A surface-modified cellulose fiber and its preparation method

Abstract

This invention provides a surface-modified cellulose fiber and its preparation method, comprising: immersing cellulose fiber in an ionic solvent and activating it at an activation temperature of 40-60°C for 10-30 min to obtain activated cellulose fiber; immersing the activated cellulose fiber in a vinyl ester modifying agent and reacting it at a reaction temperature of 40-60°C for 1-3 h to obtain grafted cellulose fiber; transferring the grafted cellulose fiber to an antisolvent for thorough washing and drying to obtain surface-modified cellulose fiber. The surface-modified cellulose fiber includes a porous outer layer and a core layer from the outside to the inside, wherein the porous outer layer forms a pore structure extending from the surface to the inside, and the hydroxyl groups of the cellulose molecular chains in the porous outer layer are grafted with acetyl groups to achieve both high strength mechanical property retention and efficient functionalized grafting performance.

Description

Technical Field

[0001] This invention relates to the field of cellulose fiber materials technology, and in particular to a surface-modified cellulose fiber and its preparation method. Background Technology

[0002] Cellulose fibers such as Lyocell fiber (NMMO system) and ionic solvent system fiber are produced by dry-jet wet spinning process. Through the synergistic assembly of non-covalent bonds in molecular chains, they form a supramolecular structure with high crystallinity and high orientation. This structure not only has excellent mechanical stability, but can also be completely degraded in natural composting or soil environments within 30-90 days, meeting the needs of green and sustainable development.

[0003] However, virgin cellulose fibers have three inherent defects that severely limit their large-scale application in high-end scenarios: First, the surface of cellulose fibers is rich in hydroxyl groups, making them extremely hydrophilic. In humid environments, they easily absorb moisture, soften, and collapse. For example, in the case of cigarette filters, moisture absorption directly leads to uncontrolled filter rod resistance and a significant decrease in filtration efficiency. Second, due to the lack of specific functional groups such as acetyl, ester, and carbonyl groups, they cannot form specific hydrogen bonds, π–π interactions, and hydrophobic interactions with harmful molecules such as phenols and aldehydes in flue gas and wastewater, resulting in insufficient selective adsorption capacity for these harmful components. Third, their low flexibility not only increases the difficulty of material molding and processing but also adversely affects their performance regulation (such as filter rod resistance control), thus making them unsuitable for application in high-end scenarios such as textiles, filter materials, food processing, and biomedicine.

[0004] Currently, the industry commonly uses chemical modification (acetylation, etherification, graft copolymerization, etc.) and physical modification (surface coating, blending, etc.) techniques to prepare cellulose fiber functional materials. However, existing solutions still cannot overcome the bottleneck of simultaneously preserving mechanical properties and achieving efficient functionality. Traditional chemical modification processes, such as the preparation of cellulose acetate, require the use of highly corrosive, irritating, and even potentially carcinogenic toxic reagents such as acetic anhydride, concentrated sulfuric acid, and dichloromethane. These reagents are difficult to fully recover, leaving some residues in the material that harm human health, limit applications in high-end scenarios, and cause environmental pollution and increase environmental protection costs. In aqueous modification systems, the modifying reagents are prone to side reactions that generate impurities, leading to irritating odors and accelerated moisture absorption in the functional materials, while also resulting in low utilization rates of the modifiers. Furthermore, due to the structural distribution characteristics of the crystalline and amorphous regions of cellulose, it is difficult for the modifying reagents to achieve uniform penetration and efficient grafting, resulting in significant fluctuations in the properties of the prepared materials, making it impossible to effectively balance excellent mechanical strength and target functional effects. Physical modification methods such as coating and blending result in poor bonding between components, which can easily lead to detachment during subsequent processing or actual use, thus affecting the stability of material properties.

