JPH0318249B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a magnetic recording medium, and an object thereof is to provide a magnetic recording medium having a magnetic layer with excellent dispersibility of magnetic powder and excellent heat resistance. Magnetic recording media are usually made by applying a magnetic paint consisting of magnetic powder, a binder component, an organic solvent, and other necessary components onto a substrate such as a polyester film, and drying the coating. As the binder component used in this case, it is desired that the binder component has excellent dispersibility of the magnetic powder, can impart excellent electrical properties such as high sensitivity and high signal-to-noise ratio to the magnetic recording medium, and has excellent heat resistance. For this reason, various studies have been conducted on the binder component used in forming the magnetic layer, and for example, a combination of a low molecular weight polyester resin and an isocyanate compound has been proposed. However, in the case of magnetic recording media using these binders, excellent electrical properties cannot be obtained because the dispersibility of the magnetic powder is not good, and magnetic paints prepared using these binders are The fluidity is poor, and this tendency is particularly pronounced when metal magnetic powder is used, making it impossible to form a good magnetic layer and causing problems such as interlayer adhesion during storage at high temperatures. In order to improve this drawback, the inventor conducted various studies on the binder component of magnetic powder, and found that at least isophorone diisocyanate, hydrogenated tolylene diisocyanate, and hydrogenated diphenylmethane diisocyanate were selected as the isocyanate component. A urethane resin with a molecular weight of 10,000 or more containing one type of hydroxyl group at the end and a molecular weight of at least two or more isocyanate groups.
It was discovered that when a binder containing an isocyanate compound of 2000 or less is used as a binder for the magnetic layer, the dispersibility of the magnetic powder is improved and the heat resistance of the magnetic layer is improved, and this invention has been completed. The urethane resin used in this invention is
Isophorone diisocyanate as an isocyanate component

