PVA-based film, preparation method thereof and polaroid

Abstract

The invention relates to the technical field of optical film processing, and discloses a preparation method of a PVA-based film, which comprises the following steps: casting a film-making stock solution containing polyvinyl alcohol onto the surface of a film-forming substrate for drying and film-forming, and stripping a cast film obtained by drying and film-forming from the film-forming substrate to obtain the PVA-based film, wherein the film-forming base material is a steel belt or a roller with the roughness Ra being less than or equal to 0.05 mu m; the drying and film forming conditions are as follows: the temperature is 75-87 DEG C, and the time is 1-25 minutes. Compared with a conventional method, the PVA-based thin film prepared by the method has the advantages that the state difference of the upper surface, the lower surface and the core layer is small, the overall optical uniformity of the thin film is remarkably improved, a polaroid with better optical uniformity can be prepared based on the PVA-based thin film, and the performance requirement of a liquid crystal display in a large-picture application scene is better met.

Description

Technical Field

[0001] The present invention relates to the technical field of optical film processing, and particularly relates to a PVA-based film, a preparation method thereof, and a polarizer. Background Art

[0002] A polarizer with light transmission and light shielding functions and a liquid crystal with a light switching function are both important components of a liquid crystal display (LCD). With the development, the application fields of liquid crystal displays have expanded from small instruments such as calculators and watches in the initial stage to fields such as notebook computers, liquid crystal monitors, liquid crystal color projectors, measuring instruments for indoor and outdoor use, mobile phones, in-vehicle navigation systems, and liquid crystal TVs. In particular, liquid crystal monitors and liquid crystal TVs are developing towards larger screen sizes. It is particularly important to find a polarizer and a PVA-based optical material with good optical uniformity in a large screen.

[0003] The industrial manufacturing process of a polarizer usually involves pulling out a rolled long strip of PVA-based film, washing it with water, swelling it, dyeing it with iodine or a dichroic dye, then uniaxially stretching it and fixing the dyed and stretched PVA-based film with a boride to obtain a PVA polarizing film. Finally, a protective film such as a triacetyl cellulose film is adhered to one or both sides of the PVA polarizing film to manufacture a polarizer. In order to obtain a polarizer with good optical uniformity, it is necessary to uniformly dye the PVA-based film with a dichroic dye and uniformly adhere the protective film. Among them, the optical uniformity of the used PVA-based film has a great influence on the optical uniformity of the polarizer. The uniformity of the PVA-based film is greatly reduced due to differences in the quality of the film-forming stock solution, the states of the upper and lower surfaces and the core layer of the film during molding. Especially when manufacturing a roll of PVA-based film with a large screen (large width, extra length), the state differences between the upper and lower surfaces and the core layer are more prominent.

[0004] CN102816339A discloses a polyvinyl alcohol-based film, and studies the distribution of plasticizers in the film-forming stock solution for preparing the PVA-based film. By controlling the maximum content difference (Ry) of the plasticizer in the TD direction of the film relative to the average value of the plasticizer content contained in the film to be 2% or less, and in the plasticizer content variation curve in the TD direction of the film, the average interval (Sm) of the variation amount concavity and convexity composed of one peak exceeding the average value of the plasticizer content and one valley adjacent to the peak in the TD direction is 5 cm or more, the optical color spots of the polarizer prepared from the PVA-based film are improved. This method has a certain effect on improving the uniform distribution of plasticizers and thus reducing optical color spots, but has no effect on the state differences between the upper and lower surfaces and the core layer of the PVA-based film.

[0005] Therefore, there is an urgent need to provide a method for effectively reducing the state difference between the upper and lower surfaces and the core layer of the PVA-based film, so as to improve the optical uniformity of the PVA-based film. Summary of the Invention

[0006] Aiming at the problem that the existing preparation method of PVA-based film is difficult to effectively control the state difference between the upper and lower surfaces and the core layer of the film, which in turn leads to poor optical uniformity of the PVA-based film, the present invention provides a PVA-based film, its preparation method and a polarizer.

