GB2057298A - Magnetic recording media - Google Patents

Abstract

A magnetic recording medium having a low coefficient of friction and good aging characteristics is formed with a lubricant which is strongly bonded to the magnetic recording medium by electron beam bombardment. The lubricant may contain a molecular chain having a lubricating property, and a double bond sensitive to dissociation products such as free radicals are coated on a magnetic layer and then an electron beam is irradiated on the magnetic layer, so that a radical reaction occurs to cause covalent bonding of the lubricant to the magnetic recording layer (e.g. polymerisation of an unsaturated monomer).

Description

SPECIFICATION Magnetic recording media This invention relates to magnetic recording media.

In any magnetic recording and reproducing system, a magnetic recording medium having a magnetic layer formed on a non-magnetizable support runs at substantial speed in contact with guide members for the medium and with magnetic heads. For this reason the magnetic recording medium must be wear resistant and have a low coefficient of friction. To reduce the coefficient of friction, the magnetic layer contains a lubricant or is coated with a layer of lubricant. However, a coating layer of lubricant on the magnetic layer is apt to come off easily, which causes a change in the coefficient of friction as the medium is used, so it is necessary to provide a substantial amount of lubricant on the magnetic layer initially.When a large amount of lubricant is provided on the magnetic recording medium, the lubricant diffuses into the magnetic layer, which softens and reduces the strength of the magnetic layer. When the lubricant is provided in the magnetic recording layer, that is, a mixture comprising magnetic powder, binder and the lubricant is coated to produce a magnetic layer, the coefficent of friction is still not stable, and moreover the lubricant exudes from the magnetic layer and causes so-called blooming. When blooming occurs, the amount of powder coming off the magnetic recording medium becomes large, which causes clogging of magnetic heads.

According to the present invention there is provided a magnetic recording medium comprising: a a non-magnetizable substrate; and a magnetic recording layer comprising finely divided magnetizable pigment and a binder formed on a major surface of said substrate; said magnetic recording medium including molecular chains having a lubricating property which molecular chains are strongly bonded to said magnetic recording medium as a result of having been exposed to a radiation.

The invention will now be described by way of example.

A magnetic recording medium comprises a non-magnetizable support and a magnetic layer, mainly composed of a finely divided ferromagnetic pigment and a binder, formed on a major surface of the support.

On the other major surface ofthe support, a non-magnetic pigment coating layer may be formed. The non-magnetic pigment coating layer, which is mainly composed of non-magnetic pigment and binder is provided to control the electric resistance and the coefficient of friction.

In embodiments of the invention, a lubricant which contains a molecular chain which acts to reduce the coefficient of friction of the medium is provided. The lubricant may be provided by adding the lubricant to the magnetic layer and/or the non-magnetic pigment layer (back coating layer), or coating the lubricant on the magnetic layer, on a back coating layer or on the surface of the non-magnetic support when the back coating layer is not provided.

The lubricant is strongly bonded to the magnetic recording medium by bombardment with radiation, such as an electron beam. The lubricant, or the binder provided together with the lubricant contains a chemical double bond which is reactive with dissociation products such as active free radicals formed by the bombardment from the lubricant itself or from the binder. The free radicals formed by the bombardment react with each other, and they also attack the double bond and react radically with the double bond to make covalent bonds, so that the lubricant is strongly bonded to the magnetic recording medium.

The lubricant can be provided in various ways. In one method, the lubricant can be provided by coating the lubricant on one of the magnetic layers, the non-magnetic pigment coating layer previously formed and the surface of the non-magnetic support. In this case the lubricant containing the double bond reactive with the dissociation products and a solvent is coated on the surface of the medium that is, on one of the surface of the support, and the layers. The lubricant may be provided on the surface of the medium, with a suitable binder and solvent therefor. In this case at least one of the lubricant and the binder must contain a double bond reactive with the dissociation products formed by the radiation bombardment, such as free radicals.

After the lubricant-containing layer is formed on the surface of the medium, an electron beam is irradiated on the lubricant-containing layer, and the above reaction occurs strongly to bond the lubricant to the medium.

