JPH10269551A - Magnetic coating material and magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the dispensability of a magnetic coating material contg. carbon black at a high degree by dispersing magnetic particles with a sulfonic acid metal base-contg. polyurethane resin, adding the carbon black of a specific average grain size of a specific value or below at specific weight % thereto in an arbitrary stage before the end of the dispersion and further dispersing the carbon black. SOLUTION: This magnetic coating material contains the sulfonic acid metal base-contg. polyurethane resin and is obtd. by first dispersing the magnetic particles with a binder and adding the carbon black of the average grain size of <=40 ×m at 7 to 30 wt.% of the magnetic particles to the resin in the arbitrary stage before the end of the dispersion and further dispersing the carbon black. The stage to add the carbon black to the resin is preferably the stage up to 1/10 to 9/10 of the entire dispersion stage. The magnetic recording medium having the magnetic layer formed of such magnetic coating material is excellent in magnetic characteristics and is improved in durability as well. The light shielding function by the carbon black is improved as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic paint containing carbon black at a high level and having excellent dispersibility, and a magnetic recording medium such as a magnetic tape and a magnetic disk obtained by using the magnetic paint.

[0002]

2. Description of the Related Art Magnetic tapes, which are general-purpose magnetic recording media,
A floppy disk is made by dispersing needle-shaped magnetic particles having a major axis of 1 μm or less together with additives such as a dispersant, a lubricant and an antistatic agent in a binder solution to form a magnetic paint, and applying this to a polyethylene terephthalate film. It is made. Characteristics required for the binder of the magnetic layer include dispersibility, filling property, orientation of the magnetic particles, durability of the magnetic layer, abrasion resistance, heat resistance, adhesiveness to a non-magnetic support, and the like. However, binders play a very important role. As a binder for the magnetic layer, a mixture of an adipate-type or polycaprolactone-type polyurethane resin and nitrocellulose or a PVC-based copolymer has been mainly used. It is known that it is effective to introduce a metal sulfonate group into a polyurethane resin or a vinyl chloride copolymer in order to improve the dispersibility of the magnetic particles. The running of the magnetic tape is controlled by light. Carbon black is included in the magnetic layer to impart light blocking properties and antistatic properties to the magnetic layer.

[0003]

In a magnetic recording medium, S
In order to improve the / N ratio (signal / ize ratio) and increase the recording density, it is necessary to smooth the surface of the magnetic layer, or to highly fill the magnetic layer with finer magnetic particles and to perform high orientation. There is a need for these and a binder that has good dispersion of magnetic particles is needed. The thickness of a magnetic layer has been reduced for the purpose of improving recording density and reducing costs. However, when the magnetic layer is thinned using a conventional magnetic paint, the light blocking property is inferior, and the tape cannot be run or stopped. A magnetic paint with a high carbon black ratio is used so that the light shielding property is maintained even when the magnetic layer is thinned.
When the ratio of carbon black is increased, the dispersion of the magnetic paint becomes worse, and the smoothness, filling property, orientation, etc. of the magnetic layer become worse,
As a result, a reduction in the S / N ratio and a reduction in the mechanical properties of the magnetic layer occur. Further, since the viscosity of the magnetic paint is increased, practical properties such as deterioration of coating aptitude and prolonged dispersion time are also inferior. Further, since the binder resin is easily adsorbed to the carbon black, the amount of the resin acting on the adhesion to the base film is reduced. Introducing a metal sulfonate group into the polyurethane is extremely effective in improving the dispersibility of the magnetic powder. However, the introduction of a metal sulfonate alone cannot satisfy the demand for improving the dispersibility of a magnetic paint having a higher carbon black ratio, and it has been difficult to obtain an excellent magnetic paint by a conventional dispersion method. An object of the present invention is to provide a method for improving the dispersibility of a magnetic paint containing carbon black at a high level, and to use the magnetic paint to obtain magnetic properties, electromagnetic conversion properties, running durability, applicability, and adhesion to a base film. An object of the present invention is to provide a magnetic recording medium having excellent properties and the like.

