A flexible UV cross-linked PMMA optical film and a method for preparing the same

Abstract

The application relates to a flexible UV cross-linked PMMA optical film and a preparation method thereof. The optical film is formed by UV bulk cross-linking of 75-88 parts of polymethyl methacrylate, 8-17 parts of a flexible bifunctional acrylate and 0.3-1.2 parts of a photoinitiator to form a single-phase solid film, wherein the weight average molecular weight of the polymethyl methacrylate is 80k-150k. The preparation method comprises solution blending, film forming, segmented multiple UV irradiation cross-linking and annealing steps. The optical film has a single glass transition temperature of 90-120 DEG C, a gel fraction of 70%-90%, a transmittance of greater than or equal to 92% at a wavelength of 550 nm, a haze of less than or equal to 1.2%, Re / Rth is all less than or equal to 10 nm, an elongation at break of greater than or equal to 15%, and residual monomers of less than or equal to 100 ppm. The optical film has excellent optical performance, flexibility and structural stability, is suitable for the winding and bending of a polarizing plate and high-definition display requirements, and has strong industrialization compatibility.

Description

Technical Field

[0001] This invention relates to the field of optical film technology, and in particular to a flexible UV crosslinked PMMA optical film and its preparation method. Background Technology

[0002] Polymethyl methacrylate (PMMA) is a commonly used substrate for polarizer protection / base films in the display industry due to its high light transmittance, low chromaticity, and excellent processability. It is widely used in polarizer fabrication and display device assembly. However, traditional pure PMMA films have inherent defects such as high brittleness and low elongation at break (≤5%). They are prone to cracking and breakage in the winding process of polarizer production and in the repeated bending scenarios of subsequent use, making it difficult to meet the bending resistance requirements of flexible displays and high-end polarizers.

[0003] In existing technologies, although there are attempts to improve the surface properties of PMMA films by hard coating or adding easy-to-adhere layers, or to achieve thinner films by directly forming films with pure UV-curable resins, these solutions only modify the surface of the film and do not address the core problem of insufficient flexibility by optimizing the internal structure of the film. Some technologies have also attempted to introduce flexible components into PMMA for blending modification to improve toughness, but they generally suffer from the technical bottleneck that it is difficult to improve flexibility and optical performance in a coordinated manner. While improving toughness, the haze of the film often increases and the visible light transmittance decreases. Furthermore, key optical indicators such as phase difference (Re, Rth) of polarizer films also deteriorate, failing to meet the optical requirements of high-definition and wide-viewing-angle displays.

[0004] Furthermore, existing modified PMMA film preparation methods often fail to construct a stable cross-linked network within the membrane, relying solely on simple blending to achieve performance adjustments. This results in insufficient dimensional stability and thermal and humidity durability of the membrane material, making it prone to deformation and optical performance degradation under high temperature and humidity environments. Moreover, there is a lack of "single-phase" structural evidence verifiable by DMA / DSC testing, making it impossible to prove the compatibility and structural stability of the blended system. Additionally, some modification methods suffer from excessively high residual monomer content, improper control of UV cross-linking process parameters leading to yellowing of the membrane material, and insufficient gelation fraction, further limiting the industrial application of modified PMMA films in the polarizer field.

[0005] In summary, existing modification schemes for PMMA films used in polarizers cannot simultaneously achieve a synergistic improvement in film flexibility, excellent optical performance, stable structural durability, and compatibility with industrial processes. There is an urgent need to develop a modification scheme that can construct a stable flexible cross-linked network within the PMMA bulk phase to obtain an optical film with a single-phase structure, low haze, low delay, and high elongation at break, meeting the comprehensive performance requirements of polarizer protection / substrate layers in the display field during winding and bending applications. Summary of the Invention

[0006] The purpose of this application is to prepare a single-phase, low-haze, low-delay, high-elongation flexible UV crosslinked PMMA film to meet the requirements of polarizer winding and bending.

[0007] To achieve the above objectives, this application provides a flexible UV-crosslinked PMMA optical film. The optical film is a single-phase solid film formed by UV bulk crosslinking of a raw material system comprising the following components: [The raw material system comprises the following components by weight after solvent removal:]

[0008] 75-88 parts of polymethyl methacrylate; 8-17 parts of flexible difunctional acrylate; Photoinitiator 0.3–1.2 parts; Wherein: the weight-average molecular weight of the polymethyl methacrylate is 80k to 150k.

[0009] As a further improvement of this application, the flexible difunctional acrylate is selected from at least one of polyethylene glycol diacrylate (PEGDA), 1,4-butanediol diacrylate (BDDA), and 1,4-butanediol dimethacrylate (BDDMA); wherein the number average molecular weight of the polyethylene glycol diacrylate is 400 to 700.

[0010] As a further improvement to this application, the raw material system also includes a compatibilizer.

[0011] As a further improvement to this application, the compatibilizer is selected from methacryloyloxypropyltrimethoxysilane or siloxane-acrylate oligomers.

[0012] As a further improvement of this application, the photoinitiator is selected from at least one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 1-hydroxycyclohexylphenyl ketone.

[0013] As a further improvement of this application, the optical film has a single glass transition temperature Tg, and Tg is between 90 and 120°C; or; there are multiple glass transition temperatures but the difference between them ΔTg ≤ 10°C, and the main Tg is between 90 and 120°C; and / or, the gelation fraction of the optical film is 70% to 90%.

[0014] As a further improvement of this application, the in-plane phase difference Re and the thickness direction phase difference Rth of the optical film are both ≤10nm at a wavelength of 550nm, and the elongation at break is ≥15%; preferably, the residual monomer content in the optical film is ≤100ppm.

[0015] To achieve the above objectives, this application also provides a method for preparing a flexible UV crosslinked PMMA optical film as described above, comprising the following steps: S1. Dissolve polymethyl methacrylate, flexible difunctional acrylate and photoinitiator in a solvent, stir and mix evenly at room temperature, filter, and let stand to remove bubbles to obtain a mixed solution. S2. Apply the mixed solution from step S1 onto the release base film, evaporate the solvent at 50-80°C to obtain a dry film precursor with a thickness of 25μm-80μm. S3. The dry film precursor is irradiated with ultraviolet light to trigger bulk crosslinking. The total irradiation dose is 0.8–1.5 J / cm². 2 A cross-linked membrane is obtained; S4. The cross-linked film irradiated with ultraviolet light is then placed in an inert or vacuum environment and annealed at 90-110°C for 30-45 minutes. After cooling, a flexible UV cross-linked PMMA optical film is obtained.

