Flame-retardant thermoplastic polyurethane composite and method for producing the same

Abstract

The application belongs to the technical field of high polymer materials, and relates to a flame-retardant thermoplastic polyurethane composite material and a preparation method thereof. The preparation method comprises the following steps: mixing carbon quantum dot water dispersion liquid with lignin to obtain a reaction liquid; adding melamine and cyanuric acid into the reaction liquid to react, and drying to obtain Lig-CQD@MCA composite flame retardant; melt blending a TPU matrix, the composite flame retardant and a polyisocyanate crosslinking agent; then, the blend is aged, hot-pressed and cold-pressed to form a flame-retardant TPU composite material. The Lig-CQD@MCA composite flame retardant is prepared, and is matched with a polyisocyanate crosslinking system and an aging treatment process, so that the uniform dispersion of the flame retardant in the matrix and the synergistic effect of the multi-component flame retardant are realized, the bonding strength of the flame retardant and the matrix is improved through interface covalent bonding, the composite material has high flame-retardant performance, and excellent mechanical properties are maintained.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology and relates to a flame-retardant thermoplastic polyurethane composite material and its preparation method. Background Technology

[0002] Thermoplastic polyurethane (TPU) is a type of thermoplastic elastomer composed of alternating oligomeric polyol soft segments and diisocyanate-chain extender hard segments. It combines the high elasticity of rubber with the processability of plastics, exhibiting excellent abrasion resistance, oil resistance, low-temperature resistance, and mechanical strength. It is widely used in wire and cable sheathing, electronics, automotive interiors, building materials, and textile coatings. However, TPU molecules contain a large number of hydrocarbon structures, resulting in a low limiting oxygen index, making it a flammable material. During combustion, it easily produces molten drips, which can ignite secondary fires and release toxic fumes, severely limiting its application in scenarios requiring high flame retardancy. Therefore, developing efficient halogen-free flame-retardant TPU materials is an important research direction in the industry.

[0003] Melamine cyanurate (MCA) is a typical halogen-free nitrogen-based flame retardant. It achieves flame retardancy through multiple mechanisms, including endothermic sublimation and decomposition, release of inert nitrogen gas to dilute flammable volatiles, and promotion of a dense char layer in the matrix. It also boasts advantages such as low smoke, low toxicity, and halogen-free environmental friendliness, making it a commonly used filler for flame-retardant modification of thermoplastic polyurethanes. However, MCA molecules can form a high-density hydrogen-bonded two-dimensional layered crystal structure, with extremely strong hydrogen bond interactions between the layers. The rigid, plate-like microcrystals easily self-stack due to intermolecular forces, forming large-sized aggregates, resulting in poor interfacial compatibility with the weakly polar TPU matrix. This defect has two negative impacts: First, filler agglomeration will significantly reduce the effective flame-retardant specific surface area, weakening the flame-retardant performance, often requiring a significant increase in the amount of filler added to meet the flame-retardant rating requirements; Second, the weak bonding between filler agglomerates and the matrix interface will generate a large number of interface defects inside the composite material, making it prone to interface debonding and stress concentration when the material is subjected to external forces, ultimately causing a significant decrease in key mechanical properties of TPU composite materials such as tensile strength and elongation at break, making it difficult to meet the requirements of practical engineering applications.

[0004] Therefore, how to break the self-aggregation behavior of MCA and achieve uniform dispersion of flame retardants in TPU matrix has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a flame-retardant thermoplastic polyurethane composite material and its preparation method, wherein the prepared thermoplastic polyurethane composite material has excellent flame-retardant properties.

[0006] The present invention employs the following technical solutions to achieve its objective: One aspect of the present invention provides a method for preparing a flame-retardant thermoplastic polyurethane composite material, comprising the following steps: (1) Preparation of composite flame retardant: Aqueous dispersion of carbon quantum dots is mixed with lignin (Lig) to obtain lignin-carbon quantum dot reaction solution; melamine (MA) and cyanuric acid (CA) are added to the lignin-carbon quantum dot reaction solution and reacted at 80~100℃ for 3~8h, followed by drying to obtain composite flame retardant of lignin-carbon quantum dots coated with melamine cyanurate (Lig-CQD@MCA). (2) Melt blending and curing: The raw materials including thermoplastic polyurethane matrix, composite flame retardant obtained in step (1) and polyisocyanate crosslinking agent are melt blended; after the blending is completed, the blend is placed at 80~110℃ for curing treatment for 6~16h. (3) Molding: The cured material is hot-pressed and cold-pressed to obtain the flame-retardant thermoplastic polyurethane composite material.

