GB2103110A - Method of making a magnetic recording medium - Google Patents

Abstract

To make a magnetic recording medium which will not curl at the edges, a non-magnetic support such as polyethylene terephthalate is coated (a) on one surface with a magnetic coating composition, and (b) on the other (rear) surface with an unsatured compound which is polymerizable by electron beams; the material is irradiated, e.g. from a curtain beam accelerator, at 100 to 1,000 Kv with electron beams at an absorbed dose of 0.5 to 20 megarads so as to polymerize the compound. Suitable polymerizable compounds have at least two -C=C- bonds, e.g. unsaturated polyesters having an acryloyl, acrylamido, allyl, vinyl ether or vinyl thioether group; preferably it has a meth(acryloyl) group on both ends of its straight chain. The magnetic coating may be of ferromagnetic particles also in a compound polymerizable by electron beams, which is irradiated separately or at the same time as the rear coating. An unsaturated -C=C- monomer and/or a thermoplastic resin may also be present in the rear coating. The medium is useful as a video tape, having low curl, low abrasion and good electrical characteristics.

Description

SPECIFICATION Method of making a magnetic recording medium The present invention relates to a magnetic recording medium that is free from curl and which has good electric characteristics and wear resistance.

Most conventional magnetic recording media are the coated type produced by dispersing ferromagnetic particles such as fine ferromagnetic iron oxide particles, fine ferromagnetic chromium dioxide particles and ferromagnetic alloy particles in an organic binder such as a vinyl chloride/vinyl acetate copolymer, a vinyl chloride/vinylidene chloride copolymer, a cellulosic resin, an acetal resin, a polyurethane resin or an acrylonitrile/butadiene resin, applying the resulting coating solution to a nonmagnetic base, and drying the coating. With the recent increasing demand for higher density recording, researchers' attention has been drawn to magnetic recording media of the thin metal film type. This type of media uses as a magnetic recording layer a thin ferromagnetic metal film that is formed by vapor deposition such as vacuum deposition, sputtering or ion plating.Either type of the recording media develops a "curl" wherein the magnetic layer assumes a generally concave form. If the magnetic tape curls, it has poor contact with the magnetic head of a video tape recorder or audio tape recorder so as to cause an appreciable decrease in the electrical characteristics. Furthermore, the tape edges are contacted by the magnetic head more frequently than the concaved center portion, so they easily wear, which causes dropouts or provides low output.

The probable reason for the curl developing in the magnetic recording medium of coated type is that the volume of the magnetic layer is decreased when the solvent evaporates. The magnetic recording medium of thin metal film type may curl as a result of the base shrinkage that occurs when the surface of the base on which the magnetic layer is formed is eventually heated. To achieve longer recording time, the demand has recently increased for developing a very thin magnetic recording medium using a thin base. However, the thinner the base, the greater the chance of developing curl.

This problem has imposed limits on efforts to develop a commercial thin magnetic recording medium.

A method for preventing curl by forming a back coating of inorganic pigment particles and a thermoplastic resin on the back side of the base is disclosed in Japanese Patent Publication No.

34324/79. However, since the thermoplastic resin is the only binder used, the back coating is easily abraded and flakes or sheds from the coating cause increased dropouts. The use of both a thermoplastic resin and a trifunctional isocyanate compound as a binder to provide a back coating having improved abrasion resistance is disclosed in Japanese Patent Application (OPI) No. 144604/76 (the symbol OPI as used herein means an unexamined published Japanese Patent Application).

However, this method is not economical since it takes a long time (several hours to several days) to heat-cure the back coating. Furthermore, a tape is subjected to the heat-curing in the form of a roll pack and this increases the chance of blocking, i.e., adhering the back coating to a magnetic layer which is in contact therewith. The primary purpose of the methods of Japanese Patent Publication No.

34324/79 and Japanese Patent Application (OPI) No. 1 44604/76 is to reduce the running tension of the tape by using inorganic pigment particles to increase the surface roughness of the back coating.

However, the particles are also effective in preventing curl. Nevertheless, the conventional back coating needs a long time to cure and has great chance of blocking.

