A production method for efficiently preparing flame-retardant monofilaments

Abstract

This invention relates to the field of fiber preparation technology and discloses a highly efficient method for preparing flame-retardant monofilaments. The method includes: surface activation of a polyamide matrix using radio frequency plasma to directionally introduce carboxyl functional groups; surface grafting modification of a composite flame-retardant masterbatch using a silane coupling agent to construct an interfacial transition layer; controlling the moisture content of the raw materials using a microwave-airflow combined drying system; achieving a gradient distribution of the flame retardant using a dual-channel spinning assembly; synergistically dispersing the nano-reinforcing phase and the flame retardant to construct a three-dimensional network structure, and forming nascent monofilaments using the spinning assembly; controlling the crystal structure through two-stage stretching involving wetting pre-orientation and dry heat main orientation; and stabilizing the fiber structure using a segmented temperature-controlled thermal relaxation process to obtain a finished monofilament product with flame-retardant properties. This invention achieves efficient and continuous production of flame-retardant monofilaments through the synergistic effect of plasma activation, gradient spinning, and staged orientation.

Description

Technical Field

[0001] This invention relates to the field of fiber preparation technology, and in particular to a production method for efficiently preparing flame-retardant monofilaments. Background Technology

[0002] Currently, flame-retardant monofilaments are mainly prepared using physical blending of flame retardants or surface coating processes. While traditional methods can achieve a certain degree of flame retardancy, they generally suffer from uneven dispersion of the flame retardant and weak adhesion to the matrix material. Monofilaments prepared by these processes often exhibit unstable flame-retardant properties and are prone to flame retardant migration and failure during long-term use.

[0003] In existing technologies, poor interfacial compatibility between flame retardants and matrix materials is a major bottleneck affecting monofilament performance. Conventional physical blending methods struggle to achieve uniform dispersion of flame retardants in the matrix, leading to a significant decrease in the mechanical properties of the monofilament. Meanwhile, while simple surface coating processes can maintain the mechanical properties of the matrix material, the durability of the flame-retardant layer is insufficient to meet long-term use requirements.

[0004] In terms of manufacturing processes, existing methods often require multiple steps to produce flame-retardant monofilaments, resulting in low production efficiency and difficulty in ensuring consistent product performance. This is especially true for polyamide materials, whose unique molecular structure makes the introduction and dispersion of flame retardants more challenging, making it difficult for traditional processes to maintain the material's mechanical properties while ensuring flame retardant efficacy.

[0005] Therefore, this invention proposes a production method for efficiently preparing flame-retardant monofilaments, which solves many problems existing in the prior art through innovative process design and material modification technology. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of poor flame retardancy, low mechanical properties, and low production efficiency in the prior art, and to propose a production method for efficiently preparing flame-retardant monofilaments.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a production method for efficiently preparing flame-retardant monofilaments, comprising the following steps:

[0008] Step S1: The polyamide matrix material is surface activated by controlled atmosphere radio frequency plasma to directionally introduce carboxyl active functional groups on the surface of the matrix material.

[0009] Step S2: The composite flame retardant masterbatch is surface grafted and modified using a reactive silane coupling agent to construct an interfacial transition layer that is chemically bonded to the polyamide matrix.

[0010] Step S3: Through a synergistic microwave-airflow combined drying system, the polyamide matrix material and the composite flame retardant masterbatch are brought to a spinnable moisture content state.

[0011] Step S4: A dual-channel spinning assembly with a differentiated flow channel structure is used to achieve a gradient distribution of flame retardant with a high outer and low inner diameter in the cross-section of the single filament.

[0012] Step S5: The topologically reinforced nano-phase is synergistically dispersed with the flame retardant to form a three-dimensional network structure in the melt, and then the nascent monofilament is formed through the spinning assembly.

[0013] Step S6: The orientation and crystal structure of the single filament is controlled by stretching processes in the wetting pre-orientation stage and the dry heat main orientation stage.

[0014] Step S7: A segmented temperature-controlled thermal relaxation process is used to stabilize the fiber structure, resulting in a finished monofilament product with flame-retardant properties.

[0015] Furthermore, step S1 also includes the following sub-steps:

[0016] S1-1, The polyamide matrix material is placed in a closed radio frequency plasma reaction chamber, and a mixed atmosphere of inert gas and oxygen-containing active gas is introduced to establish a plasma treatment environment. The polyamide matrix material includes aliphatic polyamide, semi-aromatic polyamide, aromatic polyamide, copolymerized modified polyamide and reinforced modified polyamide.

[0017] S1-2 uses an intermittent pulsed discharge mode to excite plasma, which generates active free radical sites on the surface of the substrate material and reacts in an oxygen-containing atmosphere to generate carboxyl active functional groups.

[0018] S1-3 After the treatment is completed, the matrix material is transferred to a sealed container filled with protective gas to prevent the decomposition of the carboxyl active functional groups on the surface of the matrix material.

[0019] Furthermore, step S2 also includes the following sub-steps:

[0020] S2-1, the composite flame retardant masterbatch is immersed in a treatment solution containing a reactive functional group silane coupling agent, and under the assistance of ultrasound, silane molecules are grafted onto the surface of the composite flame retardant masterbatch through chemical bonding to form a reactive interfacial transition layer. The composite flame retardant masterbatch includes a flame retardant, a dispersant and a carrier resin.

[0021] S2-2, the surface-modified composite flame retardant masterbatch and polyamide matrix are added to a twin-screw extruder, and the barrel temperature is controlled between the polyamide melting temperature and the silane coupling agent decomposition temperature for melt blending;

[0022] S2-3, in a twin-screw extruder, controls the screw speed to disperse the composite flame-retardant masterbatch in the polyamide matrix through shearing and maintains the melt residence time, so as to achieve chemical bonding between the active functional groups of the silane coupling agent and the polyamide molecular chain.

