Melt direct spinning method of polyamide 6 or polyamide 6 copolymer

Abstract

The invention discloses a melt direct spinning method of polyamide 6 or a copolymer thereof. The melt direct spinning method comprises the following steps: (1) preparing a melt of the polyamide 6 or the copolymer thereof; (2) intermittently injecting the additive fluid into the melt of the polyamide 6 or the copolymer of the polyamide 6 in a multi-point, high-frequency and multi-point manner in a micro / small droplet or micro / small bubble manner, and further uniformly dispersing the additive fluid into the melt through stirring; (3) controlling the temperature and the pressure of the mixed melt in a devolatilization device, and removing extractable micromolecular substances, so that the content of the extractable micromolecular substances in the obtained polyamide melt is less than or equal to 1.0 wt%; and (4) directly spinning or performing other subsequent processing on the polyamide melt obtained in the step (3). The method provided by the invention can promote rapid removal of volatile small-molecular substances to obtain a polymer melt suitable for direct spinning or other subsequent processing, and can shorten the process flow, reduce energy consumption and save investment.

Description

Technical Field

[0001] This invention belongs to the field of polyamide fiber preparation, and more specifically, relates to a method for direct melt spinning of polyamide 6 or its copolymers. Background Technology

[0002] Polyamide 6 fiber was the first synthetic fiber to be industrialized, and its production volume ranked first among synthetic fibers until 1972. However, due to its high raw material cost, its production volume was gradually surpassed by polyester fiber. my country has become the world's largest producer and consumer of polyamide fibers. Polyamide fibers have excellent properties such as abrasion resistance, moisture absorption, fatigue resistance, spinnability, and dyeability, and are widely used in clothing, home textiles, baby products, military industry, automobiles, and other fields. Its production volume ranks second among synthetic fibers. In recent years, the production technology of the domestic polyamide 6 fiber industry has continued to improve, significantly reducing product costs and expanding its application range. However, industrial production still adopts the chip melt spinning process. The polymer melt after monomer polymerization needs to go through processes such as casting, granulation, hot water extraction, drying, extraction water recovery, and remelting, which has problems such as long process flow, high production energy consumption, and large equipment investment. Developing and breaking through the high-efficiency and low-carbon polyamide 6 melt direct spinning process and equipment, eliminating the above-mentioned processes, and achieving low-energy consumption, high-efficiency, and low-cost production has become an important development direction for low-carbon and high-efficiency polyamide fibers.

[0003] Due to technical limitations, direct melt spinning of polyamide 6 fibers has not been industrialized. The key lies in preparing a polyamide 6 melt suitable for direct spinning. Polyamide 6 is a condensation polymer with caprolactam as the reactant monomer. Based on different polymerization mechanisms, it can be classified into hydrolytic polymerization, anionic polymerization, and solid-state polymerization, among which hydrolytic polymerization is widely used in industrial production. In the hydrolytic polymerization of polyamide 6, water is used as the initiator. The reactants are preheated and then enter the polymerization reactor for ring-opening, addition polymerization, and condensation polymerization. When the reaction reaches equilibrium, the polymerization product is approximately 90% polyamide 6 and 10% extractable low-molecular-weight substances (of which caprolactam monomer accounts for about 80% and oligomers about 20%). Due to the presence of these extractable low-molecular-weight substances, direct spinning is not possible. Currently, the main method used in industrial plants is to cool and granulate the polymer melt, then extract the chips with hot water, dry the extracted chips, and then melt spin them. The energy consumption and time required for hot water extraction and drying account for more than 50% of the entire polymerization process, and a large amount of wastewater also needs to be treated. Therefore, direct spinning of polymer melts without extraction and drying is an effective method for shortening the process, reducing production costs, and providing environmental friendliness and economy for polyamide 6 fibers. However, how to directly extract small-molecule extractable substances in the polymer melt state and meet the requirements for direct spinning remains a current technical challenge.

[0004] Patent application CN105669969A discloses a method for polymerizing nylon 6 and its melt spinning method. The method involves first preparing a polyamide 6 prepolymer at low temperature and pre-controlling the oligomer content in the melt. Then, by strengthening the kinetics of the polycondensation reaction, the polymerization is completed before a large amount of cyclic oligomers are generated, resulting in a nylon 6 polymer melt with a certain molecular weight. The extracted content of the obtained product is ≤1.5wt%, and the cyclic dimer content is ≤0.2wt%. After the polycondensation reaction kinetics are strengthened, the product is directly melt-spun into fibers.

[0005] Patent application CN109811423A discloses a technical improvement method for producing functional polyamide fibers through continuous melt direct spinning. This method involves obtaining a functional powder slurry through multi-stage grinding and dispersion, which is then uniformly mixed with caprolactam melt and a catalyst via a dynamic mixer. The mixture is then fed into a hydrolysis and polymerization reactor via a heat exchanger. Polymer melt dehydrators are installed after the open-loop reactor and the prepolymerization reactor, respectively. The resulting functional polyamide melt is then directly fed into a spinning box to obtain functional polyamide fibers after caprolactam monomer removal via a thin-film evaporation devolatilization system. The dehydrators and thin-film evaporation devolatilization system effectively reduce the oligomer content in the melt.

