Polyester protective film and preparation method thereof

Abstract

The invention discloses a polyester protective film and a preparation method thereof, the polyester protective film comprises a structure formed by co-extrusion of three layers: a surface functional layer, a core base material layer and a bottom adhesion layer, the outer side of the surface functional layer is coated with a nano transparent coating, and the nano transparent coating is formed by nano transparent paint through an ultraviolet light curing process; wherein the surface functional layer is prepared from the following components in parts by weight: 80 to 90 parts of PET (Polyethylene Terephthalate), 2 to 5 parts of benzophenone and 5 to 10 parts of nano boron nitride. According to the polyester protective film disclosed by the invention, through collaborative optimization of multiple layers and interaction of the layers, the dimensional stability, the weather resistance and the surface performance are collaboratively optimized, and the comprehensive requirements of high-end protective films are met. Compared with the prior art and a contrast embodiment, the polyester protective film has the advantages that the thermal dimensional stability is obviously improved, the MD / TD shrinkage rate is controlled to be less than or equal to 1%, the optical performance and the mechanical performance are comprehensively superior to those of a contrast group, and the weather resistance is improved to gt; meanwhile, the problems of yellowing, cracking and the like of an existing membrane material are effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, in particular to a polyester film and a preparation method thereof, and particularly to a polyester protective film and a preparation method thereof. Background Art

[0002] Protective films made of polymer materials are widely used to protect documents, cards, files and other items. They are often used in protective packaging for pictures, documents, files and office supplies. After hot stamping, they are flat and beautiful as protective films, which can keep the originals clear and not deformed. They can also be used to protect electronic equipment from impact and vibration during transportation.

[0003] Existing polymer protective films are prone to significant dimensional changes when exposed to temperature fluctuations. In particular, shrinkage in both the machine direction (MD) and transverse direction (TD) cannot be simultaneously minimized, resulting in a decrease in both protective performance and aesthetics. Some polymer protective films may yellow, crack, or experience a decrease in mechanical properties when exposed to sunlight or in humid environments for extended periods, making them difficult to meet the requirements for long-term stability. Furthermore, existing products suffer from excessive surface gloss and poor scratch resistance, compromising their performance and protective capabilities. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a polyester protective film and a preparation method thereof, so as to reduce or avoid the above-mentioned problems.

[0005] To solve the above technical problems, the present invention proposes a polyester protective film, comprising a three-layer co-extruded structure: a surface functional layer, a core substrate layer, and a bottom adhesion layer. The outer side of the surface functional layer is coated with a nano-transparent coating, and the nano-transparent coating is formed by a nano-transparent coating through a UV curing process; wherein, the surface functional layer is composed of 80-90 parts by weight of PET, 2-5 parts by weight of benzophenone, and 5-10 parts by weight of nano-boron nitride.

[0006] Preferably, the core substrate layer is composed of 95-98 parts by weight of PET, 2-5 parts by weight of terephthalic acid-isophthalic acid copolyester and 0.1-0.5 parts by weight of nano-aluminum oxide.

[0007] Preferably, the bottom adhesive layer 3 is composed of 85-90 parts by weight of PET, 5-10 parts by weight of a modified polyester containing hydroxyl groups, and 1-3 parts by weight of a block polyether antistatic agent.

[0008] Preferably, the nano transparent coating constituting the nano transparent coating is composed of 80-90 parts by weight of polyurethane resin, 5-10 parts by weight of nano alumina with a particle size of 5-20 nm, 3-5 parts by weight of triazine UV absorber, 1-2 parts by weight of fluorine-containing surfactant, 200-300 parts by weight of deionized water, and 1-3 parts by weight of photoinitiator.

[0009] Preferably, the thickness of the surface functional layer is 0.5-2 μm, the thickness of the core substrate layer is 20-180 μm, the thickness of the bottom adhesive layer is 2-6 μm, and the thickness of the nano transparent coating 4 is 0.5-2 μm.

