JPS6342030A - Production of magnetic recording medium - Google Patents

Abstract

PURPOSE:To decrease the residual solvent to a prescribed permissible limit or below while maintaining the flatness substantially and to shorten drying time by specifying respectively the b. p. of the solvent to be used for a magnetic coating compd. and the drying temp. after a decreasing drying rate process in the drying stage. CONSTITUTION:The solvent having 130 deg.C b. p. is used for the magnetic coating compd. in a coating stage and the drying temp. after the decreasing drying rate process in the drying stage is specified to 40-80 deg.C. The drying in a short time is no longer possible and the residual solvent does not decreased down to a specified level or below in spite of using the solvent having <=130 deg.C b. p. if the lower limit value of the drying temp. is the temp. below 40 deg.C. The shrinkage of a base by the effect of heat is resulted and the satisfactory flatness is longer maintained if the upper limit value exceeds 80 deg.C. The evaporation in a relatively short time is possible even when the drying temp. after the decreasing drying rate process is set at <=80 deg.C, if the solvent having the b.p. lower than 130 deg.C upper limit value of the b. p. is used.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing a magnetic recording medium, particularly a coating step of applying a magnetic coating material to a support, and a method of applying a magnetic coating material to the support under drying temperature control. The present invention relates to a method of manufacturing a magnetic recording medium, which includes a drying step of drying a magnetic paint.

(Prior Art) Magnetic recording media are generally produced by applying a magnetic paint to a moving flexible support, then aligning it, drying it, and winding it up on a return roll.

The magnetic recording medium manufactured in this way is then subjected to a calendar treatment, and further slit to a predetermined width.
Alternatively, it is punched out to a predetermined size and incorporated into a desired product.

The above-mentioned support is usually several micrometers to tens of micrometers, and even 75 micrometers.
A garment made of polyethylene terephthalate (PET) with a thickness of μ is used. In addition, various coating liquids (liquids in which Ia recording materials, undercoat materials, intermediate layer materials, bank layer materials (e.g. carbon materials), conductive materials, protective materials, etc. are dispersed or dissolved) are used as the magnetic paints. Ru. These various coating liquids use an organic solvent as a solvent, and usually a solvent such as cyclohexanone is used.

The film thickness of the magnetic paint applied to the support in the coating step is adjusted so that the dry film thickness after passing through the subsequent drying step ranges from about 0.1 microns to several microns or even tens of microns.

In the drying process, the support coated with magnetic paint is generally dried by running it in a drying oven. There is a zone where the solvent that is present in the magnetic paint is actively evaporated (hereinafter referred to as the active drying zone), and if necessary, the solvent that remains in the magnetic paint without evaporating even after passing through this active drying zone (residual solvent).
In order to minimize the amount of drying, the drying zone is comprised of a drying zone (hereinafter referred to as a post-drying zone) provided following the active drying zone.

The active drying zone is further divided into two processes. In other words, there is a constant rate drying process in which the drying rate is constant, and a decreasing rate drying process in which the drying rate decreases as the proportion of solvent contained in the magnetic material coated on the support decreases during drying. being classified.

During the constant rate drying process, the support coated with the magnetic paint is kept at a low temperature because the magnetic paint is in a wet state and the latent heat of vaporization of the solvent contained therein can be absorbed. Therefore, it is possible to set the drying temperature to a relatively high temperature. On the other hand, in the decreasing rate drying process that follows this constant rate drying process, the temperature of the support coated with the magnetic paint gradually rises as the magnetic paint continues to dry. For this reason, it is not possible to set a high temperature as in the constant rate drying process,
The drying temperature is generally set at approximately 100 degrees.

(Problem to be solved by the invention) However, the drying temperature in the lapse rate drying process is approximately 1
In the conventional manufacturing method for magnetic recording media, where the temperature is set at around 00 degrees, the magnetic recording medium that has gone through the drying process causes unfavorable phenomena in subsequent processes or in the function of the completed product. This was discovered as a result of investigation by the inventor. That is, when a floppy disk obtained by punching out a magnetic recording medium that has gone through the drying process is loaded into a recording/reproducing device, the distance between the magnetic head and the floppy disk is not constant, and effective recording and reproduction cannot be performed. It turned out that it was not possible.

It has also been found that this phenomenon occurs because the flatness of the floppy disk is insufficient. Furthermore, when the surface of a magnetic tape formed by slitting the magnetic recording medium that had undergone the drying process was inspected, it was found that the flatness was similarly insufficient.

However, in the case of conventional magnetic tapes, the magnetic head is pressed against the magnetic tape during recording and reproduction, so that the influence of deterioration in flatness has not been noticeable.

