JPH05287233A - Binder for magnetic recording medium and magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a binder excellent in an ability to disperse a magnetic powder and can form a magnetic film excellent in heat and abrasion resistances. CONSTITUTION:This binder is one containing polyvinyl acetal resin. The total content of the acetalized part is 55mol% or above based on the polyvinyl acetal resin, the content of the part acetalized with acetaldehyde is 50-100wt.%, based on the total content of the acetalized part, and the content of the part containing at least one kind of hydrophilic groups selected from among COOM, SO3M, OSO3M, PO(OM)2 (wherein M is H, Li, Na or K), a tert. amine, a quat. ammonium salt and a betaine is 0.01-5mol%, based on the entire polyvinyl acetal resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a binder for a magnetic recording material and a magnetic recording material using the binder, and in particular, it has excellent dispersibility of magnetic powder and has excellent wear resistance and heat resistance. The present invention relates to a binder for a magnetic recording body capable of forming a magnetic coating film having excellent properties, and a magnetic recording body using the binder.

[0002]

2. Description of the Related Art In recent years, various magnetic recording bodies such as magnetic recording tapes and magnetic disks have been widely used. A magnetic recording material is generally prepared by applying a magnetic paint obtained by mixing magnetic powder and a binder to a substrate such as polyester to orient the magnetic powder contained in the magnetic paint and at the same time to apply the magnetic paint. It is manufactured by drying to form a magnetic coating film. The magnetic recording medium is required to have excellent electromagnetic characteristics and durability such as sliding contact resistance with a magnetic head. Therefore, the binder for the magnetic recording material is required to have excellent dispersibility of the magnetic powder when the magnetic coating material is formed, and to have a property of imparting a high squareness ratio to the magnetic recording material. In addition to the above, various properties such as the ability to form a magnetic coating film having excellent abrasion resistance and heat resistance are required.

Conventionally, a vinyl chloride-vinyl acetate-vinyl alcohol polymer has been widely used as a binder satisfying the above requirements. On the other hand, with the recent increase in density of magnetic recording, it has become necessary to use magnetic powder having a smaller particle size. As a result, when the above vinyl chloride-vinyl acetate-vinyl alcohol-based polymer is used as a binder, the dispersibility of the magnetic powder in the form of fine particles is not sufficient, and therefore the squareness ratio of the magnetic recording medium produced is There was a problem of being small.

Therefore, in place of the above polymer, a polyvinyl acetal resin, which has excellent affinity with magnetic powder and is expected to exhibit good dispersibility in fine magnetic powder, is used as a binder resin. Is proposed to be used as. For example, Japanese Patent Application Laid-Open No. 61-227224 proposes a binder for a magnetic recording medium containing a polyvinyl mixed acetal acetalized with plural kinds of aldehydes. This polyvinyl mixed acetal is produced by acetalizing polyvinyl alcohol using butyraldehyde and an aldehyde other than butyraldehyde such as formaldehyde or acetaldehyde.

In the above polyvinyl mixed acetal, the heat resistance and abrasion resistance of the resin are enhanced by acetalizing with a lower aldehyde such as formaldehyde, and the dispersibility of the magnetic powder is improved, and the acetic acid is mixed with butyraldehyde and the like. By acetalizing with a higher aldehyde, the solubility in a solvent is increased and the magnetic coating film is provided with flexibility, and since it has a portion that is acetalized with acetaldehyde, each of the above properties The balance of is maintained. However, in recent years, more excellent electromagnetic characteristics have been required for magnetic recording media, and therefore, even in the binder for magnetic recording media, the dispersibility in magnetic powder, abrasion resistance, and heat resistance are better. What has more excellent characteristics is desired.

In order to meet such demands, JP-A-1-236290 proposes a binder for a magnetic recording medium containing a polyvinyl acetal resin, which is superior in heat resistance to the polyvinyl mixed acetal. here,
The degree of acetalization of the polyvinyl acetal resin is 50% by weight or more, and 85% by weight or more of the acetalized portion is composed of polyvinyl acetoacetal. However, when the binder for a magnetic recording medium described in this prior art is used, a magnetic recording medium having very excellent heat resistance can be obtained, but regarding the dispersibility in magnetic powder, there is still a demand in recent years. It was too much to fill.

