Pre-oxidation method of polyacrylonitrile fiber and pre-oxidized fiber

Abstract

The invention relates to the technical field of pre-oxidation methods of carbon fibers, and discloses a pre-oxidation method of polyacrylonitrile fibers and pre-oxidized fibers. The method comprises the following steps: S1, pretreating polyacrylonitrile fiber precursors to obtain pretreated fibers; s2, carrying out oxidation treatment on the pretreated fibers to obtain pre-oxidized fibers; according to the pre-oxidation method, the polyacrylonitrile fiber is subjected to pre-treatment and oxidation treatment, and meanwhile, it is guaranteed that the cyclization rate of the pre-treated fiber obtained after pre-treatment is 2-12%, so that the pre-oxidation time is remarkably shortened, and meanwhile, the performance of the fiber is not reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of pre-oxidation methods for carbon fibers, and particularly to a pre-oxidation method and pre-oxidized fibers for polyacrylonitrile fibers. Background Art

[0002] Polyacrylonitrile (PAN)-based carbon fibers have excellent properties such as high strength, high modulus, heat resistance, and wear resistance, and are widely used in fields such as aerospace, automotive, medical, and sporting goods.

[0003] PAN-based carbon fibers are generally prepared through three main technological processes: polymerization spinning, pre-oxidation, and carbonization. Among them, pre-oxidation is to make the thermoplastic PAN linear macromolecular chains become non-plastic heat-resistant ladder molecular structures through chemical reactions, so as to avoid the phenomenon of melting during the high-temperature carbonization process of the pre-oxidized fibers and ensure that the fibers are in a thermodynamically stable state. This link has an important impact on the performance of carbon fibers and is a key step in the entire preparation process.

[0004] Currently, the treatment time of domestic fibers in the pre-oxidation furnace is usually 70 - 120 minutes, accounting for more than 90% of the total production time, and the production cost is relatively high.

[0005] CN101260575A divides pre-oxidation into five temperature segments. The first and second temperature segments are for nitrogen cyclization, and the third to fifth temperature segments are for oxidative cross-linking processes. After low-temperature carbonization and high-temperature carbonization treatments, carbon fibers with a tensile strength of 3.62 GPa and a Young's modulus of 223 GPa can be obtained. However, the pre-oxidation process takes about 100 - 200 minutes, which is a long time and not conducive to efficient production.

[0006] CN104651980A pre-oxidizes polyacrylonitrile raw filaments in an air atmosphere within the temperature range of 180 - 280 °C for 80 - 120 minutes to obtain pre-oxidized fibers with a density of 1.34 ± 0.02 g / cm 3 whose density can meet the process requirements, but there is also the problem of a long pre-oxidation time.

[0007] Therefore, on the premise of ensuring the fiber performance, shortening the pre-oxidation time is the key to reducing the cost of carbon fibers and improving the quality. Summary of the Invention

[0008] The purpose of the present invention is to overcome the problem of too long pre-oxidation time existing in the prior art, and provide a pre-oxidation method and pre-oxidized fibers for polyacrylonitrile fibers. This pre-oxidation method pre-treats and oxidizes polyacrylonitrile fibers respectively, and at the same time ensures that the cyclization rate of the pre-treated fibers obtained after pre-treatment meets 2 - 12%, so that the pre-oxidation time is significantly reduced, while not reducing the fiber performance and meeting the basic performance indicators.

[0009] To achieve the above object, a first aspect of the present invention provides a pre-oxidation method for polyacrylonitrile fibers, wherein the method comprises:

[0010] S1. Pretreating polyacrylonitrile fiber filaments to obtain pretreated fibers;

[0011] S2. Oxidizing the pretreated fibers to obtain pre-oxidized fibers;

[0012] wherein the cyclization rate of the pretreated fibers is 2-12%.

[0013] A second aspect of the present invention provides pre-oxidized fibers prepared by the pre-oxidation method according to the first aspect of the present invention.

