Straw tar phenol-lignin epoxy carbon fiber reinforced composite material as well as preparation method and application thereof

Abstract

The invention discloses a straw tar phenol-lignin epoxy carbon fiber reinforced composite material and a preparation method and application thereof, and belongs to the technical field of polymer synthesis. Alkali lignin is subjected to hydroxymethylation treatment to obtain hydroxymethylated alkali lignin; mixing the hydroxymethylated alkali lignin with acrylic anhydride, and carrying out esterification reaction to obtain acryloylated lignin; carrying out epoxidation reaction on the acryloylated lignin, and introducing an epoxy group to form epoxy group grafted lignin (lignin epoxy resin); straw tar phenol and the epoxy group grafted lignin are mixed, and alkali lignin-based epoxy resin is prepared; carbon fibers subjected to surface modification treatment, acetone, a curing agent and alkali lignin-based epoxy resin are prepared into prepreg, and the straw tar phenol-lignin epoxy carbon fiber reinforced composite material is prepared through hot press molding, so that the problems that bisphenol A type epoxy resin pollutes the environment and is high in price are solved; therefore, the composite material prepared by the method disclosed by the invention has a wide application prospect in various fields.

Description

Technical Field

[0001] This invention belongs to the field of polymer synthesis technology, and particularly relates to a straw tar phenol-lignin epoxy carbon fiber reinforced composite material, its preparation method and application. Background Technology

[0002] The depletion of fossil fuels and increasing pollution have prompted the search for sustainable alternatives to petrochemical products. Lignin, the second largest natural polymer after cellulose, can be obtained from agricultural or pulp industry waste. Containing numerous aromatic ring structures, it can be used as a precursor for carbon fiber production, reducing costs by over 50% compared to polyacrylonitrile-based carbon fiber, offering significant advantages in cost and sustainability. Lignin contains approximately 60%-66% carbon, far exceeding cellulose (≈40%), and possesses its own aromatic skeleton, allowing for self-crosslinking during carbonization at 800-2800℃, reducing pre-oxidation time by over 30% and energy consumption by 20%. Furthermore, straw tar, a product of straw pyrolysis and other processing methods, has a low utilization rate. Extracting straw tar phenols from straw tar is one method to improve the comprehensive utilization rate of straw. However, the industry generally considers straw tar a byproduct of pyrolysis, with complex composition, numerous impurities, and high processing costs; it is mostly burned directly as fuel, making high-value utilization extremely difficult.

[0003] Globally, over 90% of carbon fiber relies on petroleum-derived polyacrylonitrile (PAN) as a precursor. Its production involves complex spinning, pre-oxidation, and carbonization processes, with energy consumption accounting for over 30% of the total cost. As a novel composite material reinforcement possessing multiple excellent properties such as high strength, low density, corrosion resistance, and high-temperature resistance, carbon fiber's core function is to overcome the technical bottlenecks of traditional materials like metals, plastics, and glass fibers in specific applications by enhancing their performance. It is widely used in aerospace, transportation, energy conservation and environmental protection, and construction engineering.

[0004] Carbon fiber reinforced epoxy polymer (CFRP), formed by combining carbon fiber (reinforcement) with epoxy resin (matrix), is one of the most widely used advanced composite materials. The combination of carbon fiber and epoxy resin is essentially a precise complementarity between the reinforcement (high strength) and the matrix (strong adhesion + easy molding): the epoxy resin makes the carbon fiber "usable, easy to use, and durable," while the carbon fiber makes the epoxy resin "stronger, stiffer, and lighter." The resulting CFRP has become one of the core materials for resolving the contradiction between lightweighting and high performance in modern industry. Currently, bisphenol A epoxy resin is the most widely used in epoxy resin production. It is made by the condensation reaction of bisphenol A and epichlorohydrin under alkaline conditions, and its consumption accounts for approximately 70% to 80% of the total epoxy resin consumption. However, due to the outdated production process and high production cost of bisphenol A epoxy resin products, it cannot actively lead the market and poses significant risks to the environment and human health. Therefore, finding low-cost, pollution-free biomass alternatives to bisphenol A through the integration of material innovation, process reform, and intelligent technology, and promoting the industry towards high performance, green development, and intelligentization, has become a current research hotspot. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a straw tar phenol-lignin epoxy-based carbon fiber reinforced composite material, its preparation method, and its applications. The carbon fiber reinforced composite material using straw tar phenol-lignin-based epoxy resin as the matrix exhibits excellent properties. Furthermore, the modified lignin particles (hydroxymethylated alkali lignin, epoxidized lignin) used in the resin matrix possess advantages such as high activity, good performance, wide availability, low cost, renewability, and biodegradability, thus solving the problems of environmental pollution and high price associated with bisphenol A type epoxy resins. This gives the composite material prepared using the method of this invention broad prospects for applications in various fields.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a straw tar phenol-lignin epoxy carbon fiber reinforced composite material: alkali lignin is subjected to hydroxymethylation treatment to obtain hydroxymethylated alkali lignin; the hydroxymethylated alkali lignin is mixed with acrylic anhydride and subjected to esterification to obtain acrylamide lignin; the acrylamide lignin is subjected to epoxidation to introduce epoxy groups to form epoxy-grafted lignin (lignin epoxy resin); straw tar phenol is mixed with the epoxy-grafted lignin to prepare an alkali lignin-based epoxy resin; surface-modified carbon fibers, acetone, curing agent, and the alkali lignin-based epoxy resin are mixed to form a prepreg, and the straw tar phenol-lignin epoxy carbon fiber reinforced composite material is prepared by hot pressing.

[0007] It is widely believed in the industry that straw tar is a byproduct of pyrolysis, with complex composition, many impurities, and high processing costs. It is mostly burned directly as fuel, making its high-value utilization extremely difficult. This invention overcomes this prejudice by separating straw tar phenol through a targeted extraction process, using it as a key monomer in epoxy resin synthesis. This not only solves the pollution problem of straw tar but also realizes the transformation from waste to high-value chemical raw material, expanding the utilization pathways of agricultural and forestry waste. This invention utilizes straw tar to prepare straw tar phenol, which contains active phenolic hydroxyl groups and can be used in epoxy resin synthesis reactions. Combining straw tar phenol with lignin epoxy resin not only helps prepare high-performance epoxy resin matrices but also provides a new approach for the high-value utilization of straw tar.

[0008] The synergistic reaction principle of acrylation-epoxidation: The first step, acrylation, not only provides double bonds for epoxidation but also alters the hydrophilicity / hydrophobicity of lignin molecules through the introduction of the acyl group, reducing molecular aggregation. The second step, under 0-5 °C ice bath conditions, uses hydrogen peroxide as a mild oxidant to epoxidize the carbon-carbon double bonds in the acrylation group, generating epoxy groups (-COC-). Furthermore, the only byproduct of this reaction is water, aligning with green chemistry principles. Compared to direct epoxidation, this two-step method allows for precise control of the epoxy group grafting density, avoiding the problems of excessive cross-linking or oxidative degradation of lignin in a single epoxidation reaction.