[0007]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a magnetic recording capable of forming a magnetic coating film having more excellent magnetic powder dispersibility and very excellent abrasion resistance and heat resistance. It is intended to provide a binder for body and a magnetic recording body using the binder for magnetic recording body.

[0008]

[Means for Solving the Problems] Claim 1 of the present application
The invention relates to a magnetic recording containing a polyvinyl acetal resin.
A binder for the body, which is contained in the polyvinyl acetal resin.
The proportion of all acetal moieties is 55 mol% or more,
Acetaldehyde by acetaldehyde in the cetalized part
The proportion of the converted portion is 50 to 100% by weight, and -C
OOM, -SO ₃M, -OSO₃M and -PO (OM)
₂(However, M shows H, Li, Na, or K.),
Select from tertiary amine, quaternary ammonium salt and betaine
The proportion of the portion containing at least one hydrophilic group
0.01 to 5 mol% of the entire polyvinyl acetal resin
A binder for magnetic recording media, characterized by being present.

The invention according to claim 2 is a magnetic recording in which a magnetic coating film made of a magnetic coating material containing a binder for a magnetic recording material containing polyvinyl acetal and magnetic powder is formed on the surface of a substrate. The proportion of all the acetalized portions of the polyvinyl acetal resin is 55 mol% or more, the proportion of the acetalized portion by acetaldehyde in the acetalized portion is 50 to 100% by weight, and -COOM, -SO ₃ M, -OSO ₃ M and -
PO (OM) ₂ (where M is H, Li, Na or K
Indicates. ), A tertiary amine, a quaternary ammonium salt, and betaine, the ratio of the portion containing at least one hydrophilic group is 0.0% of the whole polyvinyl acetal resin.
It is a magnetic recording material characterized by being 1 to 5 mol%. Betaine is a compound having a quaternary ammonium cation and a carboxylate anion in the molecule.

The details of the composition of the magnetic recording material binder according to claim 1 and the magnetic recording material according to claim 2 will be described below. The polyvinyl acetal resin used in the invention according to claims 1 and 2 has a proportion of the portion containing the hydrophilic group of 0.01 to 5 mol% and a degree of polymerization of 200 to 3000, preferably 300. It can be obtained by acetalizing ~ 2000 modified polyvinyl alcohol with acetaldehyde or with acetaldehyde and other aldehydes. As the method for obtaining the modified polyvinyl acetal, a unit having a unit having a hydrophilic group in advance is made to coexist at the formation position of the main chain, and a hydrophilic group using the hydroxyl group of the alcohol type unit after the formation of the main chain. The method of introducing is considered.

A unit having a hydrophilic group in advance is mainly used.
Method of coexisting at the time of chain formation; Polyvinyl acetal resin is usually synthesized by condensing polyvinyl alcohol obtained by hydrolyzing polyvinyl acetate with aldehyde. Therefore, to introduce a unit having a hydrophilic group into the polyvinyl acetal resin,
At the time of synthesizing polyvinyl acetate, an ethylene mono-substituted product of the hydrophilic group may be subjected to addition polymerization. However,
It is necessary that the introduced hydrophilic group does not participate in the subsequent hydrolysis and acetalization.

The hydrophilic group as described above has been described above.
-COOM, -SO₃M, -OSO ₃M or -PO
(OM)₂(However, M represents H, Li, Na or K.
You ), Tertiary amines, quaternary ammonium salts and betaines
Is mentioned. The above-mentioned hydrophilic group allows ethylene to be placed in
There are various possible substitutes, for example, parent
Taking the case where the aqueous base is -COOM as an example, the following compound
Examples include the items (1) and (2).

[0013]

[Chemical 1]

Further, as a mono-substituted product of ethylene with a hydrophilic group containing —SO ₃ M, the following compounds (3) to
(10) is mentioned.

