An optical polyester film and its preparation method

Abstract

This invention relates to the field of optical polyester film technology, specifically comprising the following steps: Step 1: using polyester chips and an opening agent as layer A; using polyester chips as layer B; melt-co-extruded layer A and layer B at 260℃~295℃, followed by rapid cooling to obtain a multilayer polyester sheet; Step 2: sequentially biaxially stretching the multilayer polyester sheet and heat-setting it to obtain an optical polyester film. The optical polyester film prepared in this application has better orientation and phase difference compared to traditional polyester films, and can be well applied in LCD and OLED display fields.

Description

Technical Field

[0001] This invention relates to the field of optical polyester film technology, specifically to an optical polyester film and its preparation method. Background Technology

[0002] LCD-TFT / OLED is a new display technology that has experienced explosive growth in recent years. Optical films, as a component of liquid crystal display technology, have also evolved rapidly alongside this advancement. Recently, with the trend towards larger TV sizes, LCD displays have placed higher demands on polarizers, the core material of the screen: such as higher mechanical properties and lower moisture absorption. TAC film, a core material, is made from wood pulp or cotton products. It easily absorbs moisture, affecting the alignment of PVA dyes in the polarizer, leading to light leakage, egg mura, and other defects. Furthermore, TAC film undergoes significant dimensional changes in humid and hot environments, causing butterfly patterns or panel bending. Therefore, there is an urgent need to develop PVA protective materials that can replace TAC film. Currently, COP film and PMMA film have partially replaced TAC on the market. However, COP is more expensive than TAC, and PMMA is brittle and prone to breakage during both raw material production and polarizer lamination processes.

[0003] On the other hand, in existing technologies, PET film is mainly used in the backlight modules of liquid crystal displays as a low-value-added product, such as prism film, brightness enhancement film, polarizer release film, and protective film. Using polyester, such as PET film, as a partial alternative to TAC is becoming a new research direction.

[0004] However, while polyester materials such as PET film possess advantages such as high mechanical strength, high transparency, good dimensional stability, and low water permeability, they have always exhibited a rainbow effect when used as protective film materials for PVA polarizers due to birefringence. This rainbow effect can negatively impact the visual quality of LCD TVs, causing color deviations. Similarly, when used as fingerprint unlocking screen protectors for OLED phones, the optical axis angle and rainbow effect of the PET film can distort the fingerprint image, thus affecting the phone's unlocking function.

[0005] In conclusion, the preparation of an optical polyester film is of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide an optical polyester film and its preparation method to solve the problems mentioned in the background art.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A method for preparing an optical polyester film includes the following steps:

[0009] Step 1: Use polyester chips and opening agent as layer A; use polyester chips as layer B; melt co-extrude layer A and layer B at 265℃~295℃, and rapidly cool to obtain multilayer polyester sheet;

[0010] Step 2: The multilayer polyester sheet is biaxially stretched and heat-set sequentially to obtain an optical polyester film.

[0011] Ideally, the structure of the multilayer polyester sheet is an A / B / A structure.

[0012] In this process, the extrusion structure can be an A / B / A structure, where the B layer is the core layer, mainly composed of bright polyester chips, such as Sinopec FG600 / FG604, etc. An opening agent is added to the A layer material. The main function of the opening agent is to form small protrusions on the surface of the film to prevent the formation of a vacuum between the film layers, which would cause uneven winding. The extrusion structure can also be an A / B / C structure. For the corresponding A / B / C structure, three extruders are selected. The A / C layers can be the same or different, so opening agents with different compositions or particle sizes can be added to the A / C layers.

[0013] In a more optimized manner, the biaxial stretching process is as follows: the multilayer polyester sheet is longitudinally stretched at a temperature of 70℃~130℃ with a longitudinal stretching ratio of 1~3.3 times, and transversely stretched at a temperature of 85℃~150℃ with a transverse stretching ratio of 3.8~6 times; the ratio of the transverse stretching ratio to the longitudinal stretching ratio is 1.2~6.

[0014] In the scheme, the larger the ratio of transverse stretching ratio to longitudinal stretching ratio (TD / MD), the easier it is to form a smaller optical axis angle and a larger phase difference (R0). However, when the stretching ratio TD / MD increases, the film-forming properties of the optical polyester film deteriorate: the thickness uniformity deteriorates and stretching spots occur. Studies have found that by improving the stretchability of polyester chips and reducing the stretching stress, a larger TD / MD stretching ratio can be obtained.

