JPH04239540A - White polyester film - Google Patents

Abstract

PURPOSE:To obtain a white polyester film capable of obtaining brighter screen having higher percentage of reflectance when used as a substrate for liquid crystal display reflecting sheet and optimum as the substrate for liquid crystal display reflecting sheet. CONSTITUTION:The objective white polyester film in which average percentage of reflectance in wavelength area of light of 400-700nm on the surface is >=90% and (maximum value - minimum value) of percentage of reflectance in the wavelength area is <=10% and the objective white polyester film in which average percentage of reflectance in wavelength area of light of 330-380nm on the surface is >=90%.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a white polyester film, and in particular, to a white polyester film that is most suitable for use as a base material for a reflective plate for a liquid crystal display. The present invention relates to a white polyester film that can be used as a base material for a reflector that provides a brighter screen.

0002

[Prior Art] Conventionally, when illuminating a liquid crystal display, a backlight method has been adopted in which light is applied from the back of the display, but in recent years,
The side light system as shown in Publication No. 4 has become widely used because of its thin design and ability to provide uniform illumination. The sidelight method involves halftone dot printing on one side of a transparent substrate such as a certain thickness of acrylic board.
By applying illumination from a cold cathode tube or the like from the edge of the acrylic plate, the illumination light is evenly dispersed for halftone dot printing, and a screen with uniform brightness can be obtained. Additionally, since the lighting is placed on the edge of the screen rather than on the back, it can be made thinner than the backlight method. In addition, in order to prevent illumination light from escaping to the back of the screen, it is necessary to install a reflector on the back of the screen, but since this reflector needs to be thin and have high light reflectivity, titanium oxide Films containing white pigments such as

The installation position of the reflector in this sidelight system will be explained in advance with reference to FIG. Figure 3 shows an example of the sidelight method, with halftone dots printed on one side.
A reflective plate 11 is provided on one side of a transparent light guide plate 14 made of a transparent base material coated with a transparent light guide plate 5, and a diffuser plate 13 and a liquid crystal screen 12 are placed on the other screen. Light from the cold cathode tubes 16 is introduced from the edge of the transparent light guide plate 14, is uniformly dispersed by the halftone dot printing 15, and is reflected by the reflection plate 11, which brightly illuminates the screen.

0004

[Problems to be Solved by the Invention] However, although the film whitened by adding titanium oxide or the like as described above can increase the light reflectance to a certain level, there is a limit to the improvement of the reflectance. , there is a problem that the screen brightness is not sufficient. In addition, titanium oxide, etc.
Since it absorbs light of a specific wavelength, the overall reflectance decreases in a certain wavelength range, causing the problem that a sufficiently bright screen cannot be obtained. In terms of market demands, there is a trend toward brighter screens, and there is a strong demand for reflectors with higher reflectance over a wide wavelength range.

The object of the present invention is to solve these problems and to provide a white polyester film that is optimal for use as a substrate for a liquid crystal display reflector, which provides a higher reflectance and a brighter screen.

[0006]

[Means for Solving the Problems] The white polyester film of the present invention, which meets this objective, has a surface of 400 to 700
The average reflectance in the wavelength range of nm light is 90% or more, and the reflectance (maximum value - minimum value) in the wavelength range is 10% or less. (Hereinafter, this may also be referred to as the first invention.)

Furthermore, the present invention provides a white polyester film with high reflectance in another specific wavelength range. That is, the present invention provides a white polyester film whose surface has an average reflectance of more than 90% in the light wavelength range of 330 to 380 nm. (Hereinafter, this may also be referred to as the second invention.)

