WO1987003413A1

WO1987003413A1 - Magnetic recording media

Info

Publication number: WO1987003413A1
Application number: PCT/US1986/002488
Authority: WO
Inventors: Mitsutoshi Miyasaka; Makoto Inoue; Neyoshi Ishida; Norio Miyata
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1985-11-22
Filing date: 1986-11-19
Publication date: 1987-06-04
Also published as: JPS62124624A; KR880700996A

Abstract

Magnetic recording medium comprising a non-magnetic base layer carrying a magnetic recording layer on one side and a back coating containing carbon black. The preferred carbon black has a surface area of between 18 and 50 m2/g and an average crystallite size, Lc, of between 18 and 60 Angstroms. Runability, edge damage and dropout characteristics are improved.

Description

MAGNETIC RECORDING MEDIA

Technical Field

This invention relates to magnetic recording media, such as magnetic recording tape. In detail, the invention relates to magnetic recording media excellent in electromagnetic conversion properties, drop-out properties and running durability.

Magnetic and physical properties of magnetic layer coated on one side of non-magnetic supporting material is crucial for performance of a magnetic recording medium in general. However, a tape without any treatment on the other side of the magnetic layer may lose a part of its non-magnetic supporting material while running, which may adhere to the magnetic layer, resulting in disturbance of magnetic recording and reproduction (drop out) at the part. At the same time, the tape easily becomes charged with static electricity, for the magnetic layer is generally insufficient in terms of conductivity. As the result, the tape attracts dust and chips peeled from the magnetic layer more by static electricity, which also causes drop out.

Background Art

In the conventional magnetic tape, the back side of a magnetic layer is coated with carbon black and binder (hereinafter, the coated layer is called "back coating layer") in order to improve the performance. This prevents the non-magnetic supporting layer from peeling off and forms what is called "an antistatic layer", both of which help reduce the occurrence of drop out. (Examples are Japanese Patent Kokai (Laid-Open Publication) 111828/82, 3722/84, 38725/85 and 66317/85). (Problems that the invention is going to solve). Regarding carbon black conventionally used, one with a specific surface area which is too small deteriorates conductivity, reduces Young's modulus of tape due to its heterogeneous dispersion in coating material and produce surface roughness (roughness) on a back coating - layer, which leads to deteriorations of electromagnetic conversion property and running durability and becomes susceptible to drop out. On the other hand, one with a specific surface area which is too large prolongs the time to disperse because it makes dispersion difficult in the coating material. This decreases productivity or produces heterogeneous dispersion which result in a decrease in

Young's modulus, increase in coefficient of friction, etc.

These cause deterioration of running durability and increase occurrences of drop out. In addition, most carbon5 black has high hygroscopicity and a back coating layer made from such carbon affects negatively the hardenability of a film. This in turn adversely affects the quality of the layer and running durability of a tape. Various impurities found in carbon black are also factors which may decrease ' conductivity and then increase drop out. They change various physical properties of back coating composition and subsequently damage quality of a magnetic tape in terms of stability. Conventional technology was not free from many of the problems mentioned above.

Disclosure of Invention

This invention solves the above problems accompanying conventional technology and provides magnetic recording media, such as a magnetic tape, etc., which possess good electromagnetic conversion property (quality of picture), reduced drop out and excellent running durability.

This invention solves the above problems and it provides a magnetic recording medium composed of a magnetic recording layer which is coated on one side of non-magnetic base and a resin film layer containing carbon black which is coated on the other side and characteri zed in that

2 specific surface area of said carbon black is 18-50 m /g

Q and its average crystallite size, L value as defined o hereinafter, is 18-60 A.

