Modification method of unsaturated polyester

Abstract

The invention discloses a modification method of unsaturated polyester. When the alkali lignin modified unsaturated polyester is selected, methanol insoluble lignin particles are treated by utilizing epoxypropane under a low-temperature condition, after propylene oxide lignin reacts with bio-based binary acid to obtain esterified lignin particles, the esterified lignin particles are ultrasonically dispersed into an unsaturated polyester matrix, and a modified unsaturated polyester solid can be obtained after normal-temperature curing. Therefore, the problems of low mechanical strength, low fracture toughness, high volume shrinkage, high brittleness and poor structural stability of the cured unsaturated polyester, high energy consumption and complicated process of a modification method at the present stage, unobvious improvement effect on mechanical properties of the modified resin, poor dispersity and chemical compatibility of modified particles and the like are effectively solved. The modification method can effectively improve the structural stability and durability of the unsaturated polyester resin, prolongs the service life of the unsaturated polyester resin and the composite material thereof, and has a good market prospect.

Description

Technical Field

[0001] This invention belongs to the field of polymer modification technology, specifically relating to a method for modifying unsaturated polyesters. Background Technology

[0002] Unsaturated polyesters are among the most widely used thermosetting resins, possessing advantages such as a wide curing temperature range, chemical resistance, and low cost, making them widely applied in fiber-reinforced composites and other fields. However, the curing mechanism of unsaturated polyesters is free radical chain polymerization, a reaction characterized by rapid molecular chain growth and termination. This leads to increased disorder in the resin molecular structure, resulting in volume shrinkage, high brittleness, low mechanical strength and fracture toughness, and poor structural stability. These negatively impact the functionality and durability of unsaturated polyesters, causing economic losses and safety hazards. Therefore, developing unsaturated polyesters with high strength and toughness is crucial for extending the service life of unsaturated polyesters and their composites.

[0003] Researchers have proposed various methods to improve the mechanical properties of unsaturated polyester resins, such as introducing rigid particles, bio-based rigid molecules, and synthetic crosslinking agents into the resin matrix. Rigid particles possess high mechanical rigidity and dimensional stability. Some bio-based rigid molecules, such as lignin, cellulose, and bamboo fiber, can provide a rigid skeletal structure, reducing the free movement of resin molecular chains and thus improving the structural stability of the resin. Simultaneously, synthetic crosslinking agents can undergo crosslinking reactions with resin molecules during curing, forming stable chemical bonds between resin molecules and constructing a three-dimensional network structure. This crosslinked structure can effectively reduce stress concentration effects in the resin. However, these modification methods still suffer from problems such as poor particle dispersibility and poor compatibility with unsaturated polyesters, negatively impacting the mechanical properties of unsaturated polyester resins.

[0004] Patent CN120005364A discloses a method for preparing a nano-montmorillonite-reinforced unsaturated polyester resin-based composite material. This method involves introducing a multifunctional intercalating agent into the nano-montmorillonite to improve material uniformity. Simultaneously, the multifunctional intercalating agent reacts with the montmorillonite and resin, enhancing interfacial bonding and effectively transferring stress, thereby improving the mechanical properties of the composite material. However, the nano-montmorillonite used in this method is a non-renewable material, limiting the widespread application of the resin. Furthermore, nanoparticles can easily harm human respiratory and lung health. Additionally, the temperatures used in the montmorillonite modification and resin curing processes are generally between 60℃ and 100℃, further increasing energy consumption and preparation costs. Moreover, this patent does not compare the mechanical properties of the resin itself with those of the modified resin, leaving the extent of improvement unknown, and the dispersion behavior of montmorillonite in the resin is not characterized in detail.

[0005] Patent CN115785638A discloses a method for preparing a low-viscosity bio-based unsaturated polyester prepolymer system using itaconic acid, butanediol, and isosorbide as monomers. It further describes how, by adjusting the initiator content, adding nano-lignin, and controlling its addition amount, a composite material is ultimately obtained with the introduction of a small amount of nano-lignin. However, this method involves a long preparation time, high cost, and complex process for nano-lignin. Furthermore, nano-lignin does not significantly improve the tensile strength and toughness of the resin, nor does it address the inherent mechanical property defects of unsaturated polyester.

