A method for preparing high-flowability reclaimed polyamide 6 from waste polyamide 6

Abstract

The application discloses a method for preparing high-fluidity regenerated polyamide 6 from waste polyamide 6, and the method comprises the following steps: hydrolyzing the waste polyamide 6 to obtain oligomeric polyamide 6, and then performing a branching reaction on the oligomeric polyamide 6 and a branching agent to obtain the high-fluidity regenerated polyamide 6, wherein the number-average molecular weight of the oligomeric polyamide 6 is 2000-5000 g / mol. The application can realize controllable degradation of the number-average molecular weight of the polyamide by adding a small amount of water, so that the polyamide can be fully contacted with the branching agent to occur the branching reaction, and the prepared high-fluidity regenerated polyamide 6 can improve the fluidity while ensuring the strength of the regenerated polyamide 6, and can be widely applied to the recycling, upgrading and reuse of waste polyamide 6 plastics and fibers.

Description

Technical Field

[0001] This invention belongs to the field of waste polyamide recycling and reuse, and relates to a method for preparing high-flowability recycled polyamide 6 from waste polyamide 6. Background Technology

[0002] Polyamide 6 is one of the most widely used polyamide varieties due to its high strength, high toughness, wear resistance, and good moisture absorption, performing exceptionally well in engineering plastics and fiber textiles. During the production of polyamide 6 products, the presence of bubbles or impurities can cause a decline in product performance, especially in fiber applications, where this effect is particularly severe. This necessitates that the hot water extractable content in the polyamide 6 melt be less than 2 wt%. However, the conversion rate of caprolactam hydrolysis polymerization is around 90%, resulting in approximately 10 wt% of hot water extractable (containing caprolactam and its oligomers) remaining in polyamide 6 chips. This significantly impacts the processing of polyamide 6, necessitating extraction treatment of the polyamide 6 chips, which greatly increases the production cost of polyamide 6.

[0003] To simplify the extraction process, removing hot-water extractables during the polymerization of polyamide 6 is one solution. Improving the melt flowability of polyamide 6 without affecting its melt strength is beneficial for removing hot-water extractables. Compared to linear polyamide 6 with the same relative molecular weight, the four-armed and eight-armed star-shaped polymers synthesized by Flory through condensation polymerization exhibit lower melt viscosity and smaller flow kinetic volume. Therefore, branched polyamide 6 can meet the requirements for solving the problem of polyamide 6 melt devolatilization.

[0004] The above-mentioned problems also exist in the recycling process of waste polyamide 6. Therefore, the preparation of a highly fluid recycled polyamide 6 using a branching scheme is of great significance for the recycling of waste polyamide 6.

[0005] CN115466386A discloses a modified polyamide-6, modified polyamide-6 products, their preparation method, and applications. This technology involves melting and mixing polyamide-6 and a chain extender (possessing three or more epoxy groups and a molecular weight less than 500) at a mass ratio of 100:0.1-5 to prepare modified polyamide-6 with a specific branched structure. However, the method described in CN115466386A, which uses a screw to mix polyamide-6 and a chain extender to prepare modified polyamide-6 with a specific branched structure, cannot be directly applied to the recycling of waste polyamide-6. This is because the molecular chains of waste polyamide-6 have undergone varying degrees of breakage after use, and the molecular weights of waste polyamide-6 products from different sources also vary significantly. Directly preparing branched modified polyamide-6 with a wide molecular weight distribution makes it difficult to ensure the relative consistency of the lengths of the branched chains on the same macromolecule and the consistency of the molecular weights of different macromolecules. This results in a polymer with a still wide molecular weight distribution, and the lower molecular weight molecules become macroscopic defects in the polymer, affecting its application. Summary of the Invention

[0006] The purpose of this invention is to solve the problems existing in the prior art and to provide a method for preparing high-flowability recycled polyamide 6 from waste polyamide 6.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 involves hydrolyzing the waste polyamide 6 to obtain oligomeric polyamide 6, and then reacting the oligomeric polyamide 6 with a branching agent to obtain high-flowability recycled polyamide 6.

[0009] The formula for calculating the total amount of water added during the hydrolysis treatment of waste polyamide 6 is as follows:

[0010] m0(H2O)=m(H2O)+m(gasH2O);

[0011]

[0012]

[0013] In the formula, m0(H2O) is the total amount of water added, g; m(H2O) is the mass of water to be added according to the chemical reaction equilibrium, g; m(gas H2O) is the mass of gaseous water in the reaction vessel under hydrolysis conditions, g; m(PA6) is the mass of waste polyamide 6, g; M n (PA6) is the number-average molecular weight of waste polyamide 6, g / mol; N is the average degree of polymerization of waste polyamide 6; x is the theoretical average degree of polymerization of oligomeric polyamide 6; ρ T(gas H2O) is the density of gaseous water at the hydrolysis reaction temperature, in g / cm³. 3 V0 is the volume of the reaction vessel, in cm³. 3 ;ρ T (PA6) represents the density of polyamide 6 at the hydrolysis reaction temperature, in g / cm³. 3 ;ρ T (liquidH2O) is the density of liquid water at the hydrolysis reaction temperature, in g / cm³. 3 ;

[0014] The number average molecular weight of oligomeric polyamide 6 is 2000–5000 g / mol.

