Polyvinyl alcohol film and preparation method therefor

Abstract

The present application provides a polyvinyl alcohol film and a preparation method therefor. The saturated water absorption amount of an amorphous structure in the polyvinyl alcohol film is x, and the water absorption amount increased due to the damage of a crystal surface defect structure in the polyvinyl alcohol film is y, and it is satisfied that x / (x+y) is equal to 0.82 to 0.93; and the softening point of the polyvinyl alcohol film is 65°C to 75°C, and the swelling degree of the polyvinyl alcohol film is 180% to 220%.

Description

Polyvinyl alcohol film and its preparation method

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 202411507815.9, filed on October 28, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of polyvinyl alcohol film technology, and in particular to a polyvinyl alcohol film and its preparation method. Background Technology

[0004] Polyvinyl alcohol (PVA) film is mainly used to prepare polarizing films, and its end products are used in various liquid crystal displays, such as display panels for televisions, computer monitors, mobile phones, automotive navigation systems, and wearable electronic devices. The retention rate of optical properties of PVA polarizing films under high temperature and high temperature and humidity conditions is a key indicator for evaluating the durability of PVA polarizing films. As display panels develop towards thinner, more durable, and higher transmittance, especially in automotive applications, increasingly higher demands are being placed on the durability of polyvinyl alcohol polarizing films and their upstream optical films. Summary of the Invention

[0005] The main objective of this application is to provide a polyvinyl alcohol film designed to improve the durability of polyvinyl alcohol polarizing films.

[0006] To achieve the above objectives, this application proposes a polyvinyl alcohol film, wherein the saturated water absorption of the amorphous structure in the polyvinyl alcohol film is x, and the water absorption of the polyvinyl alcohol film increased due to the destruction of the crystal surface defect structure is y, satisfying that x / (x+y) is 0.82 to 0.93;

[0007] The softening point of the polyvinyl alcohol film is 65°C to 75°C, and the swelling degree of the polyvinyl alcohol film is 180% to 220%.

[0008] In one embodiment, x / (x+y) is 0.84 to 0.89.

[0009] In one embodiment, the polyvinyl alcohol film has a crystallinity of 35% to 45% and a melting point of 210°C to 230°C.

[0010] In one embodiment, the test step for the saturated water absorption of the amorphous structure in the polyvinyl alcohol film includes: immersing at least one portion of the polyvinyl alcohol film in deionized water at a preset temperature for a preset time t1; after the amorphous structure in the polyvinyl alcohol film has absorbed water, removing the polyvinyl alcohol film, scraping off the water droplets on the surface of the polyvinyl alcohol film, and weighing it, which is recorded as m. x1 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m.x2 The saturated water absorption capacity x of the amorphous structure in the polyvinyl alcohol film is (m x1 -m x2 ) / m x2 *100%;

[0011] The test steps for determining the sum of the saturated water absorption of the amorphous structure in the polyvinyl alcohol (PVA) film and the water absorption increase due to the destruction of the crystal surface defect structure in the PVA film include: immersing at least one portion of the PVA film in deionized water at a preset temperature for a preset time t2, allowing the crystal surface defect structure in the PVA film to be destroyed; after the destroyed crystal surface defect structure has finished absorbing water, removing the PVA film, scraping off the water droplets on the surface of the PVA film, and weighing it, recorded as m. x3 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m. x4 The sum of the saturated water absorption of the amorphous structure in the polyvinyl alcohol film and the increased water absorption due to the destruction of the crystal surface structure in the polyvinyl alcohol film is x + y = (m x3 -m x4 ) / m x4 *100%.

[0012] In one embodiment, the preset temperature is 30°C, t1 is 1 min, and t2 is 15 min.

[0013] In one embodiment, this application also provides a method for preparing a polyvinyl alcohol film, comprising:

[0014] Polyvinyl alcohol and additives are dissolved in a solvent to obtain the casting film stock solution;

[0015] After degassing, casting and pre-drying the casting solution, a pre-dried film is obtained.

[0016] The pre-dried film is subjected to heat treatment to obtain a polyvinyl alcohol film.

[0017] In the step of obtaining a pre-dried film after degassing, casting and pre-drying the casting solution, the water content of the pre-dried film is 8% to 15%.

[0018] And / or, in the step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film, the water content of the polyvinyl alcohol film is 0.1% to 5%;

[0019] And / or, in the step of obtaining a pre-dried film after degassing, casting and pre-drying the casting solution, the water content of the cast film is 15% to 35%.

[0020] In one embodiment, the water content of the polyvinyl alcohol film is 0.5% to 3.5%;

[0021] And / or, the water content of the liquid film after casting is 18% to 30%.

[0022] The step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film includes: heat-treating the pre-dried film sequentially at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments shows a trend of first rising and then falling, and 2≤n≤20.

[0023] The step of sequentially heat-treating the pre-dried film at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments exhibits a trend of first increasing and then decreasing, includes:

[0024] The heat treatment time of the pre-dried film is t. Under the condition of 0%t to 20%t, the pre-dried film is heat treated with a first temperature of 50°C to 80°C.

[0025] The pre-dried film is heat-treated at a second temperature of 55°C to 95°C under conditions of 20%t to 40%t.

[0026] The pre-dried film is heat-treated at a third temperature of 70°C to 100°C under conditions of 40%t to 60%t.

[0027] The pre-dried film is heat-treated at a fourth temperature of 100°C to 130°C under conditions of 60%t to 80%t.

[0028] The pre-dried film is heat-treated at a fifth temperature range of 50°C to 98°C under conditions of 80%t to 100%t to obtain a polyvinyl alcohol film.

[0029] The step of heat-treating the pre-dried film sequentially using n temperature segments includes drying the pre-dried film using a fan;

[0030] The fan speed in the first temperature range is 5.9 m / s to 12.4 m / s, the fan speed in the second temperature range is 6.6 m / s to 13.1 m / s, the fan speed in the third temperature range is 13.2 m / s to 19.7 m / s, the fan speed in the fourth temperature range is 10.5 m / s to 17.0 m / s, and the fan speed in the fifth temperature range is 11.5 m / s to 18.0 m / s.

[0031] In one embodiment, the wind speed of the fan in the first temperature stage is 8.1 m / s to 10.3 m / s, the wind speed of the fan in the second temperature stage is 8.8 m / s to 10.9 m / s, the wind speed of the fan in the third temperature stage is 15.4 m / s to 17.5 m / s, the wind speed of the fan in the fourth temperature stage is 12.7 m / s to 14.9 m / s, and the wind speed of the fan in the fifth temperature stage is 13.7 m / s to 15.8 m / s.