The characteristic of the first invention is shown, for example, as a characteristic H1 shown by a solid line in FIG. In other words, the average reflectance is 90% in the main wavelength range 400 to 700 nm.
These are the characteristics. When this white polyester film is used as a substrate for a liquid crystal display reflector, if the reflectance decreases, the screen will take on a yellowish tinge in the low wavelength range.
Although the screen has a bluish tinge in the high wavelength range, by setting the average reflectance to 90% or more, this can be prevented and a sufficiently bright screen can be obtained. The characteristics of the second invention are as follows:
For example, the characteristic H2 is indicated by a dashed line. In other words, when this white polyester film is used as a substrate for a liquid crystal display reflector, in addition to the above-mentioned H1 property, it also exhibits 9.
This makes it possible to achieve a reflectance of 0% or more.

[0009] Polyester as used in the present invention is a polymer obtained by condensation polymerization from diol and dicarboxylic acid, and dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, adipic acid, sebacic acid, Diols include ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol, and the like. Specific examples include polymethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, and polyethylene-2,6-naphthalene dicarboxylate. In the case of the present invention, polyethylene terephthalate and polyethylene naphthalate are particularly preferred.

Of course, these polyesters may be homopolyesters or copolyesters, and examples of copolymerization components include diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, adipic acid, and sebacic acid. , phthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid,
Examples include dicarboxylic acid components such as 5-sodium sulfoisophthalic acid.

[0011] Furthermore, various known additives such as antioxidants and antistatic agents may be added to the polyester. As the polyester used in the present invention, polyethylene terephthalate is preferable. Polyethylene terephthalate film is water resistant, durable,
It has excellent chemical resistance.

Conventionally, titanium oxide or the like has been added to whiten the polyester film, but as mentioned above, there is a limit to the improvement of reflectance, and it is difficult to obtain a brighter liquid crystal screen. In the present invention, whitening is achieved by containing fine air bubbles inside the film and scattering light with the air bubbles. As a result, as shown in Examples below, a high reflectance that cannot be obtained with conventional films can be achieved.

[0013] The formation of these fine bubbles is caused by the film base material,
For example, this can be achieved by finely dispersing an incompatible polymer with a high melting point in polyester and stretching it (for example, biaxial stretching). During stretching, voids (bubbles) are formed around the incompatible polymer particles, which scatter light, resulting in whitening and a high reflectance. Incompatible polymers include poly-3-methylphthene-1, poly-4-methylpentene-1, polyvinyl-t-butane, 1,4-trans-poly-2,3-dimethylbutadiene, polyvinylcyclohexane, polystyrene, polymethylstyrene, polydimethylstyrene,
Polyfluorostyrene, poly-2-methyl-4-fluorostyrene, polyvinyl-t-butyl ether, cellulose triacetate, cellulose tripropionate,
It is a polymer with a melting point of 200°C or higher selected from polyvinyl fluoride, polychlorotrifluoroethylene, etc. Among these, polyolefins, particularly polymethylpentene, are preferred for polyester base materials.

The amount of the incompatible polymer (for example, polyolefin) added is preferably 2% by weight or more and 25% by weight or less. If it is too low, the whitening effect will be weakened and it will be difficult to obtain high reflectance, and if it is too high, the mechanical properties such as the strength of the film itself may become too low.

It is preferable that the incompatible polymer is uniformly dispersed. Due to uniform dispersion, bubbles are uniformly formed inside the film, and the degree of whitening and therefore the reflectance are uniform. In the present invention, in order to suppress variations in screen brightness, the value of (maximum value - minimum value) of reflectance in the wavelength range of 400 to 700 nm is set to 10% or less. As a result, a screen with uniform brightness can be obtained as a liquid crystal screen.

In order to uniformly disperse incompatible polymers, it is effective to add a specific gravity lowering agent as a dispersion aid. A specific gravity lowering agent is a compound that has the effect of lowering the specific gravity, and only certain compounds have this effect. For example, for polyester, polyalkylene glycols such as polyethylene glycol, methoxypolyethylene glycol, polytetramethylene glycol, and polypropylene glycol, ethyleneoxide/propylenoxide copolymers, and even sodium dodecylbenzenesulfonate and alkylsulfonates are used. sodium salt,
Representative examples include glycerin monostearate and tetrabutylphosphonium para-aminobenzenesulfonate. In the case of the film of the present invention, polyalkylene glycols, especially polyethylene glycols, are particularly preferred. The amount added is preferably 0.1% by weight or more and 5% by weight or less. If it is too small, the effect of addition will be diminished, and if it is too large, the original properties of the film base material may be impaired. Such a specific gravity lowering agent can be prepared as a master polymer (master chip) by adding it to the film base polymer in advance.

