JP2011216156A - Binder for magnetic recording medium, magnetic recording medium, and new polyurethane resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium in which various kinds of powders are highly dispersed, and which is suitable for high-density recording.SOLUTION: A binder for a magnetic recording medium is a polyurethane resin having a cyclic acetal structure represented by formula (I) and an ionic polar group. In formula (I), Rand Rrepresent individually independently a hydrogen atom or an alkyl group, or linked to each other to form a ring structure. At least one of R-Ris a site bonded to a main chain of the polyurethane resin, or the ring structure formed by linking two or more of them has a site bonded thereto, and others represent individually independently a hydrogen atom, an alkyl group, or an alkoxyl group.

Description

The present invention relates to a binder for a magnetic recording medium and a magnetic recording medium containing the same, and more particularly suitable for producing a magnetic recording medium in which various powders such as ferromagnetic powder and nonmagnetic powder are highly dispersed. The present invention relates to a magnetic recording medium binder and a magnetic recording medium including the same.
The present invention further relates to a novel polyurethane resin, and more particularly to a novel polyurethane resin suitable as a binder for magnetic recording media.

  In recent years, means for transmitting information at a high speed have remarkably developed, and it has become possible to transfer images and data having enormous information. Along with the improvement of this data transfer technique, recording and reproducing devices and recording media for recording, reproducing and storing information are required to have higher density recording.

  In order to obtain good electromagnetic conversion characteristics in a high-density recording region, it is known that it is effective to use fine magnetic particles and to disperse the fine magnetic materials to a high degree to improve the smoothness of the magnetic layer surface. ing. In addition, by increasing the dispersibility of the magnetic material, a magnetic recording medium having a high glossiness can be obtained. Furthermore, as a means for increasing the smoothness of the magnetic layer surface, it is also effective to increase the dispersibility of the nonmagnetic powder contained in the nonmagnetic layer located below the magnetic layer.

As a method for improving the dispersibility of powder used in magnetic recording media such as magnetic powder, for example, a method of incorporating an ionic polar group (adsorbing functional group) such as SO ₃ Na group into a binder (Patent Document 1). See). Various binders such as vinyl chloride resins, polyurethane resins, polyester resins, and acrylic resins are used as binders for magnetic recording media. Among them, polyurethane resins are coated by intermolecular hydrogen bonding using urethane bonds. Since strength can be increased, it is widely used (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2003-132931 Japanese Patent Publication No. 7-85305

However, the inventors of the present invention have studied a magnetic recording medium containing, as a binder, a polyurethane resin into which an ionic polar group such as an SO ₃ Na group has been introduced. As a result, high dispersibility required for a high-density recording medium is achieved. Was found to be insufficient.

  Accordingly, an object of the present invention is to provide a magnetic recording medium suitable for high-density recording in which various powders such as ferromagnetic powder and nonmagnetic powder are highly dispersed.

As a result of intensive investigations to achieve the above object, the present inventors have obtained the following knowledge.
For ionic polar groups such as SO ₃ Na groups, the polyurethane resin becomes more hydrophilic as the amount introduced increases in order to improve dispersibility, so the solubility in organic solvents used for media production decreases. To do. As the solvent solubility of the binder resin in the coating solution decreases, the coating solution becomes highly viscous. For example, depending on the treatment by a dispersing machine such as a ball mill or a sand mill as described in Patent Documents 1 and 2 above, the coating solution is applied. It becomes difficult to sufficiently disperse the magnetic recording medium powder (ferromagnetic powder, nonmagnetic powder, etc.) in the liquid, and as a result, the surface smoothness of the obtained magnetic recording medium is lowered.
On the other hand, the inventors of the present application have a high dispersibility without impairing the solvent solubility of the binder resin if the polyurethane resin has a side chain with a cyclic acetal structure such as the following partial structure together with an ionic polar group. I found out that it could be realized.

  This is presumably due to the following reasons.

  Since the cyclic acetal structure has an oxygen atom bound by a ring, Lewis basicity is higher than that of a general ether bond. However, in a polyurethane resin having an acetal ring in the main chain (for example, a polyurethane resin obtained by using spiroglycol disclosed in JP-A-62-212485 as a diol component), the acetal ring is present at the center of the polymer, so The degree of freedom of the ring is small, and it is impossible to contact the surface of the magnetic recording medium powder efficiently.

  On the other hand, a polyurethane resin having a cyclic acetal structure in the side chain as described above can be efficiently adsorbed to the powder by having an acetal ring having a high degree of freedom. In addition, since acetal is a kind of ether, it has a smaller polarity than the ionic polar group described in Patent Document 1 and is excellent in solvent solubility. And since the said cyclic acetal structure is a nonionic polar group, it can improve dispersibility, without inhibiting adsorption | suction of an ionic polar group.

As described above, a polyurethane resin having a cyclic acetal structure that is nonionic and highly Lewis basic with an ionic polar group can be used as a magnetic recording medium powder without impairing solubility in organic solvents. It is considered that high dispersibility can be realized by adsorption.
As a result of further studies based on the above knowledge, the present inventors have completed the present invention.

[一般式(I)中、Ｒ¹およびＲ²は、それぞれ独立に水素原子もしくはアルキル基を表すか、または互いに連結して環構造を形成する。Ｒ³〜Ｒ⁶は、少なくとも１つがポリウレタン樹脂の主鎖との結合部位であるか、または２つ以上が連結して形成される環構造中に結合部位を有し、他はそれぞれ独立に水素原子、アルキル基、またはアルコキシル基を表す。]
［２］前記イオン性極性基はスルホン酸（塩）基である［１］に記載の磁気記録媒体用結合剤。
［３］１０〜１０００μｅｑ／ｇのスルホン酸（塩）基を含有する［２］に記載の磁気記録媒体用結合剤。
［４］一般式(I)で表される環状アセタール構造を１０〜２０００μｅｑ／ｇ含有する［１］〜［３］のいずれかに記載の磁気記録媒体用結合剤。
［５］ウレタン基濃度が１．０〜３．５ｍｍｏｌ／ｇの範囲である［１］〜［４］のいずれかに記載の磁気記録媒体用結合剤。
［６］ガラス転移温度が５０〜１５０℃の範囲である［１］〜［５］のいずれかに記載の磁気記録媒体用結合剤。
［７］非磁性支持体上に強磁性粉末および結合剤を含む磁性層を有する磁気記録媒体であって、
前記結合剤は、［１］〜［６］のいずれかに記載の結合剤を構成成分として含むことを特徴とする磁気記録媒体。
［８］非磁性支持体上に非磁性粉末および結合剤を含む非磁性層と強磁性粉末および結合剤を含む磁性層とをこの順に有する磁気記録媒体であって、
前記磁性層に含まれる結合剤および／または前記非磁性層に含まれる結合剤は、［１］〜［６］のいずれかに記載の結合剤を構成成分として含むことを特徴とする磁気記録媒体。
［９］下記一般式(I)で表される環状アセタール構造とスルホン酸（塩）基とを有することを特徴とするポリウレタン樹脂。

[一般式(I)中、Ｒ¹およびＲ²は、それぞれ独立に水素原子もしくはアルキル基を表すか、または互いに連結して環構造を形成する。Ｒ³〜Ｒ⁶は、少なくとも１つがポリウレタン樹脂の主鎖との結合部位であるか、または２つ以上が連結して形成される環構造中に結合部位を有し、他はそれぞれ独立に水素原子、アルキル基、またはアルコキシル基を表す。]
［１０］１０〜１０００μｅｑ／ｇのスルホン酸（塩）基を含有する［９］に記載のポリウレタン樹脂。
［１１］一般式(I)で表される環状アセタール構造を１０〜２０００μｅｑ／ｇ含有する［９］または［１０］に記載のポリウレタン樹脂。
［１２］ウレタン基濃度が１．０〜３．５ｍｍｏｌ／ｇの範囲である［９］〜［１１］のいずれかに記載のポリウレタン樹脂。
［１３］ガラス転移温度が５０〜１５０℃の範囲である［９］〜［１２］のいずれかに記載のポリウレタン樹脂。 That is, the above object was achieved by the following means.
[1] A binder for a magnetic recording medium, which is a polyurethane resin having a cyclic acetal structure represented by the following general formula (I) and an ionic polar group.