[0009] Further, the preparation method of the hydroxymethylated alkali lignin is as follows: alkali lignin is dispersed in water to obtain an alkali lignin dispersion; NaOH solution and ammonia are added to the alkali lignin dispersion, and after stirring evenly, formaldehyde is added, and the reaction is carried out under heating conditions. Acid is added to adjust the pH of the solution to acidic conditions to precipitate the precipitate. The precipitate is centrifuged, washed, and freeze-dried to obtain the hydroxymethylated alkali lignin.

[0010] Further, the preparation method of the acrylamide lignin is as follows: hydroxymethylated alkali lignin is dispersed in an organic solvent to form a uniform suspension, acrylic anhydride and pyridine are added, and the reaction is carried out under heating conditions in a protective atmosphere. After the reaction is completed, the resulting reaction mixture is poured into ice water to precipitate the precipitate, filtered, washed, and vacuum dried to obtain the acrylamide lignin.

[0011] Furthermore, the mass ratio of the hydroxymethylated alkali lignin, acrylic anhydride, and pyridine is 1:2:2.

[0012] Further, the preparation method of the epoxy-grafted lignin is as follows: Acrylamide lignin is dispersed in dichloromethane, cooled in an ice bath, hydrogen peroxide is added, and the reaction is carried out in an ice bath. After the reaction is completed, the pH is adjusted to 6.0-6.5, and then sodium chloride solution is added to precipitate the solid. The residual acid is neutralized with saturated sodium bicarbonate solution, washed with water, filtered, and vacuum dried to obtain the epoxy-grafted lignin.

[0013] The principle of lignin modification and activation: Natural lignin molecules contain a large number of phenolic hydroxyl groups, but the dense intermolecular hydrogen bonds and stable aggregate state result in low reactivity and poor solubility, making it difficult to use directly in epoxy resin synthesis. This invention employs hydroxymethylation pretreatment, under alkaline conditions (catalyzed by NaOH and ammonia (NH3·H2O)), to induce a nucleophilic addition reaction between the ortho- and ortho-position phenolic hydroxyl groups of lignin and formaldehyde, introducing hydroxymethyl groups (-CH2OH), breaking the intermolecular hydrogen bond network, and simultaneously increasing the number of active reaction sites. In the subsequent acrylation reaction, acrylic anhydride acts as the acylation reagent, undergoing a nucleophilic substitution esterification reaction with the lignin hydroxyl groups under pyridine catalysis, introducing acryloyl groups (-OOC-CH=CH2), further reconstructing the lignin molecular structure and providing key carbon-carbon double bond sites for the epoxidation reaction.

[0014] Further, the preparation method of the straw tar phenol is as follows: add sodium hydroxide solution to straw tar, react under heating conditions, filter, collect the supernatant, adjust the pH of the supernatant to 3, let stand until a precipitate is formed, centrifuge and collect the precipitate, which is the straw tar phenol.

[0015] Furthermore, in the preparation method of straw tar phenol, the heating temperature is 80 ℃.

[0016] Extraction and Utilization Principle of Straw Tar Phenol: Straw tar has a complex composition, containing impurities such as polycyclic aromatic hydrocarbons and oxygen-containing heterocycles. The effective component, tar phenol, is dispersed within it and difficult to utilize directly. This invention employs an alkaline extraction-acidification-solvent extraction process. Utilizing the acidic characteristics of tar phenol, it is converted into phenolate salts dissolved in the aqueous phase under alkaline conditions at 80 °C, separating it from insoluble impurities. Then, by adjusting the pH to 3 through acidification, the phenolate salts are reduced to solid tar phenol and precipitated, achieving the enrichment and purification of the target product. The active phenolic hydroxyl groups contained therein can serve as key monomers in epoxy resin synthesis, undergoing a cross-linking reaction with lignin epoxy resin.

[0017] Further, the preparation method of the alkali lignin-based epoxy resin is as follows: straw tar phenol and the epoxy-grafted lignin are mixed in N,N-dimethylformamide, stirred and dissolved at 60 °C, then heated to 85 °C, sodium hydroxide solution is added to maintain pH=9, and after reacting for 3 h, the mixture is neutralized to neutral with hydrochloric acid, the N,N-dimethylformamide is removed by rotary evaporation, the residue is dissolved with toluene, the salt is washed off, dried with anhydrous sodium sulfate, and concentrated to obtain a brown viscous resin. The obtained brown viscous resin is the alkali lignin-based epoxy resin.

[0018] Furthermore, in the preparation method of alkali lignin-based epoxy resin, the molar ratio of straw tar phenol to epoxy-grafted lignin is 1:0.3.

[0019] Furthermore, the mass percentage of the alkali lignin-based epoxy resin in the total mass of the alkali lignin-based epoxy resin and the surface-modified carbon fiber is 40%. The amount of curing agent used is 30% of the mass of the alkali lignin-based epoxy resin.

[0020] For example, the curing agent is T31 curing agent.

[0021] The present invention also provides a straw tar phenol-lignin epoxy carbon fiber reinforced composite material prepared by the above method.

[0022] This invention also provides applications of the above-mentioned straw tar phenol-lignin epoxy carbon fiber reinforced composite material in the fields of aerospace, transportation, energy and environmental protection, and construction engineering.

