WO2013101703A1

WO2013101703A1 - Diffuse reflective laminate comprising nonwoven sheet

Info

Publication number: WO2013101703A1
Application number: PCT/US2012/071148
Authority: WO
Inventors: Weifeng DAI; Zuocheng Zhou; Qiuju Wu; Bo Xu
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2011-12-31
Filing date: 2012-12-21
Publication date: 2013-07-04
Also published as: CN103182819B; CN103182819A

Abstract

This invention related to diffuse reflective laminate comprising (a) a polyethylene layer, (b)a polyethylene terephthalate layer, and (c) a nonwoven HDPE sheet, wherein the polyethylene layer(a) is facing toward a display panel. These diffuse reflective laminates have utility in light management in optical displays such as backlight unit (BLU) in LCD displays for lap top computer and televisions.

Description

TITLE

DIFFUSE REFLECTIVE LAMINATE COMPRISING NONWOVEN SHEET

FIELD OF THE INVENTION

This invention relates to a diffuse reflective laminate comprising a nonwoven high density polyethylene (HDPE) sheet. The diffuse reflective laminates can reflect light uniformly and have utility in optical displays such as backlight unit (BLU) in liquid crystal displays for laptop computers and televisions.

BACKGROUND OF THE INVENTION

Brightness (image intensity) is a critical factor in direct view displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs)).

To improve the display's brightness, one method is to increase the number or the power of the light sources. However, this method suffered from higher usage of electricity and increased weight problems. Another method is to use a reflector to maximize the image uniformity and brightness. The reflector is positioned within the optical cavity of an optical display for reflecting light from the light source toward the display panel. Depends on where the light source situated relative to the display panel, there are edge-lit backlight unit (BLU) as illustrated in Figure 1, or direct view BLU as illustrated in Figure 2.

Reflectivity is particularly critical in battery powered equipment, where reflectivity improvements directly relate to smaller required light sources and thus lower power demands. US Pat. Application No. 2007/0048499 disclosed that according to a real test, when reflectivity of a BLU module increases by 3-5%, the brightness of the same module will increase by 8-10 %.

There is a wide variety of commercially available plastic based white reflectors. For example, PET film filled with titanium dioxide (T1O₂) or barium sulfate and then bi-axially stretched to create microvoids conducive to light reflection is a known reflector material, such as Lumirror^® film E6SR available from Toray Plastics (America), Inc. (North Kingstown, R.I.). Another known reflector material is a reflective microcellular foamed polymer sheet, such as reflective microcellular foamed polyester sheet available as MC-PET from Furukawa Electric Co. Ltd. (Tokyo, Japan). Another known reflector material is a highly reflective film made by expanding polytetraffuoroethylene (ePTFE) to create inter- tangled fibrils with voids that reflect light, such as those available from W. L. Gore & Associates, Inc. (Newark, Del).

Recently, great interest was aroused in using nonwoven fabric as backlight reflector because of its inherent high reflectance and cost effectiveness. It offers high light-shielding effect, high diffuse reflectance (>90%) and luminance compensation when using in LCD backlight unit or lighting fixtures. US Pat. No. 7,660,040 first disclosed a diffuse light reflector comprising a nonwoven polymer sheet having fibers and inter-fiber pores, wherein the polymer is, inter alia, polyolefin. More specifically, in the examples, single nonwoven sheet or multilayer nonwoven sheets of flash-spun high density polyethylene (HDPE) comprising a plurality of plexifilamentary film-fibrils of HDPE were employed. The diffuse light reflectors are useful in direct view and edge-lit optical display backlight applications.

More recently, US Pat Application No. 2010/0238665 also disclosed a diffuse light reflector comprising a light reflective nonwoven sheet and a polymer layer consisting of polyolefin, polyester, polyacrylate, and blends thereof. These diffuse light reflectors are useful for lighting fixtures formed from coil steel, or alumina, for example, fluorescent office or general lighting fixtures. The diffuse light reflectors have an excellent reflectance from about 94% to about 97% measured at 550 nm and a gloss of less than 10%. However, the stiffness and thermal stability of them are less than desirable which limited their further applications, for example, in LCD backlight units.

Improved diffuse reflectors having high reflectivity (>94%) and good thermal stability with sufficient stiffness are needed for visible light management applications that will allow for production of more affordable and energy efficient optical displays.

