Coating method for applying a coating composition onto a running flexible support

A coating method for applying a coating composition mainly containing an organic solvent onto a running flexible support 11 by ejecting the coating composition in an oversupply state from a slit 9 under pressure. A back edge top end surface 14 on the upstream side in the direction of running of the support with respect to the slit is set so as to be substantially parallel to the surface of the support. A liquid-detached position in which the coating composition ejected from the slit to overflow the slit is detached from a surface of the support 11 is set so as to be placed on the back edge surface opposite to the support.

BACKGROUND OF THE INVENTION 
The present invention relates to a coating method for coating a flexible 
support with a coating composition mainly containing an organic solvent, 
and particularly to a coating method in which a running flexible support 
laid between pass rolls is coated with a coating composition while a 
coating head having a back edge and a doctor edge is pressed to the 
flexible support. 
Heretofore, various apparatuses have been proposed as coating apparatuses 
of the type of coating a running support with a coating composition. As 
one of this type of coating apparatuses, there is a coating apparatus in 
which a coating composition mainly containing an organic solvent is 
applied. The coating apparatus for application of a coating composition 
mainly containing an organic solvent is used, for example, for cleaning a 
support or forming an undercoating layer thereof before application of a 
magnetic dispersion or for forming a back layer in the case of a magnetic 
tape or the like. 
As a method for forming an undercoating layer as described above or the 
like, there is a method as disclosed in Japanese Patent Unexamined 
Publication No. Sho-57-156066, in which after a running support is coated 
with a coating composition of a coating composition tank through a 
rotating coating drum, the thickness of a coating film is adjusted by a 
coating film thickness adjusting member to obtain a desired coating layer. 
Here, a metering means (quantification means) such as wire knife, blade, 
wire bar, etc. is used as the coating film thickness adjusting means. 
Further, a rod member which rotates in a direction reverse to the 
direction of running of the support can be used in the apparatus. 
On the other hand, Japanese Patent Unexamined Publication No. Sho-62-60750 
has disclosed an apparatus in which a slit is provided to be capable of 
supplying an organic solvent to a running support and in which a surface 
of the running support is cleaned by scraping off an organic solvent type 
coating composition with use of a rod member rotatably provided at a top 
portion of the slit while rotating the rod member suitably and ejecting 
the coating composition from the slit. 
As described above, various apparatuses using a process of application of 
an organic solvent type coating composition, for example, as a process 
before a process of application of a magnetic dispersion have been 
proposed and employed conventionally. 
Of the coating apparatuses for application of an organic solvent type 
coating composition, the apparatus for application of a coating 
composition from a slit under suitable pressure (liquid pressure) as 
disclosed in Japanese Patent Unexamined Publication No. Sho-62-60750 has 
an advantage in that the apparatus is adapted to high coating speed. That 
is, air which is associated with the support as the liquid pressure of the 
coating composition ejected from the slit is increased can be removed 
smoothly. That is, the apparatus contributes to stabilization of 
high-speed coating. 
In a coating head used in the conventional coating apparatus, there is 
however a large problem when the liquid pressure of the coating 
composition is increased. This problem is in that a mist of the solvent of 
the coating composition is produced in the upstream side in the direction 
of running of the support (in the upstream side of the slit) because the 
internal liquid pressure of the coating composition ejected from the slit 
of the coating head is instantaneously released to atmospheric pressure 
when the coating composition is ejected from the slit. When a large 
quantity of the mist is produced, there arises a problem that a solvent 
atmosphere is increased so that work environment is made undesirable. 
In the method of performing application of a magnetic layer after 
application of a pre-coating composition mainly containing an organic 
solvent in a process of producing a magnetic recording medium as described 
in Unexamined Japanese Patent Publication (Kokai) No. Sho. 63-20069, there 
arises a problem that the mist of the pre- coating composition is 
deposited onto the magnetic layer again to cause coating film failure. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a coating method in which 
when a running support is coated with an organic solvent type coating 
composition under predetermined liquid pressure, such a solvent mist is 
prevented from being produced in the upstream side in the direction of 
running of the support, of a coating head. 
