JPS6319941Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to magnetic recording media, and more particularly to magnetic recording media having a magnetic recording layer doped with carbon black to reduce surface reflectance for optical reading. In recent years, magnetic cards combined with optical reading have begun to be used. In this case, since the conventional magnetic recording layer also serves as an optical reading section (for example, a bar code), it is made of a composition in which carbon black is added for the purpose of reducing its reflectance. This magnetic recording layer (also serving as an optical reading layer) is formed on the medium substrate by coating or printing. However, paints or inks to which carbon black is added have a large yield value due to the large structural viscosity of carbon black, and their applicability and printability are significantly inferior to those without carbon black. Conventionally, therefore, when applying or printing paints or inks containing carbon black, it was necessary to set conditions within a very narrow range, which often led to a decrease in productivity. In this type of magnetic recording medium that can be used in conjunction with conventional optical reading, as shown in FIG. It consists of a single composition containing. Since carbon black 2A has a large structural viscosity, it has the effect of significantly increasing the yield value of the paint or ink, thereby degrading the coating suitability and printing suitability. In order to obtain appropriate suitability for coating and printing, it is sufficient to reduce the solid content, but this leads to deterioration of properties. For example, if the content of carbon black is reduced, the reflectance will be reduced, and if the content of magnetic material is reduced, the magnetic properties (output, etc.) will be reduced. Therefore, in the past, compositions were determined by considering both coating/printing suitability and properties, so it was impossible to escape from setting contradictory conditions such that if the former was satisfied, the latter would be inferior, and if the latter was satisfied, the former would be inferior. I couldn't do it. An object of the present invention is to eliminate the above-mentioned conventional drawbacks and provide a magnetic recording medium with good productivity without deteriorating either optical reading characteristics or magnetic characteristics. In developing a magnetic recording medium that can achieve these objectives, the present inventor investigated the relationship between the film thickness and reflectance of a carbon black-containing magnetic coating, and found, for example, the results shown in Figure 3. As illustrated in , the reflectance of a magnetic coating film containing 5% carbon black decreases as the coating thickness increases, and when the film thickness is approximately
It was found that it reached a certain value at 3 μ and did not change much thereafter. Therefore, the present inventor realized that, from the viewpoint of optical reading characteristics, it is not very meaningful to increase the thickness of the carbon black-containing magnetic coating to a predetermined value or more. The present invention was developed with attention to this point, and includes carbon black in the upper layer of the magnetic recording layer, but not carbon black in the lower layer. In other words, by providing a carbon black-containing layer on top of the magnetic recording layer, which has the minimum necessary and sufficient thickness for optical reading, the drawbacks of the conventional method can be overcome and the separation performance can be improved. It can be used as a magnetic recording layer. In the magnetic recording medium according to the present invention, as shown in FIG. It is said that only carbon black 2A is contained. In this way, since it is possible to use a material that does not contain carbon black and has excellent coating and printing suitability for the main portion 32 that determines the magnetic recording characteristics below the upper layer 31, a uniform coating film can be obtained with high productivity. is possible. In addition, since the main portion 32 does not contain carbon black, it is possible to increase the content of magnetic material compared to the case where it contains carbon, forming a coating film with excellent magnetic properties and a high residual magnetic flux density (Br). can do. This is because carbon black has a large structural viscosity, so even the addition of a small amount increases the yield value and significantly deteriorates the suitability for coating and printing. Generally, as the solid content of a paint or ink increases, its yield value increases and its suitability for coating and printing deteriorates. Therefore, when carbon black is contained, it is difficult to increase the solid content. On the other hand, when carbon black is not contained, even if the magnetic material is increased, the coating/printing suitability does not deteriorate as drastically as carbon black, so the magnetic material content can be increased by a considerable amount. In the case of the present invention, since the carbon-containing layer is provided in the upper layer portion 31 only to the minimum necessary extent, even if coating/printing suitability is poor and film thickness variations occur, the effect on the entire coating film can be suppressed to a small extent. Another possibility is to form a coating film containing only carbon black on the magnetic recording layer in order to reduce the reflectance, but it is not possible to provide a non-magnetic layer between the magnetic recording layer and the magnetic head in this way. This results in spacing loss, leading to deterioration of magnetic properties. In the present invention, since the upper layer of the magnetic recording layer contains not only carbon black but also as much magnetic material as possible, deterioration of magnetic properties can be minimized. Next, we actually produced a magnetic recording medium having a magnetic recording layer with a conventional structure as shown in FIG. 1 and a magnetic recording medium with a magnetic recording layer having a structure according to the present invention as shown in FIG. I will explain the results of a comparative study of their characteristics. First, to form the carbon black-containing layer which is the upper layer 31 of the conventional single-layer magnetic recording layer shown in FIG. 1 and the two-layer magnetic recording layer according to the present invention shown in FIG. A black-containing magnetic paint was used. Magnetic paint containing carbon black Carbon black
#2400B (Mitsubishi Chemical Industries) 2% rFe ₂ O ₃ MRM400 (Toda Industries) 17% Binder resin VAGH (UCC) 15% Solvent MEK/Toluene: 1/1 66% Two-layer structure according to the present invention shown in Figure 2 To form the carbon black-free layer which is the lower layer 32 of the magnetic recording layer, a carbon black-free magnetic paint having the following paint composition was used. Carbon black-free magnetic paint rFe ₂ O ₃ MRM400 (Toda Kogyo) 20% Binder resin VAGH (UCC) 13% Solvent MEK/toluene: 1/1 67% These magnetic paints can be dispersed in a ball mill for 48 hours. Dispersed. A 10μ thick conventional single-layer magnetic recording layer formed using the above-mentioned carbon black-containing magnetic paint, and a 7μ thick lower layer 32 formed using the above-mentioned carbon black-free magnetic paint. layer and a 3μ thick upper layer 31 formed using the above-mentioned carbon black-containing magnetic paint.
Table 1 below summarizes the results of measuring the composition and other properties of the two-layered magnetic recording layer of the present invention comprising a carbon black-containing layer and a carbon black-containing layer.

