Patent Publication Number: US-2006013968-A1

Title: Optical compensation film and manufacturing method thereof

Description:
RELATED APPLICATIONS  
      The present application is based on, and claims priority from, Taiwan Application Serial Number 93120872, filed Jul. 13, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety.  
     BACKGROUND  
      1. Field of Invention  
      The present invention relates to a liquid crystal display panel. More particularly, the present invention relates to an optical compensation film and the manufacturing method thereof.  
      2. Description of Related Art  
      Liquid crystal display (LCD) has many advantages over other conventional types of displays including high display quality, small volume, light weight, low driving voltage and low power consumption. Hence, LCDs are widely used in small portable televisions, mobile telephones, video recording units, notebook computers, desktop monitors, projector televisions and so on, and have gradually replaced the conventional cathode ray tube (CRT) as a mainstream display unit. Therefore, the market is mainly occupied by LCDs due to the high display quality and the low power consumption of the LCDs. Large size, high resolution, wide view and rapid response time are the main demands on the LCDs.  
      Some popular wide view techniques have been developed, such as In-Plane Switching (IPS), Optical Compensated Birefringence (OCB), Multi-Domain Vertical Alignment (MVA), wide view optical compensation films and any combination thereof. The simplest one of these wide view techniques is to insert the wide view optical compensation films into a liquid crystal display panel, which increases the view-angle of the LCD to between about 140 and 160 degrees. This kind of wide view technique is available to liquid crystal displays in different sizes, and only involves insertion of the wide view optical compensation films into the LCD without changing the manufacturing processes thereof.  
      The prior art generally uses two main methods, such as electrical pulling and liquid crystal spreading, to manufacture conventional optical compensation films.  FIG. 1A  illustrates a flow chart of the traditional liquid crystal spreading method, and  FIG. 1B  illustrates a schematic view of the process of  FIG. 1A . The following descriptions are made reference with  FIG. 1A  and  FIG. 1B .  
      A substrate  111  is provided (step  101 ), and then an alignment layer  112  is spread on the substrate  111  by an alignment layer spreading device  122  (step  102 ). After being spread on the substrate  111 , the alignment layer  122  is baked (step  103 ), aligned (step  104 ), and cleaned to remove residues thereon (step  105 ), thereby ensuring that liquid crystal molecules subsequently spread thereon are arranged in an orderly manner. The optical compensation film thus has a retardation value to compensate for view-angles and chromatic aberration.  
      As illustrated in  FIG. 1B , the spread alignment layer  112  is baked by an alignment layer baking device  123 , and is aligned and cleaned of residues by a mechanical roller alignment and residue cleaning device  124 . After the foregoing steps, the surface of the alignment layer  112  has many slots generated by the rubbing of the mechanical roller. The slots are oriented in the same direction and are suitable for aligning liquid crystal molecules.  
      After that, a liquid crystal material having liquid crystal molecules is spread on the alignment layer  112  by a liquid crystal spreading device  126  to form a liquid crystal (LC) layer  116  (step  106 ). The liquid crystal molecules in the liquid crystal layer  112  are aligned to be oriented in the same direction by the slots. The liquid crystal layer  116  is baked to remove the solvent therein by a liquid crystal layer baking device  127  (step  107 ), and then is cured by a UV light device  128  (step  108 ).  
      Finally, protection layers  119  are separately adhered onto two sides of the substrate  111  having the alignment layer  112  and the liquid crystal layer  116  (step  109 ), and thus completing the conventional optical compensation film.  FIG. 1C  provides a schematic, cross-sectional view of the optical compensation film manufactured by the method in  FIG. 1A . As illustrated in  FIG. 1C , the optical compensation film  130  comprises the protection layer  119 , the substrate  111 , the alignment layer  112 , the liquid crystal layer  116  and the other protection layer  119  in order.  
      However, the traditional method requires many prior steps, such as spreading the alignment layer and aligning the same by mechanical rubbing, for subsequently spreading the liquid crystal material. Therefore, under considerations of manufacturing efficiency, yields and cost, the traditional method is not ideal. Moreover, the slots of the alignment layer surface are generated by irregular mechanical damage, and therefore decrease the alignment uniformity of the liquid crystal molecules, and make enhancement of the compensation effect and optical performance of the optical compensation films difficult.  
