JPH0449517A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve electromagnetic conversion characteristics and durability by using a ferromagnetic metal powder having a specified axial ratio and X-ray particle size and using a binder resin having specified metal-sulfonate group to form a magnetic layer. CONSTITUTION:A magnetic layer formed on a nonmagnetic supporting body essentially consists of a needle-like ferromagnetic metal powder and a binder resin. The metal powder has 5 - 7 axial ratio and <=170 Angstrom X-ray particle size. The binder resin contains metal-sulfonate group in the molecule to 0.1 - 0.9 wt.% sulfer content and has 10000 - 50000 mol.wt. The binder resin is incorpo rated by 16 - 19 pts.wt. to 100 pts.wt. of the metal powder. Thus, superfine particles of ferromagnetic metal magnetic powder can be highly dispersed, and the obtd. medium has high video S/N and excellent durability.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to magnetic recording media such as magnetic tapes and magnetic disks,
In particular, it relates to a magnetic recording medium using a binder resin containing a sulfonic acid metal salt in its molecule.

Conventional technology Coated magnetic recording media such as magnetic tapes and magnetic disks are generally composed of magnetic powder and small amounts of abrasives, antistatic agents, lubricant dispersants, hardeners, etc. dispersed in a binder resin. There is.

The properties required of the binder resin for coated magnetic recording media include factors that affect electromagnetic conversion properties such as high density packing of magnetic powder, high orientation, and easy calendering properties, and factors that affect electromagnetic conversion properties such as high density packing of magnetic powder, high orientation, and ease of calendering. In this case, there are various factors that play a very important role, including factors related to the durability of the tape so that its characteristics do not deteriorate, and factors related to adhesiveness with the polyester film, which is a non-magnetic support.

In recent years, with the development of magnetic recording media, especially tapes for VTRs, as typified by high-band 8 mm video, there has been a remarkable trend toward single-wavelength recording for the purpose of high-density recording. Furthermore, next-generation VTRs such as high-definition or digital VTRs will increasingly use single-wavelength recording, and it is expected that the shortest recording wavelength will be 0.4 μm or less in the future.

In order to realize such high-density recording, ferromagnetic metal powders with large coercive force Hc and residual magnetic flux density Br are currently attracting attention as magnetic powders, and each tape manufacturer is using them for magnetic recording. Media development is actively underway. With the release of high-band 8mm video, the characteristics of coating-type magnetic recording media have improved markedly, and high characteristics beyond conventional concepts have been achieved. This is not only due to improvements in the manufacturing method of magnetic recording media, but also to significant improvements in the properties of the ferromagnetic metal powders used.

Recently, high coercivity exceeding 17,000 e, and X-ray particle diameter in the short axis direction of less than 170 angstroms (
A ferromagnetic and ultrafine metal magnetic powder with a BET value of 60%/g or more has also been developed. Furthermore, in order to improve the backing of the magnetic powder in the magnetic coating film, the axial ratio tends to become smaller, and some magnetic powders have an axial ratio of 5.

However, when such ferromagnetic and ultrafine magnetic powder is used, the dispersibility of the magnetic powder in the binder resin solution deteriorates,
This causes problems such as increased viscosity of the magnetic paint.

In particular, in the case of magnetic powders that are ferromagnetic and have a small axial ratio, in addition to the magnetic attraction force, there is also a large intermolecular attraction force, so the dispersion technology requires extremely advanced materials technology and coating technology.

On the other hand, as the shortest recording wavelength becomes shorter as the recording density improves, the relative speed between the tape and the head generally increases, and therefore durability becomes even more demanding, and a higher level of performance is required.

In particular, when using fine particles of magnetic powder with a small axial ratio, it is extremely difficult to bind these particles firmly into the magnetic coating and create a strong tape that does not adhere to the head or cylinder under various environments. The design and development of the binder resin for this purpose is the most important elemental technology in the development of coated magnetic recording media.