[In General Formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. R ^{3 to} R ⁶ each have at least one bonding site with the main chain of the polyurethane resin, or have a bonding site in a ring structure formed by linking two or more, and the others are each independently hydrogen. An atom, an alkyl group, or an alkoxyl group is represented. ]
[2] The binder for magnetic recording media according to [1], wherein the ionic polar group is a sulfonic acid (salt) group.
[3] The binder for magnetic recording media according to [2], which contains 10 to 1000 μeq / g of a sulfonic acid (salt) group.
[4] The binder for magnetic recording media according to any one of [1] to [3], which contains 10 to 2000 μeq / g of a cyclic acetal structure represented by the general formula (I).
[5] The binder for magnetic recording media according to any one of [1] to [4], wherein the urethane group concentration is in the range of 1.0 to 3.5 mmol / g.
[6] The binder for magnetic recording media according to any one of [1] to [5], wherein the glass transition temperature is in the range of 50 to 150 ° C.
[7] A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support,
The binder includes the binder according to any one of [1] to [6] as a constituent component.
[8] A magnetic recording medium having a nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder in this order on a nonmagnetic support,
The binder contained in the magnetic layer and / or the binder contained in the nonmagnetic layer contains the binder described in any one of [1] to [6] as a constituent component. .
[9] A polyurethane resin having a cyclic acetal structure represented by the following general formula (I) and a sulfonic acid (salt) group.

[In General Formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. R ^{3 to} R ⁶ each have at least one bonding site with the main chain of the polyurethane resin, or have a bonding site in a ring structure formed by linking two or more, and the others are each independently hydrogen. An atom, an alkyl group, or an alkoxyl group is represented. ]
[10] The polyurethane resin according to [9], which contains 10 to 1000 μeq / g of sulfonic acid (salt) groups.
[11] The polyurethane resin according to [9] or [10], which contains 10 to 2000 μeq / g of a cyclic acetal structure represented by the general formula (I).
[12] The polyurethane resin according to any one of [9] to [11], wherein the urethane group concentration is in the range of 1.0 to 3.5 mmol / g.
[13] The polyurethane resin according to any one of [9] to [12], which has a glass transition temperature in the range of 50 to 150 ° C.

  According to the present invention, it is possible to provide a magnetic recording medium suitable for high density recording in which various powders such as ferromagnetic powder and nonmagnetic powder are highly dispersed.

[Binder for magnetic recording media]
The binder for magnetic recording media of the present invention (hereinafter also referred to as “the binder of the present invention”) is a polyurethane resin having a cyclic acetal structure represented by the following general formula (I) and an ionic polar group.

[一般式(I)中、Ｒ¹およびＲ²は、それぞれ独立に水素原子もしくはアルキル基を表すか、または互いに連結して環構造を形成する。Ｒ³〜Ｒ⁶は、少なくとも１つがポリウレタン樹脂の主鎖との結合部位であるか、または２つ以上が連結して形成される環構造中に結合部位を有し、他はそれぞれ独立に水素原子、水酸基、アルキル基、またはアルコキシル基を表す。]

[In General Formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. R ^{3 to} R ⁶ each have at least one bonding site with the main chain of the polyurethane resin, or have a bonding site in a ring structure formed by linking two or more, and the others are each independently hydrogen. An atom, a hydroxyl group, an alkyl group, or an alkoxyl group is represented. ]

Since the polyurethane resin has a binding site to the main chain of the polyurethane resin in any of R ^{3 to} R ⁶ , the acetal ring has a high degree of freedom as described above, and it can be efficiently used as a magnetic recording medium powder. Can be adsorbed. In addition, the acetal ring has low polarity and excellent solvent solubility compared to ionic polar groups that have been introduced into binder resins for the purpose of improving dispersibility, so the viscosity increases when used in combination with ionic polar groups. Further improvement in dispersibility can be achieved without causing any problems. Moreover, since it is a nonionic polar group, dispersibility can be improved without inhibiting adsorption | suction of an ionic polar group. Thus, according to the binder of the present invention, a magnetic recording medium in which various powders are highly dispersed can be obtained.
Hereinafter, the binder of the present invention will be described in more detail.

In general formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. In the polyurethane resin represented by the general formula (I), R ¹ and R ² do not include a binding site with the main chain of the polyurethane resin. This is because when R ¹ and R ² contain a binding site to the main chain, the degree of freedom of the acetal ring is lowered, and good adsorptivity cannot be obtained.

  The “alkyl group” in the present invention includes cyclic alkyl groups such as a cycloalkyl group and a bicycloalkyl group. In addition, when a group such as an alkyl group described below has a substituent, examples of the substituent include an alkyl group (for example, an alkyl group having 1 to 6 carbon atoms), a hydroxyl group, and an alkoxyl group (for example, an alkoxyl having 1 to 6 carbon atoms). Group), halogen atoms (for example, fluorine atom, chlorine atom, bromine atom), carboxyl group and the like. In addition, the “carbon number” of a group such as an alkyl group, when the group has a substituent, has a carbon number of a portion not including the substituent.

Examples of the alkyl group represented by R ¹ and R ² include linear, branched, and cyclic substituted or unsubstituted alkyl groups. When a large steric hindrance occurs around the acetal ring, the adsorption to the magnetic recording medium may be excluded, so a linear alkyl group having 1 to 30 carbon atoms such as a methyl group, an ethyl group, and n-propyl is preferable. Group, propyl group, butyl group, n-octyl group, eicosyl group, 2-chloroethyl group, 2-cyanoethyl group.

In general formula (I), R ¹ and R ² may be the same or different. R ¹ and R ² can also be linked to each other to form a ring structure. In this case, the formed ring structure may be saturated or unsaturated, may be a carbocyclic ring or a heterocyclic ring, and may be a monocyclic ring or a condensed ring. Examples of the carbocycle include saturated or unsaturated 4- to 7-membered carbocycles such as a cyclohexyl ring, a cyclopentyl ring, a cyclohexene ring, and a benzene ring. Examples of the heterocyclic ring include saturated or unsaturated 4- to 7-membered heterocyclic rings such as a piperidine ring, piperazine ring, morpholine ring, tetrahydrofuran ring, furan ring, thiophene ring, pyridine ring, and pyrazine ring. it can. These ring structures may have a substituent.

In the general formula (I), at least one of R ^{3 to} R ⁶ is a bonding site with the main chain of the polyurethane resin, or has a bonding site in a ring structure formed by linking two or more. And the others each independently represent a hydrogen atom, an alkyl group, or an alkoxyl group.

The cyclic acetal structure represented by the general formula (I) has a high degree of freedom by having a binding site with a main chain (polymer chain including a plurality of urethane bonds) in at least one of R ^{3 to} R ^6. An acetal ring can be present on the side chain of the polyurethane resin. The number of binding sites contained in R ^{3 to} R ⁶ is preferably 1 to 3, and the number can be controlled by the structure of the raw material compound used in the synthesis of the polyurethane. For example, the following diol component (1,2: 5,6-di-O-isopropylidene-D-mannitol):

を使用しポリイソシアネートとのウレタン化反応により得られるポリウレタン樹脂では、点線で囲んだ２つの水酸基がイソシアネート基と反応しウレタン結合を形成し主鎖との結合部位となる。したがって、この場合１つの環状アセタール構造あたり１つの結合部位が含まれることになる。

In the polyurethane resin obtained by urethanation reaction with polyisocyanate using the two, the two hydroxyl groups surrounded by the dotted line react with the isocyanate group to form a urethane bond and become a bonding site with the main chain. Therefore, in this case, one binding site is included per one cyclic acetal structure.

The binding site may be directly connected to the acetal ring, but the acetal ring may be bonded to the main chain via a divalent linking group. As the linking group in this case, —O—, —CO—, and —NR ¹¹ —R ¹¹ represent a hydrogen atom or an alkyl group), an alkylene group, an arylene group, or a combination thereof. Carbon number of an alkylene group becomes like this. Preferably it is 1-10, More preferably, it is 1-8, More preferably, it is 1-6. Specific examples of preferable alkylene groups include a methylene group, an ethylene group, a trimethylene group, a tetrabutylene group, and a hexamethylene group. The carbon number of the arylene group is preferably 6 to 24, more preferably 6 to 18, and particularly preferably 6 to 12. Specific examples of preferable arylene groups include phenylene groups and naphthalene groups. These groups can also have a substituent.

The binding site to the main chain may be contained in a ring structure formed by connecting two or more of R ^{3 to} R ⁶ . The details of the ring structure in this case are as described for the ring structure formed by connecting R ¹ and R ² to each other.

