Abstract:
A process for making retarder film characterized by laminating onto a substrate in sequence an alignment layer and a retardation material. In the process where the alignment layer of the retarder film is cured by ultraviolet light to undergo crosslinking reaction, the alignment layer is exposed to air or inert gas (with oxygen content no less than 1%) and provided with O.5 wt %˜10 wt % photoinitiator to achieve better adhesion to the substrate. At the same time, proper amount of active acrylate residue is left on the surface of alignment layer to facilitate subsequently the adhesion of retardation material thereon. This process results in polarizer with built-in retarder. Consequently, not only the polarizer has larger viewing ranges and better displaying quality because of the effect of optical compensation, the thickness of the polarizer is also smaller, and its transparency and optic characteristics are better than prior art.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a process for making retarder film and a polarizer having the same, in particular a kind of retarder film suitable for the polarizer of LCD device and able to provide dual compensation for visual range and chromatic polarization, and its process.  
         [0003]     2. Description of the Prior Art  
         [0004]     Liquid crystal display (LCD) is now used by all kinds of electronic devices, such as television, computer, mobile handset, and personal digital assistant (PDA). Due to its characteristics of fast response and high contrast ratio of direct viewing angle, thin-film resistor LCD (TFT-LCD) has become the mainstream LCD technology.  
         [0005]      FIG. 1  depicts the sectional view of a conventional LCD  10 , which typically comprises a liquid crystal element  11  and two polarizers  12 ,  13  disposed respectively on each surface of liquid crystal element  11 . The liquid crystal element  11  is constituted by a glass substrate and a plurality of liquid crystal cells adhered to both surfaces of the glass substrate. Polarizer  12  (or  13 ) is made of a polarizing film  123  (or  133 ) sandwiched between two transparent substrates  121 ,  122  (or  131 ,  132 ) that provides compensation for polarization.  
         [0006]     If we look at the contrast curve of the visible range of a conventional LCD  10  ( FIG. 1A ) as shown in  FIG. 1B , it is clear that conventional LCD offers good visual effect in vertical and horizontal directions only. At 45° or 135° angle, the contrast ratio drops and the hues shift, which seriously affects the display quality of LCD.  
         [0007]     Later on LCDs are added with a retarder film to enhance the visual effect of oblique angles.  FIG. 2  shows the flow process of adding a retarder film to a conventional LCD polarizer  20 . Conventional LCD polarizer  20  is an independent plate or sheet structure made of a polarizing film  22  sandwiched between two transparent substrates  211 ,  212  (as shown in step  291 ). In addition, an independent structure of first phase retarder  24  is formed by coating on a substrate  241  in sequence an alignment layer  242  and liquid crystal material  243  (as shown in step  292 ). Similarly a second phase retarder  25  can also be formed independently (as shown in step  293 ). The first and the second phase retarders  24 ,  25  work to retard certain wavelengths at predetermined angles and directions, thereby achieving the effect of optical compensation and improving the oblique-angle display quality of LCD. In the prior art, polarizer  20 , first phase retarder  24  and second phase retarder  25  are three separate elements that are respectively produced, stored, shipped and sold. By coating a layer of pressure sensitive adhesive (PSA)  231  on one of the transparent substrates  211  of polarizer  20 , the first phase retarder  24  is laminated to the transparent substrate  211  in a manner with its liquid crystal material  243  facing down and its substrate  241  facing up (as shown in step  294 ). Subsequently, substrate  241  is stripped, and the combination of alignment layer  242  and liquid crystal material  243  is referred to as first phase retarder  24 , which is adhered to the polarizer  20  via the PSA  231  (as shown in step  295 ). Subsequently, the first phase retarder  24  and second phase retarder  25  are respectively coated with PSA  232 ,  233  to adhere second phase retarder  25  to first phase retarder  24 . As shown in step  296 , a conventional LCD polarizer  20  having the function of optical compensation is formed and can be used in liquid crystal element  11  as shown in  FIG. 1  to constitute a liquid crystal display. For example, U.S. Pat. No. 6,717,642 discloses a technology of improving the viewing angle and display quality of LCD by adding a retarder plate.  
