Ultraviolet cumulative discoloration indicating anti-uv bopet film and method for manufacturing the same

Abstract

The application discloses an ultraviolet cumulative discoloration indicating anti-ultraviolet BOPET film and a preparation method thereof. The film comprises, in sequence along a light receiving direction, a light receiving PET surface layer, a first anti-ultraviolet sublayer, a second anti-ultraviolet sublayer, a third anti-ultraviolet sublayer, an irreversible discoloration indicating layer and a mounting functional layer. The three anti-ultraviolet sublayers all contain lignin nanoparticles, and the content of the lignin nanoparticles decreases along the direction away from the light receiving side; the irreversible discoloration indicating layer contains PET modified by a diarylethene structure unit copolymerization or grafting and a photooxidation retardant. The film is prepared through multilayer co-extrusion, bidirectional stretching and heat setting. The film can produce stable color prompts according to the attenuation of the anti-ultraviolet layer caused by ultraviolet accumulation while maintaining transparency and anti-ultraviolet performance.

Description

Technical Field

[0001] This invention relates to the technical field of functional polyester films, and in particular to a UV-accumulated color change indicator UV-resistant BOPET film and its preparation method. Background Technology

[0002] Biaxially oriented polyethylene terephthalate (PET) films possess excellent mechanical properties, high dimensional stability, good transparency, and strong processing adaptability, and are widely used in automotive window films, architectural films, optical protective films, and packaging composite films. In automotive window films and architectural films, the films typically need to have high UV blocking capabilities to reduce the damage caused by ultraviolet rays to human skin, vehicle interior parts, and interior materials.

[0003] Existing UV-resistant polyester films typically enhance their UV shielding capabilities by adding benzotriazole, triazine, or benzophenone UV absorbers, or by incorporating anti-aging layers or UV-resistant coatings. For example, existing technologies disclose the addition of UV absorbers and heat stabilizers to BOPET films to improve their anti-aging properties. Other technologies disclose photochromic film materials that change color under UV irradiation to indicate UV exposure status.

[0004] However, in actual use, the effective protection status of UV-resistant films depends not only on the UV blocking rate at the time of manufacture, but also on factors such as the region of use, solar radiation intensity, storage environment, installation orientation, and service life. The cumulative UV dose experienced by the same type of film varies significantly under different usage scenarios, and its UV protection performance decays at significantly different rates. Existing products typically rely on fixed service life, experience-based judgment, or professional instrument testing to estimate replacement time. Ordinary users find it difficult to promptly and intuitively assess the film's effective protection status after long-term service, easily leading to premature replacement resulting in waste, or delayed replacement resulting in insufficient UV protection.

[0005] Therefore, how to make the UV-resistant BOPET film reflect its effective protection status intuitively after long-term service has become a technical problem that needs to be solved when further improving this type of film product. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and provide an ultraviolet cumulative color change indicator anti-ultraviolet BOPET film and its preparation method, so that the anti-ultraviolet BOPET film can intuitively reflect its effective protection status through color change after long-term service.

[0007] To achieve the above objectives, the first aspect of the present invention provides an ultraviolet cumulative color change indicator anti-ultraviolet BOPET film, comprising a light-receiving PET surface layer, a first anti-ultraviolet sublayer, a second anti-ultraviolet sublayer, a third anti-ultraviolet sublayer, an irreversible color change indicator layer, and an installation function layer arranged sequentially along the light-receiving direction; The first, second, and third UV-resistant sublayers are all based on PET and contain lignin nanoparticles, and the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers decreases in the direction away from the light-receiving PET surface. The irreversible color-changing indicator layer contains PET copolymerized or grafted with diarylethylene structural units and a photo-oxidation retardant. The photo-oxidation retardant is composed of hindered amine light stabilizers and / or hindered phenolic antioxidants. The mass ratio of the diarylethylene structural units to the photo-oxidation retardant is 8:1 to 35:1. Under xenon lamp aging conditions, a test sample layer with the same composition and thickness as the first, second, and third anti-UV sublayers was subjected to UV-A average transmittance testing. When the UV-A average transmittance of the test sample layer in the 315–400 nm range was less than 3%, the color difference ΔEab of the irreversible color-changing indicator layer was less than 3; when the UV-A average transmittance of the test sample layer in the 315–400 nm range was 3%–6%, the color difference ΔEab of the irreversible color-changing indicator layer was 8–18.

[0008] By having ultraviolet light pass through the light-receiving PET surface layer and three progressively decreasing UV-resistant sublayers before reaching the irreversible color-changing indicator layer, the color-changing response can be transformed from a simple light-receiving color development into a cumulative indication process related to the service life degradation of the UV-resistant layer.

[0009] In the first UV-resistant sublayer, the higher content of lignin nanoparticles preferentially undertakes UV absorption and photo-oxidation consumption, while the lower content of lignin nanoparticles in the middle layer and near the color-changing layer continues to maintain light transmittance stability and UV release continuity, preventing a rapid increase in UV-A transmittance in the initial stage of use. Furthermore, the photo-oxidation retarder can capture active free radicals formed under low-dose UV leakage, suppressing early color development caused by scattered leakage. After the UV-resistant sublayer has undergone cumulative aging, the average UV-A transmittance enters the range of 3% to 6%, and the UV energy and oxidative activity entering the color-changing indicator layer reach the sustained color-development initiation level of the diarylethylene structural unit, thus forming a more obvious color change.

[0010] The advantage of this is that the film is less likely to miscolor when it is still in a good state of effective protection, and can provide clear, stable and easily identifiable color indications when its UV protection capability enters the decay stage that needs attention.

[0011] Preferably, the mass content of lignin nanoparticles in the first UV-resistant sublayer is 3.5% to 6.5%, the mass content of lignin nanoparticles in the second UV-resistant sublayer is 2% to 4.5%, and the mass content of lignin nanoparticles in the third UV-resistant sublayer is 0.8% to 2.5%, and the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers decreases by at least 0.8 percentage points in that order.