In R ^{3 to} R ⁶ , those not including a bonding site with the main chain each independently represent a hydrogen atom, an alkyl group, or an alkoxyl group. Details of the alkyl group represented by R ^{3 to} R ⁶ are as described above for the alkyl group represented by R ¹ and R ² . The alkoxyl group is preferably a substituted or unsubstituted alkoxy group having 1 to 30 carbon atoms, such as a methoxy group, an ethoxy group, an isopropoxy group, a t-butoxy group, an n-octyloxy group, or a 2-methoxyethoxy group. Can be mentioned.

  Specific examples of the cyclic acetal structure represented by the general formula (I) are shown below. However, the present invention is not limited to the following specific examples. In the following, the wavy line indicates the binding site with the main chain of the polyurethane resin.

  The polyurethane resin having a cyclic acetal structure represented by the general formula (I) is: (1) a method of introducing the cyclic acetal structure represented by the general formula (I) into the side chain of the polyurethane resin by an addition reaction; It can be synthesized by any of the methods using a compound having an acetal ring in the general formula (I) as a polyol and / or polyisocyanate which is a raw material compound of a polyurethane resin. Method (2) is preferable from the viewpoint of easy availability of raw materials and synthetic reaction, and it is more preferable to use a polyol having an acetal ring in general formula (I) in method (2).

  The polyol having an acetal ring can be synthesized by a known method, and some are available as commercial products. Specific examples include the following polyols.

Polyisocyanate as a raw material compound for polyurethane synthesis may be a bifunctional or higher functional isocyanate compound. Specific examples thereof include MDI (diphenylmethane diisocyanate), 2,4-TDI (tolylene diisocyanate), and 2,6-TDI. , 1,5-NDI (naphthalene diisocyanate), TODI (tolidine diisocyanate), p-phenylene diisocyanate, XDI (xylylene diisocyanate) and other aromatic diisocyanates, transcyclohexane 1,4-diisocyanate, HDI (hexamethylene diisocyanate), IPDI (Isophorone diisocyanate), H ₆ XDI (hydrogenated xylylene diisocyanate), H ₁₂ MDI (hydrogenated diphenylmethane diisocyanate) An isocyanate etc. can be mentioned.

  As a raw material compound for polyurethane synthesis, a polyol not having the cyclic acetal structure can be included together with the polyol and polyisocyanate having the cyclic acetal structure. The polyol used in combination can serve as a chain extender. As the polyol used in combination, various polyols generally used as a polyurethane raw material, specifically, ethylene glycol, propylene glycol, trimethylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, mono Repeating units of alicyclic structure such as linear aliphatic diols such as olein, monoacetin, pentaerythritol distearate, pentaerythritol dioleate, 1,4-cyclohexanedimethanol, hydrogenated bisphenol A, tricyclodecane dimethanol Aromatic diols such as bisphenol A and xylylene diol, diethylene glycol, triethylene glycol, polyethylene glycol, polytetramethylene glycol and other (poly) ether glycols Lumpur, mention may be made of polycarbonate polyols and the like.

  The content of the cyclic acetal structure in the binder (polyurethane resin) of the present invention can be controlled by the blending ratio of the raw material compounds. A polyurethane resin having a cyclic acetal structure represented by the general formula (I) containing 10 to 2000 μeq of a ring is preferable because it can be efficiently adsorbed on various powders and can improve dispersibility. The content of the cyclic acetal structure represented by the general formula (I) in the binder of the present invention is more preferably in the range of 20 to 1200 μeq / g, further preferably 30 to 1000 μeq / g. The content of the polyol and polyisocyanate used together as the raw material compound may be appropriately set. For example, the content of the polyol used in combination in the raw material is 45.0 to 70.0% by mass, and the content of the polyisocyanate is 23.0. It can be set to -49.0 mass%.

The binder of the present invention has a cyclic acetal structure represented by the general formula (I) as an adsorptive functional group together with an ionic polar group. Thus, the cyclic acetal structure represented by the general formula (I) can play a role of imparting adsorptivity in a system in which it is difficult to obtain a sufficient dispersibility improvement effect only by introduction of an ionic polar group. . Further, since the cyclic acetal structure represented by the general formula (I) is a nonionic polar group, the dispersibility can be enhanced without inhibiting the adsorption of the ionic polar group. Here, the ionic polar group is capable of forming an ionic interaction with the surface of the ferromagnetic powder or non-magnetic powder, and includes —SO ₃ M, —SO ₄ M, —PO (OM) ₂ , — _{OPO (OM) 2, -COOM,} > NSO 3 M,> NRSO 3 M, -NR 21 R 22, -N + R 21 R 22 R 23 X - and the like. Here, M represents hydrogen or an alkali metal such as Na or K, R represents an alkylene group, R ²¹ , R ²² and R ²³ represent an alkyl group, a hydroxyalkyl group or a hydrogen atom, and X represents a halogen atom such as Cl or Br. The amount of the ionic polar group in the binder of the present invention is preferably 10 to 2000 μeq / g from the viewpoint of achieving improvement in dispersibility when used in combination with the cyclic acetal structure represented by the general formula (I). More preferably, ˜1200 μeq / g.

Examples of ionic functional groups that are preferably introduced into the binder of the present invention include sulfonic acid groups (—SO ₃ H) and sulfonic acid groups such as SO ₃ Na groups, SO ₃ K groups, SO ₃ Li (the present invention). The “sulfonic acid (salt) group” in FIG. 4 includes a sulfonic acid group and a sulfonic acid group. Thereby, the adsorptivity between the binder of the present invention and the powder can be further enhanced. When introducing a sulfonic acid (salt) group as a functional group into the binder of the present invention, the amount introduced is preferably 10 to 1000 μeq / g, and preferably 10 to 100 μeq / g, from the viewpoint of improving dispersibility. Is more preferable.

  The ionic polar group can be introduced into the polyurethane resin by a known method. Examples of the introduction method include a method using a polyol having the functional group as a part of the raw material polyol, and a method of introducing the polar group by polymer reaction after polymerizing polyurethane. For example, by using a sulfonic acid (salt) group-containing polyol as the polyol used in combination, a polyurethane resin containing a sulfonic acid (salt) group can be obtained. As the sulfonic acid (salt) group-containing polyol, sulfonic acid (salt) group-containing diols described in paragraphs [0015] to [0043] of JP-A-2009-96798 are preferably used. Usually, the polyurethane synthesis reaction is carried out in an organic solvent. However, the sulfonic acid (salt) group-containing polyol compound is generally poor in solubility in an organic solvent, and therefore has a problem of poor reactivity. On the other hand, the sulfonic acid (salt) group diol described in JP-A-2009-96798 is suitable as a raw material compound for polyurethane because of its excellent solubility in organic solvents. Regarding the method for synthesizing the sulfonic acid (salt) group-containing diol, reference can be made to paragraphs [0028], [0029] and [0045] of JP-A-2009-96798 and the examples of the same publication. Specific examples of the sulfonic acid (salt) group-containing diol include exemplified compounds (S-1) to (S-70) described in the publication.

  The polyurethane resin which is the binder of the present invention can be obtained by reacting the aforementioned raw materials by a known method. Regarding the synthesis method, examples described later can also be referred to.

  Next, various physical properties of the binder (polyurethane resin) of the present invention will be described.

  The molecular weight of the binder of the present invention is preferably in the range of 15,000 to 200,000 as a mass average molecular weight, preferably 30,000 or more from the viewpoint of head stain suppression, and 180,000 from the viewpoint of dispersibility. It is preferable that it is below, More preferably, it is 50,000-150,000. A polyurethane resin having a mass average molecular weight within the above range can form a magnetic recording medium having high solvent solubility and good dispersibility and high coating strength. The mass average molecular weight / number average molecular weight ratio (Mw / Mn) is preferably 5 or less, more preferably in the range of 1.4 to 4.0. The average molecular weight in the present invention refers to a value determined in terms of standard polystyrene. The molecular weight of the binder of the present invention can be controlled by the raw material composition, reaction conditions, and the like.

  The urethane group concentration of the binder of the present invention is preferably 1.0 mmol / g to 3.5 mmol / g, and more preferably 1.0 mmol / g to 3.0 mmol / g. When the urethane group concentration is 1.0 mmol / g or more, a coating film having a high glass transition temperature (Tg) and good durability can be formed, and the dispersibility is also good. Moreover, it is preferable if the urethane group concentration is 3.5 mmol / g or less because good solvent solubility can be obtained.

