Patent Publication Number: US-9427767-B2

Title: Apparatus for applying coating liquid and coating bar

Description:
FIELD OF THE INVENTION 
     The present invention relates to a coating apparatus and a coating bar used for the same. 
     BACKGROUND OF THE INVENTION 
     In a known coating technique, an antireflective coating and a wavelength tunable film for interrupting specific wavelength light are applied over a wide area for solar cells, display panels, and lighting apparatuses. 
     A representative technique is, for example, a bar-coating method. An invention of the bar-coating method is described in Japanese Utility Model Laid-Open No. 62-183586.  FIG. 6  is a schematic diagram for explaining the conventional bar-coating method. 
     As illustrated in  FIG. 6 , in the bar-coating method, a long bar  111  is first set in a coating width direction, and a coating liquid  114  is then supplied onto a substrate  113  from a separately provided dispenser nozzle  112 . After that, the substrate  113  and the bar  111  in contact with the substrate  113  are relatively moved in a lateral direction to spread an excessive coating liquid in a scraping manner, leaving a predetermined volume of the coating liquid  114  on the substrate  113  so as to evenly form a thin film. Hereinafter, the dispenser nozzle  112  is located upstream while the coating liquid  114  scraped into a uniform film is located downstream with respect to the bar  111 . 
     The surface of the bar  111  has small asperities. The coating liquid  114  is left on the substrate  113  such that the coating liquid  114  is as thick as gaps between the asperities and a substrate surface. Thus, a film thickness is adjusted by changing the size of the asperities. 
     As a bar used for the bar-coating method, a known bar shape is shown in, for example, Japanese Patent Laid-Open No. 2004-148204.  FIGS. 7A and 7B  are schematic diagrams illustrating the structure of a conventional bar.  FIG. 7A  is a side view of the bar.  FIG. 7B  is a cross-sectional view of the bar. A bar  111  illustrated in  FIGS. 7A and 7B  includes a wire  116  wound around a shaft  115 , the wire  116  having a predetermined diameter. A coating liquid  114  is left on a substrate  113  according to a gap  117  formed between asperities on the wire  116  and the substrate  113 , enabling coating with a constant thickness. 
     DISCLOSURE OF THE INVENTION 
     In the bar-coating method, however, the bar needs to be in contact with the substrate in the width direction of the bar. 
     As is understood from  FIG. 7A , a necessary condition for the bar-coating method is that the substrate  113  is in contact with the asperities formed by the wire  116  on the surface of the bar  111  and only the gap  117  is opened. For the necessary condition, the overall bar  111  needs to be in contact with the substrate  113 . To be specific, the substrate  113  coated with a coating liquid needs to be less rigid like a film and extend along the bar  111 , the substrate  113  needs to be flat, or a curve on the substrate  113  needs to be corrected by, for example, suction to a stage. In other words, a space other than the gap  117  may be formed between the substrate  113  and the bar  111  in the conventional bar-coating method. Thus, it is difficult to apply the conventional bar-coating method to a substrate having high rigidity and low flatness, e.g., a thick glass substrate. 
     Hence, the tracking of the bar  111  to the substrate  113  may be improved by reducing the rigidity and cross-sectional area of the bar. 
     However, in the case where the bar  111  has a small diameter in cross section, the coating liquid  114  supplied onto the substrate  113  may flow over the bar  111  as illustrated in  FIG. 7B . 
     Specifically, in the case where the bar  111  has a small diameter, the coating liquid  114  flows upward (arrow  120  in  FIG. 7B ) along grooves on the surface of the bar  111  because of the surface tension of the coating liquid  114  and a pressure for scraping the coating liquid  114 . The coating liquid  114  then flows to a downstream side  119  of the bar  111  and reaches a coating surface (arrow  121  in  FIG. 7B ). Hence, a coating film may be varied in thickness or variations in thickness may increase. 
