A method for preparing surface-modified polyimide fibers

Abstract

The application relates to a preparation method of surface-modified polyimide fibers, comprising the following steps: (S1) polyimide fibers are subjected to plasma surface treatment; (S2) a dianhydride monomer, a diamine monomer and a capping agent are added into an organic solvent, the molar ratio of the dianhydride monomer, the diamine monomer and the capping agent is n:n+1:2-3, wherein n is 6-10, and the fluorine-containing monomer accounts for 30-80% of the total amount of substance of the dianhydride monomer and the diamine monomer, so that a polyamic acid solution is prepared; (S3) the polyimide fibers treated in the step (S1) are soaked in the polyamic acid solution prepared in the step (S2), and are reacted at 80-120 DEG C, washed, dried and heat-treated at 300-350 DEG C, so that the surface-modified polyimide fibers are obtained. The application solves the problem of poor interfacial bonding performance of polyimide fibers and resin, and widens the application range of the polyimide fibers.

Description

Technical Field

[0001] The invention belongs to the technical field of polyimide polymer materials, and particularly relates to a preparation method of surface-modified polyimide fibers. Background Art

[0002] Polyimide (PI) fiber filament is a PI material used in fiber form. It fully retains the performance characteristics of PI material such as high and low temperature resistance, aging resistance, low water absorption, low dielectric, and high insulation. Combined with its outstanding tensile strength and modulus, it can be used to prepare lightweight, high-strength, wave-transparent and other advanced composite materials, and has broad application prospects in aerospace and other fields.

[0003] In the existing technology, the surface modification methods of polyimide fibers aimed at improving interfacial properties mainly include plasma treatment, ozone treatment, irradiation treatment, coupling agent treatment, and chemical grafting. Plasma and active particles can interact with the fiber surface and introduce functional groups. At the same time, the sputtering and etching effects of plasma can produce fine concave and convex structures on the fiber surface, increasing the surface roughness and specific surface area. However, plasma treatment still faces key challenges in the field of fiber surface modification, such as the time-limited nature of plasma treatment. Chemical grafting is one of the important methods for fiber surface modification. It can react with the fiber surface to form covalent bonds and introduce specific groups to achieve long-term stability of fiber surface functionalization. However, chemical grafting of polyimide fibers usually requires surface activation first.

[0004] CN201510712302.6, a prior art document, mentions the use of plasma and chemical grafting to treat material surfaces. This method aims to introduce carbenes by reacting diaryldiazomethane derivatives with reactive groups obtained through plasma treatment. The carbenes are then used as active sites to further introduce amino, carboxyl, and other functional groups to achieve functionalization. This method involves the preparation of diaryldiazomethane derivatives, modification of the material surface using the diaryldiazomethane derivatives, and the introduction of functional groups onto the material surface, resulting in a relatively complex process.

[0005] The inventor's previous patent CN103319890A discloses a polyimide fiber fabric reinforced polyimide resin composite material, the preparation method of which includes the following steps: (1) selecting polyimide fiber as reinforcement and polyimide resin as matrix material; (2) weaving the polyimide fiber into fiber fabric, modifying the surface of the woven fabric to obtain a surface-modified polyimide fiber fabric; (3) determining the amount of dianhydride, end-capping agent, diamine and alcohol used based on the principle that the total molar number of acid anhydride groups and amino groups is equal, the total amount of resin, the solid content and the designed number average molecular weight; dissolving the dianhydride and end-capping agent in anhydrous ethanol and performing an esterification reaction for 6 hours (4) using the resin solution obtained in step (3) to hand-paste the surface-modified polyimide fiber fabric obtained in step (2) into a thickness of 2 mm, then drying the solvent at 60-100° C. for 6 hours, and performing a cyclization reaction at 200° C. for 1 hour to obtain a prepreg; (5) keeping the prepreg obtained in step (4) at a temperature of 250-270° C. and a pressure of 2-3 MPa for 30 minutes, and then keeping the temperature and pressure at a temperature of 320-370° C. for 1 hour to obtain a polyimide fiber fabric reinforced polyimide resin-based composite material.

[0006] CN104233777A discloses a method for preparing surface-modified polyimide fibers, comprising the following steps: 1) treating polyimide fibers in a plasma atmosphere to obtain plasma-treated polyimide fibers; and 2) mixing the plasma-treated polyimide fibers with a solution containing a grafting agent, wherein the grafting agent is one or more of acrylic acid, methacrylic acid, acrylamide, acryl alcohol, methacrylic anhydride, maleic anhydride, silane, graphene, and carbon nanotubes, thereby obtaining surface-modified polyimide fibers.

