Patent Publication Number: US-2012038859-A1

Title: Polarizing thin film, polarizing plate and liquid crystal display device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a polarizing film which comprises a polyvinyl alcohol-based resin layer including dichroic materials, a polarizing plate having the polarizing film, and a liquid crystal display device including the polarizing film or the polarizing plate. 
     DESCRIPTION OF RELATED ART 
     Conventionally, polarizing films comprise polyvinyl alcohol-based resin layers respectively including dichroic materials are known (For instance, Japanese Unexamined Patent Application Publication No. 2001-343521). 
       FIG. 5  is a diagram showing one example of a conventional polarizing film  50 . In the conventional polarizing film  50 , the case where natural light  53  is incident on the polarizing film  50  which comprises a polyvinyl alcohol-based resin layer  52  including dichroic materials  51  is considered. It is possible to decompose natural light  53  into a component of an absorption axis direction  54  and a component of a transmission axis direction (not shown in figure) in the polarizing film  50 . As for the polarizing film  50 , vibrating surfaces of light absorb light in the absorption axis direction  54  and then the vibrating surfaces of light allow light  55  in the transmission axis direction to transmit. While light whose vibrating surfaces are in the absorption axis direction  54  is generally referred to as extraordinary light, light  55  whose vibrating surfaces are in the transmission axis direction is generally referred to as ordinary light. The absorption axis direction  54  is perpendicular to the transmission axis direction. Since light energy then absorbed is converted into thermal energy, at least 57% of the incident natural light  53  is lost. 
     SUMMARY OF THE INVENTION 
     The conventional polarizing film  50  absorbs extraordinary light to convert it into thermal energy and thus at least 57% of the incident natural light  53  is lost. It is an object of the present invention to provide a polarizing film having functions of partially reflecting extraordinary light. According to the present invention, it is possible to reduce a loss of light in the polarizing film. 
     The summary of the present invention is as follows: 
     In a first preferred embodiment, a polarizing film according to the present invention comprises a polyvinyl alcohol-based resin layer including dichroic materials. The polarizing film according to the present invention has: 
     (a) a film thickness of 0.5 μm to 5 μm, and 
     (b) an absorbance of 1.5 to 7.0 for extraordinary light per film thickness of 1 μm relative to monochromatic light having a wavelength of 550 nm. 
     In a second preferred embodiment of the polarizing film according to the present invention, the dichroic materials are iodine. 
     In a third preferred embodiment of the polarizing film according to the present invention, the polyvinyl alcohol-based resin layer consists of polyvinyl alcohol or an ethylene vinyl alcohol copolymer. 
     In a fourth preferred embodiment, a polarizing plate according to the present invention comprises: a polarizing film according to any one of the aforementioned polarizing films; and a transparent substrate to support the polarizing film from one side. And the transparent substrate in an absorption axis direction of the polarizing film has a refractive index of less than 1.54. 
     In a fifth preferred embodiment, a liquid crystal display device according to the present invention includes a liquid crystal cell and a back light, wherein the polarizing film is interposed between the liquid crystal cell and the back light. 
     In a sixth preferred embodiment, the liquid crystal display device according to the present invention includes a liquid crystal cell and a back light, wherein the polarizing plate is interposed between the liquid crystal cell and the back light. 
       FIG. 1  schematically shows a polarizing film  10  of the present invention. The inventors of the present invention have found out as below. The polarizing film  10  including dichroic materials  11  respectively having a high concentration and comprising a polyvinyl alcohol-based resin layer  12  partially reflects extraordinary light. Extraordinary light is light  15  whose vibrating surfaces are located in an absorption axis direction  13 . High concentration means a concentration having an absorbance (absorbance per unit) of 1.5 or greater for extraordinary light per film thickness of 1 μm relative to monochromatic light having a wavelength of 550 nm. When natural light  14  enters the polarizing film  10  of the present invention, the polarizing film  10  partially reflects extraordinary light to allow ordinary light to transmit. Ordinary light is light  16  whose vibrating surfaces are located in a transmission axis direction. 
     Reflectivity R e  of extraordinary light in the polarizing film  10  is represented as follows when n 1  is a refractive index of a material in contact with a surface of a polarizing film, n e  is a refractive index of extraordinary light of a polarizing film, and k is an extinction coefficient: 
         R   e =[( n   1   −n   e ) 2   +k   2 ]/[( n   1   +n   e ) 2   +k   2 ]  (1)
 
