A release film for a polarizing plate and a method for producing the same

Abstract

The present application relates to the technical field of high polymer materials and optical film, in particular to a release film for polarizing plate and a preparation method thereof; the preparation method comprises the following steps: preparation of hyperbranched polysiloxane-polyester copolymer modifier, melt blending and granulation of functional master batch, and film forming through biaxial stretching and heat setting; the core is a modifier with hyperbranched polyester as the core skeleton, peripheral connection of carboxyl and hydroxyl groups, and terminal alkenyl polysiloxane segments; the modification is realized through the space barrier effect of the hyperbranched skeleton, the in-situ ester exchange effect of the reactive functional groups and polyethylene terephthalate, and the surface segregation effect of the terminal alkenyl polysiloxane segments; the present application overcomes the defects in the prior art that the cyclic oligomers in the polyethylene terephthalate matrix migrate and precipitate out under heat, which leads to the increase of haze, and the poor compatibility of the surface chemical inertness with the release agent; through the synergy of internal barrier and surface segregation, the precipitation of oligomers is effectively inhibited, and the optical performance of the film and the release layer adhesion are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and optical thin film technology, specifically to a release film for polarizers and its preparation method. Background Technology

[0002] In the polarizer manufacturing process, polyethylene terephthalate release film is a key process consumable. As display technology continues to demand reductions in base film thickness and higher transmittance parameters, more stringent technical requirements are being placed on the optical and interfacial properties of polyethylene terephthalate base films. The existing biaxially oriented polyethylene terephthalate (PET) films have the following technical defects when processed at high temperatures and subsequently coated with silicone release agents and cured at high temperatures: the cyclic oligomers inside the PET matrix migrate to the film surface and crystallize under thermodynamic drive, resulting in increased film haze, decreased light transmittance, and damage to the continuity of the release layer. Polyethylene terephthalate has a chemically inert surface and poor chemical compatibility with addition-type silicone release agents. Traditional processes require offline coating of the base layer or the addition of nano-silica barrier agents. The former is complicated and carries the risk of the base layer being damaged by oligomers, while the latter is prone to nanoparticle aggregation, leading to the deterioration of the optical properties of the film. Summary of the Invention

[0003] The purpose of this invention is to provide a release film for polarizers and its preparation method, aiming to solve the problems in the prior art where the cyclic oligomers inside the polyethylene terephthalate matrix easily migrate and precipitate when heated, leading to increased haze and damage to the release layer, and the surface of polyethylene terephthalate is chemically inert, resulting in poor compatibility with release agents. Furthermore, traditional improvement processes are complex or lead to optical performance exceeding standards. Specifically, the technical solution of this invention is as follows: A release film for polarizing film, wherein the base film resin composition for preparing the release film comprises, by weight, the following components: 100 parts of polyethylene terephthalate resin; 0.5-5 parts of hyperbranched polysiloxane-polyester copolymer modifier; and 0.01-0.1 parts of transesterification catalyst; wherein the hyperbranched polysiloxane-polyester copolymer modifier has a hyperbranched polyester as its core skeleton, and the ends of the core skeleton are connected with carboxyl groups, hydroxyl groups, and terminal alkenyl polysiloxane segments.

[0004] Preferably, the transesterification catalyst is selected from tetrabutyl titanate or antimony acetate.

[0005] Preferably, after baking at 150°C for 24 hours, the amount of cyclic trimer precipitated on the surface of the release film is ≤0.12mg / m², the haze is ≤0.65%, and the light transmittance is ≥91.5%.

[0006] Preferably, it includes the following steps: Step S1: Prepare the hyperbranched polysiloxane-polyester copolymer modifier; Step S2: The hyperbranched polysiloxane-polyester copolymer modifier is melt-blended with a portion of the polyethylene terephthalate resin and granulated to obtain a functional masterbatch. Step S3: The functional masterbatch is mixed with the remaining polyethylene terephthalate resin and the transesterification catalyst, and after vacuum drying, it is melt extruded, cast, biaxially stretched and heat-set to obtain the release film for polarizing film.