[0004]

Means for Solving the Problems The present inventors have made intensive studies on magnetic paints containing a polyurethane resin as a binder component, and have reached the present invention. That is, in the present invention, the magnetic particles are dispersed in the sulfonic acid metal base-containing polyurethane resin, and the average particle diameter is 40 at an arbitrary stage until the dispersion is completed.
A magnetic coating obtained by adding 7 to 30% by weight of magnetic particles of carbon black and further dispersing the same, and a magnetic recording medium having a magnetic layer formed from the magnetic coating.

The polyurethane resin used in the present invention is preferably obtained from a high molecular weight polyol (A), a diisocyanate compound and, if necessary, a low molecular weight diol.
Examples of the polymer polyol (A) include polyester polyols such as aromatic polyesters, aliphatic polyesters, and alicyclic polyesters, polyether polyols, polycarbonate polyols, and copolymers thereof, and polyester diols are particularly desirable. Examples of the dibasic acid component of the polyester diol include terephthalic acid, isophthalic acid, orthophthalic acid, 1,5-naphthalic acid, 2,6-naphthalic acid, 4,4′-diphenyldicarboxylic acid, and 2,2′-diphenyldicarboxylic acid , 4,4 '
-Aromatic dibasic acids such as diphenyl ether dicarboxylic acid, adipic acid, azelaic acid, sebacic acid, 1,4-
Cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid,
Aliphatic and alicyclic dicarboxylic acids such as 4-methyl-1,2-cyclohexanedicarboxylic acid and dimer acid are mentioned, and in particular, terephthalic acid, isophthalic acid, orthophthalic acid,
2,6-Naphthalic acid and adipic acid are preferred.

The glycol component is ethylene glycol.
Propylene glycol, 1,3-propanedio-
2-methyl-1,3-propanediol, 1,2-
Butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentaneddiol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentylglycol -Diethylene glycol,
Dipropylene glycol, 2,2,4-trimethyl-
1,3-pentaneddiol, cyclohexanedimethanol
, Neopentyl hydroxypivalic acid ester, ethylene oxide adduct and propylene oxide adduct of bisphenol A, ethylene oxide adduct and propylene oxad adduct of hydrogenated bisphenol A, 1,9-nonanediol, -Methyloctanediol, 1,10-dodecanediol, 2-butyl-2-
Ethyl-1,3-propanediol, tricyclodecane dimethanol, etc., among which ethylene glycol, propylene glycol, 2-methyl-1,3-
Preferred are propanediol, diethylene glycol, neopentyl glycol and cyclohexanedimethanol. Examples of the polyether polyol include polyethers such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Examples of the high molecular weight polyol include polycarbonate diols such as poly (hexamethylene carbonate).

The organic diisocyanate component of the polyurethane resin used in the present invention includes 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, p
Phenylene diisocyanate, diphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3'-dimethoxy-4,4 '-Biphenylenediisocyanate, 1,5-naphthalenediisocyanate, 2,6-naphthalenediisocyanate, 3,
3'-dimethyl-4,4'-diisocyanate, 4,
4'-diisocyanatodiphenyl ether, 1,5-
Xylylene diisocyanate, 1,3-diisocyanate
Tomethylcyclohexane, 1,4-diisocyanatomethylcyclohexane, 4,4'-diisocyanatocyclohexane, 4,4'-diisocyanatocyclohexylmethane, isophorone diisocyanate and the like.