  The glass transition temperature (Tg) of the binder of the present invention is preferably 50 ° C. to 150 ° C., and more preferably 60 ° C. to 130 ° C. A glass transition temperature of 50 ° C. or higher is preferable because a coating film excellent in durability and storage stability can be obtained. A glass transition temperature of the polyurethane resin of 150 ° C. or lower is preferable because a calendering property is good and a magnetic recording medium having good electromagnetic conversion characteristics can be obtained. The glass transition temperature in the present invention is a value determined by dynamic viscoelasticity measurement. For specific measurement methods, the description of Examples described later can be referred to.

  The binder of the present invention is preferably used in combination with a polyisocyanate curing agent. Thereby, a magnetic recording medium having high coating strength can be obtained. Examples of polyisocyanates include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate and the like. Moreover, the product of these isocyanates and a polyalcohol, and polyisocyanate more than bifunctional, such as the polyisocyanate produced | generated by condensation of isocyanate, can be used. Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Taketake D-102, Takenate D-110N, There are Takenate D-200, Takenate D-202, Sumitomo Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc., either alone or by utilizing the difference in curing reactivity. Each layer can be used in the above combination.

  From the viewpoint of improving the coating film strength, it is preferable to use a triisocyanate or higher polyisocyanate as the polyisocyanate. Specific examples of the trifunctional or higher functional polyisocyanate include a compound obtained by adding 3 mol of TDI (tolylene diisocyanate) to trimethylolpropane (TMP), a compound obtained by adding 3 mol of HDI (hexamethylene diisocyanate) to TMP, and IPDI to TMP. (Isophorone diisocyanate) 3 mol addition compound, TMP 3 mol addition XDI (xylylene diisocyanate) 3 adduct type polyisocyanate compound, TDI condensation isocyanurate type trimer, TDI condensation isocyanurate 5 amount , Condensed isocyanurate heptamers of TDI, and mixtures thereof, isocyanurate type condensates of HDI, isocyanurate type condensates of IPDI, and crude MDI. The amount of polyisocyanate used can be, for example, 0 to 80 parts by mass with respect to 100 parts by mass of the binder of the present invention, and is preferably 10 to 50 parts by mass from the viewpoint of improving the coating film strength.

[Magnetic recording medium]
The magnetic recording medium of the present invention includes the following embodiments 1 and 2.
Aspect 1: A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support, wherein the binder contains the binder of the present invention as a constituent component.
Aspect 2: A magnetic recording medium having a nonmagnetic layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder in this order on a nonmagnetic support,
A magnetic recording medium in which the binder contained in the magnetic layer and / or the binder contained in the nonmagnetic layer contains the binder of the present invention as a constituent component.

  The magnetic recording medium of the present invention contains the binder of the present invention as a constituent component of the binder in the magnetic layer and / or the nonmagnetic layer. The phrase “containing the binder of the present invention as a constituent component of the binder” means that the binder of the present invention itself or a reaction product of the binder of the present invention and other binder components is contained in the binder. That means.

  The magnetic recording medium of the present invention can also contain a known thermoplastic resin, thermosetting resin, or reactive resin as a constituent component of the binder of the magnetic layer and / or nonmagnetic layer. In this case, as a constituent component of the binder of the magnetic layer and / or the nonmagnetic layer, a reaction product in which the binder of the present invention and the resin are cross-linked by polyisocyanate can be included. Thermoplastic resins include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, vinyl ether , Etc. as constituent units, and polymers and copolymers, and various rubber resins. Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins. And a mixture of an isocyanate prepolymer, a mixture of a polyester polyol and a polyisocyanate, a mixture of a polyurethane and a polyisocyanate, and the like. These resins are described in detail in “Plastic Handbook” published by Asakura Shoten. It is also possible to use a known electron beam curable resin for each layer. These examples and their production methods are described in detail in JP-A No. 62-256219. The above resins can be used alone or in combination.

  In the nonmagnetic layer and the magnetic layer, a binder can be used in the range of, for example, 5 to 50 parts by mass, preferably 10 to 30 parts by mass with respect to 100 parts by mass of the ferromagnetic powder or nonmagnetic powder. Moreover, it is preferable that the usage-amount of the binder (polyurethane resin) of this invention shall be 5-30 mass parts with respect to 100 mass parts of ferromagnetic powder or nonmagnetic powder from the point of a dispersibility improvement, More preferably, it is 20 parts by mass.

  Next, the magnetic recording medium of the present invention will be described in more detail.

(Magnetic layer)
In the magnetic recording medium of the present invention, examples of the ferromagnetic powder contained in the magnetic layer include hexagonal ferrite powder and ferromagnetic metal powder.

The average plate diameter of the hexagonal ferrite is preferably 10 to 100 nm, more preferably 10 to 60 nm, and particularly preferably 10 to 50 nm. In particular, when reproducing with an MR head in order to increase the track density, it is necessary to reduce the noise, so that the average plate diameter is preferably 60 nm or less, more preferably 50 nm or less. If it is smaller than 10 nm, stable magnetization cannot be expected due to thermal fluctuation. If it exceeds 100 nm, the noise is high and none of them is suitable for high-density magnetic recording. The average plate ratio (plate diameter / plate thickness) is preferably 1-15. More preferably, it is 1-7. When the plate ratio is small, the filling property in the magnetic layer is preferably high, but it is difficult to obtain sufficient orientation. When it is larger than 15, noise increases due to stacking between particles. The specific surface area according to the BET method in this particle size range is 10 to 100 m ² / g. The specific surface area is generally signified as an arithmetic calculation value from the particle plate diameter and plate thickness. The distribution of the particle plate diameter and plate thickness is generally preferably as narrow as possible. Although numerical conversion is difficult, it can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known. It is preferable to optimize the water content of the hexagonal ferrite powder depending on the type of the binder.

In general, hexagonal ferrite powder having a coercive force Hc of about 500 to 5000 oersted (40 to 398 kA / m) can be produced. A higher Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. The Hc of the hexagonal ferrite used in the present invention is preferably about 2000 to 4000 Oe (160 to 320 kA / m), more preferably 2200 to 3500 Oe (176 to 280 kA / m). When the saturation magnetization of the head exceeds 1.4 Tesla, it is preferably 2200 Oe (176 kA / m) or more. Hc can be controlled by the particle size (plate diameter / plate thickness), the type and amount of contained elements, the substitution site of the elements, the particle generation reaction conditions, and the like. The saturation magnetization σs is preferably 40 to 80 A · m ² / kg. σs is preferably higher, but tends to be smaller as the particles become finer. In order to improve σs, it is well known to combine spinel ferrite with magnetoplumbite ferrite and to select the type and amount of elements contained. It is also possible to use W-type hexagonal ferrite. When the hexagonal ferrite is dispersed, the surface of the hexagonal ferrite powder is treated with a material suitable for the dispersion medium and the polymer. As the surface treatment agent, an inorganic compound or an organic compound can be used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The amount can be 0.1 to 10% by mass relative to the hexagonal ferrite powder. The pH of the hexagonal ferrite powder is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium.

The specific surface area by BET method of the ferromagnetic metal powder employed in the magnetic layer is preferably 45~100m ² / g, more preferably 50-80 m ² / g. If it is 45 m ² / g or more, low noise is obtained, and if it is 100 m ² / g or less, good surface properties can be obtained. The crystallite size of the ferromagnetic metal powder is preferably 80 to 180 mm, more preferably 100 to 180 mm, and still more preferably 110 to 175 mm. The average major axis length of the ferromagnetic metal powder is preferably 0.01 μm or more and 0.15 μm or less, more preferably 0.02 μm or more and 0.15 μm or less, and further preferably 0.03 μm or more and 0.12 μm or less. is there. The average needle ratio of the ferromagnetic metal powder is preferably 3 or more and 15 or less, more preferably 5 or more and 12 or less. Σs of the ferromagnetic metal powder is preferably from 100~180A · m ² / kg, more preferably 110~170A · m ² / kg, and more preferably from 125~160A · m ² / kg. The coercive force of the ferromagnetic metal powder is preferably 2000 to 3500 Oe (160 to 280 kA / m), more preferably 2200 to 3000 Oe (176 to 240 kA / m).

It is preferable to optimize the moisture content of the ferromagnetic metal powder depending on the type of the binder. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range can be 4-12, preferably 6-10. The ferromagnetic metal powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount thereof can be 0.1 to 10% with respect to the ferromagnetic metal powder, and the surface treatment is preferable because the adsorption amount of a lubricant such as a fatty acid is 100 mg / m ² or less. The ferromagnetic metal powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they hardly affect the characteristics. The ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less. As for the shape, any of needle shape, rice grain shape, or spindle shape may be used as long as the above-mentioned characteristics regarding the particle size are satisfied.

