A high-extinction-ratio linearly polarizing film and a method for preparing the same

Abstract

This application relates to the technical field of optical thin films, specifically disclosing a high extinction ratio linear polarizing film and its preparation method. The film comprises a multilayer stacked structure with alternating high-refractive-index optical layers and low-refractive-index optical layers. The high-refractive-index optical layer is made of polyethylene terephthalate modified with 4,4'-biphenyl dicarboxylic acid monomer, wherein the molar content of BB monomer is 15%-35%. At a wavelength of 550 nm, the three-dimensional refractive index of the high-refractive-index optical layer satisfies: n y With n z The difference Δn yz ≤0.01, where n y n is the longitudinal refractive index. z Here, n represents the refractive index in the thickness direction. This application involves longitudinal stretching, followed by transverse stretching and simultaneous longitudinal shrinkage, thereby increasing the n of the high refractive index layer. y and n z Adjusting the values ​​to be very close to match the truly isotropic low refractive index layer effectively eliminates polarization chromatic aberration and rainbow patterns caused by refractive index mismatch.

Description

Technical Field

[0001] This application relates to the technical field of optical thin films, and in particular to a high extinction ratio linear polarizing film and its preparation method. Background Technology

[0002] Linear polarizing films are core optical components in liquid crystal displays (LCDs) and organic light-emitting diode (OLED) display modules, with their extinction ratio being a key performance indicator. A high extinction ratio requires the polarizing film to have extremely high transmittance along the transmission axis and extremely low transmittance along the blocking axis. To achieve this, multilayer film technology is widely used, which involves alternately stacking high-refractive-index and low-refractive-index layers, utilizing the interference effect of light at the interface to enhance polarization performance.

[0003] Traditional high-performance linear polarizing films often use polyethylene naphthalate (PEN) or copolyesters with high naphthalene content as the high refractive index layer material due to their high intrinsic birefringence. However, PEN materials are expensive, and the refractive index difference (Δn) between PEN and common low refractive index materials (such as PETG, COC, etc.) in the thickness direction (z-axis) is significant. z The refractive index mismatch is often quite large. This mismatch can lead to polarization problems such as rainbow patterns and color shifts when viewed at large angles, severely affecting the uniformity and clarity of the displayed image.

[0004] To reduce costs and improve performance, some researchers have proposed copolymerizing 4,4'-biphenyl phthalic acid (BB) monomers with polyethylene terephthalate (PET) to prepare BBPET materials as high-refractive-index layers. The introduction of BB enhances the in-plane birefringence potential of PET. However, traditional uniaxial stretching processes highly oriented molecular chains in the stretching direction (usually defined as the y-axis), although this can increase the longitudinal refractive index n... y In-plane birefringence Δn xy（ =n x -n y However, this also leads to an increase in the refractive index n perpendicular to the film surface direction (z-axis). z A sharp drop. This results in a refractive index difference Δn between the y-axis and z-axis directions of the high-refractive-index layer. yz (=|n) y -n z |) is too large, when it is combined with an isotropic or weakly anisotropic low refractive index layer (n y '≈n z When stacked, significant optical anisotropy mismatch is introduced in the yz plane, becoming the root cause of visual angular aberration and reduced extinction ratio.

[0005] Therefore, a new preparation method should be developed to effectively improve the in-plane birefringence Δn of BBPET materials. xyAt the same time, it actively suppresses and reduces the refractive index difference Δn between its thickness direction and the in-plane non-stretch direction. yz This allows its refractive index distribution to better match that of the low refractive index layer, which is key to obtaining a high extinction ratio and low chromatic aberration linear polarization film. Summary of the Invention

[0006] This application provides a high extinction ratio linear polarizing film and its preparation method, aiming to solve the problem of the current use of high refractive index copolyester materials such as BBPET, which, after uniaxial stretching, results in a low refractive index difference (Δn) between the y-axis and z-axis. yz Excessive polarization leads to poor matching with the low refractive index layer, resulting in significant polarization color difference in the linear polarization film and difficulty in further improving the extinction ratio.