[0005] To address the aforementioned issues, the industry has successively developed various cellulose modification and optimization schemes. One type of scheme uses superalkaline ionic liquid as a solvent to completely dissolve cellulose and then perform transesterification modification, aiming to prepare highly substituted cellulose ester derivative powders or films. However, this process completely destroys the hydrogen bond network between cellulose molecular chains, causing the fiber to lose its original high orientation and crystalline structure, and completely lose the high-strength mechanical skeleton of the original fiber, making it unsuitable as a structural fiber material for applications such as filtration and textiles. Another type of scheme uses a single aqueous solution as the reaction medium and employs vinyl ester reagents to perform surface esterification modification of cellulose. However, water molecules have extremely weak wetting ability on cellulose and cannot penetrate into the amorphous region of the fiber. The esterification reaction is limited to the outermost surface of the fiber, which is a shallow adhesion modification that cannot achieve deep functionalization or build a through-hole structure inside the fiber, resulting in limited functional improvement.

[0006] In summary, current cellulose modification technologies either fail to achieve efficient and deep functionalization of cellulose fibers, or severely damage the fiber's crystalline structure and mechanical framework. They have consistently failed to resolve the technical contradiction between "preserving high-strength mechanical properties and efficient functional grafting," and also struggle to balance the green and environmentally friendly nature of the process with its industrial feasibility. Summary of the Invention

[0007] The purpose of this invention is to provide a surface-modified cellulose fiber and its preparation method. Based on a preferred ionic solvent, the controllable modification of acetyl groups and the sustained improvement of fiber mechanical strength are achieved by adjusting process parameters such as reaction temperature and reaction time. This results in the preparation of cellulose fibers with a multi-level network structure on the surface, excellent comprehensive performance, and biodegradability, thus combining the retention of high-strength mechanical properties with efficient functional grafting performance.

[0008] To achieve the above objectives, this technical solution provides a method for preparing surface-modified cellulose fibers, comprising the following steps:

[0009] Cellulose fibers are immersed in an ionic solvent and activated at an activation temperature of 40-60 °C for 10-30 min to obtain activated cellulose fibers.

[0010] Activated cellulose fibers are immersed in vinyl ester modifying agents and reacted at a reaction temperature of 40-60 °C for 1-3 h to obtain grafted cellulose fibers.

[0011] The grafted cellulose fibers were transferred to an antisolvent, thoroughly washed, and dried to obtain surface-modified cellulose fibers.

[0012] This method controls the temperature and time after immersing cellulose fibers in an ionic solvent to achieve multiple effects: directional swelling, hydroxyl site activation, and mesoporous structure construction. Specifically, the ionic solvent only penetrates and swells the surface and loosely structured amorphous regions of the cellulose fibers, exposing highly reactive hydroxyl sites within the amorphous regions, while preserving the highly dense crystalline regions inside the cellulose fibers. Furthermore, this invention utilizes an ionic solvent for surface activation modification of cellulose fibers, effectively solving problems such as reliance on toxic and harmful organic solvents, frequent side reactions in aqueous systems, and low modifier utilization in existing modification processes, while avoiding the damage to the fiber structure caused by violent reactions.

[0013] Preferably, the ionic solvent includes proton-type organic strong base ionic liquids and hydrogen-bonded eutectic solvents, both of which are bifunctional hydrogen bond donor-acceptor systems. That is, they have acceptor sites and donor sites that can form hydrogen bonds with the hydroxyl groups of cellulose. The hydrogen bond forces can be precisely controlled by the combination of components, which is the core chemical basis for achieving controllable swelling of cellulose.

[0014] Preferably, the ionic solvent is one or more combinations of 1,8-diazabicyclo[5.4.0]undec-7-ene acetate, 1,8-diazabicyclo[5.4.0]undec-7-ene methoxyacetate, 1,5-diazabicyclo[4.3.0]non-5-ene succinate, 1,5-diazabicyclo[4.3.0]non-5-ene ethoxyacetate, tetramethylguanidine (TMG)-acetic acid, tetramethylguanidine (TMG)-urea, or choline chloride-acetic acid.

