Method for preparing flame-retardant polyamide 66 composite material by utilizing waste PET (Polyethylene Terephthalate)

Abstract

The invention relates to the technical field of polymer composite materials, and discloses a method for preparing a flame-retardant polyamide 66 composite material by using waste polyethylene glycol terephthalate (PET), and the flame-retardant polyamide 66 composite material comprises 90-99.5% of polyamide 66; the invention relates to a carbon dot composition, which comprises the following components in percentage by weight: 0.5 to 10 percent of carbon dots, the carbon dots are prepared by reaction of waste PET and an amine compound, the amine compound is ethanolamine, and the carbon dots are prepared by reaction of the waste PET and the amine compound at the reaction temperature of 250 to 290 DEG C for 10 to 64 hours. According to the carbon dots (t-CDs and T-CDs) prepared by two different processes, the melt spinning and injection molding processing temperatures of the composite material are effectively reduced, and the processing process window is widened. The two kinds of carbon dots can significantly improve the processability of polyamide 66, and the problem that the processability of the material is degraded due to introduction of the filler is solved.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials technology, specifically a method for preparing flame-retardant polyamide 66 composite materials using waste PET. Background Technology

[0002] Polyamide 66 (PA66), a key semi-crystalline engineering plastic and synthetic fiber material, holds a crucial position in automotive parts, electronic and electrical housings, industrial gears, and high-end apparel fabrics due to its excellent tensile strength, abrasion resistance, chemical resistance, and thermal stability. However, the abundant methylene groups in PA66's molecular chain structure make it inherently a flammable polymer, with a limiting oxygen index (LOI) typically only in the range of 23-25%, making it difficult to meet fire safety standards in many fields. More seriously, during combustion, PA66 melts rapidly, producing incandescent molten droplets. These droplets act as mobile ignition sources, easily igniting surrounding materials, leading to rapid spread and expansion of the fire, posing a serious fire hazard. This characteristic significantly limits its deployment in high-end applications with stringent safety standards.

[0003] To broaden the application range of PA66, effective flame-retardant modification is essential. With increasingly stringent environmental regulations, halogenated flame retardants that release toxic and corrosive gases have gradually been phased out of the market, and halogen-free flame-retardant technology has become the mainstream development in this field. Current research mainly focuses on halogen-free flame-retardant systems, especially nitrogen-phosphorus system flame retardants, such as melamine cyanurate (MCA), ammonium polyphosphate (APP), and their compound systems. The mechanism of action of these flame retardants mainly relies on physicochemical processes such as gas-phase dilution and condensed-phase charring for heat and oxygen insulation. However, these traditional halogen-free flame retardants generally have inherent limitations, such as insufficient flame-retardant efficiency. To achieve higher flame-retardant ratings for PA66, such as UL-94V-0, the amount of flame retardant added typically needs to reach 15wt% to 25wt% or even higher.

[0004] Such a high addition amount is tantamount to introducing a large number of inorganic or organic small molecule phases into the polymer matrix. These additive particles often have poor compatibility with the PA66 matrix, forming interfacial defects within the material and becoming stress concentration points. Under external forces, these defects can induce the generation and propagation of microcracks, ultimately leading to a significant decrease in key mechanical properties of the composite material, such as tensile strength, elongation at break, and impact toughness, making it unsuitable for applications as structural components or high-performance fibers. In terms of material processing, the addition of a large amount of solid powder drastically increases the viscosity of the PA66 melt, resulting in poor material flowability. This not only requires higher processing temperatures and screw shear forces, increasing energy consumption, but may also cause processing temperatures to approach or even exceed the thermal decomposition temperature of PA66, triggering polymer degradation, yellowing, and gelation, thus damaging the performance of the final product. Especially in melt spinning processes, which are extremely sensitive to melt rheology, high-viscosity melts are difficult to form stable, continuous fine streams, easily leading to problems such as fiber breakage, fuzzing, and uneven fiber diameter, severely restricting the industrial production of high-performance flame-retardant PA66 fibers.

[0005] Meanwhile, another serious global problem is the disposal of polyethylene terephthalate (PET) plastic waste. As one of the most consumed plastics globally, its waste places enormous pressure on the environment. Existing recycling strategies mostly focus on physical degradation (such as making filling cotton, packaging materials, etc.) or chemical depolymerization to prepare basic chemicals. These methods generally suffer from low added value of recycled products and fail to achieve the high-value upgrading and utilization of resources.

[0006] Therefore, there is an urgent need in this field for a completely new technical approach. This approach should be able to break free from the constraints of traditional high-filler-content flame-retardant modification, and endow PA66 with excellent flame retardancy with only extremely low addition amounts, while maintaining or even improving its original mechanical and processing properties. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for preparing flame-retardant polyamide 66 composite materials using waste PET. This method solves the problem that existing technologies, when modifying polyamide 66 for flame retardancy, cannot simultaneously achieve both mechanical and processing properties due to excessive amounts of flame retardant added, and also lack the ability to utilize waste PET resources in a high-value manner.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a flame-retardant polyamide 66 composite material, comprising 100% of the total weight of the composite material, and containing the following components:

[0009] Polyamide 66: 90% to 99.5%;

[0010] Carbon point: 0.5% to 10%;

[0011] The carbon dots are prepared by reacting waste polyethylene terephthalate with amine compounds.

[0012] Preferably, the amine compound is ethanolamine.

[0013] Preferably, the carbon dots are prepared by reacting the waste PET with the amine compound at a reaction temperature of 250°C to 290°C for 18 to 56 hours to obtain high-temperature carbon dots.

[0014] Preferably, the carbon dots are prepared by reacting the waste PET with the amine compound at a reaction temperature of 250°C to 270°C for 50 to 60 hours to obtain low-temperature long-lasting carbon dots.

[0015] Preferably, the composite material is in the form of fiber, filament, granulated chips, or injection molded parts.

[0016] A method for preparing flame-retardant polyamide 66 composite materials using waste PET includes the following steps:

[0017] S1. Provide carbon dots, said carbon dots being obtained by reacting waste polyethylene terephthalate with an amine compound under heating conditions;

[0018] S2. The carbon dots obtained in step S1 are melt-blended with polyamide 66 to obtain the flame-retardant polyamide 66 composite material.

[0019] Preferably, step S2 specifically includes:

[0020] S2a. First, the carbon dots are melt-blended with a portion of polyamide 66 to prepare carbon dot masterbatch;

[0021] S2b. The carbon dot masterbatch is then melt-blended with the remaining polyamide 66.

[0022] Preferably, in step S1, the reaction is carried out at a temperature of 250°C to 270°C for 50 to 60 hours to obtain long-term carbon points (t-CDs); the reaction is carried out at a temperature of 270°C to 290°C for 15 to 20 hours to obtain high-temperature carbon points (T-CDs).

[0023] Preferably, the amine compound is ethanolamine.