[0012] The aforementioned content distribution ensures that the light-receiving side has a sufficiently strong initial UV absorption capacity, while preventing the side near the color-changing indicator layer from being blocked from the color-changing response due to excessive lignin content. A decrease of at least 0.8 percentage points between each layer allows the UV-resistant layer to form a relatively clear thickness-direction consumption sequence during the aging process: the outer layer consumes first, the middle layer takes over, and the inner layer slowly releases the UV transmission signal. This helps to transform the continuous aging process into a more concentrated color-triggering range.

[0013] Preferably, the lignin nanoparticles have a D50 particle size of 60-250 nm and a D90 particle size of no more than 500 nm; the lignin nanoparticles are nano-lignin particles with at least one compatible group selected from hydroxyl, carboxyl, or ester groups on their surface.

[0014] By controlling lignin within the nanoparticle size range, the scattering effect of large particles on visible light can be reduced, allowing the film to maintain good transparency and uniform appearance while retaining its UV protection function. The hydroxyl, carboxyl, or ester groups on the surface can improve the interfacial bonding and dispersion stability between lignin and the PET matrix, making it less likely for lignin nanoparticles to form obvious agglomeration points during biaxial stretching, thereby reducing local stress concentration and light-transmitting spots.

[0015] Preferably, the PET copolymerized or grafted with diarylethylene structural units is formed by copolymerization, transesterification or melt grafting of diarylethylene derivatives containing hydroxyl, carboxyl or transesterification reactive groups with polyester monomers, polyester oligomers or PET.

[0016] Incorporating diarylethene structural units into the PET system through copolymerization or grafting can reduce the migration, volatilization, or phase separation of small-molecule color-changing components during melt extrusion and biaxial stretching. After the diarylethene structural units form a relatively stable bond with the polyester segments, the color-changing response is more easily fixed within the irreversible color-changing indicator layer, preventing significant precipitation due to subsequent thermal processing or long-term use. This ensures that the indicator layer can withstand the processing temperature of BOPET film and also makes the color development location more concentrated and stable, facilitating the formation of a clear lifetime indication signal.

[0017] Preferably, the mass content of the diarylene ethylene structural unit in the irreversible color-changing indicator layer is 0.2% to 2.5%; and the mass content of the photo-oxidation retardant in the irreversible color-changing indicator layer is 0.03% to 0.18%.

[0018] When the content of diarylethene structural units is within the aforementioned range, the color-changing indicator layer has sufficient color development foundation, and the indication will not be weak due to insufficient chromotropic groups; at the same time, it will not be overly sensitive to low-dose ultraviolet leakage due to excessive chromotropic groups. The photo-oxidation retarder is controlled within a low content range, mainly responsible for early free radical capture and low-dose color development delay, without completely suppressing the later color development process. With both components combined, the color-changing layer exhibits a practical delayed response characteristic, meaning it does not easily change color under normal conditions, but becomes sufficiently clear when an indication is needed.

[0019] Preferably, the irreversible color-changing indicator layer further contains phosphite antioxidants, the total mass content of the photo-oxidation retardant and the phosphite antioxidants is 0.05% to 0.22%, and the mass ratio of the diarylethylene structural unit to the total amount of the photo-oxidation retardant and the phosphite antioxidants is 10:1 to 30:1.

[0020] Phosphite antioxidants primarily help improve thermo-oxidative stability during melt processing, reducing non-targeted oxidation of diarylethene structural units before and after extrusion and stretching. When used in conjunction with photooxidation retarders, they can control thermo-oxidative interference during processing and low-dose photooxidative interference in the initial stages of use to a low level, allowing irreversible color development to occur more frequently after UV accumulation reaches the set range.

[0021] Preferably, the irreversible color-changing indicator layer has a color difference ΔE After the ab value reaches 8 or above, the color difference retention rate after being placed at room temperature in the dark for 72 hours or exposed to visible light for 72 hours is not less than 85%.

[0022] Achieving the aforementioned color difference retention rate indicates that the color signal formed by the color-changing indicator layer has good retention and will not fade significantly after short-term placement or under normal visible light conditions. For window film products, after a user observes a color change, the signal should remain stable for a period of time to facilitate judgment and replacement planning.

[0023] Preferably, the thickness of the light-receiving PET surface layer is 3-8 μm, the thickness of the first UV-resistant sublayer is 8-18 μm, the thickness of the second UV-resistant sublayer is 8-18 μm, the thickness of the third UV-resistant sublayer is 5-15 μm, the thickness of the irreversible color-changing indicator layer is 3-8 μm, and the thickness of the mounting functional layer is 5-20 μm.

[0024] The thickness of the light-receiving PET surface layer ensures basic wear resistance, weather resistance, and processing protection on the outer surface, while not excessively weakening the controlled degradation process of UV light after it enters the UV-resistant sublayer. The thicknesses of the three UV-resistant sublayers, in accordance with the decreasing lignin content, form a relatively stable UV absorption thickness and aging release pathway. The irreversible color-changing indicator layer is maintained within the range of 3–8 μm, which is beneficial for obtaining clear color changes without significantly increasing the overall film haze or affecting biaxial tensile stability. The thickness of the mounting functional layer ensures structural support during lamination, attachment, or subsequent use, allowing the entire film to balance indicator function and practical application strength.

[0025] A second aspect of the present invention provides a method for preparing the UV cumulative color change indicator UV-resistant BOPET film as described above, comprising the following steps: S1: Lignin nanoparticles are mixed with PET and melt-granulated to obtain a first UV-resistant masterbatch, a second UV-resistant masterbatch, and a third UV-resistant masterbatch, respectively; wherein the mass content of lignin nanoparticles in the first UV-resistant masterbatch, the second UV-resistant masterbatch, and the third UV-resistant masterbatch decreases sequentially. S2: Prepare polyester melt for light-receiving PET surface layer, polyester melt for first anti-UV sublayer, polyester melt for second anti-UV sublayer, polyester melt for third anti-UV sublayer, polyester melt for irreversible color-changing indicator layer, and polymer melt for mounting functional layer, respectively; wherein, the polyester melt for first anti-UV sublayer, polyester melt for second anti-UV sublayer, and polyester melt for third anti-UV sublayer respectively contain first anti-UV masterbatch, second anti-UV masterbatch, and third anti-UV masterbatch, and the polyester melt for irreversible color-changing indicator layer contains PET copolymerized or grafted with diarylethylene structural units and photooxidation retardant; S3: The polyester melt for the light-receiving PET surface layer, the polyester melt for the first UV-resistant sublayer, the polyester melt for the second UV-resistant sublayer, the polyester melt for the third UV-resistant sublayer, the polyester melt for the irreversible color-changing indicator layer, and the polymer melt for the installation functional layer are co-extruded through a multilayer distributor to obtain a multilayer co-extruded casting sheet. S4: The multilayer co-extruded sheet is subjected to longitudinal stretching, transverse stretching and heat setting to obtain a UV cumulative color change indicator UV-resistant BOPET film.

