CN111118894A

CN111118894A - Method for modifying carbon fiber

Info

Publication number: CN111118894A
Application number: CN202010037420.2A
Authority: CN
Inventors: 何敏; 张道海; 秦舒浩; 徐国敏; 刘玉飞; 张凯; 龙丽娟
Original assignee: Guizhou University
Current assignee: Guizhou University
Priority date: 2020-01-14
Filing date: 2020-01-14
Publication date: 2020-05-08
Anticipated expiration: 2040-01-14
Also published as: CN111118894B

Abstract

The invention discloses a method for modifying carbon fibers, which is characterized by comprising the following steps: firstly, supercritical CO is adopted₂And (3) carrying out surface treatment on the CF, then carrying out oxidation treatment on the cleaned CF by adopting a nitric acid solution, and finally grafting ethylenediamine containing amino groups on the oxidized CF surface to obtain the modified carbon fiber. (1) Supercritical CO₂Is a physical cleaning mode, and can effectively clean the CF meterThe surface sizing agent can also cause damage to the strength of the CF to a small extent. (2) Nitric acid solution oxidation through supercritical CO₂The CF surface after fluid cleaning can effectively increase the roughness and active functional groups of the fiber surface, and is beneficial to surface grafting. (3) The ethylene diamine is used for surface grafting of oxidized CF, so that the strength of the fiber monofilament can be improved by repairing defects through surface grafting, and the interface performance of the CF/epoxy composite material can be improved.

Description

Method for modifying carbon fiber

Technical Field

The invention relates to a method for modifying carbon fibers, and particularly relates to the field of carbon fiber modification.

Background

Carbon Fiber (CF) is a fibrous carbon material with carbon content of more than 90 percent, the carbon atoms on the surface of the fibrous carbon material are in a six-membered ring honeycomb shape, the structure is stable, and the fibrous carbon material is known as the king of materials since the birth of the world and is one of three high-performance fibers (aramid fiber, CF and high-density polyethylene). The CF can be divided into polyacrylonitrile CF, pitch CF, viscose CF and lignin CF according to the source, wherein the polyacrylonitrile CF has simple production process, excellent performance and wide application.

The manufacturers of the polyacrylonitrile CF mainly take Dongli, Dongpo and Mitsubishi in Japan, which account for more than 50% of the CF market share in the world, and the produced CF is far ahead in the world in both quantity and quality, and the Dongli company in Japan is a leading sheep for producing high-performance CF. It is feared that in recent years, the CF of China is brought under the guidance of Mr. Changchang, a college of China, and the like, and the production process is rapidly developed from zero breakthrough.

The diameter of the CF protofilament applied to military industry and life in the market is about 7 mu m, the density is 1.79g/m3, the carbon atoms on the surface of the CF protofilament can not reach 1/4 of steel, and the surface carbon atoms are in a six-membered ring honeycomb structure, so that the CF has good chemical stability; the tensile strength of the monofilament is more than 3,500MPa, which is about 7-9 times of that of steel, and the monofilament is currently used in various fields such as aerospace, automobile traffic, wind power generation and the like. Global CF demand is expected to grow from 5,800 tons in 2015 to 10,000 tons in 2020. The surface of the CF is coated with a layer of sizing agent which takes epoxy resin as a main raw material when the CF leaves a factory, and the sizing agent is mainly used for preventing the CF finished product from generating broken filaments in the transportation process, so that the CF is easy to form bundles. In the interface body formed by compounding CF and a resin-based matrix, the epoxy sizing agent cannot effectively provide good interface engaging force. Epoxy resin pastes on CF surfaces are a disadvantageous factor for laboratory studies and analyses. In addition, untreated CF has low surface roughness and the lack of reactive functional groups does not form a good interface with the resin-based matrix. Surface modification of CF has been receiving much attention in view of this.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the method solves the technical problems that untreated CF has low surface roughness and lacks active functional groups and cannot form a good interface with a resin matrix.

The technical scheme of the invention is as follows: a method for modifying carbon fiber includes such steps as supercritical CO₂And (3) carrying out surface treatment on the CF, then carrying out oxidation treatment on the cleaned CF by adopting a nitric acid solution, and finally grafting ethylenediamine containing amino groups on the oxidized CF surface to obtain the modified carbon fiber.

