EP0520006A1

EP0520006A1 - Magnetic composition and its applications

Info

Publication number: EP0520006A1
Application number: EP19910906498
Authority: EP
Inventors: Pascale Escaffre; Joseph Dussaud; René Massart; Valérie CABUIL; Sophie Neveu
Original assignee: CIMENTS VICAT SA
Current assignee: CIMENTS VICAT SA
Priority date: 1990-03-12
Filing date: 1991-03-11
Publication date: 1992-12-30
Also published as: FR2659478B1; WO1991014260A1; FR2659478A1; AU7541991A

Abstract

Composition magnétique comprenant une émulsion de microgouttelettes d'un liquide magnétique constitué d'une suspension de nanoparticules magnétiques; lesdites microgouttelettes étant éventuellement microencapsulées. Procédé de fabrication d'un support magnétique et support magnétique obtenu à partir de ladite composition magnétique.A magnetic composition comprising an emulsion of microdroplets of a magnetic liquid consisting of a suspension of magnetic nanoparticles; said microdroplets possibly being microencapsulated. Method of manufacturing a magnetic support and magnetic support obtained from said magnetic composition.

Description

Composition ^• magnetic-and-its -applications.-

The present invention relates to a new magnetic composition and its applications. More specifically, it relates to a magnetic composition comprising an emulsion of microdroplets of a magnetic liquid. A magnetic liquid is defined as a colloidal suspension in a solvent, of magnetic particles whose size is of the order of a nanometer, so that the whole of the fluid behaves like a paramagnetic body.

For the manufacture of magnetic liquids, it is known to grind magnetite (Fe ^ 0 ₄ ) for several weeks so as to obtain grains of suitable size (10 nm) then at the end of this grinding, a surfactant is added. active in the presence of solvent (surfactant ferrofluid). Magnetic liquids can thus be stabilized in different types of liquids: oils, organic solvents, but much more difficult, polar solvents. The duration of the manufacturing process is, among other things, ^'the origin of the high price of liquid obtained by this method.

A more recent method developed by R. ASSART and which was the subject of French patent application 79/18842 (FR 2 461 521) makes it possible to obtain magnetic liquids by a simple chemical synthesis: alkaline condensation d 'an aqueous mixture of ferric ions and a divalent metal leads to the formation of ferrimagnetic macroanions which are none other than magnetite particles which are electrostatically charged on the surface. This chemical synthesis leads directly to an aqueous ionic magnetic liquid.

Magnetic materials or substances and in particular those made up of magnetic microparticles are used very widely.

Thus, the recording of various data in very varied fields (sound, image, IT) is a very good process. known and widely used which is implemented by means of supports (discs, tapes, etc.) coated with a magnetic substance.

However, the magnetic sensitivity of the medium and the definition of the recorded data depend closely on the size, the polarization and the mobility of the magnetic particles.

In addition, given the problems associated with the miniaturization of recording and playback devices, we now wish to increase the storage capacity of information at the media level while maintaining or increasing their level of performance (level of output, signal / noise ratio, resistance over time ...).

This implies in particular the development of new writing and reading techniques using improved magnetic media.

- The coding methods and authentication also use magnetic materials and improving their efficiency goes particularly through the development of new magnetic materials with ^"erasable or rewritable coding capabilities more important.

- Magnetic filtering as well as the production of magnetic display supports are implemented from materials whose magnetic density is sought to be increased. The purpose of the present invention is to provide a technical solution to the problems posed in the fields mentioned above.

This object is achieved in accordance with the object of the invention which is a magnetic composition comprising an emulsion of microdroplets of a magnetic liquid consisting of a suspension of magnetic nanoparticles.

According to a particularly advantageous embodiment, the microdroplets are microencapsulated.

To compensate for the attraction between the nanoparticles and avoid their agglomeration, the nanoparticles in suspension in the magnetic liquid are surfactated or else ionized. To fix the orientation of the nanoparticles, it is preferably carry out said ^'suspension as a. crosslinkable or hot-melt composition.

According to a particular embodiment of the invention, said suspension is a photopolymerizable composition comprising ethylenically unsaturated organic compounds and a photosensitive system.

The magnetic composition of the invention can be used in a magnetic medium intended for recording and restoring information.

The magnetic composition of the invention can also be used in a magnetic support intended for coding and authentication.

