Carbon quantum dots prepared from waste and a preparation method thereof

Abstract

The present application belongs to the field of nanometer carbon material preparation, discloses a kind of carbon quantum dots and preparation method thereof prepared by waste recovery, suitable for UV curing coating application, aims at solving the problems such as poor batch consistency of carbon dots prepared from waste and insufficient compatibility with coating, realizes the high value closed loop of electrochemical aluminum / transfer film waste coating waste material.The present application uses the waste coating waste material containing nitrogen polymer in the industry as raw material, after pre-classification and alkali alcohol mixed solvent pretreatment, hydrothermal carbonization and purification, surface modification is completed by UV photochemical grafting specific UV curing monomer, and 2-10nm nitrogen-doped graphene quantum dot powder is prepared;The process parameters such as hydrothermal and grafting are optimized, to realize the covalent bonding of carbon quantum dots and UV curing coating.The raw material of the present application is green and low cost, the batch consistency of the product is significantly improved, the compatibility and storage stability of the coating are greatly improved, and after adding, the fluorescence anti-counterfeiting and curing tracing of UV curing coating can be realized, which meets the requirements of double carbon target and new material industry development.

Description

Technical Field

[0001] This invention belongs to the field of nano-carbon material preparation technology, specifically relating to a method for preparing nitrogen-doped graphene quantum dot powder by recycling nitrogen-containing polymer waste coatings generated during the production of electroplated aluminum, hot stamping materials and transfer films. Background Technology

[0002] Carbon quantum dots, as quasi-zero-dimensional fluorescent carbon nanomaterials with a particle size of less than 10 nm, have shown promising application prospects in various fields such as fluorescent anti-counterfeiting, ion detection, bioimaging, optoelectronic devices, and coating additives due to their excellent fluorescence properties, low toxicity, good biocompatibility, and ease of surface functionalization. Existing methods for preparing carbon quantum dots are mainly divided into two categories: top-down and bottom-up methods. The top-down method uses carbon materials such as graphite and carbon nanotubes as precursors, which are cut into quantum dots using physical or chemical methods. This method suffers from high equipment requirements, low yield, and wide product size distribution. The bottom-up method uses small-molecule organic matter or biomass as carbon sources, preparing quantum dots through hydrothermal or microwave pyrolysis. This method has become the mainstream preparation method due to its inexpensive raw materials and mild process. In recent years, in response to the dual-carbon goals and circular economy policies, researchers have begun to explore the preparation of carbon quantum dots using industrial or domestic waste as precursors. Examples include using waste plastics and waste biomass to prepare carbon quantum dots, or using waste polymer materials to prepare carbon materials. However, these technologies mostly use general waste as raw materials, lacking deep integration with specific high-value industrial chains, making it difficult to form a closed loop for high-value utilization of waste, thus limiting economic value. Furthermore, due to the complex and variable composition of industrial waste, existing technologies for preparing carbon quantum dots from waste suffer from poor batch-to-batch consistency, large fluctuations in quantum yield, and significant emission peak drift, failing to meet the stability requirements for industrial applications. Regarding the application of carbon quantum dots, current technologies for UV-curable coatings often employ physical blending or simple silane and PEG coating methods, which are prone to problems such as agglomeration, phase separation, migration, and fluorescence quenching, resulting in poor long-term storage stability. Moreover, existing applications of carbon quantum dots are mostly concentrated in general fields, with very little targeted optimization for UV fluorescent anti-counterfeiting labels or fluorescence tracers in the curing process of industries such as electroplated aluminum and transfer films, making it difficult to generate direct economic benefits in these industries. Electroplated aluminum and transfer film manufacturers generate a large amount of nitrogen-containing polymer waste coatings during the production process. These wastes are mainly composed of acrylates, polyurethanes, and gelatin / proteins, and are rich in carbon and nitrogen elements. However, traditional treatment methods are mostly incineration or landfill, which not only causes serious waste of resources but also brings environmental problems. At present, there is still no high-value utilization technology for this type of industry-specific waste. Summary of the Invention

[0003] In view of this, the present invention proposes a method for preparing carbon quantum dots from waste recycling, aiming to solve the technical problems of the prior art in carbon quantum dot preparation, such as the generalization of precursor sources, poor batch consistency, insufficient compatibility with UV-curable coating substrates, and weak application specificity in the electroplated aluminum / transfer film industry. At the same time, it realizes the high-value recycling and utilization of nitrogen-containing polymer waste coating materials in the electroplated aluminum / transfer film industry, forming a resource closed loop within the industry.

