JPH0570207B2 - - Google Patents

Description

[Detailed description of the invention]

1. Field of Industrial Application The present invention relates to magnetic recording media such as magnetic tapes, magnetic sheets, and magnetic disks. 2. Prior Art In general, magnetic recording media consist of a support made of polyethylene terephthalate or the like in the form of a tape or sheet, and a magnetic layer formed by coating this support with a magnetic paint containing magnetic powder and a binder as main components. It is formed by. Regarding the magnetic powder contained in the magnetic layer, metal magnetic powder is superior to iron oxide magnetic powder in terms of squareness ratio, saturation magnetization, coercive force, etc. However, when metal magnetic powder has a particle size of 1 μm or less, it not only has poor dispersibility but also becomes susceptible to oxidation and rust, which deteriorates the saturation magnetization over time and reduces storage stability and still durability. Sexuality decreases. Additionally, metal magnetic powders have a tendency to ignite in the atmosphere, even at temperatures around room temperature, under certain conditions. As described above, the reason why the metal magnetic powder becomes unstable against oxidation is that the properties of the metal magnetic powder itself, that is, the chemical properties change in the temperature range from room temperature to 70°C. In addition to this, pinholes existing on the surface of the metal magnetic powder are also considered to be a factor. On the other hand, if a thermally stable metal-based magnetic powder were available, it would be possible to solve the above-mentioned problems, but it was found that the following drawbacks could not be avoided. In other words, metal-based magnetic powder has a higher saturation magnetization (σ _S ) than oxide-based magnetic powder such as iron oxide, so the interaction between magnetic particles is large, making it extremely difficult to disperse, especially in a binder. Dispersion stability is also not very good. This is related to the fact that the metal-based magnetic powder has a large coercive force and particles tend to aggregate with each other. For this reason, it is conceivable to use various additives for the purpose of improving the dispersion of the metal-based magnetic powder or improving the wear resistance. Examples of such additives include higher fatty acids, fatty acid amides, fatty acid esters, higher alcohols, metal soaps, and polyethylene oxide. However, even with the addition of these additives, it has been difficult to obtain a magnetic recording layer with desirable characteristics. For example, if a large amount of these additives is used, the mechanical strength of the magnetic recording layer may decrease, and a blooming phenomenon in which the additives gradually ooze out after the magnetic recording layer is formed may be observed. It was hot too. For this reason, the characteristics and dispersion of the magnetic recording layer were never satisfactory. Generally, in order to improve recording density and short wavelength recording, it is necessary to improve the surface smoothness of magnetic powder and make it finer to improve its dispersibility. However, if the particle size of the magnetic powder is made smaller (in other words, the specific surface area is increased) to meet these requirements, the dispersibility of the magnetic powder becomes even worse, making it impossible to obtain the desired electromagnetic properties. . 3. Purpose of the Invention The purpose of the present invention is to improve oxidation stability, still durability,
The objective is to improve the storage stability and further the dispersibility of the magnetic material. 4 Structure of the invention and its effects That is, the present invention provides a copolymer that contains at least one monomer unit having a negative group as a copolymer component, and in which the negative group forms a salt. It is characterized by being treated with metal-based magnetic powder whose thermal analysis curve does not substantially change up to at least 80°C, and containing the metal-based magnetic powder treated with the copolymer in the magnetic layer. This relates to a magnetic recording medium. Here, the above-mentioned "differential thermal analysis curve" refers to the measurement of the temperature difference that occurs between a reference material and a sample (metallic magnetic material) while simultaneously heating them at a constant rate, and the horizontal axis is the temperature (or It is a curve that shows the change in temperature difference or calorific value difference, with temperature difference or calorific value difference plotted on the vertical axis. This differential thermal analysis curve is generally DTA (Differential Thermal Analysis)
It may be called a curve or a thermogram, but this is a DSC (Differential Scanning
This corresponds to the curve measured with a Calorimeter). According to the present invention, as a metal-based magnetic material, the differential thermal analysis curve is
℃ or below or higher than 80℃), the magnetic material is less likely to be oxidized and the electromagnetic conversion characteristics are less likely to deteriorate even if the magnetic recording medium is used repeatedly. In addition, when recording or reproducing from a magnetic recording medium, the magnetic material does not change at the heat generated by friction between the magnetic head and the magnetic recording medium (equilibrium temperature 60°C to 75°C), and the still It has excellent durability. Since the magnetic material is thermally stable, the catalytic effect on the binder in the magnetic layer is also suppressed.
The features of the binder used in the present invention do not change over time, and are sufficiently maintained and exhibited.
In order to obtain the remarkable effects described above, it is essential that the differential thermal analysis curve of the metal-based magnetic material does not change up to at least 80°C. Further, according to the present invention, it is also important that the metal magnetic material is (pre)treated with the copolymer. In other words, the above copolymer is
Because it forms a salt of the negative group, the hydrocarbon residue part of the copolymer is well compatible with the binder.
