A method for plasma enhanced carbon nanotube surface functionalization

Abstract

The application relates to the technical field of nanomaterial surface modification and plasma treatment, in particular to a plasma-enhanced carbon nanotube surface functionalization method. The method comprises the following steps: pre-dispersing and pre-drying treatment of the carbon nanotubes to be treated, removal of adsorbed water and surface impurities, and obtaining pretreated carbon nanotubes; continuously conveying the pretreated carbon nanotubes to a fluidized bed plasma reaction cavity, making the carbon nanotubes in a suspended and tumbled state under the action of a carrier gas; introducing reaction gas into the plasma reaction cavity, and exciting a low-temperature plasma by applying a radio frequency power source to activate and introduce functional groups on the surface of the carbon nanotubes, so that at least one of oxygen-containing functional groups and nitrogen-containing functional groups is formed on the surface of the carbon nanotubes. In the application, the fluidized bed plasma continuous treatment structure is adopted, the carbon nanotubes are uniformly exposed to the plasma area in a suspended and tumbled state, and the surface treatment uniformity and the functional group introduction efficiency are significantly improved.

Description

Technical Field

[0001] This invention relates to a plasma-enhanced method for surface functionalization of carbon nanotubes, belonging to the field of nanomaterial surface modification and plasma treatment technology. Background Technology

[0002] Carbon nanotubes possess excellent electrical conductivity, mechanical properties, thermal stability, and high aspect ratio, making them promising candidates for applications in fields such as conductive agents for lithium batteries, resin reinforcement materials, flexible electronic devices, thermal conductive materials, and sensors.

[0003] However, carbon nanotubes have a high degree of graphitization and strong chemical inertness, and their significant van der Waals forces make them prone to aggregation. This results in poor dispersibility and insufficient interfacial bonding in resins, aqueous slurries, or other matrices, thus limiting the full potential of carbon nanotubes. Therefore, it is usually necessary to perform surface functionalization on carbon nanotubes by introducing active functional groups such as carboxyl, hydroxyl, and amino groups onto their surface to improve their wettability, dispersibility, and interfacial compatibility.

[0004] In existing technologies, acid oxidation, wet chemical grafting, and plasma treatment are widely used for the surface modification of carbon nanotubes. Among them, plasma treatment has attracted attention due to its advantages such as fast processing speed, no need for large amounts of chemical reagents, low environmental impact, and suitability for continuous production.

[0005] However, existing plasma treatment methods still have the following problems: First, the treatment parameters mostly rely on empirical settings, and fluctuations in power, gas flow rate, time, and pressure can easily lead to unstable functional group introduction and poor batch consistency. Second, the surface-active groups after treatment are prone to migration, rearrangement, or deactivation in the air, resulting in functional group aging and decay. Third, traditional static reaction methods do not treat powdered carbon nanotubes evenly, easily leading to local over-treatment or under-treatment. Fourth, some processes can damage the conductive network of carbon nanotubes while improving surface activity, affecting the final application performance. Therefore, it is urgent to improve a plasma-enhanced carbon nanotube surface functionalization method to solve the above-mentioned problems. Summary of the Invention

[0006] This invention provides a plasma-enhanced method for functionalizing carbon nanotube surfaces. Addressing the problems of functional group aging and degradation, poor batch-to-batch consistency, insufficient processing uniformity, and difficulty in continuous production in existing carbon nanotube surface modification processes, this invention achieves efficient introduction and long-term stable maintenance of functional groups on the carbon nanotube surface through a continuous process of "pretreatment—fluidized bed plasma functionalization—online detection—digital closed-loop regulation—in-situ stable grafting—low-oxygen encapsulation and storage".