[0032] The polyvinyl alcohol (PVA) film provided in this application has a saturated water absorption of x in its amorphous structure and a water absorption of y due to the destruction of the crystal surface structure, satisfying x / (x+y) = 0.82 to 0.93. The PVA film has a softening point of 65°C to 75°C and a swelling degree of 180% to 220%. Under these conditions, the durability of the PVA polarizing film prepared using this PVA film is improved. Detailed Implementation

[0033] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the examples. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0034] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0037] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0038] One of the factors affecting the durability of polarizing films is the stability of the PVA-iodine complex in the polarizing film. The stability of the PVA-iodine complex is further affected by the structure of the PVA film. The specific surface area, stability, and crystal content of the crystals in the PVA film determine the content and stability of the fibrous PVA-polyiodine ion composite structure formed during the polarizing film processing. Polyvinyl alcohol films that meet the following conditions of this application have a crystal network with high crystal specific surface area, sufficient crystal content, and suitable crystal stability, which is beneficial to obtaining polarizing films with excellent durability.

[0039] The specific conditions are: x / (x+y) is 0.82 to 0.93, x is the saturated water absorption of the amorphous structure in the polyvinyl alcohol film, y is the water absorption of the polyvinyl alcohol film due to the destruction of the crystal surface structure, the softening point of the polyvinyl alcohol film is 65℃ to 75℃, and the swelling degree of the polyvinyl alcohol film is 180% to 220%.

[0040] Specifically, during the processing of PVA polarizing films, the PVA optical film undergoes dyeing and stretching to form dichroic dye molecules, primarily composed of iodine tri- and iodine penta-ions, aligned along the PVA chain orientation. This structure is crucial to the optical performance of the polarizing film. During use, under prolonged high-temperature or high-temperature-high-humidity conditions, the complexed polyiodide ions undergo decomposition, dissociation, or oxidation, diffusing out of the system as elemental iodine. This alters the ratio of iodine tri- to iodine penta-ions within the polarizing film, macroscopically manifesting as red and blue light transmission, overall color fading, and display failure. These defects severely impact the imaging performance of the display panel, even leading to white screen phenomena. Therefore, improving the stability of polyiodide ions (PVA-iodine tri- and PVA-iodine penta-ions) in the polarizing film is key to enhancing the durability of polarizing films, especially automotive polarizing films.

[0041] Current solutions for improving the durability of PVA polarizing films mainly involve adjusting the composition of the protective film, adhesive bonding process, dyeing process, and boric acid crosslinking agent during the polarizing film processing. However, while these solutions can address the poor durability issue to some extent, they inevitably introduce other problems, such as reduced polarizing film transmittance, decreased mechanical properties, increased thickness, difficulties in production lines, the need for large-scale equipment modifications, and significantly increased costs. It is therefore difficult to obtain PVA polarizing films with excellent performance across all aspects.

[0042] This application considers that the structural factors determining the durability of polarizing films include the stability of the PVA-iodine complex, which is determined to some extent by the chemical composition and microstructure of the upstream PVA optical film. Specifically, the larger the specific surface area of ​​the crystal and the more amorphous regions surrounding the crystal, the more stable the formed PVA-iodine tri- and PVA-iodine penta-ions. Adjusting the polarizing film processing technology can only partially improve durability; however, optimizing durability from the upstream PVA optical film material level has the advantage of more significant results and less impact on the downstream polarizing film processing technology, equipment, and product performance.

[0043] This application proposes a polyvinyl alcohol film, wherein the saturated water absorption of the amorphous structure in the polyvinyl alcohol film is x, and the water absorption of the polyvinyl alcohol film increased due to the destruction of the crystal surface structure is y, satisfying that x / (x+y) is 0.82 to 0.93, the softening point of the polyvinyl alcohol film is 65°C to 75°C, and the swelling degree of the polyvinyl alcohol film is 180% to 220%.

[0044] The amorphous region of a polymer material refers to the region where polymer chain segments are not arranged in a regular parallel pattern.

[0045] Polymer crystal structure refers to the structure formed by the regular parallel arrangement of molecular chain segments.

[0046] Crystal defects in polymers refer to crystalline regions with irregular molecular chain segments and poor stability. They are generally located at the boundary between crystalline and amorphous regions, i.e., on the crystal surface.

[0047] x represents the saturated water absorption of the amorphous structure in the polyvinyl alcohol film, y represents the increased water absorption due to the destruction of the crystal surface structure in the polyvinyl alcohol film, x / (x+y) is 0.82 to 0.93, the softening point of the polyvinyl alcohol film is 65℃ to 75℃, and the swelling degree of the polyvinyl alcohol film is 180% to 220%. Under the above conditions, the durability of the polyvinyl alcohol polarizing film prepared using this polyvinyl alcohol film is improved.

[0048] It is understandable that polyvinyl alcohol (PVA) is a semi-crystalline polymer composed of crystalline and amorphous structures. Crystals exist as physical cross-linking points between amorphous regions, forming a crystalline cross-linking network together with the amorphous structure. The saturated water absorption *x* of the amorphous structure in the PVA film is related to the content of the amorphous structure. The increased water absorption *y* due to the destruction of the crystalline surface defect structure in the PVA film is related to the content, stability, and specific surface area of ​​the crystalline structure. It is understandable that because defects exist on the crystalline surface, under certain temperature and water conditions, the defective structures gradually dissolve, becoming amorphous regions. The increased water absorption *y* due to the destruction of the crystalline surface defect structure in the PVA film... The water absorption refers to the increase in water absorption due to the amorphous structure of this part. Experiments have shown that polarizing films prepared using polyvinyl alcohol films that meet the above conditions (x / (x+y) is 0.82 to 0.93) have good durability. It can be understood that the specific surface area, stability and crystal content of crystals in the polyvinyl alcohol film determine the content and stability of the fibrous PVA-polyiodide ion composite structure formed during the polarizing film processing. A crystal network with high crystal specific surface area, sufficient crystal content and suitable crystal stability is beneficial to obtaining a polarizing film with excellent durability.

[0049] It is understandable that when a polyvinyl alcohol (PVA) film absorbs water and swells in water, the amorphous structure first undergoes water absorption and swelling, a relatively rapid process. Subsequently, water molecules gradually break the hydrogen bonds of the defect structures on the surface of the PVA crystals, disrupting the less stable defect structures until they encounter stable crystals. This process is slower. The less stable defect structures are essentially wrapped around the stable crystals. Therefore, the higher the proportion of less stable defect structures, the larger the specific surface area of ​​the crystal particles and the smaller the size of individual crystals. Simultaneously, the softening point and swelling degree of the PVA film meet the aforementioned ranges. A PVA film that meets the above conditions possesses an appropriate amount of crystal content and stability, as well as a large crystal specific surface area.

[0050] It is understandable that the softening point and swelling degree of the polyvinyl alcohol film meet the above range. The polyvinyl alcohol film has a high crystal specific surface area (small and numerous crystals), sufficient crystal content, and a stable crystal network, which is beneficial to improving the dyeing efficiency, dyeing color and durability when preparing polarizing film, and reducing the risk of film breakage when stretching during polarizing film preparation.

[0051] It is understandable that if the softening point of a polyvinyl alcohol film is high and its swelling degree is low, it is more likely that its crystallinity is high, which will lead to reduced dyeing efficiency, poor color and durability when preparing polarizing films. If the softening point is low and the swelling degree is high, it is more likely that its crystallinity is low, which will cause an increase in the amount of polyvinyl alcohol dissolved during the preparation of polarizing films and a decrease in the elongation at break, making the film prone to breakage during stretching.

