment particles and agglomeration of said particles takes place.

It is known to add various substances, such as silicon compounds and aluminum compounds, to the reactants in order to improve the pigmentary properties of the final product. Aluminum trichloride added during the process has been found to increase rutile in the final product, and silicon tetrachloride that becomes silica in the final product has been found to improve carbon black undertone (CBU), particle size and pigment abrasion. It is useful to be able to add elements to the titanium dioxide particles. However, the process and materials to be added to improve properties of the titanium dioxide particles may be hazardous.

One method of adding elements to the surface of a particle is by impregnation with a solution containing the element. This is difficult to do with pyrogenically prepared metal oxide particles since the properties of the pyrogenically produced metal oxides change upon contact with a liquid medium.

A need exists for a low cost approach for preparing paper laminates comprising pyrogenically prepared metal oxide particles, particularly titanium dioxide particles, comprising elements such as tungsten that provide improved photostability without changing the color of the product.

SUMMARY OF THE DISCLOSURE

In a first aspect, the disclosure provides a resin-impregnated, opaque, cellulose pulp-based sheet comprising inorganic particles, typically inorganic metal oxide or mixed metal oxide particles, more typically titanium dioxide (TiO₂) particles, comprising at least about 0.002 % of tungsten, more typically at least about 0.004 % of tungsten, and still more typically at least about 0.01 % of tungsten, and most typically at least about 0.05 % of tungsten, based on the total weight of the inorganic particles, wherein the inorganic particles, have a photostability ratio (PSR) of at least about 2, more typically at least about 4, and still more typically at least 10, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L* of at least about 97.0, more typically at least about 98, and most typically at least about 99.0, and b* of less than about 4, and more typically less than about 3. Typically the inorganic particles, more typically inorganic metal oxide or mixed metal oxide particles, and most typically titanium dioxide particles, comprising tungsten may further comprise alumina in the amount of about 0.06 to about 5 % of alumina, more typically about 0.2 % to about 4 % of alumina, still more typically about 0.5 % to about 3% of alumina, and most typically about 0.8 % to about 2 %, based on the total weight of the inorganic particles.

In a second aspect, the disclosure provides a paper laminate comprising a resin-impregnated, opaque, cellulose pulp-based sheet, wherein the resin-impregnated, opaque, cellulose pulp-based sheet comprises inorganic particles, typically inorganic metal oxide or mixed metal oxide particles, more typically titanium dioxide (ΤΊΟ2) particles, comprising at least about 0.002 % of tungsten, more typically at least about 0.004 % of tungsten, and still more typically at least about 0.01 % of tungsten, and most typically at least about 0.05 % of tungsten, based on the total weight of the inorganic particles, wherein the inorganic particles, have a photostability ratio (PSR) of at least about 2, more typically at least about 4, and still more typically at least 10, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L* of at least about 97.0, more typically at least about 98, and most typically at least about 99.0, and b* of less than about 4, and more typically less than about 3. Typically the inorganic particles, more typically inorganic metal oxide or mixed metal oxide particles, and most typically titanium dioxide particles, comprising tungsten may further comprise alumina in the amount of about 0.06 to about 5 % of alumina, more typically about 0.2 % to about 4 % of alumina, still more typically about 0.5 % to about 3 % of alumina, and most typically about 0.8 % to about 2 %, based on the total weight of the inorganic particles. The paper laminate further comprises a dried overlay.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic illustration showing the process for preparing titanium dioxide (T1O2).

