JPH0666089B2 - Magnetic recording medium - Google Patents

Description

The present invention relates to a magnetic tape and other magnetic recording media.

[Prior Art] In recent years, high density recording and high SN ratio of video tapes have been promoted, and the magnetic powder used has also been made finer, and the specific surface area by the nitrogen adsorption method is 30m ² / g or more. A small one is used. On the other hand, as the type of magnetic powder, iron oxide-based magnetic powders such as γ-Fe ₂ O ₃ and Co-containing γ-Fe ₂ O ₃ are generally used for recording / reproducing with an appropriate coercive force and for convenience of handling and other points. Is being used for.

However, these iron oxide-based magnetic powders are brown to brown and have a color, and when such magnetic powders are made into fine particles, the light transmittance of the magnetic layer increases, and the optical density of VHS system video tape recorders is increased. With the method of detecting the end of the tape, malfunctions are likely to occur.

In order to solve this drawback, in the past, measures were taken to increase the amount of pigments with high light shining and opacity such as carbon black, but the dispersibility of the above pigments was poor and the proportion in the magnetic layer was large. As a result, the surface smoothness and electromagnetic conversion characteristics of the magnetic layer deteriorate. It is also conceivable to increase the thickness of the magnetic layer to reduce the light transmittance, but the magnetic tape has a standard tape thickness that depends on each application. It is necessary to reduce the thickness of the support as described above, and the mechanical strength of the tape is reduced, which causes troubles.

Therefore, the inventor focused on the color of the iron oxide-based magnetic powder,
Fe ²⁺ content of this magnetic powder is lighter brown as the content is lower, and conversely as Fe ²⁺ content is higher as it tends to be brown and color to black, the latter Fe ²⁺ content is included. When a magnetic tape was made using a large amount of iron oxide-based magnetic powder such that the content of Fe ²⁺ becomes 7.0 wt% or more in terms of FeO, the above-mentioned measures such as increasing the amount of pigment were not taken. It was found that the light transmittance of the magnetic layer can be reduced.

However, as a result of subsequent studies, using the above-mentioned magnetic powder containing a large amount of Fe ²⁺ can reduce the light transmittance of the magnetic layer, but deteriorates the erasing characteristics of the magnetic tape at the beginning and after storage (magnetic recording). Was difficult to erase), and it was found that there was a problem that this was difficult to put into practical use as a magnetic tape. Therefore, in order to solve this problem, it was attempted to mix a magnetic powder having a small Fe ²⁺ content, such as 2.0% by weight or less in terms of FeO, together with a magnetic powder having a large Fe ²⁺ content as described above. If a large amount of magnetic powder with a low Fe ²⁺ content is blended, the light transmittance cannot be satisfied, and conversely, if a large amount of magnetic powder with a high Fe ²⁺ content is blended, the erasing property will deteriorate, and various blending ratios will be investigated. Even looking at it, I could not satisfy both characteristics.

[Problems to be solved by the invention]

From the above viewpoints, the present invention solves the problem of deterioration of the erasing property at the initial stage and after aging when the Fe ²⁺ content of finely divided iron oxide-based magnetic powder is increased to reduce the light transmittance. And to obtain a magnetic recording medium satisfying both the light transmittance and the erasing characteristics.

[Means for solving problems]

The present inventor, as a result of diligent studies to achieve the above-mentioned object, first of all, in order to achieve both light transmittance and erasing characteristics by setting the Fe ²⁺ content, the magnetic layer has a single-layer structure as described above. Based on this idea, the magnetic layer has a two-layer structure, and magnetic powder with a low Fe ²⁺ content is used for the inner layer on the side of the support where it is difficult to erase magnetic recording.
When using a magnetic powder with a high Fe ²⁺ content in the surface layer that is relatively easy to erase, the light transmittance was low and was not possible with the conventional single layer structure, and the initial and after aging The inventors have completed the present invention, knowing that a magnetic recording medium having excellent erasing characteristics can be obtained.

