A composite modifier, a preparation method thereof, and an anti-urine-yellowing chinlon 6 fiber and a preparation method thereof

Abstract

This invention belongs to the field of chemical synthesis and fiber preparation technology, and particularly relates to a composite modifier and its preparation method, as well as a urine-resistant nylon 6 fiber and its preparation method. This invention utilizes a triple synergistic mechanism of "amine hydrolysis inhibition - urate chelation - pigment adsorption" to block the chemical and physical causes of yellowing, fundamentally solving the problem of urine-resistant yellowing. Epichlorohydrin is used for crosslinking, and amino groups are introduced at the ends to form amino-terminated cyclodextrin derivatives. Urate chelation is achieved by grafting polyethylene glycol with sodium citrate, and nano-hydroxyapatite is introduced as an auxiliary component to reduce the generation of chromophores at the source. The fiber prepared by this invention maintains excellent mechanical properties while being formaldehyde-free and bisphenol A-free, fully complying with the safety standards for children's textiles. The innovative combination of in-situ polymerization and spray drying processes ensures uniform dispersion of functional components, solving the long-standing industry problem of agglomeration.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis and fiber preparation technology, and particularly relates to a composite modifier and its preparation method, and nylon 6 fiber with anti-urine yellowing and its preparation method. Background Technology

[0002] Nylon 6 fiber, with its excellent softness, high specific strength, and outstanding abrasion resistance, has become an indispensable basic material in the textile and apparel industry, especially in areas such as underwear and sportswear. However, in specific applications such as children's underwear and elderly care products, the fabric inevitably comes into repeated contact with urine, causing significant and irreversible yellowing of the fibers. This yellowing phenomenon not only seriously damages the appearance and aesthetics of the products but also directly restricts the lifespan and consumer experience of related textile products, becoming a technical bottleneck that has plagued the industry for many years.

[0003] Research has revealed that the yellowing mechanism of nylon 6 caused by urine contact differs significantly from conventional photothermal oxidation or phenolic yellowing. The core reasons are twofold: First, urea in urine is easily hydrolyzed to ammonia by urease. Ammonia molecules, acting as nucleophiles, launch nucleophilic attacks on the amide bonds (-CO-NH-) on the main chain of nylon 6 macromolecules in a humid and hot environment, initiating an ammonolysis reaction. This reaction leads to the breakage and degradation of the polymer chain, generating a series of chromophores containing carbonyl or conjugated structures, causing the fiber to yellow. Second, uric acid in urine forms insoluble urate crystals when it comes into contact with calcium and magnesium ions in human sweat or water. These crystals not only contribute to the color themselves but also deposit and embed themselves on or inside the fiber surface, further exacerbating the yellowing. Patent CN104294583A discloses a method for preparing an antibacterial, UV-resistant, deodorizing, and self-cleaning composite functional fabric. This technology employs a two-step treatment process involving an activated modified treatment liquid and a nano-titanium dioxide dispersion to impart photocatalytic self-cleaning properties to the fabric. While its deodorizing function effectively removes substances like ammonia, its mechanism relies on photocatalytic degradation—specifically, the decomposition of odor molecules by nano-titanium dioxide under light conditions. However, for nylon 6 fibers, urine stain yellowing is a chemical chain-breaking process, not a physical or chemical elimination of odors through surface adsorption. This patented deodorizing function depends on light conditions and cannot effectively prevent ammonia molecules from attacking the fiber's molecular chains in dark environments (such as the inside of clothing) or humid conditions. Furthermore, this technology does not address the deposition and discoloration issues associated with urate crystals.

[0004] Currently, the industry's protective technologies for yellowing of nylon 6 fibers mainly fall into two categories: one is to introduce general-purpose antioxidants (such as hindered phenols) or light stabilizers in situ during the polymerization stage, aiming to inhibit photothermal oxidative aging of the fibers during processing or use; the other is to use anti-phenolic yellowing agents in conjunction with acid regulators for padding-baking treatment during fabric finishing to resist yellowing caused by phenolic contaminants (such as BHT) during packaging and storage. However, existing technologies do not effectively block the core chemical pathway of urine stain yellowing, "ammonia-induced hydrolysis," and also lack specific inhibition of urate crystal formation. In addition, existing finishing agents are prone to detachment after repeated washing, resulting in insufficient long-term protective performance and failing to meet the actual needs of underwear in repeated wearing and washing scenarios.

