Magnetic disc cartridge

Abstract: Disclosed herein is a magnetic disc cartridge comprising a cartridge case, a magnetic disc housed rotatably in said cartridge case, and a liner, which serves as a cleaning sheet and is secured on the inner surface of said cartridge case, characterized in that said magnetic disc has a magnetic layer containing a ferromagnetic metal powder, a reinforcing powder, whose content is defined to the range of 5 to 40% by weight, and a fatty acid ester with a melting point of 0°C or below, and that at least the disc side surface layer of said cleaning sheet, which is brought into sliding contact directly with the magnetic layer, is almost entirely composed of rayon fiber. The magnetic disc cartridge of the present invention can provide a highly reliable magnetic disc cartridge having a magnetic disc which is excellent in duration and of non-error occurrence, due to the disclosed structure.

Description: TECHNICAL BACKGROUND Technical Field This invention relates to a magnetic disc cartridge having a magnetic disc housed rotatably in a cartridge case molded from a hard synthetic resin or in a flexible cartridge case. More specifically the invention relates to a cleaning sheet which contacts slidingly with the magnetic layer of said magnetic disc. It is also envisaged in this invention to provide a magnetic disc having excellent durability and reliability and fit for use in the magnetic disc cartridge according to this invention. Description of the Prior Art A magnetic disc cartridge consists mainly of a cartridge case having an opening for a magnetic head and an opening for disc drive, a flexible magnetic disc housed rotatably in said cartridge case, and a cleaning sheet or liner secured to the inside of said cartridge case. In the prior art, a cleaning sheet of a magnetic disc cartridge has been composed of a non-woven fabric of thermoplastic fiber such as polyethylene terephthalate fiber or acrylic fiber and fixed to the inside of cartridge case by means of, for instance, ultrasonic deposition or an adhesive. Recently, in the art of magnetic disc, use is made of a magnetic layer containing powder of a ferromagnetic metal such as iron, cobalt, cobalt-nickel, cobalt-phosphorus or the like for enabling high-density recording. Such magnetic metal powders, however, are low in hardness as compared with magnetic powders of metal oxides such as γ-Fe₂O₃, so that the magnetic layer comprising said magnetic metal powder is vulnerable to wear in sliding contact with magnetic head. For lessening such a tendency, attempts have been made to mix a reinforcing powder such as aluminum oxide powder in the magnetic layer to increase its hardness. Since thermoplastic fibers are used for the cleaning sheet and a hard reinforcing powder is contained in the magnetic layer as said above, the thermoplastic fibers constituting the cleaning sheet are softened and worn by the frictional heat in the course of long-term sliding contact of said cleaning sheet with the rotating magnetic disc, and the produced wear dust deposits on the magnetic head via the magnetic disc to cause errors in recording and reproducing operations. In use of magnetic disc cartridge, the magnetic disc housed therein is rotated at high speed in sliding contact with magnetic head under a high pad pressure to perform recording or reproduction. Therefore, the magnetic layer of magnetic disc or the liner made of a non-woven fabric secured to the inner surface of cartridge case tends to be worn, and wear dust of the magnetic layer or liner deposits on the magnetic disc surface or magnetic head to cause an error. Recently, there is seen a tendency to greater capacity of magnetic disc cartridge with concomitant narrowing of track width and increase of maximum line recording density, and this has brought about the problem that even very fine wear dust of magnetic layer or linear, which wouldn't cause an error in the prior art devices, is now liable to induce an error when it deposits on the magnetic disc surface. Magnetic disc cartridges also have a tendency to smaller size in spite of increase of capacity. Accordingly, magnetic disc cartridge drive is being made smaller in size, with the drive motor used therefor being switched into a low power consumption and low torque type, so that the magnetic head tends to stick to the magnetic layer or rotate unevenly unless the coefficient of friction of the magnetic layer is sufficiently reduced. As a measure for minimizing the possibility of inducing errors and for improving durability and reliability, attempts have been made to mix a lubricant in the magnetic layer. For instance, it is proposed to mix a higher fatty acid, higher fatty acid ester, liquid paraffin, squalane, fluorine type lubricant or silicone type lubricant in the magnetic layer (Japanese Patent Application Kokai (Laid-Open) Nos. 70807/80, 124707/79 and 28560/80). SUMMARY OF THE INVENTION An object of this invention is to eliminate the defects of the prior art such as mentioned above and to provide a magnetic disc cartridge having excellent durability and also safe from or