Brightening aliphatic TPU base film for invisible car cover and preparation method of brightening aliphatic TPU base film

Abstract

The invention relates to a brightening aliphatic TPU base film for invisible car clothing and a preparation method of the brightening aliphatic TPU base film. The base film comprises the following components in parts by weight: 100 parts of aliphatic TPU, 0.5-3 parts of an optical-grade modifier, 1-5 parts of a polyester polyol-siloxane copolymer compatibilizing and brightening agent, an anti-hydrolysis stabilizer, an ultraviolet light absorber and a hindered amine light stabilizer; the modifier is nano SiO2 and / or nano Al2O3 treated by amino silane with the average particle size of 10nm-50nm, and the refractive index difference between the modifier and TPU is less than or equal to 0.02. The preparation method comprises the following steps: dispersing nanoparticles in an alcohol solvent, dropwise adding a silane coupling agent at 40-60 DEG C, replenishing water for in-situ hydrolysis, completing coupling and curing grafting under the condition that the pH value is 4-5, and then carrying out melt blending, filtering and curtain coating and stretching to form the film. The base film provided by the invention has the advantages of high light transmittance, low haze and brightening effect, the hydrolysis resistance and weather resistance stability are improved, and the batch stability is realized.

Description

Technical Field

[0001] This application belongs to the technical field of polymer functional films and automotive surface protection materials, specifically relating to an aliphatic TPU base film for enhancing brightness in invisible car wraps and its preparation method. Background Technology

[0002] As a crucial material for protecting automotive paint, paint protection film (PPF) typically requires the base film to possess high light transmittance, low haze, high surface gloss, and good image clarity, while maintaining resistance to yellowing, hydrolysis, and UV aging under long-term outdoor use. Aliphatic TPU is widely used as a base film for PPF due to its relatively excellent resistance to yellowing; however, in practical applications, it still faces the challenge of simultaneously achieving multiple objectives: on the one hand, to enhance the visual effects of brightening and deepening after PPF coverage, functional components that improve interfacial compatibility and optical uniformity are often introduced; on the other hand, while nano-inorganic particles can improve the wear resistance and stability of the film, their dispersion and interfacial matching within the TPU system are difficult. If particles aggregate or have a large difference in refractive index with the matrix, it can easily lead to enhanced light scattering, resulting in increased haze, decreased light transmittance, and deteriorated image clarity.

[0003] In addition, paint protection film is prone to aging behaviors such as soft segment hydrolysis, photo-oxidation and chain segment breakage in environments with high temperature and humidity and ultraviolet radiation, which can lead to yellowing, haze, gloss reduction or mechanical property degradation of the film.

[0004] In the prior art, Chinese patent application CN118578620A discloses a method for preparing a car cover protective film. This technology involves premixing TPU masterbatch with antioxidants and UV absorbers, drying and dehumidifying the mixture, then melting and plasticizing it using a screw press and removing impurities from the melt. Matting powder is added to the melt and mixed evenly before extrusion and casting into a film. The finished product is then completed through cold pressing, roller thickness adjustment, edge trimming, and winding. The technical concept lies in using matte powder to create a matte, frosted film layer to reduce reflection, and improving film thickness uniformity and dimensional consistency through cold pressing, roller thickness adjustment, and edge trimming.

[0005] Chinese patent CN 118599462B discloses a high-yellowing-resistant, high-adhesion TPU hot melt adhesive for paint protection film, its preparation method, and its application. This technical solution uses a polymeric diol and aliphatic diisocyanate as the main components, and incorporates a curing system composed of a small-molecule diol dioxime and trimethylolpropane in a predetermined molar ratio, along with plasticizers and other components. The technical concept involves embedding oxime ester bonds into the structure using dioxime, allowing the oxime ester bonds to gradually dissociate and decrease viscosity at temperatures above 90°C. The plasticizer further reduces the system viscosity to facilitate coating and film application. Simultaneously, the design of aliphatic isocyanate and TMP balances yellowing resistance and adhesive strength.

[0006] It is evident that existing technologies primarily focus on improving the adhesive layer material system or the base film molding process. For example, they address viscosity control and yellowing resistance within the bonding temperature window through hot melt adhesive formulation design, or improve appearance consistency and surface reflectivity through drying, dehumidification, melt sieving, and the introduction of matting agents during TPU melt extrusion casting. However, without systematic control over the surface chemical state of nanoparticles, their refractive index matching with TPU, and filtration and impurity removal during melt processing, problems such as large fluctuations in optical properties, insufficient batch stability, and unsatisfactory long-term weather resistance may still occur. Therefore, there is an urgent need for an aliphatic TPU base film technology that synergistically optimizes material formulation, nanoparticle surface modification processes, melt blending, and precision filtration film-making processes. This technology could achieve stable brightening effects while maintaining high transparency and low haze, and also ensure hydrolysis resistance and weather resistance. Summary of the Invention

[0007] This application addresses the problems of existing aliphatic TPU base films for paint protection films, which still suffer from insufficient brightening and depth, unstable dispersion of nanofillers leading to scattering and fogging, optical performance degradation under outdoor ultraviolet and humid conditions, and difficulty in maintaining batch consistency over a long period of time, despite the pursuit of high light transmittance and low haze. It proposes a brightening aliphatic TPU base film for paint protection films and its preparation method, with refractive index matching and in-situ interface coupling as the core. This application improves optical uniformity and surface gloss by introducing refractive index-matched surface-treated nano-silica and / or nano-alumina, and compounding them with polyester polyol siloxane copolymer compatibilizers and brighteners. Simultaneously, ultrasonic and shear dispersion are employed in the water-alcohol system, and in-situ hydrolysis coupling and grafting of aminosilanes are carried out at 40 to 60°C. The water-to-silane molar ratio is controlled at 13:1 to 25:1, and the pH is 4 to 5 to complete the coupling and grafting, reducing agglomeration and interfacial mismatch at the source. During the film formation process, vacuum drying, inert premixing, melt blending, precision filtration, and process constraints such as casting, biaxial stretching, heat setting, and slow cooling curing are used to weaken the amplification effect of gels and micro-aggregates on optical defects and stabilize the orientation structure. This allows the base film to achieve high clarity and brightening performance while also ensuring hydrolysis resistance and weather resistance.