[0052] The values ​​from 0.82 to 0.93 mentioned above include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, etc., as well as the range values ​​between any two of the above point values.

[0053] In one embodiment, when x / (x+y) is between 0.84 and 0.89, the polyvinyl alcohol polarizing film prepared using this polyvinyl alcohol film has better durability.

[0054] The values ​​from 0.84 to 0.89 mentioned above include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, etc., as well as the range values ​​between any two of the above point values.

[0055] In one embodiment, the polyvinyl alcohol film has a crystallinity of 35% to 45% and a melting point of 210°C to 230°C.

[0056] Crystallinity is an important parameter for measuring the proportion of crystalline components in a material.

[0057] The melting point of a crystal refers to the temperature at which a polymer crystal melts at the fastest rate of melting under a given heating rate. Differential scanning calorimetry (DSC) can be used to test the melting point of a material. DSC is a thermal analysis technique that measures the heat difference between a sample and a reference material under a precisely controlled temperature program. During heating, the sample absorbs heat as it melts, resulting in endothermic peaks. By analyzing the peak shape characteristics, information such as the melting point and enthalpy of fusion can be obtained.

[0058] The moisture content of polyvinyl alcohol films was determined using a Mettler Toledo HE-53 halogen lamp rapid moisture meter or other equipment based on the same principle, at 120°C for 40 minutes.

[0059] DSC melting point and crystallinity were determined using a differential scanning calorimeter. The sample was heated from 40℃ to 240℃ at a rate of 10℃. The endothermic peak observed was the melting peak of polyvinyl alcohol crystals. The melting point was determined, and the integral enthalpy was calculated. The enthalpy ΔHc of 100% crystallized PVA is 138.6 J / g. The formula for calculating crystallinity is as follows:

[0060] The softening point is the temperature at which polyvinyl alcohol (PVA) begins to soften and become fluid during heating. The softening point can be tested using the American Society for Testing and Materials (ASTM) standard D36-95, "Determination of the Softening Point of Bituminous Pitch," specifically for testing the softening point of PVA films.

[0061] Fix the polyvinyl alcohol film in a copper ring of a specified shape and size, place a 3.5g steel ball in the center of the copper ring, immerse it in deionized water, and heat it at a rate of 5℃ / min until the polyvinyl alcohol film softens and droops, and the steel ball droops more than 25mm. Record the water temperature at this point.

[0062] The softening point measures the size, perfection, and abundance of a crystal. A high softening point generally indicates a larger crystal size, a more perfect crystal structure, and a higher abundance; a low softening point generally indicates a smaller crystal size, more defects in the crystal structure, and a lower abundance.

[0063] Swelling degree refers to a parameter describing the property of a material to absorb and expand in a liquid. The test method for swelling degree is as follows: Cut a 10*200mm film sample, swell it in 500mL of water at 30℃ for 30 minutes, centrifuge at 3000 rpm for 5 minutes, and weigh it (m10). Dry it in a 120℃ oven for 2 hours, and weigh it (m20). Swelling degree = m10 / m20*100%. Swelling degree usually represents the content of amorphous regions in polyvinyl alcohol. A higher swelling degree corresponds to more amorphous regions and lower crystallinity; a lower swelling degree corresponds to fewer amorphous regions and higher crystallinity.

[0064] It is understandable that the crystallinity and melting point of the polyvinyl alcohol film meet the above range, which is beneficial to obtaining a crystal crosslinking network with high crystal specific surface area, suitable crystal stability and sufficient crystal content, and reduces the risk of film breakage during the stretching process of preparing polarizing film.

[0065] The values ​​in the range of 35% to 45% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, etc., as well as the range values ​​between any two of the above point values.

[0066] The values ​​in the range of 210℃ to 230℃ include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 210℃, 211℃, 212℃, 213℃, 214℃, 215℃, 217℃, 219℃, 220℃, 221℃, 224℃, 225℃, 227℃, 228℃, 229℃, 230℃, etc., as well as the range values ​​between any two of the above point values.

[0067] The values ​​in the range of 65℃ to 75℃ include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, etc., as well as the range values ​​between any two of the above point values.

[0068] The values ​​in the range of 180% to 220% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 180%, 181%, 183%, 184%, 187%, 189%, 190%, 191%, 192%, 194%, 196%, 199%, 200%, 201%, 205%, 210%, 211%, 215%, 218%, 220%, etc., as well as the range values ​​between any two of the above point values.

[0069] In one embodiment, the process of testing x / (x+y) can be carried out by the following steps: specifically, at least two polyvinyl alcohol films to be tested are prepared, at least one of which is used to test and obtain the x value, and at least the other is used to test and obtain the x+y value.

[0070] The steps to obtain the x value include: immersing at least one polyvinyl alcohol (PVA) film in deionized water at a preset temperature for a preset time t1; after the amorphous structure in the PVA film has absorbed water, removing the PVA film, scraping off the water droplets on the surface of the PVA film, and weighing it, which is recorded as m. x1 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m. x2 The saturated water absorption capacity of the amorphous structure in the polyvinyl alcohol film is x = (m x1 -m x2 ) / m x2 *100%.

[0071] The steps to obtain the x+y value include: immersing at least one of the polyvinyl alcohol (PVA) films in deionized water at a preset temperature for a preset time t2, causing the crystal surface defect structure in the PVA film to be destroyed; after the destroyed crystal surface defect structure has finished absorbing water, removing the PVA film, scraping off the water droplets on the surface of the PVA film, and weighing it, recording the weight as m. x3 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m. x4 The sum of the saturated water absorption of the amorphous structure in the polyvinyl alcohol film and the increased water absorption due to the destruction of the crystal surface structure in the polyvinyl alcohol film is x + y = (m x3 -m x4 ) / m x4 *100%.

[0072] In the above test process, the preset temperature can be from 15℃ to 45℃, t1 and t2 can be from 0.1min to 30min, and the thickness of the polyvinyl alcohol film to be tested can be from 30um to 60um. It is understandable that as the temperature increases, the water absorption rate of the polyvinyl alcohol film will increase and the water absorption time will shorten. During the test of the water absorption ratio, the water absorption time should be controlled according to the actual test conditions.

[0073] In one embodiment, the preset temperature is 30°C, t1 is 1 min, and t2 is 15 min.

[0074] It is understandable that the water absorption rate of polyvinyl alcohol (PVA) films varies at different temperatures. Theoretically, the higher the temperature, the higher the water absorption rate. In this application, the water absorption rate of the PVA film in 1 minute at 30°C can be used as the water absorption rate of the amorphous structure, and the water absorption rate in 15 minutes can be used as the water absorption rate of the amorphous structure in the PVA film and the water absorption rate increased due to the destruction of the crystal surface defect structure in the PVA film. That is, the ratio of the water absorption rate of the PVA film in 1 minute to the water absorption rate in 15 minutes at 30°C can be considered to be 0.82 to 0.93. It is understandable that the durability of the polarizing film is better in the range of 0.84 to 0.89.

