JPH1045990A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a a magnetic recording medium which contains a polar polyvinyl acetal resin as the binder and has excellent magnetostatic characteristics and electromagnetic transduction characteristics and stable durability and traveling properties by maintaining the ratio of triad hydroxyl groups to hydroxyl groups of the resin at a specified value or lower. SOLUTION: This medium is prepd. by using a polar polyvinyl acetal resin represented by the formula (wherein X is a polar group; and (k), (l), (m), and (p) are mol fractions of acetal unit, vinyl acetate unit, vinyl alcohol unit, and polar monomer unit, respectively, provided k+l+m+p=100, 65<=k<=85, 1<=l<=20, 5<=m<=30, and 0.1<=p<=2) as the binder for a magnetic layer. The ratio of triad hydroxyl groups to hydroxyl groups of vinyl alcohol units of the resin is 70% or lower. A favorable magnetic recording medium is the one contg. the polar polyvinyl acetal resin and a polyester polyurethane resin as the binder for a magnetic layer.

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, and more particularly to a magnetic recording medium containing a polar group-containing polyvinyl acetal as a binder of a magnetic layer.

[0002]

2. Description of the Related Art In a coating type magnetic recording medium, a magnetic paint in which a magnetic powder, a binder, a dispersant, a lubricant, an abrasive and the like are kneaded in an organic solvent and a magnetic powder in which the magnetic powder is dispersed is used as a polyester film or the like. A magnetic layer is formed by coating on a non-magnetic support. In such a coating type magnetic recording medium, as a binder, vinyl chloride / vinyl acetate copolymer, vinyl chloride / vinyl acetate / vinyl alcohol copolymer, vinyl chloride / vinylidene chloride copolymer, polyether polyurethane resin, polyester Polyurethane resin, acrylonitrile-butadiene copolymer, nitrocellulose, epoxy resin, acrylic resin and the like are used. Among these resins, vinyl chloride resin or nitrocellulose is used as a resin for imparting coating strength to the obtained magnetic recording medium. However, in recent years, the demand for high-density recording has led to the development of fine particles of magnetic powder and thinner tapes as non-magnetic supports, and binders, nitrocellulose, and vinyl chloride resins conventionally used have been used. , A urethane resin or the like alone or in a composite system thereof may result in insufficient dispersibility of the magnetic powder and insufficient coating film strength.

[0003] In response to this tendency, the present applicant has disclosed in
In the “magnetic recording medium” according to Japanese Patent No. 128486,
It is disclosed that a polyvinyl acetal resin containing a polar group is a very excellent binder as a binder for a magnetic layer for increasing the density. In other words, it has been found that the polar group-containing polyvinyl acetal resin is extremely excellent as a binder in terms of magnetic powder dispersibility, coating film strength, electromagnetic conversion characteristics, etc., and as a binder replacing nitrocellulose and vinyl chloride resins. Expected.

[0004]

However, when the magnetic recording medium containing the above-mentioned polar group-containing polyvinyl acetal resin as a binder was further deeply studied, the durability and running properties of the magnetic recording medium sometimes varied. When the produced magnetic tape was run on a VTR, the magnetic head drum and the guide pins did not fall off in some cases.

Accordingly, it is an object of the present invention to provide a magnetic recording medium which contains a polar group-containing polyvinyl acetal resin as a binder, has excellent magnetostatic characteristics and electromagnetic conversion characteristics, and is stable in durability and running properties. .

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, in a magnetic recording medium containing a polyvinyl acetal resin containing a polar group as a binder, the hydroxyl group contained in the resin is not included. The inventors have found that the above problems can be solved by setting the ratio of triad hydroxyl groups, that is, triad-OH, to 70% or less, thereby completing the present invention.

[0007] Polyvinyl acetal resin is a general term for resins obtained by further reacting polyvinyl alcohol obtained by saponifying polyvinyl acetate with various aldehydes. Polyvinyl alcohol is intramolecularly cyclized to form an acetal group. The polar group-containing polyvinyl acetal resin is obtained using, as a starting material, a polar group-containing polyvinyl acetate obtained by, for example, copolymerizing a required amount of sodium vinyl sulfonate, and has a molecular structure represented by the following [Formula 1]. .

