Method for preparing polyamide 6 fine denier fiber through anionic polymerization

Abstract

The invention relates to a method for preparing polyamide 6 fine denier fibers through anionic polymerization. Caprolactam and a catalyst are added into a drying device A to remove system moisture and then placed in a storage tank A. Caprolactam is added into a drying device B to remove moisture and then placed in a storage tank B together with an initiator and a cyclic oligomer inhibitor. Caprolactam liquid in the storage tank A and the storage tank B is continuously injected into a screw extruder for anionic ring-opening polymerization reaction to prepare a polyamide 6 basic melt, the basic melt is further conveyed to a tandem devolatilization reaction system to remove monomers and part of cyclic oligomers to obtain a polyamide 6 final polymer, and finally, the polyamide 6 final polymer can be directly spun. The polyamide 6 fine denier fiber is obtained.

Description

Technical Field

[0001] The present invention belongs to the field of fiber manufacturing, and particularly relates to a method for preparing polyamide 6 fine denier fibers by anionic polymerization. Background Art

[0002] Polyamide 6 (PA6) fibers are widely used in the fields of clothing, home textiles and industrial applications due to their excellent mechanical properties, abrasion resistance and chemical stability. Fine denier fibers have a soft touch, delicate luster and high covering property, and are the fibers closest to the human skin. The preparation of fine denier fibers has high requirements for the quality of the fiber-forming polymer, and there are high requirements for the content of residual small molecules in the polyamide 6 fine denier fiber system.

[0003] In the conventional production of polyamide 6, when the polymer reaches equilibrium, the conversion rate of caprolactam is generally about 90%, and about 10% of residual caprolactam monomers and cyclic oligomers and other small molecules (also called hot water extractables) need to be removed by a continuous hot water extraction process for dozens of hours, which consumes a large amount of energy and water resources. Anionic polymerization to prepare polyamide 6 has the advantages of rapid and efficient reaction, low pollution and low energy consumption, etc., but there are still many problems in the existing technologies, such as residual monomers and oligomers still need to be removed to meet the requirements of fiber processing.

[0004] A small part of monomers and oligomers can be removed by vacuum devolatilization. Hyoungsan Kye et al. (Journal of Applied Polymer Science 1994, 52(9), 1249-1262) used vacuum devolatilization to remove monomers and oligomers in anionic polyamide 6, and then added a spinneret at the outlet of the screw extruder to directly carry out melt spinning. However, due to a large amount of residual oligomers, the fibers produced by them are on the millimeter scale and do not meet the usage requirements of daily fibers. Efficiently removing the residual monomers and oligomers in anionic polymerization polyamide 6 is expected to shorten the production time, reduce the production energy consumption, and promote the green and low-carbon development of the polyamide 6 material industry. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing polyamide 6 fine denier fibers by anionic polymerization, which uses a cyclic oligomer inhibitor to reduce the generation of monomers and cyclic oligomers during the polymerization process, combines a series of devolatilization reactors to remove monomers and oligomers, and the obtained polyamide 6 melt can be directly spun into polyamide 6 fine denier fibers.

[0006] Introducing a cyclic oligomer inhibitor during the anionic polymerization of polyamide 6 has the following effects: On the one hand, the metal ions in the cyclic oligomer inhibitor coordinate with the amide bonds of polyamide 6 to prevent the amide bonds from being attacked and cyclized during the polymerization process, reducing the formation of cyclic oligomers, thereby lowering the content of cyclic oligomers in the polymer.

[0007] On the other hand, usually the anionic polymerization rate of polyamide 6 is extremely fast, releasing a large amount of polymerization heat in a short time, resulting in an increase in the system temperature and the content of small molecules such as monomers and cyclic oligomers. The cyclic oligomer inhibitor added in the present invention will appropriately reduce the reaction rate, making the whole anionic polymerization system milder and more controllable, and the content of small molecules is controlled.

[0008] The caprolactam monomer accounts for the vast majority of the small molecules remaining in polyamide 6. Although caprolactam has a low boiling point and is easy to remove by volatilization, the difficulty of simultaneously removing cyclic oligomers will increase, and the accumulation of cyclic oligomers (especially cyclic dimers) will affect the product quality of the processing stabilizer of polyamide 6. According to the relationship between the physical properties of caprolactam monomer and cyclic oligomers in polyamide 6 and the temperature and pressure of the devolatilization reactor, the present invention proposes a series devolatilization reaction system in which a first devolatilization reactor and a second devolatilization reactor are used in series. Since the boiling point of the caprolactam monomer is relatively low, the reaction temperature of the first devolatilization reactor of the series devolatilization reaction system is set to 240 - 280 °C and the reaction pressure is 200 - 4000 Pa. Under this condition, more than 90% of the caprolactam monomer can be selectively removed, effectively reducing the influence of high-content monomers on the next-stage devolatilization reaction while reducing the energy consumption required for vacuum power. After a large amount of caprolactam monomer is removed, according to the physical property parameters of cyclic oligomers, the reaction temperature of the second devolatilization reactor of the series devolatilization reaction system is set to 240 - 280 °C and the reaction pressure is 20 - 600 Pa, which can achieve the enhanced removal of the remaining small amount of caprolactam monomer and cyclic oligomers, so that the monomer residue in the final polymer is lower than 0.1 wt%, the content of cyclic oligomers is lower than 1.2 wt% (where the content of cyclic dimers in cyclic oligomers is lower than 0.1 wt%), and the content of hot water extractables is lower than 0.4 wt%. This method omits steps such as cooling and pelletizing, hot water extraction, drying and re-melting compared with the traditional hot water extraction method, saving a large amount of time and energy.

