A low-refractive-index modified copolyester and its preparation method

Abstract

This application relates to the field of polyester materials, specifically disclosing a low-refractive-index modified copolyester and its preparation method. The application selects terephthalic acid and alicyclic dicarboxylic acids as carboxylic acid monomers, and ethylene glycol and cyclodiol as diol monomers. Through esterification, polycondensation, and other steps, the modified copolyester possesses both low refractive index and high glass transition temperature. The refractive index of the copolyester is controlled at approximately 1.53, and the glass transition temperature is controlled between 58℃ and 85℃. This copolyester has a low refractive index and a glass transition temperature similar to that of PET resin. It is suitable for the preparation of multilayer optical films.

Description

Technical Field

[0001] This application relates to the technical field of polyester materials, and more specifically, to a low-refractive-index modified copolyester and its preparation method. Background Technology

[0002] Multilayer optical films are generally manufactured through processes such as lamination and stretching of at least two resins with different refractive indices. The greater the difference in refractive index between adjacent layers, the higher the reflectivity of the multilayer optical film. Therefore, in the design of multilayer optical films, it is usually necessary to select two resins with a refractive index difference greater than 0.05 for lamination. Commonly used high refractive index materials include polyethylene naphthalate (PEG) with a refractive index of 1.75 or polyethylene terephthalate (PET) with a refractive index of 1.65. PEG has good impact resistance, good barrier properties, and electrical insulation properties, making it the preferred material for multilayer optical films.

[0003] Currently, low refractive index materials generally chosen are polymethyl methacrylate (PMMA) with a refractive index of 1.49 or polyethylene terephthalate-1,4-cyclohexanediethanol (PET) with a refractive index of 1.65. The glass transition temperature (GTH) of PMMA is typically 105℃, while that of PET is typically 63.8℃, a significant difference from the 80℃ of PMMA. This large difference in GTH between high and low refractive index resins can easily lead to mismatched melt flow between layers during the processing of multilayer optical films due to significant temperature differences.

[0004] Based on the above, the industry urgently needs to develop a modified copolyester with a glass transition temperature close to that of polyethylene terephthalate and a low refractive index. Summary of the Invention

[0005] This application develops a low-refractive-index modified copolyester and its preparation method. The modified copolyester is adapted to polyethylene terephthalate and has a glass transition temperature similar to that of polyethylene terephthalate and a lower refractive index, making it suitable for the preparation of multilayer optical films.

[0006] In a first aspect, this application provides a low-refractive-index modified copolyester, employing the following technical solution: A low-refractive-index modified copolyester is copolymerized from dicarboxylic acid monomers and diol monomers; The dicarboxylic acid monomer is composed of terephthalic acid and alicyclic dicarboxylic acid, and the diol monomer is composed of ethylene glycol and cyclodiol. Among them, terephthalic acid accounts for 30% to 70% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 30% to 70% of the total molar amount of dicarboxylic acid monomers; Ethylene glycol accounts for 40%–80% of the total molar amount of the glycol monomers; cyclodiols account for 20%–60% of the total molar amount of the glycol monomers. The molar ratio of dicarboxylic acid monomer to diol monomer is 1:(1.20~1.80).

[0007] Furthermore, the terephthalic acid is terephthalic acid and / or terephthalic acid.

[0008] Furthermore, the alicyclic dicarboxylic acid is one or more of 1,4-cyclohexanedicarboxylic acid and / or cis-1,3-cyclopentanedicarboxylic acid.

[0009] Furthermore, the cyclodiol is one or more selected from 1,4-cyclohexanediol, isosorbide, hydrogenated bisphenol A, 2,2,4,4-tetramethyl-1,3-cyclobutanediol and spirodiol.

[0010] Furthermore, the molar ratio of the dicarboxylic acid monomer to the diol monomer is 1:(1.30 to 1.50).

[0011] Furthermore, the terephthalic acid accounts for 45% to 60% of the total molar amount of the dicarboxylic acid monomers; the alicyclic dicarboxylic acid accounts for 40% to 55% of the total molar amount of the dicarboxylic acid monomers.

