Food-grade metal surface blackening liquid, blackening process and application

Abstract

The invention discloses food-grade metal surface blackening liquid as well as a blackening process and application. The blackening liquid comprises a basic blackening component and a modified graphene solution, wherein the basic blackening liquid comprises 15-25 g / L of sodium pyrophosphate, 8-12 g / L of sodium molybdate, 20-30 g / L of sodium gluconate, 2-5 g / L of sodium erythorbate, a citric acid / sodium citrate buffer solution, 10-15 ml / L of polyethylene glycol and 0.5-1 ml / L of polysorbate 80; the modified graphene is PEI modified graphene oxide. According to the method, stubborn dirt is cleaned and peeled off through ultrasonic waves, and grease is degraded through photocatalysis of nano TiO2, so that the surface hydrophilicity is improved, and the adsorption uniformity of blackening liquid is enhanced; according to the method, Si-O-Si network crosslinking is optimized through staged heating (80 DEG C to 120 DEG C to 140 DEG C), the hardness and the adhesive force are improved, thermal stress concentration in a traditional process is avoided, meanwhile, raw material decomposition can be avoided at the temperature, and the safety of blackening liquid is further guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of surface treatment of metal products, and particularly to a blackening liquid for food-grade metal surfaces, a blackening process and applications thereof. Background Art

[0002] In the field of cooking in human daily life, cookware occupies an indispensable position. For a long time, cast iron and carbon structural steel plates have become the mainstream materials for manufacturing cookware due to their good heat conduction performance, durability and other characteristics, and are widely used in various kitchen scenarios. However, after thousands of years of development, even though numerous people of insight have devoted themselves to relevant research, the thorny problem of "rusting" of cookware has still not been properly solved.

[0003] Currently, the cookware market presents a rich and diverse situation. There are more than 100 types of iron pans and frying pans alone, covering different sizes, functions and design styles to meet the diverse needs of consumers, and its market prospect is quite broad. However, it cannot be ignored that rust problems frequently occur both in the production process of iron pans and in the daily use by consumers, seriously affecting the service life, appearance of iron pans and the cooking experience of users. To effectively address this problem, manufacturers have racked their brains and proposed many solutions to prevent iron pans from rusting. Among them, a relatively common method is to coat an anti-rust layer on the surface of the iron pan, hoping to build a barrier between the iron pan and the external environment to prevent corrosive media such as oxygen and moisture from contacting the iron pan substrate, thereby delaying the rusting process.

[0004] Currently, the cookware market presents a rich and diverse situation. There are more than 100 types of iron pans and frying pans alone, covering different sizes, functions and design styles to meet the diverse needs of consumers, and its market prospect is quite broad. However, it cannot be ignored that rust problems frequently occur both in the production process of iron pans and in the daily use by consumers, seriously affecting the service life, appearance of iron pans and the cooking experience of users. To effectively address this problem, manufacturers have racked their brains and proposed many solutions to prevent iron pans from rusting. Among them, a relatively common method is to coat an anti-rust layer on the surface of the iron pan, hoping to build a barrier between the iron pan and the external environment to prevent corrosive media such as oxygen and moisture from contacting the iron pan substrate, thereby delaying the rusting process. However, these existing solutions often have various limitations in practical applications. For example, in Chinese Patent CN101744525A, the manufacturing process of a rust-proof non-coated iron pan uses a nitride layer to improve corrosion resistance. However, the nitride layer formed by salt bath nitriding in this patent may gradually wear during long-term use, especially under frequent friction and cleaning, resulting in a decline in the rust-proof effect. Moreover, the nitride layer may be unstable under high-temperature cooking and release harmful substances, especially under long-term high temperature or sudden heating and cooling.

