WO2008127738A1

WO2008127738A1 - Reflective article

Info

Publication number: WO2008127738A1
Application number: PCT/US2008/004974
Authority: WO
Inventors: Ralph H. Staley; John W. Hallam
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2007-04-16
Filing date: 2008-04-16
Publication date: 2008-10-23

Abstract

A reflective article, an optical display and a method for improving light reflectivity in a device requiring reflectivity of light is disclosed herein that has a reflector (10) that includes a microporous polyacrylate film containing a plurality of inorganic particles and having a high photopic reflectance of visible light. The reflector can be a diffuse reflector.

Description

TITLE REFLECTIVE ARTICLE

FIELD OF INVENTION

The present invention relates to a reflective article having a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles and having a high reflectance of visible light.

BACKGROUND

Reflectivity of visible light is important in many applications. Direct view displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs)), whether relying on supplemental lights (e.g., backlight) or ambient light, require reflectant back surfaces to maximize image quality and intensity. Reflectivity is particularly important with backlit direct view displays in battery powered equipment, where reflectivity improvements directly relate to smaller required light sources and thus lower power demands. In many of these applications diffuse reflectivity is required.

Portable computer LCDs are a substantial and demanding market requiring high levels of reflection of visible light from very thin materials. For certain markets it is important that the backlight reflector is relatively thin, i.e., less than 300 μm and sometimes less than 150 μm, to minimize the thickness of the completed display.

The reflective material used in LCD backlights has a significant effect on the brightness, uniformity, color and stability of the backlight unit and, ultimately, the LCD module. For a direct view LCD backlight, requirements for the reflector can include high photopic reflectance (e.g., sometimes greater than 95%) and stability under use conditions including cavity temperatures of 50⁰C to 70⁰C, stability to ultraviolet (UV) light from cold cathode fluorescent lamp (CCFL) sources, as well as to humidity and temperature cycling. In direct view backlights, the reflector is an integral part of the backlight unit and, therefore, the physical properties of the material are also important. Requirements for an edgelit backlight differ in that the operating temperature is typically lower and the need for UV stability can be less in instances where there is UV absorption in the light guide. However, additional requirements on edgelit backlight reflectors include the need to make uniform contact with the light guide without damaging it, and minimizing reflector thickness.

Due to the many different applications that exist for reflectant materials, it is not surprising that there are a wide variety of commercially available products with an array of diffuse and specular reflective properties. Major industrial efforts are underway to fabricate reflector sheet stock used to enhance the image quality of LCD screens in a variety of evolving electronic optical display devices. Methods used to increase reflectance while maintaining thinness and stability include the addition of fillers such as fibrils and inorganic oxides, and the use of bubbles or microvoids.

An industry standard diffuse reflective material is described in U.S. Patent Number 4,912,720 and sold under the trademark SPECTRALON^® by Labsphere, Inc., North Sutton, NH, USA. This material comprises lightly packed granules of polytetrafluoroethylene having a void volume of about 30% to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Patent Number 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with photopic reflectance over the visible wavelengths of light of better than 99%. Despite the advantages of such material, it is not generally available in very thin films of less than 250 μm, such as those needed for the laptop LCD market, and furthermore at these thickness levels, adequate reflection performance is not obtained.

Other methods to prepare a film or sheet containing micropores or microvoids are described in U.S. Patent Appln. 2004/0266930, U.S. Patent Nos. 5,672,409, 5,976,686, 5,269,977, 5,982,548 and 5,710,856.

Co-pending U.S. Patent Application No. 20060262310 filed May 15, 2005, for the present assignee, discloses an article containing a diffuse reflector of light comprising a nonwoven sheet containing a plurality of pores. The pores are disclosed to have a certain specific pore volume and high visible light scattering efficiency and the diffuse reflector is disclosed as having a high photopic reflectance of visible light. Multiple layers of this nonwoven sheet as a laminated multilayer reflector offers a low cost alternative to established film-based reflectors. However, weaknesses of this approach center on the basic nonwoven sheet thickness non- uniformity, visual surface appearance and dimensional stability relative to polyester film competitors.

Thus, unique opportunities exist to further enhance the performance of reflectors. Improved and inexpensive reflectors are needed for visible light management applications that will allow for production of more affordable and energy efficient optical displays.