minimized in risk of error in operation. The present invention is also intended to solve the problem that in a small-sized, large-capacity magnetic disc cartridge, sufficient durability and reliability won't be obtained even when mixing a lubricant such as mentioned above in the magnetic layer, and that especially under a low-temperature environment, the lubricant is poor in fluidity and becomes viscous so that fine wear dust of a magnetic layer or liner produced in sliding motion of magnetic disc tends to adhere to the magnetic disc surface or magnetic head to cause errors. The term "magnetic disc" used in the present specification refers to hard and flexible (floppy) discs, and the term "cartridge" itself is used to represent hard and soft housing cases (jacket). When the term "magnetic disc cartridge" is used in this specification, it means cartridge-loaded magnetic disc products, which include jacketed floppy disc products (floppy disc jackets). Also, the term "cleaning sheet" means a liner on the inner surface of cartridge case, which has a function to clean the magnetic disc. Our EP-A-0 194 649 provides a magnetic disc cartridge as defined in the precharacterising portion of claim 1. This prior construction uses thermoplastic fibres as at least a major portion of the surface of the cleaning sheet with the problem referred to the above. As compared therewith, and as defined in claim 1, the present invention has the surface of the cleaning sheet which contacts the disc at least substantially entirely consisting of a non-thermoplastic fibre such as rayon fibre; the magnetic layer contains a reinforcing powder, for example aluminium oxide powder, whose content is defined to the range of 5 to 40% by weight of the magnetic layer. While the provision of reinforcing powder in the magnetic layer is known from our EP-A-0 244 868, there is no appreciation therein of its need specifically in conjunction with the cleaning sheet surface of non-thermoplastic fibres as used in the present invention. Preferably, a fatty acid ester with a melting point of 0°C or below is contained in the magnetic layer of the magnetic disc housed rotatably in said magnetic disc cartridge, thereby to realise a marked improvement of wear resistance of the magnetic layer in a small-sized, large-capacity magnetic disc cartridge with maximum linear recording density of more than 35 kfci (kilo flux change per inch) and a track density of more than 150 TPI (track per inch) while preventing generation of wear dust from the magnetic layer or liner to inhibit the occurrence of error even under a low-temperature environment. The present invention also makes it possible to prevent sticking of the magnetic head or non-uniformity of rotation in case of using a miniaturised magnetic disc cartridge drive to realise improved durability and reliability under all possible practical use environments. Further, in the present invention, a non-woven fabric formed mainly from rayon fibre may be used as liner on the inner surface of jacket substantially to eliminate the risk of inducing error by liner wear dust thereby to attain a further improvement of durability and reliability of small-sized, large-capacity magnetic disc cartridge. A cartridge case and magnetic disc in accordance with this invention will be described below with reference to the accompanying drawings. The drawings attached hereto illustrate a magnetic disc cartridge according to an embodiment of this invention, in which: Figure 1 is an exploded perspective view of the magnetic disc cartridge, Figure 2 is a bottom view of the cartridge showing a cleaning sheet secured to an upper case member, Figure 3 is a plane view of the cartridge showing a cleaning sheet secured to a lower case member, Figure 4 is an enlarged sectional view of the portion around the magnetic head inlet, in a mode of use of said magnetic disc cartridge, Figure 5 is an enlarged sectional view of an elastic member and its neighbourhood in said magnetic disc cartridge, and Figure 6 is an enlarged sectional view of the cleaning sheet. For making the cartridge case according to this invention, a thermoplastic synthetic maaterial such as ABS resin, polyacetal resin, polystyrol resin or the like is used and such resin is injection moulded into a desired form of cartridge case. The magnetic disc set in said cartridge case comprises, for example, a polyester or polyimide film coated on its one side or both sides with a magnetic layer and has a moderate degree of flexibility. Referring to Figures 1 to 6, there are shown schematic drawings illustrating a magnetic disc cartridge according to an embodiment of the present invention. Figure 1 is an exploded perspective view of the disc cartridge, and Figure 2 is a bottom view thereof, showing the manner in which a cleaning sheet is secured to an upper case. Figure 3 is a plane view of the cartridge, showing the state in which a cleaning sheet is secured to a lower case. FIG. 4 is an enlarged sectional view showing the portion around the magnetic head inserting opening, showing a mode of use of said magnetic disc cartridge, and FIG. 