A method for depolymerizing waste polyamide 6 materials

Abstract

This invention belongs to the field of polyamide recycling technology, and specifically relates to a depolymerization method for waste polyamide 6 materials. The depolymerization method includes the following steps: S1: Passing steam containing caprolactam into the waste polyamide 6 melt for hydrolysis and depolymerization; S2: The caprolactam and water vapor obtained from depolymerization are condensed to obtain an aqueous caprolactam solution. This depolymerization method uses caprolactam as a depolymerization aid in the superheated steam used for hydrolysis, which can effectively reduce the reaction viscosity of the depolymerization system and increase the depolymerization rate and monomer yield. Furthermore, since the added depolymerization aid is itself the target product of hydrolysis and depolymerization, this method does not introduce additional impurities and does not affect the product purity or subsequent purification processes. At the same time, this process does not suffer from the deactivation / deactivation problems commonly encountered with traditional acidic catalysts when processing impure polyamide 6 materials.

Description

Technical Field

[0001] This invention belongs to the field of polyamide recycling technology, and specifically relates to a method for depolymerizing waste polyamide 6 materials. Background Technology

[0002] Polyamide 6 is a polymer synthesized under certain conditions using caprolactam as a monomer, with —N(H)—(CH2)5—C(O)— as the repeating unit. With the increasing use of polyamide 6 year by year, the recycling of waste polyamide 6 products has gradually become a hot topic and a challenge.

[0003] Currently, the main method for recycling polyamide 6 is physical methods. However, physical methods result in low added value, and the performance of recycled products cannot meet virgin standards. Besides physical methods, chemical methods can also be used to depolymerize waste polyamide 6 materials. Through chemical depolymerization reactions, polyamide 6 is depolymerized into monomeric forms that can be repolymerized and are easily purified, such as caprolactam. Depending on the technical route and depolymerization reagent, the depolymerization of polyamide 6 can be divided into ammonolysis, alcoholysis, alkaline hydrolysis, and hydrolysis. Among these, hydrolysis is the most promising technical route because it uses water as the depolymerization agent, does not use other organic solvents, and produces simple and easily purified depolymerization products.

[0004] The hydrolysis method (steam depolymerization) is generally carried out as follows: superheated steam at 270-400°C is continuously introduced into the polyamide 6 melt from the bottom. The depolymerization temperature is generally controlled at 250-350°C. Water molecules act as a depolymerizing agent and stripping gas. The caprolactam produced by hydrolysis passes through the polyamide 6 melt with the superheated steam and is continuously discharged from the top. After condensation, the caprolactam aqueous solution is obtained.

[0005] The hydrolysis method still suffers from slow depolymerization rates and low caprolactam yields. Chinese invention patent application CN103467378A discloses the use of phosphoric acid as a catalyst, which can significantly improve the hydrolysis and depolymerization rate of polyamide 6. However, when processing polyamide 6 materials containing impurities, these impurities can easily react with acidic catalysts, leading to deactivation. For example, waste polyamide 6 carpets contain a large amount of calcium carbonate filler; during depolymerization, calcium carbonate reacts with acidic catalysts such as phosphoric acid, causing depolymerization failure. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a depolymerization method for waste polyamide 6 materials with fast depolymerization speed and high yield.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for depolymerizing waste polyamide 6 material, comprising the following steps:

[0008] S1: Passing steam containing caprolactam into waste polyamide 6 melt for hydrolysis and depolymerization;

[0009] S2: The caprolactam and water vapor obtained from depolymerization are condensed to obtain an aqueous solution of caprolactam.

[0010] The beneficial effects of this invention are as follows: In the depolymerization method of this invention, caprolactam is doped into the superheated steam used for hydrolysis as a depolymerization aid, which can effectively reduce the reaction viscosity of the depolymerization system and increase the depolymerization rate and monomer yield. Furthermore, since the added depolymerization aid is itself the target product of hydrolysis and depolymerization, this method does not introduce additional impurities and does not affect the product purity or subsequent purification processes. Simultaneously, this process does not suffer from the deactivation / deactivation problems commonly encountered with traditional acidic catalysts when processing materials containing heteropolymer amide 6. Detailed Implementation

[0011] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments.