Another object of the invention is therefore a method of manufacturing a magnetic support, characterized in that a substrate forming a base is formed, an emulsion of microdroplets of an agnetic liquid consisting of a suspension, of nanoparticles is prepared. magnetic, said emulsion is applied to said substrate and, if necessary, after drying, -a protective coating is applied covering at least one face of said substrate.

In a particular embodiment, said magnetic recording or coding support comprises a substrate forming a base on which is disposed a layer consisting essentially of microcapsules containing a crosslinkable suspension of magnetic nanoparticles and, where appropriate, a protective coating allowing the crosslinking means to pass and covering the layer of microcapsules.

In another embodiment, said magnetic support comprises a substrate forming a base in which are incorporated microcapsules containing a crosslinkable suspension of magnetic nanoparticles, and a transparent protective coating allowing the crosslinking means to pass and covering at least one face of said substrate. can also use the composition of the invention in a coating for a bulletin board or in a magnetic filter.

Yet another object of the invention is a magneto-geometrical coding method characterized in that the assembly of a support at least partially coated with a magnetic composition consisting of a plurality of microcapsules each containing a crosslinkable suspension of magnetic nanoparticles to an optionally alternating magnetic field so as to orient the nanoparticles in suspension in the microcapsules and the orientation of the nanoparticles is selectively frozen in the suspension by crosslinking of the suspension so as to that after suppression of the magnetic field, there is coexistence of fixed oriented zones and non-oriented zones. The invention will be better understood on reading the description which follows, accompanied by the drawings in which:

Figure 1 shows a cross section of an embodiment of a magnetic support according to the invention.

Figure 2 shows a cross section of another embodiment.

Microencapsulation is a process by which an active agent in liquid or solid phase is coated by a coating or by a membrane forming the wall of the capsule. The preparation of microcapsules is well known and the manufacturing methods well mastered. The potential applications of microencapsulation know practically no limits.

The chemical encapsulation processes differ in terms of the formation of the wall of the microcapsule, which can be done either by coacervation or by interfacial polymerization. It is therefore possible to use different procedures for micro-encapsulation of magnetic liquids.

In general, the magnetic liquid containing the nanoparticles in suspension is prepared in a solvent with a high boiling point. The solution thus obtained is dispersed with stirring in a non-miscible secondary solvent (water for example) and possibly stabilized in the presence of a surfactant thus forming an emulsion or a microemulsion. In this way, droplets of magnetic liquid are obtained, the diameter of which is on the order of a micron, finely dispersed in an immiscible solvent .

Then, if the chemical composition of the various elements of the emulsion is suitable, chemical agents which will come, by reaction at the interface, to form an insoluble macromolecular film around the microdroplets forming the emulsion. The magnetic composition can be used directly in the form of an emulsion of microdroplets of magnetic liquid or else in the form of microcapsules obtained by the above method. The microcapsules themselves can be used as such or separated by physical means from the secondary solvent and dried. The encapsulation of the magnetic liquid makes it possible to preserve all the properties of the magnetic liquid, namely: fluid and magnetic liquid.

The manufacture of microcapsules requires a point of solvent of ^one high boiling, low vapor pressure, in close enough density of a ^"low -d'absorption in the ultraviolet and visible (max 320 nm) immiscible in polar solvents such as water, alcohols, etc. Mention may be made, without limitation, of aromatic derivatives, hydrogenated or not (for example hydrogenated terphenyl, cymene, tetralin, decalin • • •) _ι esters which are sometimes used as plasticizers for plastics (for example dioctyl phthalate, butyl laurate, butyl stearate ...), ethers (for example diglyme, diethylene glycol dibutyl ether ...).

In general, the magnetic nanoparticles in suspension are chosen from the group consisting of magnetic oxides based on ferrite such as, for example, ferrite ferrite, magite ite ( or manganese ferrite (MnFe _? 0 ₄ ). The particle size is generally between ^'3 and 500 nm and is preferably 10 nm and the volume concentration of magnetic nanoparticles in the magnetic liquid varies between 1 and 33%.

For example, the following two procedures can be cited: Example -1-: A magnetic liquid is used which consists of a suspension of magnetic nanoparticles in hydrogenated terphenyl (SANT0S0L - registered trademark - from the company MONSANTO).

In 180 g of 11% gelatin in aqueous solution at

55 ° C, 40 ml of the aforementioned magnetic liquid are also incorporated under vigorous stirring at 55 ° C. 180 g of 11% aqueous gum arabic at a pH of 9 are added to the emulsion thus formed.

