A polyamide 6 with low cyclic dimer content and a method for its preparation and use

Abstract

The application belongs to the technical field of polymer material synthesis, and discloses a polyamide 6 with low cyclic dimer content and a preparation method and application thereof, wherein the number average molecular weight of the polyamide 6 with low cyclic dimer content is 17200-22600 g / mol, the PDI is less than 1.5, and the cyclic dimer content is less than 0.3 wt%; the preparation method is as follows: caprolactam is subjected to ring-opening polymerization under the action of an initiator, the initiator is a diamine initiator, the diamine initiator is a linear diamine with a primary amino group at the end, the molar ratio of the initiator to the caprolactam is 1:100-200, and the water content of the caprolactam and the initiator is not more than 50 ppm; and the application is as follows: the polyamide 6 with low cyclic dimer content is used to prepare polyamide 6 fibers. The application can be subjected to ring-opening polymerization under anhydrous conditions, the generation of cyclic dimers is significantly inhibited, and the cyclic dimer content in the product is low.

Description

Technical Field

[0001] This invention belongs to the field of polymer material synthesis technology, and relates to a polyamide 6 with low cyclic dimer content, its preparation method and application. Background Technology

[0002] Polyamide 6, as an important engineering plastic, has wide applications in many fields such as fibers, films, and injection molded products. Traditionally, polyamide 6 is prepared using a hydrolytic ring-opening polymerization process. However, this process has significant drawbacks: the hydrolytic polymerization process generates 8-10 wt% oligomers and 3-4 wt% cyclic oligomers, which have a significant negative impact on the processing properties of nylon 6. In particular, if the content of cyclic dimers with a melting point as high as 348℃ is too high, they are prone to accumulation in the fibers, leading to a decrease in the mechanical properties of the prepared nylon 6 fibers under external forces, and even breakage.

[0003] To reduce the content of cyclic dimers, hot water extraction is usually required. However, this method is not only energy-intensive and time-consuming, but also wastes monomers and causes environmental pollution.

[0004] To overcome the shortcomings of traditional hydrolysis polymerization processes, the industry has explored various technical approaches, but all have certain limitations. In recent years, researchers have developed various enhanced devolatilization technologies and equipment, such as staged devolatilization reactors or falling film devolatilization reactors, attempting to selectively remove monomers and oligomers by optimizing conditions such as temperature and pressure. For example, patent application CN117248288A designed an external falling film devolatilization reactor to match the rheological properties of polyamide 6 melt. To meet the requirements of high-quality melt direct spinning, it is generally believed in the industry that the content of cyclic dimers in polyamide 6 needs to be significantly reduced, preferably not exceeding 0.3 wt%, and more preferably not exceeding 0.1 wt%. However, existing processes using physical devolatilization or hydrothermal extraction, even under enhanced devolatilization conditions, still struggle to stably control the cyclic dimer content below 0.3 wt% while maintaining molecular weight and molecular weight distribution, let alone further reduce it to below 0.1 wt%.

[0005] Due to the limitations of physical devolatilization methods, existing technologies also reduce the proportion of stable, infusible cyclic dimers by adding specific amino acids. For example, patent CN119285933B proposes adding specific amino acids (amino acids other than aminocaproic acid) during the ring-opening and polycondensation stages, respectively. These amino acids react with active end groups to generate unstable cyclic dimers, thereby reducing the proportion of stable, infusible cyclic dimers. However, while this method can achieve some results, it involves the staged addition of foreign amino acids, leading to a relatively complex process and potentially causing unknown effects on the polymer chain structure.

[0006] In summary, existing technologies have failed to fundamentally and effectively solve the problem of excessively high cyclic dimer content in the synthesis of polyamide 6, and have even led to new problems such as complex processes, difficult control, and high costs. Therefore, developing a polyamide 6 with low cyclic dimer content, its preparation method, and its applications, so as to effectively suppress the formation of cyclic dimers from the source of the polymerization reaction, and at the same time achieve short-process, low-energy-consumption, green, and efficient production of high-quality polyamide 6, is of great significance. Summary of the Invention

[0007] The purpose of this invention is to solve the problems existing in the prior art and to provide a polyamide 6 with low cyclic dimer content, its preparation method and application.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing polyamide 6 with low cyclic dimer content involves carrying out a ring-opening polymerization reaction of caprolactam under the action of an initiator, wherein the initiator is a diamine initiator, and the diamine initiator is one or more of aliphatic diamines, alicyclic diamines, polyether diamines, and aromatic diamines.