[0008]

Embedded image

That is, vinyl acetate units remain depending on the degree of saponification reaction of polyvinyl acetate, and the above-mentioned acetalization reaction of polyvinyl alcohol requires two adjacent hydroxyl groups in the same main chain. (Ratio of acetalized hydroxyl group) is generally limited to 85 mol%, and if it exceeds 90 mol%, it is difficult to synthesize it, so that a vinyl alcohol unit is present. Therefore, the number of hydroxyl groups derived from the vinyl alcohol unit in the polar group-containing polyvinyl acetal resin is larger than that of the vinyl chloride resin conventionally used as a binder. However, since these are secondary hydroxyl groups and are directly bonded to the main chain, the degree of freedom is small and the crosslinking reactivity is relatively low. For this reason, a magnetic recording medium containing a polar group-containing polyvinyl acetal resin as a binder is likely to cause a problem in durability.

The distribution of acetal groups and hydroxyl groups in the above-mentioned polar group-containing polyvinyl acetal resin varies depending on the reaction conditions for acetalization, that is, the reaction temperature, reaction time, type and amount of catalyst, and the like.

Among them, focusing on the hydroxyl group, [Chemical formula 2], [Chemical formula 3], [Chemical formula 4]

[0012]

Embedded image

As shown in FIG. 1, for one hydroxyl group —OH, an isolated (Is
There are an acrylated (OH) -OH, a diad (OH) in which both sides are a hydroxyl group and an acetal group, and a triad (Triad) -OH in which both sides are a hydroxyl group. The ratio of these varies depending on the acetalization reaction conditions.
These ratios can be determined by NMR (Yamada, Suzuki, Asahina, “Polymer Papers,” Vol. 50, N.
o. 1, 49-56).

In the magnetic recording medium of the present invention, the ratio of triad-OH in the hydroxyl group consisting of isolated OH, dyad-OH and triad-OH in a polyvinyl acetal resin containing a polar group as a binder contained in the magnetic layer is determined. 70
% Or less.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the magnetic recording medium according to the present invention will be described below.

As described above, the magnetic recording medium is made by kneading and dispersing a magnetic powder, a binder, a dispersant, a lubricant, an abrasive in an organic solvent, and the like. The magnetic recording medium of the present invention is manufactured by applying a magnetic layer by coating on the magnetic layer. Among the polar group-containing polyvinyl acetal resins represented by [Chemical Formula 1], the hydroxyl group contained in the resin is used. Triad-O in
Those having a H ratio of 70% or less are included as binders.

The degree of polymerization of the above-mentioned polar group-containing polyvinyl acetal resin is preferably from 200 to 600, and the molar composition of the acetal unit (acetalization degree), vinyl acetate unit, hydroxyl unit and polar group unit is k. , L, m, p, k + 1 + m + p = 10
0, 65 ≦ k ≦ 85, 1 ≦ l ≦ 20, 5 ≦ m ≦ 30,
Those having a ratio of 0.1 ≦ p ≦ 2 are preferably used. That is, when k is less than 65, the hardness is insufficient and the durability of the magnetic recording medium cannot be sufficiently ensured, and when k is more than 90, it is difficult to synthesize and actually cannot be obtained. l affects compatibility with other resins, durability, and softening properties when mixed with other resins, and is preferably in the above range in order to maintain their properties well. Also,
m is preferably in the above range from the viewpoint of compatibility with other resins, water resistance, and reactivity with isocyanate as a crosslinking agent.
Further, p needs the above range in view of the dispersibility of the magnetic powder.

The polar group X is -SO ₃ Na, -SO ₃ K,-
_{SO 3 H, -OSO 3 Na,} -OSO 3 K, -COO
H, -OPO (OH) ₂ , -OPO (OLi) ₂ , -O
PO (ONa) ₂ , -OPO (OK) ₂ , -PO (O
H) ₂ , -PO (OLi) ₂ , -PO (ONa) ₂ ,-
_{PO (OK) 2, -NR 2} , -NR 3 Br, -NR 3 I
Is selected from The alkyl group -R = -C _n H
_{2n + 1} is preferably n = 0 to 6. When n = 0, it becomes -H.