[0004] The technical solution of the present invention is as follows: The present invention provides a carbon quantum dot prepared by waste recycling. The carbon quantum dot is a nitrogen-doped graphene quantum dot powder with a particle size of 2-10nm. It has a graphene edge / in-plane defect structure and is prepared from nitrogen-containing polymer waste coating waste generated from the production of electroplated aluminum, hot stamping materials or transfer films. Its preparation method includes the following steps: (1) Waste pretreatment: The waste coating waste is classified into color layer type and transfer film type according to its source. It is mechanically crushed to 40-80 mesh. It is cleaned with a mixture of 1-3wt% NaOH aqueous solution and ethanol / acetone volume ratio of 1:1 at 40-60℃ with ultrasonic or mechanical stirring at 200-400W for 1-3h. After washing with water until pH is neutral, it is dried at 80-100℃ for 12-24h to obtain pretreated waste powder.

[0005] (2) Hydrothermal carbonization: Mix the pretreated waste powder with urea or melamine at a mass ratio of 1:(1.2-1.8), add deionized water to a filling degree of 60-80%, and place it in a stainless steel hydrothermal reactor lined with 100-200mL polytetrafluoroethylene. React at 190±5℃ for 6-10h. After natural cooling, filter through a 200-mesh sieve to obtain a crude product dispersion. (3) Post-processing and purification: Centrifuge the crude product dispersion at 8000-12000 rpm for 15-20 min, repeat 2-3 times, take the supernatant, filter it through a 0.22 μm microporous membrane, and then dialyze it against deionized water for 24-72 h with a dialysis bag with a molecular weight cutoff of 1000-3500 Da, changing the water every 8 h. The concentrate is then freeze-dried or spray-dried under vacuum to obtain nitrogen-doped graphene quantum dot powder. (4) UV photochemical grafting modification: The nitrogen-doped graphene quantum dot powder was dispersed in a solvent with an ethanol / water volume ratio of 1:1 to prepare a dispersion of 5-20 mg / mL. 5-20 wt% of UV-curable monomer and 0.5-2 wt% of photoinitiator (based on the dry weight of the powder) were added. Nitrogen gas was purged for 10-15 min to remove oxygen. The mixture was then subjected to a power of 20-50 mW / cm². 2Irradiate the carbon quantum dots under a 365nm UV lamp for 30-90 minutes, stirring or shaking continuously during the irradiation process. After the reaction, remove unreacted monomers by centrifugation or dialysis, and then dry to obtain the carbon quantum dots. The UV-curable monomer is one or two of 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, and hydroxyethyl acrylate. The photoinitiator is benzoin dimethyl ether or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0006] In some embodiments, the waste pretreatment in step (1) uses 300W ultrasonic power for 2 hours. The combination of ultrasonic power and cleaning time can ensure the cleaning effect while avoiding excessive crushing of waste powder due to excessive ultrasonic power and time. At the same time, it allows the alkali-alcohol mixed solvent to fully contact the waste powder, effectively removing floating color, adhesive residue and inorganic filler on the surface of the waste, and achieving homogenization of the waste. This lays the raw material foundation for the batch stability of the subsequent hydrothermal carbonization reaction. However, if the ultrasonic power is too low and the time is too short, the impurities cannot be effectively removed, which will still lead to batch differences in the raw material composition.

[0007] In some embodiments, the hydrothermal carbonization reaction temperature in step (2) is 190°C, and the reaction time is 8 hours. This temperature is the optimal temperature for carbonizing nitrogen-containing polymer waste coatings to form graphene-like structures. This ensures that the polymers in the waste undergo sufficient pyrolysis, cyclization, and condensation reactions to form graphene-like structures. 2 The hybrid structure of the carbon core can avoid excessive graphitization and aggregation of the carbon core due to excessive temperature, thus weakening the quantum confinement effect; the 8-hour reaction time allows the carbonization reaction to proceed completely. If the reaction time is insufficient, the waste material will not be fully carbonized and a complete nitrogen-doped graphene quantum dot structure cannot be formed, affecting the fluorescence performance.

[0008] In some embodiments, urea is selected as the nitrogen source in step (2), and the mass ratio of pretreated waste powder to urea is 1:1.5. Urea has the characteristics of high nitrogen content and uniform release of nitrogen source during carbonization. Compared with melamine, it is easier to combine with carbon source in waste to achieve nitrogen doping. The mass ratio of 1:1.5 is the optimal ratio for nitrogen source overcompensation, which can effectively offset the fluctuation of natural nitrogen source content in different batches of waste, ensure the stability of nitrogen doping amount, and avoid nitrogen agglomeration on quantum dot surface caused by excessive nitrogen source, which affects fluorescence emission performance.

[0009] In some embodiments, the concentrated liquid is dried by vacuum freeze drying at -50°C in step (3) for 24-48 hours. Vacuum freeze drying can achieve solvent sublimation at low temperature, avoiding the destruction of the graphene defect structure of quantum dots due to high temperature in traditional drying methods. At the same time, it prevents the agglomeration of quantum dot powder and ensures the dispersibility of the product. The drying temperature of -50°C can take into account both drying efficiency and product structural integrity.