[0015]

[Chemical 2]

Alcohol-type units after formation of the main chain
A method of introducing a hydrophilic group using the hydroxyl group of the above; typical examples of this method include a method of condensing a halogen-substituted compound and a method of using a bifunctional isocyanate. As a method of using a halogen substitution product, for example,
Examples include a method of reacting the following compounds (11) to (14) with a hydroxyl group of an alcohol type unit to introduce a hydrophilic group with the elimination of the halogen X.

[0017]

[Chemical 3]

The above-mentioned reaction is carried out in a solvent such as dimethylformamide or dimethylsulfoxide, but in order to neutralize by-produced halogen, amines such as pyridine, picoline or triethylamine or ethylene oxide or propylene oxide. It is preferable to coexist with an epoxy compound such as. The method of using a bifunctional isocyanate is a method in which a hydroxyl group remaining after hydrolysis and a terminal hydroxyl group of the following compounds (15) to (18) are converted into 4,4′-diphenylmethane diisocyanate, tolylene diisocyanate or hexamethylene diisocyanate. The cross-linking is carried out via the bifunctional isocyanate.

[0019]

[Chemical 4]

To carry out the above-mentioned cross-linking, the compounds (15) to (18) and the bifunctional isocyanate are reacted in an equimolar amount in a solvent such as dimethylformamide or dimethylsulfoxide to give one of the hydroxyl group and the bifunctional isocyanate of the compound. To form a urethane bond with the -N = C = O group, and then the other -N = of the bifunctional isocyanate.
A urethane bond may be generated between the C = O group and the hydroxyl group in the acetal resin.

In the polyvinyl acetal resin according to the first and second aspects of the present invention, at least one kind of the hydrophilic group is contained in a total amount of 0.01 to 5 mol%, whereby the hydrophilic group is contained. The dispersibility of the magnetic powder is enhanced. When the modified polyvinyl alcohol is used, the modified polyvinyl alcohol may be used alone or a modified polyvinyl alcohol and an unmodified polyvinyl alcohol may be mixed as long as the modification degree is in the range of 0.01 to 5 mol%. The polyvinyl acetal resin finally obtained has a hydrophilic group content of 0.01.
You may make it to be 5 mol%.

In the polyvinyl acetal resin, if the proportion of the portion containing the hydrophilic group exceeds 5 mol%,
Solubility in a solvent after acetalization is reduced. On the other hand, if it is less than 0.01 mol%, the effect of introducing the hydrophilic group, that is, the effect of enhancing the dispersibility of the magnetic powder cannot be sufficiently obtained. In the invention described in claims 1 and 2, the degree of polymerization of the polyvinyl acetal resin is in the range of 200 to 3000, preferably 300 to 2000, as described above. When the degree of polymerization is less than 200, the strength of the magnetic coating film such as abrasion resistance becomes insufficient, and the degree of polymerization becomes 3000.
If it exceeds, the viscosity becomes too high when a magnetic coating material is prepared by kneading with a magnetic powder in a solvent, and the dispersibility of the magnetic powder and the like decreases.

Aldehyde used for acetalization In the invention described in claims 1 and 2, the main aldehyde used for constituting the acetalization portion is acetaldehyde. Other aldehydes that can be used include, for example, formaldehyde, propionaldehyde,
Butyraldehyde, amyl aldehyde, hexyl aldehyde, heptyl aldehyde, 2-ethylhexyl aldehyde, cyclohexyl aldehyde, furfural, glyoxal, glutaraldehyde and the like,
It is not limited to these, and aldehydes other than those exemplified can be used. In addition, claim 1,
In the invention described in 2, acetalization may be performed by using three or more aldehydes in combination. The formaldehyde includes paraformaldehyde,
Acetaldehyde shall include paraacetaldehyde.

