KR101888132B1 - Polyester film for polarzing reflective sheet - Google Patents

Abstract

The embodiment relates to a polyester film for a polarizing reflective sheet. The polyester film according to the embodiment minimizes a difference in orientation angle between the center part and the end part of a film while generating a difference in tensile strength in the length/width direction of the film, thereby improving mechanical strength and brightness such as modulus and minimizing the generation of polarization mura. Therefore, it can be usefully used as a base film of a polarizing reflective sheet. Further, by using a single-layer film, a working process is easier and thickness is thinner. So, it can be advantageously applied to a miniaturized, thin film display device.

Description

POLYESTER FILM FOR POLARIZING REFLECTIVE SHEET [0002]

An embodiment relates to a polyester film for polarized light reflection sheet.

In the information society, various displays such as a liquid crystal display (LCD), a plasma display panel (PDP), and an electrophoretic display (ELD) are being developed or commercialized. Indoor display displays are becoming increasingly larger and thinner, and outdoor portable displays are becoming smaller and lighter. In order to further improve the function of such a display, various optical films have been used since early.

The optical film may have a high transmittance, optical isotropy, defect free surface, high heat resistance and moisture resistance, high flexibility, high surface hardness, low shrinkage, easy processability, high brightness, contrast, , And luminous efficiency.

In particular, a brightness enhancement film or the like may be used to enhance the luminance, contrast, and luminous efficiency of a display. As the brightness enhancement film, a reflection type polarizing film is mainly used. In general, the high refractive index layer and the low refractive index layer are alternately repeatedly laminated and attached to a backlight unit (BLU) . Commercially available brightness enhancement films include 3M's Dual Brightness Enhancement Film (DBEF).

However, such a brightness enhancement film has a high surface resistance, and foreign substances or the like are easily attached to the surface due to the generation of static electricity. Further, in the case of a large liquid crystal display device, there is a problem that the liquid crystal is broken by an external electric charge. In addition, there is a problem that the surface hardness is low and mechanical properties such as scratch resistance are deteriorated, thereby deteriorating transparency.

In order to solve this problem, a surface treatment method such as laminating a coating layer on the surface of a film has been introduced. For example, there has been known an example in which a hard coat layer is coated on one surface of a film to strengthen the surface (Korean Patent Publication No. 2007-0028826 number). However, although the surface-treated film as described above can improve the brightness, mechanical properties, and appearance characteristics, it is composed of a plurality of layers and is limited in application to the thin display and miniaturization display in terms of thickness.

Korean Patent Publication No. 2007-0028826

Thus, in the embodiment, the difference in orientation angle between the center portion and the end portion of the film is minimized while generating the difference in the length / width direction strength of the single-layer film, and the polarizing reflection sheet To provide a polyester film.

According to one embodiment, there is provided a film comprising a polyester base film and a polarized-light reflection portion disposed on the polyester base film, wherein the polarized-light reflection portion is formed by alternately laminating an isotropic resin layer and a birefringent resin layer, An in-plane retardation of from 3,000 nm to 30,000 nm, the film has a transverse tensile strength of from 25 to 35 kgf / mm ^{2 and} a transverse tensile strength of from 8.0 to 19 kgf / mm ² , Directional tensile strength of 0.25 to 0.7, the film has a width modulus of 450 to 560 kgf / mm ² and a longitudinal modulus of 220 to 380 kgf / mm ² .

The polyester film for a polarizing reflection sheet according to the embodiment minimizes the difference in orientation angle between the center portion and the end portion of the film while causing a difference in tensile strength in the length / width direction of the film, thereby improving the mechanical strength and brightness, . Further, by using a single-layer film, the working process is easier and the thickness is slimmer, so that it can be usefully applied to a miniaturized, thin film display device.

1 is a schematic diagram showing an orientation angle measurement system used in Test Example (3).
Fig. 2 is a schematic view of the test example (4), which is used for confirming whether polarized light is generated, and in which a base film is mounted on a display.
3 is a schematic view of a polarizing reflective sheet according to an embodiment.

Hereinafter, the present invention will be described in more detail.

An embodiment includes a polyester base film and a polarized-light reflection portion disposed on the polyester base film, wherein the polarized-light reflection portion is formed by alternately laminating an isotropic resin layer and a birefringent resin layer, wherein the in- Wherein said film has a transverse tensile strength of from 25 to 35 kgf / mm ² and a longitudinal tensile strength of from 8.0 to 19 kgf / mm ² , said longitudinal tensile strength to said transverse tensile strength Of the film is 0.25 to 0.7, the film has a width modulus of 450 to 560 kgf / mm ² and a longitudinal modulus of 220 to 380 kgf / mm ² .