[0005] Therefore, there is an urgent need to develop a dual-effect protection technology that can simultaneously inhibit aminolysis and urate residue. Through innovative modifier design and preparation process, the problem of urine staining and yellowing of nylon 6 fiber can be fundamentally solved, filling the gap in the field of targeted protection in existing technologies. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a composite modifier and its preparation method, and an anti-urine yellowing nylon 6 fiber and its preparation method. First, a composite modifier is prepared and then used to prepare anti-urine yellowing nylon 6 fiber. Through the triple synergistic mechanism of "amine hydrolysis inhibition-urate chelation-pigment adsorption", the problem of urine yellowing of nylon 6 fiber is fundamentally solved, and a breakthrough in the durability and stability of the protective effect is achieved.

[0007] The technical solution adopted is as follows: A method for preparing a composite modifier includes the following steps: (1) Preparation of amino-modified β-cyclodextrin: β-cyclodextrin was dissolved in sodium hydroxide solution, epichlorohydrin was added under heating conditions to carry out the reaction, and then ammonia was added to maintain the temperature and continue the reaction. After the reaction was completed, the pH was adjusted to neutral, and amino-modified β-cyclodextrin was obtained by ethanol precipitation and drying. (2) Sodium citrate grafted polyethylene glycol: Polyethylene glycol, citric acid and catalyst are mixed in toluene, heated under reflux and continuously dehydrated using a water separator. After the solvent is removed, the crude product is neutralized, dialyzed and freeze-dried to obtain sodium citrate grafted polyethylene glycol product. (3) Preparation of composite modifier: The amino-modified β-cyclodextrin obtained in step (1) and nano-hydroxyapatite are ultrasonically dispersed in deionized water, and the sodium citrate-grafted polyethylene glycol obtained in step (2) is added. The mixture is heated and stirred to form a homogeneous colloid, and finally spray-dried to obtain a microsphere-shaped composite modifier.

[0008] Preferably, in step (1), the mass fraction of the sodium hydroxide solution is 3-6%, the mass fraction of the ammonia is 20-28%, the reaction temperature is 50-60℃, and the reaction time after adding epichlorohydrin is 5-6 hours; ammonia is added dropwise to the reaction system, and the reaction time after adding ammonia is 5-6 hours. The mass ratio of β-cyclodextrin, epichlorohydrin and ammonia is 100:(5-10):(2-4).

[0009] Preferably, in step (2), the dehydration reaction time is 10 to 12 hours; the mass ratio of polyethylene glycol to citric acid is 1:(0.2 to 0.3).

[0010] Preferably, in step (3), ultrasonic dispersion is performed for 30-40 minutes, the temperature of the heated water bath for stirring is 50-60°C, the stirring time is 2-3 hours, the inlet temperature of the spray drying is 180-200°C, and the particle size of the nano-hydroxyapatite is 50-100 nm. The mass ratio of amino-modified β-cyclodextrin, sodium citrate-grafted polyethylene glycol, and nano-hydroxyapatite is (3-5):(2-4):1.

[0011] The present invention also provides a composite modifier, the composite modifier comprising a composite modification system, wherein the mass ratio of amino-modified β-cyclodextrin, sodium citrate-grafted polyethylene glycol and nano-hydroxyapatite in the composite modification system is (3-5):(2-4):1.

[0012] This invention provides a nylon 6 fiber resistant to urine stains and yellowing, using the composite modifier of this invention. Based on the mass of caprolactam, the mass ratio range of each component in the raw materials is as follows: Caprolactam: 92-95%; Composite modification system: 5-8%; Tetrabutyl titanate: 0.1-0.3%.

[0013] The present invention provides a method for preparing nylon 6 fiber resistant to urine stains and yellowing, comprising the following steps: S1. Composite modifier dispersion polymerization system: Add caprolactam to the polymerization reactor, introduce inert gas for displacement, and heat to completely melt caprolactam; Tetrabutyl titanate was added to the polymerization reactor and mechanically stirred to disperse it evenly; then the composite modifier was added and stirring continued. S2. In-situ polymerization: Heat up and add deionized water to carry out the reaction (first, to initiate ring-opening polymerization, and second, to promote the hydrolysis of tetrabutyl titanate), so that tetrabutyl titanate is hydrolyzed to generate nano-titanium dioxide. S3. Post-processing and spinning: After polymerization, excess water is removed by depressurization, and the polymer melt is directly transported to the spinning system. After extrusion, cooling, oiling, drawing, and winding by the spinneret, urea-resistant yellowing nylon 6 fiber is obtained.