A polyvinyl acetal resin is obtained by acetalizing polyvinyl alcohol with the above-mentioned various aldehydes. In the inventions described in claims 1 and 2, the degree of acetalization is about 55 mol% or more, and the acetal is obtained. It is necessary that about 50 to 100% by weight of the activated portion consist of polyvinyl acetoacetal. In addition, the acetalized portion means a portion in which polyvinyl alcohol is acetalized by the above various aldehydes, and the expression that about 50 to 100% by weight of the acetalized portion is polyvinyl acetoacetal is the whole acetalized portion. The acetoacetalized portion of,
That is,

[0025]

[Chemical 5]

It means that the weight ratio of the parts is about 50 to 100% by weight. When the degree of acetalization of the polyvinyl acetal resin is less than about 55 mol%, and when the proportion of the portion consisting of polyvinyl acetoacetal in the acetalized portion is less than about 50% by weight, the resistance of the obtained magnetic coating film is improved. The wear resistance and heat resistance deteriorate, and the dispersibility of the magnetic powder also deteriorates, resulting in a smaller squareness ratio. Further, in the polyvinyl acetal resin of the present invention, the proportion of all acetalized moieties in all the resins is about 55 mol% or more, but regarding the remaining vinyl acetate moieties, the less the vinyl acetate moieties are, the heat resistance is Is increased. Usually, the remaining vinyl acetal part is
The amount is 15 mol% or less, preferably 5 mol% or less.

It is speculated that the above-mentioned excellent heat resistance is due to polyvinyl acetoacetal having a high glass transition temperature. Similarly, polyvinyl formal having a high glass transition point has characteristics similar to polyvinyl acetoacetal in terms of heat resistance. However,
Polyvinyl formal is inferior to polyvinyl acetoacetal in solubility in a solvent. Thus, essentially, it is not possible to carry out a high proportion of acetalization with formaldehyde. In addition, if formaldehyde alone is used as the aldehyde and an attempt is made to make it highly acetal, intermolecular formalization cannot be ignored. Therefore,
Also in this case, since the solubility in the solvent is not sufficient, it is not possible to acetalize formaldehyde by itself, considering the overall performance of the obtained polyvinyl acetal resin.

Furthermore, when acetalization is carried out using a large amount of butyraldehyde, it is presumed that a sufficient effect cannot be exerted on the binding of the magnetic powders and the binding to the substrate. That is, when butyraldehyde is used as the main acetalizing material, the propyl group having 3 carbon atoms bonded to the acetal ring shields the residual hydroxyl group adjacent to the acetal ring from a steric hindrance, so that the residual hydroxyl group is It is presumed that the hydroxyl groups are not used effectively and that the hydroxyl groups cannot exert a sufficient effect when binding the magnetic powders and binding to the substrate.

However, when acetalization is mainly carried out using acetaldehyde as in the present invention, it is the methyl group having 1 carbon atom which is bonded to the acetal ring, and the residual hydroxyl group adjacent to the acetal ring is three-dimensional. It is presumed that since the magnetic powders are hard to be shielded from each other, the magnetic powders are well bound to each other, and that the magnetic powders also exert a remarkable effect in binding to the substrate. That is, by acetalizing with various aldehydes, it is possible to increase the wear resistance and heat resistance, etc. as described above, but at the same time by increasing the affinity of the non-acetalized hydroxyl group with the magnetic powder. It plays an important role in the dispersibility and binding of the magnetic powder, and it is presumed that this hydroxyl group is effectively used by acetalization with acetaldehyde.

The above-mentioned acetalization can be carried out by using an acetalization reaction which has heretofore been widely used. For example, an acid catalyst is added to an aqueous solution of polyvinyl alcohol,
Adopting a precipitation method in which the polyvinyl acetal resin formed is precipitated while adding an aldehyde compound to the reaction to cause the reaction, a dissolution method in which polyvinyl alcohol is dispersed in an organic solvent, and an acid catalyst and an aldehyde compound are added and reacted. You can Examples of the acid catalyst include hydrochloric acid, sulfuric acid,
An inorganic acid such as nitric acid, phosphoric acid or hydrobromic acid, or an organic acid such as an organic sulfonic acid can be used.

The precipitation method will be described more specifically. Usually, first, polyvinyl alcohol is dissolved in warm water and then cooled. After reaching a predetermined temperature by cooling, an appropriate acid catalyst and aldehyde are added to the aqueous polyvinyl alcohol solution. After the completion of the charging, the acetalized product is precipitated while maintaining a constant temperature for about 30 minutes or more, and then the acid catalyst is neutralized by going through a temperature aging step,
Purify according to conventional methods.

