Continuous preparation method of PVB powder for photovoltaic film and PVB powder

Abstract

The invention discloses a continuous preparation method of PVB powder for a photovoltaic film and the PVB powder, and belongs to the technical field of high polymer material preparation. The method comprises the following steps: selecting a PVA (Polyvinyl Alcohol) raw material with specific end group modification, and purifying to reduce the content of metal impurities; mixing the purified PVA with an aldehyde compound with a specific structure, a functional modifier and a catalyst, and carrying out staged acetalation and steric regulation reaction in a continuous reaction device, including primary reaction, steric curing and in-situ purification; in the reaction process, closed-loop control is realized through an integrated online detection module; performing granulation, crushing and grading on the reaction product to obtain PVB powder with a target particle size; and finally, carrying out drying and impurity removal treatment to obtain the PVB powder with the characteristics of high transparency, low flowability and ultra-high purity. According to the method, raw material purification, stereochemical regulation and control, functional modification, continuous production and intelligent quality control are integrated, and the problems of performance splitting, impurity residues and low production efficiency in the prior art are solved.

Description

Technical Field

[0001] This invention relates to the field of polymer resin synthesis technology, specifically to a method for preparing polyvinyl butyral (PVB) resin powder. Background Technology

[0002] Polyvinyl butyral (PVB) resin has become a key matrix material for interlayer films used in photovoltaic module encapsulation due to its excellent optical transparency, bonding strength, flexibility, and weather resistance. Photovoltaic modules are typically designed to have a lifespan exceeding 25 years and operate for extended periods in complex outdoor environments, which places core requirements on encapsulation materials, including high transparency, low flow, and ultra-high purity.

[0003] In realizing this invention, the inventors discovered at least the following problems in the prior art: traditional acid-catalyzed batch processes struggle to balance purity and stereoregulation, while continuous processes face challenges in impurity control and performance uniformity. For example, patent document US10875941B2 discloses a method for preparing a general-purpose low-flow PVB, which uses a specific catalyst system to regulate polymer stereoregulation to reduce flowability. However, this technical solution has significant shortcomings: first, it focuses on general material properties and does not perform functional modification design for the high transmittance and UV aging resistance required for photovoltaic applications; second, its production process is still a batch reaction, resulting in poor batch stability and low production efficiency, and it does not involve in-situ purification technology for the reaction process, making it difficult to meet photovoltaic-grade requirements for metal impurity content. Furthermore, while using carbon dioxide to replace traditional acid catalysts can reduce impurity residues, the reaction kinetics are slow, requiring high equipment pressure, and it is difficult to achieve precise control of polymer stereoregularity, thus failing to guarantee the stable achievement of low flowability.

[0004] Therefore, existing technologies have not yet solved the problem of unifying the three requirements of "high transparency, low flow, and ultra-high purity" for PVB powder used in photovoltaic films, and lack efficient, stable, and environmentally friendly integrated technology solutions suitable for large-scale continuous industrial production. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a continuous preparation method for PVB powder for photovoltaic films that can take into account high transparency, low fluidity and ultra-high purity, while solving technical problems such as high residual impurities due to acid catalysis, difficulty in achieving both low fluidity and high transparency, low efficiency of intermittent production and poor batch stability.

[0006] The technical solution adopted by this invention to solve its technical problem is as follows: In the first aspect, the present invention provides a continuous preparation method for PVB powder for photovoltaic films, the core of which lies in the integrated innovation of deep purification of raw materials, precise control of catalytic stereochemistry, synergistic effect of functional modifiers, continuous reaction engineering and intelligent closed-loop quality control system.

[0007] The method mainly includes the following steps: 1) PVA (Polyvinyl alcohol) raw material modification and deep purification: PVA raw materials with specific end-group modifications are selected. These end-group modifications aim to improve the reactivity of PVA and the stereoregularity of subsequent PVB formation. Purification treatment of PVA significantly reduces its metal impurity content, laying the foundation for obtaining ultra-high purity products.

