A heat-resistant modifier for PVC sheet material and a method for preparing the same

Abstract

The application belongs to the technical field of heat-resistant modifiers, and particularly relates to a PVC sheet heat-resistant modifier and a preparation method thereof. The heat-resistant modifier is prepared by Diels-Alder reaction of a furan group-containing lignin derivative and N-sulfonyl maleimide; wherein, carboxymethylated lignin is used as a rigid skeleton, which not only improves the compatibility with the PVC sheet matrix, but also the grafted furan group thereof participates in the reaction as a diene; the sulfonyl group in the N-sulfonyl maleimide significantly improves the electrophilicity of the double bond through strong electron-withdrawing effect, so that the N-sulfonyl maleimide becomes a highly efficient dienophile; the two are connected through a thermally reversible Diels-Alder reaction to build a dynamic crosslinking network, so that the contradiction between the heat resistance and processability of the material is solved; the network is dissociated at a high processing temperature to ensure good fluidity; the network is reformed at a use temperature, so that the heat resistance temperature, Vicat softening point and heat distortion temperature of the PVC sheet are significantly improved. The modifier realizes the unification of renewable resource utilization and high performance, and has environmental protection and practical value.

Description

Technical Field

[0001] This invention belongs to the field of heat-resistant modifier technology, specifically relating to a heat-resistant modifier for PVC sheets and its preparation method. Background Technology

[0002] Polyvinyl chloride (PVC) is a widely used thermoplastic, but its heat distortion temperature is low and its long-term service temperature generally does not exceed 60°C. This characteristic limits the application of PVC in applications requiring high heat resistance. In order to improve the heat resistance of PVC products and expand their application range, adding heat-resistant modifiers has become an effective and commonly used method.

[0003] Currently, commonly used PVC heat-resistant modifiers mainly include N-substituted maleimide copolymers, methylstyrene polymers, heat-resistant engineering plastics, and inorganic fillers. However, these modifiers have common drawbacks. First, while improving the heat resistance of the matrix, they may reduce the material's impact strength and processing fluidity. Second, the compatibility between heat-resistant modifiers and the PVC matrix is ​​not ideal, potentially leading to phase separation and affecting material uniformity and final performance. The invention patent with publication number CN110407978B uses homopolymers or copolymers containing N-(fluorophenyl)maleimide monomers to improve the heat resistance of various plastics; however, this approach requires an additional irradiation treatment process, increasing the complexity of the production process. Furthermore, fluoropolymers may face challenges in thermal stability during high-temperature processing.

[0004] N-substituted maleimide homopolymers are the most commonly used heat-resistant modifiers; however, their poor processing flowability can lead to poor mechanical properties and processing difficulties in the finished material. Lignin, as an environmentally friendly renewable resource, possesses an inherent rigid aromatic structure that provides excellent thermal stability and mechanical strengthening potential. However, lignin is highly polar and has strong intermolecular hydrogen bonds, making it prone to aggregation and difficult to disperse in the PVC matrix, potentially negatively impacting the material's mechanical properties. Therefore, addressing the compatibility of heat-resistant modifiers in the matrix and their flowability during thermal processing to synergistically improve the final material properties has become a crucial issue that urgently needs to be resolved. Summary of the Invention

[0005] The purpose of this invention is to provide a heat-resistant modifier for PVC sheets and its preparation method, so as to solve the above-mentioned technical problems.

[0006] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows:

[0007] A method for preparing a heat-resistant modifier for PVC sheets includes the following steps:

[0008] S1. Add carboxymethylated lignin to anhydrous DMF and stir to dissolve. Then add furfurylamine, EDC·HCl and 4-dimethylaminopyridine in sequence. Stir for 18-24 h under a nitrogen atmosphere to obtain a reaction solution. Pour the reaction solution into ether at 2-6℃ and stir for 4-5 min. Filter and collect the solid. Wash three times with ether and dry under vacuum at 40-65℃ for 6-10 h to obtain furan-grafted lignin.

[0009] S2. Add furan-grafted lignin and N-sulfonyl maleimide to anhydrous DMF, stir and dissolve at 60°C, remove oxygen by purging with nitrogen, raise the temperature to 70~85°C, stir for 36~48h, after the reaction is completed, concentrate under reduced pressure at 50°C to obtain a viscous product, and vacuum dry the viscous product at 70~80°C for 12~24h to obtain a heat-resistant modifier for PVC sheets.

[0010] The method for preparing carboxymethylated lignin is as follows: dry lignin is dispersed in an ethanol-water solution, sodium hydroxide solution is slowly added dropwise in an ice-water bath while continuously stirring. After the addition is completed, the mixture is stirred at 20-30°C for 1 hour to obtain an alkalized solution. Chloroacetic acid is added to the alkalized solution, the temperature is raised to 65-70°C, and the mixture is stirred and refluxed for 2-3 hours. After the reaction is completed, the mixture is cooled to room temperature and concentrated under reduced pressure to obtain a concentrated solution. The concentrated solution is poured into an ethanol-acetone solution, the pH is adjusted to 2-3 with dilute hydrochloric acid, the mixture is filtered, the solid is collected, washed three times with an ethanol-water solution, and dried under vacuum at 60°C for 8-12 hours to obtain carboxymethylated lignin.

[0011] The number-average molecular weight of the lignin is 3000~8000 g / mol.