[0026] First, lignin nanoparticles with varying contents are prepared into UV-resistant masterbatches, which are then used in three UV-resistant sublayers. This improves the pre-dispersion of lignin in PET, making it easier to control the particle content and distribution in each layer. Multilayer co-extrusion enables the formation of a continuous structure in a single film-forming process, consisting of a light-receiving surface layer, a decreasing UV-resistant layer, a color-changing indicator layer, and an mounting functional layer. This reduces interlayer bonding instability issues caused by post-coating or post-lamination. Subsequent biaxial stretching and heat setting processes impart good dimensional stability, mechanical strength, and surface smoothness to the film, ensuring the stable retention of the positional relationships between the functional layers in the finished product.

[0027] Preferably, the extrusion temperature of the polyester melt for the first, second, and third UV-resistant sublayers is 255–280°C; the extrusion temperature of the polyester melt for the irreversible color-changing indicator layer is 250–275°C; the longitudinal stretching ratio is 2.8–3.5; the transverse stretching ratio is 3.2–4.2; and the heat-setting temperature is 210–235°C.

[0028] The aforementioned temperature range allows for sufficient plasticization of the PET matrix and ensures uniform dispersion of lignin nanoparticles in the melt, while avoiding thermo-oxidative damage to the color-changing structural units caused by excessively high temperatures. The irreversible color-changing indicator layer utilizes a relatively controlled extrusion temperature, which helps maintain the effectiveness of the diarylethylene structural units and the delayed-stabilization system. The longitudinal and transverse stretch ratios, combined with the heat-setting temperature, enable the film to achieve a better orientation structure and thickness uniformity, and also reduce the impact of localized thickness fluctuations on UV-A transmittance and color uniformity.

[0029] The present invention, by adopting the above technical solution, has the following beneficial effects: (1) The present invention arranges a light-receiving PET surface layer, multiple UV-resistant sub-layers, an irreversible color-changing indicator layer, and an installation function layer in sequence along the light-receiving direction, so that ultraviolet rays first pass through the UV-resistant sub-layers and then reach the color-changing indicator layer. This layer sequence enables the color-changing response to be correlated with the service degradation process of the UV-resistant layer, so that the film can intuitively reflect the effective protection status through color change after long-term use, making it convenient for users to judge when to replace it.

[0030] (2) In this invention, the content of lignin nanoparticles in the first, second, and third anti-UV sublayers decreases in the direction away from the light-receiving PET surface, and the particle size and surface compatibility groups of the lignin nanoparticles are controlled. The higher content of lignin nanoparticles on the light-receiving side preferentially absorbs ultraviolet light and undertakes the initial photo-oxidation consumption, while the lower content of lignin nanoparticles on the side closer to the color-changing indicator layer maintains light transmittance and continuous ultraviolet release, thereby taking into account the anti-UV performance, transparent appearance, tensile stability, and controllability of the decay process.

[0031] (3) In this invention, PET copolymerized or grafted with diarylethylene structural units is used in the irreversible color-changing indicator layer, and a photo-oxidation retardant is introduced. The diarylethylene structural units are stably bound in the polyester system, which helps to reduce migration, volatilization or phase separation; the photo-oxidation retardant can suppress early false color development caused by low-dose ultraviolet leakage, so that the color-changing layer produces obvious and stable color change after the UV-A transmittance enters the set range.

[0032] (4) This invention limits the average UV-A transmittance and color difference ΔE. The correspondence between ab ensures that the film is less prone to obvious color misjudgment when it is in a good protective state, while providing easily identifiable color indicators when the UV protection performance enters the degradation stage that requires attention. Combined with the processes of preparing UV-resistant masterbatch first, multilayer co-extrusion, biaxial stretching, and heat setting, BOPET films with good interlayer bonding, high functional consistency, and suitability for large-scale production can be stably obtained. Detailed Implementation

[0033] The specific embodiments of the present invention will be described in detail below. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0034] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0037] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0038] Example 1 This embodiment discloses an ultraviolet-cumulative color-changing indicator UV-resistant BOPET film. The film, along the light-receiving direction, consists of a light-receiving PET surface layer, a first UV-resistant sublayer, a second UV-resistant sublayer, a third UV-resistant sublayer, an irreversible color-changing indicator layer, and a mounting functional layer, with a total thickness of 60 μm. Specifically, the light-receiving PET surface layer has a thickness of 5 μm, the first UV-resistant sublayer has a thickness of 12 μm, the second UV-resistant sublayer has a thickness of 12 μm, the third UV-resistant sublayer has a thickness of 10 μm, the irreversible color-changing indicator layer has a thickness of 6 μm, and the mounting functional layer has a thickness of 15 μm.

[0039] In this embodiment, the intrinsic viscosity of the Dayouguang PET chips is 0.648 dL / g, the terminal carboxyl group content is 27.6 mol / t, and the melting point is 255℃. The lignin nanoparticles are prepared from alkali lignin. Specifically, alkali lignin is added to an aqueous sodium hydroxide solution to prepare an alkaline dispersion with a lignin mass fraction of 3.5%. The pH of the system is adjusted to 12.3, stirred and dissolved for 45 min, filtered to remove insoluble matter, and then the pH is adjusted to 2.7 with 1 mol / L hydrochloric acid to precipitate the lignin. After centrifugation and washing with deionized water until the pH of the washing solution is 6.8, the precipitate is redispersed in deionized water and wet-milled for 2 h using 0.3 mm diameter zirconia beads as the milling medium. Subsequently, it is spray-dried at an inlet air temperature of 160℃ and an outlet air temperature of 82℃ to obtain lignin nanoparticles. The lignin nanoparticles have a D50 particle size of 146 nm and a D90 particle size of 382 nm. These D50 and D90 particle sizes were measured using a laser particle size analyzer after the spray-dried powder was ultrasonically dispersed in deionized water for 10 min. The lignin nanoparticles contain hydroxyl and carboxyl groups on their surface; these hydroxyl and carboxyl groups originate from the structure of alkali lignin itself and are located near 3400 cm⁻¹ and 1710 cm⁻¹, respectively, as determined by infrared spectroscopy. -1 There is a corresponding absorption peak nearby.

