JPH0612654A - Binder for magnetic recording medium and magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve dispersibility and to obtain good electromagnetic conversion characteristic by using urethane having sulfonic acid groups or carboxyl groups bonded to urethane groups as polyurethane. CONSTITUTION:This binder for a magnetic recording medium is used for a binder and a magnetic recording medium to produce a magnetic recording medium and it consists of polyurethane which is the reaction product of polyols and polyisocyanates as the main source material. Sulfoalkyl groups or carboxyalkyl groups are bonded to nitrogen atoms of the polyurethane bond in the polyurethane. The obtd. polyurethane has a structure formula expressed by the formula. In the formula, X is SO3M or COOM, wherein M is H, alkali metal, alkaline earth metal, or ammonium group, R is an alkylene group. It is preferable that the carbon number of the alkylene group is 2 to 4 and M is sodium.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a binder for a magnetic recording medium, specifically a binder for a magnetic layer of a magnetic recording medium, a binder for a non-magnetic layer (intermediate layer), a binder for a back layer, and a protection. The present invention relates to a binder suitable as a layer binder and a magnetic recording medium using the same.

[0002]

2. Description of the Related Art Magnetic recording media are widely used as recording tapes, video tapes, floppy disks and the like. In a magnetic recording medium, a magnetic layer in which ferromagnetic powder is dispersed in a binder (binder) is laminated on a non-magnetic support. The magnetic recording medium is required to be at a high level in various characteristics such as electromagnetic conversion characteristics, running durability and running performance. That is, in audio tapes for recording and reproducing music, a higher level of original sound reproduction capability is required. Further, the video tape is required to have excellent electromagnetic conversion characteristics such as excellent original image reproduction capability. In addition to having such excellent electromagnetic conversion characteristics, the magnetic recording medium is required to have good running durability as described above. Then, in order to obtain good running durability, an abrasive and a lubricant are usually added to the magnetic layer.

However, in order to obtain excellent running durability with the abrasive, it is necessary to increase the addition amount to some extent, which reduces the content of the ferromagnetic powder.
Further, when an abrasive having a large particle size is used in order to obtain excellent running durability, the abrasive tends to excessively project on the surface of the magnetic layer. Therefore, the improvement of running durability by the abrasive often causes the above-mentioned deterioration of the electromagnetic conversion characteristics, which is a problem. When the running durability is improved by the lubricant, it is necessary to increase the addition amount of the lubricant, so that the binder is likely to be plasticized and the durability of the magnetic layer tends to decrease.

Further, in order to improve the durability and electromagnetic conversion characteristics, the binder, which is one of the main components of the magnetic layer, naturally also plays an important role. Conventionally used vinyl chloride-based resins, cellulose-based resins, urethane-based resins, acrylic-based resins and the like have a problem that the wear resistance of the magnetic layer is poor and the running system members of the magnetic tape are contaminated. As a method of improving such a problem, a method of increasing the hardness of the magnetic layer by using a hard binder is used. However, when the hardness of the magnetic layer is increased, the brittleness of the magnetic layer becomes remarkable, and there are problems that dropout occurs due to contact with the magnetic head and that the still characteristics are deteriorated.

Further, the fine magnetic powder to increase the magnetic force of the ferromagnetic powder or increase the packing density of the ferromagnetic powder brings about an increase in the cohesive force due to an increase in the magnetic force or the specific surface area. There is a problem that the dispersion is lowered and sufficient magnetic properties and surface properties cannot be obtained.

In a magnetic recording medium having a non-magnetic layer (intermediate layer) between a magnetic layer and a non-magnetic support, a bond conventionally used as a binder for dispersing an abrasive or carbon black. With the agent, the dispersibility of the abrasive and carbon black was poor, and good surface properties of the magnetic layer located on the smooth upper layer could not be obtained.

[0007]

Therefore, as a binder for a magnetic recording medium made of polyurethane having excellent dispersibility, it is preferable to use a polyurethane binder having --SO ₃ M using polyester polyol as a polyol. 58-41565 and the use of a polyurethane resin containing sulfobetaine as a binder for a magnetic recording medium is described in JP-A-2-56719. However, these materials have a low solubility of the polar group-containing raw material in a solvent, so that the introduction amount of the polar group is limited and the dispersibility is insufficient.