[0015] This invention controls the molecular weight of polyamide 6 hydrolysis products by adding a small amount of water. This method is designed based on research into the hydrolysis process of polyamide 6 and reaction equilibrium. The hydrolysis of polyamide 6 can be divided into three stages: the first stage involves the hydrolysis of polyamide 6 into oligomeric polyamide 6 with a certain molecular weight, during which very little caprolactam and its water-soluble oligomers are produced; the second stage involves the further hydrolysis of the aforementioned oligomeric polyamide 6 into even lower molecular weight oligomeric polyamide 6, while caprolactam and its water-soluble oligomers begin to be produced; the third stage involves the oligomeric polyamide 6 molecular chain remaining essentially unchanged, while the rate of caprolactam and its water-soluble oligomer production surges until the hydrolysis to caprolactam reaches equilibrium; then, based on... Figure 1 The reaction produces N / x mol of oligomeric polyamide 6 with an average degree of polymerization of x, which requires 1 mol of waste polyamide 6 with an average degree of polymerization of N and N / x mol of water. According to chemical equilibrium, when the waste polyamide 6 reacts completely to produce oligomeric polyamide 6 with an average degree of polymerization of x, the theoretical molar ratio of waste polyamide 6 to water is x / N. The number of moles of waste polyamide 6 is expressed as m(PA6) / M. n (PA6) represents the number of moles of water, expressed as m(H2O) / M(H2O), where M(H2O) = 18 g / mol, i.e., [m(PA6) / M n [(PA6)] / [m(H2O) / M(H2O)]=x / N, after rearranging the terms, we can obtain the theoretical mass ratio of waste polyamide 6 to water as follows: Under hydrolysis conditions, water will vaporize to form gaseous water, providing pressure for the reaction system. Therefore, some of the water obtained from the above formula will become gaseous water and cannot participate in the hydrolysis reaction in the liquid phase. Thus, it is necessary to replenish this part of gaseous water to make up for the water lost in hydrolysis due to the reaction temperature. The mass of gaseous water can be calculated using the basic physical formula: mass of an object = density of an object × volume of an object. The density of saturated water vapor at the reaction temperature can be obtained from relevant manuals. The volume occupied by the vapor can be approximated by subtracting the volume of waste polyamide 6 and liquid water from the volume of the container. The volume of waste polyamide 6 and liquid water can be obtained by their mass and density at the reaction temperature.

[0016] This invention achieves the separation of hydrolysis products and water by changing the pressure. During the hydrolysis stage, the reaction vessel is in a sealed state. Since the amount of water added is greater than the mass of water vaporized at this temperature, the water in the reaction vessel is in a liquid-gas mixed state. After the hydrolysis is completed, the reaction vessel is reduced to atmospheric pressure and maintained at atmospheric pressure. At this temperature, water will continuously leave the system in a gaseous state, thereby achieving the separation of water from hydrolysis products.

[0017] Current research on the branching of polyamide 6 is limited, and most studies involve the simultaneous addition of branching agents and caprolactam. No research has been found on the reaction of oligomeric polyamide 6 as a branching unit with branching agents. This invention reveals that oligomeric polyamide 6 with a number-average molecular weight of 2000–5000 g / mol, due to its shorter molecular chain and lower viscosity, allows for sufficient contact between its end groups and branching agents, resulting in a branching reaction.

[0018] The number-average molecular weight of oligomeric polyamide 6 should not be too high. Otherwise, on the one hand, it will affect the full contact between oligomeric polyamide 6 and the branching agent, which is not conducive to the reaction between the branching agent and the end groups of the oligomeric polyamide 6 molecular chain. On the other hand, it is not conducive to the homogenization of the molecular weight of waste polyamide 6, that is, there will still be a difference between the high molecular weight part and the low molecular weight part in waste polyamide 6. Compared with the recycled polyamide 6 prepared by oligomeric polyamide 6 with a number-average molecular weight greater than 5000 g / mol and the branching agent, the recycled polyamide 6 prepared by oligomeric polyamide 6 with a number-average molecular weight of 2000-5000 g / mol and the branching agent has a lower MI, higher hot water extractability, higher cyclic dimer content and higher PDI at 260℃ and 2160g load.

[0019] The number average molecular weight of oligomeric polyamide 6 should not be too low, otherwise it will lead to energy waste. The chemical recycling products of waste polyamide 6 can be divided into three categories: caprolactam (monomer of polyamide 6), oligomeric polyamide 6, and derivatives of oligomeric polyamide 6 (such as amide esters obtained by alcohol recovery). If waste polyamide 6 is still to be used for regenerated polyamide 6, recycling to oligomeric polyamide 6 is the most energy-efficient way, because it retains more of the energy consumed in the initial polymerization for regenerated polyamide 6. On the other hand, it goes against the original intention of this invention. The purpose of improving the fluidity of regenerated polyamide 6 in this invention is to meet the requirements of melt devolatilization of polyamide 6. When the number average molecular weight of oligomeric polyamide 6 is too low, it becomes the object to be removed by devolatilization - hot water extractable.

