GB2069371A

GB2069371A - Magnetic Recording Media

Info

Publication number: GB2069371A
Application number: GB8104863A
Authority: GB
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1980-02-19
Filing date: 1981-02-17
Publication date: 1981-08-26
Also published as: GB2069371B; FR2476362B1; JPH0156452B2; DE3106228A1; NL8100802A; DE3106228C2; FR2476362A1; JPS56117336A

Abstract

A method of orientating magnetic powder in manufacturing a magnetic recording medium, the method comprising the steps of, applying magnetic paint composed mainly of acicular magnetic powder, binder and solvent to a non-magnetic substrate to form a magnetic coating layer, passing the magnetic coating layer through a magnetic field to apply an orientation magnetic field to align the magnetic powder along a predetermined direction while the coating layer is still wet and the powder is still movable in the paint, and drying the magnetic coating layer, the orientation magnetic field applied to the magnetic coating layer being larger than the coercive force of the magnetic powder and the polarity of the orientation magnetic field alternating periodically.

Description

SPECIFICATION Magnetic Recording Media This invention relates to methods of manufacturing magnetic recording media, and to magnetic recording media manufactured by such methods.

Magnetic recording media, such as magnetic tapes used in various magnetic recording and/or reproducing apparatus, for example, audio and/or video tape recorders are formed of a non-magnetic substrate with a magnetic coating layer on the surface thereof. The magnetic coating layer is formed by coating a magnetic paint containing acicular magnetic powder and binder uniformly dispersed in organic solvent. After the magnetic paint has been applied to the surface of the substrate, and while the paint is still wet and the magnetic powder in the coating layer is still movable, the magnetic tape is passed through a magnetic field to align the particles of the magnetic powder along a direction of the magnetic field, and then the coating layer is dried to fix the magnetic powder.By this orientation treatment, the magnetic characteristics along the predetermined direction are improved, for example, the rectangular ratio, which is a ratio of residual magnetic flux density to saturation magnetic flux density, is increased.

Such orientation treatment has been carried out by applying a steady magnetic field to the magnetic coating layer by a permanent magnet or a direct current electromagnet. However, even with a strong magnetic field, the orientation effect has not been sufficiently obtained, and contrary to expectations, the surface smoothness of the coating layer is apt to be deteriorated.

Several methods have been proposed to improve the orientation treatment. In one method, there is proposed an orientation apparatus including a main orientation magnet formed by a permanent magnet or direct current electromagnet generating a unidirectional magnetic field, and an electromagnetic supplied with alternating current to generate a supplementary magnetic field the polarity of which alternates periodically and which is superimposed on the main magnetic field.In another method, it is proposed that the orientation is carried out by applying a steady magnetic field along a predetermined direction and a supplementary alternating magnetic field superimposed on the main magnetic field but in a direction perpendicular to the direction of the main magnetic field, or alternatively that mechanical vibration is applied together with the main magnetic fields to improve the alignment of the magnetic powder. However, in these proposed methods, the orientation is substantially achieved by the steady main magnetic field, and the alternating magnetic field is relatively weak and just intended to cause vibration of the magnetic powder so that the particles are more easily moved during the orientation treatment.

None of these proposed methods give wholly satisfactory results. It is now thought that this is because no switching of the direction of magnetization of the magnetic powder occurs in the process of orientation. Switching here means changing the direction (that is, the polarity) of the magnetization from pointing in one direction to pointing in the opposite direction.

As shown in Figure 1 of the accompanying drawings, a magnetic recording medium 1 coated with a magnetic paint containing acicular magnetic powder, binder and solvent is passed through an orientation magnetic field apparatus 2 in the direction of an arrow a while the magnetic paint is still wet and the magnetic powder is still movable in the paint. A steady magnetic field generated by the orientation magnetic field apparatus 2 orientates the acicular magnetic powder along the direction of the magnetic field. In this case, the steady magnetic field acting on the magnetic powder on the magnetic recording medium 1 does not suddenly increase to the desired strength HOR for the orientation at the entrance to the orientation magnetic field apparatus 2, but gradually increases as the magnetic recording medium approaches the entrance.

