JPH05342559A - Magnetic recording medium and its production - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium excellent in magnetostatic characteristics, etc., by incorporating a binder having a residue of a tert. amine and oxycarboxylic acid into a magnetic layer. CONSTITUTION:A magnetic layer 2 is formed on one side of a nonmagnetic substrate 1 and a back coat layer 3 is formed on the other side if necessary. The magnetic layer 2 contains magnetic powder and a binder having a residue of a tert. amine. The magnetic layer 2 further contains oxycarboxylic acid and the amt. of the acid contained in the layer 2 is regulated to 0.7-1.4 times as large as the amt. of the acid adsorbed on the magnetic powder in a said. state. A curing agent having a multifunctional isocyanate group is preferably used for curing the binder. The objective magnetic recording medium excellent in magnetostatic characteristics, video characteristics, durability, coatability, etc., is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium such as a magnetic tape or a magnetic disk and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a coating type magnetic recording medium, a magnetic paint prepared by dispersing and kneading a ferromagnetic powder, a binder, a dispersant, a lubricant and the like in an organic solvent is coated on a non-magnetic support such as a polyester film. Thus, the magnetic layer is formed.

A technique is known in which such a magnetic layer is used in combination with an oxycarboxylic acid and a binder containing a tertiary amine residue (for example, a vinyl chloride copolymer).

That is, regarding a binder containing a tertiary amine residue, the tertiary amine residue causes a curing reaction between the binder molecules (specifically, urethane through a polyfunctional isocyanate as a curing agent). (Crosslinking by chemical reaction). Further, since the oxycarboxylic acid acts as an inhibitor of the above-mentioned curing reaction, it is greatly concerned with the stability (pot life) of the magnetic coating material and the durability of the magnetic coating film.

However, no report has been made on the optimum amount of oxycarboxylic acid added to the magnetic paint. Therefore, depending on the amount added, the curing reaction of the binder may be hindered and durability may be reduced, or the viscosity of the magnetic coating material after the curing agent is added may increase, resulting in uneven coating.

Further, when the surface of the magnetic powder is made basic by using a strong alkali such as NaOH in the process of producing the magnetic powder, the oxycarboxylic acid becomes magnetic when the addition amount of the oxycarboxylic acid is small. The surface of the powder is not well covered. Therefore, the surface of the magnetic powder lacks the affinity for the binder containing the basic tertiary amine residue due to the residual of the basic portion, and the dispersion of the magnetic powder in the binder becomes insufficient. I will end up.

On the contrary, when the amount of oxycarboxylic acid added is large, the surface of the magnetic powder is completely covered (therefore, the surface is transformed into acid), and excess oxycarboxylic acid is present in the magnetic paint. Therefore, this excess oxycarboxylic acid may be directly adsorbed on the tertiary amine residue of the binder, which may worsen the compatibility with the magnetic powder and lead to poor dispersion of the magnetic powder.

[0008]

The objects of the present invention are as follows.
To achieve (1)-(3). (1) To obtain a magnetic recording medium having excellent durability without inhibiting the curing reaction of the binder. (2) It is possible to suppress the increase in viscosity of the magnetic paint after adding the curing agent, and to prevent defects in the coating process of the magnetic layer, such as coating unevenness due to changes in the viscosity of the paint. (3) Dispersion of magnetic powder by increasing the adsorption amount of oxycarboxylic acid to the magnetic powder to improve the dispersibility of the magnetic powder and variation in the saturated adsorption amount of oxycarboxylic acid between the product lots of the magnetic powder. To eliminate gender variations,
To obtain stable media characteristics.

[0009]

That is, according to the present invention, in a magnetic recording medium having a magnetic layer containing magnetic powder and a binder, the binder having a tertiary amine residue and the oxycarboxylic acid have the above-mentioned magnetic properties. The content of the oxycarboxylic acid contained in the magnetic layer is 0.7 to 1.4 times the saturated adsorption amount of oxycarboxylic acid on the magnetic powder, and more preferably, a curing agent having a polyfunctional isocyanate group is used. The present invention relates to a magnetic recording medium, which is used for curing a binder.