Further, Japanese Patent Laid-Open Publication No. 3-203811 discloses that a tertiary amino group-containing polyurethane is reacted with sultone to modify the tertiary amino group into a sulfobetaine group to be used as a binder for a magnetic recording medium. Although described, the dispersibility was insufficient because the tertiary amino group was contained in the chain extender. The present invention has extremely high dispersibility,
An object of the present invention is to provide a magnetic recording medium having excellent electromagnetic conversion characteristics and excellent durability, and a binder for magnetic recording medium.

[0009]

A magnetic layer having ferromagnetic fine powder and a binder dispersed therein, or a nonmagnetic layer or back layer having nonmagnetic powder and a binder dispersed therein is provided on a nonmagnetic support. In the binder and the magnetic recording medium for producing the magnetic recording medium, the polyurethane of the binder for the magnetic recording medium, which is the reaction product of polyol and polyisocyanate as the main raw materials, is polyurethane-bonded. A binder for a magnetic recording medium, which is a polyurethane having a sulfoalkyl group or a carboxyalkyl group bonded to a nitrogen atom, wherein the polyurethane having the sulfoalkyl group or the carboxyalkyl group bonded to the nitrogen atom of the polyurethane bond is as follows: A magnetic recording medium having the structural formula

[0010]

[Chemical 2]

However, X is SO ₃ M or COOM,
M is H, an alkali metal, an alkaline earth metal or an ammonium group, and R is an alkylene group. The alkylene group preferably has 2 to 4 carbon atoms and M is sodium.

In the present invention, the binder for the magnetic recording medium is used because it can be used in any of the magnetic layer, nonmagnetic layer (intermediate layer), back layer and protective layer of the magnetic recording medium. is there. In particular, since it has polyurethane in which a sulfoalkyl group or a carboxyalkyl group is bonded to the nitrogen atom of the polyurethane bond and has excellent dispersibility of the ferromagnetic powder,
It is effective when used in the magnetic layer.

Further, in a magnetic recording medium in which a magnetic layer containing a ferromagnetic powder and a binder is formed on at least one surface of a non-magnetic support, the binder is a sulfoalkyl group or a carboxyalkyl group at the nitrogen atom of the polyurethane bond. A magnetic recording medium comprising a group-bonded polyurethane, further comprising a lower magnetic layer or a lower non-magnetic layer provided on at least one surface of a non-magnetic support, and an upper magnetic layer provided thereon. At least one of the magnetic layer or the non-magnetic layer is a magnetic recording medium containing a polyurethane in which a binder for binding the ferromagnetic powder or the non-magnetic powder has a sulfoalkyl group or a carboalkyl group bonded to the nitrogen atom of the polyurethane bond. is there.

That is, in the binder of the present invention, after forming polyurethane, N of urethane bond --NHCOO-- is formed.
Since the sulfoalkyl group or the carboxyalkyl group is introduced into the H portion, the hydrophilic group having a further excellent hydrophilicity is bonded to the hydrophilic group portion of the urethane bond, which has an extremely large hydrophilic effect. On the other hand, in the conventional polyurethane having a sulfonic acid group, since the sulfonic acid group is bonded to the lipophilic carbon chain portion, the hydrophilicity is not sufficiently increased.

The hydrophilic group-containing polyurethane of the present invention is prepared by adding a solution of sodium hydride (NaH) in dimethylformamide (DMF) to a solution in which polyurethane is dissolved, and sulfonation with propane sultone or butane sultone. You can Further, an arbitrary amount of sulfonic acid or carboxylic acid can be introduced by arbitrarily selecting the amount of propane sultone or butane sultone to react.

The content of the polar group in the binder of the present invention is preferably 1 × 10 ⁻⁶ to 1 × 10 ⁻³ eq / g, and more than 1 × 10 ⁻³ eq / g. When it is less than 1 × 10 ⁻⁶ eq / g, the effect of improving the dispersibility is small.

