Magnetic recording medium

Abstract: This invention provides a magnetic recording medium having excellent corrosion resistance and mobility, and a process for producing such recording medium. More particularly, the invention provides a magnetic recording medium having a ferromagnetic metallic layer provided on a non-magnetic substrate, which comprises an anticorrosive deposited on said ferromagnetic metallic layer containing a benzotriazole derivative as a main component and a magnetic recording medium having provided on a substrate a ferromagnetic metallic layer containing cobalt as a main component, a passivated cobalt layer on said ferromagnetic metallic layer and an anticorrosive layer on said passivated cobalt layer, said anticorrosive layer comprising at least one anti­ corrosive selected from benzothiazole derivatives, benzoimidazole derivatives having at least one of amino group, nitro group, chlorine and mercapto group as substituent in the benzoimidazole skeleton, benzotriazole, compounds containing nitrosonaphthol as a main component, oxime compounds, phytic acid and its derivatives, quinaldinic acid and its derivatives, quinoline com­ pounds, naphthaldehyde and its derivatives and a process for producing thereof.

Description: This invention relates to a magnetic recording medium. More particularly, the invention relates to a magnetic recording medium having high corrosion resistance and excellent runnability (capability to run smoothly). Description of the Prior Art As the anticorrosives used for the purporse of improving corrosion resistance of ferromagnetic metallic films of magnetic recording media, there have been proposed aromatic carboxylic acids having amino groups (Japanese Patent Application Kokai (Laid-Open) No. 58-194131), cupferrons (Japanese Patent Application Kokai No. 58-194138), polyamines (Japanese Patent Application Kokai No. 59-3724), benzotriazoles (Japanese Patent Application Kokai No. 58-194135), and some of them have been put to practical use. JP-A-59 19 235 discloses a magnetic tape comprising a metallic thin film, wherein a halogen element and a rust preventing agent, for example benzotriazole, are applied on the surface of the metallic film. JP-A-61 131 219 discloses a magnetic recording medium, wherein a first inorganic protective layer and a second inorganic protective film are provided on a ferromagnetic metallic thin film layer. JP-A-60 89 815 relates to a magnetic recording medium by forming a thin ferromagnetic metallic film on a non-magnetic substrate and sticking a rust preventive agent which consists essentially of a compound having an SH group, for example benzothiazole, and is added with Al ion to said film. There are various environmental factors, such as humidity, sulfur dioxide (sulfurous acid gas), nitrogen oxides, which are causative of corrosion of said ferromagnetic metallic films. Any of the above-mentioned anticorrosives does not have an all-round anticorrosive potency against said corrosive factors, and it can hardly be said that they show a satisfactory anticorrosive effect in use under the present-day complex corrosive environments. In case the ferromagnetic metallic film is covered with a higher-valent oxide which has a relatively high anticorrosiveness in itself, use of an anticorrosive having a reducing effect could prove detrimental to the protection of metallic film as such anticorrosive has an aptitude to break the anticorrosive oxide layer. In the prior art, however, little regard has been paid to such matter in the selection of anticorrosive. Thus, various problems such as mentioned above have remained unsolved in the prior art regarding corrosion protection of ferromagnetic metallic films. As a result of studies on the corrosion mechanism of cobalt-based ferromagnetic metallic films and the method for preventing corrosion thereof, the present inventors had previously found that when the surface of such ferromagnetic metallic film is mildly oxidized in the presence of oxygen and moisture to form a passivated cobalt layer which is essentially a cobalt oxyhydroxide layer containing three- or higher-valent cobalt ions, remarkable improvements can be attained in corrosion resistance, running stability and durability (Japanese Patent Kokai (No. 62-275316). It was found, however, that the provision of such passivated cobalt coat alone is insufficient to cope with the deterioration of corrosion resistance which would be caused when such cobalt layer is damaged due to takeup of dust or abnormal sliding contact against the magnetic head during running of the tape in actual use thereof. Summary of the Invention The