Further, the present invention, in the production of a magnetic recording medium having a magnetic layer containing magnetic powder and a binder, contains a binder having a tertiary amine residue and an oxycarboxylic acid. A step of preparing a magnetic coating composition having a carboxylic acid content of 0.7 to 1.4 times the saturated adsorption amount of oxycarboxylic acid on the magnetic powder; and applying the magnetic coating composition as a magnetic coating on a non-magnetic support right before,
And a step of adding a curing agent having a polyfunctional isocyanate group; also provided is a method for producing a magnetic recording medium.

The present inventors have found that a tertiary amine residue (--NR ¹ R
² : R ¹ and R ² may be alkyl groups. As a result of various studies on the content of oxycarboxylic acid added to the magnetic layer in combination with a binder having), the optimum range was 0.7% of the saturated adsorption amount of oxycarboxylic acid on the magnetic powder.
The inventors arrived at the present invention by finding that it was about 1.4 times. That is, it was found that the conventional problems as described above can be solved all at once only by specifying the content of the oxycarboxylic acid to be 0.7 to 1.4 times the saturated adsorption amount.

First, the minimum amount or more (above the saturated adsorption amount)
When the amount of oxycarboxylic acid (0.7 times or more) adequately advances the curing reaction of the binder to obtain a medium with excellent durability, and the surface of the magnetic powder is basic as described above. In addition, since the surface can be coated with a sufficient amount of oxycarboxylic acid, it is considered that the magnetic powder can be sufficiently dispersed in the binder containing the tertiary amine residue.

Further, since the amount of oxycarboxylic acid is suppressed to a necessary amount or less (1.4 times or less of the saturated adsorption amount described above), a medium excellent in durability can be obtained without inhibiting the curing reaction of the binder. It is considered that the viscosity of the magnetic coating material can be suppressed even after the curing agent is added to stabilize the magnetic coating material and the coating defects due to the coating unevenness can be eliminated. Moreover, even when the surface of the magnetic powder is basic, even if the surface is sufficiently covered, excess oxycarboxylic acid does not exist in the paint, and as a result, the tertiary amine of the binder is used. It is considered that the residue is not adsorbed by the oxycarboxylic acid and is better compatible with the magnetic powder that has been acidified by the oxycarboxylic acid, thus improving the dispersibility of the magnetic powder.

In this way, the optimum amount of oxycarboxylic acid improves the dispersibility of the magnetic powder, but even if there is a variation in the saturated adsorption amount of oxycarboxylic acid on the magnetic powder between product lots, this will result in different dispersions. If oxycarboxylic acid is always contained at a fixed ratio (0.7 to 1.4 times) to the saturated adsorption amount, the dispersibility of the magnetic powder is surely improved and the dispersion of the dispersibility is eliminated, and the electromagnetic conversion characteristics are always stable. Can be obtained.

In the present invention, the above-mentioned amount of the oxycarboxylic acid should be 0.7 to 1.4 times the saturated adsorption amount for the above reason, but it is preferably 0.8 to 1.3 times, more preferably 0.9 to 1.1 times.

By the addition of the above-mentioned oxycarboxylic acid, the surface of the metal- or alloy-based magnetic powder can be acidified, and the adsorption of fatty acid to the magnetic powder is hindered. This makes it easier to control the amount of fatty acid present on the medium surface without inhibiting the binding between the resin and the magnetic powder. Therefore, the friction of the medium is reduced, the shearing force due to sliding with the head such as the VTR is reduced, and the abrasion of the head is drastically reduced without lowering the abrasion resistance of the medium. In this case, since vinyl chloride resin as a binder, urethane resin and the like have a neutral hydrophilic group in addition to the above-mentioned tertiary amine residue, this hydrophilic group should also be bonded to the magnetic powder that has been acidified. Therefore, the durability of the magnetic layer is not affected.