[0007] Preferably, in step S1, the specific process for preparing the hyperbranched polysiloxane-polyester copolymer modifier includes: S11, pentaerythritol and 2,2-dimethylolpropionic acid are mixed, p-toluenesulfonic acid is added as a catalyst, and melt polycondensation reaction is carried out under nitrogen protection. During the reaction, the by-product water is removed by vacuum to obtain hydroxyl-terminated hyperbranched polyester. S12, the hydroxyl-terminated hyperbranched polyester is dissolved in anhydrous N,N-dimethylformamide, succinic anhydride and 4-dimethylaminopyridine are added, and a half-esterification ring-opening reaction is carried out. After cooling, precipitation and vacuum drying, an intermediate with carboxyl and hydroxyl groups is obtained. S13, the intermediate is mixed with monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane in toluene, tetrabutylammonium bromide is added, and the mixture is refluxed. After the reaction is completed, the solvent is removed by rotary evaporation under reduced pressure, and the mixture is purified by precipitation in methanol and dried under vacuum to obtain the hyperbranched polysiloxane-polyester copolymer modifier.

[0008] Preferably, in step S11, the molar ratio of pentaerythritol to 2,2-dimethylolpropionic acid is 1:6-1:12, the amount of p-toluenesulfonic acid added is 0.1wt% of the total mass of pentaerythritol and 2,2-dimethylolpropionic acid; the temperature of the melt polycondensation reaction is 140°C, the reaction time is 4-6 hours, and the vacuum pressure is controlled at 10Pa-100Pa.

[0009] Preferably, in step S12, the molar amount of succinic anhydride is 40% of the total molar amount of terminal hydroxyl groups in the hyperbranched polyester; the temperature of the semi-esterification ring-opening reaction is 100°C, and the reaction time is 3 hours.

[0010] Preferably, in step S13, the intermediate and the monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane are mixed at a molar ratio of carboxyl group on the intermediate to monoepoxy group of 1:0.5-1:0.8; the reflux reaction is carried out at a temperature of 120°C for 6 hours.

[0011] Preferably, in step S2, the mass ratio of the hyperbranched polysiloxane-polyester copolymer modifier to the partially polyethylene terephthalate resin is 5:95; the partially polyethylene terephthalate resin is polyethylene terephthalate chips with an intrinsic viscosity of 0.68 dL / g; the melt blending is carried out in a twin-screw extruder at a blending temperature of 265℃-280℃.

[0012] Preferably, in step S3, the mass ratio of the functional masterbatch to the remaining polyethylene terephthalate resin is 1:4; vacuum drying is performed before melt extrusion at a temperature of 120°C for 6 hours; the melt extrusion temperature is 285°C; the biaxial stretching includes longitudinal stretching and transverse stretching, the longitudinal stretching has a stretch ratio of 3.2 times at a temperature of 90°C, the transverse stretching has a stretch ratio of 3.5 times at a temperature of 110°C; and the heat setting temperature is 220°C.

[0013] The beneficial effects of this invention are as follows: This invention provides a release film for polarizers and its preparation method. The base film resin composition is modified by adding a hyperbranched polysiloxane-polyester copolymer modifier. The modification utilizes the spatial blocking effect of the hyperbranched polyester core skeleton in the modifier, the in-situ transesterification effect of carboxyl and hydroxyl groups with polyethylene terephthalate resin, and the surface segregation effect of the terminal alkenyl polysiloxane segments to effectively reduce the amount of cyclic trimer precipitation on the release film surface, maintain low haze and high light transmittance, and improve the adhesion stability of the release layer. Detailed Implementation