[0014] Preferably, in step S1, nitrogen purging is used to ensure that the oxygen content is below 5 ppm; the heating temperature is 80-90°C, the stirring speed is controlled at 200-300 rpm, and the temperature is maintained and stirring is continued for 1-2 hours after the composite modifier is added.

[0015] Preferably, in step S2, the temperature is raised to 250-260°C, the pressure is controlled at 0.2-0.5 MPa, and the reaction time is 4-6 hours; wherein the amount of deionized water added is 0.3-0.5% of the mass of caprolactam.

[0016] Preferably, in step S3, the stretching ratio is 3.5 to 4.5 times, and the heat setting temperature is controlled at 160 to 180°C.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention addresses the problem of urine stain yellowing by blocking the chemical and physical causes of yellowing through a triple synergistic mechanism of "amine hydrolysis inhibition - urate chelation - pigment adsorption". This invention uses epichlorohydrin crosslinking and introduces amino groups at the ends to form an amino-terminated cyclodextrin derivative (NH2-β-CD). This derivative, with its unique cavitary structure of cyclic oligosaccharides and the high amino affinity of its surface amino groups, can effectively capture ammonia molecules released from urine, encapsulating them within a hydrophobic cavity (also adsorbing pigments), blocking their contact with the amide bonds of nylon 6. The hydroxyl groups on the outer edge of the cyclodextrin molecule remain hydrophilic, ensuring good compatibility between the overall structure and the nylon 6 matrix. The urate chelation mechanism is achieved by grafting polyethylene glycol (PEG-6000) with sodium citrate. This graft copolymer, through an acid-catalyzed esterification reaction, grafts citric acid molecules onto polyethylene glycol chains. The carboxyl groups of the resulting sodium citrate moiety, after neutralization, form stable water-soluble complexes with calcium and magnesium ions in urine, preventing these ions from binding with uric acid to form insoluble urate crystals. Simultaneously, the steric hindrance effect of the PEG segments further inhibits the deposition of urate microcrystals on the fiber surface. After 50 accelerated urine stain simulation tests, the fiber whiteness retention rate remained above 92% (GB / T 8424.2-2001), and the yellowing grade was ≥4 (refer to GB / T29778-2013), far superior to ordinary nylon 6 and commercially available anti-yellowing products.

[0018] (2) The composite modifier prepared in this invention also incorporates nano-hydroxyapatite (particle size 50-100nm) as an auxiliary component. Its unique porous structure and huge specific surface area can effectively adsorb pigment precursors in urine, such as bilirubin degradation products, thereby reducing the generation of chromophores from the source.

[0019] (3) The composite modifier prepared in this invention assembles three functional components into microspheres through spray drying, forming a microscopic "reaction chamber". This structure can form a uniform microscopic "protective island" in the fiber. When urine invades, the hydrophilic layer (PEG) on the surface of the microsphere guides the liquid to penetrate, and the active ingredients (cyclodextrin, hydroxyapatite) inside gradually and continuously exert their effects, achieving the durability and stability of the protective effect, rather than the one-time consumption of the surface coating.

[0020] (4) In the preparation of the fiber of the present invention, the addition of tetrabutyl titanate not only acts as a catalyst for the ring-opening polymerization of caprolactam and promotes the polymerization reaction; its hydrolysis product TiO2 also acts as a coupling / crosslinking promoter, which helps to improve the interfacial bonding force between the composite modifier (organic-inorganic hybrid system) and the nylon 6 matrix, so that the modifier is more firmly anchored in the fiber, further enhancing the wash resistance and durability.

[0021] (5) The fibers prepared by this invention maintain excellent mechanical properties while being formaldehyde-free and bisphenol A-free, fully complying with the safety standards for children's textiles. The innovative combination of in-situ polymerization and spray drying processes ensures uniform dispersion of functional components, solves the long-standing problem of modifier agglomeration in the industry, and achieves a breakthrough in the durability and stability of protective effects. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the entire process technology of the present invention.