630 g of water, all at 55 ° also.

Still with stirring, the mixture is rapidly cooled to 10 ° C. in the presence of 10 ml of 25% aqueous glutaraldehyde solution.

After one hour, 15 ml of a solution of random anhydride polymethylvinyl ether copolymer are added and the whole is stirred for one hour before adding 5 ml of a 20% sodium carbonate solution and the quantity required. 10% aqueous sodium hydroxide to bring the pH to 10.

The dispersion obtained is then ready to be used. Example -2 ^• :

A mixture A consists of:

95 g of urea-formaldehyde precondensate with 45% active resin and 35% solid content.

60 g of melamine-formaldehyde precondensate, approximately 76% active resin and 71% solid content.

240 g of 20% aqueous solution of acrylamide and acrylic acid copolymer (40%). 850 g of distilled water.

To 800 g of this mixture A, 200 g of water are added then, with rapid stirring, 800 g of the magnetic liquid of Example 1 are incorporated. The rest of the mixture A and 1400 g of distilled water are then added as diluent. The mixture is stirred for 30 minutes and then the pH is adjusted to 4.7 using acetic acid. We still stir for 30 minutes. The whole is water bath at 55 ° C and stirred for 2 hours at ^'this temperature. It is allowed to cool slowly for 5 to 6 hours. It is then adjusted to pH 10. This gives a liquid microcapsule solution magnetic ready to use.

The magnetic liquid can also be an aqueous suspension of magnetic particles.

The microcapsules can then be applied to the substrate serving as a base for the support or else be incorporated directly into the material of the substrate either from its manufacture, or subsequently by impregnation. This can be achieved for example by intimately mixing the microcapsules and the substrate material before the manufacture of the support, in particular in the case of a paper-type substrate made of fibers. It is also possible to envisage making the magnetic liquid in a very specific medium such as for example a photosensitive composition crosslinkable under radiation (UV or IR) or by electron beam or even a gelable formulation. With regard to the photosensitive composition, any encapsulable erisable photopoly composition can be used provided that the magnetic liquid can be produced in one of the components of the formulation.

In the most typical case, the photosensitive composition comprises one or more compounds which polymerize by exposure to radiation. The "compound" may be inherently sensitive to actinic radiation or may require a radical or ionic photoinitiator.

Photopolymerizable compositions are preferred which include ethylenically unsaturated organic compounds having at least one terminal ethylene group per molecule and a photosensitive system.

The photosensitive system can contain prepolymers (oligomers) and reactive diluents (polyfunctional monomers). The reactive prepolymers are preferably resins epoxyacrylates, urethane acrylate, polyester acrylate, polyether acrylate. The diluents are polyols _ acrylates (hexanedioldiacrylate (HDPA), tripolylene glycol diacrylate (TPGD)) Pentaerythitol triacrylate (PETIA), trimethylolpropane triacrylate. The photopolymerization induced by visible radiation or IR can bring into play a phenomenon of photosensitization of the photoinitiators of the radical polymerization reaction (originally in UV). Example ^* 3 ^* : Crosslinkable formulation under UV (UCB)

Ebecryl 1608 (epoxyacrylate resin) 34 P TPGDA (tripolylene glycûl triacrylate) 26 P OTA 480 (oligotriacrylate) 26 P benzophenone 3 P The magnetic liquid then consists of a suspension of magnetic nanoparticles (10 nm) of cobalt ferrite volume of 6% in ebecryl 1608 or TPGDA or OTA 480 or in all of the acrylate derivatives. Anti UV agents can be added to this formulation, such as IRGACURE 651 from CIBA or DAROCURE 1173 from Merck in an amount of 2 parts. Example ^* 4-:

Another advantageous formulation is the following: Ebecryl 586 60 P TMPTA 40 P benzophenone 2 P Irgacure 4 P

The magnetic liquid then consists of magnetic nanoparticles suspended in the Ebecryl at a volume concentration of 4%.

The new magnetic composition of the invention can be advantageously used in a process for recording and restoring information from supports made up of discs or tapes (sounds, images, computer data). The possible applications mainly concern digital recordings such as: longitudinal magnetic recording, perpendicular magnetic recording or magneto-thermal recording with magneto-optical reading, but also analog recording.

In these cases, the magnetic composition of the invention is used in the coating of the support.

The new magnetic composition of the invention can also be used in a coding and authentication process using "anti-theft devices" and "control systems".