     In the equation (1), when the concentration of each of the dichroic materials  11  and the film thickness t of the polarizing film  10  is in a certain range, a square value of the extinction coefficient is greater. To meet the requirements, in the polarizing film  10  of the present invention, the reflectivity R e  of extraordinary light is considered to be large. 
     On the other hand, the conventional polarizing film  50  ( FIG. 5 ) only shows ordinary interface reflection because the concentration of each of the dichroic materials and the film thickness of the polarizing film  50  do not meet the conditions defined in the present invention. In the conventional polarizing film  50 , the square value (k 2 ) of the extinction coefficient is small, so that R e  is a substantially constant value in the range of less than 5%. As a result, it is not possible for the polarizing film  50  to obtain advantages of the present invention. 
     Advantages of the Invention 
     The polarizing film  10  of the present invention partially reflects extraordinary light. Extraordinary light typically has a reflectivity of 5% or higher relative to incident light. This reflected light may be recycled by being reflected again with other reflecting material because this reflected light remains as light energy. 
     It is possible to improve brightness of a liquid crystal display device without the use of a high-priced existing brightness enhancement film by interposing the polarizing film  10  or a polarizing plate  20  of the present invention between a liquid crystal cell and a back light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a polarizing film of the present invention; 
         FIG. 2  is a schematic view of a polarizing plate of the present invention; 
         FIG. 3  is a schematic view of a liquid crystal display device of the present invention; 
         FIG. 4  is a schematic view of a liquid crystal display device of the present invention; and 
         FIG. 5  is a schematic view of a conventional polarizing film. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [Polarizing Film] 
       FIG. 1  shows one example of a polarizing film  10  of the present invention. The polarizing film  10  of the present invention comprises a polyvinyl alcohol-based resin layer  12  including dichroic materials  11 . The polarizing film  10  of the present invention exhibits absorption anisotropy and reflection anisotropy at any wavelengths in a visible light region (wavelength of 380 nm to 780 nm). 
     In the case where the polarizing film  10  of the present invention comprises the polyvinyl alcohol-based resin layer  12  including the dichroic materials  11 , the polarizing film  10  may further include an appropriate additive. Typical examples of such an additive include a surfactant and an antioxidant. 
     The polarizing film  10  of the present invention is characterized by a film thickness t thereof and a high concentration of the dichroic materials  11 . The polarizing film  10  has a film thickness of 0.5 μm to 5 μm and preferably has a film thickness of 0.5 μm to 3 μm. When the thickness t of the polarizing film  10  is over 5 μm, the reflectivity of extraordinary light is lower. On the other hand, when the film thickness t of the polarizing film  10  is smaller than 0.5 μm, it becomes difficult to stretch the polyvinyl alcohol-based resin layer  12 , resulting in insufficient orientation of the dichroic materials  11 . 
     The polarizing film  10  of the present invention preferably has an absorbance per unit of 1.5 to 7.0 relative to monochromatic light having a wavelength of 550 nm, more preferably 1.8 to 7.0. Absorbance per unit is an absorbance of extraordinary light per film thickness t=1 μm. In the polarizing film  10  of the present invention, the absorbance per unit is in the aforementioned range under the conditions that the film thickness t is 0.5 μm to 5 μm. Accordingly, the polarizing film  10  of the present invention partially reflects extraordinary light when natural light  14  enters to allow ordinary light to transmit the polarizing film  10 . 
     [Dichroic Material] 
     Typically, the dichroic materials  11  to be used in the present invention are respectively iodine, an organic dye, and a mixture thereof. Iodine is preferable as dichroic materials  11 . 
     [Polyvinyl Alcohol-Based Resin Layer] 
     The polyvinyl alcohol-based resin layer  12  to be used in the present invention is formed by forming a polyvinyl alcohol-based resin in the form of a layer. 
     A polyvinyl alcohol-based resin is typically obtained by saponifying a polyvinyl acetate-based resin. The polyvinyl alcohol-based resin to be used in the present invention has a saponification degree of 85 mole % to 100 mole % and has a polymerization degree of 1,000 to 10,000. Typical examples of the polyvinyl alcohol-based resin include polyvinyl alcohol or an ethylene vinyl alcohol copolymer. 
     [Method for Producing Polarizing Film] 
     It is possible to obtain the polarizing film  10  of the present invention by immersing the stretched polyvinyl alcohol-based resin layer  12  in a stain solution containing the dichroic materials  11  to be dyed. Alternatively, it is possible to obtain the polarizing film  10  of the present invention by immersing the polyvinyl alcohol-based resin layer  12  in a stain solution containing the dichroic materials  11  to be dyed and then stretching the dyed polyvinyl alcohol-based resin layer  12 . 
     Any stretching processing methods, such as roll stretching and tenter stretching or the like are used as a method for stretching the polyvinyl alcohol-based resin layer  12 . The stretching ratio of the polyvinyl alcohol-based resin layer  12  is three times to seven times relative to the original length thereof. The stretching temperature of the polyvinyl alcohol-based resin layer  12  is generally 30° C. to 160° C. The layer thickness of the polyvinyl alcohol-based resin layer  12  before stretching is set so that the layer thickness t after stretching may be 0.5 μm to 5 μm. 
     The concentration of a water solution containing the dichroic materials  11  for dyeing the polyvinyl alcohol-based resin layer  12  is preferably over 1 weight part of the dichroic materials  11  and 5 weight parts or smaller relative to 100 weight parts of water. 
     When the dichroic materials  11  are iodine, it is possible to adjust the absorbance per unit of the polarizing film  10  by the addition of boric acid, urea, and potassium iodine or the like. For instance, when the added quantity of potassium iodine in the stain solution is increased, the absorbance per unit is greater. On the other hand, when the added quantity of boric acid in the stain solution is increased, the absorbance per unit is smaller. 
     The temperature and the immersion time of the stain solution are determined in accordance with the concentration of the stain solution and the layer thickness of the polyvinyl alcohol-based resin layer so that characteristics defined in the present invention may be satisfied. The temperature of the stain solution is preferably 10° C. to 40° C. and the immersion time is preferably 20 seconds to 300 seconds. It is possible to set the absorbance per unit of the polarizing film at 1.5 to 7.0 under such conditions. 
     To provide water resistance, it is preferable to immerse the stretched and dyed polyvinyl alcohol-based resin layer in a boric-acid water solution to be boric-acid treated. The boric-acid treated polarizing film is generally washed in water and is dried. 
     [Polarizing Plate] 
     As shown in  FIG. 2 , a polarizing plate  20  comprises: a polarizing film  10 ; and a transparent substrate  21  to support the polarizing film  10  from one side. The transparent substrate  21  has a refractive index of less than 1.54 when measured in the absorption axis direction  13  of the polarizing film  10 . 
     When natural light  22  enters the polarizing plate  20  of the present invention, the polarizing plate  20  partially reflects extraordinary light to allow ordinary light to transmit. 
     To increase the reflectivity R e  of extraordinary light in the polarizing film  10 , it is preferable that there is a big difference between the refractive index of the transparent substrate  21  and the refractive index of extraordinary light in the polarizing film  10 . The polarizing film  10  has a refractive index (refractive index of the absorption axis direction  13 ) of about 1.54 for extraordinary light. Accordingly, the transparent substrate  21  preferably has a refractive index of 1.53 or lower when measured in the absorption axis direction  13  of the polarizing film  10 . 
     The transparent substrate  21  is not particularly limited as long as the aforementioned conditions of the refractive index are met, however, is preferably a transparent polymer film. 
     In the polarizing plate  20 , the polarizing film  10  may be directly formed on the transparent substrate  21 , alternatively, the polarizing film  10  may be laminated on the transparent substrate  21  via an adhesion layer. 
     In the case where an adhesion layer is placed between the polarizing film  10  and the transparent substrate  21 , the adhesion layer preferably has a refractive index of 1.53 or lower, either, when measured in the absorption axis direction  13  of the polarizing film  10 . 
     The polarizing plate  20  preferably has a transmittance of 25% to 44%, more preferably 35% to 42%. 
     The polarizing plate  20  preferably has a polarization degree of 99% or higher. 
     According to the present invention, it is possible for the polarizing plate  20  to have a reflectivity R e  of 5% or higher for extraordinary light, preferably 5% to 15%. 
     Further, according to the present invention, it is possible for the polarizing plate  20  to have an increase rate of brightness of 1.4% or higher, preferably 1.4% to 3.0%. 
     [Liquid Crystal Display Device] 
     As shown in  FIG. 3 , a liquid crystal display device  30  of the present invention comprises: a liquid crystal cell  31 ; a back light  32 ; and the polarizing film  10  of the present invention interposed between the liquid crystal cell  31  and the back light  32 . 
     Alternatively, as shown in  FIG. 4 , a liquid crystal display device  40  of the present invention comprises: a liquid crystal cell  41 ; a back light  42 ; and the polarizing plate  20  of the present invention interposed between the liquid crystal cell  41  and the back light  42 . 
     Out of incident light  33  emitted from light sources  32   a  of the back light  32 , ordinary light transmits the polarizing film  10  to be used for the liquid crystal display device  30  of the present invention and extraordinary light is partially reflected by the polarizing film  10 . 
     Light  35  reflected by the polarizing film  10  is reflected again by a back reflective film  32   b  of the back light  32  to be light  36  with vibrating surfaces rotating by 90°. The light  36  passes through the polarizing film  10  to be light  37 , resulting in entering the liquid crystal cell  31 . 
     As a result, compared to the case of using a conventional polarizing film  50 , the brightness of the liquid crystal display device  30  is improved because light entering the liquid crystal cell  31  is increased by light  37 . 
     In the liquid crystal display device  40 , light entering the liquid crystal cell  41  is increased by the similar mechanism as the liquid crystal display device  30  of the present invention, so that the brightness of the liquid crystal display device  40  is improved. 
     When the polarizing plate  20  of the present invention is used for the liquid crystal display device  40  of the present invention, as shown in  FIG. 4 , incident light  43  emitted from the back light  42  enters from the transparent substrate  21  side. Although there is no illustration in figures, on the contrary, the incident light  43  emitted from the back light  42  may enter from a side of the polarizing film  10  by positioning the polarizing film  10  on the back light  42  side. 
     As shown in  FIG. 4 , when the incident light  43  enters the transparent substrate  21  side, there is an advantage that no impact is given to display properties of the liquid crystal display device  40 , even though the transparent substrate  21  has birefringence. 
     On the other hand, when the incident light  43  enters from the polarizing film  10  side, it is possible to make the reflectivity R e  of extraordinary light in the polarizing film  10  greater. 
     In general, the liquid crystal cells  31  and  41  respectively have two substrates and a liquid crystal layer sandwiched between the substrates. Typically, a color filter, an opposite electrode, and an oriented film are formed on one substrate and a liquid crystal driving electrode, a wiring pattern, a thin-film transistor, and an oriented film are formed on the other substrate. 
     Examples of an operation mode of the liquid crystal cells  31  and  41  include a Twisted Nematic mode and an electrically controlled Birefringence mode. Examples of the electrically controlled Birefringence mode include a Vertical Alignment system, an Optically Compensated Bend (OCB) system, and an IPS (In-Plane Switching) system or the like. 
     Any systems, such as a direct irradiation system, a side light system, and a surface light source system or the like are used for the back light  32  and  42  to be used in the present invention. Generally, the back light  32  includes the light sources  32   a  and a back reflective film  32   b , and the back light  42  includes light sources  42   a  and a back reflective film  42   b.    
     EXAMPLES 
     Example 1 
     (1) A surface of a transparent substrate which comprises a norbornene-based resin film with a thickness of 150 μm (manufactured by JSR Corporation, product name: ARTON Film) was corona-treated. 
     (2) A water solution of 7% by weight of polyvinyl alcohol 
     (manufactured by NIPPON SYNTHETIC CHEMICAL INDUSTRY CO., LTD., product name: NH18) is applied onto one side of the norbornene-based resin film to form a laminated film composed of a norbornene-based resin film and a polyvinyl alcohol film. 
     (3) The laminated film composed of a norbornene-based resin film and a polyvinyl alcohol film is heated by drying at 100° C. for 10 minutes to form a polyvinyl alcohol film with a thickness of 5 μm on one surface of the norbornene-based resin film. 
     (4) The laminated film was roll-stretched at 150° C. in a stretch ratio of 5 times as long as the original length of the laminated film. 
     (5) The stretched laminated film was immersed in a stain solution (liquid temperature: 20° C.) including a water solution containing iodine and potassium iodine for 30 seconds to absorb and orient iodine on a polyvinyl alcohol layer. 
     (6) The laminated film was immersed in a boric-acid solution (liquid temperature: 55° C.) with a concentration of 10% by weight and then was immersed in a potassium iodine water solution with a concentration of 4% by weight for 10 seconds. 
     (7) The laminated film was dried at 60° C. for 4 minutes. 
     As mentioned above, a polarizing plate was prepared including a polarizing film and a transparent substrate to support the polarizing film form one side. The polarizing film was a polyvinyl alcohol layer and had a transmittance of 41% and a polarization degree of 99.8% or higher. The transparent substrate was a norbornene-based resin film. Table 1 shows characteristics of the polarizing film and the polarizing plate. 
     The content of iodine contained in the stain solution was 1.1 weight parts relative to 100 weight parts of water. Further, the content of potassium iodine was 10 weight parts relative to 100 weight parts of water. 
     The stretched norbornene-based resin film had a refractive index of 1.52 measured in an absorption axis direction of the polarizing film. 
     Example 2 
     A polarizing plate composed of a polarizing film and a transparent substrate to support the polarizing film from one side was prepared in the same manner as in Example 1 except that the polyvinyl alcohol layer before stretching had a thickness of 3 μm. The polarizing film was a polyvinyl alcohol layer and had a transmittance of 41% and a polarization degree of 99.8% or higher. The transparent substrate was a norbornene-based resin film. Table 1 shows characteristics of the polarizing film and the polarizing plate. 
     Comparative Example 1 
     A polarizing plate available in the market (manufactured by NITTO DENKO CORPORATION, product name: NPF-SEG1224 with a transmittance of 43% and a polarization degree of 99.8% or higher) was evaluated. 
     Comparative Example 2 
     A polarizing plate composed of a polarizing film and a transparent substrate to support the polarizing film from one side was prepared in the same manner as in Example 1 except that the immersion time in a stain solution (liquid temperature: 20° C.) was changed to 10 seconds. The polarizing film was a polyvinyl alcohol layer and had a transmittance of 41% and a polarization degree of 99.8% or higher. The transparent substrate was a norbornene-based resin film. Table 1 shows characteristics of the polarizing film and the polarizing plate. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Polarizing film 
                 Polarizing plate 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Thick- 
                 Absorbance 
                 reflectivity Re 
                 Increase rate 
               
