JPH07120737A - Polymer dispersion type liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide a polymer dispersion type liquid crystal display device having high reliability and long life with little deterioration with time. CONSTITUTION:A pair of transparent substrates 11, 12 having transparent electrodes 13, 14 are joined with a frame-like sealing material 15 to form a cell 10. A polymer dispersion liquid crystal layer 16 in which the liquid crystal is dispersed in a resin is formed in the cell 10 to constitute the polymer dispersion type liquid crystal display device. The polymer dispersion liquid crystal layer 16 contains an adsorbent. The adsorbent has <0.65mum average particle size, and is added by <=1wt.%amt. of the liquid crystal and/or the resin. The adsorbent is, for example, silica gel or zeolite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly, to a polymer dispersed liquid crystal (PDLC) liquid crystal display device with little deterioration over time.

[0002]

2. Description of the Related Art A polymer dispersion type liquid crystal display device is a polymer in which a liquid crystal is dispersed in a polymer resin in a cell in which a pair of transparent substrates provided with transparent electrodes are joined via a frame-shaped sealing material. A dispersed liquid crystal layer is provided. As the liquid crystal, for example, nematic liquid crystal having positive dielectric anisotropy is used.

This polymer-dispersed liquid crystal display device is driven for display by applying a voltage between the transparent electrodes of both substrates. The liquid crystal molecules in the polymer dispersed liquid crystal layer are oriented in various directions when no voltage (electric field) is applied, and the incident light is scattered by the light scattering action of the polymer dispersed liquid crystal layer. Therefore, the screen becomes cloudy (dark). When a voltage equal to or higher than the threshold voltage is applied between the transparent electrodes, the liquid crystal molecules are evenly arranged perpendicularly to the substrate surface, and the refractive index for light traveling in the direction of the director of the liquid crystal, that is, the ordinary light refractive index The refractive index of the polymer resin becomes almost equal, and the incident light is transmitted through the polymer dispersed liquid crystal layer with almost no light scattering effect. Therefore, the screen becomes transparent (bright).

That is, the polymer-dispersed liquid crystal display device realizes a display by controlling the amount of transmitted light and the amount of scattered light, does not require a polarizing plate, and is commonly used in TN type liquid crystal display devices and the like. Compared with this, it has the advantage that the screen is bright.

[0005]

A polymer-dispersed liquid crystal display device is conventionally assembled by joining a pair of substrates to assemble a cell and injecting a mixed solution of a liquid crystal and a polymer material into the cell by a vacuum injection method. It is manufactured by a method of photopolymerizing a polymer material to form a polymer dispersed liquid crystal layer.

At the production stage of the polymer dispersion type liquid crystal display device,
When impurities such as water and ionic substances are mixed in the cell, ions are generated in the polymer dispersed liquid crystal layer at the time of using the device, the specific resistance of the liquid crystal layer is lowered, and leakage current flows between the electrodes. There is a risk. This problem is also the same as when moisture or other impurities permeate into the cell through the substrate or the sealing material, or when the polymer resin in the cell is decomposed by light, heat, or the like. When the specific resistance of the liquid crystal layer is reduced, the voltage applied to the liquid crystal layer is reduced and the display quality is degraded. In addition, the leakage current chemically decomposes the liquid crystal and the polymer resin, which causes a vicious cycle of promoting deterioration.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a polymer dispersion type liquid crystal display device which is less deteriorated with time. Another object of the present invention is to provide a polymer-dispersed liquid crystal display device that is less affected by impurities such as water and ions in the polymer-dispersed liquid crystal layer. A further object of the present invention is to provide a highly reliable polymer dispersion type liquid crystal display device.

[0008]

In order to achieve the above object, a polymer dispersion type liquid crystal display device according to the present invention comprises a polymer composed of a composite film of a resin and a liquid crystal between a pair of substrates each having an electrode. A polymer dispersed liquid crystal display device provided with a dispersed liquid crystal layer is characterized in that an adsorbent is added to at least one of a resin and a liquid crystal. The adsorbent preferably has an average diameter of 0.65 μm or less, and the addition amount is 1% or less by weight of the liquid crystal and / or the resin. As the adsorbent, silica gel, zeolite or the like can be used.

