EP0440716A1

EP0440716A1 - Magnetic recoding medium

Info

Publication number: EP0440716A1
Application number: EP89912248A
Authority: EP
Inventors: Antoine Laine; Bernard Cadot
Original assignee: Kodak Pathe SA
Current assignee: Kodak Pathe SA
Priority date: 1988-10-18
Filing date: 1989-10-17
Publication date: 1991-08-14
Also published as: WO1990004842A1; EP0365448A3; JPH04501334A; FR2639647B1; ES2043077T3; ATE92662T1; EP0365448B1; EP0365448A2; FR2639647A1; DE68908077D1; DE68908077T2

Abstract

The invention relates to a magnetic recording medium which is comprised of magnetic particles and a binder. The binder is a highly cross-linked tridimensional polyurethane formed of a branched polyol including at least 8 % by mass of OH groups and having a molecular weight lower than 4000 and of an isocyanate prepolymer having at least 3 isocyanate functions. In one embodiment, the lubricant is polytetrafluoroethylene in powder form of which at least 90 % of the particles have a size lower than 3 mu m and is present for less than 2 % by mass based on the mass of magnetic particles. Application to the production of a magnetic medium which is on the one hand hard and resistant and on the other hand coherent and little abrasive.

Description

The present invention relates to a magnetic recording medium comprising a specific binder, magnetic particles and a lubricant; this medium has specific characteristics of hardness, cohesion, surface roughness, resistance to abrasion and scratches, abrasiveness and slippage making it particularly suitable, without overcoating or external protection, for uses on semi supports -rigid such as discs or cards.

Most magnetic recording media have either an overlay to protect the magnetic layer during handling, scratches, abrasions and other environmental attacks, or protection by pockets or cassettes.

If you want to get rid of external protection and make a high density recording, you must use an overlay to guarantee the total conservation of information.

However, the presence of an overlay increases the head / layer spacing, which should on the contrary be reduced when it is desired to achieve high density recording. The two objectives, on the one hand of physical protection of the recording medium, and therefore of the information recorded there, and on the other hand, of reduction of the head / layer spacing, are contradictory,

The removal of the desirable overlay is therefore only conceivable on condition that a magnetic medium is used which additionally has the properties required for the overlay. In other words, this medium must be super hard here enough to resist scratches and other attacks from the environment, have a sufficient cohesion to resist wear and tearing, not being brittle or cracking; it must withstand weathering, solvents, the temperatures of use of the magnetic support, as well as those of any finishing treatments for these supports; finally, it must generate a reduced surface friction coefficient between itself and the write / read head, in order to reduce wear and tear on both, and have good sliding properties. This implies the choice of a suitable binder for the magnetic medium and the possibility of incorporating a lubricant in this binder.

Binders commonly used for magnetic recording media are formed from a mixture (a) of a high molecular weight elastomeric polyurethane obtained by reaction of a polyol with isocyanates and containing chain extenders which give the. polyurethane its elasticity and (b) a vinyl copolymer based on vinyl chloride containing crosslinkable OH groups, which serves both as a dispersing agent for magnetic particles and participates in crosslinking with isocyanates. This set gives the layer the desired rigidity.

These kinds of binders are not, however, hard enough for use without an overcoat or protection, as they would not withstand scratches and other environmental aggressions sufficiently to guarantee the total conservation of the information.

Another kind of binder for magnetic recording medium, described in French patent 1020808, comprises a non-elastomeric polyurethane, crosslinked by an isocyanate prepolymer containing at least 3 functional groups so as to obtain a three-dimensional network. This type of binder is too hard and tends to break and crack. French patent 1311588 describes the binder of French patent 1020808 associated with a copolymer based on highly polymerized vinyl chloride which is used here as an elastomeric compound; in this case it does not contain a crosslinkable OH group and it is used in an amount of 10 to 20% in the mixture. This type of binder also lacks strength and does not meet the requirements mentioned above.