[0016] As a further improvement of this application, step S1 further includes dissolving the compatibilizer, polymethyl methacrylate, difunctional acrylate, and photoinitiator simultaneously in a solvent.

[0017] As a further improvement of this application, in step S3, the irradiation is carried out in segments and multiple times, specifically in 3 to 10 sessions of ultraviolet irradiation, with each irradiation lasting 7 to 15 seconds and the interval between two adjacent irradiations being 1 to 5 seconds.

[0018] The specific benefits of this application are as follows: The optical film of this application has a single-phase solid structure. By precisely controlling the raw material composition ratio and preparation process, it retains the high light transmittance of PMMA, achieving a transmittance of ≥92% and haze of ≤1.2% at a wavelength of 550nm. It also achieves a break elongation of ≥15% by using a flexible cross-linked network, and the in-plane phase difference Re and the thickness direction phase difference Rth are both ≤10nm. This successfully solves the technical bottleneck of the high brittleness of traditional PMMA films and the difficulty in achieving both optical and mechanical properties of existing modified films, and fully meets the core requirements of polarizer winding and bending and high-definition display.

[0019] Meanwhile, the optical film has a single glass transition temperature Tg (90-120℃) and a gelation fraction of 70-90%, ensuring system compatibility and structural stability. The residual monomer content is ≤100ppm. Combined with the annealing process, it effectively releases internal stress, significantly improving the dimensional stability and thermal and moisture durability of the film material. There is no risk of yellowing or shrinkage during long-term use.

[0020] Furthermore, the raw material composition ratio of this application is clear and the preparation process is controllable. The segmented UV irradiation and roll-to-roll coating process is compatible with existing polarizer production lines, and industrial mass production can be achieved without major equipment modifications, thereby reducing production costs and having broad application prospects. Detailed Implementation

[0021] As is known from the background art, the purpose of this application is to obtain a single-phase, low-haze, low-delay, and high-elongation flexible UV-crosslinked PMMA film to meet the requirements of polarizer winding and bending. Based on the above objective, this application provides a flexible UV-crosslinked PMMA optical film, wherein the optical film is a single-phase solid film formed by UV bulk crosslinking of a raw material system containing the following components. The raw material system comprises the following components, based on the solid weight parts after solvent removal: 75-88 parts of polymethyl methacrylate; 8-17 parts of flexible difunctional acrylate; Photoinitiator 0.3–1.2 parts; Wherein: the weight-average molecular weight of the polymethyl methacrylate is 80k to 150k.

[0022] Based on the above technical solution, the "single-phase solid film" in the optical film of this application is the core structural feature and the key feature that distinguishes this optical film from traditional modified PMMA films. Specifically, polymethyl methacrylate with a weight-average molecular weight of 80k to 150k is selected as the main phase of the film material. Polymethyl methacrylate in this molecular weight range can take into account excellent film-forming properties, optical transmittance, and compatibility with other components in blending and processing, avoiding the problems of insufficient mechanical strength of the film material due to too low molecular weight, or excessive viscosity of the solution and difficulty in uniform dispersion of various components due to too high molecular weight. The mass percentage of polymethyl methacrylate is controlled at 75 to 88 parts. As the main phase of the system, it can retain its core optical advantages of high transmittance and low chromaticity. It is combined with 8 to 17 parts of flexible difunctional acrylate as a flexible modifying phase. This ratio can realize the flexible difunctional acrylate in polymethyl methacrylate. The uniform dispersion of methyl methacrylate (MMA) molecular chains ensures that the inherent brittleness of MMA is not compromised due to an insufficient proportion of flexible modified phase, nor does it cause microphase separation due to an excessively high proportion. 0.3 to 1.2 parts of photoinitiator provide triggering conditions for UV bulk phase crosslinking. Under ultraviolet light, the photoinitiator decomposes to generate active free radicals, which in turn trigger grafting and crosslinking reactions between the flexible difunctional acrylate and MMA molecular chains. Ultimately, a uniform crosslinking network is constructed within the MMA bulk phase, forming a single-phase solid film without microphase separation. This achieves a synergistic improvement in both the rigidity of the substrate and the flexibility of the modified phase, while retaining the excellent optical properties of MMA.

[0023] In an optional embodiment, the flexible difunctional acrylate is selected from at least one of polyethylene glycol diacrylate (PEGDA), 1,4-butanediol diacrylate (BDDA), and 1,4-butanediol dimethacrylate (BDDMA); wherein the number-average molecular weight of the polyethylene glycol diacrylate is 400-700. Polyethylene glycol diacrylate, 1,4-butanediol diacrylate, and 1,4-butanediol dimethacrylate are all acrylate monomers with a bifunctional structure. Their molecular chains contain two acrylate double bonds that can participate in UV bulk crosslinking reactions. Under the action of a photoinitiator, they can form a stable bulk crosslinking network with polymethyl methacrylate (PMMA) molecular chains. Simultaneously, the molecular chains of these monomers possess good flexibility, effectively alleviating the defects of high rigidity and brittleness of PMMA molecular chains. Limiting the number-average molecular weight of polyethylene glycol diacrylate to 400-700 is because the molecular chain length of polyethylene glycol diacrylate in this molecular weight range is moderate. When the number average molecular weight is below 400, the molecular chain flexibility is insufficient, resulting in limited toughening effect on polymethyl methacrylate (PMMA). When the number average molecular weight is above 700, the excessively long molecular chain reduces its compatibility with PMMA, easily causing microphase separation in the blend system, leading to increased haze and decreased light transmittance of the membrane material. 1,4-Butanediol diacrylate and 1,4-Butanediol dimethacrylate have regular molecular chain structures and good compatibility with PMMA. They can be used alone or in combination with polyethylene glycol diacrylate. After combination, the structural characteristics of different monomers can be utilized to further optimize the synergy between membrane material flexibility and optical performance.

