A lignin-based modified polyester material with branched structure and a preparation method thereof

Abstract

The application discloses a kind of lignin-based modified polyester materials with branched structure and preparation method thereof, belong to the technical field of polyester material processing, for solving the technical problems that the mechanical properties and thermal stability of polyester material prepared by blending lignin and polyester in the prior art need to be further improved, specifically comprising the following components by weight parts: PBCT 40-60 parts, epoxy modified PBCT 25-35 parts, hyperbranched lignin 20-30 parts, 2,4,6-tris (4-aminophenyl) -1,3,5-triazine 4-5 parts and 1-3 parts of additive. The present application is to construct hyperbranched structure after carbonylation of lignin, hyperbranched lignin, epoxy modified PBCT and polyamine triazine cooperate with each other, effectively improve the mechanical properties and heat resistance of the material, while maintaining the inherent biodegradable properties of polyester system.

Description

Technical Field

[0001] This invention relates to the field of polyester material processing technology, specifically to a lignin-based modified polyester material with a branched structure and its preparation method. Background Technology

[0002] Lignin, as the second largest biomass resource in nature after cellulose, is a highly promising renewable aromatic polymer. Its molecular structure is rich in various active groups such as aromatic groups, phenolic hydroxyl groups, alcoholic hydroxyl groups, and carbonyl groups. These active groups can theoretically participate in material construction through various chemical reactions, providing possibilities for its application in the field of polymer materials.

[0003] However, lignin faces many challenges in practical applications. Raw wood lignin is insoluble in water and most organic solvents and cannot be hydrolyzed into monomers. Furthermore, its amorphous and disordered structure makes lignin poorly compatible in organic systems, and direct blending easily leads to phase separation.

[0004] Poly(1,4-Butanediol-Co-terephthalate) (PBCT) is a copolymer prepared by the polycondensation reaction of 1,4-butanediol, dimethyl carbonate, and dimethyl terephthalate. As a carbon dioxide-based polymer, it meets the current requirements of sustainable development for green and environmentally friendly materials. However, PBCT is difficult to meet the needs of high-end applications in terms of strength and toughness. In addition, PBCT is relatively brittle, which limits its use in applications that require high flexibility and impact resistance. Secondly, although the thermal stability of PBCT is better than that of pure PBC, it is still inferior to that of traditional engineering plastics such as polycarbonate, which also limits its use in high-temperature environments.

[0005] Currently, blending lignin with PBCT polyester materials is an effective way to improve lignin utilization and PBCT performance. However, the method of blending lignin with polyester materials such as PBCT has many drawbacks. For example, patent application CN101555311A involves a method for preparing lignin composite polyester materials. It uses simple pretreated alkaline lignin and lignin sulfonate to simply blend with polybutylene succinate (PBS) in a mixer or high-temperature mixer. Without adding 2,4-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate or hexamethylene diisocyanate as crosslinking agents, as the proportion of lignin in the composite material increases from 5% to 50%, the strength and elongation at break of the composite material decrease by 30% and 70%, respectively. Adding crosslinking agents further reduces the elongation at break of the composite material. This method does not effectively increase the compatibility of the two phases after simple pretreatment of lignin, and the choice of crosslinking agent is not matched with the composite material.

[0006] For example, patent application CN116144187A discloses a method for preparing lignin and PBAT composite materials. First, polylactic acid is modified using a vinyl silane coupling agent to obtain vinyl-modified polylactic acid. Then, lignin is modified using cysteine ​​and glutaraldehyde to obtain modified lignin. Finally, the two modified substances are dissolved in an organic solvent and subjected to a click reaction with azobisisobutyronitrile to obtain a polylactic acid-lignin composite. The obtained composite is then mixed evenly with PBAT, modified calcium carbonate whiskers, barium stearate, and an antioxidant, and fed into a mixer to obtain a composite material with excellent lignin and PBAT compatibility. This composite material possesses strength, toughness, and biodegradability. While this method of improving the compatibility of lignin and polyester by introducing multiple components as compatibilizers effectively improves the compatibility of the two phases, its process is extremely complex, requires a high degree of material pretreatment, and significantly increases production costs.

