RETARDATION FILM, A CIRCULAR POLARIZER AND AN ELECTROLUMINESCENT DISPLAY COMPRISING THE SAME

Abstract

A retardation film is provided. The retardation film comprises a water-soluble polymer matrix and at least a water-soluble dichroic dye dispersed in the water-soluble polymer matrix. The water-soluble dichroic dye has a molecular long-axis aligned along a stretching direction of the water-soluble polymer matrix. The water-soluble dichroic dye has a maximum absorption wavelength between 550 nm and 650 nm, and has a dichroic ratio greater than 10 at the maximum absorption wavelength. The refractive index and the corresponding retardation value of the retardation film in long-wavelength band can be adjusted individually by adding the water-soluble dichroic dye with the maximum absorption wavelength between 550 nm and 650 nm in the water-soluble polymer matrix and arranging the water-soluble dichroic dye in an order. Therefore, reverse wavelength dispersion is generated.

Claims

1. A retardation film, comprising: a water-soluble polymer matrix; at least a water-soluble dichroic dye dispersed in the water-soluble polymer matrix, wherein the water-soluble dichroic dye has a molecular long-axis aligned along a stretching direction of the water-soluble polymer matrix, wherein the water-soluble dichroic dye has a maximum absorption wavelength between 550 nm and 650 nm, and has a dichroic ratio greater than 10 at the maximum absorption wavelength; wherein the retardation film satisfies a formula as follows:
R.sub.0(650 nm)/R.sub.0(550 nm)>1 wherein the R.sub.0(550 nm) is an in-plane retardation at a wavelength of 550 nm, and the R.sub.0(650 nm) is an in-plane retardation at a wavelength of 650 nm.

2. The retardation film as claimed in claim 1, wherein the retardation film preferably satisfies a formula as follows:
1.30>R.sub.0(650 nm)/R.sub.0(550 nm)>1.05

3. The retardation film as claimed in claim 1, wherein the retardation film preferably satisfies a formula as follows:
R.sub.0(450 nm)/R.sub.0(550 nm)≤1 wherein the R.sub.0(450 nm) is an in-plane retardation at a wavelength of 450 nm.

4. The retardation film as claimed in claim 1, wherein the water-soluble polymer matrix is selected from one of the group consisting of polyol polymer, polyester polyol, polyurethane and polysiloxane, or combinations thereof.

5. The retardation film as claimed in claim 1, wherein the water-soluble dichroic dye is an azo compound or a salt thereof.

6. The retardation film as claimed in claim 1, wherein the amount of the water-soluble dichroic dye ranges from 0.1 wt % to 0.5 wt % based on the total weight of the water-soluble polymer matrix.

7. The retardation film as claimed in claim 1, wherein the thickness of the retardation film ranges from 5 μm to 100 μm.

8. A circular polarizer, comprising a retardation film as claimed in claim 1.

9. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 8 disposed on a light-emitting side of the electroluminescent display panel.

10. A circular polarizer, comprising a retardation film as claimed in claim 2.

11. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 10 disposed on a light-emitting side of the electroluminescent display panel.

12. A circular polarizer, comprising a retardation film as claimed in claim 3.

13. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 12 disposed on a light-emitting side of the electroluminescent display panel.

14. A circular polarizer, comprising a retardation film as claimed in claim 4.

15. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 14 disposed on a light-emitting side of the electroluminescent display panel.

16. A circular polarizer, comprising a retardation film as claimed in claim 5.

17. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 16 disposed on a light-emitting side of the electroluminescent display panel.

18. A circular polarizer, comprising a retardation film as claimed in claim 6.

19. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 18 disposed on a light-emitting side of the electroluminescent display panel.

20. A circular polarizer, comprising a retardation film as claimed in claim 7.

21. An electroluminescent display, comprising: an electroluminescent display panel; and a circular polarizer as claimed in claim 20 disposed on a light-emitting side of the electroluminescent display panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 illustrates a cross-sectional view of a circular polarizer according one embodiment of this present invention.