The polyester film for polarized light reflection sheet is a film comprising a polyester resin such as polyethylene terephthalate (PET) resin, which is stretched 4.0 to 8.0 times in the width direction (TD) and 1.0 times in the length direction (MD) To 3.0 times the original length. Specifically, the polyester film may be stretched 4.1 to 6.0 times in the width direction and 1.2 to 1.7 times, or 1.2 to 1.5 times in the length direction.

The film stretched at the above magnification may have an orientation angle deviation (optical axis) of not more than ± 2.8 deg. (°) with respect to the full width, and more specifically, an orientation of ± 0.5 to ± 2.8 deg. Or ± 0.5 to ± 2.5 deg. You can have each deviation. The closer the orientation angle deviation is to 0 deg., The more the brightness of the film is improved. In the above range, the brightness can be improved by 5% or more while suppressing the occurrence of polarization unevenness and preventing color distortion.

The following methods can be used to improve the mechanical strength of the film. Specifically, a method of increasing the stretching load (stress) at the time of stretching can be used. When the PET film is stretched at a ratio of the longitudinal direction to the transverse direction, the stretching temperature is lowered, and if the preheating amount of the film is reduced, the mechanical stress can be improved because the stress applied at the stretching increases and the orientation of the crystal increases. More specifically, the film has a tensile strength of 8.0 to 19kgf / mm ² tensile strength, transverse direction 25 to 35kgf / mm ² in the longitudinal direction. The ratio of the longitudinal tensile strength to the transverse tensile strength may be 0.25 to 0.7, 0.25 to 0.6, 0.25 to 0.4, or 0.25 to 0.35. Further, the film width direction is 450 to 560kgf / mm ^2, or 500 to 550 kgf / mm ^2, a longitudinal direction 220 to 380kgf / mm ^2, 220 to 280kgf / mm ^2, or 220 through the modulus of 270kgf / mm ² Lt; / RTI >

In addition, a method of crystallizing by elevating the heat treatment temperature (TMS temperature) upon heat fixation after stretching can be used. If the TMS temperature is increased, the crystal growth in the film is accelerated and the amount of crystal increases, so that the mechanical strength can be improved.

The in-plane retardation (Re) of the film may be 3,000 nm or more, 7,000 nm or more, or 7,000 nm to 30,000 nm.

Further, the film may have a retardation deviation of 500 nm / m or less, and a film thickness of 10 nm / m to 300 nm / m or 10 nm / m to 200 nm / m. When the retardation of the film and the deviation thereof are within the above range, color distortion such as rainbow shadows can be prevented.

The thickness of the film may be 30 to 300 탆, 30 to 200 탆, 60 to 200 탆, 60 to 190 탆, 80 to 200 탆, 80 to 190 탆, or 80 to 188 탆. When the thickness is within the above range, it is easy to apply to a thin film type and / or miniaturized display device.

When the heat shrinkage is measured for 24 hours at 85 DEG C for a sample of 200 mm x 200 mm, the film is 0.05 to 0.6% or 0.05 to 0.3% in the longitudinal direction, 0.05 to 0.6% in the width direction, or 0.05 to 0.6% It may have a shrinkage ratio of 0.3%.

Further, the elongation at break of the film may be about 7% to about 20% in the MD direction and about 75% to about 100% in the TD direction. Since the polyester film has the above-described elongation at break, it can have a smooth cut surface.

In addition, the orientation angle of the polyester film may be within about ± 5 degrees with respect to the width direction. More specifically, the orientation angle (optical axis) of the polyester film may be within about ± 3 ° based on the width direction. More specifically, the orientation angle of the polyester film may be within ± 2 [deg.] With respect to the width direction. When the orientation angle of the polyester film is in the above range, the polarizing plate to which the polyester film is applied may have improved polarization degree and brightness. In particular, the smaller the deviation between the orientation angle and the width direction, the more the alignment direction of the polarizing plate and the orientation angle of the polyester film coincide with each other, and the polarizing plate according to the embodiment can have improved brightness and polarization degree.

The polyester film for the polarizing reflection sheet of the embodiment can be produced by the following method.

First, a resin that can be used as a raw material for a film, such as a polyester resin such as PET resin, is melt-extruded to prepare an unstretched sheet, and then the unstretched sheet is stretched in the width direction and stretched in the longitudinal direction to produce a film .