[0007] Preferably, the lignin is alkali-degraded lignin.

[0008] Preferably, carbon quantum dots are dispersed in water to obtain a carbon quantum dot aqueous dispersion; the concentration of carbon quantum dots in the carbon quantum dot aqueous dispersion is 5~100 mg / mL.

[0009] The average particle size of carbon quantum dots is 1-8 nm, and their surface contains carboxyl or hydroxyl groups. Carbon quantum dots can be obtained directly from commercially available carbon quantum dot products, or they can be prepared from raw materials including a carbon source via a solvothermal reaction. The carbon source includes, but is not limited to, one or more of straw, fruit pomace, citric acid, glucose, sucrose, starch, cellulose, urea, ethylenediamine, and amino acids; the solvent includes water and / or organic solvents (organic solvents may include ethanol, acetone, isopropanol, N,N-dimethylformamide, etc.); in the preparation process, the carbon source is dissolved or dispersed in a solvent, and carbon quantum dots are formed through a solvothermal reaction. The carbon quantum dots are then obtained after centrifugation, filtration, dialysis, and drying.

[0010] Preferably, in the lignin-carbon quantum dot reaction solution, the mass ratio of lignin to carbon quantum dots is 30~300:1. While lignin itself has poor solubility in water, studies have found that because the surface of carbon quantum dots has abundant hydrophilic functional groups, it can form π-π stacking and hydrogen bonding with lignin molecules, promoting the dispersibility of lignin in water. Therefore, the reaction solution formed by mixing lignin and carbon quantum dots macroscopically appears to be in a dissolved state, with no visible suspended particles.

[0011] Preferably, the mass ratio of lignin to melamine is 1:2~5, and the molar ratio of melamine to cyanuric acid is 1:0.9~1.1.

[0012] MA and CA undergo a high-temperature in-situ reaction in a lignin-carbon quantum dot reaction solution to generate melamine cyanurate, yielding a lignin-carbon quantum dot-coated melamine cyanurate composite flame retardant. Preferably, the reaction is carried out at 85-95°C for 4-7 hours. This temperature range is beneficial for the synthesis of MCA in an aqueous phase, ensuring the reaction rate while avoiding excessive growth and aggregation of MCA sheets due to excessively high temperatures.

[0013] The Lig-CQD@MCA flame retardant prepared in this invention comprises carbon quantum dots, lignin, and MCA nanosheets, which can form multiple π-π stacking interactions. The carbon quantum dots act as a bridge, bringing the lignin and MCA sheets closer together and significantly promoting the π-π interactions between them. Simultaneously, the carbon quantum dots are extremely small (a few nanometers). During MCA formation, the carbon quantum dots restrict the proximity and stacking of MCA sheets through steric hindrance, and their surface functional groups such as hydroxyl and carboxyl groups can form hydrogen bonds with MCA surface groups, regulating the growth behavior of MCA, inhibiting MCA sheet aggregation, and promoting the formation of a sheet structure with a thickness of approximately 40-50 nm.

[0014] Preferably, the raw materials of the flame-retardant thermoplastic polyurethane composite material include a thermoplastic polyurethane matrix, a composite flame retardant, and a polyisocyanate crosslinking agent; the polyisocyanate crosslinking agent accounts for 0.1~5 wt% of the total mass of the raw materials, more preferably 0.5~3 wt%; the composite flame retardant accounts for 1~25 wt% of the total mass of the raw materials, more preferably 5~20 wt%. The addition range of the composite flame retardant can achieve a balance between flame retardant performance and mechanical properties. When the addition amount exceeds 20%, the improvement of flame retardant performance slows down, while the decline in mechanical properties intensifies.

[0015] Preferably, the polyisocyanate crosslinking agent is one or more of diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), and isophorone diisocyanate (IPDI).

[0016] Further preferably, the polyisocyanate crosslinking agent is diphenylmethane diisocyanate. MDI has moderate reactivity, which is beneficial for a gentle crosslinking reaction.

