JPH0542733B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to magnetic recording media, and more particularly to a magnetic recording medium having improved runnability and durability of the back coat layer in a magnetic tape having a magnetic layer on the surface of the support and a back coat layer on the back surface of the magnetic tape. . BACKGROUND OF THE INVENTION Magnetic recording media such as magnetic tapes, magnetic sheets, and magnetic disks are widely used in the audio, video, and computer fields. Among these, for example, in the case of magnetic tape in the video field, when recording and playing back images, the cassette is attached to a video deck, and the tape is guided by a guide ball or guide roller. While running, the magnetic head rubs and scans the magnetic head. In order to record images on magnetic tape or reproduce them in this way, in order to improve the sensitivity, especially the output in the high frequency range, it is necessary to prevent the sliding condition of the magnetic tape against the magnetic head from changing. The surface of the magnetic layer is finished smooth. The magnetic layer is designed to have improved runnability and durability with respect to magnetic heads, guide rollers, and the like. However, when this magnetic tape is run on a video deck, not only the front surface of the magnetic tape but also the back surface is rubbed by the guide balls and guide rolls, so that the magnetic layer on the front side has poor running performance against the guide poles and guide rolls. Even if the magnetic tape has good durability, if the part of the back side of the magnetic tape that is rubbed does not have good running performance or durability, excessive tension will be applied to the running magnetic tape, and this will cause the magnetic layer to be excessively sensitive to the magnetic head. This not only causes damage to the magnetic layer and peeling off of the magnetic powder in the magnetic layer, but also causes the strength of the tension at which the magnetic tape is wound to fluctuate, resulting in fluctuations in the winding pressure and the appearance of the winding. This disarray causes the edges of the tape to become uneven, resulting in uneven running of the tape when it is reused. When these things occur, the image or electromagnetic characteristics such as skew, jitter, and S/N deteriorate. For these improvements, it has been proposed to provide a back coat layer on the back side of the magnetic tape. As mentioned above, the back coat layer has been designed to improve its runnability and durability against guide balls and guide rolls, and the main one is a resin layer containing an inorganic powder.
This is done by roughening the surface of the back coat layer to reduce the area of contact with guide balls and the like, thereby reducing the coefficient of friction. For example, Tokukai Akira
57-130234, JP-A-58-161135, JP-A-57-53825, and JP-A-58-2415 all show examples using amorphous powder, and furthermore, In many cases, the particle size is limited. However, even tapes using these inorganic powders not only do not provide sufficient slipperiness, but also scrape the pins when the back coat layer comes into contact with guide pins, etc., making it difficult to guide the tape smoothly while running. Not only may the back coat layer containing this coat be damaged, but when the back coat layer containing it is rubbed, the bonding strength of the particles to the binder is insufficient to resist this, making it easy for powder to fall off, especially when large particles are mixed in. When the magnetic tape is wound, powder fall-off is more likely to occur, and when the magnetic layer and back coat layer come into contact with each other when the magnetic tape is wound, the protrusions of the back coat layer may damage the magnetic layer, and the unevenness may be transferred to the magnetic layer. , when the magnetic layer is scanned to reproduce an image, it may impair electromagnetic characteristics such as chroma S/N, which indicates the degree of color reproducibility of the image. This is because, in general, inorganic powders not only have a wide variety of particle shapes and are not constant, but also have a wide distribution of particle sizes. Generally, when the particle shape is spherical, the particles tend to be arranged regularly, and if these particles are arranged regularly on the surface of the back coat layer, when the back coat layer contacts, for example, a guide pole, these particles will make point contact, so the coefficient of friction can be reduced. Further, when the particle size distribution of the particles is narrow, it is preferable that even if the average particle size is the same, no particularly large particles are mixed in, but this is difficult to expect in the case of inorganic powder. Therefore, when inorganic particles are used, it is desirable to use particles with a relatively large particle size in order to improve the runnability of the back coat layer surface. damage the magnetic layer or harm the electromagnetic properties. From this, it is difficult to improve the slipperiness by setting the center line average roughness Ra of the surface roughness of the back coat layer at a cutoff of 0.08 mm to 0.010 to 0.026 μm and the dynamic friction coefficient μ to 0.36 or less. Moreover, the center line maximum roughness of cut off is 0.08mm.
