Nylon with low molecular weight and its preparation method

Abstract

This invention relates to the field of polyamide resin technology, and in particular to a low-molecular-weight nylon and its preparation method. The method for preparing low-molecular-weight nylon includes the following steps: polymerizing a concentrated polyamide salt solution to obtain nylon; in the polymerization reaction, a catalyst is added to the reaction system in the later stage of polymerization; the catalyst includes a main catalyst, which is selected from at least one of organophosphorus nitrile bases, rare earth metal complexes, and alkylaluminoxanes. The nylon preparation method of this invention not only effectively consumes cyclic small molecules and significantly reduces their content, but also enables the reuse of cyclic small molecules, further increasing the molecular weight of the nylon.

Description

Technical Field

[0001] This invention relates to the field of polyamide resin technology, and in particular to a nylon with low molecular weight and its preparation method. Background Technology

[0002] During nylon polymerization, the formation of cyclic dimers or trimers is common. Because these cyclic molecules have high ring-opening activation energies, they are difficult to participate in the reaction, hindering the polymerization process and making it difficult to further increase the molecular weight of the product. Furthermore, these cyclic molecules remain in the product. These residual cyclic molecules can cause problems such as precipitation, blooming, and excessive VOC content in the finished nylon product, thereby impairing its mechanical and processing properties and limiting its application areas.

[0003] To address the aforementioned problems, several solutions have been proposed in the prior art. For example, Chinese patent application CN120230282A utilizes metal ions in a cyclic oligomer inhibitor to coordinate with the amide bonds of polyamide 6, preventing the amide bonds from being attacked and cyclized during polymerization, thus reducing the formation of cyclic oligomers and consequently lowering the content of cyclic oligomers in the polymer. However, this preparation process differs significantly from conventional polyamide production processes, making it unsuitable for industrialization. Another example is Chinese patent application CN1225954A, which processes the polymer melt into slices and then extracts the caprolactam monomer and oligomers from the slices using water. While this method is currently the mainstream approach in nylon preparation, it has significant drawbacks: firstly, it requires sophisticated equipment; secondly, the yellow index of the processed product increases, and its moisture content approaches saturation, necessitating further drying before use.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide nylon with low small molecule content and its preparation method. The preparation method of this invention not only effectively consumes cyclic small molecules and significantly reduces their content, but also enables the reuse of cyclic small molecules, further increasing the molecular weight of nylon.

[0006] To achieve the above-mentioned objective of the present invention, the first aspect of the present invention provides a method for preparing nylon with low molecular weight content, comprising the following steps: polymerizing a polyamide salt concentrate to obtain nylon; In the polymerization reaction, a catalyst is added to the reaction system in the later stage of polymerization; the catalyst includes a main catalyst, which is selected from at least one of organophosphorus nitrile bases, rare earth metal complexes and alkylaluminoxanes.

[0007] Furthermore, the amount of the main catalyst added is 0.01 wt% to 0.12 wt% of the total amount of nylon, preferably 0.02 wt% to 0.1 wt%.

[0008] Furthermore, the organophosphazene base includes at least one of phosphazene ligand P1-tert-butyl, phosphazene ligand P2-tert-butyl, and phosphazene ligand P4-tert-butyl.

[0009] Furthermore, the rare earth metal complex includes at least one of tris(cyclopentadienyl)scandium and tris(cyclopentadienyl)yttrium.

[0010] Furthermore, the alkylaluminoxane includes at least one of methylaluminoxane and modified methylaluminoxane.

[0011] Furthermore, the catalyst also includes a co-catalyst; the co-catalyst comprises straight-chain or branched alcohols having 2 to 10 carbon atoms.

[0012] Furthermore, the boiling point of the alcohol is 120~240℃.

[0013] Furthermore, the co-catalyst includes at least one of propylene glycol, ethylene glycol, and dipropylene glycol.

[0014] Furthermore, the amount of the co-catalyst added is 0.05 wt% to 0.4 wt% of the total amount of nylon, preferably 0.1 wt% to 0.3 wt%.

[0015] Furthermore, in the later stage of polymerization, when the pressure is reduced to 1 / 5 to 1 / 3 of the holding pressure, the catalyst is added to the reaction system.

[0016] Furthermore, the polymerization reaction includes a sequentially connected heating and pressurizing stage and a pressurizing stage, a first-stage depressurization stage, a catalytic reaction stage, a second-stage depressurization stage, a post-polymerization reaction stage, and a discharge stage; the catalyst is added during the catalytic reaction stage.

