JPH0680529B2 - Magnetic recording medium - Google Patents

Description

The present invention relates to a magnetic tape or other magnetic recording medium having a back coat layer and using magnetic metal powder as a magnetic recording element.

[Conventional technology]

For magnetic recording media such as video tapes and audio tapes, a magnetic layer formed using a smooth non-magnetic support to improve its electromagnetic conversion characteristics such as output and signal-to-noise ratio, and on its main surface. In recent years, the average surface roughness Ra of the magnetic layer is about 0.002 to 0.010 μm (the maximum surface roughness Rmax is 0.015 to 0.09 μm) for applications requiring high density recording. A very good smoothness of the magnetic layer surface has been achieved.

However, as the surface becomes smoother as described above, the contact area with the object to be contacted increases and the friction coefficient increases, so that the support between the back surface of the support and the guide portion of the recording / reproducing device or in the wound state. The contact resistance increases between the back surface and the surface of the magnetic layer, which deteriorates the running property and prevents normal recording / reproduction. Therefore, there has been adopted a means for improving runnability by providing a back coat layer containing non-magnetic powder on the back surface of a smoother non-magnetic support to roughen the surface.

However, when roughening is performed by such a back coat layer, the rough surface of the back coat layer slides on the surface of the magnetic layer by sliding the rough surface of the back coat layer while the magnetic recording medium is wound and running. The surface of the magnetic layer is scratched by being pressed perpendicularly to the surface of the magnetic layer under the condition that the magnetic layer is wound and left stationary. There is a problem that the surface smoothness is impaired and the electromagnetic conversion characteristics are deteriorated.

For this reason, conventionally, for example, the non-magnetic powder contained in the back coat layer has to be composed of plate-like particles having a Mohs hardness of 3 or more (JP-A-59-92436), and polyisocyanate is used as a binder for the back coat layer. It has been proposed to use a compound and to set the porosity of the layer to 12.5% or less (JP-A-59-116930) so that the back coat layer itself has a composition in which the surface state thereof is less likely to transfer to the surface of the magnetic layer. ing.

[Problems to be solved by the invention]

However, in the case of using metal magnetic powder as the magnetic powder to be contained in the magnetic layer, particularly when using fine particle-shaped metal magnetic powder having a large specific surface area with a particle diameter of less than 0.3 μm that can meet the recent demand for high density recording, Since the magnetic layer is much softer and more likely to be scratched compared to the one using oxide magnetic powder such as γ-Fe ₂ O ₃ , the average surface roughness Ra of the magnetic layer is about 0.002 to 0.010 μm as described above. When high surface smoothness is imparted, it is difficult to uniformly prevent the surface state of the back coat layer from transferring to the surface of the magnetic layer only by the composition of the back coat layer as in the proposed method. On the other hand, if the smoothness of the back coat layer is increased, the above-mentioned problem of transfer is solved, but the function of improving the running property due to the roughening of the back coat layer is lost.

Therefore, the present invention has a magnetic layer containing metal magnetic powder in the form of very fine particles and having high surface smoothness, and suppresses the above-mentioned transition to a minimum, and further exhibits the original function of the back coat layer. By doing so, it is an object of the present invention to provide a magnetic recording medium excellent in electromagnetic conversion characteristics and running properties.

[Means for solving problems]

As a result of extensive studies conducted by the present inventors for the above purpose, the phenomenon that the surface state of the back coat layer is transferred to the surface of the magnetic layer is apt to occur when the metal magnetic powder is used. The transition phenomenon becomes remarkable when the surface roughness of the back coat layer is above a specific ratio with respect to the surface roughness of the magnetic layer, and when the surface roughness of the back coat layer is below a specific value, the running property is reduced. The inventors have found that the improvement effect is impaired and have completed the present invention.