In another method, the lubricant containing a chemical double bond reactive with dissociation products such as free radicals formed by the radiation bombardment can be provided by mixing the lubricant with binder and magnetic pigment or non-magnetic pigment to form magnetic paint or non-magnetic pigment paint respectively, then coating the paint on the non-magnetic support. After the magnetic layer containing the lubricant or the non-magnetic pigment coating layer containing the lubricant is formed, an electron beam is irradiated on the layer, to cause the above-mentioned reaction to occur. Thus the molecular chains which have a friction reducing function in the lubricant are strongly bonded to the magnetic recording medium.

The lubricant used contains a molecular chain which contributes to reducing the coefficient of friction of the medium. The preferred examples of such molecular chains are straight chains or branched hydrocarbon groups containing 7 to 23 carbon atoms in the group, or straight chains or branched fluorocarbon groups containing 3 to 15 carbon atoms in the group. Such molecular chains can be provided in the form of compounds, such as higher alcohols (ROH), higher fatty acids (R'COOH), esters of alcohols and higher fatty acids or higher alcohols and fatty acids, olefin modified silicone oil (for example KF-412-416 made by Shin-Etsu Chemical Co. Ltd.) and organo silicon compounds such as

where n is an integerfrom 1 to 3.

Moreover, compounds that contain a double bond in the forms of:

can be used.

Examples of such lubricants are represented by the formulae

which can be obtained by the reaction of acrylic acid or methacrylic acid and a higher alcohol; CH2=CH-CH2OCOR, which is an ester of allyl alcohol and a higher fatty acid; CH2=CHCONH CH2OCOR which is an ester of N-methylolacrylamide and a higher fatty acid; and

In the above formulae R represents a straight chain or branched hydrocarbon group.When the lubricant contains fluorine atoms in the molecular chains which exhibit the lubricating property, the general formulae are for example

where Rf is a fluorine-containing group represented for example by CnF2n+1#, CnF2n+1(CH2)m- (where m is an integerfrom 1 to 5),

In the above examples of the lubricants, when the number of carbon atoms in the hydrocarbon group is less than 7, the hydrocarbon group does not contribute to reducing the coefficient of friction, while when the number exceeds 23, the melting point of the lubricant becomes high and the solubility of the lubricant in a solvent is lowered.

When the number of carbon atoms in the fluorocarbon group is less than 3, the molecular chain does not contribute to reducing the coefficient of friction, while if it exceeds 15, the melting point of the lubricant becomes high, and the solubility is lowered, both of which are undesirable.

In embodiments of the invention, the lubricant preferably contains a double bond reactive with dissociation products formed by radiation, and as explained above, the double bond may be an unsaturated bond in the hydrocarbon group. Moreover, it is desirable that the double bond is provided by an acrylic group, when considering the reactivity. However, when the lubricant is coated on the surface of the magnetic recording medium together with a binder containing a double bond reactive with dissociation products formed by radiation, the lubricant does not necessarily contain a double bond.

The resinous material usable as a binder in the embodiments will now be described. The binder is used in the magnetic layer, in the non-magnetic pigment layer, or to form a top coating layer together with the lubricant. Examples of suitable materials, which do not contain the double bond reactive with dissociation products formed by radiation, are 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-acryionitrile copolymers, acrylic acid ester-acrylonitrile copolymers, acrylic acid ester-vinylidene chloride copolymers, methacrylic acid esters-vinylidene chloride copolymers, methacrylic acid ester-styrene copolymers, thermoplastic polyurethane resins, phenoxy resins, polyvinyl fluoride resins, vinylidene chloride-acrylontrile copolymers, butadiene-acrylonitrile copolymers, acrylonitrile-butadieneacrylic acid copolymers, acrylonitrile-butadiene-methacrylic acid copolymers, polyvinyl butyrals, polyvinyl acetal, cellulose derivatives, styrene-butadiene copolymers, polyester resin, phenolic resins, epoxy resins, thermosetting polyurethane resins, urea resins, melamine resins, alkyd resins, urea formaldehyde resins and mixtures of these materials.