In addition, the above salt portion has a dissociation constant that is suitably larger than that of a mere (free, non-salt-forming) organic group. It is fully bonded to the hydrophilic surface of the magnetic material. In addition, since it is more hydrophilic than free organic groups, when the surface-treated magnetic material is dispersed in a binder or solvent, there is less tendency for it to be desorbed and transferred to the binder or solvent phase. As a result, the above-mentioned copolymer, on the one hand, binds well to the surface of the magnetic material, and on the other hand, it adapts well to the binder of the magnetic layer without being desorbed, so that the magnetic material treated with this copolymer The dispersibility of the body is greatly improved, and various properties such as squareness ratio, output, and surface seepage are significantly improved. First, the thermal characteristics of the metal magnetic material used in the present invention will be explained. As a means of investigating the oxidation stability of metal-based magnetic powder, the present inventor investigated the electron microscope observation of the surface state of metal-based magnetic powder and the stability of the differential thermal analysis curve. We found that there is an extremely close relationship as shown below. Conventionally known metal magnetic powder (as shown in Figure 1,
The surface condition of the metal magnetic powder whose differential thermal analysis curve changes from 20℃ to 70℃ (magnified 30,000 times using an electron microscope) is that acicular powder particles overlap as shown in Figure 2. It has been confirmed that the unit acicular particles have relatively smooth surfaces. On the other hand, in the case of the metal magnetic powder of the present invention whose differential thermal analysis curve does not change up to at least 80°C as shown in Fig. 1, as shown in Fig. 3 (electron micrograph), the unit acicular particle The surface condition is quite rough, and it is thought that there are fewer pinholes, which are considered to be closely related to the oxidation stability of metal magnetic powder. It is thought that this reduction in pinholes improves the thermal stability (and ultimately the oxidation stability) of the differential thermal analysis curve of the metal magnetic powder. Magnetic powders that can be used in the magnetic layer of the magnetic recording medium of the present invention include Fe-Ni-Co alloy, Fe-Mn-Zn alloy, Fe-Co-Ni-P alloy, Fe-Ni-Zn alloy,
Fe-Ni-Cr-P alloy, Fe-Co-Ni-Cr alloy,
Fe-Co-P alloy, Fe-Ni alloy, Fe-Ni-Mn alloy, Co-Ni alloy, Co-Ni-P alloy, Fe-Al alloy, Fe-Mn-Zn alloy, Fe-Al-P alloy, etc. Examples include metal-based magnetic powders containing Fe, Ni, and Co as main components. Methods for obtaining stable metal-based magnetic powder whose differential thermal analysis curve does not change up to at least 80°C include the following methods (1) to (3). (1) As shown in FIG. 4, a method in which the outer surface of metal-based magnetic powder 1 is coated with a thermally stable polymer compound (for example, polyamide resin) 2. (2) As shown in Fig. 5, the outer surface of the metal-based magnetic powder 1 is slowly oxidized to form a stable oxide layer 3 shown as dots.
(The degree of oxidation increases toward the surface of the magnetic powder: the degree of oxidation increases continuously). (3) Various additives,
For example, a method of adding various elements such as nickel, aluminum, silicon, magnesium, copper, phosphorus, etc. and/or compounds thereof by a method such as inclusion or deposition. The metal-based magnetic powder contained in the magnetic layer of the magnetic recording medium of the present invention may be obtained by any of the above methods, but it is preferable that the magnetic powder obtained mainly by the method (3) is used as a basis. Depending on the situation, it is preferable to adopt either or both of methods (1) and (2) as an auxiliary method. That is, in the case of magnetic powder obtained by method (1), the proportion of metal-based magnetic powder (magnetic material) per unit volume of magnetic powder is relatively small, and the amount of magnetization per unit volume of the magnetic layer is ,
It is smaller than when uncoated metal magnetic powder is used. For example, in a coated magnetic recording medium in which the magnetic layer is mainly formed of magnetic powder and a binder, the metal-based magnetic powder that should be contained in the magnetic layer improves thermal stability (or oxidation stability) as described above (1). ), the substantial filling rate of the magnetic material in the magnetic layer tends to be low. Furthermore, the magnetic powder obtained by method (1) requires special measures for the binder system (use of a special binder, special dispersant, etc.), which may complicate production. Similarly to the magnetic powder obtained by method (1), the metal-based magnetic powder obtained by method (2) above also contains portions that are not slowly oxidized (in other words, non-oxidized portions that contribute to magnetization). ) is smaller than the volume of the magnetic powder, so the filling rate of the magnetic material within the magnetic layer tends to decrease. Therefore, the magnetic powder used in the magnetic layer of the magnetic recording medium of the present invention is an acidic powder other than one coated with a slowly oxidized film, and the metal magnetic material itself is thermally stable (or oxidized). (1) or (1) as necessary.
(2), or a combination of these methods is preferred. In addition, methods (1) and (2) above, as well as other methods, such as using magnetic powder with a substance (whether a low-molecular compound or a high-molecular compound) that has the property of filling the pinholes of the magnetic powder, can be used. ), or by forming a thin film on the surface of the magnetic powder by immersion or the method described in (1) above (thermal stability may not be sufficient),
A thermally or oxidatively stable magnetic powder can also be obtained by chemically or physically reacting this. Further, antirust treatment using silicone oil or the like can also be combined. In short, the temperature must be at least 80°C to the extent that the metal-based magnetic powder does not significantly impair its magnetic properties.
The metal-based magnetic powder of the present invention is a magnetic powder that has been modified to be thermally or oxidatively stable up to 80° C. and has no change in differential thermal analysis curve up to at least 80°C. Among the above-mentioned methods (1) to (3) and other methods, method (3), which is mainly used, will be explained in detail. Various elements and/or compounds are added to the metal components of the metal-based magnetic powder (here, metal components mainly mean components other than carbon, hydrogen, oxygen, etc. that are not detected by an X-ray microanalyzer). In the method of addition (3), preferred additives include aluminum, silicon, nickel, and compounds thereof. Differential thermal analysis curves are created by measuring, for example, the temperature difference or change in calorific value that occurs when metal-based magnetic powder is heated at a constant rate with a reference material such as thermally stable alumina or quartz. The method of measurement is as follows:
For example, it is described in detail in "New Experimental Chemistry Course 2, Basic Technology" edited by the Chemical Society of Japan. When the additive is Al and/or its compound, the amount added is the Al atom (for example, if the additive is Al ₂ O ₃₎ in the metal components of the metal-based magnetic powder.
The proportion of Al ₂ ) is in the range of 0.5 to 20 atomic weight %, and particularly preferably in the range of 1 to 20 atomic weight %, based on the atomic weight of all metal components.
If the Al atom content is less than 0.5 atomic weight percent, the thermal or oxidative stability of the obtained metal-based magnetic powder as described above will be insufficient, and the characteristics of the magnetic recording medium prepared using this magnetic powder, such as storage stability. performance, playback output, and still durability are likely to be insufficient. In addition, when using Si element and/or its compound as an additive, the above Al element and/or
Or, in the same way as when the compound is used as an additive, the metal component part of the metal magnetic powder (Al, Si,
S, Fe, Ni, Co, Ra, Hf, Cu, P, Zn, Mn,
(occupied by atoms such as Cr, Bi, Mg, etc.)
The weight percentage occupied by atoms is 1% by weight or less, preferably 0.5% by weight or less, and in some cases may be 0.1% or less. When Si is outside this range, it may be difficult to modify the magnetic powder. Furthermore, when containing Ni element and/or its compound as an additive, the upper limit is 30 atomic weight % or less, preferably 20 atomic weight % from the viewpoint of magnetic properties.
The following is desirable. FIG. 6 is a characteristic curve a showing the relationship between the still durability (minutes) of the magnetic recording medium (magnetic tape) of the present invention and the stable temperature of the differential thermal analysis curve of the metal magnetic powder used in the tape. From this figure, it can be confirmed that when the lowest temperature at which a change appears in the differential thermal analysis curve is 80°C or higher, the still durability of the tape increases dramatically. On the other hand, when magnetic powder is used without being treated with a dispersant as in the present invention, the curve becomes as shown in curve b.
It can be seen that the still characteristics are significantly reduced. Next, the above-mentioned copolymer used for pretreating the surface of magnetic powder in the present invention will be explained in detail. In the anionic group-containing monomer (hereinafter referred to as monomer unit A) constituting this copolymer, the anionic group includes, for example, a carboxyl group,
Examples include phosphoric acid residues and sulfonic acid residues, and among these, carboxyl groups and phosphoric acid residues are preferred, and their salts include ammonium salts, alkali metal salts, and the like, with ammonium salts being preferred. Examples of the monomer unit A include acrylic acid, methacrylic acid, maleic anhydride, and 2-hydroxyethyl acryloyl phosphate, with acrylic acid and maleic anhydride being preferred. The negative group is preferably a carboxyl group or a phosphoric acid residue, and the salt is preferably an ammonium salt. Acrylic acid and maleic anhydride are preferred as monomer unit A because they have particularly excellent storage stability and dispersibility. In the case of additives conventionally used to improve the blooming phenomenon, the blooming phenomenon can be improved to some extent, but the storage stability is poor, pecking is likely to occur, and agglomeration is likely to occur during dispersion. However, the copolymer used in the present invention is excellent in these points. In this copolymer, to explain the effect of the salt of the negative group, the dissociation constant is different for a simple negative group (e.g., free -COOH) and its salt (e.g., ammonium salt, Na salt). There is. [Dissociation constant K] −COOH<COO ⁻ N ⁺ H ₄ <−COONa Then, using magnetic powders surface-treated with each copolymer containing monomer unit A having each of these groups, as described below, It was confirmed that the squareness ratio (Bm/Br) of a magnetic recording medium having a magnetic layer prepared by the method described in detail is as shown in FIG. 7, for example. That is, compared to the case where a copolymer having simple -COOH is used, -
When using a copolymer in which COOH is a salt,
It can be seen that the squareness ratio is improved. This is −COOH
When the copolymer is made into a salt, it has a large dissociation constant and therefore has greater hydrophilicity, and during surface treatment of the magnetic powder, the copolymer is easily bonded to the hydrophilic surface of the magnetic powder, and the adsorbed material is This is thought to be because there is little tendency for desorption during dispersion (ie, good hydrophilicity). On the other hand, -COOH has the property of easily adhering to or bonding to the surface of magnetic powder. That is, although it is easy to adhere or bond, it has higher lipophilicity than salt and is easily desorbed during dispersion in an organic solvent. Furthermore, among the above negative groups according to the present invention,
From Figure 7, the magnetic properties of ammonium salts are better than those of alkali metal salts, and the use of ammonium salts is the highest, and before and after that, the squareness ratio tends to decrease as the dissociation constant decreases or increases. It turns out that there is. The reason for this is that alkali metal salts have greater hydrophilicity (possibly due to a considerably larger dissociation constant), which makes them easier to bond to the magnetic powder surface during surface treatment in aqueous systems, as well as being more likely to detach. This is thought to be because ammonium salts preferentially bond to the magnetic powder surface due to their moderate dissociation constants. Also,
Less desorption in organic solvents. Note that when the magnetic powder is not subjected to any surface treatment, the characteristics of the medium become even worse (see FIG. 7). As the ammonium salt, the above −COO ⁻ N ⁺ H ₄