[0012] Furthermore, the ethylene glycol accounts for 50% to 70% of the total molar amount of the diol monomers; the cyclodiol accounts for 30% to 50% of the total molar amount of the diol monomers.

[0013] Secondly, this application provides a method for preparing a low-refractive-index modified copolyester, employing the following technical solution: A method for preparing a low-refractive-index modified copolyester includes the following steps: Esterification: Terephthalic acid, alicyclic dicarboxylic acid, ethylene glycol and cyclodiol are added to a reaction vessel, a catalyst and a stabilizer are added, and after mixing, the mixture is heated in stages under an inert atmosphere and a pressure of 0.25-0.35 MPa to esterify and dehydrate, thus obtaining the esterified product. Polycondensation: The esterification product is polycondensed at a pressure of -0.1 MPa to 0 MPa for 30 to 60 min, and then heated to 270 to 283 °C at an absolute pressure of <70 Pa for 60 to 180 min to obtain the modified copolyester.

[0014] Furthermore, the catalyst used in the esterification step is one or more of antimony acetate, antimony trioxide, germanium dioxide, antimony glycolate, tetrabutyl titanate, and isopropyl titanate.

[0015] Furthermore, the stabilizer used in the esterification step is one or more of triethyl phosphoroacetate, triphenyl phosphate, trimethyl phosphate, and triphenyl phosphite.

[0016] Furthermore, the segmented heating process in the esterification step is as follows: the first esterification temperature is 200-230℃, and the holding time is 20-40 min; the second esterification temperature is 230-245℃, and the holding time is 50-70 min.

[0017] By adopting the above technical solution, this application has at least the following advantages: First, this application selects terephthalic acid and alicyclic dicarboxylic acid as carboxylic acid monomers. The alicyclic dicarboxylic acid replaces part of the terephthalic acid, reducing the content of aromatic rings in the copolyester and thus lowering the refractive index of the polyester material. For the glycol monomers, ethylene glycol and cyclodiol are selected. Ethylene glycol serves as both a solvent and a reactant. Through the combination of ethylene glycol and cyclodiol, the rigid cyclic structure of the cyclodiol increases steric hindrance, thereby restricting the molecular chain movement of the copolyester and thus increasing the glass transition temperature. The glass transition temperature is controlled at 58–85°C. The cyclodiol increases the glass transition temperature of the copolyester, solving the defect of a decrease in glass transition temperature caused by the reduction of terephthalic acid content.

[0018] Measurements showed that the refractive index of the copolyester was controlled at around 1.53, and the glass transition temperature was controlled between 58℃ and 85℃. This copolyester has a low refractive index and a glass transition temperature similar to that of PET resin. It is suitable for the preparation of multilayer optical films.

[0019] Second, the copolyester produced in this application was laminated with PET material. After testing, the light transmittance of the laminated film reached 90%, and the haze was reduced to 0.1%. Attached Figure Description

[0020] Figure 1 This is a differential scanning calorimeter (DSC) of Embodiment 1 of this application. Detailed Implementation

[0021] This application is further illustrated by the following embodiments, comparative examples, and test data.

[0022] Unless otherwise specified, the raw materials used in the embodiments of this application are all commercially available and have a purity of 99% or higher.