[0006] Due to the limitations of reaction equilibrium conditions and the high melting point and low solubility of oligomers, industrial equipment uses hot water extraction to remove small molecules. After extraction, the product needs to be dried and remelted before spinning. Moreover, the extract needs to be evaporated and concentrated before the extract can be recycled. The process is long and energy-intensive.

[0007] Several existing methods for reducing small molecule substances in melts have problems that hinder the industrial application of direct spinning of the polymerized melt due to the particularly difficult removal of oligomers. Therefore, this invention is proposed.

[0008] In view of this, the present invention is proposed. Summary of the Invention

[0009] The technical problem to be solved by this invention is to overcome at least one of the shortcomings of the prior art and provide a method for direct melt spinning of polyamide 6 or its copolymers. The method of this invention can efficiently remove small molecule extractables from the polyamide 6 melt, effectively control the content of polymer monomers and oligomers, meet the requirements of direct spinning, shorten the process flow, reduce energy consumption, and save investment.

[0010] To solve the above-mentioned technical problems, the basic concept of the technical solution adopted by the present invention is as follows:

[0011] In a first aspect, the present invention provides a method for direct melt spinning of polyamide 6 or its copolymers, comprising:

[0012] (1) Prepare a melt of polyamide 6 or its copolymer;

[0013] (2) The additive fluid is injected into the melt of polyamide 6 or its copolymer in a multi-point, high-frequency, and intermittent manner in the form of micro / small droplets or micro / small bubbles, and further uniformly dispersed in the melt by stirring;

[0014] (3) The mixed melt is subjected to temperature and pressure control in a devolatilization device to remove extractable small molecules. The content of extractable small molecules in the obtained polyamide melt is ≤1.0wt%.

[0015] (4) The polyamide melt obtained in (3) is directly spun or otherwise processed.

[0016] In existing technologies, the polymerized polyamide 6 melt is directly fed into a devolatizer to remove most of the extractable substances. However, the devolatization effect of existing technologies cannot meet the requirements for direct spinning of polymer melts, thus limiting their industrial application. The method of this invention involves injecting an additive fluid as micro / small droplets or micro / small bubbles into the melt of polyamide 6 or its copolymers at multiple points, high frequency, and intermittently. The fluid is further uniformly dispersed in the melt through stirring. Then, temperature and pressure are controlled in the devolatizer to remove extractable small molecules. This creates a large number of uniformly dispersed micro / small bubbles or droplets within the polymer melt, significantly increasing the gas-liquid mass transfer interface. Polymer monomers and poorly soluble oligomers can rapidly diffuse into the bubbles or droplets. Combined with the high surface renewal rate of the devolatizer, volatiles can be efficiently removed in a short time, resulting in a polymer melt suitable for direct spinning or other subsequent processing. In addition, by adjusting the types of additives, the uniformity of the polymer's molecular weight distribution, the rapid increase of molecular weight, and its physical and chemical properties can be effectively regulated and controlled, enabling the polymer melt to meet the requirements for direct spinning or direct subsequent processing, thereby further shortening the process flow and reducing product costs.

[0017] Regarding step (1), this invention does not limit the specific method of implementing this step, and different processes, equipment, and process parameters can be adopted according to different product requirements. The process of preparing polyamide 6 or its copolymer melt generally includes batching and polymerization. The specific methods of batching and polymerization can adopt existing conventional technologies or improved technologies.

[0018] A further step, step (1), includes:

[0019] S1 Ingredients: Mix water, caprolactam, comonomers, and additives in proportion, preheat, and prepare a reaction slurry.

[0020] S2 polymerization: The prepared reaction slurry is fed into a polymerization reactor to carry out a polymerization reaction, and a melt of polyamide 6 or its copolymer is prepared.

[0021] In a further embodiment, the mass ratio of water, caprolactam, comonomer, and additive is (0.012–0.035):1:(0.001–0.06).

[0022] The caprolactam and comonomer may also include recycled caprolactam and oligomers.

[0023] In step S1, the additives are selected from one or more of the following: catalysts, matting agents, molecular weight regulators, antioxidants, UV stabilizers, colorants, flame retardants, and antibacterial agents.

[0024] In a further proposed solution, the equipment used in the S1 batching step includes a batching tank for caprolactam and comonomers, a preparation system for each additive, a mixing tank, a slurry preheater, etc.

[0025] As a preferred method, the metered and prepared additives are stirred evenly in a mixing tank before being sent to a slurry preheater, where the preheating temperature of the reaction slurry is 130–230°C.

[0026] In a further embodiment, the polymerization reaction is a ring-opening polymerization stage, and the method used can be selected from one or more of atmospheric pressure continuous polymerization, two-stage polymerization, and multi-stage continuous polymerization.