[0010] The molar ratio of initiator to caprolactam is 1:100-200;

[0011] The water content of both caprolactam and initiator does not exceed 50 ppm. In addition to the initiator and caprolactam, no other substances are added to the ring-opening polymerization reaction. By controlling the water content of caprolactam and initiator to be low, the water content of the ring-opening polymerization reaction system can be kept low.

[0012] As a preferred technical solution:

[0013] The preparation method of polyamide 6 with low cyclic dimer content as described above involves a ring-opening polymerization reaction carried out under nitrogen or inert gas protection at a temperature of 230-250℃ for 2-4 hours.

[0014] The method for preparing polyamide 6 with low cyclic dimer content as described above uses a diamine initiator with a number average molecular weight of 100-400 g / mol.

[0015] The preparation method of polyamide 6 with low cyclic dimer content as described above, wherein the water content of both caprolactam and initiator does not exceed 50 ppm, is achieved by vacuum distillation; the vacuum distillation temperature is 80-100℃, the time is 1-3h, and the vacuum degree is 10-100Pa.

[0016] In the preparation method of polyamide 6 with low cyclic dimer content as described above, after the ring-opening polymerization reaction, the polyamide 6 obtained by the ring-opening polymerization reaction is subjected to vacuum devolatilization treatment to remove some of the oligomers; before the vacuum devolatilization treatment, the cyclic dimer content in polyamide 6 is ≤0.41wt%.

[0017] The method for preparing polyamide 6 with low cyclic dimer content as described above involves using a twin- or triple-screw extruder with a negative pressure device for vacuum devolatilization. The vacuum devolatilization temperature is 260-280℃, and the vacuum degree is 10-200Pa. The screw shearing frequency of the extruder is 15-45Hz, the screw length-to-diameter ratio is 28-56:1, and the screw channel filling rate is 30%-70% to improve the devolatilization efficiency and further reduce the oligomer content in polyamide 6. The number of vacuum exhaust ports of the extruder is 1-3, and they are set in the range of 40%-80% of the total screw length, so that the material is in a partially filled state in the devolatilization zone where the vacuum exhaust port is located.

[0018] The polyamide 6 with low cyclic dimer content provided by the present invention is prepared by a method for preparing polyamide 6 with low cyclic dimer content as described above; the polyamide 6 has a number average molecular weight of 17200-22600 g / mol, a relative viscosity of 2.45-3.15, a melting point of 218.0-222.0°C, a PDI (molecular weight distribution index) <1.5, a hot water extractable content of <3.0 wt%, and a cyclic dimer content of <0.3 wt%.

[0019] The present invention also provides a method for preparing polyamide 6 fiber, wherein the raw material is polyamide 6 with low cyclic dimer content as described above.

[0020] As a preferred technical solution:

[0021] The preparation method of polyamide 6 fiber as described above involves a spinning temperature of 240-280℃ and a spinning speed of 3000-5000 m / min; the monofilament fineness of the polyamide 6 fiber is 0.55-2.5 dtex, and the number of monofilaments is 36-256; the breaking strength of the polyamide 6 fiber is ≥4 cN / dtex, the dyeing rate is not less than 90%, and the evenness CV value is ≤1.5%.

[0022] Invention principle:

[0023] The preparation of polyamide 6 from caprolactam requires the addition of an initiator to initiate the polymerization reaction. The role of the initiator is to break the lactam bonds in the caprolactam molecule, causing it to open the ring and gradually connect to form a polymer chain. Based on the different catalytic initiation systems, initiators can generally be classified into the following three categories:

[0024] (1) Hydrolytic polymerization initiator (mainly water)

[0025] Mechanism of action: Under high temperature conditions, water reacts with caprolactam, causing the lactam bond to open and generating 6-aminohexanoic acid. Subsequently, 6-aminohexanoic acid gradually forms high molecular weight polyamide 6 through condensation and addition reactions with caprolactam.

[0026] (2) Anionic polymerization initiator (basic catalyst)

[0027] Mechanism of action: The initiator reacts with caprolactam under anhydrous conditions to generate an active anionic intermediate, thereby initiating the polymerization reaction.

[0028] (3) Positive ionic polymerization initiators (hydrogen chloride, amine salts, metal halides, etc.)

[0029] Mechanism of action: These initiators react with caprolactam under anhydrous conditions to generate a positively charged intermediate, thereby initiating the polymerization reaction.