[0005] Chinese Patent CN1457737A, an iron pot with a tin-nickel alloy protective layer and a manufacturing method thereof. There is a tin-nickel alloy protective layer on the surface of the pot body. The tin-nickel alloy protective layer is attached to the surface of the pot body by electroplating or hot melting, and its thickness is 30-45μm. Although the oxide film can improve corrosion resistance, the chemicals used in the chemical treatment process may contain harmful components, and the residues may precipitate during cooking, threatening food safety. Summary of the Invention

[0006] Therefore, the present invention provides a food-grade metal surface blackening solution, a blackening process and an application thereof to solve the problems in the prior art.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A food-grade metal surface blackening solution provided according to the first aspect of the present invention, wherein the blackening solution includes a basic blackening component and a modified graphene solution;

[0009] Among them, the basic blackening solution includes 15-25 g / L of sodium pyrophosphate, 8-12 g / L of sodium molybdate, 20-30 g / L of sodium gluconate, 2-5 g / L of sodium erythorbate, a citric acid / sodium citrate buffer solution, 10-15 ml / L of polyethylene glycol, and 0.5-1 ml / L of polysorbate 80;

[0010] The modified graphene is PEI-modified graphene oxide. Specifically, the method is to first vacuum-dry the GO powder, remove the adsorbed moisture, add it to the PEI deionized water, and obtain a PEI-GO dispersion after ultrasonic dispersion. Centrifuge to remove the unpeeled aggregates, and the supernatant is the modified PEI-GO dispersion. Freeze-dry to obtain the modified PEI-GO solid.

[0011] Further, the addition amount of the modified graphene is 0.01-0.2%. Graphene can form a physical barrier layer to reduce the penetration of H2O / O2 and delay the corrosion of the substrate. The nano-sheet structure of graphene can improve the hardness of the film layer (the HV can be increased by 20-30%). For materials such as cast iron, graphene can improve the heat distribution and reduce the film layer peeling caused by local overheating. The maximum addition amount does not exceed 0.2 wt% to avoid roughness of the film layer caused by agglomeration.

[0012] Further, the preparation method of the basic blackening solution is to first dissolve sodium pyrophosphate in hot deionized water, add sodium molybdate while it is hot, stir until it is clear, cool down, and then continue to add sodium gluconate and sodium erythorbate. Use the citric acid / sodium citrate buffer solution to adjust the pH value, and then continue to add polyethylene glycol and polysorbate 80 in sequence. Stir until the reaction is complete and then age in the dark. Filter to obtain the basic blackening solution.

[0013] Further, the temperature of the hot deionized water is 45 - 55°C; it is cooled to below 40°C; the temperature for light-avoiding aging is room temperature.

[0014] Further, the pH is controlled at 3.5 - 4.5.

[0015] According to a preparation method of the food-grade metal surface blackening liquid provided in the second aspect of the present invention, the method includes first adding modified PEI-GO into deionized water and redissolving it by ultrasonic waves, controlling the temperature to be less than 40°C, adding it to the basic blackening liquid, and performing mechanical stirring and pulsed ultrasonic waves. After mixing evenly, the food-grade metal surface blackening liquid is obtained.

[0016] An application of a food-grade metal surface blackening liquid in blackening the surface of cookware according to a third aspect of the present invention.

[0017] Further, the cookware surface blackening process includes first using an alkaline nanoparticle emulsion to clean the metal by ultrasonic wave assistance; then soaking the cleaned metal in the food-grade metal surface blackening liquid, and performing post-treatment after soaking to obtain the blackened cookware. As an example, the soaking temperature is preferably 50 - 55°C.

[0018] Since the blackening treatment time is different for metals with different thicknesses, a mathematical model of the metal thickness (t / mm) and the treatment time is established through experiments: T(min) = 15 + 5×(t - 1) (t≥1mm).

[0019] Further, the alkaline nanoparticle emulsion is prepared by sodium silicate, nano-titanium dioxide, surfactant, auxiliary agent, and deionized water. The present invention uses a special alkaline nanoparticle emulsion to clean dirt, which can dissolve the grease and oxides on the metal surface in an alkaline environment (pH 10 - 12) to form a silica colloid protective layer. The added nano-TiO2 can photocatalytically degrade organic pollutants (such as oil stains), enhance surface hydrophilicity, and promote the adsorption of the subsequent blackening liquid. The surfactant used can reduce the surface tension, help the nanoparticles penetrate micropores, and peel off stubborn dirt.