[0009] Based on the above series devolatilization reaction system, the present technical solution provides a method for preparing polyamide 6 fine denier fibers by anionic polymerization, including the following steps: (1) Prepare the basic active material A: Mix the catalyst and caprolactam evenly to obtain a first mixture, and store the first mixture in storage tank A after vacuum distillation. The inside of storage tank A is protected by an inert atmosphere and kept at a constant temperature of 80 - 150 °C; (2)Prepare the basic active material B: Heat caprolactam until it melts, then conduct vacuum distillation. Mix the dehydrated caprolactam evenly with an initiator and a cyclic oligomer inhibitor to obtain a second mixture, and store the second mixture in storage tank B, where storage tank B is protected by an inert atmosphere and maintained at a constant temperature of 80-150 °C; (3)Inject the basic active materials A and B into a screw extruder in a volume ratio of 1-10:10 for anionic polymerization, and obtain a polyamide 6 basic melt through reactive extrusion; (4)Transport the polyamide 6 basic melt to a first devolatilization reactor to remove caprolactam monomer and obtain a polyamide 6 intermediate. Input the polyamide 6 intermediate into a second devolatilization reactor to remove a small amount of caprolactam monomer, cyclic dimer, partial cyclic trimer, and partial cyclic tetramer to obtain a polyamide 6 end polymer. The reaction temperature of the first devolatilization reactor is set at 240-280 °C, and the reaction pressure is 200-4000 Pa. The reaction temperature of the second devolatilization reactor is 240-280 °C, and the reaction pressure is 20-600 Pa; (5)Transport the polyamide 6 end polymer to a spinning machine for direct spinning to obtain polyamide 6 fine denier fibers.

[0010] In some embodiments, in (1), the catalyst is one or more of sodium caprolactam, sodium ethoxide, LiH, NaH, KH, LiOH, NaOH, KOH.

[0011] In step (1), the inert atmosphere is one or more of N2, CO2, He, Ne, Ar.

[0012] In step (1), by mass, mix 0.3-1.2 parts of the catalyst and 10-100 parts of caprolactam to obtain a first mixture.

[0013] In step (1), conduct vacuum distillation on the first mixture to fully remove moisture. The conditions for vacuum distillation are a temperature of 80-150 °C and an absolute reaction pressure of 5-95 kPa.

[0014] In step (2), the initiator is one or more of N-acetylcaprolactam (AcCL), bisacylated lactam-1,6-caprolactam, terephthaloyl biscaprolactam (TBCL), isophthaloyl biscaprolactam, toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), 1,6-hexamethylene diisocyanate (HMDI).

[0015] Similarly, in step (2), conduct vacuum distillation on caprolactam to remove moisture. The conditions for vacuum distillation are a temperature of 80-150 °C and an absolute reaction pressure of 5-95 kPa.

[0016] In step (2), the inert atmosphere is one or more of N2, CO2, He, Ne, and Ar.

[0017] In step (2), according to the mass ratio, 100 parts of caprolactam, 1.0 - 2.4 parts of initiator, and 0.01 - 2.0 parts of cyclic oligomer inhibitor are mixed to obtain a second mixture.

[0018] In step (2), the cyclic oligomer inhibitor is a metal ion compound MY or a mixture of two or more metal ion compounds MY, where M is a metal cation and Y is an inorganic anion or an organic anion.

[0019] Specifically, the metal cation M is selected from one of transition metal ions, lanthanide metal ions, and Group IIA metal ions. When the metal cation M is selected from transition metal ions, the metal cation M is Sc 3+ , Ni 2+ , Zn 2+ , Y 3+ , Zr 4+ , Ru 4+ , Rh 3+ ; when the metal cation M is selected from lanthanide metal ions, the lanthanide metal ion is La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Lu 3+ , Yb 3+ ; when the metal cation M is selected from Group IIA metal ions, the Group IIA metal ion is one of Be 2+ , Mg 2+ , Ca 2+ ; the metal cation M can also be one of Li + , Al 3+ .