[0008] 2) Construction of the functionalized reaction system: An acetalization reaction was carried out on PVA using an aldehyde compound with a specific branched structure. This specific structure facilitates the control of the reaction pathway and product stereochemistry. Simultaneously, functional modifiers were introduced to impart properties such as resistance to UV aging to the product. A suitable acid catalyst was selected to drive the reaction.

[0009] 3) Continuous Catalytic Stereotyping and In-situ Purification: In a continuous reaction apparatus, the acetalization reaction is carried out in stages. The initial reaction stage triggers the reaction under mild conditions; the stereotactic solidification stage precisely guides the molecular chains to form a high proportion of regular syndiotactic sequences under specific heating conditions, which is the key to achieving low flowability; the in-situ purification stage removes byproducts and unreacted substances and neutralizes residual catalyst without interrupting the process, controlling impurities from the source.

[0010] 4) Intelligent closed-loop quality control and directional powder formation: Integrating multiple online detection sensors, the system monitors key parameters affecting product performance in real time and automatically feeds back and adjusts them through a control system to ensure process stability. The reaction product is granulated, pulverized, and classified to obtain powder with uniform particle size.

[0011] 5) Post-processing and cleanliness assurance: Moisture is thoroughly removed through an optimized drying process, followed by an efficient impurity removal step to ensure the ultra-high cleanliness of the final product.

[0012] Secondly, embodiments of the present invention provide a photovoltaic film PVB powder prepared according to the continuous preparation method of photovoltaic film PVB powder described above. The PVB powder has a light transmittance (400-800 nm) ≥93%, haze ≤0.08%, melt flow rate (100 ℃ / 21.6 kg) ≤50 mg / 10min, acetalization degree of 75-84 mol%, anti-aging group grafting rate ≥90%, alkali metal content ≤15 ppm, halogen content ≤5 ppm, and light transmittance decrease of ≤0.5% after xenon lamp aging for 1000 hours.

[0013] The beneficial effects of the technical solutions of the embodiments of the present invention are as follows: 1) Synergistic Improvement of Product Performance: Through the core technology combination of "specific end-group PVA + specific structure aldehyde + precise stereochemical control", high transmittance (≥93%), low melt flow rate (≤50 mg / 10min) and high acetalization degree are synergistically achieved. Combining deep purification and in-situ purification, alkali metal and halogen impurities are controlled at extremely low levels (≤15 ppm and ≤5 ppm, respectively). Grafting with functional modifiers endows the product with excellent UV aging resistance (transmittance decrease ≤0.5% after 1000h xenon lamp aging), fully meeting the long-term service requirements of photovoltaic modules.

[0014] 2) Quality and efficiency innovation in the production process: The adoption of continuous production technology significantly improves production efficiency compared to batch processes. The intelligent closed-loop quality control system greatly improves production stability and batch consistency by real-time monitoring and automatic adjustment of key parameters, solving the inherent problems of large fluctuations and low pass rates in batch processes.

[0015] 3) Significant environmental and economic advantages: The integrated "in-situ purification" process eliminates the cumbersome subsequent washing steps in the traditional process, greatly reduces wastewater discharge, achieves a cleaner production method, and reduces overall production costs. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Those skilled in the art can select and adjust the raw materials, parameters, equipment, etc., within the scope outlined in the claims, and all such adjustments fall within the scope of protection of this invention.

[0017] The following is a detailed description of the technical solution of the present invention and various possible implementation methods: 1. Optional solutions for PVA raw material modification and deep purification: The base raw material can be PVA with hydroxyl-terminated capping and tert-butyl end-group modification. An example is as follows: hydroxyl-terminated rate 5–10%, tert-butyl end-group modification rate 8–12%, average degree of polymerization 1500–2000, and degree of hydrolysis ≥98%. The acetyl content of the PVA can be adjusted through a pre-saponification process, for example, to 0.2–2.0 mol%, with the pre-saponification temperature controlled at 40–60℃ and the time at 30–60 min.

[0018] The purification process can employ multi-stage countercurrent purification, such as one of the following solutions: Option 1: Wash with 0.08-0.12 mol / L acetic acid aqueous solution for 10-20 min, wash with chelating solution containing 40-60 ppm EDTA tetrasodium for 15-25 min, and rinse with ultrapure water 2-4 times, each rinse lasting 5-10 min. The goal is to reduce the alkali metal content in PVA to ≤30 ppm.