present invention has for its object to solve said problems in the prior art and to provide a magnetic recording medium with improved corrosion resistance or a magnetic recording medium also improved in its runnability or the ability to run smoothly by incorporating means for countervailing the possible deterioration of such runnability due to the anticorrosive measures. Said object can be attained by a magnetic recording medium having a ferromagnetic metallic layer provided on a non-magnetic substrate, which comprises an anticorrosive deposited on said ferromagnetic metallic layer containing a benzotriazole derivative as a main component, characterized in that said benzotriazole derivative has at least one of amino group, nitro group, chlorine or mercapto group as substituent in the benzotriazole skeleton or is a salt of benzotriazole and a carboxylic acid. Further studies by the present inventors for attaining said object have revealed the fact that an even greater improvement can be realized by providing a passivated cobalt layer on the ferromagnetic metallic film containing cobalt as a main component and further providing on said passivated cobalt layer a layer of at least one anticorrosive selected from said benzotriazole derivatives. For instance, when a layer of anticorrosive was provided on said passivated cobalt film by coating it with one or more of the above mentioned anticorrosives, it was found to be possible to prevent deterioration of corrosion resistance should it be caused by damage to said passivated cobalt layer due to takeup of dust, abnormal friction against the magnetic head or other causes during running of the tape in actual use, thus realizing a remarkable improvement of corrosion resistance and durability of the ferromagnetic metallic film. It was also found that corrosion resistance and durability can be further improved by providing a lubricant layer on said anticorrosive layer formed on the passivated coabalt layer. Said anticorrosives using said benzotriazole derivatives as main constituent are a so-called cathode inhibitor containing nitrogen. Their anticorrosive effect is based mainly on their inhibitory action against the hydrogen-generating reaction in the corrosion reactions caused by moisture and corrosive gases in the atmosphere. Cupferrons and polyamines mentioned above can also serve as a cathode inhibitor. The difference between cupherrons or polyamines and said anticorrosives in their anticorrosive effect under the same corrosive conditions is considered due to the difference in their adsorptive affinity to ferromagnetic metals (weak adsorptive affinity provides little anticorrosive effect, but too strong adsorptive affinity rather helps to promote corrosion) as well as the differences in molecular structure (such as difference in covering rate due to stereo hindrance). The prominent anticorrosive effect of said benzotriazole derivatives owes to their advantages in these points. Incorporation of said groups (amino group, nitro group, chlorine, or mercapto group) as substituent has the effect of controlling the electron donating activity of the adsorbed atoms (nitrogen). Said benzotriazole derivatives have a greater protective effect for the higher-valent oxide layer on the ferromagnetic metallic film surface than benzotriazole itself and provide a greater improvement of corrosion resistance than the oxide layer per se. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of the vacuum evaporator used in this invention. FIGS. 2 and 3 are the partial enlarged sectional views of the magnetic tapes obtained according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The magnetic recording medium according to this invention will hereinafter be described with reference to the examples thereof along with the comparative examples. EXAMPLE 1 On a 9 µm thick polyethylene terephthalate film substrate was deposited Co-Ni (80/20) to a thickness of 1.5 x 10⁻⁵ cm (1.500 Å) in an oxygen atmosphere to form a thin ferromagnetic metallic layer. This metallic layer was then coated with a 0.05 wt% methyl isobutyl ketone solution of benzotriazole stearate and dried to prepare a specimen. EXAMPLE 2 The procedure of Example 1 was followed except that 5-nitro-benzotriazole was used instead of benzotriazole stearate to prepare a similar specimen. EXAMPLE 3 The procedure of Example 1 was followed except that a passivated cobalt layer was formed on the surface of said ferromagnetic metallic layer before coating methyl isobutyl ketone solution to prepare another specimen. Said passivated cobalt layer was formed by placing the Co-Ni deposited film in another vessel, evacuating