General formula containing: -COO ^- N ⁺ (R ¹ ) (R ² ) (R ³ ) (R ⁴ ) (However, R ¹ , R ² , R ³ , and R ⁴ are each a hydrogen atom, or are the same or (different lower alkyl groups) are applicable. Here, when the above R ¹ , R ² , R ³ , and R ⁴ are lower alkyl groups, the total number of carbon atoms of R ¹ to ^{R 4} is 6 or less because of steric hindrance. This is desirable because it does not impair the sex. The above-mentioned copolymer used in the present invention is expressed as [-A] using the above-mentioned monomer unit A (also written as [-A]-) and monomer unit A (also written as [-B]-). _−n [−B]−n. However, m and n are each positive real numbers. The average value of (m+n) is 100 or less, preferably 50 or less. If it exceeds 100, it becomes difficult to disperse uniformly in the magnetic layer of the magnetic recording medium, and the performance (for example, output, etc.) of the recording medium tends to become partially non-uniform, which is not preferable. Further, (m+n) is particularly preferably 30 or less, and the dispersion effect in this case is particularly excellent, and the performance of the magnetic recording medium according to the present invention is particularly significantly improved. Note that the average value of (m+n) is, for example, from the viewpoint of preventing the blooming phenomenon.
It is preferable that it is 4 or more. Here, by selecting the values of m and n and the type of salt of the organic group in unit A, the copolymer can have both hydrophilic and hydrophobic properties.
Appropriate control of HLB (Hydrophile Lipophile Balance) is possible. In addition, monomer units other than monomer unit A (hereinafter referred to as monomer unit B) of this copolymer are
It is called. ) Examples include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-n-butylstyrene, p-tert-
Butylstyrene, p-n-hexylstyrene, p
-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene, p-n-dodecylstyrene, p-methoxystyrene, p-phenylstyrene, p-chlorostyrene, 3,4-dichlorostyrene Examples include styrene and styrene derivatives such as styrene and styrene derivatives. Other vinyl monomers other than these include, for example, ethylenically unsaturated monoolefins such as ethylene, propylene, butylene, and isobutylene; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl bromide, and vinyl fluoride; ;
Vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate, vinyl butyrate; methyl acrylate, ethyl acrylate, acrylic acid n
-butyl, isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate,
Phenyl acrylate, methyl α-chloroacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n methacrylate
-butyl, n-isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate,
α-methylene aliphatic monocarboxylic acid esters such as dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate; derivatives of acrylic acid or methacrylic acid such as acrylonitrile, methacrylonitrile, and acrylamide;
vinyl methyl ether, vinyl ethyl ether,
Vinyl ethers such as vinyl isobutyl ether; Vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, and methyl isopropenyl ketone; N-vinyl compounds such as N-vinylpyrrole, N-vinylcarbazole, N-vinylindole, and N-vinylpyrrolidone. ; Examples include vinylnaphthalenes. In order to surface-treat the magnetic powder with the above-mentioned copolymer, an aqueous solution of the copolymer may be added to the magnetic powder, kneaded with a kneader or the like for a predetermined period of time, and further water may be removed by filtration and/or drying. The treated magnetic powder after filtration and/or drying can be pulverized to a desired particle size and classified. Alternatively, the above copolymer is dissolved in a solvent such as ylene, methyl ethyl ketone, ethyl cellosolve, acetone, methanol, etc.
Magnetic powder is immersed in the solution at a predetermined ratio, stirred and mixed, and then filtered or the solvent is evaporated, and if necessary, it can be further dried. It has also been found that when the specific surface area of the magnetic powder to be treated is set to 30 m ² /gγ or more, the surface treatment effect by the above-mentioned copolymer is well exhibited. That is, if the specific surface area is 30 m ² /gγ or more, the particle size is small and the magnetic properties are improved to some extent in terms of high-density recording, but the dispersibility of the magnetic powder tends to be poor. As shown in Figure 8, when the magnetic powder is not subjected to any surface treatment, its properties improve to some extent as the specific surface area increases; Although the properties of the magnetic powder treated with (treated magnetic powder) are improved, the improvement is only approximately linear with the specific surface area of the magnetic powder. On the other hand, when magnetic powder surface-treated with a copolymer (treated magnetic powder) according to the present invention is used, the specific surface area is 30
It has been confirmed that at m ² /gγ or higher, the properties are rapidly improved compared to treated magnetic powder. This clearly shows that not only the deterioration of the dispersibility of magnetic powder can be effectively prevented by the surface treatment with the copolymer according to the present invention, but also the dispersibility can be significantly improved. Note that if the specific surface area of the magnetic powder used is increased more than necessary, poor dispersion will occur, so it is desirable that the upper limit is 100 m ² /gγ. In addition, "specific surface area" in the above refers to the surface area per unit weight, and is a physical quantity that is completely different from the average particle diameter. Some have small surface areas. To measure the specific surface area, for example, first, the magnetic powder is degassed while being heated at around 250°C for 30 to 60 minutes to remove what is adsorbed to the powder, and then introduced into a measuring device. Set the initial pressure of nitrogen to 0.5Kgγ/m ² and perform adsorption measurement using nitrogen at liquid nitrogen temperature (-195℃) (Specific surface area measurement method generally referred to as BET method. For details, see J.Ame. Chem.Soc, 60 309
(1938)). As the device for measuring the specific surface area (BET value), it is possible to use the "powder measuring device (Cantersoap)" jointly manufactured by Yuasa Battery Co., Ltd. and Yuasa Ionics Co., Ltd. A general explanation of the specific surface area and its measurement method can be found in ``Powder Measurement'' (JMDALLAVALLE,
Co-authored by CLYDE ORR Jr., translated by Benta and others; published by Sangyo Toshosha), and also in "Chemistry Handbook" (Application Edition, Sections 1170-1171, edited by the Chemical Society of Japan, Maruzen Co., Ltd., April 30, 1966). published). (In addition, in the above-mentioned "Chemistry Handbook," the specific surface area is simply described as surface area (m ² /gγ), but it is the same as the specific surface area in this specification.) It is desirable to use polyurethane as the binder, particularly a rigid polyurethane having a tensile strength of 200 kg/cm ² or more and an elongation at break of less than 900%. This hard polyurethane reduces stickiness and stiffness (S-tiffness).