Conventionally used binder resins include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinylidene chloride copolymer, polyurethane resin, polyester resin, acrylonitrile-butadiene. copolymer, nitrocellulose,
There are epoxy resins, etc.

Among these resins, vinyl chloride resin has excellent dispersibility of magnetic powder due to its relatively low molecular weight and the ability to easily introduce functional groups such as hydroxyl or carboxyl groups into the molecule. In addition, polyurethane resin has extremely superior toughness and abrasion resistance compared to other resins due to intermolecular hydrogen bonding caused by urethane bonds, so most coated magnetic recording media contain these two types. A combination of different resins are used.

With the recent trend toward finer particles of magnetic powder, various efforts have been made to improve the dispersibility of both vinyl chloride resins and polyurethane resins. The representative example is the Japanese Patent Publication No. 5
As seen in JP-A-4-157603 and JP-A-57-165464, this is a method of introducing a sulfonic acid metal salt, which is a highly adsorptive functional group, into the resin. Furthermore, JP-A-1-59646 discloses in detail the actual composition of a magnetic recording medium using a binder resin into which a sulfonic acid metal salt is introduced.

In order to improve the dispersion of magnetic powder, there are other methods such as surface treatment of the surface of metal magnetic powder with an aqueous solution containing a surfactant, as disclosed in JP-A-61-14705, or a special method. There are various organic and inorganic surface treatment methods, such as a method of surface treatment with a titanium coupling agent as disclosed in JP-A-60-107731 and JP-A-60-127526.

In addition, from the perspective of paint processing methods, magnetic powder is hard-kneaded using a mixing device such as a kneader, internal mixer, Banbury mixer 1, or planetary mixer to promote resin adsorption. There is a way to achieve high dispersion. In particular, tape manufacturers have recently put a lot of effort into coating technology, and various new methods have been announced. JP-A-1-106338 describes in detail a method for highly dispersing metal magnetic powder using a biaxial continuous kneading mixer.

Problems to be Solved by the Invention However, it is extremely difficult to disperse metal magnetic powder, especially the recent ultrafine ferromagnetic particles with a small axial ratio, using conventional methods, and it is also difficult to disperse in terms of durability. Various problems are arising from this.

The method of introducing a sulfonic acid metal salt with the strongest adsorption into vinyl chloride resin or urethane resin is effective in improving the dispersibility of magnetic powder, but the concentration of the sulfonic acid metal salt introduced and the sulfonic acid The molecular weight and usage amount of the binder resin into which the metal salt is introduced must be determined by fully considering the magnetization size, shape, and particle size of the magnetic powder. If the amount of sulfonic acid metal salt introduced is increased unnecessarily,
Since the sulfonic acid groups themselves are strong electron-withdrawing functional groups, the sulfonic acid groups interact with each other, causing the resin to gel. In this case, the viscosity of the magnetic paint increases significantly, and as a result, its dispersibility with respect to magnetic powder deteriorates. On the other hand, if the amount of sulfonic acid metal salt introduced is small, the adsorption to magnetic powder becomes weak, resulting in poor dispersion.

Furthermore, the amount of binder resin must be determined by taking into account the shape, particle size, etc. of the magnetic powder. If the amount of binder resin is too large, the excess resin will rise to the surface of the magnetic layer during calendering after coating, forming a thin resin layer. This increases head spacing loss and degrades electromagnetic conversion characteristics. If the calculation is performed assuming that the shortest recording wavelength is 0.4 μm and the thickness of the resin layer on the surface of the magnetic layer is 300 angstroms, the output drop in the electromagnetic conversion characteristics due to spacing loss will actually be more than 4 dB. On the other hand, if the amount of binder resin is small, not only will the dispersion of the magnetic powder deteriorate, but it will also become difficult to cover the magnetic powder sufficiently with the resin, causing problems in terms of durability. In other words, the binding force between the magnetic powder and the resin becomes weaker, which may lead to an increase in dropouts due to powder falling off when the deck is running, powder adhesion to the head in high-temperature environments, and, in the worst case, head clogging. .