Rm is relatively high, causing problems such as damage to the magnetic layer as described above. In the case of a back coat layer using inorganic powder,
When Ra is set to 0.018 μm or more, the sliding property becomes good, but due to the influence of transfer to the magnetic layer surface, Chroma S/
N, Lumi, which deteriorates electromagnetic conversion characteristics such as S/N. Therefore, it is necessary to make Ra smaller than 0.018μm, but in order to achieve this, it is necessary to improve the dispersion state of the inorganic powder in the paint used when forming the back coat layer, and to maintain the dispersion state until the paint is used. Conditions need to be maintained stably. Generally inorganic powders, such as carbon black,
Titanium oxide, calcium carbonate, etc. have relatively good dispersibility in organic solvents among inorganic powders, but
In addition, the particles of these dispersions may aggregate, and if these dispersions are stored for a long time, the particles have a large specific gravity, and most of them, except for carbon black, have a specific gravity of 2 to 4, so they settle. Sometimes. In particular, the average particle size of inorganic powder is 0.2μ
In the following cases, poor dispersion will occur, and the dispersion stability will be lacking even after dispersion, resulting in agglomerated particles being scattered in the back coat layer formed using this paint, making Ra smaller than 0.018 μm. It's difficult. On the other hand, when the average particle size of the inorganic powder is 0.2μ or more, the dispersion state in the paint is relatively good, but since the raw material particles themselves are large, the particles tend to settle, resulting in poor dispersion stability. It is difficult to make Ra smaller than 0.018μm,
If this is to be achieved, the filling rate of the inorganic powder in the binder must be lowered. If the filling rate of the inorganic powder is lowered in this manner, so-called interlayer adhesion, in which the tapes stick to each other when the tape is wound, may occur, the abrasion resistance and slipperiness may be impaired, and stake slip may easily occur. In this way, if the Ra of the back coat layer cannot achieve the above target value and sufficient wear resistance and durability are not obtained, problems such as powder falling off of the magnetic layer will occur, and not only will the output fluctuate, but the chroma Electromagnetic conversion characteristics such as S/N cannot be improved either. In particular, recently, video equipment such as VHS video movies and β movies has become smaller and more dense, and has become easier to carry. It has become the case that video cameras are being taken out and videotaped under various circumstances.
It is hoped that a magnetic tape that meets these requirements will also emerge. In other words, the miniaturization and high density of video equipment has made the running system of the magnetic tape more complex, and the number of parts that come into contact with guide poles, guide rolls, etc. of the tape has increased, causing more frequent friction, and the running speed has become even more difficult. There is a need for improved wear resistance and durability to prevent powder falling off, etc., and for this purpose, it has been desired to achieve the above target values of Ra and μ. Therefore, a method using fluororesin powder as the organic powder was devised. For example, JP-A-57-154638
~ JP-A No. 57-154644 discloses that fluororesin powders are treated with aliphatic compounds, fluorinated alkyl esters, perfluoroalkyl carboxylates, perfluoroalkyl betaines, perfluoroalkyl ethylene oxide adducts, and perfluorocarbon resin powders, respectively. Examples are shown in which they are used together with dispersants of alkyltrimethylammonium salts and perfluoroalkyl phosphate esters. However, these dispersants
