A method for recycling PA6 by multiple hydrolysis and preparation method and closed-loop recycling method thereof

Abstract

The application belongs to the technical field of polymer material preparation and recycling, and relates to a PA6 that can be hydrolyzed and recycled multiple times, a preparation method and a closed-loop recycling method. The structural formula of the PA6 that can be hydrolyzed and recycled multiple times is as follows: wherein n and m are positive integers, -OC-R1-CO- is a binary acid segment, and -O-R2-O- is a binary alcohol segment. The preparation method is to perform an ester exchange reaction on PA6 oligomer esterification to obtain the PA6 that can be hydrolyzed and recycled multiple times. The closed-loop recycling method is to perform an ester bond hydrolysis reaction on the PA6 that can be hydrolyzed and recycled multiple times, cool and collect the precipitate to obtain recycled double-carboxyl PA6 oligomer, and then sequentially perform esterification and ester exchange reactions on the recycled double-carboxyl PA6 oligomer to obtain regenerated PA6 that can be hydrolyzed and recycled multiple times. The application constructs an efficient closed-loop hydrolysis recycling path of "PA6 double-carboxyl PA6 oligomer PA6", and the method is simple, low in cost and easy to be applied in large-scale industrial production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polymer material preparation and recycling, and particularly relates to a PA6 that can be hydrolyzed and recycled multiple times, a preparation method thereof and a closed-loop recycling method. BACKGROUND

[0002] PA6 (polyamide 6) is the most commonly used polyamide engineering plastic and fiber raw material. Since PA6 cannot be degraded in the natural environment, the continuous accumulation of waste PA6 in the environment will cause microplastic and toxin pollution problems, and it is necessary to study the recycling and reuse of PA6.

[0003] Chemical recycling can depolymerize waste PA6 into caprolactam, oligomers and other derived chemicals at the molecular level, which can be reused after separation and purification, and can realize "cyclic utilization" or "upgrading utilization". It is currently recognized as the only recycling method that can achieve sustainable development. Water, as one of the products of the polymerization reaction of PA6, can also be used as a nucleophile to depolymerize PA6. After hydrolysis, no other chemical components are introduced, so water is an ideal PA6 chemical depolymerization agent. However, under the condition of no catalyst, PA6 hydrolysis needs to be carried out under the subcritical or supercritical state of water (374℃, 22.1MPa) to achieve a high proportion (>98%) of caprolactam conversion. Small molecule acids or bases as catalysts can reduce the PA6 hydrolysis conditions, but under acidic conditions, uncyclized aminocaproic acid will form aminocaproic acid salt, and the monomer recovery rate will be reduced (<85%), and the acid and alkali waste liquid will cause secondary pollution.

[0004] For example, patent application CN101423487A discloses a method for catalytically degrading and recycling epsilon-caprolactam from waste nylon 6. The method uses water as the reaction medium and phosphotungstic heteropoly acid as the catalyst. After reacting at 280-330℃ for 0.5-3h, the product is subjected to alkali neutralization, organic solvent extraction, distillation of the extraction solvent, deionized water washing, and dehydration and drying to obtain caprolactam solid with a yield of 70%-81%. However, the hydrolysis process of this method requires high temperature, and the purification process of the hydrolysis product caprolactam is complicated. The alkali neutralization and organic solvent extraction also cause environmental pollution, which limits the large-scale industrial application of PA6 hydrolysis recycling.

[0005] Therefore, there is an urgent need to develop a new PA6 hydrolysis recycling path with mild hydrolysis conditions, simple operation and less environmental pollution to help microplastic management and environmental protection. SUMMARY

[0006] The purpose of the present application is to solve the above-mentioned problems existing in the prior art, and to provide a PA6 that can be hydrolyzed and recycled multiple times, a preparation method thereof and a closed-loop recycling method.

[0007] To achieve the above-mentioned purpose, the technical solutions adopted by the present application are as follows:

[0008] A PA6 which can be hydrolytically recycled multiple times, having the following structural formula:

[0009]

[0010] wherein n and m are positive integers, -OC-R1-CO- is a diacid segment, and -O-R2-O- is a diol segment.

[0011] As a preferred technical solution,

[0012] The PA6 which can be hydrolytically recycled multiple times as described above, wherein 13≤n≤70 and 4≤m≤20.

[0013] The PA6 which can be hydrolytically recycled multiple times as described above, wherein the diacid segment corresponds to a linear aliphatic diacid with a carbon chain length of no more than 16, such as adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, 1,13-tridecanedioic acid (CAS No. 505-52-2), or 1,16-hexadecanedioic acid (CAS No. 505-54-4), and the diol segment corresponds to a linear diol with a carbon chain length of no more than 6, such as ethylene glycol, propylene glycol, butylene glycol, pentanediol, or hexanediol.

[0014] The PA6 which can be hydrolytically recycled multiple times as described above, wherein the relative viscosity of the PA6 which can be hydrolytically recycled multiple times is 2.4-4.0, the hot water extractable content is 0.05-2wt%, the melting point is 190-250℃, and the number average molecular weight is 17000-32000g / mol.

