BACKGROUND OF THE INVENTION
[0001] The present invention relates to a coating method and a coating apparatus, and in
particular, to a coating method and a coating apparatus employing a coater head which
conducts high speed thin layer coating of a coating solution extruded continuously
toward the surface of a substrate to be coated which moves relatively to the coating
head on the surface of the substrate to be coated so that the thin layer may be of
a uniform thickness.
[0002] A method for coating a prescribed coating solution on the surface of a flexible substrate
to be coated has so far been studied in many ways and has been put to practical use.
Among these various methods, a coating apparatus of an extrusion type which conducts
high speed thin layer coating of a coating solution extruded continuously toward the
surface of a moving substrate to be coated on the surface of the substrate to be coated,
for example, is superior to other roll type coating methods such as, for example,
reverse roll, kiss roll and gravure roll methods in terms of uniformity of coating,
layer thinness and a range of coating speed. Further, the coating method of an extrusion
type can conduct simultaneous multi-layer coating on the so-called wet-on-wet basis,
and it is extremely effective in terms of cost and performance for the application
to manufacture recent coated products with high value added. For attaining the multi-layer
system, on the other hand, there has been used a method to form the aforesaid coated
layers of a multi-layer type by coating a coating solution and drying it for each
layer on the substrate to be coated, as disclosed in TOKKAISHO Nos. 51-119204, 52-51908
and 53-16604. In this method wherein steps of coating and drying are repeated, however,
there have been problems that productivity is poor and it is difficult to make the
uppermost layer to be thin. So, TOKKAISHO Nos. 48-98803 and 61-111168 disclose manufacturing
methods of a type of wet-on-wet simultaneous multi-layer coating for magnetic recording
media. However, both methods represent a method wherein coating solutions superposed
in layers in advance are coated on the aforesaid substrate to be coated which is supported
on a back roll and is running continuously, which has caused problems that uneven
and nonuniform coating tends to be caused in the longitudinal coating direction because
of insufficient rotational accuracy of the back roll, a distance between a substrate
to be coated and the tip of a coater which can not be narrowed makes high speed thin
layer coating to be difficult, and manufacture of optimum magnetic recording media
is difficult.
[0003] From the background stated above, there have been proposed a method disclosed in
TOKKAISHO No. 62-124631 to coat the upper layer with a single layer extrusion coater
on a substrate to be coated that is not supported by a back roll while a lower layer
is wet, and coater heads disclosed in TOKKAISHO No. 63-88080 and TOKKAIHEI No. 2-251265
each having a slit from which two coating solutions are extruded. However, the problem
of this extrusion type coating method is that it is impossible to thoroughly eliminate
defects called the so-called pencil lines which are caused in the coating direction
by foreign materials or dried coating solutions caught at the tip of an edge of a
coater.
[0004] When conducting coating between two support rolls without conducting coating on a
substrate to be coated that is held firmly on a back roll, bending of the substrate
to be coated in its lateral direction and that in its longitudinal direction become
pressure distribution in the direction of the coater, and become layer thickness distribution
in the lateral direction and that in the longitudinal direction, resulting in the
occurrence of serious deterioration of coating quality. On the contrary, TOKKAIHEI
Nos. 1-203075 and 6-254466 disclose a method to counteract a twist of the substrate
in its lateral direction by changing a diameter of a support roll located just in
front of or just behind an extrusion coater, and TOKKAIHEI No. 1-224071 discloses
a method to make the twisted portion to be uniform by providing a means to apply fluid
pressure from the reverse side of the twisted substrate to be coated. However, it
is difficult for the method of this type to cope with a twist of several tens centimeters
or less, though it can cope with a twist with a larger cycle, and it is substantially
possible for the method to cope only with a twist with a cycle of a few meters. Though
there has been proposed a method to uniformalize the amount of flowing coating solution
in the lateral direction by changing shapes of a pocket portion and of a slit portion
of an extrusion coater in the lateral direction as in TOKKAIHEI No. 6-508571, in addition
to the foregoing, no effect is expected for the twist which fluctuates cyclically
in the longitudinal direction as stated above.
[0005] As a countermeasure for the drawbacks mentioned above, there is disclosed in TOKKAISHO
No. 6320070 an extrusion type coater for coating by letting a coating solution layer
fall from a coater provided with its outlet facing downward while holding a substrate
to be coated on a back roll, wherein pressure loss is lowered by using a coating apparatus
whose slit is gradually narrowed toward the coating solution outlet, and thereby,
coating of a solution having relatively high viscosity is made to be possible even
through a slit which is relatively narrow. However, under the structure wherein the
tip portion is gradually narrowed, pressure is too high when conducting high speed
coating and when coating a coating solution with high viscosity, and it is impossible
to thoroughly eliminate the problems that layer thickness distribution is not uniform
because accuracy of a sharp edge portion at the tip is not obtained, and pencil lines
are caused because a distance between the tip portion of a coater and a substrate
to be coated is not actually broadened very much as in other extrusion type coaters.