[0024] In an optional implementation, the raw material system further includes a compatibilizer. Preferably, the compatibilizer is 0-1 parts by weight. Although polymethyl methacrylate (PMMA) and flexible difunctional acrylate have a certain degree of compatibility, their molecular chains have inherent differences in polarity and structure, making them prone to local microphase separation during blending. The addition of the compatibilizer can effectively reduce the interfacial tension between the two and promote the uniform dispersion of flexible difunctional acrylate in the PMMA bulk phase. The compatibilizer can interact with the molecular chains of PMMA and flexible difunctional acrylate through intermolecular van der Waals forces, hydrogen bonding, or grafting reactions, acting as an interfacial "bridge" to reduce interfacial defects in the blend system. This further ensures the single-phase solid structure of the film material after UV bulk phase crosslinking, avoiding optical performance deterioration due to phase separation. At the same time, the compatibilizer can also improve the uniformity of the crosslinking network and enhance the mechanical and dimensional stability of the film material.

[0025] In an optional embodiment, the compatibilizer is selected from methacryloyloxypropyltrimethoxysilane or siloxane-acrylate oligomers. Among them, the methacryloyloxypropyltrimethoxysilane molecule contains both methacryloyloxy and trimethoxysilyl groups. The methacryloyloxy group can participate in the UV bulk crosslinking reaction and form covalent bonds with the molecular chains of polymethyl methacrylate and flexible difunctional acrylate. The trimethoxysilyl group hydrolyzes to form silanol groups, which can undergo condensation reactions to further improve the crosslinking density and dimensional stability of the film. The siloxane-acrylate oligomer molecule contains siloxane segments and acrylate double bonds. The siloxane segments have good flexibility and compatibility, which can effectively improve the interfacial compatibility between polymethyl methacrylate and flexible difunctional acrylate. The acrylate double bonds can participate in the UV bulk crosslinking reaction, allowing the oligomer to be grafted into the crosslinking network of polymethyl methacrylate. This will not introduce additional optical defects, and the flexibility of the siloxane segments can further improve the bending performance of the film. At the same time, the siloxane components can also improve the temperature and moisture resistance of the film, extending its service durability in the field of polarizers.

[0026] In an optional embodiment, the photoinitiator is selected from at least one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 1-hydroxycyclohexylphenyl ketone. Both 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 1-hydroxycyclohexylphenyl ketone are highly efficient free radical photoinitiators and have good absorption performance in the ultraviolet light band of 365-405 nm, which matches the wavelength of the light source used for UV bulk crosslinking in this application, thus ensuring photoinitiation efficiency. Among them, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide is a single-component photoinitiator with high decomposition efficiency and strong free radical activity, which can quickly initiate the crosslinking reaction of flexible difunctional acrylates. Moreover, this photoinitiator has a low yellowing coefficient, which can prevent the film material from yellowing after UV irradiation and ensure the optical transmittance of the film material. 1-hydroxycyclohexylphenyl ketone has stable photoinitiation efficiency, a wide applicable UV dose range, no harmful by-products after decomposition, and good compatibility with polymethyl methacrylate and flexible difunctional acrylates. It will not cause defects in the crosslinking network due to uneven dispersion of photoinitiators. The two can be used alone or in combination. When used in combination, they can achieve complementary photoinitiation efficiencies, further ensuring the uniformity of UV bulk crosslinking and optimizing the degree of crosslinking of the film material.

[0027] In an optional embodiment, the optical film has a single glass transition temperature Tg, which is between 90 and 120°C; or, it has multiple glass transition temperatures, but the difference between them, ΔTg, is ≤10°C, and the main Tg is between 90 and 120°C; and / or, the gelation fraction of the optical film is 70% to 90%. Wherein, "single glass transition temperature Tg (90–120°C)" is a direct structural characterization of a single-phase system formed by uniform blending of polymethyl methacrylate and flexible difunctional acrylate followed by UV bulk crosslinking. If microphase separation exists in the system, two or more glass transition temperatures will appear. The Tg range of 90–120°C is lower than the glass transition temperature of pure polymethyl methacrylate, ensuring good flexibility of the film material, and higher than the conventional operating temperature of polarizers, ensuring dimensional stability of the film material and preventing deformation during use. The gelation fraction refers to the UV bulk phase... After cross-linking, the optimal degree of cross-linking is a gel fraction of 70%–90% of the mass percentage of the cross-linked portion in the membrane material that is insoluble in acetone. When the gel fraction is below 70%, the cross-linking density of the membrane material is insufficient, the cross-linking network in the bulk phase is incomplete, and it cannot effectively improve the bending performance and dimensional stability of the membrane material. Cracking is likely to occur under winding and repeated bending. When the gel fraction is above 90%, the cross-linking density of the membrane material is too high, the mobility of molecular chains is restricted, the membrane material will become brittle again, the elongation at break will decrease, and at the same time, the excessively high cross-linking density will lead to an increase in the internal stress of the membrane material, resulting in an increase in phase difference and deterioration of optical performance.

[0028] In an optional embodiment, the in-plane phase difference Re and the thickness direction phase difference Rth of the optical film are both ≤10nm at a wavelength of 550nm, and the elongation at break is ≥15%; preferably, the residual monomer content in the optical film is ≤100ppm. The in-plane phase difference Re and the thickness direction phase difference Rth are the core indicators for evaluating the birefringence performance of the optical film. The protective / base film for polarizers requires low birefringence characteristics, and Re and Rth are both ≤10nm at a wavelength of 550nm, which can ensure that the film material does not introduce additional optical delay and avoid affecting the polarization effect of the polarizer. This application can effectively suppress the generation of birefringence in the film material by controlling the uniform blending of polymethyl methacrylate and flexible difunctional acrylate, constructing a single-phase solid crosslinked network, and releasing internal stress through subsequent annealing processes. An elongation at break ≥15% is a core quantitative indicator for improving the flexibility of the membrane material. Compared to the traditional polymethyl methacrylate (PMMA) film's elongation at break ≤5%, this improvement ensures that the membrane material will not crack or break during the winding process in polarizer production and subsequent repeated bending. A residual monomer content ≤100ppm is crucial for the long-term stability of the membrane material. Excessive residual monomer content can lead to monomer migration and volatilization during use, causing shrinkage, yellowing, and even affecting adhesion to other layers of the polarizer. This application utilizes the synergistic effect of UV bulk crosslinking and annealing processes to further engage unreacted monomers in the crosslinking reaction while removing a small amount of free monomers, controlling the residual monomer content below 100ppm to ensure the long-term stability and odorlessness of the membrane material. Preferably, the elongation at break of the optical film is 18%–25%.