[0019] Option 2: First wash with 0.1-0.15 mol / L hydrochloric acid aqueous solution for 5-15 min, then wash with chelating solution containing 50-70 ppm aminotrimethylenephosphonic acid (ATMP) for 20-30 min, and finally rinse with ultrapure water 3-4 times.

[0020] Option 3: Use a two-step method of "chelating solution + ultrapure water". The chelating solution is a composite chelating agent containing 50 ppm tetrasodium EDTA and 30 ppm citric acid. The washing time is 25-35 min, and the ultrapure water is used for rinsing 3 times.

[0021] 2. Optional configurations for the reaction system Aldehydes can be mixtures with specific branched structures. One example is a mixture of isobutyraldehyde and 2-ethylhexanal, with a molar ratio of 6.5:3.5 to 7.5:2.5, purity ≥99.9%, and acid value ≤1 mgKOH / g. The functional modifier can be 4-hydroxycinnamaldehyde, added at 0.5% to 1.5% of the PVA mass, with a purity ≥99.5%. The acid catalyst can be hydrochloric acid, with a concentration of 28% to 32%, controlling the molar ratio of acid to PVA to ≥9.0 mol H₂. + / kg PVA.

[0022] Example of an alternative: The aldehyde mixture can be a mixture of isobutyraldehyde and 2-propylheptanal (molar ratio 7:3 to 8:2), and the functional modifier can be 3-hydroxycinnamaldehyde. Another alternative: The aldehyde mixture can be a ternary mixture of isobutyraldehyde, 2-ethylhexanal, and n-butyraldehyde (molar ratio 6:3:1), and the functional modifier can be a compound of 4-hydroxycinnamaldehyde and caffeic acid. An additional 0.1 to 0.3 wt% of a hindered phenolic antioxidant (such as BHT) can also be added to the system to enhance aging resistance.

[0023] 3. Optional schemes for continuous reaction processes Continuous reaction equipment can be, for example, a twin-screw extruder with a screw length-to-diameter ratio (L / D) of 35–45 and a screw diameter of 30–60 mm. An example of the reaction process is as follows: Initial reaction stage: PVA can be mixed with a mixed solvent (e.g., water and ethylene glycol dimethyl ether in a volume ratio of 8:2 to 9.5:0.5) to prepare a 10-20 wt% solution, which is then injected into the reaction apparatus at 10-20 °C. Aldehydes, modifiers, and catalysts are added simultaneously, and the reaction is initiated by maintaining the temperature for 20-30 min. The solvent can also be replaced with a mixture of water / propylene glycol dimethyl ether or water / diethylene glycol dimethyl ether, or simply ultrapure water. The equipment can also be a planetary screw extruder or a series of continuous stirred reactors.

[0024] Stereosetting stage: The system can be heated to 70–78 °C and held for 210–300 min to regulate the stereostructure of PVB molecules. The target is to control the molar ratio of ternary continuous meta-isosteryl vinyl alcohol sequences ≥ 0.152, and the OH blockiness parameter VV / (VB+BB) > 0.37. The molar ratio of pentagonal rrrr-type meta-isosteryl sequences is > 0.23, and the molar ratio of mrrm-type sequences is < 0.3. Gradient heating or isothermal heating can be used. 0.05–0.1 wt% of a nucleating agent (such as nano-silica) can be added to assist in stereoalignment.

[0025] In-situ purification section: Degassing can be performed at 75–80 °C and a vacuum of -0.08–-0.09 MPa for 30–60 min, while simultaneously injecting an alkaline solution (such as a 4–6% sodium bicarbonate solution, added at 0.5–1.0 wt%) to neutralize residual acid. The neutralizing agent can also be replaced with sodium carbonate solution, or a two-step neutralization method (first sodium bicarbonate, then ammonium hydroxide) can be used. An additional 0.1–0.2 wt% adsorbent (such as activated alumina) can be added for further purification.