5 is an enlarged sectional view of the portion in the vicinity of elastic member of said disc cartridge. FIG. 6 is an enlarged sectional view of a cleaning sheet. The magnetic disc cartridge in accordance with this invention consists essentially of a cartridge case 1, a magnetic disc 2 housed rotatably therein, and a shutter 3 supported slidably in said cartridge case 1. Cartridge case 1 consists of an upper case member 1a and a lower case member 1b. Both case members are injection molded from a hard synthetic resin such as ABS resin. Substantially centrally of said lower case member 1b is formed an opening 4 for inserting a drive shaft, and an oblong head inserting opening 5 is also formed near said opening 4. Head inserting opening 5 is also provided in the upper case member 1a. Near the frontage of the upper and lower case members 1a and 1b are provided recesses 6 designed to define the range of slidable movement of said shutter 3, and said magnetic head inserting opening 5 is formed midway between said recesses 6 as shown in FIG. 1. On the inside of said upper case member 1a and on both sides of said magnetic head inserting opening 5 are formed spot facing recesses 7, 7 as shown in FIG. 2. The longitudinal width W1 of said recesses 7 is almost equal to the longitudinal width W2 of magnetic head inserting opening 5, and the transverse width W3 of said recesses 7 is defined to be not less than 0.3 times, preferably 0.5 to 1.5 times the width W4 in the direction orthogonal to the longitudinal direction of said magnetic head inserting opening 5. A plurality of protrusions 8 extending on the straight lines are provided on the upstream side in the direction of disc rotation of said magnetic head inserting opening 5 in the upper case member 1a. The two protrusions 8 positioned on both sides of the line of protrusions are slightly higher than the other protrusions. On the inside of the upper case member 1a are provided arcuate ribs 9 designed to define the position for housing the magnetic disc 2. A part of rib 9 passes an end of the corresponding one of said recesses 7 so that the ribs 9 can serve as a reinforcement of the surrounding of said recesses 7. On the inside of said ribs 9 is fitted a substantially C-shaped cleaning sheet 10. In this cleaning sheet 10 is formed an opening 11 in alignment with the magnetic head inserting opening 5. The transverse width W5 of said opening 11 is slightly larger than the transverse width W4 of the magnetic head inserting opening 5. Cleaning sheet 10 is placed on the inner surface of upper case member 1a so that said recesses 7 and protrusions 8 are covered, too. The periphery of said cleaning sheet 10 is fixed to the upper case member 1a by ultrasonic deposition. Ample spacing of each recess 7 in the upper case 1a makes it possible to perform ultrasonic deposition of the edge of opening 11 of cleaning sheet 10 in said recess 7. A support rib 13 and an adherence-preventing rib 14 are provided at a position on the inside of lower case 1b and substantially opposing the protrusion 8 on upper case 1a as shown in FIGS. 3 and 5. There is also provided an elastic member 15 formed by bending a plastic sheet, and as shown in FIGS. 3 and 5, the proximal portion 16 of said elastic member 15 is secured by suitable means such as bonding or heat fusion to a part on the inside of the lower case 1b and near the support rib 13. The free end 17 of said elastic member 15 is held in a slant state by said support strip 13. Regulating ribs 9 are also provided on the inner surface of lower case 1b, and cleaning sheet 10 is disposed inside of said ribs. In said cleaning sheet 10 is formed an opening 11 at a position in alignment with the magnetic head inserting opening 5. The transverse width W5 of said opening 11 is slightly larger than the transverse width W4 of said magnetic head inserting opening 5. Elastic member 15 is covered by said cleaning sheet 10 as it is placed on the inside of lower case 1b as shown in FIG. 3, and the periphery of said cleaning sheet 10 is secured to lower cause 1b by ultrasonic deposition 12. No such ultrasonic deposition is applied to the neighborhood of elastic member 15. When a magnetic disc cartridge is assembled up by combining upper and lower case members 1a and 1b, cleaning sheet 10 is partly raised up from lower case member 1b by elastic member 15 as shown in FIG. 5, and the raised-up sheet is slightly pushed up by protrusions 8 provided on upper case member 1a so that magnetic disc 2 is elastically held between upper and lower cleaning sheets 10, 10. As magnetic disc 2 rotates, its surface is cleaned by said cleaning sheets 10. FIG. 4 illustrates a mode of use of the magnetic disc cartridge. In the drawing, numeral 18 denotes magnetic head. Said cleaning sheet 10 has a three-layer structure as shown in FIG. 6. It consists of a non-woven fabric layer 10 opposing magnetic disc 2, a non-woven fabric layer 20 opposing cartridge case 1 and an intermediate non-woven fabric layer 21 joining said disc side non-woven fabric layer 19 and case side non-woven fabric layer 20. Said disc side non-woven fabric