GB2064371A

GB2064371A - Magnetic Recording Medium

Info

Publication number: GB2064371A
Application number: GB8035609A
Authority: GB
Original assignee: TDK Corp
Current assignee: TDK Corp
Priority date: 1979-11-15
Filing date: 1980-11-05
Publication date: 1981-06-17
Also published as: DE3043024C2; GB2064371B; JPS5671822A; DE3043024A1

Abstract

A master tape consists of a high polymer film support and a magnetic coating thereon comprising in a binder resin ferromagnetic metal powder formed by wet reduction and having a saturated magnetization of at least 90 emu/g, a coercive force of at least 1,800 Oe and a square ratio of at least 0.5. The ratio of metal powder to the binder resin is in the range 100:15 to 100:33, and the surface roughness of the magnetic coating is at most 0.10 mu m. The master tape has superior properties suiting it for magnetic printing or magnetic transfer, particularly for reproducing video tapes.

Description

SPECIFICATION Magnetic Recording Medium The present invention relates to a magnetic recording medium and more particularly to a master recording medium used for duplicating magnetic recordings.

In a method for duplicating magnetic recordings, known as the contact transfer method, a recorded magnetic recording tape or sheet (master tape or sheet) which bears an original recording to be transferred is brought into contact at its magnetic surface with the magnetic surface of an unrecorded magnetic recording tape or sheet (slave tape or sheet) which has a lower coercive force than that of the master tape or sheet. Then a magnetic field is applied to the contacting magnetic tapes or sheets in such a manner that the magnetic field attenuates from its maximum value sufficiently greater than the coercive force of the slave tape or sheet but not sufficient to saturate the master tape or sheet, whereby the magnetic recording of the master tape or sheet is transferred to the slave tape or sheet.The requirements for this magnetic transfer method are that the original recording of the master tape or sheet is left unchanged or unaffected by the applied magnetic field, that the transferred recording is a copy of fidelity of the original recording, and that the quality of the transferred recording is superior. These requirements become stricter when the magnetic transfer method is applied to a high density recording, for example, a video recording. Although the purpose of magnetic transfer can be achieved by a master tape or sheet having a coercive force more than 2.5 times greater than that of the slave tape or sheet, there has not been proposed a master tape or sheet which is satisfactory for high density recordings, particularly as video tapes.

The present invention is directed to a master tape particularly but not exclusively for video tape recording, and aims to solve several problems. First of all, it must be compatible with a video slave tape having a coercive force of about 700 Oe for high density recording. This we find requires that the coercive force for the master tape is at least about 1,800 Oe. Further, magnetic transfer from the master tape to a video tape is carried out while feeding the master tape and accordingly the running property of the master tape must meet high standards. Also, drop-outs of the master tape must be as small as possible. Moreover, the magnetic properties of the master tape must be sufficiently high.

However, some of these properties are not compatible and thus no really satisfactory master tape has been proposed hitherto. Another requirement for a master tape is that its magnetic surface shall be able to make intimate contact with the corresponding slave tape surface when a high density recording such as a video recording is magnetically transferred from the master tape. Otherwise, a significant spacing loss arises resulting in a lowering of N/S (noise-to-signal) ratio and local variation in the output of the video tape copy. The surface roughness of the master tape must, we find, be less than about 0.10 ym as the surface roughness of video tapes is generally of that order of magnitude.

According to the present invention, there is provided a master tape suitable for use in a magnetic transfer method consisting of a high polymer film having a magnetic layer coated thereon, the magnetic layer comprising a ferromagnetic powder dispersed in a binder resin, and having a saturated magnetization (#s) if at least 80 emu/g, a coercive force (Hc) of at least 1,800 Oe and a square ratio ( r /a5) of at least 0.5, the weight ratio of the magnetic metal powder to the binder resin being in the range 100:15 to 100:33 and the surface roughness of the magnetic layer being at most 0.10 Mm.