[0007] To achieve the above object, the first aspect of the present invention provides a method for preparing a PVA-based film, comprising:

[0008] Casting a film-forming stock solution containing polyvinyl alcohol onto the surface of a film-forming substrate and drying it to form a film, and peeling the cast film obtained by the drying and film-forming from the film-forming substrate to obtain a PVA-based film;

[0009] wherein, the film-forming substrate is a steel belt or a roller with a roughness Ra≤0.05μm;

[0010] The conditions for the drying and film-forming include: the temperature is 75-87°C, and the time is 1-25 min.

[0011] The second aspect of the present invention provides a PVA-based film prepared by the method described in the first aspect above.

[0012] The third aspect of the present invention provides a polarizer containing the PVA-based film described in the second aspect above.

[0013] The method for preparing a PVA-based film provided by the present invention, on the basis of the conventional solution casting film-making process, by improving the material and surface condition of the film-forming substrate, and combining with controlling the drying parameters during the drying and shaping of the film, enables the upper and lower surfaces of the film to have more consistent structures and appearance morphologies, and significantly reduces the state difference between the upper and lower surfaces and the core layer of the film, making the optical properties of the obtained PVA-based film uniform. Further, by controlling the peeling process parameters of the film after shaping, the surface smoothness (roughness) of the film is made more consistent, reducing optical distortion, and thus making the surface of the PVA-based film smoother, more transparent, and more excellent in optical uniformity. Compared with the conventional method, the state difference between the upper and lower surfaces and the core layer of the PVA-based film prepared by the method provided by the present invention is small, and the overall optical uniformity of the film is significantly improved. Using this PVA-based film, a polarizer with better optical uniformity can be prepared, which better meets the performance requirements of liquid crystal displays in large-screen application scenarios. Detailed Embodiments

[0014] The endpoints and any values in the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0015] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0016] The first aspect of the present invention provides a method for preparing a PVA-based film, including:

[0017] Casting a film-forming stock solution containing polyvinyl alcohol onto the surface of a film-forming substrate and drying it to form a film, and peeling the cast film obtained by the drying and film-forming from the film-forming substrate to obtain a PVA-based film;

[0018] Wherein, the film-forming substrate is a steel belt or a roller with a roughness Ra≤0.05μm;

[0019] The conditions for the drying and film-forming include: the temperature is 75-87°C, and the time is 1-25 min.

[0020] In the existing PVA-based film preparation process, by adding additives to the film-forming stock solution to improve the performance of the film-forming stock solution, it can play a role in improving the optical properties of the PVA-based film to a certain extent. For example, CN102816339A has a certain effect on reducing optical color spots in the film by improving the distribution of plasticizers in the film-forming stock solution, but it fails to fundamentally solve the problem of large differences in the states of the upper and lower surfaces and the core layer of the film during the film-forming process, and it is difficult to significantly improve the optical uniformity of the PVA-based film. The inventors of the present invention found in the research on the solution casting film-forming process that by controlling the surface roughness Ra≤0.05μm of the film-forming substrate (steel belt or roller), it is more conducive to maintaining the same structure and appearance morphology between the surface of the film-forming stock solution in contact with the film-forming substrate (i.e., the lower surface of the film) and the upper surface of the film (i.e., the contact surface of the film-forming stock solution with air), thereby improving the optical uniformity of the upper and lower surfaces of the film. Further, by controlling the drying temperature during the drying and shaping of the film to 75-87°C and the drying time to 1-25 min, the molecular motion states of the upper and lower surfaces and the core layer of the film can be made more consistent during the drying process, and the drying degree of the film can be made more consistent, thereby significantly reducing the state differences between the upper and lower surfaces and the core layer of the film and achieving the improvement of the overall optical uniformity of the PVA-based film. In addition, by using the above drying temperature and drying time, the moisture content of the shaped film can be controlled within the range of 10-30%, which is conducive to the subsequent smooth peeling from the film-forming substrate.

[0021] According to the present invention, when the drying temperature during film drying and shaping is not within the range of 75-87°C, the state difference between the upper and lower surfaces and the core layer of the film obtained after drying and shaping becomes larger, which is manifested in the appearance of skin-core structure, bubbles and other defects between the core layer and the upper and lower surfaces of the film. In addition, drying and shaping outside the above temperature range will also cause other defects in the film. For example, when the temperature is too high and exceeds 87°C, a large amount of hot steam will not have time to escape, and it is easy to form aerosols on the film-forming substrate, resulting in a large number of point defects on the film surface. When the temperature is lower than 75°C, poor drying will cause difficulty in film peeling. When the drying time is too short and less than 1 minute, the drying effect is poor. When the drying time is longer than 25 minutes, it causes excessive drying, resulting in greater stress in the film, poor peeling or decreased film surface flatness.