The supercritical CO₂The method for processing the CF surface comprises the following steps: placing CF in a reaction kettle of a supercritical extraction instrument, adding acetone, closing the reaction kettle, and performing supercritical CO₂The critical temperature of the fluid is 45-50 deg.C, the critical pressure is 14-20MPa, and the treatment time is 30-40 min.

Preferred supercritical CO₂The critical temperature point of the fluid is 45 ℃, the critical pressure is 14MPa, and the processing time is 40 min.

The oxidation treatment of the washed CF: fixing a three-neck flask provided with a stirrer, a condenser tube and a thermometer in an oil bath pan, adding a nitric acid solution and the CF cleaned by the supercritical fluid into the flask, heating to 60-70 ℃, reacting for 3-4h, taking out the CF, cleaning the surface by acetone, and drying in an oven.

Preferably, the reaction temperature is 70 ℃ and the reaction time is 4 h.

The method for grafting ethylenediamine containing amino groups on the oxidized CF surface comprises the following steps: fixing a three-neck flask provided with a stirrer, a condenser tube and a thermometer in an oil bath kettle, adding 100ml of ethylenediamine solution and CF cleaned by supercritical fluid into the flask, heating to 60-70 ℃, reacting for 5-6h, taking out the CF, cleaning the surface by acetone, and putting the CF into an oven for drying.

Preferably, the reaction temperature is 70 ℃ and the reaction time is 5 h.

The invention has the beneficial effects that: (1) supercritical CO₂The method is a physical cleaning mode, can effectively clean the CF surface slurry, and can damage the CF strength to a small extent.

(2) Nitric acid solution oxidation through supercritical CO₂The CF surface after fluid cleaning can effectively increase the roughness and active functional groups of the fiber surface, and is beneficial to surface grafting.

(3) The ethylene diamine is used for surface grafting of oxidized CF, so that the strength of the fiber monofilament can be improved by repairing defects through surface grafting, and the interface performance of the CF/epoxy composite material can be improved.

Drawings

FIG. 1 shows supercritical CO at different temperatures₂Influence of treatment on weight loss of CF;

FIG. 2 shows supercritical CO at different temperatures₂A treated CF scanning electron micrograph; (0-Uncleanned 1-35 deg.C 8MPa10 min; 2-40 deg.C 8MPa10 min; 3-45 deg.C 8MPa10 min; 4-50 deg.C 8MPa10 min; 5-55 deg.C 8MPa10min)

FIG. 3 is a Weibull distribution plot of the tensile strength of CF filaments under cleaning conditions at an optimum temperature of 45 ℃;

FIG. 4 is a graph showing the effect of an optimal temperature 45 ℃ cleaning process on the roughness profile of a CF surface;

FIG. 5 is a CF Raman electron micrograph under different cleaning process conditions; performing fitting primary Raman peak separation treatment on the CF before and after cleaning treatment;

FIG. 6 shows different pressures of supercritical CO₂Influence on weight loss of CF;

FIG. 7 shows different pressures of supercritical CO₂A treated CF scanning electron micrograph; 0-Uncleanned; 8MPa for 10min at 6-45 ℃; 11MPa for 10min at 7-45 ℃; 14MPa for 10min at 8-45 ℃; 17MPa for 10min at 9-45 ℃; 20MPa at 10-45 deg.C for 10min

FIG. 8 shows supercritical CO at different times₂Influence on weight loss of CF;

FIG. 9 shows supercritical CO at different times₂CF scanning electron microscope under treatmentA drawing;

FIG. 10 is a scanning electron micrograph of a CF oxidized surface at different times;

FIG. 11 is a scanning electron micrograph of a CF oxidized surface at different temperatures;

FIG. 12 is a photoelectron spectrum of a CF surface before and after oxidation;

FIG. 13 is a scanning electron micrograph of grafted ethylene diamine on the CF surface at different times; 1-oxidation of CF; 60 ℃ for 2-1 h; 60 ℃ for 3-2 h; 60 ℃ for 4-3 h; 60 ℃ for 5-4 h; 60 ℃ for 6-5 h;

FIG. 14 is a scanning electron diagram of a CF surface grafted ethylenediamine;

FIG. 15 is a graph showing the effect of grafting treatment on the photoelectron spectroscopy of CF surfaces;

FIG. 16 is a scanning electron microscope of the CF/epoxy composite matrix interface.