These special dispersants are essential for improving the dispersibility of the fluororesin powder, and because these special dispersants are added, these dispersants tend to bleed onto the surface. This bleeding dispersant may stain the guide rolls, guide balls, etc. of the running system that come into contact with it, and when the tape is wound, these dispersants may be transferred to the magnetic layer, which may prevent the tape from being reused. This may cause poor running when the motor is running, and may impair the electromagnetic characteristics of various S/N ratios. As mentioned above, conventional back coat layers have problems due to the use of inorganic powder or fluororesin together with dispersants, and improvements have been desired. OBJECTS OF THE INVENTION The first object of the present invention is to provide a back coat that has excellent abrasion resistance so that the powder contained in the back coat layer does not fall off, and that there is no interlayer adhesion between the back coat layer and the magnetic layer when they come into contact with each other. An object of the present invention is to provide a magnetic recording medium having a layer. The second object of the present invention is to provide a magnetic material having a back coat layer that maintains slipperiness at, for example, Ra=0.010 to 0.026 μm without increasing the surface roughness, and that can reduce the coefficient of dynamic friction to 0.50 or less. The goal is to provide recording media. A third object of the present invention is to provide a magnetic recording medium having a back coat layer formed by coating with a coating liquid having dispersion stability. Structure of the Invention The above object is achieved by making the powder contained in the back coat layer a porous organic powder with a true specific gravity of 0.8 to 2.5. Unlike conventional fluororesin powder, this porous organic powder is compatible with various binders, and since it is porous, wetting with the binder is further promoted, so the binding force of the binder is improved. Since it is large, it is possible to increase the resistance force when rubbed, and even when a dispersant is not used, the dispersibility can be maintained well, so that abrasion resistance and durability can be improved. Furthermore, since it is organic and porous, its wettability with organic solvents is further improved, and its low specific gravity makes it less likely to settle, which also improves the dispersion stability of the coating liquid used to form the back coat layer. . In addition, as a result of experiments, it is easy to disperse the particles using a ball mill, and the dispersed particles can be easily converted into primary particles, and the variation in the particle size distribution of the particles can also be reduced, so it is not necessary to increase Ra much compared to the above-mentioned inorganic powder. , the friction coefficient μ can be reduced. In this way, the porous organic powder has excellent properties not found in the above-mentioned inorganic powder, and the important point of the present invention lies in the novel use of this in place of the above-mentioned inorganic powder. Able to perform functions effectively. From this, the magnetic recording medium of the present invention has a magnetic layer on one side of a support and a back coat layer on the other side of the support, in which the back coat layer has a true specific gravity. is 0.8~2.5
It is characterized by containing porous organic powder. Next, the present invention will be explained in detail. In the present invention, the back coat layer is formed by applying a dispersion suspension obtained by dispersing at least the above-mentioned organic powder in a solvent to an object to be coated, and applying the reactive groups of this resin powder to the object to be coated. It is composed of a fixed material, or at least a binder resin and the above-mentioned organic powder. The organic powder used in the present invention is porous. The significance of this porosity is that it indicates that the particle surface is porous, and this can be easily observed using an electron microscope or the like. In other words, it means that the bulk specific gravity is smaller than the true specific gravity, and the ratio of the bulk specific gravity to the true specific gravity (true specific gravity / bulk specific gravity) is 1.2 ~
Refers to something that is 10. A true specific gravity of 0.8 to 2.5 is used. If it is smaller than this, it tends to float on the surface of the back coat layer, and is likely to be released from the back coat layer, causing the film strength of the layer to deteriorate. On the other hand, if it is larger than 2.5, it becomes difficult to exist on the surface of the magnetic layer,
The effect of reducing the frictional force is reduced, and sedimentation is more likely to occur in the paint. The average particle diameter of this organic powder is preferably 0.01 to 0.5 μm, more preferably 0.02 to 0.3 μm. In addition, it is possible to cut the surface roughness of the back coat layer using this organic powder.