[0008] This application provides the following technical solution: a brightening aliphatic TPU base film for invisible car wraps, comprising, by weight, 100 parts of aliphatic thermoplastic polyurethane particles; 0.5-3 parts of optical grade modifier, wherein the optical grade modifier is surface-treated nano-silica and / or nano-alumina with an average particle size of 10nm-50nm, and the absolute value of the difference between its refractive index and that of the aliphatic TPU is not greater than 0.02; 1-5 parts of high molecular weight compatibilizer and brightener, wherein the brightener is a polyester polyol-siloxane copolymer with a number average molecular weight of 5000g / mol-20000 g / mol; 0.3-1.5 parts of hydrolysis stabilizer; 0.5-2 parts of ultraviolet absorber; and 0.3-1.5 parts of light stabilizer.

[0009] Furthermore, the aliphatic thermoplastic polyurethane is a polycaprolactone-type or polycarbonate-type thermoplastic polyurethane.

[0010] Furthermore, the polycaprolactone-type aliphatic thermoplastic polyurethane is one or more of the following: PCL-type TPU based on hexamethylene diisocyanate, PCL-type TPU based on hydrogenated diphenylmethane diisocyanate, and PCL-type TPU based on isophorone diisocyanate.

[0011] Furthermore, the polycarbonate-type thermoplastic polyurethane is a thermoplastic polyurethane using polycarbonate diol as the soft segment. The polycarbonate diol is one of the following: polycarbonate diol obtained from linear aliphatic diols, polycarbonate diol obtained from branched aliphatic diols, polycarbonate diol obtained from cyclic diols, polycarbonate diol obtained from aromatic diols or their hydrogenated derivative diols, or copolymerized polycarbonate diol obtained by copolymerizing two or more diols. The straight-chain aliphatic diols include one or more of 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,8-octanediol. The branched aliphatic diols include one or more of neopentyl glycol, 2-methyl-1,3-propanediol, and 2,2-dimethyl-1,3-propanediol; The cyclic diols include one or more of 1,4-cyclohexanediol, cyclohexanediol, and bisphenol cyclohexanediol. The aromatic diol or its hydrogenated derivative diol includes one or more of bisphenol A diol and hydrogenated derivative diol of bisphenol A; The diol is any combination of any of the aforementioned diols.

[0012] Furthermore, the surface treatment agent of the optical grade modifier is an aminosilane coupling agent, which is one or more of 3-aminopropyltriethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, and 3-aminopropyltrimethoxysilane.

[0013] Furthermore, the preparation method of the optical-grade modifier includes the following steps: P1: Water and an alcohol solvent are mixed in a volume ratio of 1:9 and stirred until homogeneous to form an alcohol solvent; the alcohol is one or more of anhydrous ethanol, isopropanol, or n-propanol; P2: Nano-sized silica and / or nano-sized alumina powder with an average particle size of 10nm-50nm are mixed with the alcohol solvent at a volume ratio of 50%-90% of the total volume of the mixture. The mixture is then ultrasonically dispersed at an ultrasonic frequency of 20kHz-40kHz, an ultrasonic power density of 200W / L-800W / L, and an ultrasonic time of 10min-30min. This is combined with high-speed shear dispersion at a speed of 3000rpm-10000rpm and a shearing time of 5min-20min to obtain a stable primary nano-suspension. P3: The primary nano-suspension is stirred at a speed of 300-800 rpm and a temperature of 40-60℃. Under the stirring condition, an aminosilane coupling agent is added dropwise to the primary nano-suspension, and deionized water is added simultaneously to hydrolyze the aminosilane coupling agent in situ to form silanol. Deionized water is added to make the molar ratio of water to silane coupling agent 13:1-25:1. The amount of silane coupling agent is 1%-10% of the weight of the nano-silica and / or nano-alumina powder. The pH of the mixture is adjusted to 4-5, and hydrolysis and preliminary coupling reaction are carried out for 1-3 hours. P4: After the hydrolysis and preliminary coupling reaction in step P3 are completed, the mixture is heated to 70℃-85℃ and kept at 200rpm-600rpm for stirring to further mature the mixture for 2-6 hours, so that the silanol after the silane coupling agent is fully hydrolyzed can undergo a condensation reaction with the hydroxyl groups on the surface of the nano silica and / or nano alumina powder to complete the surface grafting. P5: After the reaction is complete, the product is centrifuged and the precipitate is repeatedly washed with the alcohol solvent of the concentration prepared in step P1. Finally, the washed solid is vacuum dried at 60℃-80℃ for 6-12 hours. The vacuum degree of vacuum drying in step P5 is -0.08MPa to -0.095MPa to obtain the optical grade modifier.

[0014] Further, the polyester polyol-siloxane copolymer is one of polybutylene adipate-polydimethylsiloxane block copolymer, polycaprolactone-polydimethylsiloxane block copolymer, polybutylene carbonate-polydimethylsiloxane block copolymer, or polybutylene terephthalate-polyether modified siloxane copolymer; the hydrolysis stabilizer is one of polycarbodiimide, monocarbodiimide, oxazoline compounds, or epoxy compounds; the ultraviolet absorber is one of benzotriazole, triazine, benzophenone, or cyanoacrylate ultraviolet absorbers; and the light stabilizer is a hindered amine light stabilizer.