[0017] Preferably, the raw materials of the flame-retardant TPU composite material also include other additives, the amount of which accounts for 0-10 wt% of the total mass of the raw materials, and the lower limit can be any value from 0.01 wt%, 0.02 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, to 10 wt%. Other additives can include nitrogen and phosphorus flame retardants, antioxidants, reinforcing agents, lubricants, colorants, etc. Antioxidants can be listed as one or more of hindered phenolic primary antioxidants and phosphite secondary antioxidants; reinforcing agents can be listed as one or more of inorganic fillers, such as talc, calcium carbonate, wollastonite, mica powder, and kaolin; lubricants can be listed as one or more of stearic acid, calcium stearate, zinc stearate, polyethylene wax, and EVA wax; colorants can be listed as one or more of titanium dioxide, carbon black, and iron oxide.

[0018] In the system of this invention, the polyisocyanate crosslinking agent can play a triple role: first, it reacts with the terminal hydroxyl groups of TPU molecules to achieve molecular chain extension, increase the molecular weight and crosslinking density of the matrix, and enhance the mechanical properties of the material; second, it reacts with the phenolic hydroxyl groups on the surface of the flame retardant to form covalent bridges between the filler and the matrix, which greatly improves the interfacial bonding force and avoids interfacial debonding under stress; and third, it forms a slight crosslinking between adjacent filler particles, stabilizes the dispersion network of the filler, and inhibits secondary agglomeration during processing.

[0019] Preferably, the aging process is carried out at a temperature of 85-100°C for 8-12 hours.

[0020] The curing process is one of the key processes in this invention. By conducting a pre-treatment at a constant temperature below the hot-pressing temperature, the -NCO groups of the polyisocyanate crosslinking agent react smoothly and fully with the various hydroxyl components under mild conditions. Direct high-temperature hot pressing without a curing process will cause concentrated and violent reactions of MDI, easily generating a large number of CO2 bubbles, resulting in pores and delamination inside the sample, and a precipitous drop in mechanical and flame-retardant properties.

[0021] Preferably, in step (2), the raw materials are melt-blended for 3 to 10 minutes at a temperature of 150 to 200°C and a rotation speed of 20 to 200 r / min.

[0022] Preferably, the hot-press-cold-press forming in step (3) includes: hot pressing for 8 to 15 minutes at 170 to 210°C and 3 to 8 MPa, taking it out, and then cold pressing for 1 to 5 minutes at 15 to 30°C and 3 to 8 MPa in a cold press.

[0023] A second aspect of the present invention provides a flame-retardant thermoplastic polyurethane composite material, which is prepared by the above-described preparation method.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention provides an MCA hybrid flame retardant based on lignin-carbon quantum dot synergistic coating. Relying on the rich conjugated carbon skeleton of carbon quantum dots as a molecular bridge, the self-assembly of lignin, carbon quantum dots and MCA nanosheets is realized through π-π conjugation and hydrogen bonding. This restricts the self-aggregation behavior of MCA nanosheets from the source and promotes the formation of thinner MCA sheet structure.

[0025] (2) The ternary composite flame retardant of the present invention forms a triple carbon precursor synergistic flame retardant system consisting of a carbon quantum dot high conjugated carbon skeleton, a lignin aromatic carbon chain, and an MCA decomposition carbon source; wherein the carbon quantum dots have both molecular bridge and catalytic carbonization functions, and their conjugated carbon structure has catalytically active sites, which can catalyze the dehydration and cross-linking of lignin and TPU matrix into carbon at low temperature, forming a dense and continuous protective layer in advance, effectively blocking heat and oxygen transfer, inhibiting melting and dripping, and greatly improving flame retardant efficiency. UL94 V-1 flame retardancy can be achieved with an addition of 15%.

[0026] (3) This invention innovatively introduces a constant-temperature curing process after melt blending, enabling the added crosslinking agent to react gently and fully with the terminal hydroxyl groups of TPU and the phenolic hydroxyl groups on the surface of the flame retardant. Compared with the traditional "mixing + direct molding" process, this curing process not only constructs covalent bridges between the composite flame retardant and TPU, avoiding interfacial debonding voids under stress, but also effectively prevents internal pores and delamination defects caused by the violent gas generation of unreacted MDI under high temperature hot pressing, thereby improving the mechanical properties and flame retardant properties of the composite material.