The binder materials containing a double bond reactive with the dissociation products formed by the electron bombardment are monomers, oligomers and polymers containing an acrylic group and can be obtained by modifying a polyol, polyester or polyurethane having various molecular weights with a compound having an acrylic group. Examples of monomers are acrylic acid, methacrylic acid acrylamide, methacrylamide and hydroxyethylacrylate. Examples of the acryl compounds usable for the modifications are acrylic acid, methacrylic acid, acrylamide, methacrylamide, hydroxyethlacrylate and hydroxyethylmethacrylate.The above--mentioned polyol, polyester, and polyurethane may be obtained by reacting phenols, such as (poly)ethyleneglycol, propyleneglycol, trimethiolethane, trimetheolpropane, pentaen/thri- tol, polycarboxylic acids, such as adipic acid, telephtalic acid, and polyisocianate compounds such as tolylene-diisocianate or hexamethylenediisocyanate.

Suitable ferromagnetic material powders or particles which can be used in embodiments of the invention include any ferromagnetic material which finds use in a magnetic recording medium. Examples are gamma Fe2O3, Fe304, a spinel structure forming an intermediate phase between gamma Fe2O3 and Fe304, cobalt-doped gamma Fe2O3, cobalt-doped Foe30,, a cobalt-doped spinel structure as mentioned above, 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.

Suitable non-magnetic pigments contained in the non-magnetic pigment layer (back coating layer) are carbon powder (for example channel black, furnace black or graphite), and powders of ZnO, TiO2, SiO2, Awl203 or FeOOH.

The non-magnetic base 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 or cellulose diacetate, polycarbonates, polyvinyl chlorides, polyimides, metallic materials such as aluminium or copper, as well as paper can be used.

Upon preparing the magnetic paint, various materials can be used as an organic solvent. Examples include compounds having a ketone groups such as acetone, methyl ethyl ketone, methyl isobutyl ketone or 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 or ethylene glycol acetate monoethylether. It may contain a glycol ether group such as ethylene glycol dimethyl ether, ethylene glycol monoethyl ether or 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. For the purposes of embodiments of the invention, these solvents can be used individually or in combination.

The magnetic 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 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.

In keeping with modern practice, carbon black may be used as an antistatic agent and lecithin can be used as a dispersing agent.

To irradiate the magnetic recording medium, ionizing radiations such as an electron beam, a neutron beam, or gamma-rays can be used, preferably at a dose of 1 to 10 Mrad and more preferably at a dose of 2 to 7 Mrad with a radiation energy preferably of 100 keV or more. The above radiation dose is enough to start a desirable radical reaction.

When the lubricant is provided by coating on surfaces of the recording medium, the lubricant is coated together with a suitable solvent such as those sold under the registered trademark Freon (fluorocarbons compound marketed by E I du Pont de Nemours and Company) or ethyl acetate, which does not affect the magnetic layer. The desirable coating amount of the lubricant is from 30 to 1500 mg/m2. When the lubricant is coated together with the binder, the desirable coating amounts of the lubricant and the binder are from 30 to 1500 mg/m2. When the lubricant is added to the magnetic paint, a suitable amount is from 0.1 to 5 parts by weight for 100 parts by weight of the magnetic powder. When the lubricant is added to the non-magnetic pigment paint, a suitable amount is from 0.4 to 20 parts by weight for 100 parts by weight of the binder.

Specific examples will now be described: A magnetic paint was prepared as follows: Acicular magnetic pigment of gamma Fe2O3 40 weight parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer ("Vinylite VAGH", Trade Mark of Union Carbide Corporation) 50 weight parts Thermoplastic polyurethane resin ("Estane 5702" Trade Mark, made by B B F Goodrich Company) 50 weight parts Lecithin (dispersion agent) 1 weight part ~ cur203 2 weight parts Methyl ethyl ketone (solvent) 600 weight parts Methyl isobutyl ketone (solvent) 600 weight parts The above materials were charged into a ball mill and mixed therein for 20 hours, and 15 weight parts of Desmodur L (isocyanate compound made by Bayer AG) was added to the mixture to produce a magnetic paint.The magnetic paint was coated on a base film made of polyethylene terephthalate having a thickness of 16 microns to produce a magnetic layer having a dry thickness of 4 microns. The magnetic layer was dried and subjected to a calender process.

