The present invention relates to titanium oxide particles, a substrate for a magnetic recording medium and a magnetic recording medium using the substrate. More particularly, the present invention relates to titanium oxide particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which uses magnetic particles containing iron as a main ingredient, and even more particularly, to titanium oxide particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which uses magnetic particles containing iron as a main ingredient, which show an excellent dispersibility in a binder resin, which contain only a small amount of soluble sodium salt and soluble sulfate, and have a pH value of not less than 8, a substrate for the magnetic recording medium, and a magnetic recording medium using the substrate.
With a development of miniaturized and lightweight video or audio magnetic recording and reading-out apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic layer in order to improve these properties of a magnetic recording medium.
The enhancement of the properties of magnetic particles will first be described.
The properties which magnetic particles are required to have in order to satisfy the above-described demands on a magnetic recording medium, are a high coercive force and a large saturation magnetization.
As magnetic particles suitable for high-output recording and high-density recording, acicular magnetic particles containing iron as a main ingredient which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas, are widely known.
Although acicular magnetic particles containing iron as a main ingredient have a high coercive force and a large saturation magnetization, since the acicular magnetic particles containing iron as a main ingredient used for a magnetic recording medium are very fine particles having a particle size of not more than 0.3 .mu.m, particularly, 0.01 to 0.2 .mu.m, such particles easily corrode, and as a result, magnetic characteristics thereof are deteriorated, especially, the saturation magnetization and the coercive force are reduced.
Therefore, in order to maintain the characteristics of a magnetic recording medium which uses magnetic particles containing iron as a main ingredient as the magnetic particles over a long period, it is strongly demanded to suppress the corrosion of acicular magnetic particles containing iron as a main ingredient as much as possible.
A reduction of the thickness of a magnetic recording layer will now be described.
Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the carrier signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
For the purpose of high-density recording, it is necessary to maintain the output characteristics, to reduce noise, and especially, to improve the S/N ratio with respect to signals having a short wavelength as well. In a magnetic recording medium composed of a substrate and a magnetic recording layer formed on the substrate, it have been conducted to reduce the thickness of the magnetic recording layer. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co. Ltd. (1982), " . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness. . . . "
Development of a thinner film for a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the substrate must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as Materials for Synthetic Technology (1987), published by the publishing department of Technology Information Center, " . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of a substrate (back surface roughness) so largely as to be approximately proportional, . . . since the magnetic layer is formed on the substrate, the more smooth the surface of the substrate is, the more uniform and larger head output is obtained and the more the S/N ratio is improved."
Secondly, there has been caused a problem in the strength of a non-magnetic substrate such as a base film with a tendency of the reduction in the thickness of a non-magnetic substrate which has been conventionally used in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a substrate . . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded."
The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium, at which the light transmittance is large by a video deck. As acicular magnetic particles containing iron as a main ingredient used for high-density recording, very fine particles are used as described above. With such a tendency of the reduction in the particle size of magnetic particles, and the thickness of the magnetic recording layer and substrate, the light transmittance of the magnetic recording medium tends to be larger, and as a result, it is difficult to detect the end of the magnetic tape by the video deck. For reducing the light transmittance of the magnetic recording medium, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.
However, addition of non-magnetic carbon black impairs not only the enhancement of the recording density but also the reduction of the thickness of the magnetic recording layer. It is, therefore, strongly demanded that the light transmittance of a magnetic recording layer should be small even if the amount of the carbon black or the like which is added thereto is reduced to small or zero. From this point of view, improvements in the substrate are now in strong demand.
On the other hand, there is no end to a demand for a higher performance in recent magnetic recording media. With the above-described reduction in the thickness of a magnetic recording layer and a non-magnetic substrate, since the durability of the surface of the magnetic recording layer and the magnetic recording medium itself lowers, an improvement of the durability of the surface of the magnetic recording layer and the magnetic recording medium itself is in strong demand.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679, " . . . With the recent development in magnetic recording, recording with a high picture quality and a high sound quality has increasingly been required. It is further required to improve the signal recording property, especially, to reduce noise and raise the C/N by making the surface of a magnetic tape smooth with the reduction of the particle size of ferromagnetic particles and the enhancement of the recording density. . . . However, the coefficient of friction between the magnetic tape and an apparatus during the travel of the magnetic tape increases, so that there is a tendency of the magnetic layer of the magnetic tape (magnetic recording medium) being damaged or exfoliated even in a short time. Especially, in a video tape, since the magnetic recording medium travels at a high speed in contact with the video head, the ferromagnetic particles are apt to be dropped from the magnetic layer, which may cause clogging on the magnetic head. Therefore, improvement in the running stability of the magnetic layer of a magnetic recording medium is expected. . . . "
Various attempts have been made to improve the substrate for a magnetic recording layer. A magnetic recording medium having at least one undercoat layer (hereinunder referred to as "non-magnetic undercoat layer") obtained by forming the dispersion composed of non-magnetic particles such as titanium oxide particles and a binder resin on a non-magnetic substrate such as a base film, has been proposed and put to practical use (Japanese Patent Publication No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), 8-45062 (1996), etc.).