SUMMARY OF THE INVENTION

This invention provides a diffuse reflective laminate having high thermal stability comprising:

(a) a nonwoven high density polyethylene (HDPE) sheet having a thickness of about 100-400 pm and basis weight of about 30-210 g/m²;

(b) a polyethylene terephthalate (PET) film containing 0-50 weight % of filler particles (A) and having a thickness of about 25-150 μιη; and (c) a polyethylene (PE) film containing 0-30 weight % of filler particles (B) and having a thickness of about 10-100 um;

wherein

the diffuse reflective laminate has a configuration of (a)/(b)/(c);

the PE layer (c) of the diffuse reflective laminate is facing toward a display panel, embossed with patterns having a depth ranging from 1 um to 10 um, and the pattern is random or ordered;

the filler particles (A) and (B) are independently selected from iOz, BaS0₄, polymethylmethacrylate (PMMA), and mixtures thereof; and

the diffuse reflective laminate has a photopic reflectance of at least about 94% at 550 nm as measured according to ASTM E 1349-06, and the gloss value is less than 10% as measured according to ASTM D523. In one embodiment, in the diffuse reflective laminate of the present invention, the nonwoven HDPE sheet (a) and the PET film(b) are bound through a first adhesive, and the PET film(b) and PE film (c) are bound through a second adhesive.

h another embodiment, the diffuse reflective laminate of the present invention further comprises (d) a second polyethylene terephthalate (PET) film containing 0-30 weight % of filler particles (C) having a thickness of about 25-150 μιη, wherein the diffuse reflective laminate has a configuration of (d)/(a)/(b)/(c) that the second PET film(d) is bound through a third adhesive to the nonwoven HDPE sheet (a) on the side opposite to the first PET film(b), and the filler particles (C) are selected from T1O2 , BaS0 , PMMA, and mixtures thereof.

In a further embodiment, in the diffuse reflective laminate of the present invention, the first, second, and third adhesives are independently selected from polyurethanes, polyacrylates, polyepoxides, and mixtures thereof.

In one embodiment, in the diffuse reflective laminate of the present invention, the first, second, and third adhesives contain 0-30% of filler particles (D), each independently selected from ΤΪΟ2, BaS0₄, PMMA, and mixtures thereof, where the filler particles (D) are the same or different from filler particles (A), (B) or (C).

This invention also provides a diffuse reflective article comprising the diffusive reflective laminate of the present invention.

This invention further provides an optical display, comprising:

(i) a structure defining an optical cavity;

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

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

(iv) a diffuse reflector comprising the diffusive reflective laminate of the

present invention;

wherein the diffuse reflector is positioned within the optical cavity to reflect light from the light source off the surface of the diffuse reflector toward the display panel.

This invention also provides a method for improving light reflectivity in a device requiring diffuse reflectivity of light comprising:

providing a diffuse reflector comprising the diffusive reflective laminate of the present invention; and

positioning the diffuse reflector within the device to cause light to reflect off the

surface of the diffuse reflector.

Various other features, aspects, and advantages of the present invention will become more apparent with reference to the following description, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is schematic cross-sectional view of an embodiment of the optical display of the invention with an edge-lit BLU. Figure 2 is schematic cross-sectional view of another embodiment of the optical display of the invention in a direct view BLU.

Figure 3 is schematic diagram of a method suitable for manufacturing the diffuse reflective laminates of the invention.

DETAILS OF THE INVENTION

All publications, patent applications, patents and other references mentioned herein, if not otherwise indicated, are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control.

Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.

As used herein, the term "produced from" is synonymous to "comprising". As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having," "contains" or "containing," or any other variation thereof, are intended to cover a nonexclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

The transitional phrase "consisting of excludes any element, step, or ingredient not specified. If in the claim, such a phrase would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consisting of appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The transitional phrase "consisting essentially of is used to define a composition, method or apparatus that includes materials, steps, features, components, or elements, in addition to those literally discussed, provided that these additional materials, steps features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting essentially of occupies a middle ground between "comprising" and "consisting of.

The transitional phrase "essentially no" components or "essentially free" of components, it is meant that the compositions of the invention should contain less than 1 % by weight, preferably less than 0.1% by weight, of the components, based on the total weight of the compositions.

The term "comprising" is intended to include embodiments encompassed by the terms "consisting essentially of and "consisting of. Similarly, the term "consisting essentially of is intended to include embodiments encompassed by the term "consisting of. When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. For example, when a range of "1 to 5" is recited, the recited range should be construed as including ranges "1 to 4", "1 to 3", "1-2", " 1-2 & 4-5",^' "1-3 & 5", and the like. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

When the term "about" is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to.

Further, unless expressly stated to the contrary, "or" refers to an inclusive "or" and not to an exclusive "or". For example, a condition A "or" B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Also, the indefinite articles "a" and "an" preceding an element or component of the invention are intended to be nonrestrictive regarding the number of instances (i.e. occurrences) of the element or component. Therefore "a" or "an" should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

In describing and/or claiming this invention, the term "homopolymer" refers to a polymer derived from polymerization of one species of monomer; "copolymer" refers to a polymer derived from polymerization of two or more species of monomers. Such copolymers include dipolymers, terpolymers or higher order copolymers.

In describing certain polymers it should be understood that sometimes applicants are referring to the polymers by the monomers used to produce them or the amounts of the monomers used to produce the polymers. While such a description may not include the specific nomenclature used to describe the final polymer or may not contain product-by- process terminology, any such reference to monomers and amounts should be interpreted to mean that the polymer comprises those monomers (i.e. copolymerized units of those monomers) or that amount of the monomers, and the corresponding polymers and compositions thereof.