[0023] Compared with the prior art, the present invention has the following advantages and technical effects: This invention modifies alkali lignin by hydroxymethylation, and then freeze-dries it to obtain hydroxymethylated alkali lignin. Solid phenolic compounds are collected from a complex mixture produced by straw pyrolysis (i.e., straw tar) through alkali extraction and acidification-solvent extraction. The hydroxymethylated alkali lignin undergoes an esterification reaction with acrylic anhydride. The hydroxymethylated alkali lignin molecule contains a large number of active hydroxyl groups (-OH), which are the key active sites for the reaction. Acrylic anhydride ((CH2=CH-CO)2O) acts as an acylation reagent, and under pyridine catalysis, it undergoes a nucleophilic substitution esterification reaction with the hydroxyl groups in the hydroxymethylated alkali lignin. Finally, an acryloyl group (-OOC-CH=CH2) is introduced into the lignin molecule to achieve modification, generating acrylamide lignin. The carbon-carbon double bonds in the prepared acrylamide lignin are epoxidized to introduce epoxy groups, preparing a lignin epoxy resin. The core reaction of epoxidation is the epoxidation of carbon-carbon double bonds. Under low-temperature conditions in an ice bath, hydrogen peroxide (H2O2) is added dropwise as an oxidant to undergo an epoxidation reaction with the carbon-carbon double bonds. Using carbon fibers (which have undergone surface modification) as reinforcement, a prepreg is prepared with alkali lignin-based epoxy resin as the matrix. Straw tar phenol-lignin epoxy-reinforced carbon fiber composites are then prepared by hot pressing. Simultaneously, different types of carbon fiber reinforced composites were prepared using different types of epoxy resins as matrices, demonstrating that the straw tar phenol-lignin epoxy-reinforced carbon fiber composites prepared using the method of this invention possess excellent properties. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart illustrating the overall design of the present invention. Figure 2 The infrared (IR) spectra of alkali lignin and hydroxymethylated alkali lignin in Example 1 are compared. Figure 3 Differential scanning calorimetry (DSC) images of alkali lignin before and after hydroxymethylation treatment; Figure 4 Thermogravimetric analysis (TG) graphs of alkali lignin before and after hydroxymethylation treatment; Figure 5 The differential thermogravimetric analysis (DTG) plots show the alkali lignin before and after hydroxymethylation treatment; Figure 6 This is the total ion chromatogram of straw tar before extraction (i.e., raw straw tar) in Example 1; Figure 7 The total ion chromatogram after straw tar extraction in Example 1 is shown. Figure 8The infrared spectra of lignin before and after acrylylation in Example 1 are shown below. Alkali lignin refers to lignin before acrylylation, and acrylated lignin refers to lignin after acrylylation. Figure 9 The infrared spectra of acrylamide lignin before and after epoxidation in Example 1 are shown below. Acrylamide lignin refers to acrylamide lignin before epoxidation, and epoxidized lignin refers to acrylamide lignin after epoxidation. Figure 10 This is a schematic diagram of carbon fiber surface modification in Example 1; Figure 11 This is a total distribution spectrum of EDS layered images of carbon fiber before surface modification in Example 1; Figure 12 This is a total distribution spectrum of EDS layered images after carbon fiber surface modification in Example 1; Figure 13 This is a scanning electron microscope image of the carbon fiber surface before modification in Example 1; Figure 14 The image shows the surface morphology of the carbon fiber before surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 64.3 nm. Figure 15 The image shows the three-dimensional surface morphology of the carbon fiber before surface modification in Example 1, with a scanning area of ​​1 μm × 1 μm. Figure 16 The image shows the surface morphology of the carbon fiber before surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 10.4 nm. Figure 17 This is a scanning electron microscope image of the carbon fiber surface modification in Example 1; Figure 18 The image shows the surface morphology of the carbon fiber surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 348 nm. Figure 19 This is a three-dimensional surface morphology image of the carbon fiber surface modification in Example 1, with a scanning area of ​​1 μm × 1 μm. Figure 20 The image shows the surface morphology of the carbon fiber surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 187 nm. Figure 21 This is a scanning electron microscope image of the carbon fiber surface after modification in Example 1; Figure 22The image shows the surface morphology of the carbon fiber surface after surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 134.9 nm. Figure 23 The image shows the three-dimensional surface morphology of the carbon fiber surface after surface modification in Example 1, with a scanning area of ​​1 μm × 1 μm. Figure 24 The image shows the surface morphology of the carbon fiber surface after surface modification in Example 1, with a scanning area of ​​200.0 nm × 200.0 nm and a height difference of approximately 35.7 nm.

[0025] Figure 25 This is a measurement diagram of the water contact angle of the unmodified carbon fiber in Example 1; Figure 26 This is a measurement diagram of the water contact angle after carbon fiber modification in Example 1; Figure 27 The shear stress-strain curves are shown for the straw tar phenol-lignin epoxy carbon fiber composite material and the carbon fiber composite material prepared by commercially available E-51 in Example 1. Figure 28 The image shows the bending stress-strain curve of the straw tar phenol-lignin epoxy carbon fiber composite material in Example 1. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] Embodiments of the present invention provide a method for preparing straw tar phenol-lignin epoxy carbon fiber reinforced composite materials, the overall design process of which is as follows: Figure 1 As shown: Alkali lignin is hydroxymethylated to obtain hydroxymethylated alkali lignin; hydroxymethylated alkali lignin is mixed with acrylic anhydride and subjected to esterification to obtain acrylamide lignin; acrylamide lignin is subjected to epoxidation to introduce epoxy groups to form epoxy-grafted lignin (lignin epoxy resin); straw tar phenol is mixed with epoxy-grafted lignin to prepare alkali lignin-based epoxy resin; surface-modified carbon fibers, acetone, curing agent and alkali lignin-based epoxy resin are made into a prepreg, and straw tar phenol-lignin epoxy carbon fiber reinforced composite material is prepared by hot pressing.

[0032] For example, the curing agent is T31 curing agent.

[0033] In a preferred embodiment of the present invention, the preparation method of hydroxymethylated alkali lignin is as follows: alkali lignin is dispersed in water to obtain an alkali lignin dispersion; NaOH solution and ammonia are added to the alkali lignin dispersion, and after stirring evenly, formaldehyde is added, and the reaction is carried out under heating conditions. Acid is added to adjust the pH of the solution to acidic conditions to precipitate the precipitate, which is then centrifuged, washed, and freeze-dried to obtain hydroxymethylated alkali lignin.

[0034] In a preferred embodiment of the present invention, in the method for preparing hydroxymethylated alkali lignin, the reaction process under heating conditions is as follows: the reaction is carried out in an oil bath at 70 °C for 4 h.

[0035] In a preferred embodiment of the present invention, in the method for preparing hydroxymethylated alkali lignin, the centrifugation process is as follows: centrifuging at a rate of 7000 r / min for 20 min using a high-speed centrifuge.

[0036] In a preferred embodiment of the present invention, the preparation method of acrylamide lignin is as follows: hydroxymethylated alkali lignin is dispersed in an organic solvent to form a uniform suspension, acrylic anhydride and pyridine are added, and the reaction is carried out under heating conditions in a protective atmosphere. After the reaction is completed, the resulting reaction mixture is poured into ice water to precipitate the precipitate, filtered, washed, and vacuum dried to obtain acrylamide lignin.

[0037] In a preferred embodiment of the present invention, the organic solvent in the method for preparing acryloyl lignin is N,N-dimethylformamide (DMF).

[0038] In a preferred embodiment of the present invention, in the method for preparing acrylated lignin, the mass ratio of hydroxymethylated alkali lignin, acrylic anhydride and pyridine is 1:2:2.

[0039] In a preferred embodiment of the present invention, the reaction process under a protective atmosphere and heating conditions in the preparation method of acrylamide lignin is as follows: under nitrogen protection, the reaction is stirred at 80 °C for 12 h.

[0040] In a preferred embodiment of the present invention, the preparation method of epoxy-grafted lignin is as follows: Acrylamide lignin is dispersed in dichloromethane, cooled in an ice bath, hydrogen peroxide is added, and the reaction is carried out in an ice bath. After the reaction is completed, the pH is adjusted to 6.0-6.5, and then sodium chloride solution is added to precipitate the solid. The residual acid is neutralized with saturated sodium bicarbonate solution, washed with water, filtered, and vacuum dried to obtain epoxy-grafted lignin.

[0041] In a preferred embodiment of the invention, the ice bath is cooled to 0-5°C.

[0042] In a preferred embodiment of the present invention, the concentration of the sodium chloride solution is 5 wt%.

[0043] In a preferred embodiment of the present invention, the preparation method of straw tar phenol is as follows: sodium hydroxide solution is added to straw tar, the reaction is carried out under heating conditions, the mixture is filtered, the supernatant is collected, the pH of the supernatant is adjusted to 3, the mixture is allowed to stand until a precipitate is formed, the precipitate is centrifuged and collected, which is straw tar phenol.

[0044] In a preferred embodiment of the present invention, in the method for preparing straw tar phenol, the heating temperature is 80 °C and the reaction time is 4 h.

