Polyamide 66, process for its preparation and use

Abstract

The application provides a polyamide 66 and a preparation method and application thereof, and the preparation method comprises the following steps: adding raw materials, diamine, diacid and an auxiliary into a polyamide 66 salt by metering pumps in a certain proportion, controlling pressure and temperature, forming the polyamide 66 salt, slowly increasing the temperature to 250 DEG C, pre-polymerizing, discharging water, increasing the temperature to 280 DEG C, pre-polymerizing, keeping the temperature at 280 DEG C, vacuumizing, final polymerization, finally drawing and cutting to obtain the polyamide 66. The process does not need to add water, the liquid level changes little, has the advantages of fast reaction rate, short production cycle, flexible operation, convenient product category switching, low energy consumption, and the polyamide 66 polymer prepared by the method has the advantages of good mechanical property, stable thermal property, low gel content and few black spots, is suitable for wide application fields, and is especially suitable for the spinning field.

Description

Technical Field

[0001] This invention relates to the field of polyamide preparation technology, and in particular to a method for preparing polyamide 66, the polyamide 66 obtained therefrom, and its applications. Background Technology

[0002] With breakthroughs in domestic hexamethylenediamine production technology, planned production capacity has reached over 1.7 million tons per year, leading to a planned production capacity of over 5 million tons per year for polyamide 66. The domestic market demand for polyamide 66 is estimated at 2.5 million tons per year. Traditional processing methods inevitably suffer from drawbacks such as high energy consumption, high costs, long product conversion cycles, high energy consumption, and high gel content in the product. While some technologies attempt to reduce energy consumption, such as reducing water addition to 5-15%, the increased energy consumption due to water addition has not been completely resolved.

[0003] US Patent 3402152 discloses a continuous polymerization process for nylon 66, with key steps including concentration, high-pressure prepolymerization, flash evaporation, atmospheric compression polymerization, and vacuum polymerization. This polymerization process has an excessively long initial reaction time, leading to increased gel content and making subsequent processing or spinning difficult. Furthermore, current technologies involve salt formation and prepolymerization processes with reaction times of 2-4 hours. The unstable liquid level in the prepolymer reactor results in a large area of ​​gel adhering to the reactor wall, leading to the formation of a large amount of gel at high temperatures. This instability also significantly increases the number of black spots, affecting the product's appearance and mechanical properties. The amount of gel and black spots has an even greater impact on spinning-grade products.

[0004] Currently, most domestic enterprises adopt the continuous polymerization method. Although this method has the advantages of high automation, relatively large production capacity, and low raw material consumption, and is suitable for mass production of single-brand products, it also has disadvantages such as long production cycle, numerous equipment, large investment, and inconvenience in switching brands. Summary of the Invention

[0005] This invention rapidly forms a salt from solid hexamethylenediamine and adipic acid, eliminating the need for water addition. The amount of water added is reduced from 45-48 wt% in continuous processes to 0%, thereby reducing the energy consumption required for evaporating this water. No additional wastewater treatment is needed for the evaporated water, further reducing the energy consumption in polyamide 66 production and making it more environmentally friendly. Furthermore, this invention employs a batch polymerization method (as opposed to continuous polymerization). Compared to continuous polymerization (where monomers are continuously added to the reactor and polymers are continuously removed), this invention offers greater operational flexibility, a shorter production cycle, easier product switching, and lower energy consumption. Polyamide 66 prepared using this method exhibits advantages such as good mechanical properties, stable thermal properties, low gel content, and a small number of black spots, making it suitable for a wide range of applications, especially in the spinning industry.

[0006] One of the objectives of this invention is to provide a method for preparing polyamide 66.

[0007] The second objective of this invention is to provide a polyamide 66.

[0008] The third objective of this invention is to provide an application of polyamide 66.

[0009] In a first aspect, the present invention provides a method for preparing polyamide 66, comprising the following steps:

[0010] Solid adipic acid, hexamethylenediamine, and additives are added to a reactor to mix and form a salt, which is then polymerized to obtain polyamide 66.

[0011] The following is a detailed explanation:

[0012] There are no particular restrictions on the types of additives. Additives known in the art for the preparation of polyamide 66, such as catalysts, stabilizers, molecular weight regulators, etc., can be used, but are not limited to these.

[0013] In some embodiments, the preparation method of polyamide 66 includes the following steps:

[0014] (1) Take the initial amount of adipic acid and hexamethylenediamine and add them to the reaction vessel to mix. During the mixing process, add the catalyst, stabilizer and molecular weight regulator to mix and form a salt.