[0007] For example, patent application CN113736129A involves a method for preparing a biodegradable polyester composite bead foam material with a high crystallization rate containing lignin. First, polybutylene adipate terephthalate is melt-blended with anhydride compounds and peroxide crosslinking agents to obtain MAH-g-PBAT particles. Second, the particles, biodegradable resin, lignin and chain extender are blended and granulated to obtain biodegradable polyester composite beads. Finally, the biodegradable polyester composite beads are placed in a high-pressure mold and foamed using foaming gas as a foaming agent. This invention also improves the compatibility of lignin and polyester by performing complex pretreatment of the material, and its industrialization value is not high.

[0008] In summary, traditional blending processes rely on chain extenders or crosslinking agents to improve the compatibility of lignin and polyester phases. However, these methods are prone to triggering gelation side reactions, leading to uncontrolled crosslinking of the copolyester matrix. This not only further reduces the mechanical properties of the composite material but also results in insufficient interfacial bonding. Therefore, a solution is proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a lignin-based modified polyester material with a branched structure and its preparation method, which solves the technical problem that the mechanical properties and thermal stability of polyester materials prepared by blending lignin and polyester in the prior art need to be further improved.

[0010] The objective of this invention can be achieved through the following technical solution: a lignin-based modified polyester material with a branched structure, comprising the following components by weight: 40-60 parts of PBCT, 25-35 parts of epoxy-modified PBCT, 20-30 parts of hyperbranched lignin, 4-5 parts of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine, and 1-3 parts of additives;

[0011] The hyperbranched lignin is prepared by carbonylating lignin with carbonate under the action of an alkaline catalyst using lignin as raw material, and then hyperbranching the carbonylated lignin with 1,4-butanediol and citric acid as hyperbranching modifiers.

[0012] The epoxy-modified PBCT is a PBCT with epoxy side chain modification.

[0013] Furthermore, the preparation method of epoxy-modified PBCT is as follows: PBCT, glycidyl methacrylate, initiator, and antioxidant are added to a twin-screw extruder, melt-mixed for 2-3 minutes, extruded, and granulated after natural cooling to obtain epoxy-modified PBCT.

[0014] Furthermore, the weight ratio of PBCT, glycidyl methacrylate, initiator, and antioxidant is 100:3-4:0.2-0.3:0.1, the initiator is dicumyl peroxide, the antioxidant is hindered phenolic antioxidant, and the temperature of the twin-screw extruder is 170℃ in zone I, 175℃ in zone II, 175℃ in zone III, 175℃ in zone VI, 175℃ in zone V, 180℃ in zone IV, and the die temperature is 185℃.

[0015] Furthermore, the preparation method of carbonylated lignin is as follows: lignin, dimethyl carbonate solution and alkaline catalyst are mixed and stirred, the reaction system is heated to 70-90℃, the pressure is increased to 0.1-0.5MPa, the reaction is maintained at temperature and pressure for 3-5h, and then post-processed to obtain carbonylated lignin.

[0016] The synthetic reaction formula for carbonylated lignin is as follows:

[0017]

[0018] In the formula: It is lignin, and R is or .

[0019] Furthermore, the ratio of lignin, dimethyl carbonate solution, and alkaline catalyst is 1g:3-5mL:0.01-0.05g. The dimethyl carbonate solution is composed of anhydrous ethanol, purified water, and dimethyl carbonate in a weight ratio of 8:3:45-75. The alkaline catalyst is one or more of potassium carbonate, lithium carbonate, cesium carbonate, potassium hydroxide, and sodium hydroxide. The post-treatment includes: after the reaction is complete, the reaction solution is placed in a 3000Da dialysis bag, and then the dialysis bag is placed in anhydrous ethanol at a temperature of 50-60℃ for dialysis and soaking for 20-24 hours. During the dialysis process, the anhydrous ethanol soaking solution is replaced every 4-5 hours. After dialysis, the residue in the dialysis bag is placed in a drying oven at a temperature of 60-70℃ and dried to constant weight to obtain carbonylated lignin.