[0014] The technical solutions in the embodiments of the present invention will be clearly and completely described below; the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0015] Example 1: This example provides a release film for polarizers, specifically including the following steps: S1. Preparation of hyperbranched polysiloxane-polyester copolymer modifier: S11. Preparation of hydroxyl-terminated hyperbranched polyester: Pentaerythritol and 2,2-dimethylolpropionic acid were added to a reactor at a molar ratio of 1:8. The amount of p-toluenesulfonic acid added was 0.1 wt% of the total raw material mass. Nitrogen gas was introduced for protection. The temperature was raised to 140℃ for melt polycondensation, and the vacuum was gradually reduced to 80 Pa to remove the by-product water. The reaction was maintained at this temperature for 5 hours to obtain hydroxyl-terminated hyperbranched polyester. This hydroxyl-terminated hyperbranched polyester serves as the core framework of the hyperbranched polysiloxane-polyester copolymer modifier, providing a large number of reaction sites. Its three-dimensional structure can effectively hinder the migration of oligomers. Gel permeation chromatography showed that the number-average molecular weight (Mn) of this hydroxyl-terminated hyperbranched polyester was 2400, the molecular weight distribution index (PDI) was 1.35, and the yield was 92%. S12. Partial Carboxylation Modification: The hydroxyl-terminated hyperbranched polyester obtained in step S11 was dissolved in anhydrous N,N-dimethylformamide at a mass-to-volume ratio of 1 g:5 mL. Succinic anhydride was added in a molar amount sufficient to convert 40% of the hydroxyl groups, and 4-dimethylaminopyridine was added as a catalyst at 1% of the mass of succinic anhydride. The reaction was carried out at 100 °C for 3 hours. After cooling, the reaction solution was poured into deionized water to precipitate, filtered, and vacuum dried at 80 °C for 12 hours to obtain an intermediate containing both carboxyl and hydroxyl groups. This intermediate, by introducing carboxyl groups, provides a covalently anchored reaction site for subsequent transesterification with the polyethylene terephthalate matrix. The Fourier transform infrared spectrum showed a carbonyl absorption peak of the carboxyl group at 1710 cm⁻¹, with a yield of 88%. S13, Grafted end-alkenyl polysiloxane: The intermediate with both carboxyl and hydroxyl groups obtained in step S12 and the monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane are added to toluene at a molar ratio of carboxyl groups to monoepoxy groups of 1:0.6, wherein the ratio of the total mass of the reactants to the volume of toluene is 1 g:5 mL. 0.5% of the total mass of the reactants is added as a catalyst, and the mixture is refluxed at 120 °C for 6 hours. Under the catalysis of tetrabutylammonium bromide, the carboxyl groups preferentially nucleophilically attack the less sterically hindered end carbon atoms of the epoxy groups to undergo a ring-opening addition reaction, generating secondary alcohol structures with high selectivity, thereby obtaining a specific spatial structure; After the reaction was completed, toluene was removed under reduced pressure, and the mixture was slowly added dropwise to methanol for precipitation and purification. After filtration, the mixture was vacuum dried at 70°C for 10 hours to obtain a hyperbranched polysiloxane-polyester copolymer modifier. This hyperbranched polysiloxane-polyester copolymer modifier not only has a core skeleton that blocks the precipitation of oligomers, but its peripheral terminal alkenyl polysiloxane segments can also achieve surface segregation and form a chemical bond with the release agent. Gel permeation chromatography determined that the final product had a number-average molecular weight (Mn) of 12000 and a PDI of 1.45. The characteristic peak of the epoxy group at 910 cm⁻¹ disappeared in the Fourier transform infrared spectrum, while a characteristic peak of the vinyl group appeared at 1600 cm⁻¹. The proton nuclear magnetic resonance (NMR) spectrum showed a grafting rate of 95% and an overall yield of 85%. S2. Preparation of functional masterbatch: The modifier obtained in step S1 is mixed with polyethylene terephthalate chips with an intrinsic viscosity of 0.68 dL / g at a mass ratio of 5:95. The mixture is melt-blended and granulated in a twin-screw extruder under the following temperature conditions: Zone 1 265℃, Zone 2 270℃, Zone 3 275℃, and Die Head 280℃ to obtain the functional masterbatch. S3. Base film preparation: The functional masterbatch and the remaining polyethylene terephthalate resin are mixed at a mass ratio of 1:4, and 0.05 parts of tetrabutyl titanate, a transesterification catalyst, are added. The mixture is vacuum dried at 120°C for 6 hours and then fed into the main extruder. It is melt-extruded at 285°C and cooled by casting to obtain a cast sheet. The cast sheet is longitudinally stretched at a stretch ratio of 3.2 and a temperature of 90°C. Then it is transversely stretched at a stretch ratio of 3.5 and a temperature of 110°C. Finally, it is heat-set at 220°C to obtain a release film for polarizers with a thickness of 25 μm.