[0015] Furthermore, the molar ratio of tetramethylguanidine (TMG) to acetic acid is 1:1, the molar ratio of choline chloride to acetic acid is 1:1, and the molar ratio of tetramethylguanidine (TMG) to urea is 1:2.

[0016] More preferably, the ionic solvent is 1,8-diazabicyclo[5.4.0]undec-7-ene methoxyacetate. In this ionic solvent, the cationic 1,8-diazabicyclo[5.4.0]undec-7-ene acts as an organic superbase, which can effectively improve the reactivity of cellulose. The ether bond and carboxyl group in the anionic methoxyacetate group bring good amphiphilicity and better compatibility with the modifying agent, which can make the modifying agent better enter the amorphous region of the fiber.

[0017] Preferably, the activation temperature is 50 °C and the activation time is 10 min. It should be noted that most traditional methods utilize superalkaline ionic liquids to completely dissolve cellulose, thereby disrupting the hydrogen bond network between cellulose molecular chains and causing the fiber to lose its original high orientation and crystalline structure. This method, however, controls the activation time and temperature to allow the ionic solvent to precisely penetrate the surface and amorphous regions of the cellulose fiber, fully breaking the intermolecular hydrogen bonds in these areas and exposing highly reactive hydroxyl sites. Simultaneously, it does not erode the dense crystalline regions inside the fiber, achieving efficient activation while completely preserving the fiber's original high-strength mechanical framework, laying the optimal foundation for subsequent functional grafting and hierarchical channel construction.

[0018] Preferably, the ratio of cellulose fiber to ionic solvent is 1:10.

[0019] In this method, activated cellulose fibers are then impregnated with vinyl ester modifying agents. Since the surface and amorphous regions of the activated cellulose fibers have been penetrated and swollen by ionic solvents, the vinyl ester modifying agents can also penetrate into the interior of the cellulose fibers and undergo a highly efficient transesterification-enol isomerization tandem reaction with the highly reactive hydroxyl sites.

[0020] It should be noted that in this scheme, the activated cellulose fibers are immersed in vinyl ester modifying agents to undergo transesterification-enol isomerization tandem reaction. During the reaction, volatile acetaldehyde is released, which promotes the forward reaction and achieves successful grafting of target functional groups on the fiber surface and construction of a multi-level pore network.

[0021] Preferably, the vinyl ester modifying agent contains a vinyl ester group. It is precisely the unique structure of this group that allows it to undergo a transesterification-enol isomerization tandem reaction with the hydroxyl groups of cellulose activated by an ionic solvent. The transesterification process grafts functional groups such as acetyl groups onto the cellulose, while the generated vinyl alcohol rapidly isomerizes to acetaldehyde. The volatilization of acetaldehyde not only drives the reaction forward but also constructs hierarchical channels on the fiber surface through kinetic physical etching. In other words, this method utilizes the kinetic effect of acetaldehyde gas escape to produce internal-to-external physical etching, enabling the cellulose fiber to achieve chemical hydrophobic modification while significantly increasing its physical specific surface area.

[0022] Preferably, the vinyl ester modifying agent is selected from one or more combinations of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl laurate, vinyl stearate, vinyl acrylate, vinyl methacrylate, vinyl bromoacetate, vinyl azide acetate, and vinyl adipate.

[0023] like Figures 1 to 4 The diagram illustrates the reaction process of activated cellulose fibers with vinyl ester modifying agents. Figure 1 This is a schematic diagram of the reaction between activated cellulose fibers and vinyl acetate. Figure 2 This is a schematic diagram illustrating the reaction between activated cellulose fibers and vinyl propionate. Figure 3 This is a schematic diagram illustrating the reaction between activated cellulose fibers and vinyl lauryl ester. Figure 4 This is a schematic diagram of the reaction between activated cellulose fibers and vinyl acrylate.

