Optical grade low defect laminated film and clean forming method thereof

Abstract

The present application relates to the technical field of polymer functional film processing, and particularly relates to an optical low-defect laminated film and a clean forming method thereof.The technical scheme comprises that the prepared laminated film is an integrally formed three-layer structure, which is composed of a core layer and surface layers symmetrically arranged on the upper and lower surfaces of the core layer, and the method comprises raw material pretreatment, melt blending and devolatilization, branch gradient deep layer filtration, three-layer co-extrusion air floating film forming, inert gas cooling and shaping, online detection and closed-loop regulation and control, and clean winding.The present application greatly reduces internal and external defects of the film, inhibits molecular orientation and birefringence, realizes high-precision thickness control, and the obtained film has excellent optical performance, is suitable for various resin systems, and can stably and continuously produce high-end optical laminated film in industrial production.

Description

Technical Field

[0001] This invention relates to the field of polymer functional film processing technology, and in particular to an optical-grade low-defect laminated film and its clean forming method. Background Technology

[0002] Laminated films are the core adhesive and functional layer materials in laminated safety glass and optical display components, and are widely used in automotive windshields, building explosion-proof glass, and optical bonding of flat panel displays.

[0003] Traditional methods for forming laminated films mainly include casting and calendering. Casting involves casting a resin solution or melt onto a polished metal roller surface using a die, followed by heating to evaporate the solvent or cooling before peeling to form a film. Calendering utilizes the gaps between multiple heated rollers to extrude the film. While these methods are technologically mature, they suffer from several insurmountable drawbacks: Firstly, residual moisture and low-molecular-weight volatiles in the raw materials can generate bubbles and degradation spots during high-temperature processing. Conventional single-stage or coarse filtration cannot effectively remove submicron-level impurities, resulting in a high density of internal defects in the film. Secondly, during film formation and cooling, the film surface must be in continuous contact with multiple solid guide rollers and cooling rollers, making it highly susceptible to indentations, scratches, spots, and localized thickness fluctuations due to roller surface contamination, mechanical vibration, and uneven thermal stress. Furthermore, existing processes often employ air or cold roller contact cooling, resulting in poor cooling uniformity and making it difficult to meet the requirements for optical-grade applications in terms of in-plane birefringence and thickness tolerance.

[0004] To address the aforementioned issues, some improved technologies have attempted to produce in cleanrooms, increase the number of filtration stages, or employ single-sided air cushion support. However, these still cannot systematically and simultaneously solve the comprehensive problems of internal ultrafine particle contamination, surface contact defects, stress birefringence caused by uneven cooling, and closed-loop control of full-width thickness. Therefore, there is an urgent need for a laminated film forming method that can achieve clean processing and contactless transfer throughout the entire process, from raw material handling, melt purification, film formation, cooling and shaping to winding. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the background art by proposing an optical-grade low-defect laminated film and its clean forming method.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a clean forming method for optical-grade low-defect laminated films, wherein the resulting laminated film is an integrally formed three-layer structure, consisting of a core layer and surface layers symmetrically disposed on the upper and lower surfaces of the core layer; the method includes the following steps: S1. Raw material pretreatment: The resin raw material is dried to a moisture content of ≤50ppm; when the surface layer contains nanofillers, the nanofillers are pre-dispersed with the resin matrix or prepared as masterbatch before being added. S2. Melt blending and devolatilization: The dried resin raw material is fed into a twin-screw extruder for melt blending. At the same time, volatiles and dissolved gases are removed by vacuum devolatilization to obtain a melt without visible bubbles and with uniformly dispersed components. S3. Divided gradient deep filtration: The melt is divided into two paths: core melt and surface melt; wherein, the core melt is purified by two-stage deep filtration, and the surface melt is purified by three-stage deep filtration. S4. Three-layer co-extrusion air flotation film formation: The purified core layer melt and surface layer melt are fed into a three-layer co-extrusion slit die for composite extrusion. The core layer is located in the middle, and the surface layer is symmetrically distributed on the upper and lower sides. After extrusion, a nascent film is formed under the support of clean gas air flotation. The nascent film does not come into contact with the solid guide roller during the film formation process. S5. Inert gas cooling and shaping: The nascent film is introduced into a sealed cooling chamber and cooled by airflow using inert gas as the cooling medium, so that the film can be quickly and uniformly shaped. S6. Online detection and closed-loop control: The dark field optical detection system is used to perform full-area online detection of the film, identify defect information including impurity particles, bubbles and surface defects, and provide real-time feedback to adjust the die temperature, extrusion flow rate and traction speed. S7. Clean winding: In a clean environment, the film is wound up using a non-contact air shaft to obtain the optical-grade low-defect laminated film.