Detailed Description

1. Supercritical CO₂For CF surface treatment

Selecting CF with a certain length, drying and weighing. Then, the CF is wound on a stainless steel or glass frame, the frame wound with the CF is placed in a reaction kettle of a supercritical fluid extraction apparatus, 10ml of acetone is added, and the acetone is used as a polar additive, so that the dissolving capacity of the supercritical fluid to polar substances can be greatly improved. And closing the reaction kettle, setting the temperature and the pressure of the reaction device, and controlling the reaction time. In the process of the experiment, attention is paid to the display of an instrument pressure gauge, and the pressure can exceed a preset pressure value along with the rise of the temperature, so that the stable temperature and pressure can be adjusted through multiple experiments.

For supercritical CO₂The extraction apparatus is set with different temperatures to study the clear condition of the epoxy resin slurry on the CF surface. Supercritical CO₂The critical temperature point of the fluid is 31.26 ℃ and the critical pressure is 7.29 MPa. The temperatures set in the tests all exceeded the supercritical CO₂The critical temperature point of (2). In the experiment, acetone is used as a polar additive, so that the cleaning effect of the slurry is better improved.

TABLE 1 utilization of supercritical CO₂Temperature setting for CF surface cleaning process

In order to verify whether the pressure has an influence on the cleaning of the CF surface slurry, the experiment is carried out by controlling two variables of temperature and time to be unchanged, wherein the pressure is in a range of 8MPa to 20MPa, as shown in Table 2.

TABLE 2 utilization of supercritical CO₂Pressure setting for CF surface cleaning process

Serialnumber	Temperature(℃)	Time(min)	Pressure(MPa)
				6	45	40	8
7	45	40	11
				8	45	40	14
9	45	40	17
				10	45	40	20

In a supercritical environment, in order to verify whether time can influence the cleaning effect of the CF surface slurry, the influence of the time on the cleaning effect is systematically analyzed by fixing two factors of temperature and pressure. The time range is 10min-50 min.

TABLE 3 utilization of supercritical CO₂Time setting for CF surface cleaning treatment

Serialnumber	Temperature(℃)	Time(min)	Pressure(MPa)
				11	45	10	14
12	45	20	14
				13	45	30	14
14	45	40	14
				15	45	50	14

Supercritical CO at different temperatures₂The change curve of the weight loss rate of CF is shown in FIG. 1, and the weight loss rate of CF is increased and then decreased with the increase of the treatment temperature, which shows that the supercritical CO at different temperatures₂The treatment has an effect on the weight loss of the fiber when supercritical CO is used₂The maximum weight loss rate is 0.80% at a treatment temperature of 45 ℃.

FIG. 2-0 is an untreated CF surface topography, which shows that the fiber surface is coated with a layer of slurry, which is CF coated on the surface at the time of shipment; after the CF is treated, a certain amount of epoxy resin sizing agent is remained on the surface and distributed in a fish scale shape, which shows that the supercritical CO is₂The fluid washes away some of the slurry from the CF surface. As shown in fig. 2 at 1, 2, 3, 4 and 5, the slurry on the CF surface was gradually washed away as the treatment temperature increased, and the fiber surface began to be exposed, but the CF surface slurry content was less for the treatment of fig. 3.2-3 at 45 ℃.

By comprehensively comparing the SEM surface morphology analysis and the weight loss rate of CF, the optimal processing temperature for CF cleaning is set to be 45 ℃.

Good fiber monofilament strength can provide effective force loading for the composite material thereof. Testing the tensile strength of the monofilaments by using an XQ-1 type fiber strength tester for CF before and after cleaningThe data obtained are summarized by Weibull function fitting as shown in FIG. 3, where A is the Weibull distribution curve of the tensile strength of the unwashed CF monofilaments and B is the supercritical CO₂Weibull distribution curve of tensile strength of CF monofilaments treated with a wash at 45 ℃ for 10min at 8 MPa.