[0027] (4) In the preparation process of this invention, the composite flame retardant is prepared by in-situ self-assembly in the aqueous phase. The reaction conditions are mild and the operation is simple. The melt blending, oven curing and plate hot pressing are all common equipment in the polymer industry, and no complicated equipment is required. The modular process can be steadily scaled up from small-scale to extrusion and injection molding mass production, and has strong industrial adaptability.

[0028] (5) The composite flame retardant of the present invention uses biomass-based raw materials such as dealkalized lignin and carbon quantum dots as the core carbon source. Lignin is an agricultural and forestry by-product of papermaking and biorefining. The material is low in toxicity and low in irritation, and can replace some petroleum-based raw materials. It is in line with the development direction of halogen-free and green flame retardant, and has outstanding environmental friendliness and economy. It is suitable for TPU products with high environmental protection requirements such as cables and sheaths. Attached Figure Description

[0029] Figure 1 This is a SEM image of Lig-CQD@MCA prepared in Example 1 of this invention.

[0030] Figure 2 This is a SEM image of Lig-CQD@MCA prepared in Comparative Example 2 of this invention. Detailed Implementation

[0031] In the description of this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, and includes both a and b. "Multiple" includes two or more types, and can be two, three, four, five, or more.

[0032] The technical solution of the present invention will be further described and illustrated below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for the purpose of helping to understand the present invention and are not intended to limit the specific scope of the present invention. Furthermore, the accompanying drawings used herein are merely for better illustrating the content disclosed in the present invention and do not limit the scope of protection. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used in the art, and the methods used in the embodiments are all conventional methods in the art.

[0033] In the following examples and comparative examples, the sources of raw materials are as follows: The lignin was dealkalized lignin, purchased from Maclean Chemical Reagent Co., Ltd., L849279; Carbon quantum dots: Maclean Chemical Reagent Co., Ltd., C699178, particle size: 2-5nm; Melamine (MA): Maclean Chemical Reagent Co., Ltd., 99% purity; Cyanuric acid (CA): Maclean Chemical Reagent Co., Ltd., purity 98%; Polyurethane (TPU): Elastollan, C70A10P, BASF, Germany; Diphenylmethane diisocyanate (MDI): Maclean Chemical Reagent Co., Ltd., 98% purity; Toluene diisocyanate (TDI): Maclean Chemical Reagent Co., Ltd., purity >98%; Isophorone diisocyanate (IPDI): Maclean Chemical Reagent Co., Ltd., purity 99%.

[0034] The room temperature is 25~27℃.

[0035] Example 1 The preparation method of the TPU composite material in this embodiment is as follows: (1) Preparation of flame retardant Lig-CQD@MCA: Weigh 0.05g of carbon quantum dots and disperse them in 200mL of deionized water to form a carbon quantum dot aqueous dispersion; weigh 5.0g of dealkalized lignin and add it to the carbon quantum dot aqueous dispersion, stir at 500r / min for 24h at room temperature to obtain Lig-CQD reaction solution (no visible particles suspended); transfer the Lig-CQD reaction solution to a constant temperature water bath, heat it to 90℃, stir at 200r / min for 10min to make the system temperature uniform; slowly add 12.5g of MA and 12.7g of CA, and keep the reaction at 90℃ under constant temperature reflux for 5h with stirring at 200r / min; freeze dry the entire reaction solution to obtain the light yellow powder product Lig-CQD@MCA.

[0036] (2) Place the TPU in a forced-air drying oven and dry it at 100℃ for 12h. Place 42g TPU, 7.5g Lig-CQD@MCA and 0.5g MDI in a torque rheometer and melt-blend them at 180℃ and 60r / min for 6min. After blending, place them in a forced-air drying oven at 90℃ for 10h. Then, use a flat vulcanizing machine to hot-press at 190℃ and 5MPa for 10min, remove them, and cold-press them in a cold press at room temperature and 5MPa for 3min to obtain the TPU composite material.