[0030] This invention proposes a novel initiator—a diamine initiator. The mechanism of action is as follows: under essentially anhydrous conditions, the primary amino group in the diamine initiator first undergoes nucleophilic attack with caprolactam, causing the lactam bond of caprolactam to open, generating a ring-opening addition product with the diamine as the chain end; this addition product has an active amino or amide active center at one or both ends, which can continue to undergo ring-opening addition reaction with caprolactam, gradually forming a linear polyamide chain with the diamine as the initiation end.

[0031] In addition to changing the initiator, this invention also controls the amount of initiator and the water content of the ring-opening polymerization reaction system to effectively suppress the formation of oligomers while significantly increasing the molecular weight of polyamide 6 and significantly narrowing its molecular weight distribution range.

[0032] High concentrations of initiators can rapidly initiate a large amount of caprolactam to undergo a ring-opening reaction in the early stages of polymerization, forming a high concentration and uniformly distributed amino active chain end with diamine as the initiation end. This allows the monomer to be rapidly converted into linearly growing polyamide chains in the early stages of the reaction, kinetically reducing the number of free monomers and oligomers that can participate in the "biting back" cyclization side reaction. At the same time, the high concentration of the above-mentioned amino active chain ends guides the polymerization to preferentially proceed along the intermolecular linear growth path rather than intramolecular cyclization, thermodynamically blocking or significantly inhibiting the formation of oligomers such as cyclic dimers.

[0033] High concentrations of initiators provide a large number of uniform active centers, enabling the ring-opening polymerization of caprolactam to start almost synchronously. The initiation and growth stages of each molecular chain are more concentrated in time, reducing the chain length differences caused by asynchronous initiation and growth, which is the basis for obtaining a narrow distribution.

[0034] The bifunctional nature of the initiator allows it not only to initiate polymerization but also to act as interchain coupling points during the polymerization process. In the later stages of the reaction, these active chain ends undergo efficient coupling reactions through diamine bridging agents, thereby inhibiting cyclization while powerfully driving further chain growth, resulting in a significant increase in the molecular weight of polyamide 6.

[0035] A strictly anhydrous environment fundamentally eliminates water-induced hydrolysis chain transfer and degradation side reactions, avoids random chain breakage and uncontrollable reduction in molecular weight, and ensures stable molecular weight growth to the target range.

[0036] This strategy, which synergistically controls the reaction pathway through "high concentration of active centers" and "bifunctional coupling mechanism," combined with subsequent vacuum devolatilization treatment, ultimately achieved a high molecular weight and narrow molecular weight distribution range at low cyclic dimer content, meeting the stringent requirements of high-quality melt direct spinning.

[0037] Beneficial effects:

[0038] (1) The diamine initiator used in this invention contains two amino functional groups, which can initiate the growth of two polymer chains simultaneously or sequentially, thereby rapidly forming a high concentration and uniformly distributed active chain end in the early stage of polymerization, promoting the efficient conversion of monomers into polymer chains, and inhibiting the "biting back" cyclization of monomers or oligomers under strict control of amino concentration and anhydrous reaction conditions, significantly reducing the content of oligomers such as cyclic dimers in polyamide 6.

[0039] (2) By using a high concentration of diamine initiator, the present invention carries out ring-opening polymerization under strict anhydrous conditions, which significantly inhibits the formation of cyclic dimers, making the content of cyclic dimers in the product less than 0.3wt%, and even further reduced to ≤0.1wt%, with a number average molecular weight of 17200-22600g / mol and a narrow molecular weight distribution (PDI<1.5). It can be directly used for melt spinning, and the resulting fiber has excellent properties. It also realizes a highly efficient and green process that can be directly spun without extraction. In addition, compared with the traditional hydrothermal extraction process, the present invention eliminates the extraction step, greatly reduces energy consumption and production costs, improves production efficiency, and has good prospects for industrial application. Attached Figure Description

[0040] Figure 1 The image shows the infrared spectrum of polyamide 6 prepared in Example 3 of this invention. Detailed Implementation

[0041] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0042] To ensure that the performance of the substances used in each embodiment and comparative example is fully disclosed, the manufacturers and brands of the substances are specified. Other manufacturers and brands that conform to the limitations of this invention are also feasible.

[0043] The test methods for the relevant performance indicators in the following embodiments and comparative examples are as follows:

[0044] Number-average molecular weight and PDI: The number-average molecular weight and PDI of the samples were determined using a GPC-50 gel permeation chromatograph from PL (equipped with a differential refractive index detector and a PL gel column (5µm mixed-C), with 1,1,1,3,3,3-hexafluoro-2-propanol as the eluent and a flow rate of 1 mL / min. Before testing, the samples were dried and dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol to prepare a solution with a concentration of 1.0 mg / mL. The test was performed when the column temperature reached 40 °C.

[0045] Relative viscosity: The relative viscosity of the test sample was tested in accordance with GB / T 38138-2019 "Test Method for Fiber Grade Polycaprolactam (PA6) Slices".

[0046] Hot water extractable content: The melting point of the sample was tested according to Method A (gravimetric method) in 5.3.1 of GB / T 38138-2019 "Test Method for Fiber Grade Polycaprolactam (PA6) Slices".

[0047] Melting point: The melting point of the sample to be tested was determined according to Method A (differential scanning calorimetry) in 5.7.1 of GB / T 38138-2019 "Test Method for Fiber Grade Polycaprolactam (PA6) Slices".

[0048] Cyclic dimer content: 50 mg of sample was weighed using an electronic balance and placed in a 25 mL volumetric flask. 3 mL of hexafluoroisopropanol was added, the flask was stoppered, and then sealed with sealing film. The flask was then placed in a constant-temperature shaking water bath and heated to approximately 35 °C to completely dissolve the sample. After cooling to 25 °C, the solution was diluted to volume with methanol, and the mixture was shaken every 5 minutes at 25 °C. After 20 minutes, 1 mL of the solution was taken and filtered to obtain the test sample. The cyclic dimer in the test sample was then qualitatively and quantitatively analyzed using a Shimadzu LC-16 high-performance liquid chromatograph (HPLC) equipped with a WondaSil C18-WR (200 mm, 5 μm packed particle size) column and a UV detector (detection wavelength: 200 nm, detection temperature: 30 °C). A binary gradient method was used, with acetonitrile and water as the mobile phases. The test method is shown in the table below.

[0049] 0 95% 5% 12.0 70% 30% 12.5 0% 100% 15.5 0% 100% 15.6 95% 5% 20.0 95% 5%

[0050] Tensile strength: The tensile strength of the samples was determined in accordance with GB / T 14344-2008 "Test Method for Tensile Properties of Chemical Fiber Filaments".

[0051] Dye uptake rate: The dye uptake rate of the sample was determined in accordance with GB / T 23976.1-2009 "Determination of dye uptake rate curve and dye uptake rate". The measured dye uptake rate is the dye uptake rate.

[0052] Evenness coefficient (CV) value: The evenness coefficient (CV) value of the sample was determined in accordance with GB / T 3292.1-2008 "Textiles - Test Methods for Evenness of Yarn - Part 1: Capacitance Method".

[0053] Example 1

[0054] A method for preparing polyamide 6 fiber, comprising the following steps:

[0055] (1) The water content of caprolactam was controlled to 48 ppm and the water content of 1,6-hexanediamine was controlled to 45 ppm by vacuum distillation; wherein the vacuum distillation temperature was 80℃, the time was 3h, and the vacuum degree was 10Pa.

[0056] (2) Under nitrogen protection, caprolactam was subjected to ring-opening polymerization at 230°C for 4 hours in the presence of 1,6-hexanediamine to obtain polyamide 6; wherein the molar ratio of 1,6-hexanediamine to caprolactam was 1:100.

[0057] The cyclic dimer content in polyamide 6 is 0.41 wt%.

[0058] (3) The polyamide 6 obtained after the ring-opening polymerization reaction was fed into a twin-screw extruder with a negative pressure device and vacuum devolatilized for 40 min at a temperature of 260℃, a vacuum degree of 200 Pa, and a screw shear frequency of 15 Hz. The screw length-to-diameter ratio of the screw extruder was 28:1, and the screw channel filling rate was 30%. The screw extruder had one vacuum exhaust port, which was located at 40% of the total length of the screw.

[0059] The polyamide 6 after vacuum devolatilization treatment had a number-average molecular weight of 17200 g / mol, a relative viscosity of 2.45, a melting point of 218.5°C, a PDI of 1.48, a hot water extractable content of 2.05 wt%, and a cyclic dimer content of 0.21 wt%.

[0060] (4) Polyamide 6 is fed to a spinning device and then melt-spun to obtain polyamide 6 fiber; wherein the spinning temperature is 240℃ and the spinning speed is 3000m / min.

[0061] The final polyamide 6 fiber has a single filament fineness of 2.5 dtex and a single filament count of 36; the polyamide 6 fiber has a breaking strength of 4.2 cN / dtex, a dyeing rate of 90.5%, and a yarn unevenness CV value of 1.45%.

[0062] Example 2

[0063] A method for preparing polyamide 6 fiber, comprising the following steps:

[0064] (1) The water content of caprolactam was controlled to 40 ppm and the water content of 1,4-cyclohexanediamine was controlled to 38 ppm by vacuum distillation; wherein the vacuum distillation temperature was 90℃, the time was 2h, and the vacuum degree was 50Pa.

[0065] (2) Under nitrogen protection, caprolactam was subjected to ring-opening polymerization at 240°C for 3 h in the presence of 1,4-cyclohexanediamine to obtain polyamide 6; wherein the molar ratio of 1,4-cyclohexanediamine to caprolactam was 1:150.

[0066] The cyclic dimer content in polyamide 6 is 0.34 wt%.

[0067] (3) The polyamide 6 obtained after the ring-opening polymerization reaction was transported to a three-screw extruder with a negative pressure device and vacuum devolatilized for 50 min at a temperature of 270℃, a vacuum degree of 100 Pa, and a screw shear frequency of 30 Hz. The screw of the extruder had a length-to-diameter ratio of 42:1 and a screw channel filling rate of 50%. The extruder had two vacuum exhaust ports, with the first vacuum exhaust port located at 40% of the total screw length and the second vacuum exhaust port located at 70% of the total screw length.

[0068] The number-average molecular weight of polyamide 6 after vacuum devolatilization treatment was 19800 g / mol, the relative viscosity was 2.72, the melting point was 220.2°C, the PDI was 1.35, the hot water extractable content was 1.4 wt%, and the cyclic dimer content was 0.15 wt%.

[0069] (4) Polyamide 6 is fed to a spinning device and then melt-spun to obtain polyamide 6 fiber; wherein the spinning temperature is 260℃ and the spinning speed is 4000m / min.

[0070] The final polyamide 6 fiber has a single filament fineness of 1.5 dtex and a single filament count of 128; the polyamide 6 fiber has a breaking strength of 5.6 cN / dtex, a dyeing rate of 93.8%, and a yarn unevenness CV value of 1.10%.

[0071] Example 3

[0072] A method for preparing polyamide 6 fiber, comprising the following steps:

[0073] (1) The water content of caprolactam was controlled to 15 ppm and the water content of polyether diamine with a number average molecular weight of 400 g / mol was controlled to 20 ppm by vacuum distillation; wherein the vacuum distillation temperature was 100℃, the time was 1h, and the vacuum degree was 100Pa.

[0074] (2) Under nitrogen protection, caprolactam was subjected to ring-opening polymerization at 250°C for 2 hours in the presence of polyether diamine to obtain polyamide 6; wherein the molar ratio of polyether diamine to caprolactam was 1:200.

[0075] The cyclic dimer content in polyamide 6 is 0.23 wt%.

[0076] (3) The polyamide 6 obtained after the ring-opening polymerization reaction was fed into a three-screw extruder with a negative pressure device and vacuum devolatilized for 60 min at a temperature of 280℃, a vacuum degree of 10 Pa, and a screw shear frequency of 45 Hz to obtain polyamide 6; wherein, the screw length-to-diameter ratio of the screw extruder was 56:1, and the screw channel filling rate was 70%; the number of vacuum exhaust ports of the screw extruder was 3, the first vacuum exhaust port was set at 40% of the total screw length, the second vacuum exhaust port was set at 60% of the total screw length, and the third vacuum exhaust port was set at 80% of the total screw length;

[0077] The number-average molecular weight of polyamide 6 after vacuum devolatilization treatment was 22600 g / mol, the relative viscosity was 3.15, the melting point was 221.9°C, the PDI was 1.28, the hot water extractable content was 0.55 wt%, and the cyclic dimer content was 0.08 wt%.

[0078] Infrared spectroscopy was performed on polyamide 6 after vacuum devolatilization treatment, and the results are as follows: Figure 1 As shown in the figure, it is located at 3304cm. -1 The absorption peak at 1540 cm⁻¹ is the stretching vibration absorption peak of amide (NH₃), located at 1540 cm⁻¹. -1 The bending vibration peak of amide (NH) is located at 1638 cm⁻¹. -1 The absorption peak represents the stretching vibration of the amide bond (C=O). The structure of the product was determined by infrared spectroscopy.