[0023] Furthermore, step S3 also includes the following sub-steps:

[0024] S3-1 uses microwave-assisted vacuum drying to remove moisture from the polyamide matrix material and composite flame retardant masterbatch;

[0025] S3-2 controls the temperature distribution of the polyamide matrix material and composite flame retardant masterbatch during the drying process by adjusting the gas flow rate and microwave power density in the drying environment, thus avoiding local overheating that could lead to degradation.

[0026] S3-3, the moisture content of the polyamide matrix material and the composite flame retardant masterbatch is monitored in real time using an online moisture detection device. Drying is terminated when the spinnable moisture content standard is reached, which is a moisture content of less than 300 ppm.

[0027] Furthermore, step S4 also includes the following sub-steps:

[0028] S4-1 delivers the flame retardant masterbatch melt to the outer channel of the dual-channel spinning assembly, controls the rheological properties of the melt to maintain a stable flow state, and promotes the enrichment of flame retardant to the outer layer through the channel structure design.

[0029] S4-2, the polyamide matrix melt is transported through independent core layer flow channels to maintain the laminar flow state of the melt and form a continuous core layer structure;

[0030] S4-3, adjust the flow ratio of the outer layer and the core layer melt, and utilize the difference in rheological properties between the flame retardant melt and the matrix melt to achieve a gradient distribution of flame retardant concentration that decreases from the outside to the inside.

[0031] Furthermore, step S5 also includes the following sub-steps:

[0032] S5-1 involves adding a nano-reinforced phase with a topological structure and a flame retardant to a co-rotating twin-screw extruder, and forming a uniformly dispersed system through melt blending.

[0033] S5-2, under the process conditions of controlling the mixing temperature, shear rate and residence time, melt blending is carried out to form a preliminary dispersed network structure of the nano-reinforcing phase in the matrix. The mixing temperature is maintained between the melting temperature of the matrix material and the thermal stability temperature of the nano-reinforcing phase. The shear rate can overcome the van der Waals forces between nanoparticles. The residence time can fully disperse the nano-reinforcing phase.

[0034] S5-3, through the synergistic action of the kneading block and the anti-thread element in the screw assembly, applies a shear-stretch composite flow field during the twin-screw extrusion process, induces the nano-reinforced phase to be oriented along the melt flow direction, and utilizes the chemical interaction of particle surfaces to achieve inter-phase overlap, forming a three-dimensional continuous network structure that runs through the melt.

[0035] S5-4, a hydrodynamically optimized spinning assembly is used to extrude the composite melt to obtain a nascent monofilament with a complete three-dimensional network structure. The spinning assembly includes a spinneret micro-orifice, a conical spinneret inlet, and a tapered outlet.

[0036] Furthermore, step S6 also includes the following sub-steps:

[0037] S6-1, the nascent monofilament is introduced into a wetting environment, and the ambient temperature is controlled to be lower than the glass transition temperature of the polyamide. The wetting environment includes steam and a constant temperature water bath.

[0038] S6-2, apply initial tensile force to cause the molecular chains to partially align along the fiber axis, forming a preliminary oriented crystal nucleus structure;

[0039] S6-3, the pre-oriented monofilaments are transferred to a dry heat environment, and the ambient temperature is controlled to be higher than the glass transition temperature of polyamide but lower than the melting point temperature of polyamide. Secondary stretching is performed to promote the growth of crystalline regions and form a stable crystalline network structure. The dry heat environment includes a hot air box and a heating roller group.

[0040] Furthermore, step S7 also includes the following sub-steps:

[0041] S7-1 involves a gradient heating-holding-cooling heat treatment process on oriented fibers under inert gas protection, which allows the molecular chains to reach a thermodynamically stable state through segmented relaxation.

[0042] S7-2 coordinates the matching relationship between fiber tension and ambient temperature during the thermal relaxation process, realizes the gradual release of residual stress, and completes the final shaping of the fiber structure.

[0043] S7-3 employs a three-stage cooling system to cool the heat-treated fibers to room temperature while maintaining tension, thereby fixing the three-dimensional flame-retardant network structure and producing a flame-retardant monofilament product.

[0044] The beneficial effects of the technical solution provided by this invention include at least the following:

[0045] This invention utilizes plasma activation and silane coupling agent modification to directionally introduce carboxyl active functional groups onto the surface of polyamide, and constructs a strong interfacial transition layer through chemical bonding, thereby enhancing the bonding force between the flame retardant and the matrix and preventing interfacial delamination.

[0046] This invention achieves a non-uniform distribution of flame retardant with a high outer layer and a low inner layer through a dual-channel spinning gradient distribution. This reduces the total amount of flame retardant used while maximizing the surface flame retardant efficiency and maintaining the mechanical properties of the core layer.

[0047] This invention constructs a three-dimensional flame-retardant network, which can form a three-dimensional interpenetrating network that runs through the melt through the synergistic dispersion of the topologically structured nano-reinforced phase and the flame retardant, thereby improving both flame retardancy and tensile strength.

[0048] This invention enables efficient and continuous production of flame-retardant monofilaments through optimized staged stretching and thermal relaxation processes, significantly improving production efficiency while ensuring product performance stability. Attached Figure Description

[0049] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a flowchart of a method provided in an embodiment of the present invention. Detailed Implementation

[0051] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effects of a method for efficiently preparing flame-retardant monofilaments according to the present invention. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0053] The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0054] The following describes in detail, with reference to the accompanying drawings, a specific scheme for a production method of efficiently preparing flame-retardant monofilaments provided by the present invention.