[0037] Figure 1 The image shows a SEM image of the flame retardant Lig-CQD@MCA prepared in Example 1. This SEM image can intuitively reflect the microstructure of the lignin / carbon quantum dot pre-assembled hybrid phase and the secondary self-assembly of MCA nanosheets: the main body of the sample is an ultrathin, wrinkled two-dimensional MCA nanosheet with a thickness of only 46.30 nm. The sheets are spread out and dispersed, without the severe stacking and agglomeration typical of pure MCA. The surface of the MCA sheet is covered with a uniform flocculent coating layer. This flocculent layer is a lignin-carbon quantum dot composite phase pre-bonded by π-π conjugation. The transition between it and the MCA sheets is smooth, without obvious gaps, shedding, or macroscopic phase separation phenomena of two-phase separation. This confirms that the lignin-carbon quantum dot hybrid can be tightly anchored to the surface of the MCA nanosheet through π-π conjugation and hydrogen bonding. Relying on the steric hindrance effect of carbon quantum dots, the aggregation of the sheets is prevented, and finally an integrated self-assembled composite filler with an organic hybrid layer uniformly coating the inorganic nanosheets is formed.

[0038] Table 1. List of EDS distribution maps and total spectral data for Lig-CQD@MCA

[0039] The EDS data of Lig-CQD@MCA in Table 1 show that MCA is a high-nitrogen component. The extremely high nitrogen content (37.00 wt%) in the table proves that there are a large number of MCA nanosheets in the product, confirming that MA and CA react in situ to generate MCA. Carbon (45.00 wt%) and oxygen (12.03 wt%) come from the phenolic hydroxyl or carboxyl groups of the lignin aromatic skeleton and carbon quantum dots, proving that the Lig-CQD organic phase exists stably in the system. Trace amounts of Na and S impurities are present in lignin itself and do not affect the judgment of the main structure.

[0040] Example 2 The difference between Example 2 and Example 1 is that in Example 2, 44.5g of TPU, 5g of Lig-CQD@MCA and 0.5g of MDI were melt-blended in a torque rheometer, while the rest was the same as in Example 1.

[0041] Example 3 The difference between Example 3 and Example 1 is that in Example 3, 39.5g TPU, 10g Lig-CQD@MCA and 0.5g MDI were melt-blended in a torque rheometer, while the rest was the same as in Example 1.

[0042] Example 4 The difference between Example 4 and Example 1 is that Example 4 uses TDI instead of MDI in Example 1, while the rest is the same as Example 1.

[0043] Example 5 The difference between Example 5 and Example 1 is that Example 5 uses IPDI instead of MDI in Example 1, while the rest is the same as Example 1.

[0044] Example 6 The preparation method of the TPU composite material in this embodiment is as follows: (1) Preparation of flame retardant Lig-CQD@MCA: Weigh 0.1g of carbon quantum dots and disperse them in 200mL of deionized water to form a carbon quantum dot aqueous dispersion; weigh 6.0g of dealkalized lignin and add it to the carbon quantum dot aqueous dispersion, stir at room temperature for 20h at 600r / min to obtain Lig-CQD reaction solution; transfer the Lig-CQD reaction solution to a constant temperature water bath, heat it to 85℃, stir at 300r / min for 15min to make the system temperature uniform; slowly add 13g of MA and 13.2g of CA, and keep the reaction at 95℃ under constant temperature reflux for 4h with stirring at 300r / min; freeze dry the entire reaction solution to obtain the light yellow powder product Lig-CQD@MCA.

[0045] (2) Place the TPU in a forced-air drying oven and dry it at 100℃ for 12h. Place 43.5g TPU, 6g Lig-CQD@MCA and 0.5g MDI in a torque rheometer and melt-blend them at 190℃ and 80r / min for 5min. After blending, place them in a forced-air drying oven at 95℃ for 12h. Then, use a flat vulcanizing machine to hot-press at 190℃ and 6MPa for 15min, remove them, and cold-press them in a cold press at room temperature and 6MPa for 4min to obtain the TPU composite material.

[0046] Comparative Example 1 The preparation method of the TPU composite material in Comparative Example 1 is as follows: (1) Preparation of flame retardant MCA: 12.5g MA and 12.7g CA were added to 200mL of deionized water and refluxed at 90℃ for 5h with stirring at 200r / min; the entire reaction solution was freeze-dried to obtain MCA.

[0047] (2) Replace 7.5g Lig-CQD@MCA in step (2) of Example 1 with 7.5g MCA, and follow the same steps as step (2) of Example 1 to obtain TPU composite material.