As mentioned above, since the white polyester film contains fine air bubbles, the apparent specific gravity of the polyester film is lower than that of a normal polyester film. If a specific gravity lowering agent is further added, the specific gravity will be further lowered. In other words, a white and light film can be obtained. In order to make this white polyester film lightweight while maintaining its mechanical properties as a substrate for a liquid crystal display reflector, the apparent specific gravity is preferably 0.5 or more and 1.2 or less.

In the first aspect of the present invention, a laminated film structure may be used. For example, it has a two-layer structure of layer A/layer B, or a three-layer structure of layer A/layer B/layer A. In this case, layer B is a layer containing microbubbles, and layer A
The layer is preferably a layer in which polyester contains inorganic particles (for example, calcium carbonate) in an amount of 5% by weight or more and 25% by weight or less. With such a configuration, while the target high reflectance is obtained by the B layer, the surface characteristics of the laminated base material can be maintained to the characteristics of the polyester itself by the A layer,
Adhesive properties (for example, ink adhesion and lamination properties) of the substrate surface can be maintained well. Furthermore, by adding inorganic particles such as calcium carbonate to the surface layer, a matting effect on the surface can be obtained, and since calcium carbonate itself has a void forming effect, the reflection characteristics of the surface layer can also be improved.

[0019] The second invention of the present invention further provides 330 to 38
This is a white polyester film with an average reflectance of more than 90% even in the 0 nm wavelength range. Such a white polyester film is basically obtained by incorporating a fluorescent whitening agent into the white polyester film of the first invention described above. That is, when a white polyester film is made of a film containing fine air bubbles inside the film, a fluorescent whitening agent is contained in the film, and when the film is made of a multilayer laminated polyester film, a fluorescent whitening agent is contained within the film. It is sufficient that at least the outermost layer of the laminated film layer contains an optical brightener. Further, the film may include a white polyester film layer and an outermost coating layer, and at least the coating layer may contain a fluorescent whitening agent. This fluorescent whitening agent can sufficiently improve the reflectance simply by adding it to the outermost layer.

[0020] Types of fluorescent brighteners include OB-1 (manufactured by Eastman), Uvitex-OB (manufactured by Ciba Geigy), Uvitex-MD (manufactured by Ciba Geigy), and J.
Examples include P-Conc (manufactured by Nihon Kagaku Kogyo Co., Ltd.).

[0021] The amount of the optical brightener added is 0.001% by weight or more and 0.001% by weight or more when added to the film.
1% by weight or less, preferably 0.005-0.05% by weight
, more preferably 0.01 to 0.05% by weight. If it is less than 0.001% by weight, the reflectance in the wavelength range of light from 330 to 380 nm will be lower than the above average reflectance, and the illuminance will not be sufficient when used as a reflector. 0.1% by weight
Exceeding this is not preferable because the characteristic color of the optical brightener will appear. In the case of coating, the solid content of the coating material contains 0.01% to 2% by weight, preferably 0.05 to 1.5% by weight, more preferably 0.1 to 1% by weight.
．． It is 5% by weight. The reason is the same as the above reason.

[0022] As the binder resin for the coating material, various resins such as thermoplastic resins or thermosetting resins can be used. Examples include polyester, polyamide, polyesteramide, polyvinyl chloride, poly(meth)acrylate, polyurethane, polyvinyl chloride, polystyrene, polyolefin, and copolymers and blends thereof. Among these, polyester, poly(meth)acrylic acid ester, and polyurethane are preferred. Further, it may be crosslinked by adding a crosslinking agent.