The urethane group concentration is 1 × 10 ^-4 to 1
X10 ^-2 eq / g is preferable and 1 x 10 ^-4 e
When it is more than q / g, the solubility in the solvent is low and the viscosity is high, and as a result, the dispersibility is lowered. If it is too small, the mechanical strength of the coating film is weak and the durability is reduced. Urethane having a hydrophilic group such as a sulfonic acid group in a urethane bond can be produced from a raw material containing a polyol and a polyisocyanate as main raw materials and various additives such as a chain extender, and an aliphatic polyol is , Preferably a molecular weight of 500-500
It is a polyol having an OH group at the end of the hydrocarbon chain of 0, and the OH group may be at a position other than the end of the carbon chain. Also,
The preferable range of the molecular weight is 800 to 4000. When the molecular weight is small, the dispersibility is not preferable, and when the molecular weight is too large, the solubility in a solvent is lowered and the dispersibility is lowered. The carbon chain may have a linear or branched structure, and the carbon chain may have a saturated or unsaturated structure. As the polyol, polyester polyol, polyether polyol, polycarbonate polyol, acrylic polyol, polyolefin polyol, polybutadiene polyol, or a mixture thereof can be used.

When an unsaturated polyolefin polyol or polybutadiene polyol is used as the aliphatic polyol, it is preferable that the unsaturated double bond is small.

The content of the polyolefin polyol in the polyurethane is preferably 20 to 90% by weight, and when the content is low, the dispersibility is unfavorable and the durability effect is small. On the other hand, if the amount is too large, the glass transition temperature (Tg) decreases,
It becomes difficult to adjust the physical properties of the resulting composition. The weight average molecular weight of polyurethane is preferably from 1 to 150,000, and if it is higher than this, the viscosity is high and the dispersibility is lowered.
If it is smaller than this, the mechanical strength is low and the durability deteriorates.

Further, the polyisocyanate contains MDI.
(4,4'-diphenylmethane diisocyanate), T
Aromatic polyisocyanates such as DI (tolylene diisocyanate) and XDI (xylylene diisocyanate) are preferable, but aliphatic or alicyclic ones may also be used. Since the polyolefin polyol segment is soft, the physical properties of the aromatic diisocyanate can be adjusted more easily.

Additives such as a chain extender may be added in addition to the polyol and polyisocyanate. The chain extender may be ethylene glycol, 1,4-butanediol,
Glycols such as 2,3-butanediol, bisphenol A, dihydroxyethyl ether and hydroquinone dihydroxyethyl ether, and diamines such as diphenylmethanediamine and m-phenylenediamine can be used. The terminal of the molecule of the polyurethane is preferably an OH group terminal. The OH group reacts with the isocyanate curing agent in the magnetic layer to crosslink to strengthen the magnetic layer coating film. Further, the glass transition temperature (Tg) of polyurethane is -20 to +5.
0 ° C is preferred. When the glass transition temperature is low, the magnetic coating film before curing has a low blocking property, and when it is high, it becomes brittle, which is not preferable in terms of durability.

When the binder of the present invention is used in the magnetic layer, a vinyl chloride-based synthetic resin may be used in combination with the polyurethane of the present invention. The polymerization degree of the vinyl chloride resin that can be used in combination is preferably 200 to 600, and 250 to 45.
0 is particularly preferred. The vinyl chloride resin may be a copolymer of vinyl monomers such as vinyl acetate, vinyl alcohol, vinylidene chloride and acrylonitrile.

In addition to polyurethane and vinyl chloride resin, various synthetic resins can be used for forming each magnetic layer. Examples thereof include ethylene / vinyl acetate copolymers, cellulose derivatives such as nitrocellulose resins, acrylic resins, polyvinyl acetal resins, polyvinyl butyral resins, epoxy resins, and phenoxy resins. These can be used alone or in combination.

When other synthetic resin is used in combination, the urethane contained in the magnetic layer is preferably contained in the binder in an amount of 10% by weight or more, more preferably 20% by weight or more. . The vinyl chloride resin is preferably contained in the binder in an amount of 80% by weight or less, more preferably 70% by weight or less.

