JP2515686B2 - Optical element dispersion, optical element using the same, and method of driving the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element used for a light valve, a display device, a dimming window, etc., and particularly, an element for controlling light transmittance and reflectance by applying a voltage, The present invention relates to a driving method of this element and a dispersion used in this element.

[0002]

2. Description of the Related Art An optical element (called a colloidal light valve) that controls the light transmittance and reflectance by dispersing anisotropic particles in a medium and applying an AC voltage to the particles to change the orientation of the particles. The principle has been known for a long time (see, for example, US Pat. No. 1,955,923). As shown in FIG. 1, this optical element 1 typically has a cell 4 formed by joining two opposing transparent walls (supports 2) with a sealing material 3,
A suspension 7 in which anisotropic particles 5 are dispersed in a dispersion medium 6 is enclosed, and an AC voltage is applied from a power source 9 through a transparent electrode 8 provided on the inner surface of the cell 4 wall to obtain anisotropic particles. The orientation of No. 5 is changed. At this time, at least one of the electrodes and the cell wall must be transparent, and an oxide conductor is usually used as the transparent electrode.

In such an optical element, particles are distributed in a random direction when no voltage is applied, and when a voltage is applied, the particles are oriented in the direction of the electric field and light is easily transmitted. I'm taking advantage of that. Further, since not only the transmittance but also the reflectance and the refractive index are changed, it is also used as a reflection mirror. On the other hand, an optical element very similar to the colloidal light valve is called an electrophoretic display element. The difference between the electrophoretic display element and the above optical element is mainly in the particle shape and the voltage. That is, in an electrophoretic display element, a suspension in which spherical particles are dispersed is injected into a cell, a DC voltage is applied, and a change in the refractive index when particles are attracted to one electrode by using an electrophoretic phenomenon is observed. It is used for display. The shape of the particles is not particularly limited, but spherical particles or isotropic particles are generally used. When the particles are not colored, it is common to add a coloring material to the solvent.

[0004]

However, when a voltage is applied to the above optical element for a long time, ON-OFF is repeated, or switching is repeated between the anode and the cathode, particles adhere to the electrode. Agglomeration of the spot pattern occurs. This aggregation is irreversible, and once generated, the light control performance is significantly reduced. Such an aggregation phenomenon
This is a problem common to both optical elements. Since the behavior of particles in the two elements is different, the mechanism of aggregation may be different in detail, but both are basically the same phenomenon in that the particles collide with and adhere to the electrodes. The various cohesive improvement measures up to now are also based on a common technical idea. For example, a method of coating an electrode (see, for example, JP-A-59-171931), a method of vibrating the electrode by attaching a piezoelectric element to the back surface of the electrode (eg,
JP-A-56-66830), a method of applying a high frequency voltage (see, for example, JP-A-46-4464), and a method of using a copolymer as a dispersant (for example, JP-A-62-62).
No. 232623) is disclosed.

The general contents of the particle-dispersed optical element, which is the object of the present invention, and particularly the suspension thereof will be described below. A particle-dispersed optical element is one in which a DC or AC voltage is applied to a suspension in which particles are dispersed to move the particles, causing the particles to change to a dispersed state in some form, and to change the light transmittance and the refractive index. Its characteristic is to change the optical function.
Therefore, suspensions are generally required to have the following properties. (1) Electrically insulating (2) There is a difference in dielectric constant between particles and solvent (3) Viscosity is low enough to allow particles to move freely (4) Particles are stably dispersed for a long time (5) Does not change due to external stimuli such as temperature or UV rays (6) Particles are larger than the wavelength of visible light (7) Volatility of solvent is small To meet these requirements In addition, the solvent is xylene,
It is common to use non-aqueous solvents such as silicones, aliphatic oils, esters and halogenated resins. When specifically selecting the type of solvent, in addition to the above properties, it is necessary to consider economic properties in addition to physical properties such as balance with specific gravity of particles and chemical properties such as polarity.
A dispersant is used to disperse the particles stably, and a surfactant or a polymer is often used as the dispersant. Both homopolymers and copolymers are used as macromolecules. In order for the dispersant to exert its function, it needs to be dissolved in the solvent and strongly adsorbed to the particles. Dispersion stability is maintained by electrostatic repulsion between particles and steric hindrance due to polymers.To this end, it is necessary to select a specific dispersant according to the specific combination of particles and solvent. is there.