[0022] According to the present invention, preferably, the steel belt and roller are made of mirror-finished stainless steel.

[0023] According to the present invention, preferably, the film-forming substrate is a steel belt or roller with a roughness Ra≤0.03 μm, which can make the structure and appearance of the upper and lower surfaces of the film more consistent, and further improve the optical uniformity of the prepared PVA base film.

[0024] According to the present invention, preferably, the drying film forming conditions include: temperature of 78-85°C and time of 1.5-5 minutes. The preferred drying film forming conditions can make the molecular motion states of the upper and lower surfaces of the film and the core layer more consistent during the drying process, further reduce the state difference between the upper and lower surfaces and the core layer of the prepared PVA base film, and further improve the optical uniformity.

[0025] According to the present invention, preferably, the surface temperature difference of the film-forming substrate is controlled to be ≤0.5°C, which is conducive to obtaining a PVA-based film with better optical uniformity. In particular, for a steel belt or roller with a width of ≥4m, a surface temperature difference of ≤0.5°C is more conducive to improving the optical uniformity of the PVA-based film. In the present invention, the surface temperature difference of the film-forming substrate refers to the difference between the highest temperature and the lowest temperature in each area on the surface of the film-forming substrate.

[0026] According to the present invention, the thickness of the cast film is not particularly limited. For example, the thickness of the cast film may be 30-75 μm.

[0027] According to the present invention, preferably, the peeling is performed by using a peeling roller.

[0028] According to the present invention, in the method for preparing the PVA-based film, the linear velocity of the peeling roller can be 2-12 m / min, and the linear velocity of the forming substrate can be 1.95-12.05 m / min. Further, control the linear velocity of the peeling roller: the linear velocity of the film-forming substrate is (0.998-1.002):1. By strictly controlling the above process parameters during the peeling process of the film after shaping, the surface smoothness (roughness) of the film is made consistent, no peeling marks are generated, no optical distortion occurs, and the surface of the obtained PVA-based film is smoother, more transparent, and has more excellent optical uniformity.

[0029] According to the present invention, in the method for preparing the PVA-based film, the definition of the film-forming stock solution is relatively broad and can be an aqueous solution containing polyvinyl alcohol used in various methods for preparing PVA films in the art. For example, it can be an aqueous solution with a polyvinyl alcohol concentration of 10-35 wt%, and the film-forming stock solution can contain additives.

[0030] According to the present invention, the polyvinyl alcohol in the film-forming stock solution can be unmodified polyvinyl alcohol or modified polyvinyl alcohol, that is, the polyvinyl alcohol can be prepared by alcoholysis, washing and purification, pulverization, and pressing of a polyvinyl ester resin obtained by solution polymerization of vinyl esters or a modified polyvinyl ester resin obtained by solution copolymerization of vinyl esters and other modified monomers. Among them, the vinyl resin can be selected from at least one of vinyl acetate, vinyl formate, vinyl laurate, vinyl propionate, vinyl butyrate, trimethylacetic acid vinyl ester, versatic acid vinyl ester, vinyl stearate, and vinyl benzoate, preferably vinyl acetate; monomers copolymerizable with vinyl esters, for example, can be olefins, acrylic acids, methacrylates, acrylamide derivatives, vinyl ethers, nitriles, vinyl halides, allyl compounds, unsaturated carboxylic acids and their derivatives, vinylsilyl compounds, etc. Preferably, the degree of polymerization of polyvinyl alcohol in the film-forming stock solution containing polyvinyl alcohol is 1700-3500, the degree of alcoholysis ≥99.5 mol%, and the weight average molecular weight is 75000-155000 g / mol.

[0031] According to the present invention, preferably, the film-forming stock solution containing polyvinyl alcohol can also contain a plasticizer, a surfactant, and an optional antioxidant to further improve the performance of the film-forming stock solution and be more conducive to film formation.

[0032] According to the present invention, in the film-forming stock solution, the plasticizer can improve the extensibility of the film. The present invention does not particularly limit the plasticizer, and conventional selections in the art can be used. For example, it can be a polyhydric alcohol, and further preferably at least one of glycerol, diglycerol, triglycerol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, polyalkylene glycols, and trimethylolpropane, and more preferably glycerol.