[0015] The application also provides a method for preparing the PA6 which can be hydrolytically recycled multiple times as described above, wherein a PA6 oligomer ester is subjected to a transesterification reaction to obtain the PA6 which can be hydrolytically recycled multiple times.

[0016]

[0017] As a preferred technical solution,

[0018] The method as described above, wherein a catalyst A (a titanium-based catalyst or / and an antimony-based catalyst, such as one or more of tetrabutyl titanate, titanium glycolate, antimony trioxide, antimony acetate, and antimony glycolate) and an ether inhibitor (such as sodium acetate) are added during the transesterification reaction.

[0019] The method as described above, wherein the catalyst A is 200-500ppm of the mass of the PA6 oligomer ester, the ether inhibitor is 300-600ppm of the mass of the PA6 oligomer ester, the temperature of the transesterification reaction is 250-280℃, the pressure of the transesterification reaction is 200Pa or less, and the time of the transesterification reaction is 2-4h.

[0020] The preparation process of PA6 oligomer esters is as follows: PA6 oligomers with dicarboxylic acid are mixed and then subjected to esterification reaction to obtain PA6 oligomer esters.

[0021] The structural formula of the dicarboxylated PA6 oligomer is as follows:

[0022]

[0023] The structural formula of the diol is as follows:

[0024] HO-R2-OH.

[0025] As described above, the molar ratio of diol to dicarboxylated PA6 oligomer is 1.3:1-1.8:1; the esterification reaction temperature is 220-260℃, the esterification reaction pressure is 0.01-0.5MPa, and the esterification reaction time is 2-4h.

[0026] The preparation process of dicarboxylated PA6 oligomers, as described above, involves mixing caprolactam, a diacid, deionized water, and catalyst B (e.g., 6-aminohexanoic acid), followed by a hydrolysis ring-opening reaction to obtain the dicarboxylated PA6 oligomers. The structural formula of the diacid is as follows:

[0027]

[0028] As described above, the amounts of dicarboxylic acid, deionized water, and catalyst B are 5-30 wt%, 2-5 wt%, and 0.1-1 wt% of the amount of caprolactam, respectively; the mixing temperature is 60-100℃; the hydrolysis ring-opening reaction temperature is 210-260℃; the hydrolysis ring-opening reaction pressure is 0.1-3 MPa; and the hydrolysis ring-opening reaction time is 3-5 h.

[0029] The present invention also provides a closed-loop method for the multiple hydrolysis recovery of PA6 as described in any of the preceding claims, comprising the following steps:

[0030] (1) The PA6 that can be hydrolyzed and recovered multiple times is placed in an alkaline aqueous solution to carry out an ester bond hydrolysis reaction;

[0031] (2) After cooling, a precipitate forms. The precipitate is collected to obtain the recovered dicarboxylated PA6 oligomer. The structural formula of the recovered dicarboxylated PA6 oligomer is as follows:

[0032]

[0033] (3) The recovered dicarboxylated PA6 oligomers are esterified with a diol to obtain PA6 oligomer esters. The PA6 oligomer esters are then subjected to transesterification to obtain regenerated PA6 that can be hydrolyzed and recovered multiple times. The structural formula of the diol is as follows:

[0034] HO-R2-OH.

[0035] The main existing technology for recovering PA6 is a monomeric closed-loop recovery path with hydrolysis as the depolymerization condition: "PA6 → caprolactam (monomer) → PA6". This requires high temperature and high pressure. Typically, the macromolecular chain of PA6 is hydrolyzed under conditions where the hydrolysis temperature exceeds 300°C, and then recovered in the form of caprolactam monomer. Because of the high cost, this recovery method is difficult to implement industrially, and the caprolactam monomer obtained by hydrolysis is usually no longer used in PA6 synthesis.

[0036] The closed-loop recycling method of the present invention adds ester bonds between PA6 chain segments as low-temperature hydrolysis response sites, which can achieve selective ester bond cleavage below 160°C, depolymerizing into a powdery intermediate state of dicarboxylated PA6 oligomer. The amide bonds in the PA6 molecular chain will not be broken, thereby constructing a closed-loop recycling pathway of "PA6 → dicarboxylated PA6 oligomer → PA6", which significantly reduces the conditions required for PA6 hydrolysis recycling and provides a new pathway for PA6 recycling.

[0037] Due to the structural differences between the ester bond and its surroundings, the ester bond position cannot form a lattice protection, and all ester bonds are exposed in the amorphous region, making them susceptible to attack by water molecules, which causes the ester bonds to break. This allows for the selective breakage of ester and amide bonds within the molecular chain. In a mild aqueous environment, the ester bonds in the amorphous region break, forming diols and dicarboxylated PA6 oligomers. The diols dissolve in the aqueous phase, and the dicarboxylated PA6 oligomers become solid precipitates after cooling. The dicarboxylated PA6 oligomers can be recovered through filtration and other methods. Since the dicarboxylated PA6 oligomers still retain the dicarboxylated structure, they can be further esterified with diols to obtain PA6 oligomer esters. The PA6 oligomer esters can then be used to prepare regenerated PA6 that can be hydrolyzed and recovered multiple times through transesterification, thus realizing a closed-loop hydrolysis and recovery process of "PA6 → dicarboxylated PA6 oligomers → PA6" in a mild aqueous environment.