DETAILED DESCRIPTION OF THE DISCLOSURE

This disclosure relates to a paper laminate comprising a resin- impregnated, opaque, cellulose pulp-based sheet, wherein the resin- impregnated, opaque, cellulose pulp-based sheet comprisies inorganic particles, typically inorganic metal oxide or mixed metal oxide particles, more typically titanium dioxide (T1O2) particles, comprising at least about 0.002 % of tungsten, more typically at least about 0.004 % of tungsten, and still more typically at least about 0.01 % of tungsten, and most typically at least about 0.05 % of tungsten, based on the total weight of the inorganic particles. These inorganic particles have a photostability ratio (PSR) of at least about 2, more typically at least about 4, and still more typically at least 10, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L^* of at least about 97.0, more typically at least about 98, and most typically at least about 99.0, and b^* of less than about 4, and more typically less than about 3. Typically the inorganic particles, more typically inorganic metal oxide or mixed metal oxide particles, and most typically titanium dioxide particles, comprising tungsten may further comprise alumina in the amount of about 0.06 to about 5 % of alumina, more typically about 0.2 % to about 4 % of alumina, still more typically about 0.5 % to about 3 % of alumina, and most typically about 0.8 % to about 2 %, based on the total weight of the inorganic particles, and the paper laminate made therefrom.

The paper laminate typically comprises a dried overlay and a base sheet wherein at least one of the dried overlay and the base sheet can comprise a resin-impregnated, opaque, cellulose pulp-based sheet. The base sheet can comprise a phenolic core or engineered wood comprising substrate such as particle or fiber board. The dried overlay and the base sheet can be laminated together utilizing a low pressure or a high pressure lamination process. The paper laminate may further comprise components to make it abrasion resistant.

Resin-Impregnated, Opaque, Cellulose Pulp-Based Sheet:

The resin-impregnated, opaque, cellulose pulp-based sheet is also known in the industry as Decor paper. The cellulose pulp used in the pulp- based sheet comprises pulp predominantly from hardwoods such as eucalyptus, sometimes in combination with minor amounts of softwood pulps. Pigments (such as titanium dioxide, more typically rutile titanium dioxide comprising tungsten and in addition alumina) and fillers can be added in amounts generally up to and including about 60 wt. %, more typically about 20 % to about 40 % , (based on the total dry weight prior to resin impregnation) to obtain the required opacity. Other additives such as wet-strength, retention, sizing (internal and surface) and fixing agents may also be added as required to achieve the desired end properties of the Decor paper. Resins used to impregnate the papers are typically thermosetting resins. Examples of suitable thermosetting resins include, without limit, polymers of diallyl phthalate, epoxide, urea formaldehyde, urea-acrylic acid ester copolyester, melamine formaldehyde, melamine phenol formaldehyde, dicyandiamide-formaldehyde, urethane, unsaturated polyester, curable acrylic and phenol formaldehyde and mixtures thereof. In some situations, the resin used to impregnate this decorative sheet may contain abrasive inorganic particles selected from the group consisting of aluminum oxide or silicon oxide and mixtures thereof.

This resin impregnated, opaque, cellulose pulp-based sheet may contain a print, pattern design or solid color and these are generated using known techniques. Some such techniques include various well-known analog and digital printing methods to impart desired coloration and designs as required for the particular end use. Analog printing methods such as screen printing are particularly suitable for large runs and repetitive patterns. Digital printing methods such as inkjet printing are particularly suitable for short runs and customized patterning. Some suitable resin-impregnated, opaque, cellulose pulp-based sheets are available from Mead Westvaco (1 1013 West Broad Street, Glen Allen, Virginia 23060), as, solid colored Duoply® papers or printbase Primebase® papers.

Dried Overlay

The dried overlay can be wear resistant and the dried overlay can be used in both low pressure and high pressure lamination processes to provide improved resistance to abrasive wear. The dried overlay can be of varying thickness and can be low opacity, more typically substantially optically transparent.

The dried overlay can comprise a thermosetting resin or can be a resin-impregnated, opaque, cellulose pulp-based sheet as described above. The thermosetting resin used in the dried overlay can be subjected to a pre-cure step prior to the lamination process which also includes a curing step. The term "pre-cure" is used to mean that the cure of the resin particles has been advanced either to the maximum degree possible or at least to a stage of cure where the melt viscosity of the cured resin particles is sufficiently high to prevent these particles from melting and flowing under usual laminating conditions and thus undesirably saturating into the decor paper or other resin-impregnated, opaque, cellulose pulp-based sheet, during the lamination step to form the paper laminate.