A magnetic medium has been found according to the invention having the required properties, comprising a specific binder. The subject of the present invention is a magnetic medium comprising magnetic particles and a binder formed from a crosslinked three-dimensional polyurethane, and it is characterized in that the crosslinked three-dimensional polyurethane is formed from a branched polyol comprising at least 8% by weight of OH groups, having an average molecular weight of less than 4000, and of an isocyanate prepolymer comprising at least 3 isocyanate functions.

^* One can use as polyol aromatic or aliphatic polyester polyols, or. polyether-polyol. Polyester polyols are compounds containing hydroxyl groups prepared from dicarboxylic acids or derivatives of these acids such as their anhydrides, such as adipic acid, terephthalic, phthalic, isophthalic, tetrahydrophthalic, hexahydrophthalic acids, maleic acid, and polyfunctional alcohols such as glycols, glycerol, hexanediol, hexanetriol, neopentylglycol, trimethylolpropane, pentaerythrol, etc.

Polyether polyols are also compounds containing hydroxyl groups prepared from alkylene oxides and the same alcohols as those described above for polyesters.

The amount of -COOH groups from acids and -OH groups from alcohols is such that the OH / COOH ratio is greater than 1. By condensation, all the COOH groups react, while there remain groups free hydroxyls. The polyols used according to the invention comprise at least 8% by weight of OH groups. Preferably, the dicarboxylic acid is a hydrogenated phthalic acid, optionally in admixture with other acids, and the polyfunctional alcohol is trimethylopropane, optionally in admixture with other alcohols.

The expression "isocyanate prepolymer", commonly used in polyurethane chemistry, denotes a compound with at least three isocyanate functions, that is to say a functionality of at least three. Such compounds can be obtained, for example, from 3 moles of an aliphatic or aromatic diisocyanate and 1 mole of a trivalent alcohol, such as glycerol, hexanetriol or trimethylol propane. The respective amounts of polyol and isocyanate are such that the total NCO / OH ratio is greater than 0.9, and preferably equal to or greater than 1. The expression: total OH, means that if the magnetic medium comprises another source of OH groups as polyol, it must be taken into account for the calculation of the isocyanate prepolymer content, so that the medium is fully crosslinked.

Without being bound by theoretical considerations, it is believed that the polyfunctionality of -1 * isocyanate, the specific polyol and the stoichiometry used contribute to increasing the number of crosslinking bonds of the polyurethane, which makes it possible to obtain a binder which has the properties a film-forming lacquer, that is to say which is both hard and rigid, but not brittle. The recording medium according to the invention can comprise any known magnetic particles, doped or not, such as metal oxides, such as iron, chromium oxides, metal particles such as iron particles, hexaferrites of Ba or of Sr, etc. In one embodiment, the magnetic particles are particles of barium or strontium hexaferrite having a coercive field greater than 4000 Oe (300 kA / m) and preferably greater than 5000 Oe (400 kA / m) a saturation magnetization greater than 55 emu / g and of preferably greater than 60 emu / g and a ratio of the distribution of the switching fields to the coercive field (SFD / Hc) less than 0.45 and preferably less than 0.40.

A process for the preparation of such particles is described in French patent application 8814221 entitled "Process for the preparation of magnetic particles of hexaferrites, particles obtained and product containing them".

The SFD characterizes the width of the distribution of the switching fields. From the hysteresis cycle

(magnetic flux M as a function of the magnetizing field H), the curve dM / dH is plotted as a function of H, (derived from the hysteresis cycle) and the width at half height ΔH of the peak observed when H ≈ Hc is calculated; this value is called SFD. This SFD value is divided by the coercive field of the particles, which gives the dimensionless SFD / Hc ratio.

The magnetization and the coercive field are measured with VSM (Vibrating Sa ple Magnetometer) under a maximum magnetizing field of 20,000 Oe (1590 kA / m).

The recording medium according to the invention can contain the additives commonly used in such a medium.

It can in particular contain lubricants commonly used in magnetic compositions, in order to reduce the coefficient of friction between the layer and the head, to reduce the wear of both and to improve the sliding properties . Such lubricants include, for example, fine particles of graphite or molybdenum sulfide or tungsten, silicone oils, unsaturated aliphatic hydrocarbons, fatty acid esters of monoalcohols, carbonic esters such as those described in French patent 2,094,663, fluorinated hydrocarbons or fine polymer particles such as polyethylene, polypropylene, polytetrafluoroethylene, etc.