[0040] Diarylethylene structural unit copolymerized modified PET is a copolyester obtained by esterification and polycondensation reactions of a dihydroxydiarylethylene derivative with terephthalic acid and ethylene glycol. The dihydroxydiarylethylene derivative is a thiophene- and perfluorocyclopentene diarylethylene derivative with hydroxyethyl reactive groups at both ends of the molecule. The preparation conditions for diarylethylene structural unit copolymerized modified PET are as follows: terephthalic acid, ethylene glycol, and the dihydroxydiarylethylene derivative are added to a reactor at a molar ratio of ethylene glycol to terephthalic acid of 1.25:1, with a diarylethylene structural unit mass content of 8%. Antimony glycolate (0.035% by mass of terephthalic acid) is added as a polycondensation catalyst. The mixture is heated to 240℃ under nitrogen protection for 2.5 h for esterification. Subsequently, vacuum is gradually applied, and polycondensation is carried out at 275℃ and a vacuum degree below 120 Pa for 3.2 h to obtain diarylethylene structural unit copolymerized modified PET. The mass content of the diarylethylene structural unit was calculated based on the amount of dihydroxydiarylethylene derivative fed and the mass of the copolyester after polycondensation, and verified by UV-Vis absorption spectroscopy. The intrinsic viscosity of the copolymerized modified PET is 0.612 dL / g, and the mass content of the diarylethylene structural unit is 8%.

[0041] The photo-oxidation retardant is a mixture of hindered amine light stabilizer and hindered phenolic antioxidant in a mass ratio of 2:1. The hindered amine light stabilizer is bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, and the hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid].

[0042] The film was prepared according to the following steps.

[0043] S1: Lignin nanoparticles were vacuum dried at 105℃ for 6 hours to reduce the moisture content to 0.18%. High-gloss PET slices were vacuum dried at 160℃ for 5 hours, and PET modified by copolymerization of diarylethylene structural units was vacuum dried at 145℃ for 6 hours.

[0044] The dried lignin nanoparticles were premixed with dried glossy PET chips in a high-speed mixer for 8 minutes at a speed of 900 r / min. The mixture was then fed into a twin-screw extruder for melt extrusion, cooling, and pelletizing to obtain the first, second, and third UV-resistant masterbatches.

[0045] The first UV-resistant masterbatch contains 13% lignin nanoparticles by mass, the second UV-resistant masterbatch contains 8% lignin nanoparticles by mass, and the third UV-resistant masterbatch contains 5% lignin nanoparticles by mass.

[0046] The temperatures of the twin-screw extruder in zones one through six are 240℃, 250℃, 260℃, 265℃, 265℃, and 260℃ respectively. The screw speed is 180 r / min, and the vacuum exhaust pressure is -0.085 MPa.

[0047] The drying conditions for PET chips, diarylene ethylene structural unit copolymerized PET, and UV-resistant masterbatch not specifically described in Examples 2-5 and Comparative Examples 1-6 are the same as those in this example.

[0048] S2: Prepare the melts for each layer separately. The polyester melt for the light-receiving PET surface layer is formed by melting 100 parts by weight of glossy PET chips.

[0049] The first UV-resistant sublayer polyester melt is formed by melting and mixing 40 parts by weight of the first UV-resistant masterbatch and 60 parts by weight of glossy PET chips, wherein the final mass content of lignin nanoparticles is 5.2%.

[0050] The second UV-resistant sublayer is formed by melting and mixing 40 parts by weight of the second UV-resistant masterbatch and 60 parts by weight of glossy PET chips, wherein the final mass content of lignin nanoparticles is 3.2%.

[0051] The third UV-resistant sublayer polyester melt is formed by melting and mixing 30 parts by weight of the third UV-resistant masterbatch and 70 parts by weight of glossy PET chips, wherein the final mass content of lignin nanoparticles is 1.5%.

[0052] The polyester melt for the irreversible color-changing indicator layer is formed by melt mixing 13.75 parts by mass of PET copolymerized with diarylethylene structural units, 86.17 parts by mass of glossy PET chips, and 0.08 parts by mass of photooxidation retardant. The mass content of diarylethylene structural units is 1.1%, the mass content of photooxidation retardant is 0.08%, and the mass ratio of diarylethylene structural units to photooxidation retardant is 13.75:1.

[0053] The polymer melt for mounting the functional layer is formed by melting 100 parts by weight of large glossy PET chips.

[0054] S3: The polyester melt for the light-receiving PET surface layer, the polyester melt for the first UV-resistant sublayer, the polyester melt for the second UV-resistant sublayer, the polyester melt for the third UV-resistant sublayer, the polyester melt for the irreversible color-changing indicator layer, and the polymer melt for the installation functional layer are fed into a six-layer distributor by six extruders and co-extruded onto the surface of a 25°C chilled roller through a T-die to form a multi-layer co-extruded sheet.

[0055] The thickness of each layer is controlled by the corresponding metering pump speed of the extruder. The melt flow ratio of the light-receiving PET surface layer, the first UV-resistant sublayer, the second UV-resistant sublayer, the third UV-resistant sublayer, the irreversible color-changing indicator layer, and the mounting functional layer is controlled at 5:12:12:10:6:15. The extrusion temperature of the light-receiving PET surface layer is 270℃, the extrusion temperature of the first, second, and third UV-resistant sublayers is 268℃, the extrusion temperature of the irreversible color-changing indicator layer is 262℃, the extrusion temperature of the mounting functional layer is 270℃, the die temperature is 268℃, and the linear speed of the chiller roller is 18m / min.

