Chemical depolymerization-repolymerization synthesis method for recycling melamine waste

Abstract

The invention discloses a chemical depolymerization-repolymerization method for recycling melamine waste, and relates to the technical field of recycling and recycling of resin solid waste. The method comprises the following steps: crushing the melamine waste, performing pretreatment and surface modification, and then attacking methylene bridges in the melamine waste by using OH <-> as a nucleophilic reagent under an alkaline condition to initiate C-N bond breakage in the melamine waste, so that the melamine waste is gradually degraded to generate melamine; adding the obtained melamine into a formaldehyde solution, and reacting again to obtain new melamine resin.

Description

Technical Field

[0001] This application relates to the field of resin solid waste recycling and resource utilization technology, and in particular to a chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste. Background Technology

[0002] Melamine-formaldehyde resin (melamine resin) is widely used in tableware, decorative panels, and transportation equipment due to its excellent rigidity, temperature resistance, and flame retardancy. However, its highly cross-linked three-dimensional network structure makes it difficult to degrade after disposal, and the resulting solid waste poses a serious threat to soil, groundwater, and the ecological environment.

[0003] Currently, melamine waste treatment mainly relies on landfill, incineration, and mechanical recycling, but all have significant drawbacks: landfill occupies land resources and may cause heavy metal migration; incineration releases highly toxic gases; and mechanical recycling can only pulverize the waste for use as filler, resulting in a significant decrease in the performance of recycled materials and limiting their application. Chemical depolymerization, as a greener solution, depolymerizes melamine resin to obtain melamine through chemical methods. Then, through mature production methods, formaldehyde and the depolymerized melamine are reacted to obtain new melamine resin, achieving the recycling and resource recovery of resin solid waste.

[0004] Chinese patent application CN111875843A discloses a method for selectively breaking CO bonds in melamine resin. The method involves crushing melamine resin material into blocks, immersing them in a liquid-phase reaction system of an acidic catalyst and solvent, and heating them for reaction. During the reaction, the acidic catalyst effectively acts on the CO bonds, causing specific CH2-O-CH2 bonds in the material to break. After the reaction is complete, the mixture is cooled to room temperature, filtered, and washed to obtain the product, melamine prepolymer. However, this technical solution involves chemical degradation under an acidic system. Summary of the Invention

[0005] This application provides a chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste. The melamine waste is crushed, pretreated, and surface-modified. Then, under alkaline conditions, OH- acts as a nucleophile to attack the methylene bridges in the melamine waste, initiating the breakage of the CN bonds and causing the melamine waste to gradually degrade, generating melamine. The obtained melamine is added to a formaldehyde solution for a re-reaction, thereby obtaining new melamine resin.

[0006] To achieve the above objectives, this application provides a chemical depolymerization-repolymerization synthesis method for recycling melamine waste, comprising the following steps:

[0007] S1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, heat and reflux it, filter and dry it to obtain pretreated melamine powder.

[0008] S2. Add the pretreated melamine powder to an ethanol-water mixed solution containing an aminosilane coupling agent, heat and reflux again, filter and dry to obtain the polymelamine powder to be dissolved.

[0009] S3. Add the polymelamine powder to be depolymerized and the strong alkali solution together into the reaction vessel, and react under nitrogen atmosphere while stirring, heating and pressurizing to obtain the depolymerization mixture;

[0010] S4. Filter the depolymerized mixture while it is still hot to remove the residue that is not completely depolymerized, and obtain a mixture.

[0011] S5. After adjusting the pH of the mixture, cool it and filter to obtain the precipitated melamine.

[0012] S6. Add the obtained melamine to the formaldehyde solution and react again to obtain new melamine resin powder.

[0013] This application achieves closed-loop recycling and reuse of melamine waste through a chemical depolymerization process in an alkaline system. This not only avoids the production of highly toxic substances such as hydrogen cyanide from traditional pyrolysis, but also degrades melamine waste into melamine, which is then reacted with formaldehyde to synthesize high-performance melamine resin, thus realizing resource recycling.