[0012] The molar ratio of total hydroxyl groups in chloroacetic acid and lignin is 1.5~2.5:1; the mass concentration of sodium hydroxide solution is 30%, and the molar ratio of sodium hydroxide in chloroacetic acid and sodium hydroxide solution is 1:1.2~1.5; the volume ratio of ethanol to acetone in ethanol-acetone solution is 1:1; and the volume ratio of ethanol to water in ethanol-acetone aqueous solution is 4:1.

[0013] The preparation method of the N-sulfonyl maleimide is as follows: maleic anhydride, sulfonamide, anhydrous acetic anhydride and triethylamine are taken and refluxed in an oil bath at 120~150℃ for 4~6h with continuous stirring; after the reaction is completed, the mixture is cooled to room temperature, poured into ice water, filtered, and a solid is obtained. The solid is washed three times with ice water, added to anhydrous ethanol, heated and refluxed with stirring to dissolve, cooled to room temperature, placed in an ice water bath to crystallize, filtered, and the white crystals are collected. The crystals are then dried under vacuum at 50~60℃ for 4~6h to obtain N-sulfonyl maleimide.

[0014] The molar ratio of sulfonamide, maleic anhydride, anhydrous acetic anhydride, and triethylamine is 1:1~1.2:2~2.5:1.2~1.3; the sulfonamide is benzenesulfonamide or methanesulfonamide; the volume of anhydrous ethanol is 3~5 times the volume of anhydrous acetic anhydride.

[0015] The molar ratio of carboxyl groups in furfurylamine, EDC·HCl, 4-dimethylaminopyridine, and carboxymethylated lignin is 2~3:1.5~2:0.2~0.5:1.

[0016] The mass ratio of furan-grafted lignin to N-sulfonyl maleimide is 1:0.5~1.

[0017] A heat-resistant modifier for PVC sheets.

[0018] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0019] (1) The modification of lignin by introducing carboxymethyl groups effectively breaks the hydrogen bond network between or within lignin molecules, significantly reduces the aggregation tendency of lignin, increases the compatibility of lignin with PVC matrix, solves the technical pain point of poor compatibility between lignin and PVC matrix, creates conditions for uniform dispersion of lignin in PVC matrix, and provides active sites for subsequent grafting of furan groups.

[0020] (2) Maleimide is modified by N-substitution, and N-sulfonyl maleimide is used as a dienophile: the sulfonyl group is introduced into the N atom of maleimide, and the electron cloud density of the maleimide double bond is reduced by electron-withdrawing group, which increases the electrophilicity of the maleimide double bond, making it more susceptible to attack by the furan ring, thereby increasing the Diels-Alder forward reaction rate with furan and increasing the temperature at which the Diels-Alder reverse reaction occurs, so that the crosslinking network of maleimide and lignin remains stable at the PVC sheet usage temperature; in addition, the sulfonation modification also improves the intrinsic heat resistance of maleimide;

[0021] (3) The click chemical Diels-Alder reaction of furan-grafted lignin and N-sulfonyl maleimide solved the contradiction between material processability and performance: at the high temperature required for processing, the reverse Diels-Alder reaction is dominant, which causes the crosslinking network to temporarily dissociate. This not only reduces the melt viscosity, but the shorter chain segments formed after dissociation also play the role of internal plasticizer and flow promoter in the melt, thus solving the processing difficulties usually faced by high-filled and high-crosslinking systems. When the material is cooled to the service temperature, the forward Diels-Alder reaction proceeds spontaneously, the crosslinking network is re-established, and the reconstructed three-dimensional network constitutes the crosslinking structure in the entire PVC sheet matrix, which greatly restricts the thermal movement of molecular chains in the PVC sheet, thereby effectively resisting deformation and giving the PVC sheet excellent heat resistance. Attached Figure Description

[0022] Figure 1 This is the ATR-FTIR spectrum of the N-sulfonyl maleimide prepared in Example 1;

[0023] Figure 2 This is the ATR-FTIR image of the heat-resistant modifier for PVC sheets prepared in Example 1;

[0024] Figure 3 These are the TG curves of Example 2, Comparative Example 4, Comparative Example 5, and the blank control sample. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. 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. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] Example 1: A method for preparing a heat-resistant modifier for PVC sheets, the specific preparation process is as follows:

[0027] S1. Preparation of N-sulfonyl maleimide: In a dry round-bottom flask, 9.8 g of maleic anhydride, 15.7 g of benzenesulfonamide, 18.9 mL of anhydrous acetic anhydride, and 16.7 mL of triethylamine were added sequentially. A reflux condenser and an anhydrous calcium chloride drying tube were installed. The mixture was refluxed in an oil bath at 120 °C for 4 h with magnetic stirring. After the reaction was completed, the mixture was cooled to room temperature, and the reaction solution was poured into 200 mL of ice water with continuous stirring. A solid precipitated out. The solid was filtered using a Buchner funnel, washed three times with ice water, and added to 56.7 mL of anhydrous ethanol. The mixture was heated to reflux and stirred to dissolve the solid. After cooling to room temperature, the solid was placed in an ice-water bath to crystallize. The white crystals were collected by filtration and placed in a vacuum drying oven at 50 °C for 6 h to obtain N-sulfonyl maleimide. The ATR-FTIR results of the N-sulfonyl maleimide prepared in this example are as follows: Figure 1 As shown; the reaction equation is as follows:

[0028] ,

[0029] In this embodiment, R represents phenyl;