[0007] This invention employs a fluidized bed reaction structure to keep carbon nanotubes in a suspended and tumbling state during processing, improving plasma contact uniformity. It utilizes a mixed discharge of oxygen-containing and nitrogen-containing gases to simultaneously introduce active groups such as carboxyl, hydroxyl, and amino groups onto the carbon nanotube surface. A comprehensive quality evaluation model is established by online monitoring of parameters such as surface oxygen-to-carbon ratio, nitrogen-to-carbon ratio, contact angle, resistance, and structural defect ratio, allowing for real-time closed-loop adjustment of plasma power, time, flow rate, pressure, and temperature. Immediately after functionalization, in-situ stabilization grafting is performed to form a stable protective layer on the carbon nanotube surface, reducing functional group migration and deactivation. Finally, it is stored using low-temperature, low-oxygen, or vacuum encapsulation methods.

[0008] To achieve the above objectives, the present invention provides the following specific technical solutions: A plasma-enhanced method for functionalizing the surface of carbon nanotubes includes the following steps: Step 1: Pre-disperse and pre-dry the carbon nanotubes to be treated to remove adsorbed moisture and surface impurities, and obtain pretreated carbon nanotubes. Step 2: The pretreated carbon nanotubes are continuously transported to the fluidized bed plasma reaction chamber, where they are suspended and tumbled under the action of a carrier gas. Step 3: Introduce reactive gas into the plasma reaction chamber and apply radio frequency power to excite and form low-temperature plasma to activate and introduce functional groups on the surface of carbon nanotubes, so that at least one of oxygen-containing functional groups and nitrogen-containing functional groups is formed on the surface of carbon nanotubes. Step 4: Perform online detection on the treated carbon nanotubes to obtain detection parameters characterizing the surface functionalization quality. The detection parameters include at least two of the following: surface oxygen-carbon atom ratio, surface nitrogen-carbon atom ratio, contact angle, surface resistance, and structural defect ratio. Step 5: Establish a functionalized quality evaluation model based on the detection parameters, calculate the comprehensive quality index after standardizing each detection parameter, compare the comprehensive quality index with the preset target value, and adjust at least one of the plasma power, processing time, reaction gas flow rate, reaction pressure and processing temperature in a closed loop according to the comparison results. Step 6: Perform in-situ stabilization grafting on the carbon nanotubes that have been adjusted to achieve the target quality index to form a stable protective layer on the surface of the carbon nanotubes. Step 7: Encapsulate and store the treated carbon nanotubes under low temperature, low oxygen or vacuum conditions.

[0009] Furthermore, the pre-drying treatment temperature is 80-120℃, the treatment time is 2-6h, and the water content of the carbon nanotubes after treatment is not higher than 0.3%.

[0010] Furthermore, the carrier gas is argon, nitrogen, or a mixture thereof, and the carrier gas flow rate is higher than the minimum fluidization velocity of the carbon nanotubes, so as to make the carbon nanotubes continuously tumble in the reaction chamber and be uniformly exposed to the plasma region.

[0011] Furthermore, the reaction gas includes at least two of argon, oxygen, ammonia, and nitrogen, and the plasma processing parameters meet the following requirements: power of 100-800W, reaction pressure of 30-300Pa, processing time of 30-600s, and processing temperature of 25-80℃.

[0012] Furthermore, the formula for calculating the comprehensive quality index Q is as follows: In the formula, This is the standardized value of the surface oxygen-to-carbon atomic ratio; This is the standardized value of the nitrogen-carbon atomic ratio on the surface; This is the standardized value of the contact angle; This is the standardized value of surface resistivity; This is the standardized value for the structural defect ratio; For the corresponding weight coefficients, and satisfying .

[0013] Furthermore, the prediction model between the comprehensive quality index Q and the process parameters is as follows: In the formula, P is the plasma power, t is the processing time, F is the reactant gas flow rate, p is the reaction pressure, and T is the processing temperature. ; For constant terms, is the regression coefficient.

[0014] Furthermore, the real-time detection comprehensive quality index is recorded as... The preset target quality index is denoted as The control error is: Based on the aforementioned control error, a PID algorithm is used to output the adjustment amount: ; In the formula, For output adjustment amount; This is the proportionality coefficient; The integral coefficient is... These are the differential coefficients; Furthermore, the output adjustment amount is used to adjust at least one of the power, time, flow rate, pressure, and temperature in step 3.