The average particle size of the ferromagnetic powder can be measured by the following method.
The ferromagnetic powder is photographed with a transmission electron microscope H-9000, manufactured by Hitachi, at a photographing magnification of 100,000, and printed on a photographic paper so that the total magnification is 500,000 times to obtain a particle photograph. The target magnetic material is selected from the particle photograph, the outline of the powder is traced with a digitizer, and the particle size is measured with the image analysis software KS-400 manufactured by Carl Zeiss. Measure the size of 500 particles. The average particle size measured by the above method is defined as the average particle size of the ferromagnetic powder.

In the present invention, the size of the powder such as a magnetic material (hereinafter referred to as “powder size”) is (1) the shape of the powder is needle-like, spindle-like, or columnar (however, the height is the bottom) In the case of larger than the maximum major axis, etc., it is represented by the length of the major axis constituting the powder, that is, the major axis length. (Smaller than the maximum major axis of the plate surface or the bottom surface), and (3) the shape of the powder is spherical, polyhedral, unspecified, etc. When the major axis constituting the body cannot be specified, it is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.
The average powder size of the powder is an arithmetic average of the above powder sizes, and is obtained by carrying out the measurement as described above for 500 primary particles. Primary particles refer to an independent powder without aggregation.

The average acicular ratio of the powder is determined by measuring the short axis length of the powder in the above measurement, that is, the short axis length, and calculating the arithmetic average of the values of (long axis length / short axis length) of each powder. Point to. Here, the minor axis length is defined by the above-mentioned powder size in the case of (1), the length of the minor axis constituting the powder, and in the case of (2), the thickness or height, respectively. In the case of (3), since there is no distinction between the major axis and the minor axis, (major axis length / minor axis length) is regarded as 1 for convenience.
When the shape of the powder is specific, for example, in the case of the definition (1) of the powder size, the average powder size is referred to as an average major axis length, and in the case of the definition (2), the average powder size Is called the average plate diameter, and the arithmetic average of (maximum major axis / thickness to height) is called the average plate ratio. In the case of definition (3), the average powder size is referred to as an average diameter (also referred to as an average particle diameter or an average particle diameter).

  The ferromagnetic powder described above is described in detail, for example, in paragraphs [0024] and [0028] to [0030] of JP-A-2009-88293.

(Nonmagnetic layer)
The magnetic recording medium according to aspect 2 has a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer, and the binder of the magnetic layer and / or the nonmagnetic layer is the present one. Inventive binders are included as constituents.
The nonmagnetic layer is not particularly limited as long as it is substantially nonmagnetic, and may contain magnetic powder as long as it is substantially nonmagnetic. “Substantially non-magnetic” means that the non-magnetic layer is allowed to have magnetism within a range that does not substantially reduce the electromagnetic conversion characteristics of the magnetic layer. For example, the residual magnetic flux density is 0.01T. It shows below or that a coercive force is 7.96 kA / m (100 Oe or less), Preferably it has no residual magnetic flux density and a coercive force.

  The nonmagnetic powder used for the nonmagnetic layer can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Examples of inorganic compounds include α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, and nitridation with an α conversion rate of 90% or more. Silicon, titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. are used alone or in combination. be able to. Particularly preferred are titanium dioxide, zinc oxide, iron oxide, and barium sulfate because the particle size distribution is small and there are many means for imparting functions, and more preferred are titanium dioxide and α-iron oxide. The average particle size of these non-magnetic powders is preferably 0.005 to 2 μm. However, if necessary, non-magnetic powders having different particle sizes may be combined, or single non-magnetic powders may have the same particle size distribution. The effect of can also be given. The average particle size of the nonmagnetic powder is particularly preferably 0.01 μm to 0.2 μm. In particular, when the nonmagnetic powder is a granular metal oxide, the average particle diameter is preferably 0.08 μm or less, and when it is an acicular metal oxide, the average major axis length is 0.3 μm or less. Is preferably 0.2 μm or less. The tap density is preferably 0.05 to 2 g / ml, more preferably 0.2 to 1.5 g / ml. The pH of the non-magnetic powder can be 2 to 11, and is particularly preferably between 5.5 and 10.

Preferably the specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, more preferably 5~80m ² / g, more preferably from 10 to 70 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.004 μm to 1 μm, and more preferably 0.04 μm to 0.1 μm. The oil absorption amount using DBP (dibutyl phthalate) is preferably 5 to 100 ml / 100 g, more preferably 10 to 80 ml / 100 g, and still more preferably 20 to 60 ml / 100 g. The specific gravity is preferably 1 to 12, more preferably 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape. The Mohs hardness is preferably 4 or more and 10 or less. It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO, and Y ₂ O ₃ exist on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ , and more preferred are Al ₂ O ₃ , SiO ₂ and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of treating the surface layer with silica after first making alumina exist, or vice versa can be employed. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense. Specific examples of the nonmagnetic powder include those described in paragraph [0066] of JP-A-2009-99240.

(Additive)
Additives can be added to the magnetic layer and the nonmagnetic layer as necessary. As the additive, a compound having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect, and the like can be used. Specifically, molybdenum disulfide, tungsten disulfide graphite, boron nitride, graphite fluoride, silicone oil, silicone with a polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol , Alkyl phosphates and alkali metal salts thereof, alkyl sulfates and alkali metal salts thereof, polyphenyl ether, phenylphosphonic acid, α-naphthyl phosphoric acid, phenyl phosphoric acid, diphenyl phosphoric acid, p-ethylbenzenephosphonic acid, phenylphosphinic acid and the like Compounds, carboxylic acid compounds such as benzoic acid, cinnamic acid, oleic acid, phenols such as phenol and catechol, aminoquinones, various silane coupling agents, titanium coupling agents, fluorine-containing compounds Kill sulfates and alkali metal salts thereof, monobasic fatty acids having 10 to 24 carbon atoms (which may contain unsaturated bonds or may be branched), and metal salts thereof (Li, Na, K, Cu, etc.) or a monovalent, divalent, trivalent, tetravalent, pentavalent, hexavalent alcohol having 12 to 22 carbon atoms (which may contain an unsaturated bond or may be branched), carbon number 12 to 22 alkoxy alcohols, monobasic fatty acids having 10 to 24 carbon atoms (which may contain an unsaturated bond or branched) and monovalent, divalent or trivalent carbon atoms having 2 to 12 carbon atoms, Mono-fatty acid ester, di-fatty acid ester, tri-fatty acid ester, alkylene oxide polymer comprising any one of tetravalent, pentavalent, and hexavalent alcohols (which may contain an unsaturated bond or may be branched) of Monoalkyl ether fatty acid esters, fatty acid amides having 8 to 22 carbon atoms, aliphatic amines having 8 to 22 carbon atoms, and the like can be used. For specific examples of these additives, reference can be made, for example, to paragraph [0113] of JP2009-96798A. As the additive, for example, a surfactant described in paragraph [0112] of JP-A-2009-96798 can also be used.

(Organic solvent)
Organic solvents include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and methylcyclohexanol, acetic acid Esters such as methyl, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate and glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane, aromatics such as benzene, toluene, xylene, cresol and chlorobenzene Hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dic Chlorinated hydrocarbons such as Rubenzen, N, N- dimethylformamide, a known organic solvent such as hexane may be used in any ratio. Since the binder of this invention is excellent in the solubility with respect to these organic solvents, the low viscosity coating liquid which can disperse | distribute powder efficiently can be obtained.

(Carbon black)
The magnetic recording medium of the present invention can contain carbon black in the magnetic layer and / or the nonmagnetic layer. Examples of usable carbon black include furnace for rubber, thermal for rubber, black for color, acetylene black and the like. The specific surface area is preferably ⁵ to 500 m ² / g, the DBP oil absorption is 10 to 400 ml / 100 g, and the average particle size is 5 to 300 nm, preferably 10 to 250 nm, more preferably 20 to 200 nm. The pH is preferably 2 to 10, the water content is preferably 0.1 to 10%, and the tap density is preferably 0.1 to 1 g / cc. When carbon black is used, it is preferably used in an amount of 0.1 to 30% by mass based on the ferromagnetic powder or nonmagnetic powder. Regarding the carbon black that can be used in the present invention, for example, “Carbon Black Handbook” (edited by the Carbon Black Association) can be referred to.