[0007] Furthermore, in S3: The two-stage deep filtration system of the core melt includes: a first stage of stainless steel woven mesh filter with a filtration accuracy of 20-30 μm; and a second stage of metal fiber sintered felt filter with a filtration accuracy of 5-10 μm. The three-stage deep filtration system of the surface melt includes: the first stage is a stainless steel woven mesh filter with a filtration accuracy of 20-30 μm; the second stage is a metal fiber sintered felt filter with a filtration accuracy of 5-10 μm; and the third stage is a precision deep filtration membrane with an electrostatic adsorption layer with a filtration accuracy of 0.1-0.3 μm.

[0008] Furthermore, in step S5, the inert gas is helium with a purity ≥99.999%, the cooling gas flow is injected in a transverse or vertical direction relative to the film travel direction, the gas flow velocity is 15-25 m / s, and the internal temperature of the cooling chamber is set according to the glass transition temperature Tg of the resin matrix and controlled within the range of 5-15°C below the Tg of the resin matrix; when the resin matrix is ​​polyvinyl butyral, the internal temperature of the cooling chamber is preferably 10-15°C.

[0009] Furthermore, in S4, the opening adjustment accuracy of the three-layer co-extrusion slit die is ±0.2μm, the internal flow channel of the die is electrolytically polished, and the surface roughness Ra of the inner wall is ≤0.05μm; the air flotation support uses clean and dry gas, and the air flotation gap is 0.5~1mm.

[0010] Furthermore, in S2, the twin-screw extruder is equipped with 2 to 3 vacuum devouring ports, with a vacuum degree ≤ -0.095MPa, a melt temperature controlled at 15 to 25°C above the resin melting point, and a screw speed of 200 to 350 rpm.

[0011] Furthermore, in S6, the minimum identifiable defect size of the dark field optical detection system is 0.3 μm, and the response time of the closed-loop control system is ≤100 ms. The closed-loop control is achieved by adjusting the servo actuator of the flow blocking block inside the die head and the speed of the traction system, which is used to improve the uniformity of film thickness.

[0012] An optical-grade low-defect laminated film is disclosed. The laminated film is a three-layer structure integrally co-extruded, consisting of a core layer and surface layers symmetrically arranged on the upper and lower surfaces of the core layer. The number of impurity particles with a particle size ≥ 5 μm in the core layer is ≤ 20 particles / m², and the number of impurity particles with a particle size ≥ 1 μm in the surface layer is ≤ 5 particles / m². The overall thickness tolerance of the laminated film is within ± 0.8 μm, and the visible light haze is ≤ 0.3%.

[0013] Furthermore, the thickness of the core layer accounts for 70% to 85% of the total thickness of the film, and the thickness of the single surface layer accounts for 7.5% to 15% of the total thickness of the film. Moreover, the core layer and the surface layer are an integral structure formed by co-extrusion molding of the same resin matrix.

[0014] Furthermore, the laminated film has a transmittance of ≥91.5% in the 400-780nm wavelength range, an in-plane birefringence value of ≤10nm, and a maximum height difference of surface defects of the film of ≤0.5μm.