     The present invention is devised to solve the conventional problem. An object of the present invention is to stably apply a uniform film even on a curved or wavy substrate having high rigidity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram illustrating the shape of a coating bar according to a first embodiment; 
         FIG. 1B  is a schematic diagram illustrating the shape of the coating bar according to the first embodiment; 
         FIG. 2  is a table showing the relationship between a groove forming angle and a coating film according to the first embodiment; 
         FIG. 3A  is a schematic diagram illustrating the shape of a coating bar covered with resin according to a second embodiment; 
         FIG. 3B  is a schematic diagram illustrating the shape of the coating bar covered with the resin according to the second embodiment; 
         FIG. 4A  is a schematic diagram illustrating the shape of a coating bar covered with rubber according to the second embodiment; 
         FIG. 4B  is a schematic diagram illustrating the shape of the coating bar covered with the rubber according to the second embodiment; 
         FIG. 5A  illustrates the shape of a groove end according to the present invention; 
         FIG. 5B  illustrates the shape of a groove end according to the present invention; 
         FIG. 6  is a schematic diagram for explaining a conventional bar-coating method; 
         FIG. 7A  is a schematic diagram illustrating the structure of a conventional bar; 
         FIG. 7B  is a schematic diagram illustrating the structure of the conventional bar; and 
         FIG. 8  is a perspective view illustrating the configuration of a coating apparatus including a coating bar. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     First Embodiment 
       FIGS. 1A and 1B  are schematic diagrams illustrating the shape of a coating bar according to a first embodiment.  FIG. 1A  is a cross-sectional view taken along line X-X′ of  FIG. 1B .  FIG. 1B  is a side view. In  FIGS. 1A and 1B , grooves  2  formed in a circumferential direction are arranged over a coating width on the shaft surface of a coating bar  1 . The grooves  2  are formed only on a part of the outer surface of the shaft in the cross section of the shaft. In other words, the groove  2  is shorter than the circumference of the shaft. In this case, a method for forming the grooves  2  is not particularly limited, and thus the grooves  2  can be formed by cutting, electroforming, or plating on the shaft. 
     A structural example of a coating apparatus including the coating bar will be specifically described below. 
       FIG. 8  is a perspective view illustrating the configuration of the coating apparatus including the coating bar. The coating apparatus includes a feeding device  201  that feeds a required amount of a coating liquid  204  in the width direction of a substrate  203  onto the substrate  203  installed on a substrate holding stage  202 . The feeding device  201  is connected to a ball screw  208  whose ends are held by vertically moving support members  207 . With this configuration, the feeding device  201  can be moved in the width direction of the substrate  203 , enabling coating in the width direction of the substrate  203 . The end of the feeding device  201  in  FIG. 8  has a dispenser provided with a feeding needle  206  on the end of a syringe  205 . The feeding device  201  may include a coating liquid dispenser nozzle (e.g., a die coat) extended in the width direction of the substrate. 
     A coating bar  209  that scrapes an excessive amount of the coating liquid  204  in the longitudinal direction is installed in the width direction of the substrate  203 . A holding/pressing device  211  is provided to press the coating bar  209  to the surface of the substrate  203 . According to a specific example, the coating bar  209  is held by a plurality of holding chucks  210 , each including a pressing device (not shown) capable of pressing the coating bar  209  with a predetermined pressure by means of an elastic material such as rubber, a spring, and an air pressure. 
     Elevating devices  212  are provided to move up and down the holding/pressing device  211  such that the coating bar  209  and the substrate  203  are not in contact with each other when the coating liquid  204  is not applied, for example, during replacement of the substrate  203 . 
     According to the present invention, the coating bar  209  in contact with the substrate  203  on a specific contact point is drawn without being rotated, thereby spreading the coating liquid  204  on the flat substrate  203  in a scraping manner. Grooves are partially formed around the outer surface of the coating bar  209  and are spaced at regular intervals. The grooves are formed so as to cover the contact point between the coating bar  209  and the substrate  203 . The contact point is located on the outer surface of the coating bar  209  and on a straight line extended in parallel with the shaft of the coating bar  209 . The contact point is a group of surfaces between the grooves on the coating bar  209  that comes into contact with the substrate  203  when the coating bar  209  is drawn. 
     In the following explanation, the substrate  203  is a cover glass for a solar cell. The cover glass for a solar cell has a large thickness of about 2 mm to 4 mm and thus cannot be easily bent like a glass substrate of 1 mm or less. Generally, a cover glass for a solar cell is formed by cooling molten glass pressed with a roller die, forming asperities. Thus, a stress is applied to the glass so as to form large curves or waves on the glass. Only a glass surface may be rapidly cooled from a high temperature to reinforce the glass, which may apply a thermal stress so as to form curves or waves on the cover glass for a solar cell on the order of millimeters. 