         [0008]     In the prior art LCD as shown in  FIG. 2 , polarizer  20  and phase retarders  24 ,  25  are separately produced and then laminated together with PSA. In light that the separately produced polarizer  20  and phase retarders  24 , 25  require respectively at least one transparent substrate  211  or substrate  241  to provide adequate structural strength and rigidity, and at least three layers of PSA  231 ,  232 ,  233  for adhesion, the use of many substrates and layers of PSA increase the thickness of LCD and affect adversely its transparency and optic characteristics, hence leaving room for improvement.  
       SUMMARY OF INVENTION  
       [0009]     The primary object of the present invention is to provide a process for making retarder film, characterized in which in the process of irradiating the alignment layer with ultraviolet light to produce crosslinking reaction, the alignment layer is exposed to air or inert gas (with oxygen content no less than 1% of volume percentage) and provided with 0.5 wt %˜10 wt % of weight percentage of photoinitiator. As such, proper amount of active acrylate residue is left on the surface of alignment layer to facilitate the lamination of retardation material thereon.  
         [0010]     Another object of the present invention is to provide a process for making polarizer with retarder film, where the retardation layer in the retarder film is directly built in the polarizer without the use of pressure sensitive adhesive for adhesion. As such, the polarizer achieves better viewing angle and display quality due to the effect of optical compensation, and is reduced in thickness with at least one less layer of transparent substrate, hence offering better transparency and optic characteristics.  
         [0011]     Yet a further object of the present invention is to provide a polarizer with retarder film, where the retardation layer in the retarder film is directly built in the polarizer according to the aforesaid process. As such, the polarizer achieves better viewing angle and display quality due to the effect of optical compensation, and is reduced in thickness as compared to prior art, hence offering better transparency and optic characteristics.  
         [0012]     To achieve the aforesaid objects, the present invention provides a process for making retarder film, comprising the steps of:  
         [0013]     providing a transparent substrate for coating the polarizing film;  
         [0014]     coating an alignment layer on said transparent substrate;  
         [0015]     irradiating the alignment layer with ultraviolet light using 0.5 wt %˜10 wt % (weight percentage) of photoinitiator and in an air or inert gas environment with oxygen content of at least 1% of volume percentage such that the incompletely reacted active acrylate is left on the surface of alignment layer;  
         [0016]     coating retardation material on the alignment layer where the active acrylate residue thereon brings about closer adhesion of retardation material to the surface of alignment layer; and  
         [0017]     under an air environment or inert gas, curing the alignment layer and retardation material with ultraviolet light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The details of the present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures.  
         [0019]      FIG. 1A  shows the sectional view of a conventional LCD.  
         [0020]      FIG. 1B  shows the contrast curve of viewing angles of conventional LCD in  FIG. 1A .  
         [0021]      FIG. 2  shows the flow process of adding a retarder film to a conventional LCD polarizer.  
         [0022]      FIG. 3  shows the flow process for making an retarder film according to the present invention.  
         [0023]      FIG. 4A  and  FIG. 4B  are diagrams showing the actions in  FIG. 3 .  
         [0024]      FIG. 5  is a diagram showing the adhesion of a plurality of retardation particles to the alignment layer coated on a transparent substrate under UV irradiation and &gt;10 wt % photoinitiator in pure nitrogen.  
         [0025]      FIG. 6  is a diagram showing the adhesion of a plurality of retardation particles to the alignment layer coated on a transparent substrate under UV irradiation and 0.5 wt %˜10 wt % photoinitiator in air.  
         [0026]      FIG. 7  is a diagram showing the process for directly disposing the retarder film made according to the process flows depicted in  FIG. 3 ,  FIG. 4A  and  FIG. 4B  on a polarizer replacing one of the transparent substrates thereon.  