Further, a curing agent such as a polyisocyanate compound can be used together with the binder of the present invention.
Examples of the polyisocyanate compound include a reaction product of 3 mol of tolylene diisocyanate and 1 mol of trimethylolpropane (eg, Desmodur L-75 (manufactured by Bayer)), xylylene diisocyanate or diisocyanate 3 such as hexamethylene diisocyanate. Mole reaction product of 1 mole of trimethylolpropane, buret addition compound with 3 moles of hexamethylene diisocyanate, isocyanurate compound of 5 moles of tolylene diisocyanate, isocyanurate addition of 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate Mention may be made of the compounds, polymers of isophorone diisocyanate and diphenylmethane diisocyanate. The polyisocyanate compound contained in the magnetic layer is preferably contained in the binder in an amount of 10 to 50% by weight, more preferably 20 to 20% by weight.

Further, when the curing treatment is carried out by electron beam irradiation, a compound having a reactive double bond (eg urethane acrylate) can be used. The total weight of the resin component and the curing agent (that is, the binder) is preferably in the range of usually 15 to 40 parts by weight, more preferably 20 to 30 parts by weight, based on 100 parts by weight of the ferromagnetic powder. Is. The ferromagnetic powder used in the magnetic recording medium of the present invention is a ferromagnetic iron oxide, a cobalt-containing ferromagnetic iron oxide or a ferromagnetic alloy powder having an S _BET specific surface area of 40 m ² / g or more (preferably 50 m ² / g or more). ), The crystallite size is 35
nm or less, preferably 25 nm or less. As the ferromagnetic powder, Fe, Ni, Fe-Co, Fe-Ni, Co-
Ni, Co-Ni-Fe, and the like are included, and a metal component of 20
Within the range of not more than wt%, aluminum, silicon, sulfur,
Scandium, titanium, vanadium, chromium, manganese, copper, zinc, yttrium, molybdenum, rhodium,
Palladium, gold, tin, antimony, boron, barium,
Examples thereof include alloys containing tantalum, tungsten, rhenium, gold, mercury, lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, and bismuth. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide.

The methods for producing these ferromagnetic powders are already known, and the ferromagnetic powders used in the present invention can also be produced according to known methods. The shape of the ferromagnetic powder is not particularly limited, but needle-shaped particles, granular particles, dice-shaped particles, rice particles, and plate-shaped particles are usually used. In particular, it is preferable to use needle-shaped ferromagnetic powder.

The above resin component, hardener and ferromagnetic powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone, ethyl acetate or the like which is usually used in the preparation of magnetic paint to obtain a magnetic paint. Kneading and dispersion can be performed according to a usual method. In addition to the above components, the magnetic paint contains α-Al ₂ O ₃ , Cr ₂
Abrasives such as O ₃ , antistatic agents such as carbon black, lubricants such as fatty acids, fatty acid esters, silicone oils,
It may contain a commonly used additive or filler such as a dispersant. A magnetic coating material prepared from the above materials is applied onto a non-magnetic support to form a magnetic layer.

In the method for producing the magnetic recording medium of the present invention, for example, the coating solution for the magnetic layer is preferably applied to the surface of the non-magnetic support under running, and the layer thickness after the magnetic layer is dried is within the range of 0.5 to 10 μm. , And more preferably 1.5 to 7.0 μm. Here, a plurality of magnetic paints may be sequentially or simultaneously applied in multiple layers. Multi-layer magnetic layer or lower non-magnetic layer,
Regarding the composition, physical properties, materials, manufacturing method, etc. of each layer of a magnetic recording medium having two or more layers such as an upper magnetic layer, those described in JP-A-2-240824, Japanese Patent Application No. 4-21782, etc. Can be used. As a coating machine for coating the above magnetic paint, an air doctor coat, a blade coat, a rod coat, an extrusion coat, an air knife coat, a squeeze coat, an impregnation coat, a reverse roll coat, a transfer roll coat, a gravure cord, a kiss coat, a cast coat, a spray coat. A coat or spin coat can be used. For these coating methods, refer to "Coating Engineering" Asakura Shoten (1972) or "Progress of Coating Technology" General Technology Center Co., Ltd. (19
1988).

A back layer (backing layer) may be provided on the surface of the non-magnetic support used in the present invention which is not coated with the magnetic coating material. Usually, the back layer is provided by applying a back layer forming coating in which a granular component such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent on the surface of the non-magnetic support which is not coated with the magnetic coating. It is a layer. An adhesive layer may be provided on the surface of the non-magnetic support on which the magnetic paint and the back layer forming paint are applied.