Example 1 The magnetic layer thus produced was then coated with Freon solution containing 5 weight % of an organosilicon compound with the formula: (C17H33COO)2 -Si-CH3 CH2CH2CF3 at an amount of 250 mg!m2. Then an electron beam having an energy of 200 KeV was irradiated on the coating layer on the magnetic layer at a dose of 2 Mrad.

Example 2 An organosilicon compound with the formula:

was used instead of the organosilicon compound of Example 1. Example 3 An organosilicon compound with the formula (C17H33C0O)2Si(CH3)2 was used instead of the organosilicon compound of Example 1.

Example 4 Stearyl methacrylate was used instead of the organosilicon compound of Example 1.

The following Table I shows a dynamic coefficients of friction (lid) of magnetic tape applied with the top coating layers according to Examples 1 to 4. Table I also shows, for the purpose of comparison, dynamic coefficients of friction of the top coated magnetic tape without electron beam irradiation. The value is measured by Euler's method on a drum made of aluminium.

TABLE I With electron beam Without electron beam irradiation irradiation lid udafter udas pud after prepared aging * prepared aging* Example 1 0.175 0.18 0.18 0.31 Example2 0.18 0.195 0.18 0.36 Example 3 0.20 0.22 0.21 0.40 Example 4 0.21 0.25 0.22 0.45 * + Dynamic coefficients of friction were measured on the samples kept at 600C for 10 days.

It is apparent from the results that with embodiments of the invention, it is possible to obtain a magnetic recording medium having a low coefficient of friction, and the coefficient of friction does not change witu aging.

Next, a magnetic recording layer was formed on the base film as described above. On the magnetic layer, top coating paints shown in the following Examples 5 to 8 were coated at an amount 250 mg/m2. An electron beam having an energy of 300 KeV was irradiated on the layers at a dose amount of 3 Mrad.

Example 5 Freon solution containing 0.5 weight % acryl oligomer (molecular weight of about 2000, having 3 acrylic groups) and 0.5 weight % of olive oil (mainly composed of ester of oleic acid and glycerine).

Example 6 Ethyl acrylate (molecular weight 100) was employed instead of the acryl oligomer of Example 5.

Example 7 Polymer having a molecular weight of about 20000, obtained by condensation polymerization of 20 weight parts of diacrylpentaerythritol, 30 weight parts of 1 ,6-hexanediol and 50 weight parts of adipic acid, was employed instead of the acryl oligomer of Example 5.

Example 8 5.0 weight % olive oil was employed instead of 0.5 weight % in Example 5.

Comparison Freon solution containing 2 % by weight of olive oil was coated on the magnetic layer. However, in this case no electron beam irradiation was used.

The characteristics obtained are shown in Table II.

TABLE II Life* Amount of podwer coming off 1st run 2nd run (%) (mg) Example 5 0.085 0.172 92 0.6 Example 6 0.183 0.175 91 0.7 Example 7 0.173 0.172 94 0.7 Example 8 0.170 0.170 94 0.3 Comparison 0.197 0.328 63 1.3 *ratio of output from the magnetic tapes at 1 KHZ, between the bvalue after 100 times run on a video tape recorder and the initial value.

As apparent from the results, the Examples are superior in coefficient of friction, life and the amount of powder coming off.

Example 9 A magnetic paint of the following composition was prepared: gamma - Fe2O3 400 weight parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH made by U.C.C.) 50 weight parts Estane 5702 (binder 50 weight parts Lecithin (dispersion agent) 1 weight part CH2 = CH2COOC13H27 (lubricant) 1 weight pårt W DesmodurL l5weight parts Methyl ethyl ketone (solvent) 600 weight parts Cyclo hexanone (solvent) 600 weight parts The magnetic paint was coated on a polyethyleneterephthalate film having a thickness of 16 microns to form a magnetic layer having a dry thickness of 6 microns.Thus formed magnetic tape was subjected to a calendertreatment. Then an electron beam having an energy of 150 Kevwas irradiated on the magnetic layer with a dose of 5 Mrad.