SUMMARY OF THE INVENTION

Briefly stated and in accordance with one aspect of the present invention there is provided a reflective article comprising a reflector of light positioned within a structure defining an optical cavity, wherein said reflector comprises a microporous polyacrylate film containing a plurality of inorganic particles, wherein the film has a R550 reflectance of greater than about 80%. The article can be a diffuse reflector, and the polyacrylate film can be polymethyl(meth)acrylate.

Pursuant to another aspect of the present invention, there is provided an optical display, comprising: (i) a structure defining an optical cavity; (ii) a light source positioned within said optical cavity; (iii) a display panel through which light from said light source passes; and (iv) a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles wherein the film has a R550 reflectance of greater than about 80%.

Pursuant to another aspect of the present invention, there is provided a method of improving light reflectivity in a device requiring reflectivity of light comprising: (i) providing a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles wherein the film has a R550 reflectance of greater than about 80%; and (ii) positioning said reflector within the device to cause light energy to reflect off of one face of the reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross sectional view of a side-lit liquid crystal optical display utilizing a reflector. Fig. 2 is a cross sectional view of a backlit liquid crystal optical display with a cold cathode fluorescent lamp light source utilizing a reflector. Fig. 3 is a cross sectional view of a backlit liquid crystal optical display with a light emitting diode light source utilizing a reflector.

Fig. 4 is a graph depicting the reflectance of PMMA films from Example 4 as a function of TΪO2 concentration over a range of wavelengths.

DETAILED DESCRIPTION

Disclosed herein is a reflective article comprising a reflector of light positioned within a structure defining an optical cavity, wherein said reflector comprises a microporous polyacrylate film containing a plurality of inorganic particles, wherein the film has a R550 reflectance of greater than about 80%. The reflector can be a diffuse reflector.

The term "light" as used herein means electromagnetic radiation in the visible light portion of the spectrum, from 380 nm to 780 nm wavelength. Unless stated otherwise, "R550 reflectance" is defined as the average percentage reflectance values from 500 nm to 600 nm.

Optical cavity" refers herein to an enclosure designed to receive light from a light source, and condition and direct such light toward an object benefiting from illumination. Optical cavities include structures for integrating, redirecting and/or focusing light from a source onto a receiver and may use air or high refractive index elements as the cavity medium. The geometrical shape of the structure is not limited. Example structures containing optical cavities include luminaires, copying machines, projection display light engines, integrating sphere uniform light sources, sign cabinets, light conduits and backlight assemblies. In certain embodiments, such as backlight units for liquid crystal displays (LCDs), the optical cavity may include a lightguide or waveguide. Where the reflective article is a component of an optical display, optical cavity refers to an enclosure designed to contain a light source and direct the light from the light source toward a display panel. Display panels include static and dynamic (addressable) display types.

The reflective article or optical display disclosed herein contains a light source positioned within the optical cavity. "Light source" refers herein to emitters of visible light and can be a single light source within an optical cavity or multiple light sources within an optical cavity. Example light sources include bulb and tube lamps of type incandescent, mercury, metal halide, low pressure sodium, high pressure sodium, arc, compact fluorescent, self ballasted fluorescent, cold cathode fluorescent lamp (CCFL), light emitting diode (LED) and similar apparatus capable of emitting visible light.

The present reflective article or optical display can contain a display panel through which light from the light source passes. "Display panel" refers herein to transmissive devices that modulate the transmission of light from the light source, and in certain embodiments, modulate the light for the purpose of conveying an image in the form of visible light to a viewer. In the embodiment where the structure defining the optical cavity is a luminaire or sign cabinet system for the purpose of conveying a static image to a viewer, example display panels include polymer or glass panels with a static image contained thereon (e.g., a text or pictorial image) or alternately, no image (e.g., a fluorescent light diffuser). In the embodiment where the structure defining an optical cavity is a backlight unit for a liquid crystal display for the purpose of conveying static and/or changing images to a viewer, an example display panel includes a liquid crystal with an image which changes in response to an electronic signal.