The applied coating layer of the magnetic coating material is dried after the ferromagnetic powder contained in the coating layer of the magnetic coating material is subjected to the magnetic field orientation treatment. After being dried in this way, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calendar roll or the like is used. By performing the surface smoothing treatment, the voids generated by the removal of the solvent during drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer improves, so that a magnetic recording medium with high electromagnetic conversion characteristics can be obtained. it can.

The recording layer formed by selecting the specific ferromagnetic powder and the binder as described above is subjected to the calendering treatment. As calendering conditions, a calender roll is used in the temperature range of 60 to 100 ° C.
It is preferably carried out by operating at a pressure in the range of 100 to 400 kg / cm ² . The laminate thus cured is then formed into the desired shape.
The cutting can be performed under normal conditions using a normal cutting machine such as a slitter.

[0033]

In the binder for magnetic recording media of the present invention, polyurethane having a urethane group having a sulfonic acid group or a carboxyl group bonded to urethane is used. Since the dispersibility is extremely higher than that of polyurethane in which is bonded or polyurethane in which a sulfobetaine group is bonded to the chain extender, the dispersibility of ferromagnetic powder or non-magnetic powder when used in a magnetic recording medium. Is good,
It is possible to obtain a magnetic recording medium having high glossiness, good electromagnetic conversion characteristics, and small surface roughness.

[0034]

【Example】

Synthesis Example 1 Synthesis of Polyurethane In a 1-liter three-necked flask equipped with a condenser and a stirrer and previously purged with nitrogen, the polyol (0.
(07 mol) and a chain extender (0.03 mol) were added, and the mixture was dissolved in 226 g of a mixed solvent of toluene / cyclohexanone = 1/1. Diisocyanate (0.1mo
1) was further added, and 0.03 lg (0.049 mol) of di-n-butyltin dilaurate was added as a catalyst, and the mixture was heated and stirred at 90 ° C for 6 hours under a nitrogen stream to obtain a polyurethane solution.

The molecular weight of the obtained polyurethane is shown in Table 1.

[0036]

[Table 1]

Synthesis Example 2 Introduction of Hydrophilic Group into Polyurethane Bond To the polyurethane solution obtained in Synthesis Example 1, a 1% solution of sodium hydride (NaH) in dimethylformamide (DMF) was added, and the mixture was placed in an argon stream. The mixture was stirred for 45 minutes while maintaining the temperature at -5 to 0 ° C. Then, γ-propane sultone or β-propiolactone was added, and the mixture was stirred at room temperature for 2 hours and reacted as shown below.
Got

Here, NaH and γ-propanesultone or β-propiolactone were added so that the polar groups shown in Table 2 were obtained.

[0039]

[Chemical 3]

Further, sample 12 is a sample obtained by reacting γ-propane sultone without adding a hydrophilic group and sample 13 is not added with sodium hydride. As described in Kahei 2-56719, the following sulfobetaines are formed. As a result, since it forms an intramolecular salt and exhibits a cyclic structure, the degree of adsorption with the ferromagnetic powder is weak and the dispersibility is poor.

[0041]

[Chemical 4]

Example 1 Single-layer magnetic layer Ferromagnetic alloy powder (composition: 94% Fe, 4% Zn, 2% Ni2)
%, Hc 1500 Oe, crystallite size 20 nm) 100
Parts (parts by weight, the same applies below) were ground for 10 minutes in an open kneader, and then a vinyl chloride / vinyl acetate / glycidyl methacrylate = 86/9/5 copolymer was added with hydroxyethyl sulfonate sodium salt. The added compound (SO ₃ Na = 6 × 10 ⁻⁵ eq / g, epoxy = 10 ⁻³ eq / g, Mw 30,000) was kneaded with 10 parts and 60 parts of methyl ethyl ketone for 60 minutes, and then polyurethane (Sample No. 6 -13) 10 parts (solid content) Abrasive (Al ₂ O ₃ particle size 0.3 μm) 2 parts Carbon black (particle size 40 nm) 2 parts Add methyl ethyl ketone / toluene = 1/1 200 parts Disperse for 120 minutes in a sand mill did.