[0010] In some embodiments, the UV-curable monomer in step (4) is 1,6-hexanediol diacrylate. This monomer is a bifunctional acrylate monomer. Compared with monofunctional or trifunctional monomers, it can form stable covalent bonds with the active functional groups on the surface of carbon quantum dots during photografting, and can also provide sufficient crosslinking points for the combination of carbon quantum dots and UV-curable coatings, so that carbon quantum dots can participate in the crosslinking reaction of UV-curable coatings and achieve covalent bonding with the coating matrix. Monofunctional monomers cannot provide enough crosslinking sites, while trifunctional monomers are prone to quantum dot aggregation during the grafting process.

[0011] In some embodiments, the photoinitiator in step (4) is diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and its addition amount is 1 wt% of the dry weight of the quantum dot powder. This photoinitiator has high initiation efficiency under 365nm UV light, can quickly generate free radicals, trigger the grafting reaction between the active sites on the carbon quantum dot surface and the UV-cured monomer, and its decomposition products leave no residue, which will not have a negative impact on the fluorescence performance of the carbon quantum dots. The addition amount of 1 wt% can ensure the complete initiation reaction, while avoiding product contamination caused by excessive photoinitiator.

[0012] In some embodiments, the irradiation time for UV photochemical grafting in step (4) is 60 min, and nitrogen protection is used throughout the irradiation process. The 60 min irradiation time can ensure that the grafting reaction between the UV-cured monomer and the carbon quantum dots is complete, and the carbon quantum dot surface is fully modified. If the irradiation time is too short, the grafting rate is too low, and the compatibility between the carbon quantum dots and the UV-cured coating cannot be effectively improved. Nitrogen protection can effectively remove oxygen from the system, prevent oxygen from quenching the free radicals generated by photoinitiation, ensure the continuous progress of the grafting reaction, and improve the grafting efficiency.

[0013] In some embodiments, the dispersion concentration of carbon quantum dot powder in the ethanol / water mixed solvent in step (4) is 10 mg / mL. This concentration can ensure that the carbon quantum dot powder forms a stable dispersion in the solvent, so that the photoinitiator and UV curing monomer can uniformly contact the carbon quantum dot surface and achieve uniform grafting. If the concentration is too high, the carbon quantum dots are prone to agglomeration, resulting in uneven grafting. If the concentration is too low, the production efficiency will be reduced and the preparation cost will be increased.

[0014] In some embodiments, the nitrogen-containing polymer waste coating waste mentioned in step (1) is fragments of electroplated aluminum color layer curing film or waste of transfer film peeling. This type of waste is a special waste of the electroplated aluminum / transfer film industry. The acrylate, polyurethane and gelatin / protein substances in its composition can provide sufficient carbon source and some natural nitrogen source for the preparation of carbon quantum dots. Moreover, its molecular structure is easy to form graphene-like defect structure during carbonization. It is a high-quality raw material for the preparation of nitrogen-doped graphene quantum dots. Compared with general waste plastics and biomass waste, this type of waste is highly compatible with the preparation process of the present invention.

[0015] In some embodiments, the UV lamp power for UV photochemical grafting in step (4) is selected as 30mW / cm². 2 This power of UV light can provide suitable energy for the grafting reaction, ensuring efficient decomposition of the photoinitiator while avoiding self-polymerization of UV-cured monomers due to excessive light power. It also prevents excessive polymerization of monomer chains grafted onto the carbon quantum dot surface, which could affect its compatibility with UV-cured coatings.

[0016] The present invention has the following advantages over the prior art: This invention abandons the existing method of preparing carbon quantum dots using general waste materials. Instead, it specifically selects nitrogen-containing polymer waste coating materials from the electroplated aluminum / transfer film industry as raw materials, achieving high-value recycling of such industry waste. This changes the traditional waste treatment model of incineration and landfill, forming an industrial resource closed loop of "waste-carbon quantum dots-industry main products," significantly improving the economic and environmental value of waste. Through a standardized waste pretreatment and nitrogen source excess compensation process, this invention effectively solves the problem of poor batch consistency of carbon quantum dots caused by the complex composition of industrial waste, significantly improving the industrial repeatability of the preparation process. Compared with preparation methods without targeted batch control, the fluorescence performance stability of the product is greatly improved. This invention uses UV photochemical grafting of specific UV-curable monomers to achieve surface modification of carbon quantum dots. Compared with traditional physical blending or silane / PEG coating methods, this allows the carbon quantum dots to... The UV-curable coating matrix forms stable covalent bonds, fundamentally solving the problems of carbon quantum dots' easy aggregation, migration, and fluorescence quenching in coatings, and significantly improving the compatibility and long-term storage stability of carbon quantum dots with coatings. This invention realizes the targeted application of carbon quantum dots in the electroplated aluminum / transfer film industry, enabling it to meet the industry's specific needs for UV-curable coatings' ultraviolet fluorescent anti-counterfeiting labels and fluorescence tracing during the curing process. It fills the gap in the existing technology for targeted applications of carbon quantum dots in this industry. Compared with the general applications of existing carbon quantum dots, the product of this invention can bring direct economic benefits to the industry. At the same time, the preparation process of this invention is mild, requiring no complex production equipment, and the use of self-produced waste materials significantly reduces the preparation cost of carbon quantum dots. Moreover, the product is defined as graphene powder, which precisely meets the development requirements of strategic emerging industries, has good policy adaptability, and combines green environmental protection value, industrial economic value, and technological innovation value. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] The following examples all use nitrogen-containing polymer waste coating material of electroplated aluminum / transfer film produced by Yangzhou Xianghua New Material Technology Co., Ltd. as raw material. The same raw material composition detection standard and process operation specification are adopted. Only one variable is changed. The remaining process conditions are consistent with the standard process. The products of all examples are nitrogen-doped graphene quantum dot powder with a particle size of 2-10nm and graphene edge / in-plane defect structure.