SUMMARY OF THE INVENTION
[0006] Objects of the invention are to solve the problems of the prior art stated above
and to provide a coating method and a coating apparatus wherein thin layer coating
that is free from an influence of a twist of a substrate to be coated and from the
occurrence of pencil lines caused by surrounding dust or by condensed products in
a coating solution is made possible without being restricted by the coating speed,
while making the most of advantageous points of an extrusion type coating system.
[0007] The objects stated above can be attained by the following coating apparatus.
[0008] A coating apparatus having the following structure:
a conveyance means for conveying a substrate to be coated;
a coater which is equipped with a slit having two faces which face each other, and
extrudes a coating solution, the coating solution being injected through an inlet
on the slit and passing through the clearance between the two faces facing each other
and being extruded from an outlet on the slit,
clearance d of the outlet on the slit satisfying the following expression of 0 < d
≤ 50 x 10-5 (m), and
at least a part of the portion in the vicinity of the outlet of the two faces facing
each other is formed with a water-repelling member.
[0009] Further, the objects mentioned above are attained by a preferable embodiment shown
in either one of the following structures (1) - (18).
(1) A coating apparatus wherein a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating.
(2) A coating apparatus in which a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating, wherein a prescribed portion of the surface which comes in contact
with a coating solution on the coater is constituted with a member that repels the
coating solution.
(3) The coating apparatus according to Structure (2), wherein a prescribed portion
of the surface which comes in contact with a coating solution on the coater is constituted
with a member whose contact angle with the coating solution is 45° or more.
(4) A coating apparatus in which a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating, wherein a prescribed portion of the surface which comes in contact
with a coating solution on the coater is constituted with a plane whose surface roughness
Rmax satisfies the relation of Rmax ≤ 0.8 (µm).
(5) The coating apparatus according to either one of Structures (1), (2) and (4),
wherein a gap of the slit at the portion of an inlet for a coating solution is broader
and that at the portion of an outlet for a coating solution is narrower, and gap d
of the outlet on the slit satisfies the relation of d ≤ x 10-5 (m).
(6) The coating apparatus according to Structures (5), wherein the slit has on its
outlet portion a section where two confronting surfaces are in parallel each other.
(7) The coating apparatus according to Structures (5), wherein the narrowest portion
of the slit is constituted with a member whose Vickers hardness is 280 or more.
(8) The coating apparatus according to Structure (2) or (4), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is constituted with at least one kind of metal, ceramic, resin and glass.
(9) The coating apparatus according to Structure (2), wherein a prescribed portion
of the surface which comes in contact with a coating solution on the coater is given
a repelling property by means of covering.
(10) The coating apparatus according to Structure (4), wherein a prescribed portion
of the surface which comes in contact with a coating solution on the coater is covered
with a member whose surface roughness RMax satisfies the relation of Rmax ≤ 0.8 (µm).
(11) The coating apparatus according to Structure (9) or (11), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is covered with at least one kind of fluorocarbon resin, silicon type resin and ceramic.
(12) The coating apparatus according to Structure (2) or (4), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is mirror-finished by means of grinding or the like.
(13) The coating apparatus according to Structure (2) or (4), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is a slit surface including at least a slit outlet portion on at least one bar constituting
the coater.
(14) The coating apparatus according to Structure (2) or (4), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is a portion including at least an edge of at least one bar constituting the coater
and a portion facing outside from the edge portion.
(15) The coating apparatus according to Structure (2) or (4), wherein a prescribed
portion of the surface which comes in contact with a coating solution on the coater
is a slit surface including at least a slit outlet portion, an edge and a portion
facing outside from the edge on at least one bar constituting the coater.
(16) A coating method wherein a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating.
(17) A coating method in which a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating, wherein coating is conducted by the use of a coating apparatus
in which a prescribed portion of the surface which comes in contact with a coating
solution on the coater is constituted with a member that repels the coating solution.