[0020] The inorganic anion or organic anion Y is selected from F - , Cl - , NO3 - , SO4 2- , PO4 3-, one of citrate ion, salicylate ion, 3-hydroxybutyrate ion, L-aspartate ion, lactate ion, malate ion, 2-hydroxypropionate ion, organic monocarboxylate ion H(CH2) n COO - (n = 0~12), benzoate ion, naphthoate ion, organic dicarboxylate ion COO - (CH2) m COO - (m = 0~12), terephthalate ion, phthalate ion, isophthalate ion or naphthalenedicarboxylate ion, saturated fatty acid root ion C x H 2x+1 COO - (x = 13~20), unsaturated fatty acid root ion, 6-aminohexanoate ion NH2C5H 10 COO - 、, one of amino acid root ions. When the inorganic anion or organic anion is an unsaturated fatty acid root ion, the unsaturated fatty acid root ion is oleate ion C 17 H 33 COO - 、linoleate ion C 17 H 31 COO - 、α-linolenate ion C 17 H 29 COO - 、arachidonate ion C 19 H 31 COO - 、palmitoleate ion C 15 H 29 COO - ; when the inorganic anion or organic anion is an amino acid ion, the amino acid ion is any one of glycinate ion, alaninate ion, valinate ion, leucinate ion, isoleucinate ion, prolinate ion, phenylalaninate ion, methioninate ion, serine ion, threonine ion, asparagine ion, glutamine ion, aspartate ion, glutamate ion, cysteinate ion, tyrosine ion, selenocysteinate ion.

[0021] In some embodiments, the addition amount of the cyclic oligomer inhibitor is 0.01 wt% to 2.0 wt% of caprolactam.

[0022] In step (3), the feeding temperature of the screw extruder is set to 90~160 °C, the discharging port temperature is set to 220~240 °C, the temperature of the middle section of the screw is set to gradually increase from the feeding port temperature to the discharging port temperature, the number of heating zones is not less than 1, and the rotation speed is 20~300 rpm.

[0023] In step (3), the relative viscosity of the polyamide 6 base melt is 2.0 - 3.5, the monomer content is less than 3.9 wt%, the cyclic oligomer content is less than 2.0 wt%, the hot water extractable content is less than 4.6 wt%, and the cyclic dimer content in the cyclic oligomer is less than 0.5 wt%.

[0024] In step (4), the reaction temperatures of the first devolatilization reactor and the second devolatilization reactor can be independently controlled, and both are connected to a vacuum system. The vacuum system is used to control the reaction pressures of the first devolatilization reactor and the second devolatilization reactor. The first devolatilization reactor is selected from one of a horizontal cage reactor, a horizontal disk reactor, a falling strip devolatilizer, and a vertical falling film devolatilization reactor. The second devolatilization reactor is a vertical falling film devolatilization reactor or a horizontal twin-screw devolatilization reactor. The reaction pressure of the first devolatilization reactor is controlled by the vacuum system to be 200 - 4000 Pa, and the reaction temperature is set to 240 - 280 °C. The reaction pressure of the second devolatilization reactor is controlled by the vacuum system to be 20 - 600 Pa, and the reaction temperature is set to 240 - 280 °C.

[0025] In step (4), the relative viscosity of the polyamide 6 end-polymer is 2.0 - 3.5, the monomer content is less than 0.1 wt%, the cyclic oligomer content is less than 1.2 wt%, the cyclic dimer content in the cyclic oligomer is less than 0.1 wt%, and the hot water extractable content is less than 0.4 wt%.

[0026] For a method for preparing polyamide 6 fine denier fibers by anionic polymerization as described above, the polyamide 6 end-polymer can be directly spun to obtain polyamide 6 fine denier fibers. The spinning temperature is 245 - 300 °C, and the spinning speed is 2500 - 6000 m / min.

[0027] Figure 1 、 Figure 2 and Figure 3 provide a schematic structural diagram of three methods for preparing polyamide 6 fine denier fibers by anionic polymerization, as shown in Figure 1 、 Figure 2 and Figure 3As shown in the figure, caprolactam and a catalyst are mixed and then added to drying device A for vacuum distillation, and then stored in storage tank A. The caprolactam is heated and melted and then placed in drying device B for vacuum distillation. The water-removed caprolactam is mixed evenly with an initiator and a cyclic oligomer inhibitor to obtain a second mixture, and the second mixture is stored in storage tank B; Basic active materials A and B are injected into a screw extruder in a volume ratio of 1-10:10 for anionic polymerization, and a polyamide 6 basic melt is prepared by reactive extrusion; The polyamide 6 basic melt is transported to devolatilization reactor 1 to selectively remove more than 90% of the caprolactam monomer. After a large amount of caprolactam monomer is removed, it enters devolatilization reactor 2 to intensively remove the remaining small amount of caprolactam monomer and cyclic oligomers to obtain a polyamide 6 end polymer. The difference lies in: Figure 1 In it, devolatilization reactor 1 adopts a vertical falling film devolatilization reactor, and devolatilization reactor 2 also adopts a vertical falling film devolatilization reactor; Figure 2 In it, devolatilization reactor 1 adopts a horizontal devolatilization reactor, and devolatilization reactor 2 adopts a vertical falling film devolatilization reactor, Figure 3 In it, devolatilization reactor 1 adopts a horizontal devolatilization reactor, and devolatilization reactor 2 adopts a horizontal twin-screw devolatilization reactor.