[0029] In an optional embodiment, the optical film has a transmittance of ≥92% and a haze of ≤1.2%.

[0030] This application also provides a method for preparing a flexible UV crosslinked PMMA optical film as described above, comprising the following steps: S1. Dissolve polymethyl methacrylate, flexible difunctional acrylate and photoinitiator in a solvent, stir and mix evenly at room temperature, filter, and allow to stand to remove bubbles to obtain a mixed solution; preferably, the solid content of the mixed solution is 18% to 28% by mass percentage, the filtration can be carried out using a 0.2 μm filter membrane, and the standing time for degassing is 20 min to 40 min. S2. Apply the mixed solution from step S1 onto the release base film, and evaporate the solvent at 50-80°C to obtain a dry film precursor. The thickness of the dry film precursor is 25μm-80μm; preferably, the evaporation time is 2min-10min. S3. The dry film precursor is irradiated with ultraviolet light to trigger bulk crosslinking. The total irradiation dose is 0.8–1.5 J / cm². 2A cross-linked film is obtained; preferably, the wavelength of the ultraviolet light is 365–405 nm, and the ultraviolet light irradiance is 80–150 mW / cm². 2 ; S4. The cross-linked film irradiated with ultraviolet light is then placed in an inert or vacuum environment and annealed at 90-110°C for 30-45 minutes. After cooling, a flexible UV cross-linked PMMA optical film is obtained.

[0031] Based on the above technical solution, in step S1, the solution blending step uses room temperature stirring to achieve molecular-level uniform dispersion of each raw material in the solvent. Filtration removes impurities from the raw materials, and static degassing eliminates air bubbles in the solution, preventing optical defects such as pinholes and impurities from appearing in the film material after film formation, thus ensuring the optical uniformity of the film material. In step S2, the film formation step involves applying the mixed solution onto the release base film and then gently evaporating the solvent at 50–80°C. This temperature range avoids surface skinning, internal air bubbles, or internal stress caused by rapid solvent evaporation. Simultaneously, the thickness of the dry film precursor is controlled between 25 μm and 80 μm. This thickness range is suitable for the industry requirements of polarizer protection / base films. If the film is too thin, the mechanical strength is insufficient; if it is too thick, the flexibility decreases and it is not conducive to the development of thinner polarizers. In step S3, the UV bulk crosslinking step controls the total UV dose to 0.8–1.5 J / cm². 2 At this dosage, the photoinitiator can fully decompose to generate free radicals, initiating a bulk cross-linking reaction between the flexible difunctional acrylate and polymethyl methacrylate molecular chains, constructing a uniform cross-linked network. If the dosage is too low, the photoinitiator will not decompose sufficiently, resulting in insufficient cross-linking density; if the dosage is too high, it will lead to yellowing of the film material, degradation of the molecular chains, and deterioration of optical and mechanical properties. The annealing step in step S4 places the cross-linked film in an inert or vacuum environment, which can prevent the film material from oxidizing and yellowing at high temperatures. The annealing temperature of 90-110°C is close to the glass transition temperature of the film material, which can promote the movement of molecular chains, release the internal stress generated in the film formation and cross-linking process, and at the same time allow the unreacted functional groups to further undergo cross-linking reactions, improving the stability of the cross-linked network. The annealing time of 30-45 minutes can ensure that the internal stress is fully released and the cross-linking reaction is fully carried out. If the time is too short, the effect will be poor; if the time is too long, it will lead to thermal aging of the film material. Finally, after cooling, a structurally stable and high-performance flexible UV cross-linked PMMA optical film is formed.

[0032] In an optional implementation, in step S1, the ultraviolet light uses an LED cold light source with an emission wavelength of 365nm to 405nm. In the polarizer industry, using LED cold light sources to replace traditional mercury lamps is a core process for avoiding thermal damage and achieving roll-to-roll mass production.

[0033] In an optional implementation, step S1 further includes dissolving the compatibilizer, polymethyl methacrylate, difunctional acrylate, and photoinitiator simultaneously in a solvent. Dissolving the compatibilizer and other raw materials simultaneously in a solvent and stirring at room temperature allows the compatibilizer to be uniformly dispersed among the components in the solution stage, fully utilizing its interfacial compatibility and suppressing the phase separation tendency between polymethyl methacrylate and flexible difunctional acrylate from the source. If the compatibilizer is added in subsequent steps, it will lead to uneven dispersion, with excessively high or low compatibilizer content in some areas, failing to achieve compatibility modification of the entire system, and may even introduce new optical defects due to excessive local compatibilizer. Simultaneously, mixing the compatibilizer with other raw materials allows for early intermolecular interactions with polymethyl methacrylate and flexible difunctional acrylate during stirring, laying the foundation for subsequent UV bulk crosslinking to form a uniform single-phase solid network, further ensuring the synergy of the film material's optical and mechanical properties.

[0034] In an optional implementation, in step S3, the irradiation is carried out in multiple segments, specifically in 3 to 10 segments of ultraviolet irradiation, with each segment lasting 7 to 15 seconds and the interval between two adjacent irradiations being 1 to 5 seconds. Segmented, multiple irradiations are a key process design to avoid yellowing and excessive internal stress in membrane materials during UV bulk crosslinking. If a single, continuous irradiation is used, the membrane material will absorb a large amount of UV energy locally, leading to rapid decomposition of the photoinitiator, violent crosslinking reactions, and the generation of a large amount of heat locally. This causes yellowing and molecular chain degradation. Simultaneously, the violent crosslinking reaction will drastically increase the internal stress of the membrane material, resulting in increased phase difference and decreased flexibility. Irradiating in 3–10 segments, with each irradiation lasting 7–15 seconds, allows the crosslinking reaction to proceed slowly and uniformly. Each irradiation allows the membrane material a brief period to release reaction heat, preventing localized overheating. An interval of 1–5 seconds between adjacent irradiations ensures the continuity of the crosslinking reaction and effective heat dissipation. Furthermore, multiple low-dose irradiations allow for the full and uniform decomposition of the photoinitiator, ensuring the uniformity of the crosslinking network within the membrane material. Ultimately, this achieves fully crosslinked, yellowing-free, and low-stress membrane material, ensuring optimal optical and mechanical properties.