[0009]

According to the polymer-dispersed liquid crystal display device having the above structure, impurities admixed in the polymer-dispersed liquid crystal layer at the manufacturing stage or generated or permeated during use are adsorbed by the adsorbent.
Therefore, the specific resistance of the liquid crystal and the resin is maintained at a high value.
As a result, leakage current and the like are suppressed to be small, the reliability of the device is improved, and the life of the device is extended. Further, since the applied voltage (electric field) of the liquid crystal can be maintained at a high value by maintaining the specific resistance of the liquid crystal at a high value, the operating speed of the device can be increased. In addition, unnecessary scattering of light can be prevented by reducing the value of the average diameter of the adsorbent described above.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of the polymer dispersed liquid crystal display device of this embodiment. As shown in the figure, in the polymer dispersed liquid crystal display device of this embodiment, a pair of transparent substrates (for example, glass substrates) 11 and 12 provided with transparent electrodes (for example, ITO electrodes) 13 and 14 are formed into a frame-shaped sealing material. The structure is such that a polymer dispersed liquid crystal layer (composite film of polymer resin and liquid crystal) 16 is provided in a cell 10 formed by bonding via 15.

2A and 2B show a polymer dispersed liquid crystal layer 1.
6 is an enlarged sectional view of FIG. The polymer dispersed liquid crystal layer 16 is shown in FIG.
A composite film in which a liquid crystal pool 18 composed of capsules of liquid crystal 19 as shown in FIG. 2A is dispersed in a polymer resin 17, or a mesh-like polymer resin 17 as shown in FIG. A composite film in which a pool 18 of liquid crystal 19 (liquid crystal pool) is dispersed and the liquid crystal 19 forms a continuous phase may be used.
The polymer-dispersed liquid crystal 16 shown in FIGS. 2A and 2B is manufactured by, for example, an emulsion method or a polymer phase separation method.

An adsorbent 20 is mixed in the liquid crystal reservoir 18 and the polymer resin 17. As the adsorbent 20,
For example, fine particles of silica gel for fixing water and fine particles of synthetic zeolite for adsorbing ions can be used. Adsorbent 20
Has an average diameter d of 0.65 μm or less, preferably 0.5 μm or less, and more preferably 0.45 μm or less. Further, the addition amount thereof is 1% by weight or less with respect to the weight of the liquid crystal 19 in the cell 10, but may be 1% by weight or less with respect to the weight of the polymer dispersed liquid crystal layer 16.

In the polymer-dispersed liquid crystal display device having the above structure, when no voltage is applied between the transparent electrodes 13 and 14, the liquid crystal molecules 22 have various directions as shown in FIG. 3 (A). Orientation, and its average refractive index is about 1.7. Due to the difference between the refractive index of the liquid crystal 19 in the light traveling direction and the refractive index of the polymer resin 17 (about 1.5), incident light is reflected and scattered at the interface between the two. Therefore, the display is cloudy (dark).
It becomes a state. Further, when a voltage equal to or higher than the threshold voltage is applied between the electrodes 13 and 14, the liquid crystal molecules 22 are uniformly arranged perpendicular to the substrate surface, as shown in FIG. Therefore, the refractive index of the liquid crystal 19 with respect to the traveling direction of the light and the refractive index of the polymer resin 17 become substantially equal, and the transmitted light passes through the polymer dispersed liquid crystal layer 16 with almost no light scattering effect.
The screen becomes transparent (bright).

According to this embodiment, impurities (ionic substances and water) are mixed in the liquid crystal 19 during manufacturing of the polymer dispersion type liquid crystal display device, or water is mixed in the liquid crystal 19 with the lapse of time. , Liquid crystal 19 and polymer resin 17
Even when chemically decomposed by light or the like, these are adsorbed by the adsorbent 20 as schematically shown in FIG. Therefore, the impurities do not adversely affect the operation of the polymer-dispersed liquid crystal display device, the deterioration of the device over time can be reduced, and stable operation can be ensured.

The adsorbent 20 has an average particle size d of 0.
It is 65 μm or less, and does not scatter light having a wavelength longer than that of red (center wavelength is about 6.1 μm). Therefore, unnecessary scattering of light can be prevented. Further, by setting the average particle diameter d of the adsorbent 20 to 0.5 μm or less and 0.45 μm or less, it is possible to prevent scattering of green and blue light, and it is possible to support multicolor display.