[0017] Furthermore, the heating and pressurization stage and the pressure holding stage include: heating at a rate of 2~10℃ / min, monitoring the pressure until the pressure stabilizes at P1±0.1 MPa, where P1=1.6~2.0 MPa; maintaining the pressure at P1±0.1 MPa, continuing to heat until the temperature reaches T1, and controlling the pressure holding time to be 45~90 min, where T1=260~280℃.

[0018] Furthermore, the first-stage pressure relief includes: when the temperature reaches T1, opening the pressure relief valve to perform the first-stage pressure relief operation; when the pressure drops to P2, closing the pressure relief valve, where P2 = 0.35~0.55 MPa.

[0019] Furthermore, the catalytic reaction stage includes: maintaining the pressure at P2, the temperature at T1±5℃, and reacting for 15~35 min after adding the catalyst.

[0020] Furthermore, the second-stage depressurization includes: opening the pressure relief valve to perform a second-stage depressurization operation until the pressure drops to atmospheric pressure.

[0021] Furthermore, the post-polymerization reaction and discharge stage includes devolatilization for 20-40 minutes under pressure P3 and temperature T2, followed by discharge through a bottom valve, water cooling, and pelletizing to obtain nylon 66 resin, with P3 = -0.050~-0.070MPa and T2 = 280~300℃.

[0022] Furthermore, the polyamide salt concentrate is mainly obtained by concentrating a polyamide salt solution.

[0023] Furthermore, the solid content of the polyamide salt solution is 45 wt% to 65 wt%.

[0024] Furthermore, the solid content of the polyamide salt concentrate is 75 wt% to 85 wt%.

[0025] Furthermore, the polyamide salt solution includes a polyamide 66 salt solution.

[0026] Furthermore, the polyamide salt solution also includes additives.

[0027] Furthermore, the adjuvant includes sodium hypophosphite.

[0028] The second aspect of the present invention provides a nylon with low molecular weight content, the raw materials for preparation including a polyamide salt and a main catalyst, wherein the main catalyst includes at least one of an organophosphorus nitrile base, a rare earth metal complex and an alkylaluminoxane, and is prepared by the method for preparing nylon with low molecular weight content provided in the first aspect of the present invention.

[0029] Furthermore, the content of substances with a molecular weight lower than 1048 g / mol in the nylon is ≤810 mg / kg.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: The nylon preparation method of this invention not only effectively consumes cyclic small molecules and significantly reduces their content, but also enables the reuse of cyclic small molecules, further increasing the molecular weight of nylon. Furthermore, the nylon preparation method of this invention eliminates the need for traditional post-processing steps such as prolonged high-temperature hydrolysis or hot water extraction, thereby effectively avoiding the degradation and oxidation of nylon molecular chains. This results in a bright white nylon product with a significantly reduced yellow index, expanding the application prospects of nylon products in high-end fibers, food packaging, and light-colored engineering plastics. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0032] The polymerization reaction of polyamide salt concentrate includes a sequentially connected heating and pressurization stage, a pressure holding stage, a pressure release stage, a post-polymerization reaction stage, and a discharge stage. The later stage of polymerization corresponds to the pressure release and post-polymerization reaction stage.

[0033] In this invention, the total amount of nylon refers to the theoretical output of nylon in the final production, i.e., the theoretical output.

[0034] The first aspect of the present invention provides a method for preparing nylon with low molecular weight, comprising the following steps: polymerizing a polyamide salt concentrate to obtain nylon; In the polymerization reaction, a catalyst is added to the reaction system in the later stage of polymerization; the catalyst includes a main catalyst, which is selected from at least one of organophosphorus nitrile bases, rare earth metal complexes and alkylaluminoxanes.

[0035] In the later stages of nylon polycondensation, the oligomers such as cyclic dimers and trimers in the system are in thermodynamic equilibrium with the linear polymer. This invention utilizes the characteristic that the amide bond ring strain of cyclic dimers and trimers in nylon is higher than that of the linear backbone. In the later stages of polymerization, a catalyst is added to the reaction system that can selectively activate the amide bonds in the cyclic small molecules without damaging the linear nylon backbone. This catalyzes the ring-opening of the cyclic small molecules, exposing terminal amino and carboxyl groups, allowing them to re-participate in the polycondensation reaction and attach to the linear backbone, thereby shifting the equilibrium towards the formation of a higher molecular weight linear polymer. In summary, the preparation method of this invention not only effectively consumes cyclic small molecules and significantly reduces their content, but also enables the reuse of cyclic small molecules, further increasing the molecular weight of nylon.