Production of the binder for a magnetic recording medium according to claim 1.
The binder for a magnetic recording medium according to the invention of claim 1 is:
Usually, the polyvinyl acetal resin obtained as described above, and a polyfunctional polyisocyanate if necessary,
It is prepared by dissolving in an organic solvent. As the organic solvent that can be used, any organic solvent that has been conventionally used in the preparation of a binder for a magnetic recording material or a magnetic coating material can be used. For example, one or a mixed solvent of two or more of toluene, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone is preferably used, but the organic solvent that can be used is not limited to these.

Method for producing magnetic recording medium according to claim 2 To obtain the magnetic recording medium according to claim 2, first, the binder for magnetic recording medium according to claim 1 is prepared. After that,
A magnetic coating material is prepared using the binder for magnetic recording medium.
Preparation of the magnetic coating, for example, low cobalt-γ-iron oxide magnetic powder, together with additives such as a surfactant, if necessary,
It is carried out by adding to the above binder for magnetic recording medium and kneading. In the preparation of the magnetic paint, the polyvinyl acetal resin is usually used in the range of 10 to 100 parts by weight, and usually 5 to 100 parts by weight of the magnetic powder.
It is contained in the binder in the range of ˜30 wt%.

The magnetic coating material obtained as described above is coated on the surface of a substrate according to a commonly used method such as a roll coater, and the magnetic powder contained in the magnetic coating material is oriented and the magnetic coating material is dried to form a substrate. A magnetic coating film is formed on the surface. In this way, a magnetic recording body can be manufactured. The substrate that can be used is not particularly limited, but, for example, those made of polyester, triacetylcellulose, polyamide, polyimide, polyamideimide, polystyrene, polyvinyl alcohol, nitrocellulose, etc. are usually used, and particularly preferable. The material used is polyester.

In the magnetic recording medium according to the second aspect of the present invention, a back coat layer mainly containing a non-magnetic powder and a binder is formed on the surface of the substrate opposite to the surface on which the magnetic layer is formed. The polyvinyl acetal resin used in the binder according to the first aspect of the invention can be used as the binder for the back coat layer.

[0036]

In the binder for a magnetic recording material according to claim 1 and the magnetic recording material according to claim 2, the specific polyvinyl acetal resin is used, and the specific polyvinyl acetal resin is acetalized. About 50-100 of the part
Weight% is said to be the acetalized portion with acetaldehyde. Therefore, since the content of polyvinyl acetoacetal having a high glass transition point is increased, the heat resistance of the binder and the heat resistance of the magnetic coating film obtained by using the binder are increased. Further, as described above, it is considered that the action of the residual hydroxyl groups that have not been acetalized in the polyvinyl alcohol enhances the binding property between the magnetic powders and the binding property to the substrate. Furthermore, in the above-mentioned specific polyvinyl acetal resin, since the above-mentioned specific hydrophilic group is introduced, the dispersibility of the magnetic powder is effectively enhanced by the action of the hydrophilic group.

[0037]

The present invention will be clarified below by giving non-limiting examples and comparative examples of the present invention. Example 1 Preparation of polyvinyl acetal resin: 2790 g of water was put into a 5 liter separable flask, and denatured polyvinyl alcohol (polymerization degree 300, saponification number 98.5) was added thereto.
220 g of (mol%, itaconic acid modification degree 2 mol%) was added and completely dissolved. Next, the liquid temperature of the obtained aqueous solution is set to 20.
650 g of hydrochloric acid having a concentration of 35% by weight in the aqueous solution
After the addition, the liquid temperature was lowered to 11 ° C., 143 g of acetaldehyde was added, and colorless powder was deposited. In addition,
The time from the addition of acetaldehyde to the deposition of colorless powder was 2 hours.

Next, the temperature of the reaction system is raised to 30 ° C., and the reaction is performed for 3 hours for 5 hours.
After being kept at 0 ° C., it was neutralized by washing with water to remove the catalyst and unreacted aldehyde, and a polyvinyl acetal resin was obtained. The obtained polyvinyl acetal resin was treated with DMSO-
dissolved in d ^6, was measured degree of acetalization with ¹³ C-NMR, the degree of acetalization was 74 mol%.