     Hence, in order to apply the bar-coating method to a cover glass for a solar cell, the coating bar  209  needs to track curves or waves on the order of millimeters. In response to curves and waves on the substrate  203 , materials such as Al and Cu having lower rigidity are more desirably used than SUS to reduce the rigidity of the coating bar  209 . The coating bar  209  is desirably circular in cross section with a diameter of about 2 mm to 6 mm to secure elasticity. This is because when the coating bar  209  is smaller in diameter than 2 mm, the coating bar  209  is hard to hold with the chucks  210 , whereas when the coating bar  209  is larger than 6 mm in diameter, the coating bar  209  becomes too rigid to track curves or waves on the substrate  203 . Another reason is that the coating bar  209  needs to be strongly pressed to the substrate  203  in response to curves or waves when the coating bar  209  is extremely rigid, leading to large wear on the coating bar  209 . The rigidity of the coating bar  209  can be effectively reduced by optionally forming a hollow at the center of the shaft. The depth of the groove is determined according to the thickness of a coating film. 
     A mechanism (not shown) for pressing the coating bar  209  at predetermined intervals or pressing the overall coating bar  209  in the width direction with an elastic material such as rubber is provided to fix the coating bar  209  to the coating apparatus in a direction orthogonal to the coating surface of the substrate  203 . With this configuration, the coating bar  209  can easily track curves or waves in the width direction of the substrate  203 . 
     According to the coating bar and the coating apparatus of the present invention, the grooves  2  in  FIGS. 1A and 1B  are provided partially on the surface of the coating bar  209 , thereby preventing an overflow of a coating liquid in a portion not including the grooves  2 . Hence, a uniform coating film can be formed even on a curved or wavy rigid substrate having a large width of about 300 mm to 2000 mm. Even in the case where the coating bar is reduced in diameter to further track the substrate, an overflow of a coating liquid can be prevented so as to form a uniform coating film. 
     A positional relationship for forming the grooves on the coating bar will be described below. As illustrated in  FIG. 1A , an angle α is formed by the formation region of the grooves  2  in the relative traveling direction (upstream) of the coating bar  1  from a contact point between the coating bar  1  and a substrate  3  while an angle β is formed by the grooves  2  in a direction opposite to the relative traveling direction of the coating bar  1  (downstream) from the contact point between the coating bar  1  and the substrate  3 . Referring to  FIG. 2 , visual observation results on the quality of a coating film with varying angles α and β will be described below. 
       FIG. 2  is a table showing the relationship between a groove forming angle and a coating film according to the first embodiment. In this table, a used coating liquid contained a material that forms an antireflective coating after drying and burning, according to the formation conditions of an antireflective coating in a typical solar cell. Moreover, a prime solvent was a solution containing a solvent with a viscosity of 2 mPa·s to 10 mPa·s. The coating bar in contact with the substrate on the contact point of the coating bar was drawn with a coating speed, that is, a constant relative traveling speed of 10 mm/s to 50 mm/s between the coating bar and the substrate. 
     First, the angle α was changed that is formed by the grooves  2  in the relative traveling direction (upstream) of the coating bar  1  from a contact point between the coating bar  1  and the substrate  3 . As shown in  FIG. 2 , every time the angle α was changed to 30°, 60°, 90°, and 120° with the angle β fixed at 60°, the occurrence of a flow of a coating liquid  4  over the coating bar  1  was reduced unlike in the case where the grooves  2  are formed around the outer surface of the coating bar  1 . Moreover, it was confirmed that uneven coating caused for the reason was eliminated. In the case where the angle α was 30° and 60°, unfortunately, a coating film was likely to trap air bubbles. 
     Air bubbles are trapped as follows: first, ends  5  of the grooves  2  in contact with the coating liquid  4  are covered with the coating liquid  4 , and then air bubbles trapped in the grooves  2  are contained in the coating liquid  4 . The air bubbles are likely to remain on the ends  5 , and the remaining air bubbles may be trapped on the surface of a coating film by vibrations or the like during coating. 
     In the case where the angle α is 90° and 120°, however, the ends  5  of the grooves  2  are always exposed upward (in the atmosphere) from the coating liquid  4 , allowing air bubbles trapped in the grooves  2  to flow out of the ends  5 . The ends  5  are located perpendicularly to the substrate  3  and thus are more likely to release air bubbles than in the case where the angle α is 30° and 60°. Air bubbles are left when the angle α is 30° and 60°, whereas air bubbles are not left when the angle α is 90° and 120°. When the angle α is larger than 120°, the coating liquid  4  is likely to flow over the coating bar  1  and adhere to the chucks, which may reduce the holding power of the chucks. 
     According to the results, under conditions equivalent to the formation conditions of an antireflective coating of a typical solar cell, the coating liquid contains a material that forms an antireflective coating after drying and burning, a prime solvent is a solution containing a solvent with a viscosity of 2 mPa·s to 10 mPa·s, and the coating bar is drawn with a coating speed, that is, a relative traveling speed of 10 mm/s to 50 mm/s between the coating bar and the substrate. In this case, the angle α of 90° to 120° is desirably formed by the grooves  2  in the relative traveling direction of the coating bar  1  from a contact point between the coating bar  1  and the substrate  3 . 
     In the following explanation, the angle β was changed that forms the grooves  2  in the direction opposite to the relative traveling direction of the coating bar  1  (downstream) from a contact point between the coating bar  1  and the substrate  3 . In the case where the angle α was fixed at 90° and the angle β was changed to 30°, 60°, 90°, and 120°, a uniform coating film was obtained when the angle β was 30° and 60°, whereas a coating film tended to vary in thickness when the angle β was 90° and 120°. 
     The reason may be considered as follows: the coating liquid  4  is moved upward along the grooves  2  by capillarity and is transported to ends  6  of the grooves  2  in a protruding manner, and the amount of a liquid pool  7  downstream of the coating bar  1  is varied by vibrations or the like during coating, resulting in uneven coating. When the angle β is 0°, the coating liquid  4  in contact with the substrate  3  does not open the grooves, precluding stable coating. 
     Thus, under conditions equivalent to the formation conditions of an antireflective coating of a typical solar cell, the coating liquid contains a material that forms an antireflective coating after drying and burning, a prime solvent is a solution containing a solvent having a viscosity of 2 mPa·s to 10 mPa·s, and the coating bar is drawn with a coating speed, that is, a relative traveling speed of 10 mm/s to 50 mm/s between the coating bar and the substrate. In this case, the angle β of 0° to 60° is desirably formed by the grooves  2  in the direction opposite to the relative traveling direction of the coating bar  1  (downstream) from a contact point between the coating bar  1  and the substrate  3 . 
     As has been discussed, the coating bar of the coating apparatus is a cylindrical shaft having a small diameter in cross section. The grooves are partially formed on the surface of the shaft in the relative traveling direction of the coated substrate or in a direction opposite to the relative direction from a contact point between the coating bar and the substrate, and the grooves are arranged in the width direction of the shaft. Thus, a coating film is less varied in thickness or uneven coating is less likely to be formed by an overflow of the coating liquid or the liquid pool, so that the film can be stably and evenly applied to a curved or wavy substrate having high rigidity. 
     Second Embodiment 
     According to a second embodiment, a coating bar used for a coating apparatus has gaps between wires wound around the coating bar. The gaps serve as the grooves of the first embodiment. The grooves are partially covered with a coating material such as resin and rubber so as to form an exposed region on the gaps serving as the grooves. 
     Referring to  FIGS. 3A, 3B, 4A, and 4B , the coating bar having a different structure from the coating bar of the first embodiment will be described below. 
       FIGS. 3A and 3B  are schematic diagrams illustrating the shape of the coating bar covered with resin according to the second embodiment.  FIG. 3A  is a cross-sectional view, and  FIG. 3B  is a side view.  FIGS. 4A and 4B  are schematic diagrams illustrating the shape of the coating bar covered with rubber according to the second embodiment.  FIG. 4A  is a cross-sectional view showing an initial state.  FIG. 4B  is a cross-sectional view showing a state after the rubber is moved. 
     Referring to  FIGS. 3A and 3B , the structure and forming method of the coating bar will be described below. The coating bar is provided with grooves partially formed on the surface of the coating bar as in the first embodiment. 