Example 10 A magnetic paint of the following composition was prepared: gamma - Fe203 400 weight parts Nitrocellulose 30 weight parts Aronix M-8030 (binder) 30 weight parts (Oligomer containing 3 acrylic groups in a molecule and having a mean molecularweight of 2000 made by Toa Gosei Kagaku-Company) Estane 5702 (binder) 50 weight parts Lecithin (dispersion agent) 1 weight part CH2 = CH-CH2CH2OCOC,7H35(1ubricant) 1 weight part Methyl ethyl ketone (solvent) 600 weight parts Cyclohexanone (solvent) 600 weight parts A magnetic paint was coated similarly to Example 9, calendered and then an electron beam having an energy of 150 KeV was irradiated with a dose of 5 Mrad.

Example 11 A magnetic tape was manufactured by using a lubricant of CH2 = CHCOOCH2CH2C8F17 instead of the lubricant of Example 9.

Example 12 A magnetic tape was manufactured by using a lubricant of

instead of the lubricant of Example 10.

Dynamic coefficients of friction of the magnetic recording layers of Examples 9 to 12 were measured and the results obtained are shown in the following Table Ill.

TABLE III d as prepared 'id after aging* 1st run 100th run 1st run 100th run Example 9 0.160 0.195 0.170 0.220 Example 10 0.158 0.178 0.165 0.200 Example 11 0.155 0.169 0.180 0.210 Example 12 0.190 0.200 0.230 0.250 * Fd was measured on the tape samples which were at 80 % relative humidity at 450C for 1 week.

As apparent from the above results, the Examples are low in dynamic coefficient of friction and stable in aging characteristics of the coefficient of friction.

Example 13 A paint of the following composition was prepared for the non-magnetic pigment coating layer (back coating layer): Carbon powder 100 weight parts ZnO powder 20 weight parts VAGH (binder) 25 weight parts Estane 5702 (binder) 25 weight parts Desmodur L 5 weight parts CH2 = CHCOOC13H27 (lubricant) 2 weight parts Methyl ethyl ketone (solvent) 200 weight parts Cyclohexanone (solvent) 200 weight parts The above composition was mixed in a ball mill for 72 hours and passed a filter having a 5 micron opening.

The paint was coated on a polyethylene terephthalate film having a thickness of 16 microns to form a backcoating layer having a dry thickness of 2 microns. An electron beam having an energy of 150 KeV was irradiated on the back coating layer with a dose of 5 Mrad.

Example 14 A back coating layer was formed by using a lubricant of CH2 = CHCOOCH2CH2CH2C8F17 instead of the lubricant of Example 13.

Dynamic coefficients of friction of the back coating layers of Examples 13 and 14 were measured, and the results obtained are shown in Table IV.

TABLE IV ssd as prepared Fd after aging 1st run 100th run 1st run 100th run Example 13 0.220 0.280 0.360 0.370 Example 14 0.200 0.250 0.280 0.380 As apparent from Table IV, the Examples are low in dynamic coefficient of friction and superior in aging characteristics.

Example 15 A paint of the following composition was prepared.

Butyral resin (binder) 10 weight parts Isopropyl alcohol (solvent) 100 weight parts CH2 = CHCOOC15H31 (lubricant) 10 weight parts.

The paint was coated on a base film of polyethyleneterephthalate and dried to form a coating layer. Then a electron beam having an energy of 150 KeV was irradiated on the coating layer with a dose of 5 Mrad.

Example 16 A paint of the following composition was prepared: Carbon powder 100 weight parts ZnO powder 20 weight parts VAGH (binder) 50 weight parts Estane 5702 (binder) 25 weight parts Desmodur L 5 weight parts Methyl ethyl ketone (solvent) 200 weight parts Cyclohexanone (solvent) 200 weight parts.