As the solvent for the coating material, organic solvents such as toluene, ethyl acetate, methyl ethyl ketone, and mixtures thereof can be used, and water may also be used as the solvent.

Further, particles may be added to the coating material in order to change the roughness, gloss, etc. of the coating surface. Particles that do not absorb light are preferred since they do not prevent the effects of optical brighteners. For example, calcium carbonate, silica, barium sulfate, talc, clay, etc. are preferred. The amount added is preferably 0.005% to 25% by weight, more preferably 0.01 to 25% by weight in the solid content of the coating material.
It is 20% by weight. If the amount is less than 0.005% by weight, the particle addition has no effect, and if it exceeds 25% by weight, the coating film becomes brittle and weak.

Furthermore, as a coater, a usual gravure coater, coating using a metering bar, etc. can be used. The thickness of the coating film is 0.1μ after solvent drying.
m or more and 10 μm or less, more preferably 0.5
~5 μm. If it is less than 0.1 μm, the effect of the fluorescent whitening agent will not be exhibited, and if it exceeds 10 μm, the color change due to the binder resin etc. will be noticeable, which is not preferable.

[0026] In the white polyester film according to the first and second inventions of the present invention, the gloss level on at least one side thereof is 80% or less, preferably 60%.
Below, it is more preferably 50% or less. By setting the gloss level to 80% or less, the uniformity of the brightness of the screen can be further improved without reducing the brightness of the screen.

Next, the method for producing a white polyester film of the present invention will be explained, but the method is not limited to this example. Extruder A in which polymethylpentene as an incompatible polymer and polyethylene glycol as a specific gravity lowering agent are mixed with polyethylene terephthalate, thoroughly mixed and dried, and heated to a temperature of 270 to 300 ° C.
supply to. If necessary, feed polyethylene terephthalate containing inorganic additives such as CaCO3 to extruder B using a conventional method, and make it so that the polymer of extruder B layer is on both surface layers in the T-die 3-layer mouthpiece B/A. /B may be laminated into three layers.

This molten sheet is cooled and solidified by electrostatic force on a drum cooled to a drum surface temperature of 10 to 60°C, and the unstretched film is guided to a group of rolls heated to 80 to 120°C, and the longitudinal The film is longitudinally stretched 2.0 to 5.0 times in the direction and cooled with a roll group at 20 to 50°C. Subsequently, the longitudinally stretched film is held at both ends with clips and introduced into a tenter, where it is laterally stretched in a direction perpendicular to the longitudinal direction in an atmosphere heated to 90 to 140°C. The stretching ratio is 2 to 5 times in both length and width, and the area ratio (longitudinal stretching ratio x lateral stretching ratio) is preferably 6 to 20 times. If the area magnification is less than 6 times, the resulting film will have poor whiteness, while if it exceeds 20 times, it will tend to tear easily during stretching and the film formability will tend to be poor. In order to impart flatness and dimensional stability to the biaxially stretched film, it is heat-set at 150 to 230°C in a tenter, uniformly cooled slowly, and then cooled to room temperature and wound to obtain the film of the present invention. .

[Method for Measuring Physical Properties and Evaluating Effects] The methods for evaluating physical property values and evaluating effects of the present invention are as follows. (1) Attach an integrating sphere to an average reflectance spectrophotometer (UV-260 manufactured by Shimadzu Corporation), and set the reflectance to 40% when the MgO white plate is taken as 100%.
Measure over 0-700 nm and 330-380 nm. The reflectance was read at 5 nm intervals from the obtained chart, the average value was calculated, and the result was taken as the average reflectance.

(2) Evaluation method of physical property value (maximum value - minimum value) of reflectance at light wavelength of 400 to 700 nm Chart obtained by measuring reflectance over 400 to 700 nm using the method described in item 1. Then, read the maximum value and minimum value of reflectance, and calculate (maximum value - minimum value).