[0016] Example 2: This example provides a release film for polarizers. The specific steps are basically the same as in Example 1, except that: In step S11, the molar ratio of pentaerythritol to 2,2-dimethylolpropionic acid was adjusted to 1:6, the vacuum pressure was 95 Pa, and the reaction time was 4 hours; hydroxyl-terminated hyperbranched polyester was obtained, and its number-average molecular weight (Mn) was 1800, PDI was 1.28, and the yield was 90% as determined by gel permeation chromatography. In step S13, the grafting rate of the finally obtained hyperbranched polysiloxane-polyester copolymer modifier was 93%, and the yield was 82%. In step S3, to change the amount of modifier added to the base film resin composition to 0.5 parts, the mass ratio of functional masterbatch to the remaining polyethylene terephthalate resin is adjusted to 10:90.5, and the transesterification catalyst is 0.01 parts of tetrabutyl titanate; the remaining operation steps and process parameters are the same as in Example 1.

[0017] Example 3: This example provides a release film for polarizers. The specific steps are basically the same as in Example 1, except that: In step S11, the molar ratio of pentaerythritol to 2,2-dimethylolpropionic acid was adjusted to 1:10, the vacuum pressure was 70 Pa, and the reaction time was 6 hours; hydroxyl-terminated hyperbranched polyester was obtained, and its number-average molecular weight (Mn) was 3100, PDI was 1.42, and the yield was 94% as determined by gel permeation chromatography. In step S13, the grafting rate of the finally obtained hyperbranched polysiloxane-polyester copolymer modifier was 96%, and the yield was 86%. In step S3, to change the amount of modifier added to the base film resin composition to 3 parts, the mass ratio of functional masterbatch to the remaining polyethylene terephthalate resin is adjusted to 60:43, and the transesterification catalyst is changed to 0.06 parts of antimony acetate; the remaining operation steps and process parameters are the same as in Example 1.

[0018] Example 4: This example provides a release film for polarizers. The specific steps are basically the same as in Example 1, except that: In step S11, the molar ratio of pentaerythritol to 2,2-dimethylolpropionic acid is adjusted to 1:12, the vacuum pressure is 60 Pa, and the reaction time is 6 hours. In step S3, the amount of modifier added to the base film resin composition is 5 parts, the mass ratio of functional masterbatch to the remaining polyethylene terephthalate resin is adjusted to 100:5, and the transesterification catalyst is changed to 0.10 parts of antimony acetate; the remaining operation steps and process parameters are the same as in Example 1.

[0019] Comparative Example 1: The difference between this comparative example and Example 1 is that the hyperbranched polysiloxane-polyester copolymer modifier in steps S1 and S2 is omitted, and the base film resin composition uses only 100 parts of polyethylene terephthalate resin and 0.05 parts of tetrabutyl titanate. Other operating steps and process parameters are exactly the same as in Example 1.

[0020] Comparative Example 2: The difference between this comparative example and Example 1 is that step S12 is omitted, the succinic anhydride semi-esterification treatment is not performed, and the hydroxyl-terminated hyperbranched polyester obtained in step S11 is directly grafted with monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane. Other operating steps and process parameters are exactly the same as in Example 1.