[0015] Furthermore, the resin matrix is ​​selected from one of polyvinyl butyral, ethylene-vinyl acetate copolymer, polymethyl methacrylate or thermoplastic polyurethane; the surface layer uniformly disperses 0.1% to 0.5% by mass of nano-silica particles, the nano-silica particles having a particle size of 20 to 50 nm.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: By combining deep drying of raw materials to a moisture content of ≤50ppm with multi-stage vacuum devolatilization, bubbles and degradation spots are eliminated at the source. Further employing a multi-stage gradient deep filtration process, with the core layer undergoing secondary filtration and the surface layer undergoing tertiary precision filtration with an electrostatic adsorption layer, submicron particles on the surface can be effectively captured. The resulting film exhibits ≤20 impurity particles ≥5μm in the core layer and ≤5 impurity particles ≥1μm in the surface layer, far exceeding the levels achieved by traditional processes. The film is formed by three-layer co-extrusion air flotation, with the nascent film running on a clean gas cushion without contact with solid guide rollers throughout the process; the winding end uses a non-contact air expansion shaft. This completely eliminates scratches, indentations, and particle adhesion caused by roller surface contact, and the maximum height difference of film surface defects is ≤0.5μm; Inert gas (high-purity helium) cooling and shaping provides a rapid, uniform, and isotropic thermal conduction environment. Combined with multi-temperature gradient cooling control, it effectively suppresses thickness inhomogeneity and molecular orientation caused by thermal stress. By combining online dark-field detection and multivariable closed-loop control, the overall film thickness tolerance can be stably controlled within ±0.8 μm, and the in-plane birefringence value ≤10 nm. Thanks to its extremely low internal defects, uniform thickness, and tiny surface undulations, the laminated film produced has a transmittance of ≥91.5% in the 400–780 nm wavelength range and a visible light haze of ≤0.3%, fully meeting the stringent requirements for clarity and transmittance in high-end optical-grade laminated applications. The method is applicable to various resin matrices such as PVB, EVA, PMMA, and TPU, and can stably obtain low-defect films regardless of whether nano-functional fillers are introduced on the surface. The entire filtration system is equipped with online differential pressure monitoring and non-stop switching, with a closed-loop control response time of ≤100ms, ensuring consistency and high yield in long-term continuous production. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides a clean forming method for optical-grade low-defect laminated films. This method involves deep drying of raw materials, melt devolatilization, multi-stage gradient deep filtration, three-layer co-extrusion air flotation film formation, inert gas cooling and shaping, online detection and closed-loop control, and clean winding to produce a low-defect, high-optical-quality integrated three-layer laminated film. Figure 1 As shown, it includes the following steps: Step S1: Raw material pretreatment The selected resin matrix was deeply dried in a vacuum drying oven using a multi-stage heating program, maintaining a vacuum level below 100 Pa. The drying time was dynamically adjusted according to the initial moisture content of the resin, and the moisture content of the discharged gas was monitored using an online dew point meter to ensure that the moisture content was strictly controlled at ≤50 ppm. For example, polyvinyl butyral (PVB) was vacuum dried at 80°C for 6 hours, and the measured moisture content was 30 ppm using the Karl Fischer coulometric method; ethylene-vinyl acetate copolymer (EVA) was vacuum dried at 55°C for 8 hours, and the moisture content was approximately 40 ppm. When nanofillers were introduced into the surface formulation, the nanoparticles were not directly added to the main extruder. Instead, nano-silica (average particle size 30 nm, range 20–50 nm) was premixed with a small amount of the same resin powder in a high-speed mixer at 1500 rpm for 5 minutes, and then fed into a twin-screw extruder for granulation at a temperature below the resin melting point to produce a masterbatch with a nanofiller content of 15–25 wt%. This masterbatch method can effectively avoid secondary agglomeration of nanoparticles, enabling the particles to achieve primary particle-level dispersion in the surface layer.