(3) Cut the apparent specific gravity film into a 100 x 100 mm square, and place it on a dial gauge (No. 2109-10 manufactured by Mitoyo Seisakusho) with a diameter of 10 mm.
The thickness is measured at a minimum of 10 points using a measuring element (No. 7002) of mm, and the average value d (μm) of the thickness is calculated. In addition, this film was weighed using a direct reading balance,
Read the weight w (g) to the nearest 10-4 g. At this time, apparent specific gravity = w/d x 100.

(4) Brightness of the screen After setting a film as the reflector (11) of the device shown in Figure 3, measure the illuminance on the screen (12) with an illuminance meter (manufactured by Hioki Electric, 3421). and adjust the brightness of the screen. To measure illuminance, attach a cylindrical piece of 20 mm wide black drawing paper wrapped to the size of the photoreceptor to the photoreceptor.
Measurement is performed with the distance between 12) and the photoreceptor being 20 mm.

(5) Color of the screen Judging the screen visually, we judge white light as "white", yellowish light as "yellow", and reddish light as "red".
, those with a bluish tinge were designated as "blue".

(6) Glossiness Using a gloss meter VG-107 manufactured by Nippon Denshoku Kogyo Co., Ltd.,
According to JIS-Z-8741, the incident angle and acceptance angle are 6.
Evaluation was made according to 0°.

[0035]

EXAMPLES The present invention will be explained based on examples. Example 1 A polyethylene terephthalate chip and a master chip to which polyethylene glycol with a molecular weight of 4000 was added during polymerization of polyethylene terephthalate were
After vacuum drying at ℃ for 3 hours, 89% by weight of polyethylene terephthalate, 1% by weight of polyethylene glycol:
Mix to make polymethylpentene 10% by weight, 2
The mixture was supplied to an extruder heated to 70 to 300°C and molded into a sheet using a T-die. Furthermore, this film was cooled and solidified in a cooling drum with a surface temperature of 25°C.
It is guided into a group of rolls heated to 5 to 98°C, and 3.
It was longitudinally stretched 4 times and cooled with a roll group at 25°C. continue,
While holding both ends of the longitudinally stretched film with clips, the film was introduced into a tenter and laterally stretched 3.6 times in the direction perpendicular to the longitudinal direction in an atmosphere heated to 130°C. After that, 2 in the tenter
The film was heat-set at 30°C, uniformly cooled slowly, and then cooled to room temperature to obtain a film having a thickness of 188 μm. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Example 2 In Example 1, a fluorescent brightener (OB manufactured by Eastman Co., Ltd.) was used.
-1) The master chip contains 0.03% by weight of optical brightener.
The thickness was 18 mm using the same method as in Example 1.
A film of 8 μm was obtained. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Comparative Example 1 After drying a master chip made of polyethylene terephthalate compounded with titanium dioxide particles and a polyethylene terephthalate chip at 180°C for 3 hours,
The titanium dioxide particles were mixed to a concentration of 14% by weight, and a film having a thickness of 188 μm was obtained in the same manner as in Example 1. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Comparative Example 2 After drying a master chip made of polyethylene terephthalate compounded with titanium dioxide particles and a polyethylene terephthalate chip at 180°C for 3 hours,
A film having a thickness of 188 μm was obtained in the same manner as in Example 1 by mixing 5% by weight of titanium dioxide particles and 15% by weight of polypropylene. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Example 3 Using two extruders, polyethylene terephthalate chips were extruded from extruder 1 as in Example 1, to which 14% by weight of calcium carbonate particles ("Softon 3200" manufactured by Shiraishi Calcium Co., Ltd.) were added. After vacuum drying as in Example 1, it was extruded from extruder 2, and coextrusion was performed to form a three-layer structure, with extruder 1 serving as the inner layer and extruder 2 serving as the outer layer. Thereafter, a film with a thickness of 188 μm was obtained using the same method as in Example 1. The laminated structure of the obtained film was 12/164/12 μm. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Example 4 In Example 3, Example 2 was added to the raw material supplied to the extruder 2.
The same method as in Example 3 was carried out except that the same fluorescent whitening agent was added to give a concentration of 0.03% by weight.
A film of m was obtained. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2.