[0045] In a preferred embodiment of the present invention, the preparation method of alkali lignin-based epoxy resin is as follows: straw tar phenol and epoxy-grafted lignin are mixed in N,N-dimethylformamide, stirred and dissolved at 60 °C, then heated to 85 °C, sodium hydroxide solution is added to maintain pH=9, and after reacting for 3 h, the mixture is neutralized to neutral with hydrochloric acid, N,N-dimethylformamide is removed by rotary evaporation, the residue is dissolved with toluene, the salt is washed off, dried with anhydrous sodium sulfate, and concentrated to obtain a brown viscous resin. The obtained brown viscous resin is the alkali lignin-based epoxy resin.

[0046] In a preferred embodiment of the present invention, the mass of the alkali lignin-based epoxy resin accounts for 40% of the total mass of the alkali lignin-based epoxy resin and the surface-modified carbon fiber; The amount of curing agent used is 30% of the mass of the alkali lignin-based epoxy resin.

[0047] In a preferred embodiment of the present invention, the carbon fiber surface modification process is as follows: Carbon fibers are cut into rectangular samples of 300 mm × 100 mm, and immersed in analytical grade acetone solution in a constant temperature water bath at 40 °C for 48 h. The dried carbon fibers are then immersed in concentrated nitric acid and reacted in a constant temperature water bath at 50 °C for 160 min. After the reaction, the carbon fibers are washed again with distilled water until neutral (pH=7), and finally dried to constant weight in a vacuum drying oven at 90 °C.

[0048] In a preferred embodiment of the present invention, the prepreg is prepared as follows: surface-modified carbon fiber is mixed with alkali lignin-based epoxy resin, diluted with acetone, and T31 curing agent is added to make it uniformly mixed. The impregnated laminate is placed in a mold, vacuum bag pressure is performed, and after sealing, it is left to stand at 30°C for 8 hours to finally obtain the prepreg.

[0049] In a preferred embodiment of the present invention, the curing temperature during hot pressing is 130 °C, the curing pressure is 2 MPa, and the curing time is 30 min.

[0050] An embodiment of the present invention also provides a straw tar phenol-lignin epoxy carbon fiber reinforced composite material prepared by the above method.

[0051] The embodiments of the present invention also provide applications of the above-mentioned straw tar phenol-lignin epoxy carbon fiber reinforced composite material in the fields of aerospace, transportation, energy and environmental protection, and construction engineering.

[0052] Unless otherwise specified, the room temperature in this invention is 25±2℃.

[0053] All raw materials used in the embodiments of this invention were purchased commercially. Alkali lignin was purchased from Shanghai Aladdin Technology Co., Ltd., model number 8068-05-1; straw tar was purchased from Hubei Maidehao Chemical Co., Ltd., model number 8001-58-9; and carbon fiber was purchased from Suzhou Qicai New Material Technology Co., Ltd., model number T300.

[0054] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0055] The technical solution of the present invention will be further illustrated by the following embodiments.

[0056] Example 1 A method for preparing a straw tar phenol-lignin epoxy carbon fiber reinforced composite material, comprising the following steps: Step 1: Preparation of hydroxymethylated alkali lignin 5 g of alkali lignin was dispersed in 100 mL of distilled water and stirred for 1 h to obtain an alkali lignin dispersion. 10 mL of 12.5 mol / L NaOH solution and 10 mL of 13.33 mol / L NH3·H2O were added to the above alkali lignin dispersion, and the mixture was shaken at 30 °C for 2 h. 7 g of 13.16 mol / L analytical grade formaldehyde solution was added, and the mixture was reacted at 70 °C for 4 h. 1.0 mol / L HCl solution was added until the pH of the solution was 3, at which point the precipitate was formed. The solid was separated by centrifugation at 7000 r / min for 20 min, washed three times with excess distilled water, and freeze-dried for 24 h to obtain hydroxymethylated alkali lignin.

[0057] Comparison of infrared spectra of unmodified alkali lignin and alkali lignin pretreated with hydroxymethylation (i.e., hydroxymethylated alkali lignin). Figure 2 As shown, after comparison, at 1043 cm -1 The absorption peak at this position represents the CO production in hydroxymethyl (-CH2OH). Under the catalysis of NaOH and NH3·H2O, the phenolic hydroxyl groups in alkali lignin react with formaldehyde at the ortho position, promoting the hydroxymethylation reaction. (Control) Figure 2 The peaks of alkali lignin and hydroxymethylated alkali lignin at this point show a significant increase in absorption peaks after hydroxymethylation modification, indicating a substantial increase in the number of hydroxymethyl (-CH2OH) groups. (3400 cm⁻¹) -1 The absorption peak at 2918 cm⁻¹ is attributed to the stretching vibration absorption peak of the hydroxyl group -OH in the lignin structure. -1 2845 cm -1 The absorption peaks at 1600 cm⁻¹ are respectively attributed to the antisymmetric stretching vibration of the CH bond in the methylene group and the stretching vibration of the CH bond in the methoxy group. -1 1500 cm -1 1425 cm -1 The peak at 1240 cm⁻¹ represents the absorption peak of the aromatic ring skeleton vibration. -1 The absorption peak at 790 cm⁻¹ corresponds to the methoxy group in guaiac lignin. The presence of this peak in alkali lignin after hydroxymethylation treatment demonstrates a significant increase in methoxy structural units. -1 The peak value at this point represents the out-of-plane deformation vibration of CH on the benzene ring in the alkali lignin structure. The weakening of the peak value in hydroxymethylated alkali lignin at this point is due to the substitution of hydrogen on the benzene ring by hydroxymethyl groups.

[0058] DSC results before and after alkali lignin hydroxymethylation treatment are as follows: Figure 3 As shown, the TG and DTG results before and after alkali lignin hydroxymethylation treatment are as follows: Figure 4 and Figure 5As shown, due to the high molecular weight of lignin, the increase in active functional groups, and the unique high stability of the benzene ring, the heat resistance temperature of hydroxymethylated alkali lignin increases by 11 ℃, the first thermal decomposition temperature increases by 76 ℃, and the carbon residue increases by 28.3%. The maximum thermal decomposition temperature increases by approximately 142 ℃.

[0059] Step 2: Extraction of phenolic compounds from straw tar Take 100 g of straw tar and add 300 mL of 20% (mass percentage, the same below) sodium hydroxide solution. React at 80 ℃ for 4 h. After filtering the solution, centrifuge to separate insoluble impurities and collect the supernatant. Place the supernatant in a beaker and adjust the pH of the solution to 3 with 2 mol / L hydrochloric acid. Let it stand for 30 minutes until precipitation is observed. Centrifuge at 8000 r / min and collect the precipitate to finally obtain solid straw tar phenolic compounds (referred to as straw tar phenols).

[0060] Total ion chromatograms before and after straw tar extraction are shown below. Figure 6 and Figure 7 As shown, the chromatogram before extraction is as follows ( Figure 6 The presence of numerous dense chromatographic peaks within 12-20 minutes corresponds to a variety of complex organic compounds (such as polycyclic aromatic hydrocarbons and oxygen-containing heterocycles), indicating that the original straw tar has a wide variety of components and high complexity; the extracted chromatogram ( Figure 7 The number of peaks was significantly reduced, with only a few characteristic peaks retained (such as phenols at 2-8 min and polycyclic aromatic hydrocarbons at around 18 min), indicating that the extraction process effectively simplified the compositional complexity of straw tar, enriched the target compounds, and removed a large number of impurities.