[0020] As a preferred technical solution:

[0021] The method described above for preparing high-flowability recycled polyamide 6 from waste polyamide 6 involves hydrolyzing the waste polyamide 6 to obtain oligomeric polyamide 6. The specific process is as follows: the waste polyamide 6 is mixed with water in a reaction vessel, and then hydrolyzed at 240–260°C for 1–3 hours under stirring and nitrogen or inert gas protection. After that, the pressure is released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6.

[0022] As described above, in a method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the stirring speed is 100-300 rpm. The stirring speed is relatively slow during the hydrolysis stage because the viscosity of the system is relatively high at this time. The waste polyamide 6 still exists in the system as a large molecule. Although the system is in a homogeneous state at the hydrolysis temperature, if the stirring speed is too high, the liquid will splash onto the reactor wall, the droplets will vaporize, and the waste polyamide 6 will remain on the reactor wall, unable to fully contact with water, and undergo thermal degradation, affecting the quality of the hydrolysis product.

[0023] In the method described above for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the mass of the branching agent is 1-5% of the mass of the waste polyamide 6. When the amount of branching agent added is low, many polyamide 6 molecular chain ends still cannot undergo branching reaction and exist as straight-chain molecules. This cannot fully exert the effect of branched polyamide 6 in enhancing flowability. Instead, the presence of straight-chain molecules will cause the phenomenon of connecting multiple branched molecules. When the amount of branching agent added is high, the branching agent will become a capping agent, so that each molecular chain segment is connected with the branching agent, and it is impossible to achieve the effect of one branching agent connecting 3-4 molecular chains to achieve branching. At the same time, it will also hinder the reaction between polyamide 6 molecular chains and prevent the increase of molecular weight.

[0024] The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 as described above, wherein the branching agent is one or more of the following: pyromellitic acid, pyromellitic trimethylol chloride, diphenylmethane triisocyanate, triaminoethylamine, tetracarboxylic acid cyclohexanone, ethylenediaminetetraacetic acid, star-shaped polyethyleneimine, and lysine.

[0025] The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, as described above, involves the following specific steps in the branching reaction of oligomeric polyamide 6 with a branching agent:

[0026] (a) After mixing oligopolyamide 6 with a branching agent, the mixture is stirred and reacted at 260-270°C for 2-4 hours under nitrogen or inert gas protection. During this process, the branching agent and oligopolyamide 6 are fully mixed, and the terminal groups of oligopolyamide 6 are connected to the branching agent. The terminal amino or carboxyl groups of oligopolyamide 6 react with the reaction end groups of the branching agent to produce water.

[0027] (b) Keeping the temperature constant, under stirring conditions, vacuum the system to less than 100 Pa within 10–30 min, and continue the reaction until the relative viscosity of the system is 2.0–3.0, thus obtaining high-flowability regenerated polyamide 6. During this process, vacuuming is used to remove as much water as possible, thereby achieving rapid growth in the molecular weight of polyamide 6 segments. The water generated in the previous step will affect the growth of the molecular weight of polyamide 6. Although most of the water will be removed during the depressurization to atmospheric pressure, the residual trace water will also affect the rapid growth of the molecular weight in process b. The polymerization process of polyamide 6 involves a reverse reaction that produces hot water extractable substances such as caprolactam. Vacuuming can remove the hot water extractable substances produced above, so as to achieve the purpose of polyamide 6 chips not having to undergo a hot water extraction process.

[0028] As described above, in a method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, in step (a), the stirring speed is 600-1000 rpm. The purpose of this step is to ensure that the branching agent and the oligomeric polyamide 6 are fully mixed so as to facilitate the reaction between the branching agent and the end groups of the oligomeric polyamide 6 chain. Therefore, a certain stirring speed is required. However, a large stirring speed is prone to melt splashing, which is not conducive to the stable progress of the branching reaction.

[0029] As described above, in a method for preparing highly fluid recycled polyamide 6 from waste polyamide 6, in step (b), the stirring speed is 800-1200 rpm. The purpose of this step is to achieve the growth of the recycled polyamide 6 molecular chain by vacuum extraction of water from the system, while removing the hot water extractable substances generated by the side reaction. This requires increasing the renewal frequency of the liquid surface in the system to increase the efficiency of vacuum extraction. Therefore, the stirring speed is appropriately increased.

[0030] The method described above for preparing high-flowability recycled polyamide 6 from waste polyamide 6 has the following characteristics: the high-flowability recycled polyamide 6 has a melting point of 218–222 °C, a number-average molecular weight of 14,000–30,000 g / mol, a PDI of 1.60–1.70, a hot water extractability of 1.0–1.2 wt%, a cyclic dimer content of 0.1–0.5 wt%, and an MI of 40–45 g / 10 min at 260 °C and a load of 2160 g.

[0031] Invention Mechanism

[0032] This invention controls the number-average molecular weight of polyamide 6 hydrolysis products by breaking quantitative amide bonds through the addition of a fixed amount of water. Simultaneously, the reduction of water molecules in the system during the later stages of hydrolysis provides conditions for amide exchange reactions, which is beneficial for homogenizing the molecular weight of hydrolysis products. This solves the problem of uneven molecular weight distribution in recycled products caused by uneven molecular weight distribution in waste polyamide 6 products. After hydrolysis, the hydrolysis products and water are separated by changing the pressure. Then, a branching agent is directly added, and after a high-pressure-vacuum reaction, a one-pot method for preparing high-flowability recycled polyamide 6 is achieved.