The binder containing the polar group-containing polyvinyl acetal resin includes a magnetic powder, a dispersant, a lubricant,
The mixture is kneaded in an organic solvent together with an abrasive or the like to form a magnetic paint, which is applied on a non-magnetic support to form a magnetic recording medium. That is, the magnetic recording medium of the present invention is basically formed by laminating one or more magnetic layers 12 on a non-magnetic support 11, as shown in FIG. If necessary, as shown in FIG. 2, one or more magnetic layers 22 are formed on one surface of the non-magnetic support 21, and solid particles such as an inorganic pigment and a binder are formed on the other surface. As shown in FIG. 3, a magnetic layer 32 a, a magnetic layer 32 a,
32b is formed. In any of the above cases, a configuration in which an intermediate layer as a so-called undercoat layer is provided between the nonmagnetic support and the magnetic layer is also employed.

[0020]

EXAMPLES Hereinafter, the magnetic recording medium of the present invention will be described in detail with reference to examples.

0.8 mol% of --SO ₃ Na as a polar group
Was prepared by saponifying polyvinyl acetate having a degree of polymerization of 300, and then acetalizing with acetaldehyde. By changing conditions such as reaction temperature, reaction time, type and amount of catalyst during acetalization, the degree of acetalization is 72 mol%, hydroxyl group 20
As shown in Table 1, the polar group-containing polyvinyl acetal resins A, B, C, and D differ in the ratio of triad-OH in the hydroxyl group consisting of isolated-OH, dyad-OH, and triad-OH. , E.

[0022]

[Table 1]

Each of the resins A, B, C, D and E obtained above was mixed at a mixing ratio shown in Table 2 in a sand mill for 6 hours to prepare a magnetic paint.

[0024]

[Table 2]

To the magnetic paint prepared as described above, 20 parts by weight of a polyisocyanate (trade name: Coronate L, Nippon Polyurethane) was added as a crosslinking agent, and a 14 μm-thick polyethylene terephthalate film was dried to form a magnetic layer. It was applied to a thickness of 2.5 μm, oriented in a magnetic field, dried and wound up. After this was subjected to a super calender treatment, it was cured at a temperature of 70 ° C. for a predetermined time to form a magnetic layer. Next, this was cut into イ ン チ inch widths to prepare sample tapes A, B, C, D, and E corresponding to the respective resins A, B, C, D, and E.

The sample tapes A, B, C, D, and E produced as described above were evaluated for magnetostatic properties, electromagnetic conversion properties, and durability as follows.

Magnetostatic characteristics: The residual magnetic flux density and the squareness ratio were measured. Electromagnetic conversion characteristics: Record 5MHz signal on tape,
The ratio of the reproduced output to the carrier and noise of the luminance signal, Y
-C / N was compared with in-house standard tape. Durability: Evaluated from the shuttle characteristics and the amount of powder falling. * Shuttle Characteristics A 120-minute long sample tape on which a 5 MHz signal was recorded was run 20 times with a VTR, and the degree of deterioration of the playback output after the shuttle with respect to the initial signal playback output was compared. * Amount of powder fall The amount of powder fall to the magnetic head drum, guide pins, etc. after the shuttle run was visually observed, and those with little powder fall were scored as 0 points, and those with much powder fall were scored as 5 points and evaluated. .

The measurement was performed on sample tapes A, B, C, D,
E was performed for each of 10 lines, and their average values are shown in Table 3.

[0029]

[Table 3]

As is clear from the evaluation results in Table 3, the sample tapes C, D, and E using a polar group-containing polyvinyl acetal resin having a ratio of triad-OH in the hydroxyl groups contained in the resin of 70% or less were magnetostatic. Good results were obtained in all of the characteristics, electromagnetic conversion characteristics, and durability. On the other hand, sample tapes A and B
Showed good magnetostatic properties and electromagnetic conversion properties, but no good results were obtained in the durability of the magnetic layer.