That is, the present invention, a magnetic layer containing a metal magnetic powder is formed on the main surface of the non-magnetic support, a back coat layer is formed on the back surface, the average surface roughness of the magnetic layer is Ra ^M , the average surface of the back coat layer. The magnetic recording medium is characterized in that the surface roughness of both layers when the roughness is Ra ^B is in the range of 0.010 μm ≦ Ra ^B <7 × Ra ^M ≦ 0.070 μm.

[Constitution / Operation of Invention]

As described above, the magnetic recording medium of the present invention has the magnetic layer containing the metallic magnetic powder on the main surface of the non-magnetic support and the back coat layer for improving the running property on the back surface of the support. The average surface roughness Ra ^B of the back coat layer is less than 7 times the average surface roughness Ra ^M of the magnetic layer. That is, in the case of the magnetic layer containing the metal magnetic powder, the phenomenon in which the surface state of the back coat layer is transferred to the surface of the magnetic layer is more likely to occur as the surface smoothness of the magnetic layer becomes higher, and the surface smoothness of the magnetic layer becomes higher. Is required to enhance the smoothness of the back coat layer, and when the surface smoothness is set to be relatively low, the smoothness of the back coat layer may be relatively low, but the back coat with respect to the average surface roughness Ra ^M of the magnetic layer is If the average surface roughness Ra ^B of the layer is more than 7 times, the transition phenomenon becomes extremely remarkable, which is not preferable.

Further, in the present invention, the average surface roughness Ra ^M of the magnetic layer is 0.010 μm.
m or less and the average surface roughness Ra ^B of the back coat layer
It is important that the thickness is 0.010 μm or more. That is, as described above, in order to meet the recent demand for higher recording density, it is necessary to set the surface smoothness of the magnetic layer to a high region capable of realizing high output and high SN ratio, and the above Ra ^M is 0.010. If it is larger than μm, the electromagnetic conversion characteristics become insufficient. The transition phenomenon becomes a particularly serious problem in the region where the surface of the magnetic layer has high surface smoothness. On the other hand, when the surface of the back coat layer is too smooth, the friction coefficient becomes high and the original effect of improving the running property is impaired. Therefore, Ra ^B is 0.010 μm.
It is necessary to do above. By setting such a Ra ^B value, although there is some variation depending on the type of magnetic recording medium, the friction coefficient measured by the method described in the examples below is usually suppressed to 0.3 or less. It becomes possible to do.

Average surface roughness Ra of the above-mentioned magnetic layer and back coat layer
The relationship between ^M and Ra ^B can be summarized as follows.

0.010 μm ≦ Ra ^B <7 × Ra ^M ≦ 0.070 μm That is, in the present invention, the surface roughness of the magnetic layer and the back coat layer may satisfy the above formula. The average surface roughness referred to here means the center line average roughness Ra (μm) measured using a stylus roughness meter.

In this invention, as the magnetic powder contained in the magnetic layer,
In order to achieve high recording density, fine particles of metallic magnetic powder with an average particle diameter (average major axis diameter) of less than 0.3 μm and a specific surface area of 40 m ² / g or more by nitrogen adsorption method (hereinafter referred to as BET method). Is preferred. That is, when using a metal magnetic powder having a larger particle diameter than the above, it becomes difficult to increase the recording density to a certain degree or more, as in the case of using an oxide magnetic powder such as γ-Fe ₂ O ₃ . Not desirable.
However, this does not mean that the use of metal magnetic powder having a particle size larger than the above is completely excluded. In some cases, such metal magnetic powder can be used.

Examples of the metal magnetic powder include Fe, Ni, Co, alloys of these metals, and various metal magnetic powders conventionally known as magnetic recording elements such as alloys containing these metals and other metals or a small amount of non-metal elements. Both can be used. The particularly preferred particle size of these metal magnetic powders is that the average major axis diameter is 0.
Average axial ratio (major axis diameter / minor axis diameter) of about 1 to 0.3 μm is 5
About 10 is preferable, and the BET specific surface area is 40 m ² / g or more as described above.