[0024] More preferably, the vinyl ester modifier is selected from vinyl acetate or vinyl propionate. These two low-carbon chain reagents have less steric hindrance and can better penetrate into the amorphous region swollen by the ionic solvent. At the same time, they have high reactivity and can undergo more efficient transesterification reactions in a short time.

[0025] Preferably, the ratio of activated cellulose fibers to vinyl ester modifying agents is 1:10.

[0026] Preferably, the reaction temperature is 40 °C and the reaction time is 2 h.

[0027] Preferably, the activated cellulose fibers are immersed in a vinyl ester modifying agent and reacted at a reaction temperature of 50 °C for 3 h to obtain grafted cellulose fibers.

[0028] After obtaining the grafted cellulose fibers, this method requires post-processing purification, which involves transferring the grafted cellulose fibers to an antisolvent for thorough washing to terminate the reaction and remove impurities from the system. The ionic solvent is then recovered and reused. Subsequently, the washed fibers are placed in a dry environment to dry, removing residual solvent and moisture, and finally obtaining surface-modified cellulose fibers.

[0029] Preferably, the antisolvent is a 50% aqueous solution of ethanol. It should be noted that highly polar antisolvents should not be used, as this could cause the slightly dissolved fiber sheath to regenerate rapidly, forming a film on the fiber surface that would cover the existing porous structure.

[0030] Preferably, the washing conditions are: first wash with a 50% ethanol aqueous solution, then gradually transition to water washing until the ionic solvent is completely removed, and the drying conditions are: drying at 60 °C for 30 min.

[0031] Preferably, this method can recover ionic solvents by adding 2 times the amount of ethanol at 100 °C and a vacuum of 0.008 MPa.

[0032] Preferably, the method for preparing surface-modified cellulose fibers proposed in this scheme is applicable to the modification of highly crystalline and highly oriented cellulose fibers, i.e., cellulose fibers with a crystallinity ≥45% and an orientation ≥80%.

[0033] More preferably, the cellulose fiber is one of the highly crystalline, highly oriented cellulose fibers formed by a dry-jet wet spinning process using a nonionic NMMO (N-methylmorpholine-N-oxide) system, an imidazole / pyridine plasma solvent system, an alkali / urea (or thiourea) aqueous solution system, a DMAc / LiCl (dimethylacetamide / lithium chloride) system, and a copper ammonia solution system (Schweizer reagent).

[0034] Furthermore, the cellulose fibers are non-ionic NMMO system fibers or ionic solvent system fibers.

[0035] It is worth mentioning that the surface-modified cellulose fiber preparation method provided by this solution does not require harsh conditions such as high temperature and high pressure, nor does it require special auxiliary equipment. It can realize the functionalization of different substrates such as Lyocell fiber and ionic solvent-spun cellulose fiber. After the ionic solvent completes fiber activation and functional grafting, it can be quickly separated by thorough washing with anti-solvent. The recovery efficiency is excellent, and the purity and reactivity of the recovered ionic solvent remain stable. It can be recycled for subsequent modification processes, which significantly reduces the cost of ionic solvent consumption. Moreover, the entire process is green and environmentally friendly, highly controllable, and the overall cost is controllable.

[0036] Secondly, this solution provides a surface-modified cellulose fiber prepared by the first aspect of modification, comprising a porous outer layer and a core layer from the outside to the inside, wherein the porous outer layer forms a pore structure extending from the surface to the inside, and the hydroxyl groups of the cellulose molecular chains of the porous outer layer are grafted with acetyl groups.

[0037] Preferably, the diameter of the surface-modified cellulose fibers is 35-45 μm.

[0038] More preferably, the diameter of the surface-modified cellulose fibers is 40 μm.

[0039] Preferably, the pore size of the porous skin layer forming the pore structure is 50 nm-1.5 μm.

[0040] Preferably, the thickness of the porous skin layer is 1 μm-2.5 μm, and the thickness of the core layer is 30 μm-40 μm.

