JPH03113718A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve traveling durability, still durability and electromagnetic conversion characteristics by depositing several magnetic layers on a nonmagnetic supporting body and making the crosslinking rate in the uppermost magnetic layer layer than in any other magnetic layers. CONSTITUTION:The binder used in the uppermost magnetic layer has superior dispersibility compared to binders used in other magnetic layers so that the crosslinking rate in the uppermost layer is made larger than in other magnetic layers. When vinyl chloride-vinyl acetate copolymers are used for the uppermost magnetic layer, the crosslinking rate can be increased by raising the polymn. rate of the copolymers or by using smaller amts. of lubricant and dispersant than those in other magnetic layers so as to decrease the amt. of soluble matter in MEK. Thus, the obtd. medium has improved traveling durability and still durability and as well as electromagnetic conversion characteristics.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a magnetic recording medium, and more specifically, a magnetic recording medium with improved electromagnetic conversion characteristics and improved still durability and running durability. Regarding.

[Prior art 3 and the problem to be solved by the invention] Magnetic recording media useful as recording/reproducing elements in information processing equipment must have advanced and precise electromagnetic conversion characteristics, good runnability, durability, and service life. It is required that the composition has properties that guarantee properties, and in particular, it is required that these properties be maintained in a well-balanced manner.

As electromagnetic conversion characteristics become more sophisticated, attempts have been made to smooth the surface of magnetic recording media in order to maintain the balance of characteristics.

However, if the surface of the magnetic recording medium is made smooth, the coefficient of friction during running increases, so if the strength of the magnetic coating is low, peeling of the magnetic coating will occur. Therefore, when the strength of the magnetic coating is increased, a new problem arises in that the adhesiveness between the support and the magnetic coating decreases.

On the other hand, it is known that fatty acids may be present on the surface of the magnetic layer in order to lower the friction coefficient and improve the running durability of a magnetic recording medium during running.

However, if the amount of fatty acid added is excessive, there is a problem that the fatty acid may seep onto the surface of the magnetic layer, or the coating near the surface may become softer than necessary, leading to peeling of the magnetic coating. Furthermore, since the fatty acids present on the surface of the magnetic layer are gradually lost during the running of the magnetic recording medium, there is also the problem that the coefficient of friction gradually increases and the running may eventually stop.

Taking still durability as an example, it is known that in order to improve still durability, fatty acid ester may be present on the surface of the magnetic layer.

By the way, since fatty acid esters useful for improving still durability have polarity, it is known that they tend to adhere to highly polar ferromagnetic powder in large quantities in the magnetic layer coating solution.

Therefore, in order to increase the presence of fatty acid ester on the surface of the magnetic layer and improve still durability, it is necessary to add a large amount of fatty acid ester to the magnetic layer coating solution.

However, if a large amount of fatty acid ester is added to the magnetic layer coating solution, the coating film becomes softer than necessary, and while still durability improves, running durability, especially running durability under high temperature and high humidity, deteriorates. There is an inconvenience that there is a marked tendency to

Another problem with magnetic recording media in which a large amount of fatty acid ester exists on the surface of the magnetic layer is that when running for a long time, the fatty acid ester present on the surface gradually disappears, making it impossible to maintain the improved still durability. There is also.

The present invention has been made based on the above circumstances.

An object of the present invention is to provide a magnetic recording medium that has improved electromagnetic conversion characteristics and also improved running durability and still durability.

[Means for Solving the Problems] As a result of extensive studies by the present inventors in order to solve the problems described above, the present invention has a plurality of magnetic layers.

It has been found that certain magnetic recording media in which the degree of crosslinking of the topmost magnetic layer is greater than the degree of crosslinking of other magnetic layers have improved electromagnetic conversion characteristics, as well as improved running durability and still durability. With this discovery, we have arrived at the present invention.

The invention according to claim 1 is characterized in that the degree of crosslinking of the uppermost magnetic layer among the plurality of magnetic layers laminated on the nonmagnetic support is greater than the degree of crosslinking of any other magnetic layer. The present invention is a magnetic recording medium, and the invention according to claim 2 is the magnetic recording medium according to claim 1, wherein the degree of crosslinking of each magnetic layer is 60% or more.