[0023] Figure 2 This is a schematic diagram of the structure of the amino-modified β-cyclodextrin prepared in this invention.

[0024] Figure 3 This is a planar structural diagram of sodium citrate-grafted polyethylene glycol prepared according to the present invention; wherein, -O - Na + For Ca 2+ / Mg 2+ Chelation site.

[0025] Figure 4 This is a schematic diagram of the structure of the composite modifier prepared in this invention; wherein, A: amino-modified β-cyclodextrin; B: sodium citrate grafted with polyethylene glycol; C: nano-hydroxyapatite. Detailed Implementation

[0026] The accompanying drawings are for illustrative purposes only; to make the technical solution of the present invention clearer, the present invention will be fully described below in conjunction with the accompanying drawings and specific embodiments. Unless otherwise specified, all reagents or instruments used are conventional products that can be purchased through market channels.

[0027] Example 1: Preparation of a composite modifier and a nylon 6 fiber resistant to urine staining and yellowing.

[0028] (1) Raw materials and proportions The raw materials required for implementing this invention include: caprolactam, modified β-cyclodextrin (NH2-β-CD), sodium citrate grafted polyethylene glycol (PEG-6000, grafting rate >85%), nano hydroxyapatite (particle size 50-100nm, specific surface area ≥80m² / g), tetrabutyl titanate (chemically pure), and deionized water.

[0029] The mass ratio range of each component is as follows: Caprolactam: 93.8%; Composite modified system: 6.0%; Tetrabutyl titanate: 0.2% (based on caprolactam mass).

[0030] (2) Preparation process The first step is to prepare amino-modified β-cyclodextrin (NH2-β-CD).

[0031] ① Take a certain amount of β-cyclodextrin (200g), add an appropriate amount of 5% sodium hydroxide solution (400ml), and stir continuously at room temperature until the β-cyclodextrin is completely dissolved; ② While stirring continuously, slowly add 15g of epichlorohydrin dropwise using a constant pressure dropping funnel, controlling the dropping rate at 10-15ml / min, and maintain a stable temperature in the reaction system. After the addition is complete, heat the reaction mixture to 60℃ and continue the reaction at this temperature for 6 hours. During this process, epichlorohydrin and β-cyclodextrin undergo cross-linking polymerization. ③ Keep the temperature at 60℃, slowly add 5g of 25% ammonia water to the reaction system, control the dropping rate at 8ml / min, and continue stirring for 6 hours to allow the ammonia water to fully react with the epoxy groups at the ends of the crosslinked polymer; ④ After the reaction is complete, cool the reaction system to room temperature, adjust the pH of the solution to neutral (pH≈7.0) with dilute hydrochloric acid, and slowly pour the neutralized reaction solution into a large amount of anhydrous ethanol (usually about 3 times the volume of the reaction solution) while stirring. At this time, the modified product NH2-β-CD will precipitate out as a white precipitate; ⑤ Collect the white precipitate by vacuum filtration. Wash the precipitate three times with a small amount of ethanol to remove residual salts and unreacted impurities. Finally, place the solid product in a vacuum drying oven and dry it at 60°C to constant weight to obtain the final product—amino-modified β-cyclodextrin (NH2-β-CD). Figure 2 The diagram shows the structure of the amino-modified β-cyclodextrin prepared by the invention.

[0032] The second step is to prepare sodium citrate-grafted PEG.

[0033] ① Take a certain amount of PEG-6000 (50g), citric acid (12g) and catalyst p-toluenesulfonic acid (1.0g) and place them in a dry three-necked flask equipped with a water separator and a reflux condenser, and then add an appropriate amount of toluene (300mL).

[0034] ② Heat the reaction system to toluene reflux (approximately 110°C) and continue the reaction under these conditions for 10 hours. During the reaction, use a water separator to continuously separate and remove the water generated during esterification to ensure complete reaction.