[0038] The prior art and the present invention differ in both the recycling pathway and the recycling hydrolysis principle (the chemical bonds targeted by hydrolysis).

[0039] As a preferred technical solution:

[0040] As described above, in step (1), the mass ratio of the alkaline aqueous solution to the PA6 that can be repeatedly hydrolyzed and recovered is 1:5-1:20, the pH value of the alkaline aqueous solution is 8-11, and the solute in the alkaline aqueous solution can be hydroxides such as sodium hydroxide and potassium hydroxide or carbonates such as sodium bicarbonate and sodium carbonate; the temperature of the ester bond hydrolysis reaction is 100-140℃, the pressure of the ester bond hydrolysis reaction is 1.0-1.7MPa, the time of the ester bond hydrolysis reaction is 2-4h, and the ester bond hydrolysis rate is ≥98%.

[0041] The molar ratio of the dihydric alcohol to the recovered double-carboxyl PA6 oligomer in step (3) of the method described above is 1.3:1-1.8:1; the temperature of the esterification reaction is 220-260℃, the pressure of the esterification reaction is 0.01-0.5MPa, and the time of the esterification reaction is 2-4h;

[0042] A catalyst A (a titanium-based catalyst or / and an antimony-based catalyst, such as one or more of tetrabutyl titanate, titanium glycolate, antimony trioxide, antimony acetate, and antimony glycolate) and an ether inhibitor (such as sodium acetate) are further added in the transesterification reaction; the catalyst A is 200-500ppm of the mass of the PA6 oligomer esterification product; the ether inhibitor is 300-600ppm of the mass of the PA6 oligomer esterification product; the temperature of the transesterification reaction is 250-280℃, the pressure of the transesterification reaction is 200Pa or less, and the time of the transesterification reaction is 2-4h;

[0043] The regenerated PA6 that can be hydrolyzed multiple times has a relative viscosity of 2.4-4.0, a hot water extract content of 0.05-2wt%, a number average molecular weight of 17000-32000g / mol, and a melting point of 190-250℃, meeting the processing requirements of injection molding and melt spinning.

[0044] Advantages:

[0045] (1) According to the different hydrolysis conditions of ester bonds and amide bonds, the present application realizes the separation of solid-phase PA6 segments by directional hydrolysis of ester bonds, and does not introduce other chemical structures during hydrolysis, which is beneficial to ensuring the purity and structural integrity of the recycled PA6 segments, and the recycled PA6 segments can be used for copolymerization reaction again to prepare PA6 again, thereby constructing an efficient closed-loop hydrolysis recycling path of “PA6→double-carboxyl PA6 oligomer→PA6”.

[0046] (2) The hydrolysis temperature of the present application is low, and the separation and recovery of the hydrolysis product PA6 segments can be realized through filtration and other operations, which is simple in operation, low in cost, and easy to mass industrial production.

[0047] (3) The preparation method and the closed-loop recycling method of the present application do not use organic solvents and strong acids, which has small environmental pollution and is beneficial to environmental protection. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a schematic diagram of the principle of the preparation method and the closed-loop recycling method of the PA6 that can be hydrolyzed multiple times provided by the present application. DETAILED DESCRIPTION

[0049] The application will be further described below in connection with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content of the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0050] The following is the detection method of the relevant performance indicators in each embodiment:

[0051] (1) Detection method of relative viscosity

[0052] Referring to the test method of China Textile Industry Standard FZ / T 51004-2011: dissolve the sample in concentrated sulfuric acid with a concentration of 96wt% to prepare a 0.01g / ml test solution, and use an Ubbelohde viscometer (capillary diameter is 1.03mm) to test in a constant temperature water bath with water temperature of 25±0.1℃, repeat 5 parallel tests and take the average value.

[0053] (2) Detection method of hot water extractable content

[0054] Referring to the test method of China Textile Industry Standard FZ / T 51004-2011: place the sample in a vacuum oven at 105℃ and dry for 24h to constant weight, then take a certain mass (m0) of the sample, add deionized water with a bath ratio of 20:1, use a Soxhlet extractor to extract the sample in boiling water (water bath temperature≥98℃) for 8h, take out and dry the sample in a vacuum oven at 105℃ for 24h, then weigh and record the mass as (m1), the hot water extractable content is shown in the following formula:

[0055] E=(m0-m1) / m0;

[0056] In the formula, E is the hot water extractable content, wt%; m0 and m1 are the sample masses before and after extraction, respectively, g.

[0057] (3) Detection method of number average molecular weight

[0058] The number average molecular weight of the sample is tested by using a GPC-50 type gel permeation chromatograph of British PL company The GPC-50 type gel permeation chromatograph is equipped with a differential refractive index detector and a PL gel column (5μm mixed-C), 1,1,1,3,3,3-hexafluoro-2-propanol is used as the eluent with a flow rate of 1mL / min; when testing, the sample is dried and dissolved in hexafluoroisopropanol to prepare a 1.0mg / mL solution, and the test is carried out when the column temperature reaches 40±1℃.