The ability to adsorb impurities increases as the amount of adsorbent 20 increases. However, as a result, the amount of liquid crystal relatively decreases, and the applied voltage required to obtain a desired display becomes high. In this example,
Since the amount of the adsorbent 20 added is set to about 1% by weight or less (preferably 0.5% to 0.8%) of the weight of the liquid crystal, it is possible to appropriately adjust the two in a trade-off relationship.

Next, a method of manufacturing the polymer dispersion type liquid crystal display device having the above structure will be described. (1) First, a method of manufacturing a polymer dispersion type liquid crystal display device using a liquid crystal capsule as shown in FIG. 2A will be described. First, for example, 0.7 wt% with a particle size of 0.5 μm
A certain amount of adsorbent 20 is added to the liquid crystal 19. Next, this is agitated, and in a state where the adsorbent 20 is almost evenly dispersed in the liquid crystal 19, a commonly known method is used (Japanese Patent Publication No. 3-5).
2843), the microcapsules 18 of the liquid crystal 19 are formed.

Next, polyvinyl alcohol (PVA) to which the adsorbent 20 is added is added to the liquid crystal microcapsules 18.
Etc. are dispersed in an aqueous solution of a polymerization material (mixture of monomers, oligomers, adjusting materials, etc.) obtained by polymerization, this is applied to one substrate, and then the other substrate is bonded via a spacer and a sealing material 15. It Further, the liquid crystal device is irradiated with light or heated to polymerize the polymerized material. As a result, as shown in FIG. 2A, the polymer dispersion type liquid crystal display device including the adsorbent 20 in the microcapsulated liquid crystal reservoir 18 is completed. In addition, the adsorbent 20 in the PVA
Without addition of the adsorbent 20 only inside the liquid crystal pool 18.
Can be obtained.

(2) On the other hand, the polymer-dispersed liquid crystal display device having the network-shaped polymer resin structure shown in FIG. 2B is constructed as follows, for example. First, FIG.
As shown in (A), the cell 10 is assembled by joining a pair of transparent substrates 11 and 12 with a sealant 15 interposed therebetween. The sealing material 15 has an injection port 15a.

Liquid crystal 19 such as nematic liquid crystal having positive dielectric anisotropy (60 to 80% by weight with respect to the entire polymer dispersed liquid crystal layer) and a polymer material are mixed with each other in a mixed solution of about 0.8% by weight of an adsorbent. Add 20, stir and mix evenly. In this state, the mixed solution A including the liquid crystal 19, the polymer material, and the adsorbent 20 is injected into the cell 10 by the vacuum injection method. As in the conventional vacuum injection method, the mixed solution A is injected by setting the cell 10 in a vacuum tank (not shown) to reduce the pressure in the tank to a vacuum state, and then the injection port 15a of the cell 10 is mixed in the solution dish 28. It is immersed in the solution A, and thereafter, the pressure in the vacuum chamber is raised to atmospheric pressure or slightly higher than atmospheric pressure.

When the mixed solution A is injected into the cell 10, the pressure of the injected solution A is lowered and its liquid crystal solubility is lowered. At this time, the liquid crystal 19 in an amount exceeding the lowered solubility is deposited, and as shown in FIG. 5B, the excess liquid crystal a is scattered in the mixed solution A.

Next, the cell 10 into which the mixed solution A has been injected is taken out of the vacuum chamber, and the cell 10 is heated to raise the mixed solution A therein to a predetermined temperature. When the temperature of the mixed solution A is increased, the liquid crystal solubility is also increased, and the excess liquid crystal a is redissolved as shown in FIG.

The temperature of the mixed solution A may be higher than the temperature at which the precipitated liquid crystal re-dissolves. If the temperature of the liquid crystal NI point is higher than the re-dissolving temperature, it may be higher than the NI point temperature. It is desirable to raise to. After that, the temperature of the mixed solution A is kept so that the liquid crystal 19 is not re-precipitated, and in that state, the cell 10 is irradiated with ultraviolet rays from the light source 30 as shown in FIG. By photopolymerizing, the photocurable resin 17 and the liquid crystal 19 are phase-separated, and the polymer dispersed liquid crystal layer 16 having the structure shown in FIG. 2B is formed. Although the amount of the liquid crystal 19 is set to 60 to 80% by weight, it is possible to reduce the amount of the liquid crystal 19 as shown in FIG.
A structure similar to that of (A) can also be formed.

