Abstract:
This invention relates to a solution casting process capable of decreasing uneven coating of a functional layer upon applying the functional layer onto a film surface, which comprises casting a solution of polymer dissolved in organic solvent into a film by extruding from a die onto a support, wherein the length of the film from the die opening to the landing of the film on the support is controlled to 3 to 40 mm.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a solution casting process capable of decreasing uneven coating of a functional layer upon applying the functional layer onto a film surface. 
     Films used in optical field are produced using cellulose triacetate film or the like as the support film, and the cellulose triacetate film and a the like are produced through the solution casting process. In the solution casting process, a polymer solution dissolved in organic solvent is cast from a die onto a support, such as a drum or an endless band, while a ribbon (a film of the solution between the die opening and the landing point of the film at the support) is allowed to adhere closely to the support by a vacuum chamber located in the vicinity of the die. Various improvements have been made in the solution casting process, and for example, Japanese Patent KOKOKU 49-36946 discloses a casting process of a liquid composition so as to contact the flow of the liquid composition closely with a roller by providing a vacuum chamber having two suction rooms, in order to prevent air entrapping. Japanese Patent KOKOKU 62-38133 and 63-57222 disclose a uniform pressing apparatus for a web capable of stabilizing end beads by providing two vacuum zones separated by a partition wall. Japanese Patent KOKAI 5-66212 discloses a casting process which prevents the generation of skimming by using a dope containing a good solvent and a poor solvent at a specific ratio. 
     However, in the above conventional processes, the ribbon vibrates at a certain frequency caused by the wind pressure vibration, mechanical vibration or the like due to the air column vibration in a vacuum chamber, the air column vibration in a suction duct or the vibration of the vacuum chamber by the vibration of a blower, and thereby, continuous or periodic step-shaped thickness variation occurs. 
     Incidentally, films used in optical field have an antiglare coating in order to add a hard coating or reflection-reducing to the film. Uneven thickness existing on a support film induces uneven coating of the antiglare layer to degrade appearance and functions of the films. Nevertheless, it is very difficult to remove the above vibration sources. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a process for solution coating capable of producing a film having a small unevenness in thickness, and thereby, not inducing uneven coating of various functional layers on the film. 
     The inventors investigated eagerly in order to achieve the above object, and found that the length-of the ribbon influences the generation of the uneven thickness, and the unevenness in thickness can be reduced to the degree of not appreciating uneven coating by adjusting the length of ribbon to a particular range. 
     That is, concerning step-shaped uneven thickness with a pitch in the range of 5 to 30 mm in the longitudinal direction in the films produced by the solution casting process, the inventors investigated the relationship between the pitch a mm of uneven thickness and (b/c)×100 (uneven thickness rate) wherein b μm (uneven thickness) is the difference between the highest position and the lowest position in the uneven thickness, and c μm is the (mean) thickness of the support film. The results obtained are shown in FIG.  1 . In FIG. 1, ◯ indicates that uneven coating cannot be seen, Δ indicates that =uneven coating can be seen slightly, and × indicates that uneven coating can be seen, evaluated by visual observation. From the results, they found that, in the range of a pitch of uneven thickness of 5 to 30 mm, uneven coating cannot be seen by rendering the length of a ribbon 3 to 40 mm to complete the invention. 
     Thus, the present invention provides a process for solution casting which comprises casting a solution of polymer dissolved in organic solvent into a film by extruding from a die onto a support, wherein the length of the film from the die opening to the landing of the film on the support is controlled to 3 to 40 mm. 
     In the above results of FIG. 1, the vibration sources around the ribbon are necessary to restrict the vibration quantity of the die to 0.02% or less of the distance between the die and the support, the irregularity of the velocity of the support to 0.02% or less of the velocity of the support, and the pressure pulsation in the delivery of the dope to 0.3% or less of the irregularity of the frequency which makes the pitch of uneven thickness 30 mm or more. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph indicating the relationship between uneven thickness rate and pitch of uneven thickness with respect to the appreciation of uneven coating through visual observation. 
     FIG. 2 is a schematic side view illustrating casting state of a dope from a die. 
     FIG. 3 is a schematic side view of a solution casting apparatus to which process of the invention is applied. 
     FIG. 4 is a schematic side view of another solution casting apparatus to which process of the invention is applied. 
     FIGS. 5-7 are a schematic section of various casting dies. 
    