The present invention will be explained in detail below.

One Magnetic Layer (1) Layer Structure The magnetic layer in the magnetic recording medium of the present invention is formed by laminating a plurality of magnetic layers on a non-magnetic support.

In the present invention, a back coat layer may be provided on the surface of the non-magnetic support on which the magnetic layer is not provided (back surface) for the purpose of improving the running properties of the magnetic recording medium, preventing electrification, preventing transfer, etc. good.

Furthermore, an intermediate layer (for example, an adhesive layer) may be provided on the surface of the non-magnetic support on which the magnetic layer is provided, for the purpose of improving the adhesion between the magnetic layer and the non-magnetic support.

(2) Top Magnetic Layer One of the important points in the present invention is that the degree of crosslinking of the topmost magnetic layer among the plurality of magnetic layers is greater than the degree of crosslinking of the lower magnetic layer.

This top magnetic layer is then formed by dispersing ferromagnetic powder in a binder.

The coercive force of the top magnetic layer is usually 500 to 2,000 Oe, preferably 600 to 1,700 Oe.

Further, the thickness of the uppermost magnetic layer is usually 0.1 to 2 Lm, preferably 0.3 to 1.01 Lm. If the thickness of the uppermost magnetic layer exceeds the above range, it may adversely affect the five-color output and the audio output.

■-1 Ferromagnetic Powder As the ferromagnetic powder, for example, Go-containing γ-FeJ
a powder, CO-containing FezO4 powder, Co-containing Fed.

(4/3 <x<3/2) Iron oxide magnetic powder such as powder: F
e powder, Ni powder, Go grain powder, Fe-Al1 alloy powder,
Fe-Ni alloy powder, Fe-Al-Ni alloy powder, F
e -Ai-P alloy powder, Fe-Ni-3i-11 alloy powder, Fe-Nt-31-Al-Mn alloy powder, N1
-Goo gold powder, Fe-Mn-Zn alloy powder, Fe-
Examples include ferromagnetic alloy powders such as Ni-Zn alloy powder, Fe-Co-Ni-Cr alloy powder, Fe-Co-N1-P alloy powder, Co-Ni alloy powder, and Go-P alloy powder.

Among these, preferred is Go-containing -Fe20.
It is a powder.

The shape of the ferromagnetic powder is preferably acicular, but may also be spherical or ellipsoidal.

The specific surface area (by BET method) of the ferromagnetic powder is usually 25 m"/g or more, preferably 30 to 80 m"/g, and the coercive force of the ferromagnetic powder is 450 to 2,000 oersted, preferably 550 to 1,7 (10 oersteud).

The content of the ferromagnetic powder in the top magnetic layer is usually 60 to 90% by weight, particularly 70 to 80% by weight.

@-2 Binders and other components Generally speaking, the binders are as follows:
The average molecular weight is about 10. Resins within the range of OQQ to 200,000 can be used.

Specifically, for example, urethane polymer, vinyl chloride
Vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivatives (e.g. cellulose acetate butyrate, cellulose diacetate) ,
cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymers, polyester resins, various synthetic rubber binders, phenolic resins, epoxy resins, urea resins, melamine resins, silicone resins, acrylic reaction resins, high molecular weight polyesters Examples include mixtures of resins and isocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, urea formaldehyde resins, mixtures of low molecular weight glycols and high molecular weight diol compounds, and mixtures thereof.

In the present invention, a resin having a negative functional group can also be preferably used together with these resins or alone.

As the resin having this negative functional group, -3O3M'',
-03O,Ml (In the formula, Ml is a hydrogen atom or an alkali metal, and N2 and N3 are each a hydrogen atom, an alkali metal, or an alkyl group. Also, N2 and M'' are different from each other. (It may be the same, or it may be the same.)

The resin having the negative functional group can be obtained by, for example, modifying a resin such as a vinyl chloride resin, a polyester resin, a polyurethane resin, and introducing the negative functional group.

The amount of negative functional groups in the resin having negative functional groups is
It is desirable that the amount is 0.01 to 1.00 mg/g.