[0020] Further, in the ultrasonic wave-assisted cleaning, the frequency is 40 kHz, and the power density is 0.5 W / cm 2 , and the time is 5 - 10 minutes. After ultrasonic wave-assisted cleaning, UV irradiation (wavelength 365 nm, intensity 10 mW / cm 2 ) for 2 minutes to enhance surface hydrophilicity.

[0021] Further, the post-treatment includes a sealing process and gradient temperature rise curing.

[0022] Before the post-treatment, a drying process is also included: hot air drying at 60°C for 20 minutes to remove surface free moisture; the sealing process includes spraying a food-grade silicone oxide sealing agent (solid content 8-12%), with a film thickness of 3-5μm.

[0023] Gradient temperature curing: The first stage: 80°C / 40min → pre-crosslinking; The second stage: 120°C / 30min → complete condensation (Si-O-Si network); The third stage: 140°C / 15min → hardness strengthening (reaching 3H pencil hardness).

[0024] The present invention has the following advantages:

[0025] All raw material components of the present invention meet the food safety standards, are non-toxic and harmless; avoid using heavy metals or toxic substances (such as nickel and chromium in traditional processes), ensuring the safety of the cookware in direct contact with food.

[0026] The present invention uses PEI-modified graphene oxide (PEI-GO) to form a dense physical barrier layer, reducing the penetration of H2O / O2 and delaying the corrosion of the substrate. The sealing process uses a food-grade silicone oxide (solid content 8-12%) to further isolate environmental corrosive media.

[0027] The present invention enhances the film hardness by adding a graphene nanosheet layer structure, with the HV (pencil hardness) increased by 20-30% to reach the 3H level (after gradient curing), and has stronger scratch resistance. Graphene improves heat conduction and reduces the film peeling caused by local overheating, especially suitable for materials with uneven heat conduction such as cast iron.

[0028] The present invention uses ultrasonic cleaning to remove stubborn dirt, combined with the photocatalytic degradation of oil by nano-TiO2 to improve surface hydrophilicity and enhance the adsorption uniformity of the blackening solution; the temperature is raised in stages (80°C → 120°C → 140°C) to optimize the crosslinking of the Si-O-Si network, improving hardness and adhesion, avoiding thermal stress concentration in traditional processes, and at the same time, the decomposition of raw materials can be avoided at this temperature, further ensuring the safety of the blackening solution. Description of the Drawings

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only exemplary, and for those of ordinary skill in the art, without creative efforts, other implementation drawings can also be obtained based on the provided drawings.

[0030] The structures, ratios, sizes, etc. shown in this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the limited conditions for the implementation of the present invention. Therefore, they do not have substantial technical significance. Any modification of the structure, change in the ratio relationship, or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope that can be covered by the technical content disclosed in the present invention.

[0031] Figure 1 This is a comparative diagram of the salt spray resistance experiment provided by the present invention. Among them, A - Example 1; B - Comparative Example 1; C - Comparative Example 3; D - Comparative Example 6. Detailed implementation manners

[0032] The following specific embodiments illustrate the implementation manners of the present invention. Those familiar with this technology can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0033] Sodium pyrophosphate: CAS 7722-88-5;

[0034] Sodium molybdate: CAS 7631-95-0;

[0035] Sodium gluconate: CAS 527-07-1;

[0036] Sodium erythorbate: CAS 6381-77-7;

[0037] Citric acid / sodium citrate buffer solution: Nanjing Senbeijia Biotechnology Co., Ltd., product number SBJ-1191;

[0038] Polyethylene glycol 600: CAS25322-68-3; for medicine and PEG-600 USP-NF; free of ethylene glycol / diethylene glycol residues (limit ≤ 0.1%);

[0039] Polysorbate 80: CAS 9005-65-6;

[0040] Graphene powder: CAS1034343-98-0;

[0041] Polyetherimide PEI: Suzhou Lanshen Plastic Co., Ltd., product number: 1000F-7101;

[0042] Sodium silicate: CAS1344-09-8;

[0043] Nano titanium dioxide: CAS 13463-67-7;

[0044] Carboxymethyl cellulose CMC: CAS 9004-32-4;

[0045] Sodium bicarbonate: CAS 144-55-8;

[0046] Polyglycerol fatty acid ester PGFE: CAS 67784-82-1;

[0047] Food-grade silicone sealant: Wacker (Wacker Chemie, Germany), STP-E 15 / 80 with a solid content of 8-12%;

[0048] When purchasing all raw materials, food-grade ones are purchased.