[0014] By the above technical solution, the pre-oxidation method and pre-oxidized fibers of polyacrylonitrile fibers provided by the present invention achieve the following beneficial effects: The pre-oxidation method respectively pretreats and oxidizes polyacrylonitrile fibers, and at the same time ensures that the cyclization rate of the pretreated fibers obtained after pretreatment satisfies 2-12%, so that the pre-oxidation time is significantly reduced without reducing the performance of the fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 FIG. is the infrared spectrum of the pretreated fibers and polyacrylonitrile raw filaments obtained after pretreatment in Example 6.

[0016] Figure 2 FIG. is the DSC spectrum of the pretreated fibers and polyacrylonitrile raw filaments obtained after pretreatment in Example 6. DETAILED DESCRIPTION

[0017] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0018] A first aspect of the present invention provides a pre-oxidation method for polyacrylonitrile fibers, wherein the method comprises:

[0019] S1. Pretreating polyacrylonitrile fiber filaments to obtain pretreated fibers;

[0020] S2. Oxidizing the pretreated fibers to obtain pre-oxidized fibers;

[0021] wherein the cyclization rate of the pretreated fibers is 2-12%.

[0022] In the present invention, the cyclization rate of the pretreated fiber is controlled within the above range, and combined with the oxidation treatment in step S2, the pre-oxidation time of the fiber can be significantly shortened. The cyclization rate is measured by infrared spectroscopy, and the cyclization rate of the fiber is calculated based on the absorption intensities of the characteristic peaks of -C≡N at 2240 cm -1 and -C=N at 1590 cm -1 to characterize the reaction degree of the fiber.

[0023] According to the present invention, in step S1, the cyclization rate of the pretreated fiber is 8.75 - 11.75%.

[0024] According to the present invention, in step S1, the process of the pretreatment includes N-stage heat treatment, where N is a positive integer from 5 to 8.

[0025] In the present invention, when N satisfies the above range, it can ensure that the cyclization rate of the pretreated fiber meets the requirements, thereby reducing the pre-oxidation time of the fiber.

[0026] According to some preferred embodiments of the present invention, the bulk density of the pretreated fiber is 1.199 - 1.213 g / cm 3 .

[0027] According to the present invention, along the direction from the first-stage heat treatment to the N-stage heat treatment, the temperature of adjacent heat treatments increases step by step, and the increasing temperature difference is 5 - 20 °C.

[0028] In the present invention, when the increasing temperature difference of adjacent heat treatments satisfies the above range, it can reduce the pre-oxidation time on the premise of not reducing the fiber properties.

[0029] Further, along the direction from the first-stage heat treatment to the N-stage heat treatment, the temperature of adjacent temperature zones increases step by step, and the increasing temperature difference is 5 - 10 °C.

[0030] According to the present invention, the temperature of the first-stage heat treatment is 255 - 275 °C.

[0031] According to some embodiments of the present invention, the pretreatment is carried out at 255 - 305 °C.

[0032] According to the present invention, in step S1, the time of the pretreatment is 1 - 2 min, preferably 1.12 - 1.54 min.

[0033] According to the present invention, in step S2, the process of the oxidation treatment includes M-stage thermo-oxidation treatment, where M is a positive integer from 1 to 3.

[0034] According to the present invention, along the direction from the first-stage thermo-oxidation treatment to the M-stage thermo-oxidation treatment, the temperature of adjacent thermo-oxidation treatments increases step by step, and the increasing temperature difference is 2 - 30 °C.

[0035] In the present invention, through the research of the inventor, it is found that when the number of heat treatment stages of the pretreatment is controlled to be 5-8, and at the same time the number of hot oxygen treatment stages of the oxidation treatment is controlled to be 1-3, they can cooperate with each other to further shorten the pre-oxidation time. At the same time, by controlling the increasing temperature difference between the temperatures of adjacent heat treatments (or hot oxygen treatments) in the pretreatment and the oxidation treatment respectively, the performance of the fiber can be further ensured to meet the requirements.