FIG. 4-A is an atomic force electron microscope image of CF without cleaning treatment, which shows that the surface has a large number of protruding structures, which are caused by coating the CF surface with the sizing agent. 4-B is in supercritical CO₂The atomic force electron microscope image of the CF surface treated under the process condition of 45 ℃ and 8MPa for 10min shows that the fiber surface is relatively flat and a groove structure along the fiber direction appears, which indicates that part of the sizing agent on the fiber surface is cleaned and the original fiber surface is exposed.

In supercritical CO₂Under the condition of fluid, the optimum processing temperature is 45 ℃, the processing time is 10min, and the supercritical CO with different pressures₂The change curve of the weight loss rate of CF is shown in FIG. 6, and the weight loss rate of CF is increased and then decreased with the increase of the processing pressure, which shows that the supercritical CO at different temperatures₂The treatment has an effect on the weight loss of the fiber when supercritical CO is used₂When the processing pressure is 14MPa, the weight loss rate is 0.90 percent at most.

In supercritical CO₂Under the condition of fluid, the optimal processing temperature is 45 ℃ and the processing time is 10min, so that the influence of different pressures on the surface appearance of the CF is discussed, and FIG. 7 is a scanning electron microscope image of the CF under different temperature processing. Generally, under the condition that the reaction time and the reaction temperature are determined, the higher the pressure is, the more thoroughly the CF surface is cleaned; however, it can be seen from the figure that when the pressure is increased to 14MPa, the surface of CF is less different from the surface of CF treated at 20MPa, which indicates that when the pressure reaches a certain degree, the slurry on the surface of CF cannot be effectively removed, and other conditions are required. It was concluded that better cleaning of the fibre surface was obtained at a treatment pressure of 14 MPa. The influence of different pressures on the cleaning weight loss rate and the surface appearance analysis of the CF processed by different pressures are comprehensively considered, and the optimal processing pressure is set to be 14 MPa.

FIG. 8 is a graph of supercritical CO at different times₂Change curve of CF weight loss rate with treatment temperatureThe increase in the degree, the weight loss of CF increased first and then decreased, indicating supercritical CO at different temperatures₂The treatment has an effect on the weight loss of the fiber when supercritical CO is used₂When the treatment time is 40min, the maximum weight loss rate is 1.05%.

As shown in fig. 9, the CF surface tended to be smooth with time, the surface impurity spots and the fish scale-like raised structures disappeared, and the surface slurry was found to have been completely removed. The CF surface morphology has basically no change when the treatment time is 40 min.

FIG. 10-1 is a supercritical CO₂The surface topography of the CF after cleaning treatment, fig. 10-2 is the surface topography of the CF after reaction for 1h in a nitric acid solution, fig. 10-3 is the surface topography of the CF after soaking for 2h in a nitric acid solution, fig. 10-4 is the surface topography of the CF after soaking for 3h in a nitric acid solution, fig. 10-5 is the surface topography of the CF after soaking for 4h in a nitric acid solution, and fig. 10-6 is the surface topography of the CF after soaking for 5h in a nitric acid solution. The cleaned CF has smooth surface and no structural protrusions, because the carbon atoms on the surface of the fiber are in a six-membered ring honeycomb structure, and the structure can provide good chemical core physical stability for the CF. Carbon atoms in the CF can be oxidized with nitric acid in the nitric acid solution to form oxygen-containing functional groups, and weak interface layers exist on the surface of the CF, and can be corroded and peeled off under the action of strong acid and strong oxidizing property. As shown in FIG. 10, as the treatment time increases, the smoothness of the CF surface decreases, a fish scale structure starts to be generated, the groove structure deepens, and when the oxidation time is 4h, the fiber surface is greatly different from the untreated fiber surface. Thus, the preferred time for optimizing nitric acid oxidation is 4 hours.

