JPH05205249A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium where the surface of a magnetic layer is not deformed, an improved running durability can be maintained, and improved magnetic characteristics can be achieved even if repetition slide is performed against a guide roll etc. CONSTITUTION:A binder which is contained in a magnetic layer of a magnetic recording medium, having a stress relaxation time of 5000 seconds or longer which is required in a stress relaxation test using a single Maxwell model, is used. Polyurethane resin with a low molar weight constituent of 2.5wt.% or less can be listed as such kind of binder. When using the polyurethane resin where the low-molecular weight constituent is 2.5wt.% or less, a curing temperature of a magnetic coating should be 75 degrees or more and a non-magnetic support with a glass transition point of 100 degrees C or higher should be used. Furthermore, a ferromagnetic metal particle containing Co is used as a magnetic powder for preventing deterioration of magnetic characteristics when heating and curing the magnetic coating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium suitable for use in a data cartridge (helical scan system).

[0002]

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

In recent years, in such a coating type magnetic recording medium, the surface of the magnetic layer has become more mirror-finished in response to the demand for higher recording density, which results in running durability. Problems are starting to come up. That is, deterioration of a magnetic recording medium such as a magnetic tape due to running is caused by repeated sliding of the magnetic tape with respect to a guide roll or a head, whereby the surface is deformed or destroyed, but the surface of the magnetic layer is mirror-finished. Then, the frictional force with respect to the guide rolls and the head becomes large, and the above deterioration progresses quickly. For this reason, in such a magnetic recording medium, achieving both a mirror finish and running durability is a major issue in improving the recording density.

By the way, in a magnetic recording medium such as a magnetic tape, one of the causes for increasing the friction with respect to the guide roll and the head is that the magnetic tape is stretched when it is running,
It is considered that this increases the contact area with the guide roll and the head. In magnetic tape,
It is the binder that constitutes the magnetic layer that greatly affects the extensibility, and it is required that the binder have a high Young's modulus in order to suppress the extensibility. Therefore, in such a magnetic recording medium, as a resin conventionally used as a binder, a good electromagnetic conversion characteristic can be imparted to the magnetic recording medium,
It is required to have a high Young's modulus in order to have excellent wear resistance and heat resistance and to reduce friction against guide rolls.

For example, as a resin having such requirements, vinyl chloride such as vinyl chloride-vinyl acetate copolymer, vinyl chloride-propionic acid copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, etc. Examples thereof include polyurethane resins and polyurethane resins. Among them, polyurethane resins are generally used as a binder for the magnetic layer because they have a relatively low glass transition point Tg and can impart flexibility and impact resistance to the coating film. It is used. This polyurethane resin is synthesized, for example, by binding an ester and a diol as a chain extender using a bifunctional isocyanate to make a high molecular weight.

[0006]

However, when the magnetic recording medium containing the above-mentioned polyurethane resin as a binder is run a large number of times, the coefficient of friction with respect to the guide rolls and the head is small at first, and good running properties are exhibited. However, as the running is repeated, the surface is gradually deformed and the friction coefficient increases, so that good running performance cannot be obtained. Such running deterioration due to repeated running is particularly caused in the magnetic recording medium of the data cartridge (helical scan system).
This is a big problem because it is necessary to repeatedly drive a specific section when searching.

Therefore, the present invention has been proposed in view of such a conventional situation, and provides a magnetic recording medium which is excellent in running durability without deforming the surface even after repeated running. The purpose is to

[0008]

DISCLOSURE OF THE INVENTION As a result of the studies by the inventors of the present invention to achieve the above-mentioned object, the surface deformation of the magnetic recording medium due to the repeated running of the magnetic recording medium is a guide roller or the like. It was found that the shear stress is generated when sliding against the magnetic field, which causes the plastic flow of the magnetic layer. That is, when traveling
It takes a very short time for a certain area on the magnetic tape to slide against the guide rollers, etc., but when it is repeatedly run, minute deformation due to plastic flow of the magnetic layer accumulates, leading to large deformation and running. This leads to deterioration of sex. Therefore, in order to obtain running durability, it was considered that it is important to suppress such plastic flow that causes surface deformation of the magnetic layer, that is, to select a binder with a small plastic deformation. It was

The magnetic recording medium of the present invention has been proposed on the basis of these findings, and is a magnetic recording medium comprising a non-magnetic support and a magnetic layer mainly composed of magnetic powder and a binder. In the above, the stress relaxation time obtained by a stress relaxation test using the single Maxwell model of the binder is 5000 seconds or more.

Further, in a magnetic recording medium in which a magnetic layer mainly composed of magnetic powder and a binder is formed on a non-magnetic support, 2.5 parts by weight of a low molecular weight component having a molecular weight of 400 to 1300 is used as the binder. % Or less of the polyurethane resin. Further, the curing temperature of the magnetic layer is 75 ° C. or higher.