[0008] A polyol compound having a molecular weight of 500 or less, which is used as necessary, adjusts the urethane group concentration in the polyurethane resin to impart a toughness peculiar to the polyurethane resin. It has the effect of increasing the crosslink density. For example, ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentaneddiol, 1,6-hexanediol, neopentyl glycol , Diethylene glycol, dipropylene glycol
2,2,4-trimethyl-1,3-pentanedio-
, Cyclohexane dimethanol, neopentyl hydroxypivalic acid ester, ethylene oxide adduct and propylene oxide adduct of bisphenol A,
Diol compounds such as ethylene oxide adducts and propylene oxad adducts of hydrogenated bisphenol A,
Trifunctional or higher functional polyol compounds such as trimethylolpropane, glycerin, pentaerythritol, dipentaerythritol and the like, especially neopentyl glyco-
, Neopentyl hydroxypivalate, trimethylolpropane, and glycerin are preferred.

The polyurethane resin used in the present invention preferably has a number average molecular weight of 10,000 or more. If the molecular weight is less than 10,000, the mechanical strength is insufficient, and the running durability of the magnetic recording medium is poor. If the number average molecular weight exceeds 100,000, the solution viscosity of the magnetic paint becomes large, and the workability and the dispersibility of magnetic particles, abrasives, carbon black and the like are undesirably deteriorated.

The sulfonic acid metal base-containing polyurethane resin used in the present invention preferably contains 3 to 500 equivalents, more preferably 20 to 150 equivalents, of the sulfonic acid metal base per 1 ton of the resin. Things.

As a method for introducing the metal sulfonate group, it is desirable to use a metal sulfonate-containing compound as a raw material as a part of the raw material of the polyester polyol. As the raw material therefor, 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, sodium sulfoterephthalic acid, 2-sodium sulfo-
Examples thereof include dicarboxylic acids and glycols such as 1,4-butanediol and 2,5-dimethyl-3-sodiumsulfo-2,5-hexanediol, and 5-sodium sulfoisophthalic acid and 5-potassium sulfoisophthalic acid are preferred.

The carbon black used in the present invention is used for imparting a light blocking property and an antistatic property to the magnetic layer. Examples of the carbon black include acetylene black, channel black, furnace black and the like, and those having a small particle diameter, a small agglomerate and a high degree of black pitting are preferable. Average particle size of carbon black is 4
A material having a particle size of 0 mμ or less, preferably 30 mμ or less is used. If it is 40 mμ or more, it can be easily dispersed, but since the ratio of carbon black in the magnetic paint must be increased in order to block the light of the magnetic layer and to prevent electrification, the magnetic properties,
Electromagnetic conversion characteristics and the like decrease. Carbon black is used in the range of 7 to 30% by weight of the magnetic particles. If it is less than 7%, the light-blocking property when the magnetic layer becomes thin is inferior, and if it exceeds 30%, not only the dispersibility is deteriorated, but also the viscosity of the magnetic paint becomes high and the applicability is reduced.

The magnetic coating material of the present invention contains, as a binder component of the magnetic particles, a polyurethane resin containing a metal sulfonate having a number average molecular weight of 10,000 or more, and the magnetic particles are first dispersed with the binder. It can be suitably obtained by adding carbon black having an average particle diameter of 40 mμ or less at 7 to 30% by weight of the magnetic particles and dispersing the carbon black at an arbitrary stage until the completion.

The sulfonic acid metal base-containing polyurethane resin is excellent not only in magnetic particles but also in dispersibility of carbon black. The simultaneous dispersion of magnetic particles and carbon black is thought to be due to the high surface activity and large specific surface area of the carbon black. Dispersibility decreases. In order to satisfy the properties resulting from carbon black, such as light-shielding properties and antistatic properties, without lowering the dispersion of the magnetic particles, first disperse the magnetic particles with a sulfonic acid metal base-containing polyurethane resin, and until the dispersion is completed. It is effective to add carbon black at the stage of and continue the dispersion. The step of adding carbon black is preferably from 1/10 to 9/10 of the total dispersion step. Outside this range, the dispersion of the magnetic particles may be insufficient or the carbon black may remain undispersed.