Particularly, Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) describes as follows:
"The inorganic particles usable in the present invention include, for example, metals, metal oxides, metal carbonates, metal sulfates, metal nitrates, metal carbides and metal sulfides. Concretely, TiO.sub.2, (rutile, anatase), TiO.sub.x, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, a alumina having not less than 90% of .alpha.-formation ratio, .beta.-alumina, .gamma.-alumina, .alpha.-iron oxide, goethite, corundum, silicon nitrate, titanium carbide, magnesium oxide, boron nitrate, molybdenum disulfide, copper oxide, MgCO.sub.3, CaCO.sub.3, BaCO.sub.3, SrCO.sub.3, BaSO.sub.4, silicon carbide, titanium carbide, etc. are used singly or in combination."
"As the inorganic particles, those satisfying the following conditions are preferable. The tap density is 0.05 to 2 g/cc, preferably 0.2 to 1.5 g/cc. The water content is 0.1 to 5 wt %, preferably 0.2 to 3 wt %. The pH value is 2 to 11, preferably 4 to 10. The specific surface area is 1 to 100 m.sup.2 /g, preferably 5 to 70 m.sup.2 /g, more preferably 7 to 50 m.sup.2 /g. The preferable crystal grain size is 0.01 to 2 .mu.m. In the case of granular particles, the average particle size is not more than 0.1 .mu.m, preferably not more than 0.08 .mu.m. In the case of acicular particles, the major axial diameter is 0.05 to 1.0 .mu.m, preferably 0.06 to 0.5 .mu.m, the acicular ratio (aspect ratio) is 3 to 30, preferably 5 to 15. The oil absorption using DBP is 5 to 100 ml/100 g, preferably 10 to 80 ml/100 g, more preferably 20 to 60 ml/100 g. The SA (stearic acid) adsorption is 1 to 20 .mu.mol/m.sup.2, more preferably 2 to 15 .mu.mol/m.sup.2. The roughness factor of the particle surfaces is preferably 0.8 to 1.5. The heat of wetting to water at 25.degree. C. is preferably 200 to 600 erg/cm.sup.2. A solvent having the heat of wetting in the above range is also usable. The appropriate quantity of water molecules on the surface at 100 to 400.degree. C. is in the range of 1 to 10/100 .ANG.. The pH value in water at the isoelectric point is preferably 3 to 9. The specific gravity is 1 to 12, preferably 3 to 6. The ignition loss is preferably not more than 20%."
"As the non-magnetic inorganic particles used in the present invention, titanium oxide (particularly, titanium dioxide) is preferable. The process for preparing titanium oxide will be described in detail in the following.
As the process for preparing titanium oxide, a sulfuric acid method and a chlorine method are mainly used. In the sulfuric acid method, a raw ore such as ilmenite is distilled in sulfuric acid to extract Ti, Fe, etc. in the form of sulfates. After the iron sulfate is removed by crystallization separation, the remaining titanyl sulfate solution is filtered, purified and hydrolyzed under heating to precipitate hydrated titanium oxide. After filtering the precipitated hydrated titanium oxide out and washing it with water, the impurities are washed out, and a particle size regulator is added and calcined at a temperature of 80 to 1,000.degree. C., thereby obtaining crude titanium oxide. The obtained crude titanium oxide is classified into the rutile titanium oxide and the anatase titanium oxide by the nuclear material added at the time of hydrolysis. The resultant crude titanium oxide is pulverized. After the dressing of grain and the surface treatment, the target titanium oxide is obtained.
In the chlorine method, natural rutile as a raw ore and synthesized rutile are used. The ore is chlorinated in the state of reduction at a high temperature, to that Ti is changed into TiCl.sub.4 and Fe into FeCl.sub.2. The iron oxide which is cooled into a solid is separated from liquid TiCl.sub.4. After the crude TiCl.sub.4 obtained is refined by fractionating, a nucleating agent is added thereto and instantaneously reacted with oxygen at a temperature not lower than 1000.degree. C. to obtain crude titanium oxide. The finishing process for imparting a pigment quality to the crude titanium oxide produced in the oxidization separation process is the same as in the sulfuric acid method."
