KR102061215B1 - Optical polyester film - Google Patents

Abstract

Example relates to an optical polyester film, the optical polyester film according to the embodiment minimizes the difference in the orientation angle between the film center portion and the end while generating a difference in tensile strength in the length / width direction of the film, mechanical strength And luminance can be improved, and the occurrence of polarized light can be suppressed. Furthermore, since a single layer film is used, the processing process is easier and the thickness is thinner, and thus it may be usefully applied to a miniaturized and thin film display device.

Description

Optical polyester film {OPTICAL POLYESTER FILM}

Examples relate to an optical polyester film.

As the information society enters, various displays such as liquid crystal display (LCD), plasma display panel (PDP), and electrophoretic display (ELD) are being developed or commercialized. Displays for indoor exhibitions are becoming increasingly large and thin, 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 has a high light transmittance, optical isotropy, defect-free surface, high heat resistance and moisture resistance, high flexibility, high surface hardness, low shrinkage, processability, high brightness, and contrast, depending on the type and / or use of the display. And physical properties such as luminous efficiency.

In particular, a brightness enhancing film or the like may be used to increase the brightness, contrast, and luminous efficiency of the display. A reflective polarizing film is mainly used as the brightness enhancing film, and the film is generally formed by alternately stacking a high refractive index layer and a low refractive index layer, and may be attached to a backlight unit (BLU) by a light shielding tape. Can be. Commercially available brightness enhancing films include 3M's Dual Brightness Enhancement Film (DBEF).

However, the brightness enhancing film as described above has high surface resistance, and foreign matters are easily attached to the surface due to the generation of static electricity. In addition, in the case of a large liquid crystal display, there is a problem such that the liquid crystal is broken by an external charge. In addition, there is a problem that the surface hardness is low, mechanical properties such as scratch resistance is lowered, thereby impairing transparency.

In order to solve such a problem, a surface treatment method such as laminating a coating layer on the surface of the film has been introduced, and for example, a hard coating layer is coated on one surface of the film to strengthen the surface (for example, Korean Patent Publication No. 2007-0028826). number). However, the film treated as described above can improve the brightness, mechanical properties and appearance characteristics, but is composed of a plurality of layers in view of thickness, there is a limit to apply to the display of the thinning, miniaturization trend.

Korean Laid-Open Patent No. 2007-0028826

Accordingly, the embodiment minimizes the difference in the orientation angles of the film center portion and the end portion while generating a difference in the length / width direction strength of the single layer film, thereby improving the mechanical strength and appearance characteristics while improving luminance and no polarizing mura. To provide an ester film.

According to one embodiment, a uniaxial or biaxially stretched polyester film having an in-plane retardation of 3,000 to 30,000 nm, wherein the film has a width direction tensile strength of 25 to 35 kgf / mm ² and a length direction of 8.0 to 19 kgf / mm ² . Has a tensile strength, the ratio of the longitudinal tensile strength to the widthwise tensile strength is 0.25 to 0.7, the width modulus of the film is 450 to 560 kgf / mm ² , the longitudinal modulus is 220 to 380 kgf / It provides an optical polyester film of mm ² .

According to one embodiment, melt extruding a polyester resin to prepare an unstretched sheet; Stretching the unstretched sheet in a width direction and a length direction, respectively; And heat-setting the stretched sheet to produce a polyester film.

The unstretched sheet is stretched in a width direction of 4.0 times to 8.0 times, in a length direction of 1.0 times to 3.0 times, and the heat setting temperature is 180 to 230 ° C., and the stretched sheet is lengthwise after starting heat setting. , 2% to 4% relaxed in the width direction or both, the heat setting further provides a method for producing an optical polyester film, further comprising the step of lowering the temperature stepwise to 100 to 150 ℃.

The optical polyester film according to the embodiment minimizes the difference in the orientation angles of the film center portion and the end portion while generating a difference in tensile strength in the length / width direction of the film, thereby improving mechanical strength and luminance, and suppressing polarization mura generation. Can be. Furthermore, since a single layer film is used, the processing process is easier and the thickness is thinner, and thus it may be usefully applied to a miniaturized and thin film display device.

1 is a schematic diagram showing an orientation angle measuring system used in Test Example (3).
FIG. 2 is a schematic diagram in which a film (sample film) is mounted on a display, which is used to confirm whether or not polarization mura occurs in Test Example (4).
3 is a schematic view of a liquid crystal display device including the polarizing plate according to the embodiment.
4 is a schematic diagram of an organic light emitting display device including a polarizing plate according to an embodiment.