[0023] Example

[0024] Example 1 A modified copolyester with a low refractive index is prepared according to the following steps: Esterification: Weigh out 550g of terephthalic acid (3.31mol), 466.39g of 1,4-cyclohexanedicarboxylic acid (2.71mol), 186.81g of ethylene glycol (3.01mol), 173.61g of 1,4-cyclohexanediethanol (1.21mol), 263.90g of isosorbide (1.81mol), 0.499g of antimony glycol, 0.2g of triethyl phosphoroacetate, and 186.81g (3.01mol) of ethylene glycol solvent and place them in a reaction vessel; Of these, terephthalic acid accounts for 55% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 45% of the total molar amount of dicarboxylic acid monomers. Ethylene glycol accounts for 66.7% of the total molar amount of the glycol monomers; cyclodiols account for 33.3% of the total molar amount of the glycol monomers. The molar ratio of dicarboxylic acid monomer to diol monomer is 1:1.5; A certain amount of nitrogen gas was introduced into the esterification reactor. The first stage of the esterification reaction was carried out at 216℃ for 20 min at a rotation speed of 1500 rpm. The second stage was heated to 242℃ for 70 min. Then the pressure was released to atmospheric pressure to obtain the esterification product. Condensation polymerization: The obtained esterification product was kept at 1500 rpm in the reactor and the temperature in the reactor was increased to 280℃. The reactor was then kept at this temperature for 45 min for low vacuum polycondensation. The vacuum degree of the system was gradually reduced from 0 MPa to -0.1 MPa. Excess ethylene glycol and a small amount of oligomers were removed from the reaction. Then, high-vacuum polycondensation is carried out, and the system is kept at a temperature of less than 70 Pa for 90 minutes to obtain the copolyester.

[0025] Example 2

[0026] A modified copolyester with a low refractive index differs from Example 1 in that the choice of dicarboxylic acid monomer is different, as detailed below: In Example 2, equimolar amounts of cis-1,3-cyclopentanedicarboxylic acid were used instead of 1,4-cyclohexanedicarboxylic acid.

[0027] Examples 3-6 A modified copolyester with a low refractive index differs from Example 1 in that the choice of cyclodiol is different, as detailed below: In Example 3, an equimolar amount of hydrogenated bisphenol A was used instead of isosorbide.

[0028] In Example 4, equimolar amounts of 2,2,4,4-tetramethyl-1,3-cyclobutanediol were used instead of isosorbide.

[0029] In Example 5, equimolar amounts of spirodiol were used instead of isosorbide.

[0030] In Example 6, equimolar amounts of 1,4-cyclohexanediethanol were used instead of isosorbide.

[0031] Examples 7-11 A modified copolyester with a low refractive index differs from Example 1 in that the molar proportions of each component in the diol monomer are different, as detailed below: In Example 7, ethylene glycol accounted for 53.4% ​​of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounted for 13.3% of the total molar amount of the glycol monomers; and isosorbide accounted for 33.3% of the total molar amount of the glycol monomers.

[0032] In Example 8, ethylene glycol accounts for 80% of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounts for 10% of the total molar amount of the glycol monomers; and isosorbide accounts for 10% of the total molar amount of the glycol monomers.

[0033] In Example 9, ethylene glycol accounts for 40% of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounts for 30% of the total molar amount of the glycol monomers; and isosorbide accounts for 30% of the total molar amount of the glycol monomers.

[0034] In Example 10, ethylene glycol accounts for 50% of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounts for 25% of the total molar amount of the glycol monomers; and isosorbide accounts for 25% of the total molar amount of the glycol monomers.

[0035] In Example 11, ethylene glycol accounts for 70% of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounts for 15% of the total molar amount of the glycol monomers; and isosorbide accounts for 15% of the total molar amount of the glycol monomers.

[0036] Examples 12-15 A modified copolyester with a low refractive index differs from Example 1 in that the molar proportions of each component in the dicarboxylic acid monomer are different, as detailed below: In Example 12, terephthalic acid accounts for 30% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 70% of the total molar amount of dicarboxylic acid monomers.

[0037] In Example 13, terephthalic acid accounts for 70% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 30% of the total molar amount of dicarboxylic acid monomers.

[0038] In Example 14, terephthalic acid accounted for 45% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids accounted for 55% of the total molar amount of dicarboxylic acid monomers.

[0039] In Example 15, terephthalic acid accounted for 60% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids accounted for 40% of the total molar amount of dicarboxylic acid monomers.

[0040] Examples 16-18 A modified copolyester with a low refractive index differs from Example 1 in that the total molar ratio of the carboxylic acid monomer and the diol monomer is different, as detailed below: In Example 16, the total molar ratio of the carboxylic acid monomer to the diol monomer was 1:1.2.