[0015] Furthermore, the in-situ stabilization grafting treatment involves directly transporting the carbon nanotubes to the stabilization grafting cavity without air exposure, and introducing at least one of silane coupling agent vapor, unsaturated carboxylic acid vapor, amine monomer vapor, or polymerizable organic monomer vapor to form a stable grafting layer on the surface of the carbon nanotubes, thereby improving the functional group retention rate and reducing the surface activity decay during storage.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention employs a fluidized bed plasma continuous processing structure, allowing carbon nanotubes to be uniformly exposed to the plasma region in a suspended and tumbling state, significantly improving surface treatment uniformity and functional group introduction efficiency. By establishing an online detection and digital closed-loop control model, key parameters such as power, time, gas flow rate, pressure, and temperature are adjusted in real time, effectively reducing batch fluctuations and improving product consistency. In-situ stabilization grafting after functionalization forms a protective layer on the carbon nanotube surface, significantly inhibiting functional group aging and degradation, and improving long-term storage stability. Simultaneously, while maintaining high surface activity, it reduces structural damage and conductivity degradation of the carbon nanotubes. Using this method, the dispersion performance, interfacial bonding performance, and long-term reliability of carbon nanotubes are significantly improved, making it suitable for industrial continuous production. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall structure of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0019] like Figure 1 As shown in the figure, this embodiment provides a method for plasma-enhanced functionalization of carbon nanotube surfaces, including the following steps: Step 1: Raw material pretreatment Take 1000g of multi-walled carbon nanotubes, place them in a high-speed disperser for mechanical loosening for 15min, and then put them into a vacuum drying oven and dry them at 100℃ for 4h to reduce the moisture content to 0.25%. Step 2: Continuous conveying and fluidization Carbon nanotubes are fed into the fluidized bed plasma reaction chamber at a rate of 8 kg / h using a screw feeder. Argon gas is introduced as the carrier gas at a flow rate of 120 sccm to keep the carbon nanotubes in a uniform suspension and tumbling state. Step 3: Plasma surface functionalization A mixture of Ar, O2, and NH3 gases in a volume ratio of 70:15:15 was introduced into the reaction chamber. A 13.56MHz radio frequency power supply was used to excite the low-temperature plasma, and the control parameters were as follows: Power: 300W; Pressure: 80Pa; Time: 120s; Temperature: 35℃; Step 4: Online Testing The test results are as follows: oxygen-to-carbon atomic ratio: 0.142; nitrogen-to-carbon atomic ratio: 0.067; contact angle: 38° Surface resistivity change rate: +6%; ID / IG: 1.09; Step 5: Closed-loop adjustment The overall quality index has reached the target value of 0.93, requiring no further adjustment. Step 6: In-situ stable grafting After treatment, the material is directly transported to the stable grafting cavity, and γ-aminopropyltriethoxysilane vapor is introduced and treated for 90 seconds to form a stable grafting layer on the surface. Step 7: Packaging and Storage Store in a sealed container at 10°C and 3% oxygen content.

[0020] The above embodiments demonstrate that the functional group retention rate is 89% after 30 days, exhibiting excellent dispersion performance, making it suitable for conductive plastic masterbatches. Example 2

[0021] like Figure 1 As shown in the figure, this embodiment provides a method for plasma-enhanced functionalization of carbon nanotube surfaces, including the following steps: Step 1: Raw material pretreatment 1000g of multi-walled carbon nanotubes were mechanically dispersed for 20 min and then dried under vacuum at 110℃ for 3 h to reduce the moisture content to 0.21%. Step 2: Continuous conveying and fluidization The carbon nanotubes are fed into the fluidized bed reaction chamber by a quantitative feeder at a feeding rate of 10 kg / h and 140 sccm of argon carrier gas is introduced to continuously tumble and fluidize the carbon nanotubes. Step 3: Plasma surface functionalization A mixture of Ar, O2, and N2 gas with a volume ratio of 65:20:15 was introduced, and the following control parameters were used: power: 450W; pressure: 120Pa; time: 180s; temperature: 45℃. Step 4: Online Testing The test results are as follows: oxygen-carbon atomic ratio: 0.186; nitrogen-carbon atomic ratio: 0.031; contact angle: 29°; surface resistivity change rate: +8%; ID / IG: 1.14; Step 5: Closed-loop adjustment The initial Q value was 0.88, lower than the target value of 0.92; the power was automatically increased to 480W and the processing time was extended to 210s, and the Q value reached 0.94 in the second test. Step 6: In-situ stable grafting Acrylic acid vapor is introduced for 120 seconds to form a carboxyl-stabilized layer on the surface; Step 7: Packaging and Storage It is packaged and stored at 8°C and under vacuum of 45Pa.