[0007] The above patent partially solves the problem of poor interface bonding performance between polyimide fiber and resin, but the timeliness of the interface bonding performance still cannot meet the requirements, which limits the scope of application of polyimide fiber. Summary of the Invention

[0008] To address the technical issues of poor interfacial bonding between polyimide fibers and resins, as well as the poor long-term effectiveness of interfacial bonding, in the prior art, the present invention proposes a surface-modified polyimide fiber and a method for preparing the same. Specifically, the present invention first utilizes plasma treatment to introduce reactive functional groups or free radicals, such as carboxyl and amino groups, onto the surface of the polyimide fiber. Double-bond-containing compounds are then grafted onto the fiber through a free radical reaction and polymerization reaction. A suitable polyimide monomer for grafting modification is then selected and modified. The resulting modified polyimide fiber exhibits significantly improved interfacial bonding with the resin due to its enhanced surface roughness.

[0009] The present invention provides the following technical solutions to solve the above technical problems:

[0010] A method for preparing surface-modified polyimide fiber comprises the following steps:

[0011] (S1) Polyimide fiber was plasma surface treated;

[0012] (S2) adding a dianhydride monomer, a diamine monomer, and a capping agent to an organic solvent, wherein the molar ratio of the dianhydride monomer, the diamine monomer, and the capping agent is n:n+1:2-3, wherein n is 6-10, and the fluorine-containing monomer accounts for 30-80% of the total amount of the dianhydride monomer and the diamine monomer, to prepare a polyamic acid solution;

[0013] (S3) taking the polyimide fiber after the ion treatment in step (S1) and soaking it in the polyamic acid solution prepared in step (S2), reacting it at 80-120°C, washing it, drying it, and heat-treating it at 300-350°C to obtain a surface-modified polyimide fiber.

[0014] Furthermore, in step (S1), the polyimide fibers include, but are not limited to, chopped fibers, filaments, and fabrics, and fiber strength is not required. The plasma surface treatment is performed in an atmosphere of one or a mixture of oxygen, nitrogen, ammonia, and air, at a power of 20-300W and a plasma treatment rate of 0.24 m / min-2.4 m / min. The plasma-treated polyimide surface is enriched with active functional groups or free radicals such as carboxyl, hydroxyl, and amino groups, facilitating subsequent grafting modification.

[0015] Furthermore, in step (S2), the polyamic acid solution is prepared by stirring at 0-5°C for 2-5 hours.

[0016] Further, in step (S2), the dianhydride monomer is selected from at least one of 3,3,4,4-biphenyltetracarboxylic dianhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-anthracenetetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, bisphenol A type diether dianhydride, and pyromellitic dianhydride; the diamine monomer is selected from at least one of diaminodiphenyl ether, 4,4,-diamino-2,2,-bis(trifluoromethylbiphenyl), 2,2-bis(4-amino-3,5,6-trifluorophenyl)hexafluoropropane, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, and p-phenylenediamine; The end-capped monoanhydride is selected from at least one of maleic anhydride, nadic anhydride, acrylic anhydride, fumaric anhydride, 2-methylenesuccinic anhydride, and phenylethynylphthalic anhydride; the fluorine-containing monomer is a fluorine-containing dianhydride and / or a fluorine-containing diamine, and the fluorine-containing dianhydride is selected from 9,9-bis(trifluoromethyl)-2,3,6,7-anthracenetetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride; the fluorine-containing diamine is selected from 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, 2,2-bis(4-amino-3,5,6-trifluorophenyl)hexafluoropropane, and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene.

[0017] The present invention specifies a specific content of fluorine-containing monomer in the dianhydride monomer and diamine monomer, which can significantly improve the time-effectiveness of the interfacial bonding performance of the prepared polyimide fiber and enhance the mechanical properties of the composite material at high temperatures. However, the fluorine content should be neither too high nor too low. Too low a fluorine content will affect the mechanical bonding between the grafted chains and the resin matrix due to uneven distribution of the fluorine-containing side groups, failing to effectively improve the interfacial bonding performance of the material. Too high a fluorine content will lead to the accumulation of grafted chains, excessive thickness of the interface layer, and reduced interfacial performance.

[0018] Preferably, the dianhydride monomer is a compound of fluorine-free dianhydride and fluorine-containing dianhydride in a molar ratio of 3:7 to 1:1, and the diamine monomer is a compound of fluorine-free diamine and fluorine-containing diamine in a molar ratio of 3:7 to 1:1.