[0024] This invention provides a method for preparing flame-retardant polyamide 66 composite materials using waste PET. It has the following beneficial effects:

[0025] 1. This invention provides a novel method for converting waste PET polyester into functionalized carbon dots. This method is achieved through a specific solvothermal reaction. This not only opens up a high-value reuse pathway for the vast quantities of PET waste but also fully aligns with the development direction of green chemistry and the circular economy. Compared to existing technologies that primarily treat PET waste through downgrading or direct landfilling, this invention solves the fundamental problems of low added value from recycling and failure to fully exploit its chemical potential, transforming the environmental burden into a valuable precursor for high-performance materials.

[0026] 2. This invention achieves exceptional flame retardant performance by introducing T-CDs prepared at higher temperatures into polyamide 66. Only a very low addition of 2-3% is required to achieve a UL-94V-0 flame retardant rating, with no dripping during combustion. This is achieved through the efficient catalysis of T-CDs to form a dense carbon layer with a high degree of graphitization in the matrix. In contrast, traditional intumescent or nitrogen-based flame retardant solutions typically require additions of over 15% to achieve similar results, which severely compromises the integrity of the polymer's molecular chain structure. Therefore, this invention overcomes the technical bottleneck of excessively high flame retardant additions in traditional flame retardant modification, which leads to a significant sacrifice in the material's mechanical and processing properties.

[0027] 3. The two types of carbon dots (t-CDs and T-CDs) prepared in this invention can significantly improve the processing performance of polyamide 66. These nanoparticles play an excellent plasticizing role in the matrix, effectively reducing the melt spinning temperature of the composite material, widening the processing window, and improving the stability of the spinning process. This is in stark contrast to the effects of most inorganic fillers or flame retardants in the prior art. Additives in the prior art often cause a sharp increase in polymer melt viscosity, making processing (especially spinning) difficult and increasing energy consumption. This invention solves the common problem that the introduction of fillers usually leads to the deterioration of material processability, and instead optimizes the processing conditions.

[0028] 4. This invention prepares t-CDs through a mild, long-term reaction pathway and applies them to polyamide 66 fibers, achieving synergistic enhancement of the material's mechanical properties. The resulting fibers show simultaneous improvement in both breaking strength and elongation at break, meaning that both the material's strength and toughness are improved simultaneously. This effect breaks through conventional understanding of material modification. In existing technologies, whether adding reinforcing fillers or plasticizers, a trade-off is usually made between strength and toughness, with the two exhibiting an inverse relationship. This invention cleverly solves the inherent contradiction of the difficulty in simultaneously achieving strength and toughness by utilizing the multifunctional properties of t-CDs, endowing the material with more balanced and comprehensive superior mechanical properties. Attached Figure Description

[0029] Figure 1These are TEM images and particle size distribution diagrams of t-CDs in this invention;

[0030] Figure 2 The infrared spectrum of t-CDs in this invention;

[0031] Figure 3 This is a graph showing the t-CDs thermogravimetric curve in this invention;

[0032] Figure 4 This is a differential scanning calorimetry curve of PA66 / t-CDs in this invention;

[0033] Figure 5 Thermogravimetric curves of PA66 and PA66 / t-CDs composites in this invention are shown.

[0034] Figure 6 The CONE curves of PA66 and PA66 / t-CDs complex in this invention are shown. Figure 1 ;

[0035] Figure 7 The CONE curves of PA66 and PA66 / t-CDs complex in this invention are shown. Figure 2 ;

[0036] Figure 8 Digital photographs and SEM images of PA66 and PA66 / 3% t-CDs carbon residues after the cone calorimetry experiment in this invention;

[0037] Figure 9 This is the Raman spectrum of the residual carbon of PA66 / t-CDs after cone mass testing in this invention;

[0038] Figure 10 The above are TG-IR images of PA66 and the PA66 / t-CDs complex in this invention.

[0039] Figure 11 The images show actual PA66 / t-CDs composite fibers with different draw ratios in this invention.

[0040] Figure 12 This is the XRD diffraction pattern of PA66 / t-CDs fiber in this invention;

[0041] Figure 13 This is a diagram showing the orientation properties of PA66 / t-CDs fibers at different draw ratios in this invention;

[0042] Figure 14 The TEM and particle size distribution diagrams of T-CDs in this invention are shown.

[0043] Figure 15 The infrared spectrum of T-CDs in this invention;

[0044] Figure 16 This is a thermogravimetric curve of rPET-CDs in this invention;

[0045] Figure 17 This is a differential scanning calorimetry curve of PA66 and PA66 / T-CDs complex in this invention;

[0046] Figure 18 This is a thermogravimetric curve of the PA66 / T-CDs composite in this invention;

[0047] Figure 19 The CONE curves for PA66 and the PA66 / T-CDs complex in this invention are shown. Figure 1 ;

[0048] Figure 20 The CONE curves for PA66 and the PA66 / T-CDs complex in this invention are shown. Figure 2 ;

[0049] Figure 21 This is the Raman spectrum of the residual carbon of PA66 / T-CDs after cone mass testing in this invention;

[0050] Figure 22 This is the TG-IR spectrum of the PA66 / T-CDs complex in this invention;

[0051] Figure 23 The images show actual PA66 / T-CDs composite fibers with different draw ratios in this invention.

[0052] Figure 24 This is the XRD diffraction pattern of PA66 / t-CDs fiber in this invention;

[0053] Figure 25 This is a diagram showing the orientation properties of PA66 / T-CDs fibers at different draw ratios in this invention. Detailed Implementation

[0054] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] Example:

[0056] Please see the appendix Figure 1 - Appendix Figure 25This invention provides a method for preparing flame-retardant polyamide 66 composite material using waste PET. The core of this method is to prepare carbon dots (CDs) with a specific structure and add them to the PA66 matrix as a highly efficient and multifunctional additive.

[0057] Example 1: Preparation of long-lasting carbon dots (t-CDs) and their application in PA66:

[0058] This embodiment describes in detail a carbon dot (t-CDs) prepared by extending the reaction time at a relatively mild temperature, and illustrates its effect on the properties of PA66 composite materials.

[0059] 1.1 t-CDs kilogram-scale preparation

[0060] 1500g of waste PET bottle flakes were loaded into a 5L high-pressure quick-release reactor. These flakes could be recycled beverage bottles, etc., and had undergone routine cleaning and crushing. Then, 975ml of ethanolamine was added to the reactor. The reactor was sealed, and the stirring and heating programs were started. The reaction temperature was set at 260°C. At this temperature, the reaction continued for 56 hours. After the reaction was completed, heating was stopped, and the reactor was allowed to cool naturally to approximately 80°C. The reactor was opened, and an appropriate amount of ethanol was added to disperse the product. Larger particles or unreacted matter larger than 220 nm were filtered out using a filtration device. The filtrate was collected, and the ethanol solvent was recovered using a rotary evaporator. The final solid product was thoroughly dried in a vacuum drying oven and then pulverized to obtain dark-colored t-CDs powder.

[0061] 1.1.1 Performance Analysis of Long-Term Carbon Dot (t-CDs) System

[0062] 1.1.1.1 Morphology of t-CDs:

[0063] The microstructure of t-CDs was observed using transmission electron microscopy (TEM). The results are as follows: Figure 1 As shown in Figure -a, the prepared t-CDs appear as well-dispersed, near-spherical nanoparticles. No obvious hard agglomeration was observed. Statistical analysis of a large number of particles revealed that their particle size distribution ranged from 1.15 nm to 3.85 nm, as shown in the figure. Figure 1 As shown in -b, the calculated average particle size is approximately 2.07 nanometers. This small size and good dispersibility are prerequisites for its use as a functional nanofiller in polymer matrices.