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the specific embodiments of this invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] To verify the superior performance of the technical solution of this application, the following embodiments are also provided.

[0037] Example 1 This embodiment provides a flexible UV crosslinked PMMA optical film with a thickness of 50 μm. The raw material system composition, based on the solid parts after removing the solvent, is as follows: 84.0 parts of polymethyl methacrylate with a weight average molecular weight of 100 kJ, 14.0 parts of polyethylene glycol diacrylate with a number average molecular weight of 500 kJ, and 0.8 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO).

[0038] The preparation method is as follows: S1, solution mixing: Dissolve the above raw materials in a toluene / acetone mixed solvent (the volume ratio of toluene to acetone in the mixed solvent is 1:0.6), stir at room temperature for 45 min, filter through a 0.2 μm filter membrane, and let stand for degassing for 30 min to obtain a mixed solution. The solid content of the mixed solution is 22% by weight. S2. Film formation: The mixed solution is applied to the siliconized PET release film by slit coating and the solvent is evaporated at 60°C for 5 minutes to obtain a dry film precursor with a corresponding dry film thickness of 50 μm. S3, UV bulk crosslinking, the dry film precursor is irradiated in stages with 365nm ultraviolet light at an irradiance of 120mW / cm². 2 The irradiation was performed in 5 sessions, each lasting 10 seconds, with a total radiation dose of 1.2 J / cm². 2 A cross-linked membrane is obtained; S4. Annealing: Place the crosslinked film in a nitrogen inert environment and anneal at 100°C for 40 minutes. After cooling, a flexible UV crosslinked PMMA optical film is obtained.

[0039] Example 2 This embodiment provides a flexible UV crosslinked PMMA optical film with a thickness of 50 μm. The raw material system composition, based on the solid parts after removing the solvent, is as follows: 84.0 parts of polymethyl methacrylate with a weight average molecular weight of 100k, 14.0 parts of polyethylene glycol diacrylate with a number average molecular weight of 500, 0.8 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO), and 0.2 parts of methacryloyloxypropyltrimethoxysilane (MPTS).

[0040] The preparation method is as follows: S1, solution blending: Dissolve the above raw materials in a toluene / acetone mixed solvent (the volume ratio of toluene to acetone in the mixed solvent is 1:0.6), stir at room temperature for 45 min, filter through a 0.2 μm filter membrane, and let stand for degassing for 30 min to obtain a mixed solution. The solid content of the mixed solution is 22.2% by weight. S2. Film formation: The mixed solution is applied to the siliconized PET release film by slit coating and the solvent is evaporated at 60°C for 5 minutes to obtain a dry film precursor with a corresponding dry film thickness of 50 μm. S3, UV bulk crosslinking, the dry film precursor is irradiated in stages with 365nm ultraviolet light at an irradiance of 120mW / cm². 2 The irradiation was performed in 5 sessions, each lasting 10 seconds, with a total radiation dose of 1.2 J / cm². 2 A cross-linked membrane is obtained; S4. Annealing: Place the crosslinked film in a nitrogen inert environment and anneal at 100°C for 40 minutes. After cooling, a flexible UV crosslinked PMMA optical film is obtained.

[0041] Example 3 This embodiment provides a flexible UV crosslinked PMMA optical film with a thickness of 80 μm. The raw material system composition, based on the solid parts after removing the solvent, is as follows: 88 parts of polymethyl methacrylate with a weight average molecular weight of 100k, 14.0 parts of 1,4-butanediol diacrylate, 3.0 parts of polyethylene glycol diacrylate with a number average molecular weight of 400, 1.2 parts of 1-hydroxycyclohexylphenyl ketone, and 1.0 part of siloxane-acrylate oligomer.

[0042] The preparation method is as follows: S1, solution blending: Dissolve the above raw materials in an ethyl acetate / acetone mixed solvent (the volume ratio of ethyl acetate to acetone in the mixed solvent is 1:0.8), stir at room temperature for 40 min, filter through a 0.2 μm filter membrane, and let stand for degassing for 30 min to obtain a mixed solution. The solid content of the mixed solution is 25% by weight. S2. Film formation: The mixed solution is applied to the siliconized PET release film by a blade coating method, and the solvent is evaporated at 70°C for 6 minutes to obtain a dry film precursor with a corresponding dry film thickness of 80 μm. S3, UV bulk crosslinking, the dry film precursor is irradiated in stages with 385nm ultraviolet light at an irradiance of 140mW / cm². 2 The irradiation was performed in six sessions, each lasting seven seconds, with a total radiation dose of 1.5 J / cm². 2 A cross-linked membrane is obtained; S4. Annealing: Place the crosslinked film in a nitrogen inert environment and anneal at 110°C for 45 min. After cooling, a flexible UV crosslinked PMMA optical film is obtained.

[0043] Example 4 This embodiment provides a flexible UV crosslinked PMMA optical film with a thickness of 25 μm. The raw material system composition, based on the solid parts after removing the solvent, is as follows: 75 parts of polymethyl methacrylate with a weight average molecular weight of 100 kJ, 8 parts of polyethylene glycol diacrylate with a number average molecular weight of 600 kJ, 0.3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, and 0.2 parts of methacryloyloxypropyltrimethoxysilane.