[0020] Step S2: Melt blending and devolatilization A co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1 is used. The barrel features 10 independent heating zones along its length, precisely controlling the temperature of each section. A forced-cooling feeding section prevents powder bridging. Three heating zones and three vacuum devouring ports are sequentially arranged along the screw direction, located at the end of the melting section, the mixing section, and the metering section, respectively, to ensure that volatiles fully escape from the high specific surface area of ​​the melt. The dried resin (and the required masterbatch) is fed into the extruder via a loss-in-weight metering feeder. For example, PVB resin has a melting point of approximately 140°C, and the melting section temperature is controlled at 160°C (20°C above the melting point); EVA resin has a melting point of approximately 78°C, and the melting temperature is set at 98°C; polymethyl methacrylate (PMMA) has a melting point of approximately 160°C, and the melting temperature is set at 180°C. The screw speed is adjustable between 220 and 320 rpm, and the vacuum degree at each devouring port is maintained at ≤-0.095 MPa. The vacuum pipeline is equipped with an online-cleanable condenser to prevent volatile substances such as plasticizers from entering the vacuum pump. Residual moisture, monomers, and dissolved gases in the melt are removed through multi-stage vacuum removal, resulting in a transparent melt with no visible bubbles and uniform dispersion of all components.

[0021] Step S3: Split gradient deep filtering The homogeneous melt output from the twin-screw extruder is pressurized by a high-precision gear melt pump. The entire delivery pipeline is insulated with a heat-conducting oil jacket, controlling temperature fluctuations within ±1℃. The melt is divided into two streams: the core melt and the surface melt, each entering an independent deep filtration system. The core melt passes through: a first-stage stainless steel woven mesh filter with a filtration accuracy of 25μm and a filtration area designed to be more than three times the melt flow rate to extend its service life; and a second-stage metal fiber sintered felt filter with a filtration accuracy of 5μm and a pleated structure to increase dirt-holding capacity. Through these two stages of filtration, impurities with a diameter ≥5μm in the core layer are effectively removed.

[0022] Because the surface melt is in direct contact with air or glass, it is more sensitive to defects. Therefore, a three-stage deep filtration system is employed: a first-stage stainless steel woven mesh filter (20μm), a second-stage metal fiber sintered felt filter (5μm), and a third-stage precision deep filtration membrane with an electrostatic adsorption layer. This membrane is composed of a modified polytetrafluoroethylene porous membrane with a permanently positive charge and a charge density of not less than 2.0 × 10⁻⁻⁻⁴. 4 With a filter density of C / m² and an average pore size of 0.2μm (selectable within the range of 0.1 to 0.3μm), this system utilizes both electrostatic adsorption and physical trapping to capture submicron-sized particles, ensuring ultra-high cleanliness of the surface melt. The entire filtration system is equipped with an online differential pressure sensor and a non-stop switching valve assembly. If the differential pressure exceeds the limit, it automatically switches to the backup filter, ensuring continuous and stable production.

[0023] Step S4: Three-layer co-extrusion air flotation film formation The core melt and surface melt, after deep filtration, are precisely metered separately. The metering pump is driven by an independent servo motor, with flow fluctuations of less than 0.5%, and fed into a three-layer co-extrusion slit die. The internal flow channels of the die are electrolytically polished and feature an optimized coat hanger design to eliminate melt stagnation zones. The measured surface roughness Ra of the inner wall is 0.03μm (≤0.05μm). Multiple independent temperature control zones and differential bolt-type fine-tuning push rods are set in the width direction of the die. The opening gap is adjusted by a servo mechanism with an adjustment accuracy of ±0.2μm. The core melt is extruded from the middle flow channel, and the upper and lower surface melts are symmetrically composited on both sides of the core layer to form an integrated three-layer melt film.

[0024] The extruded nascent film then enters the air flotation support zone. Clean, dry nitrogen or compressed air is used. The gas is pre-filtered with a precision of 0.01μm and kept at a constant temperature close to the melt temperature to avoid stress caused by sudden cooling upon contact. A uniform air cushion is formed through a porous air flotation plate, and the air flotation gap is maintained at 0.8mm (adjustable within the range of 0.5 to 1mm). The length of the air flotation support zone is not less than 300mm, allowing the nascent film to move smoothly without contact with solid guide rollers, completely avoiding surface scratches, indentations, and contamination caused by contact rollers.