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

Examples are uniaxial or biaxially stretched polyester films having an in-plane retardation of 3,000 nm to 30,000 nm, wherein the film has a tensile strength in the width direction of 25 to 35 kgf / mm ² and a longitudinal tensile strength of 8.0 to 19 kgf / mm ² . Wherein the ratio of the longitudinal tensile strength to the tensile strength in the width direction is 0.25 to 0.7, the width modulus of the film is 450 to 560 kgf / mm ² , and the longitudinal modulus is 220 to 380 kgf / mm ^2. It provides the polyester film for phosphorus optics.

The optical polyester film is a film containing a polyester resin, for example, polyethylene terephthalate (PET) resin, stretched from 4.0 times to 8.0 times in the width direction (TD), and 1.0 times to 3.0 in the longitudinal direction (MD). It can be prepared by stretching the vessel. In detail, the polyester film is stretched 4.1 times to 6.0 times in the width direction, it may be stretched 1.2 times to 1.7 times, or 1.2 times to 1.5 times in the longitudinal direction.

The stretched film may have an orientation angle deviation (optical axis) of ± 2.8 deg. (°) or less with respect to the full width, and in detail, an orientation of ± 0.5 to ± 2.8 deg., Or ± 0.5 to ± 2.5 deg. Each deviation can have. As the orientation angle deviation is closer to 0 deg., The luminance of the film is improved, and when within the range, the luminance may be improved by 5% or more while suppressing polarization mura and preventing color distortion.

The film can be used the following method to improve the mechanical strength. Specifically, a method of increasing the stretching load (stress) during stretching may be used. When the PET film is stretched at a ratio in the longitudinal direction / width direction, when the stretching temperature is lowered to reduce the amount of preheating received by the film, the stress of the film is increased and the orientation of the crystal is increased, thereby improving mechanical strength. More specifically, the film has a tensile strength of 8.0 to 19 kgf / mm ^{2 in} the longitudinal direction, 25 to 35 kgf / mm ^{2 in} the width direction. The ratio of the longitudinal tensile strength to the widthwise 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 is 450 to 560 kgf / mm ² in the width direction, or 500 to 550 kgf / mm ² , 220 to 380 kgf / mm ² , 220 to 280 kgf / mm ² , or 220 to 270 kgf / in the longitudinal direction It can have a modulus of mm ² .

In addition, a method of increasing the heat treatment temperature (TMS temperature) at the time of heat setting after stretching may be used. Increasing the TMS temperature accelerates crystal growth in the film and increases the amount of crystals, thereby improving mechanical strength.

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.

Furthermore, the film may have a phase difference deviation of 500 nm / m or less, and the detail may be 10 nm / m to 300 nm / m or 10 nm / m to 200 nm / m. When the phase difference of the film and the deviation thereof are within the range, color distortion such as rainbow mura can be prevented.

The film may have a thickness of 30 to 300 μm, 30 to 200 μm, 60 to 200 μm, 60 to 190 μm, 80 to 200 μm, 80 to 190 μm, or 80 to 188 μm. Within this range, it is easy to apply to thin film type and / or miniaturized display devices.

In addition, the film is 0.05 to 0.6% in the longitudinal direction, or 0.05 to 0.3%, 0.05 to 0.6% in the width direction, or 0.05 to 0.6 when the heat shrinkage was measured for 24 hours at 85 ℃ for a sample of 200mm X 200mm It may have a shrinkage 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 such a breaking elongation as above, it may have a smooth cut surface.

In addition, the orientation angle of the polyester film may be within about ± 5 ° based on the width direction. In more detail, the orientation angle (optical axis) of the polyester film may be within about ± 3 ° based on the width direction. In more detail, the orientation angle of the polyester film may be within ± 2 ° based on the width direction. When the orientation angle of the polyester film has a range as described above, the polarizing plate to which the polyester film is applied may have an improved polarization degree and luminance. In particular, the smaller the deviation between the alignment angle and the width direction, the polarization direction of the polarizing plate and the alignment angle of the polyester film is matched with each other, the polarizing plate according to the embodiment may have an improved brightness and polarization degree.

Film according to the embodiment may be prepared by the following method.

First, a polyester resin, for example, a resin that can be used as a raw material for a film 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 directly prepared or commercially available by directly esterifying and polymerizing a diol component such as ethylene glycol and a dicarboxylic acid component such as terephthalic acid. The resin is melt-extruded and then cooled to prepare an unstretched sheet, and the unstretched 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, and in the longitudinal direction, for example 3.0 times. Hereinafter, it can extend | stretch to 1.0 times-3.0 times, 1.2 times-1.7 times, or 1.2 times-1.5 times. Further, in the stretching in the width direction, the stretching speed in the width direction may be 200% / min to 800% / min, 250% / min to 600% / min, or 250% / min to 300% / min have.