The present reflective article or optical display contains a reflector positioned within the optical cavity for reflecting light toward an object benefiting from illumination. The reflector can be a diffuse reflector. The reflector is positioned within the optical cavity so that it reflects back toward the object light within the optical cavity which is not directed toward the object. In an optical display, the reflector is positioned behind the optical display light source illuminating the display panel. The light scattering and diffuse reflection characteristics of diffuse reflectors disclosed herein provides more overall diffuse lighting, e.g., a more overall diffuse light source and therefore a more evenly lit or uniformly illuminated optical display. The diffuse reflector can also have some specular reflectance component along with the diffuse reflectance component. Schematic figures of several embodiments of optical displays utilizing reflective articles disclosed herein are shown in FIGS. 1-3. FIG. 1 is a cross sectional view of a side-lit liquid crystal optical display utilizing a reflector disclosed herein. In FIG. 1 , an optical display 1 is shown having a fluorescent light source 2 coupled to an optical cavity containing a plastic light guide 3. A diffuser 4, an optional brightness enhancing film 5, such as films described in U.S. Patents numbered 4,906,070 and 5,056,892 and available from Minnesota Mining and Manufacturing Co. (3M), Minneapolis, MN, USA, and an optional reflective polarizer film 6 (also available from 3M) as described in PCT publication WO 97/32224, is placed on top of the guide 3 and act to redirect and reflectively polarize the light emitted from the guide 3 toward a liquid crystal display panel 7 and a viewer. A liquid crystal display panel 7 is placed on top of the reflective polarizing film 6 and is typically constructed of a liquid crystal 8 contained between two polarizers 9.

The light guide 3 directs light towards the display panel 7 and ultimately a viewer. Some light is reflected from the back surface of the light guide. A reflector 10 as disclosed herein is placed behind the light guide 3 and reflects light towards the liquid crystal display panel 7. It also reflects and randomizes the polarization of the light reflected from the reflective polarizing film 6 and brightness enhancing film 5 layers. The reflector 10 is a highly reflective surface that can have high diffusivity and enhances the optical efficiency of the optical cavity. The reflector 10 scatters and/or reflects light either diffusely, specularly, or a combination thereof, depolarizes the light, and has high reflectance over the visible wavelength range.

The reflector 10 is an element of a light recycling system. The reflector (i) reflects light rejected from the reflective polarizing film 6 and/or from the brightness enhancement film 5, and (ii) gives that light another opportunity to reach the liquid crystal display panel 7 and ultimately a viewer. This rejecting and recycling can occur numerous times increasing the luminance of the optical display (i.e., the amount of light directed towards the viewer). This increased optical efficiency of the reflector can be used to reflect incident light between layer 5 and the reflector 10 to increase display luminance by controlling the angles over which light is emitted. For instance, brightness enhancing film 5 transmits light within a specific, and narrow angular range and reflects light over another, specific and wider angular range. The reflected light is reflected and/or scattered by the reflector 10 into all angles. The light within the transmission angles of the brightness enhancing layer 5 is transmitted towards the viewer. Light in the second angular range is reflected by layer 5 for additional reflection and/or scattering by the reflector 10. The increased optical efficiency of the reflector 10 can be used to reflect incident light between the reflective polarizer film 6 and the reflector 10 to increase display luminance by controlling the polarization state of the light transmitted through the reflective polarizer film 6. Most displays have an absorbing polarizer 9 applied to the back of the display panel 8. At least one half of the available light is absorbed when the display is illuminated by unpolarized light. As a result, display luminance is decreased and the display polarizer 9 is heated. Both adverse situations are overcome with the use of a reflective polarizer film 6, because the reflective polarizer film 6 transmits light of one linear polarization state and reflects the other linear polarization state. If the transmission axis of the reflective polarizer film 6 is aligned with the absorbing polarizer transmission axis, the transmitted light is only weakly absorbed by the absorbing polarizer. Also, the light in the reflected polarization state is not absorbed at all by the absorbing polarizer. Instead, it is reflected towards the reflector 10. The reflector 10 depolarizes the light, creating a polarization state that has equal polarization components in the reflective polarizer film transmission and reflection states. One half of the light transmits through the reflective polarizer layer 6 towards the viewer. Light in the reflected polarization state, or "undesirable" state, is again reflected and/or scattered by the reflector 10, providing yet another chance for additional polarization conversion.

