Method for improving compactness of polyacrylonitrile precursor

Abstract

The invention discloses a method for improving the compactness of a polyacrylonitrile precursor. According to the method, the compactness of the polyacrylonitrile precursor is improved by controlling the process conditions that a polyacrylonitrile spinning stock solution is extruded through a spinneret plate, drafted through an air layer and then enters a coagulating bath, the length-diameter ratio of the spinneret plate is 3, the temperature of the coagulating bath is 18-20 DEG C, and the DMSO concentration in the coagulating bath is 35 wt.%-45 wt.%. According to the method, the compactness of the precursor and the stability in the spinning process are effectively improved, the high-performance precursor is produced, the volume density of the prepared polyacrylonitrile precursor is larger than or equal to 1.18 g / cm < 3 >, the swelling degree is smaller than or equal to 130%, and therefore the performance of the carbon fiber is improved.

Description

Technical Field

[0001] The invention relates to a method for improving the compactness of polyacrylonitrile precursor, and belongs to the technical field of carbon fiber precursor preparation. Background Art

[0002] The spinneret is an essential precision component in the production of precursor. By squeezing a certain amount of viscous spinning solution from the spinneret holes, a fiber bundle with a certain diameter and density is formed in the coagulation bath. The spinneret has a great influence on the structure and properties of polyacrylonitrile-based carbon fiber (PANCF) precursor. The aspect ratio (L / D) of the spinneret is small, and it cannot effectively solve the orifice expansion phenomenon generated during the spinning process, which affects the spinning speed, the formation of the filaments and the properties of the fiber. The significant extrusion expansion effect can easily lead to too many radial defects in the bundle, reduce the density and even break the wire. The density of PAN precursor is closely related to its internal defects. The precursor with fewer defects is denser and its strength is greater. The high-density precursor has fused internal holes, uniform pore distribution and high orientation along the axis of the precursor, high body density, fewer surface defects, uniform internal and external structures, and basically eliminates the skin-core structure. These high-quality precursor structures are inherited by carbon fibers, which improves the strength and modulus of carbon fibers. Therefore, obtaining high-quality precursors with dense structures is the primary condition for producing high-quality carbon fibers.

[0003] The PAN / DMSO solution system is a typical non-Newtonian pseudoplastic fluid. The viscosity of the system decreases with the increase of shear rate. The non-Newtonian index n can be expressed by the Ostwal power law formula τ = kγ n The material, pore size and pore distribution of the spinneret, and the aspect ratio of the micropores have a great influence on the spinning conditions and the physical and mechanical properties of the fiber: The shear rate distribution of the non-Newtonian fluid in the circular tube is given by the formula Calculate, where du / dr is the shear rate of the fluid; the extrusion expansion equation of the viscoelastic fluid is d / D = [1 + 0.5 (N1 / 2τ) γ ] 1 / 6 + A; where d / D is the extrusion expansion ratio of the viscoelastic fluid according to the shear rate and diameter relationship d0 = 2(4Q i / πγ i ) a ; Calculation shows that the extrusion expansion ratio is positively correlated with the shear rate, and the spinneret aspect ratio and aperture will simultaneously affect the shear rate and lead to different extrusion expansion ratios.

[0004] The spinning solution is in a constrained flow state in the micropores of the spinneret, and is in a free flow state after exiting the micropores. The elastic energy is formed by the stress difference in the entrance area of ​​the stock solution before entering the micropores. As the length of the spinneret increases, the residence time of the spinning solution in the spinneret channel becomes longer, which is conducive to the full relaxation of the elastic energy, and the shearing effect and extrusion expansion effect at the outlet are reduced, which is conducive to fiber refinement and surface smoothness, and the spinning is relatively stable. However, when the aspect ratio is too large, due to the high axial orientation of the carbon fiber, the stress generated by shrinkage during the carbonization process causes the fiber to split, and these splits will lead to an increase in structural defects of the carbon fiber and a decrease in density. The spinneret diameter will also affect the extrusion expansion ratio. Therefore, choosing a spinneret with a suitable aspect ratio is crucial to the production of high-density precursors and high-performance carbon fibers. Summary of the invention

[0005] The purpose of the present invention is to provide a method for improving the density of polyacrylonitrile precursor. The method improves the density of polyacrylonitrile carbon fiber precursor by regulating the spinneret aspect ratio to alleviate the spinneret extrusion swelling effect and the coagulation bath conditions.

