Abstract:
Disclosed is a casting belt used for manufacturing an optical film by means of a solvent casting process, in more detail, a casting belt for producing a transcendental wide width film which is used to produce a film in a gel state by casting a dope when manufacturing a film that is used for a polarizer of liquid crystal displays or optical compensation films.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. application Ser. No. 13/515,623 filed Jul. 11, 2012, which is a national entry of International Application No. PCT/KR2010/008090, filed on Nov. 16, 2010, which claims a priority to and benefits of Korean Patent Application No. 10-2009-0123684 filed on Dec. 14, 2009, the contents of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a casting belt used for manufacturing an optical film by means of a solvent casting process, in more detail, a casting belt for producing a transcendental wide width film which is used to produce a film in a gel state by casting a dope when manufacturing a film that is used for a polarizer of liquid crystal displays or optical compensation films. 
     BACKGROUND ART 
     Cellulose acylate films have high transparency and mechanical properties, and also have dimensional stability that is little dependent on humidity and temperature. Therefore, they are widely used as supports of optical materials requiring those features. In general, the cellulose acylate films are manufactured by casting a dope made by melting a solvent with polymer onto a continuous support. 
     The casting method can be largely divided into belt casting and drum casting in accordance with the types of continuous supports. The belt casting is a method that casts a dope on a belt, dries and separates a solvent, and conveys a film to the next process, while the drum casting is a method that casts a dope on a drum, separates it without drying, and conveys a film to the next process. 
     In general, the belt casting can manufacture a variety of films because it can control dry conditions of films, while the drum casting can be used for mass production, because it can perform high-speed casting. Both of the methods should accurately machine the surfaces of the drum and the belts, which are continuous supports, to implement a clean surface required for liquid crystal displays. 
     It is required to accurately polish the surfaces of both the belt and drum in order to accurately machine the surface of the continuous supports. The belt is manufactured by grinding and polishing a belt made of stainless steel while the drum is manufactured by plating nickel and hard chrome on a drum made of carbon steel and polishing it. 
     Although it is not a problem in the drum, referring to  FIG. 1 , it is required for the belt to forming a joint  11  connecting both ends of the belt to achieve a continuous support. Welding is widely used to form the joint, and TIG welding and laser welding are commonly used. The welding may be performed before polishing, and may be performed after polishing. The welded portion should be polished to prevent a problem in manufacturing a film, because the welded portion may remain on the belt after the welding. However, it is impossible to completely remove the welded portion and it is possible to see the welded portion with naked eyes. Therefore, some products are used with the welded portion cut off, when a polarizer or an optical compensation film is manufactured by using produced cellulose acylate. 
     Recently, the size of liquid crystal displays gradually increases, and accordingly, the width of polarizers and optical compensation films increases. Therefore, it needs to increase the width of cellulose acylate to come up with the increase in width and to increase yield in manufacturing the polarizers and optical compensation films. 
     Since a drum having a width of 2030 mm or more can be manufactured by technologies that have been developed at the present time, a wide film can be manufactured, whereas it is known that a belt having a width of maximum 2030 mm can be manufactured. Therefore, two belts can be longitudinally welded to achieve a wide belt having a width over 2030 mm; however, a joint is formed at the center longitudinally between two belts by welding, such that the central joint is imprinted on a manufactured film and the film cannot be used itself. 
     The dope casted on the belt is separated and manufactured in a film by a tender and a drier. The film contacts in the width direction from casting to separating, such that necessary extension is performed by the tender. The dimensions of the film are not largely changed in the drying process using the drier. In general, a trimming process that cuts off both ends of the film is performed to smoothly convey the film and keep the properties of the entire film, and the trimming process is performed at one to two times after the casting process, the tender process, and the drying process. 
     Therefore, when a belt having a width of 2030 mm is used, the width of an available film is 2030 mm or less. Considering stability, extension in the tender, and cutting-off of both ends of the film in the substantial casting, the available maximum width is 1800 mm, such that it is difficult to manufacture a film according to the increase in width. 
     SUMMARY OF THE INVENTION 
     The present invention provides a casting belt for producing a transcendental wide width film which makes it possible to manufacture a transcendental wide width film without imprinting a welded portion on a film in manufacturing the film. 
     The present invention provides a casting belt for producing a transcendental wide width film which makes it possible to increase quality of a welded portion by using a jig for transverse welding with laser. 
     The present invention provides a casting belt where a polymer solvent is casted to form a gel-state film in manufacturing an optical film by means of a solvent casting process, wherein the casting belt  100  includes transverse welded portions  111  formed in the transverse direction of the casting belt  100  to connect adjacent casting belt sections  110  such that the adjacent casting belt sections are longitudinally connected without welded portions longitudinally formed, the thickness t w  of the transverse welded portions is 0.95 t b ≦t w ≦1.05 t b  when the thickness of the casting belt sections is t b , and the size of pin holes formed in the transverse welded portions  111  is 20˜50 μm and the depth is within 50 μm. 
     In the present invention, the width W w  of the transverse welded portions  111  maybe within 2 mm, the transverse welded portions  111  may make an angle of 20° with the transverse cross-section of the casting belt  100 , and the width of the casting belt  100  maybe 2000 mm or more. 
     According to the present invention, it is possible to manufacture a wide film having a width of 2000 mm or more without imprinting welded portions, because casting belt sections that are longitudinally adjacent to each other are connected by transverse welded portions, not longitudinal welded portions that connects casting belt sections that are transversely adjacent to each other. 