As a result of various studies to eliminate these defects, the present inventors have found that they can be eliminated by forming on the side of the base opposite the magnetic layer a back coating containing a polymer formed by exposure to electron beams.

Therefore, one object of the present invention is to provide a magnetic recording medium having good electrical characteristics.

Another object of the present invention is to provide a magnetic recording medium having high wear resistance.

Still another object of the invention is to provide a magnetic recording medium that requires a very short curing time.

According to the present invention a magnetic recording medium has a magnetic layer on one surface of a non-magnetic base and a back coating on the other surface that is formed by exposing to electron beams a layer containing a compound that is polymerized by electron beams.

Thus, the method of the invention comprises forming on a non-magnetic support on one surface a magnetic coating composition and on the other (rear) surface a layer of a compound which is polymerizable with electron beams, and irradiating at least the rear surface with electron beams to cause polymerization of said compound.

In the present invention, there can be used any type of magnetic layer, e.g. as described in U.S.

Patents 2,581,414, 2,855,1 56, 3,240,621, 3,526,598, 3,728,262, 3,790,407 and 3,836,393. In the most preferred embodiment, the magnetic layer is formed by exposing to electron beams a layer containing a compound polymerizable with electron beams and ferromagnetic particles. Such magnetic recording medium can be produced by any of the following methods: (1) a magnetic recording layer is formed on the surface of a base opposite a back coating that has been exposed to electron beams, and the magnetic layer is then irradiated with electron beams; (2) a back coating and a magnetic recording layer each containing a compound polymerizable with electron beams are formed on opposite sides of a base sequentially or simultaneously, and are simultaneously cured by polymerization upon exposure to electron beams; and (3) a magnetic recording layer is first formed on one side of a base and exposed to electron beams, then a back coating is formed on the other side of the base, and exposed to electron beams.The second method wherein the back coating and the magnetic recording layer are simultaneously irradiated with electron beams to achieve polymerization and curing is preferred since it provides high energy utilization. Methods of irradiating with electron beams are described in, for example, S. V. Nabro et. al. Adhesive Age, vol. 22 p. 24 (1979) and W.

Deninger, J. Oil. Col. Chem. Assoc., vol. 52 p. 930 (1969). The back coating according to the present invention is applicable to any type of magnetic recording medium, i.e. heat-curable coated type or thin metal film type, and achieves remarkably improved electric characteristics. In either type of magnetic recording medium, the order of forming the magnetic layer and the back coating is not critical.

The compound polymerizable with electron beams is a compound having an unsaturated bond capable of polymerization by electron beams. The compound preferably has two or more carbon carbon double bonds such as in vinyl or vinylidene. Examples of these compounds include unsaturated polyesters having an acryloyl group, acrylamido group, allyl group, vinyl ether group or vinyl thioether group. Particularly preferred are those compounds which have an acryloyl or methacryloyl group on both ends of the straight chain, and described in A. Vrancken, Fatipec Congress, viol. 11, p. 19 (1972), British Patents 1,234,684,1,162,721,1,162,722,1,162,723 and 1,162,724, and U. S. Patent 3,530,100.One example is CH2=CH-C02-CH2CHCH0- OH

and the polyester unit of the illustrated compound may be replaced by a polyurethane unit, epoxy resin unit, polyester unit or polycarbonate unit, or mixture of these units. The acryloyl group at both ends of the straight chain may be replaced by a methacryloyl group. The compound polymerizable by electron beams according to the present invention preferably has a molecular weight of about 500 to 20,000.