However, in embodiments such as cards or disks, in which it is particularly advantageous not to use an overlay, on the one hand, and on the other hand in which the magnetic medium must be able to withstand temperatures of use or high treatment, for example above 100 ° C, and even at 150 ° C, solid lubricants are used, in which at least 90% and preferably at least 99% of the particles have a size equal to or less than 3 μm , at a rate of less than 2% by mass relative to the mass of the magnetic particles.

In fact, liquid lubricants tend to exude from the magnetic medium by raising the temperature, and in particular when the final treatment of the magnetic support requires high temperatures.

Fine particles of graphite, molybdenum or tungsten sulfide, or polyethylene can be used as the solid lubricant. Preferably, polytetrafluoroethylene (PTFE) is used. PTFE is sold in the form of powders by different manufacturers. One can find it for example at Du Pont de Nemours under the trade name of Vydax.

It is important that the solid lubricant is compatible with the other constituents of the recording medium, and that it does not contain more than 10% of particles having a size greater than 3 μ and, preferably not more than 1%, d on the one hand, and on the other hand, that its content in the magnetic medium does not exceed 2% relative to the mass of the magnetic particles. We can obtain such a size distribution of lubricant particles by sieving, filtration or any other known technique.

Without being bound by theoretical considerations, it is believed that the crosslinked structure of the binder is such that it makes it possible to trap the particles of solid lubricant with dimensions less than 3 μm.

The solid lubricant based on polytetrafluoroethylene can be combined with an inert polymer such as a polyolefin, for example polyethylene.

It is possible to use, in addition to PTFE as described above, known lubricants such as silicone oils, in an amount less than 1 part per 100 parts of magnetic particles. One or more surfactants can be used in the magnetic recording medium according to the invention to ensure the wettability, the dispersibility and the spreading of the dispersion. It is possible to use, for example, a polycarboxylic acid ester or a complex organic ester of phosphoric acid, lecithin, fatty acids such as palmitic acid or oleic acid, or mixtures of these surfactants. assets. A vinyl copolymer based on vinyl chloride comprising OH groups can also be used as a dispersing agent. Such a copolymer generally comprises from 70 to 90% of vinyl chloride, the other monomers being chosen from vinyl acetate, polyvinyl alcohol, and acrylic or methacrylic esters optionally carrying hydroxyl groups. In this case, account must be taken of the OH groups of the vinyl copolymer, which must also be completely crosslinked in the three-dimensional polyurethane network and the amount of isocyanate prepolymer must be adjusted accordingly. In a preferred embodiment, a alkylphenoxypolyethoxyethyl ester of phosphoric acid, at a rate of 3 to 8 g per 100 g of magnetic particles and a vinyl copolymer at a rate of less than 10% by mass relative to the total binder. In addition, the binder composition can comprise the usual additives such as abrasive powders such as Al 2O3 or Cr2O3, pigments, in the form of a fine powder or colloid, such as carbon black for coloring the layer and silica for improve the quality and stability of the dispersion.

Finally, it may optionally contain agents to promote crosslinking, depending on the binder used and the kinetics that are desired.

A coating dispersion is formed by mixing the magnetic particles, the various components of the binder, and the additives, in the presence of solvents. Solvent mixtures known in the art of magnetic dispersions are used.

Among the useful solvents, there may be mentioned esters such as methyl acetate, ethyl acetate, isopropyl acetate, tetrahydrofuran, ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, nitromethane, nitroethane, dichloroethane, benzene solvents such as toluene, etc. The useful quantities of solvents can be determined by the technician depending on the desired results, the particular compounds of the dispersion and the equipment used. Particularly useful dispersions are described in the examples. As is known in the art, the magnetic particles are first impasted with the surfactant and the vinyl copolymer, this paste is ground for example in a ball mill, then all the other constituents of the medium formula are added magnetic. A magnetic coating composition is obtained which is applies to a support in a known manner. The crosslinking forming the polyurethane takes place after coating by any known means and, in general, by heating, with, as indicated above, a crosslinking promoter, if necessary.