[0049] Preparation method of modified graphene:

[0050] Pretreatment of graphene oxide: GO powder (1 part by mass) is vacuum dried at 60 °C for 2 hours to remove adsorbed moisture.

[0051] Dispersion process: The dried GO is added to deionized water (1250 parts) containing 0.1% PEI, and ultrasonically dispersed at 500 W for 2 hours.

[0052] Functional modification: The temperature is raised to 60 °C, and the reaction is stirred for 4 hours. p-Phenylenediamine is directly added without high temperature dropping to directly obtain a PEI-GO dispersion.

[0053] Purification: Centrifuge (8000 rpm, 15 min) to remove unpeeled aggregates, and the supernatant is the modified PEI-GO dispersion; after pre-freezing (-80 °C), vacuum freeze-drying (below -50 °C, 24-48 h) gives the modified PEI-GO solid.

[0054] Preparation method of alkaline nanoparticle emulsion: Sodium silicate 5%, nano titanium dioxide 0.5%, surfactant Tween 80 2%, CMC food grade 0.2%, pH regulator sodium bicarbonate 1%, PGFE 0.1%, deionized water, the balance.

[0055] Preparation method: ① Mix sodium silicate and water: Slowly add sodium silicate to deionized water while stirring to fully dissolve it to form a uniform alkaline solution.

[0056] ② Disperse nano titanium dioxide: Add nano titanium dioxide powder to the above solution, and use a high-speed stirrer or ultrasonic dispersion equipment to fully disperse it to ensure that the nanoparticles are evenly distributed in the solution.

[0057] ③Add surfactants and additives: Add surfactants and other additives to the mixed solution in sequence, and continue stirring to make them fully and evenly mixed.

[0058] ④Adjust the pH value: According to the need, use an appropriate amount of acid or base to adjust the pH value of the solution to a suitable range (generally an alkaline environment, with a pH value of about 10 - 11).

[0059] ⑤Filtration: Filter the prepared degreasing agent solution through a filter to remove possible impurities or nanoparticles that are not fully dispersed, and thus obtain an alkaline nanoparticle emulsion.

[0060] Example 1

[0061] This example provides a process for blackening the surface of a baking tray:

[0062] 1. Preparation of the blackening solution: First, dissolve 15 kg of sodium pyrophosphate in 990 L of hot deionized water at 45 °C. While it is still hot, add 8 kg of sodium molybdate and stir until it becomes clear. After cooling, continue to add 20 kg of sodium gluconate and 2 kg of sodium erythorbate. Use a citric acid / sodium citrate buffer solution to adjust the pH value to about 3.5. Then, sequentially add 10 L of polyethylene glycol 600 and 0.5 L of polysorbate 80 and stir until the reaction is complete. After that, let it cool to room temperature and age in the dark for 24 h. Filter it through a 5 - micron filter membrane to obtain the basic blackening solution.

[0063] 2. Add 0.15 kg of modified PEI - GO solid to 10 L of water, ultrasonicate with phenolic acid, and then add it to the basic blackening solution while controlling the temperature at 35 - 40 °C. Use mechanical stirring (300 rpm) + pulsed ultrasound (5 s on / 5 s off, for a total of 30 min) to obtain a food - grade metal surface blackening solution after mixing evenly.

[0064] 3. Process:

[0065] ①Pretreatment: Mix the alkaline nanoparticle emulsion with deionized water at a ratio of 1:10, then add it to an ultrasonic instrument. Place the baking tray (with a thickness of 2 mm) in the ultrasonic instrument. The ultrasonic - assisted cleaning is carried out at a medium frequency of 40 kHz and a power density of 0.5 W / cm 2 , for 5 - 10 minutes. After ultrasonic - assisted cleaning, irradiate it with UV light (wavelength 365 nm, intensity 10 mW / cm 2 ) for 2 minutes.