That is, the present invention provides a magnetic recording medium comprising a non-magnetic support and a two-layer magnetic layer comprising a first layer adjacent to the support and a second layer adjacent to the support, The magnetic powder contained in both layers is made of iron oxide magnetic powder having a specific surface area of 30 to 60 m ² / g by the nitrogen adsorption method, and the proportion of Fe ^{2+ in} the magnetic powder in the first layer Is
The present invention relates to a magnetic recording medium, characterized in that it is 0.1 to 3.0% by weight in terms of FeO, and the proportion of Fe ^{2+ in} the magnetic powder of the second layer is 3.5 to 10% by weight in terms of FeO.

[Constitution / Operation of Invention]

The magnetic powders used in the first and second magnetic layers in this invention are both specific surface areas (hereinafter referred to as BET
It is an iron oxide-based magnetic powder with a specific surface area of 30 to 60 m ² / g. In addition to iron oxide magnetic powder, iron oxide magnetic powder may be mixed with other metals such as Co by various methods. A seed metal-containing iron oxide magnetic powder is included. By using the fine powder as described above, it is possible to easily cope with high density recording and high SN ratio.

The proportion of Fe ^{2+ in} the magnetic powder used in the first layer is 0.1 to 3.0% by weight in terms of FeO (hereinafter, simply referred to as Fe ²⁺ content). If it is more than 3.0% by weight, the erasing property at the initial stage and after aging becomes poor. Also, if less than 0.1% by weight,
The light transmittance tends to increase.

The magnetic powder used for the second layer is the same Fe ²⁺
It is necessary that the content is 3.5 to 10% by weight, preferably 3.5 to 9% by weight. That is, when the content is less than 3.5% by weight, the light transmittance becomes high and it becomes difficult to detect the tape end by light. On the other hand, if the content exceeds 10% by weight, the erasing property at the initial stage and after aging deteriorates.

To set the Fe ²⁺ content of the iron oxide magnetic powder as described above, for example, γ-Fe ₂ O ₃ powder is reduced to a desired degree by an appropriate means, or Fe ₃ O ₄ powder is added by an appropriate means. It can be easily carried out by a conventionally known method such as an oxidation treatment to a desired degree, and commercially available products having various Fe ²⁺ contents can be easily obtained.

To obtain the magnetic recording medium of the present invention, for example, a magnetic coating material for each layer is prepared by mixing and dispersing the above-mentioned specific magnetic powders used for the first and second layers in an appropriate binder, and The magnetic coating material for the layer is applied to a non-magnetic support such as polyester film by an arbitrary coating means and dried to form a first layer, and calendering is carried out by an ordinary method. The magnetic coating material for layer (2) may be applied and dried in the same manner as described above and calendered to form the second layer.

The thickness of the magnetic layer composed of the first and second layers is preferably about 3 to 8 μm, and the thickness ratio of the first layer / second layer is generally 1/4 to 6/1, preferably 3/2. It is good to set it to 4/1. In particular, by setting the thickness ratio of the first layer / the second layer as described above, good results can be obtained for both the light transmittance and the erasing characteristics at the initial stage and after aging.

As the binder used in the above, vinyl chloride resin, fibrin resin, polyurethane resin, polyester resin,
Any conventionally known binder such as an isocyanate compound can be used. Each magnetic paint, carbon black, and _{α-Fe 2 O 3, Al} 2 O 3, Cr 2 O 3 or the like conventionally known coating reinforcing filler, fatty acid, fatty acid ester, a silicone-based lubricants, fluorine-based Various additives such as lubricants such as lubricants can be optionally added. Further, the magnetic paint may be an organic solvent type or a water dispersion type.

〔The invention's effect〕

As described above, in the present invention, when the fine iron oxide magnetic powder is used as the magnetic powder, the magnetic powder has a two-layer structure and is used for the inner layer adjacent to the support. because of being configured to increase the Fe ²⁺ content of the magnetic powder used for the layer on the surface side to reduce reverse Fe ²⁺ content, thereby the light transmittance is low and the erase characteristics of the initial stage and after aging An excellent magnetic recording medium can be obtained.