[0015] This application also provides a method for preparing a brightening aliphatic TPU base film for paint protection film as described above, comprising the following steps: S1: Vacuum dry each raw material component at 75℃-85℃ until the moisture content is less than 100 ppm; the vacuum degree of vacuum drying in step S1 is -0.10MPa to -0.12MPa; S2: All dried components are mixed at high speed under inert gas protection to obtain a premix; wherein the high-speed mixing speed is 500rpm-1500rpm and the mixing time is 3min-10min; S3: The premixed material is melt-blended using a twin-screw extruder, and the melt is then filtered through a precision filter with a pore size of 5μm-15μm. The twin-screw extruder has a barrel temperature of 160℃-220℃, a screw speed of 150rpm-400rpm, a melt residence time of 1min-3min, and applies vacuum venting in the extruder's exhaust section, with a vacuum degree of -0.09MPa to -0.11MPa. The precision filter is either a melt filter or a candle filter. The filtration temperature of the precision filter is the same as the melt temperature, the melt pressure at the filter inlet is 5MPa-20MPa, and the filtration pressure difference is controlled between 0.2MPa and 2.0MPa. S4: The filtered melt is granulated by water cooling and stretching or by underwater granulation to obtain composite granules, and then subjected to secondary vacuum drying; the secondary vacuum drying temperature is 70℃-90℃, the vacuum degree is -0.085MPa to -0.095MPa, and the drying time is 4h-10h. S5: The composite granules after secondary drying are extruded and cast into molten sheets, and then biaxially stretched with a total stretch ratio of 2.5-4.0 times; wherein the extrusion temperature is 170℃-230℃, the die temperature is 180℃-230℃, the casting cooling roller temperature is 20℃-40℃, and the casting linear speed is 5m / min-25m / min; S6: Heat-set the stretched membrane and then perform programmed cooling. S7: After slow cooling, the film is wound into a roll under constant tension. The winding tension is 5N-25N and the winding speed is 5m / min-30m / min. After winding, the film roll is placed in an environment of 40℃-60℃ for 24h-72h to release internal stress and stabilize dimensions and optical performance.

[0016] Furthermore, in step S5, the biaxial stretching is either step-by-step biaxial stretching or synchronous biaxial stretching, with the stretching temperature being 10°C-20°C higher than the softening point of the aliphatic thermoplastic polyurethane used, and the stretching rate being 50% / second-200% / second; the step-by-step biaxial stretching is either longitudinal first and then transverse, or transverse first and then longitudinal.

[0017] Furthermore, in step S6, the heat setting temperature is 5℃-15℃ higher than the stretching temperature, and the heat setting time is 10 seconds-30 seconds; the programmed cooling is a multi-stage gradient cooling, with 3-5 stages, and the set temperature difference between adjacent stages is 10℃-20℃, and the constant temperature holding time for each stage is 30 seconds-120 seconds, and finally the film is cooled to 25℃-35℃ before being wound up.

[0018] The beneficial effects of this application are as follows: 1. In this application, aliphatic TPU is used as the continuous phase in the material system. Nano-silica and / or nano-alumina with controlled refractive index difference are introduced as optical-grade modifiers. Surface organic treatment is used to reduce particle surface energy and aggregation tendency, making the dispersion of the inorganic phase in the TPU more uniform. Since the particle size is between 10nm and 50nm and the refractive index difference is no greater than 0.02, the scattering caused by interfacial mismatch during light propagation within the film is significantly weakened. While maintaining transparency, the haze of the base film is not easily amplified by processing fluctuations, thus providing a stable microstructure basis for high-definition imaging.

[0019] 2. This application employs an aminosilane coupling system and introduces an in-situ hydrolysis and coupling process. Silane is added dropwise under a stirring condition at 40-60°C with simultaneous water replenishment. This allows the silane to form silanol, which then condenses with the hydroxyl groups on the nanoparticle surface, establishing a stable organic coating layer and chemical bonding interface. This interface layer enhances the shear depolymerization resistance of the nanoparticles during melt blending, reducing secondary agglomeration. Furthermore, it provides more interaction sites for subsequent interactions with TPU soft segments, transforming the inorganic phase from a potential defect source into a controllable optical homogenization unit, thus reducing the risk of localized haze and bright spots caused by particle agglomeration.

[0020] 3. This application further introduces a polyester polyol siloxane copolymer with a number average molecular weight of 5000 g / mol to 20000 g / mol as a compatibilizer and brightener, which balances the compatibility with polyurethane and the orientation regulation of low surface energy segments at the molecular scale. This copolymer more easily forms a uniformly distributed flexible interface layer along the film surface during melt casting and biaxial stretching, improving microscopic refractive index fluctuations and enhancing the stable presentation of surface gloss. This ensures that the visual depth and gloss retention after coating the car paint come from the synergistic optimization of the material's internal structure, rather than relying on the unstable effects of later coating or short-term brightener additions.

[0021] 4. This application employs vacuum drying and inert gas premixing in the processing path to suppress moisture and oxidation interference. After twin-screw melt blending, a 5μm to 15μm precision filter is used to reduce gel impurities and microagglomerates entering the film-forming stage. Subsequently, a controlled process of casting, biaxial stretching, heat setting, and multi-stage slow cooling releases internal stress and stabilizes the orientation structure. This process makes the optical and dimensional properties of the base film more consistent across the roll-to-roll scale. Combined with the synergistic protection of anti-hydrolysis stabilizers, UV absorbers, and hindered amine light stabilizers, it can delay yellowing and fogging under humid heat and UV conditions, maintaining clarity and brightness performance during long-term use. Attached Figure Description

[0022] The present application will be described in more detail below based on embodiments and with reference to the accompanying drawings. Wherein: Figure 1 These are comparison graphs showing the transmittance of different samples from Examples 1-3 and Comparative Examples 1-2 of this application; Figure 2 These are comparison diagrams of haze levels for different samples from Examples 1-3 and Comparative Examples 1-2 of this application. Figure 3 These are comparison images of the 60° gloss of different samples from Examples 1-3 and Comparative Examples 1-2 of this application; Figure 4 These are comparison images of the DOI of the imaging clarity of different samples in Examples 1-3 and Comparative Examples 1-2 of this application; Figure 5 The tensile stress-strain curves of different samples from Examples 1-3 and Comparative Examples 1-2 of this application are shown. Figure 6 This is a comparison chart showing the changes in yellowing index of different samples after xenon lamp aging in Examples 1-3 and Comparative Examples 1-2 of this application. Detailed Implementation

[0023] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all 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.

[0024] In this application, "TPU" is an abbreviation for Thermoplastic Polyurethane.