[0061] Step 3: Preparation of Acrylate-based Lignin Take 5 g of the hydroxymethylated alkali lignin from step 1 and disperse it in 100 mL of DMF (N,N-dimethylformamide). Stir until a uniform suspension is formed. Add 10 g of acrylic anhydride and 10 g of pyridine, and purge with nitrogen as a protective gas to remove oxygen. Heat to 80 °C and stir continuously for 12 h. Pour the resulting reaction mixture into ice water to precipitate the precipitate. Filter the precipitate and wash it successively with ethanol and deionized water until neutral. Dry under vacuum to obtain acrylamide lignin.

[0062] Step 4: Lignin epoxidation reaction 5 g of acrylamide lignin was dispersed in 100 mL of dichloromethane and cooled to 0 °C in an ice bath. 10 mL of 5% (w / w) hydrogen peroxide was added dropwise. The reaction was maintained at this low temperature with stirring for 4 h (i.e., stirring in an ice bath). The pH was adjusted to 6.2, and then 5 wt% NaCl solution was added to precipitate the solid. The residual acid was neutralized with saturated sodium bicarbonate solution, washed with water until neutral, filtered, and vacuum dried to obtain epoxy-grafted lignin (abbreviated as epoxy lignin).

[0063] Infrared spectra (transmission mode, KBr tableting method) of lignin before and after acrylization in Example 1 are as follows: Figure 8 As shown, unmodified alkali lignin and acrylamide lignin are compared; the infrared spectra (ATR mode, thin film preparation) of acrylamide lignin before and after epoxidation are shown below. Figure 9 As shown, acryloyl lignin is reacted with epoxidized acryloyl lignin ( Figure 9 In comparison, epoxidized lignin is used to represent epoxidized acrylamide lignin; by Figure 8 , Figure 9 It can be seen that after acrylation, the C=C and C=O bond peaks of lignin are significantly absorbed. After oxidation with H2O2, the double bond peaks weaken, forming an epoxy three-membered ring. The byproduct is water, which is clean and environmentally friendly.

[0064] Step 5: Synthesis of alkali lignin-based epoxy resin Straw tar phenol and epoxy lignin were dissolved in 30 mL of DMF at a molar ratio of 1:0.3 and stirred at 60 °C. The temperature was then raised to 85 °C, and 40% NaOH solution (total amount equal to 1.1 equivalents of the phenolic hydroxyl groups) was added dropwise, controlling the addition rate and maintaining pH = 9. After reacting for 3 h, the solution was neutralized with hydrochloric acid. DMF was removed by rotary evaporation (60 °C, -0.08 MPa). The residue was dissolved in toluene and washed three times to remove salts. The solution was dried over anhydrous sodium sulfate and concentrated to obtain a brown, viscous resin, which is an alkali lignin-based epoxy resin with an epoxy value of 0.25–0.38 mol / 100 g.

[0065] Step 6: Synthesis of composite materials Carbon fiber surface activity modification treatment (see schematic diagram of carbon fiber surface modification) Figure 10As shown): Carbon fibers were cut into rectangular samples of 300 mm × 100 mm, laid flat, and treated with analytical grade acetone in a sealed water bath at 40 ℃ for 48 h. The samples were then washed with distilled water until pH=7, dried in a vacuum drying oven at 70 ℃, soaked in 150 mL of concentrated nitric acid (65%~68% concentration), heated in a water bath to 50 ℃ for 160 min, washed with distilled water until pH=7, and finally dried in a vacuum drying oven at 90 ℃ until constant weight. The total number of EDS layered images before and after surface modification of the carbon fibers in Example 1 is shown below. Figure 11 and Figure 12 As shown, the oxygen content of the carbon fiber increased from 2.21% to 7.21% after modification. The increase in surface oxygen content indicates that the water-based epoxy resin sizing agent adhering to the carbon fiber surface is beneficial to the interfacial adhesion of the reinforcement.

[0066] Preparation of composite prepreg and molding of carbon fiber composite: Surface-modified carbon fibers were cut to 100 mm × 100 mm. 6 g of surface-modified carbon fibers were mixed with the alkali lignin-based epoxy resin prepared in step 5 at a ratio of 60 wt% and 40 wt%, diluted with 50 mL of acetone, and T31 curing agent (30 wt% of the alkali lignin-based epoxy resin) was added and mixed. The mixture was spread with a brush, and the impregnated layers were placed in a mold. Vacuum bag treatment was applied, and the mixture was vacuum sealed at 30 ℃ for 8 h to obtain the composite prepreg. The obtained prepreg was placed in the mold to fill the mold cavity, and the mold was closed to seal it. The sealed mold was preheated to 80 ℃ and placed in a double-layer flat vulcanizing hydraulic press for curing at 130 ℃ (curing pressure 2 MPa, curing time 30 min). After cooling to room temperature and demolding, the composite material was obtained. The resulting composite material is a straw tar phenol-lignin epoxy carbon fiber reinforced composite.

[0067] For comparative purposes, this invention uses commercially available E-51 epoxy resin to prepare E-51 epoxy resin-based carbon fiber reinforced composite materials. The specific steps are as follows: Commercially available bisphenol A type E-51 epoxy resin (epoxy value 0.48~0.54 mol / 100g) was selected as the matrix, and T31 curing agent was used (resin to curing agent mass ratio of 100:30, consistent with the straw tar phenol-lignin system). The carbon fiber shaped fabric was the same as in step 6 (300mm×100mm). Carbon fiber surface modification: The modification process of step 6 was completely followed. Prepreg preparation: Carbon fiber and E-51 epoxy resin were mixed at a mass ratio of 60:40, diluted with acetone to a uniform viscosity, and T31 curing agent was added dropwise and stirred evenly; the mixture was evenly coated on the surface of the modified carbon fiber fabric, placed in a mold, vacuum-sealed, and left to stand at 30 ℃ for 8 h to obtain the prepreg. Curing and molding: The same curing process as in step 6 was used to obtain the E-51 epoxy resin-based carbon fiber reinforced composite material.

[0068] Example 2 The preparation process in this embodiment is basically the same as that in Example 1, mainly changing the reaction temperature, time, and rotation speed in each step. The specific steps are as follows: Step 1: Lignin hydroxymethylation Take 4 g of alkali lignin, add 80 mL of distilled water, and stir to disperse evenly; then add 8 mL of 12.5 mol / L NaOH solution and 8 mL of 13.33 mol / L NH3·H2O solution in sequence, mix evenly, add 6 g of 13.16 mol / L formaldehyde solution, shake at 28 ℃ for 1.8 h, and then raise the temperature to 68 ℃ for 3.8 h; after the reaction is completed, centrifuge at 6500 r / min for 18 min to obtain hydroxymethylated alkali lignin.

[0069] Step 2: Extraction of phenolic compounds from straw Take 80 g of straw tar and add it to 250 mL of 18 wt% NaOH solution. React at 78 ℃ for 3.8 h. After the reaction is complete, adjust the pH of the system to 3.2 with 2 mol / L hydrochloric acid, and then centrifuge at 7500 r / min to obtain tar phenol extract.

[0070] Step 3: Preparation of Acrylate-based Lignin This step is exactly the same as in Example 1.

[0071] Step 4: Lignin epoxidation reaction This step is exactly the same as in Example 1.

