JPS6173234A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve wear resistance in a high-temp. atmosphere and low-temp. atmosphere by incorporating a polyisocyanate compd. and active hydrogen-contg. polymer into both of a primer coating layer and magnetic layer as essential components and using a polymer having a high number average mol.wt. for the magnetic layer. CONSTITUTION:The coated film of the double-layered structure consisting of the nonmagnetic primer coating layer and magnetic layer is formed on one or both surfaces of a flexible base. The binders to be used for the primer coating layer and magnetic layer contain the polyisocyanate compd. and active hydrogen-contg. polymer as the essential components. The active hydrogen-contg. polymer is a polymer having a hydroxyl group, carboxyl group and amino group as the group contg. active hydrogen. The number average mol.wt. of the active hydrogen-contg. polymer to be used for the magnetic layer is made higher than the number average mol.wt. of the active hydrogen-contg. polymer to be used for the primer coating layer. The degree thereof is made such that the ratio of the number average mol.wt. between the finish coating layer and the primer coating layer is made at least >=1.1 times.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnetic disk or other magnetic recording medium having a base with a magnetic layer on at least one surface.

[Prior Art] In this type of magnetic recording medium, since the magnetic layer comes into sliding contact with a magnetic head etc. at high speed, a lubricant such as a higher fatty acid or higher fatty acid ester is contained in the magnetic layer, thereby improving the sliding contact. (Reference unknown) However, in recent years, with the demand for high-density recording, thinning of the magnetic layer has been considered, and in this case, the absolute amount of lubricant contained in the magnetic layer is small (as a result, the wear resistance of the magnetic layer is reduced). There was a problem that a sufficient improvement effect could not be obtained.

In other words, when reducing the thickness of the magnetic layer, if the same amount of lubricant is included in the magnetic layer as was included in a conventional thick magnetic layer, the plasticization of the binder will cause the magnetic layer to become thinner. The cohesive force decreases, which actually impairs wear resistance, and a large amount of lubricant bleeds out onto the surface of the magnetic layer, increasing the spacing between the magnetic head and the magnetic layer, resulting in a decrease in output. Otherwise, the magnetic head and the magnetic layer may stick together. Therefore, the amount of lubricant used must be reduced compared to when the magnetic layer is thick, which causes a reduction in the effect of improving wear resistance.

In order to overcome the above drawbacks, the inventors provided a non-magnetic undercoat layer between the base and the magnetic layer, contained a large amount of lubricant in this undercoat layer, and used the lubricant to fill the voids in the magnetic layer. The lubricant effect can be sustained over a long period of time by gradually oozing and transferring to the surface of the magnetic layer.With this method, it is not necessary to include a large amount of lubricant in the magnetic layer from the beginning, so I learned that even when the magnetic layer is made thinner, there are no problems such as a decrease in the cohesive force of the magnetic layer or a decrease in output.

However, in the inventors' subsequent research, a magnetic recording medium provided with the above-mentioned non-magnetic undercoat layer:
It has been found that although it exhibits good wear resistance at high temperatures of 20°C or higher, there is a problem in that the effect of improving wear resistance cannot be sufficiently obtained in a low temperature atmosphere of, for example, about 5°C.

[Problems to be Solved by the Invention] From the above viewpoint, the present invention provides a magnetic recording medium having a structure in which a non-magnetic undercoat layer containing a large amount of lubricant is provided between the base and the magnetic layer on the low temperature side. The object of the present invention is to solve the problem of reduced wear resistance and to obtain a magnetic recording medium that has a large effect of improving wear resistance not only in high-temperature atmospheres but also in low-temperature atmospheres.

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the inventors first obtained the following knowledge. In other words, whether or not the lubricant in the undercoat layer contributes to improving the wear resistance of the magnetic layer depends on whether the lubricant leaches and transfers well to the surface of the magnetic layer, and this leaching transfer depends on the ambient temperature. Therefore, if the ambient temperature is high, it oozes out onto the surface of the magnetic layer, regardless of the gap structure of the magnetic layer, contributing to improving the wear resistance. However, when the ambient temperature decreases, unless the magnetic layer has a void structure that allows the lubricant to easily penetrate, the above leaching becomes insufficient. In this case, the lubricant stagnates at the interface between the undercoat layer and the magnetic layer, and moreover, As the hardness of the binder in each layer increases, stress concentrates on the interface when the magnetic head runs, making it easier to cause interfacial delamination, and these factors prevent the wear resistance from being sufficiently improved. understood.

Therefore, in order to obtain good wear resistance even in a low-temperature atmosphere, it is essential that the magnetic layer has a void structure that allows the lubricant to easily penetrate. On the other hand, for both the magnetic layer and the undercoat layer, a crosslinking type binder is preferred from the viewpoint of increasing the strength of the coating film, and usually an active hydrogen-containing polymer is used as the base polymer, and a HoIJ ison anate compound is used as the crosslinking agent. It will be done. This crosslinking type binder increases the crosslinking density (this results in a dense coating and increases its strength, but the dense structure impregnates and impregnates the fecal lubricant).