[0006] The technical solution for achieving the purpose of the present invention is:

[0007] A method for improving the compactness of polyacrylonitrile precursor is achieved by controlling the process conditions of the polyacrylonitrile spinning stock solution extruded through a spinneret, stretched through an air layer, and then entering a coagulation bath, wherein the aspect ratio of the spinneret is 3, the temperature of the coagulation bath is 18-20°C, and the concentration of dimethyl sulfoxide (DMSO) in the coagulation bath is 35wt.%-45wt.%.

[0008] The spinneret aspect ratio described in the present invention refers to the ratio of the spinneret micropore length to the micropore diameter.

[0009] Preferably, the spinneret aperture is 0.08-0.25 mm, and the spinneret material is 304 stainless steel, 316 stainless steel or Hastelloy.

[0010] Preferably, the air layer drafting multiple is 1.0 to 3.0 times.

[0011] Preferably, the polyacrylonitrile spinning solution consists of a polyacrylonitrile copolymer and a polar aprotic solvent, and in terms of mass percentage, the content of the polyacrylonitrile copolymer is not less than 15%, and the content of the polar aprotic solvent is not more than 85%.

[0012] Preferably, the polyacrylonitrile copolymer is a polyacrylonitrile high molecular polymer having an intrinsic viscosity of 1.6 to 2.0 obtained by free radical solution polymerization of 95 to 99 wt.% acrylonitrile and 1 to 5 wt.% itaconic acid in a polar aprotic solvent.

[0013] Preferably, the polar aprotic solvent is selected from one or a mixture of dimethyl sulfoxide, dimethylformamide and dimethylacetamide.

[0014] Preferably, the method further comprises the steps of washing, hot water drawing, oiling, drying and steam drawing the fibers formed after the coagulation bath to obtain polyacrylonitrile precursor.

[0015] Compared with the prior art, the present invention has the following technical effects:

[0016] The present invention alleviates the extrusion swelling effect of the spinneret by regulating the spinneret aspect ratio, thereby improving the density of the polyacrylonitrile carbon fiber precursor. Increasing the spinneret aspect ratio is beneficial to reducing the shear rate, reducing the inlet elastic effect, alleviating the extrusion swelling effect, and contributing to fiber refinement and surface smoothness, thereby achieving stable spinning. At the same time, adjusting the spinneret hole aspect ratio can alleviate the shearing effect on the spinning stock solution when it flows through the spinneret hole, prolonging the residence time of the spinning solution in the spinneret hole, enhancing the orientation effect of the fiber, and making the radial structure denser. In addition, the present invention further improves the performance of the fiber by regulating the coagulation bath conditions to match the spinneret, so that the density of the prepared precursor is ≥1.18g / cm 3 , swelling degree ≤130%. DETAILED DESCRIPTION

[0017] The present invention is further described below in conjunction with specific embodiments.

[0018] Example 1

[0019] (1) A mixed solution consisting of 97 wt.% acrylonitrile and 3 wt.% itaconic acid and a dimethyl sulfoxide solution with a solid content of 20.5 wt.% was subjected to free radical solution polymerization to obtain a polyacrylonitrile copolymer having an intrinsic viscosity [η] of 1.8.

[0020] (2) The polyacrylonitrile copolymer is firstly de-singed in a de-singling kettle, and then de-aerated in a 3 μm primary filtration kettle, and then de-aerated in a 1 μm secondary filtration kettle. The refined spinning solution is pressurized and transported to the spinning unit, and the spinning solution temperature is maintained at 55°C.