Binder for magnetic recording medium and magnetic recording medium
Preparation: The polyvinyl acetal resin obtained as described above is dissolved in a mixed solvent (containing methyl ethyl ketone, methyl isobutyl ketone and toluene in a volume ratio of methyl ethyl ketone / methyl isobutyl ketone / toluene = 1/1/2), A binder for magnetic recording material having a concentration of 18% by weight was obtained. 2.5 parts by weight of the binder obtained as described above
A double amount of magnetic powder (particle size 0.8 μm) made of cobalt-γ-iron oxide was added, and the mixture was kneaded for 4 hours with a paint conditioner.

The magnetic paint prepared as described above was added to
It was coated on a polyethylene terephthalate (PET) film having a thickness of 14 μm so that the thickness after drying would be 5 μm. Next, after subjecting the coating film to magnetic field orientation treatment, the coating film was dried, and the PET film having the coating film formed on one surface was wound up. Furthermore, the P on which the magnetic coating film is formed
The ET film was calendered and cut into a 1/2 inch width to prepare a magnetic recording material.

Examples 2 to 7 As shown in Tables 1 and 2 below, Example 1 was repeated except that the amount of acetaldehyde, the amount and type of other aldehydes, and the type and proportion of modified moieties were changed. In the same manner as above, to prepare the binder for each magnetic recording body of Examples 2 to 7,
A magnetic recording material was obtained in the same manner as in Example 1. The degree of acetalization of the mixed polyvinyl acetal resin was calculated using ¹³ C-NMR.

Comparative Example 1 As the resin used for the binder for the magnetic recording medium, SLECH A (vinyl chloride / vinyl acetate / vinyl alcohol) manufactured by Sekisui Chemical Co., Ltd. was used in place of the polyvinyl acetal resin. A binder for a magnetic recording material was obtained in the same manner as in Example 1 except for the above, and a magnetic recording material was prepared in the same manner as in Example 1 using the binder for the magnetic recording material.

Comparative Examples 2-5 As shown in Tables 1 and 2 below, the amount of acetaldehyde, the amount and type of aldehyde other than acetaldehyde,
Alternatively, the binders for magnetic recording bodies of Comparative Examples 2 to 5 were produced in the same manner as in Example 1 except that the degree and type of the modified portion were changed. Further, a magnetic recording material was produced in the same manner as in Example 1 using the binder for magnetic recording material. The degree of acetalization of the mixed polyvinyl acetal resin was calculated using ¹³ C-NMR.

[0044]

[Table 1]

[0045]

[Table 2]

Evaluation of Examples and Comparative Examples Examples 1 to 7 and Comparative Examples 1 to 1 produced as described above.
The glossiness, squareness ratio, abrasion resistance and heat resistance of each magnetic recording medium of No. 5 were measured and evaluated according to the following procedures. The results are shown in Table 3 below. Glossiness ... A glossiness meter (gloss meter, manufactured by Murakami Color Research Laboratory Co., Ltd.) was used, and both the incident angle and the reflection angle were measured at 60 °. Squareness ratio ... Residual magnetic flux density (Br) and saturation magnetic flux density (B) when measured with an external magnetic field of 5 kOe using a DC magnetization characteristic automatic recorder (BHH-50 manufactured by Riken Denshi Co., Ltd.)
The ratio m) was determined and used as the squareness ratio.

Abrasion resistance: The magnetic recording medium is caused to run at a running speed of 50 cm / sec while being in sliding contact with the magnetic head.
The amount of wear loss was measured by the running test after running 00 times. In Table 3, the wear reduction of Comparative Example 1 is set to 100%, and the wear reductions of Examples 1 to 7 and Comparative Examples 2 to 5 are shown as values relative to the wear reduction of Comparative Example 1. Heat resistance: evaluated by a heat adhesion test. That is,
After preparing a plurality of magnetic recording media and heating them at a temperature of 120 ° C. for 15 minutes, a plurality of magnetic recording media were superposed on each other and the block property was examined. The meanings of the evaluation symbols in Table 3 are as follows. ◎ ... extremely good, ○ ... good, × ... bad