[0006] Patent CN115232457B discloses a method for preparing a bio-based unsaturated polyester / nanocellulose UV-resistant composite material. The method uses nanocellulose as a substrate and cinnamic acid derivatives as modifying monomers to prepare a cellulose graft with good UV absorption capacity. Then, small molecules are covalently fixed onto the cellulose macromolecular chain and introduced into the unsaturated polyester to prepare the bio-based unsaturated polyester / nanocellulose UV-resistant composite material. This composite material exhibits good UV resistance and low volume shrinkage. However, the tensile strength of the modified resin is not significantly improved and may even decrease, and the thermal decomposition temperature is significantly reduced. Furthermore, the method involves a long preparation time for nanocellulose, high cost, and complex processes. The modified unsaturated polyester also needs to be cured at high temperatures, further increasing energy consumption and cost.

[0007] Patent CN120082184A discloses an unsaturated polyester resin and its preparation method. The method involves esterifying and polycondensing 1,6-hexanediol, itaconic acid, and a UV-absorbing monomer to obtain an unsaturated polyester; reacting naringenin and 4-isocyanate-tetramethylpiperidine oxide to obtain a functional crosslinking agent; reacting 3-(4-chlorophenyl)phosphine, 2,2'-thiobis(4-chlorophenol), and 2-butene-1,4-diamine to polymerize and coat the mixture onto silica to obtain pre-modified silica; reacting the pre-modified silica, n-butylamine, and nickel acetate to obtain modified silica; and mixing the unsaturated polyester, functional crosslinking agent, and modified silica uniformly, followed by injection molding to obtain the unsaturated polyester resin. However, the resin preparation process of this patent is cumbersome, and the resin curing temperature exceeds 180℃, increasing energy consumption and production costs, while the tensile strength of the resin is not significantly improved.

[0008] Researchers have proposed various methods for modifying unsaturated polyester resins, but these methods still have drawbacks, such as high energy consumption during preparation, complex modification processes, insignificant improvement in the mechanical properties of the resin, and poor particle distribution in the resin without quantitative characterization. These drawbacks affect the application of unsaturated polyesters in fiber-reinforced composites and structural reinforcement. Summary of the Invention

[0009] To address the problems of low mechanical strength and fracture toughness, large volume shrinkage, and high brittleness of cured unsaturated polyesters, as well as the high energy consumption, complex processes, and limited improvement in the mechanical properties of the modified resin, and poor dispersibility and compatibility of the modified particles, this invention aims to provide a method for modifying unsaturated polyesters. The method involves treating alkali lignin with propylene oxide at low temperature, reacting the resulting propoxylated lignin with a bio-based dicarboxylic acid to obtain esterified lignin powder. After ball milling, the powder is homogeneously dispersed into liquid unsaturated polyester using ultrasonic treatment to obtain modified unsaturated polyester resin. The modified unsaturated polyester resin prepared by this invention exhibits significantly improved tensile strength and toughness, and possesses UV shielding, acid and alkali resistance, seawater resistance, and shrinkage resistance. Furthermore, the esterified lignin shows high compatibility with the unsaturated polyester resin matrix, allowing for uniform dispersion within the resin, effectively improving the structural stability and durability of the unsaturated polyester resin, and extending the service life of the unsaturated polyester resin and its composites.

[0010] To address the technical problems existing in the prior art, the present invention is implemented through the following solution: A method for modifying unsaturated polyester includes the following steps: Step 1: Dissolve the residual alkali lignin from the papermaking industry in methanol (analytical grade ≥99.5%), and magnetically stir the mixture at 20~25℃ until the reaction is complete; Step 2: Perform solid-liquid separation on the mixture from Step 1, and dry the resulting solid phase at 40°C for 24 hours to obtain methanol-insoluble lignin; Step 3: Add the methanol-insoluble lignin obtained in Step 2 to a three-necked flask containing sodium hydroxide solution, then add propylene oxide (analytical grade ≥99.5%), control the reaction temperature at 40℃, maintain the reaction for 6~12h, and further obtain propylene oxide lignin solution. Step 4: Add dilute hydrochloric acid to the lignin-oxidized solution obtained in Step 3 above to adjust the pH and precipitate the product. After centrifugation, take the solid precipitate and dry it to obtain lignin-oxidized solution. Step 5: Dissolve the solid propionyl oxylignin obtained in Step 4 in an organic solvent (analytical grade ≥99.5%) and react it with a bio-based dicarboxylic acid under vacuum (vacuum degree -0.09) and catalyst conditions; after the reaction is completed, methanol is added to the reaction solution to precipitate the precipitate, and esterified lignin is obtained after solid-liquid separation; Step 6: Dry and ball mill the esterified lignin obtained in Step 5 above, then add it to liquid unsaturated polyester in proportion, and after ultrasonic dispersion, obtain esterified lignin-based unsaturated polyester liquid, i.e. modified unsaturated polyester prepolymer solution. Step 7: Add accelerator and curing agent to the modified unsaturated polyester prepolymer solution in proportion, and ultrasonically disperse the modified unsaturated polyester prepolymer solution evenly. After mixing evenly, pour it into a silicone mold for curing to finally obtain esterified lignin-based unsaturated polyester solid, i.e., modified unsaturated polyester.