[0033] To reduce energy consumption and time in the regeneration process and fully utilize the energy consumed in the initial polymerization of polyamide 6, a method was adopted to prepare oligomeric polyamide 6 with a number-average molecular weight of 2000–5000 g / mol based on chemical equilibrium as the branching unit. Furthermore, due to its short molecular chain, highly reactive end groups, and low viscosity, oligomeric polyamide 6 can effectively exchange matter and energy, thus allowing for a thorough reaction with the branching agent. Additionally, the shorter time required for the condensation polymerization of oligomeric polyamide 6 to form large molecular chains significantly shortens the polymerization reaction time, which helps reduce the formation of cyclic dimers and decrease devolatilization pressure.

[0034] From a microscopic perspective, the flow of polymer melts involves the movement of large molecular chains. This movement differs from the instantaneous movement of small molecules; instead, it involves a shift in the center of gravity of the molecular chain caused by the combined movement of numerous chain segments. This results in a lag between the application of forces to the molecular chain and its subsequent movement, which is the fundamental reason for the poorer fluidity of polymer melts compared to small-molecule melts. Furthermore, linear polymers exist in a random coil state within the melt, occupying a larger volume and easily becoming entangled with surrounding molecular chains, hindering chain movement and thus affecting the macroscopic flow of the melt. With a constant total molecular weight, the molecular weight of each arm of a branched polymer is smaller than that of a linear polymer, equivalent to an increased number of simultaneously moving chain segments. This reduces the difficulty of shifting the center of gravity of the entire molecular chain. Simultaneously, the molecular chains of branched polymers are closer to a spherical shape in the melt, occupying a smaller volume and exhibiting less entanglement with surrounding molecular chains. Therefore, branched molecules have better fluidity than linear molecules.

[0035] Beneficial effects:

[0036] (1) The addition of trace amounts of water in this invention can achieve the controllable degradation of the number-average molecular weight of polyamide, enabling it to fully contact the branching agent to undergo a branching reaction, and the conversion between the hydrolysis stage and the branching stage can be achieved by controlling the pressure.

[0037] (2) The present invention avoids caprolactam residue caused by the branching reaction by further branching reaction of oligomeric polyamide 6 obtained by hydrolysis treatment, and at the same time shortens the reaction time and reduces the generation of cyclic dimers, resulting in significant improvements in mechanical properties and processing.

[0038] (3) A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to the present invention is to use the low-polymer polyamide 6 obtained after hydrolysis as a branching unit and carry out a branching reaction with a branching agent to obtain high-flowability recycled polyamide 6, and greatly improve the devolatilization efficiency during the vacuum stage of the branching reaction.

[0039] (4) The high-flowability recycled polyamide 6 prepared by the present invention improves its flowability while ensuring the strength of recycled polyamide 6, which is of great significance for realizing direct spinning of recycled polyamide 6 melt. Attached Figure Description

[0040] Figure 1 The chemical equation for the hydrolysis of polyamide 6 to oligomeric polyamide 6 is given. Detailed Implementation

[0041] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0042] The formulas for calculating the total amount of water added in step (2) of the following embodiments are as follows:

[0043] m0(H2O)=m(H2O)+m(gasH2O);

[0044]

[0045]

[0046] In the formula, m0(H2O) is the total amount of water added, g; m(H2O) is the mass of water to be added according to the chemical reaction equilibrium, g; m(gas H2O) is the mass of gaseous water in the reaction vessel under hydrolysis conditions, g; m(PA6) is the mass of waste polyamide 6, g; Mn (PA6) is the number-average molecular weight of waste polyamide 6, g / mol; N is the average degree of polymerization of waste polyamide 6; x is the theoretical average degree of polymerization of oligomeric polyamide 6; ρ T (gas H2O) is the density of gaseous water at the hydrolysis reaction temperature, in g / cm³. 3 V0 is the volume of the reaction vessel, in cm³. 3 ;ρ T (PA6) represents the density of polyamide 6 at the hydrolysis reaction temperature, in g / cm³. 3 ;ρ T (liquidH2O) is the density of liquid water at the hydrolysis reaction temperature, in g / cm³. 3 ;

[0047] The testing methods for the relevant performance indicators in the following embodiments are as follows:

[0048] Number-average molecular weight and PDI: The number-average molecular weight and PDI of the samples were determined using a GPC-50 gel permeation chromatograph from PL (UK). This instrument is equipped with a differential refractive index detector and a PL gel column (5 μm mixed-C). 1,1,1,3,3,3-hexafluoro-2-propanol was used as the eluent, and the flow rate was 1 mL / min. Before testing, the sample was dried and dissolved in hexafluoroisopropanol to prepare a solution with a concentration of 1.0 mg / mL. The test was performed when the column temperature reached 40 ± 1 °C. PDI = weight-average molecular weight / number-average molecular weight; the weight-average molecular weight was tested in the same way as the number-average molecular weight.

[0049] Relative viscosity: Refer to GB / T 38138-2019 Test method for fiber grade polycaprolactam (PA6) slices, section 5.2, relative viscosity.

[0050] Melting point: The crystallization and melting behavior of the sample were tested using a TA-Q20 differential scanning calorimeter. The melting point was obtained by heating and cooling approximately 5 mg of the dry sample at a rate of 10 °C / min under a nitrogen atmosphere.