The deterioration of the flatness of the magnetic recording medium as described above is thought to be due to the heat applied during the drying process, particularly after the lapse rate drying process.

Therefore, it is possible to improve the flatness of the magnetic recording medium by lowering the drying temperature after the lapse rate drying process.

However, if the drying temperature after the reduction rate process is lowered, the solvent contained in the magnetic paint will not evaporate sufficiently within the specified drying time, and as a result, the residual solvent will prevent clogging of the magnetic head, and the organic solvent When the magnetic recording medium is wound up into a roll after drying, it may cause adhesion failure between the coated surface and the support, between the coated surfaces, between the coated surface and the back surface, etc. I don't like it.

In order to solve this problem, the reduction rate drying process J'1. It is necessary to lengthen the drying time to keep the residual solvent below a specified level, but this requires a very long drying zone, which increases both equipment and maintenance costs and increases product costs. This is undesirable. Furthermore, as the transport speed of the magnetic recording medium increases in the manufacturing process, the drying zone needs to be longer, and the above-mentioned inconvenience becomes even greater.

The present invention has been developed in view of the above circumstances, and is capable of ensuring sufficient flatness of a manufactured magnetic recording medium, and that is capable of ensuring sufficient flatness of a manufactured magnetic recording medium, and that is capable of ensuring sufficient flatness of a magnetic recording medium that is not contained in the magnetic paint applied to the support of the magnetic recording medium. An object of the present invention is to provide a method for manufacturing a magnetic recording medium that can reduce the amount of residual solvent contained in the magnetic recording medium to a predetermined allowable limit ω or less, and can achieve this without requiring a long drying time.

(Means for Solving the Problems) The method for producing a magnetic recording medium according to the present invention focuses on the fact that the boiling point of the solvent contained in the magnetic paint differs depending on the type. As a result of repeated experiments using various combinations of high and low temperatures, it was found that the above problems could be solved by applying and drying under the following conditions. It is something. That is, it is characterized in that the boiling point of the solvent used for the magnetic paint in the coating process is 130°C or lower, and the drying temperature after the lapse rate process in the drying process is 40°C to 80°C.

The values of the lower limit of the drying temperature after the reduction rate process of 40"C and the upper limit of 80°C, and the upper limit of the boiling point of 130°C were obtained as a result of experimental studies in the examples mentioned above, but the values were limited to these values. The basis is as follows.

First of all, the lower limit of the drying temperature after the lapse rate process was set at 40°C.
This is because even if a temperature below 0° C. is used, drying cannot be carried out in a short time and the residual solvent does not fall below a certain level.

Next, the upper limit of the lapse rate drying temperature is set to 80° C. because if the temperature is higher than this, the support will shrink due to heat and cannot maintain sufficient flatness. Note that polyethylene terephthalate (PE) is generally used as a support.
T) is used, but the glass transition temperature of this PET is 69°C, which is lower than the above-mentioned 80°C, and therefore it is considered that the lapse rate drying temperature should also be 69°C or lower.

However, even if the lapse rate drying temperature is 69℃ or higher, 8
If it is below 0°C, the thermal shrinkage rate is very small, and it takes time for the temperature of the support itself to rise to 80°C during drying, so the residence time after the decreasing rate drying process is relatively short. Since the temperature cannot reach 80° C., it is considered that the flatness of the magnetic recording medium in the dried film can be sufficiently ensured.

The reason for setting the upper limit of the boiling point of the solvent to 130°C is that if a solvent with a boiling point lower than this is used in magnetic paint, the drying temperature after the lapse rate process is set to 80°C or lower. This is because it can be evaporated in a relatively short time. Therefore, the boiling point here is 130
℃ or less means that when two or more types of solvents are used, the boiling point of the solvent having the highest boiling point is 130°C or less.

Incidentally, the need for the drying temperature after the rate reduction process and the boiling point of the solvent to be within the above ranges increases as the thickness of the support increases, and also increases as the dry film thickness of the magnetic coating material increases. Regarding the thickness of the support, for example, if the support is thinner than 30μ, its rigidity will be relatively low and it will easily maintain its flatness even if it undergoes some heat shrinkage, which will cause problems in the return process. However, when the thickness of the support is 30μ or more, the difference in the thickness of the support in the width direction and the length direction is large, and therefore the heat shrinkage rate of each part of the support is also greatly different, thus maintaining flatness. It becomes difficult to do. Furthermore, since the rigidity increases, it becomes difficult to maintain flatness. Regarding the dry film thickness of magnetic paint, for example, if the dry film thickness is thinner than 1μ,
Since the coating amount is small and the diffusion distance of the residual solvent in the magnetic paint is short, there is little need for high temperature drying, and the so-called bimetallic effect between the support and the coating film is small, making it easy to maintain flatness. This is because if the dry film thickness is 1 μm or more, not only will high-temperature drying become necessary, but the movement of the coating film that inhibits thermal shrinkage of the support will increase, which is thought to promote deterioration of flatness.