The photopolymerization is carried out by keeping the temperature of the mixed solution A as low as possible within a range in which the liquid crystal 19 can be sufficiently dissolved. This is because the higher the temperature of the mixed solution A, the greater the amount of ultraviolet light required for photopolymerization. Since the deterioration of the liquid crystal 19 and the polymer resin 17 is accelerated when a large amount of ultraviolet rays are irradiated, it is desirable to carry out photopolymerization with a small amount of ultraviolet rays by lowering the temperature as much as possible. However, in this embodiment, even if the liquid crystal 19 and the polymer resin 17 are decomposed by ultraviolet rays,
Since they are fixed by the adsorbent 20, adverse effects due to deterioration are reduced.

When the polymer material is photopolymerized while the liquid crystal is in the I phase to form the polymer dispersed liquid crystal layer 16,
Since the driving voltage applied between the transparent electrodes 13 and 14 becomes high, it is desirable to perform the photopolymerization after lowering the temperature of the mixed solution A to a temperature at which the liquid crystal returns to the N phase. However, when the NI temperature of the liquid crystal is lower than the liquid crystal melting temperature of the mixed liquid crystal A, if the cell temperature is lowered to the NI temperature or less, the liquid crystal 19 in the mixed solution A will be precipitated again. In this case, the photopolymerization is performed while the liquid crystal 19 is dissolved at the temperature of the mixed solution A and the liquid crystal 19 is in the I phase.

After the polymer dispersed liquid crystal layer 16 is formed in this manner, the injection port 15a of the cell 10 is sealed with a photo-curable resin or the like, and the polymer dispersed liquid crystal display device shown in FIG. 1 is obtained. Complete.

When manufacturing a polymer dispersion type liquid crystal device of the type using a liquid crystal capsule, a solution containing the liquid crystal capsule may be vacuum-injected into the cell. Further, when manufacturing a phase separation type polymer dispersion type liquid crystal device, a gel-like mixed solution A is applied onto one substrate by using a spin coater, and the other substrate is bonded thereto via a spacer and a sealing material. You may do it. Further, a thermopolymerizable resin may be used as the polymer resin.

A polymer cooker using no adsorbent and a polymer dispenser using 1% by weight of adsorbent were subjected to a pressure cooker test under UV irradiation, and the leakage current due to deterioration was measured. It was measured. The test conditions are pressure 2 atm, temperature 100 ° C, humidity 100%,
Is. The results of this experiment are shown in FIG.

As is apparent from FIG. 6, in the polymer dispersion type liquid crystal display device which does not use the adsorbent, the leakage current occurs relatively quickly. On the other hand, in the polymer dispersion type liquid crystal display device using the adsorbent, the generation of leakage current is delayed,
The leakage current value itself is small. This is because deterioration of the polymer resin and permeation of water occur under ultraviolet light, high pressure, high temperature and high humidity, and ions are generated in the device, and leakage current occurs in the conventional polymer dispersion type liquid crystal display device. However, in the polymer dispersed liquid crystal display device of the present embodiment, it is presumed that these impurities are fixed by the adsorbent and the movement of ions and the like is eliminated, so that the leakage current is suppressed to a small value.

If the average particle diameter d of the adsorbent 20 is too large, the adsorbent 20 scatters incident light, and the display is always turbid, which is not preferable. Therefore, the adsorbent 2
It is desirable that the average particle diameter d of 0 be equal to or less than the wavelength of light required for display. For example, in the case of a full-color liquid crystal display device, the particle size is 0.43, which is the center wavelength of blue light.
It is desirable that it is smaller than 5 μm, and in the case of monochromatic display, it is desirable that it is smaller than 0.61 μm which is the center wavelength of red light. Further, the function of the adsorbent 20 is influenced by its surface area, and for the same weight%, the smaller the particle size, the larger the surface area. From this viewpoint as well, it is desirable that the particle diameter is small, and from this viewpoint, the average particle diameter d is 0.5.
It is desirable that the thickness is μm or less.

