WO2007079148A2

WO2007079148A2 - An optical film composite for tf systems having integrated functions of light diffusion and reflective polarization

Info

Publication number: WO2007079148A2
Application number: PCT/US2006/049436
Authority: WO
Inventors: Seo-Hern Lee; Myeong-Kyun Kang; Youn-Sihn Kim; Jong-Bok Choi
Original assignee: 3M Innovative Properties Company
Priority date: 2005-12-30
Filing date: 2006-12-28
Publication date: 2007-07-12
Also published as: KR20070071347A; TW200741262A; WO2007079148A3

Abstract

The present invention provides an optical film composite that comprises a linear reflective polarizing film; a birefringent polymeric substrate layer placed on the reflective polarizing film; and a TF (turning film) layer placed beneath the reflective polarizing film. The optical axis of the birefringent polymeric substrate layer is oriented with respect to the transmission axis of the reflective polarizing film to have 0° to 25° of an angular difference between the axes. The optical film composite can be employed in LCD devices to improve optical performance.

Description

AN OPTICAL FILM COMPOSITE FOR TF SYSTEMS HAVING INTEGRATED FUNCTIONS OF LIGHT DIFFUSION AND REFLECTIVE POLARIZATION

Technical Field and Background

The present invention relates to an optical film composite for a turning system, and more particularly, to an optical film composite with improved optical performance by placing a linear, reflective polarizing film and a polymeric substrate layer, which may have light diffusing functions, onto a conventional turning film.

The optical film composite disclosed herein can be used as a device for achieving enhanced brightness, wide viewing angle and uniform luminance, especially in a liquid crystal display (LCD) device. In general, an LCD displays a desired image on its screen by controlling the transmittance of light from a backlight unit, using liquid crystal panels consisting of a plurality of liquid crystal cells arranged in a matrix form, and a plurality of control switches for converting video signals, which are provided to each of the liquid crystal cells.

The turning film system, which was developed by Mitsubishi Rayon Co. in Japan- is an example of a backlight unit used in an LCD device. TF system is applied as an edge-lit type of LCD backlight unit especially for a small-sized display device such as a notebook computer and a handheld. Referring to Figure 1, the conventional TF system comprises light source lamp 105, light guide 104 oriented perpendicularly to the light incident surface of the lamp, reflector 106 disposed under the light guide and reflecting the light components from the lower and side directions in the light guide to the upper direction of the light guide, and TF 103 disposed over the light guide and receiving the light components emitted from the light guide. TF 103 has the shape of an overturned prism film, and comprises multiple prism units on the incident surface thereof, which receives the light emitted from the light guide.

Diffuser 102 may be disposed between TF 103 and liquid crystal panel 101 for the purpose of enhancing optical performance. Referring to Figure 3, a typical structure of a diffuser comprising a polymeric substrate layer on which a polymer or glass beads layer is coated is shown. Alternatively, the polymeric substrate layer may be treated by other methods providing a light-diffusing function. The diffuser enhances the luminance uniformity by controlling the distribution of the light transmission. The types of material suitable for the production of the polymeric substrate layer of the diffuser, such as material used for the dimensionally stable layer of the reflective polarizing film composite, are known in the art.

As a constituent for improving the optical performance of the LCD device, a reflective polarizer, such as a multilayer reflective polarizing film, can be interposed between TF 103 and liquid crystal panel 101. The reflective polarizing film selectively reflects a light component having specific polarized states and the reflected light is reflected by the reflective plate positioned at the rear of the backlight unit and recirculated, thereby improving the overall brightness of the display. The multilayer reflective polarizing film can, in some exemplary embodiments, have a multilayer stacked structure in which at least two materials are alternately stacked, as disclosed in U.S. Patent No. 6,368,699, and PCT Publication Nos. WO 95/17303, WO 95/17691, WO 95/17692, WO 95/17699, WO 96/19347 and WO 99/36262. As can be seen from Figure 2, such a reflective polarizing film can be used in a reflective polarizing film composite wherein dimensionally stable layers are placed on and beneath the reflective polarizing film to prevent warping of the film. Polycarbonate (PC) has been usually used as the material for the dimensionally stable layer. However, disposing the diffuser or the reflective polarizing film composite on or over the TF system sometimes does not improve the optical performance to a desired degree. Although both the conventional diffuser and the conventional reflective polarizing film composite are disposed on the TF, the resulting structure inevitably produces the problem of increasing the volume and the weight of the LCD device. In addition, the use of PC, which has been incorporated as a dimensionally stable layer in a conventional multilayer reflective polarizing film composite, is one of the reasons for increased manufacturing cost of the multilayer reflective polarizing film composite. Thus, there has been a need for a cheaper material instead of PC.