In the magnetic recording medium according to the present invention, the binder is a tertiary amine residue (--NR ¹ R ² : R ¹ and R).
^In addition to (alkyl group ² ), it further contains a hydroxyl group (—OH) or the like that causes a curing reaction, but these functional groups may no longer exist after the curing reaction and may have a different form. The same applies to a curing agent having a polyfunctional isocyanate group. Therefore, in the above, the hydroxyl group, the isocyanate group and the like substantially represent the existing form at the raw material stage before curing (the same applies in the following description).

Further, the above-mentioned R ^{1 of the} tertiary amine residue,
R ² is a methyl group, a propyl group, a butyl group or the like, which may be the same or different. The content of this tertiary amine residue is preferably 0.05 to 2 mmol / g, 0.2
~ 0.6 mmol / g is even better.

The binder having such a tertiary amine residue (further, active hydrogen such as hydroxyl group) may be a vinyl chloride copolymer, polyester polyurethane or polycarbonate. Alternatively, a polyester resin, a fibrin resin, a phenoxy resin, or a thermoplastic resin having a specific usage system, a thermosetting resin, a reactive resin, an electron beam irradiation curable resin, or the like may be used in combination.

The above-mentioned oxycarboxylic acid is a compound having in its molecule one or more proton-donating acidic group, --COOH and --OH, respectively. Such an oxycarboxylic acid is a monobasic acid of an aliphatic oxycarboxylic acid,
There are dibasic acids of aliphatic oxycarboxylic acids, lactones of aliphatic oxycarboxylic acids and others.

Illustrating these concretely, the monobasic acid of the fatty acid oxycarboxylic acid is glycolic acid, glyceric acid, lactic acid, 3-hydroxypropionic acid, α-hydroxyisobutyric acid, β-hydroxy-n-butyric acid, α. −
Hydroxy-iso-caproic acid, α-hydroxy-n-
Caprylic acid, α-hydroxy-butyric acid, 2,2-bis-hydroxymethylpropionic acid, oxyvaleric acid, oxylauric acid, oxymyristic acid, oxypalmitic acid,
Examples thereof include oxystearic acid and oxybehenic acid.

Dibasic acids of aliphatic oxyacids include tartaric acid, malic acid, tartronic acid, dioxymyristic acid,
Examples thereof include dioxypalmitic acid, dioxystearic acid, trioxypalmitic acid, and the lactone is L-
Examples thereof include ascorbic acid.

Other oxy acids include citric acid, gallic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, o-hydroxybenzoic acid, parasorbic acid, ricinoleic acid, salicylic acid, oxyphenylacetic acid,
Protocatechuic acid, gentisic acid, α-resorcylic acid,
β-resorcylic acid, γ-resorcylic acid, orseric acid,
Caffeic acid, umbellic acid, 3-oxyphthalic acid and the like can be mentioned.

Since the oxycarboxylic acid used in the present invention does not function as a dispersant or a lubricant by itself, it preferably has 9 or less carbon atoms. However, this is not particularly the case for aromatic oxycarboxylic acids.

In the present invention, the above-mentioned "saturated adsorption amount of oxycarboxylic acid on magnetic powder" can be determined based on the saturated adsorption amount of myristic acid. First, the surface of the magnetic powder was previously treated with myristic acid (one carboxyl group), and myristic acid was adsorbed to the basic points on the surface until it was saturated. The surface-treated magnetic powder thus obtained was treated with methyl ethyl ketone: toluene: cyclohexanone = 5: 3:
The mixture was immersed in a mixed solvent of 2 and the supernatant containing myristic acid released from the magnetic powder was subjected to liquid chromatography. From the adsorption isotherm obtained, the amount of myristic acid (that is, the saturated adsorption amount of myristic acid on the magnetic powder) was determined. I asked.

Then, in order to obtain the saturated adsorption amount of oxycarboxylic acid on the magnetic powder, the theoretical oxycarboxylic acid adsorption amount is the same as myristic acid when the number of carboxyl groups in the oxycarboxylic acid molecule is one. It is assumed that the saturated adsorption amount of myristic acid is equivalent to the saturated adsorption amount of myristic acid. Further, when the number of carboxyl groups in the molecule of oxycarboxylic acid is 2 or more, it is assumed that 1/2 equivalent number of the saturated adsorption amount of myristic acid is the saturated adsorption amount of oxycarboxylic acid.