layer 19 and case side non-woven fabric layer 20 are both composed of rayon fiber alone, and their front and rear sides are indistinguishable. Examples of rayon fiber usable here are viscose rayon, cupro-ammonium rayon and acetate rayon. The rayon fiber (staple) used in this invention has a tensile strength of about 2.5 to 3.1 g/D, a rate of elongation of about 16 to 22%, a stretch modulus (at 3% elongation) of about 55 to 80% and a specific gravity of about 1.50 to 1.52. Said intermediate non-woven fabric layer 21 is composed of a mixture of rayon fiber and polyamide fiber. The rayon to polyamide fiber mixing ratio is properly selected from the range of about 1:9 to 9:1, preferably 3:7 to 7:3. Polyamide fiber used in this invention is the one made by using a polycondensate of adipic acid and hexamethylenediamine as starting monomer. Such polyamide fiber should have a tensile strength of about 4.5 to 7.5, a rate of elongation of about 25 to 60%, a stretch modulus (at 3% elongation) of about 95 to 100% and a specific gravity of about 1.14. Cleaning sheet 10 may be composed of rayon fiber alone, but when an intermediate layer 21 of a non-woven fabric containing thermoplastic fibers is provided between the disc side and case side non-woven fabric layers 19 and 20 as in the described embodiment of this invention, the sheet can be fusion bonded very favorably to the cartridge case 1. Beside said mixture of rayon and polyamide fibers, various types of thermoplastic fibers such as polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polyvinyl chloride and acrylic resin can be favorably used for forming said intermeidate non-woven fabric layer 21. It is especially preferred to compose the intermediate non-woven fabric layer 21 from a fiber mixture containing rayon fiber in view of good affinity for both side layers 19 and 20 in which rayon fiber is used. The basis weight of said cleaning sheet 10 is not specifically defined in this invention, but usually it is preferably in the range of about 20 to 50 g/m². The term "basis weight" used herein is the average of the measurements of weight of 5 samples with a size of 50 x 50 cm. The thickness of cleaning sheet 10 is preferably in the range of 150 to 300 µm. Typical properties of cleaning sheet 10 according to an embodiment of this invention are shown below. Cleaning sheet 10 is composed of a three-layer structure as described above, but when a magnetic disc cartridge is actually assembled by using such a cleaning sheet, there are occasions where not all of the fibers which contact slidingly with magnetic disc 2 are rayon fibers, and a very small portion of thermoplastic fibers (polyamide fibers) in the intermediate layer 21 may come out on the surface side of cleaning sheet 10 as a result of their entanglement and contact with magnetic disc 2. Even in such a case, however, the ratio of thermoplastic fibers which contact slidingly with magnetic disc 2 is less than 10%, and substantially the fibers contacted with magnetic disc 2 can be regarded as almost entirely rayon fibers. Now, the composition of the magnetic layer is described. As ferromagnetic metal powder, there can be used, for instance, powder of iron, cobalt, cobalt-nickel, cobalt-phosphorus and the like. Iron powder is used in the Examples of the invention described later. The properties of iron powder used in the Examples are as shown in Table 2 below. Hc (Oe) 1,650 σs (emu/g) 130 BET surface area (m²/g) 44.8 Average particle diameter (µm) 0.25 Examples of reinforcing powder usable in this invention has more than 6 of Mohs' hardness and include powder of aluminum oxide, chromium, silicon carbide, silicon nitride and the like. In the Examples of this invention shown below, pulverized aluminum oxide was used as reinforcing powder. The particle size of reinforcing powder is 0.1 to 1.0 µm, preferably 0.4 to 0.5 µm. A preferred example of composition of the magnetic layer is shown in Table 3 below. Composition Wt part Wt% Magnetic metal (iron) powder 100 63.9 Vinyl chloride resin 12 7.7 Urethane resin 7 4.5 Crosslinking agent 5 3.1 Aluminum oxide (particle size: 0.4 - 0.5 µm) 20 12.8 Carbon black 2 1.3 Oleyl oleate 9 5.8 Butyl cellulose stearate 1 0.6 Zinc stearate 0.5 0.3 The magnetic properties of the magnetic disc are as shown in Table 4 below. Hc (Oe) 1,500 Br (G) 1,500 Bm (G) 2,500 The results of examination of the relation between content of reinforcing powder (aluminum oxide, particle size: 0.4 - 0.5 µm) in the magnetic layer and cleaning sheet material are shown in Table 5 below. "Component fiber" in the above table is the fiber composing the surface layer of cleaning sheet which contacts slidingly with the magnetic disc. "PET" represents polyethylene terephthalate. "Mixture 1" is a mixture of rayon (50%) and polyethylene terephthalate (50%), "Mixture 2" is a mixture of rayon (50%) and acryl (50%), and "Mixture 3" is a mixture of rayon (50%) and polyamide (50%). "Accelerated durability" is the durability tested under the conditions of head load = 40 g and peripheral speed = 30 m/sec. In the column of "Error", there is shown whether no error