[0019] Example 1 Step 1, Waste Pretreatment: Collect 500g of electroplated aluminum color layer cured film fragments, classify them as color layer waste according to their source, and mechanically crush them to 60 mesh; prepare a 2wt% NaOH aqueous solution, mix it with acetone at a volume ratio of 1:1 to prepare a mixed solvent, put the crushed waste into the mixed solvent, and clean it at 50℃ and 300W ultrasonic power for 2h. After cleaning, filter it with deionized water until the pH of the washing liquid is 7. Place the washed waste in a 90℃ vacuum drying oven and dry it for 20h to obtain 420g of pretreated waste powder; Step 2, hydrothermal carbonization: Weigh 20g of the pretreated waste powder and mix it with urea at a mass ratio of 1:1.5. Add 120mL of deionized water, stir evenly, and then transfer it to a 150mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene. Add deionized water to the reactor filling degree of 70%, seal it, and place it in a constant temperature oven. React at 190℃ for 8 hours. After naturally cooling to room temperature, filter it through a 200-mesh nylon sieve to remove large particulate impurities and obtain a brownish-black crude product dispersion. Step 3, Post-processing and purification: The crude product dispersion was placed in a high-speed refrigerated centrifuge and centrifuged at 10,000 rpm for 18 min, repeated 3 times, and the supernatant was collected. The supernatant was filtered through a 0.22 μm aqueous cellulose acetate microporous membrane and then transferred to a dialysis bag with a molecular weight cutoff of 2000 Da. Dialysis was performed with deionized water as the dialysis medium for 48 h, with the water changed every 8 h. The concentrated solution after dialysis was dried in a vacuum freeze dryer at -50℃ for 36 h to obtain 1.8 g of nitrogen-doped graphene quantum dot powder, with a yield of 9.0%. Step 4, UV photochemical grafting modification: Take 1g of the above nitrogen-doped graphene quantum dot powder and disperse it in a mixed solvent of ethanol / water at a volume ratio of 1:1 to prepare a dispersion of 10mg / mL. Sonicate the dispersion for 30min until monodisperse. Add 15wt% (by dry weight of the powder) of 1,6-hexanediol diacrylate (HDDA) and 1wt% (by dry weight of the powder) of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) to the dispersion and stir until completely dissolved. Purge the dispersion with nitrogen gas at a flow rate of 20mL / min for 12min from the bottom. Place the dispersion in a 30mW / cm² power system. 2 Irradiate under a 365nm UV lamp for 60 minutes, with continuous magnetic stirring at 300rpm during the irradiation process, and maintain nitrogen protection throughout. After the reaction is completed, centrifuge at 10000rpm for 15 minutes to remove unreacted monomers, and then dialyze through a dialysis bag with a molecular weight cutoff of 2000Da for 24 hours. After drying, surface-grafted functionalized carbon quantum dots are obtained.

[0020] Example 2 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Weigh 20g of pretreated waste powder and mix it with melamine at a mass ratio of 1:1.5. The remaining operations are the same as in Example 1. Step 3, Post-processing and purification: Same as in Example 1, yielding 1.7g of nitrogen-doped graphene quantum dot powder, with a yield of 8.5%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0021] Example 3 Step 1, Waste pretreatment: Collect 500g of transfer membrane peeling waste, classify it as transfer membrane waste according to its source, and perform the remaining crushing, washing and drying operations as in Example 1 to obtain 415g of pretreated waste powder; Step 2, hydrothermal carbonization: Same as in Example 1, weigh 20g of pretreated waste powder and mix it with urea, the rest of the operation is the same; Step 3, Post-processing and purification: Same as in Example 1, yielding 1.75 g of nitrogen-doped graphene quantum dot powder, with a yield of 8.75%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0022] Example 4 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Same as in Example 1; Step 3, Post-processing and purification: Same as in Example 1; Step 4, UV photochemical grafting modification: Add a composite monomer consisting of 10 wt% trimethylolpropane triacrylate (TMPTA) and 5 wt% hydroxyethyl acrylate (HEA) by dry weight of the powder to the dispersion. The photoinitiator and other operations are the same as in Example 1.