[0030] S2. Preparation of carboxymethylated lignin: Take lignin with a number-average molecular weight of 3000 g / mol and determine its total hydroxyl content to be 5 mmol / g; in a 250 mL three-necked flask, add 20 g of dry lignin, 80 mL of ethanol, and 20 mL of deionized water. In an ice-water bath, slowly add 24 mL of 30% sodium hydroxide solution while continuously stirring; after the addition is complete, remove the ice-water bath and stir the reaction at 20 °C for 1 h to obtain an alkalized solution; add 14.2 g of chloroacetic acid to the alkalized solution, and... The mixture was heated to 65℃ and refluxed with stirring for 2 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of an ethanol-acetone solution (ethanol to acetone volume ratio of 1:1), and the pH was adjusted to 2 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution (ethanol to water volume ratio of 4:1). The solid was then dried in a vacuum drying oven at 60℃ for 8 hours to obtain carboxymethylated lignin. The reaction equation is as follows:

[0031] ;

[0032] S3. Preparation of furan-grafted lignin: The total carboxyl content of carboxymethylated lignin was determined to be 2.8 mmol / g. In a 100 mL dry round-bottom flask, 10 g of carboxymethylated lignin and 50 mL of anhydrous DMF were added and stirred to dissolve. Then, 5.44 g of furfurylamine, 8.0 g of EDC·HCl, and 0.78 g of 4-dimethylaminopyridine were added sequentially to the round-bottom flask. Anhydrous calcium chloride drying tube was attached to the mouth of the flask, and the mixture was magnetically stirred for 18 h at room temperature under a nitrogen atmosphere to obtain the reaction solution. The reaction solution was stirred at high speed and poured into 200 mL of ether at 2 °C, and stirred continuously for 4 min until fibrous precipitation occurred. The solid was collected by suction filtration and washed three times with ether to remove unreacted furfurylamine, byproducts, and DMF. The solid was then dried under vacuum at 40 °C for 6 h to obtain furan-grafted lignin. The reaction equation is as follows:

[0033] ;

[0034] S4. In a 50 mL pressure-resistant reaction tube, add 6 g of furan-grafted lignin, 3 g of N-sulfonyl maleimide, and 50 mL of anhydrous DMF. Stir at 60 °C to dissolve, purge with nitrogen to remove oxygen, raise the temperature to 70 °C, and stir for 36 h. After the reaction is complete, cool to 50 °C and concentrate under reduced pressure at 50 °C using a rotary evaporator to obtain a viscous product. Transfer the viscous product to a petri dish and vacuum dry at 70 °C for 12 h in a vacuum drying oven to obtain a heat-resistant modifier for PVC sheets. The reaction equation is as follows:

[0035] ;

[0036] The ATR-FTIR results of the PVC sheet heat-resistant modifier prepared in this embodiment are as follows: Figure 1 As shown.

[0037] Example 2: A method for preparing a heat-resistant modifier for PVC sheets, the specific preparation process is as follows:

[0038] S1. Preparation of N-sulfonyl maleimide: In a dry round-bottom flask, add 11.77 g maleic anhydride, 15.7 g benzenesulfonamide, 23.63 mL anhydrous acetic anhydride and 18.13 mL triethylamine in sequence. Install a reflux condenser and an anhydrous calcium chloride drying tube. Reflux in an oil bath at 150 °C for 6 h with magnetic stirring. After the reaction is complete, cool to room temperature and pour the reaction solution into 200 mL of ice water while continuously stirring. A solid precipitates out. Filter using a Buchner funnel to obtain the solid. Wash three times with ice water and add to 118 mL of anhydrous ethanol. Heat under reflux and stir to dissolve. Cool to room temperature and place in an ice-water bath to crystallize. Collect the white crystals by filtration and place in a vacuum drying oven at 60 °C for 4 h to obtain N-sulfonyl maleimide.

[0039] S2. Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 3000 g / mol was tested, and its total hydroxyl content was determined to be 5 mmol / g. In a 250 mL three-necked flask, 20 g of dry lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 49.9 mL of a 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the mixture was stirred at 30 °C for 1 h to obtain an alkalized solution. 23.6 g of [unspecified substance] was added to the alkalized solution. Chloroacetic acid was heated to 70°C and stirred under reflux for 3 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of ethanol-acetone solution (ethanol to acetone volume ratio 1:1), and the pH was adjusted to 3 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution (ethanol to water volume ratio 4:1). The solid was dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxymethylated lignin.

[0040] S3. Preparation of furan-grafted lignin: The total carboxyl content of carboxymethylated lignin was determined to be 2.8 mmol / g. In a 100 mL dry round-bottom flask, 10 g of carboxymethylated lignin and 50 mL of anhydrous DMF were added and stirred to dissolve. 8.16 g of furfurylamine, 10.8 g of EDC·HCl and 1.72 g of 4-dimethylaminopyridine were added sequentially to the round-bottom flask. Anhydrous calcium chloride drying tube was installed at the mouth of the flask, and the mixture was magnetically stirred for 24 h at room temperature and under a nitrogen atmosphere to obtain a reaction solution. The reaction solution was stirred at high speed and poured into 200 mL of 6 °C diethyl ether, and stirred continuously for 5 min until fibrous precipitation occurred. The solid was collected by filtration and washed three times with diethyl ether to remove unreacted furfurylamine, byproducts and DMF. The solid was dried under vacuum at 65 °C for 10 h to obtain furan-grafted lignin.

[0041] S4. In a 50 mL pressure-resistant reaction tube, add 6 g of furan-grafted lignin, 6 g of N-sulfonyl maleimide and 50 mL of anhydrous DMF, stir at 60 °C to dissolve, remove oxygen by purging with nitrogen, raise the temperature to 85 °C, stir for 48 h, after the reaction is completed, cool to 50 °C, concentrate by vacuum at 50 °C using a rotary evaporator to obtain a viscous product, transfer the viscous product to a petri dish, and vacuum dry at 80 °C for 24 h in a vacuum drying oven to obtain a heat-resistant modifier for PVC sheets.