The resins are typically thermosetting resins. Examples of suitable thermosetting resins include, without limit polymer of diallyl phthalate, epoxide, urea formaldehyde, urea-acrylic acid ester copolyester, melamine formaldehyde, melamine phenol formaldehyde, dicyandiamide- formaldehyde, urethane, curable acrylic, unsaturated polyester and phenol formaldehyde and mixtures thereof. More typically the resin used in the dried overlay is a formaldehyde-melamine polymer.

Especially when the dried overlay is not a resin impregnated, opaque, cellulose pulp-based sheet, the resin used to impregnate the resin-impregnated opaque cellulose pulp-based sheet typically has the same or substantially the same index of refraction as the resin in the dried overlay. More typically, the resin used in the dried overlay is the same resin used to impregnate the resin-impregnated opaque cellulose pulp- based sheet.

The dried overlay further comprises a binding material, selected from a group consisting of microcrystalline cellulose, carboxyl methyl cellulose, sodium alginate and mixtures thereof.

Optionally, the dried overlay further comprises mineral particles, usually ranging is size from about 20 to about 35 μιτι, comprising aluminum oxide, silicon oxide, or the mixture thereof, to further improve abrasion resistance.

The dried overlay can be transparent after curing.

The dried overlay can be made by processes well known in the paper making industry, by forming a suspension of the resin and the binding material together and drying the suspension to form the dried overlay. Optionally additional ingredients can be employed such as the mineral particles and opacifier, if the dried overlay is to be opaque.

The dried overlay can also be made by applying a thick layer of pre- cured thermosetting resin particles to the decorative sheet, as disclosed in US Patent No. 5,545,476.

Some suitable dried overlays, specifically the melamine-containing overlays are commercially available form Wilsonart International of

Fletcher North Carolina.

Other Components of the Paper laminate

The paper laminate can comprise other components such as a phenolic core sheet, engineered wood sheet, such as particle board or fiber board or plywood. The phenolic core sheet typically comprises a plurality of phenolic resin-impregnated Kraft papers which are laminated together. Glues can also be included usually as seam sealants, for example, a hot wax-oil emulsion. Other suitable glues are made of acrylic polymer, polyvinylacetate, and polychloroprene and commercially available from Wilsonart International of Fletcher North Carolina. Treated Particle:

It is contemplated that any inorganic particle, and in particular inorganic particles that are photoactive, will benefit from the treatment of this disclosure. By inorganic particle it is meant an inorganic particulate material that becomes dispersed throughout a final product such as a polymer melt or coating or paper laminate composition and imparts color and opacity to it. Some examples of inorganic particles include but are not limited to ZnO, ZnS, BaSO₄, CaCO3, T1O2, Lithopane, white lead, SrTiO3, etc.

In particular, titanium dioxide is an especially useful particle in the processes and products of this disclosure. Titanium dioxide (T1O2)

particles useful in the present disclosure may be in the rutile or anatase crystalline form. They are commonly made by either a chloride process or a sulfate process. In the chloride process, TiCI₄ is oxidized to T1O2 particles. In the sulfate process, sulfuric acid and ore containing titanium are dissolved, and the resulting solution goes through a series of steps to yield TiO₂. Both the sulfate and chloride processes are described in greater detail in "The Pigment Handbook", Vol. 1 , 2nd Ed., John Wiley & Sons, NY (1988), the teachings of which are incorporated herein by reference. The particle may be a pigment or nanoparticle.

By "pigment" it is meant that the titanium dioxide particles have an average size of less than 1 micron. Typically, the particles have an average size of from about 0.020 to about 0.95 microns, more typically, about 0.050 to about 0.75 microns and most typically about 0.075 to about 0.50 microns. By "nanoparticle" it is meant that the primary titanium dioxide particles typically have an average particle size diameter of less than about 100 nanometers (nm) as determined by dynamic light scattering that measures the particle size distribution of particles in liquid suspension. The particles are typically agglomerates that may range from about 3 nm to about 6000 nm.