[0025] Step S5: Inert gas cooling and shaping The nascent thin film is introduced into a fully enclosed cooling chamber made of stainless steel with a mirror-polished inner wall and an external insulation layer. High-purity helium (≥99.999%) is used as the cooling medium, employing a closed-loop circulation system. During circulation, the helium passes sequentially through a cold trap and a 0.1μm filter to remove potential volatiles and particles, and is then recooled to the set temperature. Multiple rows of slit nozzles, evenly distributed laterally above and below the film, are used to inject helium gas perpendicular to the film's direction of travel at a velocity of 20 m / s (15–25 m / s). The interior of the cooling chamber is divided into multiple temperature zones along the film's direction of travel, creating a gradient cooling effect and effectively preventing internal stress caused by rapid cooling. Helium's high thermal conductivity and high heat capacity ratio ensure rapid and uniform cooling. The temperature within the cooling chamber is set according to the glass transition temperature (Tg) of the resin matrix used, controlled within a range of 5–15°C below Tg. High-precision PID control achieves a temperature stability of ±0.5°C. For PVB resin with a Tg of approximately 25°C after adding plasticizers, the cooling chamber temperature is preferably controlled at 12°C (within the range of 10-15°C); for EVA with a Tg of approximately -30°C, the cooling chamber temperature is set to -40°C; and for PMMA with a Tg of approximately 105°C, the cooling chamber temperature is set to 95°C. The residence time of the film in the chamber is set according to the thickness calculation to ensure that the core layer center temperature also drops below Tg. Through rapid cooling, the film shape and size are precisely locked, effectively suppressing subsequent shrinkage and deformation.

[0026] Step S6: Online Detection and Closed-Loop Control After cooling and setting, a dark-field optical inspection system is installed to perform 100% full-width real-time scanning of the entire film. This system, using high-brightness laser dark-field illumination at a wavelength of 405nm and an 8K high-speed linear array camera with a line frequency of 40kHz, can reliably identify impurity particles, bubbles, and surface irregularities as small as 0.3μm, and can automatically classify and statistically analyze defects by size and type. The detection signal is transmitted to the central control system within 100ms. The control system incorporates a film thickness prediction model, which, in conjunction with data from an online infrared thickness gauge, detects local thickness deviations or an increasing trend in defect density. When this occurs, the system uses a closed-loop control mechanism to adjust the servo actuator of the flow-blocking block within the die head, fine-tuning the die lip opening. Simultaneously, it dynamically adjusts the extruder flow rate and traction speed, forming a multi-variable collaborative control system. This ensures that the overall film thickness tolerance is controlled within an extremely narrow range, while keeping the number of defects below the target level.

[0027] Step S7: Clean winding The entire film formation, cooling, and testing area is maintained in an ISO 5 (Class 100) clean environment. Vertical unidirectional airflow is created through a full array of FFUs (Fan Filter Units) and a positive pressure differential of 25 Pa is maintained. The shaped film is wound onto a non-contact air-expanding shaft. The shaft surface is perforated to form an air cushion, and the air cushion pressure is zoned to accommodate changes in roll diameter. There is no physical contact between the film and the shaft, avoiding indentations, scratches, and particle adhesion during winding. Winding employs a center-driven mode, using a precision servo motor and tension sensor in a closed-loop control system to achieve constant tension or linear taper tension with a tension control accuracy of ±1%, resulting in a neat, wrinkle-free laminated film roll.

[0028] The laminated film prepared by the above method is a three-layer co-extruded structure with a core layer in the middle and two surface layers symmetrically distributed on the upper and lower surfaces of the core layer. The core layer and surface layers are integral structures formed by co-extrusion of the same resin matrix, eliminating the risk of interlayer separation. The core layer accounts for 75%–82% of the total film thickness, and the single surface layer accounts for 9%–12.5%. High-precision microscopy and particle counter testing in a cleanroom showed that the number of impurity particles with a diameter ≥5μm in the core layer was ≤15 particles / m², and the number of impurity particles with a diameter ≥1μm in the surface layer was ≤4 particles / m². The overall film thickness tolerance is consistently within ±0.6μm. Visible light haze is ≤0.25%, transmittance in the 400–780nm wavelength range is ≥91.8%, in-plane birefringence is ≤8nm, and the maximum height difference of surface defects is ≤0.4μm, fully meeting the requirements for optical-grade laminated applications.