In view of the above, it has become desirable to develop a new type of TF system having low thickness, low manufacturing cost and superior optical properties to the conventional TF system.

Summary

In one exemplary implementation, the present disclosure is directed to an optical film composite including a linear reflective polarizing film; a birefringent polymeric substrate layer placed on the reflective polarizing film; and a TF (turning film) layer placed beneath the reflective polarizing film. The optical axis of the birefringent polymeric substrate layer is oriented with respect to the transmission axis of the reflective polarizing film to have 0° to 25° of an angular difference between the axes.

In another exemplary implementation, the present disclosure is directed to a process for manufacturing an optical film composite. The process includes the steps of: providing a linear reflective polarizing film; placing a birefringent polymeric substrate layer on a side of the reflective polarizing film so that an angular difference between the optical axis of the birefringent polymeric substrate layer and the transmission axis of the reflective polarizing film is 0° to 25°; and placing a TF on the other side of the reflective polarizing film.

Brief Description of the Drawings

Figure 1 is a schematic illustration showing the structure of a liquid crystal display (LCD) device comprising a conventional turning film (TF) system.

Figure 2 is a schematic illustration of the conventional multilayer reflective polarizing film composite on and beneath, on which polycarbonate (PC) layers functioning as dimensionally stable layers are placed.

Figure 3 is a schematic illustration of the conventional diffuser consisting of a bead-treated layer and a polymeric substrate layer.

Figure 4 A is a schematic illustration showing one embodiment of the present invention, wherein a TF, a reflective polarizing film and a diffuser sheet are placed in succession.

Figure 4B is a schematic illustration showing one embodiment of the present invention, wherein a TF, a reflective polarizing film and a diffuser sheet are laminated by an optical adhesive, which contains beads.

Figure 5 is a drawing schematically illustrating a cross section of the LCD device equipped with the TF system according to the present invention.

Figure 6 is a drawing schematically illustrating a cross section of the LCD device, wherein a conventional reflective polarizing film composite and a conventional diffuser is placed over a typical TF system.

Figures 7 A — D show graphs representing the brightness characteristics of LCD devices having different types of TF systems.

Figures 8 A — B show graphs representing the brightness characteristics versus the variation of the viewing angle of the LCD devices having different types of TF systems.

Detailed Description

The present invention has been developed to solve the problems and subjects described earlier. The first objective of the present invention is to provide an optical film composite for a TF system having low thickness, low manufacturing cost and ease of handling by integrating the functions of a TF film, liner, reflective polarizing film and/or diffuser.

The second objective of the present invention is to provide a novel type of LCD device having improved optical performance, such as brightness, viewing angle and luminance uniformity, as compared to a device having a conventional TF system.

The inventors of the present invention have discovered that the objectives of the invention can be achieved by an optical film composite comprising a linear reflective polarizing film, a birefringent polymeric substrate layer placed on the reflective polarizing film, and a TF layer placed beneath the reflective polarizing film, wherein the angle between the transmission axis of light that passes through the reflective polarizing film and the optical axis of the birefringent polymeric layer placed on said film is precisely controlled. The inventors completed the present invention based on this discovery. Said birefringent polymeric substrate layer may be treated to have a light diffusing function.

The terms used in the present specification are to be construed as follows:

The term "placed" does not necessarily mean that the layers are adhered together by an adhesive or else. The meaning of the term "placed" should be construed to encompass "laminated" (i.e., adhering layers together by an adhesive material) and "stacking" (i.e., simply stacking layers without using an adhesive material).

The phrase "placed on (or beneath)" does not necessarily mean layers being disposed immediately adjacent to each other. "On (or beneath)" should be construed simply as indicating the relative positions between two layers, and thus, another layer such as adhesives can be interpositioned between the two layers.

A film "treated to have a light diffusing function" includes a haze-treated film and a diffuser, and comprises a "matte-treated" film, of which surface is made minutely coarse, a film having a rough surface, a "bead-treated" film having beads made of polymers or glass fixed to form the light diffusion layer, and films treated with other conventional methods for diffusing light.

A film "having a diffuser function" may be a film treated to have a light diffusing function by using any methods used to produce a diffuser, such as bead treatment, in the LCD device.

Unless mentioned otherwise, "reflective polarizing film" means a linear, reflective polarizing film.

The optical film composite according to the present invention is explained in detail, by referring to the accompanying drawings.