It is possible to prevent a decrease in viscosity, suppress stickiness, and strengthen the stiffness of the medium. In addition, in order to facilitate calendering and increase the adhesion of the magnetic layer, the tensile strength must be less than 300 kg/cm ² and the elongation at break must be less than 900 kg/cm2.
% or more of soft polyurethane can be used in combination. The mechanical properties (tensile strength, elongation at break) of the above polyurethane are based on Japanese Industrial Standards (JIS).
It is specified in K6301-1975, and the elongation at break corresponds to the "elongation at break" in the same standard. Polyurethane, particularly hard polyurethane, used as a binder for the magnetic layer in the magnetic recording medium of the present invention can be synthesized by reacting a polyol with a polyisocyanate. Polyols that can be used include organic dibasic acids such as phthalic acid, adipic acid, dimerized linoleic acid, and maleic acid;
Glycols such as ethylene glycol, propylene glycol, butylene glycol, and diethylene glycol; polyhydric alcohols such as trimethylpropane, hexanetriol, glycerin, trimethylolpropane, hexanetriol, glycerin, trimethylolethane, and pentaerythritol; and their glycols. and a polyester polyol synthesized by reaction with any two or more polyols selected from polyhydric alcohols; or s-caprolactam, α-methyl-1-caprolactam,
Examples include lactone polyester polyols synthesized from lactams such as s-methyl-s-caprolactam and γ-butyrolactam; and polyether polyols synthesized from ethylene oxide, propylene oxide, butylene oxide, and the like. These polyols are reacted with isocyanate compounds such as tolylene diisocyanate, hexamethylene diisocyanate, methylene diisocyanate, metaxylylene diisocyanate, etc.
In this way, urethanized polyester polyurethanes and polyether polyurethanes are synthesized. These polyurethanes according to the present invention are usually mainly produced by the reaction of polyisocyanates and polyols, and free isocyanate groups and/or hydroxyl groups are synthesized. It may be in the form of a urethane resin or urethane prepolymer containing these, or it may be in the form of a urethane elastomer that does not contain these reactive end groups. Since the methods for producing polyurethane, urethane prepolymers, urethane elastomers, effective crosslinking methods, etc. are well known, detailed explanation thereof will be omitted. The polyurethane described above can be either hard or soft depending on its components. For example, rigid polyurethane can be synthesized by using the same type of polyol, by increasing the amount of isocyanate added, by using isocyanate that is easily crystallized, by using isocyanate that has a benzene ring in the molecule, etc. . Also,
Soft polyurethane can be obtained by having multiple types of polyester polyol components randomly present in the molecule, or by using as isocyanate an isocyanate whose molecular chain between isocyanate groups consists of chain aliphatic hydrocarbon groups. I can do it. As a binder for the magnetic layer, in addition to the above-mentioned polyurethane, thermosetting resins, thermoplastic resins,
A mixture of a reactive resin and an electron beam curable resin may be used. The thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and after coating and drying, the molecular weight becomes infinite due to reactions such as condensation and addition. Moreover, among these resins, those which do not soften or melt before the resin is thermally decomposed are preferable. Specifically, for example, phenoxy resin, phenolic resin, epoxy resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic reaction resin, mixture of methacrylate copolymer and diisocyanate prepolymer, urea formaldehyde resin, polyamine. resins, mixtures thereof, etc. Among these, phenoxy resin is preferred. As a thermoplastic resin, the softening temperature is 150℃ or less,
Average molecular weight is 10000~200000, degree of polymerization is 200~
About 2000, for example, acrylic ester.
Acrylonitrile copolymer, acrylic ester-vinylidene chloride copolymer, acrylic ester-styrene copolymer, methacrylic ester-acrylonitrile copolymer, methacrylic ester-vinylidene chloride copolymer, methacrylic ester-styrene copolymer Coalescence, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, styrene-butadiene copolymer, polyester resin, chlorovinyl ether-acrylic acid ester copolymer, amino resin , various synthetic rubber-based thermoplastic resins, and mixtures thereof. Examples of electron beam irradiation-curable resins include unsaturated prepolymers such as maleic anhydride type, urethane acrylic type, polyester acrylic type,
Polyether acrylic type, polyurethane acrylic type, polyamide acrylic type, etc., and polyfunctional monomers include ether acrylic type, urethane acrylic type, phosphate ester acrylic type, aryl type, hydrocarbon type, etc. In addition, if a cellulose resin and/or a vinyl chloride copolymer is also included as a desirable binder used in combination with the above-mentioned polyurethane, the dispersibility of the magnetic powder in the magnetic layer will be improved and its mechanical strength will be increased. do. However, if the cellulose resin and vinyl chloride copolymer are used alone, the layer becomes too hard, but this can be prevented by containing the above-mentioned polyurethane. Usable cellulose resins include cellulose ether, cellulose inorganic acid ester, cellulose organic acid ester, and the like. Examples of cellulose ethers include methylcellulose, ethylcellulose, propylcellulose, isopropylcellulose, butylcellulose, methylethylcellulose, methylhydroxyethylcellulose, ethylhydroxyethylcellulose, carboxymethylcellulose, carboxymethylcellulose sodium salt, hydroxyethylcellulose, benzylcellulose, cyanoethylcellulose, vinylcellulose, Nitrocarboxymethylcellulose, diethylaminoethylcellulose, aminoethylcellulose, etc. can be used. As the cellulose inorganic acid ester, nitrocellulose, cellulose sulfate, cellulose phosphate, etc. can be used. Cellulose organic acid esters include acetylcellulose, propionylcellulose, butyrylcellulose, methacryloylcellulose, chloroacetylcellulose, β-oxypropionylcellulose, benzoylcellulose, p-toluenesulfonate cellulose, acetylpropionylcellulose, acetylbutyrylcellulose. etc. can be used. Among these cellulose resins, nitrocellulose is preferred. Specific examples of nitrocellulose include Cellnova BTH1/2 manufactured by Asahi Kasei Corporation;
Examples include nitrocellulose SL-1 and nitrocellulose RS1/2 manufactured by Daicel Corporation. The viscosity of nitrocellulose (specified in JIS, K-6703 (1975)) is preferably 2 to 1/64 seconds, particularly preferably 1 to 1/4 seconds. If it is outside this range, the film adhesion and film strength of the magnetic layer will be insufficient. In addition, the above-mentioned vinyl chloride copolymers that can be used have the general formula:

There is something expressed by [ ]. in this case,

The molar ratio derived from l and m in the [Formula] unit and [-X]-m unit is 95 to 50 mol% for the former unit and 5 to 50% mol for the latter unit. In addition, X represents a monomer residue copolymerizable with vinyl chloride, including vinyl acetate, vinyl alcohol,
Maleic anhydride, maleic anhydride, maleic acid, maleic ester, vinylidene chloride,
Acrylonitrile, acrylic acid, acrylic ester, methacrylic acid, methacrylic ester, vinyl propionate, glycidyl methacrylate,
Represents at least one member selected from the group consisting of glycidyl acrylate. The degree of polymerization expressed as (l+m) is preferably from 100 to 600; when the degree of polymerization is less than 100, the magnetic layer etc. tend to become sticky, and when it exceeds 600, dispersibility deteriorates. The vinyl chloride copolymer described above may be partially hydrolyzed. The vinyl chloride copolymer is preferably a copolymer containing vinyl chloride-vinyl acetate (hereinafter referred to as "vinyl chloride-vinyl acetate copolymer"). Examples of vinyl chloride-vinyl acetate copolymers include vinyl chloride-vinyl acetate alcohol, vinyl chloride-vinyl acetate-
Maleic anhydride, vinyl chloride - vinyl acetate - vinyl alcohol - maleic anhydride, vinyl chloride - vinyl acetate - vinyl alcohol - maleic anhydride -
Examples include copolymers of maleic acid, and among vinyl chloride-vinyl acetate copolymers, partially hydrolyzed copolymers are preferred. Specific examples of the above-mentioned vinyl chloride-vinyl acetate copolymers include "VAGH" and "VYHH" manufactured by Union Carbide;
"VMCH", "Esretsu A" manufactured by Sekisui Chemical Co., Ltd.,
“Eslec A-5”, “Eslec C”, “Eslec M”, “Denkabinil” manufactured by Denki Kagaku Kogyo Co., Ltd.
1000G", "Denkabinir 1000W", etc. can be used. The above-mentioned vinyl chloride copolymer and cellulose resin may be used in any blending ratio, but for example,
As shown in the figure, vinyl chloride resin in terms of weight ratio:
The ratio of the cellulose resin is preferably 90/10 to 5/95, and more preferably 80/20 to 10/90. In addition, the ratio of polyurethane to other resins (especially the total amount of cellulose resin and vinyl chloride copolymer) is 90/10 by weight, as shown in Figure 10.
~50/50 is good, 85/15~60/40 is even better. The mixing ratio of the metal magnetic powder and the binder according to the present invention is 5 to 400 parts by weight, preferably 10 to 200 parts by weight, per 100 parts by weight of the magnetic powder. If the amount of binder is too large, the recording density of the magnetic recording medium will be reduced, and if it is too small, the strength of the magnetic layer will be poor, resulting in undesirable situations such as decreased durability and powder falling off. Furthermore, in order to improve the durability of the magnetic recording medium according to the present invention, in addition to the above-mentioned isocyanates, triphenylmethane triisocyanate, tris-(p-isocyanate phenyl) thiophosphite, and polymethylene polyphenyl isocyanate are used as crosslinking agents. may be included. The paint used to form the magnetic layer paint may contain other dispersants, lubricants, abrasives,
Additives such as antistatic agents may also be included. Other dispersants that may be used include lecithin, phosphoric acid esters, amine compounds, alkyl sulfates, fatty acid amides, higher alcohols, polyethylene oxide, sulfosuccinic acid esters, known surfactants, and salts thereof. , salts of polymeric dispersants having negative organic groups (e.g. -COOH, _-PO3H ) can also be used. These dispersants may be used alone or in combination of two or more. These dispersants are magnetic powders
It is added in an amount of 1 to 20 parts by weight per 100 parts by weight. These dispersants may be used to pre-treat the magnetic powder in advance. In addition, as lubricants, silicone oil, graphite, carbon black graft polymer, molybdenum disulfide,
Tungsten disulfide, lauric acid, myristic acid, fatty acid esters consisting of a monobasic fatty acid with 12 to 16 carbon atoms and a monohydric alcohol with a total number of carbon atoms of 21 to 23 carbon atoms, etc. can also be used. These lubricants are added in an amount of 0.2 to 20 parts by weight per 100 parts by weight of the magnetic powder. Abrasives that may be used include commonly used materials such as fused alumina, silicon carbide, chromium oxide,
Corundum, artificial corundum, diamond, artificial diamond, garnet, emery (main ingredients:
corundum and magnetite) etc. are used. These abrasives have an average particle diameter of 0.05 to 5μ, particularly preferably 0.1 to 2μ. These abrasives are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the magnetic powder. Antistatic agents that may be used include carbon black, graphite, tin oxide-antimony oxide compounds,
Conductive powders such as titanium oxide-tin oxide-antimony oxide compounds; natural surfactants such as saponin; nonionic surfactants such as alkylene oxide, glycerin, and glycidol; higher alkylamines, quaternary ammonium salts , pyridine, other heterocycles, phosphonium or sulfonium; anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, phosphate ester group; amino acids, amino acids Examples include amphoteric activators such as sulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohols, and the like. The carbon black used includes carbon black (hereinafter referred to as CB ₁ ) that imparts conductivity, and in some cases carbon black (hereinafter referred to as CB ₂ ) that imparts sufficient light-shielding properties to the magnetic layer.
) is preferably added. Generally, when static electricity is accumulated during use of a magnetic recording medium, discharge occurs between the medium and the magnetic head, which tends to generate noise, and dust and the like may be attracted, causing dropouts. Furthermore, for video, a method is known in which the running of the tape is adjusted by detecting the difference in light transmittance between a tape portion having a magnetic layer and a leader tape portion. For these reasons, it is generally required that the surface electrical resistance of the magnetic layer be 10 ³ Ω-cm or less, and that the light transmittance of the tape portion where the magnetic layer is located be 0.05% or less. For this purpose, carbon black particles are usually added to the magnetic layer. In this case, the carbon black CB ₁ mentioned above,
When using CB ₂ , the specific surface area of both carbon blacks should be 200 to 500 m ² /g (and
200-300m ² /g), and 40-200m ² /g for the latter.
It is desirable to do so. That is, as shown in Figure 11, if the specific surface area of CB ₁ is less than 200 m ² /g, the particle size is too large and the conductivity is insufficient even with the addition of carbon black, and if the specific surface area exceeds 500 m ² /g. If the particle size is too small, the dispersibility of carbon black tends to deteriorate. This carbon black
CB ₁ is preferably one in which the particles are connected like a cluster of grapes, and is porous and has a large specific surface area.
A material with a high so-called structure level is desirable. Examples of such carbon black include Conductex 975 (specific surface area 270 m ² /g, particle size
46 mμ), Conductex 950 (specific surface area 245 m ² /
g, particle size 46 mμ), Cabot Vulcan (Cabot
Vulcan) XC-72 (specific surface area 257m ² /g, particle size
18mμ) etc. can be used. Regarding CB ₂ , as shown in Figure 11, the specific surface area is 40m ² /
If the particle size is less than 200 m ² /g, the particle size is too large and the light-shielding properties tend to deteriorate, and the amount added needs to be increased more than necessary. Dispersibility tends to deteriorate. As such carbon black CB ₂ for light shielding, the particle size is too small and the structure level is relatively low.
Furthermore, those with relatively low specific surface areas, such as Raven 2000 (specific surface area 180 m ² /g, particle size 19 mμ) manufactured by Columbia Carbon, 2100, 1170,
1000, #100, #75, #44, #40, #35, #30, etc. are available. There is a certain preferable range for the mixing ratio (weight ratio) of each of the above carbon blacks, and CB ₁ / CB ₂
=90/10 to 50/50 is good, and 80/20 to 60/40 is even better. If this mixing ratio is larger than 90/10, the proportion of conductive carbon black CB ₁ will be large, resulting in insufficient light shielding properties, and if it is smaller than 50/50, the surface resistivity will be low due to the small amount of conductive carbon black CB ₁ . It will increase. However, in the magnetic recording medium of the present invention, the light-shielding property of the magnetic layer can be sufficiently obtained by containing metal-based magnetic powder at a high density, so in this case, the above CB ₂
The addition of is not necessary. Further, a curing agent can be added to the magnetic paint to promote curing of the magnetic layer. Examples of usable curing agents include polyfunctional isocyanates and adducts of these with active hydrogen compounds. Examples of isocyanate compounds include those shown in Table 1 below.

