High-refractive-index bio-based resin, modified bio-based high-reflection multilayer film and preparation method of modified bio-based high-reflection multilayer film

Abstract

The invention relates to the technical field of optical thin films, and particularly discloses high-refractive-index bio-based resin, a modified bio-based high-reflection multilayer film and a preparation method of the modified bio-based high-reflection multilayer film. The high-refractive-index bio-based resin is formed by copolymerization of furandicarboxylic acid and 4, 4 '-biphenyl dicarboxylic acid through esterification and condensation polymerization under the action of a catalyst, the molar ratio of the 4, 4'-biphenyl dicarboxylic acid is 10-30% of the total molar weight of the furandicarboxylic acid and the 4, 4 '-biphenyl dicarboxylic acid, the refractive indexes nx and ny in two in-plane vertical directions of a film formed by biaxially stretching the high-refractive-index bio-based resin meet the following conditions: nx is more than or equal to 1.66 and less than or equal to 1.68, and nx-ny is less than or equal to 0.02. The modified bio-based high-reflection multilayer film prepared by the invention has high reflection performance, excellent light transmission and good mechanical properties.

Description

Technical Field

[0001] This application relates to the technical field of optical thin films, and in particular to a high refractive index bio-based resin, a modified bio-based high reflectivity multilayer film, and a method for preparing the same. Background Technology

[0002] Optical reflective multilayer films are functional thin films designed based on the principles of reflection and interference of light waves at the interface of media with different refractive indices. They achieve selective high reflectivity of light in specific wavelengths by alternately stacking two or more optical resin materials to form a periodic structure. These films have wide applications in LCD backlight modules, energy-saving architectural glass, anti-counterfeiting packaging, and various optical devices. Traditional high-reflectivity multilayer films typically use petroleum-based polymers as raw materials, such as polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), and polyethylene naphthalate (PEN), prepared through precision co-extrusion and stretching processes. Although these material systems have mature processes and stable optical properties, their raw materials are derived from non-renewable fossil resources, resulting in high energy consumption during production and difficulty in natural degradation after disposal.

[0003] To address resource and environmental challenges, the development of bio-based polymer materials based on renewable resources has become an important research direction. Polyethylene 2,5-furandicarboxylate (PEF) is a fully bio-based polyester polymerized from biomass-derived 2,5-furandicarboxylic acid (FDCA) and ethylene glycol (EG) monomers. It possesses advantages such as good gas barrier properties, high mechanical strength, and renewable raw materials, and is considered a potential green alternative to PET. However, directly applying PEF to optical reflective multilayer films faces a core bottleneck: its maximum in-plane refractive index after stretching and orientation is approximately 1.62, which is relatively small compared to the refractive index difference (Δn) of commonly used low-refractive-index resins (such as PMMA, with a refractive index of approximately 1.49). According to optical thin film theory, reflection bandwidth and reflectivity intensity are directly and positively correlated with the refractive index difference between adjacent layers. A small Δn will lead to low reflection efficiency and insufficient bandwidth, making it difficult to meet the performance requirements of high-reflectivity optical films, severely limiting the application of PEF in this high-end field.

[0004] To improve the optical properties of bio-based polyesters, existing technologies attempt to introduce highly polarizable groups through chemical modification. For example, while introducing benzene ring structures into polylactic acid (PLA) can increase the refractive index to some extent, it is often accompanied by severe phase separation and changes in crystallinity, leading to a significant increase in film haze (typically >15%). This creates an irreconcilable contradiction of "high refractive index - high haze," failing to meet the light transmittance requirements of optical-grade applications. Furthermore, current research on the modification of PEF mainly focuses on improving its thermal properties or gas barrier properties. Systematic research specifically targeting its optical properties, particularly achieving a synergistic improvement in high refractive index and low haze through molecular design, especially copolymerization modification using biphenyl dicarboxylic acid monomers with rigid planar conjugated structures, has not yet been reported.

[0005] Therefore, developing a pure bio-based or high bio-based optical multilayer film that combines high reflectivity, excellent light transmittance, and good mechanical properties is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] In order to enable bio-based multilayer films to possess high reflectivity, excellent light transmittance, and good mechanical properties, this application provides a high-refractive-index bio-based resin, a modified bio-based high-reflectivity multilayer film, and a method for preparing the same.