[0015] In a more optimized manner, the optical polyester film has an angle between its optical axis and the transverse stretching direction of 0°~6° or 84°~90°; and the phase difference (R0) of the optical polyester film is >8000nm.

[0016] Among them, the traditional biaxially stretched polyester film has the following problems: (1) Its optical axis angle range is very large, ranging from 0° to 50°; while the OLED screen has a circular polarizer with an optical axis angle of 45°. Because the molecular chain of polyester film such as BOPET has an optical rotation effect, it will destroy the polarization state; (2) When the optical axis angle of the polyester film and the optical axis angle of the polarizer cannot be consistent, the original light path is destroyed by the BOPET film. Furthermore, the rainbow pattern generated on the surface of the polyester film causes the fingerprint chip to be unable to collect the real fingerprint image, resulting in poor fingerprint unlocking sensitivity or no unlocking at all; (3) When the BOPET film is applied to the polarizer of the liquid crystal display, because the BOPET film has a serious birefringence problem after biaxial stretching, the transmittance of light of different wavelengths is inconsistent, thus forming a rainbow halo.

[0017] Therefore, in this solution, by controlling the biaxial stretching process, when the angle between the optical axis of the PET film and the TD direction is between 0° and 6° or between 84° and 90°, and the phase difference of the optical polyester film reaches more than 8000nm, the problems generated in traditional biaxially stretched polyester films can be effectively suppressed.

[0018] In a more optimized manner, the heat setting process is as follows: first, the material is set at a temperature of 180℃~250℃, and then cooled at 180℃.

[0019] In a more optimized manner, the polyester chips are pre-dried before use. The drying process is as follows: the polyester chips are dried at 100℃~180℃ for 2~6 hours, and the moisture content is controlled to be <200ppm.

[0020] In the proposed solution, the preferred drying temperature is 150℃~170℃, the preferred moisture content of the polyester chips is <100ppm, and the optimal moisture content of the polyester chips is <50ppm.

[0021] More preferably, the polyester chips include one or both of polyethylene terephthalate and polyethylene 2,6-naphthalate.

[0022] In this scheme, the intrinsic viscosity of the polyester chips is typically in the range of 0.5 dl / g to 0.8 dl / g, preferably in the range of 0.55 to 0.75 dl / g, and more preferably in the range of 0.6 to 0.645 dl / g.

[0023] In order to remove foreign matter from the polyester chip raw material and prevent defects in the optical polyester film, it is preferable to use high-efficiency filtration during melt extrusion. The filter equipment has a pore size of less than 25 μm, and more preferably 15 μm.

[0024] More preferably, the opening agent includes one or more of silicon dioxide, barium sulfate, and calcium carbonate; the proportion of the opening agent added to the A layer material is 300ppm to 1500ppm.

[0025] In this scheme, the particle size of the opening agent is typically ≤5μm, preferably 0.2μm~3μm.

[0026] In this design, the particle size and content of the opening agent affect the anti-sticking performance and appearance of the optical polyester film. Larger particle sizes result in dot-like defects, while smaller particle sizes degrade anti-sticking performance. Furthermore, excessively high opening agent content reduces overall transmittance. The opening agent can be distributed along the entire thickness direction or only on the surface layer of the film. To improve film transparency, it is preferable to add the particles to the surface layer. When an A / B / A structure extruder is selected, the opening agent is added to both surface layers. When an A / B / C structure extruder is selected, the opening particles are added to any one or both of layers A and C. When the opening agent is added to only one surface layer, while ensuring opening performance, it also reduces overall haze and roughness on the other side, meeting the requirements for low haze and low roughness in optical films.

[0027] In addition, UV absorbers can be added to the polyester chips during the preparation of optical polyester films to improve the UV resistance of the optical polyester films. The UV absorbers can be benzophenone-based UV absorbers, salicylic acid-based UV absorbers, benzotriazole-based UV absorbers, and other UV absorbers.

[0028] Compared with the prior art, the advantages of this application are as follows:

[0029] (1) The optical polyester film prepared in this application has higher orientation degree and phase difference than traditional polyester film, and can be well applied in the fields of LCD and OLED display.

[0030] (2) In order to solve the rainbow pattern problem of biaxially stretched polyester film (such as PET film), the solution uses specific intrinsic viscosity and limited biaxial stretching process to effectively control the angle between the optical axis of the optical polyester film and the TD direction to be 0°~6° or 84°~90°, while making the phase difference (R0) of the optical polyester film >8000nm, so as to effectively suppress the optical axis angle and rainbow pattern problem of polyester film. Detailed Implementation

[0031] The following are preferred embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. For those skilled in the art, all other embodiments obtained by those skilled in the art without creative effort without departing from the principles of the embodiments of the present invention are within the scope of protection of the present invention.