[0021] Comparative Example 3: The difference between this comparative example and Example 1 is that in step S13, the monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane is replaced with monoepoxy-terminated polydimethylsiloxane, which does not contain vinyl end groups. Other operating steps and process parameters are exactly the same as in Example 1.

[0022] Comparative Example 4: The difference between this comparative example and Example 1 is that the amount of modifier added in step S3 is replaced by 0.2 parts instead of 1 part, which is lower than the limit of the present invention. Other operating steps and process parameters are exactly the same as those in Example 1.

[0023] Comparative Example 5: The difference between this comparative example and Example 1 is that no transesterification catalyst is added in step S3, while the other operating steps and process parameters are exactly the same as in Example 1.

[0024] Performance Testing and Datasheets Cyclic trimer precipitation test: The sample was baked at 150℃ for 24 hours, and the surface precipitates were eluted with chloroform. The amount of cyclic trimer precipitated was quantitatively calculated by high performance liquid chromatography, and the unit is mg / m². Optical performance testing: Haze and transmittance were measured using conventional thin-film optical testing methods; Surface elemental analysis: The percentage of Si atoms within a 5 nm range on the thin film surface was determined using X-ray photoelectron spectroscopy. Mechanical property testing: Determining the longitudinal and transverse tensile strength of the film; Release layer adhesion test: A solvent-free addition-type silicone release agent was directly coated on the film surface, cured at 120°C for 30 seconds, and then a 3M 610 tape cross-cut peel test was performed. The residual adhesion rate was also measured after aging at 85°C and 85% relative humidity for 500 hours. Table 1 Performance test results of each embodiment and comparative example As can be seen from the comparison of the test results of Example 1 and Comparative Example 1 in Table 1, after omitting the hyperbranched polysiloxane-polyester copolymer modifier, the amount of cyclic trimer precipitation increased from 0.12 mg / m² to 1.85 mg / m², the haze increased from 0.65% to 0.90%, the peeling rate of the cross-cut coating increased from 0 to 78%, and the SAS decreased from 96.8% to 28.5%. The underlying mechanism is that, without the hyperbranched polyester backbone, the polyethylene terephthalate matrix no longer has a dense spatial structure that can restrict the migration of oligomers. Under heat treatment conditions, cyclic trimers are more likely to migrate to the surface and crystallize out, resulting in enhanced light scattering and increased haze. At the same time, the film surface lacks terminal alkenyl polysiloxane segments, which cannot form an effective chemical bond with addition-type silicone release agents, resulting in a significant decrease in adhesion. As can be seen from the comparison of the test results of Example 1 and Comparative Example 2 in Table 1, after omitting the carboxylation treatment in step S12, the amount of cyclic trimer precipitation increased to 0.47 mg / m², the peeling rate of the cross-cut stripping coating increased to 6%, and the SAS decreased to 82.4%. The underlying mechanism is that the carboxyl groups introduced by the semi-esterification treatment are important reaction sites for subsequent transesterification reactions with polyethylene terephthalate (PET) molecular chains. Without this step, the covalent anchoring degree between the modifier and the PET matrix decreases, the fixation ability of the hyperbranched structure in the matrix weakens, and the oligomer barrier effect decreases. At the same time, the bonding strength between the surface segregation layer and the interior of the base film is insufficient, making it more prone to interfacial stress release during coating curing and humid heat aging, resulting in decreased adhesion stability. As can be seen from the comparison of the test results of Example 1 and Comparative Example 3 in Table 1, after replacing the terminal alkenyl polysiloxane with vinyl-free polydimethylsiloxane, there was no significant difference in the amount of cyclic trimer precipitation and haze, but the peeling rate of the cross-cut stripping coating increased to 61% and the SAS decreased to 34.8%. The underlying mechanism is that the comparative example still retains the hyperbranched framework and the surface segregation ability of polysiloxane, so the oligomer barrier and optical properties are close to those of Example 1; however, since the surface segments do not contain vinyl groups, they cannot undergo hydrosilylation reaction with the hydrogen-containing silicone oil in the release agent, and the interface mainly relies on weak polar interaction. After curing, the silicone layer is easy to detach from the surface of polyethylene terephthalate under peeling and humid heat conditions. As can be seen from the comparison of the test results of Example 1 and Comparative Example 4 in Table 1, after reducing the amount of modifier added to 0.2 parts, the percentage of Si atoms on the surface decreased to 4.6%, the amount of cyclic trimer precipitation increased to 0.73 mg / m², the peeling rate of the cross-cut coating increased to 14%, and the SAS decreased to 71.2%. The underlying mechanism is that when the amount of modifier added is too low, the number of hyperbranched structures in the polyethylene terephthalate matrix is ​​insufficient, making it difficult to form a continuous and effective oligomer migration barrier region; at the same time, the number of surface segregable vinyl polysiloxane segments decreases, the density of surface active sites decreases, resulting in insufficient coverage of the surface anchoring layer, and a simultaneous decrease in adhesion and aging stability. As can be seen from the comparison of the test results of Example 1 and Comparative Example 5 in Table 1, without the addition of the transesterification catalyst, the amount of cyclic trimer precipitation increased to 0.56 mg / m², the haze increased to 0.75%, the peeling rate of the cross-cut stripping coating increased to 9%, and the SAS decreased to 79.5%. The underlying mechanism is that the transesterification catalyst can promote the reaction between the carboxyl and hydroxyl groups on the modifier and the polyethylene terephthalate molecular chain, thereby increasing the degree of fixation of the modifier in the matrix. In the absence of a catalyst, the in-situ reaction between the modifier and polyethylene terephthalate is insufficient, and some modifiers exist only in a physical blend state. This weakens the effect of limiting oligomer migration and reduces the connection strength between the surface segregation layer and the matrix, resulting in a decrease in the adhesion retention rate after humid heat aging. The test results from Examples 1 to 4 show that, within the scope of this invention, adjusting the degree of branching of the hyperbranched polyester, the amount of modifier, and the type of transesterification catalyst can all result in lower oligomer precipitation and good interfacial adhesion performance. In Example 2, the amount of modifier and catalyst added was relatively low, and the density of surface active sites and the degree of in-situ reaction were both reduced. Therefore, the amount of cyclic trimer precipitation and the adhesion index were slightly lower than those in Example 1. In Example 3, the amount of modifier added was increased to 3 parts, and the oligomer barrier ability was still maintained in an effective barrier state, and the adhesion was close to that in Example 1. In Example 4, when the amount of modifier added was increased to 5 parts, the surface Si content increased further. However, due to the increase in the proportion of polysiloxane segments, the uniformity of the refractive index inside the film was slightly affected, the haze increased, and the transmittance decreased slightly, but overall it still met the target index requirements. The above results indicate that the resin composition can simultaneously improve the problems of oligomer thermal precipitation and release layer adhesion through the spatial barrier effect of the hyperbranched skeleton, the in-situ transesterification between reactive functional groups and polyethylene terephthalate, and the surface segregation effect of the terminal alkenyl polysiloxane segments.