Supercritical CO₂The fluid can effectively clean the surface of the fiber and has small influence on the loss of the tensile strength of the fiber. By using supercritical CO₂The cleaning mode of the cleaning agent is used for discussing CF cleaning under different temperatures, time and pressures, and the following conclusion is obtained by testing the weight loss rate and the monofilament strength of the CF and characterizing by utilizing scanning electron microscope analysis, Raman analysis, atomic force scanning electron microscope analysis and photoelectron spectroscopy analysis:

(1) when the pressure is 8MPa and the processing time is 10min, the weight loss rate of the fiber is increased and then reduced along with the increase of the cleaning temperature, and when the temperature is 45 ℃, the weight loss rate is 0.80 percent at the maximum; the fiber surface is relatively clean when the temperature is 45 ℃ through observation of a scanning electron microscope; the strength of the CF monofilament is 4.80GPa, and the reduction of the strength is smaller compared with that of the CF which is not cleaned; the average roughness of the CF surface is 31.4nm, and is reduced compared with the original CF roughness; the integral intensity ratio of the CF Raman spectrum is 2.49, which is increased by 0.81 percent compared with that before treatment, and the disordered carbon structure is increased. The optimum treatment temperature is 45 ℃.

(2) When the temperature is 45 ℃ and the processing time is 10min, the fiber weight loss rate is increased and then reduced along with the increase of the reaction pressure, and when the processing pressure is 14MPa, the weight loss is 0.90 percent at the maximum; the analysis of a scanning electron microscope can find that the surface of the fiber is cleaner when the reaction pressure is 14 MPa; the strength of the CF monofilament is 4.78GPa, and the reduction of the strength is smaller compared with that of the CF which is not cleaned; the average roughness of the CF surface is 22.8nm, and is reduced compared with the original CF roughness; the integral intensity ratio of the CF Raman spectrum is 2.51, which is increased by 1.62 percent compared with that before treatment, and the disordered carbon structure is increased. The optimum processing pressure is 14 MPa.

(3) When the temperature is 45 ℃ and the processing pressure is 14MPa, the fiber weight loss rate is increased and then reduced along with the increase of the reaction time, and when the processing time is 40min, the weight loss rate is the maximum and is 1.05 percent; analysis by a scanning electron microscope can find that the surface of the fiber is cleaner when the treatment time is 40 min; the strength of the CF monofilament is 4.71GPa, and the reduction of the strength is smaller compared with that of the CF which is not cleaned; the average roughness of the CF surface is 12.2nm, and is reduced compared with the original CF roughness; the integral intensity ratio of the CF Raman spectrum is 2.52, which is increased by 2.02 percent compared with that before treatment, and the disordered carbon structure is increased. The optimal treatment time is 40 min.

(4) By using supercritical CO₂The optimal process condition for cleaning CF is 14MPa at 45 ℃ for 40 min. The content of oxygen-containing functional groups on the CF surface is reduced, carboxyl and ester groups are reduced to almost zero, and fiber surface sizing agent is cleaned up through photoelectron spectroscopy analysis.

2. Oxidation treatment of washed CF under different process conditions

The CF surface is cleaned to expose the carbon atom surface, and T700CF is drawn by the dry-jet-wet spinning process to form weak ravine structures on the CF surface parallel to the CF axis, but these structures do not effectively improve the CF surface roughness. Under the condition of fixed acid solution types, the main factors influencing the oxidation effect of the fiber surface mainly include temperature and time. Influence CF surface this experiment sets the influence of temperature on the oxidation effect of CF at a certain time. The time is fixed to be 3h, and the temperature setting range is 30-70 ℃.

TABLE 4 Experimental setup for nitric acid solution on oxidation of washed CF surfaces at different times

The specific treatment steps are as follows: fixing a three-neck flask (250ml) provided with a stirrer, a condenser and a thermometer in an oil bath pan, adding 80ml of nitric acid solution and CF cleaned by supercritical fluid into the flask, raising the temperature to the required reaction temperature, observing the color of the solution every 1h of cadmium, taking out the CF after reacting for 3h, cleaning the surface by acetone, and drying in an oven at 50 ℃ for 12 h.

To verify the effect of oxidation time on the oxidation effect of the CF surface at a fixed temperature, different oxidation treatment times were set. The time mainly affects the contact area of the nitric acid solution to the fiber surface.