[Formula] Hydrogenated oxygen Range isocyanate

[Formula] or

[Formula] Hydrogenated tolylene diisocyanate Hydrogenated diphenylmethane diisocyanate A suitable urethane resin contains 5 to 30% by weight of at least one of the following, has a hydroxyl group at its terminal, and has a molecular weight of over 10,000, preferably 10,000 to 30,000. Specific examples include Takerakku E-551 manufactured by Takeda Pharmaceutical Co., Ltd., and the like. This type of urethane resin has good solubility in solvents, so it can be used with low paint viscosity, and as a result, it has excellent dispersibility of magnetic powder and good threadability, so it can be used for preparation. This improves the fluidity of the magnetic paint, forms a good magnetic layer, and improves the heat resistance of the magnetic layer. The isocyanate compound used in combination in this invention preferably has at least two or more isocyanate groups and has a molecular weight of 2000 or less, and is a linear bifunctional low molecular weight isocyanate compound having free isocyanate groups at both ends; Usually, a trifunctional low molecular weight isocyanate compound obtained by reacting 1 mol of triol with 3 mol of diisocyanate is preferably used. Specific examples of such bifunctional molecular weight isocyanate compounds include tolylene diisocyanate, diphenylmethane diisocyanate,
Examples include hexamethylene diisocyanate,
Further, specific examples of the trifunctional low molecular weight isocyanate compound include Coronate L manufactured by Nippon Polyurethane Industries, Desmodyur L manufactured by Bayer, and Takenate D102 manufactured by Takeda Pharmaceutical Company. These low-molecular-weight isocyanate compounds contain difunctional or trifunctional isocyanate groups that form the terminal OH of the urethane prepolymer.
Since it reacts with the group and forms a crosslinking bond, the strength of the magnetic layer is increased, its heat resistance is improved, and its flexibility is also improved. In particular, since the urethane resin used in the present invention has a relatively small molecular weight, its crosslinking density is high and this effect is remarkable. Furthermore, when a trifunctional low molecular weight isocyanate compound having a trifunctional isocyanate group is used, the trifunctional isocyanate group is crosslinked with the OH group of the urethane resin in a network-like manner, so that the heat resistance of the magnetic layer is further improved. The blending ratio of the urethane resin and isocyanate compound is preferably in the range of 9:1 to 5:5 by weight; if the amount of urethane prepolymer used is too large, the dispersibility of the magnetic powder will decrease. If there is too little isocyanate compound, heat resistance and abrasion resistance will decrease. Examples of the magnetic powder used in the present invention include γ-Fe ₂ O ₃ powder, Fe ₃ O ₄ powder, Co-containing Fe ₂ O ₃ powder, Co-containing Fe ₃ O ₄ powder, CrO ₂ powder, Fe powder,
Various conventionally known magnetic powders such as metal powders such as Co powder are widely included. Further, as the organic solvent, cyclohexanone, methyl ethyl ketone, tetrahydrofuran, dioxane, dimethylformamide, etc. may be used alone or in combination, or a mixed solvent of these solvents and methyl isobutyl ketone, toluene, etc. may be used. The magnetic recording medium of the present invention can be produced by a conventional method, for example, by applying a magnetic powder, a binder component consisting of the above-mentioned urethane and isocyanate compound, an organic solvent and other materials onto a substrate such as a polyester film. The magnetic paint containing the additives may be applied by any means such as spraying or roll coating, and dried. Next, embodiments of the invention will be described. Example 1 700 parts by weight of γ-Fe ₂ O ₃ powder Takerak E-551 (urethane resin manufactured by Takeda Pharmaceutical Co., Ltd.) 123 〃 Coronate L (manufactured by Nippon Polyurethane Industries, Ltd., trifunctional low molecular weight isocyanate compound) 70 〃 Bengara 14 Parts by weight Higher fatty acid 10 〃 Silicone oil 5 〃 Cyclohexanone 840 〃 Toluene 250 〃 A mixture consisting of this composition was heated in a ball mill to 120%
A magnetic paint was prepared by time mixing and dispersion. This magnetic paint was applied onto a 12 μm thick polyester film to a dry thickness of approximately 5 μm, dried, surface treated, and then cut to a predetermined width to produce a magnetic tape. Example 2 In the composition of the magnetic paint in Example 1, γ
A magnetic tape was made in the same manner as in Example 1, except that the same amount of metallic iron magnetic powder was used in place of the -Fe ₂ O ₃ powder. Comparative Example 1 A magnetic tape was prepared in the same manner as in Example 1, except that in the composition of the magnetic paint in Example 1, the same amount of Desmohen-800 (low molecular weight polyester resin manufactured by Nippon Polyurethane Co., Ltd.) was used in place of Takerakku E-551. I made it. Comparative Example 2 In the composition of the magnetic paint in Comparative Example 1, γ
A magnetic tape was made in the same manner as in Comparative Example 1, except that the same amount of metallic iron magnetic powder was used in place of the -Fe ₂ O ₃ powder. As a result of preparing magnetic paints in each Example and Comparative Example, the dispersibility of the magnetic powder and the fluidity of the magnetic paint were observed, and the cases where the results were good were evaluated as (○) and the cases where they were not good were evaluated as (x). Furthermore, the magnetic tapes obtained in each example and comparative example were measured for square shape Br/Bs and maximum magnetic flux density Bs, and tested for heat resistance and flexibility. In the heat resistance test, each magnetic tape was wound around a glass tube with a tension of 1 kg, left at 45℃ and 80% RH for 24 hours, and then left at room temperature for 24 hours, after which the magnetic tape was unwound from the glass tube. The state of adhesion between the winding layers of the magnetic tape was observed, and the case where almost no adhesion was observed was evaluated as (○), and the case where adhesion was observed was evaluated as (x). In the flexibility test, the damping frequency of the tape was recorded when a twisting force was applied to the tape and this force was removed, and calculations were made from this frequency. The table below shows the results.

[Table] As is clear from the above table, the magnetic paints prepared in Examples 1 and 2 had better fluidity and better dispersibility of magnetic powder than the magnetic paints prepared in Comparative Examples and 2. As a result, compared to the magnetic tape obtained by this invention (Comparative Examples 1 and 2), it has a square shape, a higher maximum magnetic flux density, better heat resistance, and better flexibility. It can be seen that the recording medium has excellent dispersibility of magnetic powder and heat resistance.

Claims (1)

[Claims] 1 As an isocyanate component, isophorone diisocyanate, hydrogenated xylene diisocyanate, hydrogenated tolylene diisocyanate,
A urethane resin containing at least one selected from hydrogenated diphenylmethane diisocyanate and having a hydroxyl group at the end and having a molecular weight of more than 10,000, and an isocyanate compound having at least two or more isocyanate groups and having a molecular weight of 2,000 or less. A magnetic recording medium having a magnetic layer consisting of.