[0029] The following examples further illustrate the implementation of the method of the present invention under different resin systems and formulations.

[0030] Example 1 The resin matrix is ​​PVB containing plasticizer, using powder with a number average molecular weight of approximately 250,000, containing 28% triethylene glycol di(2-ethylbutyrate) plasticizer, with a glass transition temperature (Tg) of 25℃ and a melting point of approximately 140℃. No nanofillers are added to the surface layer. Process parameters: In S1, PVB is dried to a moisture content of 30ppm at 80℃ and a vacuum of 80Pa; in S2, the melting temperature is 160℃, the measured melt temperature fluctuation is ±1℃, the screw speed is 280rpm, and the vacuum degree of the three devouring ports is -0.098MPa; in S3, the core layer filtration precision is 20μm woven mesh and 5μm metal fiber sintered felt, and the surface layer filtration precision is 20μm woven mesh, 5μm metal fiber sintered felt, and 0.2μm precision membrane with electrostatic adsorption; in S4, the die head gap is controlled by servo closed loop, and the air flotation gap is 0.8mm; in S5, 99.999% helium is used for cooling, the airflow velocity is 20m / s, the cooling chamber temperature is 12℃, and the chamber is divided into 3 temperature zones along its length; in S6, the minimum identification size for dark field detection is 0.3μm, and the closed loop response time is 80ms; in S7, non-contact winding is carried out in an ISO5 cleanroom, and surface static electricity is eliminated by an ion fan before winding. The resulting film has a total thickness of 100 μm, a core layer thickness of 75 μm, and a single surface layer thickness of 12.5 μm. The average results obtained from 10 samples taken from the beginning, middle, and end of the same roll are as follows: 11 particles ≥5 μm in the core layer / m², 3 particles ≥1 μm in the surface layer / m², thickness tolerance ±0.5 μm, haze 0.22%, light transmittance 92.1%, in-plane birefringence 6 nm, and maximum surface height difference 0.3 μm.

[0031] Example 2 The resin matrix remains PVB (Tg 25℃), but nano-silica is added to the surface layer to improve anti-blocking properties. During masterbatch preparation, nano-SiO2 is first pretreated with a liquid-phase coating, a small amount of silane coupling agent is sprayed in and stirred in a high-speed mixer, and then it is blended and granulated with PVB powder. SiO2 with an average particle size of 30nm is first mixed with PVB to form a masterbatch containing 20wt% SiO2, and then added at a ratio of 0.3wt% of the total surface resin content of SiO2. In S1, the surface resin and masterbatch are mixed and dried together to a moisture content of 35ppm; in S2, the melting temperature is 165℃, and the screw speed is 300rpm; the third stage of surface filtration uses a 0.1μm electrostatic adsorption filter membrane, and the remaining conditions are the same as in Example 1. The finished membrane has a total thickness of 150μm, with a core layer of 120μm and surface layers of 15μm each. The core layer has 14 particles ≥5μm / m², the surface layer has 2 particles ≥1μm / m², the thickness tolerance is ±0.6μm, the haze is 0.25%, the light transmittance is 91.9%, the birefringence is 7nm, the maximum surface height difference is 0.4μm, and there is no adhesion between the layers after the film is wound up, and the unwinding is smooth.

[0032] Example 3 The resin matrix was EVA (melt index 25 g / 10 min, 190℃, 2.16 kg, vinyl acetate content 28%), Tg approximately -30℃, melting point 78℃. No nanofillers were added. S1 drying was performed to a moisture content of 40 ppm; S2 melting temperature was 98℃, screw speed 250 rpm; S5 cooling medium was 99.999% helium, airflow velocity 18 m / s, with liquid nitrogen pre-cooling of helium to achieve a cooling chamber temperature of -40℃; other parameters were consistent with the principles of Example 1. The finished film had a total thickness of 200 μm, a core layer of 160 μm, and surface layers of 20 μm each. The core layer had 18 particles ≥5 μm / m², the surface layer had 5 particles ≥1 μm / m², thickness tolerance ±0.7 μm, haze 0.28%, transmittance 91.5%, and birefringence 9 nm. This indicates that the method is also effective for the EVA system.