Example 5 A 188 μm film was obtained in the same manner as in Example 3, and subjected to corona discharge treatment. Next, acrylic resin (“Kotax” manufactured by Toray) / silicon dioxide (particle size 1 μm) / isocyanate / optical brightener (OB-1 manufactured by Eastman) = 1
It was diluted using toluene/methyl ethyl ketone = 1/1 as a solvent so that 00/5/20/1 was 20%, and coated with a gravure coater to obtain a coating film with a thickness of 3 μm. The physical properties of the obtained film as a substrate for a liquid crystal display reflector are shown in Table 1 and FIG. 2, and almost the same physical properties as the film of Example 3 were obtained.

[0042]

[Table 1]

[0043]

Effects of the Invention: The white polyester film of the present invention has an extremely high average reflectance of 90% or more in a specific wavelength range of light, and the variation in reflectance has been reduced, making it suitable for liquid crystal display reflectors. When the white polyester film of the present invention is used as a base material, it is possible to obtain a bright and easy-to-see liquid crystal screen that is unprecedented in the sidelight system. In addition, since the white polyester film of the present invention has an excellent average reflectance in a specific wavelength range of light, it can be used not only as a substrate for liquid crystal display reflectors but also in other applications that require high reflectance. It can exhibit excellent characteristics.

[Brief explanation of the drawing]

FIG. 1 is a reflectance characteristic diagram of a white polyester film according to the present invention as a base material for a liquid crystal display reflector.

FIG. 2 is a reflectance characteristic diagram of Examples and Comparative Examples.

FIG. 3 is a schematic cross-sectional view of a liquid crystal screen incorporating a reflector.

[Explanation of symbols]

H1 Reflectance characteristics showing an example of the white polyester film of claim 1 H2 Reflectance characteristics showing an example of the white polyester film of claim 10

Claims (13)

[Claims] 1. The average reflectance of the surface in the wavelength range of light from 400 to 700 nm is 90% or more, and the (maximum value - minimum value) of the reflectance in the wavelength range is 10% or less. White polyester film. 2. The white polyester film according to claim 1, wherein the white polyester film is a film used as a main component of a substrate for a liquid crystal display reflector. 3. The white polyester film according to claim 1, wherein the white polyester film is whitened by containing fine air bubbles inside the film. 4. The white polyester film according to claim 3, wherein the white polyester film has an apparent specific gravity of 0.5 or more and 1.2 or less. 5. The white polyester film contains polyester and polyolefin in an amount of 2% by weight or more and 25% by weight.
The white polyester film according to any one of claims 1 to 4, which contains the following and is biaxially stretched. 6. The white polyester film according to claim 5, wherein said polyolefin is polymethylpentene. 7. The white polyester film according to claim 5 or 6, wherein the polyester contains 0.1% by weight or more and 5% by weight or less of polyalkylene glycol and its derivatives. 8. The white polyester film comprises A
It consists of a two-layer structure of layer/B layer or a three-layer structure of A layer/B layer/A layer, where the B layer is a layer containing the above-mentioned microbubbles, and the A layer is a polyester containing 5 inorganic particles. Weight% or more2
The white polyester film according to any one of claims 3 to 7, which is a layer containing 5% by weight or less. 9. The white polyester film according to claim 8, wherein the inorganic particles are calcium carbonate. 10. A white polyester film characterized in that its surface has an average reflectance of more than 90% in a light wavelength range of 330 to 380 nm. 11. The white polyester film comprises:
11. The white polyester film according to claim 10, comprising a white polyester film containing fine air bubbles and a fluorescent whitening agent inside the film. 12. The white polyester film comprises:
11. The white polyester film according to claim 10, which comprises a plurality of laminated polyester films, and at least the outermost layer of the laminated film layers contains a fluorescent whitening agent. 13. The white polyester film comprises:
11. The white polyester film according to claim 10, which has a coating layer as the outermost layer, and at least the coating layer contains an optical brightener.