The strength of the magnetic field acting on the magnetic powder is shown in Figure 2 of the accompanying drawings. Even if the orientation magnetic field HOR is stronger than the coercive force Hc of the magnetic powder, the magnetic powder is acted on by a magnetic field of less than the coercive force Hc for a certain, although short, period. Under the action of the magnetic field of less than the coercive force Hc, switching of the direction, that is the polarity, of the magnetization does not occur, however, the particles of the magnetic powder begin to rotate due to interaction between the existing magnetization of the magnetic powder and the magnetic field.In this case, when the existing magnetization, indicated by an arrow a in Figure 3A, of the magnetic powder 3 is inclined to the direction of the magnetic field, the magnetic powder 3 is relatively easily orientated along the direction of the magnetic field by a rotation of angle sl which is smaller than 900. While as shown in Figure 4A of the accompanying drawings, when the existing magnetization indicated by an arrow b is inclined oppositely to the direction of the magnetic field, the magnetic powder 3 has to be rotated through a large angle #, which may be as much as 1800. In this case, to complete the orientation substantial movement and a long time is necessary.Moreover, even if the orientation magnetic field is applied for a long enough time, the magnetic powder particles are apt to become entangled with each other and may get stuck in a half-way position as shown in Figure 4B of the accompanying drawings. This tendency is greater when the powder employed is poor in the dispersion characteristics.

According to the present invention there is provided a method of manufacturing a magnetic recording medium, the method comprising: preparing a magnetic paint comprising acicular magnetic powder and binder uniformly dispersed in solvent; coating said magnetic paint on a non-magnetic substrate to form a magnetic coating layer on a surface of said substrate; applying an orientation magnetic field to said magnetic coating layer, while said magnetic paint is still wet and said magnetic powder is movable in said paint, to orientate said magnetic powder along one direction; and drying said magnetic coating layer until said magnetic powder is fixed; wherein the polarity of said orientation magnetic field alternates periodically and the strength of said magnetic field is larger than the coercive force of said magnetic powder.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of orientation treatment in manufacturing a magnetic recording tape; Figure 2 is a graph for explaining an orientation magnetic field; Figures 3A, 3B, 4A, 4B and 4C are diagrams for explaining orientation; Figures 5 and 7 are graphs for showing the relation between the orientation magnetic field and the rectangular ratio; Figures 6 and 8 are graphs for showing the relation between the orientation magnetic field and the orientation ratio; Figures 9Aa, 9A2 and 9B are further diagrams for explaining the orientation; and Figure 10 is a graph for showing the relation between the powder binder ratio and the rectangular ratio.

In a method according to the invention of manufacturing a magnetic recording medium, orientation treatment is applied to a magnetic coating layer formed on a non-magnetic substrate to orientate magnetic powder in the coating layer along one direction by applying a magnetic field the polarity of which alternates periodically and which has a strength larger than the coercive force Hc of the magnetic powder. The coating layer is dried during or just after the application of the alternating magnetic.

A non-magnetic substrate, such as polyethylene terephthalate film coated with a magnetic paint containing mainly acicular magnetic powder, binder and solvent is passed through an orientation magnetic field apparatus 2 as shown in Figure 1. An orientation magnetic field generated by the apparatus 2 may be applied to the magnetic coating layer just after the coating step or during the coating step. The apparatus 2 is formed of a solenoid coil supplied by an alternating current source, so that the magnetic field which is generated alternates in polarity periodically, and has a strength larger than the coercive force Hc of the magnetic powder. At the exit side of the apparatus 2 there is provided a dryer (not shown) to dry the magnetic coating layer until the magnetic powder in the coating layer is fixed in position.Thus, the downstream end of the apparatus 2 and the upstream end of the dryer partially overlap with each other. For example, the solenoid may partially cover an outer side of the dryer. Various methods can be used to dry the coating layer, and one method is to blow warm air over the coating layer.

The frequency of the alternating magnetic field should be more than 40 Hertz, and higher frequencies result in more preferably orientation effects. The magnetic coating layer should have the alternating magnetic field greater than the coercive force of the magnetic powder applied for at least 70 milliseconds, and more preferably for at least 100 milliseconds. When the frequency is less than 40 Hertz, each period of application of magnetic field of one polarity to the magnetic powder is sufficient, even although the period is much shorter than the period in the case of a steady magnetic field, to cause undesirable rotation of the magnetic powder, since the magnetic powder is acted on by a magnetic field of less than the coercive force of the magnetic powder before being acted on by the orientation magnetic field larger than the coercive force of the magnetic powder.When the time period during which the magnetic coating layer has applied to it the alternating orientation magnetic field which is larger than the coercive force of the magnetic powder is shorter than the above described period, the orientation effect cannot be sufficiently obtained, as the period is not sufficient to rotate the magnetic powder into the direction of the orientation magnetic field. The time period can, of course, be controlled by changing the speed of the magnetic recording medium through the orientation magnetic field apparatus and the length of the orientation magnetic field along the direction of tape movement.