[0027] As one method, the two-stage polymerization method includes prepolymerization and postpolymerization. The reaction temperature of prepolymerization is 180-280℃, the reaction pressure is 0.1-3.2MPa, and the reaction time is 2-15 hours. The reaction temperature of postpolymerization is 230-270℃, the reaction pressure is 10-150kPa, and the reaction time is 1-10 hours.

[0028] In this process, the prepolymerization is carried out under pressure, while the postpolymerization is carried out under reduced pressure or normal pressure. The prepolymerization completes the ring-opening and prepolymerization of polyamide 6 monomer, and when the postpolymerization reaction is completed, a polyamide 6 or its copolymer melt with a relative viscosity of 1.5 to 2.8 is obtained.

[0029] A further proposed approach involves equipping the tops of both the pre-polymerization reactor and the post-pre-polymerization reactor with a steam condensation system to separate and recover the distilled water and caprolactam.

[0030] As another approach, in the multi-stage continuous polymerization method, the reaction temperature in the ring-open reactor is 250–280℃, the reaction pressure is 0.2–5.6 MPa, and the reaction time is 5–40 minutes. The recovered caprolactam and oligomers are directly added to the ring-open reactor after fractionation and metering.

[0031] Preferably, after the polymerization reaction is completed, the relative viscosity of the obtained polyamide 6 or its copolymer melt is 1.5 to 2.8.

[0032] In a further embodiment, in step (2), the additive fluid is injected into the melt of polyamide 6 or its copolymer through an additive injection port provided on the mixing device or devolatilization device, wherein the diameter or width of the additive injection port is 0.1 to 3 mm.

[0033] Preferably, the diameter or width of the additive injection port is 0.3-1.5 mm.

[0034] In this invention, step (2) can be performed with a separate mixing device, or with only a devolatilization device. That is, the additive can be injected into the melt in the mixing device or directly into the melt in the devolatilization device. The specific steps depend on the process and equipment requirements.

[0035] Additive injection ports can be installed on the mixing device or the devolatilization device. The additive injection port can be a small-diameter pipe, a small hole, or a slit. Its diameter or width needs to be controlled to be 0.1-3mm. This allows the additive fluid to form a large number of small liquids or small bubbles after passing through the narrow additive injection port, thereby fully mixing with the melt. At the same time, it is beneficial for small molecules in the melt to diffuse into the bubbles / droplets, and then be quickly removed.

[0036] An additive distributor is also provided at the additive injection port. The additive distributor includes a delivery pipe and multiple distribution pipes. One end of each distribution pipe is connected to the delivery pipe, and the other end is connected to a corresponding additive injection port. The diameter of the delivery pipe is much larger than the diameter of the distribution pipes, and the diameter of the distribution pipes is much larger than the diameter of the additive injection port. This allows the additive fluid to form a large number of micro / small bubbles or droplets inside the melt, facilitating mixing and devolatilization. A pulse flow generator is installed on the delivery pipe, which can generate a pulse flow of the additive fluid at a certain frequency. The pulse flow sequentially passes through the delivery pipe, distribution pipes, and additive injection port of the additive distributor, injecting into the mixing device or devolatilization device in a high-frequency, intermittent manner.

[0037] As an alternative, the additive is added to the high-efficiency mixer (mixing device) before the melt inlet of the devolatilizer, and the additive injection port is set on the rotating or stationary part of the high-efficiency mixer; the additive injection port can also be set directly at one or more places on the body of the devolatilizer, in which case the agitator of the devolatilizer also serves as the mixing part of the additive.

[0038] As a preferred embodiment, the high-efficiency mixer is a dynamic mixer with built-in rotating components. After the additive is injected into the mixer, it is continuously decomposed into small bubbles or droplets between rotating components and between rotating and stationary components, and uniformly dispersed into the melt, so that the extractable substances in the polymer melt can quickly diffuse into the small bubbles or droplets.

[0039] As a preferred embodiment, the devolatileizer is a dynamic rotating type, which has excellent melt surface renewal capability and uniform heating capability, so that the volatiles on the melt surface are continuously released, while the small bubbles inside the melt are exposed and broken, carrying the volatiles in the small bubbles out of the polymer melt.

[0040] As a preferred embodiment, additive injection ports can be distributed on the rotating or stationary parts of the dynamic rotary devourer, and an additive dispenser can be connected to them. In this case, a high-performance dynamic mixer is not required.

[0041] As a preferred option, the devolatiles can also be static types such as falling film type or strip type, in which case an independent high-efficiency mixer needs to be installed in front of the devolatiles.

[0042] In a further embodiment, in step (2), when the additive fluid is injected into the melt of polyamide 6 or its copolymer in a high-frequency, intermittent manner, the pulse frequency of the additive fluid is 6 to 1200 times / second.

[0043] Preferably, the pulse frequency of the additive fluid is 20 to 300 times per second.

[0044] In a further embodiment, in step (2), the additive is a gas, liquid, or solid, or a combination of two or three states of matter.

[0045] Preferably, when the additive is a solid, the average particle size is 0.5 to 5 μm and it has good dispersibility.