[0010] The present invention also proposes a preparation method for the above-mentioned polyester protective film, comprising the following steps: first, drying the raw materials corresponding to the surface functional layer separately, and then weighing them according to the ratio and feeding them into a first twin-screw extruder to prepare a surface functional layer masterbatch; drying the raw materials corresponding to the core substrate layer separately, and then weighing them according to the ratio and feeding them into a second twin-screw extruder to prepare a core substrate layer masterbatch; drying the raw materials corresponding to the bottom adhesion layer separately, and then weighing them according to the ratio and feeding them into a third twin-screw extruder to prepare a bottom adhesion layer masterbatch; then, feeding the prepared surface functional layer masterbatch into the first extruder corresponding to the surface layer, feeding the core substrate layer masterbatch into the second extruder corresponding to the core layer, and feeding the bottom adhesion layer masterbatch into the third extruder corresponding to the bottom layer; cooling the three-layer film obtained by co-extrusion through a cooling mechanism to form a three-layer thick sheet; first stretching the three-layer thick sheet longitudinally and then stretching it transversely; and passing the stretched film through a heat setting device to eliminate internal stress and further reduce the shrinkage rate.

[0011] Preferably, the preparation method further includes a nano-transparent coating coating step: according to the formula of the nano-transparent coating, polyurethane resin, nano-aluminum oxide, UV absorber, surfactant, and photoinitiator are mixed in deionized water, and stirred evenly by a stirring mechanism to prepare the coating; on the surface of the film after heat setting, the coating is coated online on the outside of the surface functional layer by a coating machine; and the coating is cured using a UV curing device.

[0012] The polyester protective film of this invention achieves multi-layer synergistic optimization, with each layer interacting to synergistically optimize dimensional stability, weather resistance, and surface properties, meeting the comprehensive requirements of high-end protective films. Compared with existing technologies and comparative examples, the polyester protective film of this invention exhibits significantly improved thermal dimensional stability, with MD / TD shrinkage controlled to ≤1%. Its optical and mechanical properties are superior to those of the comparative examples, with weather resistance increased to >1050 hours. It also effectively addresses the yellowing and cracking issues common in existing films. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The following drawings are only intended to illustrate and explain the present application, and are not intended to limit the scope of the present invention.

[0014] Figure 1 Shown is a schematic structural diagram of a polyester protective film according to a specific embodiment of the present invention. DETAILED DESCRIPTION

[0015] In order to have a clearer understanding of the technical features, objectives and effects of the present invention, the specific embodiments of the present invention are now described with reference to the accompanying drawings, wherein the same components are marked with the same reference numerals.

[0016] In view of the defects of existing polymer protective films, the present invention proposes a polyester protective film with multiple balanced functions, such as Figure 1 shown.

[0017] See also Figure 1 The polyester protective film of the present invention comprises a three-layer co-extruded structure: a surface functional layer 1, a core substrate layer 2, and a bottom adhesive layer 3. The outer surface of the surface functional layer 1 is coated with a nano-transparent coating 4, which is formed by a nano-transparent coating through a UV curing process.

[0018] The surface functional layer 1 provides preliminary surface functionality (such as optical properties and basic wear resistance). The core substrate layer 2, as the main body of the protective film, provides primary mechanical properties and thermal stability. The bottom adhesive layer 3 enhances the adhesion of the protective film to other materials (such as adhesive layers or printed layers). The nano-transparent coating 4 provides sufficient wear resistance, anti-fouling, and UV resistance without adversely affecting optical properties (such as transmittance and haze).

[0019] In one specific embodiment, the surface functional layer 1 preferably has a thickness of 0.5-2 μm. A thinner thickness avoids compromising transparency while meeting the flexibility and strength requirements of the protective film. The core substrate layer 2 has a thickness of 20-180 μm, a thickness range that covers the requirements of common protective films, from thin films to thicker ones. The bottom adhesive layer 3 has a thickness of 2-6 μm, a moderate thickness range that provides sufficient bonding without significantly increasing the film thickness or compromising transparency. The nano-transparent coating 4 has a thickness of 0.5-2 μm, a thickness range suitable for meeting the application requirements of functional transparent coatings.

[0020] In another specific embodiment, the surface functional layer 1 is composed of 80-90 parts by weight of PET, 2-5 parts by weight of benzophenone, and 5-10 parts by weight of nano-boron nitride. The particle size of the nano-boron nitride is preferably 5-20 nm.