[0056] S4: The multilayer co-extruded sheet is subjected to longitudinal stretching, transverse stretching, and heat setting. The longitudinal stretching temperature is 88℃, and the longitudinal stretching ratio is 3.2; the transverse stretching preheating temperature is 104℃, and the transverse stretching ratio is 3.8; the heat setting temperature is 225℃, and the heat setting time is 8s. After heat setting, the film is cooled, drawn, and wound to obtain a UV-accumulated color change indicator UV-resistant BOPET film.

[0057] Example 2 This embodiment discloses an ultraviolet cumulative color change indicator UV-resistant BOPET film. Compared with Example 1, the difference lies in the lignin nanoparticle mass content of the first, second, and third UV-resistant sublayers being 4%, 2.4%, and 1%, respectively.

[0058] Specifically: In S1, the first UV-resistant masterbatch contains 10% lignin nanoparticles by mass, the second UV-resistant masterbatch contains 6% lignin nanoparticles by mass, and the third UV-resistant masterbatch contains 4% lignin nanoparticles by mass. In S2, the first UV-resistant sublayer is formed by melting and mixing 40 parts by mass of the first UV-resistant masterbatch and 60 parts by mass of glossy PET chips; the second UV-resistant sublayer is formed by melting and mixing 40 parts by mass of the second UV-resistant masterbatch and 60 parts by mass of glossy PET chips; and the third UV-resistant sublayer is formed by melting and mixing 25 parts by mass of the third UV-resistant masterbatch and 75 parts by mass of glossy PET chips. Other raw materials, layer thicknesses, and process parameters are the same as in Example 1.

[0059] Example 3 This embodiment discloses an ultraviolet cumulative color change indicator UV-resistant BOPET film. Compared with Example 1, the difference lies in that the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers are 6.2%, 4.2%, and 2.3%, respectively.

[0060] Specifically: In S1, the lignin nanoparticle content in the first UV-resistant masterbatch is 15.5% by mass, the lignin nanoparticle content in the second UV-resistant masterbatch is 10.5% by mass, and the lignin nanoparticle content in the third UV-resistant masterbatch is 5.75% by mass. In S2, the polyester melt for the first UV-resistant sublayer is formed by melting and mixing 40 parts by mass of the first UV-resistant masterbatch and 60 parts by mass of glossy PET chips; the polyester melt for the second UV-resistant sublayer is formed by melting and mixing 40 parts by mass of the second UV-resistant masterbatch and 60 parts by mass of glossy PET chips; and the polyester melt for the third UV-resistant sublayer is formed by melting and mixing 40 parts by mass of the third UV-resistant masterbatch and 60 parts by mass of glossy PET chips. Other raw materials, layer thicknesses, and process parameters are the same as in Example 1.

[0061] Example 4 This embodiment discloses an ultraviolet cumulative color change indicator anti-ultraviolet BOPET film. Compared with Embodiment 1, the difference lies in that the mass content of diarylethylene structural units in the irreversible color change indicator layer is 0.45%, the mass content of photooxidation retarder is 0.045%, and the mass ratio of diarylethylene structural units to photooxidation retarder is 10:1.

[0062] Specifically, the irreversible color-changing indicator layer in S2 is formed by melt mixing 5.625 parts by mass of diarylethene structural unit copolymerized modified PET, 94.33 parts by mass of glossy PET chips, and 0.045 parts by mass of photooxidation retardant. Other raw materials, layer thickness, and process parameters are the same as in Example 1.

[0063] Example 5 This embodiment discloses an ultraviolet-cumulative color-changing indicator anti-UV BOPET film. The difference from Example 1 is that the irreversible color-changing indicator layer also contains a phosphite antioxidant. The phosphite antioxidant is tris(2,4-di-tert-butylphenyl) phosphite, added as a thermo-oxidative stabilizing auxiliary component and not included in the photo-oxidation retarder content.

[0064] Specifically, the irreversible color-changing indicator layer in S2 is formed by melt-mixing 15 parts by mass of PET copolymerized with diarylethylene structural units, 84.895 parts by mass of glossy PET chips, 0.055 parts by mass of bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, 0.025 parts by mass of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.025 parts by mass of tris(2,4-di-tert-butylphenyl) phosphite. The irreversible color-changing indicator layer contains 1.2% by mass of diarylethylene structural units, 0.08% by mass of photooxidation retardant, and 0.025% by mass of phosphite antioxidants. The mass ratio of diarylethylene structural units to the total amount of photooxidation retardant and phosphite antioxidants is 11.43:1. Other raw materials, layer thickness, and process parameters are the same as in Example 1.

[0065] Comparative Example 1 This comparative example discloses a BOPET film. Compared with Example 1, the difference is that the lignin nanoparticle mass content in the first, second, and third UV-resistant sublayers is 3.3%, and a lignin nanoparticle content distribution that decreases along the direction away from the light-receiving PET surface is not formed.

[0066] Specifically, in S1, an anti-UV masterbatch with a lignin nanoparticle mass content of 8.25% was prepared. In S2, the polyester melts for the first, second, and third anti-UV sublayers were all formed by melting and mixing 40 parts by mass of the anti-UV masterbatch and 60 parts by mass of glossy PET chips. Other raw materials, layer thicknesses, and process parameters were the same as in Example 1.

[0067] Comparative Example 2 This comparative example discloses a BOPET film. Compared with Example 1, the difference lies in that the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers are 4%, 3.5%, and 3%, respectively, and the decrease in the mass content of lignin nanoparticles between adjacent UV-resistant sublayers is less than 0.8 percentage points.

[0068] Specifically, in S1, the lignin nanoparticles in the first, second, and third UV-resistant masterbatches contain 10%, 8.75%, and 7.5% by mass, respectively. In S2, the polyester melt for the first, second, and third UV-resistant sublayers is formed by melting and mixing 40 parts by mass of the corresponding UV-resistant masterbatch and 60 parts by mass of glossy PET chips. Other raw materials, layer thicknesses, and process parameters are the same as in Example 1.

[0069] Comparative Example 3 This comparative example discloses a BOPET film. The difference from Example 1 is that no photo-oxidation retarder is added to the irreversible color-changing indicator layer.