Further, the glass transition point of the non-magnetic support is 100 ° C. or higher.
The magnetic powder is Co-containing ferromagnetic metal particles.

The magnetic recording medium of the present invention comprises a non-magnetic support and a magnetic layer containing magnetic powder and a binder as main constituents.

In the present invention, in order to prevent plastic flow of the magnetic layer and improve running durability, stress relaxation determined by a stress relaxation test using a single Maxwell model as the binder is performed. It is assumed that the time is 5000 seconds or more.

Stress relaxation means that when a certain strain is applied to a viscoelastic material and the material is left as it is, the material undergoes plastic flow and is deformed accordingly, whereby the stress of the material decreases (relaxes) with time. Say a phenomenon. Here, in a viscoelastic material, a value τ (η
/ E = τ) is called stress relaxation time, which is an index of the ease of deformation of the material due to plastic flow. The longer the stress relaxation time, the less plastic deformation the material exhibits.

The stress relaxation time τ can be measured by observing the time-dependent change in stress of the material when a viscoelastic material is assumed to be a single Maxwell model and the material is left with a certain strain. You can

That is, the single Maxwell model is a model in which a viscoelastic body is assumed to have a spring component 11 and a dashpot component 12 connected in series, as shown in FIG. Considering based on this single Maxwell model, the stress σ and the strain ε can be expressed as a formula 1 as a function of time.

[0017]

[Equation 1]

When considering stress relaxation, since the strain applied to the viscoelastic body is constant, dε / dt = 0, and dε / dt
By integrating Equation 1 with = 0, Equation 2 is obtained.

[0019]

[Equation 2]

By substituting τ for t in the equation 2, the equation 3 is obtained.

[0021]

[Equation 3]

This 0.368σ ₀ is the stress of the viscoelastic body when the stress relaxation time has elapsed since the beginning of strain application, that is, the time at which the stress becomes 36.8% of the initial stress is measured. Therefore, the stress relaxation time is required.

In order to determine the stress relaxation time of the binder by the stress relaxation test using the Maxwell model, for example, the sample binder is molded into a cast film so that it has an appropriate length and width. It is punched out into a film for measurement. Then, by using a precision tensile tester, the above-mentioned measurement film is held in tension, the change in stress of the measurement film with the holding time is examined, and the holding time at which the stress becomes 36.8% of the initial stress may be examined. ..

The magnetic recording medium using as a binder a material having a stress relaxation time of 5000 seconds or more, which is obtained in this manner, has little surface deformation due to repeated running, and thus has good running durability.

When selecting the binder, it is preferable to consider Young's modulus, glass transition point and the like together with the stress relaxation time. That is, in order to obtain better running durability, the Young's modulus of the binder is 140 kgf / mm ²
(1.4 × 10 ⁹ N / m ² ) or more, glass transition point is 60
It is desirable that the temperature is ℃ or higher.

As a material satisfying the above requirements, for example, a low molecular weight component having a molecular weight of 400 to 1300 has a concentration of 2.5.
A polyurethane resin or the like in an amount of up to wt% can be used. That is, in the polyurethane resin, the plastic flow is induced by the contained low molecular weight component. Therefore, if the concentration of the low molecular weight component is suppressed, plasticization is prevented and the binder is suitable for ensuring the durability of the magnetic recording medium.

Examples of the polyurethane resin include polyester polyurethane, polyether polyurethane,
Examples thereof include polyurethane resins such as polycarbonate polyurethane.

Incidentally, such a polyurethane resin includes
A suitable polar group may be introduced. The type of this polar group is
Whether it is basic, neutral, or acidic, depending on the properties such as the degree of acid dissociation of the hydroxyl groups present on the surface of the magnetic powder used,
It is preferable to appropriately select so that the resin component is adsorbed on the interface of the magnetic powder as much as possible. If specifically exemplified, a hydroxyl group, phosphate group, -SO ₃ M (M is hydrogen, an alkali metal or an alkyl group), - OSO ₃ M (M is hydrogen,
Alkali metal or alkyl group), -COOM (M is hydrogen, alkali metal or alkyl group), or -NR
₂ (R is an alkyl group) and the like.

As a method for reducing the concentration of the low molecular weight component in the polyurethane resin, for example, a polyurethane resin synthesized by an ordinary method is reprecipitated in an organic solvent such as toluene for purification, or an organic solvent such as toluene is previously prepared. A method of using as a raw material a polyester purified by reprecipitation in a solvent, or a method of synthesizing a raw material polyester, a method of transforming a diol as a constituent component into a structure in which a low molecular weight component is difficult to be produced, and the like can be considered.

