Patent Publication Number: US-6709515-B2

Title: Gravure coating apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gravure coating apparatus, and more particularly to a gravure coating apparatus for applying a coating liquid to a flexible substrate (hereinafter referred to as a “web”) which runs continuously, the gravure coating apparatus being of a type such that no doctor blade is used. 
     2. Description of the Related Art 
     One of coating apparatuses for applying a coating liquid to a continuously running web is a gravure coating apparatus. In gravure coating, an excess amount of coating liquid supplied to a gravure roll so as to exceed a desired amount of coating liquid is usually scraped off by a doctor blade. When the doctor blade is used, however, the coating liquid forms a thin film at a coating point at which the gravure roll comes into contact with a backup roll via the web, so that a coating liquid surface in a cell of the gravure roll is liable to become of a concave shape. This presents a drawback in that the coating film applied to the web sometimes becomes irregular. To eliminate this drawback, a gravure coating apparatus without the use of a doctor blade has been contrived. 
     FIG. 3 shows a general construction of a conventional gravure coating apparatus without any use of a doctor blade. As shown in FIG. 3, in this gravure coating apparatus, a gravure roll  3  whose lower part is immersed in a coating liquid  2  in a liquid pan  1  is rotated to take up the coating liquid  2  from the liquid pan  1 , by which the coating liquid  2  is supplied to the gravure roll  3 . On the other hand, a continuously running web  4  is pressed on the gravure roll  3  by using a backup roll  6  while forming a bead portion  5  between the gravure roll  3  and the web  4 , by which a desired amount of coating liquid  2  is applied to the web  4 . 
     In the case where the gravure coating apparatus without any use of a doctor blade as described above, steady formation of bead is important for forming a satisfactory coating surface shape. For this purpose, it is necessary to excessively supply the coating liquid more than the desired coating amount to the bead portion  5 . The excess coating liquid  2  that is not used for coating in the bead portion  5  flows in the direction reverse to a direction of rotation of the gravure roll  3  as a liquid return. As shown in a schematic view of FIG. 4, this liquid return forms a streak-shaped flow depending on the physical properties of the coating liquid  2  and coating speed, which poses a problem in that a streak trouble is caused on the coating surface on the web  4  due to the liquid return. Specifically, if a streak-shaped liquid return occurs, a flow in the widthwise direction is created in the bead portion  5 , so that variation of size of the bead portion  5  is formed. As a result, when the web  4  is pressed on the gravure roll  3  by the backup roll  6 , a force applied to the coating liquid  2  becomes nonuniform, so that unevenness of film thickness is caused on the web  4 , causing the streak trouble. 
     As countermeasures against the streak trouble caused by the streak-shaped liquid return, Japanese Patent Application Publication No. 63-194766 discloses a method in which the coating liquid taken up from a liquid pan is brought up to the coating portion along a coating liquid guide plate, and an excess coating liquid is discharged through an excess liquid discharge guide surface, by which the liquid return flowing the roll surface is prevented. 
     However, in the countermeasure method disclosed in Japanese Patent Application Publication No. 63-194766, although no liquid return occurs on the roll surface of gravure roll, the coating liquid flowing along the excess liquid discharge guide surface becomes of a streak shape, which exerts an influence on the bead portion as well, so that the streak trouble is caused. 
     Therefore, in the conventional gravure coating apparatus of a type such that no doctor blade is used, it is an actual situation that any streak trouble cannot be solved radically. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the above situation, and accordingly an object thereof is to provide a gravure coating apparatus in which a streak trouble can be prevented from being caused on a coating surface on a flexible substrate because a liquid return of excess coating liquid that is not used for coating at a bead portion is less liable to occur. 
     To attain the above-described object, the present invention is directed to a gravure coating apparatus in which while a bead portion of a coating liquid is formed between a continuously running flexible substrate and a roll surface of a gravure roll to which the coating liquid more than a desired coating amount is excessively supplied, the flexible substrate is pressed on the gravure roll by using a backup roll, whereby the desired amount of coating liquid is applied to the flexible substrate, wherein a liquid supply device is provided at a position near the bead portion on an upstream side in a direction of rotation of the gravure roll to supply the coating liquid from the liquid supply device to the gravure roll. 
     The inventor paid attention to the fact that a force by which an excess coating liquid that is not used for coating in the bead portion is pulled in the direction reverse to the direction of rotation of roll under gravity is strong, and as the distance increases, a streak-shaped liquid return is liable to occur, and obtained a knowledge that the coating liquid is not supplied to the gravure roll by scraping the coating liquid from a liquid pan as before, but the coating liquid is supplied to a position near the bead portion on the upstream side in the direction of rotation of the gravure roll, by which the liquid return can be inhibited, so that even if the liquid return occurs, a streak-shaped liquid return can be prevented from occurring. 