[0015] (2) Continue heating to carry out polymerization. During the polymerization process, the remaining amount of adipic acid and / or hexamethylenediamine may be added to obtain polyamide 66.

[0016] In some embodiments, the molar ratio of the final amounts (initial amount + optionally the remaining amount) of adipic acid and hexamethylenediamine is 1:1.001 to 1.1, more preferably 1:1.005 to 1.05.

[0017] In some embodiments, in step (1), the molar ratio of adipic acid to hexamethylenediamine is 5:1 to 1:5, more preferably 1:1, 5:1 to 1.1:1, or 1:5 to 1:1.1.

[0018] In some embodiments, the initial amounts of adipic acid and hexamethylenediamine account for 70-100% of the final amount, preferably 70-80%.

[0019] In some embodiments, the amount of catalyst added is 0.1-0.3 wt%, such as 0.12%, 0.15%, or 0.18%, but not limited thereto, based on the final amount of adipic acid and hexamethylenediamine; the amount of stabilizer added is 0.05-0.5 wt%, such as 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or 0.2%, but not limited thereto; and the amount of molecular weight regulator added is 0.1-0.5 wt%, such as 0.1%, 0.2%, or 0.3%, but not limited thereto.

[0020] In some embodiments, the catalyst is one or more selected from sodium phosphate, magnesium phosphate, calcium phosphate, magnesium phosphite, calcium phosphite, zinc phosphite, and sodium hypophosphite, preferably sodium hypophosphite;

[0021] In some embodiments, the stabilizer is one or more selected from cuprous iodide, copper acetate, manganese acetate, and potassium iodide, preferably a combination of copper acetate and potassium iodide;

[0022] In some embodiments, the molecular weight regulator is selected from one or more of benzoic acid, terephthalic acid, adipic acid, stearic acid, acetic acid, and propionic acid, preferably acetic acid.

[0023] In some embodiments, the salt formation temperature is 120–220°C, for example 160–220°C, 120–160°C, or 150–190°C, the salt formation pressure is 0.1–0.3 MPa, and the salt formation time is 1–30 min.

[0024] In some implementations, polymerization includes prepolymerization and final polymerization;

[0025] The prepolymerization temperature is 200–260℃, for example 230–260℃, 200–250℃, or 210–260℃; the prepolymerization pressure is 0.3–1.6 MPa, for example 1.1–1.6 MPa or 0.3–1.0 MPa; and the prepolymerization time is 1–4 hours.

[0026] The final polymerization consists of two stages. In the first stage, the temperature is raised to 260–280°C, and the pressure inside the reactor is maintained at 1.0–2.0 MPa by venting water vapor. The constant pressure reaction lasts for 0.5–2 hours. In the second stage, the temperature is maintained or raised to 280–300°C. The pressure is continuously released to atmospheric pressure over 0.5–2 hours, and a vacuum is drawn at a pressure of 10–40 kPa for 5–30 minutes.

[0027] The process parameters for salt formation and polymerization are related to the initial addition ratio of hexamethylenediamine and adipic acid, specifically including:

[0028] In one embodiment, the method for preparing polyamide 66 includes the following steps:

[0029] Adipic acid and hexamethylenediamine, initially in a molar ratio of 1:1, are added to a reaction vessel and mixed. During mixing, a catalyst, stabilizer, and molecular weight regulator are added to form a salt. The reaction temperature is 160–220℃ (preferably 180–200℃), the pressure is 0.1–0.3 MPa (preferably 0.2–0.3 MPa), and the reaction time is 1–30 min (preferably 5–15 min). Then, the temperature is raised to 230–260℃ and the pressure is maintained at 1.1–1.6 MPa for 1–4 h. The temperature is then raised to 260–280℃, and the pressure inside the vessel is maintained at 1.5–2.0 MPa by venting water vapor. The constant pressure reaction is carried out for 0.5–2 h. Then, the temperature is maintained or the temperature is raised to 280–300℃ and the pressure is continuously released to atmospheric pressure over 0.5–2 h. Vacuum is then applied at a pressure of 10–40 kPa for 5–30 min. The mixture is then stretched and granulated to obtain polyamide 66.