(18) A coating method in which a coating solution is extruded in a film shape from
a slit formed with at least two or more bars constituting a coater, and the coating
solution extruded in the shape of a film is collided with a substrate to be coated
which is set or conveyed through the prescribed clearance from the coater on a non-contact
basis, for coating, wherein coating is conducted by the use of a coating apparatus
in which a prescribed portion of the surface which comes in contact with a coating
solution on the coater is constituted with a plane whose surface roughness Rmax satisfies
the relation of Rmax ≤ 0.8 (µm).
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
Fig. 1 is a side sectional view of a coater used in the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] An embodiment of the invention will be explained with reference to the side sectional
view of the coater in Fig. 1.
[0012] A base is conveyed upward from the lower portion in Fig. 1 by paired rollers representing
a conveyance means for conveying a base representing a substrate to be coated. A coating
solution is injected into a slit through an inlet of the slit provided on a coater,
then, it passes through the inside of the slit including two confronting surfaces
(a) and (b), and is extruded through a slit outlet. Then, the coating solution extruded
through the slit outlet forms a film of extruded coating solution between a portion
where a coating solution comes in contact with a base and the coater, whereby, the
film of extruded coating solution thus formed is coated on the base.
[0013] The inventors of the invention found that no dust is caught between the tip of a
coater and a substrate to be coated, resulting in no occurrence of streak defects,
and a substrate to be coated, when it is flexible, does not affect the coating, and
thereby thin-layer coating is possible, when extruding a coating solution in a shape
of an extremely thin layer by narrowing slit outlet clearance d of the coater to 50
µm or less, preferably 18 µm or less which is much narrower than that of the conventional
extrusion type coater, and thereby coating by colliding the extruded coating solution
film with a substrate to be coated which is conveyed or held to be away from the coater
by the distance that is greater than that for the conventional extrusion type coater.
It has also been cleared in this coating method that coating can be carried out independently
of the coating speed once an extruded solution layer is formed, and the higher the
coating speed is, the thinner the solution layer is, resulting in a thinner layer
coating.
[0014] As a result of intensive studies for conditions which make it possible to coat to
be thinner than that of the conventional coater, excepting the higher speed, the relation
of

holds in the coating method of the invention when

) represents an amount of extruded coating solution per a unit width to a substrate
to be coated, Hw (m) represents a wet coating thickness of a coated layer, and U (m/min)
represents conveyance speed for a substrate to be coated, and it is preferable to
lower amount of extruded coating solution Q for thin layer coating. Namely, it is
preferable either to lower the speed for extruding a coating solution or to lower
the thickness of an extruded layer. It has also been found that it is preferable to
make a coater to repel a coating solution and to make the coating solution to tend
to leave the coater, by constituting a portion where the coater comes in contact with
a coating solution with a member whose contact angle for a coating solution is large,
or by mirror-finishing the surface of the portion. There are given a method to water-repellent-finish
at least a portion of two confronting surfaces (a) and (b) of the slit, preferably
to water-repellent-finish a portion that is closest to the outlet among (a) and (b)
surfaces, and a method to form a portion of two confronting surfaces (a) and (b) of
the slit with a water repellent member, preferably to form a portion that is closest
to the outlet among (a) and (b) surfaces with a water repellent member. In this case,
it is preferable that a contact angle of at least a portion of the two confronting
surfaces (a) and (b) is 45° or more. It is also possible to attain by making surface
roughness Rmax to be 0.8 µm or more. Even in this case, the surface roughness on the
portion which is the same as the place for water repellent finishing can take the
aforesaid value.
[0015] When a contact angle between a coater and a coating solution is 45° or more, water
repellency is satisfactory and a thin extruded layer can be formed. However, when
activators are added in a coating solution for the low surface tension or when a coating
solution is of a solvent type, the surface tension of the coating solution itself
is low and repellency is poor. Therefore, it is necessary to make the coater surface
tension to be lower.
[0016] A portion where a coating solution comes in contact with a coater is enough as a
place on the coater where a coating solution is repelled, and in particular, a fall
of the speed of a running coating solution caused by the contact with a slit surface
can be lessened at the portion where the slit clearance is smallest in the vicinity
of an outlet of a slit which is a path for a coating solution to run out, which is
effective because a thin extruded layer can easily be formed. Further, giving water
repellency to the place near an edge section where a solution leaves a coater and
starts jetting is also effective for forming of a thin extruded layer.
[0017] Means to cause a coater to repel a coating solution include a method to constitute
a coater with highly repellent ceramic, Teflon resin, silicone resin and acrylate
resin, a method of coating of other silicone type resins such as fluorine type one
and silane, and a method of mirror finishing by means of grinding such as buffing
and lapping. However, the invention is not limited to the aforesaid methods, and can
employ any method provided that the method can offer the same effect.