[Table] First, regarding the optical reflectance of the magnetic recording layer in Table 1, as mentioned above, as shown in FIG. 3, the reflectance of the coating film containing 5% carbon decreases as the coating thickness increases; It reaches a certain value at a film thickness of 3μ. Therefore, it can be seen that the coating film with the structure shown in Table 1 has the same reflectance of 6% as the conventional single-layer structure and the coating film of the present invention, and the optical reading performance is equivalent. . It should be noted that these light reflectance measurements were made at a reflection angle of 30° and a measurement light wavelength of 600 to 900 nm. However, when comparing the magnetic properties, the coating film according to the present invention is superior to the coating film with the conventional structure. The former has an average two-layer residual magnetic flux density of 910 Gauss, while the latter has only 700 Gauss. Therefore, even though the coating thickness is the same at 10μ,
The output showed different values, 6V and 4.5V. This means that the coating according to the present invention requires a thinner film thickness in order to obtain the same output as the coating film with the conventional structure, and is also preferable from the point of view of reducing film thickness loss and improving resolution. That's true. To measure these outputs, we formed a coating on a 50μ thick PET film, bonded it to a 0.6mm thick PVC substrate, punched it out in the shape of a magnetic credit card, and inserted it into a magnetic card reader (FC-1). Recording and playback (recording density: 75FCI) were performed at Fujitsu. Furthermore, the results of measuring the viscosity characteristics of each paint were as shown in Table 2.

[Table] Containing paint
carbon black 600 300
Non-containing paint As shown in Table 2, the carbon black-containing paint shows a large yield value, indicating poor coating suitability. Coating suitability varies depending on coating equipment, film thickness, etc., but in this case, the yield value was 250~
Desirably, it is 300dyne/ ^cm2 . As described above, a high degree of skill is required to uniformly apply carbon paint, which has poor coating suitability, and is subject to many restrictions such as coating speed, resulting in an increase in costs. In the present invention, this coating film with poor coating suitability is applied only to the necessary minimum amount, so even if the coating film is somewhat non-uniform, the effect on the properties of the entire coating film can be minimized. Examples have been described above, but in this example, the composition, such as the carbon content, was determined in consideration of the coating, optical reading, and magnetic properties of a conventionally structured coating film. However, with the present invention, since the non-uniformity of the carbon-containing coating has little effect on the entire coating, it is possible to further increase the amount of carbon black, creating a coating with lower reflectance and excellent optical readability. It is also possible to form Next, the effects obtained by the present invention will be summarized. (1) Since carbon black-containing paint, which has poor coating (printing) suitability, was applied to the upper layer of the paint film to the minimum necessary thickness, the effect of uneven film thickness on the entire paint film was reduced, and productivity was reduced. improves.
Note that the conventional structure has a single layer and can be coated or printed once, but the present invention has a two-layer structure and thus requires two coats or printing. but,
In the case of printing, multi-color printing equipment is common, and if coating equipment is used that has two or three coating units, which have become popular in recent years, production can be done in one pass, and productivity will not be compromised. do not have. (2) For the above reasons, it is possible to provide a coating film with a further increased carbon black content, so it is possible to provide a coating film with low reflectance and excellent optical reading performance. (3) Since most of the coating film does not contain carbon black, it is possible to increase the content of magnetic material, and a coating film with excellent magnetic properties can be obtained. Further, the coating film structure according to the present invention is very effective when using an ultraviolet curable coating film. Since a coating film containing carbon black absorbs ultraviolet rays, it will not cure sufficiently unless it is made into a thin film. Therefore, in order to obtain a predetermined film thickness, thin film coating and drying must be repeated, resulting in poor productivity. However, according to the present invention, since most of the predetermined film thickness can be a coating film that does not contain carbon, it is possible to obtain a considerable film thickness at one time, which improves productivity. The carbon-containing layer, which has poor productivity, may be provided in the uppermost layer in a necessary and sufficient amount.

[Brief explanation of the drawing]

FIG. 1 of the accompanying drawings is a schematic sectional view showing an example of the structure of a conventional magnetic recording medium, FIG. 2 is a schematic sectional view showing an example of the structure of a magnetic recording medium as an embodiment of the present invention, and FIG. FIG. 3 is a diagram showing the relationship between the thickness of a black-containing magnetic coating and reflectance. 1...Base, 2A...Carbon black, 2B
... Magnetic material, 3 ... Magnetic recording layer, 31 ... Upper layer, 32 ... Lower layer.

Claims (1)

[Claims for Utility Model Registration] (1) A magnetic recording medium characterized by having a magnetic recording layer that contains carbon black in the upper layer but does not contain carbon black in the lower layer. (2) The magnetic recording medium according to claim (1), wherein the upper layer has a thickness of about 3 μm or less. (3) The upper layer is formed of a magnetic paint containing carbon black, and the lower portion is formed of a magnetic paint that does not contain carbon black. The magnetic recording medium described in (2).