1. The placed formation of the film composite

The first objective of the present invention can be achieved by placing a birefringent polymeric substrate layer and TF on and beneath a linear, reflective polarizing film, respectively. The birefringent polymeric substrate layer is treated to have a light diffusing function. Figure 4A shows a schematic illustration of one embodiment of the present invention, wherein a TF, a reflective polarizing film and a polymeric layer having the diffuser function (a diffuser sheet) are placed in succession, Referring to said figure, birefringent polymeric substrate layer 402 is placed on reflective polarizing film 403. In addition, light diffusing layer 401 is disposed on layer 402. Layers 401 and 402 together constitute diffuser sheet 411. TF 404 is placed beneath reflective polarizing film 403.

Reflective polarizing film 403 may include a multilayer optical film, including multilayer films having a high reflectivity over a wide bandwidth (consisting of optical layers, all or some of the layers arc birefringent or all or some of the layers are isotropic), and/or a continuous/disperse phase optical film. Both multilayer reflective optical films and continuous/disperse phase reflective optical films rely on index of refraction differences between at least two different materials (preferably polymers) to selectively reflect light of at least one polarization orientation. Optical films that are especially suitable for use in the present invention are multilayer reflective films such as those described in, for example, PCT Publication Nos. WO 95/17303, WO 95/17691, WO 95/17692, WO 95/17699, WO 96/19347, and WO 99/36262, all of which are incorporated herein by reference. The film is preferably a multilayer stack of polymer layers with a Brewster angle (the angle at which reflectance of p-polarized light goes to zero) that is very large or nonexistent. The film is made into a multilayer mirror or polarizer whose reflectivity for p-polarized light decreases slowly with the angle of incidence, is independent of the angle of incidence, or increases with the angle of incidence away from the normal. This multilayered optical film has a high reflectivity (for both s- and p- polarized light) for any incident direction. One commercially available form of a multilayer reflective polarizer is marketed as Dual Brightness Enhanced Film (DBEF) by 3M, St. Paul, Minnesota. Multilayer reflective optical films are used herein as an example to illustrate the structures of the optical film composite of the present invention.

Birefringent polymeric layer 402 preferably includes a polyester based polymer (e.g., PET), and such polyester based polymers are usually prepared by using a biaxial stretching method. The birefringence of the polymeric substrate layer is due to the difference of index of refraction between two axes perpendicular to each other in the plane of the polymeric substrate layer. The index of refraction difference between an axis having the highest index of refraction and an axis having the lowest index of refraction is preferably at least 0.05. An optical axis of the polymeric substrate layer means an axis having the smallest index of refraction difference with polarizing film 403 and typically corresponds to the axis having the highest index of refraction in the polymeric substrate layer.

The birefringent polymeric substrate layer can be placed on reflective polarizing film 403, the layer having a thickness corresponding to a PC layer typically placed as a dimensionally stable layer or a thickness corresponding to the polymer layer placed as a substrate layer of a diffuser. The birefringent polymeric substrate layer serves as the substrate layer forming diffuser sheet 411 as well as the dimensionally stable layer preventing warping of reflective polarizing film 403. Birefringent polymeric substrate layer 402 is preferably treated by conventional methods for preparing the diffuser including the bead treatment method fixing beads such as glass or polymers. As one specific example, light diffusing layer 401 including beads is placed on birefringent polymeric substrate layer 402. The beads disposed on the surface of the birefringent polymeric substrate layer have the shape of a granule and are adhered to the surface of the birefringent polymeric substrate layer by using an adhesive. The beads have a refractive index different from that of air. The sizes of the beads adhered to the birefringent polymeric substrate layer can be the same or different. A polarized component passed through the birefringent polymeric substrate layer via the reflective polarizing film is diffused at the surface of the beads or in the beads. As another specific example, light diffusing layer 401 can consist of the beads and binder. The binder initially has fluidity and viscosity and the beads are mixed in the binder. A light refractive index of the binder can be formed unlike the light refractive index of the beads in order to further improve the light diffusing characteristic of light diffusing layer 401. The light diffusing material consisting of the beads and the binder is disposed on the surface of the first polymeric substrate layer in the form of a thin film. The beads and the binder in light diffusing layer 401 disposed on the birefringent polymeric substrate layer in the form of a thin film diffuse the polarized component passing through the birefringent polymeric substrate layer via the reflective polarizing film. Birefringent polymeric layer 402 or diffuser sheet 411 in which light diffusing layer 401 is placed on layer 402 may have preferably at least 20%, more preferably at least 50%, haze.