[0033] According to the present invention, in the film-forming stock solution, the surfactant has the function of inhibiting adhesion during film curling. The present invention has no particular limitation on the surfactant, and conventional selections in the art can be adopted. For example, it can be selected from anionic surfactants and / or non-ionic surfactants, and is further preferably at least one of potassium laurate, lauric acid diethanolamide, octyl salicylate, dodecyl benzene sulfonate, polyethylene oxide oil ether, polyethylene oxide octyl phenyl ether, polyoxyethylene lauric acid, polyethylene oxide-polypropylene oxide, oleic acid diethanolamide, and polyalkylene oxide allyl phenyl ether.

[0034] According to the present invention, to improve the yellowing resistance and storage stability of the film, an antioxidant can be added to the film-forming stock solution. The present invention has no particular limitation on the antioxidant, and conventional selections in the art can be adopted. For example, it can be selected from at least one of 2,6-di-tert-butyl-p-cresol, 2,2-methylenebis(4-methyl-6-tert-butylphenol), pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl) phosphite, and tris(2,4-di-tert-butylphenyl) phosphite.

[0035] According to the present invention, in the film-forming stock solution containing polyvinyl alcohol, preferably, based on 100 parts by weight of polyvinyl alcohol, the plasticizer is 5-30 parts by weight, the surfactant is 0.001-1 part by weight, and the antioxidant is 5-50 ppm.

[0036] According to the present invention, the method for preparing the PVA-based film further includes: dehydrating and drying the PVA-based film in sequence, and then winding it up by traction. Among them, the dehydration and drying can be further divided into dehydration treatment and heat treatment. Preferably, the conditions of the dehydration treatment include: temperature 70-95°C, time 2-15 min; the conditions of the heat treatment include: temperature 100-140°C, time 0.2-15 min. After the film after dehydration and drying passes the thickness, moisture, and defect detection, it is wound up into a PVA-based film product.

[0037] The thickness of the PVA-based film prepared by the preparation method provided by the present invention is 30-75 μm, the thickness difference is 1-6 μm, and the surface roughness Ra≤0.05 μm; the PVA-based film has a core layer and shell layers located on the upper and lower surfaces of the core layer. Among them, the average crystallinity of the core layer is 45-55%, the average crystallinity of the shell layer is 46-57%, the core layer has good compactness and no skin-core or bubble defects, and the shell layer has good compactness and no skin-core or bubble defects. Compared with the conventional method, the state difference between the upper and lower surfaces and the core layer of the prepared PVA-based film becomes significantly smaller, the overall optical uniformity of the film is excellent, and it has excellent high-fold stretching performance.

[0038] In the present invention, the thickness of the film is measured by an infrared on-line thickness gauge; the thickness range of the film is measured by an infrared on-line thickness gauge. The thickness of different positions of the film is measured 10,000 times. Among them, the difference between the maximum value and the minimum value of all measured thickness values is the thickness range; the surface roughness (Ra) of the film is measured by a roughness gauge; the crystallinity of the core layer and the shell layer of the film is measured by X-ray diffraction (XRD); the compactness of the core layer and the shell layer of the film is characterized by observing the skin-core structure and bubble conditions in the core layer and the shell layer through a polarized light microscope.

[0039] The second aspect of the present invention provides a PVA-based film prepared by the method described in the first aspect above.

[0040] The PVA-based film provided by the present invention has a thickness of 30-75 μm, a thickness range of 1-6 μm, and a surface roughness Ra≤0.05 μm; the PVA-based film has a core layer and shell layers located on the upper and lower surfaces of the core layer. Among them, the average crystallinity of the core layer is 45-55%, the average crystallinity of the shell layer is 46-57%, the compactness of the core layer is good, and there are no skin-core and bubble defects. The compactness of the shell layer is good, and there are no skin-core and bubble defects. The upper and lower surfaces and the core layer state of the PVA-based film have small differences, and the overall optical uniformity is excellent, and it has excellent high-fold stretching performance, and can be used as a good film material in the optical field.

[0041] The third aspect of the present invention provides a polarizer containing the PVA-based film described in the second aspect above.