[0055] Please see Figure 1 The diagram illustrates a process flow chart of an efficient method for preparing flame-retardant monofilaments according to an embodiment of the present invention. The method includes the following steps:

[0056] Step S1: The polyamide matrix material is surface activated by controlled atmosphere radio frequency plasma to directionally introduce carboxyl active functional groups on the surface of the matrix material.

[0057] Step S1 further includes the following sub-steps:

[0058] S1-1, The polyamide matrix material is placed in a closed radio frequency plasma reaction chamber, and a mixed atmosphere of inert gas and oxygen-containing active gas is introduced to establish a plasma treatment environment. The polyamide matrix material includes aliphatic polyamide, semi-aromatic polyamide, aromatic polyamide, copolymerized modified polyamide and reinforced modified polyamide.

[0059] S1-2 uses an intermittent pulsed discharge mode to excite plasma, which generates active free radical sites on the surface of the substrate material and reacts in an oxygen-containing atmosphere to generate carboxyl active functional groups.

[0060] S1-3 After the treatment is completed, the matrix material is transferred to a sealed container filled with protective gas to prevent the decomposition of the carboxyl active functional groups on the surface of the matrix material.

[0061] It should be noted that the polyamide matrix material: polyamide (commonly known as nylon) is a class of polymeric materials whose main chain contains repeating amide groups (-CONH-). As the matrix material of this invention, its characteristic is that the polar amide bonds in the molecular structure provide good reactivity, including aliphatic polyamide, semi-aromatic polyamide, aromatic polyamide, copolymerized modified polyamide and reinforced modified polyamide.

[0062] Aliphatic polyamides include PA6 (polycaprolactam), PA66 (polyhexamethylene adipamide), PA610 (polyhexamethylene sebacate), PA612 (polyhexamethylene dodecanoate), PA11 (polyundecanoamide), and PA12 (polydodecanoamide).

[0063] Semi-aromatic polyamides include PA6T (polyhexamethylene terephthalamide), PA9T (polynonyl terephthalamide), and PA10T (polydecyl terephthalamide).

[0064] Aromatic polyamides include fully aromatic polyamides (PPTA, poly(p-phenylene terephthalamide)).

[0065] Copolymer-modified polyamides include PA6 / 66 copolymers, PA6 / 12 copolymers, and other multi-component copolymer polyamides.

[0066] Reinforced and modified polyamides: glass fiber reinforced polyamides, mineral-filled polyamides, and other polyamide materials modified with other reinforcement systems.

[0067] The properties of polyamide matrix materials include: melt index (MI) in the range of 200-400 g / 10 min (2.16 kg, 230 degrees Celsius); relative viscosity in the range of 2.4-3.2; melting point between 200-280 degrees Celsius; and glass transition temperature in the range of 40-80 degrees Celsius.

[0068] The polyamide matrix materials particularly suitable for this invention are PA6, PA66 and their copolymers, because they have: good melt processing properties, excellent mechanical strength, good compatibility with flame retardant systems, moderate moisture absorption properties and high thermal stability.

[0069] Radio frequency plasma reaction chambers refer to specialized vacuum containers used to generate plasma. They are usually made of stainless steel, equipped with radio frequency electrodes (13.56MHz), and include gas inlets and outlets, vacuum systems, and matching networks. The operating pressure is maintained at a low pressure environment of 10-50Pa. They adopt a parallel plate electrode structure, and the electrode spacing is controlled at 50-100mm to ensure plasma uniformity.

[0070] Inert gas: In this process, it refers to a chemically stable gas, commonly argon (Ar) or nitrogen (N2). Its main function is to maintain plasma discharge and transfer energy. The purity requirement is greater than or equal to 99.99% (to avoid impurities interfering with the reaction).

[0071] Oxygen-containing reactive gases: gaseous components that participate in surface chemical reactions, usually oxygen or nitrogen-doped oxygen, used to provide reactive oxygen atoms to react with free radicals.

[0072] Mixed atmosphere: The gas mixture of inert gas and oxygen-containing gas. The optimal volume ratio of inert gas to oxygen-containing gas should be in the range of 4:1 to 9:1, which can ensure plasma stability and provide sufficient active oxygen.

[0073] Plasma treatment environment: refers to the reaction conditions for material surface modification, with parameters including electron temperature: 1-10 eV, ion density: 10 9 -10 11 / cm 3 The substrate temperature is less than 80 degrees Celsius (to prevent heat damage).

[0074] Intermittent pulsed discharge mode: an energy control method with a working cycle of 1-10ms pulse width and 0.5-5ms interval. This mode avoids material overheating caused by continuous discharge, improves the utilization efficiency of high-energy particles, and extends the lifetime of active free radicals.

[0075] The parameters for pulsed discharge are: frequency 13.56MHz (industrial standard frequency), duty cycle controlled at 30%-70% to avoid thermal damage to materials, and power density maintained at 0.5-2W / cm². 2 .

[0076] Active free radical sites: unpaired electronic structures generated on the material surface, formed by plasma bombardment causing CH / CC bonds to break, including alkyl free radicals (R·) and peroxy free radicals (ROO·).

[0077] Carboxyl active functional group: The key surface group (-COOH) introduced has strong polarity (the contact angle can be reduced to below 30 degrees), pKa=4.5 (can participate in acid-base reactions), and has high reactivity with silane coupling agents.

[0078] Processing time for generating carboxyl active functional groups in an oxygen-containing atmosphere: Adjusted according to the material thickness, usually 5-30 minutes to form sufficient carboxyl density.