[0025] In the following embodiments of the present invention, nano-silica and nano-alumina powders were purchased from Shandong Guoci Functional Materials Co., Ltd. (Guoci Materials); aminosilane coupling agents KH-550, KH-792, and KH-540 were purchased from the silane coupling agent product line and distribution channels of Nanjing Shuguang Chemical Group Co., Ltd.; hindered amine light stabilizers were all purchased from Qingdao Jiedejia New Material Technology Co., Ltd.; anhydrous ethanol, isopropanol, and n-propanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Sinopharm Reagent); deionized water was prepared by a laboratory pure water system; and acetic acid used for pH adjustment was prepared from glacial acetic acid supplied by Sinopharm Chemical Reagent Co., Ltd. at a concentration of 5 wt%–10 wt%. wt% acetic acid aqueous solution; the ultrasonic dispersion equipment used was the ultrasonic processor of Ningbo Xinzhi Biotechnology Co., Ltd., and the high-speed shear dispersion equipment used was the high-shear emulsification dispersion equipment of Shanghai Fluke Technology Development Co., Ltd.; the twin-screw melt blending extrusion used was the twin-screw extrusion equipment of Nanjing Keya Chemical Complete Equipment Co., Ltd.; the extrusion casting and traction winding line was configured using the casting film production line of Jiangsu Jwell Machinery Co., Ltd.; the melt precision filter and filter element assembly were purchased from polymer melt filter products of Xinxiang Liefert Filter Co., Ltd. and selected according to the pore size specifications of 5μm–15μm.

[0026] Example 1 This embodiment provides a gloss-enhancing aliphatic TPU base film for paint protection film, which, by weight, comprises the following raw materials: 100 parts of aliphatic thermoplastic polyurethane particles, wherein the TPU is a PCL-type TPU based on hexamethylene diisocyanate (HDI).

[0027] 0.5 parts of optical grade modifier, which is nano-silica surface-treated with aminosilane coupling agent, with an average particle size of 10 nm, and the absolute value of the difference between its refractive index and the refractive index of the TPU is 0.01.

[0028] One part of high molecular weight compatibilizer and brightener is polycaprolactone-polydimethylsiloxane block copolymer (PCL-PDMS block copolymer) with a number average molecular weight of 5000 g / mol.

[0029] 0.3 parts of hydrolysis-resistant stabilizer, which is polycarbodiimide.

[0030] 0.5 parts of ultraviolet absorber, a benzotriazole ultraviolet absorber, purchased from Weihai Jinwei Chemical Co., Ltd., product number UV-328.

[0031] 0.3 parts of light stabilizer, which is hindered amine light stabilizer (HALS): JADEWIN LS 292.

[0032] The preparation method of the optical grade modifier in this embodiment includes the following steps: P1: Mix water and anhydrous ethanol in a volume ratio of 1:9 and stir until homogeneous to form an alcohol solvent; P2: Nano-sized silica with an average particle size of 10 nm is mixed with the alcohol solvent obtained in step P1, with the volume of the alcohol solvent accounting for 50% of the total volume of the mixture (the mixture of nano-sized silica and the alcohol solvent). The mixture is ultrasonically dispersed at an ultrasonic frequency of 20 kHz, an ultrasonic power density of 200 W / L, and an ultrasonic time of 10 min. It is then combined with high-speed shear dispersion at a rotation speed of 10,000 rpm for 20 min to obtain a stable primary nano-suspension. P3: The surface treatment agent used is 3-aminopropyltriethoxysilane (KH-550); the primary nano-suspension is stirred at 300 rpm and 60°C, and KH-550 is added dropwise to the primary nano-suspension while stirring, and deionized water is added simultaneously to make the molar ratio of water to KH-550 in the system 16:1, so as to promote the in-situ hydrolysis of KH-550 to form silanol; the amount of KH-550 used is 10% of the weight of nano-silica; the pH of the mixture is adjusted to 4.0 with dilute acetic acid, and the hydrolysis and preliminary coupling reaction are carried out for 1 hour; P4: After the hydrolysis and preliminary coupling reaction in step P3 are completed, the mixture is heated to 85°C and kept at 200 rpm for stirring to further mature the mixture for 6 hours, so that the silanol after the silane coupling agent is fully hydrolyzed can undergo a condensation reaction with the hydroxyl groups on the surface of the nano silica powder to complete the surface grafting. P5: After the reaction is complete, the product is centrifuged and the precipitate is washed three times repeatedly with the alcohol solvent prepared in step P1 to remove unreacted coupling agent and byproducts. Finally, the washed solid is vacuum dried at 60°C for 12 hours. The vacuum degree of vacuum drying in step P5 is -0.08 MPa to obtain the surface-organically modified, loose optical grade modifier, which is a powder solid.

[0033] This embodiment uses the raw materials with the above-mentioned proportions and the prepared optical-grade modifier to prepare a TPU base film. The preparation method includes the following steps: S1: Vacuum dry each raw material component at 75℃ and a vacuum degree of -0.10MPa until the moisture content is less than 100 ppm; S2: Mix all the dried components at 1500 rpm for 3 minutes under inert gas protection to obtain a premix. S3: After the premixed material is melt-blended in a twin-screw extruder, the melt is filtered through a melt filter with a pore size of 15μm. The barrel temperature of the twin-screw extruder is 160℃, the screw speed is 400rpm, the melt residence time is 3min, and vacuum exhaust is applied in the exhaust section of the extruder, with a vacuum degree of -0.09MPa. The filtration temperature of the melt filter is the same as the melt temperature, the melt pressure at the filter inlet is 5MPa, and the filtration pressure difference is controlled at 0.2MPa. S4: The filtered melt is granulated by water cooling and stretching or by underwater granulation to obtain composite granules, and then subjected to secondary vacuum drying at 70℃ and -0.085MPa for 10 hours. S5: The composite granules after secondary drying are extruded and cast into molten sheets, and then simultaneously stretched in the transverse and longitudinal directions, with a total stretching ratio of 4.0 times; the extrusion temperature is 230℃, the die temperature is 180℃, the casting cooling roller temperature is 40℃, and the casting linear speed is 5m / min; the stretching temperature is 10℃ higher than the softening point of the TPU used, and the stretching rate is 200% / second; S6: Heat set the stretched film at a temperature 15°C higher than the stretching temperature for 10 seconds. Then, perform a programmed cooling process with three stages. The temperature difference between adjacent stages is set to 20°C. Each stage is kept at a constant temperature for 120 seconds. Finally, the film is cooled to 25°C and then wound up. S7: After slow cooling, the film is wound into a roll under constant tension. The winding tension is 25N and the winding speed is 5m / min. After winding, the film roll is placed in a 60℃ environment for 24h to release internal stress and stabilize the size and optical performance.