[0036] Furthermore, the preparation method of this invention does not require traditional post-processing steps such as prolonged high-temperature hydrolysis or hot water extraction, thereby effectively avoiding the degradation and oxidation of nylon molecular chains. This results in nylon products with a bright white color and a significantly reduced yellow index, expanding the application prospects of nylon products in high-end fibers, food packaging, and light-colored engineering plastics.

[0037] The nylon preparation method of this invention is well compatible with existing nylon production lines, requiring no equipment modification. Furthermore, the preparation method of this invention does not generate any toxic or harmful byproducts, easily meeting the comprehensive requirements of cost, environmental protection, and efficiency for large-scale industrial production.

[0038] In some embodiments, the amount of the main catalyst added is 0.01 wt% to 0.12 wt% of the total nylon, specifically within the range of 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.12 wt%, or any combination thereof, preferably 0.02 wt% to 0.1 wt%. This invention has experimentally verified that controlling the amount of the main catalyst within the above range achieves a good balance between the physical properties of the nylon product and the content of cyclic small molecules. If the amount of the main catalyst is too low, the effect on reducing the content of cyclic small molecules in the nylon product is limited; if the amount of the main catalyst is too high, it will lead to a decrease in the physical properties of the nylon product.

[0039] In some embodiments, the organophosphazene base includes at least one of phosphazene ligand P1-tert-butyl, phosphazene ligand P2-tert-butyl, and phosphazene ligand P4-tert-butyl.

[0040] In some embodiments, the rare earth metal complex includes at least one of scandium trifluoromethanesulfonate, tris(cyclopentadienyl)scandium, and tris(cyclopentadienyl)yttrium.

[0041] In some embodiments, the alkylaluminoxane includes at least one of methylaluminoxane (MAO) and modified methylaluminoxane (MMAO).

[0042] In some embodiments, the catalyst further includes a co-catalyst; the co-catalyst includes straight-chain or branched alcohols with 2 to 10 carbon atoms, specifically 2, 3, 4, 6, 8, 10, etc.

[0043] This invention introduces a co-catalyst into the reaction system simultaneously during the later stages of polymerization, further reducing the content of cyclic small molecules in the resulting nylon product. The reason for this effect may be that, by adding the co-catalyst, it can act as a proton donor or nucleophile, promoting the reaction towards ring-opening, thereby breaking the cyclic structure and incorporating it into the linear backbone; additionally, it introduces additional end groups into the system, accelerating the polycondensation reaction between the ring-opening product and the linear backbone. In other words, the "selective activation" effect of the co-catalyst and the main catalyst works synergistically, jointly shifting the reaction equilibrium towards the formation of higher molecular weight linear nylon, reducing the oligomer content in the final product.

[0044] In some embodiments, the boiling point of the alcohol is 120~240°C.

[0045] Using alcohols with boiling points that meet the above conditions as co-catalysts helps to further increase the molecular weight of nylon. The main reason is that if the boiling point is too low, it will affect the actual concentration of the substance in the molten system during the preparation of polyamide, and it will not be able to continuously play its role as a co-catalyst; while if the boiling point is too high, it will be difficult to completely remove it in the subsequent vacuum devolatilization operation, thus affecting the performance of the polymer.

[0046] In some embodiments, the cocatalyst includes at least one of propylene glycol, ethylene glycol, and dipropylene glycol.

[0047] In some embodiments, the amount of the co-catalyst added is 0.05 wt% to 0.4 wt% of the total nylon, specifically within the range of 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, or any combination thereof, preferably 0.1 wt% to 0.3 wt%. This invention has experimentally verified that controlling the amount of co-catalyst within the above range achieves a good balance between the molecular weight, cyclic small molecule content, and performance of the nylon product. If the amount of co-catalyst is too low, the effect on reducing the cyclic small molecule content and increasing the molecular weight in the nylon product is not significant; if the amount of co-catalyst is too high, it will cause a decrease in the strength of the nylon product.

[0048] In some embodiments, the catalyst is added to the reaction system during the later stages of polymerization, when the pressure is reduced to 1 / 5 to 1 / 3 of the holding pressure.

