JP4960601B2 - Light diffusion plate film - Google Patents

Description

  The present invention relates to a light diffusion plate film used in a liquid crystal display device.

In recent years, many liquid crystal displays have been distributed as TV receivers. The liquid crystal display includes a light diffusing plate for reflecting light from a light source installed behind the liquid crystal element in the display. In order to increase the brightness of the liquid crystal display, it is necessary to increase the amount of light. Usually, the light diffusing plate bends due to heat generated from the light source. Due to this deflection, luminance spots appear on the entire screen of the liquid crystal display.
Conventionally, a solution to this problem has been expected, and as one of means for improving the luminance unevenness problem due to the enlargement of the screen, for example, the use of a film having a low coefficient of thermal expansion has been proposed.

JP 2001-220432 A JP 2001-247696 A JP 2002-019064 A JP 2002-350617 A JP 2003-266622 A JP 2004-067853 A JP 2004-133021 A

  However, the above conventional technology alone is still insufficient to suppress the deflection of the light diffusion plate, the balance between the thermal shrinkage rate and the refractive index in the MD direction and the TD direction of the film is poor, and the thermal expansion Even if the coefficient is sufficiently low, deflection occurs when used under relatively high humidity conditions.

  An object of the present invention is to provide a film for a light diffusing plate used in a liquid crystal display device, which has excellent deflection resistance due to wet heat and can increase the brightness of the liquid crystal display device.

That is, in the present invention, the glass transition temperature (Tg) of the polyester constituting the biaxially oriented polyester film is 30 minutes to 72 hours at a temperature in the range of −40 ° C. to Tg with respect to the biaxially oriented polyester film after heat setting. It is a film for a light diffusion plate made of a biaxially oriented polyester film having a wet heat deflection rate of less than 20% , which is obtained by performing an off-annealing treatment at a humidity of 40 to 90% RH . The wet heat deflection rate is a numerical value measured using the method defined in the example column of this specification.

  ADVANTAGE OF THE INVENTION According to this invention, the polyester film useful as a light-diffusion board which is excellent in the bending-proof characteristic by wet heat and can raise the brightness | luminance of a liquid crystal display device can be provided.

Hereinafter, the present invention will be described in detail.
In the present invention, it is important that the wet heat deflection rate of the biaxially oriented polyester is less than 20%. A polyester film for a light diffusing plate having a deflection rate of less than 20% is not conventionally known, but this deflection rate is obtained by forming a polyester film by biaxial stretching and then performing off-annealing under the conditions described later. Obtainable.

[polyester]
In the present invention, polyethylene terephthalate is used as the polyester constituting the biaxially oriented film. The polyethylene terephthalate may be a homopolymer or a copolymer containing a copolymer component, but a homopolymer is preferred.

  In the case of a copolymer, a polymer in which the repeating unit of ethylene terephthalate accounts for preferably 90 mol% or more, more preferably 95 mol% or more of all repeating units is used. When the proportion of the copolymer component exceeds 10 mol%, the light transmittance of the film is lowered, and thermal stability and dimensional stability are lowered, which is not preferable.

  The intrinsic viscosity of polyethylene terephthalate (calculated from the measured value in orthochlorophenol at 35 ° C.) is preferably 0.4 or more, more preferably 0.5 to 1.2, and particularly preferably 0.55 to 0.85. is there. If it is less than 0.4, the mechanical strength of the film is lowered, which is not preferable. An intrinsic viscosity exceeding 1.2 is not preferable because the burden on the extruder increases, the resin generates heat, and the degree of polymerization decreases.

  Polyethylene terephthalate, for example, polycondensation reaction of terephthalic acid and ethylene glycol, polycondensation reaction after transesterification of lower alkyl ester of terephthalic acid and ethylene glycol, polycondensation of bisglycol ester of terephthalic acid It can manufacture by methods, such as the method of making it. Examples of the catalyst used for the polycondensation reaction include an antimony compound (Sb compound), a titanium compound (Ti compound), a germanium compound (Ge compound), and the like.