[0042] Example 3: A method for preparing a heat-resistant modifier for PVC sheets, the specific preparation process is as follows:

[0043] S1. Preparation of N-sulfonyl maleimide: In a dry round-bottom flask, add 10g maleic anhydride, 15.7g benzenesulfonamide, 20mL anhydrous acetic anhydride and 17mL triethylamine in sequence. Install a reflux condenser and an anhydrous calcium chloride drying tube. Under magnetic stirring, reflux in an oil bath at 135℃ for 5h. After the reaction is completed, cool to room temperature and pour the reaction solution into 200mL of ice water while stirring continuously. A solid precipitates out. Filter using a Buchner funnel to obtain the solid. Wash three times with ice water and add to 80mL of anhydrous ethanol. Heat under reflux and stir to dissolve. Cool to room temperature and place in an ice-water bath to crystallize. Collect the white crystals by filtration and place in a vacuum drying oven at 55℃ for 5h to obtain N-sulfonyl maleimide.

[0044] S2. Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 3000 g / mol was tested, and its total hydroxyl content was determined to be 5 mmol / g. In a 250 mL three-necked flask, 20 g of dried lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 32 mL of 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the mixture was stirred at 25 °C for 1 h to obtain an alkalized solution. 18 g of chloroacetic acid was added to the alkalized solution. The temperature was raised to 68℃, and the mixture was stirred and refluxed for 2.5 h. After the reaction was completed, the mixture was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of ethanol-acetone solution (ethanol to acetone volume ratio 1:1), and the pH was adjusted to 2.5 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution (ethanol to water volume ratio 4:1). The solid was dried in a vacuum drying oven at 60℃ for 10 h to obtain carboxymethylated lignin.

[0045] S3. Preparation of furan-grafted lignin: The total carboxyl content of carboxymethylated lignin was determined to be 2.8 mmol / g. 10 g of carboxymethylated lignin and 50 mL of anhydrous DMF were added to a 100 mL dry round-bottom flask and stirred to dissolve. 6.5 g of furfurylamine, 9.0 g of EDC·HCl, and 1.32 g of 4-dimethylaminopyridine were added sequentially to the round-bottom flask. Anhydrous calcium chloride drying tube was attached to the mouth of the flask, and the mixture was magnetically stirred for 20 h at room temperature and under a nitrogen atmosphere to obtain a reaction solution. The reaction solution was stirred at high speed and poured into 200 mL of 4 °C diethyl ether, and stirred continuously for 4.5 min until fibrous precipitation occurred. The solid was collected by filtration and washed three times with diethyl ether to remove unreacted furfurylamine, byproducts, and DMF. The solid was then vacuum dried at 55 °C for 8 h to obtain furan-grafted lignin.

[0046] S4. In a 50 mL pressure-resistant reaction tube, add 6 g of furan-grafted lignin, 5 g of N-sulfonyl maleimide and 50 mL of anhydrous DMF. Stir at 60 °C to dissolve, remove oxygen by purging with nitrogen, raise the temperature to 80 °C and stir for 40 h. After the reaction is complete, cool to 50 °C and concentrate under reduced pressure at 50 °C using a rotary evaporator to obtain a viscous product. Transfer the viscous product to a petri dish and vacuum dry at 75 °C for 18 h in a vacuum drying oven to obtain a heat-resistant modifier for PVC sheets.

[0047] Example 4: A method for preparing a heat-resistant modifier for PVC sheets, the specific preparation process is as follows:

[0048] S1. Preparation of N-sulfonyl maleimide: In a dry round-bottom flask, add 11.77 g maleic anhydride, 10.92 g methanesulfonamide, 23.63 mL anhydrous acetic anhydride and 18.13 mL triethylamine in sequence. Install a reflux condenser and an anhydrous calcium chloride drying tube. Reflux in an oil bath at 150 °C for 6 h with magnetic stirring. After the reaction is complete, cool to room temperature and pour the reaction solution into 200 mL of ice water with continuous stirring. A solid precipitates out. Filter using a Buchner funnel to obtain the solid. Wash three times with ice water and add to 118 mL of anhydrous ethanol. Heat under reflux and stir to dissolve. Cool to room temperature and crystallize in an ice-water bath. Collect the white crystals by filtration and place in a vacuum drying oven at 60 °C for 4 h to obtain N-sulfonyl maleimide.

[0049] S2. Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 8000 g / mol was tested, and its total hydroxyl content was determined to be 4 mmol / g. In a 250 mL three-necked flask, 20 g of dry lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 35 mL of 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the mixture was stirred at 30 °C for 1 h to obtain an alkalized solution. 18.9 g of chlorine was added to the alkalized solution. Acetic acid was heated to 70°C and stirred under reflux for 3 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of an ethanol-acetone solution with a volume ratio of 1:1. The pH was adjusted to 3 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution with a volume ratio of 4:1. The solid was then dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxymethylated lignin.