         [0027]      FIG. 8  is a diagram showing the process for laminating another retarder film on a polarizer with built-in retarder film made according to the flow process depicted in  FIG. 7 .  
         [0028]      FIG. 9  is a diagram showing the process for making polarizer with built-in retarder film with is an integration of the processes described in  FIG. 3 ˜ FIG. 8 . 
     
    
     DETAILED DESCRIPTION  
       [0029]     Referring to  FIG. 3 ,  FIG. 4A  and  FIG. 4B , the process for making retarder film  80  (also called a compensation film) according to the invention can be selectively implemented on a transparent substrate to form an independent retarder film or directly on a transparent substrate coated with polarizing film (i.e. directly implemented on the polarizer) to form a polarizer with retarder film. The process for making retarder film  80  as shown in  FIG. 3  comprises the following steps:  
         [0030]     Step  71 : Coating an alignment layer  82  (as shown in  FIG. 4A ) on a transparent substrate  81 . The alignment layer  82  contains at least oligomer. The transparent substrate  81  uses one of the transparent resin materials, including triacetyl cellulose (TAC), propionyl cellulose, and transparent resin, such as polyamide, polycarbonate, polyester, polystyrene, polyacrylate, norbomene-based polymer, and polyethyl acetate (PET). In considering that retarder film  80  is to be directly employed on a transparent substrate coated with polarizing film (i.e. directly employed on polarizer) or that retarder film  80  made according to the invention is to be used for coating or laminating a polarizing film, the transparent substrate  81  is preferably made of TAC due to its superior structural strength and rigidity that can support the entire polarizer structure and protect the polarizing film from scratches. In this embodiment, alignment layer  82  is mainly solvent-diluted oligomer (the solvent can be EAC, MeOH, IPA, MEK, or toluene on the market). The oligomer material can be Urethane or ester polymer based, such as UV-cured acrylate having an average molecular weight of 200˜4500, viscosity of 5000 cp˜100000 cp, and the number of functional group ranging preferably from two (bifunctional) to six (hexafunctional). Producers of such oligomer include UCB, Sartomer, and Kyoeisha.  
         [0031]     In another embodiment, alignment layer  82  is added with an reactive monomer, or other additives, such as stabilizer or humectant. The monomer is in general mono- or bi-functional, UV-cured acrylate resin.  
         [0032]     Step  72 : Curing the alignment layer  82  with ultraviolet light  84  to cause crosslinking reaction. In this embodiment, UV curing takes place under radiation intensity of 30 mj/cm 2 ˜1000 mj/cm 2 , 0.5 wt % ˜10 wt % (weight percentage) of photoinitiator, and in air or inert gas with oxygen content of no less than 1% (volume percentage). Under those conditions, some incompletely reacted active acrylate is left over on the surface of alignment layer  82 . The photoinitiator used can be a product available on the market, such as Irgacure 907, Irgacure 184, and Irgacure 369 by Ciba.  
         [0033]     Step  73 : Coating retardation material  83  on alignment layer  82  (as shown in  FIG. 4B ). The residual active acrylate on the surface of alignment layer  82  enables the plurality of retardation particles (e.g. liquid crystal cells) contained in the retardation material  83  (e.g. liquid crystal material) to adhere more easily to the surface of alignment layer  82 . The retardation material  83  may be made of smectic or nematic UV-cured polymerizable mono- or bi-functional liquid crystal polymers. The technologies for retardation material  83  and plurality of liquid crystal cells are disclosed in UK patent GB2324382A.  
         [0034]     Step  74 : In an air or inert gas environment with oxygen content of no less than 1% of volume percentage, curing the alignment layer  82  and retardation material  83  with 30 mg/cm 2 ˜1000 mj/cm 2  UV radiation.  