[0048] Comparative Example 2 The preparation method of the TPU composite material in Comparative Example 2 is as follows: (1) Preparation of flame retardant Lig@MCA: Weigh 5.0g of dealkalized lignin and add it to 200mL of deionized water. Stir at 500r / min for 24h at room temperature to obtain a dealkalized lignin solution (suspended particles visible to the naked eye). Transfer the dealkalized lignin solution to a constant temperature water bath, heat it to 90℃, and stir at 200r / min for 10min to make the system temperature uniform. Slowly add 12.5g of MA and 12.7g of CA, and reflux at 90℃ for 5h with stirring at 200r / min. Freeze-dry the entire reaction solution to obtain Lig@MCA.

[0049] (2) Replace 7.5g Lig-CQD@MCA in step (2) of Example 1 with 7.5g Lig@MCA, and follow the same steps as step (2) of Example 1 to obtain TPU composite material.

[0050] Figure 2This is a SEM image of the flame retardant Lig@MCA prepared in Comparative Example 2. In the SEM image of the Lig@MCA sample without carbon quantum dots, it is clearly observed that the MCA sheet-like microcrystals undergo large-scale, face-to-face, tight stacking, forming numerous dense clusters. There are almost no loosely separated ultrathin sheets within the field of view. The sheets are mutually adhered and compressed by strong intermolecular hydrogen bonds and intermolecular π-π conjugation interactions. The internal pores of the clusters are sparse and the boundaries are blurred. This is due to the lack of nanoscale spatial isolation sites and competitive hydrogen bonding provided by CQDs. CQDs themselves can form bonds with the conjugated structures of lignin and MCA. Inter-molecular π-π stacking disrupts the regular parallel stacking of MCA sheets. In contrast, in the Lig@MCA system without CQD, MCA sheets can be parallelly bonded over a large area. The π-π conjugation between molecules fully enhances the interlayer attraction, further aggravating the self-aggregation trend. This severe agglomeration morphology causes the filler to form a large number of interface defects in the TPU matrix, significantly weakening the interfacial bonding force between the filler and the matrix, causing stress concentration, and reducing the effective specific surface area of ​​the flame retardant component. Ultimately, this leads to the simultaneous deterioration of the tensile strength, elongation at break, and limiting oxygen index of the composite material.

[0051] Comparative Example 3 The preparation method of the TPU composite material in Comparative Example 3 is as follows: (1) Preparation of flame retardant CQD@MCA: Weigh 0.05g of carbon quantum dots and disperse them in 200mL of deionized water to form a carbon quantum dot aqueous dispersion; transfer the carbon quantum dot aqueous dispersion to a constant temperature water bath, heat it to 90℃, and stir it at 200r / min for 10min to make the system temperature uniform; slowly add 12.5g of MA and 12.7g of CA, and keep the reaction at 90℃ under constant temperature reflux for 5h with stirring at 200r / min; freeze dry the entire reaction solution to obtain CQD@MCA.

[0052] (2) Replace 7.5g Lig-CQD@MCA in step (2) of Example 1 with 7.5g CQD@MCA, and follow the same steps as step (2) of Example 1 to obtain TPU composite material.

[0053] Comparative Example 4 The preparation method of the TPU composite material in Comparative Example 4 is as follows: (1) Preparation of the mixture of Lig-CQD and MCA: Weigh 0.05g of carbon quantum dots and disperse them in 100mL of deionized water to form a carbon quantum dot aqueous dispersion; weigh 5.0g of dealkalized lignin and add it to the carbon quantum dot aqueous dispersion, stir at 500r / min for 24h at room temperature to obtain Lig-CQD reaction solution; add 12.5g of MA and 12.7g of CA to 200mL of deionized water, stir at 200r / min and keep at 90℃ for 5h to obtain MCA reaction solution; stir the Lig-CQD reaction solution and MCA reaction solution at 200r / min for 30min at room temperature and freeze dry to obtain a mixture of Lig-CQD and MCA.

[0054] (2) Replace 7.5g Lig-CQD@MCA in step (2) of Example 1 with a mixture of 7.5g Lig-CQD and MCA, and follow the same steps as in step (2) of Example 1 to obtain TPU composite material.

[0055] Comparative Example 5 The preparation method of the TPU composite material in Comparative Example 5 is as follows: (1) Obtain Lig-CQD@MCA by following step (1) of Example 1.