When the amount of negative functional groups is within the above range, the dispersibility of the ferromagnetic powder is improved, and as a result, the output of the magnetic recording medium is increased and the running stability is also improved.

On the other hand, if the content is outside the above range, these effects may not be sufficiently achieved.

For example, when using polyurethane having the above-mentioned negative functional group, its molecular weight is 5, usually 2.000 to 70.000,
Preferably 4. DOG~so, ooo. If this molecular weight exceeds 7 (+, (100), the viscosity of the magnetic coating material becomes unacceptably large, and the object of the present invention may not be achieved.

On the other hand, if the molecular weight is less than 2000, unreacted portions will occur during the step of applying the magnetic paint onto a non-magnetic support and curing it using a curing agent, resulting in low molecular weight components remaining. May deteriorate the physical properties of the film.

When using the resin having the negative functional group, the amount thereof is usually 2 to 2 parts by weight per 100 parts by weight of the ferromagnetic powder.
The amount is 15 parts by weight, preferably 3 to 10 parts by weight.

If this blending ratio is less than 2 parts by weight, the desired effect that should be achieved when the resin having the negative functional group is blended may not be sufficiently achieved.

If the amount exceeds 15 parts by weight, deterioration of sliding noise and head cloudiness may occur.

In the present invention, by using a polyisocyanate curing agent together with the resin having the negative functional group,
It is also possible to improve the durability of the magnetic layer.

Examples of the polyisocyanate curing agent include bifunctional isocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, and hexane diisocyanate; Trifunctional isocyanates (manufactured by Bayer AG), or polyisocyanates that have been conventionally used as curing agents, such as urethane prepolymers containing isocyanate groups at both ends, and polyisocyanates that can also be used as curing agents. You can use any of them.

The amount of curing agent used is usually 5 to 80 parts by weight of the total amount of binder.

In the present invention, in addition to the various binders described above, optional dispersants such as lecithin, phosphoric acid esters, fatty acids, amine compounds, alkyl sulfates, fatty acid amides, higher alcohols, polyethylene oxide, sulfosuccinic acid, and sulfosuccinic acid esters are used. , known surfactants and their salts, negative organic groups (e.g. -〇〇OH,
-PO,) l) Salts of polymer dispersants, etc. can be used.

You can use one of these alone or two! j or more may be used in combination.

Among the dispersants mentioned above, lecithin can be preferably used in the present invention.

Furthermore, in the present invention, a fatty acid ester can be used as an optional plasticizer. Examples of the fatty acid ester include oleyl oleate, oleyl stearate, isocetyl stearate, dioleyl maleate, butyl stearate, butyl palmitate, butyl myristate, octyl myristate, octyl palmitate, amyl stearate, and amyl palmitate. Tate.

Stearyl stearate, lauryl oleate, octyl oleate, isobutyl oleate, ethyl oleate, isotridecyl oleate, 2-ethylhexyl stearate, 2-ethylhexyl myristate, ethyl stearate, 2-ethylhexyl palmitate, isopropyl palmitate , isopropyl myristate, butyl laurate 2cetyl-2-ethyl hexalate, dioleyl adipate, diethyl adipate, diisobutyl adipate.

Examples include diisodecyl adibate. Among these, butyl stearate and butyl palmitate are particularly preferred.

The aforementioned various fatty acid esters may be used alone or in combination of two or more.

By reducing the amount of a dispersant such as lecithin or a plasticizer such as a fatty acid ester as described above, it is possible to improve the running durability of a magnetic recording medium especially under high temperature and high humidity conditions.

The magnetic layer in the magnetic recording medium of the present invention usually contains a lubricant.

Examples of lubricants include fatty acids, silicone lubricants, fatty acid-modified silicone lubricants, fluorine-based lubricants, liquid paraffin, squalane, carbon black, graphite, carbon black graft polymers, molybdenum disulfide, and tungsten disulfide. It will be done.

These may be used alone or in combination of two or more.

In the present invention, fatty acids can be preferably used among the lubricants.