[0042] The PVA-based film provided by the present invention has excellent optical uniformity and excellent high-fold stretching performance, which can make the polarizer prepared with it as a material have excellent optical uniformity and can better meet the performance requirements of liquid crystal displays in large-screen application scenarios.

[0043] The present invention will be described in detail below through examples. In the following examples and comparative examples,

[0044] Film-forming stock solution: Unmodified PVA (degree of polymerization is 2200, degree of alcoholysis is 99.87 mol%, weight-average molecular weight is 96800 g / mol), glycerol (plasticizer), lauric acid diethanolamide (surfactant), tris(2,4-di-tert-butylphenyl) phosphite (antioxidant) and water are placed in a reaction kettle and fully mixed and dissolved; among them, the weight ratio of unmodified PVA: glycerol: lauric acid diethanolamide (surfactant) is 100:10:0.5; the feeding amount of tris(2,4-di-tert-butylphenyl) phosphite (antioxidant) is 30 ppm (based on unmodified PVA); the concentration of PVA in the film-forming stock solution is 26 wt%.

[0045] Unless otherwise specified, the materials used are ordinary commercially available products.

[0046] Example 1

[0047] The film-forming stock solution was extruded through an extruder and a T-die, and the formed melt curtain was uniformly cast on a forming roll (surface roughness Ra is 0.02 μm). A heat medium sandwich was provided inside the forming roll, and the heat medium (thermal oil) uniformly heated the surface of the forming roll to 78 °C, and the surface temperature difference of the forming roll surface was controlled to be ≤ 0.5 °C. Under the heating action of the forming roll (the linear velocity of the forming roll is 11.50 m / min), the melt curtain was preliminarily dried and shaped on the surface of the forming roll (the drying temperature of the film is 78 °C, and the time is 5 min), and a cast film (thickness is 39 μm, water content is 15%) was obtained;

[0048] During the above drying and film-forming process, air was continuously evacuated between the die head and the forming roll (specifically, in the vicinity of the area where the lower surface of the film contacts the forming roll), and the aerosol generated by drying was removed in a timely manner; hot air was uniformly blown on the upper surface of the film (i.e., the surface of the film in contact with air during the drying and film-forming process) to uniformly dry the upper surface of the film, and at the same time, air was continuously evacuated to remove the aerosol generated by hot air drying;

[0049] The cast film obtained by the above drying was peeled off from the forming roll by a peeling roll (the linear velocity of the peeling roll is 11.52 m / min) (the linear velocity of the peeling roll: the linear velocity of the forming roll is 1.002:1), dehydrated by a drying drum (temperature is 80 °C, drying time is 10 min), and then sent to an oven for heat treatment (heat treatment temperature is 130 °C, heat treatment time is 5 min). After the film after heat treatment passed the thickness detection, moisture detection and defect detection, it was wound into a PVA-based film product (denoted as P1).

[0050] The thickness of P1 is 40 μm, the thickness range is 1 μm, and the surface roughness Ra is 0.019 μm;

[0051] After XRD testing, the average crystallinity of the core layer in P1 is 46%, and the average crystallinity of the shell layer is 46%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core and bubble defects, the compactness of the shell layer is good, without skin-core and bubble defects.

[0052] Example 2

[0053] The film-forming stock solution is extruded through an extruder and a T-die, and the formed melt curtain uniformly flows onto a forming roll (with a surface roughness Ra of 0.03 μm). A heat medium sandwich is provided inside the forming roll, and the heat medium (heat-conducting oil) uniformly heats the surface of the forming roll to 85 °C and controls the surface temperature difference on the surface of the forming roll ≤ 0.5 °C. Under the heating action of the forming roll (the linear speed of the forming roll is 11.50 m / min), the melt curtain is preliminarily dried and shaped on the surface of the forming roll (the drying temperature for film formation is 85 °C and the time is 5 min), obtaining a cast film (with a thickness of 40 μm and a moisture content of 10%);

[0054] During the above drying and film-forming process, air is continuously evacuated between the die and the forming roll (specifically, in the area near where the lower surface of the film contacts the forming roll), and the aerosol generated by drying is promptly removed. Hot air is uniformly blown on the upper surface of the film (i.e., the surface of the film in contact with air during the drying and film-forming process) to uniformly dry the upper surface of the film, and at the same time, air is continuously evacuated to remove the aerosol generated by hot air drying;

[0055] The cast film obtained from the above drying is peeled from the forming roll by a peeling roll (the linear speed of the peeling roll is 11.52 m / min) (the linear speed of the peeling roll: the linear speed of the forming roll is 1.002:1), dehydrated by a drying drum (the drying temperature is 80 °C and the drying time is 10 min), and then sent to an oven for heat treatment (the heat treatment temperature is 130 °C and the heat treatment time is 5 min). After the film after heat treatment passes the thickness detection, moisture detection, and defect detection, it is wound into a PVA-based film product (denoted as P2).