[0059] (4) Detection method of melting point

[0060] TA-Q20 differential scanning calorimeter was used to test the crystallization and melting behavior of the sample: about 5 mg of dry sample was heated and cooled at a rate of 10℃ / min under nitrogen atmosphere.

[0061] (5) Characterization method of polymer molecular structure

[0062] AVANCE-600 nuclear magnetic resonance spectrometer was used to test the molecular structure of the sample: 5-10 mg of dry sample was weighed before testing, deuterated sulfuric acid (D2SO4) was used as the test solvent, the testing temperature was 25℃, and the frequency was 600MHz;

[0063] Nicolet 6700 Fourier transform infrared spectrometer was used for FTIR spectral analysis: the sample was made into a potassium bromide wafer before testing, and ATR accessory was used for total reflection testing, the testing scan range was 4000-600cm -1 ;

[0064] Tests show that the structural formula of the PA6 that can be repeatedly hydrolyzed and recovered in each of the following embodiments is as follows:

[0065]

[0066] In the formula, n and m are positive integers, -OC-R1-CO- is a diacid segment, and -O-R2-O- is a diol segment.

[0067] (6) Detection method of ester bond hydrolysis rate

[0068] AVANCE-600 nuclear magnetic resonance spectrometer was used to test the molecular structure of the sample: 5-10 mg of dry sample was weighed before testing, deuterated sulfuric acid (D2SO4) was used as the test solvent, the testing temperature was 25℃, and the frequency was 600MHz; the integral area of the ester bond position characteristic peak before and after hydrolysis was tested, (1-ester bond characteristic peak area after hydrolysis) / ester bond characteristic peak area before hydrolysis=ester bond hydrolysis rate.

[0069] Example 1

[0070] A preparation method of PA6 that can be repeatedly hydrolyzed and recovered, as shown in Figure 1 , the specific steps are as follows:

[0071] (1) Preparation of dicarboxyl PA6 oligomer;

[0072] The caprolactam, adipic acid, deionized water, and 6-aminohexanoic acid are mixed, and incubated at 60°C with mechanical stirring at a speed of 100 r / min for 15 min. The amounts of adipic acid, deionized water, and 6-aminohexanoic acid are 30 wt%, 2 wt%, and 1 wt% of the amount of caprolactam, respectively. Then, a hydrolysis ring-opening reaction is performed at a temperature of 210°C and a pressure of 0.1 MPa for 5 h, to obtain the dicarboxyl PA6 oligomer;

[0073] (2) Preparation of PA6 oligomer esterification product;

[0074] The dicarboxyl PA6 oligomer is mixed with hexanediol, and an esterification reaction is performed at a temperature of 220°C and a pressure of 0.5 MPa for 4 h, with a molar ratio of hexanediol to dicarboxyl PA6 oligomer of 1.3:1, to obtain the PA6 oligomer esterification product;

[0075] (3) Preparation of PA6 that can be recycled by multiple hydrolysis;

[0076] The PA6 oligomer esterification product, tetrabutyl titanate, and sodium acetate are mixed, and an ester exchange reaction is performed at a temperature of 250°C and a pressure of 100 Pa for 4 h, with the tetrabutyl titanate being 200 ppm of the mass of the PA6 oligomer esterification product and the sodium acetate being 300 ppm of the mass of the PA6 oligomer esterification product, to obtain the PA6 that can be recycled by multiple hydrolysis.

[0077] The PA6 that can be recycled by multiple hydrolysis has a relative viscosity of 4.0, a hot water extract content of 0.05 wt%, a melting point of 190°C, and a number average molecular weight of 32,000 g / mol. In the structural formula of the PA6 that can be recycled by multiple hydrolysis, n is 13 and m is 20.

[0078] The closed-loop recycling method of the PA6 that can be recycled by multiple hydrolysis described above, as shown in Figure 1 , specifically comprises the following steps:

[0079] (1) The PA6 that can be recycled by multiple hydrolysis is placed in an alkaline aqueous solution to perform an ester bond hydrolysis reaction, with a mass ratio of the alkaline aqueous solution to the PA6 that can be recycled by multiple hydrolysis of 20:1, a pH value of the alkaline aqueous solution of 11, a solute in the alkaline aqueous solution being sodium hydroxide, a temperature of the ester bond hydrolysis reaction of 100°C, a pressure of the ester bond hydrolysis reaction of 1.0 MPa, and a time of the ester bond hydrolysis reaction of 3 h, with an ester bond hydrolysis rate of ≥98%;

[0080] (2) Cooling to precipitate, collecting the precipitate by filtration with a filtration accuracy of 40-200 μm, and drying to obtain the recycled dicarboxyl PA6 oligomer;

[0081] (3) The recovered double carboxyl PA6 oligomer is subjected to esterification reaction with hexanediol, the molar ratio of hexanediol to the recovered double carboxyl PA6 oligomer is 1.3:1, the esterification reaction temperature is 220℃, the esterification reaction pressure is 0.5MPa, the esterification reaction time is 4h, and PA6 oligomer esterification product is obtained; then the PA6 oligomer esterification product, tetrabutyl titanate and sodium acetate are mixed and subjected to ester exchange reaction, the tetrabutyl titanate is 200ppm of the mass of the PA6 oligomer esterification product, the sodium acetate is 300ppm of the mass of the PA6 oligomer esterification product, the ester exchange reaction temperature is 250℃, the ester exchange reaction pressure is 100Pa, and the ester exchange reaction time is 4h, and the regenerated PA6 that can be recycled and hydrolyzed multiple times is obtained.