Examples of the fatty acids include caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, valmitic acid, stearic acid, isostearic acid, linolic acid, linoleic acid, oleic acid, elaidic acid, dibehenic acid, malonic acid, and succinic acid. , maleic acid, glutaric acid, adipic acid,
Examples include pimelic acid, azelaic acid, sebacic acid, 1,12-dodecanedicarboxylic acid, and octanedicarboxylic acid.

Among these, particularly preferred are myristic acid, oleic acid, and stearic acid.

The blending ratio of the lubricant is usually 220 parts by weight or less, preferably 10 parts by weight or less, per 100 parts by weight of the ferromagnetic powder. If this blending ratio exceeds 20 parts by weight,
If the amount of lubricant becomes excessive, dirt may easily adhere to the surface of the magnetic layer.

The uppermost magnetic layer may further contain an abrasive and an antistatic agent in addition to the various components described above.

Note that the antistatic agent or the above-mentioned dispersant does not have a sole function; for example, a - compound may act as a lubricant and an antistatic agent.

Therefore, the above-mentioned classifications in this invention indicate the main actions, and the actions of the classified compounds are not limited by the actions shown in the classification.

(2)-3 Degree of Crosslinking In the present invention, as described above, the degree of crosslinking of the uppermost magnetic layer among the plurality of magnetic layers must be greater than the degree of crosslinking of the other magnetic layers.

That is, in the present invention, by making the degree of crosslinking of the uppermost magnetic layer larger than the degree of crosslinking of the other magnetic layers, it is possible to improve dispersibility, improve electromagnetic conversion characteristics, and increase the strength of the two coating films. It is possible to prevent the paint film from peeling off due to repeated running.

Here, the degree of crosslinking of the magnetic layer can be measured, for example, as follows.

■ Prepare a sample by leaving the coating at a temperature of 60°C for 24 hours.

(2) Put the coating film sample Ig into a 100 mjL eggplant flask, add 50 ml of methyl ethyl ketone (MEK), and boil under reflux for 4 hours.

(2) After cooling to room temperature, it is filtered and the resulting filtrate is evaporated to dryness.

(2) Next, wash with a small amount of methyl ethyl ketone, place in a 10 ml eggplant flask weighed in advance, and evaporate to dryness in the same manner as in (2) above, and measure the remaining amount.

■ The degree of crosslinking can be determined by the following formula.

Degree of crosslinking (%) *l: (A); Weight of solid content other than (ferromagnetic powder + A-2030 carbon black) There is no particular restriction as long as the degree of crosslinking is greater than that of the top magnetic layer, but it is usually 60% or more, preferably 70 to 95%. The coating strength of the layer can be maintained at a level that does not cause peeling, and deterioration in running durability can be avoided, especially in high temperature and high humidity environments.

In order to make the degree of crosslinking of the top magnetic layer greater than that of other magnetic layers, for example, a binder for the top magnetic layer that has better dispersibility than binders for other magnetic layers is used. or, if a vinyl chloride-vinyl acetate copolymer is used, increase the degree of polymerization, and furthermore, the lubricant,
The amount of dispersant used may be lower than the amount used in other magnetic layers to reduce the amount of methyl ethyl ketone solubles.Furthermore, the amount of polyfunctional isocyanate used may be greater than the amount used in other magnetic layers. The degree of crosslinking can also be increased by increasing the drying temperature higher than the drying temperature of other magnetic layers to promote crosslinking.

■ Other magnetic layers Other magnetic layers other than the top magnetic layer (hereinafter collectively referred to as the bottom magnetic layer) are made by dispersing ferromagnetic powder in a binder in the same way as the top magnetic layer. It is formed.

The coercive force of the lower magnetic layer is usually 500 to 2.0.
00 oersted, preferably 600 to 1.700 oersted.

Further, the thickness of the lower magnetic layer is usually 1 to 44 m,
Preferably it is 2 to 3.5 gm. If the thickness of the lower magnetic layer exceeds the above range, for example, when the magnetic recording medium of the present invention is made into a video tape, the tape and V
Contact with TR head may worsen.