Here, the diol having a structure in which a low molecular weight component is hard to be produced is a diol having a relatively long chain, a cyclic diol, or the like. Because of the high reactivity of the long-chain diol and the low degree of freedom of deformation of the cyclic diol, the production of low molecular weight components can be suppressed to a low level during the polymerization reaction regardless of which one is used.

Examples of the long-chain diol include 1,4-
Butanediol, 1,5-pentanediol, 1,6-
Hexanediol, diethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol and ε-
Examples include lactone-based polyester diols obtained by ring-opening polymerization of lactones such as caprolactone. Examples of the cyclic diol include cyclohexanedimethanol, an ethylene oxide adduct of bisphenol A and a propylene oxide adduct, an ethylene oxide adduct of hydrogenated bisphenol A and a propylene oxide adduct, and the like.

In the magnetic recording medium of the present invention, in the magnetic layer, for example, magnetic powder is dispersed in the above-mentioned binder, and ethers, esters, ketones, aromatic hydrocarbons, and aliphatic carbons are used depending on the kind of the binder. Obtained by applying a magnetic coating material prepared by kneading with an organic solvent selected from hydrogen, chlorinated hydrocarbons, etc., to the surface of a non-magnetic support to form a magnetic coating film, and heating and curing this magnetic coating film. Be done.

Here, it is desirable that the curing temperature at the time of heat-curing the magnetic coating film is appropriately selected depending on the binder used. For example, the above-mentioned polyurethane resin in which the concentration of the low molecular weight component is controlled is used as the binder. If you do
The curing temperature is 75 ° C. or higher, more preferably 90 ° C. or higher. By curing the polyurethane resin at 75 ° C. or higher, the crosslinking reaction proceeds rapidly, and plasticization of the magnetic layer, which is induced by the remaining uncrosslinked resin, is prevented.

The magnetic powder used in the present invention may be an oxide magnetic powder or a metal magnetic powder, but it has a high coercive force and a high saturation magnetic flux density and is the main object of the present invention, high density recording. A metal magnetic powder is preferable because it is suitable for use as a powder.

Here, since the metallic magnetic powder is generally easily oxidized, there is a possibility that the magnetic characteristics such as saturation magnetization and residual magnetization may be deteriorated when heated and cured at the above curing temperature. When Co-containing ferromagnetic metal particles are used among the powders, the stability against oxidation is high, and thus sufficiently good magnetic properties are maintained even after the heat curing treatment.

In particular, the Co-containing ferromagnetic metal in which the surface layer of the particles disclosed in JP-A-3-174704 is laminated and coated as a film of a ferrite compound having a spinel structure composed of divalent cobalt and trivalent iron. The particles have high stability against oxidation and excellent shape retention, and are suitable as the magnetic powder. Further, the Co-containing ferromagnetic metal particles having such a ferrite compound coating having a spinel structure on the surface layer of the particles are needle-like, have a major axis length of at least 0.3 μm or less, and have a specific surface area of 45.
Those with m ² / g or more are excellent. When Co-containing ferromagnetic metal particles whose shape and size satisfy the above conditions are used, high output and low noise are exhibited in a wide recording frequency band.

In order to produce Co-containing ferromagnetic metal particles having a film of a ferrite compound having a spinel structure on the surface layer of particles, first, divalent valent metal oxide particles in a slurry in which oxyhydroxide particles containing iron as a main component are dispersed. Simultaneously neutralizes cobalt and trivalent iron. Then, at the same time as the neutralization, a gel which is directly formed as a spinel compound is immediately formed as a film in which the surfaces of the oxyhydroxide particles are laminated and coated. Further, the Co-containing ferromagnetic metal particles are obtained by heating and reducing the particles having the coating film as a starting material.

The Co-containing ferromagnetic metal particles shown above are used.
The most suitable powder is magnetic powder, but of course
Uses the used metal magnetic powder and oxide magnetic powder
There is no problem in doing so. Examples of metallic magnetic powder
For example, Fe, Co, Ni, Fe-Co, Fe-Ni, F
e-Co-Ni, Co-Ni, Fe-Co-B, Fe-
Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co
-V, etc., and further improve these various characteristics.
Al, Si, Ti, Cr, Mn, Cu, Zn, etc. for the purpose of
The metal component may be added. Also,
Hexagonal ferrite such as lithium ferrite, iron nitride, etc.
It can be used. As the oxide magnetic powder, for example, γ
-Fe₂O₃, Co-containing γ-Fe₂O₃, Co deposition γ-
Fe₂O ₃, Fe₃O_Four, Co-containing γ-Fe₃O_Four, C
o Deposition γ-Fe₃O_Four, CrO₂Etc.

On the other hand, the non-magnetic support for forming the above-mentioned magnetic coating film is required to have a glass transition point higher than the curing temperature, and it should be selected from the conventionally known non-magnetic supports in consideration of this point. Is desirable. For example, a curing temperature of 75
When the temperature is higher than or equal to ℃, a glass transition point of 100 ℃ or higher, specifically polyethylene naphthalate (PE
N), aramid, etc. are preferably used.