Additionally, a reflector 11 as disclosed herein may be placed behind or around the light source 2, such as a cold cathode fluorescent lamp (CCFL) to increase light coupling efficiency into the plastic light guide 3. When the reflector 11 is a diffuse reflector it may be used alone, or in combination with a specular reflector to increase the total reflectance of the construction. When such a specular reflector is used, it is positioned behind the diffuse reflector 11 such that the diffuse reflector remains facing the light source 2.

The increased optical efficiency of the reflector disclosed herein can be used to increase the reflective efficiency of an optical cavity and/or to mix discrete wavelengths of light to make a uniform colored or white light source. FIG. 2 is a cross sectional view of a backlit liquid crystal optical display with a cold cathode fluorescent lamp light source utilizing a reflector disclosed herein. In the optical display 1 shown in FIG. 2, three fluorescent lamps 12 are depicted in an optical cavity 13. All of the lamps may be white or each lamp may be a selected color, such as red, green and blue. FIG. 3 is a cross sectional view of an alternate configuration of a backlit liquid crystal optical display with a light emitting diode light source utilizing a reflector as disclosed herein. In the optical display 1 shown in FIG. 3, the liquid crystal optical display device is shown with two light emitting diodes (LEDs) 14 as the light source providing light to an optical cavity 13. The diodes may be colored or white. In both FIGs. 2 and 3, the optical cavity 13 is lined with a reflector 15. Reflector 15 both increases reflectance and mixes the discrete light colors adequately to form a white light source with good spatial light emitting uniformity for illumination of the liquid crystal display panel 7.

By "microporous", also known as "microvoid", it is meant that the film contains pores that are microscopic in size. The pores are typically, but not exclusively, interconnected and open to the outside of the film. The pores are typically substantially about 30 nm to about 1 micron in diameter.

The film contains scattering centers that are typically about 50 nm to about 500 nm, more typically about 200 nm. By "scattering center" it is meant a spatial region that has an index of refraction differing from surrounding regions where at least one dimension of the region is in the range of 50 nm to 500 nm, or more typically about 200 nm, for the scattering of visible light. The scattering center can be either a lower index region surrounded by higher index regions, e.g. a pore or microvoid surrounded by polymer, or a higher index region surrounded by lower index regions, e.g. a wall or filament of polymer between pores or microvoids.

Herein the terms "acrylate", "acrylate resin", "(meth)acrylate resins", "polyacrylates" and "acrylate polymers", are synonymous unless specifically defined otherwise. These terms refer to the general class of addition polymers derived from the conventional polymerization of ethylenically unsaturated monomers derived from the alkyl and substituted-alkyl esters of methacrylic and acrylic acids. The terms encompass specifically the homopolymers and copolymers of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, (meth)acrylic acid and glycidyl (meth)acrylate. The term (meth)acrylate encompasses methacrylate and acrylate.

The term polymethyl(meth)acrylate (PMMA) herein encompass homopolymers and copolymers derived from the esters of methacrylic acid. The microporous polyacrylate film can be a homopolymer or copolymer comprising polyacrylate. The term copolymer encompasses polymers derived from polymerization of two or more monomers, unless specifically defined otherwise. These polymers can be prepared by any method known in the art, such as those disclosed in "Acrylic Ester Polymers", Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., 2003, DOI:10.1002/0471238961.1921182214152201. aO1.pub2, and "Methacrylic Ester Polymers", Encyclopedia of Polymer Science and Technology John Wiley & Sons, Inc., 2002, DOI: 1002/0471440264.pst196. The film, which can also be known as sheet or membrane, can be free standing, suspended in a frame or supported on a solid support, described further hereinbelow. Typically the film will have an average thickness of about 20 to about 500 microns.

Most of the techniques known in the art can be utilized to prepare the polyacrylate film as a microporous film. These include sintering (described in U.S. Patent No. 4,912,720) and stretching (described in U.S. Patent No. 5,710,856). One suitable technique is by solvent precipitation, reviewed in Kahne, C. F., Industrial & Engineering Chemistry Research, 2001 , 40(1), 33-36. Solvent precipitation can be performed using several methods, such as the quenching process, thermal precipitation, and solvent evaporation. The solvent mixture is determined by the polymer of interest; it will comprise at least one solvent that is a good solvent for the instant polymer and at least one nonsolvent. For polyacrylate films suitable solvents and nonsolvents include water, DMF (dimethylformamide), acetone, and alcohols such as butanol.