[0072] Step 5: Synthesis of alkali lignin-based epoxy resin The tar phenol extract obtained in step 2 was mixed with the hydroxymethylated alkali lignin obtained in step 1 at a molar ratio of 1:0.25 to tar phenol and epoxy lignin. 28 mL of DMF was added as a solvent, followed by 40 wt% NaOH solution with 1.05 equivalents of phenolic hydroxyl groups. The mixture was first stirred and dissolved at 58 °C, and then heated to 84 °C for 2.8 h. After the reaction was completed, DMF was removed by rotary evaporation at 58 °C and 0.07 MPa to obtain epoxy resin.

[0073] Step 6: Synthesis of composite materials Carbon fiber surface activity modification treatment (see schematic diagram of carbon fiber surface modification) Figure 10 As shown): Take a 300 mm × 100 mm carbon fiber shaping cloth, immerse it in analytical grade acetone, and treat it at 38 ℃ for 45 h; then take out the carbon fiber shaping cloth, add 120 mL of concentrated nitric acid, and react at 48 ℃ for 150 min; after the reaction is completed, dry the carbon fiber shaping cloth in an 88 ℃ vacuum oven to obtain modified carbon fiber.

[0074] Preparation of composite prepreg and molding of carbon fiber composite: The modified carbon fiber obtained in step 4 and the epoxy resin obtained in step 3 were mixed at a mass ratio of carbon fiber to resin of 62:38. T31 curing agent with a mass ratio of 28:100 to resin was added and stirred evenly. The mixture was then allowed to stand at 28 ℃ for 7.5 h to prepare the prepreg. The prepreg was then placed in a mold and cured at 128 ℃ and 1.8 MPa for 32 min to obtain straw tar phenol-lignin epoxy carbon fiber reinforced composite.

[0075] Example 3 Compared with the preparation process of Example 1, this embodiment adjusts the alkaline solution ratio and formaldehyde concentration for lignin hydroxymethylation, and also changes the process parameters for subsequent steps. The specific steps are as follows: Step 1: Preparation of hydroxymethylated alkali lignin Take 5 g of alkali lignin, add 100 mL of distilled water, and stir to disperse evenly; then add 12 mL of 10 mol / L NaOH solution and 8 mL of 15 mol / L NH3·H2O solution in sequence, mix evenly, add 7.5 g of 12 mol / L formaldehyde solution, shake at 32 ℃ for 2.2 h, and then raise the temperature to 73 ℃ for 4.2 h; after the reaction is completed, centrifuge at 7200 r / min for 22 min to obtain hydroxymethylated alkali lignin.

[0076] Step 2: Extraction of phenolic compounds from straw tar Take 110 g of straw tar, add 320 mL of 22wt% NaOH solution, and react at 81 ℃ for 4.2 h. After the reaction is completed, adjust the pH of the system to 2.9 with 2 mol / L hydrochloric acid, and then centrifuge at 8200 r / min to obtain tar phenol extract.

[0077] Step 3: Preparation of Acrylate-based Lignin This step is exactly the same as in Example 1.

[0078] Step 4: Lignin epoxidation reaction This step is exactly the same as in Example 1.

[0079] Step 5: Synthesis of alkali lignin-based epoxy resin The tar phenol extract obtained in step 2 was mixed with the hydroxymethylated alkali lignin obtained in step 1 at a molar ratio of 1:0.35 to tar phenol and epoxy lignin. 32 mL of DMF was added as a solvent, followed by 40 wt% NaOH solution with 1.15 equivalents of phenolic hydroxyl groups. The mixture was first stirred and dissolved at 61 °C, and then heated to 87 °C for 3.2 h. After the reaction was completed, DMF was removed by rotary evaporation at 61 °C and 0.08 MPa to obtain epoxy resin.

[0080] Step 6: Synthesis of composite materials Carbon fiber surface activity modification treatment (see schematic diagram of carbon fiber surface modification) Figure 10 As shown): This step is completely consistent with Example 1.

[0081] Composite material molding: The modified carbon fiber obtained in step 4 and the epoxy resin obtained in step 3 were mixed at a mass ratio of carbon fiber to resin of 59:41. T31 curing agent with a mass ratio of 31:100 to resin was added and stirred evenly. The mixture was then allowed to stand at 31 °C for 8.5 h to prepare a prepreg. The prepreg was then placed in a mold and cured at 132 °C and 2.1 MPa for 29 min to obtain straw tar phenol-lignin epoxy carbon fiber reinforced composite material.

[0082] The scanning electron microscope image of carbon fiber before modification in Example 1 is shown below. Figure 13 As shown, the atomic force microscope (AFM) image of the carbon fiber before modification in Example 1 is as follows. Figure 14-16 As shown, the surface of the carbon fiber before modification is relatively flat and the texture is clear, which is the typical state of the carbon fiber precursor.

[0083] The scanning electron microscope image of carbon fiber modification in Example 1 is shown below. Figure 17 As shown, the atomic force microscope (AFM) image of carbon fiber modification in Example 1 is as follows. Figure 18-20As shown, the surface roughness of the carbon fiber increases significantly, with obvious large-scale undulations and local depressions. This is a direct result of concentrated nitric acid etching, which oxidizes and etches more active sites onto the surface.

[0084] The scanning electron microscope image of the modified carbon fiber in Example 1 is shown below. Figure 21 As shown, the atomic force microscope (AFM) image of the modified carbon fiber in Example 1 is as follows. Figure 22-24 As shown, the roughness of the carbon fiber is reduced compared to the modified state, but it is still higher than that before modification. A uniform rough structure is formed on the surface, which is beneficial for subsequent interfacial bonding with epoxy resin.

[0085] By comparison Figure 13-24 The surface morphology and three-dimensional height distribution at different treatment stages visually demonstrate the impact of modification on the surface structure of carbon fibers.

[0086] The water contact angle measurement diagrams before and after carbon fiber modification in Example 1 are shown below. Figure 25 and Figure 26 As shown, the modification treatment effectively improves the surface wettability of carbon fibers, changing them from hydrophobic to hydrophilic. In addition, the modification treatment successfully introduces polar groups (hydroxyl, carboxyl, etc.) into the surface of carbon fibers. These groups enhance the polarity and reactivity of the surface, thereby improving its compatibility and bonding force with polar media.

[0087] The shear stress-strain curve of the composite material in Example 1 is shown below. Figure 27 As shown, the straw tar phenol-lignin epoxy composite material has a higher maximum load, indicating a stronger ultimate load-bearing capacity. The larger initial slope of the blue curve indicates that the straw tar phenol-lignin epoxy composite material has higher initial stiffness, enabling it to withstand larger loads under small displacements. Meanwhile, the straw tar phenol-lignin epoxy composite material can serve as a potential green alternative to E-51 resin.

[0088] The bending stress-strain curve of the composite material in Example 1 is shown below. Figure 28 As shown, the composite material has high flexural strength and elastic modulus, with a flexural strength of approximately 724 MPa, indicating that the carbon fiber and matrix interface are well bonded and the structure has strong overall integrity.

[0089] Comparative Example 1 This comparative example provides a carbon fiber reinforced composite material, prepared in the same way as in Example 1, except that in step 5, straw tar phenol is directly replaced by straw tar in equimolar form.