[0064] 1.1.1.2 Structure of t-CDs

[0065] The surface functional groups of t-CDs were analyzed using Fourier transform infrared spectroscopy (FTIR). Please refer to the appendix. Figure 2In the spectrum, at 3291 cm⁻¹ -1 A stretching vibration peak belonging to the NH / OH group can be observed at 2964 cm⁻¹. -1 The peak at 1538 cm⁻¹ represents the stretching vibration of the CH bond. -1 The peak at 1371 cm⁻¹ represents the bending vibration of the C=O group. -1 and 1414cm -1 The absorption peak at 1274 cm⁻¹ is related to the C=C bond. -1 The peak at 1107 cm⁻¹ represents the stretching vibration peak of the COC and CN bonds. -1 The peak of bending vibration in the ROR structure is located at 870 cm⁻¹. -1 and 737cm -1 The peaks at the position represent out-of-plane bending vibrations of NH; these spectral features confirm that the t-CDs surface has been successfully grafted with abundant oxygen- and nitrogen-containing functional groups, such as hydroxyl (-OH) and amino (-NH2), which are crucial for their subsequent role in the composite material.

[0066] 1.1.1.3 Thermal stability of t-CDs

[0067] The thermogravimetric curves of t-CDs under air and nitrogen atmospheres are as follows: Figure 3 As shown, Figure 3 -a, Under air atmosphere, the initial decomposition temperature (T5 wt%) of t-CDs is 225.24℃, which is related to the oxidative degradation of functional groups on the t-CDs surface, and the maximum thermal weight loss rate temperature (Tmax) is 418.43℃. For example... Figure 3 -b, under a nitrogen atmosphere, the temperature (T5 wt%) corresponding to a 5% weight loss is 313.71℃. Its maximum thermal weight loss rate occurs at 409.44℃. This result indicates that t-CDs possess good thermal stability, fully capable of withstanding the melt processing temperature of PA66 at 270-290℃, ensuring that they do not decompose during blending and spinning and can stably exist within the matrix.

[0068] 1.2 Preparation of PA66 / t-CDs composite material

[0069] The composite material was prepared using a melt blending method. First, t-CDs powder and a portion of PA66 powder were melt-blended in a twin-screw extruder, extruded, and granulated to prepare a masterbatch with a t-CDs content of 20%. Then, the masterbatch was melt-blended a second time with pure PA66 chips in the desired proportions (e.g., to prepare composites with t-CDs contents of 1%, 2%, and 3%) in an extruder to obtain the final PA66 / t-CDs composite chips.

[0070] Table 1: Granulation Temperature of Screw Extruder

[0071] Zone 1 / ℃ Zone 2 / ℃ Three zones / ℃ Zone 4 / ℃ Zone 5 / ℃ Zone 6 / ℃ 250 250 245 240 230 230

[0072] 1.2.1 Thermal properties, flame retardancy and flame retardant mechanism of PA66 / t-CDs composite

[0073] 1.2.1.1 Thermal properties of the PA66 / t-CDs composite

[0074] The effect of t-CDs on the crystallization behavior of PA66 was investigated using differential scanning calorimetry (DSC). The test results are shown below. Figure 4 The relevant data is summarized in Table 2. Figure 4 As shown in Table 2, compared with pure PA66, the melting temperature (Tm) of the PA66 / t-CDs composite did not change significantly, indicating that the addition of t-CDs had little impact on the thermal properties of PA66. The melting crystallization temperature (Tmc) showed an upward trend, while the supercooling (ΔTmc) showed a downward trend, decreasing from 31.24℃ to 29.47℃. This indicates that the addition of t-CDs promoted the crystallization process of PA66. As a heterogeneous nucleating agent, t-CDs provided additional crystallization sites, lowered the nucleation energy barrier, and enabled crystallization to occur at conditions closer to the melting temperature, thereby significantly improving the crystallization ability of PA66.

[0075] Table 2: Differential scanning calorimetry parameters of PA66 / t-CDs composite

[0076] Samples T m / ℃]] T mc / ℃]] Delta T mc / Celsius PA66 256.49 225.25 31.24 PA66 / 1% t-CDs 257.77 227.48 30.29 PA66 / 2% t-CDs 258.49 229.02 29.47 PA66 / 3% t-CDs 258.19 228.61 29.58

[0077] 1.2.1.2 Thermal stability of the PA66 / t-CDs composite

[0078] Depend on Figure 5 As shown in Table 3, the T5 wt% of pure PA66 is 381.63℃, and T... max The temperature was 442.76℃, compared to pure PA66, the Tc of PA66 / t-CDs was... max The decrease indicates that the addition of t-CDs promoted the high-temperature degradation of PA66; when the amount of t-CDs added reached 2wt%, its residual mass increased by 27% compared with pure PA66; when the amount of t-CDs added reached 3wt%, its residual mass increased by 11% compared with pure PA66. The reason for the increase in residual mass is that the presence of t-CDs promoted the carbonization reaction of PA66 at high temperature, generating more carbonaceous residues.

[0079] M theo. (T) X+Y = (1-wT%)×M exp. (T) X +wT%×M exp. (T) Y ;

[0080] Calculate the theoretical residual mass of the PA66 / t-CDs composite, where X and Y represent the pure polymer and flame retardant, respectively, and M... theo. (T) X+Y M represents the theoretical mass of the polymer and flame retardant. exp. (T) X For the experimental quality of pure polymers, M exp. (T) Y The experimental mass of the flame retardant is calculated and summarized in Table 3. As can be seen from Table 3, the theoretical residual mass at 800℃ is higher than the residual mass at 800℃, which indicates that the carbon points may have undergone oxidative decomposition, resulting in a decrease in the actual residual carbon content.

[0081] Table 3: TG-DTG analysis data of PA66 and PA66 / t-CDs

[0082]

[0083] 1.2.1.3 Flame retardant properties and flame retardant mechanism of PA66 / t-CDs composite

[0084] LOI and UL-94 tests of the PA66 / t-CDs complex:

[0085] To investigate the effect of t-CDs on the flame retardant properties of PA66, LOI and UL-94 tests were conducted on the PA66 / t-CDs composite, and the data are summarized in Table 4. Pure PA66 is flammable, with an LOI value of 26% and a UL-94 fire rating of V-2. The LOI value of the PA66 composite with 3% t-CDs increased to 29%, but the UL-94 fire rating remained at V-2, and the second self-extinguishing time decreased from 7.86s for pure PA66 to 1.66s. The results showed that the LOI value of the composite increased with the increase of t-CDs addition. Although t-CDs could significantly shorten the self-extinguishing time of combustion, they failed to inhibit the behavior of molten droplets igniting the absorbent cotton. In the UL-94 test, all PA66 / t-CDs composites continued to ignite the absorbent cotton, maintaining the V-2 rating. The hydroxyl and carboxyl groups on the surface of t-CDs can catalyze the partial cross-linking of PA66 into char in the early stage of combustion, forming a residual char layer, which to some extent slows down heat transfer. However, the char layer is not dense enough and cannot completely encapsulate the molten matrix. At the same time, due to the low thermal stability of t-CDs themselves, their catalytic activity in the high-temperature region is reduced. In addition, the interfacial bonding between their hydrophilic functional groups and the hydrophobic PA66 molecular chains is weak, which makes the char layer easy to crack. High-temperature molten droplets can penetrate the char layer and drip down, igniting the absorbent cotton below.