In an actual suspension, the solvent is often a mixed solution of a plurality of solvents in order to adjust the electric resistance, the viscosity or the solubility of the polymer. In such a situation, the theoretical basis for determining the composition of the suspension is extremely weak, and it is almost always determined based on experience and trial and error. The reason why a spot pattern is formed on the electrode when a voltage is applied to such a suspension is not known in detail, but is generally understood as follows. When a voltage is applied, whether direct current or alternating current, the particles move in the direction of the electric field and collide with the electrodes. Since there is a certain adhesive force between the particles and the electrode, the particles cannot be separated from the electrode even by a reverse voltage during repeated collisions, and the particles develop into agglomerates. Since the dispersant is adsorbed not only on the surface of the particles but also on the electrodes, in order for the particles to adhere to the electrodes, they must overcome the repulsive force between the dispersants and collide with each other.

The above-mentioned inventions relating to the improvement of aggregation are all made from such a viewpoint.
Since the adhesive force of the electrode is achieved by reducing the polarity of the electrode material, a method of covering the electrode surface with a non-polar substance is disclosed in, for example, JP-A-59-171931. Patents relating to the use of a dispersant having a high dispersibility are disclosed in JP-B-62-40389 and JP-A-62-1
32623 and the like. A method of coating the particles themselves with an organic substance is also disclosed in JP-A-62-183439.
No. Further, Japanese Patent Application Laid-Open No. 46-4464 discloses a method of applying a high frequency for the purpose of reducing the moving distance of particles to reduce the number of collisions with electrodes.
However, although all of these methods have certain effects, they have not yet achieved practical performance, and optical elements utilizing these methods have not yet been put to practical use. The present invention has been made in view of the above circumstances, and an object thereof is to provide means for effectively obtaining the effect of preventing particle aggregation.

[0008]

The present invention is made from a technical point of view completely different from the above-mentioned conventional method. As already mentioned, the suspension is generally non-aqueous. However, since the substances constituting the suspension including the solvent are not completely non-water-absorbing, the suspension raw material contains some water due to moisture absorption. Although not a raw material, various reagents used in the suspension manufacturing process also contain water. For these reasons, a slight amount of water (for example, about 1400 to 1800 ppm) remains in the suspension in most cases. The inventors of the present invention have earnestly studied to find a solution to aggregation, and as a result,
Focusing on the slight amount of water present in the suspension, it was thought that this might affect the aggregation of the particles, and the present invention was carried out. It is not clear by what mechanism a small amount of water affects the aggregation in the non-aqueous suspension, but it is generally considered as follows. The water molecules exist in a state of being bound to the functional groups on the surface of the particles or the functional groups of the dispersant molecule, and as a result, the reactivity of these functional groups is changed. Also, it is adsorbed on the electrode surface to form a thin film of water molecules. In such a state, the polarities of the particles, the solvent, and the dispersant necessary for achieving good dispersion are lost, so that aggregation easily occurs.

More specifically, for example, the adsorption of water on the functional groups of the polymer dispersant may cause the branches of the polymer to be easily entangled with each other. Further, since a water film is formed on the electrode surface, the adhesive force of particles is increased. When water is adsorbed on the surface of particles, the particles are more likely to adhere to the electrodes. As a result of these, the particles easily aggregate on the electrode. Therefore, the present inventors tried to solve the agglomeration of the particles by reducing the water content contained in the suspension used for the particle-dispersed optical element as much as possible within a practical range. As a result, the inventors have invented that the amount of water in the suspension is 1200 ppm or less (preferably 800 ppm or less) in terms of weight fraction, and coagulation can be practically solved by driving with a high frequency voltage of 100 Hz or more. (Japanese Patent Application No. 4-8497)
No. 5). Various experiments have been repeated since the application, and the water content is newly 500 ppm or less (preferably 200 ppm or less).
By doing so, it was found that agglomeration does not occur even when driven with a frequency voltage of about 30 Hz, and the present invention has been completed.