The polyacrylate reflectors can further comprise conventional polymer additives, such as plasticizers, stabilizing agents, deterioration inhibitors, dispersants, antistatic agents, curing agents, leveling agents, ultraviolet absorbers, anti-oxidizing agents, viscosity modifying agents, lubricants, light stabilizers and the like.

The microporous polyacrylate film contains a plurality of inorganic particles. These particles can include silicates, alkali metal carbonates, alkali earth metal carbonates, alkali metal titanates, alkali earth metal titanates, alkali metal sulfates, alkali earth metal sulfates, alkali metal oxides, alkali earth metal oxides, transition metal oxides, metal oxides, alkali metal hydroxides and alkali earth metal hydroxides. Specific examples include titanium dioxide, calcium carbonate, clay, mica, talc, hydrotalcite, magnesium hydroxide, silica, silicates, hollow silicate spheres, wollastonite, feldspar, kaolin, magnesium carbonate, barium carbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide, calcium oxide, magnesium oxide, alumina, asbestos powder, glass powder, silicon carbide, and zeolites. The particles can also be a mixture of two or more different types. Typically the particles comprise titanium dioxide or zinc oxide, or a combination thereof; more typically titanium dioxide. Suitable particles typically have an index of refraction of greater than or equal to about 1.7, or greater than or equal to about 2. The microporous polyacrylate film disclosed within containing a plurality of inorganic particles can have a R550 reflectance of greater than about 80%, greater than about 90%, greater than about 95%, greater than about 97%, or greater than about 98%. Substantially all of the inorganic particles will typically have a mean particle diameter of about 30 to about 500 nm; more typically about 100 to about 300 nm.

The reflectors disclosed herein can further comprise backing support material to maintain the shape of the reflector during reflective article assembly and use. Such backing support material is positioned on the face of the reflector facing away from the light source. Backing support materials of utility include polyester films (e.g., Mylar®), aramid fiber (e.g., Kevlar®), both available from E. I. du Pont de Nemours & Co., Wilmington, DE, USA, as well as paper, fabric or wovens, nonwoven sheets, foamed polymer, polymer films, metal foil or sheet and metallized film. The backing support material can be selected so as to increase the total reflectance of the reflector (e.g., metal foil or sheet and metallized film). The backing support material and reflector may be laminated to one another with the aforementioned adhesives by conventional techniques. In addition, to create reflectors of complex shapes, reflectors disclosed herein can be bonded to a rigid support material and then formed as a composite into shapes, such as parabolic or ellipsoidal domes.

The reflectors disclosed herein can further comprise a specular reflective layer positioned on the face of a diffuse reflector. Positioning a specular reflector as such increases the photopic reflectance of the diffuse reflector. In one embodiment, a face of the diffuse reflector may be metallized. Representative metals include aluminum, tin, nickel, iron, chromium, copper, silver or alloys thereof, with aluminum preferred. Metals may be deposited by known vacuum metallization techniques in which metal is vaporized by heat under vacuum, and then deposited on one face of the reflector in a thickness from about 75 angstroms to about 300 angstroms. In another embodiment, the specular reflective layer comprises a metallized polymer sheet, for example aluminized MYLAR®, which can be laminated to a diffuse reflector, with a metallized face of the metallized polymer sheet facing a face of the PMMA film. In another embodiment, the specular reflective layer comprises a metal foil, for example aluminum foil, which can be laminated to a face of a diffuse reflector, resulting in a stiffened diffuse reflector. The reflectors of this embodiment can be formed by laminating a metal foil to a reflector using an adhesive. In these embodiments where a reflector contains a metallized face or is laminated to a metallized polymer sheet or metal foil, the remaining (metal-free) face of the reflector is positioned in the optical cavity facing the light source. Light emitting diodes (LEDs) are useful light sources for small liquid crystal display (LCD) devices such as cell phones and hand held devices. LEDs provide the advantages of small size and lower energy consumption, but they have relatively low luminance. The optical efficiency of designs using LED illumination is increased when a reflector is used as a back reflector in combination with the aforementioned brightness enhancing and reflective polarizer films. LEDs can replace fluorescent lamps as the preferred backlight source for small LCDs such as cell phones, hand held devices, medical monitors and automotive displays. The advantage of using LEDs is their low price, small size and low energy consumption. The disadvantage of LEDs is their relatively low brightness. With the use of a diffuse reflector as a back reflector along with known specular reflective film layers, the brightness of LED displays can be increased.