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

The stretching temperature may be in the range of Tg + 5 to Tg + 50 ° C., and the lower the stretching temperature, the better the stretchability, but the breakage may occur. In particular, in order to improve the brittleness may be a stretching temperature range of Tg + 10 to Tg + 40 ℃. For example, the extending | stretching temperature to a longitudinal direction is 75-85 degreeC, and the extending | stretching temperature to a width direction is 80-120 degreeC, and it can extend | stretch, raising a temperature step by step within the said range.

Furthermore, the method may further comprise heat setting the stretched film. For example, the heat setting temperature is 180 to 230 ℃, the heat setting time may be 1 to 2 minutes. After starting the heat setting, the film is relaxed in the longitudinal direction and / or the width direction (relaxation rate of 2 to 4%), and then the film can be produced by gradually lowering the temperature 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 luminance and mechanical strength without polarizing mura can be produced. In addition, the film according to the embodiment may include various additives such as conventional electrostatic agent, antistatic agent, antiblocking agent and other inorganic lubricants within a range that does not impair the effects of the present embodiment.

In addition, the polyester film according to the embodiment may be applied to the polarizing plate. The polarizing plate includes a polarizer and a polarizer protective film adjacent to at least one of upper and lower surfaces of the polarizer. Specifically, as shown in Figure 2, the protective film applied to the viewer side may be applied to the polyester film according to this embodiment. In addition, a tri-acetyl cellulose film (TAC film) may be used as a protective film applied to the opposite of the polyester film.

The polarizer is configured to polarize natural light incident on the polarizing plate into light vibrating only in one direction while vibrating in various directions.

The polarizer may be polyvinyl alcohol (PVA) stained with iodine or the like. The polyvinyl alcohol (PVA) molecules included in the polarizer may be aligned in one direction.

The polarizing plate according to the embodiment may be applied to a display device such as a liquid crystal display or an organic light emitting display.

The display device includes a display panel and the polarizer disposed on at least one of upper and lower surfaces of the display panel.

3 is a schematic diagram schematically illustrating a liquid crystal display device as an example of a display panel including a polarizing plate according to an embodiment.

The liquid crystal display device includes a liquid crystal panel 31 and a backlight unit 32.

The backlight unit 32 emits light to the liquid crystal panel 31. The liquid crystal panel 31 displays an image by using light incident from the backlight unit 32.

The liquid crystal panel 31 includes an upper polarizer 311, a color filter substrate 312, a liquid crystal layer 313, a TFT substrate 314, and a lower polarizer 311 ′.

The TFT substrate 314 and the color filter substrate 312 are opposed to each other.

The TFT substrate 314 includes a plurality of electrodes corresponding to each pixel, a thin film transistor connected to the electrode, a plurality of gate wires for applying a driving signal to the thin film transistor, and a data signal to the electrode through the thin film transistor. It may include a plurality of data wires to be applied.

The color filter substrate 312 includes a plurality of color filters corresponding to each pixel. The color filter is configured to implement red, green, and blue by filtering the transmitted light. The color filter substrate may include a common electrode facing the electrode.

The liquid crystal layer 313 is interposed between the TFT substrate and the color filter substrate. The liquid crystal layer may be driven by the TFT substrate. In more detail, the liquid crystal layer may be driven by an electric field formed between the electrode and the common electrode. The liquid crystal layer may adjust the polarization direction of the light passing through the lower polarizer. That is, the TFT substrate may adjust the potential difference applied between the electrode and the common electrode on a pixel basis. Accordingly, the liquid crystal layer may be driven to have different optical characteristics in units of pixels.

The upper polarizer 311 is disposed on the color filter substrate 312. The upper polarizer 311 may be attached to an upper surface of the color filter substrate 312.

The lower polarizer 311 ′ is disposed under the TFT substrate 314. The lower polarizer 311 ′ may be attached to a lower surface of the TFT substrate 314.

Polarization directions of the upper polarizer 311 and the lower polarizer 311 ′ may be the same or perpendicular to each other.

4 is a schematic diagram schematically illustrating an organic electroluminescence display, which is an example of a display panel having a polarizing plate according to an embodiment.

The organic light emitting display device includes a front polarizer 42 and an organic EL panel 41.

The front polarizer 42 may be disposed on the front surface of the organic EL panel 41. In more detail, the front polarizing plate 42 may be attached to the surface on which the image is displayed on the organic EL panel 41. The front polarizer 42 may have substantially the same configuration as the polarizer described above.