To this, polyisocyanate 5 parts (solid content) (Coronate 3041 manufactured by Nippon Polyurethane) sec-butyl stearate 1 part butoxyethyl stearate 1 part stearic acid 1 part methyl ethyl ketone 50 parts were added, and after further stirring and mixing for 20 minutes. A magnetic paint was prepared by filtering using a filter having an average pore size of 1 μm. The thickness of the obtained magnetic paint after drying is 2.5μ
It was coated on the surface of a polyethylene terephthalate support having a thickness of 10 μm so as to have a thickness of m using a reverse roll.

The non-magnetic support coated with the magnetic paint was subjected to magnetic field orientation with a magnet of 3000 gauss while the magnetic paint was undried, and after drying, metal roll-metal roll-metal roll-metal roll-metal. Calender treatment by combination of roll-metal roll-metal roll (speed 100m
/ Min, linear pressure of 300 kg / cm, temperature of 90 ° C.), and then slit into a width of 8 mm to prepare an 8 mm video tape. The characteristics of the obtained video tape were measured by the following measuring methods, and the results are shown in Table 2.

Measurement method Electromagnetic conversion characteristics: Hi8-VTR (Son
The signal was recorded and reproduced by using y-TR: 705). The S / N ratio at this time was measured with a noise meter, and the value of Comparative Example 4 was set to 0 dB and expressed as a relative value. Glossiness: The glossiness of the magnetic layer surface was measured at a reflection angle of 45 degrees using a gloss meter. Surface roughness Ra: cut-off of 0. by optical interference method using a digital optical profilometer (manufactured by WYKO).
It was determined as the center line average roughness Ra under the condition of 25 mm.

Comparative Example 1 Further, a video tape was prepared in the same manner as in Example 1 by using a sample obtained by reacting γ-propane sultone without adding sodium hydride with a sample in which no hydrophilic group was introduced. make,
Table 2 shows the measurement results obtained by the following measurement method.

[0047]

[Table 2]

Example 2 Overlay of upper magnetic layer and lower nonmagnetic layer An undercoating liquid prepared by stirring for 12 hours with a Dispa stirrer was prepared according to the following formulation. Polyester resin (containing -SO ₃ Na group) 100 parts by weight Tg 65 ° C Na content 4600 ppm Cyclohexanone 9900 parts by weight Using the obtained undercoat liquid, polyethylene terephthalate (thickness 10 μm, F5 value: MD direction 20 Kg / m)
m ² , TD direction 14 Kg / mm ² , Young's modulus: MD direction 750 Kg / mm ² , TD direction 470 Kg / m
m ² ), and a dry thickness of 0.1 μm was applied on the non-magnetic support composed of m ² ).

On the other hand, a coating solution for the upper magnetic layer and a coating solution for the lower non-magnetic layer were prepared with the following formulations. Formulation of coating liquid for upper magnetic layer Ferromagnetic powder: Fe alloy powder (Fe-Co-Ni) 100 parts by weight Composition; Fe: Co: Ni = 92: 6: 2 Al ₂ O ₃ is used as a sintering inhibitor Hc 1600 Oe , Σ _s 119 emu / g major axis length 0.13 μm, acicular ratio 7 crystallite size 17.2 nm, water content 0.6 wt% vinyl chloride copolymer 13 parts by weight —SO ₃ Na 8 × 10 ⁻⁵ eq / g, -OH, epoxy group-containing Tg 71 ° C, degree of polymerization 300, number average molecular weight (Mn) 12000 weight average molecular weight (Mw) 38000 polyurethane resin (described in Table 3) 5 parts by weight α-alumina (average particle size 0.15 µm ) 6 12 parts by weight ^{_{S BET 8.7m 2 / g, pH}} 8.2, a water content of 0.06 wt% cyclohexanone 150 parts Methyl ethyl ketone 150 parts by weight the composition in a sand mill After between mixed and dispersed,
Polyisocyanate (Coronate L Nippon Polyurethane Co., Ltd.) and 5 parts by weight of oleic acid, 7 parts by weight of stearic acid, and 15 parts by weight of butyl stearate were added to obtain a coating liquid for the upper magnetic layer.