[0070] Specifically, the polyester melt used for the irreversible color-changing indicator layer in S2 is formed by melting and mixing 13.75 parts by mass of PET copolymerized with diarylethylene structural units and 86.25 parts by mass of glossy PET chips, wherein the mass content of diarylethylene structural units is 1.1%. Other raw materials, layer thickness and process parameters are the same as in Example 1.

[0071] Comparative Example 4 This comparative example discloses a BOPET film. Compared with Example 1, the difference lies in the excessively high content of photooxidation retarder in the irreversible color-changing indicator layer, and the mass ratio of diarylethene structural units to photooxidation retarder is less than 8:1.

[0072] Specifically, the irreversible color-changing indicator layer in S2 is formed by melt-mixing 13.75 parts by mass of PET copolymerized with diarylethylene structural units, 86 parts by mass of glossy PET chips, and 0.25 parts by mass of photooxidation retardant. The mass content of diarylethylene structural units is 1.1%, the mass content of photooxidation retardant is 0.25%, and the mass ratio of diarylethylene structural units to photooxidation retardant is 4.4:1. Other raw materials, layer thickness, and process parameters are the same as in Example 1.

[0073] Comparative Example 5 This comparative example discloses a BOPET film. Compared with Example 1, the difference is that the irreversible color-changing indicator layer uses a small molecule diarylethene derivative physically blended with PET, instead of using PET copolymerized or grafted with diarylethene structural units.

[0074] Specifically, the small molecule diarylethylene derivative is a small molecule diarylethylene derivative without hydroxyl, carboxyl, and transesterification reaction groups, specifically 1,2-bis(2-methyl-5-phenylthiophene-3-yl)perfluorocyclopentene; the 5% thermogravimetric loss temperature of this small molecule diarylethylene derivative under nitrogen atmosphere is 286°C. The polyester melt used for the irreversible color-changing indicator layer in S2 is formed by melt mixing 1.1 parts by mass of the small molecule diarylethylene derivative, 98.82 parts by mass of glossy PET chips, and 0.08 parts by mass of photo-oxidation retardant; the small molecule diarylethylene derivative is premixed with the glossy PET chips in a high-speed mixer for 5 minutes at a speed of 700 r / min, and then fed into the extruder corresponding to the irreversible color-changing indicator layer. The composition of the photo-oxidation retardant is the same as in Example 1. Other raw materials, layer thickness, and process parameters are the same as in Example 1.

[0075] Comparative Example 6 This comparative example discloses a BOPET film. The difference from Example 1 is that the irreversible color-changing indicator layer is disposed before the first UV-resistant sublayer.

[0076] Specifically, the film consists of, along the light-receiving direction, a light-receiving PET surface layer, an irreversible color-changing indicator layer, a first UV-resistant sublayer, a second UV-resistant sublayer, a third UV-resistant sublayer, and an installation functional layer. The formulation and thickness of each layer are the same as in Example 1. The flow rate ratio of each melt channel in the six-layer distributor is adjusted to 5:6:12:12:10:15 according to the layer sequence of the light-receiving PET surface layer, irreversible color-changing indicator layer, first UV-resistant sublayer, second UV-resistant sublayer, third UV-resistant sublayer, and installation functional layer. The irreversible color-changing indicator layer is still extruded at 262°C, and the first, second, and third UV-resistant sublayers are still extruded at 268°C. All other process parameters are the same as in Example 1.

[0077] Performance testing To verify the technical effectiveness of the present invention, the performance of the BOPET films obtained in the examples and comparative examples was tested. Before testing, all samples were placed in an environment at 23°C and 50% relative humidity for 24 hours. Unless otherwise specified, five parallel samples were taken from each group of samples, and the test results were taken as the arithmetic mean.

[0078] 1. Tests for light transmittance, haze, and initial average UV-A transmittance. Visible light transmittance and haze were tested according to GB / T 2410-2008 "Determination of transmittance and haze of transparent plastics". The whole film sample was tested using a haze meter method, with the incident light direction being the side of the PET surface exposed to light. UV-A average transmittance was measured using a UV-Vis spectrophotometer, with a test wavelength of 315–400 nm, a scanning interval of 1 nm, air as a reference, a slit width of 2 nm, a scanning speed of 600 nm / min, and the incident light direction being the side of the PET surface exposed to light. The test sample was kept flat and perpendicular to the incident light. The UV-A average transmittance was the arithmetic mean of the transmittance within the 315–400 nm range. To correspond to the UV-A release state of the anti-UV layer, test samples with the same composition and thickness as the first, second, and third anti-UV sublayers in each sample were prepared. The test sample consisted of a first UV-resistant sublayer, a second UV-resistant sublayer, and a third UV-resistant sublayer along the light-receiving direction, with a total thickness of 34 μm. It was prepared using the same formulation, layer thickness ratio, co-extrusion, and biaxial stretching conditions as the corresponding film.

[0079] Table 1 Initial Optical Performance Test Results As shown in Table 1, Examples 1-3 and Example 5 all exhibited low initial average UV-A transmittance, indicating that the decreasing lignin nanoparticle UV-resistant sublayer can form an effective UV barrier in the initial stage. Example 2 had a low lignin nanoparticle content and high visible light transmittance; Example 3 had a high lignin nanoparticle content and even lower initial average UV-A transmittance, while the haze remained within an acceptable range, indicating that the particle size and dispersion mode of the lignin nanoparticles can balance UV resistance and transparent appearance. The initial average UV-A transmittance of Comparative Example 5 was close to that of Example 1, indicating that the difference in subsequent color development stability mainly stemmed from the presence mode of the color-changing component, rather than differences in initial UV resistance.

[0080] 2. Xenon lamp aging and detection of the relationship between UV-A transmittance and color difference Xenon lamp aging was conducted according to GB / T 16422.2-2022 "Laboratory Light Source Exposure Test Methods for Plastics - Part 2: Xenon Arc Lamp". The aging test used an air-cooled xenon arc lamp and a daylight filter. The irradiance at 340 nm was set to 0.55 W / (m²·nm), the black standard temperature was 65℃, and the relative humidity inside the chamber was 50%. No water circulation was performed, and the light was incident from the illuminated PET surface. Samples were taken every 120 hours during the aging process to test the average UV-A transmittance of the test sample layer in the 315–400 nm range, as well as the color difference in the corresponding area of ​​the irreversible color change indicator layer. The color difference was calculated according to GB / T 7921-2008 "Uniform Color Space and Color Difference Formula". A D65 standard light source and a 10° observation angle were used. The sample was placed on a white standard backing plate, and measurements were taken from the illuminated PET surface. The L value of the same sample before aging was used as the reference value. a Using b as a benchmark, ΔEab is calculated. Table 2 lists the color difference results for the test sample layer when the average UV-A transmittance is below 3% and between 3% and 6%.