【table】

【table】

【table】

[Table] Solvents for magnetic paint or solvents used when applying magnetic paint include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols such as methanol, ethanol, propanol, and butanol; methyl acetate, and acetic acid. Esters such as ethyl, butyl acetate, ethyl lactate, ethylene glycol monoacetate:
Ethylene glycol dimethyl ether, diethylene glycol monoethyl ether, dioxane,
Ethers such as tetrahydrofuran; benzene,
Aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, and dichlorobenzene can be used. In addition, materials for the support forming the above-mentioned magnetic layer include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, polycarbonate, etc. plastics, metals such as Al and Zn, glass,
Silicon nitride, silicon carbide, ceramics such as porcelain, earthenware, etc. are used. The thickness of these supports is approximately 3 to 100 μm, preferably 5 to 50 μm in the case of a film or sheet, approximately 30 μm to 10 mm in the case of a disk or card, and cylindrical in the case of a drum. The type is determined depending on the recorder used. Coating methods for forming a magnetic layer by coating the magnetic paint on the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, and gravure coating. , kiss coat, cast coat, spray coat, etc., and other methods are also possible. The magnetic layer coated on the support by such a method is optionally treated to orient the metal-based magnetic powder in the layer, and then the formed magnetic layer is dried.
Further, if necessary, the magnetic recording medium of the present invention is manufactured by subjecting it to surface smoothing processing or cutting it into a desired shape. 12 to 14 illustrate the specific structure of the magnetic recording medium according to the present invention. The magnetic layer containing the metal magnetic powder, binder, etc. described above is provided as a layer 12 on the support 11 in FIG. A back coat layer (BC layer) 13 is provided on the back surface. Also, the first
The magnetic recording medium shown in FIG. 2 includes a magnetic layer 12 and a support 11.
An undercoat layer (not shown) may be provided between the two, or an undercoat layer may not be provided (the same applies hereinafter). Non-magnetic powders contained in the BC layer 13 in FIG. 12 include carbon black, silicon oxide, titanium oxide, aluminum oxide, chromium oxide, silicon carbide, calcium carbide, zinc oxide, α-Fe ₂ O ₃ ,
Examples include those made of talc, kaolin, calcium sulfate, boron nitride, zinc fluoride, molybdenum dioxide, calcium carbonate, etc., preferably carbon black or titanium oxide. If these non-magnetic powders are included in the BC layer, the surface of the BC layer can be appropriately roughened (matted) to improve the surface properties, and in the case of carbon black, the surface properties can be improved.
By imparting conductivity to the BC layer, an antistatic effect can be obtained. It is advantageous to use carbon black and other non-magnetic powders in combination, since both the effects of improving surface properties (stabilizing runnability) and improving conductivity can be obtained. However, the surface roughness of the BC layer is such that the average roughness or height (Ra) of the center line of the surface unevenness is 0.01 to 0.1 μm, preferably 0.025 μm.
or less, and the maximum roughness (Rmax) is 0.20~
It is preferable to set it to 0.80 μm. Regarding Ra, it is desirable to set Ra≦0.025μ in order to improve the chroma S/N. If the value of Ra or Rmax is too small, running stability and the winding appearance during tape winding will be insufficient, and if it is too large, transfer from the BC layer to the magnetic layer (during tape winding) will occur, making the surface even rougher. Put it away. The particle size of the filler (including non-magnetic powder) in the BC layer 13 is 0.5 μm in order to obtain the above surface roughness.
Hereinafter, it is preferably 0.2 μm or less. In addition, the BC layer 13 can be formed by coating in the same manner as described above, but the film thickness after coating and drying is 0.1
~3.0 μm, preferably 1 μm or less, more preferably 0.6 μm or less. The amount of non-magnetic powder added to the BC layer is generally 100 to 400 mg/m ² , preferably 200 to 300 mg/m ² . Further, as the binder for the BC layer 13, the same binder as that for the magnetic layer 12 can be used. Note that although the BC layer 13 is necessary for a high-quality tape, it does not necessarily need to be provided. FIG. 13 shows another magnetic recording medium in which an overcoat layer (OC layer) 14 is provided on the magnetic layer 12 of the medium in FIG. This OC layer 14 is provided to protect the magnetic layer 12 from damage, etc., and for this purpose, it needs to have sufficient slipperiness. Therefore, as the binder for the OC layer 14, it is preferable to use the polyurethane used for the magnetic layer 12 described above (preferably in combination with a cellulose resin and/or a vinyl chloride copolymer). FIG. 14 shows a magnetic recording medium configured as a magnetic disk, in which magnetic layers 12 similar to those described above are provided on both sides of a support 11, respectively. An OC layer similar to that shown in FIG. 13 may be provided on each of these magnetic layers. The magnetic recording medium according to the present invention produced as described above has extremely good wear resistance compared to conventionally known magnetic recording media, has excellent dispersibility and surface properties, and has little dropout. moreover,
The magnetic recording medium according to the present invention has a significantly improved S/N ratio than conventional magnetic recording media, and can also obtain higher reproduction output than conventional magnetic recording media. Hereinafter, the present invention will be explained with reference to specific examples. The components, proportions, order of operations, etc. shown below may be changed in various ways without departing from the spirit of the invention. In addition, all "parts" in the following examples represent "parts by weight." Example First, iron-based metal magnetic powder was surface-treated with each of the following copolymers (a) to (i) to obtain correspondingly treated magnetic powder. In other words, for 100 parts of metal magnetic powder, the solid content is
An aqueous solution of each copolymer was added at a concentration of 2.5 parts, and kneaded for 2 hours using a kneader (manufactured by Inoue Seisakusho). Each treated magnetic powder that has been surface-treated in this way is transferred to a rotary vibration dryer to evaporate the moisture contained therein.
Furthermore, the magnetic powder was loosened in a colloid mill and classified through a mesh sieve to obtain treated magnetic powder with a desired particle size. (a) Ammonium salt of a copolymer of acrylic acid and butyl acrylate (b) Ammonium salt of a copolymer of acrylic acid and N-octylacrylamide (c) Sodium salt of a copolymer of acrylic acid and butyl acrylate (d) Ammonium salt of copolymer of maleic anhydride and styrene (e) Octylamine salt (ammonium salt) of copolymer of methacrylic acid and methyl methacrylate (f) 2-hydroxyethyl acryloyl phosphate and lauryl Ammonium salt of a copolymer with acrylate (g) Ammonium salt of a copolymer of acrylic acid with propylene (h) Butylamine salt (ammonium salt) of a copolymer with methacrylic acid and butylene (i) With acrylic acid Ammonium salt of ethylene copolymer These (a) to (i) all have the structure -[A] _n -[B] _o - described on page 18 (however, m=2,
n=3), and the average molecular weight of (a) is
546, the average molecular weight of (b) is 711, and the average molecular weight of (c) is
551, the average molecular weight of (d) is 526, and the average molecular weight of (e) is
564, the average molecular weight of (f) is 1020, the average molecular weight of (g) is
288, the average molecular weight of (h) is 246, the average molecular weight of (i) is 242
It is. In addition, the iron-based metal magnetic powder used is as shown in Table 2 below, and the BET value of the magnetic powder is
It was 45 to 55 m ² /gr. In addition, the method for setting the stable upper limit temperature of these iron-based metal magnetic powders to 85°C or higher by DSC is the method (2) described on page 9 for Nos. 1 to 5. For No. 6 to No. 9, use the method (3) described on page 9.