The removal of water is extremely effective for preventing the aggregation of particles, and can be freely combined with the various aggregation countermeasures cited above to provide a more effective aggregation improving method. For example, when combined with hydrophobic treatment of the electrode, a significant improvement effect can be realized. In addition, in the cited document
In the detailed description of No. 4464, it is stated that agglomeration can be prevented by applying a high frequency of 300 kHz or more, but in the case of the present invention, it is only necessary to apply an AC voltage having a much lower frequency of 30 Hz or more. Therefore, practically sufficient coagulation resistance can be realized. Here, the frequency is set to 30 Hz or higher because the cohesion resistance is insufficient at a frequency lower than 30 Hz. Further, it is considered that the higher the frequency of the applied voltage, the more effective the prevention of aggregation is. However, in the present invention, the aggregation prevention effect is dramatically improved by removing water, and the improvement of the aggregation prevention effect by increasing the frequency is not so great. On the contrary, the higher the applied voltage is, the more the power consumption of the element increases, which causes the element to generate heat and the durability is impaired. Therefore, it is desirable to suppress the frequency to 2000 Hz. Furthermore, in practice, it is more desirable to set the frequency of the applied voltage to less than 100 Hz for the same reason as above.

In order to reduce the water content of the suspension, it is necessary not only to reduce the water content in the raw materials but also to take care of the water removal in the series of steps for preparing the suspension. is there. In fact, even with such consideration, it is often impossible to reduce the concentration to 500 ppm or less, and it is more effective to perform dehydration treatment such as adding molecular sieves after the suspension is prepared. However, the present invention does not limit these individual reducing means, and includes any method for finally reducing the water content of the suspension to a certain value or less. That is, the present invention is a non-aqueous dispersion having a water content of 500 ppm or less by weight and a non-aqueous dispersion for use in all optical elements for controlling light transmittance and refractive index by applying voltage regardless of the means. The present invention relates to an optical element using a dispersion and a driving method thereof. Further, in another invention, by driving an optical element having a water content of 500 ppm or less with a low frequency applied voltage, economic efficiency is improved without impairing the effect of preventing aggregation of particles, and durability of the element is improved. It is what makes me.

By the way, the effect of the present invention has been described above with respect to the element in which the suspension is directly enclosed in the cell. However, the embodiments of the present invention are not limited to devices having such a structure. There are various inconveniences such as pressure adjustment in directly enclosing a liquid suspension. In order to avoid such a point, it is also possible to adopt a manufacturing method in which the suspension is dispersed in a support such as a polymer, or the dispersion surrounded by the support is sandwiched between two pieces of glass. In particular, for the former, methods known to those skilled in the art such as the microcapsule method and the polymerization method (for example, Japanese Patent Laid-Open No. 2-2
34541, JP-A-63-303325, JP-A-3.
No. 18831) can be adopted. Also,
The support is a fibrous body or a porous body, and it is also possible to use a dispersion obtained by impregnating this with a suspension. In such a case, the particles in suspension will not collide with the electrodes, but will collide with the support. However, the phenomenon that particles collide with and adhere to the surrounding walls when a voltage is applied is common to the aggregation of particles in a particle dispersion type optical element. Reducing water content is only one embodiment of the present invention. Further, the method of forming the support and the selection of the material are not limited to the above examples in the present invention.