[0046] In this invention, there is no limitation on the specific type of additive; the additive can be any compound or composition that needs to be added to the polymer melt. For example, the additive can be selected from polymer monomers, solvents, functional additives or modifiers, additives that improve light, heat, climate, chemical stability and physical properties, etc.

[0047] As an alternative, the additive is selected from one or more of nucleating agents, foaming agents, stripping agents, extractants, initiators, molecular weight stabilizers, chain extenders, and additives that improve light, heat, climate, chemical stability, and physical properties.

[0048] In a further step, in step (3), the temperature is controlled at 240–280°C, the pressure is controlled at 0.05–10 kPa, and the duration is 0.5–90 min when removing extractable small molecules.

[0049] In a further embodiment, the relative viscosity of the polyamide melt obtained in step (3) is 2.0 to 5.0.

[0050] In a further step, in step (3), a vacuum condensation system is installed after the devolatilization device to condense and recycle all the small molecule extractable substances volatilized from the melt.

[0051] In a further embodiment, in step (4), the temperature of the spinning box is 230-300℃ and the spinning speed is 500-6000m / min.

[0052] In this invention, the spinning box is equipped with a vacuum suction system to extract and condense the small molecule gas released during the spinning process for recycling and reuse.

[0053] A further approach is to recycle and reuse the small molecules released during the polymerization of polyamide 6 or its copolymer in step (1), the extractable small molecules released in step (3), and the small molecules released in step (4).

[0054] Preferably, after the small molecule substances in each step are recycled, they are returned to step (1) to prepare the melt of polyamide 6 or its copolymer;

[0055] Preferably, the recycling process includes recycling and reusing the small molecule substances from each step through methods such as condensation collection, separation, pyrolysis, or depolymerization.

[0056] As an alternative specific embodiment, the devolatilization device includes a housing, inside which is a rotating element, including a rotating shaft and a rotating film-forming assembly mounted on the rotating shaft, the rotating film-forming assembly rotating with the rotating shaft; a fixed film-stretching component is mounted on the housing wall and extends towards the interior of the housing, cooperating with the rotating film-forming assembly. The rotating film-forming assembly includes a rotating disc and a rotating film-stretching component. The rotating disc is sleeved on the rotating shaft and rotates with the shaft; the radial direction of the rotating disc is perpendicular to the axis of the rotating shaft; the rotating film-stretching component is mounted on the rotating disc. The rotating film-stretching component is configured as one or a combination of several of the following shapes: bar-shaped, plate-shaped, C-shaped, U-shaped, L-shaped, T-shaped, and F-shaped. The fixed film-stretching component is configured as one or a combination of several of the following shapes: bar-shaped, plate-shaped, C-shaped, U-shaped, L-shaped, T-shaped, and F-shaped. In this invention, the fixed film-stretching component and the rotating film-stretching component are shaped correspondingly and are mutually engaged. For example, both the fixed and rotating membrane-stretching components are C-shaped, and the C-shapes of the two components interlock. A channel of a certain width is formed between the side plates of the two components to allow the melt to pass through, increasing the shearing and stretching effects on the melt. This devolatilization device has a high surface renewal rate.

[0057] Secondly, the present invention provides a polyamide 6 or copolymer fiber prepared by the melt direct spinning method of polyamide 6 or its copolymers as described above. The prepared fiber can be a civilian fiber or an industrial fiber, or it can be used for processing other polymer composite fibers.

[0058] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art:

[0059] 1. The direct melt spinning method for polyamide 6 or its copolymers of the present invention involves injecting an additive fluid into the melt of polyamide 6 or its copolymers in a multi-point, high-frequency, and intermittent manner as micro / small droplets or micro / small bubbles. Under the action of a stirring element, the fluid is uniformly distributed within the melt. Then, temperature and pressure are controlled in a devolatilization device to remove extractable small molecules. This creates a large number of uniformly dispersed micro / small bubbles or droplets within the polymer melt, significantly increasing the gas-liquid mass transfer interface. Polymer monomers and poorly soluble oligomers can rapidly diffuse into the bubbles or droplets. Combined with the high surface renewal rate of the devolatilization device, volatiles can be efficiently removed in a short time, resulting in a polymer melt suitable for direct spinning or other subsequent processing.

[0060] In the devolatilization unit, a large number of small bubbles expand rapidly under a high vacuum environment, or droplets vaporize into bubbles. Due to the film-pulling effect of the rotating elements inside the devolatilization unit, it has a strong surface renewal capability. Not only are volatiles continuously removed from the renewed surface, but these bubbles are also constantly exposed to the melt surface and burst. Small molecules such as polymer monomers and oligomers inside the bubbles also enter the gas phase space and are extracted. At this time, the bursting liquid of the bubbles promotes the surface renewal of the melt, realizing the efficient removal of small molecules and obtaining a polymer melt suitable for direct spinning or other subsequent processing. This reduces the need for polymer fibers and products to process the polymer melt into slices and remelt them, further shortening the process flow, reducing energy consumption, and saving investment.