[0079] (4) Polyamide 6 is fed to a spinning device and then melt-spun to obtain polyamide 6 fiber; wherein the spinning temperature is 280℃ and the spinning speed is 5000m / min.

[0080] The final polyamide 6 fiber has a single filament fineness of 0.55 dtex and a single filament count of 256; the polyamide 6 fiber has a breaking strength of 6.8 cN / dtex, a dyeing rate of 96.5%, and a yarn unevenness CV value of 0.70%.

[0081] Comparative Example 1

[0082] A method for preparing polyamide 6 fiber is basically the same as in Example 3, except that the molar ratio of polyether diamine to caprolactam used in step (2) is 1:80.

[0083] The number-average molecular weight of polyamide 6 after vacuum devolatilization in step (3) is 15000 g / mol, the relative viscosity is 2.20, the melting point is 217.5°C, the PDI is 1.60, the hot water extractable content is 3.8 wt%, and the cyclic dimer content is 0.45 wt%.

[0084] The final polyamide 6 fiber had a breaking strength of 3.0 cN / dtex, a dyeing rate of 85.0%, and a yarn unevenness CV value of 2.20%.

[0085] Comparing Comparative Example 1 and Example 3, it can be seen that the number-average molecular weight, relative viscosity, and melting point of polyamide 6 after vacuum devolatilization treatment in this comparative example decreased, while the PDI increased. The content of hot water extractables and cyclic dimers increased, resulting in decreased breaking strength and dyeing rate of polyamide 6 fibers prepared from it, and an increased CV value of yarn unevenness. This is because excessive initiator dosage leads to an excessive number of active centers, with monomers being dispersed on a large number of growing chains, limiting single-chain growth and causing a decrease in molecular weight. At the same time, high concentration of amino end groups exacerbates the "biting back" cyclization side reaction, increasing the content of cyclic dimers. Excessive short chains and uneven growth also lead to an increase in oligomers and a wider molecular weight distribution (increased PDI), ultimately deteriorating the properties of polyamide 6 chips and reducing fiber mechanics, dyeing, and yarn evenness.

[0086] Comparative Example 2

[0087] A method for preparing polyamide 6 fiber is basically the same as in Example 3, except that the molar ratio of the initiator to caprolactam used in step (2) is 1:220.

[0088] The number-average molecular weight of polyamide 6 after vacuum devolatilization in step (3) is 26000 g / mol, the relative viscosity is 3.45, the melting point is 223.5°C, the PDI is 1.75, the hot water extractable content is 3.50 wt%, and the cyclic dimer content is 0.52 wt%.

[0089] The final polyamide 6 fiber had a breaking strength of 3.5 cN / dtex, a dyeing rate of 86.0%, and a yarn unevenness CV value of 2.00%.

[0090] Comparing Comparative Example 2 and Example 3, it can be seen that the number-average molecular weight, relative viscosity, and melting point of polyamide 6 after vacuum devolatilization treatment in this comparative example increased, as did the PDI, the content of hot water extractables, and the content of cyclic dimers. The breaking strength and dyeing rate of the polyamide 6 fibers prepared from it decreased, while the evenness (CV) value increased. This is because insufficient initiator dosage leads to incomplete polymerization initiation, delaying the initiation time of some polymer chains, resulting in excessive growth of some chains and insufficient growth of others. This, in turn, increases the number-average molecular weight, relative viscosity, and melting point of polyamide 6, and the molecular weight... The content distribution widened significantly (PDI reached 1.75). At the same time, due to the decrease in initiation efficiency and the lengthening of the polymerization process, the residual time of monomers and oligomers in the system was prolonged, making it more prone to cyclization side reactions. This resulted in the content of hot water extractables increasing to 3.50 wt% and the content of cyclic dimers increasing to 0.52 wt%. As a result, the polyamide 6 fiber obtained from it had excessively high melt viscosity and poor flowability. The stretching and forming stability during melt direct spinning was significantly lower than that in Example 3. Consequently, the breaking strength and dyeing rate of the obtained polyamide 6 fiber decreased, and the evenness CV value increased.

[0091] Comparative Example 3

[0092] A method for preparing polyamide 6 fiber is basically the same as in Example 3, except that in step (1), the water content of both caprolactam and polyether diamine is controlled to be 60 ppm.