The term "light" as used herein means electromagnetic radiation in the visible light portion of the spectrum, from 400 nm to 780 nm wavelength. Unless stated otherwise, "photopic reflectance" of light herein means the reflectance (i.e., diffuse and specular reflectance) as seen by a human observer over the visible light wavelength range of 400 nm to 780 nm. Photopic reflectance is calculated from total reflectance spectral data using illnminant D65 and the CIE standard photopic observer described in "Bilhneyer and Saltzman Principles of Color Technology", 3rd Edition.

Embodiments of the present invention as described in the Summary of the Invention include any other embodiments described herein, can be combined in any manner, and the descriptions of variables in the embodiments pertain not only to the diffuse reflective laminates of the present invention but also to the diffuse reflective articles and the optical displays comprising the diffuse reflective laminates of the present invention.

The invention is described in detail hereinunder.

( a) Nonwoven HDPE Sheet

The nonwoven HDPE sheet (a) may be included in the diffuse reflective laminate of the present invention is a plexifilamentary filxn-fibril sheet made from flash spun high density polyethylene.

The term "plexifilamentary" as used herein means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibrils of random length and with a mean fibril thickness of less than about 4 um and a median width of less than about 25 um. In plexifilamentary structures, the film-fibrils are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form a continuous three-dimensional network. Such structures are described in further detail in U.S. Pat. No. 3,081,519 and in U.S. Pat. No. 3,227,794.

A lightly consolidated flash-spun HDPE nonwoven sheet can be produced by the general procedure of Steuber, U.S. Pat. No. 3,169,899. A high-density polyethylene is flash spun from a solution of the polyethylene in a solvent. The solution is continuously pumped to spinneret assemblies. The solution is passed in each spinneret assembly through a first orifice to a pressure let-down zone and then through a second orifice into the surrounding atmosphere. The resulting film -fibril strand is spread and oscillated by means of a shaped rotating baffle, is electrostatically charged and then is deposited on a moving belt. The spinnerets are spaced to provide overlapping, intersecting deposits on the belt to form a wide batt which is then lightly consolidated.

Suitable nonwoven HDPE sheet (a) has a thickness is in the range of from about

100-400 um, or about 120-300 μηι. Suitable nonwoven HDPE sheet (a) has a basis weight in the range of from about 30-210 g/ m², or about 40-160 g/ m², or about 50-1 10 g/ m².

The nonwoven HDPE sheet (a) are commercially available from DuPont Co. under the trade name Tyvek® with different types, preferably is the "type 10" style which has a smooth, stiff, paper-like sheet structure.

(b) PET Film Multiple layers of polymer films can be applied on top of the nonwoven HDPE sheet

(a) to increase opacity and reflectance. Multiple thin layers also have higher thermal stability on the resulting diffuse reflective laminate than a single thick layer.

To increase opacity and reflectance, the additional polymer layers typically are applied to the nonwoven HDPE sheet (a) on the side facing toward a light source. Suitable PET film

(b) has a thickness is in the range of from about 25-150 pm, or about 50-120 μιη.

White polyethylene terephthalate (PET) films are known to have good light reflectivity and are widely available. A suitable PET film for the present invention is a PET film containing with 0-50 weight % of filler particles (A) selected from Ti0_2; BaSO^ PMMA, and mixtures thereof. Preferably, the filler particles (A) are selected from Τί0₂> BaS0₄, and mixtures thereof. More preferably, the filler particles (A) are Ti0₂.

Although it is known that the amount of the filler particles corresponds to the opacity and reflectance of the PET films. When factoring the overall reflectivity and weight of the diffuse reflective laminate, the PET film (b) preferably contain 3-30% of filler particles (A).

In one embodiment, the PET film (b) of the present diffuse reflective laminate contains

3-30% of filler particles (A) selected from Ti0₂, BaS0₄, PMMA, and mixtures thereof.

In another embodiment, the PET film (b) of the present diffuse reflective laminate contains 3-10% of filler particles (A) selected from Ti0₂, BaS0₄, and mixtures thereof.

In a further embodiment, the PET film (b) of the present diffuse reflective laminate contains 3-10% of Ti0₂ as filler particles (A).

There are many commercially suppliers for clear or white PET films of various thickness. For example, Kunshan Jiantong Electronic Co., Ltd.; DuPont Teijin Films Co., Ltd.; DuPont Hongji Films Foshan Co., Ltd.; and Foshan Energetic Film Co., Ltd.. ic) PE Film

One issue of having PET film as the top layer of a laminated reflector is the inherent hardness of the PET film surface, which may scratch the delicate surface of the light guide panel during working condition when the temperature of the optical cavity increased, consequently will decrease the image uniformity for an edge-lit BLU in particular.

To avoid such problem, a third layer which is a polyethylene (PE) film (c) is applied on top of the PET film (b) in the inventive diffuse reflective laminates. Suitable PE film (c) has a thickness in the range of about 10-100 pm, or about 30-80 pm, and selected from low density polyethylene (LDPE), high density polyethylene (HDPE), or blends thereof. Preferably, the PE film (c) is derived from a blend of LDPE and HDPE.