In order to form such a magnetic layer, it may be carried out in the same manner as in the conventional method. A magnetic paint containing the above magnetic powder, a binder, and various additives to be mixed as necessary is prepared, and this magnetic paint is used as a polyester base. After coating and drying on a non-magnetic support such as a film, calendering or other necessary surface treatment may be performed. The thickness of the magnetic layer thus formed is preferably about 0.5 to 2.0 μm. The surface smoothness of the magnetic layer is determined by the particle size and amount of the metal magnetic powder, the type and cross-linking degree of the binder component, the particle size and amount of other solid additives, the magnetic metal powder and other solid additives in the magnetic paint. It is determined by the dispersion state of the agent, the surface roughness and applied pressure of the roll used for calendering, the surface roughness of the non-magnetic support, etc., but those skilled in the art can easily set it within the above range by appropriately changing these elements. It is possible.

As the binder to be added to the magnetic paint, any of conventionally known ones such as vinyl chloride-vinyl acetate copolymer, fibrin resin, polyvinyl butyral resin, polyester resin, polyurethane and polyisocyanate compound can be used. it can. As additives to be added to the magnetic coating as needed, various known additives for magnetic recording media of this kind, such as lubricants, abrasives, antistatic agents, dispersants, fillers and colorants, can be used. Can be used.

The back coat layer in the present invention can be formed in the same manner as in the prior art. Usually, a paint containing a non-magnetic powder and a binder that binds the non-magnetic powder is prepared, and the paint is applied to the side of the non-magnetic support opposite to the magnetic layer. It can be formed by applying on the back surface, drying, and if necessary, surface processing such as calendering. The back coat layer may be formed either before or after the formation of the magnetic layer. The thickness of the back coat layer is
It is preferable that the thickness is about 0.5 to 2.0 μm. The surface smoothness of such a back coat layer is similar to that of the magnetic layer in terms of particle size and blending amount of non-magnetic powder and dispersion state, binder type and degree of crosslinking, and surface roughness of roll used for calendering. It is determined by the surface roughness of the non-magnetic support, etc., and can be easily set empirically by those skilled in the art by appropriately changing these elements.

As the non-magnetic powder contained in the back coat layer, a hard inorganic pigment having a Mohs hardness of 5 or more and a Mohs hardness of 5 are used.
One or both of the soft inorganic pigments of less than 1 can be used, and the combination of both is particularly preferable. That is, by using the hard inorganic pigment and the soft inorganic pigment in combination, the back coat layer has appropriate flexibility and hardness, and the wear resistance of itself and the contact object such as the guide portion of the recording / reproducing apparatus. Both the non-wearing property against

Examples of the hard inorganic pigment include aluminum oxide (Al ₂ O ₃ ), α-Fe ₂ O ₃ , titanium oxide (TiO ₂ ), chromium oxide (Cr ₂ O ₃ ), titanium carbide (TiC), silicon oxide (SiO ₂ ). ₂ )
And the like, and those having an average particle diameter of 0.8 μm or less are particularly preferable. Examples of the soft inorganic pigment include barium sulfate (BaSO ₄ ), calcium silicate (Ca ₂ SiO ₄ ), calcium carbonate (CaCO ₃ ), calcium sulfate (CaSO ₄ ), magnesium carbonate (MgCO ₃ ), zinc carbonate ( ZnCO ₃ ), zinc oxide (ZnO) and the like are mentioned, and those having an average particle diameter of 0.2 μm or less are particularly preferable. When the hard inorganic pigment and the soft inorganic pigment are used in combination, the hard inorganic pigment preferably accounts for 1 to 50% by weight, and more preferably 5 to 30% by weight in the total amount of both.

Further, the binder used in the back coat layer is not particularly limited, polyurethane resin, vinyl chloride-vinyl acetate copolymer, polyvinyl butyral resin, fiber resin,
Any conventionally known polyvinyl acetal resin, polyester resin or the like can be used.