[0066] ②Blackening treatment: Place the cleaned baking tray in a food-grade metal surface blackening solution at a controlled temperature of 50°C, soak for 20 minutes, then take it out and dry it with hot air at 60°C for 20 minutes. After removing the surface free moisture, spray a food-grade silicone oxide sealant with a spraying thickness of 3 μm. After spraying, perform gradient temperature rise treatment: the first stage: 80°C / 40 min; the second stage: 120°C / 30 min; the third stage: 140°C / 15 min, and the blackened baking tray is obtained.

[0067] Example 2

[0068] This example provides a blackening process for the surface of a baking tray:

[0069] 1. Preparation of the blackening solution: First, dissolve 20 kg of sodium pyrophosphate in 900 L of hot deionized water at 50°C. While it is still hot, add 10 kg of sodium molybdate and stir until it is clear. After cooling, continue to add 25 kg of sodium gluconate and 3 kg of sodium erythorbate. Use a citric acid / sodium citrate buffer solution to adjust the pH value to about 4. Then, continue to add 12 L of polyethylene glycol 600 and 0.8 L of polysorbate 80 in sequence and stir until the reaction is complete. Then, cool it to room temperature and age it in the dark for 24 h. Filter it through a 5-μm filter membrane to obtain the basic blackening solution.

[0070] 2. Add 1.5 kg of modified PEI-GO solid to 100 L of water, ultrasonically treat it with phenolic acid, and then add it to the basic blackening solution while controlling the temperature at 35 - 40°C. Use mechanical stirring (300 rpm) + pulsed ultrasound (5 s on / 5 s off, for a total of 30 min). After mixing evenly, obtain the food-grade metal surface blackening solution.

[0071] 3. Process:

[0072] ①Mix the alkaline nanoparticle emulsion with deionized water at a ratio of 5:10, add it to an ultrasonic instrument, place the baking tray (with a thickness of 4 mm) in the ultrasonic instrument, and perform ultrasonic-assisted cleaning at a medium frequency of 40 kHz and a power density of 0.5 W / cm 2 , for 5 - 10 minutes. After ultrasonic-assisted cleaning, perform UV irradiation (wavelength 365 nm, intensity 10 mW / cm 2 ) for 2 minutes.

[0073] ②Place the cleaned baking tray in a food-grade metal surface blackening solution at a controlled temperature of 55°C, soak for 30 minutes, then take it out and dry it with hot air at 60°C for 20 minutes. After removing the surface free moisture, spray a food-grade silicone oxide sealant with a spraying thickness of 4 μm. After spraying, perform gradient temperature rise treatment: the first stage: 80°C / 40 min; the second stage: 120°C / 30 min; the third stage: 140°C / 15 min, and the blackened baking tray is obtained.

[0074] Example 3

[0075] This embodiment provides a process for blackening the surface of a baking tray:

[0076] 1. Preparation of the blackening solution: First, dissolve 25 kg of sodium pyrophosphate in 500 L of hot deionized water at 55 °C. While it is still hot, add 12 kg of sodium molybdate and stir until it becomes clear. After cooling, continue to add 30 kg of sodium gluconate and 5 kg of sodium erythorbate. Use a citric acid / sodium citrate buffer solution to adjust the pH value to about 4. Then, sequentially add 15 L of polyethylene glycol 600 and 1 L of polysorbate 80 and stir until the reaction is complete. After that, let it cool to room temperature and age in the dark for 24 h. Filter it through a 5-μm filter membrane to obtain the basic blackening solution;

[0077] 2. Add 3 kg of modified PEI-GO solid to 500 L of water, ultrasonicate with phenolic acid, and then add it to the basic blackening solution while controlling the temperature at 35 - 40 °C. Use mechanical stirring (300 rpm) + pulsed ultrasound (5 s on / 5 s off, for a total of 30 min) to obtain a food-grade metal surface blackening solution after mixing evenly.