[0012] A method for depolymerizing waste polyamide 6 material includes the following steps:

[0013] S1: Passing steam containing caprolactam into waste polyamide 6 melt for hydrolysis and depolymerization;

[0014] S2: The caprolactam and water vapor obtained from depolymerization are condensed to obtain an aqueous solution of caprolactam.

[0015] As can be seen from the above description, the beneficial effects of this invention are as follows: Addressing the problems of low depolymerization rate and easy deactivation of the depolymerization catalyst in the current steam depolymerization process of waste polyamide 6, this invention provides a depolymerization method for extracting caprolactam from waste polyamide 6 materials. Specifically, steam containing added caprolactam is passed into the waste polyamide 6 melt for hydrolysis and depolymerization. The resulting caprolactam and water vapor are condensed and collected to obtain a caprolactam aqueous solution of a certain concentration. The addition of caprolactam to superheated steam effectively reduces the viscosity of the depolymerization system, thereby significantly improving the depolymerization rate and the yield of depolymerized monomers. Furthermore, since the added substance is the target depolymerization product, it does not lead to a decrease in the purity of the depolymerization product. Simultaneously, this process does not react with fillers such as calcium carbonate when processing heterogeneous polyamide 6 materials, avoiding the deactivation / failure problems commonly encountered with traditional acidic catalysts.

[0016] Furthermore, in step S1, the mass percentage of caprolactam in the steam is 1–50 wt%.

[0017] Furthermore, in step S1, the mass percentage of caprolactam in the steam is 5–30 wt%.

[0018] As described above, a low proportion of caprolactam has little effect on improving the depolymerization rate and monomer yield; a high proportion of caprolactam not only increases depolymerization costs but also reduces the proportion of water in the steam, thus decreasing the depolymerization rate. Therefore, the mass percentage of caprolactam in the steam must be within the range of 1–50 wt%, preferably 5–30 wt%, to effectively increase the depolymerization rate and improve the monomer yield.

[0019] Furthermore, the hydrolysis and depolymerization temperature is 270–350℃.

[0020] As described above, too low a depolymerization temperature leads to a slow depolymerization rate, while too high a temperature increases side reactions, resulting in a decrease in the purity of the target product and increasing the pressure on subsequent separation and purification. Therefore, the optimal depolymerization rate and effect for waste polyamide 6 are achieved within the temperature range of 270–350℃.

[0021] Furthermore, the hydrolysis and depolymerization pressure is 0–2 MPa.

[0022] As can be seen from the above description, pressure affects the depolymerization rate; both excessive and insufficient depolymerization pressure will reduce the depolymerization efficiency.

[0023] Furthermore, the caprolactam added to the steam is caprolactam with industrial-grade purity or caprolactam with lower purity.

[0024] As can be seen from the above description, caprolactam with lower purity includes crude caprolactam obtained by hydrolysis and depolymerization of polyamide 6 without purification. Using caprolactam with lower purity can effectively reduce costs.

[0025] Furthermore, the temperature of the steam is 270–320°C.

[0026] As can be seen from the above description, if the steam temperature is too low, caprolactam cannot be vaporized; if the steam temperature is too high, it is easy to cause side reactions.

[0027] Furthermore, caprolactam is obtained by purifying an aqueous solution of caprolactam. The purification method includes at least one of extraction, ion exchange resin purification, potassium permanganate oxidation, hydrogenation reduction, distillation, activated carbon adsorption, and recrystallization.

[0028] Further, the extraction was performed as benzene extraction / water back-extraction.

[0029] As can be seen from the above description, caprolactam aqueous solution can be directly polymerized into polyamide 6 products after dehydration and concentration; or it can be purified and refined before being repolymerized into polyamide 6 products, the latter having higher purity.

[0030] Embodiment 1 of the present invention is: a method for depolymerizing waste polyamide 6 material, comprising the following steps:

[0031] S1: A 10 wt% caprolactam aqueous solution is introduced into the heating device of the steam generator at a flow rate of 10 mL / min and is eventually converted into steam containing 10 wt% caprolactam at 300℃; wherein the caprolactam is caprolactam with industrial-grade purity.