[Lubricant]
The biaxially oriented polyester film in the present invention preferably has a large number of fine protrusions on the surface in order to ensure the productivity. In order to impart fine protrusions to the film, inert particles may be dispersed and contained as a lubricant in the polyester film, or an easy-sliding layer containing inert particles may be provided on the polyester film. Excellent slipperiness can be obtained by having fine protrusions.

  Inactive particles include calcium carbonate, magnesium carbonate, calcium oxide, zinc oxide, magnesium oxide, silicon oxide, sodium silicate, aluminum hydroxide, iron oxide, zirconium oxide, barium sulfate, titanium oxide, tin oxide, and antimony trioxide. Examples thereof include inorganic fine particles such as carbon black and molybdenum disulfide, and organic fine particles such as acrylic cross-linked polymers, styrene cross-linked polymers, cross-linked silicone resins, fluororesins, benzoguanamine resins, phenol resins, and nylon resins.

  The average particle size of the inert particles is preferably 50 to 150 nm. If the thickness is less than 50 nm, the slipperiness does not appear, and if it exceeds 150 nm, the slipping off occurs, which is not preferable. The specific gravity of the inert particles is preferably 3 or less. If it exceeds 3, it precipitates in the aqueous dispersion, which is inconvenient in the production process and is not preferred.

  When the inert particles are incorporated in the film, the amount is, for example, 0.0025 to 0.036 parts by weight, preferably 0.0025 to 0.012 parts by weight per 100 parts by weight of the polyester. When providing a slippery layer and making it contain in a slippery layer, Preferably it is 5 to 30 weight% per 100 weight part of compositions which comprise a slippery layer. In addition, when using an inert particle with an average particle diameter of 100-150 nm, it is good to use in the range of 5-10 weight%, and when using an inert particle with an average particle diameter of 50-90 nm, it is good to use in the range of 8-30 weight%. . This average particle diameter is an average particle diameter of primary particles, and is an average obtained by tracing or projecting an image of primary particles that can be observed with a magnified photograph at a magnification of 100,000 by a transmission electron microscope to obtain an area equivalent circle diameter by an image analyzer. The particle size.

[Heat heat deflection rate]
In the present invention, the biaxially oriented polyester film has a wet heat deflection rate of less than 20%, preferably 15% or less. When the wet heat deflection rate is 20% or more, when used as a light diffusing plate in a liquid crystal display for a long time, deflection occurs due to heat from the backlight. Further, if the wet heat deflection rate is 20% or more, it causes brightness spots and the quality of the liquid crystal display is impaired. The wet heat deflection rate is defined as described in the section of the deflection rate in the examples.

[Refractive index]
The absolute value of the difference between the refractive index in the MD direction and the refractive index in the TD direction of the biaxially oriented polyester film is preferably 0.01 or less, more preferably 0.008 or less, and particularly preferably 0.006 or less. If the absolute value of the difference exceeds 0.01, a remarkable difference is caused in the behavior of MD and TD when the temperature is applied, so that the balance is easily lost. Deflection is not preferable because luminance spots appear on the liquid crystal screen and the quality is significantly impaired.
MD is the longitudinal direction of the film, and TD is the direction orthogonal to the MD.

[Heat shrinkage]
In the biaxially oriented polyester film, the absolute value of the difference between the thermal shrinkage rate in the MD direction and the thermal shrinkage rate in the TD direction at 150 ° C. for 30 minutes is preferably 0.5 or less, more preferably 0.4 or less, Especially preferably, it is 0.3 or less. If the absolute value of the difference exceeds 0.5, the film tends to be out of balance when the temperature is applied. Deflection is not preferable because luminance spots appear on the liquid crystal screen and the quality is significantly impaired.

[Thickness]
The thickness of the biaxially oriented polyester film is preferably 20 to 300 μm, more preferably 20 to 250 μm, and particularly preferably 25 to 200 μm. If the thickness is less than 20 μm, the waist is weak and flatness is lost during processing, and scratches are liable to occur. If the thickness exceeds 300 μm, the waist is too strong, the workability is poor, and the transparency is lowered, which is not preferable.

[Haze]
The haze value of the biaxially oriented polyester film is preferably 3% or less, more preferably 2% or less, and particularly preferably 1.5% or less. If the haze value exceeds 3%, the luminance required for the light diffusion plate cannot be obtained, which is not preferable.