(1) Ferromagnetic Powder The ferromagnetic powder contained in the lower magnetic layer can be appropriately selected from among the various ferromagnetic powders exemplified for the ferromagnetic powder in the uppermost magnetic layer.

The specific surface fi (according to the BET method) of the ferromagnetic powder is:
Usually 25 m”/g or more, preferably 30 to 80 m27
It is g.

The coercive force of the ferromagnetic powder is between 450 and 2,000 oersted, preferably between 550 and 1,700 oersted.

The content of the ferromagnetic powder in the lower magnetic layer is usually 6
0 to 90% by weight, especially 70 to 85% by weight.

(2) Binder and other components The binder and other optional components used to form the lower magnetic layer are almost the same as those described for the upper@1m magnetic layer.

However, in the present invention, the degree of crosslinking of the lower magnetic layer must be smaller than the degree of crosslinking of the uppermost magnetic layer.

Electromagnetic conversion characteristics can be improved by making the degree of crosslinking of the lower magnetic layer smaller than the degree of crosslinking of the uppermost magnetic layer.

While improving running durability and two-still durability, it is possible to prevent a decrease in adhesiveness between a non-magnetic support and a lower magnetic layer, which will be described later.

In order to make the degree of crosslinking of the lower magnetic layer smaller than that of the uppermost magnetic layer, as described above, a binder with lower dispersibility than that of the uppermost magnetic layer is used, or a binder with vinyl chloride- When a vinyl acetate copolymer is used, the degree of polymerization may be lowered or the amount of the lubricant or dispersion aid used may be greater than that used in the uppermost magnetic layer.

In addition, the degree of crosslinking in the lower magnetic layer can be increased by reducing the amount of polyfunctional isocyanate used in the uppermost magnetic layer or by lowering the drying temperature lower than that in the uppermost magnetic layer. The degree of crosslinking can be made smaller than the degree of crosslinking of the magnetic layer.

In any case, in the present invention, the degree of crosslinking of each magnetic layer below the uppermost magnetic layer is usually 60% or more, preferably 70 to 95%. If the degree of crosslinking of the magnetic layer is less than 60%, the strength of the entire coating film including the uppermost magnetic layer may decrease and the coating film may become brittle, leading to deterioration in running durability, especially in high-temperature environments. Sometimes it happens.

In the present invention, it is preferable that the lower magnetic layer contains a fatty acid and a fatty acid ester. Since the lower magnetic layer contains a suitable amount of fatty acids and fatty acid esters, fatty acids lost from the uppermost magnetic layer due to repeated running, for example, can be replenished from the lower magnetic layer to the uppermost magnetic layer.

Similarly, fatty acid esters lost from the uppermost magnetic layer, for example due to repeating of the Still Mort, can be replenished from the lower magnetic layer to the uppermost magnetic layer.

The content of fatty acid in the lower magnetic layer to achieve such effects is usually 0.5 to 20 parts by weight, particularly 1.0 parts by weight, based on 100 parts by weight of the ferromagnetic powder in the lower magnetic layer.
~5.0 parts by weight.

Similarly, the content of fatty acid ester in the lower magnetic layer is
For 100 parts by weight of ferromagnetic powder in the lower magnetic layer,
Usually from 0.5 to 20 parts by weight, especially from 1.0 to 10 parts by weight.

The lower magnetic layer is laminated on a non-magnetic support which will be described in detail next.

- Nonmagnetic Support - Examples of materials for forming the nonmagnetic support on which the plurality of magnetic layers are laminated include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate; polyolefins such as polypropylene; cellulose triacetate, and Mention may be made of cellulose derivatives such as cellulose diacetate: as well as plastics such as polycarbonate. Furthermore, Cu, Al
Metals such as L, Zn, glass, and various ceramics such as so-called new ceramics (for example, boron nitride, silicon carbide, etc.) can also be used.

There is no particular restriction on the form of the non-magnetic support, and it may be tape-like, sheet-like, card-like, disc-like, drum-like, etc., and various materials can be used depending on the form and if necessary. It can be selected and used.

