Magnetic recording medium

Abstract: A magnetic recording medium comprises a substrate, and a magnetic recording layer composed of a coating composition comprising a binder resin and magnetic particles, the binder resin being a polyester having an iminosulfonyl group. The magnetic recording medium has excellent magnetic characteristics.

Description: The present invention relates to a magnetic recording medium which comprises a substrate and a magnetic recording layer composed of a coating composition comprising a binder resin and magnetic particles. With progressive miniaturization, weight reduction and recordable-time prolongation of video or audio magnetic recording and reproducing devices in recent years, magnetic recording media such as a magnetic tape and a magnetic disk have been strongly required to have a higher performance, namely to record at a higher recording density and to have a lowered noise level. Since the demand for video tapes having a better picture quality has been increasingly strong, the frequencies of carrier signals recorded are higher than in a conventional video tape. In other words, the carrier signals recorded are shifted to a short-wavelength region and, as a result, the magnetization depth from the surface of a magnetic tape becomes much smaller. Accordingly, efforts to keep and further improve the high output characteristics, in particular, the CN ratio of short-wavelength signals, have been made. For this purpose, it is necessary (1) to produce very fine magnetic particles, (2) to make the surface of the magnetic coating film smooth, (3) to make the magnetic coating film thin, and (4) to improve the dispersibility of the magnetic particles. This is described in, for example, DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER (1982), published by K.K. Sogo Gijutsu Kaihatsu Center, p. 312: "The condition for high-density recording in a coating type tape is that it is possible to keep the high output characteristics with respect to a short-wavelength signal at a low noise level. For this purpose, it is necessary that both the coercive force (Hc) and the residual magnetism (Br) are large, and that the thickness of the coating film is thin"; and Nikkei Electronics (1976), May 3, pp. 82 to 105: "As the physical quantities related to the SN ratio (CN ratio) of a luminance signal, there are average number of particles per unit volume, the dispersing state (dispersibility) and the smoothness of the surface thereof. If the smoothness and the dispersibility are constant, since the SN ratio becomes higher in proportion to the root of the average number of particles, the smaller volume and the higher packing density magnetic particles have, the more advantageous." As described above, in order to improve the high output characteristics with respect to short-wavelength signals, not only are magnetic particles required to have a high dispersibility but also the magnetic coating film is required to be thin. When the magnetic coating film is made thin, it is essential to improve the packing property of the magnetic particles in order to keep or improve the other properties. For this purpose, it is necessary to reduce the amount of components other than the magnetic particles, in particular the dispersant and lubricant, as much as possible, or in some cases to dispense with any such component. Thus, the development of a binder having an excellent resin adsorptivity to the magnetic particles is strongly demanded. On the other hand, it is known that the noise level of a magnetic recording medium has a tendency to be lowered as the particle size of the magnetic particles used becomes smaller, namely, the specific surface area becomes larger, as shown, for example, in Fig. 3 on page 27, lines 23 to 29 in the REPORTS OF TECHNICAL RESEARCHES OF THE INSTITUTES OF ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN MR 81-11. Recently, the particle size of magnetic particles has been increasingly reduced. In this report, Fig. 3 shows the relationship between the particle size of Co-coated acicular maghemite particles and the noise level, and it is clear that as the particle size becomes smaller (the specific surface area of the particles becomes larger), the noise level lowers linearly. It is known, however, that as the particle size of the magnetic particles becomes smaller, the dispersibility of the magnetic particles in a vehicle is lowered, resulting in deterioration of the orientation property and the packing property in the coating film. As a method of improving the dispersibility of magnetic particles in a vehicle, adding various kinds of dispersants, using various