[0044] The preparation method is as follows: S1, solution mixing: Dissolve the above raw materials in a toluene / methyl ethyl ketone mixed solvent (the volume ratio of toluene to methyl ethyl ketone in the mixed solvent is 1:0.5), stir at room temperature for 35 min, filter through a 0.2 μm filter membrane, and let stand for degassing for 30 min to obtain a mixed solution. The solid content of the mixed solution is 18% by weight. S2. Film formation: The mixed solution is applied to the siliconized PET release film by spin coating (first spin coating at 800 rpm for 15 s, then spin coating at 1500 rpm for 30 s). The solvent is evaporated at 60°C for 3 min to obtain a dry film precursor with a thickness of 25 μm. S3, UV bulk crosslinking, the dry film precursor is irradiated in stages with 405nm ultraviolet light at an irradiance of 95mW / cm². 2 The irradiation was performed in 10 sessions, each lasting 10 seconds, with a total radiation dose of 0.8 J / cm². 2 A cross-linked membrane is obtained; S4. Annealing: Place the crosslinked film in an inert environment and anneal at 90°C for 30 minutes. After cooling, a flexible UV crosslinked PMMA optical film is obtained.

[0045] Comparative Example 1 The difference between this comparative example and Example 2 is that the raw material system does not contain a photoinitiator.

[0046] Comparative Example 2 The difference between this comparative example and Example 2 is that the proportion of flexible difunctional acrylate was increased and adjusted to 20 parts.

[0047] Comparative Example 3 The difference between this comparative example and Example 2 is that the proportion of flexible difunctional acrylate was reduced to 5 parts.

[0048] Comparative Example 4 The difference between this comparative example and Example 2 is that in step S3, the UV bulk crosslinking is performed by continuously irradiating the dry film precursor with 365nm ultraviolet light at an irradiance of 120mW / cm². 2 Total radiation dose: 1.2 J / cm 2 A cross-linked membrane is obtained.

[0049] Comparative Example 5 The difference between this comparative example and Example 2 is that no annealing treatment is performed.

[0050] The optical films prepared in Examples 1-4 and Comparative Examples 1-5 were laminated with PVA polarizing films at 60–90°C and 0.2–0.8 MPa to obtain the finished products. The performance parameters of the optical films and the finished products were tested, and the results are shown in Table 1. The test standards or test procedures for each performance parameter are as follows: Glass transition temperature (Tg, unit °C): Using a differential scanning calorimeter (DSC) or dynamic mechanical analyzer (DMA), the temperature is increased from 30 °C to 150 °C at a heating rate of 10 °C / min under nitrogen atmosphere protection. The temperature corresponding to the baseline abrupt change in the curve is Tg.

[0051] Glass transition temperature difference (ΔTg, unit °C): The thermal analysis curve is obtained by DSC / DMA test. If multiple glass transition peaks appear, the Tg difference corresponding to each peak is calculated, and the maximum value is taken as ΔTg.

[0052] Gelation fraction (unit: %): Accurately weigh approximately 0.5g of optical film sample, extract with acetone using Soxhlet extraction for 24h, dry to constant weight and weigh, and calculate the gelation fraction as (mass after extraction / initial mass) × 100%.

[0053] Residual monomer content (unit: ppm): The concentration of residual monomers was quantitatively determined by dissolving the optical film sample in an organic solvent and injecting it using a gas chromatography-mass spectrometry (GC-MS) system, calibrated by external standard method.

[0054] Light transmittance (unit: %): According to GB / T 2410-2008, the visible light transmittance of the finished sample was tested at a wavelength of 550nm using a UV-Vis spectrophotometer with air as a reference.

[0055] Haze (unit: %): Refer to GB / T 2410-2008, use a haze meter to test the haze value of the finished sample at a wavelength of 550nm, and take the average value of 3 parallel tests.

[0056] Re / Rth (unit: nm): The in-plane phase difference (Re) and thickness direction phase difference (Rth) of the finished sample were measured using a phase difference meter at a wavelength of 550 nm and an environment of 23℃.

[0057] Elongation at break (unit: %): Prepare dumbbell-shaped specimens according to ASTM D882 standard, stretch the finished specimens at a rate of 50 mm / min using a tensile testing machine, and calculate the elongation at break.

[0058] Stress resistance test: Fix the finished sample on the bending test machine, set the bending radius r=4mm, and perform repeated bending test. Record the maximum number of bends that cause cracks and whitening. The maximum number of repeated bends is limited to 5000.

[0059] Table 1

[0060] Comparative analysis of Comparative Example 1 and Example 2 revealed that the only difference between them was the presence or absence of a photoinitiator. Example 2 included 0.8 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide photoinitiator, while Comparative Example 1 did not. All other raw materials, proportions, and processes were identical. This missing core variable resulted in a fundamental difference in their performance and structure.

[0061] The optical film of Example 2 is cross-linked with UV to form a standard single-phase solid film with a single glass transition temperature of 104°C, a gel fraction of 80% which is within the optimal range of 70% to 90%, a visible light transmittance of 92.8% and a haze of 0.9% at 550 nm, an in-plane phase difference Re of 6 nm and a thickness direction phase difference Rth of 8 nm. The optical properties meet the requirements of low haze and low delay for polarizer films. The elongation at break is 19%, and there are no cracks after 5000 bends with r=4 mm. The residual monomer content is 90 ppm. It achieves excellent synergy in optical performance, flexibility, cross-linking structure stability and bending durability, which meets the requirements of polarizer protection / base film.

[0062] Comparative Example 1, lacking a photoinitiator, could not trigger a bulk cross-linking reaction under UV irradiation, resulting in no obvious cross-linking network within the film. The gelation fraction was 10%, and DSC analysis revealed a slight shoulder peak and a glass transition temperature difference (ΔTg) of 14℃, indicating it was not a single-phase solid film. While its optical performance did not deteriorate significantly (92.0% transmittance and 1.0% haze at 550nm), Re and Rth increased to 12nm and 15nm respectively, exceeding the ≤10nm limit. Mechanical properties showed a precipitous decline, with an elongation at break of only 11%. After 2000 bends at r=4mm, whitening and edge microcracks appeared, and the residual monomer content of 120ppm also exceeded the preferred requirement of ≤100ppm.

[0063] Therefore, the addition of a photoinitiator is a core prerequisite for achieving bulk crosslinking of PMMA and constructing a stable flexible crosslinked network, and it is also a key technical feature for forming a single-phase solid film. This application limits its content to 0.3–1.2 parts, which ensures sufficient initiation of crosslinking and avoids yellowing of the film material. This comparative example verifies that bulk crosslinking cannot be achieved without a photoinitiator, and the film material still suffers from brittleness. It also confirms that the limiting of raw material components and the design of the formulation in this application are well-coordinated. By adopting the above range, it is possible to better prepare flexible UV crosslinked PMMA optical films that meet the requirements for polarizer films.