[0011] Preferably, in step one, the particle size of lignin is 20~100μm, the volume of methanol is 450ml, the reaction time is 12~24h, and the magnetic stirrer speed is 200~300rpm.

[0012] Preferably, in step three, the concentration of the sodium hydroxide solution is 0.5M and the volume is 100~200 ml; the volume of propylene oxide is 20~50 ml; and the speed of the magnetic stirrer is 100~200 rpm.

[0013] Preferably, in step four, the concentration of dilute hydrochloric acid is 2M, the pH is adjusted to 4.3~5.5, the centrifugation time used in solid-liquid separation is 15min, and the centrifuge speed is 10000~20000 rpm.

[0014] Preferably, in step four, after the propylene oxide lignin solid collected by centrifugation is placed in a fume hood for 24 hours, it is dried in a drying oven at 40°C for 24 hours.

[0015] Preferably, in step five, the mass ratio of propionic lignin solid to organic solvent is 1:10~20; the vacuum degree of the vacuum environment is 0.09; the bio-based dicarboxylic acid used in the reaction is itaconic acid, 2,5-furandicarboxylic acid, sebacic acid or adipic acid; the catalyst is 95% concentrated sulfuric acid, and its addition amount is 0.1% of the mass of the bio-based dicarboxylic acid; the ratio of hydroxyl / carboxyl functional groups in the reactants is 1:1~1.2; the reaction temperature is 50°C; the reaction time is 12h; and the volume ratio of methanol to reaction liquid is 1:1.

[0016] Preferably, in step six, after drying the esterified lignin at 20-25°C for 24 hours, it is ball-milled for 1-10 hours, and the particle size of the esterified lignin after ball milling is 5-50 μm. After adding the esterified lignin to liquid unsaturated polyester, it is ultrasonically treated for 20-60 minutes, and the mass ratio of esterified lignin to unsaturated polyester is 1-7:100.

[0017] Preferably, in step seven, the accelerator is cobalt isooctanoate, the curing agent is methyl ethyl ketone peroxide, the volume of the accelerator and the curing agent are 1~2% of the liquid unsaturated polyester solution, the curing temperature is 20~25℃, and the curing time is 20~24h. Beneficial effects

[0018] Compared with existing technologies, this invention provides a method for modifying unsaturated polyesters. When using alkali lignin to modify unsaturated polyesters, propylene oxide is used to treat methanol-insoluble lignin particles at low temperatures. Then, propylene oxide lignin is reacted with a bio-based diacid to obtain esterified lignin particles, which are then ultrasonically dispersed into an unsaturated polyester matrix. After curing at room temperature, the modified unsaturated polyester solid is obtained. This effectively solves the problems of low mechanical strength, low fracture toughness, large volume shrinkage, high brittleness, and poor structural stability of cured unsaturated polyesters, as well as the problems of high energy consumption, complex processes, insignificant improvement in the mechanical properties of the modified resin, and poor dispersion and chemical compatibility of the modified particles in current modification methods. Specific advantages are as follows: 1. This invention utilizes propylene oxide to lengthen the aliphatic carbon chains in the side chains of alkali lignin, providing a large number of alcohol hydroxyl groups for its esterification reaction with diacids. This allows the prepared esterified lignin to have more similar chemical functional groups to unsaturated polyesters, thereby significantly improving the dispersibility of esterified lignin in the unsaturated polyester matrix and effectively improving the chemical compatibility between esterified lignin particles and the unsaturated polyester matrix. Ultimately, the tensile strength of the modified unsaturated polyester can be significantly increased by 16.74%~27.73%, the elongation can be increased by 38.96%~87.13%, and the toughness index can be increased by 59.43%~161.27%.

[0019] 2. The modification process of unsaturated polyester in this invention is carried out at 50°C and below, and the curing temperature of the unsaturated polyester is room temperature (20~25°C). The volume shrinkage rate after curing can be reduced by 28.04%~41.90%. Therefore, the modification method and related processes of unsaturated polyester are simple and efficient, with no process exceeding 24 hours, thus effectively solving the problems of high energy consumption, complex processes, and long time consumption in the modification method of unsaturated polyester.