The titanium dioxide particle can be substantially pure titanium dioxide or can contain other metal oxides, such as alumina. Other metal oxides may become incorporated into the particles, for example, by co- oxidizing, post-oxidizing, co-precipitating titanium compounds with other metal compounds or precipitating other metal compounds on to the surface of titanium dioxide particles. These are typically hydrous metal oxides. If co-oxidized, post-oxidized, precipitated or co-precipitated the amount of the metal oxide is about 0.06 to about 5 %, more typically about 0.2 % to about 4 %, still more typically about 0.5 % to about 3 %, and most typically about 0.8 % to about 2 %, based on the total weight of the titanium dioxide particles. Tungsten may also be introduced into the particle using co-oxidizing, or post-oxidizing. If co-oxidized or post- oxidized at least about 0.002 wt. % of the tungsten, more typically, at least about 0.004 wt. %, still more typically at least about 0.01 wt. % tungsten, and most typically at least about 0.05 wt. % may be present, based on the total particle weight.

Process for Preparing Treated Titanium dioxide Particles

The process for producing titanium dioxide particle comprises: a) mixing of chlorides of, titanium, tungsten or mixtures thereof; wherein at least one of the chlorides is in the vapor phase;

(b) oxidizing the chlorides of, titanium, tungsten or mixtures thereof; and

(c) forming titanium dioxide (TiO₂) particles comprising at least about 0.002 % of tungsten, more typically at least about 0.004 % of tungsten and still more typically at least about 0.01 % of tungsten, and most typically at least about 0.05 % of tungsten, based on the total weight of the titanium dioxide particles. These titanium dioxide particles have a photostability ratio (PSR) of at least 2, more typically at least 4, and still more typically at least 10, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L* of at least about 97.0, more typically at least about 98, and most typically at least about 99.0, and b* of less than about 4, and more typically less than about 3. Typically the titanium dioxide particles comprising tungsten further comprise alumina in the amount of about 0.06 to about 5 % of alumina, more typically about 0.2 % to about 4 % of alumina, still more typically about 0.5 % to about 3 % of alumina, and most typically about 0.8 % to about 2 %, based on the total weight of the titanium dioxide particles.

Methods known to one skilled in the art may be used to add tungsten to the titanium dioxide particles. In one specific embodiment, tungsten may be added to the titanium dioxide particle from an alloy comprising tungsten. As shown in Figure 1 , the alloy 1 1 and chlorine 12 are added to the generator 10. This reaction can occur in fluidized beds, spouting beds, packed beds, or plug flow reactors. The inert generator bed may comprise materials such as silica sand, glass beads, ceramic beads, T1O2 particles, or other inert mineral sands. The alloy comprising aluminum, titanium or mixtures thereof and tungsten, 1 1 , reacts in the generator 10 according to the following equations:

2AI + 3 Cl₂ 2AICI₃ + heat

Ti + 2 Cl₂ -> TiCI₄ + heat

W + 3 CI₂ ^ WCIe + heat

AI12W + 21 Cl₂ 12AICI₃ + WCIe + heat

The heat of reaction from the chlorination of the aluminum or titanium metal helps provide sufficient heat to drive the kinetics of the reaction between chlorine and one or more of the other elements.

Titanium tetrachloride 17 may be present during this reaction to absorb the heat of reaction. The chlorides formed in-situ comprise chlorides of the tungsten and chlorides of aluminum such as aluminum trichloride, chlorides of titanium such as titanium tetrachloride or mixtures thereof. The temperature of the reaction of chlorine with the alloy should be below the melting point of the alloy but sufficiently high enough for the rate of reaction with chlorine to provide the required amount of chlorides to be mixed with the TiCI₄.

Typical amounts of chlorine used in step (a) are about 0.4 % to about 20 %, more typically about 2 % to about 5 %, by weight, based on the total amount of all reactants. Typical amounts of titanium tetrachloride are about 75 % to about 99.5 % added in step (a) and (b), and more typically about 93 % to about 98 %, by weight, based on the total amount of all reactants.