[0064] Comparative analysis of Comparative Example 2 and Example 2 showed that the only difference between the two was the proportion of flexible difunctional acrylate. Example 2 had a proportion of 14 parts, which was within the range of 8 to 17 parts in this application. Comparative Example 2 increased to 20 parts, which exceeded this range. The other raw materials, proportions and processes were the same. This change resulted in a significant difference in performance between the two.

[0065] The optical film of Example 2 is a standard single-phase solid film with a single glass transition temperature of 104°C, a gel fraction of 80% which is within the optimal range of 70% to 90%, a visible light transmittance of 92.8% and a haze of 0.9% at 550nm, and Re and Rth of 6nm and 8nm, respectively. The optical properties meet the requirements of low haze and low delay for polarizers. The elongation at break is 19%, and there are no cracks after 5000 bends at r=4mm. The residual monomer is 90ppm. It achieves excellent synergy in optical properties, flexibility and bending durability, which meets the requirements for polarizer protection / base film use.

[0066] Although Comparative Example 2 appears to have better mechanical flexibility with a gel content of 88% and an elongation at break of 23%, its excessively high content of flexible monomers leads to microphase separation in the blend system. DSC analysis shows a shoulder peak and a ΔTg of 12°C, indicating a quasi-two-phase structure, which is not the single-phase solid film required by this application. Its optical performance deteriorates significantly, with transmittance dropping to 90.6% at 550nm, haze increasing to 2.3%, and Re and Rth reaching 15nm and 18nm respectively, exceeding the ≤10nm limit. After 3000 bends with r=4mm, the edges show micro-whitening, and cracks appear after 5000 bends, indicating that its bending durability is far inferior to Example 2.

[0067] Therefore, it can be seen that limiting the content of flexible difunctional acrylate to 8 to 17 parts in this application is a necessary technical range for achieving uniform dispersion of PMMA in bulk and constructing a single-phase cross-linked network. This limitation is the key to achieving synergistic improvement in optics and flexibility. Only by following this range can an optical film that meets the comprehensive performance requirements of polarizer films be prepared.

[0068] Comparative analysis of Comparative Example 3 and Example 2 showed that the only difference between the two was the proportion of flexible difunctional acrylate. Example 2 had a proportion of 14 parts, which is within the technical range of 8 to 17 parts in this invention and a compatibilizer was added. Comparative Example 3 had a proportion of 5 parts. The other raw materials, proportions and processes were the same. This change directly led to a significant difference in the performance of the two.

[0069] The optical film of Example 2 is a single-phase solid film with a single glass transition temperature of 104°C, a gel fraction of 80% which is within the optimal crosslinking range of 70% to 90%, a visible light transmittance of 92.8% and a haze of 0.9% at 550 nm, an in-plane phase difference Re of 6 nm and a thickness direction phase difference Rth of 8 nm. It has excellent optical performance and meets the requirements of low haze and low delay. The elongation at break reaches 19%, and it can withstand 5000 bends with r=4 mm without cracking. The residual monomer content is 90 ppm. All properties meet the requirements for use as an optical film for polarizers.

[0070] Although the optical film of Comparative Example 3 has a glass transition temperature of 118°C which is within the limited range, a transmittance of 93.5% and a haze of 0.6% at 550nm, and Re and Rth of 5nm and 7nm respectively, its optical performance is slightly better than that of Example 2. The residual monomer of 75ppm also meets the preferred requirements. However, its gelation fraction is only 65%, which does not meet the limited standard of 70% to 90%. Its elongation at break is only 8%, which is far below the index requirement of ≥15%. After bending 800 times with r=4mm, edge cracks appear. After 1000 times, the cracks extend to the film surface to form visible cracks. It is completely unable to meet the application scenarios of polarizer winding and repeated bending.

[0071] Therefore, the content range of 8-17 parts of flexible difunctional acrylate is the necessary technical window for this invention to achieve a synergistic improvement in the flexibility and cross-linking structure stability of the PMMA optical film. If the content is below this lower limit, the bulk phase of the film material cannot construct a sufficient flexible cross-linking network, and the insufficient cross-linking density leads to a lack of toughening effect. Even with excellent optical properties, the mechanical properties and bending durability will be significantly reduced, failing to meet the comprehensive performance requirements proposed in this application for flexible UV cross-linked PMMA optical films for polarizers.

[0072] Comparative Example 4 and Example 2 show that the only difference between them is the UV bulk crosslinking process. Example 2 uses segmented multiple irradiation, while Comparative Example 4 uses continuous irradiation. All other raw materials, ratios and processes are the same. This difference directly leads to a significant divergence in performance.

[0073] The optical film of Example 2 is a single-phase solid film with a single glass transition temperature of 104°C, a gel fraction of 80%, a transmittance of 92.8% and a haze of 0.9% at 550nm, Re and Rth are both ≤8nm, the elongation at break is 19%, and no cracks are found after bending 5000 times at r=4mm. All performance characteristics meet the requirements of the technical solution.

[0074] While Comparative Example 4, with a gel fraction of 82% and a Tg of 102℃ still within the specified range, and a residual monomer of 95ppm also meeting the preferred requirements, continuous irradiation caused localized overheating of the membrane material, resulting in slight yellowing and a sharp increase in internal stress. At 550nm, the transmittance dropped to 91.5%, the haze increased to 1.8%, Re and Rth exceeded 10nm, and the elongation at break was only 14%, failing to meet the ≥15% index. Microcracks appeared after 2500 bends with r=4mm, and the cracks propagated after 3500 bends, failing to meet the usage requirements.

[0075] This result confirms that segmented multiple irradiation is the key process design of this application, which allows the crosslinking reaction to proceed uniformly and effectively dissipate heat, avoiding internal stress and yellowing problems. Continuous irradiation would destroy the synergy between optical and mechanical properties, which fully demonstrates that the limitation of the UV irradiation process in this application is not arbitrary, but a necessary technical feature to achieve the excellent comprehensive performance of the film material.