[0056] The thickness of P2 is 40 μm, the thickness range is 2 μm, and the surface roughness Ra is 0.02 μm;

[0057] After XRD testing, the average crystallinity of the core layer in P2 is 47%, and the average crystallinity of the shell layer is 47%; through observation with a polarizing microscope, the compactness of the core layer is good, without skin-core or bubble defects, and the compactness of the shell layer is good, without skin-core or bubble defects.

[0058] Example 3

[0059] The film-forming stock solution is extruded through an extruder and a T-die, and the formed melt curtain uniformly flows onto a forming roll (with a surface roughness Ra of 0.04 μm). A heat medium sandwich is provided inside the forming roll, and the heat medium (heat-conducting oil) uniformly heats the surface of the forming roll to 75 °C and controls the surface temperature difference on the surface of the forming roll ≤ 0.5 °C. Under the heating action of the forming roll (the linear speed of the forming roll is 11.50 m / min), the melt curtain is preliminarily dried and shaped on the surface of the forming roll (the drying temperature for film formation is 75 °C and the time is 5 min), obtaining a cast film (with a thickness of 40 μm and a moisture content of 25%);

[0060] During the above-mentioned drying and film-forming process, air is continuously extracted between the die head and the forming roller (specifically, in the vicinity of the contact area between the lower surface of the film and the forming roller) to promptly remove the aerosol generated during drying; hot air is evenly blown on the upper surface of the film (i.e., the surface of the film in contact with air during the drying and film-forming process) to uniformly dry the upper surface of the film, and at the same time, air is continuously extracted to remove the aerosol generated by hot air drying;

[0061] The cast film obtained by the above drying is peeled from the forming roller by a peeling roller (the linear speed of the peeling roller is 11.52 m / min) (the linear speed of the peeling roller: the linear speed of the forming roller is 1.002:1), dehydrated by a drying drum (the drying temperature is 80 °C, and the drying time is 10 min), and then sent to an oven for heat treatment (the heat treatment temperature is 130 °C, and the heat treatment time is 5 min). After passing the thickness detection, moisture detection, and defect detection, the film is wound into a PVA-based film product (denoted as P3).

[0062] The thickness of P3 is 40 μm, the thickness range is 3 μm, and the surface roughness Ra is 0.035 μm;

[0063] After XRD testing, the average crystallinity of the core layer in P3 is 45%, and the average crystallinity of the shell layer is 46%; through observation with a polarizing microscope, the compactness of the core layer is good, without skin-core and bubble defects, and the compactness of the shell layer is good, without skin-core and bubble defects.

[0064] Example 4

[0065] The film-forming stock solution is extruded through an extruder and a T-shaped die head, and the formed melt curtain evenly flows onto a forming roller (the surface roughness Ra is 0.05 μm). There is a heat medium sandwich inside the forming roller, and the heat medium (thermal oil) evenly heats the surface of the forming roller to 87 °C and controls the surface temperature difference on the surface of the forming roller ≤ 0.5 °C. Under the heating of the forming roller (the linear speed of the forming roller is 11.50 m / min), the melt curtain is preliminarily dried and shaped on the surface of the forming roller (the drying and film-forming temperature is 87 °C, and the time is 5 min) to obtain a cast film (the thickness is 40 μm, and the moisture content is 11%);

[0066] During the above-mentioned drying and film-forming process, air is continuously extracted between the die head and the forming roller (specifically, in the vicinity of the contact area between the lower surface of the film and the forming roller) to promptly remove the aerosol generated during drying; hot air is evenly blown on the upper surface of the film (i.e., the surface of the film in contact with air during the drying and film-forming process) to uniformly dry the upper surface of the film, and at the same time, air is continuously extracted to remove the aerosol generated by hot air drying;

[0067] The cast film obtained by the above drying is peeled from the forming roller by a peeling roller (the linear velocity of the peeling roller is 11.52 m / min) (the linear velocity of the peeling roller: the linear velocity of the forming roller is 1.002:1), dehydrated by a drying drum (the drying temperature is 80 °C and the drying time is 10 min), and then sent to an oven for heat treatment (the heat treatment temperature is 130 °C and the heat treatment time is 5 min). After the film after heat treatment passes the thickness detection, moisture detection and defect detection, it is wound into a PVA-based film product (denoted as P4).