[0023] Example 5 Step 1, Waste pretreatment: Collect 1 kg of electroplated aluminum color layer cured film fragments, mechanically crush them to 60 mesh, use a 2wt% NaOH aqueous solution and acetone 1:1 mixed solvent, clean them at 50℃ and 300W ultrasonic for 2 hours, wash them with water until neutral, and dry them at 90℃ to obtain 835 g of pretreated waste powder. Step 2, hydrothermal carbonization: Weigh 500g of pretreated waste powder, mix it with urea at a mass ratio of 1:1.5, add 3L of deionized water, stir evenly, and then transfer it to a 5L stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with a filling degree of 70%. React at a constant temperature of 190℃ for 8 hours. After natural cooling, filter through a 200-mesh sieve to obtain a crude product dispersion. Step 3, Post-processing and purification: The crude product dispersion was centrifuged at 10,000 rpm for 18 min, repeated 3 times. The supernatant was filtered through a 0.22 μm microporous membrane, dialyzed with a dialysis bag with a molecular weight cutoff of 2000 Da for 72 h, and then freeze-dried under vacuum at -50℃ for 36 h to obtain 44.2 g of nitrogen-doped graphene quantum dot powder, with a yield of 8.84%. Step 4, UV photochemical grafting modification: Take 50g of the above quantum dot powder, prepare a 10mg / mL ethanol / water dispersion, add 15wt%HDDA and 1wt%TPO, purge with nitrogen for 12min to remove oxygen, then irradiate with a 365nm UV lamp for 60min, with nitrogen protection throughout. Subsequent purification operations are the same as in Example 1.

[0024] Example 6 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Same as in Example 1; Step 3, Post-processing and purification: After centrifugation, filtration, and dialysis, the crude product dispersion was concentrated and dried using a spray dryer with an inlet air temperature of 160℃, an outlet air temperature of 80℃, and a feed rate of 5mL / min, yielding 1.78g of nitrogen-doped graphene quantum dot powder, with a yield of 8.9%. Step 4, UV photochemical grafting modification: Same as in Example 1.

[0025] The following comparative examples are all based on the standard process of Example 1, with only the core technical features under question changed. The remaining process conditions, operating procedures, and testing standards are completely consistent with Example 1, in order to highlight the inventiveness of the technical solution and the uniqueness of the technical effect of the present invention.

[0026] Comparative Example 1 Step 1, Waste treatment: Collect 500g of electroplated aluminum color layer cured film fragments, mechanically crush them to 60 mesh, do not clean with alkali-alcohol mixed solvent, only wash with deionized water and dry to obtain 485g of waste powder. Step 2, hydrothermal carbonization: Weigh 20g of the above-mentioned untreated waste powder and mix it with urea at a mass ratio of 1:1. The remaining operations are the same as in Example 1. Step 3, Post-processing and purification: Same as in Example 1, yielding 1.9g of carbon quantum dot powder, with a yield of 9.5%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0027] Comparative Example 2 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Same as in Example 1; Step 3, Post-processing and purification: Same as in Example 1, 1.8g of nitrogen-doped graphene quantum dot powder was obtained; Step 4, Physical blending: Take 1g of the above quantum dot powder and directly disperse it in a mixed solvent of ethanol / water at a volume ratio of 1:1 to prepare a dispersion of 10mg / mL. No UV curing monomer or photoinitiator was added, and no UV irradiation was performed. The product was directly used for performance testing.

[0028] Comparative Example 3 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Same as in Example 1; Step 3, Post-processing and purification: Same as in Example 1, 1.8g of nitrogen-doped graphene quantum dot powder was obtained; Step 4, Silane coating modification: Take 1g of the above quantum dot powder, disperse it in a mixed solvent of ethanol / water volume ratio of 9:1 to prepare a dispersion of 10mg / mL, add 3wt% of γ-methacryloxypropyltrimethoxysilane (KH-570), adjust the pH to 4-5 with glacial acetic acid, reflux and stir at 70℃ for 4h, after the reaction is completed, centrifuge, dialysis purification, and dry to obtain silane-modified carbon quantum dots.