[0014] In step S1, pulverizing melamine waste to obtain powder can increase the specific surface area of ​​melamine waste and improve the contact efficiency of subsequent processing. Anhydrous ethanol, as a polar organic solvent, can dissolve oil stains, uncured small molecule impurities, and dust adhering to the surface of melamine waste. Heating to reflux at 80-90℃ can enhance the dissolving power of anhydrous ethanol, promote the rapid removal of impurities from the waste surface, and avoid impurities interfering with subsequent coupling modification and depolymerization reactions.

[0015] In step S2, the aminosilane coupling agent molecule contains siloxane groups, which hydrolyze in an ethanol-water mixed solution to produce silanol groups, which can chemically react with the functional groups on the surface of melamine powder to form a stable surface modification layer. The modified melamine powder has increased surface polarity and an increased number of amino groups, making it easier to disperse in a strong alkaline solution, thereby enhancing the contact between the strong base and the melamine polymer chain. This promotes the attack of OH- on the methylene bridges in the polymelamine powder to be degraded, initiating the breakage of CN bonds in the polymelamine powder to be degraded, and improving the degradation efficiency of the polymelamine powder to be degraded.

[0016] In step S3, the nitrogen atmosphere can prevent the polymelamine powder to be depolymerized or the depolymerization product from being oxidized under the high temperature and high pressure environment, thereby reducing the generation of by-products. Furthermore, the high temperature and high pressure conditions can increase the energy of the reaction system, accelerate the breaking of chemical bonds, shorten the depolymerization time, and enhance the dissociation ability of strong bases, thereby further improving the depolymerization efficiency.

[0017] In step S4, the depolymerization mixture contains solid residues that are not completely depolymerized. Hot filtration can prevent premature precipitation of melamine due to cooling, and at the same time reduce the possibility of it being adsorbed or entrained by the solid residues that are not completely depolymerized.

[0018] In step S5, the mixture is strongly alkaline due to the presence of a strong alkali. Adjusting the pH can reduce the alkalinity of the solution, allowing melamine to reach a suitable dissolution equilibrium in the solution. Rapidly cooling the temperature to 20°C at a rate of 10–20°C / min can cause a sharp decrease in the solubility of melamine, resulting in the rapid precipitation of crystals. After filtration, high-purity melamine monomer is obtained, which meets the raw material requirements for subsequent recombination reaction with formaldehyde.

[0019] In one feasible implementation, the parameters for the heating and reflux treatment in step S1 are: reflux at 80-90°C for 2-4 hours; and the parameters for the drying treatment are: vacuum drying at 80-100°C for 2-4 hours.

[0020] The above-mentioned reflux temperature range allows anhydrous ethanol to be fully refluxed, dissolving and removing impurities such as oil stains from the surface of melamine waste; the vacuum drying environment can lower the boiling point of anhydrous ethanol and quickly remove residual anhydrous ethanol.

[0021] In one feasible implementation, in step S2, the aminosilane coupling agent includes any one of γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, bis(trimethoxysilylpropyl)amine, and bis(triethoxysilylpropyl)amine.

[0022] The aminosilane coupling agent used in this application contains amino and siloxane groups; the siloxane groups hydrolyze to generate Si-OH, which combines with the surface of the pretreated melamine powder; and the further introduction of amino groups can promote the dispersion of the pretreated melamine powder in an alkaline environment and improve the depolymerization efficiency.

[0023] In one feasible implementation, the mass ratio of the aminosilane coupling agent to the ethanol-water mixture in the ethanol-water mixture is (5-10):(90-95); and the mass ratio of ethanol to water in the ethanol-water mixture is (70-80):(20-30).

[0024] Excessive concentration of aminosilane coupling agent can lead to increased solution viscosity and uneven dispersion; while insufficient concentration can result in inadequate surface coating of the pretreated melamine powder. Furthermore, the ethanol-water mixture exhibits good compatibility with the aminosilane coupling agent, promoting the hydrolysis of its siloxane groups to generate Si-OH, which then binds to the surface of the pretreated melamine powder.

[0025] In one feasible implementation, the parameters for the heating and reflux treatment in step S2 are: reflux at 80-90°C for 1-2 hours; and the parameters for the drying treatment are: vacuum drying at 80-100°C for 2-4 hours.