In the present invention, in addition to the sulfonic acid metal group-containing polyurethane resin used in the present invention, another resin is added for the purpose of controlling flexibility, improving cold resistance and heat resistance, and It is desirable to mix a crosslinking agent. Other resins include vinyl chloride resins, cellulosic resins, polyester resins, epoxy resins, phenoxy resins, polyvinyl butyral, acrylonitrile-butadiene copolymers, and general-purpose polyurethane resins such as adipate type and polycaprolactone type. On the other hand, examples of the crosslinking agent include a polyisocyanate compound, an epoxy resin, a melamine resin, a urea resin, an acid anhydride and the like, and among these, a polyisocyanate compound is particularly preferable.

The ferromagnetic particles used in the magnetic layer of the magnetic recording medium of the present invention include γ-Fe ₂ O ₃ and γ-Fe
Mixed crystal of ₂ O ₃ and Fe ₃ O ₄ , γ-F coated with cobalt
Examples include ferromagnetic oxides such as e ₂ O ₃ or Fe ₂ O ₄ and barium ferrite, and ferromagnetic alloy powders such as Fe—Co and Fe—Co—Ni.

In the magnetic recording medium of the present invention, if necessary, a plasticizer such as dibutyl phthalate or triphenyl phosphate, dioctyl sodium sulfosuccinate, t-butylphenol polyethylene ether, sodium ethylnaphthalene sulfonate, dilauryl succinate Lubricants such as zinc stearate, soybean oil lecithin and silicone oil, and various antistatic agents can also be added.

[0018]

The magnetic recording medium of the present invention uses a sulfonic acid metal base-containing polyurethane resin as a binder component of the magnetic layer to disperse magnetic particles and carbon black. The sulfonic acid metal base-containing polyurethane resin is excellent not only in magnetic particles but also in dispersibility of carbon black. However, when magnetic particles and carbon black are dispersed at the same time,
This is probably due to the high surface activity and wide specific surface area of the carbon black.However, the polyurethane resin containing a metal sulfonate preferentially adsorbs to the carbon black, and the adsorbent layer necessary for stabilizing the magnetic particles is required. Cannot be formed. Therefore, magnetic particles are easy to aggregate,
The dispersion of the magnetic particles decreases. When magnetic particles are dispersed in a sulfonic acid metal base-containing polyurethane resin, the resin adsorbs to the magnetic particles. Once adsorbed, the polyurethane resin does not easily desorb. Therefore, it is possible to stabilize the magnetic particles that are easily aggregated, and to prevent aggregation. It is effective for a magnetic coating material having a high carbon black content ratio to first disperse magnetic particles in a sulfonic acid metal base-containing polyurethane resin, add carbon black until the dispersion is completed, and continue dispersion.

[0019]

The present invention will be specifically illustrated by the following examples. In the examples, “parts” means “parts by weight”. Table 1 shows the binders used in Examples and Comparative Examples. The magnetic recording media obtained in Examples and Comparative Examples were evaluated by the following methods. Magnetic Layer Gloss The 60-degree gloss of the magnetic layer before calendering was measured with a gloss meter manufactured by Tokyo Denshoku Co., Ltd. The larger the gloss value, the better the dispersibility of the magnetic particles.

Squareness Ratio Magnetic hysteresis was determined using a vibrating sample magnetometer manufactured by Toei Industry Co., Ltd., and the squareness ratio was determined from the amount of residual magnetization and the amount of saturation magnetization. The closer to 1, the better the dispersion of the magnetic particles and the higher the magnetic output. Adhesion to magnetic layer A cellophane tape made of Nichiban was stuck to the magnetic layer, and a polyester film was applied at a pulling speed of 100 mm / min.
It was bent at 0 degree and peeled off. The measurement temperature was 20 ° C.