[0082] The relative viscosity of the finally prepared regenerated PA6 that can be recycled and hydrolyzed multiple times is 4.0, the hot water extract content is 0.05wt%, the number average molecular weight is 32000g / mol, and the melting point is 190℃, which meets the processing requirements of injection molding and melt spinning.

[0083] Example 2

[0084] A preparation method of a PA6 that can be recycled and hydrolyzed multiple times, and the specific steps are as follows:

[0085] (1) Double carboxyl PA6 oligomer is prepared;

[0086] Caprolactam, suberic acid, deionized water and 6-aminohexanoic acid are mixed, and the mixture is incubated at 80℃ and subjected to mechanical stirring at a speed of 150r / min for 18min, followed by hydrolysis ring-opening reaction at a temperature of 225℃ and a pressure of 2.4MPa for 3.6h, wherein the amounts of suberic acid, deionized water and 6-aminohexanoic acid are 17wt%, 2.4wt% and 0.4wt% of the amount of caprolactam respectively, and the double carboxyl PA6 oligomer is obtained;

[0087] (2) PA6 oligomer esterification product is prepared;

[0088] The double carboxyl PA6 oligomer is mixed with pentanediol and subjected to esterification reaction, the molar ratio of pentanediol to the double carboxyl PA6 oligomer is 1.7:1, the esterification reaction temperature is 235℃, the esterification reaction pressure is 0.2MPa, and the esterification reaction time is 2.5h, and the PA6 oligomer esterification product is obtained;

[0089] (3) The PA6 that can be recycled and hydrolyzed multiple times is prepared;

[0090] The PA6 oligomer esterification product, antimony acetate and sodium acetate are mixed to perform the ester exchange reaction, the antimony acetate is 400 ppm of the mass of the PA6 oligomer esterification product, the sodium acetate is 530 ppm of the mass of the PA6 oligomer esterification product, the ester exchange reaction temperature is 265°C, the ester exchange reaction pressure is 50 Pa, and the ester exchange reaction time is 3 h, to obtain the PA6 which can be repeatedly hydrolyzed and recovered.

[0091] The relative viscosity of the finally prepared PA6 which can be repeatedly hydrolyzed and recovered is 3.2, the hot water extract content is 1.2 wt%, the melting point is 225°C, and the number average molecular weight is 27000 g / mol; in the structural formula of the PA6 which can be repeatedly hydrolyzed and recovered, n is 33 and m is 7.

[0092] The closed loop recovery method of the above-mentioned PA6 which can be repeatedly hydrolyzed and recovered includes the following specific steps:

[0093] (1) The PA6 which can be repeatedly hydrolyzed and recovered is placed in an alkaline aqueous solution to perform an ester bond hydrolysis reaction, the mass ratio of the alkaline aqueous solution to the PA6 which can be repeatedly hydrolyzed and recovered is 13:1, the pH value of the alkaline aqueous solution is 9, the solute in the alkaline aqueous solution is sodium carbonate, the ester bond hydrolysis reaction temperature is 135°C, the ester bond hydrolysis reaction pressure is 1.6 MPa, the ester bond hydrolysis reaction time is 2.5 h, and the ester bond hydrolysis rate is ≥98%;

[0094] (2) Cooling to precipitate and separating, collecting the precipitate by filtration with a filtration precision of 40-200 μm, drying, and obtaining the recovered double-carboxyl PA6 oligomer;

[0095] (3) The recovered double-carboxyl PA6 oligomer is subjected to an esterification reaction with pentanediol, the molar ratio of the pentanediol to the recovered double-carboxyl PA6 oligomer is 1.7:1, the esterification reaction temperature is 235°C, the esterification reaction pressure is 0.2 MPa, the esterification reaction time is 2.5 h, and the PA6 oligomer esterification product is obtained; then the PA6 oligomer esterification product, antimony acetate and sodium acetate are mixed to perform the ester exchange reaction, the antimony acetate is 400 ppm of the mass of the PA6 oligomer esterification product, the sodium acetate is 530 ppm of the mass of the PA6 oligomer esterification product, the ester exchange reaction temperature is 265°C, the ester exchange reaction pressure is 50 Pa, and the ester exchange reaction time is 3 h, to obtain the regenerated PA6 which can be repeatedly hydrolyzed and recovered.

[0096] The relative viscosity of the finally prepared regenerated PA6 which can be repeatedly hydrolyzed and recovered is 3.2, the hot water extract content is 1.2 wt%, the number average molecular weight is 27000 g / mol, and the melting point is 225°C, which meets the requirements of injection molding and melt spinning processing.