[Hue]
In the biaxially oriented polyester film, the b * value in the L * a * b * color system is preferably −1 or more and 1.5 or less, more preferably −0.5 or more and 1.5 or less, and particularly preferably − 0.5 or more and 1.3 or less. When the b * value is less than −1, the color is bluish, and when it exceeds 1, the color is yellowish.

[Production method]
The above-mentioned physical properties of the biaxially oriented polyester film in the present invention can be achieved by subjecting the polyester film to a biaxial stretching method and then an off-annealing treatment.

  The biaxially oriented polyester film itself subjected to off-annealing can be produced by a conventionally known method for producing a biaxially oriented film. For example, it can be produced by a sequential biaxial stretching method, a simultaneous biaxial stretching method, and an inflation method. Of these, the sequential biaxial stretching method is preferred.

  In order to produce a biaxially oriented polyester film by the sequential biaxial stretching method, for example, the following may be performed. First, polyester blended with inert particles as necessary is melt-extruded through a die and rapidly cooled and solidified on a casting drum set to about 20 to 40 ° C. to obtain an unstretched film. The unstretched film is then stretched at 90 to 140 ° C., 3.0 to 4.5 times in the film running direction (longitudinal direction), and 3.0 to 4.5 times in the direction perpendicular to the film (lateral direction) The film is stretched 9 to 20 times in area magnification.

  In the present invention, the film is preferably heat-set after biaxial stretching. The heat setting temperature is preferably 180 to 235 ° C, more preferably 180 to 225 ° C, and particularly preferably 180 to 220 ° C. When the heat setting temperature exceeds 235 ° C., the polyester is excessively crystallized, and the total light transmittance may be lowered. If it is less than 180 degreeC, the balance of MD * TD of a thermal contraction rate will collapse | crumble and a deflection may not be suppressed and it is unpreferable.

  The haze value is mainly the type of inert particles in the polyester film, the average particle size, the content, the degree of voids generated around the particles, the type of inert particles contained in the easy adhesion layer, the average particle size, the content. Therefore, it is only necessary to adjust these so as to satisfy a desired haze value.

In order to obtain a biaxially oriented polyester film having the physical properties of the present invention, an off-annealing treatment is further performed on the biaxially oriented polyester film after heat setting. That is, with respect to the biaxially oriented polyester film obtained by film formation by the above method, the glass transition temperature (Tg) of the polyester of the film is from 40 ° C. to Tg for 30 minutes to 72 hours for 40 to 40 hours. treated at a humidity of 90% R H. This is referred to as off-annealing in this specification. At this time, if the heat treatment is performed at a temperature equal to or higher than the Tg of the film, the film is deformed by heat and the flatness necessary for the light diffusion plate is lost. If the heat treatment temperature is lower than Tg−40 ° C., the residual stress in the film cannot be relaxed, and the effect of off-annealing treatment cannot be obtained.

Hereinafter, the present invention will be described in more detail with reference to examples.
“Parts” means parts by weight. Moreover, the measuring method of each characteristic value is as follows.

(1) Film thickness 100 points were measured with an external micrometer, and the average value was obtained as the film thickness.

(2) Refractive index The film formed was measured with an Abbe refractometer (D line 589 nm). The refractive index in the MD direction was n _MD , and the refractive index in the TD direction was n _TD .

(3) Heat Shrinkage The heat shrinkage when a film sample was placed at a temperature of 150 ° C. for 30 minutes was measured according to JIS-Z1709, and each average value of N = 3 was taken. The MD direction is the longitudinal direction of the film, and TD is the direction orthogonal to the MD.

(4) Haze The haze of the film was measured based on JISK-7136 using the Nippon Denshoku Industries Co., Ltd. haze measuring device (NDH-2000).

(5) Thickness of easy-adhesive coating film Samples were dyed with osmic acid, embedded in epoxy resin, and sliced with a thickness of 100 nm so that the film end face could be seen with a microtome. Transmission electron microscope (manufactured by JEOL Ltd.) JEM-1200EX) was used, observed at a magnification of 5 to 100,000 times so as to close up the easy-adhesion layer, measured at 100 arbitrary positions, and an arithmetically averaged thickness was adopted.