Japanese Patent Application Laid-Open (KOKAI) No. 5-347017 (1993) describes as follows:
"In the present invention, it is possible to appropriately select various known non-magnetic particles. Examples of the usable non-magnetic particles are carbon black, graphite, TiO.sub.2, barium sulfate, ZnS, MgCO.sub.3, CaCO.sub.3, ZnO, CaO, tungsten disulfide, molybdenum disulfide, boron nitride, MgO, SnO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, .alpha.-Al.sub.2 O.sub.3, .alpha.-Fe.sub.2 O.sub.3, .alpha.-FeOOH, SiC, cerium oxide, corundum, artificial diamond, .alpha.-iron oxide, garnet, silica rock, silicon nitride, boron nitride, silicon carbide, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, tripoli, diatom and dolomite. Among these are preferable inorganic particles such as carbon black, CaCO.sub.3, TiO.sub.2, barium sulfate, .alpha.-Al.sub.2 O.sub.3, .alpha.-Fe.sub.2 O.sub.3, .alpha.-FeOOH and Cr.sub.2 O.sub.3, and polymer particles such as polyethylene powder."
"The major axial diameter of the non-magnetic particles is ordinarily not more than 0.50 .mu.m, preferably not more than 0.40 .mu.m, more preferably not more than 0.30 .mu.m. The minor axial diameter of the non-magnetic particles is ordinarily not more than 0.10 .mu.m, preferably not more than 0.08 .mu.m, more preferably not more than 0.06 .mu.m. The aspect ratio of the non-magnetic particles is ordinarily 2 to 20, preferably 5 to 15, more preferably 5 to 10. The aspect ratio here means the ratio (major axial diameter/minor axial diameter) of the major axial diameter to the minor axial diameter. The specific surface area of the non-magnetic particles is ordinarily 10 to 250 m.sup.2 /g, preferably 20 to 150 m.sup.2 /g, more preferably not more than 30 to 100 m.sup.2 /g."
As is clear from the above description, various inorganic particles are known as the non-magnetic particles for a non-magnetic undercoat layer. Especially, known titanium oxide particles which are fine particles having an excellent chemical resistance, are widely used.
The non-magnetic particles for a non-magnetic undercoat layer which are capable of not only reducing the thickness of a magnetic recording layer but also producing a substrate having as smooth a surface as possible and a high strength, which enable a thinner magnetic recording layer having a small light transmittance, an excellent surface smoothness and a uniform thickness to be formed on the substrate, and which are capable of suppressing a corrosion of the magnetic particles containing iron as a main ingredient which are dispersed in the magnetic recording layer, are now in the strongest demand, but no such non-magnetic particles have ever been obtained.
When commercially available titanium oxide particles or titanium oxide particle produced by coating on the surface of the available titanium oxide particles with an aluminum compound, are used as non-magnetic particles for a non-magnetic undercoat layer, it is impossible to sufficiently enhance the surface smoothness of the non-magnetic undercoat layer formed on the non-magnetic base film. As a result, when a magnetic recording layer is formed on such a non-magnetic undercoat layer, it is difficult to make a thin layer having a smooth surface and a uniform thickness. In addition, due to its process, the soluble sodium salt and the soluble sulfate contained in the titanium oxide particles necessarily cause the corrosion of the magnetic particles containing iron as a main ingredient which are dispersed in the magnetic recording layer, thereby greatly reducing the magnetic characteristics.
The fact that titanium oxide particles contain soluble sodium salt, soluble sulfate, etc. are described on page 77 of Titanium Oxide-Physical Properties and Applied technique (1991), (published by Gihodo Co., Ltd.) as follows: " . . . In titanium oxide, K, Na, Li, Mg, PO.sub.4, SO.sub.4 or Cl which is contained in conditioning agents and flocculants, remain as a water-soluble matter. In surface-treated titanium oxide, Na, SO.sub.4, Cl as a by-product of the production of a surface-treating hydrate adsorbs to the hydrate.
Especially, when titanium oxide is treated with an alumina hydrate, it is apt to become difficult to remove the acid radical such as SO.sub.4 due to the base of the alumina. On the other hand, when titanium oxide is treated with a silica hydrate, the silica firmly connects with the alkali metal ions Na, and the complete removal thereof is very difficult. . . . "