As a method of interposing the above-mentioned oxycarboxylic acid between the magnetic powder and the binder (that is, containing it in the magnetic layer), a method of adding it when a magnetic layer coating material (magnetic coating material) is prepared, or There is a method of previously surface-treating the magnetic powder with the above-mentioned oxycarboxylic acid.

To form a magnetic layer in the magnetic recording medium of the present invention, for example, magnetic powder is dispersed in a binder, and ethers, esters, ketones, aromatic hydrocarbons, A magnetic paint is prepared by kneading with an organic solvent selected from aliphatic hydrocarbons, chlorinated hydrocarbons and the like, and the magnetic paint is applied to the surface of a non-magnetic support, dried and calendered.

A hardener having a polyfunctional isocyanate group is added as a binder hardener to this magnetic coating immediately before coating, and the addition amount is preferably 5 to 30% by weight based on the total amount of the binder. Examples of such curing agents include isocyanates such as aromatic isocyanates and aliphatic isocyanates.

Examples of aromatic isocyanates include tolylene diisocyanate (TDI) and adducts with these isocyanate active hydrogen compounds, and those having an average molecular weight of 100 to 3,000 are preferable.

Examples of the aliphatic isocyanate include hexamethylene diisocyanate (HMDI) and adducts of these isocyanates with active hydrogen compounds. Among these aliphatic isocyanates and adducts of these isocyanates and active hydrogen compounds, those having a molecular weight of 100 to 3,000 are preferable. Among the aliphatic isocyanates, non-alicyclic isocyanates and adducts of these compounds and active hydrogen compounds are preferable.

As the magnetic powder used in the present invention, any conventionally known magnetic powder can be used. For example, metal magnetic powder can be used, which includes Fe,
Co, Ni, Fe-Co, Fe-Ni, Fe-Co-Ni, Co-Ni, Fe-Co
-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co-V and the like, and further, Al, Si, Ti, Cr, Mn for the purpose of improving these various characteristics. Alternatively, a metal component such as Cu, Cu, or Zn may be added.

It is also possible to use oxide magnetic powder as the magnetic powder, for example, γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Co-deposited γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing Fe ₃ O
₄ , Co-deposited Fe ₃ O ₄ , CrO _{2 and the} like. Also, hexagonal ferrite such as barium ferrite, iron nitride, etc. can be used.

In the magnetic recording medium of the present invention,
In addition to the above binder and magnetic powder, if necessary, a dispersant such as lecithin, a lubricant such as stearic acid, an antistatic agent such as carbon black, an abrasive such as alumina, and a rust preventive are added. Good. These dispersants, lubricants, antistatic agents,
Any conventionally known material can be used as the rust preventive agent, and the rust preventive agent is not limited at all.

The non-magnetic support usable in the present invention includes polyethylene terephthalate and polyethylene-
Polyesters such as 2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate,
Examples include plastics such as polyamide and polycarbonate. Further, metals such as Cu, Al and Zn, glass, boron nitride, ceramics such as Si carbide and the like can also be used.

An intermediate layer or an undercoat layer may be provided on the surface of the non-magnetic support in order to improve the adhesiveness of the magnetic layer.

Further, on the surface of the non-magnetic support opposite to the magnetic layer, a non-magnetic powder (eg silica, carbon black) and a binder (similar to those described above) are used to improve the running property of the medium. Back coat layer consisting of
It can be provided in a thickness of 0.4 to 2.5 μm.

FIG. 1 shows an example (magnetic tape for video) of the magnetic recording medium of the present invention. That is, a magnetic layer 2 containing magnetic powder, a binder and the like is provided on one surface of the non-magnetic support 1. Further, the other surface may have a back coat layer 3 mainly composed of a non-magnetic powder and a binder as indicated by a chain line.