[0033] Example 4 The resin matrix is ​​PMMA, using optical-grade granules with a measured Tg of 110℃ and a melting point of 160℃. 0.5wt% nano-SiO2 (SiO2 surface-modified with methacryloxysilane, particle size 30nm, added as masterbatch) is added to the surface layer. S1 drying is performed to a moisture content of 25ppm; S2 melting temperature is 180℃, screw speed is 220rpm; S5 cooling medium is helium, airflow velocity is 22m / s, cooling chamber temperature is 95℃; the rest is as described in Example 2. The finished film has a total thickness of 120μm, with a core layer of 96μm and surface layers of 12μm each. The core layer has ≥5μm particles (12 particles / m²), the surface layer has ≥1μm particles (4 particles / m²), a thickness tolerance of ±0.4μm, a haze of 0.18%, a transmittance of 92.5%, birefringence of 5nm, and a surface height difference of 0.2μm, exhibiting excellent optical properties.

[0034] Example 5 The resin matrix is ​​thermoplastic polyurethane (TPU), polyether type, with a hardness of 85A, a Tg of approximately -15℃, and a melting point of approximately 160℃. No nanofillers are added to the surface layer. S1 is vacuum dried at 100℃ for 4 hours until the moisture content reaches 45ppm; S2 has a melting temperature of 180℃ and a screw speed of 260rpm; S5 has a cooling chamber temperature of -25℃ and a helium flow rate of 21m / s; the rest is the same as in Example 1. The total film thickness is 80μm, with a core layer of 64μm and each surface layer of 8μm. The core layer has 16 particles ≥5μm / m², and the surface layer has 4 particles ≥1μm / m², with a thickness tolerance of ±0.6μm, a haze of 0.24%, a light transmittance of 91.7%, and birefringence of 8nm. The film is flexible and has high optical clarity.

[0035] The above embodiments demonstrate that the clean molding method of the present invention is applicable to various resin matrices such as PVB, EVA, PMMA, and TPU, and can obtain optical-grade low-defect laminated films regardless of whether nanofillers are introduced into the surface layer. The films exhibit few impurities and defects, excellent thickness uniformity, low haze, high light transmittance, and low birefringence, meeting the stringent requirements of high-end laminated glass, optical displays, and other fields.

[0036] The above specific embodiments are merely several further embodiments of the present invention. Based on the technical solutions of the present invention and the relevant teachings of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

[0037] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A clean forming method for optical-grade low-defect laminated films, characterized in that... The resulting laminated film is a three-layer structure integrally formed, consisting of a core layer and surface layers symmetrically arranged on the upper and lower surfaces of the core layer; the method includes the following steps: S1. Raw material pretreatment: The resin raw material is dried to a moisture content of ≤50ppm; when the surface layer contains nanofillers, the nanofillers are pre-dispersed with the resin matrix or prepared as masterbatch before being added. S2. Melt blending and devolatilization: The dried resin raw material is fed into a twin-screw extruder for melt blending. At the same time, volatiles and dissolved gases are removed by vacuum devolatilization to obtain a melt without visible bubbles and with uniformly dispersed components. S3. Divided gradient deep filtration: The melt is divided into two paths: core melt and surface melt; wherein, the core melt is purified by two-stage deep filtration, and the surface melt is purified by three-stage deep filtration. S4. Three-layer co-extrusion air flotation film formation: The purified core layer melt and surface layer melt are fed into a three-layer co-extrusion slit die for composite extrusion. The core layer is located in the middle, and the surface layer is symmetrically distributed on the upper and lower sides. After extrusion, a nascent film is formed under the support of clean gas air flotation. The nascent film does not come into contact with the solid guide roller during the film formation process. S5. Inert gas cooling and shaping: The nascent film is introduced into a sealed cooling chamber and cooled by airflow using inert gas as the cooling medium, so that the film can be quickly and uniformly shaped. S6. Online detection and closed-loop control: The dark field optical detection system is used to perform full-area online detection of the film, identify defect information including impurity particles, bubbles and surface defects, and provide real-time feedback to adjust the die temperature, extrusion flow rate and traction speed. S7. Clean winding: In a clean environment, the film is wound up using a non-contact air shaft to obtain the optical-grade low-defect laminated film.

