Abstract:
The present invention provides a preform coating device ( 10 ) provided with a cylindrical transfer roller ( 11 ) for rotating about a center shaft ( 111 ), and an application device for applying a coating liquid in a predetermined thickness to an external peripheral face of the transfer roller ( 11 ), the external peripheral face of the transfer roller ( 11 ) rotating so as to touch an external peripheral face of a preform ( 1 ), and the coating liquid thereby being transferred from the external peripheral face of the transfer roller ( 11 ) to the external peripheral face of the preform ( 1 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a preform coating device for coating a preform of a plastic bottle with a coating solution, a method for manufacturing a preform, and a method for manufacturing a plastic bottle. 
       BACKGROUND ART 
       [0002]    Today, plastic containers made of polyethylene terephthalate (PET bottles) and other plastic bottles are being widely used for holding beverages or food. The plastic bottles are formed by expanding the test tube shaped preforms by stretch blow molding. 
         [0003]    As shown in Japanese Patent Publication No. 2012-250771, to reduce the passage of gases such as oxygen and carbon dioxide between the inside and outside of a plastic bottle, it is known to form a barrier coating on the outer circumferential surface of the preform. The barrier coating is formed by applying a coating solution to the outer circumferential surface of the preform and drying the outer circumferential surface of the preform. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    However, a coating solution is typically applied, as shown in  FIG. 6 , by dipping the preform in the coating solution in the vertical direction. In this case, at the outer circumferential surface of the preform, due to gravity, the film thickness of the coating solution gradually increases the further downward in the vertical direction. 
         [0005]    For this reason, the barrier performance reducing the passage of gases becomes uneven on the outer circumferential surface of the preform and in turn the plastic bottle. Further, if trying to make the coating solution completely dry in the state where the film thickness is uneven, the parts where the film thickness is thin are sometimes excessively heated and whitened. In particular, if dipping the preform in a coating solution in the vertical direction, the bottom part of the preform is also applied with the coating solution, so the drying time of the coating solution becomes longer. For this reason, if making the coating solution of the bottom part with a thick film thickness dry, the cylindrical body part of the preform with a thin film thickness becomes excessively heated, so tends to whiten. These whitened preforms or plastic bottles often are deemed as defects in the final inspection process and cannot be used. Further, if the preform whitens when blow molding the plastic bottle, it results in poor stretching in the blowing process and a predetermined size of plastic bottle cannot be formed or cracks end up occurring in some cases. 
         [0006]    Therefore, the present invention was made in consideration of the above problem and has as its object to make the film thickness of the coating solution applied to the outer circumferential surface of a preform for a plastic bottle thin and uniform so as to reduce the occurrence of defects of a preform or plastic bottle. 
       Solution to Problem 
       [0007]    In a first embodiment of the present invention, there is provided a preform coating device comprising a columnar shaped transfer roller rotating about its center axis and an application device for applying a coating solution with a predetermined thickness to an outer circumferential surface of the transfer roller, wherein the outer circumferential surface of the transfer roller rotates in contact with an outer circumferential surface of a preform whereby the coating solution is transferred from the outer circumferential surface of the transfer roller to the outer circumferential surface of the preform. 
         [0008]    In the first embodiment of the present invention, preferably, the application device is a die coater formed with a slot and configured so as to discharge coating solution from the slot to the outer circumferential surface of the transfer roller. 
         [0009]    In the first embodiment of the present invention, preferably, the preform coating device further comprises a conveyor device conveying the preform, and a conveyance path of the preform includes a substantially arc shaped path along the outer circumferential surface of the transfer roller. 
         [0010]    In the first embodiment of the present invention, preferably, the coating solution is transferred from the outer circumferential surface of the transfer roller to only a cylindrical body part of the preform. 
         [0011]    In the first embodiment of the present invention, preferably, the preform coating device further comprises a scraper configured so as to scrape off the coating solution not transferred to the outer circumferential surface of the preform from the outer circumferential surface of the transfer roller. 
         [0012]    In the first embodiment of the present invention, preferably, the preform coating device further comprises a solution recovery tank recovering the coating solution scraped off by the scraper and a defoaming device removing bubbles from the recovered coating solution. 
         [0013]    In a second embodiment of the present invention, there is provided a method for manufacturing a preform comprising: a step of applying a coating solution with a predetermined thickness to an outer circumferential surface of a columnar shaped transfer roller rotating about its center axis by an application device, a step of conveying a preform on the outer circumferential surface of the transfer roller and transferring the coating solution from the outer circumferential surface of the transfer roller to the outer circumferential surface of the preform, and a step of drying the outer circumferential surface of the preform. 