[0022] The above embodiments can shorten the slurry dispersion time by 42% and achieve a 30-day retention rate of 91%, making it suitable for lithium battery conductive agents. Example 3

[0023] like Figure 1 As shown in the figure, this embodiment provides a method for plasma-enhanced functionalization of carbon nanotube surfaces, including the following steps: Step 1: Raw material pretreatment Take 1000g of multi-walled carbon nanotubes, loosen them mechanically for 10 minutes, and then dry them in a vacuum environment at 95℃ for 5 hours to reduce the moisture content to 0.28%. Step 2: Continuous conveying and fluidization The feed rate is 7 kg / h. The feed is introduced into the fluidized bed reaction chamber and a nitrogen / argon mixed carrier gas is introduced with a total flow rate of 130 sccm to stabilize the fluidization. Step 3: Plasma surface functionalization A mixture of Ar, NH3, and N2 gas was introduced at a volume ratio of 60:25:15, with the following control parameters: power: 380W; pressure: 95Pa; time: 150s; temperature: 40℃. Step 4: Online Testing The test results are as follows: oxygen-carbon atomic ratio: 0.096; nitrogen-carbon atomic ratio: 0.102; contact angle: 41°; surface resistivity change rate: +5%; ID / IG: 1.07; Step 5: Closed-loop adjustment The Q value reached 0.92 for the first time, meeting the target requirement, and no adjustment is needed.

[0024] Step 6: In-situ stable grafting Introduce glycidyl methacrylate vapor for 100 seconds.

[0025] Step 7: Packaging and Storage Store in an environment of 12°C and 4% oxygen.

[0026] The above embodiments can increase the tensile strength of epoxy resin composite materials by 26% and the 60-day retention rate by 84%.

[0027] Comparative Example 1 Traditional carbon nanotube plasma treatment methods follow the procedure below: Step 1: Raw material pretreatment Take 1000g of multi-walled carbon nanotubes and dry them in a conventional oven at 80℃ for 2 hours; Step 2: Conveying and Processing Carbon nanotubes are directly laid flat in a tray and placed into a static reaction chamber, without fluidization structure; Step 3: Plasma Treatment Pure O2 gas was introduced, with the following control parameters: power: 300W; pressure: 90Pa; time: 120s; temperature: room temperature. Step 4: Detection The test results are as follows: oxygen-to-carbon atomic ratio: 0.118; contact angle: 52°; surface resistivity change rate: +14%; ID / IG: 1.23; Step 5: Control Method There is no online detection, no quality evaluation model, and no closed-loop regulation; Step 6: Post-processing No in-situ stable grafting treatment; Step 7: Storage After treatment, store directly exposed to air at room temperature. Final results: 51% functional group retention rate after 30 days, batch variation CV of 16.8%, and average dispersion performance.

[0028] Compared with existing technologies, this invention significantly improves the long-term stability of functional groups on the surface of carbon nanotubes through an in-situ stabilization grafting process, increasing the 30-day functional group retention rate from 51% in traditional processes to over 89%. By combining online detection with a closed-loop control strategy, it effectively reduces batch production fluctuations, lowering the batch variation coefficient (CV) from 16.8% to below 5%, significantly improving product consistency. Simultaneously, while achieving a high degree of surface functionalization, it maintains a low resistivity change rate, minimizing damage to the conductivity of carbon nanotubes. The contact angle of the treated carbon nanotube surface can be reduced to approximately 30°, significantly improving its wetting and dispersion performance in aqueous systems and resin matrices. Furthermore, this invention employs a continuous plasma processing flow, possessing excellent scalability and suitable for stable production on ton-scale industrial carbon nanotube production lines.