[0036] Furthermore, along the direction from the first-stage hot oxygen treatment to the M-stage hot oxygen treatment, the temperatures of adjacent hot oxygen treatments increase step by step, and the increasing temperature difference is 10-22 °C.

[0037] According to the present invention, the temperature of the first-stage hot oxygen treatment is 220-230 °C.

[0038] According to the present invention, the time of the oxidation treatment is 15-35 min, preferably 16-32 min.

[0039] In the present invention, the pre-oxidation includes two steps of pretreatment and oxidation treatment. By pre-oxidizing through the method of the present invention, the pre-oxidation time can be significantly shortened.

[0040] The second aspect of the present invention provides a pre-oxidized fiber prepared by the pre-oxidation method described in the first aspect of the present invention.

[0041] According to some embodiments of the present invention, the bulk density of the pre-oxidized fiber is 1.25-1.40 g / cm 3 , preferably 1.29-1.37 g / cm 3 .

[0042] According to some embodiments of the present invention, the single filament strength of the pre-oxidized fiber is ≥0.3 GPa and the modulus is ≥8.5 GPa.

[0043] According to a particularly preferred embodiment of the present invention, the method for pre-oxidizing polyacrylonitrile fiber includes:

[0044] S1. Subjecting the polyacrylonitrile fiber raw filament to 7 heat treatments, that is, performing pretreatment in 7 temperature zones to obtain pretreated fiber;

[0045] S2. Subjecting the pretreated fiber to 2 hot oxygen treatments, that is, performing oxidation treatment in 2 temperature zones to obtain pre-oxidized fiber;

[0046] Among them, the cyclization rate of the pretreated fiber is 10.75-11.75%;

[0047] In step S1, the 7 temperature zones are the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone, the sixth temperature zone and the seventh temperature zone respectively;

[0048] The temperature increase difference of the second temperature zone compared to the first temperature zone is 10 °C, the temperature increase difference of the third temperature zone compared to the second temperature zone is 5 °C, the temperature increase difference of the fourth temperature zone compared to the third temperature zone is 5 °C, the temperature increase difference of the fifth temperature zone compared to the fourth temperature zone is 5 °C, the temperature increase difference of the sixth temperature zone compared to the fifth temperature zone is 5 °C, and the temperature increase difference of the seventh temperature zone compared to the sixth temperature zone is 5 °C;

[0049] The initial temperature of the pretreatment is 255 - 265 °C;

[0050] The time of the pretreatment is 1.54 min;

[0051] In step S2, the two temperature zones are the first temperature zone and the second temperature zone respectively;

[0052] The temperature increase difference of the second temperature zone compared to the first temperature zone is 22 °C;

[0053] The time of the oxidation treatment is 32 min;

[0054] The initial temperature of the oxidation treatment is 220 - 225 °C.

[0055] The present invention will be described in detail below through embodiments. In the following embodiments, the cyclization rate of the pretreated fiber is measured by infrared spectroscopy;

[0056] The thermal properties of the pretreated fiber are measured by DSC;

[0057] The bulk density of the pre-oxidized fiber is measured by the floating and sinking method;

[0058] The mechanical properties of the single filaments of the pre-oxidized fiber are measured by a single filament strength tester.

[0059] Example 1

[0060] The 12K polyacrylonitrile raw fiber is pretreated and oxidized according to the operating conditions in Table 1 and Table 2.

[0061] Table 1

[0062]

[0063] Note: Temperature of Zone 1 a Indicates the temperature of the first temperature zone, and the same applies to others

[0064] * Example number

[0065] Table 2

[0066]

[0067] Note: Temperature1 Denoted as the temperature of the first temperature zone, and the time is the same

[0068] Example 9

[0069] In the same manner as in Example 1, except that in step S1, the pretreatment is carried out in two temperature zones, the temperature of the first temperature zone is 260 °C, and the temperature of the second temperature zone is 270 °C. The pretreatment time is 1.54 min.