〔Example〕

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

Example 1 Co-containing γ-Fe ₂ O ₃ powder (BET specific surface area 40 m ² / g, Fe ²⁺ 3.0% by weight) 100 parts Carbon black (average particle size 24 mμ) 4.5 parts Granular α-Fe ₂ O ₃ powder ( Average particle size 1.0 μm) 5.2 parts Zinc stearate 0.5 parts Nitrified cotton H1 / 2 second type (16 wt% cyclohexanone / toluene solution) 56 parts Cyclohexanone 90 parts Toluene 90 parts The above ingredients are mixed with a high-speed stirrer for 5-10 hours. Premix,
Then, the following components were added to the composition whose dispersion was completed with a sand mill in an appropriate order and mixed with a high-speed stirrer to prepare a magnetic coating material for the first layer.

Polyurethane resin (20% by weight cyclohexanone / toluene solution) 40 parts Trifunctional low molecular weight isocyanate (75% by weight cyclohexanone / toluene solution) 4 parts Myristic acid 2 parts Cyclohexanone 40 parts Toluene 40 parts Next, the above magnetic paint with a pore size of 1 μm After passing through the filter of No. 1 above, it was coated and dried on a polyester film having a thickness of 13 μm and good surface smoothness, and then calendered to form a first magnetic layer having a thickness of 2.0 μm.

On the other hand, the following ingredients were premixed with a high speed stirrer for 5 to 10 hours, and then well dispersed with a sand mill.

Co-containing γ-Fe ₂ O ₃ powder (BET specific surface area 40 m ² / g, Fe ²⁺ content 3.5 wt%) 100
Part carbon black (average particle size 24 mμ) 4.5 parts granular α-Fe ₂ O ₃ powder (average particle size 1.0 μm) 5.2 parts Cr ₂ O ₃ powder (average particle size 1.0 μm) 0.8 part Al ₂ O ₃ powder (average particle size) Diameter 0.4 μm) 2.0 parts Zinc stearate 0.5 parts Nitrified cotton H1 / 2 second type (16 wt% cyclohexanone / toluene solution) 56 parts Cyclohexanone 90 parts Toluene 90 parts In the obtained composition, the following ingredients are mixed in an appropriate order. Was added and mixed with a high speed stirrer to prepare a magnetic coating material for the second layer.

Polyurethane resin (20 wt% cyclohexanone / toluene solution) 40 parts Trifunctional low molecular weight isocyanate (75 wt% cyclohexanone / toluene solution) 4 parts Myristic acid 2 parts n-Butyl stearate 1 part Cyclohexanone 40 parts Toluene 40 parts This magnetic paint Was passed through a filter having a pore size of 1 μm, and then coated and dried on the first layer, followed by calendering to form a second magnetic layer having a thickness of 3.0 μm.
Then, slitting was carried out to a width of 1/2 inch to produce the magnetic tape of the present invention.

Example 2 The thickness of the first layer was changed from 2.0 μm to 3.0 μm, and the Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the second layer was 3.5% by weight to 4.5% by weight. And its thickness is 3.0 μm to 2.0 μm
A magnetic tape was produced in the same manner as in Example 1 except that the above was changed.

Example 3 The Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the first layer was 3.0.
From 2.0% by weight to 2.0% by weight, and its thickness from 2.0μm to 3.0μ
m, and the Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the second layer was changed from 3.5% by weight to 6.0% by weight, and the thickness was changed.
A magnetic tape was produced in the same manner as in Example 1 except that the thickness was changed from 3.0 μm to 2.0 μm.

Example 4 The Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the first layer was 3.0.
From 0.3% by weight to 0.3% by weight, the thickness is 2.0μm to 3.0μ
m, and the Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the second layer was changed from 3.5% by weight to 9.0% by weight, and the thickness was changed.
A magnetic tape was produced in the same manner as in Example 1 except that the thickness was changed from 3.0 μm to 2.0 μm.

Example 5 Same as Example 1 except that the BET specific surface area of the Co-containing γ-Fe ₂ O ₃ powder used for the first and second layers was changed from 40 m ² / g to 30 m ² / g. Then, a magnetic tape was produced.

Comparative Example 1 The Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the second layer was 3.5.
A magnetic tape for comparison was prepared in the same manner as in Example 1 except that the weight% was changed to 2.5% by weight.

Comparative Example 2 The Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the first layer was 3.0.
From 0.3% by weight to 0.3% by weight, the thickness is 2.0μm to 3.0μ
Change to m, and the second a Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the layer 3.5 wt% to 12.0 wt%, except for changing the thickness from 3.0μm to 2.0μm A magnetic tape for comparison was manufactured in the same manner as in Example 1.