[0093] The number-average molecular weight of polyamide 6 after vacuum devolatilization in step (3) is 16000 g / mol, the relative viscosity is 2.30, the melting point is 217.8°C, the PDI is 1.70, the hot water extractable content is 4.2 wt%, and the cyclic dimer content is 0.60 wt%.

[0094] The final polyamide 6 fiber had a breaking strength of 2.8 cN / dtex, a dyeing rate of 83.0%, and a yarn unevenness CV value of 2.50%.

[0095] Comparing Comparative Example 3 and Example 3, it can be seen that the excessively high water content of caprolactam and polyether diamine in this comparative example leads to a decrease in the number-average molecular weight, relative viscosity, and melting point of polyamide 6 after vacuum devolatilization treatment, an increase in PDI, and an increase in the content of hot water extractables and cyclic dimers. Consequently, the breaking strength and dyeing rate of the polyamide 6 fibers prepared from it decrease, and the evenness (CV) value increases. This is because when the water content of caprolactam and polyether diamine is too high, the excess water acts as a chain transfer agent during high-temperature polymerization, initiating hydrolysis and degradation reactions, thus reducing the already formed polyamide... Random chain breakage leads to a decrease in number-average molecular weight, relative viscosity, and melting point, deviating from the reasonable range of Example 3. On the other hand, water participates in ring-opening hydrolysis to generate intermediates such as 6-aminohexanoic acid, which increases the concentration of monomers / oligomers in the system and makes it easier for "biting back" cyclization to occur, thereby significantly increasing the amount of cyclic dimers generated, which in turn leads to an increase in PDI, hot water extractable content, and cyclic dimer content. When polyamide 6 fibers are prepared using polyamide 6 in this comparative example, the breaking strength and dyeing rate of the polyamide 6 fibers are reduced, and the evenness CV value is increased.

[0096] Example 4

[0097] A method for preparing polyamide 6 fiber, comprising the following steps:

[0098] (1) The water content of caprolactam was controlled to 45 ppm and the water content of p-phenylenediamine was controlled to 42 ppm by vacuum distillation; wherein the vacuum distillation temperature was 85℃, the time was 2.5h, and the vacuum degree was 30Pa.

[0099] (2) Under nitrogen protection, caprolactam was subjected to ring-opening polymerization at 235°C for 3.5 h in the presence of p-phenylenediamine to obtain polyamide 6; wherein the molar ratio of p-phenylenediamine to caprolactam was 1:120.

[0100] The cyclic dimer content in polyamide 6 is 0.37 wt%.

[0101] (3) The polyamide 6 obtained after the ring-opening polymerization reaction was transported to a twin-screw extruder with a negative pressure device and vacuum devolatilized for 45 min at a temperature of 265℃, a vacuum degree of 150 Pa, and a screw shear frequency of 20 Hz. The screw of the extruder had a length-to-diameter ratio of 36:1 and a screw channel filling rate of 40%. The extruder had two vacuum exhaust ports, with the first vacuum exhaust port located at 45% of the total screw length and the second vacuum exhaust port located at 75% of the total screw length.

[0102] The number-average molecular weight of polyamide 6 after vacuum devolatilization treatment was 18,500 g / mol, the relative viscosity was 2.58, the melting point was 219.4°C, the PDI was 1.42, the hot water extractable content was 1.56 wt%, and the cyclic dimer content was 0.18 wt%.

[0103] (4) Polyamide 6 is fed to a spinning device and then melt-spun to obtain polyamide 6 fiber; wherein the spinning temperature is 250℃ and the spinning speed is 3500m / min.

[0104] The final polyamide 6 fiber has a single filament fineness of 2 dtex and a single filament count of 72; the polyamide 6 fiber has a breaking strength of 4.9 cN / dtex, a dyeing rate of 91.2%, and a yarn unevenness CV value of 1.35%.

[0105] Example 5

[0106] A method for preparing polyamide 6 fiber, comprising the following steps:

[0107] (1) The water content of caprolactam was controlled to 28 ppm and the water content of the initiator was controlled to 25 ppm by vacuum distillation. The initiator was composed of polyether diamine (polyether amine D230, number average molecular weight of 230 g / mol) and p-phenylenediamine in a mass ratio of 1:1. The vacuum distillation temperature was 95℃, the time was 1.5 h, and the vacuum degree was 80 Pa.