[0029] Comparative Example 4 Step 1, Waste pretreatment: Collect 500g of PET waste plastic bottle fragments, mechanically crush them to 60 mesh, wash and dry them with the alkali-alcohol mixed solvent of Example 1, and obtain 405g of pretreated PET powder; Step 2, hydrothermal carbonization: Weigh 20g of pretreated PET powder and mix it with urea at a mass ratio of 1:1.5. The remaining operations are the same as in Example 1. Step 3, Post-processing and purification: Same as in Example 1, yielding 1.6g of carbon quantum dot powder, with a yield of 8.0%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0030] Comparative Example 5 Step 1, Waste pretreatment: Same as in Example 1, 420g of pretreated waste powder is obtained; Step 2, hydrothermal carbonization: Weigh 20g of pretreated waste powder and mix it with urea at a mass ratio of 1:1. The remaining operations are the same as in Example 1. Step 3, Post-processing and purification: Same as in Example 1, yielding 1.82 g of nitrogen-doped graphene quantum dot powder, with a yield of 9.1%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0031] Comparative Example 6 Step 1, Waste Pretreatment: Same as Example 1; Step 2, hydrothermal carbonization: Weigh 20g of pretreated waste powder and mix it with urea at a mass ratio of 1:2.5. The remaining operations are the same as in Example 1. Step 3, Post-processing and purification: Same as in Example 1, yielding 1.95g of carbon quantum dot powder, with a yield of 9.75%; Step 4, UV photochemical grafting modification: Same as in Example 1.

[0032] Performance verification Performance verification steps All products from the examples and comparative examples underwent performance verification using standardized testing instruments, conditions, and calculation methods to ensure the comparability and accuracy of the test results. The specific verification steps are as follows: 1. Particle size test: The morphology and particle size distribution of the product were observed using a transmission electron microscope (TEM, JEM-2100F), and the particle size range of 200 randomly selected particles were statistically analyzed. At the same time, the hydrated particle size of the product in the aqueous dispersion was tested using a dynamic light scattering instrument (DLS, Malvern Zetasizer Nano ZS90) at a concentration of 0.1 mg / mL. Each sample was tested 3 times and the average value was taken.

[0033] 2. Fluorescence performance testing: The fluorescence emission spectrum of the product was tested using a fluorescence spectrophotometer (F-7000) with an excitation wavelength of 340 nm, a scanning range of 380-600 nm, and a slit width of 5 nm. The quantum yield (QY) was calculated using the quinine sulfate reference method, with the reference being quinine sulfate in 0.5 mol / L sulfuric acid (quantum yield 0.54). The formula was: QY = (sample fluorescence integral / reference fluorescence integral) × (sample absorbance / reference absorbance) × reference quantum yield. Each sample was tested 3 times and the average value was taken.

[0034] 3. Batch consistency test: Select three different production batches of the same raw material, prepare products according to the same process, test the quantum yield and emission peak position of each batch of products, and calculate the relative deviation of quantum yield (±%) and the drift range of emission peak position (nm).

[0035] 4. Storage stability test: The product was added to the UV-curable coating of the electroplated aluminum color layer at a ratio of 1 wt% to prepare a coating sample film with a thickness of 50 μm. The sample film was placed in a constant temperature and humidity chamber and stored under accelerated aging conditions of 60℃ / 85%RH for 6 months. The fluorescence intensity of the sample film was tested at 0, 1, 3 and 6 months, and the fluorescence intensity retention rate (%) after 6 months was calculated.

[0036] 5. Coating compatibility test: The product was added to the UV-curable coating of the electroplated aluminum color layer at a ratio of 1 wt%. After stirring evenly, the dispersion state of the product in the coating was observed by TEM to evaluate the dispersibility (monodisperse / slight agglomeration / severe agglomeration). At the same time, the migration test method was used. The sample film was placed on filter paper and placed at 50°C for 72 h. The presence of fluorescent migration spots on the filter paper was observed to evaluate the migration (no migration / slight migration / severe migration).

[0037] 6. Coating application performance test: The product was added to the UV-curable coating of the electroplated aluminum color layer at a ratio of 0.5 wt% to prepare a sample film with anti-counterfeiting pattern. After curing at 180℃ for 30 min, the clarity (clear / blurred / no fluorescence) of the fluorescent anti-counterfeiting pattern was observed at a distance of 10 cm from the sample film using a 365 nm handheld UV lamp. The abrasion resistance of the sample film was tested using an abrasion tester with a load of 500g and 500 abrasion cycles. The change in fluorescence intensity was observed. The sample film was soaked in ethanol and ethyl acetate for 24 h respectively, and the fluorescence intensity retention rate after soaking was tested.

[0038] Performance verification results Table 1 Statistical Table of Performance Verification Results of Examples

[0039] Table 2 Statistical Table of Comparative Performance Verification Results

[0040] Table 3 Summary of Core Performance Comparison between Examples and Comparative Examples

[0041] All the above embodiments and comparative examples were prepared using the same process operation specifications, and various performance verifications were completed using uniform testing instruments, testing conditions and calculation methods. Example 1 is the basic standard process, while Examples 2-6 are variable extensions for different nitrogen sources, raw material sources, UV curing monomers, drying methods and industrial scale-up scenarios. Comparative Examples 1-6 are based on Example 1, with targeted omissions or changes to the core technical features of the present invention, thereby verifying the rationality, effectiveness and inventiveness of the technical solution of the present invention.