In addition, in the present invention, the drying temperature is limited to the above range in the lapse rate drying step J'! , the constant rate drying temperature in the constant rate drying process that precedes the decreasing rate drying process may exceed 80°C. This is because, as already mentioned, the temperature of the support is maintained at a low temperature due to the latent heat of vaporization during the constant rate drying process. The constant rate drying temperature therefore depends on other factors (e.g.
It can be arbitrarily set in consideration of surface roughness, occurrence of Punnert cells, and slipperiness@).

Now, the relationship between the drying process and the drying zone will be described in detail. The drying process includes, as described above, a constant rate drying process, a decreasing rate drying temperature, and, if necessary, a process of lowering the residual solvent below a certain level.

On the other hand, the drying zone (equipment) is usually divided into multiple zones and the temperature is controlled for each zone.

In the drying process, the point at which the constant rate changes to the decreasing rate process varies greatly depending on the coating amount, coating speed, drying temperature, drying air volume, etc. Therefore, the division point of the dry zone and the constant rate→
It is rather rare for the transition points of the decreasing rate to coincide, and the constant rate → decreasing rate often diverges within one zone. Figure 1-A
) Furthermore, if the coating amount is small and the coating speed is relatively slow, a transition point from constant rate to decreasing rate may occur in the first drying zone.

(Figure 1-B) As mentioned above, the web is heated depending on the drying temperature after the decreasing rate drying process, but as shown in Figure 1, even in the constant rate process, the drying temperature in the drying zone is 40°C to 80°C. It is preferable that

(Function) With the above configuration, the drying temperature after the lapse rate drying step in the drying process is performed at a relatively low temperature range of 40°C to 80°C, so that thermal shrinkage occurs in the support. There is no fear. Therefore, the flatness of the magnetic recording medium after drying is sufficiently ensured. Also, magnetic paint has a boiling point of 130
Since the solvent is used at a temperature below .degree. C., the solvent evaporates in a short time even at the above-mentioned lapse rate drying temperature, making it possible to keep the residual solvent below the predetermined allowable limit a.

(Effects of the Invention) Therefore, according to the present invention, the residual solvent can be removed in a short period of time until it reaches an amount that does not cause problems such as clogging of the magnetic head, odor of organic solvent, and failure of adhesion of the magnetic recording medium. It can be evaporated, and after drying, a high quality magnetic recording medium with excellent flatness can be obtained.

(Example) Hereinafter, experiments conducted in completing the present invention and their results will be described in detail.

Table 1 shows the boiling points of the solvents used in this experiment.

Table 1 (from "Solvent Handbook" published by Kodansha) Table 2 shows the first example in which magnetic paint was applied to a support and dried.

Table 2 The magnetic paint used in this example had the formulation shown in Table 3, and cyclohexanone with a boiling point of 155.65°C was used as solvent A in the table.

Table 3 C○-containing magnetic iron oxide (Ss T 35TIL/9)
100 parts nitrocellulose
iQn polyurethane resin (product name [Niraporan 230
4J) (manufactured by Nippon Polyurethane Co., Ltd.) 8 Polyisocyanate 81/Cr2O32” Carbon black (average particle size 20 μm) 2rr
Stearin M 1〃Butyl stearate in methyl ethyl ketone 2001! Solvent A

100// Also, as a support, the glass transition temperature is 69
℃ PET is used, and its thickness is 15μ and 38μ.
.

Experiments were conducted on three types of 75μ. The magnetic paint was applied to each of these supports with a coating width of 500m5+ and a coating thickness of 2μ in dry film thickness (however, for a support with a thickness of 15μ, some parts were 0.5μ and 0.9μ), and the coating method was as follows: A known method disclosed in JP-A-58-104666 and JP-A-59-94657 was used. The magnetic paint coated on the support in this way is dried (drying in an active drying zone (both constant rate drying and decreasing rate drying)) by changing the drying temperature from 30°C to 100°C in 10°C increments. The drying time was 10 seconds. The magnetic recording medium formed by this drying is wound into a roll at 1
A tension of 10 υ to 30 K9 was applied per m width.

The magnetic recording medium formed through the above steps was evaluated as follows. Namely, residual solvent (1), crystal formation between the coated surface and the support after winding (presence or absence of adhesion);

Regarding the flatness of the magnetic recording medium after winding, it is OK for practical use.
), Δ (slightly worse but no practical problem), and × (practically unacceptable).