[0041] It should be noted that, as mentioned earlier, surface-modified cellulose fibers exhibit a distinct core-sheath structure. The outer layer is a porous sheath with adsorption and hydrophobic properties, while the inner layer maintains the original mechanical strength. Therefore, on the one hand, the porous sheath effectively endows surface-modified cellulose fibers with hydrophobic properties and selective adsorption performance for phenols and aldehydes, meeting the target application requirements. On the other hand, because the crystalline structure of the core layer is not destroyed, the original supramolecular structure of the cellulose fiber is preserved to the greatest extent. The core layer ensures that the fiber has excellent mechanical strength, thus effectively solving the problem in traditional modification processes where mechanical strength and modification depth are difficult to balance due to random penetration of reagents and destruction of crystalline regions. At the same time, it preserves the high-strength skeleton and biodegradable properties of cellulose fiber materials.

[0042] Thirdly, this solution provides an application of surface-modified cellulose fibers in textiles, filter materials, food processing, and biomedicine.

[0043] Preferably, it is used in the fields of cigarette filters, air purification, water treatment filtration, or other functional materials.

[0044] Compared with existing technologies, this technical solution has the following characteristics and beneficial effects:

[0045] This solution addresses the challenge of balancing mechanical properties and modification effects in traditional cellulose modification by constructing a synergistic mechanism between ionic solvents and vinyl ester modifying agents. During the cellulose fiber activation stage, the ionic solvent rapidly penetrates the surface and amorphous regions of the cellulose fibers. Utilizing the specific hydrogen bonding interactions between its cations and hydrogen atoms in the cellulose hydroxyl groups, and between its anions and oxygen atoms in the hydroxyl groups, the intermolecular hydrogen bonds in the amorphous regions of the fibers are gradually broken. Subsequently, the ionic solvent molecules continue to penetrate and further act on the surface of the crystalline regions, disrupting the hydrogen bond network on the surface of the crystalline regions. This causes the fiber volume to expand and its structure to loosen. Simultaneously, it activates the active hydroxyl sites on the cellulose molecular chains (especially at the C2, C3, and C6 positions in the amorphous regions), lowering the hydroxyl reaction energy barrier, while avoiding damage to the dense structure inside the crystalline regions, thus laying the foundation for preserving the fiber's mechanical properties. In the cellulose fiber modification and grafting stage, the transesterification-enol isomerization tandem reaction is used, with the release of volatile acetaldehyde driving the reaction forward, thereby achieving the grafting of functional groups such as acetyl groups on the surface and amorphous region of cellulose fibers and the construction of a multi-level pore network. At the same time, the degree of acetyl substitution can be adjusted by controlling process parameters such as reaction temperature and reaction time. This system achieves full grafting on the surface and amorphous region of cellulose fibers while preserving the complete crystalline structure.

[0046] In addition, the surface-modified cellulose fiber prepared by this method has a porous outer skin with adsorption and hydrophobic properties on the outside and a core layer that maintains the original mechanical strength on the inside. This skin-core structure is very suitable for the comprehensive performance requirements of high specific surface area, moisture-absorbing and softening resistance, collapse resistance and excellent mechanical strength in applications such as cigarette filters. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the reaction between cellulose and vinyl acetate.

[0048] Figure 2 This is a schematic diagram of the reaction between cellulose and vinyl propionate.

[0049] Figure 3 This is a schematic diagram of the reaction between cellulose and vinyl lauryl ester.

[0050] Figure 4 This is a schematic diagram of the reaction between cellulose and vinyl acrylate.

[0051] Figure 5 The Fourier transform infrared images are those obtained in Examples 1-7 and Comparative Examples 1-2.

[0052] Figure 6 This is a scanning electron microscope image of the surface of the original cellulose fibers.

[0053] Figure 7 This is a scanning electron microscope image of the surface of the surface-modified cellulose fibers in Example 1.