[0018] When a coater is made of a metal, in particular, it is almost possible to make a
contact angle to be 45° or more by making surface roughness Rmax to be 0.8 µm or less,
preferably to be 0.1 µm or less, in a surface grinding method.
[0019] An effect of the invention can be obtained also on a coater for the so-called multi-layer
coating which is structured with three bars or more and has two or more slits. In
that case, the invention can be applied naturally to a front edge located at the uppermost
stream side and a back edge located at the extremely downstream side on the coater
for a substrate to be coated, and can be applied even to one or more center edges
located between the front edge and the back edge.
[0020] In the invention wherein the tip of a coater is completely away from a substrate
to be coated, pressure distribution caused by bending and twist does not occur even
when the substrate to be coated is flexible, and coating thickness which is extremely
uniform can be obtained. In the invention, excellent coating can be conducted without
using a back roll if the twist of a substrate to be coated is corrected to a certain
extent by relatively high tension, though it is preferable to spread a flexible substrate
to be coated in the vicinity of the coating portion by using a back roll.
[0021] In the case of a high viscosity solution, slit resistance in a narrow outlet clearance
of a slit like that in the invention is extremely high, namely, pressure loss is great,
which requests fluid-forwarding pressure that is extremely great, and a liquid-forwarding
means such as a gear pump used commonly has caused a fall of the amount of flowing
solution, pulsation and pump troubles. As a measure to solve this problem, it is effective
to attain the slit form wherein the slit clearance is gradually reduced in the direction
from a slit inlet to a slit outlet by broadening the clearance of a slit on the part
of an inlet for a coating solution instead of narrowing the clearance of a slit on
the part of an outlet. For the thin layer coating, slit clearance on the part of an
outlet of 50 µm or less is required, and that of 18 µm or less is preferable, and
in that case, it is preferable to make the slit clearance on the part of an inlet
to be a broad clearance of 100 µm or more, taking a fall of pressure into consideration.
[0022] As a method for this reduction stated above, the gradual reduction by tapering has
been employed for forming. In this method, however, the tip portion of a coater becomes
a sharp edge at the slit outlet, and in the case of a coater for single-layer coating,
for example, it has been difficult to process accurately the height between the tip
on the part of a front bar and the tip on the part of a back bar and parallelism,
which has worsened coating thickness distribution in the direction of a coating width.
However, when a portion where a slit surface of a front bar and a slit surface of
a back bar are in parallel with each other without being tapered is provided at the
position of an outlet of a slit, processing is easy and accuracy of straightness is
improved, an extruded layer which is more uniform can be formed, and distribution
of coating thickness in the coating width direction can be made excellent.
[0023] It has been cleared that when abrasives and metal powder are contained in a coating
solution under the condition that a slit clearance on the part of an outlet is made
to be narrower than that in a conventional extrusion type coater, the abrasives and
metal powder grind and roughen an inner wall of the slit while they are flowing at
high speed through the inside of the slit, causing deteriorated distribution of coating
thickness and streak defects, to shorten life of the coater. However, it has been
found, in terms of slit materials, that the problem stated above can be prevented
by using a member having high hardness on the portion where the slit clearance in
the vicinity of the slit outlet is narrowest. The hardness which represents Vickers
hardness of 280 or more in practical use is sufficient, though it depends on a coating
solution to be used. Although it is preferable that all surfaces of the slit are constituted
with members having this hardness, it is also acceptable in practical use that only
parallel portions in the slit outlet where the slit clearance is narrow are constituted
with a member having Vickers hardness of 280 or more. Further, even when only the
parts of the aforesaid parallel portions which are closest to the slit outlet are
constituted with a member having Vickers hardness of 280° or more, deterioration of
coating thickness distribution and streak defects cause by grinding and roughening
can be prevented to a certain extent. This method can attain cost reduction when processing
for higher hardness is expensive. It is preferable for its purposes that both slit
surfaces of two bars structuring one slit are constituted with a member having Vickers
hardness of 280° or more.
[0024] A coating apparatus or a coating method of the invention is naturally effective for
direct coating on a substrate to be coated such as a flexible support such as PET,
TAC, PEN, paper and aluminum plate and non-flexible support such as a glass plate,
and is also capable of being applied even when coating a coating solution indirectly
on a coating roll or a belt and transferring them onto a support. It is also possible
to coat on a substrate which relatively lacks flatness.
EXAMPLE
Example 1
[0025]
Substrate to be coated: PET support web having thickness 100 µm and width 1000 mm
was used.