[0021] (3) The spinning solution is passed through a 0.5 μm candle wick filter and extruded from a spinneret into a coagulation bath. The total number of holes in the spinneret is 12,000, the hole diameter is 0.08 mm, and the aspect ratio is 3. The coagulation bath is composed of 28 wt.% DMSO and 72 wt.% water, and the coagulation bath temperature is 10°C. The spinning solution is extruded from the spinneret and drawn through an air layer into the coagulation bath. The extrusion speed is 6 m / min, and the drawing multiple is 1.5 times.

[0022] (4) The nascent fiber is washed with desalted water at 30°C in multiple stages to remove the solvent, and then stretched 1.3 times in hot water at 80°C, and then 2.0% oil is applied through an oil bath. The oiled fiber is dried and densified and stretched in saturated steam at 0.45MPa at a stretching multiple of 4.0 times. The total stretching multiple of the fiber in the entire process is about 32 times. After steam stretching, it is rolled up to obtain PAN raw yarn, and process samples are taken to test the swelling degree.

[0023] Examples 2 to 4 and Comparative Examples 1 to 4

[0024] The steps and conditions of Example 1 were followed, and other conditions were kept unchanged, except for changing the aspect ratio of the spinneret and the coagulation bath conditions. The specific process operating conditions and raw yarn density parameters are shown in Table 1.

[0025] Table 1 Spinneret process conditions and raw yarn density parameters

[0026]

[0027] As can be seen from Table 1, the swelling degree of water-drawn yarn first decreases and then increases with the increase of the spinneret aspect ratio, that is, the density of the original yarn first increases and then decreases. The increase of the spinneret aspect ratio is beneficial to reduce the shear rate, reduce the inlet elastic effect, alleviate the extrusion expansion effect, help the fiber refinement and surface smoothness, and achieve stable spinning. At the same time, adjusting the spinneret hole aspect ratio can alleviate the shearing effect on the spinning solution when it flows through the spinneret hole, the residence time of the spinning solution in the spinneret hole becomes longer, the orientation effect of the fiber is enhanced, and the radial structure is denser. However, when the aspect ratio is too high, it needs to be realized through a smaller diameter micropore. On the one hand, the extrusion expansion effect of the small hole is more obvious. On the other hand, the high aspect ratio is difficult to form in the micropore process, and there are many defects in the hole wall and more impurities, which affect the fiber forming, so the density of the original yarn decreases.

Claims

1. A method for improving the density of polyacrylonitrile precursor, characterized in that: The method is achieved by controlling the process conditions that the polyacrylonitrile spinning stock solution is extruded through a spinneret, stretched through an air layer, and then enters a coagulation bath, wherein the aspect ratio of the spinneret is 3, the temperature of the coagulation bath is 18-20° C., and the concentration of DMSO in the coagulation bath is 35wt.%-45wt.%.

2. The method according to claim 1, characterized in that The spinneret aperture is 0.08~0.25mm, and the spinneret material is 304 stainless steel, 316 stainless steel or Hastelloy.

3. The method according to claim 1, characterized in that The air layer drafting multiple is 1.0~3.0 times.

4. The method according to claim 1, characterized in that: The polyacrylonitrile spinning stock solution consists of a polyacrylonitrile copolymer and a polar aprotic solvent, and in terms of mass percentage, the content of the polyacrylonitrile copolymer is not less than 15%, and the content of the polar aprotic solvent is not more than 85%.

5. The method according to claim 4, characterized in that The polyacrylonitrile copolymer is a polyacrylonitrile polymer with an intrinsic viscosity of 1.6 to 2.0 obtained by free radical solution polymerization of 95 to 99 wt.% acrylonitrile and 1 to 5 wt.% itaconic acid in a polar aprotic solvent.

6. The method according to claim 5, characterized in that The polar aprotic solvent is selected from one or a mixture of dimethyl sulfoxide, dimethylformamide and dimethylacetamide.

7. The method according to claim 1, characterized in that The method also includes the steps of washing, hot water drawing, oiling, drying and steam drawing the fibers formed after the coagulation bath to obtain polyacrylonitrile raw fibers.

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