(Non-magnetic support)
Non-magnetic supports include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aromatic polyamide, polybenzoxazole and other known films. it can. It is preferable to use a support having a glass transition temperature of 100 ° C. or higher, and it is particularly preferable to use a high-strength support such as polyethylene naphthalate or aramid. Further, as the non-magnetic support, the center surface average surface roughness (Ra) measured with a light interference type surface roughness meter HD-2000 manufactured by WYKO is 8.0 nm or less, preferably 4.0 nm or less, more preferably It is preferable to use one having a thickness of 2.0 nm or less. For details of the nonmagnetic support, reference can be made, for example, to paragraphs [0073] to [0075] of JP-A-2009-99240.

(Layer structure)
In the magnetic recording medium of the present invention, the thickness of the nonmagnetic support is, for example, 2 to 100 μm, preferably 2 to 80 μm. In the case of a computer tape, the thickness of the nonmagnetic support is preferably from 3.0 to 6.5 μm, more preferably from 3.0 to 6.0 μm, particularly preferably from 4.0 to 5.5 μm.

  An undercoat layer may be provided between the nonmagnetic support and the nonmagnetic layer or magnetic layer to improve adhesion. The thickness of the undercoat layer is, for example, 0.01 to 0.5 μm, preferably 0.02 to 0.5 μm. The magnetic recording medium of the present invention may be a disk-shaped medium in which a nonmagnetic layer and a magnetic layer are provided on both sides of a support, or a tape-shaped medium or a disk-shaped medium provided only on one side. In this case, a backcoat layer may be provided on the side opposite to the nonmagnetic layer and the magnetic layer in order to obtain effects such as antistatic and curl correction. This thickness is, for example, 0.1 to 4 μm, preferably 0.3 to 2.0 μm. Known undercoat layers and backcoat layers can be used. For example, as for the backcoat layer, reference can be made to paragraph [0139] of JP-A-2009-96798. The magnetic recording medium of the present invention can also contain the binder of the present invention in these layers.

  The thickness of the nonmagnetic layer is usually 0.2 to 5.0 μm, preferably 0.3 to 3.0 μm, more preferably 0.4 to 2.0 μm.

  The thickness of the magnetic layer is preferably 30 to 150 nm, more preferably 50 to 120 nm, still more preferably 60 to 100 nm, and can be optimized depending on the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used. preferable. Further, the thickness variation rate of the magnetic layer is preferably within ± 50%, more preferably within ± 30%. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

(Manufacture of coating solution)
The steps for producing a coating solution for forming each layer such as a magnetic layer coating solution, and further a non-magnetic layer coating solution and a backcoat layer coating solution are usually at least a kneading step, a dispersion step, and before and after these steps as necessary. The mixing step. Each process may be divided into two or more stages. All raw materials such as ferromagnetic powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant and solvent used in the present invention may be added at the beginning or middle of any process. Absent. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. In the case of using a kneader, the kneading treatment may be performed using 15 to 500 parts by mass of a binder (preferably 30% by mass or more of the total binder) with respect to 100 parts by mass of the ferromagnetic powder or nonmagnetic powder. preferable. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse each layer forming coating solution, and zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are preferably used. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  In the method of manufacturing a magnetic recording medium, for example, a magnetic layer coating solution is formed on the surface of a nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, or a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. The coating machines that apply the coating liquid for forming each layer include air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, and cast coat. Spray coating, spin coating, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

  The medium after the coating step can be used after being subjected to post-treatment as necessary, and then cut into a desired size using a cutting machine, a punching machine, or the like.

  Examples of post-processing include magnetic layer orientation processing, surface smoothing processing (calendar processing), thermo processing, and the like. For the orientation treatment of the magnetic layer, reference can be made, for example, to paragraph [0146] of JP2009-96798A. As the calendering conditions, the temperature of the calender roll can be set in the range of, for example, 60 to 100 ° C., preferably in the range of 70 to 100 ° C., and particularly preferably in the range of 80 to 100 ° C. It is in the range of 500 kg / cm (98 to 490 kN / m), preferably in the range of 200 to 450 kg / cm (196 to 441 kN / m), particularly preferably in the range of 300 to 400 kg / cm (294 to 392 kN / m). It can be. Moreover, in order to improve the smoothness of the magnetic layer surface, the nonmagnetic layer surface can be calendered. The calendar treatment for the nonmagnetic layer is also preferably performed under the above conditions. Apart from this, the magnetic recording medium obtained after the calendering process can be thermo-processed to allow thermosetting to proceed. Such a thermo treatment can be performed, for example, at a temperature of 35 to 100 ° C., preferably 50 to 80 ° C., for example, for 12 to 72 hours, preferably 24 to 48 hours. For details of these post-processing, reference can be made to paragraphs [0091], [0092], and [0094] of Japanese Unexamined Patent Application Publication No. 2009-99240.

Physical Properties The surface smoothness of the magnetic recording medium of the present invention is preferably in the range of 0.1 to 4 nm, more preferably 1 to 3 nm, in terms of the center plane average roughness of the magnetic layer surface. The 10-point average roughness Rz on the surface of the magnetic layer is preferably 30 nm or less. The surface property of the magnetic layer can be controlled by controlling the surface property with the filler of the support or the roll surface shape of calendering. The curl is preferably within ± 3 mm.

  The saturation magnetic flux density of the magnetic layer is preferably 100 to 400 mT. The coercive force (Hc) of the magnetic layer is preferably 143.2 to 318.3 kA / m (1800 to 4000 Oe), more preferably 159.2 to 278.5 kA / m (2000 to 3500 Oe). The coercive force distribution is preferably narrow, and SFD and SFDr are preferably 0.6 or less, more preferably 0.3 or less.

The friction coefficient with respect to the head of the magnetic recording medium of the present invention is, for example, 0.50 or less, preferably 0.3 or less, in the temperature range of −10 to 40 ° C. and humidity of 0 to 95%. The surface resistivity is preferably a magnetic surface of 10 ⁴ to 10 ⁸ Ω / sq, and the charged position is preferably within −500 V to +500 V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 to 70 kg). / Mm2), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1500 kg / mm ² ) in each in-plane direction, and the residual spread is preferably 0.5% or less and 100 ° C. or less. The thermal contraction rate at any temperature is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The glass transition temperature of the magnetic layer (maximum point of loss elastic modulus measured by dynamic viscoelasticity measured at 110 Hz) is preferably 50 to 180 ° C, and that of the nonmagnetic layer is preferably 0 to 180 ° C. The loss elastic modulus is preferably in the range of 1 × 10 ⁷ to 8 × 10 ⁸ Pa (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. These thermal characteristics and mechanical characteristics are preferably almost equal within 10% in each in-plane direction of the medium.

The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, more preferably 20% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, a larger void ratio is often preferable for running durability.

  When the magnetic recording medium of the present invention has a nonmagnetic layer and a magnetic layer, these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer can be increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer can be made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

[Polyurethane resin]
The polyurethane resin of the present invention has a cyclic acetal structure represented by the general formula (I) and a sulfonic acid (salt) group. The details are as described above. Since the polyurethane resin of the present invention has a cyclic acetal structure represented by the general formula (I) together with a sulfonic acid (salt) group as an adsorptive functional group, a polyurethane resin having only a sulfonic acid (salt) group responsible for improving dispersibility In comparison, a coating solution having a low viscosity can be prepared, whereby a magnetic recording medium in which various powders are highly dispersed can be obtained.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the components, ratios, operations, order, and the like shown here can be changed without departing from the spirit of the present invention, and should not be limited to the following examples. Further, “parts” in the examples represents parts by mass unless otherwise specified.

1. Example of polyurethane resin synthesis

[Example 1]
Synthesis of polyurethane resin containing sulfonate group and acetal ring (1)
(1) Synthesis of sulfonate group-containing diol 100 parts of taurine (manufactured by Wako Pure Chemical Industries) and 44.8 parts of potassium hydroxide (manufactured by Wako Pure Chemical Industries) were added to 500 parts of methanol and stirred at 25 ° C. for 30 minutes. To the obtained methanol solution, 207 parts of butyl glycidyl ether (manufactured by Tokyo Chemical Industry) was added, and the mixture was further stirred at 30 ° C. for 2 hours. The reaction solution was introduced into a vacuum distillation apparatus, concentrated and dried to obtain the following sulfonate group-containing diol.