As disclosed is a method of improving light reflectivity in a device requiring reflectivity of light comprising: (i) providing a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles wherein the film has a R550 reflectance of greater than about 80%; and (ii) positioning said reflector within the device to cause light energy to reflect off of one face of the reflector. The reflector can be a diffuse reflector.

TEST METHODS

Reflectance Spectra R550 ^was obtained as the average of the reflectance values from

500 nm to 600 nm. Reflectance spectra of the reflector films were obtained by measurements made using an X-rite SP64 and X-rite Color Master software from X-Rite Incorporated, Grand Rapids, Ml USA. The instrument was calibrated for white and black with the supplied calibration standard. The output is a percent reflectance at each wavelength and the spectral range measured is 400 nm to 700 nm in 10 nm intervals.

Thickness

Film thickness measurements were made with an Ono Sokki EG- 225 thickness gauge with a 0.95 cm (3/8 inch) measurement probe affixed to a Ono Sokki ST-022 ceramic base gauge stand, both available from

Ono Sokki, Addison, IL, USA.

EXAMPLES

Preparation of Acrylate Films

Example 1

Quenching Process

Microporous polymer reflector films were fabricated in a four-step process: (1) A 25% solution of 120K MW poly(methyl methacrylate) (PMMA) from Aldrich, Milwaukee, Wl in dimethylformamide (DMF) was drawn down onto a glass plate to give a 15 mil (381 microns) thick film. (2) The glass plate and film were immediately placed into a bath of DMF- water in an 80:20 ratio at room temperature for 10 minutes. During this time the film turned white and separated from the glass. (3) The film was transferred to a bath with water but no DMF for 10 minutes at room temperature. (4) The film was removed and dried in air at room temperature. The thickness of the film was determined to be 218 microns and the R550 was 98.2%. The procedure was repeated using a 50:50 ratio of DMF-water. The resulting film had a thickness of 207 microns and a R550 of 96.4%.

Example 2

Thermal Precipitation Process

A microporous polymer reflector was fabricated in a two-step process: (1) A 20% solution of 120K MW poly(methyl methacrylate) (PMMA) from Aldrich, Milwaukee, Wl was prepared in solvent mixture of acetone-water-butanol in an 80:16:4 ratio at 50⁰C. The solution was drawn down onto a clear cellulose acetate film to give a 15 mil (381 microns) thick film which immediately turned white. (2) The film was air dried. The thickness of the film was determined to be 154 microns and a R550 of 94.0%..

Example 3

Solvent Evaporation Process A microporous polymer reflector was fabricated in a two-step process: (1) An 18% solution of 120K MW poly(methyl methacrylate) (PMMA) from Aldrich, Milwaukee, Wl was prepared in solvent mixture of acetone-water in an 86:14 ratio. The solution was drawn down onto a clear cellulose acetate film to give a 15 mil (381 microns) thick film. (2) The film was air dried. The film turned white as it dried. The thickness of the film was determined to be 139 microns and the R550 of the film was 94.8%. Alternately, a microporous polymer reflector was fabricated in a two-step process: (1) An 8% solution of 410K MW Elvacite® 2041 poly(methyl methacrylate) (PMMA) from Lucite International, Cordove, TN was prepared in solvent mixture of acetone-water in an 88:12 ratio. The solution was drawn down onto a clear cellulose acetate film to give a 40 mil (1016 microns) thick film. (2) The film was air dried. The film turned white as it dried. The thickness of the film was determined to be 174 microns and a R550 of 97.7%.

Example 4

Pigmented Film Pigmented microporous polymer reflector films were fabricated in a four-step process: (1) Tiθ2 (DuPont TiPure® R101) was added to a 25% solution of 120K MW poly(methyl methacrylate) (PMMA) from Aldrich, Milwaukee, Wl in dimethylformamide (DMF). A series of solutions were prepared with 1%, 10%, 30% and 100% Tiθ2 by weight with respect to the weight of the polymer in the solution. The resulting solutions were drawn down onto a glass plate to give 15 mil (381 microns) thick films. (2) The glass plates and films were immediately placed into a bath of DMF-water in an 80:20 ratio at room temperature for 10 minutes. During this time the films separated from the glass. (3) The films were transferred to a bath with water but no DMF for 10 minutes. (4) The films were removed and dried in air. R550. determined by the aforementioned method, and thickness data for the resulting films are given in Table 1 , and spectra are given in Figure 4.