The magnetic medium obtained is very hard; this hardness can be evaluated by the scratch test described below. It has excellent cohesion, has both high wear resistance and low abrasiveness, and is resistant to solvents. In general, it itself presents, without overlay, wear corresponding to a loss of electrical signal equal to or less than 5% for 2000 passes, an abrasiveness less than 600 μm and a scratch resistance less than 1.5 μ. In addition, in the case where PTFE is used as described above, the magnetic medium is insensitive to temperature variations between -20 ° C and 130 ° C.

The recording medium according to the invention can be used in all cases where. We wish a rigid or semi-rigid medium without overlay, for example for discs or cards.

In a particular embodiment, the magnetic recording medium according to the invention is used in a large capacity magnetic memory card such as that described in French patent application 8814217 entitled: "Large capacity magnetic memory card and manufacturing process".

The following examples particularly relate to a card comprising a high energy and high density recording medium, having a coercive field of between 4500 and 6000 Oe, a magnetization at saturation greater than 60 emu / cm, and preferably greater than 80 emu / cm, and a ratio of the distribution of the switching fields to the coercive field less than 0.50, and preferably less than 0.30. Like on powders, the magnetization and the coercive field are measured at VSM under a maximum magnetizing field of 20,000 Oe (1590 kA / m). The invention is not however limited to such a card. The results given in the examples relate to the electrical wear, the abrasiveness, the roughness and the scratch resistance of the magnetic recording medium. These parameters are measured by the following tests: Electrical wear test

On a magnetic recording medium, a number of tracks are chosen which are distributed uniformly over the entire magnetic layer.

On each track we record a signal which we re-read a determined number of times. (Number of passages).

The amplitude of the read signal is measured using an oscilloscope. The loss of signal corresponds to the difference between the first reading and the last, and it is expressed in% of the signal collected at the first reading.

Roughness test

The magnetic medium to be measured is displaced at 40 μm / s over a distance of 2 mm. Using a converter, the displacement of a sensor is measured, the part of which in contact with the medium is a ball with a radius of 5 μm. The force applied to this ball is 0.03 mN. The analog voltage at the output of the converter, proportional to the oscillations of the sensor, is processed digitally using a computer which calculates the roughness criteria. We will use the mean arithmetic roughness (arithmetic mean deviation of the profile from the mean line, integrated over the base length). Abrasiveness test The method used to assess the abrasiveness of a layer consists in running a Vickers 52 hardness copper core, having a wire diameter of 1.6 mm and an inside loop radius of 8 mm, on the layer to be measured, over a length of 5 cm, with a force constant application F = 0.6 N and a speed of 8 cm / s, 1400 times back and forth.

One obtains on the torus an abrasion figure represented by a pseudo-ellipse whose major axis is all the greater as the layer is more abrasive. The length of the major axis of this pseudo-ellipse is measured on a microscope equipped with a micrometric target. This length is used as a parameter for defining the abrasiveness. Scratch test The scratch test is carried out by passing a spherical diamond - of radius R = 15 μm under a load of 4 g over the surface of the magnetic recording medium. The scratch resistance is expressed in scratch depth in μm, calculated from the half-width a of the imprint generated by the diamond on the layer, according to the formula: depth = R - The following commercial products are used in the examples:

- Desmophen, commercial brand for a group of polyesters and polyethers crosslinkable by isocyanates, sold by Bayer AG.

- Desmodur ®, trade mark for a group of isocyanates and isocyanate prepolymers sold by Bayer AG.

- Gafac, trademark for a group of anionic surfactants which are complex organic phosphate esters, sold by General Aniline and Film Corp.

- Vinnol brand for polyvinyl alcohol copolymers, sold by Wacker- Chemie Gesellschaft.

- Vydax, commercial brand for fluorocarbon telomeres, sold by E.I. Du Pont de Nemours.

- Lexan trademark for polycarbonates, sold by General Electric Co. The particles of barium hexaferrite used in all the examples are prepared according to the method of French patent application 8814221. They have an acicularity of between 1.5: 1 and 3: 1 and a length of 0.1 to 0.3. μm. Their saturation magnetization is 62 ± 2 emu / g, their coercive fields are between 4700 Oe and 5300 Oe, their SFD is between 1800 and 2100 Oe and their SFD / Hc ratios are between 0.34 and 0.45 .