dispersing machines, improving various properties of the magnetic particles, and improving various properties of a binder resin (varying the kinds of binder resin) are known. The kind of binder resin is an important factor for influencing the dispersion of magnetic particles in a vehicle. This is described in DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER, p. 65: "The influence of a binder on the dispersion of magnetic powder is large. For example, even if the dispersing machine used has a high dispersion efficiency, in the case where the dispersibility of the binder is low, the magnetic particles in the coating do not disperse ... The dispersibility is greatly different depending upon the kinds of a binder." As a conventional binder resin, nitrocellulose, a polyvinylidene chloride, a vinyl chloride-vinyl acetate copolymer, a styrene-butadien copolymer, a polymethyl methacrylate, a vinylidene chloride-methyl methacrylate copolymer, a polyurethane and an epoxy resin are generally used. As a magnetic recording medium with magnetic particles having an improved dispersibility in a vehicle, a magnetic recording medium having a magnetic recording layer which contains fine magnetic particles uniformly dispersed in a polyester which has a reducing viscosity of not less than 0.2 and which is produced by using a dicarboxylic acid having a sulfonic acid metal base as a copolymerizing component in an amount of 0.3 to 7.0 mol% based on the total carboxylic acid component, is known [JP-A-54-28603 (1979)]. The strongest demand in the manufacture of a magnetic recording medium is an improvement of the dispersibility of magnetic particles in a vehicle and an increase in the resin content adsorbed to the magnetic particles. Even if the polyester produced by using as a copolymerizing component a dicarboxylic acid containing a sulfonic acid metal base [described in JP-A-54-28603 (1979)] is used, the dispersibility of the magnetic particles in a vehicle cannot be said to be satisfactory and the resin adsorption is small. Furthermore, the method of improving the dispersibility of the magnetic particles in a vehicle by adding a large amount of dispersant is disadvantageous in that it is impossible to improve the packing property of the magnetic particles and in that the dispersant remaining in the coating film (magnetic recording layer) influences the surface property of the coating layer such as the viscosity, which may lead to defective operation due to a change in the ambient temperature. Therefore, a magnetic recording medium and a method of producing the same which allows the amount of dispersant used in the manufacturing process to be reduced as much as possible, or in some cases dispensed with, due to an excellent dispersibility of magnetic particles in a vehicle and the excellent packing property of the magnetic particles in the coating film (magnetic recording layer) which is brought about by the large resin adsorption of the magnetic particles, is strongly demanded. JP-A-59-172149 discloses a magnetic recording medium wherein a magnetic powder is coated together with a binder resin on a substrate. The magnetic powder is subjected to a surface treatment with a copolymerized polyester resin containing both a sulfonic acid metallic salt group and a nitrogen atom. The binder resin may be any known binder resin. As a result of the present inventors' studies on a magnetic recording medium which allows the amount of dispersant used in the manufacturing process to be reduced as much as possible, or in some cases dispensed with, it has been found that when a substrate is coated with a coating composition containing a polyester having an iminosulfonyl group as a binder and magnetic particles so as to form a magnetic recording layer thereon, the dispersibility of the magnetic particles in a vehicle is excellent and the packing property of the magnetic particles in the resultant coating film (magnetic recording layer) is also excellent. The present invention provides a magnetic recording medium comprising a substrate and a magnetic recording layer composed of a coating composition comprising a binder resin and magnetic particles, the binder resin being a polyester having an iminosulfonyl group. Fig. 1 of the accompanying drawing shows the relationship between the ratio of various binder resins added to acicular Co-coated maghemite particles having a specific surface area of 40 m²/g and the resin adsorption of the Co-coated maghemite particles. As the magnetic particles in the present invention, any magnetic iron oxide particles such as maghemite particles; magnetite particles; Berthollide compounds ( FeO x ·Fe₂O₃, 0 < x < 1); these magnetic iron oxide particles containing an element other than Fe, e.g., Co, B, Al, Ni, P, Si and/or Zn; these magnetic iron oxide particles coated with, for example, Co; iron-based metal magnetic particles; iron-based alloy magnetic particles containing an element other than Fe, e.g., Co, Ni, Al, P and/or B; plate-like Ba ferrite particles; and plate-like composite ferrite particles obtained by adding a bivalent metal such as Co, Ni, and/or Zn, and a tetravalent metal such as Ti, Sn and/or Zr as a coercive reducing agent to the plate-like Ba ferrite particles may be used. The shape of the magnetic particles is not specified and may be acicular, spindle-like, cubic or plate-like. The size of the magnetic particles is not restricted but it is ordinarily not more than 0.5 µm, preferably not more than 0.3 µm, more preferably 0.05 to 0.20 µm. The specific surface area of the magnetic particles is not particularly restricted, but is preferably not less than 30 m²/g, more preferably 40 to 70 m²/g. As a binder resin in the present invention, a polyester having an iminosulfonyl group is used. The polyester is preferably an unsaturated polyester obtainable by condensing a dicarboxylic acid having a double bond in a molecule such as maleic acid or an anhydride thereof, and fumaric acid with a dihydric alcohol such as ethylene glycol and propylene glycol. A dicarboxylic acid having an iminosulfonyl group may be used as a condensation ingredient. The average number molecular weight of the polyester is preferably 10,000 to 40,000, more preferably 10,000 to 20,000. The melt viscosity at 200°C of the polyester is preferably 1,000 to 50,000 poise, more preferably 2,000 to 42,000 poise. The glass transition temperature of the polyester is preferably 20 to 70°C, more preferably 25 to 60°C. Examples of the polyester are AQ29S, AQ55S and AQ38S produced by Eastman Kodak. The weight ratio of the magnetic particles to the binder resin in the magnetic recording layer is preferably 75:25 to 92:8, more preferably 80:20 to 85:15. Ordinarily used additives such as a lubricant, a polishing agent and an antistatic agent may be added to the coating composition. The content of the additives in the recording layer is preferably not more than 7 wt%, more preferably not more than 5 wt%. As the material for the substrate in the present invention, a synthetic resin film such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamidoimide, polyimide and polysulfone, a metal foil or plate such as aluminium and stainless steel and various kinds of papers, which are now generally used in the manufacture of a magnetic recording medium, are usable. A magnetic recording medium according to the present invention can be obtained by forming a coating film on a substrate by coating the substrate with a coating composition containing the binder resin and magnetic particles by an ordinary method. Fig. 1 shows the relationship between the ratio of various binder resins added to acicular Co-coated maghemite particles having a specific surface area of 40 m²/g, a major axis diameter of 0.18 µm, an aspect ratio (major axis/minor axis) of 6.0, a Co content (Co/total) of 4.2 wt%, a coercive force of 840 Oe and a saturation magnetization of 76.8 emu/g, and the resin adsorption of the Co-coated maghemite particles. In Fig. 1, the straight line A shows the theoretical value of the resin adsorption when the total amount of binder resin added is adsorbed to the Co-coated maghemite particles, and the straight lines B to D show the values when a polyester having an iminosulfonyl group (AQ55S, trade name, produced by Eastman Kodak), a polyester produced by using a dicarboxylic acid having a sulfonic acid metal base as a copolymerizing component (Vyron-280, trade name, produced by Toyo Spinning Co., Ltd.), and a vinylidene chloride resin containing sodium sulfonate (VAGH, trade name, produced by U.C.C., Ltd.), respectively, were used. The resin adsorption in Fig. 1 was measured without adding any dispersant, as will be described later in detail. Since the resin adsorption was as large as possible and approximate to the theoretical value means that it is possible to reduce the amount of binder resin added so as to obtain the same resin adsorption. In other words, it means that the amount of magnetic particles packed per unit volume is large. As shown in Fig. 1, in the case of using a polyester having an iminosulfonyl group in accordance with the present invention, a resin adsorption approximate to the theoretical value is obtained even without adding a dispersant. This means that the dispersibility of the magnetic particles