Specifically, the PET resin may be prepared by directly producing a diol component such as ethylene glycol and a dicarboxylic acid component such as terephthalic acid through esterification reaction or polymerization, or a commercially available product may be used. The resin is melt-extruded and then cooled to produce an unoriented sheet. The unoriented sheet is stretched in the width direction, for example, 4.0 times or more, 4.0 times to 8.0 times, or 4.1 times to 6.1 times, Or less, 1.0 times to 3.0 times, 1.2 times to 1.7 times, or 1.2 times to 1.5 times. Further, in the stretching in the width direction, the stretching speed in the width direction may be from 200% / min to 800% / min, from 250% / min to 600% / min, or from 250% have.

The melt extrusion may be performed at a temperature ranging from Tm + 30 占 폚 to Tm + 60 占 폚. When melt extrusion is carried out within the above-mentioned temperature range, smooth melting can be performed and the viscosity of the extrudate can be properly maintained. In addition, the cooling can be carried out at a temperature of 30 DEG C or less, specifically 15 to 30 DEG C or less.

The stretching temperature may be in the range of Tg + 5 DEG C to Tg + 50 DEG C, and the lower the stretching temperature, the better the stretchability, but the breaking may occur. In particular, to improve the brittleness, it may be a stretching temperature range of Tg + 10 ° C to Tg + 40 ° C. For example, the stretching temperature in the longitudinal direction is from 75 to 85 占 폚, and the stretching temperature in the width direction is from 80 to 120 占 폚, and the stretching can be performed while raising the temperature stepwise within the above range.

Further, the method may further comprise the step of heat setting the stretched film. For example, the heat-setting temperature may be 180 to 230 ° C, and the heat-setting time may be 1 to 2 minutes. After initiating the heat setting, the film is relaxed in the longitudinal and / or width direction (relaxation rate 2 to 4%), and then the film can be prepared by lowering the temperature stepwise to 100 to 150 ° C.

According to the above production method, a film having an appropriate thickness and a low in-plane retardation and having improved brightness and mechanical strength without generating polarized light can be produced. In addition, the film according to the embodiment may contain various additives such as ordinary electrostatic charge, antistatic, anti-blocking agent and other inorganic lubricant within the range that does not impair the effect of this embodiment.

In addition, the polyester film according to the embodiment can be applied to a polarizing reflective sheet.

Specifically, the polarizing reflection sheet may include a polarizing reflection portion and a protective film sandwiching the polarizing reflection portion.

The polarizing reflector may be a multilayer stretched film. The multilayer stretched film is composed of a laminated film formed by alternately laminating a first layer and a second layer, and is stretched in the uniaxial direction (x direction).

The first layer and the second layer may be alternately stacked to form a laminate, and the laminate may have a thickness of 250 to 1,000, preferably 300 to 500.

Wherein the first layer of the multilayer stretched film is made of polyester whose refractive index is changed by stretching and the second layer is made of polyester having a small change in refractive index due to stretching, (X direction), the optical interference which selectively reflects or transmits light of a specific wavelength due to the difference in refractive index with respect to the second layer in the stretching direction is caused to exhibit the reflection characteristic .

The first layer may be a single polymerized polyethylene naphthalate, and the second layer may be a copolymerized polyethylene naphthalate having a low crystallinity.

In addition, the optical thicknesses of the first layer and the second layer may be 1/4 of the visible light wavelength. Accordingly, the film has a reflection ability due to the difference in refractive index in the stretching direction (x direction), but does not have a refractive power difference in a direction perpendicular to the uniaxial stretching direction (y direction) , It is possible to exhibit polarization characteristics that reflect only light in a certain direction.

The stretching in the x direction may be 3 to 7 times, preferably 4 to 6 times.

The difference in refractive index between the first layer and the second layer in the x direction by the stretching may be 0.1 to 0.5, preferably 0.2 to 0.4. When the refractive index difference in the x direction is in the above range, the reflection performance can be efficiently increased.

The refractive index difference in the film plane of the first layer and the second layer in the direction perpendicular to the uniaxial stretching direction (y direction) and the film thickness direction (z direction) may be 0.05 or less, preferably 0.04 or less, May be 0.03 or less. When the difference in refractive index between the y direction and the z direction is 0.05 or less, the color deviation when the polarized light is incident in the oblique direction can be suppressed.