[0061] In addition, by adjusting the types of additives, the molecular weight, physical and chemical properties of polymers can be effectively regulated and controlled, enabling polymer melts to meet the requirements for direct spinning or direct subsequent processing, thereby further shortening the process flow and reducing product costs.

[0062] 2. The method of the present invention can also add various other functional additives and modifying additives, and can also replace various functional masterbatches, eliminating the processes of masterbatch manufacturing and remelting, resulting in significant energy saving and cost reduction.

[0063] 3. The polymer mixing or devolatilization device of the present invention can be improved based on existing devices, and is suitable for mixing, devolatilization and direct spinning of various polymer melts, and is applicable to the field of fibers and products of various types of polymers.

[0064] The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0065] The accompanying drawings, as part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation of the invention. Obviously, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without creative effort. In the drawings:

[0066] Figure 1 This is a schematic flowchart of the melt direct spinning method for polyamide 6 or its copolymers according to the present invention.

[0067] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0068] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0069] Example 1

[0070] (1) Ingredients

[0071] Water, caprolactam, recycled caprolactam, and various additives are mixed in proportion, preheated, and then the required reaction slurry is prepared.

[0072] The slurry contains 2.0 wt% water, 0.4 wt% PTA (molecular weight regulator), 0.3 wt% NH2(CH2)4COOH (catalyst), and 2.5 wt% titanium dioxide.

[0073] After being measured and prepared, each component is stirred evenly in a mixing tank and then sent to a slurry preheater. The preheating temperature of the slurry is >190℃.

[0074] (2) Ring-opening polymerization reaction

[0075] The prepared slurry is fed into an open-loop reactor. The reaction temperature of the open-loop reactor is 260℃ and the reaction pressure is 1.2MPa. Most of the open-loop reaction and the hydrolysis reaction of the oligomers are completed in a short time.

[0076] The open-loop reactor's discharge port is connected to a pressure regulating valve, which reduces the pressure and sends the material to the prepolymerization reactor. The prepolymerization reactor has a flash chamber at the top, heat exchange tubes in the upper middle section, and radial mixing elements at the bottom. The material undergoes ring-opening addition and prepolymerization reactions within the prepolymerization reactor. A dehydrator at the top of the prepolymerization reactor removes moisture and recovers the distilled monomers. The prepolymerized melt exits from the bottom of the prepolymerization reactor. The prepolymerization reaction temperature is 235℃, the reaction pressure is 0.2 MPa, and the melt viscosity of polyamide 6 at discharge reaches approximately 1.7.

[0077] The discharge port of the prepolymerization reactor is connected to a pressure regulating valve, which reduces the pressure and sends the material to the postpolymerization reactor. The postpolymerization reactor has a film distributor at the top, heat exchange tubes in the upper middle section, and coils or radial mixing elements at the bottom. The material undergoes polycondensation in the postpolymerization reactor. A dehydrator is located at the top of the postpolymerization reactor to remove moisture and recover the distilled monomers. After polycondensation, the melt is discharged from the bottom of the postpolymerization reactor by a discharge pump. The reaction temperature for postpolymerization is 250℃, and the reaction pressure is 60 kPa. Upon completion of the postpolymerization reaction, a polyamide 6 melt with a relative viscosity of 2.0–3.8 is obtained.

[0078] (3) Addition and mixing of additives

[0079] The discharge pump at the bottom of the post-polymerizer delivers the polyamide 6 melt into the high-performance mixer. The high-performance mixer has multiple small holes, each 0.5 mm in diameter, regularly distributed on its inner wall and the surface of its stationary components. Distribution and delivery pipes are connected to these holes, and a pulse pressure generator is installed on the delivery pipes. Caprolactam is melted and preheated to 200°C. Then, a pressure wave with a frequency of 100 times / s is generated within the pipe by the pulse pressure generator. This wave is then injected intermittently and at high frequency into the high-performance mixer through the small holes distributed inside. Under the action of the rotating components of the high-performance mixer, the caprolactam liquid is dispersed into tiny droplets and uniformly dispersed within the melt delivered from the post-polymerization reactor. The droplets quickly vaporize to form uniformly distributed small bubbles, allowing the extractable components in the polyamide 6 melt to rapidly diffuse into these small bubbles.

[0080] (4) Removal of volatiles

[0081] The polyamide 6 melt exiting the high-performance mixer is conveyed to a devolatifier under differential pressure. The devolatifier operates at a temperature of 265°C and a pressure of 100 Pa. The devolatifier is a horizontal rotary type, with rotating components exhibiting excellent film-stretching properties and a high melt surface renewal rate. Under high temperature and vacuum conditions, on the one hand, the continuously renewed melt surface allows extractable small molecules within the melt to diffuse out continuously; on the other hand, small bubbles introduced by the high-performance mixer or mixing components expand and are exposed to the melt surface, subsequently rupturing and releasing the extractable substances diffused into the small bubbles. Through this combined effect, most of the extractable small molecules in the polyamide melt can be removed, achieving an extractable small molecule content of ≤1.0 wt%. After devolatation, the relative viscosity of the polyamide 6 melt is 2.0–5.0.