The foregoing object of the present invention can be achieved by a coating 
method in which while a coating head having a slit positioned between a 
back edge and a doctor edge is pressed to a running flexible support laid 
between pass rolls, a coating composition mainly containing an organic 
solvent is ejected from the slit under pressure and in an oversupply 
condition to thereby coat the flexible support with the coating 
composition, characterized in that: a surface opposite to the support, of 
the back edge on the upstream side in the direction of running of the 
flexible support with respect to the slit is set so as to be substantially 
parallel to the surface of the support; the coating composition ejected 
from the slit is made to overflow toward the back edge surface opposite to 
the support; and a liquid-detached position in which the coating 
composition is detached from a surface of the support is set so as to be 
placed on the back edge surface opposite to the support.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described below by using a 
coating apparatus for carrying out the coating method of the present 
invention with reference to the accompanying drawings. 
FIG. 1 shows an embodiment of a coating apparatus for forming an 
undercoating layer while performing cleaning and metering in a process of 
producing a magnetic recording medium. 
As shown in FIG. 1, the coating apparatus 7 in this embodiment is disposed 
in the downstream side of a support 11 which is running in a predetermined 
direction (the direction of the arrow A). A coating composition F receives 
pressure from a pressure device not shown so that the coating composition 
F is supplied to a pocket 8 through a liquid feeding system P and ejected 
from the pocket 8 toward the support 11 through a slit 9. A rotary rod 6 
is provided in the downstream side of a discharge outlet of the slit 9 so 
that the rotary rod 6 is driven by a driver not shown to be rotated in a 
direction reverse to the direction of running of the support 11. 
The feature of this embodiment is in a structure in which a back edge 13 on 
the upstream side in the direction of running of the support 11 with 
respect to the slit 9 has a surface 14 opposite to the support and 
substantially parallel to the surface of the support 11 (i.e., the angle 
of incidence .theta..sub.w of the support 11 and the angle of inclination 
.theta..sub.d of the back edge surface 14 are substantially the same). A 
liquid-detached position T in which the coating composition F ejected from 
the slit 9 to overflow the slit 9 is detached from the support 11 is set 
so as to be placed on the surface 14 opposite to the support. 
The length L.sub.d of the surface 14 opposite to the support is preferably 
selected to be not smaller than 2 mm when, for example, the liquid 
pressure at the outlet of the slit 9 is not lower than 0.02 Kg/cm.sup.2 
(if the discharge liquid pressure is lower than 0.02 Kg/cm.sup.2, the 
solvent mist is little produced). The difference t1 between the surface 14 
opposite to the support and an upper end portion of the rotary rod 6 can 
be selected to be in a range of from 0.02 to 00.5 mm. 
As described above, a flow of the coating composition F from the slit 9 to 
the upstream side (back edge side) forms a liquid reservoir in the 
distance t2 (substantially equal to the distance t1) formed between the 
running support 11 and the surface 14 opposite to the support and then 
forms a drop flow F.sub.0. Therefore, even in the state where the liquid 
pressure in the slit 9 is high, the drop flow F.sub.0 is formed after the 
liquid pressure is once lowered in the liquid reservoir. As a result, the 
rapid change of the pressure of the coating composition F from high liquid 
pressure to atmospheric pressure is avoided. Accordingly, the rapid 
volatilization of the organic solvent in the coating composition is 
suppressed so that the solvent mist as produced conventionally can be 
prevented from being produced. 
The drop flow F.sub.0 may be recovered suitably by a recovering system not 
shown or may be filtrated to be recycled if necessary. 
The coating composition F applied by the coating apparatus 7 mainly 
contains an organic solvent. For example, the coating composition F can 
contain as the organic solvent an arbitrary percentage of a material 
selected from ketones such as acetone, methyl ethyl ketone, methyl 
isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, 
tetrahydrofuran, etc.; alcohols such as methanol, ethanol, propanol, 
butanol, isobutyl alcohol, isopropyl alcohol, methyl cyclohexanol, etc.; 
esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl 
acetate, ethyl lactate, glycol acetate, etc.; glycol ethers such as glycol 
methyl ether, glycol monoethyl ether, dioxane, etc.; aromatic hydrocarbons 
such as benzene, toluene, xylene, cresol, chlorbenzene, etc.; chlorinated 
hydrocarbons such as methylene chloride, ethylene chloride, carbon 
tetrachloride, chloroform, ethylene chlorhydrin, dichlorbenzene, etc.; and 
others such as N- N-dimethyl formamide, hexane, etc. It is not always 
necessary that these organic solvents are provided in the 100% purity. 