[0049] In some embodiments, the polymerization reaction includes a sequentially connected heating and pressurization stage and a pressure holding stage, a first-stage depressurization stage, a catalytic reaction stage, a second-stage depressurization stage, a post-polymerization reaction stage, and a discharge stage; the catalyst is added during the catalytic reaction stage.

[0050] Inserting a catalytic reaction stage into the conventional depressurization phase, utilizing the high temperature and high shear reaction environment of the depressurization stage, is more conducive to the catalyst fully exerting its catalytic effect on the ring-opening reaction of cyclic small molecules, thereby further reducing the content of cyclic small molecules. Furthermore, adding the catalyst when the pressure is reduced to 1 / 5 to 1 / 3 of the holding pressure further helps to shift the equilibrium towards the formation of linear polymers with higher molecular weights. If the pressure is too high, the molecular weight of the linear polymer has not yet increased significantly, and the difference in ring strain between the linear polymer and the amide bond of the cyclic small molecules is small, resulting in relatively low selectivity for cyclic small molecules; if the pressure is too low, the polymer viscosity increases, the flowability deteriorates, and it is not conducive to the dispersion of the catalyst within it.

[0051] In some embodiments, the heating and pressurizing stage and the pressure holding stage include: heating at a rate of 2~10℃ / min, monitoring the pressure until the pressure stabilizes at P1±0.1MPa, where P1=1.6~2.0 MPa; maintaining the pressure at P1±0.1 MPa, continuing to heat until the temperature reaches T1, and controlling the pressure holding time to be 45~90 min, where T1=260~280℃. In different embodiments, the heating rate can be a range of 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, or any combination thereof; P1 can be a range of 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, 2.0 MPa, or any combination thereof; T1 can be a range of 260℃, 265℃, 270℃, 275℃, 280℃, or any combination thereof; and the holding time can be a range of 45 min, 50 min, 60 min, 70 min, 80 min, 90 min, or any combination thereof.

[0052] In some embodiments, the first-stage pressure relief includes: opening the pressure relief valve to perform a first-stage pressure relief operation when the temperature reaches T1, and closing the pressure relief valve when the pressure drops to P2, where P2 = 0.35~0.55 MPa. In different embodiments, P2 can be a range of 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa, 0.55 MPa, or any combination thereof.

[0053] In some embodiments, the catalytic reaction stage includes: maintaining a pressure of P2 and a temperature of T1±5°C, and reacting for 15~35 min after adding the catalyst. In different embodiments, the reaction time for the catalytic reaction stage can be a range of 15 min, 20 min, 25 min, 30 min, 35 min, or any combination thereof.

[0054] In some embodiments, the two-stage depressurization includes: opening the pressure relief valve to perform a two-stage depressurization operation until the pressure drops to atmospheric pressure.

[0055] In some embodiments, the post-polymerization reaction and discharge stage includes devolatilization for 20-40 minutes at pressure P3 and temperature T2, followed by discharge through a bottom valve, water cooling, and pelletizing to obtain nylon 66 resin. P3 = -0.050~-0.070 MPa, T2 = 280~300℃. In different embodiments, P3 can be a range of -0.050 MPa, -0.055 MPa, -0.060 MPa, -0.065 MPa, -0.070 MPa, or any combination thereof; T2 can be a range of 280℃, 285℃, 290℃, 295℃, 300℃, or any combination thereof; and the devolatilization time can be a range of 20 min, 25 min, 30 min, 35 min, 40 min, or any combination thereof.

[0056] In some embodiments, the polyamide salt concentrate is mainly obtained by concentrating a polyamide salt solution.

[0057] In some embodiments, the solid content of the polyamide salt solution is 45 wt% to 65 wt%, specifically a range of 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, or any combination thereof.

[0058] In some embodiments, the solid content of the polyamide salt concentrate is 75 wt% to 85 wt%, specifically it can be a range of 75 wt%, 78 wt%, 80 wt%, 82 wt%, 85 wt%, or any two of these.

[0059] In some embodiments, the polyamide salt solution includes, but is not limited to, a polyamide 66 salt solution.

[0060] In some embodiments, the polyamide salt solution also includes additives.

[0061] In some embodiments, the additive includes sodium hypophosphite. Further, the amount of sodium hypophosphite used is 0.001 wt% to 0.005 wt% of the total nylon.