[0041] In Example 17, the total molar ratio of the carboxylic acid monomer to the diol monomer was 1:1.8.

[0042] In Example 18, the total molar ratio of the carboxylic acid monomer to the diol monomer was 1:1.3.

[0043] Example 19

[0044] A modified copolyester with a low refractive index differs from Example 1 in that the first stage of the esterification reaction is replaced with 40 min and the second stage with 50 min.

[0045] Example 20

[0046] A modified copolyester with a low refractive index differs from Example 1 in that the catalyst is replaced with germanium oxide in an equimolar amount.

[0047] Example 21

[0048] A modified copolyester with a low refractive index differs from Example 1 in that the stabilizer is replaced with trimethyl phosphate in an equal molar amount.

[0049] Comparative Example

[0050] Comparative Example 1 A modified copolyester is prepared according to the following steps: Esterification: Weigh out 550g of terephthalic acid (3.31mol), 466.39g of 1,4-cyclohexanedicarboxylic acid (2.71mol), 298.9g of ethylene glycol (4.82mol), 173.61g of 1,4-cyclohexanediethanol (1.21mol), 0.499g of antimony glycol, 0.2g of triethyl phosphoroacetate, and 186.81g (3.01mol) of ethylene glycol solvent and place them in a reaction vessel; Of these, terephthalic acid accounts for 55% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 45% of the total molar amount of dicarboxylic acid monomers. Ethylene glycol accounts for 86.6% of the total molar amount of the glycol monomers; cyclodiols account for 13.4% of the total molar amount of the glycol monomers. The molar ratio of dicarboxylic acid monomer to diol monomer is 1:1.5; A certain amount of nitrogen gas was introduced into the esterification reactor. The first stage of the esterification reaction was carried out at 216℃ for 20 min at a rotation speed of 1500 rpm. The second stage was heated to 242℃ for 70 min. Then the pressure was released to atmospheric pressure to obtain the esterification product. Condensation polymerization: The obtained esterification product was kept at 1500 rpm in the reactor and the temperature in the reactor was increased to 280℃. The reactor was then kept at this temperature for 45 min for low vacuum polycondensation. The vacuum degree of the system was gradually reduced from 0 MPa to -0.1 MPa. Excess ethylene glycol and a small amount of oligomers were removed from the reaction. Then, high-vacuum polycondensation is carried out, and the system is kept at a temperature of less than 70 Pa for 90 minutes to obtain the modified copolyester.

[0051] Comparative Example 2 A modified copolyester is prepared according to the following steps: Esterification: Weigh 1000g of terephthalic acid (6.02mol), 186.81g of ethylene glycol (3.01mol), 434.03g of 1,4-cyclohexanediethanol (3.01mol), 0.499g of antimony glycol, 0.2g of triethyl phosphoroacetate, and 186.81g (3.01mol) of ethylene glycol solvent and place them in a reaction vessel; Among them, terephthalic acid accounts for 100% of the total molar amount of dicarboxylic acid monomers; Ethylene glycol accounts for 66.7% of the total molar amount of the glycol monomers; cyclodiols account for 33.3% of the total molar amount of the glycol monomers. The molar ratio of dicarboxylic acid monomer to diol monomer is 1:1.5; A certain amount of nitrogen gas was introduced into the esterification reactor. The first stage of the esterification reaction was carried out at 216℃ for 20 min at a rotation speed of 1500 rpm. The second stage was heated to 242℃ for 70 min. Then the pressure was released to atmospheric pressure to obtain the esterification product. Condensation polymerization: The obtained esterification product was kept at 1500 rpm in the reactor and the temperature in the reactor was increased to 280℃. The reactor was then kept at this temperature for 45 min for low vacuum polycondensation. The vacuum degree of the system was gradually reduced from 0 MPa to -0.1 MPa. Excess ethylene glycol and a small amount of oligomers were removed from the reaction. Then, high-vacuum polycondensation is carried out, and the system is kept at a temperature of less than 70 Pa for 90 minutes to obtain the copolyester.

