Magnetic recording disc

Abstract: A magnetic recording medium with improved durability is provided and comprises a non-magnetic substrate and a magnetic layer formed thereon containing magnetic powder consisting of platelet hexagonal system ferrite particles, granular non-magnetic powder, a lubricant and a binder, the granular non-magnetic powder being contained in an amount of 0.1 to 30 parts by weight per 100 parts by weight of the platelet hexagonal system ferrite particles and also a lubricant.

Description: The present invention relates to a magnetic recording disc. More particularly, it relates to a magnetic recording disc comprising a magnetic layer containing, as magnetic recording element, platelet hexagonal system ferrite powder. Generally, a magnetic recording medium such as a magnetic tape comprises a non-magnetic substrate and a magnetic layer thereon which contains magnetic powder as a magnetic recording element and a binder. Conventionally, as magnetic powder, magnetic powder consisting of acicular particles such as γ- Fe₂O₃, Fe₃O₄, Co-containing γ-Fe₂O₃ or metallic Fe is used. In recording and reproduction, usually a magnetic component in the direction parallel to a plane of a magnetic layer is used by orientating the magnetic powder along the surface of said non-magnetic substrate during the formation of the magnetic layer. Recently, high recording density is highly desired. For this end, magnetic powder consisting of platelet hexagonal system ferrite particles having a magnetic easy axis perpendicular to a granular plate, for example, Ba, Sr and Pb ferrite particles, is increasingly used so as to utilize a magnetic component perpendicular to the plane of the magnetic layer in order to obtain high output power in a short wavelength region. Such a magnetic recording medium is required to have good durability of a magnetic layer, since the medium slidingly contacts with a magnetic head and the like at a large relative rate during recording or reproduction. Conventionally, for improving such durability, it is proposed and practiced to incorporate a lubricant such as higher fatty acids and their esters, silicone oils and fluorinated oils in a magnetic layer to impart lubricity to the surface of the magnetic layer and decrease its friction coefficient. DE-A-34 43 566 discloses magnetic tapes comprising a non-magnetic substrate and formed thereon a magnetic layer. The magnetic layer consists of platlet Co-substituted Ba-ferrite particles, a lubricant, a binder and a chromeoxide as an abrasive. It is described that the layer structure is prepared by a dispersion of the above mentioned particles in a device containing a magnet. When the platelet hexagonal system ferrite particles are used as magnetic powder, orientation and a filling ratio are improved in comparison to the magnetic powder comprising the conventional acicular particles, effects of the lubricant are decreased, so that the magnetic layer and in turn the magnetic recording medium have insufficient durability. As a result of the extensive study by the present inventors to overcome the above problems of the platelet hexagonal system ferrite particles, it has been found that when a magnetic paint comprising them is coated on a non-magnetic substrate to orientate them, surfaces of the platelet particles tend to easily overlap each other because of their platelet shapes, and the particles contact intimately with each other with little space by magnetic drawing force since each particle acts as a granular magnet which generates opposite poles on two platelet surfaces but not as simple platelet particles such as pigment particles. In case of a magnetic recording medium utilizing the above magnetic powder, the volume of the voids in the magnetic layer for containing the lubricant is much smaller than that in case of one utilizing the magnetic powder comprising conventional acicular particles, then effects of the lubricant are not exhibited and the durability is deteriorated. An object of the present invention is to provide a magnetic recording disc which comprises a magnetic layer containing platelet hexagonal system ferrite particles as magnetic powder and has enough voids in the magnetic layer to keep the lubricant and good electromagnetic conversion properties and durability. This and other objects are accomplished by a magnetic recording disc which comprises a non-magnetic substrate and a magnetic layer formed thereon containing magnetic powder consisting of platelet hexagonal system ferrite particles, granular non-magnetic powder being present between adjacent surfaces of said platelet hexagonal system ferrite particles, a lubricant and a binder, the granular non-magnetic powder being contained in an amount of 10 to 25 parts by weight per 100 parts by weight of the platelet hexagonal system ferrite particles. The present invention has been completed based on the finding that when