[0028] Adopting the technical solution of the present invention can achieve the following beneficial effects: (1) In the anionic polymerization process of polyamide 6 of the present invention, a cyclic oligomer inhibitor is introduced. The metal ions in the cyclic oligomer inhibitor coordinate with the amide bonds of polyamide 6 to prevent the amide bonds from being attacked and cyclized during the polymerization process, reduce the formation of cyclic oligomers, and thus reduce the content of cyclic oligomers that are difficult to devolatilize in the polymer.

[0029] (2) The present invention controls the reaction temperature and pressure of the devolatilization reactor respectively according to the physical property differences between the monomer and the cyclic oligomer. The first devolatilization reactor can selectively remove more than 90% of the caprolactam monomer, effectively reducing the influence of the high-content monomer on the next-stage devolatilization reaction. After a large amount of monomer is removed, according to the physical property characteristics of the cyclic oligomer, the reaction temperature and reaction pressure of the second devolatilization reactor are set, and the intensive removal of the remaining small amount of caprolactam monomer and cyclic oligomers can be realized. Finally, a high-quality polyamide 6 melt with a relative viscosity of 2.0-3.5, a monomer content of less than 0.1 wt%, a cyclic oligomer content of less than 1.2 wt% (where the content of cyclic dimer is less than 0.1%), and a hot water extractable content of less than 0.4 wt% is prepared, so as to prepare polyamide 6 fine denier fiber. Description of the Drawings

[0030] Figure 1 、 Figure 2 and Figure 3 are the schematic structural diagrams of the anionic polymerization for preparing polyamide 6 fine denier fiber provided by the present invention. Detailed Embodiments

[0031] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0032] To further illustrate the present invention, a method for preparing high-quality polyamide 6 fibers by anionic polymerization provided by the present invention will be described in detail below in conjunction with embodiments, but they cannot be understood as limiting the protection scope of the present invention.

[0033] The present invention uses an ultra-high performance liquid chromatograph to measure the contents of monomers and cyclic oligomers in the polyamide 6 base melt and the final polymer, and the specific condition parameters are as follows: - Chromatographic column: T3 column, 100 mm × 2.1 mm (inner diameter) × 1.7 μm, or equivalent; - Column temperature: 30 °C; - Flow rate: 0.3 mL / min; - Detection wavelength: 200 nm; - Injection volume: 2 μL; - Elution program (as shown in the following table): Mobile phase A is water, and mobile phase B is acetonitrile.

[0034]

[0035] Comparative Example A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1) Preparation of basic active material A: By mass, 0.8 parts of NaOH is added to 100 parts of caprolactam and mixed evenly, and the water is removed by vacuum distillation at 120 °C and 5 kPa, and then stored in a storage tank A protected by Ar and maintained at a constant temperature of 120 °C; (2) Preparation of basic active material B: By mass, 100 parts of monomer caprolactam is heated and melted, the water is removed by vacuum distillation at 120 °C and 5 kPa, and then 1.5 parts of N-acetylcaprolactam is added and mixed evenly, and then stored in a storage tank B protected by Ar and maintained at a constant temperature of 120 °C; (3) Inject the base reactive materials A and B into a screw extruder in a volume ratio of 1:1 for anionic polymerization. The feeding temperature of the screw extruder is set at 100 °C, the discharging temperature of the screw extruder is set at 230 °C, the temperature of the middle section of the screw is set to gradually increase from 100 °C to 230 °C, there are 2 heating zones, the rotation speed is 30 rpm, and a polyamide 6 melt with a relative viscosity of 2.81, a monomer content of 4.13 wt%, a cyclic oligomer content of 2.49 wt% (where the cyclic dimer content is 0.75 wt%), and a hot water extractable content of 5.35 wt% is prepared by reactive extrusion. (4) Transport the polyamide 6 melt to a spinning machine for direct spinning. The spinning temperature is 270 °C and the winding speed is 4200 m / min. A large amount of caprolactam volatilized gas appears during the spinning of polyamide 6, and fibers cannot be formed.