These compounds may be used with a monomer having an unsaturated carbon-carbon bond in the molecule. Examples of such monomer include acrylic acid, methacrylic acid, itaconic acid, acrylic acid alkyl esters such as methyl acrylate, methacrylic acid alkyl esters such as methyl methacrylate, styrene and its derivative such as a-methylstyrene and P-chlorostyrene, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate and vinyl propionate. Two or more unsaturated bonds may be present in the molecule. Examples of such compounds are found in KankoseijushiData-shu FA list of Data on Photosensitive Resins), published by Sogo Kagaku kenkyusho, pp. 235-236, December 1968, British Patents 834,733, and 854,980, and U.S.Patents 2,760,863, 2,791,504 and 2,927,023. Particularly preferred compounds are unsaturated esters of polyols such as ethylene diacrylate, diethylene glycol diacrylate, glycerol trimethacrylate, ethylene dimethacrylate, pentaerythritol tetramethacrylate and glycidyl methacrylate having an epoxy ring. A compound having one unsaturated bond in the molecule may be used in combination with a compound having two or more unsaturated bonds. If these monomers are used, the weight ratio of the polymerizable compound to monomer is preferably 2/8 or more. If the polymer to monomer ratio is less than 2/8, more energy is necessary for curing the polymer.

The back coating of the present invention may optionally contain fine particulate inorganic pigments, such as carbon black, graphite, zinc oxide, titanium oxide, barium sulfate, talc, kaolin, chromium oxide, cadmium sulfate, geothite, silica, fine particulate anhydrous alumina, calcium carbonate, molybdenum disulfide and carbon fluoride. These pigments preferably have a particle size of 3 FL Or less.

The back coating of the present invention may further contain one or more thermplastic resins such as a vinyl chloride/vinyl acetate copolymer, cellulosic resin, acetal resin, vinyl chioride/vinylidene chloride resin, polyurethane resin, and acrylonitrile/butadiene resin.

The back coating of the present invention may also contain a suitable additive such as a lubricant, dispersant, abrasive, corrosion inhibitor or antistat, as described in U.S. Patents 2,923,642, 2,997,451,3,007,892, 3,041,196,3,115,420, 3,166,688 and 3,761,311. Examples of the applicable lubricant are a saturated or unsaturated higher aliphatic acid, aliphatic acid ester, higher aliphatic acid amide, higher alcohol, silicone oil, mineral oil, edible oil, animal and vegetable oils, and fluorinecontaining compounds.

A paint composition for the back coating is prepared by charging a mixer with the compound polymerizable by electron beams and all other ingredients simultaneously or sequentially. Various mixers may be used to achieve thorough mixing of the paint composition. Examples are a two-roll mill, three-roll mill, ball mill, pebble mill, trommel, sand grinder, Szegvari attritor, high-speed impeller disperser, high-speed stone mill, high-speed impact mill, disperser, kneader, high-speed mixer homogenizer, and ultrasonic disperser. For details of the kneading and dispersing technique, see T. C.

Patton, Paint Flow and Pigment Dispersion, John Wiley 8 Sons,1964, as well as U.S. Patents 2,581,414 and 2,855,156.

The back coating is formed from the paint composition on the base by air doctor coating, blade coating, air knife coating, squeeze coating, impregnate coating, reverse roll coating, transfer roll coating, gravure coating, kiss coating, cast coating, spray coating and spin coating. For details of these and other coating techniques, see Coating Kogaku (Coating Engineering), published by Asakura Shoten, March 20, 1971, pp. 253-277. The paint composition is applied to the base to form a back coating having a dry thickness of from 0.1 to 5 4.

Suitable bases on which the back coating is formed include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate; polyolefins such as polyethylene and polypropylene; cellulose derivatives such as cellulose triacetate, cellulose diacetate, cellulose acetate butyrate and cellulose acetate propionate; vinyl resins such as polyvinyl chloride and polyvinylidene chloride; other plastics such as polycarbonate, polyimide and polyamideimide; non-magnetic metals such as aluminum, copper, tin, zinc and non-magnetic alloys including the same; paper, baryta paper, and paper coated or laminated with ct~-polyolefins having 2 to 10 carbon atoms such as polyethylene, polypropylene and ethylene-butene copolymer. Polyethylene terephthalate film is preferably used for the base.These non-magnetic bases may be transparent or opaque depending upon the specific use.

They may be in the form of a film, tape, sheet, disc, card, drum or other suitable shape, and a suitable material is selected from various materials depending upon their form. These non-magnetic bases have a thickness of about 1 to 50 ym, preferably 2 to 25 Mm, when they are in the form of a film, tape or sheet.