[0019] While the inventors have discovered that controlling the fluorinated monomer content in the total monomer composition can improve the interfacial bonding performance and long-term stability of polyimide fibers, evenly dispersing the fluorinated monomer as a repeating unit is more beneficial to the overall material performance. For example, when the fluorinated monomer accounts for 50-70% of the total monomer composition, the interfacial bonding performance and long-term stability of the material are improved. Furthermore, when the ratio of fluorinated dianhydride in the dianhydride monomer is consistent with the ratio of fluorinated diamine in the diamine monomer, the performance improvement is optimal.

[0020] Furthermore, in step (S2), the organic solvent is selected from at least one of dimethylformamide and dimethylacetamide; the amount of the organic solvent is such that the solid content of the obtained polyamic acid solution is 10-20 wt%, such as 15 wt%.

[0021] Furthermore, in step (S3), the reaction time at 80-120°C is 5-10h, such as 6h, 7h, 8h, or 9h; the heat treatment at 300-350°C is to place the modified fiber in a nitrogen oven, heating it from room temperature to the target temperature at a rate of 2-10°C / min for heat treatment for 0.5-2h.

[0022] This invention utilizes a combined plasma and chemical grafting method, which not only meets the requirement for pre-activation of the fiber surface required by chemical grafting, but also overcomes the time-sensitive nature of plasma treatment. By utilizing the free radicals and active functional groups generated by plasma treatment, long-term stable functional groups are introduced to the PI fiber surface, further improving its interfacial bonding with the resin. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a SEM image of the composite material obtained in Example 1;

[0024] Figure 2 is a SEM image of the composite material obtained in Example 2;

[0025] Figure 3 is a SEM image of the composite material obtained in Example 3;

[0026] Figure 4 is a SEM image of the composite material obtained in Example 5;

[0027] Figure 5 is a SEM image of the composite material prepared in Comparative Example 1;

[0028] Figure 6 is an SEM image of the composite material obtained in Comparative Example 2;

[0029] Figure 7 is an SEM image of the composite material of Example 1 after micro-debonding test;

[0030] Figure 8 is an SEM image of the composite material of Example 2 after micro-debonding test;

[0031] Figure 9 is an SEM image of the composite material of Example 3 after micro-debonding test;

[0032] Figure 10 is an SEM image of the composite material of Example 5 after micro-debonding test;

[0033] Figure 11 This is the SEM image of the composite material of Comparative Example 1 after micro-debonding test.

[0034] Figure 12 This is the SEM image of the composite material of Comparative Example 2 after micro-debonding test. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further explained below with specific implementation methods.

[0036] The polyimide fiber used in the embodiment of the present invention is a high-strength and high-modulus polyimide fiber purchased from Jiangsu Xiannuo New Material Technology Co., Ltd.

[0037] Example 1

[0038] (S1) The polyimide fiber is ultrasonically cleaned in ethanol, acetone, and deionized water, followed by drying in a vacuum oven at 60° C. for 8 hours to obtain a clean polyimide fiber; the cleaned polyimide fiber is placed in an oxygen plasma atmosphere, wherein the plasma equipment has a processing power of 30 W and a plasma nozzle movement speed of 0.24 m / min to obtain a plasma-treated polyimide fiber;

[0039] (S2) adding a dianhydride monomer, a diamine monomer, and a capping agent, maleic anhydride, in a molar ratio of 10:11:3, wherein the dianhydride monomer is a compound of 3,3,4,4-biphenyltetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride in a molar ratio of 3:7, and the diamine monomer is a compound of diaminodiphenyl ether and 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine in a molar ratio of 3:7, and adding the above monomers and the capping agent to a dimethylacetamide solvent, reacting in an ice-water bath for 4 hours to obtain a polyamic acid (PAA) solution having a solid content of 15 wt%;

[0040] (S3) The plasma-treated polyimide fiber obtained in step (S1) is immersed in the PAA solution obtained in step (S2), the oil bath temperature is raised to 80°C, the reaction is carried out at a constant temperature for 6 hours, the fiber is taken out, and then washed three times with dimethylacetamide. The fiber is placed in an oven, the temperature is raised to 350°C at 2°C / min, and the temperature is maintained for 2 hours to obtain a surface-modified polyimide fiber.

[0041] Example 2

[0042] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride monomer, the diamine monomer and the end-capping agent maleic anhydride are added in a molar ratio of 6:7:2.

[0043] Example 3

[0044] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride monomer is a mixture of 3,3,4,4-biphenyltetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride in a molar ratio of 1:1, and the diamine monomer is a mixture of diaminodiphenyl ether and 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine in a molar ratio of 1:1.