Coater: One shown in a side sectional view in Fig. 1 was used.
[0026] A slit was reduced at a step at d = 40 µm and slit surfaces in the vicinity of outlet
were parallel.
[0027] Distance between the coater and a substrate to be coated: 0.3 mm
Coating solution: Following magnetic coating (surface tension approx. 30 dyn/cm, viscosity
approx. 50 cp · density approx. 1000 kg/m3)
Coating length: 10,000 m
Composition of magnetic coating:
[0028]
Co-γ-Fe2O3 (Hc:900 oersted, BET value: 45 m2/g) |
10 parts |
Diacetyl cellulose |
100 parts |
α-alumina (average particle size: 0.2 µm) |
5 parts |
Stearic acid |
3 parts |
Carnauba wax |
10 parts |
Cyclohexanone |
100 parts |
Acetone |
200 parts |
[0029] Under the conditions described above, materials of a member of a coater contacting
a solution were changed variously, and coating thickness and distribution of coating
thickness for various coating speeds were measured to confirm the limit of thin layer
coating.
[0030] Table 1 shows the results of the example stated above.

[0031] As a result, thin layer coating was possible when water repellency was given to the
coater, and coating thickness distribution proved to be excellent.
[0032] In the case that the coating speed was 100 m/min., on comparison with the conventional
type coater, by the coater whose edge surface is made of Teflon resin according to
the present invention, for example, the lower limit thickness of coating layer was
made thinner to the thickness of 7 µm. Further, the coating layer thickness distribution
was improved to 1%. This improvement in the coating layer thickness distribution is
the appreciable effect of the present invention.
[0033] Incidentally, under all conditions, neither streak defect nor coating mottle was
caused, resulting in excellent results.
[0034] When coating conditions were established based on expressions of the invention, coating
was not adversely affected by partial slackness or twist of a substrate to be coated,
and it was possible to coat thin layer at high speed stably in a coating method which
is free from streak defects. Further, by employing a coater slit shape of the invention,
it was possible to obtain uniform coating thickness easily. Further, since the distance
between the coater and the substrate to be coated is relatively large, roundness of
a back roll, fluttering and twist of a substrate to be coated, straightness and bending
of the coater tip hardly affect the coating thickness adversely. Whereby accuracy
of them has nothing to do with coating, and reduction of apparatus cost, easy management
and easy operation and work have been realized.
1. A coating apparatus, comprising:
conveyance means for conveying a substrate to be coated;
a coater including two surfaces opposite to each other so that a slit is formed therebetween,
wherein a coating solution is introduced from an inlet of the slit, passes through
the slit between the two surfaces and is discharged from the outlet of the slit,
the gap distance d of the slit at the outlet being made so as to satisfy the following
formula:

and
at least a part of the outlet section of the two surfaces being formed by a member
having a water-repelling property.
2. The coating apparatus of claim 1, wherein the contact angle of the part of the outlet
section of the two surfaces is 45° or more.
3. The coating apparatus of claim 1, wherein the roughness (Rmax) of the part of the
outlet section of the two surfaces is 0.8 µm or more.
4. The coating apparatus of claim 1, wherein the gap distance of the slit at the outlet
is made narrower than that at the inlet.
5. The coating apparatus of claim 4, wherein the slit has on its outlet section a section
where the two surfaces are parallel to each other.
6. The coating apparatus of claim 4, wherein the narrowest portion of the slit is constructed
with a member whose Vickers hardness is 280 or more.
7. The coating apparatus of claim 1, wherein a predetermined portion of the surface of
the coater which comes in contact with the coating solution is constructed with at
least one kind of metal, ceramic, resin and glass.
8. The coating apparatus of claim 1, wherein a predetermined portion of the surface of
the coater which comes in contact with a coating solution is given a repelling property
by a covered layer.
9. The coating apparatus of claim 3, wherein a predetermined portion of the surface of
the coater which comes in contact with a coating solution is covered with a member
whose surface roughness Rmax satisfies the relation of Rmax ≤ 0.8 (µm).
10. The coating apparatus of claim 8, wherein the predetermined portion of the surface
of the coater is covered with at least one kind of fluorocarbon resin, silicon type
resin and ceramic.
11. A coating apparatus, comprising
a coater including two bars forming a slit therebetween;
a conveyor for conveying a substrate to be coated in the vicinity of the coater such
that a predetermined gap distance is kept between the substrate and the outlet of
the sit; and
jetting means for jetting a coating solution in the form of a layer from the outlet
of the slit through the gap onto the substrate.