[0082] The top of the devoluting unit is equipped with a vacuum condensation system, which condenses all the small molecule extractables volatilized from the melt and sends them to the recycling unit for reuse. The bottom of the devoluting unit has a discharge port, which is connected to a discharge device and is pumped to the spinning unit by a booster pump.

[0083] (5) Spinning

[0084] The polyamide 6 melt that has undergone de-volatileization is transported by a booster pump to the spinning box for direct spinning.

[0085] The spinning box is equipped with a vacuum suction system to extract and condense the small molecule gases released during the spinning process, and send them to the recycling unit for reuse.

[0086] (6) Volatile matter recovery:

[0087] The volatile components collected by condensation in steps (2), (4), and (5) are further recovered and returned to the batching process. The recovery unit includes collection, separation, and purification, separating out water and solid residues, and sending caprolactam and its oligomers back to step (1) for reuse.

[0088] Example 2

[0089] (1) Ingredients

[0090] Water, caprolactam, recycled caprolactam, and various additives are mixed in proportion, preheated, and then the required reaction slurry is prepared.

[0091] The slurry contains 1.5 wt% water, 0.15 wt% PTA (molecular weight adjuster), 0.2 wt% nylon 66 salt (catalyst), and 0.14 wt% antioxidant 1010 (antioxidant).

[0092] After being measured and prepared, the components are stirred evenly in a mixing tank and then sent to a slurry preheater. The slurry is preheated at a temperature of 200°C.

[0093] (2) Ring-opening polymerization reaction

[0094] The prepared slurry is fed into the prepolymerization reactor. The prepolymerization reactor is equipped with a liquid distributor at the top, heat exchange tubes in the upper middle section, and radial mixing elements at the bottom. The material undergoes ring-opening, addition, and prepolymerization reactions within the prepolymerization reactor. A dehydrator is located at the top of the prepolymerization reactor to remove moisture and recover the distilled monomers. The prepolymerized melt exits from the bottom of the prepolymerization reactor. The prepolymerization reaction temperature is 240℃, the reaction pressure is 0.2 MPa, and the melt viscosity of polyamide 6 at discharge reaches approximately 1.7.

[0095] A gear pump is connected to the outlet of the prepolymerization reactor to deliver the material to the postpolymerization reactor. The postpolymerization reactor is equipped with a film distributor at the top, heat exchange tubes in the upper middle section, and coils or radial mixing elements at the bottom. The material undergoes polycondensation in the postpolymerization reactor. A dehydrator is located at the top of the postpolymerization reactor to remove moisture and recover the distilled monomers. After polycondensation is complete, the melt is discharged from the bottom of the postpolymerization reactor by a discharge pump. The reaction temperature for postpolymerization is 260℃, and the reaction pressure is 50 kPa. Upon completion of the postpolymerization reaction, a polyamide 6 melt with a relative viscosity of 2.0–3.8 is obtained.

[0096] (3) Addition of adjuvants and mixing and dispersion

[0097] The discharge pump at the bottom of the post-polymerizer delivers the polyamide 6 melt into the high-performance mixer. The inner wall of the high-performance mixer and the surface of the internal stationary parts are regularly distributed with multiple small holes, each with a diameter of 1.2 mm. The holes are connected to distribution pipes and conveying pipes, and the conveying pipes are equipped with pulse pressure generators.

[0098] Nitrogen gas containing a small amount of moisture is preheated to 180-260°C, and then a pressure wave with a pulse frequency of 100 times / s is generated in the pipeline through a pulse pressure generator. The pressure wave is then injected into the high-performance mixer at high frequency and intermittently through small holes distributed inside the high-performance dynamic mixer. At the same time, under the action of the rotating parts of the high-performance mixer, the nitrogen gas is dispersed into tiny bubbles and evenly dispersed in the melt delivered from the post-polymerization reactor, so that the extractable substances in the polyamide 6 melt can quickly diffuse into the tiny bubbles.

[0099] (4) Removal of volatiles

[0100] The polyamide 6 melt exiting the high-performance mixer is conveyed to the devolatizer under pressure differential. The devolatizer operates at a temperature of 270°C and a pressure of 100 Pa. The devolatizer is a vertical rotary type, and the rotating components have a good scraping effect, resulting in a high melt surface renewal rate and heat exchange efficiency. Under high temperature and vacuum conditions, on the one hand, the continuously renewed melt surface allows extractable small molecules inside the melt to continuously diffuse out; on the other hand, small bubbles added by the high-performance mixer or mixing components expand and are exposed to the melt surface and then rupture, releasing the extractable substances diffused into the small bubbles. Under the combined effect, most of the extractable small molecules in the polyamide melt can be removed, bringing the content of extractable small molecules to ≤1.0 wt%. After devolatation, the relative viscosity of the polyamide 6 melt is 2.0 to 5.0.