[0090] This comparative example failed to successfully prepare the composite material. The core reasons are: the straw tar was not purified by alkali extraction and acidification, and the straw tar phenols it contained were encapsulated by a large number of inert impurities such as polycyclic aromatic hydrocarbons and oxygen-containing heterocycles, making it difficult for the active phenolic hydroxyl groups to be exposed. This prevented the cross-linking reaction with epoxy lignin and the formation of a cohesive epoxy resin matrix. Furthermore, the numerous hydrophobic impurities in the straw tar had poor compatibility with DMF and epoxy lignin, resulting in stratification and agglomeration after mixing, which disrupted the homogeneity of the reaction system and prevented the formation of a continuous resin matrix. Finally, the non-phenolic impurities in the straw tar occupied reaction sites, hindering the nucleophilic addition reaction between epoxy groups and phenolic hydroxyl groups. Ultimately, the resin could not be cured and molded, resulting only in a mixture of carbon fibers and impurities, rather than a structurally complete composite material.

[0091] Comparative Example 2 This comparative example provides a carbon fiber reinforced composite material, prepared in the same way as in Example 1, except that in step 5, alkali lignin is directly used to replace epoxy lignin in equimolar amounts.

[0092] The reasons for the performance degradation of the composite material prepared in this comparative example are: alkali lignin was not modified by acrylation-epoxidation, and its molecular structure lacks epoxy groups, making it unable to undergo cross-linking reactions with the phenolic hydroxyl groups of straw tar phenol. The resin matrix is ​​only bound by intermolecular forces, resulting in a loose structure. Natural alkali lignin has dense intermolecular hydrogen bonds and a stable aggregate state, resulting in low solubility and reactivity. Its interaction with the oxygen-containing functional groups (hydroxyl and carboxyl groups) on the carbon fiber surface is weak, leading to poor interfacial bonding between the reinforcement and the matrix, and easy interfacial delamination under stress. Unmodified alkali lignin has a low thermal decomposition temperature and low carbon residue. Figure 4 , Figure 5 As shown, the thermal stability was significantly improved after hydroxymethylation modification, while the performance of the unmodified sample was worse. As a matrix, it could not provide good structural support and heat resistance for the composite material, resulting in a significant decrease in overall mechanical properties and thermal stability.

[0093] Comparative Example 3 This comparative example provides a carbon fiber reinforced composite material, prepared in the same way as in Example 1, except that in step 5, the acryloyl lignin prepared in step 3 of Example 1 is used to directly replace the epoxy lignin in equimolar amounts.

[0094] The reasons for the performance degradation of the composite material prepared in this comparative example are as follows: Acrylamide lignin only introduces carbon-carbon double bonds and does not undergo epoxidation. It lacks epoxy groups that can react with the phenolic hydroxyl groups of straw tar phenol, so the cross-linking reaction of the resin matrix cannot occur, and only a physical blend system can be formed, resulting in poor structural stability. Although acrylamide lignin improves the hydrophilicity and hydrophobicity of lignin, it does not introduce epoxy groups, resulting in a low matching degree between the reaction sites of straw tar phenol. After the two are mixed, it is difficult to form a uniform resin network, leading to defects in the matrix. The lack of strong polar effects of epoxy groups results in weak interaction between the polar functional groups on the surface of the resin matrix and carbon fibers, low interfacial bonding strength, and the inability of stress to be effectively transferred from the matrix to the carbon fiber reinforcement under stress, leading to a significant decrease in the mechanical properties of the composite material.

[0095] Comparative Example 4 This comparative example provides a carbon fiber reinforced composite material, prepared in the same way as in Example 1, except that in step 5, the hydroxymethylated alkali lignin prepared in step 1 of Example 1 is used to directly replace the epoxy lignin in equimolar form.

[0096] The reasons for the performance degradation of the composite material prepared in this comparative example are as follows: Hydroxymethylated alkali lignin only introduces hydroxymethyl (-CH2OH) groups and lacks epoxy groups, thus it cannot undergo cross-linking reactions with the phenolic hydroxyl groups of straw tar phenol. Therefore, the resin matrix cannot be cured into a continuous phase and is merely a physical mixture of lignin and straw tar phenol. Although hydroxymethylated alkali lignin breaks some intermolecular hydrogen bonds, it still lacks sufficient active sites and polar groups, making the molecules prone to re-aggregation. This leads to defects such as pores and agglomeration within the resin matrix, which become stress concentration points under stress, reducing mechanical properties.

[0097] Taking the straw tar phenol-lignin epoxy carbon fiber reinforced composite material prepared in Example 1 and the carbon fiber reinforced composite materials prepared in Comparative Examples 1-4 as examples, the following performance tests were conducted: Test Example 1: Testing the mechanical properties of straw pyrophenol-lignin epoxy carbon fiber reinforced composites. Referring to the tensile strength standard GB / T 1447-2005 "Test Method for Tensile Properties of Fiber Reinforced Plastics", the flexural strength standard GB / T 1449-2005 "Test Method for Flexural Properties of Fiber Reinforced Plastics", and the heat distortion temperature standard GB / T 1634.2-2004 "Determination of Load Deflection Temperature of Plastics - Part 2: Plastics, Hard Rubber and Long Fiber Reinforced Composites", the mechanical properties of the straw tar phenol-lignin epoxy carbon fiber reinforced composite material in Example 1 and the carbon fiber reinforced composite materials in Comparative Examples 1-4 were tested. The results are shown in Table 1.

[0098] Table 1 As shown in Table 1, the straw tar phenol-lignin epoxy carbon fiber reinforced composite material prepared in Example 1 exhibits the best mechanical properties, with tensile strength and flexural strength reaching 586.3 MPa and 724.1 MPa, respectively, and a heat distortion temperature as high as 187.5℃. It also demonstrates excellent high-temperature stability and molding precision. In Comparative Example 1, the unpurified straw tar prevented the resin matrix from crosslinking and curing, thus failing to form an effective composite material. Comparative Examples 2-4, due to the incomplete acrylylation-epoxidation modification of lignin, lacked sufficient epoxy groups, resulting in insufficient crosslinking density and weak interfacial bonding, leading to a significant decrease in all mechanical and thermal properties. Comparative Example 4 (with hydroxymethylated alkali lignin as a substitute) showed the worst performance, while Comparative Example 3 (with acrylylated lignin as a substitute) had a superior molecular structure and thus performed better than Comparative Examples 2 and 4.

[0099] Test Example 2: Testing the Degradability of Straw Tar Phenol-Lignin Epoxy Carbon Fiber Reinforced Composite Material. The specific procedure for the degradation test is as follows: 1. Sample preparation: Cut each composite material into 10mm×10mm×3mm samples and dry them to constant weight (recorded as initial mass m0).

[0100] 2. Degradation environment: A soil composting degradation system is adopted to simulate the natural humus environment (temperature 25±2℃, humidity 60±5%, soil pH=6.5~7.5, with 10wt% humus microbial agent added).

[0101] 3. Experimental period: Samples were taken out after 30 days, 60 days, 90 days and 180 days of degradation, respectively. The surface soil was rinsed with distilled water and vacuum dried to constant weight (denoted as m1).

[0102] 4. Performance testing: Calculate the mass loss rate (mass loss rate = (m0-m1) / m0 × 100%).