[0050] S3. Preparation of furan-grafted lignin: The total carboxyl content of carboxymethylated lignin was determined to be 2.1 mmol / g. In a 100 mL dry round-bottom flask, 10 g of carboxymethylated lignin and 50 mL of anhydrous DMF were added and stirred to dissolve. 6.12 g of furfurylamine, 8.05 g of EDC·HCl and 1.28 g of 4-dimethylaminopyridine were added sequentially to the round-bottom flask. Anhydrous calcium chloride drying tube was installed at the mouth of the flask, and the mixture was magnetically stirred for 24 h at room temperature and under a nitrogen atmosphere to obtain a reaction solution. The reaction solution was stirred at high speed and poured into 200 mL of 6 °C diethyl ether, and stirred continuously for 5 min until fibrous precipitation occurred. The solid was collected by filtration and washed three times with diethyl ether to remove unreacted furfurylamine, byproducts and DMF. The solid was dried under vacuum at 65 °C for 10 h to obtain furan-grafted lignin.

[0051] S4. In a 50 mL pressure-resistant reaction tube, add 6 g of furan-grafted lignin, 6 g of N-sulfonyl maleimide and 50 mL of anhydrous DMF, stir at 60 °C to dissolve, remove oxygen by purging with nitrogen, raise the temperature to 85 °C, stir for 48 h, after the reaction is completed, cool to 50 °C, concentrate by vacuum at 50 °C using a rotary evaporator to obtain a viscous product, transfer the viscous product to a petri dish, and vacuum dry at 80 °C for 24 h in a vacuum drying oven to obtain a heat-resistant modifier for PVC sheets.

[0052] Comparative Example 1: A method for preparing a heat-resistant modifier for PVC sheets, differing from Example 2 in that it does not contain lignin units, i.e., it uses N-sulfonyl maleimide as the heat-resistant modifier. The specific preparation process is as follows:

[0053] Preparation of N-sulfonylmaleimide: In a dry round-bottom flask, 11.77 g of maleic anhydride, 15.7 g of benzenesulfonamide, 23.63 mL of anhydrous acetic anhydride, and 18.13 mL of triethylamine were added sequentially. A reflux condenser and an anhydrous calcium chloride drying tube were installed, and the mixture was refluxed in an oil bath at 150 °C for 6 h under magnetic stirring. After the reaction was completed, the mixture was cooled to room temperature, and the reaction solution was poured into 200 mL of ice water while continuously stirring. A solid precipitated out and was filtered using a Buchner funnel. The solid was washed three times with ice water, added to 118 mL of anhydrous ethanol, heated under reflux and stirred to dissolve it, cooled to room temperature, and placed in an ice-water bath to crystallize. The white crystals were collected by filtration and placed in a vacuum drying oven at 60 °C for 4 h to obtain N-sulfonylmaleimide.

[0054] Comparative Example 2: A method for preparing a heat-resistant modifier for PVC sheets, differing from Example 2 in that it does not contain an N-sulfonyl maleimide unit, i.e., it uses carboxymethylated lignin as the heat-resistant modifier. The specific preparation process is as follows:

[0055] Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 3000 g / mol was measured, and its total hydroxyl content was determined to be 5 mmol / g. In a 250 mL three-necked flask, 20 g of dried lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 49.9 mL of 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the reaction was stirred at 30 °C for 1 h to obtain an alkalized solution. 23.6 g of ethyl chloride was added to the alkalized solution. The acid was heated to 70℃ and stirred under reflux for 3 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of ethanol-acetone solution with a volume ratio of 1:1. The pH was adjusted to 3 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution with a volume ratio of 4:1. The solid was dried in a vacuum drying oven at 60℃ for 12 hours to obtain carboxymethylated lignin.

[0056] Comparative Example 3: A method for preparing a heat-resistant modifier for PVC sheets, differing from Example 2 in that the maleimide was not modified with N-sulfonyl substitution. The specific preparation process is as follows:

[0057] S1. Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 3000 g / mol was tested, and its total hydroxyl content was determined to be 5 mmol / g. In a 250 mL three-necked flask, 20 g of dried lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 49.9 mL of a 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the mixture was stirred at 30 °C for 1 h to obtain an alkalized solution. 23.6 g of [unspecified substance] was added to the alkalized solution. Chloroacetic acid was heated to 70°C and stirred under reflux for 3 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of ethanol-acetone solution (ethanol to acetone volume ratio 1:1), and the pH was adjusted to 3 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution (ethanol to water volume ratio 4:1). The solid was then dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxymethylated lignin.

[0058] S2. Preparation of furan-grafted lignin: The total carboxyl content of carboxymethylated lignin was determined to be 2.8 mmol / g. 10 g of carboxymethylated lignin and 50 mL of anhydrous DMF were added to a 100 mL dry round-bottom flask and stirred to dissolve. 8.16 g of furfurylamine, 10.8 g of EDC·HCl, and 1.72 g of 4-dimethylaminopyridine were added sequentially to the round-bottom flask. Anhydrous calcium chloride drying tube was attached to the mouth of the flask, and the mixture was magnetically stirred for 24 h at room temperature under a nitrogen atmosphere to obtain the reaction solution. The reaction solution was stirred at high speed and poured into 200 mL of 6 °C diethyl ether, and stirred continuously for 5 min until fibrous precipitation occurred. The solid was collected by filtration and washed three times with diethyl ether to remove unreacted furfurylamine, byproducts, and DMF. The solid was then vacuum dried at 65 °C for 10 h to obtain furan-grafted lignin.

[0059] S3. In a 50 mL pressure-resistant reaction tube, add 6 g of furan-grafted lignin, 6 g of maleimide monomer and 50 mL of anhydrous DMF, stir at 60 °C to dissolve, remove oxygen by purging with nitrogen, raise the temperature to 85 °C, stir for 48 h, after the reaction is completed, cool to 50 °C, concentrate by vacuum at 50 °C using a rotary evaporator to obtain a viscous product, transfer the viscous product to a petri dish, and vacuum dry at 80 °C for 24 h in a vacuum drying oven to obtain a PVC heat-resistant modifier.