off 0.08mm center line average roughness Ra is 0.010~0.026μm
Make it so. To do this, the amount is 1 to 50 parts by weight, preferably 2 to 30 parts by weight, based on 100 parts by weight of the binder. The thickness of the back coat layer thus formed is preferably 0.5 to 1 .mu.m. Examples of organic powders that can be used include benzoguanamine resin powder, polyolefin resin powder, polyester resin powder, polyamide resin powder, and polyimide resin powder. It is important that the organic powder used in the present invention is porous, and has extremely good compatibility with, for example, urethane resins often used in magnetic recording media, and has excellent dispersibility. Further, the particles of these organic powders are preferably spherical for the reasons mentioned above. However, the shape is not limited to this, and any shape such as an ellipse or a rectangle may be used, and particles of different shapes may be mixed and used. The benzoguanamine resin will be explained in more detail as an example of the organic powder used in the present invention. An example of this benzoguanamine-based resin powder is a resin whose basic structure is derived from a reaction between a compound having the following structure and formaldehyde, and which is obtained by reactions such as methylolization, methylenization, and alkyl etherification of benzoguanamine. Also, it may be a resin powder made by copolymerizing benzoguanamine with urea, melamine, phenol, etc. In addition to this, benzoguanamine resins manufactured from similar compounds having the same properties as the benzoguanamine resins of the following compounds are also available. included. The true specific gravity of this benzoguanamine resin powder is
1.1 to 1.5 is preferred. Also, the true specific gravity/bulk specific gravity is
1.3-8 is preferable. What is particularly important about the properties of this benzoguanamine-based resin powder is that it has high mechanical strength, and when dispersed with a binder resin and other additives in a dispersion machine, it can withstand severe dispersion conditions such as a ball mill. Its properties are superior to those of polyfluorinated ethylene resin powder. The benzoguanamine resin powder preferably has a maximum particle size of 2.0 μm or less. More preferably, the average particle size is 0.01
~0.5 μm, particularly preferably 0.02-0.4 μm. In this case, it is preferable for the particle size distribution to have little variation for the reasons mentioned above. However, those with a wide particle size distribution can also be used, and two with different particle size distributions can be used.
A mixture of more than one species of particles may be used. This benzoguanamine-based resin powder is well wetted with binders, especially polyurethane resins, so it can be well dispersed in binders without using a dispersant, which is also one of the major features of the present invention. A specific example of the benzoguanamine-based resin powder is Epostor (manufactured by Nippon Shokubai Kagaku Co., Ltd.). When carbon black is further added to the back coat layer constructed as described above, an antistatic effect is imparted, but when this carbon black is dispersed together with the organic powder, the dispersion effect of this carbon black is surprising. It is also a major feature of the present invention that it should be significantly improved. This improved dispersion effect also makes it possible to reduce the amount of carbon black added compared to the case where the organic powder is not used in combination. Although the reason for this is not clear, it is understood that the dispersion of the carbon black is aided by the dispersion of the organic powder in the hydrophobic binder. The above-mentioned organic powder and carbon black may be mixed in any proportion, but the total amount thereof is preferably 1 to 50 parts by weight based on 100 parts by weight of the binder. When it is less than 1, running properties become unstable, and when it is more than 50, powder falls off easily. To explain the addition of carbon black in detail, carbon black is added to reduce the surface electrical resistance and light transmittance of the magnetic tape, but if the amount added is large, it will significantly deteriorate the mechanical properties of the layer. Usually, it is necessary to add 5 to 35% by weight (preferably 10 to 25% by weight) of the binder. However, by using the organic powder according to the present invention as described above, it is possible to maintain the mechanical properties of the back coat layer well so that the powder does not fall off, and also to maintain the desired surface electrical resistance (10 ⁹ Ω).・cm or less) and/or light transmittance (0.05