A steady magnetic field may be applied to the magnetic recording layer before the alternating orientation magnetic field is applied to the magnetic coating layer. In this case, the magnetic powder in the magnetic coating layer is substantially oriented by the steady magnetic field, and the time to complete the magnetic orientation by the alternating orientation magnetic field is reduced.

The magnetic powder employed in manufacturing the magnetic recording medium may be any one of gamma-Fe203, Fe304, a spinel structure forming an intermediate phase between gamma-Fe203 and Fe304, cobalt doped gamm0--Fe2O#, cobalt doped Fe304, a cobalt doped spinel structure as aforementioned, chromium dioxide, barium ferrite, various alloys or particles, such as Fe-Co, Co-Ni, Fe Co-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al or Fe-Co-V, iron nitride or mixtures thereof. These powders usually have an acicular shape.

The resinous material which is used as a binder can also be one of an extremely wide variety of binders used in the magnetic recording art, for example, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinyl acetate-maleic acid copolymers, vinyl-chloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrile copolymers, acrylic acid esteracrylonitrile copolymers, acrylic acid ester-vinyliden chloride copolymers, methacrylic acid estervinyliden chloride copolymers, methacrylic acid ester-styrene copolymers, thermoplastic polyurethane resins, phenoxy resins, polyvinyl fluoride resins, vinyliden chloride-acrylonitrile copolymers, butadieneacrylonitrile copolymers, acrylonitrile-butadiene-acrylic acid copolymers, acrylonitrile-butadiene- methacrylic acid copolymers, polyvinyl butyrals, polyvinyl acetals, cellulose derivatives, styrenebutadiene copolymers, polyester resin, phenolic resins, epoxy resins, thermosetting polyurethane resins, urea resins, melamine resins, alkyd resins, urea formaldehyde resins and mixtures of these materials.

When a polyisocyanate curing agent is used as a cross-linking agent for the binder, it is preferred that the amount of curing agent constitutes from 10 to 40 weight percent relative to the total amount of binder.

The non-magnetic base for the magnetic recording medium can also be any of an extremely wide variety of materials. For example, materials containing polyester groups such as polyethylene terephthalate, polyolefin groups such as polypropylene cellulose derivatives such as cellulose triacetate and cellulose diacetate, polycarbonates, polyvinyl chlorides, polyimides, and metallic materials such as aluminium and copper, as well as paper, can be used.

Upon preparing the magnetic paint various materials can be used as an organic solvent. For example, compounds having a ketone group such as acetone, methylethylketone, methylisobutylketone and cyclohexanone. There may be alcohol groups present such as in methanol, ethanol, propanol or butanol. The solvent may include ester groups such as in methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, ethylene glycol acetate and monothylether. It may contain a glycol ether group such as in ethylene glycol-dimethylether, ethylene glycol-monoethylether and dioxane. The solvent may be an aromatic hydrocarbon such as benzene, toluene or xylene. It may be an aliphatic hydrocarbon such as hexane or heptane. Substituted hydrocarbons such as nitropropane can be used.

These solvents can be used individually or in combination.

The magnetic coating layer of the recording medium may include an abrasive agent such as aluminium oxide, chromic oxide or silicon oxide, which materials can be used separately or in combination.

The magnetic coating layer may contain a lubricant such as a higher fatty acid, an ester of a higher fatty acid, an alcohol or silicone oil.

The magnetic coating layer may further contain an anti-static agent such as carbon black, and a dispersion agent, such as lecithin.

Next, two specific examples of the invention will be described.