Comparative Example 3 The Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder used in the first layer was 3.0.
From 4.0% by weight to 2.0% by thickness of 3.0μ
m and change the thickness of the second layer from 3.0 μm to 2.0 μm
A magnetic tape for comparison was prepared in the same manner as in Example 1 except that the magnetic tape was changed to.

Comparative Example 4 The BET specific surface area of the Co-containing γ-Fe ₂ O ₃ powder used for the first layer was measured.
40m ² / g to 30m ² / g, Fe ²⁺ content from 3.0% by weight to 1
Co-containing γ-F used in the second layer, changed to 2.0% by weight
The BET specific surface area of e ₂ O ₃ powder was changed from 40 m ² / g to 30 m ² / g
²⁺ content was changed from 3.5% by weight to 0.3% by weight,
A magnetic tape for comparison was prepared in the same manner as in Example 1.

Comparative Example 5 Example 1 except that the Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder was changed from 3.5 wt% to 0.3 wt% without providing the first layer.
A magnetic coating material having the same composition as that of the magnetic coating material for the second layer is prepared and directly applied on the same polyester film as in Example 1 by the same operation and dried to be subjected to calendar treatment to give a thickness of 5.0. A μm magnetic layer was formed, and a magnetic tape for comparison was prepared.

Comparative Example 6 Example 1 except that the Fe ²⁺ content of the Co-containing γ-Fe ₂ O ₃ powder was changed from 3.5 wt% to 9.0 wt% without providing the first layer.
A magnetic coating material having the same composition as that of the magnetic coating material for the second layer is prepared, and this is directly applied on the same polyester film as that of the agent 1 by the same operation and dried, and calendered to give a thickness of 5.0. A μm magnetic layer was formed, and a magnetic tape for comparison was prepared.

For each of the magnetic tapes of Examples 1 to 5 and Comparative Examples 1 to 6 described above, the light transmittance and the erasing characteristics at the initial stage and after aging were measured and evaluated by the following methods. The results were as shown in the table below. The measurement conditions conformed to the VHS system video cassette system standard.

<Light transmittance> A magnetic tape was placed between the 9,000 Å light source and the photo sensor, and the light transmitted through the magnetic tape was measured by the photo sensor.

<Erase characteristics> VHS method VTR with specified bias current 10% from specified level
A signal of dB higher than 1 KHz was recorded, and a part of the signal was erased immediately after that, and the difference between the output of the erased part and the reproduced output of the non-erased part was measured.

<Change of erasing characteristics with time> Using a VHS type VTR, with a specified bias current and a specified level
A signal of 10 kHz higher than 1 KHz was recorded, and the difference between the erasure rate of the part immediately erased and the erasure rate of the part erased 100 hours later was measured.

As is clear from the above results, the BET specific surface area 30-60m
^{When 2} / g of iron oxide magnetic powder is used, the Fe ²⁺ content of the magnetic powder used in the first layer is 0.1 to 3.0% by weight, and the Fe ²⁺ content of the magnetic powder used in the second layer is The magnetic tapes (Examples 1 to 5) having a content of 3.5 to 10% by weight have low light transmittance and good erasing characteristics at the initial stage and after aging. In contrast,
The magnetic tape in which the Fe ²⁺ content of any of the layers is out of the range of the present invention (Comparative Examples 1 to 4) and the magnetic tape having a single magnetic layer (Comparative Examples 5 and 6) have high light transmittance. Alternatively, the erasing characteristics are deteriorated at the initial stage and after aging.

Claims (1)

[Claims] 1. A magnetic recording medium comprising a non-magnetic support and a two-layer magnetic layer comprising a first layer adjacent to the support and a second layer adjacent to the support. The magnetic powders contained in both layers are iron oxide-based magnetic powders having a specific surface area of 30 to 60 m ² / g according to the nitrogen adsorption method, and the proportion of Fe ^{2+ in} the magnetic powders in the first layer is FeO equivalent 0.1 ~
3.0% by weight, and the ratio of Fe ^{2+ in} the magnetic powder of the second layer is 3.5 to 10% by weight in terms of FeO.