Coating liquid formulation for lower non-magnetic layer TiO ₂ 85 parts by weight Average particle size 0.035 μm Crystalline rutile TiO ₂ content 90% or more Surface treatment layer Al ₂ O ₃ S _BET 35-45 m ² / g True specific gravity 4 .1 pH 6.5-8.0 Carbon black 5 parts by weight Average particle diameter 160 nm DBP oil absorption 80 ml / 100 g pH 8.0 S _BET 250 m ² / g Coloring power 143% Vinyl chloride copolymer 13 parts by weight -SO ₃ Na 8 × 10 ⁻⁵ eq / g, —OH, epoxy group-containing Tg 71 ° C., degree of polymerization 300, number average molecular weight (Mn) 12000 weight average molecular weight (Mw) 38000 polyurethane resin (Table 3) 5 parts by weight cyclohexane 100 parts by weight Parts Methyl ethyl ketone 100 parts by weight After mixing and dispersing the above composition in a sand mill for 4 hours,
5 parts by weight of polyisocyanate (Coronate L), 5 parts by weight of oleic acid, 5 parts by weight of stearic acid and 15 parts by weight of butyl stearate were added to obtain a coating liquid for the lower non-magnetic layer.

The above coating solution was applied in a wet state by using two doctors having different gaps, and after orientation treatment with a permanent magnet of 3500 gauss and then with an electromagnet of 1600 gauss, it was dried. Then, a metal roll and a super calender treatment with the metal roll were performed at a temperature of 80 ° C.
The coating thickness was 0.3 μm for the magnetic layer and 3.0 μm for the non-magnetic layer. Next, a coating solution for the back coat layer was prepared according to the following formulation.

Backcoat layer formulation Carbon black 100 parts by weight S _BET 220 m ² / g Average particle size 170 nm DBP oil absorption 75 ml / 100 g Volatile content 1.5% pH 8.0 Bulk density 240.2 kg / m ³ Nitrocellulose RSI / 2 (Daicel Chemical Industry Co., Ltd.) 100 parts by weight Polyurethane 30 parts by weight Dispersant Copper oleate 10 parts by weight Copper phthalocyanine 10 parts by weight Barium sulfate (precipitation) 5 parts by weight Methyl ethyl ketone 500 parts by weight Toluene 500 parts by weight.

The above composition was pre-kneaded and kneaded with a roll mill, and 100 parts by weight of the obtained composition, 100 parts by weight of carbon black S _BET 200 m ² / g average particle size 0.2 μm DBP oil absorption 36 ml / was dispersed in a sand grinder at _{100g pH 8.5 α-Al 2 O} 3 ( average particle diameter 0.2 [mu] m) composition was added 0.1 part by weight,
After filtration, the following composition was added to 100 parts by weight of the above composition to prepare a coating liquid. Methyl ethyl ketone 120 parts by weight Polyisocyanate 5 parts by weight The obtained coating liquid was applied on the side of the non-magnetic support opposite to the side on which the magnetic layer was provided to a dry thickness of 0.5 μm. Table 3 shows the results of the same method as in Example 1 in which the raw fabrics produced by using the samples 15 to 21 having different compositions thus obtained were cut into 8 mm widths to prepare 8 mm video tapes. .

Comparative Example 2 A video tape similar to that of Example 1 was prepared except that a sample 22 using polyester polyurethane was used as polyurethane for both the upper magnetic layer and the lower non-magnetic layer.
Table 3 shows the results of measuring the characteristics as in Example 1.

[0055]

[Table 3]