[0078] 3. Process:

[0079] ① After mixing the alkaline nanoparticle emulsion with deionized water at a ratio of 10:10, add it to an ultrasonic instrument. Place the baking tray (with a thickness of 3 mm) in the ultrasonic instrument. The ultrasonic-assisted cleaning is carried out at a medium frequency of 40 kHz and a power density of 0.5 W / cm 2 , for 5 - 10 minutes. After ultrasonic-assisted cleaning, irradiate it with UV light (wavelength 365 nm, intensity 10 mW / cm 2 ) for 2 minutes.

[0080] ② Place the cleaned baking tray in the food-grade metal surface blackening solution, control the temperature at 55 °C, soak for 25 minutes, then take it out, dry it with hot air at 60 °C for 20 minutes to remove the surface free moisture, and then spray a food-grade silicone sealant with a spraying thickness of 4 μm. After spraying, carry out gradient temperature rising treatment: the first stage: 80 °C / 40 min; the second stage: 120 °C / 30 min; the third stage: 140 °C / 15 min to obtain the blackened baking tray.

[0081] Comparative Example 1

[0082] This comparative example provides a process for blackening the surface of a baking tray:

[0083] In this comparative example, during the pretreatment, industrial alcohol is used for cleaning, and the rest is exactly the same as in Example 2.

[0084] Comparative Example 2

[0085] This comparative example provides a process for blackening the surface of a baking tray:

[0086] In this comparative example, only the basic blackening solution is used for the blackening solution, and modified PEI-GO is not added. The rest is exactly the same as in Example 2.

[0087] Comparative Example 3

[0088] This comparative example provides a process for blackening the surface of a baking tray:

[0089] 0.3% of modified PEI-GO is added to the blackening solution in this comparative example, and the others are exactly the same as in Example 2.

[0090] Comparative Example 4

[0091] This comparative example provides a process for blackening the surface of a baking tray:

[0092] This comparative example does not use a sealing agent, and the others are exactly the same as in Example 2.

[0093] Comparative Example 5

[0094] This comparative example provides a process for blackening the surface of a baking tray:

[0095] This comparative example uses a fixing and curing temperature of 150 °C for 30 min, and the others are exactly the same as in Example 2.

[0096] Comparative Example 6

[0097] This comparative example provides a process for blackening the surface of a baking tray:

[0098] No treatment is adopted.

[0099] Experimental Example 1

[0100] The products obtained in Examples 1-3 and Comparative Examples 1-5 were subjected to safety tests, and the results are shown in Table 1.

[0101] Table 1

[0102]

[0103] As can be seen from Table 1, the TOC value of this example is significantly lower than that of Comparative Example 4 (without a sealing agent), indicating that the silicone sealing agent effectively blocks the migration of organic components (such as polyethylene glycol and polysorbate 80) in the blackening solution. There is no heavy metal migration in the examples of the present invention, but in the traditional high-temperature blackening process, Cr 6+ , indicating that the sodium pyrophosphate-molybdate system of the present invention has no heavy metal risk. The examples of the present invention avoid local overheating due to gradient curing (up to 140 °C), and the furfural residue is lower than that of Comparative Example 5 (cured at 150 °C);

[0104] Experimental Example 2

[0105] The products obtained in Example 2 and Comparative Examples 1-5 were subjected to performance tests, and the specific results are shown in Table 2.

[0106] Abrasion resistance: Taber weight loss / mg, 1000 revolutions.

[0107] Table 2

[0108]