[0021] Specifically, the PET in the surface functional layer 1 can be Toray Industries' Lumirror T60 series PET products, or Mitsubishi Chemical's Hostaphan RN series PET products. Benzophenone can be BASF's Uvinul 3008 or SONGWON's SONGNOX UV 234. Nano-boron nitride can be Saint-Gobain's Combat BN10020 (particle size 5-20 nm), 3M's 3M Boron Nitride Cooling Fillers (particle size 10-20 nm), or Momentive Performance Materials' Boron Nitride Grade PTX (particle size 10-15 nm).

[0022] In another specific embodiment, the core substrate layer 2 is composed of 95-98 parts by weight of PET, 2-5 parts by weight of terephthalic acid-isophthalic acid copolyester, and 0.1-0.5 parts by weight of nano-alumina. The particle size range of the nano-alumina is preferably between 10-50 nm. PET, as the main substrate, provides excellent mechanical properties and dimensional stability. The terephthalic acid-isophthalic acid copolyester is used to improve flexibility and heat aging resistance. Nano-alumina (Al2O3) is mainly used to enhance the mechanical strength, wear resistance and thermal stability of the film.

[0023] Specifically, the PET in the core substrate layer 2 can be a Lumirror T60 series PET product from Toray Industries, or a Hostaphan RN series PET product from Mitsubishi Chemical. The terephthalic acid-isophthalic acid copolyester can be a TPA-IPA copolyester (TPA:IPA molar ratio of 90:10) from Mitsui Chemicals, or Yisheng IPA-CoPET (terephthalic acid (TPA) and isophthalic acid (IPA) molar ratio of 85:15 to 95:5) from Yisheng Petrochemical, or a Valox copolyester series from SABIC. Nano-alumina (Al2O3) can be Alpha Alumina Powder T10 series (particle size: 10-50nm) from Saint-Gobain, or NanoAlumina NPA20 (particle size 20nm) from Nanomaterials Technology Pte Ltd, or JH-Al2O3-20 (particle size 10-30nm) from Shanghai Junhua New Materials Technology Co., Ltd.

[0024] In another specific embodiment, the bottom adhesive layer 3 is composed of 85-90 parts by weight of PET, 5-10 parts by weight of a modified polyester containing hydroxyl groups, and 1-3 parts by weight of a block polyether antistatic agent.

[0025] Specifically, the PET in the bottom adhesive layer 3 can be a PET product of the Lumirror T60 series from Toray Industries, or a PET product of the Hostaphan RN series from Mitsubishi Chemical. The modified polyester containing hydroxyl groups imparts hydroxyl activity to the bottom adhesive layer, enhancing its bonding with other layers or substrates. For example, Joncryl ADR-4368 from BASF or DR-Polyester-721H from Dongguan Dongrong Chemical Co., Ltd. can be used. Block polyether antistatic agents provide antistatic properties, reduce surface resistance, and improve the processing and dustproof properties of the film. For example, Clariant's HS1, the main component is polyether block-ester compound, or Nouryon 2000 block polyether based antistatic agent.

[0026] In another specific embodiment, the nano-transparent coating material constituting the nano-transparent coating 4 comprises 80-90 parts by weight of a polyurethane resin, 5-10 parts by weight of nano-alumina with a particle size of 5-20 nm, 3-5 parts by weight of a triazine UV absorber, 1-2 parts by weight of a fluorosurfactant, 200-300 parts by weight of deionized water, and 1-3 parts by weight of a photoinitiator. The polyurethane resin is suitable for transparent coatings and can provide excellent flexibility and weather resistance; it has good compatibility with nanofillers and photoinitiators; and it exhibits outstanding scratch and chemical resistance. The nano-alumina provides abrasion resistance and thermal stability, maintaining the transparency of the coating. The triazine UV absorber has excellent light stability and durability; it absorbs UV rays efficiently within the wavelength range of 290-400 nm without affecting the transparency of the coating. The fluorosurfactant can significantly reduce the surface tension of the coating and improve coating uniformity. The coating exhibits excellent stain resistance, water and oil repellency, while maintaining its transparency. Deionized water is a solvent that does not contain organic components and is more environmentally friendly. Industrial-grade deionized water can be used, and the amount used can be slightly increased or decreased according to actual conditions.