Example 3 Overlay of upper magnetic layer and lower magnetic layer A coating solution for the upper magnetic layer and a coating solution for the lower magnetic layer having the following compositions were prepared. Upper layer liquid CO-γFeOx 100 parts by weight (x = 1.45, major axis length 0.20 μm, Hc950Oe, Br1600 gauss) Vinyl chloride copolymer 10 parts by weight (containing sulfonic acid group 0.25% by weight) Polyurethane 5 parts by weight Polyisocyanate 6 parts by weight Stearic acid (for industrial use) 1 part by weight Butyl stearate (for industrial use) 1 part by weight α-alumina (particle size 0.2 μm) 10 parts by weight Conductive carbon (particle size 70 nm) 1 part by weight Methyl ethyl ketone / Cyclohexanone = 7/3 solvent 200 parts by weight Lower layer liquid (liquid B) CO-γFeOx 100 parts by weight (x = 1.45, major axis length 0.25 μm, Hc850Oe, Br1400 gauss) Vinyl chloride copolymer 11 parts by weight (sulfone Containing 0.25% by weight of acid group) Polyurethane (described in Table 10) 4 parts by weight Polyisocyanate ( Roneto L) for 6 parts by weight of stearic acid (Industrial) 1 for parts by weight of butyl stearate (Technical) 1 part by weight of conductive carbon (particle size 20 nm) 5 parts by weight Methyl ethyl ketone / cyclohexanone = 7/3 solvent 200 parts by weight.

The coating solution was adjusted using a kneader, sand grinder or the like, and simultaneous multilayer coating was performed. The non-magnetic support has a center line average surface roughness (Ra) of 4 nm and 14 μ.
An m-thick polyethylene terephthalate film was used. After coating, it was oriented in the longitudinal direction and dried to obtain a bulk roll. The obtained bulk roll was further calendered. Calendar roll treatment is Shore A hardness 80
Degree, 7 layers of metal rolls with Ra 0.5 nm, temperature 8
It was treated at 0 ° C. and a linear pressure of 300 kg / cm ² . After the above-mentioned calender roll treatment, a bulk thermo treatment was carried out at 60 ° C. for 24 hours to cure the magnetic layer and then slit to a 1/2 inch width to obtain a video tape. When the surface roughness of the obtained video tape was measured, all were 5 to 6 nm. The obtained video tape was incorporated into an S-VHS type cassette to prepare a video cassette tape, and the video cassette tapes of Examples 1-2 were prepared.
Tables 10 and 11 show the results of measurement performed by the same method as described above.

Comparative Example 3 A video tape similar to that of Example 18 was prepared except that a sample 29 using polyester polyurethane was used as polyurethane for both the upper magnetic layer and the lower magnetic layer, and the same procedure as in Example 1 was performed. The results of measuring the properties are shown in Table 4.

[0059]

[Table 4]

[0060]

According to the present invention, as the polyurethane, a urethane group having a sulfonic acid group or a carboxyl group bonded to urethane is used. Therefore, the sulfonic acid group or the like is bonded to the polyester polyol which is the skeleton of the conventional polyurethane. As compared with polyurethane or polyurethane in which a sulfobetaine group is bonded to a chain extender, a magnetic recording medium having extremely large dispersibility, high gloss, good electromagnetic conversion characteristics and small surface roughness can be obtained.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Tsutomu Okita 2-12-1, Ogimachi, Odawara-shi, Kanagawa Fuji Photo Film Co., Ltd.

Claims (4)

[Claims] 1. A binder for a magnetic recording medium comprising polyurethane, which is a reaction product of a polyol and a polyisocyanate as main raw materials, wherein the polyurethane has a sulfoalkyl group or a carboxyalkyl group bonded to the nitrogen atom of the polyurethane bond. A binder for a magnetic recording medium, which is polyurethane. 2. The polyurethane having a sulfoalkyl group or a carboxyalkyl group bonded to the nitrogen atom of the polyurethane bond has the following structural formula:
A binder for a magnetic recording medium as described above. [Chemical 1] However, X is SO ₃ M or COOM M is H, an alkali metal, an alkaline earth metal or an ammonium group, and R is an alkylene group. 3. On at least one surface of the non-magnetic support,
A magnetic recording medium in which a magnetic layer containing a ferromagnetic powder and a binder is formed, wherein the binder contains polyurethane in which a sulfoalkyl group or a carboxyalkyl group is bonded to a nitrogen atom of the polyurethane bond. 4. At least one surface of the non-magnetic support,
In a magnetic recording medium having a lower magnetic layer or a lower non-magnetic layer and an upper magnetic layer provided thereon, at least one of the magnetic layer or the non-magnetic layer is a binder for binding a ferromagnetic powder or a non-magnetic powder. A magnetic recording medium characterized by containing polyurethane having a sulfoalkyl group or a carboalkyl group bonded to the nitrogen atom of the polyurethane bond.