         [0014]    In the second embodiment of the present invention, preferably, the coating solution is transferred from the outer circumferential surface of the transfer roller to only a cylindrical body part of the preform. 
         [0015]    In the second embodiment of the present invention, preferably, the preform is continuously conveyed on the outer circumferential surface of the transfer roller. 
         [0016]    In the second embodiment of the present invention, preferably, the coating solution not transferred to the outer circumferential surface of the preform is recovered, is deformed and is again applied to the outer circumferential surface of the transfer roller by the application device. 
         [0017]    In the second embodiment of the present invention, preferably, the coating solution has a viscosity of 100 mPa·s or more. 
         [0018]    In a third embodiment of the present invention, there is provided a method for manufacturing a plastic bottle including stretch blow molding a preform manufactured by the second embodiment of the present invention. 
       Advantageous Effects of Invention 
       [0019]    According to the present invention, it is possible to make the film thickness of the coating solution applied to the outer circumferential surface of a preform for a plastic bottle uniformly thin and in turn reduce the occurrence of defects of a preform or plastic bottle. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  shows a preform for a plastic bottle. 
           [0021]      FIG. 2A  to  FIG. 2D  shows a stretch blow molding method for forming a plastic bottle from a preform. 
           [0022]      FIG. 3  shows a plastic bottle formed from a preform. 
           [0023]      FIG. 4  is a schematic view of a preform coating device according to an embodiment of the present invention. 
           [0024]      FIG. 5  is a schematic perspective view of a die coater. 
           [0025]      FIG. 6  shows a conventional method of applying a coating solution to a preform. 
           [0026]      FIG. 7  shows another method for supplying a coating solution to an outer circumferential surface of a transfer roller. 
           [0027]      FIG. 8  shows the state in the preform coating device of  FIG. 7  where excess coating solution is dropped into a solution dipping tank by a blade. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Below, embodiments of the present invention will be explained with reference to the attached drawings. Note that, these embodiments do not limit the present invention. Further, the components of the present embodiment include components which a person skilled in the art could use instead or which would be easy to use instead and components which are substantially the same. 
         [0029]    First, referring to  FIG. 1  to  FIG. 3 , the method of forming a plastic bottle will be explained. Note that, in this Description, a “plastic bottle” means a bottle made of a plastic like polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE) and is not limited to a PET bottle. 
         [0030]      FIG. 1  shows a preform  1  for a plastic bottle. The preform  1  is formed from a resin by the injection molding method or the PCM (preform compression molding) method. The preform  1  is comprised of a mouth part  1   a  fitting with a cap of the plastic bottle, a cylindrical body part  1   b  adjoining the mouth part  1   a , and a bottom part  1   c  closing one end of the cylindrical body part  1   b  and has a shape like a test tube. At the outer circumferential surface of the mouth part  1   a , a male thread for engaging with the female thread of the cap is formed. The end part of the mouth part  1   a  side of the preform  1  is open. 
         [0031]    After forming the preform  1 , the outer circumferential surface of the preform  1  is formed with a barrier coating. The barrier coating is formed by applying a coating solution to the outer circumferential surface of the preform  1  and drying the outer circumferential surface of the preform  1 . By doing this, the passage of a gas such as oxygen and carbon dioxide between the inside and outside of the plastic bottle formed from the preform  1  is reduced and storage life of the beverage etc. contained can be extended. Further, the scratch resistance, moisture resistance, etc. of the plastic bottle can be improved. 
         [0032]    A plastic bottle is formed from the preform  1  by stretch blow molding.  FIGS. 2A to 2D  show the stretch blow molding method for forming a plastic bottle  3  from the preform  1 . 
         [0033]    First, as shown in  FIG. 2A , the preform  1  is heated by a heater  40 . Next, as shown in  FIG. 2B , the preform  1  is inserted into a mold  2  and the mold  2  is closed. Next, as shown in  FIG. 2C , the preform  1  is stretched by a stretch rod (not shown) in the vertical direction and is stretched by pressurized air in the horizontal direction. Next, as shown in  FIG. 2D , if the preform  1  is expanded to the desired shape, the inside surface of the plastic bottle  3  is cooled by cooling air. Finally, the plastic bottle  3  is taken out from the mold  2 .  FIG. 3  shows a plastic bottle  3  formed from the preform  1 . 
         [0034]    Below, referring to  FIG. 4  and  FIG. 5 , a preform coating device according to an embodiment of the present invention will be explained. 