[0035] ③ After the reaction is complete, the system is cooled to about 60°C. A rotary evaporator is then connected, and most of the toluene is evaporated under reduced pressure in a 60°C water bath to obtain a viscous, brownish-yellow crude product (this is citric acid-grafted PEG). ④ Dissolve the crude product in an appropriate amount (about 100 mL) of deionized water, transfer it to a beaker, and slowly adjust the pH to 7.0-8.0 (neutral to weakly alkaline) with 5% NaOH aqueous solution while cooling in an ice-water bath and stirring vigorously. Note: This process is exothermic, and the temperature and dropping rate must be strictly controlled. ⑤ Place the neutralized solution into a pretreated dialysis bag and dialyze it with deionized water in a 4°C refrigerator. Change the water every 6-8 hours for 48 hours to completely remove small molecule impurities such as sodium chloride and unreacted citrate. ⑥ Transfer the liquid from the dialysis bag to a lyophilization bottle and freeze-dry it to obtain a white or off-white loose solid foam, which is the final product—sodium citrate grafted with PEG-600. Figure 3 The diagram shown is a planar structural diagram of sodium citrate-grafted polyethylene glycol prepared according to the present invention.

[0036] The third step is the preparation of the composite modifier.

[0037] Take an appropriate amount of amino-modified β-cyclodextrin (NH2-β-CD) (20g), add an appropriate amount of deionized water (150ml), stir at 350rpm / min for about 20 minutes until NH2-β-CD is completely dissolved; then add a certain amount of nano-hydroxyapatite powder (5g), increase the stirring speed to 600rpm / min for about 15 minutes to initially wet and disperse the nano-powder in the solution; sonicate for 30 minutes at 400w to obtain a fine and uniform milky white suspension without obvious particles; then add sodium citrate-grafted PEG (15g) to the suspension, and mechanically stir at 500rpm for 3 hours in a constant temperature water bath at 60°C, gradually transforming the milky white suspension into a uniform, transparent or translucent viscous colloid. This signifies that the molecular chains of sodium citrate-grafted PEG have fully extended and interwoven under thermal action, encapsulating and bridging NH2-β-CD and nano-hydroxyapatite together to form a stable ternary composite system; finally, after spray drying, a dry, white powdery microsphere composite modifier is obtained (see...). Figure 4 (As shown), seal the package and place it in a desiccator for later use.

[0038] A method for preparing urine-resistant yellowing nylon 6 fiber includes the following steps: Step 1: Preparation of the polymerization system. Add caprolactam to the polymerization reactor, purge with nitrogen three times to ensure the oxygen content is below 5 ppm. Heat to 85°C to completely melt the caprolactam.

[0039] Step 2: Modifier Dispersion. Tetrabutyl titanate is added to the above melt and mechanically stirred for 30 minutes to ensure uniform dispersion. Then, the pre-prepared composite modifier is added, and the stirring speed is controlled at 250 rpm, the temperature is maintained at 85±5℃, and stirring is continued for about 1 hour.

[0040] Step 3: In-situ polymerization. The temperature is slowly increased to 250°C, and deionized water is added (to initiate ring-opening polymerization). The pressure is maintained at 0.3 MPa, and the reaction is carried out for 5 hours. During this process, tetrabutyl titanate gradually hydrolyzes to generate nano-titanium dioxide, which serves as a dispersion medium to ensure uniform distribution of the functional components.

[0041] Step 4: Post-processing and spinning. After polymerization, excess moisture is removed under reduced pressure, and the polymer melt is directly fed into the spinning system. It is then extruded through a spinneret, cooled, oiled, drawn, and wound to obtain urea-resistant yellowing nylon 6 fiber. The draw ratio is 4, and the heat setting temperature is controlled at 160℃.

[0042] Example 2 Preparation of composite modifiers.

[0043] Take 200g of β-cyclodextrin, add it to 400mL of 5% sodium hydroxide solution, and stir to dissolve. Slowly add 15g of epichlorohydrin and react at 60℃ for 6 hours.

[0044] Then, 5g of 25% ammonia solution was added, and the reaction was continued at 60℃ for 6 hours. After the reaction was completed, the pH was adjusted to neutral, and the product was precipitated with ethanol and dried under vacuum to obtain amino-modified β-cyclodextrin (NH2-β-CD).

[0045] 50g of PEG-6000, 12g of citric acid, and 1.0g of p-toluenesulfonic acid were placed in 300mL of anhydrous toluene. A water separator was installed, and the mixture was heated to reflux (approximately 110℃) for 12 hours, continuously separating the water produced during the reaction. After the reaction was complete, the toluene was evaporated, and the crude product was dissolved in water. The pH was adjusted to 7.5 with sodium hydroxide solution, and after dialyzing and freeze-drying, sodium citrate-grafted PEG was obtained with a grafting rate of 89%.