2. The clean forming method for an optical-grade low-defect laminated film according to claim 1, characterized in that: In S3: The two-stage deep filtration system of the core melt includes: a first stage of stainless steel woven mesh filter with a filtration accuracy of 20-30 μm; and a second stage of metal fiber sintered felt filter with a filtration accuracy of 5-10 μm. The three-stage deep filtration system of the surface melt includes: the first stage is a stainless steel woven mesh filter with a filtration accuracy of 20-30 μm; the second stage is a metal fiber sintered felt filter with a filtration accuracy of 5-10 μm; and the third stage is a precision deep filtration membrane with an electrostatic adsorption layer with a filtration accuracy of 0.1-0.3 μm.

3. The clean forming method for an optical-grade low-defect laminated film according to claim 1, characterized in that: In step S5, the inert gas is helium with a purity ≥99.999%, and the cooling gas flow is injected in a transverse or vertical direction relative to the film's travel direction, with a gas flow velocity of 15-25 m / s. The internal temperature of the cooling chamber is set according to the glass transition temperature Tg of the resin matrix and controlled within a range of 5-15°C below the Tg of the resin matrix. When the resin matrix is ​​polyvinyl butyral, the internal temperature of the cooling chamber is preferably 10-15°C.

4. The clean forming method for an optical-grade low-defect laminated film according to claim 1, characterized in that: In S4, the opening adjustment accuracy of the three-layer co-extrusion slit die is ±0.2μm, the internal flow channel of the die is electrolytically polished, and the surface roughness Ra of the inner wall is ≤0.05μm; the air flotation support uses clean and dry gas, and the air flotation gap is 0.5~1mm.

5. The clean forming method for an optical-grade low-defect laminated film according to claim 1, characterized in that: In S2, the twin-screw extruder is equipped with 2 to 3 vacuum devouring ports, with a vacuum degree ≤ -0.095MPa, a melting temperature controlled at 15 to 25°C above the resin melting point, and a screw speed of 200 to 350 rpm.

6. The clean forming method for an optical-grade low-defect laminated film according to claim 1, characterized in that: In S6, the minimum identifiable defect size of the dark field optical detection system is 0.3 μm, and the response time of the closed-loop control system is ≤100 ms. The closed-loop control is achieved by adjusting the servo actuator of the flow blocking block inside the die head and the speed of the traction system, which is used to improve the uniformity of film thickness.

7. An optical-grade low-defect laminated film, characterized in that: The laminated film is a three-layer structure integrally co-extruded, consisting of a core layer and surface layers symmetrically arranged on the upper and lower surfaces of the core layer; the number of impurity particles with a particle size ≥ 5 μm in the core layer is ≤ 20 particles / m², and the number of impurity particles with a particle size ≥ 1 μm in the surface layer is ≤ 5 particles / m²; the overall thickness tolerance of the laminated film is within ±0.8 μm, and the visible light haze is ≤ 0.3%.

8. The optical-grade low-defect laminated film according to claim 7, characterized in that: The core layer accounts for 70% to 85% of the total film thickness, and the single surface layer accounts for 7.5% to 15% of the total film thickness. The core layer and the surface layer are integral structures formed by co-extrusion molding of the same resin matrix.

9. The optical-grade low-defect laminated film according to claim 7, characterized in that: The laminated film has a transmittance of ≥91.5% in the 400-780nm wavelength range, an in-plane birefringence value of ≤10nm, and a maximum height difference of surface defects of the film of ≤0.5μm.

10. The optical-grade low-defect laminated film according to claim 7, characterized in that: The resin matrix is ​​selected from one of polyvinyl butyral, ethylene-vinyl acetate copolymer, polymethyl methacrylate or thermoplastic polyurethane; the surface layer is uniformly dispersed with nano-silica particles at a mass ratio of 0.1% to 0.5%, and the particle size of the nano-silica particles is 20 to 50 nm.

Images