[0097] Example 3

[0098] A preparation method of the PA6 which can be repeatedly hydrolyzed and recovered includes the following specific steps:

[0099] (1) preparing double carboxyl PA6 oligomer;

[0100] Mixing caprolactam, sebacic acid, deionized water and 6-aminohexanoic acid, keeping at 100℃ and mechanical stirring, the speed of mechanical stirring is 200r / min, the time of mechanical stirring is 20min, then carrying out hydrolysis ring-opening reaction, the temperature of hydrolysis ring-opening reaction is 245℃, the pressure of hydrolysis ring-opening reaction is 3MPa, the time of hydrolysis ring-opening reaction is 3h, the amount of sebacic acid, deionized water and 6-aminohexanoic acid is 13wt%, 5wt% and 0.1wt% of the amount of caprolactam respectively, thus obtaining double carboxyl PA6 oligomer;

[0101] (2) preparing PA6 oligomer esterification product;

[0102] Mixing double carboxyl PA6 oligomer and propylene glycol, then carrying out esterification reaction, the molar ratio of propylene glycol to double carboxyl PA6 oligomer is 1.4:1, the temperature of esterification reaction is 245℃, the pressure of esterification reaction is 0.3MPa, the time of esterification reaction is 3h, thus obtaining PA6 oligomer esterification product;

[0103] (3) preparing PA6 which can be hydrolyzed and recycled for multiple times;

[0104] Mixing PA6 oligomer esterification product, antimony trioxide and sodium acetate, then carrying out ester exchange reaction, the amount of antimony trioxide is 500ppm of the mass of PA6 oligomer esterification product, the amount of sodium acetate is 600ppm of the mass of PA6 oligomer esterification product, the temperature of ester exchange reaction is 270℃, the pressure of ester exchange reaction is 200Pa, the time of ester exchange reaction is 2.5h, thus obtaining PA6 which can be hydrolyzed and recycled for multiple times.

[0105] The relative viscosity of the finally prepared PA6 which can be hydrolyzed and recycled for multiple times is 2.4, the content of hot water extractable substance is 1.5wt%, the melting point is 230℃, and the number average molecular weight is 17000g / mol; in the structural formula of the PA6 which can be hydrolyzed and recycled for multiple times, n is 37 and m is 4.

[0106] The closed loop recycling method of the above-mentioned PA6 which can be hydrolyzed and recycled for multiple times includes the following specific steps:

[0107] (1) placing the PA6 which can be hydrolyzed and recycled for multiple times in an alkaline aqueous solution to carry out ester bond hydrolysis reaction, the mass ratio of the alkaline aqueous solution to the PA6 which can be hydrolyzed and recycled for multiple times is 16:1, the pH value of the alkaline aqueous solution is 8, the solute in the alkaline aqueous solution is sodium carbonate, the temperature of the ester bond hydrolysis reaction is 120℃, the pressure of the ester bond hydrolysis reaction is 1.3MPa, the time of the ester bond hydrolysis reaction is 2h, and the ester bond hydrolysis rate is ≥98%;

[0108] (2) cooling to precipitate, collecting the precipitate by filtration with a filter precision of 40-200 pm, drying, and obtaining the recovered double carboxyl PA6 oligomer;

[0109] (3) esterifying the recovered double carboxyl PA6 oligomer with propylene glycol, the molar ratio of propylene glycol to the recovered double carboxyl PA6 oligomer being 1.4:1, the esterification temperature being 245°C, the esterification pressure being 0.3 MPa, and the esterification time being 3 h, to obtain a PA6 oligomer esterification product; then performing a transesterification reaction on the PA6 oligomer esterification product, sodium acetate, and antimony trioxide, the amount of antimony trioxide being 500 ppm of the mass of the PA6 oligomer esterification product, the amount of sodium acetate being 600 ppm of the mass of the PA6 oligomer esterification product, the transesterification temperature being 270°C, the transesterification pressure being 200 Pa, and the transesterification time being 2.5 h, to obtain the regenerated PA6 that can be hydrolyzed and recovered multiple times.

[0110] The regenerated PA6 that can be hydrolyzed and recovered multiple times obtained has a relative viscosity of 2.4, a hot water extract content of 1.5 wt%, a number average molecular weight of 17000 g / mol, and a melting point of 230°C, meeting the requirements for injection molding and melt spinning processing.

[0111] Example 4

[0112] A method for preparing a PA6 that can be hydrolyzed and recovered multiple times, the specific steps being as follows:

[0113] (1) preparing a double carboxyl PA6 oligomer;

[0114] mixing caprolactam, 1,13-tridecanedioic acid (CAS No. 505-52-2), deionized water, and 6-aminohexanoic acid, keeping at 75°C and mechanically stirring at a speed of 240 r / min for 5 min, then performing a hydrolysis ring-opening reaction at a temperature of 255°C and a pressure of 1.7 MPa for 4 h, the amounts of 1,13-tridecanedioic acid, deionized water, and 6-aminohexanoic acid being 24 wt%, 3.4 wt%, and 0.7 wt% of the amount of caprolactam, respectively, to obtain the double carboxyl PA6 oligomer;

[0115] (2) preparing a PA6 oligomer esterification product;

[0116] mixing the double carboxyl PA6 oligomer with ethylene glycol, and performing an esterification reaction at a temperature of 230°C and a pressure of 0.1 MPa for 3 h, the molar ratio of ethylene glycol to the double carboxyl PA6 oligomer being 1.6:1, to obtain the PA6 oligomer esterification product;

[0117] (3) preparing a PA6 that can be hydrolyzed and recovered multiple times;

[0118] The PA6 oligomer esterification product, antimony glycolate and sodium acetate are mixed and then subjected to a transesterification reaction, the antimony glycolate is 350 ppm by mass of the PA6 oligomer esterification product, the sodium acetate is 450 ppm by mass of the PA6 oligomer esterification product, the temperature of the transesterification reaction is 255°C, the pressure of the transesterification reaction is 150 Pa, and the time of the transesterification reaction is 3.5 h, to obtain the PA6 that can be repeatedly hydrolyzed and recovered.