When the support is in the form of a tape or sheet, the thickness of the support is as follows:
Usually from 3 to 1100 gm, preferably from 5 to 50 gm. In addition, in the case of disk-shaped or card-shaped, usually 30
~1100 IL. Furthermore, in the case of a drum shape, it can be made into a cylindrical shape, etc., depending on the recorder used.

Note that the nonmagnetic support may be subjected to adhesion-facilitating treatment, for example, by a known method.

-Method for manufacturing a magnetic recording medium- Generally speaking, the magnetic recording medium of the present invention is produced by cross-dispersing the ferromagnetic powder, a binder such as polyurethane having a negative functional group, and other magnetic layer forming components in a solvent. After preparing a magnetic paint, the magnetic paint can be applied onto the adhesion-promoting surface of the non-magnetic support and dried.

The solvent used for kneading and dispersing the magnetic layer forming components is as follows:
For example, acetone, methyl ethyl ketone (MEK),
Ketones such as methyl isobutyl ketone (MIBK) and cyclohexanone; alcohols such as methanol, ethanol, propatool and butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, propyl acetate and ethylene glycol monoacetate; Ethers such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran, and dioxane; aromatic hydrocarbons such as benzene, toluene, and xylene; methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, etc. Halogenated hydrocarbons and the like can be used.

Composition of the magnetic paint components When mixing the magnetic paint components, the ferromagnetic powder and other magnetic paint components (hereinafter sometimes referred to as raw materials) are charged into a mixer simultaneously or individually. For example, first, the ferromagnetic powder is added to a solution containing a dispersant, and after kneading for a predetermined time, the remaining components are added, and mixing is continued to obtain a magnetic paint.

Various types of crosstalk devices can be used to disperse crosstalk. Examples of this kneading machine include a two-roll mill, a three-roll mill, a pressure kneader, a continuous kneader, an open kneader, a ball mill, a pebble mill, a side grinder Sqegvari attritor, a high-speed impeller disperser, a high-speed stone mill, and a high-speed impact mill. , Disper Kneader-1 high speed mixer, homogenizer, ultrasonic disperser, etc.

In the present invention, the magnetic layer is formed by applying the coating solution for the lower magnetic layer forming component and the coating solution for the uppermost magnetic layer forming component prepared as described above by a wet-on-wet coating method. A magnetic recording medium may be manufactured by coating the lower magnetic layer-forming component on the adhesion-promoting surface of the non-magnetic support, drying, and then applying the uppermost magnetic layer-forming component. A magnetic recording medium may be manufactured by forming a magnetic layer by applying a liquid onto the lower magnetic layer using a so-called wet-on-dry method.

Coating methods that can be used to apply the lower magnetic layer forming component coating solution and the uppermost magnetic layer forming component coating solution include, for example, gravure roll coating, Meyer bar coating, doctor blade coating, reverse roll coating, and dip coating. coatings, air knife coatings, calendar coatings, squeegee coatings, kiss coatings, extrusion coatings and fantine coatings.

The thickness of the magnetic layer coated in this way is such that the dry thickness of the topmost magnetic layer is 0.1 part 5 lm, particularly 0.1 part 2 lm.
The dry thickness of the lower magnetic layer is usually 1 part 6 m.

After the magnetic layer-forming components have been applied in this way, a magnetic field orientation treatment is performed if necessary in an undried state.

Furthermore, surface smoothing treatment is usually performed using a super calender roll or the like.

Next, a magnetic recording medium can be obtained by cutting into a desired shape.

The magnetic recording medium of the present invention can be used as a magnetic tape such as a video tape or audio tape by cutting it into a long length, or as a two-floppy disk by cutting it into a disk shape. Furthermore, like a normal magnetic recording medium, it can also be used in a card-like, cylindrical, or other form.

[Example] Next, an example of the present invention and a comparative example will be shown to further specifically explain the present invention.

(Example 1) A lower magnetic layer composition and a top magnetic layer set having the compositions shown below! After stirring and mixing each of the above ingredients separately in a ball mill, 5 parts of each polyfunctional isocyanate (manufactured by Nippon Polyurethane Industries ■, "Coronate Shi") were added,
This was filtered through a filter having an average particle size of 1 gm to prepare a paint for the lower magnetic layer and a paint for the uppermost magnetic layer.

