Modified polyester with characteristics of low yellowness value and high glass transition temperature and preparation method thereof

Abstract

The invention relates to the field of polyester materials, and particularly discloses modified polyester with low yellowness value and high glass transition temperature characteristics and a preparation method thereof, and the modified polyester is prepared by taking terephthalic acid, diphenyldicarboxylic acid and fluorenyl aromatic dicarboxylic acid as carboxylic acid monomers, respectively esterifying with ethylene glycol, and carrying out co-polycondensation on corresponding esterification products. According to the present invention, the biphenyldicarboxylic acid with the conjugated aromatic ring structure and the fluorenyl aromatic dicarboxylic acid monomer are selectively introduced to modify the PET, such that the regularity of the polyester chain segment is broken so as to ensure the optical performance of the polyester; the movement of a polyester chain segment is limited so as to improve the Tg value of polyester; and by matching with a novel preparation method, the yellowness value of the polyester material is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polyester materials, and more particularly to a modified polyester with low yellowness value and high glass transition temperature and a preparation method thereof. BACKGROUND

[0002] A multilayer optical film is made by stacking and stretching at least two resins with different refractive indexes. The resins used for the multilayer optical film require: first, the refractive index difference between adjacent layers is greater than 0.05 to improve the infrared reflectivity of the multilayer optical film; second, the resin has high transparency and low yellowness value to ensure that the multilayer optical film has excellent optical performance under the influence of the environment for a long time.

[0003] To meet the above two requirements, the multilayer optical film generally stacks PET resin and PMMA resin. Most of the PET on the market is synthesized by the PTA method, and the refractive index of the PET resin is generally 1.65, and the refractive index of the PMMA resin is generally 1.49. However, the glass transition temperature Tg of the PET resin is generally about 65℃, and the Tg value of the PMMA resin is generally about 105℃, and the glass transition temperature values of the two resins differ greatly, which leads to a large difference in processing temperature of the multilayer optical film, mismatched interlayer melt flow, and reduced interfacial bonding strength and interlayer bonding force between different resin materials, and the multilayer optical film is prone to delamination.

[0004] Therefore, in order to improve the glass transition temperature of the PET resin, a related patent uses terephthalic acid, 4,4'-diphenyl dicarboxylic acid and ethylene glycol as monomers to prepare a modified copolyester by one-pot melting method. The Tg value of the modified copolyester can only be increased to 96℃, which still has a certain gap with the Tg value of PMMA. At the same time, the esterification reaction temperatures of terephthalic acid and 4,4'-diphenyl dicarboxylic acid in the modified polyester are different. In order to ensure sufficient reaction, the reaction is carried out at a high temperature, thereby increasing the content of chromophores generated by side reactions in the reaction system and increasing the yellowness value of the modified polyester.

[0005] Based on the above situation, there is an urgent need in the industry to develop a modified polyester with low yellowness value and high glass transition temperature to adapt to PMMA and other high Tg value and low refractive index resin materials to make high reflectivity multilayer optical films with excellent stability. SUMMARY

[0006] In order to solve the problems of low glass transition temperature and high yellowness value of the polyester material, the present application provides a modified polyester with low yellowness value and high glass transition temperature and a preparation method thereof.

[0007] In a first aspect, this application provides a modified polyester that combines low yellowness value and high glass transition temperature characteristics, using the following technical solution: A modified polyester with both low yellowness value and high glass transition temperature is formed by esterifying terephthalic acid, biphenyl dicarboxylic acid and fluorenyl aromatic dicarboxylic acid as carboxylic acid monomers with ethylene glycol, and then polycondensing the corresponding esterification products. Of which, terephthalic acid accounts for 63-69% of the total molar amount of carboxylic acid monomers; biphenyl dicarboxylic acid accounts for 27-31% of the total molar amount of carboxylic acid monomers; and fluorenyl aromatic dicarboxylic acid accounts for 4-6% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:(1.2-1.7); the molar ratio of biphenyl dicarboxylic acid to ethylene glycol is 1:(1.5-2.0); and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:(1.4-2.0).

[0008] Furthermore, the biphenyl dicarboxylic acid is 3,4'-biphenyl dicarboxylic acid and / or 4,4'-biphenyl dicarboxylic acid.

[0009] Furthermore, the fluorenyl aromatic dicarboxylic acid is any one or more selected from 9H-fluoren-2,7-dicarboxylic acid, 9,9-dimethylfluoren-2,7-dicarboxylic acid, 9,9-diethylfluoren-2,7-dicarboxylic acid, 9,9-dibutylfluoren-2,7-dicarboxylic acid, and 9,9-diphenylfluoren-2,7-dicarboxylic acid.

[0010] Furthermore, the molar ratio of terephthalic acid to ethylene glycol is 1:1.2 to 1.3; the molar ratio of biphenyl dicarboxylic acid to ethylene glycol is 1:1.6 to 1.8; and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:1.8 to 1.9.

[0011] Secondly, this application provides a method for preparing a modified polyester that combines low yellowness value and high glass transition temperature, employing the following technical solution: A method for preparing a modified polyester with both low yellowness value and high glass transition temperature includes the following steps: Esterification: Terephthalic acid, biphenyl acid, and fluorenyl aromatic dicarboxylic acid are esterified with ethylene glycol in a specific molar ratio; the esterification temperature of terephthalic acid with ethylene glycol is 230–245℃, the esterification temperature of biphenyl acid with ethylene glycol is 240–255℃, and the esterification temperature of fluorenyl aromatic dicarboxylic acid with ethylene glycol is 245–260℃. Polycondensation: The above esterification products are blended and heated to 275-300℃ under a pressure of 0MPa to -0.1 MPa, and the reaction is maintained for 30-60 min to remove ethylene glycol and oligomers. Then, the reaction is maintained at an absolute pressure of <70Pa for 48-70 min to obtain modified polyester.

[0012] Furthermore, in the esterification step, terephthalic acid, biphenyl dicarboxylic acid, and fluorenyl aromatic dicarboxylic acid are all esterified with ethylene glycol under a pressure of 0.25–0.35 MPa.

[0013] Furthermore, in the esterification step, terephthalic acid, biphenyl dicarboxylic acid, and fluorenyl aromatic dicarboxylic acid react with ethylene glycol under an inert atmosphere.

[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] By adopting the above technical solution, this application has at least the following advantages: First, this application selects terephthalic acid, biphenyl dicarboxylic acid and fluorenyl aromatic dicarboxylic acid as carboxylic acid monomers, and esterifies them with ethylene glycol monomers in different reaction vessels. The resulting esterification products are then subjected to low vacuum polycondensation, removal of small molecules, and high vacuum polycondensation to obtain modified polyester with low yellowness value and high glass transition temperature. The principle is as follows: Biphenyl dicarboxylic acid and fluorenyl aromatic dicarboxylic acid, among the carboxylic acid monomers, possess conjugated aromatic ring structures with high electron cloud density and high molar refractive index. After polymerization with ethylene glycol, they can increase the refractive index of the polyester material. Simultaneously, the three carboxylic acid monomers can break the regularity of the polyester chain segments, thereby obtaining an amorphous copolyester, ensuring the optical properties of the polyester material. The conjugated aromatic ring structure contained in the carboxylic acid monomers generates significant steric hindrance, greatly restricting the movement of molecular chain segments, thus increasing the glass transition temperature of the modified polyester material.

[0017] Because the esterification reaction temperatures of terephthalic acid, biphenyl phthalic acid, and fluorenyl aromatic dicarboxylic acid differ significantly, the applicant adds the three carboxylic acid monomers to different reactors and controls the esterification temperature in each reactor to reduce the possibility of degradation and oxidation of terephthalic acid monomers at lower reaction temperatures during esterification, thereby reducing the generation of byproducts. The three esterification products are then blended and polycondensed under low vacuum. After a period of polycondensation, excess ethylene glycol and oligomers and other small molecule impurities are removed to reduce the possibility of degradation of small molecule impurities during the later polycondensation reaction, thereby reducing the yellowness value of the final modified polyester.

[0018] Second, the glass transition temperature of the modified polyester produced in this application is increased to 100°C, which is close to that of PMMA. When the two materials are laminated, the light transmittance of the laminated film reaches 87% and the haze is reduced to 0.7%. The weather resistance of the optical film is tested using QUV 2000h, and the yellowing index is only 9.36. Attached Figure Description

[0019] Figure 1 This is a differential scanning calorimeter (DSC) of Embodiment 1 in this application.

[0020] Figure 2 This is the differential scanning calorimetry (DSC) of Comparative Example 2 in this application.

[0021] Figure 3 Differential scanning calorimetry (DSC) of PMMA used in this application.

[0022] Figure 4 This is the thermogravimetric analysis (TGA) chart of Example 1 in this application. Detailed Implementation

[0023] This application is further illustrated by the following embodiments, comparative examples, and test data.

[0024] 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.

[0025] Example

[0026] Example 1

[0027] A modified polyester exhibiting both low yellowness value and high glass transition temperature is prepared according to the following steps: Esterification: Prepare three esterification reactors; Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.135g of antimony glycolate, and 0.1g of triethyl phosphoroacetate and place them in the second esterification reactor; Weigh 84.8g of 9,9-dimethylfluorene-2,7-dicarboxylic acid, 33.5g of ethylene glycol, 0.03g of antimony glycolate, and 0.05g of triethyl phosphoroacetate and place them in the third esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 5% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.8. A certain amount of high-purity nitrogen gas (nitrogen purity > 99.99%) was introduced into three esterification reactors. The first esterification reactor was heated to 230°C, the second esterification reactor was heated to 250°C, and the third esterification reactor was heated to 255°C. All three esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The three esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum level of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0028] Example 2

[0029] A modified polyester exhibiting both low yellowness value and high glass transition temperature is prepared according to the following steps: Esterification: Prepare three esterification reactors; Weigh 648.0g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.135g of antimony glycolate, and 0.1g of triethyl phosphoroacetate and place them in the second esterification reactor; Weigh 93.1g of 9,9-diethylfluorene-2,7-dicarboxylic acid, 33.5g of ethylene glycol, 0.03g of antimony glycolate, and 0.05g of triethyl phosphoroacetate and place them in the third esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers; and fluorenyl aromatic dicarboxylic acid accounts for 5% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:1.8. A certain amount of high-purity nitrogen gas (nitrogen purity > 99.99%) was introduced into three esterification reactors. The first esterification reactor was heated to 230°C, the second esterification reactor was heated to 250°C, and the third esterification reactor was heated to 255°C. All three esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The three esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum level of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0030] Example 3

[0031] A modified polyester exhibiting both low yellowness value and high glass transition temperature is prepared according to the following steps: Esterification: Prepare three esterification reactors; Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.135g of antimony glycolate, and 0.1g of triethyl phosphoroacetate and place them in the second esterification reactor; Weigh 110.0g of 9,9-dibutylfluorene-2,7-dicarboxylic acid, 33.5g of ethylene glycol, 0.03g of antimony glycolate, and 0.05g of triethyl phosphoroacetate and place them in the third esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers; and fluorenyl aromatic dicarboxylic acid accounts for 5% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:1.8. A certain amount of high-purity nitrogen gas (nitrogen purity > 99.99%) was introduced into three esterification reactors. The first esterification reactor was heated to 230°C, the second esterification reactor was heated to 250°C, and the third esterification reactor was heated to 255°C. All three esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The three esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum level of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0032] Example 4

[0033] A modified polyester exhibiting both low yellowness value and high glass transition temperature is prepared according to the following steps: Esterification: Prepare three esterification reactors; Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.135g of antimony glycolate, and 0.1g of triethyl phosphoroacetate and place them in the second esterification reactor; Weigh 122.1g of 9,9-diphenylfluorene-2,7-dicarboxylic acid, 33.5g of ethylene glycol, 0.03g of antimony glycolate, and 0.05g of triethyl phosphoroacetate and place them in the third esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers; and fluorenyl aromatic dicarboxylic acid accounts for 5% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:1.8. A certain amount of high-purity nitrogen (nitrogen purity > 99.99%) was introduced into three esterification reactors. The first esterification reactor was heated to 230°C, the second esterification reactor was heated to 250°C, and the third esterification reactor was heated to 260°C. All three esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 3 hours to obtain the esterification product. Condensation polymerization: The three esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum level of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0034] Example 5

[0035] A modified polyester with both low yellowness value and high glass transition temperature characteristics, differing from Example 1 in that antimony glycol is replaced with an equimolar amount of antimony acetate.

[0036] Example 6

[0037] A modified polyester with both low yellowness value and high glass transition temperature characteristics, differing from Example 1 in that triethyl phosphoroacetate is replaced with an equimolar amount of trimethyl phosphate.

[0038] Example 7

[0039] A modified polyester exhibiting both low yellowness and high glass transition temperature differs from Example 1 in that: terephthalic acid accounts for 63% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 31% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 6% of the total molar amount of the carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.8.

[0040] Example 8

[0041] A modified polyester exhibiting both low yellowness and high glass transition temperature differs from Example 1 in that: terephthalic acid accounts for 69% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 27% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 4% of the total molar amount of the carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.8.

[0042] Example 9

[0043] A modified polyester exhibiting both low yellowness and high glass transition temperature differs from Example 1 in that: terephthalic acid accounts for 65% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 5% of the total molar amount of the carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.7; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:2.0; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:2.0.

[0044] Example 10

[0045] A modified polyester exhibiting both low yellowness and high glass transition temperature differs from Example 1 in that: terephthalic acid accounts for 65% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 5% of the total molar amount of the carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.5; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.4.

[0046] Example 11

[0047] A modified polyester exhibiting both low yellowness and high glass transition temperature differs from Example 1 in that: terephthalic acid accounts for 65% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 5% of the total molar amount of the carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.3; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.8; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.9.

[0048] Comparative Example Comparative Example 1 A modified polyester is prepared according to the following steps: Esterification: Prepare two esterification reactors; Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 510g of 4,4'-biphenyldicarboxylic acid, 208.5g of ethylene glycol, 0.145g of antimony glycolate, and 0.15g of triethyl phosphoroacetate and place them in the second esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 35% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6. A certain amount of nitrogen gas was introduced into two esterification reactors. The first esterification reactor was heated to 230°C and the second esterification reactor was heated to 250°C. Both esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The two esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum degree of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0049] Comparative Example 2 A modified polyester is prepared according to the following steps: Esterification: Prepare two esterification reactors; Weigh 698.0g of terephthalic acid, 312.8g of ethylene glycol, 0.315g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.145g of antimony glycolate, and 0.15g of triethyl phosphoroacetate and place them in the second esterification reactor; Of these, terephthalic acid accounts for 70% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6. A certain amount of nitrogen gas was introduced into two esterification reactors. The first esterification reactor was heated to 230°C and the second esterification reactor was heated to 250°C. Both esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The two esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum degree of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0050] Comparative Example 3 A modified polyester is prepared according to the following steps: Esterification: Prepare two esterification reactors; Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate and place them in the first esterification reactor; Weigh 593.6g of 9,9-dimethylfluorene-2,7-dicarboxylic acid, 235g of ethylene glycol, 0.21g of antimony glycolate, and 0.35g of triethyl phosphoroacetate and place them in the second esterification reactor; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 35% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.8. A certain amount of nitrogen gas was introduced into two esterification reactors. The first esterification reactor was heated to 230°C and the second esterification reactor was heated to 255°C. Both esterification reactors were continuously heated and stirred at a speed of 1500 rpm for 2 hours to obtain the esterification product. Condensation polymerization: The two esterification products were introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285°C. The reactor was then kept at this temperature for 40 minutes for low-vacuum polycondensation. The vacuum degree of the system gradually decreased 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 55 minutes to obtain the modified polyester.

[0051] Comparative Example 4 A modified polyester is prepared according to the following steps: Esterification: Weigh out 647.9g of terephthalic acid, 290.5g of ethylene glycol, 0.293g of antimony glycolate, and 0.2g of triethyl phosphoroacetate; Weigh out 436.6g of 4,4'-biphenyl dicarboxylic acid, 179g of ethylene glycol, 0.135g of antimony glycolate, and 0.1g of triethyl phosphoroacetate; Weigh out 84.8g of 9,9-dimethylfluorene-2,7-dicarboxylic acid, 33.5g of ethylene glycol, 0.03g of antimony glycolate, and 0.05g of triethyl phosphoroacetate; Of these, terephthalic acid accounts for 65% of the total molar amount of carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 30% of the total molar amount of carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 5% of the total molar amount of carboxylic acid monomers. The molar ratio of terephthalic acid to ethylene glycol is 1:1.2; the molar ratio of 4,4'-biphenyl dicarboxylic acid to ethylene glycol is 1:1.6; and the molar ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid to ethylene glycol is 1:1.8. The above materials were added to the same esterification reactor, a certain amount of nitrogen gas was introduced, the esterification reactor was heated to 255°C, and the esterification reactor was continuously heated and stirred at a speed of 1500 rpm for 4 hours to obtain the esterification product. Condensation polymerization: The obtained esterification product was introduced into a polycondensation reactor, the rotation speed was maintained at 1500 rpm, and the temperature in the polycondensation reactor was increased to 285℃. The reactor was then kept at this temperature for 40 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 55 minutes to obtain the modified polyester.

[0052] Comparative Example 5 A modified polyester differs from Example 1 in that: terephthalic acid accounts for 58% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 32% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 10% of the total molar amount of the carboxylic acid monomers.

[0053] Comparative Example 6 A modified polyester differs from Example 1 in that: terephthalic acid accounts for 98% of the total molar amount of the carboxylic acid monomers; 4,4'-biphenyl dicarboxylic acid accounts for 1% of the total molar amount of the carboxylic acid monomers; and 9,9-dimethylfluorene-2,7-dicarboxylic acid accounts for 1% of the total molar amount of the carboxylic acid monomers.

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

[0055] 1.2. Colorimetric value test: GB / T 14190-2017 Test method for fiber grade polyester (PET) chips: Test method for 5.5 colorimetric value.

[0056] 1.3. Intrinsic viscosity test: GB / T 14190-2017 Test method for fiber grade polyester (PET) chips: 5.1 Test method for intrinsic viscosity.

[0057] The test results of Examples 1-11 and Comparative Examples 1-6 are shown in Table 1.

[0058] Table 1. Performance test data of Examples 1-11 and Comparative Examples 1-6

[0059] in conclusion Based on the above test data, it can be seen that: Comparative Examples 1-3 form a single comparison with Example 1. In Comparative Example 1, 4,4'-biphenyldicarboxylic acid was used in an equimolar amount to replace 9,9-dimethylfluorene-2,7-dicarboxylic acid. The resulting modified polyester had a glass transition temperature of only 92.3°C, indicating that modifying PET with only 4,4'-biphenyldicarboxylic acid has a limited effect on improving the glass transition temperature (Tg) of the polyester material. Furthermore, when the molar percentage of 4,4'-biphenyldicarboxylic acid in the carboxylic acid monomer reached 35%, the resulting modified polyester chips exhibited a fogging phenomenon.

[0060] In Comparative Example 2, terephthalic acid was used in equimolar amounts to replace 9,9-dimethylfluorene-2,7-dicarboxylic acid, reducing the molar amount of 4,4'-biphenylcarboxylic acid, and consequently lowering the Tg value of the modified polyester. See also... Figures 1-4 It can be seen that the introduction of 9,9-dimethylfluorene-2,7-dicarboxylic acid can increase the Tg value of the modified polyester to 100.35℃, which is closer to the Tg of PMMA (110.94℃). Furthermore, the TGA (thermal weight loss analysis) graph shows that the modified polyester loses less than 2% of its weight at 400℃, indicating that the introduction of 9,9-dimethylfluorene-2,7-dicarboxylic acid gives the modified polyester excellent heat resistance.

[0061] In Comparative Example 3, 9,9-dimethylfluorene-2,7-dicarboxylic acid was used in equimolar amounts to replace 4,4'-biphenylcarboxylic acid. The modified polyester produced had a Tg of 128.42°C, which was higher than the Tg of 100.35°C in Example 1. Furthermore, the excessively high replacement ratio of 9,9-dimethylfluorene-2,7-dicarboxylic acid resulted in the modified particles appearing yellowish and opaque, making them unsuitable for use in the preparation of optical multilayer films.

[0062] Comparative Example 4 forms a single comparison with Example 1. Comparative Example 4 uses a one-pot method to produce modified polyester. During the esterification reaction, the time required for the esterification rate of Comparative Example 4 to reach more than 90% is much longer than that required for Example 1. Furthermore, the high esterification temperature leads to the decomposition of some raw materials, resulting in a yellowness value of 10.31 for the modified polyester, a yellowish tint to the modified polyester chips, and a decrease in the glass transition temperature.

[0063] Comparative Examples 5-6 form a single comparative example with Example 1, the difference being the content of fluorenyl aromatic dicarboxylic acid. In Comparative Example 5, the molar percentage of 9,9-dimethylfluorene-2,7-dicarboxylic acid reaches 10%, causing a sharp increase in the yellowness value of the modified polyester; in Comparative Example 6, the molar percentage of 9,9-dimethylfluorene-2,7-dicarboxylic acid is 1%, although the yellowness value of the modified polyester is significantly reduced, the glass transition temperature is also significantly reduced. Based on this, it is necessary to reasonably control the molar percentage range of the three carboxylic acid monomers so that the yellowness value of the modified polyester is low and the glass transition temperature is close to the Tg value of PMMA.

[0064] Examples 1-4 of this application form a single comparison, the difference being that the substituent at the 9-position of the fluorenyl aromatic dicarboxylic acid is different; in Example 2, the substituent at the 9-position of the fluorenyl aromatic dicarboxylic acid is ethyl, while in Example 3, the substituent at the 9-position of the fluorenyl aromatic dicarboxylic acid is butyl. According to the test data in Table 1, the flexible branch can offset the rigidity of the fluorene structure, resulting in a smaller increase in Tg value; furthermore, the longer the flexible branch, the smaller the increase in Tg value. In Example 4, the substituent at the 9-position of the fluorenyl aromatic dicarboxylic acid is phenyl, which increases the rigidity of the branch, making the overall structure more rigid and resulting in a greater increase in Tg than methyl. However, because the rigidity is too high, it leads to poor meltability, high reaction temperature, and long reaction time, resulting in an increased yellowness value of the modified polyester and poor melt flowability, making it difficult to process.

[0065] Examples 1 and 5 of this application form a single comparison, the difference being that different catalysts were used. According to the test data in Table 1, it can be seen that antimony acetate as a catalyst helps to reduce the yellowness value and Tg value of the modified polyester, but has little effect on the Tg value of the modified polyester.

[0066] Examples 1 and 6 of this application form a single comparison, the difference being that different stabilizers were used. According to the test data in Table 1, it can be seen that trimethyl phosphate as a stabilizer helps to reduce the yellowness value and Tg value of the modified polyester.

[0067] Examples 1 and 9-11 of this application form a single comparison. The difference lies in the different molar ratios of terephthalic acid, 4,4'-biphenyl dicarboxylic acid, and 9,9-dimethylfluorene-2,7-dicarboxylic acid with ethylene glycol. According to the test data in Table 1, it can be seen that the different molar ratios of the three carboxylic acid monomers with ethylene glycol have an impact on the yellowness value of the modified polyester.

[0068] Application examples Application Example 1 A multilayer optical film is prepared according to the following steps: After the modified polyester chips and PMMA resin chips (brand name CP-80) 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 PMMA 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 87%. The haze of the sample was measured using a haze meter, and the haze was 0.7%. The interlayer bonding strength of the multilayer optical film was 8.1 MPa. The weather resistance of the optical film was tested using QUV 2000h, and the yellowing index was 9.36.