[0079] Protective gas: The gas used to prevent the modified surface from being oxidized. High-purity nitrogen (purity greater than or equal to 99.999%) or inert gases such as argon are preferred.

[0080] Transfer: The transfer should be completed within 10 minutes after treatment. The dew point of the transfer environment should be less than or equal to -40 degrees Celsius, and the oxygen content in the container should be controlled to be less than or equal to 50 ppm.

[0081] Decomposition of carboxyl active functional groups: They are easily decarboxylated at temperatures above 150 degrees Celsius, undergo photolysis under UV irradiation, and are catalyzed by transition metal ions.

[0082] Step S2: The composite flame retardant masterbatch is surface grafted and modified using a reactive silane coupling agent to construct an interfacial transition layer that is chemically bonded to the polyamide matrix.

[0083] Step S2 further includes the following sub-steps:

[0084] S2-1, the composite flame retardant masterbatch is immersed in a treatment solution containing a reactive functional group silane coupling agent. Under the assistance of ultrasound, silane molecules are grafted onto the surface of the composite flame retardant masterbatch through chemical bonding to form a reactive interfacial transition layer. The composite flame retardant masterbatch includes flame retardant, dispersant and carrier resin.

[0085] S2-2, the surface-modified composite flame retardant masterbatch and polyamide matrix are added to a twin-screw extruder, and the barrel temperature is controlled between the polyamide melting temperature and the silane coupling agent decomposition temperature for melt blending;

[0086] S2-3, in a twin-screw extruder, controls the screw speed to disperse the composite flame-retardant masterbatch in the polyamide matrix through shearing and maintains the melt residence time, so as to achieve chemical bonding between the active functional groups of the silane coupling agent and the polyamide molecular chain.

[0087] It should be noted that the core of step S2 is to construct a chemical bonding interface through a reactive silane coupling agent. The technological breakthrough lies in realizing the transformation of flame retardant-matrix from physical mixing to chemical bonding; ultrasonic assistance increases the silane grafting rate by more than 40%; and forms an interface transition layer with a thickness of nanometers (about 10-50 nm).

[0088] Composite flame retardant masterbatch: A pre-dispersed flame retardant functional material, consisting of cylindrical particles with a particle size of 1-3 mm and a melt index 10-20% higher than the matrix resin. It includes a core structure: flame retardant (30-70 wt%) as the functional host to impart flame retardant properties; a dispersion system: dispersant (1-5 wt%) to ensure uniform distribution and improve the dispersibility of the flame retardant; and a carrier framework: carrier resin (25-69 wt%) to provide processing performance.

[0089] Flame retardants include inorganic flame retardants, organic flame retardants, and organic-inorganic hybrid flame retardants, among which:

[0090] Inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, zinc borate, and expanded graphite;

[0091] Organic flame retardants include ammonium polyphosphate, melamine cyanurate, and phosphazene compounds;

[0092] Organic-inorganic hybrid flame retardants include organosilicon-encapsulated nano-metal oxides and phosphorus-nitrogen synergistically modified layered silicates.

[0093] Reactive functional group silane coupling agents: a class of bifunctional molecules with the general structural formula YR-Si(OR')3, where Y: reactive functional group (amino, epoxy), R: short-chain alkyl (-(CH2)3-), and OR': hydrolyzable group (methoxy or ethoxy).

[0094] Functional group silane coupling agents refer to silane compounds containing active functional groups that can chemically react with the polyamide matrix, including:

[0095] Aminosilanes: 3-aminopropyltriethoxysilane (KH550), N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane (KH792);

[0096] Epoxysilane: γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560);

[0097] Mercaptosilane: γ-mercaptopropyltrimethoxysilane (KH590);

[0098] Methacryloxysilane: γ-methacryloxypropyltrimethoxysilane (KH570).

[0099] The preferred silane coupling agent is an aminosilane (KH-550) or an epoxysilane, which can react the amino group with the carboxyl group at the end of the polyamide.

[0100] The treatment solution refers to a liquid mixture of ethanol and water (volume ratio 95:5), which can promote the hydrolysis of silanes to form Si-OH.

[0101] Ultrasonic-assisted: Utilizing the physical effects of ultrasound (20-100kHz), it is possible to generate localized high temperatures of 5000K and high pressures of 1000atm, with speeds reaching 100m / s and accelerations greater than 10. 5 g.

[0102] The parameters for this process are set as follows: frequency 40±5kHz, power density 50-200W / L, and processing time 10-30min.

[0103] Reactive interfacial transition layer: refers to the chemically bonded region constructed between the flame retardant and the matrix, with a thickness of 10-50 nm. Its structural features include silane molecules anchored by Si-OM bonds (M = metal on the surface of the flame retardant), and organic ends connected to the matrix by covalent bonds. Its performance indicators are an interfacial shear strength greater than or equal to 15 MPa and thermal stability greater than 300 degrees Celsius.

[0104] Control the barrel temperature: the polyamide melting temperature is greater than or equal to 260 degrees Celsius, and the silane decomposition temperature is less than 270 degrees Celsius.

[0105] Screw speed: 300-500 rpm.

[0106] Shear: A key rheological behavior in polymer processing, with shear rates of 500-1500 s⁻¹ -1 .

[0107] Residence time: refers to the length of time the material stays in the extruder. The total residence time is 2-5 minutes, and the residence time in the reaction zone is greater than or equal to 1 minute.

[0108] Step S3: Through a synergistic microwave-airflow combined drying system, the polyamide matrix material and the composite flame retardant masterbatch are brought to a spinnable moisture content state.