The PE film (c) may contain 0-30 weight % of filler particles (B) selected from T1O2,

BaS0₄, PMMA, and mixtures thereof.

Due to the PE film (c) being the top layer facing toward the display panel, when embossed with patterns, the gloss level will lower while maintaining the reflectivity of the diffuse reflective laminate. Gloss levels can be less than 10%, including less than 9%, 8%, 7%, 6%, and 5%.

The patterns embossed on the PE film (c) may be random or ordered and have a depth of about 1- 10 μπι. The embossed PE film imparts more diffuse reflectance of the inventive diffuse reflective laminate than a corresponding laminate without the patterns.

There are many commercial sources for the embossed PE film. For example, clear or white PE films (i.e. having filler particles (B)) of various thickness with a rhombus pattern are available from Hangzhou Quanxing Plastic Co. Ltd. (China).

(d) Supporting Film- a second PET Film

It's been found that additional polymer layers may be applied to the nonwoven HDPE sheet (a) on the side opposite to the light source to provide extra stiffness or structural stability of the diffuse reflective laminates. Thus, it is referred as the "supporting film" herein. It's particular useful when the diffuse reflective laminates are used for large dimension optical displays such as LCD TV screens above 32 inches.

There is no limitation on the polymer material suitable for the supporting film as long as the thickness of the supporting film is in the range of about 25-150 μηι, or about 50-120 μηι.

The second PET film (d) may contain 0-30 weight %, or 0-10 weight % of filler particles (C), wherein the filler particles (C) are selected from T1O₂, BaS0₄, PMMA, and mixtures thereof. As the main function of the second PET film (d) is to add stiffness to the inventive diffuse reflective laminate, the second PET film (d) may be the same or different from the first PET film (b) in terms of composition and thickness. For example, the second PEF film (d) can be a clear film without any filler particles, which is cheaper than a white PET film.

In one embodiment, the second PET film (d) of the present diffuse reflective laminate contains 0-10% of filler particles (C) selected from Ti0₂, BaS0₄, PMMA, and mixtures thereof.

Adhesives

The 3- or 4-layered diffuse reflective laminate of present invention, where the nonwoven HDPE sheet (a), the PET film(b), PE film (c) and optionally the second PET film(d) are bound with the adjacent layers through adhesives. For a 3 -layered diffuse reflective laminate, i.e. a structure of (a)/(b)/(c), the nonwoven HDPE sheet (a) and the PET filmfb) are bound through a first adhesive, and the PET film(b) and PE film (c) are bound through a second adhesive.

For the 4-layered diffuse reflective laminate of present invention, the second PET film

(d) is bound to the HDPE sheet (a) on the side opposite to the PET film (b) through a third adhesive. Suitable adhesives are those that maintain sufficient structural integrity of the diffuse reflective laminate during normal handling and use, for example, LCD assembly and use. Adhesives are preferred to have thermal expansion properties similar to those of the nonwoven HDPE sheet, PET film and PE layer so that temperature variation does not lead to separation of the diffuse reflective laminate by differential expansion.

Known adhesives including polyurethanes, polyacrylates, polyepoxides, and mixtures thereof are preferred. However, considering manufacturing ease, the first, second and/or third adhesives are preferably the same.

In one embodiment, the first, second and/or third adhesives are each independently selected from polyurethanes, polyacrylates, polyepoxides, and mixtures thereof.

The suitable adhesives may contain 0-30% of filler particles (D) selected from Ti0₂, BaS0₄, PMMA, and mixtures thereof, which may be the same or different from the filler particles (A), (B) or (C).

Specific examples of adhesives of utility include polyacrylic adhesive available from Nanjing Jiu Hang Chemical Ltd. Co, and polyurethane adhesive available from Shanghai Lieyin Chemical Ltd. Co.

Process of Manufacturing the Diffuse Reflective Laminate

The diffuse reflective laminate of the invention can be manufactured by dry lamination method using a laminator. Typically, as illustrated in Figure 3, a PET film (b) can be surface roughed on both side by corona and/or plasma treatment 22 to assist adhering the film to the nonwoven HDPE sheet (a) in the first step. The PET film (b) is surface coated with a thin layer of adhesive by passing through a tank 23 of the first adhesive solution in the 2^nd step. The adhesive coated PET film (b) then runs through an oven 26 for solvent evaporation and leaving the adhesive coated PET film (b) surface in a slightly tacky stage at the 3^rd step. Lastly, laminating the adhesive coated PET film (b) surface with the nonwoven HDPE sheet (a) through a combination nip 27 in a laminator. The bonding between the nonwoven HDPE sheet (a) and PET film (b) is achieved by applying suitable pressure on the combination nip 27 to provide a 2-layered laminate having a configuration of: [(a)/l^st adhesive/(b)] . By repeating a similar procedure, the 2- layered laminate is coated with a second adhesive solution on the PET film (b) surface. After solvent evaporation, laminate the adhesive coated PET film (b) with the PE film (c) to provide a 3-layered diffuse reflective laminate of the invention, which has a configuration of: [(a)/l^st adhesive/(b)/2^nd adhesive/(c)] . If the PE film (c) is embossed with pattern, then the lamination is carried out between the PET film (b) of the 2-layered laminate and the smooth surface of the embossed PE film (c).