Furthermore, in the coating material for forming the back coat layer, if necessary, an antistatic agent such as carbon black or graphite,
Additives such as lubricants and dispersants may be blended appropriately. In particular, carbon black has an antistatic effect as well as an effect of improving mechanical strength and providing light-shading properties for a video tape, and preferably accounts for 3 to 95% by weight in the total amount with the inorganic pigment. It can be used in proportion.

〔The invention's effect〕

The magnetic recording medium according to the present invention makes the surface roughness of the magnetic layer containing the metal magnetic powder not more than a specific value, and the surface roughness of the back coat layer is not more than a fixed ratio to the surface roughness of the magnetic layer, and Since the magnetic layer has a specific value or more, the magnetic layer is soft and easily scratched, and has a high surface smoothness. The phenomenon that the surface condition of the layer is transferred to the magnetic layer hardly occurs, the deterioration of the electromagnetic conversion characteristics is prevented, and good running property is exhibited in the back coat layer.

〔Example〕

Hereinafter, examples of the present invention will be described in comparison with comparative examples. In the following, "parts" means "parts by weight".

Example Needle-like Fe magnetic metal powder (average major axis diameter 0.29 μm, average axial ratio 7,
Specific surface area by BET method 47 m ² / g) 100 parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (product name by UCC, USA VAGH 10 parts Polyurethane (product name by Dainippon Ink and Chemicals Pandex T
−5201) 12 parts α-Fe ₂ O ₃ powder (average particle size 0.5 μm) 6 parts Toluene-cyclohexanone-methylethylketone mixed solvent (weight ratio 1: 1: 1) 230 parts The above composition is mixed and dispersed in a ball mill for 96 hours. After that,
A magnetic coating material was prepared by adding 3 parts of a polyisocyanate compound (trade name: Desmodule L, manufactured by Sumitomo Bayer Co., Ltd.) as a curing agent and mixing them uniformly. This magnetic paint is applied to the main surface of a 15 μm thick polyester base film with a thickness of 1.
The magnetic layer was applied so as to have a thickness of 0 μm, oriented and dried to form a magnetic layer, which was subjected to surface treatment by calendering. During this surface treatment, calender rolls with different surface smoothness were used to obtain a surface roughness Ra ^M of 0.010 μm as shown in the table below.
A number of different samples were made below.

Next, a back coat layer was formed on the back surface of the polyester base film of each of the samples obtained as described above in the following manner.

Barium sulfate powder (average particle diameter 0.08 μm) 50 parts α-Fe ₂ O ₃ powder (average particle diameter 0.5 μm) 5 parts Carbon black (average particle diameter 0.24 μm, DBP oil absorption 60 m
l / 100g, volatile content 5%) 45 parts Nitrocellulose 40 parts Polyurethane (Dainippon Ink and Co., trade name HI-2000) 29 parts Stearic acid 3 parts Toluene-cyclohexanone-methyl ethyl ketone mixed solvent (weight ratio 1: 1: 1) 620 parts of the above composition in a ball mill to obtain a plurality of different dispersion states by changing the dispersion time in the range of 12 to 72 hours,
A plurality of paints were prepared by adding 12 parts of a polyisocyanate compound (described above) as a curing agent. Apply these coatings to the back surface of the samples with different surface roughness Ra ^{M so} that the thickness after drying becomes 1.0 μm, dry to form a back coat layer, and then cut into 1/2 inch width. Thus, a plurality of different magnetic tapes were prepared as shown in the table below, in which the surface roughness Ra ^B of the back coat layer was 0.010 μm or more and less than 7 × Ra ^M.

Comparative Example The surface roughness Ra ^B of the back coat layer was adjusted to 0.010 as shown in the table below by changing the dispersion time at the time of preparing the back coat coating composition.
except that it is outside the range of μm ≦ Ra ^B <7 × Ra ^M
A plurality of magnetic tapes were produced in the same manner as in the example.