[0020] Furthermore, the preparation method of hyperbranched lignin is as follows: under the protection of an inert gas atmosphere, carbonylated lignin, 1,4-butanediol, citric acid and esterification catalyst are mixed and stirred, the reaction system is heated to 150-180℃, and the reaction is maintained at this temperature for 5-8 hours. After post-treatment, hyperbranched lignin is obtained.

[0021] The synthesis reaction formula for hyperbranched lignin is as follows:

[0022]

[0023] Furthermore, the weight ratio of the carbonylated lignin, 1,4-butanediol, citric acid, and esterification catalyst is 2:3-4:7-8:0.03-0.05, and the esterification catalyst is either tetrabutyl titanate or titanium isopropoxide. The post-treatment includes: after the reaction is complete, the reaction system is cooled to room temperature, the reaction solution is placed in a 5000 Da dialysis bag, and then the dialysis bag is placed in anhydrous ethanol at a temperature of 50-60℃ for dialysis and soaking for 20-24 hours. During the dialysis process, the anhydrous ethanol soaking solution is replaced every 4-5 hours. After dialysis, the residue in the dialysis bag is placed in a drying oven at a temperature of 60-70℃ and dried to constant weight to obtain hyperbranched lignin.

[0024] The present invention also proposes a method for preparing a lignin-based modified polyester material with a branched structure, comprising the following steps: adding PBCT, epoxy-modified PBCT, hyperbranched lignin, 4,4'-diaminodiphenylmethane and additives into a twin-screw extruder, melting and mixing for 3-6 minutes, then melting and extruding and granulating to obtain the modified polyester material.

[0025] Furthermore, the additives are composed of lubricant, dispersant, heat stabilizer and antioxidant in a weight ratio of 1:2:1:1. The lubricant is ethylene bis-stearamide, the dispersant is stearate, the heat stabilizer is triethyl phosphite, and the antioxidant is any one or more of antioxidant 264, antioxidant RD and antioxidant NBC. The twin-screw extruder has a zone temperature of 165℃, a zone temperature of 170℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a zone temperature of 175℃, a die temperature of 180℃.

[0026] The present invention has the following beneficial effects:

[0027] 1. This invention carbonylates lignin and further constructs a hyperbranched structure, transforming lignin from an inert filler into a functional filler with reactive network nodes. This allows the hyperbranched lignin to be uniformly dispersed in the PBCT matrix in a multi-point anchoring manner, avoiding severe phase separation of lignin due to polarity differences. This enables it to form stable interfacial bonds within the material, thereby effectively reducing interfacial defects and stress concentration, improving tensile strength and notched impact performance, and providing support for structural stability under high-temperature loads.

[0028] 2. This invention also combines epoxy-modified PBCT with hyperbranched lignin. During melt extrusion, epoxy-modified PBCT provides reactive epoxy sites that undergo esterification, transesterification, or ring-opening reactions with the terminal carboxyl groups or hydroxyl groups unique to hyperbranched lignin. By removing small molecule byproducts, chemical bonds are formed. The reactive bridging mechanism transforms the simple physical contact between the lignin phase and the polyester continuous phase into a chemically bonded interface, significantly enhancing stress transfer efficiency and interface stability. This results in a simultaneous increase in tensile strength and simply supported beam impact strength on a macroscopic scale, and effectively suppresses performance degradation caused by interface failure.

[0029] 3. This invention also introduces a triazine compound with a polyamine structure. The polyamine groups in the triazine molecule can react with epoxy-modified PBCT, serving as multifunctional reaction nodes to further improve the branched / lightly crosslinked network structure. Its rigid aromatic structure helps to improve the material's ability to restrict chain segment movement under load, thereby significantly increasing the heat distortion temperature. At the same time, this network structure is mainly formed through interfacial reactions and chain branching, without introducing a large number of non-degradable main chain structures, enabling the material to maintain a high biodegradability under industrial composting conditions. Thus, the triazine compound with the polyamine structure works synergistically with epoxy-modified PBCT and hyperbranched lignin to improve the material's mechanical properties, heat resistance, and biodegradability. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] In this application, the lignin is hardwood lignin, softwood lignin or herbaceous lignin, with a purity ≥90%, a weight-average molecular weight of 1000~5000g / mol, and a hydroxyl content of 5.0-8.0mmol / g;

[0032] In this application, PBCT is poly(butylene terephthalate-co-butylene adipate), with an intrinsic viscosity of 0.8-1.2 dL / g, a melt index of 10-25 g / 10 min (190℃, 2.16 kg), and a terminal carboxyl group content ≤30 mg KOH / kg.

[0033] Example 1

[0034] This embodiment provides a method for preparing a lignin-based modified polyester material with a branched structure, specifically including the following steps:

[0035] Step S1: Preparation of carbonylated lignin

[0036] Anhydrous ethanol, purified water and dimethyl carbonate were mixed evenly in a weight ratio of 8:3:45 to obtain a dimethyl carbonate solution.

[0037] Weigh out 100g of lignin, 300mL of dimethyl carbonate solution and 1g of potassium hydroxide and add them to a reaction flask. Stir the mixture and seal the flask. Heat the flask to 70℃ and maintain the pressure at 0.1MPa for 3 hours. Cool the flask to room temperature and place the reaction solution in a 3000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 50℃ for dialysis and soak for 20 hours. Replace the anhydrous ethanol soaking solution every 4 hours during dialysis. After dialysis, place the residue in the dialysis bag in a drying oven at 60℃ and dry to constant weight to obtain carbonylated lignin.

[0038] During the reaction, under alkaline catalysis, the phenolic and aliphatic hydroxyl groups on the lignin molecule are partially deprotonated to form a strongly nucleophilic oxygen anion. This oxygen anion nucleophilically substitutes the carbonyl carbon of dimethyl carbonate, and the lignin oxygen bond is attached to the carbonate group. At the same time, small molecule byproducts such as methanol are released, resulting in the carbonated structure of lignin. The mixed solvent of ethanol and water mainly plays the roles of swelling, mass transfer and mild medium, making it easier for lignin to expose hydroxyl groups and improving the uniformity of the reaction. Subsequently, dialysis is used to remove unreacted small molecules, salts and byproducts to obtain purer carbonylated lignin.

[0039] Carbonylation does not directly build a network, but it makes the lignin surface more susceptible to reaction with the subsequent polycondensation system, providing a reaction basis for the formation of branched structures.

[0040] Step S2: Preparation of hyperbranched lignin

[0041] Weigh out 100g of carbonylated lignin, 150g of 1,4-butanediol, 350g of citric acid, and 0.15g of tetrabutyl titanate and add them to an argon-protected reaction flask. Stir the mixture and heat the flask to 150℃. Maintain the temperature for 5 hours. Cool the flask to room temperature and place the reaction solution in a 5000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 50℃ for dialysis and soak for 20 hours. During dialysis, change the anhydrous ethanol soaking solution every 4 hours. After dialysis, place the residue in the dialysis bag in a drying oven at 60℃ and dry it to constant weight to obtain hyperbranched lignin.

[0042] During the reaction, citric acid contains multiple carboxyl and hydroxyl groups, and 1,4-butanediol provides dihydroxyl groups. Under the catalysis of tetrabutyl titanate, the hydroxyl groups condense with the carboxyl groups to form ester bonds or undergo transesterification with the carbonate on the carbonylated lignin molecules, removing small molecule byproducts such as water. Because citric acid is multifunctional, the reaction does not grow linearly but continuously generates branching points, gradually constructing a three-dimensional branched framework and forming a multi-branched chain structure with lignin as the core or node. The hyperbranched structure of lignin brings a large number of terminal carboxyl and hydroxyl groups, giving lignin a higher density of reaction sites, enabling it to form more chemical bonds with polyester segments during extrusion. Moreover, branched molecules are more likely to form a fine dispersion in polyester, reducing the agglomeration and interfacial debonding common in original lignin, reducing notch-sensitive interfacial defects from the source. Furthermore, hyperbranched lignin in the system is no longer just a filler but more like a reactive three-dimensional node, which is conducive to transferring stress from the continuous polyester phase to the lignin phase and redispersing it back to the matrix, achieving synergy between load-bearing and energy dissipation.

[0043] Step S3: Preparation of epoxy-modified PBCT

[0044] Weigh out the following components by weight: 100 parts PBCT, 3 parts glycidyl methacrylate, 0.2 parts dicumyl peroxide, and 0.1 parts antioxidant 1076. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder as follows: Zone I: 170℃, Zone II: 175℃, Zone III: 175℃, Zone VI: 175℃, Zone V: 175℃, Zone IV: 180℃, and the die temperature to 185℃. After melting and mixing for 2 minutes, extrude the mixture, allow it to cool naturally, and then granulate to obtain epoxy-modified PBCT.

[0045] During the preparation process, dicumyl peroxide, acting as an initiator, decomposes at the extrusion temperature to generate free radicals. These free radicals abstract hydrogen atoms from polyester segments or react with unsaturated bonds to generate polyester chain free radicals. Glycidyl methacrylate contains carbon-carbon double bonds that can be polymerized by free radicals, forming a graft structure that fixes epoxy-containing side chains onto the polyester molecules, thus preparing epoxy-modified PBCT. In subsequent blending, the epoxy groups can undergo ring-opening reactions with carboxyl, hydroxyl, and amine groups, improving the chemical compatibility of the material. When the epoxy groups react with the terminal carboxyl or hydroxyl groups of hyperbranched lignin, covalent bonds are formed between the lignin and polyester phases, significantly reducing the tendency for phase separation, improving melt strength and high-temperature modulus retention, and making the material less prone to deformation due to chain slip during heat distortion testing.

[0046] Step S4: Preparation of modified polyester material

[0047] Lubricant ethylene bis-stearamide, dispersant zinc stearate, heat stabilizer triethyl phosphite and antioxidant 264 are mixed evenly in a ratio of 1:2:1:1 to obtain the additive.

[0048] Weigh out the following components by weight: 40 parts of PBCT, 25 parts of epoxy-modified PBCT, 20 parts of hyperbranched lignin, 4 parts of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine, and 1 part of additives. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder to 165℃ in zone I, 170℃ in zone II, 175℃ in zone III, 175℃ in zone VI, 175℃ in zone V, and 175℃ in zone IV. Set the die temperature to 180℃. After melt mixing for 3 minutes, melt extrude the mixture and granulate it to obtain the modified polyester material.

[0049] During the preparation process, hyperbranched lignin contains a large number of terminal carboxyl and hydroxyl groups. At the extrusion temperature, it can undergo esterification with the terminal hydroxyl groups of the polyester chain to generate new ester bonds between lignin and polyester, while removing small molecule byproducts such as water. In addition, terminal hydroxyl or carboxyl groups can also undergo transesterification with ester or carbonate groups in the polyester backbone, causing local rearrangement of polyester chain segments and forming new covalent linkages between lignin and polyester. This allows lignin to gradually transform from a dispersed phase into a component that is "chemically entangled" with the continuous phase, thereby reducing interfacial debinding. Epoxy-modified PBCT carries epoxy groups, hyperbranched lignin provides carboxyl and hydroxyl groups, and triazine provides multiple amino groups. During extrusion, epoxy groups can be attacked by carboxyl, hydroxyl, or amino groups to undergo ring-opening, forming new hydroxyl groups and generating linkage structures such as ester bonds, ether bonds, or amino alcohol bonds. The multiple amino groups of triazine amino groups can easily form multi-point linkages, further improving the network.

[0050] Under external force, the load is no longer concentrated at the interface defect, but is distributed and transferred between the lignin nodes and the polyester matrix through covalent connections, which improves the tensile strength of the material. The improved network and interface reduce the brittle crack initiation caused by phase separation. At the same time, the branched structure and multi-point connection make the crack propagation path more tortuous and dissipate more energy, thereby improving the notched impact strength. The cross-linked network restricts the chain segment movement at high temperature, the epoxy reaction and polyamine nodes improve the ability to maintain high temperature modulus, and the lignin aromatic rigid structure further enhances the resistance to deformation. The multi-component system works together to improve the heat distortion temperature of the material.

[0051] Example 2

[0052] This embodiment provides a method for preparing a lignin-based modified polyester material with a branched structure, specifically including the following steps:

[0053] Step S1: Preparation of carbonylated lignin

[0054] Anhydrous ethanol, purified water and dimethyl carbonate were mixed evenly in a weight ratio of 8:3:60 to obtain a dimethyl carbonate solution.

[0055] Weigh out 100g of lignin, 400mL of dimethyl carbonate solution, and 3g of sodium hydroxide and add them to a reaction flask. Stir the mixture and seal the flask. Heat the flask to 80℃ and maintain the pressure at 0.3MPa for 4 hours. Cool the flask to room temperature and place the reaction solution in a 3000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 55℃ for dialysis and soak for 22 hours. Replace the anhydrous ethanol solution every 4.5 hours during dialysis. After dialysis, place the residue in the dialysis bag in a drying oven at 65℃ and dry to constant weight to obtain carbonylated lignin.

[0056] Step S2: Preparation of hyperbranched lignin

[0057] Weigh out 100g of carbonylated lignin, 175g of 1,4-butanediol, 375g of citric acid, and 0.20g of titanium isopropoxide and add them to an argon-protected reaction flask. Stir the mixture and heat the flask to 165℃. Maintain the temperature for 6.5h. Cool the flask to room temperature and place the reaction solution in a 5000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 55℃ for dialysis and soak for 22h. During dialysis, change the anhydrous ethanol soaking solution every 4.5h. After dialysis, place the residue in the dialysis bag in a drying oven at 65℃ and dry to constant weight to obtain hyperbranched lignin.

[0058] Step S3: Preparation of epoxy-modified PBCT

[0059] Weigh out the following components by weight: 100 parts PBCT, 3.5 parts glycidyl methacrylate, 0.25 parts dicumyl peroxide, and 0.1 parts antioxidant 1076. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder to 170℃ in zone I, 175℃ in zone II, 175℃ in zone III, 175℃ in zone VI, 175℃ in zone V, and 180℃ in zone IV. Set the die temperature to 185℃. After melting and mixing for 2.5 minutes, extrude the mixture. After natural cooling, granulate the extruded mixture to obtain epoxy-modified PBCT.

[0060] Step S4: Preparation of modified polyester material

[0061] Lubricant ethylene bis-stearamide, dispersant calcium stearate, heat stabilizer triethyl phosphite and antioxidant RD are mixed evenly in a ratio of 1:2:1:1 to obtain the additive.

[0062] Weigh out the following components by weight: 50 parts of PBCT, 30 parts of epoxy-modified PBCT, 25 parts of hyperbranched lignin, 4.5 parts of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine, and 2 parts of additives. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder as follows: Zone I: 165℃, Zone II: 170℃, Zone III: 175℃, Zone VI: 175℃, Zone V: 175℃, Zone IV: 175℃, and the die temperature to 180℃. After melt mixing for 4.5 minutes, melt extrude the mixture and granulate it to obtain the modified polyester material.

[0063] Example 3

[0064] This embodiment provides a method for preparing a lignin-based modified polyester material with a branched structure, specifically including the following steps:

[0065] Step S1: Preparation of carbonylated lignin

[0066] Anhydrous ethanol, purified water and dimethyl carbonate were mixed evenly in a weight ratio of 8:3:75 to obtain a dimethyl carbonate solution.

[0067] Weigh out 100g of lignin, 500mL of dimethyl carbonate solution, and 5g of potassium carbonate and add them to a reaction flask. Stir the mixture and seal the flask. Heat the flask to 90℃ and maintain the pressure at 0.5MPa for 5 hours. Cool the flask to room temperature and place the reaction solution in a 3000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 60℃ for dialysis and soak for 24 hours. Replace the anhydrous ethanol solution every 5 hours during dialysis. After dialysis, place the residue in the dialysis bag in a drying oven at 70℃ and dry to constant weight to obtain carbonylated lignin.

[0068] Step S2: Preparation of hyperbranched lignin

[0069] Weigh out 100g of carbonylated lignin, 200g of 1,4-butanediol, 400g of citric acid, and 0.25g of tetrabutyl titanate and add them to an argon-protected reaction flask. Stir the mixture and heat the flask to 180℃. Maintain the temperature for 8 hours. Cool the flask to room temperature and place the reaction solution in a 5000Da dialysis bag. Then, place the dialysis bag in anhydrous ethanol at 60℃ for dialysis and soak for 24 hours. During dialysis, change the anhydrous ethanol soaking solution every 5 hours. After dialysis, place the residue in the dialysis bag in a drying oven at 70℃ and dry it to constant weight to obtain hyperbranched lignin.

[0070] Step S3: Preparation of epoxy-modified PBCT

[0071] Weigh out the following components by weight: 100 parts PBCT, 4 parts glycidyl methacrylate, 0.3 parts dicumyl peroxide, and 0.1 parts antioxidant 1076. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder to 170℃ in zone I, 175℃ in zone II, 175℃ in zone III, 175℃ in zone VI, 175℃ in zone V, and 180℃ in zone IV. Set the die temperature to 185℃. After melting and mixing for 3 minutes, extrude the mixture. After natural cooling, granulate to obtain epoxy-modified PBCT.

[0072] Step S4: Preparation of modified polyester material

[0073] Lubricant ethylene bis-stearamide, dispersant sodium stearate, heat stabilizer triethyl phosphite and antioxidant NBC are mixed evenly in a ratio of 1:2:1:1 to obtain the additive.

[0074] Weigh out the following components by weight: 60 parts of PBCT, 35 parts of epoxy-modified PBCT, 30 parts of hyperbranched lignin, 5 parts of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine, and 3 parts of additives. Add these components to a twin-screw extruder. Set the temperatures of the twin-screw extruder as follows: Zone I: 165℃, Zone II: 170℃, Zone III: 175℃, Zone VI: 175℃, Zone V: 175℃, Zone IV: 175℃, and the die temperature to 180℃. After melt mixing for 6 minutes, melt extrude the mixture and granulate it to obtain the modified polyester material.

[0075] Comparative Example 1

[0076] The difference between this comparative example and Example 3 is that the lignin prepared in step S1 is used instead of the hyperbranched lignin in step S4.

[0077] Comparative Example 2

[0078] The difference between this comparative example and Example 3 is that the carbonylated lignin prepared in step S1 is used instead of the hyperbranched lignin in step S4.

[0079] Comparative Example 3

[0080] The difference between this comparative example and Example 3 is that step S3 is omitted, and the epoxy-modified PBCT in step S4 is replaced by an equal amount of PBCT.

[0081] Comparative Example 4

[0082] The difference between this comparative example and Example 3 is that 2,4,6-tris(4-aminophenyl)-1,3,5-triazine was not added in step S4.

[0083] Performance testing:

[0084] The tensile strength of the modified polyester materials prepared in Examples 1-3 and Comparative Examples 1-4 was determined in accordance with the standard GB / T 1040.1-2025 "Determination of tensile properties of plastics - Part 1: General".

[0085] Referring to standard GB / T 1043.2-2018 "Determination of impact properties of simply supported beams of plastics - Part 2: Instrumented impact test", the modified polyester material samples prepared in Examples 1-3 and Comparative Examples 1-4 were prepared into notched impact test specimens, and the simply supported beam impact strength of the specimens was determined.

[0086] The heat distortion temperature of the modified polyester materials prepared in Examples 1-3 and Comparative Examples 1-4 under a load of 0.45 MPa was determined in accordance with the standard GB / T 1634.1-2025 "Determination of Deflection Temperature of Plastics under Load - Part 1: General Test Method".

[0087] The biodegradability of the modified polyester materials prepared in Examples 1-3 and Comparative Examples 1-4 was determined under industrial composting conditions for 6 months in accordance with the standard GB / T 41010-2021 "Degradation Performance and Labeling Requirements of Biodegradable Plastics and Products". The specific test data are shown in Table 1 below.

[0088] Table 1 - Performance Test Data of Samples

[0089]

[0090] Data Analysis:

[0091] Comparative analysis of the data in Table 1 shows that the modified polyester material prepared by this invention has a tensile strength of 38.5-41.2 MPa, a simply supported beam impact strength of 15.6-163 kJ / m2, and a heat distortion temperature of 105-108℃. Under industrial composting conditions, the biodegradation rate reaches 92-94% after 6 months. All performance test data are superior to the comparative example. This indicates that the present invention constructs hyperbranched lignin by carbonylating lignin and then using citric acid and 1,4-butanediol to undergo esterification and polycondensation with its hydroxyl groups under catalytic conditions. The hyperbranched lignin, together with epoxy-modified PBCT and polyamine triazine, forms a stable cross-phase chemical bond and branched network structure, which significantly improves the dispersion and interfacial bonding state of lignin in the polyester matrix, enhances the mechanical properties and heat resistance of the material, and maintains the inherent biodegradable characteristics of the polyester system, achieving a synergistic balance between performance improvement and green attributes.