[0081] Table 2. Detection Results of Correspondence Between UV-A Average Transmittance and Color Difference of Irreversible Color-Changing Indicator Layer As shown in Table 2, in Examples 1-5, when the average UV-A transmittance of the test sample layer was below 3%, ΔEab was less than 3, making it difficult for the naked eye to produce obvious misjudgment. When the average UV-A transmittance entered the 3%-6% range, ΔEab reached above 8, forming a clear and identifiable color indication. Comparative Example 1 did not form a decreasing distribution of lignin nanoparticles, and Comparative Example 2 had insufficient decreasing amplitude, with high color difference in the low transmittance stage and insufficient color difference in the critical stage, indicating that the decreasing absorption path of the UV-resistant sublayer plays an important role in delayed triggering. Comparative Example 3 did not add a photo-oxidation retarder, and obvious color development appeared when the average UV-A transmittance was below 3%, indicating that low-dose leaked UV can easily cause premature miscoloration. In Comparative Example 4, the photo-oxidation retarder was excessive. Although early color development was suppressed, color development was insufficient after the average UV-A transmittance entered the 3%-6% range. Comparative Example 5 used a physically blended small-molecule diarylethene derivative. The high color difference in the low-transmittance stage and insufficient color development in the critical stage indicate that fixing the diarylethene structural units within the PET system is beneficial for obtaining stable indication. In Comparative Example 6, after changing the layer sequence, the color-changing indicator layer was directly located before the UV-resistant sublayer. Even though the average UV-A transmittance of the test sample was still below 3%, a significant color difference was already observed, indicating that the layer sequence setting in the light-receiving direction has a direct effect on preventing premature color development.

[0082] 3. Color retention rate test Color retention was tested using Examples 1, 5, Comparative Example 5, and Comparative Example 6. Samples were aged under a xenon lamp to the aging time corresponding to an average UV-A transmittance of 3%–6% as shown in Table 2, and then placed at room temperature in the dark for 72 hours and irradiated with visible light for 72 hours, respectively. Visible light irradiation used a D65 simulated light source with an illuminance of 10000 lx, irradiating the light-receiving PET surface. L was measured again after treatment. a The b-value was calculated, and ΔEab was determined. During testing, the sample remained on a white standard backing board, and measurements were taken from the light-exposed PET surface side. Color difference retention was calculated as the ratio of ΔEab after treatment to ΔEab before treatment.

[0083] Table 3. Results of color retention rate test after color development of samples. As shown in Table 3, Example 1 exhibits good color difference retention after color development, indicating that the copolymerization modification of PET with diarylethylene structural units can stably retain the color signal. In Example 5, the addition of phosphite antioxidants further improved the color retention after color development, demonstrating that the thermo-oxidative stabilizing auxiliary components help reduce non-targeted oxidation interference during processing and aging. Comparative Example 5 shows a significantly lower color difference retention, indicating that small-molecule diarylethylene derivatives are prone to migration or unstable local distribution after physical blending, leading to decreased color signal retention. Although Comparative Example 6 initially showed strong color development, its retention rate after visible light irradiation was lower than that of Examples 1 and 5, indicating that abnormal layer sequence can cause the color-changing layer to produce a strong but relatively unstable color development state under early exposure.

[0084] 4. Color uniformity test Example 1, Example 5, Comparative Example 1, and Comparative Example 5 were selected for color uniformity testing. After aging the samples under a xenon lamp until the average UV-A transmittance of the test sample layer was 3%–6%, nine test points were uniformly selected within a 100mm × 100mm area in the middle of the entire film sample. ΔEab was calculated for each point according to GB / T 7921-2008 "Uniform Color Space and Color Difference Formula". During testing, a D65 standard light source and a 10° observation angle were used. The sample was placed on a white standard backing plate, and measurements were taken from the light-receiving PET surface side. The average ΔEab and standard deviation of the nine test points were calculated, and the standard deviation was used to evaluate color uniformity.

[0085] Table 4. Results of color development uniformity test during the critical transmission stage of the samples. As shown in Table 4, Examples 1 and 5 exhibited lower color development standard deviations during the critical transmission stage, indicating a more stable distribution of diarylethene structural units in the color-changing indicator layer and more uniform color development. Comparative Example 1, lacking a decreasing UV-resistant sublayer structure, showed an increased color development standard deviation, indicating a less balanced UV release state in the UV-resistant layer. Comparative Example 5, using a physically blended small-molecule diarylethene derivative, further increased the color development standard deviation, suggesting that the small-molecule color-changing component is more prone to localized distribution differences during high-temperature co-extrusion and biaxial stretching.

[0086] 5. Tensile property testing Tensile properties were tested using Examples 1, 2, 3, and Comparative Example 5. Tensile properties were determined according to GB / T1040.3-2006, "Determination of tensile properties of plastics – Part 3: Test conditions for films and sheets". Samples were cut longitudinally and transversely, with a sample width of 15 mm, an effective clamping distance of 100 mm, and a testing speed of 100 mm / min. Five parallel samples were tested in each group, and the results were taken as the arithmetic mean.

[0087] Table 5 Tensile property test results As shown in Table 5, Examples 1-3 all maintained good longitudinal and transverse tensile properties, indicating that lignin nanoparticles, after pre-dispersion in the masterbatch, can adapt to the co-extrusion and biaxial stretching processes of BOPET. In Example 3, the lignin nanoparticle content was higher, resulting in a slight decrease in tensile properties, but they remained within a relatively stable range. Comparative Example 5, due to the physical blending of small-molecule diarylethylene derivatives, showed a significant decrease in tensile strength and elongation at break, presumably related to insufficient local compatibility of the small-molecule component in the polyester matrix.