[0007] In a first aspect, this application provides a high extinction ratio linear polarizing film, employing the following technical solution: A high extinction ratio linear polarizing film comprises a multilayer stacked structure of alternating high-refractive-index optical layers and low-refractive-index optical layers. The high-refractive-index optical layers are made of polyethylene terephthalate modified with 4,4'-biphenyl diphthalate monomer, wherein the molar content of BB monomer is 15%-35%. At a wavelength of 550 nm, the three-dimensional refractive index of the high-refractive-index optical layer satisfies: n y With n z The difference Δn yz ≤0.01, where n y n is the longitudinal refractive index. z denoted as the refractive index in the thickness direction.

[0008] By adopting the above technical solution, the high refractive index layer is limited to BBPET with a BB monomer molar content of 15%-35%, and the refractive index difference Δn between its longitudinal and thickness directions is limited by the three-dimensional refractive index relationship. yz ≤0.01, this solution fundamentally solves the problem of severe refractive index imbalance in BBPET along the y and z axes caused by traditional uniaxial stretching processes. It significantly reduces the optical anisotropy of the high-refractive-index layer in the non-polarizing functional direction, thereby achieving precise refractive index matching with the low-refractive-index layer and completely eliminating the problem caused by Δn. yz Excessive polarization causes polarization axis shift, rainbow patterns, and large visual aberration, laying a decisive foundation for achieving ultra-high extinction ratio.

[0009] Optionally, the lateral refractive index n of the high refractive index optical layer x The in-plane birefringence Δn is 1.66-1.83. xy ≥0.15.

[0010] Optional, Δn yz ≤0.006.

[0011] Optionally, the material of the low-refractive-index optical layer is selected from PETG, MS resin, and COC, having a refractive index of 1.53-1.57 at a wavelength of 550 nm, and its n y 'and n z The difference is ≤0.005.

[0012] Optionally, the total number of optical layers of the linear polarizing film is 40-100 layers, and the extinction ratio is ≥2000:1.

[0013] Secondly, this application provides a method for preparing a high extinction ratio linear polarization film, employing the following technical solution: A method for preparing a high extinction ratio linear polarization film includes the following steps: S1. Raw material casting: BBPET resin and low refractive index resin are melted separately, and cast into a cast sheet by multi-layer co-extrusion and quenching. S2. Longitudinal stretching: After preheating the cast sheet shown, longitudinal stretching is performed along the extrusion direction; S3. Synergistic control of lateral stretching and longitudinal contraction: The membrane after stretching in step S2 is stretched laterally, and during this process, the membrane is controlled to undergo 10%-30% active contraction in the longitudinal direction. S4. Heat setting and post-treatment.

[0014] By employing the above technical solution, a two-step preparation method comprising longitudinal stretching-transverse stretching with longitudinal shrinkage synergistic regulation is provided, deconstructing and recombining traditional uniaxial and biaxial stretching processes: the first step, longitudinal stretching, establishes an appropriate longitudinal orientation of molecular chains; the second step, through the synergistic effect of transverse stretching and active longitudinal shrinkage, while continuing to enhance transverse orientation, actively releases excessive longitudinal orientation and inhibits the orderly stacking of molecular chains in the thickness direction, achieving n x Significantly improved, n y Moderate decline, n z The target for a significant rebound in refractive index control is to make Δn yz The fundamental means to reduce it to below 0.01.

[0015] Optionally, in step S2, the longitudinal stretching ratio is 1-3 times, and the stretching temperature is the T value of the BBPET resin. g To T g +15℃.

[0016] Optionally, in step S3, the transverse stretching ratio is 3-6 times, and the process temperature is the T value of the BBPET resin. g +5℃ to T g +20℃.

[0017] Optionally, in step S3, the lateral stretching and longitudinal contraction are synchronized by controlling the track shape and movement speed of the tenter frame clamp.

[0018] Thirdly, this application provides an application of a high extinction ratio linear polarizing film, employing the following technical solution: A display device comprising the above-described high extinction ratio linear polarizing film.