That is, these organic solvents may contain impurities such as unreacted 
product, side-reaction product, decomposition product, oxide, water, etc. 
as other components than main components. The amount of these impurities 
is selected to be preferably not larger than 30%, more preferably not 
larger than 10%. 
The thickness (the quantity of coating) of the coating layer 12 formed on 
the support 11 can be adjusted in accordance with conditions such as the 
tension of the support 11, the running speed of the support, the size of 
the rotary rod 6, the rotational speed of the rotary rod 6, the viscosity 
of the coating composition, and so on. 
The coating apparatus for carrying out the coating method of the present 
invention is not limited to the structure in which the apparatus has a 
rotary rod 6 as described in the aforementioned embodiment. That is, 
various changes can be made. For example, an extrusion type coating 
apparatus 27 with a doctor edge 7a as shown in FIG. 2 may be used. In FIG. 
2, constituent parts the same as constituent parts shown in FIG. 1 are 
referenced by like numerals for the purpose of omitting the description 
thereof. 
As described above, in the coating method according to the present 
invention, not only the opposite to the support, of the back edge on the 
upstream side in the direction of running of the support with respect to 
the slit is set so as to be substantially parallel to the surface of the 
support, but the liquid-detached position in which the coating composition 
ejected from the slit to overflow the slit is detached from a surface of 
the support is set so as to be placed on the surface opposite to the 
support. Accordingly, because a flow the coating composition from the slit 
to the upstream side forms a liquid reservoir in the distance produced 
between the support and the surface opposite to the support before the 
flow forms a drop flow, the liquid pressure is once reduced in the liquid 
reservoir. As a result, the rapid change of the liquid pressure of the 
coating composition from high liquid pressure to atmospheric pressure is 
avoided. Accordingly, the rapid volatilization of the organic solvent in 
the coating composition is suppressed so that the solvent mist as produced 
conventionally can be prevented from being produced. 
As a result, not only the worsening of environment caused by the 
volatilization of the solvent can be eliminated but, for example, even in 
the case where another coating film is successively formed in the 
downstream side of the coating apparatus, the coating film failure caused 
by the re-deposition of the mist onto the coating film can be eliminated. 
[EXAMPLES] 
The effects of the present invention will become clearer from the following 
specific examples of the present invention. 
(Example 1) 
The condition of production of the mist in the upstream side in the case 
where methyl ethyl ketone which was a solvent was applied onto the support 
11 by using the coating apparatus 7 shown in FIG. 2 was examined by eyes. 
The width of the slit 9 was 0.3 mm. The viscosity .eta. of methyl ethyl 
ketone, the flow rate of methyl ethyl ketone and the blowout pressure of 
methyl ethyl ketone were in a range of from 0.5 to 10 cp, 5 LT/min and 
about 0.02 kg/cm.sup.2, respectively. The tension of the support 11 was 15 
kg/m. The running speed of the support was 200 m/min. The diameter of the 
rotary rod 6 and the rotational speed of the rotary rod 6 were set to be 4 
mm and 100 rpm, respectively. 
The angle .theta..sub.w of incidence of the support 11 and the distance t2 
between the outlet of the slit 9 and the support 11 were set to be 
15.degree. and 0.05 mm, respectively. The influence on the prevention of 
the production of the mist, of the length L.sub.d of the surface 14 
opposite to the support and the angle .theta..sub.d of inclination of the 
surface 14 opposite to the support was examined. 
As a comparative example, an experiment was made by using a coating 
apparatus having an upstream side inclined surface 17 having no upstream 
side parallel portion, as shown in FIG. 3. The angle .theta..sub.d of 
inclination of the upstream side inclined surface 17 used was 45.degree.. 
Further, an experiment was made by using a coating apparatus in which the 
coating film thickness adjusting method using a rotary rod was not 
employed in the downstream side of the coating apparatus but a doctor edge 
surface with a curvature radius R=10 mm as shown in FIG. 2 was employed. 