[0119] The relative viscosity of the PA6 that can be repeatedly hydrolyzed and recovered obtained finally is 3.0, the hot water extractable content is 0.3 wt%, the melting point is 205°C, and the number average molecular weight is 24000 g / mol; in the structural formula of the PA6 that can be repeatedly hydrolyzed and recovered, n is 17 and m is 12.

[0120] The closed loop recycling method of the PA6 that can be repeatedly hydrolyzed and recovered described above has the following specific steps:

[0121] (1) The PA6 that can be repeatedly hydrolyzed and recovered is subjected to an ester bond hydrolysis reaction in an alkaline aqueous solution, the mass ratio of the alkaline aqueous solution to the PA6 that can be repeatedly hydrolyzed and recovered is 9:1, the pH value of the alkaline aqueous solution is 9.5, the solute in the alkaline aqueous solution is potassium hydroxide, the temperature of the ester bond hydrolysis reaction is 115°C, the pressure of the ester bond hydrolysis reaction is 1.2 MPa, and the time of the ester bond hydrolysis reaction is 4 h, and the ester bond hydrolysis rate is ≥98%;

[0122] (2) Cooling to precipitate and collecting the precipitate by filtration with a filtration precision of 40-200 μm, drying to obtain the recovered double carboxyl PA6 oligomer;

[0123] (3) The recovered double carboxyl PA6 oligomer is subjected to an esterification reaction with ethylene glycol, the molar ratio of the ethylene glycol to the recovered double carboxyl PA6 oligomer is 1.6:1, the temperature of the esterification reaction is 230°C, the pressure of the esterification reaction is 0.1 MPa, and the time of the esterification reaction is 3 h to obtain a PA6 oligomer esterification product; the PA6 oligomer esterification product, antimony glycolate and sodium acetate are mixed and then subjected to a transesterification reaction, the antimony glycolate is 350 ppm by mass of the PA6 oligomer esterification product, the sodium acetate is 450 ppm by mass of the PA6 oligomer esterification product, the temperature of the transesterification reaction is 255°C, the pressure of the transesterification reaction is 150 Pa, and the time of the transesterification reaction is 3.5 h, to obtain the regenerated PA6 that can be repeatedly hydrolyzed and recovered.

[0124] The relative viscosity of the regenerated PA6 that can be repeatedly hydrolyzed and recovered obtained finally is 3.0, the hot water extractable content is 0.3 wt%, the number average molecular weight is 24000 g / mol, and the melting point is 205°C, which meets the requirements of injection molding and melt spinning processing.

[0125] Example 5

[0126] A preparation method of PA6 which can be recycled by multiple hydrolysis, and the specific steps are as follows:

[0127] (1) Preparation of dicarboxylic PA6 oligomer;

[0128] Mix caprolactam, 1, 16-hexadecanedioic acid (CAS No.: 505-54-4), deionized water and 6-aminohexanoic acid, incubate at 90℃ and mechanically stir, the stirring speed is 60r / min, the stirring time is 10min, then perform hydrolysis ring-opening reaction, the hydrolysis ring-opening reaction temperature is 260℃, the pressure is 0.5MPa, the time is 4.3h, the amount of 1, 16-hexadecanedioic acid, deionized water and 6-aminohexanoic acid is 5wt%, 4.2wt% and 0.5wt% of the amount of caprolactam respectively, and the dicarboxylic PA6 oligomer is obtained;

[0129] (2) Preparation of PA6 oligomer esterification product;

[0130] Mix the dicarboxylic PA6 oligomer with butanediol and perform esterification reaction, the molar ratio of butanediol to dicarboxylic PA6 oligomer is 1.8:1, the esterification reaction temperature is 260℃, the esterification reaction pressure is 0.01MPa, and the esterification reaction time is 2h, and the PA6 oligomer esterification product is obtained;

[0131] (3) Preparation of PA6 which can be recycled by multiple hydrolysis;

[0132] Mix the PA6 oligomer esterification product, titanium glycolate and sodium acetate and perform ester exchange reaction, the amount of titanium glycolate is 250ppm of the mass of PA6 oligomer esterification product, the amount of sodium acetate is 370ppm of the mass of PA6 oligomer esterification product, the ester exchange reaction temperature is 280℃, the ester exchange reaction pressure is 100Pa, and the ester exchange reaction time is 2h, and the PA6 which can be recycled by multiple hydrolysis is obtained.