the vessel to 2.66 kPa (20 Torr), then charging 101.3 kPa (1 atm.) oxygen gas into it and keeping said film under this condition for 3 days. COMPARATIVE EXAMPLE 1 A specimen was prepared by following the same procedure as Example 1 except that no coating of a methyl isobutyl ketone solution of benzotriazole stearate was made. COMPARATIVE EXAMPLE 2 Another specimen was prepared by following the same procedure as Example 1 except that a cupferron was used instead of benzotriazole stearate. COMPARATIVE EXAMPLE 3 Still another specimen was prepared by following the same procedure as Example 3 except that no coating of a methyl isobutyl ketone solution of benzotriazole stearate was made. Said six specimens were subjected to an accelerated corrosion tests to compare their corrosion resistance. In these tests, 2 items were used to do this comparison, that is the reduction rate of saturation magnetization of each specimen after it has been kept in a 60°C and 90% RH atmosphere for one week, and the sulfur content (atom %) in the magnetic layer of each specimen after it has been kept in a 35°C and 75% RH atmosphere with 1.0 ppm of SO₂, 0.5 ppm of H₂S and 1.0 ppm of NO₂ for 100 hours. The results are shown in Table 1. Specimen Reduction rate of saturation magnetization (%) Sulfur content (atom%) Example 1 9.3 4.6 Example 2 8.9 5.0 Example 3 4.1 5.8 Comp. Example 1 31.2 55.6 Comp. Example 2 23.2 48.9 Comp. Example 3 5.6 54.8 The results of Table 1 attest to far higher corrosion resistance of the specimens prepared according to this invention (Examples 1-3) than the comparative specimens either when the specimens were kept in a high-temperature and high-humidity condition or kept in a corrosive gas atmosphere. As regards the build-up of anitcorrosive, it has been experimentally confirmed that a similar effect is produced when the build-up is in the range of 0.5 to 100 mg/m², the most preferred range being 1 to 50 mg/m². In the above examples, there were taken up the cases where a salt of benzotriazole with a carboxylic acid and a benzotriazole derivative having a nitro group as substituent were used as anticorrosive, but the same result can be obtained when using benzotriazole derivatives having amino group, chlorine or mercapto group as substituent. As described above, in accordance with this invention, an anticorrosive principally composed of a benzotriazole derivative having at least one of amino group, nitro group, chlorine and mecapto group as substituent or a salt of benzotriazole with a carboxylic acid is deposited on the ferromagnetic metallic layer formed on a non-magnetic substrate to provide a magnetic recording medium which is free of the problems of the prior art and shows an amazingly high corrosion resistance in use under the complex corrosive environments. The invention will be further described below regarding its other significant embodiments. In accordance with a preferred embodiment of this invention, the anticorrosive layer is formed on a passivated cobalt layer. This can be achieved, for instance, by dissolving the above specified anticorrosive in a proper solvent such as toluene, methyl isobutyl ketone, methyl ethyl ketone, cyclohexane, ethyl acetate, ethyl alcohol, isopropyl alcohol, Freon, tetrahydrofuran, dimetylformamide or dioxane, and dipping in this solution the ferromagnetic metallic film having formed thereon said passivated cobalt layer, or by applying said solution (by atomizing or other means) onto said ferromagnetic metallic film. Also, the anticorrosive may be deposited by vacuum evaporation or other means on the ferromagnetic metallic film having formed thereon said passivated cobalt layer. In other methods, the anticorrosive, instead of applying it directly onto the passivated cobalt layer, may be coated on the back coat or impregnated in the back coat provided on the back side of the substrate having a ferromagnetic metallic layer on its upper side, and the anticorrosive on or in this back coat may be transferred to the passivated cobalt layer when this laminate is wound up. The anticorrosives used in the present invention have quite a remarkable anticorrosive potency, and they never cause any change of quality or deterioration of the passivated cobalt layer when they are applied on said layer. Nor do they affect wear resistance of the passivated cobalt layer and ferromagnetic metallic layer. Therefore, when an anticorrosive surface layer is formed on the passivated cobalt layer, an admirable corrosion protective effect is provided. Even if the passivated cobalt layer should be demaged due to takeup of dust or abnormal frictional contact against