     SUMMARY  
      It is therefore an objective of the present invention to provide a method for manufacturing an optical compensation film, which uses substrate extension to replace a conventional alignment layer technique; the manufacturing efficiency and yield are thereby raised, and the cost is reduced.  
      It is another objective of the present invention to provide an optical compensation film, of which the uniformity of liquid crystal molecules is substantially improved, and thus effectively enhances the compensation and optical performance thereof.  
      In accordance with the foregoing and other objectives of the present invention, an optical compensation film and the manufacturing method thereof are provided. A substrate is pulled to a stretch ratio, and a liquid crystal material is then spread on a surface of the substrate to form a liquid crystal layer. Next, a protection layer is adhered onto the liquid crystal layer.  
      According to one preferred embodiment of the present invention, the method further comprises adhering a second protection layer onto a second surface of the substrate after pulling the substrate, spreading the liquid crystal material and then baking the liquid crystal layer and curing the liquid crystal layer by UV light. Moreover, the substrate is mechanically pulled to the stretch ratio by a pulling machine.  
      A material of the substrate is polyvinyl alcohol (PVA), triacetyl cellulose (TAC), ARTON, cyclic olefin copolymer (coc), cyclic olefin polymer (cop) or polyethylene terephthalate (PET). The liquid crystal material is nematic liquid crystal or discotic liquid crystal. When the material of the substrate is polyvinyl alcohol, the stretch ratio is between 5 and 12. In addition, materials of the first and second protection layers are triacetyl cellulose, cyclic olefin copolymer, cyclic olefin polymer or polyethylene terephthalate.  
      Compared with the conventional methods, the method of the present invention does not require the several prior steps, such as spreading the alignment layer, baking the alignment layer, aligning by mechanical rubbing and residue removal. Manufacturing efficiency and yields are thus increased, and the manufacturing cost is reduced. Moreover, the device for spreading the liquid crystal material can be placed behind the device for pulling the substrate; that is, the liquid crystal material can be spread immediately after the substrate is pulled. Therefore, the manufacturing processes are coherent and easily completed, in addition to proper maintenance of the stretch ratio of the substrate.  
      In another aspect, uniform and continuous striped slots are easily formed on the surface of the substrate, which are oriented in the pulling direction, because the substrate is pulled by skilled mechanical pulling with good uniformity. The uniform slots greatly help the uniform orientation of the polar liquid crystal molecules. In contrast to the conventional alignment layer with irregular slots formed by mechanical rubbing, the invention substantially improves the arrangement uniformity of liquid crystal molecules, and thus effectively enhances the compensation and optical performance of the optical compensation film.  
      It is to be understood that both the foregoing general description and the following detailed description are examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
       FIG. 1A  is a flow chart of the traditional liquid crystal spreading method;  
       FIG. 1B  is a schematic view of the process described in  FIG. 1A ;  
       FIG. 1C  is a schematic, cross-sectional view of the optical compensation film manufactured by the method described in  FIG. 1A ;  
       FIG. 2  is a flow chart of one preferred embodiment of the present invention;  
       FIG. 3A  is a flow chart of another preferred embodiment of the present invention;  
       FIG. 3B  is a schematic view of the process described in  FIG. 3A ; and  
       FIG. 3C  is a schematic, cross-sectional view of an optical compensation film manufactured by the method described in  FIG. 3A .  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
       FIG. 2  is a flow chart of one preferred embodiment of the present invention. As illustrated in  FIG. 2 , a substrate is pulled to a stretch ratio (step  201 ), and a liquid crystal material is then spread on a surface of the substrate to form a liquid crystal layer (step  206 ). Next, a protection layer is adhered onto the liquid crystal layer (step  209 ).  
      A material of the substrate is polyvinyl alcohol (PVA), triacetyl cellulose (TAC), ARTON, cyclic olefin copolymer (coc), cyclic olefin polymer (cop) or polyethylene terephthalate (PET). The liquid crystal material is nematic liquid crystal or discotic liquid crystal; the nematic liquid crystal has a better compensation effect. A material of the protection layers is triacetyl cellulose, cyclic olefin copolymer, cyclic olefin polymer or polyethylene terephthalate.  
