Process for producing a polyimide composite fiber

Abstract

The application relates to the technical field of fiber preparation and discloses a preparation process of a polyimide composite fiber, which comprises the following steps: surface activation treatment is performed to introduce active functional groups into the fiber surface layer; continuous fibers are divided into three functional zones, i.e., an A section phosphorus system, a B section nitrogen system and a C section silicon system, through segmented differential grafting; a section-to-section barrier layer is constructed to prevent the migration of flame retardants between different functional sections; a layer-by-layer self-assembly nano coating is deposited and in-situ cured to form a multilayer barrier structure; and a circulating atmosphere gradient treatment is performed to realize the differential curing of the functional sections. Through segmented functionalization and a multilayer synergistic flame-retardant structure, the technical problems of the easy migration and loss of flame retardants in traditional flame-retardant fibers, the poor adaptability of single flame-retardant mechanism and the weak physical barrier capacity of the flame-retardant layer are solved; the prepared polyimide composite fiber has the characteristics of multi-temperature zone intelligent response, the limiting oxygen index (LOI) reaches 60-68%, the flame-retardant performance is long-acting and stable, and the mechanical property loss is less than 10%.

Description

Technical Field

[0001] This invention relates to the field of fiber preparation technology, and more specifically, to a preparation process for polyimide composite fibers. Background Technology

[0002] Polyimide fibers possess excellent high-temperature resistance and mechanical strength, making them widely used in aerospace, fire protection, and high-temperature filtration. However, polyimide fibers still pose a risk of combustion under extreme high temperatures or direct flame contact, limiting their application in scenarios with stringent flame-retardant requirements.

[0003] Existing flame-retardant modification technologies mainly include blending and surface treatment. Blending involves adding flame retardants during fiber preparation, but it suffers from problems such as flame retardant migration and loss, and damage to the fiber molecular structure leading to a 15-30% decrease in mechanical properties. While surface treatment avoids damage to the internal fiber structure, it faces difficulties such as thin surface flame-retardant layers, low loading capacity, and limited flame-retardant effect.

[0004] More importantly, existing technologies generally use a single type of flame retardant, which can only function within a limited temperature range (usually 300-500℃) through a single flame retardant mechanism. This is insufficient to cope with the complex environment of temperature gradient changes and multi-stage combustion in actual fires. Furthermore, when segmented functional modification is used, the flame retardants between different functional segments can migrate through diffusion, causing the functional zones to fail within 3-6 months, resulting in a uniformly mixed fiber distribution. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a process for preparing polyimide composite fibers, comprising the following steps:

[0006] Surface activation treatment: Plasma treatment or ozone oxidation is performed on polyimide fibers to introduce active functional groups on the fiber surface;

[0007] Segmented Differentiated Grafting: The continuous fiber is divided into three functional regions: A, B and C. Phosphate ester flame retardants are grafted onto section A to form a phosphorus-based flame retardant functional layer, melamine derivatives are grafted onto section B to form a nitrogen-based flame retardant functional layer, and organosilicon flame retardants are grafted onto section C to form a silicon-based flame retardant functional layer.

[0008] Construction of inter-segment barrier layer: A barrier layer solution composed of polyvinyl alcohol, nano-layered silicate and crosslinking agent is applied at the junction of segments AB and BC to form an inter-segment barrier layer;

[0009] Layer-by-layer self-assembled nano-coating deposition and in-situ curing: Fibers are sequentially immersed in a positively charged polyelectrolyte solution and a negatively charged nanomaterial dispersion for layer-by-layer deposition. In-situ curing is performed after every 2-3 bilayers are deposited.

[0010] Circulating atmosphere gradient treatment: sequentially performing ammonia atmosphere treatment, nitrogen-sulfur dioxide mixed atmosphere treatment, and nitrogen-organosilicon vapor mixed atmosphere treatment;

[0011] Post-treatment: Heat treatment at 120-150℃ for 2-4 hours.

[0012] Preferably, in the segmented differentiated grafting, the phosphate ester flame retardant in segment A is selected from at least one of diphenyl phosphate and tri(2-chloroethyl) phosphate; the melamine derivative in segment B is selected from at least one of melamine phosphate and melamine cyanurate; and the organosilicon flame retardant in segment C is selected from at least one of aminopropyltriethoxysilane and phenyltrimethoxysilane.

[0013] Preferably, in the segmented differentiated grafting, the length ratio of each segment is 1:1:1 or 2:1:1, the grafting reaction temperature is 50-110℃, and the reaction time is 30-100 minutes.

[0014] Preferably, the intersegment barrier layer solution is composed of: 10-15% polyvinyl alcohol, 5-10% sodium carboxymethyl cellulose, 5-8% nano-layered silicate, and 2-3% crosslinking agent, with water or a water-ethanol mixture as the solvent.

[0015] Preferably, the nanolayered silicate is selected from at least one of sepiolite nanotubes, halloysite nanotubes, and montmorillonite, and the crosslinking agent is selected from at least one of glutaraldehyde, citric acid, and borax.

[0016] Preferably, in the layer-by-layer self-assembled nanocoating deposition, the positively charged polyelectrolyte solution comprises chitosan-phosphate complex, gelatin phosphorylation modifier, or polyallylamine hydrochloride, with a concentration of 2-5 g / L and a pH of 4-5; the negatively charged nanomaterial dispersion comprises montmorillonite, graphene oxide, layered bimetallic hydroxide, or cellulose nanocrystals, with a concentration of 1-3 g / L and a pH of 8-9.

[0017] Preferably, the in-situ curing involves pre-adding ammonium persulfate thermal initiator to a positively charged polyelectrolyte solution, and curing at 80-120°C for 2-5 minutes after every 2-3 double layers are deposited.

[0018] Preferably, in the cyclic atmosphere gradient treatment, the first round of ammonia atmosphere treatment is at a temperature of 180-220℃ for 30-50 minutes; the second round of nitrogen-sulfur dioxide mixed atmosphere treatment is at a temperature of 220-260℃ for 35-55 minutes; and the third round of nitrogen-organosilicon vapor mixed atmosphere treatment is at a temperature of 260-320℃ for 40-60 minutes.

[0019] Preferably, the layers self-assemble to form 20-30 double layers, with a total coating thickness of 200-800 nanometers.

[0020] A polyimide composite fiber prepared by the above-mentioned preparation process has a limiting oxygen index (LOI) of 60-68%, achieves a V-0 rating in vertical burning, and retains ≥90% of its tensile strength.

[0021] The beneficial effects of this invention are as follows:

[0022] Unlike existing technologies that use a single flame retardant for uniform distribution, this invention divides the continuous fiber into three functional zones: Segment A (phosphorus-based), Segment B (nitrogen-based), and Segment C (silicon-based). Each segment is covalently grafted with a different type of flame retardant. This technology enables the construction of a spatial distribution of multiple flame-retardant functions on a single continuous fiber, allowing the fiber to sequentially activate different flame-retardant functions such as phosphorus-based char formation catalysis, nitrogen-based gas-phase dilution, and silicon-based high-temperature protection in three temperature ranges: 300-500℃, 400-600℃, and above 600℃.