[0026] In one feasible implementation, in step S3, the strong base solution includes a strong base solute and a solvent; the strong base solute includes any one of 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]non-5-ene, and tetramethylguanidine; the solvent includes any one of an ethanol-water mixture, acetone, and dimethylformamide; in the ethanol-water mixture, the mass ratio of ethanol to water is (70-80):(20-30).

[0027] In this application, a strong base solute ionizes in water or a mixed solvent to release hydroxide ions, which then attack the methylene bridges in the polymelamine powder to be degraded, thereby promoting the degradation of the polymelamine powder. Furthermore, the ethanol-water mixture, acetone, and dimethylformamide increase the swelling degree of the polymelamine powder, making it easier for the strong base solute to penetrate into its cross-linked network and improving its depolymerization efficiency.

[0028] In one feasible implementation, the mass ratio of the strong base solute to the solvent is (15-25):(75-85).

[0029] In this application, excessively high concentrations of strong alkali solutes lead to increased system viscosity, while excessively low concentrations result in insufficient degradation efficiency. Furthermore, the solvent ratio ensures a homogeneous degradation reaction system, promotes mass and heat transfer, and prevents the agglomeration of the polymelamine powder to be degraded.

[0030] In one feasible implementation, the parameters for the reaction in step S3, which involves stirring, heating, and pressurizing, are: stirring rate 300–500 rpm, 120–160 °C, 5–10 MPa, and reaction time 6–10 h.

[0031] In this application, high temperature can provide activation energy to promote the breaking of methylene bridges in melamine powder; high pressure can suppress water evaporation, maintain the liquid phase environment and promote the penetration of strong alkali; appropriate stirring rate can make the solid and liquid of the reaction system fully mixed, avoid local concentration unevenness, and at the same time prevent the polymelamine powder to be dissolved from settling, thereby improving its reaction uniformity.

[0032] In one feasible implementation, in step S5, the pH range is 5 to 8; the cooling parameters are: 10 to 20°C / min cooling down to 20°C.

[0033] In this application, the depolymerization mixture is strongly alkaline due to the presence of a strong alkali solute. Adjusting the pH to 5-8 can reduce the alkalinity of the solution. At the same time, rapid cooling can cause the solubility of melamine to decrease sharply, resulting in rapid precipitation. After filtration, melamine raw material is obtained, which meets the raw material requirements for subsequent repolymerization reactions.

[0034] In one feasible implementation, the concentration of the formaldehyde solution in step S6 is 30% to 40%.

[0035] During the reaction of formaldehyde and melamine to form melamine resin, excessive formaldehyde concentration will lead to over-crosslinking, making the melamine resin brittle; while insufficient formaldehyde concentration will result in insufficient crosslinking, reducing the heat resistance of the melamine resin.

[0036] Beneficial technical effects:

[0037] The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste provided in this application is based on the realization of closed-loop resource recycling through chemical depolymerization-repolymerization process, which is both environmentally friendly and highly efficient.

[0038] From an environmental perspective, this application employs chemical depolymerization in an alkaline system. OH- groups attack the methylene bridges in the melamine waste, triggering the breakage of CN bonds and avoiding the generation of highly toxic hydrogen cyanide and other harmful substances during pyrolysis, thus significantly reducing the risk of environmental pollution. From a resource recycling perspective, the method provided in this application can degrade melamine waste into melamine, which can then react with formaldehyde to generate new melamine resin, forming a closed-loop recycling chain of melamine waste → synthetic monomer → new melamine resin, achieving efficient resource reuse. Furthermore, in the method provided in this application, anhydrous ethanol is refluxed during the pretreatment stage to remove surface impurities, avoiding interference with subsequent reactions. The melamine powder is surface-modified with an aminosilane coupling agent to increase its polarity and the number of surface amino groups, enhancing its dispersibility in strong alkaline solutions and promoting its contact with strong alkali. Further, by using appropriate reaction conditions, the degradation efficiency is improved, yielding melamine. Finally, by controlling the formaldehyde concentration, excessive or insufficient cross-linking during its reaction with the obtained melamine is avoided, ensuring that the regenerated melamine resin possesses both good mechanical properties and heat resistance. Attached Figure Description

[0039] Figure 1 This is a photograph of the melamine resin powder re-obtained after the depolymerization of melamine waste in Example 1.

[0040] Figure 2 This is a schematic diagram of the chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste. Detailed Implementation

[0041] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0042] This application provides a chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste. The process of this method is as follows: Figure 2 As shown.

[0043] The following describes, in conjunction with different embodiments, a chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste provided in this application.

[0044] Example 1

[0045] like Figure 2 As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0046] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 80°C for 2 hours, filter, and vacuum dry at 80°C for 2 hours to obtain pretreated melamine powder.

[0047] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing γ-aminopropyltrimethoxysilane, refluxed at 80°C for 1 hour, filtered, and then vacuum dried at 80°C for 2 hours to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of γ-aminopropyltrimethoxysilane to the ethanol-water mixed solution was 5:95; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 80:20.

[0048] 3. The polymelamine powder to be depolymerized and the ethanol-water mixed solution of 1,8-diazabicyclo[5.4.0]undec-7-ene were added together into a reaction vessel and reacted for 6 hours under a nitrogen atmosphere, a stirring speed of 300 rpm, a temperature of 120°C, and a pressure of 5 MPa to obtain a depolymerized mixture; wherein, the mass ratio of 1,8-diazabicyclo[5.4.0]undec-7-ene to the ethanol-water mixed solvent was 15:85; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 80:20.

[0049] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0050] 5. After adjusting the pH of the mixture to 5, cool it to 20℃ at a rate of 10℃ / min, and filter to obtain the precipitated melamine;

[0051] 6. The obtained melamine is added to a 30% formaldehyde solution to react again and obtain new melamine resin powder, such as... Figure 1 As shown.

[0052] Example 2

[0053] like Figure 2 As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0054] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 85°C for 3 hours, filter, and vacuum dry at 90°C for 3 hours to obtain pretreated melamine powder.

[0055] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing γ-aminopropyltriethoxysilane, refluxed at 85°C for 1.5 h, filtered, and then vacuum dried at 90°C for 3 h to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of γ-aminopropyltriethoxysilane to the ethanol-water mixed solution was 7:93; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 75:25.

[0056] 3. The polymelamine powder to be depolymerized and the dimethylformamide solution of 1,5-diazabicyclo[4.3.0]non-5-ene were added together into a reaction vessel and reacted for 8 hours under a nitrogen atmosphere, with a stirring speed of 400 rpm, a temperature of 140°C, and a pressure of 7 MPa to obtain a depolymerization mixture; wherein, the mass ratio of 1,5-diazabicyclo[4.3.0]non-5-ene to dimethylformamide solvent was 20:80;

[0057] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0058] 5. After adjusting the pH of the mixture to 6, cool it to 20°C at a rate of 15°C / min, and filter to obtain the precipitated melamine;

[0059] 6. The obtained melamine is added to a 40% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0060] Example 3

[0061] like Figure 2 As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0062] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 90°C for 4 hours, filter, and vacuum dry at 100°C for 4 hours to obtain pretreated melamine powder.

[0063] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, refluxed at 90°C for 2 hours, filtered, and then vacuum dried at 100°C for 4 hours to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of N-(β-aminoethyl)-γ-aminopropyltriethoxysilane to the ethanol-water mixed solution was 10:90; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 70:30.

[0064] 3. Add the polymelamine powder to be depolymerized and the acetone solution of tetramethylguanidine to a reaction vessel. React for 10 hours under a nitrogen atmosphere, stirring speed of 500 rpm, 160℃ and 10 MPa to obtain a depolymerization mixture; wherein the mass ratio of tetramethylguanidine to acetone solvent is 25:75.

[0065] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0066] 5. After adjusting the pH of the mixture to 8, cool it to 20℃ at a rate of 20℃ / min, and filter to obtain the precipitated melamine;

[0067] 6. The obtained melamine is added to a 50% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0068] Example 4

[0069] like Figure 2 As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0070] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 85°C for 2.5 hours, filter, and vacuum dry at 100°C for 2.5 hours to obtain pretreated melamine powder.

[0071] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing bis(trimethoxysilylpropyl)amine, refluxed at 90°C for 1.5 h, filtered, and then vacuum dried at 90°C for 2 h to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of bis(trimethoxysilylpropyl)amine to the ethanol-water mixed solution was 6:94; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 78:22.

[0072] 3. The polymelamine powder to be depolymerized and the dimethylformamide solution of 1,8-diazabicyclo[5.4.0]undec-7-ene were added together into a reaction vessel and reacted for 7 hours under a nitrogen atmosphere, with a stirring speed of 350 rpm, a temperature of 130°C, and a pressure of 6 MPa to obtain a depolymerized mixture; wherein, the mass ratio of 1,8-diazabicyclo[5.4.0]undec-7-ene to dimethylformamide solvent was 18:82;

[0073] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0074] 5. After adjusting the pH of the mixture to 6, cool it to 20°C at a rate of 15°C / min, and filter to obtain the precipitated melamine;

[0075] 6. The obtained melamine is added to a 35% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0076] Example 5

[0077] like Figure 2 As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0078] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 88°C for 3.5 hours, filter, and vacuum dry at 95°C for 3 hours to obtain pretreated melamine powder.

[0079] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing bis(triethoxysilylpropyl)amine, refluxed at 90°C for 1 hour, filtered, and then vacuum dried at 95°C for 2.5 hours to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of bis(triethoxysilylpropyl)amine to the ethanol-water mixed solution was 8:92; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 73:27.

[0080] 3. The polymelamine powder to be depolymerized and the ethanol-water mixed solution of 1,5-diazabicyclo[4.3.0]non-5-ene were added to a reaction vessel and reacted for 9 hours under a nitrogen atmosphere, with a stirring speed of 450 rpm, a temperature of 150°C, and a pressure of 8 MPa to obtain a depolymerized mixture; wherein, the mass ratio of 1,5-diazabicyclo[4.3.0]non-5-ene to the ethanol-water mixed solvent was 22:78; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 70:30.

[0081] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0082] 5. After adjusting the pH of the mixture to 7, cool it to 20℃ at a rate of 16℃ / min, and filter to obtain the precipitated melamine;

[0083] 6. The obtained melamine is added to a 45% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0084] Example 6

[0085] like Figure 2As shown, a chemical depolymerization-repolymerization synthesis method for recycling melamine waste includes the following steps:

[0086] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 85°C for 3 hours, filter, and vacuum dry at 90°C for 3 hours to obtain pretreated melamine powder.

[0087] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing γ-aminopropyltrimethoxysilane, refluxed at 85°C for 1.5 h, filtered, and then vacuum dried at 90°C for 3 h to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of γ-aminopropyltrimethoxysilane to the ethanol-water mixed solution was 9:91; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 76:24.

[0088] 3. The polymelamine powder to be depolymerized and the dimethylformamide solution of 1,5-diazabicyclo[4.3.0]non-5-ene were added together into a reaction vessel and reacted for 8 hours under a nitrogen atmosphere, with a stirring speed of 400 rpm, a temperature of 140°C, and a pressure of 7 MPa to obtain a depolymerization mixture; wherein, the mass ratio of 1,5-diazabicyclo[4.3.0]non-5-ene to dimethylformamide solvent was 20:80;

[0089] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0090] 5. After adjusting the pH of the mixture to 7.5, cool it to 20℃ at a rate of 15℃ / min, and filter to obtain the precipitated melamine;

[0091] 6. The obtained melamine is added to a 40% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0092] Comparative Example 1

[0093] A chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste includes the following steps:

[0094] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 80°C for 2 hours, filter, and vacuum dry at 80°C for 2 hours to obtain the polymelamine powder to be dissolved.

[0095] 2. The polymelamine powder to be depolymerized and the ethanol-water mixed solution of 1,8-diazabicyclo[5.4.0]undec-7-ene were added together into a reaction vessel and reacted for 6 hours under a nitrogen atmosphere, a stirring speed of 300 rpm, a temperature of 120°C, and a pressure of 5 MPa to obtain a depolymerized mixture; wherein, the mass ratio of 1,8-diazabicyclo[5.4.0]undec-7-ene to the ethanol-water mixed solvent was 15:85; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 80:20.

[0096] 3. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0097] 4. After adjusting the pH of the mixture to 5, cool it to 20℃ at a rate of 10℃ / min, and filter to obtain the precipitated melamine;

[0098] 5. Add the obtained melamine to a 30% formaldehyde solution and react again to obtain new melamine resin powder.

[0099] Comparative Example 2

[0100] A chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste includes the following steps:

[0101] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 90°C for 4 hours, filter, and vacuum dry at 100°C for 4 hours to obtain pretreated melamine powder.

[0102] 2. The pretreated melamine powder was added to an ethanol-water mixed solution containing N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, refluxed at 90°C for 2 hours, filtered, and then vacuum dried at 100°C for 4 hours to obtain the polymelamine powder to be dissolved; wherein, the mass ratio of N-(β-aminoethyl)-γ-aminopropyltriethoxysilane to the ethanol-water mixed solution was 10:90; and the mass ratio of ethanol to water in the ethanol-water mixed solution was 70:30.

[0103] 3. Add the polymelamine powder to be depolymerized and acetone together into the reaction vessel, and react for 10 hours under a nitrogen atmosphere, stirring speed of 500 rpm, 160℃ and 10 MPa to obtain the depolymerized mixture.

[0104] 4. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0105] 5. After adjusting the pH of the mixture to 8, cool it to 20℃ at a rate of 20℃ / min, and filter to obtain the precipitated melamine;

[0106] 6. The obtained melamine is added to a 50% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0107] Comparative Example 3

[0108] A chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste includes the following steps:

[0109] 1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, reflux at 85°C for 3 hours, filter, and vacuum dry at 90°C for 3 hours to obtain pretreated melamine powder.

[0110] 2. Add the polymelamine powder to be depolymerized and dimethylformamide together into the reactor, and react for 8 hours under a nitrogen atmosphere, stirring speed of 400 rpm, 140℃, and 7 MPa to obtain the depolymerized mixture.

[0111] 3. Filter the depolymerized mixture while it is still hot to remove any residue that has not been completely depolymerized, and obtain a mixed liquid;

[0112] 4. After adjusting the pH of the mixture to 7.5, cool it to 20℃ at a rate of 15℃ / min, and filter to obtain the precipitated melamine;

[0113] 5. The obtained melamine is added to a 40% formaldehyde solution and reacted again to obtain new melamine resin powder.

[0114] The degradation efficiency of melamine waste was calculated by weighing the amount of melamine waste fed into the above examples and comparative examples, as well as the mass of the residue that was not completely depolymerized. The test results are shown in Table 1 below.

[0115] Table 1. Test results of melamine resins re-prepared after depolymerization in the examples and comparative examples.

[0116]

[0117]

[0118] As shown in Table 1, when the amount of melamine waste is the same, the degradation efficiency of the melamine waste in Examples 1 to 6 is significantly better than that in Comparative Examples 1 to 3.

[0119] This is because, in Examples 1-6, chemical depolymerization under an alkaline system is employed. The CN bonds in the melamine waste are broken by OH- attacking the methylene bridges, thus avoiding the generation of highly toxic hydrogen cyanide and other harmful substances during pyrolysis, significantly reducing the risk of environmental pollution. From a resource recycling perspective, the method provided in this application can degrade melamine waste into melamine, which can then react with formaldehyde to generate new melamine resin, forming a closed-loop recycling chain from melamine waste to synthetic monomers and then to new melamine resin, achieving efficient resource reuse. Furthermore, in the method provided in this application, the pretreatment stage uses an ethanol-water mixed solution for reflux to remove surface impurities, avoiding interference with subsequent reactions; the melamine powder is surface-modified with an aminosilane coupling agent to increase its polarity and the number of surface amino groups, enhancing its dispersibility in a strong alkaline solution and promoting its contact with the strong alkali. Further, by using appropriate reaction conditions, the degradation efficiency is improved, yielding melamine. Finally, by controlling the formaldehyde concentration, excessive or insufficient cross-linking during its reaction with the obtained melamine is avoided, ensuring that the regenerated melamine resin possesses both good mechanical properties and heat resistance.

[0120] In Comparative Example 1, after the melamine waste was recycled and cleaned, no further surface treatment with an aminosilane coupling agent was used. Therefore, its polarity and the number of surface amino groups were not improved, and consequently, its dispersibility was not improved. Its contact with strong alkali was not promoted, and thus its degradation efficiency was reduced.

[0121] In Comparative Example 2, although the pretreated melamine powder underwent surface treatment with an aminosilane coupling agent, a strong alkali was not used, thus preventing the construction of chemical depolymerization in an alkaline system and the inability to break CN bonds by attacking the methylene bridges in the melamine waste with OH-. As a result, the degradation efficiency of the melamine waste was significantly reduced.

[0122] In Comparative Example 3, after the melamine waste was recycled and cleaned, neither further surface treatment with aminosilane coupling agent nor strong alkali was used. Therefore, the dispersibility of the melamine waste was not improved, and chemical depolymerization under alkaline system was not constructed. In the end, the degradation efficiency of the melamine waste was the lowest.

[0123] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0124] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. A chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste, characterized in that, Includes the following steps: S1. After crushing the melamine waste to obtain waste powder, add the waste powder to anhydrous ethanol, heat and reflux it, filter and dry it to obtain pretreated melamine powder. S2. Add the pretreated melamine powder to an ethanol-water mixed solution containing an aminosilane coupling agent, heat and reflux again, filter and dry to obtain the polymelamine powder to be dissolved. S3. Add the polymelamine powder to be depolymerized and the strong alkali solution together into the reaction vessel, and react under nitrogen atmosphere while stirring, heating and pressurizing to obtain the depolymerization mixture; S4. Filter the depolymerized mixture while it is still hot to remove the residue that is not completely depolymerized, and obtain a mixture. S5. After adjusting the pH of the mixture, cool it and filter to obtain the precipitated melamine. S6. Add the obtained melamine to the formaldehyde solution and react again to obtain new melamine resin powder.

2. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S1, the parameters for the heating and reflux treatment are: reflux at 80-90℃ for 2-4 hours; the parameters for the drying treatment are: vacuum drying at 80-100℃ for 2-4 hours.

3. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S2, the aminosilane coupling agent includes any one of γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, bis(trimethoxysilylpropyl)amine, and bis(triethoxysilylpropyl)amine.

4. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In the ethanol-water mixed solution containing the aminosilane coupling agent, the mass ratio of the aminosilane coupling agent to the ethanol-water mixed solution is (5-10):(90-95); in the ethanol-water mixed solution, the mass ratio of ethanol to water is (70-80):(20-30).

5. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S2, the parameters for the heating and reflux treatment are: reflux at 80-90℃ for 1-2 hours; the parameters for the drying treatment are: vacuum drying at 80-100℃ for 2-4 hours.

6. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S3, the strong base solution includes a strong base solute and a solvent; the strong base solute includes any one of 1,-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]non-5-ene, and tetramethylguanidine; the solvent includes any one of an ethanol-water mixture, acetone, and dimethylformamide; in the ethanol-water mixture, the mass ratio of ethanol to water is (70-80):(20-30).

7. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 6, characterized in that, The mass ratio of the strong base solute to the solvent is (15-25):(75-85).

8. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S3, the parameters for the reaction while stirring, heating, and pressurizing are: stirring rate 300-500 rpm, 120-160℃, 5-10 MPa, and reaction time 6-10 h.

9. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S5, the pH range is 5 to 8; the cooling parameters are: 10 to 20°C / min to 20°C.

10. The chemical depolymerization-repolymerization synthesis method for recycling and reusing melamine waste according to claim 1, characterized in that, In step S6, the concentration of the formaldehyde solution is 30% to 40%.

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