[0109] From Table 2 and the rusting result graph of salt spray resistance as Figure 1 ( Figure 1 In the figure are the salt spray resistance test graphs of Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 6), it can be seen that alcohol cleaning will cause residual organic matter on the surface, reduce the wettability of the blackening solution, and the rusting area in the salt spray test increases by 60% (8% vs 5%), and the adhesion decreases (1 - level peeling). The examples of the present invention adopt alkaline nano - emulsion + UV cleaning, which can generate a silica colloid protective layer and improve the binding force of the subsequent film layer. Without graphene (Comparative Example 2): The film hardness decreases by 24% (220 HV), and the abrasion resistance decreases by 47% due to the lack of nano - reinforcement phase. Excessive graphene (Comparative Example 3): 0.3% addition causes agglomeration, the roughness increases by 55% (0.85 μm), and local pitting appears in the salt spray test (the barrier of the agglomeration area fails). 0.01 - 0.2% PEI - GO in the examples of the present invention: Through pulsed ultrasonic dispersion, the graphene sheets are uniformly arranged. Without a sealant: Although the initial adhesion is grade 0, the rusting area in the salt spray test soars to 40% (siloxane sealing can reduce the H2O permeability by more than 90%). Gradient curing sealant: Forms a dense Si - O - Si network, and the pencil hardness reaches 3H. Constant temperature curing at 150 °C: Causes excessive cross - linking of the siloxane resin, increases the internal stress in the film layer, and micro - cracks appear (SEM shows that the crack width is about 2 μm), and the abrasion resistance decreases by 125%. Gradient curing: Segmented curing at 80 °C → 120 °C → 140 °C, which gradually increases the cross - linking degree.

[0110] Although the present invention has been described in detail above with general descriptions and specific examples, based on the present invention, some modifications or improvements can be made, which are obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

Claims

1. A food-grade metal surface blackening solution, characterized in that, The blackening solution includes a basic blackening component and a modified graphene solution; Among them, the basic blackening solution includes 15-25 g / L of sodium pyrophosphate, 8-12 g / L of sodium molybdate, 20-30 g / L of sodium gluconate, 2-5 g / L of sodium erythorbate, a citric acid / sodium citrate buffer solution, 10-15 ml / L of polyethylene glycol, and 0.5-1 ml / L of polysorbate 80; The modified graphene is graphene oxide modified by PEI. The specific method is to first vacuum-dry the GO powder to remove adsorbed moisture, then add it to deionized water containing PEI, and after ultrasonic dispersion, obtain a PEI-GO dispersion. Centrifuge to remove unpeeled aggregates, and the supernatant is the modified PEI-GO dispersion. Freeze-dry to obtain the modified PEI-GO solid.

2. The food-grade metal surface blackening liquid according to claim 1, characterized in that The addition amount of the modified graphene is 0.01-0.2%.

3. A food-grade metal surface blackening liquid according to claim 1, characterized in that, The preparation method of the basic blackening solution is to first dissolve sodium pyrophosphate in hot deionized water, add sodium molybdate while it is hot, stir until clear, cool down, then continue to add sodium gluconate and sodium erythorbate, adjust the pH value with a citric acid / sodium citrate buffer solution, and then sequentially add polyethylene glycol and polysorbate 80 and stir until the reaction is complete. After aging in the dark, filter to obtain the basic blackening solution.

4. A food-grade metal surface blackening solution according to claim 3, characterized in that, The temperature of the hot deionized water is 45-55°C; cool down to below 40°C; the temperature for aging in the dark is room temperature.

5. A food-grade metal surface blackening liquid according to claim 3, characterized in that, The pH is controlled at 3.5-4.

5.

6. A preparation method of a food-grade metal surface blackening solution as claimed in any one of claims 1-5, characterized in that, The method includes first ultrasonically redissolving the modified PEI-GO solid with deionized water, controlling the temperature to be less than 40°C, adding it to the basic blackening solution, and performing mechanical stirring and pulsed ultrasound. After mixing evenly, a food-grade metal surface blackening solution is obtained.

7. Application of a food-grade metal surface blackening solution in blackening the surface of cookware.

8. Application of a food-grade metal surface blackening liquid in blackening of the surface of cookware, characterized in that, The process for blackening the surface of the cookware includes first using an alkaline nanoparticle emulsion to ultrasonically assist in cleaning the metal; then immersing the cleaned metal in the food-grade metal surface blackening solution, and performing post-treatment after immersion to obtain the blackened cookware.

9. Use of a food-grade metal surface blackening solution in blackening of the surface of cookware according to claim 8, characterized in that, The alkaline nanoparticle emulsion is prepared from sodium silicate, nano-titanium dioxide, a surfactant, an auxiliary agent, and deionized water.

10. The application of the food-grade metal surface blackening liquid according to claim 9 in the blackening of the surface of cookware, characterized in that, The post-treatment includes a sealing process and gradient temperature rise curing.

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