[0027] Specifically, the polyurethane resin can be Bayhydrol UH340 / 1 from Bayer MaterialScience (Covestro). Nanoalumina can be NanoAlumina T10 (particle size 10nm) from Saint-Gobain. Triazine UV absorber can be Tinuvin 400 from BASF. Fluorinated surfactant can be Novec FC-4430 from 3M. Photoinitiator can be IGM Resins. 2959.

[0028] For example, the nano transparent coating material constituting the nano transparent coating 4 may be (in parts by weight): polyurethane resin: 85 parts (Bayhydrol UH 340 / 1), nano alumina: 8 parts (NanoAlumina T10), triazine UV absorber: 4 parts (Tinuvin 400), fluorine-containing surfactant: 1.5 parts (Novec FC-4430), deionized water: 250 parts, photoinitiator: 2 parts ( 2959).

[0029] The preparation process of the polyester protective film of the present invention is described in detail below.

[0030] First, the raw materials corresponding to the surface functional layer 1 are dried separately, weighed according to the ratio, and fed into a first twin-screw extruder to produce a surface functional layer masterbatch. The raw materials corresponding to the core substrate layer 2 are dried separately, weighed according to the ratio, and fed into a second twin-screw extruder to produce a core substrate layer masterbatch. The raw materials corresponding to the bottom adhesive layer 3 are dried separately, weighed according to the ratio, and fed into a third twin-screw extruder to produce a bottom adhesive layer masterbatch.

[0031] The PET in each layer's raw materials is dried at a temperature of 150-160°C for 6-8 hours. The drying temperature for nano-boron nitride and nano-alumina is 60-80°C for 8-12 hours. The drying temperature for terephthalic acid-isophthalic acid copolyester and hydroxyl-containing modified polyester is 110-120°C for 4-6 hours. The drying temperature for the remaining raw materials is 30-40°C for 3-4 hours.

[0032] The prepared surface functional layer masterbatch was then fed into the first extruder corresponding to the surface layer, the core substrate layer masterbatch was fed into the second extruder corresponding to the core layer, and the bottom adhesive layer masterbatch was fed into the third extruder corresponding to the bottom layer. The melt temperature of the surface layer was set at 260-270°C, the core layer at 270-280°C, and the bottom layer at 250-260°C. The extrusion temperature of the three-layer extrusion die was set at 260-270°C, and the casting speed was 80-120 m / min.

[0033] The three-layer film obtained by co-extrusion is cooled by a cooling mechanism (cooling temperature 10-20° C.) to form a three-layer thick sheet.

[0034] The three-layer thick sheet is first stretched longitudinally and then stretched transversely, with a longitudinal (MD) stretching ratio of 3.0-3.5 times and a transverse (TD) stretching ratio of 3.5-4.0 times; the temperature range of longitudinal stretching and transverse stretching is 80-120°C.

[0035] The stretched film is passed through a heat setting device to eliminate internal stress and further reduce shrinkage. The setting temperature is 200-230°C and the setting time is 10-20 seconds.

[0036] The coating process of the nano transparent coating is as follows.

[0037] According to the formula of the nano transparent coating, polyurethane resin, nano aluminum oxide, UV absorber, surfactant and the like are mixed in deionized water, and stirred evenly by a stirring mechanism to prepare the coating.

[0038] Online coating: After heat setting, the coating is applied online to the outer surface of the surface functional layer 1 by a coating machine using a roller coating method. The coating amount is controlled within the thickness range of 0.5-2μm.

[0039] Use UV curing equipment to cure the coating, UV lamp power: 100-200W / cm; wavelength range: 250-400nm; curing speed: 10-30m / min.

[0040] It should be noted that the thickness of the surface layer, core layer, base layer and coating needs to be precisely controlled to ensure that the total thickness is within the range of 23-188μm. The thickness distribution of each layer is as follows: surface functional layer 1: 0.5-2μm; core substrate layer 2: 20-180μm; base adhesion layer 3: 2-6μm; nano transparent coating: 0.5-2μm.