[0086] Table 4: LOI and UL-94 of PA66 and PA66 / t-CDs complex

[0087]

[0088] (2) Cone calorimetry analysis of PA66 / t-CDs complex

[0089] Peak heat release rate (pHRR) refers to the highest heat release rate during combustion, directly reflecting the severity of the fire. Total smoke production (TSP) represents the total amount of smoke produced during material combustion. Smoke is not only a major suffocating factor in fires but also severely impairs escape and rescue visibility. Total heat release (THR) reflects the total heat energy released by the material from the start to the end of combustion, usually measured per unit area or per unit mass. Figure 4-6 The curves showing the changes in heat release rate (HRR), total smoke generation (TSP), total heat release (THR), and smoke generation rate (SPR) of PA66 / t-CDs composite material are presented in Table 5.

[0090] Table 5: TTI and heat release data of PA66 and PA66 / t-CDs complex

[0091]

[0092] Depend on Figure 6 As shown in Table 6, the pHRR, CO production (COP), and carbon dioxide production (CO2P) of the PA66 / t-CDs composite all decreased, indicating a reduction in the release of smoke and gases during combustion. pHRR is an important indicator for evaluating the combustion performance of materials; a higher pHRR value indicates a greater fire risk. With increasing t-CDs content, the pHRR of the composite material gradually decreased. When the mass fraction of t-CDs was 1%, the pHRR was 5.49% lower than that of the pure PA66 sample; when the content increased to 3%, the pHRR decreased by 10.64%. The decrease in pHRR is mainly attributed to the promotion of surface char layer formation by t-CDs during combustion. This char layer covers the material surface, acting as a heat and oxygen barrier, effectively inhibiting the heat release rate. Compared to pure PA66, the pCO2P of PA66 / 3% t-CDs decreased by 12.74%, while the decrease in pCOP and pCO2P is due to the effect of t-CDs in promoting matrix degradation and forming a shielding char layer.

[0093] Table 6: Smoke, CO, and CO2 emission data of PA66 and PA66 / t-CDs complex

[0094] Samples pSPR / m 2 ·s -1 ]]> <![CDATA[TSP / m 2 ]]> <![CDATA[COP / g·s -1 ]]> <![CDATA[CO2P / g·s -1 ]]> PA66 0.1031 10.12 0.0048 0.4032 PA66 / 1% t-CDs 0.1491 11.41 0.0052 0.3606 PA66 / 2% t-CDs 0.1208 11.62 0.0052 0.3684 PA66 / 3% t-CDs 0.1117 12.39 0.0047 0.3518

[0095] The Fire Growth Index (FGI) is an important parameter for measuring the rate of fire growth of a material in the early stages of a fire; a higher value indicates a faster fire spread. The Fire Performance Index (FPI) is used to assess a material's ability to spread fire and its potential risk under fire conditions; a higher FPI indicates that the material is less likely to cause fire spread. The Flame Retardancy Index (FRI) comprehensively reflects a material's combustion behavior and flame retardancy in a fire; a higher FRI value indicates better flame retardancy. To determine its safety factor and evaluate its fire hazard and flame retardancy, the test results are calculated using the formula, where PHRR is the peak heat release rate, t-PHRR is the time to reach the peak heat release rate, TTI is the ignition time, and THR is the total heat release.

[0096] The fire hazard index is calculated based on FGI = PHRR / t-PHRR, FPI = TTI / PHRR, and FRI = (THR*PHRR / TTI)PA66 / (THR*PHRR / TTI)FRPA66, as shown in Table 7.

[0097] Sample <![CDATA[FGIkW / (m 2 ·s)]]> <![CDATA[FPI(m 2 ·s) / kW]]> FRI PA66 2.77 0.14 - PA66 / 3% t-CDs 2.25 0.17 1.19

[0098] The FGI of pure PA66 is 2.77 kW / (m³). 2 ·s), FPI is 0.14 (m 2 ·s) / kW. When the addition amount of t-CDs is 3wt%, the FGI of PA66 / 3% t-CDs is 2.25kW / (m²). 2 ·s), FPI is 0.17 (m) 2 With a fire hazard rate of 1.19 / kW, the fire risk is slightly reduced, and the FRI is 1.19 > 1, indicating improved flame retardant performance.

[0099] (3) Flame retardant mechanism of PA66 / t-CDs composite

[0100] Please refer to the appendix. Figure 8 The attached figure shows the physical image and SEM image of the char residue of the samples after CONE testing. As can be seen from the image, no expanded char layer formed on the surface of the char residue of pure PA66 and the PA66 / 3% t-CDs composite material. The SEM image further shows that the char layer of pure PA66 contains a large number of interconnected pores, while when the t-CDs content increases to 3%, the pores in the char layer are significantly reduced, the structure becomes more compact, and it exhibits superior thermo-oxidative shielding performance. This has a positive effect on inhibiting the diffusion of heat and combustible gases, thus contributing to improving the flame retardant properties of the material.

[0101] Please refer to the appendix. Figure 9 The attached figure shows the Raman spectra of the char residues after CONE testing of PA66 and the PA66 / 3% t-CDs complex. The pure PA66 char residues show a Raman spectrum at 1368 cm⁻¹.-1 (D peak) and 1593cm -1 Two characteristic peaks were observed (G peak), with an ID / IG value of 3.35, while the carbon residue of PA66 / 3% t-CDs was at 1380 cm⁻¹. -1 (D peak) and 1589cm -1 Two characteristic peaks were observed (G peak), with an ID / IG value of 4.19, indicating that the residual carbon graphitization degree of PA66 / 3% t-CDs was reduced and did not play a better shielding role.

[0102] In summary, the addition of t-CDs to PA66 reduces the porosity and continuity of the char residue after combustion, thus exerting a certain solid-phase flame retardant effect. However, the densification of the char residue is reduced, so the improvement in flame retardant performance is limited.

[0103] PA66 and the PA66 / t-CDs complex were subjected to thermogravimetric-infrared spectroscopy (TGA) under nitrogen atmosphere. The results are shown in the appendix. Figure 10 As shown in Figure (a), pure PA66 begins to show a significant infrared absorption peak at approximately 400℃, indicating that the material undergoes thermal decomposition at this temperature, gradually releasing gaseous volatile products. At 2931 cm⁻¹... -1 The characteristic peak of stretching vibration of CH bonds can be observed at 2352 cm⁻¹, while at 2352 cm⁻¹... -1 The presence of a characteristic CO2 absorption peak further indicates the generation of combustible gases during the pyrolysis of PA66. Figure (b) shows that the peaks of the PA66 / t-CDs complex are not prominent, indicating that the addition of t-CDs reduces the gaseous products during PA66 combustion.