[0070] Comparative Example 1

[0071] In the same method as in Example 1, except that the residence time during the pretreatment process is 1.12 min, and the oxidation treatment step is not carried out at the same time.

[0072] Comparative Example 2

[0073] In the same method as in Example 1, except that no pretreatment is carried out, and the polypropylene fiber is directly subjected to oxidation treatment in four temperature zones. The temperature of zone 1 is 229 °C, the time is 20 min, the temperature of zone 2 is 234 °C, the time is 20 min, the temperature of zone 3 is 240 °C, the time is 20 min, and the temperature of zone 4 is 252 °C, the time is 20 min, to obtain pre-oxidized fibers.

[0074] Test Example 1

[0075] Taking Example 6 as an example, the pretreated fibers obtained in Example 6 were subjected to infrared spectrogram analysis and compared with the infrared spectrogram results of 12K PAN raw filaments. From Figure 1 It can be seen that the 12K PAN raw filament has a significant -C≡N structural characteristic peak at 2240 cm -1 , while the results of Example 6 show that when the PAN raw filament is pretreated, the -C≡N structural characteristic peak at 2240 cm -1 and the characteristic peak intensity of -CH2 at 1450 cm -1 are significantly weakened, and a characteristic peak of -C=C-C=N conjugated structure starts to appear at 1590 cm -1 and the peak shape is relatively wide. It shows that during the pretreatment process, the PAN fiber has undergone a cyclization reaction.

[0076] Test Example 2

[0077] The bulk density and cyclization rate of the pretreated fibers obtained during the preparation of the examples and comparative examples were tested, and the results are shown in Table 3.

[0078] Sample <![CDATA[Body density (g / cm 3 )]]> Cyclization rate (%) Example 1 1.213 11.75 Example 2 1.213 11.75 Example 3 1.213 11.75 Example 4 1.213 11.75 Example 5 1.213 11.75 Example 6 1.207 6.8 Example 7 1.200 5.1 Example 8 1.209 6.9 Example 9 1.199 1.50 Comparative Example 1 1.198 1.53 Comparative Example 2 1.37 72.2

[0079] Test Example 3

[0080] The thermal properties of the pretreated fibers obtained in Example 1, Example 6 and Comparative Example 1 were analyzed. The results are shown in Table 4 andFigure 2 as shown

[0081] Table 4

[0082] Sample Onset / ℃ Tp1 / ℃ Tp2 / ℃ ΔH / J / g AI / % Polyacrylonitrile fiber precursor 234.83 275.16 325.11 2455.5 / Comparative Example 1 199.81 277.60 326.10 2410.7 1.82 Example 1 194.83 277.48 324.51 1934.1 21.23 Example 6 195.72 279.04 324.55 2053.7 16.36

[0083] Where: Onset is the starting temperature of the exothermic peak, T P1 is the peak temperature of the first exothermic peak, T P2 is the peak temperature of the first exothermic peak, ΔH is the reaction enthalpy change of the fiber, and AI is the aromatization index

[0084] Test Example 4

[0085] The monofilament mechanical properties of the pre-oxidized fibers prepared in the examples and comparative examples were tested, and the results are shown in Table 5. To illustrate its stability, Example 6 and Comparative Example 2 were selected to calculate the coefficient of variation of their mechanical properties, and the results are shown in Table 5 (continued).

[0086] Table 5

[0087]

[0088] Table 5 (continued)

[0089]

[0090] It can be seen from the results of Tables 1-5 and Table 5 (continued) that Examples 1-9 using the pre-oxidation method provided by the present invention achieved good results. The cyclization rate of the PAN fiber precursor was not less than 2% after pretreatment to ensure a shorter subsequent oxidation treatment time, and at the same time, the prepared pre-oxidized fiber had mechanical properties that met the production requirements.