Suitable electron beam accelerators include a Van de Graaff scanning accelerator, double scanning accelerator and curtain beam accelerator. A curtain beam accelerator is preferred for its low price and high power output. The acceleration voltage is generally from 100 to 1,000 kilovolts, preferably from 1 50 to 300 kilovolts. The absorbed dose is generally from 0.5 to 20 megarads, preferably from 2 to 10 megarads. An acceleration voltage lower than 100 kilovolts causes insufficiency of energy transmission, and a voltage higher than 1,000 kilovolts reduces the energy efficiency used in polymerization.If the absorbed dose is less than 0.5 megarads, the curing reaction is not sufficient to provide a strong magnetic layer, and if the dose is greater than 20 megarads, the efficiency of energy used for curing is reduced, or the web being irradiated generates heat, and in particular, the plastic base deforms.

The present invention is now described in greater detail by reference to the following examples and comparative examples and wherein all parts are by weight.

Example 1 Preparation of paint composition A for magnetic layer A composition having the formulation indicated below was agitated in a ball mill for 20 hours.

1,-Fe203 100 parts Nitrocellulose (viscosity RS=-21H) 10 parts Polyester polyurethane (MN 30,000) 5 parts Ester acrylate oligomer ("Aroniks M 6100" of Toagosei Chemical Co., Ltd.) 4 parts Diethylene glycol diacrylate 2 parts Butoxyethyl acrylate 4 parts Stearic acid 1 part Silicone oil [poly(dimethyl siloxane) ] 1 part Methyl ethyl ketone 250 parts Preparation of paint composition B for back coating A composition having the formulation indicated below was agitated in a ball mill for 10 hours.

Carbon black (av. particle size=0.1 ,um) 50 parts Calcium carbonate (av. particle size=0.08 #m) 100 parts Polyester polyurethane (mol. wt. ca. 20,000) 30 parts Nitrocellulose (viscosity=RS2H) 40 parts Ester acrylate oligomer ("Aroniks M 6100" of Toagosei Chemical Co., Ltd.) 10 parts Diethylene glycol diacrylate 10 parts Butoxyethyl acrylate 20 parts Butyl stearate 1 part Methyl ethyl ketone 500 parts Magnetic paint composition A was applied onto a 1 5 m thick polyethylene terephthalate base to a magnetic layer in a dry thickness of 5,us. The magnetic particles were oriented in a magnetic field, and the web was dried. The dried web was super-calendered to provide a smooth magnetic surface.Paint composition B was applied to the other side of the base to form a back coating in a dry thickness of 1 ,um, and the coating was dried. The web was irradiated with 5 megarads of electron beams at an acceleration voltage of 165 kilovolts and a beam current of 15 milliamperes. The resulting magnetic recording medium was referred to as Sample No. 1.

Example 2 A magnetic recording medium was prepared as in Example 1 except that a magnetic layer was formed from Paint composition C (whose formulation is indicated below) and subjected to super-calendering and heat treatment (600C, 48 hr.). Paint composition B was applied to the other side of the base to form a back coating which was irradiated with electron beams as in Example 1. The recording medium was referred to as Sample No. 2.

Paint composition C y-Fe203 100 parts Nitrocellulose (viscosity=RS 2 H) 15 parts Polyester polyurethane 5 parts Adduct of 1 mol trimethylolpropane and 3 mol tolylene diisocyanate 5 parts Stearic acid 1 part Butyl stearate 1 part Methyl ethyl ketone 250 parts Example 3 Example 1 was repeated except that Paint composition B was replaced by Paint composition D having the following formulation. The resulting magnetic recording medium was referred to as Sample 3.

Paint composition D Carbon black (av. particle size=0.1 #m) sold under the Registered Trademark 30 parts Silica/Aerosil (av. particle size=0.08 ym) 100 parts Ester acrylate oligomer ("Aroniks M 6100" of Toagosei Chemical Co., Ltd.) 40 parts Hexanediol diacrylate 15 parts 2-Ethylhexyl acrylate 15 parts Acrylic acid 20 parts Butyl stearate 1 part Methyl ethyl ketone 500 parts Example 4 Example 1 was repeated except that a magnetic cobalt film (2000 A thick) was formed on a polyethylene terephthalate base by using electron beams to condense the vapor of cobalt which was directed onto the PET base at an angle of 700 from the normal. The resulting magnetic medium was referred to as Sample No. 4.