    
       1  . . . Casting die 
       11 ˜ 13  . . . Manifold 
       14  . . . Slot 
       15  . . . End opening (die opening) 
       16  . . . Feed block 
       17  . . . Passage 
       2  . . . Support 
       21  . . . Band 
       22  . . . Drum 
       23  . . . Rotary drum 
       3  . . . Vacuum chamber 
       4  . . . Ribbon 
       5  . . . Suction duct 
       6  . . . Buffer tank 
       7  . . . Blower 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the process of the invention, the length of the ribbon is controlled to 3 to 40 mm, preferably 7 to 25 mm, more preferably 8 to 15 mm. The ribbon length of less than 3 mm makes the ribbon unstable caused by the steep casting. The ribbon length of exceeding 40 mm renders uneven coating conspicuous. The shape of the ribbon is usually curved, and the length of the ribbon is measured by taking a photograph or the like. 
     The control method of the ribbon length is explained with reference to FIG.  2 . In FIG. 2,  1  is a casting die,  2  is a support, such as a band or a drum,  3  is a vacuum chamber, and  4  is a ribbon. 
     As the factors for controlling the length of the ribbon, there are {circle around (1)} the gap h between the casting die  1  and the support (band or drum)  2 , {circle around (2)} suction pressure p of the vacuum chamber  3 , {circle around (3)} viscosity η of dope (ribbon)  4 , {circle around (4)} lip clearance cl of the casting die  1 , {circle around (5)} casting speed v (traveling speed of a support  2 , {circle around (6)} thickness t of a cast (support) film  4 , and the like. 
     The length of the ribbon  4  can be shortened by shortening the gap h between the casting die  1  and the support  2 , raising the suction pressure p of the vacuum chamber  3 , increasing the dope viscosity η, narrowing the die lip clearance cl, lowering the casting speed v, thickening the thickness t of the cast film  4 , or the like. The length of the ribbon  4  can be lengthened by operating opposite to above. The length of the ribbon is controlled to 3 to 40 mm by employing one or more of the above operations. Moreover, any other means can be employed for the control of the ribbon length. 
     A suitable gap h between the casting die and the support is, in general, 1 to 10 mm, preferably 1.5 to 6 mm. 
     A suitable suction pressure p of the vacuum chamber is, in general, −1 to −50 mm Aq (1 mm Aq=9.80665 Pa), preferably −2 to −30 mm Aq. 
     A suitable viscosity η is in general, 10 to 1,000 poises (at the casting temperature of dope), preferably 100 to 800 poises 
     A suitable lip clearance cl is, in general, 0.2 to 3 mm, preferably 0.5 to 2 mm. 
     A suitable casting speed v is, in general, 3 to 150 m/min, preferably 10 to 100 m/min. 
     A suitable film thickness t is, in general, 20 to 500 μm, preferably 30 to 300 μm, more preferably 35 to 200 μm. 
     Illustrative of the polymers suitable for the invention are cellulose triacetate, cellulose diacetate, and the like, and illustrative of the organic solvents are methylene chloride, methanol, butanol, acetone, and the like. 
     The solution casting apparatus for carrying out the invention may be a solution casting apparatus using a mirror-finished cast band or a solution casting apparatus using a mirror finished cast drum. A solution casting apparatus using a cast band is shown in FIG. 3, and a solution casting apparatus using a mirror-finished cast drum is shown in FIG.  4 . 
     The solution casting apparatus shown in FIG. 3 comprises a casting die  1 , a pair of rotary drums  23 , an endless band  21  traveling between the rotary drums  23 , a vacuum chamber  3  located adjacent to the casting die  1 , and a suction duct  5 , a buffer tank  6  and a blower  7  connected to the vacuum chamber  3  in this order. 
     The solution casting apparatus shown in FIG. 4 uses a cast drum  22  instead of the cast band  21  and a pair of the rotary drums  23 . 
     The casting die  1  in the apparatus of FIG. 3 and 4 has a schematic structure shown in FIG.  5 . The die  1  is used for casting a single layer film, and has a manifold  11  at the center and a slot  14  connecting the manifold  11  and the end opening  15  of the die. The casting dies  1  shown in FIG. 6,  7  are also usable. The die  1  of FIG. 6 is used for casting a multiplayer (triple layer) film, and has three manifolds  12 ,  12 ,  12  and slots  14 ,  14 ,  14 , which meet on their midway to be integrated into one slot, connecting the manifolds and the end opening  15 . The die  1  of FIG. 7 is also used for casting a multiplayer (triple layer) film, and has a feed block  16  where three passages  17  are met at the inside of the feed block  16 . 
     The above dies are coathanger type dies, and any other type die, such as T die, is also applicable. 
     According to the invention, even if it is very difficult to remove vibrations around the ribbon through casting a film, i.e. vibration remains, uneven thickness can be reduced by controlling the length of the ribbon in the range of 3 to 40 mm, preferably 7 to 25 mm. As a result, uneven coating of functional layers, such as an antiglare layer and an orientation layer, becomes small to improve appearance and functions. 
     EXAMPLES 
     Materials and conditions employed in the examples are as follows: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Dope (Solution of Polymer) 
                 (parts by weight) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cellulose triacetate 
                 100 
               