[0101] The top of the devoluting unit is equipped with a vacuum condensation system, which condenses all the small molecule extractables volatilized from the melt and sends them to the recycling unit for reuse. The bottom of the devoluting unit has a discharge port, which is connected to a discharge device and is pumped to the spinning unit by a booster pump.

[0102] (5) Spinning

[0103] The polyamide 6 melt that has undergone de-volatileification is transported by a booster pump to the spinning box for direct spinning. The spinning box is equipped with a vacuum suction system to extract and condense the small molecule gases released during the spinning process, and send them to the recycling unit for reuse.

[0104] (6) Volatile matter recovery:

[0105] The volatiles collected by condensation in steps (2), (4), and (5) are further recovered and returned to the batching process. The recovery unit includes collection, separation, hydrolysis, or pyrolysis to separate water, decompose caprolactam oligomers into caprolactam monomers, and send them back to step (1) for reuse.

[0106] Example 3

[0107] (1) Ingredients

[0108] Water, caprolactam, recycled caprolactam, and various additives are mixed in proportion, preheated, and then the required reaction slurry is prepared.

[0109] The slurry contains 2.5 wt% water, 0.2 wt% PTA (molecular weight adjuster), 0.15 wt% nylon 66 salt (catalyst), and 0.12 wt% antioxidant 1010 (antioxidant).

[0110] After being measured and prepared, each component is stirred evenly in a mixing tank and then sent to a slurry preheater. The preheating temperature of the slurry is >190℃.

[0111] (2) Ring-opening polymerization reaction

[0112] The prepared slurry is fed into the polymerization reactor. The reactor is equipped with a stirrer and heat exchange tubes at the top, heat exchange tubes in the upper middle section, and radial mixing elements and heat exchange coils in the middle and lower sections. The material undergoes ring-opening and addition reactions in the upper part of the pre-polymerization reactor, addition and prepolymerization reactions in the upper middle section, and polymerization in the lower middle section. A dehydrator is installed at the top of the polymerization reactor to remove moisture and recover distilled monomers. The polymerized melt is discharged from the bottom of the polymerizer. The operating temperature of the polymerization reactor is 250℃, the operating pressure is 120 kPa, and the relative viscosity of the polyamide 6 melt at discharge is approximately 2.0–2.8.

[0113] (3) Addition of adjuvants and mixing and dispersion

[0114] The discharge pump at the bottom of the post-polymerizer delivers the polyamide 6 melt into the high-performance mixer. Multiple small holes, each 3.0 mm in diameter, are regularly distributed on the inner wall and surface of the stationary components inside the high-performance mixer. Pipes and a pulse pressure generator are connected to the outside of these holes. The melt or solution containing functional additives is preheated to 250°C. Then, a pressure wave with a frequency of 50 times / s is generated within the pipes by the pulse pressure generator. This wave is then injected intermittently and at high frequency into the high-performance mixer through the distributed small holes. Simultaneously, under the action of the rotating components of the high-performance mixer, the melt or liquid is dispersed into small droplets and uniformly dispersed within the melt delivered from the post-polymerization reactor. At this point, the fine particles or droplets within the droplets act as bubble nucleating agents, generating a large number of small bubbles. This allows the extractable components in the polyamide 6 melt to rapidly diffuse into these small bubbles.

[0115] (4) Removal of volatiles

[0116] The polyamide 6 melt exiting the high-performance mixer is conveyed to a devolatizer under differential pressure. The devolatizer operates at a temperature of 270°C and a pressure of 100 kPa. The devolatizer is a horizontal, biaxially rotating type. The rotating components have excellent film-stretching and kneading effects, resulting in a high melt surface renewal rate and heat exchange efficiency. Functional additives are uniformly dispersed into the melt under the stretching and shearing action of the rotating components. Simultaneously, under high temperature and vacuum conditions, the continuously renewed melt surface allows extractable small molecules to diffuse out continuously. Furthermore, small bubbles in the melt expand and rupture upon reaching the melt surface, releasing the extractable substances diffused into the bubbles. This combined effect removes most of the extractable small molecules from the polyamide melt, achieving an extractable small molecule content of ≤1.0 wt%. After devolatation, the relative viscosity of the polyamide 6 melt is 2.0–5.0.

[0117] The top of the devoluting unit is equipped with a vacuum condensation system, which condenses all the small molecule extractables volatilized from the melt and sends them to the recycling unit for reuse. The bottom of the devoluting unit has a discharge port, which is connected to a discharge device and is pumped to the spinning unit by a booster pump.

[0118] (5) Spinning

[0119] The polyamide 6 melt that has undergone de-volatileization is transported by a booster pump to the spinning box for direct spinning.

[0120] The spinning box is equipped with a vacuum suction system to extract and condense the small molecule gases released during the spinning process, and send them to the recycling unit for reuse.

[0121] (6) Volatile matter recovery:

[0122] The volatiles collected by condensation in steps (2), (4), and (5) are further recovered and returned to the batching process. The recovery unit includes collection, separation, hydrolysis, or pyrolysis to separate water, decompose caprolactam oligomers into caprolactam monomers, and send them back to step (1) for reuse.