Comparative Example 1 A magnetic recording medium was prepared as in Example 1 except that no back coating was formed. The medium was referred to as Sample No. C-1.

Comparative Example 2 t ilegnetic recording medium was prepared as in Example 1 except that Paint composition B was replaced by Paint Composition E having the formulation indicated below. The medium was referred ~o 8s Sample No. C-2.

Paint compcsition E Carbon black (av. particle size=0.1 ym) 50 parts Calcium carbonate (av. particle size=0.08 #m) 100 parts Polyester polyurethane (mol. wt. 20,000) 30 parts Nitrocellulose (viscosity=RS2H) 90 parts Butyl stearate 1 part Methyl ethyl ketone 500 parts The six samples of magnetic recording medium were subjected to the measurement of the degree of curl, "still" abrasion and electric characteristics.

(1) Degree of curl Each sample was slit to T inch wide and 2 mm long, set erect on a glass plate, and exposed to a warm and humid atmosphere (230C, 65% r.h.). The distance (a) by which the top of the curling tape lowered was measured.

(2) "Still" abrasion Each sample was slit to T inch wide and caused to run in a still mode on a VHS video tape recorder (HR 3600 of Victor Company of Japan). After one-hour running, the surface of the tape was checked for any abrasion.

(3) Electric characteristics To color signal-to-noise ratio for each sample was measured, using Sample No. 1 as a reference.

The results of the measurement are shown in Table 1 below.

Table 1 Sample Electric No. Curl (a) "still" abrasion characteristics 1 0.3 mm none +0 2 0.2 mm do -0.5dB 3 0.1 mm do +0.2dB 4 0.0 mm do C-i 2.0 mm Magnetic layer was -1.0 dB removed at the edges of O. 1 mm wide C-2 1.5 mm do -1.0dB From the data in Table 1, it can be seen that the back coating of the present invention effectively prevents a tape from curling. Further, since the back coating can prevent increasing of the friction between tape-guide poles and the back surface of the tape (i.e., opposite to the surface on which a magnetic layer is provided), the running tension of the tape is reduced, resulting in no abrasion of the magnetic surface.

Claims (12)

Claims

1. A method of making a magnetic recording medium, comprising forming on a non-magnetic support on one surface a magnetic coating composition and on the other (rear) surface a layer of a compound which is polymerizable with electron beams, and irradiating at least the rear surface with electron beams to cause polymerization of said compound.

2. A method as claimed in Claim 1 , wherein said compound is an unsaturated polyester having an acryloyl, acrylamido, alkyl, vinyl ether or vinyl thioether group.

3. A method as claimed in Claim 2, wherein the compound has an acryloyl or methacryloyl group at both ends of the chain.

4. A method as claimed in Claim 1,2 or 3, wherein the compound in the rear layer is coated admixed with a monomer containing an unsaturated -C=C- bond, in a weight ratio of said compound to the monomer of at least 2:8.

5. A method as claimed in any preceding claim, wherein said coating on the rear surface has a thickness of 0.1 to 5,us.

6. A method as claimed in any preceding claim, wherein the irradiation is at a voltage of 100 to 1,000 kilovolts.

7. A method as claimed in any preceding claim, wherein the adsorbed dose of electrons is 0.5 to 20 megarads.

8. A method as claimed in any preceding claim, wherein the support is made of polyethylene terephthalate.

9. A method as claimed in any preceding claim, wherein the thickness of the support is 1 to 50 ,um.

10. A method as claimed in any preceding claim, wherein the magnetic coating composition comprises a compound polymerizable with electron beams in which is dispersed ferromagnetic particles, and this composition is also irradiated with electron beams.

11. A method as claimed in Claim 1, substantially as hereinbefore described in any of Examples 1 to4.

12. A magnetic recording medium comprising a rear coating which has been polymerized by electron beams, when made by a method as claimed in any preceding claim.