[0045] Example 4

[0046] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride monomer is a compound of 3,3,4,4-biphenyltetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride in a molar ratio of 2:8, and the diamine monomer is a compound of diaminodiphenyl ether and 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine in a molar ratio of 2:8.

[0047] Example 5

[0048] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride monomer is 4,4'-(hexafluoroisopropylene) diphthalic anhydride, and the diamine monomer is diaminodiphenyl ether.

[0049] Example 6

[0050] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride monomer is a mixture of 3,3,4,4-biphenyltetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride in a molar ratio of 2:8, and the diamine monomer is a mixture of diaminodiphenyl ether and 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine in a molar ratio of 8:2.

[0051] Comparative Example 1

[0052] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride and diamine monomers are 3,3,4,4-biphenyltetracarboxylic dianhydride and diaminodiphenyl ether, respectively, that is, no fluorine-containing monomer is added.

[0053] Comparative Example 2

[0054] Other conditions are the same as those in Example 1, except that in step (S2), the dianhydride and diamine monomers are 4,4'-(hexafluoroisopropylene) diphthalic anhydride and 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine, respectively, that is, all monomers are fluorine-containing monomers.

[0055] Comparative Example 3

[0056] Other conditions were the same as those in Example 1, except that the polyimide fibers were not treated in any way, that is, the original fibers were used for the preparation and testing of the composite materials.

[0057] Figure 1-4 The following are SEM images of the modified fibers obtained in Example 1, Example 2, Example 3, and Example 5, respectively. Figure 5-6SEM images of the fibers produced in Comparative Examples 1 and 2 are shown. The results show that the grafted materials on the plasma-grafted fibers adhere to the fiber surface in a large, clumping pattern, and that plasma etching marks are still present to some extent on the fiber surface. The grafted short chains adhere uniformly to the fiber surface in the form of particles, demonstrating that plasma-grafting modification can improve the fiber surface roughness while achieving uniform distribution.

[0058] Figure 7-8 is the SEM image of the fibers of Example 1 and Example 2 after micro-debonding test; Figure 9 This is the SEM image of the fiber in Example 3 after micro-debonding test. Figure 10 This is the SEM image of the fiber in Example 5 after micro-debonding test. Figure 11-12 It is an SEM image of the fibers of Example 1 and Example 2 after micro-debonding test. It can be seen that the embodiment shows obvious fiber skin tearing phenomenon, that is, part of the fiber skin is separated from the fiber body structure along with the resin microspheres, indicating that the fiber-resin interface bonding strength has exceeded the cohesive strength of the fiber cortex, further illustrating the interface enhancement effect of the modified system. The uniform distribution of fluorine-containing monomers also has an impact on shear force. The uniform dispersion of the repeating units of fluorine-containing monomers in the entire polymer chain is more conducive to improving the interface stability of the composite material at high temperature. The unbalanced distribution causes the generation of defects in the composite material, resulting in poor interface performance. In Example 1 and Example 2, the debonded samples showed a smooth fiber surface, which indicates that the grafted chains of fluorine-free or perfluorinated monomers failed to play the role of linking the resin matrix and the reinforcement.

[0059] Microsphere debonding samples and composite material plates were prepared from the fibers obtained in the above examples and comparative examples, and the performance tests of interfacial shear strength and flexural strength were performed, respectively. The results are shown in Table 1 below.

[0060] Interface shear strength:

[0061] (1) Epoxy resin was added dropwise to polyimide fiber and cured under programmed temperature conditions of 60°C / 2h, 80°C / 2h, and 135°C / 3h to obtain a microsphere debonding sample with polyimide fiber as reinforcement and epoxy resin as matrix.

[0062] The interfacial shear strength (IFSS) between the epoxy resin matrix and the fiber reinforcement was measured using a composite material interface performance evaluation device (TOHEI SANGYO, HM410). In this experiment, the diameter of the droplets used was in the range of 50 to 70 μm. During the experiment, the resin microspheres were fixed using the built-in clamping tool of the device while the fiber was kept in tension, and a tensile force was applied to the sample. At the moment when the fiber and the resin microsphere separated, the maximum load force (F max), fiber diameter (d), and droplet length (L). In this way, the interfacial shear strength (IFSS) can be calculated. To ensure the accuracy and reliability of the data, 10 valid tests were performed in each experiment, and the average of these test values ​​was calculated as the final result. The interfacial shear strength (IFSS) was calculated using the following formula:

[0063] IFSS=F max / πdL

[0064] Flexural strength of composite materials Test as follows:

[0065] (1) A thermosetting PI resin with a solid content of 60% was used as the raw material and coated layer by layer on a PI fiber fabric by hand lay-up. The resin-coated fabric was then vacuum-insulated at 80°C for 1 hour and then at 200°C for 2 hours to obtain a PI / PI prepreg. The PI / EP prepreg was prepared by alternately stacking epoxy film and fabric to obtain a PI / PI prepreg.