[0026] 4. Optional solutions for intelligent quality control and powder production The intelligent quality control system integrates multiple online detection modules, such as: an online moisture analyzer in the feeding section; an online carbon-13 nuclear magnetic resonance (¹³C-NMR) rapid detection module in the reaction section to monitor stereochemical parameters; an online ion chromatography detection module in the purification section to monitor halogen content; and an online thickness and density analyzer in the extrusion section. A closed-loop control system of "detection-feedback-regulation" is achieved through a PLC control system.

[0027] An example of the powdering process is as follows: The reaction product is granulated underwater (e.g., at a water temperature of 55–65 °C, with a particle size of 1.5–3.5 mm), then pulverized by air jet milling (pressure 0.5–0.7 MPa) and classified to obtain PVB powder with the target particle size (e.g., D50 of 80–220 μm). Alternative methods: Mechanical pulverization or direct powdering via spray drying can be used.

[0028] 5. Post-processing options Drying can be performed in two stages, for example, first drying at 75–85 °C with forced air for 2–3 hours, then vacuum drying at 95–105 °C for 1–2 hours, with inert gas introduced for protection during the process. The moisture content after drying can be controlled to ≤0.5%. Impurity removal can be performed sequentially by electrostatic removal (voltage 25–35 kV) and machine vision re-inspection (recognition accuracy ≥3 μm). For high-cleanliness environments, it can be simplified to single-end vacuum drying. A magnetic separation device can be added to remove ferromagnetic impurities.

[0029] 6. Coordinated control of key process parameters Examples of core process parameters are as follows: Acetalization degree controlled at 75–84 mol%, anti-aging group grafting rate ≥90%; twin-screw extruder speed 30–50 r / min; no emulsifier added throughout the reaction. The acetalization degree range (e.g., 73–86 mol%) can be adjusted according to product requirements (e.g., photovoltaic film thickness). When the acetalization degree increases, the temperature of the stereocuring section can be reduced or the holding time shortened accordingly to maintain low flow properties.

[0030] Example 1 Raw material pretreatment: PVA with 8% hydroxyl end-capping rate, 10% tert-butyl end-capping rate, average degree of polymerization of 1800, and degree of hydrolysis of 98.5% was selected. The acetyl content was adjusted to 0.8 mol% through a pre-saponification process (temperature 50 ℃, time 45 min). A multi-stage countercurrent purification process was adopted: washing with 0.1 mol / L acetic acid aqueous solution for 15 min, followed by washing with a chelating solution containing 50 ppm ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na) for 20 min, and finally rinsing three times with ultrapure water (8 min each time). The alkali metal content of the PVA after treatment was 25 ppm.

[0031] The reaction system was configured as follows: The aldehyde compound used was a mixture of isobutyraldehyde and 2-ethylhexanal (molar ratio 7:3), with a purity of 99.95% and an acid value of 0.8 mg KOH / g. The functional modifier was 4-hydroxycinnamaldehyde (purity ≥99.5%), added at 1.0%. Additionally, 0.2% of 2,6-di-tert-butyl-p-cresol (BHT) was added as an antioxidant. The acid catalyst was hydrochloric acid (concentration 30%, purity ≥99.9%), used at 9.2 mol H₂. + / kg PVA.

[0032] Continuous reaction: A twin-screw extruder was used as the continuous reaction unit, with a screw length-to-diameter ratio (L / D) of 40 and a screw diameter of 45 mm. The initial reaction section temperature was 15 ℃, held for 25 min; the stereolithography section temperature was 75 ℃, held for 270 min; the in-situ purification section had a vacuum of -0.085 MPa, was injected with 0.8% of a 5% sodium bicarbonate solution, and degassed for 45 min.

[0033] Intelligent quality control and powder formation: An online Karl Fischer moisture analyzer (accuracy ±0.1%) is installed in the feeding section; an online ¹³C-NMR rapid detection module (detection cycle 8 min) is integrated in the stereolithography section to monitor the molar ratio of ternary homologous sequences in real time; an online ion chromatography detection module (detection limit 0.5 ppm) is set at the purification section outlet to monitor chloride ions. All signals are connected to a PLC controller to form a closed-loop control of "detection-feedback-control". The product is pelletized underwater at 60 ℃, pulverized by airflow at 0.6 MPa, and laser particle size classification to obtain PVB powder with D50=150 μm.