occurred or an error was caused in 24-hour seeking. As seen from Table 5, when the content of reinforcing powder (aluminum oxide) in the magnetic layer is low, such as 1 to 3% by weight, the magnetic disc is unsatisfactory in durability due to use of relatively soft magnetic metal powder. However, magnetic disc durability is markedly improved when the content of reinforcing powder in the magnetic layer is higher than 5% by weight, especially when it is higher than 10% by weight. It is to be noted that an increase of content of reinforcing powder naturally leads to a corresponding reduction of other components such as magnetic metal powder and binder, even to such an extent that the desired properties will not be obtained. Therefore, it is necessary to limit the content of reinforcing powder to 40% by weight at most. The preferred range of reinforcing powder content is 10 to 30% by weight. It is possible to improve durability of the magnetic disc by defining the content of reinforcing powder within the range of 5 to 40% by weight. However, when polyethylene terephthalate fiber alone is used (Comparative Examples 5 - 11) or when thermoplastic fiber is contained in the component fibers of cleaning sheet, as in the case of rayon/polyethylene terephthalate mixture (Comparative Example 12), rayon/acryl mixture (Comparative Example 13) and rayon/polyamide mixture (Comparative Example 14), such thermoplastic fiber is worn by heat of friction with the magnetic disc and the produced wear dust becomes a cause of error. On the other hand, in the case of Examples 1 to 7 of this invention, the magnetic disc is excellent in durability and no error occurs. It has been experimentally confirmed that the same effect can be obtained when using other reinforcing powder. Now, the relation between surface roughness of magnetic disc and wear of cleaning sheet is discussed. In order to minimize wear of cleaning sheet, it is necessary to take into account surface roughness of magnetic disc. When surface roughness of magnetic disc is relatively large, such as 0.020 - 0.025 µm as in the conventional discs, cleaning sheet is naturally prone to wear. It was found, however, that wear of cleaning sheet could be kept minimized for a long time by reducing surface roughness Ra of magnetic disc to less than 0.01 µm, preferably less than 0.005 µm. In the present invention, surface roughness of magnetic layer was measured by Tayler - Habon Co.'s surface roughness tester under the following conditions: stylus diameter = 2 µm, feeler load = 25 mg, cut-off = 0.08 mm, scanning speed = 0.03 mm/sec. It is possible to maintain surface roughness of magnetic layer to a desired level by properly changing the surface treating conditions (temperature, pressure and number of calender rolls in calendering apparatus). In case a binder comprising urethane resin is contained in the magnetic layer as shown in Table 3, even when wear dust is produced from cellulose fiber such as rayon fiber, such wear dust scarcely deposits on the magnetic disc because of no affinity for such wear dust, thus minimizing the risk of causing error. While rayon fiber was used as non-thermoplastic resin in the above-described embodiment of the invention, the similar effect can be obtained as well by using other cellulose fibers such as cotton fiber. Selection and use of lubricant in the magnetic layer of magnetic disc in magnetic disc cartridge according to this invention is also an important feature of the present invention. It will be described below by showing examples. Fatty acid esters having a melting point below 0°C, especially 2-heptylundecyl oleate obtainable from an ester reaction of 2-heptylundecyl alcohol and oleic acid are preferably used as lubricant to be contained in magnetic layer of magnetic disc set in cartridge case. As these types of fatty acid esters have a branched structure and unsaturated bonds at the same time, they are low in melting point, assuming a liquid state at 0°C, and also low in viscosity, show good fluidity at low temperatures and have excellent lubricating performance. When such type of fatty acid ester is contained in magnetic layer of magnetic disc, the coefficient of friction of the magnetic layer surface is significantly reduced owing to the excellent lubricating performance, resulting in a marked improvement of wear resistance of said magnetic layer or the liner secured to the inner surface of cartridge case. Accordingly, generation of wear dust of magnetic layer or liner is arrested or minimized. Further, since this type of fatty acid esters assume a liquid state at 0°C, are low in viscosity and show good fluidity at low temperatures, fine wear dust of magnetic layer or liner little deposits on the magnetic disc surface or magnetic head even under low-temperature environments to markedly lessen the possibility of causing error. Moreover, because of low coefficient of friction of the magnetic layer surface, there takes place no sticking of magnetic head to magnetic layer nor non-uniform rotation of magnetic head even if the disc drive motor is a low power consumption and low torque