In the case of codings such as “magneto geometrical” coding, a support, for example a sheet, at least partially coated with the composition of the invention, is subjected to an almost uniform magnetic field so as to orient the nanoparticles in suspension in the microcapsules and the orientation of the nanoparticles is frozen at the surface to be coded on the support by crosslinking of the suspension so that after removal of the magnetic field, there is coexistence of oriented and non-oriented areas.

This coding is erasable and rewritable as in the case of the coding of traditional magnetic supports based on ferromagnetic substances.

Crosslinking is obtained for example with UV radiation applied above a window mask placed on the support. Reading is done by detection of oriented and non-oriented areas which can correspond respectively to logical 0 and 1.

It is thus possible to code information digitally. This type of writing with UV radiation can be used during the manufacture of the sheet (or of a strip) just before the drying of the layer of magnetic composition. We then speak of a magnetic watermark which constitutes security supports. But with the composition of the invention it is also possible to use writing with UV radiation at any time after manufacture since the wall of the microcapsules isolates the magnetic particles and leaves them within the suspension not yet crosslinked any freedom of movement.

During reading, the position, shape and size of the oriented zones will be detected, which offers many possible applications.

In the case of "alternative coding", the UV source and the alternating magnetic field are fixed, it is the band to be coded which scrolls in relation to them. UV crosslinking is done on the surface to be coded, exposed or not to a magnetic field and the reading is still done by detection of oriented and non-oriented areas.

It is also possible to envisage producing an emulsion of magnetic liquid in a polymer medium and of depositing the mixture on a paper such as a polymer layer. In this case, the coding is done by action of the field after polymerization.

One can also use a gelatin as a polymer, in this case, the message is rewritable by heating the coded area (fusion -of gelatin - random orientation of the particles).

According to the invention, a magnetic support is made from a substrate forming a base on which an emulsion of microdroplets optionally encapsulated with a magnetic liquid, consisting of a suspension of magnetic nanoparticles in a crosslinkable or change-over medium, is applied. viscosity (for example hot-melt) or even evaporable (for example solvent). After drying in the field, a homogeneous layer of magnetic nanoparticles is obtained forming a magnetic support. In the case where a microdroplet emulsion is used directly, an aqueous emulsion will preferably be produced. The magnetic liquid will then be formed in a medium such as a wax, an oil, a solvent, a crosslinkable formulation. in which cobalt ferrite nanoparticles are suspended at a concentration of approximately 10% in the presence sodium dodecyl sulfate (SDS) at 0.05 to 1 mole / liter.

In the case where microencapsulated microdroplets are applied to the substrate, a crosslinkable suspension and a protective coating allowing the crosslinking means to be used.

In general, the coercivity and the persistence can be controlled by the nature, the size and the shape of the particles used.

FIG. 1 represents a cross section of a magnetic recording or coding support according to the invention comprising a substrate 1 forming a base, on which is disposed a layer 2 consisting essentially of microcapsules 3 containing a crosslinkable suspension 4 of magnetic nanoparticles 5 whose diameter is of the order of 10 nm, and a protective coating 6 allowing the crosslinking means to pass. This coating will for example be transparent in the case where the crosslinking means is UV radiation.

The protective coating 6 is formed as in Figure 1 as a layer in contact with the substrate 1 in which are embedded the microcapsul ^'are or in the form of a separate layer overlying the layer of microcapsules and without contact with the substrate.

In the embodiment of Figure 2, the microcapsules 3 are incorporated into the substrate from its manufacture by intimately mixing them with the material of the substrate from the manufacture of the support or later by impregnation. In this case, a protective coating may also be provided covering at least one face of the substrate.

The composition of the invention can also be used in a magnetic filter.

The filter is a stack of sheets coated with microcapsules containing a magnetic liquid. By the action of a magnetic field, the sheets are then magnetized and attract dust aim ^'antables (whether in liquid or gaseous medium). Cleaning the filter is particularly simple. In fact, when the magnetic field is removed, the particles which are in a liquid medium lose their orientation. The leaves no longer attract the dust and it falls and is collected. It is also possible to produce a magnetic table consisting of a support coated with microcapsules of magnetic liquid (magnetic support) which adheres to any magnetized surface or else a magnetic table consisting of a magnetic coated support which adheres to any magnetizable surface. The magnetic board can be produced from an emulsion of droplets or microcapsules by drying in the field, the suspension or core being able to be crosslinkable, hot-melt or polymer.