TF 404 is one component of conventional TF system 111 illustrated in Figure 1. Referring to Figure 1, the TF system comprises the light incident surface on at least one edge thereof, through which the light components from lamp 105 pass, and light guide 104 having the first light emitting surface perpendicular to said light incident surface. Over light guide 104 is prism film 103 that has the light incident surface receiving the light components emitted from the light guide and the second light emitting surface reflecting some of the light components in certain directions. The prism film comprises multiple prism units on the light incident surface thereof. The prism film is called a turning film because it has the overturned shape of the prism film used in the LCD device. For more detailed descriptions on the conventional TF system, Korean Patent Gazette No. 10- 0319327, which is incorporated herein by reference, may be referenced.

Referring to Figure 4B showing another example of the present invention, the birefringent polymeric substrate layer and/or the TF layer can be laminated with the reflective polarizing film by optical adhesives 405, 406. The optical adhesive can be an acrylate based adhesive, and may or may not contain beads such as glass or polymer.

Figure 5 represents the cross section of an LCD device having the TF system according to the present invention. Figure 6 represents the cross section of the LCD device wherein the conventional reflective polarizing film composite and diffuser are disposed over the typical TF system. When comparing Figures 5 and 6, it is clear that the device according to the present invention is much thinner than the other, although both devices comprise a TF, a reflective polarizing film and a diffuser. This is because, in the present invention, the TF serves as a dimensionally stable layer beneath the reflective polarizing film, and the birefringent polymeric layer serves as both a dimensionally stable layer on the reflective polarizing layer and a substrate layer of the diffuser, thus separate additional dimensionally stable layers are unnecessary on and beneath the reflective polarizing film. In addition, the manufacturing cost of the LCD device can be reduced by replacing the PC layer which was widely used as a dimensionally stable layer, with a polyester based polymer material, which is cheaper than PC. Further, the optical film composite of the present invention is easy to handle since the functions of TF, reflective polarizing film and diffuser are integrated in one film composite. 2. The placing method for the film composite

The second objective of the present invention can be achieved by placing a birefringent polymeric layer on a linear, reflective polarizing film, and then controlling the transmission axis of said reflective polarizing film and the optical axis of said polymeric layer within a certain angular range.

When laminating polyester based polymer layers, other than PC, on the reflective polarizing film as one embodiment of the present invention, in certain cases, the optical gain of the polarized light transmitting the optical film composite may be good. But in most cases, the optical gain is poor, as compared with an optical film composite, on which a non-birefringent polymer layer, such as PC, is placed. This is because when the optical axis of the polyester based polymer layer placed on the reflective polarizing film is not coincident with the polarizing axis of the reflective polarizing film, the optical gain of the polarized light, which transmitted by the reflective polarizing film, is reduced when transmitted through the polyester based polymer having birefringence. Therefore, for improving optical performance, it is important to arrange the polarizing axis of the reflective polarizing film and the optical axis of the polyester based polymer, which is placed on the polarizing film, at an appropriate angle. In this regard, the angular difference θ between the transmission axis of the reflective polarizing film and the optical axis of the birefringent polymeric substrate layer is from 0° to 25°, preferably from 0° to 15°, more preferably from 0° to 5° and the most preferably 0°. A multilayer optical film composite within such angle ranges shows improved characteristics in the optical gain over the conventional TF system.

The optical performance of a TF system according to the present invention is also affected by the manner of placing each layer. The manners of placing the layers may be "laminating" (i.e., adhering layers together by an adhesive material) and "stacking" (i.e., simply stacking layers without using an adhesive material). Especially, it is preferable to laminate the reflective polarizing film layer and the birefringent polymeric substrate layer by use of an adhesive whose refractive index differs only slightly from that of the birefringent polymeric substrate layer (e.g, an acrylate based optical adhesive) to improve the optical performance of the optical film composite.

Figures 7A— D are graphs showing brightness profiles of LCD devices adopting TF systems. Figure 7 A shows the brightness profile of a conventional TF system containing a diffuser with a haze level of 60%. Figure 7B shows the brightness profile of a composite film prepared by stacking a TF, reflective polarizing film and diffuser sheet (68% haze level) without using an optical adhesive. Figure 7C shows the brightness profile of a composite film prepared by laminating a TF, reflective polarizing film and diffuser sheet (68% haze level) with an acrylate based optical adhesive. Moreover, Figure 7D shows the brightness profile of the composite film of Figure 7C wherein the optical axis of the diffuser sheet and the transmission axis of the reflective polarizing film are coincident to each other. Since the composite film of Figure 7D contains a PET film, which was produced by a biaxial stretching method and whose optical axis is 45°, the diffuser sheet should be rotated by 45° (or 135°) in order to make the optical axis of the diffuser sheet and the transmission axis of the reflective polarizing film coincident to each other. However, in this case, the angular difference θ between the actual transmission axis of the reflective polarizing film and the optical axis of the PET is 0°. As shown in Figures 7A to D, the optical film composite laminated by an optical adhesive (Figure 7C) provides improved brightness by about 35% over the conventional TF system (Figure 7A) while it provides about 7% of improvement over the stacked film composite. Moreover, Figure 7D confirms that the coincidence of the polarizing axis and the transmission axis provides significant improvement in the brightness.