The above is a basic description of the magnetic recording medium of the present invention. In the above recording medium, for example, various dispersants, lubricants, antistatic agents, rust preventives, etc. may be added to the magnetic coating film as required. Additives such as agents may be added. Any conventionally known material can be used as these additives, and the additives are not limited at all.

[0041]

In the magnetic recording medium, the surface deformation of the magnetic layer due to repeated running is caused by the fact that plastic flow occurs when the magnetic layer makes sliding contact with the guide roll or the drum. When a binder having a stress relaxation time of 5000 seconds or more is used as the binder contained in the magnetic layer, such plastic flow of the magnetic layer is prevented and surface deformation due to repeated running is suppressed.

For example, in the case of polyurethane resin,
Plastic flow is caused by the presence of 400-1300 low molecular weight components due to the raw material. Therefore, in the polyurethane resin, by suppressing the concentration of the low molecular weight component, the stress relaxation time becomes 5000 seconds or more, and the binder becomes a plastic layer in which the magnetic flow hardly occurs. Here, when using the above-mentioned polyurethane resin in which the concentration of the low molecular weight component is controlled as a binder, the curing temperature of the magnetic coating film is 75 ° C.
With the above, the crosslinking reaction proceeds rapidly in the magnetic layer. Therefore, the plasticization of the magnetic layer due to the residual uncrosslinked resin is suppressed, and the magnetic recording medium has more excellent running durability.

When the magnetic coating film is heat-cured at the above-mentioned curing temperature, if Co-containing ferromagnetic metal particles are used as the magnetic powder, the stability against oxidation is high, and therefore the magnetic characteristics do not deteriorate during heat-curing. It will exhibit good characteristics.

[0044]

EXAMPLES The present invention will be described below with reference to specific examples, but it goes without saying that the present invention is not limited to these examples.

Example 1 This example is an example of a magnetic tape using a resin component A containing no low molecular weight component in a polyester polyurethane resin as a binder. First, the resin component A was synthesized. That is, a polyester polyurethane resin synthesized by an ordinary method (content of low molecular weight component: 2.6% by weight, hereinafter referred to as resin component a) is reprecipitated in toluene, and the obtained undissolved content is obtained. Separation and purification gave a resin component A. Using this resin component A, a magnetic paint was prepared with the following composition.

Composition of magnetic paint Metal magnetic powder (specific surface area 50 m ² / g) 100 parts by weight Resin component A 8 parts by weight Vinyl chloride-vinyl acetate copolymer 8 parts by weight Carbon black 4 parts by weight Alumina 8 parts by weight Butyl stearate 2 parts by weight Myristic acid 1 part by weight Methyl ethyl ketone 150 parts by weight Toluene 70 parts by weight The above materials were placed in a container and dispersed in a sand mill for 4 hours.
A magnetic paint was prepared by mixing.

Then, 4 parts by weight of the curing agent Coronate L was added, and this magnetic paint was applied onto one surface of polyethylene terephthalate having a thickness of 9.5 μm to form a magnetic coating film.
The magnetic coating film is annealed at a curing temperature of 60 ° C. for 20 hours. Further, a back coat was applied on the other surface of the polyethylene terephthalate and cut into a predetermined width to prepare a magnetic tape.

Example 2 A resin component B containing 2.2% by weight of a low molecular weight component in the resin was used in place of the resin component A used in Example 1 above.
A magnetic tape was manufactured in the same manner as in Example 1 except that.

The resin component B was synthesized by urethanizing only polyester (the same one used in the resin component a) and diisocyanate without using diol as a chain extender as a raw material. is there.

Example 3 In place of the resin component A used in Example 1 above, a resin component C containing 1.6% by weight of a low molecular weight component in the resin
A magnetic tape was manufactured in the same manner as in Example 1 except that.

The resin component C was obtained by reprecipitating the polyester in toluene to remove the oligomer component derived from the ester, and then urethanizing the undissolved component together with the diol and diisocyanate as chain extenders. It is a composite.

Example 4 In place of the resin component A used in Example 1 above, a resin component D containing 0.7% by weight of a low molecular weight component in the resin
A magnetic tape was manufactured in the same manner as in Example 1 except that.

When the raw material polyester is synthesized, the resin component D is changed so that the diol, which is a constituent component of the ester, has a structure that makes it difficult to form an oligomer component, and only the polyester and diisocyanate are urethanized. It was synthesized by.

Comparative Example 1 A magnetic tape was produced in the same manner as in Example 1 except that the resin component A used in Example 1 was replaced with the resin component a.