[0042] From the perspective of the core fluorescence performance of the products, the quantum yield of the nitrogen-doped graphene quantum dot powder obtained in each embodiment of the present invention reaches more than 20.0%, and the quantum yield can reach 22.4% under the optimal standard process. Moreover, the emission peak position of different embodiments is stable at 442-447nm, and the batch quantum yield fluctuation is only ±1.8%~±2.5%, which shows excellent fluorescence performance and batch consistency. The comparative examples, lacking the core process of this invention or deviating from the technical parameter range, exhibited significant degradation in fluorescence performance. Comparative Example 1, due to the absence of standardized waste pretreatment and nitrogen source excess compensation, had a quantum yield of only 8.2%, with no fixed emission peak position and batch-to-batch fluctuations reaching ±68.0%. Comparative Example 5, which only underwent pretreatment without nitrogen source excess compensation, had a quantum yield of 15.6%, with batch-to-batch fluctuations still reaching ±32.1%. Comparative Example 6, with a nitrogen source ratio exceeding the limits of this invention, experienced nitrogen agglomeration, resulting in a quantum yield of only 17.5%. These results fully demonstrate that the combined process of waste pre-sorting + alkali-alcohol mixed solvent pretreatment + nitrogen source excess compensation adopted in this invention is the core for achieving stable batch performance of the product. Furthermore, the 1:(1.2-.8) nitrogen source mass ratio limited by this invention is the optimal ratio for industry-specific waste, and the parameter selection is clearly targeted and reasonable.

[0043] Regarding product storage stability and coating compatibility, the products obtained in each embodiment, when added to the UV-cured coating of the electroplated aluminum color layer, can all achieve a monodisperse state without agglomeration or migration problems. After being stored for 6 months under accelerated aging conditions of 60℃ / 85%RH, the fluorescence retention rate remains above 86.0%, and the fluorescence retention rate can reach 89.2% under the standard process. Comparative Example 2, which uses a common physical blending method in the prior art to replace the UV photochemical grafting modification of the present invention, had a higher initial quantum yield, but severe agglomeration and obvious fluorescence migration occurred in the coating, with a fluorescence retention rate of only 52.0% after 6 months. Comparative Example 3, which uses silane coating modification to replace UV photochemical grafting, showed slight agglomeration of the product in the coating, with a fluorescence retention rate of 71.2% after 6 months, which was significantly lower than that of the embodiment of the present invention. This result confirms that the UV photochemical grafting technique of the present invention, which involves grafting specific UV-curable monomers, can achieve covalent bonding between the product and the UV-curable coating matrix, fundamentally solving the technical problems of poor compatibility between carbon quantum dots and coatings and low storage stability in the prior art. Furthermore, the bifunctional monomers such as HDDA selected in the present invention, along with the matching photoinitiator and irradiation conditions, can achieve efficient grafting, which is far superior to the modification methods of the prior art.

[0044] From the perspective of raw material compatibility, Examples 2-3, using melamine as the nitrogen source and transfer film stripping waste as the raw material respectively, maintained excellent performance, proving that the process of the present invention has good universality within the scope defined by the claims. However, Comparative Example 4, using commonly used PET general waste plastics instead of the industry-specific nitrogen-containing polymer waste coating waste of the electroplated aluminum / transfer film of the present invention, even with the exact same preparation process, resulted in a quantum yield of only 12.3% and batch fluctuations of ±45.2%, with severe agglomeration in the coating and extremely poor anti-counterfeiting effect. This fully demonstrates that the industry-specific waste material selected in the present invention is not a simple replacement of general waste materials in the prior art; the composition of this type of waste material is highly compatible with the preparation process of the present invention, which is an important foundation for achieving excellent performance. Meanwhile, Example 5 completed a 1kg-level industrial pilot-scale preparation. The fluorescence properties, batch consistency, and application effects of the obtained product were basically consistent with the laboratory small-scale test, with a quantum yield of 22.1% and batch fluctuations of ±2.0%, proving that the technical solution of the present invention can achieve scale-up from laboratory small-scale to industrial production without complex equipment modifications, and has practical industrial application value.

[0045] In terms of practical application performance, the products obtained in each embodiment of the present invention, when added to the UV-curable coating of the electroplated aluminum color layer at a ratio of 0.1-wt%, can form clear and bright fluorescent anti-counterfeiting patterns after curing at 180°C, and have good abrasion resistance and solvent resistance, fully meeting the practical application needs of the electroplated aluminum / transfer film industry; while the comparative examples, due to defects in fluorescence performance, stability or compatibility, all have problems such as blurred anti-counterfeiting patterns, weak fluorescence or even no clear patterns, and cannot adapt to the specific application needs of the industry.

