Liquid application apparatus and application method

1. Field of the Invention

The present invention relates to a liquid application apparatus and application method.

2. Related Background Art

Liquid application apparatuses including a guide roll for guiding a sheet for application; and a slit die having a plurality of openings extending in the axial direction of the guide roll and applying two liquid layers (hereinafter referred to as an upper layer and a lower layer) to a surface of the sheet for application guided by the guide roll have been conventionally known (see, for example, Japanese Patent Laid-Open No. 2001-300394, Japanese Patent Publication No. 6-49171, and the like).

However, in the conventional liquid application apparatuses, the interface between the upper layer and the lower layer is not flat and undulates in many cases. When the interface undulates in this manner, the thickness of the lower layer is significantly nonuniform, which is not preferred, particularly when the lower layer is relatively thin (for example, 5 μm or less).

The present invention has been made in view of the above problem. It is an object of the present invention to provide a liquid application apparatus that can suppress undulation at the interface between the upper layer and the lower layer, and a liquid application method using the same.

An application apparatus according to the present invention includes a guide roll for guiding a sheet for application; and a slit die having an opening extending in the axial direction of the guide roll and applying a liquid to a surface of the sheet for application guided by the guide roll. The slit die has two flow paths spaced in the rotational direction of the guide roll and joining at the opening. In a cross section perpendicular to the axis of the guide roll, 1: an angle α formed by the center line of the upper layer flow path on the front side in the rotational direction of the guide roll, of the two flow paths, and the center line of the lower layer flow path on the back side in the rotational direction of the guide roll, of the two flow paths, is 0.5 to 25°, 2: an angle β formed by a line connecting the intersection point of the center line of the upper layer flow path and the center line of the lower layer flow path with the axis of the guide roll, and by the center line of the upper layer flow path is 0 to 70° as measured from this connecting line toward the center line of the upper layer flow path in the direction opposite to the rotational direction of the guide roll, and 3: the distance γ between a point where the two flow paths join and the opening is 0.05 to 2.8 mm.

According to the present invention, α, β, and γ satisfy the above-described requirements, and therefore, when liquids different from each other are fed to the two flow paths to form a laminate of two liquid layers on the surface of the sheet for application, undulation at the interface between the two layers can be suppressed.

Here, in the cross section perpendicular to the axis of the guide roll, the width of the upper layer flow path: the width of the lower layer flow path is preferably 3:2 to 3:1. Thus, when the lower layer is relatively thin, the undulation at the interface is easily suppressed.

Also, preferably, the slit die has an edge portion, opposed to the guide roll, on the front side in the rotational direction with respect to the opening, and the length δ of the edge portion along the rotational direction of the guide roll is 0.01 to 0.05 mm.

Thus, the undulation at the interface can be further suppressed.

Also, a liquid application method according to the present invention includes feeding a liquid having a solid concentration of 20 to 30 wt % to the lower layer flow path in the above-described liquid application apparatus.

In particular, when such a liquid having high solid concentration is used for the lower layer, the undulation at the interface easily becomes severe, and the lower layer often disappears in part. Even in such a case, a lower layer having high uniformity can be formed.

According to the present invention, it is possible to provide a liquid application apparatus that can suppress undulation at the interface between the upper layer and the lower layer, and a liquid application method using the same.

FIG. 1 is a schematic configuration view of an application apparatus according to an embodiment; and

FIG. 2 is an enlarged cross-sectional view of a portion where a guide roll and a slit die in FIG. 1 are opposed to each other.

The preferred embodiment of the application apparatus of the present invention will be described below in detail with reference to the drawings. In the following description, like numerals indicate like or corresponding parts, and redundant description is omitted.

The application apparatus of the present invention mainly has liquid storage tanks 1 a and 1 b , liquid feed pumps 3 a and 3 b , a slit die 20 , a feed reel 5 of a sheet for application, a guide roll 6 , a take-up reel 7 , and a dryer 8 , as shown in FIG. 1 , and simultaneously applies a laminate L having an upper layer Lu and a lower layer Ld to a sheet for application.