[0039]

EXAMPLES The present invention will be described below with reference to specific examples, but the present invention is not limited to the following examples. In the following examples, "part" means "part by weight".

The following magnetic powders A and B were prepared. Magnetic powder A: iron oxide type (Co-γ-Fe ₂ O ₃ ) specific surface area 30.0m ² / g myristic acid saturated adsorption amount 0.51mg / m ² theoretical citric acid saturated adsorption amount 0.64 parts / magnetic powder 100 parts magnetic powder B : Iron oxide-based specific surface area 30.0m ² / g Myristic acid saturated adsorption amount 0.70mg / m ² Theoretical citric acid saturated adsorption amount 0.88 parts / Magnetic powder 100 parts

Regarding the above-mentioned "saturated adsorption amount of myristic acid", the surface of the magnetic powder was previously treated with myristic acid, and the myristic acid was adsorbed to the basic points on the surface until it was saturated. The surface-treated magnetic powder thus obtained was treated with methyl ethyl ketone: toluene: cyclohexanone = 5: 3:
The mixture was dipped in a mixed solvent of 2, and the supernatant containing myristic acid released from the magnetic powder was subjected to liquid chromatography, and the adsorption isotherm was used to determine the amount of myristic acid (that is, the saturated adsorption amount of myristic acid on the magnetic powder). I asked.

Although citric acid was used as the oxycarboxylic acid, the above-mentioned "citric acid saturated adsorption amount" is as follows.
It was assumed that 1/2 equivalent number of the theoretical adsorption amount of myristic acid was the saturated adsorption amount of citric acid.

Example 1 The following composition was prepared. Magnetic powder A 100 parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer 12.5 parts (copolymerization ratio 84: 13: 3, -NR ₂ group amount 0.2 mmol / g) Polyester polyurethane 12.5 parts Citric acid 0.64 parts (theoretical saturated adsorption) 1.0 times the amount) myristic acid 0.5 parts n-heptyl stearate 1.5 parts α-Al ₂ O ₃ 10.0 parts carbon black 5.0 parts methyl ethyl ketone 100 parts toluene 100 parts cyclohexanone 50.0 parts

The above composition was mixed and dispersed in a ball mill for 48 hours, 15 parts of a trifunctional isocyanate compound (Coronate L-50EBT manufactured by Nippon Polyurethane) was added, and the mixture was dispersed in an agitator for 15 minutes and then passed through a filter having a pore size of 5 μm. Then, a magnetic paint was prepared. This magnetic paint has a thickness of 20 μm
A polyester terephthalate film with a dry thickness of 5.
A magnetic layer was formed by coating and drying so as to have a thickness of 5 μm, and was mirror-finished by calendering, and then cut into a predetermined width to obtain a video tape.

Example 2 In Example 1, magnetic powder B was used in place of magnetic powder A,
A video tape was produced in the same manner as in Example 1 except that the amount of citric acid was changed to 0.88 part (1.0 times the theoretical saturated adsorption amount).

Example 3 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.448 parts (0.7 times the theoretical saturated adsorption amount).

Example 4 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 0.616 parts (0.7 times the theoretical saturated adsorption amount).

Example 5 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.512 parts (0.8 times the theoretical saturated adsorption amount).

Example 6 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 0.704 parts (0.8 times the theoretical saturated adsorption amount).

Example 7 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.576 parts (0.9 times the theoretical saturated adsorption amount).

Example 8 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 0.792 part (0.9 times the theoretical saturated adsorption amount).

Example 9 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.768 part (1.2 times the theoretical saturated adsorption amount).

Example 10 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 1.056 parts (1.2 times the theoretical saturated adsorption amount).

Example 11 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.896 parts (1.4 times the theoretical saturated adsorption amount).

Example 12 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 1.232 parts (1.4 times the theoretical saturated adsorption amount).

Example 13 A video tape was prepared in the same manner as in Example 1 except that lactic acid was used as the oxycarboxylic acid and the theoretical saturated adsorption amount of 0.6 part / 100 parts of magnetic powder was added. It was made.