[0075] It is understandable that the ratio of water absorption in 1 minute to water absorption in 15 minutes at 30°C represents the ratio of the rapidly swelling portion (amorphous structure) to the slowly swelling portion (amorphous structure + increased water absorption due to structural damage from crystal surface defects) in the polyvinyl alcohol film. These ratios correspond to the swelling of the amorphous region and the dissolution of the crystal surface defects, respectively. A smaller ratio indicates a larger specific surface area, a smaller average volume, and less stable crystals; a larger ratio indicates a smaller specific surface area, a larger average volume, and more stable crystals. In the polyvinyl alcohol network structure... The size, stability, and number of crystal structures affect the complexation stability of hydroxyl and iodide ions in the polyvinyl alcohol (PVA) network structure. A PVA network structure that satisfies the above ratio has a large number of small crystal structures and an appropriate amount of crystal structures. The complexation of iodide ions with hydroxyl groups in the PVA network structure is more stable, making it difficult for iodine to detach from the PVA network structure. In other words, the more stable the polyiodide ions formed by PVA and iodine are, the less likely they are to be destroyed by high temperature and high humidity environments, thus improving the durability of the PVA polarizing film prepared using this PVA film.

[0076] Understandably, when polyvinyl alcohol (PVA) swells in water, the amorphous regions first absorb water and swell. For PVA films with a thickness of 30µm to 60µm, this process mainly occurs within one minute. Subsequently, water molecules gradually disrupt the hydrogen bonds of the PVA crystal surface defect structures, breaking down the less stable crystal surface defect structures until a stable crystal is encountered. This process is relatively slow, and it is currently believed to be completed within 15 minutes at 30°C. The less unstable crystal surface defect structures are essentially wrapped around the stable crystal; therefore, the higher the proportion of less unstable crystal surface defect structures, the larger the specific surface area of ​​the crystal particles and the smaller the size of individual crystals.

[0077] Understandably, in basic research, the crystallization state of polymer films is primarily determined using DSC (Differential Scanning Calorimetry) and X-ray crystallography. These methods are costly, complex, and prone to data processing errors, and are susceptible to significant errors. This application evaluates the crystallization properties of polyvinyl alcohol (PVA) films by assessing the water absorption and swelling performance of amorphous states and defective structures on the crystal surface. The determination of crystal state is mainly based on kinetic analysis of the swelling rate. Furthermore, conventional evaluation methods such as softening point and degree of swelling are combined to characterize the polymer crystals.

[0078] The values ​​in the range of 15℃ to 45℃ include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 15℃, 18℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, etc., as well as the range values ​​between any two of the above point values.

[0079] The values ​​in the range of 0.1 min to 30 min include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 0.1 min, 0.5 min, 1 min, 5 min, 10 min, 12 min, 16 min, 19 min, 20 min, 22 min, 25 min, 28 min, 30 min, etc., as well as the range values ​​between any two of the above point values.

[0080] The values ​​in the range of 30um to 60um include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 30um, 32um, 35um, 38um, 40um, 45um, 50um, 55um, 60um, etc., as well as the range values ​​between any two of the above point values.

[0081] In one embodiment, this application also provides a method for preparing a polyvinyl alcohol film, comprising: dissolving polyvinyl alcohol and additives in a solvent to obtain a casting solution; degassing the casting solution, casting it into a film, and pre-drying it to obtain a pre-dried film; and heat-treating the pre-dried film to obtain a polyvinyl alcohol film.

[0082] For example, polyvinyl alcohol and additives are dissolved in a solvent to prepare a casting solution. After extrusion and degassing, the solution is cast through a die, pre-dried by a drying roller, dried and shaped in a heat treatment oven, and finally wound up to obtain a polyvinyl alcohol film.

[0083] In one embodiment, in the step of obtaining a pre-dried film after degassing, casting, and pre-drying the casting solution, the water content of the cast liquid film is 15% to 35%. It is understood that if the water content of the cast liquid film is too high, it will be difficult to peel off, and the load on the subsequent pre-drying stage will increase; if the water content of the cast liquid film is too low, the equipment load will be too high, and productivity will decrease.

[0084] In one embodiment, the casting solution is degassed, cast, and pre-dried to obtain a pre-dried film with a water content of 8% to 15%. It is understood that excessively low water content is detrimental to the formation of a uniform and dense polyvinyl alcohol crystal network, resulting in increased crystallinity and a smaller specific surface area in the prepared polyvinyl alcohol optical film. This leads to decreased stability of the crystal crosslinking network, which is detrimental to improving the durability of the polarizing film and also deteriorates its tensile properties. For example, if the water content of the pre-dried film is too low, it may lead to uneven crystal growth, forming a loose network structure and affecting the material's tensile properties.

[0085] Excessive moisture content is detrimental to subsequent heat treatment processes. For example, if the pre-dried film is not sufficiently dried, the residual moisture will turn into steam during heat treatment, which may cause dripping. This will affect the appearance and quality of the product. If plasticizers (such as glycerin) are added to the pre-dried film, if the moisture content is too high during heat treatment, the plasticizers may migrate to the material surface with the moisture and evaporate. They will gradually condense into droplets on the film surface, which will also lead to a deterioration in the film's appearance and tensile properties.

[0086] In one embodiment, in the step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film, the water content of the polyvinyl alcohol film is 0.1% to 5%. It is understood that too low a water content can easily lead to high crystallinity, a smaller crystal specific surface area, and difficulties in dyeing during the polarizing film processing; while too high a water content can lead to too low crystallinity, easily resulting in increased polyvinyl alcohol leaching during production, leading to reduced film tensile strength and easy film breakage during stretching.

[0087] The values ​​in the range of 8% to 15% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., as well as the range values ​​between any two of the above point values.

[0088] The values ​​in the range of 0.1% to 5% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 0.1%, 0.3%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, etc., as well as the range values ​​between any two of the above point values.

[0089] The values ​​in the range of 15% to 35% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 15%, 17%, 19%, 20%, 22%, 25%, 28%, 30%, 31%, 33%, 34%, 35%, etc., as well as the range values ​​between any two of the above point values.

[0090] In one embodiment, the water content of the liquid film after casting is 18% to 30%.

[0091] In one embodiment, the water content of the polyvinyl alcohol film is 0.5% to 3.5%.

[0092] In one embodiment, the step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film includes: heat-treating the pre-dried film sequentially at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments shows a trend of first rising and then falling, and 2≤n≤20.

[0093] It is understandable that the heat treatment of the pre-dried film can be carried out in one or more stages. For example, in one embodiment, a multi-stage heat treatment method is used, in which the pre-dried film is heat-treated sequentially at n temperature stages to obtain a polyvinyl alcohol film. The temperature distribution of the n temperature stages shows a trend of first rising and then falling, where 2≤n≤20. It is understood that as the casting, drying and heat treatment processes proceed step by step, the polyvinyl alcohol casting solution gradually becomes a polyvinyl alcohol film with low water content. In this process, the drying and crystallization processes of polyvinyl alcohol occur simultaneously. Under different water content and temperature conditions, polyvinyl alcohol tends to form crystals of different sizes and quantities. That is, the above heat treatment steps help to finally obtain a polyvinyl alcohol film with a large crystal specific surface area and moderate crystallinity.