[0019] FIG. 2 illustrates a cross-sectional view of an electroluminescent display according one embodiment of this present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0021] In the following description, numerous specific details are described in detail in order to enable the reader to fully understand the following examples. However, embodiments of the present invention may be practiced in case no such specific details. In other cases, in order to simplify the drawings the structure of the apparatus known only schematically depicted in figures.

[0022] This invention provides a retardation film, comprising a water-soluble polymer matrix and at least a water-soluble dichroic dye dispersed in the water-soluble polymer matrix, wherein the water-soluble dichroic dye has a molecular long-axis aligned along a stretching direction of the water-soluble polymer matrix, and the water-soluble dichroic dye has a maximum absorption wavelength between 550 nm and 650 nm, and has a dichroic ratio greater than 10 at the maximum absorption wavelength, and the in-plane retardation of the retardation film satisfies a formula as follows:

R.sub.0(650 nm)/R.sub.0(550 nm)>1

wherein R.sub.0(550 nm) is the in-plane retardation of the retardation film at the wavelength of 550 nm, and R.sub.0(650 nm) is the in-plane retardation of the retardation film at the wavelength of 650 nm.

[0023] The term “in-plane retardation R.sub.0” used herein refers to a product of the thickness d and the difference between the extraordinary refractive index n.sub.e and the ordinary refractive index n.sub.o of the retardation film calculated by the formula: R.sub.0=(n.sub.e−n.sub.o)*d, wherein R.sub.0(550 nm) representing the in-plane retardation measured at the wavelength of 550 nm is a typical in-plane retardation. However, R.sub.0(550 nm) can't represent all compensation results of a retardation at various wavelengths because the polymer forming the retardation film has different dispersion wavelength depending on the wavelength of the incident light. When a selected polymer material is stretched and the in-plane retardation R.sub.0 at a wavelength of 550 nm and the in-plane retardation R.sub.0 at a wavelength of 650 nm thereof are almost equal, that is R.sub.0(650 nm)/R.sub.0(550 nm)=1±0.01, then the polymer can be classified as a material having a flat dispersion at wavelength between 550 nm and 650 nm.

[0024] Suitable water-soluble polymer matrix for the retardation film of this present invention is selected from one of the group consisting of polyol polymer, polyester polyol, polyurethane and polysiloxane, or combinations thereof. Since the water-soluble polymer matrix is soluble in water, which can be stretched uniaxially or biaxially without adding any organic solvent, to serve as a carrier for water-soluble dichroic dyes and as an alignment layer for aligning the water-soluble dichroic dyes, wherein the axis with a higher stretching ratio is defined as the main stretching direction of the water-soluble polymer matrix. The conventional polymer materials, especially those with positive wavelength dispersion, have larger retardation deviations in long-wavelength bands. As the in-plane retardation of the stretched water-soluble polymer matrix reaches a flat wavelength dispersion satisfying the formula: R.sub.0(650 nm)/R.sub.0(550 nm)≈1 in the long-wavelength band, the polymer material is sufficient to be used for the retardation film of the present invention. In the short-wavelength band, the water-soluble polymer matrix preferably has a flat wavelength dispersion or a reverse wavelength dispersion to further satisfy the relational of formula: R.sub.0(450 nm)/R.sub.0(550 nm)≤1 when the retardation film is applied to electroluminescent displays to maintain or reduce the reflectivity in the short-wavelength band, where R.sub.0(450 nm) is the in-plane retardation corresponding to the wavelength of 450 nm. Moreover, because the theoretical value of the ration of the center wavelength 450 nm of blue light over the center wavelength 550 nm of green light is 0.82, the in-plane retardation of the retardation film according to an embodiment of this present invention preferably satisfies the relationship of the formula: 0.82<R.sub.0(450 nm)/R.sub.0(550 nm)≤1 in order to avoid the retardation in short-wavelength band too low to provide the electroluminescent display with more leakage of short-wavelength light of high energy in dark state.