[0051] Hot water extractability: Refer to GB / T 38138-2019 Fiber grade polycaprolactam (PA6) slice test method 5.3 Hot water extractable content.

[0052] Cyclic dimer content: The liquid obtained after the hot water extractable content test was used to perform qualitative and quantitative analysis of cyclic dimers in the sample using a Shimadzu LC-16 high performance liquid chromatograph (HPLC) equipped with a WondaSil C18-WR (200 mm, 5 μm packed particle size) column and a UV detector. The detection wavelength was 210 nm, the detection temperature was 40 °C, and a binary gradient test method was used. The mobile phases were methanol and water, and the test method is shown in the table below.

[0053]

[0054] MI at 260℃ and 2160g load: Refer to GB / T 3682.1-2018 Plastics Thermoplastics Determination of melt mass flow rate and melt volume flow rate.

[0055] Example 1

[0056] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0057] (1) Preparation of raw materials;

[0058] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0059] water;

[0060] Protective gas: nitrogen or inert gas;

[0061] Branching agent: pyromellitic acid;

[0062] (2) Preparation of oligomeric polyamide 6;

[0063] Waste polyamide 6 was mixed with water (the calculated mass ratio of waste polyamide 6 to water was 15.9:1; assuming the volume of the reaction vessel was 1000 mL, the mass of waste PA6 was 27.5 g) in a reaction vessel. The mixture was then hydrolyzed at 240 °C for 1 h under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 100 rpm.

[0064] The number-average molecular weight of the obtained oligomeric polyamide 6 was 5000 g / mol;

[0065] (3) Preparation of high-flowability recycled polyamide 6;

[0066] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 270°C for 4 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 1% of the mass of the waste polyamide 6, and the stirring speed was 1000 rpm.

[0067] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 60 Pa within 10 min, and continue the reaction until the relative viscosity of the system is 3, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 1200 rpm.

[0068] The final high-flowability recycled polyamide 6 has a melting point of 222℃, a number-average molecular weight of 30000 g / mol, a PDI of 1.6, a hot water extractability of 1 wt%, a cyclic dimer content of 0.1 wt%, and an MI of 40 g / 10 min at 260℃ and a load of 2160 g.

[0069] Comparative Example 1

[0070] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 is basically the same as in Example 1, except that the hydrolysis reaction time in step (2) is 20 min.

[0071] The number-average molecular weight of the oligomeric polyamide 6 obtained in step (2) is 8000 g / mol.

[0072] The final high-flowability recycled polyamide 6 had a melting point of 219℃, a number-average molecular weight of 28000 g / mol, a PDI of 1.7, a hot water extractability of 1.2 wt%, a cyclic dimer content of 0.7 wt%, and an MI of 38 g / 10 min at 260℃ and a load of 2160 g.

[0073] Comparing Comparative Example 1 and Example 1, it can be seen that the PDI, hot water extractability, and cyclic dimer content of the high-flowability recycled polyamide 6 obtained in Comparative Example 1 are all higher than those in Example 1, while the number-average molecular weight is lower. This is because the molecular weight of the oligomeric polyamide 6 obtained in Comparative Example 1 is too high, resulting in excessively high melt viscosity and poor devolatilization. Consequently, the cyclic dimer content of the recycled polyamide 6 obtained is higher than that in Example 1. At the same time, the reactivity of the end groups is reduced, which is not conducive to the contact and reaction between the branching agent and the end groups. Therefore, the branching effect is poor. The small number of molecular chains that have undergone branching reaction and the unreacted molecular chains result in a slightly higher PDI of the recycled polyamide 6 compared to Example 1. The high molecular weight of the oligomeric polyamide 6 means that under the same polymerization conditions, Comparative Example 1 will reach a high molecular weight earlier. Subsequently, as the reaction time increases, side reactions such as end group biting occur for a longer period of time, resulting in an increase in the content of hot water extractables and a decrease in the number-average molecular weight.

[0074] Comparative Example 2

[0075] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 is basically the same as in Example 1, except that the hydrolysis reaction time in step (2) is 5 hours.

[0076] The number-average molecular weight of the oligomeric polyamide 6 obtained in step (2) is 1000 g / mol.

[0077] The final high-flowability recycled polyamide 6 had a melting point of 218℃, a number-average molecular weight of 14000 g / mol, a PDI of 1.90, a hot water extractability of 3.0 wt%, a cyclic dimer content of 0.9 wt%, and an MI of 48 g / 10 min at 260℃ and a load of 2160 g.

[0078] Comparing Comparative Example 2 and Example 1, it can be seen that the PDI, hot water extractability, and cyclic dimer content of the high-flowability recycled polyamide 6 obtained in Comparative Example 2 are all higher than those in Example 1, while the number-average molecular weight is lower. This is because the molecular weight of the oligomeric polyamide 6 obtained in Comparative Example 2 is too low. Although the branching agent reacts well with the end groups and can quickly complete the branching reaction of polyamide 6, it also leads to more oligomeric polyamide 6 becoming branched chains, making it impossible for the oligomeric polyamide 6 chains to react with each other. In the vacuum stage, sufficient chain growth cannot be achieved, resulting in a lower number-average molecular weight of the obtained high-flowability recycled polyamide 6. The limited free oligomeric polyamide 6 can only achieve chain growth of partially branched chains, resulting in a larger PDI of the obtained high-flowability recycled polyamide 6. Although the lower molecular weight leads to a decrease in viscosity and good flowability, which is beneficial to the devolatilization effect, the obtained high-flowability recycled polyamide 6 itself has poor performance and contains more hot water extractables, which cannot improve the hot water extractability of the high-flowability recycled polyamide 6. When the molecular weight of the oligomeric polyamide is lower, it is more likely to undergo side reactions such as end group biting and cyclization to generate cyclic oligomers, resulting in a higher cyclic dimer content in the high-flowability recycled polyamide 6 compared to Example 1.