[0036] Example 1 A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1) Preparation of base reactive material A: By mass, add 0.8 parts of NaOH to 100 parts of caprolactam and mix evenly. Distill off water under reduced pressure at 120 °C and 5 kPa, and then store it in a storage tank A protected by Ar and maintained at a constant temperature of 120 °C. (2) Preparation of base reactive material B: By mass, heat 100 parts of monomer caprolactam to melt, distill off water under reduced pressure at 120 °C and 5 kPa, then add 1.5 parts of N-acetylcaprolactam and 1.5 parts of magnesium chloride and mix evenly, and then store it in a storage tank B protected by Ar and maintained at a constant temperature of 120 °C. (3) Inject the base reactive materials A and B into a screw extruder in a volume ratio of 1:1 for anionic polymerization. The feeding temperature of the screw extruder is set at 100 °C, the discharging temperature of the screw extruder is set at 230 °C, the temperature of the middle section of the screw is set to gradually increase from 100 °C to 230 °C, there are 2 heating zones, the rotation speed is 30 rpm, and a polyamide 6 base melt with a relative viscosity of 2.80, a monomer content of 3.41 wt%, a cyclic oligomer content of 1.49 wt% (where the cyclic dimer content is 0.39 wt %), and a hot water extractable content of 3.82 wt% is prepared by reactive extrusion. (4)The above-mentioned basic melt is transported to two series-connected devolatilization reactors through a pipeline. The first devolatilization reactor is a horizontal squirrel-cage reactor, and the second devolatilization reactor is a vertical falling-film devolatilization reactor. Each of the two devolatilization reactors is connected to a vacuum system. The temperature of the first devolatilization reactor is 240 °C, and the reaction pressure is 1000 Pa. The temperature of the second devolatilization reactor is 270 °C, and the reaction pressure is 60 Pa. Finally, a high-quality polyamide 6 melt with a relative viscosity of 2.81, a monomer content of 0.01 wt%, a cyclic oligomer content of 0.69 wt% (where the cyclic dimer content is 0.07 wt %), and a hot water extractable content of 0.26 wt% is prepared; (5)The high-quality polyamide 6 melt is transported to a spinning machine for direct spinning. The spinning temperature is 270 °C, and the winding speed is 4200 m / min to obtain high-quality polyamide 6 fine denier fibers. The fiber specification is 50D / 136F, and the fiber breaking strength is 5.2 cN / dtex, and the elongation at break is 23%.

[0037] Example 2 A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1)Preparation of basic active material A: By mass, 1.0 part of NaOH is added to 10 parts of caprolactam and mixed evenly. Water is removed by vacuum distillation at 120 °C and 50 kPa, and then it is stored in a storage tank A protected by N2 and kept at a constant temperature of 130 °C; (2)Preparation of basic active material B: By mass, 100 parts of monomer caprolactam are heated and melted. Water is removed by vacuum distillation at 120 °C and 50 kPa, and then 1.3 parts of terephthaloyl biscaprolactam (TBCL) and 1.5 parts of zinc citrate are added and mixed evenly, and then it is stored in a storage tank B protected by N2 and kept at a constant temperature of 130 °C; (3)The basic active materials A and B are injected into a screw extruder in a volume ratio of 1:10 for anionic polymerization. The feeding temperature of the screw extruder is set at 120 °C, the discharging temperature of the screw extruder is set at 220 °C, the temperature of the middle section of the screw is set to gradually increase from 120 °C to 220 °C with 3 heating zones, the rotation speed is 40 rpm, and a polyamide 6 basic melt with a relative viscosity of 3.30, a monomer content of 3.76 wt%, a cyclic oligomer content of 1.52 wt% (where the cyclic dimer content is 0.41 wt %), and a hot water extractable content of 3.84 wt% is prepared by reactive extrusion; (4) The above-mentioned basic melt is transported to two series-connected devolatilization reactors through pipelines. The first devolatilization reactor is a horizontal disk reactor, and the second devolatilization reactor is a vertical falling film devolatilization reactor. Each of the two devolatilization reactors is connected to a vacuum system. The temperature of the first devolatilization reactor is 260 °C, and the reaction pressure is 2500 Pa. The temperature of the second devolatilization reactor is 250 °C, and the reaction pressure is 60 Pa. Finally, a high-quality polyamide 6 melt with a relative viscosity of 3.31, a monomer content of 0.02 wt%, a cyclic oligomer content of 0.57 wt% (where the cyclic dimer content is 0.08 wt %), and a hot water extractable content of 0.29 wt% is obtained. (5) The high-quality polyamide 6 melt is transported to a spinning machine for direct spinning. The spinning temperature is 275 °C, and the winding speed is 3000 m / min to obtain high-quality polyamide 6 fine denier fibers. The fiber specification is 50D / 96F, the fiber breaking strength is 7.2 cN / dtex, and the elongation at break is 20%.