[0025] The water-soluble dichroic dye of the retardation film according to this present invention has a maximum absorption wavelength between 550 nm and 650 nm, and has a dichroic ratio greater than 10 at the maximum absorption wavelength. When the maximum absorption wavelength of the water-soluble dichroic dye is between 550 nm and 650 nm, the difference of refractive index is also more significant at the wavelength band thereof. When the dichroic ratio of the water-soluble dichroic dye at the maximum absorption wavelength is greater than 10, the refractive index difference between the stretch direction and the direction perpendicular to the stretch direction can be easily changed by the stretching and alignment of the water-soluble polymer matrix to enhance the in-plane retardation of the retardation film in long-wavelength band to sufficiently cover the visible light of 565 m to 715 nm, thereby a retardation film with reverse wavelength dispersion in long-wavelength band can be obtained.

[0026] Suitable dichroic dyes used in the retardation film of this invention can be for example but not limited to azo compounds or salts thereof, and preferably the azo compounds represented by a formula as follows:

Ar.sub.1—N═NAr.sub.2—N═N.sub.n—Ar.sub.3

wherein Ar.sub.1 is a naphthyl group substituted with a sulfonyl hydroxide group and/or a sulfonic acid alkoxy group, wherein the alkoxy group is a C.sub.1-C.sub.5 alkoxy group; Ar.sub.2 is phenyl group substituted with a C.sub.1-C.sub.4 alkyl group and/or C.sub.1-C.sub.4 alkoxy group; Ar.sub.3 is a naphtholsulfonic acid group substituted with anilino group or naphtholsulfonic acid group substituted with a methoxyaniline group; n is an integer from 1 to 3.

[0027] The amount of the water-soluble dichroic dye of the present retardation film is 0.1 wt % to 0.5 wt % based on the weight of the water-soluble polymer matrix. Such an amount of the water-soluble dichroic dye is sufficient to generate effective retardation values for the long-wavelength band. Because the amount of the water-soluble dichroic dye is low (<1 wt %), the effect on the intensity of the bright-state images of electroluminescent displays is very minor. Unlike the conventional modified polymer materials, the phase difference in the short-wavelength band will be excessively reduced to result in a color-shift of reflection light in dark state again. Moreover, because the ideal value of the ration of the center wavelength 650 nm of red light over the center wavelength 550 nm of green light is 1.18, the in-plane retardation of the retardation film according to an embodiment of this present invention preferably satisfies the formula: 1.30>R.sub.0(650 nm)/R.sub.0(550 nm)>1.05 to provide the electroluminescent display with a suitable retardation in long-wavelength band to reduce the light leakage at long-wavelength band.

[0028] In another embodiment of this invention, the thickness of the retardation film ranges from 5 μm to 100 μm to provide the retardation film with a predetermined retardation value.

[0029] This invention also disclose a method of manufacturing a retardation film comprising the steps of coating a solution of a water-soluble polymer mixed with a dichroic dye onto a carrier substrate to form a coating layer, curing the coating layer and dry stretching thereafter, and peeling off the carrier substrate to generate a retardation film.

[0030] In a preferred embodiment of the method of manufacturing a retardation film according to this invention, the dried coating layer is stretched three-times in a stretching speed of 0.1 m/min to avoid broken caused by over stretching and undesired retardation caused by insufficient stretching. In another preferred embodiment of the method of manufacturing a retardation film according to this invention, the coating layer is preferably dried at 80° C. to 110° C., and more preferably dried at 85° C. to 110° C.

[0031] The retardation film of this invention can be bonded to a linear polarizer to generate a circular polarizer. As shown in FIG. 1, a cross-sectional view of a circular polarizer according to this invention is illustrated. In FIG. 1, the circular polarizer 1 comprises one of the above-mentioned retardation films 2 and a linear polarizer 3 disposed on the light-exit side of the retardation film 2.