[0007] In a first aspect, this application provides a high refractive index bio-based resin, employing the following technical solution: A high-refractive-index bio-based resin is copolymerized from furanyl dicarboxylic acid, ethylene glycol, and 4,4'-biphenyl dicarboxylic acid through esterification and polycondensation reactions under the action of a catalyst. The molar percentage of 4,4'-biphenyl dicarboxylic acid is 10-30% of the total molar amount of furanyl dicarboxylic acid and 4,4'-biphenyl dicarboxylic acid. A film formed by biaxially stretching the high-refractive-index bio-based resin has a refractive index n in two perpendicular in-plane directions. x n y Satisfies: 1.66≤n x ≤1.68, and |n x –n y |≤0.02.

[0008] By employing the above technical solution, a high-refractive-index bio-based resin was prepared from furanyl dicarboxylic acid, ethylene glycol, and 4,4'-biphenyl dicarboxylic acid. This resin, with bio-based furanyl dicarboxylic acid as its backbone and modified by copolymerization with a specific ratio of 4,4'-biphenyl dicarboxylic acid, utilizes the rigid, planar, and symmetrical molecular structure of 4,4'-biphenyl dicarboxylic acid to successfully achieve an in-plane refractive index n after biaxial stretching of the material. x While significantly improving to 1.66-1.68, it ensures extremely low optical anisotropy (|n x -n y(≤0.02), which not only solves the technical bottleneck of insufficient refractive index of pure bio-based PEF resin, but also provides a key high refractive index and optically uniform basic material for the preparation of high-performance optical films.

[0009] Optionally, the molar percentage of 4,4'-biphenyl dicarboxylic acid is 15-25% of the total molar amount of furanyl dicarboxylic acid and 4,4'-biphenyl dicarboxylic acid, and the high refractive index bio-based resin, after uniaxial stretching, has an orientation refractive index of 1.66-1.72 parallel to the stretching direction.

[0010] By adopting the above technical solution, the molar ratio of 4,4'-biphenyldicarboxylic acid is further optimized to 15-25%, achieving the best balance between modification effect and processing performance. Within this preferred range, the bio-based resin can obtain a higher orientation refractive index of 1.66-1.72 in a specific direction after uniaxial stretching, which is suitable for polarization optical elements with strict requirements for high refractive index in a single direction. At the same time, it avoids the haze increase and processing difficulties that may be caused by excessive addition, making the optical performance control of the material more precise and reliable.

[0011] Optionally, the intrinsic viscosity of the high refractive index bio-based resin is 0.85-0.90 dL / g, and for every 1% increase in the molar percentage of 4,4'-biphenyl dicarboxylic acid in the high refractive index bio-based resin, the haze increase rate of the film made therefrom is less than 0.05%.

[0012] By adopting the above technical solution, the intrinsic viscosity is controlled within the range of 0.85-0.90 dL / g, and the upper limit of its haze growth rate is clearly defined, ensuring that the material has excellent melt processability and film-forming properties. While obtaining a high refractive index, the common contradiction between "refractive index and haze" is suppressed, ensuring that the optical film made from this material has both high reflectivity and high light transmittance clarity, meeting the stringent requirements of low haze in high-end optical applications.

[0013] Secondly, this application provides a modified bio-based high-reflectivity multilayer film, employing the following technical solution: A modified bio-based high-reflectivity multilayer film comprises at least one high-refractive-index layer made of the aforementioned high-refractive-index bio-based resin, and at least one low-refractive-index layer made of a low-refractive-index resin, wherein the high-refractive-index layer and the low-refractive-index layer are alternately stacked.

[0014] Optionally, the low-refractive-index resin is selected from polymethyl methacrylate, polycarbonate, and polyethylene terephthalate-1,4-cyclohexanediol ester, and the difference between its intrinsic viscosity and that of the high-refractive-index bio-based resin is within ±0.05 dL / g.

[0015] Optionally, the refractive index difference between the high refractive index layer and the adjacent low refractive index layer in the same direction is ≥0.19.

[0016] Optionally, the modified bio-based high-reflectivity multilayer film has an average reflectivity of ≥90% in the wavelength range of 380-780nm and an overall haze of ≤2.0%.

[0017] Optionally, the modified bio-based high-reflection multilayer film is a uniaxially stretched film, wherein the refractive index of the high-refractive-index layer in the stretching direction is ≥1.70; or, the modified bio-based high-reflection multilayer film is a biaxially stretched film, wherein the refractive index of the high-refractive-index layer in both in-plane perpendicular directions is ≥1.65.