[0030] In another embodiment, the method for preparing polyamide 66 includes the following steps:

[0031] Adipic acid and hexamethylenediamine, with an initial molar ratio of 5:1 to 1.1:1, are added to a reaction vessel and mixed. During mixing, a catalyst, stabilizer, and molecular weight regulator are added to induce salt formation. The reaction temperature is 120–160°C (preferably 120–150°C), the pressure is 0.1–0.3 MPa (preferably 0.15–0.2 MPa), and the reaction time is 1–30 min (preferably 5–15 min). Then, the temperature is raised to 200–250°C, and the pressure is 0.3–1.0 MPa, under which adipic acid and hexamethylenediamine are further added. Add 50% of the remaining hexamethylenediamine (calculated in equimolar ratio) and react for 1–4 hours. Continue heating to 260–280°C, add 51% of the remaining hexamethylenediamine (calculated in equimolar ratio of adipic acid to hexamethylenediamine), and react for 1–2 hours. Maintain the pressure inside the reactor at 1.0–2.0 MPa by venting water vapor. Then maintain the temperature or continue heating to 280–300°C and continuously release the pressure to atmospheric pressure over 0.5–2 hours. Apply vacuum at a pressure of 10–40 kPa for 5–30 minutes. Pulverize and cut into pellets to obtain polyamide 66.

[0032] In another embodiment, the method for preparing polyamide 66 includes the following steps:

[0033] Adipic acid and hexamethylenediamine, with an initial molar ratio of 1:5 to 1:1.1, are added to a reaction vessel and mixed. During mixing, a catalyst, stabilizer, and molecular weight regulator are added to induce salt formation. The reaction temperature is 150–190°C (preferably 150–170°C), the pressure is 0.1–0.3 MPa (preferably 0.2–0.25 MPa), and the reaction time is 1–30 min (preferably 5–15 min). Then, the temperature is raised to 210–260°C, and the pressure is 0.3–1.0 MPa, under which adipic acid and hexamethylenediamine are further added. Add 50% of the remaining adipic acid (calculated in equimolar ratio) and react for 1–4 hours. Continue heating to 260–280°C, add 49% of the remaining adipic acid (calculated in equimolar ratio of adipic acid to hexamethylenediamine), and react for 1–2 hours. Maintain the pressure inside the reactor at 1.0–2.0 MPa by venting water vapor. Then maintain the temperature or continue heating to 280–300°C and continuously release the pressure to atmospheric pressure over 0.5–2 hours. Apply vacuum at a pressure of 10–40 kPa for 5–30 minutes. Pulverize and granulate to obtain polyamide 66.

[0034] Secondly, the present invention provides a polyamide 66, which is prepared by the preparation method described above.

[0035] Thirdly, the present invention provides an application of polyamide 66 in engineering plastics or spinning.

[0036] Spinning includes, but is not limited to, industrial yarn and civilian yarn.

[0037] Beneficial effects:

[0038] 1. The preparation method of this invention is intermittent, which is flexible in operation, has a short production cycle, and is convenient for switching grades.

[0039] 2. In traditional processes, the salt solution is prone to oxidation, yellowing, and decomposition when the salt concentration exceeds 60%. However, this invention uses solid raw materials and does not add water, which significantly accelerates the reaction rate from raw materials to salt formation and from salt formation to prepolymer compared to existing methods that add water. Therefore, a prepolymer that is not easily oxidized or decomposed can be formed quickly, which can effectively prevent the volatilization of hexamethylenediamine and avoid oxidation, yellowing, decomposition reactions and the generation of by-products of high-concentration salt, thereby effectively controlling and reducing the subsequent gel formation.

[0040] 3. Because no water is added to the raw materials, the liquid level change in the batch reactor is small, greatly reducing the probability of gelation and black spot formation, thus improving product quality. The polyamide 66 polymer prepared by this method has advantages such as good mechanical properties, stable thermal properties, low gel content, and few black spots, making it suitable for a wide range of fields, especially the spinning industry.

[0041] The present invention has been described in detail above; however, the above embodiments are merely illustrative in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments.

