WO2002001288A1

WO2002001288A1 - Switchable reflective film

Info

Publication number: WO2002001288A1
Application number: PCT/EP2001/006119
Authority: WO
Inventors: Friedrich Georg Schmidt; Andreas Pawlik
Original assignee: Creavis Gesellschaft Für Technologie Und Innovation Mbh
Priority date: 2000-06-27
Filing date: 2001-05-30
Publication date: 2002-01-03
Also published as: DE10031294A1; AU2001272432A1

Abstract

The invention relates to composite films with electrically switchable, optically reflective properties, made from two control electrodes and a micro-compartment film with cavities which contain electrophoretically mobile particles suspended in liquid. The cavities in the micro-compartment film have a conical or cone-shaped structure. The ratio of the cavities of the outer area to the base is more than 1.5 and the electrophoretic mobile particles enable total reflection of the incident light. Preferably, the geometry of the particles is flat with a form factor, thickness to diameter, of 1: 5 - 1: 1.000. The reflective composite films can be used, for example, for flexible reflective areas or for the production of window panes, coverings, greenhouse roofing, packaging, reflectors, textiles, glasses, wind screens, signals, car mirrors, headlights, toys or sun protecting devices.

Description

Switchable mirror film

The invention relates to electrically switchable composite films with optically reflective properties using electrophoretically mobile particles.

Smooth surfaces are referred to as mirrors [Kirk-Othmer (4.) 22, 176, 192; 23, 1074; Ullmann (4th) 21, 633-636; (5.) A 16, 641-647; Winnacker-Küchler (4.) 3, 138.], of which most of the incident light is reflected by reflection (reflection). Here, the reflected light can be polarized, which is why glare effects can often be switched off with the help of polarized glasses. Glass mirrors (silver mirrors) are produced by applying thin layers of silver to the surface of suitable glass: With the help of a reducing agent (glucose, fructose, galactose), an even layer of metallic silver is deposited on the previously carefully cleaned glass surface from an ammoniacal silver nitrate solution , today mainly in the so-called silver spray process, earlier in the so-called swing process. Silvering is usually followed by copper plating. The applied, very sensitive metal layers are then protected against damage by the so-called mirror coating with special coatings. For special purposes, aluminum (aluminum mirror) is applied to the glass surfaces by vapor deposition in a high vacuum instead of silver. Before the silvering was introduced by J. von Liebig (1856), mercury in amalgam form (so-called mirror amalgam, consisting of approx. 77% Hg and 23% Sn) was used as the mirror metal. With the help of modern mirror covering tapes, mirrors with edge lengths of approx. 3 4.5 m can be produced continuously. In household and Automotive Ind. plastics are also used as carrier materials (plastic metallization and galvanization). Mirror foils can also be made from plastics.

Carrier-free metal mirrors are obtained by polishing metal surfaces, which may be protected with colorless lacquer coatings to protect against corrosion.

Modern telescopic mirrors consist of alloys of 70% Cu u. 30% Sn. The galvanic mirror alloy of the Tin Research Inst contains 55% Cu u. 45% Sn; they are harder than Ni u. retain their light reflectivity better than Ag. The metal mirrors (mirror bronzes) represent the oldest group of mirrors. For example, the so-called speculum metal is a hard, tough leg. made of 65-67% Cu, up to 5% Ni, rest Sn, polished gives a beautiful shine and achieves about 70% of the light reflection from silver (slightly intensified red reflection, which gives warm shimmer). A mirror of the Romans consisted of an alloy of 64% Cu, 19% Sn u. 17% Pb, an ancient Egypt. Mirror made of 85% Cu, 14% Sn, 1% Fe, an ancient Greek. 68% Cu u. 32% Sn.

The decisive factor for the usability as a mirror is the reflection of light [Kohlrausch, Praktische Physik 3, p. 416, Stuttgart: Teubner 1996; Gauert u. Bode, reflection measuring device for serial examinations of vapor deposition layers (DFVLR-Mitt. 85-18), Cologne-Porz: DFVLR 1985 in Hecht, Optik, New York: McGraw-Hill 1987; Kohlrausch, Practical Physics 2, Stuttgart: Teubner 1996 in Lerner u. Trigg (ed.), Encyclopedia of Physics, Weinheim: VCH Verlagsges. 1991; Spectrum Scientific 1986, No. 6, 152-157], whereby depending on the physical nature of the interfaces, reflection (directional reflection) or remission (non-directional or diffuse reflection) occurs. The ratio of remitted to radiated energy in percent is called the albedo value.