From the above point of view, the inventors constructed both the undercoat layer and the magnetic layer with a crosslinked binder consisting of an active hydrogen-containing polymer and a polyisocyanate compound in order to improve the strength of the coating film. When the molecular weight of the hydrogen-containing polymer is set to be higher than that of a similar polymer in the subbing layer, thereby reducing the crosslinking density of the magnetic layer to be lower than that of the subbing layer, the impregnability of this magnetic layer increases.

This invention was completed after learning that permeability was improved and good results were obtained in improving wear resistance even in a low-temperature atmosphere.

That is, the present invention provides a magnetic recording medium in which a magnetic layer is provided on at least one surface of a base via a non-magnetic undercoat layer containing a lubricant, in which both the undercoat layer and the binder of the magnetic layer are made of polyamide. Relating to a magnetic recording medium containing an isocyanate compound and an active hydrogen-containing polymer as essential components, and containing a polymer having a higher number average molecular weight as the active hydrogen-containing polymer of the magnetic layer than the active hydrogen-containing polymer of the undercoat layer. It is something.

[Structure and operation of the invention] In the present invention, a coating film having a two-layer structure consisting of a non-magnetic undercoat layer and a magnetic layer is formed on one or both sides of a polyester film or other preferably flexible base. The thickness of the magnetic layer is preferably 2F or less, usually up to 0.5/'', and practically about 1p, in order to meet the demands for high-density recording.The thickness of the undercoat layer is: It is preferable to make this layer as thick as possible so that a large amount of lubricant can be contained in this layer, but generally 0.1 of the magnetic layer thickness is used.
It is preferable to increase the amount by at least 0.3 times, preferably from 0.3 times to about 5 times. The total thickness of the undercoat layer and magnetic layer is:
Generally, it is desirable that the thickness be about 1.3 to 3.0 μ.

The non-magnetic undercoat layer may be formed by, for example, applying and drying an undercoat paint made by dissolving or dispersing a binder and a lubricant in an organic solvent on one or both sides of the base, and forming the magnetic layer by: Various conventionally known magnetic paints containing a binder and magnetic powder may be coated and dried on the undercoat layer according to a conventional method. The magnetic paint may contain the same lubricant as the undercoat paint in an amount within a range that does not cause a decrease in the cohesive force of the magnetic layer.

The binder used for the undercoat layer and the magnetic layer both contains a polyisocyanate compound and an active hydrogen-containing polymer as essential components. Typical examples include hydroxyl group-containing vinyl chloride-vinyl acetate copolymers, polyurethane resins, polyester resins, cellulose resins, polyvinyl butyral resins, and polyacetal resins. Further, as the polyisocyanate compound, a low molecular weight compound consisting of a heptamer or an oligomer having at least two isocyanate groups in the molecule is used, and examples of commercially available products include Coronate L manufactured by Nippon Polyurethane Kogyo Co., Ltd.

In this invention, the number average molecular weight of the active hydrogen-containing polymer used in the magnetic layer needs to be higher than the number average molecular weight of the active hydrogen-containing polymer used in the undercoat layer.
As for the extent of this, it is preferable that the ratio of the number average molecular weight of the topcoat layer/undercoat layer is at least 11 times or more, preferably 1.5 times or more. However, if the molecular weight of the polymer in the magnetic layer becomes too high, the crosslinking density of the magnetic layer becomes too low, which is disadvantageous in terms of coating strength. o
o.

It is good that it is within the range of . The number average molecular weight of the polymer in the undercoat layer ranges from io, ooo to 20,000 degrees.

Note that the active hydrogen-containing polymer used for the magnetic layer and the undercoat layer does not need to be one type, and two or more types may be used in combination. In this case, it is necessary that the active hydrogen-containing polymer used in the magnetic layer has a higher molecular weight than that of the above-mentioned polymers in the undercoat layer, which have higher molecular weights. Of course, if two or more types of active hydrogen-containing polymers are used in the magnetic layer, at least one of them satisfies the above relationship, and the other polymers may have the same molecular weight as the polymer with the highest molecular weight in the undercoat layer. However, in some cases, such as when the amount used is small, a polymer having a lower molecular weight than the higher molecular weight polymer of the undercoat layer may be contained to some extent.

The proportion of the active hydrogen-containing polymer and the polyisocyanate compound in the magnetic layer and the undercoat layer is such that the proportion of the polyisocyanate compound in the total amount of the polymer and the compound in each layer is 5 to 40% by weight, preferably 10 to 40% by weight. It is preferable to set the amount of electricity to 30%.

By changing the type and amount of the polyisocyanate compound contained in the magnetic layer and the undercoat layer, for example, by making the amount of the above compound contained in the magnetic layer smaller than that of the undercoat layer, crosslinking of the magnetic layer can be achieved. The density can be lowered even further.

However, by this means alone, that is, by making the molecular weight of the active hydrogen-containing polymer the same between both layers and adopting the above-mentioned means, the impregnation and permeability of the lubricant cannot be sufficiently improved to improve the wear resistance of the magnetic layer. It is difficult to raise it to that level.