[0038] Example 3 A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1) Preparation of basic active material A: By mass, 0.5 parts of sodium ethoxide are added to 100 parts of caprolactam and mixed evenly. Water is removed by vacuum distillation at 100 °C and 20 kPa, and then it is stored in a storage tank A protected by Ar and kept at a constant temperature of 100 °C. (2) Preparation of basic active material B: By mass, 100 parts of monomer caprolactam are heated and melted. Water is removed by vacuum distillation at 100 °C and 20 kPa, and then 0.5 parts of terephthaloyl biscaprolactam (TBCL) and 1.1 parts of magnesium chloride are added and mixed evenly. Then it is stored in a storage tank B protected by Ar and kept at a constant temperature of 100 °C. (3) The basic active materials A and B are injected into a screw extruder in a volume ratio of 1:1 for anionic polymerization. The feeding temperature of the screw extruder is set at 150 °C, the discharging temperature of the screw extruder is set at 230 °C, the temperature in the middle section of the screw is gradually increased from 150 °C to 230 °C, there are 3 heating sections, the rotation speed is 40 rpm, and a polyamide 6 basic melt with a relative viscosity of 2.40, a monomer content of 3.47 wt%, a cyclic oligomer content of 1.33 wt% (where the cyclic dimer content is 0.42 wt %), and a hot water extractable content of 3.76 wt% is prepared by reactive extrusion. (4) The above-mentioned basic melt is transported to two series-connected devolatilization reactors through pipelines. The first devolatilization reactor is a falling-strip devolatilizer, and the second devolatilization reactor is a horizontal twin-shaft devolatilizer. Each of the two devolatilization reactors is connected to a vacuum system. The temperature of the first devolatilization reactor is 250 °C, and the reaction pressure is 2000 Pa. The temperature of the second devolatilization reactor is 260 °C, and the reaction pressure is 60 Pa. Finally, a high-quality polyamide 6 melt with a relative viscosity of 2.45, a monomer content of 0.02 wt%, a cyclic oligomer content of 0.63 wt% (where the cyclic dimer content is 0.05 wt %), and a hot water extractable content of 0.19 wt% is prepared; (5) The high-quality polyamide 6 melt is transported to a spinning machine for direct spinning. The spinning temperature is 280 °C, and the winding speed is 4200 m / min to obtain high-quality polyamide 6 fine denier fibers. The fiber specification is 20D / 24F, the fiber breaking strength is 4.5 cN / dtex, and the elongation at break is 25%.

[0039] Example 4 A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1) Preparation of basic active material A: By mass, 0.7 parts of NaH are added to 100 parts of caprolactam and mixed evenly. Water is removed by vacuum distillation at 120 °C and 10 kPa, and then stored in a storage tank A protected by N2 and maintained at a constant temperature of 130 °C; (2) Preparation of basic active material B: By mass, 100 parts of monomer caprolactam are heated and melted. Water is removed by vacuum distillation at 120 °C and 10 kPa, and then 1.7 parts of N-acetyl caprolactam (AcCL) and 0.9 parts of lanthanum aminocaproate are added and mixed evenly, and then stored in a storage tank B protected by N2 and maintained at a constant temperature of 130 °C; (3) The basic active materials A and B are injected into a screw extruder in a volume ratio of 1:1 for anionic polymerization. The feeding temperature of the screw extruder is set at 130 °C, the discharging temperature of the screw extruder is set at 230 °C, the temperature in the middle section of the screw is set to gradually increase from 130 °C to 230 °C, there are 5 heating sections, the rotation speed is 50 rpm, and a polyamide 6 basic melt with a relative viscosity of 2.54, a monomer content of 3.59 wt%, a cyclic oligomer content of 1.33 wt% (where the cyclic dimer content is 0.47 wt %), and a hot water extractable content of 3.69 wt% is prepared by reactive extrusion; (4) The above-mentioned basic melt is transported to two series-connected devolatilization reactors through pipelines. The first devolatilization reactor is a vertical falling film devolatilization reactor, and the second devolatilization reactor is also a vertical falling film devolatilization reactor. Each of the two devolatilization reactors is connected to a vacuum system. The temperature of the first devolatilization reactor is 280 °C, and the reaction pressure is 1000 Pa. The temperature of the second devolatilization reactor is 250 °C, and the reaction pressure is 40 Pa. Finally, a high-quality polyamide 6 melt with a relative viscosity of 2.56, a monomer content of 0.04 wt%, a cyclic oligomer content of 0.77 wt% (where the cyclic dimer content is 0.02 wt %), and a hot water extractable content of 0.21 wt% is obtained. (5) The high-quality polyamide 6 melt is transported to a spinning machine for direct spinning. The spinning temperature is 280 °C, and the winding speed is 4500 m / min to obtain high-quality polyamide 6 fine denier fibers. The fiber specification is 70D / 136F, the fiber breaking strength is 4.8 cN / dtex, and the elongation at break is 24%.