[0042] Unless otherwise expressly stated, numerical ranges throughout the application include any subranges therein and any numerical values ​​incremented by the smallest subunit of a given value. Unless otherwise expressly stated, numerical values ​​throughout the application represent an approximate measure or limitation of the range of embodiments including minor deviations from a given value and having approximately the mentioned value as well as having the mentioned precise value. Except in the detailed description of the working embodiments provided at the end, all numerical values ​​of parameters (e.g., quantities or conditions) in this application (including the appended claims) should in all cases be understood to be modified by the term “approximately,” regardless of whether “approximately” actually precedes the numerical value. “Approximately” indicates that the stated numerical value allows for slight inaccuracies (some close to precision at that value; approximately or reasonably close to the value; approximate). If the inaccuracy provided by “approximately” is not understood in this common sense in the art, then “approximately” as used herein at least indicates a variation that can be produced by common methods of measuring and using these parameters. For example, “approximately” can include variations less than or equal to 10%, less than or equal to 5%, less than or equal to 4%, less than or equal to 3%, less than or equal to 2%, less than or equal to 1%, or less than or equal to 0.5%. Detailed Implementation

[0043] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0044] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0045] Example 1 (Diic acid and diamine fed in equal proportions)

[0046] Step (1): 730.7g adipic acid and 581g hexamethylenediamine are added to a 5L polymerization reactor. The initial temperature of the polymerization reactor is 200℃. At the same time, acetic acid, copper acetate and sodium hypophosphite are added to the polymerization reactor. The amounts of the three added are 2.62g, 0.66g and 2.62g respectively.

[0047] Step (2): under the conditions of reaction temperature of 200℃ and pressure of 0.15MPa, react for 5 minutes, and then heat up to 230℃ and pressure of 1.2~1.4MPa and keep warm for 2 hours.

[0048] Step (3): Continue heating to 260℃, and maintain the pressure inside the reactor at 1.5-1.7MPa by venting the water vapor inside the reactor, and react under constant pressure for 1 hour.

[0049] Step (4): Continue heating to 280℃ and continuously depressurize to atmospheric pressure within 0.5h.

[0050] Step (5): Vacuum is drawn, pressure is 20-30 kPa, time is 5-10 min, and the product is stretched and granulated to obtain polyamide 66 polymer.

[0051] Example 2 (Initially, acid was in excess; the remaining amine was gradually added).

[0052] Step (1): Add 730.7g of adipic acid and 194.05g of hexamethylenediamine to the polymerization reactor. The initial temperature of the polymerization reactor is 150℃. At the same time, add acetic acid, copper acetate and sodium hypophosphite to the polymerization reactor. The amounts of the three added are 2.62g, 0.66g and 2.62g respectively. The reaction time is 5min.

[0053] In step (2), the temperature was raised to 240°C and the pressure was 0.8-1.0 MPa. 193.47 g of hexamethylenediamine was added and the reaction was carried out for 1.5 h.

[0054] In step (3), the temperature was raised to 275°C, and 197.31 g of hexamethylenediamine was added. The reaction was allowed to proceed for 1 hour. The pressure inside the reactor was maintained at 1.5–1.7 MPa by removing water vapor from the reactor.

[0055] Step (4): Continue heating to 280℃ and continuously depressurize to atmospheric pressure within 0.5h.

[0056] Step (5): Vacuum is drawn, pressure is 20-30 kPa, time is 5-10 min, and the product is stretched and granulated to obtain polyamide 66 polymer.

[0057] Example 3 (Initially, amine was in excess; the remaining acid was added gradually thereafter)

[0058] Step (1): Add 244.05g of adipic acid and 592.62g of hexamethylenediamine to the polymerization reactor. The initial temperature of the polymerization reactor is 180℃. At the same time, add acetic acid, copper acetate and sodium hypophosphite to the polymerization reactor. The amounts of the three added are 2.62g, 0.66g and 2.62g respectively. The reaction time is 5min.

[0059] In step (2), the temperature was raised to 250℃ and the pressure was 0.9-1.0MPa. 243.32g of adipic acid was added and the reaction was carried out for 1.5h.

[0060] In step (3), the temperature is raised to 280℃, and 243.32g of adipic acid is added. The reaction is carried out for 1 hour. The pressure inside the reactor is maintained at 1.5-1.7MPa by venting the water vapor inside the reactor.

[0061] Step (4): Continue to keep warm at 280℃ and continuously release pressure to normal pressure within 0.5h.

[0062] Step (5): Vacuum is drawn, pressure is 20-30 kPa, time is 5-10 min, and the product is stretched and granulated to obtain polyamide 66 polymer.

[0063] Comparative Example 1 (blank) (salt polymerization upon addition of water)

[0064] Step (1), salt formation reaction: adipic acid, hexamethylenediamine, sodium hypophosphite, copper acetate, acetic acid and water were added to a 5L polymerization reactor in the following amounts: 730.7g, 581g, 2.62g, 0.66g, 2.62g and 1311.7g respectively. The mixture was kept at 50℃ for 1 hour.