Example 1 A magnetic paint having the following composition was prepared: gamma-Fe203 (magnetic powder) 100.0 parts by weight vinylchloride-vinylacetate-vinylalcohol copolymer (VAGH; trade name manufactured by Union Carbide Corporation) 15.0 parts by weight polyurethane resin (Estane 5702; trade name, manufactured by B.F. Goodrich) 1 5.0 parts by weight lecithin (dispersion agent) 1.0 part by weight methyl ethyl ketone (solvent) 150.0 parts by weight methyl isobutyl ketone (solvent) 150.0 parts by weight The magnetic paint was coated on a non-magnetic substrate, such as polyethylene terephthalate film to form a magnetic coating layer.The non-magnetic substrate coated with the magnetic coating layer, referenced 1 in Figure 1, was introduced into the orientation magnetic field apparatus 2 formed of a solenoid coil and an electric power supply for the coil to generate a variable magnetic field along the longitudinal direction of the film. The magnetic field was of between 0 and 5 KOe at the centre of the solenoid coil and alternated in direction at a frequency of 50 Hertz, the magnetic field being applied while the magnetic paint was still wet, and the magnetic powder in the paint was still movable. After passing through the apparatus 2, the magnetic coating layer was dried quickly by a dryer (not shown).

In this example, the solenoid coil had a length of 40 cm and the magnetic tape was run at a speed of 100 m/min., thus the magnetic coating layer was subjected to the orientation magnetic field for about 240 milliseconds.

The strength of the alternating magnetic field at 50 Hertz was varied between 0 and 5 KOe and a plurality of magnetic tapes were prepared. The rectangular ratio Rs (the ratio of the residual magnetic flux density Br to the maximum magnetic flux density Bm) and the orientation ratio MR (the ratio of the residual magnetic flux density along the direction of the magnetic field By, to the residual magnetic flux density along the transverse direction of the magnetic field Br1) were measured on the tapes so prepared and the measured results are shown in Figures 5 and 6.

The solid lines 4 and 5 show the rectangular ratio and the orientation ratio respectively wherein the alternating magnetic field was applied. For the purpose of comparison, the rectangular ratio Rs, and the orientation ratio MR measured on magnetic tapes prepared by application of a previously proposed orientation magnetic field, in which direct current was supplied to the solenoid coil, are shown by dotted lines 6 and 7 in Figures 5 and 6 respectively.

Example 2 Ferromagnetic chromium dioxide CrO2 powder was employed as the magnetic powder in a magnetic point otherwise as described in Example 1, and the magnetic paint so prepared was coated on polyethylene terephthalate film and the orientation magnetic field was applied as in Example 1.

The rectangular ratio Rs and the orientation ratio MR were measured and the measured results are shown in Figures 7 and 8 respectively. The solid lines 8 and 9 are the results when an alternating magnetic field of 50 Hertz was applied, while the dotted lines 10 and 1 1 are the results when a steady magnetic field as previously proposed was applied to the magnetic coating layer.

The following Table I shows the magnetic characteristics of the powder itself.

Table I Magnetic Hc. (Oe) #s (e.m. u/g) Sr/6s Powder gamma-Fe203 390 73.6 0.44 CrO2 480 80.4 0.40 Fe-Co alloy 1100 170.0 0.45 Hc: coercive force Ss: saturation magnetization Sr: residual magnetization As shown in Figure 5, when the orientation is carried out by the application of an alternating magnetic field having a strength of less than about 390 Oe, which is nearly the same as the coercive force of the magnetic powder, gamma-Fe203, the rectangular ratio Rs obtained is lower than the rectangular ratio when no orientation treatment is applied. While, when orientation by an alternating magnetic field stronger than the coercive force of the magnetic powder is used, the rectangular ratio increases suddenly.When an orientation magnetic field larger than 2 KOe, was applied, the rectangular ratios obtained both with an alternating magnetic field and the previously proposed steady magnetic field have similar values. However, in case of the steady magnetic field substantial unevenness of the magnetic and loss of surface smoothness occurred. Similar behaviour can be observed in the results of the orientation ratio MR shown in Figure 6. It is considered that when an alternating orientation magnetic field smaller than the coercive force of the magnetic powder is applied, the switching of the direction of magnetization does not occur, and it is even observed that this occurs with powder oriented perpendicular to the direction of the magnetic field.However, when an alternating magnetic field larger than the coercive force of the magnetic powder is applied, switching of the direction of the magnetization occurs to point the direction along the magnetic field, which results in the sudden increase of the rectangular ratio. Thus a high rectangular ratio can be obtained by applying an alternating magnetic field a little larger than the coercive force of the magnetic powder, which avoids the loss of surface smoothness.