[0032] Example 1

[0033] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0034] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) 3.3 times at 83℃, and then stretched transversely (TD) 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0035] Example 2 is based on Example 1, and adopts an A / B / C structure;

[0036] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the chips evenly with silica to obtain layer A; the polyester chips are layer B. Use three extruders to melt co-extrude layer A, layer B, and layer C (only layer A has the same amount of silica opening agent added, and layers C and B are pure PET polyester) at 280℃, and rapidly cool at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets; wherein, the polyester chips are polyethylene terephthalate, and the silica content added to layer A is 500ppm;

[0037] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) 3.3 times at 83℃, and then stretched transversely (TD) 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0038] Example 3 is based on Example 1, with a longitudinal stretching ratio of 2.8 times;

[0039] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0040] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 2.8 times at 83℃, and then stretched transversely (TD) by 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0041] Example 4 is based on Example 1, with a longitudinal stretching ratio of 2.5 times;

[0042] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0043] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 2.5 times at 83℃, and then stretched transversely (TD) by 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0044] Example 5 is based on Example 4, with a lateral stretching ratio of 4.6 times;

[0045] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0046] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 2.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0047] Example 6 is based on Example 5, with a longitudinal stretching ratio of 2 times and an optical polyester film thickness of 50 μm;

[0048] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0049] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) twice at 83℃, and then stretched transversely (TD) 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 50μm is obtained.

[0050] Example 7 is based on Example 6, with a longitudinal stretching ratio of 1.5 times;

[0051] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0052] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 50μm is obtained.

[0053] Example 8 is based on Example 7, with the thickness of the optical polyester film being 100 μm;

[0054] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0055] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0056] Example 9 is based on Example 8, with a longitudinal stretching ratio of 1.3 times, and adopts a three-layer structure of A / B / C, with the opening agent added only to layer A;

[0057] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the chips evenly with silica to obtain layer A; the polyester chips are layer B. Use three extruders to melt co-extrude layer A, layer B, and layer C (only layer A has the same amount of silica opening agent added, and layers C and B are pure PET polyester) at 280℃, and rapidly cool at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets; wherein, the polyester chips are polyethylene terephthalate, and the silica content added to layer A is 500ppm;

[0058] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.3 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0059] Example 10 is based on Example 9, using an A / B / A structure, with an opening agent content of 300ppm in layer A, followed by longitudinal stretching and single-sided coating with a primer material;

[0060] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A; the polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃, and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets; wherein the polyester chips are polyethylene terephthalate.

[0061] Step 2: The multilayer polyester sheet is longitudinally stretched (MD) 1.3 times at 83℃. A base coating material is applied to one side of the sheet using a gravure roller or wire rod. The function of the base coating material is mainly to improve the transparency of the film. Then, it is transversely stretched (TD) 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0062] The primer material consists of the following components by weight: 4% acrylic, polyurethane, or polyester resin, 0.8% epoxy curing agent, and 1% sulfonate.

[0063] Example 11 is based on Example 10, after longitudinal stretching, a primer material is coated on both sides of the film;

[0064] Step 1: Dry polyester chips with an intrinsic viscosity of 0.675 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate. The opening agent content in layer A is 300ppm.

[0065] Step 2: The multilayer polyester sheet is longitudinally stretched (MD) 1.3 times at 83℃. A base coating material is then applied to both sides of the sheet using a gravure roller or wire rod. The function of the base coating material is mainly to improve the transparency of the film. Then, it is transversely stretched (TD) 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0066] The primer material consists of the following components by weight: 4% acrylic, polyurethane, or polyester resin, 0.8% epoxy curing agent, and 1% sulfonate.

[0067] Example 12 is based on Example 1, and the intrinsic viscosity of the polyester chips is 0.645 dl / g;

[0068] Step 1: Dry polyester chips with an intrinsic viscosity of 0.645 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0069] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) 3.3 times at 83℃, and then stretched transversely (TD) 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0070] Example 13 is based on Example 8, and the intrinsic viscosity of the polyester chips is 0.645 dl / g;

[0071] Step 1: Dry polyester chips with an intrinsic viscosity of 0.645 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize them until the moisture content is ≤25ppm. Mix them uniformly with silica to obtain layer A. The polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃ and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0072] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0073] Example 14 is based on Example 12, and the intrinsic viscosity of the polyester chips is 0.6 dl / g;