TABLE 5 Experimental setup for nitric acid oxidation of CF surfaces at different times

The surface topography of the CF surface oxidized by the nitric acid solution at different temperatures is shown in fig. 11, fig. 11-7 are the surface topography of the fiber after surface cleaning, fig. 11-8 are the surface topography of the CF after treatment for 4h in the nitric acid solution at 30 ℃, fig. 11-9 are the surface topography of the CF after treatment for 4h in the nitric acid solution at 40 ℃, fig. 11-10 are the surface topography of the CF after treatment for 4h in the nitric acid solution at 50 ℃, fig. 11-11 are the surface topography of the CF after treatment for 4h in the nitric acid solution at 60 ℃, and fig. 11-12 are the surface topography of the CF after treatment for 4h in the nitric acid solution at 70 ℃. As shown in fig. 11, as the treatment time increases, more and more fish-scale protrusions appear on the fiber surface because the carbon atoms on the fiber surface react with the nitric acid and fall off the fiber surface, and the nitric acid also etches the original groove structure of CF to be deeper. It can be seen from the figure that when the temperature is raised to 70 ℃, the surface structure of the fiber is rough, forming more scale-shaped protrusions and gully structures, which can greatly increase the roughness of the surface of the fiber. Therefore, 70 ℃ is optimized as the preferred processing temperature.

The CF surface can be etched by using a nitric acid solution, so that the surface roughness is increased, and the surface activity is improved. The influence of different time and different temperature on the washed CF is discussed by using nitric acid solution, and the following results are obtained by testing means such as an electron scanning microscope, an atomic force microscope, a fiber strength test, a Raman electron microscope analysis, a photoelectron spectroscopy and the like:

(1) when the treatment temperature is 50 ℃, scanning electron microscope detection shows that when the oxidation reaction time is 4 hours, larger grooves appear on the fiber surface, and the increase trend of the surface grooves is not obvious along with the increase of the reaction time; the average strength of the CF monofilament is 4.60GPa, which is reduced by 2.34 percent compared with the strength of the CF monofilament after cleaning of 4.71 GPa; the integral intensity ratio of the CF Raman is 2.64, and is increased by 4.55 percent compared with the original integral intensity ratio of the CF of 2.52, which shows that the carbon disordered structure of the CF is increased after the oxidation treatment; the average roughness of the CF surface is 35.9nm, and the average roughness of the CF surface after cleaning is increased by 194.26% compared with the average roughness of the CF surface after cleaning at 12.2 nm. The preferred treatment time is 4 hours.

(2) When the oxidation time is 4 hours, a scanning electron microscope shows that when the oxidation temperature is 70 ℃, a large number of obvious groove structures are formed on the surface of the fiber; the strength of the CF monofilament is 4.52GPa, which is reduced by 4.03 percent compared with the strength of the CF monofilament after cleaning by 4.71GPa, which shows that the strength of the fiber can be reduced by the oxidation of a nitric acid solution; the CF roughness is increased from 12.2nm of unoxidized treatment to 32.2nm, and the acid solution etches the surface to increase the roughness; CF Raman R (I)_D/I_G) The value is 2.83, the CF Raman integral value after cleaning is increased by 10.95 percent, and the fiber disordered carbon structure after treatment is increased, and the surface is etchedThe effect is to increase the activity of the grain boundaries. The preferred treatment temperature is 70 ℃.

(3) Combined with the loss of bulk strength of the CF, the preferred process conditions for oxidation of CF with nitric acid solution are 70 ℃ for 4 h. The increase in oxygen-containing functional groups on the CF surface and the increase in carboxyl and ester groups to 14.08% and 7.08%, respectively, was determined by XPS analysis, indicating that a certain amount of oxygen-containing functional groups had formed on the fiber surface under nitric acid oxidation.

3. Preparation of grafted CF

After the smooth surface of the CF is oxidized by a nitric acid solution, the surface roughness is increased, and the oxidized CF surface is grafted with ethylenediamine containing amino groups, so that the interface compatibility between fibers and a resin matrix and the strength of fiber monofilaments can be improved. The systematic study in this chapter examined the preparation of CF grafted ethylene diamine at different times and different temperatures.