[0109] Step S3 further includes the following sub-steps:

[0110] S3-1 uses microwave-assisted vacuum drying to remove moisture from the polyamide matrix material and composite flame retardant masterbatch;

[0111] S3-2 controls the temperature distribution of the polyamide matrix material and composite flame retardant masterbatch during the drying process by adjusting the gas flow rate and microwave power density in the drying environment, thus avoiding local overheating that could lead to degradation.

[0112] S3-3 utilizes an online moisture detection device to monitor the moisture content of the polyamide matrix material and the composite flame retardant masterbatch in real time. Drying is terminated when the spinnable moisture content standard is reached, which is a moisture content of less than 300 ppm.

[0113] It should be noted that microwave-assisted vacuum drying is a composite drying technology that combines the effects of two types of energy.

[0114] Microwave energy: 2.45GHz±50MHz electromagnetic waves cause water molecules to rotate and generate heat (penetration depth 5-20cm).

[0115] Vacuum environment: pressure 0.08-0.1MPa (absolute pressure), lowering the boiling point of water to 45-60 degrees Celsius.

[0116] Gas flow rate: the volume of gas passing through a unit cross-sectional area per unit time (m³). 3 / (m 2 • min), velocity range: 0.5-2 m / s (material layer), laminar flow state (Reynolds number Re < 2100) and microwave power ratio: 1 W microwave power corresponds to 0.1-0.3 m 3 / h air flow rate.

[0117] Microwave power density: a key parameter characterizing microwave energy intensity. In the early stage of drying, microwave power density is 1.5-2 W / g for rapid heating. In the middle stage of drying, microwave power density is 0.8-1.2 W / g for constant-rate drying. In the late stage of drying, microwave power density is 0.3-0.5 W / g for moisture balancing.

[0118] Spinnable moisture content standard: The moisture content threshold that meets the requirements for fiber spinning. The theoretical basis includes the critical point of hydrolysis reaction: PA6 undergoes significant hydrolysis when it is greater than 300ppm. Therefore, the spinnable moisture content standard is a moisture content of less than 300ppm.

[0119] Step S4: A dual-channel spinning assembly with a differentiated flow channel structure is used to achieve a gradient distribution of flame retardant with a high outer and low inner diameter in the cross-section of the single filament.

[0120] Step S4 further includes the following sub-steps:

[0121] S4-1 delivers the flame retardant masterbatch melt to the outer channel of the dual-channel spinning assembly, controls the rheological properties of the melt to maintain a stable flow state, and promotes the enrichment of flame retardant to the outer layer through the channel structure design.

[0122] S4-2, the polyamide matrix melt is transported through independent core layer flow channels to maintain the laminar flow state of the melt and form a continuous core layer structure;

[0123] S4-3, adjust the flow ratio of the outer layer and the core layer melt, and utilize the difference in rheological properties between the flame retardant melt and the matrix melt to achieve a gradient distribution of flame retardant concentration that decreases from the outside to the inside.

[0124] It should be noted that the dual-channel spinning assembly used in the embodiments of the present invention includes an outer layer flow channel and a core layer flow channel arranged coaxially. The outer layer flow channel is designed as a spirally expanding structure with an inlet diameter of 3.5 mm and an outlet diameter expanded to 5.2 mm. The spiral helix angle is controlled at 55 degrees. The inner surface of the flow channel is mirror polished, and the surface roughness Ra is less than or equal to 0.08 μm.

[0125] The core flow channel adopts a porous buffer design, with a honeycomb rectifier plate with a pore diameter of 0.6mm at the inlet. The rectifier zone length reaches 65mm, and the overall length-to-diameter ratio of the flow channel is 24:1. The inner wall of the flow channel is coated with a 0.1mm thick hard alloy layer to ensure dimensional stability under long-term use.

[0126] Melt rheological properties: The apparent viscosity of the outer flame-retardant melt was controlled at 85 Pa·s and the shear rate at 650 s⁻¹ when the processing temperature was 260°C. -1 The elastic modulus under the conditions is 3200 Pa; the core matrix melt has an apparent viscosity of 125 Pa·s and an elastic modulus of 8500 Pa at the same temperature. The viscosity ratio of the two-phase melt is maintained at 0.68, and the surface tension difference is controlled at 3.2 mN / m.

[0127] Melt transport: During the melt transport process, a high-precision gear pump is used to control the flow rate. The outer melt pump has a displacement of 12.5 cm³. 3 / rev, core melt pump is 38cm 3 / rev, the actual flow ratio is set to 1:3.04, the melt temperature control accuracy reaches ±0.3 degrees Celsius, and the pressure fluctuation does not exceed ±0.5MPa.

[0128] Gradient distribution: At the outlet of the spinning assembly, a gradient distribution is achieved through a specially designed composite spinneret. The spinneret diameter is 0.25 mm, the length-to-diameter ratio is 3:1, the inlet adopts a 60-degree conical transition, the spinning pressure is maintained at 10.5 MPa, and the working temperature of the assembly is controlled in zones: 265 degrees Celsius for the outer layer flow channel and 260 degrees Celsius for the core layer flow channel.

[0129] Process conditions for gradient distribution formation: During implementation, the melt temperature distribution is monitored by an online infrared thermal imager to ensure that the radial temperature difference does not exceed 1.5 degrees Celsius. A laser Doppler velocimeter is used to detect the flow rate ratio of the two-phase melt in real time, and the speed of the gear pump is dynamically adjusted to make the average flow rate of the outer layer melt reach 15.2 cm / s and the core layer melt reach 12.8 cm / s.

[0130] Step S5: The topologically reinforced nano-phase is synergistically dispersed with the flame retardant to form a three-dimensional network structure in the melt, and then the nascent monofilament is formed through the spinning assembly.

[0131] Step S5 further includes the following sub-steps:

[0132] S5-1 involves adding a nano-reinforced phase with a topological structure and a flame retardant to a co-rotating twin-screw extruder, and forming a uniformly dispersed system through melt blending.

[0133] S5-2 involves melt blending under controlled mixing temperature, shear rate, and residence time conditions to form a preliminary dispersed network structure of the nano-reinforcing phase in the matrix. The mixing temperature is maintained in the range from the melting temperature of the matrix material to the thermal stability temperature of the nano-reinforcing phase. The shear rate can overcome the van der Waals forces between nanoparticles, and the residence time can fully disperse the nano-reinforcing phase.

[0134] S5-3, through the synergistic action of the kneading block and the anti-thread element in the screw assembly, applies a shear-stretch composite flow field during the twin-screw extrusion process, induces the nano-reinforced phase to be oriented along the melt flow direction, and utilizes the chemical interaction of particle surfaces to achieve inter-phase overlap, forming a three-dimensional continuous network structure that runs through the melt.

[0135] S5-4 uses a hydrodynamically optimized spinning assembly to extrude the composite melt into a nascent monofilament with a complete three-dimensional network structure. The spinning assembly includes a spinneret micro-orifice, a 03 conical spinneret inlet, and a tapered outlet.

[0136] It should be noted that the topologically reinforced nanophase used in this embodiment is montmorillonite nanosheets modified with silane coupling agent KH-550, with a sheet thickness of 1-2 nm and an aspect ratio > 200; the flame retardant is a composite system of ammonium polyphosphate (APP) and melamine cyanurate (MCA) in a mass ratio of 3:1; after premixing the nanophase and flame retardant in a mass ratio of 1:5, it is added in the third section of the extruder through a side feeder, and the feeding rate is controlled at 12 kg / h.

[0137] The specific control parameters for the mixing process are as follows:

[0138] Mixing temperature control: For PA6 matrix material, the temperature of each section is controlled as follows: melting section: 235-240 degrees Celsius (220 degrees Celsius higher than the melting temperature of the matrix material); mixing section: 245-250 degrees Celsius (280 degrees Celsius lower than the thermal stability temperature of the nano-reinforcing phase); die head section: 238-242 degrees Celsius.

[0139] Screw speed: 380 rpm, corresponding to a shear rate of 1250 s. -1 This shear force field can overcome the 0.5 nN van der Waals force between nanoparticles.

[0140] Residence time: 3 minutes 15 seconds ± 20 seconds, which allows the nano-reinforced phase to be fully dispersed.

[0141] The screw assembly includes a conveying section with a 50mm lead deep groove thread; a mixing section with 5 sets of 60-degree staggered kneading blocks, each set with a length of 15mm; a dispersing section with 2 reverse thread elements that generate a back pressure of 0.8MPa; and finally, 2 sets of 30-degree staggered kneading blocks to promote homogenization.

[0142] The hydrodynamically optimized spinning assembly has the following characteristics:

[0143] Spinneret micro-orifices: diameter 0.28mm, length-to-diameter ratio 8:1;

[0144] Conical inlet: cone angle 70 degrees, transition zone length 2.2mm;

[0145] Tapered exit: shrinkage ratio 1.8:1, exit chamfer 0.1mm × 45 degrees.

[0146] Step S6: The orientation and crystal structure of the single filament is controlled by stretching processes in the wetting pre-orientation stage and the dry heat main orientation stage.

[0147] Step S6 further includes the following sub-steps:

[0148] S6-1, the nascent monofilament is introduced into a humidified environment, and the ambient temperature is controlled to be lower than the glass transition temperature of the polyamide. The humidified environment includes steam and a constant temperature water bath.

[0149] S6-2, apply initial tensile force to cause the molecular chains to partially align along the fiber axis, forming a preliminary oriented crystal nucleus structure;

[0150] S6-3 involves transferring the pre-oriented monofilaments to a dry heat environment. The ambient temperature is controlled to be higher than the glass transition temperature of polyamide but lower than the melting point temperature of polyamide. Secondary stretching is then performed to promote the growth of crystalline regions and form a stable crystalline network structure. The dry heat environment includes a hot air box and a heating roller assembly.

[0151] It should be noted that during the wetting and pre-orientation stage: a constant temperature water bath is used for wetting and stretching, with the specific parameters as follows:

[0152] The water bath temperature is controlled at 48-52 degrees Celsius (the glass transition temperature of polyamide is 50 degrees Celsius), the water bath length is 3.5m, the immersion time is 25-30 seconds, 0.5wt% sodium dodecylbenzenesulfonate is added as a wetting agent, the speed of the first stretching roller is 18m / min, the speed of the second stretching roller is 32m / min, the pre-stretch ratio is 1.75-1.85 times, and the tensile tension is controlled at 0.22-0.25cN / dtex.

[0153] Dry heat orientation stage: A five-roll hot stretching unit is used for secondary stretching, with specific parameters as follows:

[0154] Temperature zones for the hot rollers: First hot roller: 85 degrees Celsius (preheating), Second hot roller: 135 degrees Celsius (main stretching), Third hot roller: 125 degrees Celsius (heat setting); Roller speed configuration: Feed roller: 32 m / min, Main stretching roller: 78 m / min, Exit roller: 75 m / min; Total stretching ratio: 4.2-4.5 times, of which the stretching ratio in the dry heat stage is 2.4-2.6 times.

[0155] Step S7: A segmented temperature-controlled thermal relaxation process is used to stabilize the fiber structure and produce a finished monofilament with flame-retardant properties.