The reaction of chlorine with the alloy occurs at temperature of above 190 °C, more typically at temperature of about 250 °C to about 650 °C , and most typically at temperatures of about 300 °C to about 500 °C. In one specific embodiment where the metal is Ti the reaction occurs at temperature of above 50 °C (bp of TiCI₄= 136 °C), more typically at temperature of about 200 °C to about 1000 °C, and most typically at temperatures of about 300 °C to about 500 °C.

The chlorides formed in the in-situ step 13 flows into an oxidation reactor 14 and titanium tetrachloride 15 is then added to the chlorides, such that titanium tetrachloride is present in a major amount. Vapor phase oxidation of the chlorides from step (a) and titanium tetrachloride is by a process similar to that disclosed, for example, in U.S. Pat. Nos. 2,488,439, 2,488,440, 2,559,638, 2,833,627, 3,208,866, 3,505,091 , and 7,476,378. The reaction may occur in the presence of neucleating salts such as

potassium chloride, rubidium chloride, or cesium chloride.

Such reaction usually takes place in a pipe or conduit, wherein oxygen 16, titanium tetrachloride 15 and the in-situ fomned chlorides comprising chlorides of tungsten and chlorides of aluminum such as aluminum trichloride, chlorides of titanium such as titanium tetrachloride or mixtures thereof 13 are introduced at a suitable temperature and pressure for production of the treated titanium dioxide. In such a reaction, a flame is generally produced.

Downstream from the flame, the treated titanium dioxide produced is fed through an additional length of conduit wherein cooling takes place. For the purposes herein, such conduit will be referred to as the flue. The flue should be as bng as necessary to accomplish the desired cooling. Typically, the flue is water cooled and can be about 50 feet (15.24 m) to about 3000 feet (914.4 m), typically about 100 feet (30.48 m) to about 1500 feet (457.2 m), and most typically about 200 feet (60.96 m) to 1200 feet (365.76 m) long.

The following Examples illustrate the present disclosure. All parts, percentages and proportions are by weight unless otherwise indicated. EXAMPLES

Photostabilitv ratio (PSR) is the rate of photoreduction of Ag+ by T1O2 particles without tungsten (control samples) divided by the rate of photoreduction of Ag+ by the otherwise same ΤΊΟ2 particles comprising tungsten. The rate of photoreduction of Ag+ can be determined by various methods. A convenient method was to suspend the ΤΊΟ2 particles in 0.1 M AgNO3 aqueous solution at a fixed ratio of ΤΊΟ2 to solution, typically 1 :1 by weight. The suspended particles were exposed to UV light at about 0.2 mW./cm² intensity. The reflectance of visible light by the suspension of TiO₂ particles was monitored versus time. The reflectance decreased from the initial value to smaller values as silver metal was formed by the photoreduction reaction, Ag⁺ -> Ag°. The rate of reflectance decrease versus time was measured from the initial reflectance ( 100 % visible reflectance with no UV light exposure) to a reflectance of 90 % after UV exposure; that rate was defined as the rate of Ag⁺ photoreduction.

Color as measured on the CIE 1976 color scale , L*, a*, and b* , was measured on pressed pellets of dry TiO₂ powder. Comparative Example 1 .

Titanium dioxide made by the chloride process comprising 1 .23 % alumina by weight and having an L*a*b* color index of (99.98, 0.60, 2.13) and a rate of Ag⁺ photoreduction of 0.0528 sec^"1 was fired under flowing oxygen at 4 °C/min to 1000 °C and held at temperature for 3 hours;

furnace cooled to 750 °C and held at temperature for 1 hour; furnace cooled to 500 °C and held at temperature for 3 hours; furnace cooled to 250 °C and held at temperature for 3 hours; and finally furnace cooled to room temperature. After firing the sample had an L*a*b* color index of (99.15, -0.45, 2.17) and a rate of Ag⁺ photoreduction of 0.1993 sec^"1.

Comparative Example 2.