Here, in order to investigate the molecular weights and molecular weight distributions of the resin components A to D and the resin component a used in the above-mentioned respective examples and comparative examples, analysis was carried out by gel permeation chromatography (GPC). It was The measurement conditions were as follows: Shodex KF-80 as a column.
1 and KF-802 (both are trade names), tetrahydrofuran was used as a developing solvent, and the flow rate was 1.0 ml /
Minutes RI was used as the detector. As a result, GPC charts as shown in FIGS. 2 to 6 were obtained.

FIG. 2 is a GPC chart of the resin component A used in Example 1. In FIG. 2, the horizontal axis represents the outflow time, and the faster the outflow time, the larger the molecular weight of the component. Since the exclusion limit of the above column is 5000 or less, the polymers having a molecular weight of more than 5000 cannot be separated, and appear together in the peaks at around 10 to 12 minutes in FIG. Further, when the outflow time was 16 minutes or longer, the molecular weight was 200 or less, and a peak due to the solvent appeared, so data of 16 minutes or longer was deleted.
The vertical axis represents the intensity of the RI detector, which is proportional to the content of the component (hereinafter, the same applies to FIGS. 2 to 5).

As shown in FIG. 2, except for the above-mentioned peaks at around 10 to 12 minutes, no remarkable peaks appeared, so that it was found that the resin had almost no low-molecular weight component. Further, FIG. 3 is a GPC chart of components eluted in toluene when the resin component A was reprecipitated from toluene. As shown in FIG. 3, the outflow time is 13.5.
Peak a, peak b, and peak c appeared near the minutes, around 14.5 minutes, and slightly after 16.5 minutes, respectively.

Then, the molecular weight of the polymer derived from these peaks a to c was determined using a polystyrene molecular weight standard sample, and it was 1008 for peak a, 715 for peak b and 531 for peak c.

Furthermore, when the above-mentioned peaks a to c were analyzed by NMR and MS, peak a was a dimer of diol and diisocyanate as a chain extender produced during urethanization, and peak b was a raw material polyester synthesis. It was found that the diol and the acid produced at times were attributed to the trimer, and the peak c was also attributed to the diol and the acid dimer.

From these facts, when the resin component A was reprecipitated from toluene, the low molecular weight components such as the ester oligomer component and the urethane oligomer component were dissolved in toluene, and the resin component A separated and purified contained in the resin component A. It was confirmed that these low molecular weight components hardly remained.

FIG. 4 is a GPC chart of the resin component B used in Example 2, and FIG. 5 is a GPC chart of the resin component D used in Example 4. As shown in FIG. 4, the resin component B containing no chain extender was
The peak a attributed to the dimer formed at the time of urethanization of the raw material diol and diisocyanate, which was observed in Example 1, disappeared, and only the peaks b and c appeared. Furthermore, when the chain extender was not used and the diol in the raw material polyester was modified, the peak a disappeared, and the other peaks b and c hardly appeared.

On the other hand, FIG. 6 is a GPC chart of the resin component a used in Comparative Example 1. As shown in FIG. 5, in the polyester polyurethane resin obtained by the conventional synthesis method, the above-mentioned peaks a to c were clearly observed, which revealed that a large amount of low molecular weight components were contained.

Based on the above results, the content of the low molecular weight component in each resin component was determined, and the results are shown in Table 1.
Here, the content of the low molecular weight component is the peak (sum of peaks a to c) area of the low molecular weight component having a molecular weight of 400 to 1300 with respect to the total peak (peak around 10 to 16 minutes) area of each resin component. The ratio was used. In addition, Table 1 also shows the number average molecular weight M _N , the weight average molecular weight M _W of the polymer as the main component of each resin component, and the molecular weight distribution obtained from the ratio thereof.

[0064]

[Table 1]

Next, the Young's modulus, the glass transition temperature and the stress relaxation time of the resin component A, the resin component B, the resin component D and the resin component a were measured. Young's modulus and glass transition temperature were measured by a usual measuring method, and stress relaxation time was measured by a stress relaxation measurement test based on a single Maxwell model as follows. First, in order to measure the stress relaxation time, each resin component was molded into a cast film having a thickness of about 50 μm and punched into a length of 50 mm × width of 4 mm to obtain a measurement sample. Then, with respect to this measurement sample, using a precision tensile tester, the measurement length was 30 mm, the tensile stress was 12.25 MPa (1.25 kg weight / mm ² ).
The sample was held under tension under the condition (1) and the temporal change in stress (stress relaxation) was examined.

Table 2 shows the measured Young's modulus, glass transition temperature, and stress relaxation time of each resin component.