[0020] 3. The modified unsaturated polyester of this invention, after being soaked in acid, alkali solutions and seawater, has a tensile strength loss rate of no more than 5%, and the ultraviolet light transmittance of the modified unsaturated polyester can be reduced by 33.94%~71.20%. Therefore, it has excellent resistance to acid and alkali, seawater corrosion and ultraviolet shielding performance. Attached Figure Description

[0021] Figure 1 The images show (a) fluorescence of unmodified unsaturated polyester and (b) fluorescence of esterified lignin in Example 1 (where the bright blue fluorescent dots represent the distribution of esterified lignin). Figure 2 The images show the infrared spectra of the unsaturated polyester before and after modification in Example 1. Detailed Implementation

[0022] The unsaturated polyester resin solutions used in the various embodiments and comparative examples of this invention are sourced from Xinxin New Materials Technology Co., Ltd.

[0023] Example 1

[0024] A method for modifying unsaturated polyester includes the following steps: 1) Dissolve 30 g of residual alkali lignin from the papermaking industry in 450 ml of methanol (analytical grade ≥99.5%). The lignin particle size is 100 μm. Stir the mixture at 200 rpm using a magnetic stirrer at 20℃ to ensure complete reaction for 12 h. Separate the mixture into solid and liquid phases. Place the obtained solid phase in a drying oven and dry it at 40℃ for 24 h to obtain methanol-insoluble lignin. 2) The methanol-insoluble lignin obtained was added to a three-necked flask containing 100 ml of 0.5 M sodium hydroxide solution, followed by 20 ml of propylene oxide (analytical grade ≥99.5%). A thermometer, a condenser, and a magnetic stirrer were connected. The magnetic stirrer was set to 100 rpm, the reaction solution temperature was controlled at 40 °C, and the reaction time was 6 h to further obtain a propylene oxide lignin solution. 3) Add 2 M dilute hydrochloric acid to the oxypropionate lignin solution to adjust the pH of the solution to 4.3 to precipitate the product. Use a centrifuge to separate the solid and liquid in the precipitated mixture at a speed of 10,000 rpm. Then place the solid in a fume hood for 24 hours and then place it in a drying oven for 24 hours at a drying temperature of 40°C to obtain oxypropionate lignin. 4) Solid lignin was dissolved in the organic solvent 1,4-dioxane (analytical grade ≥99.5%) at a mass ratio of 1:10. The solution was then reacted with itaconic acid under vacuum (-0.09) and 95% concentrated sulfuric acid catalyst conditions. The catalyst addition was 0.1% of the itaconic acid mass. The ratio of hydroxyl groups in the lignin to carboxyl groups in the itaconic acid was 1:1. The reaction temperature was 50 °C, and the reaction time was 12 h. After the reaction, the product was precipitated in 50 ml of methanol. The solid-liquid separation was performed by centrifugation for 15 min at a speed of 10,000 rpm, yielding esterified lignin. 5) After drying the esterified lignin at 20 ℃ for 24 h, it was ball-milled for 1 h. The particle size of the esterified lignin after ball milling was 14 μm. Then, the esterified lignin was added to the liquid unsaturated polyester resin solution at a mass ratio of 3:100. The solution was then ultrasonically dispersed for 20 min to obtain the esterified lignin-based unsaturated polyester liquid, i.e., the modified unsaturated polyester prepolymer solution. 6) Add the accelerator and curing agent to the modified unsaturated polyester prepolymer solution obtained by ultrasonic dispersion in proportion. The accelerator is cobalt isooctanoate and the curing agent is methyl ethyl ketone peroxide. The volume of the accelerator and the curing agent are 1% of the liquid unsaturated polyester solution. After mixing evenly, pour it into a silicone mold for curing. The curing temperature is 20℃ and the curing time is 20h. Finally, esterified lignin-based unsaturated polyester solid is obtained, that is, modified unsaturated polyester.

[0025] Comparative Example 1 Take 100g of unmodified unsaturated polyester resin solution, add cobalt isooctanoate as an accelerator and methyl ethyl ketone peroxide as a curing agent, and the amount of accelerator and curing agent added is 1% of the volume of unsaturated polyester resin. After stirring thoroughly, pour into a mold and cure at 20 ℃ for 20 h to obtain the control sample.

[0026] Figure 1 The distribution of esterified lignin in unsaturated polyester shows that lignin is uniformly distributed in unsaturated polyester.

[0027] Figure 2 The infrared spectra of unsaturated polyester before and after modification show that the main functional groups of the unsaturated polyester did not change significantly after the addition of lignin, indicating that the two have good compatibility. Therefore, it can effectively solve the problems of poor dispersibility and chemical compatibility of modified particles in unsaturated polyester.