In the case of the diffuse reflective laminate of this invention further comprising a second PET film (d), then 4-layered diffuse reflective laminate can be made by subjecting the second PET film (d) through the 4 step process of surface roughed by corona and/or plasma treatment, surface coated with a third adhesive, dried the adhesive, then laminated with the nonwoven HDPE sheet (a) of a 3 -layered diffuse reflective laminate. The 4-layered diffuse reflective laminate produced by the method describe above has a configuration of:

[(d)/3^rd adhesive/(a)/l^st adnesive/(b)/2^nd adhesive/(c)].

In view of cost and process ease, the first, second and third adhesives are preferably the same. Therefore, there is no need to have multiple adhesive tanks or extra laminators to manufacture the diffuse reflective laminates of the present invention.

The theoretical thickness of the adhesives after drying is preferably about 25 μτη to provide sufficient bonding between layers. Therefore, one skilled in the art can adjust the concentration of adhesive solution, the coating speed (or residence time of film passing through the adhesive solution tank), and drying condition for a set of laminating equipments accordingly.

Characterization of the Diffuse Reflective Laminate

The diffuse reflective laminate of the present invention can provide a reflectivity of more than 94% in the visible wavelength range from 400 nm to 780 nm, and thus can effectively enhance the brightness of backlight modules. For comparison purpose, reflectance at 550 nm was used for comparison between samples.

"Gloss" is the visualization property for evaluating if the surface of an article, i.e. a diffuse reflective laminate, is smooth or not. If the surface of an article reflects mainly due to specular reflection, the gloss of the article will be higher. The inventive reflective laminate is preferred to reflect lights mainly through diffusive reflection to obtain uniform image. When a light source projects with an incidence angle of 85°, the gloss of the surface as measured according to ASTM D523 standard method, is preferably lower than 10%.

The present diffuse reflective article or optical display comprises a diffuse reflective laminate positioned within a structure defining an optical cavity. "Optical cavity" refers herein to an enclosure designed to receive light from a light source; to condition and to 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.

The diffuse reflective article or optical display of the present invention 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 diffuse reflective article or optical display contains 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 diffuse reflective article or optical display contains a diffuse reflective laminate positioned within the optical cavity for reflecting light toward an object benefiting from illumination. The diffuse 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 diffuse reflector is positioned behind the light source illuminating the display panel. The light scattering and diffuse reflection characteristics of diffuse reflectors according to the present invention provides more overall diffuse lighting, e.g., a more overall diffuse light source and therefore a more evenly lit or uniformly illuminated optical display.

Schematic figures of several embodiments of optical displays utilizing diffuse reflective articles according to the present invention are shown in FIGS. I -2.

FIG. 1 is a cross sectional view of an edge-lit liquid crystal optical display utilizing a diffuse reflector 10 comprising the reflective laminate of the present invention. In FIG. 1, an optical display 1 is shown having a fluorescent light source 2 coupled to an optical cavity containing a light guide panel 3. A diffuser 4, an optional brightness enhancing film 5, such as films described in U.S. Pat. Nos. 4,906,070 and 5,056,892 and available from Minnesota Mining and Manufacturing Co. (3M), Minneapolis, Minn., USA, and an optional reflective polarizer film 6 (also available from 3M) as described in PCT publications WO 91/5327 and WO 97/32224, are placed on top of the light guide panel 3 and act to redirect and reflectively polarize the light emitted from the light guide panel 3 toward a liquid crystal display panel 7 and ultimately 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. A diffuse reflector 10 is placed behind the light guide panel 3 within an optical cavity 13 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 diffuse reflector 10 has a highly reflective, high diffusivity surface that enhances the optical efficiency of the optical cavity. The diffuse reflector 10 scatters and reflects hght diffusely, depolarizes the light, and has high reflectance over the visible wavelength range.

FIG. 2 is a cross sectional view of a backlit liquid crystal optical display with a cold cathode fluorescent lamp light source utilizing a diffuse reflector according to the present invention. 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. The optical cavity 13 is lined with a diffuse reflector 10 comprising the reflective laminate of the present invention. Diffuse reflector 10 both increases reflectance and mixes the discrete lights adequately to form a light source with good spatial light emitting uniformity for illumination of the liquid crystal display panel 7.

Therefore, the present invention includes a diffuse reflective article comprising a diffusive reflective laminate of this invention.

The present invention also includes an optical display, comprising:

(i) a structure defining an optical cavity;

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

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

(iv) a diffuse reflector comprising the diffusive reflective laminate of this invention;

wherein the diffuse reflector is positioned within the optical cavity to reflect light from the light source off of the surface of the diffuse reflector toward the display panel.