[0079] Comparative Example 3

[0080] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 is basically the same as in Example 1, except that: in step (2), the mass ratio of waste polyamide 6 to water is 1:1 (that is, under the assumption that the reaction vessel is 1000mL, the mass of water and the mass of waste PA6 are both 27.5g). After obtaining oligomeric polyamide 6, it is filtered and dried.

[0081] The final high-flowability recycled polyamide 6 has a melting point of 220℃, a number-average molecular weight of 26000 g / mol, a PDI of 2.0, a hot water extractability of 2.9 wt%, a cyclic dimer content of 0.9 wt%, and an MI of 40 g / 10 min at 260℃ and a load of 2160 g.

[0082] Comparing Comparative Example 3 and Example 1, it can be seen that because the amount of water added in Comparative Example 3 does not meet the formula, the excess water causes step (2) to require additional filtration and drying before step (3). The resulting high-flowability recycled polyamide 6 has a lower number-average molecular weight, a higher PDI, and higher hot water extractability and cyclic dimer content. This is because the amount of water added in Comparative Example 3 is larger, the hydrolysis reaction is faster, and the waste polyamide 6 will enter the stage of hydrolysis of oligomeric polyamide 6 into caprolactam more quickly. More polyamide 6 components become aqueous phase products. More water molecules increase the probability of nucleophilic attack, and the number of break points of each molecular chain is more random. Macroscopically, this is manifested as a larger PDI in the solid phase product. During the branching reaction, the smaller molecular weight fraction reacts more readily with the branching agent, while the larger molecular weight fraction has poor end-group reactivity and is more difficult to attach to the branched molecular chain. This results in a lower number-average molecular weight compared to Example 1, and a higher PDI. In addition, the oligomeric polyamide 6 that has not reacted to the branched molecular chain undergoes side reactions such as end-group biting, generating oligomers with even smaller molecular weights. This leads to a higher hot water extractability and cyclic dimer content, as well as a higher PDI. The low molecular weight fraction of the recycled polyamide 6 acts as a plasticizer, so the MI of Comparative Example 3 is comparable to that of Example 1. However, the lower molecular weight fraction of the recycled polyamide 6 becomes a "weak point" during stress, causing a decrease in the mechanical properties of the finished material.

[0083] Example 2

[0084] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0085] (1) Preparation of raw materials;

[0086] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0087] water;

[0088] Protective gas: nitrogen or inert gas;

[0089] Branching agent: pyromellitic acid chloride;

[0090] (2) Preparation of oligomeric polyamide 6;

[0091] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 243°C for 1.4 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 130 rpm.

[0092] The number-average molecular weight of the obtained oligomeric polyamide 6 was 4600 g / mol;

[0093] (3) Preparation of high-flowability recycled polyamide 6;

[0094] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 270°C for 4 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 1% of the mass of the waste polyamide 6, and the stirring speed was 1000 rpm.

[0095] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 60 Pa within 10 min, and continue the reaction until the relative viscosity of the system is 2.8, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 1200 rpm.

[0096] The final high-flowability recycled polyamide 6 had a melting point of 221℃, a number-average molecular weight of 28000 g / mol, a PDI of 1.62, a hot water extractability of 1 wt%, a cyclic dimer content of 0.1 wt%, and an MI of 41 g / 10 min at 260℃ and a load of 2160 g.

[0097] Example 3

[0098] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0099] (1) Preparation of raw materials;

[0100] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0101] water;

[0102] Protective gas: nitrogen or inert gas;

[0103] Branching agent: diphenylmethane triisocyanate;

[0104] (2) Preparation of oligomeric polyamide 6;

[0105] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 245°C for 1.6 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 150 rpm.

[0106] The number-average molecular weight of the obtained oligomeric polyamide 6 was 4400 g / mol;

[0107] (3) Preparation of high-flowability recycled polyamide 6;

[0108] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 268°C for 4 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 2% of the mass of the waste polyamide 6, and the stirring speed was 900 rpm.

[0109] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 70 Pa within 15 min, and continue the reaction until the relative viscosity of the system is 2.6, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 1100 rpm.

[0110] The final high-flowability recycled polyamide 6 has a melting point of 220℃, a number-average molecular weight of 26000 g / mol, a PDI of 1.63, a hot water extractability of 1 wt%, a cyclic dimer content of 0.2 wt%, and an MI of 41 g / 10 min at 260℃ and a load of 2160 g.

[0111] Example 4

[0112] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0113] (1) Preparation of raw materials;

[0114] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0115] water;

[0116] Protective gas: nitrogen or inert gas;

[0117] Branching agent: Triaminoethylamine;

[0118] (2) Preparation of oligomeric polyamide 6;

[0119] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 247°C for 1.8 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 170 rpm.