[0094] In the above 2≤n≤20, the values ​​include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 2, 3, 5, 7, 9, 10, 11, 12, 15, 17, 19, 20, etc., as well as the range values ​​between any two of the above point values.

[0095] In one embodiment, the step of sequentially heat-treating a pre-dried film at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments shows a trend of first increasing and then decreasing, includes: heat-treating the pre-dried film for a time t; heat-treating the pre-dried film at a first temperature segment of 50°C to 80°C under conditions of 0%t to 20%t; heat-treating the pre-dried film at a second temperature segment of 55°C to 95°C under conditions of 20%t to 40%t; heat-treating the pre-dried film at a third temperature segment of 70°C to 100°C under conditions of 40%t to 60%t; heat-treating the pre-dried film at a fourth temperature segment of 100°C to 130°C under conditions of 60%t to 80%t; and heat-treating the pre-dried film at a fifth temperature segment of 50°C to 98°C under conditions of 80%t to 100%t; thereby obtaining a polyvinyl alcohol film.

[0096] In one embodiment, a five-stage heat treatment process can be adopted. The heat treatment time for the pre-dried film is t, which refers to the time from the start to the end of the heat treatment. The end of the heat treatment can be judged by the water content of the polyvinyl alcohol film being 0.1% to 5%.

[0097] Understandably, if the temperature in the second to fourth stages is too high, the crystal size will be too large, and if the temperature is too low, the total water content will be too high. Furthermore, the temperature in the fifth stage is slowly reduced to ambient temperature to avoid the precipitation of plasticizers (such as glycerin) due to excessively low temperatures.

[0098] The values ​​in the range of 50°C to 80°C include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 50°C, 55°C, 60°C, 68°C, 70°C, 80°C, etc., as well as the range values ​​between any two of the above point values.

[0099] The values ​​in the range of 55℃ to 95℃ include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 55℃, 60℃, 68℃, 70℃, 80℃, 90℃, 95℃, etc., as well as the range values ​​between any two of the above point values.

[0100] The values ​​in the range of 70°C to 100°C include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 70°C, 80°C, 90°C, 95°C, 100°C, etc., and the range values ​​between any two of the above point values.

[0101] The values ​​in the range of 100℃ to 130℃ include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, as well as 100℃, 110℃, 115℃, 120℃, 130℃, etc., and the range values ​​between any two of the above point values.

[0102] The values ​​in the range of 50°C to 98°C include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 50°C, 60°C, 68°C, 70°C, 80°C, 90°C, 98°C, etc., as well as the range values ​​between any two of the above point values.

[0103] In one embodiment, the step of sequentially heat-treating the pre-dried film using n temperature stages includes drying the pre-dried film using a fan; the fan speed for the first stage temperature stage is 5.9 m / s to 12.4 m / s, the fan speed for the second stage temperature stage is 6.6 m / s to 13.1 m / s, the fan speed for the third stage temperature stage is 13.2 m / s to 19.7 m / s, the fan speed for the fourth stage temperature stage is 10.5 m / s to 17.0 m / s, and the fan speed for the fifth stage temperature stage is 11.5 m / s to 18.0 m / s. Furthermore, in another embodiment, the wind speed of the first stage temperature fan is 8.1 m / s to 10.3 m / s, the wind speed of the second stage temperature fan is 8.8 m / s to 10.9 m / s, the wind speed of the third stage temperature fan is 15.4 m / s to 17.5 m / s, the wind speed of the fourth stage temperature fan is 12.7 m / s to 14.9 m / s, and the wind speed of the fifth stage temperature fan is 13.7 m / s to 15.8 m / s.

[0104] Understandably, the airflow rate during heat treatment affects the crystal size and quantity of the polyvinyl alcohol (PVA) film. Therefore, employing the aforementioned airflow rate steps helps to obtain a PVA film with a larger quantity, smaller size, and moderate overall crystallinity. For example, the airflow rate during heat treatment can be adjusted by regulating the fan frequency, using the actual airflow rate measured during PVA film testing for calibration. Too high an airflow rate results in a rapid decrease in water content and reduced crystallinity, while too low an airflow rate leads to a slower decrease in water content and increased crystallinity.

[0105] Understandably, the heat treatment effect of polyvinyl alcohol (PVA) film can be referenced by the water content of the PVA film after final heat treatment.

[0106] The values ​​from 5.9 m / s to 12.4 m / s mentioned above include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 5.9 m / s, 6 m / s, 6.1 m / s, 6.7 m / s, 7 m / s, 7.5 m / s, 8 m / s, 9 m / s, 9.6 m / s, 10 m / s, 11 m / s, 12 m / s, 12.4 m / s, etc., as well as the range values ​​between any two of the above point values.

[0107] The values ​​from 6.6 m / s to 13.1 m / s mentioned above include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 6.6 m / s, 6.7 m / s, 7 m / s, 7.5 m / s, 8 m / s, 9 m / s, 9.6 m / s, 10 m / s, 11 m / s, 12 m / s, 13.1 m / s, etc., as well as the range values ​​between any two of the above point values.

[0108] The values ​​from 13.2 m / s to 19.7 m / s mentioned above include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 13.2 m / s, 13.7 m / s, 14 m / s, 14.5 m / s, 15 m / s, 16 m / s, 16.6 m / s, 17 m / s, 18 m / s, 19 m / s, 19.7 m / s, etc., as well as the range values ​​between any two of the above point values.

[0109] The values ​​from 10.5 m / s to 17.0 m / s mentioned above include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 10.5 m / s, 10.7 m / s, 11 m / s, 12.5 m / s, 13 m / s, 14 m / s, 15.6 m / s, 16 m / s, 17 m / s, etc., as well as the range values ​​between any two of the above point values.

[0110] The values ​​from 11.5 m / s to 18.0 m / s mentioned above include the minimum and maximum values ​​of this range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 11.5 m / s, 11.7 m / s, 12 m / s, 12.5 m / s, 13 m / s, 14 m / s, 15.6 m / s, 16 m / s, 17 m / s, 18.0 m / s, etc., as well as the range values ​​between any two of the above point values.

[0111] In one embodiment, the degree of polymerization of polyvinyl alcohol is between 1200 and 4000. It is understood that if the degree of polymerization is too low, the stability of the molecular chain entanglement network of the prepared polyvinyl alcohol polarizing film will be worse, the polyiodide ions will be less stable, and it will be less conducive to improving durability; if the degree of polymerization is too high, it will cause excessive tensile tension during the polarizing film processing, excessive internal stress in the polarizing film product, and severe shrinkage problems will also occur under high temperature and high humidity.

[0112] In one embodiment, the degree of hydrolysis of polyvinyl alcohol is 98% to 99.9%. It is understood that if the degree of hydrolysis is too low, the hydrogen bond network stability of the prepared polyvinyl alcohol polarizing film will be worse, the polyiodide ions will be less stable, and the film durability under high temperature and high humidity will be worse.