     According to the present invention, the liquid supply device is provided at a position near the bead portion on the upstream side in the direction of rotation of the gravure roll to supply the coating liquid from the liquid supply device to the gravure roll. Therefore, even if a liquid return of excess coating liquid that is not used for coating in the bead portion is going to occur, the liquid return is blocked by the liquid supply device at a position near the bead portion, so that the liquid return in the direction reverse to the direction of rotation of the roll is less liable to occur. Therefore, the force by which the excess coating liquid is pulled in the direction reverse to the direction of rotation of roll under gravity can be weakened. Further, by supplying the coating liquid to a position near the bead portion, the coating liquid is made in a state of riding on the upper face of the roll surface and is less liable to flow down, which makes the liquid return more difficult to occur. Thereby, the occurrence of streak-shaped return liquid can be prevented effectively, so that a streak trouble can be prevented from being caused on the coating surface formed on the flexible substrate. 
     Also, as a preferred mode of the present invention, it is preferable that the liquid supply device has an inclined surface that is inclined downward toward the roll surface of the gravure roll to supply the coating liquid by causing the coating liquid to flow down on the inclined surface, and also has a gap larger than 0 μm and not larger than 400 μm between the tip end of the inclined surface and the roll surface. By causing the coating liquid to flow down on the inclined surface, the coating liquid can be supplied stably to the roll surface, and by forming the gap larger than 0 μm and not larger than 400 μm between the tip end of the inclined surface and the roll surface, the tip end of the inclined surface does not come in contact with the roll surface, and also the above-described effect of blocking the coating liquid can be achieved. In this case, a gap larger than 0 μm and not larger than 200 μm is more desirable. By making the upper limit of gap not larger than 200 μm, the coating liquid can surely be prevented from flowing down through the gap, so that the liquid return of coating liquid can surely be prevented. 
     Thus, in the present invention, it is important that even if the liquid return of excess coating liquid in the bead portion is going to occur, the liquid return be blocked immediately to prevent the coating liquid from flowing down to the lower part of gravure roll. Therefore, it is preferable that the tip end on the roll side of the inclined surface would be located at a position such that a central angle formed between a line drawn from the center of the gravure roll to a contact point at which the flexible substrate comes into contact with the gravure roll and a line drawn from the center of the gravure roll toward the tip end on the roll surface side of the inclined surface is not larger than 45°. Also, if the angle of the inclined surface of the liquid supply device is too large, the coating liquid flowing on the inclined surface is disordered, so that it is preferable that the angle of downward inclination of the inclined surface do not exceed 15° with respect to the horizontal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
     FIG. 1 is a schematic view for illustrating an embodiment of a gravure coating apparatus in accordance with the present invention, showing an example in which an extrusion type die coater and an inclined plate are used as a liquid supply device; 
     FIG. 2 is a schematic view for illustrating an embodiment of a gravure coating apparatus in accordance with the present invention, showing an example in which a slide type die coater is used as a liquid supply device; 
     FIG. 3 is a schematic view of a conventional gravure coating apparatus; and 
     FIG. 4 is an explanatory view for illustrating a streak-shaped liquid return. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of a gravure coating apparatus in accordance with the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic view for illustrating an embodiment of a general construction of a gravure coating apparatus in accordance with the present invention. 
     As shown in FIG. 1, a gravure coating apparatus  10  is mainly composed of a gravure roll  12 , a backup roll  14 , and a liquid supply device  18  for a coating liquid  16 . 
     The gravure roll  12  has a roll surface  12 A, which is engraved to stick and hold the coating liquid  16 . By the rotation of the gravure roll  12 , the coating liquid  16 , which is supplied from the liquid supply device  18  to the roll surface  12 A is transferred and applied to a continuously running web  20 . Under the gravure roll  12  is provided a pan  22 . This pan  22  serves to receive the coating liquid  16  dropping from the gravure roll  12 , and does not serve to supply the coating liquid  16  to the gravure roll  12  unlike the conventional gravure coating apparatus. Therefore, the pan  22  does not store the coating liquid  16 . The backup roll  14  is arranged adjacently on the upper side of the gravure roll  12  to press the continuously running web  20  on the gravure roll  12  in such a manner that the web  20  is held between the gravure roll  12  and the backup roll  14 . While a bead portion  24  of the coating liquid  16  is formed between the roll surface  12 A of the gravure roll  12  to which the coating liquid  16  is excessively supplied from the liquid supply device  18  and the continuously running web  20 , the web  20  is pressed on the gravure roll  12  by using the backup roll  14 , by which a desired amount of coating liquid  16  is applied to the web  20 . 
     The liquid supply device  18  has an inclined surface  26  that is inclined downward toward the roll surface  12 A, and also has a gap  28  too narrow for the coating liquid  16  to flow down between the roll surface  12 A and the inclined surface  26 . The liquid supply device  18  is disposed so that the inclined surface  26  is located at a position near the bead portion  24  on the upstream side in the direction of rotation of the gravure roll  12 . The liquid supply device  18  is not subject to any special restriction, and may be a device that can supply the coating liquid  16  uniformly in the widthwise direction (back and face direction of FIG. 1) and can supply the coating liquid  16  to the gravure roll  12  while causing the coating liquid  16  to flow down on the inclined surface  26 . The liquid supply devices  18  shown in FIGS. 1 and 2 can be used suitably. 