      In the preferred embodiment, according to different requirements and specifications, the optical compensation film can be made of various substrate materials, stretch ratios and liquid crystal materials to achieve the required compensation effect and obtain good optical performance.  
       FIG. 3A  is a flow chart of another preferred embodiment of the present invention, and  FIG. 3B  is a schematic view of a process in  FIG. 3A . The following descriptions are made with reference to  FIG. 3A  and  FIG. 3B .  
      A substrate  311  is pulled to a stretch ratio by a pulling device  322 , such as a pulling machine (step  201 ). When the material of the substrate  311  is polyvinyl alcohol (PVA), the stretch ratio is between 5 and 12, and preferably is 10. A protection layer  319  is adhered onto the backside of the substrate  311  for protecting the pulled substrate  311  and giving enough supporting to prevent the substrate  311  from shrinking back (step  302 ).  
      A liquid crystal layer spreading device  326 , such as a die, a wire bar, a gravure or other spreading device, is used to spread a liquid crystal material on the other surface of the substrate  311  to form a liquid crystal layer (step  206 ). In the preferred embodiment, the liquid crystal material comprises 25% BASF liquid crystal molecules, chiral dopant, photoinitiator and p-xylene, which is used as a solvent.  
      The chiral dopant cooperates with the liquid crystal molecules to form spiral structures, and the weight percent thereof is about 10%. Furthermore, according to other preferred embodiments of the present invention, the range of the weight percent of the liquid crystal molecules is between about 10% and 50%, depending on different types of liquid crystal and the required compensation effect.  
      After being pulled, the substrate  311  has uniform and continuous striped slots oriented in the pulling direction. The uniform slots substantially help the arrangement uniformity of polar liquid crystal molecules, such as the BASF liquid crystal molecules used in the preferred embodiment.  
      Therefore, the liquid crystal molecules in the liquid crystal layer  316  are aligned in the same direction by the slots. A liquid crystal layer baking device  327 , such as an oven, is used to bake the liquid crystal layer  316  to remove the p-xylene solvent (step  307 ), and a UV light device  328  is used to cure the liquid crystal layer  316  (step  308 ). Finally, a protection layer  319  is adhered onto the liquid crystal layer  316  (step  209 ), thus completing the optical compensation film.  FIG. 3C  is a schematic, cross-sectional view of an optical compensation film manufactured by the method in  FIG. 3A . As illustrated in  FIG. 3C , the optical compensation film  330  comprises the protection layer  319 , the substrate  311 , the liquid crystal layer  316  and the other protection layer  319 , in order.  
      In addition, a comparison of refractive indexes and retardations between a pulled PVA substrate with the stretch ratio of 10 and a pulled PVA substrate further with a BASF liquid crystal layer are listed in Table 1. In the embodiments in Table 1, a thickness of the cured liquid crystal layer  316  is about 1.3 mm, and the surface roughness thereof is about 5-6 nm. N x  is a refractive index in x direction, N y  is a refractive index in y direction, N z  is a refractive index in z direction, Ro is an in-plane retardation and R th  is an out-of-plane retardation.  
               TABLE 1                          A comparison of refractive indexes and retardations between a       pulled PVA substrate and a pulled PVA substrate further with a BASE liquid       crystal layer.                                         N x     N y     N z     R 0     R th                                                   Pulled PVA substrate   1.502-1.508   1.502-1.509   1.487-1.495    80-200   360-440       Pulled PVA substrate/   1.503-1.508   1.502-1.511   1.486-1.496   100-220   310-500       BASF liquid crystal layer                  
 
      From Table 1, it is evident that the refractive indexes of the pulled PVA substrate with or without the BASF liquid crystal layer are almost unchanged. The range of in-phase retardation of the pulled PVA substrate with the BASF liquid crystal layer is not further changed more, either. However, the range of out-of-phase retardation of the pulled PVA substrate with the BASF liquid crystal layer is obviously greater than the range of out-of-phase retardation of the pulled PVA substrate without the BASF liquid crystal layer. The range of out-of-phase retardation is substantially increased from 80 to 210, and thus effectively enhances the compensation and optical performance of the optical compensation film.  
      It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.