[0032] S2: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 280℃, turn off the nitrogen, and continuously introduce steam (containing 10wt% caprolactam) from the bottom of the reactor. The steam temperature is 300℃, the flow rate is 10mL / min, and the depolymerization temperature inside the reactor is controlled at 300℃ and the pressure is 1MPa.

[0033] S3: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution. The yield of caprolactam obtained by depolymerization is calculated.

[0034] Embodiment 2 of the present invention is: a method for depolymerizing waste polyamide 6 material, comprising the following steps:

[0035] S1: A 20wt% caprolactam aqueous solution is introduced into the heating device of the steam generator at a flow rate of 10mL / min and is eventually converted into steam containing 20wt% caprolactam at 300℃; wherein the caprolactam is caprolactam with industrial-grade purity.

[0036] S2: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 280℃, turn off the nitrogen, and continuously introduce steam (containing 20wt% caprolactam) from the bottom of the reactor. The steam temperature is 300℃, the flow rate is 10mL / min, and the depolymerization temperature inside the reactor is controlled at 300℃ and the pressure is 1MPa.

[0037] S3: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution. The yield of caprolactam obtained by depolymerization is calculated.

[0038] Embodiment 3 of the present invention is: a method for depolymerizing waste polyamide 6 material, comprising the following steps:

[0039] S1: A 1 wt% caprolactam aqueous solution is introduced into the heating device of the steam generator at a flow rate of 10 mL / min and is eventually converted into steam containing 1 wt% caprolactam at 270°C; wherein the caprolactam is caprolactam with industrial-grade purity.

[0040] S2: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 270℃. Turn off the nitrogen and continuously introduce steam (containing 1wt% caprolactam) from the bottom of the reactor. The steam temperature is 270℃ and the flow rate is 10mL / min. Control the depolymerization temperature inside the reactor to 270℃ and the pressure to 2MPa.

[0041] S3: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution.

[0042] S4: Caprolactam aqueous solution is purified by one or more of the following purification processes: benzene extraction / water back-extraction, potassium permanganate oxidation, hydrogenation reduction and recrystallization.

[0043] Embodiment 4 of the present invention is: a method for depolymerizing waste polyamide 6 material, comprising the following steps:

[0044] S1: A 50 wt% caprolactam aqueous solution is introduced into the heating device of the steam generator at a flow rate of 10 mL / min and is eventually converted into steam containing 50 wt% caprolactam at 320℃; wherein the caprolactam is caprolactam with industrial-grade purity.

[0045] S2: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 280℃, turn off the nitrogen, and continuously introduce steam (containing 50wt% caprolactam) from the bottom of the reactor. The steam temperature is 320℃, the flow rate is 10mL / min, and the depolymerization temperature inside the reactor is controlled at 350℃ and the pressure is 0MPa.

[0046] S3: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution.

[0047] S4: Caprolactam was obtained by benzene extraction / water back-extraction, ion exchange resin purification, activated carbon adsorption and distillation purification of caprolactam aqueous solution.

[0048] Comparative Example 1 of the present invention is:

[0049] The difference between Comparative Example 1 and Example 1 is that caprolactam was not added to the steam, and the steam flow rate was 10 g / min.

[0050] A method for depolymerizing waste polyamide 6 material includes the following steps:

[0051] S1: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 280℃, turn off the nitrogen, and continuously introduce steam from the bottom of the reactor. The steam temperature is 300℃ and the flow rate is 10mL / min (the steam is prepared by a superheated steam generator). Control the depolymerization temperature inside the reactor to 300℃ and the pressure to 1MPa.

[0052] S2: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution. The yield of caprolactam is calculated.

[0053] Comparative Example 2 of the present invention is as follows:

[0054] The difference between Comparative Example 2 and Example 1 is that the weight percentage of caprolactam in the steam is 60 wt%, and the steam flow rate is 10 mL / min.

[0055] A method for depolymerizing waste polyamide 6 material includes the following steps:

[0056] S1: A 60wt% caprolactam aqueous solution is introduced into the heating device of the steam generator at a flow rate of 10mL / min and is eventually converted into steam containing 60wt% caprolactam at 300℃; wherein the caprolactam is caprolactam with industrial-grade purity.

[0057] S2: Add 250g of polyamide 6 chips to a 1L reactor. Under nitrogen protection, raise the temperature inside the reactor to 280℃, turn off the nitrogen, and continuously introduce steam (containing 60wt% caprolactam) from the bottom of the reactor. The steam temperature is 300℃, the flow rate is 10mL / min, and the depolymerization temperature inside the reactor is controlled at 300℃ and the pressure is 1MPa.

[0058] S3: The caprolactam and water vapor discharged from the top of the reactor are collected in liquid form after passing through the condenser to obtain a caprolactam solution. The yield of caprolactam obtained by depolymerization is calculated.

[0059] The yields of caprolactam obtained from depolymerization in Examples 1 and 2 and Comparative Examples 1 and 2 were calculated. The calculation method was as follows: the yield of caprolactam was calculated by periodically measuring the weight (volume) and concentration (caprolactam concentration was determined by GC). For Examples 1 and 2, where caprolactam was added to the steam, the yield of caprolactam obtained from depolymerization was the yield after deducting the caprolactam added to the steam. The calculation results are shown in Table 1.

[0060] Table 1

[0061]

[0062] As shown in Table 1, adding a certain amount of caprolactam to water vapor can effectively improve the depolymerization rate of polyamide 6 and the yield of depolymerized monomers.

[0063] In summary, this invention provides a method for depolymerizing waste polyamide 6 materials. Specifically, steam containing a certain proportion of caprolactam is passed into the waste polyamide 6 melt for hydrolysis and depolymerization. The caprolactam and water vapor obtained from the depolymerization are collected after condensation to obtain a caprolactam aqueous solution of a certain concentration. This aqueous solution can be dehydrated and concentrated before being directly polymerized into polyamide 6 products, or it can be purified and refined before being repolymerized into polyamide 6 products.

[0064] Adding a certain amount of caprolactam to superheated steam can effectively reduce the viscosity of the depolymerization system, thereby significantly improving the depolymerization rate and monomer yield. The proportion of caprolactam should not be too low, as too little caprolactam will not have a significant effect on improving the depolymerization rate and monomer yield; the proportion should also not be too high, as too high a proportion of caprolactam will not only lead to an increase in depolymerization costs, but will also reduce the proportion of water in the steam, thus causing a decrease in the depolymerization rate. At the same time, it is also necessary to control the depolymerization temperature and pressure to prevent the formation of side reactions.

[0065] Introducing caprolactam into superheated steam does not reduce the purity of the depolymerization product because the added substance is the target product for depolymerization. At the same time, this process does not have the deactivation / failure problem that traditional acid catalysts are prone to when processing materials containing heteropolymer amide 6.

[0066] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made using the present invention specification, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for depolymerizing waste polyamide 6 material, characterized in that, Includes the following steps: S1: Steam containing caprolactam is passed into waste polyamide 6 melt for hydrolysis and depolymerization; the mass percentage of caprolactam in the steam is 10~20 wt%. S2: The caprolactam and water vapor obtained from depolymerization are condensed to obtain an aqueous solution of caprolactam.

2. The depolymerization method for waste polyamide 6 material according to claim 1, characterized in that, The hydrolysis and depolymerization temperature is 270~350℃.

3. The depolymerization method for waste polyamide 6 material according to claim 1, characterized in that, The temperature of the steam is 270~320℃.

4. The method for depolymerizing waste polyamide 6 material according to claim 1, characterized in that, The caprolactam aqueous solution is purified to obtain caprolactam, and the purification method is at least one of extraction, ion exchange resin purification, potassium permanganate oxidation, hydrogenation reduction, distillation, activated carbon adsorption and recrystallization.

5. The depolymerization method for waste polyamide 6 material according to claim 4, characterized in that, The extraction method is benzene extraction / water back-extraction.

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