[0054] Figure 8 This is a scanning electron microscope image of the multi-level mesh structure on the surface of the surface-modified fiber in Example 1. Detailed Implementation

[0055] In the following description, different "an embodiment" or "an embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form.

[0056] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0057] Unless otherwise specified, the methods in the following examples are conventional methods; unless otherwise specified, the materials or reagents in the following examples are commercially available.

[0058] The present invention will be further described below with specific embodiments, but these are not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0059] Example 1:

[0060] First, weigh 100 parts by weight of 1,8-diazabicyclo[5.4.0]undec-7-enemethoxyacetate, heat to 40 °C, and then add 10 parts of cellulose fiber to completely immerse it, activating for 10 min. Then, add the activated cellulose fiber to 100 parts of vinyl acetate and react at 40 °C for 1 h. After the reaction is complete, remove the fiber, wash it thoroughly with 50% ethanol aqueous solution, and dry it at 60 °C for 30 min.

[0061] Finally, the activating and modifying reagents were collected, and the ionic solvent was recovered by adding 2 times the amount of ethanol at 100 °C and a vacuum of 0.008 MPa.

[0062] Example 2

[0063] Same as Example 1, with the activation and modification times remaining unchanged, the difference being that the activation and modification temperatures are increased to 50 °C.

[0064] Example 3

[0065] Same as Example 1, with the activation and modification times remaining unchanged, the difference being that the activation and modification temperatures are increased to 60 °C.

[0066] Example 4

[0067] Same as Example 1, with the same activation temperature and modification temperature, and the same modification time, the only difference being that the activation time is increased to 20 min.

[0068] Example 5

[0069] Same as Example 1, with the same activation temperature and modification temperature, and the same modification time, the only difference being that the activation time is increased to 30 min.

[0070] Example 6

[0071] Same as Example 1, the activation temperature, activation time and modification temperature remain unchanged, the only difference is that the modification time is increased to 2 h.

[0072] Example 7

[0073] Same as Example 1, the activation temperature, activation time and modification temperature remain unchanged, the only difference is that the modification time is increased to 3 h.

[0074] Comparative Example 1

[0075] Same as Example 1, except that it was directly placed into the modifying reagent for modification without activation.

[0076] Comparative Example 2

[0077] Same as Example 1, except that the activation and modification are performed at 30 °C.

[0078] Comparative Example 3

[0079] Same as Example 1, except that the activation and modification are performed at 70 °C.

[0080] Comparative Example 4

[0081] Same as Example 1, with the same activation temperature and modification temperature, and the same modification time, the only difference being that the activation time is increased to 40 min.

[0082] Comparative Example 5

[0083] Same as Example 1, the activation temperature, activation time and modification temperature remain unchanged, the only difference is that the modification time is increased to 4 h.

[0084] Based on the conditions of the above embodiments and comparative examples, the surface-modified cellulose fibers obtained under various conditions were tested, and the test performance is shown in Table 1 below. The Fourier transform infrared spectra obtained from Examples 1-7 and Comparative Examples 1-2 are shown below. Figure 5 As shown.

[0085] Table 1. Performance of surface-modified cellulose fibers in various embodiments and comparative examples.

[0086]

[0087] As shown in Table 1, under the condition that other process parameters are constant, when the reaction temperature is gradually increased from a lower temperature to a higher temperature (corresponding to Examples 1 to 3), the diameter of the modified fiber gradually increases (from the original 35 μm to 45 μm). This is mainly attributed to the enhanced penetration ability of the ionic solvent during the heating process, which produces a stronger swelling effect on the amorphous region of the fiber, causing the fiber to expand more significantly in the transverse direction.

