Process for the synthesis of a polyester and its use

Abstract

The application discloses a polyester synthesis method and application thereof. The polyester synthesis method comprises the following steps: preparing a sol containing cellulose nanocrystals, and an alcohol solution containing an aluminum alkoxide; adding the alcohol solution into the sol to react; separating a solid phase to obtain a composite catalyst; and synthesizing polyester under the co-catalysis of the composite catalyst and diisopropyl bis(acetylacetonyl) titanate. The polyester synthesized by the method has low carboxyl content, high light transmittance, high mechanical property and hydrolysis resistance, and can be used for preparing a polyester film. The application also provides application of the polyester prepared by the method.

Description

Technical Field

[0001] This invention relates to the field of polyester materials technology, and more specifically to a method for synthesizing polyester and its application. Background Technology

[0002] Conventional polyester generally refers to polyethylene terephthalate (PET), which has good mechanical properties, transparency, insulation and processability, and is widely used in packaging, optical, electronic, electrical, architectural or other decorative films.

[0003] Polyester is a semi-crystalline material, and improving light transmittance by inhibiting crystallization is a major research direction in polyester modification. While introducing a third monomer to reduce the regularity of the polyester molecular chain can decrease crystallinity and increase light transmittance, it easily leads to a loss of strength. Furthermore, due to the lack of consolidation in crystalline regions, excessive molecular chain orientation can affect the stretchability of biaxially oriented polyester films, hindering film processing and mechanical property improvement. In addition, since esterification is a reversible reaction, even with excess alcohol, carboxyl groups are difficult to fully convert. Studies have shown that the presence of residual carboxyl groups significantly affects the hydrolytic resistance of polyester, making it more susceptible to degradation in aqueous environments and limiting its applications.

[0004] Therefore, there is still a lack of polyesters with high light transmittance, good mechanical properties, and hydrolysis resistance. Improving these properties is of great significance for expanding the application of polyesters, especially polyester film products. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for synthesizing polyester, the prepared polyester having a low carboxyl content and possessing the characteristics of high light transmittance, high mechanical properties, and hydrolysis resistance.

[0006] The present invention also provides polyesters prepared by the above-described synthesis method.

[0007] The present invention also provides a polyester film made using the above-described polyester.

[0008] The present invention also provides a method for preparing the above-mentioned polyester film.

[0009] The present invention also provides applications of the above-described polyester film.

[0010] Specifically, the first aspect of the present invention relates to a method for synthesizing a polyester, comprising the following steps:

[0011] S1. Disperse cellulose nanocrystals in water to form a sol;

[0012] S2. Dissolve the aluminum alkoxide in alcohol to prepare an alcohol solution;

[0013] S3. Under heating and pressurization conditions, the alcohol solution is added to the sol to react, the solid phase is separated, and a composite catalyst is obtained.

[0014] S4. Terephthalic acid, ethylene glycol, 1,4-cyclohexanediethanol, diisopropyl di(acetylacetonyl)titanate and the composite catalyst are mixed and esterified. The reaction is carried out until the water content reaches more than 93% of the theoretical value. Trimethylolpropane is added to the product and reacted. Vacuum is drawn, the temperature is raised and polycondensation reaction is carried out to obtain polyester.

[0015] The method for synthesizing polyester according to the first aspect of the present invention has at least the following beneficial effects:

[0016] Previous studies have shown that the hydrolysis of aluminum alkoxides can generate aluminum oxides (aluminum hydroxyacid, AlOOH). The in-situ formed aluminum oxides have high activity and strong adsorption capacity. At the same time, cellulose nanocrystals have a polyhydroxy structure and nano-size effect, which can induce the growth of aluminum oxides. Under pressure, the growth of aluminum oxides is limited, thus effectively utilizing cellulose nanocrystals as a template agent to refine the dispersed particle size of aluminum oxides and improve catalytic activity.

[0017] Aluminum in aluminum oxide and titanium in di(acetylacetonyl)titanate diisopropyl can both serve as catalytic active sites. Studies have found that using di(acetylacetonyl)titanate diisopropyl with a composite catalyst is more conducive to promoting the reaction. The resulting product has higher viscosity and lower end carboxyl group content, indicating the acquisition of a higher molecular weight polyester, which can significantly improve mechanical properties and also improve hydrolysis resistance. The hydroxyl groups in cellulose nanocrystals can also participate in the reaction, further increasing the polyester molecular weight, reducing crystallinity, and improving light transmittance and mechanical properties.

[0018] 1,4-Cyclohexanediethanol reduces the regularity of the molecular chain, while the carbon in the cyclohexyl group has a certain degree of rotational freedom, which gives the molecular chain a certain degree of flexibility, thereby improving the stretchability and better balancing light transmittance, toughness and processability. It can be used to prepare high-performance polyester films.

[0019] The esterification reaction temperature is relatively lower, while the polycondensation reaction temperature is higher, which is conducive to the stable progress of the reaction. At the same time, under vacuum conditions, the hydroxyl groups in trimethylolpropane and cellulose nanocrystals participate in the reaction, which can further reduce the regularity of polyester molecular chains, improve crystallinity, increase polyester molecular weight, reduce residual carboxyl content, and improve mechanical properties and hydrolysis resistance.

[0020] The polyester synthesis method described in this embodiment is easy to implement and conducive to industrial production. The synthesized polyester has excellent mechanical properties, light transmittance, hydrolysis resistance and other characteristics.

[0021] According to some embodiments of the present invention, the diameter of the cellulose nanocrystals is 1 to 10 nm.

[0022] According to some embodiments of the present invention, the length of the cellulose nanocrystals is 100-1000 nm.

[0023] Controlling the length and diameter of cellulose nanocrystals within a suitable range is beneficial for loading aluminum oxides while also taking into account nucleation properties.

[0024] According to some embodiments of the present invention, the sol contains 0.5%-2% by mass of fibrous nanocrystals. This lower concentration results in a stable dispersion, which facilitates uniform loading of aluminum oxide.

[0025] According to some embodiments of the present invention, the aluminum alkoxide is aluminum isopropoxide, which can be hydrolyzed to obtain aluminum oxide with high catalytic activity.

[0026] According to some embodiments of the present invention, the mass percentage of aluminum alkoxide in the alcohol solution is 5%-10%. The concentration of aluminum alkoxide should not be too high in order to refine the particle size of aluminum oxide and improve its dispersion uniformity with cellulose nanocrystals.

[0027] According to some embodiments of the present invention, the alcohol in the alcohol solution is selected from at least one of ethylene glycol and 1,2-propanediol.

[0028] According to some embodiments of the present invention, the mass ratio of Al in the alcohol solution to the cellulose nanocrystals in the sol is 1:15-35.

[0029] By controlling the mass ratio of Al to cellulose nanocrystals, the cellulose nanocrystals are less likely to be covered by aluminum oxide, thus balancing the compatibilizing and nucleation effects of cellulose nanocrystals and improving the catalytic and nucleation performance of the composite catalyst.

[0030] According to some embodiments of the present invention, in step S3, the heating temperature is 85-90°C.

[0031] According to some embodiments of the present invention, in step S3, the pressure applied is 0.2-0.5 MPa.

[0032] According to some embodiments of the present invention, in step S3, during the reaction process, the pH is controlled to be 8-9, specifically between 8-8.5.

[0033] The reaction temperature, pH, and pressure are controlled to obtain aluminum oxides with higher catalytic activity.

[0034] Under suitable temperature and pH conditions, cellulose nanocrystals exhibit better stability, inhibiting self-aggregation and hydrolysis of cellulose nanocrystals.

[0035] According to some embodiments of the present invention, in step S3, after the alcohol solution is added to the sol, the reaction continues for 0.5-1 h.

[0036] According to some embodiments of the present invention, in step S3, the alcohol solution is added to the sol by dropwise addition.

[0037] The slow, dropwise addition method provides instantaneous dilution of the alcohol solution. Furthermore, stirring during the dropwise addition reduces the dispersed particle size of the aluminum oxide and improves the uniformity of the loading on the cellulose nanocrystals.

[0038] According to some embodiments of the present invention, in step S3, the method for separating the solid phase includes at least one of microporous filtration, centrifugation, vacuum distillation, and spray drying. Further, vacuum distillation and spray drying are performed sequentially. Vacuum distillation removes water and low-boiling-point alcohols for concentration, followed by spray drying to improve drying efficiency and better ensure the dispersion morphology of the solid phase.

[0039] According to some embodiments of the present invention, in step S4, the molar ratio of alcohol to acid in the esterification reaction is 1.4-1.6:1.

[0040] According to some embodiments of the present invention, in step S4, the molar ratio of ethylene glycol to 1,4-cyclohexanediethanol is 20-30:1. The amount of 1,4-cyclohexanediethanol should not be too high, otherwise it may have too great an impact on the regularity of the molecular chain, which is not conducive to the formation of microcrystalline regions and may also affect the full reaction of carboxyl groups.

[0041] According to some embodiments of the present invention, in step S4, the mass of the diisopropyl di(acetylacetonyl)titanate is 10-30 ppm, based on the mass of terephthalic acid.

[0042] According to some embodiments of the present invention, in step S4, the mass of Al in the composite catalyst is 0.005%-0.02% of the mass of terephthalic acid.

[0043] According to some embodiments of the present invention, in step S4, the mixing process includes: dispersing the composite catalyst and diisopropyl di(acetylacetonate)titanate in ethylene glycol, and then adding 1,4-cyclohexanediethanol and terephthalic acid to mix, which facilitates the interaction between diisopropyl di(acetylacetonate)titanate and the composite catalyst and enhances the overall catalytic activity.

[0044] According to some embodiments of the present invention, in step S4, the mass of the trimethylolpropane is 0.4-1% of the mass of the product obtained from the esterification reaction.

[0045] According to some embodiments of the present invention, in step S4, the temperature of the esterification reaction is 240-245°C and the pressure is 2.5-4 kPa.

[0046] According to some embodiments of the present invention, in step S4, the reaction time for adding trimethylolpropane is 30-40 min.

[0047] According to some embodiments of the present invention, in step S4, the temperature of the polycondensation reaction is 270-275°C and the time is 1-2 hours.

[0048] According to some embodiments of the present invention, in step S4, the vacuum is drawn until the pressure reaches below 100 Pa.

[0049] The second aspect of the present invention relates to polyesters obtained by the above-described synthesis method.

[0050] A third aspect of the present invention relates to a polyester film, which is made from raw materials including the polyester described above.

[0051] According to some embodiments of the present invention, the polyester film is a biaxially oriented polyester film.

[0052] The fourth aspect of this invention relates to a method for preparing the above-mentioned polyester film, comprising the steps of:

[0053] The raw materials, including the polyester, are obtained by melt extrusion, cooling, and biaxial stretching.

[0054] This preparation method is simple, has high production efficiency, and can achieve continuous industrial production. The prepared polyester has good comprehensive properties such as mechanical properties and light transmittance.

[0055] According to some embodiments of the present invention, the cooling temperature is 40-50°C.

[0056] According to some embodiments of the present invention, the longitudinal stretching temperature of the biaxial stretching is 90-100°C, and the stretching ratio is 3-4:1.

[0057] According to some embodiments of the present invention, the transverse stretching temperature of the biaxial stretching is 110-120°C, and the stretching ratio is 3-4:1.

[0058] By adjusting the longitudinal and transverse stretch ratio and the corresponding stretching temperature, higher mechanical properties and light transmittance can be obtained.

[0059] The fifth aspect of the present invention relates to the use of the above-described polyester or polyester film in the preparation of packaging products, optical products, electronic products or decorative products.

[0060] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Detailed Implementation

[0061] The embodiments of the present invention are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0062] Specifically, a first aspect of the present invention provides a method for synthesizing polyester, comprising the following steps:

[0063] S1. Disperse cellulose nanocrystals in water to form a sol;

[0064] S2. Dissolve the aluminum alkoxide in alcohol to prepare an alcohol solution;

[0065] S3. Under heating and pressurization conditions, the alcohol solution is added to the sol to react, the solid phase is separated, and the composite catalyst is obtained.

[0066] S4. Terephthalic acid, ethylene glycol, 1,4-cyclohexanediethanol, diisopropyl di(acetylacetonyl)titanate and composite catalyst are mixed and esterified. The reaction is carried out until the water content reaches more than 93% of the theoretical value. Trimethylolpropane is added to the product and reacted. Vacuum is drawn, temperature is raised and polycondensation reaction is carried out to obtain polyester.

[0067] Aluminum alkoxide hydrolysis can generate aluminum oxide (aluminum hydroxyaluminate, AlOOH). The in-situ formed aluminum oxide has high activity and strong adsorption capacity. At the same time, cellulose nanocrystals have a polyhydroxy structure and nano-size effect, which can induce the growth of aluminum oxide. Under pressure conditions, the growth of aluminum oxide is limited, thus effectively utilizing cellulose nanocrystals as a template agent to refine the dispersed particle size of aluminum oxide and improve catalytic activity.

[0068] Aluminum in aluminum oxide and titanium in diisopropyl di(acetylacetonate)titanate can both serve as catalytic active sites. Studies have found that using diisopropyl di(acetylacetonate)titanate in conjunction with a composite catalyst is more conducive to promoting the reaction. The resulting product has higher viscosity and lower end-carboxyl group content, indicating the acquisition of a higher molecular weight polyester, which significantly improves mechanical properties and also enhances hydrolysis resistance. The mechanism of the combination of the two catalysts may be:

[0069] (1) Diisopropyl di(acetylacetonyl)titanate (CAS: 17927-72-9, structural formula as follows) contains a weakly polar side chain. On the one hand, it helps to regulate the coordination ability of titanium within a suitable range, ensure appropriate catalytic performance, and make the early polymerization reaction proceed smoothly. If the catalytic efficiency is not matched, it is easy to cause uneven polymerization and affect the steady growth of polyester molecular chains. On the other hand, the weakly polar side chain has a certain binding force to polar groups, which improves the reaction selectivity.

[0070]

[0071] (2) The presence of cellulose nanocrystals can improve the interfacial compatibility between polyester molecules and aluminum oxides, making it easier for aluminum oxides to interact with polyester molecules and promoting the reaction. At the same time, after loading aluminum oxides onto the surface of cellulose nanocrystals, the aggregation of cellulose nanocrystals during the drying process is inhibited (the surface interaction of cellulose nanocrystals leads to the easy aggregation of cellulose nanocrystal dispersions during the drying process in conventional methods), so as to maintain their dispersed morphology and thus utilize cellulose nanocrystals to play a heterogeneous nucleation role, forming fine crystal nuclei.

[0072] (3) The composite catalyst, in combination with diisopropyl di(acetylacetonyl)titanate, achieves a co-catalytic effect, which is beneficial to increasing the molecular weight of polyester.

[0073] In addition, the hydroxyl groups in cellulose nanocrystals can participate in the reaction, further increasing the molecular weight of polyester, reducing crystallinity, and improving light transmittance and mechanical properties.

[0074] 1,4-Cyclohexanediethanol (CAS: 105-08-8) is used as a comonomer to reduce the regularity of the molecular chain. At the same time, the carbon in the cyclohexyl group has a certain degree of rotational freedom, which gives the molecular chain a certain degree of flexibility, thereby improving the stretchability and better balancing light transmittance, toughness and processability. It can be used to prepare high-performance polyester films.

[0075] The esterification reaction temperature is relatively lower, while the polycondensation reaction temperature is higher, which is conducive to the stable progress of the reaction. At the same time, under vacuum conditions, the hydroxyl groups in trimethylolpropane (CAS: 77-99-6) and cellulose nanocrystals participate in the reaction. On the one hand, this can further reduce the regularity of the polyester molecular chain and improve crystallinity. On the other hand, it is conducive to increasing the polyester molecular weight, reducing the residual carboxyl content, and improving mechanical properties and hydrolysis resistance.

[0076] The polyester synthesis method in this embodiment is easy to implement and conducive to industrial production. The synthesized polyester has excellent mechanical properties, light transmittance, hydrolysis resistance and other characteristics, which can expand the application fields of polyester.

[0077] In some embodiments, the diameter of the cellulose nanocrystals is 1 to 10 nm, specifically 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm or any value between them.

[0078] In some embodiments, the length of the cellulose nanocrystals is 100-1000 nm, specifically 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm or any value between them.

[0079] Cellulose nanocrystals are nanoscale fiber crystals extracted from natural fibers. They contain polar groups on their surface and exhibit a nanoscale size effect. Controlling the length and diameter of cellulose nanocrystals within a suitable range is beneficial for loading aluminum oxides and also takes into account nucleation properties.

[0080] In some embodiments, the fibrous nanocrystals in the sol constitute 0.5%-2% by mass. This lower concentration facilitates the formation of a stable dispersion, which is beneficial for uniform loading of aluminum oxide.

[0081] In some embodiments, the aluminum alkoxide is aluminum isopropoxide (CAS: 555-31-7), which can be hydrolyzed to obtain aluminum oxide with high catalytic activity.

[0082] In some embodiments, the mass percentage of aluminum alkoxide in the alcohol solution is 5%-10%. The concentration of aluminum alkoxide should not be too high in order to refine the particle size of aluminum oxide and improve its dispersion uniformity with cellulose nanocrystals.

[0083] In some embodiments, the alcohol in the alcohol solution is selected from at least one of ethylene glycol and 1,2-propanediol.

[0084] In some embodiments, the mass ratio of Al in the alcohol solution to the cellulose nanocrystals in the sol is 1:15-35. Specifically, it can be 1:15, 1:20, 1:25, 1:30, 1:35, or any ratio between them.

[0085] With a higher content of cellulose nanocrystals and a larger specific surface area, aluminum oxide can be well loaded. At the same time, by controlling the mass ratio of Al to cellulose nanocrystals, the cellulose nanocrystals are not easily covered by aluminum oxide, which can give full play to the compatibilizing and nucleation effects of cellulose nanocrystals and improve the catalytic performance and nucleation performance of the composite catalyst.

[0086] In some embodiments, the heating temperature in step S3 is 85-90°C.

[0087] In some embodiments, the pressure applied in step S3 is 0.2-0.5 MPa.

[0088] Within this pressure range, composite catalysts with good performance can be prepared. However, if the pressure is further increased, the equipment requirements become too high, which is not conducive to industrial production.

[0089] In some embodiments, during step S3, the pH is controlled at 8-9, specifically between 8 and 8.5.

[0090] The reaction temperature, pH, and pressure were controlled to obtain aluminum oxides with higher catalytic activity. Meanwhile, cellulose nanocrystals exhibited better stability under suitable temperature and pH conditions, while higher temperatures or strong acid / base conditions promoted the self-polymerization or hydrolysis of cellulose nanocrystals.

[0091] In some embodiments, in step S3, after adding the alcohol solution to the sol, the reaction continues for 0.5-1 h.

[0092] In some embodiments, in step S3, the alcohol solution is added to the sol by dropwise addition.

[0093] Specifically, stirring is performed during the dropwise addition process. There are no special requirements for the stirring rate; 100-300 rpm can be selected. The slow dropwise addition method has an instantaneous dilution effect on the alcohol solution. Combined with stirring, this reduces the dispersed particle size of the aluminum oxide and improves the uniformity of its loading on the cellulose nanocrystals.

[0094] In some embodiments, in step S3, the method for separating the solid phase includes at least one of microfiltration, centrifugation, vacuum distillation, and spray drying.

[0095] In some embodiments, in step S3, the method for separating the solid phase is to sequentially perform vacuum distillation and spray drying.

[0096] Water and low-boiling-point alcohols are removed by vacuum distillation and then concentrated, followed by spray drying to improve drying efficiency and better ensure the dispersion of the solid phase.

[0097] In some embodiments, in step S4, the molar ratio of alcohol to acid in the esterification reaction is 1.4-1.6:1.

[0098] In some embodiments, in step S4, the molar ratio of ethylene glycol to 1,4-cyclohexanediethanol is 20-30:1, for example, it can be 25-30:1.

[0099] The amount of 1,4-cyclohexanediethanol should not be too high, otherwise it may have too much impact on the regularity of the molecular chain, which is not conducive to the formation of microcrystalline regions. In addition, the reactivity of 1,4-cyclohexanediethanol is not as good as that of ethylene glycol. If the content is too high, it may affect the full reaction of the carboxyl group.

[0100] In some embodiments, in step S4, the mass of diisopropyl di(acetylacetonyl)titanate is 10-30 ppm, based on the mass of terephthalic acid.

[0101] In some embodiments, in step S4, the mass of Al in the composite catalyst is 0.005%-0.02% of the mass of terephthalic acid.

[0102] In some embodiments, step S4, the mixing process includes: dispersing the composite catalyst and diisopropyl di(acetylacetonyl)titanate in ethylene glycol, and then adding 1,4-cyclohexanediethanol and terephthalic acid for mixing.

[0103] First, the composite catalyst and diisopropyl di(acetylacetonate) titanate are mixed and dispersed in ethylene glycol to ensure that the two catalysts are pre-dispersed evenly, which is beneficial for the interaction between diisopropyl di(acetylacetonate) titanate and the composite catalyst (diisopropyl di(acetylacetonate) titanate can be adsorbed by aluminum oxide in the composite catalyst or physically bonded to the surface of cellulose nanocrystals by hydrogen bonding), thereby improving the overall catalytic activity.

[0104] In some embodiments, in step S4, the mass of trimethylolpropane is 0.4-1% of the mass of the product obtained from the esterification reaction.

[0105] In some embodiments, in step S4, the esterification reaction is carried out at a temperature of 240-245°C and a pressure of 2.5-4 kPa.

[0106] In some embodiments, in step S4, the reaction time for adding trimethylolpropane is 30-40 minutes. The process parameters such as temperature and pressure in this reaction stage are the same as those for the esterification reaction.

[0107] In some embodiments, in step S4, the temperature of the polycondensation reaction is 270-275°C and the time is 1-2 hours.

[0108] The esterification reaction has a relatively low temperature and suitable catalyst activity, making the reaction more stable. On this basis, the condensation reaction is used to steadily increase the molecular weight and promote the full reaction of the carboxyl groups.

[0109] In some embodiments, in step S4, a vacuum is drawn until the pressure reaches below 100 Pa.

[0110] A second aspect of the present invention provides a polyester prepared by the above-described synthesis method.

[0111] The polyester produced by this method has high mechanical properties and light transmittance, good hydrolysis resistance and certain stretchability, and is suitable for preparing polyester films, such as biaxially oriented polyester films.

[0112] A third aspect of the present invention provides a polyester film, which is prepared from raw materials including the polyester prepared in the foregoing embodiments.

[0113] Given the excellent properties of the prepared polyester, the polyester film made from this polyester also has advantages such as good mechanical properties and high light transmittance.

[0114] It is understandable that polyester is the base resin of polyester film. In addition to polyester, depending on the actual application scenario, additives can be added to better meet the actual use requirements. For example, when used outdoors, additives such as ultraviolet absorbers can be added.

[0115] In some embodiments, the polyester film is a biaxially oriented polyester film.

[0116] The fourth aspect of this invention relates to a method for preparing the above-mentioned polyester film, comprising the steps of:

[0117] The raw materials, including polyester, are prepared by melt extrusion, cooling, and biaxial stretching.

[0118] Polyester is melt-extruded into sheets, and crystalline regions are formed during the cooling process. After biaxial stretching, the molecular chains are oriented or slipped, which improves the mechanical properties. At the same time, the small size of the crystalline regions ensures light transmittance.

[0119] This preparation method is simple, has high production efficiency, and can achieve continuous industrial production.

[0120] In some embodiments, the cooling temperature is 40-50°C.

[0121] Controlling the cooling temperature range can improve crystallization uniformity and enhance the overall performance of the film.

[0122] In some embodiments, the longitudinal stretching temperature of the biaxial stretching is 90-100°C, and the stretching ratio is 3-4:1.

[0123] In some embodiments, the transverse stretching temperature of the biaxial stretching is 110-120°C, and the stretching ratio is 3-4:1.

[0124] In biaxial stretching, longitudinal stretching is generally performed first, followed by transverse stretching. The temperature for transverse stretching is appropriately higher than that for longitudinal stretching to reduce internal defects caused by stretching.

[0125] By adjusting the longitudinal and transverse stretch ratio and the corresponding stretching temperature, higher mechanical properties and light transmittance can be obtained.

[0126] The fifth aspect of the present invention relates to the use of polyester or polyester film in the preparation of packaging products, optical products, electronic products or decorative products.

[0127] Specifically, packaging products include transparent packaging films (including packaging films with partial printed patterns or anti-counterfeiting patterns), optical products include reflective films or lamps, electronic products include liquid crystal displays, and decorative products include architectural or furniture decorative films, etc.

[0128] The following specific examples provide further details.

[0129] In the following examples, the cellulose nanocrystal dispersion was purchased from Zhejiang Jinjiahao Green Nanomaterials Co., Ltd., 4wt% aqueous dispersion, with fiber diameter of 1-10nm and fiber length of 200-800nm.

[0130] Unless otherwise specified, all other raw materials are commercially available standard products.

[0131] Example 1

[0132] This embodiment prepares a polyester, and the specific steps are as follows:

[0133] (1) Dilute the cellulose nanocrystal dispersion with distilled water to a mass concentration of 1% to prepare a sol.

[0134] (2) Dissolve aluminum isopropoxide in ethylene glycol to prepare an alcohol solution with a mass percentage of 10%.

[0135] (3) Add the sol to the reactor, heat to 85°C, pressurize to 0.5 MPa, start stirring to 200 rpm, and then add the alcohol solution dropwise to the hot sol (the mass ratio of Al in the alcohol solution to the mass of cellulose nanocrystals in the sol is 1:28). During the dropwise addition, maintain the pH of the system at 8-8.5 with sodium hydroxide solution. After the dropwise addition is completed, stir at constant temperature, constant pressure and constant speed for 50 min, remove most of the water and isopropanol by vacuum distillation, obtain the concentrated solution, and spray dry to obtain the composite catalyst.

[0136] (4) Ethylene glycol was added to a reaction vessel, along with a composite catalyst and diisopropyl di(acetylacetonate)titanate, which were then dispersed evenly. 1,4-cyclohexanediethanol and terephthalic acid were then added and mixed. The molar ratio of ethylene glycol to 1,4-cyclohexanediethanol was 25:1, and the total alcohol-acid molar ratio was 1.4:1. Based on the mass of terephthalic acid, the amount of diisopropyl di(acetylacetonate)titanate added was 25 ppm, and the amount of Al added to the composite catalyst was 0.008%. The temperature was raised to 245℃, and the reaction chamber pressure was 3.5 kPa. The reaction proceeded until the water output reached 93% of the theoretical output, yielding the intermediate product.

[0137] (5) Add trimethylolpropane to the obtained intermediate product (the mass ratio of intermediate product to trimethylolpropane is 100:0.8), keep the reaction temperature and pressure constant, continue the reaction for 30 min, then evacuate to 50 Pa, react at 275 °C for 1.5 h, purge with nitrogen to restore the pressure of the reactor to normal pressure, terminate the reaction, open the bottom discharge valve of the reactor, purge with nitrogen to extrude the melt, cast into strips, cut into pellets, and obtain polyester.

[0138] Example 2

[0139] This embodiment prepares a polyester, and the specific steps are as follows:

[0140] (1) Dilute the cellulose nanocrystal dispersion with distilled water to a mass concentration of 1.5% to prepare a sol.

[0141] (2) Dissolve aluminum isopropoxide in ethylene glycol to prepare an alcohol solution with a mass percentage of 5%.

[0142] (3) Add the sol to the reactor, heat to 90°C, pressurize to 0.2 MPa, start stirring to 200 rpm, and then add the alcohol solution dropwise to the hot sol (the mass ratio of Al in the alcohol solution to the mass of cellulose nanocrystals in the sol is 1:26). During the dropwise addition, maintain the pH of the system at 8-8.5 with sodium hydroxide solution. After the dropwise addition is completed, stir at constant temperature, constant pressure and constant speed for 30 min, remove most of the water and isopropanol by vacuum distillation, obtain the concentrated solution, and spray dry to obtain the composite catalyst.

[0143] (4) Ethylene glycol was added to a reaction vessel, along with a composite catalyst and diisopropyl di(acetylacetonate)titanate, which were then evenly dispersed. 1,4-cyclohexanediethanol and terephthalic acid were then added and mixed. The molar ratio of ethylene glycol to 1,4-cyclohexanediethanol was 28:1, and the total alcohol-acid molar ratio was 1.35:1. Based on the mass of terephthalic acid, the amount of diisopropyl di(acetylacetonate)titanate added was 30 ppm, and the amount of Al added to the composite catalyst was 0.012%. The temperature was raised to 245℃, and the reaction chamber pressure was 3.5 kPa. The reaction proceeded until the water output reached 93% of the theoretical output, yielding the intermediate product.

[0144] (5) Add trimethylolpropane to the obtained intermediate product (the mass ratio of intermediate product to trimethylolpropane is 100:0.9), keep the reaction temperature and pressure constant, continue the reaction for 30 min, then evacuate to 50 Pa, react at 275 °C for 2 h, purge with nitrogen to restore the pressure of the reactor to normal pressure, terminate the reaction, open the bottom discharge valve of the reactor, purge with nitrogen to extrude the melt, cast into strips, cut into pellets, and obtain polyester.

[0145] Comparative Example 1 (Preparation of composite catalyst without pressure)

[0146] This comparative example prepared a polyester, which differs from Example 1 in that step (3) is as follows:

[0147] The sol was added to the reactor and heated to 85°C. Stirring was started at 200 rpm. Then, an alcohol solution was added dropwise to the hot sol (the mass ratio of Al in the alcohol solution to the mass of cellulose nanocrystals in the sol was 1:25). During the dropwise addition, the pH of the system was maintained at 8-8.5 with sodium hydroxide solution. After the dropwise addition was completed, the mixture was stirred at a constant temperature and speed for 50 min. Most of the water and isopropanol were removed by vacuum distillation to obtain a concentrated solution, which was then spray-dried to obtain the composite catalyst.

[0148] Comparative Example 2 (without trimethylolpropane)

[0149] This comparative example prepared a polyester that differed from Example 1 in that step (5) did not include the addition of trimethylolpropane.

[0150] Example 3

[0151] A biaxially oriented polyester film was prepared by vacuum drying the polyester at 100°C to constant weight, melt extruding it (die temperature 285°C) into sheets, cooling it with a casting roller at 40°C, and then biaxially stretching it with a longitudinal stretching temperature of 90°C and a stretching ratio of 3:1, and a transverse stretching temperature of 115°C and a stretching ratio of 3:1, to produce films with thicknesses of 0.05 mm and 0.02 mm.

[0152] Thin film samples are labeled as follows:

[0153] Film 1: Polyester prepared in Example 1;

[0154] Film 2: Polyester prepared in Example 2.

[0155] Comparative Example 3:

[0156] Thin films were prepared using the polyesters prepared in Comparative Examples 1-3, and the preparation method was the same as in Example 3.

[0157] Thin film samples are labeled as follows:

[0158] Film D1: Polyester prepared in Comparative Example 1;

[0159] Film D2: Polyester prepared in Comparative Example 2;

[0160] Test case

[0161] The intrinsic viscosity and end carboxyl group content of the polyesters prepared in Examples 1-2 and Comparative Examples 1-2 were tested, as were the tensile strength (0.05 mm) and light transmittance (0.02 mm) of the polyester films prepared in Example 3 and Comparative Example 3.

[0162] The testing method is as follows:

[0163] Intrinsic viscosity: GB / T14190-2017, capillary viscometer method.

[0164] Terminal carboxyl group content: GB / T14190-2017, titration method.

[0165] Tensile strength: According to GB / T1040.3-2006, type 2 specimen, specimen size: 15mm*150mm, tensile rate 100mm / min.

[0166] Transmittance: Tested with a UV-Vis spectrophotometer.

[0167] The test results are shown in Table 1.

[0168] Table 1

[0169]

[0170] The results above show that the polyesters prepared in Examples 1 and 2 have high intrinsic viscosity and low end carboxyl group content, indicating a more complete reaction and the ability to obtain polyesters with higher molecular weights. Furthermore, the reduced carboxyl group content is beneficial for improving the hydrolysis resistance of the polyester, extending its durability and service life. In terms of performance, the film samples (film 1 and film 2) prepared using the polyesters from Examples 1 and 2 exhibit higher tensile strength and light transmittance. This may be related to the increased molecular weight and improved crystallinity of the polyester. It is understandable that the refinement of the crystalline regions and a certain degree of crystallinity can reduce the impact on light transmission while ensuring mechanical properties, thus balancing light transmittance. Comparatively, film 2 has higher light transmittance, possibly due to a higher degree of crosslinking, which reduces crystallinity. Simultaneously, the more complete esterification reaction and increased overall molecular weight slightly enhance mechanical properties.

[0171] The performance comparison between Example 1 and Comparative Example 1 shows that applying pressure during the preparation of the composite catalyst significantly increases the intrinsic viscosity and end-carboxyl group content of the polyester prepared using this composite catalyst, resulting in a significant improvement in the strength and transmittance of the prepared film 1. This may be because, under pressure, the growth of aluminum oxide is limited, resulting in smaller particle sizes that can be more uniformly loaded onto the surface of cellulose nanocrystals. This improves the catalytic activity of the aluminum oxide and inhibits the agglomeration of cellulose nanocrystals during drying, facilitating the combination of the composite catalyst with diisopropyl di(acetylacetonate)titanate, thereby enhancing the catalytic effect and nucleation performance of the composite catalyst. Thus, while increasing the molecular weight of the polyester, it induces the formation of fine crystalline regions, improving mechanical properties and transmittance.

[0172] The performance comparison between film 1 and film D2 shows that the addition of trimethylolpropane significantly improves light transmittance. This is because trimethylolpropane has high reactivity, providing crosslinking points and reducing the regularity of the polyester molecular chains, thus decreasing crystallinity. Without the addition of trimethylolpropane, the overall reaction process is also slightly affected, resulting in a slightly higher end-carboxyl group content in the polyester prepared in Comparative Example 2. Furthermore, the lack of trimethylolpropane's crosslinking effect hinders the formation of a large-scale network structure, leading to a decrease in the intrinsic viscosity of the polyester prepared in Comparative Example 2. However, the tensile strength of film D2 is slightly higher than that of film 1, which may be related to the higher crystallinity of film D2.

[0173] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for synthesizing polyester, characterized in that: Includes the following steps: S1. Disperse cellulose nanocrystals in water to form a sol; S2. Dissolve the aluminum alkoxide in alcohol to prepare an alcohol solution; S3. Under heating and pressurization conditions, the alcohol solution is added to the sol to react, the solid phase is separated, and a composite catalyst is obtained. S4. Terephthalic acid, ethylene glycol, 1,4-cyclohexanediethanol, diisopropyl di(acetylacetonyl)titanate and the composite catalyst are mixed and esterified. The reaction is carried out until the water content reaches more than 93% of the theoretical value. Trimethylolpropane is added to the product and reacted. Vacuum is drawn, the temperature is raised and polycondensation reaction is carried out to obtain polyester.

2. The method for synthesizing polyester according to claim 1, characterized in that: The cellulose nanocrystals have a diameter of 1-10 nm; and / or, the cellulose nanocrystals have a length of 100-1000 nm; and / or, the cellulose nanocrystals in the sol have a mass percentage content of 0.5%-2%.

3. A method for synthesizing polyester according to claim 1 or 2, characterized in that: The alcohol solution contains 5%-10% by mass of aluminum alkoxide; and / or the alcohol in the alcohol solution is selected from at least one of ethylene glycol and 1,2-propanediol; and / or the mass ratio of Al to the cellulose nanocrystals in the sol in the alcohol solution is 1:15-35.

4. The method for synthesizing polyester according to claim 1, characterized in that: In step S3, the heating temperature is 85-90℃; and / or the pressurization pressure is 0.2-0.5MPa; and / or the pH is controlled at 8-9 during the reaction; and / or the reaction continues for 0.5-1h after the alcohol solution is added to the sol; and / or the alcohol solution is added to the sol by dropwise addition; and / or the method for separating the solid phase includes at least one of microporous filtration, centrifugation, vacuum distillation, and spray drying.

5. The method for synthesizing polyester according to claim 1, characterized in that: In step S4, the molar ratio of alcohol to acid in the esterification reaction is 1.4-1.6:1; and / or, the molar ratio of ethylene glycol to 1,4-cyclohexanediethanol is 20-30:1; and / or, based on the mass of terephthalic acid, the mass of diisopropyl di(acetylacetonyl)titanate is 10-30 ppm; and / or, the mass of Al in the composite catalyst is 0.005%-0.02% of the mass of terephthalic acid; and / or, the mass of trimethylolpropane is 0.4-1% of the mass of the product obtained from the esterification reaction. %; and / or, the temperature of the esterification reaction is 240-245℃ and the pressure is 2.5-4kPa; and / or, the reaction time for adding trimethylolpropane is 30-40min; and / or, the temperature of the polycondensation reaction is 270-275℃ and the time is 1-2h; and / or, the vacuuming is carried out until the pressure reaches below 100Pa; the mixing process includes: dispersing the composite catalyst and diisopropyl di(acetylacetonate)titanate in ethylene glycol, and then adding 1,4-cyclohexanediethanol and terephthalic acid for mixing.

6. A polyester, characterized in that: The polyester is prepared by the synthesis method described in any one of claims 1-5.

7. A polyester film, characterized in that: The polyester film is made from the polyester as described in claim 6.

8. The method for preparing the polyester film as described in claim 7, characterized in that: Including the following steps: The raw materials, including the polyester, are obtained by melt extrusion, cooling, and biaxial stretching.

9. The method for preparing the polyester film according to claim 8, characterized in that: The longitudinal stretching temperature of the biaxial stretching is 90-100℃, and the stretching ratio is 3-4:1; and / or, the transverse stretching temperature of the biaxial stretching is 110-120℃, and the stretching ratio is 3-4:1; and / or, the cooling temperature is 40-50℃.

10. The use of the polyester as described in claim 6 or the polyester film as described in claim 7 in the preparation of packaging products, optical products, electronic products or decorative products.

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