[0068] The thickness of P4 is 40 μm, the thickness range is 4 μm, and the surface roughness Ra is 0.045 μm;

[0069] After XRD testing, the average crystallinity of the core layer in P4 is 48%, and the average crystallinity of the shell layer is 50%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core or bubble defects, the compactness of the shell layer is good, without skin-core or bubble defects.

[0070] Example 5

[0071] According to the method of Example 4, the difference is that the drying film-forming temperature is 80 °C, and other steps and conditions are the same as those in Example 4, to obtain a PVA-based film product (denoted as P5).

[0072] The thickness of P5 is 40 μm, the thickness range is 3 μm, and the surface roughness Ra is 0.04 μm;

[0073] After XRD testing, the average crystallinity of the core layer in P5 is 48%, and the average crystallinity of the shell layer is 48%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core or bubble defects, the compactness of the shell layer is good, without skin-core or bubble defects.

[0074] Example 6

[0075] According to the method of Example 4, the difference is that the drying film-forming time is 15 min (the linear velocity of the forming roller is 3.90 m / min, and the linear velocity of the peeling roller is 3.91 m / min), and other steps and conditions are the same as those in Example 4, to obtain a PVA-based film product (denoted as P6).

[0076] The thickness of P6 is 40 μm, the thickness range is 5 μm, and the surface roughness Ra is 0.049 μm;

[0077] After XRD testing, the average crystallinity of the core layer in P6 is 48%, and the average crystallinity of the shell layer is 51%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core or bubble defects, the compactness of the shell layer is good, without skin-core or bubble defects.

[0078] Example 7

[0079] According to the method of Example 4, the difference is that the surface roughness Ra of the forming roller is 0.03 μm, and other steps and conditions are the same as those in Example 4, to obtain a PVA-based film product (denoted as P7).

[0080] The thickness of P7 is 40 μm, the thickness range is 3 μm, and the surface roughness Ra is 0.03 μm;

[0081] After XRD testing, the average crystallinity of the core layer in P7 is 48%, and the average crystallinity of the shell layer is 49%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core and bubble defects, the compactness of the shell layer is good, without skin-core and bubble defects.

[0082] Example 8

[0083] According to the method of Example 4, the difference is that in the peeling stage, the linear speed of the peeling roller is 11.70 m / min, that is, the linear speed of the peeling roller: the linear speed of the forming roller is 1.017:1, and other steps and conditions are the same as those in Example 4, to obtain a PVA-based film product (denoted as P8).

[0084] The thickness of P8 is 40 μm, the thickness range is 5 μm, and the surface roughness Ra is 0.05 μm;

[0085] After XRD testing, the average crystallinity of the core layer in P8 is 48%, and the average crystallinity of the shell layer is 54%; through polarized light microscopy observation, the compactness of the core layer is good, without skin-core and bubble defects, the compactness of the shell layer is good, without skin-core and bubble defects.

[0086] Comparative Example 1

[0087] According to the method of Example 4, the difference is that the temperature for drying and forming the film is 95 °C, and other conditions are the same as those in Example 1, to obtain a PVA-based film product (denoted as D1).

[0088] The thickness of D1 is 40 μm, the thickness range is 10 μm, and the surface roughness Ra is 0.1 μm;

[0089] After XRD testing, the average crystallinity of the core layer in D1 is 56%, and the average crystallinity of the shell layer is 59%; through polarized light microscopy observation, skin-core and bubble defects exist in the core layer, and skin-core and bubble defects exist in the shell layer.

[0090] Comparative Example 2

[0091] According to the method of Example 4, the difference is that the time for controlling drying and forming the film is 0.5 min, and other steps and conditions are the same as those in Example 1 (in Comparative Example 2, the linear speed of the peeling roller: the linear speed of the forming roller is the same as the ratio in Example 4, which is 1.002:1), to obtain a PVA-based film product (denoted as D2).