[0088] Regarding mechanical properties, when the reaction temperature is at a medium to low temperature (corresponding to Examples 1-2), the tensile strength of the modified fiber (179-230 MPa) is basically equivalent to that of the original fiber (191-233 MPa), indicating that the supramolecular structure of the fiber is not destroyed under mild reaction conditions, and the mechanical properties are effectively maintained. However, when the temperature is higher (corresponding to Example 3), the tensile strength of the fiber drops sharply (123-166 MPa). This is because at excessively high temperatures, the ionic solvent causes more severe damage to the fiber surface and even gradually destroys the deep crystalline structure of the fiber, resulting in deterioration of mechanical properties.

[0089] Regarding surface wettability, the water contact angle of the fiber gradually increased from 85° to 100° with increasing temperature, indicating that the higher temperature provided sufficient reaction activation energy, promoted efficient acetyl grafting, effectively replaced the hydrophilic hydroxyl network, and formed a richer hierarchical network and rough surface, making the modified cellulose fiber more hydrophobic.

[0090] In terms of thermal stability, the maximum thermal degradation temperature of the modified fiber increased from 360 °C to 370 °C, both significantly higher than that of the original fiber (346 °C). This is because the dense surface acetylation modification layer constructs a physical and thermodynamic barrier (core-sheath structure), which effectively inhibits the transfer of heat to the fiber interior and the escape of volatile small molecules, thus slowing down the thermal decomposition process of the fiber skeleton.

[0091] Corresponding to Examples 1, 4, and 5, as the activation time of the ionic solvent increased (from 10 min to 30 min), the diameter of the modified fiber slowly expanded from 37 μm to 40 μm, and the water contact angle increased from 85° to 93°. This indicates that extending the activation time can effectively promote the diffusion of the ionic solvent into the deeper amorphous regions of the fiber, thereby exposing more reaction sites and resulting in better modification. Meanwhile, its tensile strength remained at a high level (177~200 MPa) over time.

[0092] Corresponding to Examples 1, 6, and 7, as the vinyl acetate grafting reaction time increased (from 1 h to 3 h), the diameter of the modified fiber stabilized at around 37 μm, and the tensile strength remained basically within the range of 183–234 MPa. This indicates that, under the condition that the activation time and activation temperature remain unchanged, simply extending the chemical grafting time does not cause additional damage to the physical skeleton of the fiber. Furthermore, its water contact angle reached 89° after 2 h of modification and tended to saturate (88° in Example 7), and the maximum thermal degradation temperature also stabilized between 363 and 364 °C. This indicates that the optimal chemical equilibrium can be reached within 1–2 h of the reaction.

[0093] The water contact angle of the comparative examples (Comparative Examples 1-2) was only 40°~41°, very close to that of the original fiber (35°), and the maximum thermal degradation temperature was slightly lower than that of the original fiber. This indicates the necessity of ionic solvent activation treatment, and that under conventional treatment without suitable process conditions, the acetyl grafting reaction is difficult to trigger effectively. This demonstrates that the specific combination of process parameters in this invention is a necessary prerequisite for achieving efficient surface acetylation of fibers and simultaneously improving hydrophobicity and heat resistance.

[0094] Comparative Examples 3-5 showed excessive dissolution of the fibers, resulting in the loss of the fiber's original morphology and structure. This illustrates the importance of controlling temperature and time in this method. When the temperature is too high or the time is too long, the ionic solvent intensifies the damage to the cellulose fibers, causing the fibers to lose their original supramolecular structure and macroscopic fiber morphology.

[0095] In addition, the surface-modified cellulose fibers treated in Example 1 were scanned using this method to obtain a scanning electron microscope (SEM) image of the surface-modified cellulose fibers, as shown below. Figure 7 As shown in the image, the scanning electron microscope (SEM) image of the multi-level mesh structure on the surface of the surface-modified fiber is as follows. Figure 8 As shown, comparison Figure 6 As can be seen from the original cellulose fibers, the surface-modified cellulose fibers of this scheme do indeed achieve the modification of the porous skin layer.