[0062] The second aspect of the present invention provides a nylon with low molecular weight content, the raw materials for preparation including a polyamide salt and a main catalyst, wherein the main catalyst includes at least one of an organophosphorus nitrile base, a rare earth metal complex and an alkylaluminoxane, and is prepared by the method for preparing nylon with low molecular weight content provided in the first aspect of the present invention.

[0063] In some embodiments, the content of substances with a molecular weight of less than 1048 g / mol in the nylon is ≤810 mg / kg, specifically 810 mg / kg, 700 mg / kg, 600 mg / kg, 500 mg / kg, 400 mg / kg, 350 mg / kg, 320 mg / kg, or any combination thereof.

[0064] Example 1 This embodiment provides a method for preparing nylon 66 with low molecular weight, including the following steps: (1) 116 kg hexamethylenediamine, 146 kg adipic acid and 214.36 kg deionized water were mixed and 6 g sodium hypophosphite was added as a catalyst to obtain a polyamide salt solution with a solid content of 55 wt%. The polyamide salt solution was added to a polymerization reactor for heating and concentration to obtain a polyamide salt concentrate with a solid content of 80 wt%. After concentration, the material temperature in the polymerization reactor was 160℃ and the pressure was 0.30 MPa.

[0065] (2) Heat the polymerization reactor containing the polyamide salt concentrate in step (1) at a rate of 2℃ / min until the pressure inside the polymerization reactor rises to 2.0 MPa. At this time, the temperature of the material inside the polymerization reactor is 225℃.

[0066] (3) Continue heating the polymerization reactor from step (2) to raise the temperature of the material inside the polymerization reactor to 280°C within 60 min. During this stage, the pressure inside the polymerization reactor is kept constant at 2.0 MPa.

[0067] (4) Depressurize the polymerization reactor from step (3). Within 25 minutes, the pressure inside the polymerization reactor is uniformly reduced from 2.0 MPa to 0.45 MPa. At this time, the temperature of the material inside the polymerization reactor is approximately 285°C. Stop depressurizing and add phosphazene ligand P2-tert-butyl and dipropylene glycol to the polymerization reactor. Maintain the pressure at 0.45 MPa (by adding a small amount of nitrogen and depressurizing a small amount) and 285°C. After stirring the reaction for 25 minutes, continue depressurizing. Within 25 minutes, the pressure inside the polymerization reactor is uniformly reduced from 0.45 MPa to atmospheric pressure. The amounts of phosphazene ligand P2-tert-butyl and dipropylene glycol added are 0.05 wt% and 0.12 wt% of the theoretical output, respectively.

[0068] (5) Vacuum the polymerization reactor in step (4), control the pressure inside the polymerization reactor to -0.070 MPa, and the material temperature inside the polymerization reactor to 300℃, and carry out post-polymerization for 30 min.

[0069] (6) The melt obtained in step (5) is discharged through the bottom valve, and water-cooled and pelletized at around 20°C to obtain Nylon 66.

[0070] Example 2 This embodiment provides a method for preparing nylon 66 with low molecular weight, including the following steps: (1) 116 kg hexamethylenediamine, 146 kg adipic acid and 174.67 kg deionized water were mixed and 6 g sodium hypophosphite was added as a catalyst to obtain a polyamide salt solution with a solid content of 60 wt%. The polyamide salt solution was added to a polymerization reactor for heating and concentration to obtain a polyamide salt concentrate with a solid content of 80 wt%. After concentration, the material temperature in the polymerization reactor was 160℃ and the pressure was 0.30 MPa.

[0071] (2) Heat the polymerization reactor containing the polyamide salt concentrate in step (1) at a rate of 10℃ / min until the pressure inside the polymerization reactor rises to 1.6 MPa. At this time, the temperature of the material inside the polymerization reactor is 200℃.

[0072] (3) Continue heating the polymerization reactor from step (2) to raise the temperature of the material inside the polymerization reactor to 260°C within 60 min. During this stage, the pressure inside the polymerization reactor is kept constant at 1.6 MPa.