         [0035]      FIG. 5  and  FIG. 6  are diagrams showing the adhesion of a plurality of retardation particles  831  (e.g. liquid crystal cells) to alignment layer  82  coated on a transparent substrate  81  under UV irradiation and &gt;10 wt % photoinitiator in pure nitrogen, and under UV radiation and 0.5 wt %˜10 wt % (weight percentage) photoinitiator in air respectively. As shown in  FIG. 5 , when the transparent substrate  81  coated with alignment layer  82  is cured by UV in pure nitrogen and &gt;10 wt % photoinitiator, there is little active acrylate left over on the surface of alignment layer  82  due to the complete crosslinking reaction of the oligomer. Thus although the resulting surface of alignment layer is flat and smooth, the adhesion of the plurality of retardation particles  831  to the surface is made more difficult. In comparison as shown in  FIG. 6 , when UV curing of transparent substrate  81  coated with alignment layer  82  takes place in air or a little inert gas (with oxygen content greater than 1% of volume percentage) with 0.5 wt %˜10 wt % (weight percentage) photoinitiator, the incompletely reacted active acrylate  821  would form tiny dents on the surface of alignment layer  82  that make it easier for retardation particles  831  (e.g. liquid crystal cells) to adhere to. In addition, retardation particles  831  adhere more strongly and more likely to arrange in a specific direction (e.g. vertical direction). As such, it becomes easier for the retardation particles  831  to adhere to alignment layer  82  in some kind of perpendicular orientation and results in retarder film  80  (or compensation film) of higher quality and greater stability. The present invention allows retarder film  80  to be directly configured on the polarizer without the need to produce it separately or worrying that the unstable quality of retarder film  80  might affect the yield of polarizer. Moreover, the present invention makes use of the incompletely reacted active acrylate  821  to make it easier for retardation particles  831  to adhere to alignment layer  82  in a specific direction without the need to add surfactant to alignment  82  for enhancement of adhesion. Thus as compared to the prior art disclosed in patent GB2324382A that employs surfactant, the present invention offers the advantages of lower cost and higher yield.  
         [0036]     The inventor finds in experiments that under process conditions of 2 wt %˜5 wt % photoinitiator and nearly 20% oxygen content in air, the adhesion between retardation particles  831  and alignment layer  82  is further enhanced, hence resulting in retarder film  80  with more superior stability and optic characteristics.  
         [0037]      FIG. 7  is a diagram showing the process for directly disposing retarder film  80  made according to the process flows depicted in  FIG. 3 ,  FIG. 4A  and  FIG. 4B  on a polarizer  90  replacing one of the transparent substrates thereon. As shown in  FIG. 7 , the retarder film  80  made is rolled into a tube. For the preparation of polarizing film, a PVA polarizing film  91  is first soaked in dichromatic dye  911  (e.g. iodine, potassium iodide, and other dichromatic dye) and then stretched in a predetermined direction and deformation range with a stretching device  912  to give polarizing film  91  specific polarizing effect. The aforesaid retarder film  80  in tube shape and a transparent substrate  92  (e.g. TAC substrate) are respectively laminated to each surface of polarizing film  91  in an oven  913 ,  914  to result in a polarizer  90  with built-in retarder film  80 . In light that the retarder film  80  (including transparent substrate  81  coated with alignment layer  82  and retardation material  83 ) is directly disposed on polarizer  90 , there is no need to employ an additional transparent substrate, or PSA, or any other material between retarder film  80  and polarizer  90 . The polarizer  90  provided by the invention not only offers optical compensation with its built-in retarder film  80 , the transparent substrate  81  of the retarder film  80  can provide a protective layer to the surface of polarizing film  91 . Thus the polarizer  90  with built-in retarder film  80  according to the invention has at least one less layer of PSA or TAC substrate as compared to prior art shown in  FIG. 2 . It is therefore thinner and offers better transparency and optic characteristics.  
         [0038]     In this embodiment, the retarder film  80  is preferably a retarder film that satisfies the conditions of nx=ny&lt;nz and Rth=−10˜−300 nm (referred to as C+Plate). In addition, the retarder film  80  possesses even better optic characteristics when its Rth is confined to Rth=−30˜−80 nm, where nx denotes the refractive index along x-axis of film surface; ny denotes the refractive index along y-axis of film surface; nz is thicknesswise refractive index along z-axis; Rth={(nx+ny)/2−nz}*d; and d is film thickness.  