The results of the mechanical and magnetic measurements on the cards obtained are collated in the table below for all the examples.

It is further observed that the cards obtained in all the examples have excellent resistance to solvents, to temperature and to external aggressions. Repeated bending stresses do not cause any damage to the magnetic layer. Example 1

A magnetic dispersion is prepared comprising: Ba hexaferrite particles 100.0 g

GAFAC ^® RE 610 surfactant 7.0 g

Vinnol E 22/40 A 1.0 g

Carbon black 3.0 g Desmophen ® 651 15.8 g Desmodur ® L 18.2 g Vydax ^® AR 0.4 g

The following mixture of solvents is added to this dispersion, at the rate of 55 parts of dispersion for 45 parts of mixture of solvents:

Ethyl acetate 50 parts THF 25 parts

Methyl acetate 25 parts

After stirring and filtering, the dispersion is applied to a LEXAN® 8800 polycarbonate sheet from 40 to

.60.μm thick, and on. dry to obtain a layer 6 μm thick in the dry state. Apply a rectangle of the film obtained on a stack of polycarbonate sheets to obtain a thickness of 600 μm, make four ultrasonic welding points to hold the sandwich in place, apply the sandwich on the magnetic film side on a plate glass and welded at a temperature between 160 ° C and 180 ° C, under a pressure of 100 to 1000 N / cm. We cut a card to ISO standards (54 x 86 mm) and we determine the magnetic and mechanical properties of this card. The electrical wear test is carried out on a track 200 μm wide, at a recording speed of 200 mm / s, and a frequency of 14.6 kHz. (This corresponds to a density of 1457 bits / cm). The number of passages is 2000. Example 2 A magnetic dispersion is prepared containing:

Ba hexaferrite particles 100.0 g

GAFAC ^® RE 610 3.0 g

Vinnol ^® 22/40 2.0 g Si0 ₂ 1.5 g

A1 ₂ 0 ₃ 15.0 g

Carbon black 3.0 g Desmophen ® 651 20.8 g Desmodur ® L 27.2 g Polytetrafluoroethylene / polyethylene copolymer (50/50) (particle size equal to or less than 3 μm) 1.0 g

The following mixture is used as solvent, at a rate of 53 parts of dispersion for 47 parts of mixture of solvents:

Isopropyl acetate 55

Nitroethane 15

Dichloroethane 20

Methylisobutyl ketone 10 A card is prepared as in the previous example, and the electrical wear test is carried out under the same conditions as in the previous example.

Example 3

A magnetic dispersion is prepared containing: Particles of barium hexaferrite 100 g

Polycarboxylic acid ester 6 g

Vinnol ^® E22 / 40A 1 g

Si0 ₂ 2 g

A1 ₂ 0 ₃ 15 g Carbon black 3 g

® Desmophen 651 21.4 g

® Desmodur L 27.9 g

Polytetrafluoroethylene / polyethylene (50/50)

(particle size equal to or less than 3 μm) 1.5 g The following solvent mixture is used, at the rate of 62 parts of dispersion for 38 parts of solvent mixture: Ethyl acetate 50

Methyl isobutyl ketone 25 Nitroethane 25

A card is prepared as in Example 1 and the electrical wear test is carried out under the same conditions as in Example 1. Example 4 A magnetic dispersion is prepared containing:

Barium hexaferrite particles 100 g

GAFAC ^® RE 610 3 g

Silicone Byk ^® -310 0.1 g

Vinnol ^® E22 / 40A 2.0 g Si0 ₂ 1.5 g

Carbon black 3 g Desmophen ® 651 20.2 g Desmodur ® L 27.5 g Polytetrafluoroethylene / polyethylene copolymer

(50/50) (particle size equal to or less than 3 μm) 1.0 g

The following mixture is used as solvent, at a rate of 58 parts of dispersion for 42 parts of mixture of solvents:

Cyclohexanone 22

Ethyl acetate 9

Methyl ethyl ketone 60

Toluene 9 A card is prepared as in Example 1, and the electrical wear test is carried out under the same conditions, except that the number of passes is 24,000.