% or less). Furthermore, carbon black with small particle size is advantageous for improving the surface resistivity and/or light transmittance of the coating film when the content is the same. However, on the contrary, it becomes difficult to disperse into the coating film, and surface roughness and pinholes occur due to poor dispersion, which increases surface resistivity and light transmittance, and causes deterioration of electromagnetic conversion characteristics. However, this can also be suppressed by using the above-mentioned organic powder in combination. In the above, conductive carbon black can be used to reduce the surface resistivity of the back coat layer, etc. to a sufficient range, but such carbon black has particles connected to each other like a bunch of grapes. A material with a high structure level, which is porous and has a large specific surface area, is preferable. Examples of such carbon black include Conductex 975 (specific surface 270 m ² /
g, particle size 46 mμ), Conductex 950 (specific surface area
245m ² /g, particle size 46mμ), Cabot Vulcan XC-72 (specific surface area 257m ² /g,
Particle size 18mμ) etc. can be used. Although these carbon blacks have a large specific surface area, when applied to a back coat layer, they can be dispersed surprisingly well when used in combination with the above-mentioned organic powder. On the other hand, as light-shielding carbon black, those with small particle size, relatively low structure level, and relatively low specific surface area are used, such as Raven 2000 manufactured by Columbia Carbon Co., Ltd. (specific surface area 180 m ² /g). , particle size 19mμ), 2100, 1170, 1000,
#100, #75, #44, #40, #35, manufactured by Mitsubishi Kasei Corporation
#30 etc. can be used. The back coat layer may further contain additives such as inorganic powders, which will be described later. Then, a back coat layer consisting of a blend of these components is formed by coating the dispersion described later on the support described later by the method described later. Inorganic powders used together with the above porous organic powder include silicon oxide, titanium oxide, aluminum oxide,
Chromium oxide, silicon carbide, calcium carbide, zinc oxide, α-Fe ₂ O ₃ , talc, kaolin, calcium sulfate, boron nitride, zinc fluoride, molybdenum dioxide,
Examples include calcium carbonate. Binder resins that can be used in the back coat layer include thermoplastic resins, thermosetting resins, reactive resins, electron beam curable resins, and mixtures thereof. Thermoplastic resins used as binder resins have a softening temperature of 150℃ or less and an average molecular weight of 10,000~
200000, with a degree of polymerization of about 200 to 2000, such as vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic acid ester-acrylonitrile copolymer, Acrylic acid ester
Vinylidene chloride copolymer, acrylic ester
Styrene copolymer, methacrylic ester-acrylonitrile copolymer, methacrylic ester-
Vinylidene chloride copolymer, methacrylic acid ester-styrene copolymer, urethane elastomer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral,
Cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer, polyester resin, chlorovinyl ether
Acrylic ester copolymers, amino resins, various synthetic rubber-based thermoplastic resins, and mixtures thereof are used. As the thermosetting resin or reactive resin, one is used that has a molecular weight of 200,000 or less in the state of a coating liquid and becomes insolubilized by a reaction such as condensation or addition after coating and drying. Among these resins, those that do not soften or melt before the resin is thermally decomposed are preferred. Specifically, for example, phenolic resin, phenoxy resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic reaction resin, vinyl chloride-vinyl acetate resin, methacrylate copolymer. and diisocyanate prepolymers, mixtures of high molecular weight polyester resins and isocyanate prepolymers, urea formaldehyde resins, mixtures of polyester polyols and isocyanates,
Polycarbonate-type polyurethane, polyamide resin, mixture of low molecular weight glycol, high molecular weight diol, triphenylmethane triisocyanate,
These include polyamine 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 order to improve the durability of the back coat layer of the magnetic recording medium according to the present invention, the back coat layer can contain various hardening agents, for example, isocyanate. Aromatic isocyanates that can be used include, for example, tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), metaxylylene diisocyanate (MXDI), and combinations of these isocyanates with active hydrogen compounds. There are adducts of
The average molecular weight is preferably in the range of 100 to 3000. On the other hand, examples of aliphatic isocyanates include hexamethylene diisocyanate (HMDI), lysine isocyanate, trimethylhexamethylene diisocyanate (TMDI), and adducts of these isocyanates with active hydrogen compounds. Among these aliphatic isocyanates and adducts of active hydrogen compounds of these isocyanates, those having a dispersion amount of 100 to 3000 are preferred.