Claims

CLAIMS 1. Magnetic composition comprising an emulsion of microcapsules containing a magnetic liquid consisting of a suspension of magnetic nanoparticles.

2. Magnetic composition according to claim 1, characterized in that the nanoparticles in suspension are surfactated.

3. Magnetic composition according to claim 1, characterized in that the nanoparticles in suspension are ionized.

4. Magnetic composition according to one of the preceding claims, characterized in that said suspension is a crosslinkable composition.

5. Magnetic composition according to claim 3, characterized in that said suspension is a photopolymerizable composition comprising ethyléπiquement unsaturated organic compounds and a photosensitive system.

6. Magnetic composition according to claim 4, characterized in that said photosensitive system contains oligomers as prepolymers, polyfunctional monomers as reactive diluents and optionally a radical or ionic photoinitiator.

7. Magnetic composition according to claim 5, characterized in that said oligo beings are chosen from the group consisting of epoxyacrylate resins, urethane acrylate, polyester acrylate, polyether acrylate.

8. Magnetic composition according to claim 5 or 6, characterized in that said monomers are chosen from the group consisting of polyol acrylates (hexanedioldiacrylate (HDPA), tripolylene glycol diacrylate (TPGD)) Pentaerythitol triacrylate (PETIA), trimethylolpropane triacrylate.

9. Magnetic composition according to claim 2 or 3, characterized in that said suspension is a composition crosslinkable by electron beam.

10. Magnetic composition according to claim 2 or 3, characterized in that said suspension is a composition

11. magnetic composition according to claim 2 or 3, characterized in that said suspension is a composition with a change in viscosity.

12. Magnetic composition according to claim 2 or 3, characterized in that said suspension consists essentially of a polymeric medium.

13. Composition according to claim 2 or 3, characterized in that said suspension consists essentially of a solvent.

14. Composition according to one of the preceding claims, characterized in that the nanoparticles are particles of ferrite, cobalt whose diameter is of the order of lOπm.

15. Composition according to one of the preceding claims, characterized in that the volume concentration of magnetic nanoparticles in said suspension varies from 1 to 33%.

16. A method of manufacturing a magnetic support, characterized in that a base-forming substrate is formed, an emulsion of microdroplets of a magnetic liquid consisting of a suspension, of magnetic nanoparticles is prepared, said emulsion is applied to said substrate and, if necessary, after drying, a protective coating is applied covering at least one face of said substrate.

17. The method of claim 15, characterized in that said suspension is crosslinkable.

18. The method of claim 15, characterized in that said suspension is hot melt.

19. The method of claim 15, characterized in that said suspension is evaporable.

20. The method of claim 15, 16 or 17, characterized in that the microdroplets are microencapsulated.

21. The method of claim 15 or 19, characterized in that said protective coating allows the crosslinking means to pass.

22. Magnetic recording or coding support comprising a substrate forming a base, on which is disposed a layer consisting essentially of microcapsules containing a if necessary a protective coating allowing the crosslinking means to pass and covering the layer of microcapsules.

23. Magnetic recording or coding support comprising a substrate forming a base in which are incorporated microcapsules containing a crosslinkable suspension of magnetic nanoparticles and a protective coating allowing the crosslinking means to pass and covering at least one face of said substrate.

24. Magneto-geometric coding method characterized in that a support at least partially coated with a magnetic composition according to one of claims 1 to 15 is subjected to an optionally alternating magnetic field so as to orient the nanoparticles in suspension in the microcapsules and the orientation of the nanoparticles is selectively frozen in the areas to be coded by the support by crosslinking of the suspension so that after removal of the magnetic field, there are coexistence of fixed oriented zones and zones not oriented.

25. Coding method according to claim 23, characterized in that the suspension is frozen by crosslinking under the action of UV radiation.

26. Coding method according to claim 23 or 24, characterized in that the suspension is selectively frozen at any time after manufacture of the support at the areas to be coded by means of a window mask.

27. Use of the magnetic composition according to one of claims 1 to 14 in a magnetic medium intended for recording and restoring information.

28. Use of the magnetic composition according to one of claims 1 to 14 in a magnetic medium intended for coding and authentication.

29. Use of the magnetic composition according to one of claims 1 to 14, in a coating for a display board.

30. Use of the magnetic composition according to one of claims 1 to 14 in a magnetic filter.

31. Use of the magnetic composition according to one of claims 1 to 14 in a paper intended for erasable coding.