[0029] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0030] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for plasma-enhanced functionalization of carbon nanotube surfaces, characterized in that, Includes the following steps: Step 1: Pre-disperse and pre-dry the carbon nanotubes to be treated to remove adsorbed moisture and surface impurities, and obtain pretreated carbon nanotubes. Step 2: The pretreated carbon nanotubes are continuously transported to the fluidized bed plasma reaction chamber, where they are suspended and tumbled under the action of a carrier gas. Step 3: Introduce reactive gas into the plasma reaction chamber and apply radio frequency power to excite and form low-temperature plasma to activate and introduce functional groups on the surface of carbon nanotubes, so that at least one of oxygen-containing functional groups and nitrogen-containing functional groups is formed on the surface of carbon nanotubes. Step 4: Perform online detection on the treated carbon nanotubes to obtain detection parameters characterizing the surface functionalization quality. The detection parameters include at least two of the following: surface oxygen-carbon atom ratio, surface nitrogen-carbon atom ratio, contact angle, surface resistance, and structural defect ratio. Step 5: Establish a functionalized quality evaluation model based on the detection parameters, calculate the comprehensive quality index after standardizing each detection parameter, compare the comprehensive quality index with the preset target value, and adjust at least one of the plasma power, processing time, reaction gas flow rate, reaction pressure and processing temperature in a closed loop according to the comparison results. Step 6: Perform in-situ stabilization grafting on the carbon nanotubes that have been adjusted to achieve the target quality index to form a stable protective layer on the surface of the carbon nanotubes. Step 7: Encapsulate and store the treated carbon nanotubes under low temperature, low oxygen or vacuum conditions.

2. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The pre-drying treatment temperature is 80-120℃, the treatment time is 2-6h, and the water content of the carbon nanotubes after treatment is not higher than 0.3%.

3. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The carrier gas is argon, nitrogen, or a mixture thereof, and the carrier gas flow rate is higher than the minimum fluidization velocity of the carbon nanotubes, so as to make the carbon nanotubes continuously tumble in the reaction chamber and be uniformly exposed to the plasma region.

4. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The reaction gas includes at least two of argon, oxygen, ammonia, and nitrogen, and the plasma processing parameters meet the following requirements: power of 100-800W, reaction pressure of 30-300Pa, processing time of 30-600s, and processing temperature of 25-80℃.

5. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The formula for calculating the comprehensive quality index Q is as follows: ； In the formula, This is the standardized value of the surface oxygen-to-carbon atomic ratio; This is the standardized value of the nitrogen-carbon atomic ratio on the surface; This is the standardized value of the contact angle; This is the standardized value of surface resistivity; This is the standardized value for the structural defect ratio; For the corresponding weight coefficients, and satisfying .

6. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The prediction model between the comprehensive quality index Q and the process parameters is as follows: In the formula, P is the plasma power, t is the processing time, F is the reactant gas flow rate, p is the reaction pressure, and T is the processing temperature. ; For constant terms, is the regression coefficient.

7. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The real-time comprehensive quality index is denoted as... The preset target quality index is denoted as The control error is: ； Based on the aforementioned control error, a PID algorithm is used to output the adjustment amount: ； In the formula, For output adjustment amount; This is the proportionality coefficient; The integral coefficient is... These are the differential coefficients; The output adjustment amount is used to adjust at least one of the power, time, flow rate, pressure, and temperature in step 3.

8. The method for surface functionalization of plasma-enhanced carbon nanotubes according to claim 1, characterized in that: The in-situ stabilization grafting treatment involves directly transporting carbon nanotubes to a stabilization grafting cavity without air exposure, and introducing at least one of silane coupling agent vapor, unsaturated carboxylic acid vapor, amine monomer vapor, or polymerizable organic monomer vapor to form a stable grafting layer on the surface of the carbon nanotubes. This process is used to improve the retention rate of functional groups and reduce the decay of surface activity during storage.

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