[0028] Example 2

[0029] The difference from Example 1 is that the process parameters in the preparation of esterified lignin are different. In this case, the bio-based dicarboxylic acid used is sebacic acid, and the mass ratio of esterified lignin to unsaturated polyester in the modified unsaturated polyester resin is 2:100. The volumes of the accelerator and the curing agent are 1.5% of the liquid unsaturated polyester solution, respectively.

[0030] A method for modifying unsaturated polyester includes the following steps: 1) Dissolve 30g of residual alkali lignin from the papermaking industry in 450ml of methanol (analytical grade ≥99.5%). The lignin particle size is 100μm. Stir the mixture at 300 rpm using a magnetic stirrer at 25℃ to ensure complete reaction for 24h. Separate the mixture into solid and liquid phases. Place the obtained solid phase in a drying oven and dry it at 40℃ for 24h to obtain methanol-insoluble lignin. 2) The methanol-insoluble lignin obtained was added to a three-necked flask containing 200 ml of 0.5 M sodium hydroxide solution, followed by 50 ml of propylene oxide (analytical grade ≥99.5%). A thermometer, a condenser, and a magnetic stirrer were connected. The magnetic stirrer was set to 200 rpm, the reaction solution temperature was controlled at 40 °C, and the reaction time was 12 h to further obtain a propylene oxide lignin solution. 3) Add 2M dilute hydrochloric acid to the oxypropionate lignin solution to adjust the pH of the solution to 5.5 to precipitate the product. Use a centrifuge to separate the solid and liquid in the precipitated mixed solution at a speed of 20,000 rpm. Then place the solid in a fume hood for 24 hours and then place it in a drying oven for 24 hours at a drying temperature of 40°C to obtain oxypropionate lignin. 4) Solid lignin oxide was dissolved in 1,4-dioxane (analytical grade ≥99.5%) at a mass ratio of lignin oxide solid to organic solvent of 1:20. The mixture was then reacted with sebacic acid under vacuum (vacuum degree -0.09) and 95% concentrated sulfuric acid catalyst conditions. The catalyst addition was 0.1% of the mass of sebacic acid. The ratio of hydroxyl groups in the lignin to carboxyl functional groups in itaconic acid was 1:1.2. The reaction temperature was 50°C, and the reaction time was 12 h. After the reaction, the product was precipitated in 100 ml of methanol. The solid-liquid separation was performed by centrifugation for 15 min at a speed of 20,000 rpm, yielding esterified lignin. 5) After drying the esterified lignin at 25 ℃ for 24 h, it was ball-milled for 10 h. The particle size of the esterified lignin after ball milling was 7 μm. Then, the esterified lignin was added to the liquid unsaturated polyester resin solution at a mass ratio of 2:100. The solution was then ultrasonically dispersed for 60 min to obtain the esterified lignin-based unsaturated polyester liquid, i.e., the modified unsaturated polyester prepolymer solution. 6) Add the accelerator and curing agent to the modified unsaturated polyester prepolymer solution obtained by ultrasonic dispersion in proportion. The accelerator is cobalt isooctanoate and the curing agent is methyl ethyl ketone peroxide. The volume of the accelerator and curing agent is 1.5% of the liquid unsaturated polyester solution. After mixing evenly, pour it into a silicone mold for curing. The curing temperature is 25℃ and the curing time is 24h. Finally, esterified lignin-based unsaturated polyester solid is obtained, that is, modified unsaturated polyester.

[0031] Comparative Example 2 Take 100g of unmodified unsaturated polyester resin solution, add cobalt isooctanoate as an accelerator and methyl ethyl ketone peroxide as a curing agent, and the amount of accelerator and curing agent added is 1.5% of the volume of unsaturated polyester resin. After stirring thoroughly, pour into a mold and cure at 25℃ for 24h to obtain the control sample.

[0032] Example 3

[0033] The difference from Example 1 is that the process parameters in the preparation of esterified lignin are different. The bio-based dicarboxylic acid used is 2,5-furandicarboxylic acid. At the same time, the mass ratio of esterified lignin to unsaturated polyester in the modified unsaturated polyester resin is 4:100, and the volumes of accelerator and curing agent are 2% of the liquid unsaturated polyester solution.