[0013]

In other words, according to the present invention, the water content in the suspension is reduced as much as possible, so that the polarities of the particles, the solvent and the dispersant are kept in good balance, and the adhesive force of the particles is not increased. Therefore, the aggregation of particles is very effectively prevented.
Furthermore, the effect of the present invention is not limited to the prevention of aggregation. Suspensions can be roughly classified into those in which inorganic particles are dispersed and those in which organic particles are dispersed.In the latter case, in particular, the constituent substances of the suspension are altered by heat or ultraviolet rays. In many cases, the suspension deteriorates by reacting with each other or with each other. Even in the former case, the solvent and the dispersant may react. Many of the alterations and reactions are related to the oxidation reaction, which are important. The residual water content in the suspension means that an oxygen source is present. For example, deterioration due to ultraviolet rays can be regarded as promotion of oxidation reaction due to ultraviolet rays, and thus removal of water improves the ultraviolet resistance of the suspension. As described in the examples, in fact, the suspension of the present invention exhibits a life of 5 times or more as compared with the conventional suspension. Further, the same effect can be observed in heat resistance.

[0014]

EXAMPLES Examples of the present invention will be described below.
For the colloidal light valve, the effect of the amount of water on the aggregation of particles was examined using the five types of suspensions A to E in Table 1.
The composition of each suspension in Table 1 is g in 100 g of the suspension.
It is shown as a weight component. However, the amount of solvent is 10
It is the remaining amount in 0 g. The water content was adjusted by subjecting the main constituents of the suspension including the solvent to a humidification treatment or a dehydration treatment, and the water content was measured by the Karl Fischer method for the suspension. These suspensions were injected into a cell with a gap of 0.1 mm made of ITO sputtered glass to fabricate a light valve. This light valve has an effective voltage of 100V, 5
Apply 0Hz AC voltage and turn on / off every 5 seconds
Was repeated. An index representing the degree of aggregation was previously determined, and the number of repetitions in which the same aggregation state was obtained was recorded. However, the test was terminated after the number of repetitions of 200,000 times.
The result of the aggregation test is shown in FIG. When the water content of each suspension is 500 ppm (weight fraction) or less, the number of repetitions up to aggregation rapidly increases, and the effect of preventing aggregation by dehydration treatment is clear. The water content is preferably 200 ppm or less.

[0015]

[Table 1]

[0016]

As described above, according to the dispersion for optical elements of the present invention, the water content of the suspension is reduced as much as possible, so that the aggregation of particles is effectively prevented and the suspension is further suppressed. The deterioration of the liquid is prevented. By constructing an optical element using this dispersion, it is possible to obtain an optical element in which aggregation of particles and deterioration of a suspension are prevented, and performance does not deteriorate for a long period of time. Further, even if this optical element is driven with an alternating voltage of 30 Hz or higher, no aggregation occurs.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a typical structure of an anisotropic particle dispersion type optical element.

FIG. 2 is a graph showing the effect of water in suspension on the aggregation of colloidal light valves.

[Explanation of symbols]

 1 Optical Element 4 Cell 5 Anisotropic Particle 6 Dispersion Medium 7 Suspension 8 Transparent Electrode

Claims (7)

(57) [Claims] 1. An optical element dispersion in which particles are dispersed in a medium and a voltage is applied to an electrode to enclose or sandwich the cells in a cell of an optical element for controlling light transmittance or reflectance. A dispersion for an optical element, which is a suspension and has a water content of 500 ppm or less in weight fraction. 2. The dispersion for optical element according to claim 1, wherein the main component of the solvent of the suspension is a non-aqueous solvent composed of at least one of halogenated resin, aliphatic oil and ester. 3. The dispersion for an optical element according to claim 1, wherein the suspension contains one or more kinds of a surfactant and a polymer as a dispersant. 4. The optical element according to claim 1, wherein the dispersion comprises a suspension and a support, and the suspension is dispersed or wrapped in the support. For dispersion 5. An optical element comprising the dispersion for optical element according to claim 1 enclosed or sandwiched in a cell. 6. The optical element according to claim 5, wherein the electrode surface of the cell is covered with a hydrophobic substance. 7. An optical element according to claim 5 or 6
Driving method of optical element characterized by driving with frequency voltage of 0 Hz or more and less than 100 Hz