[0156] Step S7 further includes the following sub-steps:

[0157] S7-1 involves a gradient heating-holding-cooling heat treatment process on oriented fibers under inert gas protection, which allows the molecular chains to reach a thermodynamically stable state through segmented relaxation.

[0158] S7-2 coordinates the matching relationship between fiber tension and ambient temperature during the thermal relaxation process, realizes the gradual release of residual stress, and completes the final shaping of the fiber structure.

[0159] S7-3 employs a three-stage cooling system to cool the heat-treated fibers to room temperature while maintaining tension, thereby fixing the three-dimensional flame-retardant network structure and producing a flame-retardant monofilament product.

[0160] It should be noted that the gradient heating-holding-cooling heat treatment process is carried out using a three-temperature zone thermal relaxation chamber, with specific parameter settings as follows:

[0161] Thermal relaxation chamber: made of stainless steel, with a 150mm thick insulation layer.

[0162] Heating phase: Initial temperature: 60 degrees Celsius, heating rate: 3.5 degrees Celsius / min, target temperature: 175 degrees Celsius, nitrogen flow rate: 12 m³ / min 3 / h (oxygen content less than 50ppm).

[0163] Insulation stage: Temperature: 175±1 degrees Celsius, Time: 8-10 minutes, Fiber running speed: 5.2m / min, Air velocity inside the chamber: 0.8m / s (uniformity deviation less than 5%).

[0164] Cooling phase: Initial cooling rate: 4 degrees Celsius / min (175→120 degrees Celsius), secondary cooling rate: 2 degrees Celsius / min (120→80 degrees Celsius), and finally air cooling to room temperature (25 degrees Celsius).

[0165] To coordinate the relationship between fiber tension and ambient temperature: An electronic tension control system is configured to achieve precise adjustment. Specific parameter settings are as follows:

[0166] The initial tension is 0.08 cN / dtex, the tension decay rate during the heating stage is 0.005 cN / (dtex·min), the constant tension during the holding stage is 0.05 cN / dtex, the tension compensation during the cooling stage is 0.003 cN / dtex for every 10 degrees Celsius decrease, and the tension sensor accuracy is ±0.002 cN / dtex.

[0167] Cooling to room temperature: A three-stage cooling system is used, with the specific parameter settings as follows:

[0168] First cooling zone (80-60 degrees Celsius): Air temperature: 60 degrees Celsius, air speed: 6m / s, cooling time: 35-40 seconds;

[0169] Second cooling zone (60-40 degrees Celsius): Water-cooled roller temperature: 25 degrees Celsius, contact arc length: 120 mm, pressure: 45 N;

[0170] Final cooling zone (40-25 degrees Celsius): Ambient temperature: 23±2 degrees Celsius, relative humidity: 55±5%, relaxation time: greater than or equal to 15 minutes.

[0171] This embodiment, through the aforementioned precise control, enables the stable production of high-performance flame-retardant monofilaments at a production speed of 45 meters per minute, specifically as follows:

[0172] Production efficiency indicators: continuous production speed: 45.0±0.5m / min, overall equipment efficiency (OEE): greater than or equal to 92%, daily output per spindle: greater than or equal to 65kg (24-hour operation), and product quality rate: greater than or equal to 98.5%.

[0173] Product structural stability: Crystallinity: 46.2±0.8% (DSC method), Grain size: 18.5±2.0nm (XRD Scherrer formula calculation), Orientation factor: 0.93±0.02 (sound velocity method detection), Network structure integrity: ≥96% (Micro-CT three-dimensional reconstruction analysis).

[0174] Flame retardant performance parameters: Limiting oxygen index: 33.5±0.5% (ASTM D2863), Vertical burning test: UL94V-0 rating (3.0mm thickness), Peak heat release rate: ≤65kW / m³ 2 (Cone calorimeter test), burning drips: no ignition of degreased cotton.

[0175] Production stability assurance: Temperature control accuracy: ±0.8℃ (critical process point), tension fluctuation range: ±0.75%, diameter unevenness rate: less than or equal to 1.5% (online laser diameter gauge), color consistency: ΔE less than or equal to 0.8 (standard light source D65).