[0060] Comparative Example 4: A method for preparing a heat-resistant modifier for PVC sheets, differing from Example 2 in that N-sulfonyl maleimide and carboxymethylated lignin are not chemically linked but physically blended. The specific preparation process is as follows:

[0061] S1. Preparation of N-sulfonyl maleimide: In a dry round-bottom flask, add 11.77 g maleic anhydride, 15.7 g benzenesulfonamide, 23.63 mL anhydrous acetic anhydride and 18.13 mL triethylamine in sequence. Install a reflux condenser and an anhydrous calcium chloride drying tube. Reflux in an oil bath at 150 °C for 6 h with magnetic stirring. After the reaction is complete, cool to room temperature and pour the reaction solution into 200 mL of ice water while stirring continuously. A solid precipitates out. Filter using a Buchner funnel to obtain the solid. Wash three times with ice water, add to 118 mL of anhydrous ethanol, heat under reflux and stir to dissolve. Cool to room temperature and crystallize in an ice-water bath. Collect the white crystals by filtration and place in a vacuum drying oven at 60 °C for 4 h to obtain N-sulfonyl maleimide.

[0062] S2. Preparation of carboxymethylated lignin: Lignin with a number-average molecular weight of 3000 g / mol was tested, and its total hydroxyl content was determined to be 5 mmol / g. In a 250 mL three-necked flask, 20 g of dry lignin, 80 mL of ethanol, and 20 mL of deionized water were added. In an ice-water bath, 49.9 mL of a 30% sodium hydroxide solution was slowly added dropwise while continuously stirring. After the addition was complete, the ice-water bath was removed, and the mixture was stirred at 30 °C for 1 h to obtain an alkalized solution. 23.6 g of [unspecified substance] was added to the alkalized solution. Chloroacetic acid was heated to 70°C and stirred under reflux for 3 hours. After the reaction was completed, it was cooled to room temperature and concentrated under reduced pressure using a rotary evaporator to remove most of the ethanol, obtaining a concentrated solution. The concentrated solution was poured into 200 mL of ethanol-acetone solution (ethanol to acetone volume ratio 1:1), and the pH was adjusted to 3 using 1 mol / L dilute hydrochloric acid. The solution was filtered using a Buchner funnel, and the solid was collected. The solid was washed three times with an ethanol-water solution (ethanol to water volume ratio 4:1). The solid was then dried in a vacuum drying oven at 60°C for 12 hours to obtain carboxymethylated lignin.

[0063] S3. Mix 6g of carboxymethylated lignin and 6g of N-sulfonyl maleimide to obtain a heat-resistant modifier for PVC sheets.

[0064] Comparative Example 5: A method for preparing a heat-resistant modifier for PVC sheets. The specific preparation method is as follows: the heat-resistant modifier for PVC sheets is a commercially available SAM-I heat-resistant modifier, which is a terpolymer of styrene, acrylonitrile and N-phenylmaleimide.

[0065] Performance testing

[0066] Preparation of experimental materials: Take 100 parts of PVC resin S1000, 10 parts of the heat-resistant modifier from Examples 1-4 or Comparative Examples 1-5, 2.5 parts of organotin heat stabilizer, 0.8 parts of calcium stearate, 0.5 parts of stearic acid, and 1.5 parts of ACR processing aid. Add all the above raw materials to a high-speed mixer and mix for 5 minutes at room temperature until the material temperature reaches 55°C. Add the premixed material to a torque rheometer. Set the torque rheometer parameters to 175°C, 60 rpm, and 8 minutes for mixing. Record the equilibrium torque and melting time. Take an appropriate amount of material and place it into a preheated flat vulcanizing mold. Pressing conditions: temperature 180°C, preheating for 5 minutes, then hot pressing at 10 MPa for 5 minutes. Subsequently, maintain a pressure of 15 MPa and use a large blower to cool the PVC sheet until the temperature drops to 25°C. Finally, a PVC sheet of uniform thickness is obtained for subsequent testing.

[0067] Preparation of blank PVC sheet material: without the heat-resistant modifiers of Examples 1-4 or Comparative Examples 1-5, other materials and processing steps are the same as the preparation steps of the experimental materials above.

[0068] (1) Thermal stability performance

[0069] The PVC sheets prepared above were subjected to Vicat softening temperature, heat distortion temperature, and TGA tests: the Vicat softening temperature was determined according to GB / T 1633-2000 "Determination of Vicat softening temperature (VST) of thermoplastic plastics", with a load of 10 N, a heating rate of 120 °C / h, and the endpoint being the temperature at which the indenter penetrates 1 mm into the sample surface; the heat distortion temperature was determined according to GB / T 1634 "Determination of load distortion temperature of plastics", with a load of 1.82 MPa, a heating rate of 120 °C / h, and the endpoint being the temperature at which the sample bends to a depth of 0.25 mm; the TGA test was performed according to GB / T 33047.1 "Thermogravimetric analysis (TG) of plastic polymers Part 1: General Rules", using a thermogravimetric analyzer under a nitrogen atmosphere, with the heating range set to 25 °C~800 °C and the heating rate set to 20 °C / min.

[0070] The results of the Vicat softening temperature and heat distortion temperature are shown in Table 1; the results of the TGA determination are shown in Table 1. Figure 3 As shown.

[0071] Table 1 Thermal stability test results

[0072]

[0073] Vicat softening temperature and heat distortion temperature are two of the most commonly used indicators for evaluating the heat resistance of materials. Vicat softening temperature is one of the heat resistance indicators of polymers. It represents the ability of a material to deform under external force and is usually used to evaluate the temperature range of polymer materials. Heat distortion temperature is another important heat resistance indicator. It can reflect the highest temperature at which a material product can maintain its shape and size stability under short-term heating conditions without additional external force.

[0074] As can be seen from Table 1, the Vicat softening temperature and heat distortion temperature of all examples are significantly higher than those of the blank example and the comparative examples. Among them, the performance of Example 2 is the most outstanding, with a Vicat softening temperature and heat distortion temperature of 113.7℃ and 105.8℃, respectively, which is more than 35℃ higher than that of the blank example. Its excellent heat resistance is attributed to the reinforcing effect of the quality rigid aromatic skeleton and the dense cross-linked network formed by the efficient Diels-Alder reaction between furan and N-sulfonyl maleimide. This network greatly restricts the thermal movement of molecular chains within the PVC sheet. The performance of the examples is also comprehensively better than that of the comparative examples. The indicators of the examples are significantly higher than those of comparative examples 3 and 4. This result confirms the key role of N-sulfonyl groups in improving the cross-linking reaction activity and network perfection, as well as the absolute advantage of chemical cross-linking over physical blending. In addition, the examples also surpass the commercially available product comparative example 5, highlighting the technical advantages of this solution.

[0075] from Figure 3 As can be seen from the TGA curve, the above conclusions are further corroborated. Compared with Comparative Example 4, Comparative Example 5 and the blank sample, the TGA curve of Example 2 shows advantages in both the initial decomposition temperature and the maximum decomposition temperature, and the char residue rate is significantly improved. This indicates that its crosslinking network not only improves the heat resistance deformation temperature, but also effectively delays the thermal decomposition process of the material by forming a thermal barrier, thus synergistically improving the thermal stability of PVC sheet products from different dimensions.

[0076] (2) Measurement of torque rheological properties

[0077] During the preparation of experimental materials, a torque rheometer was used for measurement. The parameters were set at a temperature of 175℃, a rotation speed of 60 rpm, and a mixing time of 8 min. The maximum torque, equilibrium torque, and plasticizing time were recorded. The maximum torque is the peak resistance generated when the sample is compressed, compacted, and begins to melt under heating and shearing. A higher value indicates greater difficulty in plasticizing. The equilibrium torque is the stable value of the torque reached after the sample is completely melted and plasticized. A lower value indicates better melt flowability and is more conducive to processing. The plasticizing time is the time taken from the start of feeding to the first time the torque reaches the equilibrium torque value. A shorter time indicates faster material plasticization and higher production efficiency. The test results are shown in Table 2.

[0078] Table 2 Test results of torque rheological properties

[0079]

[0080] Torque rheometers simulate and record the entire process of materials undergoing heating and shearing, from solid powder or granules to melt and plasticize, ultimately reaching dynamic equilibrium. This process directly reflects the actual behavior of materials within processing equipment. Evaluating the processing rheological behavior of PVC resin is extremely important for predicting resin processing performance, guiding the determination of molding processes and formulation components, and designing plastic processing molds.

[0081] As can be seen from Table 2, the heat-resistant modifier for PVC sheets prepared in the examples exhibits excellent processing fluidity. The equilibrium torque of all examples is significantly lower than that of the comparative examples, especially the equilibrium torque of Example 2, which is only 24.6 N·m, lower than that of Comparative Examples 1-3, and even better than that of Comparative Example 5 of the commercially available product. This result proves that the crosslinking network constructed by the present invention through the dynamic Diels-Alder reaction undergoes reversible dissociation at the processing temperature, effectively reducing the melt viscosity and thus significantly improving the processing fluidity of the material. Although the maximum torque of the examples is slightly higher than that of the blank example, it is still significantly lower than that of all comparative examples. This indicates that although the addition of the modifier slightly increases the initial plasticization difficulty, it does not cause substantial obstacles to processing due to the characteristics of its dynamic covalent bonds. In addition, the plasticization time of the examples is closest to that of the blank example and is significantly shorter than that of the comparative examples, indicating that the modifier of the present invention does not significantly delay the plasticization process while ensuring excellent processing fluidity, thus ensuring production efficiency.

[0082] (3) Mechanical property determination

[0083] The tensile strength and impact strength of the PVC sheets prepared above were tested:

[0084] Tensile strength was determined according to the method in GB / T 1040.1-2025 "Determination of Tensile Properties of Plastics". The tensile rate was 10 mm / min.

[0085] The tensile strength (σ) is calculated using the following formula: σ = F / b•d (MPa); where: F is the maximum tensile load on the specimen (N); b is the specimen width (mm); d is the specimen thickness (mm); and the test result is the average value of 5 tensile specimens.

[0086] Impact performance was determined according to the method in GB / T1043-2008 "Determination of Impact Performance of Plastic Simply Supported Beams".

[0087] Impact strength (α) is calculated using the following formula: α = A / b•d × 10 3 (kJ / m 2 ); where: A is the work consumed by the unnotched specimen (J); b is the specimen width (mm); d is the thickness of the unnotched specimen (mm); the test result is the average value of 5 impact specimens.