[0019] In summary, this application includes at least one of the following beneficial technical effects: 1. By employing a process of longitudinal stretching followed by transverse stretching and simultaneous longitudinal shrinkage, the excessive orientation along the y-axis was controllably mitigated, and the ordered arrangement of molecular chains along the z-axis was suppressed, thereby enhancing the n-axis orientation of the high-refractive-index layer. y and n z By adjusting the values ​​to be very close, the high refractive index layer is nearly isotropic in the yz plane, achieving a good optical match with the isotropic low refractive index layer, eliminating polarization chromatic aberration and rainbow patterns caused by refractive index mismatch. 2. Due to the excellent refractive index matching, light leakage and mode coupling when light passes through the film interface under non-ideal viewing angles are greatly suppressed. Experiments show that the extinction ratio of the linear polarization film prepared in this application can stably reach more than 2000:1, and the preferred scheme can reach 3500:1 to 5000:1, which is an order of magnitude improvement over the extinction ratio of traditional uniaxially stretched BBPET / PETG laminated film. 3. Because the process greatly improves the in-plane birefringence of the high refractive index layer, the optical efficiency per unit thickness is enhanced. Therefore, under the premise of achieving the same or better polarization performance, the total number of optical layers required can be greatly reduced, which significantly reduces raw material costs, processing complexity and overall film thickness, and enhances product competitiveness. Attached Figure Description

[0020] Figure 1 This is a schematic diagram illustrating the process principle of transverse stretching and simultaneous longitudinal shrinkage in the preparation method of this application. Detailed Implementation

[0021] Example

[0022] This application discloses a high extinction ratio linear polarizing film.

[0023] Example 1

[0024] A high extinction ratio linear polarizing film is prepared by the following steps: BBPET resin with a BB molar content of 15% was prepared. The dried BBPET resin and PETG resin (refractive index 1.567) were fed into two separate extruders and co-extruded through a multi-layer distributor to form a melt with PETG as the outer protective layers and a 100-layer alternating internal structure. This 100-layer alternating structure comprised 50 cycles of BBPET and PETG, with a BBPET:PETG thickness ratio of 1.08:1 within each cycle. The initial cycle had a thickness of 2 μm, and the final cycle had a thickness of 5.2 μm. The melt was then extruded through a T-die onto a 25°C chill roll to form a uniform cast sheet with a thickness of approximately 200 μm.

[0025] The cast film was preheated at 92°C for 30 seconds, then longitudinally stretched at 95°C with a stretch ratio of 1.2. Immediately afterwards, the film was fed into a tenter frame. The tenter frame preheating section temperature was set to 98°C, and the stretching section temperature to 100°C. (Refer to...) Figure 1 The clamps are controlled to open at a certain rate in the transverse direction (x-direction) to achieve a transverse stretching ratio of 5 times. The clamps hold the diaphragm on both sides in the longitudinal direction. While stretching in the transverse direction, the diaphragm is actively contracted in the longitudinal direction (y-direction) by adjusting the clamp track and clamping force, with the longitudinal contraction rate controlled at 20%. This process is completed simultaneously.

[0026] Finally, the film was heat-set at 210°C for 15 seconds and then cooled to obtain a high extinction ratio linear polarizing film.

[0027] Optical performance testing: High refractive index layer (15% BBPET) refractive index n x =1.719, n y =1.565, n z =1.548, Δn xy =0.154, Δn yz =0.017, the extinction ratio of the polarizing film is 2500:1.

[0028] Example 2

[0029] BBPET resin with a BB molar content of 35% was prepared. The process was the same as in Example 1, with the following adjustments to the process parameters: longitudinal stretching ratio of 1.4 times (temperature 98℃); lateral stretching and longitudinal shrinkage synergistic control stage: lateral stretching ratio of 5.0 times, longitudinal shrinkage rate of 30%, and process temperature of 102℃.

[0030] Optical performance testing: High refractive index layer (35% BBPET) refractive index n x =1.809, n y =1.568, n z =1.562, Δn xy =0.241, Δn yz=0.006. The total number of optical layers of the polarizing film is reduced to 80 layers, the thickness of the optical region is about 15μm, and the extinction ratio is as high as 4800:1.

[0031] Comparative Example

[0032] Comparative Example 1 Using the same 15% BBPET and PETG raw materials as in Example 1, and employing a conventional unidirectional stretching process, the cast sheets were preheated and stretched only in the transverse direction (stretching ratio 3.5 times, temperature 95°C), without active longitudinal shrinkage control. The resulting polarized film was obtained after heat setting.