[0025] The above are merely specific embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any conventional modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the protection scope of the technical solution of the present invention.

Claims

1. A release film for polarizing films, characterized in that, The base film resin composition for preparing the release film comprises, by weight, the following components: 100 parts of polyethylene terephthalate resin; 0.5-5 parts of hyperbranched polysiloxane-polyester copolymer modifier; 0.01-0.1 parts of transesterification catalyst; the hyperbranched polysiloxane-polyester copolymer modifier has a hyperbranched polyester as the core skeleton, and the ends of the core skeleton are connected with carboxyl, hydroxyl and terminal alkenyl polysiloxane segments. The preparation process of the hyperbranched polysiloxane-polyester copolymer modifier includes: S11, pentaerythritol and 2,2-dimethylolpropionic acid are mixed, p-toluenesulfonic acid is added as a catalyst, and melt polycondensation reaction is carried out under nitrogen protection. During the reaction, the by-product water is removed by vacuum to obtain hydroxyl-terminated hyperbranched polyester. S12, the hydroxyl-terminated hyperbranched polyester is dissolved in anhydrous N,N-dimethylformamide, succinic anhydride and 4-dimethylaminopyridine are added, and a half-esterification ring-opening reaction is carried out. After cooling, precipitation and vacuum drying, an intermediate with carboxyl and hydroxyl groups is obtained. S13, the intermediate is mixed with monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane in toluene, tetrabutylammonium bromide is added, and the mixture is refluxed. After the reaction is completed, the solvent is removed by rotary evaporation under reduced pressure, and the mixture is purified by precipitation in methanol and dried under vacuum to obtain the hyperbranched polysiloxane-polyester copolymer modifier.