Example 14 In Example 13, 0.42 parts of lactic acid (of theoretical saturated adsorption amount)
A videotape was produced in the same manner as in Example 13 except that 0.7 times the amount was added.

Example 15 In Example 13, 0.54 parts of lactic acid (of theoretical saturated adsorption amount) was used.
A video tape was produced in the same manner as in Example 13 except that the addition amount was 0.9 times.

Example 16 In Example 13, 0.72 parts of lactic acid (of theoretical saturated adsorption amount) was used.
A videotape was produced in the same manner as in Example 13 except that the addition amount was 1.2 times.

Example 17 In Example 13, 0.84 parts of lactic acid (of theoretical saturated adsorption amount) was used.
A videotape was produced in the same manner as in Example 13 except that 1.4 times was added.

Comparative Example 1 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 0.25 part (0.39 times the theoretical saturated adsorption amount).

Comparative Example 2 A video tape was produced in the same manner as in Example 1 except that the amount of citric acid in Example 1 was changed to 1.25 parts (1.95 times the theoretical saturated adsorption amount).

Comparative Example 3 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 0.5 part (0.57 times the theoretical saturated adsorption amount).

Comparative Example 4 A video tape was produced in the same manner as in Example 2 except that the amount of citric acid in Example 2 was changed to 1.5 parts (1.7 times the theoretical saturated adsorption amount).

Comparative Example 5 In Example 13, the amount of lactic acid was 0.3 part (the theoretical saturated adsorption amount).
A video tape was produced in the same manner as in Example 13 except that the ratio was changed to 0.5 times.

Comparative Example 6 The amount of lactic acid in Example 13 was 1.0 part (the theoretical saturated adsorption amount of 1.
A video tape was produced in the same manner as in Example 13 except that the video tape was changed to 67 times.

The following measurements were carried out on the video tapes of the above examples, and the results are shown in FIGS. Viscosity and gloss: The viscosity and gloss (incident angle: 45 °) of the coating material were measured before, after 60 minutes, after 120 minutes, and after 180 minutes, before and after addition of the curing agent L-50.

The following measurements were performed on the video tapes of the above examples, and the results are shown in Table 1 below. Magnetostatic property: Using VSM (manufactured by Toa Kogyo Co., Ltd.), the maximum magnetic flux density (Bm) and the squareness ratio (Br / Bm) were measured.

Electromagnetic conversion characteristics and durability: C-format VTR: Sony BVH-2000 electromagnetic conversion characteristics (R
F-out, YS / N) and Still durability were measured. Electromagnetic conversion characteristics are values for Sony Reference, and durability is 5
In still measurement under the environment of ℃ / 15% RH, it was evaluated by the time until the output decreased by 6 dB from the initial output.

[0070]

Further, Table 2 below shows Coronate L-50EB.
The coating surface state and the audio sensitivity unevenness (1 KHz, 0 dB input) when the coating material is coated 2 to 3 hours after T is added are shown. The coating surface state means ◯: very good with no coating unevenness, Δ: good with little coating unevenness, ▲: with coating unevenness and slightly defective, ×: bad with many coating unevenness.

[0072]

The relationship between the addition amount of citric acid and the dispersibility is shown in FIG. 2 for each of the magnetic powders A and B, and the stability of the paint after addition of the curing agent Coronate L-50EBT is shown in FIG. It is shown in FIG. These results and Table-1 and Table-2
From the result of, the following is clear.

(1) From the comparison between Examples 1 to 12 and Comparative Examples 1 to 4, as long as citric acid was added in the range of 0.7 to 1.4 times the theoretical citric acid saturated adsorption amount, Table 1 shows that the magnetostatic characteristics It can be seen that the video characteristics and durability all have excellent characteristics.

(2) When the amount of citric acid added exceeds 1.4 times the theoretical saturated adsorption amount, as shown in Comparative Examples 2 and 4, the magnetostatic characteristics and the video characteristics are considerably good. This is because the dispersibility is quite good as seen in FIGS. However, the durability of magnetic powder A is 1.4 times higher than that of the theoretical saturated adsorption amount of citric acid.
It decreases to about 56% at 1.95 times the theoretical saturated adsorption amount of citric acid.
With the magnetic powder B, it decreases to about 73% with the addition amount of 1.7 times. Also, as seen in FIGS. 4 and 6, the addition of excess citric acid
It can be seen that the curing reaction (in the case of FIGS. 4 and 6, the increase in viscosity after addition of L-50 can be read as the progress of the crosslinking reaction in the paint) is suppressed.

(3) As shown in FIGS. 4 and 6, if the amount of addition is less than 0.7 times the theoretical adsorption amount of citric acid, the viscosity rises too much, so that coating unevenness is likely to occur as shown in Table 2. ,
As a result, the audio sensitivity unevenness is large.

(4) From the comparison between Examples 13 to 17 and Comparative Examples 5 to 6, as long as lactic acid was added in the range of 0.7 to 1.4 times the theoretical adsorption amount of saturated lactic acid, it was confirmed from Tables 1 and 2 that Magnetic properties,
It can be seen that the video characteristics, durability, and coating properties (audio sensitivity unevenness) all exhibit excellent characteristics.

Examples 18 to 23 Polyester polyurethanes having a tertiary amine residue (-NR ₂ ) were used in Examples 1, 3, 5, 5, 7, 9 and 11 to prepare vinyl chloride copolymers. A corresponding video tape was prepared in the same manner as above except that no tertiary amine residue was used.

The performance of each of these video tapes was good as shown in Table 3 below.

[0080]

[0081]

As described above, according to the present invention, a binder having a tertiary amine residue and oxycarboxylic acid are contained in the magnetic layer, and the saturated adsorption amount of oxycarboxylic acid to the magnetic powder is within the range of 0.7 to 1.4 times. Since oxycarboxylic acid is added, all of the static magnetic properties, video characteristics, durability, and coating properties are excellent.

On the other hand, if the amount of oxycarboxylic acid added to the magnetic powder is too small, sufficient characteristics cannot be obtained,
Further, uneven coating is likely to occur due to increase in viscosity after addition of the curing agent. On the other hand, if the amount of oxycarboxylic acid added to the magnetic powder is too large, not only sufficient characteristics cannot be obtained, but also the curing reaction due to the acid is strongly suppressed, and sufficient durability cannot be obtained.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing one structural example of a magnetic recording medium to which the present invention is applied.

2A and 2B are graphs showing the relationship between the amount of citric acid added to various magnetic powders and dispersibility.

FIG. 3 is a graph showing the relationship between the amount of citric acid added to magnetic powder and the gloss change after addition of a curing agent.

FIG. 4 is a graph showing the relationship between the amount of citric acid added to the magnetic powder and the change in viscosity after addition of a curing agent.

FIG. 5 is a graph showing the relationship between the amount of citric acid added to another magnetic powder and the gloss change after addition of a curing agent.

FIG. 6 is a graph showing the relationship between the amount of citric acid added to the magnetic powder and the change in viscosity after addition of a curing agent.

[Explanation of symbols]

 1 ... Non-magnetic support 2 ... Magnetic layer 3 ... Back coat layer

Claims (3)

[Claims] 1. A magnetic recording medium having a magnetic layer containing magnetic powder and a binder, wherein the binder having a tertiary amine residue and oxycarboxylic acid are contained in the magnetic layer.
The content of the oxycarboxylic acid in the magnetic layer is 0.7 to 1.
Magnetic recording medium characterized by 4 times. 2. The magnetic recording medium according to claim 1, wherein a curing agent having a polyfunctional isocyanate group is used for curing the binder. 3. A magnetic recording medium having a magnetic layer containing magnetic powder and a binder, which comprises a binder having a tertiary amine residue and an oxycarboxylic acid. A step of preparing a magnetic coating composition whose amount is 0.7 to 1.4 times the saturated adsorption amount of oxycarboxylic acid on the magnetic powder; A step of adding a curing agent having a functional isocyanate group;