     When a transcendental wide width casting belt is manufactured by longitudinal welding using laser, it is difficult to fix belts in welding and it is required to weld a long belt, such that it is difficult to satisfy quality required in manufacturing an optical film. However, a transcendental wide width casting belt is manufactured only by transverse welding, a jig can be used, such that, in the present invention, it is possible to perform transverse welding, with the casting belt sections fixed; therefore, it is possible to improve quality of the transverse welded portions 
     Further, according to the present invention, it is possible to easily manufacture a film having a necessary width while satisfying limited conditions in an in-surface phase difference, an on-surface phase difference, and a thickness of a film, because it is possible to manufacture a wide film without increasing elongation of the film in the extending process of the manufacturing process of an optical film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a casting belt of the related art. 
         FIG. 2  is a perspective view of an embodiment of the present invention. 
         FIG. 3  is a plan view of an embodiment of the present invention. 
         FIG. 4  is a side view of a casting belt of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     An embodiment of the present invention relates to a casting belt for producing a transcendent wide width film according to the present invention. 
       FIG. 2  is a perspective view of an embodiment of the present invention.  FIG. 3  is a plan view of an embodiment of the present invention.  FIG. 4  is a side view of a casting belt of the present invention. 
     Manufacturing an optical casing by means of solvent casting performs a casting process that casts a dope containing a polymer solution on a belt to manufacture a film, using the polymer solution, a separating process that separates a gel-state film formed in the casting process, an extending process that extends the separated film with a tender, a drying process that dries the extended film, and a winding process that winds the dried film. 
     An embodiment of the present invention relates to a belt that is used in a casting process for manufacturing an optical film by means of solvent casting, that is, a casting belt for manufacturing a transcendental wide width where a dope containing a polymer solvent is casted to form a gel-state film. 
     Referring to  FIG. 2 , a casting belt  100  for manufacturing a transcendental wide width film is composed of five casting belt sections  110 . The sizes and shapes of the casting belt sections  110  may be the same. 
     Referring to  FIG. 2 , the casting belt sections  110  adjacent to each other in the longitudinal direction of the casting belt  100  is connected by transverse welded portions  111 . That is, the transverse welded portions  111  are joints that connect the adjacent casting belt sections  110  longitudinally arranged, to make a continuous belt from the casting belt sections  110 . The transverse welded portions  111  are formed by welding, for example, YAG laser welding or common laser welding. 
     Referring to  FIGS. 3 and 4 , when the transverse welded portion is formed by the YAG welding, the width W w  of the transverse welded portion  111  can be within 1 mm, and when it is formed by the common laser welding, the width W w  of the transverse welded portion  111  can be within 2 mm. When the width of the transverse welded portion  111  is large, the welded portion is likely to be imprinted on a film, such that it is preferable that the width W w  of the transverse welded portion  111  is within 2 mm. 
     Referring to  FIGS. 3 and 4 , the transverse welded portion  111  is formed at an angle of θ, 20°, from the transverse cross-section of the casting belt  100 . This is for preventing the transverse welded portion  111  from breaking by maximally distributing the force applied to the casting belt sections  110  in an operation with tension exerted in the casting belt  110  for manufacturing a transcendental wide width film. 
     Thought not shown in the drawings, pin holes are formed in the transverse welded portion  111 , and the size of the pin hole is 20˜50 μm and the depth is within 50 μm. Blowholes are formed in welding, and they are made in the pin holes. It is known that the maximum size of the pine hole which can be seen by naked eyes of common people is 20 μm. Therefore, the size may not case a problem up to 50 μm, but larger sizes have a limit in being used for films for LCDs. 
     On the other hand, though not shown in the drawings, when the thickness of the casting belt sections  110  is t b , the thickness t w  of the transverse welded portion is 0.95 t b ≦t w ≦1.05 t b . That is, the difference in thickness of the transverse welded portion  111  and the other portions is within 5%. This is for prevent the welded portion from being imprinted on the film in solvent casting. 
     Meanwhile, the width of the casting belt  100  is 2000 or more, for example, 2000 mm or 2030 mm, which is the maximum belt width in the related art. 
     The present invention has the advantage that it is possible to manufacture a transcendental wide width film having a width of 1800 mm or more, without imprinting the transverse welded portions  111  on the film, by reducing the size and depth of the pin holes formed in the transverse welded portions  111  and reducing the difference in thickness of the transverse welded portions  111  and the other portions. That is, when an optical film is manufactured by solvent casting, the surface state of the film is largely influenced by the surface state of the belt, as compared with common extrusion casting; however, according to the present invention, it is possible to achieve a wide film without a welded portion imprinted. 
     When the transcendental wide width casting belt  100  is manufactured by longitudinal welding using laser, it is impossible to fix the left belt at the transversely left side and the right belt connected to the left belt, at the transversely right side, because a jig cannot be used, such that defects are easily generated in the welded portion due to bad longitudinal welding. The present invention manufactures the casting belt for producing a transcendental wide width film by performing transverse welding that transversely welds the casting belt  100 , not longitudinal welding that longitudinally welds the casting belt  100 . On the other hand, when the transcendental wide width casting belt  100  is manufactured by transverse welding using laser, a jig can be used, such that, in the present invention, it is possible to perform transverse welding, with the casting belt sections  110  fixed; therefore, it is possible to improve quality of the transverse welded portions  111 . 
     The present invention has the advantage of easily manufacturing a film having a necessary width while satisfying limited conditions in an in-surface phase difference, an on-surface phase difference, and a thickness of a film, because it is possible to manufacture a wide film without increasing elongation of the film in the extending process of the manufacturing process of an optical film.