[0040] Example 5 A method for preparing polyamide 6 fine denier fibers by anionic polymerization, the specific steps are as follows: (1) Preparation of basic active material A: By mass, 0.9 parts of NaOH is added to 100 parts of caprolactam and mixed evenly. Water is removed by vacuum distillation at 130 °C and 30 kPa, and then it is stored in a storage tank A protected by Ar and kept at a constant temperature of 120 °C. (2) Preparation of basic active material B: By mass, 100 parts of monomer caprolactam is heated and melted. Water is removed by vacuum distillation at 130 °C and 30 kPa, and then 1.4 parts of m-phthaloyl bis-caprolactam and 0.5 parts of magnesium nitrate are added and mixed evenly. Then it is stored in a storage tank B protected by Ar and kept at a constant temperature of 120 °C. (3) The basic active materials A and B are injected into a screw extruder in a volume ratio of 1:1 for anionic polymerization. The feeding temperature of the screw extruder is set at 160 °C, the discharging temperature of the screw extruder is set at 240 °C, the temperature of the middle section of the screw is set to gradually increase from 160 °C to 240 °C, there are 5 heating sections, the rotation speed is 60 rpm, and a polyamide 6 basic melt with a relative viscosity of 2.92, a monomer content of 3.57 wt%, a cyclic oligomer content of 1.43 wt% (where the cyclic dimer content is 0.36 wt %), and a hot water extractable content of 3.82 wt% is prepared by reactive extrusion. (4)The above-mentioned basic melt is transported through a pipeline to two series-connected devolatilization reactors. The first devolatilization reactor is a vertical falling film devolatilization reactor, and the second devolatilization reactor is also a vertical falling film devolatilization reactor. Each of the two devolatilization reactors is connected to a vacuum system. The temperature of the first devolatilization reactor is 260 °C, and the reaction pressure is 1000 Pa. The temperature of the second devolatilization reactor is 260 °C, and the reaction pressure is 50 Pa. Finally, a high-quality polyamide 6 melt with a relative viscosity of 2.93, a monomer content of 0.01 wt%, a cyclic oligomer content of 0.73 wt% (where the cyclic dimer content is 0.06 wt%), and a hot water extractable content of 0.23 wt% is obtained. (5)The high-quality polyamide 6 melt is transported to a spinning machine for direct spinning. The spinning temperature is 285 °C, and the winding speed is 3000 m / min to obtain high-quality polyamide 6 fine denier fibers. The fiber specification is 50D / 96F, the fiber breaking strength is 6.2 cN / dtex, and the elongation at break is 22%.

[0041] Perform performance characterization on the polyamide 6 basic melt, polyamide 6 end polymer, and polyamide 6 fibers of the comparative example and Examples 1 to 5, and obtain the performance characterization table as shown in Table 1 below: Table 1 Performance characterization table of the polyamide 6 basic melt, polyamide 6 end polymer, and polyamide 6 fine denier fibers of the comparative example and Examples 1 to 5

[0042] Those skilled in the art should understand that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combinations of these technical features do not conflict, they should be considered to be within the scope described in this specification.

[0043] The above embodiments only represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A method for preparing polyamide 6 fine denier fibers by anionic polymerization, comprising the following steps: (1) Prepare the basic active material A: Mix the catalyst and caprolactam evenly to obtain the first mixture, subject the first mixture to vacuum distillation and store it in the storage tank A, wherein the inside of the storage tank A is protected by an inert atmosphere and maintained at a constant temperature of 80 - 150 °C; (2) Prepare the basic active material B: Heat caprolactam to melt it and then perform vacuum distillation, mix the water - removed caprolactam with the initiator and the cyclic oligomer inhibitor evenly to obtain the second mixture, and store the second mixture in the storage tank B, wherein the inside of the storage tank B is protected by an inert atmosphere and maintained at a constant temperature of 80 - 150 °C; (3) Inject the components of the basic active materials A and B into a screw extruder in a volume ratio of 1 - 10:10 for anionic polymerization, and extrude and react to obtain the polyamide 6 basic melt; (4) Transport the polyamide 6 basic melt to the first devolatilization reactor to remove the caprolactam monomer to obtain the polyamide 6 intermediate, and input the polyamide 6 intermediate into the second devolatilization reactor to remove a small amount of caprolactam monomer, cyclic dimer, part of cyclic trimer and part of cyclic tetramer to obtain the polyamide 6 final polymer, wherein the reaction temperature of the first devolatilization reactor is set at 240 - 280 °C and the reaction pressure is 200 - 4000 Pa, and the reaction temperature of the second devolatilization reactor is 240 - 280 °C and the reaction pressure is 20 - 600 Pa; (5) Transport the polyamide 6 final polymer to a spinning machine for direct spinning to obtain the polyamide 6 fine denier fibers.