[0076] Comparative Example 5 and Example 2 were analyzed, and the only difference between them was whether or not annealing was performed. Example 2 was annealed in an inert environment of 90-110°C for 40 minutes, while Comparative Example 5 was not annealed. All other raw materials, proportions and processes were the same. This difference directly led to a significant difference in the performance of the membrane material.

[0077] The optical film of Example 2 meets all the technical requirements. It is a single-phase solid film with a Tg of 104℃, a gel fraction of 80%, a transmittance of 92.8% and a haze of 0.9% at 550nm, Re and Rth are both ≤10nm, an elongation at break of 19%, a residual monomer of 90ppm, and no cracks after 5000 bends at r=4mm.

[0078] While Comparative Example 5, being a single-phase solid membrane, had a Tg of 106℃ and a gel fraction of 75% still within the acceptable range, and its transmittance of 92.2% and haze of 1.1% were also close to the standard, the lack of annealing prevented the release of internal stress generated during film formation and cross-linking. Unreacted functional groups also failed to cross-link further, resulting in Re and Rth values ​​rising to 11nm and 12nm, respectively, exceeding the low delay index. The elongation at break was 14%, failing to meet the ≥15% requirement, and the residual monomer content of 115ppm exceeded the standard. Microcracks appeared after 2000 bends at r=4mm, and room-temperature warping was also observed. These results confirm that annealing is a key process in this application, not only releasing internal stress and reducing phase difference but also reducing residual monomers and improving cross-linking stability. It is a necessary step to achieve excellent synergistic optical and mechanical properties of the membrane material.

[0079] In summary, this application successfully prepared a single-phase solid-state flexible UV crosslinked PMMA optical film by precisely controlling the raw material composition ratio and preparation process. The core beneficial effects are as follows: First, it achieves excellent synergy between flexibility and optical performance, with an elongation at break ≥15%, no cracks after bending r=4mm and 5000 cycles, and a transmittance ≥92% and haze ≤1.2% at a wavelength of 550nm, with Re / Rth both ≤10nm, meeting the requirements of high-definition display and bending resistance of polarizers; Second, it has a stable and durable structure, with a single-phase structure of Tg (90-120℃) and a gelation fraction of 70-90%, combined with an annealing process, resulting in low internal stress and residual monomers ≤100ppm, and stable performance under high temperature and high humidity conditions; Third, it has strong process compatibility, with segmented multiple UV irradiation, roll-to-roll coating and other processes suitable for industrial production, and can be directly composited with PVA polarizing layers.

[0080] Furthermore, comparative verifications show that specific content ranges of photoinitiators and flexible difunctional acrylates, as well as segmented UV irradiation and annealing processes, are key to achieving the above effects, effectively solving the technical bottlenecks of high brittleness of traditional PMMA films and the difficulty in achieving both optical and mechanical properties of existing modified films.

[0081] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A flexible UV crosslinked PMMA optical film, characterized in that, The optical film is a single-phase solid film formed by UV bulk crosslinking of a raw material system comprising the following components, which, by weight of solids after solvent removal, comprise the following components: 75-88 parts of polymethyl methacrylate; 8-17 parts of flexible difunctional acrylate; Photoinitiator 0.3–1.2 parts; Wherein: the weight-average molecular weight of the polymethyl methacrylate is 80k to 150k.

2. The flexible UV crosslinked PMMA optical film according to claim 1, characterized in that, The flexible difunctional acrylate is selected from at least one of polyethylene glycol diacrylate (PEGDA), 1,4-butanediol diacrylate (BDDA), and 1,4-butanediol dimethacrylate (BDDMA); wherein the number average molecular weight of the polyethylene glycol diacrylate is 400 to 700.

3. The flexible UV crosslinked PMMA optical film according to claim 1, characterized in that, The raw material system also includes a compatibilizer.

4. The flexible UV crosslinked PMMA optical film according to claim 3, characterized in that, The compatibilizer is selected from methacryloyloxypropyltrimethoxysilane or siloxane-acrylate oligomers.

5. The flexible UV crosslinked PMMA optical film according to claim 1, characterized in that, The photoinitiator is selected from at least one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 1-hydroxycyclohexylphenyl ketone.

6. The flexible UV crosslinked PMMA optical film according to claim 1, characterized in that, The optical film has a single glass transition temperature Tg, which is between 90 and 120°C; or, it has multiple glass transition temperatures, but the difference between them is ΔTg ≤ 10°C, and the main Tg is between 90 and 120°C; and / or, the gelation fraction of the optical film is 70% to 90%.

7. The flexible UV crosslinked PMMA optical film according to claim 1, characterized in that, The in-plane phase difference Re and the thickness direction phase difference Rth of the optical film are both ≤10nm at a wavelength of 550nm, and the elongation at break is ≥15%.

8. A method for preparing a flexible UV crosslinked PMMA optical film as described in claim 1, characterized in that, Includes the following steps: S1. Dissolve polymethyl methacrylate, flexible difunctional acrylate and photoinitiator in a solvent, stir and mix evenly at room temperature, filter, and let stand to remove bubbles to obtain a mixed solution. S2. Apply the mixed solution from step S1 onto the release base film, evaporate the solvent at 50-80°C to obtain a dry film precursor with a thickness of 25μm-80μm. S3. The dry film precursor is irradiated with ultraviolet light to trigger bulk crosslinking. The total irradiation dose is 0.8–1.5 J / cm². 2 A cross-linked membrane is obtained; S4. The cross-linked film irradiated with ultraviolet light is then placed in an inert or vacuum environment and annealed at 90-110°C for 30-45 minutes. After cooling, a flexible UV cross-linked PMMA optical film is obtained.

9. The preparation method according to claim 8, characterized in that, Step S1 also includes dissolving the compatibilizer, polymethyl methacrylate, difunctional acrylate, and photoinitiator simultaneously in a solvent.

10. The preparation method according to claim 8, characterized in that, In step S3, the irradiation is carried out in multiple stages, specifically in 3 to 10 stages of ultraviolet irradiation, with each irradiation lasting 7 to 15 seconds and the interval between two adjacent irradiations being 1 to 5 seconds.

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