type. This leads to a remarkable enhancement of reliability of magnetic disc. Especially when it is desired to reduce only the coefficient of friction of magnetic layer, it is required to give particular consideration to molecular structure of fatty acid ester. In case of employing an ester of branched alcohol and fatty acid, it is possible to reduce the coefficient of friction without regard to melting point. It is especially effective to use a fatty acid ester represented by the following formula (1): wherein R₁ and R₂ represent independently a straight-chain or branched saturated alkyl group having 1 to 15 carbon atoms, but the sum of the carbon numbers of R₁ and R₂ is 16, and R₃ represents a straight-chain or branched saturated or unsaturated alkyl group having 17 carbon atoms. Typical examples of said fatty acid esters are 2-ethylhexadecyl stearate, 2-hexyldodecyl stearate, 2-octyldecyl stearate, 2-heptylundecyl isostearate, 2-ethylhexadecyl oleate, 2-hexyldodecyl oleate, and 2-octyldecyl oleate. Among them, the esters of the formula (1) wherein R₁ is a straight-chain or branched alkyl group having 7 carbon atoms and R₂ is a straight-chain or branched alkyl group having 9 carbon atoms are preferred, the examples of such esters being 2-heptylundecyl stearate and 2-heptylundecyl oleate. Needless to say, in the fatty acid esters having said molecular structure, those having a melting point below 0°C are excellent synthetically. In this sense, 2-heptylundecyl oleate is the most preferred as it has a large molecular weight and shows high surface tension, hence excellent durability of the product using it. Such fatty acid esters with melting point below 0°C may be used singly, but they may be used with a known lubricant such as higher fatty acids, higher fatty acid esters, liquid paraffin, squalane, fluorine type lubricants, silicone type lubricants and the like. In this case, in order to maximize the desired effect, it is desirable to add the lubricant at such a rate that the total amount thereof will be less than 10% by weight of the amount of the fatty acid ester with melting point below 0°C. Several methods are available for containing a fatty acid ester with melting point below 0°C in the magnetic layer. For instance, said fatty acid ester is mixed and dispersed with a magnetic powder, binder resin or the like to prepare a magnetic coating material and this magnetic coating material is applied on a non-magnetic substrate and dried to form a magnetic layer, or said fatty acid ester is dissolved in a solvent such as toluene, n-hexane or the like and this solution is coated or sprayed on the surface of the already formed magnetic layer or this magnetic layer is immersed in said solution. As magnetic powder used for forming the magnetic layer in this invention, there can be employed various types of magnetic powder generally utilized for magnetic recording media, such as γ-Fe₂O₃ powder, Fe₃O₄ powder, Co-containing γ-Fe₂O₃ powder, Co-containing Fe₃O₄ powder, CrO₂ powder, Fe powder, Co powder, Fe-Ni powder, barium ferrite powder, etc. As binder resin, one may use any of those generally employed for magnetic recording media, such as vinyl chloride-vinyl acetate polymer, cellulose resin, polyester resin, isocyanate compounds, radiation-cured resin and the like. As for organic solvent, it is possible to use any of those commonly employed for magnetic recording media, such as methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, benzene, toluene, xylene, ethyl acetate, tetrahydrofuran, dioxane and the like. It is also possible to properly blend various types of commonly used additives such as dispersant, abrasive, antistatic agent, etc., in the magnetic coating material. The liner bonded to the inside of cartridge case housing magnetic disc of this invention is preferably made of a non-woven fabric which is mainly composed of rayon fibre. Such a liner is highly resistant to abrasion even when contacted slidingly with a magnetic disc rotating at high speed and there is produced little wear dust from the liner. Thus, the possibility is minimised that the error can be caused by the deposition of liner wear dust on the disc surface or magnetic head, making it possible to obtain a magnetic disc cartridge having excellent durability and reliability. The non-woven fabric used as liner may not necessarily be composed of rayon fibre alone. It is possible to mix a minor proportion of other types of fibre such as nylon, polyester etc, with rayon fibre. preferably not less than 60% by weight based on the total weight of liner. As described above, the exemplified magnetic disc cartridge in which at least one fatty acid ester with a melting point below 0°C is contained in the magnetic layer of magnetic disc housed rotatably in cartridge case and a non-woven fabric mainly composed of rayon fibre is used as liner secured on the inside of the cartridge case, has excellent durability and reliability under all possible use conditions, and even when a small-sized and a large-capacity magnetic disc having a maximum linear recording density of greater than 35 kfci and