[0018] Thirdly, this application provides a method for preparing a modified bio-based high-reflectivity multilayer film, employing the following technical solution: A method for preparing a modified bio-based high-reflectivity multilayer film includes the following steps: S1: The dried high-refractive-index bio-based resin particles and low-refractive-index resin particles are melt-extruded separately; S2: The molten resin is combined through a multi-layer co-extrusion distributor to form an alternating layered melt flow; S3: The melt flow is extruded through a die and cooled to form a cast sheet, resulting in an unstretched cast sheet; S4: The unstretched casting is stretched unidirectionally or bidirectionally, then heat-set and wound up to obtain a modified bio-based high-reflectivity multilayer film.

[0019] Optionally, in step S4, the process conditions for uniaxial stretching are: stretching along the longitudinal direction, stretching temperature of 90-100℃, and stretching ratio of 5-7 times. Alternatively, the process conditions for biaxial stretching are as follows: first, longitudinal stretching is performed at 90-95℃ with a stretching ratio of 3.5-4.0 times, and then transverse stretching is performed at 115-125℃ with a stretching ratio of 3.5-4.0 times.

[0020] In summary, this application includes at least one of the following beneficial technical effects: 1. A high-refractive-index bio-based resin, modified by copolymerization of furanyl dicarboxylic acid with a specific ratio of 4,4'-biphenyl dicarboxylic acid, was prepared from furanyl dicarboxylic acid, ethylene glycol, and 4,4'-biphenyl dicarboxylic acid. Utilizing the rigid, planar, and symmetrical molecular structure of 4,4'-biphenyl dicarboxylic acid, the in-plane refractive index n of the material after biaxial stretching was successfully achieved. x While significantly improving to 1.66-1.68, it ensures extremely low optical anisotropy. This not only solves the technical bottleneck of insufficient refractive index of pure bio-based PEF resin, but also provides a key high-refractive-index and optically uniform basic material for the preparation of high-performance optical films. 2. By controlling the intrinsic viscosity within the range of 0.85-0.90 dL / g and defining the upper limit of its haze growth rate, the material is ensured to have excellent melt processability and film-forming properties. While obtaining a high refractive index, the common contradiction between refractive index and haze is suppressed, ensuring that the optical film made from this material has both high reflectivity and high light transmittance clarity, meeting the stringent requirements for low haze in high-end optical applications. Detailed Implementation

[0021] Preparation Example Preparation Example 1 A PEF-BP bio-based resin is prepared by the following steps: Feeding: Add 4.50 mol (747.5 g) of furanyl dicarboxylic acid, 5.50 mol (341.0 g) of ethylene glycol, 0.50 mol (121.1 g) of 4,4'-biphenyl dicarboxylic acid, and 0.95 g of tetrabutyl titanate as a catalyst to a 2 L reactor equipped with a stirrer, a fractionating column, a nitrogen inlet, and a thermometer. Esterification reaction: Under continuous nitrogen protection, the mixture is heated to 200°C at a rate of 10°C / min and stirred at this temperature for 3 hours. During this process, the water generated by the esterification reaction is distilled off and measured. Polycondensation reaction: After the esterification reaction is completed, the temperature of the reaction system is gradually increased to 265°C within 30 minutes. Then, the system pressure is slowly reduced to below 50 Pa. Under these conditions, the polycondensation reaction continues for 4 hours. Discharge and pelletizing: After the polycondensation reaction is completed, the reactor is slowly restored to normal pressure with nitrogen gas. The molten polymer is extruded into strips through the bottom valve of the reactor. After cooling in a water bath, the strips are pelletized to obtain PEF-BP bio-based resin particles with an intrinsic viscosity (IV) of 0.87 dL / g, which are designated as PEF-BP10.

[0022] Preparation Example 2 The difference between Preparation Example 2 and Preparation Example 1 is that the proportions of the raw materials are different. In Preparation Example 2, 4.25 mol (706.3 g) of furanyl dicarboxylic acid, 5.50 mol (341.0 g) of ethylene glycol, 0.75 mol (181.7 g) of 4,4'-biphenyl dicarboxylic acid and 0.98 g of tetrabutyl titanate were added.

[0023] PEF-BP bio-based resin particles with an intrinsic viscosity (IV) of 0.88 dL / g were obtained and designated as PEF-BP15.

[0024] Preparation Example 3 The difference between Preparation Example 3 and Preparation Example 1 is that the proportions of the raw materials are different. In Preparation Example 3, 3.75 mol (623.0 g) of furanyl dicarboxylic acid, 5.50 mol (341.0 g) of ethylene glycol, 1.25 mol (302.8 g) of 4,4'-biphenyl dicarboxylic acid and 1.05 g of tetrabutyl titanate were added.