Titanium dioxide made by the chloride process comprising 0.06 % alumina by weight and having an L*a*b* color index of (99.43, -0.58, 1 .36) and a photoractivity rate of 0.3322 was fired under flowing oxygen at 4 °C/min to 1000 °C and held at temperature for 3 hours; furnace cooled to 750 °C and held at temperature for 1 hour; furnace cooled to 500 °C and held at temperature for 3 hours; furnace cooled to 250 °C and held at temperature for 3 hours; and finally furnace cooled to room temperature. After firing the sample had an L*a*b* color index of (97.71 , -0.03, 1 .89) and a photoactivity rate of 0.2229 sec^"1.

Example 3.

Titanium dioxide similar to that described in Comparative Example

1 was well mixed with various amounts of ammonium tungstate,

(NH )ioWi2O₄i -5H₂O, to give samples having the W contents listed below. These samples were fired as described in Comparative Example 1 . After firing the samples had L*a*b* color and photostability ratios (PSR) as given in the following table:

The increased incorporation of W clearly enhanced photostability up to roughly a factor of 200 while the color was only minimally affected.

Example 4.

Titanium dioxide similar to that described in Comparative Example 1 was impregnated via incipient wetness with various amounts of ammonium tungstate, (NH )i₀Wi2O₄i -5H₂O, to give samples having the W contents listed below. These samples were fired as described in

Comparative Example 1 . After firing the samples had L*a*b* color and photostability ratios as given in the following table: W (wt. %) L* a* b* PSR

0.0 98.16 0.02 2.09 1.0

0.34 97.97 -0.02 2.53 2.2

1.72 97.52 -0.15 2.79 10.0

3.44 97.41 -0.53 3.34 67.4

The increased incorporation of W clearly enhanced photostability up to roughly a factor of 67 while the color index was only minimally affected. Example 5.

Titanium dioxide similar to that described in Comparative Example 2 was well mixed with amounts of ammonium tungstate,

(NH )ioWi2O₄i -5H₂O, to give samples having the W contents listed below. These samples were fired as described in Comparative Example 1 . After firing the samples had L^*a^*b^* color and photostability ratios as given in the following table:

The increased incorporation of W clearly enhanced photostability up to roughly a factor of 140 while the color index was only minimally affected.

Comparative Example 6.

Titanium dioxide similar to that described in Comparative Example 1 was well mixed with various amounts of ammonium molybdate,

(ΝΗ )6Μθ7θ2₄-4Η₂Ο, to give samples having the Mo contents listed below. These samples were fired as described in Comparative Example 1 . After firing the samples had L^*a^*b^* color and photostability ratios as given in the following table: Mo (wt. %) L* a* b* PSR

0.0 98.76 -0.37 2.48 1

0.18 94.08 -3.45 17.96 314.8

0.91 93.77 -4.47 30.45 no rate

1.83 91.89 -5.27 35.82 no rate

The increased incorporation of Mo clearly enhanced photostability to the point where, at the higher Mo concentrations, the photostability ratio could not be determined. However, the material took on a decidedly yellow coloration clearly compromising its use as a white pigment.

Comparative Example 7.

Titanium dioxide similar to that described in Comparative Example 1 was impregnated via incipient wetness with various amounts of ammonium molybdate, (ΝΗ )₆Μθ7θ₂₄-4Η₂Ο, to give samples having Mo to Al atomic ratios of 0.1 , 0.5, and 1 .0 versus 0.0 for the undoped control. These samples were fired as described in Comparative Example 1 . After firing the samples had L^*a^*b^* color and photostability ratios as given in the following table:

The incorporation of Mo clearly enhanced photostability to the point where, at the highest Mo concentration, the photostability ratio could not be determined. However, the material took on a decidedly yellow coloration clearly compromising its use as a white pigment. Example 8:

Whatman #1 filter paper is impregnated with a slurry consisting of 10 wt. % of the dry titanium dioxide samples having W contents as listed in Example 3 and a 50 wt. % aqueous solution of Kauramin® 773

impregnating resin (melamine formaldehyde powder). The impregnated paper is dried in a convection oven at 230 °F. Laminate lay-ups are constructed between two steel caul plates. From the bottom up, the construction is as follows: a) single overlay sheet (LK2050FK MELAMINE IMPREGNATED 20# BASE WEIGHT, WHITE OVERLAY) b) single white backing sheet (L2028050 MELAMINE

IMPREGNATED 80# BASE WEIGHT, WHITE BACKER) c) three sheets of Kraft paper (C99N50CG PHENOLIC

IMPREGNATED 99# BASE WEIGHT, KRAFT PAPER) single white backing sheet (see (b) above) e) two sheets of the Whatman #1 filter paper impregnated with T1O2 slurry f) single overlay sheet (see (a) above)

The laminate is formed using a Carver press heated to 300 °F under a force of 36,000 pounds for six minutes.

Claims

CLAIMS What is claimed is:

1 . A resin-impregnated, opaque, cellulose pulp-based sheet comprising an inorganic particle, wherein the inorganic particle comprises at least about 0.002 % of tungsten, based on the total weight of the inorganic particle, and has a photostability ratio (PSR) of at least about 2, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L* of at least about 97.0, and b* of less than about 4.

2. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 1 wherein the inorganic particle is an inorganic metal oxide or mixed metal oxide particle.

3. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 2 wherein the inorganic metal oxide particle is titanium dioxide.

4. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 further comprising a resin.

5. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 4 wherein the resin is a thermosetting resin.

6. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 5 wherein the thermosetting resin is a polymer of diallyl phthalate, epoxide, urea formaldehyde, urea-acrylic acid ester copolyester, melamine formaldehyde, melamine phenol formaldehyde, dicyandiamide- formaldehyde, urethane, unsaturated polyester, curable acrylic and phenol formaldehyde or mixtures thereof.

7. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein the amount of the titanium dioxide in the resin- impregnated, opaque, cellulose pulp-based sheet is up to and including about 60 wt. %, based on the total dry weight of the cellulose pulp-based sheet prior to resin impregnation.

8. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 7 wherein the amount of the titanium dioxide in the the resin- impregnated, opaque, cellulose pulp-based sheet ranges from about 20 to about 40 wt. %, based on the total dry weight of the cellulose pulp-based sheet prior to resin impregnation.

9. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein tungsten is present in the amount of at least about 0.004 %, based on the total weight of the inorganic particle.

10. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein the photostability ratio (PSR) is at least about 4.

1 1 . The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein L* is at least about 98.

12. resin-impregnated, opaque, cellulose pulp-based sheet of claim

3 wherein b* is less than about 3.

13. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein tungsten is added to the titanium dioxide particle by cooxidation or post-oxidation.

14. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein tungsten is added to the titanium dioxide particle from an alloy comprising tungsten.

15. The resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 wherein the titanium dioxide particle further comprises alumina in the amount of about 0.06 to about 5 % based on the total weight of the titanium dioxide particle.

16. A paper laminate prepared from a resin-impregnated, opaque, cellulose pulp-based sheet comprising an inorganic particle, wherein the inorganic particle comprises at least about 0.002 % of tungsten, based on the total weight of the inorganic particle, and has a photostability ratio (PSR) of at least about 2, as measured by the Ag⁺ photoreduction rate, and color as depicted by an L* of at least about 97.0, and b* of less than about 4.

17. The paper laminate of claim 16 further comprising a dried overlay.

18. The paper laminate of claim 16 wherein the inorganic particle in the coating composition is an inorganic metal oxide or mixed metal oxide particle.

19. The paper laminate of claim 18 wherein the inorganic metal oxide particle is titanium dioxide.

20. The paper laminate of claim 16 wherein tungsten is present in the amount of at least about 0.004 %, based on the total weight of the inorganic particle.

21 . The paper laminate of claim 19 wherein the titanium dioxide particle further comprises alumina in the amount of about 0.06 to about 5 % based on the total weight of the titanium dioxide particle.