TABLE 6 CF grafted ethylene diamine after Oxidation at different times

TABLE 7 CF grafted ethylene diamine after Oxidation at different temperatures

The method comprises the following specific steps: a three-necked flask (250ml) equipped with a stirrer, a condenser and a thermometer was fixed in an oil bath, 100ml of an ethylenediamine solution and CF after supercritical fluid cleaning were added to the flask, as shown in tables 6 and 7, the temperature was raised to the desired reaction temperature, the color of the solution was observed at 1 hour intervals, CF was taken out according to the reaction time, and the surface was cleaned with acetone and placed in an oven to dry at 50 ℃ for 12 hours. And taking out a sample, and performing performance detection and structural characterization.

As shown in FIG. 13, as the CF grafting treatment time increases, the number of particulate grafts on the surface of the fiber increases continuously, the terminal group of the ethylenediamine contains amino functional groups, and the amino groups and the oxidized CF surface carboxyl groups undergo dehydration condensation reaction to form amino groups and are grafted on the surface of the fiber. And after fig. 13-2 the CF surface began to appear as more distinct grafted particles, with the increase in grafting time, the fiber surface grafted particles increased significantly. The surface grafts reached the maximum when the reaction was 5 h. The grafting reaction time was optimized to 5 h.

Scanning electron microscope images of the surface morphology of CF grafted ethylenediamine at different temperatures are shown in FIG. 14, FIG. 14-1 is the surface morphology of CF oxidized by nitric acid solution, FIG. 14-2 is the surface morphology of CF reacted for 5h in 40 ℃ ethylenediamine solution, FIG. 14-3 is the surface morphology of CF reacted for 5h in 50 ℃ ethylenediamine solution, FIG. 14-4 is the surface morphology of CF reacted for 5h in 60 ℃ ethylenediamine solution, FIG. 14-5 is the surface morphology of CF reacted for 5h in 70 ℃ ethylenediamine solution, and FIG. 14-6 is the surface morphology of CF reacted for 5h in 80 ℃ ethylenediamine solution. As can be seen from fig. 14, as the reaction time increases, more and more grafted particulate structures appear on the fiber surface, because the CF surface oxidized by the nitric acid solution contains a large amount of oxygen-containing functional groups such as carboxyl groups, which undergo a glycidyl polymerization reaction with the terminal hydroxyl groups of ethylenediamine to form ester groups and connect the fibers and the grafted monomers. The tendency of the grafted particulate matter on the surface of the fibers is not very great when the reaction temperature is 70 ℃. The ethylenediamine graft on the CF surface may agglomerate under the temperature change to form larger granular substances. Therefore, the optimal 70 ℃ is the better reaction temperature of the CF grafted ethylene diamine.

And analyzing the surface morphology of the fracture interface of the CF composite material by using a scanning electron microscope. The SEM image of the wire drawing fracture of the CF composite material can reflect the difference of CF before and after grafting, FIG. 16 is the fracture surface of the CF composite material, and A image is the surface morphology of the fracture surface of the oxidized CF composite material, so that the large gap between the fiber and the resin matrix and the uneven fracture surface can be found, which indicates that the interfacial property of the composite material is poor; the graph C shows the surface appearance of the fracture surface of the surface grafted ethylene diamine CF composite material, and can find that the fracture surface is regular, the gap between the fiber and the resin matrix is small, and the composite material has partial CF fracture under the action of stress, which can indicate that the grafted CF composite material has good interface performance.