As the binder for the magnetic layer and undercoat layer, in addition to the active hydrogen-containing polymer and polyisocyanate compound as described above, other polymers, such as a polymer not containing active hydrogen, may be used in combination, if necessary. The amount of these other polymers used is generally preferably kept to 50% by weight or less of the total amount of the binder.

The magnetic powder included in the magnetic layer includes Ding-Fe203 powder,
Fe50. Any conventionally known magnetic powder such as powder, cobalt-containing iron oxide magnetic powder, iron powder, cobalt powder, oxide magnetic powder such as iron-cobalt alloy powder, or metal or alloy magnetic powder can be used. On the other hand, as the lubricant to be included in the undercoat layer, any conventionally known lubricant that is liquid or semi-solid at room temperature can be used, such as higher fatty acids, higher fat 11 esters, hydrocarbons, silicone oils, fluorine oils, and polyethers.

As mentioned above, the lubricant can be included in the magnetic layer if necessary, but the total amount thereof is usually 2201 ng or more, preferably 250 rng or more and up to 700 rnq per unit surface area (irrl) of the magnetic layer. You can do it like this. Of course, it may be outside the above range.

The above lubricant content is strictly for the undercoat layer and magnetic layer provided on one side of the base. Therefore, if the undercoat layer and magnetic layer are provided on both sides, the amount of lubricant on each side should be as shown above. You just have to make it a value. Further, the magnetic recording medium of the present invention includes a base and a non-magnetic undercoat layer and a magnetic layer provided on at least one surface thereof as essential components, but the base and undercoat layer may be combined as necessary. An appropriate layer such as an adhesive layer may be further interposed therebetween.

〔Effect of the invention〕

As described above, in the present invention, a magnetic layer is coated on at least one surface of the base via a non-magnetic undercoat layer containing a lubricant in order to easily meet recent demands for high-density recording. In the magnetic recording medium provided, the binder of the undercoat layer and the magnetic layer is composed of a specific crosslinked binder, and the molecular weight of the binder polymer is set so that the lubricant in the undercoat layer easily oozes and transfers to the surface of the magnetic layer. Because of this structure, it is possible to avoid the problem of the wear resistance of the magnetic layer decreasing at a low temperature of about 5°C, and to obtain a magnetic recording medium that has a large effect of improving wear resistance not only in a high temperature atmosphere but also in a low temperature atmosphere. can.

[Examples] Examples of the present invention will be described below in comparison with comparative examples. In addition, in the following, "part" and "Mn" respectively mean parts by weight and number average molecular weight.

Example The following undercoat paint was applied to both sides of a polyester film having a thickness of 75 mm to form a pair of undercoat layers having a dry thickness of 2.0 mm on the front and back sides, respectively.

Undercoating paint> Polymer) Polyurethane resin) Coronate product manufactured by Nippon Polyurethane Industries Co., Ltd. 20
Part name: trifunctional low molecular weight incyanate compound) fatty acid ester (oleic acid 10 parts oleylalcole ester) 210 parts ncrohexanone 210 parts methyl ethyl ketone.The following magnetic paint was applied on both of the above undercoat layers. The dry thickness is 1.
A pair of magnetic layers of 07I7+ were formed. Thereafter, it was calendered and punched out into a disk shape to produce a magnetic disk.

Magnetic paint> Co-containing T-Fe203 270 parts VAG
H (see above) 40 parts polyurethane resin) Coronate (see above) 20 parts α-Fe
203 powder 10 parts Methyl isobutyl ketone 500 parts
500 parts Comparative Example The same procedure as in the example was carried out, except that the same amount of Bandex T-5250 (a polyurethane resin having a terminal hydroxyl group of 20,000 as described above) was used in place of Nistan 5702 in the magnetic paint. A magnetic disk was created.

Each of the magnetic disks obtained in the above examples and comparative examples was subjected to the following durability test using a magnetic disk durability tester manufactured by Hitachi Maxell, and the results were as shown in the table below. .

Durability Test> Recording was performed under the conditions of a disc rotation speed of 30, Orpm 1 relative speed of 1 m/sec, and a writing frequency of 125 KHz, and the running time until the playback output became 70% of the initial output was defined as the durability time. The test was conducted at room temperature (18°C) and 5°C.

As is clear from the above results, it can be seen that the magnetic disk of the present invention exhibits excellent durability both in a high temperature atmosphere (room temperature) and in a low temperature atmosphere (5° C.).

Claims (1)

[Claims] (1) In a magnetic recording medium in which a magnetic layer is provided on at least one surface of a base via a nonmagnetic undercoat layer containing a lubricant, the undercoat layer and the binder of the magnetic layer are both polyisocyanate compounds. 1. A magnetic recording medium comprising an active hydrogen-containing polymer as an essential component, and a polymer having a higher number average molecular weight than the active hydrogen-containing polymer of the undercoat layer as the active hydrogen-containing polymer of the magnetic layer.