[0034] Post-processing: The particles were first dried in a forced-air dryer at 80 ℃ for 2.5 hours, and then vacuum dried at 100 ℃ for 1.5 hours (under nitrogen protection throughout). They were then subjected to 30kV electrostatic purification and machine vision re-inspection (5 μm accuracy).

[0035] Product performance test results: transmittance 93.5%, haze 0.06%, acetalization degree 78.2 mol%, molar ratio of ternary homologous sequences 0.185, OH blockiness 0.40, melt flow rate 35 mg / 10min, alkali metal content 12 ppm, chloride ion content 2 ppm, transmittance decreased by 0.4% after xenon lamp aging for 1000h.

[0036] Example 2 Raw material pretreatment: PVA with a hydroxyl end-capping rate of 10%, a terminal tert-butyl modification rate of 12%, a degree of polymerization of 2000, and a degree of hydrolysis of 99% was selected; after the pre-saponification process, the acetyl content was 0.5 mol%. A multi-stage countercurrent purification process was adopted: washing with 0.12 mol / L hydrochloric acid aqueous solution for 10 min, followed by washing with a chelating solution containing 60 ppm aminotrimethylenephosphonic acid (ATMP) for 25 min, and rinsing three times with ultrapure water. The alkali metal content of the purified PVA was 20 ppm.

[0037] Reaction system configuration: Aldehyde compound (isobutyraldehyde: 2-ethylhexanal = 7:3) purity 99.9%, acid value 0.6 mgKOH / g. 4-Hydroxycinnamaldehyde addition 1.2%. Hydrochloric acid dosage 9.5 mol H₂. + / kg PVA.

[0038] Continuous reaction: initial reaction section temperature 18 ℃, held for 22 min; stereolithography solidification section temperature 76 ℃, held for 260 min; in-situ purification section vacuum degree -0.088 MPa, 1.0% of 5% sodium bicarbonate solution injected.

[0039] Powdering and post-processing: PVB powder with D50=180 μm was obtained after air jet milling, and then dried and removed in two stages.

[0040] Product performance test results: transmittance 93.2%, haze 0.07%, acetalization degree 80.5 mol%, molar ratio of ternary homologous sequences 0.192, OH blockiness 0.42, melt flow rate 28 mg / 10min, alkali metal content 10 ppm, chloride ion content 1 ppm, transmittance decreased by 0.3% after xenon lamp aging for 1000h.

[0041] Comparative Example 1 Except for changing the temperature of the stereocuring section to 50 ℃ and the holding time to 60 min, the other conditions were the same as in Example 1. Due to insufficient stereocuring, the molar ratio of the three components with the same sequence was only 0.12, the OH blockiness was 0.28, and the melt flow rate was as high as 230 mg / 10 min. Severe edge overflow occurred during lamination, which could not meet the processing requirements.

[0042] Comparative Example 2 A traditional batch reactor was used with hydrochloric acid catalysis. After the reaction, three water washes and one neutralization wash were performed to remove impurities. The raw materials and proportions used were the same as in Example 1. The resulting product had an alkali metal content of 78 ppm and a chloride ion content of 20 ppm. Due to the large fluctuations caused by batch operation, the transmittance varied by 1.5% between batches. The production efficiency of this process was only 35% of that of the continuous process in Example 1, and the wastewater discharge was large.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A continuous preparation method for PVB powder for photovoltaic films, characterized in that, Includes the following steps: (1) Raw material pretreatment: PVA is selected as the basic raw material, and the alkali metal content in PVA is reduced to below 30 ppm through purification process; (2) Reaction system configuration: Aldehyde compounds with specific branched structures are compounded into the purified PVA as acetalizing agents, functional modifiers are added, and acid catalysts are selected. (3) Continuous acetal and stereoregulation reaction: The reaction system prepared in step (2) is continuously injected into the continuous reaction device for staged reaction: (31) Initial reaction stage: The acetalization reaction is triggered at 10-20 ℃, and the reaction time is 20-30 min; (32) Stereoscopic curing stage: Under the heating condition of 70-78 ℃, the PVB molecules are regulated to form a high proportion of synisosteric sequences, and the reaction time is 210-300 min; (33) In-situ purification section: During the reaction at 75-80 °C, unreacted substances and volatile impurities are removed by vacuum degassing, and residual catalyst is neutralized; (4) Intelligent quality control and powder formation: The key parameters of the reaction process are monitored in real time through the online detection module to form a closed-loop control; the reaction products are granulated, crushed and classified to obtain PVB powder with the target particle size; (5) Post-processing: The obtained PVB powder is dried and impurity removed to obtain the final PVB powder product.

2. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (1), the PVA is hydroxyl-terminated polyvinyl alcohol with tert-butyl end-capping, wherein the hydroxyl-termination rate is 5-10%, the tert-butyl end-capping rate is 8-12%, the average degree of polymerization is 1500-2000, and the degree of hydrolysis is ≥98%; the purification process is a multi-stage countercurrent purification process, which reduces the alkali metal content in PVA to ≤30 ppm.

3. The continuous preparation method of PVB powder for photovoltaic films according to claim 2, characterized in that, Step (1) further includes a pre-saponification process, wherein the pre-saponification temperature is controlled at 40-60 °C and the time is 30-60 min, and the acetyl content of the PVA is adjusted to 0.2-2.0 mol.

4. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (2), the aldehyde compound is a mixture of isobutyraldehyde and 2-ethylhexanal, with a molar ratio of 6.5:3.5 to 7.5:2.5, a purity ≥99.9%, and an acid value ≤1 mgKOH / g; the functional modifier is 4-hydroxycinnamaldehyde, added at 0.5 to 1.5% of the PVA mass, with a purity ≥99.5%; the acid catalyst is hydrochloric acid, with a concentration of 28 to 32%, a purity ≥99.9%, and the molar ratio of acid to PVA is controlled to be ≥9.0 mol H₂. + / kg PVA.

5. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (32), the molar ratio of the ternary continuous metaisopropyl vinyl alcohol sequence in the PVB molecule is controlled to be ≥0.152, the OH blockiness parameter VV / (VB+BB)>0.37, the molar ratio of the 5-membered rrrr type metaisopropyl sequence is >0.23, and the molar ratio of the mrrm type sequence is <0.

3.

6. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (33), the in-situ purification is carried out under vacuum conditions, with a vacuum degree of -0.08 to -0.09 MPa; and an alkaline solution is injected into the reaction system to neutralize the residual acid catalyst. The alkaline solution is a sodium bicarbonate solution with a concentration of 4 to 6%, and the amount added is 0.5 to 1.0% based on the total mass of the reaction system.

7. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (4), the online detection module includes at least an online moisture detector, an online carbon-13 nuclear magnetic resonance rapid detection module, and an online ion chromatography detection module; the granulation is underwater granulation, the pulverization is air jet pulverization, and the grading is particle size screening to obtain PVB powder with a particle size D50 of 100-200 μm.

8. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, In step (5), the drying is a two-stage drying process, which includes first air drying and then vacuum drying; the impurity removal process includes electrostatic impurity removal and / or machine vision re-inspection.

9. The continuous preparation method of PVB powder for photovoltaic films according to claim 1, characterized in that, The continuous reaction device is a twin-screw extruder with a screw length-to-diameter ratio (L / D) of 35–45 and a screw diameter of 30–60 mm.

10. A PVB powder for photovoltaic films, characterized in that, The PVB powder is prepared by the continuous preparation method for photovoltaic films according to any one of claims 1 to 9. The PVB powder has a transmittance (400-800 nm) ≥93%, haze ≤0.08%, melt flow rate (100 ℃ / 21.6 kg) ≤50 mg / 10min, acetalization degree of 75-84 mol%, anti-aging group grafting rate ≥90%, alkali metal content ≤15 ppm, halogen content ≤5 ppm, and transmittance decrease of ≤0.5% after aging in a xenon lamp for 1000 hours.