[0056] (2) Place the TPU in a forced-air drying oven and dry it at 100℃ for 12h. Place 42.5g TPU and 7.5g Lig-CQD@MCA in a torque rheometer and melt-blend them at 180℃ and 60r / min for 6min. After blending, place them in a forced-air drying oven at 90℃ for 10h. Then, use a flat vulcanizing machine to hot-press them at 190℃ and 5MPa for 10min, remove them, and cold-press them in a cold press at room temperature and 5MPa for 3min to obtain the TPU composite material.

[0057] Comparative Example 6 The preparation method of the TPU composite material in Comparative Example 6 is as follows: (1) Obtain Lig-CQD@MCA by following step (1) of Example 1.

[0058] (2) Place the TPU in a forced-air drying oven and dry it at 100℃ for 12h. Place 42g TPU, 7.5g Lig-CQD@MCA and 0.5g MDI in a torque rheometer and melt-blend them at 180℃ and 60r / min for 6min. Then, hot-press the TPU composite material at 190℃ and 5MPa for 10min using a flat vulcanizing machine. Remove the TPU and cold-press it at room temperature and 5MPa for 3min in a cold press to obtain the TPU composite material.

[0059] The TPU composite materials prepared in the examples and comparative examples were subjected to the following performance tests, and the results are shown in Table 2: Tensile properties: Tensile strength and elongation at break were tested according to GB / T 528-2009; Limiting Oxygen Index (LOI): Tested according to GB / T 2406.2-2009; Vertical burning (UL94): Tested according to GB / T 2408-2008, with a sample thickness of 3.2 mm.

[0060] Table 2 Mechanical and flame retardant properties of TPU composite materials prepared in the examples and comparative examples

[0061] Comparative examples and comparative examples show that when the composite flame retardant of the present invention is added to the TPU matrix, the flame retardant performance of the material is improved with the increase of the amount of flame retardant added. Although the mechanical properties are lower than those of pure TPU, they can still be maintained at an excellent practical level.

[0062] A comparison between Example 1 and Comparative Example 1 shows that Comparative Example 1 uses pure MCA as a flame retardant. Due to the strong van der Waals forces and hydrogen bonds between MCA layers, it is prone to stacking and agglomeration, failing to fully realize its flame retardant efficiency and significantly reducing mechanical properties. Consequently, the flame retardant and mechanical properties of Comparative Example 1 are significantly lower than those of Example 1. A comparison between Example 1 and Comparative Example 2 shows that without carbon quantum dots as "molecular bridges," lignin and MCA are only bonded by weak hydrogen bonds, failing to form a stable π-π conjugated coating structure. The MCA layers are still prone to agglomeration. Furthermore, lignin itself has poor dispersibility in water, easily forming large agglomerates, significantly reducing the flame retardant and mechanical properties of the material. A comparison between Example 1 and Comparative Example 3 shows that without lignin as the main carbon source, the trace carbon skeleton of carbon quantum dots cannot provide sufficient char precursors, reducing flame retardant performance. Simultaneously, the lack of an organic coating layer of lignin makes the flame retardant prone to agglomeration, resulting in a simultaneous decrease in mechanical properties. A comparison of Example 1 and Comparative Example 4 shows that the "simple physical mixing" of carbon quantum dots, lignin, and MCA cannot promote the formation of a tight π-π stacked and hydrogen-bonded structure of carbon quantum dots, lignin, and MCA as in the "in-situ high-temperature reaction," and the flame retardant still agglomerates. The above data demonstrates the necessity of using the "carbon quantum dot-lignin-MCA in-situ self-assembly" structure as a flame retardant in this invention.

[0063] By comparing Example 1 and Comparative Example 5, it can be seen that when MDI is missing, the flame retardant and TPU matrix are only physically bonded, with weak interfacial bonding force. Under stress, interfacial debonding and stress concentration are prone to occur, resulting in deterioration of mechanical properties. At the same time, the flame retardant is prone to secondary agglomeration during processing, resulting in decreased dispersibility and a corresponding decrease in flame retardant efficiency.

[0064] By comparing Example 1 and Comparative Example 6, it can be seen that although MDI was added to the system, no curing treatment step was added accordingly. MDI reacted intensively in a short time under high temperature and high pressure, and the generated CO2 gas could not be discharged in time, resulting in defects inside the material and thus affecting its performance.