The molecular weight must also be selected within a certain range. If the molecular weight is too large, the viscosity of the coating material will rapidly increase for ultrafine magnetic powder having an X-ray particle size of 170 angstroms or less, and the dispersion of the magnetic powder will be inhibited. On the other hand, if the molecular weight is small, when the magnetic paint is applied onto a support and then cured using a curing agent, unreacted components will be generated and low molecular weight components will remain in the magnetic coating, deteriorating the physical properties of the coating. This is because it makes you

As described above, the amount of sulfonic acid groups introduced into vinyl chloride resins and polyurethane resins, the amount of resin used, and even the molecular weight have a very large influence on tape properties, and these settings must be made with great care.

The method of organically or inorganically treating the surface of magnetic powder to improve dispersibility can be expected to be somewhat effective for iron oxide magnetic powders, which have relatively large particle sizes and low magnetization strength, but Furthermore, it has not been possible to obtain satisfactory dispersibility for ultrafine metal magnetic powder particles having an X-ray particle diameter of less than 170 angstroms. Furthermore, in the case of organic surface treatment, if the surface treatment agent remains in the magnetic coating without reacting with the magnetic powder, the physical properties of the coating will deteriorate, especially powder falling off in high-temperature environments, or deterioration of still characteristics at low temperatures. invite. In the case of inorganic surface treatment, water is often used, which is the most unsuitable material for magnetic paint (because of this, it is necessary to thoroughly wash and dry the magnetic powder after surface treatment, which complicates the process. Magnetic powder becomes expensive and difficult to use regularly.

Recently, there has been remarkable progress in paint conversion methods, and it has become possible to disperse fine particles of metal magnetic powder with large magnetization. However, these can only be highly dispersed by using vinyl chloride resin or polyurethane resin into which sulfonic acid groups have been introduced as described in the present invention, and no matter how advanced the coating technology is, it will not be possible to achieve high dispersion. At present, without a binder resin, dispersion that satisfies the required high electromagnetic conversion properties is not possible.

Means for Solving the Problems In order to solve the above problems, the configuration of the present invention has an axial ratio of 5 to 5.
7 and has an X-ray particle size in the short axis direction of 170 angstroms or less, a sulfonic acid metal base is added so that the sulfur content in the molecule is 0.1 to 0.9% by weight. and a molecular weight of 10,000 to 50,000, in an amount of 16 to 100 parts by weight of metal powder.
This is achieved by using 19 parts by weight.

Effects According to the structure of the present invention, since a specific amount of a sulfonic acid metal base that has a strong adsorption power to magnetic powder is used in the binder resin, the resin viscosity is such that a high shear force is applied during dispersion, which makes it difficult to absorb magnetic powder. It becomes possible to adsorb the binder resin to the magnetic powder by using the magnetic attraction and intermolecular force that cause the powder to coagulate strongly.

In addition, since the adsorption force is strong, the resin does not separate from the magnetic powder in the magnetic paint, and the tailing resin in the magnetic paint acts as a steric hindrance between the magnetic powders, preventing the magnetic powder from re-agglomerating. Therefore, a magnetic coating material with excellent dispersion stability can be obtained.

Furthermore, since the minimum amount of binder resin is used, the resin layer covering the magnetic powder is thin, and a magnetic recording medium with excellent durability and electromagnetic conversion properties can be obtained in a wide range of environments.

Furthermore, since the molecular weight of the binder resin is specified within a preferable range of 10,000 to 50,000, it has excellent workability during paint preparation, no unreacted components remain after reaction with the curing agent, and is highly reliable for storage.