Among the aliphatic isocyanates, non-alicyclic isocyanates and adducts of these compounds with active hydrogen compounds are preferred. Examples of the adduct of the above-mentioned isocyanate and an active hydrogen compound include an adduct of a diisocyanate and a trivalent polyol. Polyisocyanates can also be used as curing agents, including, for example, a pentamer of diisocyanate, a decarboxylation compound of 3 moles of diisocyanate and water, and the like. Examples of these include an adduct of 3 moles of tolylene diisocyanate and 1 mole of trimethylolpropane, an adduct of 3 moles of metaxylylene diisocyanate and 1 mole of trimethylolpropane, a pentamer of tolylene diisocyanate, and 3 moles of tolylene diisocyanate. and 2 moles of hexamethylene diisocyanate. The magnetic layer of the magnetic recording medium of the present invention may be a coated magnetic layer using magnetic powder, a binder, a dispersant, a lubricant, etc., or may be formed by a vapor deposition method, a sputtering method, a vapor deposition method, etc. The magnetic layer may be a thin film type magnetic layer formed by using the same method. The binder for the magnetic layer can be the same as the binder for the back coat layer, and
Examples of magnetic materials include γ-Fe ₂ O ₃ and Co-containing γ-
Fe ₂ O ₃ , Co adhesion γ- Fe ₂ O ₃ , Fe ₃ O ₄ , Co containing
Oxide magnetic materials such as Fe ₃ O ₄ , Co-coated Fe ₃ O ₄ , CrO ₂ ,
For example, Fe, Ni, Co, Fe-Ni alloy, Fe-Co alloy,
Fe-Ni-P alloy, Fe-Ni-Co alloy, Fe-Mn-
Zn alloy, Fe-Ni-Zn alloy, Fe-Co-Ni-Cr alloy, Fe-Co-Ni-P alloy, Co-Ni alloy, Co-
Examples include various ferromagnetic materials such as P alloy, Co-Cr alloy, and metal magnetic powder containing Fe, Ni, and Co as main components.
Additives to these metal magnetic materials include Si,
Elements such as Cu, Zn, Al, P, Mn, and Cr, or compounds thereof may be included. Hexagonal ferrite magnetic materials such as barium ferrite are also used. The magnetic paint forming the magnetic layer may contain additives such as a dispersant, a lubricant, an abrasive, an antistatic agent, and the like, if necessary. Examples of dispersants include lecithin; fatty acids (R R represented by -COOH is a saturated or unsaturated alkyl group having 7 to 17 carbon atoms); alkali metals of the above fatty acids (Li, Na, K
etc.) or alkaline earth metals (Mg, Ca, Ba, etc.)
Examples include metal soaps made of. In addition, higher alcohols having 12 or more carbon atoms, sulfuric esters, and the like can also be used. Moreover, commercially available general surfactants can also be used. These dispersants may be used alone or in combination of two or more. Further, these dispersants can also be used in the above-mentioned back coat layer. When these dispersants are used in the magnetic layer, they are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the magnetic layer, and when used in the back coat layer, they are added in an amount of 2 to 20 parts by weight per 100 parts by weight of the binder. May be added. In addition, as lubricants, silicone oil, graphite, molybdenum disulfide, tungsten disulfide, fatty acid esters consisting of monobasic fatty acids with 12 to 16 carbon atoms and monohydric alcohols with 3 to 12 carbon atoms, carbon Fatty acid esters made of monobasic fatty acids having 17 or more monobasic fatty acids and monohydric alcohols having a total of 21 to 23 carbon atoms are used. These lubricants are added in an amount of 0.2 to 20 parts by weight based on 100 parts by weight of the binder. In addition, commonly used abrasive materials include fused alumina, silicon carbide, chromium oxide, corundum, artificial corundum, diamond, artificial diamond, garnet, emery (main ingredients are corundum and magnetite), titanium dioxide, etc. be done.
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. In addition, carbon black is used as an antistatic agent.