Magnetic Layer Filling Property A precisely weighed magnetic tape (weight W1) was immersed in silicone oil for 30 minutes. After wiping off the silicone oil adhering to the surface of the magnetic tape, the weight (W
2) was precisely weighed. After wiping the magnetic layer with a solvent, the film was precisely weighed (W3) to determine the weight of the magnetic layer. The weight increase rate by immersion in the silicone oil with respect to the weight of the magnetic layer, that is, (W2-W1) / (W1-W3) was determined.
Silicone oil penetrates into the voids in the magnetic layer, so the weight increase by silicone oil immersion (expressed as a percentage)
Is smaller, the filling rate of the magnetic layer is higher.

Example 1 A composition having the following composition ratio was placed in a paint shaker manufactured by Toyo Seiki and dispersed for 6 hours, and 2.5 parts of furnace black "Raven 1255" manufactured by Columbia Carbon Co. was added.
The dispersion was further performed for 60 minutes to obtain a magnetic paint. Next, as a polyisocyanate compound of a curing agent, Millionate M
1 part of R (manufactured by Nippon Polyurethane Industry Co., Ltd.), 0.1 part of stearic acid and 0.1 part of butyl stearate as a lubricant were added, and mixing was continued for 30 minutes. This was applied on a 15 μm-thick polyethylene terephthalate film so that the thickness after drying became 4 μm, dried while applying a magnetic field of 2000 gauss, passed through a calender roll at 80 ° C. and a linear pressure of 200 kg / cm, and then passed through a calender roll. And a magnetic tape was prepared. The obtained magnetic tape was left at 60 ° C. for 1 day, and then slit to イ ン チ inch. The surface gloss of the magnetic layer before calendering and the squareness ratio of the obtained tape were measured. Further, the adhesion between the magnetic layer and the base film and the filling property of the magnetic layer were measured by the methods described above. The wear state of the magnetic layer after running 100 times at 10 and 40 ° C. on a commercially available VTR deck was observed. Table 1 shows the results.

Solution 5 of polyurethane resin (1) (30% solution of MEK / toluene / cyclohexanone = 1/1/1) Solution 5 of polyurethane resin (2) (MEK / toluene / cyclohexanone = 1/30 of 1/1/1) % Solution) Magnetic particles (magnetite specific surface area 35 m ² / g) 13.5 Cyclohexanone 8 Toluene 8 MEK 8 Alumina (average particle size 0.05 μm) 0.5

Polyurethane resin (1) Composition: polyester-A / NPG / MDI = 100/7
/ 26 (weight ratio) Number average molecular weight: 26,000 (Polyester-A: terephthalic acid / isophthalic acid / 5
-Sodium sulfoisophthalic acid / ethylene glycol / neopentyl glycol = 50 / 47.5 / 2.5 /
50/50 mol%, number average molecular weight = 2,500)

Polyurethane resin (2) Composition: Polyester-B / NPG / MDI = 100/7
/28.8 (weight ratio) Number average molecular weight: 32,000 (Polyester-B: adipic acid / 5-sodium sulfoisophthalic acid / 1,6-hexanediol / neopentyl glycol = 97.5 / 2.5 / 70) / 30 mol%, number average molecular weight = 2,000)

Examples 2 to 3 The same dispersion procedure as in Example 1 was used, except that the dispersion time of the magnetic particles and the dispersion time after the addition of carbon black were changed. The total dispersion time was 7 hours in each case. After mixing the same curing agent and lubricant as in Example 1, a magnetic tape was obtained in the same manner as in Example 1. Table 1 shows the characteristics.

Comparative Examples 1 to 3 In Comparative Examples 1 and 2, the same materials as in Example 1 were used, except that the carbon black used in Example 1 was charged together with the magnetic particles. The dispersion was simultaneously dispersed for 7 hours and 15 hours in Comparative Example 2 to obtain a magnetic paint. After mixing the same curing agent and lubricant as in Example 1,
A magnetic tape was obtained in the same manner as described above. Table 2 shows the characteristics.