the granular non-magnetic powder is used in addition to a lubricant, voids are formed in the magnetic layer so that a sufficient amount of the lubricant is kept in the magnetic layer and effectively exhibits its functions to give a magnetic recording disc having high electromagnetic conversion properties and good durability. According to the present invention, the granular non-magnetic powder is used in an amount of 10 to 25 parts by weight per 100 parts by weight of the platelet hexagonal system ferrite particles. Since the magnetic layer contains the non-magnetic powder in such the specific amount and also the lubricant, the effect of the lubricant is greatly exhibited, and the durability of the magnetic recording disc is significantly improved. The reason why the durability of the magnetic recording disc is improved is not clear, but may be as follows: When the magnetic layer is formed by coating a magnetic paint on the non-magnetic substrate, the particles of the granular non-magnetic powder contained in the specific amount are present between adjacent surfaces of platelet hexagonal system ferrite particles so as to prevent the surfaces from contacting intimately. Then, the voids are formed between said platelet particles and the lubricant is contained in the voids so that the lubricant is contained in the formed magnetic layer in a sufficient amount. Therefore, the surface of the magnetic layer generally exhibits good lubrication and has a long life due to the lubricant provided from the inside of the magnetic layer even if the magnetic recording disc is repeatedly subjected to recording and reproduction for a long time. On the contrary, when the non-magnetic powder consists of platelet particles, surfaces of the particles of the non-magnetic powder and of the magnetic powder contact intimately each other even if the non-magnetic powder is present between the platelet hexagonal system ferrite particles. Since, therefore, the voids for containing the lubricant are rarely formed, little lubricant is contained in the magnetic layer and most of the lubricant floats on the surface of the magnetic layer. As the result, lubricity of the magnetic layer can be good at the beginning of the use but lost in a short time. When an amount of the non-magnetic powder is less than 0.1 parts by weight per 100 parts by weight of platelet hexagonal system ferrite particles, the number of the voids formed by the non-magnetic powder is small, the substantial space in the whole magnetic layer decreases so that the sufficient amount of the lubricant is not contained in the magnetic layer and then the magnetic recording disc has unsatisfactory durability. When the amount of the non-magnetic exceeds 30 parts by weight, the magnetic characteristics are degraded due to the decrease of relative amount of the magnetic powder and it is not good for forming the voids. Further, the binding property of the magnetic layer is degraded and effect of improving the durability is not achieved. As the granular non-magnetic powder, various kinds of inorganic and organic powder may be used. Suitable is that having larger hardness than the platelet hexagonal system ferrite particles. The granular non-magnetic powder having larger hardness not only acts as an abrasive which increases strength of the magnetic layer and enhances abrasion resistance, but also, when it is present between the ferrite particles and contacts with the ferrite particles, exhibits such a special function that it forms the voids for containing the lubricant so as to prevent decrease of the voids due to its deformation. Thus, the use of the granular non-magnetic powder enhances the durability of the magnetic recording disc. Examples of the preferable granular non-magnetic powder includes powder which comprises particles such as α-Aℓ₂O₃ (Vickers hardness: 2,500 kg/mm²), Cr₂O₃ (2,800 kg/mm²), α-SiC (2,500 kg/mm²), TiC (3,200 kg/mm²), WC (2,000 kg/mm²), Si₃N₄ (2,200 kg/mm²), ZrO₂ (1,400 kg/mm²) and the like and a mixture of two or more thereof, since the platelet hexagonal system ferrite particles generally have about 1,000 to 3,500 kg/mm² of Vickers hardness. The granular non-magnetic powder has preferably an average particle size of 10 to 75% of the thickness of magnetic layer formed. When the average particle size is too large in comparison with the thickness of the magnetic layer, a large number of the particles protrude from the magnetic layer surface to deteriorate smoothness of the magnetic layer surface and to degrade electromagnetic conversion property, and the particles drop off from the magnetic layer to degrade the durability of the disc. When the average size is too small, the particles can not sufficiently act as the abrasion and then can hardly improve the durability which results from the improvement of abrasion resistance of the magnetic layer. As the platelet hexagonal system ferrite particles used as