(2) Synthesis of polyurethane resin 1,2: 5,6-di-O-isopropylidene-D-mannitol 1.0 part, polyether (ADEKA polyether BPX-1000 manufactured by ADEKA Corporation) 3.23 parts, tricyclo [5,2,1,0 (2,6)] decanedimethanol (Tokyo Chemical Industry Co., Ltd.) 1.85 parts, sulfonate group-containing diol 0.28 parts synthesized in the above (1), crosslinking catalyst ( Nitto Kasei Neostan U-600) 0.01 part was added to 15.8 parts of cyclohexanone and stirred at room temperature for 30 minutes to complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 4.16 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The polystyrene obtained by gel permeation chromatography (GPC) using a DMF solvent containing 0.3% by mass of lithium bromide with a weight average molecular weight and a weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane. Calculated by conversion. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. Moreover, when the glass transition temperature (Tg) and sulfonic acid (salt) group content of the obtained polyurethane were determined by the following methods, respectively, they were Tg = 83 ° C. and —SO ₃ K = 60 μeq / g. Since it was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%), the cyclic acetal structure content and the urethane group concentration were determined from the charged amount as described later. The cyclic acetal structure = 360 μeq / g and the urethane group concentration = 3.1 mmol / g.

(A) Glass transition temperature (Tg)
The obtained polyurethane was prepared by diluting with a solution having a ratio of methyl ethyl ketone: cyclohexanone of 50:50 (mass ratio) to a concentration of 22% by mass. Then, it was apply | coated on the aramid base so that the thickness after drying might be set to 20 micrometers, and it was made to dry and the clear film | membrane was obtained. The obtained clear film was cut to a width of 3.35 mm and a length of 5 cm, and the dynamic viscoelasticity measuring device (leovibron made by TOYO BALDWIN, heating rate 2 ° C./min, measuring frequency 110 Hz) up to 30 to 140 ° C. The peak temperature of the loss modulus (E ″) was taken as the glass transition temperature (Tg) of the polyurethane.

(B) Measurement of content of sulfonic acid (salt) group in polyurethane by fluorescent X-ray The polyurethane solution obtained in Synthesis Example 1 was diluted to 1.5% by mass with methyl ethyl ketone (MEK). 200 μl of the diluted solution was dropped onto a filter paper with a circle (manufactured by Shimadzu Corporation) and dried under vacuum conditions for 10 hours. After drying, the sulfur content contained in the filter paper with a circle was quantified with a fluorescent X-ray analyzer. The sulfonic acid (salt) group was quantified by measuring the sulfur content using a fluorescent X-ray measuring apparatus LAB CENTER XRF-1700 manufactured by Shimadzu Corporation and using a calibration curve based on copper sulfate pentahydrate.

(C) Content of cyclic acetal structure The obtained polyurethane was dissolved in a heavy DMSO solvent, and ¹ H NMR was measured. For the measurement, 400 MHz NMR (AVANCE II-400 manufactured by BRUKER) was used. From this measurement, the following 4.7 ppm acetal ring peak was detected, and it was confirmed that the acetal ring was retained in the polyurethane. Therefore, the content of the cyclic acetal structure in the polyurethane was calculated from the charged amount of the acetal ring-containing compound.

(D) Urethane group concentration The urethane group concentration was calculated from the charged amount of diphenylmethane diisocyanate.

[Example 2]
Synthesis of polyurethane resin containing sulfonate group and acetal ring (2)
(1) Synthesis of sulfonate group-containing polyester 159.7 parts of dimethyl sodium 5-sulfoisophthalate (Tokyo Kasei), 275.2 parts of ester glycol (Mitsubishi Chemical), zinc acetate dihydrate (manufactured by Wako Pure Chemical Industries, Ltd.) ) 2.4 parts were heated at 245 ° C. The resulting distillate was stirred for 6 hours while distilling off using a Dean-Stark tube. The resulting solid was removed. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the resulting sulfonate group-containing polyester were determined in terms of standard polystyrene by GPC using a THF solvent. The weight average molecular weight was 1000 and Mw / Mn = 1.85.

(2) Synthesis of polyurethane resin 1,2: 5,6-di-O-isopropylidene-D-mannitol 1.0 part, polyether (ADEKA polyether BPX-1000 manufactured by ADEKA Corporation) 3.23 parts, tricyclo [5,2,1,0 (2,6)] decane dimethanol (manufactured by Tokyo Chemical Industry Co., Ltd.) 1.85 parts, sulfonate group-containing polyester 0.28 parts synthesized in the above (1), crosslinking catalyst ( Nitto Kasei Neostan U-600) 0.01 part was added to 15.6 parts by mass of cyclohexanone and stirred at room temperature for 30 minutes for complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 4.06 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
When various physical properties were measured and calculated for the obtained polyurethane in the same manner as in Example 1, glass transition temperature Tg = 85 ° C., —SO ₃ Na = 60 μeq / g, cyclic acetal structure = 360 μeq / g, urethane group concentration = 3.0 mmol / g.

[Example 3]
Synthesis of sulfonate group and acetal ring-containing polyurethane resin (3)
2,3-O-isopropylidene-D-threitol 1.0 part, polyether (Adeka Polyether BPX-1000 manufactured by Adeka Corporation) 3.23 parts, tricyclo [5,2,1,0 (2,6 )] 1.85 parts of decanedimethanol (Tokyo Chemical Industry Co., Ltd.), 0.28 part of a sulfonate group-containing diol similar to Example 1, and 0.01 part of a crosslinking catalyst (Neostan U-600 made by Nitto Kasei) The mixture was added to 6.5 parts of cyclohexanone, and stirred at room temperature for 30 minutes for complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 4.63 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
When various physical properties were measured and calculated for the obtained polyurethane in the same manner as in Example 1, glass transition temperature Tg = 90 ° C., —SO ₃ K = 60 μeq / g, cyclic acetal structure = 560 μeq / g, urethane group concentration = 3.3 mmol / g.

[Example 4]
Synthesis of sulfonate group and acetal ring-containing polyurethane resin (4)
1,2-O-isopropylidene-α-D-xylofuranose (Tokyo Kasei) 1.0 part, polyether (Adeca Polyether BPX-1000 manufactured by Adeka Corporation) 3.23 parts, tricyclo [5,2,1 , 0 (2,6)] decanedimethanol (Tokyo Chemical Industry Co., Ltd.) 1.85 parts, sulfonate group-containing diol 0.28 parts similar to Example 1, cross-linking catalyst (Nitto Kasei Neostan U-600 ) 0.01 part was added to 16.3 parts of cyclohexanone and stirred at room temperature for 30 minutes to complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 4.50 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. Various physical properties were measured and calculated in the same manner as in Example 1. Glass transition temperature Tg = 89 ° C., —SO ₃ K = 60 μeq / g, cyclic acetal structure = 560 μeq / g, urethane group concentration = 3.2 mmol / g.

[Comparative Example 1]
Synthesis of polyurethane resin not containing sulfonic acid (salt) group and acetal ring Polyether (Adeka Polyether BPX-1000 manufactured by Adeka Co., Ltd.) 3.23 parts, tricyclo [5,2,1,0 (2,6)] de 1.85 parts of candimethanol (manufactured by Tokyo Chemical Industry Co., Ltd.) and 0.01 part of a crosslinking catalyst (Neostan U-600 made by Nitto Kasei) were added to 11.2 parts of cyclohexanone and stirred at room temperature for 30 minutes for complete dissolution. . The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 3.06 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined in terms of standard polystyrene using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
When various physical properties of the obtained polyurethane were measured and calculated in the same manner as in Example 1, the glass transition temperature Tg = 81 ° C. and the urethane group concentration = 3.1 mmol / g.

[Comparative Example 2]
Synthesis of polyurethane resin containing sulfonate group and no acetal ring Polyether (Adeka Polyether BPX-1000 manufactured by Adeka Co., Ltd.) 3.23 parts, Tricyclo [5,2,1,0 (2,6)] decandimethanol 1.85 parts (manufactured by Tokyo Chemical Industry Co., Ltd.), 0.46 part of a sulfonate group-containing diol similar to Example 1, 0.01 part of a crosslinking catalyst (Neostan U-600 made by Nitto Kasei), 12.2 parts of cyclohexanone And stirred at room temperature for 30 minutes to complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 3.32 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
When the various properties of the obtained polyurethane were measured and calculated in the same manner as in Example 1, glass transition temperature Tg = 83 ° C., —SO ₃ K = 60 μeq / g, cyclic acetal structure = 0 μeq / g, urethane group concentration = 3.1 mmol / g.