Liquids to be applied are respectively stored in the liquid storage tanks 1 a and 1 b . The liquids are not particularly limited, but, for example, a liquid for forming the active material containing layer of an electrochemical device, such as a lithium ion secondary battery, a liquid for forming a recording medium, and the like can be used. Examples of the liquid for forming an active material containing layer include a liquid including an active material, and a conductive aid, a binder, and a solvent as required. Examples of the active material include lithium containing metal oxide and a carbon powder. Examples of the conductive aid include carbon black and the like. Examples of the binder include PVDF and the like. Examples of the solvent include N-methylpyrrolidone (NMP), tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and the like. The viscosity of the liquids can be, for example, in the range of 1 P to 500 P. The solid concentration of the application liquid fed to the liquid storage tank 1 a for the upper layer, and the solid concentration of the liquid fed to the liquid storage tank 1 b for the lower layer are not particularly limited. Generally, when the solid concentration is high, the interface between the upper layer Lu and the lower layer Ld described later undulates easily. This embodiment can be sufficiently implemented even if the solid concentration of the application liquids for the upper layer and for the lower layer is 20 to 30 wt %.

The liquid storage tank 1 a and the inlet 21 a of the slit die 20 are connected by a line L 1 a , and the liquid storage tank 1 b and the inlet 21 b of the slit die 20 are connected by a line L 1 b . The liquid feed pumps 3 a and 3 b , which respectively feed a constant amount of the liquids in the liquid storage tanks 1 a and 1 b to the slit die 20 , are connected to the lines L 1 a and L 1 b . The liquid feed pumps 3 a and 3 b are not particularly limited, but precision gear pumps are preferred. Valves 2 a and 2 b are respectively connected to the lines L 1 a and L 1 b.

The guide roll 6 is a cylindrical rotatable roll. A sheet for application S fed from the feed reel 5 of a sheet for application and taken up around the take-up reel 7 is placed over the circumferential surface of this guide roll 6 , and the guide roll 6 is rotated in the direction of an arrow A shown and guides the sheet for application S. The diameter of the guide roll 6 is not particularly limited, but, for example, the outer diameter can be 10 to 250 mm. The rotational speed of the guide roll 6 is not particularly limited, but is preferably set so that the linear velocity on the circumferential surface of the guide roll 6 is equal to the line speed (the flow speed of the sheet for application S).

The sheet for application S is not particularly limited, but examples of the sheet for application S include PET, PEN, aramid, and the like. Also, the thickness and width are not particularly limited, but can be, for example, 5 to 300 μm and 50 to 2000 mm, respectively.

One opening 24 is formed in the slit die 20 along the axial direction of the guide roll 6 , as shown in FIG. 1 and FIG. 2 . This slit die 20 spreads the liquids flowed in from the inlets 21 a and 21 b of the slit die 20 in the width direction of the sheet for application S respectively in manifolds 22 a and 22 b , which are provided inside the slit die 20 as cavities and extend in the axial direction of the guide roll 6 , passes the liquids through a slit-like upper layer flow path (a flow path on the front side in the rotational direction of the guide roll) 23 a and a slit-like lower layer flow path (a flow path on the back side in the rotational direction of the guide roll) 23 b to provide a layer-like upper layer and lower layer respectively, further joins the upper layer and the lower layer before the opening 24 so that the laminate L in which these two layers are laid on each other is fixated, then discharges this laminate in a sheet shape from the one opening 24 , and applies the laminate L onto the sheet for application S moving on the guide roll 6 . The laminate L formed on the sheet for application S is dried by the dryer 8 in midstream while being moved by the take-up reel 7 . Examples of the dryer include a hot wire heater, a steam heater, an infrared heater, and the like.

Next, the vicinity of the opening 24 of the slit die 20 will be described in detail, referring to FIG. 2 in which the slit die 20 and the guide roll 6 are cut in a cross section perpendicular to the axis 6 a of the guide roll 6 .