[0065] Step (2): Heat to 200℃, control the pressure at 1.4~1.6MPa, and react for 2h.

[0066] Step (3): Continue heating to 275-280℃, maintain the pressure inside the reactor at 1.7-1.8MPa by venting the steam inside the reactor, react at constant pressure for 1.5h, and then continuously release the pressure to atmospheric pressure within 30min.

[0067] Step (4): Maintain the temperature at 275-280℃, evacuate for 5-10 minutes, control the vacuum degree to be less than 500Pa, observe the current to reach the specified value, discharge, granulate, and dry to obtain polyamide 66 polymer.

[0068] Performance Evaluation

[0069] Mechanical performance testing:

[0070] The tests were conducted according to the standards GB / T1042-92 for tensile strength and GB / T1843-1996 for impact.

[0071] Melting point and crystallization temperature tests:

[0072] Refer to JBT8630-1997, "Test method for determining the heat of fusion, melting point, heat of crystallization, and crystallization temperature of electrical insulating materials by differential scanning calorimetry".

[0073] Gel content test:

[0074] (1) Take a certain mass of M0 nylon 66 resin slices to be tested and dissolve them in 98% anhydrous formic acid for 24 hours.

[0075] (2) Select a microporous filter membrane of appropriate specifications with a minimum pore size of 0.22 μm. Soak it in formic acid solution until the mass is constant. The mass of the filter membrane is recorded as M1.

[0076] (3) Install a sand core filter and a circulating water vacuum pump. During the filtration process, it is necessary to prevent contamination by foreign matter, and when the filtrate level is low, add a certain amount of formic acid and repeat several times.

[0077] (4) After filtration, rinse with a certain amount of deionized water and anhydrous ethanol, remove the filter membrane and dry it to constant weight. Its mass is recorded as M2.

[0078] The amount of gel, G, is calculated using the following formula: G = (M2 - M1) / M0 × 100%.

[0079] Number of black spots: Visual inspection method, take 1000g of sample and visually inspect the number of black spots in the particles.

[0080] The results are shown in Table 1.

[0081] Table 1

[0082]

[0083] 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 polyamide 66, characterized in that, Includes the following steps: (1) Take the initial amount of solid adipic acid and hexamethylenediamine and add them to the reaction vessel to mix. During the mixing process, add the catalyst, stabilizer and molecular weight regulator to mix and form a salt. The molar ratio of adipic acid to hexamethylenediamine is 5:1 to 1:

5. The catalyst is selected from one or more of sodium phosphate, magnesium phosphate, calcium phosphate, magnesium phosphite, calcium phosphite, zinc phosphite, and sodium hypophosphite. The stabilizer is selected from one or more of cuprous iodide, copper acetate, manganese acetate, and potassium iodide; The molecular weight regulator is selected from one or more of benzoic acid, terephthalic acid, stearic acid, acetic acid, and propionic acid; The salt-forming temperature is 120–220℃; the salt-forming pressure is 0.1–0.3 MPa; and the salt-forming time is 1–30 min. (2) Continue heating to carry out polymerization. During the polymerization process, the remaining amount of adipic acid and / or hexamethylenediamine may be added to obtain polyamide 66. Polymerization includes prepolymerization and final polymerization; The prepolymerization temperature is 200–260℃, the prepolymerization pressure is 0.3–1.6 MPa, and the prepolymerization time is 1–4 h. The final polymerization consists of two stages. In the first stage, the temperature is raised to 260–280°C, and the pressure inside the reactor is maintained at 1.0–2.0 MPa by venting water vapor. The constant pressure reaction lasts for 0.5–2 hours. In the second stage, the temperature is maintained or raised to 280–300°C. The pressure is continuously released to atmospheric pressure over 0.5–2 hours, and a vacuum is drawn at a pressure of 10–40 kPa for 5–30 minutes. The final molar ratio of adipic acid to hexamethylenediamine is 1:1.001 to 1.1, and the initial amount of adipic acid and hexamethylenediamine accounts for 70-100% of the final amount. Based on the final amounts of adipic acid and hexamethylenediamine, the amount of catalyst added is 0.1-0.3 wt%; the amount of stabilizer added is 0.05-0.5 wt%; and the amount of molecular weight regulator added is 0.1-0.5 wt%.

2. The preparation method according to claim 1, characterized in that, The final molar ratio of adipic acid to hexamethylenediamine is 1:1.005 to 1.05; the initial amount of adipic acid and hexamethylenediamine accounts for 70-80% of the final amount.