[0034] A method for modifying unsaturated polyester includes the following steps: 1) Dissolve 30g of residual alkali lignin from the papermaking industry in 450ml of methanol (analytical grade ≥99.5%). The lignin particle size is 50μm. Stir the mixture at 250rpm using a magnetic stirrer at 22℃ to ensure complete reaction for 18h. Separate the mixture into solid and liquid phases. Place the obtained solid phase in a drying oven and dry it at 40℃ for 24h to obtain methanol-insoluble lignin. 2) The methanol-insoluble lignin obtained was added to a three-necked flask containing 150 ml of 0.5 M sodium hydroxide solution, followed by 35 ml of propylene oxide (analytical grade ≥99.5%). A thermometer, a condenser, and a magnetic stirrer were connected. The magnetic stirrer was set at 150 rpm, the reaction solution temperature was controlled at 40 °C, and the reaction time was 9 h to further obtain a propylene oxide lignin solution. 3) Add 2M dilute hydrochloric acid to the oxypropionate lignin solution to adjust the pH of the solution to 4.9 to precipitate the product. Use a centrifuge to separate the solid and liquid in the precipitated mixed solution at a speed of 15000 rpm. Then place the solid in a fume hood for 24 hours and then place it in a drying oven for 24 hours at a drying temperature of 40°C to obtain oxypropionate lignin. 4) Solid lignin was dissolved in 1,4-dioxane (analytical grade ≥99.5%) at a mass ratio of lignin solid to organic solvent of 1:15. The mixture was then reacted with 2,5-furandicarboxylic acid under vacuum (vacuum degree -0.09) and 95% concentrated sulfuric acid catalyst conditions. The catalyst addition was 0.1% of the mass of 2,5-furandicarboxylic acid. The ratio of hydroxyl groups in the lignin to carboxyl functional groups in the 2,5-furandicarboxylic acid was 1:1.1. The reaction temperature was 50°C, and the reaction time was 12 h. After the reaction, the product was precipitated in 100 ml of methanol. The solid-liquid separation was performed by centrifugation for 15 min at a speed of 15000 rpm, yielding esterified lignin. 5) After drying the esterified lignin at 22℃ for 24 hours, it was ball-milled for 5 hours. The particle size of the esterified lignin after ball milling was 10 μm. Then, it was added to liquid unsaturated polyester in a ratio of 4:100. The lignin was then ultrasonically dispersed for 40 minutes to obtain the esterified lignin-based unsaturated polyester liquid, i.e., the modified unsaturated polyester prepolymer solution. 6) Add the accelerator and curing agent to the modified unsaturated polyester prepolymer solution obtained by ultrasonic dispersion in proportion. The accelerator is cobalt isooctanoate and the curing agent is methyl ethyl ketone peroxide. The volume of the accelerator and curing agent is 2% of the volume of the liquid unsaturated polyester solution. After mixing evenly, pour it into a silicone mold for curing. The curing temperature is 22℃ and the curing time is 22h. Finally, esterified lignin-based unsaturated polyester solid is obtained, that is, modified unsaturated polyester.

[0035] The mechanical, volume shrinkage, and durability behavior of the unsaturated polyester were characterized according to the relevant standards and test methods in Example 1.

[0036] Comparative Example 3 Take 100g of unmodified unsaturated polyester resin solution, add cobalt isooctanoate as an accelerator and methyl ethyl ketone peroxide as a curing agent, and the amount of accelerator and curing agent added is 2% of the volume of unsaturated polyester resin. After stirring thoroughly, pour into a mold and cure at 22℃ for 22h to obtain the control sample.

[0037] Example 4

[0038] The difference from Example 1 is that the process parameters in the preparation of esterified lignin are different. In this case, the bio-based dicarboxylic acid used is adipic acid, and the mass ratio of esterified lignin to unsaturated polyester in the modified unsaturated polyester resin is 5:100.