[0023] To address the key technical challenge of functional zone failure caused by flame retardant migration during segmented grafting, this invention constructs a barrier layer at the functional segment boundaries, composed of high molecular weight polyvinyl alcohol and nano-layered silicate. In this barrier layer, the long polymer chains and nanosheets form a labyrinthine diffusion path, which, combined with chemical cross-linking anchoring, reduces the diffusion coefficient of small molecule flame retardants by 80-90%, effectively preventing component migration between different functional segments.

[0024] Unlike traditional layer-by-layer self-assembly followed by overall curing, this invention employs an in-situ curing method immediately after depositing 2-3 bilayers. By pre-adding ammonium persulfate thermal initiator to the polyelectrolyte solution, a free radical cross-linking reaction is initiated under infrared irradiation at 80-120℃, achieving layer-by-layer in-situ curing of the coating. This method increases the coating peel strength from 2-4 MPa in traditional processes to 8-12 MPa, eliminating the quality instability of the coating in its uncured state.

[0025] Breaking through the limitations of traditional single-atmosphere overall curing, this invention employs a three-round cyclic atmosphere treatment: the first round uses an ammonia atmosphere (180-220℃) to react with the phosphorus-based A section to generate phosphoramide compounds; the second round uses a nitrogen-sulfur dioxide mixed atmosphere (220-260℃) to introduce sulfur into the nitrogen-based B section to form nitrogen-sulfur compounds; and the third round uses a nitrogen-organosilicon vapor mixed atmosphere (260-320℃) to promote the formation of Si-N bond structures in the silicon-based C section. The incremental temperature and targeted selection of atmosphere components in each round ensure that the curing degree of each functional section reaches 85-92%.

[0026] This invention constructs a five-layer integrated structure consisting of a polyimide matrix, a flame retardant grafted layer, an intersegment barrier layer, a nano-self-assembled coating, and an atmosphere-modified layer. Each layer is covalently bonded and physically interlocked to form a unified structure. During combustion, this structure sequentially forms a phosphorus-based catalytic char layer, a nitrogen-based gas dilution layer, a silicon-based glassy protective layer, and a nanosheet ceramic barrier layer, achieving a total char residue rate of 50-60%, thus realizing multiple protections through physical barrier and chemical flame retardancy. Attached Figure Description

[0027] Figure 1 This is a comparison of the limiting oxygen index of the present invention;

[0028] Figure 2 This is a comparison of the thermogravimetric analysis curves of the present invention;

[0029] Figure 3 This is an analysis of the synergistic effect of the flame-retardant mechanism of the present invention;

[0030] Figure 4 This is a comparison of the stability of the functional areas of the present invention;

[0031] Figure 5 This is a comparison of the migration distance of the flame retardant in this invention;

[0032] Figure 6 This is a heat map showing the distribution of elements in the barrier-free layer of the present invention (after 180 days);

[0033] Figure 7 This is a comparison of the interfacial bonding strength of the present invention;

[0034] Figure 8 This is a comparison of the surface characteristics of the present invention. Detailed Implementation

[0035] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples. Example

[0036] This embodiment proposes a preparation process for polyimide composite fibers, including the following steps:

[0037] Surface activation treatment: Plasma treatment is performed on polyimide fibers to introduce active functional groups on the fiber surface;

[0038] Segmented Differentiated Grafting: The continuous fiber is divided into three functional regions: A, B and C. Phosphate ester flame retardants are grafted onto section A to form a phosphorus-based flame retardant functional layer, melamine derivatives are grafted onto section B to form a nitrogen-based flame retardant functional layer, and organosilicon flame retardants are grafted onto section C to form a silicon-based flame retardant functional layer.

[0039] In the segmented differentiated grafting, diphenyl phosphate is selected as the phosphate ester flame retardant in segment A, melamine phosphate is selected as the melamine derivative in segment B, and aminopropyltriethoxysilane is selected as the organosilicon flame retardant in segment C.

[0040] In segmented differential grafting, the length ratio of each segment is 1:1:1, the grafting reaction temperature is 80℃, and the reaction time is 65 minutes.

[0041] Construction of inter-segment barrier layer: A barrier layer solution composed of polyvinyl alcohol, nano-layered silicate and crosslinking agent is applied at the junction of segments AB and BC to form an inter-segment barrier layer;

[0042] The intersegment barrier layer solution is composed of: 12% polyvinyl alcohol, 8% sodium carboxymethyl cellulose, 7% nano-layered silicate, 2.5% crosslinking agent, and water as solvent;

[0043] The nanolayered silicate is sepiolite nanotubes, and the crosslinking agent is glutaraldehyde.

[0044] Layer-by-layer self-assembled nano-coating deposition and in-situ curing: Fibers are sequentially immersed in a positively charged polyelectrolyte solution and a negatively charged nanomaterial dispersion for layer-by-layer deposition, and in-situ curing is performed after every two bilayers are deposited;

[0045] In-situ curing involves pre-adding ammonium persulfate thermal initiator to a positively charged polyelectrolyte solution, and curing at 100°C for 3 minutes after every two bilayers are deposited.

[0046] In the layer-by-layer self-assembled nanocoating deposition, the positively charged polyelectrolyte solution contains a chitosan-phosphate ester complex at a concentration of 3.5 g / L and a pH of 4.5; the negatively charged nanomaterial dispersion contains montmorillonite at a concentration of 2 g / L and a pH of 8.5.

[0047] The layers self-assemble to form 25 double layers, with a total coating thickness of 500 nanometers.

[0048] Circulating atmosphere gradient treatment: sequentially performing ammonia atmosphere treatment, nitrogen-sulfur dioxide mixed atmosphere treatment, and nitrogen-organosilicon vapor mixed atmosphere treatment;

[0049] In the cyclic atmosphere gradient treatment, the first round of ammonia atmosphere treatment is at a temperature of 200℃ for 40 minutes; the second round of nitrogen-sulfur dioxide mixed atmosphere treatment is at a temperature of 240℃ for 45 minutes; and the third round of nitrogen-organosilicon vapor mixed atmosphere treatment is at a temperature of 290℃ for 50 minutes.

[0050] Post-treatment: Heat treatment at 135℃ for 3 hours. Example

[0051] The difference between this embodiment and Embodiment 1 is that:

[0052] Surface activation treatment: ozone oxidation is performed on polyimide fibers to introduce active functional groups on the fiber surface;

[0053] In the segmented differentiated grafting, the phosphate ester flame retardant in segment A is tri(2-chloroethyl) phosphate, the melamine derivative in segment B is melamine cyanurate, and the organosilicon flame retardant in segment C is phenyltrimethoxysilane.

[0054] In segmented differential grafting, the length ratio of each segment is 2:1:1, the grafting reaction temperature is 50℃, and the reaction time is 30 minutes.

[0055] The intersegment barrier layer solution is composed of: 10% polyvinyl alcohol, 5% sodium carboxymethyl cellulose, 5% nano-layered silicate, and 2% crosslinking agent, with water-ethanol mixed solvent as the solvent.

[0056] The nanolayered silicate is selected from at least one of sepiolite nanotubes, halloysite nanotubes, and montmorillonite, and the crosslinking agent is selected from at least one of glutaraldehyde, citric acid, and borax.

[0057] Layer-by-layer self-assembled nano-coating deposition and in-situ curing: In-situ curing is performed after every 3 bilayers are deposited;

[0058] After depositing three double layers, cure at 80°C for 2 minutes;

[0059] In the layer-by-layer self-assembled nanocoating deposition, the positively charged polyelectrolyte solution contains a chitosan-phosphate ester complex at a concentration of 2 g / L and a pH of 4; the negatively charged nanomaterial dispersion contains graphene oxide at a concentration of 1-3 g / L and a pH of 8-9.