[0069] The test results above show that the optical film has high light transmittance, low haze, high interlayer bonding strength 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 modified polyester possessing both low yellowness value and high glass transition temperature, characterized in that, The mixture is prepared by esterifying terephthalic acid, biphenyl phthalic acid, and fluorenyl aromatic dicarboxylic acid with ethylene glycol, and then co-polymerizing the esterification products. The terephthalic acid comprises 63%–69% of the total molar amount of the carboxylic acid monomers; biphenyl phthalic acid comprises 27%–31% of the total molar amount of the carboxylic acid monomers; fluorenyl aromatic dicarboxylic acid comprises 4%–6% of the total molar amount of the carboxylic acid monomers; the molar ratio of terephthalic acid to ethylene glycol is 1:(1.2–1.7); the molar ratio of biphenyl phthalic acid to ethylene glycol is 1:(1.5–2.0); and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:(1.4–2.0).

2. The modified polyester with both low yellowness value and high glass transition temperature as described in claim 1, characterized in that: The biphenyl dicarboxylic acid is 3,4'-biphenyl dicarboxylic acid and / or 4,4'-biphenyl dicarboxylic acid.

3. The modified polyester with both low yellowness value and high glass transition temperature as described in claim 1, characterized in that: The fluorenyl aromatic dicarboxylic acid is any one or more selected from 9H-fluoren-2,7-dicarboxylic acid, 9,9-dimethylfluoren-2,7-dicarboxylic acid, 9,9-diethylfluoren-2,7-dicarboxylic acid, 9,9-dibutylfluoren-2,7-dicarboxylic acid, and 9,9-diphenylfluoren-2,7-dicarboxylic acid.

4. The modified polyester with both low yellowness value and high glass transition temperature as described in claim 1, characterized in that: The molar ratio of terephthalic acid to ethylene glycol is 1:1.2 to 1.3; the molar ratio of biphenyl dicarboxylic acid to ethylene glycol is 1:1.6 to 1.8; and the molar ratio of fluorenyl aromatic dicarboxylic acid to ethylene glycol is 1:1.8 to 1.

9.

5. The method for preparing the modified polyester with both low yellowness value and high glass transition temperature characteristics as described in any one of claims 1-4, characterized in that: Includes the following steps: Esterification: Terephthalic acid, biphenyl acid, and fluorenyl aromatic dicarboxylic acid are esterified with ethylene glycol in a specific molar ratio; the esterification temperature of terephthalic acid with ethylene glycol is 230–245℃, the esterification temperature of biphenyl acid with ethylene glycol is 240–255℃, and the esterification temperature of fluorenyl aromatic dicarboxylic acid with ethylene glycol is 245–260℃. Polycondensation: The above esterification products are blended and heated to 275-300℃ under a pressure of 0MPa to -0.1 MPa, and the reaction is maintained for 30-60 min to remove ethylene glycol and oligomers. Then, the reaction is maintained at an absolute pressure of <70Pa for 48-70 min to obtain modified polyester.

6. The method for preparing the modified polyester with both low yellowness value and high glass transition temperature characteristics as described in claim 5, characterized in that: In the esterification step, terephthalic acid, biphenyl dicarboxylic acid, and fluorenyl aromatic dicarboxylic acid are all esterified with ethylene glycol under a pressure of 0.25–0.35 MPa.

7. The method for preparing the modified polyester with both low yellowness value and high glass transition temperature as described in claim 5, characterized in that: In the esterification step, terephthalic acid, biphenyl dicarboxylic acid, and fluorenyl aromatic dicarboxylic acid react with ethylene glycol under an inert atmosphere.

8. The method for preparing the modified polyester with both low yellowness value and high glass transition temperature as described in claim 5, 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 the modified polyester with both low yellowness value and high glass transition temperature characteristics as described in claim 5, characterized in that: The stabilizer used in the esterification step is one or more of triethyl phosphate, triphenyl phosphate, trimethyl phosphate, and triphenyl phosphite.

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