EXAMPLE The sulfonic acid metal base used in the present invention is introduced into the side chain in the binder resin in the form of -303M. Here, M is a hydrogen atom, lithium, sodium, or potassium. In addition, the binder resin according to the present invention into which these metal bases are introduced include vinyl chloride-vinyl acetate copolymer,
Suitable examples include vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl maleate-vinyl alcohol copolymer, polyester resin, polyurethane resin, etc. used as something.

Further, the ferromagnetic metal powder in the present invention is Fe, Fe-G
It is possible to use needle-shaped metal powder containing trace amounts of additive elements such as Al, Cr, St, etc. in consideration of weather resistance, prevention of sintering during manufacturing, etc. The X-ray particle size and axial ratio of these ferromagnetic metal powders can be controlled by specifying the crystal particle size of the starting material, hydrated iron oxide, and the temperature, time, atmosphere, etc. during reduction with H2. can.

A polyisocyanate is preferably used as the curing agent. Particularly representative are di-, tri-, and tetraisocyanates selected from aliphatic, aromatic, or alicyclic compounds having two or more -N=, C=O groups in the molecule. These isocyanates include hexamethylene diisocyanate, xylylene diisocyanate,
These include 4-4' diphenylmethane diisocyanate, isophorone diisocyanate, and tolylene diisocyanate.

In addition, aluminum oxide, chromium oxide, iron oxide, and silicon oxide may be added as reinforcing agents and abrasives, higher fatty acids and fatty acid esters may be added as lubricants, and carbon black may be added as antistatic agents to the magnetic layer. is also possible.

Examples of the present invention will be described in detail below.

Example 1 Magnetic powder (iron metal powder, major axis 0.13μm, short axis 0.017μm)
m Hc15500 e) Carbon black 2 parts by weight (manufactured by Tokai Carbon Co., Ltd., GEST S) Sodium sulfonate group-containing vinyl chloride resin (sulfur content 0.5%, molecular weight 30000) 9 parts by weight Sodium sulfonate group-containing polyurethane resin (Sulfur content 0.5%, molecular weight 35000) 9 parts by weight Solvent methyl ethyl ketone 24 parts by weight Toluene 16 parts by weight cyclohexanone 8 parts by weight The above composition was mixed under a nitrogen atmosphere (
02 (concentration: 2% or less) was mixed and kneaded for 2 hours in a 10 liter pressure kneader to obtain a kneaded product.

Materials having the composition shown below were further added to the obtained kneaded material, diluted with Daysilver, and then dispersed in a 10-liter sand grinder to obtain a magnetic paint.

Kneaded product 160 parts by weight a
A I z O3 (0.3 um granules) 7 parts by weight Solvent methyl ethyl ketone 150 parts by weight Toluene 100 parts by weight Cyclohexanone 40 parts by weight Materials having the following composition were further added to the obtained magnetic paint, and coating was performed.

Stearic acid 3 parts by weight n-butyl stearate 1 part by weight Coronate
5 parts by weight The coating was carried out on a polyester base film having a thickness of 15 μm so that the dry thickness was 4 μm, and then an orientation magnetic field was applied to orient it, and then it was dried with hot air. After further calendering at 80°C, it was kept in an oven at 60"C for 24 hours for curing. After curing, a back coat layer of 0.6
It was coated in micrometers and cut into 1/2 inch width to make metal video tape.

Example 2 The same procedure as in Example 1 was carried out except that instead of 9 parts by weight of the sodium sulfonate group-containing polyurethane resin of Example 1, a sulfonic acid group-containing polyester resin (sulfur content 0.5%, molecular weight 25,000) was used. We obtained a magnetic paint and used it to create a metal videotape.

Comparative Examples 1-2 Magnetic paint was prepared in the same manner as in Example 1, except that the parts by weight of the sodium sulfonate group-containing vinyl chloride resin and the sodium sulfonate group-containing polyurethane resin of Example 1 were changed to the parts by weight shown in Table 1. was obtained and used to create a metal videotape.