[0060] The layers self-assemble to form 20 double layers, with a total coating thickness of 200 nanometers.

[0061] In the cyclic atmosphere gradient treatment, the first round of ammonia atmosphere treatment is at a temperature of 180℃ for 30 minutes; the second round of nitrogen-sulfur dioxide mixed atmosphere treatment is at a temperature of 220℃ for 35 minutes; and the third round of nitrogen-organosilicon vapor mixed atmosphere treatment is at a temperature of 260℃ for 40 minutes.

[0062] Post-treatment: Heat treatment at 120℃ for 2 hours. Example

[0063] The difference between this embodiment and Embodiment 1 is that:

[0064] In the segmented differentiated grafting, the flame retardants for segment A are diphenyl phosphate and tri(2-chloroethyl) phosphate; the flame retardants for segment B are melamine derivatives such as melamine phosphate and melamine cyanurate; and the flame retardants for segment C are aminopropyltriethoxysilane and phenyltrimethoxysilane.

[0065] In the segmented differential grafting, the grafting reaction temperature is 110℃ and the reaction time is 100 minutes.

[0066] The intersegment barrier layer solution is composed of: 15% polyvinyl alcohol, 10% sodium carboxymethyl cellulose, 8% nano-layered silicate, and 3% crosslinking agent;

[0067] The nano-layered silicate is selected from montmorillonite, and the crosslinking agent is selected from borax.

[0068] After depositing three double layers, cure at 120°C for 5 minutes;

[0069] In the layer-by-layer self-assembled nanocoating deposition, the positively charged polyelectrolyte solution contains polyallylamine hydrochloride at a concentration of 5 g / L and a pH of 5; the negatively charged nanomaterial dispersion contains graphene oxide at a concentration of 3 g / L and a pH of 9.

[0070] The layers self-assemble to form 30 double layers, with a total coating thickness of 800 nanometers.

[0071] In the cyclic atmosphere gradient treatment, the first round of ammonia atmosphere treatment is at a temperature of 220℃ for 50 minutes; the second round of nitrogen-sulfur dioxide mixed atmosphere treatment is at a temperature of 260℃ for 55 minutes; and the third round of nitrogen-organosilicon vapor mixed atmosphere treatment is at a temperature of 320℃ for 60 minutes.

[0072] Post-treatment: Heat treatment at 150℃ for 4 hours. Example

[0073] In this embodiment, a polyimide composite fiber prepared by the above-described preparation process is proposed, which has a limiting oxygen index (LOI) of 60-68%, achieves a V-0 rating in vertical burning, and retains ≥90% of its tensile strength. Example

[0074] This embodiment presents a process for preparing polyimide composite fibers, including the following steps:

[0075] Step 1: Surface activation treatment

[0076] Polyimide fibers are activated by introducing active functional groups such as hydroxyl, carboxyl, and amino groups into the fiber surface.

[0077] Technical specifications of raw material polyimide fiber:

[0078] Fiber diameter: 10-25 micrometers (preferably 15-20 micrometers);

[0079] Tensile strength: ≥2.5 GPa (typical value 2.8-3.2 GPa);

[0080] Elastic modulus: ≥120 GPa (typical value 125-135 GPa);

[0081] Density: 1.38-1.45 g / cm³ - Melting point: >400℃ (does not melt);

[0082] Glass transition temperature: 360-380℃;

[0083] Limiting oxygen index: 28-32% (unmodified);

[0084] Surface energy: 38-45 mJ / m² (before activation);

[0085] Surface roughness: Ra 0.8-1.5 micrometers (before activation).

[0086] One of the following two methods can be used:

[0087] Plasma treatment: Atmospheric pressure low-temperature plasma equipment is used, with a power of 100-300W (preferably 200W), a treatment time of 3-10 minutes (preferably 5-8 minutes), and the working gas is oxygen (purity ≥99.5%) or compressed air, with a gas flow rate of 50-150 mL / min and a treatment distance of 5-15 mm. After treatment, the fiber surface contact angle decreases from the initial 80-90° to 30-50°, indicating a significant improvement in surface hydrophilicity.

[0088] Ozone oxidation: Ozone gas is prepared using an ozone generator with an ozone concentration of 10-30 mg / L (determined by iodometric titration, preferably 15-25 mg / L), a treatment temperature of 60-100℃ (preferably 70-90℃), a treatment time of 10-30 minutes (preferably 15-25 minutes), and a relative humidity of 40-60%.

[0089] This step forms an active site layer with a density of 5-10 sites / nm² (verified by X-ray photoelectron spectroscopy (XPS) to determine the O / C atomic ratio, which increases from 0.15-0.20 to 0.35-0.45 after activation), providing reaction sites for subsequent covalent grafting of flame retardants. Criteria for intermediate product identification: surface contact angle ≤50°, O / C ratio ≥0.35 as determined by XPS.

[0090] Step 2: Segmented Differentiated Grafting

[0091] This step involves segmented grafting of continuous polyimide fibers using a three-stage impregnation system. Unlike existing technologies that use a single flame retardant for uniform treatment, this step constructs three distinct flame-retardant functional zones on a single continuous fiber:

[0092] Grafting of phosphorus-based flame retardant in section A: Immerse section A of fiber in a solution containing phosphate ester flame retardant. The flame retardant is selected from diphenyl phosphate (CAS No. 1241-94-7, molecular weight 250.2, phosphorus content 12.4%), tri(2-chloroethyl) phosphate (CAS No. 115-96-8, molecular weight 285.5, phosphorus content 10.9%) or its derivatives. The solution concentration is 5-15% (preferably 8-12%), the solvent is N,N-dimethylformamide or tetrahydrofuran, the reaction temperature is 60-100℃ (preferably 70-90℃), and the reaction time is 30-90 minutes (preferably 45-75 minutes).

[0093] Heating is performed in a constant-temperature oil bath with a magnetic stirrer at 100-200 rpm. In the presence of an alkaline catalyst (triethylamine concentration 0.1-0.5% or sodium hydroxide concentration 0.05-0.3%), phosphate ester molecules undergo a de-alcoholization condensation reaction with the hydroxyl groups on the fiber surface to form POC covalent bonds. The reaction is considered complete when the phosphorus atom content on the fiber surface is ≥2.5 at%, as determined by XPS, and a characteristic P=O peak appears in the infrared spectrum at 1240-1280 cm⁻¹. Unreacted flame retardant is removed by washing three times with acetone, followed by vacuum drying at 80-120℃ for 30 minutes.

[0094] Section B nitrogen-based flame retardant grafting: The fiber section B is immersed in a solution containing melamine derivatives. The flame retardants selected are melamine phosphate (CAS No. 20208-95-1, nitrogen content 33.3%), melamine cyanurate (CAS No. 37640-57-6, nitrogen content 66.7%), etc. The solution concentration is 8-20% (preferably 12-18%), the solvent is deionized water or water-ethanol mixed solvent (volume ratio 1:1), the reaction temperature is 70-110℃ (preferably 80-100℃), and the reaction time is 40-100 minutes (preferably 60-90 minutes).