The organic EL panel 41 displays an image by self light emission in units of pixels. The organic EL panel 41 includes an organic EL substrate 411 and a driving substrate 412.

The organic EL substrate 411 includes a plurality of organic electroluminescent units respectively corresponding to pixels. The organic electroluminescent units each include a cathode, an electron transport layer, an emission layer, a hole transport layer, and an anode. Detailed description of the configuration of the cathode and the like will be omitted below.

The driving substrate 412 is operatively coupled to the organic EL substrate 411. That is, the driving substrate 412 may be coupled to apply a driving signal such as a driving current to the organic EL substrate 411. In more detail, the driving substrate 412 may drive the organic EL substrate 411 by applying a current to the organic electroluminescent units, respectively.

The polyester film according to the embodiment satisfies the in-plane retardation, tensile strength and modulus all at once in the appropriate ranges described above. That is, the polyester film according to the embodiment has an in-plane retardation of 3,000 nm to 30,000 nm, and the film has a tensile strength in the width direction of 25 to 35 kgf / mm ² and a longitudinal tensile strength of 8.0 to 19 kgf / mm ² . , The ratio of the longitudinal tensile strength to the widthwise tensile strength is 0.25 to 0.7. At the same time the widthwise modulus of the film is 450 to 560 kgf / mm ² , and the longitudinal modulus is 220 to 380 kgf / mm ² . In addition, the polyester film according to the embodiment may satisfy the above in-plane retardation, tensile strength and modulus, and at the same time, may have an orientation angle within about ± 5 ° based on the width direction.

Thus, the polyester film according to the embodiment may have improved mechanical strength and optical performance simultaneously. In order to realize such optical and mechanical properties, the polyester film according to the embodiment, for example, the draw ratio in the longitudinal direction is 1.0 times to 3.0 times, 1.2 times to 1.7 times, or 1.2 times to 1.5 times, in the width direction The draw ratio of may be 4.0 times to 8.0 times, or 4.1 times to 6.0 times. Further, the stretching speed in the width direction may be 200% / min to 800% / min, 250% / min to 600% / min, or 250% / min to 300% / min. In addition, the extending | stretching temperature in a longitudinal direction is 75-85 degreeC, and the extending | stretching temperature in a width direction is 80-120 degreeC, and it can extend | stretch while raising a temperature step by step within the said range.

The heat setting temperature may be 1 to 2 minutes at 180 to 230 ° C., and then the film may be prepared by decreasing the relaxation rate to 2 to 4% and gradually decreasing the temperature to 100 to 150 ° C.

As described above, the optical polyester film according to the embodiment minimizes the difference in the orientation angles of the film center portion and the end portion while generating a difference in tensile strength in the length / width direction of the film, thereby improving mechanical strength and luminance such as modulus. It is possible to minimize the occurrence of polarization mura. Furthermore, since a single layer film is used, the processing process is easier and the thickness is thinner, and thus it may be usefully applied to a miniaturized and 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 only for illustrating the present invention, and the scope of the present invention is not limited thereto.

Example  One

Polyethylene terephthalate resin (100 mol% of ethylene glycol + 100 mol% of terephthalic acid, IV 0.61 dl / g, SKC) was melt extruded through an extruder at about 280 ° C, cooled on a casting roll at about 25 ° C, and unstretched sheet Was prepared. The pre-stretched sheet was preheated to 100 ° C., and then stretched 4.16 times while gradually increasing the temperature from 80 ° C. to 1.20 times in the longitudinal direction and 80 ° C. to 120 ° C. in the width direction. Thereafter, the stretched sheet was heat set at a temperature of about 210 ° C. for about 90 seconds, and the temperature was lowered from about 150 ° C. to about 100 ° C. while relaxing at a relaxation rate of about 3% to obtain a monolayer film having a thickness of 188 μm. Prepared.

Example  2 to 5 and Comparative example  One

A monolayer film was prepared in the same manner as in Example 1, except that the draw ratios in the longitudinal and width directions and the thickness of the final film were changed as shown in Table 2 below.

Comparative example  2

Commercially available uniaxially stretched PET film (SRF, Toyobo Co., Ltd.) was used.

< Test Example >

(One) Modulus  evaluation

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

(2) tensile strength evaluation

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

(3) optical axis evaluation

The orientation angle of a film (sample film) was measured using the orientation angle measuring system as shown in FIG. 1, and the deviation (optical axis) of the angle between the measured orientation angle and the width direction was calculated | required. The results are shown in Table 1 below.