[0091] As shown in Table 2, at the same pretreatment temperature, when the pretreatment time was extended to 1.54 min, the bulk density of the fiber could reach 1.213 g / cm 3 , and the cyclization rate could reach 11.75%. It can be seen from Comparative Example 1 and Example 1 that at the same temperature, when the pretreatment time was increased, the bulk density of the fiber increased by 0.014 g / cm 3 , and the cyclization rate increased by 10.22%. By comparing Comparative Example 1 and Example 6, it was found that at the same residence time, when the temperature of each zone increased, the bulk density of the fiber increased by 0.008 g / cm 3 , and the cyclization rate increased by 5.27%. This is because after the PAN fiber is pretreated by the method provided by the present invention, a cyclization reaction of the molecular chain occurs, forming a rigid ring structure, and the fiber structure becomes denser.

[0092] As shown in Table 3, compared with the PAN fiber precursor, the starting temperature of the first exothermic peak of the pretreated fiber decreases, indicating that the subsequent cyclization and oxidation reactions can occur at a lower temperature after being treated by the pretreatment method provided by the present invention. For the heat release, the heat release of the pretreated fiber in Comparative Example 1 is close to that of the PAN fiber precursor. The heat release of Examples 1 and 6 decreases, and the heat release of Example 1 is the lowest. From Figure 2 it can be seen that the exothermic peak of the 12K PAN precursor is relatively sharp and the heat release is relatively concentrated. After pretreatment, the shape of the exothermic peak becomes significantly broadened. This shows that the pretreatment means can improve the exothermic behavior during the pre-oxidation process and effectively avoid local heat accumulation in the tow during production.

[0093] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A pre-oxidation method for polyacrylonitrile fibers, characterized in that, The method includes: S1. Pretreating a PAN fiber filament to obtain a pretreated fiber; S2. Subjecting the pretreated fiber to an oxidation treatment to obtain a pre-oxidized fiber; wherein the cyclization rate of the pretreated fiber is 2-12%.

2. The pre-oxidation method according to claim 1, wherein, In step S1, the cyclization rate of the pretreated fiber is 8.75-11.75%.

3. The pre-oxidation method according to claim 1 or 2, wherein, In step S1, the pretreatment process includes N stages of heat treatment, where N is a positive integer from 5 to 8; Preferably, along the direction from the first stage of heat treatment to the Nth stage of heat treatment, the temperature of adjacent heat treatments increases step by step; Preferably, the increasing temperature difference between adjacent heat treatments is 5-20°C, preferably 5-10°C.

4. The pre-oxidation method according to claim 3, wherein, The temperature of the first stage of heat treatment is 255-275°C.

5. The pre-oxidation method according to any one of claims 1-4, wherein, In step S1, the pretreatment time is 1-2 min, preferably 1.12-1.54 min.

6. The pre-oxidation method according to any one of claims 1-5, wherein, In step S2, the oxidation treatment process includes M stages of thermal oxidation treatment, where M is a positive integer from 1 to 3; Preferably, along the direction from the first stage of thermal oxidation treatment to the Mth stage of thermal oxidation treatment, the temperature of adjacent thermal oxidation treatments increases step by step; Preferably, the increasing temperature difference between adjacent thermal oxidation treatments is 2-30°C, preferably 10-22°C; Preferably, the temperature of the first stage of thermal oxidation treatment is 220-230°C.

7. The pre-oxidation method according to claim 6, wherein, In step S2, the oxidation treatment time is 15-35 min, preferably 16-32 min.

8. A pre-oxidized fiber prepared by the pre-oxidation method according to any one of claims 1-7.

9. The pre-oxidized fiber according to claim 8, wherein the bulk density of the pre-oxidized fiber is 1.25 - 1.4 g / cm 3 , preferably 1.29 - 1.37 g / cm 3 .

10. The single fiber strength of the pre-oxidized fiber according to claim 8 or 9 is ≥0.3 GPa, and the modulus is ≥8.5 GPa.

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