[0120] The number-average molecular weight of the obtained oligomeric polyamide 6 was 4000 g / mol;

[0121] (3) Preparation of high-flowability recycled polyamide 6;

[0122] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 268°C for 3 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 2% of the mass of the waste polyamide 6, and the stirring speed was 850 rpm.

[0123] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 70 Pa within 15 min, and continue the reaction until the relative viscosity of the system is 2.4, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 1100 rpm.

[0124] The final high-flowability recycled polyamide 6 has a melting point of 220℃, a number-average molecular weight of 22000 g / mol, a PDI of 1.65, a hot water extractability of 1.1 wt%, a cyclic dimer content of 0.2 wt%, and an MI of 42 g / 10 min at 260℃ and a load of 2160 g.

[0125] Example 5

[0126] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0127] (1) Preparation of raw materials;

[0128] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0129] water;

[0130] Protective gas: nitrogen or inert gas;

[0131] Branching agent: Tetracarboxylic acid cyclohexanone;

[0132] (2) Preparation of oligomeric polyamide 6;

[0133] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 250°C for 2 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 200 rpm.

[0134] The number-average molecular weight of the obtained oligomeric polyamide 6 was 3600 g / mol;

[0135] (3) Preparation of high-flowability recycled polyamide 6;

[0136] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 265°C for 3 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 3% of the mass of the waste polyamide 6, and the stirring speed was 800 rpm.

[0137] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 80 Pa within 20 min, and continue the reaction until the relative viscosity of the system is 2.4, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 1000 rpm.

[0138] The final high-flowability recycled polyamide 6 has a melting point of 220℃, a number-average molecular weight of 22000 g / mol, a PDI of 1.65, a hot water extractability of 1.1 wt%, a cyclic dimer content of 0.3 wt%, and an MI of 42 g / 10 min at 260℃ and a load of 2160 g.

[0139] Example 6

[0140] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0141] (1) Preparation of raw materials;

[0142] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0143] water;

[0144] Protective gas: nitrogen or inert gas;

[0145] Branching agent: ethylenediaminetetraacetic acid;

[0146] (2) Preparation of oligomeric polyamide 6;

[0147] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 253°C for 2.4 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 230 rpm.

[0148] The number-average molecular weight of the obtained oligomeric polyamide 6 was 3200 g / mol;

[0149] (3) Preparation of high-flowability recycled polyamide 6;

[0150] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 263°C for 3 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 3% of the mass of the waste polyamide 6, and the stirring speed was 700 rpm.

[0151] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 80 Pa within 25 min, and continue the reaction until the relative viscosity of the system is 2.3, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 900 rpm.

[0152] The final high-flowability recycled polyamide 6 has a melting point of 220℃, a number-average molecular weight of 20000 g / mol, a PDI of 1.66, a hot water extractability of 1.1 wt%, a cyclic dimer content of 0.3 wt%, and an MI of 43 g / 10 min at 260℃ and a load of 2160 g.

[0153] Example 7

[0154] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0155] (1) Preparation of raw materials;

[0156] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0157] water;

[0158] Protective gas: nitrogen or inert gas;

[0159] Branching agent: star-shaped polyethyleneimine;

[0160] (2) Preparation of oligomeric polyamide 6;

[0161] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 255°C for 2.6 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 250 rpm.

[0162] The number-average molecular weight of the obtained oligomeric polyamide 6 was 3000 g / mol;

[0163] (3) Preparation of high-flowability recycled polyamide 6;

[0164] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture was reacted at 263°C for 2 hours under stirring and protective gas conditions; wherein the mass of the branching agent was 4% of the mass of the waste polyamide 6, and the stirring speed was 750 rpm.

[0165] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 90 Pa within 25 min, and continue the reaction until the relative viscosity of the system is 2.2, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 900 rpm.

[0166] The final high-flowability recycled polyamide 6 had a melting point of 220℃, a number-average molecular weight of 18000 g / mol, a PDI of 1.67, a hot water extractability of 1.2 wt%, a cyclic dimer content of 0.4 wt%, and an MI of 44 g / 10 min at 260℃ and a load of 2160 g.

[0167] Example 8

[0168] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0169] (1) Preparation of raw materials;

[0170] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0171] water;

[0172] Protective gas: nitrogen or inert gas;

[0173] Branching agent: a mixture of pyromellitic acid and pyromellitic chloride in a mass ratio of 1:1;

[0174] (2) Preparation of oligomeric polyamide 6;

[0175] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 257°C for 2.8 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 270 rpm.

[0176] The number-average molecular weight of the obtained oligomeric polyamide 6 was 2400 g / mol;

[0177] (3) Preparation of high-flowability recycled polyamide 6;

[0178] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture is reacted at 260°C for 2 hours under stirring and protective gas conditions; wherein the mass of the branching agent is 5% of the mass of the waste polyamide 6, and the stirring speed is 600 rpm.

[0179] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 90 Pa within 30 min, and continue the reaction until the relative viscosity of the system is 2.1, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 800 rpm.