The thickness of the first layer and the second layer may be continuously varied so that the maximum layer thickness for each minimum layer thickness is 1.2 to 1.5 times.

At least one of the protective films may be a polyester film as described above.

The width direction of the protective film and the main stretching direction of the multilayer stretched film may correspond to each other. That is, the width direction of the polyester film and the stretching direction of the multilayer stretched film can be substantially matched.

Accordingly, a polarizing reflective sheet comprising a polyester film according to an embodiment can have improved brightness.

As described above, the polyester film for polarizing reflection sheet minimizes the difference in orientation angle between the center portion and the end portion of the film while generating a difference in tensile strength in the length / width direction of the film, thereby improving the mechanical strength and brightness such as modulus , It is possible to minimize the occurrence of polarization unevenness and thus can be usefully used as a base film of a polarizing reflection sheet. Further, by using a single-layer film, the working process is easier and the thickness is slimmer, so that it can be usefully applied to a miniaturized, thin film display device.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are intended to illustrate the present invention, but the scope of the present invention is not limited thereto.

Example  One

Extruded from a polyethylene terephthalate resin (ethylene glycol 100 mol% + terephthalic acid 100 mol%, IV 0.61 dl / g, SKC) through an extruder at about 280 캜 and then cooled in a casting roll at about 25 캜 to obtain an unstretched sheet . The unstretched sheet was preheated at 100 ° C and stretched 4.16 times while raising the temperature stepwise at 80 ° C in the longitudinal direction at 1.20 times and in the transverse direction at 80 ° C to 120 ° C. The stretched sheet was then heat set at a temperature of about 210 캜 for about 90 seconds and relaxed at a relaxation rate of about 3% while lowering the temperature from about 150 캜 to about 100 캜 to obtain a single layer film having a thickness of 188 탆 .

Example  2 to 5 and Comparative Example  1 and 3

A single-layer film was prepared in the same manner as in Example 1, except that the stretching ratio in the longitudinal direction and the transverse direction and the thickness of the final film were changed as shown in Table 2 below.

Manufacturing example . Polarized reflective sheet manufacture.

A Vikuiti (TM) multi-layer stretched film of 3M Company was bonded to the base film of Examples and Comparative Examples in a cross-section or on both sides to prepare a polarizing reflection sheet.

< Test Example >

(One) Modulus  evaluation

In accordance with ASTM D 882, longitudinal / transverse elastic modulus was measured using universal tester 4206-001 (Instron). The results are shown in Table 1 below.

(2) Evaluation of tensile strength

The tensile strength was measured by applying a load to the film according to KS B 5521 and dividing the maximum load at the stretched value by the original cross-sectional area of the film. The results are shown in Table 1 below.

(3) Optical axis evaluation

The orientation angle of the film (sample film) was measured using the orientation angle measurement system as shown in Fig. 1, and the deviation (optical axis) of the angle between the measured orientation angle and the width direction was obtained. The results are shown in Table 1 below.

(4) In-Plane Phase difference  evaluation

The refractive index (nx, ny) and the refractive index (nz) in the thickness direction of the orthogonal biaxial were measured using Abbe's refractive index meter (NAR-4T, measurement wavelength 589 nm, Atago) The measurement was carried out using a meter (Millitron 1245D, Pahrung Phosphor) and the unit was converted to nm. The in-plane retardation Re was determined by multiplying the measured refractive indexes of the biaxials by the absolute value (| nx-ny |) and multiplying the film thickness d (nm) by (nxy x d) Respectively.

(5) Polarized light  Occurrence

(Sample film) obtained in the above example or comparative example was attached to one side of a polarizer made of PVA and iodine so that the absorption axis of the polarizer and the main axis of orientation of the film were perpendicular to each other and a TAC film 80 Mu m, FUJIFILM Corporation) was pasted to produce a polarizing plate. The polarizing plate was provided on the outgoing light side of a liquid crystal display device using a white LED composed of a light emitting element in combination of a blue light emitting diode and a yttrium aluminum garnet yellow phosphor as a light source (NSPW500CS, Nichia Corporation) . The liquid crystal display had a polarizing plate having two polarizing plates as a polarizer protective film on the incident light side of the liquid crystal cell (see Fig. 2). The polarizing plate of the liquid crystal display was visually observed from the front and the oblique direction of the polarizing plate to observe the occurrence of polarization unevenness, and the results are shown in Table 1 below.