(Left below) Table 1 Table 2 Comparative Example 3 and Comparative Example 4 In Example 1, the sulfonic acid group content of the vinyl chloride resin containing sodium sulfonate group was adjusted to the amount shown in Table 2 in terms of sulfur content. A magnetic paint was obtained in the same manner as in Example 1 except for the changes, and a metal videotape was made using this.

(Margin below) Comparative Example 5 and Comparative Example 6 A magnetic paint was obtained in the same manner as in Example 1 except that the molecular weight of the sodium sulfonate group-containing polyurethane resin was changed as shown in Table 3. A metal videotape was created using

(Margin below) Table 3 Example 1 is shown as OdB.

Table 4 Paint viscosity of magnetic paint obtained in Table 4, surface roughness of metal videotape, glossiness (after coating), video S/N
The results of the ratio measurements are shown.

Here, the paint viscosity is a value measured at 60 rpm using a B-type viscometer for the paint just before coating. Also, the surface roughness is wyc
It shows the average surface roughness measured using a non-contact microsurface roughness meter manufactured by Company O. Moreover, the glossiness shows the value of 45°-45° cross. The video S/N ratio was measured using a commercially available MIr deck, and Table 5 shows the amount of resin eluted and the degree of head powder adhesion when a durability test was conducted using these metal video tapes. .

Here, the amount of resin eluted is the value obtained by measuring the amount of resin eluted when the prepared tape is left in a tetrahydrofuran solution at 60° C. for 1 hour. Durability test was conducted using commercially available MI
100 in an environment of 40°C, 180RH% using an I-deck.
The state of powder adhesion to the head when the bus was running was visually evaluated on a five-point scale.

(Left below) Table 5 ■ Videotape is obtained. In terms of durability, since the resin has a strong adsorption power to magnetic powder, the amount eluted into the tetrahydrofuran solution is small, and as a result, there is little adhesion of powder to the head.

In Comparative Example 1, since the amount of resin was small, the magnetic powder was insufficiently dispersed, resulting in poor surface roughness and low video S/N ratio. In addition, regarding durability, since the magnetic powder is not sufficiently coated with resin, the magnetic coating film is brittle and has a lot of dregs attached. On the other hand, in Comparative Example 2, since the amount of resin was too large, a resin layer was present on the surface of the magnetic layer, resulting in spacing loss. the result,
Dispersion is good and surface roughness is good, but the video S/N ratio is low.

In Comparative Example 3 and Comparative Example 5, the viscosity of the magnetic paint was extremely high because the content of sodium sulfonate in the resin was high or the molecular weight of the resin was large. Therefore, the dispersibility deteriorates and the video S/N ratio becomes low. On the contrary, in Comparative Example 4, the content of sodium sulfonate was low, so the dispersibility of the resin was very poor, and as a result, the video S/N ratio was low. In Comparative Example 6, since the molecular weight of the resin is low, the viscosity of the magnetic paint is low and the dispersibility is good, but many unreacted low molecular weight components remain in the magnetic layer. As a result, the amount of powder adhesion is large and the durability is significantly inferior.

Effects of the Invention As described below, according to the present invention, it is possible to highly disperse ferromagnetic and ultrafine metal magnetic powder, thereby producing a magnetic recording medium with a high video S/N ratio and excellent durability. can be obtained.

Claims (1)

[Claims] A magnetic recording medium comprising a magnetic layer mainly composed of an acicular ferromagnetic metal powder and a binder resin on a non-magnetic support, the metal powder having an axial ratio of 5 to 7 and a short axis. The X-ray particle size in the direction is 170 angstroms or less, and the binder resin has a sulfur content of 0.1 to 0.9% by weight in the molecule.
It has a sulfonic acid metal base such that the molecular weight is 1.
0,000 to 50,000, and the binder resin is contained in an amount of 16 to 19 parts by weight per 100 parts by weight of the metal powder.