the magnetic powder, examplified are particles which have the magnetic easy axis perpendicular to the plate of the particle, such as barium ferrite, strontium ferrite or lead ferrite. The particles having about 2 to 30 of an average platelet ratio (average platelet diameter/average platelet thickness) is particularly suitable in view of magnetic property or electromagnetic conversion property of the magnetic recording disc. Preferably, the average platelet diameter of the platelet hexagonal system ferrite particles is not less than 0.06 µm, since it has a certain effect on the durability of the magnetic recording disc. The larger the average platelet diameter, the better the durability. In addition to the platelet hexagonal system ferrite particles, magnetic powder of conventional acicular particles such as γ-Fe₂O₃, Fe₃O₄, Co-containing γ-Fe₂O₃, metalic iron or the like can be used in an amount of less than 60 % by weight based on the whole magnetic powder weight. The lubricant used according to the present invention may be any of the conventional lubricants which are known as lubricants contained in the magnetic layer of the magnetic recording disc. For example, higher aliphatic acids and the esters thereof, silicone oils or fluorinated oils can be used. The lubricant in a liquid or semi-solid state at normal temperature is particularly suitable. Preferred examples of the lubricant are oleyl oleate, 2-ethylhexyl oleate, octyl oleate, hexyl oleate, isoamyl stearate, butyl stearate, 2-ethylhexyl myristate, butylcellosolve stearate, butylcellosolve oleate, glyceryl trioleate, trimethylolpropane trioleate, isocetyl stearate, isocetyl oleate, myristic acid, lauric acid palmitic acid, stearic acid, dimethylpolysiloxane, Crytox® 143, Crytox® 1500, Crytox® 1600 (DuPont), Fomblin® Z (Montedison) and the like. A mixture of two or more of them can be used. Preferably, the lubricant is used in an amount of 2 to 25 parts by weight per 100 parts of the magnetic powder containing the the platelet hexagonal system ferrite particles. When the amount of the lubricant is more than 25 parts by weight, the magnetic layer surface becomes tacky so that the lubricant adheres to and contaminates a contacting subject such as a magnetic head or guide of a recording and reproducing machine. When it is less than 2 parts by weight, the smoothness of the surface of the magnetic layer is not satisfactory and then the durability of the magnetic recording disc is not sufficiently improved. The binder may be any of the conventional ones known to be used in the magnetic layer of the magnetic recording disc and includes vinyl chloride-vinyl acetate copolymers, cellulose type resins, butyral resins, polyurethane resins, polyester resins, isocyanate compounds, and a mixture of two and more of them. Other conventional additives such as a dispersant, an antistatic agent or a filler may be used. The magnetic recording disc of the present invention may be produced by a per se conventional method. For example, the magnetic powder containing the hexagonal system ferrite and optionally other magnetic particles is mixed with the granular non-magnetic powder, the lubricant, the binder and optional other additives to prepare a magnetic paint. Then the magnetic paint is coated on one side or both sides of the non-magnetic substrate made of, for example, a polyester film, and dried. The coated magnetic layer may be surface treated by, for example, calendering and the coated substrate is cut in a desirable shape and size. The substrate may have an electrically conductive layer comprising electrically conductive material such as carbon black. When only one side of the non-magnetic substrate is provided with the magnetic layer, the other side may be provided with a back coat layer. The magnetic recording disc according to the present invention has good durability, as described in the above, since the lubricant in the magnetic layer can be contained in the voids which are formed by the particles of the granular non-magnetic powder present between plate surfaces of platelet hexagonal system ferrite particles, and then the lubricant is dispersed homogeneously in the magnetic layer. Preferably, the magnetic layer has a void volume of more than 20% by volume. The void volume can be desirably changed according to the size and ratio of the platelet hexagonal system ferrite particles and granular non-magnetic powder, ratio of the both to the binder, and temperature and pressure of calendering after the formation of the magnetic layer. When the void volume is too small, an amount of the lubricant contained in the magnetic layer is so small to improve the durability of the magnetic recording disc. The present invention will be hereinafter explained further in detail by following examples, wherein parts are by weight unless otherwise indicated. Examples 1-21 Following components were compounded homogeneously in