3. The preparation method according to claim 1, characterized in that, The catalyst is sodium hypophosphite; and / or The stabilizer is a combination of copper acetate and potassium iodide; and / or the molecular weight regulator is acetic acid.

4. The preparation method according to claim 1, characterized in that, The method includes one of the following methods: Method 1: Adipic acid and hexamethylenediamine with an initial molar ratio of 1:1 are added to a reaction vessel and mixed. During the mixing process, a catalyst, stabilizer, and molecular weight regulator are added to form a salt. The reaction temperature is 160–220 °C, the pressure is 0.1–0.3 MPa, and the reaction time is 1–30 min. Then, the temperature is raised to 230–260 °C and the pressure is maintained at 1.1–1.6 MPa for 1–4 h. The temperature is then raised to 260–280 °C, and the pressure inside the vessel is maintained at 1.5–2.0 MPa by venting water vapor. The constant pressure reaction is carried out for 0.5–2 h. Then, the temperature is maintained or the temperature is raised to 280–300 °C and the pressure is continuously released to atmospheric pressure over 0.5–2 h. Vacuum is then applied at a pressure of 10–40 kPa for 5–30 min. The mixture is then stretched and granulated to obtain polyamide 66. Method 2: Adipic acid and hexamethylenediamine, with an initial molar ratio of 5:1 to 1.1:1, are added to a reaction vessel and mixed. During the mixing process, a catalyst, stabilizer, and molecular weight regulator are added to induce salt formation. The reaction temperature is 120–160 °C. The reaction was carried out at ℃ and pressure of 0.1-0.3 MPa for 1-30 min; then the temperature was raised to 200-250℃ and the pressure was 0.3-1.0 MPa, and 50% of the remaining hexamethylenediamine (calculated in the equimolar ratio of adipic acid to hexamethylenediamine) was added, and the reaction was carried out for 1-4 h; the temperature was raised to 260-280℃ and 51% of the remaining hexamethylenediamine (calculated in the equimolar ratio of adipic acid to hexamethylenediamine) was added, and the reaction was carried out for 1-2 h. The pressure inside the reactor was maintained at 1.0-2.0 MPa by venting water vapor from the reactor. Then the temperature was maintained or the temperature was raised to 280-300℃ and the pressure was continuously released to atmospheric pressure over 0.5-2 h. Vacuum was drawn at a pressure of 10-40 kPa for 5-30 min, and the mixture was stretched and granulated to obtain polyamide 66. Method 3: Adipic acid and hexamethylenediamine, with an initial molar ratio of 1:5 to 1:1.1, are added to a reaction vessel and mixed. During mixing, a catalyst, stabilizer, and molecular weight regulator are added to induce salt formation. The reaction temperature is 150–190 °C. The reaction was carried out at ℃ and pressure of 0.1-0.3 MPa for 1-30 min; then the temperature was raised to 210-260℃ and the pressure was 0.3-1.0 MPa, and 50% of the remaining adipic acid (calculated as an equimolar ratio of adipic acid to hexamethylenediamine) was added, and the reaction was carried out for 1-4 h; the temperature was further raised to 260-280℃, and 49% of the remaining adipic acid (calculated as an equimolar ratio of adipic acid to hexamethylenediamine) was added, and the reaction was carried out for 1-2 h. The pressure inside the reactor was maintained at 1.0-2.0 MPa by venting water vapor from the reactor. Then the temperature was maintained or the temperature was raised to 280-300℃ and the pressure was continuously released to atmospheric pressure over 0.5-2 h. Vacuum was then applied at a pressure of 10-40 kPa for 5-30 min, and the mixture was stretched and granulated to obtain polyamide 66.

5. The preparation method according to claim 1, characterized in that, In Method 1, the salt formation reaction temperature is 180–200℃, the pressure is 0.2–0.3 MPa, and the reaction time is 5–15 min; In Method 2, the preferred salt formation reaction temperature is 120–150℃, the pressure is 0.15–0.2 MPa, and the reaction time is 5–15 min. In Method 3, the salt formation reaction temperature is 150–170℃, the pressure is 0.2–0.25 MPa, and the reaction time is 5–15 min.

6. A polyamide 66, prepared by the method for preparing polyamide 66 according to any one of claims 1-5.

7. The use of the polyamide 66 of claim 6 in engineering plastics or spinning.

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