[0095] Using a water bath for heating and a mechanical stirrer at 80-150 rpm, the primary amino groups in the melamine molecule undergo an amidation reaction with the carboxyl groups on the fiber surface to form CN covalent bonds. The reaction is considered complete when: XPS measurement shows a nitrogen atom content of ≥3.0 at%, and the infrared spectrum shows a CN stretching vibration peak at 1550-1650 cm⁻¹. The fiber is then washed with deionized water until neutral and dried at 60-80℃ for 45 minutes.

[0096] C-segment silicone flame retardant grafting: The C-segment of fiber is immersed in a solution containing organosilicon flame retardant. The flame retardant is a compound of aminopropyltriethoxysilane (CAS No. 919-30-2, molecular weight 221.4), phenyltrimethoxysilane (CAS No. 2996-92-1, molecular weight 198.3) and phosphate ester (mass ratio 1:1). The solution concentration is 5-12% (preferably 7-10%), the solvent is anhydrous ethanol, the reaction temperature is 50-90℃ (preferably 60-80℃), and the reaction time is 30-80 minutes (preferably 40-70 minutes).

[0097] An acid catalyst, dilute hydrochloric acid, was added to the reaction system to adjust the pH to 4-6, and the mixture was heated using a constant-temperature water bath. The organosilane hydrolyzed to generate silanol groups, which then dehydrated and condensed with hydroxyl groups on the fiber surface to form Si-OC covalent bonds. The reaction was considered complete when XPS analysis showed a silicon atom content on the fiber surface ≥ 1.8 at%,² 9 Si NMR showed a T³ structure signal at -65 to -70 ppm. The sample was washed twice with anhydrous ethanol and dried at 50-70°C for 60 minutes.

[0098] The length ratio of each segment can be adjusted according to application requirements, with typical ratios being 1:1:1 or 2:1:1.

[0099] Intermediate product processing procedures and solvent recovery:

[0100] After the A-stage treatment is completed, immediately wash with acetone 3 times (soaking for 2-3 minutes each time) to remove unreacted phosphate esters;

[0101] Acetone recovery: The cleaning solution is distilled using a rotary evaporator (40-50℃, -0.08 MPa), with a recovery rate of ≥95% and a acetone purity of ≥99%, which can be reused;

[0102] Phosphate ester recovery: The distillation residue contains 2-8% phosphate esters. It is purified by recrystallization with methanol, and the recovery rate is 60-75%.

[0103] Vacuum drying (80-120℃, 30 minutes), solvent vapor is collected in a condenser and recycled;

[0104] Quality inspection before Section B treatment: Phosphorus content on the surface of Section A ≥ 2.5 at% (XPS test), pass rate requirement ≥ 95%;

[0105] After the B-stage treatment is completed, rinse with deionized water until neutral (pH 6.5-7.5).

[0106] Melamine derivative recovery: The cleaning solution is concentrated to 1 / 5 of its original volume by vacuum evaporation, and then cooled to crystallize and precipitate melamine derivatives, with a recovery rate of 55-70% and a purity of ≥92%.

[0107] Process water recovery: Distilled water can be reused, and its conductivity is controlled to <10 μS / cm;

[0108] Hot air drying (60-80℃, 45 minutes);

[0109] Quality inspection before C-stage treatment: Nitrogen content in B-stage ≥ 3.0 at%;

[0110] After the C-section is processed, wash twice with anhydrous ethanol.

[0111] Ethanol recovery: Separation via a distillation column (boiling point 78.4℃), recovery rate ≥92%, recovered ethanol purity ≥99.5%;

[0112] Organosilane recovery: High-boiling-point components contain 5-15% organosilanes, which can be recovered by vacuum distillation (0.01-0.05 MPa, 50-80℃), with a recovery rate of 40-60%.

[0113] Hot air drying (50-70℃, 60 minutes).

[0114] Economic benefits of recycling: Total solvent recovery rate ≥90%, flame retardant recovery rate 50-70%, raw material cost savings of 2000-3500 yuan per ton of product.

[0115] Quality control checkpoints: After each stage of processing is completed, the content of the corresponding elements and the grafting effect must be tested. Unqualified products are returned for reprocessing or disposed of as waste.

[0116] Step 3: Construction of Inter-segment Barrier Layer

[0117] This step is crucial for resolving functional zone failure caused by flame retardant migration during segmented grafting. An inter-segment barrier layer solution is applied at the junctions of segments AB and BC by impregnation or spraying.

[0118] The composition of the barrier layer solution is as follows:

[0119] Main barrier polymer: Polyvinyl alcohol (PVA, molecular weight 100,000-200,000, preferably 150,000, degree of hydrolysis 87-89%, viscosity 28-32 mPa·s) 10-15% (preferably 12-14%) as the main barrier polymer;

[0120] Auxiliary barrier polymer: Sodium carboxymethyl cellulose (CMC-Na, degree of substitution 0.7-1.2, viscosity 300-800 mPa·s) 5-10% (preferably 6-9%);

[0121] The physical maze composition consists of 5-8% (preferably 6-7%) of nano-layered silicates, which may include: sepiolite nanotubes (aspect ratio 20-50, preferably 30-40, specific surface area 180-220 m² / g); halloysite nanotubes (aspect ratio 10-30, preferably 15-25, tube diameter 20-50 nm); and montmorillonite (sheet diameter 100-200 nm, preferably 120-180 nm, cation exchange capacity 80-120 mmol / 100g).

[0122] Crosslinking agent: glutaraldehyde (concentration 25%, purity ≥99%, must be used in a fume hood with personal protective equipment), citric acid monohydrate (purity ≥99.5%, non-toxic and safe) or borax decahydrate (purity ≥99%, avoid oral ingestion) 2-3% (preferably 2.5%).

[0123] Safety Precautions: Glutaraldehyde is irritating and toxic; protective gloves and goggles must be worn during handling, and the operation must be carried out in a fume hood. It is chemically compatible with PVA, but contact with strong oxidizing and reducing agents should be avoided. Citric acid and borax are both weakly alkaline in aqueous solution and are chemically compatible with PVA and nano-silicates.

[0124] Solvent: Deionized water (conductivity <10 μS / cm) or water-ethanol mixed solvent (anhydrous ethanol ≥99.7%, volume ratio 1:1).

[0125] The application process involves the following specific steps:

[0126] Precision impregnation equipment should be used, with the impregnation speed controlled at 5-15 mm / min (preferably 8-12 mm / min), and the width of each barrier zone precisely controlled at 1-2 cm (ensuring ±0.2 mm accuracy through a mechanical positioning system). Pneumatic spraying equipment can also be used, with a spraying pressure of 0.2-0.5 MPa, a nozzle diameter of 0.5-1.0 mm, and a spraying distance of 8-15 cm.

[0127] Solution preparation: First, dissolve PVA at 80-90℃ for 30-45 minutes until completely transparent. After cooling to room temperature, add CMC-Na and nano-silicate (pre-dispersed ultrasonically for 15 minutes) sequentially. Finally, add the crosslinking agent and mechanically stir for 30 minutes until uniformly dispersed. The solution viscosity is controlled at 500-1500 mPa·s (measured by a rotational viscometer NDJ-8S).