         [0035]      FIG. 4  is a schematic view of a preform coating device  10  according to an embodiment of the present invention. The preform coating device  10  applies a coating solution to the outer circumferential surface of the preform  1 . The preform coating device  10  comprises a transfer roller  11 , a conveyor device  12  conveying a preform  1  on the outer circumferential surface of the transfer roller  11 , a die coater  13  discharging a coating solution to the outer circumferential surface of the transfer roller  11 , a solution holding tank  14  holding the coating solution to be supplied to the die coater  13 , and a pump  15  supplying the coating solution from the solution holding tank  14  to the die coater  13 . 
         [0036]    The transfer roller  11  has a columnar shape and rotates about its center axis  111 . The transfer roller  11  is rotated by turning a handle connected to the center axis  111  by hand or by using a motor etc. to make the transfer roller  11  rotate about the center axis  111 . 
         [0037]    The conveyance path of the preform  1  is shown by the arrows in  FIG. 4 . The conveyance path of the preform  1  by the conveyor device  12  includes a substantially arc shaped path along the outer circumferential surface of the transfer roller  11 . If the preform  1  is conveyed by the conveyor device  12  on the outer circumferential surface of the transfer roller  11 , the outer circumferential surface of the preform  1  continuously rotates in contact with the outer circumferential surface of the transfer roller  11 , to which the coating solution is applied, along the substantially arc shaped path along the outer circumferential surface of the transfer roller  11 . Due to this, the preform  1  can be made to move while transferring the coating solution from the outer circumferential surface of the transfer roller  11  to the outer circumferential surface of the preform  1 . The circumferential direction length of the transfer roller  11  of the part contacting the preform  1  is the circumferential direction length of the preform  1  or more so that the coating solution is transferred over the entire circumference of the preform  1 . In this example, the outer circumferential surface of the preform  1  rotates in contact with the outer circumferential surface of the transfer roller  11  over substantially half the circumference of the transfer roller  11 . Note that, the path of conveyance of the preform  1  on the transfer roller  11  may be a path other than the substantially arc shaped path along the outer circumferential surface of the transfer roller  11  so long as the preform  1  can continuously rotate in contact with the transfer roller  11 . 
         [0038]    The conveyor device  12  can rotatably fasten the preform  1 . As shown by the arrows in  FIG. 4 , preforms  1  are conveyed by the conveyor device  12  one at a time on the outer circumferential surface of the transfer roller  11 . Further, a plurality of preforms  1  may be rotatably fastened at predetermined intervals to the conveyor device  12  and be continuously conveyed by the conveyor device  12  on the outer circumferential surface of the transfer roller  11 . Further, preforms  1  may be rotated so as to move along the outer circumferential surface of the transfer roller  11  one at a time by manual operation. 
         [0039]    The die coater  13  is arranged with its front end adjoining or approaching the outer circumferential surface of the transfer roller  11 .  FIG. 5  is a schematic perspective view of the die coater  13 . As shown in  FIG. 5 , the die coater  13  is formed with an elongated slot  131 . The coating solution is discharged in a sheet shape from the elongated slot  131  to the outer circumferential surface of the transfer roller  11 . Note that, the preform coating device  10  may be provided with another application device configured to apply the coating solution with a predetermined thickness to the outer circumferential surface of the transfer roller  11 , instead of the die coater  13 . 
         [0040]    Another method may also be considered as the method for supplying the coating solution to the outer circumferential surface of the transfer roller  11 . For example, as shown in  FIG. 7 , it is also possible to arrange the solution dipping tank  31 , in which the coating solution is poured, below the transfer roller  11  and directly dip the transfer roller  11  in the coating solution. In this case, if using the handle  32  etc. to make the transfer roller  11  rotate, the coating solution is deposited on the outer circumferential surface of the transfer roller  11 , then, if the outer circumferential surface of the transfer roller  11  rotates in contact with the outer circumferential surface of the preform  1 , the coating solution is transferred from the outer circumferential surface of the transfer roller  11  to the outer circumferential surface of the preform  1 . 
         [0041]    However, in this method, as shown in  FIG. 8 , when making the excess coating solution drop into the solution dipping tank  31  by the blade  33  so as to make the film thickness of the coating solution supplied to the outer circumferential surface of the transfer roller  11  constant, the coating solution draws air along with it and sometimes is formed with bubbles inside the solution dipping tank  31 . In this case, the coating solution containing the bubbles  50  is supplied from the solution dipping tank  31  to the outer circumferential surface of the transfer roller  11  to be transferred to the outer circumferential surface of the preform  1 , so the film thickness of the coating solution becomes uneven on the outer circumferential surface of the preform  1 . This problem easily occurs in particular when using a high viscosity coating solution, since the bubbles are difficult to remove from the coating solution. 
         [0042]    On the other hand, in the present embodiment, the minimum extent of coating solution is constantly discharged from the elongated slot  131  of the die coater  13  directly to the outer circumferential surface of the transfer roller  11  so that the solution becomes a predetermined film thickness, so bubbles do not easily form in the coating solution. Therefore, it is possible to make the film thickness of the coating solution coated on the outer circumferential surface of the preform  1  uniformly thin and in turn reduce the occurrence of defects of the preform  1  or plastic bottle. 
         [0043]    As shown in  FIG. 4 , the die coater  13  is connected through a first hose  16  to the pump  15 . The pump  15  is connected through a second hose  17  to the solution holding tank  14 . To further reduce the formation of bubbles at the coating solution, the pump  15  is preferably a non-pulsation pump. The solution holding tank  14  is filled with the desired coating solution before starting the coating. Further, as explained later, the coating solution not transferred to the outer circumferential surface of the preform  1  is returned to the solution holding tank  14  for reuse. If the amount of the coating solution inside the solution holding tank  14  becomes smaller than a predetermined amount, the coating solution is refilled in the solution holding tank  14 . 
         [0044]    In general, the mouth part  1   a  and bottom part  1   c  of the preform  1  are sufficiently low in gas barrier property even without the coating. Further, the bottom part  1   c  is easily abraded compared with other parts, so if a coating is formed on the bottom part  1   c , the coating on the bottom part  1   c  is liable to crack. 
         [0045]    As the coating solution, for example, a polyvinyl alcohol (PVA) solution having a gas barrier property and a non-water soluble coating agent protecting this are used. Usually the outer circumferential surface of the preform  1  is first formed with the PVA coating, then is given the protective coating to cover the PVA coating. In this case, the problem can arise that since PVA is hydrophilic, if the protective coating of the bottom part  1   c  cracks, the exposed PVA coating will end up being redissolved in the water. 
         [0046]    Therefore, the coating solution is preferably transferred from the outer circumferential surface of the transfer roller  11  to only the cylindrical body part  1   b  of the preform  1 . Due to this, it is possible to shorten the drying time of the coating solution coated on the preform  1 . Further, cylindrical body part  1   b  has a simple cylindrical shape, so the coating of the coating solution by the transfer roller  11  becomes easier than other parts. 
         [0047]    As shown in  FIG. 4 , the preform coating device  10  further comprises a solution receiving tank  18  receiving the coating solution dropped from the transfer roller  11 , a scraper  19  scraping off the coating solution not transferred to the outer circumferential surface of the preform  1  from the outer circumferential surface of the transfer roller  11 , and a solution recovery tank  20  recovering the coating solution scraped off by the scraper  19 . 
         [0048]    The scraper  19  contacts the transfer roller  11  over the entire axial direction of the transfer roller  11 . Due to this, the majority of the coating solution not transferred to the outer circumferential surface of the preform  1  can be recovered. Further, the scraper  19  extends downward at a slant from the outer circumferential surface of the transfer roller  11  toward the solution recovery tank  20 . Due to this, the coating solution scraped off by the scraper  19  flows into the solution recovery tank  20  by gravity. At this time, the coating solution draws air along with it and sometimes is formed with bubbles inside the solution recovery tank  20 . 
         [0049]    As shown in  FIG. 4 , the preform coating device  10  further comprises a defoaming device  21  removing bubbles from the recovered coating solution. The defoaming device  21  is connected through a first pipe  22  to the solution recovery tank  20  and through a second pipe  23  to the solution holding tank  14 . The defoaming device  21  removes the bubbles  50  formed in the solution recovery tank  20 . The defoamed coating solution is returned to the solution holding tank  14  and is again supplied by the pump  15  to the die coater  13 . 
         [0050]    Below, the methods for manufacturing the preform  1  and plastic bottle  3  will be explained. 
         [0051]    First, the injection molding method or PCM method is used to form a preform  1  from a resin. Next, the coating solution is applied to the outer circumferential surface of the formed preform  1 . Note that, to improve the adhesion of the coating solution on the preform  1 , it is possible to treat the surface of the outer circumferential surface of the preform  1  before applying the coating solution. The surface treatment is, for example, plasma treatment, corona treatment, or electron beam treatment. 
