JP2540079B2 - Method of manufacturing magnetic recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a magnetic recording medium in which a plurality of organic solvent-based magnetic layers are simultaneously overlaid on a continuously running non-magnetic support. Is.

(Prior Art) In recent years, the density and thickness of magnetic recording media have increased, and along with that, the magnetic layer conventionally coated on a non-magnetic support was a single layer, but now it is shifting to a multilayer. is there.

This is because a magnetic recording medium having a multi-layer magnetic layer can significantly improve magnetic recording characteristics such as an increase in magnetic data storage capacity as compared with a magnetic recording medium having a single magnetic layer. As for the layer structure, it has become necessary to form multiple layers such as two layers to several layers.

On the other hand, in order to achieve the above-mentioned multilayer structure, for example, Japanese Patent Publication No.
No. 43362, No. 58-43816, JP-A No. 51-119204, No. 52
As disclosed in JP-A-51908 and JP-A-53-16604, a method of forming a multilayer coating layer by coating and drying the coating liquid one by one on the support has been conventionally performed. It was

(Problems to be Solved by the Invention) However, in this method, since the steps such as coating and drying are repeated, the productivity is poor, and the apparatus (equipment) becomes large and the equipment cost becomes high. In addition, irregularities of the magnetic recording element may occur at the internal interface between the coating layers, which is apt to cause undesirable tape modulation noise and the like.

Therefore, there has been a demand for a method of forming a multi-layer coating layer by a single coating / drying step. However, as a method for coating a plurality of magnetic layers simultaneously, for example, Japanese Patent Laid-Open No. 62-2129
In the coating method disclosed in No. 33 and No. 62-124631, the organic solvent-based non-magnetic coating liquid and the magnetic coating liquid, or when the magnetic coating liquid is simultaneously multi-layer coating, sequential coating and drying steps Color unevenness and vertical streaks are more likely to occur even at low-speed application compared to the repeated method, and deterioration in quality becomes noticeable from the viewpoint of electromagnetic conversion characteristics or appearance. Depending on the combination of the liquid formulations of the coating liquid, the upper layer coating may be applied. There were problems that the liquid could not be applied uniformly and two layers were not formed, or that the upper layer coating liquid could not be applied at all onto the lower layer coating liquid. This phenomenon was more remarkable as the amount of the coating liquid was smaller (the thinner the layer was coated) or the coating speed of the coating liquid was higher. Furthermore, even if the coating can be applied with a certain thickness, the surface property of the tape surface is deteriorated, and when it is used for a video tape, for example, there is a problem that noise of video characteristics becomes high.

On the other hand, as a multi-layer coating method, the same color unevenness and vertical streaks occur in the case of a magnetic recording medium even in the production process of a photographic light-sensitive material. And, it is estimated that one of the causes of this problem is a phenomenon due to the disturbance of the interface between the liquid layers due to the difference in the viscosity of each liquid. By bringing them close to each other, the manufacturing problem due to the difference in viscosity was solved. That is, in the Newtonian fluid such as the photographic light-sensitive material, its physical properties are greatly influenced by the static viscosity (which can be easily determined by the formulation). Therefore, the solution can be solved by formulating the coating solution so that the static viscosity is adjusted. done.

However, the magnetic coating liquid of the magnetic recording medium is a mixture of an inorganic powder such as magnetic powder and a binder in a relatively large amount of an organic solvent, and a thixotropy which causes an extremely large viscosity change depending on the shear rate applied to the coating liquid. It is a so-called non-Newtonian fluid that is a tropic solution, and its viscosity is not constant and changes depending on various conditions, so it is extremely difficult to control it, and how much adjustment is performed solves the above problems. It was almost an empirical area to be able to do it. Under such circumstances, it was extremely difficult to obtain more stable quality and higher productivity than ever before.

The object of the present invention is to solve the above-mentioned problems in the multi-layer coating method and to stably produce a magnetic recording medium having good magnetic recording characteristics without the occurrence of color unevenness or vertical streaks in multi-layer simultaneous coating. It is to provide a method of manufacturing a magnetic recording medium that can be performed.

Therefore, as a result of intensive research and development, the present inventors have found that the value relating to the viscosity of the coating liquid in each slit or the liquid pool has a great influence on the formation of a thin film, and further research will be conducted on this. As a result of stacking, the present invention can be achieved.