[0088] The test results are shown in Table 3:

[0089] Table 3 Mechanical Performance Test Results

[0090]

[0091] As can be seen from Table 3, the PVC sheet heat-resistant modifier prepared by this invention exhibits significant advantages in mechanical properties: Example 2 demonstrates the best overall performance, with a tensile strength reaching 58.8. The impact strength was 26.8 kJ / m², representing increases of 30.1% and 25.8% respectively compared to the blank example. This result fully demonstrates the synergistic reinforcing effect of the dynamic Diels-Alder crosslinking network and the rigid lignin skeleton. Furthermore, while maintaining high tensile strength, the impact strength of the examples was also significantly improved, overcoming the drawback of traditional modifiers leading to decreased toughness. In particular, the impact strength of Example 3 reached 28.4 kJ / m², the highest among all samples, indicating that further improvement in toughness can be achieved by adjusting the crosslinking density. Compared to the comparative examples, the mechanical properties of the examples were comprehensively superior to the physical blend system of Comparative Example 4 and the unsulfonated system of Comparative Example 3, confirming the necessity of chemical crosslinking and N-sulfonyl modification. Moreover, the mechanical properties of all examples were superior to the commercially available product Comparative Example 5, especially in impact strength, demonstrating the unique technical effect of this invention in achieving strength improvement while maintaining material toughness. This characteristic of simultaneously improving both strength and toughness makes the modifier of this invention have broad application prospects in the field of PVC products requiring high mechanical properties.

[0092] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a heat-resistant modifier for PVC sheets, characterized in that, Includes the following steps: S1. Add carboxymethylated lignin to anhydrous DMF and stir to dissolve. Then add furfurylamine, EDC·HCl and 4-dimethylaminopyridine in sequence. Stir for 18-24 h under a nitrogen atmosphere to obtain a reaction solution. Pour the reaction solution into ether at 2-6℃ and stir for 4-5 min. Filter and collect the solid. Wash three times with ether and dry under vacuum at 40-65℃ for 6-10 h to obtain furan-grafted lignin. S2. Add furan-grafted lignin and N-sulfonyl maleimide to anhydrous DMF, stir and dissolve at 60°C, remove oxygen by purging with nitrogen, raise the temperature to 70~85°C, stir for 36~48h, after the reaction is completed, concentrate under reduced pressure at 50°C to obtain a viscous product, and vacuum dry the viscous product at 70~80°C for 12~24h to obtain a heat-resistant modifier for PVC sheets. The method for preparing carboxymethylated lignin is as follows: dry lignin is dispersed in an ethanol-water solution, sodium hydroxide solution is slowly added dropwise in an ice-water bath while continuously stirring. After the addition is completed, the mixture is stirred at 20-30°C for 1 hour to obtain an alkalized solution. Chloroacetic acid is added to the alkalized solution, the temperature is raised to 65-70°C, and the mixture is stirred and refluxed for 2-3 hours. After the reaction is completed, the mixture is cooled to room temperature and concentrated under reduced pressure to obtain a concentrated solution. The concentrated solution is poured into an ethanol-acetone solution, the pH is adjusted to 2-3 with dilute hydrochloric acid, the mixture is filtered, the solid is collected, washed three times with an ethanol-water solution, and dried under vacuum at 60°C for 8-12 hours to obtain carboxymethylated lignin. The preparation method of the N-sulfonyl maleimide is as follows: maleic anhydride, sulfonamide, anhydrous acetic anhydride and triethylamine are taken and refluxed in an oil bath at 120~150℃ for 4~6h with continuous stirring; after the reaction is completed, the mixture is cooled to room temperature, poured into ice water, filtered, and a solid is obtained. The solid is washed three times with ice water, added to anhydrous ethanol, heated and refluxed with stirring to dissolve, cooled to room temperature, placed in an ice water bath to crystallize, filtered, and the white crystals are collected and dried under vacuum at 50~60℃ for 4~6h to obtain N-sulfonyl maleimide.

2. The method for preparing a heat-resistant modifier for PVC sheets according to claim 1, characterized in that, The number-average molecular weight of the lignin is 3000~8000 g / mol.

3. The method for preparing a heat-resistant modifier for PVC sheets according to claim 1, characterized in that, The molar ratio of total hydroxyl groups in chloroacetic acid and lignin is 1.5~2.5:1; the mass concentration of sodium hydroxide solution is 30%, and the molar ratio of sodium hydroxide in chloroacetic acid and sodium hydroxide solution is 1:1.2~1.5; the volume ratio of ethanol to acetone in ethanol-acetone solution is 1:1; and the volume ratio of ethanol to water in ethanol-acetone aqueous solution is 4:

1.

4. The method for preparing a heat-resistant modifier for PVC sheets according to claim 1, characterized in that, The molar ratio of sulfonamide, maleic anhydride, anhydrous acetic anhydride, and triethylamine is 1:1~1.2:2~2.5:1.2~1.3; the sulfonamide is benzenesulfonamide or methanesulfonamide; the volume of anhydrous ethanol is 3~5 times the volume of anhydrous acetic anhydride.

5. The method for preparing a heat-resistant modifier for PVC sheets according to claim 1, characterized in that, In step S1, the molar ratio of carboxyl groups in furfurylamine, EDC·HCl, 4-dimethylaminopyridine and carboxymethylated lignin is 2~3:1.5~2:0.2~0.5:

1.

6. The method for preparing a heat-resistant modifier for PVC sheets according to claim 1, characterized in that, In step S2, the mass ratio of furan-grafted lignin to N-sulfonyl maleimide is 1:0.5~1.

7. The PVC sheet heat-resistant modifier prepared by the method of any one of claims 1 to 6.

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