a track density of higher than 150 TPI is incorporated in said magnetic disc cartridge, error seldom occurs, and also sticking of the magnetic head or non-uniformity of rotation is prevented. The present invention will be further described below with reference to the examples thereof. Example 8 100 parts by weight of magnetic powder of α-Fe (coercive force: 1,650, Oe; saturation magnetization: 135 emu/g; BET specific surface area: 45 m²/g), 14.1 parts by weight of vinyl chloride-vinyl acetate-vinyl alcohol copolymer, 8.5 parts by weight of polyurethane resin, 5.6 parts by weight of trifunctional isocyanate compound, 20 parts by weight of alumina (average particle diameter: 0.43 µm), 2 parts by weight of carbon black, 10 parts by weight of 2-heptylundecyl oleate, 150 parts by weight of cyclohexanone and 150 parts by weight of toluene were mixed and dispersed in a ball mill to prepare a magnetic coating material. This coating material was applied on both sides of a 75 µm thick polyester film and dried so that the dry coat thickness would become 2.5 µm, and the coated film was calendered to form a magnetic layer. This magnetic layer was hot cured at 80°C for 16 hours, then punched into a disc shape and polished to obtain a magnetic disc. The thus obtained magnetic disc was incorporated in a cartridge case lined with a Kendall Co.'s 149-246 non-woven fabric (made of 100% rayon at the side contacted with magnetic disc and a mixture of 80% rayon and 20% nylon at the side contacted with cartridge case) to produce a magnetic disc cartridge. Example 9 A magnetic disc was produced in the same way as Example 8 except that in the composition of magnetic coating material, magnetic powder of barium ferrite was used in place of magnetic powder of α-Fe in the same amount, and a magnetic disc cartridge was made according to Example 1 by using said magnetic disc. Example 10 A magnetic disc cartridge was produced in the same way as Example 8 except for use of a cartridge case lined with Mitsubishi Rayon's VA450D (non-woven fabric of 100% acrylic fiber) in place of the cartridge case lined with Kendall Co.'s 149-246. Example 11 A magnetic disc cartridge was produced in the same way as Example 8 except for use of a jacket lined with Kendall Co.'s 149-007 (100% polyester non-woven fabric) in place of the cartridge case lined with Kendall Co.'s 149-246. Example 12 A magnetic disc cartridge was produced in the same way as Example 8 except for use of cartridge case lined with Kendall Co.'s 149-188 (mixed non-woven fabric of 50% rayon and 50% polyester) in place of the cartridge case lined with Kendall Co.'s 149-246. Example 13 A magnetic disc cartridge was produced in the same way as Example 8 except for use of a cartridge case lined with Mitsubishi Rayon's VK450C (mixed non-woven fabric of 50% rayon and 50% acryl) in place of the cartridge case lined with Kendall Co.'s 149-246. Comparative Example 15 A magnetic disc was produced by using a magnetic coating material of the same composition as Example 8 except that oleyl oleate was used in place of 2-heptylundecyl oleate in the same amount, and by using said magnetic disc, a magnetic disc cartridge was produced according to Example 8. Comparative Example 16 A magnetic disc was produced with the same composition of magnetic coating material as used in Example 8 except that n-butyl stearate was used in place of 2-heptylundecyl oleate in the same amount, and a magnetic disc cartridge was produced according to Example 8 by using said magnetic disc. Comparative Example 17 A magnetic disc and a magnetic disc cartridge were produced in the same way as Example 8 except that in the composition of magnetic coating material, glycerin trioleate was used in place of 2-heptylundecyl oleate in the same amount. Comparative Example 18 A magnetic disc and a magnetic disc cartridge were produced in the same way as Example 8 except that in the composition of magnetic coating material, liquid paraffin was used in place of 2-heptylundecyl oleate in the same amount. The probability of occurrence of error, durability and starting torque were examined for each of the magnetic disc cartridges obtained in the Examples and Comparative Examples. The probability of occurrence of error was determined by loading each magnetic disc cartridge with a magnetic disc drive with a recording capacity of 12.5 megabytes, maximum linear recording density of 35 kfci, track density of 406 TPI and disc speed of 360 r.p.m., and a magnetic disc drive with a recording capacity of 4 megabytes, maximum linear recording density of 35 kfci, track density of 135 TPI and disc speed of 300 r.p.m., recording and reproducing by using an amorphous magnetic head of sendust with gap length of 0.35 µm, and after confirming no dropout, conducting 2-hour seeking at 0°C and 50°C to determine the rate of occurrence of error. Durability was determined by loading each magnetic disc cartridge with a 3.5 inch magnetic disc drive, bringing the magnetic disc into sliding contact with magnetic head at 0°C and 50°C, measuring the drop of reproducing output level due to wear of magnetic layer, and