in a vehicle is excellent and that the packing property of the magnetic particles in the coating film (magnetic recording layer) is excellent due to the high resin adsorption of the magnetic particles. It is possible to reduce the amount of dispersant or dispense with the dispersant in the manufacture of a magnetic recording medium. Although Fig. 1 shows the resin adsorption using Co-coated maghemite particles as the magnetic particles a similar trend is observed for other magnetic particles such as iron-based metal or iron-based alloy magnetic particles and plate-like Ba ferrite particles. The resin adsorption to the magnetic particles in the present invention is preferably not less than 1.65 wt%, more preferably not less than 1.75 wt%. The squareness ratio (Br/Bm) of the magnetic recording medium is preferably not less than 0.850. In the case of using acicular magnetic particles, the degree of orientation thereof is preferably not less than 2.70, more preferably not less than 2.90. The resin adsorption to the binder resin used in the present invention is larger than that to a conventional binder resin. The magnetic recording medium of the present invention has a superior residual flux density, squareness ratio (Br/Bm) and degree of orientation to that in which the magnetic recording layer contains a conventional binder resin. In the manufacturing process of the magnetic recording medium of the present invention, the amount of dispersant can be reduced, or in some cases, no dispersant is necessary, because the dispersibility of the magnetic particles in a vehicle is excellent and the packing property of the magnetic particles in the coating film (magnetic recording layer) is excellent due to the high resin adsorption to the magnetic particles. In addition, when fine magnetic particles having a specific surface area of not less than 30 m²/g, in particular, not less than 40 m²/g are used for a magnetic recording medium according to the present invention, the orientation property of the magnetic particles in the coating film (magnetic recording layer) and the packing property of the magnetic particles are ever better, so that the magnetic recording medium produced is more suitable as a magnetic recording medium for high-density recording and for recording at a low noise level. [Examples] The present invention is further described hereinbelow in the following Examples and Comparative Examples. The major axial diameter and the aspect ratio (major axis : minor axis) of the particles in each of the following Examples and Comparative Examples are expressed by the average values measured in electron microphotographs and the specific surface area are expressed by the value measured by a BET method. The magnetic characteristics of the magnetic particles were measured using a vibrating sample magnetometer (VSM-3S-15, produced by Toei Kogyo K.K.) and applying an external magnetic field of up to 10 KOe. The resin adsorption based on the weight of the magnetic particles was calculated from the difference between the amount of the resin in the resin liquid having the following composition and the amount of the resin in the supernatant liquid of the coating obtained by charging 20 g of magnetic particles, 48 g of the resin liquid and 50 g of 2-mm glass beads into a 140 ml glass bottle, dispersing them with a paint conditioner for 60 minutes and subjecting the mixture to centrifugal separation. The amounts of resins were measured as the weights of the nonvolatile contents in the liquids. Composition of Resin Liquid Resin/particle 0.7 wt% 1.4 wt% 2 wt% Solid resin content 0.14 g 0.28 g 0.40 g Cyclohexanone 47.86 g 47.72 g 47.60 g In the above-described experiments shown in Fig. 1, the resin adsorptions were plotted when the resin/particle ratios in the resin liquid were 0.7 wt%, 1.4 wt% and 2 wt%, respectively. In the following Examples and Comparative Examples, the resin adsorptions are shown when the resin/particle ratio was 2 wt%. The coercive force, the residual flux density and the squareness ratio in the case of manufacturing a magnetic recording medium produced by using plate-like Ba ferrite fine particles as the magnetic particles were measured in the perpendicular direction of the magnetic recording medium with the magnetic field oriented perpendicularly to the recording surface. The coercive force was expressed by the measured value, and the residual flux density and the squareness ratio were expressed by the values obtained by the diamagnetic field correction of the measured values. Example 1 A binder resin, AQ55S produced by Eastman Kodak, and Co-coated acicular γ-Fe₂O₃ particles (Co/total amount = 4.21 wt%) having a major axial diameter of 0.18 µm, an aspect ratio (major axis : minor axis) of 6.0, a coercive force of 840 Oe, a saturation magnetization of 76.8 Am²/kg (76.8 emu/g) and the (AQ55S) resin adsorption of 1.89 wt% were used. 100 parts by weight of the above-described Co-coated acicular γ-Fe₂O₃ particles, 20 parts by weight of AQ55S produced by Eastman Kodak, 60 parts by weight of cyclohexanone, 1 part by weight of Al₂O₃ powder and 2 parts by weight of carbon black were kneaded by a kneader for 90 minutes. Thereafter, 90 parts by weight of cyclohexanone was added to the kneaded material to dilute it, and mixed and dispersed by a sand grinder for 3 hours. 