The slit width 24 W of the opening 24 of the slit die 20 is not particularly limited, but can be, for example, 30 to 500 μm. The slit width 24 W is preferably equal to or less than the sum of the width 23 a W of the upper layer flow path 23 a and the width 23 b W of the lower layer flow path 23 b described later.

The upper layer flow path 23 a and the lower layer flow path 23 b are in communication with the opening 24 and join before the opening 24 in the liquid flow direction. In other words, a point 23 e where the upper layer flow path 23 a and the lower layer flow path 23 b join is formed before the opening 24 in the liquid flow direction. Here, the joining point 23 e is a point where two flows are first mixed and is the coupling portion of the side walls. The angle of the joining point 23 e is preferably equal to α.

Here, the slit die 20 in this embodiment meets the following three requirements in a cross section perpendicular to the axis 6 a of the guide roll 6 .

1: An angle α formed by the center line 23 aa of the upper layer flow path 23 a and the center line 23 bb of the lower layer flow path 23 b is 0.5 to 25°.

2: An angle β formed by a line 23 xx connecting the intersection point 23 x of the center line 23 aa of the upper layer flow path 23 a and the center line 23 bb of the lower layer flow path 23 b with the axis 6 a of the guide roll 6 and by the center line 23 aa of the upper layer flow path 23 a is 0 to 70° as measured from the connecting line 23 xx toward the center line 23 aa of the upper layer flow path 23 a in the direction opposite to the rotational direction A of the guide roll 6 .

3: The distance γ between the joining point 23 e of the upper layer flow path 23 a and the lower layer flow path 23 b and the opening 24 is 0.05 to 2.8 mm. Here, the distance between the joining point 23 e and the opening 24 is the length of a perpendicular line from the joining point 23 e to a line defining the slit width W of the opening 24 , that is, a line connecting one end and the other end of the opening 24 (or its extension line), in FIG. 2 .

The width 23 a W of the upper layer flow path 23 a and the width 23 b W of the lower layer flow path 23 b are preferably in the range of the width of the upper layer flow path 23 a : the width of the lower layer flow path 23 b =3:2 to 3:1. Here, the width of the flow path is width in the direction perpendicular to the flow of the flow path in a plane perpendicular to the axis of the guide roll 6 . Specifically, for example, the width of the upper layer flow path 23 a can be 150 to 300 μm, and the width of the lower layer flow path 23 b can be 50 to 100 μm.

An edge portion 20 E opposed to the guide roll 6 is formed on the downstream side with respect to the opening 24 of the slit die 20 (the front side in the rotational direction of the guide roll). The present invention can be implemented even if the length 6 of this edge portion 20 E in the rotational direction of the guide roll 6 (hereinafter referred to as the length δ of the edge portion 20 E) is, for example, 0.5 mm or more. But, the length δ of the edge portion 20 E is preferably 0.01 to 0.05 mm in terms of further reducing undulation at the interface between the upper layer and the lower layer. Here, the length δ of the edge portion 20 E is length along the circumferential surface of the guide roll 6 .

In such an application apparatus 100 , when the liquids from the liquid storage tanks 1 a and 1 b are fed to the slit die 20 by the liquid feed pumps 3 a and 3 b , the liquids are discharged, as the laminate L in which the upper layer Lu is laminated on the lower layer Ld, from the opening 24 via the manifolds 22 a and 22 b , and the upper layer flow path 23 a and the lower layer flow path 23 b , and applied in a sheet shape on the sheet for application S guided by the guide roll 6 .

According to this embodiment, the above-described angles α and β and distance γ are suitable, and therefore, the undulation at the interface between the upper layer Lu and the lower layer Ld applied on the surface of the sheet for application S can be suppressed. The reason why such effect is obtained is not clear, but one reason is considered to be, for example, that the turbulent flow caused when the two liquids join is suppressed.

Therefore, the thickness of the upper layer Lu and the lower layer Ld, and the sum of these, i.e. the thickness of the laminate L, can be easily made uniform. On the other hand, when the angles α and β and the distance γ do not satisfy the above-described conditions, the undulation at the interface between the upper layer Lu and the lower layer Ld cannot be suppressed, and portions where the lower layer Ld is not formed are produced in some cases. In particular, when the average thickness after drying set for the lower layer Ld is about 1 to 10 μm, it is significantly difficult to uniformly apply the lower layer by a conventional application apparatus, but in this embodiment, a sufficiently uniform lower layer can be formed. Also, this embodiment can be implemented even if the viscosity of the upper layer and the lower layer is largely different, for example, the difference is about 8000 to 10000 cp.

In particular, in forming the electrode of an electrochemical device, it is necessary to form a layer of about 5 μm or less as a lower layer when a layer having higher resistance and thinner thickness than the current collector is formed between the main active material layer and the current collector to interrupt current. This embodiment is particularly suitable, for example, in such a case.

The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, two liquid layers are laminated and discharged from the one opening 24 , but three or more layers may be laminated.

(Application Liquid for Upper Layer)

89 parts by mass of a ternary positive electrode material (LiNiMnCoO 2 -lithium nickel manganese cobalt composite oxide) and 3 parts by mass of graphite (trade name: KS-6, manufactured by Lonza) as active materials, 3 parts by mass of carbon black (trade name: DAB, manufactured by DENKI KAGAKU KOGYO KABUSHIKI KAISHA) as a conductive aid, and 5 parts by mass of polyvinylidene fluoride (trade name: KYNAR 761, manufactured by ATFINA) as a binder were mixed and dispersed, and then, a suitable amount of N-methyl-pyrrolidone (NMP) as a solvent was introduced to adjust viscosity so that the solids were 25 wt %, to prepare a slurry application liquid for the upper layer.

(Application Liquid for Lower Layer)

90 parts by mass of iron lithium phosphate (LiFePO 4 ) and 10 parts by mass of polyvinylidene fluoride (trade name: KYNAR 761, manufactured by ATFINA) as a binder were mixed and dispersed, and then, N-methyl-pyrrolidone (NMP) as a solvent was introduced so as to obtain a solid concentration of 15, 20, 25, and 30 wt % for viscosity adjustment to prepare four types of slurry application liquids for the lower layer.

Examples 1 to 22 and Comparative Examples 1 to 4

Using a guide roll having a radius of 60.2 mm, and using a slit die, as shown in FIG. 2 , having a slit width 24 W of 300 μm, the application liquid for the upper layer (dry thickness: 80 μm) and the application liquid for the lower layer (dry thickness: 5 μm) were simultaneously applied to a surface of an A1 sheet having a thickness of 21 μm and a width of 150 mm and dried to form a multilayer electrode. Here, α, β, γ, δ, and the combination of the width 23 a W of the upper layer flow path and the width 23 b W of the lower layer flow path in the slit die, and the solid concentration of the application liquid for the lower layer were as shown in Table 1. Also, the sheet feed speed was 15 m/min. In Example 3, the angle α was 0.5° by dividing two flow paths by a PET film. In Comparative Example 1, β was −1°, and this means that the center line 23 aa of the upper layer flow path 23 a was inclined on the front side in the rotational direction of the guide roll 6 with respect to the connecting line 23 xx . In Comparative Example 4, two flow paths were not joined in one opening, and the layers were discharged from two independent openings.

(Evaluation)

SEM micrographs in cross sections parallel to the lamination direction of the multilayer electrode and parallel to the sheet feed direction were taken, and the interface between the upper layer and the lower layer, and the surface of the A1 sheet were extracted by image processing. The distribution of the position of the interface was obtained, and the standard deviation of the position of the interface was obtained. The results are shown in Table 1.

In the Examples, the undulation at the interface was sufficiently suppressed. On the other hand, in the Comparative Examples, dripping in which the liquid for the lower layer was squeezed out on the back side in the rotational direction of the roll occurred, and the lower layer was extremely nonuniform.