counting the number of times of run that lasted until the reproducing output was reduced to 70% of the initial output. Starting torque was determined by setting each magnetic disc cartridge in a measuring device made by remodelling a magnetic disc drive, and measuring the torque after one-hour run at 0°C and 50°C. The results are shown in Table 6. As seen from Table 6, the magnetic disc cartridges obtained in accordance with this invention are low in probability of occurrence of error especially at 0°C, high in durability at 0°C and 50°C and also small in starting torque at 0°C and 50°C. These facts confirm that the magnetic disc cartridges obtained according to this invention are minimized in occurrence of error and show excellent durability and reliability under all possible use conditions. Further examples of this invention where esters of various types of branched alcohols and fatty acids were contained in the magnetic layer for reducing the coefficient of friction of magnetic disc are described below. Examples 14-20 By using the branched fatty acid esters shown in Table 7 as lubricant, there were prepared samples of magnetic coating material comprising the following composition: Magnetic powder of metallic iron (average particle diameter along major axis: 0.25 µm, average axial ratio: 8 SBET 50 m²/g, HC = 1,630 Oe, σs = 120) 100 parts Vinyl chloride-vinyl alcohol copolymer (VAGH, produced by UCC Inc.) 12.5 parts Polyurethane resin (HI-200, produced by Dai-Nippon Ink Co., Ltd.) 7.5 parts Polyisocyanate compound (COLONATE-L, produced by Nippon Polyurethane Co., Ltd.) 5 parts α-Al₂O₃ (ave. particle diameter: 0.4 µm) 20 parts Lubricant (shown in Table 1) 10 parts Cyclohexanone 157 parts Toluene 157 parts Each sample of magnetic coating material was applied on both sides of a 75 µm thick polyester film so that the coating thickness after dried would become 2.5 µm, and then subjected to a calendering treatment to form a magnetic layer. This was then hot cured at 80°C for 16 hours and the coated film was blanked into a disc shape to make a magnetic disc. The coefficient of friction of each of the thus obtained magnetic discs was measured after one-hour running at 20°C and 45°C by using a measuring device made by remodeling a 3.5 inch magnetic disc cartridge drive. The results are shown in Table 7. Example No. Lubricant Coefficient of friction 20°C 45°C 14 2-heptylundecyl stearate 0.20 0.24 15 2-heptylundecyl oleate 0.20 0.23 16 2-hexyldodecyl stearate 0.21 0.24 17 2-hexyldodecyl oleate 0.21 0.24 18 Oleyl oleate 0.25 0.27 19 Butyl cellosolve oleate 0.24 0.25 20 Liquid paraffin 0.28 0.30

Claims: A magnetic disc cartridge comprising a cartridge case (1), a magnetic disc (2) housed rotatably in said cartridge case (1), and a cleaning sheet (10) secured on the inner surface of said cartridge case (1), said magnetic disc (2) having a magnetic layer containing a ferromagnetic metal powder or barium ferrite powder and a reinforcing powder, characterised in that the reinforcing powder comprises 5 to 40% by weight of the magnetic layer, and in that at least the disc side surface layer (19) of the cleaning sheet (10), which is brought into sliding contact directly with the magnetic layer consists substantially entirely of a non-thermoplastic fibre. A magnetic disc cartridge according to claim 1, wherein said ferromagnetic metal powder is iron powder. A magnetic disc cartridge according to claim 1 or 2, wherein said reinforcing powder is aluminium oxide powder. A magnetic disc cartridge according to claim 1, 2 or 3, wherein said non-thermoplastic fibre is rayon fibre. A magnetic disc cartridge according to claim 1, 2 or 3, wherein said cleaning sheet is composed of three layers, a surface layer (19) and a backside layer (21) of the sheet being composed of rayon fibre alone and an intermediate layer (20) being composed of a mixed fabric of a rayon fibre and a thermoplastic fibre. A magnetic disc cartridge according to any one of claims 1 to 5, wherein said magnetic disc has a surface of not more than 0.01 µm in the roughness (Ra). A magnetic disc cartridge according to any preceding claim, wherein said magnetic layer contains urethane resin as a binder. A magnetic disc cartridge according to any preceding claim, wherein the magnetic layer containing a magnetic powder and a binder resin is provided on a disc-shape non-magnetic substrate with at least one fatty acid ester with a melting point of 0°C or below being contained in the magnetic layer. A magnetic disc cartridge according to claim 8, wherein the fatty acid ester contained in the magnetic layer is 2-heptylundecyl oleate. A magnetic disc cartridge according to claim 8 or 9, wherein a maximum linear recording density is at least 35 kfci. A magnetic disc cartridge according to any one of claims 8 to 10, wherein track density is at least 150 TPI.

Inventors: MIYAKE AKIRA, MIYATA TERUHISA, MIZUSHIMA KUNIO, UMEBAYASHI NOBUHIRO, MIYAKE, AKIRA, MIYATA, TERUHISA, MIZUSHIMA, KUNIO, UMEBAYASHI, NOBUHIRO
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1989
Main classification: G11B23
IPC codes: G11B23, G11B5