80 parts by weight of cyclohexanone was further added to the mixed and dispersed material and mixed for 30 minutes. The mixture was filtered and the filtrate was mixed with 3 parts by weight of Coronate L (trifunctional low-molecular isocyanate compound, produced by Nihon Polyurethane Kogyo) to obtain a magnetic paint. The magnetic paint was applied to a polyester film having a thickness of 12 µm and dried to produce a magnetic layer 4 µm thick. The polyester film was calendered and then cut into a width of 12.7 mm. The magnetic characteristics of the magnetic recording medium obtained are shown in Table 1. Examples 2 to 4, Comparative Examples 1 to 8 Magnetic recording media were produced in the same way as in Example 1 except that magnetic particles and binder resins shown in Table 1 were used in the manufacture of the magnetic recording media. The magnetic characteristics of the magnetic recording media obtained are shown in Table 1. When AQ29S or AQ38S was used as a binder resin instead of AQ55S, similar results were obtained.

Claims: A magnetic recording medium comprising a substrate and a magnetic recording layer composed of a coating composition comprising a binder resin and magnetic particles, said binder resin being a polyester having an iminosulfonyl group. A magnetic recording medium according to claim 1, wherein the weight ratio of said magnetic particles to said binder resin is from 75:25 to 92:8. A magnetic recording medium according to claim 1 or 2, wherein the average number molecular weight of said polyester is 10,000 to 40,000. A magnetic recording medium according to any one of the preceding claims, wherein said magnetic recording layer further comprises not more than 7 wt% of additives A magnetic recording medium according to any one of the preceding claims, wherein the specific surface area of said magnetic particles being not less than 30 m²/g. A magnetic recording medium according to any one of the preceding claims, wherein the magnetic particles are maghemite particles; magnetite particles; particles of Berthollide compounds ( FeO x ·Fe₂O₃, 0 < x < 1); magnetic iron oxide particles containing an element selected from Co, B, Al, Ni, P, Si and/or Zn; magnetic iron oxide particles coated with Co; iron-based metal magnetic particles; iron-based alloy magnetic particles containing Co, Ni, Al, P and/or B; plate-like Ba ferrite particles; or plate-like composite ferrite particles obtained by adding a bivalent metal selected from Co, Ni and/or Zn and a tetravalent metal selected from Ti, Sn and/or Zr as a coercive reducing agent to the plate-like Ba ferrite particles. A magnetic recording medium according to any one of the preceding claims, wherein the polyester is an unsaturated polyester obtained by condensing a dicarboxylic acid or anhydride thereof having a double bond in the molecule with a dihydric alcohol. A magnetic recording medium according to claim 7, wherein the dicarboxylic acid or anhydride thereof is maleic acid, maleic anhydride or fumaric acid and the dihydric alcohol is ethylene glycol or propylene glycol. A magnetic recording medium according to any one of the preceding claims, wherein the melt viscosity at 200°C of the polyester is from 100 to 5,000 Pas (1,000 to 50,000 poise) and the glass transition temperature of the polyester is from 20 to 70°C. A magnetic recording medium according to any one of the preceding claims, wherein the substrate is a synthetic resin film selected from a polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamidoimide, polyimide or polysulfone film; a metal foil or plate composed of aluminium or stainless steel; or a suitable paper.

Inventors: HAYASHI KAZUYUKI, ITAMOCHI HIROKO, IWASAKI KEISUKE, NISHIMURA AKIHIRO, TANAKA YASUYUKI, HAYASHI, KAZUYUKI, ITAMOCHI, HIROKO, IWASAKI, KEISUKE, NISHIMURA, AKIHIRO, TANAKA, YASUYUKI
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1990
Main classification: G11B5
IPC codes: G11B5, C09D5, C08G63, C09D167