Conductive powders such as graphite, tin oxide-antimony oxide compounds, titanium oxide-tin oxide-antimony oxide compounds, carbon black graft polymers; natural surfactants such as saponin; alkylene oxide-based, glycerin-based, glycidol-based, etc. Nonionic surfactants; cationic surfactants such as pyridine and other heterocycles, phosphoniums or sulfoniums; carboxylic acids, sulfonic acids,
Examples include anionic surfactants containing acidic groups such as phosphoric acid, sulfuric ester groups, and phosphoric ester groups; amphoteric surfactants such as amino acids, aminosulfonic acids, and sulfuric or phosphoric esters of amino alcohols. Solvents to be added to the above paint or diluting solvents when applying this 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 ethyl acetate. , esters such as butyl acetate, ethyl lactate, and ethylene glycol monoacetate; ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, and tetrahydrofuran; aromatic hydrocarbons such as benzene, toluene, and xylene; methylene chloride, ethylene chloride, Halogenated hydrocarbons such as carbon tetrachloride, chloroform, and dichlorobenzene can be used. Examples of the support include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polyamide and polycarbonate. However, metals such as Cu, Al, Zn, glass,
Ceramics such as BN, Si carbide, porcelain, and earthenware can also be used. The thickness of these supports is approximately 3 to 100 μm in the case of a film or sheet, preferably 5 to 50 μm.
and 30 μm to 10 in the case of disks and cards.
mm, and in the case of a drum, it is used in a cylindrical shape, and its shape is determined depending on the recorder used. An intermediate layer for improving adhesion may be provided between the support and the back coat layer according to the present invention. Application methods for forming the above-mentioned back coat layer on the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss coating, and cast coating. , spray coat, etc. can be used, but are not limited to these. Effects of the Invention As described above, in the present invention, since the porous organic powder is contained in the back coat layer, it is possible to improve the adhesion force within the layer.For example, when used together with the binder resin, the It has good wettability and is firmly fixed within the layer, so even when the back coat layer is rubbed against a guide pole, for example, it reduces powder falling off and increases running stability. It is possible to reduce the need to scrape off guide rolls, etc. Furthermore, since the organic powder can be formed into spherical particles and its particle size distribution can have less variation, when contained in the back coat layer, Ra can be reduced and its friction coefficient μ can be reduced. This improves the running properties of the magnetic recording medium, making it possible, for example, to wind a magnetic tape uniformly.Moreover, when the magnetic tape is wound, the unevenness of the back coat layer is transferred to the magnetic layer, resulting in a chroma S/N ratio. It does not impair the electromagnetic conversion characteristics such as, etc., and the surface of the magnetic layer is less likely to be damaged. In addition, since the organic powder used in the present invention is organic and porous, it has good dispersibility in binder resins and organic solvents, and can improve dispersion stability in the coating liquid. Since it can be omitted, running instability and interlayer adhesion caused by bleed-out of the dispersant can be avoided, and it can also contribute to improved productivity. Examples Next, examples of the present invention will be described, but the present invention is not limited thereto. Example 1 A magnetic layer was previously coated on a polyethylene terephthalate support as follows. The composition in the column of Magnetic layer coating liquid 1 in Table 1 (parts by weight is shown; the same applies hereinafter) is charged into a ball mill and dispersed to prepare a dispersion liquid. This dispersion is filtered through a 1 μm filter, and 5 parts by weight of a polyfunctional isocyanate is added to the filtrate and mixed to prepare a magnetic paint. This magnetic paint was coated onto a support to a thickness of 5 μm using a reverse roll coater and dried, followed by supercalender treatment. Next, a coating solution was prepared by dispersing the composition in the column of back coat layer coating solution 1 in Table 1 for 5 hours in a ball mill. This coating solution was applied to the back surface of the support using a reverse roll and dried to form a back coat layer with a dry film thickness of 0.8 μm, which was then slit into 1/2 inch width to produce the magnetic tape of Example 1. Example 2 In Example 1, the composition of magnetic layer coating liquid 2 and Table 1 were used instead of the composition of magnetic layer coating liquid 1 in Table 1.