The paint was coated on a polyethylene terephthalate film to form a back coating layer having a dry thickness of 2 microns. The paint of Example 15 was coated on the back coating layer and dried. Then an electron beam the same as in Example 15 was irradiated on the layer.

Example 17 The following paintwas prepared: C7H15CH2CH2NHCH2CH2OCOCH = CH2 1 weight part Freon TF (Trade Mark of E.l. du Pont do Nemours and Company) 100 weight parts.

The paint was coated on a back surface of the polyethylene terephthalate film and the electron beam was irradiated as in Example 13.

Example 18 On the back coating layer of Example 15 the paint of Example 17 was coated and the electron beam was irradiated on the coating layer as in Example 16.

Dyanamic coefficients of friction were measured for the magnetic tapes of Examples 15 to 18 and the results are shown in Table V.

TABLE V Ftd as prepared Ftd after aging istrun 100th run Iso run 100th run Example 11 0.155 0.170 0.170 0.220 Example 12 0.220 0.260 0.270 0.300 Example 13 0.157 0.160 0.165 0.180 Example 14 0.200 0.245 0.250 0.270 As apparent from Table V, the Examples are low in dynamic coefficient of friction and superior in aging characteristics.

Claims (17)

1. A magnetic recording medium comprising: a non-magnetizable substrate; and a magnetic recording layer comprising finely divided magnetizable pigment and a binder formed on a major surface of said substrate; said magnetic recording medium including molecular chains having a lubricating property which molecular chains are strongly bonded to said magnetic recording medium as a result of having been exposed to a radiation.

2. A magnetic recording medium according to claim 1 further comprising a back coating layer comprising a non-magnetizable pigment and a binder formed on the surface of said substrate opposite to said major surface.

3. A magnetic recording medium according to claim 1 or claim 2 wherein said molecular chains are bonded to said magnetic recording layer.

4. A magnetic recording medium according to claim 1 wherein said molecular chains are bonded to the surface of said substrate opposite to said major surface.

5. A magnetic recording medium according to claim 2 wherein said molecular chains are bonded to said back coating layer.

6. A magnetic recording medium according to claim 1 or claim 2 wherein said molecular chains are provided by coating a compound containing a double bond sensitive to dissociation products formed by the irradiation, and a molecular chain having a lubricating property.

7. A magnetic recording medium according to claim 1 or claim 2 wherein said molecular chains are provided by coating a compound having a double bond sensitive to the dissociation products formed by the irradiation, and a molecular chain having a lubricating property, together with a binder and said pigment.

8. A magnetic recording medium according to claim 1 or claim 2 wherein said molecular chains are provided by coating a compound containing a molecular chain having a lubricating property and a binder containing a double bond sensitive to dissociation products formed by the irradiation.

9. A magnetic recording medium according to claim 1 or claim 2 wherein said molecular chains are provided by coating a compound containing a molecular chain having a lubricating property and a double bond sensitive to dissociation products formed by the irradiation together with a binder.

10. A magnetic recording medium according to any one of claims 6,7,8 or 9 wherein said double bond is provided by an acrylic group.

11. A magnetic recording medium according to any one of claims 6,7,8, or 9 wherein said molecular chain having a lubricating property is an aliphatic hydrocarbon group containing 7 to 21 carbon atoms.

12. A magnetic recording medium according to any one of claims 6,7,8 or 9 wherein said molecular chain having a lubricating property is an aliphaticfluorocarbon containing 3 to 15 carbon atoms.

13. A magnetic recording medium according to any one of the preceding claims wherein said radiation is an electron beam.

14. A magnetic recording medium according to any one of the preceding claims wherein said radiation has an energy of not less than 100 keV.

15. A magnetic recording medium according to any one of the preceding claims wherein said radiation is at a dose between 1 and 10 Mrad.

16. A magnetic recording medium substantially as hereinbefore described with reference to any one of Examples 1 to

17.