(6) Moisture heat deflection rate The moisture heat deflection rate was determined by the following method. Samples were cut out from the film into a size of 24 × 24 cm, and lines were drawn in a lattice pattern at intervals of 2 cm as shown in FIG. Four sides of this sample were fixed to a glass plate with a double-sided adhesive tape. As the glass plate, a glass plate having a surface subjected to sand mat treatment was used in order to prevent the glass surface and the film from sticking to each other. The position fixed with the adhesive tape was the edge of the film, and the width was 1 cm on each side. The fixed state is shown in FIG. In this state, the sample was left in a constant temperature and humidity oven at a temperature of 60 ° C. and a relative humidity of 90% RH for 1 hour. Thereafter, the film was taken out from the oven and allowed to stand at room temperature for 1 hour. When the state of the film was stabilized, the degree of deflection of the film was visually observed, and the wet heat deflection rate was determined according to the following criteria.

[How to determine the heat-heat deflection rate]
The sample fixed to the glass plate was reflected by the fluorescent light, and the number of bent 20 mm × 20 mm was counted according to the following rules.
1) The reflected fluorescent light is distorted by the deflection of the film, and the film floats from the glass plate.
2) Although the film is not lifted from the glass plate, the reflected fluorescent lamp light appears to be distorted by the deflection of the film.
3) Although the reflected fluorescent lamp is not distorted, it is 20 mm x 20 mm raised from the glass plate → △
4) The light of the reflected fluorescent light is not distorted and is not lifted off the glass plate.
Each x was counted as 1 and Δ was counted as 0.5. The total number of flexures of 20 mm × 20 mm bent was divided by the total number of 100 (excluding because the ends were fixed with adhesive tape) to obtain the deflection rate.
Deflection rate = Deflection (pieces) / 100 (pieces) x 100 (%)
○: Less than 20% (good)
×: 20% or more (defect)
This experiment was performed with n = 5, and the arithmetic average was taken as the result.

[Example 1]
96 parts of dimethyl terephthalate, 58 parts of ethylene glycol and 0.038 part of manganese acetate were charged into the reactor, respectively, and the ester exchange reaction was conducted while distilling methanol until the internal temperature reached 240 ° C. with stirring. After completion, 0.097 parts of trimethyl phosphate and 0.041 parts of antimony trioxide were added. Subsequently, the temperature of the reaction product was raised, and finally, polycondensation was performed under a high vacuum condition at 280 ° C. to obtain a polyester chip having an intrinsic viscosity ([η]) of 0.64.

  The polyester chip was dried at 160 ° C. for 3 hours, melt-extruded at 280 ° C., and rapidly cooled and solidified on a cooling drum maintained at 20 ° C. to obtain an unstretched film. This unstretched film was stretched 3.6 times in the longitudinal direction at 95 ° C., and then the following coating agent P (coating liquid) was applied on the lower surface and further on the upper surface so that the thickness after drying was 0.06 μm, The film was stretched 3.6 times in the transverse direction at 120 ° C. and then heat-treated at a heat setting temperature of 220 ° C. to obtain a biaxially oriented film having a thickness of 100 μm.

The obtained biaxially oriented film was cut into a size of 240 mm × 240 mm. This film was hung by being wrapped in a Teflon (registered trademark) sheet in order to prevent sticking between the films. In this state, off-annealing treatment was performed under conditions of 75 ° C., 50% R H , and 1 hour. The physical properties of the obtained film are shown in Tables 1 and 2.

[Coating agent P]
(P component 1) Copolyester resin (Tg: 68 ° C.) 60% by weight.
Acid component: terephthalic acid (90 mol%), isophthalic acid (6 mol%), potassium 5-sulfoisophthalate (4 mol%).
Glycol component: ethylene glycol (95 mol%), neopentylene glycol (5 mol%).
(P component 2) N, N′-ethylenebiscaprylic acid amide 5% by weight.
(P component 3) Acrylic copolymer (number average molecular weight: 248,000) 20% by weight.
Composition: methyl acrylate (65 mol%), ethyl acrylate (28 mol%), 2-hydroxyethyl methacrylate (2 mol%), N-methylol methacrylamide (5 mol%).
(P component 4) Acrylic resin fine particles (average particle size 0.03 μm) 10% by weight.
(P component 5) 5% by weight of polyoxyethylene nonylphenyl ether.