[0104] 1.3 Preparation and Properties of PA66 / t-CDs Composite Fibers

[0105] 1.3.1 Spinning and Post-processing Technology

[0106] Melt processing of polymers typically requires a temperature 20°C above their melting point. DSC data shows that PA66 has a melting point of 256°C, therefore its melt spinning temperature needs to be above 276°C. Furthermore, PA66 contains a large number of amide bonds in its molecular chain, which are prone to hydrolysis or oxidative degradation at high temperatures. Therefore, the spinning temperature of PA66 usually needs to be below 300°C. This is because although PA66 can maintain stability for a short time at 300°C, high temperatures or prolonged thermal stagnation easily trigger β-elimination reactions in the PA66 molecular chain, generating low-molecular-weight byproducts such as cyclopentanone. These cyclopentanone molecules cross-link with the amino or hydroxyl groups at the ends of the PA66 molecular chain through keto-enol tautomerism, forming a three-dimensional network gel structure. The gel in PA66 not only leads to an abnormally high melt viscosity but also gradually deposits on the inner wall of the spinneret. Even a small amount of gel can clog the spinneret during PA66 spinning, causing filament breakage and resulting in significant economic losses.

[0107] Determining the spinning temperature and the temperature of each screw zone during polymer melt spinning is a crucial step in the entire spinning process. Based on the stability during spinning, the screw temperature and spinning temperature of PA66 and t-CD modified PA66 composites were determined, as shown in Table 8. The results show that the spinning temperature gradually decreases with increasing t-CDs content, and the temperature of each screw zone also decreases to some extent. This helps to inhibit and reduce the thermal degradation and gelation of PA66 during spinning, which is significant for improving the stability of PA66 melt spinning. This may be because the abundant oxygen-containing functional groups on the surface of t-CDs form hydrogen bonds with the PA66 molecular chains, reducing the hydrogen bond interactions between PA66 molecular chains, lowering the apparent viscosity of the melt, lowering the spinning temperature, and moving it away from the thermal degradation sensitive zone. Furthermore, the free radical scavenging function of t-CDs can capture free radicals formed during the thermal degradation of PA66, blocking the cross-linking pathway.

[0108] Table 8: Spinning parameters of PA66 / t-CDs fiber

[0109] Zone 1 / ℃ Zone 2 / ℃ Three zones / ℃ Zone 4 / ℃ Spinning temperature / ℃ PA66 250 280 285 285 285 PA66 / 1% t-CDs 250 270 270 275 275 PA66 / 2% t-CDs 250 270 270 270 270 PA66 / 3% t-CDs 250 270 270 270 270

[0110] Determination of drawing temperature: The temperature of the first hot roller is set at 65℃, which is higher than the glass transition temperature of PA66 to ensure that the molecular chains are fully untangled and to avoid stress concentration caused by chain segment freezing during the stretching process; the temperature of the second hot roller is set at 80℃, which is lower than the temperature at which PA66 crystallizes the fastest, to suppress brittle fracture caused by over-crystallization; the hot box setting temperature is set at 190℃, which is higher than the temperature at which PA66 crystallizes the maximum, to compensate for the heat transfer hysteresis effect of the slit-type hot box and to simultaneously realize molecular chain orientation and crystallization phase transition. Figure 11Images show PA66 fibers with different t-CDs additions after being drawn to different ratios. As can be seen from the images, the fibers with added t-CDs are pale golden yellow, and the color gradually deepens with increasing t-CDs content. When the draw ratio exceeds 2, the nascent PA66 fibers with 3% t-CDs content cannot be stably drawn. This is because the surface of t-CDs is rich in hydrophilic functional groups such as hydroxyl groups. When the addition amount is large, it introduces more water molecules, leading to high-temperature hydrolysis of PA66 during melt spinning, thus resulting in a lower draw ratio during subsequent processing.

[0111] 1.3.2 Mechanical properties of PA66 / t-CDs fiber

[0112] Table 9: Tensile breaking strength and elongation at break of PA66 / t-CDs fiber

[0113]

[0114] Table 9 shows the tensile breaking strength and elongation at break of PA66 / t-CDs fibers. The results indicate that the addition of t-CDs synergistically improves both the breaking strength and elongation at break of PA66 fibers, which may be related to the abundant -OH / -NH2 groups on the t-CDs surface. The functional groups on the t-CDs surface form hydrogen bonds with the amide bonds of the PA66 molecular chains, creating cross-linking points between the PA66 molecular chains and enhancing the mechanical properties of the fiber. Furthermore, t-CDs act as a plasticizer in PA66, increasing the elongation at break of the PA66 fibers. The decrease in spinning temperature also indirectly proves that t-CDs function as a plasticizer in PA66.

[0115] 1.3.3 PA66 / t-CDs composite fiber aggregate structure

[0116] (1) Crystal structure analysis of PA66 / t-CDs composite fibers

[0117] Please refer to the appendix. Figure 12 The figure shows (a) fibers with a draw ratio of 2.5, (b) fibers with a draw ratio of 3.0, and (c) fibers with a draw ratio of 3.25. Figure 12 It can be seen that when the t-CDs addition amount is 0%, the XRD curves of PA66 fibers at different draw ratios all show a characteristic diffraction peak around 21°. When the addition amount increases, the XRD diffraction peak of PA66 / t-CDs composite fibers is still around 21°, indicating that the addition of t-CDs has little effect on the crystal form of PA66 / t-CDs composite fibers.

[0118] (2) Orientation structure analysis of PA66 / t-CDs fibers

[0119] Table 10: Orientation data for PA66 / t-CDs

[0120]

[0121] The orientation degree of fibers is influenced by both the composition of the blend system and the draw ratio. Its value reflects the degree of orderliness of the axial arrangement of polymer chain segments within the fiber. A higher orientation degree indicates a more regular orientation of the molecular chains along the fiber axis, which helps improve the fiber's mechanical properties. Table 10 lists the test results of the orientation structure of PA66 / t-CDs composite fibers at different draw ratios. (Table 10 and...) Figure 13 It can be seen that the orientation degree of the fiber increases with the increase of the draw ratio when the same amount of fiber is added. When the draw ratio increases, the stress applied to the fiber increases, and the macromolecular chains in the fiber are better arranged along the fiber axis, thus increasing the orientation degree of the fiber.

[0122] Example 2: Application of low-temperature long-term carbon dots (t-CDs) at the minimum effective addition amount

[0123] This embodiment aims to verify the technical effectiveness of the present invention under the mildest preparation conditions and the lowest addition amount.

[0124] 2.1 Preparation of carbon dots

[0125] 1500 g of cleaned and crushed waste PET bottle flakes were added to a clean, dry 5 L high-pressure reactor. 975 mL of ethanolamine was then added. The reactor was sealed, and mechanical stirring was started. The heating program was initiated, slowly raising the temperature of the reaction system to 250 °C. Once the temperature inside the reactor stabilized at 250 °C, timing was started, and the reaction was maintained at this temperature for 50 hours. After the reaction time was completed, the heating device was turned off, and the reactor was allowed to cool naturally to room temperature. Subsequent dilution, filtration, solvent recovery, and drying / pulverizing steps were performed as described above to obtain t-CDs powder.

[0126] 2.2 Preparation of composite materials

[0127] In this embodiment, a composite material with a carbon dot content of 0.5% was prepared. The obtained t-CDs were melt-blended with PA66 resin using a masterbatch method to finally obtain PA66 / t-CDs composite material slices with a carbon dot content of 0.5%.

[0128] Example 3: Preparation of High-Temperature Carbon Dots (T-CDs) and Their Application in PA66

[0129] This embodiment describes carbon dots (T-CDs) prepared at relatively high temperatures, a method designed to achieve the ultimate flame retardant properties.

[0130] 3.1 Preparation of T-CDs

[0131] The preparation method was similar to that of Example 1. 1500 g of waste PET bottle flakes and 975 mL of ethanolamine were added to a 5 L high-pressure quick-release reactor. The difference lay in the adjustment of the process parameters: the reaction temperature was increased to 280 °C, and the reaction time was shortened to 18 hours. The post-processing steps were exactly the same as in Example 1. Finally, T-CDs powder was obtained.

[0132] 3.2 Structural and performance characterization of T-CDs

[0133] Please refer to the appendix. Figure 14 -a, TEM images show that T-CDs are also spherical and relatively uniformly distributed. Transmission electron microscopy reveals that the particle size distribution of T-CDs is relatively uniform, similar in size to t-CDs. Please refer to the attached image. Figure 14 -b As shown in the figure, the particle size range of T-CDs is between 1.15nm and 3.85nm, with an average particle size of 2.32nm. It can be seen that the size of T-CDs is relatively uniform, which is beneficial to the performance of various properties.

[0134] 3.2.1 Structure of T-CDs

[0135] Please refer to the appendix. Figure 15 The infrared spectrum of T-CDs exhibits multiple characteristic absorption peaks, located at 3292 cm⁻¹. -1 The absorption peak corresponds to the stretching vibration of the NH or OH bond, indicating the presence of amino or hydroxyl functional groups in the sample; 2963 cm⁻¹ -1 The peak at 1537 cm⁻¹ represents the stretching vibration of the CH bond. -1 The appearance of a bending vibration peak of the C=O bond at 1409 cm⁻¹ indicates that the material contains a carbonyl structure; -1 The peak at 1269 cm⁻¹ is a characteristic absorption peak associated with the C=C bond. -1 The vibration at this point is attributed to the stretching vibrations of the COC and CN bonds; 1108 cm⁻¹ -1 The peak value at 864 cm⁻¹ represents the bending vibration peak of the ROR structure. -1 and 733cm -1 The peaks at this location represent out-of-plane bending vibrations of NH; these characteristic peaks indicate that the surface of T-CDs is rich in various oxygen- and nitrogen-containing functional groups, which is beneficial to its compatibility with matrix materials and its functional modification.

[0136] 3.2.2 Thermal stability analysis of T-CDs

[0137] Please refer to the appendix. Figure 16 As shown in -a, the initial decomposition temperature (T5) of T-CDs in air atmosphere. wt％ The temperature at which the maximum rate of thermal weight loss is 341.27℃ is also mentioned. max The temperature was 415.45℃, and the residual amount was 0.22%; please refer to the appendix. Figure 16 As shown in -b, under a nitrogen atmosphere, the T of T-CDs 5wt％ The temperature was 321.62℃, T max The temperature was 424.1℃; this indicates that T-CDs have high thermal stability. Furthermore, T-CDs exhibit higher thermal stability than t-CDs, especially in air, because T-CDs have fewer surface functional groups than t-CDs, resulting in better thermal stability.

[0138] Table 11: TGA-DTG Analysis Data of rPET-CDs

[0139]

[0140] 3.3 Thermal properties, flame retardancy and flame retardant mechanism of PA66 / T-CDs composite

[0141] PA66 / T-CDs complex was prepared using the same method as in Example 1.

[0142] 3.3.1 Thermal properties of the PA66 / T-CDs composite

[0143] The thermal properties of the PA66 / T-CDs composite were characterized by DSC, and the results are as follows: Figure 17 As shown in Table 12:

[0144] Table 12: Differential scanning calorimetry parameters of PA66 and PA66 / T-CDs complex

[0145] Samples <![CDATA[T m / ℃]]> <![CDATA[T mc / ℃]]> <![CDATA[ΔT mc / ℃ <!-- 11 -->]]> PA66 256.49 225.25 31.24 PA66 / 1% T-CDs 258.39 228.59 29.80 PA66 / 2% T-CDs 259.20 227.89 31.31 PA66 / 3% T-CDs 255.90 228.25 27.65 PA66 / 4% T-CDs 255.25 227.85 27.40 PA66 / 5% T-CDs 256.45 227.05 29.40

[0146] Depend on Figure 17 As shown in Table 13, the T of pure PA66 m The temperature was 256.49℃, T mc The T value of the PA66 / T-CDs complex is 225.25℃. m T mc and ΔT mc The differences are not significant, indicating that the thermal properties of the material do not change much after the PA66 / T-CDs composite is blended into PA66.

[0147] 3.3.2 Thermal stability of the PA66 / T-CDs composite

[0148] Table 13: TG-DTG analysis data of PA66 and PA66 / T-CDs complex

[0149] Samples <![CDATA[T 5wt％ / ℃]]> <![CDATA[T max / ℃]]> 800℃ char / wt% Mtheo.Cw / wt% PA66 381.63 442.76 6.41 6.41 PA66 / 1% T-CDs 391.93 447.83 6.39 6.50 PA66 / 2% T-CDs 393.73 448.35 7.46 7.65 PA66 / 3% T-CDs 388.13 442.93 6.27 6.59 PA66 / 4% T-CDs 389.02 444.13 9.25 9.56 PA66 / 5% T-CDs 382.59 433.34 7.14 7.64

[0150] Depend on Figure 18As shown in Table 13, compared with pure PA66, the T of the PA66 / T-CDs complex is... max The residual mass of PA66 increased by 27% when the addition of T-CDs reached 2 wt% compared with pure PA66; when the addition of T-CDs reached 3 wt%, the residual mass of PA66 increased by 11% compared with pure PA66. The reason for the increase in residual mass is that the presence of T-CDs promotes the carbonization reaction of PA66 at high temperature, generating more carbonaceous residue.

[0151] 3.3.3 Flame retardant properties and flame retardant mechanism of PA66 / T-CDs composite

[0152] (1) LOI and UL-94 tests of PA66 / T-CDs complex

[0153] To investigate the effect of T-CDs on the flame retardant properties of PA66, LOI and UL-94 tests were conducted on PA66 / T-CDs composites, and the data are summarized in Table 14. Pure PA66 is flammable, with an LOI value of 26% and a UL-94 fire rating of V-2, accompanied by ignition of absorbent cotton by molten droplets. When the T-CDs addition amount reached 3% and 4%, the LOI value of the composite increased to 30%, and further increased to 31% when the addition amount was 5%. Moreover, the UL-94 fire rating of PA66 / 3%T-CDs, PA66 / 4%T-CDs, and PA66 / 5%T-CDs composites all reached V-0, with no molten droplets and a self-extinguishing time ≤10 seconds. The results showed that the LOI value of the composite gradually increased with the increase of T-CDs addition. The introduction of T-CDs significantly improved the flame retardant performance limit of the PA66 composite material through catalytic densification and synergistic gas-solid phase flame retardancy mechanism. When the T-CDs addition was 5%, the LOI value jumped from 26% of pure PA66 to 31%, and the UL-94 rating also improved from V-2 to V-0. The epoxy groups enriched on the surface of T-CDs efficiently catalyzed the cross-linking of PA66 molecules, forming a continuous, non-porous, dense char layer. This char layer effectively isolated oxygen permeation and blocked the escape path of combustible gases. The complete disappearance of the dripping phenomenon in the UL-94 test further confirmed that T-CDs have higher thermal stability than t-CDs. This high thermal stability ensures that its catalytic efficiency does not decay at high temperatures and forms a strong interfacial bond with the PA66 matrix, thereby maintaining the integrity and barrier effect of the char layer.

[0154] Table 14: LOI and UL-94 test results of PA66 and PA66 / T-CDs complex

[0155]

[0156]

[0157] (2) Cone calorimetry analysis of PA66 / T-CDs complex

[0158] pHRR and THR of PA66 / T-CDs complex Figure 19 As shown, the specific values ​​are shown in Table 15.

[0159] Table 15: TTI and heat release data of PA66 and PA66 / T-CDs complex

[0160]

[0161] Table 16: Smoke, CO, and CO2 Emission Data of PA66 and PA66 / T-CDs Complex

[0162] Samples <![CDATA[pSPR / m 2 ·s -1 ]]> <![CDATA[TSP / m 2 ]]> <![CDATA[COP / g·s -1 ]]> <![CDATA[CO2P / g·s -1 ]]> PA66 0.1077 11.46 0.0045 0.3733 PA66 / 1% T-CDs 0.1050 11.79 0.0044 0.3527 PA66 / 2% T-CDs 0.1130 10.30 0.0043 0.2899 PA66 / 3% T-CDs 0.1049 10.62 0.0042 0.3126 PA66 / 4% T-CDs 0.1010 11.38 0.0041 0.2810 PA66 / 5% T-CDs 0.1058 12.19 0.0037 0.2689

[0163] Please refer to the appendix. Figure 20 and Table 16, Figure 20 In the figures, (a) is the SPR curve; (b) is the weight curve; (c) is the COP; and (d) is the CO2P. The pHRR, CO production (COP), and carbon dioxide production (CO2P) of the PA66 / T-CDs complex all decreased. The pHRR of pure PA66 was 641.76 kW·m. 2 The THR was 109.62 MJ·m -1 The COP is 0.0045 g·s⁻¹. -1 CO2P is 0.3733 g·s -1 As the amount of T-CDs added increased, the pHRR gradually decreased. When the mass fraction of T-CDs reached 1%, the pHRR decreased by 7.8%; when the mass fraction was 3%, the pHRR decreased by 14.8%, and the THR decreased by 13.6%; when the mass fraction of T-CDs reached 5%, the pHRR decreased by 22.29% compared to the pure PA66 sample, the pCOP decreased by 17.77%, and the pCO2P decreased by 27.97%. In summary, the heat release of PA66 / 3% T-CDs... It exhibits a balanced performance in terms of suppression, smoke toxicity control, and ignition delay, representing an optimal balance between flame retardancy and smoke toxicity. Its TSP is reduced by 7.3%, and pCOP and pCO2P decrease by 6.7% and 16.3%, respectively, indicating that the char layer effectively traps smoke particles. This verifies the increase in incomplete combustion and the reduction in the release of toxic gases. Its TTI is extended by 3.7%, indicating that the material is more difficult to ignite. These results show that the introduction of T-CDs helps to form a heat-insulating char layer during combustion, effectively suppressing the release of combustible gases, thereby improving the flame retardant performance of the material.

[0164] To assess the fire risk and flame retardant properties of the materials and further determine their safety factor, the fire hazard index of the samples was calculated and analyzed. The summary results of each index are shown in Table 17.

[0165] Table 17: Fire Hazard Index of PA66 and PA66 / T-CDs Compound

[0166] Sample <![CDATA[FGIkW / (m 2 ·s)]]> <![CDATA[FPI(m 2 ·s) / kW]]> FRI PA66 2.79 0.12 - PA66 / 5% T-CDs 1.87 0.13 1.13

[0167] The FGI of pure PA66 is 2.79 kW / (m³). 2 ·s), FPI is 0.12 (m 2 The FGI of PA66 / 5% t-CDs composite material is 1.87 kW / (m·s) / kW when the addition amount of T-CDs is 5 wt%. 2 ·s), FPI is 0.13 (m 2 The flame retardant ratio (FGI) was 32.97% lower than that of pure PA66, indicating a significant reduction in the growth rate of the material in the early stages of a fire, effectively suppressing the fire hazard. Its flame retardant index (FRI) was 1.13 > 1, indicating that the composite material possesses excellent flame retardant properties.

[0168] (3) Flame retardant mechanism of PA66 / T-CDs composite

[0169] Raman spectroscopy can effectively reflect the degree of graphitization of char residues. The integral area ratio (ID / IG) is used to evaluate the proportion of ordered graphite structures in the char residues; the smaller the ratio, the higher the degree of graphitization. The degree of graphitization of char residues from pure PA66 and the PA66 / 3% T-CDs composite material with the best overall performance was analyzed using Raman spectroscopy after testing with a cone calorimeter. The results are as follows: Figure 21 As shown. Pure PA66 char residue at 1368 cm⁻¹ -1 (D peak) and 1593cm -1 Two distinct characteristic peaks appeared at (G peak), with an ID / IG value of 3.36; in contrast, the characteristic peaks of PA66 / 3% T-CDs char residues were located at 1593 cm⁻¹. -1 (D peak) and 2859cm -1 (G peak), with the ID / IG value decreasing to 2.62, indicating an improved degree of graphitization. This phenomenon suggests that the addition of T-CDs results in a denser char layer forming in the composite material during combustion, enhancing its shielding effect against heat and oxygen, thereby effectively improving its flame-retardant properties.

[0170] PA66 and the PA66 / T-CDs complex were subjected to thermogravimetric-infrared spectroscopy (TGA) under nitrogen atmosphere, and the results are as follows: Figure 22As shown in Figures (a) and (b), the pure PA66 sample begins to show a significant infrared absorption peak at 400℃, indicating that the material undergoes thermal decomposition at this point, gradually releasing gaseous volatile products. Specifically, at 2931 cm⁻¹... -1 A distinct absorption peak appeared at 2352 cm⁻¹, corresponding to the stretching vibration of the C–H bond; -1 The characteristic infrared absorption peak of CO2 appeared at 400℃, reflecting the generation of carbon dioxide during combustion. Figures (c) and (d) show that the PA66 / T-CDs complex also showed an absorption peak near 400℃, and its absorption peak wavenumber was almost the same as that of PA66. This indicates that T-CDs have no effect on the thermal decomposition products of PA66.

[0171] 3.4 Preparation and Properties of PA66 / T-CDs Composite Fibers

[0172] 3.4.1 Spinability:

[0173] T-CDs can also improve the spinnability of PA66. With increasing addition, the spinning temperature can be gradually reduced from 285°C to 260°C (Table 18), which is also attributed to its plasticizing effect.

[0174] Table 18: Spinning parameters of PA66 / T-CDs fiber

[0175]

[0176]

[0177] The addition of T-CDs can appropriately lower the spinning temperature of PA66, as shown in Table 18. The decrease in spinning temperature indicates that T-CDs act as a plasticizer during the spinning process of PA66. Actual images of the fibers after different draw ratios are shown below. Figure 23 As shown in the figure, (a) 2.5 times; (b) 2.75 times; (c) 3.0 times; (d) 3.25 times; (e) 3.5 times. The results show that as the amount added increases, the fiber gradually changes from colorless to golden yellow.

[0178] 3.4.2 Mechanical properties:

[0179] Unlike t-CDs, the addition of T-CDs has a different effect on the mechanical properties of fibers (Table 19). At the same draw ratio, the breaking strength of the fiber decreases with increasing T-CD content, while the breaking elongation increases. This may be because T-CDs, prepared at a higher temperature, have relatively fewer surface functional groups, resulting in weaker hydrogen bond crosslinking with the PA66 molecular chain, thus having a less significant effect on strength improvement. However, its plasticizing effect still exists, leading to increased breaking elongation and greater fiber flexibility.

[0180] Table 19: Tensile breaking strength and elongation at break of PA66 / T-CDs composite fibers

[0181]

[0182]

[0183] 3.4.3 Fiber aggregate structure of the PA66 / T-CDs complex

[0184] (1) Crystal structure analysis of PA66 / T-CDs composite fibers

[0185] Figure 24 XRD patterns of PA66 / T-CDs composite fibers are shown, where (a) fibers with a draw ratio of 2.5, (b) fibers with a draw ratio of 2.75, (c) fibers with a draw ratio of 3.0, (d) fibers with a draw ratio of 3.25, and (e) fibers with a draw ratio of 3.5. Figure 24 It can be seen that when the amount of T-CDs added is 0%, the XRD curves of PA66 fibers at different draw ratios all show a characteristic diffraction peak around 21°. When the amount of T-CDs added is increased, the XRD diffraction peak of PA66 / T-CDs composite fibers is still around 21°, indicating that the addition of T-CDs has little effect on the crystal form of PA66 / T-CDs composite fibers.

[0186] (2) Orientation structure analysis of PA66 / T-CDs composite fibers

[0187] Table 20: Orientation data of PA66 / T-CDs

[0188]

[0189]

[0190] The acoustic orientation test results of PA66 / T-CDs fibers at different draw ratios are shown in Table 20 and... Figure 25 As shown in the figure. The results indicate that, under the same T-CDs addition conditions, the fiber orientation gradually increases with the increase of the draw ratio. The elastic properties of fibers are closely related to the crystallinity and orientation of their macromolecular chains. A higher orientation usually means that the molecular chains are more regularly arranged along the fiber axis, thereby improving the mechanical properties of the fiber. The higher the fiber orientation, the more crystallization induced by stretching, and the better the elastic recovery rate of the fiber. The maximum orientation is achieved with PA66 / 4% T-CDs at a draw ratio of 3.5, which forms the optimal molecular chain orientation structure.

[0191] Example 4: Flame retardant application of high-temperature short-time carbon points (T-CDs) at high addition levels

[0192] This embodiment aims to explore the performance limits of the present invention under the highest preparation conditions and the highest addition amount.

[0193] 4.1 Preparation of carbon dots

[0194] The preparation process was basically the same as in Example 1. Similarly, 1500g of waste PET bottle flakes and 975ml of ethanolamine were added to a 5L high-pressure reactor. The key process parameter was adjusted as follows: the reaction temperature was raised to 290℃ and the reaction was continued at this temperature for 25 hours. Subsequent steps were the same as above, finally yielding T-CDs powder.

[0195] 4.2 Preparation of composite materials

[0196] In this embodiment, a composite material with a carbon dot content of 10% was prepared. The prepared T-CDs were melt-blended with PA66 chips using a masterbatch method to obtain PA66 / T-CDs composite chips with a final T-CDs content of 10%.

[0197] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A flame-retardant polyamide 66 composite material, characterized in that, The composite material comprises, by weight 100%, the following components: Polyamide 66: 90% to 99.5%; Carbon point: 0.5% to 10%; The carbon dots are prepared by reacting waste polyethylene terephthalate with amine compounds.

2. The flame-retardant polyamide 66 composite material according to claim 1, characterized in that, The amine compound is ethanolamine.

3. The flame-retardant polyamide 66 composite material according to claim 1, characterized in that, The carbon dots are prepared by reacting the waste PET with the amine compound at a reaction temperature of 250°C to 290°C for 10 to 64 hours.

4. The flame-retardant polyamide 66 composite material according to claim 1, characterized in that, The carbon dots are prepared by reacting the waste PET with the amine compound at a reaction temperature of 250°C to 270°C for 50 to 60 hours to obtain low-temperature long-lasting carbon dots (t-CDs) or by reacting the waste PET with the amine compound at a reaction temperature of 270°C to 290°C for 15 to 25 hours to obtain high-temperature carbon dots (T-CDs).

5. The flame-retardant polyamide 66 composite material according to claim 1, characterized in that, The composite material is in the form of fibers, filaments, granulated chips, or injection molded parts.

6. A method for preparing flame-retardant polyamide 66 composite material using waste PET, characterized in that, The flame-retardant polyamide 66 composite material according to any one of claims 1-5 is applied to the following steps: S1. Provide carbon dots, said carbon dots being obtained by reacting waste polyethylene terephthalate with an amine compound under heating conditions; S2. The carbon dots obtained in step S1 are melt-blended with polyamide 66 to obtain the flame-retardant polyamide 66 composite material.

7. The method for preparing flame-retardant polyamide 66 composite material from waste PET according to claim 6, characterized in that, Step S2 specifically includes: S2a. First, the carbon dots are melt-blended with a portion of polyamide 66 to prepare carbon dot masterbatch; S2b. The carbon dot masterbatch is then melt-blended with the remaining polyamide 66.

8. The method for preparing flame-retardant polyamide 66 composite material from waste PET according to claim 6, characterized in that, In step S1, the reaction is carried out at a temperature of 250°C to 270°C for 50 to 60 hours to obtain long-term carbon points (t-CDs); the reaction is carried out at a temperature of 270°C to 290°C for 15 to 20 hours to obtain high-temperature carbon points (T-CDs).

9. A method for preparing flame-retardant polyamide 66 composite material from waste PET according to claim 6, characterized in that, The amine compound is ethanolamine.

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