The other structure was the same as that of the coating apparatus in FIG. 
1. 
Results of the experiments were shown in Table 1. 
TABLE 1 
______________________________________ 
Experi- 
Coating Mist 
ment Apparatus .theta. 
L Condition Level 
______________________________________ 
1 FIG. 1 15.degree. 
10 mm No observation 
.largecircle. 
by eyes 
2 FIG. 1 15.degree. 
2 mm A very small 
.DELTA.-.largecircle. 
quantity 
3 FIG. 1 20.degree. 
10 mm A small .DELTA. 
quantity 
4 FIG. 2 10.degree. 
10 mm A small .DELTA. 
quantity 
5 FIG. 3 45.degree. 
0 mm Observation on 
x 
(Compar- the whole 
ative 
Example) 
______________________________________ 
It was apparent from Table 1 that the production of the mist could be 
effectively prevented by a parallel portion 14 which was provided on the 
upstream side of the blowout outlet of the coating apparatus so as to be 
parallel to the support 11. 
In the case where the discharge liquid pressure at the outlet of the slit 
was not higher than 0.02 Kg/cm.sup.2, the solvent mist was little produced 
even in the conventional apparatus (FIG. 3). As a result of examination of 
the length of the surface 14 opposite to the support to eliminate the 
solvent mist in the case where the discharge liquid pressure was not lower 
than 0.02 Kg/cm.sup.2 in which the production of the solvent mist was 
started, it became apparent that the mist was eliminated effectively when 
the length was not smaller than 2 mm. 
(Example 2) 
There were shown results of experiments in which coating was carried out by 
using a magnetic dispersion in a position near to the coating apparatus of 
Example 1. 
The magnetic dispersion used was prepared by mixing and dispersing 
components shown in Table 2 in a ball mill for 10.5 hours. As the support 
used was polyethylene terephthalate film with a thickness of 8 .mu.m and a 
width of 300 mm. As the coating apparatus used was an extrusion type 
apparatus. The other conditions were the same as those in Example 1. 
TABLE 2 
______________________________________ 
Coating composition 
______________________________________ 
Fe/Zn/Ni (weight proportion 92:4:4) powder 
300 parts 
(needle-like particles of average grain size in 
by weight 
the direction of length: 0.20 .mu.m, length/width 
ratio: 10, coercive force: 1600 oersted) 
vinyl chloride-vinyl acetate copolymer 
30 parts 
(copolymerization ratio: 87:13, 
by weight 
copolymerization degree: 400) 
conductive carbon 20 parts 
by weight 
polyamide resin (amin-valent: 300) 
15 parts 
by weight 
lecithin 6 parts 
by weight 
silicon oil (dimethyl polysiloxane) 
3 parts 
by weight 
cyclohexanone 300 parts 
by weight 
methyl isobutyl ketone 300 parts 
by weight 
n-butanol 100 parts 
by weight 
______________________________________ 
As a result of measurement, the viscosity of the aforementioned coating 
composition was 0.9 poise at the shear rate of 5.times.10 sec. 
TABLE 3 
______________________________________ 
Roughness of 
Experi- 
Coating Magnetic 
ment Apparatus .theta. 
L Surface Level 
______________________________________ 
6 FIG. 1 15.degree. 
10 mm No observation 
.largecircle. 
by eyes 
7 FIG. 1 15.degree. 
2 mm No observation 
.largecircle. 
by eyes 
8 FIG. 1 20.degree. 
10 mm A very small 
.DELTA.-.largecircle. 
quantity 
9 FIG. 2 10.degree. 
10 mm A very small 
.DELTA.-.largecircle. 
quantity 
10 FIG. 3 45.degree. 
0 mm Observation on 
x 
(Compar- the whole 
ative 
Example) 
______________________________________ 
It was apparent from the aforementioned experimental results that the 
coating method in which the support was set so as to be parallel to the 
surface opposite to the support, of the back edge on the upstream side of 
the slit was effective for the prevention of the production of the solvent 
mist and for the prevention of the bad influence of the solvent mist on 
the surface of the coating film of the magnetic dispersion.