[0046] 20g of NH2-β-CD, 15g of sodium citrate-grafted PEG and 5g of nano-hydroxyapatite (average particle size 80nm) were added to 150mL of deionized water and stirred in a 60℃ water bath to form a homogeneous colloid. Finally, the microsphere composite modifier was obtained by spray drying.

[0047] Preparation of nylon 6 fiber resistant to urine stains and yellowing.

[0048] 45 kg of caprolactam was added to a 50 L polymerization reactor and heated to 85 °C under nitrogen protection to melt it. 135 g of tetrabutyl titanate was added and stirred for 30 minutes. Then, 3.2 kg of the composite modifier prepared in Example 1 was added, and the mixture was stirred at 90 °C and 250 rpm for 90 minutes. The temperature was raised to 255 °C, 150 mL of deionized water was added, and the reaction was carried out at 0.3 MPa pressure for 5 hours. After polymerization, the fiber was spun, cooled, wound at 1200 m / min, and then stretched 3.8 times and heat-set at 170 °C to obtain nylon 6 fiber resistant to urine stains and yellowing.

[0049] Other areas not mentioned are the same as in Example 1.

[0050] Test example: Performance comparison experiment.

[0051] To verify the overall effectiveness of the present invention, the nylon 6 fiber (sample C) prepared in Example 2 of the present invention was compared with commercially available ordinary nylon 6 fiber (sample A) and commercially available anti-yellowing nylon 6 fiber (sample B). The test methods included mechanical property testing and a custom-designed "urine stain simulation-accelerated yellowing experiment". The specific data comparison is shown in Table 1.

[0052] Table 1. Comparison of the nylon 6 fiber prepared by this invention with commercially available ordinary nylon 6 fiber and commercially available anti-yellowing nylon 6 fiber. Test methods and conditions description: (1) Test for resistance to yellowing due to urine stains: The “Urine stain simulation-accelerated yellowing test” was conducted in accordance with the principle of GB / T 3922-2013. The samples were immersed in urine stain simulation solution (urea 2.5%, uric acid 0.05%, sodium chloride 0.8%, calcium chloride 0.02%) for 30 minutes and then dried at 60℃. This process was recorded as one cycle. After all samples completed 50 cycles, the whiteness retention rate was measured and the yellowing grade was evaluated.

[0053] (2) Results analysis: As shown in the table above, the product of the present invention (sample C) has a much better resistance to yellowing than the comparative sample after undergoing a rigorous urine stain simulation test. At the same time, its mechanical properties are not only undamaged, but also slightly improved, which fully demonstrates the synergistic enhancement and protection effect of the composite modification system of the present invention.

[0054] This invention discloses a urine-resistant nylon 6 fiber and its preparation method, belonging to the field of functional fiber technology. Addressing the ineffectiveness of existing technologies in resolving the yellowing problem of nylon 6 caused by urine, this invention employs a dual-mechanism targeted protection strategy of "amine hydrolysis inhibition + urate chelation." Amino-modified β-cyclodextrin is used as an amine hydrolysis inhibitor to specifically capture ammonia molecules; sodium citrate-grafted PEG is used as a urate chelating agent to prevent the formation of insoluble crystals; and nano-hydroxyapatite is used to adsorb pigment precursors. The above-mentioned composite modification system is introduced into the nylon 6 matrix through in-situ polymerization. After 50 urine stain simulation experiments, the prepared fiber still maintains a whiteness retention rate of over 92% and a yellowing level ≥4, far superior to existing products. It also possesses excellent mechanical properties, making it particularly suitable for high-requirement textiles such as children's underwear.