[0123] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for direct melt spinning of polyamide 6 or its copolymers, characterized in that, Includes the following steps: (1) Prepare a melt of polyamide 6 or its copolymer; (2) The additive fluid is injected into the melt of polyamide 6 or its copolymer in a multi-point, high-frequency, and intermittent manner in the form of micro / small droplets or micro / small bubbles, and further uniformly dispersed in the melt by stirring; (3) The mixed melt is subjected to temperature and pressure control in a devolatilization device to remove extractable small molecules. The content of extractable small molecules in the obtained polyamide melt is ≤1.0wt%. (4) The polyamide melt obtained in (3) is directly spun or otherwise processed.

2. The melt direct spinning method for polyamide 6 or its copolymers according to claim 1, characterized in that, In step (2), the additive fluid is injected into the melt of polyamide 6 or its copolymer through an additive injection port provided on the mixing device or devolatilization device, wherein the diameter or width of the additive injection port is 0.1 to 3 mm; Preferably, the diameter or width of the additive injection port is 0.3-1.5 mm; Preferably, the mixing device is located upstream of the devolatilization device, and the additive fluid is injected into the melt of polyamide 6 or its copolymer through one or more additive injection ports on the mixing device.

3. The melt direct spinning method for polyamide 6 or its copolymers according to claim 1, characterized in that, In step (2), when the additive fluid is injected into the melt of polyamide 6 or its copolymer in a high-frequency, intermittent manner, the pulse frequency of the additive fluid is 6 to 1200 times / second. Preferably, the pulse frequency of the additive fluid is 20 to 300 times per second.

4. The melt direct spinning method for polyamide 6 or its copolymers according to claim 1, characterized in that, In step (2), the additive is a gas, liquid, or solid, or a combination of two or three states of matter; Preferably, when the additive is a solid, the average particle size is 0.5–5 μm; Preferably, the additive is selected from one or more of nucleating agents, foaming agents, stripping agents, extractants, initiators, molecular weight stabilizers, chain extenders, and additives that improve light, heat, climate, chemical stability, and physical properties.

5. The method for direct melt spinning of polyamide 6 or its copolymers according to any one of claims 1-4, characterized in that, In step (3), the temperature is controlled at 240-280℃, the pressure is controlled at 0.05-10kPa, and the duration is 0.5-90min when removing extractable small molecules.

6. The method for direct melt spinning of polyamide 6 or its copolymers according to any one of claims 1-4, characterized in that, The relative viscosity of the polyamide melt obtained in step (3) is 2.0 to 5.

0.

7. The method for direct melt spinning of polyamide 6 or its copolymers according to any one of claims 1-4, characterized in that, In step (1), water, caprolactam, comonomers, and additives are prepared in proportion, mixed, preheated, and formulated into a reaction slurry; then polymerization is carried out to prepare a melt of polyamide 6 or its copolymer. Preferably, the mass ratio of water, caprolactam, comonomer, and additive is (0.012-0.035):1:(0.001-0.06). Preferably, the additive is selected from one or more of the following: catalyst, matting agent, molecular weight regulator, antioxidant, UV stabilizer, colorant, flame retardant, and antibacterial agent; Preferably, the preheating temperature of the reaction slurry is 130–230°C.

8. The melt direct spinning method for polyamide 6 or its copolymers according to claim 7, characterized in that, The polymerization process is selected from one or more of the following: atmospheric pressure continuous polymerization, two-stage polymerization, and multi-stage continuous polymerization. Preferably, when using a two-stage polymerization method, it includes prepolymerization and postpolymerization. The reaction temperature of the prepolymerization is 180-280℃, the reaction pressure is 0.1-3.2MPa, and the reaction time is 2-15 hours; the reaction temperature of the postpolymerization is 230-270℃, the reaction pressure is 10-150kPa, and the reaction time is 1-10 hours. Preferably, when using the multi-stage continuous polymerization method, the reaction temperature of the open-loop reactor is 250–280°C, the reaction pressure is 0.2–5.6 MPa, and the reaction time is 5–40 minutes; Preferably, after the polymerization reaction is completed, a polyamide 6 or its copolymer melt with a relative viscosity of 1.5 to 2.8 is obtained.

9. The method for direct melt spinning of polyamide 6 or its copolymers according to any one of claims 1-4, characterized in that, The small molecules released during the polymerization of polyamide 6 or its copolymer in step (1), the extractable small molecules released in step (3), and the small molecules released in step (4) are recycled and reused. Preferably, after the small molecule substances in each step are recycled, they are returned to step (1) to prepare the melt of polyamide 6 or its copolymer; Preferably, the recycling process includes recycling and reusing the small molecule substances from each step through methods such as condensation collection, separation, pyrolysis, or depolymerization.

10. A polyamide 6 fiber prepared by melt direct spinning of polyamide 6 or its copolymers as described in any one of claims 1-9.

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