[0066] (2) Place the PI / PI prepreg in a mold with a stopper, heat it to 340°C and hold it there for 10 minutes; then heat it to 350°C, apply pressure to 2.5 MPa, and vent twice every 5 minutes; then maintain the pressure at 2.5 MPa and slowly heat it to 360°C, hold it there for 2 hours, and then cool it down to room temperature while holding it there to obtain a PI / PI composite material board.

[0067] (3) High temperature test: The specimens were cut into 40 mm × 12.5 mm strips according to GB / T 3356-2014 standard, placed in a three-point bending fixture, and subjected to high temperature bending tests in an Instron 3365 universal testing machine.

[0068] Table 1 Composite material performance test results

[0069]

[0070]

Claims

1. A method for preparing surface-modified polyimide fiber, characterized in that: The following steps are involved: (S1) Polyimide fiber was plasma surface treated; (S2) adding a dianhydride monomer, a diamine monomer, and a capping agent to an organic solvent, wherein the molar ratio of the dianhydride monomer, the diamine monomer, and the capping agent is n:n+1:2-3, wherein n is 6-10, and the fluorine-containing monomer accounts for 30-80% of the total amount of the dianhydride monomer and the diamine monomer, to prepare a polyamic acid solution; the dianhydride monomer is a compound of a fluorine-free dianhydride and a fluorine-containing dianhydride in a molar ratio of 3:7 to 1:1, and the diamine monomer is a compound of a fluorine-free diamine and a fluorine-containing diamine in a molar ratio of 3:7 to 1:1; (S3) soaking the polyimide fiber treated in step (S1) in the polyamic acid solution prepared in step (S2), reacting at 80-120°C, washing, drying, and heat-treating at 300-350°C to obtain surface-modified polyimide fiber.

2. The preparation method according to claim 1, characterized in that In step (S1), the polyimide fiber includes short fibers, filaments or fabric forms; the plasma surface treatment is carried out in an atmosphere of one or more mixed gases of oxygen, nitrogen, ammonia and air, with a treatment power of 20-300W and a treatment rate of 0.24m / min-2.4m / min.

3. The preparation method according to claim 1, characterized in that In step (S2), the polyamic acid solution is prepared by stirring at 0-5°C for 2-5 hours.

4. The preparation method according to claim 1, characterized in that In step (S2), the dianhydride monomer is selected from at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-anthracenetetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, bisphenol A type diether dianhydride, and pyromellitic dianhydride; the diamine monomer is selected from at least one of diaminodiphenyl ether, 4,4'-diamino-2,2'-bis(trifluoromethylbiphenyl), 2,2-bis(4-amino-3,5,6-trifluorophenyl)hexafluoropropane, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, and p-phenylenediamine; and the blocked monoanhydride is selected from at least one of maleic anhydride, nadic anhydride, acrylic anhydride, fumaric anhydride, 2-methylenesuccinic anhydride, and phenylethynylphthalic anhydride.

5. The preparation method according to claim 1, characterized in that The fluorine-containing monomer is a fluorine-containing dianhydride and / or a fluorine-containing diamine, the fluorine-containing dianhydride is selected from 9,9-bis(trifluoromethyl)-2,3,6,7-anthracenetetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylene) diphthalic anhydride; the fluorine-containing diamine is selected from 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, 2,2-bis(4-amino-3,5,6-trifluorophenyl)hexafluoropropane and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene.

6. The preparation method according to claim 1, characterized in that In step (S2), the organic solvent is selected from at least one of dimethylformamide and dimethylacetamide.

7. The preparation method according to claim 6, characterized in that The amount of organic solvent used is such that the solid content of the obtained polyamic acid solution is 10-20 wt %.

8. The preparation method according to claim 1, characterized in that In step (S3), the reaction time at 80-120°C is 5-10 hours; the heat treatment at 300-350°C is to place the modified fiber in a nitrogen oven, heat it from room temperature to the target temperature at 2-10°C / min, and heat treat it for 0.5-2 hours.

9. The surface-modified polyimide fiber prepared by the preparation method according to any one of claims 1 to 8.

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