Carbon black used in the back coating layer of this invention is produced by thermal decomposition of saturated hydrocarbon, such as methane, ethane, etc., unsaturated hydrocarbon, such as ethylene, acetylene, naphthalene, etc., or mixtures thereof, or material containing these. Among them, carbon black produced from acetylene is the most desirable. Carbon black used in this

2 invention has specific surface area of 18-50 m /g,

2 preferably 30-40 m /g, according to BET method and size of

O » crystallite (L_r value) of 18-60 A, preferably 20-40A. Carbon black with specific surface area of 17 m 2/g or less makes a back coating layer with a rougher face and less electromagnetic conversion property and antistatic effect.

2 Carbon black with the surface area of 51 m /g or more makes a back coating layer with a heterogeneous dispersion of carbon black in the coating material, lower Young's modulus, reduced conductivity and rougher face, which further deteriorates electromagnetic conversion property, running durability and drop-out property. On the other hand, carbon black with an L_ value of 17 A or less not only makes a back coating layer with less conductivity and worse drop out, but also deteriorates electromagnetic conversion property and running durability due to increased hygroscopicity. Besides, other problems are raised concerning production and control of magnetic recording

0 media, while one with L_ value of 18 A or more improves conductivity and decreases drop out. These highly crystallized carbon black materials contain less amount of impurities, improving conductivity, diminishing fluctuation of various physical properties of a black coating layer and stabilizing the quality of a magnetic tape. One with L_r value of 61 A or more decreases conductivity because of a tendency of less relation between carbon black particles. L_c value, an average size of crystallite perpendicular to the face represented by (0, 0, 2) corresponding to (h, k,l) measured by X-ray diffraction, is calculated from the following Scherrer's equation.

__ - K

£-cos θ

. o

^m. '. wavelength of X-ray used for measurement (A)

J : half-width of diffraction line according to size of crystallite (FWHM) fl : Bragg angle of diffraction line K - 0.9

Binder used in this invention in a back coating layer is thermoplastic, thermosetting or reaction type resin which is conventionally used for magnetic recording media, or the composite material of these. Thermosetting resin is especially preferable in view of the film strengths, etc., obtained.

Thermoplastic resin used has to show softening temperature of 150°C. or less, average molecular weight of 10,000-200,000 and degree polymerization of ca. 200-2,000. Examples are copolymers of vinyl chloride and vinyl acetate, copolymers of vinyl chloride, vinyl acetate and vinyl alcohol, copolymers of vinyl chloride and vinylidene chloride, copolymers of vinyl chloride and acrylonitrile, copolymers of acrylic acid ester and acrylonitrile, copolymers of acrylic acid ester and vinylidene chloride, copolymers of acrylic acid ester and styrene, copolymers of methacrylic acid ester and vinylidene chloride, copolymers of methacrylic acid ester and styrene, urethane elastomer, nylonsilicon type resin, nitrocellulose-polyamide resin, polyvinyl fluoride, copolymers of butadiene and acrylonitrile, polyamide resin, polyvinyl butyral, cellulose derivatives, copolymers of styrene and butadiene, polyester resin, copolymers of chlorovinyl ester and acrylic acid ester, amino resin, thermoplastic resins of various kinds of synthetic rubber and mixtures of these.

Thermosetting or reaction type resins used have to have a molecular weight of 200,000 or less in the form of coating liquid. The molecular weight becomes infinite through reactions, such as condensation, addition, etc., by heating after coating and drying. Among these resins, those that are not softened or melted until the resin becomes decomposed are preferable. Examples are phenol resin, epoxy resin, thermosetting type polyurethane resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic-type reaction resin, epoxy-polyamide resin, nitrocellulose-melamine resin, mixture of macromolecular weight polyester resin and isocyanate prepolymer, mixture of metacrylate copolymer and diisocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, mixture of low molecular weight glycol, macromolecular weight diol and triphenylmethane isocyanate, polyamine resin and the mixture of these.

Hardeners useful in this invention are those in w applicable for these thermosetting resins, among them the isocyanate hardener being preferable. These compounds are, for example, toluene diisocyanate adducts such as KRYSPON 4565 and 4560 of Dainnippon Ink & Chemicals Inc., CORONATE

L of Nippon Polyurethane Industry Co., Ltd. and TAKENATE

25

L-1007 OF Takeda Chemical Industries, Ltd.