[0046] In summary, the technical solution of this invention features coordinated processes and scientifically matched optimal parameters. Through the selection of industry-specific waste materials, standardized batch control processes, targeted UV photochemical grafting modification, and precise regulation of product structure, it achieves a comprehensive improvement in the fluorescence performance, batch consistency, storage stability, and coating compatibility of carbon quantum dots. Compared with existing technologies that rely on general waste material preparation, simple process superposition, and conventional surface modification, this invention represents a qualitative breakthrough. Furthermore, the combination of technical features is not a simple combination of existing technologies or a conventional transplantation of known technologies; it possesses outstanding substantive characteristics and significant progress. At the same time, this invention enables the high-value recycling and utilization of waste materials from the electroplated aluminum / transfer film industry, forming a resource closed loop within the industry, and possessing both significant green environmental protection value and industrial economic value.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A carbon quantum dot prepared from recycled waste, characterized in that, The carbon quantum dots are nitrogen-doped graphene quantum dot powders with a particle size of 2-10 nm. They have graphene-type edge defect and in-plane defect structures and are prepared from nitrogen-containing polymer waste coatings generated during the production of electroplated aluminum, hot stamping materials, or transfer films. Its preparation method includes the following steps: (1) Waste pretreatment: The waste coating waste is sorted into color layer type and transfer film type according to its source, mechanically crushed to 40-80 mesh, and then cleaned with a mixed solvent of 1-3wt% NaOH aqueous solution and ethanol / acetone volume ratio of 1:1 at 40-60℃ with ultrasonic or mechanical stirring at 200-400W for 1-3h. After washing with water until pH neutral, it is dried at 80-100℃ for 12-24h to obtain pretreated waste powder. (2) Hydrothermal carbonization: Mix the pretreated waste powder with urea or melamine at a mass ratio of 1:(1.2-1.8), add deionized water to a filling degree of 60-80%, and place it in a stainless steel hydrothermal reactor lined with 100-200mL polytetrafluoroethylene. React at 190±5℃ for 6-10h. After natural cooling, filter through a 200-mesh sieve to obtain a crude product dispersion. (3) Post-processing and purification: Centrifuge the crude product dispersion at 8000-12000 rpm for 15-20 min, repeat 2-3 times, take the supernatant, filter it through a 0.22 μm microporous membrane, and then dialyze it against deionized water for 24-72 h with a dialysis bag with a molecular weight cutoff of 1000-3500 Da, changing the water every 8 h. The concentrate is then freeze-dried or spray-dried under vacuum to obtain nitrogen-doped graphene quantum dot powder. (4) UV photochemical grafting modification: The nitrogen-doped graphene quantum dot powder was dispersed in a solvent with an ethanol / water volume ratio of 1:1 to prepare a dispersion of 5-20 mg / mL. 5-20 wt% of UV-curable monomer and 0.5-2 wt% of photoinitiator (based on the dry weight of the powder) were added. Nitrogen gas was purged for 10-15 min to remove oxygen. The mixture was then subjected to a power of 20-50 mW / cm². 2 Irradiate under a 365nm UV lamp for 30-90 minutes, stirring or shaking continuously during the irradiation process. After the reaction, remove unreacted monomers by centrifugation or dialysis, and obtain the carbon quantum dots after drying. The UV-curable monomer is one or two of 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, and hydroxyethyl acrylate; the photoinitiator is benzoin dimethyl ether or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

2. The carbon quantum dot according to claim 1, characterized in that, The hydrothermal carbonization reaction temperature in step (2) is 190°C and the reaction time is 8 hours.

3. The carbon quantum dot according to claim 1, characterized in that, The UV-curable monomer mentioned in step (4) is 1,6-hexanediol diacrylate.

4. The carbon quantum dot according to claim 1, characterized in that, The irradiation time for the UV photochemical grafting modification in step (4) is 60 min, and nitrogen gas is used for protection throughout the irradiation process.

5. The carbon quantum dot according to claim 1, characterized in that, The ultrasonic power for waste pretreatment in step (1) is 300W, and the cleaning time is 2h.

6. The carbon quantum dot according to claim 1, characterized in that, The vacuum freeze-drying process conditions described in step (3) are drying at -50℃ for 24-48 hours.

7. The carbon quantum dot according to claim 1, characterized in that, The photoinitiator in step (4) is diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and its addition amount is 1 wt% of the dry weight of the powder.

8. The carbon quantum dot according to claim 1, characterized in that, The nitrogen source mentioned in step (2) is urea, and the mass ratio of pretreated waste powder to urea is 1:1.

5.

9. The carbon quantum dot according to claim 1, characterized in that, The waste coating material mentioned in step (1) is fragments of electroplated aluminum color layer cured film or waste material of transfer film peeling.

10. The application of carbon quantum dots in UV-curable coatings according to any one of claims 1-9, characterized in that, The carbon quantum dots are added at a ratio of 0.1-1 wt% to UV-curable coatings for electroplated aluminum color layer coatings or transfer film coatings for use as fluorescent tracers in the preparation or curing process of ultraviolet fluorescent anti-counterfeiting labels.

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