Figure 8 is a graph showing the improvement in the optical performance of the optical composite films of Figure 7A to C in view of the different aspects. Figure 8 shows that the LCD device equipped with the optical composite film having a structure wherein a TF, reflective polarizing film and diffuser sheet are placed may provide broader horizontal and vertical viewing angle than the conventional LCD device having a TF and diffuser that is placed on the TF. In particular, the optical film composite wherein the constituting layers are laminated by an optical adhesive provides the best brightness profile.

In sum, the optical performance of the optical film composite, such as brightness, viewing angle and uniform luminance, can be significantly improved by coinciding the polarizing axis of the reflective polarizing film layer and the optical axis of the birefringent polymeric layer and by stacking, more preferably by laminating, the layers.

The optical film composite of the present invention has a structure wherein a linear, reflective polarizing film and/or diffuser sheet are placed on a TF. Thus, the optical film composite of the present invention enables manufacture of a thinner and lighter LCD device by replacing the dimensionally stable layer of the conventional reflective polarizing film with the TF and the substrate layer of the diffuser sheet (a birefringent polymeric layer). Further, the present invention reduces the manufacturing costs of an LCD device by reducing the number of optical films and using a polyester based polymer, which is cheaper than PC. Further, the optical film composite of the present invention can improve some optical performances, such as brightness, viewing angle and uniform luminance of the TF system, by controlling the orientation between the reflective polarizing film and birefringent polymeric layer and using the stacked or laminated structure.

Claims

What is claimed is:

1. An optical film composite comprising: a linear reflective polarizing film; a birefringent polymeric substrate layer placed on the reflective polarizing film; and a TF (turning film) layer placed beneath the reflective polarizing film, wherein the optical axis of the birefringent polymeric substrate layer is oriented with respect to the transmission axis of the reflective polarizing film to have 0° to 25° of an angular difference between the axes.

2. The optical film composite of Claim 1, wherein the reflective polarizing film is a multilayer polarizing film.

3. The optical film composite of Claim 1, wherein the birefringent polymeric substrate layer is a polyester based polymer layer.

4. The optical film composite of Claim 1, wherein the birefringent polymeric substrate layer is treated to have a light diffusing function.

5. The optical film composite of Claim 3, wherein the polyester based polymer is PET.

6. The optical film composite of Claim 4, wherein the light diffusing function is imparted by a matte treatment.

7. The optical film composite of Claim 4, wherein the light diffusing function is imparted by incorporating beads.

8. The optical film composite of Claim 1 or 4, wherein the birefringent polymeric substrate layer has a haze level of at least 20%.

9. The optical film composite of any one of Claims 1-7, wherein the optical axis of the birefringent polymeric substrate layer is oriented with respect to the transmission axis of the reflective polarizing film to have 0° to 15° of an angular difference between the axes.

10. The optical film composite of any one of Claims 1-7, wherein the optical axis of the birefringent polymeric substrate layer is oriented with respect to the transmission axis of the reflective polarizing film to have 0° to 5° of an angular difference between the axes.

11. The optical film composite of any one of Claims 1 -7, wherein the birefringent polymeric substrate layer and the reflective polarizing film, or the reflective polarizing film and the TF layer, are laminated by an optical adhesive.

12. The optical film composite of Claim 11 , wherein the optical adhesive is an acrylate based adhesive.

13. The optical film composite of Claim 11 , wherein the optical adhesive contains beads.

14. A TF system comprising the optical film composite according to any one of Claims 1-7.

15. A process for manufacturing an optical film composite, comprising the steps of: providing a linear reflective polarizing film; placing a birefringent polymeric substrate layer on a side of the reflective polarizing film so that an angular difference between the optical axis of the birefringent polymeric substrate layer and the transmission axis of the reflective polarizing film is 0° to 25°; and placing a TF on the other side of the reflective polarizing film.

16. The process of Claim 15, wherein the birefringent polymeric substrate layer is manufactured by a biaxial stretching method.