[0034] Example 2 This embodiment provides a gloss-enhancing aliphatic TPU base film for paint protection film, which, by weight, comprises the following raw materials: 100 parts of aliphatic thermoplastic polyurethane particles, wherein the TPU is a polycarbonate-type TPU (the soft segment is polycarbonate diol).

[0035] Three parts of optical grade modifier, which is nano-alumina surface-treated with aminosilane coupling agent, with an average particle size of 50 nm, and the absolute value of the difference between its refractive index and the refractive index of the TPU is 0.02.

[0036] Five parts of high molecular weight compatibilizer and brightener were selected, which were polybutylene carbonate-polydimethylsiloxane block copolymers (PCDL-PDMS block copolymers) with a number average molecular weight of 20,000 g / mol.

[0037] 1.5 parts of hydrolysis stabilizer, an oxazoline compound, 2,2'-(1,3-phenylene)-dioxazoline (1,3-PBO), was purchased from Wuhan Yuancheng Chemical Co., Ltd.

[0038] Two parts of ultraviolet absorber, which is a triazine ultraviolet absorber, purchased from Tianjin Lianlong New Material Co., Ltd., RIASORB series, product number RIASORB UV-1577.

[0039] 1.5 parts of light stabilizer, which is hindered amine light stabilizer (HALS): JADEWIN LS 944.

[0040] The preparation method of the optical grade modifier in this embodiment includes the following steps: P1: Mix water and isopropanol in a volume ratio of 1:9 and stir until homogeneous to form an alcohol solvent; P2: Nano-alumina with an average particle size of 50 nm is mixed with the alcohol solvent obtained in step P1, with the volume of the alcohol solvent accounting for 90% of the total volume of the mixture (nano-alumina and the alcohol solvent). The mixture is ultrasonically dispersed at an ultrasonic frequency of 35 kHz, an ultrasonic power density of 800 W / L, and an ultrasonic time of 30 min. It is then combined with high-speed shear dispersion at a rotation speed of 3000 rpm for 5 min to obtain a stable primary nano-suspension. P3: The surface treatment agent selected is N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane (KH-792); the primary nano-suspension is stirred at 800 rpm and 40°C, and KH-792 is added dropwise to the primary nano-suspension while stirring, and deionized water is added simultaneously to make the molar ratio of water to KH-792 in the system 13:1, so as to promote the in-situ hydrolysis of KH-792 to form silanol; the amount of KH-792 used is 1% of the weight of nano Al2O3; the pH of the mixture is adjusted to 5.0 with dilute acetic acid, and the hydrolysis and preliminary coupling reaction are carried out for 3 hours; P4: After the hydrolysis and preliminary coupling reaction in step P3 are completed, the mixture is heated to 70°C and kept at 600 rpm for stirring to further mature the mixture for 2 hours, so that the silanol after the silane coupling agent is fully hydrolyzed can undergo a condensation reaction with the hydroxyl groups on the surface of the nano alumina powder to complete the surface grafting. P5: After the reaction is complete, the product is centrifuged and the precipitate is washed four times with the alcohol solvent prepared in step P1 to remove unreacted coupling agent and byproducts. Finally, the washed solid is vacuum dried at 80°C for 6 hours. The vacuum degree of vacuum drying in step P5 is -0.095 MPa to obtain the surface-organically modified, loose optical grade modifier, which is a powder solid.

[0041] This embodiment uses the raw materials with the above-mentioned proportions and the prepared optical-grade modifier to prepare a TPU base film. The preparation method includes the following steps: S1: Vacuum dry each raw material component at 85℃ and a vacuum degree of -0.12MPa until the moisture content is less than 100 ppm; S2: Mix all the dried components at 500 rpm for 10 min under inert gas protection to obtain a premix. S3: After the premixed material is melt-blended in a twin-screw extruder, the melt is filtered through a candle filter with a pore size of 5μm. The barrel temperature of the twin-screw extruder is 220℃, the screw speed is 150rpm, the melt residence time is 1min, and vacuum exhaust is applied in the exhaust section of the extruder, with a vacuum degree of -0.11MPa. The filtration temperature of the melt filter screen is the same as the melt temperature, the melt pressure at the filter inlet is 20MPa, and the filtration pressure difference is controlled at 2.0MPa. S4: The filtered melt is granulated by water cooling and stretching or by underwater granulation to obtain composite granules, and then subjected to secondary vacuum drying at 90℃ and -0.095MPa for 4 hours. S5: The composite granules after secondary drying are extruded and cast into molten sheets, and then biaxially stretched with a total stretch ratio of 2.5 times. The biaxial stretching is carried out in steps (first longitudinally and then transversely). The extrusion temperature is 170℃, the die temperature is 230℃, the casting cooling roller temperature is 20℃, and the casting linear speed is 25m / min. The stretching temperature is 20℃ higher than the softening point of the TPU used, and the stretching rate is 50% / second. S6: Heat set the stretched film at a temperature 5°C higher than the stretching temperature for 30 seconds. Then, perform a programmed cooling process with 5 segments. Set a temperature difference of 10°C between adjacent segments and maintain a constant temperature for 30 seconds in each segment. Finally, cool the film to 35°C and then rewind it. S7. After slow cooling, the film is wound into a roll under constant tension of 5N and a winding speed of 30m / min. After winding, the film roll is placed in an environment of 40℃ for 72h to release internal stress and stabilize dimensions and optical performance.