(Means for Solving the Problems) That is, the object of the present invention is to extrude a plurality of organic solvent-based coating liquids which are non-Newtonian fluids from a plurality of slits of an extrusion type coating head, and immediately and continuously. In the method of manufacturing a magnetic recording medium applied on a running non-magnetic support, the viscosity difference in the slit of the adjacent coating liquid in the plurality of coating liquid,
A method for manufacturing a magnetic recording medium characterized by being set to 50 cp or less, and the extrusion type coating head, a liquid pool is provided on the upstream side of the slit, and the liquid pool of the adjacent coating liquid is provided. It is achieved by a method for manufacturing a magnetic recording medium, characterized in that the viscosity difference in the inside is set to 500 cp or less.

The setting of the viscosity of each coating solution can be easily measured by using a Rotovisco viscometer, and the inherent viscosity curve of each coating solution at an arbitrary shear rate can be obtained.

On the other hand, the average γ _s of the shear rates γ in the slits of the coating head is, where slit clearance is d, slit width is W, and flow rate in slit (coating amount) is Q,
formula; Thus, even a non-Newtonian fluid can be approximately calculated. Further, the shear rate γ ₁ on the coating edge is also an equation, where v is the traveling speed of the support. Can be approximated by

Further, the shear rate r _p in the pocket can be represented by the state when the coating solution flows into the pocket. Letting Q _{i be} the flow rate of the coating solution into the pocket and D be the pocket diameter, an equation; Can be approximated by

From the above equations (1) and (2), it is possible to roughly estimate the slit clearance d that can reduce the difference in viscosity of the coating liquids to be simultaneously coated. Therefore, when, for example, two types of coating liquids to be coated are drawn on the same graph, a region where the difference in viscosity is small can be widened, and desired coating can be performed within a certain range of the difference in viscosity.

The viscosity difference, as a result of diligent research, first, by setting the viscosity difference in the slit within 50 cp, it is possible to create a condition within 10 cp viscosity difference of the coating liquid on the edge, as a result, at the interface of the coating liquid contacting each other at the time of coating. It is considered that the behavior of the liquid can be stabilized and good coating can be performed. In this way, by clarifying the guideline of the setting range that enables good coating, it is possible to always guarantee a stable coating state. The multi-layer coating in which the viscosity difference setting condition in the slit is the dominant condition of the coating conditions is when the thickness of each layer is approximately the same.

When the thickness of each coating layer is significantly different, for example, in order to further increase high-density recording or to reduce cost, in the coating in which it is necessary to thin the uppermost layer (downstream side during coating), Further, by setting the viscosity difference between the adjacent coating liquids to be 500 cp or less in the liquid pool of the coating head, extremely stable and favorable coating can be performed.

The reason why the difference in viscosity in the liquid pool becomes a major factor as a coating condition is that the difference in the liquid properties due to the difference in the shear history of the coating liquid forming the adjacent layer causes some influence on the liquid behavior at the layer interface. Is estimated to give. In particular, in the case of coating in which the coating thickness of each layer is greatly different, since the slit clearance is generally different and the size of the pocket diameter is also changed accordingly, the coating liquid up to the coating on the support is It is considered that the shear history becomes relatively larger than other conditions as a dominant factor of the physical properties related to the liquid behavior during coating.

As a result of repeated examinations and experiments based on these estimations, it became clear that the change in the physical properties of the liquid due to the shear history is greatly influenced by the shear when flowing into the slit. That is, the shear history can be represented by the state of the liquid immediately before the slit. In this way, as a result of research focusing on the viscosity difference in the liquid pool (pocket) located immediately before the slit, the viscosity difference between the adjacent coating liquids in the liquid pool was set to 500 cp or less, Since the difference in shear history can be made small, the liquid properties due to the difference in shear history can be approximated, and the behavior of the liquid at the interface of the coating liquids that are in contact with each other during coating can be stabilized, which is extremely good. It is considered that it can be applied.

The present invention mainly adjusts the viscosity difference in the slit when the film thicknesses of the multiple coating layers are substantially the same, and adjusts the viscosity difference in the slit and in the liquid pool when the film thicknesses greatly differ. . By clarifying the target of the coating head in this way, it is possible to make extremely precise adjustments so that the liquid physical properties between adjacent layers approach each other according to the desired purpose of coating, and stabilize the liquid behavior at the liquid interface (layer interface). Since it can be reliably maintained, the conditions such as the viscosity of the coating liquid can be set accurately rather than the empirical area as in the past, and there is almost no difference in the thickness of the overlapping layers, so the difference in the coating thickness is large. In this case, the coating conditions can be made favorable, there is no color unevenness or vertical streaks, etc., and it is possible to easily achieve an improvement in the coating speed that could not be achieved in the past, and magnetic recording characteristics with good surface properties. It is possible to stably supply a good multi-layer (including three or more layers) magnetic recording medium.