[0180] The final high-flowability recycled polyamide 6 had a melting point of 219℃, a number-average molecular weight of 16000 g / mol, a PDI of 1.68, a hot water extractability of 1.2 wt%, a cyclic dimer content of 0.4 wt%, and an MI of 45 g / 10 min at 260℃ and a load of 2160 g.

[0181] Example 9

[0182] A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, the specific steps of which are as follows:

[0183] (1) Preparation of raw materials;

[0184] Waste polyamide 6: average degree of polymerization is 142, number average molecular weight is 16000 g / mol;

[0185] water;

[0186] Protective gas: nitrogen or inert gas;

[0187] Branching agent: Lysine;

[0188] (2) Preparation of oligomeric polyamide 6;

[0189] Waste polyamide 6 was mixed with water in a reaction vessel and then hydrolyzed at 260°C for 3 hours under stirring and protective gas conditions. The pressure was then released to an absolute pressure of 101 kPa to obtain oligomeric polyamide 6. The stirring speed was 300 rpm.

[0190] The number-average molecular weight of the obtained oligomeric polyamide 6 was 2000 g / mol;

[0191] (3) Preparation of high-flowability recycled polyamide 6;

[0192] (3.1) After mixing oligomeric polyamide 6 with the branching agent, the mixture is reacted at 260°C for 2 hours under stirring and protective gas conditions; wherein the mass of the branching agent is 5% of the mass of the waste polyamide 6, and the stirring speed is 600 rpm.

[0193] (3.2) Keep the temperature constant, and under stirring conditions, evacuate to 100 Pa within 30 min, and continue the reaction until the relative viscosity of the system is 2, thus obtaining high-flowability recycled polyamide 6; wherein, the stirring speed is 800 rpm.

[0194] The final high-flowability recycled polyamide 6 had a melting point of 218℃, a number-average molecular weight of 14000 g / mol, a PDI of 1.7, a hot water extractability of 1.2 wt%, a cyclic dimer content of 0.5 wt%, and an MI of 45 g / 10 min at 260℃ and a load of 2160 g.

Claims

1. A method for preparing high-flowability recycled polyamide 6 from waste polyamide 6, characterized in that, Waste polyamide 6 is hydrolyzed to obtain oligomeric polyamide 6. Then, the oligomeric polyamide 6 is subjected to a branching reaction with a branching agent to obtain high-flowability recycled polyamide 6. The formula for calculating the total amount of water added during the hydrolysis treatment of waste polyamide 6 is as follows: ； ； ； In the formula, The total amount of water added, in grams; m(H2O) is the mass of water to be added according to the chemical reaction equilibrium, in g; m(gas H2O) is the mass of gaseous water in the reaction vessel under hydrolysis conditions, in g; m(PA6) is the mass of waste polyamide 6, in g. M n (PA6) is the number-average molecular weight of waste polyamide 6, g / mol; N is the average degree of polymerization of waste polyamide 6; x is the theoretical average degree of polymerization of oligomeric polyamide 6; ρ T (gas H2O) is the density of gaseous water at the hydrolysis reaction temperature, in g / cm³. 3 V0 is the volume of the reaction vessel, in cm³. 3 ; The density of polyamide 6 at the hydrolysis reaction temperature, in g / cm³. 3 ; The density of liquid water at the hydrolysis reaction temperature, in g / cm³. 3 ; The number average molecular weight of oligomeric polyamide 6 is 2000~5000 g / mol; The branching agent is one or more of the following: pyromellitic acid, pyromellitic trimethylol chloride, diphenylmethane triisocyanate, triaminoethylamine, tetracarboxylic acid cyclohexanone, ethylenediaminetetraacetic acid, star-shaped polyethyleneimine, and lysine. The high-flowability recycled polyamide 6 has a melting point of 218~222℃, a number-average molecular weight of 14000~30000 g / mol, a PDI of 1.60~1.70, a hot water extractability of 1.0~1.2wt%, a cyclic dimer content of 0.1~0.5wt%, and an MI of 40~45 g / 10 min at 260℃ and a load of 2160 g.

2. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 1, characterized in that, The specific process of hydrolyzing waste polyamide 6 to obtain oligomeric polyamide 6 is as follows: waste polyamide 6 is mixed with water in a reaction vessel, and then hydrolyzed at 240~260℃ for 1~3 hours under stirring and nitrogen or inert gas protection. After that, the pressure is released to an absolute pressure of 101KPa to obtain oligomeric polyamide 6.

3. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 2, characterized in that, The stirring speed is 100~300 rpm.

4. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 1, characterized in that, The mass of the branching agent is 1-5% of the mass of the waste polyamide 6.

5. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 1, characterized in that, The specific steps for the branching reaction of oligomeric polyamide 6 with a branching agent are as follows: (a) After mixing oligomeric polyamide 6 with the branching agent, react at 260~270℃ for 2~4h under stirring and nitrogen or inert gas protection; (b) Keep the temperature constant, and under stirring conditions, evacuate to less than 100 Pa within 10 to 30 minutes, and continue the reaction until the relative viscosity of the system is 2.0 to 3.0, thus obtaining high-flowability recycled polyamide 6.

6. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 5, characterized in that, In step (a), the stirring speed is 600~1000 rpm.

7. The method for preparing high-flowability recycled polyamide 6 from waste polyamide 6 according to claim 5, characterized in that, In step (b), the stirring speed is 800~1200 rpm.

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