Crystals of fuchsin or methyl violet u. a. Dyes reflect light similarly to metals; they show strong reflectivity in the same radiation area (green or yellow) in which they absorb light. Therefore, their color in the striking light is green (or yellow), whereas their solution in the transmitted light shows the complementary color (red or violet). Metals show anomalous reflection due to the interaction of photons with conduction electrons; they reflect the light almost completely. In the case of glass, in particular optical glass, unwanted reflection can be reduced by tempering (anti-reflective coating using anti-reflective coatings). In contrast, the reflection z. B. desirable in reflective films and gloss pigments. By selective amplification or attenuation of the reflection in certain wavelength ranges by means of interference, mirrors with a high degree of reflection are produced (eg laser mirrors).

These mirror systems are static, ie the reflectivity cannot be influenced. To change the color or transparency, ie not the reflectivity of large areas, various techniques such as. B. known thermochromism or LCD.

Color changes in small areas e.g. B. in pixel size can be used to display information.

A new development for the representation of electronically changeable information is the "electronic ink" by Prof. J. Jacobson et al. This technique uses the orientation of single or multi-colored pigment particles in an electric field to display image information. Details can e.g. B. in J. Jacobson et al., IBM System Journal 36, (1997), pages 457-463 or B. Comiskey et al., Nature, Vol. 394, July 1998, pages 253-255.

For the production of corresponding bipolar, one or two-colored particles in various embodiments and their use in electrophoretically working displays, for. B. be referred to WO 98/03896. It describes how these particles are suspended in an inert liquid and encapsulated in small bubbles of a carrier material. This technique allows the macroscopic display of two colors by rotating a two-color particle depending on the applied electric field.

WO 98/19208 describes a similar electrophoretic display, in which electrophoretically mobile particles in an optionally colored liquid can be moved by an electric field within a microcapsule. Depending on the direction of the field, the particles orient themselves towards an electrode and thus macroscopically represent yes / no color information (either the color of the particles or the color of the liquid is visible).

WO 98/41899 discloses electrophoretic displays, which are also based on the principles described above, but are particularly suitable for multi-color displays. It describes how the color impression of differently colored particles can be improved by providing the particles with reflective materials. This technique is also used to improve a white background color. It is liier not a complete reflection of the incident light, ie the entire wavelength range, but only an improved color effect, in particular the color white, due to partial reflection of the light (wavelength range). The use of a suspension with liquid-crystalline behavior is also described. The liquid crystals block or enable the electrophoretic migration of the particles depending on the applied electric field.

Such an LCD effect is an electro-rheological effect, which in suspensions of immiscible liquid-crystalline substances such as z. B. by Tajiri (Tajiri et al., J. Rheol., 41 (2), 335 (1997)) can occur. Mixtures of the surrounding (suspension) matrix and liquid-crystalline substances lead to phase-separated morphologies, in which the liquid-crystalline phase forms a higher aspect ratio (length / diameter) in the field direction when an electric field is applied. The phase separation of the liquid crystalline substances can lead to an undesirable change in the optical properties of the suspension.

The literature describes systems of liquid, inhomogeneous blends which are not based on liquid-crystalline substances but have a higher dielectric constant (e.g. Kimura et al., J. Non-Newtonian Fluid Mech. 76 (1998) 199-211) , wherein the optical properties and the dielectric constant can be modified as known to the person skilled in the art.

The electrophoretic display system described in WO 98/41898 can be produced by its special arrangement by a printing process, in particular by ink jet printing technology. Both the electrodes and the electrophoretic display itself can advantageously be produced in successive printing steps.

WO 99/56171 describes a "shutter mode" display based on the electrophoretic

Migration of particles in a suspension is described. In order to obtain a better contrast from "on" to the "off" state of the display, the cavities are conical here. The conical design enables the particles to be brought together at the smallest point of the Cavity, so that light can exit the cavity almost unhindered in this case. The viewer only perceives a small area as a point of interference. The mode of operation of the displays consisting of conical cavities corresponds to the electrophoretic displays known from the literature mentioned above.

WO 99/56171 generally relates to the representation of electronically changeable information by changing the color of pixels. A change in the reflection behavior influenced by an electric field is not described, presumably because extensive total reflection of incident light is completely unsuitable for displays. The particles disclosed in WO 98/41899 and WO 99/56171, which are equipped with reflective materials, are used to improve the color impression, i. H. only certain wavelength ranges of the incident light are to be reflected; total reflection is not desired.

In this context, US 6 017 584 and WO 99/10767 describe the use of retroreflective particles in electrophoretic displays. Again, the retroreflective particles and the corresponding cavities are produced as displays according to the intended use, i.e. the change in the macroscopically perceptible color is more important than the mirror properties. Correspondingly, cavities with a conical depth profile that would increase the mirror effect are not described.

Areas whose reflection behavior can be controlled electronically are of great economic importance. So, e.g. B. electrically switchable, mirrored window areas in winter let sunlight through, but reflect in summer.

It was therefore an object of the present invention to develop composite films which work with electrophoretic mobile particles and which have a reflectivity which can be influenced by an electric field and which enable a flat construction.

The present invention therefore relates to composite films with electrically switchable, optically reflective properties, composed of two control electrodes and one Microcompartment film with cavitates which contain electrophoretically mobile particles in a suspension liquid, the cavitates in the microcompartment film show a conical or conical depth profile, the ratio of the surface area of the cavitates to their base area is greater than 1.5 and the electrophoretically mobile particles enable total reflection of the incident light.

The electrically mobile particles are said to have total reflection, i.e. H. enable extensive reflection of the entire wavelength range of the incident light. In the ideal case, the reflectivity of the composite film according to the invention in the "on" state corresponds to that of a conventional metal / glass mirror, in the "off" state there is almost no reflection, ie. H. the color of the composite film becomes macroscopically visible (see Fig. 5b). This can be done for mirror systems such. B. be matt black. With appropriate transparent composite films, the radiation can even largely pass through the composite film (see FIG. 5a). The reflectivity of the composite film according to the invention should be almost complete, but scatter losses cannot be avoided. Therefore, 60-90% of the incident light is preferably reflected.

Composite films according to the invention can be switched between the states “specular” and “non-specular”. In the “non-reflecting” state, the composite films can be transparent or non-transparent. Of course, intermediate states are also possible, so that the mirror effect of the films can be dimmed. Composite films according to the invention are therefore particularly suitable for all mirror systems whose reflectivity should be able to be influenced, such as, for example, Another advantage can be seen in the inherent corrosion protection of the mirror systems, since the sensitive metal surfaces of the particles are encapsulated airtight.

The electrophoretically mobile particles preferably have a flat geometry with a shape factor (ratio of thickness to diameter) of 1: 5 to 1: 1,000, preferably 1:20 to 1: 500, particularly preferably 1:50 to 1: 500 (see FIG . 6 and 7).

The total reflection of the electrophoretically mobile particles preferably takes place on Metal surfaces. This can be achieved by coating the particles with one or more metal layers and / or metal oxide layers, e.g. B. by vapor deposition or electrochemical deposition z. B. of silver or nickel in the form of a coating of the particles or one or more metals and / or metal oxides in the form of embedded flakes or tinsel.

Depending on the manufacturing process, the shape of the platelets varies between circular and irregular. The thickness of the platelets influences the mechanical stability in e.g. the dispersion. A subsequent comminution that occurs during the dispersion can lead to losses in the optical quality.

Various surface-modified gloss pigments can be used as electrophoretically active and mobile particles. In addition to interference pigments and pearlescent pigments, metallic effect pigments are used, which are generally platelet-shaped particles made of non-ferrous metals, which are predominantly produced by the so-called Hall process, in which e.g. pure (approx. 99.5%) aluminum is atomized to a fine aluminum powder. This intermediate product is sifted, comminuted and deformed into ball-shaped particles in ball mills with the addition of low-molecular paraffin mixtures and process additives. The additives prevent i.a. a cold welding of the platelets (literature: textbook of coating technology, Brock et al. 1998, Vincentz Verlag Hannover). The choice of additives can also be used to produce hydrophilic (e.g. by using stearic acid) or oleophilic (e.g. by using oleic acid) particle surfaces. Depending on the surface coverage of the platelets by the appropriate additives and the matrix used, the distribution of the platelets in the suspension and the possible attachment to the wall of the cavities can be influenced. By creating an oleophilic particle surface, the dispersion in a non-polar suspension liquid is increased. A more hydrophilic surface modification increases the buildup on a wall of cavities with a more polar character than the suspension liquid.

Corresponding aluminum pigments are commercially available, for example, from Schlenk Metallpulver GmbH & Co. KG, Roth-Barnsdorf, Germany, under the name Decomet. The form factor and the orientation of the particles in the electric field are of crucial importance for the mirror effect. 7a shows a good alignment of the particles in the electric field with a resulting good mirror effect without an interference pattern. The particles or the cavity in FIG. 7b, on the other hand, show a mirror effect, but a bad reflection pattern due to the incomplete alignment of the particles in the electrical field, ie the reflected radiation has interference losses.

Pearlescent and interference pigments are also platelet-shaped. The incident light is partially reflected at the phase bounds of the suspending agent / pigment or, if applicable, at phase boundaries inside the particles - depending on the refractive index difference of the phases involved. In contrast to the metallic effect pigments, the pearlescent pigments are translucent. This enables special effects of a 3D-like shine to be created. The schematic structure is shown in Fig. 8, wherein a) a metal oxide and b) z. B. called mica.

The mica flakes are covered with a transparent metal oxide layer (eg TiO ₂ ) of a precisely defined layer thickness. By choosing the thickness, different color effects can be achieved through interference effects. Interference can be avoided with thin layers. Layers, for example TiO ₂ rutile, with a thickness between 30 and 60 μm, particularly preferably between 40 and 50 μm, are preferred here.

Furthermore, metal pigments or inorganic pigments such as fuchsin, methyl violet, mica or glasses can be used as the reflective material.

The mirror effects can also by the use of effect pigments such. B. from the product series Iriodin® or Afflair® from Merck KG aA, Darmstadt or with a special angle-dependent changing of the color effect by pigments of the Colorstream® series from Merck KG aA, Darmstadt.

The conical or conical depth profile of the cavities is shown in FIG. 1. It is a particular feature of the present invention that is the eye of the beholder the side of the cavities facing ("top surface", a in Fig. 1) is larger than the side facing away from the eye ("base surface", b in Fig. 1). The ratio of the surface area to the base area of the cavities should be greater than 1.5, preferably greater than 25, particularly preferably greater than 100, very particularly preferably greater than 250. 1c shows an example of a selection of depth profiles of the cavities in a side view.

The size distribution of the cavities or compartments can be monomodal, unimodal, bimodal or multimodal, each with a stochastic or regular arrangement in the carrier material of the micro-compartment film.

It is advisable to arrange the cavities in columns and rows. However, this arrangement does not necessarily have to be rectangular or even square, also e.g. B. oblique arrangement of the rows and columns or hexagonal arrangements of the cavities are possible. 2 shows an exemplary selection of arrangements of cavities in a top view.

The present invention also relates to a method for producing the composite films according to the invention, the cavities in the micro-compartment film being produced by eroding or machining processes. A suitable eroding process uses laser radiation.

The cavitates can e.g. B. by needling, embossing, 3D printing, eroding, etching, molding with molding compounds, injection molding, photographic or photolithographic processes or interference methods in a carrier material or in the micro-compartment film. How such microstructured surfaces can be produced is e.g. B. in DE 29 29 313, WO 97/06468, US 4,512,848, DE 41 35 676, WO 97/13633 or EP 0 580 052. Younan Xia and George M. Whitesides in Angew describe further methods for producing small structures. Chem. 1998, 110 568-594. These methods, called "soft lithography", enable the production of very thin structures in the range from below 1 μm to approximately 35 nm. Another method is the micro-milling of a master, with which plates or foils with the desired microstructure can be produced. The master represents a negative form. This can then be embossed, cast or Injection molding processes are molded.

Alternatively, an unstructured film can be provided with cavities of the desired dimensions and shapes. There are also eroding or machining methods such as laser radiation or drilling / milling z. B. with a CNC machine.

The carrier material of the cavitates, i.e. H. the micro-compartment film can be optically transparent, colorless or colored. The control electrodes are attached above and below the cavitates on the carrier layer, the one arranged above the cavitates, i. H. The electrode lying between the viewer and the cavity is of course as transparent or colored as the support material can be. In order to keep the voltages of the electrodes low, the control electrode located below the cavities is usually placed between the lighting unit and the cavities and should then be transparent.

If the carrier material, suspension liquid and electrodes are transparent, the mirror composite film according to the invention can be switched between at least two different optically transparent states.

Ideally, this means switching the composite film between "optically transparent" and "reflective".

Optical transparency or non-transparency, i. H. The "extreme" switching states represent "specular". In practice, extensive transparency / non-transparency, for example for darkening or dimming windows, for example for improved thermal insulation, will be sufficient.

All mechanically or lithographically editable polymers such as thermoplastics, polycarbonates, polyurethanes, polysiloxanes, polyolefins such as, for example, are suitable as carrier material for the cavitates, ie as a micro-compartment film. B. polyethylene, polypropylene, COC (cyclo-olefinic copolymers), polystyrene, or ABS polymers, PMMA, PVC, polyester, polyamides, thermoplastic elastomers or crosslinking materials, such as UV-curing Acrylate paints, but also polytetrafluoroethylene, polyvinylidene fluoride or polymers made from perfluoroalkyoxy compounds, be it as a homo- or copolymer or as a component of a blend of a polymer blend.

Apart from the conical or conical shape of the depth, the cavities can have any shape when viewed from above. 2 shows an exemplary selection. The cavities expediently have a round, oval, triangular, right-angled, square, hexagonal or octagonal surface on the side facing the viewer's eye (viewing surface).

The surface area of the cavities should be larger than 10,000 μm ² , preferably larger than 40,000 μm ² , particularly preferably larger than 62,500 μm ² and very particularly preferably larger than 250,000 μm ² .

The depth of the cavities can be between 20 and 250 μm, preferably between 30 and 200 μm, very particularly preferably 50 to 100 μm, regardless of the visible area.

The web widths between the individual cavities on the top of the micro-compartment film should be kept as small as possible; webs with a width of 2-50 μm are preferred, particularly preferably 5-25 μm. The top of the web of the micro-compartment film can be opaque coated or mirrored. So z. B. an aluminum lamination, metal bedding or a TiO ₂ coating can be made. This prevents unwanted light from escaping from the webs when the light from the cavities is blocked by the electrophoretically mobile particles.

After the carrier layer has been provided with the desired cavitates, the cavitates are filled with the electrophoretically mobile particles and the suspension liquid. This can e.g. B. by slurrying and scraping off the excess suspension, by direct knife-coating / coating of the suspension, by means of ink-jet technology in one printing process or by self-filling by means of capillary forces. Through these measures, the particle suspensions are introduced directly into the cavities. The cavities must then be encapsulated or sealed. The filling can also be done through the Capillary forces occur via fine channels, the cavities being closed before the filling process. This is expediently carried out with a cover film which is tightly connected to the micro-compartment film or to the webs of the cavities. Various methods can be used to seal the cavities, e.g. B.:

- Gluing or thermal fusion (microwave heating, contact or friction welding, hot melt adhesive, hot lamination)

- Reactive resins, in particular UV-curing (e.g. acrylate dispersions) or 2-component systems (e.g. polyurethane coating systems) that do not mix with the pigment suspension, interfacial polymerization, interfacial polycondensation and other processes that z. B. also be applied in the field of microencapsulation technologies, such as. B. in "Microencapsulation: methods and industrial applications", Ed. S. Benita, Marcel Dekker, Inc. NY / 1996 for the encapsulation of spherical particles.

Suspensions of electrophoretically mobile particles which have already been encapsulated can also be used. H. prepared capsules are used. As shown in FIG. 3, these prepared capsules can be pressed or pressed into the cavities of the micro-compartment film. The cavities filled in this way must then be sealed again with a cover film. With a correspondingly adjusted ratio between capsule size and micro-compartment size, this technique significantly reduces the requirements for the stability of the capsule wall material for practical use, since the capsules are enclosed by the webs of the micro-compartment film. Furthermore, the classification of the capsules in the prepared cavities forces a regular arrangement of the capsules.

It is important in both variants that there is no air or other gas inclusion when sealing, that there are no reactions between the suspension medium or the microparticles of the suspension and the capsule layer and that there are no leaks to the environment or connections between the individual cavities.

The cavitates or the prepared capsules can be mixed with a suspension or melu Suspensions that can be optically transparent or non-transparent are filled.

Non-transparent suspensions can be colored or matt black. This is of particular interest in the case of mirror systems for motor vehicles, since it enables a continuous dimming effect to be produced in rear-view mirrors or headlights.

Furthermore, it is possible to dispense with coloring by the suspension, i. H. fill the cavities with the particles with an optically transparent and colorless suspension liquid. As optically transparent and colorless liquids such. B. non-polar organic liquids such as paraffin or isoparaffin oils, low molecular or low viscosity silicone oils.

The suspension liquids can also be optically transparent and colored. Colored suspensions must have a lightfast color and must not react with the material of the micro-compartment film or the top layer. They can also contain fluorescent or phosphorescent substances. The use of fluorescent or phosphorescent substances enables special optical effects. As fluorescent dyes are such. B. Coumarin 314T from Acros Organics or Pyromethene 580.

The production of the electrophoretically mobile particles which are between 0.1 and 120 μm, preferably between 3 and 70 μm, particularly preferably between 5 and 60 μm in diameter, or at the widest point when using particles with a form factor, can be based on WO 98/41898, WO 98/41899 or WO 98/0396. This includes the already mentioned coating of the pigments with organic and / or polymeric materials and / or the use of the pure pigments, which, for. B. have been provided with electrical charges by treatment of charge-controlling additives (see in particular WO 98/41899).

The particles must be able to move freely in the suspension liquid so that the particles move to one of the electrodes due to their charge, depending on the applied electric field can. The "off'7" on "state of a cavity, ie the macroscopically perceptible color or the reflectivity is therefore determined by the spatial arrangement of the particles and can be controlled by the electric field.

4 shows an exemplary structure of a composite film according to the invention, in which a) top layer b) front electrode c) microcompartment film with cavities and d) counter electrode.

If the particles are localized by the electric field on the side of the cavities facing away from the viewer (base area, "b" in FIG. 1), the particles are not or only slightly visible to the viewer, and the incident light can be almost unimpeded by the The suspension liquid and the carrier material pass through or there is largely no reflection of the light on the particles (cavity f in FIG. 4). The particles are located in cavity g in FIG. 4 on the side of the cavities facing the viewer and thus reflect incident light. The result is a mirror surface, with only webs of the carrier material being visible. The webs of the micro-compartment film should therefore be made as thin as possible and / or have an opaque or mirrored coating.

To control the cavities or the particles, two electrodes (b and e in FIG. 4) are necessary, of which the front electrode (b in FIG. 4) should preferably be largely transparent to the incident light.

The control of the electrodes, i. H. in extreme cases, the addressing of individual cavities can e.g. B. by a row / column arrangement of switch units according to WO 97/04398. If the cavities are too small for a single control, several cavities are switched per switch unit.

It would be desirable to have the switching states of the composite films without external electrical Make the field visible over a longer period of time.

Rheologically controllable suspensions, which contain electrophoretically mobile particles, are suitable for displaying switching states that persist over a longer period of time even without an external electric field. Such suspensions are e.g. B. described in DE 100 21 984.5.

In a further embodiment of the present invention, the suspension with electrophoretically mobile particles additionally contains electrorheologically active additives. The electrorheological effect is preferably negative here.

With the help of the suspension with a negative electrorheological effect, bistable composite films are obtained. When an electric field is applied, the electrophoretically mobile particles orient themselves according to their charge in the field, i.e. H. the external viewer perceives either the reflection of the particles or the color of the suspension liquid. The particles can move freely in the suspension when an electric field is applied. If the electric field is removed, the viscosity of the electrorheological suspension rises sharply and the particles are largely fixed in the order state they have just taken. The set reflectivity of the composite film is fixed accordingly so that it remains stable even without an external electrical field.

The composite films of the invention can be very thin (2 to 5 mm) and are therefore particularly suitable for three-dimensionally shaped objects, such as. B. the inside of concave cavities such as headlights for motor vehicles.

In order to obtain suspensions with a negative electrorheological effect used in the invention, either a dissolved substance or electrophoretically mobile particles with a negative electrorheological effect can be used as the electrorheologically active additive.

It is possible that the suspension contains several types of particles, at least one of which negative electrorheological and at least one other particle type shows the electrophoretic effect.

Suspensions of liquids that change their viscosity in the presence or absence of an electrical field (electrorheological effect, ER) are known. A distinction is made in the literature between positive and negative ERs, the viscosity of the positive ER increasing with increasing electric field strength and decreasing of the negative ER. The cause of positive and negative ER are not yet fully known (e.g. BT Uemura et al., Polym. Prep. ACS, Div. Polym. Chem., 1994, 35 (2), 360-361; K. Minagawa et al ., Journal of Intelligent Material Systems and Structures, Vol 9 8/1998, 626-631; HC Conrad et al., J. Rheol. 41 (2) 1997, 267-281; O. Quadrat et al., Langmuir 2000, 16, 1447-1449; C. Zukowski IV et al. J. Chem. Soc, Faraday Trans. I, 1989, 85 (9), 2785-2795; T. Hao et al., Langmuir 1999, 15, 918-921 ); are attributed to a reordering of molecules due to van der Waals interactions, which are overcome when an electric field is applied.

For an additive dissolved in the suspension liquid with a negative electrorheological effect, z. B. hydrated polycondensates of phenyl isocyanate and polytetramethylene glycol or p-chlorophenyl isocyanate and polytetramethylene glycol or polymethyl methacrylate as the alkali salt or as a blend with polystyrene block (polyethylene-co-propylene).

Only the negative electrorheological effect of the suspension is important for the present invention. Substances, liquids or suspensions are known which, in addition to a theological, i.e. H. viscosity-changing effect show a liquid crystal effect (LCD) when an electric field is applied.

This additional LCD effect, which has a negative influence on the optical properties of the suspension or the composite film, has nothing in common with the negative electrorheological effect of the suspension and is not desired here.

Such an LCD effect is an electrorheological effect, which is not the case with suspensions miscible liquid-crystalline substances, such as, for. B. by Tajiri (Tajiri et al., J. Rheol., 41 (2), 335 (1997)) can occur. Mixtures of the surrounding (suspension) matrix and liquid-crystalline substances lead to phase-separated morphologies, in which the liquid-crystalline phase forms a higher aspect ratio (length / diameter) in the field direction when an electric field is applied. The phase separation of the liquid crystalline Susb punch can lead to an undesirable change in the optical properties of the suspension.

In the literature, systems of liquid, inhomogeneous blends are described which are not based on liquid-crystalline substances but have a higher dielectric constant (e.g. Kimura et al., J. Non-Newtonian Fluid Mech. 76 (1998) 199-211) , wherein the optical properties and the dielectric constant can be modified as known to the person skilled in the art.

In the embodiment of the present invention with electrorheological suspensions, therefore, only suspensions with a negative electrorheological effect can be used which show no or only slight optical changes when an electrical field is applied.

In order to obtain the suspensions used in the invention with a negative electrorheological effect, the suspension ala additive can either contain a dissolved substance or electrophoretically mobile particles which show the negative electrorheological effect.

The substances dissolved in the suspension are generally polymeric in nature and therefore only soluble in the suspension liquid up to a certain molecular weight. Which substance is sufficiently soluble in which liquid can easily be determined by means of orientation tests.

It is also possible to use the abovementioned substances in particle form, ie as particles which are not electrophoretically mobile, as an electrorheological control substance (rheological control agent, RCA). Furthermore, the electrophoretically mobile particles, which allow total reflection of the incident light, can themselves show the required negative electrorheological effect. This can e.g. B. hydrated by coating electrophoretically mobile particles with polycondensates of phenyl isocyanate and polytetramethylene glycol or p-chlorophenyl isocyanate and polytetramethylene glycol or polymethyl methacrylate as alkali salt or as a blend with polystyrene block (polyethylene-co-propylene).

In addition to the coating with RCA substances, the particles can have a coating with polyacrylates, polymethacrylates, polyurethanes or polyamides, either above or expediently below the RCA substance.

The electrophoretically mobile particles themselves can contain inorganic or organic pigments such as e.g. B. TiO ₂ , Al ₂ O ₃ , ZrO ₂ , FeO, Fe ₂ O ₃ , carbon black, fluorescent pigments, phthalocyanides, porphyrins or azo dyes. Such particles can again have a coating made of polyacrylates, polymethacrylates, polyurethanes or polyamides.

Finally, the use of the composite films according to the invention is the subject of the present invention. Due to the switchability, as well as the flat and optionally flexible design, the mirror composite films can be used in particular for novel designs of mirror surfaces, in particular for the production of window panes, covers, greenhouse roofs, reflectors, packaging, textiles, glasses, signals, car mirrors, car headlights, toys or Sun protection devices are used.