the magnetic head during running of the tape, said anticorrosive layer can effectively control or minimize the deterioration of corrosion resistance caused by said damage, allowing maintenance of high enough corrosion resistance and durability of said film. Further, this type of anticorrosive gives no adverse effect to the lubricant layer which may be formed thereon, so that when a lubricant layer is formed on the anticorrosive layer, the lubricant can perform its activity to the fullest measure, providing a further improvement of corrosion resistance and durability. The thickness of the layer of anticorrosive formed on the passivated cobalt layer should be in the range of 2 to 20 nm (20 to 200 Å). When this thickness is less than 2 nm (20 Å), the desired effect of the layer may not be obtained, while when the thickness is greater than 20 nm (200 Å), a too large spacing loss may be caused to give an ill effect to the electro-magnetic conversion characteristics. To form a passivated cobalt layer on the ferromagnetic metallic film, the surface of the ferromagnetic metallic film mainly composed of cobalt is subjected to a moisture absorption treatment and a dehydrating-oxidization treatment either simultaneously or separately. In case the both treatments are conducted simultaneously, since the contradictory steps of moisture absorption and dehydration are performed at the same time, the operations must be carried out at a low humidity below 50% RH under heating at a temperature above 40°C. In this case, therefore, the operations tend to become difficult to control. In case the two treatments are carried out separately, such difficulty of control is eliminated. The moisture absorption treatment can be achieved by exposing the ferromagnetic metallic layer to a moist atmosphere with a vapor pressure above 0.133 Pa (10⁻³ Torr). In case said layer is exposed under saturated vapor pressure, the moisture absorption is adjusted in the ensuing dehydration and drying step. However, considering the ordinary workability and other factors, this treatment is preferably performed by leaving said metallic layer in an atmosphere of 30-80% RH and room temperature for 0.1 to 24 hours. In the dehydration and oxidation treatment, in order to accelerate the desired oxidation reaction, it is necessary to drive moisture out of the oxidation reaction system. It is therefore desirable to subject a ferromagnetic metallic layer to the oxidation reaction in a closed system and in a dry state. For performing the oxidation reaction in a dry state, such reaction may be carried out in vacuo under an atmosphere mostly composed of oxygen such as pure oxygen gas or in the presence of a dessicant. Such reaction may be also conducted by passing a heated oxygen gas. In case of carrying out the reaction in a closed system by using, for example, a closed vessel, this vessel is once evacuated to a vacuum below 13.33 kPa (100 Torr), preferably below 6.66 kPa (50 Torr), and then the reaction is conducted in an atmosphere containing 50% or more of oxygen gas by partial pressure or under pressure below 202.65 kPa (2 atm). It is not necessary to supply 100% oxygen gas in the course of oxidation reaction; it is possible to use a mixed gas containing 50% or more of oxygen gas by partial pressure, the rest consisting of inert gases such as Ar gas, or N₂ gas. With said moisture absorption treatment and dehydrating/oxidation treatment, a passivated cobalt layer which is essentially a cobalt oxyhydroxide layer containing three- or higher-valent cobalt ions such as expressed by the chemical formula Co₃O₄·nH₂O or Co₂O₃·nH₂O is formed on the ferromagnetic metallic film. This passivated cobalt layer serves as protecting barrier for the ferromagnetic metallic layer against corrosion. So the corrosion resistance runnability and durability of the magnetic recording medium are greatly improved. As described above, the magnetic recording medium is greatly improved in durability and corrosion resistance by providing an anticorrosive layer on the passivated cobalt layer provided on the ferromagnetic metallic layer. When a lubricant layer is further formed on said anticorrosive layer, the coefficient of friction is reduced owing to the lubricating effect of said lubricant layer, providing an even greater improvement of wear resistance. Such lubricant layer can be formed in various ways. For instance, a lubricant is dissolved in a proper solvent such as toluene, methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, ethyl acetate, tetrahydrofuran, dimethylformamide or dioxane, and said anticorrosive layer is dipped in this solution, or said solution is applied or atomized onto said anticorrosive layer. Also, a lubricant may be deposited by vacuum evaporation on the anticorrosive layer. Such lubricant layer formed on said anticorrosive layer in the manner described above can well display its lubricating effect to reduce the coefficient of friction, realizing a further improvement of wear resistance. The lubricant used in this invention may be aliphatic, fluorine, silicone and hydrocarbon type, all of which can be favorably used for the purpose of this invention. Aliphatic lubricants include fatty acids and metal salts thereof, fatty acid esters, fatty acid amides or aliphatic alcohols. Preferred examples of fatty acids are lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid. As the metal salts of fatty acids, there can be favorably used magnesium salt, aluminum salt, lithium salt, sodium salt, calcium salt, cobalt salt, zinc salt, barium salt and lead salt of said fatty acids. Aliphatic acid esters include butyl stearate, octyl myristinate, monoglyceride stearate, monoglyceride palmitate or monoglyceride oleate. As for fatty acid amides, caproic acid amide, capric acid amide, lauric acid amide, palmitic acid amide, behenic acid amide, oleic acid amide, linoleic acid amide or methylenebisstearic acid amide can be favorably used. Stearyl alcohol and myristyl alcohol are preferably used as aliphatic alcohols. Chlorides such as trimethylstearylammonium chloride or stearoyl chloride, and amines such as stearylamine, stearylamine acetate or stearylamine hydrochloride, are also usable as aliphatic lubricant. The fluorine lubricants suited for use in this invention include trichlorofluoroethylene, perfluoropolyether, perfluoroalkylpolyether or perfluoroalkylcarboxylic acids. Among the commercial products of this type of lubricant are Daifluoil #20 by Daikin Corp., Krytox M, Krytox H and Vydax AR by Du Pont, and Fomblin Z, Fomblin Z DOL by Montedison, and Fomblin V84/20 by Hoechst. The silicone lubricants usable in this invention include silicone oil or modified silicone oil. Paraffin, squalane or wax, can be suitably used as hydrocarbon lubricant in this invention. The thickness of the lubricant layer formed by using said type of lubricant is preferably not less than 2 nm (20 Å) for producing the enough effect of lubrication to noticeably improve wear resistance. It should be noted, however, that when the total thickness of the lubricant layer and the anticorrosive layer formed on said passivated cobalt layer exceeds 100 nm (1,000 Å), a large spacing loss may be caused to deteriorate the read-write characteristics. Therefore, the lubricant layer thickness should be so selected that the total thickness of both lubricant and anticorrosive layers won't exceed 100 nm (1,000 Å). The magnetic recording medium of this invention may have, if necessary, a back coat on the underside of the substrate which has on its upper side a ferromagnetic metallic film. A coating material for such back coat is prepared by mixing and dispersing a filler such as red iron oxide, barium sulfate, calcium carbonate or carbon black along with a binder and an organic solvent, and such coating material is applied to the back side of the substrate and dried to form a back coat. Various types of commonly used additives such as dispersant, lubricant, abrasive, or antistatic agent, may be properly added in said coating material. The ferromagnetic metallic layer is preferably made of Co or a Co alloy such as Co-Cr alloy, Co-Ni alloy, Co-P alloy, Co-Fe alloy or Co-Fe-Cr alloy, in which Co is the primary constituent. Such Co-based magnetic metallic layer is formed on the substrate film by such means as vacuum deposition, ion plating, sputtering and plating. The magnetic recording media provided according to this invention include the magnetic tapes and magnetic discs using a synthetic resin film such as polyester film or polyimide film, as base; magnetic discs and magnetic drums using as base a disc or drum formed from a synthetic resin film, aluminum plate or glass plate; magnetic cards, and all other types of recording media having a structure designed to contact slidingly with the magnetic head. REFERENCE EXAMPLE 1 By using a vacuum deposition apparatus shown in FIG. 1, a 12 µm thick polyester film 1 was delivered out from a stock roll 3 so that said film would move along the periphery of a cylindrical can 4 and be wound up on a take-up roll 5, all said elements being set in a vacuum chamber 2. Also, a cobalt-nickel alloy (80:20 by atomic ratio) 7 was set in a ferromagnetic material evaporation source 6 disposed at the bottom of said vacuum chamber 