[0032] The linear polarizer 3 of the circular polarizer 1 can be one of the linear polarizer generally used on the display field, such as polymer films dyed with aligned dichroic dyes. Suitable linear polarizer can be hydrophilic polymer films dyed with dichroic iodine, wherein the hydrophilic polymer films can be for example but not limited to polyvinyl alcohol films, ethylene-vinyl acetate copolymer films, ethylene vinyl alcohol copolymer films, cellulous films and/or partial saponified cellulous films, or polyene alignment films such as dehydrated polyvinyl alcohol films, dechlorinated polyvinyl alcohol films or analogues thereof. In a preferred embodiment of this invention, the linear polarizer is a polyvinyl alcohol film dyed with dichroic iodine. The linear polarizer can be obtained by any conventional processes such as coating, and dry stretching or wet stretching thereafter, and the sequence of the processes and repeating times are not limited.

[0033] The circular polarizer can be used in the electroluminescent display. As shown in FIG. 2, a cross-sectional view of an electroluminescent display according to this present invention is illustrated. In FIG. 2, the electroluminescent display 11 comprises an above-mentioned circular polarizer 1 and an electroluminescent display panel 4, and the circular polarizer 1 is disposed on the light-exit side of the electroluminescent display panel 4 to reduce the external reflected light and act as a protective cover.

[0034] In an embodiment of the electroluminescent display 11 according to this present invention, the electroluminescent display panel 4 can be for example but not limited to a millimeter-scaled small-pitch light-emitting diode display panel (Small-pitch LED Display), a sub-millimeter-scaled light-emitting diode display panel (Mini LED Display), a micron light-emitting diode display panel (Micro LED Display) or organic light-emitting diode display panel (OLED Display). For the types of self-luminous display panels with highly reflective electrodes and resonant cavity, or even self-luminous display panels with integrated touch modules, conventional adhesion methods can be used, without the need to change the laminated structure of the electroluminescent display panel or by changing the packaging process to increase the internal quantum efficiency, and the effect of reducing the degree of light reflection and low color-shift can be achieved by attaching the circular polarizer 1 of the present invention onto the light-existing side of the electroluminescent display panel 4.

[0035] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

EXAMPLE

Example 1

[0036] A solution prepared by dissolving 50 g of polyvinyl alcohol (PVA) powder (JC-40, having an average degree of polymerization of 4000, and a degree of saponification of 99 mol %, commercially obtained from Japan Vam & Poval Co., Ltd.) and 0.05 g of water-soluble azobiphenyl dichroic dye (Purple-150682, having a maximum absorption wavelength at 560 nm and a dichroic ratio of 18.7, commercially obtained from Orgchem Technologies Inc.) in water was coated onto a polyethylene terephthalate (PET) film to form a coating layer. The coating layer was cured in an oven at 90° C. for 15 minutes, and uniaxially stretched in a speed of 0.1 m/min at 90° C. for three times. After peeling off the PET film, a retardation film with a thickness of 8 μm was obtained.

[0037] The in-plane retardations of the obtained retardation film at various wavelengths were measured by a polarmeter (Axoscan, commercially obtained from Axometrics) and the measured in-plane retardations were shown in Table 1.

Example 2

[0038] A retardation film with a thickness of 8 μm was manufactured by the process the same as described in Example1 except that the solution for coating was prepared by dissolving 50 g of polyvinyl alcohol (PVA) powder (JC-40) and 0.15 g of water-soluble azobiphenyl dichroic dye (Purple-150682) in water. The in-plane retardations of the obtained retardation film at various wavelengths were measured by the same manner as described in Example 1 and the measured in-plane retardations were shown in Table 1.

Example 3

[0039] A retardation film with a thickness of 8 μm was manufactured by the process the same as described in Example 1 except that the solution for coating was prepared by dissolving 50 g of polyvinyl alcohol (PVA) powder (JC-40) and 0.25 g of water-soluble azobiphenyl dichroic dye (Purple-150682) in water. The in-plane retardations of the obtained retardation film at various wavelengths were measured by the same manner as described in Embodiment 1 and the measured in-plane retardations were shown in Table 1.