The viscosity can be adjusted mainly by changing the liquid formulation, changing the liquid temperature, and changing the shear rate.
In the method of changing the liquid formulation, for example, by changing the amount of the solvent of the coating liquid (magnetic dispersion liquid), a coating liquid having a viscosity characteristic similar to that of the viscosity curve can be produced. When the amount is changed, it is possible to obtain a coating liquid showing viscosity curves having different inclination characteristics. The viscosity adjustment by adjusting the liquid temperature does not require changing the liquid composition. The method of changing the shear rate is extremely easy and effective in the practice of the present invention. That is, the shear rate in the slit can be changed by changing the slit spacing of the coating head and the liquid flow rate, and the shear rate on the edge can be maintained at a desired value by adjusting the coating thickness and the coating speed (support running speed). Not only can this be done, but in particular, if a portion of the coating liquid stored in the pocket is forcibly withdrawn, the amount of withdrawal can be adjusted to adjust the amount of liquid in the pocket without changing the pocket diameter and liquid composition. The viscosity can be adjusted by changing the shear rate.

In addition, in the method of changing the slit spacing, for example, the tip end portion of the coating head, that is, the minimum tip portion directly involved when the coating liquid adheres to the support is separated from other head portions. Since the tip portion is made detachable and is made of ceramic or cemented carbide, good coating can be realized and the workability of changing the slit spacing can be improved at low cost.

(Effect of the invention) The method for producing a magnetic recording medium of the present invention is a coating method for a magnetic recording medium in which multiple layers are simultaneously coated using an extrusion type coating head, and the difference in viscosity between at least adjacent coating layers is In the slit of the coating head
By setting it to 50 cp or less, it becomes possible to clarify the setting of good coating conditions and always maintain accurate coating conditions.
Since the difference in viscosity of the coating liquid on the edges can be reduced and the liquid behavior during coating is stable, it is possible to stably perform favorable coating without mixing of the coating liquid layers and color unevenness or vertical stripes.

Further, according to the present invention, when the thickness of each coating layer is largely different, the viscosity difference in the liquid pool in addition to the viscosity condition in the slit is set to 500 cp or less, thereby substantially shearing history of adjacent coating liquids. Since it is possible to reduce the difference between the two, it is possible to approach the liquid physical properties caused by the difference in the shearing history of the adjacent layers, it is possible to stabilize the behavior of the liquid at the interface of the coating liquid in contact with the coating, in particular, Even when the slit spacing is reduced to make the coating layer thinner, it is possible to set the viscosity accurately rather than the empirical range as in the past, to make the coating conditions favorable, and to prevent color unevenness and vertical length. It is possible to efficiently manufacture a multi-layer magnetic recording medium that has no streaks and the like, has good surface properties, can improve the coating speed, and has good magnetic recording characteristics.

Hereinafter, the effects of the present invention can be clarified by examples.

(Example-1) Hereinafter, one example of the present invention will be described in detail.

Coating Liquid A magnetic coating liquid having the composition components shown below was prepared to prepare a coating liquid.

(Composition) Co-γ-Fe ₂ O ₃ (Hc550 Oe) 100 parts by weight Vinyl chloride, vinyl acetate, vinyl alcohol copolymer X
Parts by weight Polyurethane resin "Nipporan-2301" 8 parts by weight (Nippon Polyurethane Co., Ltd.) Polyisocyanate "Coronate L" 8 parts by weight (Nippon Polyurethane Co., Ltd.) Carbon black 12 parts by weight (average particle size 20 μm) Stearic acid 1 part by weight Butyl stearate 1 part by weight Solvent Methyl ethyl ketone Y parts by weight Butyl acetate Z parts by weight Five vinyl alcohol copolymers, methyl ethyl ketone and butyl acetate as the compositions shown in Table 1 (A, B, C, D,
The coating solution of E) was prepared. In this case, butyl acetate was kept constant.

The viscosity of each coating solution prepared as described above is measured with a Rotovisco viscometer, and each of the coating solutions A, B, C, D and E showing the relationship between the shear rate and the viscosity is prepared as a viscosity curve specific to the solution. To do. Then, a multi-layer coating solution comprising a combination of the coating solution A as a lower layer and the upper layers of the coating solutions B, C, D and E was applied on a support of polyethylene terephthalate having a thickness of 15 μm to obtain a sample.
NO.1 to NO.4 were prototyped.