Claims

claims:

1. Composite foils with electrically switchable, optically reflective properties, made up of two control electrodes and a micro-compartment foil with cavities which contain electrophoretically mobile particles in a suspension liquid, characterized in that the cavities in the micro-compartment foil show a conical or conical depth profile, whereby the ratio of the surface area of the cavities to their base area is greater than 1.5 and the electrophoretically mobile particles allow total reflection of the incident light.

2. Composite films according to claim 1, characterized in that the micro-compartment film is optically transparent.

3. Composite films according to claims 1 or 2, characterized in that the suspension liquid is optically transparent and colorless.

4. Composite films according to claims 1 or 2, characterized in that the suspension liquid is optically transparent and colored.

5. Composite films according to one of claims 1 to 4, characterized in that the electrophoretically mobile particles which allow total reflection of the incident light have a flat geometry with a form factor (thickness: diameter) of 1: 5 to 1: 1,000.

6. Composite film according to one of claims 1 to 5, characterized in that the electrophoretically mobile particles that allow total reflection of the incident light are coated with one or more metal layers and / or metal oxide layers or contain one or more metals and / or metal oxides in the form of flakes.

7. Composite films according to one of claims 1 to 6, characterized in that the surface area of the cavities is larger than 10,000 μm ² .

8. Composite films according to one of claims 1 to 7, characterized in that the cavities have a depth of 20 to 250 microns.

9. Composite films according to one of claims 1 to 8, characterized in that the cavities in the micro-compartment film are separated from one another at the top by webs with a width of 2 to 50 μm.

10. Composite films according to one of claims 1 to 9, characterized in that additives with an electrorheological effect are contained in the suspension liquid.

11. Composite films according to claim 10, characterized in that the additive is a substance homogeneously dissolved in the suspension liquid with an electrorheological effect.

12. Composite films according to claim 11, characterized in that as a substance dissolved in the suspension liquid with an electrorheological Effect polycondensates of phenyl isocyanate and polytetramethylene glycol or p-chlorophenyl isocyanate and polytetramethylene glycol or polymethyl methacrylate hydrated as alkali salt or used as a blend with polystyrene block (polyethylene-co-propylene).

13. Composite films according to claim 10, characterized in that as an additive, electrophoretically mobile particles which allow total reflection of the incident light and have an electrorheological effect are used.

14. Composite films according to claim 13, characterized in that several types of particles are present in the suspension, at least one type of particles showing the electrorheological effect and at least one type of particles showing the electrophoretic effect.

15. Composite films according to one of claims 13 or 14, characterized in that the electrophoretically mobile particles with polycondensates of phenyl isocyanate and polytetramethylene glycol or p-chlorophenyl isocyanate and polytetramethylene glycol,

Polymethyl methacrylate is hydrated as an alkali salt or coated with polystyrene block (polyethylene-co-propylene) as a blend.

16. Composite films according to one of claims 13 or 14, characterized in that the electrophoretically non-mobile particles of phenyl isocyanate and polytetramethylene glycol or p-chlorophenyl isocyanate and polytetramethylene glycol, hydrated polymethyl methacrylate as the alkali salt or as a blend with polystyrene block (polyethylene co-propylene) consist.

17. Composite films according to one of claims 1 to 16, characterized in that the suspension is contained in regularly or stochastically arranged compartments with a unimodal, bimodal, monomodal or multimodal size distribution.

18. Composite films according to one of claims 1 to 17, characterized in that the electrophoretically mobile particles contain inorganic or organic pigments.

19. Composite films according to claim 18, characterized in that the inorganic or organic pigments contain TiO ₂ , Al ₂ θ ₃ , ZrO ₂ , FeO, Fe ₂ O ₃ , carbon black, fluorescent pigments, phthalocyanines, porphyrins or azo dyes.

20. Composite films according to one of claims 1 to 19, characterized in that the electrophoretically mobile particles which allow total reflection of the incident light are coated with polyacrylates, polymethacrylates, polyurethanes or polyamides.

21. A method for producing composite films with electrically switchable optical reflective properties according to one of claims 1 to 20, characterized in that the cavities in the microcompartment film by eroding or chipping

Procedures are generated.

22. Use of the composite films with electrically switchable optical reflective properties according to one of claims 1 to 20 for flexible mirror surfaces.

23. Use of the composite films with electrically switchable optical reflective properties according to one of claims 1 to 20 for the production of window panes, covers, greenhouse roofs, reflectors, packaging, textiles, glasses, signals, car mirrors, car headlights, toys or sun protection devices.