Table 1

Reflectance of Films with Tiθ2 Pigment (25% PMMA in DMF quenched in 20:80 H2θ:DMF)

Example 5

Pigmented Film

Pigmented microporous polymer reflector films were fabricated in a two-step process: (1) Tiθ2 (DuPont TiPure® R101) was added to a 8% solution of 410K MW Elvacite® 2041 poly(methyl methacrylate) (PMMA) from Lucite International, Cordova, TN in solvent mixture of acetone-water in an 88:12 ratio. A series of solutions were prepared with 50%, 75%, and 100% Tiθ2 by weight with respect to the weight of the polymer in the solution. The resulting solutions were drawn down onto a cellulose acetate film to give 30 mil (762 microns) thick films. (2) The film was air dried. The film turned white as it dried. R550. determined by the aforementioned method, and thickness data for the resulting films are given in Table 2, and spectra are given in Figure 5. Table 2

Reflectance of Films with TΪO2 Pigment (8%PMMA in acetone-water in an 88:12 ratio)

It is therefore, apparent that there has been provided in accordance with the present invention, a reflective article that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A reflective article comprising a reflector of light positioned within a structure defining an optical cavity, wherein said reflector comprises a microporous polyacrylate film containing a plurality of inorganic particles, wherein the film has a R550 reflectance of greater than about 80%.

2. The reflective article of claim 1 , wherein the reflector is a diffuse reflector.

3. The reflective article of claim 1 , wherein the microporous polyacrylate film is polymethyl (meth)acrylate.

4. The reflective article of claim 1 , wherein the microporous polyacrylate film has a reflectance of greater than about 90%.

5. The reflective article of claim 4, wherein the microporous polyacrylate film has a reflectance of greater than about 95%.

6. The reflective article of claim 1 , wherein the inorganic particles have an index of refraction of greater than or equal to 2.

7. The reflective article of claim 1 , wherein the inorganic particles are selected from the group of materials consisting of titanium dioxide, and zinc oxide or combinations thereof.

8. The reflective article of claim 7, wherein the inorganic particles consist essentially of titanium dioxide.

9. The reflective article of claim 1 , wherein substantially all of the inorganic particles have a mean particle diameter of about 30 nm to about 500 nm.

10. The reflective article of claim 1 , wherein the microporous polyacrylate film has an average thickness of about 20 to about 500 microns.

11. The reflective article of claim 1 , wherein the microporous polyacrylate film additionally comprises a solid support.

12. The reflective article of claim 1 , further comprising: (i) a light source positioned within the optical cavity; and (ii) a display panel through which light from the light source passes, wherein the reflector is positioned within the optical cavity for reflecting light from the light source toward the display panel.

13. The reflective article of claim 12, wherein said display panel is a liquid crystal.

14. The reflective article of claim 12, wherein said reflector lines at least a portion of the optical cavity and partially wraps around the light source so as to direct light from the light source into the optical cavity.

15. The reflective article of claim 12, wherein said optical cavity includes a light guide.

16. The reflective article of claim 12, wherein said reflector reflects light from the light source into the light guide.

17. An optical display, comprising:

(i) a structure defining an optical cavity;

(ii) a light source positioned within said optical cavity;

(iii) a display panel through which light from said light source passes; and

(iv) a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles wherein the film has a R550 reflectance of greater than about 80%.

18. The optical display of claim 17, wherein the reflector is a diffuse reflector.

19. The optical display of claim 17, wherein the microporous polyacrylate film is polymethyl(meth)acrylate.

20. A method of improving light reflectivity in a device requiring reflectivity of light comprising: (i) providing a reflector comprising a microporous polyacrylate film containing a plurality of inorganic particles wherein the film has a R550 reflectance of greater than about 80%; and

(ii) positioning said reflector within the device to cause light energy to reflect off of one face of the reflector.

21. The method of claim 20, wherein the reflector is a diffuse reflector..

22. The method of claim 20, wherein the microporous polyacrylate film is polymethylmethacrylate.