[0133] The relative viscosity of the finally prepared PA6 which can be recycled by multiple hydrolysis is 3.8, the hot water extractable content is 2wt%, the melting point is 250℃, and the number average molecular weight is 32000g / mol; in the structural formula of the PA6 which can be recycled by multiple hydrolysis, n is 70 and m is 4.

[0134] The above-mentioned closed loop recycling method of PA6 which can be recycled by multiple hydrolysis, and the specific steps are as follows:

[0135] (1) Put the PA6 which can be recycled by multiple hydrolysis into an alkaline aqueous solution to perform ester bond hydrolysis reaction, the mass ratio of the alkaline aqueous solution to the PA6 which can be recycled by multiple hydrolysis is 5:1, the pH value of the alkaline aqueous solution is 10, the solute in the alkaline aqueous solution is sodium hydroxide, the ester bond hydrolysis reaction temperature is 140℃, the ester bond hydrolysis reaction pressure is 1.7MPa, the ester bond hydrolysis reaction time is 3.5h, and the ester bond hydrolysis rate is ≥98%.

[0136] (2) cooling to precipitate, collecting the precipitate by filtration, the filtration precision is 40-200 μm, drying, obtaining the recovered double carboxyl PA6 oligomer;

[0137] (3) esterification reaction of the recovered double carboxyl PA6 oligomer and butanediol, the molar ratio of butanediol to the recovered double carboxyl PA6 oligomer is 1.8:1, the esterification reaction temperature is 260℃, the esterification reaction pressure is 0.01 MPa, the esterification reaction time is 2h, obtaining PA6 oligomer esterification product; then ester exchange reaction of the PA6 oligomer esterification product, titanium ethylene glycol and sodium acetate, the amount of titanium ethylene glycol is 250ppm of the mass of the PA6 oligomer esterification product, the amount of sodium acetate is 370ppm of the mass of the PA6 oligomer esterification product, the ester exchange reaction temperature is 280℃, the ester exchange reaction pressure is 100Pa, the ester exchange reaction time is 2h, obtaining the regenerated PA6 which can be hydrolyzed and recovered multiple times.

[0138] The relative viscosity of the finally prepared regenerated PA6 which can be hydrolyzed and recovered multiple times is 3.8, the hot water extractable content is 2wt%, the number average molecular weight is 32000g / mol, and the melting point is 250℃, which meets the requirements of injection molding and melt spinning processing.

Claims

1. A closed loop recycling method of PA6 which can be hydrolytically recycled multiple times, characterized in that, The method comprises the following steps: (1) placing the multiple hydrolysis recyclable PA6 in an alkaline aqueous solution to perform an ester bond hydrolysis reaction; the structure of the multiple hydrolysis recyclable PA6 is as follows: ； wherein n and m are positive integers, -OC-R1-CO- is a diacid segment, and -O-R2-O- is a diol segment; The mass ratio of the alkaline aqueous solution to the multiple hydrolysis recyclable PA6 is 5:1-20:1, the pH value of the alkaline aqueous solution is 8-11, the temperature of the ester bond hydrolysis reaction is 100-140℃, the pressure of the ester bond hydrolysis reaction is 1.0-1.7 MPa, and the time of the ester bond hydrolysis reaction is 2-4 h; (2) precipitating and separating after cooling, collecting the precipitate, and obtaining the recycled double-carboxyl PA6 oligomer, wherein the structure of the recycled double-carboxyl PA6 oligomer is as follows: ； (3) performing an esterification reaction on the recycled double-carboxyl PA6 oligomer and a diol to obtain a PA6 oligomer esterification product, and performing an ester exchange reaction on the PA6 oligomer esterification product to obtain the regenerated multiple hydrolysis recyclable PA6, wherein the structure of the diol is as follows: 。 2. The closed loop recycling process of claim 1, wherein, In step (3), the molar ratio of the diol to the recycled double-carboxyl PA6 oligomer is 1.3:1-1.8:1, the temperature of the esterification reaction is 220-260℃, the pressure of the esterification reaction is 0.01-0.5 MPa, and the time of the esterification reaction is 2-4 h; The catalyst A and the ether inhibitor are further added during the ester exchange reaction; the catalyst A is 200-500 ppm of the mass of the PA6 oligomer esterification product; the ether inhibitor is 300-600 ppm of the mass of the PA6 oligomer esterification product; the temperature of the ester exchange reaction is 250-280℃, the pressure of the ester exchange reaction is 200 Pa or less, and the time of the ester exchange reaction is 2-4 h; The regenerated multiple hydrolysis recyclable PA6 has a relative viscosity of 2.4-4.0, a hot water extract content of 0.05-2 wt%, a number average molecular weight of 17000-32000 g / mol, and a melting point of 190-250℃.

3. The closed loop recycling method of claim 1, wherein, 13≤n≤70, 4≤m≤20; the diacid corresponding to the diacid segment is a linear aliphatic diacid with a carbon chain length of not more than 16, and the diol corresponding to the diol segment is a linear diol with a carbon chain length of not more than 6.

4. The closed loop recycling process according to claim 1 or 3, characterized in that, The multiple hydrolysis recyclable PA6 has a relative viscosity of 2.4-4.0, a hot water extract content of 0.05-2 wt%, a melting point of 190-250℃, and a number average molecular weight of 17000-32000 g / mol.

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