When the specific gravity of the solution of the liquid crystal 19 and the specific gravity of the solution of the polymerization material are greatly different from the specific gravity of the adsorbent 20, the adsorbent 20 may not be uniformly dispersed in the mixed solution when the mixed solution A is injected into the cell 10. . Therefore, it is desirable to select the adsorbent 20 that has substantially the same specific gravity as these. Moreover, you may make it inject while agitating. Further, the method of manufacturing the polymer dispersed liquid crystal display device according to the present invention is not limited to the above embodiment. For example, at the stage of assembling the cell 10, the adsorbent 20 may be dispersed in the cell 10, and then the mixed solution of the liquid crystal 19 and the polymer material may be injected into the cell 10 by using a vacuum injection method or the like.

As the adsorbent 20, one kind of adsorbent such as silica gel or synthetic zeolite may be used.
Also, a plurality of types of adsorbents may be mixed and used. Also, other commonly known adsorbents may be used. Also,
As the liquid crystal, a liquid crystal used for ordinary TN, STN, ferroelectric, antiferroelectric liquid crystal or the like may be used. The present invention can be applied to both a simple matrix type polymer dispersed liquid crystal display device and an active matrix type polymer dispersed liquid crystal display device such as a TFT (thin film transistor) polymer dispersed liquid crystal display device. Also, liquid crystal 1
9 is not limited to nematic liquid crystal, cholesteric liquid crystal,
A smectic liquid crystal, a liquid crystal having ferroelectricity, a guest-host type liquid crystal containing a dye, or the like may be used.

[0033]

As described above, according to the present invention, the adsorbent adsorbs moisture or impurity ions that are mixed in during manufacture or that are generated due to deterioration over time or that have entered from the outside, so that leakage current is generated. Can be suppressed and the specific resistance of the liquid crystal layer can be maintained at a high value. Therefore, the life of the polymer-dispersed liquid crystal device can be extended and reliability can be improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a polymer dispersed liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of the structure of a polymer-dispersed liquid crystal layer, in which (A) is a type that uses liquid crystal microcapsules, and (B) is a network polymer resin filled with liquid crystal. Indicates the type.

3A and 3B are diagrams showing an operation of the polymer-dispersed liquid crystal display device shown in FIGS. 1 and 2, where FIG. 3A is a diagram showing a state in which a voltage is not applied between electrodes, and FIG. It is a figure which shows the state which is applying the voltage to.

FIG. 4 is a diagram showing how impurities are adsorbed by an adsorbent.

FIG. 5 is a diagram illustrating a method for manufacturing a polymer-dispersed liquid crystal display device according to an embodiment of the present invention.

FIG. 6 is a diagram showing characteristics of a polymer-dispersed liquid crystal display device according to an embodiment of the present invention and a conventional polymer-dispersed liquid crystal display device for comparison.

[Explanation of symbols]

10 ... Cell, 11, 12 ... Transparent substrate, 13, 14 ...
Transparent electrode, 15 ... Sealing material, 16 ... Polymer dispersed liquid crystal layer (composite layer of polymer resin and liquid crystal), 17 ... Polymer resin,
18 ... Liquid crystal pool (domain), 19 ... Liquid crystal, 20 ...
・ Adsorbent, 22 ... Liquid crystal molecule, 28 ... Solution dish, 30 ... Light source

Claims (5)

[Claims] 1. A polymer-dispersed liquid crystal display device comprising a polymer-dispersed liquid crystal layer comprising a composite film of resin and liquid crystal between a pair of substrates each having electrodes, wherein the polymer-dispersed liquid crystal layer is constituted. At least one of the resin or the liquid crystal that contains an adsorbent is a polymer-dispersed liquid crystal display device. 2. The adsorbent according to claim 1, wherein the adsorbent has an average diameter of 0.65 μm or less, and an amount of addition is 1% or less by weight of the liquid crystal and / or resin. Polymer dispersed liquid crystal display device. 3. The polymer dispersed liquid crystal display device according to claim 1, wherein the adsorbent contains at least one of silica gel and zeolite. 4. A pair of substrates, electrodes formed on opposing surfaces of the pair of substrates, a layer disposed between the pair of substrates and containing a resin and a liquid crystal, and disposed in the resin and / or the liquid crystal. And fine particles that permeate the layer or fix impurities generated in the layer,
A liquid crystal display device characterized by reducing deterioration over time. 5. The liquid crystal display device according to claim 4, wherein the fine particles are an adsorbent that absorbs moisture and / or ions, and has an average diameter of 0.65 μm or less.