[0073] (4) Depressurize the polymerization reactor from step (3). Within 25 minutes, the pressure inside the polymerization reactor is uniformly reduced from 1.6 MPa to 0.35 MPa. At this time, the temperature of the material inside the polymerization reactor is approximately 275°C. Stop depressurizing and add phosphazene ligand P2-tert-butyl and propylene glycol to the polymerization reactor. Maintain the pressure at 0.35 MPa (by adding a small amount of nitrogen and depressurizing a small amount) and 275°C. After stirring the reaction continuously for 30 minutes, continue depressurizing. Within 25 minutes, the pressure inside the polymerization reactor is uniformly reduced from 0.35 MPa to atmospheric pressure. The amounts of phosphazene ligand P2-tert-butyl and propylene glycol added are 0.02 wt% and 0.1 wt% of the theoretical output, respectively.

[0074] (5) Vacuum the polymerization reactor in step (4), control the pressure inside the polymerization reactor to -0.050 MPa, and the material temperature inside the polymerization reactor to 285℃, and carry out post-polymerization for 30 min.

[0075] (6) The melt obtained in step (5) is discharged through the bottom valve, and water-cooled and pelletized at around 20°C to obtain Nylon 66.

[0076] Example 3 This embodiment provides a method for preparing nylon 66 with low molecular weight, including the following steps: (1) 116 kg hexamethylenediamine, 146 kg adipic acid and 262 kg desalinated water were mixed and 6 g sodium hypophosphite was added as a catalyst to obtain a polyamide salt solution with a solid content of 50 wt%. The polyamide salt solution was added to a polymerization reactor and heated and concentrated to obtain a polyamide salt concentrate with a solid content of 80 wt%. After the concentration was completed, the material temperature in the polymerization reactor was 160℃ and the pressure was 0.30 MPa.

[0077] (2) Heat the polymerization reactor containing the polyamide salt concentrate in step (1) at a rate of 2℃ / min until the pressure inside the polymerization reactor rises to 2.0 MPa. At this time, the temperature of the material inside the polymerization reactor is 225℃.

[0078] (3) Continue heating the polymerization reactor from step (2) to raise the temperature of the material inside the polymerization reactor to 270°C within 60 min. During this stage, the pressure inside the polymerization reactor is kept constant at 1.6 MPa.

[0079] (4) Depressurize the polymerization reactor from step (3). Within 25 minutes, the pressure inside the polymerization reactor is reduced from 1.6 MPa to 0.55 MPa at a constant rate. At this time, the temperature of the material inside the polymerization reactor is approximately 280°C. Stop depressurizing and add phosphazene ligand P2-tert-butyl and ethylene glycol to the polymerization reactor. Maintain the pressure at 0.55 MPa (by adding a small amount of nitrogen and depressurizing a small amount) and 280°C. After stirring and reacting for 35 minutes, continue depressurizing. Within 25 minutes, the pressure inside the polymerization reactor is reduced from 0.55 MPa to atmospheric pressure at a constant rate. The amounts of phosphazene ligand P2-tert-butyl and ethylene glycol added are 0.1 wt% and 0.3 wt% of the theoretical output, respectively.

[0080] (5) Vacuum the polymerization reactor in step (4), control the pressure inside the polymerization reactor to -0.060 MPa, and the material temperature inside the polymerization reactor to 280℃, and carry out post-polymerization for 30 min.

[0081] (6) The melt obtained in step (5) is discharged through the bottom valve, and water-cooled and pelletized at around 20°C to obtain Nylon 66.

[0082] Example 4 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 3. The only difference is that in step (4), the phosphazene ligand P1-tert-butyl is used to replace the phosphazene ligand P2-tert-butyl in Example 3 by the same mass. All other aspects are the same as in Example 3.

[0083] Example 5 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that in step (4), the phosphazene ligand P4-tert-butyl is used to replace the phosphazene ligand P2-tert-butyl in Example 1 by the same mass. All other aspects are the same as in Example 1.

[0084] Example 6 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that scandium trifluoromethanesulfonate (CAS No. 144026-79-9) is used in step (4) to replace the phosphazene ligand P2-tert-butyl in Example 1. All other steps are the same as in Example 1.

[0085] Example 7 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that in step (4), methylaluminoxane (CAS: 206451-54-9, toluene solution of methylaluminoxane, mass concentration of 10 wt%; the amount used is calculated based on the amount of methylaluminoxane in the solution) is used to replace the phosphazene ligand P2-tert-butyl in Example 1. All other steps are the same as in Example 1.

[0086] Example 8 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that the amount of phosphazene ligand P2-tert-butyl in step (4) is different. All other aspects are the same as in Example 1.