The CF surface grafted ethylenediamine can effectively increase the interface performance of the composite material, repair the surface defects of the fibers and improve the tensile strength of the fibers. The influence of ethylenediamine grafting on CF at different time and different temperature is systematically discussed in this section, and the following results are obtained by utilizing the characterization means such as scanning electron microscope analysis, surface roughness analysis, Raman spectrum analysis, photoelectron spectroscopy analysis and interlaminar shear strength:

(1) when the reaction temperature is 60 ℃, the grafting time of the ethylenediamine and the ethylenediamine particles on the surface of the fiber is increased along with the increase of the reaction time through the analysis of a scanning electron microscope atlas, and the generation of a large amount of uniform ethylenediamine grafts on the surface of the fiber is 5 hours. The strength of the CF monofilament is increased from 4.52GPa to 4.85GPa after oxidation, which shows that the grafting of the ethylenediamine on the CF can fill up the defects of the fiber caused by oxidation, and simultaneously, a compact protective layer is formed on the surface of the fiber, so that the effect of improving the strength of the fiber monofilament is achieved; raman R value (I) of CF_D/I_G) The reduction to 2.36 indicates that the disordered carbon structure content of the fiber is reduced after grafting; the carboxyl content on the CF surface is reduced, while the ester group content is increased, which indicates that the fiber surface is grafted with ethylenediamine; the interfacial property of the grafted CF composite material is enhanced and the interlaminar shear strength is improved to 55.64MPa by the interlaminar shear strength and the scanning of the fracture surface of the composite material.

(2) At the reaction time of 5h, the ethylenediamine grafted particles on the surface of the fiber increased with the increase of the reaction temperature, and reached the maximum at the temperature of 70 ℃. The monofilament strength of the CF is 4.92GPa, which is increased by 8.85 percent compared with the CF4.52GPa after oxidation, so that the strength of the CF body is maintained at about the original filament; CF Raman R (I)_D/I_G) The value decreases to 2.41, which indicates that the grafted CF disordered carbon structure decreases and the fiber strength increases under the protection of the ethylenediamine graft; the CF carboxyl content decreased from 14.08% to 2.09% of the oxidized CF, indicating that the ethylenediamine was grafted on the fiber surface. Through the analysis of a scanning electron microscope and the interlaminar shear strength of the fracture surface of the composite material, the fracture surface of the grafted CF composite material is uniform, the interface performance is enhanced, and the interlaminar shear strength of the grafted CF is improved to 56.32 MPa.

Claims

1. A method for modifying carbon fibers, a process for producing the same,the method is characterized in that: firstly, supercritical CO is adopted₂And (3) carrying out surface treatment on the CF, then carrying out oxidation treatment on the cleaned CF by adopting a nitric acid solution, and finally grafting ethylenediamine containing amino groups on the oxidized CF surface to obtain the modified carbon fiber.

2. A method of modifying carbon fibers as recited in claim 1, wherein: the supercritical CO₂The method for processing the CF surface comprises the following steps: placing CF in a reaction kettle of a supercritical extraction instrument, adding acetone, closing the reaction kettle, and performing supercritical CO₂The critical temperature of the fluid is 45-50 deg.C, the critical pressure is 14-20MPa, and the treatment time is 30-40 min.

3. A method of modifying carbon fibers as recited in claim 2, wherein: preferred supercritical CO₂The critical temperature point of the fluid is 45 ℃, the critical pressure is 14MPa, and the processing time is 40 min.

4. A method of modifying carbon fibers as recited in claim 1, wherein: the oxidation treatment of the washed CF: fixing a three-neck flask provided with a stirrer, a condenser tube and a thermometer in an oil bath pan, adding a nitric acid solution and the CF cleaned by the supercritical fluid into the flask, heating to 60-70 ℃, reacting for 3-4h, taking out the CF, cleaning the surface by acetone, and drying in an oven.

5. A method of modifying carbon fibers as recited in claim 4, wherein: preferably, the reaction temperature is 70 ℃ and the reaction time is 4 h.

6. A method of modifying carbon fibers as recited in claim 1, wherein: the method for grafting ethylenediamine containing amino groups on the oxidized CF surface comprises the following steps: fixing a three-neck flask provided with a stirrer, a condenser tube and a thermometer in an oil bath kettle, adding 100ml of ethylenediamine solution and CF cleaned by supercritical fluid into the flask, heating to 60-70 ℃, reacting for 5-6h, taking out the CF, cleaning the surface by acetone, and putting the CF into an oven for drying.

7. A method of modifying carbon fiber as claimed in claim 6, wherein: preferably, the reaction temperature is 70 ℃ and the reaction time is 5 h.