[0074] Step 1: Dry polyester chips with an intrinsic viscosity of 0.6 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the dried chips with silica to obtain layer A; the polyester chips are layer B. Melt co-extrude layer A and layer B at 280℃, and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0075] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) 3.3 times at 83℃, and then stretched transversely (TD) 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0076] Example 15 is based on Example 13, with the intrinsic viscosity of the polyester chips being 0.6 dl / g;

[0077] Step 1: Dry polyester chips with an intrinsic viscosity of 0.6 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the dried chips with silica to obtain layer A; the polyester chips are layer B. Melt co-extrude layer A and layer B at 280℃, and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0078] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0079] Example 16 is based on Example 14: the intrinsic viscosity of the polyester chips is 0.55 dl / g;

[0080] Step 1: Dry polyester chips with an intrinsic viscosity of 0.55 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the dried chips with silica to obtain layer A; the polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃, and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0081] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) 3.3 times at 83℃, and then stretched transversely (TD) 3.8 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0082] Example 17 is based on Example 15: the intrinsic viscosity of the polyester chips is 0.55 dl / g;

[0083] Step 1: Dry polyester chips with an intrinsic viscosity of 0.55 dl / g at 160℃ for 3 hours to remove moisture and pre-crystallize, drying until the moisture content is ≤25ppm. Mix the dried chips with silica to obtain layer A; the polyester chips are layer B. Melt and co-extrude layer A and layer B at 280℃, and rapidly cool them at 25℃ to below the glass transition temperature Tg to obtain multilayer polyester sheets. The polyester chips are polyethylene terephthalate, and the silica content in layer A is 500ppm.

[0084] Step 2: The multilayer polyester sheet is stretched longitudinally (MD) by 1.5 times at 83℃, and then stretched transversely (TD) by 4.6 times at 115℃. After being shaped at 230℃ and then cooled at 180℃, an optical polyester film with a thickness of 100μm is obtained.

[0085] Testing Experiment: Evaluation Indicators and Methods:

[0086] (1) Phase difference: R0 is calculated as R0=(nx-ny)×d;

[0087] R0 represents the phase difference of the optical polyester film. The phase difference R0 is measured by taking samples every 100 mm along the direction perpendicular to the width using the American Axoscan phase difference measuring instrument, and bringing the refractive index and sample thickness into the measuring instrument.

[0088] Wherein, nx is the refractive index in the direction of maximum refractive index within the film plane, ny represents the refractive index perpendicular to the nx direction, and d represents the film thickness; the refractive indices nx and ny in the plane of the optical polyester film were measured at a wavelength of 589 nm using an Abbe refractometer (manufactured by ATAGOCO.,LTD, NAR-4T).

[0089] (2) Orientation angle: Using the KOBRA-WPR optical property measuring machine of Prince Metrology, samples are taken every 100 mm along the direction perpendicular to the width and placed on the measuring stage of the instrument to measure the optical axis angle of the sample; the angle between the optical axis angle and the longitudinal tension (TD) direction is 0°~6° or 84°~90°.

[0090] (3) Transmittance: The transmittance of the optical polyester film was measured at 100 mm intervals along the direction perpendicular to the width using an NDH 4000 Haze meter.

[0091] (4) Haze: Using an NDH 4000 Haze meter, measure haze every 100 mm along the direction perpendicular to the width of the instrument.

[0092] (5) Thickness: The thickness was measured every 100 mm along the direction perpendicular to the width of the optical polyester film using a Mitutoyo thickness gauge;

[0093] (6) Evaluation method of rainbow pattern: Cut the test optical polyester film into A4 or other size and shape samples along the MD and TD (MD: longitudinal stretching, TD: transverse stretching) directions. Place the sample on the backlight, place a polarizer on the backlight, place the sample on the polarizer, and then place another polarizer on the sample. The absorption axes of the two polarizers should be kept at 90° orthogonal. Then rotate the sample to check whether rainbow pattern occurs during the rotation.