[0025] PEF-BP bio-based resin particles with an intrinsic viscosity (IV) of 0.89 dL / g were obtained and designated as PEF-BP25.

[0026] Preparation Example 4 The difference between Preparation Example 4 and Preparation Example 1 is that the proportions of the raw materials are different. 3.50 mol (581.4 g) of furanyl dicarboxylic acid, 5.50 mol (341.0 g) of ethylene glycol, 1.50 mol (363.4 g) of 4,4'-biphenyl dicarboxylic acid and 1.08 g of tetrabutyl titanate were added.

[0027] PEF-BP bio-based resin particles with an intrinsic viscosity (IV) of 0.88 dL / g were obtained and designated as PEF-BP30.

[0028] Comparative preparation example Comparative Preparation Example 1 The difference between Comparative Preparation Example 1 and Preparation Example 1 is that the raw materials of Comparative Preparation Example 1 do not contain 4,4'-biphenyldicarboxylic acid, and 5.00 mol (830.6 g) of furanyldicarboxylic acid, 5.50 mol (341.0 g) of ethylene glycol and 1.17 g of tetrabutyl titanate are added.

[0029] Unmodified PEF resin particles with an intrinsic viscosity (IV) of 0.88 dL / g were obtained and denoted as PEF-BP0.

[0030] The refractive indices of films made from PEF-BP resin obtained in Preparation Examples 1-4 and Comparative Preparation Example 1 after uniaxial stretching are shown in Table 1 below.

[0031] Table 1

[0032] The refractive indices of films made from PEF-BP resin obtained in Preparation Examples 1-4 and Comparative Preparation Example 1 after biaxial stretching are shown in Table 2 below.

[0033] Table 2

[0034] Example

[0035] Example 1

[0036] A modified bio-based high-reflectivity multilayer film is prepared by the following steps: Raw material pretreatment: PEF-BP10 obtained in Preparation Example 1 was used as a high refractive index layer material and dried at 85°C and -0.1MPa vacuum for 8 hours until the moisture content was reduced to below 25ppm. Commercially available optical grade polymethyl methacrylate resin (intrinsic viscosity 0.90dL / g) was used as a low refractive index layer material and dried at 90°C and -0.1MPa vacuum for 8 hours until the moisture content was reduced to below 20ppm. Melt co-extrusion: Two independent single-screw extruders (screw diameter 30 mm, L / D=28) were used to melt-dry PEF-BP10 and PMMA resins respectively. The melt temperature was set to 250℃. The two melt flows were merged and precisely distributed into a linearly gradient thickness structure with a total of 200 layers arranged in an alternating AB pattern. PEF-BP10 was the A layer and PMMA was the B layer. The thickness of each AB cycle increased continuously from the outermost to the innermost part of the film. In each cycle, the thickness ratio of the A layer to the B layer was 1.2:1. Casting: The laminated melt flow is extruded through a T-die with a width of 300 mm, rapidly cooled and rolled on a cooling steel roller with a surface temperature maintained at 25°C to obtain a uniform unstretched casting with a thickness of approximately 800 μm. Biaxial stretching and heat setting: The cast film is fed into a longitudinal stretching machine and stretched longitudinally at 92°C with a stretching ratio of 3.8 times and a stretching rate of 40 mm / s. Then, the film is fed into a transverse stretching machine and stretched transversely at 120°C with a stretching ratio of 3.8 times and a stretching rate of 30 mm / s. Finally, the film is heat-treated for 5 seconds in a heat setting zone at 200°C, then cooled and wound up to obtain a modified bio-based high-reflectivity multilayer film with a thickness of approximately 55 μm.

[0037] The spectral reflectance of the obtained film in the wavelength range of 380-780 nm was measured using a UV-Vis spectrophotometer, and its average reflectance was calculated. The haze of the film was measured using a haze meter. The results showed that the average reflectance of the film was 76%, and the haze was 0.6%.

[0038] Example 2

[0039] The difference between Example 2 and Example 1 is that the high refractive index layer in Example 2 uses the PEF-BP15 resin prepared in Preparation Example 2.

[0040] The resulting modified bio-based high-reflectivity multilayer film achieved an average reflectivity of 82% in the 380-780nm wavelength range and a haze of 0.8%.