Go content - Fe*03 ferromagnetic fine powder 100 parts [specific surface area 43 m''/g (BET value), coercive force 650 oersted] Polyurethane resin [4,4'-diphenylmethane diisocyanate (MDI), 1,4-butanediol (
1,4-BG) and adipic acid (AD8). Glass transition temperature Tg 10℃, molecular weight 30.0001 Vinyl chloride-vinyl acetate copolymer (degree of polymerization 300) Myristic acid Stearate Butyl palmitate Butyl stearate Lecithin Cyclohexanone Methyl ethyl ketone Co content - FetOz Ferromagnetic fine powder [Specific surface area 45 ”/g (BET value), coercive force 850 oersted] Negative functional group-containing polyurethane resin [consisting of 11DT, 1.4-BG, ^D^.

-3O, Na content 0-05 mnol/g, glass transition temperature TgO°C, molecular weight 30.000] Vinyl chloride-vinyl acetate copolymer (degree of polymerization 350) Alumina powder carbon black myristate stearate butyl palmitate cyclohexanone methyl ethyl ketone 0 Part 5 Part 5 Part 3 Part 1 Part 1 Part 1 Part 100 Parts 150 parts The obtained magnetic paint for the lower magnetic layer was coated on a polyethylene terephthalate nonmagnetic support having a thickness of 14 μm to a dry film thickness of 3.5 μm. After coating to an OILm, a magnetic paint for the top magnetic layer was applied to a dry film thickness of 0.5 lm6.Next, a magnetic field orientation treatment was performed, and after further drying at a temperature of 60°C, Supercalendering was carried out at a temperature of 70° C. and a processing pressure of 300 kg/cm2 to form a magnetic layer consisting of two layers, a lower magnetic layer and an uppermost magnetic layer, to obtain a wide magnetic tape film.

The obtained wide magnetic tape film was cut into a 172 inch width to make a video tape.

Various objects were measured for this video tape.

The results are shown in Table 1.

In addition, various characteristics were measured as follows.

(a) RF-output, Lumi-3/N; Using a color video noise meter (manufactured by Sivak Co., Ltd., '925D/IJ), the value (dB) relative to the reference tape was determined using a HR-57000J type deck manufactured by Victor Company of Japan. Ta.

The frequencies of each signal are as follows.

RF-output +6 MHz Lumi-9/N: 6 MHz (b) (C) (d) Squareness ratio: Measurement magnetic field Under the condition of 5 Woe, residual The ratio (Bm/Br) between the magnetic flux density (Br) and the saturation density (am) was determined.

Still durability: The time (minutes) required for the RF specific output to reach -2 dB in still mode was determined using rHR-37000 model deck manufactured by Victor Company of Japan.

Driving durability under high temperature and high humidity: Below conditions of temperature 40℃ and humidity 80% We conducted an evaluation.

(d)-1 Dropout increase; tsILs for a 100% white signal before running for 100 hours, drop alert corresponding to a magnitude of -12 dB or more was measured for 1 minute using a 'HR-37000J deck manufactured by Victor Japan Co., Ltd. The measured value was compared with the value measured for 1 minute before dropout after 100 hours of running, and if it increased, it was marked as ×, and if it did not increase, it was marked as O.

(d)-2 Helad stain; The stain on the head after running for 100 hours using a "rHR-3700" type deck manufactured by Victor Company of Japan was visually evaluated on a four-level scale (first grade).

A: No stains.

B: Some stains.

C: Dirty.

D: Extremely dirty.

(d)-3 Edge damage: After running for 100 hours using a 'HR-37000゜ model deck manufactured by Victor Japan, the tape was visually observed, and those with broken edges were marked with X, and those with no breaks were marked with X. was set as O.

(Example 2) Using a lower magnetic layer composition, an intermediate magnetic layer composition, and an uppermost magnetic layer composition having the compositions shown below, the paint for the lower magnetic layer and the intermediate magnetic layer were prepared in the same manner as in Example 1. A paint for the magnetic layer and a paint for the top magnetic layer were prepared.