[0096] Example 8

[0097] Similar to Example 1, the activation temperature, activation time, modification temperature, and modification time remain unchanged, except that the modifying agent is replaced with vinyl propionate. This reaction provides a pathway for preparing tunable cellulose blends. By controlling the ratio of vinyl acetate to vinyl propionate, a continuous spectrum of materials ranging from rigid (acetyl-dominant) to flexible (propionyl-dominant) can be obtained, making it ideal for developing films, coatings, or fiber products requiring specific flexibility.

[0098] Example 9

[0099] Similar to Example 1, the activation temperature, activation time, modification temperature, and modification time remain unchanged. The difference is that the modifying agent is replaced with vinyl laurate. The introduction of long lauric acid chains is equivalent to grafting flexible segments similar to polyethylene onto the cellulose backbone, which can significantly change the surface properties and bulk mechanical properties of the material, making it very suitable for developing superhydrophobic coatings or thin film flexible packaging materials.

[0100] Example 10

[0101] Similar to Example 1, the activation temperature, activation time, modification temperature, and modification time remain unchanged. The difference is that the modifying agent is replaced with vinyl acrylate. The introduction of cellulose acrylate transforms the originally thermoplastic cellulose ester into a reactive functional polymer, which is very suitable for developing hydrogel or composite material interface compatibilizers.

[0102] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the features in the above embodiments have been described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0103] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for preparing surface-modified cellulose fibers, characterized in that, Includes the following steps: Cellulose fibers are immersed in an ionic solvent and activated at an activation temperature of 40-60 °C for 10-30 min to obtain activated cellulose fibers. The ionic solvent is one or a combination of 1,8-diazabicyclo[5.4.0]undec-7-ene acetate, 1,8-diazabicyclo[5.4.0]undec-7-ene methoxyacetate, 1,5-diazabicyclo[4.3.0]non-5-ene succinate, 1,5-diazabicyclo[4.3.0]non-5-ene ethoxyacetate, tetramethylguanidine-acetic acid, tetramethylguanidine-urea, or choline chloride-acetic acid. Activated cellulose fibers are immersed in vinyl ester modifying reagents to undergo transesterification-enol isomerization tandem reaction, and the reaction is carried out at a reaction temperature of 40-60 °C for 1-3 h to obtain grafted cellulose fibers. Grafted cellulose fibers are transferred to an antisolvent, thoroughly washed, and dried to obtain surface-modified cellulose fibers. The surface-modified cellulose fibers include a porous outer layer and a core layer from the outside to the inside. The porous outer layer forms a pore structure extending from the surface to the inside, and the hydroxyl groups of the cellulose molecular chains in the porous outer layer are grafted with acetyl groups.

2. The method for preparing surface-modified cellulose fibers according to claim 1, characterized in that, The vinyl ester modifiers are selected from one or more combinations of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl laurate, vinyl stearate, vinyl acrylate, vinyl methacrylate, vinyl bromoacetate, vinyl azide, and vinyl adipate.

3. The method for preparing surface-modified cellulose fibers according to claim 1, characterized in that, The activation temperature was 50°C and the activation time was 10 min.

4. The method for preparing surface-modified cellulose fibers according to claim 1, characterized in that, The reaction temperature was 50°C and the reaction time was 2 h.

5. The method for preparing surface-modified cellulose fibers according to claim 1, characterized in that, Cellulose fibers are either non-ionic NMMO system fibers or ionic solvent system fibers.

6. A surface-modified cellulose fiber, characterized in that, The surface-modified cellulose fiber prepared according to any one of claims 1 to 5 comprises a porous outer layer and a core layer from the outside to the inside, wherein the porous outer layer forms a pore structure extending from the surface to the inside, and the hydroxyl groups of the cellulose molecular chains of the porous outer layer are grafted with acetyl groups.

7. The surface-modified cellulose fiber according to claim 6, characterized in that, The diameter of the surface-modified cellulose fibers is 35-45 μm.

8. The surface-modified cellulose fiber according to claim 6, characterized in that, It is used in textiles, filter materials, food processing and biomedicine.

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