[0041] Example 3

[0042] The difference between Example 3 and Example 1 is that the high refractive index layer in Example 3 uses the PEF-BP25 resin prepared in Preparation Example 3.

[0043] The resulting modified bio-based high-reflectivity multilayer film achieved an average reflectivity of 94% and a haze of 1.5% in the 380-780nm wavelength range.

[0044] Example 4

[0045] The difference between Example 4 and Example 1 is that the high refractive index layer in Example 4 uses the PEF-BP30 resin prepared in Preparation Example 4.

[0046] The resulting modified bio-based high-reflectivity multilayer film further improved the average reflectivity to 98% in the 380-780nm wavelength range, but the haze also increased to 2.9%.

[0047] The refractive index-BPDA content model is shown in Table 3 below, where n is the refractive index of the PEF-BP film, crystallinity is the crystallinity of the PEF-BP film, and haze is the haze of the corresponding PEF-BP film used to make a modified bio-based high-reflection multilayer film.

[0048] Table 3

[0049] Data analysis shows that the refractive index (n) of the PEF-BP film and the molar ratio of BPDA (x) conform to the following relationship: n = 1.620 + 0.0032x - 5.6 × 10⁻ 5 x² (R²=0.988).

[0050] Critical point analysis shows that when x > 28%, the second derivative turns negative, indicating that excessive BPDA leads to molecular chain entanglement, and the increase in haze exceeds the refractive index gain. Therefore, the performance inflection point is when x ≈ 28%. When the BPDA proportion is below this point, the refractive index steadily increases and the haze is controllable; when the proportion exceeds this point, the haze increases sharply. Therefore, the molar proportion of BPDA is preferably 10%-30%, more preferably 15%-25%, achieving the best balance of optical performance within this range.

[0051] Comparative Example Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the high refractive index layer of Comparative Example 1 uses the PEF-BP0 resin prepared in Comparative Preparation Example 1.

[0052] The average reflectivity of the obtained multilayer reflective film was only 58%, and the haze was 0.5%. This proves that the unmodified PEF, due to its inherent low refractive index, has too small a refractive index difference with the PMMA layer to form an efficient reflective interface, thus highlighting the necessity of chemical modification.

[0053] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the high refractive index layer of Comparative Example 2 uses 2,2'-biphenyl dicarboxylic acid modified PEF resin. That is, referring to the molar amount and process of Preparation Example 1, 4,4'-biphenyl dicarboxylic acid is replaced with 2,2'-biphenyl dicarboxylic acid in equal molar amounts to synthesize modified PEF resin.

[0054] The steric hindrance of 2,2'-biphenyl dicarboxylic acid leads to a decrease in molecular chain regularity, resulting in a limited increase in the refractive index of the obtained thin film (n x =1.598), and light scattering is severe, resulting in an average reflectivity of less than 65% and a haze greater than 2.0% for the multilayer film.

[0055] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the high refractive index layer of Comparative Example 3 uses 3,3'-biphenyl dicarboxylic acid modified PEF resin. That is, referring to the molar amount and process of Preparation Example 1, 4,4'-biphenyl dicarboxylic acid is replaced with 3,3'-biphenyl dicarboxylic acid in equal molar amounts to synthesize modified PEF resin.

[0056] The asymmetry of 3,3'-biphenyldicarboxylic acid results in low molecular chain stacking efficiency, which is not effective in improving the refractive index. Furthermore, the haze of the resulting multilayer film is as high as 2.8%, and its optical performance is unqualified.

[0057] Comparative Example 4 A modified bio-based multilayer membrane is prepared by the following steps: Resin modification: 10 kg of polylactic acid resin and 1.2 kg of 4-styrene monomer were melt-blended in a twin-screw extruder at 180 °C, and reactive extrusion modification was carried out, followed by granulation. Thin film preparation: Modified PLA granules and PMMA granules were dried, then co-extruded in multiple layers and biaxially stretched (longitudinal / transverse stretching ratio 3.0 / 2.8) to obtain modified bio-based multilayer films.

[0058] The modified bio-based multilayer film has a refractive index of only 1.58, a haze of up to 15.2%, and reduced thermal performance. The multilayer reflective film prepared in this way has an average reflectivity that is more than 38% lower than that of Example 2 (reflectivity 82%), and cannot meet the application requirements of high-reflectivity optical films.

[0059] The experimental results of Examples 3 and Comparative Examples 1-4 are shown in Table 4 below. The in-plane refractive index and crystallinity refer to the refractive index and crystallinity of the high refractive index layer in Examples 3 and Comparative Examples 1-4, and the haze refers to the haze of the modified bio-based multilayer film in Examples 3 and Comparative Examples 1-4.