[0065] In summary, this invention effectively solves the problems of easy stacking and agglomeration and weak interfacial compatibility of ordinary flame retardants in thermoplastic polyurethane matrices by preparing a composite flame retardant of lignin-carbon quantum dots in situ coated with melamine cyanurate, and combining it with a polyisocyanate crosslinking system and curing process. It achieves both uniform dispersion of the flame retardant in the matrix and synergistic flame retardant effect of multiple components, and improves the bonding strength between the flame retardant and the matrix through interfacial covalent bonding, so that the composite material can obtain high-level flame retardant performance while maintaining excellent mechanical properties.

[0066] All aspects, embodiments, and features of this invention should be considered illustrative in all respects and not limiting of the invention; the scope of the invention is defined only by the claims. Other embodiments, modifications, and uses will become apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0067] In the preparation method of this invention, the order of the steps is not limited to the listed order. For those skilled in the art, variations in the order of the steps without creative effort are also within the scope of protection of this invention. Furthermore, two or more steps or actions can be performed simultaneously.

[0068] Finally, it should be noted that the specific embodiments described herein are merely illustrative examples of the invention and are not intended to limit the implementation of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them; it is neither necessary nor possible to exemplify all embodiments here. However, these obvious variations or modifications derived from the essential spirit of the invention still fall within the scope of protection of the invention, and interpreting them as any additional limitation would contradict the spirit of the invention.

Claims

1. A method for preparing a flame-retardant thermoplastic polyurethane composite material, characterized in that, Includes the following steps: (1) Preparation of composite flame retardant: Mix carbon quantum dot aqueous dispersion with lignin to obtain lignin-carbon quantum dot reaction solution; add melamine and cyanuric acid to lignin-carbon quantum dot reaction solution, react at 80~100℃ for 3~8h, and then dry to obtain lignin-carbon quantum dot coated melamine cyanurate composite flame retardant; (2) Melt blending and curing: The raw materials including thermoplastic polyurethane matrix, composite flame retardant obtained in step (1) and polyisocyanate crosslinking agent are melt blended; after the blending is completed, the blend is placed at 80~110℃ for curing treatment for 6~16h. (3) Molding: The cured material is hot-pressed and cold-pressed to obtain the flame-retardant thermoplastic polyurethane composite material.

2. The preparation method according to claim 1, characterized in that, Carbon quantum dots are dispersed in water to obtain an aqueous dispersion of carbon quantum dots; the concentration of carbon quantum dots in the aqueous dispersion is 5~100 mg / mL.

3. The preparation method according to claim 1, characterized in that, In the lignin-carbon quantum dot reaction solution, the mass ratio of lignin to carbon quantum dots is 30~300:

1.

4. The preparation method according to claim 1, characterized in that, The mass ratio of lignin to melamine is 1:2~5.

5. The preparation method according to claim 1, characterized in that, The molar ratio of melamine to cyanuric acid is 1:0.9~1.

1.

6. The preparation method according to claim 1, characterized in that, The raw materials of the flame-retardant thermoplastic polyurethane composite material include a thermoplastic polyurethane matrix, a composite flame retardant, and a polyisocyanate crosslinking agent; the polyisocyanate crosslinking agent accounts for 0.1 to 5 wt% of the total mass of the raw materials, and the composite flame retardant accounts for 1 to 25 wt% of the total mass of the raw materials.

7. The preparation method according to claim 1 or 6, characterized in that, The polyisocyanate crosslinking agent is one or more of diphenylmethane diisocyanate, toluene diisocyanate, and isophorone diisocyanate.

8. The preparation method according to claim 1, characterized in that, In step (2), the raw materials are melt-blended for 3 to 10 minutes at a temperature of 150 to 200°C and a rotation speed of 20 to 200 r / min.

9. The preparation method according to claim 1, characterized in that, Step (3) hot-press-cold-press forming includes: hot pressing for 8-15 minutes at 170-210℃ and 3-8MPa, taking it out, and then cold pressing for 1-5 minutes at 15-30℃ and 3-8MPa in a cold press.

10. A flame-retardant thermoplastic polyurethane composite material, characterized in that, It is prepared by the preparation method as described in claim 1.

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