[0042] Example 3 This embodiment provides a gloss-enhancing aliphatic TPU base film for paint protection film, which, by weight, comprises the following raw materials: 100 parts of aliphatic thermoplastic polyurethane particles, wherein the TPU is a PCL-type TPU based on isophorone diisocyanate (IPDI). 1.8 parts of optical-grade modifier, which is a composite powder of nano-silica and nano-alumina (nano-SiO2:nano-Al2O3 mass ratio of 1:1) surface-treated with an aminosilane coupling agent, with an average particle size of 30 nm, and an absolute value of the difference between its refractive index and that of the TPU being 0.018.

[0043] Three parts of high molecular weight compatibilizer and brightener, which is polybutylene adipate-polydimethylsiloxane block copolymer (PBA-PDMS block copolymer), with a number average molecular weight of 12000 g / mol.

[0044] 0.9 parts of hydrolysis stabilizer, an epoxy compound: BASF Joncryl® ADR-4468, purchased from Shanghai Shengdian New Materials Co., Ltd.

[0045] 1.2 parts of ultraviolet absorber, which is a benzophenone-based ultraviolet absorber: 2-hydroxy-4-n-octyloxybenzophenone, product number UV-531, purchased from Jinan Haisheng Chemical Co., Ltd.

[0046] 0.9 parts of light stabilizer, which is hindered amine light stabilizer (HALS): JADEWIN LS 770.

[0047] The preparation method of the optical grade modifier in this embodiment includes the following steps: P1: Mix water and n-propanol in a volume ratio of 1:9 and stir until homogeneous to form an alcohol solvent; P2: The composite powder of nano-SiO2 and nano-Al2O3 with an average particle size of 30nm and a mass ratio of 1:1 is mixed with the alcohol solvent obtained in step P1, with the volume of the alcohol solvent accounting for 70% of the total volume of the mixture (the nanoparticle composite powder and the alcohol solvent). The mixture is ultrasonically dispersed at an ultrasonic frequency of 40kHz, an ultrasonic power density of 500W / L, and an ultrasonic time of 20min. It is then combined with high-speed shear dispersion at a rotation speed of 6500rpm for 12min to obtain a stable primary nano suspension. P3: The surface treatment agent used is 3-aminopropyltrimethoxysilane (KH-540); the primary nano-suspension is stirred at 50°C and kept at a speed of 550 rpm; KH-540 is added dropwise to the primary nano-suspension while stirring, and deionized water is added simultaneously to make the molar ratio of water to KH-540 in the system 25:1, so as to promote the in-situ hydrolysis of KH-540 to form silanol; the amount of KH-540 used is 6% of the weight of the nanoparticle compound powder; the pH of the mixture is adjusted to 4.5 with dilute acetic acid, and the hydrolysis and preliminary coupling reaction are carried out for 2 hours; P4: After the hydrolysis and preliminary coupling reaction in step P3 are completed, the mixture is heated to 78°C and kept at 350 rpm for stirring to further mature the mixture for 4 hours, so that the silanol after the silane coupling agent is fully hydrolyzed can undergo a condensation reaction with the hydroxyl groups on the surface of the nanoparticle composite powder to complete the surface grafting. P5: After the reaction is complete, the product is centrifuged and the precipitate is washed four times with the alcohol solvent prepared in step P1 to remove unreacted coupling agent and byproducts. Finally, the washed solid is vacuum dried at 70°C for 9 hours. The vacuum degree of vacuum drying in step P5 is -0.088 MPa to obtain the surface-organically modified, loose optical grade modifier, which is a powder solid.

[0048] This embodiment uses the raw materials with the above-mentioned proportions and the prepared optical-grade modifier to prepare a TPU base film. The preparation method includes the following steps: S1: Vacuum dry each raw material component at 80℃ and a vacuum degree of -0.11MPa until the moisture content is less than 100 ppm; S2: Mix all the dried components at 1000 rpm for 6 minutes under inert gas protection to obtain a premix. S3: After the premixed material is melt-blended in a twin-screw extruder, the melt is filtered through a candle filter with a pore size of 10μm. The barrel temperature of the twin-screw extruder is 190℃, the screw speed is 280rpm, the melt residence time is 2min, and vacuum exhaust is applied in the exhaust section of the extruder, with a vacuum degree of -0.10MPa. The filtration temperature of the melt filter screen is the same as the melt temperature, the melt pressure at the filter inlet is 12MPa, and the filtration pressure difference is controlled at 1.1MPa. S4: The filtered melt is granulated by water cooling and stretching or by underwater granulation to obtain composite granules, and then subjected to secondary vacuum drying at 80℃ and -0.090MPa for 7 hours. S5: The composite granules after secondary drying are extruded and cast into molten sheets, and then biaxially stretched with a total stretch ratio of 3.2 times. The biaxial stretching is carried out in steps (first transverse and then longitudinal). The extrusion temperature is 200℃, the die temperature is 205℃, the casting cooling roller temperature is 30℃, and the casting linear speed is 15m / min. The stretching temperature is 15℃ higher than the softening point of the TPU used, and the stretching rate is 120% / second. S6: Heat set the stretched film at a temperature 10°C higher than the stretching temperature for 18 seconds. Then, perform a programmed cooling process with four stages. The temperature difference between adjacent stages is set to 15°C. Each stage is kept at a constant temperature for 75 seconds. Finally, the film is cooled to 30°C and then wound up. S7. After slow cooling, the film is wound into a roll under constant tension of 15N and a winding speed of 18m / min. After winding, the film roll is placed in a 50℃ environment for 48h to release internal stress and stabilize dimensions and optical performance.

[0049] The dilute acetic acid used in this application embodiment is of a fixed concentration. It can be achieved by adjusting the amount of dilute acetic acid added to a 5 wt%–10 wt% aqueous solution of glacial acetic acid according to the pH value to be achieved.

[0050] Comparative Example 1 Comparative Example 1 is identical to Example 1 except for the following differences: In Comparative Example 1, no high molecular weight compatibilizer or brightening agent was added to the raw materials used to prepare the aliphatic TPU base film for paint protection film (i.e., the amount of added polyester polyol-siloxane copolymer was 0 parts); the remaining components remained the same as in Example 1.