For the magnetic tapes obtained in the above Examples and Comparative Examples, the video SN ratio was measured, and then an accelerated storage test was conducted to investigate the phenomenon in which the surface state of the back coat layer was transferred to the surface of the magnetic layer. Video SNR was measured. The table below shows the difference (dB) in the video SN ratio before and after this accelerated storage test. In the table, the area surrounded by the alternate long and short dash line is the magnetic tape of the example, and the others are the magnetic tape of the comparative example. On the other hand, for these magnetic tapes, the relationship between the surface roughness Ra ^B of the back coat layer and the friction coefficient was investigated, and the results are shown in the drawings. However, the accelerated storage test, the surface roughness measurement, and the friction coefficient of the back coat layer were measured as follows.

<Accelerated Storage Test> A magnetic tape was incorporated in a video cassette as a reproduction length of 120 minutes, and was wound in a reproduction state at a winding tension of about 100 g and stored for 120 hours at 60 ° C. and 80% RH.

<Measurement of surface roughness> As described above, the centerline average roughness Ra (μ
m) was measured. In addition, the indicated value is the stylus speed 0.03 mm /
Second, run 0.25mm under the condition of 0.08mm of cutoff and Ra
The operation for obtaining is repeated 10 times, and the obtained Ra is shown as the average value.

<Friction coefficient of back coat layer> A 4 mm diameter SUS304 cylinder (surface roughness 0.2S) is supported horizontally, and magnetic tape is applied to this at an angle of 90 ° with the back coat layer in contact with the cylinder surface. The stress (T) g when the other end was pulled horizontally at a speed of 1.4 cm / sec while applying the load was calculated, and this stress (T) g was applied to the following formula to calculate the friction coefficient μ.

As is clear from the above table, the surface roughness Ra of the back coat layer is
^{When B} exceeds 7 times the surface roughness Ra ^M of the magnetic layer, the deterioration of the video SN ratio becomes noticeable after the magnetic tape is stored in a wound state. This is because the surface state of the back coat layer is transferred to the surface of the magnetic layer during the above storage and the surface smoothness of the magnetic layer is impaired. On the other hand, as is clear from the drawing, when the surface roughness Ra ^B of the back coat layer is smaller than 0.010 μm, the friction coefficient sharply increases, which causes a problem in running property.

In the examples and comparative examples described above, the surface roughness Ra ^M of the magnetic layer was set by the surface smoothness of the calender roll in calendering, and the surface roughness of the back coat layer was also set.
Although Ra ^B is set in the dispersed state of the back coat paint, these may be set by changing other factors that influence the surface roughness as described above. For example, in the magnetic layer, the setting means by changing the particle size, blending amount, dispersion state and combinations of the magnetic powder and other solid additives, and in the back coat layer, the particle size of the non-magnetic powder is changed and the surface smoothness is different. It is possible to adopt setting means such as calendering with rolls.

[Brief description of drawings]

The drawing is a characteristic diagram showing the relationship between the surface roughness Ra ^B of the back coat layer of the magnetic tapes obtained in Examples and Comparative Examples of the present invention and the friction coefficient.

Claims (3)

[Claims] 1. A magnetic layer containing metallic magnetic powder is formed on the main surface of a non-magnetic support, and a back coat layer is formed on the back surface. The average surface roughness of the magnetic layer is Ra ^M , and the average surface of the back coat layer. A magnetic recording medium characterized in that the surface roughness of both layers when the roughness is Ra ^B is in the range of 0.010 μm ≦ Ra ^B <7 × Ra ^M ≦ 0.070 μm. 2. The magnetic recording medium according to claim 1, wherein the average particle diameter of the metal magnetic powder is less than 0.3 μm. 3. The specific surface area of the magnetic metal powder measured by the nitrogen adsorption method is
The magnetic recording medium according to claim (1) having a size of 40 m ² / g or more.