         [0039]      FIG. 8  is a diagram showing the process for laminating another retarder  93  on a polarizer  90  with built-in retarder film  80  made according to the flow process depicted in  FIG. 7 . First, another retarder film  93  (referred to as first phase retarder  93  hereunder) is provided. The first phase retarder  93  is a film that satisfies the conditions of nx&gt;ny=nz and Ro=0.1˜220 nm (called A-Plate), where Ro=(nx−ny)*d. In a preferred embodiment, the first phase retarder  93  possesses even better optic characteristics when its Ro is further confined to Ro=80˜130 nm. The retarder film  80  and first phase retarder  93  can retard specific wavelengths at predetermined angles and directions to achieve the purpose of optical compensation, and hence better display quality in oblique angles. Next applying a layer of pressure sensitive adhesive  941  (e.g. PSA) on the top surface of polarizer  90  with built-in retarder film  80  (e.g. the surface having retarder film  80 ) and a layer of pressure sensitive adhesive  942  on the surface of first phase retarder  93  opposite the other surface of polarizer  90 . Then laminating first phase retarder  93  onto the polarizer  90  with built-in retarder film  80  to form a polarizer suitable for use in in-plane switching (IPS)LCD. In this embodiment, the polarizer  90  has two layers of retarder film  80 ,  93  to provide even better optical compensation effect.  
         [0040]      FIG. 9  is a diagram showing the process for making polarizer with built-in retarder with is an integration of the processes described in  FIG. 3 ˜ FIG. 8 .  
         [0041]     As shown in  FIG. 9 , the process for polarizer with optical compensation function according to the invention includes the following steps:  
         [0042]     Step  61 : Coating an alignment layer  82  on a transparent substrate  81 , and curing the alignment layer  82  with 30 mj/cm 2 ˜1000 mj/cm 2  UV and 0.5 wt %˜10 wt % photoinitiator, and in an air or inert gas environment with oxygen content of no less than 1% of volume percentage to leave some incompletely reacted active acrylate on the surface of alignment layer  82 .  
         [0043]     Step  62 : Coating retardation materials  83  (e.g. liquid crystal material) on alignment layer  82  where the residue of active acrylate on the surface of alignment layer  82  makes the adhesion of a plurality of retardation particles (e.g. liquid crystal cells) contained in retardation material  83  to the alignment layer  82  easier. Subsequently, UV curing the alignment layer and retardation material under inert gas or air environment. The alignment layer coated with retardation material can retard specific wavelengths at predetermined angles and directions to achieve the effect of optical compensation.  
         [0044]     Step  63 : Providing a dye-containing polarizing film  91 . Subsequently stretching said polarizing film in a predetermined direction and deformation range to give it specific polarizing effect.  
         [0045]     Step  64 : Laminating transparent substrate  81  coated with alignment layer  82  and retardation material  83  and another transparent substrate  92  onto the top and bottom surfaces of polarizing film  91  respectively. The two transparent substrates  81 ,  92  provide relatively high rigidity and structural strength as the protective layers of polarizing film  91 .  
         [0046]     Step  65 : Providing another retarder film  93  (i.e. first phase retarder  93 ).  
         [0047]     Step  66 : Laminating the first phase retarder  93  onto transparent substrate  81  coated with alignment layer  82  and retardation material  83  using a pressure sensitive adhesive  941 . The side of first phase retarder  93  farther away from the polarizer  90  is also coated with pressure sensitive adhesive  942  for adhering glass and polarizer.  
         [0048]     As such, a polarizer  90  with built-in retarders  80 ,  93  that offers optical compensation as shown in step  67  is completed.  
         [0049]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, that above disclosure should be construed as limited only by the metes and bounds of the appended claims.