The present invention also includes a method of improving light reflectivity in a device requiring diffuse reflectivity of light comprising:

providing a diffuse reflector comprising the diffusive reflective laminate of this invention; and

positioning the diffuse reflector within the device to cause light energy to reflect off of the surface of the diffuse reflector.

The materials, methods, and examples herein are illustrative only and, except as specifically stated, are not intended to be limiting. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein. EXAMPLES

The abbreviation "E" stands for "Example" and "CE" stands for "Comparative Example" is followed by a number indicating in which example the diffusive reflective laminate is prepared. The examples and comparative example were all prepared and tested in a similar manner. Percentages are by weight unless otherwise indicated.

Table 1

Material ID Supplier Product code and Specification

Nonwoven Tyvek^® 4077, a 231 + 19 μπι thick film, with a basis weight is 92

DuPont USA

HDPE (al) g m².

Nonwoven Tyvek^® 1056D,a 162 + 15 μιιι thick film, with a basis weight is

DuPont USA

HDPE (a2) 55 g/m².

Nonwoven A no -commercial HDPE film, has a thickness of 133 + 18 μιη,

DuPont USA

HDPE (a3) with a basis weight is 69.5 g/m².

DuPont Teijin Films A white PET film, has a thickness of 101 +2 μηι and a

PET film (bl)

Co., Ltd. reflectance>90%.

Kunshan Jiantong A white PET film, has a thickness of 75 + 1 μηι and a PET film (bl)

Electronic Co., Ltd. reflectance>90%.

Foshan DuPont Hongji

PET film (b3) A clear PET film, has a thickness of 78 + 1 μπι.

Films Co., Ltd.

Foshan DuPont Hongji

PET film (dl) A clear PET film, has a thickness of 52 + 1 μηι.

Films Co., Ltd.

Hangzhou Quanxing A white 109 +2 μηι thick film, with a basis wei ht of 100 g m² PE film (cl)

Plastic Co. Ltd. and embossed with rhombus pattern.

Hangzhou Quanxing A white 67+2 μηι thick film, with a basis weight of 50 g/m² and PE film (c2)

Plastic Co. Ltd. embossed with rhombus pattern.

Hangzhou Quanxing A white 36 + 1 μπι thick film, with a basis weight of 20 g/m² and PE film (c3)

Plastic Co. Ltd. embossed with rhombus pattern.

Hangzhou Quanxing A clear 34 + 1 μηι thick film, with a basis weight of 20 g m² and PE film (c4)

Plastic Co. Ltd. embossed with rhombus pattern.

Hangzhou Quanxing A clear 50 + 1 μιη thick film, with a basis weight of 45 g/m² and PE film (c5)

Plastic Co. Ltd. smooth surface without embossed pattern.

Nanjing Jiu Hang Pressure sensitive acrylic adhesive, ethyi acetate based single Adh-1

Chemical Ltd. Co. component, clear viscous liquid, solid content is 45%.

Shanghai Lieyin PU Adhesive , Ethyl acetate based two components adhesive,

Adh-2

Chemical Ltd. Co. LY-50A and LY-50AH; clear viscous liquid, content is 50%.

Tr0₂ DuPont USA R-105, average particle size is 350 nm; density is 4.0 g/cm³. Adh-3 Added Ti0₂ to Adh-1 to make a Adh-3 containing 5% Ti0₂. Adh-4 Added Ti0₂ to Adh-2 to make a Adh-4 containing 5% TiO,. General Method for Manufacturing of the Diffuse Reflective Laminates

The diffuse reflective laminates were manufactured by a dry lamination method using a laminator (Meansun optoelectronics technology co., ltd, China). The parameters for the lamination process are listed in Table 2.

Table 2

*To configuration of the diffuse reflective laminates is represented by abbreviating the nonwoven HDPE sheet (a) as abbreviated as "(a)", the PET file (b) as "(b)", the PE film (c) as "(c)", the second PET film (d) as "(d)", and the adhesive as "adh". A number "n" is followed the abbreviation to indicate the exact materi l used in the example. For example, (al) means the Tyvek^® sheet listed in Table 1 with the material ID of nonwoven HDPE (al).

**Drying tumiel is a series of 6-section ovens; each section is 6 m long with different temperature setting.

The adhesive- 1 or adhesive-2 was diluted with ethyl acetate to a solution having a solid content of about 30% and filled in the adhesive tank. The web roller speed was set at 15 m/min. As illustrated in Figure 3, a PET film (b) was fed on the 1st web roller 21 unwound and was roughed on both surfaces by corona treatment 22, then the PET film (b) was coated with an adhesive solution in the tank 23 and run through a 6-section oven 26 at the temperatures as specified in Table 2 to leave the adhesive coated PET film (b) surface in a slightly tacky stage. Lastly, laminating the adhesive coated PET film (b) surface with the nonwoven HDPE sheet (a) which was fed from the 2nd web roller 28, through a combination nip 27 in the laminator at a nip pressure as specified in Table 2 and wound on a roller 29 to obtain a 2-layered laminate having a configuration of: [ (a)/adh/(b)j .