     A coating bar  12  according to the second embodiment has a wire  8  that is wound around the outer surface of a substrate  3  and is coated with resin  10  partially covering the wire  8  over the width of the coating bar  12 . The wire  8  of the coating bar  12  and the substrate  3  are in contact with each other in a region where the wire  8  is exposed from the resin  10  without being coated with the resin  10 . The wire  8  is substantially circular in cross section and has a predetermined diameter. 
     As has been discussed, the wire  8  is exposed in the region where the wire  8  is not coated with the resin  10 . Gaps  9  on the wire  8  have the same effect as the grooves  2  of the first embodiment (see  FIGS. 1A and 1B ). As in the first embodiment, the resin  10  is not provided in a region from a contact point between the coating bar  12  and the substrate  3  in the relative traveling direction (upstream) of the coating bar  12  (the range of an angle α in  FIG. 3A ) and a region from the contact point between the coating bar  12  and the substrate  3  in a direction opposite to the relative traveling direction (downstream) of the coating bar  12  (the range of an angle β in  FIG. 3A ). As in the first embodiment, a coating liquid contains a material that forms an antireflective coating after drying and burning, a prime solvent is a solution containing a solvent having a viscosity of 2 mPa·s to 10 mPa·s, and the coating bar  12  in contact with the substrate  3  on the contact point of the coating bar is drawn with a coating speed, that is, a constant relative traveling speed of 10 mm/s to 50 mm/s between the coating bar  12  and the substrate  3 . In this case, the angle α is desirably 90° to 120° while the angle β is desirably larger than 0° and is equal to or smaller than 60°. 
     A material selected as the used resin  10  needs to be resistant to corrosion against an applied coating liquid. 
     As has been discussed, the coating bar  12  of the coating apparatus includes the wire  8  wound around the surface of the cylindrical shaft, and the resin  10  provided so as to open a region from the contact point between the coating bar  12  and the substrate  3  in the relative traveling direction of the coated substrate  3  and in the direction opposite to the relative traveling direction, thereby exposing the gaps  9  on the wire  8 . Hence, the resin  10  suppresses an overflow of the coating liquid so as to prevent an overflow of the coating liquid or a liquid pool from causing variations in the thickness of a coating film or uneven coating. This allows the curved or wavy substrate  3  having high rigidity to be stably coated with a uniform film. 
     Even in the case where the coating bar is reduced in diameter to further track the substrate, an overflow of the coating liquid can be prevented. Furthermore, the winding of the wire  8  and the formation of the resin  10  are easier than the formation of grooves on the shaft, thereby easily applying a uniform film at low cost without causing uneven coating. 
     As illustrated in  FIGS. 4A and 4B , the resin  10  may be replaced with elastic rubber  13  bonded so as to cover the gaps  9  on the wire  8  (see  FIGS. 3A and 3B ). In this case, the rubber  13  on the coating bar  14  can be shifted so as to shift a contact point  11  on the substrate  3 . Specifically, in the case where a surface  15  of the wire  8  wears on the contact point  11  on the substrate  3  after a predetermined number of times of coating (a state in  FIG. 4A ), the rubber can be slightly shifted to expose another surface  16  of the wire  8  (a state in  FIG. 4B ) as the contact point  11  (see  FIGS. 3A and 3B ) for use. Hence, the life of the coating bar  14  can be extended while lower running cost for equipment can be expected. 
     As illustrated in  FIGS. 5A and 5B , the coating bars according to the first embodiment and the second embodiment may have a tilted end in the groove or exposed gap.  FIGS. 5A and 5B  illustrate the shape of a groove end according to the present invention.  FIG. 5A  illustrates the shape of the groove while  FIG. 5B  illustrates the shape of a groove formed in a gap. 
     As illustrated in  FIG. 5A , the end region of a groove  2  decreases in depth toward the end of the groove  2 . As illustrated in  FIG. 5B , the resin  10  or the rubber  13  is provided such that the end region of the gap  9  formed on the wire decreases in depth toward the end of the gap  9 . Since the end of the groove  2  or the gap  9  gradually decreases in depth, air bubbles in a coating liquid  4  are easily released and the protrusion of the coating liquid  4  is suppressed, thereby stably applying a uniform film.