[Comparative Example 3]
Synthesis of sulfonate group and main chain type acetal ring-containing polyurethane resin 1.0 part of the following spiroglycol (manufactured by Nippon Finechem), 3.23 parts of polyether (ADEKA polyether BPX-1000 manufactured by ADEKA CORPORATION), tricyclo [5, 2,1,0 (2,6)] decanedimethanol (Tokyo Chemical Industry Co., Ltd.) 1.85 parts by mass, 0.28 parts by mass of the sulfonate group-containing polyester obtained in Example 2 (1), dilauric acid 0.01 part of dibutyltin was added to 14.5 parts of cyclohexanone and stirred at room temperature for 30 minutes for complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 4.14 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
When the various properties of the obtained polyurethane were measured and calculated in the same manner as in Example 1, the glass transition temperature Tg = 73 ° C., —SO ₃ Na = 60 μeq / g, main chain type acetal ring = 400 μeq / g, urethane The group concentration was 2.4 mmol / g.

[Comparative Example 4]
Synthesis of sulfonic acid (salt) group-free, acetal ring-containing polyurethane resin 1,2: 5,6-di-O-isopropylidene-D-mannitol 1.0 part, polyether (Adeka Polyether BPX manufactured by Adeka Corporation) -1000) 3.23 parts, 1.85 parts tricyclo [5,2,1,0 (2,6)] decanedimethanol (manufactured by Tokyo Chemical Industry Co., Ltd.), 0.01 part dibutyltin dilaurate and 15 parts cyclohexanone .1 part was added and stirred at room temperature for 30 minutes to complete dissolution. The moisture in the flask was measured with a Karl Fischer moisture meter, and 1-fold mol of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the water contained. After setting the internal temperature to 70 ° C., 3.98 parts of diphenylmethane diisocyanate (Millionate MT manufactured by Nippon Polyurethane Industry Co., Ltd.) was added. The mixture was stirred at an internal temperature of 70 ° C. to 90 ° C. for 4 hours and then cooled to room temperature. The weight average molecular weight and the weight average molecular weight / number average molecular weight ratio (Mw / Mn) of the obtained polyurethane were determined by standard polystyrene conversion by GPC using a DMF solvent containing 0.3% by mass of lithium bromide. The weight average molecular weight was 70,000 and Mw / Mn = 1.90. It was confirmed by the GPC analysis that the raw material monomer was introduced in the charged amount (residual monomer: less than 0.5%).
For the obtained polyurethane, various physical properties were measured and calculated in the same manner as in Example 1. Glass transition temperature Tg = 82 ° C., sulfonic acid (salt) group = 0 μeq / g, cyclic acetal structure = 380 μeq / g, urethane The group concentration was 3.1 mmol / g.

Evaluation method 1. Evaluation of adsorptivity to magnetic powder One part of the polyurethane resin synthesized in Examples and Comparative Examples was suspended in a solution comprising 11.9 parts of cyclohexanone and 17.7 parts of 2-butanone together with 7.3 parts of barium ferrite powder described below. . 90 parts of zirconia beads (Nikkato) were added to the suspension and dispersed for 6 hours. The abundance ratio of the barium ferrite powder in the obtained dispersion / the binder in the solution was measured by the following method.
Ferromagnetic hexagonal barium ferrite powder composition excluding oxygen (molar ratio): Ba / Fe / Co / Zn = 1/9 / 0.2 / 1
Hc: 176 kA / m (2200 Oe), average plate diameter: 25 nm, average plate ratio: 3
BET specific surface area: 65 m ² / g
σs: 49 A · m ² / kg (49 emu / g)
pH: 7
Binder abundance ratio Barium ferrite powder and solution were centrifuged under conditions of 100,000 rpm and 80 minutes in a Hitachi separation microcentrifuge CS150GXL. 3 ml of the supernatant was weighed and the mass was measured. After drying at 40 ° C. for 18 hours, it was dried at 140 ° C. for 3 hours under vacuum. The mass of the dried product was defined as the non-adsorbing solid content of the binder, and the abundance ratio of the binder in the barium ferrite powder surface / solution was calculated.

2. Evaluation of dispersibility The suspension prepared in the same manner as described above was applied onto a polyethylene naphthalate resin support having a thickness of 6 μm and a center surface average roughness of 0.003 μm using a 25 μm blade, and dried at room temperature for 2 hours. Thus, a magnetic tape was obtained. The magnetic layer surface roughness of the obtained magnetic tape was measured by the following method.
Surface roughness of the magnetic layer The center plane average roughness Ra of the magnetic layer is set to 5 μm in scan length by scanning white light interferometry using a general-purpose three-dimensional surface structure analyzer NewView 5022 manufactured by ZYGO, objective lens: 20 times, zoom The lens was 1.0 times, the measurement field of view was 260 μm × 350 μm, and the measured surface was obtained by filtering with HPF: 1.65 μm and LPF: 50 μm.

  The above evaluation results are shown in Table 1 below.

Evaluation results From the results shown in Table 1, in Examples 1 to 4 using a polyurethane resin having an ionic polar group (sulfonic acid (salt) group) together with the cyclic acetal structure represented by the general formula (I), Compared with the powder used in the examples, it can be confirmed that the binder adsorbability to the magnetic powder is higher. As a result of using these polyurethanes, it was confirmed that a magnetic layer having a low Ra and a high smoothness could be formed as a result of using these polyurethanes.
On the other hand, in Comparative Examples 1-4, the binder adsorption amount to powder was low compared with the said Example, and it was inferior to dispersibility. As a result, in Comparative Example 3, the surface smoothness of the formed magnetic layer was greatly inferior to that of the example. Furthermore, in Comparative Examples 1, 2, and 4, the dispersibility of the coating material was extremely low, so it was not possible to obtain a sample capable of evaluating Ra.
From the above results, it was shown that the polyurethane used in Examples 1 to 4 showed an excellent effect of improving dispersibility, and thereby a magnetic recording medium having high smoothness was obtained.

  According to the present invention, it is possible to provide a magnetic recording medium suitable for high-density recording in which various powders such as ferromagnetic powder and nonmagnetic powder are highly dispersed.

Claims (13)

A binder for a magnetic recording medium, which is a polyurethane resin having a cyclic acetal structure represented by the following general formula (I) and an ionic polar group.

[In General Formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. R ^{3 to} R ⁶ each have at least one bonding site with the main chain of the polyurethane resin, or have a bonding site in a ring structure formed by linking two or more, and the others are each independently hydrogen. An atom, an alkyl group, or an alkoxyl group is represented. ] The binder for magnetic recording media according to claim 1, wherein the ionic polar group is a sulfonic acid (salt) group. The binder for magnetic recording media according to claim 2, comprising 10 to 1000 μeq / g of sulfonic acid (salt) group. The binder for magnetic recording media according to any one of claims 1 to 3, which contains 10 to 2000 µeq / g of a cyclic acetal structure represented by the general formula (I). The binder for magnetic recording media according to any one of claims 1 to 4, wherein the urethane group concentration is in the range of 1.0 to 3.5 mmol / g. The binder for magnetic recording media according to any one of claims 1 to 5, wherein the glass transition temperature is in the range of 50 to 150 ° C. A magnetic recording medium having a magnetic layer comprising a ferromagnetic powder and a binder on a nonmagnetic support,
The magnetic recording medium, wherein the binder contains the binder according to any one of claims 1 to 6 as a constituent component. A magnetic recording medium having a nonmagnetic layer containing a nonmagnetic powder and a binder on a nonmagnetic support and a magnetic layer containing a ferromagnetic powder and a binder in this order,
7. The magnetic recording medium according to claim 1, wherein the binder contained in the magnetic layer and / or the binder contained in the nonmagnetic layer contains the binder according to claim 1 as a constituent component. . A polyurethane resin having a cyclic acetal structure represented by the following general formula (I) and a sulfonic acid (salt) group.

[In General Formula (I), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, or are linked to each other to form a ring structure. R ^{3 to} R ⁶ each have at least one bonding site with the main chain of the polyurethane resin, or have a bonding site in a ring structure formed by linking two or more, and the others are each independently hydrogen. An atom, an alkyl group, or an alkoxyl group is represented. ] The polyurethane resin according to claim 9 containing 10 to 1000 µeq / g of sulfonic acid (salt) groups. The polyurethane resin according to claim 9 or 10, which contains 10 to 2000 µeq / g of a cyclic acetal structure represented by the general formula (I). The polyurethane resin according to any one of claims 9 to 11, wherein the urethane group concentration is in the range of 1.0 to 3.5 mmol / g. The polyurethane resin according to any one of claims 9 to 12, which has a glass transition temperature in the range of 50 to 150 ° C.