         [0052]    The coating solution is applied by discharging the coating solution from a slot  131  of the die coater  13  to the outer circumferential surface of the columnar shaped transfer roller  11  rotating about its center axis  111  and by conveying the preform  1  on the outer circumferential surface of the transfer roller  11  and transferring the coating solution from the outer circumferential surface of the transfer roller  11  to the outer circumferential surface of the preform  1 . The coating solution not transferred to the outer circumferential surface of the preform  1  is recovered by the scraper  19 . After that, the recovered coating solution is cleared of bubbles by the defoaming device  21  and again discharged to the outer circumferential surface of the transfer roller  11  from the slot  131  of the die coater  13 . Note that, the coating solution may be supplied on the outer circumferential surface of the transfer roller  11  by another application device configured to apply the coating solution with a predetermined thickness to the outer circumferential surface of the transfer roller  11 , instead of the die coater  13 . 
         [0053]    As explained above, the coating solution is preferably transferred from the outer circumferential surface of the transfer roller  11  to only the cylindrical body part  1   b  of the preform  1 . Further, the circumferential direction length of the transfer roller  11  at the part contacting the preform  1  is the circumferential direction length of the preform  1  or more. Preforms  1  can be continuously conveyed by the conveyor device  12  on the outer circumferential surface of the transfer roller  11 . Due to this, it is possible to efficiently apply a coating solution over the entire circumference of the outer circumferential surfaces of a plurality of preforms  1 . 
         [0054]    After applying the coating solution, the outer circumferential surface of the preform  1  is made to dry on the conveyor device  12 . The drying is performed in two stages. In the first stage, the preform  1  is rapidly heated by a carbon heater. In the second stage, the preform  1  is dried by hot air by a far infrared ray heater to be held in temperature. As a result, a coating is formed on the outer circumferential surface of the preform  1 . 
         [0055]    As the coating solution, for example, a PVA solution having a gas barrier property and a non-water soluble coating agent protecting this are used. In this case, the outer circumferential surface of the preform  1  is first formed with the PVA coating, then, to prevent the PVA coating from being exposed, is given the protective coating to cover the PVA coating. The double layer coating can be formed by using a single preform coating device  10  while changing the coating solutions or by using two preform coating devices  10 . Note that, the non-water soluble coating agent protecting the PVA is, for example, a polyolefin dispersion solution, various modified polyolefin dispersion solutions, a polyvinyl butyral (PVB) solution, etc. 
         [0056]    Further, the non-water soluble coating agent protecting the PVA is typically low in viscosity and applied thinner than the PVA solution, so the drying time is shorter than the PVA solution. For this reason, excessive heating seldom causes the preform  1  to whiten. Therefore, the non-water soluble coating agent protecting the PVA may be applied by dipping the preform  1  in the vertical direction like in the past. In this case, the bottom part  1   c  of the preform  1  can also be formed with a protective coating, but the PVA coating is not formed on the bottom part  1   c , and therefore even if the protective coating of the bottom part  1   c  cracks, the problem of the hydrophilic PVA redissolving in water will not arise. 
         [0057]    Other examples of the coating solution are a water-soluble polyamide, water-soluble polyester, polyvinylidene chloride (PVDC), polyacrylonitrile, ethylene-vinyl alcohol copolymer resin (EVOH), polyglycolic acid, and other solutions of barrier resins and mixed solutions of these with inorganic matter, oxide absorbents, etc., an epoxy-based, urethane-based, or phenol-based thermosetting binder, a hot melt resin or other thermoplastic binder, or a coating material like an aqueous ink or solvent-based ink. 
         [0058]    As explained above, the problem of the film thickness of the coating solution becoming uneven at the outer circumferential surface of the preform  1  easily occurs in particular when using a high viscosity coating solution. For this reason, if applying the present embodiment when using the high viscosity coating solution, it is possible to obtain a more remarkable effect. For example, the viscosity of the coating solution used in the present embodiment is preferably 1 mPa·s or more, more preferably 100 mPa·s or more. 
         [0059]    The plastic bottle  3  is manufactured by stretch blow molding a preform  1  manufactured by the above method. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 . preform 
           1   a . mouth part 
           1   b . cylindrical body part 
           1   c . bottom part 
           2 . mold 
           3 . plastic bottle 
           10 . preform coating device 
           11 . transfer roller 
           111 . center axis 
           12 . conveyor device 
           13 . die coater 
           131 . slot 
           14 . solution holding tank 
           15 . pump 
           16 . first hose 
           17 . second hose 
           18 . solution receiving tank 
           19 . scraper 
           20 . solution recovery tank 
           21 . defoaming device 
           22 . first pipe 
           23 . second pipe 
           31 . solution dipping tank 
           32 . handle 
           33 . blade 
           40 . heater 
           50 . bubbles