[0087] In step (4) of this embodiment, the amount of phosphazene ligand P2-tert-butyl added is 0.01 wt% of the theoretical output.

[0088] Example 9 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that the amount of phosphazene ligand P2-tert-butyl in step (4) is different. All other aspects are the same as in Example 1.

[0089] In step (4) of this embodiment, the amount of phosphazene ligand P2-tert-butyl added is 0.12 wt% of the theoretical output.

[0090] Example 10 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1, the only difference being that dipropylene glycol is not added in step (4), and the rest is the same as in Example 1.

[0091] Example 11 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that the amount of dipropylene glycol in step (4) is different. All other steps are the same as in Example 1.

[0092] In step (4) of this embodiment, the amount of dipropylene glycol added is 0.05 wt% of the theoretical output.

[0093] Example 12 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that the amount of dipropylene glycol in step (4) is different. All other steps are the same as in Example 1.

[0094] In step (4) of this embodiment, the amount of dipropylene glycol added is 0.4 wt% of the theoretical output.

[0095] Example 13 This embodiment refers to the preparation method of low molecular weight nylon 66 in Example 1. The only difference is that in step (4), ethanol is used to replace dipropylene glycol in Example 1 by mass. All other steps are the same as in Example 1.

[0096] Comparative Example 1 Comparative Example 1 uses the same method as Example 1 for preparing nylon 66 with low molecular weight, except that the phosphazene ligand P2-tert-butyl and dipropylene glycol were not added in step (4).

[0097] Comparative Example 2 Comparative Example 2 uses the same method as Example 1 for preparing nylon 66 with low molecular weight, except that the phosphazene ligand P2-tert-butyl is not added in step (4), and all other steps are the same as in Example 1.

[0098] Experimental Example The Nylon 66 prepared in different embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0099] (1) Content of small molecules (molecular weight less than 1048 g / mol): Refer to GB / T12006.4 to extract the extractable material from nylon 66, and then use GPC to detect the content of small molecules with molecular weight less than 1048 g / mol in the extractable material, and calculate the content of substances with molecular weight less than 1048 g / mol in nylon 66. (2) Relative viscosity: Tested according to GB / T 12006.1; (3) Tensile strength and elongation at break: Tested according to GB / T 1040.1; (4) Yellow Index: Tested according to GB / T 39822.

[0100] Table 1 Performance Test Results

[0101] The test results above show that the nylon preparation method of the present invention can not only effectively consume cyclic small molecules and significantly reduce their content, but also realize the reuse of cyclic small molecules and further increase the molecular weight of nylon.

[0102] The test results from the examples and Comparative Example 1 show that by adding a catalyst to the reaction system in the later stage of polymerization, the examples can selectively activate the amide bonds in the cyclic small molecules without destroying the linear nylon backbone, thereby catalyzing the ring-opening of the cyclic small molecules, exposing the terminal amino and terminal carboxyl groups, allowing them to re-participate in the polycondensation reaction and attach to the linear backbone, thereby driving the equilibrium towards the formation of a higher molecular weight linear polymer. This not only reduces the content of small molecules but also increases the molecular weight of nylon.

[0103] The test results from Examples and Comparative Example 2 show that using only a co-catalyst without a main catalyst cannot reduce the content of small molecules.

[0104] The test results from Examples 1 and 5-7 show that using organophosphorus nitrile base as the main catalyst is more conducive to reducing the content of small molecules and increasing the molecular weight of nylon.

[0105] The test results from Examples 1 and 8-9 show that controlling the amount of the main catalyst within a certain range can achieve a good balance between the physical properties of the nylon product and the content of cyclic small molecules. If the amount of the main catalyst is too low, the effect on reducing the content of cyclic small molecules in the nylon product will be limited; if the amount of the main catalyst is too high, it will lead to a decrease in the physical properties of the nylon product.

[0106] The test results from Examples 1 and 10-12 show that adding an appropriate amount of co-catalyst can synergistically work with the main catalyst to shift the reaction equilibrium towards the formation of higher molecular weight linear nylon, thereby reducing the oligomer content in the final product. However, excessive co-catalyst will cause a decrease in the strength of the nylon product; if the amount of co-catalyst is too low, the effect on reducing the content of cyclic small molecules and increasing the molecular weight of the nylon product will be insignificant.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing nylon with low molecular weight molecule content, characterized in that, The process includes the following steps: polymerizing a concentrated polyamide salt solution to obtain nylon; In the polymerization reaction, a catalyst is added to the reaction system in the later stage of polymerization; the catalyst includes a main catalyst, which is selected from at least one of organophosphorus nitrile bases, rare earth metal complexes and alkylaluminoxanes.