[0033] Optical performance testing: Refractive index n of the high refractive index layer x =1.680, n y =1.570, n z =1.525, Δn xy =0.110, Δn yz =0.045. Because Δn yz If the polarization is too large, it will not match the PETG layer well, resulting in a significant apparent color difference in the polarized film, with an extinction ratio of only 950:1.

[0034] Comparative Example 2 Using the same 35% BBPET and PETG raw materials as in Example 2, simultaneous biaxial stretching was attempted (i.e., stretching laterally while also slightly stretching longitudinally, rather than shrinking). Process conditions: 3.5 times stretching laterally, 1.1 times simultaneous stretching longitudinally, and a temperature of 102°C.

[0035] Optical performance testing: Refractive index n of the high refractive index layer x =1.788, n y =1.625, n z =1.528, Δn xy =0.163, Δn yz =0.097. n y Further increase, n z Still very low, Δn yz It is actually larger than that of a single pull, resulting in a deterioration in optical matching.

[0036] As can be seen from Examples 1-2 and Comparative Examples 1-2, the longitudinal stretching, transverse stretching, and longitudinal shrinkage synergistic control process of this application significantly improves the in-plane birefringence Δn. xy At the same time, Δn yz By controlling the level to an extremely low level, a linear polarizing film with an ultra-high extinction ratio was ultimately obtained.

[0037] All equivalent changes made to the structure, shape, and principle of this application shall be covered within the scope of protection of this application.

Claims

1. A high extinction ratio linear polarizing film, comprising a multilayer stacked structure of alternating high-refractive-index optical layers and low-refractive-index optical layers, characterized in that: The high-refractive-index optical layer is made of polyethylene terephthalate modified with 4,4'-biphenyl diphthalate monomer, wherein the molar content of BB monomer is 15%-35%. At a wavelength of 550 nm, the three-dimensional refractive index of the high-refractive-index optical layer satisfies: n y With n z The difference Δn yz ≤0.01, where n y n is the longitudinal refractive index. z denoted as the refractive index in the thickness direction.

2. The high extinction ratio linear polarizing film according to claim 1, characterized in that: The transverse refractive index n of the high refractive index optical layer x The in-plane birefringence Δn is 1.66-1.

83. xy ≥ 0.

15.

3. The high extinction ratio linear polarizing film according to claim 2, characterized in that: Δn yz ≤0.

006.

4. The high extinction ratio linear polarizing film according to claim 1, characterized in that: The low-refractive-index optical layer is made of a material selected from PETG, MS resin, and COC, with a refractive index of 1.53-1.57 at a wavelength of 550 nm, and its n y 'and n z The difference is ≤0.

005.

5. A high extinction ratio linear polarizing film according to claim 1, characterized in that: The linear polarizing film has a total of 40-100 optical layers and an extinction ratio ≥2000:

1.

6. A method for preparing a high extinction ratio linear polarizing film according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Raw material casting: BBPET resin and low refractive index resin are melted separately, and cast into a cast sheet by multi-layer co-extrusion and quenching. S2. Longitudinal stretching: After preheating the cast sheet shown, longitudinal stretching is performed along the extrusion direction; S3. Synergistic control of lateral stretching and longitudinal contraction: The membrane after stretching in step S2 is stretched laterally, and during this process, the membrane is controlled to undergo 10%-30% active contraction in the longitudinal direction. S4. Heat setting and post-treatment.

7. The method for preparing a high extinction ratio linear polarizing film according to claim 6, characterized in that: In step S2, the longitudinal stretching ratio is 1.05-3 times, and the stretching temperature is the T value of the BBPET resin. g To T g +15℃.

8. The method for preparing a high extinction ratio linear polarizing film according to claim 6, characterized in that: In step S3, the transverse stretching ratio is 3-6 times, and the process temperature is the T value of the BBPET resin. g +5℃ to T g +20℃.

9. The method for preparing a high extinction ratio linear polarizing film according to claim 6, characterized in that: In step S3, the lateral stretching and longitudinal contraction are synchronized by controlling the track shape and movement speed of the tenter frame clamp.

10. A display device, characterized in that, It includes a high extinction ratio linear polarizing film as described in any one of claims 1-5.

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