[0113] In one embodiment, polyvinyl alcohol includes at least one of polyvinyl alcohol homopolymer and polyvinyl alcohol copolymer. The comonomer used to prepare the polyvinyl alcohol copolymer includes at least one of olefins, acrylates, methacrylates, methacrylamide derivatives, vinyl esters, and halogenated vinyl groups. The mass percentage of the comonomer to the mass of the ethylene alcohol monomer is 0% to 10%. It is understood that an excessively high mass percentage of the comonomer to the ethylene alcohol monomer can affect the solubility of polyvinyl alcohol and the optical properties of the polyvinyl alcohol film.

[0114] The procedure for testing the degree of polymerization of polyvinyl alcohol (PVA): Dissolve the PVA raw material to prepare a solution with a mass concentration of approximately 0.5%. The precise mass concentration is determined by the dry weighing method and recorded as c. Use an Orthocrite viscometer fixed in a constant temperature bath to measure the solution concentration. Measure the time required for the sample and deionized water to flow through the upper and lower graduations of the viscometer, respectively, and record these as S0 and S, in seconds. Calculate the degree of polymerization (PA) using the following formula:

[0115] Degree of alcoholysis usually refers to the percentage of hydroxyl groups in the product after the alcoholysis reaction.

[0116] The procedure for testing the degree of hydrolysis is as follows: Weigh 1–2 mg of polyvinyl alcohol (PVA) raw material, add 0.5 mL of fully deuterated dimethyl sulfoxide (DMSO-D6) to dissolve it, and measure 1H NMR. In the 1H NMR spectrum, first integrate the peak at 1.0–1.8 ppm (-CH2 in PVA), normalize the integrated area of ​​this peak to "200", then integrate the multiplet at 1.85–2.06 ppm (the unhydrolyzed portion of PVA -CH3 peak), and the integrated area is denoted as a. The formula for calculating the degree of hydrolysis is: Degree of hydrolysis (%) = 100 - a / 3.

[0117] The values ​​from 1200 to 4000 include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 1200, 1500, 1800, 2000, 2500, 3000, 3500, 4000, etc., as well as the range values ​​between any two of the above point values.

[0118] The values ​​in the range of 98% to 99.9% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments, and 98%, 98.5%, 99.0%, 99.5%, 99.9%, etc., as well as the range values ​​between any two of the above point values.

[0119] The values ​​in the range of 0% to 10% include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include, but are not limited to, the point values ​​in the embodiments and 0.1%, 0.5%, 1%, 2%, 5%, 6%, 7%, 9%, 10%, etc., as well as the range values ​​between any two of the above point values.

[0120] In one embodiment, the degree of polymerization of polyvinyl alcohol is 2000 to 3500.

[0121] In one embodiment, the degree of alcoholysis of polyvinyl alcohol is 99% to 99.9%.

[0122] In one embodiment, the olefins include nonpolar α-olefin comonomers, wherein the mass percentage of the nonpolar α-olefin comonomers to the polyvinyl alcohol monomers is 0% to 5%. It is understood that nonpolar α-olefin comonomers, such as ethylene and butene, can be added during the polymerization of polyvinyl alcohol to regulate crystallization properties. Excessive addition can easily lead to the formation of continuous nonpolar repeating units on the molecular chain, resulting in phase separation and affecting the uniformity of the optical film; insufficient addition will have no crystallization regulating effect.

[0123] In one embodiment, the mass percentage of the nonpolar α-olefin comonomer to the ethylene alcohol monomer is 0% to 3%.

[0124] In one embodiment, the additives include plasticizers, surfactants, antioxidants, and ultraviolet absorbers, used to improve the performance of the polyvinyl alcohol film.

[0125] In one embodiment, the solvent includes water or dimethyl sulfoxide.

[0126] In one embodiment, the mass fraction of the solute in the casting solution is 20% to 40%.

[0127] In one embodiment, the dissolution temperature of the film stock solution is 130°C to 170°C, and the dissolution time of the casting film stock solution is 2 hours to 10 hours.

[0128] In one embodiment, the plasticizer includes at least one of glycerol, diglycerol, polyglycerol, ethylene glycol, and propylene glycol.

[0129] In one embodiment, the surfactant includes nonionic surfactants and anionic surfactants. The anionic surfactants include potassium lauryl ester, dodecylbenzene sulfonate, etc.; the nonionic surfactants include polyoxyethylene lauryl ester, polyoxyethylene lauryl amino ester, polyoxyethylene lauryl amide, etc.

[0130] In one embodiment, the plasticizer accounts for 8% to 15% of the mass of polyvinyl alcohol, and the surfactant accounts for 1.0‰ to 5.0‰ of the mass of polyvinyl alcohol. Excessive amounts of plasticizer and surfactant can cause the plasticizer and surfactant to precipitate, resulting in a deterioration in the optical properties of the polyvinyl alcohol film.

[0131] In one embodiment, the mass fraction of the solute in the casting solution is 25% to 35%.

[0132] In one embodiment, the dissolution temperature of the casting solution is 145°C to 160°C, and the dissolution time is 4h to 7h.

[0133] In one embodiment, during the pre-drying process, m drying rollers are used sequentially to dry the cast film. The temperature of the m drying rollers gradually decreases, with 8 ≤ m ≤ 20°C, and the temperature of the drying rollers ranges from 30°C to 90°C. This drying method ensures the uniformity of the film during the drying process and reduces film damage caused by excessively high temperatures.

[0134] In one embodiment, the pre-dried film is heat-treated in an oven such that the heat treatment step is performed continuously.

[0135] In one embodiment, in the step of obtaining a pre-dried film after degassing, casting, and pre-drying the casting solution, the casting solution is degassed using an extruder with a die pressure of 1.0 MPa to 3.0 MPa. It is understood that degassing the casting solution using an extruder with a die pressure within the above range is necessary; too low a pressure results in poor degassing, while too high a pressure overloads the equipment.

[0136] In one embodiment, in the step of dissolving polyvinyl alcohol and additives in a solvent to obtain a casting film stock solution, a disc filter is used to filter the casting film stock solution. The pore size of the disc filter is 1µm to 20µm. The casting film stock solution is filtered through the disc filter to remove impurities, gels, etc. The pore size of the filter is 1µm to 20µm, for example, 2µm to 10µm. If the pore size of the filter is too large, it will not be able to remove impurities and gels. If the pore size is too small, the pressure drop will be too large, and the power load on the equipment will increase.

[0137] In this application, by using the above preparation method and coordinating each step, such as adjusting the temperature and airflow rate during the heat treatment process, the external field parameters such as the temperature and wind speed curves during the processing are matched to a certain extent with the decrease curve of the water content of the film and the internal crystallization process, thereby obtaining a polyvinyl alcohol film with a polymer structure that is numerous, small in size, and has a moderate total crystallinity.

[0138] This application, starting from the microstructure of polyvinyl alcohol (PVA), combines polymer crystallization theory with PVA film production processes to facilitate the preparation of more stable iodine-PVA complexes. This application designs the required crystal structure for the raw materials used in preparing PVA polarizing films to obtain more stable iodine-PVA complexes, resulting in more significant effects. Furthermore, it has the advantage of minimal impact on downstream polarizing film processing techniques, equipment, and product performance.