[0108] (2) Under nitrogen protection, caprolactam was subjected to ring-opening polymerization at 245°C for 2.5 h with the action of an initiator to obtain polyamide 6; wherein the molar ratio of initiator to caprolactam was 1:180.

[0109] The cyclic dimer content in polyamide 6 is 0.31 wt%.

[0110] (3) The polyamide 6 obtained after the ring-opening polymerization reaction was transported to a three-screw extruder with a negative pressure device and vacuum devolatilized for 55 min at a temperature of 275℃, a vacuum degree of 50 Pa, and a screw shear frequency of 40 Hz. The screw length-to-diameter ratio of the screw extruder was 48:1, and the screw channel filling rate was 60%. The screw extruder had three vacuum exhaust ports. The first vacuum exhaust port was set at 40% of the total screw length, the second vacuum exhaust port was set at 60% of the total screw length, and the third vacuum exhaust port was set at 80% of the total screw length.

[0111] The polyamide 6 after vacuum devolatilization treatment had a number-average molecular weight of 21200 g / mol, a relative viscosity of 2.95, a melting point of 221.2°C, a PDI of 1.31, a hot water extractable content of 1.05 wt%, and a cyclic dimer content of 0.11 wt%.

[0112] (4) Polyamide 6 is fed to a spinning device and then melt-spun to obtain polyamide 6 fiber; wherein the spinning temperature is 270℃ and the spinning speed is 4500m / min.

[0113] The final polyamide 6 fiber has a single filament fineness of 1 dtex and a single filament count of 144; the polyamide 6 fiber has a breaking strength of 6.2 cN / dtex, a dyeing rate of 95.0%, and a yarn unevenness CV value of 0.90%.

Claims

1. A method for preparing polyamide 6 with low cyclic dimer content, comprising carrying out a ring-opening polymerization reaction of caprolactam under the action of an initiator, characterized in that, The initiator is a diamine initiator, which is one or more of alicyclic diamines, polyether diamines, and aromatic diamines; The ring-opening polymerization reaction is carried out under nitrogen or inert gas protection at a temperature of 230-250℃ for 2-4 hours. The number average molecular weight of diamine initiators is 100-400 g / mol; The molar ratio of initiator to caprolactam is 1:100-200; The water content of both caprolactam and initiator does not exceed 50 ppm, and it is achieved by vacuum distillation; the temperature of vacuum distillation is 80-100℃, the time is 1-3h, and the vacuum degree is 10-100Pa. After the ring-opening polymerization reaction, the polyamide 6 obtained by the ring-opening polymerization reaction was subjected to vacuum devolatilization treatment; before the vacuum devolatilization treatment, the content of cyclic dimers in polyamide 6 was ≤0.41wt%.

2. The method for preparing polyamide 6 with low cyclic dimer content according to claim 1, characterized in that, The vacuum devolatilization process uses a screw extruder equipped with a negative pressure device; the temperature of the vacuum devolatilization process is 260-280℃, and the vacuum degree is 10-200Pa; the screw shearing frequency of the screw extruder is 15-45Hz, the screw length-to-diameter ratio is 28-56:1, and the screw channel filling rate is 30%-70%; the number of vacuum exhaust ports of the screw extruder is 1-3, and they are located in the range of 40%-80% of the total screw length.

3. Polyamide 6 with low cyclic dimer content, characterized in that, The polyamide 6 with low cyclic dimer content was prepared using the method described in claim 2. The polyamide 6 had a number-average molecular weight of 17200-22600 g / mol, a relative viscosity of 2.45-3.15, a melting point of 218.0-222.0°C, a PDI of <1.5, a hot water extractable content of <3.0 wt%, and a cyclic dimer content of <0.3 wt%, as measured according to GB / T 38138-2019 "Test Method for Fiber Grade Polycaprolactam (PA6) Slices".

4. A method for preparing polyamide 6 fiber, characterized in that, The raw material is polyamide 6 with low cyclic dimer content as described in claim 3.

5. The method for preparing polyamide 6 fiber according to claim 4, characterized in that, The spinning temperature is 240-280℃, and the spinning speed is 3000-5000m / min; the single filament fineness of polyamide 6 fiber is 0.55-2.5dtex, and the number of single filaments is 36-256. The tensile strength of polyamide 6 fiber is ≥4cN / dtex, the dyeing rate is not less than 90%, and the evenness (CV) value is ≤1.5%.

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