[0052] Comparative Example 3 A modified copolyester is prepared according to the following steps: Esterification: Weigh 1036.42g of 1,4-cyclohexanedicarboxylic acid (6.02mol), 298.9g of ethylene glycol (4.82mol), 173.61g of 1,4-cyclohexanediethanol (1.21mol), 0.499g of antimony glycol, 0.2g of triethyl phosphoroacetate, and 186.81g (1.81mol) of ethylene glycol solvent and place them in a polymerization reactor; Of which, 1,4-cyclohexanedicarboxylic acid accounts for 100% of the total molar amount of dicarboxylic acid monomers; Ethylene glycol accounts for 86.6% of the total molar amount of the glycol monomers; 1,4-cyclohexanediethanol accounts for 13.4% of the total molar amount of the glycol monomers. The total molar ratio of carboxylic acid monomers to diol monomers is 1:1.50; A certain amount of nitrogen gas was introduced into the esterification reactor. The reaction was carried out at a speed of 1500 rpm. The temperature of the first stage of the esterification reaction was 200℃ for 20 min. The temperature of the second stage was raised to 230℃ for 70 min. Then the pressure was released to atmospheric pressure to obtain the esterification product. Condensation polymerization: The obtained esterification product was kept at 1500 rpm in the reactor and the temperature in the reactor was increased to 280℃. The reactor was then kept at this temperature for 45 min for low vacuum polycondensation. The vacuum degree of the system was gradually reduced from 0 MPa to -0.1 MPa. Excess ethylene glycol and a small amount of oligomers were removed from the reaction. Then, high-vacuum polycondensation is carried out, and the system is kept at a temperature of less than 70 Pa for 90 minutes to obtain the modified copolyester.

[0053] Comparative Example 4 A modified copolyester is prepared according to the following steps: Esterification: Weigh 550g of terephthalic acid (3.31mol), 466.39g of 1,4-cyclohexanedicarboxylic acid (2.71mol), 879.68g of isosorbide (6.02mol), 0.499g of antimony glycol, 0.2g of triethyl phosphoroacetate, and 186.81g (1.81mol) of ethylene glycol solvent and place them in a polymerization reactor; Of these, terephthalic acid accounts for 55% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 45% of the total molar amount of dicarboxylic acid monomers. Ethylene glycol accounts for 23.1% of the total molar amount of the glycol monomers; isosorbide accounts for 76.9% of the total molar amount of the glycol monomers. The total molar ratio of carboxylic acid monomers to diol monomers is 1:1.50; A certain amount of nitrogen gas was introduced into the esterification reactor. The reaction was carried out at a speed of 1500 rpm. The temperature of the first stage of the esterification reaction was 200℃ for 20 min. The temperature of the second stage was raised to 230℃ for 70 min. Then the pressure was released to atmospheric pressure to obtain the esterification product. Condensation polymerization: The obtained esterification product was kept at 1500 rpm in the reactor and the temperature in the reactor was increased to 280℃. The reactor was then kept at this temperature for 45 min for low vacuum polycondensation. The vacuum degree of the system was gradually reduced from 0 MPa to -0.1 MPa. Excess ethylene glycol and a small amount of oligomers were removed from the reaction. Then, high-vacuum polycondensation is carried out, and the system is kept at a temperature of less than 70 Pa for 90 minutes to obtain the modified copolyester.

[0054] Test data 1.1. DSC analysis and testing.

[0055] 1.2. Refractive index testing (TE, TM): Measured using a prism coupler under a 636nm light source; 1.3. Intrinsic viscosity test: The fiber grade polyester (PET) slice test was performed according to GB / T 14190-2017.

[0056] The detection results of Examples 1-21 and Comparative Examples 1-4 are shown in Table 1.