Thickness of a back coating layer in this invention is generally within the range of 1-10 micrometers after coating and drying.

The amount of hardener used in this invention is typically about 5-50 parts by wt. against 100 parts by wt. of binder.

The weight ratio of binder to carbon black in this invention is generally in the range of 20-500 parts by wt., preferably 50-300 parts by wt. against 100 parts by

35 wt. of carbon. axamples of solvents used for production of a back coating layer in this invention are ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, etc.; alcohols, such as methanol, ethanol, proponal, butanol, etc.; esters, such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, acetic acid glycol monoethyl ether, etc.; glycol ethers, such as ether, tetrahydrof ran, glycol dimethyl ether, glycol monoethyl ether, dioxane, etc.; tars (aromatic hydrocarbon), such as benzene, toluene, xylene, etc.; hydrocarbon chlorides, such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, etc., which are used by about 3-30 times (wt.) of binder in general.

A magnetic layer useful in this invention can be either metallic thin film type or composite type of coating type and metallic thin film type. The former consists of a coating type ferromagnetic metallic thin film which is composed of a film containing ferromagnetic microparticles and binder. Examples of ferromagnetic material are metals, such as Fe, Co, Fe-Co, Fe-Co-Ni, Co-Ni, etc., metal oxide, metal nitride and alloy of them, and alloy of them and Al, Cr, Si, etc. Although ferromagnetic material mainly composed of ferromagnetic metal needs an antistatic countermeasure, for it causes high electric resistance of a film and drop out easily takes place, combination of the material with a back coating layer produced according to the present invention readily eliminates such a problem.

A magnetic recording layer mentioned above ^• preferably shows coercive force within the range of 500-2000 Oe. Magnetic coating material is prepared from ferromagnetic powder with various binders. Generally, thermosetting resin type binder and radiation curing type binder are preferred. Dispersant, lubricant and antistatic agent can be used in compliance with the conventional methods as other additives. Dispersants used are surfactant, organotitaniu coupling agent, etc. Binders used are copolymers of vinyi cnloride, vinyl acetate and vinyl alcohol, binders consisting of polyurethane prepolymer and polyisocyanate, etc.

According to the conventional method, ferromagnetic powder is mixed with binder, solvent and various additives to prepare magnetic coating material, which is then coated on a base material produced from polyester, etc. The coating is cured by heating or radiation to form a magnetic film, which is further subjected to a calendering process. Besides the above combination, a ferromagnetic metallic thin film or a composite film of coating type and metallic thin film above mentioned could be used as a magnetic recording layer.

Detailed Description

The following examples explain this invention in further detail.

EXAMPLE 1

100 parts by wt. of cobalt adsorption type iron oxide (Minnesota Mining and Manufacturing Co., "3M") (containing 2.5 wt% of cobalt), 7 parts by wt. of binder composed of polyester polyurethane (B.F. Goodrich, ESTANE 5703) and 10 parts by wt. of copolymer of vinyl chloride and vinyl acetate (Union Carbide), additives composed of 2 parts by wt. of soybean lecithin, 1 part by wt. of silicone oil (dimethyl polysiloxane) , 2 parts by wt. of alumina powder, and solvent composed of 120 parts by wt. of methyl isobutyl ketone and 100 parts by wt. of butyl acetate were mixed and dispersed with sand-mill grinder. The liquid was coated on one side of a polyethylene terephthlate base film which was 14 micrometers in thickness. The coated layer itself was 4 micrometers in thickness after dring. After drying, it was subjected to a calendering process.

Next, the following mixture was mixed and dispersed for four hours with a sand-mill grinder. Carbon Black (specific surface

2 area; 35.8 m /g and β c value: 23.4 A 100 parts by wt.

Polyurethane resin (B.F. Goodrich,

ESTANE 5707) 100 parts by wt.

Phenoxy resin (Union Carbide, PKHH) 80 parts by wt.