               
                   
                 ness t 
                 per unit 
                 of extraordinary 
                 of brightness 
               
               
                   
                 (*1) (μm) 
                 (*2) (1/μm) 
                 light (*3) (%) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 2.5 
                 1.8 
                 5.2 
                 1.4 
               
               
                 Example 2 
                 1.3 
                 4.9 
                 11.0 
                 2.7 
               
               
                 Comparative 
                 25.0 
                 1.4 
                 4.2 
                 0.0 
               
               
                 Example 1 
               
               
                 Comparative 
                 2.5 
                 1.1 
                 4.7 
                 0.0 
               
               
                 Example 2 
               
               
                   
               
               
                 (*1) Thickness t = thickness of the polarizing film 
               
               
                 (*2) Absorbance per unit = absorbance per thickness of 1 μm of the polarizing film for extraordinary light relative to monochromatic light having a wavelength of 550 nm 
               
               
                 (*3) Reflectivity Re of extraordinary light = reflectivity of the polarizing plate for extraordinary light relative to monochromatic light having a wavelength of 550 nm (reflectivity in an absorption axis direction) 
               
            
           
         
       
     
     [Evaluation] 
     As indicated in Table 1, the polarizing plate in Example 1 had a higher increase rate of brightness than the polarizing plates in Comparative Examples 1 and 2. The polarizing plate in Example 2 had a further higher increase rate of brightness. 
     [Measuring Method] 
     [Film Thickness} 
     The film thickness of a polarizing film or the like was measured using a digital micrometer (manufactured by ANRITSU CORP., product name: KC-351C). 
     [Absorbance Per Unit] 
     Absorbance per unit was measured by a spectrophotometer with an integrated sphere equipped with Glan-Taylor prism polarizer (manufactured by HITACHI, LTD., product name: U-4100). 
     Base line corrections were made using a stretched polyvinyl alcohol layer before dyeing as a reference. Transmittance K 2  of extraordinary light in a polarizing film relative to monochromatic light having a wavelength of 550 nm was measured to obtain an absorbance per unit from the following equation: 
       Absorbance per unit(1/μm)=−Ln(K 2 )/film thickness(μm)
 
     The absorbance per unit in Table 1 is a value obtained when entering measured light from a polarizing film side. 
     [Reflectivity of Extraordinary Light] 
     A transparent substrate side of a polarizing plate was polished by a sandpaper to reduce reflection on a back side to an acceptable level and then an acrylic black lacquer was sprayed to be well dried. Measured light entered from the polarizing film side and then the reflectivity of extraordinary light relative to monochromatic light having a wavelength of 550 nm was measured using a spectrophotometer with an integrating sphere (manufactured by HITACHI, LTD., product name: U-4100) equipped with a Glan-Taylor prism polarizer. Further, the reflectivity of a standard white plate (BaSO 4 ) was set at 100%. 
     [Increase Rate of Brightness] 
     A back light was taken out form a 32-inch liquid crystal television unit (manufactured by HITACHI, LTD., product name: Woo) available in the market. And a sample where each polarizing plate in Examples and Comparative Examples is attached to a glass plate was prepared. The sample was placed on a front surface of the back light so that light of the back light was incident from the polarizing film side. Brightness (B 1 ) from a front direction after lighting the back light was measured by a luminance meter (manufactured by TOPCON CORPORATION, product name: BM-5). 
     An increase rate (%) of brightness was obtained by the following equation, representing brightness as B o  when using the polarizing plate in the Comparative Example 1: 
       Increase rate of brightness(%)=( B   1   −B   0 )/ B   0 ×100
 
     INDUSTRIAL APPLICABILITY 
     The liquid crystal display device of the present invention is preferably used for liquid crystal television units, computer displays, car navigation systems, mobile phones, and game devices or the like. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
         
           
               10 : polarizing film;  11 : dichroic material;  12 : polyvinyl alcohol-based resin layer;  13 : absorption axis direction;  14 : natural light;  15 : reflected light;  16 : transmitted light;  20 : polarizing plate;  21 : transparent substrate;  22 : natural light;  23 : reflected light;  24 : transmitted light;  30 : liquid crystal display device;  31 : liquid crystal cell;  32 : back light;  32   a : light source;  32   b : back reflective film;  33 : natural light;  34 : transmitted light;  35 : reflected light;  36 : reflected light;  37 : transmitted light;  40 : liquid crystal display device;  41 : liquid crystal cell;  42 : back light;  42   a : light source;  42   b : back reflective film;  43 : incident light;  50 : polarizing film;  51 : dichroic material;  52 : polyvinyl alcohol-based resin layer;  53 : natural light;  54 : absorption axis direction;  55 : transmitted light