[0057] Table 1. Performance test data of Examples 1-21 and Comparative Examples 1-4

[0058] in conclusion Based on the above test data, it can be seen that: Comparative Example 1 serves as a single comparison with Example 1, the difference being that Comparative Example 1 uses ethylene glycol in an equimolar amount instead of isosorbide, and the total molar proportion of ethylene glycol in the diol monomers exceeds 80%, indicating an excessively high amount of ethylene glycol. The test data shows that when the proportion of cyclic diols is too low, the resulting modified copolyester has a low refractive index but also a low glass transition temperature of only 43.03℃, significantly different from the glass transition temperature of PET. Therefore, the amount of cyclic diols added cannot be too low; if it is too low, the cyclic structure of the cyclic diol has almost no effect on the glass transition temperature of the modified copolyester.

[0059] Comparative Example 2 serves as a single comparison with Example 1, the difference being that an equimolar amount of terephthalic acid was used instead of 1,4-cyclohexanedicarboxylic acid. The test data shows that, due to the lack of alicyclic dicarboxylic acids, the refractive index of the modified copolyester significantly increased, reaching over 1.56.

[0060] Comparative Example 3 serves as a single comparison with Example 1, the difference being that an equimolar amount of 1,4-cyclohexanedicarboxylic acid was used instead of terephthalic acid. According to the test data, although the refractive index of Comparative Example 4 is relatively low, its glass transition temperature is extremely low, only 16.05℃, which is significantly different from the glass transition temperature of PET.

[0061] Comparative Example 4 serves as a single comparison with Example 1, the difference being that an equimolar amount of isosorbide was used instead of 1,4-cyclohexanediethanol. According to the test data, isosorbide has a significant impact on the glass transition temperature of the modified copolyester, and can significantly increase the glass transition temperature of the modified copolyester.

[0062] Examples 1-2 form a single comparison, using different cyclodicarboxylic acids for modification. Cis-1,3-cyclopentanedicarboxylic acid lowers the glass transition temperature of the modified copolyester, but tends to increase the refractive index of the modified copolyester.

[0063] Examples 3-6 form a single comparison, the difference being that different cyclic diols were used. The different structures of these cyclic diols affect the refractive index and glass transition temperature of the modified copolyester. According to the test results, the material modified with 2,2,4,4-tetramethyl-1,3-cyclobutanediol has the closest glass transition temperature to PET, and its refractive index is relatively low.

[0064] Examples 7-11 form a single comparison, the difference being: the proportions of ethylene glycol and cyclodiol are different, the higher the proportion of cyclodiol, the higher the Tg, but isosorbide has a greater effect on Tg than 1,4-cyclohexanediethanol; in terms of refractive index, a higher proportion of isosorbide will slightly increase the refractive index, while the introduction of 1,4-cyclohexanediethanol will slightly decrease the refractive index.

[0065] Examples 12-15 form a single comparison, the difference being that the proportions of terephthalic acid and alicyclic dicarboxylic acids are different. When the proportion of alicyclic dicarboxylic acids is higher, the proportion of cyclohexane structure in the alicyclic dicarboxylic acid replacing the original high molar refractive index and high rigidity benzene ring structure is higher, resulting in a lower refractive index of the synthesized polyester, and the glass transition temperature of the material also decreases as its proportion increases.

[0066] Examples 16-18 form a single comparison, the difference being: the ratio of the total molar amount of carboxylic acid monomer to diol monomer. When the total molar amount of diol monomer is too high during the reaction process, the side reaction will be aggravated, generating too much diethylene glycol, resulting in poor thermal properties, decreased crystallinity, and a yellowish hue in the synthesized material; when the total molar amount of diol monomer is too low during the reaction process, the forward reaction rate slows down, the high content of terminal carboxyl groups leads to incomplete reaction, resulting in a smaller molecular weight and poor melt strength in the synthesized material.

[0067] Application examples

[0068] Application Example 1 A multilayer optical film is prepared according to the following steps: After the modified copolyester chips and PET resin chips (brand name FG605) obtained in Example 1 are melted, they are transported to the designed multilayer feeding module after passing through a melt metering pump and a filter screen. The polyester chips obtained in Example 1 form film layer I, and the PET resin chips form film layer II. Film layer I and film layer II are alternately stacked according to the designed optical thickness. The stacked film is biaxially stretched to obtain a multilayer optical film. The visible light transmittance of the optical film was measured using an LS162 solar film tester, and the visible light transmittance was 90%. The haze of the sample was measured using a haze meter, and the haze was 0.1%. The interlayer bonding strength of the multilayer optical film was 8.0 MPa. The weather resistance of the optical film was tested using QUV 2000h, and the yellowing index was 3.10.