The ferromagnetic metal powder is preferably produced by the wet reduction process and can easily possess the properties aS greater than 80 emu/g, Hc greater than 1800 Oe and a 5 greater than 0.5. However, the coefficient of friction, the occurrence of drop-outs and the surface roughness of a master tape produced using magnetic powder thus produced vary in wide ranges. The present invention selects the ratio of the magnetic powder to the binder resin to provide a master tape having a superior surface property, a low friction and minimized drop-outs.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a graph showing the relation between the coefficient of friction and the ratio of magnetic metal powder to binder resin; Fig. 2 is a graph showing the relation between the frequency of drop-outs and the ratio of magnetic metal powder to binder resin; and Fig. 3 is a graph showing the relation between the surface roughness and the ratio of magnetic powder to binder resin.

The magnetic metal powder used in the present invention is produced by employing any process known as the wet reduction process for example as is described in U.S. Patent Specification Nos.

4,096,316, 3,663,318 and 3,661,556. Briefly, the wet reduction process is a process involving reduction of a salt or salts of ferromagnetic metal such as iron, nickel and cobalt in aqueous solution by a reducing agent comprising a borohydride and/or a derivative thereof.

The magnetic metal powder is most desirably an iron-based alloy, preferably iron-cobalt alloy.

The use of iron is important as it gives a high saturated magnetization and a high remnant magnetization. The use of cobalt is important as it gives a high coercive force and a high square ratio.

The magnetic metal powder formed in an aqueous solution is separated as a slurry and is dried and heat-treated in a reducing or non-oxidizing atmosphere such as hydrogen or nitrogen at an elevated temperature between 12000 and 4500 C. The magnetic properties are greatly improved by such treatment.

The magnetic powder so produced is mixed with and thoroughly dispersed in a binder according to any conventional method. The binder resin may be selected from known materials such as disclosed in U.S. Patent Specification No. 4.096,31 6. Preferred as the binder resin is a binary or tertiary system which contains both thermosetting and thermoplastic resins. For example, a combination of nitrocellulose and polyurethane is a preferred binder in which polyurethane is formed in situ by reaction of its prepolymer and a curing agent.

Non-limiting, illustrative examples of the present invention now follow.

Example 1 A 1 M aqueous solution of an iron salt and a cobalt salt, with the atomic ratio of Fe2+ ion to CO2+ ion being 0.7 to 0.3, and a 1 M aqueous solution of a strong reducing agent comprising sodium borohydride were fed continuously at equal flow rates to a continuous reactor and were reacted therein to obtain by the wet reduction process a fine ferromagnetic metal powder.

The ferromagnetic metal powder was subjected to a heat treatment at 400by for two hours in hydrogen to improve its magnetic properties. The heat-treated ferromagnetic powder had the following properties: saturated magnetization (u5) 127.6 emu/g, remnant magnetization (a) 74.0 emu/g and coercive force (Hc) 1 950 Oe (at a packing rate of 0.3/cc.).

Using this ferromagnetic powder a magnetic tape was produced in the following manner. First, a magnetic coating composition was prepared according to the formulation tabulated hereunder.

Parts by weight Ferromagnetic metal fine powder 200 Polyurethane (having reactive-OH groups 25 (molecular weight 50,000) 25 Nitrocellulose 3 Lubricant 140 Toluene 140 Methyl ethyl ketone 140 Methyl isobutyl ketone The above composition was placed in a sand grinding mill and thoroughly mixed and dispersed.

Thereafter, 4 parts of polyisocyanate curing agent (DESMODUR L obtained from Bayer AG) was added and the composition was again mixed thoroughly to produce the magnetic coating material.

The resulting magnetic coating was then applied to a support consisting of polyethylene terephthalate film having a thickness of 20 #m so as to yield a dry thickness of 5,us. After drying, the coated film was subjected to a super-calendering treatment to finish its surface and was then heated at 8000 for 24 hours to cure the magnetic coating. The coated cured film was slit into a plurality of tapes having a width of 1/2 inch. The magnetic tape had a surface roughness of 0.05 to 0.07 #m.