By a similar procedure, the 2-layered laminate was fed from the 1st web roller without corona treatment and was coated with an adhesive solution of choice on the PET film (b) surface, which after solvent evaporation, was laminated with the PE film (c) on the smooth side without embossed patterns, which was fed from the 2nd web roller, to provide a 3- layered diffuse reflective laminate of the invention, which has a configuration of: [ (a)/adh/ (b)/adh/(c)].

For examples having 4~layered diffuse reflective laminate, the second PET film (d) was fed from on the 1 st web roller, surface roughed by corona treatment, coated with the same adhesive, which was then dried by passing through the oven, then laminated with the nonwoven HDPE sheet (a) of a 3 -layered diffuse reflective laminate, which was produced according to the procedures described above, to produce a 4-layered diffuse reflective laminate having a configuration of: [(d)/adh/(a)/adh/(b)/adh /(c)]. Test Methods

The thickness of the examples was measured by a thickness gauge (ST-022, available from ONO SO KI Co., LTD.) with a digital gauge counter (DG-4140). A test sample of 20 cm x 20 cm in size was measured at 20 positions ( 10 positions along machine direction and 10 positions along transverse direction) and averaged according to the test standard ISO 534.

Reflectance measurements were obtained using an X-rite SP64 Spectrophotometer (available from X-Rite Lnc). The luminance/visual angle was D65/10⁰ and measuring aperture was 8 mm diameter and calibrated with the X-Rite Calibration Standard. The output is a percent reflectance at each wavelength and the spectral range measured is 380 nm to 750 nm in 5 nm intervals. For each test sample, 20 readings were taken randomly across a 20 cm² area and averaged. Reflectance at 550 nm was used for comparison between samples.

Gloss was measured by a Micro-TRI-Gloss machine (available from BYK Gardner) according to ASTM D523. Gloss at 85° was used for comparison between samples.

Damp-heat test of the laminate examples was performed by placing a test sample of 21 cm (machine direction) x 29.7 cm (transverse direction) in size, which was placed between two glass plates of slightly larger size, and the sandwiched test sample was placed in a damp-heating chamber (SETH-Z-022R, Shanghai ESPEC Environmental Equipment Corp.) at 60^°C/85%RH (relative humidity) for 100 hours. The flatness of each test sample was visually assessed before and after the damp-heat treatment when the sample was laid on a plain surface and rated as follows:

means the sample is completely flat without any puckering and waviness; means the edge of the sample curled slightly but had no puckering and waviness; " ·" means the edge of the sample curled with slight puckering and waviness; and " X " means sample film curled severely and/or with severe puckering and waviness. Hardness Shore D measurements were obtained by using Instron® Shore A&D S 1 902 according to the test standard: ISO868-2003E with the dwell time 15s.

Embodiments of the present invention are further defined in the following Examples. Configurations of the examples and comparative examples as well as the evaluation results are shown in Tables 3 to 4. Table 3

*"Before": before the Weathering Test, "After": after the Weathering Test

From the results of Table 3, the following are evident:

Comparing CE1 and E1-E13, the former has a 2~layered structure; despite having a high reflectance at 550 nm (>96%), the high gloss data (about 87 %) suggests that the majority of the total reflectance is due to specular reflective instead of diffusive. In contrast, all inventive diffuse reflective laminates of El -E 33 have a total reflectance in the range of 94%-96% at 550 nm and a gloss data of between 3-8%. The much lower gloss data of the examples demonstrated that the total reflectance of the inventive laminates was due to diffusive light. The results suggest that the inventive laminates can provide uniform viewing angles when used as a diffuse reflective article or a diffuse reflector in the BLU module of an optical display.

Comparison of the results of E1-E3 suggests that the thickness of the nonwoven HDPE sheets (al), (a2) or (a3) makes little difference in the reflectance data. E2 is the only example that showed flatness degraded after the weathering test, which may be due to its being the thinnest laminate among all examples. Comparison of the reflectance data between El and E5 indicates that the white PET film (b2) is slightly better than the clear PET film (b3) of the same thickness when other layers are the same.

Comparing between E8 and CE2 that E8 has clear PE film with embossed pattern (c4) whereas CE2 has clear PE film without embossed pattern (c5), the data showed E8 has slightly higher reflectance and much lower gloss than those of CE2. In contrast, comparing between El and E8 indicates that the white PE film (c3) and clear PE film (c4) make no difference in reflectance. Furthermore, as long as the PE films are embossed with the same patterns, the gloss data are similar, too. Therefore the results strongly suggest that the PE film (c) having embossed pattern is critical to impart diffuse reflectivity of the inventive laminates.