(6) Brightness Improvement rate

After arranging the polarizing reflection sheets prepared on the back light source including the light guide plate, the brightness of light passing through the prism sheets was measured through a Minolta CS-2000. At this time, the luminance increase rate was determined based on the luminance of Comparative Example 1. [

From the results shown in Table 1, all of the films prepared in Examples 1 to 5 are excellent in tensile strength and modulus, and have an alignment angle deviation (optical axis) of 2.8 deg. And the in-plane retardation is as high as 7,000 nm or more, so that polarization unevenness is not generated, and it is confirmed that the brightness of one side and the other side are simultaneously improved. On the other hand, in the films of Comparative Examples 1 to 3, the deviation of the alignment angle was large, the in-plane retardation was low, and the polarization unevenness was strongly exhibited, or the luminance improvement rate was lower than that of the film of Example.

Therefore, it can be seen that not only the polarized light is not generated in the film produced in the embodiment, but the brightness of both sides (double) is improved and the mechanical strength is also greatly improved.

Claims (13)

Polyester based films; And
And a polarizing reflector disposed on the polyester base film,
Wherein the polarized-light reflection portion is formed by alternately laminating an isotropic resin layer and a birefringent resin layer,
Wherein the polyester base film has an in-plane retardation of 3,000 nm to 30,000 nm,
The film has a longitudinal tensile strength of 25 to 35 kgf / mm ² and a transverse direction tensile strength of 8.0 to 19 kgf / mm ^2, the
The ratio of the longitudinal tensile strength to the transverse tensile strength is 0.25 to 0.7,
The film has a modulus in the width direction of 450 to 560 kgf / mm ² , a modulus in the longitudinal direction of 220 to 380 kgf / mm ² ,
The film has a shrinkage ratio of 0.05 to 0.6% in the longitudinal direction (MD) and 0.05 to 0.6% in the transverse direction (TD) at 85 DEG C for 24 hours,
Wherein the film has an elongation at break of 7 to 20% in the longitudinal direction (MD) and 75 to 100% in the transverse direction (TD). The method according to claim 1,
The refractive index difference between the isotropic resin layer and the birefringent resin layer is 0.1 to 0.5 in the first direction and 0.1 or less in the second direction perpendicular to the first direction,
Wherein the first direction is the width direction of the polyester substrate. The method according to claim 1,
Wherein the film has a width of 占 0.5 to 占 2.8 deg. Of the polarized-light reflection sheet. The method according to claim 1,
Wherein the in-plane retardation of the film is from 7,000 nm to 30,000 nm. The method according to claim 1,
Wherein the film has a retardation deviation of 10 to 300 nm / m or less. Melt-extruding a polyester resin to produce an unstretched sheet;
Stretching the unstretched sheet in the width direction and the length direction, respectively;
Drying the stretched sheet to produce a polyester base film; And
And adhering the polyester base film to a cross section or both sides of the polarizing reflection section,
Stretching the undrawn sheet in the width direction by 4.0 to 8.0 times and extending in the longitudinal direction by 1.0 to 3.0 times,
The heat-setting temperature is 180 ° C to 230 ° C,
The stretched sheet is relaxed by 2% to 4% in the longitudinal direction, the width direction, or both, after initiating the heat setting,
Further comprising a step of lowering the temperature stepwise to 100 ° C to 150 ° C. The method according to claim 6,
Wherein the stretching temperature in the width direction is from 80 to 120 占 폚 and the stretching temperature in the longitudinal direction is from 75 to 85 占 폚. The method according to claim 6,
Wherein the stretching speed in the width direction is from 200% / min to 800% / min. The method according to claim 6,
Wherein the polarizing reflection portion is a multilayer stretched film,
The multilayer stretched film is a film in which a laminated film formed by alternately stacking a first layer and a second layer is uniaxially stretched,
The width direction of the polyester base film corresponds to the uniaxial stretching direction of the multilayer stretched film,
Wherein the first layer is made of polyester whose refractive index is changed by stretching,
Wherein the second layer is made of polyester having a small refractive index change by stretching than the first layer. 10. The method of claim 9,
Wherein the laminated film has from 250 to 1000 layers. 10. The method of claim 9,
Wherein the multilayer stretched film is produced by stretching the laminated film three to seven times. 10. The method of claim 9,
Wherein the first layer is a single-polymerized polyethylene naphthalate, and the second layer is a copolymerized polyethylene naphthalate. 10. The method of claim 9,
Wherein the difference in refractive index between the first layer and the second layer in the stretching direction by stretching the laminated film is 0.1 to 0.5.

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