[0139] This application achieves control over the crystal structure of polyvinyl alcohol (PVA) films by regulating the temperature and air velocity at each stage of the heat treatment process. This allows for precise and efficient control of the stability of the PVA film crystal network, the specific surface area of ​​the crystals, and the crystal content, thereby contributing to improved durability of the polarizing film. Furthermore, this control method, starting from the heat treatment process, is simple and effective, requires no large-scale equipment modifications, and has good adaptability.

[0140] Example

[0141] Example 1

[0142] Preparation of polyvinyl alcohol film: 32 kg of polyvinyl alcohol polymer (PVA) (degree of polymerization 2400, degree of hydrolysis 99.8%), sodium dodecylbenzenesulfonate, glycerol, and an antioxidant mixture (where the plasticizer content accounts for 15% of the PVA mass and the surfactant content accounts for 5‰ of the PVA content) were added to a reactor with 68 L of deionized water and stirred at 150 °C and 0.4 MPa for 6 h to dissolve, obtaining a PVA casting solution with a solute content of 35%. The resulting casting solution was degassed by an extruder at a die head pressure of 0.5 MPa, filtered through a 6 μm filter, and extruded from a die. After drying by a casting roller to reduce the moisture content to 35%, it was pre-dried by a drying roller to reduce the moisture content to 15%. It was then heat-treated in an oven and finally wound up to obtain the PVA film. The heat treatment section is designed as five sections, each with the same length and temperatures of 70℃, 72℃, 86℃, 118.5℃, and 65℃, respectively. By controlling the fan frequency, the wind speeds blowing onto the membrane surface are 9.2m / s, 9.8m / s, 16.4m / s, 13.8m / s, and 14.8m / s, respectively.

[0143] Preparation of polyvinyl alcohol (PVA) polarizing film: Sample preparation was performed using a self-made solution and stretching apparatus. The prepared PVA film was first treated in a constant temperature and humidity chamber for 24 hours (23±1℃, 55±5% RH). After equilibration, 60*40mm pieces of the PVA film were cut along the stretching direction for later use. The dyeing and stretching experimental parameters are shown in Table 1 below. Dyeing and washing solutions were prepared as needed, with 150mL used per PVA film for single use only. The stretching solution was prepared according to the actual volume of the stretching apparatus. The film was stretched uniformly to the specified stretch ratio within a specified time, then the length was fixed and the film removed. The remaining steps involved immersing the film in a liquid container at the corresponding temperature for a specified time. After completion, the film was dried in a 65℃ forced-air oven for 5 minutes. The swelling step was performed in deionized water.

[0144] Table 1. Dyeing and Tensile Test Process Parameters

[0145] Durability testing methods

[0146] Optical performance tests were performed on the aforementioned polyvinyl alcohol polarizing film. A spectrophotometer with polarization capability was used to measure the transmittance spectrum with the polarization direction perpendicular to the film's stretching direction, with wavelength sampling intervals less than or equal to 5 nm. The average of five transmittance values ​​at 600, 605, 610, 615, and 620 nm was taken to avoid the influence of film interference on the transmittance measurement. This transmittance was converted to absorbance. According to Beer-Lambert law, this absorbance value is proportional to the content of polyiodide ions oriented along the stretching direction. The ratio of the absorbance measured for the original sample to the absorbance measured after heat aging treatment at 95°C for 5 hours in a forced-air drying oven was calculated as an indicator of the sample's durability.

[0147] Water absorption rate curve experiment

[0148] Cut a 50*50mm polyvinyl alcohol film sample and swell it in 300mL of 30℃ water for T minutes. After the specified time, remove the film, scrape off the surface water droplets, and weigh it, recording the weight as m1. Dry the film in a 120℃ forced-air oven for 2 hours, then weigh it again, recording the weight as m2. T can be taken from 0.1min to 30min. Each water absorption test uses a separate film and cannot be reused. Water absorption = (m1-m2) / m2*100%.

[0149] This application primarily uses water absorption at two time points: 1 minute and 15 minutes, and calculates the water absorption values ​​for 1 minute and 15 minutes. These values ​​represent the ratio of the rapidly swelling portion to the slowly swelling portion in the polyvinyl alcohol film, corresponding to the swelling of the amorphous region and the dissolution of the crystal surface defect structure, respectively. A lower value indicates a larger proportion of crystal surface defect structures. When the polyvinyl alcohol film swells in water, the amorphous region first absorbs water and expands; for films of 30µm to 60µm, this process mainly occurs within one minute. Subsequently, water molecules gradually break the hydrogen bonds of the crystal surface defect structure, disrupting the less stable crystal surface defect structure until a stable crystal is encountered. This process is relatively slow, and it is currently believed to be essentially completed within 15 minutes at 30°C. The less stable crystal surface defect structure is essentially wrapped around the stable crystal; therefore, the greater the proportion of the less unstable crystal surface defect structure, the larger the specific surface area of ​​the crystal particles and the smaller the size of individual crystals.

[0150] Example 2

[0151] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0152] In the second and third sections, the wind speeds blowing onto the membrane surface are controlled to be 10.5 m / s and 17.3 m / s, respectively, by controlling the fan frequency.

[0153] Example 3

[0154] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0155] The temperatures of the second and third heat treatment sections are 84℃ and 98℃, respectively. The wind speeds blowing onto the membrane surface in the second and third sections are controlled by adjusting the fan frequency to 10.1m / s and 16.8m / s, respectively.

[0156] Comparative Example 1

[0157] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0158] The temperature of the fourth heat treatment section is 132℃. The wind speeds blowing onto the membrane surface in the second and third sections are 10.5m / s and 17.3m / s, respectively, by controlling the fan frequency.

[0159] Comparative Example 2

[0160] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0161] The temperatures of the second, third, and fourth heat treatment sections are 84℃, 98℃, and 123.5℃, respectively. The wind speeds blowing onto the membrane surface in the second and third sections are controlled by adjusting the fan frequency to 6.5m / s and 12.5m / s, respectively.

[0162] Comparative Example 3

[0163] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0164] The temperature of the fourth heat treatment section is 132℃. The wind speeds blowing onto the membrane surface in the second and third sections are 6.5m / s and 12.5m / s, respectively, by controlling the fan frequency.

[0165] Comparative Example 4

[0166] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0167] The temperature of the fourth heat treatment section is 123.5℃. The wind speeds blowing onto the membrane surface in the second and third sections are 10.5m / s and 17.3m / s, respectively, by controlling the fan frequency.

[0168] Comparative Example 5

[0169] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0170] The temperatures of the second, third, and fourth heat treatment sections are 84℃, 98℃, and 123.5℃, respectively. In the second and third zones, the wind speeds blowing onto the membrane surface are controlled to be 10.5m / s and 17.3m / s, respectively, by controlling the fan frequency.

[0171] Comparative Example 6

[0172] This embodiment was prepared using the same methods and conditions as in Example 1, with the only difference being:

[0173] The temperature of the fourth heat treatment section is 123.5℃. The wind speeds blowing onto the membrane surface in the second and third sections are controlled by adjusting the fan frequency to 9.8m / s and 16.4m / s, respectively.