Originally, the polyvinyl acetal resin containing a polar group has a low water content, and the polyisocyanate added as a crosslinking agent exists without reacting with moisture, and reacts with hydroxyl groups in the resin to form a sufficient crosslinked structure. And should give considerable durability. However, it is considered that the above-mentioned difference appears depending on the ratio of triad-OH in the contained hydroxyl group for the following reason.

Most of the polar group-containing polyvinyl acetal resin has an interaction with the magnetic powder in the magnetic paint and the magnetic coating film, and the hydroxyl group contained therein has a larger polarity than the acetal group and is oriented to the magnetic powder. When the isolated -OH in the hydroxyl group is oriented, only the OH is required. In the case of the dyad -OH, when one -OH is oriented, the other-
OH also faces in the same direction, but in the case of triad-OH, when one -OH is oriented to the magnetic powder, the other two -OH also face the magnetic powder. Although the hydroxyl group is a site to react with the polyisocyanate of the cross-linking agent to become a cross-linking point, the proportion of triad-OH in the hydroxyl group is reduced because the oriented -OH decreases the probability of reacting with the cross-linking agent to cross-link. Small sample tapes C, D, and E would have given excellent results.

From another perspective, when one isocyanate of the triad-OH is reacted with the polyisocyanate as a crosslinking agent, the same triad-O is present due to steric hindrance.
It is considered that a crosslinking reaction is unlikely to occur with other -OH in H.

In any case, it is considered that the higher the ratio of triad-OH in the hydroxyl group, the more difficult it is to improve the crosslink density. To further expand this idea, when the molar composition ratio m of the vinyl alcohol unit in the polar group-containing polyvinyl acetal resin is set small, that is, when the molar composition ratio of the hydroxyl group is small, the contained triad-O
It is desired that the ratio of H is as small as possible. Further, thereby, the hydroxyl group contained is most effectively crosslinked according to the molar composition ratio m of the vinyl alcohol unit set according to the purpose.

The embodiments of the present invention have been described above.
Of course, the present invention is not limited to this, and various modifications can be made based on the technical spirit of the present invention.

For example, in the present embodiment, as the binder,
A polyester polyurethane resin was mixed with a polar group-containing polyvinyl acetal resin having a limited triad-OH ratio, but as long as compounding is possible, conventionally known thermoplastic resins and thermosetting resins as other binders Can be mixed. In addition, triad-OH
Needless to say, a polar group-containing polyvinyl acetal resin having a limited ratio may be used alone as a binder. Further, in this example, polyvinyl acetate having polar groups already introduced was used as a starting material, but polyvinyl alcohol,
Alternatively, a polar group may be introduced at the stage of polyvinyl acetal.

In this embodiment, Fe metal powder is used as the magnetic powder, but any other material conventionally known as magnetic powder for this type of magnetic recording medium can be used. Such magnetic powders include, for example, Co,
Metal materials such as Ni, Fe-Co, Fe-Ni, Fe-
Al, Fe-Ni-Al, Fe-Al-Si, Co-N
and an alloy material such as i. Al, S added for the purpose of preventing sintering or maintaining the shape during these reductions
Even if i, P, B, etc. are contained in appropriate amounts, the effects of the present invention are not hindered.

Further, γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co
-Γ-Fe ₂ O _3, ferromagnetic iron oxide particles such as Co-Fe ₃ O _4, ferromagnetic chromium dioxide particles, such as hexagonal ferrite particles are used. In addition, iron nitride, iron carbide and the like can be used. These magnetic powders may be used alone or in combination.

In addition to the magnetic powder and the binder, a dispersant, an abrasive, an antistatic agent and the like may be added as a material constituting the magnetic layer. These dispersants, abrasives, antistatic agents and the like are not particularly limited as long as they are materials usually used for this type of magnetic recording medium.

The above-mentioned magnetic layer forming material is formed into a paint, applied to a non-magnetic support, and dried to form a magnetic layer. The organic solvent used for the paint is methyl ethyl ketone used in the present example. Other than toluene, cyclohexanone, ketones, alcohols, esters, ethers, hydrocarbons, halogenated hydrocarbons and the like can be appropriately selected and used.