[0103] The results are shown in Table 2.

[0104] Table 2 As shown in Table 2, the composite material of Example 1 exhibits the best degradability, with a mass loss rate of 15.6% after 180 days of soil composting, indicating that the biomass-based epoxy resin matrix can be decomposed by soil microorganisms. Comparative Examples 2-4, due to imperfect cross-linking structures or insufficient active groups in the resin matrix, showed lower degradation efficiencies than Example 1. Among them, Comparative Example 2 (unmodified alkali lignin) had the worst degradation performance due to its stable molecular structure. The degradability of Example 1 stems from the biomass nature of straw tar phenols and modified lignin; the ester and ether bonds in their molecules are easily decomposed by enzymes secreted by microorganisms, solving the environmental problem of the difficulty in degrading traditional bisphenol A epoxy resins.

[0105] Test Example 3: Testing the heat resistance of straw tar phenol-lignin epoxy carbon fiber reinforced composite material. 1. Thermogravimetric analysis (TG-DTG): A thermogravimetric analyzer was used under nitrogen atmosphere (flow rate 50 mL / min), heating rate 10℃ / min, and temperature range 30~800℃. The thermal decomposition temperature of the sample was recorded (T5%: the temperature at which 5% mass loss occurs; T...). 50 % (temperature at which 50% mass loss occurs) and carbon residue at 800℃.

[0106] 2. Dynamic Thermomechanical Analysis (DMA): A dynamic thermomechanical analyzer was used. The test mode was three-point bending, the frequency was 1 Hz, the heating rate was 5℃ / min, and the temperature range was 30~250℃. The glass transition temperature (T) was recorded. g ).

[0107] 3. Thermal aging performance: The sample was placed in a 150℃ oven for 1000h thermal aging. The sample was taken out periodically (200h, 500h, 1000h) to test the retention rate of tensile strength and appearance changes (whether it cracks or changes color).

[0108] The results are shown in Table 3.

[0109] Table 3 As shown in Table 3, the composite material of Example 1 exhibits excellent heat resistance, with T5% and T... 50The tensile strengths of Example 1 reached 286.7℃ and 478.2℃ respectively, significantly higher than the comparative examples, and the carbon residue at 800℃ reached 58.3%. This is attributed to the high-density cross-linked network of the aromatic ring structure of straw tar phenol and the epoxy group of lignin, forming a stable thermally stable system. Dynamic thermomechanical analysis showed that its glass transition temperature (178.5℃) was much higher than that of the comparative examples, indicating that the interfacial bonding between the matrix and carbon fibers was tight and thermal motion was suppressed. In the thermal aging test, after aging at 150℃ for 1000h, Example 1 still retained 85.7% of its tensile strength, with only slight yellowing and no cracking. In contrast, the comparative examples, due to insufficient cross-linking of the resin matrix or poor molecular structure stability, showed significant strength degradation after thermal aging and exhibited varying degrees of cracking and yellowing. Comparative Example 4 (hydroxymethylated alkali lignin substitute) exhibited the worst heat resistance, with a T5% of only 207.5℃ and a strength retention rate of less than 66% after heat aging. This further demonstrates that the thermal stability and interfacial bonding strength of lignin are significantly improved after acrylation-epoxidation synergistic modification, providing good heat resistance support for the composite material. The heat resistance performance of Example 1 meets the requirements for material stability under high-temperature environments in aerospace, transportation, and other fields.

[0110] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a straw tar phenol-lignin epoxy carbon fiber reinforced composite material, characterized in that, Alkali lignin is subjected to hydroxymethylation to obtain hydroxymethylated alkali lignin; the hydroxymethylated alkali lignin is mixed with acrylic anhydride and subjected to esterification to obtain acrylamide lignin; the acrylamide lignin is subjected to epoxidation to introduce epoxy groups to form epoxy-grafted lignin; straw tar phenol is mixed with the epoxy-grafted lignin to prepare alkali lignin-based epoxy resin; surface-modified carbon fibers, acetone, curing agent and the alkali lignin-based epoxy resin are mixed to form a prepreg, and the straw tar phenol-lignin epoxy carbon fiber reinforced composite material is prepared by hot pressing.

2. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The preparation method of the hydroxymethylated alkali lignin is as follows: alkali lignin is dispersed in water to obtain an alkali lignin dispersion; NaOH solution and ammonia are added to the alkali lignin dispersion, and after stirring evenly, formaldehyde is added. The mixture is reacted under heating conditions, and acid is added to adjust the pH of the solution to acidic conditions to precipitate the precipitate. The precipitate is then centrifuged, washed, and freeze-dried to obtain the hydroxymethylated alkali lignin.

3. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The preparation method of the acrylamide lignin is as follows: hydroxymethylated alkali lignin is dispersed in an organic solvent to form a uniform suspension, acrylic anhydride and pyridine are added, and the reaction is carried out under heating conditions in a protective atmosphere. After the reaction is completed, the resulting reaction mixture is poured into ice water to precipitate the precipitate, filtered, washed, and vacuum dried to obtain the acrylamide lignin.

4. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 3, characterized in that, The mass ratio of the hydroxymethylated alkali lignin, acrylic anhydride, and pyridine is 1:2:

2.

5. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The preparation method of the epoxy-grafted lignin is as follows: Acrylamide lignin is dispersed in dichloromethane, cooled in an ice bath, hydrogen peroxide is added, and the reaction is carried out in an ice bath. After the reaction is completed, the pH is adjusted to 6.0-6.5, and then sodium chloride solution is added to precipitate the solid. The residual acid is neutralized with saturated sodium bicarbonate solution, washed with water, filtered, and vacuum dried to obtain the epoxy-grafted lignin.

6. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The preparation method of the straw tar phenol is as follows: add sodium hydroxide solution to straw tar, react under heating conditions, filter, collect the supernatant, adjust the pH of the supernatant to 3, let stand until a precipitate is formed, centrifuge and collect the precipitate, which is the straw tar phenol.

7. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The preparation method of the alkali lignin-based epoxy resin is as follows: straw tar phenol and the epoxy-grafted lignin are mixed in N,N-dimethylformamide, stirred and dissolved at 60 °C, then heated to 85 °C, sodium hydroxide solution is added to maintain pH=9, and after reacting for 3 h, the mixture is neutralized to neutral with hydrochloric acid, the N,N-dimethylformamide is removed by rotary evaporation, the residue is dissolved with toluene, the salt is washed off, dried with anhydrous sodium sulfate, and concentrated to obtain a brown viscous resin. The obtained brown viscous resin is the alkali lignin-based epoxy resin.

8. The method for preparing the straw tar phenol-lignin epoxy carbon fiber reinforced composite material according to claim 1, characterized in that, The mass of the alkali lignin-based epoxy resin accounts for 40% of the total mass of the alkali lignin-based epoxy resin and the surface-modified carbon fiber; The amount of curing agent used is 30% of the mass of the alkali lignin-based epoxy resin.

9. A straw tar phenol-lignin epoxy carbon fiber reinforced composite material, characterized in that, It is prepared according to any one of claims 1 to 8.

10. The application of the straw tar phenol-lignin epoxy carbon fiber reinforced composite material as described in claim 9 in the fields of aerospace, transportation, energy and environmental protection, and construction engineering.

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