[0174] Table 2. List of process parameters and test results for different embodiments and comparative examples

[0175] In this application, x / (x+y) is 0.82 to 0.93, the softening point of the polyvinyl alcohol film is 65°C to 75°C, and the swelling degree of the polyvinyl alcohol film is 180% to 220%. Under these conditions, the durability of the polyvinyl alcohol polarizing film prepared using this polyvinyl alcohol film is improved.

[0176] As can be seen from Table 2, under the conditions described above in this application, the polarizing film has a better retention rate of absorbance at 610 nm, indicating that the durability of the polarizing film is improved.

[0177] Furthermore, the polyvinyl alcohol films obtained after casting, drying, and heat treatment of the casting solution in Examples 1, 2, and 3, when simultaneously meeting the preparation process conditions of this application and the condition that x / (x+y) in the polyvinyl alcohol film is 0.82 to 0.93, exhibit relatively good durability when prepared by dyeing and stretching. In Comparative Examples 1, 2, and 3, the processing parameters exceeded the range described above, making it impossible to obtain films with the desired crystal structure, resulting in poor durability.

[0178] The above are merely optional embodiments of this application and do not limit the patent scope of this application. Any equivalent structural transformations made using the content of this application's specification under the inventive concept of this application, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. A polyvinyl alcohol film, wherein, The saturated water absorption of the amorphous structure in the polyvinyl alcohol film is x, and the water absorption of the polyvinyl alcohol film increased due to the destruction of the crystal surface defect structure is y, satisfying that x / (x+y) is 0.82 to 0.93; The softening point of the polyvinyl alcohol film is 65°C to 75°C, and the swelling degree of the polyvinyl alcohol film is 180% to 220%.

2. The polyvinyl alcohol film as described in claim 1, wherein, The x / (x+y) is between 0.84 and 0.

89.

3. The polyvinyl alcohol film as described in claim 1 or 2, wherein, The polyvinyl alcohol film has a crystallinity of 35% to 45% and a melting point of 210°C to 230°C.

4. The polyvinyl alcohol film as described in claim 1 or 2, wherein, The test procedure for the saturated water absorption of the amorphous structure in the polyvinyl alcohol film includes: immersing at least one portion of the polyvinyl alcohol film in deionized water at a preset temperature for a preset time t1; after the amorphous structure in the polyvinyl alcohol film has absorbed water, removing the polyvinyl alcohol film, scraping off the water droplets on the surface of the polyvinyl alcohol film, and weighing it, which is recorded as m. x1 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m. x2 The saturated water absorption capacity x of the amorphous structure in the polyvinyl alcohol film is (m x1 -m x2 ) / m x2 *100%; The test steps for determining the sum of the saturated water absorption of the amorphous structure in the polyvinyl alcohol (PVA) film and the water absorption increase due to the destruction of the crystal surface defect structure in the PVA film include: immersing at least one portion of the PVA film in deionized water at a preset temperature for a preset time t2, allowing the crystal surface defect structure in the PVA film to be destroyed; after the destroyed crystal surface defect structure has finished absorbing water, removing the PVA film, scraping off the water droplets on the surface of the PVA film, and weighing it, recorded as m. x3 The polyvinyl alcohol film was dried and weighed, and the weight was recorded as m. x4 The sum of the saturated water absorption of the amorphous structure in the polyvinyl alcohol film and the increased water absorption due to the destruction of the crystal surface structure in the polyvinyl alcohol film is x + y = (m x3 -m x4 ) / m x4 *100%.

5. The polyvinyl alcohol film as described in claim 4, wherein, The preset temperature is 30℃, t1 is 1 min, and t2 is 15 min.

6. A method for preparing a polyvinyl alcohol film as described in any one of claims 1 to 5, wherein, The preparation method includes: Polyvinyl alcohol and additives are dissolved in a solvent to obtain the casting film stock solution; After degassing, casting and pre-drying the casting solution, a pre-dried film is obtained. The pre-dried film is subjected to heat treatment to obtain a polyvinyl alcohol film.

7. The method for preparing a polyvinyl alcohol film as described in claim 6, wherein, In the step of obtaining a pre-dried film after degassing, casting and pre-drying the casting solution, the water content of the cast film is 15% to 35%. And / or, in the step of obtaining a pre-dried film after degassing, casting and pre-drying the casting solution, the water content of the pre-dried film is 8% to 15%; And / or, in the step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film, the water content of the polyvinyl alcohol film is 0.1% to 5%.

8. The method for preparing a polyvinyl alcohol film as described in claim 7, wherein, The water content of the liquid film after casting is 18% to 30%. And / or, the water content of the polyvinyl alcohol film is 0.5% to 3.5%.

9. The method for preparing the polyvinyl alcohol film according to any one of claims 6 to 8, wherein, The step of heat-treating the pre-dried film to obtain a polyvinyl alcohol film includes: heat-treating the pre-dried film sequentially at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments shows a trend of first rising and then falling, and 2≤n≤20.

10. The method for preparing a polyvinyl alcohol film as described in claim 9, wherein, The step of sequentially heat-treating the pre-dried film at n temperature segments to obtain a polyvinyl alcohol film, wherein the temperature distribution of the n temperature segments exhibits a trend of first increasing and then decreasing, includes: The heat treatment time of the pre-dried film is t. Under the condition of 0%t to 20%t, the pre-dried film is heat treated with a first temperature of 50°C to 80°C. The pre-dried film is heat-treated at a second temperature of 55°C to 95°C under conditions of 20%t to 40%t. The pre-dried film is heat-treated at a third temperature of 70°C to 100°C under conditions of 40%t to 60%t. The pre-dried film is heat-treated at a fourth temperature of 100°C to 130°C under conditions of 60%t to 80%t. The pre-dried film is heat-treated at a fifth temperature range of 50°C to 98°C under conditions of 80%t to 100%t to obtain a polyvinyl alcohol film.

11. The method for preparing a polyvinyl alcohol film as described in claim 10, wherein, The step of heat-treating the pre-dried film sequentially using n temperature segments includes drying the pre-dried film using a fan; The fan speed in the first temperature range is 5.9 m / s to 12.4 m / s, the fan speed in the second temperature range is 6.6 m / s to 13.1 m / s, the fan speed in the third temperature range is 13.2 m / s to 19.7 m / s, the fan speed in the fourth temperature range is 10.5 m / s to 17.0 m / s, and the fan speed in the fifth temperature range is 11.5 m / s to 18.0 m / s.

12. The method for preparing a polyvinyl alcohol film as described in claim 11, wherein, The fan speed in the first temperature range is 8.1 m / s to 10.3 m / s, the fan speed in the second temperature range is 8.8 m / s to 10.9 m / s, the fan speed in the third temperature range is 15.4 m / s to 17.5 m / s, the fan speed in the fourth temperature range is 12.7 m / s to 14.9 m / s, and the fan speed in the fifth temperature range is 13.7 m / s to 15.8 m / s.

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