[0039] 1) Dissolve 30g of residual alkali lignin from the papermaking industry in 450ml of methanol (analytical grade ≥99.5%). The lignin particle size is 10μm. Stir the mixture at 200 rpm using a magnetic stirrer at 20℃ to ensure complete reaction for 12h. Separate the mixture into solid and liquid phases. Place the obtained solid phase in a drying oven and dry it at 40℃ for 24h to obtain methanol-insoluble lignin. 2) The methanol-insoluble lignin obtained was added to a three-necked flask containing 100 ml of 0.5 M sodium hydroxide solution, followed by 20 ml of propylene oxide (analytical grade ≥99.5%). A thermometer, a condenser, and a magnetic stirrer were connected. The magnetic stirrer was set to 100 rpm, the reaction solution temperature was controlled at 40 °C, and the reaction time was 6 h to further obtain a propylene oxide lignin solution. 3) Add 2M dilute hydrochloric acid to the oxypropionate lignin solution to adjust the pH of the solution to 4.3 to precipitate the product. The volume of dilute hydrochloric acid used is 50 ml. Use a centrifuge to separate the solid and liquid in the precipitated mixture at a speed of 10,000 rpm. Then place the solid in a fume hood for 24 hours and then place it in a drying oven for 24 hours at a drying temperature of 40°C. After drying, oxypropionate lignin is obtained. 4) Propylene oxide lignin solid was dissolved in 1,4-dioxane (analytical grade ≥99.5%), with a mass ratio of propionyl oxylignin solid to organic solvent of 1:10. The reaction was carried out under vacuum (vacuum degree -0.09) and 95% concentrated sulfuric acid catalyst conditions with adipic acid. The catalyst addition was 0.1% of the mass of adipic acid. The ratio between the hydroxyl groups in the lignin and the carboxyl functional groups in itaconic acid was 1:1. The reaction temperature was 50°C, and the reaction time was 12 h. After the reaction, the product was precipitated in 50 ml of methanol. The solid-liquid separation was performed by centrifugation for 15 min at a speed of 10000 rpm, yielding esterified lignin. 5) After drying the esterified lignin at 20°C for 24 hours, it was ball-milled for 1 hour. The particle size of the esterified lignin after ball milling was 14 μm. Then, the esterified lignin was added to the liquid unsaturated polyester resin solution at a mass ratio of 5:100. The solution was then ultrasonically dispersed for 20 minutes to obtain the esterified lignin-based unsaturated polyester liquid, i.e., the modified unsaturated polyester prepolymer solution. 6) Add the accelerator and curing agent to the modified unsaturated polyester prepolymer solution obtained by ultrasonic dispersion in proportion. The accelerator is cobalt isooctanoate and the curing agent is methyl ethyl ketone peroxide. The volume of the accelerator and the curing agent are 1% of the liquid unsaturated polyester solution. After mixing evenly, pour it into a silicone mold for curing. The curing temperature is 20℃ and the curing time is 20h. Finally, esterified lignin-based unsaturated polyester solid is obtained, that is, modified unsaturated polyester.

[0040] The materials prepared in Examples 1-4 and Comparative Examples 1-3 were characterized for their mechanical, volume shrinkage, and durability behavior as unsaturated polyesters according to relevant standards and test methods. The test results are shown in Tables 1 and 2.

[0041] Mechanical property testing: The tensile strength, elongation at break, and toughness index of unsaturated polyester were tested according to GB / T1447-2005 "Test Method for Tensile Properties of Fiber Reinforced Plastics". Volume shrinkage rate: Measure the mass and volume of the unsaturated polyester resin solution with added accelerator and curing agent, and calculate the density of the sample before curing; after curing at room temperature, measure the mass and volume of the unsaturated polyester resin solid again, and calculate the density of the resin after curing. At the same time, obtain the rate of change of density before and after curing, which is the volume shrinkage rate. UV, acid and alkali, and seawater resistance: The cured unsaturated polyester was placed in an accelerated UV aging chamber for 100 hours with a UV wavelength of 365-395 nm, and the change rate of tensile strength before and after UV aging was compared. In addition, the cured unsaturated polyester was immersed in 10% sodium hydroxide, 10% hydrochloric acid, and seawater solution, respectively. After 14 days, the mass and tensile strength change rate of the test samples were taken out. Ultraviolet shielding test: The transmittance of unsaturated polyester samples was tested using an ultraviolet spectrophotometer in the range of 200–800 nm.

[0042] Table 1 shows the changes in mechanical properties of unsaturated polyester before and after modification.

[0043] As shown in Table 1, the mechanical properties of unsaturated polyester modified with esterified lignin changed significantly. Compared with Comparative Example 1, Example 1 showed a 27.73% increase in tensile strength, a 39.47% increase in elongation at break, a 155.68% increase in toughness index, and a 41.49% decrease in volume shrinkage. Compared with Comparative Example 2, Example 2 showed a 26.06% increase in tensile strength, a 38.96% increase in elongation at break, a 123.23% increase in toughness index, and a 28.04% decrease in volume shrinkage. Compared with Comparative Example 3, Example 3 showed a significant increase in tensile strength. The tensile strength of Example 4 increased by 25.22%, the elongation at break increased by 87.13%, the toughness index increased by 161.27%, and the volume shrinkage decreased by 41.90% compared to Comparative Example 1. Therefore, esterified lignin can significantly improve the mechanical properties of unsaturated polyester, effectively solving the problems of low mechanical strength and fracture toughness, poor structural stability, high brittleness, and high volume shrinkage after curing of unsaturated polyester.

[0044] Meanwhile, the modified unsaturated polyester exhibits superior UV shielding performance, with its UV transmittance at 350 nm significantly reduced by 43.72% (Example 1), 33.94% (Example 2), 53.73% (Example 3), and 71.20% (Example 4).