[0060] Table 4

[0061] Compared to the comparative example, only biphenyl dicarboxylic acid with the specific configuration of 4,4'- can simultaneously achieve the goals of high refractive index, low haze, and high thermal stability on the specific bio-based framework of PEF.

[0062] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A high refractive index bio-based resin, characterized in that, The high-refractive-index bio-based resin is copolymerized from furanyl dicarboxylic acid, ethylene glycol, and 4,4'-biphenyl dicarboxylic acid through esterification and polycondensation reactions under the action of a catalyst. The molar percentage of 4,4'-biphenyl dicarboxylic acid is 10-30% of the total molar amount of furanyl dicarboxylic acid and 4,4'-biphenyl dicarboxylic acid. The film formed by biaxially stretching the high-refractive-index bio-based resin has a refractive index n in two perpendicular in-plane directions. x n y Satisfies: 1.66≤n x ≤1.68, and |n x –n y |≤0.

02.

2. The high refractive index bio-based resin according to claim 1, characterized in that: The molar percentage of 4,4'-biphenyl dicarboxylic acid is 15-25% of the total molar amount of furanyl dicarboxylic acid and 4,4'-biphenyl dicarboxylic acid. The high refractive index bio-based resin, after uniaxial stretching, has an orientation refractive index of 1.66-1.72 parallel to the stretching direction.

3. The high refractive index bio-based resin according to claim 1, characterized in that: The intrinsic viscosity of the high refractive index bio-based resin is 0.85-0.90 dL / g, and for every 1% increase in the molar percentage of 4,4'-biphenyl dicarboxylic acid in the high refractive index bio-based resin, the haze increase rate of the film made therefrom is less than 0.05%.

4. A modified bio-based high-reflectivity multilayer film, characterized in that: It comprises at least one high refractive index layer made of a high refractive index bio-based resin as described in any one of claims 1 to 3, and at least one low refractive index layer made of a low refractive index resin, wherein the high refractive index layer and the low refractive index layer are alternately stacked.

5. The modified bio-based high-reflectivity multilayer film according to claim 4, characterized in that: The low-refractive-index resin is selected from one of polymethyl methacrylate, polycarbonate, and polyethylene terephthalate-1,4-cyclohexanediol ester, and the difference between its intrinsic viscosity and that of the high-refractive-index bio-based resin is within ±0.05 dL / g.

6. The modified bio-based high-reflectivity multilayer film according to claim 4, characterized in that: The refractive index difference between the high refractive index layer and the adjacent low refractive index layer in the same direction is ≥0.

19.

7. The modified bio-based high-reflectivity multilayer film according to claim 4, characterized in that: The modified bio-based high-reflectivity multilayer film has an average reflectivity of ≥90% in the wavelength range of 380-780nm and an overall haze of ≤2.0%.

8. The modified bio-based high-reflectivity multilayer film according to claim 4, characterized in that: The modified bio-based high-reflection multilayer film is a uniaxially stretched film, wherein the refractive index of the high-refractive-index layer in the stretching direction is ≥1.70; or, the modified bio-based high-reflection multilayer film is a biaxially stretched film, wherein the refractive index of the high-refractive-index layer in both in-plane perpendicular directions is ≥1.

65.

9. The method for preparing a modified bio-based high-reflectivity multilayer film according to claim 4, characterized in that, Includes the following steps: S1: The dried high-refractive-index bio-based resin particles and low-refractive-index resin particles are melt-extruded separately; S2: The molten resin is combined through a multi-layer co-extrusion distributor to form an alternating layered melt flow; S3: The melt flow is extruded through a die and cooled to form a cast sheet, resulting in an unstretched cast sheet; S4: The unstretched casting is stretched unidirectionally or bidirectionally, then heat-set and wound up to obtain a modified bio-based high-reflectivity multilayer film.

10. The method for preparing a modified bio-based high-reflectivity multilayer film according to claim 9, characterized in that, In step S4, the process conditions for unidirectional stretching are: stretching along the longitudinal direction, stretching temperature of 90-100℃, and stretching ratio of 5-7 times. Alternatively, the process conditions for biaxial stretching are as follows: first, longitudinal stretching is performed at 90-95℃ with a stretching ratio of 3.5-4.0 times, and then transverse stretching is performed at 115-125℃ with a stretching ratio of 3.5-4.0 times.

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