2. The preparation method according to claim 1, characterized in that, The amount of the main catalyst added is 0.01 wt% to 0.12 wt% of the total amount of nylon; Preferably, the amount of the main catalyst added is 0.02 wt% to 0.1 wt% of the total amount of nylon.

3. The preparation method according to claim 1, characterized in that, The organophosphazene base includes at least one of phosphazene ligand P1-tert-butyl, phosphazene ligand P2-tert-butyl, and phosphazene ligand P4-tert-butyl; The rare earth metal complex includes at least one of scandium trifluoromethanesulfonate, tris(cyclopentadienyl)scandium, and tris(cyclopentadienyl)yttrium; The alkylaluminoxane includes at least one of methylaluminoxane and modified methylaluminoxane.

4. The preparation method according to claim 1, characterized in that, The catalyst further includes a cocatalyst; the cocatalyst comprises straight-chain or branched alcohols having 2 to 10 carbon atoms; Preferably, the boiling point of the alcohol is 120~240℃; Preferably, the cocatalyst comprises at least one of propylene glycol, ethylene glycol, and dipropylene glycol.

5. The preparation method according to claim 4, characterized in that, The amount of the co-catalyst added is 0.05 wt% to 0.4 wt% of the total amount of nylon; Preferably, the amount of the co-catalyst added is 0.1 wt% to 0.3 wt% of the total amount of nylon.

6. The preparation method according to claim 1, characterized in that, In the later stage of polymerization, when the pressure is reduced to 1 / 5 to 1 / 3 of the pressure during the holding stage, the catalyst is added to the reaction system. Preferably, the polymerization reaction includes a heating and pressurizing stage and a pressurizing stage, a first-stage depressurization stage, a catalytic reaction stage, a second-stage depressurization stage, a post-polymerization reaction stage, and a discharge stage, which are sequentially connected. The catalyst is added during the catalytic reaction stage; Optionally, the heating and pressurizing stage and the pressure holding stage include: heating at a rate of 2~10℃ / min, monitoring the pressure until the pressure stabilizes at P1±0.1 MPa, where P1=1.6~2.0 MPa; maintaining the pressure at P1±0.1 MPa, continuing to heat until the temperature reaches T1, and controlling the pressure holding time to be 45~90 min, where T1=260~280℃; Preferably, the first-stage pressure relief includes: when the temperature reaches T1, opening the pressure relief valve to perform the first-stage pressure relief operation; when the pressure drops to P2, closing the pressure relief valve, where P2 = 0.35~0.55 MPa. Preferably, the catalytic reaction stage includes: maintaining the pressure at P2, the temperature at T1±5℃, and reacting for 15~35 min after adding the catalyst; Preferably, the two-stage depressurization includes: opening the pressure relief valve to perform a two-stage depressurization operation until the pressure drops to atmospheric pressure; Preferably, the post-polymerization reaction and discharge stage includes devolatilization for 20-40 min at P3 pressure and T2 temperature, followed by discharge through a bottom valve, water cooling, and pelletizing to obtain nylon 66 resin, with P3 = -0.050~-0.070 MPa and T2 = 280~300℃.

7. The preparation method according to claim 1, characterized in that, The polyamide salt concentrate is mainly prepared by concentrating a polyamide salt solution. Preferably, the solid content of the polyamide salt solution is 45 wt% to 65 wt%. Preferably, the solid content of the polyamide salt concentrate is 75 wt% to 85 wt%.

8. The preparation method according to claim 7, characterized in that, The polyamide salt solution includes a polyamide 66 salt solution; Preferably, the polyamide salt solution further includes an auxiliary agent; Preferably, the adjuvant includes sodium hypophosphite.

9. A nylon with low molecular weight molecule content, characterized in that, The raw materials for preparation include polyamide salts and a main catalyst, wherein the main catalyst includes at least one of organophosphorus nitrile bases, rare earth metal complexes and alkylaluminoxanes, and is prepared by the preparation method described in any one of claims 1 to 8.

10. The nylon with low molecular weight according to claim 9, characterized in that, The content of substances with a molecular weight lower than 1048 g / mol in the nylon is ≤810 mg / kg.

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