[0041] By setting the heat setting parameters and the stretching ratio, the polyester protective film prepared by the present invention has an MD / TD shrinkage of ≤1%; a light transmittance of ≥90% and a haze of ≤1%; and the adhesion of the nano transparent coating is verified by a 100-grid test, with an adhesion grade of ≥5B.

[0042] According to the film layer structure of the polyester protective film of the present invention, embodiments are designed respectively.

[0043] Formula combination examples A1-A3 of surface functional layer 1.

[0044] Examples PET (parts by weight) Benzophenone (parts by weight) Nano boron nitride (parts by weight) Particle size (nm) A1 85 5 10 10 A2 90 3 7 5 A3 80 4 8 15

[0045] Formula combination examples B1-B3 of core substrate layer 2.

[0046]

[0047] Examples of formulation combinations of the bottom adhesive layer 3 are C1-C3.

[0048] Examples PET (parts by weight) Modified polyester (parts by weight) Block polyether antistatic agent (parts by weight) C1 88 8 2 C2 90 7 3 C3 85 10 1

[0049] Formula combination examples D1-D3 of nano-transparent coating 4.

[0050]

[0051] Twelve embodiments were obtained by combining examples (A1-A3) of the surface functional layer 1, examples (B1-B3) of the core substrate layer 2, examples (C1-C3) of the bottom adhesive layer 3, and examples (D1-D3) of the nano transparent coating 4.

[0052] Examples: Each surface layer formulation is matched with different combinations of core layer, base layer and coating layer, with a total of 12 examples.

[0053]

[0054] The thickness parameters of each layer of Examples 1-12 are shown in the following table.

[0055]

[0056] The performance parameters of each embodiment are shown in the following table.

[0057]

[0058] Among them, high light transmittance and low haze reflect the combined effect of the nano-transparent coating 4 and the surface functional layer 1. Due to the optimized formula, the light transmittance of Example 4 (A2-B2-C1-D1) is as high as 92.2%. All examples achieve the performance requirement of MD / TD≤1%, and Examples 4 and 11 perform best, indicating that the terephthalic acid-isophthalic acid copolyester in the core substrate layer 2 contributes greatly to the shrinkage control. The optimized formula and UV curing process of the nano-transparent coating ensure that the adhesion grade of all examples reaches 5B. Weather resistance is mainly affected by the nano-boron nitride content of the surface functional layer 1 and the ratio of UV absorbers in the coating. The weather resistance of Example 4 reaches the highest value (1050 hours). The nano-boron nitride content and particle size of the surface functional layer 1 have a significant effect on scratch resistance. Examples 4, 7, and 10 perform outstandingly, with scratch resistance up to 115-120 times. The content of the block polyether antistatic agent in the bottom adhesive layer 3 directly affects the antistatic performance. Examples 2, 4, 8, and 10 all achieved optimal performance (109Ω). The ratio of PET to terephthalic acid-isophthalic acid copolyester in core substrate layer 2 significantly contributed to tensile strength. Example 4 achieved a tensile strength (MD / TD) of 265 / 255 MPa. The terephthalic acid-isophthalic acid copolyester ratio also improved heat resistance, with Example 4 achieving the highest heat resistance value (130°C).

[0059] The following comparative examples are designed to compare with the technical solutions of the present invention.

[0060]

[0061] The performance parameters of Comparative Examples 1-7 are shown in the following table.