In this embodiment, a sand mill was used for kneading and dispersing to produce a magnetic coating material, but other roll mills, ball mills, kneaders, homogenizers, ultrasonic dispersers and the like are preferably used. In addition, a gravure coater, a knife coater, a blade coater, a reverse roll coater, a die coater, or the like is used for applying the coating material thus prepared on a nonmagnetic support.

In this embodiment, polyethylene terephthalate is used as the non-magnetic support, but any other conventionally known materials such as polyesters, polyolefins and cellulose derivatives can be used. Further, the form is not limited at all, and may be any form such as a tape form, a sheet form, and a drum form. Further, the nonmagnetic support may be subjected to a surface treatment so as to form fine irregularities for controlling the surface properties.

Although not particularly provided in the present embodiment, a back coat layer may be provided on the back side of the non-magnetic support as needed for the purpose of improving the running properties of the magnetic recording medium, preventing charge and preventing transfer. You may. The back coat layer is formed, for example, by dispersing solid particles such as an inorganic pigment in a binder, and kneading the mixture with an organic solvent selected according to the binder, and applying a coating for a back coat which is adjusted. As the solid particles to be used, any conventionally known materials can be used as long as they can impart an antistatic effect and a lubricating effect. For example, graphite, carbon black,
Examples include a carbon black graft polymer, tungsten disulfide, molybdenum disulfide, and titanium oxide.
These solid particles are used alone or in combination.

As the binder used for the back coat layer, any conventionally known binder including a thermoplastic resin and a thermosetting resin can be used. Further, as a material constituting the back coat layer, a dispersant, an abrasive, an antistatic agent, and the like may be added in addition to the solid particles and the binder described above. These dispersants, abrasives, and antistatic agents are not particularly limited as long as they are materials commonly used for this type of magnetic recording medium.

[0045]

The present invention is embodied in the form described above, and has the following effects.

In the magnetic recording medium of the present invention, the ratio of the triad hydroxyl group in the hydroxyl group in the polar group-containing polyvinyl acetal resin contained as a binder is set to 70% or less. The probability that the hydroxyl groups remain without contributing to the reaction is small and sufficiently crosslinked, and not only the magnetostatic properties and electromagnetic conversion properties, but also the reproduction output does not deteriorate during actual repeated use, and the VTR's magnetic head drum and guide There is no powder dropping off the pins and it shows extremely excellent durability.

[Brief description of the drawings]

FIG. 1 is a sectional view of a magnetic recording medium in which a magnetic layer is formed on one surface of a non-magnetic support.

FIG. 2 is a cross-sectional view of a magnetic recording medium in which a magnetic layer is formed on one surface of a non-magnetic support and a back coat layer is formed on the other surface.

FIG. 3 is a cross-sectional view of a magnetic recording medium in which magnetic layers are formed on both surfaces of a non-magnetic support.

[Explanation of symbols]

11, 21, 31... Non-magnetic support, 12, 22, 32
a, 32b: magnetic layer, 23: back coat layer.

Claims (2)

[Claims] [Chemical formula 1] Here, X is a polar group, R is an alkyl group, k is an acetal unit (acetalization degree), l (el) is a vinyl acetate unit, m is a vinyl alcohol unit,
p is the molar composition ratio of the polar group-containing monomer unit.
Then, k + 1 + m + p = 100, 60 ≦ k ≦ 90, 1
≦ l (L) ≦ 20, 5 ≦ m ≦ 30, and 0.1 ≦ p ≦ 2. In a magnetic recording medium comprising a polar group-containing polyvinyl acetal resin as a binder of a magnetic layer, a ratio of a triad hydroxyl group in a hydroxyl group derived from the vinyl alcohol unit of the polar group-containing polyvinyl acetal resin is 70% or less. A magnetic recording medium, comprising: 2. The magnetic recording medium according to claim 1, wherein the binder of the magnetic layer contains a polyester polyurethane resin together with the polar group-containing polyvinyl acetal resin.