[Example 2]
After the film formation in the same manner as in Example 1, the same process as in Example 1 was performed except that the off-annealing conditions were 60 ° C., 50% RH , and 3 hours. The physical properties of the obtained film are shown in Tables 1 and 2.

[Comparative Example 1]
A biaxially oriented film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that off-annealing was not performed. The physical properties of the obtained film are shown in Tables 1 and 2.

[Comparative Example 2]
A biaxially oriented film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the stretching ratio was changed to 3.2 times in the longitudinal direction and 4.2 times in the transverse direction. Even when off-annealing was performed, the film formation conditions were not appropriate, so that deflection occurred. The physical properties of the obtained film are shown in Tables 1 and 2.

[Comparative Example 3]
A biaxially oriented film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the off-annealing temperature was changed to 120 ° C., 50% R H and 1 hour. Since the conditions for off-annealing were not appropriate, the flatness of the film after off-annealing was lost. The physical properties of the obtained film are shown in Tables 1 and 2.

[Reference Example 1]
A biaxially oriented film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the heat setting temperature was changed to 238 ° C. Although the deflection was good, the optical characteristics deteriorated, so it was inappropriate as a film for a light diffusion plate.

  According to the present invention, it is possible to provide a film suitable as a substrate for a light diffusing plate that is used in a liquid crystal display device that has high resistance to moist heat and has little deflection.

It is a mode of a lattice of a sample in evaluation of a wet heat deflection rate.

Claims (6)

With respect to the biaxially oriented polyester film after heat setting, the glass transition temperature (Tg) of the polyester constituting the biaxially oriented polyester film is 40 to 90 hours at a temperature in the range of −40 ° C. to Tg for 30 minutes to 72 hours. A film for a light diffusing plate comprising a biaxially oriented polyester film having a wet heat deflection rate of less than 20%, which is obtained by performing an off-annealing treatment at a humidity of% RH.
(However, the wet heat deflection rate is obtained by cutting a sample from a film into a size of 24 cm × 24 cm and drawing lines in a lattice pattern at intervals of 2 cm, and the surface of the sample is sand-matted on the four sides. It is a value obtained as a ratio of the amount of bending that is observed when left in a constant temperature and humidity oven with a temperature of 60 ° C. and a relative humidity of 90% RH for 1 hour and then at room temperature for 1 hour. .) The film for light diffusing plates according to claim 1, wherein an absolute value of a difference between a refractive index in the MD direction and a refractive index in the TD direction of the biaxially oriented polyester film having a wet heat deflection rate of less than 20% is 0.01 or less. The absolute value of the difference between the thermal shrinkage rate in the MD direction and the thermal shrinkage rate in the TD direction under the condition of 150 ° C. for 30 minutes of the biaxially oriented polyester film having a wet heat deflection rate of less than 20% is 0.5 or less. The film for light diffusing plates according to claim 1 or 2.   The light diffusing plate for liquid crystal display devices which consists of a film for light diffusing plates of any one of Claims 1-3.   A liquid crystal display device comprising the light diffusing plate according to claim 4. When manufacturing a film for a light diffusing plate comprising a biaxially oriented polyester film having a wet heat deflection rate of less than 20%, the polyester glass constituting the biaxially oriented polyester film with respect to the biaxially oriented polyester film after heat setting Transition temperature (Tg) A method for producing a film for a light diffusion plate, wherein an off-annealing treatment is performed at a temperature in the range of −40 ° C. to Tg for 30 minutes to 72 hours at a humidity of 40 to 90% RH.
(However, the wet heat deflection rate is obtained by cutting a sample from a film into a size of 24 cm × 24 cm and drawing lines in a lattice pattern at intervals of 2 cm, and the surface of the sample is sand-matted on the four sides. It is a value obtained as a ratio of the amount of bending that is observed when left in a constant temperature and humidity oven with a temperature of 60 ° C. and a relative humidity of 90% RH for 1 hour and then at room temperature for 1 hour. .)

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