[0062] By comparative analysis, Comparative Example 1 does not contain nano-boron nitride, resulting in a significant decrease in scratch resistance (reduced to 75 times), indicating that the addition of nano-boron nitride is crucial to enhancing surface wear resistance. Comparative Example 2 uses UV-328 to replace benzophenone, and the weather resistance is reduced to 920 hours, indicating that benzophenone is superior to ordinary UV absorbers in ultraviolet stability. Comparative Example 3 does not add nano-aluminum oxide, resulting in a significant decrease in heat resistance and tensile strength, indicating that inorganic nanofillers significantly enhance the mechanical properties and thermal stability of the substrate. Comparative Example 4 uses PETG to replace terephthalic acid-isophthalic acid copolyester, and the tensile strength drops to 240 / 230MPa, and the thermal shrinkage rate increases, indicating that copolyester is important for improving dimensional stability and mechanical properties. Comparative Example 5 lacks an antistatic agent, resulting in a deterioration in antistatic performance (surface resistance increases to 10 12 Ω), indicating the essential role of antistatic agents in film performance. Comparative Example 6 lacks a triazine UV absorber, resulting in reduced weathering resistance to 860 hours, demonstrating the critical role of this component in the coating's UV protection. Comparative Example 7 lacks a fluorosurfactant, resulting in a slight increase in haze and deterioration in surface quality, demonstrating the importance of fluorosurfactants in improving transparency and surface smoothness.

[0063] By comparing the performance degradation of the examples, we can clearly see the performance improvement achieved by the optimized combination of components in Examples 1-12. This difference demonstrates the synergistic optimization effect of the nanofillers, functional additives, and coating formulation, resulting in Examples 1-12 demonstrating excellent technical results in terms of optical, mechanical, and weather resistance.

[0064] The core substrate layer of the present invention adopts a combination of PET, terephthalic acid-isophthalic acid copolyester and nano-alumina, which provides good mechanical strength and thermal stability for the polyester protective film, reduces the crystallinity of the molecular chain, and optimizes the thermal shrinkage behavior; the nano-alumina is evenly dispersed in the matrix to form a stable physical support, further improving the thermal dimensional stability. The test results of Examples 1-12 show that the longitudinal and transverse thermal shrinkage rates of the polyester protective film of the present invention are maintained at ≤1%, which is significantly better than Comparative Examples 3 and 4 (when there is no nano-alumina or when PETG is used, the thermal shrinkage rates increase to 0.9-1.2% respectively). The present invention ensures the dimensional stability of the film material under various temperature environments through the optimization of the material formula and the synergistic effect of the multi-layer co-extrusion process, solving the problem of high thermal shrinkage rate in the prior art.

[0065] The surface functional layer of the present invention provides excellent UV resistance by adding benzophenone. The nano-aluminum oxide and triazine UV absorbers contained in the coating formula of the nano-transparent coating 4 improve weather resistance by multiple shielding of ultraviolet rays. The coating is cured by ultraviolet light to form a cross-linked and dense network structure, which further enhances the surface protection ability. Example 6 shows excellent weather resistance of more than 1050 hours; in comparative examples 2 and 6 (lacking benzophenone or UV absorber), the weather resistance is reduced to 860-920 hours, indicating that the synergistic effect of the two is crucial for long-term stability. The present invention effectively solves the yellowing, cracking and aging problems of existing protective films through a layered anti-ultraviolet protection design, allowing the product to adapt to long-term harsh environments.

[0066] The surface functional layer of the present invention forms a uniformly distributed microscopic rough structure by adding nano-boron nitride, thereby improving the scratch resistance and regulating the surface gloss. The nano-transparent coating 4 reduces the surface friction coefficient by using a fluorinated surfactant, further improving the scratch resistance, and adjusting the surface gloss to a moderate range. The present invention maintains a light transmittance of >92% and a low haze (<1%) through reasonable formulation and thickness design. The scratch resistance test times of Examples 4 and 6 were 120 times, respectively, showing excellent surface wear resistance. In comparison with Examples 1 and 7 (without nano-boron nitride or lacking a fluorinated surfactant), the scratch resistance times dropped to 75-95 times, and the surface gloss was uneven. The present invention not only effectively improves the surface scratch resistance through innovative surface and coating design, but also adjusts the gloss and transparency, making the performance of the protective film more balanced.

[0067] In summary, the overall technical effect of the present invention is: through multi-layer collaborative optimization, the interaction between each layer, and the collaborative optimization of dimensional stability, weather resistance and surface properties to meet the comprehensive needs of high-end protective films. Compared with the prior art and comparative examples, the thermal dimensional stability of the polyester protective film of the present invention is significantly improved, and the MD / TD shrinkage rate is controlled at ≤1%, which is better than the comparative example. The optical properties (transmittance>92%, haze<1%) and mechanical properties (tensile strength>265 / 255MPa) are better than the control group in all aspects. The weather resistance is improved to>1050 hours, and at the same time, the problems of yellowing and cracking of existing film materials are effectively solved. The present invention fundamentally solves the problems of poor dimensional stability, insufficient weather resistance, excessive surface gloss or poor scratch resistance of existing polymer protective films through the collaborative design of multi-layer structure and innovative materials, reflecting technical innovation and significant technical effects.