[0055] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for preparing a composite modifier, characterized in that, Includes the following steps: (1) Preparation of amino-modified β-cyclodextrin: β-cyclodextrin was dissolved in sodium hydroxide solution, epichlorohydrin was added under heating conditions to carry out the reaction, and then ammonia was added to maintain the temperature and continue the reaction. After the reaction was completed, the pH was adjusted to neutral, and amino-modified β-cyclodextrin was obtained by ethanol precipitation and drying. (2) Sodium citrate grafted polyethylene glycol: Polyethylene glycol, citric acid and catalyst are mixed in toluene, heated under reflux and continuously dehydrated using a water separator. After the solvent is removed, the crude product is neutralized, dialyzed and freeze-dried to obtain sodium citrate grafted polyethylene glycol product. (3) Preparation of composite modifier: The amino-modified β-cyclodextrin obtained in step (1) and nano-hydroxyapatite are ultrasonically dispersed in deionized water, and the sodium citrate-grafted polyethylene glycol obtained in step (2) is added. The mixture is heated and stirred to form a homogeneous colloid, and finally spray-dried to obtain a microsphere-shaped composite modifier.

2. The method for preparing a composite modifier according to claim 1, characterized in that, In step (1), the mass fraction of the sodium hydroxide solution is 3-6%, the mass fraction of the ammonia is 20-28%, the reaction temperature is 50-60℃, and the reaction time after adding epichlorohydrin is 5-6 hours; ammonia is added dropwise to the reaction system, and the reaction time after adding ammonia is 5-6 hours. The mass ratio of β-cyclodextrin, epichlorohydrin and ammonia is 100:(5-10):(2-4).

3. The method for preparing a composite modifier according to claim 1, characterized in that, In step (2), the dehydration reaction takes 10 to 12 hours; the mass ratio of polyethylene glycol to citric acid is 1:(0.2 to 0.3).

4. The method for preparing a composite modifier according to claim 1, characterized in that, In step (3), ultrasonic dispersion is performed for 30-40 minutes, the temperature of the heated water bath is 50-60℃, the stirring time is 2-3 hours, the inlet temperature of the spray drying is 180-200℃, and the particle size of the nano hydroxyapatite is 50-100nm. The mass ratio of amino-modified β-cyclodextrin, sodium citrate-grafted polyethylene glycol, and nano-hydroxyapatite is (3-5):(2-4):

1.

5. The composite modifier prepared by the preparation method according to any one of claims 1-4, characterized in that, The composite modifier includes a composite modification system in which the mass ratio of amino-modified β-cyclodextrin, sodium citrate-grafted polyethylene glycol, and nano-hydroxyapatite is (3-5):(2-4):

1.

6. A nylon 6 fiber resistant to urine staining and yellowing, comprising the composite modifier described in claim 5, characterized in that, Based on the mass of caprolactam, the mass ratio range of each component in the raw material is as follows: Caprolactam: 92-95%; Composite modification system: 5-8%; Tetrabutyl titanate: 0.1-0.3%.

7. The method for preparing a urine-resistant yellowing nylon 6 fiber as described in claim 6, characterized in that, Includes the following steps: S1. Composite modifier dispersion polymerization system: Add caprolactam to the polymerization reactor, introduce inert gas for displacement, and heat to completely melt caprolactam; Tetrabutyl titanate was added to the polymerization reactor and mechanically stirred to disperse it evenly; then the composite modifier was added and stirring continued. S2. In-situ polymerization: Heat the water and add deionized water to react and hydrolyze tetrabutyl titanate to generate nano-titanium dioxide. S3. Post-processing and spinning: After polymerization, excess water is removed by depressurization, and the polymer melt is directly transported to the spinning system. After extrusion, cooling, oiling, drawing, and winding by the spinneret, urea-resistant yellowing nylon 6 fiber is obtained.

8. The method for preparing a urine-resistant yellowing nylon 6 fiber according to claim 7, characterized in that, In step S1, nitrogen purging is used to ensure that the oxygen content is below 5 ppm; the heating temperature is 80-90℃, the stirring speed is controlled at 200-300 rpm, and the temperature is maintained and stirring is continued for 1-2 hours after the composite modifier is added.

9. The method for preparing a urine-resistant yellowing nylon 6 fiber according to claim 7, characterized in that, In step S2, the temperature is raised to 250-260℃, the pressure is controlled at 0.2-0.5MPa, and the reaction time is 4-6 hours; wherein, the amount of deionized water added is 0.3-0.5% of the mass of caprolactam.

10. The method for preparing a urine-resistant yellowing nylon 6 fiber according to claim 7, characterized in that, In step S3, the stretching ratio is 3.5 to 4.5 times, and the heat setting temperature is controlled at 160 to 180°C.

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