[0128] The thickness of the applied barrier layer is 8-15 micrometers (measured by scanning electron microscopy SEM), and high molecular weight polyvinyl alcohol and nano-silicate form a three-dimensional network structure. Pre-curing is carried out in a forced-air drying oven at a temperature of 60-80℃ (preferably 70℃) for 20-30 minutes (preferably 25 minutes) with relative humidity controlled at 30-50%.

[0129] Intermediate product quality standards: - Barrier layer thickness: 8-15 micrometers (SEM measurement) - Surface smoothness: Roughness Ra≤0.5 micrometers (profilometer measurement) - Barrier effect: Small molecule tracer (Rhodamine B) 24-hour penetration rate≤5% - Adhesion strength: Peel strength≥3.0 MPa (tested according to GB / T 2790 standard)

[0130] The obtained segmented fibers have clear functional zones and effective inter-segment barriers, reducing the flame retardant migration coefficient by 80-90% and providing a stable substrate for subsequent self-assembled coatings.

[0131] Step 4: Layer-by-layer self-assembled nano-coating deposition and in-situ curing

[0132] First, two electrolyte solutions are prepared:

[0133] Composition and preparation of positively charged polyelectrolyte solutions:

[0134] Main ingredients (choose one of three):

[0135] Chitosan-phosphate complex: trade name Chitosan-PO4, degree of deacetylation 85-95% (determined by ¹H NMR), molecular weight 100,000-500,000 (GPC determination, polystyrene standard), degree of substitution 0.3-0.8 (³¹P NMR determination), supplier: Sigma-Aldrich or equivalent quality.

[0136] Phosphorylated modified gelatin: collagen gelatin, Bloom strength 200-250 (determined according to GB / T 6783), isoelectric point pH 4.8-5.2, degree of phosphorylation 15-25% (calculated by elemental analysis of P content), supplier: Rousselot Gelatin Co., Ltd. or equivalent.

[0137] Polyallylamine hydrochloride: Trade name PAH·HCl, molecular weight 30,000-150,000 (determined by viscosity method), nitrogen content ≥20% (elemental analysis method), purity ≥95%, supplier: Aladdin or equivalent.

[0138] Preparation conditions: Solvent: Deionized water (conductivity <5 μS / cm, pH 6.8-7.2); Concentration: 2-5 g / L (preferably 3-4 g / L); pH adjustment: Adjust to pH 4-5 (preferably 4.2-4.8) with dilute hydrochloric acid (analytical grade, concentration 0.1 mol / L); Zeta potential: +20 to +40 mV (measured by Zetasizer Nano ZS); Initiator: Ammonium persulfate (analytical grade, purity ≥98%, content 2-4%, preferably 3%); Storage: Store at 4℃ protected from light, shelf life 24 hours; Stirring conditions: Magnetic stirrer, 300-500 rpm, dissolution time 30-60 minutes.

[0139] Composition and preparation of negatively charged nanomaterial dispersions (choose one):

[0140] Montmorillonite dispersion:

[0141] Raw materials: Sodium-based montmorillonite (trade name Na-MMT, interlayer spacing 1.2-1.8 nm, cation exchange capacity 90-120 mmol / 100 g, particle size <2 μm, supplier: Zhejiang Huatai or equivalent); Pretreatment: drying at 105℃ for 2 hours, grinding through a 200-mesh sieve; Dispersion conditions: concentration 1-3 g / L, deionized water as the dispersion medium, ultrasonic dispersion (power 400 W, frequency 40 kHz) for 30 minutes.

[0142] Graphene oxide dispersion:

[0143] Raw materials: Graphene oxide (GO, sheet size 0.5-5 μm, thickness 0.8-1.2 nm, C / O ratio 2.0-3.0, prepared by modified Hummers method, supplier: Xianfeng Nano or equivalent); Purification: Dialyze to neutral using a dialysis bag (molecular weight cutoff 3500); Dispersion conditions: Concentration 1-3 g / L, ultrasonic dispersion (power 300W, frequency 25 kHz) for 20 minutes.

[0144] Layered bimetallic hydroxide dispersion:

[0145] Raw materials: Mg-Al hydrotalcite (Mg / Al molar ratio 2:1-3:1, specific surface area 80-120 m² / g, supplier: Beijing Deco Island Gold or self-made); Synthesis conditions: co-precipitation method, Mg(NO3)2·6H2O and Al(NO3)3·9H2O as precursors, pH=10±0.2, hydrothermal at 100℃ for 24 hours; Dispersion conditions: concentration 1-3 g / L, ultrasonic dispersion (power 350W) for 25 minutes.

[0146] Cellulose nanocrystal dispersion:

[0147] Raw materials: Cellulose nanocrystals (CNC, aspect ratio 10-20, width 3-8 nm, length 100-300 nm, sulfate content 200-400 μmol / g, supplier: FP Innovations, Canada or equivalent); Purification: Ion exchange resin treatment to conductivity <50 μS / cm; Dispersion conditions: Concentration 1-3 g / L (preferably 2-2.5 g / L), dispersed by gentle stirring.

[0148] Standardized preparation parameters: pH adjustment: Adjust to pH 8-9 (preferably 8.2-8.8) with NaOH solution (analytical grade, 0.1 mol / L); Zeta potential: -25 to -45 mV (Zetasizer determination); Storage: Store at 4℃, stir gently before use; Stability: Use within 24 hours, with no significant sedimentation during this period.

[0149] The layer-by-layer deposition process is carried out using automated impregnation equipment:

[0150] The fibers are passed sequentially through a positively charged polyelectrolyte solution tank and a negatively charged nanomaterial dispersion tank at a constant speed (2-8 mm / min, preferably 5 mm / min), with each immersion time being 1-3 minutes (preferably 2 minutes). The solution temperature is controlled at 20-25°C, and the solution is gently stirred in the tank (30-50 rpm) to keep it homogeneous.

[0151] Cleaning process: After each deposition, the fibers are immediately rinsed in a deionized water bath (pH 6.5-7.5, conductivity <5 μS / cm) for 30-60 seconds to remove loosely adsorbed material and retain firmly bound monolayers. Deposition quality is monitored in real time using a quartz crystal microbalance (QCM). Deposition amounts for each monolayer are: 15-25 ng / cm² for polyelectrolyte layers and 8-18 ng / cm² for nanomaterial layers.

[0152] In-situ curing: After every 2-3 bilayers (i.e., 4-6 monolayers) are deposited, the fiber is immediately passed through the curing zone for in-situ curing. The curing equipment can be selected from:

[0153] Infrared irradiation curing: Use a medium-wave infrared heater (wavelength 8-12 micrometers, power density 0.8-1.5 W / cm²), temperature controlled at 80-120℃ (preferably 100℃), irradiation time 2-5 minutes (preferably 3 minutes), irradiation distance 8-15 cm.

[0154] Hot air convection curing: Use a hot air circulating oven with a hot air temperature of 80-120℃ (preferably 90-110℃), a wind speed of 1.5-3.0 m / s, and a curing time of 2-5 minutes (preferably 4 minutes).

[0155] At the curing temperature, the pre-added ammonium persulfate thermal initiator (decomposition temperature 60-80℃) decomposes to produce sulfate free radicals (SO4). - • (Redox potential 2.6 V) initiates free radical cross-linking reactions between polyelectrolyte molecules, forming a chemical cross-linking network.