Co-containing γ-Fea03 ferromagnetic fine powder 100 parts [specific surface area 40 m''/g (BET value), coercive force 650 oersted] Polyurethane resin 10 parts [MDI
, 1.4-BG, AD^. Glass transition temperature TglO°C1 molecular weight 30.000] Vinyl chloride-vinyl acetate copolymer 15 parts (degree of polymerization 3
00) Myristic acid 2 parts Stearate Butyl palmitate Butyl stearate Lecithin Cyclohexanone Methyl ethyl ketone 2 parts 2 parts 2 parts 100 parts 15 (1 part Go-containing γ-Fe2O2 ferromagnetic fine powder [Specific surface area 43"7g (BET value ), coercive force 650 oersted] Negative functional group-containing polyurethane resin [composed of DI, 1.4-BG, ADA.

-3O3Na content 0.05 mmol/g, glass transition temperature "rgo°C1 molecular weight 30.000] Vinyl chloride-vinyl acetate copolymer (degree of polymerization 350) myristic acid 00 0 5 part part part stearic acid part butyl palmitate Butyl stearate cyclohexanone methyl ethyl ketone 1 part 1 part 100 parts 150 parts Go-containing γ-re, 03 Ferromagnetic fine powder [Specific surface area 49" 7 g (BET value) 5 Coercive force 850 oersted] Negative functional group-containing polyurethane resin [Toluene diisocyanate (TDI), neobentanediol (NPG), and ADA. -3O3Na content 0.05s
1/g, glass transition temperature Tg-5℃, molecular weight 30,00
0] Vinyl chloride-vinyl acetate copolymer (degree of polymerization 350) Alumina powder carbon black myristate butyl stearate palmitate cyclohexanone methyl ethyl ketone 1 part 1 part 100 parts 150 parts The obtained magnetic paint for the lower magnetic layer was mixed into a 14 ILm thick film. After coating the polyethylene terephthalate non-magnetic support to a dry film thickness of 2.51 Lm, the paint for the middle magnetic layer was coated to a dry film thickness of 1.0 parts m. The dry film thickness of the magnetic paint for the magnetic layer is 0.5 gm.
It was applied so that

Thereafter, the same procedure as in Example 1 was carried out.

The results are shown in Table 1.

(Examples 3, 5 and Comparative Examples 2, 3, 5, 7゜8.9.
10) In Example 2, the composition for the lower magnetic layer, the intermediate magnetic layer part ti, and the composition for the top magnetic layer shown in Table 2 were used, and the drying temperatures shown in Table 2 were used. A video tape was prepared in the same manner as in Example 2 except that the temperature was changed, and various properties of this video tape were measured.

The results are shown in Table 1.

(Example 4, 6.7 and Comparative Example 1, 4, 6゜11, 1
2) In Example 1, the compositions for the lower magnetic layer and the composition for the uppermost magnetic layer shown in Table 2 were used, and the drying temperature was changed to the temperature shown in Table 2. A video tape was prepared in the same manner as in Example 1, and various properties of this video tape were measured.

The results are shown in Table 1.

(This page, blank space below) (Evaluation) As is clear from Table 1, the magnetic recording medium of the present invention is 2
Electromagnetic conversion characteristics, running durability, and still durability have all been improved.

Furthermore, magnetic recording media in which the degree of crosslinking of each magnetic layer is less than 60% have extremely poor still durability and running durability.

[Effects of the Invention] According to the present invention, since it has a plurality of magnetic layers, it has excellent electromagnetic conversion characteristics, and the degree of crosslinking of the uppermost magnetic layer among the plurality of magnetic layers is greater than the degree of crosslinking of the other magnetic layers. It is possible to provide an industrially useful magnetic recording medium which has the advantages of improved still durability and running durability and excellent balance of properties.

Claims (2)

[Claims] (1) A magnetic recording medium characterized in that the degree of crosslinking of the uppermost magnetic layer among a plurality of magnetic layers stacked on a nonmagnetic support is greater than the degree of crosslinking of any other magnetic layer. (2) The magnetic recording medium according to claim 1, wherein the degree of crosslinking of each magnetic layer is 60% or more.