Tetrahydrofuran 800 parts by wt.

Cyclohexanone 200 parts by wt.

This dispersed liquid was filtrated through a filter with a mesh size of ca. 2 micrometers. 30 parts by wt. of polyisocyanate (a toluene diisocyanate adduct available from Nippon Polyurethane Industry Co., Ltd., CORONATE L) was added into the filtrate as a hardener and then stirred for 30 minutes to obtain back coating material.

This coating material was coated on back side of a base film with thickness of 1 micrometer, on the other side of which a magnetic layer was already produced. After drying, this was cut into 1/2 inch (approximately 1.25 cm.) width and thus a magnetic tape was obtained.

EXAMPLES 2-4 AND COMPARATIVE EXAMPLES 1-6

Except for kind of carbon black used and dispersion time, the same conditions and procedures as required in Example 1 were conducted to prepare samples.

In the following table, production conditions and properties of samples thus obtained are shown.

Each property in the table was measured as follows: (1) Electromagnetic conversion property i) Video signal to noise ratio (S/N)

Average S/N values (relative value) were obtained when a video signal of standard picture-recording electric current 50 IRE was recorded for five minutes and then reproduced with a notice meter (Shibasoku Type 925) is shown. ii) Chroma S/N

Average S/N value (relative value) obtained when 100% chromasuperimposed monocolor signal by 100% of standard picture-recording electric current 50 IRE was recorded for five minutes and then reproduced with a notice meter (Shibasoku Type 925) is shown.

(2) Drop out property i) Drop out 10 microseconds , 15 dB

Average count of samples out of 10 per minute, which took 10 microseconds or more to lower signal by 15 dB or more from average reproduction level when color bar was recorded and reproduced at 23°C. and under 60%RH with a VHS deck (Matsushita Electric Industrial Co., Ltd., 8200) is shown, ii) Drop out 5 microseconds, 12 dB

Average count of samples out of 10 per minute, which took 5 microsecond or more to lower signal by 12 dB or more from average reproduction level when color bar was recorded and reproduced at 23°C. and under 60%RH with a VHS deck (Matsushita Electric Industrial Co., Ltd., 8200) is shown. (3) Running durability i) Tension

Tension of head drum out of a VHS deck (Matsushita Electric Industrial Co., Ltd., 8200 measured at the 1st and 200th of reproduction is shown. ii) Probability of edge damage at the 200th of reproduction.

The ratio of occurrences of edge damage of video tapes out of 12 samples measured at the 200th of reproduction is shown in percentage.

In the above table, the bigger values in Video

S/N and Chroma S/N correspond to the better properties, while the smaller values in drop out property, tension and probability of edge damage at the 200th reproduction correspond to the better properties. Regarding tension, low values after repeated reproduction particularly indicates excellent running durability. Therefore, it is only a magnetic recording medium of this invention that is excellent in every property and possesses remarkably good properties as a whole.

In addition, dispersion time is relatively short and various properties are superior.

As mentioned above, the magnetic recording media of this invention reduces drop out owing to the proper selection of carbon black of a back coating layer, keeping electromagnetic conversion property equal to that of conventional tapes, which makes running durability improved. This invention provides high performance magnetic recording media due to such effects, which could be easily applied favorably in various fields, such as an audio, video, computer and magnetic tapes, etc. On the other hand, given carbon black selected decreases difficulties in preparation of homogeneous dispersion liquid of back coating material, by which control of production became easier. Therefore, this invention is practically and economically significant from various aspects mentioned above.

Claims

1. A magnetic recording medium composed of non-magnetic base with a magnetic recording layer coated on one side of the non-magnetic base and a back coating layer containing carbon black coated on the other side of the base, and wherein the specific surface area of the above carbon black is 18-50 m²/g and the L_C value is 18-60 Å.

2. A magnetic recording medium according to claim 1, wherein the specific surface area of carbon black is 30-40 m²/g and the L_C value is 20-40 Å.