     The liquid supply device  18  shown in FIG. 1 is composed of independent members of a liquid supply portion for supplying the coating liquid  16  and an inclined portion forming the inclined surface  26 . Specifically, an extrusion type die coater  32 , which is the liquid supply portion, is disposed in the downward direction above an inclined plate  30  forming the inclined surface  26 . In the die coater  32 , a pocket portion  34  which is parallel with the inclined surface  26  in the widthwise direction is formed, and also a narrow slit  38  extending from the pocket portion  34  to a discharge port  36  is formed. The coating liquid  16  supplied to the pocket portion  34  is caused to flow so as to spread in the widthwise direction of the inclined surface  26  by the pocket portion  34 , flowing down in the slit  38 , and is pushed out of the discharge port  36  onto the inclined surface  26 . Thereby, the coating liquid  16  is supplied uniformly in the widthwise direction of the inclined surface  26 , flowing down on the inclined surface  26 , and is supplied to the roll surface  12 A of the gravure roll  12 . 
     On the other hand, the liquid supply device  18  shown in FIG. 2 is constructed so that the liquid supply portion for supplying the coating liquid  16  and the inclined portion forming the inclined surface  26  are formed integrally, by using a slide type die coater  40 . This slide type die coater  40  has the inclined surface  26  on the top surface of the die coater  40  itself. Other elements such as pocket portion  34  and the slit  38  are the same as those of the extrusion type die coater  32 . In this case, the coating liquid  16  having been caused to flow so as to spread by the pocket portion  34  rises in the slit  38  and is pushed out onto the inclined surface  26 . Thereby, the coating liquid  16  is supplied uniformly in the widthwise direction of the inclined surface  26 , flowing down on the inclined surface  26 , and is supplied to the roll surface  12 A of the gravure roll  12 . 
     For both of the liquid supply devices  18  shown in FIGS. 1 and 2, a preferred position of the inclined surface  26  with respect to the roll surface  12 A of the gravure roll  12  is a position such that a central angle β formed between a line  42  drawn from the center O of the gravure roll  12  to a contact point A at which the web  20  comes into contact with the gravure roll  12  and a line  44  drawn from the center O of the gravure roll  12  toward the tip end on the roll surface side of the inclined surface  26  is not larger than 45°, preferably not larger than 30°. The reason for this is that if the position of the tip end of the inclined surface  26  is too distant downward from the bead portion  24 , the distance of a liquid return, which is a phenomenon that the excess coating liquid  26  that is not used for coating in the bead portion  24  flows down along the roll surface  12 A, increases, so that a streak-shaped liquid return is liable to occur. Therefore, the important thing is to block the liquid return at a position near the bead portion  24 . For this purpose, it is preferable that the tip end of the inclined surface  26  of the liquid supply device  18  be located at a position such that the central angle β is not larger than 45°. Further, by making the central angle β not larger than 45°, the coating liquid  16  is made in a state of riding on the upper face of the roll surface  12 A and is less liable to flow down, which makes the liquid return more difficult to occur. Since it is not possible to locate the tip end of the inclined surface  26  so that the central angle β is 0°, that is, at the contact point A at which the web  20  comes into contact with the gravure roll  12 , the lower limit of the central angle β is an angle such that the coating liquid  16  can be supplied physically. 
     Also, the angle of inclination α of the inclined surface  26  of the liquid supply device  18  does preferably not exceed 15° with respect to the horizontal, further preferably not exceed 10°. The reason for this is as follows: in order to supply the coating liquid  16  to the roll surface  12 A, the inclined surface  26  must be inclined downward toward the roll surface  12 A. If the angle of inclination α is too large, the flow-down speed of the coating liquid  16  flowing down on the inclined surface  26  becomes too high, so that the liquid film of the flowing-down coating liquid  16  is disordered. A trouble due to this disorder exerts an adverse influence on the application to the web  20 . 
     Also, the gap  28  between the tip end of the inclined surface  26  and roll surface is preferably larger than 0 μm and not larger than 400 μm, further preferably larger than 0 μm and not larger than 200 μm. The reason for this is that the roll surface  12 A must be prevented from being worn by avoiding the contact of the tip end of the inclined surface  26  with the roll surface  12 A, and also it is important that it be difficult for the coating liquid  16  to flow down through the gap  28  to inhibit the liquid return. Therefore, if the upper limit of the gap  28  between the tip end of the inclined surface  26  and roll surface is set at a value not larger than 400 μm, the coating liquid  16  can be made difficult to flow down through the gap  28 , and if the upper limit thereof is set at a value not larger than 200 μm, the coating liquid  16  scarcely flows down, so that the liquid return can be prevented surely. 
     EXAMPLE 
     By using a web of polyethylene terephthalate (PET) of 0.1 mm thick and 1000 mm wide, coating performance of the gravure coating apparatus in accordance with the present invention shown in FIG. 1 (embodiment), the conventional gravure coating apparatus shown in FIG. 3 (comparative example 1), and the conventional gravure coating apparatus disclosed in Japanese Patent Application Publication No. 63-194766 (comparative example 2) was evaluated under the coating conditions given in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Coating 
                 Amount of 
                 Surface 
                   
               
               
                   
                 speed 
                 coating 
                 tension 
                 Viscosity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Coating condition 
                 10 m/min 
                 4 cc/m 2   
                 27 mN/m 
                 2 mPa · s 
               
               
                   
               
            
           
         
       
     
     In the test, the coating length of one time on the web was set at 100 m, and the number of streak troubles occurring in a plane shape was counted by sampling 10 m of the coating end portion. The level of occurring streak trouble was evaluated by three grades of A, B and F. A designates a streak trouble within the allowable range of product, which cannot be found unless it is observed carefully. B designates a streak trouble which is slightly more obvious than A and within the allowable range of product. F designates a streak trouble which is obvious and out of the allowable range of product. In addition, the number of liquid return streaks occurring on the gravure roll was counted visually. In the case of comparative example 2, the number of liquid return streaks occurring on the excess liquid discharge guide surface was counted. The number of liquid return streaks was counted in the third test. 
     Test results are given in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Level 
                 Embod- 
                 Comparative 
                 Comparative 
               
               
                   
                 of streak 
                 iment 
                 example 1 
                 example 2 
               
               
                   
                 trouble 
                 Number 
                 Number 
                 Number 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Number of streak 
                 A 
                 0 
                 3 
                 5 
               
               
                 troubles occurring 
                 B 
                 0 
                 5 
                 4 
               
               
                 on coating surface 
                 F 
                 0 
                 13 
                 3 
               
               
                 Number of liquid re- 
                   
                 0 
                 21 
                 12 
               
               
                 turn streaks 
               
               
                   
               
            
           
         
       
     
     As seen from Table 2, when the gravure coating apparatus of comparative example 1 was used for coating, of the streak troubles occurring on the coating surface, the number of streak troubles out of the allowable range of product was as large as 13, and the number of liquid return streaks was also very large, being 21. 
     When the gravure coating apparatus of comparative example 2 was used for coating, no liquid return was found on the roll surface of the gravure roll, but the liquid film formed a streak shape on the excess liquid discharge guide surface. Therefore, although the number of streak troubles was smaller than that of comparative example 1, three streak troubles that were out of the allowable range of product were found, which was not a satisfactory result. Also, the number of liquid return streaks occurring on the excess liquid discharge guide surface was as large as 12. 
     Contrarily, when the gravure coating apparatus in accordance with the present invention was used for coating, a streak trouble did not occur at all, and a liquid return streak on the roll surface was not found at all. 
     As described above, according to the gravure coating apparatus in accordance with the present invention, a liquid return of the excess coating liquid that is not used for coating in the bead portion is less liable to occur, so that a streak trouble on the coating surface formed on the flexible substrate can be prevented from occurring. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.