Moreover, when ferromagnetic chromium dioxide (CrO2) powder is used as the magnetic powder, a high rectangular ratio and a high orientation ratio MR were obtained by the alternating magnetic field methods according to the invention, which ratios could not be obtained by using the previously proposed steady magnetic fields even by applying a steady magnetic field of several thousand Oe as shown in Figures 7 and 8.

Fe-Co magnetic alloy powder shown in Table I may, of course, be used as the magnetic powder, and a rectangular ratio as high as 81% was obtained with a method according to the invention, while the ratio obtained with the previously proposed method was 77%.

As explained above, superior magnetic orientation is achieved with methods according to the invention using an alternating orientation magnetic field larger than the coercive force applied to the magnetic coating layer. The reason is considered to be the switching of the direction of the magnetization. Thus the switching of the direction of the existing magnetization of the magnetic powder 3 occurs by the application of a magnetic field +H or -H, the absolute value of which is larger than the coercive force of the magnetic powder 3, and the angle between the spontaneous magnetization M and the magnetic field H or -H, that is 0~a is always less than 900.A torque is applied to the magnetic powder to rotate the magnetic powder to a state where the orientation energy is a minimum, that is, the long axis of the magnetic powder and the direction of magnetic field are parallel with each other, in other words O=#=0. Thus the switching of the magnetization which occurs when an alternating magnetic field larger than the coercive force of the magnetic powder is applied, the magnetic powder can be oriented to the desired direction by rotating through only a small angle. Thus entanglements between the particles of magnetic powder are avoided and movement of the magnetic powder is accelerated by the vibrations caused by the application of the alternating magnetic field, which results in improved orientation.

Methods according to the invention can also be applied with advantage to magnetic coating layers containing magnetic powder with poor dispersion characteristics or to magnetic coating layers containing large amounts of magnetic powder, that is, having a high powder to binder ratio (P/B).

Figure 10 shows the relation between the powder to binder ratio and the rectangular ratio in which the solid line 12 indicates the results with alternating magnetic field orientation according to the invention, and the dotted line 13 indicates the results with previously proposed steady magnetic field orientation. As apparent from Figure 10, there is little decrease of the rectangular ratio when the powder to binder ratio is not greater than six.

With methods according to the invention, undesirable movement or rotation of the magnetic powder which is caused by the magnetic powder being subjected to a magnetic field smaller than the coercive force of the magnetic powder can be avoided, since an alternating magnetic field is applied which changes in direction periodically, and the each fraction of the magnetic field is applied for a very short time which is not long enough to move the magnetic powder when the magnetic field is less than the coercive force of the magnetic powder.

Claims

1. A method of manufacturing a magnetic recording medium, the method comprising: preparing a magnetic paint comprising acicular magnetic powder and binder uniformly dispersed in solvent; coating said magnetic paint on a non-magnetic substrate to form a magnetic coating layer on a surface of said substrate; applying an orientation magnetic field to said magnetic coating layer, while said magnetic paint is still wet and said magnetic powder is movable in said paint, to orientate said magnetic powder along one direction; and drying said magnetic coating layer until said magnetic powder is fixed; wherein the polarity of said orientation magnetic field alternates periodically and the strength of said magnetic field is larger than the coercive force of said magnetic powder.

2. A method according to claim 1 wherein said orientation magnetic field alternately changes direction at a frequency of not less than 40 Hertz.

3. A method according to claim 1 wherein said orientation magnetic field is applied to said magnetic coating layer for not less than 70 milliseconds.

4. A method according to claim 1 wherein said orientation magnetic field is generated by a solenoid coil supplied with alternating current.

5. A method according to claim 1 wherein said drying is carried out partially simultaneously to the application of said orientation magnetic field.

6. A method according to claim 4 wherein said orientation magnetic field is applied to said magnetic coating layer by passing said substrate with said magnetic coating layer thereon through said solenoid coil.

7. A magnetic recording medium made by a method according to any one of the preceding claims.

8. A method of manufacturing a magnetic recording medium, the method being substantially as herein described with reference to Example 1 or 2.

9. A magnetic recording medium made by a method according to claim 8.