[Table] The following compositions were prepared using these treated magnetic powders. Treated metal magnetic powder (above) 100 parts Polyurethane (Nitsuporan N-2304 manufactured by Dainippon Polyurethane Co., Ltd.: Tensile strength 400 Kg/cm ² , Elongation at break
700%) (solid content 35%) 30 parts (as a solution) Phenoxy resin (PKHH) (solid content 20%)
20 parts (″) Conductex 975 (BET value 270 m ² /gr, particle size 46 mμ) 2.5 parts Lecithin 5 parts Myristic acid 2 parts Palmitic acid butyl ester 1 part Alumina 4 parts Methyl ethyl ketone 50 parts Cyclohexanone 100 parts This composition was thoroughly milled using a ball mill. At this time, samples were taken at regular intervals and applied on a glass plate, and the degree of dispersion was compared with a standard plate under a 100x microscope to determine the end point of dispersion. Polyfunctional isocyanate (curing agent)
5 parts of were added and filtered through a filter with an average pore size of 1 μm. The obtained magnetic paint was applied onto a 10 μm thick polyester film using a reverse roll coater while applying a magnetic field, and dried (dry film thickness:
3.2μm). Thereafter, the surface of the magnetic layer was treated using a super calender roll to obtain a wide magnetic film having a magnetic layer of a predetermined thickness. this film
I cut it to 12.65mm width and made a magnetic tape for video. Comparative Examples 1 and 2 Magnetic tapes were prepared in the same manner as in the examples except that two kinds of magnetic powders surface-treated with a copolymer of acrylic acid and butyryl acrylate were used in the paint composition of the examples. . Comparative Examples 3 and 4 Comparative magnetic tape 3 was prepared in the same manner as in Example except that magnetic powder whose surface was not treated was used in the coating composition of Example. Comparative tape 4 was prepared in the same manner as in Example except that magnetic powder surface-treated with an acrylate copolymer was used. The metal magnetic powders in the comparative tapes 1 to 4 were as shown in Table 3 below.

[Table] For each of the above magnetic tapes, squareness ratio, gloss,
The video characteristics were measured and the results are shown in Table 4 below. The method of these measurements was as follows. Gloss: Measured using a variable angle photometer at an angle of 60°, and expressed with the value of Comparative Example 1 as 100% (the larger the value, the better the surface smoothness). Video characteristics: Measured under the condition of 5 MHz playback output as RF, and the measured value of Comparative Example 1 is set as 0, and is expressed as a relative value. The same applies to Y-C/N. Y-C/N (brightness-C/N) means, as shown in Figure 15, when a 5 MHz single wavelength signal a is input and reproduced, noise b is generally added, but in this case, the 4 MHz sideband expressed as the ratio of the noise level to the non-noise level.

[Table] As is clear from this result, when magnetic powder pretreated with the ammonium salt of the copolymer a to i above is used, magnetic powder treated with a copolymer consisting of a simple free organic acid ( Compared to Comparative Examples 1, 2, and 4) and untreated magnetic powder (Comparative Example 3), the squareness ratio is high and the gloss is excellent.

[Brief explanation of the drawing]

The drawings are for explaining the present invention, and
Figure 1 is a graph showing the differential thermal analysis curve of metal magnetic powder, and Figure 2 is an enlarged photograph (electron micrograph) showing the particle structure (or shape) of conventional metal magnetic powder.
Figure 3 is an enlarged photograph (electron micrograph) of the particle structure (or shape) of the metal-based magnetic powder, which is contained in the magnetic layer of the magnetic recording medium of the present invention and shows no change in the differential thermal analysis curve up to at least 80°C. ) Figure 4 is a schematic diagram showing the structure of metal-based magnetic powder whose outer surface is coated with a polymer compound, Figure 5 is a schematic diagram showing the structure of metal-based magnetic powder whose outer surface is slowly oxidized, Figure 6 is a characteristic curve diagram showing the relationship between the temperature at which a change can be observed in the differential thermal analysis curve of metal-based magnetic powder contained in the magnetic layer of a magnetic recording medium and the still durability of the magnetic recording medium. A graph showing the squareness ratio of magnetic recording media depending on the type of powder surface treatment agent, Figure 8 is a graph showing changes in the output S/N of various magnetic powders depending on the specific surface area of the magnetic powder, and Figure 9 is a graph showing changes in the output S/N of various magnetic powders depending on the specific surface area of the magnetic powder. Figure 10 is a graph showing changes in tape properties depending on the blending ratio of polyurethane and other resins.
Fig. 1 is a graph showing changes in tape characteristics depending on the specific surface area of carbon black, Figs. 12, 13, and 14 are enlarged cross-sectional views of a portion of a magnetic recording medium according to each example, and Fig. 15 is a graph showing changes in tape characteristics depending on the specific surface area of carbon black. N (Y-C/N)
FIG. In addition, in the symbols used in the drawings, 1
...metal magnetic powder, 2...polymer compound layer, 3
... Oxidation layer, 11 ... Support, 12 ... Magnetic layer,
It is.

Claims (1)

[Claims] 1. A copolymer containing at least one monomer unit having a negative group as a copolymer component and in which the negative group forms a salt causes a differential thermal analysis curve to substantially change up to at least 80°C. 1. A magnetic recording medium characterized in that the magnetic layer contains metal-based magnetic powder treated with the copolymer.