Comparison of the data between El, E9, E10 and El l indicates that there is no difference in reflectance regardless of the type of adhesives (polyacrylic or polyurethane) nor if the adhesive comprises filler particles (adh-1 , adh-2 versus adh-3, adh-4).

E12 and E13 are examples of 4-layered laminates, produced by applying the second

PET film (dl) to the nonwoven HDPE sheet of El and E3, respectively. Comparison of the results between El vs. E12 and E3 vs. El 3 indicates that there is no difference in reflectance, gloss and flatness changes after damp-heat test except the total thickness of the laminates. The results suggest that when extra stuffiness or dimensional stability is required in certain applications, the second PET film, which can be a clear film, may then be incorporated.

Table 4

*The hardness was measured for the PE film (c3) surface of Example 1.

For the edge-lit backlight unit of LCD, the backlight reflector (i.e. where the inventive diffuse reflective laminates will be situated) is in contact with the plastic light guide panel uniformly and care is taken to not damage the surface of the plastic light guide. Therefore, a backlight reflector having an optimal hardness is required to avoid scratching the plastic light guide panel. It is known that PET film is too hard to contact the plastic light guide directly. Therefore, a special top coating was used for PET reflectors to avoid such a problem. However, the cost of the coated PET reflectors increased as a consequence.

In this invention, a PE film (c) was placed at the top of the diffuse reflective laminates, which has an inherent lower hardness than that of a PET film, also provides a solution to the above mentioned problem. Such the process of laminating a PE film is simpler and easier to control than applying an even coating on top of a PET film.

As shown in Table 4, the hardness of embossed white PE film is 27.4 (Shore D), which is indeed much lower than that of the white PET film. After lamination, the inventive laminate of El, has a hardness of 53.8 (Shore D), which is still lower than that of the white PET film.

While the invention has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown, since various modifications and substitutions are possible without departing from the spirit of the present invention. As such, modifications and equivalents of the invention herein disclosed may occur to persons skilled in the art using no more than routine experimentation, and all such modifications and equivalents are believed to be within the spirit and scope of the invention as defined by the following claims.

Claims

CLAIMS What is claimed is:

1. A diffuse reflective laminate having high thermal stability comprising:

(a) a nonwoven high density polyethylene (HDPE) sheet having a thickness of about. 100-400 μιη and basis weight of about 30-210 g/m²;

(b) a polyethylene terephthalate (PET) film containing 3-50 weight % of filler particles (A) and having a thickness of about 25- 150 um; and

(c) a polyethylene (PE) film containing 0-30 weight % of filler particles (B) and having a thickness of about 10- 100 μηι;

wherein

the diffuse reflective laminate has a configuration of (a)/(b)/(c);

the PE film (c) of the diffuse reflective laminate is facing toward a display panel, embossed with patterns having a depth ranging from 1 μπι to 10 μιη, and the pattern is random or ordered;

the filler particles (A) and (B) are independently selected from Ti0₂, BaSC , polymethylmethacrylate (PMMA), and mixtures thereof; and

the diffuse reflective laminate has a photopic reflectance of at least about 94% at 550 nm as measured according to ASTM E1349-06, and the gloss value is less than 10% as measured according to ASTM D523.

2. The diffuse reflective laminate of Claim 1 , wherein the nonwoven HDPE sheet (a) and the PET film(b) are bound through a first adhesive, and the PET film(b) and PE film (c) are bound through a second adhesive.

3. The diffuse reflective laminate of Claim 1 further comprising (d) a second polyethylene terephthalate (PET) film containing 0-30 weight % of filler particles (C) and having a thickness of about 25- 1 0 pm, wherein the diffuse reflective laminate has a configuration of (d)/(a)/(b)/(c) that the second PET film(d) is bound though a third adhesive to the nonwoven HDPE sheet (c) on the side opposite to the first PET film(b), and the filler particles (C) are selected from Ti0₂, BaS0₄, PMMA, and mixtures thereof.

4. The diffuse reflective laminate of Claim 2 or Claim 3, wherein the first, second, and third adhesives are independently selected from polyurethanes, polyacrylates, polyepoxides, and mixtures thereof.

5. The diffuse reflective laminate of Claim 4, wherein the first, second , and third adhesives contain 0-30% of filler particles (]¾, each independently selected from Ti0₂, BaS0₄, PMMA, and mixtures thereof, where the filler particles (D) are the same or different from filler particles (A), (B) or (C).

6. A diffuse reflective article comprising the diffusive reflective laminate of Claim 1.

7. An optical display, comprising:

(i) a structure defining an optical cavity;

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

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

(iv) a diffuse reflector comprising the diffusive reflective laminate of Claim 1 ; wherein the diffuse reflector is positioned within the optical cavity to reflect light from the light source off the surface of the diffuse reflector toward the display pane).

8. A method for improving light reflectivity in a device requiring diffuse reflectivity of light comprising:

providing a diffuse reflector comprising the diffusive reflective laminate of Claim 1 ; and

positioning the diffuse reflector within the device to cause light to reflect off the surface of the diffuse reflector.