[0094] Example Intrinsic viscosity Longitudinal stretch ratio Horizontal expansion ratio Thickness (μm) structure Primer material Transmittance (%) haze(%) R0(nm) Rainbow pattern Appearance 1 0.675 3.3 3.8 100 A / B / A none 88.2 1.4 3217 ╳ ○ 2 0.675 3.3 3.8 100 A / B / C none 88.7 1.1 3298 ╳ ○ 3 0.675 2.8 3.8 100 A / B / A none 88.3 1.5 3651 ╳ ○ 4 0.675 2.5 3.8 100 A / B / A none 88.4 1.4 3973 ╳ ○ 5 0.675 2.5 4.6 100 A / B / A none 88.5 1.5 4319 ╳ △ 6 0.675 2 4.6 50 A / B / A none 88.4 1.4 3004 ╳ △ 7 0.675 1.5 4.6 50 A / B / A none 88.2 1.4 5154 △ ╳ 8 0.675 1.5 4.6 100 A / B / A none 88.8 1.4 8807 ○ ╳ 9 0.675 1.3 4.6 100 A / B / C none 89.1 1.0 8930 ○ ╳ 10 0.675 1.3 4.6 100 A / B / A single-sided 90.8 1.2 8906 ○ ╳ 11 0.675 1.3 4.6 100 A / B / A Double-sided 92.3 1.2 8911 ○ ╳ 12 0.645 3.3 3.8 100 A / B / A none 88.2 1.4 3105 ╳ ○ 13 0.645 1.5 4.6 100 A / B / A none 88.3 1.5 8841 △ △ 14 0.6 3.3 3.8 100 A / B / A none 88.3 1.4 3307 ╳ ○ 15 0.6 1.5 4.6 100 A / B / A none 88.1 1.4 9028 ○ ○ 16 0.55 3.3 3.8 100 A / B / A none 88.3 1.4 3219 ╳ △ 17 0.55 1.5 4.6 100 A / B / A none 88.2 1.4 8986 ○ ╳

[0095] Table 1

[0096] Note: Rainbow patterns × indicates severe rainbow patterns, △ indicates visible rainbow patterns, and ○ indicates no or no rainbow patterns. Appearance × indicates very poor unevenness in pattern thickness, △ indicates moderate unevenness in pattern thickness, and ○ indicates no pattern and no unevenness in thickness.

[0097] Conclusion: By studying specific intrinsic viscosity and limiting biaxial stretching process, the angle between the optical axis of the optical polyester film and the TD direction can be effectively controlled within 0°~6° or 84°~90°, while the phase difference (R0) of the optical polyester film surface is made >8000nm. Therefore, the optical axis angle and rainbow pattern problems of the polyester film can be effectively suppressed. Among Examples 1 to 17, Example 15 is the optimal solution.

[0098] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the spirit and principles of the present invention and within the technical scope disclosed in this application should be included within the scope of protection of this application. Where there is no conflict, the embodiments and features described in the embodiments of this application can be combined with each other. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing an optical polyester film, characterized in that: The following steps are included: Step 1: Use polyester chips and opening agent as layer A; use polyester chips as layer B; melt co-extrude layer A and layer B at 265℃~295℃, and rapidly cool to obtain multilayer polyester sheet; Step 2: The multilayer polyester sheet is biaxially stretched and heat-set sequentially to obtain an optical polyester film; The optical polyester film has an optical axis angle that is 0°~6° or 84°~90° with the transverse stretching direction; the phase difference R0 of the optical polyester film is ≥8000nm. The biaxial stretching process is as follows: the multilayer polyester sheet is stretched longitudinally at a temperature of 70℃~130℃ with a longitudinal stretching ratio of 1~3.3 times, and then stretched transversely at a temperature of 85℃~150℃ with a transverse stretching ratio of 3.8~6 times; the ratio of the transverse stretching ratio to the longitudinal stretching ratio is 1.2~6. The intrinsic viscosity of polyester chips is 0.6~0.645 dl / g; The polyester chips are pre-dried before use. The drying process is as follows: the polyester chips are dried at 100℃~180℃ for 2~6 hours, and the moisture content is controlled to be <200ppm.

2. The method for preparing an optical polyester film according to claim 1, characterized in that: The structure of the multilayer polyester sheet is an A / B / A structure.

3. The method for preparing an optical polyester film according to claim 1, characterized in that: The heat setting process is as follows: first, the material is set at a temperature of 230℃~250℃, and then cooled at 180℃.

4. The method for preparing an optical polyester film according to claim 1, characterized in that: The polyester chips include one or both of polyethylene terephthalate and polyethylene 2,6-naphthalate.

5. The method for preparing an optical polyester film according to claim 1, characterized in that: The opening agent includes one or more of silicon dioxide, barium sulfate, and calcium carbonate; the addition ratio of the opening agent in the A layer material is 300ppm to 1500ppm, and the particle size of the opening agent is 0.2μm to 3μm.

6. The optical polyester film prepared by the method for preparing an optical polyester film according to any one of claims 1 to 5, characterized in that: The thickness of the optical polyester film is 50μm~100μm.