Example 5

A magnetic dispersion is prepared containing: Particles of barium hexaferrite 100 g 3.0g

3.0g

0.2g

1.5g

2 g

20.5g 28g

Polytetrafluoroethylene / polyethylene copolymer (50/50) (particle size equal to or less than 3 μm) 1.2 g

The following mixture is used as solvent, at the rate of 61 parts of dispersion for 39 parts of mixture of solvents: Cyclohexanone 20

Ethyl acetate 20

Methyl ethyl ketone 38

Toluene 22

A card is prepared as in Example 1, and the electrical wear test is carried out under the same conditions, except that the number of passes is 24,000.

Example 6

A magnetic dispersion is prepared containing:

Barium hexaferrite particles 100 g Dextrol OC 70 sold by Dexter 2.0 g

Palmitic acid 1.5 g

Si0 ₂ 1.5 g

g g Desmodur ® L 28 g

Polytetraf luoroethylene / polyethylene copolymer

(50/50) (particle size equal to or less than 3 μm) 1 g Vydax ^® 1 g The following mixture is used as solvent, at a rate of 55 parts of dispersion for 45 parts of mixture of solvents:

Cyclohexanone 12

Ethyl acetate 25

Methyl ethyl ketone 63

BOARD

Examples

Measures

Magnetic

Oe coercive field

(kA / m)

SFD Oe (kA / m)

Im emu / cm ³

SFD / Hc

00

Mechanical

Electrical wear test for 2000 passages <1% <1% <1% for 24,000 passages

Roughness (harmed)

Abrasiveness (μm)

Scratch resistance (μ)

Claims

CLAIMS - Magnetic recording medium comprising magnetic particles and a binder formed from a crosslinked three-dimensional polyurethane, characterized in that the crosslinked three-dimensional polyurethane is formed from a branched polyol comprising at least 8% by weight of OH groups, having an average molecular weight of less than 4000, and of an isocyanate prepolymer comprising at least 3 isocyanate functions. - Magnetic recording medium as claimed

1, wherein the polyol is a polyester polyol. - Magnetic recording medium of any one of claims 1 to 2, wherein the amounts of polyol and isocyanate prepolymer constituting the polyurethane are such that the total NCO / OH ratio is greater than 0.9. - Magnetic recording medium according to claim 3, in which the total NCO / OH ratio is equal to or greater than 1. - The magnetic recording medium of any one of claims 1 to 4, further comprising a solid lubricant under form of particles, at least 90% of the particles having a dimension equal to or less than 3 μm, present at a rate of less than 2% by mass relative to the mass of the magnetic particles. - Magnetic recording medium according to claim 5, in which at least 99% of the particles of solid lubricant have a dimension equal to or less than 3 μm. - Magnetic recording medium according to one of claims 5 or 6, wherein the solid lubricant particles are particles of polytetrafluoroethylene. - Magnetic recording medium according to claim 7, wherein the solid lubricant further comprises a polyolefin. - Magnetic recording medium of any one of claims 1 to 8 wherein the magnetic particles are iron oxide particles. - Magnetic recording medium of any one of claims 1 to 8, wherein the magnetic particles are particles of metallic iron. - Magnetic recording medium of any one of claims 1 to 8, wherein the magnetic particles are particles of barium hexaferrites or strontium. - Magnetic recording medium of claim 11 in which the particles of barium or strontium hexaferrites have a coercive field greater than 5000 Oe (400 kA / m), a saturation magnetization greater than 60 emu / g and a ratio .of the. distribution of the switching fields in the coercive field less than 0.40. - Magnetic recording medium of any one of claims 1 to 12, which does not have an overlay and has wear corresponding to a loss of electrical signal equal to or less than 5% for 2000 passes, an abrasiveness less than

600 μm and a scratch resistance of less than 1.5 μm. - Use of the magnetic recording medium according to any one of claims 1 to 13, in a disc or a card for magnetic recording. - Use of the magnetic recording medium according to claim 13, in a high capacity magnetic memory card.