The coating head is an extrusion type coating head 1 (not a type of head for withdrawing a part of the coating liquid from the pocket) shown in FIG. 1, and the curvature of the doctor edge 2 on the upstream side in the traveling direction of the support W is large. the radius of curvature R ₂ are both 6mm radius R ₁ and the downstream doctor edge 3, a slit clearance d _1, d ₂ used was both configured to 0.5 mm.

The coating conditions are coating speed V: 300 m / min, coating section tension: 10 kg / 500
mm width, coating width W: 500 mm, coating amount of each layer Q: 3000 cc / min (20 c
It corresponds to c / m ^{2 and} has a thickness of about 4 μm).

Then, it was visually inspected whether or not the coating of each of the manufactured sample Nos. 1 to 4 was well multilayered. The results are shown in Tables 2-5. The shear rate γ in the slit is 1,200 and the shear rate on the edge is 125,000, and the difference in viscosity between the samples can be obtained.

(Example-2) The slit clearance of the coating head was changed using the coating liquid in Example-1, and the other conditions were the same as in Example-1. The results shown in Tables 6 to 9 were obtained.

Table 6 shows the slit clearance d ₁ forming the lower layer.
0,5 mm, the slit clearance d ₂ forming the upper layer
The one configured to have a thickness of 1,0 mm was used. The shearing intensity γ in the slit is 1,200 for liquid A and 300 for liquid B, and the shearing speeds on the edges are both 125,000. See FIG. 7.

Table 7 shows the slit clearance d ₁ forming the lower layer.
0,5 mm, the slit clearance d ₂ forming the upper layer
The one configured to 0.8 mm was used. The shear rate γ in the slit is 1,200 for the liquid A and 470 for the liquid B, and the shear rates on the edges are both 125,000. See FIG. 7.

Table 8 shows the slit clearance d ₁ forming the lower layer.
0,5 mm, the slit clearance d ₂ forming the upper layer
The one configured to 0.7 mm was used. The shear rate γ in the slit is 1,200 for liquid A and 610 for liquid B, and the shear rates on the edges are both 125,000. See FIG. 7.

In Table 9, the slit clearance d ₁ forming the lower layer (liquid A) was 1.0 mm, and the slit clearance d ₂ forming the upper layer (liquid E) was 0.5 mm. still,
The shear rate γ in the slit is 300 for liquid A and 1,200 for liquid E, and the shear rates on the edges are both 125,000. See FIG. 8.

As is clear from the above tables, the viscosity difference in the slit
By adjusting to within 50 cp, it became possible to reliably perform extremely good multi-layer simultaneous coating. The evaluation in each table is as follows: ○ indicates a very good coating state without color unevenness or vertical streaks, Δ indicates a coating state that can be put to practical use but is slightly difficult, and × indicates uneven color or vertical streaks. Many indicate practically problematic coating states.

(Example-3) In the liquid composition shown in Example-1, the vinyl alcohol copolymer, methyl ethyl ketone, and butyl acetate were used as four compositions (F, G,
H, I) was prepared and used.

In this case, butyl acetate was kept constant.

The viscosity of each coating solution prepared as described above is measured by a Rotovisco viscometer, and a viscosity curve peculiar to each of the coating solutions F, G, H and I showing the relationship between the shear rate and the viscosity is prepared. Then, on a support of polyethylene terephthalate having a thickness of 15 μm, a multi-layer coating solution composed of a combination of the coating solution F as the lower layer and the upper layers of the coating solutions G, H and I was prepared, and sample No. 9
~ NO.18 was prototyped.

The coating head is an extrusion type coating head 1 (a type of head for withdrawing part of the coating liquid from a pocket) shown in FIGS. 1 and 2, and has a doctor edge 2 on the upstream side in the traveling direction of the support W. The radius of curvature R ₁ and the radius of curvature R ₂ of the doctor edge 3 on the downstream side are both 6 mm, and other head configurations and conditions are: * Pocket diameter ... Φ15 mm or Φ8 mm * Slit spacing ... 0.4 mm or 0.15 mm * Coating width: 500 mm * Coating speed: 300 m / min * Coating amount: 20 cc / m ² or 5 cc / m ² , ^2.5 cc / m ² (Note that coating amount and film thickness The relationship was 5 cc / m ² and the thickness was about 1 μm.) * Pullout rate: 300 cc / min or 900 cc / min. And visually inspected whether the manufactured sample was applied well or not. It was The results are shown in Tables 10 to 23. Note that FIG. 9 shows a viscosity characteristic curve showing the relationship between shear rate and viscosity. The coating head used in this example is for coating upper and lower two layers as shown in FIGS. 1 and 2, and includes a lower layer side pocket 6 for forming a lower layer and a lower layer side connected to the pocket 6. It has a structure in which a slit 4, an upper layer side pocket 7 and an upper layer side slit 5 connected to the pocket 7 are provided side by side, and as shown in FIG. 2, the coating liquid supplied to the both pockets 6 and 7 from one end is The slits 4, 5
A predetermined amount is discharged through the nozzle and a part of the liquid in the pocket is forcibly withdrawn from the other end side.