A magnetic tape of Example 2 was prepared in the same manner except that the composition of Back Coat Layer Coating Liquid 2 was used in place of Back Coat Layer Coating Liquid 1. Comparative Examples 1 to 3 In Example 1, magnetic layer coating liquids 1, 2, and 3 and back coat layer coating of Comparative Examples in Table 1 were used instead of magnetic layer coating liquid 1 and back coat layer coating liquid 1 of Examples in Table 1, respectively. Magnetic tapes of Comparative Examples 1, 2, and 3 were prepared in the same manner except that the compositions of Liquids 1, 2, and 3 were used, respectively. The magnetic tapes of Examples 1 and 2 and Comparative Examples 1 to 3 were tested for each item shown in Table 2, and the results are shown in the corresponding columns. The measurement method is as follows. (a) Chroma S/N (dB): Chroma signal (3.58MHz)
is set to 0.714Vp-p and recorded on the luminance signal,
This recording is played back, only the chroma signal is extracted, and the ratio of its effective value (s) to the noise level when the chroma signal is removed is shown. The magnetic tape of Example 1 is shown as a relative value with the value of the magnetic tape of Comparative Example 1 being 0, and the magnetic tapes of Example 2 and Comparative Example 3 are shown as relative values with the value of the magnetic tape of Comparative Example 2 being 0. (b) Jitter value (μsec): Measured with a VTR jitter meter manufactured by Meguro Electric Corporation. (c) Bleed-out: The magnetic tape was left at a humidity of 80% and a temperature of 45°C for 4 hours, and after being left at room temperature for 24 hours, the surface of the back coat layer was observed under a microscope. (d) Roll appearance: Repeatedly run 400 times with a video deck (using Matsushita Electric's NV300).
It was judged as good if the edges of the tape were aligned on one plane, and bad if the edges were not aligned. (e) Surface roughness: 3D roughness measuring device manufactured by Kosaka Laboratory
Measured using 3E-3RK. (f) Interlayer adhesion: 1/2 as in the above rolling test
In this case, an inch-wide magnetic tape was wound with a pressure of 1 kg and left at 45°C and 80% relative humidity for 24 hours, and then for 24 hours.
Those that showed resistance when being unwound and pulled apart after being left at room temperature for hours were evaluated as "Yes," and those that did not were evaluated as "None." (g) Dispersibility test: The coating liquid for the back coat layer is applied to a glass plate with an applicator, and the size of the dispersed particles is evaluated on a five-point scale (5 is the best, 1 is the worst) by microscopic observation. A score of 2 or less was considered good, and a score of 2 or less was considered poor. From the results in Table 2, it can be seen that in the corresponding Example 1 and Comparative Example 1, and Example 2 and Comparative Example 2, the one in Example was good in all measurement items, while the one in Comparative Examples 1 and 2 was particularly bleed-out. , the winding appearance and the interlayer adhesion were poor, and it can be seen that this was due to bleeding of the dispersant added to improve the dispersibility of the polyfluoroethylene resin powder. Furthermore, from the results of Example 2 and Comparative Example 3, Comparative Example 3 is similar to Example 2 in that there is no bleed-out and no interlayer adhesion, but it is inferior to the magnetic tape of Example 2 in other respects, and the benzoguanamine resin is There is a clear difference depending on whether it is porous or non-porous.

【table】

【table】

【table】

[Table] The dynamic friction coefficients of the back coat layers of the magnetic tapes of Examples 1 and 2 were 0.24 and 0.20, respectively.

Claims (1)

[Claims] 1. In a magnetic recording medium having a magnetic layer on one side of a support and a back coat layer on the other side of the support, the back coat layer comprises a porous organic powder having a true specific gravity of 0.8 to 2.5. A magnetic recording medium characterized by containing: