Cellulose triacetate optical film for polaroid and preparation method of cellulose triacetate optical film

Abstract

The invention provides a cellulose triacetate optical film for a polaroid and a preparation method of the cellulose triacetate optical film, and relates to the field of optical films for polaroids. The preparation method of the cellulose triacetate optical film for the polaroid comprises the following steps: grafting modification, purification treatment, preparation of a film casting solution, and film formation and curing. According to the preparation method of the cellulose triacetate optical film for the polaroid, the hardness, toughness and wear resistance of the optical film can be effectively balanced while the situation that an existing blending modification method influences the optical performance, high-temperature and high-humidity stability and overall stability of the optical film can be avoided, and the overall mechanical performance of the optical film is further optimized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical films for polarizing plates, and in particular to a triacetate cellulose optical film for polarizing plates and a preparation method thereof. BACKGROUND

[0002] Cellulose is one of the most abundant natural polymers in nature, and cellulose acetate is an acetylated derivative of cellulose. According to the degree of substitution, cellulose acetate is divided into diacetate cellulose and triacetate cellulose. Among them, triacetate cellulose optical film has good transparency, flexibility, thermal stability, and good gas and moisture permeability. Its main use is as an optical film for manufacturing polarizing plates, and it plays a crucial role in the production process of liquid crystal displays (LCD) and other electronic displays. Polarizing plates with triacetate cellulose optical film are widely used in liquid crystal displays (LCD), organic light-emitting diode displays (OLED), and other electronic displays, such as personal computers, televisions, mobile phones, wearable terminals, and vehicle instrument panels.

[0003] The existing preparation method of polymer film is generally melt extrusion film forming and solution casting film forming. Since the melting temperature of triacetate cellulose is close to the decomposition temperature, if melt extrusion film forming is used, triacetate cellulose will be severely degraded, and the optical film with the desired performance cannot be obtained. Therefore, triacetate cellulose optical film is generally prepared by solution casting film forming method. The conventional preparation method of triacetate cellulose optical film is to dissolve triacetate cellulose and additives in an organic solvent to prepare a cotton gum solution, filter out the insoluble impurities in the cotton gum solution, and then deaerate to obtain a casting solution. Then, the casting solution is used for casting operation to form a wet film containing solvent. The wet film is dried, wound, and other operations to obtain triacetate cellulose optical film. However, triacetate cellulose optical film is thin and easy to break, and problems such as film breakage may occur during processing, which directly leads to the whole roll being scrapped, seriously affecting the production efficiency and production cost.

[0004] In view of the above technical problems, the existing technology discloses a modified triacetate cellulose optical film prepared by blending modification of a modified monomer with triacetate cellulose in an organic solvent. However, due to the poor compatibility and refractive matching of the modified monomer added in the blending modification with triacetate cellulose, there are defects such as phase separation, water swelling and hydrolysis. Not only will it cause the deterioration of the optical properties of the optical film such as haze, affecting the performance of the polarizing plate using the optical film, but also will cause the decrease of the high temperature and high humidity stability of the optical film, and the yellowing, aging and optical performance attenuation phenomenon may easily occur during long-term use, and the overall stability is poor. Moreover, the single blending modification method is difficult to balance the hardness, toughness and wear resistance of the optical film, and it is difficult to meet the performance requirements of existing electronic display products, and the overall mechanical properties of the optical film need to be further improved.

[0005] Based on this, a polarizing sheet triacetate cellulose optical film and its preparation method are provided, which can effectively balance the hardness, toughness and wear resistance of the optical film while avoiding the influence of the existing blending modification method on the optical performance, high temperature and humidity stability and overall stability of the optical film, further optimize the overall mechanical properties of the optical film, and have important technical significance and research value. SUMMARY

[0006] To solve the technical problems existing in the prior art, the present application provides a polarizing sheet triacetate cellulose optical film and its preparation method, which can effectively balance the hardness, toughness and wear resistance of the optical film while avoiding the influence of the existing blending modification method on the optical performance, high temperature and humidity stability and overall stability of the optical film, and further optimize the overall mechanical properties of the optical film.

[0007] To solve the above technical problems, the technical solutions adopted by the present application are as follows: A preparation method of a polarizing sheet triacetate cellulose optical film, comprising the following steps: graft modification, purification treatment, preparation of casting solution, film formation and curing; The method of graft modification is to put triacetate cellulose into a solvent in an inert gas environment, swell after heating, and then drop 2-cyclohexyl acrylate in the presence of zinc triflate; after the dropping is completed, 60-65℃ is kept for stirring, and the graft modification product is obtained; The graft modification product is purified to obtain modified triacetate cellulose; The method of preparing the casting solution is to mix polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333, anhydrous ethanol, ethylene dichloride, and modified triacetate cellulose uniformly, and then filter and degas to prepare the casting solution; The method of film formation and curing is to coat the film with the casting solution, dry, and then cure with ultraviolet light to prepare the polarizing sheet triacetate cellulose optical film.

[0008] Preferably, in the graft modification, the weight of 2-cyclohexyl acrylate added is 10-10.5% of the weight of triacetate cellulose; The weight of zinc triflate added is 4-4.2% of the weight of triacetate cellulose.

[0009] Preferably, in the graft modification, the dropping rate of 2-cyclohexyl acrylate is 0.15-0.25mL / min; The temperature is controlled at 60-65℃ during the dropping of 2-cyclohexyl acrylate; The 2-cyclohexyl acrylate is kept at 60-65℃ for 5-5.5h after the dropping is completed.

[0010] Preferably, in the graft modification, the temperature for swelling is 48-52℃, and the time for swelling is 2-2.5h. The solvent is dimethyl sulfoxide, and the weight of the solvent is 13-15 times of the weight of the cellulose triacetate.

[0011] Preferably, in the preparation of the casting solution, the weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333, anhydrous ethanol, dichloromethane, and modified cellulose triacetate is 1.5-1.6:0.85-0.95:0.3-0.35:0.4-0.45:0.08-0.11:2-2.5:58-60:10-10.5.

[0012] Preferably, in the film formation and curing, the coating speed is controlled to be 0.2-0.3m / min, and the coating thickness is 160-180μm; after the casting solution is coated onto the surface of the substrate at 27-29℃ to form a wet film, the wet film is dried. The ultraviolet curing is performed in a nitrogen atmosphere, using ultraviolet light with a wavelength of 365nm and an intensity of 31-33mW / cm 2 for 9-11s.

[0013] Further, in the film formation and curing, the drying is performed in three stages. The temperature for the first-stage drying is 50-55℃, and the time is 3-4min. The temperature for the second-stage drying is 70-75℃, and the time is 5-6min. The temperature for the third-stage drying is 60-65℃, and the time is 2-3min.

[0014] Further, the method for the purification treatment is as follows: triethylamine is used to quench the zinc triflate in the graft modification, and then the mixture is put into a methanol aqueous solution, and the solid is separated after shearing; the solid is washed, and then put into anhydrous acetone, and the purified solution is obtained after ultrasonic treatment; the purified solution is put into anhydrous methanol to precipitate, and the solid is separated; the solid is dried and crushed to obtain the modified cellulose triacetate.

[0015] Preferably, in the purification treatment, the weight of the triethylamine is 1.6-1.8 times of the weight of the zinc triflate used in the graft modification. The volume concentration of methanol in the methanol aqueous solution is 80-85%. The shearing speed is 4000-5000rpm, and the time is 8-12min.

[0016] A cellulose triacetate optical film for a polarizing sheet is prepared by the above method.

[0017] Compared with the prior art, the present application has the following advantages: (1) The preparation method of the tri-acetyl cellulose optical film for polarizing plate of the present application, by using 2-cyclohexyl acrylate graft modification tri-acetyl cellulose (i.e. modified tri-acetyl cellulose) and blending modification monomer (polyethylene glycol diacrylate, dipentaerythritol hexaacrylate) to make casting solution, then coating with the casting solution, drying, and ultraviolet curing, the tri-acetyl cellulose optical film for polarizing plate is prepared; the above technical means cooperate with each other and synergistically, which can effectively balance the hardness, toughness and wear resistance of the optical film while avoiding the influence of the existing blending modification method on the optical performance, high temperature and humidity stability and overall stability of the optical film, further optimizing the overall mechanical properties of the optical film.

[0018] (2) The tri-acetyl cellulose optical film for polarizing plate of the present application has a light transmittance of 92.9-93.2%, a haze value of 0.07-0.08%, a pencil hardness grade of 6H, a tensile strength of 119.1-119.7 MPa, an elongation at break of 20.0-20.5%, a Young's modulus of 4.0-4.1 GPa, and a breaking work of 14.0-14.3 MJ / m 3 .

[0019] (3) The tri-acetyl cellulose optical film for polarizing plate of the present application has a yellow index change rate of 0.7% after standing in an environment with a temperature of 85℃ and a relative humidity of 85% for 500h, the light transmittance can still reach 92.6%, the haze value increase rate is 0.1%, the pencil hardness grade can still reach 6H, and the tensile strength can still reach 111 MPa.

[0020] (4) The tri-acetyl cellulose optical film for polarizing plate of the present application does not have surface cracks after 200 cycles of cold and hot impact from -35℃ to 85℃; at the same time, after 300h of ultraviolet light irradiation in an environment with a temperature of 40℃ and a relative humidity of 80%, the yellow index change rate is 0.6%, and the tensile strength can still reach 111.9 MPa. DETAILED DESCRIPTION

[0021] In order to have a clearer understanding of the technical features, objectives and effects of the present application, the specific embodiments of the present application will now be described. It should be noted that the following detailed description is exemplary and is intended to provide further description of the present application. Unless otherwise specified, all technical and scientific terms used in the present application have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0022] It is to be noted that the terms used herein are merely for describing specific embodiments and are not intended to limit exemplary embodiments according to the present application. As used herein, "first", "second", and the like are used to distinguish similar objects, not to describe a particular sequential or chronological order, and furthermore, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it means that there are features, steps, operations, devices, components, and / or combinations thereof.

[0023] In order to solve the above technical problems, the present application provides a preparation method of a triacetate cellulose optical film for polarizing plate, comprising the following steps: graft modification, purification treatment, preparation of casting solution, film formation and curing. The graft modification method is as follows: in an inert gas environment, triacetate cellulose is put into a solvent, and after swelling by heating, 2-cyclohexyl acrylate is dropped in the presence of zinc triflate; after dropping is completed, 60-65℃ is kept for stirring, and a graft modification product is obtained. The graft modification product is purified to obtain modified triacetate cellulose. The preparation method of the casting solution is as follows: polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333, anhydrous ethanol, ethylene dichloride, and modified triacetate cellulose are uniformly mixed, and then filtered and defoamed to prepare the casting solution. The film formation and curing method is as follows: after coating with the casting solution, drying and ultraviolet curing are performed to prepare the triacetate cellulose optical film for polarizing plate.

[0024] In the present application, after triacetate cellulose is swelled by heating in a solvent, the graft reaction of triacetate cellulose and 2-cyclohexyl acrylate is carried out under the catalysis of zinc triflate to obtain a graft modification product, and the graft modification product is purified to obtain modified triacetate cellulose (2-cyclohexyl acrylate graft modified triacetate cellulose). The 2-cyclohexyl acrylate is grafted onto the molecular chain of triacetate cellulose and expanded around the molecular chain of triacetate cellulose, and the hydrophobic cyclohexyl group blocks the penetration of external small molecules such as water molecules into the material, especially protects the easily hydrolyzed part (acetyl group) of triacetate cellulose from contacting with water molecules to avoid hydrolysis, and improves the hydrophobicity of triacetate cellulose and reduces water molecule adsorption.

[0025] Further, in the preparation of the casting solution, polyethylene glycol diacrylate and dipentaerythritol hexaacrylate which have high compatibility with the modified triacetate cellulose and do not affect the optical performance are selected as the blending modification monomers, and a plasticizer, a photoinitiator, a leveling agent and a cosolvent are used to prepare the casting solution; the casting solution is cured to obtain the triacetate cellulose optical film for a polarizing sheet. The polyethylene glycol diacrylate and the dipentaerythritol hexaacrylate are filled in the molecular chain gap of the modified triacetate cellulose, and after drying and ultraviolet curing, a dense interpenetrating crosslinked network is formed to further limit the possible diffusion of water molecules and bind the movement of the molecular chain of the modified triacetate cellulose, thereby further improving the hydrolysis resistance, effectively improving the high temperature and high humidity stability and overall stability of the triacetate cellulose optical film, and optimizing the optical performance.

[0026] Further, the monomer (i.e. 2-cyclohexyl acrylate) with rigid ring grafted to the modified triacetate cellulose in the casting solution improves the basic hardness of the optical film and optimizes the toughness; the flexible node effect of the flexible monomer polyethylene glycol diacrylate, combined with the plasticizer, balances the brittleness of the optical film and further improves the toughness of the optical film; and the photopolymerization monomer dipentaerythritol hexaacrylate, combined with other raw materials under the action of the photoinitiator 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, forms an interpenetrating crosslinked network to balance and optimize the hardness, toughness and wear resistance of the optical film, and further improve the overall stability of the triacetate cellulose optical film.

[0027] In the embodiment of the present application, preferably, in the graft modification, the weight of 2-cyclohexyl acrylate added is 10-10.5% of the weight of triacetate cellulose. The weight of zinc triflate added is 4-4.2% of the weight of triacetate cellulose.

[0028] In the embodiment of the present application, preferably, in the graft modification, the dropping rate of 2-cyclohexyl acrylate is 0.15-0.25 mL / min. The temperature is controlled at 60-65℃ during the dropping process of 2-cyclohexyl acrylate. After the dropping of 2-cyclohexyl acrylate is completed, the temperature is kept at 60-65℃ for 5-5.5 h.

[0029] In the embodiment of the present application, preferably, in the graft modification, the temperature for swelling is 48-52℃, and the swelling time is 2-2.5 h. The solvent is dimethyl sulfoxide; and the weight of the solvent is 13-15 times of the weight of triacetate cellulose.

[0030] In the embodiment of the present application, preferably, in the preparation of the casting solution, the weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333, anhydrous ethanol, dichloromethane, and modified cellulose triacetate is 1.5-1.6:0.85-0.95:0.3-0.35:0.4-0.45:0.08-0.11:2-2.5:58-60:10-10.5.

[0031] In the embodiment of the present application, preferably, in the film forming and curing, the coating film speed is controlled to be 0.2-0.3 m / min, the coating film thickness is 160-180 μm, and after the casting solution is coated on the surface of the substrate at 27-29 ℃ to form a wet film, the wet film is dried; The ultraviolet curing is carried out by irradiating and curing for 9-11 s under the nitrogen atmosphere environment by using the ultraviolet light with a wavelength of 365 nm and an intensity of 31-33 mW / cm 2 .

[0032] In the embodiment of the present application, further, in the film forming and curing, the drying is carried out by using three-stage drying. The temperature of the first-stage drying is 50-55 ℃, and the time is 3-4 min. The temperature of the second-stage drying is 70-75 ℃, and the time is 5-6 min. The temperature of the third-stage drying is 60-65 ℃, and the time is 2-3 min.

[0033] In the embodiment of the present application, further, the method for the purification treatment is as follows: triethylamine is used to quench the zinc triflate in the grafting modifier, and then the grafting modifier is put into the methanol aqueous solution, and after the precipitation by shearing, the solid substance is separated; the solid substance is washed, and then put into the anhydrous acetone, and after the ultrasonic treatment, the purified liquid is obtained; the purified liquid is put into the anhydrous methanol for precipitation, and then the solid substance is separated; the solid substance is dried and crushed to obtain the modified cellulose triacetate.

[0034] In the embodiment of the present application, after the triethylamine is used to quench the zinc triflate in the grafting modifier, the grafting modifier is put into the methanol aqueous solution for precipitation, and the precipitate is dispersed by shearing, so that the impurities in the solid substance are fully contacted and removed; then the solid substance is re-dissolved in the acetone, and further washed by ultrasonic treatment to remove the impurities in the solid substance, and then put into the anhydrous methanol for precipitation again; through the two times of precipitation and purification, the modified cellulose triacetate is effectively purified, and the modified cellulose triacetate meeting the subsequent application requirements is obtained.

[0035] In the embodiment of the present application, preferably, in the purification treatment, the weight of the triethylamine is 1.6-1.8 times of the weight of the zinc triflate used in the grafting modification. The methanol volume concentration of the methanol aqueous solution is 80-85%. The shearing precipitation rotation speed is 4000-5000 rpm, and the time is 8-12 min.

[0036] The application also provides a tri-acetyl cellulose optical film for a polarizing plate prepared by the above preparation method.

[0037] Specifically, the preparation method of the tri-acetyl cellulose optical film for a polarizing plate comprises the following steps: graft modification, purification treatment, preparation of a casting solution, film formation and solidification.

[0038] The method of graft modification is as follows: dimethyl sulfoxide (DMSO) is introduced into a clean and dry reaction kettle, then tri-acetyl cellulose is added into 13-15 times the weight of dimethyl sulfoxide (DMSO), under a nitrogen atmosphere, 50-80 rpm stirring is performed to heat to 48-52℃, after 2-2.5 h of heat preservation and stirring swelling, stirring is performed to cool to 40-42℃, heat preservation is continued, and zinc triflate is added, after 20-30 min of stirring, 2-cyclohexyl acrylate is added dropwise at a drop rate of 0.15-0.25 mL / min, and the system temperature is controlled to be 60-65℃ during the drop process; after the drop of 2-cyclohexyl acrylate is completed, 60-65℃ heat preservation and stirring are performed for 5-5.5 h to obtain a graft modification product.

[0039] The added weight of 2-cyclohexyl acrylate is 10-10.5% of the weight of tri-acetyl cellulose; The added weight of zinc triflate is 4-4.2% of the weight of tri-acetyl cellulose.

[0040] The method of purification treatment is as follows: triethylamine is added into the graft modification product, after stirring and quenching of zinc triflate, the graft modification product is added into 3-3.5 times the volume of a methanol aqueous solution (methanol volume concentration 80-85%), 4000-5000 rpm shearing is performed for 8-12 min, and the solid product is collected by filtration; the solid product is washed by anhydrous methanol, then is added into 5-6 times the weight of anhydrous acetone, the ultrasonic power is controlled to be 35-40 kHz, the ultrasonic power is 300-350 W, and ultrasonic treatment is performed for 20-30 min to obtain a purified liquid; the purified liquid is added into 3-3.5 times the volume of anhydrous methanol for precipitation, the solid product is collected by filtration, and the solid product is dried and crushed to obtain purified 2-cyclohexyl acrylate graft modified tri-acetyl cellulose, i.e., modified tri-acetyl cellulose.

[0041] The added amount of triethylamine is 1.6-1.8 times the weight of zinc triflate used in the graft modification.

[0042] The method for preparing the casting solution is as follows: polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, anhydrous ethanol are put into dichloromethane, stirred at 100-150 rpm for 1-2 h, then modified cellulose triacetate is continuously added and stirred for 2-3 h; after removing impurities by filtration, standing and defoaming for 6-7 h, the casting solution is prepared.

[0043] The weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, anhydrous ethanol, dichloromethane and modified cellulose triacetate is 1.5-1.6:0.85-0.95:0.3-0.35:0.4-0.45:0.08-0.11:2-2.5:58-60:10-10.5.

[0044] The method for film forming and curing is as follows: the casting solution is introduced into an automatic film coater, the film coating speed is controlled to be 0.2-0.3 m / min, the film coating thickness is 160-180 μm, after the casting solution is coated onto the surface of a substrate at 27-29℃ to form a wet film, three-stage drying treatment is performed; specifically, first-stage drying is performed at 50-55℃ for 3-4 min, then second-stage drying is performed at 70-75℃ for 5-6 min, and then third-stage drying is performed at 60-65℃ for 2-3 min; then, under a nitrogen atmosphere, the wet film is irradiated and cured by ultraviolet light with a wavelength of 365 nm and an intensity of 31-33 mW / cm 2 The polarizing sheet cellulose triacetate optical film is prepared.

[0045] The application will be further described below in combination with some specific examples.

[0046] Example 1 The application provides a preparation method of a polarizing sheet cellulose triacetate optical film, specifically: 1. Graft modification Dimethyl sulfoxide DMSO is introduced into a clean and anhydrous reaction kettle, then cellulose triacetate is added into 13 times the weight of dimethyl sulfoxide DMSO, stirring at 50 rpm, heating to 48℃, and then keeping the temperature and stirring for 2 h, then cooling to 40℃, keeping the temperature and continuously adding zinc triflate, and stirring for 20 min; under stirring, 2-cyclohexyl acrylate is added dropwise at a dropping rate of 0.15 mL / min, and the system temperature is controlled to be 60℃ during the dropping process; after the dropping of 2-cyclohexyl acrylate is completed, 5 h of 60℃ temperature and stirring are performed, and the graft modification product is obtained.

[0047] The added weight of 2-cyclohexyl acrylate is 10% of the weight of triacetate cellulose.

[0048] The added weight of zinc triflate is 4% of the weight of triacetate cellulose.

[0049] 2, purification treatment The triethylamine is added to the grafting modifier, after stirring and quenching the zinc triflate, it is added to 3 times the volume of a methanol aqueous solution (80% methanol by volume), sheared at 4000 rpm for 12 min, and the solid is collected by filtration; the solid is washed with anhydrous methanol, then added to 5 times the weight of anhydrous acetone, the ultrasonic power is controlled at 40 kHz and 300 W, and the purified liquid is obtained after ultrasonic treatment for 25 min; the purified liquid is precipitated in 3 times the volume of anhydrous methanol, the solid is collected by filtration, dried and crushed to obtain the purified 2-cyclohexyl acrylate grafting modified triacetate cellulose, i.e. the modified triacetate cellulose.

[0050] The added amount of triethylamine is 1.6 times the weight of the zinc triflate used in the grafting modification.

[0051] 3, preparation of casting solution The polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, and anhydrous ethanol are added to dichloromethane, stirred at 100 rpm for 1 h, and then the modified triacetate cellulose is continuously added and stirred for 2 h; after removing the impurities by filtration, the casting solution is prepared by standing for 6 h to remove bubbles.

[0052] The weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, anhydrous ethanol, dichloromethane, and modified triacetate cellulose is 1.5:0.85:0.3:0.4:0.08:2:58:10.

[0053] 4, film formation and curing The casting solution is introduced into an automatic film coater, the film coating speed is controlled at 0.25 m / min, and the film coating thickness is 180 μm; after the casting solution is coated onto the surface of the substrate at 28℃ to form a wet film, three-stage drying treatment is performed; specifically, the first-stage drying is performed at 50℃ for 4 min, then the second-stage drying is performed at 70℃ for 6 min, and then the third-stage drying is performed at 60℃ for 3 min; then, under a nitrogen atmosphere, the wet film is irradiated and cured by ultraviolet light with a wavelength of 365 nm and an intensity of 31 mW / cm 2 The polarizing sheet is obtained by irradiating and curing the wet film under a nitrogen atmosphere by ultraviolet light with a wavelength of 365 nm and an intensity of 31 mW / cm

[0054] The embodiment also provides the tri-acetyl cellulose optical film for a polarizing plate prepared by the above method.

[0055] Embodiment 2 The embodiment provides a preparation method of a tri-acetyl cellulose optical film for a polarizing plate, in particular to the following steps. 1. Graft modification Dimethyl sulfoxide (DMSO) is introduced into a clean and dry reaction kettle, and then tri-acetyl cellulose is added into 14 times the weight of dimethyl sulfoxide (DMSO). In a nitrogen atmosphere, the temperature is raised to 50°C under 70 rpm stirring, and then the temperature is kept and stirred for 2.2 hours. After that, the temperature is cooled to 41°C, and then zinc triflate is continuously added. After stirring for 25 minutes, 2-cyclohexyl acrylate is added at a dropping rate of 0.2 mL / min under stirring, and the temperature of the system is controlled at 62°C during the dropping process. After the dropping of 2-cyclohexyl acrylate is completed, the temperature is kept at 62°C for 5.3 hours under stirring, and then the graft modification product is obtained.

[0056] The added weight of 2-cyclohexyl acrylate is 10.3% of the weight of tri-acetyl cellulose.

[0057] The added weight of zinc triflate is 4.1% of the weight of tri-acetyl cellulose.

[0058] 2. Purification treatment After triethylamine is added into the graft modification product and the zinc triflate is quenched by stirring, the graft modification product is added into 3.2 times the volume of a methanol aqueous solution (the volume concentration of methanol is 82%) and sheared at 4500 rpm for 10 minutes. The solid product is collected by filtration. After the solid product is washed by anhydrous methanol, the solid product is added into 5.5 times the weight of anhydrous acetone. The ultrasonic power is controlled at 40 kHz and 300 W. The purified liquid is obtained by ultrasonic treatment for 25 minutes. After the purified liquid is added into 3.2 times the volume of anhydrous methanol for precipitation, the solid product is collected by filtration. The solid product is dried and crushed to obtain the purified 2-cyclohexyl acrylate graft modification tri-acetyl cellulose, i.e., the modified tri-acetyl cellulose.

[0059] The added amount of triethylamine is 1.7 times the weight of zinc triflate used in the graft modification.

[0060] 3. Preparation of casting solution Polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333 and anhydrous ethanol are added into dichloromethane. After stirring at 120 rpm for 1.5 hours, the modified tri-acetyl cellulose is continuously added and stirred for 2.5 hours. After removing the impurities by filtration, the casting solution is prepared by standing for 6.5 hours for defoaming.

[0061] The weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, leveling agent BYK-333, anhydrous ethanol, dichloromethane, and modified cellulose triacetate is 1.57:0.91:0.33:0.42:0.1:2.3:59:10.3.

[0062] 4. Film formation and curing The casting solution was introduced into an automatic film coater, and the film coating speed was controlled at 0.25 m / min, and the film coating thickness was 180 μm. After the casting solution was coated onto the surface of the substrate at 28°C to form a wet film, three-stage drying treatment was performed. Specifically, first-stage drying was performed at a temperature of 52°C for 3.5 min, then second-stage drying was performed at a temperature of 72°C for 5.5 min, and then third-stage drying was performed at a temperature of 62°C for 2.5 min. Then, under a nitrogen atmosphere, the wet film was irradiated with ultraviolet light having a wavelength of 365 nm and an intensity of 32 mW / cm 2 to cure for 10 s, and then cooled to obtain a cellulose triacetate optical film for a polarizing plate.

[0063] The present embodiment also provides a cellulose triacetate optical film for a polarizing plate prepared by the above method.

[0064] Example 3 The present embodiment provides a method for preparing a cellulose triacetate optical film for a polarizing plate, specifically: 1. Graft modification Dimethyl sulfoxide (DMSO) was introduced into a clean and anhydrous reaction kettle, and then cellulose triacetate was added to 15 times the weight of dimethyl sulfoxide (DMSO). Under a nitrogen atmosphere, the system was stirred at 80 rpm and heated to 52°C, and then the system was kept at the temperature and stirred for 2.5 h to swell. After cooling to 42°C, the system was kept at the temperature and zinc triflate was continuously added. After stirring for 30 min, 2-cyclohexyl acrylate was added dropwise at a rate of 0.25 mL / min, and the temperature of the system was controlled at 65°C during the dropping process. After the dropping of 2-cyclohexyl acrylate was completed, the system was kept at 65°C and stirred for 5.5 h to obtain a graft modification product.

[0065] The weight of 2-cyclohexyl acrylate added was 10.5% of the weight of cellulose triacetate.

[0066] The weight of zinc triflate added was 4.2% of the weight of cellulose triacetate.

[0067] 2. Purification treatment The triethylamine is put into the graft modification, after stirring to quench the zinc triflate, and then put into 3.5 times volume of methanol aqueous solution (volume concentration of methanol is 85%), sheared at 5000 rpm for 8 min, and then the solid is collected by filtration; the solid is washed by anhydrous methanol, and then put into 6 times weight of anhydrous acetone, the ultrasonic power is controlled at 40 kHz and 300 W, and the purified liquid is obtained by ultrasonic treatment for 25 min; the purified liquid is precipitated by putting into 3.5 times volume of anhydrous methanol, and then the solid is collected by filtration, and the purified 2-cyclohexyl acrylate graft modified cellulose triacetate, i.e. modified cellulose triacetate, is obtained by drying and crushing the solid.

[0068] The amount of triethylamine is 1.8 times of the weight of zinc triflate used in the graft modification.

[0069] 3. Preparation of casting solution The polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, and anhydrous ethanol are put into dichloromethane, stirred at 150 rpm for 2 h, and then the modified cellulose triacetate is continuously put in and stirred for 3 h; after removing the impurities by filtration, the casting solution is prepared by standing for 7 h to remove bubbles.

[0070] The weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, anhydrous ethanol, dichloromethane, and modified cellulose triacetate is 1.6:0.95:0.35:0.45:0.11:2.5:60:10.5.

[0071] 4. Film forming and curing The casting solution is introduced into an automatic film coater, the film coating speed is controlled at 0.25 m / min, and the film coating thickness is 180 μm; after the casting solution is coated on the surface of the substrate at 28℃ to form a wet film, three-stage drying treatment is performed; specifically, the first stage drying is performed at 55℃ for 3 min, then the second stage drying is performed at 75℃ for 5 min, and then the third stage drying is performed at 65℃ for 2 min; then the wet film is cured by irradiation with ultraviolet light with a wavelength of 365 nm and an intensity of 33 mW / cm 2 for 9 s in a nitrogen atmosphere, and then cooled to obtain the cellulose triacetate optical film for polarizing plate.

[0072] The cellulose triacetate optical film for polarizing plate prepared by the above method is also provided.

[0073] Comparative Example 1 The technical solution of Example 2 is adopted, except that the graft modification and purification treatment steps are omitted, and unmodified cellulose triacetate is used instead of modified cellulose triacetate in the preparation of the casting solution step.

[0074] Comparative Example 2 The technical solution of Example 2 is adopted, except that 1) in the preparation of the casting solution step, the addition of polyethylene glycol dipolyacrylate, dipentaerythritol hexaacrylate, and 2,4,6-trimethylbenzoylphenyl ethyl phosphonate is omitted; and 2) in the film-forming and curing step, the related operations of ultraviolet light curing are omitted.

[0075] The light transmittance, haze value, pencil hardness grade, tensile strength, elongation at break, Young's modulus, and breaking work of the cellulose triacetate optical films for polarizing sheets of Examples 1-3 and Comparative Examples 1-2 were detected. The haze value refers to the percentage of scattered light that deviates more than 2.5° from the direction of incident light. The specific results are shown in the following table:

[0076] Further, the cellulose triacetate optical films for polarizing sheets of Example 2 and Comparative Examples 1-2 were placed in a thermostat with a temperature of 85°C and a relative humidity of 85%, and after standing and aging for 500h, the yellowness index of each optical film before and after standing and aging was detected using the related method of ASTM E313, and the yellowness index change rate was calculated. The light transmittance, haze value, pencil hardness, and tensile strength of each cellulose triacetate optical film for polarizing sheets were detected, and the increase rate of the haze value compared to before standing and aging was calculated. The specific results are shown in the following table:

[0077] Further, the cellulose triacetate optical films for polarizing sheets of Example 2 and Comparative Examples 1-2 were placed in a cold and hot impact test chamber, and after cooling each optical film to -35°C, maintaining for 30min, and then heating to 85°C, maintaining for 30min, the aforementioned cooling-heating process was taken as one cold and hot impact process, and after repeating 200 times of cold and hot impact, whether surface cracks appeared in each optical film was observed. The specific results are shown in the following table:

[0078] Further, the cellulose triacetate optical films for polarizing sheets of Example 2 and Comparative Examples 1-2 were placed in a thermostat with a temperature of 40°C and a relative humidity of 80%, and after standing and ultraviolet aging for 300h under the condition of continuous irradiation of ultraviolet light with a wavelength of 340nm, the yellowness index of each optical film before and after ultraviolet aging was detected using the related method of ASTM E313, and the yellowness index change rate was calculated. The tensile strength of each cellulose triacetate optical film for polarizing sheets was detected. The specific results are shown in the following table:

[0079] It can be seen that, by using a solvent to swell triacetate cellulose at an elevated temperature, then grafting triacetate cellulose with 2-cyclohexyl acrylate under the catalysis of zinc triflate, a grafting modification product is obtained, and the grafting modification product is purified to obtain modified triacetate cellulose (i.e. 2-cyclohexyl acrylate grafting modified triacetate cellulose); wherein 2-cyclohexyl acrylate is grafted onto the molecular chain of triacetate cellulose and expands around the molecular chain of triacetate cellulose, and the hydrophobic cyclohexyl group blocks the penetration of external small molecules such as water molecules into the material, especially protects the hydrolyzable part (acetyl group) of triacetate cellulose from contacting with water molecules to avoid hydrolysis; and improves the hydrophobicity of triacetate cellulose and reduces water molecule adsorption.

[0080] Further, in the preparation of the casting solution, polyethylene glycol diacrylate and dipentaerythritol hexaacrylate, which have high compatibility with the modified triacetate cellulose and do not affect the optical performance, are selected as the blending modification monomers, and a plasticizer, a photoinitiator, a leveling agent and a cosolvent are used to prepare the casting solution; the casting solution is cured to obtain a triacetate cellulose optical film for a polarizing sheet. Among them, the polyethylene glycol diacrylate and the dipentaerythritol hexaacrylate are filled in the molecular chain gap of the modified triacetate cellulose, and after drying and ultraviolet curing, a dense interpenetrating crosslinking network is formed to further limit the possible diffusion of water molecules and bind the movement of the modified triacetate cellulose molecular chain, further improve its hydrolysis resistance, effectively improve the high temperature and high humidity stability and overall stability of the triacetate cellulose optical film, and optimize its optical performance.

[0081] Further, the monomer (i.e. 2-cyclohexyl acrylate) grafted with the modified triacetate cellulose in the casting solution improves the basic hardness of the optical film and optimizes its toughness; the flexible node effect of the flexible monomer polyethylene glycol diacrylate balances the brittleness of the optical film, further improves the toughness of the optical film; and the photopolymerization monomer dipentaerythritol hexaacrylate forms an interpenetrating crosslinking network with other raw materials under the action of the photoinitiator 2,4,6-trimethylbenzoyl phenyl phosphonic acid ethyl ester, balances and optimizes the hardness, toughness and wear resistance of the optical film, and further improves the overall stability of the triacetate cellulose optical film.

[0082] It can be seen from the comparative example 1 that, the graft modification and purification treatment steps are omitted, and the unmodified triacetate cellulose is used to replace the modified triacetate cellulose for preparing the optical film. Due to the lack of the blocking and hydrophobic effect of the modified triacetate cellulose on the water molecules, the lack of the optimization effect of the modified triacetate cellulose on the hardness and toughness of the optical film, and the lack of the cooperation of the modified triacetate cellulose and the blending modification monomers (polyethylene glycol diacrylate and dipentaerythritol hexaacrylate), the optical performance, hardness and mechanical properties of the prepared optical film are obviously deteriorated, and the overall stability of the optical film is reduced. Specifically, after the optical film is subjected to high temperature and high humidity aging, multiple cold and hot impact or ultraviolet aging, each index is obviously deteriorated.

[0083] It can be seen from the comparative example 2 that, the preparation of the casting solution step is omitted, the addition of polyethylene glycol diacrylate and dipentaerythritol hexaacrylate is omitted, and the ultraviolet curing is omitted. Due to the lack of the improvement effect of the aforementioned blending modification monomers on the toughness, hardness and wear resistance of the optical film, and the lack of the improvement effect of the cooperation of the blending modification monomers and the modified triacetate cellulose on the overall stability of the optical film, the optical performance, hardness and mechanical properties of the prepared optical film are deteriorated to a certain extent, and the overall stability of the optical film is reduced. Specifically, after the optical film is subjected to high temperature and high humidity aging, multiple cold and hot impact or ultraviolet aging, each index is deteriorated to a certain extent.

[0084] Unless otherwise specified, the percentages used in the present application are mass percentages.

[0085] Finally, it should be noted that: the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a triacetylcellulose optical film for a polarizing plate, characterized by, It comprises the following steps: graft modification, purification treatment, preparation of casting solution, film forming and curing. The method of graft modification is as follows: in an inert gas environment, triacetate cellulose is put into a solvent, and after swelling by heating, 2-cyclohexyl acrylate is added dropwise in the presence of zinc triflate; after the dropwise addition is completed, 60-65℃ is maintained for stirring to obtain the graft modification product. The graft modification product is subjected to purification treatment to obtain modified triacetate cellulose. The method of preparing the casting solution is as follows: polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, absolute ethanol, dichloromethane, and modified triacetate cellulose are uniformly mixed, filtered and defoamed to prepare the casting solution. The method of film forming and curing is as follows: after coating with the casting solution, drying and ultraviolet curing are performed to prepare the triacetate cellulose optical film for polarizing sheet.

2. The method for producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the graft modification, the weight of 2-cyclohexyl acrylate added is 10-10.5% of the weight of triacetate cellulose. The weight of zinc triflate added is 4-4.2% of the weight of triacetate cellulose.

3. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the graft modification, the dropwise addition rate of 2-cyclohexyl acrylate is 0.15-0.25 mL / min. The temperature is controlled at 60-65℃ during the dropwise addition of 2-cyclohexyl acrylate. After the dropwise addition of 2-cyclohexyl acrylate is completed, 60-65℃ is maintained for stirring for 5-5.5 h.

4. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the graft modification, the swelling temperature is 48-52℃, and the swelling time is 2-2.5 h. The solvent is dimethyl sulfoxide, and the weight of the solvent is 13-15 times the weight of triacetate cellulose.

5. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the preparation of the casting solution, the weight ratio of polyethylene glycol diacrylate, dipentaerythritol hexaacrylate, triethyl citrate, 2,4,6-trimethylbenzoyl phenyl ethyl phosphonate, leveling agent BYK-333, absolute ethanol, dichloromethane, and modified triacetate cellulose is 1.5-1.6:0.85-0.95:0.3-0.35:0.4-0.45:0.08-0.11:2-2.5:58-60:10-10.

5.

6. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the film forming and curing, the coating speed is controlled at 0.2-0.3 m / min, the coating thickness is 160-180 μm, and after the casting solution is coated onto the surface of the substrate at 27-29℃ to form a wet film, drying is performed. The UV light curing was carried out under nitrogen atmosphere, using UV light with wavelength of 365 nm and intensity of 31-33 mW / cm 2 for 9-11 s.

7. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, In the film forming and curing, drying is performed in three stages. The first stage drying temperature is 50-55℃, and the time is 3-4 min. The second stage drying temperature is 70-75℃, and the time is 5-6 min. The third stage drying temperature is 60-65℃, and the time is 2-3 min.

8. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 1, characterized by, The method of purification treatment is as follows: triethylamine is used to quench the zinc triflate in the graft modification product, and then the product is put into a methanol aqueous solution, and the solid product is separated after shearing; the solid product is washed, put into anhydrous acetone, and subjected to ultrasonic treatment to obtain a purified liquid; the purified liquid is precipitated in anhydrous methanol, and the solid product is separated; the solid product is dried and crushed to obtain modified triacetate cellulose.

9. The method of producing a triacetylcellulose optical film for a polarizing plate according to claim 8, characterized by, The weight of the triethylamine is 1.6-1.8 times of the weight of the zinc triflate used in the graft modification in the purification treatment; The volume concentration of methanol in the methanol aqueous solution is 80-85%. The shear precipitation rotation speed is 4000-5000 rpm, and the time is 8-12 min.

10. A cellulose triacetate optical film for polarizers, characterized in that, The preparation method is prepared by any one of claims 1-9.

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