Measurement of the magnetic properties in a magnetic field having a maximum value of 5KOe showed that the magnetic tape had the following properties. Hc: 18000e. Bm: 3500G and Br/Bm: 0.78.

The video output of the tape was + 10 dB over conventional magnetic video tape containing a magnetic powder of cobalt-sorbed iron oxide ("Avilin" sold by TDK Electronics Co. Ltd.).

A magnetic transfer test was done using the tape as a master tape and the "Avilin" tape as a slave tape. The S/N of the 50% gray signal was -1.5dB lower than the output from an "Avilin" tape which was recorded directly using a magnetic recording head.

Example 2 A 1 M aqueous solution of salts of iron and cobalt with the atomic ratio of iron to cobalt being 0.65 to 0.35 was prepared. This solution was reduced by sodium borohydride as reducing agent in a similar way to Example 1. The resulting magnetic metal powder had a saturated magnetization of 1 50 emu/g, a remnant magnetization of 80 emu/g and a coercive force of 21 50 Oe.

This magnetic powder was used to produce a coating composition according to the formulation set out in Example 1 and a master tape was prepared as in Example 1. The resulting tape had the following properties: Hc: 1950 Oe, Bm: 3700 G, Br/Bm: 0.80. Video output at 5 MHz was + 12 dB compared with that of an "Avilin" tape. The surface roughness was from 0.04 #m to 0.05 pm.

Using this master tape, a video signal was magnetically contact-transferred to an "Avilin" tape.

The S/N ratio of this tape at 50% gray signal was -0.5 dB compared with that of an "Avilin" tape recorded directly by a magnetic head.

Example 3 In Example 1, the weight ratio of the quantity of the magnetic metal powder (P) to the quantity of the binder (polyurethane and nitroceliulose) (B), that is P/B, was varied to produce a number of master tapes. The coefficient of friction of the magnetic surface of these tapes was measured and the results are shown in Fig. 1. Drop-outs were counted using the same samples and the results are shown in Fig.

2. The values of the coefficient of friction are expressed relative to the coefficient for P/B=3 taken as unity, and the drop-outs are counted number per minute when a recording was reproduced from the master tapes. As is seen from Figs. 1 and 2, the number of drop-outs significantly increases when P/B exceeds 100/15 and on the other hand the running property is adversely affected due to the increased frictional resistance when P/B is below 100/33. Further, the relation between the surface property and P/B was as shown in Fig. 3 from which it is indicated that the surface property is superior when the ratio P/B is between about 3(=.1 00/33) and about 7(=,1 00/1 5).

From the foregoing, an excellent master tape for contact magnetic transfer has been provided according to the present invention.

Claims

1. A master tape for use in a magnetic transfer method consisting of a high polymer film and a magnetic layer coated thereon, the magnetic layer comprising a ferromagnetic metal powder dispersed in a binder resin and having a saturated magnetization (aS) of at least 90 emu/g, a coercive force (Hc) of at least 1,800 Oe and a square ratio (a,/a5) of at least 0.5, the weight ratio of said metal powder to said binder resin being in the range 100:15 to 100:33, and the surface roughness of the magnetic layer being at most 0.10 Mm.

2. A master tape according to claim 1, wherein the magnetic layer has a saturated magnetic flux density of more than 3,000 Gauss, a remnant magnetic flux density of more than 2,000 Gauss and a coercive force of more than 1,800 Oe.

3. A master tape according to claim 1 or 2, wherein the binder resin contains a cured thermosetting polyurethane resin.

4. A master tape according to claim 3, wherein the binder resin further contains nitrocellulose.

5. A master tape according to claim 1 or 2, wherein the surface of said magnetic layer has a calendered finish.

6. A master tape according to any of the preceding claims, wherein the metal powder, uniformly dispersed throughout the binder resin, is the annealed product of a wet reduction process.

7. A master tape when made according to Example 1 or Example 2 herein.

8. A master tape for use in a magnetic transfer method of reproduction substantially as herein described by way of example.