               
                   
                 Triphenyl phosphate 
                 10 
               
               
                   
                 Biphenyl diphenyl phosphate 
                 5 
               
               
                   
                 Methylene chloride 
                 400 
               
               
                   
                 Methanol 
                 70 
               
               
                   
                 η-Butanol 
                 2 
               
               
                   
                 Film thickness after drying 
                 40-150 μ m 
               
               
                   
                 Degree of pressure reduction 
                 −2-−50 mm Aq 
               
               
                   
                 in vacuum chamber 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Dope of substream on cocasting 
                 (parts by weight) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cellulose triacetate 
                 100 
               
               
                   
                 Triphenyl phosphate 
                 10 
               
               
                   
                 Biphenyl diphenyl phosphate 
                 5 
               
               
                   
                 Methylene chloride 
                 500 
               
               
                   
                 Methanol 
                 90 
               
               
                   
                 η-Butanol 
                 3 
               
               
                   
                 Layer thickness of main stream (after drying) 
                 76 μ m 
               
               
                   
                 Layer thickness substreams (after drying) 
                 each 2 μ m 
               
               
                   
                 (both surface layers) 
               
               
                   
                   
               
             
          
         
       
     
     Example 1 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 3.5 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −20 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 200 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 0.9 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 8 mm, and b/c (uneven thickness rate) and a (pitch of uneven thickness) were 0.6% and 12 mm, respectively. Uneven coating could not be seen. 
     Example 2 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 5 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −5 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 300 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 1.0 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 23 mm, and b/c and a were 1.8% and 25 mm, respectively. Uneven coating could hardly be seen. 
     Example 3 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 3.5 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −20 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 200 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 0.9 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 40 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 10 mm, and b/c and a were 0.75% and 15 mm, respectively. Uneven coating could not be seen. 
     Example 4 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 5.0 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −2 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 300 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 1.4 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 35 mm, and b/c and a were 3.5% and 30 mm, respectively. Uneven coating could hardly be seen. 
     Example 5 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 2 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −30 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 200 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 0.7 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 5 mm, and b/c and a were 0.6% and 8 mm, respectively. Uneven coating could hardly be seen. 
     Comparative Example 6 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 12 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −20 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 300 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 1.4 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     Upon casting, the length of the ribbon was 45 mm, and b/c and a were 15% and 35 mm, respectively. Uneven coating was seen. 
     Comparative Example 7 
     Casting was carried out under the following conditions: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gap h between die and support 
                 0.5 
                 mm 
               
               
                   
                 Suction pressure p of vacuum chamber 
                 −30 
                 mmAq 
               
               
                   
                 Viscosity of Dope η 
                 200 
                 poise 
               
               
                   
                 Die lip clearance cl 
                 0.7 
                 mm 
               
               
                   
                 Casting speed v 
                 50 
                 m/min 
               
               
                   
                 Film thickness t 
                 80 
                 μ m 
               
               
                   
                   
               
             
          
         
       
     
     In this case, the ribbon was ruptured, and the cast film could not be made.