The evaluation results are shown in Table 2. As is clear from the table, when a magnetic paint containing cyclohexanone with a boiling point of 155.65°C in a solvent is coated on a support so that the dry film thickness is 2μ, the thickness of the support is 15μ, 38μ, or 75μ. It can be seen that even when the adhesive is applied to a drying temperature of 70° C. or lower, there is a large amount of residual solvent and poor adhesion occurs. On the other hand, if the temperature is 80°C or higher, the amount of residual solvent will be improved, but if the temperature is 90°C or higher for a support with a thickness of 38μ or 75μ, a problem will occur in that the flatness of the magnetic recording medium cannot be maintained after winding. I understand that. In addition, the thickness is 75
Dry film thickness of magnetic paint on μ support is 0.5μ and 0.9μ.
When the coated film was dried at 60° C., good results were obtained in terms of residual solvent, adhesion after winding, and flatness after winding.

Next, a second embodiment will be described.

In this example, in the first example, the diameter of the support was 15μ, the dry film thickness of the magnetic paint was 2μ, the drying temperature was 80℃,
The experiment was conducted under conditions of 0°C and 100°C, and the magnetic recording medium that had been scraped onto a roll was sent out again, and the back surface was coated and dried under the same conditions.

The above experiment was carried out because a support with a thickness of 75 μm is used for floppy disks, and in this case, magnetic paint is applied to both sides of the support.

As a result, the film thickness distribution in the width direction of the magnetic paint after coating and drying on the front side (80"C) was less than 0.1μ in the microwave, but when coating and drying on the back side III Ji on the back surface after application; [Distribution is at drying temperature of 80℃
Te 0.12μ, 90''C 11ooo℃Tea deteriorated to 0.25μ, and it was found that the deterioration of flatness had a negative effect on the roughness in the subsequent process.In addition, although the surface side was dried at 90'C1100℃ The flatness deteriorated further after coating on the back surface, making it unusable for practical use.

Table 4 shows the third example.

Table 4 In this example, methyl ethyl ketone, MIBK, and butyl acetate were used as the solvent A of the magnetic paint shown in Table 3 in place of cyclohexanone in Example 1, and the dry film thickness of the magnetic paint was 2μ. The experiment was otherwise conducted under the same conditions as in the first example.

From the results shown in Table 4, if the solvent A contained in the magnetic paint has a low boiling point (at most 126.11
It can be seen that even when the drying temperature is 80'C or less, IFJ is of a quality that does not cause any practical problems in terms of residual solvent, sticking of winding erosion, and flatness of winding.

Table 5 shows the fourth example.

Table 5 In this example, a magnetic paint containing carbon black with a formulation not shown in Table 6 was used, and solvent B in the table was used.
An experiment was conducted by replacing methyl ethyl ketone, MIBK, butyl acetate, and cyclohexanone. Other conditions such as coating and drying are the same as in the 16th and 3rd embodiments.

Table 6 Carbon black 150 parts (
Sebalco MI CT, average particle size 250 μm) Niraporan-730480 parts (manufactured by Nippon Polyurethane) Phenoxy resin (PKH-1) 35 parts Olein [! 1 part methyl ethyl ketone 634 parts dissolved
8,100 parts of agent The results shown in Table 5 show that the change in magnetic paint made it difficult for the residual solvent to evaporate. However, even when such a magnetic paint is used, if a solvent with a boiling point of approximately 130°C or lower is used, residual solvent, stickiness, etc. may occur at drying temperatures in the range of 40°C to 80°C. It can be seen that the flatness of the winding erosion can be obtained without any practical problems. On the other hand, when cyclohexanone with a high boiling point is used as a solvent, it is found that there is no drying temperature that simultaneously satisfies all three quality characteristics. In other words, it can be seen that when using a magnetic paint having a formulation as shown in Table 6, the advantage of using a low boiling point solvent is considerably greater.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the structure of the drying step in the method for manufacturing magnetic recording media according to the present invention.

Claims (1)

[Scope of Claims] 1) A coating step of applying a magnetic paint to a support, and a drying step of drying the magnetic paint applied to the support under drying temperature control, A method for manufacturing a magnetic recording medium, characterized in that a solvent having a boiling point of 130° C. or less is used in the paint, and the drying temperature after the lapse rate drying step in the drying step is 40° C. to 80° C. 2) The method for manufacturing a magnetic recording medium according to claim 1, wherein the support is made of polyethylene terephthalate and has a thickness of 30 μm or more. 3) When the magnetic paint is applied, the film thickness of the paint after drying is 1
2. The method for manufacturing a magnetic recording medium according to claim 1, wherein the manufacturing method is carried out so that the magnetic recording medium is greater than or equal to μ.