Sample Nos. 9, 10, and 11 show examples in which the upper layer thickness is reduced using the same coating head. This sample (NO.9 and NO.1
As can be seen from (0), when the difference in the thickness between the coating layers becomes large, the effect of the viscosity difference in the pocket greatly appears on the coating surface property. Samples Nos. 12 and 13 have narrow slits and a slit internal difference of 50 cp or less, but not so good results are obtained. Therefore, if the pocket diameter is further changed like sample Nos. 14 and 15 and the viscosity difference in the pocket is 500 cp or less, good conditions are obtained. Samples Nos. 16 and 17 are examples of obtaining good conditions by changing the physical properties of the liquid without changing the pocket. Sample No. 18 shows that the flow rate in the pocket can be changed by increasing the withdrawal amount to adjust the viscosities, which is an example of obtaining good conditions.

Sample No. 19 used the same coating head as used in Sample No. 15, and as the coating liquid, G liquid was used for the lower layer and F liquid was used for the upper layer. As a result, it was not possible to obtain a good result as in the case of sample NO.15 because the viscosity difference in the slit was large.

Sample No. 20 uses the same coating head as used in Sample No. 16, and the coating liquid is G liquid in the lower layer and H in the upper layer.
The liquid was used. As a result, a very good result could not be obtained as in the case of sample No. 16 because the viscosity difference in the slit was slightly large, but a reasonable result could be obtained.

Sample No. 21 was the same as the coating head used in Sample No. 17, and as the coating liquid, liquid I was used in the lower layer and liquid F was used in the upper layer. As a result, a good result could not be obtained as in the case of sample No. 17, because the viscosity difference in the slit was slightly large, but a decent result could be obtained.

The sample No. 22 used the same coating head as used in the sample No. 18, and the flow rate in the pocket was changed by increasing the withdrawal amount. As the coating liquid, the liquid G was used for the lower layer and the liquid F was used for the upper layer. However, due to the large viscosity difference in the slit, it was not possible to obtain good results as in the case of sample No. 17.

As is clear from the above tables and FIG. 10 (hatched area indicates good coating), the viscosity difference in the pocket is 500 cp.
By adjusting the thickness within the range, extremely good simultaneous multilayer coating can be surely performed even when the thickness of the upper layer is a fraction of the imaginary thickness and the thickness difference between the upper and lower layers is very large.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a coating head used in an example of a method for manufacturing a magnetic recording medium of the present invention, and FIG. 2 is a schematic sectional view of a coating head used in an example of a method of manufacturing a magnetic recording medium of the present invention. FIG. 3 is a perspective view, FIG. 3 to FIG. 9 are graphs showing the viscosity characteristic curve of the coating liquid used in the examples of the present invention, and FIG. 10 is a diagram showing the evaluation by the viscosity difference in the examples of the present invention. (Reference numeral in the figure) 1 ... Coating head, 2 ... Upstream doctor edge, 3 ... Downstream doctor edge, 4 ... Lower layer slit, 5 ... Upper layer slit, 6 ... Lower layer slit, 7 ... Upper layer side pocket, A, B, C, D, E, F, G, H, I ... Coating solution, W ... Support.

Claims (2)

(57) [Claims] 1. A magnetic recording medium in which a plurality of organic solvent-based coating liquids, which are non-Newtonian fluids, are extruded from a plurality of slits of an extrusion type coating head and immediately coated on a non-magnetic support that continuously runs. The method of manufacturing a magnetic recording medium according to the manufacturing method, wherein a difference in viscosity between adjacent coating liquids in the plurality of coating liquids in the slit is set to 50 cp or less. 2. The extrusion type coating head is provided with a liquid pool on the upstream side of the slit, and a viscosity difference between the adjacent coating liquids in the liquid pool is set to 500 cp or less. The method of manufacturing a magnetic recording medium according to claim 1.