2. The release film for polarizing films according to claim 1, characterized in that, The transesterification catalyst is selected from tetrabutyl titanate or antimony acetate.

3. The release film for polarizers according to claim 1, characterized in that, After baking at 150°C for 24 hours, the release film exhibits a surface cyclic trimer precipitation amount ≤0.12mg / m², haze ≤0.65%, and light transmittance ≥91.5%.

4. A method for preparing a release film for polarizers, used to prepare the release film for polarizers as described in any one of claims 1-3, characterized in that, Includes the following steps: Step S1: Prepare the hyperbranched polysiloxane-polyester copolymer modifier; Step S2: The hyperbranched polysiloxane-polyester copolymer modifier is melt-blended with a portion of the polyethylene terephthalate resin and granulated to obtain a functional masterbatch. Step S3: The functional masterbatch is mixed with the remaining polyethylene terephthalate resin and the transesterification catalyst, and after vacuum drying, it is melt extruded, cast, biaxially stretched and heat-set to obtain the release film for polarizing film.

5. The method for preparing a release film for a polarizer according to claim 4, characterized in that, In step S11, the molar ratio of pentaerythritol to 2,2-dimethylolpropionic acid is 1:6-1:12, and the amount of p-toluenesulfonic acid added is 0.1wt% of the total mass of pentaerythritol and 2,2-dimethylolpropionic acid; the temperature of the melt polycondensation reaction is 140℃, the reaction time is 4-6 hours, and the vacuum pressure is controlled at 10Pa-100Pa.

6. The method for preparing a release film for a polarizer according to claim 4, characterized in that, In step S12, the molar amount of succinic anhydride is 40% of the total molar amount of terminal hydroxyl groups in the hyperbranched polyester; the temperature of the semi-esterification ring-opening reaction is 100°C, and the reaction time is 3 hours.

7. The method for preparing a release film for a polarizer according to claim 4, characterized in that, In step S13, the intermediate and the monoepoxy-terminated poly(dimethyl-co-vinylmethyl)siloxane are mixed at a molar ratio of 1:0.5 to 1:0.8 between the carboxyl group on the intermediate and the monoepoxy group; the reflux reaction is carried out at a temperature of 120°C for 6 hours.

8. The method for preparing a release film for a polarizer according to claim 4, characterized in that, In step S2, the mass ratio of the hyperbranched polysiloxane-polyester copolymer modifier to the partially polyethylene terephthalate resin is 5:95; the partially polyethylene terephthalate resin is polyethylene terephthalate chips with an intrinsic viscosity of 0.68 dL / g; the melt blending is carried out in a twin-screw extruder at a blending temperature of 265℃-280℃.

9. The method for preparing a release film for a polarizer according to claim 4, characterized in that, In step S3, the mass ratio of the functional masterbatch to the remaining polyethylene terephthalate resin is 1:4; vacuum drying is performed before melt extrusion at a temperature of 120°C for 6 hours; the melt extrusion temperature is 285°C; the biaxial stretching includes longitudinal stretching and transverse stretching, with a stretching ratio of 3.2 times and a temperature of 90°C for longitudinal stretching, and a stretching ratio of 3.5 times and a temperature of 110°C for transverse stretching; the heat setting temperature is 220°C.

Images