[0069] The test results above show that the optical film has high light transmittance, low haze and good weather resistance, which can meet the performance requirements as a multilayer optical film.

[0070] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0071] Furthermore, the above-described embodiments merely illustrate several implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A low-refractive-index modified copolyester, copolymerized from dicarboxylic acid monomers and diol monomers; characterized in that: in, Dicarboxylic acid monomers are composed of terephthalic acid and alicyclic dicarboxylic acids, and diol monomers are composed of ethylene glycol and cyclic diols; Among them, terephthalic acid accounts for 30% to 70% of the total molar amount of dicarboxylic acid monomers; alicyclic dicarboxylic acids account for 30% to 70% of the total molar amount of dicarboxylic acid monomers; Ethylene glycol accounts for 40%–80% of the total molar amount of the glycol monomers; cyclodiols account for 20%–60% of the total molar amount of the glycol monomers. The molar ratio of dicarboxylic acid monomer to diol monomer is 1:(1.20~1.80).

2. The low refractive index modified copolyester as described in claim 1, characterized in that: The terephthalic acid is terephthalic acid and / or terephthalic acid.

3. The low refractive index modified copolyester as described in claim 1, characterized in that: The alicyclic dicarboxylic acid is 1,4-cyclohexanedicarboxylic acid and / or cis-1,3-cyclopentanedicarboxylic acid.

4. The low refractive index modified copolyester as described in claim 1, characterized in that: The cyclodiol is one or more selected from 1,4-cyclohexanediol, isosorbide, hydrogenated bisphenol A, 2,2,4,4-tetramethyl-1,3-cyclobutanediol and spirodiol.

5. The low refractive index modified copolyester as described in claim 1, characterized in that: The molar ratio of the dicarboxylic acid monomer to the diol monomer is 1:(1.30-1.50).

6. The low refractive index modified copolyester as described in claim 1, characterized in that: The terephthalic acid accounts for 45% to 60% of the total molar amount of the dicarboxylic acid monomers; the alicyclic dicarboxylic acid accounts for 40% to 55% of the total molar amount of the dicarboxylic acid monomers; the ethylene glycol accounts for 50% to 70% of the total molar amount of the diol monomers; and the cyclodiol accounts for 30% to 50% of the total molar amount of the diol monomers.

7. A method for preparing a low-refractive-index modified copolyester according to any one of claims 1-6, characterized in that: Includes the following steps: Esterification: Terephthalic acid, alicyclic dicarboxylic acid, ethylene glycol and cyclodiol are added to a reaction vessel, a catalyst and a stabilizer are added, and after mixing, the mixture is heated in stages under an inert atmosphere and a pressure of 0.25-0.35 MPa to esterify and dehydrate, thus obtaining the esterified product. Polycondensation: The esterification product is polycondensed at a pressure of -0.1 MPa to 0 MPa for 30 to 60 min, and then heated to 270 to 283 °C at an absolute pressure of <70 Pa for 60 to 180 min to obtain the modified copolyester.

8. The method for preparing a low-refractive-index modified copolyester as described in claim 7, characterized in that: The catalyst used in the esterification step is one or more of antimony acetate, antimony trioxide, germanium dioxide, antimony glycolate, tetrabutyl titanate, and isopropyl titanate.

9. The method for preparing a low-refractive-index modified copolyester as described in claim 7, characterized in that: The stabilizer used in the esterification step is one or more of triethyl phosphate, triphenyl phosphate, trimethyl phosphate, and triphenyl phosphite.

10. The method for preparing a low-refractive-index modified copolyester as described in claim 7, characterized in that: The segmented heating process in the esterification step is as follows: the first esterification temperature is 200-230℃, and the holding time is 20-40 min; the second esterification temperature is 230-245℃, and the holding time is 50-70 min.

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