[0067]

[Table 2]

Looking at the stress relaxation time in Table 2, the stress relaxation time is 5 for resin component A, resin component B, and resin component D.
While it is long at 000 seconds or more, it is 3460 for resin component
Seconds and stress relaxation time are short. Therefore, from this, it was found that in the polyurethane resin, the stress relaxation time has an appropriate value when the concentration of the low molecular weight component is within the predetermined range. In the resin component A, the resin component B, and the resin component D, the Young's modulus and the glass transition point are 1.4 × 10 ⁹ N /, which are within the ranges in which good characteristics can be obtained.
It was found that it belongs to m ² or more and 60 ° C. or more and satisfies the properties required as a binder.

Next, using the magnetic tapes obtained in the respective examples and comparative examples, a durability test conducted on a medium DDS (digital data storage) which is usually used for data storage was conducted. That is, the test data is recorded on the entire area of each magnetic tape, the save set mark X and the save set mark Y are provided at predetermined positions on these magnetic tapes, and these save set marks X and Y are set.
The space was repeatedly run 100 times. Then, the increase rate Δμ of the friction coefficient μ due to plastic flow on the surface of the magnetic layer at the save set mark X (or save set mark Y) was measured. The results are shown in Table 3.

The increase rate Δμ of the friction coefficient μ is expressed by the following equation 4.

[0071]

[Equation 4]

[0072]

[Table 3]

When the above-described save set marks X and Y are repeatedly run, the save set mark X or the save set mark Y is searched every time the tape running is reversed, and this operation is accompanied. As a result, the tape reversely runs several times near the save set mark X (save set mark Y). Therefore, the save set mark X
On the surface of the magnetic layer near the (save set mark Y), the number of times of sliding with the guide or the drum is significantly increased, and the damage to the magnetic layer is larger than that of other portions. Therefore,
These save set mark X (save set mark Y)
In the vicinity of the magnetic layer, extremely good durability is required.

From the results shown in Table 3, it was found that in Examples 1 to 4, the increase rate Δμ of the friction coefficient μ was small and the plastic flow on the surface of the magnetic layer was suppressed. From this, as a binder, the stress relaxation time is 5
It has been clarified that the running durability can be remarkably improved by using a material of 000 seconds or more, for example, a polyurethane resin containing 2.5% by weight or less of a low molecular weight component in the polyurethane resin.

The relationship between the stress relaxation time (in log) and the increase rate Δμ of the friction coefficient is shown in FIG. 7. As described above, the stress relaxation time and the increase rate Δμ of the friction coefficient are correlated. It is also suggested that regulating the stress relaxation time of the binder is effective in improving the running property of the magnetic recording medium.

Examination of Curing Temperature of Magnetic Coating Film Next, the relationship between the curing temperature of the magnetic coating film and the running durability was examined when a polyurethane resin having a controlled low molecular weight component concentration was used as a binder. First, a magnetic paint was prepared with the following composition.

Composition of magnetic paint Metal magnetic powder (specific surface area 40 m ² / g) 100 parts by weight Polyurethane resin having low molecular weight components removed 8 parts by weight Vinyl chloride-vinyl acetate copolymer 8 parts by weight Carbon black 4 parts by weight Alumina 8 Parts by weight butyl stearate 2 parts by weight myristic acid 1 part by weight methyl ethyl ketone 150 parts by weight toluene 70 parts by weight The above materials were placed in a container and stirred with a ball meal for 24 hours to prepare a magnetic paint.

Then, 4 parts by weight of the curing agent Coronate L was added, and this magnetic coating material was applied onto one surface of a non-magnetic support having a film thickness of 7.3 μm to form a magnetic coating film. Was cured by annealing for 20 hours at various curing temperatures. Further, a back coat was applied on the other surface of the non-magnetic support and cut into a predetermined width to prepare magnetic tapes (Sample Tape 1 to Sample Tape 6). The non-magnetic support used here was polyethylene terephthalate (manufactured by Teijin Ltd.,
There are three types: those having a glass transition point ≈70 ° C.), polyethylene naphthalate (made by Teijin, glass transition point ≈120 ° C.), and aramid (made by Toray, glass transition point ≈280 ° C.). Table 4 shows the curing temperatures of the non-magnetic support and the magnetic coating film used for each sample tape.

[0079]

[Table 4]

With respect to the magnetic tape thus produced, the cross-linking ratio of the magnetic layer and the running durability were examined.

In this example, the crosslinking rate was evaluated by conducting a solvent resistance test. That is, in order to carry out the solvent resistance test, a magnetic tape is stuck on a slide glass, and the magnetic layer of the magnetic tape is rubbed in a certain direction with a cotton swab impregnated with an organic solvent (methyl ethyl ketone). here,
When the crosslinking reaction of the magnetic layer has not progressed, the low-molecular component of the binder dissolves in the solvent and the magnetic layer easily peels off. Since the amount of low-molecular components is small, the magnetic layer is hard to peel off. That is,
The solvent resistance is evaluated by measuring the number of rubbing times required for the magnetic layer to peel off.