[0088] Based on the above test results, Examples 1-5, while maintaining transparency, UV resistance, and tensile properties, can maintain low color difference when the average UV-A transmittance is below 3%, and form clear and stable color indicators when the average UV-A transmittance enters the 3%-6% range. A good synergistic relationship is formed between the three-layer decreasing lignin nanoparticle UV-resistant structure, the diarylethylene structural unit copolymerized modified PET, and the photooxidation retarder, enabling the film to convert the service degradation state of the UV-resistant layer into a visible color signal.

[0089] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A UV-accumulated color-changing indicator UV-resistant BOPET film, characterized in that, It includes a light-receiving PET surface layer, a first UV-resistant sublayer, a second UV-resistant sublayer, a third UV-resistant sublayer, an irreversible color-changing indicator layer, and an installation function layer arranged sequentially along the light-receiving direction; The first, second, and third UV-resistant sublayers are all based on PET and contain lignin nanoparticles, and the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers decreases in the direction away from the light-receiving PET surface. The irreversible color-changing indicator layer contains PET copolymerized or grafted with diarylethylene structural units and a photo-oxidation retardant. The photo-oxidation retardant is composed of hindered amine light stabilizers and / or hindered phenolic antioxidants. The mass ratio of the diarylethylene structural units to the photo-oxidation retardant is 8:1 to 35:

1. Under xenon lamp aging conditions, a test sample layer with the same composition and thickness as the first, second, and third anti-UV sublayers was subjected to UV-A average transmittance testing. When the UV-A average transmittance of the test sample layer in the 315–400 nm range was less than 3%, the color difference ΔEab of the irreversible color-changing indicator layer was less than 3; when the UV-A average transmittance of the test sample layer in the 315–400 nm range was 3%–6%, the color difference ΔEab of the irreversible color-changing indicator layer was 8–18.

2. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The mass content of lignin nanoparticles in the first UV-resistant sublayer is 3.5% to 6.5%, the mass content of lignin nanoparticles in the second UV-resistant sublayer is 2% to 4.5%, and the mass content of lignin nanoparticles in the third UV-resistant sublayer is 0.8% to 2.5%, and the mass content of lignin nanoparticles in the first, second, and third UV-resistant sublayers decreases by at least 0.8 percentage points in that order.

3. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The lignin nanoparticles have a D50 particle size of 60-250 nm and a D90 particle size of no more than 500 nm; the lignin nanoparticles are nano-lignin particles with at least one compatible group selected from hydroxyl, carboxyl or ester groups on their surface.

4. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The PET copolymerized or grafted with diarylethylene structural units is formed by copolymerization, transesterification or melt grafting of diarylethylene derivatives containing hydroxyl, carboxyl or transesterification reactive groups with polyester monomers, polyester oligomers or PET.

5. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The diarylethylene structural unit has a mass content of 0.2% to 2.5% in the irreversible color-changing indicator layer; the photo-oxidation retardant has a mass content of 0.03% to 0.18% in the irreversible color-changing indicator layer.

6. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The irreversible color-changing indicator layer also contains phosphite antioxidants, the total mass content of the photo-oxidation retardant and the phosphite antioxidants is 0.05% to 0.22%, and the mass ratio of the diarylene ethylene structural unit to the total amount of the photo-oxidation retardant and the phosphite antioxidants is 10:1 to 30:

1.

7. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The irreversible color-changing indicator layer has a color difference ΔE After the ab value reaches 8 or above, the color difference retention rate after being placed at room temperature in the dark for 72 hours or exposed to visible light for 72 hours is not less than 85%.

8. The UV cumulative color change indicator UV-resistant BOPET film according to claim 1, characterized in that, The thickness of the light-receiving PET surface layer is 3–8 μm, the thickness of the first UV-resistant sublayer is 8–18 μm, the thickness of the second UV-resistant sublayer is 8–18 μm, the thickness of the third UV-resistant sublayer is 5–15 μm, the thickness of the irreversible color-changing indicator layer is 3–8 μm, and the thickness of the mounting functional layer is 5–20 μm.

9. A method for preparing a UV-cumulative color-changing indicator UV-resistant BOPET film as described in any one of claims 1 to 8, characterized in that, The steps include the following: S1: Lignin nanoparticles are mixed with PET and melt-granulated to obtain a first UV-resistant masterbatch, a second UV-resistant masterbatch, and a third UV-resistant masterbatch, respectively; wherein the mass content of lignin nanoparticles in the first UV-resistant masterbatch, the second UV-resistant masterbatch, and the third UV-resistant masterbatch decreases sequentially. S2: Prepare polyester melt for light-receiving PET surface layer, polyester melt for first anti-UV sublayer, polyester melt for second anti-UV sublayer, polyester melt for third anti-UV sublayer, polyester melt for irreversible color-changing indicator layer, and polymer melt for mounting functional layer, respectively; wherein, the polyester melt for first anti-UV sublayer, polyester melt for second anti-UV sublayer, and polyester melt for third anti-UV sublayer respectively contain first anti-UV masterbatch, second anti-UV masterbatch, and third anti-UV masterbatch, and the polyester melt for irreversible color-changing indicator layer contains PET copolymerized or grafted with diarylethylene structural units and photooxidation retardant; S3: The polyester melt for the light-receiving PET surface layer, the polyester melt for the first UV-resistant sublayer, the polyester melt for the second UV-resistant sublayer, the polyester melt for the third UV-resistant sublayer, the polyester melt for the irreversible color-changing indicator layer, and the polymer melt for the installation functional layer are co-extruded through a multilayer distributor to obtain a multilayer co-extruded casting sheet. S4: The multilayer co-extruded sheet is subjected to longitudinal stretching, transverse stretching and heat setting to obtain a UV cumulative color change indicator UV-resistant BOPET film.

10. The method for preparing the UV cumulative color change indicator anti-UV BOPET film according to claim 9, characterized in that, The extrusion temperature of the polyester melt for the first, second, and third UV-resistant sublayers is 255–280°C; the extrusion temperature of the polyester melt for the irreversible color-changing indicator layer is 250–275°C; the longitudinal stretching ratio is 2.8–3.5; the transverse stretching ratio is 3.2–4.2; and the heat setting temperature is 210–235°C.

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