[0048]

[Table 3]

As is clear from Table 3, the heat resistance of the magnetic recording materials of Comparative Examples 1 to 4 was not sufficient, and the heat resistance of the magnetic recording materials of Comparative Example 5 was comparative because they were acetalized only with acetaldehyde. On the other hand, in the magnetic recording materials of Examples 1 to 7, the acetalized portion with acetaldehyde is within the scope of the invention, and the portion modified with itaconic acid, maleic acid, or sulfonic acid is used. Since it had a ratio within the range, it showed excellent heat resistance. Further, in Example 5, the squareness ratio was as low as 0.80 probably because it did not have the above-mentioned modified portion, and it was worn at a rate of 55% compared to the wear reduction in Comparative Example 1, whereas In Examples 1 to 7, the squareness ratio is as high as 0.83 or more, the glossiness is excellent, and the abrasion resistance is also Comparative Example 1
The wear loss was 35% or less, which was very small.

[0050]

As described above, in the binder for a magnetic recording medium and the magnetic recording medium according to claims 1 and 2, the proportion of all acetalized moieties in all the resin is 55 mol% or more. ,
Moreover, since the above-mentioned specific polyvinyl acetal resin in which 50 to 100% by weight of the total acetalized portion is the acetalized portion by acetaldehyde is used, the heat resistance is increased by the action of the polyvinyl acetoacetal having a high glass transition point. Is effectively increased.

Further, in the binder for a magnetic recording medium according to claim 1, the specific hydrophilic group is contained in the polyvinyl acetal resin in a proportion of 0.01 to 5 mol%, and the acetal ring is contained. By the binding action of the residual hydroxyl group adjacent to the magnetic powder, the dispersibility in the magnetic powder is enhanced, and therefore a magnetic coating film having an excellent squareness ratio can be formed, and the abrasion resistance and heat resistance of the magnetic coating film can be improved. In addition, the binding property to magnetic powder and the binding property to a substrate made of polyester or the like can be improved.

In the magnetic recording medium according to the second aspect of the invention, since the magnetic coating film is formed by using the binder for the magnetic recording medium according to the first aspect, the squareness ratio is high and the durability is high. It is possible to provide a magnetic recording body having a magnetic coating film having excellent wear resistance and heat resistance and capable of coping with high density magnetic recording. In the inventions described in claims 1 and 2, when the binder for a magnetic recording material contains a polyisocyanate, the binder reacts with the residual hydroxyl group of the polyvinyl acetal resin to cause cross-linking. It becomes possible to further improve wear resistance and heat resistance.

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location G11B 5/702 7215-5D // C08F 216/38 MLC 6904-4J

Claims (2)

[Claims] 1. A binder for a magnetic recording medium containing a polyvinyl acetal resin, wherein the proportion of all acetalized moieties in the polyvinyl acetal resin is about 55 mol% or more, and acetaldehyde in the acetalized moieties. ratio of acetalization moiety by is about 50 to 100 wt%, and _{-COOM, -SO 3 M, -OSO 3} M and -PO (O
M) ₂ (provided that M represents H, Li, Na or K), a tertiary amine, a quaternary ammonium salt, and a portion containing at least one hydrophilic group selected from betaine. 0.01 to the total acetal resin
A binder for a magnetic recording medium, which is 5 mol%. 2. A magnetic recording body, wherein a magnetic coating film comprising a magnetic coating material containing a binder for a magnetic recording body containing a polyvinyl acetal resin and a magnetic powder is formed on the surface of a substrate, wherein the magnetic recording is performed. The polyvinyl acetal resin contained in the body binder has a total acetalization ratio of about 55 mol% or more, and the acetalization acetaldehyde conversion ratio of the acetalization part is about 50 to 100%.
Percent by weight, and _{-COOM, -SO 3 M, -OSO 3} M and -PO (O
M) ₂ (provided that M represents H, Li, Na or K), a tertiary amine, a quaternary ammonium salt and betaine, the proportion of the portion containing at least one hydrophilic group is the above. 0.01% of the entire polyvinyl acetal resin
A magnetic recording medium, characterized in that it is ˜5 mol%.