Further, the running durability was evaluated by running each magnetic tape 100 times under a hot and humid condition of a temperature of 40 ° C. and a relative humidity of 80% using a deck for data of 8 mm, before and after running. This was done by comparing the friction of the magnetic tapes. Table 5 shows the results of measuring the solvent resistance (rubbing frequency of the cotton swab) and the friction before and after repeated running.

[0083]

[Table 5]

As can be seen from Table 5, the magnetic coating film was treated at 75 ° C.
In the sample tapes 1 to 5 cured at the above curing temperatures, the magnetic layer has high solvent resistance, and the increase in friction due to repeated running is low, and the cross-linking ratio of the magnetic layer and the running durability are excellent. It has become a thing. On the other hand, in the sample tape 6 in which the magnetic coating film was cured at 75 ° C. or lower, the solvent resistance was low, the friction increased due to repeated running, and the crosslinking rate and running durability were insufficient.

From this, therefore, it was found that the curing temperature of 75 ° C. or higher is suitable when the polyurethane resin from which the low molecular weight has been removed is used as the binder. As a comparison, the curing temperature and running durability of the magnetic coating film when a polyurethane resin in which the concentration of the low molecular weight component was not controlled were used as a binder were also examined.

That is, in the above-mentioned magnetic tape, a low-molecular weight component-free polyurethane resin was used in place of the low-molecular weight component-free polyurethane resin, and the curing temperatures of the non-magnetic support and the magnetic coating film are shown in Table 6. Magnetic tapes (Comparative Tape 1 to Comparative Tape 3) were prepared by changing as shown.

[0087]

[Table 6]

With respect to the magnetic tape thus produced, the crosslinking rate and running durability were examined in the same manner as described above. The results are shown in Table 7.

[0089]

[Table 7]

As can be seen from Table 7, Comparative Tape 1 and Comparative Tape 2 have low solvent resistance and are insufficient in both the crosslinking rate and running durability. Therefore, from this, when using a polyurethane resin from which the low molecular weight component has not been removed, it is shown that running durability cannot be obtained even if the curing temperature is increased, and the low molecular weight component is not added to the polyurethane resin. The effect of removal has become clearer.

Further, in the comparative tape 3, the non-magnetic support was significantly deformed during curing and could not be cut into a predetermined width, and the running durability could not be measured. This is because the glass transition point of the polyethylene terephthalate film used as the non-magnetic support is much lower than the curing temperature, so that the non-magnetic support undergoes glass transition during curing. From this, it was found that it is necessary to select a non-magnetic support having a glass transition point higher than the curing temperature of the magnetic coating film.

Examination of Magnetic Powder Next, the relation between the magnetic powder to be used and the magnetic characteristics was examined when the curing temperature when the magnetic coating film was cured by heating was 75 ° C. or higher.

First, a magnetic paint having the following composition was prepared. Composition of magnetic paint Magnetic powder (specific surface area 50 m ² / g) 100 parts by weight Polyurethane resin with low molecular weight components removed 8 parts by weight Vinyl chloride-vinyl acetate copolymer 8 parts by weight Carbon black 4 parts by weight Alumina 8 parts by weight Butyl steer Rate 2 parts by weight Myristic acid 1 part by weight Methyl ethyl ketone 150 parts by weight Toluene 70 parts by weight The above materials were placed in a container and stirred with a ball meal for 24 hours to prepare a magnetic paint.

Then, 4 parts by weight of the curing agent Coronate L was added, and this magnetic coating material was applied onto one surface of a non-magnetic support having a film thickness of 7.3 μm to form a magnetic coating film. Was cured by annealing for 20 hours at various curing temperatures. Further, a back coat was applied on the other surface of the non-magnetic support and cut into a predetermined width to prepare magnetic tapes (Sample Tape 7 to Sample Tape 14). In each sample tape, as the non-magnetic support, one made of polyethylene terephthalate, polyethylene naphthalate, or aramid was selected and used with a glass transition point higher than the set curing temperature.

Table 8 shows the non-magnetic support, the curing temperature of the magnetic coating and the magnetic powder used for each sample tape.

[0096]

[Table 8]

With respect to the magnetic tape thus manufactured, the deterioration rate of the magnetic properties by the heat curing treatment, the crosslinking rate of the magnetic layer and the running durability were examined.

Here, the deterioration rate of the magnetic properties was determined by measuring the residual magnetization amount of the magnetic coating film before and after the heat curing treatment with a vibrating sample magnetometer and substituting the measured value into the following formula. I asked. (Residual magnetization before curing-Residual magnetization after curing) / (Residual magnetization before curing) x 100 = Deterioration rate of magnetic properties (%) Further, the crosslinking rate of the magnetic layer (solvent resistance) and running durability The sex was evaluated in the same manner as described above.