[0092] The thickness of D2 is 40 μm, the thickness range is 8 μm, and the surface roughness Ra is 0.08 μm;

[0093] After XRD testing, the average crystallinity of the core layer in D2 is 38%, and the average crystallinity of the shell layer is 45%; through polarized light microscopy observation, there are skin-core defects in the core layer and skin-core defects in the shell layer.

[0094] Comparative Example 3

[0095] According to the method of Example 4, the difference is that the surface roughness Ra of the forming roller is 0.1 μm, and other steps and conditions are the same as those in Example 1, to obtain a PVA-based film product (denoted as D3).

[0096] The thickness of D3 is 40 μm, the thickness range is 9 μm, and the surface roughness Ra is 0.12 μm;

[0097] After XRD testing, the average crystallinity of the core layer in D3 is 41%, and the average crystallinity of the shell layer is 45%; through polarized light microscopy observation, there are skin-core and bubble defects in the core layer and skin-core and bubble defects in the shell layer.

[0098] From the above examples and comparative examples, it can be seen that by using the preparation method provided by the present invention, the thickness range of the obtained PVA-based film can be 1-6 μm, the surface roughness Ra ≤ 0.05 μm, the average crystallinity of the core layer in the film is 45-55%, the average crystallinity of the shell layer is 46-57%, the state differences between the upper and lower surfaces and the core layer of the film are small, and there are no skin-core and bubbles in the core layer and shell layer, having excellent optical uniformity and denseness. While in Comparative Examples 1-3, the preparation method of the present invention is not adopted, and the state differences between the upper and lower surfaces and the core layer of the obtained film are relatively large or the denseness decreases.

[0099] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing a PVA-based film, characterized in that, comprising: casting a film-forming stock solution containing polyvinyl alcohol onto the surface of a film-forming substrate and drying to form a film, and peeling the cast film obtained by the drying film formation from the film-forming substrate to obtain a PVA-based film; wherein, the film-forming substrate is a steel belt or a roller with a roughness Ra≤0.05μm; the conditions for the drying film formation include: the temperature is 75-87°C and the time is 1-25 minutes.

2. The preparation method according to claim 1, wherein, the film-forming substrate is a steel belt or a roller with a roughness Ra≤0.03μm; and / or, the conditions for the drying film formation include: the temperature is 78-85°C and the time is 1.5-5 minutes.

3. The preparation method according to claim 1 or 2, wherein, the surface temperature difference of the film-forming substrate ≤0.5°C.

4. The preparation method according to any one of claims 1-3, wherein, the peeling is carried out using a peeling roller; preferably, the linear velocity of the peeling roller: the linear velocity of the film-forming substrate is (0.998-1.002):

1.

5. The preparation method according to any one of claims 1-4, wherein, in the film-forming stock solution containing polyvinyl alcohol, the degree of polymerization of polyvinyl alcohol is 1700-3500, the degree of alcoholysis ≥99.5mol%, and the weight average molecular weight is 75000-155000g / mol; and / or, the film-forming stock solution containing polyvinyl alcohol further contains a plasticizer, a surfactant and an optional antioxidant.

6. The preparation method according to claim 5, wherein, in the film-forming stock solution containing polyvinyl alcohol, based on 100 parts by weight of polyvinyl alcohol, the plasticizer is 5-30 parts by weight, the surfactant is 0.001-1 part by weight, and the antioxidant is 5-50ppm.

7. The preparation method according to any one of claims 1-6, wherein, the preparation method further includes: dehydrating and drying the PVA-based film in sequence and then winding it up by traction.

8. A PVA-based film prepared by the method according to any one of claims 1-7.

9. The PVA-based film according to claim 8, wherein, the PVA-based film has a core layer and shell layers located on the upper and lower surfaces of the core layer; preferably, the thickness of the PVA-based film is 30-75μm; preferably, the thickness difference of the PVA-based film is 1-6μm; preferably, the surface roughness Ra of the PVA-based film ≤0.05μm; preferably, the average crystallinity of the core layer is 45-55%, and the average crystallinity of the shell layer is 46-57%.

10. A polarizer containing the PVA-based film according to claim 8 or 9.