[0092] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A lignin-based modified polyester material with a branched structure, characterized in that, It includes the following components by weight: 40-60 parts of PBCT, 25-35 parts of epoxy-modified PBCT, 20-30 parts of hyperbranched lignin, 4-5 parts of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1-3 parts of additives. The hyperbranched lignin is prepared by carbonylating lignin with carbonate under the action of an alkaline catalyst using lignin as raw material, and then hyperbranching the carbonylated lignin with 1,4-butanediol and citric acid as hyperbranching modifiers. The epoxy-modified PBCT is a PBCT with epoxy side chain modification.

2. The lignin-based modified polyester material with a branched structure according to claim 1, characterized in that, The preparation method of epoxy-modified PBCT is as follows: PBCT, glycidyl methacrylate, initiator and antioxidant are added to a twin-screw extruder, melt-mixed for 2-3 minutes and then extruded, naturally cooled and granulated to obtain epoxy-modified PBCT.

3. The lignin-based modified polyester material with a branched structure according to claim 2, characterized in that, The weight ratio of PBCT, glycidyl methacrylate, initiator, and antioxidant is 100:3-4:0.2-0.3:0.1, wherein the initiator is dicumyl peroxide and the antioxidant is hindered phenolic antioxidant.

4. The lignin-based modified polyester material with a branched structure according to claim 1, characterized in that, The preparation method of carbonylated lignin is as follows: lignin, dimethyl carbonate solution and alkaline catalyst are mixed and stirred, the reaction system is heated to 70-90℃, the pressure is increased to 0.1-0.5MPa, the reaction is maintained at temperature and pressure for 3-5h, and then post-processed to obtain carbonylated lignin.

5. A lignin-based modified polyester material with a branched structure according to claim 4, characterized in that, The ratio of lignin, dimethyl carbonate solution, and alkaline catalyst is 1g:3-5mL:0.01-0.05g. The dimethyl carbonate solution is composed of anhydrous ethanol, purified water, and dimethyl carbonate in a weight ratio of 8:3:45-75. The alkaline catalyst is one or more of potassium carbonate, lithium carbonate, cesium carbonate, potassium hydroxide, and sodium hydroxide.

6. The lignin-based modified polyester material with a branched structure according to claim 1, characterized in that, The preparation method of hyperbranched lignin is as follows: under the protection of an inert gas atmosphere, carbonylated lignin, 1,4-butanediol, citric acid and esterification catalyst are mixed and stirred, the reaction system is heated to 150-180℃, and the reaction is maintained at the temperature for 5-8 hours. After post-treatment, hyperbranched lignin is obtained.

7. A lignin-based modified polyester material with a branched structure according to claim 6, characterized in that, The weight ratio of the carbonylated lignin, 1,4-butanediol, citric acid and esterification catalyst is 2:3-4:7-8:0.03-0.05, and the esterification catalyst is either tetrabutyl titanate or titanium isopropoxide.

8. A method for preparing a lignin-based modified polyester material with a branched structure according to any one of claims 1-7, characterized in that, The process includes the following steps: adding PBCT, epoxy-modified PBCT, hyperbranched lignin, 4,4'-diaminodiphenylmethane, and additives into a twin-screw extruder, melting and mixing for 3-6 minutes, then melt-extruding and granulating to obtain the modified polyester material.

9. The method for preparing a lignin-based modified polyester material with a branched structure according to claim 8, characterized in that, The additives consist of a lubricant, a dispersant, a heat stabilizer, and an antioxidant in a weight ratio of 1:2:1:

1. The lubricant is ethylene bis-stearamide, the dispersant is stearate, the heat stabilizer is triethyl phosphite, and the antioxidant is any one or more of antioxidant 264, antioxidant RD, and antioxidant NBC.

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