[0156] The above deposition-cleaning-curing process is repeated 8-12 times, forming a total of 20-30 bilayers with a total coating thickness of 200-800 nanometers. The resulting coating has alternating flame-retardant polyelectrolyte layers and inorganic nanosheet layers, forming a dense multilayer barrier structure with nanoscale interlayer spacing. Each bilayer is bonded by electrostatic attraction, and every 2-3 bilayers are firmly bonded by chemical cross-linking.

[0157] Step 5: Circulating Atmosphere Gradient Processing

[0158] This step is the core of achieving coordinated solidification of various functional sections, breaking through the limitations of traditional single-atmosphere treatment.

[0159] Equipment requirements: A multi-segment temperature-controlled tube furnace must be used, with a furnace length ≥80 cm and an inner diameter of 15-25 mm. It must be equipped with an independent three-segment temperature control system (temperature control accuracy ±2℃), quartz furnace tubes (purity ≥99.9%), a high-temperature resistant sealing system, a gas flow controller (accuracy ±2%), and an exhaust gas treatment device. Safety facilities include a gas leak detector, emergency shut-off valve, and ventilation system.

[0160] The coated fibers are placed in the center of the quartz furnace tube, with the fiber tension controlled at 0.5-2.0 N, and subjected to three rounds of circulating atmosphere treatment.

[0161] First round of ammonia atmosphere handling: Safety precautions: Ammonia is toxic and highly irritating. Before operation, the ventilation system must be checked, and an ammonia leak detector (alarm concentration 25 ppm) must be installed. Operators must wear gas masks, protective clothing, and protective gloves. Emergency flushing equipment and first-aid supplies must be provided.

[0162] Pure ammonia gas (purity ≥99.5%, moisture content <100 ppm) is prepared using a liquid ammonia vaporizer, with the flow rate precisely controlled at 50-100 mL / min (preferably 70 mL / min) using a mass flow meter. The furnace tubes are pre-purified with high-purity nitrogen gas (purity ≥99.999%) for 15 minutes before being switched to an ammonia atmosphere.

[0163] Heating program: Increase the temperature from room temperature to 180-220℃ (preferably 200℃) at a rate of 5-10℃ / min, and maintain the temperature for 30-50 minutes (preferably 40 minutes) after stabilization. The furnace tube outlet is treated by a two-stage absorption system: the first stage uses an alkaline gas washing bottle (5% NaOH solution) to neutralize the exhaust gas, and the second stage uses an activated carbon adsorber for further purification, ensuring that the ammonia concentration in the exhaust gas is <5 ppm before discharge.

[0164] Chemical reaction mechanism: Ammonia molecules undergo ammonolysis with the phosphate ester groups (RO-PO(OR')2) in the phosphorus-based flame retardant of section A. At a reaction temperature ≥180℃, some alkoxy groups are replaced by amino groups to form phosphoramide groups (RO-PO(OR')(NH2)), with a reaction conversion rate of 35-50%. Simultaneously, ammonia molecules diffuse into the nano-coating and dehydrate with the carboxyl groups in the polyelectrolyte at high temperature to form amide bonds (-CONH2), with an amidation degree of 20-35%.

[0165] Cooling procedure: After the treatment is completed, turn off the ammonia gas and introduce nitrogen gas for protection. Slowly cool down to room temperature at a rate of 2-5℃ / min (preferably 3℃ / min) to prevent microcracks from being generated by thermal stress.

[0166] Gas preparation: High-purity nitrogen (purity ≥99.999%) is used as the carrier gas. Sulfur dioxide (purity ≥99.9%, moisture content <50 ppm) is precisely controlled by a pressure reducing valve and a mass flow meter. The volume ratio of nitrogen to sulfur dioxide is precisely controlled at 5:1 (±0.1), and the total flow rate is 60-90 mL / min (preferably 75 mL / min).

[0167] Heating and treatment: Continue heating from the final temperature of the first round of treatment at a rate of 3-8℃ / min to 220-260℃ (preferably 240℃), and maintain the temperature for 35-55 minutes (preferably 45 minutes) after stabilization. The furnace tube outlet is equipped with a two-stage tail gas treatment system: a first-stage alkaline absorption tower (6% NaOH solution + 3% H2O2 solution) and a second-stage activated carbon adsorber, ensuring that the SO2 concentration in the tail gas is <1ppm before discharge.

[0168] Chemical reaction mechanism: At a temperature of 220-260℃, sulfur dioxide gas undergoes surface adsorption and molecular penetration with the B-segment melamine derivative. Sulfur dioxide molecules combine with the amino groups in melamine molecules through Lewis acid-base interactions, forming a sulfur-containing complex in the melamine network structure, with the amount of sulfur atoms introduced reaching 0.8-1.5 wt%.

[0169] Cooling procedure: After the treatment is completed, stop the SO2 supply, continue to purge with nitrogen for protection, and cool down to room temperature at a rate of 3-6℃ / min to ensure stable binding of sulfur compounds.

[0170] Third round of nitrogen-organosilicon vapor mixed atmosphere treatment: Safety precautions: Hexamethyldisilazane (HMDS) is flammable, and its vapor is irritating to the eyes and respiratory tract. It must be handled away from ignition sources, using explosion-proof electrical equipment and equipped with an organic gas detector. Operators must wear protective eyewear and gloves, and operate in a well-ventilated environment.

[0171] Chemical compatibility confirmation: HMDS is incompatible with residual SO2 from previous steps, but under nitrogen protection and heating conditions, the residual SO2 content is <0.01 ppm, which will not cause a hazardous reaction. The thermal decomposition products of HMDS are compatible with silicon-based flame retardants, with no risk of side reactions.

[0172] Steam generation system: An explosion-proof constant-temperature steam generator is used to vaporize hexamethyldisilazane (HMDS, CAS No. 999-97-3, purity ≥99%, boiling point 125℃, flash point -9℃) at 100-110℃. The resulting organosilicon vapor is mixed with high-purity nitrogen gas at a volumetric flow ratio precisely controlled at 10:1 (±0.2), with a total flow rate of 70-110 mL / min (preferably 90 mL / min). The steam generator is equipped with a condensation recovery device and anti-static measures to avoid waste of organosilicon compounds and fire risks.

[0173] Heating and treatment: Continue heating from the second-round treatment temperature at a rate of 2-6℃ / min to 260-320℃ (preferably 290℃), and maintain the temperature for 40-60 minutes (preferably 50 minutes) after stabilization. The furnace tube outlet is equipped with an organic solvent condensation and recovery device and an activated carbon adsorption device to treat the exhaust gas.

[0174] Chemical reaction mechanism: At high temperatures of 260-320℃, hexamethyldisilazane undergoes thermal decomposition: (CH3)3Si-NH-Si(CH3)3 → active silicon nitrogen groups + volatile small molecules. The generated active silicon nitrogen groups react with the silanol groups (Si-OH) in the C-segment silicon-based flame retardant, forming a Si-N-Si cross-linked structure through deamination, with a cross-linking degree reaching 60-80%. Simultaneously, this promotes further condensation of the siloxane network, enhancing thermal stability.

[0175] Cooling procedure: After the treatment is completed, stop the supply of HMDS, continue to introduce nitrogen for protection, and cool down to room temperature at a rate of 2-4℃ / min (preferably 3℃ / min) to ensure the stable formation of the Si-N bond structure.

[0176] The three-cycle treatment employs a strategy of increasing temperature (180-220℃→220-260℃→260-320℃) and targeted selection of atmosphere components to ensure that the three functional sections—A section (phosphorus-based), B section (nitrogen-based), and C section (silicon-based)—can complete the curing reaction and functional optimization under their respective most suitable chemical environment and temperature conditions. This results in multifunctional flame-retardant fibers with a curing degree of 85-92% for each functional section and synergistic flame-retardant performance.

[0177] Step Six: Post-processing and Quality Control

[0178] Post-processing: The fibers that have undergone circulating atmosphere gradient treatment are placed in a forced-air drying oven for post-processing. Specific process conditions:

[0179] Temperature: 120-150℃ (preferably 135℃), temperature uniformity ±3℃; Time: 2-4 hours (preferably 3 hours); Relative humidity: <5% (maintained by dry air circulation); Air velocity: 0.5-1.2 m / s, to ensure uniform heating and effective removal of volatiles.

[0180] Post-treatment functions to remove residual moisture (down to <0.5%), solvents, and unreacted small molecule compounds, stabilize the chemical structure and physical morphology of each functional layer, release internal fiber stress, and enhance the bonding strength between layers.

[0181] Verification Experiment

[0182] Experiment 1: Verification Experiment of Segmented Synergistic Flame Retardant Effect

[0183] 1. Experimental Objective

[0184] The segmented synergistic flame retardant mechanism of the fiber of the present invention in different temperature ranges was verified, and the synergistic flame retardant effect of the three functional zones of phosphorus-based (segment A), nitrogen-sulfur (segment B), and silicon-nitrogen (segment C) was confirmed.

[0185] 2. Preparation of experimental samples

[0186] The following samples were prepared according to Embodiment 1:

[0187] Sample 1: Unmodified polyimide fiber (control group);

[0188] Sample 2: Single phosphorus-based modified fiber (phosphate grafted only in segment A);

[0189] Sample 3: Single nitrogen-modified fiber (melamine derivative grafted only into segment B);

[0190] Sample 4: Single silicon-based modified fiber (C segment grafted with organosilicon flame retardant only).

[0191] Sample 5: Segmented synergistic flame-retardant fiber of the present invention (prepared using complete process);

[0192] Twenty fibers were prepared for each sample, each 100 mm in length and 15 ± 2 micrometers in diameter.

[0193] 3. Experimental conditions

[0194] Ambient temperature: 20±2℃;

[0195] Relative humidity: 65±5%;

[0196] Experimental equipment: Differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA), gas chromatograph-mass spectrometer (GC-MS), limiting oxygen index analyzer.

[0197] 4. Experimental Procedure

[0198] Thermogravimetric analysis test: The sample was heated from room temperature to 800℃ at a heating rate of 10 K / min under a nitrogen atmosphere, and the mass loss curve was recorded.

[0199] Thermal decomposition gas analysis by temperature range: GC-MS was used to analyze the composition and content of gases produced by thermal decomposition in three temperature ranges: 300-500℃, 400-600℃, and 600-800℃.

[0200] Limiting Oxygen Index (LOI) Test: The LOI value of each sample was determined according to GB / T 5454-1997 standard;

[0201] Analysis of char residue morphology: The structure and thickness of the char residue were observed using SEM after combustion;

[0202] Synergistic effect calculation: The theoretical synergistic effect is calculated based on the LOI contribution value of each individual system and compared with the measured value.

[0203] 5. Experimental Results

[0204] Table 1 Experimental data on the synergistic flame retardant effect of segmentation

[0205]

[0206] Figure 1 It is a comparison of limiting oxygen index;

[0207] Figure 2 It is a comparison of thermogravimetric analysis curves;

[0208] Figure 3 This is an analysis of the synergistic effect of flame retardant mechanisms.

[0209] 6. Analysis and Summary

[0210] The LOI value of the fiber of this invention reaches 63.2%, which is 109.3% higher than that of unmodified fiber, far exceeding the effect of a single modification method;

[0211] The synergy coefficient was 1.15, confirming that the segmented flame-retardant system has a significant synergistic effect.

[0212] In the 300-500℃ temperature range, phosphorus-based functions play a dominant role, with a carbonization rate of 42.8%; in the 400-600℃ temperature range, nitrogen and sulfur synergistic gas phase dilution effect is significant, with a gas release of 16.8 mL / g; in the high-temperature range >600℃, silicon and nitrogen synergistically form a 7.1μm thick protective layer.

[0213] The temperature-domain synergistic flame-retardant mechanism has been verified, achieving multiple protection effects across the entire temperature domain.

[0214] Experiment 2: Verification Experiment on the Anti-migration Effect of Inter-segment Barrier

[0215] 1. Experimental Objective

[0216] The study aimed to verify the inhibitory effect of the inter-segment barrier layer on flame retardant migration and to evaluate the long-term stability of the functional zones and the effectiveness of the barrier layer.

[0217] 2. Preparation of experimental samples

[0218] The following samples were prepared according to Embodiment 1:

[0219] Sample A: Segmented grafted fibers without barrier layers (Segment A: phosphorus-based + Segment B: nitrogen-based + Segment C: silicon-based, but without intersegment barrier layers).

[0220] Sample B: Fibers (including intersegment barrier layers) prepared using the complete process of this invention.

[0221] Sample C: Traditional uniformly mixed modified fiber (uniformly distributed phosphorus, nitrogen, and silicon);

[0222] Fifteen fibers were prepared for each sample, each 150 mm long and 15 ± 2 micrometers in diameter, with the length ratio of each functional segment being 1:1:1.

[0223] 3. Experimental conditions

[0224] Accelerated aging conditions: 80℃, relative humidity 65%, according to GB / T 7141-2008 standard;

[0225] Detection equipment: X-ray fluorescence spectrometer (XRF), scanning electron microscope (SEM), X-ray photoelectron spectrometer (XPS);

[0226] Testing time points: 0 days, 30 days, 90 days, 180 days.

[0227] 4. Experimental Procedure

[0228] Initial state detection: The distribution of phosphorus, nitrogen, and silicon content was detected every 5 mm along the fiber axis using XRF;

[0229] Accelerated aging treatment: The sample is placed in a constant temperature and humidity chamber for accelerated aging;

[0230] Periodic testing: Take samples at various time points and repeat the testing in step (1);

[0231] Interface diffusion analysis: SEM was used to observe the morphological changes at the interface between segments and to measure the diffusion distance;

[0232] Barrier layer integrity testing: XPS analysis was used to analyze changes in the chemical composition of the barrier layer.

[0233] 2. Experimental Results

[0234] Table 2 Experimental data on the anti-migration effect of inter-segment barrier

[0235]

[0236] Figure 4 This demonstrates a comparison of the stability of the functional areas;

[0237] Figure 5 The comparison of flame retardant migration distances was shown;

[0238] Figure 6 A heatmap showing the distribution of elements in the unobstructed layer (after 180 days) is presented.

[0239] 6. Analysis and Summary

[0240] The inter-segment barrier layer significantly inhibited the diffusion and migration of flame retardants, with a functional area retention rate of 94% after 180 days, compared to only 42% for the sample without the barrier layer.

[0241] The interfacial diffusion distance was effectively controlled within the range of 0.3-0.4 mm, which is 97.4% lower than the 15.2 mm diffusion distance without a barrier layer.