2. The method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (1), the catalyst is one or more of sodium caprolactamate, sodium ethoxide, LiH, NaH, KH, LiOH, NaOH, KOH.

3. The method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that, In step (2), the initiator is one or more of N - acetylcaprolactam, bis - acyl - lactam - 1,6 - hexanamine, terephthaloyl bis - caprolactam, isophthaloyl bis - caprolactam, toluene diisocyanate, diphenylmethane diisocyanate, 1,6 - hexane diisocyanate.

4. A method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that, The conditions for vacuum distillation in steps (1) and (2) are a temperature of 80 - 150 °C and an absolute reaction pressure of 5 - 95 kPa.

5. The method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (2), the metal cation M is selected from one of transition metal ions, lanthanide metal ions, and Group IIA metal ions. When the metal cation M is selected from transition metal ions, the metal cation M is Sc 3+ , Ni 2+ , Zn 2+ , Y 3+ , Zr 4+ , Ru 4+ , Rh 3+ ; when the metal cation M is selected from lanthanide metal ions, the lanthanide metal ion is La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Gd 3 + , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Lu 3+ , Yb 3+ ; when the metal cation M is selected from Group IIA metal ions, the Group IIA metal ion is one of Be 2+ , Mg 2+ , Ca 2+ ; the metal cation M can also be one of Li + , Al 3+ ; the inorganic anion or organic anion Y is selected from F - , Cl - , NO3 - , SO4 2- , PO4 3- , citrate ion, salicylate ion, 3-hydroxybutyrate ion, L-aspartate ion, lactate ion, malate ion, 2-hydroxypropionate ion, an organic monocarboxylate ion H(CH2) n COO - , where n = 0 to 12, benzoate ion, naphthoate ion, an organic dicarboxylate ion COO - (CH2) m COO - , where m = 0 to 12, terephthalate ion, phthalate ion, isophthalate ion or naphthalenedicarboxylate ion, a saturated fatty acid root ion C x H 2x+1 COO - , where x = 13 to 20, an unsaturated fatty acid root ion, 6-aminohexanoate ion NH2C5H 10 COO - One of the inorganic anions or organic anions; when the inorganic anion or organic anion is an unsaturated fatty acid root ion, the unsaturated fatty acid root ion is oleate ion C 17 H 33 COO - Linoleate ion C 17 H 31 COO - Alpha-linolenate ion C 17 H 29 COO - Arachidonate ion C 19 H 31 COO - Palmitoleate ion C 15 H 29 COO - ; when the inorganic anion or organic anion is an amino acid ion, the amino acid ion is any one of glycinate ion, alaninate ion, valinate ion, leucinate ion, isoleucinate ion, prolinate ion, phenylalaninate ion, methioninate ion, serine ion, threonine ion, asparagine ion, glutamine ion, aspartate ion, glutamate ion, cysteinate ion, tyrosine ion, selenocysteinate ion; the addition amount of the cyclic oligomer inhibitor is 0.01 wt% - 2.0 wt% of caprolactam.

6. A method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (3), the feeding temperature of the screw extruder is set at 90 - 160 °C, the discharging port temperature is set at 220 - 240 °C, the temperature in the middle section of the screw is set to increase gradually from the feeding port temperature to the discharging port temperature, the number of heating sections is not less than 1, and the rotation speed is 20 - 300 rpm.

7. The method for preparing polyamide 6 fine denier fiber by anionic polymerization according to claim 1, characterized in that In step (3), the relative viscosity of the polyamide 6 basic melt is 2.0 - 3.5, the monomer content is less than 3.9 wt%, the cyclic oligomer content is less than 2.0 wt%, wherein the content of cyclic dimer in the cyclic oligomer is less than 0.5 wt%, and the hot water extractable content is less than 4.6 wt%.

8. A method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (4), the reaction temperatures of the first devolatilization reactor and the second devolatilization reactor can be independently controlled, and both are connected to a vacuum system, which is used to control the reaction pressures of the first devolatilization reactor and the second devolatilization reactor. The first devolatilization reactor is selected from one of a horizontal cage reactor, a horizontal disk reactor, a falling-strip devolatilizer, and a vertical falling-film devolatilization reactor, and the second devolatilization reactor is a vertical falling-film devolatilization reactor or a horizontal twin-shaft devolatilization reactor.

9. The method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (4), the relative viscosity of the polyamide 6 end polymer is 2.0 to 3.5, the monomer content is less than 0.1 wt%, and the cyclic oligomer content is less than 1.2 wt%. Among them, the content of cyclic dimer in the cyclic oligomer is less than 0.1 wt%, and the hot water extractable content is less than 0.4 wt%.

10. The method for preparing polyamide 6 fine denier fibers by anionic polymerization according to claim 1, characterized in that In step (5), the spinning temperature is 245 to 300 °C, and the spinning speed is 2500 to 6000 m / min.

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