[0051] Comparative Example 2 Comparative Example 2 provides a brightening aliphatic TPU base film for paint protection film, except for the following differences, the remaining steps are the same as in Example 2: 1) The optical grade modifier used in the preparation of TPU base film is changed to: nano Al2O3 (average particle size of 50 nm) without aminosilane coupling treatment, and the refractive index difference between it and TPU is not controlled; the amount of optical grade modifier added is still 2.4 parts by weight.

[0052] 2) The P1–P5 steps provided in this application for preparing the optical grade modifier are no longer used. In the preparation of the TPU base film, the nano-Al2O3 (average particle size of 50 nm) that has not undergone aminosilane coupling treatment is replaced by the optical grade modifier in step S1 and vacuum dried. After premixing in step S2, it enters step S3 for melt blending with other component raw materials.

[0053] The aliphatic TPU base films for invisible car wraps prepared in Examples 1-3 and Comparative Examples 1-2 were tested for optical, mechanical, and weather resistance properties. Light transmittance and haze (sample thickness 50 μm) were determined according to GB / T 2410-2008 "Determination of Light Transmittance and Haze of Transparent Plastics," using a haze meter under specified geometric conditions, as shown in Figure 1. 60° specular gloss was determined according to the method in GB / T 9754-2007 "Determination of 20˚, 60˚ and 85˚ Specular Gloss of Paint Films Without Metallic Pigments," using a gloss meter under 60° geometric conditions, and the gloss value was expressed as GU. Image sharpness (DOI) was determined using an orange peel vividness meter, referring to the instrumental measurement method for DOI in relevant standards for automotive topcoats, and the DOI value was recorded. Tensile strength and elongation at break were determined according to GB / T The tensile strength and elongation at break were obtained by preparing a specified dumbbell-shaped specimen and then stretching it at a specified rate, in accordance with the standard GB / T 16422.2-2022 "Laboratory Light Source Exposure Test Methods for Plastics - Part 2: Xenon Arc Lamp". The xenon lamp aging was performed under specified irradiation and temperature and humidity conditions. The yellowing index YI and its change ΔYI before and after aging were performed in accordance with the standard GB / T 39822-2021 "Determination of Yellow Index and its Change Value of Plastics". The results of the YI change after 2000 hours of xenon lamp aging in Table 1 are the results of calculating the difference between the YI values ​​before and after aging.

[0054] Table 1. Test results of TPU base film performance From Table 1 and Figures 1-2 As can be seen, the TPU base films prepared in Examples 1–3 maintained a transmittance of 92.5%–93.1% and a haze of 0.5%–0.7% at a thickness of 50 μm, while the transmittance of Comparative Example 1 decreased to 90.2% and the haze increased to 1.8%. This indicates that the present application, through silane coupling modification of nano-SiO2 and / or nano-Al2O3, combined with ultrasonic and shear dispersion, and melt precision filtration, can effectively reduce particle agglomeration and interface scattering, enabling the film to maintain low haze while achieving high transmittance. Figures 3-4 As can be seen, the 60° gloss of Examples 1–3 is 104 GU-106 GU and the DOI is 95–97, while the 60° gloss of Comparative Example 1 is 98 GU and the DOI is 88. This indicates that the compatibilizing and brightening system and the stretching and shaping process in the formulation of this application are beneficial to improving surface specular reflection and imaging clarity, resulting in a brighter and clearer appearance. Figure 5As can be seen, Examples 1–3 maintain a high elongation at break (460%–500%) while achieving a tensile strength of 43 MPa–48 MPa, compared to only 38 MPa in Comparative Example 1. This demonstrates that this application improves the optical appearance without sacrificing mechanical properties, and instead enhances the strength level. Figure 6 As can be seen, after 2000 hours of xenon lamp aging, the ΔYI of Examples 1–3 was +1.5 to +1.8, while that of Comparative Example 1 was +3.5. This indicates that the present application, through the synergistic setting of anti-hydrolysis stabilizer, ultraviolet absorber and hindered amine light stabilizer, combined with vacuum drying and devolatilization control, makes the yellowing growth of the film under accelerated weathering conditions smaller and the weather resistance better.

[0055] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A brightening aliphatic TPU base film for invisible car wraps, characterized in that, By weight, the raw materials include: 100 parts aliphatic thermoplastic polyurethane particles; 0.5-3 parts optical grade modifier, wherein the optical grade modifier is surface-treated nano-silica and / or nano-alumina with an average particle size of 10nm-50nm, and the absolute value of the difference between its refractive index and that of the aliphatic TPU is not greater than 0.02; 1-5 parts high molecular weight compatibilizer and brightener, wherein the brightener is a polyester polyol-siloxane copolymer with a number average molecular weight of 5000g / mol-20000 g / mol; 0.3-1.5 parts hydrolysis stabilizer; 0.5-2 parts ultraviolet absorber; and 0.3-1.5 parts light stabilizer.

2. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 1, characterized in that, The aliphatic thermoplastic polyurethane is a polycaprolactone-type or polycarbonate-type thermoplastic polyurethane.

3. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 2, characterized in that, Polycaprolactone-based aliphatic thermoplastic polyurethane is one or more of the following: PCL-type TPU based on hexamethylene diisocyanate, PCL-type TPU based on hydrogenated diphenylmethane diisocyanate, and PCL-type TPU based on isophorone diisocyanate.

4. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 2, characterized in that, The polycarbonate-type thermoplastic polyurethane is a thermoplastic polyurethane using polycarbonate diol as the soft segment. The polycarbonate diol is one of the following: polycarbonate diol obtained from linear aliphatic diols, polycarbonate diol obtained from branched aliphatic diols, polycarbonate diol obtained from cyclic diols, polycarbonate diol obtained from aromatic diols or their hydrogenated derivative diols, or copolymerized polycarbonate diol obtained by copolymerizing two or more diols. The straight-chain aliphatic diols include one or more of 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,8-octanediol. The branched aliphatic diols include one or more of neopentyl glycol, 2-methyl-1,3-propanediol, and 2,2-dimethyl-1,3-propanediol; The cyclic diols include one or more of 1,4-cyclohexanediol, cyclohexanediol, and bisphenol cyclohexanediol. The aromatic diol or its hydrogenated derivative diol includes one or more of bisphenol A diol and hydrogenated derivative diol of bisphenol A; The diol is any combination of any of the aforementioned diols.

5. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 1, characterized in that, The surface treatment agent of the optical grade modifier is an aminosilane coupling agent, which is one or more of 3-aminopropyltriethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, and 3-aminopropyltrimethoxysilane.

6. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 5, characterized in that, The preparation method of the optical grade modifier includes the following steps: P1: Water and an alcohol solvent are mixed in a volume ratio of 1:9 and stirred until homogeneous to form an alcohol solvent; the alcohol is one or more of anhydrous ethanol, isopropanol, or n-propanol; P2: Nano-sized silica and / or nano-sized alumina powder with an average particle size of 10nm-50nm are mixed with the alcohol solvent at a volume ratio of 50%-90% of the total volume of the mixture. The mixture is then ultrasonically dispersed at an ultrasonic frequency of 20kHz-40kHz, an ultrasonic power density of 200W / L-800W / L, and an ultrasonic time of 10min-30min. This is combined with high-speed shear dispersion at a speed of 3000rpm-10000rpm and a shearing time of 5min-20min to obtain a stable primary nano-suspension. P3: The primary nano-suspension is stirred at a speed of 300-800 rpm and a temperature of 40-60℃. Under the stirring condition, an aminosilane coupling agent is added dropwise to the primary nano-suspension, and deionized water is added simultaneously to hydrolyze the aminosilane coupling agent in situ to form silanol. Deionized water is added to make the molar ratio of water to silane coupling agent 13:1-25:

1. The amount of silane coupling agent is 1%-10% of the weight of the nano-silica and / or nano-alumina powder. The pH of the mixture is adjusted to 4-5, and hydrolysis and preliminary coupling reaction are carried out for 1-3 hours. P4: After the hydrolysis and preliminary coupling reaction in step P3 are completed, the mixture is heated to 70℃-85℃ and kept at 200rpm-600rpm for stirring to further mature the mixture for 2-6 hours, so that the silanol after the silane coupling agent is fully hydrolyzed can undergo a condensation reaction with the hydroxyl groups on the surface of the nano silica and / or nano alumina powder to complete the surface grafting. P5: After the reaction is complete, the product is centrifuged and the precipitate is repeatedly washed with the alcohol solvent of the concentration prepared in step P1. Finally, the washed solid is vacuum dried at 60℃-80℃ for 6-12 hours. The vacuum degree of vacuum drying in step P5 is -0.08MPa to -0.095MPa to obtain the optical grade modifier.

7. The aliphatic TPU base film for enhancing brightness in paint protection film according to claim 1, characterized in that, The polyester polyol-siloxane copolymer is one of polybutylene adipate-polydimethylsiloxane block copolymer, polycaprolactone-polydimethylsiloxane block copolymer, polybutylene carbonate-polydimethylsiloxane block copolymer, or polybutylene terephthalate-polyether modified siloxane copolymer; the hydrolysis stabilizer is one of polycarbodiimide, monocarbodiimide, oxazoline compounds, or epoxy compounds; the ultraviolet absorber is one of benzotriazole, triazine, benzophenone, or cyanoacrylate ultraviolet absorbers; and the light stabilizer is a hindered amine light stabilizer.

8. A method for preparing an aliphatic TPU base film for enhancing brightness in paint protection film as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Vacuum dry each raw material component at 75℃-85℃ until the moisture content is less than 100 ppm; the vacuum degree of vacuum drying in step S1 is -0.10MPa to -0.12MPa; S2: All dried components are mixed at high speed under inert gas protection to obtain a premix; wherein the high-speed mixing speed is 500rpm-1500rpm and the mixing time is 3min-10min; S3: The premixed material is melt-blended using a twin-screw extruder, and the melt is then filtered through a precision filter with a pore size of 5μm-15μm. The twin-screw extruder has a barrel temperature of 160℃-220℃, a screw speed of 150rpm-400rpm, a melt residence time of 1min-3min, and applies vacuum venting in the extruder's exhaust section, with a vacuum degree of -0.09MPa to -0.11MPa. The precision filter is either a melt filter or a candle filter. The filtration temperature of the precision filter is the same as the melt temperature, the melt pressure at the filter inlet is 5MPa-20MPa, and the filtration pressure difference is controlled between 0.2MPa and 2.0MPa. S4: The filtered melt is granulated by water cooling and stretching or by underwater granulation to obtain composite granules, and then subjected to secondary vacuum drying. The secondary vacuum drying temperature is 70℃-90℃, the vacuum degree is -0.085MPa to -0.095MPa, and the drying time is 4h-10h. S5: The composite granules after secondary drying are extruded and cast into molten sheets, and then biaxially stretched with a total stretch ratio of 2.5-4.0 times; wherein the extrusion temperature is 170℃-230℃, the die temperature is 180℃-230℃, the casting cooling roller temperature is 20℃-40℃, and the casting linear speed is 5m / min-25m / min; S6: Heat-set the stretched membrane and then perform programmed cooling. S7: After slow cooling, the film is wound into a roll under constant tension. The winding tension is 5N-25N and the winding speed is 5m / min-30m / min. After winding, the film roll is placed in an environment of 40℃-60℃ for 24h-72h to release internal stress and stabilize dimensions and optical performance.

9. The preparation method according to claim 8, characterized in that, In step S5, the biaxial stretching is either step-by-step biaxial stretching or synchronous biaxial stretching. The stretching temperature is 10°C-20°C higher than the softening point of the aliphatic thermoplastic polyurethane used, and the stretching rate is 50% / second-200% / second. The step-by-step biaxial stretching is either longitudinal first and then transverse, or transverse first and then longitudinal.

10. The preparation method according to claim 8, characterized in that, In step S6, the heat setting temperature is 5℃-15℃ higher than the stretching temperature, and the heat setting time is 10 seconds-30 seconds. The programmed cooling is a multi-stage gradient cooling, with 3-5 stages, and the set temperature difference between adjacent stages is 10℃-20℃. The constant temperature holding time for each stage is 30 seconds-120 seconds. Finally, the film is cooled to 25℃-35℃ before being wound up.

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