Table 9 shows the results of measurement of solvent resistance (rubbing frequency of a cotton swab), friction before and after repeated running, and deterioration rate of magnetic properties.

[0100]

[Table 9]

As can be seen from Table 9, first, regarding running durability and solvent resistance, the sample tape 10 and the sample tape 14 in which the magnetic coating film was cured at 75 ° C. or lower were used.
In the same manner as the previous experiment, it was suggested that the curing temperature of the magnetic coating film should be 75 ° C or higher in order to obtain sufficient running durability and solvent resistance. .

Next, looking at the deterioration rate of the magnetic characteristics of the magnetic tape in which the curing temperature of the magnetic coating film is 75 ° C. or higher, the magnetic tape using Co-containing ferromagnetic metal particles as the magnetic powder (Sample Tape 7 to Sample With the tape 9), even if the curing temperature is increased to 120 ° C., the deterioration rate of the magnetic characteristics does not increase so much, and the deterioration of the magnetic characteristics due to the heat curing treatment hardly occurs.

On the other hand, in the magnetic tapes (sample tape 11 to sample tape 13) using the ferromagnetic metal particles containing no Co as the magnetic powder, the deterioration rate of the magnetic characteristics greatly increases as the curing temperature increases. However, the magnetic characteristics are likely to deteriorate due to the heat curing treatment.

Therefore, it is effective to use the Co-containing ferromagnetic metal particles as the magnetic powder for suppressing the deterioration of the magnetic properties due to the heat curing treatment, which results in the running durability, solvent resistance and magnetic properties. It was found that the characteristics can be compatible.

[0105]

As is apparent from the above description, in the magnetic recording medium of the present invention, the stress relaxation time obtained by the stress relaxation test using the single Maxwell model of the binder is 5000 seconds or more. Since the agent is used, the surface of the magnetic layer is not deformed even when repeatedly slid against a guide roll or the like, and good running durability can be obtained. In particular, when a polyurethane resin from which low molecular weight components have been removed is used as the binder, the running durability as well as the electromagnetic conversion characteristics, abrasion resistance, and heat resistance are excellent.

Further, in the present invention, when a polyurethane resin having a low molecular weight concentration regulated is used as a binder, the curing temperature of the magnetic coating film is set to 75 ° C. or higher, so that the plasticizing of the magnetic layer is effective. Is prevented, and more excellent running durability is obtained.

Furthermore, since Co-containing ferromagnetic metal particles are used as the magnetic powder, the magnetic characteristics do not deteriorate even when the curing temperature of the magnetic coating film is set high, and both running durability and magnetic characteristics are compatible. Is possible. Therefore, according to the present invention, it is possible to provide a magnetic recording medium suitable for use as a data cartridge (helical scan method) that requires particularly excellent durability.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a single Maxwell model which is a principle of measuring stress relaxation time.

FIG. 2 is a GPC chart of a polyester polyurethane resin obtained by reprecipitation in toluene by gel permeation chromatography (GPC).

FIG. 3 is a GPC chart of components eluted in toluene when a resin component was reprecipitated in toluene.

FIG. 4 is a GPC chart of a polyester polyurethane resin synthesized by urethanizing polyester and diisocyanate without introducing a diol as a chain extender.

FIG. 5 is a polyester polyurethane resin synthesized by urethanizing a polyester and a diisocyanate that are modified so that a diol that is a constituent component of an ester does not easily form an oligomer component without introducing a diol as a chain extender. It is a GPC chart of.

FIG. 6 is a GPC chart of a polyester polyurethane resin obtained by a conventional synthesis method.

FIG. 7 is a characteristic diagram showing a relationship between a stress relaxation time of a resin component and an increase rate of a friction coefficient of a magnetic recording medium.

Front page continued (72) Inventor Keiko Nihei 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation (72) Inventor Atsushi Hashimoto 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation Shares In-house (72) Inventor Honoka Momoi 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation

Claims (5)

[Claims] 1. A magnetic recording medium comprising a magnetic layer mainly composed of magnetic powder and a binder formed on a non-magnetic support, which is obtained by a stress relaxation test using the single Maxwell model of the binder. A magnetic recording medium having a stress relaxation time of 5000 seconds or more. 2. A magnetic recording medium comprising a magnetic layer mainly composed of magnetic powder and a binder formed on a non-magnetic support, wherein the binder comprises 2.5 parts by weight of a low molecular weight component having a molecular weight of 400 to 1300. % Of a polyurethane resin is contained in the magnetic recording medium. 3. The magnetic recording medium according to claim 2, wherein the curing temperature of the magnetic layer is 75 ° C. or higher. 4. The glass transition point of the non-magnetic support is 100 ° C.
4. The magnetic recording medium according to claim 3, which is the above. 5. The magnetic recording medium according to claim 1, wherein the magnetic powder is Co-containing ferromagnetic metal particles.