[0242] The elemental distribution heatmap shows that the fibers of this invention still maintain clear functional partition boundaries after long-term aging;

[0243] The labyrinth structure and chemical cross-linking mechanism of the inter-segment barrier layer effectively prevented the migration of small molecule flame retardants, verifying long-term stability.

[0244] Experiment 3: Verification Experiment of Barrier Performance of Multilayer Nanocoating

[0245] 1. Experimental Objective

[0246] To verify the gas barrier properties and interfacial bonding strength of the layer-by-layer self-assembled nanocoating, and to evaluate the effect of in-situ curing technology on improving coating performance.

[0247] 2. Preparation of experimental samples

[0248] The following samples were prepared according to Embodiment 1:

[0249] Sample I: Uncoated base fiber (only segmented grafting and barrier layer construction completed);

[0250] Sample II: Traditional layer-by-layer self-assembled coated fiber (20 double layers, no in-situ curing);

[0251] Sample III: In-situ cured self-assembled coated fiber of the present invention (20 double layers, 2-3 double layers cured in situ each).

[0252] Sample IV: Complete process fiber of this invention (25 double layers, in-situ curing + atmosphere treatment);

[0253] Twelve fibers were prepared for each sample, each 100 mm long and 15 ± 2 micrometers in diameter.

[0254] 3. Experimental conditions

[0255] Test environment: Standard atmospheric conditions (23±2℃, 50±5%RH);

[0256] Testing equipment: air permeability tester, universal testing machine, atomic force microscope (AFM), differential scanning calorimeter (DSC);

[0257] Test standards: GB / T 1038-2000 (air permeability), GB / T 2790-1995 (peel strength).

[0258] 4. Experimental Procedure

[0259] Oxygen permeability test: The oxygen permeability of each sample was determined according to GB / T 1038 standard;

[0260] Coating peel strength test: The bonding strength between the coating and the substrate is determined using a universal testing machine;

[0261] Surface morphology analysis: AFM was used to observe the surface roughness and density of the coating;

[0262] Thermal performance testing: The glass transition temperature and thermal stability of the coating were analyzed using DSC.

[0263] Coating thickness uniformity test: Observe the coating thickness distribution using SEM cross-section.

[0264] 5. Experimental Results

[0265] Table 3 Experimental data on the barrier properties of multilayer nanocoatings

[0266]

[0267] Figure 7 This demonstrates a comparison of the strength of interface integration;

[0268] Figure 8 The comparison of surface properties is shown.

[0269] 6. Analysis and Summary

[0270] The in-situ curing technology of this invention significantly improves coating performance, reducing oxygen permeability from 8.6 cm³ / (m²·day·atm) in the traditional process to 1.8 cm³ / (m²·day·atm), a reduction of 79.1%.

[0271] The peel strength was increased from 3.2 MPa in the traditional process to 11.2 MPa, an increase of 250%, resulting in a stronger interfacial bond.

[0272] The surface roughness was significantly reduced to 2.8 nm, and the coating density reached 96%, forming a high-quality nanoscale barrier structure.

[0273] With the increase of the number of coating layers, the barrier performance and bonding strength show a good linear improvement trend;

[0274] The in-situ curing mechanism enables the ordered arrangement and chemical cross-linking of polyelectrolyte molecules, eliminating structural defects in traditional processes.

[0275] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A process for preparing polyimide composite fibers, characterized in that, Includes the following steps: Surface activation treatment: Plasma treatment or ozone oxidation is performed on polyimide fibers to introduce active functional groups on the fiber surface; Segmented Differentiated Grafting: The continuous fiber is divided into three functional regions: A, B and C. Phosphate ester flame retardants are grafted onto section A to form a phosphorus-based flame retardant functional layer, melamine derivatives are grafted onto section B to form a nitrogen-based flame retardant functional layer, and organosilicon flame retardants are grafted onto section C to form a silicon-based flame retardant functional layer. Construction of inter-segment barrier layer: A barrier layer solution composed of polyvinyl alcohol, nano-layered silicate and crosslinking agent is applied at the junction of segments AB and BC to form an inter-segment barrier layer; The intersegment barrier layer solution is composed of: 10-15% polyvinyl alcohol, 5-10% sodium carboxymethyl cellulose, 5-8% nano-layered silicate, and 2-3% crosslinking agent, with water or a water-ethanol mixture as the solvent. Layer-by-layer self-assembled nano-coating deposition and in-situ curing: Fibers are sequentially immersed in a positively charged polyelectrolyte solution and a negatively charged nanomaterial dispersion for layer-by-layer deposition. In-situ curing is performed after every 2-3 bilayers are deposited. In the layer-by-layer self-assembled nanocoating deposition, the positively charged polyelectrolyte solution contains chitosan-phosphate ester complex, gelatin phosphorylation modified product, or polyallylamine hydrochloride, with a concentration of 2-5 g / L and a pH value of 4-5; the negatively charged nanomaterial dispersion contains montmorillonite, graphene oxide, layered bimetallic hydroxide, or cellulose nanocrystals, with a concentration of 1-3 g / L and a pH value of 8-9. Circulating atmosphere gradient treatment: sequentially performing ammonia atmosphere treatment, nitrogen-sulfur dioxide mixed atmosphere treatment, and nitrogen-organosilicon vapor mixed atmosphere treatment; Post-treatment: Heat treatment at 120-150℃ for 2-4 hours.

2. The preparation process according to claim 1, characterized in that, In the segmented differentiated grafting, the phosphate ester flame retardant in segment A is selected from at least one of diphenyl phosphate and tri(2-chloroethyl) phosphate; the melamine derivative in segment B is selected from at least one of melamine phosphate and melamine cyanurate; and the organosilicon flame retardant in segment C is selected from at least one of aminopropyltriethoxysilane and phenyltrimethoxysilane.

3. The preparation process according to claim 1, characterized in that, In the segmented differentiated grafting, the length ratio of each segment is 1:1:1 or 2:1:1, the grafting reaction temperature is 50-110℃, and the reaction time is 30-100 minutes.

4. The preparation process according to claim 1, characterized in that, The nanolayered silicate is selected from at least one of sepiolite nanotubes, halloysite nanotubes, and montmorillonite, and the crosslinking agent is selected from at least one of glutaraldehyde, citric acid, and borax.

5. The preparation process according to claim 1, characterized in that, The in-situ curing involves pre-adding ammonium persulfate thermal initiator to a positively charged polyelectrolyte solution, and curing at 80-120°C for 2-5 minutes after every 2-3 double layers are deposited.

6. The preparation process according to claim 1, characterized in that, In the cyclic atmosphere gradient treatment, the first round of ammonia atmosphere treatment is at a temperature of 180-220℃ for 30-50 minutes; the second round of nitrogen-sulfur dioxide mixed atmosphere treatment is at a temperature of 220-260℃ for 35-55 minutes; and the third round of nitrogen-organosilicon vapor mixed atmosphere treatment is at a temperature of 260-320℃ for 40-60 minutes.

7. The preparation process according to claim 1, characterized in that, The layers self-assemble to form 20-30 double layers, with a total coating thickness of 200-800 nanometers.

8. The polyimide composite fiber prepared by the preparation process according to any one of claims 1-7, characterized in that, It has a limiting oxygen index (LOI) of 60-68%, achieves a V-0 rating in vertical burning, and retains ≥90% of its tensile strength.

Images