In Comparative Example 3, the dispersion of carbon black was
This was performed before the dispersion of the magnetic particles. The composition of carbon black at the time of dispersion is as follows: polyurethane resin (1) / polyurethane resin (2) / carbon black / toluene / MEK / cyclohexanone = 1.5 / 1.5 / 2.5 / 10.3 / 1
At 0.3 / 10.3, the mixture was dispersed for 1 hour, and magnetic particles were added thereto and dispersed for 6 hours to obtain a magnetic paint. After mixing the same curing agent and lubricant as in Example 1, a magnetic tape was obtained in the same manner as in Example 1. Table 2 shows the characteristics.

[0029]

[Table 1]

[0030]

[Table 2]

Examples 4 to 6 The dispersion procedure was the same as in Example 1, except that the ratio of carbon black to magnetic particles was changed as shown in Table 3. In each case, the ratio of the magnetic particles to the polyurethane resin was 4.5 to 1, the solid content concentration was 40%, the solvent composition was MEK / toluene / cyclohexanone = 1/1/1, and the total dispersion time was in each case. 7 hours. After mixing the same curing agent and lubricant as in Example 1, a magnetic tape was obtained in the same manner as in Example 1. However, the thickness of the magnetic layer after drying was 2.
0 to 2.1 μm. Table 3 shows the characteristics. The light transmittance was measured at a wavelength of 700 mμ.

Comparative Examples 4 to 6 The dispersion procedure was the same as in Example 4, except that the ratio between the carbon black and the magnetic particles was changed as shown in Table 4. In Comparative Examples 4 and 5, the ratio between the carbon black and the magnetic particles is out of the range of the present invention. In Comparative Example 6, the average particle size of the carbon black used was 55 mμ, which is outside the scope of the present invention.
After mixing the same curing agent and lubricant as in Example 1, a magnetic tape was obtained in the same manner as in Example 1. However, the thickness of each of the dried magnetic layers was 2.0 to 2.1 μm. Table 4 shows the characteristics.

Comparative Example 7 A magnetic coating material was obtained by the same dispersion procedure as in Example 5. However, since the polyurethane resins (3) and (4) used as the binder did not contain a metal sulfonate group, soybean oil lecithin was used at 5% by weight of the magnetic particles for dispersing the magnetic particles. Lecithin was charged at the same time as the magnetic particles. The thickness of the dried magnetic layer was 2.1 μm. Table 4 shows the characteristics.

[0034]

[Table 3]

[0035]

[Table 4]

Polyurethane resin (3) Composition: polyester-C / NPG / MDI = 100/7
/ 26 (weight ratio) Number average molecular weight: 27,000 (Polyester-C: terephthalic acid / isophthalic acid / ethylene glycol / neopentyl glycol = 50/5
0/50/50 mol%, number average molecular weight = 2,500)

Polyurethane resin (4) Composition: polyester-D / NPG / MDI = 100/7
/28.8 (weight ratio) Number average molecular weight: 34,000 (Polyester-D: adipic acid / 1,6-hexanediol / neopentyl glycol = 100/70/30)
Mol%, number average molecular weight = 2,000)

[0038]

According to the magnetic coating composition of the present invention, even when a large amount of fine carbon black is contained, the dispersing ability of the metal sulfonate base is effectively utilized, and the dispersion of the magnetic particles and the carbon black is very good. As a result, the magnetic recording medium having the magnetic layer formed from the magnetic paint of the present invention not only has excellent magnetic properties but also has improved durability.
Further, the light blocking function by carbon black is also improved.

Claims (2)

[Claims] 1. A method for dispersing magnetic particles in a sulfonic acid metal base-containing polyurethane resin, adding carbon black having an average particle size of 40 mμ or less at an optional stage until the dispersion is completed, in an amount of 7 to 30% by weight of the magnetic particles; Magnetic paint obtained by further dispersing. 2. A magnetic recording medium having a magnetic layer formed from the magnetic paint of claim 1.