[0045] In addition, the modification method and related processes of unsaturated polyester are simple and efficient, with no process exceeding 24 hours, thus effectively solving the problems of high energy consumption, complex processes and long time consumption in the modification method of unsaturated polyester.

[0046] Table 2 shows the changes in the durability behavior of the modified unsaturated polyester before modification.

[0047] As shown in Table 2, after immersion in acid, alkali solutions, and seawater, the tensile strength loss rates were 1.5%–4% (Example 1), 1.75%–3.5% (Example 2), 1.95%–5% (Example 3), and 2%–4.8% (Example 4), all significantly lower than those of unmodified unsaturated polyester. This demonstrates that modified unsaturated polyester exhibits excellent resistance to acids, alkalis, seawater corrosion, and UV radiation.

Claims

1. A method for modifying unsaturated polyester, characterized in that, Includes the following steps: Step 1: Dissolve the residual alkali lignin from the papermaking industry in methanol, and magnetically stir the mixture at 20-25°C until the reaction is complete; Step 2: Perform solid-liquid separation on the mixture from Step 1, and dry the resulting solid phase at 40°C for 24 hours to obtain methanol-insoluble lignin; Step 3: Add the methanol-insoluble lignin obtained in Step 2 to a three-necked flask containing sodium hydroxide solution, then add propylene oxide, control the reaction temperature at 40℃, maintain the reaction for 6~12h, and further obtain propylene oxide lignin solution. Step 4: Add dilute hydrochloric acid to the lignin-oxidized solution obtained in Step 3 above to adjust the pH and precipitate the product. After centrifugation, take the solid precipitate and dry it to obtain lignin-oxidized solution. Step 5: Dissolve the solid propionyl lignin obtained in Step 4 in an organic solvent and react it with a bio-based dicarboxylic acid under vacuum and catalyst conditions; after the reaction is completed, add methanol to the reaction solution to precipitate the precipitate, and obtain esterified lignin after solid-liquid separation; Step 6: Dry and ball mill the esterified lignin obtained in Step 5 above, then add it to liquid unsaturated polyester in proportion, and after ultrasonic dispersion, obtain esterified lignin-based unsaturated polyester liquid, i.e. modified unsaturated polyester prepolymer solution. Step 7: Add accelerator and curing agent to the modified unsaturated polyester prepolymer solution in proportion, and ultrasonically disperse the prepolymer solution to obtain a uniform modified unsaturated polyester solution. Then pour the solution into a silicone mold for curing to finally obtain esterified lignin-based unsaturated polyester solid, i.e., modified unsaturated polyester.

2. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step one, the particle size of lignin is 20~100μm, the volume of methanol is 450ml, the reaction time is 12~24h, and the magnetic stirrer speed is 200~300rpm.

3. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step three, the concentration of the sodium hydroxide solution is 0.5M and the volume is 100~200 ml; the volume of propylene oxide is 20~50 ml; and the speed of the magnetic stirrer is 100~200 rpm.

4. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step four, the concentration of dilute hydrochloric acid is 2M, the pH is adjusted to 4.3~5.5, the centrifugation time used in solid-liquid separation is 15min, and the centrifuge speed is 10000~20000 rpm.

5. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step four, the solid lignin oxide collected by centrifugation is placed in a fume hood for 24 hours and then dried in a drying oven at 40°C for 24 hours.

6. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step five, the mass ratio of propionic lignin solid to organic solvent is 1:10~20; the vacuum degree of the vacuum environment is 0.09; the bio-based dicarboxylic acid used in the reaction is itaconic acid, 2,5-furandicarboxylic acid, sebacic acid or adipic acid; the catalyst is 95% concentrated sulfuric acid, which is added at 0.1% of the mass of the bio-based dicarboxylic acid; the ratio of hydroxyl / carboxyl functional groups in the reactants is 1:1~1.2; the reaction temperature is 50°C; the reaction time is 12h; and the volume ratio of methanol to the reaction solution is 1:

1.

7. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step six, the esterified lignin is dried at 20-25℃ for 24 hours, then ball-milled for 1-10 hours. After ball milling, the particle size of the esterified lignin is 5-50 μm. The esterified lignin is then added to liquid unsaturated polyester and ultrasonically treated for 20-60 minutes. The mass ratio of esterified lignin to unsaturated polyester is 1-7:

100.

8. The method for modifying unsaturated polyester according to claim 1, characterized in that, In step seven, the accelerator is cobalt isooctanoate, the curing agent is methyl ethyl ketone peroxide, the volume of the accelerator and the curing agent are 1~2% of the liquid unsaturated polyester solution, the curing temperature is 20~25℃, and the curing time is 20~24h.

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