(4) in-plane Phase difference  evaluation

The biaxial orthogonal refractive index (nx, ny) and the refractive index (nz) in the thickness direction were measured using an Abbe refractometer (NAR-4T, measurement wavelength 589 nm, Atago), and the thickness d (nm) of the film was measured using an electric micrometer. The measurement was performed using a meter (Millitron 1245D, Pineryu Corp.) and the units were converted to nm. The difference of each of the measured biaxial indices of refraction is calculated as an absolute value (| nx-ny |), and the in-plane retardation (Re) is obtained by multiplying the thickness d (nm) of the film (nxy × d). Indicated.

(5) Polarized light  Occurrence

On one side of the polarizer made of PVA and iodine, the film (sample film) obtained in the above Example or Comparative Example is affixed so that the absorption axis of the polarizer and the orientation major axis of the film are perpendicular, and on the opposite side thereof, the TAC film (80) Micrometer, FUJIFILM Corporation) were affixed, and the polarizing plate was produced. The polarizing plate is provided on the exiting light side of a liquid crystal display device using a white LED composed of a light emitting element combining a blue light emitting diode and a yttrium aluminum garnet-based yellow phosphor as a light source (NSPW500CS, Nichia Chemical). Located on the side. The liquid crystal display device had a polarizing plate having two TAC films as polarizer protective films on the incident light side of the liquid crystal cell (see FIG. 2). Observation was made with the naked eye in the front and oblique directions of the polarizing plate of the liquid crystal display, and the presence or absence of polarization mura was observed.

(6) cutting surface state

After cutting the film of the Example and the comparative example with the cutter of a square frame form, the state of the cut surface was visually confirmed with an optical microscope. When a residue such as a burr was observed on the cut surface, it was judged to be defective, and if not observed, to be good.

From the results in Table 1, the films prepared in Examples 1 to 5 were all excellent in tensile strength and modulus, and the orientation angle deviation (optical axis) was 2.8 deg. It can be seen that the polarization mura does not occur as low as below and in-plane retardation is higher than 7,000 nm. On the other hand, while the film of Comparative Example 1 was excellent in mechanical strength, the orientation angle deviation was large, and the in-plane retardation was low, the polarization mura was strong, and the film of Comparative Example 2 was poor in the cut surface.

Therefore, it can be seen that the film produced in the example not only did not generate polarization mura but also greatly improved luminance and mechanical strength.

Claims (10)

As a biaxially stretched polyester film,
In-plane retardation is 3,000nm to 30,000nm,
The draw ratio in the width direction TD is 4.0 to 8.0 times, the draw ratio in the longitudinal direction MD is 1.2 times to 1.7 times,
The film has a transverse tensile strength of 25 to 35 kgf / mm ² and a longitudinal tensile strength of 8.0 to 19 kgf / mm ² , and the ratio of the longitudinal tensile strength to the transverse tensile strength is 0.25 to 0.7 ,
Width direction modulus of the film is 450 to 560 kgf / mm ² , longitudinal modulus is 220 to 380 kgf / mm ² ,
The film has an elongation at break of 7% to 20% in the longitudinal direction and 75% to 100% in the width direction,
The polyester film for optics which has a shrinkage rate of 0.05 to 0.6% in the longitudinal direction (MD) and 0.05 to 0.6% in the width direction (TD) on the conditions of 24 hours and 85 degreeC. delete The method of claim 1,
The film was ± 2.8 deg. The polyester film for optics which has the following orientation angle deviations. The method of claim 1,
The polyester film for optics whose in-plane phase difference of the said film is 7,000-30,000 nm. The method of claim 1,
The polyester film for optics whose said phase difference is 500 nm / m or less. delete delete Melt extruding the polyester resin to prepare an unstretched sheet;
Stretching the unstretched sheet in a width direction and a length direction, respectively; And
And heat-setting the stretched sheet to produce a polyester film.
Stretching the unstretched sheet 4.0 times to 8.0 times in the width direction, stretching 1.2 times to 1.7 times in the longitudinal direction,
The heat setting is performed for 1 to 2 minutes at 180 to 230 ℃,
The stretched sheet is relaxed 2% to 4% in the longitudinal direction, the width direction, or both after starting the heat setting,
The heat setting further comprises the step of lowering the temperature step by step to 100 to 150 ℃, the method for producing an optical polyester film of claim 1. The method of claim 8,
The extending | stretching temperature to the said width direction is 80-120 degreeC,
The drawing temperature in the said longitudinal direction is 75-85 degreeC, The manufacturing method of the optical polyester film. The method of claim 8,
The manufacturing method of the optical polyester film whose extending | stretching speed to the said width direction is 200% / min-800% / min.

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