[0068] Those skilled in the art should understand that although the present invention is described in terms of multiple embodiments, not each embodiment contains only one independent technical solution. This description is provided for clarity only. Those skilled in the art should understand the description as a whole and consider the technical solutions involved in each embodiment as being combinable into different embodiments to understand the scope of protection of the present invention.

[0069] The above description is only an illustrative embodiment of the present invention and is not intended to limit the scope of the present invention. Any equivalent changes, modifications and combinations made by those skilled in the art without departing from the concept and principle of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A polyester protective film comprising a three-layer co-extruded structure: a surface functional layer, a core substrate layer, and a bottom adhesive layer. The outer surface of the surface functional layer is coated with a nano-transparent coating formed by a nano-transparent coating through an ultraviolet light curing process; characterized in that: The surface functional layer is composed of 80-90 parts by weight of PET, 2-5 parts by weight of benzophenone and 5-10 parts by weight of nano boron nitride.

2. The polyester protective film according to claim 1, wherein The core substrate layer is composed of 95-98 parts by weight of PET, 2-5 parts by weight of terephthalic acid-isophthalic acid copolyester and 0.1-0.5 parts by weight of nano-aluminum oxide.

3. The polyester protective film according to claim 1, wherein The bottom adhesive layer 3 is composed of 85-90 parts by weight of PET, 5-10 parts by weight of a modified polyester containing hydroxyl groups, and 1-3 parts by weight of a block polyether antistatic agent.

4. The polyester protective film according to claim 1, wherein The nano transparent coating comprises 80-90 parts by weight of polyurethane resin, 5-10 parts by weight of nano aluminum oxide with a particle size of 5-20 nm, 3-5 parts by weight of triazine UV absorber, 1-2 parts by weight of fluorine-containing surfactant, 200-300 parts by weight of deionized water, and 1-3 parts by weight of photoinitiator.

5. The polyester protective film according to claim 1, wherein The thickness of the surface functional layer is 0.5-2 μm, the thickness of the core substrate layer is 20-180 μm, the thickness of the bottom adhesive layer is 2-6 μm, and the thickness of the nano transparent coating is 0.5-2 μm.

6. A method for preparing a polyester protective film as described in any one of claims 1 to 5, comprising the following steps: first, drying the raw materials corresponding to the surface functional layer respectively, and then feeding them into a first twin-screw extruder according to a ratio to prepare a surface functional layer masterbatch; drying the raw materials corresponding to the core substrate layer respectively, and then feeding them into a second twin-screw extruder according to a ratio to prepare a core substrate layer masterbatch; drying the raw materials corresponding to the bottom adhesion layer respectively, and then feeding them into a third twin-screw extruder according to a ratio to prepare a bottom adhesion layer masterbatch; then, feeding the prepared surface functional layer masterbatch into the first extruder corresponding to the surface layer, feeding the core substrate layer masterbatch into the second extruder corresponding to the core layer, and feeding the bottom adhesion layer masterbatch into the third extruder corresponding to the bottom layer; cooling the three-layer film obtained by co-extrusion through a cooling mechanism to form a three-layer thick sheet; first stretching the three-layer thick sheet longitudinally and then stretching it transversely; and passing the stretched film through a heat setting device to eliminate internal stress and further reduce shrinkage.

7. The preparation method according to claim 6, further comprising the step of applying a nano-transparent coating: according to the formula of the nano-transparent coating, polyurethane resin, nano-alumina, UV absorber, surfactant, and photoinitiator are mixed in deionized water and stirred uniformly using a stirring mechanism to prepare the coating; on the surface of the heat-set film, the coating is applied online to the outside of the surface functional layer using a coating machine; and the coating is cured using a UV curing device.

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