a ball mill for 72 hours to prepare a magnetic paint. Platelet hexagonal system barium ferrite magnetic powder (See Table 1) 100 parts Non-magnetic powder (see Table 1) 10-25 parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH) 12.5 parts Polyurethane resin (Pandex® T 5250), 7.5 parts Trifunctional polyisocyanate compound (Colonate® L) 5 parts Lubricant (see Table 1) 10 parts Toluene 185 parts Cyclohexane 185 parts The magnetic paint was coated on both sides of a non-magnetic substrate made of a polyethylene terephthalate film having a thickness of 75 µm and having conductive carbon black layers each having a thickness of 1 µm on the both sides of the film to give a thickness of a magnetic layer after drying as shown in Table 1, and dried to prepare the magnetic layer followed by calendering. Then the film with the magnetic layers was blanked to produce a magnetic disc having a diameter of 20.02 cm. Comparative Examples 1-8 In the same manner as in Examples 1-21 but using the platelet hexagonal system barium ferrite magnetic powder, non-magnetic powder and a lubricant shown in Table 2 in amounts shown in Table 2 and changing a thickness of the magnetic layer as shown in Table 2, magnetic discs were produced. In Comparative Example 1, non-magnetic powder was not used. The discs produced in the Examples and Comparative Examples were tested in surface roughness and durability of the magnetic layer. The results are shown in Tables 1 and 2. The surface roughness of the magnetic layer is represented in terms of a center line average height (Ra value) which is measured by a tracer type surface roughness meter. The durability is represented in terms of the number of the revolutions at which the surface of the disc is damaged when the magnetic disc is caused to rotate at a rate of 360 rpm in a floppy disc drive unit. The void volume of the magnetic layer is expressed by weight increase of the magnetic disc when the lubricant in the magnetic layer formed from magnetic paint is washed off with Freon®-TF or hexane and then the lubricant is again completely impregnated in the magnetic layer. The results of Tables 1 and 2 show that the magnetic disc according to the present invention which contains the magnetic powder of the platelet hexagonal ferrite particles and a specific amount of the granular non-magnetic powder in the magnetic layer (Examples 1-21) has much greater effect of improving the durability than that of the magnetic disc not containing non-magnetic powder (Comparative Example 1). The magnetic discs containing the platelet non-magnetic powder (Comparative Examples 5-8) and the magnetic discs containing the granular non-magnetic powder in an amount out of the range of 0.1 to 30 parts by weight per 100 parts by weight of the platelet hexagonal system ferrite particles (Comparative Examples 2-4) have poorer effect of improving the durability. Among the magnetic discs according to the present invention, those in which the granular non-magnetic powder have an average particle size of 10-75 % of the magnetic layer thickness, the non-magnetic powder is harder than the platelet hexagonal system ferrite particles, the platelet hexagonal system ferrite particles have an average platelet diameter of more than 0.06 µm, and the magnetic layer has a void volume of more than 20 % by volume (Examples 1-5, 7-9, 11 and 15-20) have better durability than ones which do not satisfy these conditions (Example 6, 10, 12-14 and 21).

Claims: A magnetic recording disc which comprises a non-magnetic substrate and a magnetic layer formed thereon containing magnetic powder consisting of platelet hexagonal system ferrite particles, granular non-magnetic powder being present between adjacent surfaces of said platelet hexagonal system ferrite particles, a lubricant and a binder, the granular non-magnetic powder being contained in an amount of 10 to 25 parts by weight per 100 parts by weight of the platelet hexagonal system ferrite particles. The magnetic recording disc according to claim 1, wherein the magnetic layer has a void volume of not less than 20 % by volume, and the lubricant is contained in voids. The magnetic recording disc according to claim 1, wherein the granular non-magnetic powder comprises particles having a larger Vickers hardness than that of the platelet hexagonal ferrite particles. The magnetic recording disc according to claim 1, wherein the granular non-magnetic powder has an average particle size of 10 to 75 % of a magnetic layer thickness. The magnetic recording disc according to claim 1, wherein the platelet hexagonal system ferrite particles have an average platelet diameter of not less than 0.06 µm.

Inventors: KISHIMOTO MIKIO, MIYAKE AKIRA C O HITACHI MAXEL, KISHIMOTO, MIKIO, MIYAKE, AKIRA C/O HITACHI MAXELL SHATAKU B-2-1
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1987
Main classification: G11B5
IPC codes: G11B5, C09D5, C10M105, C10N40