Comparative Example

[0040] A retardation film with a thickness of 8 μm was manufactured by the process the same as described in Example 1 except that the solution for coating was prepared by dissolving 50 g of polyvinyl alcohol (PVA) powder (JC-40) in water without adding any water-soluble dichroic dye. The in-plane retardations of the obtained retardation film at various wavelengths were measured by the same manner as described in Example 1 and shown in Table

TABLE-US-00001 TABLE 1 In-plane retardation at various wavelengths and ratio thereof R.sub.0 R.sub.0 (450 nm)/ (650 nm)/ R.sub.0 R.sub.0 R.sub.0 R.sub.0 R.sub.0 (450 nm) (550 nm) (650 nm) (550 nm) (550 nm) Comparative 110.73 109.74 109.11 1.01 0.99 Example Example 1 108.29 108.23 114.71 1.00 1.06 Example 2 119.44 120.73 141.48 0.99 1.17 Example 3 122.27 124.95 158.84 0.98 1.27

[0041] As shown in Table 1, the ratios of R.sub.0(650 nm)/R.sub.0(550 nm) of the retardation films of Examples 1 to 3 in long-wavelength band are of reverse wavelength dispersion. The ratios of R.sub.0(650 nm)/R.sub.0(550 nm) of the retardation films of Examples 2 and 3 are close to and overlap with the ideal value of 1.18, and have excellent enhancing efficiency of in-plane retardation R.sub.0 at the wavelength of 650 nm to enhance R.sub.0(650 nm) to a value close to the theoretical value of ¼λ. Therefore, it provides a tolerance to adjust the in-plane retardation by increasing its thickness. Moreover, as shown in Table 1, the ratios of R.sub.0(450 nm)/R.sub.0(550 nm) of the retardation films of Examples 1 to 3 at short-wavelengths demonstrate that they still maintain the flat wavelength dispersion or slight reverse wavelength dispersion characteristics as that of the PVA substrate without adding any dichroic dye. Therefore, in the present invention, the water-soluble dichroic dye with the maximum absorption wavelength in the long-wavelength band can be individually compensated for the retardation deviation in the long-wavelength band without excessively reducing the retardation value in the short-wavelength band.

[0042] Circular polarizers were obtained by respectively disposing the retardation films of Embodiments 2 and 3 and the Comparative Example to linear polarizer, wherein the angle between the optical axis of the retardation film and the absorption axis of the linear polarizer is 45°. These circular polarizers were respectively attached to a mirrored metal plate whose reflectivity is close to the same in all wavelengths to eliminate the difference in reflectivity of OLED panel materials produced by different manufacturers to measure circular polarizer's reducing effect of reflectivities of the visible light reflected by the highly reflective electrodes of the OLED panel. The reducing effect of reflectivities of the visible light was measured by a spectrophotometer (U4100, HITACHI) to measure the reflected light of the dark state black image formed after standard D65, which is a light source close to natural light, is irradiated on the circular polarizer attached to the mirrored metal plate, and then reflectivities of reflected light of various wavelengths at 5° incident angles were measured. The measured results are shown in Table 2.

TABLE-US-00002 TABLE 2 Reflectivities of reflected light of various wavelengths R % R % R % (450 nm) (550 nm) (650 nm) Comparative 6.6 8.1 12.3 Example Example 2 6.1 7.4 7.2 Example 3 5.2 7.1 5.6

[0043] As shown in Table 2, the reflectivities in long-wavelength of 650 nm are effectively reduced by the circular polarizers of Examples 2 and 3. Similar to the measurement results of retardation shown in Table 1, the reflectivities in short-wavelength band are rarely affected by the circular polarizers of Examples 2 and 3. Consequently, the retardation film made by adding water-soluble dichroic dye to the water-soluble polymer matrix of the present invention can selectively adjust and increase the retardation in long-wavelength band to improve the color-shift of reflected light of the dark state black image of an electroluminescent display.

[0044] While the invention has been described by way of example(s) and in terms of the embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.