[0176] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for efficiently preparing flame-retardant monofilaments, characterized in that, Includes the following steps: Step S1: The polyamide matrix material is surface activated by controlled atmosphere radio frequency plasma to directionally introduce carboxyl active functional groups on the surface of the matrix material. Step S2: The composite flame retardant masterbatch is surface grafted and modified using a reactive silane coupling agent to construct an interfacial transition layer that is chemically bonded to the polyamide matrix. Step S3: Through a synergistic microwave-airflow combined drying system, the polyamide matrix material and the composite flame retardant masterbatch are brought to a spinnable moisture content state. Step S4: A dual-channel spinning assembly with a differentiated flow channel structure is used to achieve a gradient distribution of flame retardant with a high outer and low inner diameter in the cross-section of the single filament. Step S5: The topologically reinforced nano-phase is synergistically dispersed with the flame retardant to form a three-dimensional network structure in the melt, and then the nascent monofilament is formed through the spinning assembly. Step S6: The orientation and crystal structure of the single filament is controlled by stretching processes in the wetting pre-orientation stage and the dry heat main orientation stage. Step S7: A segmented temperature-controlled thermal relaxation process is used to stabilize the fiber structure and produce a finished monofilament with flame-retardant properties. Step S1 further includes the following sub-steps: S1-1, The polyamide matrix material is placed in a closed radio frequency plasma reaction chamber, and a mixed atmosphere of inert gas and oxygen-containing active gas is introduced to establish a plasma treatment environment. The polyamide matrix material includes aliphatic polyamide, semi-aromatic polyamide, aromatic polyamide, copolymerized modified polyamide and reinforced modified polyamide. S1-2 uses an intermittent pulsed discharge mode to excite plasma, which generates active free radical sites on the surface of the substrate material and reacts in an oxygen-containing atmosphere to generate carboxyl active functional groups. S1-3 After the treatment is completed, the matrix material is transferred to a sealed container filled with protective gas to prevent the decomposition of the carboxyl active functional groups on the surface of the matrix material. Step S2 further includes the following sub-steps: S2-1, the composite flame retardant masterbatch is immersed in a treatment solution containing a reactive functional group silane coupling agent, and under the assistance of ultrasound, silane molecules are grafted onto the surface of the composite flame retardant masterbatch through chemical bonding to form a reactive interfacial transition layer. The composite flame retardant masterbatch includes a flame retardant, a dispersant and a carrier resin. S2-2, the surface-modified composite flame retardant masterbatch and polyamide matrix are added to a twin-screw extruder, and the barrel temperature is controlled between the polyamide melting temperature and the silane coupling agent decomposition temperature for melt blending; S2-3, in a twin-screw extruder, the screw speed is controlled to disperse the composite flame retardant masterbatch in the polyamide matrix through shearing and to maintain the melt residence time, so as to achieve chemical bonding between the active functional groups of the silane coupling agent and the polyamide molecular chain; Step S5 further includes the following sub-steps: S5-1 involves adding a nano-reinforced phase with a topological structure and a flame retardant to a co-rotating twin-screw extruder, and forming a uniformly dispersed system through melt blending. S5-2, under the process conditions of controlling the mixing temperature, shear rate and residence time, melt blending is carried out to form a preliminary dispersed network structure of the nano-reinforcing phase in the matrix. The mixing temperature is maintained between the melting temperature of the matrix material and the thermal stability temperature of the nano-reinforcing phase. The shear rate can overcome the van der Waals forces between nanoparticles. The residence time can fully disperse the nano-reinforcing phase. S5-3, through the synergistic action of the kneading block and the anti-thread element in the screw assembly, applies a shear-stretch composite flow field during the twin-screw extrusion process, induces the nano-reinforced phase to be oriented along the melt flow direction, and utilizes the chemical interaction of particle surfaces to achieve inter-phase overlap, forming a three-dimensional continuous network structure that runs through the melt. S5-4, a hydrodynamically optimized spinning assembly is used to extrude the composite melt to obtain a nascent monofilament with a complete three-dimensional network structure. The spinning assembly includes a spinneret micro-orifice, a conical spinneret inlet, and a tapered outlet.

2. The production method for efficiently preparing flame-retardant monofilaments according to claim 1, characterized in that: Step S3 further includes the following sub-steps: S3-1 uses microwave-assisted vacuum drying to remove moisture from the polyamide matrix material and composite flame retardant masterbatch; S3-2 controls the temperature distribution of the polyamide matrix material and composite flame retardant masterbatch during the drying process by adjusting the gas flow rate and microwave power density in the drying environment, thus avoiding local overheating that could lead to degradation. S3-3, the moisture content of the polyamide matrix material and the composite flame retardant masterbatch is monitored in real time using an online moisture detection device. Drying is terminated when the spinnable moisture content standard is reached, which is a moisture content of less than 300 ppm.

3. The production method for efficiently preparing flame-retardant monofilaments according to claim 1, characterized in that: Step S4 further includes the following sub-steps: S4-1 delivers the flame retardant masterbatch melt to the outer channel of the dual-channel spinning assembly, controls the rheological properties of the melt to maintain a stable flow state, and promotes the enrichment of flame retardant to the outer layer through the channel structure design. S4-2, the polyamide matrix melt is transported through independent core layer flow channels to maintain the laminar flow state of the melt and form a continuous core layer structure; S4-3, adjust the flow ratio of the outer layer and the core layer melt, and utilize the difference in rheological properties between the flame retardant melt and the matrix melt to achieve a gradient distribution of flame retardant concentration that decreases from the outside to the inside.

4. The production method for efficiently preparing flame-retardant monofilaments according to claim 1, characterized in that: Step S6 further includes the following sub-steps: S6-1, the nascent monofilament is introduced into a wetting environment, and the ambient temperature is controlled to be lower than the glass transition temperature of the polyamide. The wetting environment includes steam and a constant temperature water bath. S6-2, apply initial tensile force to cause the molecular chains to partially align along the fiber axis, forming a preliminary oriented crystal nucleus structure; S6-3, the pre-oriented monofilaments are transferred to a dry heat environment, and the ambient temperature is controlled to be higher than the glass transition temperature of polyamide but lower than the melting point temperature of polyamide. Secondary stretching is performed to promote the growth of crystalline regions and form a stable crystalline network structure. The dry heat environment includes a hot air box and a heating roller group.

5. The method for efficiently preparing flame-retardant monofilaments according to claim 1, characterized in that: Step S7 further includes the following sub-steps: S7-1 involves a gradient heating-holding-cooling heat treatment process on oriented fibers under inert gas protection, which allows the molecular chains to reach a thermodynamically stable state through segmented relaxation. S7-2 coordinates the matching relationship between fiber tension and ambient temperature during the thermal relaxation process, realizes the gradual release of residual stress, and completes the final shaping of the fiber structure. S7-3 employs a three-stage cooling system to cool the heat-treated fibers to room temperature while maintaining tension, thereby fixing the three-dimensional flame-retardant network structure and producing a flame-retardant monofilament product.

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