2. Then the vacuum chamber 2 was evacuated to 0.00133 Pa (5 x 10⁻⁵ Torr) by an exhaust system 8 connected to said vacuum chamber 2, and oxygen gas was introduced from an oxygen gas supply pipe 10 passed between said cylindrical can 4 and a deposition preventing plate 9 disposed immediately therebelow. Cobaltnickel alloy 7 in the ferromagnetic material evaporation source 6 was heated and vacuum deposition was done in the oxygen atmosphere of 0.0199 Pa (1.5 x 10⁻⁴ Torr), thus forming a 140 nm (1,400 Å) thick ferromagnetic metallic layer on the polyester film 1. The polyester film 1 having thus deposited thereon said ferromagnetic metallic layer composed of cobalt-nickel alloy was taken out of the vacuum chamber 2 and left under the conditions of 25°C and 60% RH for 3 hours for allowing moisture absorption. Thereafter, said polyester film 1 was placed in a closed vessel and the vessel was evacuated to 1.999 kPa (15 Torr) for dehumidification. Then 121.59 kPa (1.2 atm.) dry oxygen gas was charged into the vessel. The vessel has been kept in this condition for one week to let a mild oxidation reaction proceed, forming a passivated cobalt layer on the surface of the ferromagnetic metallic layer. XPS analysis of this passivated cobalt layer showed that this layer was made of cobalt oxyhydroxide substantially comprising three- or higher-valent cobalt ions although containing a slight amount of divalent cobalt ions. The ferromagnetic metallic layer on which said passivated cobalt layer had been formed had a saturated magnetic flux density of 6,450 gausses and a coercive force of 1,050 Oe. Then a solution prepared by dissolving an anticorrosive in the solvents such as benzene, methyl isobutyl ketone, ethyl alcohol or tetrahydrofuran, to a concentration of 5 x 10⁻³ mol/l were applied on the ferromagnetic metallic layer having said passivated cobalt layer thereon and dried to form an approximately 10 nm (100 Å) thick anticorrosive layer. Thereafter a solution prepared by dissolving a 1:1 mixture of Fomblin Z DOL (a fluorine oil produced by Montedison and Fomblin V84/20 (a fluorine solid produced by Hoechst in Fluorinert FC77 (a Freon solvent produced by Sumitomo 3M) to a concentration of 0.5 g/l was applied on said anticorrosive layer and dried to form an approximately 10 nm (100 Å) thick lubricant layer composed of Fomblin Z-DOL and Fomblin V84/20. Then said coating solution for back coat was applied to the back side of each polyester film having the ferromagnetic metallic layer thereon and dried to form a 1.0 µm thick back coat. The laminate was then slit into a prescribed width to make a magnetic tape A in which a ferromagnetic metallic layer 11, a passivated cobalt layer 12, an anticorrosive layer 13 and a lubricant layer 14 were laminated successively on the polyester film 1 and a back coat 15 was formed on the back side of said polyester film 1 as shown in FIG. 2. REFERENCE EXAMPLE 2 By following the same procedure as in Reference Example 1 except that no lubricant layer was formed, the magnetic tapes B were made, in which a ferromagnetic metallic layer 11, a passivated cobalt layer 12 and anticorrosive layer 13 were laminated successively on a polyester film 1 and a back coat 15 was formed on the back side of said polyester film 1 as shown in FIG. 3.

Claims: A magnetic recording medium having a ferromagnetic metallic layer provided on a non-magnetic substrate, which comprises an anticorrosive deposited on said ferromagnetic metallic layer containing a benzotriazole derivative as a main component, characterized in that said benzotriazole derivative has at least one of amino group, nitro group, chlorine or mercapto group as substituent in the benzotriazole skeleton or is a salt of benzotriazole and a carboxylic acid. The magnetic recording medium according to claim 1, wherein before depositing said anticorrosive containing a benzotriazole derivative as a main component on said ferromagnetic metallic layer, a passivated layer is formed on the surface of said ferromagnetic metallic layer. The magnetic recording medium according to claim 1, wherein the benzotriazole derivative is at least one compound selected from benzotriazole stearic acid and 5-nitrobenzotriazole. The magnetic recording medium according to claim 1, wherein a lubricant layer is further provided on the anticorrosive layer. The magnetic recording medium according to claim 1, wherein the ferromagnetic metallic layer contains cobalt as a main component.

Inventors: MIZUMURA TETSUO, WAKAI KUNIO, MIZUMURA, TETSUO, WAKAI, KUNIO
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1988
Main classification: G11B5
IPC codes: G11B5